Studies in Surface Science and Catalysis 16 PREPARATION OF CATALYSTS III Scientific Bases for the Preparation of Heterogeneous Catalysts
Studies in Surface Science and Catalysis Volume
1
Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium held at the Solvay Research Centre, Brussels, October 14-17, 1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet
Volume 2
The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon
Volume 3
Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-Ia-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet
Volume 4
Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon
Volume
Catalysis by Zeolites. Proceedings of an International Symposium organized by the Institut de Recherches sur la Catalyse - CNRS - Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud
5
Volume 6
Catalyst Deactivation. Proceedings of the International Symposium, Antwerp, October 13-15, 1980 edited by B. Delmon and G.F. Froment
Volume 7
New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, 30 June-4 July 1980 edited by T. Seiyama and K. Tanabe
Volume 8
Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov
Volume 9
Physics of Solid Surfaces. Proceedings of the Symposium held in Bechylle, Czechoslovakia, September 29-0ctober 3, 1980 edited by M. Laznil!ka
Volume 10
Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium held in Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing
Volume 11
Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium organized by the Institut de Recherches sur la Catalvse - CNRS Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine
Volume 12
Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.1. Jager, P. Jiru and G. Schulz-Ekloff
Volume 13
Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard
Volume 14
Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, California, U.S.A.,1-4 September 1982 edited by C.R. Brundle and H. Morawitz
Volume 15
Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.1. Golodets
Volume 16
Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-Ia-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs
Studies in Surface Science and Catalysis 16
PREPARATION OF CATALYSTSm Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the Third International Symposium, Louvain-Ia-Neuve, September 6-9,1982
Editors G. Poncelet and P. Grange Universite Catholique de Louvain, Groupe de Physico-Chimie Minerale et de Catalyse, Louvain-Ia-Neuve, Belgium
and
P.A. Jacobs Katholieke Universiteit Leuven, Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Heverlee, Belgium
ELSEVIER Amsterdam - Oxford - New York 1983
ELSEVIER SCIENCE PUBLISHERS B.V. Molenwerf 1 P.O. Box 211, 1000 AE Amsterdam, The Netherlands
Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, NY 10017
ISBN 0-44442184-X (Vo\' 16) ISBN 0444-41801-6 (Series)
© Elsevier Science Publishers B.V., 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers BV., P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Printed in The Netherlands
v CONTENTS
Organizing Committee Foreword Acknowledgements Financial Support
v IX XI XIII XV
Production and thermal pretreatment of supported catalysts (J.W. Geus) Theoretical and experimental aspects of catalyst impregnation (S. Y. Lee and R. Aris)
35
Competitive adsorption of H2PtC16 and HCl on A1203 in the preparation of naphtha reforming catalysts (A.A. Castro, O.A. Scelza, E.R. Benvenuto, G.T. Baronetti, S.R. De Miguel and J.M. Parera)
47
The role of competitive adsorbate in the impregnation of platinum in pelleted alumina support (Wang Jianguo, Zhang Jiayu and Pang Li)
57
The influence of solvent nature of chloroplatinic acid used for support impregnation on the distribution, dispersity and activity of platinum hydrogenation catalysts (V. Machek, J. Hanika, K. Sporka, V. Ruzicka, J. Kunz and L. Janacek)
69
Influence of the various activation steps on the dispersion and the catalytic properties of platinum supported on chlorinated alumina (J.P. Bournonville, J.P. Franck and G. Martino)
81
Preparation and properties of platinum crystallites supported on polycrystalline tin oxide (G.B. Hoflund)
91
Synthesis and properties of Pt-Sn/A1203 catalysts by the method of molecular deposition (D.P. Damyanov and L.T. Vlaev)
101
Production of silver bimetallic catalysts by liquid-phase reduction (K.P. de Jong and J.W. Geus)
111
Preparation and characterisation of highly dispersed palladium catalysts on low surface alumina. Their notable effects in hydrogenation (J.P. Boitiaux, J. Cosyns and S. Vasudevan)
123
Preparation of colloidal particles of small size and their catalytic effect in redox processes induced by light (J. Kiwi, E. Borgarello, D. Duonghong and M. Gratzel)
135
Synthesis, surface reactivity and catalytic activity of high specific surface area molybdenum nitride powders (L. Volpe, S.T. Oyama and M. Boudart)
147
The preparation of ceramic-coated metal-based catalysts (C.J. Wright and G. Butler)
159
The design of pores in catalytic supports. Magnesia-aluminaaluminum phosphate (G. Marcelin, R.F. Vogel and W.L. Kehl)
169
Dispersed-metal/oxide catalysts prepared by reduction of high surface area oxide solid solutions (J.G. Highfield, A. Bossi and F.S. Stone)
181
Preparation of monodispersed nickel boride catalysts using reversed micellar systems (J.B. Nagy, A. Gourgue and E.G. Derouane)
193
Thioresistant flammable gas sensing elements (S.J. Gentry and P.T. Walsh)
203
VI The formation of active component layer in coated catalysts (R. Haase, U. Illgen, G. Ohlmann, J. Richter-Mendau, J. Scheve and 1. Schulz)
213
Preparation of highly active composite oxides of silver for hydrogen and carbon monoxide oxidation (M. Haruta and H. Sano)
225
The effect of preparation method upon the structures, stability and metal/support interactions in nickel/alumina catalysts (D.C. Puxley, I.J. Kitchener, C. Komodromos and N.D. parkyns)
237
An assessment of the influence of the preparation method, the nature of the carrier and the use of additives on the state, dispersion and reducibility of a deposited "nickel oxide" phase (M. Houalla)
273
Thermally and mechanically stable catalysts for steam reforming and methanation. A new concept in catalyst design (K.B. Mok, J.R.H. Ross and R.M. Sambrook)
291
Synthesis of methanation catalysts by deposition-precipitation (H. Schaper, E.B.M. Doesburg, J.M.C. Quartel and L.L. van Reijen)
301
Preparation of titania-supported catalysts by ion exchange, impregnation and homogeneous precipitation (R. Burch and A.R. Flambard)
311
Influence of phosphorus on the HDS activity of Ni-Mo!y-AI203 catalysts (D. Chadwick, D.W. Aitchison, R. Badilla-Ohlbaum and L. Josefsson)
323
Study of the influence of the preparation conditions on the final properties of a HDS catalyst (C.V. Caceres, M.N. Blanco and H. J. Thomas)
333
The evolution of Co species on the surface of y-A1203 and Si0 2 modified with the pre-transition cations (A. Lycourghiotis)
343
Criteria for the evaluation of bauxite as carrier for low-cost hydrotreating catalysts (S. Marengo, A. Iannibello and A. Girelli)
359
Preparation and properties of supported liquid phase catalysts for the hydroformylation of alkenes (H.L. Pelt, L.A. Gerritsen, G. van der Lee and J.J.F. Scholten)
369
Role of the metal-support interaction in the preparation of Fe!MgO catalysts (H. Mousty, B.S. Clausen, E.G. Derouane and H. TopsiVe)
385
New Fischer-Tropsch catalysts of the aerogel type (F. Blanchard, B. Pommier, J.P. Reymond and S.J. Teichner)
395
Selective doping of a carbon substrate transition-metal ammonia catalyst (F.F. Gadallah, R.M. Elofson, P. Mohammed and T. Painter)
409
A study of the preparation and properties of precipitated iron catalysts for ammonia synthesis (D.G. Klissurski, loG. Mitov and T. Tomov)
421
Influence of the preparation technique of pd-silica catalysts on metal dispersion and catalytic activity (G. Gubitosa, A. Berton, M. Camia and N. Pernicone)
431
Preparation of non-pyrophoric metallic catalysts (A.V. Krylova, G.A. Ustimenko and N.S. Torocheshnikov)
441
Effect of metal-support interaction on the chemisorption and CO hydrogenation activity of FeRu catalysts (L. Guczi, Z. schay and I. Bogyay)
451
VII
The palladium alumina systenl: influence of the preparation procedures on the structurp of tbe metallic phase (S. Vasudevan, J. Cosyns, E Lesage, E. Freund and H. Dexpert)
463
Bimetallic supported catalysts prepared via metal adsorption. Preparation and catalytic activity of Pd-Pt/Al203 catalysts (J. Margitfalvi, S. Szab6, F. Nagy, S. G6bolos and M. Hegedus)
473
A scientific approach to the preparation of bulk mixed oxide catalysts (Ph. courty and Ch. Marcilly)
485
Deposition of ternary oxides as active components by impregnation of porous carriers (M. Kotter, L. Riekert and F. weyland)
521
Effect of support and preparation on structure of vanadium oxide catalysts (Y. Murakami, M. Inomata, K. Mori, T. Ui, K. Suzuki, A. Miyamoto and T. Hattori)
531
Structural modifications of V-P mixed oxides during calcination in air or in a mixture of butenes-air (G. Genti, C. Galassi, 1. Manenti, A. Riva and F. Trifiro)
543
Method of impregnation with transition metal alkoxides. Vanadium-alumina and vanadium-silica systems (M. Glinski and J. Kijenski)
553
The significance of the mullite phase in a silver catalyst for the oxidation of ethylene into ethylene oxide (Lin Bing-Xiong, Zhang Wan-Jing, Yan Qing-Xin, Pan Zuo-Hua, Gui Lin-Lin and Tang You-Chi)
563
Preparation of active carbon supported oxidation catalysts (J.L. Figueiredo, M.C.A. Ferraz and J.J.M. Orfao)
571
Influence of the preparation variables on the activity and on the mechanical properties of an industrial catalyst for the propylene oxidation to acrylic acid (R. Covini, C. D'Angeli and G. Petrini)
579
Design and preparation of hydrocracking catalysts
587
(J.W. Ward)
Influence of sodium chloride on the catalytic properties of tellurium-loaded Y-zeolites (B.E. Langner and J.H. Kagon)
619
Control of the pore structure of pcrous alumina (T. Ono, Y. Ohguchi and 0. Togari)
631
The properties of commercial alumina base materials and their effect on the manufacture of active porous alumina supports by means of extrusion (W. Stoepler and K.K. Unger)
643
Influence of aluminium hydroxide peptization on physical properties of alumina extrudates (K. Jiratova, L. Janacek and P. Schneider)
653
Formation of silica gel porous structure (V.A. Fenelonov, V.Yu. Gavrilov and L.G. Simonova)
665
Study of the preparation of iron catalysts for liquefaction of coal by hydrogenation under pressure (M. Andres, H. Charcosset, P. Chiche, G. Djega-Mariadassou, J.P. Joly and S. Pregermain)
675
Impregnation of y-alumina with copper chloride. Equilibrium behaviour, impregnation profiles and immobilization kinetics (R.J. Ott and A. Baiker)
685
Preparation of copper supported on metal oxides and methanol steam reforming reaction (H. Kobayashi, N. Takezawa, M. Shimokawabe and K. Takahashi)
697
VIII
Effect of preparation methods and promoters on activity and selectivity of Cu-ZnO-A1203-K catalysts in aliphatic alcohols synthesis from CO and H2 (C.E. Hofstadt, M. Schneider, 0. Bock and K. Kochloefl)
709
Preparation of Cu-Zn-Al mixed hydroxycarbonates precursors of catalysts for the synthesis of methanol at low pressure (P. Gherardi, 0. Ruggeri, F. Trifiro, A. Vaccari, G. Del Piero, G. Manara and B. Notari)
723
Preparation and characterization of very active Cu/ZnO and Cu/ZnO/Al203 LTS catalysts using a single phase CU-Zn precursor compound (G. Petrini, F. Montino, A. Bossi and F. Garbassi)
735
Cu/kieselguhr catalysts for hydration of acrylonitrile (E. Nino, A. Lapena, J. Martinez, J.M. Gutierrez, S. Mendioroz, J.L.G. Fierro and J.A. Pajares)
747
Phase-structural characteristics of the oxide systems in their first stage of preparation using one of the ingredients of the catalyst compounds as a precipitating reagent (D.S. Shishkov, N.A. Kassabova and K.N. Petkov)
757
MINI SYMPOSIUM ON CATALYST NORMALIZATION Standardization of catalyst test methods (R.J. Bertolacini and A. Neal)
767
Progress report on the Committee on Reference Catalyst, Catalysis Society of Japan (Y. Murakami)
775
The SCI/IUPAC/NPL standard nickel-silica catalyst (R. Burch, A.R. Flambard, M.A. Day, R.L. Moss, N.D. parkyns, A. Williams J.M. Winterbottom and A. White)
787
Research group on catalysis, Council of Europe. Standard catalyst projects (J.W.E. Coenen and P.B. Wells)
801
Progress report of BCR activity in surface area and pore size reference materials (N. Pernicone)
815
Standardization of procedures for determination of activity and selectivity of commercial catalysts by comparative kinetic investigations in different laboratory reactors (M. Baerns and H. Hofmann)
821
List of participants
833
Author index
853
IX ORGANIZING COMMITTEE
President
Prof. B. DELMON, Universite Catholique de Louvain
Executive Chairmen
Dr. P. GRANGE, Universite Catholique de Louvain Dr. P.A.JACOBS, Katholieke Universiteit Leuven Dr. G. PONCELET, Universite Catholique de Louvain
Secretaries
Dr. J.M. DIEZ TASCON, U.C.L., Belgium Dr. B.K. HODNETT, U.C.L., Belgium
Scientific Committee
Dr. J.P. BRUNELLE, Rhone Poulenc, France Dr. R. CANDIA, Haldor Tops¢e, Denmark Prof. B. DELMON, U.C.L., Belgium Dr. U. DETTMEIER, Hoechst A.G., Germany Dr. P. GRANGE, U.C.L., Belgium Dr. P.A. JACOBS, K.U.L., Belgium Dr. H. NEUKERMANS,Catalysts & Chemicals Europe,Belgium Dr. N. PERNICONE, 1st. G. Donegani (Montedison), Italy Dr. G. PONCELET, U.C.L., Belgium Dr. J. SONNEMANS, AKZO Chemie, The Netherlands Dr. S. VIC BELLON, Enpetrol, Spain
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XI FOREWORD
There have now been three Symposia on the Scientific Bases for the Preparation of Heterogeneous Catalysts. The First Symposium, held in 1975, was organized in order to gather together experts from both Universities and Industry to discuss the scientific problems involved in the preparation of real, industrially used, heterogeneous catalysts. In spite of the heterogeneity of the subjects treated, the large international response demonstrated the great interest held by Universities as well as Industry in these topics. Although initially it was not the aim of the organizers to produce a series of symposia, it became clear that the First symposium had to be followed by a Second one, and that a smaller number of scientific domains concerned with the preparation of industrial catalysts should be considered. The Scientific Committee composed of representatives of Industry, Universities and Research Institutes decided, therefore, to focus the Second Symposium on two unit processes in catalyst preparation, namely impregnation and activation of supported catalysts. At the same time it appeared that normalization of catalyst test methods was a field of significant current concern. For the Second Symposium, 36 communications were selected (out of 102 submitted) that best fitted the proposed topics; 4 plenary lectures and 3 extended communications introduced the different sessions dealing with the preparation and activation of distinct groups of catalysts. In the concluding remarks of the Second symposium, the great majority of the
participants appeared to believe that the number of topics concerned with the preparation of real industrial catalysts that had not been dealt with justified a further symposium. Accordingly, when arranging the programme of the Third Symposium, the aim of the Scientific Committee was to organize self-consistent and related sessions. Therefore, the Third Symposium was devoted to reforming, hydrogenation, selective oxidation, HDS ... and also new trends in catalyst preparation. A special session on normalization of catalyst test methods was also scheduled. Out of 150 high quality papers originally proposed, 58 were selected by the Scientific Committee, keeping in mind the need for a balanced program centered on the chosen topics. The sessions were introduced by an invited lecturer. As for the previous symposia, 300 participants, representing 33 countries, attended the Third Symposium, 60% of the audience belonging to Industry, and 30% of the accepted papers coming from industrial laboratories.
XII
Twenty-one companies showed their interest in this symposium by sponsoring the social events. The increasing number of submitted papers, the steady number of participants, and the constant proportion of participants and accepted papers from Industry confirmed the view of the local organizers that fundamental research on the various and inter-disciplinary aspects involved in the preparation of industrial heterogeneous catalysts, as they have been treated by the three Symposia, corresponds to a real need and seems to be much appreciated by Industry. As a result of the demand for the continuation of such Symposia, as expressed by the participants to the organizers, there will hopefully be a Fourth Symposium organized for 1986.
P. GRANGE P.A. JACOBS G. PONCE LET
Xli ACKNOWLEDGEMENTS
The Organizing Committee is greatly indebted to Mgr. E. Massaux, Rector of the Universite Catholique de Louvain, who agreed that this Symposium could again be held in Louvain-la-Neuve, and for all facilities provided by the university. We thank very sincerely Prof.
B.
Delmon, who initiated this series of sym-
posia, for the major role he played in the assessment of the scientific content of this symposium. The organizers also greatly appreciate the constructive comments of several authorities in the field of catalyst preparation whose suggestions were largely responsible for establishing the base and scope of this Third Symposium. In this respect, we are particularly grateful to Dr. P. Andreu (INTEVEP), Dr. S.P.S. Andrew (I.C.I.), Dr. J.P. Brunelle (Rhone Poulenc), Dr. H. Charcosset (I.R.C.), Prof. J.W.E. Coenen (Unilever), Dr. G. Martino (I.F.P.), Prof. E. Matijevic (Clarkson Institute), Dr. L. Moscou (AKZO), Dr. N. Pernicone (Montedison), Prof. J. Petro (Techn. Univ. Budapest), Dr. S.T. Sie (Shell), Prof. D.L. Trimm (Univ. New South Wales), Dr. D.A. Whan
(Univ. Edinburgh).
The local organizers convey special thanks to the members of the Scientific Committee who not only selected the papers with outstanding care and conscientiousness but also, acting as chairmen, stimulated the discussion and enabled the sessions to run smoothly. The success of the minisymposium on Catalyst Normalization is due to Dr. N. Pernicone, who alone handled its organization.
The Organizing Committee is most
obliged to him and to those who contributed to this very fruitful session. The plenary lectures given by Prof. J.W. Geus, Dr. D.C. Puxley and Dr. J. Ward were most stimulating.
The Organizers gratefully acknowledge these authors
and congratulate them for achieving with such competence a difficult goal. Two excellent extended communications were given by Prof. J.J.F. Scholten and Dr. P. Courty.
Our thanks go to these authors.
The Organizing Committee acknowledges the authors of the 150 papers that were submitted for presentation at this Symposium.
Special thanks are due to
the authors of the papers included in the present Proceedings. Our congratulation and gratitude go, once more, to the team of hostesses of the REUL (Relations Exterieures de l'Universite de Louvain) , headed by Mrs F. Bex, and to Mr. J. Therrer, of the Service du Logement, for their devoted and enthusiastic assistance. The Organizing Committee wants to associate with the acknowledgements all those of the Groupe de Physico-Chimie Minerale et de Catalyse and of the Laboratorium voor Colloidale en Oppervlakte Scheikunde, K.U.L., who contributed in various degrees to the success of this symposium: C. Ancion, B. Arias, T. J.L. Dalons, M. Gennen, P. Jacques, J.P. Marcq, J. Martens, F. Melo-Faus,
Bein,
XIV M. Montes, N. Mazes, C. Pierard, D. Pirotte, M. Ruwet, A. Schutz, R. Sosa Hermandez, P. Struyf, M. Tielen, D. Van Wouwe, B. Vidick, K. Willemen, and WU Qin. Finally, we all owe our deepest appreciation to R.M. Torres and to M. O'Callghan, and more particularly to the secretaries, B.K. Hodnett and M. Diez-Tascon, who, from the very beginning right until the very end, took care of the unavoidable and least challenging parts of the organization of the symposium.
P. GRANGE
P.A. JACOBS G. PONCELET
xv FINANCIAL SUPPORT
The Organizing Committee gratefully acknowledges the financial guarantee of the "Fonds National de la Recherche Scientifique" and the "Ministere de l'Education Nationale et de la Culture Fran,
Ltd., England
procatalyse, France Societe de la Vieille Montagne, Belgium Solvay, Belgium Sud-Chemie A.G., Germany The Standard Oil Company (Sohio), U.S.A. U.C.B., Belgium Unilever N.V., The Netherlands Union Carbide Corporation, U.S.A. Universal-Matthey Products Ltd, England. We also thank Hoechst Belgium for contributing some of the conference folders and Ondah Belgium for its assistance in the organization of the cocktail party. ies:
We would also like to express our appreciation to the following brewerAbbaye de Chimay, Abbaye d'Orval, Moorgat and Palm for their generous
supply of some of Belgium's finest products!
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1
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PRODUCfION
and THERMAL PRETREA1MENT of" SUPPORTED CATALYSTS
J.W. Geus Dept. Inorganic Chemistry, University of Utrecht, Netherlands
ABSTRACT After methods
a review of the requirements solid catalysts have to meet, to produce supported catalysts are dealt with.
deficiencies
of
precipitation
the
Consideration
presently used procedures leads to the
of
conclusion
an active precursor onto a separately prepared
of
the usual
carrier
the that can
provide the best results. The cussed.
theoretical and practical aspects of deposition-precipitation are The special methods developed are surveyed.
with deposition-precipitation of Ni,
Fe,
Subsequent sections
disdeal
and Cu on silica and on alumina car-
riers. Finally
some pitfalls to be avoided when applying
deposition-precipitation
to produce supported catalysts are mentioned.
Introduction Only chemical
in
the presence of catalysts the rate of many industrially
reactions
is sufficiently high.
separated from the reaction products.
As solid catalysts can
important be
utilisation of solid catalysts
readily predomi-
nates in the chemical industry. Since the
the reactions are accelerated at the surface of the solid
catalysts,
main problem seems to be to provide a surface of the desired activity
selectivity.
The
and
specific activity of the surface determines the surface area
per unit volume that must be produced to achieve an economical rate of
produc-
tion. However, an efficient contact of a flow of reactants with a catalytically active surface calls for additional properties of solid catalysts.
As a result
the
conflicting
production
of
solid catalysts has to deal with a number
of
demands which will now be surveyed.
Requirements of Solid Catalysts (1.2.3.4) Two types of catalytic reactors are generally utilized:
fixed and fluidized
bed reactors. Catalyst pellets to be used in a fixed bed reactor have generally to
be at least about 1 mm to restrict the pressure drop over the catalyst
to acceptable values.
bed
With very small catalyst particles ,moreover. channeling
2 may occur. of
in which the reactants flow through a fraction of the cross-section
the reactor where the particles of the catalyst are moving.
catalyst
are Vigorously moving.
Now a
limited
In a fluid bed reactor all particles of the
fraction of the catalyst is used.
The friction of the fluid flowing through the
reactor is lifting the catalyst bed. The particles of a fluid bed catalyst must be
at least about
which is much smaller than the minimum dimension
30~,
fixed
bed catalyst pellets.
tance
is a prerequisite.
For a fluid bed catalyst a high attrition
Fixed bed catalysts must also have a sufficient
chanical
strength
to withstand transport and loading into
the
catalyst
must also not des integrate during thermal treatment,
of
resisme-
reactor. thermal
The
shocks
and passage of fluids often at high flow rates. Mechanical strength is generally one of the most important characteristics of solid catalysts.
RAPID SlNTER/NG SMAll CATALVST
LARGE CATALYST
PELLETS
PELLETS
SMALL UNSUPPORTED METAL PARTICLES ~
LARGEPRESSURE DROP
t
SMALL PRESSURE DROP
t
, LONG PORES
~';~
~SHORT
~
__METAL PARTICLE
PORES
THERMOSTABlE SUPPORTED METAL CATALYST
ACT(VESURFACE BADLY ACCESSIBLE
A£TIlfE SURFACE EASILY AlXESSIBLE
Fig. T. Effect of pellet-size of fixed
Fig.2. A thermostable
bed catalyst on pressure-drop and on
vents sintering of the active mate-
transport through pellets.
rial.
The
external
dimensions per
surface
carrier pre-
area of catalyst pellets or particles of
the
above
is generally too small to provide the required active surface
unit volume.
The catalysts must therefore be porous.
area
It is difficult
achieve simultaneously a high porosity and mechanical strength.
The
to
transport
of reactants and reaction products through the pores of the catalyst pellets or particles can present difficulties,
especially with long narrow pores. Whereas
a rapid transport of molecules through the catalysts calls for short pores and, hence, catalyst
small particles. pellets (fig.1).
a technically acceptable pressure drop requires large Restrictions on the pressure drop generally
predo-
3 minate; diffusion limitations in the catalyst pellets are accepted. An instance is the ammonia synthesis catalyst. Diffusion limitations can be remedied by the production of catalyst pellets with wide pores. The presence of wide pores may, however, deteriorate the mechanical strength.
When
a
catalysts
high selectivity of a catalytic reaction is needed, cannot be utilized.
reaction
products
surface.
With
must be transported rapidly from the
catalyst
highly
To prevent subsequent undesired
porous
reactions
catalytically
pellets of an appreciable porosity the
the
active
transport
of
reaction products out of the pellets cannot proceed sufficiently rapidly to get the
desired selectivity with pellets large enough to limit the pressure
In
that case low- or non-porous catalyst pellets are used.
oxidation
drop.
Instances are
of ethylene to ethylene oxide and of ammonia to nitric oxide,
the where
the catalysts have a small or no porosity. Another case where the use of highly porous where
catalysts
leads to difficulies is with strongly
exothermic
an efficient removal of the heat of reaction is needed.
reactions
An increase
in
the temperature can lead to an unfavourable shift of the thermodynamic equilibrium
or
to undesired reaction products.
An example
is
the
Fischer-Tropsch
synthesis. As the thermal conductivity of highly porous solids is low, the heat of
reaction
cannot be transported rapidly out of the catalyst bed.
requirements of selectivity or thermal conductivity are stringent,
When
the
catalyst of
a low porosity must be utilized.
Most catalytically active materials cannot be processed to pellets or particles of the required thermostable porosity. sintering
of
many
oxides) is small.
Especially the resistance
catalytically active solids (metals and
against
transition
metal
Even to produce non-porous catalysts many active solids
are
not suitable. The relatively large pellets needed to maintain the required flow pattern often sinter.
To obtain thermostable catalysts,
supports or carriers
have to be used(fig.2). A support can be produced with the desired thermostable porosity
and
mechanical
strength.
Supports are generally
indispensable
to
prepare both porous and non-porous catalysts.
When the active material is applied onto a support, bility
is
the required thermosta-
usually obtained provided the active material has
distributed over the surface of the support.
been
itself is mostly not active and the support hence dilutes the active the
material,
stabilisation of the active surface causes the thermostable active surface
area per unit volume to exceed that of the unsupported material. hand
adequately
Though the surface of the support
On the
there are some important instances where a specific activity of the
port is a prerequisite (bifunctional catalysts in catalytic reforming).
other supIt can
4 also
be
methanol
that a support is required not exhibiting any activity (e.g.
in
the
synthesis,
of
by-
no acid support can be used to prevent formation
products) •
Two different
types of supported catalysts
can be distinguished
(fig.3).
With supported noble metal catalysts the costs involved with the metal inates. cles
predom-
To utilize the precious metal most effectively very small metal parti-
having almost all their atoms at the surface are required.
To achieve
a
maximum metal dispersion loadings of the support of about 1 wt% are used, which leads
to
particles of about 1 nm.
Many noble metal catalysts have to be
bi-
functional; the support must also exhibit a (different) catalytic activity. The relatively large bare surface area of the support present in noble metal
cata-
lysts of a low loading is favourable.
TYPES of SlIPPORTED CATALYSTS NOBLE METAL Partie'" Size-2nm CATALYSTS Oftan Bifunctional
4"'"IIIIII"III"lIIuJ ellmU1OOV1WIWV11C:llllwmll
1111f1Jmnmmfimllllilfilll LOADlf\K3-1w1'/,
MAXIMUM tv'ETAL OSPERSION
(!)
Coverage
Support PlATELETS
LESS EXPENSNE METALS orOXlfES
Fig.3.
Fig.4. Model support area
as
a
function
nm 2.0 5.8
17 15
1.5
26.7 124
,~
DENSITY SUPPORT 2.3g1ml DENSITY ACTIVE MATERIAl 9g/ml
perUnitVolume
Particles
224 77
336 72
Thickness 2 nm
SURFACE AREA
Area m/g of cat
100
/~/
~
MAXIMLJ.1 ACTIVE ~~./.
AcfiveSurfoce Size Active
Support %
"'"IJI'"'"'III@ijIllIll'IIIIIIIII"
and of
active the
surface
extent
of
coverage of the carrier.
With
less expensive active metals or oxides it can be important to maximize
the activity per unit volume (1). permits
to
get
A large active surface area per unit
a considerable conversion with moderately sized
volume
reactors
at
relatively low temperatures. The thermodynamics or the selectivity of catalytic reactions
are
often more favourable at low temperatures.
surface area per unit volume, more
are attractive.
To obtain
a
large
loadings of the active component(s) of 40 wt% or
If a large surface area per unit volume
is
desired,
a
5 uniform
dense
distribution of the active component or its precursor over
the
support is a prerequisite.
With of
both porous and non-porous supported catalysts a uniform
the
active
the surface area of an active material being present
particles
distribution
is
of
paramount
To demonstrate the significance of the distribution we will
importance. sider
component or its precursor over the support
on the surface of a platelike support.
as
con-
hemi-spherical
The fraction of the
surface
area of the carrier onto which the active material had been deposited uniformly was varied between unity and 0.015 at a loading of the active component of 66.2 wt%. The density of the active material and of the support was 9 and 2.3 g/cm 3, respectively. extent
Figure
4 shows the size of the active particles as well as
of the active surface area.
the
It is evident that the active surface area
per
unit weight drops rapidly as the distribution becomes more inhomogeneous. The active surface area drops from 224 to 3.55 m2g-1, when the fraction of the
surface
area
formly,
goes from 100 to 1.5%.
The
of the support where the active material has been
applied
above calculation has demonstrated that a uniform distribution
uni-
of
the
active component over the surface of the support is essential to achieve a high active
surface area per g of the active component or per unit volume of
cata-
lyst. To utilize an expensive active component most efficiently or to arrive at a
maximum active surface area per unit volume of catalyst the distribution
the
active material over the support must be controlled carefully.
tions where the selectivity may raise problems. required pellets
to of
established Subsequent application theme
of
have
the support.
the
thermal
active material. established thermal
generally deteriorate the
original
When a precursor has been brought
component
is
brought
be
of
the often
together.
distribution.
onto
a
the
The thermal treatment must not destroy the even
The main
support,
treatment must convert the compound(s) deposited into
in the application of the precursor.
treatment
it can
active component or its precursor is therefore
this paper.
subsequent
The distribution of the active
the active precursor and the support are
treatments of
on the other hand,
the active component exclusively at the outer edge
when
of
With reac-
a the
distribution
Moreover the severity of the
required to bring about the needed
conversion
can
strongly on the extent of dispersion over the surface of the support.
depend
6 Production of Supported Catalysts
The procedures for the production of supported catalysts can be divided into two main groups: (i) application of the active precursor onto a separately produced carrier, and (ii)selective
removal
of one or more components from solids of
an
initially
small specific surface area. The
first
group
properties process.
of
of preparation methods has the advantage that
a
the support can be adapted to the requirements of
number a
of
catalytic
Especially when previously shaped pellets are utilized, the pore-size
distribution
and the mechanical strength of the pellets may be
active precursor can be applied onto the support by:
adjusted.
The
SELECTIVE REMOVAL
-adsorption
• RANEY METALS .MIXED OXALATES .COPRECIPITATES
-impregnation and drying -precipitation
SEPARATE APPLICATION Adsorption done
of
an active precursor
from liquids.
Since
generally does not proceed, be
established
result
is
multilayer
.IMPREGNAroN • PRECIPITATION
adsorption
the loadings that can
by adsorption are limited.
adsorption
.ADSORPTION
mostly
As
a
from liquids is very well suited to produce precious
catalysts that are used at low loadings.
The properties of the metal precursor
have to be adjusted to effect adsorption on the surface of the metal
chloride
complexes
interact strongly with
adsorption on silica surfaces is small; on silica (5). of
adsorption sites on the support,
sites
precursor
on
surfaces,
Noble whereas
ammine complexes are strongly adsorbed
which may be affected by the pH-level
the support can lead to an
(6).
of
With low overall loadings a high density of adsorbinhomogeneous distribution
The active material will be deposited mainly at
edge of the particles or pellets of the support. ing
alumina
support.
The distribution of the active precursor depends on the density
the impregnating solution. ing
metal
the
of
the
external
Addition of ions not contain-
active material that can compete with the active precursor for the adsorb-
ing sites of the support can improve
the uniformity of the distribution.
With
some catalytic processes it is desired to prevent deposition of active material at
the
outermost
containing brought
the
ions
not
active precursor that adsorb more strongly on the support
surface of pellets of the support.
Addition
can
about deposition of active matarial in the interior part
of
of
catalyst
pellets only (7).
Impregnation to aim
and subsequent drying is utilized to obtain higher loadings or
apply active precursors that do not markedly adsorb onto the is
to
break up the solution of the precursor
into
small
support.
The
discontinuous
7 elements
present
in
the pores of the support by
gradually
evaporating
the
solvent (8). Deposition of the small amount of active material dissolved in the small
liquid elements leads to small particles.
incipient wetness impregnation is generally used, tion
With a pelletized support in which an amount of
is added just sufficient to fill up the pore volume of the pellets.
powdered volume
supports a volume of the solution substantially larger than the can be applied.
taining
small
Since the external surface of a powdered support
particles is considerable,
deposition of the
active
an
soluWith pore con-
material
mainly on the outer surface of the small particles still can lead to relatively small particles.
With larger volumes of the impregnatlng solution it is impor-
tant that the mass is continuously stirred during drying. and
Usually impregnation
drying leads to a very broad particle-size distribution.
Figure
5
shows
electronmicrographs of silica impregnated by copper nitrate. After impregnation and drying large copper-containing particles inhomogeneously distributed in the silica support can be seen.
Fig.5.
Micrographs of an impregnated Cu/Si0 2 catalyst. Note the strongly varying size of the deposited Cu-species in the same catalyst.
a. copper deposit in void in carrier;
b. large copper deposit in between particles of the carrier; c. very well dispersed eu-species.
The
evidence
on
supported catalysts produced by impregnation
shows
that the support is covered rather inhomogeneously by the
rial.
As
a
result
the active surface area per unit
catalysts is relatively small.
volume
and active
of
drying mate-
impregnated
It has been demonstrated that the inhomogeneous
distribution is established during drying. Evidently capillary forces cause the
8 solution
to migrate to the outer edges of the pellets of the
support.
Kotter
and
Riekert obtained better results by raising the viscosity of the impregnat-
ing
solution
solution
by the addition of polymers (9).
limits the migration during drying.
The higher
viscosity
of
the
Nevertheless the desired uniform
distribution is not obtained by impregnation and drying.
Precipitation of an active precursor in the presence of a suspended is
also utilized to produce supported catalysts.
cipitation the solids are filtered, treated.
dried,
support
After completion of the pre-
processed to pellets and thermally
Since precipitation can be carried out more rapidly than drying, this
procedure has some advantages. However, the distribution of the active material throughout the support is even worse than with impregnation and drying. Evidence has e.g. provided by Servello et al.(10).
The ting
above discussion has shown that to produce supported catalysts a large active surface area starting from a separately
generally
is not viable.
removal of one or more constituents from a solid.
pertains
e.g.
support
The catalysts utilized in large chemical plants
therefore mostly produced by the second of the above main methods, tive
exhibi-
produced
to the production of Raney metals,
are
viz. selec-
This general procedure
of supported
catalysts
by
coprecipitation and subsequent thermal treatment, and by decomposition of mixed oxalates.
With
Raney
metals the starting material is a rather brittle alloy
active metal and aluminium. remove
the
aluminium.
The active metal remains finely divided together
some alumina which stabilizes the small particles. the
metal
water. the
of
particles can stabilize the metal particles against
Raney metals are not thermostable.
relatively
expensive. very
reaction
with
The consumption of aluminium and of
With small-scale slurry reactions,
suitable.
with
Some aluminium remaining in
electrical energy during the production of the alloy renders Raney
however,
the
The alloy is reacted with an alkali to selectively
Raney
metals
metals are,
In the production of fine chemicals Raney metals are
consequently widely utilized.
Mixed oxalates are prepared by coprecipitation of magnesium and one or divalent rather
active large
metal
ions with oxalate ions
crystallites are obtained.
oxalates loose water and carbon oxides. metal
component
and the gas atmosphere,
(Langenbeck
(11
».
At elevated temperatures
Generally the
Depending on the nature of the the decomposition results
more
in
mixed active small
metal or oxide particles very evenly distributed over the magnesia support. The mixed
oxalates des integrate completely on decomposition,
the resulting powder
9 has to be processed to pellets.
As this is difficult to carry out in an
inert
atmosphere, a subsequent reduction (at a more elevated temperature) is required to
produce active metals.
cally
strong
Owing to this and the difficulty to obtain mechani-
pellets mixed oxalates are not much used
to
prepare
technical
industrially used catalysts are produced by
selective
catalysts.
Many
highly loaded,
removal of mainly water and oxygen.
An old,
but still almost exclusively used
catalyst is the ammonia synthesis catalyst (12). By electrically melting magnetite and about 5 wt% of alumina a mixed oxide is obtained. crushing to particles of the required size,
After a (difficult)
the catalyst is reduced and passi-
vated. The selective removal of oxygen during reduction leads to highly porous, mechanically
strong particles.
The high iron content causes the iron
area per unit volume to be large. resulting
However,
the relatively narrow pores in the
catalyst lead to diffusion-limitation.
catalyst is rather expensive,
surface
Though the production of the
its excellent properties has brought about
that
essentially the same catalyst is utilized since about 1910.
Other
catalysts
coprecipitates. catalyst
and
used in large scale processes are produced
Instances copper
starting
from
are the nickel-on-alumina (methane) steam reforming
(oxide)-zinc oxide-alumina catalysts used
in
the
low-
temperature carbon monoxide shift and in the methanol synthesis (13,14,15). The ions of the active component(s) and of the support are precipitated together as hydroxides drying,
or hydroxy salts (mostly hydroxy carbonates).
After filtering
and
the solid is processed to pellets, calcined and reduced (alternatively
the solids are pelletized after calcination).
It is essential that the ions of
the active material and the support are intimately mixed already in the precipitation
step.
With nickel and aluminium ions coprecipitation can result in
mixed
basic carbonate.
salt.
Removal
of
Also copper and zinc ions can
react to a mixed
carbon dioxide and water during calcination and
of
a
basic oxygen
during reduction leads to the the porous catalyst. Starting from dissolved ions an
intimate
mixture of the components can be obtained,
which results
in
homogeneous distribution of the active particles in the final
catalyst.
it
distribution
is
generally not possible to establish a uniform,
dense
a
Since of
active particles by other methods, several industrially important catalysts are produced from coprecipitates (13,16).
However, tails
of
the
ferent solutions, tion,
extent of mixing of the constituents strongly depends on
the precipitation procedure (order and mode of addition of the
filtering
dedif-
temperature, aging period of precipitate in the mother soluand washing procedures).
The size of the
active
particles,
10
Fig.6. Model of preparation of a supported catalyst by selective removal of one or more constituents.
their
distribution,
the
porous structure and the mechanical strength of
the
finished (reduced) catalyst pellets vary considerably with the extent of mixing of
the
components established in the precipitation step.
coprecipitation
step,
asking
Scaling-up
of
for mixing of large volumes of solutions
the in
a
controlled way, is difficult. Moreover, the structure of the solid is prOfoundly
changed during calcination and reduction.
catalyst
depends
thermal history. difficult
to
on
The properties of the
variables such as water vapour pressure
considerable
local
variations
Production of supported catalysts by coprecipitation is
analogous to build and to push over a brick wall(fig.6). structure
resulting the
With large volumes of catalyst to be thermally treated, it is
control these variables and to prevent
over the catalyst bed.
the
and
of the solid during the removal of water,
The drastic change in carbon
dioxide
and
oxygen causes the process to be very analogous to pushing over a wall. While it is already difficult to build the wall reproducibly by coprecipitation,
it
is
hardly possible to push over the wall in a controlled way.
Theoretical Background of Deposition-Precipitation (17.18) The above generally used procedures all have disadvantages; leads
to
a
build-up of the catalyst difficult to control and
coprecipitation to
reproduce,
adsorption only works with low loadings and impregnation results in an geneous
distribution
of the active material.
It is therefore
inhomo-
worthwhile
to
consider alternative methods. The porous structure of the final catalyst can be controlled moreover,
more
easily starting from a separately
the structure of the solid is
produced
less drastically
carrier.
Since,
affected during the
11
subsequent thermal treatment,
well reproducible results can be expected if the
active precursor is applied onto a previously produced carrier. Above the methods to deposit an active precursor onto a support have been reviewed. Migration of
the solution during drying of an impregnated support appeared to be
cult
to
prevent,
diffi-
while adsorption of an active precursor generally does
not
proceed beyond a monolayer.
We therefore will consider PRECIPITATION of an active precursor more ly.
On
addition of a precipitating agent to a suspension of the support in
solution
of
precursor tated
close-
the precursor,
the precipitant initially contacts the
outside the pore system of the support.
particles of the active component,
proceed
rapidly.
Consequently,
To get very small
nucleation of the
a
dissolved precipi-
precipitate
must
small precipitated particles of the precursor
will develop outside the pores of the carrier. Provided the precipitated particles are attracted by the support, ever,
the
size.
As
they will be attached to the carrier.
diffusivity of colloidal particles rapidly drops with the a result.
the porosity of the carrier must be kept small
transport problems as much as possible. vented
by
to
limit
Transport problems are completely pre-
coprecipitation with the support,
support nucleate simultaneously.
How-
particle
if the active precursor and
Coprecipitation,
the
however, raises other prob-
lems as mentioned above.
Addition
and reaction of a precipitating agent can be separated by using
compound that slowly reacts to a precipitant.
a
The slowly reacting compound can
be added and a homogeneous solution can be established before the precipitating
agent has attained a marked concentration. Though nucleation of the active precursor can occur as rapidly as required to generate very small particles, homogenizing the suspension can be completed before the precursor starts to precippitate. The time available to homogenize the suspension can be extended considerably
by
working at different temperatures.
The solutions can be mixed
and
homogenized at a temperature where no marked formation of the precipitant takes place after which the temperature is raised and the precipitant develops rapidly. Using this procedure the active material to be applied onto the support has to be present within the pores of the carrier before formation of the
precipi-
tant sets in. Consequently the volume of the dissolved active precursor together with the inchoate precipitant can be at most equal to the pore volume of the support.
When
a highly loaded support is to be produced,
the support must be
impregnated by a concentrated solution. With concentrated solutions the precipitated thermal
particles treatment
of the active precursor are likely causes the clusters to sinter,
large active particles.
to
cluster;
which leads
to
subsequent relatively
12
I
/L
.I
" ... --........... ....,
,
t
SIZE_
,,
\
\L--S
\ BULK (F Tf£ \ SOLlfTCJN
AG
\ \ \
\
\
TEMP
L.S
I
/ /
,
r
Lis-'
a solid
;~
;/
/
/
I lL.s
CG1POSITlON-----
Top: difference in free energy
on formation of
/
/>t..s /
,"
-
-
COMPOSITON-----
Fig. 7.
"
,
"", /
particle in
Fig.B. Top: difference in free energy on
formation of a solid
particle in
the bulk of the solution as a func-
the suspension of the support as a
tion of the particle size for the 3
function of the particle size at one
regions indicated in the diagram at
concentration between L+ssupport and
the bottom.
L+s.
Bottom: equilibrium diagram for
Bottom: equilibrium diagram for a more stable bulk and surface compound
a pure solution.
with the carrier.
To
avoid clustering of active particles especially at high loadings of
carrier,
the support would be favourable.
To investigate the conditions leading to pre-
cipitation only onto the support, more
the
precipitation of the active precursor exclusively onto the surface of
closely.
The
we shall consider the precipitation
process
bottom of figure 7 shows an equilibrium diagram where
the
concentration of a saturated solution is given as a function of the temperature (solubility tween
a
curve).
At the top of figure 7 the difference in free energy
solution with a solid particle and a homogeneous
solution
of
be-
equal
overall composition is represented as a function of the particle size. When the concentration of the solution is below that of the solubility curve, energy grows on formation of a solid particle. particles
of the solubility curve,
free
Since the free energy of larger
increases linearly with the volume of the particle,
proportional to the third power of the particle size. that
the
the increase is
At concentrations
above
the free energy of a solid particle and a satu-
13 rated solution is lower than that of the homogeneous solution. ticles
with large par-
the decrease in free energy is proportional to the third power
particle size. ference
of
the
The decrease in free energy per unit volume grows with the dif-
between the concentration of the homogeneous solution and that of
solubility
curve.
the
At the concentration of the solubility curve the difference
in free energy is zero.
When the size of the precipitated particles is small, the
the surface energy of
solid can appreciably affect the change in free energy on formation
solid
particle.
of
a
At relatively elevated concentrations the drop in free energy
per unit volume is sufficiently large to cause the free energy to decrease even with
very small particles,
particles. curve,
When
though less rapidly with smaller than with
the concentration is not much beyond that of
the
larger
solubility
the surface energy brings about that the difference in free energy pas-
ses
through a (positive) maximum as the size of the solid particle
The
rate
strongly
of
nucleation
depends
of solid particles from
a
increases.
supersaturated
on the sign of the difference in free energy at
solution
very
small
particle sizes. When the difference in free energy steadily drops at increasing particle size, initially changes,
the rate of nucleation is much higher than when this difference
rises. roughly
The concentrations at which the rate of nucleation
starts to drop steadily; bility curve. energy
of
abruptly
coincides with those at which the difference in free these concentrations are indicated by the
energy
supersolu-
The position of the supersolubility curve depends on the surface
the solid in contact with the solution and on the decrease in
energy per unit volume of the bulk precipitate.
free
These quantities can vary con-
siderably with different solids and with the stoichiometry and the defect
con-
centration
with
the
of the precipitate.
The rapid increase in rate of nucleation
concentration brings about that we can distinguish the
perature range L+S in figure 7,
concentration-tem-
where large particles only are stable, whereas
very small particles are also stable in the range L+s.
When the concentration of the solution is raised homogeneously, crossing the concentration tate.
of the solubility curve does not lead to formation of a precipi-
Nucleation starts only when the concentraticn reaches that of the super-
solubility curve. With a relatively large difference between the concentrations of the solubility and the supersolubility curve the nuclei once generated
will
grow fast. Since in the first stage of the process the addition of the precipitating species usually will not keep up with the consumption of the precipitant by
the rapidly growing nuclei,
We
shall demonstrate that especially with basic Cu salts the concentration
the concentration will pass through a maximum.
the precipitating OH--ions sharply drops after the concentration of the
of
super-
14 solubility curve has been reached.
Since subsequently the concentration of the
solution remains below that of the supersolubility be
generated.
The
relatively large crystallites. locally number
curve, more nuclei will not
growth of a small number of nuclei consequently When,
on the other hand,
leads
raised considerably above that of the supersolubility curve, of nuclei results.
to
the concentration is a
large
Growth of these numerous nuclei leads to many small
crystallites.
In figure 8 the case of a finely divided carrier suspended in a solution the
active precursor is considered.
It is assumed that the ions of the active
species chemically interact with the surface of the carrier. teraction the
carrier lower than in the bulk of the solution.
able
to
figure
Owing to this in-
the concentration of the supersolubility curve is at the surface
L+ssupport in figure 8.
of
This is the curve
marked
We shall find that often the active precursor is
form a bulk compound with the carrier.
We therefore have assumed
8 that the solubility curve has also shifted to higher
of
also in
concentrations.
The top of figure 8 shows the difference in free energy for the same concentration which is between that of the two supersolubility curves. At the surface of the
support
the free energy steadily decreases.
whereas in the bulk
solution the free energy initially rises on formation of a solid concentrations active
of
particle.
between that of the curves L+ssupport and L+s of figure 8,
precursor will precipitate exclusively onto the surface of the
(DEPOSITION-PRECIPITATION).
Especially
a considerable
At the
carrier
when an active precursor is to be
plied onto a carrier having rather narrow pores,
the
ap-
concentration
difference is required to transport the precursor at a reasonable rate into the porous
support.
To be able to establish an appreciable concentration gradient
without inducing nucleation in the bulk of the solution,
the concentration
of
the two supersolubility curves must differ sufficiently. The difference in concentration is related to the bond strength to the surface of the support. erally
Gen-
a significant interaction with the carrier is required to carry out de-
position-precipitation at an acceptable rate.
Practical Aspects of Deposition-Precipitation Simple addition of a precipitating agent to a suspension of the carrier in a solution of the precursor does not lead to the homogeneous increase in· concentration required to get deposition-precipitation. When the solution of the precipitant locally
is poured into the suspension of the support,
the concentration
rise above that of the supersolubility of the bulk compound.
sult nucleation proceeds locally in the bulk of the solution. are
too
stable or have grown too large to redissolve when the
can
As a re-
Often the nuclei suspension
is
homogenized. SUbsequent growth of the nuclei in the solution cannot be avoided.
15 Local concentration differences in the suspension of the support can be minimized by the following two procedures.
The first procedure separates addition
An instance is the increase in hydroxyl
and reaction of a precipitating agent.
ion concentration by hydrolysis of urea (19). marked rate only above about 60 oC,
Since the hydrolysis occurs at a
the solution can be homogenized at a
lower
temperature and subsequently brought at a temperature where the reaction rapidly proceeds.
According to the second of the above procedures a solution of the
precipitant the
is injected into the suspension of the support below the level
liquid (20).
of
The injection tube must end below the surface of the liquid.
because no sufficiently large shear stresses can be established at a gas-liquid interface. Provisions are required to assure a without
interruption
steady flow of the
as well as high shear stresses at the
Since the suspension must thus be agitated vigorously, difficulties. volume which with
of
As indicated in figure 9,
injection
scaling up can
the
content
of
While the first procedure has been carried out
identical results in vessels of 1 and of 2000 liter,
straightforward with the second procedure. dure may take e.g.
point. present
it is possible to recirculate a large
the suspension through a relatively small vessel,
is intensively agitated.
precipitant
scaling up is
On the other hand,
24 hours to finish the precipitation,
less
the first proce-
while the
injection
procedure may be carried out more rapidly.
INJECTION into SUSPENSION of the SUPPORT kept at a constant pH leveL
DEPOSITION PRECIPITATION from HOMOGENEOUS SOlUTION
·Change In pH_level -INCREASE
N,ID! CulIIIFelIIJ CrlIJII
-DECREASE
V( V)
s-av:
.Change In Valency -OXIDATION Fe(II)--FerDI; Mn(II) -MnflVJ -REDUCTION Cr(V1)--CrfmJ
CulJII-CulJi PfllVl PdlIIJAglJi ~Mefal5
• DeComplexing REMOVAL of NH)
UREA CO(Nl-',), --NH:.CNOCNO-.3 H,O --NH:.HCQ.OHCNO-.2 H,O ~NH:.CO,.2 OHLower Temperotures More limitedreactionof SILICA CYANATE CNO-.3H,O ~NH;'+HCO;-.OH CNO-.2H,O ~NK.Co,.2 OW
No ComplexFormation of NH, NITRITE 3 NQ. H,O ~ 2 NO.NQ.20H-
Oxidation of £DTA
Fig.9.
Fig. 10
Injection procedure on a
Methods developed for deposition-
large scale.
precipitation onto suspended supports.
16 Precipitation the
method
according to the first of the above procedures corresponds to
of precipitation from a homogeneous solution used
analysis to prepare well crystallized, easy
to filter (21,22,23).
provide
extremely small particles.
number of other methods has been developed,
figure 10.
gravimetric
relatively large crystallites that
Deposition-precipitation,
urea
a
in
are
on the other hand,
can
Besides the well described utilization which are
summarized
Many active precursors can be precipitated by raising the
of in
pH-level
of a solution of the active component. Figure 10 gives some importnt instances. Cyanate is utilized when the precipitation has to be done at lower temperatures than about 70 oC, the temperature at which urea hydrolyzes rapidly. To avoid formation
of soluble ammine complexes,
nitrite can be favourably
paper will deal mainly with precipitation by raising the pH-level.
used.
This
Other meth-
ods that have also been used successfully, will be dealt with shortly.
Anionic creasing
species can be deposited onto surfaces of suspended carriers by dethe pH-level (24).
used with Mo{VI).
Besides with vanadium{V) this procedure has
Oxidation at a pH-level where the ions of the lower
been
valency
are soluble and the oxidized species insoluble, can also be utilized to precipitate active precursors.
Dissolved oxidation agents, such as nitrate ions, are
very suitable to precipitate from a homogeneous solution.
The interaction with
the surface or the bulk of the carrier can strongly depend on the pH-level. accurately
control the interaction,
can be fixed at a desired level, tion.
As
injection of alkali or e.g.
keep the pH at the chosen level.
tion
is favourable.
This is possible with
oxida-
subsequent reaction (hydrolysis) of the oxidized species can consume
hydroxyl ions,
soluble
Tb
precipitation at a constant pH-value that
The
hydrolysis of urea must be used to
oxidized ions can also react with
metal ions to insoluble compounds.
other
An important instance is the reac-
of Fe(III) with Fe(II) to insoluble magnetite;
to prevent
formation
of
more insoluble hydrated Fe(III) oxide, the Fe(III) ions must be generated homogeneously (25).
In the presence of other dissolved divalent metal ions, oxida-
tion of Fe(II) can lead to precipitation of ferrites.
Reduction With
to insoluble ionic compounds has been done with Cr,
copper hydroxy-acid complexes can be applied (26).
form a soluble complex,
can be obtained.
Since Cu(I) does
Especially with noble metals
be
reducing
not
good
Elsewhere in this volume the production of supported
alloy catalysts with extremely small particles is described. must
and Mo.
reduction brings about precipitation. Reduction to the
corresponding metal has been practiced too. results
Cu,
Generally
excluded during the reduction to prevent catalytic oxidation agent to proceed only.
Still more important is to avoid
oxygen of
the
reoxidation
17 and,
hence,
redissolution of finely divided reduced material during filtering
and washing of the loaded carrier.
Decomplexing onto
has
also been used to precipitate from
suspended carriers.
ammine
complexes,
redissolution
utilization of ammine complexes is attractive.
solution decompose
To
prevent
of the metal ions at the decreased pH obtained after removal
the ammonium ions, struction
homogeneous
Since raising the temperature suffices to
anions such as hydroxyl or
of complexing EDTA by e.g.
to produce supported catalysts (27).
carbonate have to be used.
of De-
hydrogen peroxide had also been employed With metal ions catalyzing the decomposi-
tion of hydrogen peroxide the reaction has to be carried out in a thin layer.
Evaporation of the solvent seems to be very obvious to gradually and homogeneously raise the concentration of a solution.
As dealt with above, however, a
very large fraction of the solvent has to be removed to increase the concentration sufficiently to induce crystallisation.
Consequently the impregnating so-
lution does not remain continuous during the evaporation. Transport of elements of the concentrated liquid leads to
an inhomogeneous
distribution of the
ac-
deposition-precipitation
the
tive material.
Deposition-Precipitation on Silica SUpports
In
the
production of supported catalysts by
interaction
of
the precipitating precursor with the carrier plays a
dominant
role. We will deal more in detail with the interaction with silica and alumina. Since the hydroxyl ion concentation can be monitored relatively easily, we will concentrate on precipitation by increase in hydroxyl ion concentration.
In and
an earlier volume of this series (28),
it had been shown how the extent
the strength of the interaction with the carrier can be inferred from
curves.
The pH-value continuously recorded during addition of hydroxyl ions to
solutions added.
of
the active precursor is plotted against the amount
active precursor, of
to a suspension of the carrier in pure water,
the carrier in a solution of the precursor are
experiments suspended
hydroxyl
the
of
without against
and to a sus-
measured.
concentration of the solution and the amount of
the
In
The consumption of hydroxyl ions by the
the carrier in pure water and that by the solution of a
suspended
carrier are mathematically added.
The
the sum
the
suspenprecursor
is
plotted
the amount of hydroxyl added and compared with the consumption of ions
the
carrier
as well as the volume of the liquid and the rate of addition of
hydroxyl ions are kept equal.
droxyl
of
pH-curves resulting from addition of hydroxyl ions to a solution of the
pension
sion
pH-
experimentally meaured with the suspension of the carrier in
hythe
18 solution of the precursor.
A marked interaction is evident from the suspension
taking up more hydroxyl ions at an equal pH-level than the calculated tion.
A marked interaction with the carrier brings about
libria
causing reaction of hydroxyl ions at a lower
ions
consump-
a shift in the equi-
pH-level.
When
hydroxyl
are made available by reactions of a rate difficult to assess accurately,
as e.g.
the reaction of urea or nitrite, the pH-value is plotted preferably as
a function of time. pH-versus-tirne curves can ison
of
be reproduced very well. Compar-
pH-versus-time curves recorded under identical
conditions
with
and
without a suspended carrier can also demonstrate quite clearly interaction with the carrier.
The above procedure had been used to investigate the interaction of precipitating Ni ions with fumed silica (surface area 200 and 380 m2g- 1)(28). The pHcurves
at 25 0C indicate reaction of the surface of the silica
recorded
initially
the pH-value of the suspension of the silica in the nickel
rises much more slowly than that calculated silica the
theoretically.
only;
solution
The surface of the
hence strongly reacts with precipitating nickel ions.
The point
where
pH-curve slowly approaches the calculated curve indicates that the
silica
does not markedly react beyond the surface layer. When the same experiments are carried out at 90 oC, the pH remains considerably below the calculated values, until the Ni ions (at lower loadings) or the silica (at elevated loadings) have completely from
reacted.
The different extent of reaction of the silica as evident
pH measurements has been corroborated by redissolution
experiments,
frared measurements (disappearance of the lattice vibration of silica), sis of the porous structure of the loaded carrier, tron
microscopy.
Besides the temperature,
analy-
thermal analysis and
the particle size and the
in-
electhermal
pretreatment strongly affect the extent of reaction of the silica. Silica of a smaller surface area (about 10 m2 g-1) reacts incompletely at 90 oC.
Ni(II) nickel
ions
can
hydrosilicate.
compound
whether
shows
Fe(III).
bulk nuclei
We now want to investigate
It is therefore interesting to investigate whether Fe(III)
the
to a
hydrosilicate, can be
deposited onto
increase in pH on addition of hydroxyl ions to
with ions,
silica. Figure a
solution
of
The consumption of OH--ions already at a pH of 2 indicates the stabi-
of hydrated Fe(III) oxide. be seen.
rises.
a
and
a stable bulk compound is required to get sufficient interaction
which do not react
can
compound,
Figure 7 thus applies to the Ni-silica system;
by interaction with the silica surface.
the carrier.
lity
bulk
more stable than the bulk hydroxide or hydroxy-carbonate
stabilized
11
react with hydroxyl ions and silica to a
As long as the pH is about 2 no
precipitate
The well known red-brown precipitate appears when the pH steeply
Polynuclear
complexes or
very small oxide particles that coagulate at
19
/
7
e 6 5
1
rr:: /'
5
pH
1·
/
1/
-- --T=90 ·C
4
pH 3
Water Aerosil FeCl2 FeCl2·Aerosil
3
2
2 1.0
OH/Fe
2.0
60
3.0
120
timeCmin)-
+
Fig.ll
Fig. 12
Precipitation of Fe(III) from ho-
pH-versus-time curves recorded for urea solutions at 90 oC. Curves of a pure
mogeneous solution.
Neither with
urea nor with alkali injection an
urea solution, of a suspension of silica
effect
in water, of a Fe(II) solution, and of a
of
suspended
silica was
apparent.
silica suspension in a
Fe (II)
solution
are represented.
higher pH-levels are formed at pH = 2.
The presence of suspended silica has no
any effect on the pH-curve. Apparently the species generated at pH
=
2 does not
interact with the silica surface. The absence of interaction has been confirmed by thermal analysis (dehydration), In
the
X-ray diffraction and electron
microscopy.
pores of the support large clusters (about 20 to 70 nm) of very
particles
(about 3 nm) could be observed.
Evidently precitation from
small homoge-
neous solution even with a very rapid nucleation, which leads to small crystallites, is not sufficient to apply an active component uniformly onto a carrier.
To produce supported Fe use Fe(II).
An Fe(II)
(oxide) catalysts it is therefore more attractive to
solution can be easily prepared by reacting an excess of
metallic iron with hydrochloric acid. prevents the
When air is excluded, the excess of iron
formation of Fe(III). Figure 12 shows the pH-curves recorded during of urea at 90 oC. It can be seen that the fumed silica alone
hydrolysis
consumes
a small amount of OH--ions above a pH of about 6.
react markedly above pH = 4.8.
Fe(II)
starts
to
without silica rapid nucleation sets in a pH of
6.7. The subsequent rapid growth of the nuclei leads to the drop in pH that was mentioned above. With suspended silica the maximum of the pH and the subsequent
20 level is appreciably below that of the curve without silica. the
nuclei
and
Accordingly, both
the bulk compound are mor-e stable and figure 7
applies
here
also.
7 /
6
/
/ T=45·C injection no silica injechon silica
pH
4
2
60
time(min) --------+
120
Fig.13. Precipitation of Fe(II) by injection of NaOH with and without suspended silica.
150
H2
uploke
1
UREA 900C
o.u
650
unsuppor t ed
U 90 (l)
NoOH
9O"C
190 (1)
145 (1)
1000
·c--
Fig. 14. Left:structures
resulting from deposition-precipitation of Fe(II) by different
procedures. Right:temperature-programmed reduction of Fe oxide deposited on silica by hydrolysis of urea (U 90(1)), injection at 90 0c (I 90(1)), and injection at 4SoC. For comparison bulk Fe oxide is included.
Comparison represented,
of figure 12 with 13, shows
that
where an injection experiment at 4S oC
the reaction with the support is less
extensive
is at
21 45°c. A slightly more elevated pH at the injection point brings about formation of
a less reactive Fe precipitate.
As a result the attack of the
support
is
more limited on injection at 90 o C. The structures obtained in the three differents 3xperiments are as indicated in figure 14.
The temperature-programmed re-
duction experiments of figure 14 demonstrate the different extent of hydrosilicate formation.
Previous air-drying partially oxidizes the Fe(II). As interac-
tion with silica stabilizes Fe(II), reduction step to Fe(II). hydrosilicate
Fe
the supported Fe specimens show a separate
This step is not displayed by the bulk Fe oxide. The
obtained in the urea precipitation at 90°C is fairly
and is reduced only above 650 oC.
stable
The Fe(II) precipitated at 45°C is less inti-
mately connected with the silica and consequently starts to be reduced at about
450°C. tion
Figure 14 shows that injection at 90°C leads to a less extensive to hydrosilicate.
reac-
The micrographs of figure 15 confirm the different ex-
tents of reaction of the support.
The carrier loaded at 45°C shows the
silica
particles covered by very small Fe oxide particles, while the carrier has reacted to thin hydro silicate platelets at 90°C.
Fig. IS.
Electronmicrographs of silica loaded by Fe(II). Precipitated by
a. injection of NaOH at 4SoC.
The
b. hydrolysis of urea at 90°c.
above experiments have shown that Fe(II) does and Fe(III) does not
in-
teract sufficiently with silica to precipitate exclusively onto the support. It may
be questioned whether the difference between Fe(II) and Fe(III) is due
to
the ability to form a bulk compound with hydroxyl ions and silica. Formation of
22 a surface or interfacial compound,
which must suffice,
easily than that of bulk compounds.
proceeds generaly more
The lack of interaction of Fe(III) may
be
due to the low pH at which Fe(III) precipitates. An electrostatic charge on the suspended silica particles may be instrumental in the transport of Fe(III) ions to the silica surface (29). face
with
As shown in figure 16,
reaction of an oxidic sur-
water leads to a hydroxylated surface onto which a layer
of
water
molecules is bound by hydrogen bridges. At increasing pH-levels the reaction of the surface OH-groups shifts from the left to the right-hand side in figure 16. The pH at which the surface charge of suspended oxides changes sign varies with the nature of the oxide. Silica has no charge at a pH of about 2, while alumina changes the sign of its charge at pH-values from 6 to 7 depending on the preparation and the pretreatment.
M/
~/ "0
M/ "0
M/
-,
/0
M'oH
~H
....-oH M
M/OH -,
> "0
+H20~
+H20~
M
M'oH
M -,
M/ -,
~H
-,
M/ -, 'PH
M'~
M/
"0
/0
M)'
'0 0 '0 <
/0
M
>
/
M,OH /OH
"0 /
M/
~
-,
M"
..
M/ 'OH
~: ,.oH ~OH ....-oH M'OH ,.oH ""OH M!'H
M~tj! /OHO~
...
<,
M'OH OH OHI:I
M(OH~ OH ~ M(OH O
/OH~
°
M'o Hd;\ M..-£lHOH 'OH H
! 6
MO~®
i
6
H = MOH -- Mcf3+ 9 0
:\
Eil
,
~+OHe Q
i
Fig. 16.
Reaction
of oxidic surfaces with water.
At the bottom the
reactions
charging the surface are indicated. At increasing pH the reaction shifts to the right.
Hence
silica
rises.
Presumably
tively
charged.
surfaces become increasingly negatively charged when
the
pH
the polymerized Fe(III)-species developing at pH 2 is posiElectron micrographs showed that the precipitated Fe
species
23 had clustered and had not been deposited onto the silica, negatively charged electrostatic
though the silica is
above pH 2. Hence the charge density is either too small or
attraction is not affecting the deposition on
silica.
tively the pH may affect the reactivity of surface hydroxyl groups.
AlternaAt
higher
pH-levels surface hydroxyl groups are more reactive. There is evidence that the reaction
of two hydroxyl groups on different silica particles to water and
oxygen
bridge.
silica
suspensions.
groups tween
(30).
an
The bonds between the silica particles raises the viscosity of OH--ions
catalyze the reaction of the
surface
hydroxyl
It is likely that OH--ions not only accelerate the reaction
hydroxyl groups on silica,
but also between
hydroxyl groups on
be-
silica
and bound to a dissolved metal ion:
I I
I I
I I
As
I I
-Si-O-M-
-Si-OH + HO-M-
a result the interaction at more elevated pH-levels is due to a more
reaction Fe(II)
of the surface hydroxyl groups. precipitate
strongly differ.
Tb
level at which the precipitation proceeds, more suitable. per
The pH-values at which
rapid
Fe(III)
and
investigate the influence of the
pH-
a pH-range between 3 and 5 would be
Basic copper(II) salts precipitate in this pH range. Since cop-
can also react to
hydrosilicates,
information about the requirement
of
ability of formation of bulk compounds can thus be gained.
-
o
6
t ime(H)
Fig. 17.
Precipitation
I-1/".: _
hydrolysis
.,
with
lit ! ;wrtn
of Cu(II) at 900C by
of urea.
An experiment
and without suspended
silica
is represented. SIlica
I'"
With
silica two different eu
con-
centrations have been used. o
Figure 17 shows the precipitation of CU(II) from Cu nitrate by urea hydrolysis.
It can be seen that the presence of suspended silica does not affect
pH-curve.
In
the
bulk of the solution nucleation proceeds at a
pH-level
the of
24 about
3.8.
Owing to the rapid growth of the nuclei,
the pH passes through
a
sharp maximum. Evidently the precipitating Cu species does not interact markedly with silica. Electronmicrographs show that very large (about 20 lites
of
smaller
basic Cu nitrate have precipitated. silica
nitrate.
crystal-
~)
With suspended silica the
particles have been deposited onto the large plates
of
much basic
The ability to form a bulk compound is not sufficient to cause depos-
ition-precipitation.
Since at a pH-level of about 4 the negative charge on the
silica particles is much larger than at pH 2,
an interaction
intermediate be-
tween that of Fe(III) and Ni(II) or Fe(II) had to be expected,
if the electro-
static charge was important. Consequently, the approach of partially hydrolyzed Cu(II) ions to the silica surface is not likely to determine the reaction
with
the silica surface.
By
using an anion that has a basic Cu salt of a lower stability,
which the precipitation takes place, perchlorate
can be raised.
precipitated
by urea hydrolysis,
suspended
without silica.
silica
When Cu
stable basic Cu perchlorate
is
the pH-curve with suspended silica runs sig-
nificantly below that measured without silica. with
The less
precipitates at a higher pH (about 4.2).
the pH at
Moreover, the pH-curve recorded
does not exhibit a maximum in contrast
to
the
curve
Investigation of the loaded carrier shows that the Cu has been
deposited onto the silica and even has reacted with the bulk of the carrier. small
increase in the pH of precipitation is hence sufficient to
deposition onto the silica. a By
A
about
The strong effect of a small change in pH suggests
catalytic action on the reactivity of the surface OH-groups to be operative. decreasing
the concentration of Cu nitrate the pH at which
precipitates can also be increased.
=
the
4.8 and a maximum is not displayed. silica.
Apart
basic
nitrate
One of the experiments represented in fig-
ure 20 has been done with a small Cu concentration. pH
bring
Precipitation proceeds
Again Cu appears to be deposited
from the level at which the basic salt
precipitates,
at on the
anion hence does not affect the interaction with the support.
Copper ions can be deposition-precipitated not only from a Cu(II) but also from a suspended (previously precipitated) Cu compound. trate
becomes unstable at higher pH-levels.
which
alkali was injected into a copper nitrate solution.
the precipitation of Cu(II), basic
salt
solution,
The basic ni-
Figure 18 shows an experiment
in
After completion of
the pH increases rapidly up to about 9, where the
becomes unstable and reacts to CUD.
The release of
leads to the intermediate plateau of the pH at about 5.
nitrate
Also decomposition
ions to
another basic Cu salt is possible. The stability of basic Cu carbonate is higher
than that of the basic nitrate.
dioxide
is
When during the injection of alkali carbon
passed through the solution (dashed curve of figure
18),
the
pH
25 exhibits
a lower maximum.
Now the final product contains basic Cu
During the hydrolysis of urea carbon dioxide evolves also. curve
of
carbonate.
In figure 19 the pH-
a urea precipitation is shown together with the composition
precipitate determined by X-ray diffraction.
of
the
It can be seen t9at the primarily
precipitated basic nitrate reacts to basic carbonate at a pH of about 5.
r.
CuO
,,
[cCU,IOH):f'O:!
C~OH)3N03 timelh).
0
8
10
12
,.
J
CuCO:fu{OH)
CuiOH)3N03 cUC0 3Cu(OH)2
time(h}.
0
Fig. 18.
12
18
2.
30
36
52
Fig.19
Precipitation by injection of alkali
Precipitation of Cu(II) by urea hydro-
at 90 0 c from nitrate.
lysis. The phase composition of the
Fully drawn curve without CO2
solid precipitates has been determined
dashed curve with CO2
by X-ray diffraction.
~(C~')i 6
I
-2
PH
Fig.20. -4
pH-curves measured at a low Cu con-
4-( I
,
centration
and at a higher concen-
-6
--
"
5
10
tion
"
time (H)
0
tration.
With the high
concentra-
the Cu concentration has been
-8
simultaneously determined. 15
20
26 Besides a solid state reaction with carbonate ions, intermediate dissolution of eu ions and subsequent reaction with carbonate and hydroxyl ions is also possible. sic
With an intermediate dissolution, reaction of Cu ions released by the banitrate
conceivable. tioned,
with suspended silica to a surface or bulk hydrosilicate is
also
Besides
men-
figure
the experiment with a low Cu concentration already
20 shows an experiment in which also the eu concentration
been determined during a urea precipitation.
has
During the precipitation of basic
nitrate at pH 3.8 the Cu concentration drops.
After completion of the precipi-
tation, the pH increases. At a pH of about 5 the basic nitrate becomes unstable and the Cu concentration slightly increases.
Investigation of the loaded
car-
rier shows that the Cu(II) ions have also reacted with the bulk of the silica.
100
200
300
400
500
oo
eoo
200
500
400
02
82.0
oo---.L:.... ----
660
IG
-0<
I
: : I
-OJ(
I
::
-020
I
DTG mgfmln
80.0
,_,
~
;
00
,
'; \
,----------- -66,0
I
weight
79,0
'"\/ "
78.0
, ,
770
' ,
76.0
mg
1
-02
640
rmoimG
,0." 62D
-D40
750
100
200
300
400
500
100
tefll)eroture"C"
~ CuO~
...... --silica
~
~
Second ~ reduction one step
•
400
300
500
Fig.21. Thermogravimetric reduction of Cu deposited
c~~ · t ed t'· hydrosilicate FIrs r uc IOn. two steps '" Oxidation
200 te~tlTe'C
• '"
on silica from basic Cu nitrate. Both the weight and the rate of the decrease in weight are represented. Top left:first reduction. Top right:second reduction. Bottom:structure of catalyst before and after reduction, reoxidation and second reduction.
C~
silica
Reaction
of the Cu ions with the silica can be demonstrated clearly by
duction experiments.
These experiments also show that a well dispersed precur-
sor can ask for a higher reduction temperature. metric
experiment
re-
Figure 21 shows a thermogravi-
on a carrier loaded by decomposition of basic
Cu
Besides the decrease in weight the rate of the decrease is represented.
nitrate. It can
27 seen that the reduction proceeds in two steps, one peaking at about 180 o e, o and another at about 250 e. The high temperature peak is broader. Bulk eu oxide reduces at about 180 o e, the same temperature as that of the first peak. The be
second
peak is due to
eu
that has reacted with silica.
from the second reduction of the reoxidized catalyst. not regenerated 180 o e.
The
above
groups
by the reoxidation and the
eu
This can be
eu
The
is completely reduced at
evidence indicates that the reactivity of the
surface
of silica determines whether or not deposition-precipitation
Inasmuch
concluded
hydro silicate
hydroxyl proceeds.
as OH--ions catalyze the reaction with hydroxyl groups bound to
solved metal ions,
the reactivity is strongly affected by the
is
about
dis-
pH-level.
When
ions are precipitated that are capable of forming a bulk compound with hydroxyl ions
and silica,
the bulk of the silica also reacts at elevated temperatures.
The OH--ions hence raise the reactivity of the bulk of the silica too. However, the capability to react to a bulk compound with the carrier is not required get deposition-precipitation.
Iron(III) can be deposited on silica by injection
of a weakly acid Fe(III) solution into a silica suspension, kept above 5.5 (31). is
thus obtained.
oxide
is
silica
the pH of which is
A homogeneous deposition of hydrated Fe species on silica Now the reaction of the hydroxyl groups of the hydrated
more restricted to the surface of the silica.
proceeds
to
to a smaller extent with Fe(III) at 90 0
Bulk reaction of
e
and
a
pH-level
Fe the of
about 5.5.
Deposition-Precipitation on Alumina Supports
Many alumina supports are produced by dehydration of Al hydroxides. Generally ~.
well crystallized Al hydroxides are used exhibiting particle sizes of When the hydroxide is dehydrated,
rather
narrow
crystallites. over
some
the external shape is retained and many
pores (diameter about 5 nm) are developed inside
the
original
It is extremely difficult to apply an active precursor uniformly
the large internal surface area of the alumina thus produced,
since
the
pores are long and narrow. The relatively low diffusion coefficients in liquids lead to a very slow transport into the pores of the alumina.
Often the
active
material appears to be deposited only at the outer edge of the porous particles and
has not markedly penetrated.
Though ultrasonic treatment can considerably
decrease the size of the porous alumina particles and hence improve the sibility carried
of the internal surface area, out on a large scale.
aluminas catalysts.
also
acces-
this treatment is too expensive to
The unfavourable properties of most
explains the frequent use of
coprecipitated
be
commercial
alumina-supported
28 The usual deposition-precipitation of nickel with urea at 90
0C
does not lead
to nickel being distributed uniformly over the surface of the alumina.
it was found that at about 50 distribution too
large.
over
0C
However,
hydrolysis of urea leads to the desired uniform
the alumina provided the porous alumina particles were
Precipitation by means of cyanate leads to excellent
results.
not Tb
establish the reason for the lack of interaction, the precipitation was studied more in detail.
NaOH injection 36"C Q.45mllmin O.76N
:I~
3
2
'I:'M, XJI D.'
,
Ir:TIE-;CAlC= ORET1C=lJ=Ii.;-~:- ;-v- - - 1
5
10
o
15
" 22P ultras.
o DEGUSSA
. ,, ,, ,
.. II'fT1Al.lY
D.,,_ _~ pH=3
o o
Time(hl~
o
o 22P
pH=2 ,
A1A.N"
4
INITIAlLY
D.33P pH,,2 .. 33P pH:3
40
80
-
Temperature ·C
NoOH injection AlA suspension 36"C o.78ml/min o.76N NaOH
4
2
--
----.~~/
Timefmiflj,.
a
15
30
45
60
Fig.22. Top-left: pH-curves for the Ni-alumina system indicating the interaction. Top-right: surface
area between calculated and measured pH-curve as a function
of the precipitation temperature for three different aluminas. Bottom: interaction of dissolved Al-ions with OH--ions; effect of time at pH 2
Figure 22 shows a representative experiment in which the pH-curves on addition of alkali to an alumina suspension, to
recorded
to a Ni nitrate solution,
and
a suspension of alumina in a solution of nickel nitrate are represented. The
surface
area between the theoretically calculated and the experimentally
mea-
29 sured curve is shown as a function of the temperature for three different mina
preparations.
precipitating Ni-ions varies appreciably for the three aluminas. 22P
and
Degussa
33P have been prepared by dehydration of Al
hydroxides,
large and 33P appreciable smaller porous particles.
interaction,
which
whilst
with 22P and 33P.
of 33P is
the
22P has
The extent
appreciably
of from
temperature to an almost negligible value at
Presumably due to the different preparation
the Degussa alumina exhibits a different temperature dependence.
ence
aluminas
is proportional to the above surface area calculated
the pH-curves, decreases with the 90 0 e
The
alumina has been produced by flame hydrolysis of Al chloride.
relatively
alu-
It can be seen that the capacity to interact strongly with
larger than that of 22P.
is due to the size of the porous particles.
procedure
The
capacity
It is likely that the differUltrasonic treatment of
22P
raises the extent of interaction significantly.
drop in the extent of interaction at 90 0 e could be associated with dis-
The
solution of alumina. the
precipitation
Usually the pH-value is established at 2 at the start to assure the absence of any precipitated Ni
species.
precipitation and redissolution of AI(III) has been investigated (32). precipitated alumina dissolves at pH-values below about 2.5, react
of The
Freshly
and AI(III)
with hydroxyl ions at pH levels from 2 to 4 depending on the Al
ions
concen-
tration. Though the aluminas investigated have been thermally treated, dissolution of Al when the pH of the suspension is about 2 has to be envisaged, cially at 90 o e. The right-hand side of figure 22 shows that even at 36 0
espe-
e
alumi-
na markedly dissolves. Reaction of dissolved Al ions with OH--ions brings about a
plateau in the pH-curve.
pH-level
of 3.8.
Owing to the low concentration the plateau is at a
The fully drawn curve in figure 22 is from an experiment
which the pH was immediately raised after establishment of a pH of 2.
in
The pla-
teau at pH 3.8 is rather short. The dashed curve was measured after keeping the suspension for 30 min at pH 2;
now the plateau is appreciably longer.
had been dissolved during the 30 min at pH 2. more
rapidly.
starting
The
At 90
0
e
dissolution must proceed
effect of dissolution at pH 2 at 90 0 e is
a precipitation at 90 0 e at pH 3.
More Al
As can be seen in
demonstrated figure
22,
by
the
extent of interaction is much larger. Apparently reactive Al ions dissolves and react at pH about 4 to a species not reacting with Ni ions. The above data have been
included to show the effects of the size of the porous particles,
of the
preparation of the alumina, and of the pH to which the carrier is exposed. Full details will be published.
Conclusions
The and
production of an optimal solid catalyst calls for control of
the shape of elementary particles,
the
size
of the interstices between the partic-
30 les,
as well as of the coherence of the elementary particles. Since often ele-
mentary particles of active components are of the order of some nrn, the control has
to go down to almost atomic dimensions.
An adequate
characterization
of
solid catalysts consequently requires advanced and subtle techniques.
The
procedures presently utilized to produce supported catalysts on an
dustrial
scale
catalysts
have been developed mainly empirically.
have been optimized admirably,
less serious drawbacks.
Though the
in-
resulting
all procedures suffer from
more
or
In view of the difficulty to control solid materials on
an almost atomic scale, it is obvious that the reproducibility of the catalysts is a major problem.
In
this
that can, has
review the method of deposition-precipitation has in principle,
been
presented
improve on many of the present procedures. However, it
also been shown that this method requires much research to provide
results
than the empirical procedures.
The carrier has to be characterized in
detail, and the mechanism of the teraction
better
precipitation
and the in-
of the precipitating precursor with the
must be studied carefully. the support with the active
Moreover,
support
complete reaction of
precursor must be envisaged. A
more uniform distribution of the active material and, especially, bulk reaction of the support can bring about that a more severe thermal pretreatment is required. mal pretreatment is not adapted, likely
PARTlCLE SHAPE
••••
If the ther-
the resulting catalyst is
to perform much worse than a catalyst produced
cor-ding to the present state of the art.
ac-
A thorough study
of the thermal pretreatment must accompany the
development
of the precipitation procedure.
FACETED
•••• •••• ROUfIlJED
SPHERICAL
A better distribution of the active material over the port
sup-
can lead to active particles appreciably smaller than
in the
usual catalysts. Though the active surface area can
be larger with smaller particles,
the surface structure of
very small particles may be less favourable.
As
indicated
in figure 25 the surface structure must be considered,
be-
sides the active surface area. For the effect of the particle size on the surface structure, the equilibrium shape of
Fig.23
the
active
particles in contact with the surface
of
the
carrier is important.
Figure
25
shows that the surface structure
of
completely
31 rounded
and
facetted
extremely small.
particles is not affected,
effect of the particle size is more pronounced. metals
is
and edges,
unless the particles become
With an equilibrium shape with rounded edges and corners, the Since the equilibrium shape of
between the completely rounded shape and that with rounded
corners
an effect on the surface has to be considered especially with
met-
als.
Penetration of foreign
atoms in between the surface atoms of an active com-
ponent proceeds more easily,
if the dimensions of the planes bounding the par-
ticles are small. Penetration of reactants or impurities into the surface of an active
material may also strongly affect its catalytic properties.
with oxides the catalytic activity may be due to lattice defects. fects in very small particles often anneal rapidly. lattice
Especially Lattice
de-
The small number of stable
defects in very small particles may cause the "catalytic activity to be
unexpectedly low.
It is therefore often not favourable to aim at the production of very particles
of the active component.
tions the optimim size of the active particles is about 10 nm or more. therefore
small
With a number of "important catalytic reacIt
may
be required to establish primarily the optimum particle size for the
reaction to be accelerated before endeavouring to produce very small thermostable active particles.
Deposition-precipitation thus needs much research to be capable of producing catalysts that perform better than the usually produced catalysts.
Acknowledgment The author is much indebted to his coworkers for their contributions to
the
development of the "art" of deposition-precipitation. J. v.d.Meyden and J. v.Hofwegen
studied
and J. Bartels the iron catalysts,
and
the copper catalysts, A.C. Vermeulen,
W.J.J. v.d.Wal
P. de Bokx and
A.F. Wielers the nickel-on-alumina catalysts.
References 1. Catalyst Handbook
Chapter 2 & 3
Wolf Scientific Books, London (1970) 2. Charles L Thomas Academic Press,
"Catalytic Processes and Proven Catalysts" Chapter 1 New York (1970)
3. Charles N. Satterfield "Mass Transfer in Heterogeneous Catalysis" Chapter 1 MIT Press Cambridge Massachusetts USA
(1970)
32 4. J.M. anith "Chemical Engineering Kinetics" 2nd Ed i t.i.on Chapter 8 - 13 McGrawHil1 New York (1970) 5. G. Gubitosa, A. Berton. M. Camia and N. Pernicone Preprint E 6.1 Preparation of Catalysts III (1982) 6. L.L. Hegedus. T.S. Chou. J.C. Summer and N.M.
Potter in
"Preparation of Catalysts II" (B. Delmon, P. Grange. P.
Jacobs and
G. Poncelet eds.) p.171 Elsevier Scientific Publishing Ce. Amsterdam (1979) 7. G.H. v.d.Berg and H.Th. Rijnten Ref.6 8. G.K.
p.265 Boreskov in
"Preparation of Catalysts" (B. Delmon, P. A. Jacobs and G. Poncelet eds. ) p.233 Elsevier Scientific Publishing Ce. Amsterdam (1976) 9. M. Kotter and L. Ref. 6
10. J. Cervello, Ref .8
Riekert
p.51 E. Hermana, J.F. Jimenez and F. Melo
p.251
11. W. Langenbeck, H. Dreyer and D. Nehring Naturwissenschaften 41
(1954) 332
W. Langenbeck, H. Dreyer and D. Nehring Zeitschr. Anorg.Allgem. Chem. 281
(1955) 90
12. Ref. 1 Chapter 7 1 3. Ref. 1 Chapter 5 & 6 14. S.P.S. Andrew Ref.8 15. J. R.
p.429 Rostrup-Nielsen "Steam Reforming Catalysts"
Teknisk Forlag A/S (Danish Technical Press Inc.) Copenhagen (1975) 16. C.L. Thomas "Catalytic Processes and Proven Csatalysts" Chapter 11 & 14 Academic Press New York (1970) 17. A. E. 1. Nielsen "Kinetics of Precipitation" Pergamon Press New York (1964) 18. Jaroslsav Nyvlt translated by Paul Feltham "Industrial Crystallisation from Solutions" Chapter 1 Butterworth London (1971) 19. Neth. Appl. 67 05,259
14 Apr. 1967 (Stamicarbon)
Neth. Appl. 67 12,004
31 Aug. 1967 (Stamicarbon)
20. Neth. Appl. 68 13,236
16 Sept. 1968 (Stamicarbon)
21. H.H. Willard and N.K. Tang 22. W.J. Blaedel and V.W.
Ind. Eng. Chem. Anal. ill. 9 (1937 ) 357
Meloche "Elementary Quantitative Analyses"
2nd illition p.177-184, p.719-729 Harper & Row New York (1963)
33 23. P.F.S. Cartwright, E.J. Newman and D.W. Wilson The Analyst 92 (1967) 663 24. Ger. Offen.
1,964,620
16 Jul. 1970 (Stamicarbon)
25. Ger. Offen.
1,961,760
09 Jul. 1970 (Stamicarbon)
26. Neth. Appl.
68 16,777
23 Nov. 1968 (Stamicarbon)
27. Ger. Offen.
1,963,827
09 Jul. 1970 (Stamicarbon)
28. L.A.M. Hermans and J.W. Geus Ref.6
p.111
29. J.P. Brunelle Ref.6
p.211
30. R. K. Her "The Colloid Chemistry of Silica and Silicates" Chapter 3 Ithaca
Cornell University Press 31. Neth. Appl.
70 05,647
New York (1955)
09 Apr. 1970 (Stamicarbon)
32. A.C. Vermeulen, J.W. Geus,
R.J. Stol and P.L. de Bruyn
J. Colloid and Interface Sci. 51
(1975) 449
This page intentionally left blank
35
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THEORETICAL AND EXPERIMENTAL ASPECTS OF CATALYST IMPREGNATION S.Y. LEE and R. ARIS University of Minnesota, Minneapolis, Minnesota (U.S.A.)
ABSTRACT Several aspects of catalyst preparation have been modelled and experimentally investigated.
Those included here are the process of imbibition of a solution
of catalytic precursors into a dry porous sphere, the corresponding distribution of solutes when adsorption is very rapid and the modification of this distribution by finiteness of adsorption rate, diffusion during and after imbibition and by the drying process.
INTRODUCTION The preparation of a catalyst involves a number of steps such as the impregnation, redistribution and drying which are in their several ways amenable to mathematical modelling.
Considerations of space will allow us only
to set out the assumptions that have gone into the models, their equations, their results and comparisons with experiment.
No attempt will be made to
survey the background literature and the recent publication of an excellent review by Neimark et al.
(ref. 1) makes this omission acceptable:
full
results are in the dissertation of the first author (ref. 2) and a more copious review is in progress.
The general objective is straightforward,
namely to set down such methods as may provide a rationale for the better design of supported catalysts, particularly of those where the active layer is to be buried some way beneath the surface in the so-called "egg-white" configuration. The equations of imbibition and adsorption will first be discussed, followed by a model of the hydrodynamics of dry imbibition and its experimental confirmation.
The effects of finite adsorption rate and of diffusion will be
treated and of the aftermath of impregnation. adsorbate during drying is considered.
Finally the redistribution of
It is not to be supposed that the
detailed quantitative results will always provide precise predictions, but the qualitative and relative effects of the different factors provide a useful framework for catalyst design.
36 DRY IMPREGNATION In setting up equations for the distribution of solute concentrations during impregnation it will be assumed that:
i, the pellet is a homogeneous, porous
sphere which is initially dry; ii, the support surface is completely wetted by an aqueous solution; iii, the total amount of aqueous solution is considerably more than that which would be needed to saturate the surface of the support; iv, the pores are large enough so no physical exclusion of the solute occurs; v, the immobilization of the solute on the surface is an isothermal, reversible Langmuir adsorption; vi, mass transfer at the external surface is negligible; vii, spherical symmetry is maintained throughout the process. Then if n. (r,t)
cj(r,t) is the concentration of the jth solute in the pore and
its concentration on the surface at distance
J
at time
r
from the centre and
t, dc.
'j(t)
af
(1)
+
;In.
J
k
8t where
E
=
porosity, a
imbibition, D
=
-:-n. J J
(2)
j concentration, k+
J
and
=
area per unit volume, V(t)
= effective diffusivity of solute,
n
volumetric rate of
= saturated surface
s k. = adsorption and desorption rate constants. J
Initially
a
c. (r,O) J
(3)
and at the boundaries
r=R
(4)
I
aV(t)n.
(5)
J
where
rf(t)
o
c.
is the position of the imbibed liquid front and
concentration.
J
the external
Note that
(6)
V(t)
When the diffusional relaxation time is long compared with the time of imbibition the diffusional fluxes can be neglected.
When
k~
J
and
k.
J
are
large we can assume that equilibrium is attained and
n.
J
Then we can dispense with eqn.
k.
J
+
-
(7)
k./k. J J
(2) and write eqn.
(1)
37
dc.
a at
J
{c. + J
(8)
:lr
Letting
t
J
T*
(9)
fractional volume imbibed at time t
V(t')dt'
o
and
fractional volume beyond radius
1
P
r,
(10)
we have the equations of multicomponent column chromatography with T*
as position and time.
Rhee et al.
These are well understood.
p
and
See references in
(ref. 3).
When only a single component is involved we can drop the suffix and write kc ,
u
f(u)
(11)
°
(12)
giving
au
af
+
dP
,IT*
with
°,
u(p,O)
n(O,T*)
u
°
(13)
The characteristics are straight lines of constant Since
f' (u).
f" (u)
U
and slope (dT*/dp) =
< 0, the characteristics in this problem,
from the origin, overlap and must be separated by a shock. shock between
a
The speed of a
is given by
and
(14)
[f(u) l/[u]
(dT*/dp)12
all emanating
°
o = U and u = and a = f(uo)/uo. Thus the saturation l 2 front moves inward to occupy a volume of (1/0) times the volume imbibed.
In this case
u
The solution of eqns.
°
U(p,T*)
u
U(p,T*)
°
(12)
and (13) is thus
T* > op (15) T*
< op
An egg-shell catalyst is formed with a shell thickness of R{l -
[(0_1)/Ojl/3}
R/3G
if
G
is large.
When there are two components we must set
38 v (16)
x
p
[*
y
r
p
for then dU l dX
1 + u +
2 1',2
u
l
(x,O)
dU I dy
= 1 + u
I',
= u
dU l
u
l 1',2
3Y
1',2 u
where
2
+
l
2(x,O)
dU 2
<
~
+ u 2· = 0
dU 2 dy
0
1 + u +
1',2
l
(17)
dU 2 ()y
(18)
0
Also
,
(19)
This pair of partial differential 'equations is reducible (Cf. p. 280 of ref. 4)
and, mirabile dictu, has straight characteristics.
that the solution to eqns.
(17)-(19) at a 'time'
region's represented in Fig. 1.
y=Y
consists of three
An egg-shell region, from
:
~
I::
:
It can be shown
x=O
to
x=C, in
x
Fig. 1. The distribution of two solutes in the equilibrium case.
which the concentrations u and u are those of the liquid beyond the outer 2 l 00. surface, ul' u an egg-whlte region, from x=~ to x=~, a region where the 2; more strongly adsorbed component is absent (without loss of generality we can take U l u
l
«1 u
2
= 0
and this is then beyond
K=".
u and u 2) l The values of
1 0 0 0 {K-l+K u + u + [«-1+< u 2 2 2 l
til;
an egg-yolk region with
A
ul' E,
and
~
are:
u 0)2 + 4K u o o]l/2} lu 2 l
(20)
39 (21)
Thus knowing the adsorption characteristics of a binary mixture allows the calculation of where the egg-white zone will lie and how much of the solute will lie in this region. v, the values of for
,
u
o
In fact given the adsorption characteristics
and
u
o
K
and
can be chosen to put the band in any position,
l 2 can only approach 1 as the imbibed volume fills the total pore space.
THE DYNAMICS OF IMBIBITION
.*
Though the striking thing about the results of the previous paragraph is that the transformation to rate
makes them valid for any time dependent volume
V(t) whatever, the neglect of the diffusion terms and the assumption of
adsorption equilibrium da require that we should take some notice of the rate of imbibition. Washburn studied the rate of entry of fluid into a capillary (ref. 5) showed that the length filled, x
x
2 f
f'
at time
t
and
was given by
(aycos8j2p) t
for a horizontal tUbe of radius
a, y,
8
and
W being the surface tension,
contact angle and viscosity of the fluid respectively. velocity at
t=O, but, though Bosanquet (ref. 6)
This gives an infinite
and others corrected this
anomaly, the corrections only affect the penetration velocity in a first interval 2j4w. of order pa The Washburn equation has been tested experimentally, both in capillaries (ref. 7) and porous solids (ref. B). Here we assume that in ideal imbibition a sharp, spherical front of liquid moves inward compressing the trapped air, Darcy's law is applicable, inertial and gravity forces are negligible and the capillary force can be characterised by a representative pore size.
This allows us to use a quasi-steady-state
approximation by writing Darcy's law as v(r)
(kdjw) (dPjdr)
Then, putting and
4nr
2v(r)
V(t)
and integrating between the front at
r
r=R, gives
P(R)
(22)
Now
P(R) = P the constant atmospheric pressure, while p(r is the difference f) a between the pressure of the trapped air and the capillary pressure, Pc. If
Pc = 2ycos8ja, where
a
is the characteristic pore radius, and the trapped air
40 pressure is
P
a
R3/r~
(i.e. the trapped air does not dissolve) then
1 +
(J
-
(R/r )
3
f
(23)
(R/r ) - 1 f
where
= 8- 3
(J
notation
T
P
1
= 1 -
f
=
£2
kdP a --2 t pER
~
6
+
2ycos8/a Pa.
This can be integrated and, using the
(rf/R) 3, U)f = rf/R, it gives
£3
2 (1 - w f)
~n
85
1 + 6 + 6 (l - 6)2
~tan -1 13
2w
f
6
6
Pf + 3
2
(w
f
-
-
6)
pf 1 3 1 - 6
2
2 2 w + wfS + 6 f
+ 6
136
~nll
-
tan
-1
2+6J
(24)
136
(23) it is clear that w -> 6 as 8 --+ 00. f If we take the limit of instantaneous solution of the trapped air (or its
From eqn.
escape by some mysterious means) so that the inside pressure is always
a
then
(25)
(JT
and the imbibition is complete when
t
P
X
1/60, Le.
2 pER a/12kdycos8
(26)
Fig. 2 shows the course of ideal imbibition in the two limiting cases.
When
this final time is used to make the time dimensionless 60x
3{ 1 -
(l
(27)
This universal curve is found to fit the data on water and hexadecane of Wade et al. complicated.
(ref. 9)
remarkably well, though their mathematical model was more
In the model with gas entrapment, which requires an infinite time
to reach equilibrium, the end was, somewhat arbitrarily, taken to be the time at which 99.9% of the fluid had been imbibed.
Our own experiments with the air
trapped within have been more qualitative but have produced general agreement; the detail of these will be discussed elsewhere.
Needless to say, minor
irregularities often lead to the air being expelled as a bubble, but sometimes the build-up of pressure finds a structural weakness and the pellet itself is fragmented.
For the system we have looked at
the time of imbibition about 20-25 sees.
0
is of the order of 125 and
41
1 · 0 , - - - - - - - - - , - - - - - - , - - - - - - -.....- - - - ,
Fig. 2. The course of imbibition, --- with air entrapment, --- without air entrapment.
DRY IMPREGNATION OF THE KINETICS OF ADSORPTION If the dynamics of imbibition are known we can solve the transient equations for the case of a finite rate of adsorption.
e.
u
n/n s
J
0
r
c/c j
j
1
Let (r/R)
tit
T
(28) 2 )lER /kdP
t
n.
a
un
J
s
/c~
+
+
J
0-
K.c.t J J
k. J
k.t
K. J
J
and V(t)t/41TER
Q(T)
3
The equations (1) - (4) translate into d e j} __ J + (l-r) 2 {"u. n j dT dT de. _J dT
+
K. uj(l
J
+
- zei)
Q(T)
K.
J
duo _J Clr
e.
J
(29)
°
(30)
f. J
with uj(O,r) Q(T)
ej(o,r)
is given in terms of
terms of ae. _J dT
T
f. J
by eqn. au. __ J dT
and
° (24) .
+
u (T, 0) j
(31)
1
r f = 1 - wf by eqn. (23) and
w
implicitly in f But these equations can also be written
q t t , r)
du.J __ ar
-n. f . J J
(32)
42 where
q(T,f) = Q(ff(T))/(l-f)2, and in this form it becomes clear that there
are two sets of characteristics: Ch 1:
the curves (1_f)3
Ch 2:
the straight lines
(33) f=B
These characteristics in the (f,T) and the (B,A) planes are shown in Fig. 3.
chl
.
-
, ,
,-'
-oc o
ch2 :
,, , ,,I , ,
,, '.0
Fig. 3. The characteristics and their transformations.
The details of the method of computation with these equations and the full range of results will be reported elsewhere; one typical result is given in Fig. 4.
The broken line shows the equilibrium case in which an egg-white
catalyst with a coverage of
8
1 buried beneath a layer in which
= 0.6 01
in a thin band is only 0.05.
0, 1, 2 are all with the same value of
0.106 S f S 0.111 is
The sequence of curves
but with decreasing ratio
As the relative rate of adsorption of the second species diminishes it is less effective in pushing the first into a narrow, more highly concentrated band. The sequence 0, 3, 4 are all at the relative rate
k+/k+ but of increasing 2 1 It is clear that in spite of the rate of adsorption of the second species
being greater there is no semblance of egg-whiteness once
K
gets close to 1.
The effect of diffusion during imbibition is similar to that of finite rate of adsorption, namely the blurring of otherwise sharp contours.
Space does
not permit full description of the methods and results here and they will be examplified by a single figure which links with the preceeding case.
In Fig. 5
the curve labelled 0 is the same as the curve with the same label in Fig. 4 The remaining curves in -6 2 Fig. 5 show the effect of diffusion. With D = D = 10 cm /sec only a slight l 2 -5 -6 vestige of the peak remains (curve 1); with = 10 ,D = 10 ,the peak 2 -5 is pushed a little deeper (curve 2); with D = 10 all sign of the 2 peak is gone.
that showed the effect of finite rate of adsorption.
43
I
0
I
,; ·
LS
~,
"1 "
· ,; · ,;
LS
"
:
,I ,
" · LS "
,
I
I
k; ~
0.5
"1
· " " · ,; "
2
LS
LS
" ·"
o
"· " · 0.' "
'1
1.5
'1
O.()Q
Fig. 4. Modification of the equilibrium distribution (shown dotted) by finite adsorption rate.
Fig. 5. Modification of finite adsorption rate distribution by diffusion.
THE AFTERMATH OF IMPREGNATION If the pellet is left in the impregnating solution after the impregnation is complete, the solutes will continue to diffuse in the pores of the pellet and redistribute themselves on the surface. in this period.
There is no convective transport
The concentrations and coverages now satisfy
au. J F I-f 26.
au.
__ J
_J
(34)
a~
ao.
_J
K.
J
a1 where
u.
Je
and
8.
Je
(35)
8.
J
are the profiles at the end of imbibition.
can be solved by the Crank-Nicholson or fully implicit method.
These equations Fig. 6 shows
the effect of letting the profile established as curve 3 in the previous figure relax under the continued action of diffusion. white effect is quickly wiped out.
Clearly the already small egg-
44
,060 i§ 050 ;::: ~
,0 40
,030 , 020
!z u.J 5i o
..
"
,010
Fig. 6. Modification of profile by diffusion after imbibition. distribution is curve 3 of Fig. 4.
The initial
REDISTRIBUTION DURING DRYING Since the air has been compressed in the centre of the sphere by imbibition it is reasonable to suppose that in the drying process the evaporation takes place from the surface of the sphere and the core expands so that the wetted region is always the outer annulus.
There is thus a convective and diffusive
transport through the pores and a further redistribution of the solutes.
The
drying process has been described as having four stages (refs. 10,11); in the first or preheating period the entire system is being warmed up and the drying rate increases; in the second the temperature has reached the so-called pseudowet-bulb value and, the external surface being kept wet as the liquid is drawn to the surface by capillary action, the drying rate is constant; in the third stage, dry patches appear on the surface but the liquid within'is a continuous phase of minimal connection; in the fourth, the moisture is in isolated patches and the drying rate--as in the previous stage--falls off. It goes without saying that the detailed modelling of this process in a porous solid is more than a little difficult and we approached it by first considering a single capillary tube.
For this system, assuming initial
imbibition with compression of the trapped air, the equations of heat conduction, liquid motion and vaporization were solved to determine the course of the evaporation.
The mass balance equations fer the receeding column were then
solved to describe the redistribution of solute.
Figs. 7 and 8 show two cases
in the first of which the somewhat feeble egg-whiteness is destroyed by the drying process (X
is the fractional depth of the fluid front). In the second f case however the band is well preserved. The difference between the two cases
45
Profile
t
[min]
X
f
t
[min]
_'_'
_I
,.w Fig. 7. Modification of a poor eggwhite distribution during drying.
Fig. 8. Preservation of good eggwhiteness during drying.
-6 cm2sec / "In the flrst but 0 = 10D = 10 -5 em2/ sec In ' = O = 10 1 2 1 2 the second. Otherwise the parameters are the same for both: namely, o 0 -6 2 c 0.0091 (mol/I), c = 0.091 (mol/I), n 1.88 xIO (mol/m) 111 10D 41.32, 2 l 2 s -3 + + k 2k = 1.83xIO (l/mol). k 2k = 21.11 (l/mol. sec.). l l 2/3 2/3
is that
0
ACKNOWLEDGEMENT We are indebted to the Petroleum Research Fund of the American Chemical Society for support of this work. REFERENCES 1
A. V. Neimark, L. 1. Keifez and V. B. Fenelonov, Ind. Eng. Chern. Prod. Res. Oev., 20 (1981) 439-450. 2 S-Y. Lee. "A study of the preparation of supported catalysts," Ph.D. Dissertation. University of Minnesota. 1981. 3 H-K. Rhee, R. Aris and N. R. Amundson, Phil. Trans. Roy. Soc., A267 (1970) 419-455. 4 R. Aris and N. R. Amundson. First-order partial differential equations with applications. Prentice-Hall. Englewood Cliffs. 1973. 5 E. W. Washburn, Phys. Rev., 17 (1921) 273. 6 C. H. Bosanquet, Phil. Mag., 45, ser. 6 (1923) 525-531. 7 L. R. Fisher and P. D. Lark, J. Colloid and Interface Sci., 63 (1979) 486. 8 J. VanBrakel, Powder Tech., 11 (1975) 205-236. 9 W. H. Wade, J. A. Wingrave and R. S. Schechter in J. F. Padday (Ed.), Wetting, Spreading arid Adhesion. Academic Press. New York, 1978, pp. 261-288. 10 J. M. Coulson and J. F. Richardson. Chemical Engineering, Vol. 2, Pergamon Press, Oxford, 1956, pp. 857-859. 11 D. Berger and D. C. T. Pei, Int. J. Heat and Mass Transfer, 16 (1973) 293-302.
This page intentionally left blank
47
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
cmWETITIVE ADSORPTION OF H
zPtC1 6
AND HCl ON AI Z03 IN THE PREPARATION OF NAPHTHA
REFORMING CATALYSTS
A.A. CASTRO, O.A. SCELZA, E.R. BENVENUTO, G.T. BARONETTI, S.R. DE
~IIGUEL
and
J.H. PARERA Instituto de Investigaciones en Catalisis y Petroqulmica -INCApESantiago del Estero Z654 - 3000 Santa Fe, Argentina
ABSTRACT The HCl competition iq the H deposition on AI pellets to obtain ZPtCl6 Z03 0.38% Pt was studied. The radial profiles of platinum and chlorine surface concentration for different HCI concentrations in the impregnating solution were measured. For low HCI concentrations the Pt penetrations into the pellet are incomplete and the Pt profiles sharply decrease towards the interior whereas the CI profiles increase. Both profiles are uniform for high HCl concentration. The results were interpreted through a mathematical model based on a diffusionadsorption process with very fast and irreversible HZPtCl and HCI adsorptions. 6
INTRODUCTION llany industrial catalysts consist of small crystallites dispersed on a support. Their manufacture includes impregnation of the support with a solution of the active component or metallic precursor, drying and treatments to transform the metallic precursor to the active form. The activity, selectivity and stability of these catalysts depend, among other factors, on the distribution of the active component into the porous structure of the support. For multimetallic and bifunctional catalysts those properties are also related to the relative distribution of the active components. In the preparation of these catalysts both the impregnation and the drying steps can define the surface concentration profiles. If the adsorbate-support interaction is strong, the distribution of the metallic precursor will be defined during the impregnation, with no change during drying (except when the metallic load is higher than the adsorption capacity of the support). But if the interaction is weak, the drying step will be important. For the bifunctional Pt-CI/AI
naphtha reforming catalyst, when the support is impregnated with Z03 HZPtCl6 the metal is deposited in the outer shell of the particles due the
48 strong interaction of the metallic precursor with AI
[1, Z]. The resulting Z03 high metallic surface concentration favors the sintering of the metallic phase, changing negatively the activity and selectivity. The Pt distribution can be modified by coimpregnation of the support with H and some competitor ZPtCl6 substances (such as HCI, HN0 3, etc. [1, Z, 3J). When using HCI the penetration increases [Z] and simultaneously chlorine is added to the support as active component which enhancing the acidic function of the catalysts. The competitive effect of HCI on the H adsorption on AI ZPtCl 6 Z0 3 during the preparation of the reforming catalyst is studied.
EXPERUlENTAL A commercial Y-AI Z03 (Cyanamid Ketjen CK 300) with a specific surface area of 170 mZ/g, pore volume of 0.5 cm3/g and porosity of 0.55 in extruded particles (1/16" x 5 rom), calcined in a stream of dry air during 3 h at 650 DC was used as support. The impregnations of the support were performed at 30 DC with aqueous solutions of HZPtCl6 to obtain approx. 0.38% (wt) of Pt. To study the competitive action of HCI on the H adsorption, HCI was added to the aqueous solution of ZPtCl 6 HZPtCl6 to obtain HCI concentrations of 0.1, 0.3, 0.4 and 0.6 mol/I. The volume of the impregnating solution was 1.4 ml/g AI
and the impregnation times were Z03 Z, 5, 15, 30, 60 and 600 minutes. After the impregnation the samples were
drained off and washed with distilled water. The solid and liquid fractions were analyzed for Pt and CI. A chromatic development technique with HZS was used to measure the Pt penetration into the support pellets. By the HZS action the zones occupied by the metal turn into a dark brown color. Iffien the pellets were transversally cut it was observed that for competitor concentrations of 0.1 and 0.3 mol/l the section showed a dark outer shell and a white central nucleus. On the other hand, for HCI
co~centrations
of 0.4 and 0.6 mol/l the sections were completely dark.
!1easurements of the central nucleus radius were done with a microscopy Projectina Model 4014/BK-Z. To determine the radial distribution of Pt and CI surface concentrations, impregnations of cylindric tablets of 1.5 cm diameter and 1.7 cm length were done. The tablets were obtained by compression of AI Z03 powder. Then they were calcined at 650 DC and finally impregnated during 6 h being the other conditions as mentioned above. The plane ends of the tablets were covered with stearic acid before the impregnation to avoid the axial diffusion of active species. After the impregnation several portions of the tablets corresponding to different radial positions were obtained by uniform abrassion of their lateral faces. In these portions CI and Pt were determined. The adsorption isotherms of HCI and H PtCl on y-AI were determined Z03 Z 6
49 leaving Z g of AI in contact with ZO ml of HCI or H Z03 zPtCl 6 aqueous solution of different concentrations during 6 h at 30°C. Then the liquid was separated from the solid and in both of them CI or Pt were analyzed. g of alumina was left in contact
To study the acid attack to the support,
with 1.4 ml of HCI solution in concentrations between 0.08 and 0.65 mol/l. Then the solid was drained and repeteadly washed with distilled water. The liquid 3 by a complexometric method using dithizone as
collected was analyzed for AI+ indicator.
Pt was analyzed by photocolorimetry using SnCI Chlorine in solid samples Z' was determined titrating with AgN0 the liquid obtained according to two 3 methods: dissolving the solid with H or extracting it with NH Chlorine 40H. ZS04 in liquid samples was determined by titration with AgN0 3. RESULTS Figures 1 and Z show the molar amounts (deposited plus occluded) of HCI (species A) and H (species B) referred to one gram of support as a function ZPtCl6 of the impregnation time for different initial HCI concentrations. The molar amount qi - qi is the difference between the initial amount in the impregnating solution (qi) and the molar amount in the external solution (qi) at a given impregnation time for the species i. These experimental results were correlated according to the following function:
750
e ....
o ....
0
0
....J
....J
z
17
~
1.0
'52
'52 ~
15
rn
~
0"
0"
I
om
0'
~
13
0"
0"
0
-:::
19
V
....
-: "
11
0
30
60 600
TIME (MIN)
Fig. 1. HCI molar amount (deposited plus occluded) as a function of the impregnation time for initial HCI concentrations of: 0.1 (~); 0.3 (.); 0.4 (y) and 0.6 molll (.). The curves correspond to Eq. (1).
0
30
60 600
TIME (MIN)
Fig. Z. HZPtCl6 molar amount (deposited plus occluded) as a function of the impregnation time for initial HCI concentrations of: 0 (e); 0.1 (~); 0.3 (.); 0.4 (y) and 0.6 molll (.). The curves correspond to Eq , (l).
50 t
(1)
a. + b. t 1
1
where a
and b are parameters depending on the impregnating conditions and t is i i the time. The values of a and b calculated by linear regression from Eq. (1) i i are given in Table 1.
TABLE 1 Values of the parameters of Eq. Initial HCI cone. (mol/I) 0 0.1 0.3 0.4 0.6
q~
q~ 10-
(1)
6
mol/g
10-
6
mol/g
18.76 19.00 19.00 19.30 19.65
140 423 562 833
4.38 -2.07 1. 52 0.76
7.40 2.63 2.00 1. 38
0.96 10.00 11.30 12.93 29.16
53.47 52.58 52.63 54.15 60.23
Figure 3 shows the results of the Pt penetration (F):
(2)
P
as a function of time, being Rand r
f
the pellet and the nucleus radius
respectively. The radial profiles of H2PtCI6 and HCI in tablets are shown in Figs. 4 and 5.
-
100
0
0
z
Q
~
....0:::W Z W
D-
o::
a a
30
60 600
TIME (MIN)
Fig. 3. Pt penetration as a function of the impregnation time for different initial HCI concentrations: 0 (0); 0.1 (~); 0.3 (,); 0.4 (y) and 0.6 mol/l (.). The curves were obtained from Eq. (15) with C ~ 4.59 x 10- 4 mol/g. s
51
:!.
(a)
0.6 MOUl
•
•
•
Hel
I
0.6 MOLll H Cl
5 • (a)
•
3L6
0.4 MOUl H Cl
•
(b)
•
----l
0.4 MOUl H Cl • -~-----l
4/~.
•
•
2L-
...J
C>
....... U)
0.3MOL /l H Cl
C3
n:
~
N
:x:
•
-I
0
4"/
0.3MOUl H Cl
2 o ;:[ O'--I
•
;:[
(~). ..I..-_..J
LO
'52
(d)
0.1 MOUl H Cl
0.1 MOUl H Cl
OL-_--I. 15 10 r-----=--=", 5
0.8
---l
• 0.6
0.4
0.2
0.8
0
r/R Fig. 4. HZPtCl6 surface concentration as a function of the radial position for different initial HCI concentrations. a-d: 0.38% Pt, e: 0.97% Pt
Fig. 5. HCI surface concentration as a function of the radial position for different initial HCI concentrations. a-d: 0.38% Pt, e: without Pt
10-6/CONC IN L1Q (MLlMOLl 0
(5
5
10
(5 100
..... -I
'-I
o;:[
~ 10
:> ":J 50 H Cl
o0'=-----±c
Z-----!4c-----'
10-4/CONC IN L1Q (MLlMOLl Fig. 6. Linearized plot of the adsorption isotherms for HCI and HZPtCl6
~ M
+
o
0.2
0.4
0.6
INITIAL H CI CONe (MaUll Fig. 7. Acid attack expressed as AI+ in solution as a function of initial HCI concentration
3
52 The adsorption isotherms of HCl and H follow the Langmuir model. The 2PtC1 6 4 7 values of the Langmuir coefficients are 3.1 10 ml/mol H and 6.6 10 2PtC1 6 4 ml/mol HCl and the adsorption capacities are 1.4 10- mol H and 4.7 10- 4 2PtC1 6/g mol HCl/g. Figure 6 displays the linearized adsorption isotherms. The influence of the HCl concentration on the acid attack is given in Fig. 7. INTERPRETATION AND DISCUSSION OF RESULTS Figure 3 shows that for initial competitor concentrations of 0, 0.1 and 0.3 mol/I, the Pt penetration reaches 8%, 25% and 70%, respectively. But for concentrations of D.4 and 0.6 mol/l the penetration is complete. Pavlikhin et al.
[4] have also found in impregnatio~s
203 with HCl and H2Pt C1 6 that the
of A1
penetrations of both substances are equal and they depend on the HCl concentration in the impregnating solution. The incomplete Pt and Cl penetrations are caused not only by the strong HCl and H2PtC16 interactions with the support but also by the low amounts of substances which are not enough to saturate the surface. These results show that the interactions of both species with the support are essentially irreversible. This agrees with the high Langmuir adsorption coefficient values mentioned before and the results obtained by Shyr et al.
[6].
and Van der Berg et al.
According to Sivasanker et al. adsorption on Y-A1
20 3
OH
Cl
Al
+ H C l - ll
I
[5]
[7]
and Santacesaria et al.
[8J
the HCl
can be represented as involving only one surface site:
+
(3)
Besides, the H Pt C1 adsorption on A1 can be pictured as [9J: 2 203 6 OH
+
2
I
Al -
Cl
Cl]-
0/
0
Cl~P(-Cl
t
I
Al
+ 2 Cl
+
(4)
I
Al
The released Cl- may be fixed on the alumina and therefore the adsorption of one molecule of H2PtC1 would occupy four surface sites. 6 Table 2 shows that in samples impregnated with HCl amounts lower than the adsorption capacity of the support, the chlorine content determined either by extraction with NH
or by acid dissolution are the same as the chlorine amount 40H initially present in the solution. In the samples impregnated with H 2PtC16 amounts lower than the adsorption capacity, the chlorine content measured by
extraction with NH is equal to the chlorine coming from the H added 2PtC16 40H whereas the chlorine content determined by acid dissolution is smaller. It is
53 infered from these results that a fraction of the chlorine would be bonded to the deposited Pt and would correspond to the difference between the chlorine contents measured by both methods (Table 2). Such difference referred to one atom of Pt is approximately four. This might indicate the existence of the adsorbed Pt compound shown in reaction (4). TABLE 2 Chlorine analysis in samples impregnated with R or RCl 2PtCl6 Cl added
Pt measured in solid
Cl in solid (a)
Cl in solid (b)
(umoL,'g)
(\lmol/g)
(umoL' g)
(umoL' g)
118.8(c) 660.0(c) 141.0(d)
19.8 110.0
39.6 220.0 240.0
118 650 130
Cl (b) -Cl (a) Pt 3.96 3.91
(a): measured by acid dissqlution; (b): measured by NR40R extraction; (c): samples impregnated with R2PtCl6 and (d): samples impregnated with RCl Considering that the R and RCl are adsorbed on the same sites and the 2PtCl6 reactions (3) and (4), the RCl/R molar adsorption capacities ratio will be 2PtCl6 four. This value is close to the ratio obtained from the adsorption isotherms. Sivasanker et al. [7] and Santacesaria et al. [8J have pointed out that the surface reactions of R2PtC16 and RCl with Al are very fast. So these 20 3 adsorptions will be kinetically controlled by mass transfer being the internal diffusion the controlling step. Therefore the impregnation process can be simply described as follows. There is an adsorption front in the support particle which advances as the diffusion proceeds. Behind the adsorption front there are adsorbed species which saturate the surface. Ahead the front the surface concentration of adsorbed species and the amounts of species in solution are zero. The results can be interpreted through a mathematical model based upon a diffusion-adsorption process according to the following development. The mass transport for R2PtCl and RCl in a differential element behind the 6 adsorption front, for a cylindric particle of radius R and length L»R, is: :lC i at
=
D. l
l.l
(r dCi ) r dr \ dr
(5 )
The mass balance for each solute in the external solution is given by:
V
e
(6 )
54 The advance rate of the adsorption front is expressed as:
dCAI
-D
(7)
A dr
where C. (mol/cm 3 ) is the molar concentration of i inside the porous, t (min) :L
the time, D (cmZ/min) the effective diffusivity of i, r (em) the radial i 3 position, r (em) the radial position of the front, C (mol/cm ) the molar f il 3/g) concentration in the external solution, V (cm the external solution volume e 3 per gram of support, p (g/cm ) and € are respectively the apparent density and p
porosity of the particle, C (mol sites/g) the adsorption capacity of the s support, A is the ratio between the number of sites occupied by one adsorbed H molecule and the number of sites occupied by one adsorbed HCI molecule. ZPtCl6 The initial and boundary conditions are:
t
C.
0
r
R
:L
0
r t > 0
r r
f ~
'" ~
(8)
< r < R
(9) (0)
C.
R
r
o
:L
f
0
o r dC./d :L
(11)
o
(12)
The mass conservation equation for the system is:
(13)
The first member of Eq. (13) contains the A and B amounts disappeared from the external solution. The second member contains the occluded amounts into the porous and the deposited amounts between r f and R. By
c.:L
application of a quasi steady-state approximation to Eq. (5): In rf/r (4)
Ci l In rf/R
The assumption of the quasi steady-state is valid when the rate of ffiotion of the front is slow compared with the rate of relaxation of the mobile phase molecules diffusing into the porous, that is when C
s
p /s Z C.»1 [10J. p
i:L
Introducing Eq. (14) in Eq. (13), integrating and considering qi results:
~
C V i l e,
55
ve
p
p
(15)
In rf/R
As from Eq. (1), qi-qi is related to t, Eq. (15) gives the position of the adsorption front as a function of time. With ai' b
and rf/R values (the latest i obtained from Pt penetration results) for each initial condition, and assuming
A; 4, Cs was determined from Eq. (15). The C value that best fits the results s 4 was 4.59 10- mol/g. The curves in Fig. 3 were drawn with this value showing a good concordance. Moreover, the value of C is similar to the determined from s the adsorption isotherms. The quasi steady-state approximation is adequate because the C p /S L C. > 1 even for the highest HCI concentration. s p i 1 . The punctual surface concentratlons (9 , mol/g) of both species for any i radial position behind the adsorption front (1' > r are related according to: f) C
(16)
s For a given position of the adsorption front the average surface
concentrations of Pt and CI between Rand r
(q~-qA)
f
are given by:
- =v--~'->· e
(17)
(18)
The punctual surface concentration can be approximate by:
(19)
The Pt and CI profiles, calculated according to Eq. (19) are shown together with the experimental results in Figs. 4 and 5 and a good agreement is observed. The results denote that for competitor concentrations lower than 0.4 mol/l the penetration is not complete and both elements are deposited in the outer shell. In this case the Pt radial profiles sharply decrease towards interior of the tablets whereas the CI profiles increase. For higher HCI concentrations both species completely penetrate ane: the profiles are less sharp, so, for 0.6 mol HCI/I both elpments are uniformly distributed.
56
0.2
0.4
0.6
INITIAL H CI CONC (MaUll Fig. 8. Residual "HCl and H2PtC16 (referred to q!) in solution at the end of the impregnation as a function of the initial HCl concentration.
Figure 8 shows the Pt and Cl percentages remaining in the solution at the end of the impregnation as a function of the initial HCl concentration. According to the adsorption capacity of the support there will be residual amounts of Pt and Cl in solution for initial HCl concentration higher than 0.3 mol/I. This behavior was verified for Pt, but there is a significative amount of chlorine in solution below such initial HCl concentration. Figure 7 shows that there are 3 ions in solution even for low HCl concentration and such amount increases 3 with the initial HCl concentration. Al+ must be neutralized by Cl ions in Al+
solution, and this might justify the high amounts of chlorine in solution after the impregnation. REFERENCES 1 R.W. Maatman and C.D. Prater, Ind. Eng. Chern., 49(1957)253. 2 A.A. Castro, a.A. Scelza, E.R. Benvenuto, G.T. Baronetti and J.M. Parera, in L. Nogueira, M. Schmal, R. Frety, Y. Lau Lam (Eds.), VI Simposio Iberoamericano de Catalisis, Rio de Janeiro, 1978, p. 229. 3 T.A. Nuttal, CSIR Report CENG 182, (1977). 4 B.M. Pavlikhin, L.I. Ulasova and M.E. Levinter, Izv. Vyssh. Uch. Zaved. Khim. Khim. Teknol., 19(1976)1880. 5 Yen-Shin Shyr and W.R. Ernst, J. Catal., 63(1980)425. 6 G.H. Van der Berg and H.Th. Rijnten, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet, Preparation of catalysis II, Elsevier, Amsterdam, 1979, p. 285. 7 S. Sivasanker, A.V. Ramaswamy and P. Ratnasamy, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of catalysts II, Elsevier, Amsterdam, 1979, p. 185. 8 E. Santacesaria, S. Carra and J. Adami, Ind. Eng. Chem. Prod. Res. Dev., 16 (1977)41. 9 J.F. Le Page, Catalyse de Contact, Technip, Paris, 1978, p. 598. 10 P.B. Weisz, Trans. Faraday Soc., 63(1967)1801.
57
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE ROLE OF COMPETITIVE ADSORBATE IN THE IMPREGNATION OF PLATINUM IN PELLETED ALUMINA SUPPORT vlANG JIANGUet, ZHANG JIAYU and PANG LI
Department of Chemistry, Peking University, Beijing, P.R.C.
ABSTRACT The effect of competitive adsorbate, i.e., lactic acid, citric acid etc. on the adsorption equilibrium of H2PtC16 on
~-A1203
and the
impregnation kinetics vere studied. The obtained experimental results helped to explain, why with citric acid as a competitive adsorbate, a catalyst with Pt distributing within pelleted alumina in an eggwhite (or eggyolk) zone may be resulted; while with lactic acid as a competitive adsorbate, Pt tends to be distributed in a broadened eggshell region or homogeneously.
INTRODUCTION It has been known
that the catalytic performance of supported
catalysts can be much influenced by the distribution of the active ingredient on the internal surface of the final catalyst prepared by impregnation. Therefore, the regulation of the radial concentration profile of the catalytically active ingredient within the support pellet is of practical importance. For the preparation of Pt/Al 20 3 catalyst, when the pelleted alumina is impregnated with the solution of chloroplatinic acid alone, there is a strong tendency toward exterior surface impregnation (1). By introducing a competitive adsorbate, the platinum tends to penetrate deeper into the interior of the pellet. Two kinds of competitive adsorbate effect have been observed. When using a limited amount of polybasic organic acid, i.e., citric or tartaric acid as a competitive agent, a catalyst with platinum in an eggwhite (or eggyolk) type distribution may be aChieved (2,3) • • Present address: Institute of Enviromental Chemistry, Academia Sinica P.O. Box 934, Beijing, P.R.C.
58
On the other hand, when acetic, chloroacetic, lactic or hydrochloric acid is used, the platinum can not be distributed in an eggwhite type, but tends to acquire a uniform or broadened eggshell type distribution (1,4). Our work is an experimental study of these two different kinds of effects with the consideration of adsorption equilibrium and impregnation kinetics. EXPERIMENTAL Support and impregnating compound Commercial '1-AI203 pellets, 2-2.5 mm in diameter were employed for impregnation. The characteristics of this porous support are: Sg = 245 m2/g (BET method); Vp = 0.405 ml/g (CCI4 method); true density = 3.29 g/cm 3• Chloroplatinic, citric,
~artaric,
acetic and chloroacetic acids
were A.R. Grade; lactic acid was C.P. Grade. Determination of the radial distribution of adsorbate The pellets were sectioned equatorially. The impregnated organic acid was made more visible and contrasting by the addition of dimethyl yellow solution (turns to red or orange colour). The impregnated chloroplatinic acid was bLackened by heating on a hot plate. The depth of impregnation and the location of distribution zones were determined by means of a DM reading microscope (precision to 0.01 mm) taking the mean value of 15-20 measurements. Determination of adsorption amount at equilibrium 0.5 g of the powdered alumina support was added to 20 ml of impregnating solution of known molarity and continuously shaked in a thermostated vibrator until the adsorption equilibrium was reached (ca. 6 hrs.). After centrifugal settling, the solution was drained off. The concentration of organic acid was determined by titrating with standard NaOH solution, using phenolphthalein as indicator. The concentration of chloroplatinic acid solution was determined by spectrophotometry, using 721 Model spectrophotometer. The equilibrium adso~ tion amount was calculated as follows (5),
x
ffi=
V(Cg-C*
g
(1)
where x/m is the equilibrium adsorption amount in mmol/g; Co is initial concentration of impregnating solution in M; Cadis the equilibrium concentration in M;
59
V is the volume of impregnating solution in ml; g is the total weight of alumina in g. Impregnation kinetics The wetted alumina pellets of homogeneous size (r=1.15 mm) were selected for the impregnation kinetics study. The variation in the concentration of impregnating solution C and the depth of impregnation I as a function of impregnation time t was determined. RESULTS AND DISCUSSION The effect of competitive adsorbate on the adsorption of
chloropla~
acid The adsorption isotherms of acetic, chloroacetic, lactic,tartaric, and citric acid on
are shown in Fig. 1.
f-~1203
a
b c d
o
e
2 1 3 2 (M) concentration x 10
Fig. 1. The adsorption isotherms of organic acids on f-AI203. a. citric, b. tartaric, c. lactic, d. chloroacetic, e. acetic acid. The adsorption isotherms of the organic acids mentioned above all obeyed the Langmuir adsorption isotherm (see Fig. 2 ). By means of their slopes, the saturated adsorption amountsof each acid on powdered I(-A1203 were found
(shown in Table 1).
60
coo
0 .....
X
""' 6 4 <, :x: '-" <,
u
2
2 3 can cen tra tion x 1. 0 2 (M) Fig. 2. The plot of C/(x/m) against concentration C. a. citric, b. tartaric, c. lactic, d. chloroacetic, e. acetic acid. TABLE 1 The saturated adsorption amount of organic acids on
~-Al203
Acids
Acetic
Chloroacetic
Lactic
Tartaric
Citric
(x/m)m mmol/g
0.53
0.56
0.59
0.68
0.82
The effect of the competitive adsorbate on the equilibrium adsorption amount of H2PtCl6 is shown in Fig. 3, 4, 5. It can be seen, as the competitive adsorbate was adsorbed independently, the larger its (x/m)m value attained, the less the equilibrium amount of H2PtC16 was adsorbed in the presence of the former adsorbate, no matter the impregnation was carried out stepwise or simultaneously. The relationship between the radial distribution of the competitive adsorbate and chloroplatinic acid The support pellets were immersed in 0.05N competitive adsorbate solution for a definite time. The impregnated support was separated into two portions: one was used for determining the distribution zone of competitive adsorbate; the other was further impregnated with chloroplatinic acid solution and the distribution zone was then determined. It was noted that chloroplatinic acid was distributed in two successive zones. The outer zone was lightly coloured, whereas
61
~acetic ,-, 0.15 bfJ ........
~
o s s '-'
'"d
~
chloroacetic lactic
0.10
(I)
..0
.... o
Ul
'"d
(';\
citric
o
.0.4
0.2
0.6
0.8
competitor adsorbed (mmol/g)
Fig. 3. The relationship between the adsorption amount of H2PtCl6 and that of competitive adsorbate (stepwise impregnation, concentratior of H2PtCl 6 Co=O.0042M).
a
,-,
bfJ <,
o
S
.
S 0 2
'-'
b
'"d
(I)
..0
.... oUl
o
c
'"d
(';\
1 concentration x 10 2
2 (M)
Fig. 4. The adsorption isotherms of H2PtC16 on ~-A1203 by co-impregnation with lactic acid. Concentration of lactic acid: a. Co=O, b. Co=O.0093M, c.O Co=O.0279M,. Co=O.0466M.
62
a
.-..
-
b.O
<,
o
S S 0.2
'-' '1j (j)
.0
....
o
o
U'l
~
0.1
,
o
•o
1
2
concentration x 10 2 (M) Fig. 5. The adsorption isotherms of H2PtCl6 on f-A1203 by co-impregnation with citric acid. Concentration of citric acid: a. Co=O, b. Co=0.0102M, c.O Co=0.0306M,. Co=0.0508M. the inner zone was deeply coloured. The thicknesses of these different zone are given in Table 2. TABLE 2 Thicknesses of the adsorbate distribution zones Competitive adsorbate
Ci tric acid Ci tric acid Lactic acid Tartaric acid
Thickness of the zone with competitive adsorbate in mm 0.35 0.51 0.51 0.44
Thickness of zones with H2PtCl6 in mm outer
inner
0.15 0.39 0.53 0.53 0.46
0.07 0.21 0.20 0.15
The data given in Table 2 show that the thickness of the outer H2PtCl6 zone well agreed with that of the competitive adsorbate zone. It was also observed in our experiment that using citric acid as a competitive adsorbate, the outer H2PtCl 6 zone was only very lightly coloured; whereas it was deeply coloured, if lactic acid was used as a competitive adsorbate.
63
The effect of the competitive adsorbate on the impregnation kinetics The variations in the concentration of impregnation solution (C) and the depth of penetration (1) with respect to the impregnation time (t) were determined. The average adsorption amount of chloroplatinic acid (x/m)l in the impregnated zone of thickness (1) may be calculated by the following expression (2)
where V is the volume of impregnation solution (ml); Co is the initial concentration (M); C is the concentration of impregnating solution at time t; g is the total weight of A1203; r is the radius of A1203 pellet. The calculated values are shown in Fig. 6.
1'"\
-
0,2
llII <,
0
.0
•
0
•
0
E1 E1
o.
Q
•
'V
... 0.1 ..-.
e
<,
>< 'V
50
100 time (min)
Fig. 6. The average adsorption amount of H2PtCl6 in the impregnated zone of thickness (1) at time t.O Co=O.00524M •• Co=0.Ol03M. From Fig. 6, it is seen that the average adsorption amount (x/m\ may be considered as a constant. 0.165 mmol/g, but (x/m)l is not a real thermodynamic equilibrium value, because when l=r, as the duration of impregnation being prolonged to 24 hrs, the adsorption value of chloroplatinic acid will further increase to 0.20 mmol/g. Thus under the conditions of catalyst impregnation. when l(r, the adsorption of chloroplatin1c acid may be considered only as a steady state equilib'rium. In our experimental condition the impregnation of H2PtC16 should be considered as a shell progressive process of sorption (6,7). For the spherical support, we have been established that as the solute
64
in the solution within the pore can be neglected the plot of t/(Co-C) against t should be a straight line and the slope of which approximately equals to V/(g(x/m)l) (7). For the impregnation of Al 20 3 pellets with H2PtCI 6• the variation of C with t is shown in Fig. 7.
c ,-...
::E 4
c;
0
X ~
0
...., ~
...., '"'
~ "-J
o ~
0
o
3
B
A 100
300
time (min) Fig. 7. The impregnation kinetics of H2PtC16 under various conditions. A. In the absence of competitive adsorbate; B. In the presence of lactic acid; B'. In the presence of lactic acid distributing in eggshell zone (thickness 0.28 mm); C. In the presence of citric acid; ct. In the presence of citric acid distributing in eggshell zone (thickness 0.58 mm). In the absence of competitive adsorbate (Fig. 7 A) and in the presence of lactic acid (Fig. 7 B). the value of (x/m)l calculated from the slopes (Fig. 8) is 0.16 mmol/g and 0.10 mmol/g respectively. When the support was pretreated with citric acid. the
C~
is 0.00458 M
(Fig. 7 C). in this condition the adsorption amount (x/m)l of H2PtC16 is 0.006 mmol/g. In Fig. 7 it is interesting to note that in the initial impregnating stage line B' coincides with line B. It implies that in this stage
65
15 I
0 ,...,
x 10 ""'
U
I
.s '-"
5
....,
<,
100
200
300
time (min) Fig. 8. The plot of t/(Co-C) X 10- 4 against t. 0without competor, 0 preimpregnated with lactic acid. the adsorption of H2PtCl6 referring to line B' occurs in the zone preimpregnated with lactic acid, so the kinetics of both line B and line B' are similar. From Table 2, it has been known that this corresponded to the formation of an outer H2PtC16 dispersing zone. At a certain point line B' starts to depart from line B and the depletion of the concentration of H2PtC1 6 solution is accelerated. It implies that the distribution of H2PtC1 6 in the outer zone is ended and a new inner concentrated zone on the free surface of support is started. By comparison of line B and line B' the rates of depletion of the concentration in each zone are observed. Referred to the thickness of impregnating zone and the radius of the support pellet, the adsorption amount of chloroplatinic acid in each zone can be calculated. Similarly, by comparison of line C and line C', using citric acid as a competitive adsorbate, the adsorption amount of H2PtCl6 in each zone can be calculated. The adsorption amount of H2PtCl 6 under various conditions are given in Table 3. The data in Table 3 indicate that when the competitive adsorbate distributes in an eggshell zone the adsorption amount of H2PtCl 6 in each zone well agrees with its equilibrium amount in the corresponding conditioned surface. Hence the adsorption amount on r-A1203 is determined by adsorption eqUilibrium. When the citric acid within the support pellet distributes in an eggshell zone the adsorption amount
66
TABLE 3 The adsorption amount of H2PtCl6 in each zone Competitive adsorbate and their distribution
Adsoption amount (mmol/g) in eggshell zone
in eggwhite zone
iVi thout competi tive
adsorbate
0.16
Ci tric acid uniformly distributed
0.006
Lactic acid uniformly distributed
0.10
Ci tric acid in eggshell distribution
0.006
0.15
Lactic acid in eggshell distribution
0.10
0.17
of chloroplatinic acid in this zone was negligible, the chloroplatinic acid mainly concentrated onto the inner free surface forming an eggWhite (or eggyolk) zone. Therefore, a catalyst with platinum distributed in a eggwhite zone resulted. The case was different with lactic acid, the amount of chloroplatinic acid adsorbed in the lactic acid dispersing zone was still higher and could not be neglected. Hence in this case, no catalyst with platinum concentrated in an eggwhite zone could arise. However, because the amount of chloroplatinic acid adsorbed in the eggshell zone was also reduced to a certain extent, so the distributing zone of a definite amount of chloroplatinic acid within the support pellet would be broadened. Therefore, a catalyst with platinum in a thickened distribution may be resulted. REFERENCES 1 R.W. !·1aatman and C.D. Prater, Ind. Eng. Chem., 51(1959)913-914. 2 E. Michalko, U.S. Patent, 3,259,454(1966). 3 E.R. Becker and T.A. Nuttall, Preparation of Heterogeneous Catalysts, Proe.llnd Inter.Symposium,B.Delmon et al. (Eds) ,EIsevier,1979,p.159. 4 G.N. Maslyanskii, et aI, Kinet. Katal., 12(1971)784. 5 Ting Yin-Ju and Ying FU, Acta Chimica Sinica, 21(1951)335. 6 P.B. Weisz, Trans. Faraday Soc., 63(1967)1801. 7 Wang Jianguo, MS Thesis, Department of Chemistry, Peking University, 1981.
67 DISCUSSION Did you measure the pore size distribution of applied alumina ? K. KOCHLOEFL Do you think that Y-A1203 will show the same behaviour as n-A1203 used in your study ? PANG LI No, we did not measure the pore size distribution. In our work, when 1< r, (x/m) 1 may be considered only as a steady state equilibrium. This means that the diffusion of H2PtC16 to some smaller pore is very slow and the absorbed H2PtC16 in the smaller pore can be neglected. ccording to the results reported by G.H. van den Berg (Preparation of Heterogeneous Catalysts,2nd Intern. Symposium, B. Delmon et al. (Eds.), Elsevier, 1979, p. 265), it seems that Y-A1203 shows the same behaviour as n-A1203 used in our study. J; KIWI: During adsorption of citric acid onn~1203' did you heat and, if so, how long? The risk always exists that by heating the citric acid polymerizes and may reduce H2PtC16 present.
PANG LI Our adsorption experiments were carried out at 30 ~ 0.5°C, during 6 hrs. In such conditions y the risk you mention does not exist.
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G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
THE INFLUENCE OF SOLVENT NATURE OF CHLOROPLATINIC ACID USED FOR SUPPORT IMPREGNATION ON THE DISTRIBUTION, DISPERSITY AND ACTIVITY OF PLATINUM HYDROGENATION CATALYSTS 0,.;,.; 1 J. KUNZ 2 V. MACHEK 1 , J. HANIKA 1 , K. SPORKA 1 , V. RUuIvKA, 3 and L. JANACEK
1Department of Organic Technology, Prague Institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia 2Department of Materials, Institute of Nuclear and Physical Engeenering, 115 19 Prague 1, Czechoslovakia 3Research Institute for Chemical Utilization of Hydrocarbons, 436 70 Litvinov, Czechoslovakia
ABSTRACT Charcoal and S' -alumina were impregnated by aqueous and organic solutions of chloroplatinic acid. The uniformity of the Pt distribution profiles through the support pellets was strongly influenced by the nature of H2PtC16 solvent used for impregnation. Using a mathematical model the impregnation process was described enabling the calculation of th.e platinum distribution profiles. The model predidted qualitatively the measured Pt distribution profiles.
INTRODUCTION The metal -on- support catalysts are frequently produced by the porous support impregnation with a solution of the metal compounds. The conditions applied during this step can influence the metal distribution through the support particle. Due to its technical importance and scientific interest, the Lf,terature of catalyst impregnation has been rapidly growi.ng. Attention was focused in several recent papers (e.g. 1- 7) to the formulation of the mathematical description of the porous support impregnation by an active component. This problem
69
70
was solved for both the spherical (ref. 1 - 5) and cylindrical (ref. 6 - 7) shapes of the support particle. The method of electron microanalysis enables the comparison of the model idea with the existing experimental distribution profiles (ref. 5,7) of the active component through the support particle. Sorr.e authors (ref. 5,8,9) showed that Pt can be uniformly distributed by the addition of salts or acids to the impregnating solutions (as a site blocking agents or competitors). The presented paper was aimed at finding possibilities how Pt distribution can be controlled using different solvents of chloroplatinic acid during the support impregnation. The achieved Pt distribution in the catalyst prepared was confronted with catalyst activity measured on 1-octene and nitrobenzene liquid phase hydrogenation. EXPERIMENTAL Commercial active carbon Supersorbon H8-3 (Degussa), (diameter 4 mm, lenght 6 mm), Norit RB1 (diameter 1 mm lenght 4 mm) and l-alumina Catapal S (Condea), (diameter 1.8 mm lenght 8 mm ) were employed for impregnation by aquaous and organic solutions of chloroplatinic acid. The impregnation was carried out in the apparatus enabling the circulation of H2PtC1 6 solution through the fixed bed of the support. The initial concentration of H2PtC1 6 solutions was in all cases 10 gil (it is in accordance to the 5% wt Pt in the catalyst prepared). The H2PtC1 6 concentration was monitored ape c t r-oph.ot omet r-Lca Iky . The adsorption isotherms were measured in similar way after the system had reached the eqUilibrium. The initial chloroplatinic acid concentration was changed in the range 2 - 60 gil. Both the impregnation kinetics and the equilibrium measurements were carried out at 20 °C. The catalyst preparation process was described elsewhere (ref. 10,11). After drying at 60 °c the impregnated support was calcinated in a fiKed bed in a nitrogen atmosphere and thereafter reduced in the hydrogen stream. The preparation temperature was varied between 100 and 300 °C. The catalyst activity was determined as the initial rate of liquid phase hydrogenation of nitrobenzene and 1-octene in a batch stirred tank reactor at 25 °c and atmospheric pressure. The mean size of the platinum crystallites was determined
71
by XRIJB method and H-O titration method. Platinum radial distribution profiles through catalyst pellet were measured using the electron microanalysis method. RESULTS AND DISCUSSION Impregnation kinetics and equilibrium Two types of active carbon and l-alumina were impregnated by the aqueous ~nd acetone solution of chloroplatinic acid. Moreover, active carbon Degussa was impregnated with the methanol, ethanol, methylethylketone and methylisobutylketone solutions of chloroplatinic acid. The platinum uptake measurements as a function of time on the supports in question are illustrated in Fig. 1 - J. The corresponding adsorption isotherms are shown in Fig. 4 - 6. It is evident that the solvent used has a marked effect on the rate of impregnation as well as on the adsorbed amount of chloroplatinic acid on the support. The different solvent effects on the chloroplatinic acid adsorption could be explained in terms of the different ability of the solvents concerned to compete with chloroplatinic acid in the adsorption on the supports under study; in our preceding works (ref. 11,18) this ability has been characterized by the corresponding heats of wetting. Both the adsorption rate and the adsorbed amount of chloroplatinic acid on hydrophobic active cerbon supports have been observed to decrease with increasing heat of wetting for the support used. The heats of wetting for organic solvents exceeded essentially the value for water. Naturally, the opposite situation should be expected for hydrophilic t -alumina. The competi tive sorption of the solvent and chloroplatinic acid necessarily affects also the concentration gradient of the active component within the support grain. Platinum distribution profiles A mathematical model of a diffusion-adsorption process was applied to the support impregnation by a single component. A model was formulated in a similar way as described previously by e.g. Martinez, et.al. (ref. 4), Hegedus, et.al. (ref. 5) and Melo, et.al. (ref. 6,7). Therefore, we present here a presumed model and the resulting equations only. We have considered the following assumptions: a) non-steady state b) spherical
72
shape of support grain c) prewetted support, so that its pores are initially filled with the solvent d) limited volume of impregnation solution without concentration gradients e) simultaneous diffusion in pores and adsorption on inner support surface of the active component, the concentration of which is both the function of coordinate and time f) rate of active component adsorption is more rapid than its diffusion. Using the Langmuir adsorption isotherm the time dependence of the radial active component concentration profile in support pores is described by the partial differential equation (1)
( :~) vdth
1 +
initial and boundary conditions:
follo~~ng
C(r,O) = 0
C(R,O) = C
0
(
~ C3:0. t )) = 0
R C(R,t)
C0
2dr 41fT j(C a + C) r V
(2)
0
where C resp. C is solute concentration in pores, resp. a on support surface t - time, r - radial coordinate, De f f - effective diffusion coeficient, R - support grain radius, ~ resp. E support density resp. porosity and K Langmuir i adsorption isotherm parameters. The time function f(t) expresses the concentration change in limited space around the impregnated support particlle and follows from the mass balance of the system. The equation (1) with condition (2) was solved using a finite difference method. The value of diffusion coefficient was found by trial error method applied to the fitting of the computed time dependence of H2PtCl 6 concentration in the impregnation solution to the experimental one. Comparison of the computed Pt distribution profiles with those ex!=,erimentally measured by the electron microanalysis method has been illustrated for two typical examples of active carbon impregnation by aqueous and acetone solutionsof chloroplatinic acid. Fig. 7 - 9 show the computed Pt profiles at different
73
impregnating times.The computed profiles are in qualitative agreement with the electron microanalysis measurements. The experimentally measured profiles of platinum through the Pt/active carbon Degussa catalyst grain for the various solvents of chloroplatinic acid are shown in Fig. 10. In the solvents tested there clearly appears a transition from pure surface distribution (water) to a uniform distribution of platinum within the catalyst grain (ketones). The solvent effect on the platinum distribution profile, explained in terms of the competitive sorption, is noticeable particularly in th~ case of impregnation with chloroplatinic acid in water and in ketones, respectively. With methanol, a partly non-uniform concentration profile establishes in spite of the competitive sorption. The Pt distribution profiles observed in the catalysts prepared using r-alumina support exhibit the opposite character, Le. the non-uniform, resp. uniform distribution appeared in the catalysts prepared using acetone, resp. aqueous solution of H2PtC1 6 (see Fig. 11). An explanation of this fact on the base of competitive adsorption of solvent and H2PtC1 6 on the support follows from the above discussion on the impregnation kinetics and equilibrium. Generally, apart from the sorption-diffusion mechanism, the concentration profile is determined also by the chemical nature of the support, the solute, and the solvent. Platinum dispersity and activity Based on the known concentration profile of platinum through the catalyst grain, it is possible to explain the changes in the metal dispersi ty ocourring on heat treatment of catalysts prepared by using various solutions of chloroplatinic acid. The platinum dispersity expressed as the mean size of the platinwTI crystallites is given in Table 1 for the various samples. As the dependence of the mean size of the metal crystallites on the reduction temperature indicates, a uniform distribution of platinum is favourable for the stability of its dispersity; the crystallites are thus less liable to sintering during the reduction or during application of the catalyst at higher temperatures. The reverse is true in the case of non-uniform platinum distribution. The catalyst actiVity is closely related with the dispersity
74
TABLE Properties of Pt supported catalysts prepared from different solutions of chloroplatinic acid Solvent
Cpt
%
Reduction Pt cryst. size temperature nm dXR LB
Ca t a l., activity mmol H2 / g Pt s
dHO
rON
r NB
ACTIVE CARBON DEGUSSA 5.0 100 4.0 3.7 300 17 .2 30.4 Methanol 100 4.3 4.0 5.4 300 20.0 23.2 Ethanol 100 6.0 2.7 5.6 10.0 8.1 300 Acetone 100 3.9 5.4 4.7 8.2 300 8.4 Methylethyl3.2 100 6.0 6.0 ketone 300 10.1 7.6 Methylisobutyl- 3.6 100 4.0 2.5 ketone 300 6.4 3.0
3.9 0.9 4.6 0.3 5.8 2.0 5.3 4.2 5. 1 2.1 3.9 3.2
2.4 2.2 3.1 1.0 3.8 2.6 2.9 2.8 2.6 1.7 2.2 2.1
5.6 0.8 7.4 1.2
2.4 0.8 4.0 0.7
6.9 7.9 4.4 5.8
5.5 6.9 5.4 5.4
°c Water
Water Acetone
ACTIVE C/lliBON NORIT 5.0 100 4.0 300 13.7 100 3.9 4.0 300 10.0
0'Water
3.2
Acetone
3.9
ALmUNA 100 300 100 300
3.6 12.3
of the metal on the support surface. The results of the catalytic activity measurements for hydrogenation of 1-octene and nitrobenzene are summarized in Table 1. The effect of solvent of chloroplatinic acid on the dispersity, and thereby on the catalyst activity, is apparent from the above discussion; by affecting the platinum distribution profile through the catalyst grain, the solvent influences the dispersity of the metal.
75
Fig. 1. H2PtC1 6 uptake measurements on active carbon Norit - aqueous solution, 2 - acetone solution.
20~
e Fig. 2. H2PtC1 6 uptake measurements on t-alumina 1 - acetone solution, 2 - aqueous solution.
10
2
O-_.........
_ - - L_ _.L-_.....L-_~
o
c.l
60 Fig. 3. H2PtC1 6 uptake measurements on active carbon Degussa H2PtC1 6 solution in 1-water, 2 - acetone, 3 - metr-anol, 4 - methylethylketone, 5 methylisobutylketone, 6 - ethanol.
76
a
Fig.
4.
Adsorption isotherms
for H2PtC1 on active carbon 6 Norit
1
- aqueous solution, 2 - acetone solution.
2
50 Fig. 5. Adsorption
2
a
isothel~s
t
for H2PtC1 6 on -alumina 1 - aqueous solution, 2 - acetone solution.
0.2 1
Fig. a
6. Adsorption isothelWs
for H on active carbon 2PtC1 6 Degussa H solution in1 - wa t e r; 2PtC1 6 2 - methanol, J - acetone, 4 - methylethylketone, 5 - ethanol.
50
77
Fig. 7. Pt distribution in the pellets of Pt/active carbon Degussa catalysiE prepared from aqueous solution of H2PtCl 6: comparison of theory (A) with measurements (B). Impregnating time 1 - 600 s, 2 - 7200 s.
A
0r-----''--'----'-----.l--'~---'----l
B
A
or----.L-...o"---..l----<'-L..---L----i 6
B
Fig. 8. Pt distribution in the pellets of Pt/active carbon Degussa catalysts prepared f~om acetone solution of H2PtCl 6: comparison of the ory (A) wi th mea sur-ement s ~B). Impregnating time 1 - 600 s, 2 - 3600 s, 3 - 7200 s .
r/R e
a Pt,IO2
Fig. 9. Pt distribution in the pellets of Pt/active carbon Norit catalysts pre,-:;.---I-----.J pared from aqueous and acetore solutions of H2PtC1 6: o comparison of theory (A) with I------'---O<::---...L....----'----i measur ement s (B). A
8
8
B
0'::-0----'--..-.::'-1---'-1---'-1----' r/R ..
Impregnating time 1 - water solution 2400 s, 2 - acetone solution 9000 s.
78
10
r/Rl!
Fig. 10. Pt distribution measured in the pellets of Pt/active carbon Degussa catalysts prepared from various solutions of H2PtC1 6: 1 - water, 2 - me t hano L, 3 - ethanol, 4 - acetone, 5 - methylethylketone.
2 6
o o
~_~=--..1.-_...L.-_....L...._...J
r/Re
1
Fig. 11. Pt distribution measured in the pellets of Pt/ -alumina catalysts prepared from aqueous (1) and acetone (2) solutions of H2PtC1 6 •
r
79
NOMENCLATURE
/g support) 2PtC1 6 apt (gPt/gsupport) a (gH
C (gH PtCl 11) 2 6 /gsupport) Cr (gH 2PtC16 r
(mm)
r NB (mmol H2/gpts)
Re (mm) t (s)
H2PtC1 6 uptake by support Pt concentration in catalyst prepared H2PtC16 concentration in the impregnating solution H2PtC16 equilibrium concentration Pellet radial coordinate Reaction rate of nitrobenzene hydrogenation Reaction rate of l-octene hydrogenation Equivalent pellet radius Time
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
P. Harriott, J. Catal., 14 (1969) 43. J. Cervello, J .F. Garcia de la Banda, E. Hermana, J .F. Jirrenez, Chem. Ing. Tech. 48 (1976) 520. R.C. Vincent and R.P. Merril, J. Catal. 35 (1974) 206. J. Garmendia, F. Lopez-Isunza and E. Martinez, 6 th Congress CHISA, Prague, L 3. 1., 1978. L.I,. Hegedus, T.D.Chon, J.C. Summers and N.M. Potter, in Preparation of Catalysts II, p. 171 (B. Delmon Ed.), Elsevier Sci. ruble Com]:., Amsterdam 1979. F. Melo, J. Cervello,and E. Hermana, Chern. Ing. ScL 35 (1980) 2165. F. lVIelo, J. Cervello and E. Hermana, Chem. Ing. ScL 35 (1980) 2175. R.W. Maatman, Ind. Eng. Chern. 51 (1959) 913. G.H. van den Berg and H.T. Rijnten in Preparation of Catalysts II, p , 265 (B. DeLmon Ed.), Elsevier ScL rubl. Comp., Amsterdam 1979. J. Hanika, K. Sporka, V. Ruzicka and J. Bauer, ColI. Czech. Chern. Commun. 44 (1979) 2619. V. Machek, J. Hanika, K. Sporka and V. Ruzicka, ColI. Czech. Chem. Commun. 46 (1981) 1588. M. Uhlir, J. Hanika, K. Sporka and V. Ruzicka, ColI. Czech. Chern. Commun , 42 (1977) 2791. J. Freel, J. Catal. 25 (1972) 139. J. Prasad, K.R. Hurtby and P.G. Menon, J. Catal. 52 (1978)515. J.R. Anderson, Structure of Metallic Catalysts p. 366, Academic Press, London 1975. J. Soukup and V. Zapletal, Chern. Listy 62 (1968) 991. J. Hanika, K. Sporka, V. Ruzicka and J. Deml, ColI. Czech. Chern. Commun. 37 (1972) 951. V. Machek J V. Ruzicka and M. ~ourkov~, Coll. Czech. Chern. Commun. 4b (1981) 2178.
80 DISCUSSION N.P. WILKINSON Do you have solubility curves for H2PtC16 in each of the solvents mentioned? Have you considered how solubility during the 60°C drying stage will lead to deposition of salt throughout the pellet as the solvent front retreats, particularly for the Degussa carbon? V. MACHEK We do not have the solubility curves for H2PtC16 in the solvents in question. Distribution of the active component through the pellet of a support may depend significantly upon the conditions of impregnation and drying. The final distribution of the components adsorbed from solution is determined by the combined diffusion, adsorption, and reaction effects involved. Support impregnation can result basically in two cases: 1) the interaction of the solute with the support is strong ad?orption by nature: the metal distribution is governed by the sorption-diffusion mechanism and does not change appreciably during the subsequent treatment of the catalysts (drying, thermal treatment), so-called adsorption catalysts. 2) If the extent of adsorption of active components is negligibly small as compared with their total concentration (weak adsorption), then the distribution depends primarily on the drying conditions (so-called impregnated catalysts). The catalyst type can be as;essed 1,2 by employing the criterium P ~ (Co - Cr)V/CrVpm s ' which is the ratio of the amount of the solute that passed from the solution onto the support to that present in the support pores. At P » 1 the component is primarily in an adsorbed form (adsorption catalysts) and if P « 1 adsorption is negligibly small and the catalyst is of the impregnated type. In our work, as the data in Figs. 1-6 demonstrate, P » 1 for all of the systems used; therefore, the Pt distribution profiles establishing during the impregnation can be regarded as invariable during the subsequent drying and thermal treatment. The details are also given in our preceding work 2. 1. V.B. Fenelonov, A.V. Neimark, L.I. Kheifets, A.A. Samakhov in: Preparation of Heterogeneous Catalysts, lInd Intern. Symposium, B. Delmon et al. (Eds.), Elsevier, 1979, p. 233. 2. V. Machek, J. Hanika, K. Sporka, V. Ruzicka and J. Kunz, Coll. Czech. Chern. Comm. 46, 3270 (1981). H. IIDA : What are the platinum crystallite size distributions of uniformly dispersed catalyst and non-uniform dispersed one in the state of reduction at 100°C? V. MACHEK: We used for the Pt crystallite size measurement the hydrogen-oxygen titration method and X-ray diffraction line broadening (XRLB). Both H2-02 titration and XRLB technique have not enabled us to measure the Pt crystallite size distribution. For this purpose the transmission electron microscopy (TEM) or small angle X-ray scattering (SAXS) should be used.
81
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
INFLUENCE OF THE VARIOUS ACTIVATION STEPS ON THE DISPERSION AND THE CATALYTIC PROPERTIES OF PLATINUM SUPPORTED ON CHLORINATED ALUllINA
J . P.
BOURNONVILLE, J. P. FRANCK and G. I1ARTINO
Institut Fran9ais du Petrole, B.P. 311 - 92 506 - RUEIL MALlffiISON - FRANCE -
ABSTRACT The study of the influence of the temperature and partial pressure of water, during the steps of calcination and reduction, on the accessible metallic area and on the catalytic properties of platinum
supported on chlorinated alumina
shows that: - the metallic dispersion and the catalytic properties versus the temperature of calcination or reduction vary according to a volcano curve : maxima values appear at a well defined temperature. - after impregnation,in aqueous solution,of the ionic precursor and after drying,it is absolutely necessary to perform a calcination in air
before the
reduction, in order to obtain the highest state of dispersion. - the reduction in wet hydroCjen, containing more than 200ppm volume of water, increases the sintering rate of the metallic phase. The optimal dispersion is obtained by the transformation of the ion PtCl
6
in oxidizing atmosphere into a surface compound in which platinum is always oxidized but has lost and exchanged some of
its
halic1e ligands. The
controlled reduction leads to a distribution of small particles of platinum less than one nanometer in size.
INTRODUCTION Metals supported on high surface area supports are widely used catalysts in refining and petrochemical industries (ref.
1). However, the high cost anc1
limited available amounts of noble metals have a direct involvement in the elaboration of the methods of preparation and activation of metal
supported
catalysts: metal must be deposited on the support in such a way that every metal
atom
is accessible to the reagents. Another important aspect concerns
the specific catalytic properties:activity, selectivity and stability.BOUDART (ref. 2) has shown that the activity of a metal
atom depends on the superficial
metallic structure or, more precisely, on its state of co-ordination (ref. 3).
82 When the rate of reaction is not affected by the details of structure, the reaction is called structure insensitive : benzene hydrogenation and cyclohexane
dehydrogenation may be classified in this category (refs. 4-9). If the
rates are sensitive to the mode of preparation, the pretreatments ,the change in the crystallite size or the support,the reactions are defined as structuresensitive : i t is the case of the hydrogenolysis of cyclopentane (ref. 11). The
(ref. 10) and of
~-hexane
dehydrocyclization of
normal paraffins
(a reaction of major importance in reforming) on a bifunctional
platinum suppor-
ted on acidic alumina catalyst is too complex to be only described as a function of the structure of the metallic surface. Nevertheless, relations between the size of the metal particles and the rate of aromatization (ref.12) the state of oxidation of platinum and the rate of
and between
dehydrocyclization (ref. 13)
have been established. The influence of the nature of exposed crystalline faces on the rate of aromatization has been shown by
SOMORJAI
(ref. 14). On the
other hand, selective poison, such as Hi>and CO, may suppress the structure sensibility of the reaction (refs. 15 -
16).
In many cases, it needs to have the greatest number of active sites as possible. This means that the highest accessible metallic area by a rigourous control
must be obtained
of preparation processes:the anchoring of precursors
on the support and the thermal activation under several different atmospheres. If the ionic exchange
during the impregnation step
leads to a quasi-atomic
dispersion of the precursor of the active species (ref. 1), the final dispersion of the metallic phase depends directly on the activation under oxidizing, inert and reducing atmospheres before the contact with the reagents (H The influences of treatment conditions, temperature
+ hydrocarbons) . 2 (refs. 17-19) and nature of
the atmosphere (refs. 20-25), have been separately studied without defining all the following operations which led to the highest dispersion. At most of the results have been obtained on badly dispersed (D
~0,5)
first,
platinum
catalysts and haven't taken into account the effect of the superficial chlorine concentration,which has been shown previously (ref. 26). The characteristics of activity and selectivity, depending directly on the quality of the accessible metallic phase, were studied during the activation steps. The catalysts used were very close to industrial ones.
EXPERIMENTAL TECHNIQUES The catalyst was 0.6 weight % platinum supported on f alumina extrudates 2 c purchased from R.P (BET surface area : 200 m per gram). The metal was impregnated in a chlorhydric acid aqueous solution and dried at 110°C before the thermal treatments of activation. The homogeneity of platinum and chlorine repartition through the pellet was
83 checked by scanning microprobe . Platinum and chlorine contents were determined by X-Ray fluorescence. Some samples were characterized by electron
microscopy.
Results obtained by EXAFS (ref. 27) allowed us to precise the different structure transformations of the precursor into the metallic phase during the activation stages. - the experiments were performed with an hourly space velocity of gases of 2000 liters/liter
of catalyst/h.
All qases were dried and contained less
than 15 ppm volume of water. - the metal area was measured by hydrogen-oxygen titration (ref. 28), after reduction at 450°C. Benzene hydrogenation was performed at 100°C under atmospheric pressure (ref. 29).
RESULTS To reproduce the industrial conditions of activation, we used the different atmospheres to which a catalyst is exposed during its preparation and its life. Temperatures varied from 110°C to 700°C to avoid the 103s of surface area of the support, when the experiments were performed at higher temperature. Air treatment : calcination After drying, the catalysts were calcined in dried air at different temperatures. The variations of the metallic area and hydrogenating activity as a function of the temperature of calcination are plotted in figures 1 and 2. The final chlorine
content of the samples was more than One per cent in weight.
The metallic surface area and hydrogenation activity increase from the drying temperature to 450- 500°C
and drop above this temperature. This pheno-
menom is influenced by the concentration of ionic precursor as given in table 1. Studying the effect of treatment in oxygen , WANKE (refs. 23-30)
found similar
results : but in fact he observed only a redispersion of a prereduced badly dispersed metallic phase (DiO,3)
due to oxychlorination (ref. 26).
The specific hydrogenating activity of an accessible metallic atom is constant : the structure insensitivity of the hydrogenation of benzene is confirmed (fig. 2). The transformation, in oxidizing atmosphere, of the superficial ionic species obtained after exchange,
into an oxychloroplatinum complex strongly bound
to the carrier (ref. 31), leads after reduction to the formation of a well dispersed metallic phase : when the temperature of calcination increases, the formation of this
stable
oxijized dispersed phase is faster, but above 550°C
its destruction is faster too. This last phenomenom is helped chlorine (ref. 29)
by the loss of
: equilibrium between sintering, high in oXijizing atmosphere,
(ref. 32), and redispersion may be obtained at much lower dispersion. When the temperature rises, the volatile oxychloroplatinum complex finds fewer fewer
OH groups for anchoring onto the support.
and
84
0,-
---, Dried Air, GHSV 2000
Time, 2 hours
1,0
0,5
100
200
300
400
500
600
PC Fig. 1. The variation of the dispersion as a function of the temperature of calcination.
Inert atmosphere To assure the transition between the oxidizing and the reducing atmospheres, a purge by a stream of inert gas is realized: in our case, argon was used. We examined samples previously calcined at 530°C in order to study the stability of the oxychloroplatinum complex in neutral atmosphere. Thermal treatments were performed at temperatures higher than 500°C to enhance the rate of the process. The results are given in table 2. The metallic dispersion and the halogen concentration decrease with an increase of temperature. Without oxygen, the superficial complex obtained after calcination decomposes in metallic platinum : platinum oxides
are unstable at high temperature (ref. 33). The metal sinters on the
surface of the support, which has lost a few of its binding sites by droxylation.
dehy-
85
Aclivily .10 2
Specific Aclivily .10·
mole h-1.g-1
mole h-'(m2 PIl-'
1,5
o
1,0
2,0
1,5
0,5 ---J-----..- - - . -
L--.----.-
1,0
0,5
100
200
300
400
500
600
roc Fig. 2. The evolution of the activity and the specific activity in benzene hydrogenation as a function of the temperature of calcination.
TABLE 1.
Influence of platinum content on the metallic dispersion during the calcination in air at different temperatures.
Platinum (% weight)
Dispersion after calcination 500°C-2h
0,65 1, 53 2,62
Dispersion after calcination 700°C-2h
0,91 0,92 0,79
0,69 0,48 0,18
D 700 D 500 0,76 0,52 0,22
Reduction under hydrogen The last step, before feedstock being processed, is the reduction in hydrogen, ie ,the transformation of the oXidized precursor bound
to the support
into a
metallic phase as well dispersed as possible. The calcination of the exchanged ionic species may modify the nature and the
strength of interaction between the precursor and the support and consequently its behaviour during the reduction. Therefore we studied two types
of samples.
- the first one only dried before reduction - the second one
dried and calcined at 500°C before
reduction
Fig. 3 represents the variations of the dispersion as a function of the temperature of reduction. For the two precursors states, dried and calcined, a maximum value of dispersion appears at around 450°C. Below this temperature platinum is not entirely reduced : these results have been confirmed by temperature programmed reduction (ref. 35). Above the maximum there is sintering of the metallic phase which can be slowed down if the chlorine content remains constant (ref. 26). The oxmizing decomposition at 500°C of the exchanged complex leads to higher dispersions than the single drying. The calcination removes the solvent (water) still contained in the porosity of the support and allows the stabilization of the active component before reduction. Without this treatment, the ions Pt IV, very mobile in the presence of water at high temperature, migrate at the surface of the support to give, after reduction, metallic particles whose size increases when the temperature is rising. The activity in benzene hydrogenation is directly proportional
to the acces-
sible metallic area whatever the mode of obtaining the active sites may be. In figure 4, we also plotted the results connected with the influence of the temperature of calcination. For this particular reaction, the specific activity of one site remains constant compared with the different processes which determine their realization: it will be very interesting to obtain the highest dispersion in order to get the best efficiency of the metal.
TABLE 2. Influence of the temperature of treatment in inert gas (Argon) on the metallic dispersion and the chlorine concentration.
Temperature 1°C) 500 550 600 700
Reduction under wet Until
D
0,92 0,94 0,73 0,58
Cl (Wt %) 1,38 1,22 1,09 0,88
hydroge~
now the influence of the nature of the atmosphere and of the tempera-
ture of treatment has been stUdied. But, the quality of the atmosphere is very important too : presence of impurities, such as water and carbon monoxide, may be detrimental to the final state of the metallic phase.
87
Dr-
..., Hz, GHSV 2000
Time, 2 hours
• Dried Catalyst o
Dried and calcinedCatalyst
0,
200
300
400
500
600
700
roc
Fig. 3. The variation of the dispersion as a function of the temperature of reduction. Table 3 gives the influence of the partial pressure of water on the metallic dispersion during the reduction step: improvement brought about by the use of a bimetallic formula catalyst (Pt-Ir) is suppressed by a reductiori with hydrogen containing one per cent of water. The hydrolysis of the oxychloroplatinum complex and the loss of chlorine lead to high rates of sintering of the metallic phase. TABLE 3 Influence of the moisture content of H on the metallic dispersion 2 Time of reduction : 10 hours
Temperature of reduction : 650°C
0,6 % Pt/Al D
= 0,46
203
Dried H 2 0,6% Pt + 0,2% Ir/Al D
= 0,72
20 3
H + 1 %H 2O 2 0,6% Pt + 0,2% Ir/Al;2°3 D
= 0,31
88
Activity (mole h-1 g.Pt-2>. CATALYST
•
Calcined at vorious temperatures
e Calcined at 500·C and reduced o
at various temperatures Dried and reduced at various tem eratures
2,0
e
•
o 1,0
"------:-":~----7------'D D,S
Fig. 4. The variation of the activity towards benzene hydrogenation versus metallic dispersion.
DISCUSSION-CONCLUSION The necessary sequence of the elementary operations for obtaining a well dispersed metallic phase may be conceived in the particular case of platinum supported on chlorinated alumina as follows - during the impregnation, the fixation of the metal can be represented by an ionic exchange at the surface of the support: after drying, platinum keeps the same halogenated octahedral surrounding
(ref. 27).
- during the calcination, the nature and the number of the ligands of platinum would be modified :removal of chlorine and exchange between chlorine of the complex and oxygen atoms of the support. This structure best fits the oxidation state of platinum (pt +4 ) and agrees with the work of ESCARD et al. (ref. 31). When the calcination is performed at high temperature (T)600°C) , the metal is badly dispersed : the oxychlorinated platinum complex decomposes into an oxije phase which is not stable when temperature rises and leads to
89 platinum metal weakly bound to the support. - after reduction of the superficial oxychloroplatinum complex in hydrogen at a sUfficiently high temperature, the metallic phase is made up of particles less than one nanometer in size, ie containing less than 20 atoms, in the case of a correctly processed catalyst. The results presented here have been obtained in connection with an applied research in progress on behalf of Procatalyse.
ACKNOWLEDGEMENTS The authors thank Drs H. DEXPERT and E. FREUND for the results obtained by B.X.A.F.S. and Electronic microscopy.
REFERENCES 1 J.F. Le Page and AL , "Catalyse de Contact", Technip Ed, Paris 1978. 2 M. Boudart,Aiche JournaT, Vol. 18 n03, p. 465 (1972). 3 R. Montarnal and G. Martino, Revue de l'Institut Fran~ais du Petrole XXXII n03, p. 367 (1977). 4 M. Boudart, Adv Catal 20, p. 153 (1969). 5 M. Poltorak and VS. Boronin, Ross J phys Chern, Vol. 40, nOll, p. 1436 (1966). 6 P. Marecot, Thesis University of Poitiers, (1979). 7 A.W. Aldag, L.D. Ptak, J.E. Benson and M. Boudart, J. catal. 11, p. 35 (1968). 8 Kraft and Spindler, Proceedings of the fourth International Congress on Catalysis, paper 69, Moscou (1968). 9 A. Morales, J. Barbier and R. Maurel, Rev. Port Quim 18, p. 158 (1976). 10 J.P. Boitiaux, G. Martino and R. Montarnal, Cras 281 C 48 (1975). 11 J. Barbier, P. Marecot, A. Morales and R. Maural, Bull Soc Chim Fr I 31 (1978). r 12 H.J. Maat and L. Moscou, Proceedings of the 3 International Congress on Catalysis, p. 1277 (1964). 13 M.R. Bursian, S.B. Kogan and Z.A. Davydova, Kinetika y Kataliz 8-123 (1968). 14 R.W. Joyner, B. Lang and G. A. Somorjai, J. Catal. 27-405 (1972). 15 G. Abolhamd, Thesis, Paris (1980). 16 W. Molina, Thesis, Poi tiers (1981). 17 A.Renouprez, C. Hoang Van and P.A. Compagnon, J. Catal 34, p. 411, (1974). 18 T.A. Dorling, B.W.J. Lynch and R.L. Moss, J. Catal 20, p. 190, (1971). 19 R.T.K. Baker, C. Thomas and R.B. Thomas, J. Catal 38, p. 510, (1975). 20 H. Spindler, Int Chern Eng 14, p. 725, (1974). 21 G.R. Wilson and W.K. Hall, J. Catal 17, p . 190, (1970). 22 J. Freel, J. Catal 25, p. 149, (1972). 23 R.M. Fiedorow and S.E. Wanke, J.Catal.43, p. 34, (1976). 24 Y.F. Chu and E. Ruckenstein, J. Catal 55, p. 348, (1978). 25 A.G. Graham and S.E. Wanke, J. Catal 68, p. 1, (1981). 26 J.P. Bournonville and G. Martino, in "Catalyst Desactivation" B. Delmon and G.F. Froment Ed. p. 159, (1980). 27 T. Murata, A. Fontaine, P. Lagarde, D. Raoux, J.P. Bournonville, H. Dexpert and E. Freund, unpublished results. 28 J.E. Benson and M. Boudart, J. Catal 4, p. 704, (1965). 29 J.P. Bournonville, Thesis, Paris, (1979). 30 S. E. Wanke, Conference on "Catalyst Desactivation and Poisoning", '!ay 1978 Berkeley, California. 31 J. Escard, S. Pontvianne, M.T. Chennebeaux and J. Cosyns, Bull Soc Chim 3, 349, (1976). 32 J.C. Schlatter, Mater. Sci. Res, Vol. 10, p , 141, (1975). 33 H. Blume, C. Szkibik, F. Pfeiffer, H. Klutzsche, E.R. Strich, K. Becher and G. Weindebach, Chern tech. 18, p. 449, (1966). 34 H. Scheifer and M. Frenkel, Z. Anorg. Allg. Chern., 414-437, (1975). 35 J.M. Deves, P. Dufresne and E. Freund, unpublished results.
90 DISCUSSION In the experimental part of your paper it is mentioned that K. KOCHLOEFL y-alumina extrudates were used as starting material of your Pt-catalyst. What grain size of the platinum catalyst did you use in the benzene hydrogenation ? J.P. BOURNONVILLE : The non-diffusional conditions for the measurements of the hydrogenation activity were checked according to two parameters: the pellet size and the activation energy. When pellets smaller than 1.4 millimeter in size were used, the limiting effect due to the diffusion of the reactants was suppressed; then an activation energy of 46 KJ/mole/oC was found. J. MARGITFALVI: with respect to your Fig. 1, what will be the influence of the initial concentration on the dispersity changes upon increasing the calcination temperature? Did you have a complete reduction if the reduction was carried out at 4~O°C? This concerns the catalysts with high metal loading. J.P. BOURNONVILLE: At low calcination temperature, the chlorine content will not have a detrimental effect on the metal dispersion. But at high temperature, lower is the chlorine concentration, higher is the sintering rate of the metal phase: the absence of chlorine during the oxidizing thermal treatment shifts the equilibrium between sintering ana redispersion towards the sintering. T.P.R. experiments showed that the rate of reduction did not depend on the metal loading up to 3 wt % of platinum, when the samples were reduced at sufficiently high temperature (T ) 450°C). When the metal loading increases, the overall metal-support interaction decreases; so, the reducibility of the supported metal increases but leads to a lower dispersion. H. CHARCOSSET You showed three ways for decreasing the dispersion of Pt/A1203: i) increasing the calcination temperature above a certain value; ii) increasing the reduction temperature above a certain value; iii) using wet hydrogen for reducing the catalyst. In each of these three cases, does or does not the sintering preserve the homodispersity of the Pt phase? J.P. BOURNONVILLE In accordance with other results not reported in the present paper, it appears that the nature of the atmosphere during the thermal treatments may influence the size distribution of the metal particles. In oxidizing atmosphere, a well dispersed phase remains in equilibrium with a sintered phase, due to the presence of chlorine, which leads to a bimodal particle size distribution; on the other hand, in reducing atmosphere, the size distribution remains narrower and monomodal during the sintering. H. CHARCOSSET: Is it true that the 700°C reduced catalyst keeps its normal Pt properties, as well in H2-02 titration as in benzene hydrogenation? This would indicate the absence of any metal-support interactions in your case, even at this high temperature of reduction. J.P. BOURNONVILLE: After reduction at high temperature, our samples were in contact with air at room temperature before being re-reduced at 450°c for platinum surface area and hydrogenating activity determination. According to your results and to those of Dautzenberg, the contact with oxygen destroys the interaction between platinum and partially reduced alumina. In consequence, we could not observe this phenomenon. Moreover, this phenomenon has been particularly described in the case of catalysts reduced at high temperature after drying without calcinatio~ and on which the metal phase was rather badly dispersed. R.J. BERTOLACINI: Your paper recognizes the importance of chloride on the dispersion of Pt on Cl-Al203 supported catalysts. Your paper does not indicate the same Cl- dependence for Pt-Ir/Al203 catalysts. Would you comment on the Cl- effect on Pt-Ir catalysts? What was the chloride content of your Pt-Ir catalyst ? J.P. BOURNONVILLE: After calcination at 500°C and reduction at 450°C in dried atmosphere, the Pt-Ir catalyst contains about 1.2 % chlorine in weight. As far as the activation and regeneration steps are concerned, we found that the chlorine effect was very similar for Pt and Pt-Ir catalysts.
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
91
PREPARATION AND PROPERTIES OF PLATINUM CRYSTALLITES SUPPORTED ON POLYCRYSTALLINE TIN OXIDE
GAR B. HOFLUND Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611
ABSTRACT The preparation and characterization of highly dispersed platinum supported on polycrystalline, antimQny-doped tin oxide films is discussed. The usefulness of numerous surface characterization techniques (such as ESCA, AES, ESD, SIMS, etc.) in relating catalyst surface properties to the methods of preparation is demonstrated.
INTRODUCTION Tin oxide and its modified forms I'kiich are prepared by doping or pl atinization are active catalysts for numerous reactions. Chromia-doped tin oxide (1,2) is active in nitric oxide reduction, antimony-doped tin oxide is active toward the oxidative dehydration of butene to butadiene (3) and the selectlve oxidation of propene to acrolein (4) while platinized, antimony-doped tin oxide is as much as 100 times more active per surface platinum than metallic platinum toward the electrochemical oxidation of methanol (5-9). Also, the addition of tin to alumina-supported, platinum reforming catalysts has resulted in greatly improved stability (10-16). Tin oxide is ideal as a support material in fundamental studies for highly dispersed platinum because it is conductive, it has a high transmissivity to photons over a broad range and it is fairly inert to most chemical treatments in addition to its interesting catalytic properties. These properties permit novel methods of catalyst preparation and characterization l'kIich will be discussed in this paper. CATALYST PREPARATION For this study the tin oxide films were prepared by a high temperature spray hydrolysis. This consisted of multiple sprayings of a Solutlon of 3M SnCl' 5H20
92
0.03M SbC1 3 and 1.5M HCl onto a quartz plate held at 500·C I'kiich had been ultrasonically cleaned and heated in air at 800·C for 3 hours. This results in a polycrystalline film I'kiich is several thousand angstroms thick. There are other methods which could be used to prepare the fi lms such as vapor- or 1i quid-ph ase deposition of SnC14 followed by hydrolysis, but the film's properties could vary greatly depending upon the impurities present, the oxygen-to-tin ratio and the geometrical structure. Platinum deposition can be carried out using anyone of five different methods: (l) an electrochemical deposition from a chloropl atinic acid solution, (2) chemisorption of pl at inum from a chloroplatinate solution, (3) a spray hydrolysis deposition by incorporation of a platinum salt into the spray solution, (4) deposition from a molten salt containing platinum and (5) decomposition of an adsorbed metallo-organic compound such as platinum acetylacetonate. The characteristics of the platinum can be altered in each of the five deposition methods by changing numerous variables. Catalysts prepared by methods (1) and (2) will be discussed here. A thorough discussion of catalyst preparation by method (2) has been presented by Watanabe, Venkatesan and Laitinen (17) I'kiile Katayama (8,9) has discussed both the preparation and ESCA characterization of samp1es prepared by method (3). It has been found that a caustic pretreatment of the tin oxide film enhances the deposition of platinum by methods (1) and (2). This pretreatment consists of soaking the films in a 10M NaOH solution at 90·C for 1 hour. It is believed that this process hydroxylates the surface. Indirect evidence of this has been published by Morimoto et al , (18) and Ansell et al . (19) while evidence that is more direct than the above will be presented in the next section. CATALYST CHARACTERIZATION A large number of techniques have been used to characterize tin oxide surfaces. A few selected ones including transmission electron microscopy (TEM), particle-induced X-ray emission (PIXE), Rutherford backscattering (RBS), Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), electron-stimulated desorption (ESD) and secondary ion mass spectrometry (SIMS) will be discussed here. TEM is useful in observing both the tin oxide structure and the platinum crystall ites. Figure 1 is a TEM photograph of an antimony-doped tin oxide sample showing the polycrystalline structure of the tin oxide. The tin oxide crystals are approximately 100-150A in diameter, and the light patches are thought to be a separate phase consisting primarily of SnO.
93
Fig. 1. TEM photograph showing the polycrystalline structure of the tin oxide support.
Bulk elemental analysis of the films is accomplished using RBS and PIXE. Typical spectra are shown in figure 2. RBS has been shown to be sensitive to 0.3ng of platinum, and the response is linear from 0.01 to 80\Jg Pt cm- 2 of substrate surface area (20). PIXE is particularly sensitive to intermediateatomic weight elements such as K, Ca, Fe and Zr, l'Itlich are film impurities. Antimony, which constitutes 1% of the film, is detectable with PIXE. The usefulness of RBS and PIXE in relating total platinum content of a catalyst to preparative variables of method (2) platinization of tin oxide is demonstrated in reference 17. The number of surface Pt atoms can be determined using electrochemical or chemisorption techniques so that dispersion can be cal cul ated. AES and ESCA are surface sensitive techniques used to characterize the composition and oxidation states of atoms near the surface. A typical AES scan of a platinized surface is reproduced (21) in figure 3. Impurities such as P, S, Cl, C and Ca are present. A portion of the oxygen feature at 515eV is due to contamination probably in the form of adsorbed CO. Antimony features are not observed because they are masked by the Sn peaks at 454 and 458eV. Also, the primary electron beam has been shown to modify the surface through electron stimulated desorption (22).
94
(a)
(b)
Sn KCa
Pt
E-
Fig. 2. oxide.
Spectra from (a) RBS and (b) PIXE of platinized, antimony-doped tin
~
Z
::J
> a: <{
a: !: III a: <{
zlw...
...
Pt
Fig. 3.
Sn
AES spectrum of platinized, antimony-doped tin oxide.
95
ESCA is useful in examining both the core level and valence level electrons. A high reso1 ution scan of the Pt 4f peaks of a sample prepared by method (l) is shown in figure 4. This sample has a fairly high platinum loading of about 40 wg/cm 2. The platinum is primarily present in two different oxidation states; metallic platinum and PtD (or Pt(DH)2)'
However, scans of lightly loaded
samples show only the PtO form of platinum. This supports the hypothesis that the deposition consists of a nucleation process in I'kiich Sn-O-Pt bonds are formed initi ally followed by growth of pl atinum crystallites.
~
z
::>
> a:
«
a:
I-
iii
a:
« w Z
Fig. 4. High resolution ESCA spectrum of the platinum 4f core levels from a tin oxide sample which had been platinized electrochemically. Unfortunately the oxidation state of the tin cannot be determined in the same manner because the Sn 3d peaks are located at the same position for both SnO and Sn02. However a method suggested by Lau and Wertheim (23) can be used to distinguish between SnO and Sn02. In this method an ESCA scan is taken of the valence band as shown in figure 5.
The peak at 3eV
1S
characteristic of SnD
while the peak at 5eV is characteristic of Sn02- Curve (a) is an ESCA scan immediately after insertion of the sample into the vacuum system, and curve (b) is a scan after heating the sample to 500·C. Heating causes desorption of impurities and sharpens the features. This particular sample contains more SnD than Sn02 as shown in curves (a) and (b). Curve (c) is a spectrum after heati ng the sample at about 400·C in 9xl0 5L of oxygen. This has caused the Sn02 peak to grow with respect to the SnO peak. The UPS results of Powell and Spicer (24) show similar features during the oxidation of tin. Their interpretation was not correct as pointed out in a later AES and electron-energy loss spectroscopy (EELS) study by Powell (25).
96
ESD can be an especially powerful tool for studyinq surface species particu1 arly now that it is better understood (26). It is one of the very few techniques which can be used to study hydrogen on surfaces. Fiqure 6 shows the result of a positive-ion scan over the low AMU r anqe using a 1000eV electron beam, and table 1 gives possible identifications of the ions. The 7 peak is most likely N++ since CH;+ probably is not stable. This would indicate that the peaks at 14 and 28 have a nitroqen component and possibly other components such as CH~ and CO+ respectively. Nitroqen has been identified on these surfaces using AES and probably is in the form of a nitrate. The argon peaks at 20 and 40 are due to ionization of gas phase arqon, which was present after sputtering in the system. The peaks often cannot be identified conclusively because there is a cracking process occurring on the sample's surface as 1'.I:!11 as in the ionizer of the mass spectrometer. TABLE 1 Possible identification of ESD peaks shown in fiq ure 6
m/e 1 2 6 7
8 12 14
Possible Identity H+
H~ C++ N++ , CH++ 2 0++ C+ N++ N+ CH+ 2' , 2
m/e 16 19 20 23 24 28 40
Possible Ident ity 0+ + + F ,H 3O Ar++,Ne+ Na+ Mg+ N;, CO+ Ar+
ESD is a destructive technique because the electron beam alters the surface. This means that the relative peak heiqhts in figure 6 change and, thus, are not of particular siqnificance. However, it is interesting to compare the H+ and 0+ peak heights as a function of time for an untreated vs. a caustic-treated sample. For both the H+ and 0+ peaks from the caustic-treated sample, the peak rises to a higher initial value and decays slower than the untreated sample. This result is consistent with the concept of surface hydroxylation by means of a caustic pretreatment even though no OH+ ions are detected. SIMS is another technique which is useful for characterizing catalyst surfaces because it is very sensitive to trace quantities, it can detect hydrogen and it can be used to depth profile surfaces. Its drawbacks are that SIMS is a destructive technique, and it is difficult to quantify the results. A SIMS scan of tin oxide is shown in figure 7. Curve (b) is the same as curve (a) after
97
N(E)
10
5
BINDING ENERGY
Fiq. 5. ESeA spectra of the valence band (a) immediately after sample insert i on, (b) after heat i nq and (c) after heat i nq in oxygen.
ESD POSITIVE ION SPECTRUM
o
Fig. 6. ESD positive-ion spectrum obtained by striking a tin oxide sample with lOOOeVelectrons.
98
beinq maqnified by a factor of 11. SIMS spectra usually are complex due to lts sensitivity and a detailed discussion of the SIMS spectrum will not be given here. The total metals content other than tin in the SnC14·H20 is 0.02%. SIMS identifies these impurities as Li-7, Na-23, Mg-24, K-39, Ca-40, Mn-55, and Fe-56. (b) shows a peak at 17 AMU which is identified as OW. Even more information is contained in the 100-300AMU range. Sb is observed at 123, but its height very rapidly decreases to the baseline indicating that most of the Sb is located at the surface.
SIMS POSITIVE ION SPECTRUM
_
...................M-...........(b]
(a)
75
Fig. 7. tion.
85
95
SIMS posit ive ion spectrum usi ng a 1000eV argon-ion beam for exci t a-
CONCLUSION It has been demonstrated that numerous surface techniques can be used to understand catalysts in terms of the methods used to prepare them. A better fundamental understanding of catalysts and their preparation should result in improved catalysts.
99
ACKNOWLEDGMENTS I would like to thank John Hren, Henry Van Rinsvelt and Paul Holloway for use of the transmission electron microscope, Van de Graff generator and SIMS spectrometer respectively. REFERENCES 1 F. Solymosi and J. Kiss, J. Cat., 41(1976)202. 2 M. Niwa, T. Minami, H. Kodama, 1. Hattori and Y. Murakami, J. Cat., 53(1978)198. 3 H.H. Herniman, D.R. Pyke and R. Reid, J. Cat., 58(1979)68. 4 Y. Boudeville, F. Figueras, M. Forissier, J.L. Portefaix and J.C. Vedrine, J. Cat., 58(1979)52. 5 M.M.P. Janssen and J. Moolhuysen, J. Cat., 46(1977)289. 6 M.M.P. Janssen and J. Moolhuysen, Electrochimica Acta, 21(1976)869. 7 M.R. Andrew, J.S. Drury, B.D. McNicol, C. Pinnington and R.T. Short, J. App. Electrochem., 6(1976)99. 8 A. Katayama, Chern. Lett., (1978)1263. 9 A. Katayama, J. Phys ••Chem., 84(1980)376. 10 R. Burch, J. Cat., 71(1981)348. 11 R. Burch and L.C. Garla, J. Cat., 71(1981)360. 12 R. Bacaud, P. Bussiere and F. Figueras, J. Cat., 69(1981)399. 13 J. Volter, G. Lietz, M. Uhlemann and M. Hermann, J. Cat., 68(1981)42. 14 B.H. Davis, J. Cat., fl6(1977)348. 14 B.H. Davis, J. Cat , , 46( 1977)348. 15 A.C. Muller, P.A. Engelhard and J.E. Weisang, J. Cat., 56(1979)65. 16 F.M. Dautzenberg, J.N. Hells, P. Biloen and W.M.H. Sachtler, J. Cat., 63( 1980) 119. 17 M. Watanabe, S. Venkatesan and H.A. Latinen, to be published. 18 1. Morimoto, M. Kiriki, S. Kittaka, 1. Kadota and M. Nagoa, J. Phys. Chem., 83(1979)2768. 19 R.O. Ansell, T. Dickinson, A.F. Povey and P.M.A. Sherwood, J. Electrochem. Soc., 124(1977)1360. 20 J. Rosenfarb, H.A. Laitinen, J.T. Sanders and H.A. Van Rinevelt, Anal. Chim. Acta, 108(1979)119. 21 G.B. Hoflund, D.F. Cox and H.A. Laitinen, Thin Solid Films, 83(1981)261. 22 G.B. Hoflund, D.F. Cox, G.L. Woodson and H.A. Laltinen, Thin Solid Films, 78(1981)357. 23 C.L. Lau and G.K. Wertheim, J. Vac. Sci. Technol., 15(1978)622. 24 R.A. Powell and W.E. Spicer, Surf. Sci., 55(1976)681. 25 R.A. Powell, Appl. Surf. Sci., 2( 1979)397. 26 M.L. Knotek and Peter J. Feibelman, Phys. Rev. Lett., 40(1978)964.
100 DISCUSSION I appreciate your comments concerning the value of electron D. CHADWICK stimulated desorption. However, in view of your observation of AES induced effects and the high signal-to-noise ratio of your spectra, perhaps you could comment on the current densities used to obtain your AES spectra. The accepted limit for no interogation phenomena in AES is around 5 mA/cm 2. G.B. HOFLUND The estimated electron beam current used in the slides presented was about 200 mA/cm 2 which is very large. Smaller beam currents of 24 mA/cm 2 were used in the ESD experiments. We use very small beam currents of 0.008 mA/cm 2 and pulse counting techniques for AES when we wich to minimize beam damage. However, electron beam damage occurs at all beam current densities qnd even at very low beam voltages (~ 20 eV on these samples). Since different samples are damaged at different rates and the damage may depend upon the total beam exposure, accepted limits must be considered only as rough guidelines. D. CHADWICK : With respect to your observation of a decrease in the low energy Pt Auger peak intensity with running time, have you considered segregation or sintering of the ~t? Your failure to observe Pt ions in the mass spectrometer seems inconsistent with the desorption of Pt as oxide species, since Pt ions would be expected from fragmentation of the parent ions (of course, the Pt ions may be multiple charged). A possible method for detecting segragation or sintering is to study the chemisorption of carbon monoxide by XPS before and after running the AES spectra. G.B. HOFLUND: We have not investigated this observation carefully at this time. Both segregation and sintering of the Pt are possible explanations and the suggested detection method will be one technique used in studying this phenomenon. J.W. GEUS We are using RBS to investigated the reaction between NiO and Al203. In this research we have found that the RBS spectra contain much more information than the thichness of the Pt layer: the position of the loading edge of the tin indicates whether the tin oxide is completely covered by the platinum or not. The oxygen-to-tin ratio can be calculated from the heights of the tin and oxygen peaks in the spectra. Did you use also the RBS spectra to get this information ? G.B. HOFLUND: RBS and PIXE are promising techniques in many respects. Thus far we have only used RBS to analyze the total Pt content of our samples. B.E. LANGNER: You have used different UHV techniques for the physical characterization of your catalysts. Did you carry out any reaction on the sno2/Pt or what does give you the knowledge that you have investigated a catalyst system and not only a mixture of inorganic compounds? Furthermore, a quantitative correlation between physical properties in UHV and real catalytic activity would be very important, as the activity of catalyst may change under UHV conditions. G.B. HOFLUND : The first 16 references cited in the paper document the catalytic properties of tin oxide before and after platinization and various dopings. In addition we are relating the electrochemical catalytic reactivity for several reactions to the surface characterization. In the near future gas phase reactions at intermediate presures will be run in the vacuum system. Much effort is being expended to relate UHV surface characterization to catalytic activity under realistic reaction conditions.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts II! © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SYNTHESIS AND PROPERTIES OF Pt-Sn/A1
20 J
101
CATALYSTS BY THE METHOD OF MOLECULAR
DEPOSITION
D.P. DAMYANOV
and L.T. VLAEV
Higher Institute of Chemical Technology, Bourgas 8010, Bulgaria
ABSTRACT It is shown that SnC1 Y-AI
vapours interact with the surface hydroxyls of 4 The quantity of tin deposited can be controlled by the packing density
20 3. of the hydroxyl groups. By hydrolysis of the modified samples with water vapour, followed by calcination and further treatment with SnCI
4,
complete formation of
a surface tin-containing'layer can be achieved. In synthesizing bi-metallic pt-sn/AI the
by the above method , 203-catalysts thermal stability and acidic properties for the same pore distribution of
the carrier were increased. The absence of catalytic activity with respect to hydrogenation of benzene, and high activity, stability and selectivity in dehydrocyclization of n-hexane are characteristic properties of this system.
INTRODUCTION Recently interest in bimetallic catalysts for reforming has increased considerably. Among various promotors of Pt/A1 systems,special attention has 203 been given to tin, The introduction of the latter in synthesizing Pt-sn/A1 20 3 catalysts was carried out by impregnation of the carrier with aqueous or nonaqueous solutions of its various compounds: chlorides (refs. 1, 2), tartrates (refs. 3, 4), organometallic compounds (refs. 5, 6)
and cluster compounds (refs.
7, 8). The difference observed in stability, activity and selectivity of these catalysts is directly related to their pore- and acidic properties and the state of the metal on the surface, conditioned naturally by the method employed for their formation. In literature there are no data available on the introduction of the promoting element by reaction between vapours of its volatile compounds and the surface of A1 Similar reactions have been successfully employed for the modifi203. cation of Si0 (refs. 9-11). Having in mind the potential possibilities and ad2 vantages of this method compared to conventional methods of impregnation, it is of interest to study the interactions between y-A1 ce the possibility of synthesizing pt-sn/A1
203
and SnC1
vapours and hen4 which is the aim of
20 3-catalysts, this work. Since these reactions involve deposition with atomic dispersion of the promoting ion, we define them as reactions of "molecular deposition".
102 EXPERIMENTAL The chosen support was y-A1 (Leuna, GDR) having a specific surface area 20 3 3/g. 2/g (with nitrogen) of 150 m and pore volume of 0.64 cm By calcination in air to constant weight of a fraction of 0.25-0.64 mm within the temperature range 250-750 °C, a series of samples labelled as: A-250, A-350, etc. were obtained having different contents of surface OH-groups determined thermogravimetrically. The calcined Al (or the sample of Pt/AI obtained on its basis by im20 3 20 3 pregnation with H was treated with a stream of nitrogen, saturated with 2PtCl 6) vapour of SnCI in a glass reactor at 150°C for 4 hours. After stopping the 4, flow of snCl and passing nitrogen through the samples, they were analysed for 4tin and chloride contents. Sample A-550 after modification with snCl was futher subjected to successi4 ve treatments including: hydrolysis with water vapour at 150°C for 1 hour, calcination for 6 hours at 550 °C and another treatment with SnCI
Such alternated 4. hydrolysis·-calcination operations made it possible to increase the quantity of the promoting agent deposited. Wafers
(10 x 30 mm) of thickness 12-15 mg/cm
2
°
fo fine Al powder were pre2 3 2 pared for IR spectroscopy by pressing at a pressure of 150 kg/cm . Studies were carried out in a portable quartz IR-cell with NaCI windows permitting calcination, outgassing, modification of the wafers with different quantities of snCl vapours and hydrolysis an UR-20
4 (in situ). Spectra were recorded at room temperature with
(Carl Zeiss, lena) apparatus. An optical attenuator was placed in the
reference beam. The catalytic activity of the samples thus obtained after suitable thermal activation in air and reduction with hydrogen (in situ) was studied in a flow reactor for the following reactions: hydrogenation of benzene, dehydrogenation of cyclohexane and dehydrocyclization of n-hexane
(Fluka AG). The analysis of
the reaction products was done by direct dosing of a gas sample by means of a six-way valve to a T.C.D. gas-chromatograph fitted with a 15 % squalane of chromosorb P AW column. RESULTS AND DISCUSSION Interaction between y-A1
20 3
and SnCl
4-vapours
Table 1 presents the hydroxyl group content of the samples and the quantities of CI
and tin fixed on the surface of Al
203
after its treatment with SnCl
vapours.
~Up
to 400°C coordination water is still present on the surface of y-A1 and 203 therefore, the data on OH-groups up to this temperature are approximate and
higher than the real ones.
~
4
103 TABLE 1 Dependence of the hydroxyl group ( IX ) content, the quantities of the "fixed" OR 4+ 4+ Cl and Sn ,and the Cl /Sn ratio, on the degree of dehydroxylation of
Parameters
Samples
' mgequ/g OR Cl-,mgat/g 4+,mgat/g sn 4+ Cl /Sn IX
A-250
A-350
A-450
A-550
A-650
A-750
3.17
2.64
2.00
1. 32
0.76
0.44
1. 10
1. 04
1. 01
0.97
0.91
0.84
0.38
0.36
0.31
0.28
0.25
0.22
2.89
2.89
3.36
3.46
3.64
3.82
It is obvious that with a decrease in packing density of the hydroxyl coverage, the quantity of Cl tio increase
and tin deposited regularly decreases,while their ra-
The relatively small quantities of Cl
and tin, as compared to the
total content of hydroxyl groups, can be accounted for by their incomplete inteteraction due to steric difficulties and by secondary reactions. More detailed data on the surface reactions were obtained by IR spectroscopy (Figs.
and 2).
In the spectrum of fresh Y-A1 0 dehydrated at 550°C (la-l), in conformi2 3 ty with the model of Peri (ref. 12), absorption bands were observed at 3780, 1 3744, 3733 and 3700 cm- due to stretching vibrations of isolated OR-groups. After contact of A1 0 with successively increasing quantities of snC1 these 2 3 4, bands changed and disappeared altogether (la-2, 3, 4). The most reactive OR1. groups are characterized by bands at 3780 and 3700 cmA new wide band with 1 a maximum at about 3580 cm- appeared, which in agreement with (refs. 13, 14), may be accounted for by hydrogen bonds between OR-groups and neighbouring chlorine ions. This requires a secondary reaction of RCl, evolved as a result of the chemisorption of SnC1
4:
'\ AI""
°-,/ ./'
-, AI-Cl /
+ AI-O
RCI
°\
(1)
AI-OR
/'
Further pretreatment decreased the intensity of this band and eliminated 1 partially reap-
it at 350°c. At the same time the bands at 3744 and 3733 cm-
peared, which confirms that not all the OR-groups reacted. These bands became much more pronounced in the spectrum obtained after calcination-hydrolysis-calcination (lb-3). This indicates the formation of "secondary" OR-groups which, as seen from spectrum (lb- 4), are reactive and interact with other portions of
104
----~:~l ~ 7
Q
0 1 ~5
e 0 'iii
'fUl c:
7 6
~
6
~
5 ~I
4 3
,,,,,
<:1 ....
t-
(Y>,
I
I
0
co
I~ I ~
gl
~'""
Lt'l
I
I I
0
~II
t!I
""
0
3800
2 ~I
I
~
~I ......
3400
3000
3400
1800 1400 Frequency, em'
pretreated at 550 DC (1) and after treatment with 20 3 20 DC increasing quantities of SnC1 2.6 ~mol (2), 5.2 umo I (3) at P 1 =23 4-vapours: SnC 4 Torr (4), and following pretreatment at :150 (5), 250 (6) and 350 DC (7). Fig. 1a. IR-spectra of Y-A1
lb. After additional successive treatments including : calcination and outgassing at 550 DC (1, 3, 6), hydrolysis with water vapour (2, 5) and treatment with vapours of SnC1
4
(4, 7).
snC1
The fact that the initial hydroxyl coverage after hydrolysis is not fully 4. recovered testifies to sufficient stability of the Al-O-Al bonds formed with
their participation. Under these conditions the identification of the Sn-OR 1, is impossible
groups which, according to (ref. 9), are to appear at 3665 cmas in this region the stretching vibrations of OR-groups hydrogen bonds
connected through
are observed. Repeated hydrolysis and calcination result in a
considerable decrease in the intensity of the bands of these OR-groups which is a proof that the building up of the surface layer is made at their expense. This is also verified by data from chemical analyses which point to an increase in the tin content at the second and the third cycles by 0.15 and 0.11 mgat/g, respectively, while the chloride content remains approximately constant and is within of the order
0.98 mgat/g.
After sample A-750 had been treated with snC1 and hydrolysed with 4-vapours water vapour, changes similar to those for sample A-SSO were observed (Fig. 2), the only difference being that its characteristic bands were less pronounced. Moreover, the difference between the thermal stability of the hydroxyl groups obtained by hydrolysis with water vapour and those obtained by modification must be
pointed out. With the latter (fig. 2-3) it was considerably smal-
ler because of the protonation resulting from neighbouring Cl- ions.
105
---./ 5
C
a
'iii
'<11E c
cI
....
I-
~
L:
I
0 CO
~
I
~I
~
2
~ ---~
t
,.,r--
,'",
0
"
~I
3800 3400
1800
1400
Frequency. cm' Fig. 2. IR-spectra of y-A1
pretreated at 750°c (1) and after ensuing treat20 3 20°C vapours at P = 23 Torr (2), pretreatment at 350°c for 4 snC1 4 6 hrs.(3), hydrolysis with water vapour (4) and pretreatment at 350°c (5).
ments: with SnC1
Keeping in mind what has been said above
as well as the fact that M6ssbauer
4
spectra showed only the presence of sn + , it may be assumed that, as a result of chemisorption of snC1 , surface structures of the type A, Band C were formed:
4
Cl
Cl
"Sn/
I
Sn
/1"-
°I °I °I
Ai
I" /
Ai
"-
/
(A)
°AiI
Ai
/".. /
<, / -,
° °
°
Cl I Cl-Sn-Cl
Cl
I
°AiI
(B)
°I
(c)
Ai
"-
1"-
It is questionable if part of HC1,a product of the reaction, was capable of interacting according to the equation: >Al-OH
+
HCl
->-
(2)
)Al-Cl
An affirmative answer was obtained on the basis of further studies concerning the interaction of Y-A1
203
with vapours of Cr0
2C1 2
under analagous condi-
tions (Fig. 3). As can be seen from Fig. 3 this reaction led to the appearance of a band at 1 in the spectrum, attributed to the deformation vibration of water. A si'
1580 cm-
milar band was also observed in the spectrum of y-A1 pretreated at 350°C. 20 3 1 The absence of a band at 1580 cm- in the spectrum of y-A1 after its 203 4 treatment with snC1 can be accounted for by the coordinatively filled sn + and 4 partial hydrolysis of the tin-containing structures obtained. With chromiumcontaining structures the water evolved was coordinated to surface aluminium ions or by
forming chromium ions in incomplete coordination. The possibility
106
1800
1400
Frequency. ern" Fig. 3 . IR-spectra of Y-AI
prepared at 350°C (1), 550°C (2) and after 20 3 successive contact with vapours of Cr0 1.9 ~mol (3), 3.8 ~mol (4) and 2CI 2: 20°C at P = 19 Torr (5). Cr0 2Cl 2
of chromium-containing structures is highly limited. The band at 1580 cmpearing
1
aF-
after hydrolysis with an excess of water vapour of the sample modified
with SnCI
confirmed the coordinating ability of the surface aluminium ions 4, (Figs. 1b-2 and 2-4). It may thus be concluded that by treating A1
precalcined at 250-350 °c 20 3 preference should be given to the reaction resulting in structures 4, 4+ of the A and B types. Theoretically, this requires that the CI /Sn ratio
with SnCI
should be within the limits 1 and 2, while the recorded ratio ( =2.82) confirms the occurrence of reaction (2). Type C structure is formed on the surface of Al dehydroxylated at 750°C. 203 4+ This is proved by the smaller quantities of tin and CI ,and the higher CI /Sn ratio. In this case the smaller packing density of OH-groups on the one hand, and the greater number of AI-O-AI bridges on the other hand, hindered interaction 4 according to scheme (2). Thus ratio CI-/sn + = 3.82 can be explained by additional halogenation according to equation (1). On the basis of this discussion, the reaction routes can be shown diagrammatically (Fig. 4). On the partially dehydroxylated surface of Y-AI along with the variuos 20 3, types of OH-groups (different number of neighbouring oxygen ions) there are also ones that are connected through hydrogen bonds with different configurations (Fig. 4 a). Treatment of this surface with snCl
4
vapours leads to the re-
107
o x ~
0
x
...~ ... ~
0
o
0
x 0 x ~ x ~ x
x
o
X 0
x
~
... ~
X 0
e
0
0
x
x
o x
0
o x
0
... ~
X
~ :
0"'~
0
~
•.• ~ I
~
:
0 @ x
G0 x
. * ~
OX~XO@OO
x 0
. X
a D
e... e... ~
~ .-. $"'~"'~
0
~
o ~ ... ~ ... ~ • ~ ... ~ ... ~
~ •
•
0
:l ~ ·
~ ...~
:: 181'' ~' '@
I
o x ~
x
.00 ~
... ~
x
~
~
o
e... ~
x
~
o
:
•
0
:
~ ...~ ...~ ... ~
e
x
0 x 0 x x 0 x
0 0
~
0
.
0
o x
=I ~
~ ... ~.
0
x
@
b
18l"'~"'~"'~'"
~~"'8"'~"'•~"'~ ~
0
o
X 0 X
o
x 0
•
~
o e
x
• 0
x ~ o • o
o 12>
x o • 0 x 000 x ~ x
c
0
x
o o
d
Fig. 4. A diagram of reaction routes:a)partially dehydroxylated surface of Y-AI
(X-AI ion from a lower layer, O-oxygen ions, 0-0H-groups); b) after 203 reaction with SnCl (o-tin structures of type A,B and C, 0 -AI-CI, O-"secondary" 4 OH-groups, ... -Hydrogen bonds); c) after hydrolysis with water vapour (~coordi nation water ); d)
after calcination.
moval of part of the hydroxyl coverage and the formation of tin surface structures of configuration A, Band C (Fig. 4b). Part of the HCI, evolved during the reaction, interacted with the surface, forming more OH-groups and AI-CI. The rest of the HCI, because of a lack of suitable centres, did not react with the surface, which is proved by a Cl-/Sn 4 + ratio smaller than 4. The next stage of hydrolysis (Fig. 4c) results in a dissociation-adsorption of water, the
forma~
tion of additional quantities of OH-groups, and coordination filling-up of surface aluminium ions with water molecules. The calcination stage leads to the removal of coordination water
as well as water molecules and HCI formed from the
recombination of neighbouring OH and CI
ions. If dehydroxylation was purely ac-
cidental, then chlorine ions and newly formed OH-groups remaining on the surface would, upon the second treatment with SnCl
vapours, interact in a similar way 4 and increase the quantity of tin deposited, packing in this way the surface tin-
containing structures.
108 Properties of Pt-sn/A1
catalysts
20 3-
After calcination of Pt/A1 0 at 550 aC in air and treatment with 2 3 or impregnation with a solution of SnC1 two bimetallic Pt-Sn/ 4-vapours 2, A1 0 catalysts of platinum and tin content of 0.51 and 3.3 % by wt, respective2 3 ly, were obtained. Table Z gives some of their main characteristics compared to snC1
those of the initial Pt/A1 0
Z 3
sample.
TABLE 2 Influence of the method of preparation of pt-sn/A1 0 on their struc2 3-catalysts tural characteristics. Sample
Pt/A1 0 2 3 ~t-sn/Alt3 by lmpregna lon Pt-sn/A1 with 20 3 SnC1 pours 4-va
Parameter 2/g S, m
V
3 em /g
p'
Temp. of phase transition y-a) , °C
Chemisorbe
NH , mmol/g 3
170
0.59
950
0.246
140
0.48
1050
0.330
177
0.62
1150
0.362
The results presented in Table 2 show that the method of modification affects both pore- and acidic properties, and the temperature of phase transition (y-a A1 0 The advantage of modifying with snC1 manifests itself in 2 3). 4-vapours retaining the pore distribution, increasing the phase transition temperature and the acidic properties of the initial y-A1
Furthermore, IR spectroscopy 20 3. studies of CO chemisorption showed a considerable decrease in the band frequen-
cy of CO chemisorbed on Pt-sn/A1 0
2 3,
which indicates the presence of electronic
effects in the system. All these changes as a whole specifically affect the catalytic activity of the Pt-sn/A1 0 system. As an illustration, Table 3 summarizes the results of 2 3 1 cyclohexane dehydrogenation at mass velocity 2 hmol ratio H : C ~ 6 and 2 6H12 atmospheric pressure. TABLE 3 Dependence of the conversion of C on reaction temperature. 6H1 2 Composition of reaction products (wt; %)
C 6H12 C 6H6
Temperature, °C
390
420
450
480
530
36.0
19.8
2.0
traces
traces
64.0
80.2
98.0
ca.100
ca.100
109 The only product of dehydrogenation of cyclohexane is benzene, the bimetallic catalysts displaying lower activity at lower temperatures as compared to monometallic Pt!A1
catalyst. A sim'lar tendency in the Pt-Sn!A1 system has 20 3 203 been noted by other authors also (ref. 15,16) ,but, at temperatures characteristic of reforming, conversion is selective and complete. Table 4 contains the results from catalytic tests of n-hexane dehydrocyclization under the conditions cited above. TABLE 4 Dependence of the composition of the reaction products on the temperature of
n-hexane dehydrocyclization. Temperature, °C
Composition of reaction products, C
1-C 5
2MP+CP
3MP
(wt %)
n-C H 6 14
MCP
C H 6 6
450
0.9
2.7
2.7
90.0
1.8
1.0
520
7.4
6.6
7.3
48.8
6.2
23.7
540
10.1
4.8
4.7
26.0
6.5
46.9
550
9.3
3.6
3.5
19.1
6.7
56.6
1.2
570
14.6
1.9
1.5
11. 1
5.8
64.2
0.9
570 after 5-fold regeneration
16.3
2.0
1.6
10.1
4.1
64.8
1.1
1.0
2MP= 3-Methylpentane; CP= Cyclopentane; EMP= 3-Methylpentane; MCP=Methylcyclopentane. Because of the inhibition of the cracking reactions at the expense of the ensemble effect, catalysts rich in tin showed a weaker tendency to form coke and hence, an increased stability. This is confirmed also after a 5-fold regeneration with air, the initial activity of the catalyst being preserved. The increased dehydrocyclization ability of pt-Sn!A1
can be attributed 203-catalysts to the "ligand effect" of the promotor which leads to a change of the electronic
density of the active metallic component. As regards the benzene hydrogenation,bimetallic Pt-Sn!A1 catalysts exhi20 3 bited a total lack of hydrogenating activity within the whole thermodynamically suitable temperature range (100-180 °C) ,\-,hich is attributed to the absence at these temperatures
of hydrogen free platinum centres (17, 18).
In conclusion, chemisorption of SnC1 modifiying the Pt!A1
on A1 can be successfully used for 4 203 system as well as for the formation of bimetallic cata-
203 lysts with interesting properties. This offers practical prerequisites for the
production of stable and selective discrete Pt-Sn!A1
203
catalysts.
REFERENCES 1.
V.N. Selesnev, Yu. V. Fomichev, M.E. Levinter, Neftekhimiya, 14
(1974) 205-
110
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
208. B. Davis, G. Westfall, J. Watkins and J. Pezzanite, J. Catal. ,42 (1976) 247-256. Eng. Patent 1356383 (1974). F.M. Dautsenberg, M.J. Helle and W.M.H. Sachtler, J. Catal., 63 (1980) 119128. A. Campero, M. Ruiz and R. Gomez, React. Kinet. Catal. Lett., 5 (1976) 177 182. B.N. Kuznehzov, V.K. Duplyakin, V.I. Kovalchuck, Yu.A. Ryndin and A.S. Bely, Kin. Katal., 22 (1981) 1484-1489. V. Keirn, H. Leuchs, B. Engler, Forschungsberichte den Landes Nordrhein-Westfalen, BRD, 2838 (1979) 1-36. V.I. Zaikovskii, V.I. Kovalchuck, YU. A. Ryndin, React. Kinet. Catal. Lett., 14 (1980) 99- 103. B. Camara, P. Fink, G. Pforr and B. Rackow, Z. Chern., 12 (1972) 451-455. W:Hnake, R. Bienert, H. Jerschkewitz, Z. anorg. allg. Chern, 414 (1975) 109 129. S.I. Koltzov, V.B. Aleskovskii, Silica gel, its structure and chemical properties, Goskirnizdat, Leningrad, 1963. J.B. Peri, J. Phys. Chern., 69 11965) 220·230. J.B. Peri, J. Phys. Chern., 70 (1966) 3168-3179. M. Tanaka, S. Ogasawara, J. Catal., 16(1970) 157-163. A.S. Bely, V.K. Duplykin et al. React. Kinet. Catal. Lett., 7 (1977) 461466. J/ V61ter, H. Lieske, G. Lietz, React. Kinet. Catal. Lett., 16 (1981) 87-91 A.S. Bely, V.K. Duplykin, YU. V. Fomichev et al., Sb. Kataliticheskaya konversiya uglevodorodov, Kiev, USSR, 4 (1979)29-33. J. V61ter, G. Lietz, M. Uhlemann, M. Hermann, J. Catal., 68 (1981) 42-51.
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts II! © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
111
PRODUCTION OF SILVER BIMETALLIC CATALYSTS BY LIQUID-PHASE REDUCTION K.P. DE JONG and;J.W. GEUS Department of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht, The Netherlands
ABSTRACT The possibilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts are investigated. Although the method is more generally applicable, we here concentrate on one combination of metals, viz. Pt and Ag. The catalysts are-characterized by electron microscopy and infrared spectra of adsorbed CO. PtAg/Si02 catalysts are prepared using a 6 wt.% Pt/Si0 2 catalyst as the starting material. Silver was deposited selectively onto the Pt particles by reduction of Ag(NH 3); ions dissolved in an aqueous suspension of the platinum-loaded silica. Using formalin as a reducing agent bimetallic particles having sizes from 30 to 50 ~ result, whilst infrared spectroscopy indicates the platinum surface to be covered almost completely by silver. The selective deposition of Ag onto the Pt particles during preparation appears to be due to the suppression of nucleation of Ag particles in the liquid phase on the one hand, and the catalyzing effect of Pt or metallic Ag on the reduction, on the other hand. INTRODUCTION In the production of supported bimetallic catalysts, besides a high dispersion to provide a large active surface area, an intimate contact between the two metal phases is important to bring about alloy formation at low temperatures. Impregnation and drying procedures have been successfully utilized with the preparation of supported bimetallic catalysts of two group VIII metals [1,2]. Preparation of catalysts containing both a group VIII and a group IB metal appears to be much more difficult. Boudart and coworkers [3,4] succeeded to prepare highly dispersed PdAu/Si02 catalysts by carefully applying an ion-exchange method. To this end they used a silica with a very high surface area (700 m2/g). Ion-exchange at the silica surface called for positively charged metal complexes. Many examples in the literature exist, however, where the deposition of a group VIII and IB metal onto a support using simple impregnation and drying procedures led to two separate metal phases. Preparing RhAg/Si0 2 by co-impregnation, Anderson et al. [5] observed both very small Rh particles (50 ~) and silver crystallites of
112
500 ~. X-ray patterns of PdAg/SiO Z catalysts prepared by Soma-Noto and Sachtler [6J revealed the presence of considerably larger Ag crystallites besides small PdAg particles. O'Cinneide and Gault [7] had to calcine their PtAu/SiO Z catalysts at 770 0C in air to obtain alloy particles. These examples convincingly demonstrate the problems encountered with the production of bimetallic catalysts by impregnation and drying. Anderson [8J concludes that separation of the two metals already starts during the impregnation step. Usually, the group VIII metal ions will adsorb onto the support, whereas the IB metal ions remain dissolved. After drying and hydrogen reduction the result is obvious: the transition metal has formed very small particles due to the atomic dispersion after impregnation, whereas the IB metal has formed much larger crystallites in the pores of the support. of conventional methods, we will explore the possiBecause of the drawback~ bilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts. This work deals with liquid-phase reduction of noble metal ions to the metallic state in an aqueous suspension of the support. Concerning monometallic catalysts we have reported previously on the preparation of silicaand alumina-supported silver catalysts by reduction of complex silver ions [9). Favourable preparation conditions to enhance the silver dispersion were observed to be: homogeneous addition of the reducing agent, the absence of micropores in the carrier and an appreciable interaction between the soluble metallic complex and the support. In this paper attention will be paid to the difficult task of preparing small alloy particles of a group VIII and a group 18 metal. The preparation method comprises selective deposition of the second metal onto particles of the first component already present on the support. In an aqueous suspension of the support covered by particles of the first metal, a compound of the second metal is dissolved and the corresponding metal is subsequently deposited by addition of a reducing agent, e.g. formalin, hydrazine or gaseous hydrogen. Since the particles of the first component catalyze the reduction reaction, the second metal is deposited exclusively onto the metal particles. As a result the two metal phases are now in an intimate contact after the reductive deposition and alloy formation can be easily achieved. With a Pt/SiO Z catalyst as the starting material, the preparation of PtAg/SiOZ catalysts will be described in detail. To demonstrate the general applicability of the method, the preparation of RuAg/SiOZ will also be reported. The catalysts produced will be compared with a sample prepared by impregnation and drying. The catalysts will be characterized by electron microscopy, infrared spectra of adsorbed CO, and X-ray diffraction. EXPERIMENTAL Platinum-silver alloy catalysts were prepared by using a 6 wt. % Pt/SiOZ
113
catalyst as the starting material. This catalyst has been prepared by Johnson Matthey and proposed as a common standard (Eurocat) by a research group sponsored by the Council of Europe. The Pt/SiO Z catalyst was powdered and suspended in an aqueous solution containing Ag(NH 3)z+ ions*. Unless stated otherwise, the reduction of the silver complex was performed at 50C by injection of a solution of formalin or hydrazine into the suspension through a capillary tube having its end below the level of the liquid. During the injection the suspension was vigorously agitated. After addition of the reducing agent the suspension was slowly heated to room temperature. This reduction procedure took two hours. The composition of the alloy catalyst produced was varied by depositing different q~antities of silver on an identical batch of Pt/SiOZ' A RuAg/SiOZ catalyst was produced by first preparing a Ru/SiOZ sample via impregnating silica (Aerosil ZOOV) with a RuC13 solution followed by drying and reduction with HZ at 400 0C. Next,silver was deposited onto this sample as described for Pt/SiOZ' The'reducing agent used was hydrazine. Because under the above experimental conditions oxygen is able to reoxidize the deposited metallic silver, air was excluded during the liquid-phase reduction by working in an NZ atmosphere. The concentration of silver ions in the solution duriRg precipitation was continuously measured by means of a combination of a silver-ion selective electrode (Philips IS 550-Ag+) and a reference electrode (Philips R44/Z-SD/l). Characterization of the samples dried at lZOoC by X-ray diffraction was done using a Debye ,-Scherrer camera. A Philips EM301 and a Jeol ZOOC microscope were used for detailed examination of the catalysts by electron microscopy. We estimated the surface composition of the PtAg/SiOZ catalysts from transmission infrared spectra of adsorbed CO. To that end a powdered catalyst was pressed into a self-supporting disk and transferred to an in situ infrared cell. As a standard treatment the catalyst was oxidized at 4000C in 1 atm of Oz to equilibrate the alloy particles. Subsequently the disk was reduced and evacuated at 4000C followed by oxygen adsorption (10 Torr OZ) at the same temperature. Short evacuation was followed by cooling down the catalyst to room temperature. The thus mildly oxidized catalyst was exposed to 100 Torr CO at room temperature and spectra were recorded using a Perkin-Elmer 580B spectrophotometer. Gas phase absorption was compensated for by an identical cell placed in the reference beam of the spectrophotometer. The spectra were corrected for background absorption of the SiOZ carrier.
*In the absence of stirring, silver-ammonia solutions spontaneously form extremely explosive silver-nitrogen compounds.
114
RESULTS The BET surface of the Pt/Si02 catalyst used as a starting material was established to be 189 m2/g. At room temperature the reduced platinum catalyst adsorbed 3.30 ml(STP)H2/9 catalyst (H2 pressure 10 Torr). Using a surface stoichiometry H/Pts = 1.8 [10] we arrived at a surface-mean particle size of 23 ~. An electron micrograph of the catalyst has been reproduced in fig.la. This representative micrograph ~hows a very homogeneous distribution of Pt particles of a fairly narroW size distribution, which displayed a maximum around 20 ~. Silver was deposited onto Pt/Si02 by reduction of Ag(NH3); with either formalin or hydrazine at 50C. A survey of PtAg/Si0 2 samples produced is found in table 1. Catalysts prepared by formalin or hydrazine are designated by F or H, respectively. followed by a serial number. Using formalin as a reducing agent. it is important to carry out the liquidphase reduction below room temperature. As dealt with before [9]. formalin does not react with A9(NH3); at 50C in a suspension of unloaded Si0 2. In the presence of Pt/Si02. however. the reduction takes place very fast at 50C as was inferred from a rapid decline of the silver concentration measured by the ion-selective electrode. It is obvious that the platinum particles do catalyze the reduction of the silver complex more effectively than does the pure carrier. It has to be expected that silver will be deposited very selectively on the Pt particles TABLE 1. Survey of PtAg/Si02 catalysts produced by liquid-phase reduction of Ag(NH 3); in the presence of Pt/Si0 2. One catalyst (15) has been prepared by impregnation of Aerosil 200V with a mixed solution of silver and platinum nitrate. Catalyst
Pt/Si0 2 F1* F2* F3 F6 H4 15
Loading (wt.%Ag)
Alloy composition At.%Pt At.%Ag
2.0 5.5 5.7 1.5 5.3 5.7
100 62 36 36 69 37 36
0 38 64 64 31 63 64
Particle size from TEM (~) 20 30 50 50-100 + 20-300++
t See text for explanation.
* Total amount of formalin added at once. + Some large clustered particles besides very small ones. ++ Bimodal particle size distribution.
Absorbancet at 2090 cm- 1 1.68 0.08 0.61 0.31
115
Fig.I. Transmission electron micrograph of the 6 wt.% Pt/Si0 2 catalyst (a); micrographs of the PtAg/Si02 catalysts FI (b), F2 (c), F3 (d). leaving the support uncovered. Fig.I.b shows the experimental facts supporting this reasoning. This electron micrograph of a freshly prepared and dried (I20 0C) PtAg/Si02 sample shows irregularly shaped metal particles, sometimes rod-like, whereas the original Pt particles (fig. I.a) definitely exhibit regular shapes. Obviously, metallic silver has been deposited exclusively adjacent to the Pt particles. Fig.I further reveals the absence of large silver particles (~ 300 ~) characteristic of the Ag/Si02 samples produced by liquid-phase reduction [9]. The alloy particles produced have a narrow size distribution exhibiting a maximum around 50 ~. PtAg/Si02 catalysts prepared by hydrazine reduction display a broad particle size distribution if compared with F-type catalysts (table 1). Hydrazine is so
116
Fig.2. Transmission electron micrograph of PtAg/Si02' sample 15. The catalyst has been reduced at 400 0C in H2. fast a reducing agent that it does produce Ag both on the Pt particles and in the liquid phase. TEM revealed clustered silver particles the number of which was scarce, however. For comparison, one PtAg/Si02 sample was prepared by impregnation and drying (table 1, catalyst 15). An electron micrograph of this catalyst (fig.2) revealed a bimodal particle size distribution. Both particles of 20 to 50 ~ and of 100 to 300 ~ can be seen. The X-ray pattern of catalyst 15 showed fairly sharp Ag lines and no Pt lines, while there was no evidence of alloy formation. These observations strongly suggest that the smallest particles mainly consist of Pt, whereas the larger ones consist of Ag. Adsorption of platinum ions and silver ions remaining dissolved during impregnation readily explain these phenomena. We have thus shown that the liquid-phase reduction technique is superior to the impregnation and drying method for the production of PtAg/Si02. A most elegant way to demonstrate the success of our preparation method is by infrared spectra of adsorbed CO. Besides results obtained with the PtAg/Si02 catalysts F2 and F6 results for Pt/Si0 2 have been added for comparison (fig.3 ). To appreciate the results shown it is necessary to know that spectra of CO adsorbed on reduced and mildly oxidized Pt/Si0 2 catalyst are almost identical [11,12]. Apparently, CO is able to remove adsorbed oxygen from a Pt surface already at room temperature. On the contrary, the PtAg catalysts show largely different spectra for reduced and oxidized samples [12], which demon-
117
10 _ PtAg36/64 ___ PtAg69f31 _._. Pt
•
o8
II II
jlx05
oe
I rl "
r.,
j:l
,: \ 1'1
u
oA
c:
i: 1
-" I-
': i
0
., \
0
'"
.o
\
I,"
0
I
'""
i'",
o2
I,
\,.,
I'
"
I
.: -,
I
~ ...
o.C _/ -,i 2200
~:"
.
'~ .,.",,"';...,.
wavenumber(c';;:'-
2100
2000
1900
Fig.3. Infrared spectra of CO adsorbed on oxidized catalysts. Samples F2 ( - ) , F6 (- - -) and Pt/Si0 2 (- . -). strates strongly held oxygen. This lower reducibility of PtAg compared with Pt has been used to estimate the surface composition of our alloy catalysts. The absorption band at 2090 cm- 1 shown in fig.3 is ascribed to CO adsorbed on reduced Pt sites [11,12] while the band at 2175 cm- 1 is due to CO bound to Ag+ ions [12,13]. The intensity of the 2090 cm- 1 band is positively correlated with the amount of Pt surface not covered by Ag; see fig.3 and table 1. From the intensity the almost complete covering of the Pt particles by silver in catalyst F2 can be inferred. At this point the reader should note that we start our preparation with pure Pt particles; every Pt particle not covered with Ag during the liquid-phase reduction will contribute to the band at 2090 em-I. Whereas catalyst F2 hardly exposes free Pt surface, catalyst F6 displays a significant absorption around 2090 cm- 1 (fig.3 ). Due to the low silver loading, not all of the Pt particles in catalyst F6 have been covered with Ag. Comparison of catalysts F2 and H4 (table 1) shows that hydrazine is less successful than is formalin to deposit uniformly Ag onto the Pt particles. To show that our preparation method is not limited to PtAg catalysts we produced a RuAg/Si0 2 sample by liquid-phase reduction. Transmission electron micros-
118
copy of both Ru/Si02 and RuAg/Si0 2 yielded results similar to those obtained with the PtAg catalysts. RuAg particles ranging in size from 50 to 100 ~ have been distributed homogeneously over the support. DISCUSSION Concerning the preparation of Ag/Si02 catalysts [9], l'~uid-phase reduction of Ag(NH3); with formalin at temperatures ranging from 20 to 500C typically produced a bimodal particle size distribution. Particles nucleated on the support displayed sizes of 35 to 70 ~, whereas particles formed in the liquid phase were considerably larger (200-400 ft). A more or less fundamental lower limit of the average particle size for the silver-silica system of about 60 ft was recognized and put together with the high mobility of silver particles over silica surfaces. This mobility, which is especially high in the presence of water [14], was though" to reflect a weak metal-support interaction. Clearly, a strong interaction between silver and the support is a prerequisite to keep the deposited Ag particles small. The work presented here shows that the presence of Pt (or Ru) particles effectively enhances the silver-silica interaction. The enhanced interaction can be inferred from the fast reduction of the silver ions at low temperatures (SoC) and from the very high dispersion of the silver metal deposited. We emphasize two factors contributing to this high dispersion. First of all the absence of large silver particles, which nucleate in the liquid phase, is important. The absence of the large particles (~ 300 ~) is due to the low reaction temperature utilized where the reaction proceeds exclusively on the catalytically active Pt particles. Another contribution to the silver dispersion of the Pt particles is the anchoring of metallic silver on the support. Apparently, Pt particles are strongly bound to silica and are therefore effective in stabilizing the deposit, silver. The reader will take for granted that the strong interaction between silver and the platinum-loaded support is due to strong intermetallic bonds between tr two metals. Intermetallic bonding is much stronger than the physical interacti< between Ag and Si0 2. Strong interaction between two metals leading to small nUl of the deposited metal was observed also by Wassermann and Sander [15]. These authors deposited iron onto rocksalt and gold substrates kept at 80 K. Whereas many isolated iron crystallites were observed on rocksalt, an almost continuou layer was obtained with gold as a substrate which points to a much higher dens of iron nuclei. Besides the anchoring of silver, it is worthwhile to examine more closely 1 reduction of the silver complex. From the transmission electron micrographs (cf. figs. l.a and l.b ) it appears to us that the deposition of metallic si
119
mainly occurs lateral on the support. This phenomenon can be explained by assuming adsorption of the complex silver ions on the support prior to the reduction step. An appreciable interaction between the silver complex and the support appeared to be favourable to enhance the dispersion of monometallic silver catalysts [9]. In a forthcoming paper we will show that the extent of metallic complex adsorption is an important parameter which controls the production of supported PtAu bimetallic particles [16]. The above results demonstrate that the concept of increasing the interaction of a group IB metal with a silica support by introduction of a second metal has been used to produce excellent bimetallic catalysts. PtAg and RuAg particles of 30 ~ on silica, homogeneously distributed over the support, can easily be produced using this technique. The method has been shown to be superior to impregnation and drying procedures. The reduction of a metal complex in the presence of a loaded supp~rt can be considered to be both a method to prepare bimetallic catalysts and a method to improve the dispersion and thermal stability of monometallic catalysts. The latter application calls for less expensive metals strongly adhering to the support. ACKNOWLEDGEMENTS The authors are indebted to Mr. R. Hendriks for preparing the greater part of the catalysts. The investigations were supported by the "Netherlands Foundation of Chemical Research" (SON) with financial aid from the "Netherlands Organi zat i on for the Advancement of Pure Research" (ZWO). REFERENCES 1 C.H. Bartholomew and M. Boudart, J. Catal., 25 (1973) 173-181. 2 J.H. Sinfelt and G.H. Via, J. Catal., 56 (1979) 1-11. 3 Y.L. Lam and M. Boudart, J. Catal., 50 (1977) 530-540. 4 E.L. Kugler and M. Boudart, J. Catal., 59 (1979) 201-210. 5 J.H. Anderson, P.J. Conn and S.G. Brandenberger, J. Catal., 16 (1970) 404-406. 6 Y. Soma-Noto and W.M.H. Sachtler, J. Catal., 32 (1974) 315-324. 7 A. O'Cinneide and F.G. Gault, J. Catal. 37 (1975) 311. 8 J.R. Anderson, Structure of Metallic Catalysts, Academic Press, London, 1975, p. 176. 9 K.P. de Jong and J.W. Geus, Applied Catalysis, submitted. 10 J.-P. Candy, P. Fouilloux and A.J. Renouprez, J. Chern. Soc., Faraday Trans. I, 76 (1980) 616-629. 11 H. Heyne and F.C. Tompkins, Trans. Faraday Soc., 63 (1967) 1274-1285. 12 K.P. de Jong, Ph. D. Thesis, Utrecht, 1982. 13 G.W. Keulks and A. Ravi, J. Phys. Chern., 74 (1970) 783-786. 14 L. Bachmann and H. Hilbrand, in R. Niedermeyer and H. Mayer (Eds.), Basic Problems in Thin Film Physics, Van den Hoeck and Rupprecht, Gottingen, 1966, p. 77. 15 E.F. Wassermann and W. Sander, J. Vac. Sci. Technol., 6 (1969) 537-539. 16 K.P. de Jong, R.C. Verkerk and J.W. Geus, in preparation.
120 DISCUSSION H. CHARCOSSET drag en at 400°C ?
What happens during heating your PtAg/Si02 catalyst in hyIs there some interdiffusion of Pt and Ag in these conditions?
K.P. de JONG A freshly prepared sample of PtAg/Si02 (catalyst F2) has been studied after reduction at 120°C. It was observed that adsorption of CO only led to a weak band in the IR spectrum. As Co hardly adsorbs on silver, we conclude that the Pt particles are still covered up by silver and that no interdiffusion of Pt and Ag has taken place at 120°C. However, after reduction at 400°C this sample adsorbed a considerable amount of CO, which shows the presence of Pt sites at the surface of the alloy particles. Apparently, the interdiffusian of the two elements within the alloy particles has been effected at 400°C. S. VASUDEVAN adsorption?
Why do you oxidize your catalysts before the IR study of CO Does the CO adsorb on the metal or on the metal oxide ?
K.P. de JONG 1. An extensive IR study of CO adsorption on the reduced samples showed a considerable shift of the vibrational frequency of adsorbed CO with the silver content of the alloy particles. Moreover, adsorbed CO caused a serious surface segregation of Pt. Both factors contribute to the difficulty of estimating the composition of the bimetallic particles after reduction. It was shown that oxidation caused segregation of silver (ions) to the surface. Under these conditions adsorbed CO hardly led to segregation of platinum to the surface, while the IR band of CO adsorbed on Pt sites did not shift considerably (fig. 3). Due to these three factors a fair impression of the bulk composition of the alloy particles could be obtained by studying the oxidized catalysts. 2. At room temperature CO was able to remove to a large extent adsorbed oxygen from the platinum surface. After this removal CO adsorbed on metallic Pt sites which led to the IR band at 2090 cm- 1. Co adsorption on platinum oxide would give rise to a band at 2120 cm- 1 (ref. 11). A. MIYAMOTO: On the basis of your method, the surface of Pt is covered with Ag. Then, is it possible to cover the Pt surface with Rh ? K.P. de JONG Whether a second metal, e.g. Rh, will be deposited selectively onto Pt particles already present on the support, depends on three factors: - Is there a considerable interaction between the metallic precursor and the support ? - What is the nature of the alloy produced, viz. exothermic or endothermic? Preliminary experiments with supported PtAu have shown that the preparation of this endothermic alloy is much more diffi~ult than of PtAg, which is an exothermic alloy. - Is the deposition reaction catalyzed by the metal itself? The catalytic action of Pt should be more pronounced than that of the second metal. If this is not true, the second metal will agglomerate and not be distributed homogeneously over the Pt particles. J. MARGITFALVI 1. what is the initial form of your Pt particles? Are they oxidized or are they covered with hydrogen ? 2. In what phase of the preparation will the contact between two metal phases be formed ? 3. What is the practical use of your PtAg/Si0 2 catalysts? K.P. de JONG: 1. Our starting material is been produced by ion-exchange and subsequent Because the catalyst has contacted air prior Pt particles will be covered with a layer of inferred from an induction period during the correlates with the removal of this adsorbed
the EUROPT-1 catalyst which has reduction with hydrogen at 400°C. to the use in our experiments, the adsorbed oxygen. This could be liquid-phase reduction, which oxygen.
121 2. As shown by electron microscopy, the two metal phases already are in contact after preparation and drying at 120°C (fig. 1). 3. Thepractical use of our PtAg/Si0 2 catalysts is twofold. In this study the catalysts served as a model system to elucidate general factors which control this liquid-phase reduction method. The preparation technique can now be used for other (practical) bimetallic systems. Secondly, silver is immobilized in the PtAg/Si02 catalysts by underlying platinum metal. With conventional supports, e.g. Si02 and a-A1203' silver metal is very mobile which leads to extensive sintering at elevated temperatures. Our preparation method offers the possibility to prepare thermostable, highly dispersed silver catalysts. L. GUCZI: On support not containing Pt, large silver particles are formed due to some additional migration on the surface. Bearing in mind this mechanism, some other can be proposed in addition to the migration. That is, on Pt/Si02 the nucleation rate is higher, due to the presence of Pt and the immediate formation of bimetallic particles. On the other hand, with silver alone, the nucleation rate is slower; thus, either particles can be grown prior to reduction, or, more simply, the already existing particles can be increased by being attached with non-reduced silver precursor. K.P. de JONG: Both mechanisms are indeed operative in the formation of particles in Ag/Si0 2 samples. From an extensive study of the preparation of supported monometallic Ag catalysts (1), the bimodal particle size distribution observed could be related to these two mechanisms. Large particles (~ 300 ~) are formed by reduction of silver ions from the liquid-phase at the surface of metallic silver particles present on the support and/or in the liquid-phases. The small particles (35-70 ~) have been formed from silver ions adsorbed on the support. Literature data show a similar particle size (40-80 ~) for catalysts produced by ion-exchange and H2 reduction (2). In the latter case an initial atomic distribution was obtained, but again a high mobility of the silver particles led to a particle size similar to that of particles produced by liquidphase reduction of adsorbed silver ions. (1) K.P. de JONG and J.W. GEUS, Applied Catalysis, in press. (2) M. JAF~OUI, B. MORAWECK, P.C. GRAVELLE and S.J. TEICHNER, J. Chim. Phys. Physicochim. Bioi. 75 (1978), 1060.
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123
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION AND CHARACTERISATION OF HIGHLY DISPERSED PALLADIUM CATALYSTS ON LOW SURFACE ALUMINA. THEIR NOTABLE EFFECTS IN HYDROGENATION
J.P. BOITIAUX, J. COSYNS and S. VASUDEVAN Institut Fran9ais du Petrole, Rueil-Malmaison
(FRANCE)
ABSTRACT Highly dispersed Pd catalysts (D:l00%) were prepared on low surface area aluminas 2g- 1) (9-100 m by wet impregnation technique, using palladium acetylacetonate as precursor. Characterisation of these catalysts by CO chemisorption confirms the stoichiometry of 1Pd-ICO. Electron Microscopy (EM) results confirm the presence of highly dispersed metallic particles «
lnm) and the diameter observed in EM corre-
lates with the diameter obtained by CO chemisorption. Large shifts
in I.R spectra
of adsorbed CO are observed over these catalysts, as dispersion increases. Turnover number for the hydrogenation of 1-butene is constant with dispersion but for 1,3-butadiene it diminishes sharply as dispersion increases. These results are explained by the
high
adsorption strength of small particles.
INTRODUCTION For selective hydrogenation of acetylenics in an olefin or diolefin cut, palladium based catalysts have by far proved to be the most active and selective ones (ref. 1). Literature studies indicate that on low surface aluminas the metallic dispersion (ratio of the atoms on the surface to the total number of atoms) obtained has been low (refs. 2,3). Thus only a fraction of the palladium impregnated participates in the reaction. High dispersion has been obtained over high surface
~aluminas,
(ref. 4), but these supports are avoided in selective hydrogenation reactions as they promote polymerisation reactions. Attempts made to anchor organometallic compounds in order to obtain high dispersion did not produce any significant improvements (ref. 5). Though high dispersion could be obtained over a high surface silica (ref. 6) by anchoring Pd (C attempts to impregnate low surface aluminas 3H5)2' using Pd acetylacetonates (Pd (AcAc) 2) yielded dispersions at best 15%. (ref. 7). In the present work we show that Pd
(ACAC)2 can be anchored to low surface
aluminas to yield high dispersion catalysts. The Pd catalysts thus prepared were fUlly characterised by
co
chemisorption, Electron Microscopy and I.R spectroscopy,
124 thus revealing the special properties of highly dispersed particles. These special properties are reflected in the abnormal catalytic behaviour during the liquid phase hydrogenation of 1,3 butadiene.
EXPERIMENTAL METHODS Materials Support : The aluminas used in our studies were supplied by Rhone-Poulenc (France). 2 An~Alumina of 9m g-l and 0.43 ml g-l pore volume and two ~ -aluminas of 69 and 2 -1 -1 t 104 m.g surface area and 0.56 and 1.1 ml g pore volume,respectivelY,were used. Precursor: Palladium acetylacetonate (Pd (C
5H702)2 J
was procured from Johnson
Mathey (Paris). Gases : All the gases were procured from Air Liquide (France). Hydrogen : Ultra pure
(> 99 :999%)
deoxo unit and Union Carbide CO : of purity
~99.995%
was further purified by passing through n
4A molecular sieves.
was used as such.
1 butene and 1,3 butadiene: )99.0% were purified by passing them through a bed of 4A+13 X molecular sieves, in order to eliminate traces of water and sulfur compounds. Solvents: Organic solvents for catalyst preparation (benzene,
dichloro-
ethane, carbon tetrachloride) were supplied by Merck and were of analytical grade. Heptane used as a solvent for catalytic tests was procured from Halterman (W.Germany). Its purity was )99.8% and was further passed through a 4A+13 X bed of molecular sieves. Apparatus and procedure CO chemisorption : The metal surface area was mesured by CO chemisorption under dynamic conditions at room temperature (ref. 8). The catalyst sample (1-2 g)
was
reduced with hydrogen at 250°C and desorbed at 300°C under inert gas (argon) for 2 hours respectively. The sample was then cooled under argon and known volumes (0.5 cc) of CO were injected.The volume of CO unadsorbed by the catalyst was detected by a catharometer; By supposing a stoichiometry of 1 CO-1 atom of Pd, the number of Pd atoms exposed was determined and thus the dispersion was calculated. I.R spectroscopy: The spectra of adsorbed species were recorded on a Beckmann 1
I.R 4240. In the condition of our study the resolution was better than 5 cm-
A special cell was used for the study of catalyst wafers, wherein the catalyst could be heated under H or inert gas, and gases like CO could be injected in 2 pulses under a stream of inert gas. Several pulses of CO were normally injected in order to ensure a complete saturation of the catalyst. Microscopy : CTEM : a JEOL 120 X was used to examine the catalyst particles. Under the conditions of our study
particles~1
nm could be identified.
STEM : A VG HB 5 microscope with provisions for high speed recording of X-ray
125 emission microanalysis of particles was used. In the case of metallic crystallites it was established that the signal could be resolved best when at least 50 atoms were present. Catalytic tests : Catalytic tests were done in liquid phase and in batch in a CSTR reactor at 2 I1pa (20 bars) pressure. The hydrogenation rate was followed by noting the pressure drop in the constant volume hydrogen ballast, upstream of the pressure regulator, as a function of time. Cooling water was circulated in the reactor jacket in order to maintain the reactor at 20°C.
RESULTS AND DISCUSSION I. Catalyst preparation (a)
Impregnation : The catalysts were prepared by the wet impregnation technique,
using benzene as a solvent (other solvents tried like CH or CCl gave identical 4 2Cl 2 results) . In order to establish the nature of impregnation, studies were performed on 2 two aluminas having different surface areas of 9 and 69 m g-1. The maximum Pd that could be retained by these two supports was determined as follows Solutions of Pd(ACAC)2 of varying concentration were added to known quantities of support. The solution was left in contact with the support for at least 48 hours after which the support and the liquid were separated and their Pd content analysed. Fig. 1 shows the Pd retained by the two supports as a function of Pd remaining in the solution. We can deduce from this figure that at low Pd loadings the support retains all the Pd that is present in the solution and that the maximum possible metal loading increases with the surface area of the support. Attempts were made to characterise, by I.R spectroscopy, the different species involved during the impregnation. Spectra of the support impregnated with Pd
(AcAc)2'
(fig. 2 B)
resembled more closely the spectra of the support saturated with acetylacetone, (Fig. 2 C), than the precursor Pd
(AcAc)2'
(Fig. 2 A), thus suggesting the liberation
of acetylacetone during the impregnation, which adsorbs on the alumina support. The influence of this adsorbed acetylacetone in the impregnation reaction is further demonstrated by the
fol~owing
experiments.
- A support which easily retains 0.3% Pd, is able to retain only 0.07% Pd when acetylacetone is preadsorbed on it. -When a support which has retained 0.3% Pd is immersed in a solution containing an excess of acetylacetone, more than 80% of the Pd is lost from the support to the solution. These observations along with certain other studies (refs. 5,9) lead write an equation as follows :
us to
126 J-OH
+
Pd (ACAC)2
AcAcH (ad)
--
~O-Pd
(AcAc) + AcAcH(ad)
: Acetylacetone (CSHSO) adsorbed on the support hence not detectable
in the solution after impregnation (ref. 10) .
• b't Alumina (69m 2 g-l) • ex Alumina (9m2 g-l)
1::
0 0. 0.
= 0.6
t
III
... ~
•
.= >.a
&: 0
'iii
III
's
..... III
"Q
&:
~
ftI
&:
....
'j; ~
"Q
a. ~
~
...--' ...---e-:
OL.-_-l..._ _...L--_~--"'"
o
1 2 3 4 Concentration of Pd in the solution hi/I)
Fig. 1. Pd retained by support as a function of Pd remaining in solution after impregnation.
1600
1400 1200 ~cm-1
Fig. 2. I.R spectra of (a) Pd (AcAC)2 salt, (b) support impregnated with Pd (AcAc)2 (c) support saturated with acetylacetone.
(b) Thermal treatment : The Pd impregnated aluminas were oven-dried at 120°C to eliminate the solvent. A series of catalysts were then calcined in air at temperatures varying from 200-600 oC and then reduced at 200°C. The curve representing CO chemisorbed as a function of calcination temperature shows a maximum at 300°C. Fig. 3 and Table 1 show
that the catalysts calcined at 300°C yield
disper-
sions of almost 100% when reduced under H at temperatures up to 300°C. At higher 2 temperatures of reduction or inert gas treatment there is a sharp fall in the dispersion due to metal sintering.
127
0.29% Pd/rtA1203 •, 0.33% PdNAI203 0.76% Pd/rtA12 •, 0.17% Pd/a.AI 2003
,
100 80 60
.1... w i:5
Alumina Alumina
..
1.0
d_ , CO- .
0.8
·i
/
u u
'1:1
\•
• 'e.
20
IX
-...
III III
)'t
•
~
1.2
-C Do
40
3
•
.t~
w
.D D III
/'
lJ
/'I'
0.6 c
•.. /
U
0.4 0.2
•
./"
/,/
~~
=1.5
./
/'
V
%Pd
Fig. 3. Dispersion as a function of re3uction temperature.
Fig. 4. Determination of stoichiometry of CO chemisorption over Pd catalysts.
catalyst Treatment Diameter of CO condition (OCl chemisor- Disper:- particle~.L Air H sian From CO From ption N 2 2 % chemi- electron Calc ina- reduc- desorp- cc/g N° tion tion tion sorption microsvalues cOPv 1 300 300 300 0.4 0.382 100 (1.0 2 O.17%Pd/ 300 500 500 0.268 71 0.8 3.5 3 300 700 700 0.057 15 6.0 4.9 4 'it A1 20 3 800 300 800 0.046 12 7.6 8.7 5 300 350 350 0.742 100 0.4 0.0 6 0.33%Pd/ 300 250 1.8 400 0.405 55 1.3 7 1'tA1 300 250 600 0.137 2.8 19 4.7 20 3 8 250 700 0.129 300 18 5.0 3.5 9 0.76%Pd/ 300 1.012 60 300 300 1.1 1.7 10 300 700 700 0.461 27 3.1 3.2 'tt A12 03 11 300 550* 550 49 3.3 0.835 1.6 12 0.278%Pd/ 300 40 200 0.497 80 0.7 1.7 A12 03 13 300 100 100 98 0.4 (.1.0 0.610 't 14 300 2.9 600 600 0.190 30 2.8 15 0.34%Pd/'t-A12 03 100 100 0.360 47 3.3 1.6 % Pd and support
~
SINTERED WITH WET HlrDROGEN
Table 1. Comparison electron microscopy.
of particle size as determined by CO chemisorption and
128 II. Characteriz ation (a) CO chemisorption : chemisorption of CO as a means of characterization of
pa
surface has been widely used and recommended (refs. 11,12,13). Two modes of adsorption of CO have been proposed over Pd , the linear form Pd-CO, and the bridged from "CO, and the stoichiometry depends on the ratio of these two forms. A Pd/CO Pd Pd ratio of 1 has often been used (ref. 4) to calculate the dispersion even though a ratio of 1.5 has also been suggested (refs. 12,14,15). In order to determine the ratio which is valid in our conditions of studY,we plotted on fig. 4 the volume of the CO chemisorbed by the highest dispersed catalysts for different metal loadings and for two alumina carriers
(0(,
and 'it)' From this figure we observe that
the experimental points align more closely on the straight line representing a Pd/CO ratio of 1.0 rather than 1.5. (b)
Electron microscopy:
CTEM: a certain number of catalysts,
(table 1.),
were examined in CTEM. From electron micrographs of these catalysts it was observed that the repartition of the particles was statistical without any preferential zone, and the particle size distribution was mono-modal for all the dispersions
studied,
(fig. 5.). The mean dimension based on the volume to surface ratio was calculated by the following formula :
where n is the number of particles having a diameter 0; The diameter thus obtained i was compared with the diameter determined from the CO chemisorption values, (fig. 6),by supposing a FCC structure,
(refs. 16,17);
the correlation appears to be quite
satisfactory. STEM: at dispersions close to 100%, no particles were detected in CTEM indicating that their average dimensions at these high dispersions WRre less than 1 nm.Further X-ray microanalysis of some zones in STEM reveals the spectra of Pd, thus confirming that the Pd particles were ultra dispersed.
129
o. 76%PdNA1 2 03 Dispersion: 60% IZ22Z:l Dispersion: 27%
c:n
(See tablel
100 80
-
N°9 810)
E 8 ..5 :IE
-~
loLl
6 &
III
4U
U
60 'f
•a.
-..
4
0
40
4U
.a
2
E
:::I
••
••
20 z
o
1.0
2.0 3.0 4.0 0i Particle diameter (nm)
Fig. 5. Particle size distribution for t",O catalysts.
(c)
Fig. 6. Correlation between particle size calculated from CO chemisorption and determined by electron microscopy (see also table 1 )
Infra-red spectroscopv : the IR spectra of CO adsorption over three Pd cata-
lysts of dispersion varying between 27 and 86% are presented in fig.7. The spectra sho\ 1 -1 a symmetrical band at frequencies >2000cm(HF band) and another band at <2000 cm (LF band). Attempts to classify them as linear and bridged species,respectively (ref. 18), has been contested by certain authors (refs. 19,20). It seems difficult to conclude on this matter of dispute. However,our results of CO chemisorption and EM are consistent with a
Pd/CO ratio of 1.0 in the full range of dispersions studied,
even though the I.R spectra always show
two bands (HF & LF).
interesting in our study is the remarkable displacement of both
However what is more bands towards
lower frequencies as dispersion increases. This displacement which is more pronounced for the LF band
(~40
cm -1), can be interpreted as a reinforcement of the metal-CO
bonding (ref. 21). This result explains also a part of the scatter in literature data on I.R spectra of CO on Pd
(refs. 15,22,23) because the studies were done on catalysts
of different dispersions. Two other interesting observations based on this study can be made: - examination of the two catalysts (b)
0AV were 1.7
&
& (c) of fig. 7 in EM shows that their
3.1 nm ,respectively (Fig. 5) ,and this small change in diameter of the
130 metal particle was sufficient to displace the LF band by 30 cm-
I
t
c
c
'iii ell
's
.
ell
C
III
I-
1980
2200 2000 1900 1800 ycm- t ~ig. 7. I,R spectra of co adsorption over (a) 0.44%Pd/tt-A1203 and (b) ,(c) 0. 76%Pd/V - A1 (See also fig. 5). 20 3 t
- We also observe that at higher dispersions the LF band becomes broader. This could be interpreted as co adsorption on low coordination sites, which increase
as
dispersion increases, as suggested by Blyholder (ref. 24). These results hence indicate that the adsorption strengths vary with dispersion.
III. Catalytic tests Hydrogenation of I-butene and I,3-butadiene were studied over a wide range of dispersions. I-butene is consumed in two parallel reactions: hydrogenation leading to the formation of butane and isomerisation to 2-butenes (cis & trans). I,3-butadiene on the other hand is selectively hydrogenated to I-butene and 2-butenes. Formation of butane is negligible
(~3%)
up to a conversion of 90%.
The rate of consumption of hydrogen is constant with time
for the hydrogenation
thus indicating that the reaction is zero order with resoect to hvdrocarbon.This appa_ rent
zero order is an indication that the active surface is saturated by the reagent.
There was no
deactivation of the catalyst during the run as a second charge of
reagent could be hydrogenated
at the same rate as the first charge.
131
.!J...- Butane OfJ.
1 Butene
+ H2
r-Butenes
2 (CIS+TRANS
-
• 1.3 Butadiene + H2
'j
u
~
Butenes
w
. 175 . 100 c .= 7550
~
.I E 150 ;i 125 w
:::::I
I-
25 100 Dispersion (%)
0
Fig. 8. Variation of the turn-over number as a function of dispersion in the hydrogenation of 1-butene and 1,3-butadiene.
Fig. 8 shows the specific activity, expressed in Turn-Over Number, for both these reagents
as a function of dispersion.
as well as its
~somerisation,
We
bserve that for 1 butene hydrogenation
the turn-over number does not vary with dispersion, thus
confirming that these reactions are not sensible to structure (ref. 25). However,the turn-over number of 1,3-butadiene which initially is much higher than that of 1-butene remains constant only up to a dispersion of 20%.Above this dispersion there is an abrupt drop in the specific activity and it attains very low values as dispersion, approaches 100%. Thus the kinetic results reveal two zones of dispersion. The first zone up to 20% dispersion where the hydrogenation reactions are not influenced by particle size,and the second zone above 20% dispersion where the hydrogenation of diolefin becomes more and more difficult as crystallite size diminishes. This abnormal behaviour of the small crystallites could be correlated to the very strong adsorption of these hydrocarbons on low coordination sites, whose proportion increases at high dispersion. This property of highly dispersed catalyst is very similar to homogeneous catalysts where diolefin
132 hydrogenation rate is very often slower than that of the corresponding olefins. This observation can be interpreted by a stronger complexation of the metal by the highly unsaturated hydrocarbons (ref. 26).
CONCLUSION Our studies indicate that organometallic compounds can be successfully anchored onto low-surface alumina to yield high dispersion catalysts. This method of preparation can be extended to other Group VIII metals also (ref. 27). The different characterisation procedures. and the catalytic tests performed Over the full range of dispersion highlights a special behaviour of small crystallites. The surface palladium atoms of small particles coordinate
stronqlv with the hiohlv unsaturated hydrocarbons
like 1,3-butadiene, thus acting similarly to homogeneous catalysts.ln a further publication we shall show that this concept can be generalized to the hydrogenation of other highly unsaturated hydrocarbons like 1-butyne and isoprene.
ACKNOWLEDGEMENTS One of the authors,S.V, would like to thank Engineers India Ltd (India), for having given him the necessary study leave and the French
Government for Droviding a
scholarship to do this work.
REFERENCES 1 G.C. Bond, Catalysis by 11etals, Academic Press, London, 1962. 2 Y. Lee, Y. Inoue, I. Yasumori, Bull Chern. Soc, Japan 54, 1981,13-19. 3 T. Paryjczale and J.A. Szymura, Z. Anorg. Allg.Chem, 449, 1979, 105-114. 4 M.A. Vannice and R.L. Garten,I.E.C.Process Res. Dev, Vol. 18-2, 1979, 186-91. 5 Yu. I. Yermakov, Catal. Rev. Sci. Eng, Vol. 13-l, 1976, 77-120. 6 G. Coco, G. Fagherazzi, G. Carturan and V. Gottardi, JCS Chern. Corum, 1978, 979. 7 U.S. Patent 4 038 175, 1977. 8 C.S. Brooks and V.J. Kehrer, Anal. Chern, 41, 1969, 103. 9 A.R. Siedle, P.M. Sperl and T.~·l.P.usch, Appl.. of Sur. Sci.~, 1980, 149-60. 10 F. Feigl and V. Anger, Spot tests in Organic analysis, Elsevier publishing Co, Amsterdam, 1966, 446-47. 11 H.S. Taylor and P.V.Mc.Kinny, JACS 53, 1931, 3610. 12 J.J.F. Scholten and A.Van 110ntfoort, J Catal, .1, 1962, 85-92. 13 R.J.Farrauto,A.I.Ch.E.Sym Serio 143 70, 1974, 9-72. 14 S.J. Stephens, J.Phys. Chern, 63, 1959, 188. 15 A. Vannice and S.Y. Wang, J.Phys. Chern. 85, 1981, 2543-46. 16 S. Vasudevan, J. Cosyns, E. Lesage, E. Freund and H. Dexpert III International Symposium on scientific hasis for the preparation of heterogeneous catalysis,sepl.82 17 J.P. Brunelle, A.Sugier and J.F. Le Page, J.Catal.43, 1976, 273-91. 18 R.P. Eischens and W.A. Pliskin, Adv Catal. 10, 1958, 14-25. 19 G. Blyholder and M.C. Allen J. Phys. Chern. 68 lQ, 1964, 2712. 20 R.P. Ford, Adv Catal.21, 1970, 51-147. 21 M. Primet, J.11. Basset, 11.V. Mathieu and '1. Prettre, J Cata1.29, 1973, 213-23. 22 E.L. Kugler and M. Boudart, J Catal.59, 1979, 201-210. 23 Y. Soma Noto and W.M.H. Sachtler J Catal.32, 1974, 315-324. 24 G. Blyholder, J.Phys. Chern. 68 lQ, 1964, 2272-78. 25 G. Cartuzan and G. Strukul J.Catal.57, 1979, 516-21. 26 R. Ugo, Cat. Re~ Sci. Eng, 11, 1975, 225-297. 27 J.P.Bournonville, J. Cosyns, and S. Vasudevan French Patent 81/09 055.
133 DISCUSSION L. VOLPE : A strong particle size effect for small Pd particles on Si0 2 on surface and catalytic properties has been observed. D. Doering and H. Poppa found that below a certain dimension, the Pd particles are capable of dissociating CO. We have unpublished data proving that CO hydrogenation selectivity shifts from MeOH to CH4 in going from large to small Pd/Si0 2 particles. J.P. BOITIAUX: We have shown that liquid phase hydrogenation of unsaturated hydrocarbons could be structure sensitive. This is probably the first observation of structure sensitivity in hydrogenation reactions and will be confirmed in other publications (ref. : J.P. Boitiaux, J. Cosyns & S. Vasudevan, to be pUblished shortly in Applied Catalysis). Your results confirm that hydrogenation is a structure sensitive reaction.
J.G. van OMMEN: This is a comment on your acetylacetonate method of catalyst preparation. The acetylacetonate method, we also found, is a good method for the preparation of well dispersed oxidic or metallic supported catalysts. Instead of pd(AcAc)2' we used Fe (AcAc)3 or VO(ACAc)Z dissolved in toluene to prepare well dispersed supported catalysts of FeZ03 or VZ05 on TiOZ' In these cases we were, in contrast with your findings, able to detect acetylacetone and some decomposition products of acetylacetone in the toluene after adsorption of the acetylacetonates. This with some other evidence leads us to the same conclusion as yours, that during the adsorption the acetylacetonates a reaction takes place with the OH groups of the support. S. VASUDEVAN : Thank you for your comment. It is nice to note that your experimental results agree with ours. In fact as I have mentioned in this publication, this organometallic precursor can be used to prepare many highly dispersed catalysts of group VIII metals. For the particular point of the acetylacetone detection in the solution, we do not see the reason of the difference between our respective results. It seems that alumina adsorbs more easily the acetylacetone than titanium dioxide. M.M. BHASIN: First, a general comment: aluminas of 9-100 mZ/g are not normally called low-surface area aluminas. Low surface area aluminas are normally considered to be <10 mZ/g and typically <5 or ~1 m2/g. My question deals with your conclusion that the selective hydrogenation of 1,3butadiene is a surface-structure sensitive reaction. Could the decrease in turnover number at higher dispersion be due to a different role of impurities or due to interaction of palladium with the alumina support ? S. VASUDEVAN: The nomenclature employed by us to describe a low surface area support is very relative. In our laboratory this definition is used to distinguish these supports from zeolites (S ~ 600 mZ/g) and reforming aluminas (S ~ ZOO m2/g) . We have used differeqt types of alumina supports and one silica support in our study as shown in the table below and the decrease in turnover number is dependent uniquely on metal dispersion. Alumina Silica a Yt Yt S m2g- 1 9 60 104 210 Impurities (in ppm)
Fe Na S
160 300 160
130 300 1800
< 50 <100 500
100 650 <50
Further, the impurities effect is normally more pronounced on olefin hydrogenation but in our case, we find that the turnover number for l-butene hydrogenation and isomerization is constant over the whole range or dispersion.
134 2/g) K. KOCHLOEFL : The surface concentration of OH-groups of a-A1 203 (9 m according to our measurements (titration with (CH3)2Zn,C4HSO complex) is relatively very low (our data will be sent to the authors of this paper) and so I am wondering that you can deposit about 0.24 wt % of Pd on the surface of a-A1203 exclusively by the reaction of Pd-acetylacetonate with OR groups. S. VASUDEVAN : The aluminas used in our study are of commercial grade and hence are not very pure. They contain some low quantities of other transition aluminas. It is probable that these aluminas have higher OR contents which contribute to the active surface for exchange with Pd(AcAc)2' We have not made any correlation between the OR content and the amount of exchanged metal. This study has still to be done. For example, the change of this quantity with the degree of hydration of the alimina.
J. MARGITFALVI : 1) There is one missing point in your lecture, namely the influence of the surface OR group concentration on the adsorption of Pd(AcAc)2. If one assumes a surface reaction, this fact should be of great importance. What is your opinion on this matter? 2) Did you measure the carbon contant of your catalyst and what will be the influence of this surface carbon ? 3) Can the rate dependence on Pd dispersion (see fig. 8) be attr~buted to the increased negative order with respect to butadiene? S. VASUDEVAN : 1) For this matter, see the answer to the comment of Dr. K. Kochloefl. 2) We did not analyzes the carbon content of our catalysts, but we do not think that such a deposit could be responsible for our results: the hydrogenation of butadiene is zero order for the hydrocarbon and successive runs give the same hydrogenation rate. 3) The kinetics of butadiene hydrogenation is similar for low and high dispersions: zero order with respect to hydrocarbon.
135
G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
©
PREPARATION OF COLLOIDAL PARTICLES OF SNALL SIZE AND THEIR CATALYTIC EFFECT IN REDOX PROCESSES INDUCED BY LIGHT
J. KIWI, E. BORGARELLO, D. DUONGHONG and M. GRATZEL Dept. of Chemistry, City University, Northampton Sq., London, U.K.
SUMMARY
The photochemical cleavage of water into H. and O2 has recently been investigated in our laboratory using band gap irradiation and visible light in colloidal. (microheterogeneous) TiD. systems.
Visible light driven redox pro-
cesses leading to H. production involve a transition metal RU(biPY)~' tris-bipyridyl) as sensitizer.
features of colloidal particles of TiD. used in H2 sions is discussed.
(Ruthenium-
The preparation, characterization and structural gen~ration
of water suspen-
The activity and stability of these particles are criti-
cally dependent on the preparative method employed.
INTRODUCTION
In recent years there has been a surge of interest in finding chemical processes which are capable of quantum storage of light energy as shown by Kalyanasundaram et.al. [1] and Kiwi et.al. [2]. written by Gratzel [3] On this subject.
A recent review has been
TiD. loaded with noble metals plays a
central role in these processes as recently reported by E. Borgarello et.al.[4] TiD. of truly colloidal dimensions has been reported in studies performed in our laboratory by Duonghong et.al. [5] and Kiwi [6]. The objective of the present work is to describe the preparation of colloidal TiD. particles that are efficient in photocatalytically inducing H. from water.
Mechanistic features of their intervention in these processes will be
discussed since their low scattering allows kinetic events to be followed. In these events the charge transfer takes place on their surface.
RESULTS AND DISCUSSION The preparation of the catalyst was carried out in the following way. Ti-isopropoxide Fluka p.a.
~-Ventron
4 g
puris (no. 77115) was dissolved in 50 ml isopropanol
This mixture was injected over a 4 hour period into 150 ml HCL IN.
A transparent solution of colloidal TiD. particles is obtained in this way (1.12 g TiO./200 cc).
This solution was evaporated to dryness at 50°C in a
136 rotovapor and the solution redissolved in distilled water (200 cc).
By the use
=2
of a perkin-Elmer GC, provided with a 1.20 m Porapak-Q, column,
~
made certain that isopropanol was not present in the solution.
In the column,
mm, it was
He gas was used at 150°C and the product was detected via thermal conductivity. This solution was dialysed in a Carl Roth tube 32 mm diameter (No. 2-2052). After 4 hour dialyzed in distilled water, the pH increases from 1.5 to 3.8. Loading of Ti0 2 colloidal with platinum was carried out from a H2PtC16 solution (0.5 mg/ml Pt) and 0.4cm 3 of this solution in 25 cm 3 of colloid was reduced under U.V irradiation.
pta was obtained after 1 hour with 8 mg Pto/1 loading.
The Pt-ions are reduced by conduction band electrons of Ti0 2 under band gap irradiation. The loading of Ti0 2 with Ru0 2 was carried out via decomposition of Ru04 that produced Ru0 2 and 02 under light irradiation. 0.2 cc of stock solution of 1 mg Ru04/m1 H20 was reduced under light stirring for 30 minutes.
The final level of RV02 attained was 6.4 mg Ru0 2/1.
Continuous irradiation experiments were carried out using an Osram XBO-450 watt lamp as light source with a 16 cm water jacket to eliminate IR radiation.
A
gas chromatographic method was employed for hydrogen detection as previously reported by Kalyanasundaram et a L [1]. Figure 1 shows the yields of H2 when Ti~-colloidal concentrations were irradiated with light above 320 mm. uration begins after 80 hours irradiation. and during irradiation the small concentrations of Ti+
solutions of different It is shown that sat-
The initial pH in all cases was 3.8
solution turned light blue due to the production of 3
as reported by Herrmann et
al. [7].
The impor-
tance of this experiment is twofold :
a) H2 evolves efficiently at pH values above 3.0 under UV irradiation and, b) Solutions not having noble metal loading also evolve H2.
This high activity for H2 evolution can be ascribed to the high
density of OR/nm2 on the surface of the catalyst which should render this catalyst extremely active in these processes.
Parfitt [8] points out the importance
of the density of OH groups as a function of heat treatment during catalyst preparation as a controlling factor in the catalytic power of Ti0 2. In our case this value is above a concentration of 8 OH/nm 2 since during preparation of Ti0 2 cOlloidal the temperature was kept below 50°C.
Hydrogen and oxygen have been
reported to be produced simultaneously under band gap irradiation.
02 is ad-
sorbed simultaneously under band gap irradiation and is adsorbed strongly on Ti0 2.
It does not appear in gas phase until 40 hours irradiation.
of the stoichiometric amount of 02 was then observed.
Only
~
10%
When a 3% Na3P04 solution
was stirred after irradiation for 3-4 hours in the dark with a N2 flushed sample of Ti0 2 in the conditions previously described,up to 75% of the stoichiometric 02 produced under irradiation was released.
Similar effects have been previously
reported by Boehm [9] since a competition between phosphate and 02 for the catalytic sites on Ti02 takes place the phosphate ions being very strongly bound in
137 the process and releasing the adsorbed 02 on the surface of Ti0 2. we observed that
More recently
sulfate and carbonate ions strongly adsorbed on the Ti0 2
surface accelerate photodesorption in sacrificial processes where Ag+ and Fe+ 2 have been used as water oxidants in light induced reactions. Ti0 2-Ru02 has been used as catalyst, as described previously by E. Borgarello et. al. [10]. The effect of these ions adsorbed before irradiation is to accelerate the 02 photoproduction by a factor 2-3 as compared with systems in which they are absent.
c
o 30
_ ~
oo
E ~ 20 ,
N
N ~
E 10 d
hFigure 1. - Amount of hydrogen evolved in irradiation with light >320 nm in a solution containing different concentrations of Ti0 2 colloidal per 25 ml solution at pH = 3.8. a) 130 mg Ti0 2/25 ml solution; b) 65 mg Ti02/25 ml solution; c) 32 mg Ti02/25 ml solution; d) 16 mg Ti0 2/25 ml solution. Figure 2 presents the H2 evolution of the colloidal particles used through out this work as a function of Pt loading of the catalyst. 0.33 cm
3/hour/25
The lower limit of
ml solution is given by H2 evolution in the Ti0 2 particles
which are free of noble metal loading.
The efficiency of H2 production begins
to decrease after 10 mg Pt/l loading due to the absorption of light by the Pt particles.
This result agrees with similar systems used in sustained H2 prodColloidal
uction via sacrificial systems,as reported by Kiwi and Gratzel [11].
suspensions are always in a better position than powders to adsorb ions and metals and do so more readily and uniformlY,as stated by Zsigmondy [12],and this is reflected in the good reproducibility of the H2 yield obtained during this work. Since a good contact between Pt and Ti0 2 may control the efficient evolution of H2, a
Ti0 2 suspension was irradiated in the UV for 18 hours to
138 reduce the Ti0 2 to Ti+
4
and only then H2PtC16was injected (0.4 cc in 25 ml) In this way, the cation Ti+ 4 already produced
under continuous irradiation.
allows the approach of pta and,as reported by Tauster et allow close contact between the Ti+
4
cation and platinum.
a I. [13],would not Results of Pt
(8 mg/l)-Ti0 2 (pre-irradiated sample) run under the same conditions,as shown in 1.2 cm 3 H2/h/25 ml solution, confirming that higher rates of H2 are observed when good contact between Pt and Ti0 2 exists.
Figure 2,give
I
0.9
c .2
-:::J
0
11l
E
It)
N
s:
N
J:
E
2.0
8.0mgPt/l-
Fig. 2 - Rate of hydrogen evolution evolved in irradiations with light> 320 nm in a solution containing 130 mg Ti0 2 colloidal per 25 ml solution at pH = 3.8, as function of Pt concentration in the solution In figure 3 spectrophotometric measurements for colloidal Ti0 2 are presented.
The Ti0 2 particles do not exhibit any absorption in the visible. The small baseline drift is due to scattering. The absorption rises sharply at
A < 380 nm.
This onset agrees well with the 3.2 eV band reported for anatase by
D. Duohghong et al. [5].
The size of the colloidal Ti0 2 particles was determined
by photon correlation spectroscopy according to the derivation proposed by Corti and Degiorgio [14].
Correlation functions were obtained with a Chromatix light
[15] scattering instrument.
They can be presented by a single exponential over
at least two correlation times ,indicating a low degree of polydispersity «0.20). Application of the Stokes-Einstein equation yields a
hydrodynamic radius of
a
Rh
= 200A o
215A.
for Ti0 2.
When Pt and Ru0 2 are loaded onto the particle the Rh is -
139
Ti O 2 Solution
\I
ABSORBANCE
1.5
0.320
360
400
440
),(nm)-
Fig. 3 - Visible and near UV absorption spectrum for Ti0 2 sol, at concentrations 100 mgl1 and 1 gil and pH 1.5Further information concerning the structure of the colloidal Ti0 2 particles was obtained from X-ray studies. of anatase cystals. X-ray amorphous.
The diffraction pattern shows the presence
A significant part of this material was also shown to be Figure 4 shows data obtained from the photolysis of aqueous
Ti0 2 sol loaded with Pt and simultaneously with Pt and Ru0 2 for these
meta~and
•
oxides are shown in the legend to Figure 4.
Loading conditions This result
shows the true cyclic nature of the hydrogen photogeneration under study.
After
a short induction period, the H2 generation in the case of the bifunctional catalyst becomes linear.
The process was stopped after seven hours, when pressure
had build up in the reaction vessel and the gas produced was flushed out with N2
•
Upon reillumination H2 generation resumes at the initial rate.
can be repeated many times. Pt and Ti0 2 were observed.
~(H2)
= 0.4 at 308 nm.
,
The quantum yield for H2 production was determined
by illuminating through a Balzers RUV 308 interference filter. was
This cycle
Turnover numbers of 1700, 500 and 8 for the Ru0 2
The value of H2
140 o PI, RuO,. TiO, • PI,TiO,
Fig. 4 - Water cleavage by near UV photolysis of TiO. dispersions in 25 ml samples. N. indicates that the solution has been flushed out with N. 6 Pt (1 mg). RuO. (0.2 mg) TiO. (25 mg) 0 Pt (1 mg), TiO. (25 mg) . These H. evolution results can be rationalized in terms of the model proposed by Nozik [16] presented in Figure 5.
Band gap excitation produces an ele-
tron-hole pair in the colloidal TiO. particle.
The electron is subsequently
channelled to Pt sites where hydrogen evolution occurs.
An ohmic contact be-
tween Pt and TiO. takes place as stated by Gerischer [17].
The role of RuO.
in
the water splitting process is to accelerate the hole transfer from the valence band of TiO. to the water as shown by Kiwi and Gratzel [18].
The low over-vol-
tage characteristic for oxygen evolution on RuO. renders hole capture by water highly efficient inhibiting electron-hole recombination as has been shown by Trasatti [19], Anatase has been used because its flat band potential is about 250 mV more negative than that of rutile,as determined by Rao et al.[20].
The
shift towards more negative potentials affords enough driving force to affect efficient hydrogen production from water. UV
Consistent with this interpretation
experiments were carried out with TiO. sols loaded with Pt in isopropanol.
The rate of H. generation was two times faster than in isopropanol-free solution, approaching a quantum yield of 100%.
In the presence of isopropanol ,current
doubling takes place,as measured by Dutoit et al. [21] ,from the isopropyl radical into the TiO. conduction band,increasing the observed current in TiO. crystal anodes by a factor of
~2.
141
Fig. 5 - Schematic illustration of the photoinduced events on Ti0 2 induced by UV light leading to water cleavage. In a recent study by Kiwi [6] electron injection due to electron transfer from excited RU(bipy);' (Ruthenium-tris-bipyridyl) to the semiconductor material has been demonstrated.
OscillBscope traces illustrating the time course of
events are shown in Figure 6.
Figure 6b shows that luminescence of
is strongly reduced by the TiO, colloids present.
RU(bipy)~'
This effect is more drastic
than the effect shown in Figure 6a, where only Ru(bipy);' was present.
The
faster decay is considered to be caused by the quenching process due to electron transfer from the excited
by the semiconductor material.
RU(bipy)~2
Loading
with Pt and Ru0 2 increased the quenching at room temperature since two species are involved in the electron transfer: Ti0 2 and the noble metals. Since Ti0 2 increased its Fermi level upon electron injection. and the energy flows from a higher to a lower level when equali~ation
takes place (of the levels involved).
the electron discharges to the electrolyte. forming Pt- as shown by Smith [22].
-1200I-n5
b
a
1
10mV
T
./
v
1L
if
V
/'r'"
V
V
V
J
~
/'
I
2V
V
/
J
./
V
V
V
2L
I
V
II
1/
If
Fig. 6 - Osc1lloscope traces obta1ned from the 602 nm laser photolysis of aqueous solutions a) RU(bipy);2 2 10- 4 at pH 3, luminescent signal 1 and 2 are taken at 75°C and 25°C.respectively. b) same as in a) using
142 Ti0 2 500 mg/l as quencher. 75°C as 25°C respectively. Besides colloidal
Luminescent signals 1 and 2 are taken at
Ti0 2 involved in hydrogen generation via light-in-
duced processes,Ti0 2 anatase has been prepared by thermal hydrolysis of titanium sulfate according to the Blumenfeld procedure as outlined in Barksdale's book [23].
Upon dilution a gel-like material is precipitated.
If Nb+
5
doping was
desired an appropiate amount of Nb20s was digested together with TiOS04 in sulfuric acid.
The powders obtained reveal a prolate shape for these particles,
° respectively. the short and long axis being 1500 and 300 A
Loading with Ru0 2
was achieved by adding.RuCl a.H 20 at pH6, drying at 100°C overnight in air. Higher yields of H2 are observed for these powders as compared with the Ti0 2 sols previously reported by J. Kiwi et al [2].
This is shown in Figure 7.
The H2
formation follows a monophotonic process involving one electron reduction of H+. Hydrogen evolution is proportional to the intensity of applied light.
Since the
yields of H2 are condiserably higher than at room temperature,this implies that the rate of formation of electron-hole pairs depends on two factors 1) the flux of incident photons 2) the temperature of the semiconductor which is a controlling factor of the Fermi band position of Ti0 2,as shown by E. Borgarello et al. [4].
t a
3
2 1
Fig. 7 - Effect of irradiation intensity on the amount of H2 evolved in solutions irradiated in 25 ml flask a) Ti0 2 500 mg/l. Loading 0.1% Ru0 2, 0.42% Nb20s and 60 mg Pt/l. A > 250 nm, pH 4.3 and temperature 75°C b) same conditions as in a) but irradiation was carried out with A > 400 nm and RU(bipy)t 2 2 10-4 M. Low scattering Ti0 2 colloids are also active in decomposing water by visible light induced processes.
Results obtained from visible light photolysis experi-
ments are shown in Figure 8. derivative of Ru(bipy>t 2
has
In Figure 8,Ru(bipy);2 lP as an isopropylester + 1.5V vs NHE redox potential.
This high redox
143 potential makes possible the occurrence of 02 evolution even at low pH's (though the oxygen produced is not observed since at the low concentration produced it is adsorbed on Ti0 2) and the concomitant H2 yields are shown in the lower line of Figure 8.
Using
having redox potentials at + 1.26V vs NHE there is
RU(bipy)~2
enough to drive 02 evolution at pH 3,the H2 yields are plotted in the second lower line in this figure. Rhodamin-B (Rh-B) with 1.3V vs NHE redox potential is also active in H2 evolution process at pH 3.
The hydrogen generation rate
increases significantly upon addition of methyloviologen (MV+2) to the solution. In this case electron transfer quenching takes place,as worked out by Gafney and Adamson [24], between
excited state and MV+2:
RU(bipy)~2
RU(bipy)t 2 + MV+2 ---+ RU(bipy)t 2 + MY+
(1)
that renders H2 subsequently in a dark reaction: My+2 + OH-+ 1 / 2 H2 and 02 (actually adsorbed on Ti0 2) by reaction:
(2)
RU(bipy)t 2 + H20
(3)
MV+ + H20 ~
~
2H+ +
RU(bipy)~2
:z: >
+
~
02
+
1.5 RU(bipy)~',MV
',PH=3
1.0
Fig. 3 - Water cleavage by visible light irradiation of aqueoup solutions of Ti0 2 colloidal. Sensitizer concentration 2 10- 4M and MV+ 2 = 5 10- 3M. When excited by light in the visible (400 nm cuttoff filter placed in the light beam direct band gap excitation of Ti0 2) ,excited state quenching of s~ kes place
ta-
by Ti0 2 as shown by Kiwi [6]. These electrons are channelled via the conduction band of Ti0 2 and charge the metal islands present on the surface of Ti02 mediating water reduction in a process:
144 (4) The excited sensitizer S* renders the ion S+ that has the necessary potential to inject charge through the Ru0 2 sites (in reality these are oxide islands on the Ti0 2 particle).
The rate of oxygen evolution will depend on the nature of
the sensitizer cation and the ground state redox potential of the sensitizer.
REFERENCES,
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
K. Ka1yanasundaram, J. Kiwi and M. Gratzel, He1v.Chim.Acta, 61 (1978) 2702. J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca and M. Gratzel, Angew. Chem Int. Ed. Engl. 19 (1980) 647. M. Gratze1, Disc. Faraday. Soc. 70 (1980) 359. E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca and M. Gratzel, J. Amer. Chem. Soc. 102 (1981) 6324. D. Duonghong, E. Borgarello and M. Gratzel. J. Amer. Chem. Soc., 103 (1981) 4685. J. Kiwi, Chem. Phys. Letts., 83 (1981) 594. J.M. Hermann, J. Disdier and P. Pichat, J. Chem. Soc. Faraday Trans. 1. 77 (1981) 2815. G. Parfitt. Progr. Surf. Membr. Sci. 11 (1976) 181. H. Boehm, J. Catal. 22 (1971) 347. E. Borgarel10, J. Kiwi, E. Pelizzetti, M. Visca and M. Gratzel, J. Amer. Chem. Soc. in pres 1982. J. Kiwi and M. Gratzel, J. Amer. Chem. Soc., 101 (1979) 7214. R. Zsigmondy, Kolloidchemie. Otto Spamer Verlag, Leipzig 1927. S. Tauster, S. Fung and R. Garten, J. Amer. Chern. Soc. 100 (1978) 170. M. Corti and V. Digiorgio, Ann Phys. 3 (1978) 303. Chromatix Application Note LS-8 (1978) 560 Oaxmead Parkway, Sunnyvale California 94086 U.S.A. A. Nozik, Appl. Phys. Letts., 30 (1977) 567. H. Gerischer, Pure and Appl. Chern., 52 (1980) 2649. J. Kiwi and M. Gratzel. Chimia, 33 (1979) 289. S. Trasatti, Electrodes of Conductive Metallic Oxides. Elsevier, Amsterdam 1981. M. Rao, K. Rajeshwar, V. Vernecker and J. Dubov, J. Phys. Chem., 84 (1980). 1987. E. Dutoit, F. Cardon adn W. Gomes, Ber. Bunsen. Phys. Chem., 80 (1976) 1285. R.A. Smith, Semiconductors. Cambridge University Press, Cambridge, 1976. Jelks Barksdale, Titanium. Ronald Press New York. 1966. H. Gafney and A. Adamson, J. Amer. Chem. Soc. 94 (1972) 8238.
145 DISCUSSION B. NAGY: Did you try to prepare colloidal Ti02 particles either from normal or reversed micelles? If so, what is the difference between these two types of preparations as far as the size of the particles is concerned?
J. KIWI: No preparation of colloidal Ti0 particles either from normal or 2 reversed micelles has been atterr~ted so far. L. GUCZI Have you tried other transition metal oxides for the oxygen cycle beside ruthenium? On what basis was Ru02 chosen for this step ?
J. KIWI : Yes, other transition metal oxides besides ruthenium oxide, such as Pt02 and Ir02 have been tried for the oxygen cycle. This has already been reported by J. Kiwi and M. Gratzel in Angew. Chern. Int. Ed. Engl. 17, 860,1978. The basic reason for selecting Ru0 2 for this oxidation process is that,of all the metallic oxides, Ru02 was reported to have the lowest overvoltage for the oxygen evolution process in aqueous solutions (P. Lu and S. Srinivasan, Brookhaven National Laboratory, Report 24914, New York, 1979 ; M. Miles and M. Thomason, J. Electrochem. Soc., ~, 1459, 1976). Nevertheless, the overpotential for a given current density, in Ru02 depends on the aggregation state of Ru02 crystals and becomes progressively lower as the oxide is prepared at lower temperature attaining a higher state of oxidation. Hydrated oxides of Ru02 are effective in sacrificial water oxidation and this has recently been shown in our laboratory (J. Kiwi and M. Gratzel, Chimia, iI, 289, 1979). A. BAlKER: You have shown that oxygen adsorption on the colloidal Ti02 particles can severely affect their catalytic activity by depolarizing of Pt-. I am wondering how feasible it is to replace the Ti02 by other oxides which are active, but exhibit a lower affinity for oxygen adsorption. Furthermore, what do you think about poisoning the Ti0 2 surface in order to inhibit oxygen adsorption ? J. KIWI The depolarization of Pt- when enough 02 has accumulated in the system via the reaction: Pt- + 02 + 07 + Pt is a serious inhibitor for the reaction leading to water decomposition~of the type: Pt- + H20 -.- Pt + 1/2 H2 + OH-. Other semiconductor materials like SrTi03 and CdS2 have been used in water splitting processes but with significantly lower efficiencies and are also affected in their H2 generation capacity by the 02 present. We do not think that it is feasible to poison the Ti0 2 surface (as suggested) because electron injection from species excited by visible light having the adequate redox characteristics, e.g. Ru(bip)+2 takes place on the available surface of Ti02- If this surface is hindered by ~n additive in its receptivity to affect charge transfer from solution or to the solution (via Pt clusters) then its catalytic role would be expected to decrease considerably. K.S.W. SING: In your opinion what form of Ti02 has the highest potential activity for the photochemical cleavage of the H20 into H2 and 02? Colloidal particles of Ti0 2 are likely to have a high degree of surface heterogeneity. Do they have high intrinsic catalytic activity? If so, whu is this the case?
J. KIWI: The anatase form of Ti02 with -0.6 eV cb has the best activity ~or water splitting at moderate potentials, e.g. pH 5. At this pH,about 300 rnV are needed for H+/H 2 reduction and, therefore 300 mV are available to drive the reaction producing a fast kinetics favourable to this process. The relation between the catalyst structure and hydrogen production has been studied in detail and reported (E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca and M. Gratzel, J. Am. Chern. Soc. 103, 6324, 1981). For 02 production we have to reckon with the fact that Ti02-serves as an adsorbant for the 02 produced during the photolysis. High surface area Ti02 (> 200 m2/g) which is beneficial for H2 production affording a high contact surface area between the catalyst and 02 is
146 nevertheless detrimental for 02 generation in the same process. The 02 which is strongly attached to the Ti~2 surface is reduced by conduction band electrons to 02 and in the presence of H ,the radical H02 trapped on the surface has been reported (C. Jaeger and A.J. Bard, J. Phys. Chern., 24, 3146, 1979). Through this mechanism the amount of 02 in solution is kept very low. To evolve 02 efficiently a low surface area Ti0 2 having low microporosity as base material seems necessary. Flame-hydrolyzed Ti02 (p-25 Degussa) having a small number of surface hydroxyl groups and hence a low affinity for 02 binding has been proven to be useful in our laboratory'for efficient 02 production in dark and light induced processes. We have not investigated the surface heterogeneity of Ti0 2. In Degussa p-25 sizes and shapes as shown by electron microscopy reveal that they are very heterogeneous in size as well as in form.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
147
©
Syr4THESIS, SURFACE REACTIVITY, AND CATALYTIC ACTIVITY OF HIGH SPECIFIC SURFACE AREA MOL YBDENU~l NITRI DE PO~JDERS L. VOLPE, S.T. OYAMA+ and M. BOUDART Department of Chemical Engineering, Stanford University, Stanford, CA 94305,U.S.A.
ABSTRACT High specific surface area powders of t10 2N have been prepared by temperature-programmed reaction of 1100 3 and ammonia. This topotactic transformation produces small nitride particles that have an orientation relation with the original large trioxide platelets. Unsupported stable powders have thus been obtained with as much as 224 m2g-1 as measured by N~L BET adsorption. Bulk structure was studied by x-ray diffraction, and the surfacE was characterized by selective chemisorption of CO. The powders were active catalysts for NH 3 synthesis. Turnover rates based on titration of sites by CO indicate a pronounced effect of particle size on catalytic activity. This confirms the structure-sensitive character of ammonia synthesis. IJJTRODUCTION Molybdenum is known as one of the more active catalysts for ilmmonia synthesis [lJ; the pertinent literature was reviewed recently by Ozaki and Aika [2J. During synthesis on Mo powders at atmospheric pressure and temperatures up to 900 K, dimolybdenum nitride, y-~102N, is formed as the stable phase. To use a catalyst particle efficiently, its surface-to-volume ratio must be maximized. This can be accomplished either by dispersing small catalyst particles in a highly porous support or by using an unsupported powder with a high specific surface area, Sg (m 2g-1). Because it is difficult to reduce Mo compounds on various supports [3J, the former approach seems to be less promlslng. It becomes very desirable, therefore, to prepare an unsupported nitride powder with a high Sg value. Furthermore, structural properties of the surface are expected to depend on particle size below ca. 10 nm, which in the case of Mo 2N corresponds to 2 -1 60 mg. Hence, high-S g catalysts add the opportunity of investigating the structure sensitivity of NH 3 synthesis, a phenomenon observed on both small [4J and large [5J crystals of iron. +Present address:
Catalytica Associates, Inc., Santa Clara, CA 95051, U.S.A.
148
Previous studies of ammonia synthesis were performed on nitrided ~10 samples that had low or unspecified Sg and were often insufficiently reduced. Kiperman and Temkin [6J thoroughly analyzed the synthesis kinetics on MOZN prepared by reacting ammonium molybdate and NH at 873-9Z3 K. They did not measure the 3 Sg of their catalyst. Hillis ~~. [7J studied the reaction on molybdenu~ dioxide (MoO Z) partially reduced in HZ and nitrided with NZ at temperatures around 773 K. Aika and Ozaki [8J carried out isotopic tracer investigations of NH 3 synthesis on MOZN powder with 13 mZg- l They established that the reaction's rate-determining step was the same as on Fe catalysts: dissociative adsorption of NZ. Their sample was prepared by reduction of molybdenum trioxide (Mo0 3) in HZ followed by nitridation of Mo metal in NZ and NZ/H Z mixtures, all at a constant temperature of about 773 K. Most recently, two of us [9J studied NH 3 synthesis on three molybdenum compounds: Mo, MOZC, and MOOxC y. Despite profound differences in composition and crystallography, the reaction rate on these materials reached similar steady-state values, when referred to the number of sites as titrated by CO. Steady state was achieved only after the catalysts absorbed small amounts of nitrogen that corresponded to at most three atomic layers. Thus, the catalytic activity of these materials appeared to be determined by surface molybdenum nitride layers, regardless of the structure or composition of the bulk. Among these studies, the last was the first one that used selective chemisorption [lOJ to count the number of active sites for NH~ synthesis . This work describes new ways to synthesize unsupported t10 ZN powders with Zm-gl ) ,and low (lZ mZ-l) high ( 190-ZZ4 mZ-l g ), medium (50 g values of Sg. Comparison of their activity towards NH 3 synthesis confirms the structure sensitivity of this reaction. The paper also presents catalyst characterization by selective chemisorption, physisorption, microscopy, and x-ray diffraction (XRD). ..J
EXPERIMENTAL RESULTS Catalyst Preparation The three types of catalysts used in this study had low, medium, and high values of Sg and will, hereafter, be distinguished as MOZN-L, M, and H. All of them were synthesized at atmospheric pressure by downflow of reactive gases over packed beds of Mo0 3 powder. The powder bed was contained in a quartz or Pyrex cellon top of a coarse fritted disc made out of the same material. The cell was designed both to use its packed-bed portion as a plug-flow reactor and to perform volumetric adsorption measurements. Its entire volume could be isolated by stopcocks and removed from the gas-delivery system to conduct all experimentation in situ, without exposure of its
149
contents to air. The temperature of the cell's reactor section could be varied, in a furnace or otherwise, and monitored locally with a thermocouple. The preparation of MOZN-L and Mconsisted of sequential reduction and nitridation of Mo0 3 (MC&B, 99.5%) performed isothermally [9,11J. The trioxide had its normal orthorhombic crystal structure according to the XRD pattern. The reduction was carried out at 773 K by Pd-diffused HZ. In the case of MOZN-L, HZ was passed at the rate of 91 ~mol s -1 over 1 g of Mo0 3 for 30h, and in the case of MOZN-M, 18 g of Mo0 was treated for 300h with 3 -1 the gas flowing at the rate of 35 ~mol s Then the surface of the powders was passivated in a flowing 0Z/He mixture for XRD examination. The patterns of the reduction products had only Mo metal peaks. Subsequently, both powders were treated with a mixture of about 1% NH 3 in HZ. This produced MOZN-L as a result of Z4h of nitriding at 773 K, whereas 7Zh of nitriding at 803 K yielded the MOZN-M s~mple. The MOZN-H catalyst, in turn, was synthesized from ultra-high purity Mo0 3 powder (Johnson MattheY, Puratronic, 99.998%, batch 5.86897). The BET Sg of Mo0 3, measured after evacuation at 470 K to 10-3 Pa, was 0.86 mZg-1 The Mo0 3 crystals had an orthorhombic lattice, and the anomalously strong inte9rated intensities of all the (OkO) reflections in the diffractometric pattern indicated a morphology with extensive (OkO) planes. This morphology could be destroyed by grinding the powder in a mortar with a pestle. The unground Mo0 crystals seen through an optical microscope (Fig. 1) were 3 shaped as slabs, consistent with the layer structure of the solid, and were about ZO ~m in size.
Figure 1.
Optical Micrograph of Mo0
3
(Johnson Matthey).
150
The Mo 2N-H sample was prepared, as schematically shown in Fig. 2, by temperature-programmed reaction between 1 g of Mo0 3 and NH 3 (Matheson, Anhydrous) flowing at a rate of 70 ~mol s-l The ammonia was purified by passage through a sodium trap. The temperature-time program consisted of two consecutive linear increases followed by a brief isothermal portion. After rapid heating to 690 K, the reactor temperature was raised to 740 K at the rate of 0.01 K s-l and then further to 979 K at 0.05 K s-l. Subsequently, this fjnal temperature was maintained for 0.5h.
1000
atmospheric pressure
~
<, 800 ~
600 L . . . . - - - _ . . . J . . . -
o
--..L.-
2
---l.._ _----J
3
time /h Figure 2.
Sy~thesis
of Mo 2N-H in the presence of NH 3.
Ouring the first 1inear heating interval, the Mo0 3 --:). Mo0 2 phase transition took place. This was established by XRD after the temperature program was interrupted at 740 K, in a separate experiment. The Mo0 2 diffraction peaks showed no crystal-size broadening, implying that the particle dimension could not be much less than 1 ~m. The second heating stage corresponded to the Mo0 2 ~ ~102N trans format i on. Th i s process bypasses the Mo metal phase, as proved by the compl ete absence of ~10 peaks in XRD patterns taken before its completion in separate experiments. In the latter stage of Mo 2N-H synthesis, the powder catalyzes NH 3 decomposition. Unless its flow rate is high, almost all the NH 3 decomposes by 979 K, causing concentration gradients in the powder bed. Catalyst Characterization The three types of samples were characterized by CO chemisorption and XRD. Their Sg was determined according to the standard BET method based on N2
151
adsorption isotherms obtained at liquid nitrogen (LN Z) temperature. Besides, scanning electron microscope (SEr1) pictures of the MOZN-H catalyst were taken. Importantly, all the characterization work was done only following the NH 3 synthesis studies reported in the next section. The adsorption was measured ~~. Then, after a surface passivation, bulk properties of the samples were investigated. To measure gas adsorption, the isolated cell was transferred and attached to a highly precise volumetric adsorption system, designed and described by Hanson [lZJ. Following evacuation of the cell to 10- 3 Pa at 7Z3 K, a CO (Matheson, 99.995%) adsorption isotherm was obtained at room temperature (RT); then the cell was evacuated to 10- 3 Pa again to remove the gas weakly adsorbed at RT, and finally, a second RT carbon monoxide isotherm was taken. The amount of strongly che~isorbed gas given in Table 1 was calculated as the difference between the values of uptake obtained by extrapolating the linear portions of the two isotherms to zero pressure. Strong CO chemisorption amounted to about 70% of the total adsorption for all samples. TABLE 1 Characterization of Catalysts CO number density/ 1015 cm- Z 0.13 lZ MOZN-L [9,11 J O.lZ 50 MOZN-M [11 J 0.Z8 190 (d) MOZN-H (a) Strong CO cnemi s orpt ron at room temperature. (b) Calculated from Sg values. (c) Deduced from XRD line broadening. (d) 5 = ZZ4 mZg- l prior to CO adsorption. g
Sample
CO uptake/ llmol g-1 (a) Z5 104 88Z
s/m Zg- l
Dp/nm (b) 63 lZ 3
D/nm (c) 10 10 5
Subsequently, N (Matheson, 99.998%) adsorption was carried out at LN Z Z Z -1 temperature to yield 5g values of lZ, 50, and 190 m g for MOZN-L, M, and H, respectively. The CO strongly adsorbed at RT decreased 5g of the MOZN-H sample: In a separate experiment where NZ adsorption was not preceded Z -1 by CO exposure, the value of S was found to be as high as ZZ4 m g The g particle size (Table 1) was calculated from the relation Dp = 6/5 gP . The density P was deduced from the structure and parameter of the y-MozN lattice, known from XRD data. We attempted to find the pore size of ~10ZN-H based on the N adsorption between 30 and 100 kPa at LN Z temperature. The powder Z . took up 0.123 cm 3g-1 LN Z at saturatlon.
152
Although all the nitrides were passivated at RT with a flowing mixture of 1% O2 in He, only Mo 2N-H required such treatment because it was pyrophoric. The passivation reduced S9 of the specimen by only 10 mZg- l The bulk structure of the samples was investigated by standard powder diffractometric technique. Only the peaks corresponding to y-M0 2N could be observed in the patterns. In y-MoZN, Mo atoms have the FCC arrangement, with a lattice parameter of 416 pm, where N atoms occupy every other octahedral site. Our nitride patterns agreed well with the one tabulated in the Powder Diffraction File [13J. For M0 2N-H sample, the integrated intensities of the XRD peaks corresponding to the (ZOO) and (400) reflection planes were almost 20 times greater than expected. Interestingly~ this (hOD) morphology could be easily eliminated by grinding the powder in a mortar, and, what is more, the nitride synthesized from the ground Mo0 also exhibited reflections of normal 3 intensities. An XRD film of unground MOZN-H sample taken with a DebyeScherrer camera showed no signs of the (hOD) anomaly. Twenty wm platelets can be seen in an SEM picture (Fig. 3) of the oriented MOZN. We found no influence of the morphology on the surface properties of the nitride. Thus, a sample synthesized from ground 1100 had the same high 5g, CO uptake, and 3 NH synthesis activity. 3
Figure 3.
SEM of MOZN-H (Supported on Ag Paint).
Crystallite size of the specimens was estimated from XRD line broadening using a standard procedure [14J. The average coherently diffracting domain
153
perpendicular to (hk£) planes was calculated from the Scherrer equation, Dc = KA/S hk£ cos 8, where 8 is the Bragg angle, A is the x-ray wave length, and Shkk is the peak width at half-maximum. The constant K was taken to be unity. We observed no (hkk)-dependence of Dc' The last column of Table 1 gives the average Dc values of the specimens. All MOZN samples were good electronic conductors at RT. Ammonia Synthesis The nitride powders were found to be active and stable in ammonia catalysis, and the synthesis kinetics was investigated between 567 and 773 K near atmospheric pressure. The reaction was carried out immediately following the catalyst preparation in the same reactor. Ammonia was synthesized from a stoichiometric NZ/H Z mixture (Matheson, custom made) flowing at a constant rate. Before entering the reactor, the mixture was purified by passage through an Engelhard Deoxo unit followed by a 5A molecular sieve trap at dry-ice temperature. The gas stream was analyzed for NH 3 contents with the help of an IR spectrophotometer (Beckman IR-9 or NDIR-865). Calibration of this instrument for absorbance at 966.5 cm- l was carried out with gases of known composition. The mole fraction, y, of NH 3 at the reactor exit could be monitored continuously. The total molecular flow rate per second, F, was varied and measured with a soap film flow meter before venting the reaction mixture. Steady-state data were obtained on all samples. Here we shall provide no detailed kinetic analysis, but rather use a few representative interpolated results to compare the activity of the three NH 3 catalysts expressed the number of NH 3 molecules at the as site-time yield, STY (s-l), ~., reactor exit produced per second per site as titrated by CO. The STY represents an average turnover rate and is equal to yF/L, where L is the number of sites capable of strong CO chemisorption. The rate data in Table Z are compared at the same values of temperature and efficiency. The latter is defined as the ratio of y at the reactor exit to the value of y in an equilibrated reaction mixture at the same conditions. For reactions like NH synthesis, where product concentration influences the 3 turnover rate, efficiency- is an important parameter in the comparison of rates.
154
TABLE 2 Comparison of Ammonia STY/s-l (NZ:H Z
1:3; 1 atm; Steady State).
~
MOZN-L [9,11]
MOZN-M [11]
Z.4xlO- 3
1.5xlO- 3
5.9xlO- 5
T - 6Z3 K Efficiency O.OZ
8xlO- 4
3xlO-4
3xlO-5
T = 60Z K Efficiency 0.014
5xlO-4
-'-
lxlO- 5
T = 576 K Efficiency 0.005
ZxlO- 4
--
3xlO-6
Conditions
T = 673 K Efficiency 0.1
~1oZN-H
DISCUSSION Preparation No attempts were made to optimize the synthesis conditions of HOZN-L and Mcatalysts. They were prepared following, in essence, the procedures of previous studies [7,8] at temperatures where reduction and subsequent nitridation of Mo oxide requires tens of hours. It is likely that the extent of surface oxidation during the intermediate passivation was larger for ~10ZN-t1 than for MOZN-L. The react ion with NH/H Z whi ch fo 11 owed mi ght have increased the Sg for MO ZN-t1 by a greater amount than for MOZN-L. In the synthesis of MoZN-H, we chose to use NH 3 as the sole reagent for both reduction and nitridation. Carpenter and Hallada [15] found that the first reaction, 3 Mo0 3 + Z NH 3 = 3 MOO Z + 3 HZO + NZ' proceeded faster and at lower temperature than the reduction of trioxide to dioxide with Hz. Importantly, this stage must be carried out slowly so as to avoid particle agglomeration caused by the formation of large quantities of water. If the temperature is increased at 0.03 K s-l, for instance, the particles sinter to such a severe extent that they lose the properties of a fine powder. It was unknown what changes the oxide would undergo when treated with NH 3 above 700 K. The x-ray results of this work show that MOO Z is transformed directly into y-MoZN between 740 and 979 K (Fig. Z), and the transition is accompanied by a two-orders-of-magnitude decrease in both crystal and particle size. Changes in crystal size are quite common in gas-solid reactions, but the absence of sintering of 3 nm particles at 979 K is noteworthy. As far as we know, the value of ZOO mZg- l for this nitride sample may be the highest Sg on record for unsupported metallic catalysts or powders.
155
The overall transformation between the Mo0 3 platelets shown in Fig. 1 and Mo 2N-H appears to be an ordered process such that the final nitride crystallites have an orientation relationship with the parent trioxide. The SEM picture of Mo 2N-H (Fig. 3) shows agglomerates similar to the original Mo0 3 crystals both in size and in well-developed slab-like shape -- subject to preferred orientation. The orientation features in both solids are demonstrated by the anomalous (hkt) intensity in their powder diffractometric patterns. No anomalies could be detected with the Debye-Scherrer technique, which is much less susceptible to sample orientation effects. When the (OkO) feature in Mo0 3 was removed by grinding, the product Mo 2N-H had no (hOO) anomaly. We conclude that the overall reaction results in the (OkO)//(hOO) orientation correspondence between the trioxide and nitride lattices, and therefore belongs to the class of topotactic transformations [16]. It produces very small Mo 2N crystallites aggregated in a spongy relic of the parent Mo0 3. The external shape of the platelets doesn't change in the reaction; the pore structure of the catalyst is produced during the removal of oxygen and insertion of nitrogen. Characterization According to our XRD results, the crystallites of the three powder samples had undistorted y-Mo 2N structure, and no other phases could be detected. of coherently diffracting domains, Judging by the absence of (hk~)-dependence the crystals comprising the agglomerates had isotropic or random shapes. A particles were comparison between Dp and 0c (Table 1) suggests that the Mo?N-L ~ polycrystalline, whereas the other two samples consisted of single crystallites. Carbon monoxide number density in Table 1 was calculated from the amount of strong CO chemisorption per unit area of the powder. Clean Mo metal adsorbs a full monolayer of £0 at RT in dissociated and molecular states [17]. Ko and Madix [18] have demonstrated that on the Mo(lOO) face a monolayer of C prevented dissociative but not molecular adsorption, and overlayers of 0 both suppressed CO dissociation and reduced its number density and binding energy. Surface N might also decrease the amount of CO chemisorption on Mo. The decrease of adsorption by N might be greater than that by C and smaller than that by 0 overlayers. It is assumed that one CO molecule adsorbs without dissociation on each surface metal atom. There is no information, however, about CO adsorption on M0 2N. Making the usual approximation that the surface is formed from equal proportions of the main low-index planes, one would expect the maximum number density of 1.lxl015 cm- 2 for Mo atoms on Mo 2N. Since the adsorption is likely to be hindered by nitridic N, our experimental values of 0.12-0.28xl0 15 CO molecules per cm2 appear to be reasonable. It
156
is unclear why the M0 2N-H sample has a number density more than twice as high as the other two ~atalysts do. At any rate, the trend shown in Table Z would hardly change at all if, instead of citing values of STY based on CO chemisorption, we had compared areal rates, ~., rates referred to total BET surface area. The agglomerates of M0 2N-H (Fig. 3) must be highly porous to possess S around 200 mZg- l. According to a pore-size distribution analysis [19], g most of the-pores were smaller than 1.5 nm, but, because the validity of the Kelvin equation becomes uncertain in this region, no conclusion about the average pore size could be made. The fact that CO, strongly adsorbed at RT, blocked 15% of the BET surface also indicated that some of the pores were very small. Since the powder w~s to serve as an NH 3 catalyst, we were concerned with the accessibility of the pores to N2. A theoretical estimate indicates, however, that internal diffusion limitations should be absent at the reaction conditions (T - 700 K, NZ concentration c - 1019 cm- 3). Indeed, the rate of N2 diffusion inside the nitride agglomerates of size r - 10 ~m exceeds by several orders of magnitude the measured STY of . prevai'1" - 10-4 s - 1 or rate R - 1016 cm -3 s - 1. S'tnce Knu dsen reqrme s ms t de - 1 nm pores at atmospheric pressure, the diffusivity, D, should be greater than 10- 3 cm 2 s-l. According to the criterion involving observable rates [20], no diffusion problems arise as long as the ratio r 2R/cD is far below unity. In our case, this number is on the' order of 10- 6. Thus, smaller STY values for M0 2N-H cannot be attributed to a lower extent of utilization of the internal catalyst surface. Structure Sensitivity of Ammonia Synthesis In 1975, Dumesic g1 ~ [4] discovered that the activity of supported iron particles as NH catalysts strongly depended on particle size. They ascribed 3 this effect to structure sensitiv.;ty, according to which the fraction of sites with enhanced activity for NH 3 synthesis would decrease with the reduction in particle size. These sites, they postulated, were connected with the relatively open atomic ensembles exposed on the (111) planes of Fe. The first direct demonstration of the structure sensitivity of NH 3 synthesis has been reported by Spencer et ~ [5], who showed that the (111) face of Fe was 41B times more active than the (110) face at 798 K and 20 atm. The present work provides new evidence for the structure-sensitive character of flH 3 synthesis. It shows a dependence of NH 3 turnover rates on the particle and crystal dimension of unsupported Mo 2N powder, representing the first such finding on a catalyst other than Fe. Table 2 gives STY ratios of 40:25:1 for 63, 12, and 3 nm particles, respectively, at 673 K and 10%
157
efficiency. It would be unreasonable to expect a drastic size effect for particles above 10 nm -- and we did not observe one. The STY value on M0 2N-H, on the other hand, is markedly lower than on the other two samples. Accordingly, we suggest that NH 3 synthesis is sensitive to the surface structure of M0 2N, and the fraction of the more active sites decreases as its particles become smaller. Further work is necessary to elucidate the reason for this structure sensitivity on M0 2N. ACKNOWLEDGEMENT This work was carried out with support from the Center for Materials Research at Stanford University under the NSF MRL Program. REFERENCES 1. A. Mittash, Adv. Cat. Re1. Sub. 2, (1950) 81. 2. A. Ozaki and K. Aika, "The Synthesis of Ammonia by Heter0geneous Catalysis", in R. W. F. Hardy et a1. (Eds.), A Treatise on Dinitrogen Fixation, John Wiley & Sons, New York, 1979, p. 169. 3. F. E. Massoth, J. Cata1. 30, (1973) 204. 4. J. A. Dumesic, H. Tops¢e, M. Boudart, J. Cata1. 37, (1975) 513. 5. N. D. Spencer, R. C. Schoonmaker, G. A. Somorjai, J. Catal., in press. 6. L. Kiperman, M. Temkin, Acta Phisicochim. U.R.S.S. 21, (1946) 267. 7. M. R. Hillis, C. Kemball, M. W. Roberts, Trans. Faraday Soc. 62, (1966) 3570. 8. K. Aika and A. Ozaki, J. Catal. 14, (1969) 311. 9. S. T. Oyama and M. Boudart, J. Res. Inst. Catal., Hokkaido University, 28 (1980) 305. 10. P. H. Emmett and S. Brunauer, J. Am. Chem. Soc. 59, (1937) 310 and 1553. 11. S. T. Oyama, Ph.D. Dissertation, Stanford University, 1981. 12. F. V. Hanson, Ph.D. Dissertation, Stanford University, 1975. 13. International Powder Diffraction File, W. F. McClure, Ed., 1979, Inorganic Materials Index: 25-1336. 14. S. F. Bartram, Chap. 17 in E. F. Kae1b1e (Ed.) Handbook of X-rays, McGraw-Hill, New York, 1967. 15. K. H. Carpenter and C. J. Hallada, in H. F. Barry and P. C. M. Mitchell (Eds.) Proc. Climax 3rd Intl. Conf. on the Chemistry and Uses of Molybdenum, 1979, p. 204. 16. J. M. Thomas, Philos. Trans. R. Soc. London, Ser. A, 277 (1974) 251. 17. R. R. Ford, Adv. Cat. Rel. Sub. 21, (1970) 97. 18. E. Ko and R. J. Madix, Surf. Sci. 100. (1980) L505. 19. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, p. 161. 20. P. B. Weisz and C. D. Prater, Adv. Cat. Re1. Sub. 6, (1954) 143.
158 DISCUSSION Your conclusion that "the ammonia synthesis reaction on molybM.M. BHASIN denum nitride powders is surface-structure sensitive" is based on an assumption that strong CO chemisorption sites measured are responsible for ammonia synthesis. What evidence do you have to support that assumption? L. VOLPE: By analogy with ammonia synthesis on iron catalysts (M. Boudart and G. Djega-Mariadassou, La Cinetique des Reactions en Catalyse Heterogene, Masson, Paris, 1982, p. 184), we assume that irreversible chemisorption of CO merely counts the number of surface atoms capable of strong chemisorption. By contrast, chemisorption of dinitrogen on iron seems to require special ensembles, the surface density o£ which may be smaller than that of sites titrated by CO. Indeed, if chemisorption of CO did count correctly the number of sites responsible for ammonia synthesis, the site time yield (STY) for this reaction referred to CO chemisorption sites should be independent of particle size. In fact, on iron catalysts, the STY is independent of particle size when the sites are referred to those counted by high temperature chemisorption of dinitrogen. Whether this would also be the case on our M02N catalysts remains to be determined. A. BAlKER: Based on the complete absence of Mo-refelctions in the X-ray diffraction patterns, you concluded that Mo02 is directly transformed to M02N. Would you exclude the possibility of an intermediate formation of small. Mo-clusters which do not show up in the diffraction patterns which, however, may play an important role in the reaction ? L. VOLPE: Our conclusion is based on the lack of detection of metallic Mo by X-ray diffraction. We cannot rule out the formation of nuclei, embryos, or clusters of Mo during the topotactic transformation of Mo0 2 to M02N, but believe they would play no role in the mechanism of the transformation. Yet we admit that we do not understand this mechanism at the moment. S.P.S. ANDREW: With reference to the question of topotactic transformation: when effecting the phase change of a solid by reaction with a gas resulting in the creation of porosity, considerations of geometry alone can produce a radial porr structure normal to the particle surface irrespective of true topotactic transformations on the atomic scale. I have observed this phenomenon in the reduction of magnetite crystals. Are you sure that this may not be the origin of your topotaxy ? L. VOLPE:
We refer Dr. Andrew to our answer to Dr. Baiker.
M. HAMALAlNEN "Representing the first such finding on a catalyst other than Fe ...... Therefore, are Mo-nitride catalysts going for a commercial success? L. VOLPE: material.
OUr only claim thus far is to have made a new active catalytic
D.D. SURESH Have you nitrided Mo02 instead of starting from MoO) ? If so, what is the difference ? L. VOLPE: We treated a Mo02 powder (Alfa Ventron) exactly like we treated MoO). with MoO) we obtained the M02N-H material described in our paper. But with Mo02' the product exhibited only the X-ray diffraction peaks of the starting material. K.S.W. SING The agglomerates of M02N-H are likely to be microporous and in the absence of mesoporosity the nitrogen adsorption isotherms could be analyzed in terms of external surface area and micropore volumes. Such an approach could be used to study the topotactic conversion of MOO) to M02N. If thus interconversion is truly topotactic i t should be to some extent reversible. Is there any evidence to suggest that this is the case ? L. VOLPE: We agree with Prof. Sing on his interesting suggestion that a study of porosity would throw light on the mechanism of our transformation. We do not know whether the latter is reversible. However, this is not a requirement for a transformation to be topotactic.
159
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publisbers B.V., Amsterdam - Printed in The Netberlands
THE PREPARATION OF CERAMIC-COATED METAL-BASED CATALYSTS
C.J. WRIGHT and G. BUTLER Harwell Catalyst Unit, A.E.R.E. Harwell, Didcot, Oxon OXII ORA (U.K.)
ABSTRACT The paper discusses the scientific problems of preparing catalysts from metallic honeycomb substrates.
The control of the hardness, the thickness,
the thermal stability and the adhesion of the catalyst support by modification of the support precursor is described.
Necessary properties of the metal
substrate are also examined.
INTRODUCTION Packed bed catalytic reactors are used for many commercial chemical processes. The dimensions of the catalyst pellets used in these reactors are chosen after careful optimisation of a number of factors.
Two important factors are the
variation of the effectiveness factor of a catalyst pellet with its surface to volume ratio, and the relationship between the pellet dimension and the reactor pressure drop. For a number of important processes the overall reaction rate is controlled by pore diffusion, or internal mass transfer.
Under these conditions the
catalyst effectiveness factor can be very low, if the catalyst dimensions are too large.
Typical processes which have been shown to be pore-diffusion limited,
include ammonia synthesis (ref. 1), methanation (ref. 2), hydrocarbon oxidation and steam reforming.
Consequently a catalyst designer is faced with a dilemma,
since an attempt to improve effectiveness factors by reducing the pellet size will lead inevitably to an increase in the reactor pressure drop and vice versa. It was for processes of these types that the monolith or honeycomb catalyst substrate (Fig. I) was introduced since such a substrate combines a high external surface to volume ratio, with a low pressure drop, in the same artefact. There are a number of general discussions of the preparation of catalysts by the impregnation and coating of ceramic honeycomb substrates (ref. 3).
It is the
intention of this paper to draw attention to the preparation of catalysts from metallic honeycomb substrates, and to the particular scientific problems which are associated with such preparations.
A number of advantages have been cited
for the metallic honeycomb compared to the ceramic, of which their mechanical strength is the most important.
Applications of metallic honeycomb catalysts
have been discussed in a number of patents which are tabulated in table 1.
160
Fig. 1
Ceramic and Metal Monolith Catalyst Substrates
This typical metal based honeycomb catalyst consists of three components; the metal;
a catalyst support produced by washcoating and firing;
active phase.
and the
The first two components are chosen or designed to satisfy a
number of criteria which are summarised in Table II.
These criteria are
discussed in more detail in the remainder of this paper.
TABLE I Patented applications of metallic honeycomb catalysts
Application Hydrocarbon oxidation in vehicle exhuasts Ammonia oxidation Oxidation of methanol to formaldehyde Methanation Steam reforming Methanol synthesis and shift reactions CO oxidation in CO lasers 2 Ozone decomposition
Reference
4,5 4 6 7
8 8 9
10
TABLE II Selection criteria for the metal and the ceramic used in metal based catalyts 1. The Metal
Must be chemically inert within the reactor atmosphere, and must not provide a source of ions which might diffuse into the catalyst and impair its activity and or selectivity.
161 2. 3.
Must adhere strongly to the catlyst support. Must· be mechanically strong at the reactor temperature.
1.
Must adhere strongly to the metal substrate. Must be deposited upon the metal simply and economically. Must have sufficient surface area so that the monolith activity is not surface area limited. Must have sufficient mechanical strength to withstand tensile forces created during formation and use.
2. 3. The catalyst support 4.
THE CATALYST SUPPORT (a)
Hardness The major requirements of the catalyst support are that it provides suffi-
cient surface area on which the active phase can be dispersed, and that as far as is compatible with the first criterion, it withstands erosion and attrition. To optimise the hardness of the catalyst support, it is necessary to control the properties of the support precursor, which is generally a concentrated dispersion of oxide in water, often called a washcoat, which can be applied to the metal base by dipping, removing, drying and firing. To understand how the hardness can be controlled, it is useful to look in more detail at the processes which occur during the coating procedure. the washcoat film is dried, loss of water leads initially to gelling
When as the
electrical-double-layers surrounding the individual particles in the coatingfluid overlap.
After gelling, further loss of water takes place without an
accompanying change in the area of the coating parallel to the metal substrate. This creates strain within the gel and cracks will then develop perpendicular to the substrate, if the increase in surface free energy on cracking is less than the release of stored elastic energy.
At very high stresses cracking at
the coat - substrate interface can also occur.
A number of patents have
discussed methods of formUlating coating fluids to yield crack-free gels. (ref. 11,12)
It is important to note however that i t is not necessary for
catalytic puposes for the catalyst support to be crack-free. Figs. 2 and 3 show electron micrographs of successful catalyst coats and it can be seen that in fig. 3 the catalyst coat has a typical 'mudflat' appearance due to shrinking during preparation and use.
If two sols containing the same
initial volume fractions of solids are progressively concentrated, then the one containing the smaller particles will gel first because of the greater volume occupied by their double layers.
This will cause the small particle sol to
have a greater tendency to crack when dry, because it will then be storing a greater quantity of elastic energy.
In contrast the larger particles will
gel later but they will only form a soft coating (ref. 13).
This experimental
162
Fig. 2 Electron micrograph of a catalyst coat (X640)
Fig. 3 Electron micrograph of a catalyst coat (X320)
observation, is of some interest, because it would not be expected from simple theory which indicates that the strength of a ceramic, S, containing spherical pores, can be written
S
tr [1-
(pi
) 2/3]
Pcr where p is the porosity, and Pcr the critical porosity giving zero strength and trthe ideal porosity-free strength.
This equation, which has been shown
to agree well with experimental results, indicates that the strength is independent of the pore-size (ref. 14)
The reason why small particle sols give
rise to harder gels is possibly due,therefore,to second order effects such as the detailed shape of the pores, and the larger areas of contacts between the original gel particles. A compromise solution, which can 'lead to hard, crack-free, catalyst supports, involves the formation of a gel from sols which contain spherical particles whose size distribution leads to close packing. (ref. 11)
The diameter of the
spheres in the second largest group in the formulation, should be close to 0.414 times that of the diameter of the largest group, since this enables them to pack into the interstices created by the packing of the largest group.
The
total volume of the second group should be close to .07 times the volume of the first group.
Further small particles can be added to minimise the free
volume in the dry gel.
Such formulations lead to hard coatings both because of
the low volumes of water that remain in such sols when they gel, and also to the large contact area between the spheres.
163 Although there are intrinsic limitations of the achievable hardness, for a predetermined coating voidage and composition,
(ref. 15) there are two other
routes for maximising the hardness within these limitations.
The first is to
include within the coating fluid an inorganic polymer which will precipitate at the surfaces of the particles upon drying.
Bulk gels of this type have been
shown to have transverse rupture strengths >1500 p.s.i.
(ref. 12)
The second
method is an extension of the first in so far as it requires the pH of the liquid phase to be controlled so that during the drying process some of the particulate matter dissolves and reprecipitates to form necks at the points of contact between the spheres.
The driving force for this solution - precipita-
tion process, is the negative surface free energy of the ceramic particles at the points of negative curvature.
The surface coating in Fig. 2 is one in
which considerable necking has occurred.
(b)
Thickness Another parameter which defines the catalytic coating, is its thickness t.
Clearly the total catalytic surface area within a honeycomb catalyst is much less than the potential surface area within a conventional pellet of the same external dimensions.
Neglecting the thickness of the cell walls, and assuming
square cross-section cells, this ratio is 1
4t(d-t)
~ where d is the length of a cell wall.
The activity of the catalyst will
increase with t, until t reaches a limiting value and the activity becomes diffusion controlled.
This value of t varies with reaction and temperature,
but for sulphur dioxide oxidation t is methanation t
"*
300 microns (ref. 2).
~100
microns (ref. 16), and, for
'£'he former thickness is typical of a
metal based catalyst support, and it leads to a ratio for the mass of the catalyst layer to the mass of the metallic base, of thickness of the metal sheet coating.
p'
~t
is its density, and
e
(d-t) :
For alumina on a Fecralloy* steel monolith and for d
micron coating is equivalent to
ft'd.
t' is the
is the density of the =
Imm a 100
25% of the final mass of the catalyst.
In manufacture it is clearly desirable to be able to deposit this coating in a single step, and considerable attention has been paid to the rheology of catalyst washcoats, in order to attain this objective.
(c)
Thermal stability For many applications a coating should have a high surface area which is
* Fecralloy is a registred trade mark of the U.K.A.E.A.
164 stable up to high temperatures. w,~
In many of the preceding discussions, however,
have used simple non-aggregated sols to illustrate the concepts involved.
These sols are comprised of individual particles which can close pack in the dry gel such that each particle is in contact with a large number of other particles.
As a consequence of this high coordination number however,
washcoats comprised solely of such materials can lose a significant proportion of their surface area at high temperatures because of sintering. As well as these.sols, there is another class which have been referred to as aggregated sols (ref. 17).
In these sols the individual particles have been
drawn together by the appropriate control of the concentration of ions dissolved in the liquid phase, and the degree of aggregation can be studied by light scattering (ref. 18).
Gels formed from these sols have considerably enhanced
porosity over that present in gels formed from unaggregated sols, since the aggregates have sufficient strength to ensure that their intrinsic porosity is retained on heating, even though the individual aggregates may pack together to minimise the formation of macroporosity.
(d)
Adhesion One of the most important applications of metal-based catalysts has been in
vehicle exhaust emission control, and it is here that the demands placed upon the catalyst by thermal cycling are particularly severe.
On cycling from 20
to 10000C for instance the differential expansion between a Fecralloy* steel substrate and a ~
A1 coating, can be 0.7%. 20 3 The resulting stress in the coating will lead to cracks developing perpen-
dicular to the substrate, together at high stress, with shearing within the film and spalling. The conditions for preventing material loss from the coating, are a high substrate surface energy to enhance adhesion
to the metal, and a high gel
surface energy to prevent fracture paraLLel to the metal surface, but within the gel.
Because of the first condition, it is important that the surface of
the metal is kept grease free. The 04 A1 at the surface of the metal will 20 3 adhere to a washcoat if the formulation includes a readily sinterable component such as Boehmite (ref. 4).
THE METAL The metallic substrate utilised in conjunction with a catalytic coating must be easily workable, highly resistant to corrosion and have an adequate yield strength over the temperature ranges where it is to be used.
In
addition, the substrate surface should both adhere to the ceramic coating and also provide a diffusion barrier to metal ions contained within the bulk of the metal.
This latter point is important because trace surface concentrations
165 of metals such as iron, can significantly alter catalyst activity and selectivity. For most of the applications for which metal based catalysts have been evaluated, it has been found that it is the aluminium-bearing ferritic steels, which most readily satisfy the above requirements.
Table 3 lists the composi-
-tions of some alloys that have been evaluated as catalyst materials.
(ref. 19)
Alloy
%Al
%Y
%Cr
%C
%Si
%Fe
Fecralloy (R.T.M.) In90nel (RTM) 600 Inconel (RTM) 601 Incoloy (RTM) 800 AISI 310 Kanthal (RTM) DSC
4:05.2 0
0.050.40 0
15.022.0 15.50
<0.03
0.200.40 0
Balance 80
Balance
23.00
0.05
Balance
60.0
21.00
0.04
° °
Balance
32.0
0.4 Ti
Balance Balance
20
2.0 Mn 2.0 Co
1.40
0
0.30
0
0 6.00
0 0
.25.0
0.05
.25
1.5
23.0
%Ni
Others
0
R.T.M. signifies a registered trade mark.
The first of these alloys, Fecralloy* steel (ref. 20) has been particularly successful, and further discussion of the properties of the metal, will be restricted to this example.
Prior to coating the metal, it is condi-
tioned, so that a protective and dense layer of
~A1203
forms at its external
surface by the preferential diffusion and oxidation of aluminium atoms contained within its interior.
Figure 4 shows how the thickness of this layer
varies with time as the metal is conditioned in air at 10000C.
(ref. 21)
To prevent spalling of this outer layer a grain growth inhibitor in the form of yttrium is added to the steel, and figure 5 shows the presence of this element in a transmission micrograph of the alumina surface film.
It shows
ribbons of dense material which decorate the original metal grain boundaries, and which contain Y,La and Dy in the same ratio to each other as is found in Fecralloy* steel.
The coating was rendered transparent to the electron beam,
by etching the supporting metal with 8M HCl (ref. 22).
Selected area
diffraction shows this surface film to be
~ A1 and microanalysis reveals 20 3, that it contains only a very small concentration of iron and chromium.
Once conditioned, Fecralloy* steel is resistant to corrosion in both reducing and oxidising environments (e.g. as vehicle exhuast and steam reforming catalysts).
The oxide film is wettable by aqueous dispersions,
and forms a strong bond to an applied ceramic coating. note, however, that the
~
It is important to
A1 film is not naturally porous, a property which 20 3
166
<J 0
0·6
I::>
l5
-4' 1
0 ~
)(
,
0«
N
E
U
lO
0·4
en E c '0
III III Ql
c
~
u
0·2
0·5
0'1
.....
s: ..... Ql "0 X
~
0
10
20 30 40 50 60
0
0
Time { hours} Figure 4
Figure 5
The growth ofothe oxide layer at the surface of Fecralloy steel at 1000 C
Micrograph of protective oxide on Fecralloy steel showing yttrium ribbons decorating the original grain boundaries of the metal (X2000)
167 differentiates it from the ceramic honeycomb supports. measured by water adsorption of
24%.
These have porosities
We have shown that it is advantageous
for the metal base of the catalyst to contain a minimum quantity of aluminium and yttrium.
It is also preferable that the upper limits of the major alloying
.elements (AI and Cr) are not exceeded, if the catalyst base is to have the necessary ductility.
To minimise the pressure drop across a single monolith,
without disastrously impairing the strength or its corrosion resistance, it is usually constructed of metal sheet 0.05mm thick, which is then formed and wound so that an end face of the complete artefact is intersected by cells with cross-sectional areas of the order of Isq.mm.
ACKNOWLEDGEMENTS We would like to record our indebtedness to the many people at A.E.R.E. Harwell who have contributed to the present understanding of metal-based catalysts.
The Harwell Catalyst Unit is supported by the Materials and
Chemicals Requirements Board of the U.K. Department of Industry. REFERENCES 1 G.W. Bridger in Catalyst Handbook, Wolfe Scientific Books, London, (1970) p143. 2 A.L. Hausberger, C.B. Knight and K. Attwood in L.Seglin (Ed.), Methanation of Synthesis Gas, Advances in Chemistry Series No. 146, A.C.S., Washington, (1975), p47. 3 G.J.K. Acres, A.J. Bird, J.W. Jenkins and F. King in Catalysis Vol. 4 The Chemical Society, London (1981). 4 L.A. Heathcote, W.G. Bugden and A.S. Pratt, U.K. Patent 1,492,929. 5 A.S. Pratt and J.A. Cairns, Platinum Metals Review 21 (1977) 2-11. 6 D.E. Webster and I.M. Rouse U.K. Patent 1,603,821. 7 D.T. Thompson and M. Wyatt German Patent 28,13,329. 8 J.D. Rankin and M.V. Twigg, European Patent Application 0021736. 9 M.S. Sonem and G. Faulkner, Rev.Sci.Instr. 52 (1981) 1193. 10 A.E.R. Budd, Platinum Metals Review 24 (1980) 90-94. 11 R.K. Iler U.S. Patent 2,956,958. 12 P.C. Yates, Canadian Patent 878444. 13 R.K. Iler, The Chemistry of Silica, John Wiley, New York, 1979. 14 K.K. Schiller and W.H. Walton (Ed.) "Mechanical Properties of Non-Metallic Brittle Materials" Butterworth, Lonnon , 1958, p35 and discussions by D.J. Millard p.45 and'D. Tabor p47. 15 S.P.S. Andrew in Catalyst Handbook, Wolfe Scientific Books, London, 1970, 20; 16 C.N. Kenney in Catalysis Vol. 3, Chemical Society, London (1980) 123. 17 R.L. Nelson, J.D.F. Ramsay, J.L. Woodhead, J.A. Cairns and J.A.A. Crossley, Thin Solid films 81 (1981) 329-337. 18 J.D.F. Ramsay, S.R. Daish and C.J. Wright, Discuss. Faraday Soc. 65 (1978) 65. 19 Metal Supported Catalysts for Exhaust Emission Control, John Matthey Chemicals, Royston, England. 20 S.F. Pugh, R.L. Nelson, R.S. Nelson and J.A. Cairns, U.K. Patent 1,471,138. 21 R. Williamson, unpublished work. 22 P.T. Moseley, J.S. Sears, G. Tappin, Thin Solid Films 78 (1981) 349.
168 DISCUSSION B. GRIFFE DE MARTINEZ: There is a group in the Venezuelan Institute of Scientific Research: J. Laine, Fr. Severino and myself that is working in a project in "Automotive Catalysts" due to the fact that in Caracas the CO and hydrocarbons levels are high. In our literature research we have found a comparison between alumina pellet supports and the honeycomb-ceramic supports. You have already mentioned the advantages of the second ones; however, there are some disadvantages. For example , they are more expensive, they are difficult to replace in catalytic convertors in the automotive vehicles, and lastly, but most importantly, we know very little about how they are made. Could you tell me how we can overcome these difficulties if we do not get the financial support to buy the ceramic or metallic honeycomb supports from the international companies ? G. BUTLER: I accept that honeycomb c~talysts do suffer from the disadvantages you have quoted. However, I believe that in the US (the largest market for car exhaust catalysts), the honeycomb substrates are predominantly used. I also understand that General Motors have spent many years in researching pelleted catalysts themselves and that they utilise about 50% pelleted catalysts/50 % honeycomb substrate catalysts. Thus, if sufficient resources were available, honeycomb catalysts may present the best option. If such resources are not available then pelleted systems would have to be used. M. FARINHA PORTELA: You mentioned the use of honeycomb supports for tte catalytic oxidation of methanol to formaldehyde. With what type of active species was such support used ? G. BUTLER:
The active species was a 10 wt % silver on alumina catalyst.
H. NEUKERMANS In car use, honeycomb catalysts have an acceptable life cycle, and the spalling of the coating may not be too rapid to be a disadvantage. However, in the large tonnage chemical industry, with 8000 hours per year operation, is the spalling then still on an acceptable basis ? G. BUTLER: Car exhaust catalysts operate in a very arduous environment. They are subjected to rapid thermal cycling during which the catalyst temperature may change by as much as 400°C or more within minutes. The catalysts are exposed to numerous poisons (Pb, P, S and metal species) and acidic vapours. Finally, they are operated at very high space velocities (50.000-100.000 h- 1). Under such conditions they operate successfully for over 1500 hours. In most chemical plants catalysts beds are operated under very controlled conditions in comparison to car exhaust catalysts. The bed temperatures are maintained within narrow limits and much attention is paid to plant start-up conditions and the nature and concentration of poisons. Under such conditions ~ believe that the spalling rate of the catalytic coating will be such that many large tonnage processes may employ such catalysts. I know of examples where ceramic-coated metal-based catalysts have been employed with very successful results over extended periods in industrial plants. S.P.S. ANDREW: Adhesion of the coating to the metal, other factors being constant would be improved if the metal and the ceramic had the same coefficient of thermal expansion. Have you attempted to develop an alloy haVing such characteristics ? the development G. BUTLER: I agree that this is an interesting idea. However~ of alloys possessing all of the required properties of a metal substrate for ceramic coatings is a complex area. Thus, we have not turned our attention to this specific property to date. M.M. BASIN: How serious a spalling problem have you observed with your ceramiccoated metal monolith catalysts? G. BUTLER : Spalling may be observed on both coated metallic and ceramic honeycomb substrates. From the point of view of car exhaust catalysts the problem is not serious, however, since the catalysts meet the required specifications for activity, etc. throughout their 50.000 mile lifetime.
169
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III e 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in Tbe Netherlands
THE DESIGN OF PORES IN CATALYTIC SUPPORTS. MAGNESIA-ALUMINA-ALUMINUM PHOSPHATE G. MARCEL IN , R. F. VOGEL and W.r., KEHL Gulf Research
&
Development Company, P. O. Drawer 2038, Pittsburgh, PA
15230
(U.S.A.)
ABSTRACT The
surface
area
and pore properties of composites of magnesia-alumina-
aluminum phosphate have been examined as a function of preparation parameters. Sufficiently good correlations were obtained between the composition and the BET surface area, models
could be
average pore size and median pore size, so that empirical constructed
degree of confidence. Other
preparation
to predict these
properties
with a
reasonable
Pore volume was found to be independent of composition.
variables,
such
as
pH and
calcination
temperature,
were
examined and these were found to only slightly influence the characteristics of the finished product.
INTRODUCTION Metal catalysts used in the chemical industry are typically dispersed in a support in order to improve their stability, area,
and
lower
the
cost
of
the
metal.
increase the available surface
The
dispersed
metallic particles
sometimes can interact with the support thereby modifying the reactivity or selectivity
of
the
reaction
of
interest.
For
these
reasons
the
specific
surface properties of the support, and in particular the surface area and pore size distribution, are deemed to be important parameters affecting the overall performance of a catalyst. The use of tailored pore structures for heterogeneous catalysts offers many practical advantages.
If the size of the pores and their distribution can be
controlled, specific catalysts could be designed for specific reactions. example,
diffusion limitations
molecule
reactions,
For
could be minimized in liquid phase or large
selectivity could be
controlled
in demanding reactions
which are dependent on the size of metal crystallites found wi thin the pores of the support,
or even shape-selective catalysis could be mandated through
the design of pores with the correct shape and size.
170 Several investigators have supports.
Manton
and
reported techniques for making controlled pore
Davidtz
(ref.1)
used
tetraalkylammonium
cations
of
various sizes--methyl to butyl--in the preparation of silica-alumina in order to
control
general,
the
the
pore
size
median pore
of
the
resultant
material.
They found
that,
decreased as the size of the cation used for the synthesis increased. Ruckenstein
(ref.
predetermined
2)
and
in
size and the spread in the pore size distribution
were able
Chu and
to prepare and characterize alumina foils
uniform pore
diameter
through
a
technique
involving
of the
controlled anodizing of aluminum sheets. Aluminum phosphate has received only limited attention in the open literature as a catalyst or a catalytic support.
A recent review by Moffat (ref. 3)
discusses the general sUbject of phosphates as catalysts and points out that because
of
the
structural similarity between. silica and aluminum phosphate,
the latter should be useful as a catalytic material.
Alumina-aluminum phos-
phate mixtures have been claimed useful as catalytic supports (refs. 4,
5) and
various patents exist in which said materials, sometimes in combination with silica
when
catalytic
compounded
cracking
published a
with
zeolites,
applications
series
of papers
are
(refs. 6,
found
7).
to
be
highly
Alberolas
in which they discuss
active
and Marinas
in
have
the uti Ii ty of aluminum
phosphate as a support and as a hydrocracking catalyst but provide no systematic study of mixed alumina-aluminum phosphate as a support (refs. 8-11). Kehl recently extended the utility of alumina-aluminum phosphate materials by
incorporating
(ref. 12). nesia, could
alumina, be
a
third
oxide
He demonstrated and
obtained.
that,
aluminum This
work
into
the
composite,
typically
by using mixed materials
phosphate
(MgAAP)
described
the
controlled
preparation
of
magnesia
containing magpore
properties
coprecipi tated
mixtures of the three components with diverse pore structures depending on the exact
stoichiometry
of
the
material.
Subsequent work
in
this
laboratory
demonstrated the utility of these materials in specific applications, such as catalytic cracking and the hydrotreating of heavy feeds We report composi tes
in this
of
paper a
magnesia,
materials.
We have
preparation
parameters
present
some
detailed study on
alumina,
the
(ref. 6). preparation of
and aluminum phosphate as
looked in particular at on
the
physical
empirical correlations
of
the
properties
effect of of
these
ternary
controlled pore composition and composites
the preparation-properties
and
relation-
ships.
EXPERIMENTAL The preparation of these materials was carried out according to the procedure described by Kehl
(ref. 12).
Aluminum nitrate,
magnesium nitrate,
and
171 phosphoric acid were dissolved in a common solution. ammonium hydroxide,
1:1
in
A second solution of
commercial
concentrated
prepared.
The two stock solutions were slowly added to a well-mixed vessel
distilled water,
was
containing distilled water as a stirring medium while maintaining a constant pH,
causing precipitation of a hydroxide-phosphate mixture.
The resultant
slurry was filtered, washed well with water, and the wash water tested for unprecipitated magnesium ions with a solution of ammonium phosphate, dibasic. The filter cake was then dried at 120°C for 24 hr, followed by calcination at 500°C for 10 hr. Surface areas and pore size distribution were determined for each of the calcined materials using BET nitrogen adsorption at 77°K.
X-ray diffraction
of some of the finished supports were measured in either a Picker Model 810 or a Philips APD 3100 diffractometer using CuKa-radiation.
Typically, scans were
taken between 5° and 75°.28 to check for crystallinity of the calcined material.
RESULTS AND DISCUSSION In order to determine the effect of composition on the physical properties of these composites, a series of 30 materials was prepared in which the stoichiometry was varied while maintaining all other variables, such as pH, drying and calcination temperatures, e t c , , constant.
Table 1 gi ves the composition
of nonreplicate preparations of these materials and the resultant surface area and pore size data.
Although the presentation of
the data implies that
magnesium is present only in its oxide form, experimental evidence suggests that an important interaction with phosphate exists.
The qualitative tests of
slurry filtrate showed that Mg 2+ was present for samples where Mg/P > 1. Hence the presence of phosphate affected the extent of Mg precipitation.
Because of
this, only data in which Mg/P " 1 are reported in order to be confident of the stoichiometry of the final material. a
selected
number
of
materials
The X-ray powder diffraction patterns of
expanding
the
range
of
compositions
were
examined and showed the materials to be completely amorphous, indicating the absence of any pure alumina phase.
Their amorphous nature over such a wide
range of compositions, confirms previous claims by Kehl (ref. 4) that alumina and aluminum phosphate form not merely physical mixtures, but rather composites in much the same silica-alumina.
manner as the more
familiar iso-structural analog,
The incorporation of a third component, in this case mag-
nesia, does not destroy this intimacy of mixing but serves to distort the order, causing variations in the physical properties and thermal stability (to be discussed later) of the composite.
172 TABLE Physical properties of magnesia-alumina-aluminum phosphate composites
Stoichiometry MgO Al 20 3 A1P0 4
Sample * 1 3 4 5 6 7 8 10 11 12 15 16 17 18 19 24 25 26 27 28 29
1 1 1 '3 9 3 1 2 3 5 5 3 1 8 3 1 4 1 2 4 3
* = All One
4 8 5 3 2 6 1 7 20 20 8 7 10 3 4 1 1 1 1 13 2
1 1 4 4 9 11 8 4 5 8 7 10 9 9 13 1 4 12 2 10 15
samples were prepared at pH of
the aims of
Median Pore Radius (A)
Pore Volume (cc/g)
81.8 40.8 151.9 190.3 199. f! 177.6 191.9 150.2 60.9 95.4 167.3 177.0 183.0 184.3 198.0 113.6 92.4 186.7 113.4 115.7 157.0
0.83 0.50 0.96 0.88 0.72 0.96 0.61 1.11 0.60 0.90 0.88 0.72 0.83 0.67 0.67 0.84 0.63 0.62 0.61 0.88 0.51
=9
this work was
Average Pore Radius
BET Surface Area (m 2/g)
(A)
54.4 34.0 93.1 134.2 135.3 112.6 101.4 106.4 44.4 67.7 121.8 117.5 131.4 130.2 131.9 80.6 66.9 108.4 69.3 98.4 79.0
304.5 292.7 207.2 130.4 105.7 169.8 120.9 208.0 272.0 267.2 144.2 122.5 126.5 102.7 102.1 209.0 187.1 115.1 174.9 179.6 130.1
followed by calcination at 500°C. to develop materials with predictable
surface properties which could be tailored to particular needs depending on their preparation procedure.
Because of the important role that pore sizes
play in reaction selectivity the tailoring of pore size distributions was of particular interest.
Figures
1 and
2 show the effect that varying the stoi-
chiometry can have on the pore size distributions of the finished material. Figure 1 is designed to show the effect of relative alumina concentration on pore size distribution.
It shows qualitatively that increasing the alumina
content
the
form.
in materials
of
type
under discussion causes
smaller pores
to
As the composition is changed from 1:8:1 to 4:1:4 (MgO:AI 20 3:AIP04 ) the A and a broader
median pore radius is found to increase from 40.8 A to 92.4 distribution with larger pores is obtained.
It is interesting to note that
the low alumina compositions exhibit a slightly bimodal pore size distribution as evidenced by the wave in the cummulative pore volume curve.
This feature
was reproducible for low alumina compositions and may be due to the breakup of the
composite,
thereby
forming discriminate
magnesia,
alumina,
or aluminum
phosphate entities, each with their characteristic pore size distribution.
173
UJ
%
p----------------,
~IOO --J o > w
UJ
a:
15 0..
o
CL
UJ
50
.~
>
I-
I-
:::> ~. :::> o
o
<{ --J
<{ --J
:::> ~ :::>
100
50
250
150
200
250
PORE RADIUS IAJ
Fig. 1. Cumulative pore size as a function of total pore volume for a series of composites of varying stoichiometry, showing the effect of alumina content on the pore size distribution. Molar ratios represented (MgO:Al203:AlP04) are: 0, 1:8:1;0,1:4:1; e, 1:1:1; &,2:1:2; <>, 4: 1: 4.
Fig. 2. Cumulative pore size as a function of total pore volume for a series of composi tes of varying stoichiometry, showing the effect of aluminum phosphate content on the pore size distribution. Molar ratios represented (MgO:Al203:AlP04) are: e, 1: 1: 1; . , 1: 1: 8; 0, 1: 1: 12; 0, 3:4:13.
An increase in the aluminum phosphate concentration caused an increase in
pore size only for a limited range of compositions.
Figure 2 shows that an
increase in pore sizes is indeed observed as the stoichiometry is changed from 1:1:1
to
content
3:4:13, does
but
not
further
cause
any
increase significant
in
the
relative
increase
aluminum phosphate
in either
the
pore
size
distribution or the median pore radius. In order to determine whether in fact any statistically significant correan
arbitrary
parameter which accounted for all three components was defined as
lation
exists
between
composition
and
surface
properties,
(Mg+Al) IP.
Pearson correlations between this parameter and the average physical properties
listed in Table 1--surface area,
median pore -0.73,
and
pore volume,
radius--were calculated and -0.80,
respectively.
With
found the
average pore radius,
to be equal to
exception of pore
0.83,
and
-0.16,
volume,
good
correlations exist between composition and physical properties for the range of catalysts studied. Sufficiently
good
correlations
were
obtained
between
composition
and
surface properties
(excepting pore volume) so that when fitted using linear
models
no
involving
higher
than
quadratic
terms
in
molar
fractions,
174
!:LI
!:LI
u
300
~
~
;::J
en 200 b
!:LI
rn
33 % MgO
en
;::J >---<
0200 ...::r: ~
!:LI ~
0 100 0...
Z
...::r: >---<
0
100
a
!:LI ~
100
67 0/0
[fJ
2
150
pc::
100
33
MgO
a
..
~
pc::
0 0...
50
~
CJ
0
100
~
>
67 100
33 67
33
0
% AIP04
%MgO Fig. 3. Response plots of predicted surface properties in relation to com;; posite stoichiometry. Model used was Property = A + B(\A1203) + C(\A1203) + D(\A1 20 3 x V1g0).
175 statistically significant correlations were
obtained
average pore radius, and median pore radius (ref. 13).
for
the
surface area,
As before, no signifi-
cant correlation was obtained between composition and pore volume. shows
the
predicted
surface-response
plots
obtained using
the
Figure 3
best
linear
model relating composition to each individual surface property.
In all cases
shown, correlation coefficients were in the range of 0.70-0.80.
Values of the
surface properties predicted by the use of the surface-response plots agreed reasonably well with experimental values exhibit a large discrepancy. shown
in
precludes
the
response,
Compositions in which the Mg:P ratio is >1 are
but
their formation.
(:1:15%) but two areas of the surface
in
fact
the
preparation
Composi tions with no
procedure
utilized
magnesia are also in error
since the nondisrupted AAP structure represents a discontinuity in the mathematical equation and, as such, cannot be successfully modeled by a continuous empirical model. Another variable which could affect the physical properties of catalytic supports effect
of
supports
is
the pH during the precipitation step of the preparation.
pH of
on
surface
identical
properties was
stoichiometry
Table 2 shows this dependence. for materials prepared at pH toward larger pores
for
=
probed by preparing a
(4HgO: 13Al203: 10AlP04)
=
9.
of pH.
The pore size distribution tends
a support prepared at pH = 7 resulting in a
TABLE 2 Effect of pH on the pore properties of MgAAP Sample
30
31
32
Stoichiometry, IIIOlar MgO Al 20 3 AlP0 4
4 13 10
4 13 10
4 13 10
9
8
7
115.7 0.88 98.4 179.6
110.0 0.95 76.5 249.6
165.8 0.66 90.9 144.3
6.7 55.9 31.5 4.1 1.6 0 0
6.4 50.6 31.6 4.3 3.7 3.5 0
38.2 35.9 16.4 3.3 3.2 3.0 0.3
Median Pore Radius, A Pore Volume, cc/q Average Pore Radius, A Surface Area, m2/q Distribution, Vol% 200-300 A 100-200 50-100 40-50 30-40 20-30 <20
series
varying
Almost identical distributions are observed 8 and pH
median pore radius and smaller surface area.
pH
at
The
larger
176 The addition of magnesia to the AAP system results Data presented by Kehl
stable material.
in a
less
(ref. 4) demonstrate that AAP is
stable with respect to pore properties up to at least 900°C. representative data for the MgAAP system.
thermally
Table 3 shows
It is clear from Samples 15 and 16
that thermal. stability is maintained up to at least 750°C. loss of surface area can occur after treatment at 900°C.
However, complete
Differential thermal
analysis of MgAAP composites showed endotherms at ca. 200°C, corresponding to loss of water. more
than
A sharp exotherm was observed at 950°C for samples containing
10\ MgO.
This
has
been
attributed
accompanied by the observed loss of porosity. MgO
content,
lacked
both
It appears
collapse.
the
high
that crystal
addi tion of the Mg and a
to
crystal
temperature
exotherm and
growth in MgAAP is
threshold amount is necessary.
MgAAP samples calcined at
growth which
is
Samples such as 17, with low the
structure
facilitated by
the
X-ray patterns of
1000 0C for -2 hr showed that materials containing
more than 10\ MgO readily crystallize into a rudimentary tridymite structure (AIP0 4) and a r-alumina phase. by the same heat treatment.
The low magnesia samples were not crystallized
TABLE 3 Thermal stability of magnesia-alumina-aluminum phosphate Sample Molar Composition, (MgO:AI 20 3:AIP04) Calcination Temperature, °C
500
15
16
17
5:8:7
3: 7: 10
1: 1 0: 9
750
Median Pore Radius, A 167.3 157.2 Pore Volume, ccjg 0.88 0.72 Average Pore Radius, A 121.8 119.0 Surface Area, m2 j g 144.2 121.1 Distribution, Volt 200-300 A 30.6 23.0 100-200 52.8 59.7 50-100 14.3 15.3 40-50 2.3 1.3 30-40 0 0.5 20-30 0 0 <20 0 0
*
900
*
500
750
177.0 190.0 0.72 0.62 117.5 112.4 122.5 111.2 44.4 41.0 11.0 2.1 0.9 0.6 0
900
*
44.6 38.3 12.5 2.2 1.1 1.3 0
500
750
183.0 174.3 0.83 0.76 131.4 112.1 126.5 135.8 40.7 44.3 11.9 2.2 0.8 0 0
40.9 40.6 13.5 2.1 2.0 0.9 0
900 170.0 0.53 91.5 115.2 37.2 40.9 13.4 2.1 2.4 2.7 1.2
Too small to measure by this method.
Clearly, changes
in
many these
variables
can be manipulated which could conceivably cause
materials--drying
treatments,
calcination
time
and
atmo-
sphere, amount of washing, precipitating agents, etc.--to name just a few. hope to investigate some of these effects in future work.
We
177 CONCLUSIONS The
use
of
magnesia-alumina-aluminum
material has been demonstrated.
phosphate
as
a
tailored
support
By varying the stoichiometry of the precur-
sors during the preparation step, a wide range of materials can be prepared wi th predictable surface area
and pore characteristics.
Empirical linear
models were fitted to extensive experimental data and surface-response plots were obtained which could be useful for deciding preparation procedures for specific tailored supports.
The effect of other variables, such as pH, were
also probed and found to be less important in governing the characteristics of the final material.
ACKNOWLEDGEMENTS The authors thank J. A. Tabacek, R. H. Hazlett, and N. A. White for their capable work in preparing tae composites, D. M. Regent for the thermal analyses,
and Gulf Research
&
Development Company for permission to pub Ldsh the
work.
REFERENCES
1 2 3 4 5 6 7 8
9 10 11 12 13
M. R. S. Manton and J. C. Davidtz, J. Catal. , 60 (1979) 156. Y. F. Chu and E. Ruckenstein, J. Catal. , 41 (1976) 384. J. B. Moffat, Catal. Rev.--Sci. Eng., 18(2) (1978) 199. w. L. Kehl, U.S. Patent 4,080,311 (1978). N. L. Cull, U.S. Patent 4,233,184 (1980). H. E. Swift, J. J. Stanulonis, and E. H. Reynolds, U.S. Patent 4,228,036 ( 1980). Japan. Kokai 77 03592 ( 1977). A. Alberola and J. M. Marinas, An. Quim. 65 (1969) 1001. A. Alberola and J. M. Marinas, An. Quim. 65 (1969) 1007. A. Alberola and J. M. Marinas, An. Quim. 67 ( 1971) 37. A. Alberola and J. M. Marinas, An. Quim. 70 (1974) 371. w. L. Kehl, U.S. Patent 4,210,560 (1980 i. SAS User Guide, SAS Institute Inc. , Cary, North Carolina, 1979.
178 DISCUSSION K.S.W. SING Values of pore radius and surface area in tables 1, 2 and 3 are quoted to within 0.1 A and 0.1 m2/g,respectively. In my view this degree of precision is unjustifiable in view of the uncertainties involved in the calculations. G. MARCELIN We agree with Professor Sing that the values quoted for surface area and average pore radius are probably only accurate to two significant figures. M.H. REI What are the acidic properties of MgO-AI203-AI(P04)3 and the effect of MgO on the acidity ? G. MARCELIN The measurement of acidic properties of catalysts is a difficult problem with many differences of opinion among workers in the field. We have attempted to correlate the acidic properties of AAP-type systems by the use of ammonia chemisorption and by observing the activity of these materials towards alcohol dehydration. When we used ammonia chemisorption to measure acidity we found that most AAP-type supports are capable of adsorbing ca. 7 cc/g of NH3 at 175°C, with the only exception being co~positions of high alumina content which exhibited ammonia chemisorptions of about 10-15 cc/g at the same temperature. By comparison a commercial alumina (Filtrol grade 86) is capable of adsorbing ca. 12 cc/g, indicating that within certain compositions AAP is less acidic than even pure alumina. unfortunately we do not have comparable data for magnesia-alumina-aluminum phosphate. In a series of related experiments, using a metal supported on AAP (no MgO) , we observed that alcohols readily dehydrate when the support contains about 80% alumina (4A:IAP). But, as the aluminum phosphate content is increased all activity for dehydration disappears. This agrees, at least qualitatively,with our ammonia chemisorption results, and has been interpreted to mean that moderate or strong acid sites are only found in the high alumina-containing materials. In cases where magnesia is present in the support we have not observed any alcohol dehydration activity, even in high alumina-containing materials, indicating that the magnesia serves to diminish the residual acidity of the support and as such is beneficial when used for the catalysis of labile compounds. The highest alumina stoichiometry tested has been 1 MgO: 8 Al203' 1 AI(P0 4)3' D. SURESH : X-ray spectroscopy is excellent to analyse crystalline materials. Have you used Raman spectroscopy to analyse your non-crystalline material to understand more about the significance of magnesium ? G. MARCELIN :
No, we have not.
J.P. STRINGARO Is the MAAP-support material presented in this paper also shaped i.e. extrudates (pellets) or spheres, saddles etc.? If so, could you indicate the mechanical properties such as crush strength or resistance to abrasion ? G. MARCEL IN We have successfully shaped these materials into 1.5 and 3 mm diameter extrudates using standard techniques. Typical crush strengths of the 1.5 mm materials are in the range of 6 psi. We have no data on the resistance abrasion. N.P. MARTINEZ Due to the presence of Mg and phosphates onto the surface of your support, I assume you certainly change the surface characteristics of the support. How much metal of the Mo and Co type you think you are able to put onto the catalyst by impregnation ?
179 G. MARCELIN: The supports have adsorptive properties typical of aluminabased materials. We have successfully loaded up to 20% metals by conventional impregnation. R.J. BERTOLACINI You have shown the materials to be stable thermally. Are they also stable in the presence of steam? Are they stable once they have corne in contact with metals? G. MARCELIN : A number of u.s. Patents have been recently inssued concerning the use of magnesia-alumina-aluminum phosphate as a matrix for zeolites in cracking catalysts (U.s. Patents No. 4,179,358 and 4,222,896). In this work it was shown that the composites were stable in the presence of steam to 750°C even when synthetically contaminated with vanadium and nickel to metal levels of 5000 ppm nickel eqUivalents. M.M. BHASIN : Were the data used to generate the models (and contour maps) obtained from a orthogonal set or by varying one or two variables at-a-time ? If not, what were the correlation coefficients between main effects and interaction terms? Since correlation coefficients in various terms is ~ 0.8, then one cannot be very certai~ of the models developed containing interaction terms. G. MARCEL IN : The date set was not orthogonal and it exhibited individual correlation coefficients of approximately 0.8. The rationale for inclusion of the cross term in the final model was partially based on our chemical experience with these systems. If we leave out the cross term we obtain significantly poorer correlations. More importantly, we can then make no distinction between the ternary MgAAP system and the binary AAP system. But we know from practical experience that the inclusion of magnesium makes a significant difference in the surface area and pore structure, even when added in small amounts. For these reasons we chose to include the cross term. C.J. ADAMS low temperature
To what extent are the pore properties dependent on the initial drying conditions ?
G. MARCELIN properties.
We have not studied the effect of drying conditions on the pore
C. GUEGUEN How much metal can be accepted by the support in treating heavy feedstocks with high level of metals ? G. MARCELIN supports.
We have not studied heavy feedstock demetallization using these
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G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
181
DISPERSED-METAL/OXIDE CATALYSTS PREPARED BY REDUCTION OF HIGH SURFACE AREA OXIDE SOLID SOLUTIONS J.G. HIGHFIELD, A. BOSSI* and F.S. STONE School of Chemistry, University of Bath, Bath BA2 7AY, England
ABSTRACT Superficial reduction of dilute NixM9l_xo and CoxM9l_xo solid solutions of high specific surface area yields supported Ni and Co catalysts with high metal dispersion. Reduction by bpth H2 and CO are described. With CO the Boudouard reaction intervenes at 500-600 0C, and thus reduction is self-regulating. The oxygen uptake by the reduced solid solutions, studied as a function of temperature, enables exsolved Ni and Co to be discriminated from that present in the matrix. The extent of reduction controls the selectivity in CO hydrogenation.
INTRODUCTION Solid solutions of transition metal ions dispersed in dilute form in insulating oxide solvents [lJ have aroused much interest as catalysts in fundamental studies [2J, and attenti on has recent ly been gi ven to prepari ng and characterizing high surface area (HSA) forms [3,4J. The present paper is concerned with dilute HSA solid solutions as precursors of dispersed metal catalysts. Selective reduction of the transition metal ions can be effected, leaving the solvent to act as a combination of a matrix and a support. Such reduced systems afford a new means to examine metal-support interaction. The reduced state is not necessarily equivalent electronically to that of the zerovalent bulk metal, and may therefore induce novel catalytic activity and selectivity. A further advantage is that the stability of the reduced material towards sintering is likely to be good, and possibly better than in catalysts prepared by reduction of conventional impregnated or ion-exchanged transition metal species. Dilute HSA solid solutions of cobalt and nickel ions in MgO [4J were regarded as well suited for these investigations. We report here the reduction behaviour of CoO-MgO and NiO-MgO, the characterization of the reduced solids by chemisorption, and some preliminary studies of their catalytic activity and selecti vity. *On leave from Montedison Donegani Research Institute, Novara, Italy.
182
EXPERH1ENTAL Hi gh surface area (HSA) soli d solutions of NiO-MgO and CoO-11g0 with solute concentrations of 1, 5 and 10 mol %were prepared atlOOO°C and characterized by the methods previously described [4J. These solids are designated by the nomenclature MN 1, MN 5, MN 10 and MCo 1, MCo 5 and MCo 10, respectively. Reduction, chemisorption and catalytic experiments were conducted in a fixed volume system consisting of a silica reaction vessel, a gas doser, a transducer for pressure measurements, a cold trap, and a sampler bulb for gas analysis. Reductions were effected in situ in HZ or CO (30-40 Torr) at temperatures up Z, to 700 0C. The amount of reductant consumed is expressed in ~mol m- where the surface area (BET NZ' 77 K) is that of the starting solid solution, and also for convenience as 'solute monolayer equivalents' ('sme'). This latter indicates directly the reduction extent, and is de~ined by assuming that HZ and CO reduce by removing the oxygen atom from a solute ion pair (Ni 2+0 Z- or CoZ+O Z-) in the o (100) surface of the MgO structure (ao = 4.Z A). The number of solute (Ni or Co) ions in 1 m2 of such a surface is 1.12 x 1017x , where x is the mol %of solute, and hence 1 sme of HZ or CO is 1.12 x 1017x molecules m-2 (O.186x ~mol m-2 ). Chemisorption and reaction of O2 and N20 were studied by admitting the gases to the reduced solid solution at ZOOC and raising the temperature in a stepwise fashion after certain time intervals. Chemisorption of CO and HZ on the reduced solids was studied similarly. Uptakes are expressed as ~mol m- 2 or as 'sme', the latter making possible immediate comparison with reduction 'sme' values. Catalysis experiments (ethane hydrogenolysis and CO hydrogenation) were conducted as follows. For the CZH6/HZ reaction we used CZH6:HZ = 1:1, T = 300 0C and p(initial) = 10 Torr; the reaction proceeded cleanly to CH 4 and the % CH 4 present after ZO min was taken as a qualitative measure of the rate. CO hydrogenation behaviour was explored similarly in the closed system by selecting as standard conditions T = 250 0C, p(initial) =30 Torr, CO:H 2 =1:3 (except in a few cases) and taking the pressure fall after 15 min as an index of initial rate. Mass spectrometric analysis was used in the reduction, chemisorption and catalysis experiments to ascertain gas compositions whenever necessary. RESULTS I. Reduced NiO-MgO 1. The initial solid solutions. X-ray diffraction showed only reflections due to one rock-salt structure and the lattice parameters of the 1%, 5% and 10% solutions were in agreement with previous work ~4J. The air-exposed samples contained some Ni 3+ in the surface layers as evidenced by their brownish colour and in line with the diffuse reflectance spectra of oxygen-exposed HSA NiO-MgO already reported [4J. The extent of surface oxidation was standardized by heating in 02 until there was no further uptake of oxygen {5500C was typically
183
chosen), cooled in Oz and then evacuated at zooe. Four samples (one for the 1% and 5% solutions, and two for the 10% solution) were the starting materials for the reduced catalysts, and their surface areas were: MN 1, lZ6 mZg- l; MN 5, 139 mZg- l; MN lOa, 88 mZg- l; MN lOb, 95 mZg- l Z. Reduction in hydrogen. Hydrogen was admitted at 35 Torr and zooe and the temperature was raised in stages to 650 0 e . The behaviour of MN 10 and MN 5 was similar. There was a small uptake (0.05 ~mol m- Z) below 300°C, accompanied by a colour change to bright green, but no further uptake until 5000 e, when a step occurred in the HZ consumption. There was a second step at 600-650 0e, as shown in Fig. 1 for MN 5 taken ultimately to 650°C. The stage below 300°C is regarded as reduction of surface Ni 3+ to Ni Z+. The step at about 500°C is of the order of 1 sme, which is 0.93 ~mol m- Z for MN 5 and 1 .86 ~mol m- Z for MN 10, suggesting reduction of a first topmost layer. The next step (up to 6500e) is of similar size, indicating reductio~into the next layer. The HzO product was mostly retained by the solid, probably by hydroxylation of MgZ+_O Z- surface ion pairs. At any given temperature, MN 5 was reduced more slowly than MN 10. This trend extended noticeably to MN 1, which did not reduce at all at 6000e. 3. Reduction in carbon monoxide. The same procedure was followed as with HZ' MN 10 gave a small uptake below 3000e and a colour change to green, as with HZ' However, the next reduction stage began at about 4000e, lower than with HZ' The temperature was taken to 600°C, and examination of the contents of the cold trap showed that eoz had been condensed equal in amount to the CO consumed. MN 5 behaved similarly to MN 10 at 550-6000e, with e02 again being recovered stoichiometrically. However, at 400-5000e, CO gave the surprising result of an increased rate of consumption, and the rate did not decrease with time when the 0e). temperature was held at 4700e (in contrast to the behaviour when held at 580 0e. 0 In fact, more CO was consumed in 10 h at 470 e than in 10 h at 580 The paradox was resolved by examining the trap, which showed that the e02 condensed was now less than stoichiometric. The results are consistent with the postulate that Reaction (1), which is occurring at temperatures above 4000e (albeit to a (1 )
limited extent at the surface), is the sole reaction at 5800 e, but that at 470 0e it is accompanied by Reaction (Z), namely the Boudouard reaction, catalyzed by
Z CO
~
, e + eoz
(Z)
the reduced species (written here as Nio) formed in Reaction (1). MN 1 showed no reduction below 6500 e (as with HZ)' but slight reduction was possible at 700 0e. When the solid so treated was cooled to 5000e, eo uptake was found, a result again explained by the sequence of Reactions (1) and (2).
184
4. Chemisorption characterization of reduced NiO-MgO. The solid solutions were reduced to different extents by control of the variables described above (nature of reducing gas, temperature, time). The reduced solids were next subjected to chemisorption studies with 02 (and for MN 5 also with N20, CO and H2) to characterize the chemistry of their surfaces. The reduction extent was always very small (equivalent to a few layers at most, and usually much less). Chemisorption and oxidation results will now be described, including some studies on solids subjected to reduction-oxidation-reduction-oxidation cycles. Catalytic experiments on the same solids are reported in a later section. MN 10. Five experiments (A-E) corresponding to five different reduction conditions will be described. Oxygen chemisorption was first studied at 20°C: the temperature was then raised in stages and the increase in uptake monitored up to 600°C. The chemisorption is irreversible, ~nd as the temperature is raised 'chemisorption' becomes increasingly synonymous with 'oxidation'. There is an extremely small chemisorption of oxygen on the unreduced matrix [lJ, but for the present purposes this component can be regarded as negligible. 2 550
650 ~
e
1.5
630
w
o
3-
520
51.0 615
ii: c
~O.5
c
1:[
~ r
-300'C
600 560
•
II
I
r
N
Temperature
Fig. 1. Reduction solid solution in strong conditions
of NiO.OSM90.9S0 (MN S) H2 under increasingly (T in °c on curve).
Fig. 2. Oxygen uptake by a superficially reduced solid solution (schematic).
Oxygen uptake showed the three-stage behaviour illustrated in Fig. 5. The temperatures at which the plateaux start in II and III are not always the same (see details below). but typical values would be 300 0C and 600°C, respectively. The essence of the experiments was to measure the magnitudes of the respective uptakes I, II and III when the preceding reduction conditions were changed. Thus: A. Gentle reduction in hydrogen (2 h at 590 0C). The limiting uptake (I) at 20 0 e ~nd 20 Torr in the subsequent O2 experiment was 0.058 ~mol m- 2• hereafter called V(A-20). Temperature was raised and a further limiting uptake at a total of 0.183 ~mol m- 2 (I + II + III) occurred at 580 0 e, designated as V(A-S80).
185
B. Gentle reduction in CO (1.5 h at 550°C). The limiting uptake at 20°C -2 [V(B-20)J in 20 Torr O2 was only 0.021 vmo1 m ,even though the extent of prereduction, due to the favoured reaction with CO, was 1.9 times that in A. I. Strong reduction in H2 (1.5 h at 645°C). This produced reduction 1.4 times that in A. V(C-20) was 0.075 vmol m- 2, and V(C-575) was 0.270 vmol m- 2 . Q. Strong reduction in CO (2 h at 620 oC). The extent of reduction was 2.5 times that in B (and 3.4 that in C). V(D-20) was 0.069 vmo1 m- 2, proportionately much less than V{C-20). On raising the temperature in stages a second plateau (Fig. 2, I + II) was discerned at 285°C with V(D-285) =0.21 umol m- 2 and a third plateau (Fig. 2, I + II + III) at 645°C with V(D-645) =0.40 umo l m- 2. I~ Long-term, low-temperature reduction in H (16 h at 530 oC). The oxygen 2 results were 1= V(E-20) =0.095 umo l m- 2, and I + II = V(E-300) =0.224 umol M- 2 . In experiments A and C the total oxygen uptake (I + II + III) at the highest to half the amount of the hydrogen which had been temperature studied was eq~a1 consumed in the preceding reduction. Thus the nickel was back to its initial oxidation state. Re-reduction in H2 reproduced the reduction behaviour described in Section 1.2 for virgin MN 10, indicating that the oxidation at 580°C had satisfactorily re-formed the solid solution, completing a cycle. MN 5. Oxygen uptake after H reduction (600°C, 6 h, consumption 0.5 vmo1 m- 2) 2 behaved as in Fig. 2, and the limits and uptakes (vmol m- 2) were I (20oC)=0.06, II (270°C) = 0.15 and III (420°C) =0.24. N20, CO and H2 chemisorption were also studied, each after a separate identical reduction (H 2, 600°C, 6 h). N20 at 200C (15 Torr) decomposed to release N2 and leave the oxygen chemisorbed: the reaction arrested sharply when 0.14 vmol N2 m- 2 had been released. The chemisorbed oxygen equivalent (0.07 vmol m- 2) agrees quite well with the I (20°C) limit found with O2 itself. With CO (20 Torr CO), the adsorbed amount at 20°C included adsorption on the unreduced surface (t1g0 blank): when allowance was made for this the adsorption ascribable to the reduced solute was 0.13 vmo1 m- 2. Assuming linearly-bonded CO (1 CO per site) this is equivalent to 0.065 vmol m- 2 of O2, again agreeing with I (20oC). The H2 chemisorption result (25 Torr) gave no such correlation at 20°C: the uptake was only 0.017 vmol m- 2 of H The chemisorption has an 2. activated character, since it rose to 0.042 vmo1 m- 2 at 170°C, and to a steady value of 0.066 vmol m- 2 at 450°C. This last value, however, correlates with I (20°C) for O2, MN 1. After reduction in H for 7 h at 700°C and consumption of 0.082 vmol m- 2 2 (i. e. 0.44 sme), an oxygen experiment gave I (20°C) =O.014 umo l m-2. Oxygen uptakes may also be expressed in sme, provided account is taken of the fact that the reaction is di ssoci ati ve. Thus, in the above case (MN 1, where x = 1), 0.014 umo] m- 2 of oxygen uptake is equivalent to (2 x 0.014)/0.186=0.15 sme. -
186
5. Catalytic properties of reduced NiO-MgO. The reduced solids were active in ethane hydrogenolysis and CO hydrogenation. Table 1 shows how increasing the severity of reduction conditions for MN 10 (sequence A -+ E above) affects the activity and selectivity. CO/H rates are expressed as fractions of that for the 2 most active catalyst, viz. E (H 2-reduced, 5300C, 16 h), for which AP = 16 Torr in 15 min, the initial pressure being 30 Torr (CO:H 2 = 1:3). Data for reduced r~N 5 (H 6000C, 6 h) and MN 1 (H 2, 700 0C, 7 h) are also reported. 2, TABLE 1 Catalytic behaviour of reduced NiO-MgO solid solutions Catalyst Ethane CO hydrogenation hydroInitial Conversionb Products (mol %) genolysis rate activitya .CH 4 C2fl4 C2H6 C3H6 (relative) ~jN 10 A 50 0.47 69 (1.0 h) 93.5 0.6 4.0 0.3 B 44 0.08 47 (2.5 h) 52.5 4.0 11.0 4.0 0.90 C 55 71 (1.0 h) 98.6 0.3 1.0 D 55 0.40 64 (1.5 h) 94.0 1.4 3.8 0.2 E 40 1.00 59 (1.8 h) 93.8 1.5 2.9 0.7 MN 5 50 0.90 78 (2.0 h) 98.5 0.25 0.65 0.02 MN 1 5 0.10 40 (4.0 h) 94.2 5.6
C3H8 EC 4 CO 2 l.Z O.Z 0.2 4.0 1.0 23.5 0.1 0.2 0.4 1.1 0.03 - 0.04 0.2
aActivity expressed as %CH 4 in C2H6/H2/CH4 mixture after 20 min at 300 0C. bConversion of CO into recoverable C-containing products, expressed as percentage of initial CO, during hydrogenation for period in brackets. II.
Reduced CoO-MgO 1. The initial solid solutions. X-ray diffraction showed only one phase and ao agreed with earlier work [4J. C0 3+ was formed in surface layers on exposure to oxygen (cf. [4J), and was standardized in amount as for MN. C0 3+ was more prevalent than the corresponding Ni 3+ formed with MN. Five samples (one of the 1% solution and two each of the 5% and 10% solutions) were used for reductions, and their surface areas were: MCo 1, 71 m2g-1; Meo 5a, 79 mZg- l j MCo 5b, 65m 2g-1; MCo lOa, 30 m2g-1; MCo lOb, 34 m2g-1 2. Reduction in hydrogen. The same procedure as for r·1N was fo11 owed, and results for ~1Co 10 are shown in'Fig. 3. There is a 20-fold larger initial step than with MN 10, the brown colour of the starting material becoming lilac (c0 3+ -+ C0 2+) , but reduction of the divalent solute is more difficult than with MN. This was accentuated with MCo 5, which after an initial step of 0.7 ~mol m- Z . (Co 3+ -+ Co 2+ ) showed no further reductlon, even at 6400 C. Further uptake of HZ by MCa 5 could, however, be stimulated by treatment at high temperature in CO. 3. Reduction in carbon monoxide. Results are shown in Fig. 4 for MCo 10, and MCo 5 was similar. The marked uptake at 200C produced no colour change, but the lilac colour developed at 250 0C without further uptake. Reduction of Co Z+ began
187
620
620
2
7.5
535
620
E
"0 3 E
620
l
65
.' 620
62
~
S
00
625 590
~2
IN
625
515
260
540
500
00 260
10
20
30
2.5
40
Time/hours
Fig. 3. Reduction of CoO.1M90.gO (MCo 10) in hydrogen. Numbers at points indicate temperature in °c.
5
7.5
Time/hours
10
Fig. 4. ~eduction of CoO.1M90.gO (MCo 10) in CO. Numbers at points indicate temperature in °c.
at 460 0C (SOOoC for MCo 5). A striking subsequent feature is the steady uptake at 500-515 0C (Fig. 4), which was accompanied by CO 2 condensation in the trap commensurate not with reduction but with the stoichiometry of Reaction (2). At 590°C this gave way to some genuine reduction, but this petered out after 1.0 sme, even at 625°C. When the te~perature was lowered, the Boudouard reaction reasserted itself, and a rapid and sustained CO loss was observed at 500-550 0C. To test that Reaction (2) was occurring, the residual CO gas was evacuated, the temperature was raised to 600°C and the system was kept closed. Pressure gradually developed, and mass spectrometric analysis proved that CO was being evolved. The deposited carbon is evidently capable of reducing the solid solution. A vacuum anneal at 640°C enabled all deposited C to be recovered as CO. MCo 1 could be reduced only vestigially, and at a very slow rate. 4. Chemisorption characterization of reduced CoO-MgO. Oxygen again shows a 3-stage behaviour (Fig. 2), but there are differences between reduced MCo and reduced ~IN. For MCo 10 reduced in H2 (620°C, 30 h, consumption =3.0 umo l m- 2, i.e. 1.6 sme), the oxygen uptakes were I (20 C) = 0.4 umo l m-2 , I + II (160 C) 2, and I + II + III (380°C) = 1.45 umo l m- 2, i.e. 1.56 sme. Thus = 0.75 umo l mreoxidation could .be completed at 380°C. The resulting solid was not a solid solution, since-it could be easily reduced (H 2, 470°C, 1 h), with subsequent O2 uptake as before [I + I I + I I I (400°C) = 1.5 sme l . When, however, the re-oxi di sed solid was vacuum-annealed at 570°C (4 h), re-reduction in H2 at 470°C became very slight (0.2 sme): this is because the high-temperature conditions (570°C) were leading towards regeneration of the original solid solution. For MCo 10 reduced in CO (630°C, 4 h, consumption 1.3 sme) and vacuum-annealed (660°C, 2 h) to remove any deposited carbon, subsequent O2 chemisorption gave I (20°C) =0.28 sme , I + II (190°C) =0.62 sme and I + II + III (300°C) = 1.12 sme.
°
°
188
Long-term reduction in CO, however, yielded a proportionately low value of I (ZO°C). Thus, MCo 5 reduced in CO (630°C, 40 h, reduction degree =4.0 sme) yi e1ded I (ZOoC) =O. Z sme. Nevertheless, when thi s oxygen-chemi sorbed MCo 5 was re-reduced in H2 (620°C, 2 h, extra reduction degree=0.7 sme), the subsequent Oz chemisorption gave I (20°C) =0.6 sme. The low I (ZO°C) after the CO reduction is thus not due to exsolved Co having sintered, but to a high proportion of its reduced species being inaccessible to oxygen at ZOoC for some other reason. In contrast to MN, N20 chemisorption was not suitable for measuring reduced cobalt. HZ-reduced MCo 10 (6Z00C, 30 h, consumption 1.6 sme) yielded only I (20 C) =0.2 sme and I + II (85 C) =0.35 sme; above 1300 C N20 decomposed catalytically and no further plateau could be reached. As regards CO and HZ chemisorption, exposure of reduced MCo to these gases at ZOoC failed to give significant chemisorption, although both gases showed uptakes at higher temperatures (CO at T > ZOOoc, but with Boudouard reaction; H2 at T > Z700C, ascribed to activated chemisorption). 5. Catalytic properties of reduced CoO-MgO. Reduced MCo was active for ethane hydrogenolysis and CO hydrogenation. Table 2 shows results on CO hydrogenation for MCo 10 taken through sequences of treatments. Thus in A a sample was first subjected to mild reduction in H2 at 500°C for 1 h, cooled to Z500C and tested
°
.
°
TABLE 2 Catalytic behaviour of reduced CoO.1M90.gO solid solutions in CO hydrogenation Gas preReaction Initial Conversion Products (mol %) treatment condit. rate % (time,h) :...CH....:...;.;;;.;..C:...H~..:C-H:....:...-C-H------T(oC)/t(h) (rel.) 4 24 2 6 HZ/5001l HZ/630/4 A vac/670/3 vac/7l01l6 } HZ/72512 HZ/730/5 B [H Z/610/5
~CO/515/4
C CO/630/4 } vac/660/Z
a b c a
0.11 0.07 0.09 0.30 0.05 0.09 0.30
18(lh) 15(5h) lZ(6h) 80(4h) 30(6h) 70(5h) 40(1.5h)
86 75 51 87 74 42 85
a
0.90
80(l.5h)
95
a
1.00
a a a
d
e
7 8 10 Z 2
Z
3 6 C3H8 zC 4 CO Z 4 7 0.5 0.5 6 1.5 1 8 4 3.5 Z.5 1.5
1 1.5 1
3
31 2 2 44 0.5
3.5
0.5 6 5 2 6.5
0.1
1.1
3.5
75(1h)
97.5 0.7
l.Z
0.5
0.10
40(3.5h)
55
0.01 0.70
9(Zh) 65(1 h)
~45
99.5
8
16
10
23
lZ.5 0.5
2
14
5 11.5 Z.O
a - Standard conditions [CO:H Z =1:3; T = Z50 0C; P(initial) = 30 Torr], b - CO:H Z = 1: 1; c - P(initial) = 140 Torr; d - T= ZOOoC; e - T= 300°C.
2.5
3.5
189
for CO hydrogenation in three different reaction conditions (a,b,c). The same solid was then given a stronger reducing treatment (630°C for 4 h), followed by further tests. The sequence in A is that of increasingly strong reduction treatments in hydrogen. B is an isolated experiment with a newly prepared sample aimed at confirming the result for corresponding conditions in the A sequence. C shows the effect on catalytic behaviour of reducing in CO. DISCUSSION The results show that incorporation of nickel and cobalt ions in solid solution in MgO is an effective way to control their reducibility. Ni 3+ and C0 3+ for.med superficially during oxygen contact are reduced below 300°C, but the divalent Ni 2+ and C0 2+ ions resist reduction strongly. Reduction is limited to only the first few layers even at 600-650 0C. Figs. 1 and 3 show that the actual amount is equivalent to less than two complete solute monolayers of Ni 2+ or C0 2+ being reduced to the z~rovalent state. These are indeed conditions which should favour preservation of the reduced ions in a finely-divided state. The catalytic activity developed for ethane hydrogenolysis and CO hydrogenation (which require ensembles of metal atoms) shows that metal particles have been produced, so the following processes must have occurred during the reduco( . t ion: (a) x Nl.3+(or Co 3+) -.- x N1o2+( or C02+) ; (b) y N·1 2+( or C0 2+) -.- Y N°1 or Co). 0 , (c) z Nio(or Coo) -rNi(or Co) particles. Fig. 5 illustrates a model for the initial and the reduced state. Taking MN being reduced by H2 as the example (the same will hold, mutatis mutandis, for MCo, and also for CO reduction if temperature is high), process (b) comprises: ... 0
2-
Mg
2+ 2- 02+ 2- 2+ 0 Nl 0 r~g ••• + H 2
- + .•• 0
2- Mg 2+OH - Ni 001-1 - Mg 2+
The model presumes that outermost Ni o will nucleate to form metal particles, but some (that which lies deeper) will remain isolated in the matrix, either as Ni o or as a charged species in a low oxidation state. The decrease in matrix volume I(surface metal atomsl II (interior metal atoms)
~\
.. 1.'. Initial state of solute ions
o •
Reduced state
Fig. 5. Model for the initial state (left) and reduced state (right) of NiO-MgO and CoO-MgO solid solutions. Only the Ni (or Co) species are indicated: ions of the solvent MgO are omitted. The reduced state illustrated is that after reduction of a few solute monolayers, with a metal particle supported on the oxide.
190
shown in Fig. 5 takes account of the fact that some actual extraction of oxygen 2- + CO as H20 (or CO 2) will also ensue [20H - + 2- + H20 (or C0 23 + 2)J. Reduction of divalent solute by CO has the special feature that the threshold temperature is.low enough (450-500°C) for processes (b) and (c) to be rapidly replaced by the Boudouard reaction (for the MCo case, see Fig. 4). Thus the reduction is self-regulating. The reduced species should accordingly remain highly dispersed, albeit with the complication that carbon is simultaneously deposited. By about 600 0C, however, the Soudouard reaction no longer dominates the action of CO: this is because Reaction (2) is an equilibrium which between 500° and 600°C becomes increasingly favourable for the back reaction, especially for carbon in contact with finely-divided metal [5J. Genuine reduction is then resumed. There is evidence, however, that the mechanism (at least near the 'take-over' temperature) is: 2CO + C + CO 2; C + 02- + CO + 2e; 2e + Ni 2+(or C0 2+) + Nio(or Coo). Direct reductive action (cf. MN and H2 above) will occur at higher temperature. The chemisorption results are notable for the 3-stage nature of the uptake of oxygen (Fig. 2). For MN, the 20°C stage (I) agrees in extent with that with N20 and with CO chemisorption (see data for MN 5), suggesting that only surface metal atoms are involved. The limiting uptake for oxygen on bulk nickel and cobalt at room temperature is generally held to be 2-3 monolayers [6J, but for systems (such as the present case) where heat dissipation is good the value could be lower [7J. The proposition that stage I measures surface metal atoms on particles (albeit with some possible attack beyond the outermost layer) leads to a ready explanation for plateaux II and III in Fig. 2, namely that II represents oxidation of the interior of the particles and III describes the more difficult oxidation of matrix-isolated species (Fig. 5). It follows that the ratio 1/(1 + II) is a measure of the metal dispersion in the reduced solid solution. Thus for D and E in the MN results, dispersions (maximum values) are 0.33 and 0.42, respectively. For hydrogen-reduced MCo 10, the dispersion (assuming I is one monolayer) is 0.4/0.75 =0.53. However, the lack of agreement between the N20 chemisorption and oxygen uptake suggests that there is incorporation with oxygen at 200C (i.e., I is more than 1 monolayer), so the true dispersion will be rather lower. The low I (200C) value for 10ngterm CO-reduced MCo 5 is thought to be due to carbon atoms produced by CO disproportionation having diffused into the oxide sub-surface, yielding Coo atoms at depths from which they do not easily exsolve to give Co particles. Reoxidation at 300-400 0C which yields oxide particles of NiO, CoO (or C0 304) supported on MgO can be readily distinguished from reoxidation at 6000C which re-forms the solid solution by the behaviour on re-reduction in H2: the former exhibits rapid reduction below 5000C, whilst the latter gives the same profile as Fig. 1 or Fig. 3.
°
°
191
The catalysis results (Tables 1 and 2) show that higher hydrocarbons than CH 4 are formed under certain conditions, in agreement with the results of Vannice for Ni/A1 203 and Co/A1 203 [8J, and especially Doesburg et a1. [9J for Ni/A1 203. The MgO solid solutions studied here show very clearly that the manner and extent of reduction have important consequences for the activity and selectivity in CO hydrogenation. The gentle self-regulating reduction at 500-550 0C in CO (see B in Table 1 and C in Table 2) yields a catalyst of low activity but high selectivity towards C2 and C hydrocarbons. Strong reduction, on the other hand, 3 favours methanation. The same trend can also be seen in the case of the hydrogen reductions. The less severe the reduction conditions, the greater the incidence of C2-C4 hydrocarbons in the subsequent CO hydrogenations. The chemisorption data confirm that the reduced catalysts have high dispersion, notwithstanding the uncertainties about the absolute significance of I (200C), and the gentler the reduction the higher is the dispersion [cf. I (200C) for A and C, respectively, in the MN 10 seriesJ. The catalytic results for CO hydrogenation are therefore consistent with greater departure from methanation behaviour the greater is the dispersion. However, the effect is not necessarily a particle size effect per se. The reduced solid solution provides a strongly ionic environment at the particle-support interface and even more so for any reduced species which are at the surface and not fully exso1ved. Partially-ionized (electron-deficient) Ni and Co may be the source of the departure from methanation behaviour. This is similar to the conclusion reached by Doesburg et al. [9] for CO hydrogenation on Ni/alumina. REFERENCES 1 A. Cimino, M. Schiavello and F.S. Stone, Discussions Faraday Soc., 41(1966)350. 2 J.C. Vickerman, in Catalysis, Vol. 2, p.107, Spec. Per. Rep., Chemical Society, London, 1978. 3 A.P. Hagan, C. Otero Arean and F.S. Stone, Proc. 8th Int. Symp. Reactivity of Solids, Gothenburg, 1976, Plenum Press, New York, 1977, p.69. 4 A.P. Hagan, M.G. Lofthouse, F.S. Stone and M.A. Trevethan, in Preparation of Catalysts, II, Elsevier, Amsterdam, 1979, p.417. 5 J.R. Rostrup-Nie1sen, Steam Reforming Catalysts, Teknisk For1ag A/S, Copenhagen, 1975. 6 M.W. Roberts and C.S. McKee, Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford, 1978. 7 R.M. Dell, D.F. K1emperer and F.S. Stone, J. Phys. Chern., 60(1956)1586. 8 M.A. Vannice, J. Cata1., 37(1975)449. 9 E.8.M. Doesburg, S. Orr, J.R.H. Ross and L.L. van Reijen, J.C.S. Chern. Commun., (1977)734.
192 DISCUSSION J.H.R. ROSS What happens to the carbon deposited in the Boudouard reaction? Does it not have an effect on the catalytic behaviour of the reduced catalyst ? F.S. STONE .: The amount of carbon deposited by the Boudouard reaction under our conditions is very small, of the order of 10 18 atoms m- 2. As such, it is likely to be reactive towards hydrogen, so I am not inclined to think it impairs the activity of the reduced catalyst. J.W. GEUS: Could you digress on the mechanism of the reduction of the nickel or cobalt ions inside the support? Is the reduction proceeding by migration of hydrogen or carbon monoxide to the ions and carbon dioxide or water from the reduced specie? or are you considering an indirect reduction process ? F.S. STONE: I believe the reduction process for internal transition metal ions is indirect. It is not necessary for H2 or CO to migrate to the ions. It is sufficient that they convert surface oxide ions to OH- or CO~(H2 + 20 2- ~ 20H- + 2e; CO + 20 2- ~ CO~+ 2e); the electrons thereby freed then migrate by a hopping mechanism to the ions and reduce them. S.P.S. ANDREW: In the reduction of ammonia synthesis catalyst there is a pronounced effect of the partial pressure of water vapour during reduction with hydrogen on the structure. The catalyst becomes coarser as pH20/pH2 increases. Did you notice any such effect ? F.S. STONE: We do not observe such an effect. However, this could well be because we are carrying out so little reduction that the partial pressure of water vapour present in the hydrogen is always insignificant. Indeed, most of the reacted hydrogen can be contained as OH- on the magnesia surface in the case of our dilute solutions. If, on the other hand, one were to reduce a concentrated solid solution, then I would think it quite likely that the effect you describe would result. J.W.E. COENEN: In answer to a question from Professor Ross, you expressed the opinion that carbon left behind by the Boudouard reaction in CO-reduction was rather innocuous. However, I note that in table 1 of the paper, samples A, B, C, D, E of MN 10 show alternating activity behaviour and I find that the low activities do belong to samples reduced by CO. Doesn't that indicate that the carbon blocks sites which otherwise would have been active in methanation ? F.S. STONE: It is true that one could construe from Table that low methanation activity is associated with CO reduction (experiments B and D). However, one should note that gentle hydrogen reductions (experiments A dn E) also give low methanation activity relative to experiment C. Thus it does not follow that deposited carbon is responsible, and it will be necessary to do more experiments to decide the matter.
193
G. Poncelet, P. Grange and P.A. Jacob. (Editors), Preparation orCata/yot.III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION OF MONODISPERSED NICKEL BORIDE CATALYSTS USING REVERSED MICELLAR SYSTEMS
J.B. NAGY, A. GOURGUE and E.G. DEROUANE Facultes Universitaires de Namur, Laboratoire de Catalyse, 61, rue de Bruxelles, B-5000 Namur, Belgium
ABSTRACT The reduction by NaBH of Ni(II) cation dissolved in the aqueous disconti4 nuous phase of reversed micelles provides a convenient way to produce nickel boride particles of predetermined size and size distribution, given a welldefined micellar composition and concentration of the reducing agent. obtained
The so-
particles normally in the colloidal state, can be deposited on a sup-
port if needed.
Such catalysts are nearly monodispersed and show an activity
apparently superior to that of Raney nickel in the hydrogenation of n-1-heptene.
INTRODUCTION Nickel-based catalysts, due to their low cost and high activity, are widely applied in the field of organic reductions. Among these catalysts, the so-called "nickel borides" obtained by reduction of nickel salts by sodium or potassium borohydride have been known for a long time (refs. 1,Z).
With respect to Raney
nickel, they present some advantages like better selectivity and fatigue resistance (refs. 3-6).
Their initial preparation method has been improved by chan-
ging the reaction medium.
More active catalysts may be prepared using ethanol
rather than water as solvent (refs. 7,8).
Such new catalysts show an activity
in the hydrogenation of olefins which is close to that of Raney nickel.
More
recently colloidal nickel boride particles have been obtained in ethanol using polymers as stabilizing agents (refs. 9,10).
We describe in this paper an at-
tractive new preparation method, using a "cage effect" in reversed micellar systerns, thereby controlling the size and size distribution of the catalyst particles.
EXPERIMENTAL An excess of NaBH solution is added dropwise, under N flow and vigorous Z 4 stirring at QOC,to the degassed micellar solution containing the NiCI salt. Z
194 At the end of the reaction, the temperature is raised up to room temperature until complete hydrolysis of the excess NaBH occurs. 4 The composition of the nickel boride particles was determined by chemical analysis (complexometric titration for Ni (ref. 11) and proton induced y-ray emission for boron content (ref. 12». The size of the catalyst particles, after deposition on silica, was measured using a Philips EM 301 electron microscope in the transmission mode (ref. 13). The method of Peret was used to obtain the mean-diameter of the particles (ref. 14) • The size of the micellar droplets was calculated from the 19F-chemical shift variations (ref. 15).
The 19 p_ NMR measurements were carried out at room tempe-
rature with a Bruker CXP-200 spectrometer working in the Fourier-transform mode at 188 MHz. The n-l-heptene hydrogenation catalytic tests were performed under continuous stirring in a static system by following volumetrically the consumption of hydrogen.
RESULTS AND DISCUSSION We will first discuss the factors influencing the size of the catalyst particles, such as the micellar composition and the nickel (II) and NaBH concen4 trations. Then, we will focus on the catalytic activity of the nickel boride particles. Size of the nickel boride particles The catalysts systems were prepared by reduction of NiCl CTAB-n hexanol-water reversed micellar system.
2
with NaBH
in the 4 ratio was held
The NaBH 4/NiCl 2 equal to 3, because larger particles were obtained for a lower value, the particle size remaining constant above that ratio.
The final composition of the
nickel boride particles is NixB with x close to the literature value of 2 (ref. 16). The weight-percent compositions of the different reversed micellar systems used in this study are listed in Table 1.
TABLE 1 Composition of the reversed micellar systems (wt.%) component Water n-Hexanol CTAB
Composition (wt. %) 4.0 90.0 6.0
8.0 80.0 12.0
12.0 70.0 18.0
16.0 60.0 24.0
20.0 50.0 30.0
Figures la and Ib show the variation of the mean radius of the water core as a function of water content or of the Ni(II) ion concentration respectively. The higher the water content,the greater are the dimensions of the inner water core.
195
12
18
Water (weight %): 15.8 12.6
8
...:I 4
o
rM=3.6+0.52 (% water) (R-O.9997)
10
6;-..........--.-..,..-..........--.-..,..--r--. 4
8 12 Water (weight %)
16
o
0.2 [Ni++]
0.4
0.6
0.8
(molal/water>
Fig. 1. Variation of the average radius (r of the water core as a function M) (a) of water content and (b)'of the Ni(II) ion concentration.
Figure 2 shows the dependence of the nickel boride particles size on the water content in the reversed micelle and on the Ni(II) ion concentration.
The
average size of the particles decreases with decreasing size of the inner water core (decreasing water content), while a complex behaviour is observed as a function of the Ni(II) ion concentration: a minimum is detected at an approxi2 mately 5 x 10- molal concentration. All these observations can be rationalized if one analyzes the nucleation process.
The number of nuclei (N ) formed from the reversed micellar water con
res is proportional to the number of these water cores (N It depends also M). on the integral of the gaussian distribution function (G) describing the number of Ni(II) ions per micelle (average value of N with a lower limit equal to A), the critical number of Ni(II) ions per micelle (n necessary for the formation c) of a stable nucleus. Finally, it is also proportional to an efficiency factor (Fe)' taking into account the ability of NaBH diffusion in sOlution with res4 pect to the rate of rearrangement of the micellar droplets :
196
Fig. 2. Variation of the average diameter (d in A) of the nickel boride catalyst particles as a function of water content and Ni(II} ion molal concentration The number of nuclei (N is computed from the total weight of NiCl intro2 n) duced in the reaction mixture and from the average weight of nickel boride particles determined from their size (electron microscopy), density (ref. 17) and -2 composition. From about a 4 x 10 molal NiCl concentration, the number of 2 water cores (N is independent of either the micellar composition (water conM) tent) or the total NiCl concentration (Fig. 3). For a constant NiCl concen2 2 tration, the average number of Ni(II) ions per micelle (N is a constant, henA) ce also the integral of the gaussian function G (N Therefore, the only facA). tor to be considered in this particular state is the efficiency factor (Fe). The latter factor is determined by the rate of diffusion of the reducing agent NaBH ~ncreasing with NaBH concentration and with increasing lability of the 4 4 interface) and the rate of rearrangement of the micellar droplets (which also increases with increasing lability of the interface).
The lability of the in-
terface has two opposing effects on the nucleation process,cancelling each others as a first approximation. tion effects of
NaBH~.
Therefore, we can only retain the concentra-
o
4 8 12 [NiH] x1 0 2 (molal)
16
Fig. 3. Dependence of the number of micelles per gram of the reversed micellar system (N on the Ni(II) ion concentration at different water contents. M) The lower the water content iS,the higher the local concentration of the reducing agent is when reduction occurs in the micellar system.Hence, the linear decrease of the number of nuclei per micelle (Nn!N with the water content (or M) 2 = 4 x 10- molal (Fig. 4a).
size of the water core) at [Ni(II)] The influence of the NiCl sigmoid curve (Fig. 4b).
concentration on the Nn!N ratio appears as a Z M In this case, both the integral of the gaussian dis-
tribution function
G(N and the efficiency factor Fe vary. The critical conA) centration of Ni(II) ion per micelle (n remains probably constant, but the c) average number of Ni(II) ions per micelle (N increases quasi-linearly with the A) total NiCl concentration (due to the constant value of N in Fig. 3) displaM 2 cing the maximum of the gaussian function toward higher N values. Hence, the A integral of the gaussian function itself increases following a sigmoid curve. In addition, the efficiency factor increases with increasing NiCl tion, because of the constant NaBH
4!NiCI 2
ratio.
Z
concentra-
198
15 '
...
' ... ,
10 5 ' ...
.....
'
...
~
0
x
..... ~
25
z "c:: z 20
10
5
0
15
20
25
Water (weight %)
water
20%
15 10 5
o
5
10
15
[Ni++] X 10 2 (molal)
Fig. 4. variation of the number of nuclei (N per micelle (N as a function n) M) (a) of water content and (h) of Ni(II) ion concentration.
The factors influencing the nucleation process can therefore adequately plain the experimental
data.~e
larger the amount of the nucleation centers,the
smaller the average size of the particles is (Fig.2,~nfluence concentration).
At low
ex-
of the water
NiCl
concentration, the nucleation centers N (only n 2 in a small amount) lead to rather large particles. At the optimum concentra-
tion (inflexion point in the Nn/N
a Ni(II) curve), the size reaches a minimum. M At higher concentration, because of the lower rate of increase in the Nn/N M function, the average size of the particles increases again with increasing NiCl
2
concentration (Fig. 2).
199 Catalytic activity in the n-1-heptene hydrogenation The hydrogenating activity of the nickel boride catalysts was tested in a mixture of the reversed micellar system (22% v/v) with ethanol (78% v/v) at 20 .:l:. i
-c (Fig. 5).
100 c:
o
...
CIl
Q)
> c o
o
?fi 50
o
20
10
30
Time (mln.)
Fig. 5. Time dependence of the conversion of n-1-heptene on nickel boride cata2M lysts : Ni boride catalyst (2.5x10- 2M; PH = 760 Torr; 5x10alkene) . - in
2
ethanol (a)
- in an ethanol-micellar system (b) Ni boride catalysts obtained from reversed micellar systems (4.0, 12.0 and 20.0 water wt.%) - in an ethanol-micellar system (c). The results for the newly prepared catalysts (c) are compared to the activity of nickel boride particles obtained in ethanol (95% v/v) - water (5% v/v)
,
(cal-
led Ni P-2 catalyst (refs. 5-8») ,the activity of which is close to that of Raney nickel, tests being conducted either in ethanol-water (Fig. Sa) or in ethanolmicellar systems (Fig. 5b).
An enhancement factor of ca. 3 is found with res-
pect to the catalyst of (b) in the initial rate of n-1-heptene conversion using o
a nickel boride catalyst (particles of 30-50 A diameter) obtained in the CTAB-n hexanol-water micellar system.
200 CONCLUSIONS The reduction of NiCl by NaBH in the CTAB-n hexanol-water reversed micellar 2 4 o system leads to small nickel boride particles (30-60 A) with a narrow particle size distribution. duction. ched.
The micellar water cores act as reaction cages for the re-
Nucleation can only occur if a critical number of Ni(II) ions is rea-
The efficiency of the nucleation is essentially linked to the rate of
diffusion of the reducing agent.
The latter is increased with decreasing water
content, yielding consequently smaller particles.
The hydrogenation activity
of these catalysts seems higher than those of catalysts prepared in ethanol only solutions. The formation of small catalytic particles using reversed micelles thus opens new possibilities in the preparation of heterogeneous catalysts.
REFERENCES 1 H.I. Schlesinger and H.C. Brown, U.S. Patent, 2.461.661 (1949). 2 H.I. Schlesinger, H.C. Brown, A.E. Finholt, I.R. Gilbreath, H.R. Hoekstra and E.K. Hyde, J. Amer. Chern. Soc., 75(1953)215-219. 3 R.C. Wade, D.G. Holah, A.N. Hugues and B.C. Hui, Catal. Rev. Sci. Eng., 14(1976)211-246. 4 R. Paul, P. Buisson and N. Joseph, Ind. Eng. Chern., 44(1952)1006-1010. 5 C.A. Brown, J. Org. Chern., 35(1970)1900-1904. 6 M. Kajitani, Y. Sasaki, J. Okada, K. Ohmura, A. Sugimori and Y. Urushibara, Bull. Chern. Soc., Japn., 47(1974)1203-1206. 7 H.C. Brown and C.A. Brown, J. Amer. Chern. Soc., 85(1963)1005-1006. 8 C.A. Brown and V.K. Ahuja, J. Org. Chern., 38(1973)2226-2230. 9 Y. Nakao and S. Fujishige, Chern. Lett., (1979)995-996. 10 Y. Nakao and S. Fujishige, J. catal., 68(1981)406-410. 11 Methodes d'Analyses complexometriques pour les Titriplex, 3e ed., E. MERCK, Darmstadt, p. 48. 12 Measurements carried out in Laboratoire d'Analyse par Reactions Nucleaires, Facultes Universitaires de Namur, Namur. 13 Measurements carried out at unite Interfacultaire de Microscopie Electronique, Facultes Universitaires de Namur, Namur. 14 In J.R. Anderson, structure of Metallic Catalysts, Academic Press, London, 1975, p. 364. 15 T. Nguyen and H.H. Ghaffarie, .C.R. Acad. Sci. Paris, Ser. C, 290(1980) 113-115. 16 J.A. Schreifels, P.C. Maybury and W.E. Schwarts, Jr., J. Catal., 65(1980) 195-206. 17 G.V. Samsonov and I.M. Vinitskii, Handbook of Refractory Compounds, Plenum Press, New York, 1980, p.'96.
201 DISCUSSION R. CAHEN: As this catalyst is more active than the conventional Raney is it more or less resistant to poisons such as sulfur ?
nicke~
Poisoning experiments have not been carried out on our samples. B. NAGY : Nevertheless, the effects of n-butanethiol and of thiophene were already studied on the hydrogenation of 1-hexadecene and 1-octene, on nickel boride (P-3Ni) 1 and Raney nickel catalysts. The n-butanethiol affects drastically the hydrogenation activity of 1-hexadecene on Raney nickel,while the activity on nickel boride is only slightly decreased at low poison concentration. In both cases monotonous decrease of activity is observed with n-butanethiol concentration. The poisoning effects of thiophene on the hydrogenation of 1-octene on P-3Ni and Raney nickel are more complex, but in the presence of large amount of poison, both catalysts retain approximatively 25% of their original activity. S. KALIAGUINE
Should not the size distribution of micelles be time-dependent?
B. NAGY : From the 19F_NMR data we compute an average size of the inner water cores and therefore we do not have any information on an eventual time dependent size distribution. G.W.E. COENEN: 1. you create a colloidal dispersion of high surface energy. Does it not agglomerate/sinter very fast? 2. How do you seperate the catalyst from the reaction medium? Clearly you cannot filter. 3. Did you observe any special selectiVity effects ? B. NAGY 1. The synthesized colloidal nickel boride particles are immediately deposited on silica gel in order to impede possible agglomeration. The catalyst is filtered in a glove box under nitrogen atmosphere. 2. For catalytic tests, the nickel boride catalyst is prepared in situ, in a nitrogen atmosphere by the reduction of NiC12 with NaBH4 in the reversed micelle composed by CTAB-n hexanol-water. Ethanol is then added to the colloidal system and a hydrogen atmosphere is introduced in the static reactor. Finally, the solution of 1-heptene in ethanol is added to the catalyst suspension. The catalysts previously deposited on silica gel are not active in the hydrogenation reaction. 3. The selectivity of our colloidal nickel boride catalysts on the hydrogenation reactions was not investigated yet, as the emphasis was put on the preparation and characterization of these catalysts. The selectivity study will be an interesting and important part of a later catalytic study: the interaction of the different functional groups (e.g. olefine and carbonylic) with the surface can be characterized and a relation can be looked for with the selectivity. A recent work deals with the selectivity of p-1 nickel boride catalysts doped with 2% Cr in the hydrogenation of phenol to cyclohexanone and cyclohexanol. At 150°C, a 42.5% cyclohexanone selectivity was obtained at 48.5% total conversion~ C.J. WRIGHT: Presumably .the particles of nickel boride that are prepared by your technique have a surface which os covered with adsorbed surfactant molecules. Have you been able to estimate the fraction of the surface which is covered with surfactartt, and would you expect the nature of the surfactant used in the preparation to modify the activity and selectivity of the catalysts ? B. NAGY : The surfactant molecules, or even the other components of the reversed micellar system, have a definite influence on the hydrogenation rate of n-l-heptene. The hydrogenation activity of the Ni P-2 catalyst is higher in the reaction mixture composed by ethanol-micellar system than in ethanol (Fig. 5). The specific adsorption of the surfactant molecules was not studied yet but the surface covered by the adsorbed surfactant molecules should be determined for a detailed kinetic study in these reaction mixtures.
202 R. SIGG: At what temperature you have done the experiments? mum temperature for the nickel-boride catalyst ?
What is the maxi-
The catalytic tests have been carried out at 20·C. The maximum B. NAGY : available temperature was not searched for in this study. On similar catalysts, hydrogenation reactions were carried out at temperatures as high as 160·C 2. M.H. REI : Have you tried this catalyst in reaction other than hydrogenation of olefin ? will micelle isolate Ni2B from further reactions with NaBH4 to form NaB03? Is the particle size solely responsible for the activity? B. NAGY 1. Only the reaction of hydrogenation of n-1-heptene has been carried out as the catalytic test to characterize these colloidal nickel boride catalysts. 2. The analysis of our nickel boride catalysts deposited on silica gel, by nuclear reactions (PIGE or proton induced gamma ray emission method) indicate strongly the presence of NaB02 on the silica surface. It was previously reported that Ni2B catalyzes the decomposition of NaBH4 to NaB02 in the presence of water 3. The same reaction occurs in the inner water core of the reversed micellar system and it is quite rapid at.room temperature. 3. The particle size is probably important at low catalyst concentrations. At higher concentrations where our catalytic tests were carried out, the rate of n-1-heptene conversion is not dependent on the particle size, but only on the rate of transfer of hydrogen from the solution to the active sites of the catalyst 2. Little is known on the geometric-electronic effects of these colloidal particles. A high resolution electron microscopy study is presently carried out on these catalysts at the laboratory of Professor J.M. Thomas (University of Cambridge, England). REFERENCES 1. D.G. Holah, I.M. Hoodless, A.N. Hughes and L. Sedor, J. Catal., 60,148 (1979). 2. C.C. Chang, MS Thesis, National Taiwan University, Taipei, Taiwa~ 1982 (Prof. M.H. Rei). 3. H.I. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.R. Hoekstra and E.K. Hyde, J. Amer. Chern. Soc., 75, 215 (1953).
203
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III
e 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THIORESISTANT FLAMMABLE GAS SENSING ELEMENTS S.J. GENTRY and P.T. WALSH Health and Safety Executive, Sheffield (U.K.)
ABSTRACT A number of alumina-supported noble metal catalysts (Pd, Rh, Pt and Ir), covering a wide range of intrinsic activities and porosities, have been studied.
The initial rate of poisoning by hydrogen sulphide has been shown to
depend on the porosity of the catalyst in relation to its intrinsic activity, while the equilibrium degree of poisoning is dependent on the oxide/metal ratio of the noble metal.
The role of thoria, as an additive, is to decrease
the porosity and increase the oxide/metal ratio.
INTRODUCTION The use of catalytic gas sensors for the detection and monitoring of flammable gas is widespread throughout industry (ref. 1).
However a severe
limitation on their use can be the poisoning of the catalyst by sulphur containing gases such as hydrogen sulphide.
This is particularly important in
such applications as monitoring flammable gas concentrations in coking ovens and off-shore oil rigs, where concentrations in excess of several parts per million (ppm) of sulphurous gases may be present. A typical catalytic element (ref. 2) consists of a coil of platinum wire (dia. 0.05 mm) encapsulated in a bead of low porosity, refractory oxide with a surface layer of noble metal catalyst (total dia. 1 mm).
The oxide is
commonly -AI20), deposited as a saturated solution of aluminium nitrate which is then decomposed by electrical heating of the coil.
The catalyst commonly
consists of a mixture of palladia and thoria, deposited from a solution of palladium and thorium salts and decomposed by electrical heating.
The coil
acts both as a heater and temperature sensor to detect the heat liberated during oxidation at the catalyst surface.
The element is mounted between two
posts which serve as electrical contacts to the sensor circuit. A catalytic element of this type is strongly inhibited by sulphurous gases. For example, after exposure to air contaminated by 100 ppm hydrogen sulphide the sensitivity of the catalytic element is reduced by about 70%.
Clearly
204 this performance is unsatisfactory and thus a thioresistant catalytic element suitable for applications described above is required.
This paper describes
the development of such an element.
PREPARATION OF ELEMENTS Standard elements were prepared as outlined above using«-Alz03 as the oxide and depositing a solution of 0.45 M (NH4)Z PdC14 and 1.1 M Th(N03)4.6 HZO to form the catalyst mixture.
The elements were pretreated by
heating in a 1Z% CH4 + air mixture to about 1100 K for 1 min and then in air to about 900 K for 1 min. Thioresistant catalytic elements were 'prepared (ref. 3) from slurries of a range of noble metal salt solutions and fine particulater-Alz03 (B.D.H. Ltd.) having an elementary particle size of about 0.05 pm and a specific surface area of 100-1Z0 mZg- l• The slurry was deposited dropwise onto a coil of alumina coated platinum wire (Pt wire dia. 0.05 mm, total dia. 0.088 mm) which was mounted between the posts of a T05 transistor header.
A porous, spherical
bead (dia. 1.6 mm) encapsulating the coil was formed on decomposition of the slurry by electrical heating. The noble metals investigated were Pd, Rh, Pt and Ir, prepared from the salts (NH4)2 PdC14, Rh(en)3 C13.3 HZO, Pt(NH3)4 ClZ.H20 and (NH4)3 IrC16.HzO (Johnson Matthey Ltd.), respectively. For Pd, Rh and Pt 0.11 and 0.45 M solutions were used to give noble metal contents of approximately 0.4 and 1.6 pmol.
However, due to the insolubility
of the Ir salt, only the lower loading Ir element could be produced.
The
metal loadings (% w/w) of the elements were: Pd 3.Z, 11.7; Rh 3.1, 11.4; Pt 5.6, 19.6; Ir 5.5. In order to investigate the role of thoria, elements were prepared from mixed solutions of (NH4)2 PdC14 and Th(N03)4.6HZO to achieve the same Pd content as in the elements above.
The Pd loadings were 2.7 and 6.9% w/w with
a mole ratio of Pd:Th of 0.41. the same as that in the standard element. Each element was pretreated by exposure at 1100 K to 12% CH4 + air for 5 min and then at 900 K to air for 10 min.
Four elements of each type were
manufactured. CHARACTERISATION BY TEMPERATURE PROGRAMMED REDUCTION Three samples of each type of element were characterised individually by temperature programmed reduction (t.p.r.). ref. 4.
The apparatus used is described in
A 10% HZ + NZ mixture was used as the reducing gas at a flow rate of
205
ZO ml min-I, with a heating rate of 14 K min- 1 from Z40-800 K.
Before
reduction each sample was pretreated in the reactor in flowing air at 470 K The temperature was then lowered to Z40 K before introduction of
for 10 min.
the reducing gas. The presence of noble metal in an oxidised state is indicated by positive peaks in the reductogram.
The area under the peaks is related to the amount
of HZ consumed in the reduction, and the positions of the peak maxima (Tm) are characteristic of the environment of the oxidised species. No reduction was observed for the Pt elements.
Rh and Ir elements produced
single peak reductograms, while Pd elements produced a large positive peak followed by a negative peak.
It is assumed that the negative peak arises from
the evolution of absorbed hydrogen.
Consequently the area of this peak is
subtracted from the positive peak to obtain the amount of hydrogen consumed by the reduction process. X-ray diffraction analysis showed that the oxidised noble metals in the samples were PdO, RhZ03, and IrOz, in agreement with other studies (refs. 5, 6 and 7).
Thus the amount of metal present as oxide could be derived from the
amount of hydrogen consumed.
The fraction of oxidised metal was obtained as
the ratio of the above to the total weight of metal in the element.
The
results of the t.p.r. measurements are shown in Table 1. TABLE I T.p.r. data for catalytic elements Element
% oxidised metal
Pd Rh Pt lr Pd-ThOZ Standard
31 91 0 59 69
± 9, 40 ± 8 ± 5, 77 ± 8
,
0
± 3, ± 4, 71 ± 5 70
Tm (K) 370, 360 390, 380
,
530, 360, 340 360
Values quoted are the mean of three measurements, low loading data reported first. It can be seen that the fraction of oxide increases in the order: Pt
This order is the same as that followed by the stability of the
metal oxides (ref. 8).
The addition of thoria to palladium approximately
doubles the fraction of oxide in the catalyst.
Pd, Rh, Pd-ThOZ and the
standard element have similar values for Tm whereas Ir reduces about 160 K higher. The values of Tm for Rh and Ir are in agreement with those determined in previous studies (refs. 6 and 7).
206 In the case of palladium it was possible to check the accuracy of the weighing method by reduction of an element oxidised to completion at 900 K. Good agreement was obtained. EXPERIMENTAL One sample of each type of element was tested in an atmospheric pressure flow system controlled by a microcomputer.
The elements were mounted axially
The test gases (air, 1% CH4 + air and 1% CH4 + 100 ppm H2S + air) were supplied to the flow tubes at a rate of 100 ml min- 1
within 13 rom dia. flow tubes.
via a set of valves font rolled by the computer.
Five elements could be tested
simultaneously. Each element formed one arm of a separate Wheatstone bridge network, the reference arm being a 4 ohm standard resistor.
Each bridge was completed with
a fixed 1 k ohm resistor and a variable 0-1 R ohm resistor as ratio arms. The temperature of the element is set by the ratio of the fixed and variable resistors.
Thus, for a given ratio, the bridge voltage and hence the
power supplied to the element is adjusted until balance is achieved.
At this
point the resistance of the element can be determined and the temperature obtained using platinum resistance thermometry. In the present work the elements were operated in the isothermal mode (ref. 9), i.e. the temperature of the element is maintained constant independent of reaction.
This is achieved by a feedback loop which keeps the
bridge in balance by adjusting the electrical power supplied to the element to compensate for the chemical power of the reaction.
Using this method
(ref. 10) the reaction rate (r) is related to the difference in power consumption of the element in air (P a) and fuel + air mixture (Pf) by: r bH = bP
(1)
where lI.H is the heat of reaction.
Where necessary, the value of AP should be
corrected for the difference in power consumption due to the differing thermal conductivities of air and fuel + air mixture.
Typical values for Pa and Pf for thioresistant elements at 80Q K in air and 1% CH4 + air are 290 and 240 mW respectively.
Assuming all the methane combusted is converted to carbon
dioxide and water thenll.P
= 50
mW is equivalent to a rate of 62 nmol s-1.
Effect of temperature For each e.Le.uent; the rate of methane oxidation was investigated in the temperature range 500-900 K.
At each temperature the reaction rate was
determined as the difference in the power requirement of the element in air and 1% CH4 + air as described above.
207 Effect of hydrogen sulphide The temperature of each element was then set at 750 K and the initial reaction rate (poisoning time t = 0 s), AP (0), measured as above: AP (0)
Pa (0) - Pf (0)
(2)
The elements were then exposed to 1% CH4 + 100 ppm H2S + air and their powers recorded at 10 s intervals for up to 2 hr, Pf (t).
The reaction rate as a
function of poisoning can then be expressed as: AP '(t)
Pa (0) - Pf (t)
(3)
and the degree of poisoning (expressed as a percentage), PL, becomes: PL
100 (l - AP (t)/ AP (0»
(4)
RESULTS AND DISCUSSION The rate-temperature data for the elements studied are shown in Figs. 1a and lb.
Table 2 summarises the data.
The apparent activation energy Ea and pre-exponential factor A were determined by fitting the low temperature data
to an Arrhenius plot.
R750 is the predicted rate at 750 K in the absence of
diffusional effects and Robs is the observed rate at 750 K. TABLE 2 Rate data for catalytic elements Element
Ea
Pd Rh Pt Ir Pd-Th02 Standard
67, 59, 65, 64, 68,
(kJmol- 1) 67 68 67 84 87
In
A
16.2, 14.0, 13.0, 12.9, 15.3,
(mW)
16.7 16.3 13.1 19.2 19.2
R750 (mW)
Robs (mW)
234, 386 94, 220 13, 11 14, 81, 308 190
42, 33, 14, 16, 3D,
45 45 10
62 67
Low loading data reported first. Pt and Ir elements have the least activity and show an exponential rise in rate over the entire temperature range.
At low temperatures the Pd and Rh
elements have apparent activation energies similar to those of their Pt and Ir counterparts, but have significantly greater activity.
As the temperature is
208
60
l-
• Pd Rh
I(
(a)
OPt
50 r ~
E
40 r
... CP
a:
30
I-
20
I-
10 r
o I(~:?' 500
o
Ir
A Pd - Th02 ... Standard
"..-;
-> J' /~~~o e/e ht
bf~
I-Q:l...... 600
X
I
I
700
800
900
Temperature; K
70 60
I
~X;«
40
,
s
a:
(b)
A
50 ~ E
t
01
30 20 10 0 500
600
800
900
Temperature: K Fig.1 - Effect of temperature on the reaction rate of unpoisoned sensing elements. Fuel. 1% CH 4 + air. (al low loading elements (bl high loading elements
209 increased, however, the activity of the Pd and Rh elements becomes increasingly less temperature-dependent, approaching a Tt dependence at the highest temperatures studied.
This is indicative of pore diffusion control.
The standard element shows similar behaviour at low temperature but has a higher apparent activation energy.
However, the temperature dependence
. becomes essentially linear at higher temperature.
Since, in this form of
element, the catalyst is present as an essentially non-porous shell, this behaviour suggests that the rate of reaction becomes controlled by the rate of gas diffusion across the concentration and temperature gradients adjacent to the surface of the element, rather than by the rate of diffusion through pores. The Pd-Th02 elements show some of the characteristics of both the above groups, depending on the catalyst loading.
Thus at low loading the catalyst
exhibits the same apparent activation energy at low temperature as the corresponding thoria-free elements, while exhibiting a linear temperature dependence above 650 K.
The behaviour of the high loading Pd-Th02 element is
almost indistinguishable from that of the standard element.
Thus the presence
of thoria markedly reduces the porosity of the elements. At 750 K, the temperature used in the poisoning experiments, three ratecontrol regimes exist.
The activity of the Pt and lr elements is essentially
kinetically controlled, whereas the greater activity of the Pd and Rh elements results in pore diffusion control.
The activity of the standard element is
essentially limited by bulk diffusion, while for Pd-Th02 elements control is partly through pore diffusion and partly bulk diffusion, depending on catalyst loading. The results of the poisoning experiments are shown in Figs. 2a and 2b.
The
Pt elements show no loss of activity in hydrogen sulphide, while all other elements lose activity at differing rates, eventually achieving an equilibrium level. The main reaction is kinetically controlled in the case of lr but pore diffusion controlled for Pd and Rh.
Since the initial rate of poisoning is
approximately linear for Ir and increasingly convex for Rh and Pd, it is suggested that hydrogen sulphide behaves as a non-selective poison; the results are
thu~
described by cases A and B in Hegedus and McCabe (ref. 11,
p. 422). The initial rate of poisoning of the Pd-Th02 elements is significantly faster; about 25% of the initial activity is lost in the first minute.
An
even more rapid loss of 55% in the first minute is shown by the standard element.
These rapid initial losses are a clear indication of the reduction
in porosity of the elements by the incorporation of thoria.
210
o
d do·
20
60
o
10
20
30
20
30
Time: min
o
10
Time: min
Fig.2 - Effect of hydrogen sulphide on the reaction rate of sensing elements. Fuel. 1% CH4 + 100 ppm ~ + air; temperature. 750 K. (a) low loading elements (b) high loading elements. Symbols as in Fig.l. Data recorded at 10 sec intervals. however points omitted for clarity
211 For the thoria-free elements plateaux in the poisoning curves were reached after about 20-30 min; no further loss was observed in 2 hr.
The sequence
obtained for the degree of poisoning was Pt
Considering first the Pt and Ir elements, Where the
reaction is kinetically controlled, the degree of poisoning is similar to the oxide/metal ratio determined by t.p.r.
In the case of Pd and Rh elements it
was observed that the degree of poisoning increased with concentration for Pd whilst decreasing for Rh.
A correspondingly small increase in oxide ratio for
Pd and decrease for Rh was also observed by t.p.r.
Indeed for both Pd and Rh
the ratip of the degree of poisoning for the low loading to that of the high loading is identical to the corresponding ratio of their oxide contents. Thus, for Pd the ratio for the two elements is 31/40 the degree of poisoning is 38/49 91/77
0.78.
= 0.78
and the ratio of
For Rh the corresponding ratios are
= 1.18 and 58/49 = 1.18.
For each of the thoria containing elements the t.p.r. results indicate a palladium oxide content of about 70%.
The equilibrium degrees of poisoning
were 70%, 80% and 80% for low and high loading and the standard element respectively, in reasonable agreement with a predicted value of 85% based on their oxide contents relative to thoria-free Pd elements. Similar results were obtained when thoria was incorporated in Rh elements. CONCLUSIONS A number of noble metal catalysts, exhibiting a wide range of intrinsic activities and porosities, have been studied.
The initial rate of poisoning
by hydrogen sulphide has been shown to depend on the porosity of the catalyst, while the equilibrium degree of poisoning has been shown to depend on the oXide/metal ratio in the noble metal catalyst.
REFERENCES 1 J.G. Firth, A. Jones and T.A. Jones, Ann. Occup. Hyg., 15(1972)321. 2 A.R. Baker, U.K. Patent No. 892530(1962). 3 D.W. Dabill, S.J. Gentry, N.W. Hurst, A. Jones and P.T. Walsh, U.K. Patent Applic. No. 8124036(1981). 4 S.J. Gentry and P.T. Walsh, J. Chem. Soc., Faraday I, in the press. 5 Y.L. Lam and M. Boudart, J. Catalysis, 47(1977)393. 6 H.C. Yao, S. Japar and M. Shelef, J. Catalysis, 50(1977)407. 7 N. Wagstaff and R. Prins, J. Catalysis, 59(1979)446. 8 M. Salmeron, L. Brewer and G.A. Somorjai, Surface Science, 112(1981)207. 9 A. Jones, J.G. Firth and T.A. Jones, J. Phys. E: Scient. Instr., 8(1975)37. 10 S.J. Gentry, A. Jones and P.T. Walsh, J. Chem. Soc., Faraday I, 76(1980)2084. 11 L.L. Hegedus and R.W. McCabe, Catal. Rev.-Sci. Eng., 23(1981)377.
212 DISCUSSION J.W. GEUS: Considering the results of the poisoning experiments the impression is gained that the resistance against poisoning runs parallel with the activity in the sulphur dioxide oxidation. Pt has been the first catalyst used in the production of S03 by the contact process. Have you any evidence about a parallel activity in S02 oxidation and resistance against poisoning? P. WALSH We have not examined the oxidation of sulphur dioxide over our catalysts. However we have observed that the poisoning effect of sulphur dioxide is quantitatively similar to that of hydrogen sulphide. We believe that hydrogen sulphide is ox~dised to sulphur dioxide under the conditions used and that the same poison species is formed in either case. Previous studies of sulphur dioxide oxidation over transition metal catalysts have shown that the reaction is limited by the rate of desorption of sulphur trioxide, this rate being greatest for platinum. Thus the poison resistance of the present catalysts does have a parallel in the activity for sulphur dioxide oxidation. We believe that both are limited by the formation of similar "surface sulphate" species: the ability of platinum to desorb S03 rapidly than the other metals being responsible for the greater thioresistance and S02 oxidation activity of that metal. S.P.S. ANDREW Is the poisoning you are describing temporary or permanent? In the former case the sensitivity of your device is determined by the ratio between catalytic activity and gas film diffusion rate constant. If the former is high enough and the latter low enough the device is temporary poison resistant. Lower gas velocities help poison resistance of the device. See my paper at the Antwerp Conference on Cobalt Oxide-NH3 Oxidation Catalyst. P. WALSH : The poisoning of the elements is to some extent temporary; on removal of H2S from the gas stream there is some recovery of activity. Three factors, the intrinsic catalytic rate, gas film diffusion and pore diffusion, influence the activity and poison resistance of the sensing elements to varying degrees. For the standard type element the activity is controlled mostly by the gas film diffusion rate whilst lower activity of the platinum thioresistant element is least controlled by any diffusional effects (table 2 and fig. 1). However thioresistance is greater for the platinum element (fig. 2), thus the sulphur resistance of the devices is not due to a gas film diffusion effect. In our apparatus cold gas flowing through a tube (dia 13 rom) passes over a hot sensing element (dia 2 rom). Here transport of reactants to the catalyst surface occurs mainly by gas diffusion. The gas flow rates used are much larger than either the catalytic rate or the gas diffusion rate, thus we do not observe any effect of gas velocity on either the reactivity or poison resistance of the sensing elements. N.J. GUDDE : Do these thioresistant sensors show any resistance to poisons such as silicone vapours (e.g. hexa~ethyldisiloxane) ? P. WALSH : These thioresistant sensors do show resistance to silicone vapours such as hexamethyldisiloxane (hmds). They are 100-200 times more resistant than the standard type of sensing element described in the paper. The noble metals Pd, Rh, Pt and Ir dispersed on Y-A1203 (thioresistant elements) are poisoned by hmds at similar rates and eventually lose all their activity to methane. Thus hmds appears to be a non-selective poison, under our conditions, unlike H2S or S02' The superior resistance of the supported elements to high molecular weight poisons such as hmds is a consequence of the increased metal dispersion and porosity of the catalysts.
G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
213
C)
THE FORMATION OF ACTIVE COMPONENT LAYER IN COATED CATALYSTS R. HAASE~,
U. ILLGEN, G. OHLMANN, J. RICHTER-HENDAU, J. SCHEVE
and I. SCHULZ
XVEB Chemiekombinat Bitterfeld, DDR-44 Bitterfeld Zentralinstitut fUr physikalisohe Chemie der Akademie der Wissensohaften der DDR, DDR-1199 Berlin-Adlershof
ABSTRACT A concentrated solution of vanadium oxalate, normally used for ooating the ceramic core of oatalyst granula, was mixed with different amounts of acetic, oxalic and citric acid as well as with different alcohols, detergents, water soluble polymeric substanoes like gelatine and polyvinyl aloohol. After a constant time the height of a droplet, plaoed on the flat surface of sintered ol-AI 20" and the percentage weight loss at 90 0C were measured. Spreading on the surface and evaporating are influenced by the surface tension and the viscosity of the mixtures. The additives also caused the formation of differently shaped V205 crystals on the surface of Ti0 2-V205 layers detected by SEM. The dried and at ,'OOC annealed mixtures of vanadium oxalate yield powders of V20, which differ considerably in catalytic decomposition of 2-propanol.
INTRODUCTION The manufacture of industrial coated catalysts is often very similar to the painting of walls by using a spraying device. The solution of the salts of the aotive oomponents and pigment like insoluble substanoes are better fixed on the surface of ceramic cores by adding surfactants and glue, i.e. substances whioh may optimise the surface tension, the viscosity and the adhesive properties of the slurry. Since suoh compounds can vary the properties of precipitated vanadium OXide (ref. 1), we inve. stigated the influence of such water soluble organics, which
214
strongly alter the surface tension and the viscosity of water (ref. 2). Inorganic salts do not change significantly the surface tension and the viscosity of water (ref. 2). Therefore we took the values of these parameters for our mixtures from literature (ref. 2). comparing them with the heights of one droplet of the mixture on the surface of a small sintered plate of alumina. We assume the increased evaporation of the water to be due to the growing of the surface of the droplet, caused by its spreading on the ceramic core. A rapid evaporatioa should result in many small crystals of the precursor While slow evaporation should give larger crlstale in less quantity. For investigating this problem we choose the V205 type c?ated catalyst. Formerly we got more or less active V205- Ti 0 2 catalysts for the partial oxidatioa of o-xylene without ohanging the stoichiometry (ref. ,). We felt this should be due to V205 crystallites, different in size and amount. EXPER1MENTAL Always 5 ml of a saturated solution of vanadium oxalate were prepared by resolving V205 powder in a solution of oxalic aoid. Methanol, ethanol, 1-propanol, 1-butanol, sacoharose and polyviayl aloohol were added as well as aoetic, oxalic and citric aoid. Finally the influence of surfaotants was tested by mixing the vanadium oxalate solution with 55 ethoxY-1-hexadeoanol or with & commeroial mixture of sulfonic aoids. A 20pl droplet was giveD on a small sintered plate of corundum and its height was measured every minute by a cathetometer for a time of ten minutes. The drying of the various mixtures was measured by the weight loss of 10 - '0 mg at 90 0 e With time. This process is normally finished after 10 - '0 minutes. The data of the first and the fifth minute are used to avoid m1st~es due to ooncentratioD effeots. They were obtained on a Perkin-Elmer-Thermobalance TG-1. 2-propanol decomposition was measured in a micro flow reaotor. A stream of air or nitrogen was loaded with ~2.5 vol. $ of 2propanol. 12 llh flowed through 0.2 g of V20,. 5 samples of vaa~ dium oxalate solution + 50 mass $ CH3COOH, + 7 mass $ 1~butanol, + 1 mass $ gelatine, + 21 mass $ 1-hexadecanol With 55 ethoxy + '0 mass $ saccharose groups,
215
were used. The water containing mixtures were dried at 125°C and decomposed at 350°C for 2 h. The resulting vanadium oxide was taken as catalyst for 2-propanol deoomposition. The highest temperature for thermal decomposition of vanadium oxalate mixtures were found at 31,oe (sample 5). Temperatures higher than 350°C are not suitable for detecting the influence of the additives on the reactivity of the samples due to the low melting point of V205 of 6750e. Therefore lattice defects are expected to be annealed at 400°C (ref. 4). Thermogravimetric behaviour in air above '50°C w~s used for determining the oxygen deficit of V205• The SEM micrographs were taken with a Tesla BS '00 microscope. The specimens were prepared by using a ceramic insolating ring of 8 mID in diameter as a pan for a wet paste of anatase. After drying a 5 ul droplet of the mixtures was given on the surfaoe of the anatase. After drying the sample was fired for 12 h at 400 0e in air like the commercial oatalyst. Then the sample was cooled and taken into the SEM. The pH was measured by a pH-meter manufactured by VEB Praecitronio Dresden. RESULTS From a booklet of the KRONOS Titan Company (ref. 5) we got the idea, that our vanadium oxalate solution should spread better on the surface of the ceramic support if it would be more acid. Aooording to Y. Kawashima and C.E. Capes (ref. 6) the height of a droplet is proportional to the contact angle. Therefore we took the height of the droplet for measuring the coating of the surfaoe of the support. We added 0.05, 0.19 and 1.00 mass ~ of oxalio aoid to the nearly saturated solution of vanadium oxalate. The pH of the above mentioned sequenoe of added mass ~ of oxalic aoid amounts to 0.70, 0.67 and 0.76 and for vanadium oxalate solution to 0.80. Beoause the vanadium oxalate solution 1s already v~ry aoid for itself, the effect of the added oxalic acid is not very strong, but nevertheless with deoreasing pH the height of the droplet deoreases, too (fig. 1). In another set of experiments we tested the influence of the amount of acid (wt %) on the evaporation.
216
o
mm 1.0
0.5
0/: : . /' o
0
o
•
•
•
•
0.7
F1g. 1. Droplet height versus PH of va.rious vanadium oxalateoxa.lic aoid mixtures after 1 minute (open circles) and 10 minutes (full circles).
TABLE 1
Weight loss at 90°C and droplet height change in the first minute for different mixtures of vanadium oxalate with organic acids with respeot to the pH and the conoentration.
concentration (mass pH droplet height (mm) A mass %
~)
acetic acid
oxalic acid
citric acid
1.2 0.'1 0.55 5.5
0.047 0.67 0.72
0.04 0.74 0.59 4.9
55.0 00.07 0.48 7.0
4.'
1.00 0.76 0.94 5.9
1.'4 0.72 0.59 7.2
It is to be seen that the height of the droplet decreases with decreasing pH in acetie and oxalio acid mixtures, whereas both are constant in the case of citric acid addition. The mass loss increased with increasing concentration of oxalio and cit rio acid. Because the surface tension of acetic acid solution in water varies With the concentration (ref. 2), we tried to oorrelate this
217
parameter with the layer ~ormation o~ vanadium oxalate. Aooording to (re~. 2) aloohols ohange the surface tension of water already in minor concentration. A considerable power o~ 1-butanol is demonstrated. The height of the droplet decreases proportionally to the decrease of the estimated surface tension of the mixture. The same correlation between these two parameters was found in the case o~ the other alcohols. Only 1-propanol showed after an initial decrease o~ the droplet height an increase with decreasing surface tension. A comparison of all our additives for whioh surface tension data were available in literature with the weight loss after the first and the fi~th minute due to evaporation o~ the liquids is given in fig. 2. The peroentage weight loss is strongly affected by lowering the surfa~e tension. Only gelatine does not follow the straight line o~ this oorrelation.
50 %6 mass
40
30
-",
I I-C 3H70H
'" a.'allne 31-!'i.HgOH
20
, C",HSOH
5 CH 3COOH 6C1H S(OH)3 7 Saccharose
10
Fig. 2. Peroentage weight loss at 90°C versus surfaoe tension of various aqueous vanadium oxalate mixtures after 1 minute (open circles) and 5 minutes (full circles).
218
Looking for the other properties of aqueous solution which can influence the surface covering and the drying process of our presursor solutions, we oompared the viscosity. In fig. 3 the viscosity of aqueous vanadium oxalate-sacoharose solutions is oompared with the height of the droplet on oorundum. The height inoreases with increasing visoosity. The visoosity data are taken from (ref. 2). This is oorrect, beoause the dissolving of salts in water does not change its visoosity (ref. 2).
mm 1.0
0.5
Fig. ,. Height of droplets of aqueous vanadium oxalate-saooharose solutions compared with their estimated dynamio visoosity after the first minute (open circles) and after 10 minutes (full ciroles). In fig. 4 the correlatioa between the percentage weight 10S8 at 90 0C and the viscosity obtained for different additives 18 given. A lowering in viscosity result. in an increase of peroentage weight loss, i.e. the mixture is drying faster. From the model mentioned above one could expect that detergents and polymeric substances should alter the spreading significantly. Therefore we investigated the influenoe of polyvinyl alcohol and of a commeroial mixture of sulfonio acids of paraffins (mersolat) on the height of the vanadiua oxalate droplet (fig. ,). As is to be seen already small amounts of these additives change the 00vering of the support very strongly. The polyvinyl alcohol results in a droplet height which iS,even after 10 minutes,as high as the pure vanadium oxalate droplet after 1 minute ("'" 1.25 mm). Contrar1 to this,sulfonio aoid enhanoed the spreading, so that the droplet height after 10 minutes was only 1/' of that of polyvinyl alcohol.
219
1 H;H70H
2 Scc chcross 3 C.,H50H 4 C3H5lOHl3 5 CH3COOH
1,0
2.0
?i
3
10- Nxs/m
.,
Fig. 4. Variation of percentage weight loss with different vanadium oxalate mixtures in water after 1 minute (full cirolee) and 5 minutes (open circlee).
polyvinyl alcohol
mm
o
15
1.0
mixture of 'paraffin Q
0.5
~.
sulfonic acics
-.
8
:• 0.01
0.1
_. •
1.0 %moss
Fig. ,. Droplet height versue conoentration of a surfactant and a stabilizer.
220
The above demonstrated effects were assumed to influenoe the size and the distribution of vanadium oxide crystals formed after deoomposition of vanadium oxalate. Since in oommeroial oatalyst anatase is the support, we used this material in further experiments. By soanning electron microsoopy we obtained miorographs, demonstrating differently sized orystals on the surfaoe of anatase. These crystals should be vanadium oxide, beoause pure anatase did not show orystals on its surface. Oxalate solutions without any additives, mixed with 0.005 mas8~ 1-propanol and with 20 mass ~ saooharose, were put on the surfaoe of , different anatase samples. Fig. 6 - 8 demonstrate the effect of these additives.
Fig. 6. Rods and plates on the surfaoe of anatase appearing after addition of 5 pI vanadium oxalate solution and deoomposition at 400°C for 12 h.
Fig. 7. Filaments on the surfaoe of anatase after dripptag of 5 pI vanadium oxalate + 0.005 mass ~ 1-propanol aqueous mixture on it and deoomposing the dry residue at 400 0C for 12 h.
221
Fig. 8. Filaments and'small rods on the surfaoe of anatase after putting an aqueous vanadium oxalate solution, whioh contained 20 mass % 1-butanol, on it and heating the oomplete sample at 400 0C for 12 h. It is obvious that the pure vanadium oxalate solution is forming compact orystallites, whioh scarcely cover the surface of the support. We found only 20 crystallites per 0.5 om 2 (fig. 6). As the peroentage weight loss of 1-propanol addition is the highest and that of saocharose is the lowest (fig. 2), we ohose these substanoes for investigating the effect of additives on the size of vanadium oxide orystals. The quick evaporation caused by 1-propanol results in a lot of very thin filaments on the support (fig. 7). The addition of saocharose gives bigger whiskers, as shown in fig. 8. There is a second difference between the pure vanadium oxalate solution and the mixtures. In the first case the spot of the droplet is nearly uniformly covered by the big orysta~ whereas the additives oause oiroular zones oovered with orystals 00 the border of the spot. Finally, the question arises in what manner the addition of organic substances would alter the oatalytio properties of vanadium oxide, produoed by deoomposition of aqueous solution of these oompounds and vanadium oxalate. As a test reaotion the deoomposition of 2-propanol at 150 0C was chosen. This temperature is low enough to prevent lattice defeot annealing. The results are given in table 2.
222
TABLE 2 2-propaaol decompositioa oa various vanadium oxides at 150 0C. additives
oonversion (~)
produotion rate (mole x h- 1x g-1)
N2
propene
CH,COOH '.5 1-butanol 21.7 gelatine 12.1 1-hexadecanol 24.1 + 55 ethoX1 groups saooharose 2•.7
air
aoetone
N
2
air
N2
air
22.4
1.09 6.81 1.09 7.09
3.27 7.6, 4.'6 4.90
traoe 1.5' traoe 1.31
2.18 '.60 2.89 2.72
11.7
1.09
2.20
traoe
1.47
18.2 35.2 ~5.1
The results demonstrate the great influenoe of the used additives on the oatalytio aotivity of the formed vanadium oXide. DISCUSSION AND SUMMARY The addition of aloohols, organio aoide and surfaotante alter the drying prooess of aqueous solutions of salts of active compounds of oatalysts. Larger orystals and lower surfaoe ooverage of the support are obtained if the evaporation rate is lowered. A decrease of the surface tension led to the formation of a fibrous texture. An inorease of viscosity of the aqueous mixture inversely led to the formation of bigger and less orystals OR the surfaoe. This effect together with different spreading also forms different reaotive surfaoe layers. ThiS, espeoially, is the case at low temperature reactions.
REFERENCES 1 P. Aldebert, N. Baffier, N. Gharbi, J. Livage, Mat. Res. Bull. 16(1981)946-955. 2 Handbook of Chemistry aRd Physios, 44th eda., The Chemioal Rubber Publishing Co., Cleveland, 1962. , W. Fiebig, R. Haase, U. Illgen, H. MUller t J. Soheve, H.-P. Walter, Chem. Teohn. 28(1976)673-676. 4 R. Haase, R.-G. Jersohkewitz, G. tihlmann, J. Richter-Mendau, J. Soheve, in B. Delmon, P. Grange, p. Jaoobs and G. Ponoelet CEds.), Preparation of Catalysts II, Elsevier, Amsterdam, 1979, pp.615-624. 5 Krollos-Informatioll 17, H. Rechmann, "Weohselwirkung zWisohen Ti0 2-Pigmentea und anderell Substanzea". 6 Y. Kawashima, C.E. Capee, Ind. Eng. Chem. Fundam. 1'(1980)'12'14.
223 DISCUSSION A. ANDERSSON 1) What do you think is the reason for the various activities obtained (table 2) ? Is the variation due to different specific surface areas of the active component, i.e. the vanadium oxide or is it a question of variations in the plane distribution ? 2) The nails, which can be seen on Figs 7 and 8, are they mainly crystalline .or amorphous ? J. SCHEVE:
1) According to our measurements the different activities are related to different reoxidation rates,i.e. to the ability to reconstruct the lattice by incorporation of oxygen according to the following table : Additives
Rate ratio propene / acetone
f'..m//:;t (mg/min) oxygen uptake
Conversion in N2 (%)
CH
0.20
3.5
0.30
12.1
1.09
1-butanol
0.40
21.7
4.45
Ethoxyhexadecanol
0.45
24.1
5.41
3COOH Gelatine
1.09
This is in accordance with the propene/acetone ratio, which indicates that the oxygen uptake rate parallels the acidity/basicity ratio of the surface. 2) The needles are V307 or V6013 crystallites, as measured by electron diffraction on some samples. S.P.S. ANDREW There is a theory that the rate of progression of a drop of liquid over a surface is in part the result of volatile components of the liquid moving across the surface (perhaps in the gas phase) radsorbing and preparing the surface. Was there any evidence that volatile surface active agents were more effective for a given surface tension reduction than those which were involatile ? J. SCHEVE : Because the experiments were done at room temperature and the data were taken from the first and tenth minute and the slope of these curves are parallel we assume that volatilization did not influence our results. A. R. FLAMBARD
tal artefacts ?
Are the minima indicated in fig. 5 real or are they experimenIf real, do you have any explanation for this behaviour?
J. SCHEVE : We have no explanation and believe this small maximum to be due to
experimental deviation. K.S.W. SING: Do you have any results which show the effect of changing the nature of the Ti02 surface? Have you taken into account the possible influence of adsorption at the solid/liquid interface ? J. SCHEVE : Yes we have. Using rutile we got only an enamellation of the Ti02 particles because the v+ 4 ions are incorporated into the rutile lattice and the rema~n~ng nearly pure V205 is spreading over the surface. The crystallites to be seen on the surface of anatase are V307 or V6013 ~ccording to electron diffraction measurements. The influence of adsorption was not taken into account because it seens to us that it has only a small effect on our measurements. The support is either non-porous sintered corundum and shows no adsorption capacity like fine powders or non porous coarse anatase pigment powder according to SEM micrographs.
224 K.S.W. SING:
Do you have any results which show the effect of changing the nature of the Ti02 surface? Have you taken into account the possible influence of adsorption at the solid/liquid interface ?
J. SCHEVE: Yes we have. Using rutile we got only an enamallation of the Ti0 2 particles becaus~ the V+4 ions are incorporated into the rutile lattice and the rerna~n~ng nearly pure V20S is spreading over the surface. The crystallites to be seen on the surface of anatase are V6013 according to electron diffraction measurements. The influence of adsorption was not taken into account because it seems to us it has only a small effect on our measurements. The support is either non porous sintered corundum and shows no adsorption capacity like fine powders, or non porous coarse anatase pigment powder according to SEM micrographs.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III
e 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
225
PREPARATION OF HIGHLY ACTIVE COMPOSITE OXIDES OF SILVER FOR HYDROGEN AND CARBON MONOXIDE OXIDATION M. HARUTA and H. SANO Government Industrial Research Institute of Osaka. Ikeda (Japan)
ABSTRACT Highly active oxide catalysts have been developed for low-temperature catalytic combustion of hydrogen and carbon monoxide. These oxides, which consist of silver oxide combined with manganese dioxide and/or cobalt oxide, are prepared by coprecipitation from nitrate mixtures in the presence of KMn04. A systematic investigation of the correlation between activity and atomic ratios of Mn/Ag and Co/Ag resulted in an optimum composition of Ag-Mn-Co (1:4:20 in atomic ratio). The oxide exhibits remarkably enhanced activity and good thermal stability in comparison with the simple component oxides.
INTRODUCTI ON The use of catalysts to carry out stable combustion at low temperatures without flames or at high temperatures for low air/fuel ratios has been receiving much attention in connection with pollution control (refs. 1-3), energy saving (ref. 4), and new energy technology (ref. 5). In residential utilization of hydrogen. which has been proposed as one of the most desirable and clean forms of secondary energy, low-temperature flameless combustion over catalysts appears to be preferable to conventional flame combustion from the point of view of safety from fire hazards and elimination of pollutant emission. As the likely obstacles to the wide-spread application of low-temperature catalytic combustors are the cost and the availability of catalysts. we have developed inexpensive oxide catalysts based on 3d transition metals. In this paper. preparation of highly active oxidation catalysts composed of silver, manganese. and cobalt is described with a special emphasis on the correlation between activity and composition. EXPERIt~ENTAL
Catalytic activity measurements were conducted in a small fixed-bed reactor
226
(ref. 5) with 0.30g of catalyst, that had passed between 42 and 70 mesh sieves. A standard gas consisting of 1 vol.% hydrogen balanced with air to 1 atm. was passed through the catalyst bed at a flow rate of 100 ml min- l. The temperature of the catalyst bed was continuously increased at a rate of 2°C min- l and the change of hydrogen concentration in the effluent gas was monitored by automatic gas chromatography. The rising-temperature technique adopted for simple and rapid measurement proved to give nearly identical results to those observed under steady-state conditions. The activity of a given catalyst is expressed in terms of the temperature corresponding to 50% efficiency (Tl/2), which can be obtained from the efficiency vs temperature curve. tower temperatures of Tl/2 indicate correspondingly greater catalytic activity. Specific surface areas were measured by using a flow technique using nitrogen as adsorbate (Quantasorb, Quantachrome Corp., U.S.A.). RESULTS AND DISCUSSION Screening of catalysts Several tens of oxides were screened for catalytic oxidation of hydrogen and the activity of some of them is well summarized by a volcano-like relation with the heat of formation of oxides per gram-atom of oxygen as shown in Fig. 1.
o 100
200
o
CuO
050 Hf' kcal / g. atom oxygen Fig. 1. Dependence of hydrogen oxidation activities of metal oxides on their heat of formation per gram-atom of oxygen. The volcano relation indicates that the breaking of metal-oxygen (M-O) bond is the slowest step in hydrogen oxidation over the oxides located on the right arm and thus the catalytic activity decreases with increasing bond energy.
227
Conversely, over silver oxide, the formation of M-O bond may be slower than bond breaking and therefore it is more active than gold. Based on the above generally accepted interpretation of the volcano-shaped curve, an attempt was made to develop composite oxides of silver with the transition metal oxides located on the opposite arm of the curve expecting a remarkable enhancement in catalytic activity. In fact, the mixture effect was not so appreciable among the 3d transition metal oxides, for example, in Co-Mn, Co-Ni, Co-Cu, Mn-Ni, Mn-Cu, and Ni-Cu oxides (ref. 6). Silver oxide as an oxidation catalyst , During the course of research for active catalysts for the removal of carbon monoxide from air in the past (ref. 7), silver oxide had been found to be the most active. It was indeed also active for hydrogen combustion as shown in Fig. 1. However, a serious drawback to its use as a combustion catalyst is that its activity is not stable at temperatures above 180°C. Fig. 2 shows that simple silver oxide, although thermogravimetrically stable up to 300°C, tends to lose activity from 180°C. It is far less active than C0304, Mn02' and NiO when calcined at 300°C. The need for an improvement in thermal stability of simple silver oxide has also led us to the search for its composite oxides.
o~ I
N
~O
-
III III
0
2
0
TG
-
-4 s: .2"6 CI) ~8
c
0
III
l0-
Q)
>
c 0 u
200
0
Temp.,
400
ce
~
100
150
200
Catalyst Temperature,
250
300
OC
Fig. 2. Oxidation efficiency of hydrogen vs temperature curves and thermogravimetric curve for silver oxide.
228
Binary oxides of silver Table 1 shows the activities for hydrogen oxidation of binary oxides of silver calcined at 400°C in an air stream for 5 hrs. Except for Ag-Al and Ag-Mn oxides, they were prepared by coprecipitation from aqueous nitrate mixtures with NaOH or Na2C03. Mixed oxides of Ag and Al were prepared by precipitating AgOH in a diluted solution of commercially available A1203 sol. Mixed oxides of Ag and Mn were prepared by coprecipitation in the presence of KMn04 with an amount calculated from the reaction; 2Mn04- +' 3Mn 2+ + H20 ~
5Mn02 + 4H+
The binary oxides of silver can be classified into three groups: the oxide which exhibits higher activity than simple silver oxide (Ag-Mn oxide), the oxides with equivalent activity (Ag-Co and Ag-Al oxides), and the other less active oxide systems. TABLE 1 Catalytic activities of Ag binary oxides for hydrogen oxidation Oxides
Ag content (atom%) 50 Ag-r~n 10 20 Ag-Co 2 10 Ag-Al 5 100 A92 0 (dried at 120°C)
Tl/2 (OC)
Oxides
62 126 85 95 88 126 89-103
Ag-Cr Ag-Fe Ag-Cu Ag-Ni Ag-Zn Ag-V
Ag content (atom%) 10 10
50 10 80 10 10 10
Tl/2 (OC) 108 118 131 154 150 155 175 250
A characteristic feature of Ag-Co oxide is that only 2 atom% is sufficient to obtain the initial activity of simple silver oxide and further enrichment in Ag content only slightly enhances the activity (Fig. 3). The activity was not effected by changing the precipitating reagent from Na2C03 to NaOH. The specific surface area of the Ag-Co oxide system, probably owing to larger ionic radius of Ag+ (Ag+=1.27 AO, C02+=0.88 AO), increased with an increase in Ag content up to 2 atom% and then it declined. Surface sintering of A920 may be the main cause of the decrease in surface area at high Ag contents and it compensated the increase in Ag content resulting in almost the same activity for a variety of compositions. A comparison of the Co 2Pl/2 XPS spectrum between C0304 and Ag-Co (1 :9) oxide indicated that the presence of Ag increased the concentration of C03+ ions and decreased the concentration of C02+ ions in the surface layer. It can be assumed that the change of valency of surface Co cations facilitated an efficient incorporation of Ag, while in the case of A1203 it only supported A920 on its surface
229
150 140 130 u 0
n
.. . . 1I-.-. 1I ,.' ,'"
70
..\
"".----,
\\
.,'
60
Ol
.......
50
•,,
E
\
120
1'",~ ..
'" 110
..==-
100
\
.......... ~
90 80
40 0 QJ
II
~
30 0QJ
'.
\
.) ~,
•
u
-...
20 0 ~
10
cf)
0 2 5 10 20 50 100 Ag content, atom% 05 1
0
Fig. 3. Dependence of the temperature for 50% oxidation of hydrogen and specific surface area on Ag content in Ag-Co oxide . • ,.: carbonate precursors, 0: hydrate precursors.
II
250
200
II
150
200
•
u 0
~
N <,
",,-0,
"
, " • ,,
0
~
.=-
0 QJ
\
U
0
0
100
E
100 ~
,, ,, , ,
150
01 <, N
~
50
\
~
cf)
'0
50
n
0
CV 0.5 1 2
Ag content,
5 10 20 50 100 a atom·'.
Fig. 4. Dependence of the temperature for 50% oxidation of hydrogen and specifi c surface area on Ag content in Ag-Mn oxide . • : carbonate precursors, 0,0: hydrate precursors.
230
without any specific interaction. The fact that Ag-Al oxides prepared by coprecipitation was less active than those prepared by precipitation of A920 in aqueous A1203 sol may support the above assumption. Accordingly, A920 combined with a large excess of Co oxide can provide a thermally stable and less expensive silver oxide catalyst for perfect oxidation. Fig. 4 shows the drastic changes in activity and specific surface area of Ag-Mn oxide system with composition. The activity maximum was observed at around a composition of Ag-Mn (1 :1). It can also be seen that the activity was little influenced by the precitation agents, Na2C03 or NaOH. At a composition of Mn/Ag= 1, the activity was so markedly enhanced that Tl/2 was lowered to 62°C from c.a. 90°C for simple silver oxide. This activation corresponds to about a 20 fold increase in oxidation rate. Thermogravimetric analysis of the hydroxide pr~cursor of Ag-Mn (1 :1) oxide (Fig. 5) indicated a bulk composition of AgMn02 at 400°C. This composite oxide was X-ray amorphous (Fig. 6) and the appreciable surface area increase which was observed can be interpreted as having been due to the formation of a new amorphous solid solution formed by the incorporation of Ag+ into Mn02 with consequent complete disordering of the Mn02 host lattice. The corrected XPS peak intensity ratio for Ag 3d5/2, Mn 2P3/2' and 0 1s for the Ag-Mn (1:1) oxide indicated the surface composition was A9Mn204' If this composition is assumed to be one of the species responsible for the high activity, it appears reasonable that the activity of the composite oxide increased remarkably in the region 20 to 40 atom% in Ag. In this region, the composite
•-,.! 5 III III
AgMnOz (T1/2 = sz-c i
B
-
~ 10
--_AgMn°1.5 (T1I2 = no c i
Ql
~
15
a
200
400 Temperature,
600
800
·C
Fig. 5. Thermogravimetric curve for Ag-Mn (1:1) oxide.
231
CuKoc
Ag- Mn (1: 1) oxide
20
30
50
40
2S,
60
400 "C
70
o
Fig. 6. X-ray diffraction patterns for A9ZO, MnOZ and Ag-Mn (1 :1) oxide. oxide exceeded the simple silver oxide in activity accompanied by a decreasing trend in specific surface area. Further increase in Ag content caused a continuous decrease in surface area, however, since activity increased reaching its maximum for 50 atom% in Ag, it might be concluded that this composition provided a maximum surface concentration of active species. As seen from Fig. 5, the equimolar composite oxide of Ag and Mn lost one oxygen atom at temperatures above 400°C, resulting in an appreciable decrease in activity. Although it exhibits the highest oxidation activity among oxide catalysts reported so far, it would be another constraint that a large amount of relatively expensive silver was required. Ternary oxides of silver In order to improve the thermal stability and to reduce the required amount of silver in Ag-Mn oxide, several ternary oxides were prepared and evaluated. Among those, the ternary oxide consisting of Ag, Mn and Co was found to be the only one that could satisfy the above requirements. The Ag-Mn-Co oxides were prepared by adding an aqueous mixture of Ag, Mn and Co nitrates to an aqueous mixture of Na2C03 and KMn04 followed by washing, drying, and calcination at 400°C. The optimum composition was searched for by the activity measurements of the oxides with various Mn/Ag ratios at a constant
232
Co/Ag ratio, and vice versa. The results are summarized as a contour map of Tl/2 in Fig. 7, where the shaded area shows a region of composition that can provide 50% efficiency in hydrogen oxidation at temperatures between 60°C and 70°C. Especially, Ag-Mn-Co (1:4:20) oxide is the most preferable both from the catalytic and economical point of view. It is not only as active as Ag-Mn (1 :1) oxide but also thermally more stable. An increase in calcination temperature from 400°C to 600°C hardly changed its activity while causing detrimental affect to the Ag-Mn binary oxide(Fig. 5).
40
.....
0
.... 30
0
u
E 0
0
20
01 ~
....0 u
10
o o
5
10
Mn/ Ag (atomic ratio) Fig. 7. Contour map of Tl/2 with respect to Mn/Ag and Co/Ag ratios in Ag-Mn-Co oxide. The influence of preparation methods on the activity of Ag-Mn-Co (1:4:20) oxide was investigated to try to gain insight 'into the genesis of the high activity (Table 2). All precursors were calcined at 400°C in an air stream for 20 hrs and those containing Mn were prepared by using KMn04 as an oxidizing agent of Mn 2+ ions. Coprecipitation from Ag, Mn and Co nitrates by Na2C03 and KMn04 gave the highest activity, while coprecipitation without KMn04 yielded much inferior activity. The above results indicate that complete oxidation of Mn 2+ to Mn 4+ is a prerequisite for preparing active oxides in the ternary system. Precipitation of A920 in the suspension of Mn-Co coprecipitate and the simple mixing of separately precipitated A920, Mn02 and CoC03 could only provide the same activity as that of simple silver oxide and brought about no enhancement in activity. However, the activity was higher than simple silver oxide when the
233
TABLE 2 Catalytic activities of Ag-Mn-Co (1 :4:20) oxides prepared by different methods Surface area (m 2/g)
Preparation methods Coprecipitation from nitrates mixture with NaZC03 and KMn04 Coprecipitation from nitrates mixture with NaOH Precipitation of Ag-Mn oxide in COC03 suspension Precipttation of Ag-Co carbonate in MnOz suspension Precipitation of A9ZO in Mn-Co coprecipitate mixing of separately prepared A920, Mn02 and CoC03
61
82.7
84 70
97.0
80
75.5
94
45.1
91
52.8
composite oxides were prepared by precipitating Ag binary precursors on the other component which was precipitated beforehand. Especially, precipitation of Ag-Mn oxide in CoC03 suspension was better than precipitation of Ag-Co carbonate in MnOZ suspension, which showed consistency with the activity differences in binary silver oxides between Ag-Mn oxide and Ag-Co oxide. The specific surface area of Ag-Mn/Co oxide was larger than that of Ag-Mn-Co (KMn04)' however, the former was less active. Because the surface area decreased with increasing Ag content from ZO atom% in Ag-Mn binary oxide as shown in Fig. 4, it can be imagined that Ag content·at the surface layer was higher in coprecipitated Ag-Mn-Co oxide than in Ag-Mn/Co oxide prepared by two-stage precipitation. During coprecipitation, a certain fraction of manganese would be incorporated into Co precipitate and then Ag-Mn would be precipitated. This may be the reason why coprecipitation was better than two-stage precipitation. Catalytic activity for CO oxidation and kinetic measurements for HZ oxidation Another advantageous feature of the ternary oxide of silver is that it is more active than Ag-Mn binary oxide in the oxidation of carbon monoxide. Table 3 TABLE 3 Catalytic activities of the present composite oxides of silver and commercial Hopcalite catalysts for CO oxidation Catalysts Ag-Mn-Co (l:4:Z0) oxide Ag-Mn (1 :1) oxide
Tl/Z (OC) 5
40
Catalysts Hopcalite I Hopcalite IT Hopcalite I (calcined at 400°C)
Tl/Z (OC) 47 70 88
234
shows that the ternary oxide can provide 50% efficiency of CO oxidation at a temperature as low as 5°C under the same reaction conditions as those used for HZ oxidation. It is much superior to commercially available Hopcalite catalysts (I : AnO 5, 11nOZ 50, CuD 30, CoZ03 15 wt%, n: Mn02 60, CuO 40 wt%, Ki sh i da Chemicals, Japan) for room-temperature removal of CO from air. The conventional Hopcalite catalysts are usually not calcined and they tend to exhibit decreased activity when calcined at 400°C. It was confirmed that the presence of Cu caused reduced activity of silver containing composite oxide; in any preparations the introduction of Cu lowered the activity accompanied by a marked decrease in specific surface area. Kinetic measurements of H2 oxidation over the ternary composite oxide of silver were conducted in a closed circulating system. Hydrogen oxidation was first order in hydrogen partial pressure and independent of oxygen partial pressure. The first order rate constant was 3.~5x 10- 5 sec- l g-l at O°C and the activation energy was 6.9 kcal mol- l deg. CONCLUSION 1. The binary oxides of silver could be classified into three groups: the oxide which exhibited higher activity than simple silver oxide (Ag-Mn oxide), the oxides with equivalent activity (Ag-Co and Ag-Al oxides), and the the other less active oxide systems. 2. In Ag-Mn oxide, catalytic activity was markedly enhanced only at a high content of silver of 50 atom%, while Ag-Co oxide exhibited similar activity to that of simple silver oxide for Ag contents as low as. 2 atom%. 3. Ternary oxide could provide catalysts as highly active as Ag-Mn (1 :1) oxide for atomic ratios of Co/Ag between 5 and Z5 and for Mn/Ag ratios between 1 and 5.5. It was more thermally stable and more active for CO oxidation than Ag-Mn (1 :1) oxide. 4. The use of KMn04 as an oXidizing agent for Mn 2+ ions was of a key importance for preparing the above active composite oxides by coprecipitation. REFERENCES 1 G. Perkinson, Chem. Engng. News, June 15(1981)51-55. 2 W.S. Blazowski and D.E. Walsh, Comb. Sci. Technol., 10(1975)233. 3 W.C. Pfefferle, J. Energy, 2(1978)142-146. 4 A.M. Madgavkar, R.F. Vogel and E. Swift, I&EC Prod. Res. Dev., 20(1981)628-644. 5 M. Haruta and H. Sano, Int. J. Hydrogen Energy, 6(1981)601-608. 6 M. Haruta, Y. Souma and H. Sano, Proc. 3rd World Hydrogen Energy Conf., Tokyo, 23-26 June 1980, pp.1135-l147. 7 M. Katz, in W.G. Frankenburg, V.I. Komarewsky and E.K. Redeal (Eds.), Advances in Catalysis, Vol. V, Academic Press Inc., New York, 1953, pp.177-216.
235 DISCUSSION C. BROOKS : This is a comment rather than a question. It is suggested that Au warrants examination for its low temperature oxidation catalysis, in view of ~ts interesting proximity to Ag in the correlation of T1/Z versus M-O bonding energy in Fig. 1 of your paper. Gold should provide favorable low-temperature oxidation kinetics for HZ or CO in view of the potentially low bonding and hence weak retention of oxidation products. M. HARUTA: I appreciate your valuable comments. As silver is not so expensive as gold, we have extensively studied the mixed oxide of silver. However, I agree with you that the correlation of T1/Z vs Metal-Oxygen bonding energy strongly warrants examination for mixed oxides of gold, because the weaker M-O bonding energy of Au oxide than that of Ag oxide might provide an appreciable mixture effect when combined with the base 3d transition metals. B. GRIFFE DE MARTINEZ: How stable catalysts could provide your ternary oxide Ag-Mn-Co with respect to high temperatures resistance? I am thinking in terms of their possible application as automative exhaust catalysts ? M. HARUTA: As for the therm~l stability of our ternary mixed oxides of Ag, Mn and Co, they are fairly stable up to 700°C. They also have a good resistivity to the poisoning by SOZ' Taking into consideration their high activity for the oxidation of HZ' CO, and hydrocarbons and their fairly good stability,we can say that the ternary oxides of Ag, Mn and Co are good candidates as automotive exhaust catalysts provided that the catalyst temperature is technically controlled not to exceed 700°C and that there is no need to reduce nitrogen oxides. S.P.S. ANDREW Compared with Hopcalite catalyst how resistant is your catalyst to water vapour poisoning when used for CO oxidation ? M. HARUTA: We have not get made a quantitative comparison in the resistivity to water vapour poisoning between them. However, a few experimental data have shown that our ternary oxide consisting of Ag, Mn and Co is much more resistant to water vapour poisoning than the conventional Hopcalite catalysts which are not calcined. A. KORTBEEK: In oxidation catalysis the rate of the overall process is often controlled by diffusion limitations. Could you, taking this consideration into account, comment on the pore structure of your Ag-Mn-Co catalyst and its eventual influence on the observed oxidation kinetics ? M. HARUTA : We have not yet measured the pore structure of the ternary Since the kinetic data where obtained below 150°C and no effect of the size of the catalyst was observed on the rate of hydrogen oxidation in perature region, we were sure that, the kinetic data obtained were free fusion limitations. D.D. SURESH: Cap you tween pure A9zO,binary
oxide particle this temfrom dif-
on the difference in the life characteristics beAg-Mn-Ox and Co-Ag-Mn-Ox ?
cow~ent
M. HARUTA : There is a big difference in the life characteristics among the three oxides. The catalyst life increases in the following order ; pure A9ZO < binary Ag-Mn oxide < ternary Ag-Mn-Co oxide. Pure A9ZO can be used only at temperatures below 150°C because of thermal decomposition. The binary Ag-Mn oxide tends to loose its activity at 300°C, while the ternary Ag-Mn-Co oxide is stable up to 700°C. We have been using the ternary oxide for the catalytic burner of hydrogen without serious problems of catalyst deactivation for more than half a year.
236 GUI LIN LIN C0203 layer ?
How do you make it sure that Ag20 is dispersed on the surface of
M. HARUTA: It is speculative at the present stage of our investigation to describe the structure of composite oxides of silver. However, X-ray diffraction measurement of Ag-Co (atomic ratio 2:98) oxide showed the presence of Ag20 even at such a low content'of silver. In addition to that, the activity of the binary oxide of Ag and Co is always the same as that of Ag20 for a variety of compositions. Although the XPS data did not necessarily show higher content of silver at the surface than in the bulk, we assume that silver oxide is most efficiently dispersed on the Co oxide than on the other metal oxides.
237
G. Poneelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE EFFECT OF PREPARATION METHOD UPON THE STRUCTURES, STABILITY AND METAL/SUPPORT INTERACTIONS IN NICKEL/ALUMINA CATALYSTS D.C. PUXLEY, I.J. KITCHENER, C. KOMODROMOS & N.D. PARKYNS British Gas Corporation, London Research Station, Michael Road, London SW6 2AD
ABSTRACT This paper considers the factors which affect the structure of a typical co-precipitated catalyst. The structure at each stage in the preparation of a co-precipitated Ni/A1203 catalyst is described and is shown to be the result of a competition between inherited disorder and a thermal ordering process. This accounts for the paracrystalline nature of the nickel and very extensive metal/support interaction in the final catalyst, and for the differences between catalysts made by co-precipitation and by other methods such as impregnation. Some implications concerning the structure of co-precipitated catalysts in general are suggested. 1.
INTRODUCTION Co-precipitated nickel/alumina catalysts for hydrogenation were probably
first prepared by Zelinsky in 1924 [IJ.
Co-precipitated catalysts of higher
nickel content were developed on an industrial scale by Dent et al. the steam reforming of hydrocarbons.
[2-~
for
Today such catalysts are widely used for
the steam reforming of oil feedstocks to make substitute natural gas (SNG)
~,6J,
for the methanation of synthesis gas [7J and are now attracting
attention for potential use in new energy processes such as nuclear methane reforming [8,9J, methanation of coal-derived gases [lOJ and even the steam gasification of ethanol [11]. Much work has been done on the structure of co-precipitated nickel/alumina catalysts at all stages in their preparation.
However the literature is often
contradictory and descriptions of the structure are vague.
The aim of this
paper is to explore why such co-precipitated nickel/alumina catalysts should be so different in properties from similar catalysts made by other preparative methods such as impregnation. In section 2 the general features of preparation by co-precipitation are described.
The following parts review the studies that have been carried out
on the structure and decomposition of the co-precipitate, and the oxide formed upon calcination.
Sections 4&5 include our new model for the structure of the
oxide and reduced forms, and experiments carried out to test the paracrystalline model of the reduced catalyst.
The metal/support interaction is related to this
238
model (section 6).
An attempt is made in part 7 to rationalise the factors
which determine the structure of a co-precipitated catalyst and to explain how and why its structure is so different from that of an impregnated catalyst.
We
also discuss the conditions necessary for the genesis of paracrystallinity in catalysts. 2.
PREPARATION BY CO-PRECIPITATION The rationale and procedures for designing and making catalysts of high
metal content by co-precipitation from aqueous solution have been reviewed by Andrew ~2J.
Due.to cost, these catalysts are usually limited to non-precious
transition metals such as iron, nickel, copper and zinc and are used for large scale industrial processes like the production of SNG (substitute natural gas) and methanol.
They offer the advantages of high activity and long life.
The preparation route for a typical co-precipitated nickel/alumina catalyst starts with a solution of the metal nitrates.
This is treated with an alkali,
such as sodium carbonate to give the second stage, the co-precipitate.
This is
filtered, washed and dried, and then calcined in air to produce the oxide form (stage 3).
This is pelleted and charged to the reactor for final reduction in
situ. Thus in order to fully understand the nature of the reduced, active catalyst, we must first examine the structure of the three earlier stages of the preparation.
3.
STRUCTURE AND DECOMPOSITION OF THE CO-PRECIPITATED PRECURSOR Although co-precipitation as a method for preparing nickel/alumina catalysts
has been established since 1924, the structure of the co-precipitate itself was not well understood until relatively recently. When the pR of an aqueous solution containing nickel and aluminium salts in a suitable ratio is raised, usually by the addition of alkali, a greenish blue precipitate is formed.
This is not a mixture of the separate hydroxides but a
mixed basic carbonate of nickel and aluminium.
The material is commonly called
a Feitknecht compound after the discoverer of the double layer structure found in this, and a wide variety of 'other mixed metal basic salts
[13J.
These
compounds have the general formula [14J. Me
2+
x Me
3+
-
Y (OR)2 x+ 3y- 2 z.
where Me 2+ Me 3+ A2-
2+
Mg, Mil, Fe, Co, Ni, Cu or Zn AI, Fe, Cr 3+ etc.
etc.
22C0 ' S04 (or twice as many monovalent anions ego N03, Cl , Br ) 3 Generally x/y lies between 1.5/1 and 4/1.
239 The idealised formula appears to be Me(II)6 Me(III)z (OH)16 C03.4H ZO The structure
although a truly stoichiometric material is rarely obtained.
adopted by these Feitknecht compounds seems to form remarkably readily.
Not
only is it found in the precursor phases to several types of co-precipitated chemicals and catalysts [15-l~,
it is also the structure of a number of
minerals [14,18-ZlJ (the sj5grenite-pyroaurite group) and corrosion products
~Z,Z3J
(the "green rusts").
3.1 Structural studies of the related minerals The structure has been described [l~ in terms of brucite-like layers in Z 3 which some of the divalent Me + ions have been replaced by trivalent Me + ones, giving a charged layer;-
eMe
Z+
i-x Me 3+x (OH - )Z
r
n
This charge is compensated by an interlayer containing carbonate or other anions, and water molecules.' The minerals with this double layer structure fall into two polytypes which differ only in the stacking sequence of the hydroxyl layers.
Adjacent brucite layers stack such that the hydroxyl groups
on the lower surface of one layer are directly above those on the upper surface of the layer below
[zoJ.
Thus two simple stacking sequences are possible viz.
- AB - BA - AB - (ZH polytype eg sjogrenite) (see Fig. 1) and - AB - BC - CA AB - (3R polytype eg pyroaurite). As with clay minerals, mixed layer and random sequence (turbostratic) structures are possible. with a~3.l
Both subgroups are made up of hexagonal subunits
Rand c"'7.8 R.
Interlayer
Hydroxyl stacking sequence -80H-
Brucite layer
{
--r---
xxxxxx 6
Mg
A
Z+
+
3+
ZFe
- - - 8 OH
o
7.8 A ------- C0
3
Z-
_ L=::::.
B +
4H ZO B
A
The unit cells of the sjogrenite (ZH) and pyroaurite (3R) subgroups are thus made up of Z and 3 repeats of this subunit respectively.
Crystal
structure determinations have been carried out for a number of these minerals rZO,Zl,ZtJ •
240
The Me 2+/Me3+ ratio is usually about 3/1 in minerals.
However variations
from this do occur and tend to be larger in synthetically produced specimens, probably because of poorer ordering.
Similarly, although carbonate is the
most common anion in minerals, it can be replaced by others such as chloride especially in synthetic Feitknecht compounds. There has been some discussion about whether the trivalent ions substitute in the brucite layers or form part of the inter1ayer. Work on the synthetic .. . , . (manasse~te . 0 f sJogren~te and pyroaur~te an d hy d rota1 c~te ' )
Mg 2+/ Al 3+ analogues by Ross and Kodama
[2~
suggests that the aluminium ions may be present in the
interlayer (as originally proposed by Feitknecht) leaving brucite layers with
O 2 (OH) 16 ] ~ - This will be discussed 6 further in relation to the catalyst precursors • . 2+ 3+ /Al compound has been reported to occur naturally as the minerals The N~
1 in 4 cation sites vacant;
[Mg
eardleyite [26,27J or takovite [28,29J. favoured name
The latter appears to be the currently
~lJ.
The nominal composition is Ni 6 A12(OH)16 C0 although 3.4H20 the nickel may be partly substituted by zinc and some samples also have a lower Ni/Al ratio. 3.2 Structural studies of catalyst precursors Although various groups around the world were studying co-precipitated nickel/alumina catalysts during the 1940s, it was not until 1948 that any structural data was published on the catalyst precipitate. Longuet-Escard [30-32J prepared a series of materials by co-precipitating nickel and aluminium from a solution of the chlorides or nitrates with alkali. She published the first X-ray diffraction powder patterns and from these she concluded that the precipitate was a single phase containing layers of nickel and aluminium ions separated by layers of hydroxyl ions.
However it is now
quite clear that the material which she called "nickel hydroaluminate"
was in
fact a Feitknecht compound. Longuet-Escard prepared precipitates with NiO/Al 203 ratios of 2,4,6,8,10 and IS/I. At the extremes of composition, free alumina hydrate or nickel hydroxide were precipitated in addition to the "hydroaluminate". spacing of the structures varied from 7.55 highest aluminium contents to 7.95
Rfor
R for
The basal
the precipitates with the
those with the highest nickel contents.
Teichner et al. [33J prepared a similar product and noted that it was difficult to wash out all the nitrate ion.
They also tried using ammonia as
the precipitating agent. As part of an extensive study into co-precipitated nickel/alumina catalysts,
Rubinsh~net metry NiO.A1
al. [34J examined materials of low Ni/Al ratio (spinel stoichio-
obtained by precipitation of the nitrates with ammonia. X-ray 203) patterns were obtained for samples heated to various temperatures. Those heated to temperatures up to 250°C gave patterns differing from nickel
241 hydroxide, aluminium hydroxide or the "nickel hydroaluminate" of Longuet-Escard. Rubinshtein also studied catalysts with higher nickel content [3sJ prepared in a similar way for the hydrogenation of creso1s. diffuse diffraction patterns.
For very high
a~d
The precipitates gave
low nickel contents, nickel
hydroxide and gibbsite respectively were also observed. In 1969, Ga1lezot [36J observed the existence of two types of line in the X-ray diffraction pattern of the precipitate.
Some lines were narrow and
symmetrical whilst others were broadened asymmetrically.
He interpreted the
pattern as indicative of layer disorder. Although there was considerable progress on the nature and catalytic properties of the oxide and reduced forms of co-precipitated nickel/alumina catalysts in the 1960s, it was not until 1971 that there were any significant advances concerning the precursor. for the first time,
Patents [ls-17J started to appear which,
the precipitates as Feitknecht compounds.
identifi~d
Studies in our own laboratories by X-ray powder diffractometry [37J showed that the composition, homogeneity and degree of ordering of the precipitates were dependent upon many parameters.
For a given Ni/A1 ratio, these parameters
included the nature and amount of the precipitant, (this affects the pH) the concentration of the reactants, direction of addition, time and temperature of the precipitation, the subsequent ageing treatment, and the filtration and drying methods used.
001 Reflections - - Nickel-rich phases ---- Aluminium-rich phases (\ _. - Resultant profile of
'i !\I\
~r~~~~t~~~eous
N j
i
I
I
:i
\~
(003) 7,87,5
d(0~)/dI006)
12,0/1
!f\..\ //
/\;.\~
(006) 3,93,75
28 d-spacing
hkl Reflections
11 .~ E
12001(202)
Af\ (~~~)
(205)
(208)
I
l Sharp, Symmetric - 2 8
Figs. 2a,b. Diffraction pattern of co-precipitate. Fig. 2a (top) shows the effects of inhomogeneity. Fig. 2b (below) shows the effect of random layer ordering.
242 Since aluminium is precipitated at a lower pH than nickel [38J, the coprecipitate tends to be inhomogeneous, with an aluminium-rich phase coming down first, followed by a nickel-rich one. The exact positions of the basal reflections (-7.8 R and~3.9
R) in the
X-ray pattern can be used to assess the homogeneity of the precipitate (dspacing ratios # 2.0 indicate inhomogeneity) (see Fig. 2a).
The asymmetric
reflections in the patterns of precipitates that have been aged by autoclave treatment have also been explained [39J as turbostratic ordering of the hydroxide layers in a similar fashion to that observed in clays [2lJ (see Fig. 2b). The same materials also give interesting infra-red spectra that tend to support 3 the G.J. Ross and Kodama [2sJ structure with A1 + ions in the interlayers rather 2than the brucite layers. The C0 vibrations are consistent with -Al-OCOZ-Al3 bridges between the brucite layers [40J. Th~ results of Bradley & Stencel [4lJ appear to support this suggestion.
Analogous bridging is found in hydrozincite,
[42].
Zns(OH)6(C03)Z' J.R.H. Ross et al. [8,43,44J have carried out extensive studies on the incorporation of ions other than carbonate (eg. nitrate, chloride) into the catalyst precipitates.
They observed that for nickel/aluminium ratios between
2/1 and 3/1 a single phase product was obtained.
Outside this range, the excess
nickel or aluminium formed a separate phase, Ni(OH)2 or Al(OH)3'
Our own
results agree very closely with these findings, although for precipitates that have had little or no ageing, the range for a "single phase product" may be somewhat broader. We have obtained another single phase precursor when preparing catalysts of spinel stoichiometry (NiO.A1
using ammonium bicarbonate. The chemical 203) composition agrees closely with the formula Ni (OH)3·4Al(OH)3·4H20 and the 2(N03) X-ray pattern can be indexed as a hexagonal layer structure with a'" 3.0 Rand c"'26.7 (cf , 3.04 and 23.2 R for the Feitknecht compound) (see Table 1). TABLE 1 Chemical composition Measured Ni 2+ A13+ C032N03NH + 4 0 L.O.C. (550 )
18.7 17.8 0.92 11.8 1.5 42.1
X-ray powder data (d , Calculated 19.1 17.6 10.1 42.5
(h k 1) (probable) (003) (006) (100) (102) (105) (108) (1010) (110) (113) (116) (200)
Spinel precursor 8.9 4.4 2.60 2.56
R)
Feitknecht compound 7.7 3.87 2.63 2.58 2.32 1.98
1.86 1.50 1.30
1.52 1.49 1.41 1.31
243 3.3 Thermal decomposition of the precipitates The magnesium/aluminium analogue, Mg6AlZ(OH)16.C03.4HZO, of the usual catalyst precursor has been studied by Gastuche, Brown & Mortland ~sJ.
They
showed by thermogravimetry that the thermal decomposition took place in two 0C. stages. In the first step water alone was lost up to about Z80 Between 0 0C both CO and water were evolved. Z80 and 460 Z Ross and Kodama [ZSJ showed that the first step involving loss of water was reversible and corresponded to a decrease in the basal spacing from about 7.8 to about 6.6
R
R..
The magnesium/iron analogues, the minerals sjogrenite and pyroaurite have been found to behave in a similar way [46J. They lose molecular water below 0C ZOOOC and by 4S0 water and CO are lost irreversibly. Z Teichner et al. [47] showed that the "nickel hydroaluminates" lost adsorbed 0C. water between ZOo and lZOoC, and water of crystallisation above l60 De0C. composition was complete by 4S0 Studies by high temperature X-ray diffractometry and thermogravimetric analysis [48J have shown that the usual niCkel/alumina catalyst precursor decomposes in a similar way to the Mg/Al compounds. adsorbed water (up to about 1000C).
There is an initial loss of
This is followed by a reversible loss of
structural water at temperatures varying from lSOo to ZlOoC depending upon the degree of crystallinity of the sample.
For a well-crystalline precipitate, this
corresponds to a decrease in the basal spacing from -7.7
R.
to -6.6
R.
ural water and CO are then lost irreversibly at temperatures of 290 2 The product appears to be poorly crystalline nickel oxide.
Struct0-4000C.
The decomposition of hydroxy-nitrate precursors has been shown to be slightly different [44J. 4.
STRUCTURE OF THE OXIDE FORM
4.1 Literature survey The first structural studies of the co-precipitated NiO/A1Z0 system appear 3 to be those of Milligan and Merten in 1946 [49J. They prepared materials containing O,lO,ZO .... lOO moles per cent of nickel oxide by co-precipitation from the nitrates with
so~ium
temperatures of SOOO, 700
hydroxide solution, followed by calcination in air at 0 and 10000C. The diffraction patterns of the precip-
itates heated to SOOoC were diffuse. the pattern of y-Al content increased. phous.
In materials containing 0-40 mol % of NiO,
was obtained. The lines became less sharp as the nickel Z03 For contents of SO-70 mol % of NiO the samples were amor-
80-100 mol % NiO gave only the pattern of NiO.
Diffraction lines were discernible for all samples heated to 700
0
and
It was suggested that nickel aluminate (NiAl Z04) forms a solid solution not only with y-AlZ0 3 but also with NiO. They also suggested that nickel oxide 10000C.
244
and alumina exhibit a "mutual protective action against crystallisation."
The
same conclusions were reached from magnetic susceptibility studies [sO,slJ. The Weiss constant for materials calcined at 10000e showed a marked change at 50 mol % of NiO (corresponding to NiA1
However for samples heated to 700 204). the change in Weiss constant with composition was found to be linear.
0e
Rubinshtein et al. [34,52-54J examined similar materials made by coprecipitation using ammonia.
They contained up to 60 mol % of NiO and were 0e. heated at various temperatures up to 900 Parameters such as density, surface area, catalytic activity [52J, phase composition [34,s3J and magnetic properties [34,s4J were investigated as a function of composition and calcination temperature.
They conciuded that, contrary to the work of Milligan and Merten, NiO was
only soluble in y-A1
up to 50 mol % (spinel stoichiometry) and that higher 203 contents gave rise to NiA1 plus free NiO although both were poorly crystal20 4 0C. line. They thought that spinel was formed at temperatures as low as 400
Marked changes were observed in surface area and activity between catalysts heated to 750
0
and 900 0e.
The magnetic measurements were thought to confirm the 3 structural findings and to indicate the formation of Ni + ions in the materials. 0 0 The magnetic susceptibility showed marked changes at 300 and between 700 and 0e. l200 Later work [5sJ showed that the catalyst became more difficult to reduce as the alumina content increased and suggested that the active centres of the catalysts might be at the interface of the nickel and unreduced NiO/A1 203 solid solution. ESR spectra [3sJ of catalysts of high Ni content after reduction differed markedly from that of nickel produced from pure NiO.
The lattice
parameter of the nickel oxide in these catalysts was also somewhat smaller than usual (4.15-4.17
Rcompared
to 4.177
R for
pure NiO).
In 1965, Yoshitomi et al. [s6J compared the structure and activity of nickel /alumina catalysts of low Ni content impregnation and oxide mixing.
(~20%),
prepared by co-precipitation,
They tried to assess the degree of interaction
between the two oxides by measuring the intensity of the diffraction pattern of NiA1204 and the amount of nickel oxide that was soluble in hydrochloric acid. For the co-precipitated catalysts, the amounts of spinel increased and soluble nickel oxide decreased as the calcination temperature was increased. Holm and Clark [57 ] measured reduction profiles (temperature-programmed reduction) for catalysts prepared by co-precipitation and impregnation.
They
found that the co-precipitated ones were more difficult to reduce, "apparently because of better distribution and therefore better opportunity for interaction'. Shephard [58] examined the activity and- structure of co-precipitated nicke1/ alumina catalysts of high nickel content. The catalysts were heat treated in 0e 0 nitrogen at temperatures of 340 to l160 before testing their activity, and the total and metal surface areas measured.
The crystallite sizes of the nickel
oxide and nickel subsequently produced by hydrogen reduction were measured by
245 X-ray diffraction line broadening and electron microscopy.
Crystallite size and
surface area measurements were also carried out for catalysts treated with steam and hydrogen for varying times at temperatures up to 800°C [S9J. showed a rapid initial decrease but then stabilised. ions of the catalysts were found to be bimodal.
Surface areas
The pore size distribut-
Borisova, Dzis'ko et al. published a series of papers [60-63J concerning catalysts made by co-precipitation with sodium carbonate.
They attempted to
resolve the contradictory evidence regarding the phase composition and extent of solid solution in the oxide forms.
Precipitates of various NilAl ratios were
catcined in nitrogen and examined by X-ray diffraction.
For catalysts with Nil
Al atomic ratios of 0.ZZ-0.87, the d-spacings were intermediate between those of Nio and NiAl Z04• For higher nickel contents, the d-spacings approached those of NiO. They suggested that for NilAl atomic ratios~Z.O, the calcination in nitrogen at temperatures up' to soooe gave a single phase product, described as a solid solution or weakly crystallised compound of variable composition. higher nickel contents (Nil AI;;, Z.0) a "free NiO" phase was observed. atures above 780
0e
For
At temper-
this compound of variable composition decomposed to form
spinel, NiAl Z04 with separation of excess NiO. It was suggested that this compound becomes partially depleted of nickel upon reduction of the catalyst, to form a stable support for the metallic nickel [60J.
The extent of reduction by
hydrogen was found to increase with reduction temperature and the NilAl ratio of the catalyst.
The reduction characteristics were interpreted as implying that
for NiiAl ratios :;;z.a, the oxide structure changes continuously with NilAl ratio. At higher nickel contents, the degree of reduction increases due to excess Nia. However in order to explain the differences in reducibility between these high Ni catalysts and pure Nia it was necessary to postulate that the NiO was not "free" but actually a solid solution of Al
in NiO. Z03 Measurements of the surface areas and of the crystallite size of the nickel
after reduction were made as a function of NilAl ratio of the catalyst [6lJ. The total surface area decreased and nickel crystallite size increased as the reduction temperature and nickel content were raised.
Similar measurements were
made on co-precipitated catalysts of low Ni content and on catalysts made by precipitation on boehmite, by impregnation of y-Al ides ~2,63J.
and by mixing the hydroxZ03 The method of preparation was found to have a marked effect upon
reducibility.
The co-precipitated catalyst was the most difficult to reduce and
the mixed hydroxide the easiest.
In all cases the reduction was more difficult
than for pure Nia, indicating some degree of interaction with the alumina. extent of reduction of the impregnated catalyst increased with the nickel content to approach that of the mixed hydroxides. Various attempts have been made to assess the nickel area of reduced coprecipitated catalysts.
Rudajevov~
and Pour [64J compared chemisorption of
The
246 hydrogen, oxygen, carbon monoxide, hydrogen sulphide and nitrous oxide.
Apart
from oxygen and nitrous oxide which cause deep oxidation, consistent results were obtained.
Levina et al. [65J found that nickel surface areas determined by
chemisorption of hydrogen and oxygen, X-ray diffraction and magnetic measurements (calculated from crystallite sizes) all varied widely. Levina et al. [66J also studied the phase composition of co-precipitated catalysts (NiJAl ratios of 0-0.89) calcined at temperatures up to 450°C, by X-ray diffraction, infrared spectroscopy and thermal analysis. calcined at 300
0C
and.NiJAl ratios of 0-0.3, a spinel-like phase of low nickel
content was observed.
R (cf.
For materials
The d-spacing of the (440) reflection varied from 1.40-
Rfor
Rfor
NiA1 For NiJAl = 0.37 two 204). maxima were observed in this region. One was attributed to the spinel NiA1204 (d = 1.42 and the other to a distorted NiO structure (d = 1.459 cf. 1.476 1.41
1.398
y-A1 203 and 1.423
R)
R for
R,
(220) of NiO).
Catalysts of higher NiJAl ratios were also found to be two
phase, the intensity and d-spacing of the distorted NiO phase increasing with Ni content (see Fig. 3) to approach that of pure NiO.
Similar behaviour was
observed in materials heated to 450°C.
d,A Fig. 3. Variation in d-spacing of "NiO" with NiJAl ratio for co-precipitated catalysts (after Levina et al. [66] )
1·47
1·46
1·45
1·44 L..L..-LJ-'--J.l-...1-L~ 0·20-406 ()8 Ni/AI ratio
In a series of papers [8,67,68J Ross et al. developed a two phase model for the oxide form of co-precipitated catalysts of high Ni content.
This was essen-
tially similar to those proposed by Borisova, Dzis 'ko et al. and by Levina et a I , in that one phase consisted of poorly ordered spinel and the other nickel oxide with some dissolved alumina.
They suggested that these structures gave rise to
two distinct types of nickel upon reduction;
the nickel oxide was easier to
reduce and generated bulk nickel crystallites, and the nickel aluminate phase produced isolated (monodispersed) nickel atoms that remained closely associated with the alumina structure.
The hydrogen adsorption behaviour and catalytic
activity of these two types of nickel site would be different.
These suggest-
ions were supported by the fact that the activity and selectivity of the
247
2000
1000
100010
600·C 011000 800·C 4000/0
2000
1000·C
0
NIO
Y-AI 20 3
Fig. 4a (top). X-ray diffraction patterns of oxide form of a co-precipitated low nickel catalyst heated at various temperatures. Fig. 4b (below). Expansion gf higher angle region, showing breakdown of metastable phases above 800 C.
248 catalysts varied considerably as a function of temperature of reduction.
The
model was discussed further in a later paper [69J. 4.Z The continuously variable phase model In 1979, we proposed a new model of the oxide form [70J, based upon consideration of the crystal chemistry of the NiO/AI
system and studies by X-ray Z03 diffraction and temperature-programmed reduction (TPR). This suggested that the structure cannot be described in terms of one or more oxide phases but is
continuously variable. Catalysts of spinel and other stoichiometries were prepared using sodium carbonate or ammonium bicarbonate solution as precipitant.
The precipitates
were examined by X-ray diffraction and calcined in air to temperatures ranging from 450
0
to 10000C for two hours.
X-ray diffraction and TPR (HZ/Ar, 30
The oxide forms were then characterised by o/min. ramp rate). Typical results are
given in Figures 4&5.
450" 600" 800" 1000"
Fig. 5. TPR profiles of oxide form of a co-precipitated low nickel catalyst heated at various temperatures.
The reduction profile of a catalyst .which has been calcined at 450
0C
is
completely different from those of pure nickel oxide or nickel aluminate (Fig. oC. 0C S). Reduction is occurring at all temperatures between 450 and 900 Another small peak seen at about 3S0 0C is due to the reduction of nitrate. Materials calcined at 600
0
and BOOoC also show a broad range of reduction
temperature with signs of resolution of the profile into separate peaks. The sample calcined at 10000C shows a large peak in the TPR profile at about oC, 900 close to that of pure nickel aluminate. There is also a small peak at oC, about 450-500 just above the reduction temperature of pure nickel oxide under the same conditions. The X-ray diffraction patterns (Fig. 4) of the materials (CoKa radiation)
249 also show marked changes with calcination temperature. At first sight, it is easy to see how such materials calcined at 4S0oC could be identified as "poorly ordered spinels" (see Fig. 4a). However, close inspection of the higher angle lines (~1.37--l.S0~)
(see
Fig. 4b) reveals that the peaks are broadened asymmetrically and are shifted to positions between those of spinel and nickel oxide.
It is quite impossible to
explain such a profile by any mixture of nickel oxide and nickel aluminate, whether well ordered or not.
In the catalysts calcined at higher temperatures,
some ordering process is clearly taking place, and that calcined at lOOOoC corresponds closely to a mixture of nickel aluminate and a small amount of nickel oxide. For catalysts with higher Ni/Al ratios, the same general behaviour is seen. The TPR profiles change in,shape, with a larger fraction of the oxide reducing at a lower temperature but still show the broad range of reduction temperature seen in low nickel catalysts. reduction of Ni 3+ ions.
Another small peak occurs at ISOo-ZOOoC due to
Similarly the diffraction patterns show gradual changes
in shape with overall Ni/AI ratio for the lower calcination temperatures, and a higher fraction of nickel oxide in the materials calcined at lOOOoC. oC What we are apparently seeing in catalysts calcined at 4S0 is a continuous change in structure and reducibility between nickel oxide and nickel aluminate. At lOOOoC the material has disproportionated to form discrete particles of nickel oxide and nickel aluminate in a ratio dictated by the overall composition. 4.3 Crystal chemistry of the NiO/AlZ03 system The nickel oxide structure consists of a framework of cubic close-packed oxide ions with nickel ions occupying the octahedral holes. a face-centred cubic lattice like that of sodium chloride.
This gives rise to (See Fig. 6a).
departs from the fully regular structure in two ways however [7l,7ZJ;
It
firstly
it has full cubic symmetry (Fm3m) at temperatures only above lSOoC and secondly it is non-stoichiometric due to the formation of Ni 3+ ions and cation vacancies. Non-stoichiometry accounts for its black colour. The nickel aluminate structure is a spinel [71,73J. also based upon a framework of oxide ions which are in an almost perfect cubic close-packed array.
One
half of the octahedral sites and one eighth of the available tetrahedral sites are filled by cations giving eight AB
units per unit cell. It may be regarded Z04 cubes (where B cations are in octahedral sites) and
as being built up of 4 B 404 4 A0 tetrahedra with further tetrahedral A cations at the corners and face 4 centres [7ZJ (see Fig. 6b).
In a normal spinel, such as MgAl the eight tetrahedral A sites are Z04, occupied entirely by divalent cations (Mg Z+ ) and the octahedral B sites are only 3 occupied by trivalent cations (A1 +) . In an inverse spinel, such as NiFe Z04• the Z eight divalent cations (Ni + ) are all in octahedral B sites. The trivalent Fe 3+
250
Nickel in octahedral site
ox ygen layers
08
00
Figs. 6a,b. Crystal structures of nickel oxide and nickel aluminate. In the nickel oxide structure (top) the oxide ions occur in cubic close-packed layers and all nickel ions are in octahedral sites. In the spinel structure (below) the oxide framework is the same. In an inverse spinel like nickel aluminate, all the nickel ions are in octahedral (B) sites and aluminium ions are in both tetrahedral (A) and octahedral (B) sites.
251 ions occupy eight of the remaining octahedral B sites and eight tetrahedral A sites.
Normal and inverse spinels are limiting cases.
Where the divalent and
trivalent cations have comparable affinities for tetrahedral or octahedral sites
~4,75],
the structure may be partially inverse or even random.
Nickel alumin2+ ion
ate, NiA1204, is largely inverse, [73], due to the preference of the Ni for octahedral sites. The y-alumina structure is a defective spinel.
In the regular spinel, the
sixteen octahedral and eight tetrahedral sites are filled by divalent and trivalent cations.
In a defective spinel such as y-A1 the cation sites in the 203, same oxide lattice are filled only by trivalent ions. Only 21~ ions may be
accommodated to maintain electrical neutrality. This leaves 2~octahedral 3+ vacancies per cell. The A1 ions in y-A1 203 may be partially replaced by d i~va l ' + ent cat~ons such as Mg 2 + or. 2 N~ Thus both MgA1204 and N~A. 1 2 40 canform a complete range of solid solutions with y-alumina.
Materials of stoichiometry
A1 203.xNiO where
When x>l, separation into
x~
1 have a spinel type structure.
two phases would normally be expected to occur (NiO+NiA1
204). the X-ray powder diffraction data for NiO, NiA1 4 and y-A1 20 203•
Table 2 compares
TABLE 2 Corresponding X-ray reflections for nickel oxide, nickel aluminate and y-alumina NiO 0
2.8 (CoKa)
d (R)
1/10
hkl
2.410 2.088
43.6 50.8
91 100
(111) (200)
1.476
74.7
57
(220)
NiA1 0 2 4 o d (R) 2.e (CoKa)
1/10
hkl
4.65 2.846 2.427
22.2 36.7 43.3
20 20 100
(111) (220) (311)
2.013 1. 642 1.549 1. 423
52.8 66.1 70.6 77.9
65 8 30 60
(400) (422) (511) (440)
y-A1 d
203
db
2 eO 1/10 hkl (CoKa)
4.55 2.782 2.387 2.283 1.977
22.7 37.5 44.0 46.2 53.8
10 15 35 20 100
(111) (220) (311) (222) (400)
1.521 1.398
72.1 79.6
10 90
(511) (440)
Oxygen layer-tc-layerspacings 2.41 5.
R
2.33
R
2.28
R
SUMMARY OF MODEL FOR THE STRUCTURE OF CO-PRECIPITATED NICKELlALUMINA CATALYST
5.1 The co-precipitate and its decomposition The most common precursor in the co-precipitation of nicke11a1uminacatalysts is a Feitknecht compound with the nominal composition Ni6A12(OH)16C03.4H20. Ni/Al ratios lower than -2.0 or greater than
~3.0
If
are used then aluminium hydr-
oxide or nickel hydroxide are likely to be formed additionally.
The degree of
homogeneity and ordering in the precipitate depend greatly upon the conditions of precipitation and ageing.
The initial precipitate tends to be aluminium-rich,
252
with a gradual increase in Ni/Al ratio as precipitation proceeds.
Ostwald
ripening in the solution reduces this inhomogeneity, which can be monitored by X-ray diffraction (see Fig. 2). The Feitknecht compound has a double layer structure made up of brucite-like 3 layers separated by interlayers. Any discussion as to whether Al + ions are present in the brucite layer or form part of the interlayer is probably irrelevant with respect to catalyst preparation.
Under normal preparative conditions,
the structure is not well ordered and both types of structure may well co-exist. This lack of order is reflected in the temperature ranges over which the precipitates decompose, being much broader than for the better-ordered mineral analogues.
Decomposition is a two step process.
Other anions, especially nitrate, can replace carbonate in the structure. Other co-precipitates are also possible, especially using low Ni/Al ratios and a low pH for precipitation.
These are probably nickel aluminium hydroxy-
nitrates. 5.2 Oxide form As described earlier,under conditions of thermodynamic equilibrium, one would expect to form solid solutions between nickel aluminate and y-alumina in any proportions, but not between nickel aluminate and nickel oxide.
However the
conditions pertaining upon decomposition of a co-precipitate are not ones of equilibrium.
When a precursor such as the Feitknecht compound, Ni 6A12(OH)16 C03.4HZO, decomposes to the oxide form, it leaves oxide ions with nickel and aluminium cations distributed in a ratio of about 3/1 (subject to some local variations).
This is in between the Ni/Al ratios of nickel oxide (1/0) and
nickel aluminate (l/Z).
These cations must occupy the octahedral and tetra-
hedral sites between the oxide ions.
It is possible for re-arrangement of the
ions to form nickel oxide and nickel aluminate:Ni 6Al Z (OH) 16 C03' 4HZO ~ Ni/Al ratio 3/1
"Ni 6Al Z09" ~ Ni/Al ratio 3/1
5NiO+NiA1 20 4 Ni/AI ratio
1/0
l/Z
mean 3/1 However such a re-arrangement of the lattice would involve a large decrease in entropy, and therefore require a considerable energy input.
It is therefore
not surprising that metastable structures with Ni/Al ratios between those of nickel oxide and nickel aluminate should be observed when catalyst co-precipitates are decomposed at low calcination temperatures [39,70J.
What then is the
nature of these metastable structures ? As pointed out in the previous section, nickel oxide and nickel aluminate both contain a cubic close-packed lattice of oxide ions.
The structures differ
253 only in the way in which the cation vacancies are filled.
Let us reconsider
Figure 6.
If all the tetrahedrally co-ordinated ions in spinel (A04 units) are replaced by octahedrally co-ordinated ones (B404 units) we derive the nickel
oxide structure (Fig. 6a). units by octahedral ones.
Now consider partial replacement of tetrahedral What we obtain is a disordered structure, also based
upon a cubic close-packed oxide ion framework, intermediate between nickel oxide and nickel spinel in its Ni/Al ratio.
By similar arguments we can derive a
complete continuous range of structures between nickel oxide and y-alumina (see Table 3).
We have chosen to name the metastable phases as substituted nickel
oxide, disordered oxide-spinel intermediate and spinel-like material in order of decreasing Ni/Al ratio.
However it must be stressed that these are merely
labels of convenience to describe what is a continuous structural variation between nickel oxide and nickel aluminate. TABLE 3 Range of structures possible in oxide form of co-precipitated Ni/Al Z03 catalyst Structure
Probable site occupation Octahedral
NiO Substituted nickel oxide (S.N.O.) Disordered oxide - spinel intermediate (D.O.S.I.) Spinel-like material (S.L.M.) Ni AlZ04 Ni AlZ04/y-AlZ03 y-Al Z03 Note: Ni 3+ ions may also be present
Tetrahedral
.Z+
3+ Ni +,Al Ni Z+, (A13+)
N~Z
3 Z Consider the replacement of Ni + in the nickel oxide strucutre by A1 + Z 3 The first A1 + ions can probably substitute for Ni + in octahedral sites without seriously disrupting the structure.
Cation vacancies will also be created.
This type of structure we have called substituted nickel oxide (S.N.O.).
As the
degree of aluminium substitution increases, some aluminium starts to occupy tetrahedral sites as it does in the spinel.
Since this substitution is random
and not ordered, we have referred to this type of structure as disordered oxide 3+ - spinel intermediate (D.0.S.1.). Further incorporation of Al eventually produces a stoichiometry approaching that of spinel.
However the structure will
be much more disordered than that of nickel aluminate made by reaction of the two oxides. This is therefore called spinel-like material (S.L.M.). As further 3 3 nickel is replaced by A1 + (Al when x
254 The effect of overall Ni/Al ratio in the catalyst is to change the distribution of structure types.
Obviously high nickel catalysts will show a prepon-
derance of the high nickel structures (S.N.O. & D.O.S.I.) and low nickel ones more of the spinel-like material and nickel deficient spinels. These metastable structures arise because the Ni/Al ratio in the precursor precipitate is inherited by the oxide phase.
Any local variations in Ni/Al
ratio in the precipitate will also be transmitted to the oxide phase.
Evidence
for such local variations in Ni/Al ratio has come not only from X-ray diffraction and TPR but also from direct measurements of local Ni/Al ratio in a scanning transmission electron microscope using X-ray and energy loss detectors
~6J.
Selected areas (>200
R)
of the unreduced catalyst gave very widely
varying Ni/Al ratios; At about SOOOC these metastable structures start to break down to form ordered phases, that is nickel oxide and nickel aluminate.
The diffraction
patterns and reduction profiles resolve into distinct peaks (see Figs. 4,5). Calcination at low temperatures does not provide enough thermal energy to drive the ordering reaction at a significant rate. require a high calcination temperature (~SOOoC)
These metastable structures
to order quickly.
Thus, in the
oxide form of the co-precipitated catalysts, there is little if any true yalumina or true nickel oxide present, only metastable, mixed nickel aluminium oxides.
There is no clear distinction between the nickel phase and the support
J.
material [39 5.3 Reduced form Until the work of Ross et al. [6SJ who suggested that the reduced catalyst contained both bulk nickel and monodispersed nickel atoms associated with the alumina substrate, it was never questioned that the "nickel" in the reduced catalyst was present as any phase other than crystallites of the metal. On the basis of the model suggested for the oxide form of the catalyst, we predicted that complete removal of aluminate groups from the nickel during reduction would be energetically unfavourable [77,7SJ.
That is, the nickel
would be paracrystalline, with the lattice of each nickel crystallite disrupted by defects consisting of aluminate groups, thus producing local microstrains. It is possible to test for the existence of paracrystallinity using X-ray diffraction by measuring the line broadening of multiple order reflections ego (111), (222) and (444).
Hosemann et al. [79-SlJ have shown that iron/alumina
catalyst for ammonia synthesis and copper/zinc/alumina catalyst for methanol production were paracrystalline.
However the time required for such measure-
ments with conventional X-ray sources is extremely long.
In order to test our
predictions about the existence of paracrystallinity in the nickel of reduced co-precipitated catalysts, we decided to use time-of-flight neutron diffraction [82J rather than the X-ray method used previously.
This technique has the
255 advantages of "a broad spectral distribution, so that small d-spacings may be observed, and high beam intensity.
Line broadening may be due to (a) small
crystallite size, in which case it is independent of the order of the reflection (b) crystallite strain, where it is proportional to the order of the reflection or (c) paracrystallinity which produces broadening proportional to the square of the order. A co-precipitated catalyst of relatively high nickel content which had been used for steam reforming of a hydrocarbon feedstock was examined in the LINAC at A.E.R.E. Harwell.
equi~ment
Line broadening was measured for a series of
nickel reflections, corrected for instrumental broadening, and plotted against 2+k2+l2)/3 2+k2+l2)/3 J(h and against (h to determine the origin of the broadening.
The results (see Figs. 7 and 8) showed conclusively [82J that the nickel
was paracrystalline, like the active phases in the ammonia and methanol catalysts.
0-007
Ihhol
. ..
~
0·005
~
o
Ponc"/Stoiline Model
01357911131517 (h 2.k2+r2)/3
Figs. 7,8. Line broadening measurements by time-of-flight neutron diffraction. Fig. 7 (left) The strain model Fig. 8 (right) The paracrystallinity model. What then do we mean by a paracrystalline structure?
Hosemann [83
has been the pioneer in this field, suggests the following analogy.
J, who
If we take
a tray and cover it with small, identical coins, they will close-pack to form an ordered two-dimensional lattice. material.
This is equivalent to a crystalline
However, if we replace 10% of these coins by larger ones at random,
then the close-packed ordering will be disturbed (see Fig. 9). resentation of the paracrystalline state.
This is a rep-
The paracrystalline state is inter-
mediate in nature between crystalline and amorphous states (see Fig. 10). Random point defects within the structure set up local micros trains overlapping with each other.
In the catalyst, these defects are aluminate groups trapped
256
within the nickel crystallites.
The paracrystalline defects may indeed be
especially active sites .
• Fig. 9 (left). Random point defects (here aluminate groups) produce local microstrains and distorted lattice cells. Fig. 10 (right). Representations of the crystalline (top), paracrystalline (centre) and amorphous (bottom) lattices. The de-activation process for these catalysts may be concerned, not so much with the migration and growth of nickel crystallites, as existing models suggest, as with the gradual removal of paracrystalline defects.
Recent
evidence suggests that paracrystallinity is still present after long periods of use for such catalysts. 5.4 The new model and past work This new model helps to resolve a number of anomalies in the literature. The original conflict about whether a solid solution existed between NiO and NiA1 [49 ,53J, the apparent formation of "spinel" at temperatures as low as 204 0 400 [52} the change in lattice parameter of "NiO" with Ni/Al ratio [35,66J (see Fig. 3) are readily explained by the continuously variable, metastable mixed oxides suggested.
The changes in magnetic properties and surface areas
at about BOOoe [50-52,54J can be
asc~ibed
oxides to more ordered structures.
to the breakdown of those metastable
The differences in ESR spectra [35
Jbetween
reduced, co-precipitated catalyst and nickel derived from pure NiO can be attributed to the breakdown of magnetic structure by paracrystalline defects.
257
The distinctive reduction behaviour of co-precipitated catalysts [55,57,63,70J also agrees very well with the model. 6.
METAL/SUPPORT INTERACTION AND ITS REPRESENTATION It is clear from the preceding sections that discussion of the metal/support
interaction in a reduced, co-precipitated catalyst cannot be divorced from discussion of the equivalent interaction in the precursor precipitate and oxide. A device which we have found useful for representing interaction at all stages of tRe catalyst preparation and use is the phase distribution diagram [77J. was designed for application to two-metal catalysts, such as Ni/A1
It
but can
203 The phase distri203• of each phase present against the nickel
also be used for more complicated ones such as Cu/Zn/A1 bution diagram plots the
conc~ntration
and aluminium contents of that phase.
e
i
'NiO
Ni A1
i
2 04
;
I
I I
8
,
I
I
I
I I I
I I I
1 0·0
075/ 0·25
0·51
0·331
001
0·0
0·67
10
Fig. 11 shows the phase distribution diagram for an equimolar mixture of NiO, NiA1204 and y-A1 203•
(Ni/AI) Molar ratio
A homogeneous well-ordered phase will be represented by a vertical straight line.
If the phase has slight variations in composition, the line will be
broadened to reflect the distribution of Ni/Al ratios present. If no interaction between the two metals or their compounds is present, the diagram will simply be two vertical lines at the left and right hand sides. Any area in between represents a mixed metal phase and therefore some interaction.
This device will be used to illustrate the effect of variables in the
preparation upon the structure, although the diagrams should not be interpreted as quantitatively accurate •. 7.
DISCUSSION
7.1 Hereditary and environmental factors in the development of catalyst structure The psychologists tell us that our characters are determined by two sets of influences: (a) the genetic characteristics which are inherited by successive generations and (b) the environmental factors whereby our experiences may
258
modify these inherited traits. catalyst preparation [77
J.
This analogy may be usefully applied to
The preparation of a catalyst, especially by co-precipitation, involves a series of stages or "generations".
To what extent is the structure of succeed-
ing "generations" in the preparation governed by the structure of its precursor phase or "parent"? "environment"?
Or is it determined by the heat treatment it receives, the
By examining the structure at each stage of the preparation,
can we ascertain the relative importance of these "hereditary" and "environmental" factors ? The first step in the preparation of a co-precipitated catalyst is the treatment of a mixed solution of the metals, (which we will call the "greatgrandparent" or 1st generation) with alkali. variables;
We have already introduced many
the solutions may vary in nature, concentration, temperatures etc.
The alkali may be potassium, sodium or ammonium as hydroxide, carbonate or bicarbonate. pH etc.
We can vary the nickel/aluminium ratio, total metal/alkali ratios,
However in the solution, we have complete mixing of nickel and alumin-
ium ions and the phase distribution diagram (p.d.d.) is simply a straight line representing the initial Ni/Al ratio (see Fig. 12).
We will use these diagrams
to follow how the nickel/aluminium distribution varies through the steps of the preparation. As mentioned earlier, the first material precipitated is aluminium-rich and as the pH decreases the fraction of nickel in the precipitate increases. then has happened to our idea of structural heredity?
What
We started with a homo-
geneous solution and have prepared an inhomogeneous precipitate.
The material
being precipitated at any instant is determined completely by the NijAl ratio and the pH of the "parent" solution;
it is just that the pH changes during
precipitation. Thus the p.d.d. for precipitation of a Feitknecht compound is as shown in Figure 12.
The centroid of the peak corresponds to a NijAl ratio of 3/1 and
the width is a measure of the inhomogeneity. material to become more homogeneous.
Ageing of the precipitate causes
This is illustrated by a narrowing of
the p.d.d. When catalysts of very high or very low Ni/Al ratio are made, the precipitate usually consists of two phases.
For example a low nickel precipitate may
contain a Feitknecht compound plus a free alumina phase such as boehmite.
This
phase separation can also be shown in a p.d.d. (Fig. 13). Thus the precipitate depends upon the parent solution.
It also depends upon
the thermal ageing to which the suspension has been subjected (the environment). The more extreme the ageing treatment, the more ordered are the phases formed. Thus we have a competition between the inherited disorder and the ordering caused by thermal treatment.
259
Ni/AI ratio in ........ parent solution
Ni/AI ratio in parent SOlution~1
I I
:
Precipitate after ageIng (more homogeneous)
I
!I
)
Feitknecht compound
Free alumina phase ____
~preCiPitate
before ageing (Ni/AI ratio varies)
I
I
0751 0-25
101
00
051 0·0
O{)I
033 0-67
'0
O' 5/
0·33/
OW
0'5
0'67
1·0
(NitAI) Molar ratio
(Ni/AI) Molar ratio
Fig. 12 (left). Phase distribution diagram showing changes occurring during precipitation and ageing. . Fig. 13 (right). Diagram showing phase separation in precipitation from solution of low Ni/Al ratio. So much for the "grandparent" precipitate. form.
Let us now examine the oxide
In this stage the phase distribution diagram shows marked differences
between the continuously variable phase model described here and earlier single or two phase models.
The phase distribution for the earlier models would be
two lines; one for the "nickel oxide" phase and one for the "poorly ordered spinel".
For the continuously variable phase model the p.d.d. is a broad peak
whose centroid corresponds to the overall Ni/Al ratio and whose width reflects the inhomogeneity of the phase composition.
The X-ray diffraction and TPR
data tell us that a wide range of environments is present in materials calcined at low temperatures.
For catalysts of spinel stoichiometry, the range of
structures extends continuously from almost pure NiO to almost pure y-A1 203. (see Fig. 4b). The Ni/Al ratio and structure distribution are inherited from the "parent" precipitate.
If we look at catalysts of higher nickel content,
the same metastable phases are formed but in different ratios.
Interaction .2+
between the metals is virtually complete, with nearly all the N1 present as mixed phases.
and Al
3+
ions
In both catalysts, the phase distribution is complete-
ly different from that in impregnated catalysts, where interaction is confined to the formation of a small amount of "surface spinel" (see Fig. 14). effect of calcination temperature is illustrated in Figure 15.
The
The broad dist-
ribution of structures breaks down at high temperature to form essentially pure nickel oxide and nickel aluminate.
Thus again we have a competition between
thermal ordering during calcination, and the disorder and variable Ni/Al ratios inherited from the precipitate.
An extreme environment (high temperature)
supplies enough energy to drive the ordering reaction at a significant rate. At low calcination temperatures, the ordering process is very slow.
260
- - - Impregnated catalyst ' - ' - ' High nickel co-precipitated catalyst
450°C
'OOO'C
- - Low nickel co- precipitated catalyst
i .: i
U
n ji..
§
~:
U I \N;O
\
, ,,
I
'./-
0·5/ 0·0
1·0/
0-0
NiAl204
0·331 0·67
.•
Ni/AI ratio J: in solution --1-1
1-0/ 0·0
1-0
\
j11-'" :\
\ -"~------)
O~I
-if\\
0-7S/ 0·25
O-S/ 0·5
0-33/ 0·67
"0-0/ 1·0
«NiIAI) Molar ratio
( NilAI) Molar ratiO
Fig. 14 (left). Phase distribution diagrams for two co-precipitated nickel/ alumina catalysts (high and low nickel content) ccm~ared with that of an impregnated catalyst (oxide forms). Fig. 15 (right). Diagram showing the effect of calcination temperature upon the phase distribution for the oxide form of a co-precipitated catalyst.
Gt. Grandparents
I
ENVIRONMENT
I
~
Grandparent
Inhomogeneous Ni/AI ratio varies Phase separation
~ Microparacrystals 01 aluminate substituted nickel No true nickel or l'-alumina
I
Severe
I
Sintering Nrckel, (~Alumina
Fig. 16. Diagram showing the compet1t10n between structural disorder derived from the parent phase (the hereditary factor) and thermal ordering (the environmental factor) at each stage in the preparation of a co-precipitated nickel/alumina catalyst.
261 The same factors may also be applied to the reduced state. paracrystalline because of inherited disorder.
The nickel is
High temperature sintering,
involving the gradual removal of the paracrystalline defects may also be seen as a thermal ordering process. Thus at every stage, the structure is the result of a competition between inherited disorder, and thermal ordering.
This is shown in Figure 16.
7.2 Comments on techniques for studying co-precipitated catalysts Because the metal content of these catalysts is high, and interaction is so extensive at all stages of the preparation, bulk analytical techniques are far more informative than they are for some other catalysts.
However, again because
of the extensive interaction, we have to be very careful about the interpretation of some measurements. There are a number of papers in the literature which refer to the determination of crystallite size distributions for both nickel oxide and nickel in coprecipitated Ni/AI
203 magnetic measurements.
catalysts, using X-ray diffraction line broadening and In the case of the oxide form, the line broadening is
due partly to crystallite size but also to variations in local Ni/Al ratios and hence structure.
In the reduced state, the magnetic structure will be broken up
by the paracrystalline defects, and the line broadening will be due to a combination of crystallite size and paracrystallinity.
Thus both X-ray diffraction
and magnetic measurements are valuable tools for investigating the structure, but results must be interpreted with great care. cannot be obtained at present.
Crystallite size distributions
Diffraction techniques give useful information
at every stage in the catalyst preparation, from precipitation to de-activation in use.
Investigations of paracrystallinity are carried out much more easily
using time-of-flight neutron diffraction than with X-ray diffraction. Temperature-programmed reduction, whilst it does not provide direct structural information, is an excellent technique for characterising co-precipitated materials and for investigating interaction with the support.
Provided that
normal precautions are observed concerning sample preparation and heating rates, it can give a good semi-quantitative estimate of the mixed oxides present. is probably easier to use for this purpose than X-ray diffraction.
It
Conventional
thermogravimetric analysis is also useful for investigating the decomposition of the precipitate; Chemical analysis is not always very informative because the materials tend to be inhomogeneous.
The chemical dissolution technique, ego hydrochloric acid
[:6J may be a useful empirical approach to the determination of extent of interaction.
However, since the oxide structure is continuously variable in Ni/AI
ratio, it is difficult to say exactly what is being dissolved. Although we have no direct experience, some spectroscopic techniques look potentially valuable.
EXAFS spectra obtained for the oxide form of a Ni/Al203
262 catalyst Q34J may readily be explained in terms of the model described here. Electron spin resonance (ESR) spectroscopy may be suitable for investigating metal/support interaction in the reduced catalyst [35J.
Mossbauer spectroscopy
has been used to study paracrystallinity in the iron of co-precipitated ammonia synthesis catalysts [85,86J.
Although valuable for catalysts containing Fe and
Sn, its use for other elements, such as Ni, Sb, is more difficult.
X-ray photo-
electron spectroscopy (XPS) has been applied to both impregnated [87,88J and coprecipitated catalysts [89,90J to study metal/support interaction. results for co-precipitated Ni/Al with X-ray diffraction
~esults.
The XPS
catalyst [90J were apparently at variance
Z03 However this discrepancy is readily explained
by the structural model suggested here. Electron microscopy, especially STEM with elemental analysis [9lJ, is very valuable in that it can be used to investigate
~omogeneity
in precursor stages
and the final catalyst, as well as giving information about texture and particle sizes.
Particle sizes may, in fact, be misleading if the structure is defective
or paracrystalline. Surface area measurements, such as NZ BET areas or HZ chemisorption, must like X-ray line broadening, be interpreted with great care. Assumptions concerning stoichiometry of adsorption are certainly questionable since the structure of a paracrystallite is so markedly different from that of a nickel single crystal.
Hosemann [83] talks of the "inner surface area" of paracrystal-
lites and it may be that such a defect-ridden structure has a measure of porosity to hydrogen. Our own experience is that it is dangerous to rely heavily on any single technique, and that for co-precipitated catalysts, the multiple-technique approach is necessary.
Moreover the spectroscopic techniques have not, per se,
generated a satisfactory structural model.
However once a suitable model has
been derived by other methods, they can give valuable supportive information. 7.3 Differences between co-precipitated and impregnated catalysts It is impossible to review here the vast amount of work which has been done on impregnated nickel/alumina catalysts.
We will confine our discussion to a
few papers concerned with the metal/support interaction in Ni/y-Al Z03 catalysts, which are probably those closest to the co-precipitated catalyst. Lo Jacono, Schiavello et al. (9Z-94J have shown that when a y-alumina 0 . support ~s. ~mpregnated w~t. h N~ .Z+. ~ons an d h eate d to 600 C, two react~ons • occur concurrently;
one produces a "surface spinel", and the other gives segregation
of free nickel oxide.
Spectroscopic evidence showed that the distribution of
nickel ions between tetrahedral and octahedral sites, and the extent of segregation of nickel oxide were affected by the nickel content and atmosphere of
firin~
The distribution was not necessarily the same as in a well-crystalline spinel, Z+ and was affected by the addition of small amounts of foreign ions such as Zn ,
263 Ga3+ .
Specimens containing a large fraction of nickel in tetrahedral sites were
not easily reduced.
The authors suggested that the reactions might involve
counter diffusion of the cations through the oxide lattice [95J.
The foreign
ions were thought to affect the reactions by modifying the equilibrium cationic vacancies [74,75J.
Since y-alumina has the same type of structure as nickel
aluminate and has cation vacancies it is reasonable that compound formation should occur easily by cation diffusion.
In impregnated catalysts of low nickel
loading, one would not expect the stoichiometric spinel compound, but rather solid solutions of the type x NiO. A1 where x
This "surface spinel" is probably not
radically different from the solid solution phases that can be made by solid state reaction of the oxides [9.5,97,98J. catalyst remains as the pure oxides. is shown in Figure 14.
Note that no c~ound
nickel oxide and nickel aluminate.
A considerable fraction of the
A corresponding phase distribution diagram occurs with stoichiometry between
Interaction takes place without any major
change to the structure of the support.
Calcination at high temperature would
be expected to increase the extent of interaction. Upon reduction of such a catalyst, the "surface spinel" probably does not react at normal temperatures and the free nickel oxide gives rise to conventional, small nickel crystallites.
Metal/support interaction in the reduced state
is inter-crystallite and probably not especially strong.
In impregnated
catalysts, there is a clear distinction between metal and support. This behaviour contrasts markedly with that of the co-precipitated catalyst. In the oxide form, interaction is virtually complete, with almost none of the pure oxides present. what is the support.
It is not possible to say what is the metal oxide and A complete range of mixed oxides is possible.
These
oxides are highly disordered, metastable, and cannot be prepared by conventional routes such as solid state reaction of the individual oxides. formed by the co-precipitation route.
They can only be
The degree of interaction is actually
decreased by high temperature calcination, as pure nickel oxide separates. Upon reduction,. the nickel produced is full of paracrystalline defects which take the form of aluminate groups. extensive and is intra-crystallite.
Thus metal/support interaction is very Again any division into separate active and
support phases is scarcely meaningful. The reasons for these differences in interaction between co-precipitated and impregnated catalysts can be traced back to the first steps in their preparations.
Co-precipitated catalysts are made via a mixed solution, a mixed basic
carbonate and finally a mixed oxide.
In the preparation of impregnated catalysts
the two metals are always separate, except for the formation of a limited amount
264 of mixed oxide by calcination. 7.4 Origins of paracrystallinity and the prediction of structure in co-precipitated catalysts Three types of catalyst' of industrial importance have now been shown to be paracrystalline in their active states.
These are iron/alumina for ammonia
production [79-81,85,86J, copper/zinc/alumina for methanol synthesis [8lJ and now nickel/alumina [82J.
Two of these catalysts were prepared by co-precipita-
tion, and this suggests the possibility of a link between paracrystallinity and the co-precipitation route. Paracrystallinity has been found in a wide variety of materials ranging from synthetic polymers and proteins to pyrolytic graphite [99J, where co-precipitation obviously cannot be the cause of the defects.
However we would suggest
that whenever the co-precipitation route is used, the possibility of paracrystallinity in the catalyst produced always has to be considered.
One can go
further and say that any catalyst preparation route which involves a mixed phase (in the strict sense and not a mixture of phases) may generate paracrystallinity in the final product. We shall attempt to define what conditions of composition and crystal chemistry must be fulfilled for the genesis of such a paracrystalline catalyst:(a) Firstly the two or more metals of the catalyst must fornl a mixed oxide (assuming that the final step involves reduction of an oxide precursor). (b) The fraction of the active reducible metal in this mixed .oxide must be high. If the amount of the irreducible component (eg. alumina) that forms the paracrystalline defects is too high, the structure is unlikely to be stable.
Con-
versely if it is too low, then the reduced metal will not have the resistance to sintering imparted by paracrystallinity. reduce at a relatively low temperature.
Furthermore the active metal must Otherwise the defects may be rapidly
removed during the reduction process. (c) The mixed oxide precursor may be a normal, stable solid solution such as NiO/MgO or Fe
as formed in the ammonia synthesis catalyst. Such 304/Al z0 3 precursor oxides may have various structures ego sodium chloride, perovskite, spinel etc.
The general synthesis methods for these mixed oxides are well
understood and have been reviewed by Courty & Marcilly [lOOJ. (d) Even where a stable solid solution of the required composition (ie. right fraction of reducible metal) does not form, such as the nickel/alumina system, it may still be possible to prepare a suitable metastable mixed oxide by decomposition at relatively low temperatures of a mixed salt or hydroxide. precipitation is a simple way to prepare such precursors.
Co-
Metastable mixed
oxides cannot be prepared by impregnation methods or by direct reaction of the oxides. These routes to the possible formation of a paracrystalline catalyst are
265
shown in Figure 17. Impregnation /firing
Direct reaction of oxides
t
Stable mixed salt/hydroxide ego Feitknecht compound Ni!Mg formate fLOW temperature
Stable mixed oxide eg.NiO/MgO _ Fe 0 4/A1 0 3 Z 3
Metastable mixed oxide ego NiO/A1 Z03 PARACRYSTALLINE CATALYST eg Fe/A1 Z03 ' Ni/A1 Z03
Fig. 17. Possible pathways for the genesis of paracrysta11ine catalysts Let us then consider in more detail possible synthetic routes to the metastable mixed oxide.
In principle any mixed salt or hydroxide might be used as
a precursor to a metastable mixed oxide. The possible permutations of metals which can be obtained by co-precipitation have been discussed by Andrew [12J.
The nickel aluminium hydroxy-carbonate
that is the precursor in nickel/alumina catalysts is only one of a large family of compounds.
Many divalent and trivalent metals may be co-precipitated as
Feitknecht compounds (see section 3).
The magnesium/aluminium analogue decom-
poses to form a metastable oxide, as does the zinc/aluminium one [101].
It
seems very likely that some synthesis routes for CU/A1 and CU/Zn/A1 Z03 Z03 catalysts for methanol synthesis also involve the formation of a Feitknecht precursor and then metastable mixed oxides.
The calcination temperature for
decomposition of the co-precipitate must be kept low to avoid breakdown of the metastable mixed oxide.
The facility with which the metastable mixed oxide
disproportionates depends upon a number of factors including the structure of the individual oxides, and the site preference energies and stable oxidation states of the cations present. For example, nickel oxide, magnesium oxide and y-a1umina all have the same cubic close-packed oxide ion framework. different.
The zinc oxide structure is slightly
Thus we might expect the Mg/A1 and Ni/A1 oxides to disproportionate
less readily than the Zn/A1 one. Thus to summarise, for preparations by co-precipitation, we need to consider (a) Can the metals form a true co-precipitate, ie.sing1e phase?
The pH of
precipitation of the individual hydroxides will be relevant to this question. (b) Does that single phase co-precipitate contain the correct fraction of the
266 active, reducible metal ion to yield a paracrystalline catalyst? (c) What are the optimum conditions for the precipitation of that single phase precursor? (d) What are the crystal structures (in particular, oxide frameworks) of the individual oxides and stoichiometries and structures of known stable mixed oxides? (e) What are the stable oxidation states of the metals present under calcination conditions? (f) At what temperature
~hould
the precursor be calcined to preserve the meta-
stable mixed oxide? and (g) Under what conditions should the catalyst be reduced to generate paracrystallinity? 8.
CONCLUSIONS Nickel/alumina catalysts prepared by co-precipitation differ from their
counterparts made by impregnation because the interaction is far more extensive at every stage in the preparation.
In the oxide state, this interaction is
demonstrated by the formation of metastable mixed oxides with scarcely no pure nickel oxide or y-alumina present.
In the active, reduced state, the inter-
action persists as paracrystallinity of the nickel.
The lattice of each nickel
crystallite is strained internally by defects consisting of aluminate groups. Thus the metal/support interaction is intracrystallite rather than intercrystallite as in an impregnated catalyst.
Co-precipitated nickel/alumina is
the third catalyst of industrial importance that has been shown to be paracrystalline and it seems likely that a number of other co-precipitated catalysts will be found to be so. At each stage in the preparation of the co-precipitated nickel/alumina catalyst, there is a competition between disorder inherited from its parent stage and a thermal ordering process.
If we are to fully understand the
structure and properties of the final, reduced catalyst, we must also consider the structure at each earlier stage in the preparation.
In the words of Delmon
and Grange, "Solids have a memory of the way they have been prepared.
More
generally, the way each transformation has taken place has some influence on the subsequent steps"
[102].
Acknowledgements The authors would like to thank their many colleagues at the Midlands and London Research Stations of British Gas who contributed to this work and the British Gas Corporation for permission to publish it.
267 REFERENCES 1 2 3 4 5 6 7 8
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269
72 N.N. Greenwood in "Ionic Crystals, Lattice Defects & Non Stoichiometry", Butterworths, 1970, p.166. 73 G. Blasse, Philips Research Repts.Suppl. 3 (1964). 74 A. Miller, J.Appl.Phys. 30 (1959) 24S-25S. 75 A. Navrotsky and O.J. Kleppa, J.Inorg.Nucl.Chem. 29 (1967) 2701-2714. 76 A. Howie, S. Collett, and D.C. Puxley, to be published. 77 D.C. Puxley, Roy. Soc. Chern. Conference on "Constitution and Structure of Active Sites in Catalysts", Reading, 198178 D.C. Puxley, Institute of Physics Crystallography Conference, UMIST, Manchester, 1981. 79 R. Hosemann, A. Preisinger and W. Vogel, Ber.Bunsenges. Phys.Chem. 70 (1966) 796-802. 80 H. Ludwiczek, A. Preisinger, A. Fischer, R. Hosemann, A. Schonfeld and W. Vogel, J.Cat. 51 (1978) 326-337. 81 A. Fischer, R. Hosemann, W. Vogel, J. Koutec:ky, J. Pohl and M. Ralek, Proceedings of the VIlth Int.Cong. on Catalysis, Tokyo,1980. 82 C.J. Wright, C.G. Windsor and D.C. Puxley, J.Cat., in the press. 83 R. Hosemann, W. Vogel, D. Weick and F.J. Balta-Calleja, Acta.Cryst. A37 (1981) 85-9184 R.W. Joyner in "Character~sation of Catalysts", (J.M. Thomas, R.M. Lambert, eds.) Wiley, 1980, pp.237-253. 85 F. Garbassi, G. Fagherazzi, M. Calcaterra, J.Cat. 26 (1972) 338-343. 86 G. Fagherazzi, F. Galante, F. Garbassi and N. Pernicone, J.Cat. 26 (1972) 344-347. 87 M. Wu and D.M. Hercules, J.Phys.Chem. 83 (1979) 2003-2008. 88 W.N. Delgass, T.R. Hughes and C.S. Fadley, Cat.Rev.4 (1970) 179-220. 89 R.B. Shalvoy and P.J. Reucroft, J.Electr. Spectr. Rel.Phen. 12 (1977) 351356. 90 R.B. Shalvoy, B.H. Davis and P.J. Reucroft, Surf. & Interface Anal.2 (1980) 11-16. 91 A. Howie in "Characterisation of Catalysts" (J .M. Thomas, R.M. Lambert, eds.) Wiley, 1980, pp.89-l04. 92 M. Lo Jacono, M. Schiavello and A. Cimino, J.Phys.Chem. 75 (1971) 1044-1056. 93 E. Borello, A. Cimino, G. Ghiotti, M. Lo Jacono, M. Schiavello and A. Zecchina, Disc.Faraday Soc., 52 (1971) 149-172. 94 M. Lo Jacono and M. Schiavello in "Preparation of Catalysts I" (B. Delmon, P.A. Jacobs, G. Poncelet, eds.) Elsevier, Amsterdam, 1976, pp.473-487. 95 F.S. Stone and R.J.D. Tilley in "Proc , 5th Int.Syrnp. on Reactivity of Solids, 1964" (G.M. Schwab, ed.) Elsevier, Amsterdam, 1965, pp.583-595. 96 J. Cervello, E. Hermana, J.F. Jiml!nez and F. Melo in "Preparation of Catalysts I" (B. Delmon, P.A. Jacobs, G. Poncelet, eds.) Elsevier, Amsterdam, 1976, pp.25l-263. 97 A. Lejus and R. Collongues in "Proc. 5th Int.Syrnp. on Reactivity of Solids, 1964" (G.M. Schwab ed.) Elsevier, Amsterdam, 1965, pp.373-38l. 98 A. Lejus, Rev.Hautes Temper. et Refract., 1 (1964) 53-95. 99 F.J. Balta-Calleja and R. Hosemann, J.Appl.Cryst., 13 (1980) 521-523. 100 P. Courty and C. Marcilly in "Preparation of Catalysts I" (B. Delmon, P.A. Jacobs, G. Poncelet, eds.) Elsevier, Amsterdam, 1976, pp.119-l45. 101 S.M. Bruce and D.C. Puxley, British Gas, unpublished results. 102 B. Delmon and P. Grange in "Catalyst Deactivation" (B. Delmon, G.F. Froment, eds.) Elsevier, Amsterdam, 1980, pp.507-543.
270 DISCUSSION N. PERNICONE : I was very glad to hear your comprehensive discussion of applications of paracrystallinity in catalysis as, when the concept of paracrystallinity was firtsly introduced by Hosemann and applied to ammonia synthesis catalysts, most catalyst researchers fail to recognise its importance. I would like to recall here that in 1972 at the Fifth International Congress on Catalysis (Palm Beach, U.S.A.) I presented some evidence for increase of catalytic activity due to the paracrystallinity induced by the insertion of FeAl204 in the lattice of a-Fe. D.C. PUXLEY Unfortunately, as you say, the concept of paracrystallinity has been slow to gain general acceptance. An important factor must be the experimefr tal difficulties in verifying the existence of paracrystallinity. The work that you referred to on ammonia synthesis catalyst is particularly valuable in that it provided evidence for the paracrystalline nature not only by diffraction methods, but also by M6ssbauer spectroscopy and thermal analysis (see references 85 and 86). It is a pity that our nickel catalysts are not suitable for study by Mossbauer spectroscopy. We are sure that many more examples of paracrystallinity in catalysts will be found, but more work is needed to provide both a better theoretical understanding of the physical nature and origins of paracrystallinity and also easier experimental techniques for its detection. S.P.S. ANDREW: I am not a believer in paracrystallinity. I suspect the X-ray diffraction evidence may solely be the result of strain resulting from the different coefficients of expansion of metal and oxide phases. Furthermore I doubt whether the presence of alumina micro inclusions inside the nickel particles would sufficiently stabilise the nickel against sintering caused by surface mobility of nickel atoms. D.C. PUXLEY As stated before, more work is needed on paracrystallinity. However, I would like to point out two factors which-tend to support the paracrystalline model rather than the strain model suggested. Firstly, such strain would be expected to produce broadening varying linearly with reflection order. This is the basis of the well-known Warren-Averbach method of separating the effects of crystallite size and strain. Figures 7 and 8 show that the broadening gives a much better correlation when plotted against the square of the order, as predicted by the theory of paracrystallinity. Secondly, if the role of the support was the conventional one as you suggest, rather than an internal stabilization effect, one might expect that an increase in the fraction of support would lead to enhanced stability. This is not observed. It would also be extremely difficult to understand the proven stability of the ammonia synthesis catalyst which contains only 3% alumina. C.J. WRIGHT: May I comment upon the remarks of Professor Andrew concerning the existence of paracrystallinity and the sintering behaviour of paracrystals. The difference between paracrystallinity and microstrain is a subtle one since both are descriptions of disorder within a crystallite. Microstrain is defined as a disorder in which individual crystals contain microcrystals within which the lattice parameter is constant but different to that of adjacent microcrystals. Consequently there is a distribution of lattice parameters over the microcrystals. In contrast in a paracrystal there is a random distribution of separations be~een adjacent crystal planes. Turning to the subject of the stability of a paracrystal, may I refer to a paper in the Journal of Catalysis in which a simple theory was proposed which explained this enhanced stability in terms of the additional free energy within a paracrystal caused by the defects. The strain energy and the surface energy of the paracrystals vary in opposite directions as the paracrystals sinter.
271 L. RIEKERT : Are there any quantitative kinetic data such as rates of reaction per unit surface area of nickel for paracrystalline metal particles as compared to nickel obtained by reduction of NiO? What is the basis of the claim that paracrystallinity is beneficial to catalytic properties? D.C. PUXLEY: We have not carried out any comparison of the specific activity of the nickel in these catalysts with that derived from nickel oxide. Kruissink et al. (reference 8) measured the specific activity for CO hydrogenation of a series of co-precipitated mickel/alumina catalysts. They found evidence that the specific activity varied with the method of preparation, as we would expect if they were paracrystalline. It must be pointed out that the specific activity of a paracrystalline catalyst may not be strictly comparable with those of other catalysts, because of uncertainties about the meaning of surface areas obtained by hydrogen chemisorption. We do not have any evidence that paracrystallinity engenders a higher specific activity in the catalyst. We would claim that the activity is maintained longer under industrial condit~ons. All the catalysts in which paracrystallinity has been found have been in industrial use for many years, and are of proven stability. Furthermore, all of them contain very low fractions of support. It is difficult to reconcile the proven stability and very high metal loading with the conventional model of discrete active and support phases. I am not aware of any catalysts of such high metal loading and known weak metal/support interaction that exhibit this degree of stability.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III
273
© 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
AN ASSESSMENT OF THE INFLUENCE OF THE PREPARATION l\lETHOD, THE NATURE OF THE CARRIER AND THE USE OF ADDITIVES ON THE STATE, DISPERSION AND REDUCIBILITY OF A DEPOSITED "NICKEL OXIDE" PHASE 1". HOUALLA
Groupe de Phys i co-Ch inrie f1inerale et de Ca ta lyse , Universite Catholique de Louvain, Louvain-La-Neuve (Belgium) *Present address: University of Pittsburgh, Department of Chemistry, Pittsburgh, PA 15260 (USA)
ABSTRACT The communication is an attempt to investigate by the combined use of various techniques (X-ray diffraction XRD, Analytical Electron f1icroscopy Am, XPS, Temperature programmed reduction TPR, etc.) the influence of some major factors on the state, dispersion and reducibility of a deposited "nickel oxide" phase. We have examined the influence of (i) the preparation method, (ii) the nature of the carrier and (iii) the use of additives. The systems considered in the present study consist respectively, of (i) two series of impregnated and ion-exchanged silica supported nickel oxides; (ii) a series of nickel oxides deposited on a wide range of silica-aluminas, and (iii) three series of "NiO" deposited on Li, Na and K modified aluminas.
INTRODUCTION Supported catalysts are most commonly obtained by (a) impregnation of a high and surface area carrier with the metal salt precursor followed by (b) ~, (c) decomposition of the precursor salt (e.g. calcination) and (d) activation of the supported oxide via reduction or reduction-sulfidation in the case of hydrotreating catalysts. In a previous review article presented at the Second Symposium on Preparation of Catalysts [1], a tentative claSSification of the various factors which affect the reduction of supported oxides has been proposed. In particular, it was pointed out that two major factors can be discerned, namely, (t ) the dispersion of the deposited oxide and (ii) Hs_ interaction with the carrier. These factors are in turn affected by various parameters. The present communication will examine various parameters which have been shown to have a signifiCant bearing on the dispersion of the supported oxide and the active oxide-support interaction, namely:
274
1. The choice of the preparation method 2. The nature of the carrier 3. The use of additives For the sake of consistency, the investigation of the influence of the above mentioned parameters will be restricted to the case of supported nickel oxide. The conclusions may, however, be readily extrapolated to other systems. The results discussed in this paper were obtained by ourselves and other investigators in the Groupe de Physico-Chimie Minerale et de Catalyse. The physico-chemical characterization and reactivity of the deposited nickel oxides were assessed by the combined use of several techniques, namely, XPS, Analytical Electron Microscopy AEM, X-ray Diffraction XRD in conjunction with isothermal or temperature programmed reduction experiments (TPR). We will first, briefly, discuss the methodology adopted in the course of this study and then examine, successively, the influence of the preparation method, the nature of the carrier and the use of additives on the state, dispersion and reducibility of a deposited "nickel oxide" phase.
METHODOLOGY Overa11 Remarks An adequate characterization of the surface structure of supported oxides often requires the combined use of several techniques Which complement each other. Some of these techniques adopted here are of routine use in most laboratories. Others such as TPR, XPS and AEM have been described in previous papers [2-7]. However, because of the relevance of the joint use of XPS and AEM to the present study, we will discuss the capabilities of these techniques. AEM AEM designates the joint use in the same. equipment (Jeol Temscan 100 cx) of the techniques of Transmission Electron Microscopy TEM (bright-field, darkfield and high resolution imaging electron diffraction) together with the microanalytical capabilities offered by a Kevex energy dispersive x-ray spectrometer. In the case of supported oxides the active phase (m) may be present either as crystallized particles or as a highly dispersed phase (for instance, as a surface compound). The former case is readily distinguished by TEM if the crystallite sizes are larger than a few nanometers. The latter case may be detected using X-ray microanalysis, for instance by directing the finely focused electron probe onto areas of the support(s) where no crystallized
275
particles are distinguished and measuring the characteristic X-ray intensity ratio 1m/Is' This ratio may easily be converted into concentration ratio by the use of thin film approximation which neglects absorption and fluorescence effects [7]. Another main advantage of AEM for the study of supported catalysts rests on the possibility of assessing the repartition of the supported phase by pointing the finely focused electron probe on various thin areas of the sample to measure the local active phase concentration [5,7]. XPS XPS spectra were recorded with a Vacuum Generators ESCA2 spectrometer equipped with an aluminum anode (Al Ka = 1486.6 eV) operated at 50 mA and 10 kV. The residual pressure inside the spectrometer was 10-8 Torr. A Tracor Northern NS 560 signal averager was used, in order to improve the signal to noise ratio. Several models have been proposed to relate XPS intensity ratio of two peaks, associated respectively with a supported phase and a carrier, to the dispersion of the deposited substance (i .e. the ratio of the surface area to the volume of the supported species) [8-11]. The validity of most of these models rests, however, on the assumption, not always justified [5,12] of a similar repartition (i .e. relative amount of species deposited in the inner parts of the elementary catalyst particle and those located at the mouth of the pores or on the external surface of the particle), of the supported phase in all the samples. Bearing in mind this limitation, it has been shown by Kerkhof and Hou1ijn [11] that, assuming a sheet-like structure of the carrier with cubic particles of the deposited phase in between, one may obtain different relations, according to the various configurations of the supported material over the carrier. We may write the following equations in the case of two typical catalysts structures: Monofuye/l. 06 ac,Uve pha.6e on ;the -6Uppoll-t:
I~/I~
(m/s)(Dm/Ds)(om/os)(l/psAsSs) X A [1+exp(-2/psAmSs)]/[1-exp(-2/psAmSs)]
where m/s
om,s
(1)
is the atomic ratio of the supported phase (m) and the carrier (s); is the detector efficiency for m and s. On the basis of the brightness law, it varies as l/E k [13]. Recently a variation of . Dm. s as Ek-1/2 has been found experlmenta1ly [14];
276
am,S
is the photoelectron cross-section of m and s taken from Scofield
Ps Am,s Ss A
is is is is
[15] ;
the the the the
bulk density of the carrier; mean escape depth of photoelectrons in (m) and (s); specific surface area of the carrier; asymmetry parameter [16].
(2)
where d is the length of the edge of the cubic crystallites of the deposited the predicted intensity ratio in the case of monolayer phase and I~/I~ dispersion. When dealing with large particles of the deposited phase (d > Am)' Eqn. (2) may be simplified to:
For various active phase loadings and assuming that no change of the surface area occurs during the preparation of the supported phase, taking Eqn. (1) into account, we may write (3)
Eqn. (3) shows that, in this case, the factor (Im/Is) (s/m) is directly proportional to the dispersion lid of the supported phase. When the active phase concentration is maintained constant, Eqn. (3) may be expressed as (4)
INFLUENCE OF THE PREPARATION ilETHOD 14ateria1s The influence of the preparation method has been examined in the case of two series of impregnated 1 and ion-exchanged ~ silica-supported nickel oxides. The silica carrier used (Ketjin, grade F2) had the following characteristics: surface area, 307 m2g-l, pore volume, 1.12 cm 3g-l. Series 1 was obtained by pore volume impregnation of the support with nickel nitrate solution of the concentrations required for obtaining the desired NiO content. Series ~ was obtained by 'adsorption' of the cation complex [Ni(NH 3)6]2+ onto the support. More details about the preparation method are given
277
e1sewhere [17,18]. Samples of both series were dried at 120°C and ca1cined for 4 h at 500°C. They are designated by i(or e) Ni:x:Si, where x indicates the nickel content expressed as wt. NiO per 100 g of the final supported metal oxides rounded to the nearest integer. The exact chemical composition of the samp1es and their surface area (S) are given in Tables 1 and 2. TABLE 1 Nickel content and specific surface areas of impregnated NiO/Si0 samples 2 i Ni:x:Si Ni:2:Si Ni:4:Si Ni:6:Si Ni:7:Si Ni:9:Si Wt% NiO 1.91 3.7 5.6 7.38 9.43 300 30"1 303 292 291 S m2g-1 TABLE 2 Nickel content and specific surface areas of ion-exchanged NiO/Si0 2 samples e Ni :x:Si e Ni:2:Si 3 Ni :4:Si e Ni:9:Si e Ni :6:Si Wt% NiO 2.29 3.56 5.53 9.16 S m2g-1 342 337 355 318
Results and discussion XRD. XRD patterns taken for i samples show only the presence of well crystallized NiO. Conversely, no evidence of any crystalline NiO can be deduced from XRD spectra of ion-exchanged specimens~. Only broad and asymmetrical bands attributable to an ill defined and badly crystallized nickel hydrosilicate have been observed [17]. AEM. In agreement with XRD data, TEM micrographs of impregnated samples indicate the presence of dark particles identified by electron diffraction as NiO. The mean size of NiO agglomerates (dNi) increases gradually from 15 to 70 nm as nickel content increases. The variation of the dispersion of NiO, defined as l/dNi, as a function of the Ni loading is shown in Fig. l a. EPr~A of "crystallites free" areas shows that no highly dispersed NiO phase (dNi < detection limit in TEM ~ 2.5 nm) or surface compound can be found in nickel rich samples (x = 6, 7 and 9). For i Ni :4:Si and i Ni :2:Si samples, the nickel signal which has been found corresponds to a maximum amount of dispersed or combined phase <20.25% of the overall NiO content of those samples. Contrary to impregnated samples, TEM micrographs of ion-exchanges samples show no evidence of NiO. This indicates that in all specimens, nickel containing phase is present in a highly dispersed form or a surface compound. This is illustrated by the comparison of the EPMA spectrum relative to iNi:9:Si sample and that obtained from the crystallite free areas of
278
8
Si
181
1
1
o
Ni Si
Ni
o
5
wt % NiO
10
Fig. 1 Evolution of the dispersion of NiO as a function of nickel content in impregnated (i)NiO/Si02 samples determined by (a) TEH and (b) XPS
8
4 E/kev
Fig. 2 Typical EPMA spectra of impregnated (i) and ion exchanged (en~iO/Si02 samples
impregnated sample iNi:9:Si (Fig. 2). It is clear that while a significant Ni signal is exhibited in the former case, no Ni signal is detected in the latter. XPS. Fig. 3 shows the variations of the intensity ratio I Ni 2p/ISi 2p for _~ and ~ series, with respect to the nickel content. These XPS data clearly show the dramatic difference between the two sets of solids. Ion exchanged samples give rise to I Ni 2p/I Si 2p ratios, more than an order of magnitude higher than those obtained in the case of the corresponding impregnated specimens. This indicates a much higher dispersion of ion-exchanged samples. In fact, when ANi and ASi are taken respectively, equal to 1.05 nm and 1.8 nm (average of the theoretical values given by Penn [19] and the empirical estimation by Chang [20]),the observed intensities ratios I Ni 2p/I Si 2p are in good agreement with the predicted values for an atomically dispersed nickel phase. However, when the experimental value of ASi 2p is adopted (ASi 2p = 4.5 nm [21]) or when
279
4 I Ni2P I Si2P
2
1----------".......,~-------1
O~
o
....J
--J
5
10
wt. °/oNiO F~g. 3 Variation of XPS intensity ratio INi 2p/ISi ZP with respect to the samples nlckel content of impregnated (1) and ion-exchanged (~) NiO/SiO~
is assumed to vary as E - l/ 2 instead of E - l, the calculated intensities K K for a monolayer coverage are below the measured values, which may be taken as an indication of a segregation of the nickel phase at the outer part of the catalysts particle. This phenomenon has already been observed with nickel oxide supported on sodium modified aluminas [5,12]. A semi-quantitative analysis of XPS data may be equally performed in the case of impregnated samples. Indeed,as shown in the experimental section (Eqn. 3),when the supported phase is present as large particles (d Ni > ANi)' the factor (Im/Is)(s/m), being equal to K/d Ni ,varieslinearly with the dispersion of the NiO phase. Thus, the variations of (Im/Is)(s/m) as a function of the nickel content reflect the evolution of the dispersion of impregnated samples. The results illustrated in Fig. lb indicate a decrease in the dispersion of nickel phase with increasing Ni content. It is worth noting the conspicuous similarity between the evolution of dispersion determined by XPS and that estimated from independent TEI1 measurements.
om,s
Reduction studies. The reduction experiments further illustrate the profound difference between i and e series (Fig. 4). Indeed, while 90% of the
280
impregnated sample iNi:9:Si may be reduced at 325°C, under the same conditions, the extent of reduction of the corresponding ion-exchanged sample eNi :9:Si does not exceed 13%.
1
1
0.5
-=, 8
OL.::::::::~
o
4
t
hrs
Fig. 4 Reduction isotherms of impregnated iNi:9:Si and ion-exchanged eNi:9:Si samples (T = 325°C, PH = 1 atm, 0:: fraction of NiO reduced to Ni). 2
Overall remarks. The results presented here are in agreement with the known features of pore volume impregnation and ion-exchange methods. Indeed, impregnated solids were obtained by filling the pores of the Si0 2 carrier with a generally acidic solution of nickel nitrate whereas ion-exchanged specimens were obtained by adsorption at basic (pH = 10) of the cation complex [Ni(NH 3)6]2+. Considering that only at a pH 5 or 6 units higher than its isoe1ectric point (rEPS = 1-2) [22J . Si0 adsorbs cations to any signifi2 cant extent, one may expect that Ni 2+ ions adsorbed from an acidic nickel nitrate solution are weakly held by the carrier [18,23]. Thus, migration and aggregate formation may readily take place during drying and calcination. This gives rise to poorly dispersed impregnated samples containing non-interacting, easily reduced NiO aggregates as the predominant component and a negligible fraction of Ni present in a highly dispersed state or as a surface compound. Conversely, in the case of ion-exchanged samples, adsorption of the cation
281
complex [Ni(NH 3)6 J2+ on the silica surface would occur readily at pH = 10 [22J. Bearing in ming that occluded Ni 2+ ions were removed by washing, the remaining metal ions, strongly bound to the carrier, can be expected to be atomically dispersed on the surface. Little migration will then take place during drying and calcination. However, the high dispersion achieved after impregnation and drying is likely to bring about a greater possibility of compound formation with the carrier during the subsequent preparation steps, e.g. calcination. It ensues that the resulting ion exchanged samples would contain little or no NiO agglomerates but rather I~i in a highly dispersed, strongly bound form or as a surface nickel silicate. Consequently, the reducibility of ion-exchanged samples is suppressed.
INFLUENCE OF THE NATURE OF THE CARRIER Materials In investigating the influence of the support on the state and dispersion of NiO, we chose a series of well characterized silica and silica-aluminas covering the range from pure silica to alumina rich carriers (85 wt% A1 03), 2 prepared by the same method and having comparable surface areas. By choosing such supports, we hoped to increase the value of any comparison one may make. The synthesis of the silica-alumina carriers and their properties have been described elseWhere [8,24J. The samples are identified as SA followed by their Si02 weight percentage X. Five different carriers were studied. Their composition and surface area are listed in Table 3. TABLE 3 Surface areas of silica alumina carriers SA100 450
SA95 408
SA55 388
~w
SA15
369
373
The catalysts were prepared by pore volume impregnation of the carrier with a solution of nickel nitrate, followed by drying at 110°C and calcination for 6 h at 500°C. The samples are designated as NiO-SAX. Nickel content is 10 wt% NiO. Results and Discussion AEM. In accordance with the results reported in the previous section, TEN micrograph of silica supported sample NiO-SA100 exhibit the presence of particles identified as agglomorates of NiO crystallites. As the A1 203 content increases, a significant decrease in the average size of these aggregates is observed. A rough estimate of the mean diameters of NiO particles gives 100,
282
35 and 7 nm, respectively, for NiO-SA100, NiO-SA55 and NiO-SA40 samples. The NiO-SA15 specimen exhibits very few crystallites. The fraction of nickel present as a "surface compound" or in a highly dispersed form has been carefully measured by EPMA. The results illustrate the difference between pure Si0 2 and Si0 2-A1 203 carriers. For instance, while practically no surface compound or a dispersed phase can be detected for silica supported nickel oxide, a significant Ni characteristic X-ray signal is obtained from the "crysta11 He free" areas for Si02-A1203 supported specimens (Fi g. 5).
AI NiO-SA40 Si
Ni
_"",--,_JL Si NiO-SA100
....._....,..J
o
\.--------------..-01 I
4
8 E/kev
Fig. 5 Typical EPMA spectra from "crystallite fr ee" area of silica and silicaalumina su~ported nickel oxide Figure 6 shows the variation of the fraction of NiO present in a dispersed or combined state as a function of composition of the carrier. A steady increase of the nickel dispersed species as a function of the Al?03 content of the carrier is observed.
283
10
1
3+ en
,5.-z
o
~~---5""'0---~0
wt~i02 Fig. 6 0 - Variation of the fraction of l~iO present ina "di spersed" form or as a surface compound (NiO)s as a function of the alumina content of the carrier, • - variation of XPS intensity ratio [I Ni 2P3/2/(I Si 2p + IAl 2s)] with the alumina content of the support.
50
0 wtOjo Si0 2
Fig. 7 0 - variation of the fraction of NiO present in a "dispersed" form or as a surface compound (NiO)s as a function of Al content of the carrier, • - variation of the unreducible fraction of NiO, (NiO)u, with the Al content of the support.
XPS. The variation of the intensity ratio I tji 2P3/2IISi 2p + IAl 2s) as a function of the alumina content of the carrier is shown in Fig. 6. One may observe a marked and steady increase of this ratio as the alumina content of the carrier increases. This increase can be attributed to a higher dispersion of Ni 2+ species achieved as we progressively use carriers richer in alumina. This is in general agreement with the variation of the dispersion of the nickel phase as established from AB~ data, namely, a low dispersion for silica supported nickel oxide illustrated by a low XPS intensity ratio INi/(I Si + IA1) and a steady enhancement of 'the dispersion of the nickel containing phase reflected by the observed increase of this ratio with increasing the alumina content of the carrier. It must be noted, however, that the observed behavior of XPS intensity ratio I Ni 2P/(I Si 2p + I Al 2s) may be the result of two phenomena: a) the reduction of the size of HiO aggregates and b) the concomitant gradual formation of a surface compound with the carrier. Reduction studies. Reduction isotherms of NiO-SAX specimens were conducted 1 atm. The results were discussed elsewhere [2]. One at T = 40Q·C and P H2 salient feature is the steady decrease of the extent of reduction as the
284
support gets richer in alumina. This is clearly illustrated in Fig. 7 which presents the variation of the unreducible fraction of NiO, e.g. (NiO)u as a function of Al content of the carrier. The fraction of (NiO)u increases from nearly for NiO SA100 sample to 0.85 for NiO-SA15 specimen. These results are also in remarkable agreement with those deduced from AH1 studies. Indeed, as it is clearly shown in Fig. 7 the "unreducible" fraction of the nickel phase in the various SAX carriers is directly correlated to the fraction of Ni present as a surfa~e compound and determined from AEM data.
°
Overall review. The origin of the phenomena described above can be traced to the genesis of the supported catalysts. It has been shown [18,23] that when adsorbed from a nickel nitrate solution, Ni 2+ ions are weakly held on a Si0 2 carrier and can easily be removed by washing. Conversely, under the same conditions, Ni 2+ ions are known to adsorb strongly on A1 203 [25]. One may thus infer that Al addition to Si0 leads to an increase in the fraction of Ni 2+ ions 2 strongly held by the carrier. This fraction would essentially retain its dispersion during heat treatment and may readily react with the support to form a surface compound. The remaining fraction of Ni 2+ ions in loose contact with the carrier would give rise to NiO. One must also add, in this context, that as compared to because of the higher concentration of vacancies of y-A1 203 Si0 2 an enhanced reactivity of A1 203 containing carriers toward Ni 2+ ions would be expected.
INFLUENCE OF ADDITIVES tlateri a1s The role of additives has been assessed by a systematic study of the influence of alkali metal addition (Li, Na, K) to a y-A1 203 carrier on the properties of the deposited "nickel oxide". Alkali modified aluminas were prepared by pore volume impregnation of the y-A1 203 carrier with a solution containing variable concentration of alkali nitrate, followed by drying at 110·C for 2 h and calcination ai 600·C for 12 h. The alkali modified aluminas samples were designated by the symbol A1M x; (M = Li, Na, K) where x indicates the alkali content expressed as the number of atoms of the alkali metal per 100 atoms of aluminum. The surface area of Na and Kmodified aluminas decreases from 140 m2/g for x = to 75 m2/g for x = 13. Lithium modified aluminas exhibit a smaller decrease of the surface areas with lithium addition (S = 118 m2/g for x = 12). The series of nickel oxides were obtained from A1M x series by pore volume impregnation with a solution of nickel nitrate followed by drying at 110·C and calcination for 6 h at 500·C. The nickel content is
°
285
nearly equal in all samples and corresponds to 10 wt% NiO. designated by NiA1I4 x'
The samples were
Results and Discussion AEI4. Examination of NiA1M x by TEM did not give any evidence of a significant presence of crystallized NiO particles. The repartition of the supported nickel phase as a function of the local bulk nickel concentration of the spot where it is deposited has been measured for the various NiAlI\ series. The results show that for Na(or K) modified samples the distributio~ becomes broader with increasing Na content [5]. This effect is less pronounced in the case of NiA1Li x series. Further investigation of this phenomenon has been carried out in the case of NiA1Na x series by comparing the repartition of Ni through ultra-thin sections of unground grains from samples NiA1Na l and NiA1Na 13. Fig. 8 pr-esents the variation of the EPI1A ratio INi/I Al obtained by microanalysis of successive areas along a straight line across two of these sections. The results clearly show a constant nickel profile in the case of Na deficient simple and a near complete depletion of Ni in the center of the grain for Na rich specimen.
NiAINCI1
AEM
•
o
o
NiAlNa13 •
DO
o
0
Distance
centre (".ni
Fig. 8 Variation of the INilIA1' AEfl intensity ratio, across thin section of grains from ila deficient' (NiA1Na l) and sodium rich aluminas (NiA1Na 13). XPS. The variations of XPS intensity ratio I Ni 2p/I Al 2s as a function of the alkali metal content are shown in Fig. 9. One can readily note the different behavior of Li modified samples as compared to the corresponding Na and K doped series. Indeed ,increasing the Na and K content of the carrier leads to a significant increase in the XPS intensity ratios I Ni 2p/I Al 2s' whereas a rather slight decrease is observed when similar concentration of lithium is added to the carrier. According to the methodology developed in the
286
experimental section the variations of XPS intensity ratio INi 2p/I Al 2p would reflect as a first approximation the modification of the dispersion of the deposited nickel phase. One may thus conclude that lfa and K addition to the A1 203 carrier enhances the dispersion of the supported nickel species whereas lithium enrichment of the support has a rather deleterious effect on the dispersion of nickel. However, AEM studies offer an alternative explanation of the observed increase in XPS intensity ratio I Ni 2p/IAl 2s for [fa and K rich samples. Indeed, XPS analysis being mo~e sensitive to the nickel deposited on the "external" part of the catalyst grain, one would expect that Ni segregation at the outer region of the catalyst pareicle evidenced by AEf·1 data of Na rich samples will induce and increase in the XPS intensity ratio I Ni 2p/I Al 2s'
2
•
K
ONa o Li
15
Fig. 9 Variation of XPS intensity ratio INi 2p/IAl 2s as a function of the alkali metal nature (M: Li, Na, K) content 1n the alumina carrier. Reduction measurements. Fig. 10 shows the TPR diagrams relative to selected NiA1Mx samples. Two waves of reduction Tl and T2 are exhibited by nearly all specimens. The position of the peak T has been found to coincide 2 with that corresponding to the reduction of pure NiA1 204" A close examination of Fig. 10 further illustrates the similarity between NiA1Na x and NiA1Kx series already established from XPS measurements. Indeed, one may clearly observe that Na and K addition to the A1 203 carrier brings about a steady increase in the fraction of Ni present as NiA1 204-like compound. This may be taken as a consequence of an improved dispersion of impregnated nickel species in Na or K rich samples, which leads to a higher reactivity with the carrier after calcination. Conversely, lithium doping of the alumina support leads to a progressive disappearance of the reduction wave 1 2 ascribed to NiA1 204.
287
Interpertation of the effect of alkali addition. A tentative explanation of the role of alkali metal additives rests on their eventual influence on the pH of the impregnating medium, during the deposition of the active phase. In the case of Na and K, one may reasonably conceive that Na(or K) containing species which cannot be accomodated in the A1 203 lattice or exchanged with the OH groups present on the surface, cause the alteration of the pH of the nickel nitrate solution during the second impregnation. Owing to the likely basic nature of the Na species (Na 20, NaOH, etc.), the solvated Na+ ions increase the pH of the impregnating solution and brings about through hydrolysis and precipi.tation of the metal ions, a better anchoring of the Ni on the surface of the
-
kg
:::J
cd c 0
Hatv
~
i ~
16 a:
Li
500
700
TCC
900
Fig. 10 TPR diagrams of "NiO" deposited on alkali modified aluminas modified support and consequently an improved dispersion of Ni in the final solids. On the other hand, because of the immediate hydrolysis of the nickel
288
salt, a rapid depletion in nickel of the impregnating solution will take place as it diffuses throughout the pores of the catalyst particle. Such phenomena fully accounts for the AEM, XPS and TPR results. In the case of lithium,a higher reactivity of this additive toward alumina, due to the easy diffusion of small lithium cations into the carrier lattice may be expected. The consequence would be the absence of a "free" alkali containing species required for the mechanism proposed for Na and K ions to be operative. On the other hand, the increased thickness of surface lithium-aluminum containing species (LiA1 50S' g-LiA10 2, solid solution of Li ions in alumina) hinders the diffusion of Ni 2+ species into the y-A1 203 lattice during calcination, thus inhibiting the formation of nickel aluminate-like compound and concomitantly increasing the fraction of nickel present as NiO.
CONCLUSIONS The present communication was restricted to the study of the influence of three major parameters known to affect the reducibility of a deposited oxide phase, namely, the preparation method, the nature of the carrier and the use of additives. We have attempted to develop a methodology, based upon the combined use of several techniques and the systematic study of samples containing increasingly higher amounts of the variable to be examined. The results concerning the influence of the preparation method clearly illustrate the known features of impregnated and ion exchanged samples and demonstrate that XPS measurements may offer an effective tool to assess the evolution of the dispersion of a supported oxide. The study of the influence of the nature of the carrier unveils the biphasic nature of silica-aluminas supported "nickel oxide". It shows that increasing the alumina content of a Si0 2 carrier leads to the progressive formation of a surface compound and to a substantial decrease in the average size of the NiO aggregates. The investigation of the influence of additives on the dispersion of a deposited phase and its interaction with the carrier shed more light on the intricate and complex role of alkali metal additives in the genesis of metal supported catalysts. Our results stress the care which must be exercised in analyzing the effect of additives on the physico-chemical properties of the supported phase. They clearly indicate that depending on its size and abundance, an alkali metal additive may have a beneficial or deleterious effect on the dispersion of the active phase. Iloreover, other characteristics which have been less considered in the literature, like the repartition of the active phase, may also be affected.
289
ACKNOWLEDGEMENTS We gratefully acknowledge the constant advice and guidance of Professor B. Delmon, Director of the Groupe de Physico-Chimie fli nerale et de Catalyse, who initiated and supervised this work. We also thank Dr. J. Lemaitre and F. De1annay for critical discussions and for the performance of many delicate and imaginative TPR and AEl-1 experiments. We finally acknowledge the "Services de la Programmation de la Politique Scientifique" (Belgium) and the National Science Foundation (USA) (Grant No. CHE-8020001) for their financial support. REFERENCES
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
B. Delmon and 1',1. Houalla, in B. Delmon, P. Grange, P. Jacobs and G. Pancelet (Eds.), "Preparation of Catalysts II", Louvain l a Neuve, September 4-7, 1978, Elsevier, Amsterdam, 1979, pp. 439-468. M. Houal1a and B. Delman, J. Phys. Chem., 84 (1980) 2194-2199. M. Houalla, J. Lemaitre and B. Delmon, J. Phys. Chem., in press. H. Haualla and B. Delmon. Surf. Interface Anal., 3 (1981), 103-105. M. Houal1a, F. Delannay and B. De1mon, J. Phys. Chem., 85 (1981) 1704. M. Houal1a, F. De1annay and B. Delmon, J. Electron Spectrosc. Relat. Phenom., 25 (1982) 59-65. F. Delannay, Catal. Rev., Sci. Eng., 22 (1980) 141. C. DeFosse, P. Canesson, P. G. Rauxhet and B. Delman, J. Catal., 51 (1978) 296-277 . P. J. Angevine, J. C. Vartu1i and W. N. Delgass, in G. C. Bond, P. B. Wells and F. C. Tompkins (Eds.), Proc. 6th Int. Congress on Catalysis, The Chemical Society, London, 1976, pp. 611-618. S. C. Fung, J. Catal., 58 (1979) 454-469. F. P. J. ['1. Kerkhof and J. A. r,loulijn, J. Phys , Chem., 83 (1979) 1612-1619. F. Delannay, M. Houalla, D. Pirotte and B. Delmon, Surf. Interface Anal., 1 (1979) 172-174. J. C. Helmer and N. H. Weichert, Appl. Phys. Lett., 13 (1968) 266. J. E. Castle and R. H. West, J. Electron Spectrosc. Relat. Phenom., 19 (1980) 409. J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 9 (1976) 29. R. F. Reilman, A. f1. Sezane and S. T. ttandon, J. Electron Spectrosc. Relat. Phenom., 8 (1976) 389. H. Houalla, F. De1annay, I. Matsuura and B. De1mon, J. C. S. Faraday I, 76 (1980) 2128-2141. M. Houal1a, I. Matsuura and B. Delmon, in preparation. D. R. Penn, J. Electron Spectrosc. Re l a t , Phenom., 9 (1976) 29. C. C. Chang, Surf. Sci., 48 (1975) 9. r·l. Klasson, A. Berndtsson, J. Hedman, R. Nilsson, R. Nyholm and C. Nordling, J. Electron Spectrasc. Relat. Phenom., 3 (1974) 427. J. P. Brunelle, in B. Delman, P. Grange, P. Jacobs and G. Poncelet (Eds.), "Preparation of Catalysts II", Louvain La Neuve, September 4-7, 1978, Elsevier, Amsterdam, 1979, pp 211-232. J. H. Anderson, J. Catal., 26 (1972) 277. P. O. Scokart, F. D. Declerck, R. E. Sempels and P. G. Rouxhet, J. C. S. Faraday I, 73 (1977) 359. V. A. Dzis'ko, S. P. Noskova, M. S. Borisova, V. D. Bolgova and L. G. Karakchiev, Kinet. i. Katal., 15 (1976) 751.
290 DISCUSSION
J. KIWI: You report in Fig. 9 an increase in the Ni/AI ratio as a function of alkali ion concentration. In this case Na+ and K+ had a favourable effect but Li+ in practice did not show any effect. Besides the structural reasons yoy invoke, have you tried to measure the work function of your systems? It is known that work functions decrease as heavier alkali ions are present in your systems and this observation may explain, from the electronic side, your observations reported in Fig. 9. M. HOUALLA: The influence of alkali-ion nature and content in the alumina carrier on the dispersion of nickel-containing species has been tentatively attributed to the modification of the local pH of the Ni impregnating solution induced by the presence in Na rich samples of "free" Na-containing species. other mechanisms which may account for some of the observed effects and pertaining to the influence of alkali metal on the phase transformation of the Y-A1203 and the distribution of Ni 2+ cations between the tetrahedral and octahedral sites of the alumina, have been discussed in References 3 and 5. We have not attempted to measure the work function of our systems. However, it seems reasonable to expect that modification of the work function will be most significant when alkali metal ions interact strongly with the alumina carrier (e.g. for low and medium additive concentration) and less pronounced for "free" alkali metal-containing specimens. Our results indicate that only in the latter case the influence of additives on the dispersion of nickel species is clearly manifested.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam";" Printed in The Netberlands
291
THERMALLY AND MECHANICALLY STABLE CATALYSTS FOR STEAM REFORMING AND METHANATION. A NEW CONCEPT IN CATALYST DESIGN K. B. MOKa , J. R. H. ROSS a* and R. M. SAMBROOKb aSchool of Chemistry, University of Bradford, Bradford BD7 lOP (U.K.) and bDyson Refractories Ltd., Owler Bar, Sheffield S17 3BJ (U.K.)
ABSTRACT The various catalysts for use in the steam reforming and methanation process produced by existing techniques (such as the impregnation of a relatively high surface area preformed carrier or the forming into pellets of coprecipitated materials) represent a compromise between strength, activity and stability. A process has now been developed to give catalysts which have relatively high catalytic activities but which also have the high mechanical strength of a ceramic matrix. It involves the homogeneous precipitation of precursors of the active component together with promoters and spacers within the pores of a preformed matrix of a low surface area. For example, for use in the steam reforming and methanation processes, the active phase may be derived by calcination and reduction from a coprecipitate consisting of nickel, lanthanum and aluminium species.
1. INTRODUCTION Industrial catalysts for the continuous high-pressure steam reforming of hydrocarbons to produce hydrogen and synthesis gas:
and for the methanation of residual traces of CO in ammonia or hydrogen plants:
have in common the fact that they have nickel as their active constituents.
* Present address: Department of Chemical Technology, Twente University of Technology, P. O. Box 217, 7500 AE Enschede (Netherlands).
292
For the former process, high thermal and mechanical stability are essential and there is little need for very high specific activity; in hydrogen production, exit temperatures in excess of 700 0C are considered normal. For the latter process, ease of reducibility and high generally carried out at temperatures below ~3500C, activity are the most important parameters and stability is of secondary importance. However, with the recent great interest in the methanation of high concentrations of carbon monoxide (from the gasification of coal or from the steam reforming of methane in the NFE project (ref. 1», there is a need for methanation catalysts which have relatively high activity at low temperatures as well as high stability at hjgh temperatures in the presence of steam. Previous work, reported in outline at the last symposium (ref. 2), has shown that various parameters in the preparation of coprecipitated Ni-A1 203 catalysts have a marked effect on the properties of the final catalysts (refs. 3.4) and that these materials can have high activity coupled'with reasonable thermal stability (ref. 5): this stability was attributed to the high degree of interaction between the Ni and Al of the catalyst. However, the coprecipitated materials must be formed into suitable pellets and these pellets have relatively low mechanical strength under the reaction conditions encountered in typical reactors. The main aim of the present project was to find a method of incorporating coprecipitates of this type into the pores of a preformed ceramic matrix of high mechanical strength (typically used for the preparation by impregnation techniques of catalysts for the cyclic steam reforming process (ref. 6)) in such a way that the high and stable activities characteristic of the reduced materials derived from the coprecipitates are maintained in the ceramic-based samples. The paper first discusses in outline the general method of preparation (which can also be applied to other types of catalyst) and then discusses the properties of some of the resultant materials, outlining the development of samples for commercial use. 2. EXPERIMENTAL 2.1 Catalyst Characterisation The methods by which the catalysts were prepared are described in section 3. They were characterised in a number of ways, including x-ray fluorescence (Te1sec Instruments Ltd.) for Ni content. X-ray line broadening (Phillips diffractometer) for Ni particle size. Kr (-1950C) and HZ (20oC) adsorption (pyrex adsorption system) for total and metallic areas and differential scanning calorimetry (DuPont 910 DSC) for the measurement of the activities for the methanation of CO (see ref. 7). The activities were determined in a standard reaction mixture of CO (lZ%), HZ(36%) and Ar (52%). The calcination and reduction of the catalysts were examined, using thermogravimetry (DuPont 951 Thermobalance). in a flowing atmospheres of Ar and of Ar+H2 (l:l).The extent of ageing of the catalysts in hydro-
293
thermal reducing atmospheres was estimated by examining the catalysts after exposure to a flow of H2 and H20 (4:1) at BOOoC for 4h. The performance of the samples under steam reforming conditions and, in particular, the resistance of the catalysts to carbon deposition were examined using an atmospheric-pressure continuous steam-reforming system in which the feedstock was heptane, the exit temperature of the bed was 650 0C and the steam: carbon ratio was 4:1. 2.2 Materials Analar and reagent grade chemicals, from various sources, were used throughout. Solutio~s were made up using de-ionised water and the vessels used were made of stainless steel, Pyrex or quartz as appropriate. The gases (Kr and H2) used for total and metallic areas were supplied in sealed ampoules (Grade X) and the remainder in cylinders (BOC Ltd.). The preformed ceramic matrice~ used for the samples described in this paper were composed of pure a-A1 203 with an apparent porosity in the range 50-60% and a mean pore diameter in the rangeO.5-2.0~m; they had been fired at >1400 oC. Other materials, such as silicon carbide, alumino-silicates or silica could also be used. The matrices were formed as Raschig rings of various sizes; rings of approximately 1 cm height and diameter were normally used. 3. RESULTS AND DISCUSSION The aim of this and parallel wor~ (ref. 8) was to prepare mechanically stable catalysts derived from coprecipitates of composition such as Ni6 A12(OH)16 C0 3· 4H20 (ref. 3). Preliminary experiments showed that attempts to incorporate the precipitate by normal precipitation techniques using Na2C03 or (NH4)2C03 gave deposits on the surfaces of the rings and in the bulk of the solutions but not in significant amounts in the pores of the a-A1203' When the rings were immersed and heated in a solution to which had been added urea or some other easily hydrolysable material (the homogeneous precipitation technique pioneered by Geus and coworkers (ref. 9) and used since by others (see, for example, ref. 10», significant precipitation occurre~ within the pores. A further improvement was realised when the rings were vacuum impregnated with the urea-containing deposition solution and excess solution was drained off prior to heating the impregnated rings at a temperature up to 1100C in an oven; deposition now occurred almost exclusively within the pores of the matrix. By suitable adjustment of the concentrations and compositions of the urea-containing solutions and by repeated depositions, relatively high loadings of the matrices (up to 20 wt% Ni) could be achieved. Between depositions, it is advantageous to heat the rings to approximately 310 0C for 2h to bring about partial or complete decomposition of the precipitate formed, thus increasing the capacity of the pores for further deposition solution. Washing with alkali (see section 3.3) can also improve the subsequent
294
uptake. After the deposition stages have been completed, the catalyst orecursor is "calcined" at 4500C in a stream of air for 'V4h and is then reduced in flowing H2 (preferably with a gradual increase in temperature to 600oC) prior to use. A variety of different formulations have been prepared and tested and the results of some of these tests are described below together with more detail of the compositions of the solutions used in the preparations, etc. 3.1 NiAl, NiMgAl and NiMgAl Ba Formulations Some typical formulations prepared in this work and selected results obtained with them are presented in the table. Table: Results for Selected Samples. Catalyst
A* B C D
E
F G
H I
Composition
Ni/a- A1203 Ni/a- A1203 Ni/a-A1203 Ni2.3Al/a-A1203 Ni 2.3A1/a-A1203 Ni3.5A12.5Mg/a-A1203 NilO.OA13.3La/a-A1203 Ni 7A1 5La/a-A1 203 Ni14A14.7la/a-A1203
Ni Content /wt %
5.0 4.5 4.3 7.7 6.5 4.2 6.5 7.6
Total A,ea /m 2g-
Ni Area /m 2g-1
1.0 2.5 7.6 17.8 6.5 13.5
0.47 0.42 0.63 0.56 0.98 1. 91
Ni partiile Size / Fresh Aged 300 250 257 198 235 180 117 180 107
1000
750 467 530 340 331
* Sample A was prepared by impregnation, all the others were prepared by the urea hydrolysis me thodj- signifies not determined. Sample A was prepared by a normal impregnation technique and is similar to catalysts used in cyclic steam-reforming plants (ref. 6). Sample B had a similar Ni content ('V5wt %) but was prepared by precipitation at 900C from a urea-containing "deposition" solution (350g of Ni(N03)26H20 and 217g of urea in 150g of water). Sample B had rather smaller Ni particles and there was a more marked difference between the samples aged in H2/H20 at 8000 e for 4h although both sintered significantly. Sample e is similar to B. Its total area is considerably higher than that of the untreated matrix (2.5 m2g-1 compared with 0.5m 2g-1) and the nickel area is about a quarter of the total; we infer that the nickel must have interacted to some extent with the alumina of the matrix to give
295
roughening of the interior of the pores. When an appropriate amount of aluminium nitrate (depending on the desired Ni/Al ratio) was added to the above deposition solution, a further considerable improvement was found in the particle sizes of the calcined and reduced catalysts both before and after the ageing test (samples D and E); the total area of the samples is increased considerably although the Ni areas are unchanged. Tests of the methanation activities of such samples show that they have activities which compare very closely with normal coprecipitates (ref. 11) when the relative weights of nickel are considered. Fig. 1 shows differential thermogravimetric (DTG) results for the reduction of samples C and D. While the sample without added Al (C) is G almost completely reduced at 4000C, D the presence of Al decreases the ease of reduction, temperatures III ...., >600 0C being required for complete reduction, a result typical of coprecipitates (ref. 2). Samples C and E were tested for the steam reforming of heptane. The C former had good initial activity but deactivated relatively rapidly and 300 disintegrated, presumably due to the Fig. 1. growth of carbon in the voids of the matrix. Sample E had a better activity but, although it did not disintegrate, a back-pressure built up, again presumably because of C deposition in the matrix. A further improvement in the particle sizes of the freshly reduced and aged samples was achieved by incorporating magnesium nitrate in the deposition solution (sample F). A further sample (F~ was prepared by impregnating F with barium acetate solution; the ~ddition of barium to urania-containing catalysts (the NUA series of catalysts made by Dyson Refractories Ltd; see ref. 12) is known to improve the carbon gasification activities of such catalysts and a similar property was sought here. Samples F and Fl were each tested for steam reforming activity and both gave 100% gasification; F was gradually deactivated and there was a bUild-up of back-pressure, again due presumably to carbon deposition, but sample Fl operated well with no apparent change.
-------
3.2 Ni La Al Formulations Lanthanum was then tested as a possible promoter. This was done for two reasons: (a) The NUA samples referred to above are thought to owe their success
296
to the ability of urania to dissociate molecular water and lanthana is likely to have a similar ability; (b) Wallace and coworkers (see, for example, ref. 14) have shown that catalysts derived from rare-earth intermetallics (e.g. La NiS) have high methanation activities. Sample G was therefore made as described above for Sample B but with the addition of a suitable weight of La(N03)3 to the deposition solution. Sample H was made in a similar way, but has a slightly higher metal content and was made by several deposition sequences with partial decomposition of the precipitates at 310 0C between each. Sample I had an even higher Ni content but was washed with alkali (see section 3.3) between depositions. Fig. 1 shows the DTG-trace obtained for the reduction of sample G, from which it is seen that La decreases the reducibility of the sample still further compared with the addition of only Al (Sample E). Another sample, not shown in the table, was prepared without the addition of Al to the deposition solution; it can be seen that La now has little effect on the reducibility (compare sample C, with no additives). The addition of La appears to reduce the particle size of the freshly reduced sample and also to stabilise the particles in the ageing test. Samples G and H were tested for activity and resistance to carbon deposition in the steam reforming of heptane, as discussed above.They gave 100% conversion over the period of the test and there was no evidence for C deposition. We therefore conclude that La imparts a resistance to C deposition similar to that imparted by urania. 3.3 Effect of Washing During Preparation Although the amount of urea added to the deposition solution was sufficient. if completely hydrolysed on heating, to bring about complete precipitation of the metallic species present (Ni, La, Mg, Al) the hydrolysis was not complete in a reasonable time at 9So~ or even at 110o~ as is shown by results such as those of Fig. 2. This presents DTG data for the heating in Ar of samples derived from the uncalcined precursor of sample G after washing in water and solutions of NaOH, Na2C03 and (NH4)2C03 (0.1 mol dm- 3). For the precursor washed in the carbonate solutions (and .0 s, Itl for the unwashed precursor, not shown for reasons of KOH clarity), there was a NaOH large peak in the DTG trace at 300 0C which was shown 200 300 T/0~00 500 600 to be due to unhydrolysed DTG of Calcination of Sample G after various washing treatments.
297
nitrate. Washing with water or with NaOH or KOH caused the disappearance of these peaks and the decomposition behaviour was very similar to that of pure coprecipitates (ref. 3). Washing with water reduced the Ni content, presumably by the removal of unreacted nitrate, while the hydroxides caused complete precipitation of any free nitrate. Fig. 3 shows DTG results for the reduction of these samples after decomposition. The carbonate-washed samples are reduced much more easily than the others, presumably due to the presence of discrete NiO particles formed by the decomposition of nickel nitrate. The reduction behaviour of the other samples is similar to that of the pure co~-\;"'-~-.d=---'--..,!-,.-----L-----L -l---.-'.'.....--'-----'----;~:-150 250 350 Tlot50 550 precipitates. The methanFig. 3. DTG of Reduction of samples of Fig. 2. ation activities of the samples were also determined using the DSC. The water, NaOH and KOH samples had activities which were a factor of approximately three times those of the unwashed sample and of those washed in carbonate solutions. We conclude from these and other results that the most highly dispersed and active catalysts result from those materials which are reduced with the greatest difficulty. 3.4 Use of Samples in Commercial Plants. Materials similar to Sample H have been prepared in commercial quantities and have been used successfully since mid-1981 in two plants, the first reforming a naphtha feed by the cyclic process to give a town gas and the second reforming a butane feed to produce hydrogen. Both plants have worked with high efficiency since the catalysts were installed. In the first case, the gas produced is very much leaner than that produced with conventional cyclic catalysts and the efficiency of the plant is consequently improved considerably. In the second, the catalyst is able to reform more feedstock than the design capacity of the plant; it has also survived,without apparent damag~ a major plant breakdown during which the steam-carbon ratio was very low while a competitive catalyst in a parallel plant using the same feedstock was destroyed. 4. CONCLUSIONS Mechanically and thermally stable catalysts suitable for high-temperature steam reforming, and probably also for methanation of CO-rich gases, have been
298
successfully prepared by homogeneous deposition within the pores of a-A1203 rings. Formulations containing NiMgAl Ba and also NiLaAl have been shown to have considerable advantages over NiAl alone. ACKNOWLEDGEMENTS Thanks are due to Dyson Refractories Ltd. for their sponsorship of this work and for permission to publish the paper, to C. Whitehurst for X-ray results, to D. Salt for steam reforming results and to Mr. J. Laming and Dr. B. Jackson for continued encouragement. We also thank Professor L. L. van Reijen and Mr. E. B. M. Doesburg for useful discussions. REFERENCES 1 B. H~hlein, R. Menzer and J. Range, Appl. Catal., 1 (1981), 125. 2 E.C. Kruissink, L.E. Alzamora, S.Orr, E.B.M. Doesburg, L.L.van Reijen, J.R. H. Ross and G. van Veen, Preparation of Catalysts II, Ed. B. Delmon et al Elsevier (1979), p; 143. 3 E.C.Kruissink, L.L. van Reijen and J.R.H. Ross, J. Chem. Soc. , Faraday Trans. I, 77 (1981) 649. 4 L.E. Al zamora , J. R. H. Ross, E.C. Kruissink and L.L. van Reijen, J. Chem. Soc., Faraday Trans. I, 77 (1981) 665. 5 G.van Veen, E.C. Kruissink, E.B.M. Doesburg, J.R.H. Ross and L.L. van Reijen, Rn, Kinet. Catal. Lett., 9 (1978) 143. 6 Gas Making and Natural Gas, 1972 B.P. Trading Ltd. ,Chapter 8. 7 T. Beecroft, A.W. Miller and J.R.H. Ross, J. Catal., 40 (1975) 281. 8 H. Schaper, E.B.M. Doesburg and L.L. van Reijen, Paper C.4, this symposium. 9 See L.A.M. Hermans and J.W. Geus, Preparation of Catalysts II, Ed. B. Delmon et al., Elsevier (1979), p. 113. 10 J.T. Richardson, R.J. Dubus, J.G. Crump, P. Desai, U. Osterwalder and T.S. Cale, Preparation of Catalysts II, Ed. B. Delmon et al ., Elsevier (1979}, p. 131. 11 M.R. Gelsthorpe and J.R.H. Ross, to be published. 12 T. Nicklin, F. Farrington, R.J. Whittaker, Inst. od Gas Engineers J., (1970), 151. 13 V.T. Coon, T. Takeshita, W.E. Wallace and R.S. Craig, J. Phys. Chem. ,80 (1976) 1878.
299 DISCUSSION J.W.E. COENEN: It is somewhat surpr1s1ng to find deposition-impregnation appearing at this Conference as a relatively new method. About 10 years ago Unilever applied for patents for this method. Last week the Dutch patent was granted. J.R.H. ROSS: Dyson Refractories are fully aware of the existing patent literature. Indeed, a urea hydrolysis technique was first patented in 1942. The novelty of the Dyson Refractories technique lies in the deposition of a complex multicomponent active phase within the pores of a preformed low surface area ceramic matrix. B. NIELSEN : Do you have any analyses of alkali metals in the La-promoted catalysts? It is known that alkali is able to prevent carbon formation which could be the explanation for the better resistance against carbon formation. Many of the preparations include washing with alkali metals. J.R.H. ROSS: The alkali metals content of the lanthanum catalysts is negligible, e.g. potassium less than 0.05 wt.%. ZHAO JIUSHENG : Have you been doing any research on coke deposition on catalyst with La? Is it any differentce between your catalyst and the usual commercial catalyst ? J.R.H. ROSS: Laboratory and plant data show the presence of a lanthanum species in a steam reforming catalyst confers carbon gasification activity to that catalyst. However, the level of carbon gasification activity depends greatly on the method of catalyst preparation. M.S. SCURRELL : Could you comment on the possible importance of the chemical nature of the ceramic material used? You mention that the nickel interacts at least to some extent with the a-alumina support. Is this of special significance or could any other low-area supports such as silica or silica-alumina be considered for use in this application? J.R.H. ROSS: Dyson Refractories use a pure a-alumina carrier of high stable mechanical strength in the manufacture of steam reforming catalysts. Chemical purity is important as silica and alkali metals are known to migrate under high pressure steam reforming conditions. F. GADALLAH: The role of La-Elaborate. on reactivity and stability.
When and how it is added.
Its effect
We cannot comment on when and how the lanthanum is added in the J.R.H. ROSS manufacture of these cat~lysts. The lanthanum species has a dramatic beneficial effect on the thermal and hydrothermal stability of the nickel catalyst. M. BHASIN: D'i.d-you compare the activity of your coprecipitated catalysts with those prepared by impregnation/decomposition of aluminium nitrate followed by impregnation/decomposition of the nitrate of the nitrate of nickel and lanthanum ? J.R.H. ROSS Many aspects of the preparation of the initial lanthanum catalysts have been studied including sequential impregnation deposition or decomposition of the components of the active phase. J.W. GEUS You did not completely precipitate the nickel using the urea hydrolysis. I therefore wonder what the urea (nickel and aluminium) ratio was you have used ? J.R.H. ROSS catalysts.
We cannot comment on matters concerning the manufacture of the
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G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts 111 @ 1983 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
301
SYNTHESIS OF METHANATION CATALYSTS BY DEPOSITION-PRECIPITATION
H. SCHAPER, E.B.M. DOESBURG, J.M.C. QUARTEL and L.L. VAN REIJEN Laboratory of Inorganic and Physical Chemistry, University of Technology, Delft (The Netherlands)
ABSTRACT Nickel alumina methanation catalysts were made by deposition-precipitation of nickel compounds in the pores of. commercial alumina supports. The nickel ions are provided by the thermal decomposition of nickel ammino complexes. The results show that part of the support reacts to form nickel aluminium hydroxysalts. The activity of samples based on gamma or theta alumina corresponds to the activity to be expected for nickel particles of 4 to 7 nm diameter, the thermostability in H mixtures of atmospheric pressure at BOOoC is excellent. 2/H20 It was found that alpha alumina is not a suitable support for this process.
INTRODUCTION In recent years much research has been done on the nickel alumina catalyzed methanation reaction. The main reason for the interest in this reaction is the production of synthetic natural gas from carbon monoxide and hydrogen; these gases can be produced by gasification of coal. The high exothermic reaction enthalpy of carbon monoxide methanation makes this reaction also suitable for use in a long distance energy transport process (ref. 1). In this process the heat of a nuclear reactor is used for an endothermic reaction, steam reforming of methane. The product gases are transported to energy consumer centres, where the exothermic methanation is used to release the chemically stored energy. This methanation process, where heat is the desired product instead of methane, 0 o requires catalysts that are active at 250 C and stable at 800 e . In a previous communication (ref. 2) we have reported results for the preparation of nickel alumina methanation catalysts by coprecipitation of nickel and aluminium nitrate by means of sodium hydroxide and carbonate, at constant temperature and pH, followed by calcination and reduction. It was found that the precursor obtained by coprecipitation consists of brucite-type hydroxide layers, containing nickel and aluminium ions, alternated by layers containing water and anions like carbonate and nitrate. In order to form only this compound the
302 nickel/aluminium ratio has to be larger than one; otherwise, a separate aluminium hydroxide phase is precipitated. It was found that the intimate mixture of nickel and aluminium ions in the hydroxy carbonate coprecipitate results in a much higher
of thermostability than was found for catalysts 0C made by impregnation. After a sinter test of 800 hours at 700 in a 30% steam/ de~ree
70% hydrogen mixture the mean nickel crystallite size was stabilized at 25 nm. The results obtained thus far apply to the catalyst powders prepared by laboratory methods. For industrial applications pellets or extrudates in the size range of several mm are required. Thus an extra preparation step is required in which a
suffi~ient
mechanical stability of the product, not only
under normal handling conditions, but also under conditions of the reaction, should be obtained. Preliminary experiments have shown that this is still a difficult step. Therefore it was decided to investigate other preparation methods, using preformed industrial aluminium oxide supports, which already have. a
proven satisfactory mechanical stability.
Such a method is deposition-precipitation, where a precursor of the catalytically active compound is precipitated in the pores of a support, instead of impregnation.
Deposition-precipitation is a rather new catalyst preparation
method (ref. 3). By using a preformed support no extra forming operations are necessary. Also the scaling-up of this process should be less difficult than that of a coprecipitation process. In this contribution we describe an investigation into the suitability of this method for the production of thermally stable nickel alumina methanation catalysts. The method chosen is based on a patent to McArthur (ref. 4) which claims the production of nickel alumina catalysts by deposition-precipitation of nickel hydroxide on an aluminium hydroxide support by thermal decomposition of nickel ammino complexes. Such a homogeneous precipitation method is necessary to avoid precipitation outside the pores of the support. It was found that this preparation method can be improved by some alterations, the most important being the use of aluminium oxide supports, an increase of the initial pH of the solution to activate the alumina surface and leading carbon dioxide through the solution during deposition-precipitation to avoid incorporation of nitrate ions.
EXPERIMENTAL To an aqueous nickel nitrate solution, prepared by dissolVing 20 grams of nickel nitrate hexahydrate in 20 ml of distilled water and 1 ml of concentrated nitric acid, 25 ml of concentrated ammonia were added slOWly under vigorous stirring. A 2M aqueous solution of sodium hydroxide was very slowly added to this solution until the pH was 10.5. All chemicals used were of pro analysis quality. Ten grams of preformed aluminium oxide extrudates were put into this solution. After waiting for 30 minutes, the solution was heated to gOOe, while
303 carbon dioxide was led through it. This resulted in a rapid decrease of the pH to a constant value of 7.5 + 0.5. After some hours a further decrease of the pH was observed and at that point the synthesis was stopped. The product was filtered, washed with 2 I of hot distilled water and dried for one night at 80°C. Three types of ·supports were used in this investigation: 2
(Ketjen 000-1.5E, SBET = 200 m /g, pV = 0.5 ml/g). 2 6-A1 (Ibid., heated at 900 oe, SBET 135 m /g, pV = 0.5 ml/g). 203 2/g, a-A1 (Dyson, SBET = m pV = 0.3 ml/g). 203 In order to investigate the effect of successive deposition-precipitation a
- Y-A1
203
batch of 40 grams of gamma alumina was treated up to four times; after each step the sample was calcined at 450
0e
in air. The amounts of chemicals and solute
were adjusted to the amount of support. One preliminary experiment was carried out to study the scaling-up of this process. In this experiment 400 grams of gamma alumina were used, with the corresponding amounts of chemicals and solute. For comparison one sample was made by impregnating 10 grams of gamma alumina to incipient wetness with a 1.5 M nickel nitrate solution, followed by a drying step at BOoC. Also one sample was made by coprecipitation of an aqueous solution of nickel nitrate (0.6 M) and aluminium nitrate (0.3 M) with sodium carbonate = 7 and 80oe. The product (nickel aluminium hydroxycarbonate) was filtered,
at pH
washed with 2 1 of hot distilled water and dried at BOoe. Table I gives a description of the samples studied in this investigation.
Table I Description of samples
Sample
Support
Amount (g)
1 2 3 4
'Y-A1 20 3 y- A1 20 3 y- A1 20 3
10 10 40 30 20 10 400 10 10 10
5 6 7 8 9 10
3 (calcined) 4 (calcined) 5 (calcined) y-A1 20 3 -A1 20 3 a-A1 20 3 y-A1 20 3
e
11
Samples were calcined at 450
Remarks Deposition precipitation Deposition precipitation (duplicate of 1 ) Deposition precipitation Successive deposition precipitation (2x) Successive deposition precipitation (3x) Successive deposition precipitation (4x) Deposition precipitation (scaling-up) Deposition precipitation Deposition precipitation Impregnation eoprecipitation
0C
in air for 16 hours. Reduction of both un-
calcined and calcined samples was carried out at 600
0e
(heating rate 2°C/min) in
a hydrogen flow (lB Nl/h) for 16 hours. This was done to investigate the effect of calcination on activity and stability. Reduced samples were passivated at 100°C in a 30% steam/70% nitrogen mixture (18 Nl/h) for 16 hours. In order to
304 investigate the thermostability of these catalysts samples were sintered for 20 hours at 800
0
e
in a 30% steam/70% hydrogen mixture (18 Nl/h).
Samples were characterized by X-ray diffraction (Enraf Nonius Guinier-De Wolff camera mark II), thermogravimetry (decomposition of the precursor, reduction of calcined samples) and chemical analyses: sodium and nickel content by atomic absorption spectroscopy, carbonate content by dissolving the sample in sulfuric acid, precipitating the carbon dioxide evolved with barium ions and water and titrating the hydrogen ions
released by this reaction
with sodium
hydroxide. The methanation activity of samples before and after the sinter test was determined by differential scanning calorimetry (Du Pont Instruments 910) at 250
0
e
in a 2% carbon monoxide/98% hydrogen mixture.
RESULTS Deposition-precipitation X-ray analysis of samples prepared by deposition-precipitation on gamma or theta alumina showed the diffraction lines of the support used and the presence of a compound with the same diffraction lines as nickel aluminium hydroxycarbonate (d-values 0.766, 0.384 and 0.151 nm). This was confirmed by decomposition and reduction behaviour, as investigated by thermogravimetry, and carbonate analyses. The nickel/aluminium ratio in this compound was determined from the (110) diffraction line (0.151 nm), using the results of Kruissink (ref. 5)
for
nickel aluminium hydroxycarbonates made by coprecipitation;. a value of 1.5 + 0.3 was found. It was found that the increase of the initial pH of the solution by the addition of sodium hydroxide is necessary to form this conpound; otherwise nickel hydroxide is precipitated in the pores of the support. Experiments where air was used to stir the solution instead of carbon dioxide showed that in this way nickel aluminium hydroxynitrate is formed. This is less desirable because calcination of nitrate containing samples causes sintering of nickel oxide crystallites (ref. 2). When alpha alumina was used as a support the X-ray analysis of the product did not show any nickel compounds. This could be ascribed to the low nickel content of this sample. However, catalytic behaviour of this sample indicates that no intermediate nickel aluminium compound was formed. Since sodium is a well known poison for nickel alumina methanation catalysts we investigated whether the introduction of sodium ions into the solution resulted in the incorporation of sodium compounds in the product. This is sometimes found for catalysts made by coprecipitation (ref. 6). For depositionprecipitation the sodium ion content of the products did not exceed the sodium ion content of the support.
305 Catalyst activity and stability Table II lists the nickel contents and methanation activities, expressed per gram nickel, after reduction of calcined and uncalcined samples before and after the sinter test.
TABLE II Results for reduced samples
Sample
1 2 3 4 5 6 7 8 9 10
Support
Ni content (wt %)
y- Al 20 3 y- Al 20 3 y-Al 20 3 3 (calcined) 4 (calcined) 5 (calcined) y- Al 20 3 (up-scaling) 8- Al203 a-Al203 y-Al 20 3 (impregnate)
11
Activity* of calcined samples Before After sinter sinter test test
Activity* of uncalcined samples Before After sinter sinter test test
>12.6 13.7 17.7 27.0 33.9 37.2 17.9
0.75 0.61 0.96 0.89 0.74 0.62 0.71
0.27 0.29 0.47 0.38 0.30 0.26 0.65
1. 12 0.98 1. 51 1. 25 1.00 0.97 1.00
0.43 0.54 0.46 0.45 0.27 0.25 0.64
15.5 1.6 4.0
0.28 0.00 0.10
0.21
1. 08 0.00
0.39
69.7
0.64
0.13
0.58
0.32
(coprecipitate) *Methanation activity at 250
oC,
expressed as mol CO/g Ni, h.
The first thing we note is the large activity of uncalcined samples made by deposition-precipitation compared with calcined samples. This is not observed for a sample made by coprecipitation. However, after the sinter test these differences generally disappear. Comparing the results for gamma alumina (samples 1, 2), theta alumina (sample 8)
and alpha alumina (sample 9) we note
that the use of gamma alumina gives better results than theta alumina. The sample based on alpha alumina has no activity at all. The methanation activity, expressed per gram nickel, of samples made by deposition-precipitation on gamma alumina compare favourably to the activity of the sample made by coprecipitation. The thermostability of samples made by deposition-precipitation is distinctly better than of the sample made by coprecipitation. Successive deposition-precipitations (samples 3-6) show that the activity, expressed per gram catalyst, does not increase after the second step; the activity, expressed per gram nickel is highest for the sample which was treated once. scaling-up of the process from 10 to 400 grams gave no problems. The activity
306 of the latter sample is a little higher, but the thermostability is remarkably better.
DISCUSSION Deposition-precipitation It is shown that deposition-precipitation on gamma or theta alumina results in the formation of nickel aluminium hydroxycarbonate. This is surprising since no aluminium ions were introduced into the solution. Obviously the alumina support provides aluminium ions, which react with nickel ions. Since a high initial pH of the solution.(10.S) is necessary for this process we speculate that part of the support is dissolved by hydroxide ions. From the nickel content of the product and the nickel/aluminium ratio in the nickel aluminium hydroxycarbonate we calculate that 10% of the support has reacted. One monolayer of alumina corresponds to 20%. Furthermore, since the product can be detected by X-ray diffraction, it has to be several layers thick. Combining these facts leads to the conclusion that only a small part of the surface reacts. We may imagine this as pits etched into the alumina surface by hydroxide ions. These pits are filled with a AI(OH)~
solution which reacts with nickel ions, released
by decomposition of nickel ammino complexes. Further investigations (electron microscopy) are planned to test this model. For alpha alumina this reaction probably does not occur, because of the low surface area and the stability of the support against hydroxide ions. This explains the negative results obtained for the sample based on alpha alumina. Attempts to "activate" the alpha alumina surface by a five hour treatment at 100
0c
in a 1 M sodium hydroxide solution, followed by deposition-precipitation,
were not successful.
Catalyst activity and stability Although the reproducibility of the deposition-precipitation still gives some problems, mainly for scaling-up experiments, we can point out some important results. The activity of samples made by deposition-precipitation on gamma alumina, expressed per gram nickel, is approximately the same as that of coprecipitated samples. The activity corresponds to a nickel crystallite size of 10 nm, assuming that the activity is inversely proportional to crystallite size, as was found by Kruissink (ref. 5). The thermostability of depositionprecipitation samples is distinctly better. The difference in thermostability of samples made by deposition-precipitation and coprecipitation is unexpected since the same nickel aluminium compound is formed. Probably the large interaction with the gamma alumina support causes the better thermostability of samples made by deposition-precipitation.
307 The difference in activity of uncalcined and calcined samples after reduction, that was found only for samples made by deposition-precipitation, may be explained as follows: the nickel aluminium hydroxysalt that is formed during deposition-precipitation contains mainly carbonate ions but also a small amount of nitrate ions. Upon calcination the nitrate ions decompose, resulting in the presence of nitrogen oxide vapours in the pores of the extrudates. This may lead to sintering of nickel oxide crystallites (ref. 2). However, if this decomposition is carried out in a hydrogen flow, as is the case during reduction of uncalcined samples, these nitrogen oxide vapours are probably reduced to nitrogen or ammonia and water, catalyzed by nickel oxide or aluminium oxide (ref. 7). This would prevent sintering of nickel oxide, resulting in smaller nickel crystallites (4-7 nm) and higher activities. The sample made by coprecipitation contains practica1ly no nitrate ions and is calcined in the form of powder, allowing nitrogen oxide vapours to be removed very quickly. Therefore the difference between calcined and uncalcined samples after reduction should be very small for this sample, as confirmed by experimental results. Finally, from these results we conclude that deposition-precipitation on gamma alumina in the way described in this contribution is a very promising method to produce thermally stable nickel alumina methanation catalysts.
ACKNOWLEDGEMENTS The authors wish to thank G. Hakvoort for the development of the method of activity determination by D.S.C. and J.P. Koot from the Department of Analytical Chemistry for assistance with the sodium and nickel analyses. They also thank N.M. van der Pers and J.F. van Lent from the Department of Metallurgy for assistance with the X-ray diffraction measurements. Finally they thank the Dutch Organization for Pure Scientific Research (Z.W.O.) for financial support.
REFERENCES 1 B. Hohlein, Jul. Report No. 1433, KFA Julich GmbH, 1977. 2 E.C. Kruissink, L.E. Alzamora, S. Orr, E.B.M. Doesburg, L.L. van Reijen, J.R.H. Ross and G. van Veen, in B. Delman, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts, II, Elsevier, Amsterdam, 1979, pp. 143-153. 3 J.A. van Dillen, J.W. Geus, L.A.M. Hermans and J. van der Meijden, in G.C. Bonds, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congress on Catalysis, London, 1976, The Chemical Society, London, 1977, pp. 677-685. 4 D.P. McArthur, U.S. Patent 4,042,532 (Aug. 16, 1977). 5 E.C. Kruissink, Coprecipitated Nickel Alumina Methanation Catalysts, thesis, Delft University Press, Delft, 1981, p. 25. 6 E.C. Kruissink, H.L. Pelt, J.R.H. Ross and L.L. van Reijen, Appl. Catal., 1 (1981) 23-30. 7 Gmelins Handbuch der anorganischen Chemie, 8e AUflage, 4, Stickstoff, Verlag Chemie GmbH, Berlin, 1936.
308 DISCUSSION D.C. PUXLEY Is the nitrate not associated with a lower Ni/AI ratio in the precursor phase? Could the apparent increase in sintering be associated with the lower Ni/AI ratio rather than the presence of nitrate ? H. SCHAPER: In our previous work with coprecipitated nickel alumina catalysts we have never noticed an appreciable effect of Ni/AI ratio either on nitrate content or on the activity or stability at high temperatures. For higher nickel alumina ratios we expect similar differences between calcined and uncalcined samples after reduction as observed in this work. C.M. LOK: Upon calcination of your nickel-alumina catalysts sintering of nickel oxide crystallites occurs, which may be caused, as you suggest, by the presence of nitrogen oxide vapours in the pores of the extrudates. My question is whether the effects of calcination can also be attributed to the accumulation of water vapour in the pores of the extrudates from which water is more slowly removed than from coprecipitates. H. SCHAPER: During reduction of uncalcined samples water vapour is formed at two stages. The decomposition of hydroxide ions from the Feitknecht compound takes place at around 300·C. Upon reduction of nickel oxide water vapour is formed at temperatures in the range of 450·C to 600·C. Sintering of nickel crystallites is much more severe at the higher temperatures, so we do not think that water vapour formed at lower temperatures will increase the nickel crystallite size. Furthermore, when calcination is performed in an air flow, we do not observe any difference between calcined and uncalcined samples, after reduction. J.W. GEUS You mentioned that omitting the previous calcination with nitrate containing catalysts results in smaller nickel particles. The same observation has been made previously by Eischens and van Hardeveld (Int. Catalysis Congr. Amsterdam) for silica impregnated with nickel nitrate: calcination of impregnated nickel nitrate led to rather large nickel oxide particles and consequently to larger metallic nickel particles. Have you any suggestion about the mechanism of sintering of nickel oxide by nitrogen oxides during the calcination? Is there evidence of any volatile nickel-nitrogen-oxygen compound? H. SCHAPER: We think that formation of a volatile compound is the most likely explanation. However, for nickel there is no evidence of such a compound. For copper the formation of a volatile Cu(N02) is known (M. Pospisil & P. Taras, ColI. Czech. Chern. Commun. 42 (1977) 1266). J.W.E. COENEN: You mention nickel crystallite sizes derived from methanation activity. That presupposes a one to one relation between the two quantities. Can you tell us more about that? H. SCHAPER: For coprecipitated nickel-alumina catalysts, where nickel crystallite sizes can be easily determined from X-ray diffraction line broadening, we investigated samples with crystallite sizes in excess of 2 nm and found that the activity plotted versus the reciprocal crystallite size yields a straight line. For some samples made by deposition-precipitation nickel crystallite sizes could also be determined from X-ray line broadening and for those the same relationship was found. S.P.S. ANDREW: Could it be that it is undesirable to perform calcination of nitrate melts slowly, it being better to employ a higher calcination temperature ? Thus calcination of the calcined and subsequently reduced catalyst was at 450·C, whereas when both operations were performed together, the temperature used was 600°C, so that the calcination operation would be much quicker.
309 H. SCHAPER: The nitrate ions that we are talking about are present in the form of nickel aluminium hydroxynitrate. This compound does not melt before decomposition, so there are no nitrate melts present during calcination. Your suggestion that the time of calcination is responsible for the observed differences does not explain why we only find differences for samples made by deposition-precipitation (extrudates), and not for coprecipitated samples (powder). A.R. FLAMBARD: My question concerns your observation that the direct reduction of uncalcined Ni/Si0 2 catalysts can lead to.higher metal-free surface areas; a phenomenon which has been observed on a number of previous occasions (ref. 1-5). You have mentioned that the presence of nitrate and nitrogen oxides may playa role in determining the final free metal surface area. I agree with you that these de~omposition products can cause some sintering of the nickel oxide particles produced during calcination. However, are you also aware that the heating rate used during the reduction of uncalcined samples in the presence of nitrates can also dramatically affect the subsequent metal surface areas? In agreement with Bartholomew and Farrauto (ref. 3), I have shown that above ca. 563 K the strongly exothermic reaction
+ Ni(N03)2 + Ni + 2NH3 + 6H20 ~H = - 1115.9 kJ/mol 2 predominates (ref. 6). Under fast heating rates excessive reduction to ammonia occurs, accompanied by localized heating and metal sintering. However, with a slow heating rate such as you have, I have found little reduction of the decomposition pr~ducts. Have you any information concerning the reduction of your catalysts after using different heating rates? There is also the problem that the water produced in the above equation can also sinter the metal particles. What are your comments concerning this ? Secondly, what are your comments in relation to the proposition that the direct reduction of uncalcined catalysts can lead to particles of different morphology (surface structure) as to those obtained when the catalysts have been precalcined? For example, Montarnal (ref. 7) has reported that the direct reduction of silica supported Ni(NH 3)4 (HCOO)2 gives materials of higher metal surface areas which he has interpreted in terms of microporous nickel crystallites. I have some evidence for a similar conclusion from my work with impregnated Ni/Si02 catalysts (ref. 6). I am investigating the possibility that after calcination "smooth" metal particles may be obtained upon subsequent reduction, whereas after direct reduction the particles may be "rougher". I see that you carry out your reductions under quite severe conditions which may cause an annealing of the metal surface, but I would be interested in your comments on this possibility. 9H
H. SCHAPER: First of all I have to remind you that we presented results for nickel/alumina catalysts. The nitrate ions that we are talking about are not present in the form of nickel nitrate but in the form of nickel aluminium hydroxynitrates, so your interesting remarks on the reduction of nickel nitrate do not apply in our case. We have no information on the effect of the heating rate on the reduction of uncalcined samples. All samples were heated at 2°C/min, which was found to be the best heating rate for calcined coprecipitated samples. Your second question, concerning the morphology of nickel crystallites is very interesting. We have no information on the morphology of our nickel crystallites, but it will be a subject for future research. 1) 2) 3) 4) 5) 6) 7)
Schuit, G.C.A. and van Reijen, L.L., Adv. Catal., 10, 242 (1958). van Hardeveld, R. and Hertog, F., Adv. catal., 22,~5 (1972). Bartholomew, C.H. and Farrauto, R.J., J. Catal.~45,41 (1976). This conference, paper by R. Burch and A.R. Flambard, M.A. Day et al. Burch, R. and Flambard, A.R., submitted to J. Catal. (1982). Flambard, A.R., Ph.D.thesis, Univ. Reading (1982). Montarnal, R., Proc. Ith Int. Symp. Brussels 1975, General Discussion, p.471.
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311
G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation orCata/ysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION OF TITANIA-SUPPORI'ED CATALYSTS BY ION EXCHANGE, IMPREGNATION AND
HCMJGENEDUS PRECIPITATION R. BURCH and A. R. FLAMBARD
Olenistry Department, The university, Whiteknights, Reading RG6 2AD, England
ABSTRACl'
Titania and silica-supported
cata~ysts
have been prepared by the techniques
of wet :inpregnation and ion exchange, and by precipitation either by the addition of alkali or hanogeneously by the hydrolysis of urea.
After drying,
the uncalcined materials have been investigated by X-ray diffraction, surface area rreasurernant, and tarperature-prograrrrned reduction.
Titania-supported
catalysts prepared by wet :inpregnation or ion exchange shewed little evidence of interaction with the support.
For hanogeneously precipitated catalysts, there
was sare indication of the formation of titanate-type species.
INl'roDOCTION It
is nOlrl recognised that oxide supports used to disperse metal particles are
rarely, if ever, inert.
In addition to the sanetimes extensive interaction
betv.een the deposited phases and the support which can occur during the early stages of catalyst preparation - to fonn specific canpounds such as silicates or aluminates (1,2) - it has also been found recently that a support may influence the catalytic properties of the rretal after reduction.
The most,
striking effects are found when reducible transition rretal oxides, such as titania, are used as supports. (3)
Such strong metal-support interactions (SMSI)
w:rre first noted for platinum metal/titania catalysts whose chenisorption and catalytic properties were modified after reduction at high tarperatures. (4,5) In earlier w:>rk (6,7,8) we have shown that Ni/titania catalysts can exhibit
special properties under conditions where SMSI are absent, and it became apparent that there was a need for a thorough investigation of the influence of sample history on catalytic properties. The precise way in which the active metal or its precursors are first
brought into contact with the support can affect the structure, reducibility, dispersion, and even the rrorphology of the catalyst.
The three methods of
preparation rrost carroonly encountered are wet impregnation, deposition/
312 precipitation, and ion exchange.
Impregnation may not be silrple as other effects,
such as adsorption may occur simultaneously. (9)
Frequently, poor dispersions
and broad particle size distributions are obtained. (10,11)
Often, there appears
to be little interaction between support and deposited phase, although this will depend on metal loading. (12)
The deposition/precipitation of metal precursors
onto a support may also be accanpanied by adsorption. (13, 14)
In the usual methcd
where the precipitant is added directly to a suspension of the support, local supersaturation and inharogeneity can result in poor dispersions. (14)
However,
the hcnoqenecus generation of the precipitant can yield high dispersions and
unifonn particle size distributions, often as a direct result of the fonnation of compounds with the support. (13-16)
Adsorption, or ion exchange, usually leads
to catalysts with high metal surface areas. (10-12)
Depending on the conditions
and system under investigation this mayor may not be a consequence of direct canpound fonnation between deposited phase and support.
Catalysts prepared by
this method are frequently rnuch more difficult to reduce than ilrpregnated catalysts. (10-12) In this paper
'lie
describe the preparation, using these three techniques, of
titania-supported Ni catalysts, and canpare these with silica-supported catalysts prepared as reference materials. EXPERIMENTAL
Materials and reagents The titania (Degussa P25) consisted of 80% anatase and 20% rutile, and had a surface area of 50 m2g-1. The silica (Davison grade 57) had a surface area of 2g-1, 290 m and was ground up before use. The 35-60 mesh size fraction was used. The source of nickel for all the preparations was nickel nitrate (Fisons).
Preparation of catalysts Wet impregnation.
Suspensions of the supports were contacted with solutions
of the appropriate Ni concentration for 0.25 h at 298 K before excess water was removed by rotary evaporation at 343 K.
Catalyst precursors 'J'Jere dried in air at
393 K for 16 h, and stored under vacuum until required. Deposition/precipitation. (i) addition of NaCE at 298 K. 3 200 an of 0.0144 M Ni nitrate solution 'llere added to 1. 52 g of support, and the pH adjusted to 2.4.
Snall doses of 0.01 M NaoH solution were then added by means
of a graduated burette. was allowed to
After the addition of each dose of NaaI, the solution
cx:me to equilibrium, and the
(ii) hydrolysis of urea at 353 K.
pH noted.
The reaction vessel was charged with 1.52 g
of support in 200 an3 of 0.0144 M nickel nitrate solution, and heated to 353 K. The pH was adjusted to 2.4, 4.6 g of urea powder was added, and the change in pH with time recorded autanatically.
313 Ion exchange.
Solutions of the required nickel concentration were prepared
and arnronium hydroxide added to raise the pH to 11.
The adsorption of the hex-
anmino Ni(II) CCllplex (12,17) was performed by adding this solution to a suspension of the support, in water and leaving for a period of 500 h. materials 393 K.
~
After this time, the
filtered off, washed with dilute arnronium hydroxide, and dried at
The dried materials were stored under vacuum,
Techniques used in catalyst characterisation Samples were analysed for their Ni contents by atanic adsorption spectrophotanetry, after dissolution with HF. measured using a calm microbalance.
Nitrogen adsorption isotherms were
X-ray powder patterns were measured with a
Phillips horizontal diffractaneter using nickel filtered Cu radiation. Tar[lerature-prograrmed reduction profiles were measured in the usual way.
The
heating rate was 7 K minute-I, the gas mixture contained 5 or 25% H in argon, 2 3 -1 and the gas flow rate was 10 an minute • RESULTS AND DISCUSSION Our objective in this work has been to canpare the preparation of titania and
silica-supported Ni catalysts in order to gain insight into the nature of the interactions between titania and metals.
Three methods of catalyst preparation
have been used - impregnation, deposition/precipitation, and ion exchange - in order of increasing degree of interaction between the support and the metal precursor. Wet impregnation Little adsorption of Ni onto silica or titania surfaces is to be expected fran acidic solutions, and this has been confirmed under our conditions by a spectrophotanetric investigation of the adsorption equilibrium.
Therefore, the majority
of the Ni taken up by the support during wet impregnation consists initially of a deposit of nickel nitrate.
HClINeVer, when the materials are dried to rarove
excess solvent, dehydration of the nickel nitrate crystallites may lead to an interaction with the support as the Ni tries to recover its co-ordination sphere. Figure 1 shows the nitrogen adsorption isothenns for the supports and sane of the catalysts, and the pore size distribution (PSD) for the silica samples. sunmarises the relevant surface area and porosity data.
Table 1
The Ni/Si0
2 catalyst
has the sarre BEl' surface area as the original silica, but a snaller internal
surface area. pores.
The PSD shows that the catalyst has a larger fraction of narrow
This suggests that during drying the nickel nitrate tends to wet the
support and spread out.
In the case of titania, the support is non-porous, but
the introduction of nickel nitrate results in the fonnation of mesopores.
is accanpanied by a substantial reduction in the surface area.
This
These effects
are thought to occur because the deposited nickel nitrate CCllqJOUIlds act as an 'adhesive' to hold the primary titania particles together, as shown in Figure 2.
314
b.
Q
...
11) Q)
O'l
.... 0
-
~500
....0..0
"tJ
....
Q)
.Q
Q)
~250
~ ~
{l 0
~C\j
05
1·0
1-0
P/Po Fig. 1. Nitrogen adsorption isothenns for silica (a) and titania (c) catalysts, and pore size distributions for silica samples (b).
TABLE 1 surface areas of the supports and the Ni catalysts 2 SBmI -1 m (g support)
sal 2 I -1 m (g support)
Silica
286
254
Ni9.8Si
285
230
Sample
Titania
52
30
Nil.OTi
48
29
Ni4.7Ti
46
26
Ni9.8Ti
33
22
a interna l surface area.
Fig.
2. Fonnation of a secondary titania structure after liIpregnation.
315 Thennogravimetric analysis of these catalysts shows (Table 2) that after drying, the average number of water molecules retained by the deposited Ni nitrate is in
the range 1-3, Le. the hexahydrate has becane dehydrated. TABLE 2 Experimental and calculated weight loss of Ni catalysts during reduction Sample
calculated wt , lossa/rng
Measured
wt. loss/rng
3
2
1
0
NiO.94Si
2.39
2.37
2.13
1.89
1.65
Ni4.8Si
10.12
11.42
10.26
9.11
7.96 14.77
Ni9.8Si
17.06
21.20
19.06
16.91
Nil.OTi
2.46
2.49
2.24
1.99
1.74
Ni4.7Ti
6.50
11.24
10.10
8.97
7.83
Ni9.8Ti
17.18
20.45
18.38
16.32
14.25
~umbers
refer to number of water molecules retained by the nickel.
X-ray diffraction of the Ni9.8Si catalyst indicated the presence of NiCOH)2.Ni(N03)2.2H20 (making up about 70% of the deposited material, Ni(OO)2 (about 5%), and a third phase which resenbled NiO. X-ray diffraction indicated that Ni (00) 2.Ni (00
For the Ni9.8Ti catalyst,
2.2Hp made up about 40% of the
3) total nickel, Ni (00) 2 about 30%, and Ni (N03) 2' 4H the remainder. There was no 20 evidence of direct canpound formation between the titania and the nickel.
Even allowing for the fact that the metal loading for a given surface area of
support differs for the two sets of catalysts, we conclude that for impregnated catalysts there is little interaction between nickel and titania. Deposition/precipitation ceus and co-workers (13,14) have dem:mstrated that useful information on the degree of interaction between a metal ion and a support can be obtained by monitoring the pH of the solution as hydroxide ions are gradually introduced.
We
have used their method to ccmpare the properties of silica and titania. (a) addition of NaCH Figure 3 shows the changes in pH as NaOH is added to distilled water, a nickel nitrate solution, and to suspensions of the supports in distilled water or nickel nitrate solution.
I f the addition of hydroxide ions did not lead to
the interaction of Ni ions with the support, then the measured pH curves should be equivalent to the sum of the curves for the nitrate solution and the support alone.
Figure 3 shows that the experimental curves differ markedly fran the
calculated curves (shown as broken lines in Figure 3), especially in the case of
316
g
Q
H2O
9
Ni 2 + 7
P
I
Si 02+ Ni
0-
2 +
5
3 50 Fig.
75
100 50 5 OH X 10 Imoles
75
-
100
3. 'l'itration curves for the silica and titania systems at 298 K.
titania.
It is inferred that for both supporta there is an interaction which
ccmrences at pH 5.2 for silica and pH 4.2 for titania. of Geus and Hennans (14) on Ni/Si0
By analogy with the work
catalysts we conclude that the precipitation
2 of Ni hydroxide on the silica surface began at podnt; P on Figure 3 (a) . titania, there is a similar change in slope at a pH of 5.4.
For the
These experiments
show that the adsorption of Ni ions onto a titania surface occurs at a pH of 4.0,
and that this is most probably followed by a smooth change over to give a
precipitate attached to the surface of the support.
XRD measurerrents on the
products of these preparations failed to'identify the structure of the precipitate, no lines could be detected.
Chemical analysis showed that the Ni content of roth
the silica and titani.a-suppor'ted materials was 0.9%, indicating that for the
addition of a given arrount of hydroxide the amount of nickel deposd.ted is independent of the support.. (b) hydrolysis of urea Figure 4 shows the change in pH with time as hydroxyl ions are generated by the hydrolysis of urea at 353 K.
The pH curve for water corresponds to the hydrolysis
of urea to give amronium carbonate. curve is obtained.
When silica is present, a broadly similar
With titania, however, the pH rises above the value for
317
6
HD
9
Si0
2
Ni + SiD2 +Ni 2+
2
5 4 I
c.
0·5 Fig.
1·0 Time Ih
1·5
4. Urea hydrolysis curves for the silica and titania systems at 353 K.
distilled water.
'!'his is thought to be due to the adsorption of carbonate ions
on the titania, which is known to contain basic hydroxyl groups. When the experiments are perfonned in the presence of nickel nitrate solution, quite differen\ curves are obtained, especially in the case of titania.
For both
systems the pH curves exhibit transient max.ilI1a (not illustrated in the titania case) characteristic of a nucleation barrier.
Such maxima were absent for Ni
nitrate in the absence of a support, and are stringly indicative of a supporcNi ion interaction.
XRD again failed to show evidence for any crystalline phases,
even th)ugh in this case the Ni content was about 3%. We conclude that this methcrl of preparation gives w=ll dispersed Ni both for silica (13-16) and for titania supports. Ion exchange Under the oonditions used in the ion exchange experiments it was found that the silica had adsorbed 95% and the titania 40% of the available nickel.
If
318 allowance is made for the different surface areas of the supports, these values correspond to 25% and 60% coverage. XRD
~
However, although for the silica catalyst
only very broad lines oorresponding to Ni silicate (12), in the case
of titania well defined lines oorresponding to Ni hydroxide, Ni Oxide, and Ni nitrate were observed.
It is apparent that this method of preparation leads to
extensive interaction between Ni ions and silica, presumably because under alkaline conditions the silica has a tendency to dissolve.
In contrast, the
interaction with thil titania appears to be limited to creation of a surface on which Ni oxide and Ni hydroxide can deposit.
There is no evidence for the
formation of Ni titanates. Temperature-prograrrrred reduction The influence of the method of preparation on the reducibility of sane of these catalysts has been investigated by TPR.
Fiqure 5 shows TPR profiles for
uncalcined sarrples of impregnated and ion exchanged catalysts.
These results
daronstrate rather well the differences between the Ni corpounda formed in the case of silica and titania.
In the case of the impregnated catalysts, the sharp
peaks at about 590 K have been identified as being due to the reduction of Ni oxide, the remainder of the profiles being indicative of the reduction of supported Ni oxide.
The ion exchanged Ni/Si0
catalyst is very difficult to
2 reduce, and is canparable in reduction characteristics to Ni silicate.
Significantly, however, for the ion exchanged Ni/ri0
catalyst reduction is much
2
more facile, confinning the absence of strong interactions with the support. CDNCLUSIOOS These experiments have demonstrated that a titania surface is much less reactive towards metal ions than a silica surface, which is often itself considered to be rooderately inert.
In particular, it has been shown that
titania-supported nickel catalysts, whether prepared by impregnation, deposition/precipitation, or ion exchange have little tendency for reactive interaction between the netal Lens and the support.
Even under oonditions
where there is a strong adsorption of Ni ions by the titania surface there is little direct evidence for the formation of titanates.
319
s
c 0
~
a. .~
..
E ::J Ul
C 0
u
c
~
Q)
Cl 0
L.
-g, ::c
g
473
673
873
T/K Fig. 5. 'I'E!rperature-progranrned reduction profiles for uncal.ctned , :impregnated and ion exchanged catalysts. (a), 10.7%Ni/Si0 i.rrpregnated, (b), 9.2%Ni/Si0 2 ion exchanged, 2 (c), 13.8%Ni/I'i0 inpregnated, (d), 4.0%Ni/I'i02 ion exchanged. 2
ACKNCMLEDGEMENI
R.B. thanks Amax Inc., and A.R.F. thanks the States of the Island of Jersey for financial support.
we
are grateful to Degussa and W.R. Grace for supplying
samples of the catalyst supports.
320
1 2 3 4 5 6 7 8 9
G.C.A. Schuit and L.L. van Reijen, Advances in Catalysis, 10(1958)242. K. r.brikawa, T. Shirasaki and M. Okeda, Advances in Catalysis, 20(1969)97. S.J. Tauster, S.C. FUng, R.T.K. Baker and J.A. Horsley, Science, 211(1981)1121. S.J. Tauster, S.C. FUng and R.L. Garten, J. Amer. Chem. Soc., 100(1978)170. P. ~iaudeau, B. Parmier and S.J. Teichner, C.R. Acad. Sci., C289(1979)395. R. Burch and A.R. Flambard, J. Chern. Soc. Chern. Communications, (1981)123. R. Burch and A.R. Flambard, React. Kinet. Catal. Letts., 17(1981)23. R. Burch and A.R. Flambard, suhnitted to J. Catal. (1982). J.R. Anderson, 'Structure of Metallic Catalysts', Academic Press, London, (1975)17110 V.A. DZis'ko: Kinet Catal., 21(1980)207. 11 M.S. Borisova, B.N. Kuznetsov, V.A. Dzis'ko, V.I. Kulikov and S.P. Noskova, Kinet. Catal., 16(1975)888. 12 M. Houalla, F. Dellanney, I. Matsuura and B. DelIron, J. Chern. Soc. Faraday I, 76 (1980) 2128. 13 J.A. van Dillen, J.W. Geus, L.A.M. He:rrnans and J. van der Meijden, Proc. 6th Int. Congr. Catal., London, 1976. (Eds. G.C. Bond, P.B. wells and F.C. Tompkins), 2(1976)677. 14 L.A.M. Hermans and J.W. Geus, Proc. 2nd Int. Sympositml, IJJuvain-1a-Neuve, 1978. (Eds. B. Delmon, P. Grange, P. Jacobs and G. Poncelet), (1979)113. 15 J.T. Richardson and R.J. Dubus, J. Catal., 54(1978)207. 16 J.T. Richardson, R.J. Dubls, J.G. Crump, P. Desai, U. Osterwalder and T.S. Cale, Proc. 2nd Int. Sympositml, IJJuvain-la-Neuve, 1978. (Eds. B. DelIron, P. Grange, P. Jacobs and G. Poncelet), (1979)131. 17 M. Primet, J.A. Dalmon and G.A. Martin, J. Catal., 46(1977)25. 18 M. Primet, P. Pichet and M.V. Mathieu, J. Phys. Chern., 75(1971)1221. >
321 DISCUSSION J. KIWI: 1. To which temperature did you heat the Ni(N03)2 on Ti02 and how long, since you report that no NiTi03 has been formed in your systems? 2. In Fig. 1 you report nitrogen adsorption isotherms for silica that at the left hand side evidence the outside BET area and towards the right show evidence for inner pores in this material. How did you assess the reported curve for the inner pores you report? Did you try titration methods to determine the contribution of the internal surface area since H20 has a diameter of 2.8 ~ as compared with N2 having 14 A2 in particle surface? A.R. FLAMBARD 1. The catalyst precursors discussed in this paper were all uncalcined, that is to say they were only subjected to a mild drying stage (16 h at 393 K). However, I have carried out separate experiments (Flambard A.R., Ph. D. Thesis, University of Reading 1982) into the effects of calcination on titania - supported nickel catalyst precursors and have not observed the presence of any detectable nickel titanate - type phases after calcination at temperatures up to 873 K. 2. The N2 BET isotherm for the silica shown in Fig. 1 (a) shows many of the characteristics of a Type IV isotherm, which is not uncommon for xerogels. The pore-size distribution curves of Fig. 1(b) were determined by use of the Kelvin equation, assuming cylindrical pores and a liquid-solid contact angle of zero. We did not investigated the possibility of using titration methods in order to determine microporosity. R. SIGG What is the difference between you BET surface area and the internal surface area? (Table 1). A.R. FLAMBARD: The difference between the BET surface areas and the internal surface areas listed in Table 1 arises from the use of two different models and hence equations (the BET and the Kelvin equations) for their determination. Although the surface area of a porous material is largely composed of contribution from the surfaces of the pores, the BET model will estimate the external surface as well. The Kelvin model will only estimate the surface due to the walls of the pores. J.W. JENKINS It would seem as though you are comparing the different surface reactivities of titania prepared by flame hydrolysis and silica prepared by precipitation. Have you had a chance to look at a flame hydrolyzed silica or a precipitated titania gel ? A.R. FLAMBARD: It is true that the titania and silica samples used as catalyst supports in this investigation were prepared by different methods. We have not investigated a flame hydrolyzed silica and only briefly looked at some precipitated titania gels because of the following reasons. Firstly, the precipitated titania gels that were available to us were all of a low surface area « 10 m2g- 1) and prepared from titanium (IV) sulphate. As the results presented in this paper represent only a fraction of those assimilated during an intensive investigation which was primarily geared to look at the active catalysts (i.e. after reduction), then these precipitated titanias were, for obvious reasons, considered unsuitable. Scondly, the function of the silica supported materials were to act as references. Therefore, in order to be in line with most of the literature, a precipitated silica was employed. I would consider a material prepared by flam hydrolysis to be perhaps more reactive than a material prepared by precipitation, so that it may be speculated that the use of a silica prepared by the former method would show up the differences between silica and titania even more effectively. G.C. BOND: With reference to the TPR plot of the ion-exchanged Ni/Ti02 catalyst (Fig. 5(d» ,it appears as if there may be an uptake of H2 in excess of that required to reduce Ni 2 + to Ni o• Is this your opinion also? If so, what degree of reduction of the Ti0 2 does it correspond to ?
322 A.R. FLAMBARD: We have carried out quite a detailed investigation into the reducibility of titania - supported nickel catalyst precursors, both before and after calcination. I am of the opinion that these measurements indicate that there is srnne surface reduction of the support. However, this does not imply the formation of'such phase as Ti4C7 (the Magneli phases) and indeed our results indicate that the support surface is only reduced as far as TiOl.98' This may have important consequences as far as the SMSI effect is concerned (Burch, R. and Flambard, A.R., submitted to J. catal. 1982). For the uncalcined catalyst whose TPR profile is shown in Fig. 5(d), this support reduction is difficult to observe directly because of the presence of peaks due to the reduction of nitrate decomposition products. The reduction of this nitrate is believed to account largely for the ~cess hydrogen consumption in this case.
323
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
INFLUENCE OF PHOSPHORUS ON THE HDS ACTIVITY OF Ni-Mo/y-A1
Z03
CATALYSTS
D. CHADWICK, D.W. AITCHISON, R. BADILLA-OHLBAUM and L. JOSEFSSON Department of Chemical Engineering and Chemical Technology Imperial College, London, SW7 ZBY (U.K.)
ABSTRACT A series of coimpregnated
P~Ni-Mo/y-A1Z03
catalysts have been prepared with
various phosphorus loadings and their activities for thiophene HDS measured. HDS activity is found to increase with phosphorus content reaching a maximum at about 1% wt P.
The catalysts have been characterised by a number of techniques
including XPS.
XPS studies show that the phosphorus is in monolayer form and
that it influences the repartition of Mo in the catalysts.
No evidence was
found for the involvement of phosphorus in a sulphide species.
INTRODUCTION The properties of Co-Mo and Ni-Mo catalysts have been widely investigated with respect to hydrodesulphurisation (HDS) reactions.
Efforts have also been
made to find additional promoters which can increase further the HDS activity of these catalysts.
For the Co-Mo system, no such promoters have emerged, but
phosphorus has been reported to improve the performance of Ni-Mo catalysts in HDS and in hydroprocessing (refs. 1-5).
However, the promotion effect of phos-
phorus on HDS activity has not been properly charted and the structures of phosphorus containing catalysts have not been established. The present paper describes preliminary results from a study of P-Ni-Mo/y-A1 catalysts which was undertaken in order to clarify the influence of phosphorus on HDS activity.
The results presented here are for a series of coimpregnated
catalysts with a Ni and Mo loading common in industrial catalysts.
Catalyst
activities for thiophene HDS are reported and are discussed in relation to characterisation studies by physico-chemical techniques including X-ray photoelectron spectroscopy.
Z03
324 EXPERIMENTAL Catalyst preparation A series of P-Ni-Mo/y-Al
catalysts with various P loadings were prepared Z03 by coimpregnation using the dry soaking method with a solution containing Ni(N0
(NH Mo and H In each case the initial pH of 3P04• 3)Z.6HZO, 4)6 70Z4.4HZO the impregnating solution was Z. For the catalysts containing small P loadings,
a small amount of nitric acid was added to ensure pH was Norton SA-6l75 (surface area
=
particle size range of 30-50 mesh.
~
Z59 mZg, pore volume
Z.
The y-Al support Z03 3/g) cm with a
= 0.55
All catalysts contained a constant Ni:Mo:Al
molar ratio based on 4% wt Ni (as NiO) and 15% wt Mo (as Mo0 The catalysts 3). were dried in dry air at 383 K for 7Z hours and 'calcined at 673 K for 8 hours in dry air at a flow rate of 400
m~/min.
After calcination the catalysts were
analysed for phosphorus content.
Catalyst characterisation A microbalance system was used to measure N BET surface areas, pore-size Z distributions and surface acidity using pyridine adsorption (refs. 6,7). X-ray photoelectron spectra were obtained with a VG ESCA-3 using AlK radiation. a Binding energies were referenced to Al Zp = 74.5 eV.
Catalyst activities Activities for thiophene HDS were measured in a tubular reactor at atmospheric pressure and 6Z3 K using a total flow rate at SOD molar ratio of 13.
Catalyst charge was 3 g.
m~/min
and hydrogen/thiophene
Catalysts were pre-sulphided for
1 hour at 573 K in 10% v/v HZS in HZ at a flow rate of 100
m~/min.
The steady
state conversion was determined from the disappearance of thiophene.
RESULTS Catalyst activities In order to examine the effect of phosphorus on thiophene HDS, the catalysts were prepared with a constant Ni:Mo:Al molar ratio based on 4% wt Ni (as NiO) and 15% wt Mo (as M00 This loading and P loadings in the range 0-Z.4l% wt P. 3) corresponds to a P/Mo atomic ratio in the range 0-0.74. A consequence of the constant Ni:Mo:Al molar ratio is that there is a slight reduction in the amount of Ni and Mo per gram of catalyst as the phosphorus loading increases. The apparent kinetic constants for thiophene HDS at 6Z3 K and atmospheric pressure were calculated from the measured conversions assuming first order kinetics and plug flow.
The results are shown in Fig. 1.
The HDS activity
was observed to increase steadily with phosphorus content reaching a broad
325
0·6
~
,;:::
.
+' <0
CJ
01
<, M
0
E
01
.-I<:
0·3 0
% wt P
2
Fig. 1. Apparent kinetic constants for thiophene HDS over coimpregnated P-Ni-Mo/Y-A1 catalysts with various phosphorus contents • Z03
. N
~-----------
•
E
<,
.....
~ 1'5 L-
.L.
.L-
o
---'
2
% wt P
.Fig. Z. Specific apparent kinetic constants for thiophene HDS over coimpregnated P-Ni-Mo/y-Al?O, catalysts with various phosphorus contents.
326 maximum at approximately 1% wt P.
Further increase of the phosphorus content
leads to a slight reduction in the HDS activity.
The specific apparent kinetic
constants based on the determined BET surface areas (see below) are shown in Fig. 2.
The HDS activity is observed to increase with phosphorus content
to 1% wt P.
Further increase of the phosphorus content does not result in any
significant change in HDS activity and the decrease in activity apparent in Fig. 1 is not observed ••
The present activity results are consistent with the
optimum phosphorus contents reported in the patent literature (refs. 3,5). Catalysts prepared using the same support and containing only phosphorus with loadings in the same range as the P-Ni-Mo catalysts did not have any significant HDS activity under the same conditions as the measurements reported above.
Catalyst characterisation The BET surface areas were found to decrease gradually with increasing phos2 2 phorus content from 218 m /g-cat for 0% wt P to around 180 m /g cat for 2.41% wt P.
At the same time there is a reduction in pore volume and an increase in the
mean pore radius.
For example, the catalysts containing 0% wt P and 0.72% wt P 3/g-cat were found to have pore volumes of 0.56 and 0.40 cm respectively and mean pore radii of 5.5 and 6.9 urn respectively.
Rather surprisingly there was only
a slight increase in surface acidity determined using pyridine adsorption with 2 phosphorus content. Values of 0.22 and 0.26 mg/m pyridine adsorbed were obtained for the support and the Ni-Mo catalyst respectively.
Addition of 2.41% 2
wt P only increased the amount of pyridine adsorbed by about 0.02 mg/m • Although the range of the increase was small, the trend was of slightly increased acidity with increasing phosphorus content. X-ray photoelectron spectra were obtained for the calcined P-Ni-Mo catalysts. and Ni 2P3/2 binding energies (b.e.) are given in 5/ 2 The Ni 2P3/2 and Mo 3d 2 b.e. are consistent with the presence of 5/
The measured P 2p, Mo 3d Table 1. TABLE 1
XPS binding energies (eV) for calcined P-Ni-Mo/y-A1 203 catalysts % wt P
Mo 3d 5/ 2
Ni 2P3/2
P 2p
0.0 0.24 0.50 0.72 0.95 l.15 2.41
233.3 232.8 232.8 232.9 233.0 233.2 233.5
856.0 856.1 856.1 856.3 856.3 856.2 856.3
134.4 134.2 134.3 134.2 134.4
Ni 2+ and predominantly octahedral Mo 6 + respectively.
The P 2p b.e. reported
in Table 1 for the P-Ni-Mo catalysts were identical to P 2p b.e. determined for several P/y-Al?O, catalysts.
327 0·2 .......- - - - - - - - - - - - - - - - - - - - r
% wt P and (~) P/y-AI Fig. 3. XPS PZp/AI Zp peak area ratios for (0) P-Ni-Mo/y-AI Z03 catalysts. The dashed line - has been calculated from the bulk concentrationZ03 for P/y-AI Z03•
5
4
o, N
rl
3
~
'0
C"1
~
2
2
0
3
% wt P
Fig. 4. XPS Mo 3d/AI Zp peak area ratios for P-Ni-Mo/y-Al
z0 3
catalysts.
The variation with phosphorus loading of the measured P Zp/AI Zp peak area and P/y-Al z0 3 catalysts are shown in Fig. 3. In 3 both cases the peak area ratios were observed to increase linearly with phos-
ratios for P-Ni-Mo/y-Al z0 phorus loading.
This linear behaviour is consistent with a monolayer type
catalyst.
The results for P/Y-Al z0 3 catalysts are in reasonable agreement (Fig. 4 ) with peak area ratios calculated from the bulk composition using the
model of Kerkhof and Moulijn (ref. 8) and published sensitivity factors (ref. 9).
328
Higher P Zp/Al Zp ratios were obtained for the P-Ni-Mo catalysts compared to those containing only phosphorus.
This difference can be explained almost com-
pletely by the reduction in Al Z03 content. The residual difference could be attributed to an increase in effective wall thickness of the support by Ni and Mo which are present slightly in excess of monolayer loadings.
However, this
is rather speculative since the phosphorus would have to be located on top of the Ni and Mo layer(s),whereas the linear P 2p behaviour suggests the reverse. The variation with phosphorus loading of the measured Mo 3d/AI 2p peak area ratios are shown in Fig. 4.
In general, the Mo 3d/AI 2p ratio fell with in-
creasing phosphorus content.
(Catalyst 0.7Z% wt P has not been included since
it appeared to have an anomalously high Mo 3d/AI Zp ratio.)
The Ni 2P3/2/Al 2p
ratios showed more scatter, but fell from 1.7 at 0% wt P to 0.74 at 2.41% wt P. It is interesting to note that the P/Mo atomic ratio determined from the peak area ratios is equal to the bulk composition for 2.41% wt P, but is significantly less than bulk for the other catalysts.
Since the phosphorus is behaving as a
monolayer catalyst, the above result indicates that the Mo is present in ideal monolayer form only at 2.41% wt P. work on Ni-Mo/y-A1
This conclusion is supported by previous
catalysts (ref. 10).
Under acidic impregnation conditions 203 values for Mo 3d/AI 2p and Ni Zp/AI 2p of about 2.0 and 0.7 would be expected respectively for well-dispersed catalysts. the 2.41% wt P catalyst.
These values are close to those for
This point is discussed further below.
Diffuse reflectance spectra of the calcined catalysts showed an increase in absorption around 300 nm with increasing phosphorus content.
This could be
interpreted as an increase in the relative proportion of octahedral Mo. ever, phosphorus oxides also have a weak absorption in this region.
How-
Electron
microprobe studies of several sectioned particles of each catalyst were carried out.
Some particles of the Ni-Mo and low P content catalysts showed an in-
creased concentration of Mo and P at the particle edge.
The most marked exam-
ple was the 0.72% wt P catalyst which showed pronounced enrichment of Mo and P at the particle surface.
This is consistent with the anomalously high Mo 3d/
Al 2p ratio found for this catalyst.
Catalysts containing 0.95% wt P or more
gave no significant enrichment at the edge and showed a fairly even distribution throughout the particles.
Occasional high, local concentrations of Mo were ob-
served within particles and these points also contained relatively higher P concentration.
However, these regions were not typical of the elemental distribu-
tion in these catalysts. Some preliminary investigations have been made of the sulphided state of the catalysts. dation.
No changes were noted in P 2p b.e. or P 2p/Al 2p ratios on sulphiChemical analysis of the sulphur content revealed that the
329 catalysts were not fully sulphided by the pre-sulphiding treatment.
Interest-
ingly, the sulphur content decreased slightly with increasing P content reaching a shallow minimum at around 1% wt P which corresponds to the maximum HDS activity. DISCUSSION Phosphorus 2p b.e. in a range of phosphorus compounds have been reported previously (refs. 11,12).
Comparison of the measured b.e.'s in Table 1 with these
literature values show that the catalysts have significantly higher P 2p b.e.'s than simple anions such as phosphite or orthophosphate.
The catalyst b.e.'s
corresponded closely to the b.e.'s reported for condensed phosphate structures, particularly polyphosphates.
Although the catalyst P 2p b.e.'s are significant-
ly higher than for phosphorus in heteropoly structures, their presence in the catalysts cannot be excluded since the phosphorus loading in the high P content catalysts exceeds that required to form heteropolymolybdates. In considering the influence of phosphorus on HDS activity it is important to distinguish between chemical and textural effects.
The XPS and electron
microprobe results indicate that the addition of phosphorus has a pronounced effect on the catalyst architecture and in particular the repartition of Mo. That is, the partitioning of Mo between the internal and external surface of the support.
As noted above, the XPS Mo 3d/AI 2p ratios at lower phosphorus load-
ings are considerably higher than expected for a well-dispersed catalyst indicating that there is significant enrichment of Mo on the external particle surface. The gradual decrease in Mo 3d/AI 2p ratio with increasing phosphorus loading can be understood in terms of an increasingly even distribution of Mo.
Since the
Mo loading is slightly in excess of a monolayer, this implies at the same time an improved Mo dispersion.
On this basis, the agreement between the P/Mo
atomic ratio determined by XPS for the 2.41% wt P catalyst and the bulk composition can be easily understood if it is assumed that the Mo achieves the ideal monolayer dispersion in this catalyst.
This view is consistent with claims in
the patent literature (ref. 3). Turning to the possibility of chemical promotion of HDS activity by phosphorus, we note that the insensitivity of the P 2p b.e. 's to the pre-sulphiding treatment suggests that phosphorus is not involved in a sulphided species.
Re-
cently, emission Mossbauer spectroscopy studies of Co-Mo catalysts have demonstrated that HDS activity is related to the presence of a Co-Mo-S phase (refs. 13-16).
One might speculate that a similar phase exists in the Ni-Mo catalyst
system.
The Mossbauer studies suggest that the Co-Mo-S phase has a specific
oxide precursor in which Co and Mo are highly dispersed (refs. 15,16).
A pos-
sible role of phosphorus in the Ni-Mo catalysts, therefore, may be to promote the formation of an oxide precursor for a "Ni-Mo-S" phase.
The fact that the
0.72% wt P catalyst appears to have poor Mo distribution and yet does not deviate
330 significantly from the HDS activity curve in Fig. 1 suggests that this latter form of promotion may be the more important. CONCLUSIONS The HDS activity of P-Ni-Mo/y-A1 catalysts is found to increase with phos203 phorus content reaching a maximum at about 1% wt P. XPS studies have shown that the phosphorus is present in a monolayer form.
It is concluded that phos-
phorus influences the repartition of Mo in the catalysts.
It is suggested the
phosphorus may promote the formation of an oxide precursor of a Ni-Mo-S phase similar to that reported for Co-Mo catalysts.
No evidence was obtained for the
involvement of phosphorus in a su1phided species. ACKNOWLEDGEMENT D.W.A. thanks the SERC(UK) for the award of a studentship and R.B-O. thanks the National Coal Board(UK) for financial support. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
W. Ripperger and W. Saum, J. Less Common Metals, 54(1977). U.S. Patent No. 2608534. U.S. Patent No. 3287280. U.S. Patent No. 3904550. U.S. Patent No. 4003828. R.L. Richardson and S.W. Benson, J. Phys. Chern., 61(1957)405. L. Forni, Cata1. Reviews, 8(1974)65. F.P.J.M. Kerkhof and J.A. Mou1ijn, J. Phys. Chem., 83(1979)1612. S. Evans, R.G. Pritchard, and J.M. Thomas, J. Electron Spectrosc., 14(1978)341. R. Badi11a-Oh1baum, Ph.D. Thesis, London (1979). M. Pe1avin, D.N. Hendrickson, J.M. Hollander, and W.L. Jolly, J. Phys. Chern" 74(1970)1116. J. Hedman, M. K1asson, B.J. Lindberg, and C. Nordling, in Electron Spectroscopy, (Ed.) D.A. Shirley, North Holland, Amsterdam, 1972, p.681. B.S. Clausen, S. M~rup, H. Tops~e, and R. Candia, J. Physique, 37(1976)C6-249. and H. Tops~e, J. Catalysis, C. Wive1, R. Candia, B.S. Clausen, S. M~rup, 68(1981)453. H. Tops~e, B.S. Clausen, R. Candia, C. Wivel, and S. M~rup, Bull. Soc. Chim. Be1g., 90(1981)1190. M. Breysse, B.A. Bennett, D. Chadwick" and M. Vrinat, Bull. Soc. Chim. Belg., 90(1981)1271.
331 DISCUSSION R. CANDIA : You mention that the P did not sulfide in your experiment, and of course, it was not expected to do so. This means that P must have a secondary effect, that is, it must affect some physical rather than chemical property. It is thinkable that the P043 ion will compete with the molybdate ions, resulting in a molybdenum monolayer interrupted by phosphate ions, which probably leads to the formation of smaller islands of Mo on the surface, than in the case where no P was added. After sulphiding, the P promoted catalysts will thus produce smaller crystals of MoS2' Might this be a possible effect of the P promotion ? In our paper we show that P affects the repartition of Mo in the D. CHA~WICK catalysts. However, we do not believe this to be the primary mechanism of the observed promotion effect. We agree that since the phosphorus does not appear to be involved directly in the sulphided state, the effect must be indirect or secondary. In our paper we have termed this effect "chemical" to distinguish it from the effect on repartition. There are several possible mechanisms for this "chemical" effect, one of which you have described (although you term it "physical"). Another possibility is that a proportion of the phosphorus (say 10-15 %) is involved in the formation of heteropoly structures with Ni and Mo and that these structures SUlphide readily to an active "Ni-Mo-S" species. Alternatively, the observed linear increase in pIAl ratio in XPS irrespective of the Mo and Ni distribution suggests that the P may be located adjacent to the alumina surface and beneath the Mo layer. It could be that the presence of the P layer prevents the Ni promoter from entering the alumina lattice. Experiments to test these various hypotheses are in progress. N.P. MARTINEZ In view of determining what is the role of phosphorus, I suggest you to measure the adsorption isotherms of molybdenum and phosphorus, because they both will interact with the support. What we probably are in presence of is a competition, and phosphorus will help to disperse the Mo, and that might be the reason for the increase of your MolAl ratio in XPS with increasing P content. D. CHADWICK I agree that the adsorption isotherms of P and Mo could be usefully investigated. In fact, the MolAl ratios in XPS are observed to decrease with increasing P content. Nevertheless, as explained in the paper this arises from improved distribution of Mo with increasing P content. D.D. SURESH : Have you looked at the XPS of catalysts after HDS activity and compared with the XPS of fresh catalysts ? C. GUEGUEN: How do you sulphide the catalyst? Did you study the surface after sulphidation ? D. CHADWICK : (Reply to Suresh and Gueguen). The catalysts were pre-sulphided in a 10% H2S/H2 mixture. Details are given in the paper. We have not studied the catalyst surfaces after the thiophene activity test. However, we have made some preliminary studies of the pre-sulphided states. We did not observe any change in P binding energies on sulphiding. There were only slight changes in the pIAl, MolAl and Ni/AI peak area ratios when the catalysts were sulphided A more detailed investigation of the sulphided states of the catalysts by XPS and other techniques is in progress. G. ANTOS: Have you evaluated commercial HDS catalysts in your thiophene test? How do the activities compare? Is the increase observed in your work due to P significant ? D. CHADWICK: The activities of the catalysts reported here are comparable to a commercial Ni-MO/Y-Al203 catalyst of similar composition. The catalyst activities were measured several times and were found to be enti-
332 rely reproducible. Hence, we believe the increase in HDS activity with P content to be significant. This observation is in agreement with claims in the patent literature. In addition, we have examined the performance of the catalysts for hydrotreating a model feedstock containing dibutyl sulphide, quinoline and dibenzofuran at high pressure and fine increased conversions with increasing P content. R. CAHEN: Have you examined regenerated catalysts? high activity?
Do they maintain their
D. CHADWICK We have not attempted to regenerated the catalyst. activity of regenerated catalyst would be of some.interest.
However, the
R.J. BERTHOLACINI: It has previously been shown that HDS activity is associated with small pore structure Al203' Using H3P04 as the P source in your work, lowered surface area of your support, increasing the pore size, have you tried other non-acidic sources of P so as not to alter the pore structure ? D. CHADWICK: We are currently investigating varIous methods of introducing phosphorus into the catalysts. A. SUDKAMP Can you give some details about the method of preparation of the coimpregnation solution ? D. CHADWICK : The support was added to a solution containing the correct amounts of Mo, Ni and P adjusted to pH = 2. The volume of the solution was slightly in excess of the pore volume of the support. GUI LIN-LIN: Did you measure the acidity of the Mo-Ni-P/Y-Al203 and compare it to MO-Ni/Y-AI203 catalyst? We measured the acidity of MO-Ni-P/Y-AI203 catalysts by various methods. The acidity of MO-Ni-P/y-AI203 is distinctly lower than that of MO-Ni/y-AI203 catalysts. What do you think of it ? D. CHADWICK : The acidity The trend was of slightly hydrotreatment of a model increase in hydrocracking ly increased acidity.
of the catalysts was measured by pyridine adsorption increased acidity with P content. In studies of the feedstock containing phenanthrene, we found a small with P content. This result is consistent with slight-
D.A. YOUNG: Regarding the effect of phosphorus on the acidity of HDS catalyst: several years ago I examined the effect of phosphorus on the acidity of calcined MO-Al203 using Ho and HR indicators. The experiments indicated that Bronsted sites formed by the reaction of Mo with Al203 were destroyed when phosphorus was added by impregnating with H3P04 and calcining. D. CHADWICK:
Thank you for this comment.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III
333
© 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STUDY OF THE INFLUENCE OF THE PREPARATION CONDITIONS ON THE FINAL PROPERTIES OF A HDS CATALYST C.V. CACERES, M.N. BLANCO and H.J. THOMAS Centro de Investigaci6n y Desarrollo en Procesos Cataliticos, La Plata, Argentina
ABSTRACT The effect of the operating conditions on the final properties of catalys~ of molybdenum and cobalt oxides supported on v-alumina spheres, prepared by wet impregnation, in two stages, with intermediate drying and calcination, was studied. It was observed that: a) the molybdenum was redistributed during cobalt impregof adsorbed to occluded molybdenum nation, b) those catalysts with higher ratio (at equal total molybdenum content) showed higher hydrodesulfurization activity, c) catalysts calcined at higher temperatures presented lesser activity in relation to the species observed by reflectance spectroscopy.
INTRODUCTION The extensive bibliography on the CoMo/A1 203 hydrodesulphurization catalyst actualized in a recent review by Grange (ref.I), intends to establish, through different techniques, a correlation between the surface species existing in the catalyst and the catalytic properties. A detailed reading of the published works allows to obtain information on the influence of certain operating variables (impregnation method, impregnationorde~ precursors concentration, atomic ratio Co/Co + Mo, activation temperature) on the activity of powdered catalysts. This paper presents the influence of the concentration of the impregnating solution, the presence of ammonia in this solution, the impregnation time and the calcination temperature on a) the active components distribution in the support sphere radius, b) the ratio of adsorbed to occluded molybdenum, c) the present species of molybdenum and cobalt, d) the activity as hydrodesulphur agent, and e) the mechanical and textural properties of the final catalysts. EXPERIMENTAL Catalyst preparation Commercial v-alumina spheres, of 4-4,2 mm diameter, were used as support, with
334
55% porosity, mean pore diameter 42 A and surface area 282 m2/g. The impregnant solutions were prepared using ammonium heptamolybdate Merck PA and cobalt nitrate Merck PA as precursors of the active components, ammonia Merck PA as competitor and water as solvent. The operative conditions were selected according to the different concentration profiles of molybdenum in the pellet, the total molybdenum content (10% w/w as Mo0 3), the total cobalt content (2,5% w/w as C0 304), wet successive impregnation (first molybdenum and afterwards cobalt) with intermediate drying and calcina~o~ The impregnations with molybdenum solution were made in a similar way as in the described in a previous work (ref. 2). XII, XIX and XX experim~nts Subsequently, the samples were impregnated during one hour with cobalt solution, dried in air during one day and calcined under the same conditions as in the impregnation with molybdenum. The impregnation conditions corresponding to each catalyst are pointed out in Table 1. TABLE 1 + Impregnation conditions Catalyst XIX XII XX MI MIl MII I M IV MV MVI
CMo (%
~1003'w/v)
15 15 5 15 15 5 15 15 5
CN (% NH 3,w/v)
°
10 0 0 10 0 0 10 0
ti (h)
t d (days)
T (OC)
tc (h)
CCo (% CoO,w/v)
1 1 24 1 1 24 1 1 24
1 1 1 1 1 1 1 1 1
400 400 400 400 1100 400 550 550 550
0 0 0 1 1 1 1 1 1
0
°0 4
4 ~,
4
4 4
+Abbreviations used: CMo' CN' CCo = molybdenum, ammonia and cobalt concentratioffi in the impregnant solution; t i, t t = impregnation,drying and calcination d, c times; T = calcination temperature. Quantitative analysis The molybdenum and cobalt distribution in the spheres was obtained by attrition of them, varying the spheres diameter from 0,1 to 0,2 mm and collecting the powdered fractions. These were analyzed by X-Ray spectrometry in a Philips PW 1410/10 spectrometer; the Ka. of ~10, Co and Al were measured. The concentrations were obtained by the equation of Rasberry and Heinrich (refJ)
335
which takes into account interelement effects. The constants of this equation were calculated by measuring standards prepared in the laboratory. The total molybdenum. content in the solid was determined by the precipitation method with a-benzoinoxlme; cobalt was analyzed by atomic absorption spectrosco~.
Textural characterization The surface areas of the final catalysts were determined by BET method and the pore size distribution by the Hg penetration method. The ~esistance to abrasion and attrition was measured in a horizontal rotating tube. The sizes and operating conditions stated by Dart (ref. 4) were used. The grain to grain crushing resistance was determined with an INSTRON Universal equipment model TTCM. The pellet position as well as its behaviour during the experience were observed using a stereoscopic microscope Carl Zeiss MIll. Diffuse reflectance spectroscopy The UV-visible reflectance spectra were obtained with a Varian Super Scan 3 spectrophotometer, between 200 and 850nm, using the alumina support as reference. Activity The measurements of the catalysts activity in thiophene hydrodesulphurization were carried out in a test reactor SIREM. The presulphurization was made at 350°C, PH 300 kPa and LHSV 48 h- 1.A mixture 2 of cyclohexane and toluene (50% wjw) was used, to which 15000 ppm thiophene were added. The runs were made at 280 and 350°(, PH2 3000 kPa and LHSV 67 h- 1. The mixture was the same as the one of presulphurization, but with 5000 ppm thiophene. The sample analysis was carried out by gas chromatography. RESULTS AND DISCUSSION The textural and mechanical properties of final catalysts are shown inTable 2. TABLE 2 Textural and mechanical properties Sample
~1~O3 M II MIll M IV MV MVI
Surface area (m 2jg) 282 252 248 232 221 231 263
Mean pore diameter Attrition Crushing strength (% fines) (~) (~) 42 47 46 48 52 56 45
7 5,1 5,5 6,4 4
3 6,6
3,5 3 2,5 3,5 3,5 3 3
-
4 3,5 3,5 4 4 3,5 3,5
336
A decrease of the surface area and an increase of the mean pore diameter of the catalyst with respect to the original alumina may be observed. The resistance to attrition and the grain-to-grain crushing strength shows no important differences between the original alumina and the catalysts. The distribution of molybdenum as a function of the sphere volume of the samples is presented in figs 1, 2 and 3.
10
c
M
-
o
o
-XIX )( MI IJ MN
~
~
Fig. 1.
10
Molybdenum concentration as a function of the sphere volume for samples M I, M IV and XIX.
T j(
;
•
::'. )(
I<
.
C
•
:lit
-xu )( MIT I
,
D
MV I
0,5 Fig. 2.
Molybdenum concentration as a function of the sphere volume for samples MIl, MV and XII.
337
• XX xMm D
Fig. 3.
M"2I
~10lybdenum concentration as a function of the sphere volume for samples MIll, MVI and XX.
In these figures, it may be observed that the distribution of molybdenum along the pellet is almost constant in the samples M I, MIl, M III and ~ V. The samples t~ IV and r1 VI show a concentration profile. Figure 2 shows that those experiments containing ammonia in the impregnating solution do not present any change in the molybdenum distribution in the interior of the support, during the different cobalt impregnation stages, as compared with sample XII. The effect of redistribution in the profiles is observed in figures 1 and 3. Thus, the catalysts MI and M IV with respect to sample XIX, as well as the catalysts M III and " VI with respect to sample XX, show concentration profiles less pronounced, the effect of redistribution being more marked in the samples prepared at lower calcination temperature. On the other hand, the cobalt distribution in the inside of the sphere is almost constant in all the samples. The values of total molybdenum and cobalt concentration of the catalysts are shown in Table 3. In this table, it can be observed that the atomic ratio (R = Co/Co + ~o) is nearly similar in all the catalysts, except for ~ IV which gives a value slightly higher. The reflectance spectra between 450 and 850 nm of the catalysts and of the model compounds (CoA1 204, C0 304) are shown in figure 4.The analysis of the spectra indicates the presence,in all the samples, of a broad triple band situated between 500 and 700 nm, which is assigned to tetrahedral Co(II) in CoA1 204,
338
TABLE 3 Supported molybdenum and cobalt content and atomic composition of catalysts Catalyst Supported molybdenum content (% Mo0 3 w/w)
MI M II M II I t1 IV MV MVI
10,31 9,53 10,,22 9,44 10,41 It,14
Supported cobalt content Jltomic ratio R = Co/Co+r~o (% C0 304 w/w) 0,29 2,36 0,30 2,29 0,31 2,54 0,38 3,21 0,32 2,75 0,28 2,38
ASS 0,4
01-----....l----~----....L.----__1
850
A(nm)
Fig. 4. Diffuse reflectance spectra between 450 and 850 nm. However, the intensity of the band in the samples calcined at 400°C (M I, MIl, M III) is less intense than in those calcined at 550°C (M IV, MV, MVI). This would be in accordance to what was pointed out by Grange (ref. 1), who establi~ffi that the CoA1 204 concentration increases when the calcination temperature rises. On the other hand, according to Gajardo (ref.5),the absorption band in the 650960 nm range, with a maximum at 700 nm, present in C0 304 spectrum, is presumably octahedral Co (III). This band would be superimposed, in part,with the band of CoA1 204, in those catalysts with R < 0.5. All the prepared catalysts show Co(III) in octahedral oxidic surrounding. Nevertheless, the samples calcined at 400°C
339
show greater influence of this species with respect to the corresponding ones calcined at 550°C. The reflectance spectra, in the 200-500 nm range, of the catalysts and the model compounds (Mo0 3, Na 2Mo04 . 2 H20) are shown in figure 5. In all catalysts, a broad absorption band with maximum between 200 and 300 nm is observed. Taking into account that the Mo (VI) is present in Mo0 3 in octahedral surrounding and in.Na 2Mo0 4 . 2 H20 in tetrahedral surrounding, it may be concluded that in our catalysts most of Mo (VI) is placed in tetrahedral position and only a small part is in octahedral position.
400
Alnm)
500
Fig. 5. Diffuse reflectance spectra between 200 and 500 nm. The HDS activity (thiophene conversion) of the prepared catalysts and of one commercial was measured at different times. The activity did not change noticeably after 5 hours. The results corresponding to 8 hrs are given in Table 4. TABLE 4
Hydrodesulphurization activitips Catalyst
Commercial
t~
I
MII
M II I
MVI
42 54
49
M IV 35
MV
37 54
41
45
69
48
56
69
340
The values obtained show a slightly higher activity for the catalysts prepared at 400°C than for those calcined at higher temperature. This fact is related to the species observed by reflectance. According to Richardson (ref.6), CoA1 204 is inactive and, as it was previously mentioned, sample M IV has more CoA1 204 and less Co (III) than M I. Equal behaviour is observed in MV with respect to MII and in MVI with respect to MIll. Besides, in the samples calcined at 550°C the activity decreases in the order at 400°C,in the order MIll, MIl, MI. MVI, MV, M IV,and in those ~alcined This can be explained taking into account the different impregnation conditions and that, according to de Beer e t al.I ref . 7), the active molybdenum is that which is adsorbed on the alumina as a monolayer. As concerns M III and MVI, an impregnant molybdenum solution of lower concentration (5%) was used during 24 hours, giving a greater ratio of adsorbed to occluded molybdenum, at equal total molybdenum content. On the other hand, MV and M II were prepared with NH 3 in the impregnant solution; this would lead to a ratio of adsorbed to occluded molybdenum lower than in M IV and M I, prepared with the same molybdenum concentration (15%) but without NH 3. Nevertheless, the loss of NH 3 by volatilization during calcination would make it possible, in the later impregnation with cobalt, that the occluded molybdenum present in high concentration in these samples will be adsorbed, resulting in a higher ratio of adsorbed to occluded molybdenum. From Table 4, it can also be concluded that the catalysts MVI and M III have the greatest activity,similar to that of commercial catalysts. The results obtained show that the operating conditions in the preparation of catalysts for HDS of light compounds by wet impregnation, have a greater influence on the activity than the macroscopic distribution of the precursors of the active species inside the pellet.
CONCLUS IONS 1.- The molybdenum was redistributed during cobalt impregnation. 2.- Those catalysts with higher ratios of adsorbed to occluded molybdenum (at equal total molybdenum content) showed higher hydrodesulphurization activity. 3.- Catalysts calcined at higher temperature presented lesser activity in relation to the species observed by reflectance spectroscopy. 4.- The operating conditions in the preparation of catalysts for HDS of light compounds, by wet impregnation, have greater influence on activity than the macroscopic distribution of the precursors of the active species inside the pellet.
341
ACKNOWLEDGEMENTS The authors acknowledge the experimental contribution by Tee. L. Osiglio, Lie. N.H. Firpo and Lie. D. Pena.
REFERENCES 1 P. Grange, Catal. Rev. Sci. Eng., 21 (1980) 135-181 2 C.V. C~ceres, M.N. Blanco and H.J. Thomas, Proc. 7° Simp. Iberoamericano de catalisis, La Plata, Julio 13-18, 1980, Grafos, Santa Fe, 1980, pp. 318-326. 3 S.D. Rasberry and K.F.J. Heinrich, Anal. Chern., 46 (1976) 81-89. 4 J.C. Dart, Chern. Prog., 71 (1975) 46-47. 5 P. Gajardo Sabater, Thesis, Universite Catholique de Louvain, 1978. 6 J.T. Richardson, Ind. Eng. Chern. Dundam., 3 (1969) 154-158. 7 V.H.J. de Beer, M.J.N. van der Aalst, C.J. Machiels and G.C.A. Schuit, J. Cat., 43 (1976) 78-89.
342 DISCUSSION N.P. MARTINEZ You mention in your presentation that pore size distribution was measured by mercury penetration. When you look at Table 2, one can see that the mean pore diameters of your catalysts are at the limit of the technique. In these cases i t is more advisable to determine the PSD by using nitrogen adsorption. H.J. THOMAS: Although nitrogen adsorption technique employed in order to determine pore diameters up to 10 A and the equipment utilized (Aminco, model 5/7125) permits determinations up to 30 by comparing the curves obtained the variation in pore size of the catalysts with respect to the support shows that the maximum in distrrbution curves lies within the applicable range of the equipment. Besides, tanking into account the real density of y-alumina given in bibliography, it was possible to calculate that the volume of pores smaller than 30 A is negligible with respect to the total volume.
A,
J. KIWI In Table 2, you report about 42 A as a mean pore diameter fOr your HDS catalyst, that is ~ 4 mm in diameter. To attain this pore size, which pressure of Hg have you applied? Is this the limit for pore size determination for your instrument as given by Kelvin's relation? Did you crush with the employed pressures the pellets under observation ? H.J. THO~AS: The pressure of Hg applied to attain 42 A as a mean pore diameter was 42.000 psia, whereas the maximum pressure applied was 58.000 psia (30 It was not observed that the pellets were crushed using these pressures.
A).
R. CANDIA We have observed that the type of Co species present in the catalyst is the major factor in determining its activity level. In a Co-MO/Al203 catalyst there may be 3 different Co species (R. Candia et al., 4th Intern, Conference on the Uses and Chemistry of Mo, Golden, Colorado, USA, August 1982). Therefore, catalysts with the same chemical composition but with different distribution of Co species may have quite different activities. You have prepared Mo/Al 20 3 catalysts by three different methods, which probably results in catalysts with different "surface properties". Do you think that these "different" MO/Al203 catalyst may have affected the distribution of Co species on the finished catalysts and that this is the main reason for the differences in activity? Or do you think that different Mo species also are playing a role on the activity ? H.J. THOMAS: We think, based on diffuse reflectance spectra, that the catalysts obtained under different operative conditions (concentration of the impregnation solution, time of impregnation) do not have important differences in the distribution of the Co species. Nevertheless, some differences in the distribution of the Mo species are observed. As it has been pointed out the catalysts prepared have very different adsorbed to occluded molybdenum ratios and this is, for us, the principal reason for the different activities. D. CHADWICK: Emission Mossbauer spectroscopy studies by Topsoe and coworkers and myself suggest that the oxide precursor of the active Co-Mo-S phase is rather disordered and possibly contains a distribution of Co sites. This may result in rather broad bands in the UV diffuse reflectance spectra making it difficult to detect out Co(II) or Co(III). H.J. THOMAS: Although diffuse reflectance spectroscopy does not permi~ to detect octahedral Co(II) and that the octahedral Co(III) band is partially superimposed with that of CoAl204' it is possible to detect the latter. It was observed that the broad triple band of CoAl204' which is considered inactive, is more defined in the samples activated at higher temperature and that the activity of these samples islower. That is to say, we cannot distinguish which of the other species is the active one, but it is possible to relate the decrease of activity to the formation of CoAl204'
343
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in Tbe Netherlands
THE EVOLUTION OF Co SPECIES ON THE SURFACE OF y-A1
20 3
AND Si0
2
MODIFIED WITH
THE PRE-TRANSITION CATIONS
A. LYCOURGHIOTIS
Physical Chemistry Laboratory,
Uni versi ty of Patras, Patras ,Greece
ABSTRACT A regulation of Co species formed on the surface of y-A1 20 and Si0 2 was ob3 tained by modification of these carriers by various amounts of pre-transition cations. Specifically, a quite severe control of the critical ratio "cobalt oxi2 de"/"tetrahedral C0 +" was feasible by altering the "dope parameters", such as the kind, content or the conditions of deposi tion of the modifiers on the supports. Two novel mechanistic schemes are ses observed.
proposed to describe all the suz-f ace proces-
A study of the influence of the "dope parameters"on the kinetics
of the various steps included in the mechanistic schemes, shed more light on the trends exhibited by the modifiers studied. A tentative model to interpret all the phenomena observed is developed.
INTRODUCTION Cobalt oxide non-supported or supported on y-A1 and Si0 2 constitutes theacti20 3 ve or precursor ccmponerrt of some important catalysts used in oxidation ,hydrogenation, polymerization and hydrodesulfurization reactions (refs. 1-5). of the supported catalysts mentioned above should be related to
The
reactivity
the nature of
Co species formed, in the oxidic state, on the surfaces of the carriers. The composition of the Co oxidic phase obtained under certain experimental conditions [mode of impregnation, Co loading, and temperature or atmosphere heating] is closely ne Lat ed tion of the Co species. of the cobalt phase. y-A1
20 3
or Si0
2
to
of
the mechanistic route which follows the evolu-
Co (II) nitrate is usually employed for the deposition
The thermal decomposition in air of this salt
surface can be depicted as
follows:
on
the
344 <150 oC
300-600 oC
(a)
(b)
> 650°C
I
I
y-A1
Co(N03 ) 2
~
[CoO] ~
Co 304
Co(N0 3 ) 2
~
[CoO] ~
Co 304
I
I
20 3 )
)
I
CoA1
Co-Si0
I
2
C 2+ °t
~
C 2+ °t
I
300-800 oC
< 150°C
204
~
> 800°C )
temperature
Fig. 1. Schematic representation of the evolution of the Co-species on the surface of y-AI (a) and Si0 (b). The temperature regions where the various 20 2 species prevaiX are indicated. The square bracket used in the case of the CoO accounts for the fact that this oxide is very unstable in air. Co-Si0 stands 2 for a tridimensional cobalt-silica compound in which C02+ has tetrahedral symmetry. By coi+,cobalt (II) cations diluted into the tetrahedral sites of the carriers latt~ce are symbolized. Although the above mentioned schemes (a) and (b) describe the evolution of
Co
species brought about by the increase of calcination temperature, they cannot be considered as strictly consecutive;
thus, for instance, the
co~+
ions could be
formed at the expense of [CoO]. A control of the kind of the cobalt species formed under certain experimental conditions is related the forementioned
to
kinetic
the regulation of the rates of the various steps schemes.
of
Such a kinetic regulation can be obtained
by doping the supports by suitable ions (modifiers).
Recently, quite extensive
experimentation (refs. 6-18) using various modifiers showed that the pre-transition cations [p. t.c.] namely, the alkali [a.c.J
and alkali earth cations [a.e .c~
can be used for this purpose. In the present communication we deal with the following systems
[Co oxide] ! Si0
2
[x;p.t.c.]
In particular, we examine the
pos~ibility
of obtaining a control on the evolu-
tion of the Co-species by altering the so called dope parameters, namely
the
kind and content of the p.t.c. as well as the conditions of the "p.t.c. incorporation" into the support. EXPERIMENTAL Preparation of the samples The doping of the carriers with various amounts of a p.t.c. has been perfor-
345
med by dry impregnation of y-A1 [Houdry Ho 415J or Si0 [silica precipita2 203 ted, M.B.: 60.09 B.D.H. Chemicals L.t.d] powder with aqueous solutions of a p.t.c.-nitrate.
Wet impregnation was followed in some cases where the solubili-
ty of the p.t-c.-nitrate was too low [e.g.Ba(N0
(refs. 16,17). 3)2] The deposition of the active phase on a doped or pure carrier was conducted
by dry impregnation with aqueous solutions of CO(N03)2.6H2o [Merk p.a.] .A1l the specimens
prepared contained a quantity of active phase corresponding to 2.8
C0 4 per 100g of the doped support. Each impregnation step was followed 30 drying and calcination at a given temperature, usually for 24h.
by
To examine the influence of various dope parameters in addition to p.t.c. kind, content and calcination temperature, some y-A1
based specimens were pre20a pared by inverse impregnation [first Co followed by p , t.c.J or~o-impregnation.
Moreover, in some specimens
th~
incorporation of a certain p.t.c. into the sup-
port has been obtained using aqueous solutions of CH CH CH
3COONa(aq),
NaOH(aq)] or a solution of CH
or NaOH [denoted by 3CoONa in CH [denoted by 3COONa 3COOH
3COONa(b)J. Additional details concerning the preparation of the specimens has been re-
ported previously (refs. 6-18). Notation.
The specimens prepared by normal impregnation [first p.t.c.
and
then co] will be designated using the formulas: M-X-AI and M-X-Si0 2203-Y-Co-Z Y-Co-Z where M:p.t.c., X:nominal composition as mmol of M per g of support, Y: calcination temperature after p.t.c. impregnation and Z calcination temperature after cobalt impregnation (oC).
Specimens prepared by normal impregnation using
NaOH(aq), CH
and CHaCOONa(b) will be denoted as follows: Na-CH 3COONa 3COONa(aq) (aq)-X-Al Na-CH and Na-NaOH(aq)-X-AI 203-YaCOONa(b)-X-AI 203-Y-CO-Z 203-Y-Co-Z, -Co-Z. To symbolize the samples prepared by inverse impregnation or co-impregnation the formulas: Co-AI
ly.
and M-X-Co-AI are used respective20a-Z-M-X-Y 20a-Y In all cases the symbols have the meaning mentioned before.
Diffuse Reflectance Spectroscopy The determination of cobalt species formed at each calcination temperature has been obtained using Diffuse Reflectance Spectroscopy (D.R.S.).
The electro-
nic reflectance spectra were recorded at room temperature in the region 800-400 nm, using a Cary 219- spectrophotometer equipped with a D.R.S. accessory. In all cases the corresponding doped supports were used as references.
The powdered
samples, as well as the references, were mounted in quartz cells
which provi-
de a sample thickness higher than
1 mm to secure the determination of the re-
flectance at infinite sample thickness, Roo. the Roo is related to
Under our experimental conditions,
the Kubelka-Munk function, F(Ra,), which in turnsis pro-
346 portional to the concentration of species responsible for the absorption at
a
given wave-length.
F(Roo)
E:.c
(1 )
S
In Equation (1) E: and S are the extinction and scattering coefficients, respectively . F(Roo) can thus be used to estimate the variation of the concentration of an entity through a series of specimens of similar texture.
In fact, inside a se-
ries of samples with similar texture the scattering coefficient is considered to be nearly constant. The CoO and/or C0 on SiQ2 or y-A1 exhibit two characteristic reflec304 20 3 2 tions due to octahedral C0 + , a shoulder at about 750 and a broad band at 425 Since any distinction between CoO and C0 is extremely difficult by D. 304 R.S., we will frequently refer both substances as "cobalt oxide". Therefore,
nm.
the F(Roo) determined at 750nm-denoted by F(Roo)750- can be used to estimate the concentration of the "cobalt oxide".
A triplet band centered at 600nm is attri2+. buted to the presence of tetrahedral C0 Thus, it can account for the forma-
tion of CoA1 Co-Si0 and co~+. Unfortunately, this triplet cannot be used 204, 2 for quantitative determination of the species mentioned, because a broad structurless band due to "cobalt oxide" appears in the same region. RESULTS Catalysts prepared by normal impregnation M-X-A1 20 3-600-Co-100[X:O.000, 0.392, 0.621, 0.984, 1.560 and 2.470J.
The
only phase detected on the undoped specimen was the deposited Co(N03)26H20 which developed
the
same yellow-red color as the aqueous solutions OfCo(N03)26H20.
The phenomenon observed in the Be 2+ modified aluminas was quite impressive: 2 the presence of Be + accele.rated the decomposition of the Co(N0 to "cobalt 3)2 oxide". The characteristic black color as well as the shoulder at 750nm started appearing in the samples with X>0.984. The Co(N0 decomposition increa3)2 2+ sed with Be content. Thus, the F(Roo)750 values obtained for X=O.OOO, 0.392, 0.984 and 2.470 were equal to 0.00, 0.01, 0.06 and 0.21,respectively.A similar 2 effect but much weaker has been observed with Lit and Mg + doped specimens.How2+ ever, even in the sample containing the maximum Be content, the rate of the "cobalt oxide" formation was extremely low and the CO(N0
3)2
remained the predo-
minant phase. All the other p.t.c. interaction
being in excess of 0.984 mmol/g, promoted the direct
of the salt deposited with the
y-Al?O~
surface,resulting
in the
347
t
0.2
"T' ~
..:=..J l»
o
- 0.0
400
500
600
Wavelength /nm
700
800
•
Fig. 2. D.R.S. spectra of the Sr-X-Al specimens. 203-600-Co-100 indicated. 2 formation of Co + tetrahedral species.
X values are
This was manifested by the development
of the blue color component and the appearance of the characteristic triplet in the D.R.S. spectra as well. Typical spectra of the Sr-X-Al are 20 3-600-Co-100 2 Since dilution of Co + into y-Al lattice is a very diffi20 3 cult process at 100oC, the detected tetrahedralCo 2 + is attributed to epitaxial
shown in Fig. 2. CoAl 204·
M-X-SiO T600-Co-100[x:1.l6, 2.92 and 4.63 when x:a.e.] [X:O,73, 1.84 and 2.92 when X: a.c]. The phenomena observed in the title specimens were quite analogous to those observed in the corresponding ones based on y-Al
20 3:
~t
was
. .. catlons, provok e t h e d ilssoclatlon f oun d t h at Be 2+ ,and to a lesser extent Ll. + of the cobalt(II) nitrate to "cobalt oxide". Thus, for instance, the F(Roo)750 values obtained fo~ the Be2+ doped specimens with X=1.16, 2.92 and 4.63 were 2 found to be equal to 0.00, 0.43 and 0.56, respectivel~ Mg + cations cause no appreciable effect regardless of the X values. On the other hand, the remaining p.t.c. catalyse the formation of a "surface" · Co-Sl02 compoun d 'In wh ilC h Co2+ posesses a tetra h e d ra 1 symmetry.
M-X-Al203-600-CO-ZrZ:100-700(FOr each series, X takes
),1
J
M-X-Si0 2-600-Co-Z LZ:100-900 the above mentioned values For most of the oC, samples and for temperatures lower than 500 rise of the calcination temperature accelerated the Co(II) nitrate decomposition as shown D.R.S. signal at 750nm.
by the increase of the
A probable acceleration of the parallel reaction, na-
mely the interaction between the surface
of the carriers and Co(II) nitrate can-
not be easily realized because the characteristic triplet was masked by a struc-
348 tureless broad band appeared in the same region. However, the function of this oC parallel reaction in the region 100-500 could be inferred from the variation of the F(Roo)750 with the kind of the p.t.c.: values of f(Roo)750' determined for specimens doped with a particular p.t.c., higher (smaller) than those obtained for the corresponding undoped ones, strongly suggest
that this
p.t.c. depresses (promotes) the reaction mentioned. Concern~ng
Mg
21-
.
t h e y-A1 20 3 b ase d
•.
spec~mens,~t
was found that
L~
. +,Be 21- and
inhibit the interaction between Co(II) nitrate and support surface,where-
as the other p.t.c. promote it. As for the Si0 2 based specimens the situation 2+ is more complicated because the inhibitory action of Be and Li+ is progressively \\eakened by increasing the calcinaticn temperature; ':he remaining p.t.c. catalyse the Co(II) salt-support interactions as for Y-AI 20 3 ' Further increase of the calcination
brings about various inter-
temperatu~e
actions between active phase and support resulting in the formation of [CoA1204 and/or co~+] and [Co -Si0 2 and/or co~+] in y-A1 20 3 and Si0 2 based specimens, respectively. Moreover, these interactions cause the decrease of the concentration of "cobalt oxide". These observations are reflected in the decrease of the signal at 750 nm as well as in the appearence of the characteristic
triplet in the D.R.S. spectra. At a given temperature, the amount
of cobalt oxide which remains on the support surface
depends on the content
and kind of the p.t.c. contained. As typical example, the D.R.S spectra of the Na-X-Si0 2-600-Co-610 are shown in Fig.3.
0.6
~0.3
8
II: 1~,0.2 CIt
~
0.1
300
400
500
600
700
800
Wavelength!nm - - +
Fig.3. D.R.S. spectra of the specimens of the Na-X-Si0 2-600-Co-610 series. X values are indicated. Regarding the dependence on the p.t.c. content,it must be noticed that,for
349 0.8,I
E
(a>
0.&0
c
........ Q
I'll
'8
0.4 0
,5 ~
0.2:0
• • I
0.00
undoped Li
Na
K
Rb
Cs
Be
Mg
-
Ca
I Sr
Ba
(b> 1.50
E c
Q
.. :e
,....ftI 8
1.00
It:
'-' LL
undoped
Li
Na
K
Rb
Cs
1.50
(c)
E
c 1.0
.. ~
,....I I 8
It:
'-' LL
0.50
0.00
w1doped Be Mg Ca Sr Sa V'ariation of the F(Roc,)750 for the M-2.92-Si0 M-O.9842-600-Co-610(a), (M:a.c., b) and M-O.984-A1 (M:a.e.c., c). -A1 203-600-Co-620 203-600-Co-600 Fig. 4.
350
a given time and temperature of calcination, the amount of "cobalt oxide" located on the support surface
is minimized in those specimens with medium X values,
Only a few exceptions from this general trend have been observed.
However, the
exact X value at which the minimum amount of "cobalt oxide" is observed, varies with the kind of p.Lc,
The dependence of "cobalt oxide" concentration. on the
kind of the p.t.c. at relatively high temperatures was different for the specimens based on Si0
as compared with those based on y-A1 To discuss this de203, 2 pendence,let us consider typical examples. Fig. 4 illustrates the variation of
the F(Roo)750 for the M-2.92-Si0
M-O.984-AI (M:a.c., 203-600-Co-600 2-bOO-Co-6l0(a), (M:a,e.c., c). An inspection of this figure de-
b) and M-O.984-AI 203-600-Co-620 monstrates that concerning the Si0 a decrease of . qUlte wea k'In -t -t -t Na , K , Rb , sed catalysts K-f- , Rb-t , Cs-t- ,
based specimens, ~ the p.t.c. bring about 2 the "cobalt oxide"/ [Co-Si0 -tco~-t] critical ratio. This effect is 2 d d speclmens, . becomi t h e Be2-t , Mg 2-t , L,-t lope ecomlng conslLdera bel 'In t h e 2-t 2-t 2-t Ca , Sr and Ba modified samples. With regard to y-A1 ba203 2-t] 2-t one may observe that the doping by [Li-t, Be and Mg and [Na~ . an increase and a decrease of the Ca2-t Sr 2-t and Ba 2-t] results In
"cobalt oxide"/ [CoAI -tCo~-t] ratio, respecti vely These trends concerning the 204 dependence of the ratios on the kind of the p,t.c. hold at almost all temperatures studied in excess of 600 0C.
The values of the ratios at T>600
ned by the rate of two processes, namely
0C
are determi-
the direct "CO(N03)2-support" reaction
observed at low temperatures and the decomposition of the "cobalt oxide". The 2-t rate of the first process as already mentioned, decreased after Li-t, Be and 2-t Mg doping while following modification by the other p.t.c. increased. Investigation of the second process requires to' determine
the activation energies
of the "cobalt oxide" decomposition over the various supports.
Such determina-
tions were carried out for a large number of samples using a method previously described (refs. 8,18). based specimens
The activation energies determined for the dopedy-Al 203 were smaller than those obtained for the samples prepared using
pure y-A1
Thus, for instance, the activation energies determined for the 203, M-O.OOO-Al Be-O.984-A1 Mg-O.984-A1 Ca203-600-Co-Z, 203-600-Co-Z, 203-600-Co-Z, -O.984-A1 Sr-O.984-A1 and Ba-O.984-AI were 203-600-Co-Z, 203-600-Co-Z 203-600-Co-Z found to be 118, 79, 83, 63, 32, 70 KJ,mol-~ respectively On the contrary,the
activation energies determined for the modified specimens based on Si0
were 2 higher when compared with those obtained for samples synthesized using pure Si0 2• To take an example, the activation energies determined for M-O.000-Si0 -600-Co-Z, 2 Be-4.63-Si0 2-600-Co-Z, Mg-4.63-Si0 Ca-4.63-Si0 2-600-Co-Z, Sr-4.63- Si 02 2-600-Co-Z, l, -600-Co-Z and Ba-4.63-Si0 were 8,25,83,75,91,80 KJmolrespectively, 2-600-Co-Z
M-X-AI20iY-Co-500.
To study the influence of the calcination temperature
351 after p s t c , impregnation on the "cobalt i
oXide"/[coA12olf+co~+J
critical ratio,
Some specimens have been synthesized with various Y values. In all the cases examined,it was found that the action of the p.t.c. mentioned required heating of the doped carriers
before Co deposition, above a critical temperature.(usually oC). Otherwise,the p.t.c. were found to be almost inacti-
centered at about 500 ve.
To take some typical examples {M-O.OOO-A1 Rb203-600-Co-600:F(Roo)750=1.20, -0.984-A1 20S-SOO-Co-600:F(Roo)750=1.22 and Cs-0.984-A1 20S-SOO-Co-600:F(Roo)750 = 1.2S} but {Rb-O.984-A1 20S-500-Co-600:F(Roo)750=0.61 and Cs-O.98lf-A1 20 3-500-Co-600:F(Roo)750=O.82 (ref. 14). Na-CHSCOONa(aq)-0.98lf-A1203-600-Co-600, Na-CH3COONa(b)-0.984-A1 203-600-Co-600 Na-NaOH(aq)-O.984-A1 20 3-600-Co-600.In order to examine whether the anion of
the salt used for the introduction of the p.t.c. into the support participated to the mechanism
of the Co species evolution, we synthesized some specimens
using the title compounds and the F(Roo)750 values were measured.
The results
obtained are compiled in Table 1. TABLE 1 F(Roo)750 values obtained for some specimens prepared (ref. 15)
Sample
F(Roo )
M-0.000-A1203-600-Co-600 Na-O.98lf-A1203-600-Co-600 Na-CH3COONa(aq)-0.984-AI203-600-Co-600 Na-CH 3COONa(b)-O.984-A1203-600-Co-600 Na-NaOH(aq)-0.984-A120S-600-Co-600
1.20 0.52 0.50 0.51 0.45
An inspection of this table demonstrates that there is no
important ef-
feet caused either by the negative part of the salt used or the solvent employed.
by inverse and co-impregnation (refs. 10, 14)
Catalysts prepared
To investigate whether the order of the impregnation procedure determine the "cobalt oXide"/[coA120lf+co~+J
ratio, some specimens prepared by inverse or co-
impregnation have been synthesized. is a key factor: "cobalt
It was found that the order of impregnation
even with X values> 0.984, the disturbances caused on the
oXide"/[coA1204+Co~+]
ratio ar-e negligible.
se and co-impregnation as well.
This is true for the inver-
Thus for example, the F(Roo)750 values obtained
for the catalysts M-0.OOO-Al20 3-600-Co-600, Co-A1203-600-Rb-O.984-600, Co-A120 3-600-Cs-0.98lf-600, Rb-0.984-Co-AI 20S-600 and Cs-0.984-Co-A1 203-600 were 1.20,
352 1.20, 1.21, 1.26 and 1.30, respectivelY
(ref. 14).
DISCUSSION Novel mechanistic schemes The results presented demonstrate
that the mechanistic routes proposed ear-
lier [see fig. IJ fail to describe the real evolution of the Co species on the y-A1 203 and Si0 2 surfaces. Therefore, two novel mechanistic schemes are' proposed (Fig. 5), which take into account the most important observations, notably the direct formation of tetrahedral Co2+ species [step a]. nisms a discussion of the
phenom~na
Based on these mccra-
observed will be attempted.
The effect of modifiers on the rates of the various steps of the mechanistic routes Modification of the supports by Be 2+ ,Ll. + and Mg 2+ decreases the ratio
[ ra-
te of the step(a)/rate of the step(b)], whereas doping by the remainder p.t.c. increases it.
This explains the observed ratios ["cobalt oxide'Ytetrahedral co-
balt] at relatively low temperatures where the rate of cobalt oxide decomposition [steps (d) and/or (e)J is too low.
At relatively high temperatures the ra-
te of this decomposition becomes important thus contributing to the determination of the ratio mentioned above. In Si0 2 based specimens, all the p.t.c. 's increase the activation energy of "cobalt oxide" decomposition; therefore, in the temperature region above the isokinetic point, p.t.c. increase the rate
of
steps(d) and/or (e).
This, in
conjunction with the effect caused by p.t.c. on the rate of the step(a) explains + Be2+ ,Mg 2+J and [Na +, the slight and dramatic decrease brought about by [ Li, + + + 2+ 2+ 2 + ] . [ ] [ ,respectlvely, on the "cobalt oxide" / tetraheK , Cs , Rb ,Ca ,Sr ,Ba dral cobalt] (Fig. 4a). In the y-Al based specimens ,all the p. Lc. ',s decrease the activation energy 203 and consequently the rate of "cobalt oxide" decomposition. Thus, an increase 2 of the mentioned ratio of cobalt species is expected after doping by Be +, Li+ 2t and Mg [the rate of the step (a) is decreased] (Fig. 4b, c). On the other hand,the decrease of this ratio caused after modification by the other p. t.c. is the overall effect of two antagonistic processes ,namely the augmentation of the rate of step(a) and the decrease of the velocity of the steps(d) and/or (e) (Fig. 4b,c). An interpretation of the results using a tentative model The above considerations showed that the regulation of the ratio [rate of
353 direct reaction with 5i0 (a)
2
I----------~·~Co-Si:::r
(a) reaction with 5i0
2
(b)
decomposition ~------------~CoO/Si02
direct reaction with
(b)
(b)
decomposition
Fig. 5. Two novel mechanistic schemes proposed to describe the evolution of cobalt species on the surface of Si0 (a) and y-A1 (b) based specimens. 203 2
354 the step (a)/rate of the stap (b)J is a key factor for obtaining a final control on the Co species. to
On the other hand,the experimental results related
the influence of the mode of impregnation, calcination temperature before
Co deposition and kind of the p.t.c.-salt used cies, showed that p.t.c.-entities
on the evolution of the Co spe-
formed above a critical temperature on/with
support surface are responsible for the regulating capability of the p.t.c. To explain the influence of the various "dope parameters" on the ratio
of
the rates mentioned, we propose the following model based on the existence of these p.t.c.-entities. (i) P.t.c.-entitie~ can regulate the local surface pH of the support and . . 0 f Co2+ a d sor b e d to Co2+ preclpltate " d [h consequent I y t h e ratlo t e I atter preclpitates as Co(N03)2]' (ii) During the first or second impregnation the p.t.c.-entities hydrolyse slightly the surface of the carriers and promot~
the evolution of the surface
2-
2
species (for example A1204)' These species can react with Co + to form pounds in which C0 2+ has tetrahedral symmetry (for example CoA1204)' (iii) tion
com-
P.t.c.-entities serve as centers of nucleation or blocking the adsorp-
agents.
They favour the precipitation of CO(N0
If one takes in mind that Co(N0
agglomerates. 3)2 agglomerates are transformed into "cobalt
3)2 2+ oxide" aggregates during heating whereas the C0 adsorbed react with support 2+
species to form tetrahedral C0
surface compounds, one can understand the con-
tributionof the (i), (ii) and (iii) to the control of the ratio G:'ate of the step (a)/rate of the step (b)J:
the rates of the steps(a) and (b) increase due
to (ii) and (iii),respectively.
In most cases, the rate of step (a) increases
due to (i).
In fact, the regulation of the local pH by p.t.c.-entities usually
favours adsorption over precipitation. The experimental results obtained show that mechanisms (i) and (ii) prevail . t h e case 0f ' + Rb+, Cs+ ,Ca 2+ ,Sr 2+ an d Ba2+J In speclmens mod lLf ile d by [Na+, K, 2+
whereas mechanism (iii) is predominant in the samples doped with [Li+, Be Mg
2
ij .
and
Moreover, the results obtained demonstrate that mechanism (iii), at d i f ilcatlon, i on b ecomes lmportant . , sammOl In
• t h e case 0 f L'+ 1 east In l , Be2+ an d Mg 2+
i
ples containing relatively high p.t.c. content.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
B. Dmuchovsky, M.C.Freerks and F.B.Zienty. J. Catal., 4(1965)577. D. L.Harrison, D.Nicho11s and H.Steiner, J. Catal., 7(1967)3.59. R.G.Scultz, J. Catal., 7(1967)286. S.C.Schuman and H.Shalit,4, (l9:70J 245. T.Ohtsuka, Cata1. Rev., 16(1977)291. A.Lycourghiotis, C.Defosse and D.Delmon, Rev. Chim. Miner., 16(1979)473. A.Lycourghiotis, C.Defosse, F.Delannay, J.Lemaitre and B.Delmon, J.C.S. Faraday-I, 76(1980)1677.
355
8. A.Lycourghiotis,D~attis and Ph.Aroni, Z.Phys.Chem. (N.r.), 120(1980) 211. 9. A.Lycourghiotis, D.Vattis and Ph.Aroni, Z.Phys.Chem. (N.r.), 121(1980)257. 10. A.Lycourghiotis, C.Defosse and B.De1mon, Bu11.Soc.Chim. Be1g., 89(1980)929. 11. C.Defosse, M.Houa11a, A.Lycourghiotis and r.Delannay, in T.Seiyanna and K.Tanabe (Eds.) Proc. 7th Int. Cong.Catalysis, Tokyo, 1980, Elsevier, Amsterdam, 1981, p.l08. 12. r.Delannay, C.Defosse, M.Houalla, A.Lycourghiotis and B.De1mon, in R.Langer (Ed.), Proc. 12th Swedish Symp. on Catalysis. Perspectives in Catalysis, Lund, October 11th, 1979, p.85. 13. A.Lycourghiotis, D.Vattis and N.A.Katsanos, Z.Phys.Chem. (N.r.), 125(1981) 239. 14. A.Lycourghiotis, D.Vattis, Ph.Aroni and N.A.Katsanos, Acta Chimika (in press). 15. A.LJcourghiotis, React.Kinet.Catal.Lett., 17(1981) 165. 16. A.Lycourghiotis, A.Tsiatsios and N.A.Katsanos, Z.Phys.Chem. (N.r.), 126(1981) 95. 17. A.Lycourghiotis, A.Tsiatsios and N.A.Katsanos, Z.Phys.Chem. (N.r.), 126(1981) 85. 18. A.Lycourghiotis, M.Kotinopoulos, N.A.Katsanos and G.Karaiskakis, React. Kinet.Catal.Lett. (in press).
356 DISCUSSION B. GRIFFE DE MARTINEZ: Have you tried to correlate the doping parameters of the elements that you have studied with the activity and selectivity of the catalyst on any catalytic reaction ? A. LYCOURGHIOTIS: Of course our final purpose is the extension of the control obtained on the Co-phase by altering the doping parameters to the sorptive and catalytic properties of the catalysts. Preliminary results obtained on the CO oxidation show that such a control could be achieved. N.P. MARTINEZ : I want to comment on the use of the Kubelka-Munk function and its straight line correlation with the concentration of the absorbing species. As Beer's law, K-M law is followed by low concentrations of the adsorbing material, in the case of a CoMo/alumina, we have found that K-M function is followed for the oxidic state until you have no more than 3.5% of cobalt as CoO. Have you determined the Kubelka-Munk function vs concentration? How much cobalt do you have in your catalysts ? A. LYCOURGHIOTIS I agree with you that the K-M,relationship is followed at quite low concentrations of optically active species. In the case of Co species, you have found that this relationship is followed until 3.5% of Co as CoO. Since our catalysts contain lower amount of active phase than yours the K-M relation must be held in our case. D. CHADWICK: Could you explain in more detail what you mean by accelerate and rate with respect to decomposition of Co salts. For example, how did you quantify these effects ? A. LYCOURGHIOTIS: In our text the terms acceleration and rate have the usual meaning. To understand the quantification attempted on the change of the rate of the Co (N03)2 decomposition, we must bear in mind that the solid state processes are in general kinetically controlled. Specifically, in the decomposition of the solid salts, thermodynamic equilibrium is achieved only seldom. Therefore, the difference in the concentrations of an optically active product (C0304 and/or CoO in our case) simply reflects analogous difference in the rates of the salt decomposition (Co(N03)2 in our case. Under constant time and temperature, the higher the rate of the Co(N03)2 decomposition, the higher is the concentration of the C0304 and/or CoO on the surface of the modified support and, therefore, the higher the value of the K-M function determined at 750 nm. F.S. STONE It is interesting to see the effects of pre-transition metal ions reported in this paper. However, I have two comments in regard to the reflectance measurements. First, it is doubtful whether the shoulder at 750 nm is "due to octahedral C0 2 + in CoO and/or Co 304", as is stated in the paper. Absorption in this region is more pften regarded (1) as being due to octahedral Co(III). A matter which is more relevant, however, is that absorption at 750 nm is influenced very greatly by Co(II)-Co(III) intervalence charge transfer absorption, which is intense and broad in the visible. This being so, any changes which increase the amount of CO(III) will increase F(Roo)750' and vice versa. This could therefore be a basis for using F(Roo)750 as a measure of C0304' as the author has done. However, and this is my second point, it is probably not justified to assume that Co(III), and hence Co(II)-Co(III) intervalence absorption is present only in the pure cobalt oxide phase. For example, in an aluminabased system, Co(III) could be present as a minor component in a surface aluminate phase such as the spinel Co(II) CO(III)x AI2-x04. Thus F(~)750 may not be a very precise measure of "cobalt oxide". (1)
P. Gajardo, P. Grange and B. Delmon, J. Catal., 63, 201 (1980).
357 A. LYCOURGHIOTIS: I do not fully agree with Professor stone that the absorption at 750 nm must be attributed to the presence of CO(III) rather than to Co(II) in octahedral symmetry, because a quite high value of K-M function is obtained at 750 nm even after calcination at qUite low temperatures, for instance at 140°C (1), where the oxidation of co2+ to Co3+ seems to be rather difficult. On the other hand, considerable absorption is observed at 750 nm after calcination at extremely high temperatures, where the concentration of Co3+ is quite high. Therefore, I think that we can at~ribute the absorption shoulder at 750 nm to the presence of both Co2+ and Co + in octahedral symmetry. Moreover, although the formation of CO(I'I) Co(III)x Al 2_x0 4 phase on yAl203 cannot be excluded, I believe that its concentration is to low to disturb considerably the magnitude of, the signal at 750 nm. (1) A.
~ycourghiotis
et al., unpublished results.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
359
CRITERIA FOR THE EVALUATION OF BAUXITE AS CARRIER FOR LOW-COST HYDROTREATING CATALYSTS S. MARENGO, A. IANNIBELLO and A. GIRELLI Stazione sperimentale per i Combustibili, San Donato Milanese (Italy)
ABSTRACT The determination of the porous structure after thermal activation, the measure of the anionic exchange capaci~y with respect to ~lo(VI) and W(VI) species and the study of the functionality of the catalytic systems in model reactions have proved to be efficient criteria for a preliminary evaluation of natural materials as carriers for low-cost hydrotreating catalysts. Experiments performed with two bauxite samples, from a USA ore and a Southern Italian ore respectively, have made it possible to ascertain the possibility of util i zi ng the sampl e with lower i ron content and hi gher surface area as a support for catalytic systems with properti es comparable to those of more expensive alumi na-based cata lysts.
INTRODUCTION According to the most recent forecasts, petroleum will continue to occupy a predominant role in the next 20 years with increasing production of heavy crudes (1, 2). This trend, together with the increasing demand for conversion of residuum to lighter products (2), explains the growing interest in residuum hydroprocessing and hydrodesulfurization catalysts. The irreversible poisoning of the catalyst due to metal contaminants (principally V and Ni), weigh heavily on the cost of heavy crudes hydroprocessing. In the development of more effective catalytic systems, particular attention is being devoted to the nature of the support: novel materials such as modified aluminas, manganese nodules and bauxite have been the object of recent studies (2-5). In order to define efficient criteria for a preliminary evaluation of natural materials as carriers for low-cost hydrotreating catalysts, two samples of bauxite were considered: sample A from a USA ore, and sample B - rich in iron - from an ore from South Italy. Data obtained with a commercial y-alumina are included for comparison.
360
The following properties were studied: i) surface area, pore volume and pore size distribution after thermal activation; ii) anionic exchange capacity with respect to Mo(VI) and W(VI) species to ascertain the limiting amount of catalytic components which can be dispersed on the bauxite surface after calcination at 550°C; iii) functionality of the catalytic system with respect to the content of Mo(VI), W(VI), Co(II), Ni(II) and to activation procedure. EXPERIMENTAL SECTION Materials y-alumina Ketjen 000-1.5 E; commercial bauxite from a USA ore (A); natural bauxite from a Southern Italian ore (B). Ammonium heptamolybdate, ammonium dodecatungstate, nickel nitrate and cobalt nitrate Carlo Erba, pure reagent grade. Analysis Surface area, pore volume and pore size distribution were determined with Carlo Erba Strumentazione Sorptomatic 1800 Series and Mercury Pressure Porosimeter 200 Series. Molybdenum and tungsten in the solid phase were determined by X-ray fluorescence using a Philips Model PW 1540 spectrophotometer. For gas chromatographic analysis of the products of cyclohexene and thiophene conversion, a column packed with 15% squalane on chromosorb was employed. Adsorption of Mo(VI) and W(VI) The support was impregnated with molybdenum(VI) and tungsten(VI) aqueous solution by the step addition technique (6). A glass column (500 mm high, 4 mm into diam.) filled with the test material (particle diameter 0.1-0.3 mm) was first washed with water and then fed with a 3.8 10-2 Maqueous solution of ammonium paramolybdate or ammonium paratungstate; the pH of the solution at the column outlet was recorded as a function of the amount of fed solution. Catalytic tests The catalytic activity of the systems investigated was determined in a pulse microreactor placed in the vaporization chamber of a 2350 Carlo Erba ~as chromatograph in series with a chromatographic column. The catalytic bed consisted of particles with a diameter of 0.1-0.3 mm held between two plugs of quartz wool inside a stainless steel tube (80 mm long, 4 mm into diam.). The amount of catalyst corresponded to a surface areaof20m 2; the sizeof the pulse of liquid reactant was 0.5 ~l. The tests were performed at 375°C and atmospheric pressure in a H2 flow of 30 ml/h. Activation was carried out at 375°C according to the following procedures: a) heating for 2 h in a flow of He and reducing for 4 h in H2: b) heating for 2 h in He and sulfiding for 2 h with 10% H2S in H2'
361
RESULTS AND DISCUSSION Physicochemica1 properties of bauxites The two samples of bauxite considered are characterized by quite different physicochemica1 properties (table 1). TABLE 1 Chemical composition of bauxites A 57.4 10.5
A1203 Si02 t4g0 CaO Fe003 Fe MnO Cr2 O3 NiO Ti02 * H20 ( )
2.5 0.2
1.1 28.3
B 57 1.1 0.2 0.2 28 0.2 0.2 0.1 13
(*) weight 10ss at 500°C Sample A, from a USA ore, is a commercia1 product containing gibbsite as the main component before activation. In the active form, y-alumina prevails and the surface area is comparable to that of a commercial y-alumina (Fig. 1). Sample B, 300
BAUXITE B
200
400
1000
Activation
Fig. 1. Surface area as a function of activation temperature.
362
from a Southern Ita 1i an ore, conta ins ma i nly boehmiteamong the a1umi na compounds and is characterized by a relevant content of iron oxides. The raw material is in the form of pisolytes with a diameter of 5-30 mm and very low surface area; after crushing, washing with water and heating at 500-600°C, the surface area increases considerably, but, compared with type A, remains at a lower level. The pore size distribution of bauxite A is significantly different from that of the commercial y-alumina (Fig. 2); in particular, a consistent fraction of 0.6 (-ALUMINA
0.5
O"--..L---"----...L.----~---
10
102
103
.....
105
Pore rddius,;'
Fig. 2. Pore size distribution of bauxite A and y-alumina macropores is evidenced in A, which is expected to exert a relevant influence on the performance of the finished catalyst in the hydrotreating of heavy feedstocks. Adsorption of Mo(VI) and W(VI) The step addition technique was previously used to investigate the interaction of Cr(VI), Mo(VI) and W(VI) with alumina (6). The shape of the pH profile obtained in such experiments recalls the frontal analysis and gives information on the chemical processes occurring on the solid surface. The adsorption of Cr(VI), Mo(VI) and W(VI) onto y-alumina was described through an ion exchange process between Me0 4 species and surface hydroxyl groups of alumina. In Fig. 3 the pH curves determined in the addition of tlo(VI) and W(VI) to the different supports are reported. The curve relative to high-purity y-alumina shows only one well defined step, as hydroxyl groups are the only species that take part in the ionic exchange process. The curves produced by the two bauxite samples show different steps, reflecting elution of different anionic species from the surface (7) (e.g. the presence of Cr04-- in the effluent from bauxite B was ascertained).
363
7 Addition of MoM) T=25°C
6
o
2
8
10
12
14
16
pH
9
8
7
6
Addition of W (VI) T=60oC
m
o 2 4 6 8 Q ~ lli Total Me (VI) in the Fed solution [gat/g suppJ·10 4 Fig. 3. pH of the effluent in the step addition experiments. The higher steps of the pH profile can be correlated with the exchange of Me(VI) oxospecies with the surface hydroxyl groups. The results of adsorption experiments are reported in Table 2. The correlation between the amount of Me(VI) adsorbed and the surface area of the support suggests that Mo(VI) and W(VI) oxospecies are presents as surface phases with an equivalent degree of dispersion. In accordance with recent EXAFS studies, the presence of isolated Me0 4 species can be hypothesized (8). More-
364
TABLE 2 Amount of Mo0 3 and W0 3 adsorbed in the step addition experiments Support y-alumina Bauxite A Bauxite B
f1003 adsorbed wt % mo 1ecules . nm -2 9.3 6.6 2.6
1.6 1.3 1.2
1-1°3 adsorbed wt % 21 11.6
5.9
molecules.nm-2 2.2 1.4 1.7
over, it must be observed that the specific surface of the support changes only slightly after addition of molybdenum and tungsten, as can be expected for a process involving only the surface of the porous material. Thus, step addition technique makes it possible, by an efficient and relatively simple procedure, to investigate and control the adsorption process of different chemical species on a potential catalyst support. The practical implications of this result are relevant, as the method can easily be scaled up for preparative purposes. Catalytic activity The conversion of cyclohexene and thiophene were employed as model reactions for the evaluation of the catalytic properties of the bauxite-based catalysts. In the experimental conditions considered, the following reactions of cyclohexene were evidenced: i) hydrogenation-dehydrogenation to cyclohexane and benzene; ii) isomerization to methylcyclopentenes; iii) isomerization plus hydrogenation to methylcyclopentane; iv) hydrogenolysis to lighter hydrocarbons. Selectivity to product X was defined as moles of cyclohexene converted to X .100. moles of cyclohexene converted The products of thiophene hydrodesulfurization were HZS, n-butane and butenes. The following catalytic systems were tested: bauxite A and B, the systems obtained by step addition of ~10(VI) and Iv(VI) to bauxite, and the ternary systems prepared by adding, according to the pore filling procedure (9), Z wt % CoO or NiO to the Mo0 3/bauxite and W0 3/bauxite systems. Bauxite A shows prevailing isomerization activity (Fig. 4). Incorporation of Mo(VI) and W(VI) on the surface promotes hydrogenation activity; nevertheless, the system maintains a marked bifunctional character (isomerization plus hydrogenation) more evident in the W0 3/A system. In the case of an analogous alumina-based system, this behaviour was explained by hypothesizing that acid centers are associated not only with the support, but also with molybdenum and especially with tungsten species (10). In the sulfided state, Mo(VI) and W(VI) containing systems exhibit enhanced isomerization properties,
365
Fig. 4. Catalytic properties of bauxite-based catalysts. Selectivity in cyclohexene conversion: 1 hydrocracking, 2 benzene + cyclohexane, 3 methylcyclopentane, 4 methylcyclopentenes. Desulfurization activity: 5 thiophene conversion. compared to the reduced state, and a moderate desulfurization activity. Addition of cobalt and nickel produces a further increase of the hydrogenating function; the marked difference in selectivity between molybdenum and tungsten is maintained also in the ternary systems. Hydrodesulfurization activity is significantly improved in the ternary molybdenum-containing catalysts, while the NiO-W0 3/A system results as being less active in tiophene conversion. In general, the catalytic properties of the systems obtained from bauxite A are similar to those exhibited by analogous alumina-based catalysts (10). In the reduced form bauxite B shows marked hydrogenation and hydrogenolysis activity, attributable to its relevant iron content. In the sulfided form, isomerization activity prevails. Molybdenum promotes hydrogenation and desulfurization activity of the sulfided system, while tungsten is more effective
366
in enhancing isomerization. Cobalt and nickel promote hydrogenolysis activity in the reduced form and hydrogenation and desulfurization properties in the sulfided state. Nevertheless, in all systems prepared from bauxite B, the nature of the support limits activity in cyclohexene and thiophene conversion to a remarkable extent. CONCLUSION The physicochemical properties determined for bauxite A make this material a promising support for low-cost hydrotreating catalysts. More relevant is the possib~lity as confirmed by the above described results, of establishing efficient and relatively simple criteria for screening potential catalyst supports. Step addition experiments made it possible to study the surface chemical properties of two natural materials and to prepare systems in which the active components are present as surface species with a controlled degree of dispersion. Catalytic tests indicated the possibility of preparing, from properly selected natural materials, catalytic systems with properties comparable with those of more expensive alumina-based catalysts. ACKNOWLEDGEMENTS This work was carried out with the financial support of the Consiglio Nazionale delle Ricerche. The Authors thank Carlo Erba Strumentazione - microstructure applications lab - for surface characterization. Mr. S. Scappatura is thanked for valuable contribution in performing the experiments. REFERENCES 1 G.R. Brown, CEP, Sept. (1981) 9-15. 2 D.C. Green and D.H.Broderick, CEP, Dec. (1981) 33-39. 3 C.D. Chang and A.J. Silvestri, Ind. Eng. Chem., Process Des. Develop., 13 (1974) 315-316. 4 V. Berti, A. Iannibello and S. Marengo, Riv. Combultibili, 29 (1975) 121-134. 5 A. Iannibello, S. Marengo and A. Girelli, Riv. Combustibili, 33 (1979) 373-383. 6 A. Iannibello, S. Marengo, F. TrifirO and P.L. Villa, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts II, Elsevier, Amsterdam, 1979, pp. 65-76. 7 A. Iannibello and F. Trifiro, Z. Anorg. Allg. Chem., 413 (1975) 293-304. 8 B.S. Clausen, H. Topsoe, R. Candia, J. Villadsen, B. Lengeler, J. Als-Nielsen and F. Christensen, J. Phys. Chern., 85 (1981) 3868-3872. 9 A. Iannibello and P.C.H. Mitchell, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts II, Elsevier, Amsterdam, 1979, pp. 469-478. 10 A. Iannibello, S. Marengo and P.L. Villa, in H.F. Barry and P.C.H. Mitchell (Eds.), Proc. 3rd Int. Conf. on The Chemistry and Uses of Molybdenum, Ann Arbor, August 19-23, 1979, Climax Molybdenum Co., Ann Arbor, 1980, pp. 92-98.
367
DISCUSSION H. CHARCOSSET
what about the particle size of your catalysts ?
S. MARENGO: Before the physico-chemical characterization, the samples of bauxite were ground and sieved to particles with diameter of 0.1-0.3 mm. Also in the catalytic tests the same size of the particles was utilized. H. CHARCOSSET : Have you any idea about the dispersion (crystallite size) of Fe203 in your sample B (in relation to theoretically possible use of that bauxite as a catalyst for direct hydro liquefaction of coal) ? S. MARENGO : We carried out no further investigation on sample B till now. At present, our interest is mainly centered on the hydrotreating of heavy petroleum ~actions. In this connection, bauxite A has been considered more suitable for evaluation in the hydroprocessing of a petroleum residuum in a micropilot reactor. R.J. BERTOLACINI: I would agree with Dr. Charcosset's comment, that should be a good catalyst ,for coal liquefaction. I also suggest that because of its high iron content (28%), may be a good Fischer-Tropsch I would further add that Hydrocarbon Research Inc., USA, has patented as a hydrodemetallation catalyst for reduced crude feedstocks.
bauxite B bauxite B, catalyst. Mo-bauxite
N.P. MARTINEZ: You mention the possibility of using these catalysts for hydrotreating heavy crudes. Most literature which has been published on catalysts for heavy crudes says that big pores are necessary. According to the pore size distribution presented in your paper, I assume your catalyst would not be good enough to hydrotreat heavy oils. Would you please comment on that? S. MARENGO: In our investigation we started with considering mainly the chemical properties of our materials. Bauxite A, after addition of Mo(VI) and Ni(II) or Co(II), exhibited good catalytic properties with model compounds. In view of the utilization of these systems in the hydrotreating of residua, a second phase of this study should undoubtedly include the improvement of the pore size distribution in the direction that you indicated. S. VASUDEVAN You have shown that bauxite B has low activity but higher selectivity (for certain reactions). Have you compared these selectivities at isoconversions? Or does selectivity remain constant with conversion in your study? S. MARENGO: The pulse technique was utilized in the catalytic tests with the aim of evaluating a large number of catalysts in comparable experimental conditions. Our data refer to a fixed temperature and flow rate of the carrier gas (H2); till now, we have not studied in a systematic way the change of selectivity with conversion.
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369
G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts /11 e 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
PREPARATION AND PROPERTIES OF SUPPORTED LIQUID PHASE CATALYSTS FOR THE HYDROFORMYLATION OF ALKENES
H.L. PELT, L.A. GERRITSEN, G. VAN DER LEE AND J.J.F. SCHOLTEN .X) Department of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
SUMMARY A description is presented of the preparation and characterization of supported liquid phase (SLP) rhodium catalysts for the hydroformylation of alkenes. Special attention is given to those aspects which do not play a role in classical heterogeneous catalysts, viz. the degree of pore filling and the adsorptive withdrawal of rhodium complexes at the support/liquid interface.
INTRODUCTION When a molten or dissolved catalyst is dispersed in a porous support we are dealing with a supported liquid phase catalyst (abbreviation: SLP catalyst). In the chemical literature a number of examples may be found of the industrial application of catalysts in the SLP form. One such example is the molten mixture of V
and K supported on diatomaceous pellets, used for the oxidation of 20 S 2S04 sulfur dioxide, and another is phosphoric acid impregnated on Kieselguhr, for
the gas-phase oligomerization of low molecular weight alkenes, which is probably the oldest SLP application (ref. 1). An interesting application of the principle of supported liquid phase catalysis is heterogenizing homogeneous metal-organic catalysts. In such case, the advantages of the homogeneous catalyst, viz. a high selectivity, a high degree of utilization of the precious metal and a high resistance against poisoning, are combined with the advantages of heterogeneous catalysis, viz. a continuous separation between the catalyst and the products and a circumvention of eventual corrosion problems, as is the case in the Wacker oxidation of alkenes. The .first publication about the use of a supported liquid phase catalyst in the field of organometallic complex catalysis is due to Acres c.s.
(ref. 2),
whQ isomerized 1-pentene with a solution of rhodium trichloride in ethylene glycol dispersed in Kieselguhr. In 1969 Rony (ref. 3)
reported on the hydro-
X)Author to wnom all correspul1Uence should be addressed.
370 formylation of propylene with RhCOCl(PPh
3)3 and brought into the pores of silica gel.
dissolved in benzyl butyl phtalate
Hydroformylation with a special type of supported liquid phase rhodium catalyst
was described extensively by Gerritsen and coworkers (refs. 4-8).
These investigators dissolved a Wilkinson-type catalyst, hydridocarbonyltris (triphenylphosphine) rhodium (I) , in one of the ligands of the complex, PPh
3, called the solvent-ligand. The activity and selectivity of these catalysts are high compared with known analogues. Moreover these catalysts exhibit a very high stability; in -fact nQ sign of deactivation was observed in the hydroformylation of propylene, even after runs of more than 800 hrs (refs. 9-10). Various types of support were studied , and a number of tertiary phosphines related to PPh
were used as solvent ligands. 3 It is the aim of the present article to discuss the preparation of SLP
rhodium catalysts and their characterization. Special attention will be given to a number of aspects which do not play a role in classical supported heterogeneous catalysts.
EXPERIMENTAL Materials RhHCO(PPh was prepared by the method of Ahmad (ref. 11). Triphenylphosphine 3)3 (Fluka, Switzerland, 99.5%) was used as received. Benzene (Merck, Germany, 99.7%) and toluene (Merck, Germany, 99%) were dried over molecular sieve 3A (from Union Carbide, USA). Nitrogen (from Air Products, USA, 99.98%) was freed from oxygen and water over a BASF catalyst R3-11 and molecular sieve 3A, respectively. Hydrogen (99.99%) and carbon monoxide (99.5%) were obtained from Air Products, USA. Silica 000-3E, silica-alumina LA-3D, y-alumina 005-0.75E, 000-1.5E and 000-3p were all obtained from Akzo Chemie, Amersfoort, The Netherlands. a-Alumina, type 5A-5202, is obtained from Norton, England. Silica Dll-ll was from BASF, Arnhem, The Netherlands. KieselgUhr MP-99 was obtained
from
Eagle Pitcher, USA. Silica S and silica H were silica research-samples from DSM, Geleen, The Netherlands (refs. 13-13), both of low sodium content and hydrophobic. Amberlite XAD-2 was a macroreticular resin obtained from Serva, Germany. If necessary, the support materials were crushed and sieved to the desired size fraction.
Apparatus The catalyst preparation apparatus is shown in Fig. 1. The apparatus is constructed from Pyrex glass, and provided with a thermostated
371
A
t
.
--cooling water
c
Figure 1. Catalyst preparation apparatus. A = reflux cooler, B solution holder, C support holder, D = magnetic stirrer.
catalyst
mantle in order to prevent crystallization of the PPh
solution. 3!complex A special apparatus was constructed for studying the adsorption of rhodium
complexes at the PPh
interface under the conditions of a hydroformy3!support lation experiment, i.e. in the presence of a mixture of hydrogen and carbon monoxide (Fig. 2). For the same reasons as indicated for the catalyst preparation apparatus, this pyrex glass apparatus was provided with thermostated mantles (heating jackets). The PPh
distribution across a catalyst particle was measured by means of 3 X-ray microanalysis (RMA) using a Jeol JXA-50A apparatus with a lateral solution of about 1
~m.
The catalyst particles were embedded in Woods metal, after which
thin slices of thickness 0.2 mm were cut from the material. The specific phosphorus and silicon X-ray
emissions were measured, generated by bombard-
ment with a high-energetic electron beam of 25 kV. so sruall (1
~m)
The diameter of the beam is
that a relatively high resolution is arrived at.
372
C
_
oil 70·C
C
Fig. 2. Adsorption apparatus. A material, C = heating mantle.
filter; B
+
support
BET surface areas were determined from the physical adsorption of nitrogen at -196
°c, taking for the cross-sectional area of a nitrogen molecule 16.2 ~ 2
Use was made of a semi-automatic adsorption apparatus, a Sorptomatic type 1800, from Carlo Erba, Italy. Pore size distributions were measured with the same apparatus, and the analysis of the adsorption and desorption isotherms was performed according to the methods introduced by Broekhoff et. al. The
(~efs.
14-15).
before and after adsorption of RhHCO(PPh on 3, 3)3 the various support materials, was in some cases determined by neutron activa~hodiumcontent
of PPh
tion analysis, using the "single
comparator method", with zinc as reference
material (ref. 16). In other cases use was made of a Philips PW 1450 sequential wavelength-dispersive X-ray fluorescence spectrometer. A very important characteriseic ofa catalyst is, of course, its catalytic performance. We measured the activity and selectivity in the hydroformylation of ethylene, propylene or butylene-1: R-CH=CH + co + HZ .. R-CH (normal and iso-aldehydes). 2-CH2-CHO 2 A detailed description of these measurements is to be found in refs. 4-10.
373 RESULTS General description of the system In the preparation of supported liquid phase catalysts in general, and more specifically when preparing SLP catalysts for hydroformylation, we have to reckon with a nwnber of variables which don't playa role in classical heterogeneous catalysis. This may be elucidated from the schematic representation of a partly filled pore in an SLP catalyst (Fig. 3).
Fig. 3. Schematic representation of a partly filled pore in an SLP rhodium complex catalyst. S = the support, for instance silica; L = the solvent-ligand PPh 3 ; G = the gas-phase (reactant-gases CO + HZ + alkene); open circles are the Rh complex molecules; a is a complex in the meniscus; b is a complex adsorbed at the PPh interface; ~ is a dissolved co~plex; ~ is the 3/silica meniscus.
The function of the solvent-ligand, PPh is threefold. The support material 3, is chosen in such a way that the pore wall is wetted by the PPh in doing so 3; a suction force is created according to Kelvin's law, which, for pores with Z. diameters of a few nm, may become as low as - 50 kg/cm Hence, the solution is strongly fixed in the pores. A second, chemical, function of the PPh of carbon monoxide and an excess of PPh equilibria:
co
=
3
is the followin~. In the presence 3 one may imagine the following
RhHCO(PPh C
RhH(CO)Z(PPh 3)2 D
3'Z
=co
RhH(CO)Z(PPh D
3'Z
374 In this series of complexes the selectivity for the formation of normal (linear) aldehydes increases according as the number of ligands around the rhodium increases. Both sterical and electronic arguments are advanced for this. At the same time, however, the catalytic activity decreases proceeding to the left in the series, and it is very likely that complex
~
is no longer catalyti-
cally active. Strong arguments are presented by Gerritsen et al. the rhodium complexes in the SLP catalyst being mainly in
form~,
(ref. 5)
for
and hence
complexes in solution (c, Fig, 3) are inactive. Active complexes are found in the meniscus
and in the thin layer of adsorbed PPh
these last complexes only ar~
(complexes a, Fig. 3). As 3 in direct contact with the reactant-gases (their
solubility in PPh
is very low), and as the degree of surrounding with free 3 is lower in the meniscus than in the liquid, an equilibrium shift in the
PPh
3 meniscus will occur from complex
~
to complex
~.
Complex B is likely to be the
active center, and, due to its relatively high number of PPh
ligands, it 3 exhibits a high selectivity for n-aldehydes. It follows from the foregoing that the second function of the solvent-ligand is increasing the selectivity to the economically most favourable products. For instance, in the case of propene
hydroformylation (ref.B), selectivities S for n-butyraldehyde of 30 to 40 are arrived at (S is the ratio n-aldehyde/iso-aldehyde). In homogeneous hydroformylation S is of the order of only ten in most cases. A third important function of the solvent-ligand is its stabilizing action. By the excess of PPh
ligands around the rhodium centre 3 remains high, and an activity loss through the formation of dimeric complexes 3
the number of PPh
is avoided. In practice stability problems are totally absent; for instance, in the hydroformylation of propylene at 90 DC and 16 Atm., no change in activity or selectivity is observed after test periods of more than BOO hours (ref.?).
catalyst preparation In the preparation of the catalyst the support may be impregnated with a solution of the rhodium complex or a precursor thereof in the solvent-ligand PPh
without any other sol~ents. Just such an amount of solution is then used 3 that the required loading degree is reached immediately. However, it is easier to use an inert auxiliary solvent, which means impregnating the support with a solution of the complex in a mixture of the
solvent-li~land
and a volatile
solvent, and removing the volatile solvent thereafter. The ratio between the solvent-ligand and the volatile solvent is determined by the required loading degree of the catalyst. Except for XAD-2 and silica S, the supports were dried in vacuo, first at 150 DC for 3 hrs and then at 500 DC for 16 hrs. Only silica Sand XAD-2 were dried in air at 120 DC for 16 hrs. The dried supports were placed in the cata-
375 lyst preparation apparatus shown in Fig. 1. Calculated amounts of RhHCO(PPh
3)3 were dissolved in benzene or toluene at 70°C under flowing nitrogen. 3 The total volume of the catalyst was taken exactly equal to the total pore and PPh
volume of the support (dry impregnation). The catalyst solution was added dropwise to the stirred support, which was 1 kewise held at 70°C. Next, the benzene was slowly
evaporated under flowing nitrogen at room temperature for
3 hrs and then at 90°C for 16 hrs, during which period the PPh tribute in the pore system. By varying the PPh
3
could redis-
or PPh
volume 3!toluene 3!benzene ratio in the catalyst solution, several degrees of pore filling with catalyst solqtion could be realized after evaporation of the benzene or toluene. The dry and free-flowing catalyst particles were stored at -20°C. It turned
out that two batches of catalyst, prepared in the same way, showed the same catalytic performance; the catalyst preparation is fUlly reproducible.
Catalyst characterization The PPh
distribution across a catalyst particle with a diameter of 0.423 0.50 rom, as measured by RMA, is presented in Fig. 4.
10r---------------------------------~
Intensity
(Q.U)
1 8
6
100
300
200 Particle coordinate
400
(~m)
Fig. 4. PPh distribution across a catalyst pellet. Support XAD-2 (line C); 3 degree of pore filling with PPh3 is 65%. Support silica 000-3E (lines A and B); the pore filling is 56%. Line A: Si-signal. Line B: P-signal. Line C: P-signal.
The phosphorus line-scans prove that the catalyst is not a mantle catalyst; no PPh 3 enrichment is found at the outer surface of the particles. It is seen
376 from the figure that in silica 000-3E a decrease of the silicon -signal is generally accompanied
by an increase of the phosphorus-signal. This shows the
porosity of the silica to be non-uniform, the more porous regions being filled up with more PPh as 16
~m.
The dimensions of these PPh regions are as large 3-enriched 3. XAD-2 (linescan c) shows a somewhat uniform distribution, which has
to be attributed to the relatively regular framework of microspheres in this material. The influence of the degree of liquid loading on the distribution of the catalyst solution in the support was measured by nitrogen capillary condensation at -196 °C.
with some typical silica 000-3E SLPC's
Result~obtained
are given in Fig.5.
( c
3
gm
)
0·6
0·4 ......- - - - - -
0·2
o L -_ _...L-_---L......._ - ' -_ _~-~:'::";;" 2
5
10 1 -
2 rp (nrn)
Figure 5. Pore size distribution of a silica 000-3E SLPC at various degrees of liquid loading. The cumulative pore volume Vc um is plotted as a function of the pore radius. X = hare support; 0 = degree of pore filling. 0.038; A = degree of pore filling is 0.17; degree of pore filling is 0.56; X.= degree of pore filling is 0.65; V = degree of pore filling is 0.88.
.=
377 It is clearly seen from Fig.5 that the catalyst solution in silica 000-3E is distributed as predicted by the theory of capillary condensation (ref. 1415); at low liquid loadings the walls of the pores are covered with a thin layer of physically adsorbed PPh
whereas at higher liquid loadings the thickness 3, of this layer increases, and the smaller pores get completely filled up with
capillary-condensed PPh
The BET surface area, equivalent to. the surface area 3• will therefore decrease strongly with increasing degree of
exposed by PPh
3, liquid loading, as appears from Table 1.
TABLE Surface area of the gas/PPh interface on silica 000-3E, at various degrees 3 of liqUid loading. liquid loading <5 0 0.038 0.075 0.17
S(BET) (m2/g) 203 188 177 142
liquid loading <5 0.32 0.55 0.65 0.88
S(BET) (m2/g)
liquid loading
S(BET) (m2/g)
81 39 26 10
0.96 1.00
8 4
Strong evidence is presented by Gerritsen et al. complexes at the PPh
(ref.5) that only the
interface are catalytically active, one of the
3/9as arguments being that catalysts with the solvent-ligand PPh
respectively in 3 the liqUid and in the solid state do not show any difference in apparent
activation energy and in catalytic activity in the same range of temperatures. Therefore one might expect, in the first instance, the catalytic activity to be proportional to the BET surface area, which is eqUivalent to the PPh
3 surface area. This, however, is not found experimentally, for reasons to be explained below. The influence the degree of liqUid loading has on the final activity level in the hydroformylation of ethylene, as depicted in Fig. 6, combined with the
data in Table 1, clearly shows that a simple relation between activity and S(BET) does not exist. It is clear alsqfrom Fig.6 that the influence of the degree of liqUid loading on the final activity level differs for the various supports, and so does the mean activity level. Whereas the activity decreases gradually with increasing degree of· liquid loading in the case of XAD-2 as a support, with Silica 003-3E
o~
we find an initial increase, followed by a weak decrease above
0.3. With KieselgUhr MP-99, on the other hand,
<5
has hardly any influence
on actiVity. An explanation of such behaviour can be given only when taking into account the complex-adsorption isotherms to be presented in the next paragraph.
378 500..------,=,.,--------,
400
Fig. 6. Final activity level of SLP catalysts in the hydroformylation of ethylene versus degree of liquid loading 0, for six different support materials Details are given in ref.5. T = 90 °C, P = 1.2 MPa. ~ = Silica S; + = XAD-2; fi = silica 000-3E; X = silica/alumina LA-30; 0 = Kieselguhr MP-99; 0 = alumina 005-0.75 E. The rhodium complex soncentration is 5.5 mol/m 3. The ratio p/Rh is 743 mol/mol.
C2
is ethylene.
Adsorptive withdrawal of the complex at the PPh
interface 3/support If preferential adsorption of rhodium complexes occurs at the support/PPh
3 in the pore, 3 and hence also from the meniscus where the catalytically active sites are interface,complexes are withdrawn from the bulk of the liquid PPh
located. Moreover, the extent to which adsorptive withdrawal occurs depends on the ratio PPh
area, and hence is dependent on the degree 3/support-surface of pore filling. Therefore, a separate study was made of the adsorption isotherms of RhHCO(PPh dissolved in PPh on different supports at 90 °c and 3, 3)3' higher. A 1:1 hydrogen/carbon monoxide mixture was DUbbledthrough the PPh 3 solution, in order to imitate the experi~ental hydroformylation conditions as much as possible; however, with pure hydrogen the same results were arrived at. Use was made of the adsorption apparatus depicted in Fig. 2. The amount of adsorbed rhodium complex was calculated from the change in rhodium concentration in the PPh
3
solution before and after adsorption, taking equilibration
times of about two hours. Increasing the equilibration time to more th?n 16
hrs had no influence on the position of the isotherms. Adsorption isotherms at 90 oC. on a number of silica's as adsorbents are
plotted in Fig. 7; by way of comparison the isotherm for Amberlite XAD-2 is included.
379
'"E ~'" "ll
a 20
b
E
.Q -
'--'=
~'"
"COl
o ,. s:
'"
, 3000 -----<__-
Concentration Rh in PPh3 (~g
Rh/g PPh3)
Fig. 7. Adsorptlon isotherms of the rhodium complex on various types of silica and on XAD-Z, at 90 °e. The amounts adsorbed per m2 BET surface area are plotted as a function of the rhodium complex concentration in PPh3' Sample a: Kieselguhr MP-99;S(BET) = 18 m2/g. Sample b: Silica Hi S(BET) = 25 m2/g. Sample c: Silica 000-3E; S(BET) 200 mZ/g. Sample d : Silica 5; S (BET) = 95 mZ/g. Sample e: Silica ntt-t1; S (BET) = 120 m2/g. Sample f: Amberlite XAD-2; S (BET) = 245 mZjg.
Interestingly, samples with a low surface area, like Kieselguhr and Silica H, Z show a higher adsorbability for the complex per m BET surface area than those with a high surface area, whereas the macroreticular resin XAD-Z, with its very high surface area, hardly shows any complex adsorption. A second interesting phenomenon is that supports with a high surface area, like Silica-OO 3E (ZOO mZ/g) and y-alumina 000-1.SE (198 mZ/g) , show an increase of rhodium adsorption pBr unit surface area with increasing temperature, whereas a low surface area support, like a-alumina SA-5Z02 (1.02 mZ/g) exhibits the reverse trend (see Fig. 8). We are dealing here with an effect which is also reported in adsorption experiments with molecular sieves (ref. 17): gas adsorption isobars of nitrogen on zeolite NaA show an increasing amount of adsorbed molecules with increasing temperature, whereas
~he
isobar for oxygen shows the reverse.
The explanation is that nitrogen diffuses into zeolite NaA with considerable difficulty and in any reasonable period of time does not attain equilibrium; this, however, is not the case for oxygen which has a smaller molecular diameter than nitrogen and can penetrate the cages in the molecular sieve at all temperatures. One or both of two factors appear to be operative:
(1) The process of
activated diffusion that is a function of temperature or {2) the effect of
380
;;; J' s:
40
r-----------------, 400
30
300 N
_E s:
II:
IX
'" '"
'" =
N
N
E
200
8-
. .."5 -e
.Q
.Q
..l5 II .c
I
l;
0.
".. II:
E
100
10
a
-
e .c
a:
I
Temperature of adsorption (·e)
Fig. 8. Amount of rhodium complex adsorbed per unit surface area at an equilibrium concentration of 2000 ~g/g PPh 3, as a function of temperature. Line a: y-alumina 000-1.SE (198 m2/g). Line b: Silica 000-3E (200 m2/g). Line c: a-alumina SA-S202 (1.02 m2/g).
increasing temperature on the vibration of the oxygen atoms in the zeolite crystal structure which surrounds the apertures. It is likely that in our case process (1) is operative. The diameter of the rhodium complex is about 15
R (ref.1S)
and has to penetrate into the PPh}-
filled pores. From a plot of the complex adsorption as a function of the mean pore radius for various supports we found that pore penetration at 80°C is strongly hindered for all pores with radii lower than 1200
R•. For
(see Fig. 8, line c), all pore radii are larger than 6000
R,
a-alumina
and hence no
sterical penetration hindrance occurs; indeed the amount of adsorbed rhodium complex diminishes with increasing temperature, as to be expected thermodynamically. Lines a andb in Fig. 8 are pseudo-equilibrium lines, as, due to sterical hindrance, adsorption equili.brium i.n the supports is attained only in the macroporous part of the supports in question. The influence the degree of liquid loading has an the catalytic activity (see Fig. 6) may now be explained in a qualitative manner. With increasing degree of liquid loading the PPh
surface area decreases and hence the catalytic 3 activity. This, however, is the case only when there is hardly any adsorptive
381 withdrawal of the complex, and catalysts with XAD-2 as a support are good examples for this (compare Fig. 6, upper curve, and Fig. 7, curve f). In dealing with supports which show a higher adsorptive withdrawal of the rhodium complexes, the percentage of the complexes withdrawn by adsorption decreases with an increasing PPh
ratio, and hence with increasing 3/support degree of pore filling. This makes the catalytic activity to increase, which
effect counterbalances the decrease due to the lowering PPh
surface area. 3 Silica 000-3E delivers a good example for the combined effect of decreasing surface area and increasing activity due to decreasing complex withdrawal,
which results in an activity curve with a maximum (see Fig. 6). Catalysts with Kieselguhr MP-99 as a support exhibit a low catalytic activity as compared
to
Amberlite XAD-2, due to their relatively low surface
area and the strong adsorptive withdrawal of the complexes (compare Figs. 6 and 7). We do not yet understand why the catalytic activity is hardly influenced by the degree of liquid loading in this case. Catalysts on a y-alumina support have a low catalytic activity (see Fig. 6, lowest curve, y-alumina 005-0.75E), notwithstanding the fact that adsorptive withdrawal of complexes should be low as a consequence of sterically hindered pore penetration; the diameter of the slit-shaped pores is of the order of 50
R
only. Nevertheless the adsorption level is high as compared
(see Fig. 8)
to
silica
and the same was found for silica-alumina supports (ref. 6), where
at very low complex concentration extremely large amounts of rhodium complexes are adsorbed. This undoubtedly is caused by the strong acidic surface sites on y-alumina and on silica-alumina, by which adsorptive decomposition of the complexes occurs.
other important characteristics of rhodium SLP catalysts Besides the requirements with respect to complex adsorption and pore filling described above, the supports should also meet the normal requirements for supported catalysts, like high breaking strength and low attrition. Another important factor is, however, that the support should have a low activity for unwanted side reactions in hydroformylation, for instance for the aldol condensation of the formed aldehydes. In a foregoing publication we have shown that the aldol activity has to be
ascribed mainly to the support (ref. 4), and
the lower the sodium and aluminium content of the supports,the lower is the aldol activity. This was re-established in a number of new experiments on the hydroformylation of butene-l at 90 ·C, 8 Atm. and a H ratio of 2/CO/alkene 1:1:1. When working with y-alumina 000-1.5E as a support, about 1% of the aldehyde produced was transformed into aldol products. The highly dehydrated
382 and dehydroxylated Silica H, which is an extremely pure silica (sodium content <
50 ppm), gave, under the same conditions, only 0.1% aldol formation.
Experiments with a-alumina
are in progress.
CONCLUSIONS Supported liquid phase catalysts offer interesting possibilities for the fixation or immobilization of metal-organic catalysts, which are normally applied homogeneously. New important variables, not encountered in working with classical heterogeneous catalysts, are the degree of pore filling, the complex
concentra~ion
in the liquid ( reaction rates are first order in
complex concentration; see ref. 8), the type of solvent ligand and the adsorbability of the complex on the support material, and these new variables should be studied thoroughly before formulating the recipe for the preparation of a technical catalyst. In the study of adsorptive withdrawal an interesting new phenomenon was encountered, viz. sterical hindrance of the penetration of the complexes in part of the PPh
filled pore system. In the case of 3 Amberlite XAD-2 as a support, this results in an increased concentration of complexes in the very large pores, by which the catalytic activity per gram of rhodium is very high. Two critical situations may be encountered when working with SLP catalysts. First, the catalyst can dry up by evaporation of the solvent ligand. In future publications we will show that, with PPh
as the solvent ligand, temperatures 3 up to 120 °c are still acceptable for technical use of the SLP system. For
higher working temperatures other solvent ligands may be used (see Lit. 7). Secondly, soaking of the catalyst may occur by the formation of aldol products, or by capillary condensation of the formed aldehydes in the pores of the SLP catalyst. This situation may be circumvented when working with supports with an extremely low aldol activity, and by regulation of the conversion per pass in such a way that the partial pressure of the aldehydes formed is too low for capillary condensation.
ACKNOWLEDGEMENTS The investigations were supported (in part) by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).
REFERENCES 1. J. Villadsen and H. Livbjerg, Catal. Rev.-Sci. Eng. 17(2) (1978)203-272. 2. G.J.K. Acres, G.C. Bond, B.J. Cooper and J.A. Dawson, J. Catal. 6(1966)139. 3. P.R. Rony, J. Catal. 14(1969)142.
383 4. L.A. Gerritsen, A. van Meerkerk, M.H. VreugdenhilandJ.J.F. Scholten, J.Mol.Catal. 9(1980)139-155. 5. L.A. Gerritsen, J.M. Herman, W. Klut and J.J.F. Scholten. J.Molec. Catal. 9(1980)157-168. 6. L.A. Gerritsen, J.M. HermanandJ.J.F. Scholten, J.Molec.Catal. 9(1980)241-256. 7. L.A. Gerritsen, W. Klut, M.H. VreugdenhilandJ.J.F. Scholten, J.Molec. Catal. 9(1980)257-264 8. L.A. Gerritsen, W. Klut, M.H. Vreugdenhil and J.J.F. Scholten, J.Molec.Catal. 9(1980)265-274. 9. L.A. Gerritsen and J.J.F. Scholten, US Pat. 4.193.942, d.d. Mar. 18, 1980 10. L.A. Gerritsen and J.J.F. Scholten, US Pat. 4.292.198, d.d. Sep. 29, 1981. 11. N. Ahmad, S.D. Robinson, M.F. Uttley, J.Chem.Soc., Dalton Trans. (1972)843. 12. J.J.F. Scholten, A. van Montfoort, L. van de Leemput and H.W.M. BootLemmen~, Dutch Patent Appl. No. 7707961, d.d. 18 july 197~ and Dutch Patent Appl. No. 7807220, d.d. 4 july 1978. 13. J.J.F. Scholten, A. van Montfoort, L. van de Leemput and G.H.A. Nooijen, Dutch Patent Appl. No. 8005856, d_d. 24 october 1980. 14. J.C.P. Broekhoff and B.G. Linsen, in B.G. Linsen (Ed.), physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, New York, 1970, Ch. 1. 15. J.C.P. Broekhoff, in Adsorption and Capillarity, thesis Delft University of Technology 1969, Waltman, Delft, 1969. 16. P.J.M. KOrthoven and M. de Bruin, J. Radional. Chem. 35(1977)127. 17. D.W. Breck, in Zeolite Molecular Sieves, John Wiley, New York 1974, page 638-639. 18. S.s. Bath, L. Vaska, J.Am.Chem.Soc. 85(1963)3500.
384 DISCUSSION G.C. BOND ; You observe no change in activation energy as you pass through the melting point of PPh 3 as determined directly by DSC. Could this be due to the heat of reaction raising the local temperature, so that under operating conditions the PPh 3 is always in the liquid state ? J.J.F. SCHOLTEN: Indeed, the heat of reaction for hydroformylation is relatively high, of the order of 35 kcal/mole of alkene. Our experiments were, however, conducted at a very low conversion per pass (about 2%). Moreover, heat dissipation will occur mainly through the gas phase. S.P.S. ANDREW: It is interesting to note that the evidence you present that the reaction is truly heter~geneous, in as much as there is no effect on the rate in moving through the liquid phase melting point, is also found in the case of sulphuric acid production catalyst. J.J.F. SCHOLTEN: Thank you for this interesting remark. Further experiments were performed with non-supported catalysts (solid solutions of the Wilkinson complex in PPh 3). The activation energy is equal to that found for the supported catalyst solution. Further results and explanations may be found in : L.A. Gerritsen, J.M. Herman, W. Klut and J.J.F. Scholten, Journal of Molecular Catalysis, ~, 157-168 (1980). J. MARGITFALVI; I should like to make a comment on the adsorption behaviour of your carriers. In the case of adsorption of hydrido-complexes like HRh(CO)(PPh 3)3 and HRhC12(PPh3)3 we observed (1) a loss of hydride ligand, which depended both on the acidity and on the OH content of the carrier. Have you ever tried to look after the fate of the adsorbed complexes? From a practical point of view this loss of hydride ligand will result in a decrease of the overall activity per rhodium atom. (1) J. Margitfalvi and S. Gobolos, to be published. J.J.F. SCHOLTEN: From the perfect stability of our SLP rhodium catalysts, controlled for mOre than 800 hrs one might conclude that the complexes are not deteriorated by any chemical action of the support. When, however, this chemical attack is very slow, it perhaps escaped our observation. Your suggestion is very valuable, and we will certainly investigate this problem more closely. In the past we performed some successful experiments with silica supports, the surfaces of which were modified with tri(ethoxy)phenylsilane, prior to loading the support with catalyst solution. These catalysts had a very high activity indeed, and the activation time (in many cases of the order of 100 hrs) was very short: a fex hours only. Obviously, in accord with your remark, screening of the silanol groups is very effective. Literature: L.A. Gerritsen, J.M. Herman and J.J.F. Scholten, J. Mol. Catal. 168 (1980).
~,
385
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ROLE OF THE METAL-SUPPORT INTERACTION IN THE PREPARATION OF Fe/MgO CATALYSTS H. MOUSTY a, B.S. CLAUSEN b, E.G. DEROUANE a
and H. Tops0E b
a. Facultes Universitaires de Namur, Departement de Chimie, Laboratoire de Catalyse,
bl, rue de Bruxelles, B-SOOO Namur (Belgique) b. Haldor Tops¢e Research Laboratories, P.O. Box 213, DK-2800 Lyngby (Denmark)
INTRODUCTION In catalysis, the production of highly dispersed catalysts is often desirable but one of the major problems is the achievement and maintaining of small particles.
For metallic catalysts, this may be realized by dispersing the metal on a
porous carrier.
In order to stabilize metallic particles, the metal-support inte-
raction should be optimum, i.e. neither so strong that the reduction to the metallic state is too difficult, nor so weak that high particle mobility results in rapid sintering. Boudart et al. (ref. I) found that the preparation of small iron particles can easily be achieved by using magnesium oxide as a support.
The initial step in the
generation of the iron catalysts was the impregnation of magnesium hydroxycarbonate (MHC) with an iron nitrate solution.
The genesis of such catalysts as well as of other
catalysts prepared by coprecipitation has been studied recently (ref. 2).
In par-
ticular, it was observed that the preparation method as well as the metal loading affect the particle size and the particle size distribution of the metallic iron. However the fraction of reduced metal was found to be rather independent on the type of preparation, a behaviour quite different from that observed for iron catalysts on other supports such as alumina and silica (ref. 3).
Fe on MgO catalysts
were also prepared by exposure of MgO to iron carbonyls (ref. 4) or by precipitation of iron nitrate on magnesium oxalate (ref. 5).
For these catalysts, the frac-
tion of reduced iron seemsto be greater as compared to the above preparation methods. It appears therefore interesting toestablish more clearly the type of relationships existing between the preparation procedures, the type of precursors, the activation of the
pre~ursors
and the nature and extent of the metal-support interactions
Which, in turns influence the reducibility of the metal.
The present paper des-
cribes new preparations and characterizations of Fe/MgO catalys t s .
Using the inci-
pient wetness method, magnesium oxide or hydroxycarbonate were impregnated with aqueous ammonium ferrocyanide.
The ferrocyanide complex was chosen because of its high
386 stability at basic pH values.
Impregnation of MgO by iron(III) nitrate was also
carried out and was motivated by our wish to eliminate the influence of the decomposition of the MgO precursor. The reduction of the catalysts was followed by thermal analysis (TG-DTA) under hydrogen atmosphere.
Mossbauer spectroscopy was used to provide information on
the chemical state and the degree of reduction of iron in the reduced catalysts. Particle sizes were estimated by transmission electron microscopy and by chemisorption of CO. EXPERIMENTAL The precursors of the magnesium oxide supported iron catalysts were prepared using two different
procedures: a) impregnation of MgO with iron nitrate as des-
cribed for the impregnation of MHC in ref. 1 and b) incipient wetness impregnation of magnesium hydroxycarbonate or of MgO by ammonium ferrocyanide.
We will refer
to these catalysts as FeN03-0-y, FeCN-HC-y and FeCN-O-y,respectively, where N03, CN, 0 and HC stand for iron nitrate, ammonium ferrocyanide, magnesium oxide and magnesium hydroxycarbonate, and y stands for the type of treatment. pregnation the samples were dried at 400 K for 48 h (y fused hydrogen, 9lh
-1
) (y
=
p).
After im-
Reductions(Pd dif-
R) were carried out at 750 K for 20 h, after the sam-1
pIe had been heated at 120 Kh
to this temperature.
The iron loadings of the
FeN03-0-R, FeCN-HC-R and FeCN-O-R catalysts are 3.37, 3.12 and 4.09 wt %, respectively, as measured by spectrophotometry. Chemisorption of CO and BET surface area measurements on the reduced and subsequently evacuated (673 K, I h) catalysts were carried out at 195 and 78 K respectively, using a constant volume system equipped with a Texas pressure gauge. The particle sizes for iron were estimated from the CO chemisorption experiments as described in ref. 1. Mossbauer spectra were obtained in situ with a constant acceleration spectrometer using a 57 Co in Rh source. foil of metallic iron. metallic iron spectrum.
The spectrometer was calibrated using a thin
Isomer shifts are given relative to the centroid of the The in situ ce Ll.s for carrying out experiments above and
below room temperature have been described elsewhere (refs. 6-7). Thermal analyses were carried out in a Stanton Redcroft STA 780 thermal analyzer. I
Experiments were conducted under hydrogen flow (2.5 lh- ) at a heating rate of I
120 Kh- .
X-ray diffraction patterns were obtained with a Philips PW-l050 verti-
cal goniometer.
Average particle sizes were derived using the Scherrer equation
with appropriate corrections for the Cu-Ka doublet and the instrumental broadening.
They were also investigated by transmission electron microscopy using a
Philips ESM 301 electron microscope.
387
RESULTS AND DISCUSSION Temperature programmed reduction of the precursors under H2
TG a 1/1 1/1
s
£
g, Gl
~
1 10
%
1/
DTA
~
EXO
~
1 EN DO
300
400
500
600
700
800
TEMPERA TURE (K)
Fig. I. Thermal decomposition of catalyst precursors under H2 a) FeN03-0-P ; b) FeCN-HC-P ; c) FeCN-O-P. Figure I shows the weight losses and corresponding differential thermal analysis responses for the decomposition-reduction of the three catalyst precursors. MgO used in the preparation of FeN03-0-p was converted to Mg(OH)2 as identified by X-ray diffraction during the impregnation with iron nitrate solution (pH ~.5
- 9, 363 K).
in humid air (ref.
=
This transformation has also been observed previously to occur ~).
Transmission electron microscopy (TEM) shows regular he-
xagonal platelets and some poorly defined aggregates. The decomposition of FeN03O-P occurs in one step characterized by a total weight loss of 32.5 % (from 380 to 1070 K) and by a strong endothermic peak at 605 K. This observation may be 2+ hydroxide during impregnation,
3+ explained by the formation of a mixed Fe and Mg
as the theoretical total weight loss value for such an intermediate is 32.5 %.
388 FeCN-HC-P has the typical X-ray diffraction pattern of the MHC structure and appears as platelets in TEM.
Furthermore, a cubic "Prussian-Blue"-like phase, with
an average size of 30 nm, was also detected by X-ray diffraction.
The total
weight loss occurring during the decomposition of FeCN-HC-P is 53.7 % (380-1070 K). This value should be compared to the experimental value of 56.4 % observed for MHC (under the same conditions).
Two strong endothermic peaks corresponding to
the dehydration and the decarbonatation of MHC were detected at 505 and 675 K. The cubic structure of MgO (a FeCN-O-P.
=
4.21 A) is observed by X-ray diffraction for
The cubic "Prussian-Blue"-like phase, already observed for FeCN-HC-P,
was also detected.
The
of the thermogravimetric curve is completely diffe-
shap~
rent from that observed for FeN03-0-p although both of them were prepared using MgO as the initial support.
The total weight loss is 21.0 % (380-1070) and two
different DTA signals are observed : a very broad endothermic peak at about 590 K and an exothermic one which is very intense and sharp at 635 K.
The latter tempe-
rature corresponds to the last step in the decomposition of bulk ammonium ferrocyanide into Fe
2Fe(CN)6
(ref. 9).
Characterization of the reduced phases TABLE 1 Lattice parameter of MgO in different samples Samples
a, lattice parameter (A)
MHC calc. FeNOJ-O FeCN-HC FeCN-O
4.240 4.230 4.233 4.219
Table 1 lists the lattice parameters of the MgO phases obtained after reduction of the precursors and after calcination of MHC.
The value for FeCN-O-R is typical of
crystalline MgO, with no indication of the formation of a (Fe, Mg)O or of the presence of very small MgO particles.
solid solution
In contrast, the avalue for MgO
stemming from MHC decomposed following the same heating schedule,is 0.5 % larger than that of bulk MgO, in agreement with previous data which indicate expansion taking place in small MgO particles (ref. I).
a lattice
The values of the lattice
parameters observed for FeNOJ-O-R and FeCN-HC-R are higher than the one observed for FeCN-O-R ; this may be due to the formation of a (Fe, Mg)O solid solution or to the presence of very small MgO particles. calcined MHC,
Their values being smaller than that of
the influence of small MgO particles seems to be negligible.
As they
nave nearly identical iron loadings, the similitude of their lattice parameters seems to indicate that the formation of a FeO-MgO solid solution is responsible for the lattice expansion of MgO in agreement with earlier reports (refs. 1-2).
The carrier particle
389
size of FeCN-O stays nearly constant upon reduction.
In contrast, the reduction
treatment decreases considerably the MgO particle size for the two other precursors. This effect is particularly pronounced for FeN03-0 : MgO particles (32 nm) which first increased in size (40 nm) during their conversion into Mg(OH)2(impregnation treatment) were subsequently decomposed thermally into smaller particles of MgO (7.5 nm).
As seen from Table 2, MgO particle sizes derived from X-ray line broa-
dening agree satisfactorily
with values evaluated from BET surface area measu-
rements. TABLE 2 Iron and carrier particle size Compounds Carrier particle size (nm) precursors X-ray line broadening
MgO
MHC
FeN03-0
FeCN-HC
FeCN-O
32
31
40
32
32
9 6
10 10
32 30
reduced catalysts X-ray line broadening BET isotherm
b
Iron particle size (nm) CO chemisorption Electron microscopy
2.8 4.4
3.2 4.7
13 13
Table 2 also lists the surface average particle sizes for the metallic iron particles as obtained from TEM and CO chemisorption.
Corrections for strong CO adsorp-
tion on MgO were assumed to be proportional to the total surface area of the catalysts, using the approach described previously (ref. I).
A Fe:CO surface stoi2hio-
metry of 1:1 was assumed which however may not hold for very small iron parti2 cles (ref. 10). In addition, Fe + can also chemisorb CO (refs. 11-12). In the calculation of the particle sizes from CO chemisorption data, only the fraction of reduced iron was taken into account using the data obtained by Mossbauer spectroscopy (see Table 3). of the "reduced"
Apart from metallic iron, iron carbide was also considered to be part iron fraction.
In view of the good agreement between the Fe par-
ticles sizes evaluated from the TEM and chemisorption data, the above assumptions seem to be reasonable. Room temperature Mossbauer spectra of the reduced catalysts are shown in Fig. 2. The parameters which characterize the different components are listed in Table 3. The Mossbauer spectra of all three samples indicate two main states of iron.
The
first one with an isomer shift of about 0.00 mm/s gives a magnetically split sixline component and corresponds to metallic iron. order FeN03-0-R, FeCN-HC-R, FeCN-O-R.
Its intensity increases in the
A second state of iron is characterized by
a doublet in the central region of the spectrum; it has an isomer shift of
390 2 1.00-1.11 mm/s and is attributed to Fe + in MgO (refs. 1-2). The amount of residual 2 Fe + in MgO varies from 2~ to 68 % as seen from Table 3. The Mossbauer spectra of FeCN-HC-R and FeCN-O-Rshow some additional features.
For the FeCN-HC-R sample,
a magnetically split spectral component with an isomer shift of 0.16 rom/s corresponding to iron carbide (cementite) is observed.
This component probably stems
from the decomposition of the ferrocyanide complex (refs.
9-13).
Another central
line is observed in both systems with the highest intensity observed for FeCN-O-R. The absence of magnetic splitting
of the central peak in FeCN-O-R in the presence
of an applied magnetic field indicates that this line does not correspond to metallic iron behaving superparamagnetically, but probably to some incompletely reduced ferrocyanide species.
It is likely that the central peak for FeCN-HC-R
also stems from the presence of such species •
••;.,...-:-»:....
b
-,
\ ..•.~. ,",:
-8
-6
-4
-2
0
VELOCITY
Fig. 2. In a) b) c)
2
4
6
8
(mm/s)
situ Mossbauer spectra at 298 K of reduced Fe/MgO catalysts FeNOJ-O-R FeCN-HC-R FeCN-O-R.
391 TABLE 3 Mossbauer parameters for reduced Fe/MgO samples at 298 K
Metallic iron isomer shift (mm/s) Metallic iron magnetic field (KG) Iron carbide isomer shift (mm/s) Iron carbite magnetic field (KG) 2+ Fe isomer shift (mm/s) 2+ I " Fe q~adrupo e spllttlng (mm/s) Central peak isomer shift (mm/s)
FeN03-0-R
FeCN-HC-R
FeCN-O-R
-0.01 330
0.00 331 0.16 208
0.01 319
1.06
1.11
1.06
0.81 0.05
0.70 0.16
0.63 0.01
32
38 9 8
52
68
45
28
u-Fe spectral area (%) Iron carbide spectral area (%) Central peak spectral area (%) 2+ Fe spectral area (%)
20
2 The Fe + quadrupole splitting decrease in the order FeN03-0-R, FeCN-HC-R, FeCN-O-R.
Its value for FeN03-0-R is close to that observed previously (refs.
1-2) and shows that ferrous ions are not randomly distributed in the MgO carrier (refs. 1-2). The magnetic hyperfine field for a-Fe in FeCN-O-R is significantly smaller than the bulk value which is observed for the other two samples. an external magnetic field
(H~II
Application of
KG) to the latter sample leads to a magnetic
splitting of about 320 KG, corresponding (after correction for the applied field) to a magnetic hyperfine field of 331 KG.
The latter value is identical to that
of the other two samples. This result led ustc propose that the reduced magnetic splitting in FeCN-O-R is due to collective magnetic excitations (ref. 14).
In
contrast to the CO chemisorption and electron microscopy results, Mossbauer data hence seem to lead to the conclusion that small Fe particles are also present in FeCN-O-R.
An explanation for this apparent discrepancy may be that the incomple-
tely reduced ferrocyanide observed in this catalyst partially covers the metallic iron particles.
This situation may decrease the value of the surface magnetic ani-
sotropy constant, in which case, the particles sizes evaluated by Mossbauer spectroscopy would be underestimated.
The presence of a covering layer has in fact been
observed previously to influence the surface anisotropy (ref. 3). Metal-support interaction As shown in Fig. 3a and 3b, a suitable parameter to evaluate the extent of the metal-support interaction is the specific iron loading expressed as the total iron 2
loading per unit surface area of MgO (wt. % Fe/SA (MgO. m.g the specific surface area of the final MgO.
-I
»,SA standing for
The present data can thereby be
392
compared to those obtained earlier for other systems, prepared either by impregnation of MHC or by coprecipitation of the iron and magnesium hydroxides.
It is
clear from Fig. 3a that the fraction of iron reduced to the metallic state does not depend on the preparation technique to any significant extent and that it varies within a rather restricted range (25-55 % of the total iron content).
Although
this may appear surprising, it demonstrates clearly that the reducibility of iron to the metallic state is mainly determined by the amount of iron present and by the specific surface area of the
M~O
carrier in the final state of the catalysts,
pointing to the important role of the Fe-MgO interaction.
75
f s.. I:
50
U :I
..o-Q-""
"•. ...
25
....0-=
•
0
.".
0
15
5
•
~
0
MHC (ret t)
Ii.
CP (ref.2)
•
this work
15
•
iI:
-
:10
it
•
'; I:
U
o ..
.. iii ~
.••.
5
Go
~
o
5 10 15 10 2 x wt. " Fe/SA (MilO, m2.11·1 ) _
Fig. 3. Metallic iron particle size and degree of reduction as function of the total iron loading per unit surface area,of MgO. Similar analysis of the data for the metallic iron particles sizes shows an effect of the preparation itself, as observed in Fig. 3b.
Particles sizes nor-
mally increase at high specific iron loading with correlations that hold only for catalysts stemming from the same type of precursors.
Particles sizes are observed
to increase at higher iron loading or for supports of lower specific area, and apparently also for precursors which undergo less dramatic structural rearrangements during the consecutive treatments leading to the final catalysts.
393
CONCLUSIONS The final state of Fe/MgO catalysts, i.e., the amount of iron in the metallic state and the iron particle size, have been demonstrated to depend ultimately on the specific iron loading, that is the total amount of iron present per unit surface area of the ultimate MgO carrier.
It is clear also that the structural reor-
ganization of the support precursor will be responsible for obtaining high specific surface area of the carrier. of
me~allic
MgO.
No other factors seem to affect the fraction
iron, indicating that a major effect is the dissolution of Fe ions in
These ions are thus less reducible than if they were readily accessible on
the surface.
Obviously, higher carrier surface areas and lower iron loadings will
result in a higher proportion of "dissolved" iron and, hence, less iron metal in the reduced catalyst. Metallic particle sizes show the same trend but with the additional influence of the preparation technique.
Preparations which give a non uniform distribution
of iron in the precursor and therefore higher local iron concentrations in the uppermost layers of the final MgO carrier, yield larger particles. REFERENCES
2 3 4 5 6 7 8 9 10 11 12 13 14
M. Boudart, A. Delbouille, J.A. Dumesic, S. Khammouma and H. Tops~e, J. Catal., 37(197~)486-502. H. Tops~e, J.A. Dumesic, E.G. Derouane, B.S. Clausen, J. Villadsen and Preparation of Catalysts II, Stud. Surf. Sci. Catalysis 3, N. Tops~e, Elsevier, 1979, p. 365. H. Tops~e, J.A. Dumesic and S. Morup, Application of Mossbauer Spectroscopy II, Catalysis and Surface Science, Academic Press, 1980, p. 55-188. F. Hugues, P. Bussiere, J.M. Basset, D. Commereuc, Y. Chauvin, L. Bonneviot and D. Olivier, Stud. Surf. Sci. Catalysis, 7A, 1981 p. 418-431. D.A. Storm, PhD dissertation, Stanford University, 1978. B.S. Clausen, S. Morup, P. Nielsen, N. Thrane and H. Tops¢e, J. Phys.E Sci. Instr, 12(1981)433. H. Tops¢e, B.S. Clausen, R. Candia, C. Wivel and S. Morup, J. Catal. 6l:l(1981)433. M. Faure, Bull. Soc. Chim. France, 1970, ,69-73. A. Mittasch, E. Kuss, O. Emmert, Z. Anorg. Allgem. Chern., 170(1928)193-212. H. Tops¢e, N. Tops¢e, H. Bohlbro and J. Dumesic, Stud. Surf. Sci. Catalysis 7A, 1981 p.247-265. T. G. Voroshilov, N.K•. Lunev, L.M. Roev and M.T. Rusou, Dopov. Akad. Nauk. Ukr. RSR, serie B4, 1975, Chern. Abstracts 83, 103698b. B.S. Clausen, S. Morup and H. Tops¢e, Surf. Sci., 106(1981)438. H. Mousty, A. Abou-Kais, N. Nashrallah-Abou Kais, Z. Gabelica and E.G. Derouane, Proceedings of the Second European Symposium on Thermal Analysis, Heyden, 1981, 449. S. Morup and H. Tops¢e, Appl. Phys ; , 11(1976)63.
394 DISCUSSION S.P.S. ANDREW It occurs to me that the problem with this type of preparation is the likely mobility and partial dissolution of the magnesia during the impreg~ nation operation. This leads to the ready formation of difficult reducible iron magnesium compounds upon calcination. H. MOUSTY We agree with you that this phenomenon, the partial dissolution of magnesia during the impregnation, has to be taken into account for the understanding of the genesis of the Fe/MgO catalyst. However, the degree of reduction was demonstrated to be independent of the type of preparation and it is unlikely that these dissolution processes were the same for the different methods of preparation described here or previously. M.V. TWIGG: with regard to the iron(II) species that amount to some 20% of the iron in ferrocyanide derived catalyst even after quite forcing reduction that you suggest from Mossbauer data are iron cyano complexes. Do you have any direct results in support of the iron(II) being associated with cyanide? For instance, do you have any infrared results ? H. MOUSTY: The absence of magnetic splitting of the central peak in FeCN-O-R in the presence of an applied magnetic field indicates that this line does not correspond to metallic iron behaving superparamagnetically. However, the value of the isomer shift (~ 0.0 mm/s) of the central peak is also in accordance with many diamagnetic ferrocyanides for which reason we assign it to such a species. Infrared measurements may, as you mention, give further evidence to this assignment. L. GUCZI According to CO chemisorption and TEM data, the particle size is about 4 rum. On Mossbauer data you have hyperfine splitting which indicates a much higher particle size. Could you comment on this matter ? H. MOUSTY: The hyperfine field, in the case of the catalysts containing small iron particles (4 nm), are similar to that of bulk metallic iron. This is in agreement with previous results (see ref. 1 and 5), and indicates that the magnetic anisotropy constant, which is not known for the Fe/MgO system, is higher than that observed in the Fe/Si02 system.
395
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Prepa....tion of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherland.
NEW FISCHER-TROPSCH CATALYSTS OF THE AEROGEL TYPE F. BLANCHARD, B.
POMMIE~,
J.P. REYMOND and S.J. TEICHNER
Laboratoire de Catalyse Appliquee et Cinetique Heterogene de l'Universite Claude Bernard (Lyon I) associe au C.N.R.S. (L.A. N° 231), 43 boulevard du II novembre 1918 - 69622 Villeurbanne Cedex, France
SUW1ARY Iron oxide supported on silica or on alumina aerogels were prepared in organic solvent (methanol or sec-butanol) which was evacuated in hypercritical conditions, in an autoclave. These aerogels exhibit an unusual activity, without deactivation, in the Fischer-Tropsch synthesis at 250°C, provided that iron is initially in the maximum oxidation state (Fe
and not reduced previously to 20 3), the reaction. This behaviour is not found for unsupported iron oxides or
for supported iron oxide xerogels.
INTRODUCTION The best catalysts of the Fischer-Tropsch synthesis, producing higher hydrocarbons, are based on metals Fe, Co, Ru (ref. I). They dissociate CO at the reaction temperature (ref. 2) and give easily oxides and carbides (ref. 3). The low cost of iron and its use in ammonia process gave an impetus to its industrial applications (ref. 4) and to practical developments in the Fischer-Tropsch process (ref. 5). The iron catalysts which are used on industrial scale at the present time are of fused type or precipitated and their preparation is very similar to that of ammonia catalysts. They initially contain magnetite Fe 304 and promotors like K and Al 203 but once reduced their surface area does not exceed 2 2 -I 10 m g . In order to increase the metal surface area per unit mass of metal
P
and the thermal stability against sintering it has been proposed to disperse ~ron
on refractory oxides like silica or alumina (ref. 6). But even for suppor-
ted iron it is difficult to achieve a highly divided state (refs. 7, 8) and the best results were obtained by decomposition of complex salts of iron in the presence of support (ref. 9). Now
the technique of the preparation of aerogels of
inorganic oxides is a powerful
tool for
obtaining a
highly divided sta-
te (ref. 10) of pure oxides or supported oxides (ref. II). Also an attempt has been made at the preparation of Fischer-Tropsch type catalysts, based on supported iron, of aerogel type.
396 EXPERIMENTAL METHODS AND MATERIALS The chemical analysis of solid catalysts, fresh or spent, was performed by atomic absorption for iron and by calcination in 02 for carbon (C0 2) and hydrogen (H evolved). The XRD spectra were obtained with K radiation of Co (Phi20 a lips PW 1009) in Debye-Scherrer or Guinier-Lenne cameras for the "identification of the solid phases and with a G.M. goniometric recorder (Siemens) for the particle size determination. The E.M. observations (by transmission) were realized °
with Philips EM 300 or Hitachi HU 12 A instruments (resolution 5 A). The B.E.T. surface area of solids was measured by dynamic gas chromatography (ref. 12) . The mercury porosity measurements were performed in an Aminco 57121 porosimeter. The catalysts (50 mg) were tested in
a
differential dynamic microreactor, with 3 -I H2/CO ratio equal to 9, at I atm pressure,and 60 em mn flow rate, at the reaction temperature of 250°C. After the reaction the catalysts were etched by pure H2 at 250°C and then at 500°C in order to determine the amount of carbon (as CH 4) left on the solid (ref. 13). The reactants (high purity) and products were analyzed by gas chromatography. For the preparation of aerogels high purity chemicals : iron(III)acetylacetonate, silicon tetramethoxide, aluminium sec-butylate, methanol and sec-butanol, were used. PREPARATION OF IRON OXIDE AEROGELS Unsupported iron oxide aerogels or iron oxide supported on silica or on alumina aerogels were prepared by the method of drying the solvent (methanol or secbutanol) in hypercritical conditions, in an autoclave (refs. 10,11). A typical procedure for preparation of a silica supported iron oxide consists in the formation, in the first step, of a silica aerogel. Silicon tetramethoxide in methanol (25 % by volume) is hydrolyzed by added water at 25°C (4 H 20/ Si(OCH3)4) and the system is heated in the autoclave above the critical temperature of methanol (240°C) which is then evacuated as vapour. The silica aerogel thus obtained is redispersed in methanol containing iron(III)acetylacetonate in an amount corresponding to 10 % iron content of the final aerogel. The iron salt is hydrolyzed by adding water at 25°C [3H
and the system is again 20/Fe(C 502H7)3] heated in the autoclave in order to achieve hypercritical conditions (p = 150 atm, T
=
275°C) for the evacuation of methanol. This sample is labelled
[SiFeI0(A)(2H)] where A stands for aerogel, 10 for iron % content and 2H for two steps preparation by hydrolysis. A second type of preparation involves the redispersion of silica aerogel, obtained as previously, in the methanolic solution of iron(III)acetylacetonate without adding water to hydrolyze the second partner which is merely decomposed by heating the system to hypercritical conditions. This sample is labelled [SiFel0(A)(2D)] where D stands for decomposition. Alumina supported iron oxide was made by the same procedure but with alumina aerogel prepared in the first step from aluminium sec-butylate in sec-butanol (10 % by
397 volume), hydrolyzed and evacuated in hypercritical conditions (P
=
188 atm, T
Z90°C) in the autoclave. The samples are respectively labelled [AlFe6(A)(ZH») and [AlFeIO(A)(ZD»). For the first sample the iron content was found equal to 6 %. It should be pointed out that the method of simultaneous hydrolysis of two compounds (of silicon and iron or of aluminium and iron) dissolved in alcohol, in one step, followed by the evacuation of , the alcohol in hypercritical conditions, gives catalysts in which iron oxide is screened by silica or alumina and whose catalytic performances are not as good as those of samples prepared in two steps, where iron oxide aerogel is formed on the surface of the first aerogel (silica or alumina). Iron oxide aerogels without the support were also prepared for comparison purposes, by the same method, in the autoclave, involving either the hydrolysis of the iron precursor [AcAcFe(H») or its thermal decomposition at the hypercritical
[(AcAcFe(D»). Finally for the same purposes, con-
temperatur~
ventional supported on silica or alumina xerogels were prepared by incipient wetness impregnation of the support (amorphous silica-Aerosil Degussa - ZOO mZ ti umlna-P 110 Degussa - 110 mZ g -1) b y t h e so Lu utlon g-I or d e 1 ta-a Iumi
0
f f errlC . nl.
trate. After drying in vacuo at Z5°C the solid is decomposed at 170°C in air. The samples are labelled SiFeIO(X) and AIFeIO(X) where X stands for xerogel. Table I gives the composition and the nature of phases detected (XRD) in aerogels and xerogels. TABLE 1 Composition and nature of the solid phases Nature of the solid
Weight percent of Fe
C
SiFeIO(A) (ZH)
10.8
5.6
1. 47
SiFeIO(A) (ZD)
10.4
7.4
1.8
SiFeIO(X)
9.5
AIFe6(A) (ZH)
6.7
AlFeIO(A) (ZD)
10.1
H
Crystalline phase detected by XRD
° ~Q4(a)
a-FeZ03(b)
Fe304 a-Fez03
0.15 I. 30
a-Alz03,HZO(b) ; Fe3Q4
Z.04 I. 65
a-AlZo3,HZO ;
~24
AlFelO(X)
9.6
AcAcFe(H)
71.3
0.9
<0.1
>Z7.7
a-Fez23 ; Fe304
AcAcFe(D)
67.3
1.7
'V().
I
'\,30.9
Fe3Q4 ; a-Fez03(c)
a-FeZ03(c) a-FeZ03(c)
C-AlZ03 ; a-Fez 03
(a) the most abundant iron oxide phase is underlined (b) silica is amorphous whereas alumina gives the XRD pattern of boehmite (c) in a very smali amount The carbon and hydrogen contents in aerogels are due to organic rests of precursor salts or of solvents (refs. 10,11). For all supported iron aerogels the most abundant phase is magnetite Fe
It is only for non supported iron aerogel 304. [AcAcFe(H)] that hematite (a-Fe predominates. It is also the only phase deZ03) tected in conventional xerogels (X) of table I. The size of particles of iron oxides was determined by XRD (d 104' d l l O and
398
Fig. I. Aerogel SiFeIO(A) (2D)
Fig. 2. Xerogel SiFeIO(X)
399
Fig. 3. Aerogel AlFelO(A) (2D)
Fig. 4. Xerogel AlFelO(X)
400 d l 16 for a-Fe for Fe and by electron microscopy and d d and d 304) 203 3 11, 400 440 and is given in Table 2 together with the total B.E.T. surface area of solids. TABLE 2 Dispersion of the iron oxide phase and the total surface area Nature of the solid
Oxide particles mean diameter or range (A) as determined by Electron microscopy XRD
B.E.T. total surface area (m2 g -1)
siFelO(A) (ZH)
100 to 700
94
760
SiFeIO(A) (ZD) (fig. 1)
100 to 700
110
690
SiFeIO(X) (fig. Z)
200
AIFe6(A) (ZH)
Fe304
50 to 600
Fe304 : 175
330
AIFeIO(A) (2D) (fig. 3)
Fe304
50 to 600
Fe304.: 184
300
AIFeIO(X) (fig. 4)
a-Fez03
800 to 1500
a- FeZ03
lOS
a - Fe 20 3
1000 to 3000 (a)
AcAcFe(H) AcAcFe(D)
{
Fe304 : 100 to 175 Fe304 : 200 to 300
200 >600(a)
a - Fe 223 { Fe304 : 320 260 Fe304
6.8 31.0
(a) the most abundant phase is underlined. Higher surface areas of silica supported aerogels are in good agreement with better dispersion of Fe
(determined by XRD and M.E.) on this carrier than on 304 alumina. The E.M. observation probably accounts for particles formed from many
crystallites whereas the XRD method gives the mean diameter of elementary particles. In the case of aerogels prepared in one step (not listed in the Tables) the dispersion observed by E.M. was much smaller. Unsupported iron oxides show (Tables I and 2) particles of higher diameter than for supported oxides and the resulting surface area is very low. They are not substantially different from conventional xerogel oxides. It has not been attempted to measure the dispersion of iron by titration or chemisorption of a gas because chemisorption of an adsorbate (like CO) was not found to be specific of ionic or zerovalent iron (ref. 14). The comparison of the diameter of particles in iron oxide supported aerogels with those in the conventional iron oxide supported xerogels (Table 2) shows that the dispersion of iron oxide in xerogels is smaller. Moreover, iron oxide in xerogels is under the form of hematite a-Fe
whereas practically only Z03, magnetite Fe 304 is found in aerogels. It is shown below that the crystalline form of the precursor plays a role in the catalytic activity. It may be also observed (Table 2) that supported iron oxide aerogels and xerogels exhibit higher dispersion of the precursor phase that unsupported iron oxide. Finally, all supported aerogels exhibit high macropore volume (6 to 10 cm3 g-I), as determined by mercury porosimetry, and show the absence of any microporosity which would be
401
detected by the t-plot of nitrogen adsorption isotherm. CATALYTIC ACTIVITY OF AEROGELS AND XEROGELS It is well known that the commercial fused iron catalyst has to be thoroughly reduced (60h) by H2 at 500°C in order to present the catalytic activity (ref. 15) in the Fischer-Tropsch reaction. On the contrary, unsupported aerogel catalysts and supported xerogel catalysts (Table 3) exhibit catalytic
(re-
~ctivity
corded after Zh or after ZOh time on stream and expressed as the number of micromoles of CH 4 per minute and per gram of iron, irrespective of the oxidation state of ~ron) even without a previous reduction (pretreatment in He at 250°C during Ih) or in a mildly reduced state (pretreatment in H2 at 250°C, during 15 h). The results in Table 3 give also the activities after a strong reduction (pretreatment in H at 500°C duri~g ISh) and the nature of phases in the solids, 2 detected by XRD analysis i) after the pretreatment ii) after ZOh of reaction. For a better comparison, the results for supported aerogel catalysts are shown separately in Table 4. TABLE 3 Catalytic activity and nature of solid phases for unsupported aerogels and supported xerogels Nature of the solid
AcAcFe(H)
Pretreatment
He,2s0°C,lh
Crystalline phase (XRD) after pretreatment
after 2h
a- Fe22J
170
Catalytic activity (~mole CH4 mn-lg-IFe) after 20h 64
Fe3Q4 ;
150
75
SiFeIO(X)
AlFeIO(X)
;
~Q4
H2,sOO°C,lsh
a-Fe
60
15
~O~g;
He,ZsO°C,Ih
Fe304
50
73
~Q4
160
74
5
4
a-Fe ; X-FezC
HZ,2s0°C,lsh
Fe304 ; a-Fe
HZ,sOO°C,Ish
a-Fe
He,2s0°C,lh
a-FeZ03
400
153
Fe3Q4 ; X-FeZ C
H2,Z50~C,Ish
Fe304
800
174
~Q4;
HZ'sOO°C, ISh
a-Fe
560
100
£-FezC ;
He,2s0°C,lh
a-FeZ03
660
125
Fe304 ; £- Fe2 C
(o-AlZ03) HZ,Z50°C,15h
Fe304
HZ,sOOoC,15h
a-Fe (o-Al Z03)
Fe304 ; Fe20C9 ~o~g
; a-Fe
£-FezC
(o-AlZ03) 65
60
1075
Z34
(o-Al203)
(a)
;
~Q4
X-FeZC
a-FeZ03
AcAcFe(D)
of
X-Fe2 C
Fe304 H2,ZsO°C,lsh
Crystalline phase (XRD) after 20h reaction (a)
Fe304 (o-AlZ03) FeZOC g (o-AlZ03)
the most abundant (supported in binary catalysts) phase is underlined.
402
These results show that for unsupported aerogels [AcAcFe(H) and AcAcFe(D») a previous reduction (HZ at 500°C) is rather detrimental to the catalytic activity which is higher after a non reducing pretreatment (He at 250°C) or a mild reducing pretreatment (HZ at 250°C). After ZOh time on stream a high activity always corresponds to the presence of magnetite Fe
and only to a partial carburiza304 tion of iron. This carburization is always limited (Table 3) if the catalyst is
initially formed by iron oxides (ref. 16) instead of reduced iron after a strong reducing pretreatment (HZ at 500°C). The influence of the
~rrier
is always beneficial to the catalytic activity
as shown by the results concerning supported xerogel catalysts, reduced initially or not. The decrease in the activity between Zh and ZOh on stream always corresponds to an increase of the content of iron carbides (ref. 16). With the exception of the alumina supported catalyst the highest activities are obtained with the
unreduced (He at Z50°C) or partially (HZ at Z50°C) reduced catalysts
and only iron oxide in the form of magnetite is found in the spent catalyst together with a small amount of carbide. On the contrary, only carbides are found in the highly reduced (HZ' 500°C) spent catalysts which initially contain a-Fe. Table 4 shows the same characteristics for supported aerogel catalysts. After their preparation by the autoclave method iron in all supported aerogels is under the form of magnetite Fe304 (Table Z). After a pretreatment in He at 250°C this phase is of course conserved but it is only moderately active in the Fischer-Tropsch reaction. The important point to notice is that these unreduced catalysts do not contain carbides after ZOh of reaction [with the exception of SiFeIO(A)(ZD) where only traces of FeZOC g are found]. This behaviour is different from that of catalysts of Table 3 which are pure aerogel oxides or supported xerogel oxides. Indeed, for these unreduced (He, Z500C) catalysts carbides are found after the reaction. The lack of formation of carbides is also recorded for aerogel supported catalysts (Table 4) after a mild reduction (H 2, Z50°C). A strong reduction (H2 at 500°C) which leads to a-Fe increases the catalytic activity of aerogels [except for AlFeIO(A) (2D») and simultaneously carbides are found in the solids after ZOh of reaction. But because the phase at the beginning of the reaction is now a-Fe this carburization is to be expected (ref. 16). It appears therefore that the aerogel precursors, unreduced or mildly reduced (He or HZ at 250°C), contain initially the phase Fe
which is practically not 304 modified by the reactants (CO + HZ) after ZOh of reaction, but which is only moderately active. After a strong reduction (HZ at 500°C) all catalysts of Tables 3 and 4 contain a-Fe which is transformed into carbide by the reactants. After 20h of reaction the carbide is in general the most abundant phase [except for SiFeIO(A) (ZD»). It has been found recently that iron carbides are not the active agents of the Fischer-Tropsch synthesis (ref. 17). Their formation is, on the
403 TABLE 4 Catalytic activity and nature of solid phases for supported aerogels Nature of the solid
SiFelO(A) (ZH)
Pretreatment
He,Z50·C,lh
Crystalline phase (XRD) after pretreatment (a)
Catalytic activit, (umo Ie CH4 mn-Ig- Fe) after Zh after ZOh
Crystalline phase (XRD) after ZOh of reaction (a)
Fe304; a-FeZ03
SiFelO (A) ZD)
HZ,Z50·C,15h
Fe304
50
40
HZ,500·C,15h
a-Fe
350
ZIO
He,Z50·C,lh
Fe304
40
35
HZ,500·C,15h
a-Fe
Z80
ISO
Z300
5000
Fe304
80
5Z
Fe304
°Z,500·C,Zh
AlFe6(A) (ZH)
He,Z50·C,Zh
~Q4
a-FeZ 03
Fe304 a-Fe; FezoC 9 Fe3Q4
FeZOC9 (traces' a-Fe; £-FeZC
(a-Alz03,HZO)
(a-AlZ0 3,HZO) HZ,Z50·C,15h
Fe304
HZ,500·C,15h
a-Fe
350
ZZ4
300
149
ZO
IS
(a-Alz03,H ZO) (y-AlZ03) AlFeIO(A) (ZD) He,Z50·C,lh
Fe304
HZ,Z50·C,15h
Fe304
Fe304 (ex-AIZ03. HZO)
(a-AlZ03,H ZO) 80
Z5
45
Z5
550
1900
(ex-AIZ03,HZO) HZ,500·C,15h
ex-Fe
0Z,500·C,Zh
ex-FeZ03
FeZoC g (y-AIZ03)
(y-AlZ03) y-FeZ03
Fe304 (y- AI Z0 3 )
(a) the most abundant supported phase is underlined. contrary. responsible of the deactivation of the catalyst with time on stream. This is well observed for catalysts of Table 3 which show the decrease of the catalytic activity between Zh and ZOh of time on stream. This deactivation is however less severe in the case of aerogel catalysts of Table 4 despite the formation of carbides. Also if for these catalysts the formation of carbides could be restricted or even suppressed, an unusual activity would be observed. This is the case for the aerogel precursors of Table 4 which have been oxidized (OZ' 500·C) into a-Fe
Z03
and y-Fez0
3
previously to the reaction. The catalyst then
404
acquires an exceptional activity which increases with time on stream. After ZOh of reaction the initial oxide Fe
is reduced to magnetite Fe only and no Z03 304 traces of carbide could be identified. These results are in agreement with the
chemical analysis and with hydrogen etching of spent catalysts. The formation of carbides does not
~eem
therefore to be required for a high catalytic activity of
iron catalysts in Fischer-Tropsch synthesis. The high catalytic activity could be correlated perhaps to a better dispersion of the active phase but it seems that the beneficial effect of the oxidation treatment is correlated with the oxidation degree of iron which is here maximum (Fe
after this treatment. Now Z03) the high catalytic activity is not encountered with xerogels SiFeIO(X) and
AlFeIO(X) of Table 3 which also contain a-Fe
after He pretreatment. Moreover, Z03 the oxidation pretreatment (OZ at 500°C) applied to xerogels of Table 3 does not increase their catalytic activity which is then almost the same as that recorded
after He pretreatment of these xerogels. Any interaction between iron oxides and the support (AI Z03 or SiOZ) which would lead to the formation of an aluminate or silicate has not been detected. Also, at the present time, the peculiar behaviour of preoxidized supported aerogel catalysts could be attributed to a bprter dispersion of the active phase. It is however intriguing to observe that the phase which is found on preoxidated spent aerogels is magnetite Fe304 (whereas the initial phase is Fe Z03) which is not active to the same extent when it forms the initial phase of aerogels pretreated in He or HZ at ZSO°C (Table 4). Therefore the initially present magnetite in aerogel catalysts cannot be activated during the Fischer-Tropsch synthesis (catalysts of Table 4, unreduced or moderately reduced by HZ at ZSO°C). On the contrary when magnetite is formed during the Fischer-Tropsch reaction from the initial phase Fe
(on preoxidized aerogel catalysts) the recorded Z03 activity is very high. Now, for catalysts of Table 3 which may also contain initially Fe Z03, and Fe after the reaction, the recorded activity is much lower 30 4 than for aerogel catalysts of Table 4 exhibiting the same sequence of iron oxidized phases (Fe
initially, Fe 304 after ZOh of reaction). Z03 Finally, it should be pointed out that preoxidized aerogel catalysts show an
activity
per unit mass of iron which is 300 times higher than the activity of
commercial fused iron catalyst (ref. IS). Concerning now the selectivities into various hydrocarbons of preoxidized supported aerogel catalysts, they are not very much different from those of unreduced (He, ZSO°C) supported xerogel catalysts, as it is shown in Table 5. In conclusion, a new type of supported iron oxide catalysts, in aerogel form, in which iron is active in the oxidized state, has been developed. If the precursor, instead of being reduced, is preoxidized into Fe
(the maximum oxidaZ03 tion state of iron) the reactants (CO + HZ) of the Fischer-Tropsch synthesis at ZSO°C transform this phase into Fe304 which then exhibits a particularly high
405
TABLE 5 Selectivities into various hydrocarbons Nature of the
Selectivities (%)
solid
CI
C 2
C 3
C 4
C 5
C 6
CO
siFeIO(A) (2D) preoxidated
44.5
22.5
16.5
7.5
4.5
1.5
3
52.5
AlFeIO(A) (2D) preoxidated
46
20
18.5
6.5
4.5
2.5
2
52
SiFe"]O(X) unreduced
44.5
25
14
6
2
8.5
47
AlFeIO(X) unreduced
55
25.5
11
2
7
38.5
2
EC>I
activity without any formation of iron carbides and the catalyst does not deactivate with time on stream. This high activity and the absence of deactivation are not found on the same aerogel catalysts if the precursor is directly the Fe 304 phase. For unsupported pure oxides or supported xerogel precursors the initial presence of the maximum oxidation state of iron (Fe 203) does not induce this very high activity, neither does it prevent the formation of carbides or the deactivation of the catalyst with time on stream. REFERENCES I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17
R.B. Anderson, Catalysis, 4 (1956) 29. A. Jones and B.D. McNicol, J. Catal., 47 (1977) 384. L.J. Hofer, Catalysis, 4 (1956), 373. H. Pichler, Advan. Catal., 4 (1952) 272 M.E. Dry, Ind. Eng. Chern. Prod. Res. Dev., IS (1976) 282 M.A. Vannice, J. Catal., 50 (1977) 228. M.A. Vannice, J. Catal., 37 (1975) 449. G.B. McVicker and M.A. Vannice, J. Catal., 63 (1980) 25. A. Brenner and D.A. Hucul , Inorg. Chern., 18 (1979) 2836. S.J. Teichner, G.A. Nicolaon, M.A. Vicarini and G.E.E. Gardes, Advan. Colloid Interface Sci., 5 (1976) 245. M. Astier, A. Bertrand,D. Bianchi, A. Chenard, G. Gardes, G. Pajonk, M.B. Taghavi, S.J. Teichner and B.L. Villemin, "Preparation of Catalysts", Elsevier Sc , Publ , Corp. Ed. Amsterdam, 1976, p . 315. B. Pommier, F. Juillet and S.J. Teichner, Bull. Soc. Chim. Fr. (1972) 1268. H. Matsumoto and C.O. Bennett, J. Catal., 53 (1978) 331. J.P. Reymond, B. Pommier, P. Meriaudeau and S.J. Teichner, Bull. Soc. Chim. Fr., (1981), I, 173. J.P. Reymond, P. Meriaudeau, B. Pommier and C.O. Bennett, J. Catal., 64 (1980) 163. J.P. Reymond, P. Meriaudeau and S.J. Teichner, J. Catal, in press. J.W. Niemantsverdriet and A.M. Van der Kraan, J. Catal., 72 (1981) 385.
406 DISCUSSION P.A. JACOBS 1. If you plot the data on your catalysts according to a SchulzFlory law (log molar distribution against carbon number of products) an almost perfect straight line is obtained for the data of Table 5" In contrast to what is usually found: - no high yield of C1 is found nor is a drop in the curve at the C2 level observed. Have you any idea whether this behaviour might be related to its unusual preparation procedure? 2. You also work with extremely high H2/CO ratios (~ 9) and nevertheless find a growth factor (whiCh can be derived from the slope of the semi-logarithmic plot) which is of the same order as usually found for more classical iron-based catalysts at much lower H2/CO ratios. I wonder whether you could comment on this. 3. Is there any difference in product distribution between your unreduced and/or oxidized catalyst and the same catalyst reduced in a classi~al way ? J.P. REYMOND: 1. We observed that the schulz-Flory law is observed in an usual way for the reduced catalysts. Your observation concerning the unreduced catalysts (or preoxidized) is very interesting and may be perhaps correlated to the oxidized state (initially) of these catalysts. 2. It would be of interest to examine the selectivity behaviour for normal H2/CO ratio (3/1) which will probably induce still higher selectivity into higher hydrocarbons and perhaps olefins. 3. We reported at the Bruges meeting that the length of hydrocarbon chains is increased after the oxidation pretreatment in comparison with that observed after the conventional reducing pretreatment. L. VOLPE: You have rather large catalyst particles. Catalysis is an interfacial phenomenon. XRD gives you information about the bulk. What is the state (composition and structure) of your surface? Could it be independent of the bulk under the reaction conditions ? J.P. REYMOND: The state of the surface of our supported catalysts is difficult to study in the experimental conditions of the reaction, taking into account our facilities in XPS equipment in Lyon. However, the peculiar surface composition of the unsupported oxidized catalyst in comparison with that of the reduced catalysts was well recorded and is described in J. Catal., 1982, ~, 39-48. L. GUCZI: If I understood you correctly, the exceptional activity of your aerogel supported catalyst is due to the formation of Fe304 phase. For F.T. reaction we need a dissociation of CO which - in turns- requires metallic sites. How can you accomodate your metallic sites in Fe304 ?
°
J.P. REYMOND The problem of the dissociation of CO into C and on iron and of the redox character of this reaction was envisaged also on Fe304 as it was described in the paper presented in Bruges and also in J. Catal., 1982, ~, 3948. The thermodynamic data for both equilibria are : Fe + CO t FeO + C 2 Fe304 + CO l' 3 Fe203 + C
~Go523 ~Go523
10.6 kcal 10.8 kcal
There is no restriction to have reoxidation of Fe304 by CO into Fe203' in the same way as to have oxidation of Fe by CO into FeO. M. BAERNS: Could you please comment on the activity of the aerogel type F.T.catalyst in comparison to conventional F.T.-catalysts as they are used in the SASOL plant. What would be the required space velocity to achieve conversion in the order of 70 to 80 % ? J.P. HEYMOND This comparison has been made in our laboratory. An industrial catalyst (fused iron C.C.I.) present an activity in the differential reactor conditions, described in this paper, which is 300 times smaller than that of the silica-iron oxide aerogel (preoxidized SiFel0(A) (2D». This first catalyst is described in J. Catal., 1980, 64, 163. We did not yet examine the behaviour of
407 aerogel catalysts in an integral reactor conditions.
J. KIWI
I missed the BET area for the most active aerogel-iron catalyst used to produce methane under your experimental conditions. Could you elaborate on it ?
J.P. HEYMOND: The total surface area of the silica supported aerogel iron catalyst is 690 m2g- 1• It is reduced by 10 % by the F.T. reaction and the diameter of iron oxide particles (XRD and TEM) is not changed (d ; llb A).
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409
G. Poncelet, P. Granae and P.A. Jacoba (Editors), Preparation of Catalyst. III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SELECTIVE DOPING OF A CARBON SUBSTRATE TRANSITION-METAL AMMONIA CATALYST F.F. GADALLAH,
R.M. ELOFSON,
P. MOHAMMED
and
T. PAINTER
Alberta Research Council, Edmonton, Alberta, Canada Alberta Research Council Contribution No. 1116
ABST~CT
Ammonia catalysts designed to operate at moderate temperatures and pressures were prepared by studying the effects of various techniques of impregnating carbonaceous materials with alkaline earth ions, ruthenium ions and alkali ions. The influence of the impregnation process on ruthenium dispersions is discussed. The degree of catalytic activity for the production of ammonia and the surface area studies were the two methods used to select the best catalyst. It was found that the nature of the carbon support and the order of impregnation have significant effect on catalytic activity. INTRODUCTION A high distribution of the active components of a supported catalyst does not necessarily qualify it as the best catalyst for a particular process.
The
appropriate dispersion of these active components for optimum catalytic activity for specific reactions can be controlled by the nature of the support, impregnation method, drying, order of impregnation for a multi-component catalyst, and by activation. It is understood that every step taken during the preparation of a catalyst influences its qualities.
Every step is a critical variable which controls
qualities of the catalyst such as activity and stability. The steps involved in the development of this new family of catalysts were: (1)
preparation of the carbonaceous support (charring temperature and
activation),
(2)
of impregnation
impregnation technique, impregnating salts and the sequence (ref. 1, 2, 3).
Active carbon was chosen as the catalyst support because its catalytic power (donor acceptor properties) is often quite specific and could be controlled by the method of its preparation (ref. 4, 5). EXPERIMENTAL Preparation of the Carbon Support Although commercially available forms of activated carbon (i.e. coconut charcoal) could be used as a support material, we prepared our own.
This
exercise enabled us to control parameters such as the starting material and
410
charring temperature.
Hardwood, either maple or birch, polyvinylidene chloride
and cellulose were some of the materials used for charring.
Carbonization was
carried out using known procedures (ref. 6, 7). Because charring temperature affects the size of the graphite crystallites, support carbon was prepared at two temperatures, 600°C and 800°C, to determine if the size of the crystallites has any effect on catalyst performance. The char was then ground to the desired mesh, and activated in a
fluidiz~d
bed by a mixture of steam and air, or by carbon dioxide, at 850° to 900°C. The effect of the charring temperature on catalyst performance was studied by the ESR propert.ies of the char and its activity towards nitrogen fixation. Impregnation and Drying Three methods were tried to prepare an active and homogeneous (as close as possible to achieve uniformity) catalyst: (b)
tumbling
and
(c)
(a)
soaking and drying,
impregnation under vacuum.
The most active catalysts
were obtained from the vacuum method. Typically, the activated carbon was degassed under vacuum
(70-l00~)
at a
temperature of 250°C for a period of four hours, then cooled under vacuum to room temperature.
Barium, an amount equal to 2% of the weight of the carbon,
was added under vacuum as an aqueous solution of the nitrate, in sufficient water to cover the char (5 ml per gram of carbon).
The slurry was then kept
under vacuum and the temperature increased gradually until all the water was driven out at about l50 aC.
The product was then baked under vacuum for four
hours at 250°C and cooled.
An aqueous solution of RuC1 3"3HzO, 4% of the weight of the carbon in ruthenium, was added under vacuum. Although chloride ion might exhibit a poisoning effect on the catalyst, ruthenium chloride trihydrate was used for its greater stability over other ruthenium salts. obtaining a uniform coating of the carbon support. and then cooled under vacuum.
This is important in The slurry was dried, baked
The process was repeated a third time by doping
and baking as previously described with an aqueous solution of potassium hydroxide, 12% of the weight of the carbon in potassium. In other runs, other metal ions were impregnated in the same manner, the order of the individual impregnations was altered and the concentration of each metal varied. The highest level of activities were obtained when each salt is added separately under vacuum and when the doped product is baked. In Figure I the reactivity of the new catalysts is compared to the commercial catalyst (Top Soe) operating at Nz/Hz 1/3. The surface of the resulting catalyst was black and lustrous and gave no
411
20
,,
Figure 1. Reactivity of the commercjal and a new catalyst
,/,'7 /
, ,,
15
..
,, '
#. :I:
z
,,/, /
,, , / " /'.,..,... »>:
10 ",~
~
-----
IV
,
.;/'
5
V 0
:7 20
40
60
80
BaLaRuK (4,1,4, 14) I Equil: values at 42()OC VI New Catalyst: 4200C, 6000 s.v . VII New Catalyst: 4200C, 9000 S.v. IV Top Soe: 42()OC, 5000 s.v,
100
Patm
visual indication of any precipitated salts; no attempt was made to study the surface microscopically. about 550 - 950 m2/g.
The various catalysts displayed a surface area of
They could be stored under ambient conditions prior to
activation or under nitrogen after activation.
Catalysts prepared by this
technique have reproducible activities. Since a low evaporation rate of the solvent at a low temperature causes a high salt concentration on the surface of the porous support, and also produces a non-uniform distribution of large crystallites
(ref. 8), it is reasonable to
suggest that the high activity of these catalysts is partly due to impregnation under vacuum.
Vacuum assists penetration by drawing the air from the pores on
the surface particularly at high temperature where surface tension and viscosity of the impregnating solutions are low.
Also rapid evaporation of the solvent at
high temperatures produces an even distribution of small crystallites. Baking acts to convert the salts of some doping solutions (i.e. nitrates) to their respective oxides and apparently produces a better distribution on the carbon support.
412 Activation The catalyst was placed in a stainless steel. double-walled laboratory reactor shown in Figure 2. degassed under vacuum for four hours at 400°C and then activated with hydrogen at 12 - 25 atm. for 12 hours.
Activation was
considered complete when 95% of the chloride ion (in RuCI 3) was recovered as AgCl.
8
Figure 2.
A. Cap IThr!'laded) 1. Outer Wall 2. Heating Tape 3. Annular Space 4. Inner Wall 5. Cavity (Catalyst Bed) 6. Catalyst 7. Perforated Cap 8. Thermocouple Well 9. Inlet Gas 10. Outlet Gas
7
10
RESULTS AND DISCUSSION Ideally the active components of an impregnation-type catalyst should cover uniformly the porous structure of the support. inert carrier material.
Also the support should be an
However. this is not the case in a support like active
carbon which has catalytic activity.of its own. Effect of Charring Temperatures Since ESR properties characterize the interactions of free radicals with each other and with their surroundings. we tried to establish a correlation between the number of spins in a char before and after activation and impregnation with activity.
The results are shown in Table 1.
413
TABLE 1 Effect of Charring Temperature on ESR Properties* and Catalytic Activity 800° Char
600° Char No. Spins/g x 10 18
Sample
l. NA, ND 2. A, ND 3. A, D(Ru) 4. A, D(Ba, Ru, K)
38.0 26.0 4.3 2.9
a
Cat. Act. Y .b Eff.%c "0 abs
No. Spins/g x 10 18
Cat. Act. a Y %b Eff.%c .·abs
0.07 2.4 9.6
15. 60.
20. 74.
3.1 11.7
* We thank Dr. Karla F. Schulz for the ESR measurement and helpful discussion. 1 Not activated, not doped 2 Activated, not doped 3 Activated, doped, Ru 4% 4 Activated, doped, Ba 2%, Ru 4%, K 12% a Cat. Act. at 400°C, 50 atm., 3000 space velocity and Nz/Hz b y % (moles NH 3/moles (Nz + Hz) x 100 abs c Eff. % (actual yield/equilibrium yield) x 100
1/3
As expected the number of free spins in 600° char is much higher than 800° char.
Since activation of 600° char occurred at 850 to 900°C, the expected
drop in the number of spins occurred, but the decrease was not as large as expected (compare activated not doped 600° char with a value of 26 x 10 18 to that of non-activated, not doped 800° char of 0.07 x 10 18). Whether the chars were activated or not, they had no catalytic activity in spite of their high population of free spins.
From these results, it appeared that there is no
relation between the density of free radicals and catalytic activity.
However,
the activity of 800° char was always higher than the 600° char (Fig. 2). But, for 600 0 the decrease in the number of spins from 26 to 4.3 to 2.9 x 10 18 is indicative of a strong interaction between the donor acceptor sites of the active carbon support and the doping species. Effect of Nz~ Ratio As early as 1974 'we found that the catalytic activity of ruthenium is inhibited by hydrogen adsorption (ref. 9a-d) and the efficiency of a ruthenium catalyst could be optimized by careful selection of the gas ratio.
In other
words, the gas feed composition is as important a factor as temperature, pressure and gas feed rate in assessing the efficiency of a catalyst for a particular process.
Table 2, Figures 3 and 4 show that the inhibiting effect
of hydrogen could be countered by decreasing the partial pressure in the gas feed mixture.
Later, Rambeau and Amariglio (ref. 10) suggested that the
performance of ruthenium powder in ammonia synthesis may be strongly enhanced
414 by application of a periodic feed of nitrogen then hydrogen to the catalyst. TABLE 2 Effect of N2/H 2 Ratio on Ammonia Yield Mole Ratio
Y % abs
Eff. %
1/3 1/1 3/2
8.5 11.1 7.3
42. 78. 67.
Catalyst: 600 0 char, Ba, La, Ru and K in 2,2,4, and 14% Reaction Conditions: 400°C, 68 atm., 9000 S.v.
Figure 3. Effect of gas feed ratio on efficiency. , Top Soe s.v. 3000 N/H = 1/3 " Top Soe S.v. 3000 NIH = 1/1 III BaLaRuK (6OO'C) NIH = 1/3 IV 14.1.4,14) s.v, 9000 NIH = 1/1 V BaLaRuK (8OO"C) NIH = 1/3 VI s.v, 6000 NIH = 1/1
Effect of Promoters Chemisorption data and catalytic activity of some ruthenium catalysts are shown in Table 3 (ref. 11).
The amounts of hydrogen or nitrogen chemisorbed on
the active carbon support are negligible (a). The decrease in N -BET surface area when alkali promoters are added (first column) suggests that the micro-
415
II
12
11 10
/1
9
*-
-£ z
/
I
/
"
BaRuK (2,4,121, s.v. 7500 I NIH II NIH
/
8 7
/1
6
/
5 4
/
/
Figure 4. Effect of gas feed ratio on ammonia yield.
1/
1/3, 1/1
/
380
420
400
TOC
TABLE 3 Surface Characteristics after Activation,* and Comparative Catalytic Activity of Ruthenium Catalysts
Cata1yst
a. b. c. d. e.
a
Comparative BET Hz Percent N2 Surface Area Chemisorbed Dispersion Chemisorbed Cata1ytic b Activity (mz/g) IlM/g llM/g
Active Carbon Ru/carbon Ru, K/carbon K, Ru/carbon Ba, Ru/carbon f. Ba, Ru, K/carbon g. Ru, Ba, K/carbon h. 5% Ru/carbon (comm) * a b
967 765 710 907 662 554 932
13.8 58.0 35.7 15.4 62.5 117.0 29.5
1.3 32.0 25.0 27.0 14.0 18.0 6.0 17.0
7.0 29.3 18.1 7.8 31.6 59.1 11. 9
We thank Dr. S. Parkash for making the surface area measurements. Metal concentrations are 4% Ru, 12% K, and 2% Ba. Reaction conditions, 400°C, 27 atm., 3000 s.v. and
N2/Hz
1/3.
1.0 2.0 1.4 9.5 2.4
416
pores of the supporting material have been occupied as expected.
The decrease
in surface area is more in the case of potassium than it is in the case of barium, and there is a significant increase in metal dispersion by the alkali addition (ref. 9a).
The increased dispersion is more than four-fold in the
case of potassium as in (c) and eight-fold for barium and potassium (g).
This
suggests either a reaction between the adsorbed ruthenium and the added potassium solution followed by a redistribution of the transition metal during impregnation, or a redistribution during the activation process when the anions were being removed.
The
is greater in the case of potassium than
~ispersion
barium (d and e) and very high when both metals are used as in (g). The chemisorption of nitrogen follows a different route than that of hydrogen.
Potassium has a stronger inhibiting effect than barium, and for the
doubly-promoted catalyst, the decrease in nitrogen chemisorption is additive. Barium, not only inhibits sintering (ref. 12), it also modifies the surface of the support, especially when it is added as the first impregnating metal. Unfortunately, we did not look at the nature of this modification. The results in Table 3 clearly illustrate the very important role played by potassium and barium in controlling the dispersion (the metal crystallite size) of the active species, ruthenium, to produce the exact environment for the chemisorption of the reactants.
We found that high alkalinity is still
necessary to develop a catalyst to produce ammonia up to the equilbrium yield. Effect of Order of Impregnation Changing the order in which barium and ruthenium were added (c and d, Table 4) changed the ratio of chemisorbed nitrogen and hydrogen.
This ratio
controls the reaction rate.
Catalyst (d), with a Nz/Hz ratio of 1/3.5 has higher activity (four-fold) than (c), with a ratio of 1/19.5 toward ammonia
production.
However, the Nz/H2 adsorption ratio were measured at 77°K, and it
is not at all certain that these measurements have significance in the 673 0 to 720 0K range in which we were working. It is critical to determine how and when to add the promoters to achieve the optimum conditions for a particular
r~action.
To obtain meaningful results and
to assess the role of order of impregnation, two more catalysts in this series RU,K,Ba and
K,Ru,Ba
will be prepared and the surface characteristics and
catalytic activities of the group will be studied and correlated. We found that these catalysts are also very active in hydrogenation of carbon monoxide under mild conditions.
417
TABLE 4 Effect of Order of Impregnation on Catalytic Activity Catalyst (600°) a.
Ba K Ru
b.
c.
K Ba Ru Ru Ba K
d.
Ba Ru K
Comparative Catalytic Activity
Nz/H z Chemisorbed
1.0 1.8
1/19.5 11 3.5
3.7 14.4
concentrations as in Table 3 Reaction conditions as in Table 3
Me~al
Effect of CO The commercial iron catalyst is very sensitive to poisoning by CO, especially at high pressure.
A poisoned iron catalyst regains its activity
only slowly after treatment with pure gas for several days.
The new carbon
catalysts are less sensitive to CO, (Table 5), and regain full activity quickly when CO is removed from the feed gas stream.
It appears that CO is a temporary
inhibitor for these catalysts rather than a poisoning agent. The data in Table 5 are not taken at comparable temperatures and pressures. However, since it is generally true that the poisoning effect of CO is greatest at low temperatures and high pressures, it seems reasonable to conclude that the effect of CO on the carbon catalyst is less than its effect on the iron catalyst. TABLE 5 Effect of CO Poisoning on Ammonia Yield CO % in Gas Stream
Conditions
Catalytic Activity Remaining % Top Soe
450°C, 450°C, 500°C, 500°C, 420°C, 420°C,
100 100 100 100 70 70
atm atm atm atm atm atm
0.08 0.04 0.08 0.04 1.0 0.1
Carbon Catalyst
16 34
55 85 32 78
CONCLUSION One of the most important factors influencing the reactivity of a carbon support catalyst impregnated with alkali and transition metal ions, is the order of impregnation of the metals.
More research in catalyst design and
spectral analysis is needed to clarify the relation between structure and reactivity.
418
REFERENCES 1 G. Berrebi and Ph. Bernusset, in "Preparation of Catalysts I" (B. Delmon, P. Jacobs and G. Poncelet, Eds.), Elsevier, Amsterdam, 1976. 2 G.H. van den Berg and H.Th. Rijnten, in "Preparation of Catalysts II" (B. Delmon, P. Grange, P. Jacobs and G. Poncelet, Eds.), Elsevier, Amsterdam, 1979. 3 J. R. Anderson, "Structure of Metallic Catalysts", Academic Press, New York, 1975. 4 P. Ehrburger, OvP, Mahagan and P.L. Walker, Jr., J. Cat a l . 43, 61 (1976). 5 T. Mahmood, J.a. Williams, R. Miles and B.D. McNicol, J. Catal. ~, 218 (1981). 6 John W. Hassler, "Activated Carbon", Chemical Publishing Co. Inc. New York, N.Y. 1963. 7 R.M. Elofson and F.F: Gadallah, Can. Pat. 1094532. 8 M. Kotter and L. Reibert, 2nd Int. Symp., "Scientific Bases for Preparation of Heterogeneous Catalysts", Louvain-la Neuve, Belgium, 1978. 9a C. ERTL Cat. Rev. Sc. Eng. 21, 201 (1980). b R.M. Elofson and F.F. Gada11ah, Am. Pat. 4142993 filed December 1977. c M. Boudart, Catal. Rev. Sci. Eng., 23, 1 (1981). d Anders Nielsen, ibid, 23, 17 (1981).-10 G. Rambeau and H. Amarig1io, App1. Catal., 1, 291 (1981). 11 S. Parkash, F.F. Gadallah and S.K. Chakrabartty, Carbon 17, 403 (1979). 12 G.B. McVicker, R.L. Garten and R.T.K. Baker, 5th North Amer. Catal. Symp. Abstracts 15-10 Pittsburgh (1977).
419 DISCUSSION L. VOLPE K. Aika and A. Ozaki in the mid 70's found that Ru/C catalysts, especially when promoted with alkali metals are far more active than the iron catalyst for NH synthesis. Nevertheless would you sincerely propose that an expen3 sive noble metal could compete with the rugged cheap Tops¢e catalyst ? F. GADALLAH: For the first part of the question,Ozaki et al. impregnated their carbon support with the metals (MO) which made the catalysts ,pyrophoric. OUr catalyst is impregnated with the metal ions. Concerning the composition between the new and commercial catalysts : economic evaluation made by ARC (Alberta Research Council) found that the cost/ton NH3 is very competitive due to the high reactivity of the new catalyst. Moreover, other engineering processes, besides the fixed bed used for Tops¢e catalyst, such as fluidized bed with space velocities up to 50,000 would be suitable for the new catalyst. The high yields of ammonia would offset the high cost of the noble metal Ru very comfortably. A. KORTBEEK: 1. I have difficulties to accept a comparison between the activity of a non-noble metal such as Fe with a rare and expensive metal such as Ru, which is known to exhibit a much higher activity than Fe. Have you tested Fe impregnated on your carbon support and if so how does its performance compare with the commercial catalyst ? 2. It is known that noble metals and alkali can catalyse hydrogasification of carbon into methane. Did you observe any volatilization of the carbon support during NH3 synthesis ? F. GADALLAH 1. The Fe/C catalyst prepared in our laboratories had poor performance at the low temperatures 420°C. However, the iron catalyst is at its best around 500°C and carbon would be hydrogenated at these temperatures. 2. Carbon support gasification did not occur at temperatures up to 435°C. However, it occurs above 450°C. B.E. LANGNER: It is known that the yield in ammonia synthesis is restricted not only by the intrinsic activity of the catalyst, but by transport phenomena like pore diffusion. Did you use always the same particle size for the comparison of your catalysts with the commercial catalyst? F. GADALLAH Pore size and pore diffusion are different than particle size. We did not study the pore size but we always used the same particle size (same mesh) in our comparisons. M.V. TWIGG How tolerant are your catalysts to water vapour; if there are effects, are they short or long term ? F. GADALLAH: We expect low tolerance at low temperature; however, the tolerance increases with increasing temperature. We did not study the effect of water vapour but we used commercial hydrogen and nitrogen which contain traces of water (ppm) and there ~ere no ill effects. BAlKER Do you have any experience about the thermal stability against sintering of your carbon supported Ru particles? In particular, have you observed an effect of the alkali promoters on this stability?
A.
F. GADALLAH: As the working temperatures of 420°C the catalysts were stable. Some of the catalysts were taken off stream after eight months of continuous production without loss of activity. However, higher than 450°C methanation occurs. With K at 14% by wt. the catalysts were stable. Above 16% alkali concentration, deterioration in the catalyst occurred. Whether deterioration is due mainly to the high concentration of the alkali remains to be verified.
420 D. CHADWICK:
In Table 3 of your paper you compare activity and hydrogen chemisorption data. In all the catalysts except one the activity seems to follow the dispersion. In the exceptional case, the activity differs markedly from the other catalysts. Are you sure that the chemisorption and activity data are entirely reproducible ? F. GADALLAH: The catalytic activity of this ammonia catalyst has to be related to the chemisorption of the two reactants H2 and N2. We believe that in the Case g in Table 3 that the chemisorbed H2 covers the catalyst surface and prevents N2 chemisorption. Besides, we are very sure of the reproducibility of these catalysts. NG CHING FAI: Did you de~ermine the pore size distribution of your carbon ? Do you know whether your Ru in inside the pores or not ? F. GADALLAH We did not determine the pore size distribution, but we are going to start this study. However, in the text, we discussed the effect of promoters and we mentioned that the decrease in N-BET surface area when alkali promoters are added (first column, Table 3) suggests that the micropores of the supporting material have been occupied. Since the highest values of surface areas are when Ru alone was added, we can conclude, inderectly, that Ru goes inside the pores.
G. Poncelet, P. Gran,e and P.A. Jacobs (Editora), Preparation of CataZ)'ItBIII
e 1983 Elanier Science Pub1isbera B.V., Amsterdam - Printed in The Netherlanda
A STIJDY OF 1HE
PREPARATH~
421
AND PROPERTIES OF PRECIPITATED IRON CATALYSTS FOR
AMMONIA SYN1HESIS D.G. KLISSURSKI, I.G. MITOV and T. TOMOV+ In;;titute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040 (Bulgaria) +Institute of Nuclear Energetics, Bulgarian Academy of Sciences, Sofia 1040 (Bulgaria)
ABSTRACT 'I'h e prelmration,reduction,activi ty and therm8.l stability of ~-Fe203 based precipitated catalysts have been systematically studied. An Lncrease of both:a)the rate of reduction and b) the specific activity with the increase of the atomic number of the alkali metals used as promoters was established. A method for the preparation of catalysts with a large specific surface area,s~~able porosity and high therrrlal stability is proposed.The specimens promoted with 0,8 at.% Rb or Cs have shown a specific and total actiVity comparable with that of widely used industrial melted catalysts. INTRODUCTION The precipitated catalysts for arrunonia synthesis have not yet been used industrially ,but are considered as promising(I-5). Besides,theypermit a more uniform distribution of the promoters, a larger specific' surface area and a higher porosity to be obtained.This offers a principal possibility to achieve a higher total activity. The present communication contains results concerning the influence of the preparation conditions and of the different alkali prollioters on the final properties of this type of catalysts. EXPZRINiENTAL Catalysts
422
Type A. Different specimens were prepared by Impz-egne.t Lon with alkali hydroxide solutions of ol-Fe containing A120 (3 ! 0,2 wt. 20 3 3 %) in the form of a solid solution.Before impreenation this composition ~as heat-treated for 4 h at BOOoe.The mixed Fe 0 20 3-A1 2 3 oxides were pre~ared by thermal decomposition of the coprecipitated hydroxLdes at 20 0e and pH=9.The content of the alkali promoters \";as:O,84;I,68 and 3,36 at %,respectively.For specimens containing K as promoter this corresponds to 0,5;1,0 and 2,0 wt % of K20.
Type B. Specimens \'/i th the carne c ompos Ltions were prepared by addition of alkali metal hydroxides to the fresh coprecipitated hydroxides of Fe(III) and Al and subsequent calcination for 4 h at 800 0 e. kodel reduction measurements have been pcrf'ormed \'d th 0( -2e 20 3 aarn oLea containine 10 at.% of Le (1ja'~l..i,Na,K,Rb or Cs) and -prepared in the same manner. aethods.The activity of the catalysts was tested in a pilot-plant installatiop,in which the preliminary reduction of the catalysts waa also performed(8) .Fractions of the cate,lysts with grain size of 0,4-0,6 mm were used.The reduction was perfor1,ed in a stepwise regime for 36 h in the temperature range 300-500 0C.A s-pace velocity of the N2-H 2 mixture of 3.10 4 h- I and a pressure of 50 a tmvwez-e chosen.The activity was measured in the terrJperature range 0e 350-550 at a presBure of 100 atm and a space velocity of 3.10 4
h- I.
Mercury porosiraetry and a modified .i:l-C;T method wer-e ap o.Li.ed for determination of the pore distribution and the specific surface urea of the reduced and unreduced catalysts. The I\;ossbauer spectra were taken at room temperature, the source used being e0 57 in Pd.The conatant acceleration spectrometer vias calibruted with rJ...-ie and the centroid of its spectrum was used as a reference -point forthe isomer shift measurements. ~SULTS
AND DISCUSSION
To obtain a rraximun stabilizine effect of the textural proffioter-A1 20 conditions of its incorporation in the form of a 3,the solid aolution were stuJied.As cen be seen fro~ Jig.l .,the for~u tion of a solid solution of A120 in ol-le 20 proceeds at an ap3 3 propriate rate at BOOoe.This temperature also permits the formation of a large specific surface area of the final catalysts.
423
This indicates that in both-type A and type :a catalysts,i.1 2 0 is 3 practically completely in5,425 r------------, c or-porv t od in the f orr. of a solid solution. ~ossbauer spectra of Fe(III) 6el heated at 80, 330 and 400 0 C showed its couplete transformation to after 4 h heatine at 400 0C.The DTA curves showed the exothermal ef5,405 fects of o(-Fe 20 crystalli3 zation in the temperature 0C range 460-670 dependin to on the amount of the aluminium component in the 5,395 L--._~_.r--_.r-----' system. Obviously, this com5 10 15 20 ponent increases the stabimol% Al203 lity range of the X-rays Fig.I.Variation of the lattice ronorphous component in the system,i.e.it has a retarparameter of cl-Fe203 as a function of the A120 content after 4 h dation effect with respect 3 heat-treatment at different tempeto the crystallization. It has also been estabratures:I-5000C;2-6000C;3-800oC(and 950°C) . lished that this promoter (A1 20 has a measurable 3) effect on the rate of cGtalyst reduction. However the effect of the alkali promoters 1s more pronounced.As can be seen in Eig.2 the rate of reduction increases with increasing atomic number- of the alkali metals during the first stage of reduction.On the other hand,it was found that in the case of A type catalysts the alkali metals do not change the basic mechanism of d.. -Fe 20 reduction. 3 The Illossbauer spectra of specimens reduced up to 11,33 and 92 f. corresponded to the spectra of mugnetite,and maenetite and~-Fe, respectively( 5 ),i.e. to those observed during the reduction of ~-Fe203( 6 ).The main phase transitions durinG the reduction are: Fe203----~Fe304----~o(-Fe.Only at higher temperatures(above 500°C) the formation of an intermediate ~vstite phase was established. The kinetics and mechanism of the reduction of B-type preparations showed a detectable difference.In this case the formation ~-Fe203
a
424
of ferrites was established. In the case of LiOH additive,the fornmtion of 1iFe508 was confirmed;in the case of NaOH,NaFe0 2 and Nale50a;in the case of KOH,RbOH or CsoH-IIFe 20y1.e20,where i..e=K,Rb or ca, The kinetics of the reduction ofc(-Fe 20 impregnated with al3 kt'.li hydroxide sOlutions(IO at.% of alkali liIetal in the final ~reparation} and calcined 4 h at 800 00 is illustrated in Fig.2.
_0,9 c
e
<,
o
#0,5
0,1
°
20
40
60
80
100
'1[%0 J
Fig.2.Dependence of the reduction rate on the reduction degree Of:I-o(-Fe 20 + LiOH; 3;2-o(-ie 20 3 3-i.-:Pe 20 + NaOH;4-o(-Fe 20 + KOH;5-o(-Fe20 3 3 3+RbOH; 6-0£-Fe 20 + OsOH;1- tJ. -Fe 20 + Al 20 3; 3 3 to = 300 0 C; PH =80 rom Hg 2 As c~n be scen the initial specific rate of reduction increases in the oeQuence: H(Fe
<
<
} RFe RtFe203 + LeOH) 20 3 20 3 + Al 20 3 ".:hcre i"e= Na, K,lib or Os. On the other hand,in this case again the specific rate of reduction of the alkali-promoted specimens increases with the increase of the atomic number of the alkali metals.The stepwise reduction typical for 0(-Fe 20 and A type samples in this case is 3 much less pronounced. lor the type of preparation considered, a minimum of the reduction rate corresponding to the phase tr~nsi tionc{.Fe203~e304 Vias also observed at a degree of reductio:" = li-I3 %.This minimum is,however,only slightly expressed. This is indicative of a more complex cha.racter or the process.The kinetic curves in this case are influenced by the simultaneous reduction of 0( -7e 20 and the alka.li ferrites.The appearance of the 3
V
425
lines of ~ -Fe in the tJlSssbauer spectra of specimens reduced up to II ~ degree of reduction confirm thio conclusion(]ig.3). It is worth not inc that for ~pccimenD conIi ....."""T-.....-..-....,r--...,'''·Fe tninin c.Al 20 and ~lku 3 Ii oroJ:loters the influence of A120 on the 3 reduction rate secus to be pr-cdomfna t f ng , ") """. rr-. _ r»..... ,... r2 The structural oro\,.! 1 :"" ....! ........ ':,i I,,, r • :." .:;,j :; :.} y~: moter A1 20 nnd the al~ : 3 kali prollloters influen~ ce the values of the ':. I'.!,-.,,'" (-'.. !,-..... :~ t">.\~. :~3 •. : "',., ·w V " \. s~eeific surface areas : . . .',: :; \,II'~.:-:; '. c. , , of the catalysts to 048 -8 -4 different dee:reeD am1 in ooposite directions. V [mm/sl As can be scen froln the data in Table I these ?ig.3.fuossbnuer spectra of: I-11re 20 different influences 3.K 20 2-rH'e 20 reduced up to '? =11: ~~ arc .nos t pronounc e« in 3.K 20 3- ~ -Fe 20 + KOll, reduced UO) to =11: I~ the course of tile re3 duction. I ,
:.
'1
TABLE I surface area of the catalysts at different derrees of reduction,
~pecific
Composition of the specimens
S~ecific
'l = J. -Fe 20 3 ci. -Fe 20 3
o(-Fe 20
3
+ CsOH + esOH + Al 20 3
surface area(m 2/g) at
° % 9=5-6 %
0,26 1,05
0,83 0,9
4,6
5,7
1=
'7=IO-II % '7=98-100;' I,r 0,47 10,6
5
28,4
The difference between the promoters is more pronoWlced when comparing the total and the specific activity of tho catalysts. Data on the total and the specific rate constants for A type
426
catalysts are summarized in Table 2 Table 3.
and for B type catalysts in
TABLE 2 Data on the catalytic activity of A type catalysts(K 4oo-total rate constant at 400oC,K~OO-specific rate constant at 400 oC) Alkali promoters in at.%
Specific 'surface area, m2/g
K 400
K~OO
atmO,5/h
atmO,5/hm2
Li
0,84 1,68 3,36
23,4 18,1 13,1
84 312 472
2,6 12,3 25,3
Na
0,84 1,68 3,36
25,7 17,1 6,2
156 652 366
4,3 23,9 42,2
25,6 21,2 10,7 27,4 23,2 14,4 28,9 19,9 12,6
936 827 692 2519 2262 1787 2613 2177 1869
26,1 27,9 45,8 64,7 69,4 88,1 64,7 77 ,8 105,6
15,5
2008
62
K
0,84 1,68 3,36 Rb 0,84 1,68 3,36, Cs 0,84 1,68 3,36 Industrial catalyst CA-I
These data havG shown sGveral basic tendencies.First of all it is worth noting the increase of the promoting action with increasing atonic number of the alkali luetals.This tendency is nost pronounced when compar-Ing the specific r-a t e constants of the reaction,calculated according to the Tenkin-Pizhev equa.tion.A sharp increase of the values as a result of the transition from Ii to Cs is observed.A oecond tendency is the increase of the specific rate constants ve.Lues \'/i th the increase in the alkali metal contents.On the other hand the increase in this content leads to an unfavorable decrease in the specific surface areaS of the catalysts(Table 2 and 3).As a result the total activity of K,Rb or Cs promoted catalysts,which are i;nportant for the practice,c1ecreases with the increase of the alkali metal content from 0,84 to 3,4 at. %.In the case of Li promoted catalysts the total activity increases and in the case of Na promoted apec uaens it passes tihz-ough a
427
mnxiuum.1t should be notea,however,that the latter two catalysts do not seem promising. TABLE 3
Data on the catalytic activity of B type catalysts(K 400-total rate cons terrt at 400°C ,K:OO-specific rate constent ,at 400°C) Alkali promoters in at.%
K
Spo.cific surface area, m2/g
atmO,5/h
Ke400 E'.tm0 , 5/m2h
K400
0,84 1,68 3,36
23,0 15,4
II,I
472 430 312
12,8 18,9 19,9
Rb
0,84 1,68 3,36
24,8 16,6. 13,4
1225 II05 830
34,6 47,0 43,7
CS
0,84 1,68 3.36
28,4 17,7 10.6
2177 1787 1434
54,2 72,1 96.9
Th2 Dro~oting action of the alkali metals can be related to local changes of the \York function( 7 ).Thesc changes facilitate the electron transfer to the ni·tro~en molecules adsorbed on the catalyst.On the basis of more general considerations it can be expected that the change of the wQrk function will increase wi th the increase of the at onu c number- of the alkali aletals. Cs, which has the lowest value of this parameter has also the highest promoting effect. According to some other concepts the action of the alkali promoters should be related to a neutralization of the acidity of A120 this way the strong chemisorption of m~lonia on the ca3.1n talyst surface is avoided.?or the present this concept cannot be completely ignored.However the influence of the alkali pr-on.o teron the electron transfer in the rate liutiting stace of the reaction seems to be prevailing. The results summarized in Tables 2 ~nd 3 support the view that the content of the best prollioters,i.e. Hb and Cs,should be below 0,8 at.%.Otherwise the total activity of the catalysts decreases and the sintering processes are accelerated. It has been established that the partial sintering of the catalysts at BoOoc leads to e. significant improvement of their
428
mechanical properties.Their mechanical strength remains well over 150 ke/cm 2• of this type of catalysts is their A very Lnrpoz-t r.n t pro~)erty re3i~tivity to overheating and poisoning. For an estimation of these parameters, samples were treated for 2 hrs at 700°C or in the presence of water vapour (CH20 = 0.1 vol.%). The ratios of the specific rate constents with respect to the amillonia synthesis K' and K'400/ K 400 a s well as the spe400'IK '·400 s s cific surface arena 3~S before end after heating are given in Table 4.
TABLE 4 Date. on the resistance Alkali promoters at. %
of A type ca't a.Ly a t s to overheating
.r
_K_IOO,fa
S'
K
S
100,%
K'B
K
0,8 3,4
67 39
84 78
80 50
Hb
0,8 3,4
73 57
86 77
84 74
0,8 3,4
76 65
87 81
88 80
Cs
100,%
Ks
Table 5 contains the values of the same paror:eters before and after poisoning.
TABLE 5 Data on the resistance Alkali pr-ono t e rn , at. ,.f/0
of A type cetnlysts to poisoning .,/
K'
100,5~
-"-IOO,;~
K
s
K'
__ s_, 100,%
Ks
K
0,8 3,4
85 60
91 85
93 69
Rb
0,8 3,4
89 73
93 87
96 84
92
97
96
Industrial en tf!.l.Y.:Jt
429
The following more important tendencies have been observed: a) The chemical nature of the alkali pro;;:oter influences, although to a small extent,the resistance ae-aiDS~ overheating end poisoning. The catalysts promoted with RhOH or CsOIf showed a higher stability as com9ared to those 9romoted with KOB. b) The stability decreases with an increase i~ the content of the alkali promoter. The stability of A and 3 type catalysts, obtained by imprcgnation(A type) or impregnation and consequent sinlultaneous calcinatidn of the components are close,but B tYges are more stable. It has also been established that the resistivity of 3 type catalysts ~romoted with RbOH or CSOH(0,84 at.% alkali metals) is close to that of the industrial catalysts CA-I. As can be sup~osed,overh3ating and poisonine lead to a chaneo in the porous structure of the specimena.A cOlllplete or e. partial transfer from a bidisperse to a monodisperse porosity accompanied with a remarkable decrease of the total pore volume was observed.
600l
_50
o VfO,223cmtg .VTO,161 cmtg
->40 ';f.
30 20 10 0,01
o,os
0,3
Fig.4.Pore distribution for a doubly promoted catalyst(3,0 wt.% A1 20 + 0,64 at.
% K):
3
I-before overheating 2-after overheating at 700°C A comparison of the total and specific activity of the precipitated catalysts prepared by the proposed methods shows that
430
they are comparable with and even hither than those of some widely used industrial catalysts prepared by melting. A further improvement of the precipitated catalysts can be achieved by introduction of additional promoters. In 6eneral,the results of the present study show that the precipitated and partially sintered ammonia synthesis catdlysts are promising for industrial application. REFERENCES 1.V.S.Kornarov,E.• D.2ffros,G.S.lemeshonok,A.T.lozin,Veszi Akadernii Navuk BS8R,Serya Khim.navuk(Russian),I (1978) 15 2.A.T.Rozin,V.J.KoDarov,L.D.~fross,G.3.j~G~eshonok,J.I.~rcmenko,
Veszi Akademii Navuk 33SR,Serya Khim.naVUk,No5(IgCi),35. 3.A.T.Rozin,V.~.Komarov,k.D.Efro3s,G.J.lemeshonok,Veazi Akademii Havuk 13.3Sit,Jerya khim.navuk,No2 (1900) ,27 4. O.;~. Tihonova,~. I. Zubov<:,.,rl.~'. Lvaricva , Yu.A.l.ubchenko, She 3h.Dimi trenko,Teldu1oloeiya NeorcanicheSkykh veshtestv(Procecdings of the lJ.1.kendeleev's Institute of Chemical Technology,i,coscow), vol.1l(I973), I 23(Russian). 5.D.G.Klissuraki,I.G.b.itov,and T.Tomov,Proc.8 t h Iberoamerican ~ymp.on Catalysis,Huelva,I982(in press). 6.D. G.Klissurski, 1. G.i"i tov, and T. Tomov,Can. J .Chem. ,58( 1980 )1473. 7 . L130udart ,CataLRev. -~ci.3ng. ,23( 1981), I. 8.D.Klissurski and I .G.l,:i tov - Ln press
G. Poncelet, P. GraJlle and P.A. Jacobs (Editors), Preparation of Catalysts 111
e 1983 Elsevier Science Publishers BoV., Amsterdam - Printed in The Netherlands
431
INFLUENCE OF THE PREPARATION TECHNIQUE OF Pd-SILICA CATALYSTS ON METAL DISPERSION AND CATALYTIC ACTIVITY G. Gubitosa, A. Berton, M. Camia and N. Pernicone G.Donegani Research Institute, Montedison Group, Novara (Italy)
ABSTRACT pd-silica catalysts were prepared from various precursor Pd compounds and characterized as reducibility, metal dispersion and catalvtic activity in 1octene hydrogenation. Preliminary IRS measurements allowed a better definitionof CO chemisorption stoichiometry. A strong influence of the precursor on Pd dispersion was detected. The hydrogenation of l-octene was found to be apparently structure-sensitive. A connection with hydrogen solubility in bulk Pd was proposed to explain this phenomenon.
INTRODUCTION Supported metal catalysts are widely used in catalytic hydrogenations to produce organic intermediates and fine chemicals. In this respect,noble metals show very peculiar catalytic properties, so that they are often used in such industrial processes, in spite of their high cost (ref. 1). Obviously, it is necessary to exploit as completely as possible the noble metal loaded on the carrier, namely to obtain catalysts of very high metal dispersion. However it is advisable to be well aware of the possible shortcomings of a high metal dispersion. First, for some structure-sensitive reactions the specific catalytic activity (or the turnover number) decreases when metal dispersion increases (depending on the metal). However, it may occur that this phenomenon is more than counterbalanced by the increase of dispersion so that a net advantage is still obtained for practical purposes. Second, diffusional limitations and/or steric hindrances (when microporous carriers and large organic molecules have to be dealt with) can make most of the metal content practically inaccessible. In such cases metal distribution on the external surface of the carrier particles is to be preferred, with consequent lower dispersion (the metal content being the same). Third, the spe~ial preparation techniques, which might be necessary to have high meta.l dispersions, could produce metal surfaces with physico-chemical characteristics unfavourable for a good catalytic activity (for instance, some kind of poisoning could occur as a consequence of the preparation, or the relative percentage of the exposed crystal faces could change). Though it is convenient to have in mind such limitations, nevertheless it is a matter of fact that in many cases a high noble metal dispersion is profitable. Therefore it is necessary to know the influence of the various factors involved in the procedure of preparation, including the nature of the precursor compounds. The latter aspect will be especially considered in this paper.
432
Palladium catalysts were chosen for this study, owing to their importance for industrial hydrogenations. They are usually supported on charcoal, at least for liquid phase hydrogenations, but we have used silica as carrier, because it can be more easily characterized and, in particular, for its suitability for IRS studies. However, some comparative data will be reported coqcerning one of the Ausind (Montedison Group) Pd-C commercial catalysts currenlly used for industrial hydrogenations, including dinitrotoluene to toluendiamine and benzoic acid to , hexahydrobenzoic acid.
EXPERIMENTAL Catalyst preparation The catalysts were prepared by impregnation of silica powder (F-7 from AKZO, surface area 420 m2/g, pore volume 2.0 ml/g) with aqueous solution of H PdC1 4, Pd(NH )4C12' Pd(NH and of pentane solution of Pd(C The pr~paration 3HS)2' 3)4(OH)2 proceJures were as follows. From H PdC1 Silica was suspended in aqueous alkali solution (KOH) and 4• H PdC1 s~lution was added under vigorous stirring. Then the catalyst was filt~red, 4washed with water and air dried at 110·C. From Pd(NH3)4Cl • Silica was suspended in ammonia aqueous solution at 70·C - -2 and pH 10. An aqueous solution of Pd(NH )4Cl was slowly added under stirring. The pale-yellow solid was filtered, was~ed with water and air-dried at 110·C. From Pd(NH (OH) • Silica was previously evacuated at l20·C, then suspended 3) in CO -free wat~~~ ammonia aqueous solution of Pd(NH ) (OH) was added and 2. 3 h4e wh i~te2 so l'~d was t h en kept at 90 e C for 1 hour un d er " st~rr~ng. T t h e suspens~on filtered, washed with water and air-dried at 110·C. From Pd(C ~52~. Silica was firstly evacuated at 250°C, then suspended under nitrogen in ary pentane, A pentane solution of Pd (C H) was added at O°C. The . . .3 5 for 2 3 h ours. suspens~on was kept at room temperature under st~rr~ng The grey solid was filtered under nitrogen, washed with pentane and dried at room temperature under vacuum. All the catalysts, unless otherwise specified, were reduced with hydrogen at 400°C for 2 hours. Chemical analysis Palladium was determined by atomic absorption spectrophotometry, nitrogen by the Kjeldahl method. TPR measurements The equipment used for the TPR quantitative measurements was described elsewhere (ref. 2). A correction was applied in the runs where evolution of ammonia or propane occurred. IRS measurements A Perkin-Elmer mod. 180 spectrophotometer was used. It was equipped with a
433 quartz cell with Irtran Z windows. CRtalyst disks were obtained by tableting at Z• 500 kg/cm Static reduction was used with 100 Torr hydrogen. The gas was evacuated and hydrogen reintroduced for 3 times at the chosen temperature (400°C). Metal dispersion measurements The pulsed-flow equipment used for the measurement of palladinm dispersion by CO chemisorption at room temperature and the procedure of measurenlent were described elsewhere (ref. 3). Catalytic tests The catalytic act1v1ty was measured in the hydrogenation of l-octene in noctane solution at 30°C and atmospheric pressure. The substrate initial concentration was 0.318 mol/! in all the runs. Agitation was provided by a suitable shaker. Hydrogen was firstly added to the catalyst suspension in n-octane and stirring was started. After 1 hour the n-octane solution of l-octene was added and the extent of the reaction was followed by measuring the volume of absorbed hydrogen versus time at constant pressure. The reaction was found to be first order in l-octene. Turnover numbers were calculated from the initial rates.
RESULTS AND DISCUSSION Catalytic reduction TPR experiments were performed to get information on the reducibility of the catalysts. The main data are reported in Table 1. TABLE 1 TPR data of Pd-SiO Catalyst precursor Ird(NH ) 4] (OH)2 3) D>d(NH 4] C1 2 Pd(C )Z
it
Hld21~
Z
catalysts Pd conc.% 0.88 3.47 0.95 4.26
T
s OK
416 411 3Z8 r.t.
Tm OK 453 465 373
Molar ratio H/Pd 1.13(1)
1.00
(1) Including a small additional peak with Tm Ts T m
starting reduction temperature temperature of maximum reduction rate
It may be seen that the reducibility decreases in the order (as precursors):
The catalyst having H as precursor consists mainly of PdO, which is ZPdCl 4 easily reduced by hydrogen even at room temperature. The highest reducibility of the Pd(C3HS)2 catalyst with respect to that of
434 the catalysts prepared by aminocomplexes is to be connected with the higher stability of the Pd-NH with respect to the Pd-allyl bond. According to Yermakov (ref. 4) the interacti~n of Pd(C with silica hydroxyls gives surface spe3H5)2 cies si-0-Pd'Pc H2';-CH. . ' H2 re d ' . .1S evolved from the am1nocomplex . Dur1ng the uct10n ammonia cata I ysts. Its amount agrees satisfactorily with that determined by chemical analysis. The molar ratios NH are reported in Table 2 for catalysts with different Pd con3/Pd tents prepared from Pd(NH )4(OH) • These values are slightly higher than 2,apart from some samples prepareJ with fiigh excess of NH , thus showing that the surface species obtained is probably 3
Si -
o
Si -
<, 0.......
NH • Pd'" <,
NH
3
3
The slight excess of NH could be due to the presence of a small amount of 3the 5i-0-NH groups. Hwang, on contrary, reported (ref. 5) that a tetrammino-Pd 4 surface species is present in catalyst prepared with similar procedures. However the well-known preference of Pd++ for square coordination seems to be in favour of the surface species we have proposed. The amount of hydrogen consumed for the reduction is equivalent, on a molar basis, to that of palladium (Table 1), indicating a reaction like Si- 0 5i -
./NH "Pd 3 0....... ....NH 3
Si - OH +
H_ 2
5i -
OR
+
Pd
+
is subsequent dehydration of silica is not considered. TABLE 2 Ammonia
content of catalysts prepared from Pd(NH 3)4(OH)2
Pd cone. %
0.60 0.88 1.52% 1.92* 2.55% 4.35
Molar ratio NH 3/Pd from chem. anal. 2.4 2.3 3.1 3.2 2.7 2.2
x a higher excess of NH
3
from evolved NH 3 2.17 2.98
was used in these runs.
IRS measurements While CO chemisorption has been widely used for metal dispersion measuremen~ of palladium catalysts on various carriers (refs. 6-9), the problem of different surface species, whose relative amounts often change with both coverage and metal particle size, requires careful consideration. The common practice of reference measurements on pure metal powders can give
435 serious errors When high dispersions are to be measured. We have found an average chemisorption stoichiometry Pd/CO=1.8~0.1 on pure palladium powders prepared with ditferent methods (ref. 3). On this basis unusually high metal dispersion values were obtained on the catalysts prepared from aminocomplexes. To clarify this point, IR spectra of chemisorbed CO were recorded for some of the catalysts under study, after the same pretreatment used for the dispersion measurements. The general features of the spectra are Shown in Fig. I, Quantitative measurements of the concentr~tions of linear-bound (peak between Z020 and 2150 em-I) and bridge-bound (peak between 1750 and 2020 em-I) CO species gave the results reported in Table 3 (while the details of the adopted procedure will be published elsewhere, the behaviour of the extinction coefficients is quite similar to (hat reported in ref. 10). It may be seen that there is a higher content of bridged CO species in low-dispersion catalysts. This explains the high Pd/Co ratio found in pure palladium powders (surface area around 10 m2 / g) .
2200
2100
2000
1900
1800 1700 v (em-')
Fig. 1. IR spectrum of Pd-Si0 catalyst prepared from Pd(NH3)4C12 after hydro2 gen reduction at 400°C.
On the basis of the data reported in Table 3, if we assume that no Pd atom is involved at the same time in linear and bridged chemisorption, a stoichiometry Pd/co = 1.2 should be used for well-dispersed samples. This result is partly supported by the comparison between oxygen and CO chemisorption on a catalyst prepared from Pd(NH )4(OH) : the volume of chemisorbed CO was exactly 3the two times that of oxygen. As st~ichiometric Pd/O ratio is usually considered to be nearly 1 (ref. 3), the same seems to occur for the ratio Pd/co in our well dispersed samples. Though the difference is small, to reconcile this result with that of IRS measurements one should admit that in fact a fraction of Pd atoms are involved at the same time in linear and bridged CO chemisorption. As an average value, a chemisorption stoichiometry Pd/co 1.1+0.1 was used to calculate dispersion in our well-dispersed catalysts.
436 TABLE 3 IRS data of CO chemisorption on Pd-Si0
Catalyst precursor
Pd conc.
2
catalysts
Linear CO
%
%
Bridged CO %
Volume of chemisorbed CO, cm3 j g Pd From IR data
From adsorption measurem.
[Pd(NH 3) 4] (OH)2
4.35
82
18
145.4
121.3
[Pd(NH 3) 4JC1 2
4.02
81
19
126.9
117.7
4.26
58
42
5.9
14.1
0.60
79
21
117.7
122.6
HldC1
4
[Pd(NH 3)
J
(OH)2
Metal dispersion The influence of the nature of the precursor on palladium dispersion after reduction by hydrogen at 400·C is shown in the upper part of Table 4. The dramatic effect of the precursor is clearly seen. Of special interest is the comparison between cat~lysts preftared from H (through PdO) and from 2PdC14 aminocomplexes (through ~~-O::Pd~N 3 surface species). We have found that . . . 1-0 NH Pd0 cata 1ysts are eas1ly reduced by 3hydrogen even at room temperature, glv1ng good metal dispersions, but dispersion strongly decreases when the reduction temperature is increased. On the contrary, Pd catalysts from aminocomplexes are not reduced at all below about 100·C, but, once reduced, they maintain high dispersions even at high temperatures. The different degree of dispersion of the precursor species could simply explain this behaviour. However it is also possible to explain these phenomena if we admit that absorbed hydrogen, with formation of Pd hydride species, stable only in the lower temperature range, increases the mobility of Pd atoms. Unexpected sintering phenomena of Pd crystallites when hydrogen-treated at room temperature have been recently considered from a similar point of view (ref. 11). For catalysts having to work at relatively high temperatures, there should be therefore a definite advantage to use preparation methods giving surface precursor species not easily reducible, when high dispersion is needed to have good activity. The lower dispersion of the catalyst prepared from Pd(C H ) , with respect . 1 . . . 3 W1t 5. 2h the a b ove exto t h ose prepare d f rom am1nocompexes, 1S qU1te cons1stent planation, if the TPR data (Table 1) are taken into account. Moreover, the data reported in Table 4 show that, for catalysts prepared from Pd(NH , 3)4(OH) the metal dispersion does not depend on the metal content (up to 4.5% Pd) ana that the highest dispersion is obtained after reduction at temperatures around 300·C. It is to be remarked also that the commercial catalyst at 400·C still retains a satisfactory dispersion, though supported on charcoal, where selective anchoring of well-defined surface species is a very difficult task. Catalytic activity The absence of diffusional limitations was indicated by the following results:
437
TABLE 4 Dispersion Precursor
data of Pd-Si0 Pd cone. %
H/dC1
4
Pd(C
3H5)2 Pd(NH ) C1 • 3 4 2 Pd(NH 3\(OH)2 Pd-C(2) Pd(NH 3)4(OR)2 Pd(NH 3)4(OR)2 Pd(NH 4 (OR)2 3) Pd(NH 3)4(OR)2
catalysts
2
Reduction temp. °c
Pd surface area (1) m2 / g Pd
Dispersion
4.26
400
42.8~
0.09~
0.95
400
49.9~
0.10
4.02
400
280.5
0.62
4.35
400
289.1
0.64
4.95
400
129.4
0.28
0.60
400
292.1
0.65
1.52
400
287.2
0.64
0.60
200
224.0
0.60
300
344.4
(1) On the basis of 1.26.10
19
surface Pd atoms/m
~
0.77
2
(2) Ausind (Montedison Group) MPT5 commercial catalyst x Calculated from chemisorption stoichiometry Pd/CO
=
1.4
a) straight-line dependence of reaction rate on catalyst mass; b) no difference in turnover number for two catalysts differing sevenfold in metal loading, but having the same dispersion (ref. 12). A threefold increase of turnover number was found (Fig. 2) in the dispersion range investigated (0.1-0.6). In particular, the dependence of turnover number on palladium dispersion occurs more markedly (Fig. 2) in the low dispersion range, wh~re it should not be expected, as only small metal clusters (below about 50 A, corresponding for Pd toa dispersion of 0.22) can justify structuresensitivity coming from surface geometry (ref. 13). Therefore it seems more reasonable to explain this apparent structure-sensitivity through the influence of hydrogen solubility in Pd. In fact, our experimental conditions for l-octene hydrogenation are included in the region of stability of the J3 hydride phase (ref. 14). Several authors (refs. 15-17) have found that hydrogen solubility in Pd, with formation of)9 hydride, decreases when metal dispersion increases. It is usually considered that in the presence offthydride the Pd' catalytic activity decreases (ref. 14), but some opposite results have been reported also (refs. 18-19). Data of hydrogen solubility taken from ref. 16 are reported in Fig. 2. The very similar correlations of both turnover number in l-octene hydrogenation and hydrogen solubility with Pd dispersion give support to our explanation, though it cannot be considered as a definite proof.
438
25·---------·-·--"0
07
!:: J:
0.6 ';: 05 04
=-6
.0
.
03 ~ 02 01 0+--,--------,----,-----,--,--------,----;------'-0 o 0.1 0.2 0.3 04 05 06 0.7 Dispersion
E
£
Fig. 2. Turnover number in l-octene hydrogenation (full line) and hydrogen solubility (dashed line) as function of metal dispersion in Pd-Si0 catalysts. 2 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19
C.L. Thomas, Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970, chapt. 12-13. A. Bossi, A. Catta1ani and N. Pernicone, in Thermal Analysis, Proc. 7th ICTA, Kingston, Ont., Canada, August 22-28 , 1982. N. Pernicone, U. Guacci, R. Barisone and F. Traina, Proc. 7th Ibero-American Symp. Catalysis, La Plata, Argentina, July 13-18, 1980. Y.I. Yermakov, B.N. Kuznetsov, Y.A. Rindin and A.M. Lazutkin, Kinetics Catalysis (English Trans1.), 14(1973)1413. H. Hwang, Thesis, Stanford University , 1976. J.J.F.Scholten and A. Van Ilontfoort, J. Cata1, 1(1962)85. H. Kra1, Z. Physik.Chem. N.F., 48(1967)129. E.G. Schlosser, Chem. lng. Techn., 39(1967)409. D. Pope, D.L. Smith, M.J. Eastlake and R.L. 110ss, J. Cata1., 22(1971)72. M.A. Vannice and S,Y. Wang, J. Phys.Chem., 85(1981)2543. A. Janko, W. Palczewska and 1. Szymarska, J. Cata1., 61(1980)264. E.E. Gonzo and M. Boudart, J. Cata1., 52(1978)462. R. Van Hardeveld and F. Hartog, Surf. Sci., 15(1969)189. W. Pa1czewska, Advances in Catalysis, 24(1975)245, Academic Press, New York. P.C. Aben, J. Cata1., 10(1968)224. M. Boudart and H.S. Hwang, J. Cata1., 39(1975)44. R.K. Nandi, R. Pitchai, 5.5. Wong, J.B. Cohen, R.L. Burwell Jr. and J.B. But~ J. Cata1.,70(1981)298. A. Borodzinski, R. Dus, R. Frak, A. Janko and W. Pa1czewska, in e.c. Bond, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Intern.Congr.Catalysis, London, July 12-16, 1976, The Chemical Society, London, 1977, p. ISO. I. Yasumori, in G.C.Bond, P.B. Wells and F.C, Tompkins (Eds.),Proc. 6th Intern. Congr.Catalysis, London, July-12-l6,1976, The Chem.Society,London,1977,p.15B.
439 DISCUSSION G.C. BOND: The 6-PdH phase is unstable above about 80°C. If you are right in associating NT with H2 solubility, a study of the same catalysts above this temperature (perhaps in a gas-phase reaction) should show no dependence of NT on D. It is also easy to measure directly the amount of 6-PdH formed by a TPR-type experiment: after heating in H2 to achieve reduction of the precursor, the 6-PdH phase is formed on cooling to below about 80°C, and its formation is revealed by a further H2 uptake. On a second heating, the B-PdH again decomposes. It is thus very simple to obtain direct confirmation that the solubility of H2 is dispersion-dependent, as you suggest. N.-PERNICONE : Thank you for your suggestions. I agree that some experiments of gas-phase l-octene hydrogenation over 100°C on the same catalysts could in fact give further information. That H solubility in Pd catalysts is disper2 sion-dependent was in fact not suggested by us, but experimentally demonstrated by others (see refs. 15-17), so that we concluded that no further confirmation was needed. -1
L. GUCZI: CO stretching frequency is normally 2045-2060 cm Why is your value around 2100 cm- 1 ? Acetylene hydrogenation on Pd black and Pd/A1203 catalyst proved to be dependent on wheter H2 or D2 is used. In the presence of D2' the reaction rate is 5 times lower than for H2. In accordance with Palczewska's data, we explained this difference by the absence of hydride (deuteride) formation in the presence of D2• N. PERNICONE From the most recent data (1-2) it can be concluded that at high CO coverage the high-frequency IR absorption band is located very near to 2100 cm- l and that a decrease of its frequency occurs when coverage decreases. Our experimental conditions correspond, in fact, to high CO coverage of the Pd surface. Thank you for your comment, which gives further support to our interpretation of our activity data. More direct evidence for the positive effect of the H2 pretreatment of Pd catalysts on olefin hydrogenation activity has been reported recently (3). 1) D.R. Kember and N. Sheppard, J. Chern. Soc. Farad. Trans.II, 77 (1981) ,1309. 2) M.A. Vannice, S.Y. Wang and S.H. Moon, J.Catal., 71 (1981), 152. 3) R.L. Augustine and R.W. Warner, J. Org.Chem.,46 (1981), 2614. S. VASUDEVAN: I have a comment on Prof. Guczi's remarks. We have done hydrogenation of pure 1-butene and i-butene formed selectively from 1-butyne H2 H2 (l-butyne ---- 1-butene --- n-butane) and found that the turnover number (TON) for hydrogenation of pure 1-butene and 1-butene ex 1-butyne were constant when varying dispersion, whereas hydrogenation of other highly unsaturated diolefins and acetylenics shows a strong structure-sensitivity (there is a decrease in the TON by a factor of 12 to 15 at very high dispersions). (Reference: S. Vasudevan, Doctorate Thesis, IFP/ENSPM, France, 1982). Further, I would like to point out certain similarities between your study and our work, which was presented at this Symposium (Baitiaux, Cosijns and Vasudevan), especially concerning the Pd/CO ratio of 1.0 and the form of IR spectra of CO chemisorption over highly dispersed Pf catalysts. Literature studies as well as our results show that the order of reaction with respect to the hydrocarbon is zero, but your results indicate a value of 1.0. Could this order of reaction be due to some poisoning caused by impurities in H2 or hydrocarbons? This could be the reason why you find a decrease of TON at higher dispersions, as, at higher dispersion the catalysts are more susceptible to certain poisons than low dispersion catalysts. You know that PdO reduced at low temperature gives high dispersion but catalysts prepared via Pd(NH3)4+ give high dispersion only at high reduction temperatures. Did you
440 calcine this catalyst before reducing ? It is known that pdO can be reduced easily at low temperature, but Pd(NH3>!+ has first to be decomposed to give easily reducible species. N. PERNICONE I agree with you that zero hydrocarbon order is more common in olefins hydrogenation. However your suggestion about our data cannot be accepted for the following reasons : -due to our particular hydrogenation procedure, impurities in hydrogen and/~r solvent,if any, cannot give an apparent first order, because enough time available for their interaction with pd before starting the reaction; -from the results of the experiments carried out at different initial concentrations of l-octene, the presence of poisons in the olefin can be excluded. Apparent orders lower than 1 can be found, in our opinion, when too high reaction rates are used in the kinetic experiments. We have not calcined our catalysts during their preparation. One of the results of our work is that it is possible to obtain high Pd dispersions starting from amino-complexes without any need for calcination.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
441
l'nEPARATION OF NON-PYROPHORIC ME;-l'ALLIC CATALYSTS A.V. KRYLOVA, G.A. USTILffiNKO and N.S. TOROCHESHNIKOV D.I.Llendeleyv's Institute of Chemical Engineering, Moscow (U.S.S.R~
ABS'l'RACT The data on the mechanism of passivation and pyrophoric oxidation of ammonia synthesis catalys~obtained by various physico-chemical methods have been generalized. It has been shown that the participation of weakly-bonded adsorbed oxygen in passivation is responsible for a long duration of the process and instability of the protective layer. The temperatures have been determined (-195°- -85° and 450-550 0C) at which thin protective layers with an enhanced stability to oxidation are rapidly formed on the catalyst surface. Effective methods have been proposed for the elimination of the ?yrophoric nature of the ammonia synthesis catalyst and of other catalysts.
IN'rRODUC TION Many metallic catalysts are pyrophoric. Preparation of non-pyrophoric catalysts is very important for reduction of the catalysts outside the converter, for repair work inside the converters and for removal of spent catalyst. Non-pyrophoric catalysts are prepared by passivation in a nitrogen flow with a low oxygen concentration at 20-60 0C [1J • Passivation is a long and complicated process which is carried out under emnirically chosen conditions, requires an exact content of oxygen in the nitrogen flow, and does not provide for a stability of the protective layer. Passivated catalysts undergo further reoxidation and overheating when the storage conditions are violated. The aim of this study was to consider the mechanism of oxygen interaction with the ammonia synthesis catalyst under passivation and oxidation within a wide range of temperatures and partial oxygen pressures and to develop new effective methods of preparing non-pyrophoric catalysts.
442
The samples of the industrial aWfionia synthesis catalyst contained as promoters 3 mass % of A120 , 1% of K20, 2.5% of CaO, 3 and 0.7% of 5i02• The weighed samples from 5.0 to 1300 g, fractions 2-3 rom were used in the experiments. The details of studying the interaction of oxygen with the catalyst in the course of passivation and the properties of passivated catalysts with the aid of various physico-ehemical methods have been published earlier [2-8] • To study pyrophoric properties, use was made of a flow-type installation into the reactor of which (with a reduced sample) a flow of dry air was delivered at a preset velocity. 'I'her-mograms o f heating, oxygen consumption, and change in the catalyst weight were determined during oxidation. The pyrophoric properties were characterized by a maximum temperature of pyrophoric heating of the sample (Tmax) , time of oxidation (q;) determined by completion of oxygen consumption, and the oxidation degree of the catalyst (~) calculated by add-on weight of the sample.
RESULTS The passivation mechanism According to the pulse and microcalorimetric methods [2-3] , passivation of the catalyst at room temperature is caused by adsorption of 7-10 molecular layers of oxygen as calculated for metallic surface. The oxygen of the passivating layer is non-uniform. Approximately one oxygen layer is weakly bonded to the surface and is removed from the sorption centres in helium and hydrogen flows at temperatures up to 200°C; 2-4 layers are mobile and can be removed in a helium flow at 400-500°C; 4-6 layers can be removed only by hydrogenation and are, therefore, the oxygen of oxide. The initial oxygen adsorption heat is close to the known value of the formation heat of iron oxide and is 95±5 kcal/ /mole. In the process of filling the passivating layer the heat of oxygen adsorptiori was observed to be less than 20 kcal/rnole. The formation of different states of oxygen under passivation conditions was confirmed by the methods of isotopic exchange [4] . Fig.1a illustrates the kinetic curves of a homomolecular oxygen exchange on a passivated catalyst which proceeded at a high rate and low temperature s, was charac terized by an ac ti vation energy of about 2 kcal/mole and, consequently, pointed to the participation of weakly-bonded oxygen in passivation. The exchange reaction was
443
terninated when carbon oxide, hydrogen, or water was introduced into the system because of the replacement of this form of oxygen. Fig.1b shows that the exchange rate decreased rapidly when the catalyst was kept in oxygen for 24 hours; then the decrease was slower. After contacting the catalyst with oxygen for 8 days, however, the exchange rate exceeded the rate in blank experiments by an order of magnitude.
a)
0,6
b)
7)5 ~
c:u ~
~
~
0,+
.5,0
~.... ~
I ~
'-
..... 25 1::::)'
-
,
~O,2
-
~
1
2
3
4
5 cr,mln
2
6
8 ~dogs
Fig. 1. Hornomolecular oxygen exchange on passivated catalyst: a) the effect of the impurities on the exchange kinetics; b) the effect of the catalyst exposure to oxygen on the exchange rate. This points to a low rate of transition of weakly-bonded oxygen into strongly-bonded one. The number of the molecules participating in a heteroexchangeat low temperatures, calculated by a change in isotope content, was 0.1-0.5.10 19 molecules 02/m2Fe, i.e. about half of the molecular layer in accord with the data on desorption of weakly_bonded oxygen. On the catalysts in an oxide form and on partially oxidized with oxygen at 300-500 oC passivated catalysts no" exchange was observed at low temperatures and only within the temperature range from 300 to 500°C the exchange took place with an activation energy of 30 kcal/mole. Thus, as follows from the" data on the exchange reactions, oxygen on the surface of passivated catalyst is not identical to oxygen of iron oxide. Unlike the passivation at room temperature, passivation at 80-100 oC decreased the rate of low-temperature isotopic exchange and strengthened the bond of the oxygen with the catalyst.
444
The electron work function measured upon oxygen adsorption within the temperature range -30-180 oC on an iron catalyst [5J also indicated that oxygen is present in different states: adsorbed negative dipoles, formed at the initial moment, passed into dissolved or oxide oxygen (partially at low temperatures and completely at elevated temperatures). The study of industrial passivated catalyst by thermodesorptir on and deri vatographic methods [6J has shown that passivated catalysts contain a great amount of admixtures, including more than 1 mass % of wate~, and are stable to oxidation in air below 107°C. Adsorption and other methods in which deuterium and oxygen isotopes are used have demonstrated that water may remain in the catalysts after reduction, be formed in the process of passivation and adsorbed upon contact with air. The relationships for adsorption, nature of the oxygen bond in passivated catalysts, thermal stability, evolution of the products of destruction of t)le passivated layer and admixtures found for industrial and model ammonia synthesis catalysts with a modified composition of promoters proved to be similar. The data considered allow a conclusion to be drawn that the forrnationof weakly -bonded oxygen, its slow transition into a strongly-bonded state, and possibility of its replacement with admixtures (water, hydrogen, and others) are responsible for a long duration of the industrial process of passivation and for an instability of the protective layer. Pyrophoric properties of the catalysts It is known that the contact of catalyst with air under temperature conditions of passivation results in pyrophoric oxidation. The main specific feature of pyrophoric oxidation consists in that the reaction proceeds at an increasing temperature. Using in our experiments the ammonia synthesis catalysts, low-temperature and high-temperature shift catalysts, catalysts for methanation and others, we have established that pyrophoric properties depend mainly on the concentration, dispersion and type of the metal in the catalyst; for the same catalyst it also depends on the degree of reduction and space velocity of air. Tmax may vary by hundreds of degrees depending on these factors. Pyrophoric nature of the ammonia synthesis catalyst depended slightly on the composition of the promoters and on the presence
445
of the residual hydrogen. When the catalyst fraction decreased from 4-5 n~ to 0.5-1 rom, Tmax increased by 70°C.
-1,0 2,30 2.20 ~
+~
Q
-~ .~.
,":".: "
+1,0
"
2,-to
. . .;.:.
45'OO~90 '''.\
~;
~
~
-,
4:80
~ I ..
470
' .
I •
'J. mm/s
/ .•...... ..; 4
.:..{' r" 2
'-". :
:
::
:".:: I
' . ' '. , .
2~9~ .....: " ", ..-. .'._.'. ..... ..... . ......'......3 219 ~Og _ ~ ~ ~ .{. ': .: ::,:..,\: .~"
t
~~
I r ·:~·: :·-· · /·': '·\ ~~ 1~
I
4 Fe~O~
6
Fig. 2. M~ssbauer spectra of the catalyst after pyrophoric heating up to: 1-150°, 2-225°, 3-350°, 4-400°. Fig. 2 shows Mossbauer spectra of the ammonia synthesis catalyst subjected to pyrophoric oxidation to temperatures of 150-400 0C [7J The spectra points to the formation of the magnetite phase during pyrophoric heating. The spectrum of the samples after slight heating displayed only one, the most intensive, peak 6 of magnetite corresponding to the beginning of oxidation. At higher temperatures intensity of this peak increased and the peaks IA and IB appeared. A change in the spectrum parameter H (the effective ef f magnetic field) pointed to an increase in size of crystallites as temperature of oxidation increased. ThUS, pyrophoric oxidation results in a fast formation of the oxide phase in the catalysts. Short-time heating of the catalysts in air up to 400°C did not affect the activity. Heating in deriva~ograph in an air flow up to 900°C resulted in (according to the data obtained on a scanning microscope [8] ) sintering and in a redistribution of the ingradients in the catalyst (enrichment of the surface with iron). As a whole, the data on studying passivation and pyrophoric oxidation of the catalyst with air have shown that temperature and
446
pressure of the gas are the main factors determining the nature of the bond of oxygen with the catalyst. Preparation of non-pyrophoric catalysts One may assume that in the process of passivation oxidation of the bulk of the catalyst is slowed down because of a low oxygen pressure. The same factor determines slow oxidation of the sUJ;face and instability of the protective layer. The process of surface oxidation can be accelerated and bulk oxidation slowed down either when pr~ssures of oxygen are high and temperature low or at high temperatures when the rate of surface oxidation exceeds that of oxygen diffusion into the bulk of the catalyst. In connection with this the interaction of the ammonia synthesis catalyst with oxygen was studied at temperatures being varied from 20 to -195°C and from 20 to 550°C under different concentrations of oxygen from air to pure oxygen.
x,% 4D 30
-too -100
0
iOO 200 . . 48Q SOO
18,oe
Fig. 3. The oxidation degree of the ammonia synthesis catalyst as a function of temperature of contact with air. Fig. 3 shows the oxidation degree of the industrial ammonia synthesis catalyst as a function of the temperature of contact with air. Two temperature regions (-85- -195°C and 450-550°C) were revealed in which the contact of the catalyst with air leads
447
to a low oxidation degree (3-4%) typical for the process of preparing passivated catalysts. Within the intermediate temperature region the interaction of the catalyst with air results in considerable oxidation of the catalyst. Near~5°C the transition point is established from a low (3%) to a very high (36%) oxidation degree. The maximum value is observed near 225°C. With further increase in temperature the value of ~ gradually decreases to about 4% at 500-550°C. The temperature dependence found did not change its character when oxygen content in air increased or pure oxygen was used. The results obtained show that for obtaining protective oxygen layers on the surface,it is reasonable to use the temperature regions corresponding to the minimum oxidation degree of the catalysts in which the possibility of local overheating is excluded because of a fast termination of the oxidation reaction. The study of pyrophoric properties of the catalysts has confirmed the efficiency of the processes of preparing non-pyrophoric catalysts within the above temperature range. Table 1 lists the values characterizing low-temperature interaction of the catalyst with air. '1'ABL.I:!l 1
Pyrophoric properties of the catalyst at low temperatures Temperature of primary contact wi th air T ,oC
Maximum tem¥erature of heating , °C max
Time of oxidation q: , min
Degree of iron oxidation X. , %
0 -83 -85 -87 -136 -195
372 366 35 27 30 31
36 42 1.5-2 1.5-2 1.5-2 3
33 36 3.1 3.0 3.1 3.3
0
The treatment of the catalyst with air at temperatures below -85°C makes it possible to obtain non-pyrophoric catalysts instantaneously (for 1.5-2 minutes). Passivation of the same amount of the catalyst by the traditional method requires 2-3 hours. Stabilization with air of copper-containing shift catalyst took place in a wider temperature range from -195 to -40°C for several minutes and resulted in a degree of copper oxidation in thecatalyst of about 7%. At temperatures from -39 to 25°C the reaction
448
time increased up to 20 minutes and interaction with air led to pyrophoric heating up to 230-285°C and to almost complete oxidation of copper in the catalyst. 'rable 2 lists the values which characterize the process of high-temperature interaction of the ammonia synthesis catalyst with air. Heating of the catalyst which takes place" in air flow is almost completely ruled out when air is fed portion-wise. TABLE 2 Pyrophoric properties of the catalyst at high temperatures Temperature of the primary contact wi th air To' °C
Maximum temperature of' heating T" °C max,
Oxidation time ~ , min
Degree of iron oxidationX, %
flow 500 of air 500 portions 550 of air
511
5.5 32 25
6.0
569 505
400
2
555
3.2
4.8
5.9
The catalysts treated with air at 400-550 0 C did not exhibit pyrophoric properties and the formation of the protective layers proceeded at a high rate. Activity of the catalysts stabilized both at low and high temperatures, does not differ from that of industrial passivated catalysts.
500 200
100 iO
20
30
40
Fig. 4. The temperature of oxidation the samples of the catalyst stabilized by the industrial (1), low-temperature (2) and high-temperature (3) methods.
449
Fig. 4 shows the temperature of oxidation obtained upon heating different stabilized samples in air. Contrary to the industrial samples, the initial temperature of oxidation of the catalyst after low-temperature stabilization is 117°C and that of the catalyst after high-temperature stabilization is 230-240°C. Thus, the study of the mechanism of passivation and pyrophoric oxidation made it possible to substantiate the choice of conditions for effective preparation of non-pyrophoric catalysts. Preparation of non-pyrophoric aIIlmonia synthesis catalysts by t;eating in air at low and high temperatures reduces the time of passivation, does not require pure nitrogen, and ensures high stabili ty of the catalysts to oxidation. For the low-temperature shift catalyst the possibility is shown of using one of the methods described for other catalysts.
REFERENCES 1 Catalyst Handbook with Special Heference to Unit Processes in Ammonia and Hydrogen Manuf ac tur-e , Wolfe Sci-Books, London, 1970, p. I78. 2 A.V. Krylova, V.V. LIorozov, S.S. Lachinov, N.S. Torocheshnikov, React.Kinet.Catal.Lett., 9(1978)125-130. 3 A.V. Krylova, V.I. '[sarev, B.L. Aptekar', N.S. 'l'orocheslmikov, Izv.Akad.Nauk SSSR, ser.khim., 7(1981)1658-1660. 4 A.V. Krylova, L.A. Kasatkina, V.V. Morozov, All-Union Conference "Isotopic Methods in Studyine; the Mechanism of Catalysis", Novosibirsk, December 5-9, 1980, Preprint 33. 5 G.1. Kovalev, Yu.B. Kagan, A.V. Krylova, Kinetika i Kataliz, 11(1970)1505-1506. 6 A.V. Krylova, S.S. Lachinov, N.S. Torocheshnikov, N.N. Voldunyanin, Froc. 6th Int.Congr.Catalysis, London, Jule 10-16, 1976, Elsevier, Amsterdam, 1977, pp. 717-726. 7 T. Peev, A.V. Krylova, G.A. Ustimenko, Kinetika i Kataliz, 21(1980)1346-1350. 8 L. Patyi, V.I. Tsarev, A.V. Kry10va, D. Oravets, Zs. Farkas, and N.S. Torocheslmikov, React.Kinet.Catal.Lett., 12(1979) 165-170.
450 DISCUSSION L. VOLPE: 1. On pyrophoric samples, it is the rate of 02 supply to the solid that is important rather than the Po or the total flow rate alone. Do you agree that it is the combination of P02 an~ flow rate that control the passivation process? 2. Some of our pyrophoric Mo and W samples after low-temperature treatment in air like in your Fig. 3 ignite when brought up to room temperature. Do you have similar experience ? 3. It is astonishing that you haven't bulk oxidation of NH3 catalyst in air a~ T > 450°C. How do you explain this ? A.V. KRYLOVA : 1. Yes I do. But according to our data it is the temperature of the interaction of ~he iron catalyst with oxygen which is the most important in the oxidation or passivation, and after that the combination of the pressure with the flow rate of oxygen. 2. There is no such phenomenon with the ammonia synthesis industrial catalysts, copper-containing shift-reaction catalysts and SOme others. If we observe ignition of some catalysts when the temperature ,increases to room temperature, we try to use some additives which do not change the catalytic activity but stabilize the catalysts. 3. I think that the surface oxidation rate of iron at these temperatures is higher than the rate of diffusion in the bulk and this is why we have the thin protective layer on the surface. There are different methods to stabilize this layer and to obtain the well stabilized catalysts.
451
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V 0' Amsterdam - Printed in The Netherlands
@
EFFECT OF METAL-SUPPORT INTERACTION ON THE CHEMISORPTION AND CO HYDROGENATION ACTIVITY OF FeRu CATALYSTS
L. GUCZI, Z. SCHAY and I. BOGYAY Institute of Isotopes of the Hungarian Academy of Sciences, H-15Z5 Budapest, P.O.Box 77, Hungary
ABSTRACT The formation of the active phase during different pretreatments of supported iron-ruthenium bimetallic catalysts, and correlations among the temperature programmed reduction (TPR) spectra, hydrogen TPD, 0Z' HZ and CO chemisorption, as well as catalytic activity and selectivity revealed in CO + HZ
reactio~,
have
been investigated. The catalysts prepared from aqueous solution of RuC1 and 3 Fe(N0 (I) and from RU and Fe(CO)lZ (II), were decomposed in oxygen 3)3 3(CO)1Z (570 K and 770 K), or in hydrogen and helium, prior to the hydrogen TPR measurements. Oxygen treatment causes a decrease in the adsorption of HZ' 0z and CO, and simultaneously increases the amount of the activated form of hydrogen. The analysis of the nature of the oxygen adsorbed at room temperature reveals considerable differences between surface structures obtained with different treatments. The catalytic
actiVity of CO + HZ reaction is higher on catalyst (II), which
can be understood in terms of metal-support interaction (MSI). Due to the secondary effect of MSI, the particle size can be stabilized. However, this effect is opposed by the decrease in the amount of weakly adsorbed hydrogen which is available for reaction. All these effects are reflected in the change of the nature of the catalysts, i.e. predictions can be made for the catalytic behaviour simply by studying the catalysts with the methods mentioned.
INTRODUCTION Two basic methods are used for the preparation of mono- or bimetallic catalysts : (i) impregnation of a support with aqueous solution of metal ions followed by drying, calcination and reduction, and (ii) the use of organometallic complexes in which the metal is in a zero valent state. The dispersion of the supported metal catalysts formed after the final reduction treatment is controlled by several factors. One of the most important is the initial deposition of the metal ions, as mentioned earlier (ref.1). Increased dispersion can be achieved by chemical interaction between ions and particular sites of the support,
452 e.g. hydroxyl groups (ref. 2,3). The initial distribution of the metallic ions can also be influenced by the pH value of the solution (ref.4). The ion-support interaction is also a factor here, due to coordinatively unsaturated sites which strongly hold ions, preventing their migration during the following treatments (ref. 5). During calcination, metal ions are converted into the highest' oxidation state and sometimes agglomerate to form large oxide crystallites. The extent to which this occurs depends on water content of the surface (ref. 6, 7) and oxideoxide interactions, e.g. surface spinel formation (ref. 8). For catalysts prepared from metal carbonyl clusters, metal particles should be formed after decomposition. However, due to the presence of the support, the zero valent metal atoms are oxidized in most cases (ref. 9-11). Nevertheless, the dispersion of the metal particles is higher than for the catalysts prepared in a traditional way (ion impregnation)
(ref. 12).
In both cases metal atoms react with the support, resulting in the uncomplete reduction of the metallic phase (ref. 13-15). Metal-support interaction (MSI) influences the electronic structure of the metal particles, thereby affecting the adsorption and the catalytic behaviour of the catalyst formed. For instance, hydrogen adsorption (H/metal ratio) decreases (ref. 16), or on stabilized particles, an activated form of adsorption occurs (ref. 17). The change of the adsorption properties is related to the alteration of the catalytic properties as well. In the present paper, temperature programmed reduction (TPR)
is utilized to
investigate MSI. It is combined with adsorption and catalytic reaction studies on the catalyst treated in different ways. Treatment of the catalyst precursor may be relevant in the development of different surface structures, which influence not only the particle size, but the interactions between support and metal particles.
EXPERIMENTAL Materials Catalyst (I) was prepared by impregnation of Cab-O-Sil HS5 (Cabot corporation Boston) with an aqueous solution of a mixture of RuC1
and Fe(N0 The impreg3 3)3' nated samples were dried overnight. Catalyst (II) was prepared by impregnation
of Cab-O-Sil with a mixture of RU 12 dissolved in hexane, followed by evacua3(CO) tion at 570 K overnight. The impregnated samples were treated for hr with oxygen at 570 K, oxygen at 770 K, with only hydrogen up to 770 K and with He up to 770 K denoted by °2/570, °2/770, H and He/770, respectively. Reduction 2/770 of °2/570 and °2/770 was carried out by H at 770 K for 1 hr. 2
453 Experimental Methods Temperature programmed reduction (TPR) was carried out using a 1 % H 2/Ar 1. mixture at a programming rate of 20 K minThe gas was purified by passing it through a Pd catalyst followed by molecular sieve SA. Temperature programmed desorption of H was carried out in He stream at a programming rate of 2(TPD) 1 20 K min- after reduction at 770 K and cooling in H 2. H O and CO adsorptions were measured at room temperature (further denoted 2, 2 by RT) by the pulse method. H He and CO gases were deoxygenated by passing the 2, gas through a manganeous oxide column followed by molecular sieve SA. 02 adsorption was measured occasionally by the gravimetric method using a Sartorius microbalance. The catalytic reactions were carried out with a 1:3 mixture of CO/H in a tu2 bular flow reactor, working in the differential regime at a conversion level less than 1 %. The reaction products were analysed by a Packard 427 gas chromatograph, using a n-octane on Porasil C column. A second FID was used to measure the overall hydrocarbon conversion. Metal loading, which was nominally 0.5 wt % Ru and 0.5 wt % Fe, was measured by XRF.
RESULTS Catalysts (I) and (II) show significant differences in TPR after °2/570 treatment. The main reduction peak is at a lower temperature and is much narrower for catalyst (II) than for catalyst (I). For a first approximation, it means that after oxygen treatment, the catalyst prepared from metal carbonyl cluster (MCC) has weaker interactions with the surface, and the distribution of the oxide sites formed is rather uniform (see Fig. 1a and Fig. 2a). The structure of the catalyst surface reduced by hydrogen can be studied by TPR after RT 02 adsorption. On ruthenium, RT 02 adsorption has been used to determine the metal surface area (ref. 18,19). Nevertheless, even if it is only an approximation, the oxygen atoms adsorbed at RT occupy surface sites and subsequent H TPR displays their position. These results are presented in Figs. 3 2 and 4. Decomposition of catalyst (II) in H and He reveals a "mild" treatment, 2 because RT oxygen is bonded weakly to metal sites (see Figs.4a and 4b). On catalyst (I), similar treatment shows significant differences which result in the appearance
of a second large peak at 670 K (see Fig. 3a). This second type of
site is characteristic of the catalyst prepared by oxygen treatment (°2/570 and
°
/ 770) . On catalyst (II) treated this way, the low temperature peak almost 2 completely disappeared (see Figs.4c and 4d), which means that the metal is transferreri into a different position after this "strong" treatment. The structure of the surface is similar to catalyst (I) after oxygen treatment (see Figs.3b
454
373 473 573 673 773
K
Fig. 1. TPR spectra of catalyst (I): a) after °2/570; b)
after RT 02
adsorption; c) after RT + 570 K
Fig.2. TPR spectra of catalyst (II): a)
after °2/570 , b)
after RT 02 adsorp-
tion, c) after RT + 570 K 02 adsorption.
0:, adsorption.
373 473 573 673 773 K Fig. 3. TPR spectra of catalyst (I) after RT 02 adsorption: a) after
H2 / 770 ; b) 0/770.
after °2/570; c) after
Fig. 4. TPR spectra of catalyst (II) after RT 02 adsorption: a) after He/770; b) °2/570; d)
after H c) after 2/770; after °2/770.
455 and 3c) . When RT
0z
adsorption is followed by
0z
adsorption at 570 K, a new peak ap-
pears (at or below 370 K), as shown in Fig. 1b, 1c and Fig. Zb, Zc for catalyst (I) and (II), respectively. At higher temperature, oxidation of the sub-surface layer presumably occurs, or under these circumstances, the structure of the surface is rearranged. Further information can be obtained when HZ TPD is considered. As shown is Fig. 5, the low temperature hydrogen TPD peak is the largest for catalyst (II) by He/770, but it nearly disappears when the interaction between the active metal component and the support becomes stronger. Fig. 5. Hydrogen TPD for catalyst (II)
a) Catalyst (II) , He/770 b) Catalyst (II) , H/770 c) Catalyst (II) , °Z/?70 d) Catalyst (I)
,
H/770
e) Catalyst (I) , °Z/570 f) Catalyst (I) , °Z/770
373
473
573
673
173
K
It is better documented when the ratio of the low and high temperature TPD peaks are compared (see Table 1). The stronger the interaction, the lower is this ratio. Quantitative data on the HZ'
0z
and CO adsorption measured at room temperatu-
re as well as the H/O ratio measured in the removal of oxygen are presented in Table 1. All adsorptions decrease as the treatment proceeds from mild to strong. Hydrogen adsorption is practically nil for catalysts pretreated with oxygen. The H/O ratios show that oxygen cannot be completely removed. In Fig. 6 the rate and selectivity values are summarized for the CO + HZ
456 TABLE Data on adsorption of HZ' 0Z' CO, low and high temperature HZ TPD and H/O ratio at removal of adsorbed oxygen Adsorption at RT (j.lmol/g Treatment
°z
HZ Cat.
HZ TPD
c a t)
CO
H/O 470 K
770 K
(I)
a HZ
b °Z/570 K
1.Z
Z8
0.3
C
0/770 K
Cat (II) d He a HZ b 0/570 K
10
5.Z
Z5.6
13.6
Z.8
10.4
Z
5.Z 3Z
Z Z
6.4
1.5
8.6
36.8
16
6.8
1.2
Z.O
4
ZO.8
11. Z
6.8
3.Z
Z.O
11.6
4.8
Z.8
Z.8
8.8
2.8
C
0/770 K
1.5 1.4
a)
catalyst decomposed and reduced in HZ at 770 K
b)
catalyst pretreated in oxygen for 1 hr at 570 K followed by reduction in HZ at 770 K
c)
=
catalyst pretreated in oxygen for 1 hr at 770 K followed by reduction in
HZ at 770 K d)
=
catalyst is decomposed
in helium up to 770 K
reaction . It is obvious that on catalysts treated with oxygen before reduction, methane selectiVity drastically decreases and the amount of olefins and higher hydrocarbons
increases. For a better comparison, selectivities on pure Ru and
Fe on Cab-O-Sil are also presented. The catalytic properties of the oxidized surface were also observed in the impulse regime ..In Table Z, catalysts shown are completely inactive when the surface is oxidized; the reaction starts with the reduction of the surface. TABLE Z Threshold temperature for CO + HZ reaction on catalysts with different treatments (in K)
Catalyst (I)
0Z/570 with no HZ
503
Catalyst (II)
0Z/570 + H Z/770 0Z/570 with no H 2 0/570 + Hz/nO
483
480
423
457
s
HZ 773 K T=553 K r=18><10 8
. C 1
S
C2 C3
HZ773K
Oz 573K+H Z 773K
T= 5Z7K r= 30.9 >< 10- 8
T = 505K r e 4,4>< 108
T= 5ZBK r=14><10 B
(bl
(0)
Q5
He 773K
C4
Hz 773K T=4e9K_ 8 r = 2,67><10
C1
(d)
(e)
C2 C3 0z 573K T=465K_ 8 r= Z,2>< 10
T=630K -8 r=0,074>< 10
Q5 (e)
(f)
(g)
Fig. 6. Rate and selectivity values of CO + H reactions 2 a) RU (CO) 12/cab-0-Sil; b) Cat. (II), He/770; c) Cat. (II), H/770; d) Cat. (II), 3 °2/570; e)Cat. (I), H f) Cat. (I), °2/570; g) Fe 12/cab-0-Sil. 2/770; 3(CO) DISCUSSION In one of our previous papers on the iron-ruthenium system (ref.20) the small Ru particle size was interpreted by the presence of non-reduced paramagnetic iron oxide and its stabilization effect on the ruthenium particles. The presence of strongly bonded hydrogen was also established, and was later supported by experiments on Pt/Al
(ref. 21). 20 3 In the present paper, more details on the Ru/iron oxide interaction, by means
of hydrogen TPR, can be established. First, for both catalysts, the TPR spectra, while rather
nar~ow
at the first heating up to 770 K, become much broader after
RT 02 adsorption, displayed in TPR. The appearance of the peak at 670 K especially points to the fact that part of the metal particles are transferred into different environments, and that not purely physical, but also chemical interactions are operative. The broadening of the TPR spectra may be explained in such a way that after the reduction the Ru particles are not only more strongly bonded to the support, but also a distribution of sites, with slightly different energies, exists and these are enveloped by the curves actually found. The chemical interaction regarded as MSI depends on the chemical form of the metal component applied at the impregnation. MCC decomposes at low temperature
458 (ref. 12), thus smaller particles are stabilized which results in a higher dispersion (ref. 10, 12) and in a higher adsorption, as shown in Table 1. When inorganic salt solutions are used, a second peak appears at about 570 K, which coincides with the temperature range of RUC1 decomposition in the presence of 3 oxygen. It is believed that before its decomposition, larger crystallites are formed. Therefore the particle size is not only larger (see adsorption data in Table 1) but the interaction between RU0
and Fe is also more operative. This 2 203 is supported by the decrease of hydrogen adsorption and the sharp increase in the activated form of hydLOgen. The latter is not easily available for the reaction, as shown earlier (ref. 20). The nature of the surface is well represented by TPR after RT 02 adsorption. When
MSI is not operative to a high enough extent" oxygen can be easily remo-
ved. When strong MSI exists, metal particles are in such an environment that it is difficult to remove 02' resulting in the shift of the H TPR peak. Moreover, 2 in some cases oxygen cannot be displaced simply by hydrogen TPR, i.e. part of the oxygen remains on the surface. This means that the surface consists of a mixture of RU-RuO
and RUO is probably the link between iron oxide and metallic x-Fe 203, x ruthenium, similar to the case of Fe!MgO (ref. 15). It is of interest to observe that when 02 is adsorbed at 570 K after RT 02
adsorption, a low temperature peak appears . There are two possible explanations for this: i! there exists an iron rich surface layer which bonds oxygen strongly, and below this layer there are ruthenium particles from which oxygen can be removed at around 370 K (ref. 19), or, ii! the ruthenium particles themselves are covered by a layer of iron oxide. At the present stage, a clear distinction between these explanations cannot be made. The catalytic activity and selectivity reflect the phenomena found in the adsorption and TPR experiments, i.e. these are affected by MSI (which control the amount of hydrogen weakly bonded) and by particle size. An oxidized surface itself is not active in the CO + H reaction, as shown in Table 2. when the sur2 face is oxidized, the reaction starts at a higher'temperature which coincides with the reduction temperature of the catalyst. When the catalyst is decomposed in He, the activity is the highest (ref. '12,22) due to the highest dispersion. Since MSI is not serious here and a sufficient amount of hydrogen is available, selectivity for methane is quite large (ref.22) , as indicated in Fig. 6. As MSI increases,weakly bonded hydrogen is depleted, and thus the reaction to methane is retarded and the formation of olefins and higher hydrocarbons enhanced. This may be an explanation for the experimental results found for RU!Ti0
and Ni!Ti0 2 2 systems (ref. 23,24). In our case, of course, the interaction exists not between
metal and Cab-a-Sil, but between metal and iron oxide. To summarize, it was established that H TPR to remove adsorbed oxygen is a 2
459 useful tool to monitor MSI, which is reflected in the catalytic reaction behaviour of the catalyst formed.
ACKNOWLEDGEMENT The authors
are indebted to Mrs. G. Stefler for experimental help in carrying
out the catalytic reactions.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13. 14.
15. 16. 17.
18. 19. 20. 21. 22. 23. 24.
L. Guczi, K. Matusek, J. Margitfalvi, M. Eszterle, F. Till, Acta. Min. Acad. Sci. Hung., 101 (1979) 107. H.A. Benesi, M.P. Curtis, J. studer, J. Catal., 10 (1969) 328. L. Guczi, K. Matusek, J. Margitfalvi, React. Kinet. Catal. Lett., 8 (1978) 309. ' M. Eszterle, L. Guczi, Magyar Kern. Lapja, 36 (1981) 429. R.L. Garten, D.F. Ollis, J. Catal. ,35 (1974) 232. C.H. Bartholomew, M. Boudart, J. Catal., 19 (1973) 278. G.B. Raupp, W.N. Delgass, J. Catal., 58 (1979) 278. E.G. Derouane, M. LoJacono, M. Schiavello, A. Cimino, J. Phys. ~hem., 75 (1971) 1044. K. Lazar, Z. Schay, L. Guczi, Int. Symposium on the Reaction of One Ca~bon Atom, Bruge, June-1-4, 1982. L. Guczi, Z. Schay, K. Lazar, A. Vizi, L. Mark6, Surf. Sci. 106 (1981) 516. L. Guezi, K. Lazar, K. Matusek, J. Mink. Inorg. Chern., to be published. L. Guezi, Z. Sehay, K. Matusek, I. Bogyay, G. Stefler, Proc. 7th Int. Congress on Catalysis, Part A, Kodansha Ltd., Tokyo and Elsevier Sci. Publ. Co. Amsterdam (1981) p. 211. L. Guczi, Catal. Rev. Sci. Eng., 23 (1981) 329. A. Bossi, F. Garbassi, G. Petrini, L. Zanderighi, Proe. 7th Congress on Catalysis, Part B, Kodansha Ltd., Tokyo, Elsevier Sci. Publ. Co., Amsterdam (1981) p.1468. H. Tpos¢e, J.A. Dumesic, E.G. Derouane, B.S. Calusen, S. M¢rup, J. Villadsen, N. Tops¢e, Preparation of Catalyst Vol 2, Elsevier Amsterdam (1979) p. 365. R.L. Garten, M.A. Vannice, J. Catal., 66 (1980) 242. H. Tops¢e, N. Tops¢e, H. Bohlbro, J. Dumesic, Proc. 7th Congress on Catalysis, Part A, Kodansha Ltd, Tokyo, Elsevier Sci. Publ. Co., Amsterdam (1981) p. 247. M.P. Brown, R.D. Gonzales, J. Phys. Chern., 80 (1976) 1731. H. Kubicka, React. Kinet. Catal. Lett., 5 (1976) 223. L. Guczi, K. Matusek, I. Manninger, J. Kiraly, M. Eszterle, Preparation of Catalysts Vol. 2 , Elsevier, P~sterdam (1979) P. 391. P.G. Menon, G.F.Froment, J. Catal., 59 (1979) 138. Z. Schay, t. Guczi, First Belgian-Hungarian Colloquium on Catalysis Matrafured, 19-22 October 1981, in press in Acta Chim. Acad. Sci. Hung. M.A. Vannice, R.L. Garten, J. catal., 56 (1979) 236. E.L. Kugler, Preprints, ACS Div. of Petroleum Chemistry, 25 (1980) 564.
460 DISCUSSION I agree in a general way with your conclusion that H2 TPR to H. CHARCOSSET remove adsorbed oxygen is a useful tool for studying those and similar systems. we have practized it in fact for Pt-Re and Pt-Ru supported catalysts. Coming to your own system, Fig. 1 and Fig. 2, do you think that comparison of curves ~ could indicated i) a higher dispersion of Ru in catalyst (I) than in catalyst (II), ii) a higher dispersion of Fe in catalyst (II) than in catalyst (I), from comparison of the areas of the peaks ? L. GUCZI: The two peaks are not a direc~ measure of the amount of Ru and Fe reduced. Earlier evidences showed (Preparation of catalysts II, Elsevier Sci. Publ. Co, Amsterdam, 1979, p. 391) that iron cannot be reduced, thus the second peak cannot be assigned to the amount of iron reduced. Nevertheless, the dispersion of catalyst (II) is ~igher than for catalyst (I) as indicated by the separate chemisorption data. However, the shift in the TPD peak indicates the change of the environment of the metal particles after the different treatments, which in turns, influence the reducibility and its participation to the catalytic reaction. T. BEIN: You refer to different selectivities in the CO hydrogenation reaction. with the low conversion of less then 1%, do you have indications to work in a stationary regime of reaction? Did you check the schUlz-Flory plot to characterize the product distribution ? L. GUCZI: The total conversion i.e. the total amount of hydrocarbon formed, was continuously monitored by FID. The analysis of the product was carried out when the steady state of the hydrocarbon formed was already set. In some cases the SF distribution was checked and we found deviation at the methane side. G.C. BOND: Have you allowed for the possibility that Ru may generate H atoms in the presence of H2' and that these may undergo "spillover" and may subsequently reduce Fe 3 + to lower oxidation states ? secondly, I do not understand the very low values for H2 adsorption reported in Table 1 of your paper. The H2'02 ratios seem extremely low, and I wonder what you think the explanation might be ? L. GUCZI Hydrogen spillover might help the reduction of Fe 3 + . However, we obtained direct evidences earlier that most part of iron stays in the form of Fe 3 + and only a few percents of iron can be reduced to Fe 2 + . The possible explanation is that the iron oxide particles are covered primarily with RuOx layer which makes the link between highly dispersed iron oxide and metallic Ru. This layer may prevent the reduction of iron oxide. The second explanation may be the low capability of Ru-Fe-Cab-O-Sil system to chemisorb hydrogen. That is, the H2,02 ratio never exceeded the value of 2, which means that oxygen can be removed by hydrogen but the additional adsorption of hydrogen is very low. We do not have any other explanation but the general phenomenon observed for very low hydrogen adsorption on titania supported metals. Here, of course, we may not talk about SMSI in context of Cab-O-Sil, but there are many evidences that we have a similar behaviour for Ru-Fe203-Cab-O-Sil system. Here an activated type of hydrogen adsorption was found as indicated by the hydrogen TPD data. J.B. BUTT: Have you, in experiments with the Fe-Ru carbonyl precursors, ever observed phase separations either as a function of metals loading, stoichiometry or temperature programmed decomposition rates ? L. GUCZI: We have studied the temperature programmed decomposition of Fe3(C012, RU3{CO)12 and their mixture supported on Cab-a-sil by infrared spectroscopy (to be published) . We found the formation of subcarbonyl species but phase seperation could not be
461 observed. However, we have investigated Fe3(CO)12 (I), Fe2Ru(CO)12 (II) and H2FeRu3(CO)13 (III) by M6ssbauer spectroscopy. In crystalline state, (I) gives three line, (II) two line and (III) single line spectrum. In supported state, the same character of the spectra can be observed in some distorted form, which probably means that the cluster keeps its identity. However, data are not available whether it separates into the pahses during decomposition. M.V. TWIGG: A potential problem with ruthenium-based catalysts is the loss of ruthenium during preparation and use via formation of volatile compounds. Do you observe loss of ruthenium from your catalysts during their preparation or use, and if so how this has been overcome ? L. ~UCZI The reaction temperature was too low to be able to observe any loss in the form of Ru04' However, if some water ttraces were in the Co + HZ feed, some volatilization of ruthenium was observed. The possible explanation is that, in particular after a long time-on-stream, carbonaceous deposits react with H20 and thus CO + H2 are formed. CO may react immediately with ruthenium forming volatile carbonyl compound which leaves the catalyst bed. The results obtained on such a catalyst were discarded and the kinetic data include only results when evaporated metallic film was not observed. M.L. GOOD: Just a moment. Professor Guczi's temperature programmed reduction work is in agreement with work we have done on the characterization of FeRu systemps, both unsupported (bulk) and on a number of supports including Y-zeolites. These studies using Fe and Ru M6ssbauer spectroscopy have shown that when the metals (or their salts) are dried and calcined and then reduced, the resulting mixture contains RU, Ru0 and Fe203' Small amounts of RU-Fe alloy are formed. 2 This is in contrast to the properties of each of these metals studied alone. Ru s easily oxidized to Ru02 and clearly reduced to Ru metal in H2' at relatively mild conditions. Fe oxidized easily and is always at least partially reduced to Fe metal. L. GUCZI: I am very pleased on your comment. In work in the Preprint Series of the ACS Division of work was the first direct evidence for what we had of the bimetallic system is strange indeed, but it tion between the two metals.
fact, we have read about your Petroleum Chemistry and this stated before. The behaviour points to the mode of interac-
A. BOSSI: In a recent paper we reported the strong effect of A1203 support on RU3(CO)12' On fresh impregnated samples the presence of Ru carbonyl was certified by IR spectroscopy. Large decomposition of the precursor complex was observed in samples aged at room temperature in air, giving rise to new carbonylic complexes and to the formation of oxidized Ru species. Did you observe similar behaviour in your Fe-Ru catalysts ? L. GUCZI: In the case of RU-Cab-O-Sil samples the formation of IR bands at or above 2100 cm- l ,has been observed (to be published) which may indicate indeed some oxidation of ruthenium particles. This is in agreement with your data quoted and with earlier data on iron-ruthenium system.
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463
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publisbers B.Y., Amsterdam - Printed in The Netherlands
THE PALLADIUM ALUMINA SYSTEM : INFLUENCE OF THE PREPARATION PROCEDURES ON THE STRUCTURE OF THE METALLIC PHASE S. VASUDEVAN, J. COSYNS, Institut
Fran~ais
E. LESAGE, E. FREUND and H. DEXPERT
du Petrole, B.P. 311, 92506 Rueil-Malmaison Cedex (France)
ABSTRACT Two preparation procedures for alumina supported palladium catalysts are compared using conventional and scanning transmission electron microscopy. Different structural relationships have been found between the metallic phase and the carrier with respect to the chosen impregnation method. When the alumina surface structure is damaged a maximum dispersion of only 20% is non destructive route from
ohtained. A
an organometallic precursor leads to a monomodal
higher dispersion.
INTRODUCTION Though the concept of demanding reactions in the field of supported metallic catalysts has been proposed by Boudart (ref. 1) as early as 1966, and has been correlated to such parametemas crystallite size, electronic properties varied by alloying or metal-support interactions, or even preparation procedures, fundamental studies aimed at fully characterizing the metallic particle structure and/or its relationships with the underlying carrier have been generally limited to model systems: evaporated films (ref. 2, 3), or monocrystalline samples (ref. 4) with flat or stepped surfaces. Modern
transmission electron microscopes allow lattice imaging down to
0.15 nm or even below so that lattice images of small (3-10 nm) metallic particles are rather easily obtained (see for example ref. 5). However, the information contained in these images must be interpreted with care. A major advance has been brought about by the advent of high resolution dedicated scanning transmission electron microscopes, which provide simultaneously high resolution images, microanalytical and microstructural facilities down to a few atoms (see for example ref. 6). Applied to the characterization of supported metallic catalysts, electron microscopy may give not only a detailed picture of crystallite size, composition, microstructure, crystallographic relationships between the metallic crystallites and the carrier (ref. 7), but also may help in understanding
464
the influence of each step of a given preparation procedure, or in comparing different preparation procedures. This will be exemplified in the present paper for the case of a palladium catalyst supported on a Y alumina c~rrier. The t metallic phases obtained from two different procedures will be compared : A) a classical impregnation method, which leads to a poor or moderate metal dispersion
; B) preparation by an organometallic route leading to high metallic dis-
persion. EXPERIMENTAL I. Preparation of catalysts
The carrier used is a Y alumina manufactured by Rhone Poulenc with a specit fic surface area of 69 m2/g. Procedure A (classical impregnation) : This procedure consists of dry impregnation of the support with a slight excess of palladium nitrate aqueous solution so as to fix 0.3 % wt palladium. Palladium fixes on the periphery of each pellet of the carrier (penetration depth < I rom). The catalyst is oven-dried (373 K) overnight, calcined at medium (673 K) or high (1173 K) temperature and reduced by hydrogen under atmospheric pressure at 373 or 773 K. Procedure B (organometallic route) : The detailed
procedure is reported in
another paper (J.P. Boitiaux et aI, this conference) wherein a five-fold excess (with respect to pore volume) of a palladium acetylacetonate benzenic solution is left in contact with the carrier for at least 48 hours. The palladium content of the catalyst is controlled by varying the palladium concentration in the benzenic solution. The impregnated alumina is oven-dried overnight at 393 K, then air-calcined at 573 K and reduced under hydrogen at 573 K. Metal dispersion may be varied by subsequent treatment with hydrogen or argon at temperatures varying between 373 and 1073 K. 2. Characterization of catalysts a) CO chemisorption The dynamic method is used, with a catharometric detection. The catalyst sample (1-2 g) is reduced at 523 K for 2 hours, treated with argon at 573 K for 2 hours, cooled to room temperature before
chemisorption measurements. Disper-
sion is computed assuming a 1 : I stoichiometry for CO chemisorption on palladium. b) Electron microscopy A Jeol 120 CX equipped with high resolution pole pieces is used to image lattice fringes of the metal and the carrier. X-ray emission microanalysis (Kevex detector) and microdiffraction is carried out on a V.G. Microscope
HB5 STEM
equipped with analytical pole pieces allowing a point to point resolution of 0.45 nm. The microdiffraction facility has been modified as described in reference (8). Sensitivity and spatial resolution for X-ray emission microanalysis
465
is discussed in reference {9}. c} Temperature programmed reduction Reduction by a 2 or 5 % volume H2-Ar mixture is followed with a catharometer. Temperature is increased linearly up to 873 K. (maximum temperature 1000 K). Raw data are processed using a H.P. 2645 A computer. RESULTS I. Samples obtained from procedure A The samples studied are listed in Table I together with the pretreatment conditions and the dispersion values obtained from CO chemisorption. TABLE I Procedure A Sample
Tcalcination(K)
AI A2 A3 A4 A5 A6
673 673 673 673 1173 1173
Remark
freduction(K)
373 573 373
-
Dispersion (%)
-
-
water after
373
26 16 26
wash
i~regnation
-
-
14
The series of samples calcined at 673 K is first considered. The standard procedure {reduction at 373 K} yields a catalyst of 20% dispersion
(sample A2).
Electron micrographs reveal two types of crystallites : - a few large crystals up to 20 nmin diameter. Sucherystals are present (probably as palladium oxide) in the precursor Al ; - small crystals with an average diameter of 5 nm. However palladium is also present in a highly dispersed phase - palladium metal clusters or palladium ions as revealed by microanalysis. Temperature programmed reduction of sample AI shows that reduction takes place in two steps
a peak at room temperature and another at high temperature of
613 K can be observed. The origin of this second peak is still unclear. Sample A3 has been reduced at a temperature high enough to ensure a complete reduction. A new.population of small crystallites (3-5 nm) is observed in accorddance with the above hypothesis (reduction and/or sintering of the highly dispersed palladium phase). High temperature calcination (1173 K, sample AS) leads to a poorly dispersed catalyst even when reduced at low temperatures {373 K, sample A6}. Only large palladium crystals are visible, however X-ray microanalysis still reveals a highly dispersed (may be unreduced) palladium phase (fig. I). Lastly if the precursor AI is washed before calcination, no large {palladium
466 oxide) particles are observed and a monomodal dispersion is obtained even after low temperature reduction.
Fig. I. X-ray emission evidence for a highly dispersed palladium phase. Left part (I) : zone imaged before analysis and the corresponding spectrum; Right part (2) : after analysis a palladium signal is seen coming from the dark zone formed under the electron beam. Some characteristic high resolution images obtained with tilted illumination, on sample A2 are shown in figures 2 and 3. The following conclusions may be derived from these images - the (Ill) lattice fringes
are generally obtained in the palladium crystals
images. Thus the most frequent zone axes must be [110), [123) or [211). This can be checked directly for instance on figure 2 where the (III) and (002) lattice fringes are imaged ; - the same zone axes are also obtained for the alumina carrier, as it is visible on figure 3, the [lID] zone axes being the most frequent. A much more systematic study is possible, using microdiffraction with STEM. Typical results are presented in figure 4 for sample A3. The microdiffraction
467
Fig. 2. CTEM high resolution image of a palladium crystallite oriented along the [lID] axis.
Fig. 3. CTEM high resolution image of a metal-carrier association. Alumina zone axis [\12] .
Fig. 4. Palladium typical parallel microdiffraction pattern. The diffracting area is 4 nm2.
468
interpretation yields the following results: - particles in the size range 3 to 7 nm are mono crystalline. Some larger particles exist and are polycrystalline , - the above quoted zone axes are confirmed in most cases, the layer of alumina on which the palladium is deposited is poorly crystallized or even amorphous. 2. Samples obtained from procedure B High dispersions are obtained with this procedure, for reduction temperatures not exceeding 573 K.
~ll
samples are partly reduced at room temperature. Several
high temperature peaks are seen for calcination temperatures below}73 K, confirming that palladium is partly present in the unreduced state for all the samples considered. For the higher dispersion, no
pallad~um
phase can be imaged in the CTEM
though palladium is detected by X-ray emission microanalysis and shown to be rather uniformely located on the surface of the alumina carrier. As small crystallites (characteristic dimension less than 3 nm). cannot be easily (or at all) investigated, we studie4 a sample treated under argon at 973 K. Typical microdiffraction patterns are given in figure 5. Concerning the metallic phase, the situation is roughly similar to what is observed for an equivalent sample (same overall dispersion) obtained from procedure A. All observed particles are monocrystalline and have [110), [123) or [211) ~s
zone axes. However the alumina
beneath the metal is crystalline and partial epitaxy is generally obtained (i.e.
5. [123) palladium-alumina epitaxy for the two arrowed crystallites.
Fig~
469 along one lattice plane: (III)). This conclusion is valid in the particles size range of 3-7 nm, and for all three zone axes. Furthermore, the recording system used for microdiffraction acquisition is rapid enough (down to a few tenth of a second) to follow the fluctuations of the crystallographic orientations of a -10 . . g~ven part~cle under the electron beam (beam current = 10 A in I to 10 nm2). DISCUSSION Two important points for each procedure studied can be observed and will be discussed : - the chemistry involved in the formation of the metallic phase - the crystallographic relationship between palladium particles and the carrier; In the precursors issued from procedure A, palladium is present in three phases: bulk oxide (10-50 nm)" divided
oxide (3-7 nm) and highly dispersed
oxide or palladium ions. Bulk oxides may be eliminated by carefully washing the precursor after impregnation as shown by the examination of sample A4. The hiehly divided palladium phase cannot be eliminated by air calcination, even at 1173 K. Thus, a monomodal palladium dispersion cannot be easily obtained from procedure
A. On the contrary, starting from procedure B, no bulk oxide phase is formed, most of palladium becomes reducible at low temperature for calcination temperatures above 573 K. Thus, very high monomodal dispersions may be obtained. The microstructural study by microdiffraction and high resolution conventionnal transmission microscopy demonstrates that epitaxy is a normal phenomenon in the palladium/alumina system, provided that the alumina surface remains undamaged, condition which is respected with procedure B but not with procedure A (amorphisation of the structure during impregnation and/or calcination) As a consequence of epitaxy, a rather strong interaction is produced in the case of small crystallites, which explains the very high resistance to sintering of catalysts obtained from procedure B. The preliminary results suggest that it is worthwhile : 1/ to examine if the phenomenon observed in the palladium/alumina system is general and if it
c~n
be extended to other metal/carrier systems ;
2/ to study in more detail the formation of a metallic phase from the precursor. Work is
in progress in this direc don.
ACKNOWLEDGE~ffiNTS
One of the authors (S. V.) wishes to thank Engineers India Ltd (India) for the necessary study leave and the French Governement for the scholarship to do this work.
470 REFERENCES M. Boudart, A. Aldag, J.E. Benson, N.A. Dougharty and E.G. Harking, J. of cae .; 6(1966)92. 2 M. Gillet and A. Renou, Surf. Sci., 90(1979)91. 3 M. Gillet and A. Renou, Thin Solid Films, 41(1977)15. 4 G.A. Somorjai and coli., J. de Catal., 67(1981)371, Appl. Surf. Sci., 2(1979) 352 and Surf. Sci., 92(1980)489. 5 D.J. Smith and L.D. Marks, Phil. Mag., 44(1981)735. 6 H. Dexpert, E. Freund and J.P. Lynch, Proceedings of Quantitative microanalysis with high spatial resolution, Manchester, March 1981, The Metals Society, London, 1981, p , 101. 7 A. Howie i~ J.M. Thomas and R.M. Lambert (Ed.), Characterization of Catalyst, J. Wiley and sons, 1980, p. 89. 8 J.P. Lynch, E. Lesage, H. Dexpert and E. Freund, lnst. Phys. Conf., 61(2) (1981)67. 9 H. Dexpert, J.P. Lynch and E. Freund, lnst. Phys. Conf., 61(4)(1981)171.
471 DISCUSSION J.W.E. COENEN In several laboratories (Eindhoven, Leiden) evidence was obtained that in well-dispersed supported noble metal catalysts, metal ions were present, which presumably could act as a glue layer between metal crystallite and support and thereby enhance stability against sintering. The same idea was put forward by me for Ni/Si02 already 10 years ago. E. FREUND: According to our TPR results concerning the palladium on nitrate system, the proportion of unreduced palladium is small if not'zero. As the metal dispersion is not very high (maximum 25%), the presence in sufficient number of unreduced palladium cations cannot be ruled out. However, for another system: platinum on Yc chlorinated alumina (reforming catalysts) we have carried out a det~iled EXAFS and XANES study, (to be published very shortly), which clearly shows the absence of any significant amount of unreduced platinum. In this case, the metal dispersion is 100% (maximum crystallite size 0.9 nm). R. VAN NORD STRAND : With your Pd nitrate preparation, the TPR curve was explained as a reduction of Pf at low temperature, followed by reduction of the nitrate at a considerably higher temperature. How is this possible, that the nitrate ion remains while its cation is reduced? E. FREUND / I agree that the nitrate anions located near the palladium cations are likely to be reduced at low temperature. However, because low pH solution is used for the impregnation, nitrate anions will be fixed on the alumina carrier away from palladium. These nitrate anions will not be so easily reducible. H. CHARCOSSET: 1. A short comment on TPR. The diagrams are often complicated by superimposition of H2 consumption due to reduction and of H2 desorption at the same time. We have observed this phenomenon over (Pt,Ru)/y-AI203' 2. With your techniques could you easily observe a reduction of the support at the interface Pd/AI203' if for instance you would reduce your catalysts at higher temperatures ? E. FREUND: 1. We do not observe H2 desorption (except hydride decomposition below 373K) in the temperature range considered in our study. 2. No,except if a well defined Pd-Al intermetallic compound is formed. The precision for parameter determination from an electron microdiffraction pattern (or more generally an electron diffraction pattern) is rather poor (5%), so that a Pd-Al solid solution would not be detected. H. CHARCOSSET : What experimental evidence have you to say that the high temperature TPR small peak is due to reduction of SO~or/and Fe 2 + and Fe 3 + in the support ? E. FREUND This attribution was arrived at by considering different alumina carriers having different impurity contents (especially titanium, iron and sulphate) .
This page intentionally left blank
473
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
BIMETALLIC SUPPORTED CAT/\LYSTS PREPARED VIA METAL ADSORP'I'ION. PREPARATION AND CATALYTIC ACTIVITY OI<' Pd-Pt/AI
20 3
CAcI'-
ALYSTS
J. MARGITFALVI, S. SZABO, F. NAGY, S. GOBOLOS and M. HEGEDUS
Central Research. Institute for Chemistry, Hungarian Academy of Sciences, H-l025 Budapest, Pusztaszeri ut 59-67
ABSTRACT Alumina supported pd-pt bimetallic catalysts prepared by electrochemical metal adsorption via ionization of adsorbed hydrogen. The palladium content of the catalysts can be controlled by thermal treatment processes of the parent catalysts, as well as by applying subsequent adsorption steps. The Pd-Pt catalysts were tested in n-hexane dehydrocyclization. Their catalytic properties depend strongly on the thermal treatment processes applied before the reaction. Omission of oxygen treatment results in lower activities and lower selectivities
for benzene
formation.
IN'I'RODUC'l'ION One of
the major challenges in catalysis science is the application of
scientif ic methods in the preparation of catalysts with the reg uired given catalytic properties. This aim, at least partially, has been achieved by means of the development of multimetallic systems with improved high activity and selectivity, and good activity maintenance. Different methods of preparation of bimetallic alloy or cluster type catalysts are known, however, unless bimetallic organometallic compounds are used as catalyst precursors, exclusive formation of bimetallic entities on the support cannot be guarante€,d. In this paper we present a cation of a
systematic study to demonstrate the appli-
new method for the preparation of supported bimetallic cata-
lysts by means of metal adsorption, where the formation of bimetallic entities is controlled by the chemical nature of the system itself. Preparation of different Pd-Pt/ Al catalysts is demonstrated together with test re20 3 sults in n-hexane dehydrocyclization.
474 'rHEOHETICAL BACKGI
Many papers have been devoted to the theoretical and experimental aspects of metal adsorption on foreign metal surfaces (refs. 1-5). It has been shown that adsorption takes place at a
more POSitiVE' potential than
the Nernst potential of the adsorbing ion. IVlostly electrochemical methods have been used. to investigate the un-•. derpotential deposition of metals onto metals (refs. 1-5), since they are the most suitable Jor distinguishing bulk deposition from the formation of adsorbed species, .2J.s can be seen from Fig. 1,. where A and 13 denote the ionization peaks of bulk and adsorbed palladium, respectively.
"\
"l \
«
I I
E -.
I
I
A
\ \
- - - - ---_........ ....., ,/ _---- -------
",
...'"
" .....
0,5
1,0
EIV
-1
-2
Fig. 1 Cyclic voltamogram of a smooth Pt electrode in the presence of 5x10- 4 M PdC1 2• Sweep rate 0.05 vis; the dashed line is the voItamogram of the electrode in 1"1 HCI It has also been shown that via the ionization of hydrogen adsorbed on platinum, an adsorbed metal monolayer with the same character as obtained by electrochemical methods can be formed (refs 4, 5). In this case the adsorbed hydrogen can be considered as a
source of electrons,
according to the following process:
H a
-
H+ + e
(1)
Such an experiment is shown in Fig. 2. The convergence of charging cur-v ess 1
and 2 shows that Pd is deposited. on the Pt surface in an
amount equivalent to the adsorbed hydrogen. Curves 3 and 4 demonstrate
475
1,0
EN B
0,5
a
30
15
tlmin
45
Fig. 2. (I = 0.5 rnA) 1 - Charging curve of Pt electrode in 0.2 M HCI; 2 - Charging curve ·of a Pd-covered electrode after metal deposition, in the presence of the depositing palladium ions; 3 - Charging curve of the electrode covered with Pdads in 0.2 M HCI; 4 - Charging curve of the electrode covered with Pd in 0.2 M HCI b ulk the difference between the adsorbed and bulk forms of the deposited palladium (ref. 5). From our earlier results (ref, 4) it can be concluded that, for kinetic reasons, bulk deposition should always be reckoned with if it is thermodynamically possible, and metal adsorption can be considered to include the following processes: n Me +
n t----,--a Me + f
sclu.
1
sur •
---',",=,"~--+ne 2
~l\ileads
. Me
f __
sur •
44 ~
(2)
5f
Me
b ulk
where /1/ is metal ion diffusion to the surface, /2/ is charge transfer,
13/
is metal adsorption, /4/ is bulk deposition and
/51
is conversion of
bulk metal into adsorbed species in the presence of metal ions under open circuit cohditions, when the potential rises to a value higher than the Nernst potential. It has been. found that adsorbed metal atoms inhibit hydrogen adsorption (refs. 1,2,4,5). 'The number of hydrogen adsorption sites occupied by one adsorbed metal atom can also be determined. According to our calculations based on curves 1 and 3 in Fig. 2, an adsorbed Pd atom on a Pt surface covers 1.5 hydrogen adsorption sites (refs. 2,4,5).
476 EXPERIMENTAL Supported Pt!AI catalysts modified by adsorbed palladium have been 20 3 prepared as follows. Before metal adsorption the catalyst precursors were stored either in sealed tubes under nitrogen or air. In the former case, after thermal treatment, the parent mono- or bimetallic catalysts were cooled in nitrogen or hydrogen atmosphere. Catalysts 2 pd-pt and 3 pd-pt were prepared after subsequent palladium adsorption, applying saturation with hydrogen after each palladiur:? adsorption step. The parent catalysts after heat treatment and cooling in nitrogen or hydrogen atmosphere were poured in 0.2 MHCI solution and saturated with hydrogen gas. Then the excess hydrogen was flushed out of the reactor with N 2 and a
pdCl solution was introduced into the supporting 2 result, the hydrogen adsorbed on pt!A1 was exchan20 3 ged into adsorbed Pd. To minimize the formation of bulk phase, the modi-
electrolyte. As a
fied catalysts were kept for 72 hours in a
solution containing
The catalysts prepared were carefully washed with HCI and H
pdCI~-
ions. and fi-
20 nally dried. All these manipulations were carried out under oxygen-free
conditions with the use of deoxygenated solutions. Dehydrocyclization of n-hexane has been studied in pulse and continvous-flow reactors. The reactor setup and the product analysis have catalyst had 0.5 0/0 20 3 pt loading. Pd and pt contents were. determined by flame photometry. In been described elsewhere (ref. 6) the base Pt!AI
every experiment the hydrogen to n-hexane ratio was 5:1. Hydrogen chemisorption was studied by a pulse technique and temperature-programmed desorption. Before the catalytic reaction took place, catalysts were first calcined in flowing oxygen at a
definite temperature for one hour. 'I'he temperature was
then raised or lowered in nitrogen flow to the temperature. of hydrogen treatment and the nitrogen flow was replaced by hydrogen. The catalyst samples were treated in hydrogen for. 1.5 h. In each run a
fresh catalyst
sample was used. The numbers in parentheses indicate the temperatures of oxygen and hydrogen treatments I respectively. 0 refers to catalyst samples where the oxygen treatment has been omitted.
RESULTS AND DISCUSSION Catalysts preparation In order to obtain pd-pt catalysts by metal adsorption with different palladium contents as well as to stUdy the influence of the nature of the catalyst precursor on the palladium deposition, different thermal treatment processes were used before metal adsorption. Characteristic data of the
TABLE 1 Characteristic data of the catalyst prec ursors and Pd-Pt catalysts prepared via metal adsorption Catalysts
Pd weight
Precursor
0/0
Chlorine weight 0/0
Pd!Pt atoms
H! ('Pd+pt)a
-
1.57
0.08
pte 0, 500)
-
-
1.31
0.08
pte 400, 500)
-
1.06
1.78
-
-
0.41
Pt(400, 575)
-
0.20
1.42 1.82
PtC
C
0.86
-
0.93
0.087
0.329
1.07
0.14
0.47
2Pd-Pt
1Pd-Pt
0.164
0 •.5 86
1.14
0.15
0.52
3Pd_Pt
2Pd-Pt d pt
0.215
0.806
1.28
-
0,48
0.139
0.513
1.05
0.18
0.61
0.151
0.548
1.27
-
0.58
0.084
0.307
1.00
0.1
0.46
0.152
0.549
1.12
-
0.57
1Pd-Pt
(Pd-Pt)l (Pd-Pt) 2 (Pd-Pt) 3 (Pd-Pt) 4 Pd(Pd-Pt) 1 2Pd(Pd-Pt)1 a b c d e
deterncined
H / F(b
.Pt
pte 400,570)( H
e
2) e 2) Pt(400,575)(N 2)e (Pd_Pt)d 1 , pte 0,500)( H
Pd(Pd-Pt)~
0.293
1.062
1.04
0.21
0.470
1.722
1.07
0.18
f rom hydrogen chemisorption measured by a
calculated from TPD curves, H') desorption up to
0.47 0.43
pulse method (ref. 7) at 20
0
C;
650 °C;
base catalyst prepare~ without c e Ic irie.tio n, reduced at 400 °c and stored in air; 20 3 treated in oxygen and hydrogen at 400 and 500 0 C, respectively, and cooled in hydrogen atmosphere prior to Pd adsorption; Pt!AI
the gas atmosphere applied during the cooling process
...
-J -J
478 bimetallic catalysts prepared via metal adsorption are shown in Table 1. Three types of Pd-Pt catalysts can be distinguished: Type A - prepared from the Pt/AI 0 base catalyst by subsequent adsorp2 3 tion, lPd-Pt, 2Pd-Pt and 3Pd-Pt; Type B - prepared from Pt/AI 0 (400,500) in such a way that thermal 2 3 treatment at (400,500) was applied .b eror e each' adsorption step; (Pd-Pt)l' Pd(Pd-Pl)l and 2Pd(Pd-Pt)1; Type C
Prepar'ed from the Pt/Al 0 z 3 base catdlyst applying oxygen and hydrogen tr e etme-nt at 400 and 500 °C, respectively, prior to every Pd adsorption step.
In subsequent Pd adsorption the Pd content of the catalysts increased by 0.087, 0.077 and 0.051 F'or-me.tio n of a
0/0,
respectively, after each adsorption step •
mixed Pd-Pt alloy or cluster 'by calcination and subsequ-
ent reduction steps prior to adsorption resulted in higher amounts of Pd deposited in one adsorption step ( 2Pd(Pd-Pt)1
catalysts (Pd-Pt) l' Pd( Pd-Pt) 1 and
). In this series the Pd content of these catalysts increased
by 0.14, 0.15 and 0.18
0/0,
resp,ectively, i.e. twice as much as was achi-
eved by subsequent adsorption. Neither the temperature of hydrogen treatment nor the atmosphere of cooling appeared to be of importance in controlling the palladium content ( catalyst samples lPd-Pt vs. (Pd-Pt) 3 or (Pd-Pt) l' (Pd-Pt) 2 and (Pd-Pt,) 4
). An increase of the dispersity of
catalyst precursor resulted in higher palladium concentrations (catalyst samples lPd-Pt vs. (Pd-Pt) 1)' however, the amount of palladium adsorbed was not sensitive to slight changes in the Pt dispersity (catalysts (Pd-Pt) 1 'vss,
(Pd-Pt) 2 ).
The amount of Pd adsorbed is higher than can be expected from the H!(Pd+Pt) ratios, but is lower than one would calculate from the H!Pt ratios measured in 1.'PD experiments. To solve this contradiction
an in-
dependent method is needed to measure the amount of hydrogen adsorbed on supported platinum in liquid electrolytes. n-Hexane dehydrocyclization Results obtained in a pulse reactor at 480
°c
are shown in Fig. 3,
where conversion and selectivity values obtained in the first pulses are presented. Significant differences in the catalytic performance were obtained depending on whether oxygen treatment is applied or not prior to the reaction. Without oxygen treatment the activity of the Pd-Pt catalysts was lower than that of their monometallic precursors. On this type of Pd-pt catalysts the conversions were almost constant, irrespective of the palladium content. The decrease in the activity of the Pd-Pt catalysts with respect to the base platinum sample can be due to blocking of the active
479
0,2
0,4 Pd,-I.
0,2
02
0,4 Pd,OJ.
Ss
conversion OJ.
51
70
20
8 6
50
40
20
O,4Pd:I.0
0,2
0,2
0,4 Pd:I.O
Q2
Pd,·'.
Fig. 3. Convorsi~n
and selectivity. data of Pt/A1 and Pd-Pt/Al cat20 3 20 3 alysts obtained in a pulse r e e.c tor-, Amount of catalyst: 0,10 g, feed rate:
2.8 g g
-1
h
-1
• I .., catalysts without oxygen treatment, II - catalysts with
oxygen treatment. 5
51' SMCP' 5 -C and 5 r are selectivity to benzC B, . h exenes, met h ylcyc1opentane, 1 5. ene, ISO cracking produc t s and toluene, res-
pectively• •
-
catalyst of group A;
•
-
Pt/A1
0-
A - catalyst of group B
(400,500)
20 3 (Pd-Pt)1 i
+
(Pd-Pt)2
0 -
(Pd-Pt)3
480 sites on platinum by adsorbed palladium. The constancy of the catalytic activity upon increasing the Pd content. may indicate that most of th o palladium is irie.c c e e e ib Io for n-hexane, i.e. the platinum clusters are covered, at least partly, by bulk deposited palladium. The slightly higher activity of catalysts with high Pd contents can be attributed to differences in the initial dispersity, the altered surface concentration and increased activity of their precursors. Oxygen treatment resulted in a
sharp increase in activity. The activity
of the Pd-Pt catalyst can even exceed that of the Pt/AI
z0 3
catalyst. As
has been mentioned, three groups of catalysts can be distinguished. F'r o m conversion and selectivity data obtained on catalysts treated in oxygen prior to reaction, it can be seen that the separation of these groups is quite demonstrative. Introducing palladium into the platinum crystallites resulted in drastic changes in the selectivity. Without oxygen treatment the Pd-Pt catalysts show lower selectivity to benzene and toluene. and higher selectivities to isomerization, hydrogenolysis and C
cyclization. However. with increa5 sing palladium content the trends in selectivities changed and opposite
trends for group A and I3 Pd-Pt/Al
catalysts were observed. After Z0 3 oxygen treatment the selectivity of the Pd-Pt catalysts has markedly
changed. their dehydrocyciization activity became more pronounced at the expense of cracking. isomerization and C
cyclization activities. Selectiv5 ities to hydrogenolysis products and toluene slightly increase with in-
creasing Pd content. while the selectivity to benzene has two. maxima and that to isomerization as well as. for C 5 cyclization two minima.corresponding to group A and B catalysts. These maxima and minima are obtained on catalyst samples ZPd-Pt and Pd( Pd-Pt) 1 and from the almost indentical actiVity and product distribution over these two catalysts it can be suggested that they whould have identical surface Pd/Pt ratios, which probably are not equivalent to the overall Pdl Pt ratios. The difference between these two catalysts are as follows. In the Pd( Pd-Pt) 1 catalyst after oxygen treatment the first Pd layer is alloyed with Pt. with the formation of a Pd-Pt mixed phase thus both catalysts have almost identical Pd contents in the adsorbed layer, as can be seen on comparing the Pd contents and Pd/Pt ratios presented in Table 1. 'l'he resemblance between 3Pd-Pl. and ZPd( Pd-Pt) 1 can probably be explained in a
similar
way. Thus applying oxygen treatment the formed Pd"';Pt phase at the platinum or Pd-Pt crystallites is responsible for the overall catalytic perfor-
mance. Results obtained in the continuous-f low reactor experiments are shown in Figure 4. As can be seen. increasing amounts of Pd result in a
481
·,. -i:
40
.,.
60
30
12
40
20
8
20
10
4
o
0,2 0,4
Pd, .,. 0
0,2 0,4
Pd,·'.
°
0,2 0,4 Pd'-'.
Fig. 1/.. Conversion and selectivity data obtained in a
continuous-flow re-
actor after 4 hours of experiment. Amount of catalyst: 0.2 g. Fe-=d rate: 5.7 g g
A
-
-1
h
-1
• Reaction temperature: 500
0
C. Symbols as in Fig. 3.
Pd-Pt catalyst of group A
•
Pt-Pt catalyst of group B
[J
Pt!AI 0 (400,500) 2 3
higher activity and an increased selectivity to benzene. The difference between catalysts of groups A and B is clearly seen on comparing selectivities to isomerization and hydrogenolysis. It is interesting to mention that selectivities to methane formation increased with increasing Pd content of the catalyst.
CONCLUSIONS The experimental data presented in this paper clearly prove the applicability of the metal adsorption technique to prepare Pd-Pt!AI 0 suppor2 3 ted catalysts. The catalytic activity of these bimetallic catalysts can be controlled by the amount of palladium metal deposited, as well as by thermal pretreatment processes applied before metal adsorption and prior to the catalytic reaction. 'The lower activity of the Pd-Pt!AI 0 catalysts without oxygen treat2 3 ment is in good agreement with earlier suggestions (ref. 8) that the low activity of palladium can be attributed to the decreased rate of hydride
482 abstraction from hydrocarbons such as n-hexane. The oxygen treated Pd-Pt! A1 changes to the Pd-Pt/Si0
catalysts showed opposite selectivity 20 3 catalysts (ref. 9). We observed increased
2 activity maintenance, higher selectivity to benzene formation and constant isomerization selectivity with increasing Pd content, while the trends in
selectivity to hydrogenolysis and to C
cyclization did not change. 5 Gomez et ed, have found that both the surface enrichment and the modi-
fication of the electronic properties of surface Pt atoms are im?ortant in supported Pd-Pt catalyst (ref. 10). From our results we can suggest that after oxygen treatment, formation of a
bulk Pd-Pt phase with different Pd!Pt ratios may also have a signi-
ficant contribution to the overall catalytic performance as well as to the surface enrichment process and the electronic properties of surface Pt atoms. Separation of the Pd-Pt crystallites in oxygen into distinct crystallites of metal and tetragonal
Pd~
with formation of Pt and Pd-rich particles af-
ter reduction was shown by Chen and Schmidt (ref. 11). However, according to their data, this process can take place under o ur experimental condition (at 400
0
C) only for the 2Pd( Pd-Pt) catalyst.
ACKNOWLEDGEMENT T'he authors thank Dr. L.
Koltai for the palladium determination.
F<EFERENCES 1 2 3
M.W. Breiter, Trans. Faraday Soc., 65 (1969) 2197. B.J. Bowles, Electrochim Acta, 15 (1970) 737. Dj M, Kolb, M. Przasnyski and H. Gerischer, J, Electroanal. Chem. 54 (1971) 25. 4 S. Szabo and F', Nagy, J, Eledroanal. Chem-, 70 (1976) 357. 5 S. Szabo and F. Nagy, Israel J. Chem., 18 (1979) 162. 6 J. Margitfalvi, M. HegedUs, S. G-CibCilCis and F. Nagy, Pead. Kinet. Ceited, Lett., 18 (1981) 73. 7 A, Renouprez, C. Hoang-Van and P.A. Compagnon, J. Catal.. 34 (1974) 41 L 8 J.L. Contreras, J.M. Ferreira, S. l:-~uentes and R. Gomez, React. Kinet. Catal, Lett., 4 (1977) 373. 9 Z. Karpinski and T. Koscielski, J. Catal., 63 (1980) 313. 10 R. Gomez, G. Del Angel and G. Corro, No uv, Jo ur-n, Chim. 4 (1980) 219. 11 M. Chen and L.D. Schmidt, J. Catal., 56 (1979) 218.
483 DISCUSSION G. CORDIER : Have you prepared the same catalysts in sulphuric or phosphoric acid ? Have you measured the chloride contents after the oxidation treatment, and in conclusions, what do you think about the influence of chlorides in your results?
J. MARGITFALVI: As far as your first question is concerned, the fact that our basic studies on Pd adsorption over Pt metal were done in hydrochloric acid we did not try to use different acidic media. The chloride concentrations of our bimetallic catalysts shown in Table 1 were measured after metal adsorption, while those of the precursors were determined after the pretreatment procedure. The small changes in the chlorine concentrations of catalysts (Pd-Pt)l' Pd(Pd-Pt)l and ZPd(Pd-Pt)l, which were submitted to one, two and three oxygen treatments, respectively, may indicate that the oxygen treatment does not change the chlorine content significantly, but stabilizes it at around 1 %. As there are only insignificant changes in the chlorine content of our catalyst~ I believe that these small differences will have only minor importance on the overall catalytic performances of our catalysts. However, what differences could be expected in the absence of this chlorine? I have only speculations. D. CHADWICK: In your brief comments concerning the need for deoxygenated solutions and nitrogen, you referred to palladium desorption. Isn't it more likely that dissolved oxygen provides a cathodic reaction for the anodic dissolution of adsorbed palladium. In addition, it may remove preadsorbed hydrogen during the palladium deposition part of the preparation.
J. MARGITFALVI: In my brief comment I should like to stress out the importance of the deoxygenated environment in the different steps of preparation. ObViously, the oxygen can remove the preadsorbed hydrogen. We have experimental evidences that in the presence of oxygen and water the dissolution of the adsorbed metal can take place and the reoxidation of the adsorbed metal can be written as follows : (1)
The dissolution of Pd via reaction (1) resulted not only in a decrease in the total Pd concentration, but it also changed the Pd-Pt ratio within the catalyst pellet as the dissolution process was most enhanced in the outer sphere of the catalyst pellet. J.W. GEUS: Is it possible to apply electrons on the noble metal particles not from adsorbed hydrogen but from an external electrode inserted into the stirred suspension of the loaded support ?
J. MARGITFALVI
I believe that it is impossible.
J. KIWI: You mentioned you had 1.0% CIon the surface of AlZ03 coming from the noble metal chlorides of the metals you deposited. Have you tried to eliminate these Cl- ions by any of the known methods, e.g., dialysis in order to avoid segregation effects on the surface of AlZ03 ?
J. MARGITFALVI:
We did not try to eliminate the chloride content of our monoand bimetallic catalysts as these reforming type catalysts should have a certain bifunctionality. The only thing we tried to do is to prepare catalysts with almost identical chlorine contents. However, it will be really very interesting to prepare chlorine-free bimetallic catalysts and if the dialyse can be used in our system we certainly will try to adopt it.
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485
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A SCIENTIFIC APPROACH TO THE PREPARATION OF BULK MIXED OXIDE CATALYSTS Ph. COURTY and Ch. MARCILLY Institut Franqais du Petrole, B.P. 311 - 92506
RUEIL-MALMAISON (FRANCE)
Procatalyse, "Les Miroirs", 18 boucle d'Alsace
Paris La Defense zone 3
92400 - COURBEVOIE (FRANCE)
I. INTRODUCTION The purpose of this paper is to present an approach, as scientific as possible, to the selective synthesis of mixed oxide catalysts, i.e. those, which are uncommon ,
made of mixed oxides or true solid solutions and those, which
are more numerous, made of a complex association of mixed oxides (or solid solutions) and of single oxides.
We will restrict our discussion to bulk
mixed oxide catalysts. In our earlier publication of 1975 (1, 2), we had pointed out that homogeneity is a necessary condition for selectively obtaining mixed
oxide catalysts.
However, this condition is not always sufficient to characterize the microscopic composition of the catalysts. neity (on a 1 to 10 nm scale).
Some of them
~ndeed
show a controlled heteroge-
This is the case of the multicomponent
ammoxidation catalysts made of the association of Bi, Fe, Co, Mg and/or Zn molybdates in various compositions (3) and of some methanol synthesis catalysts made of the association of Zn aluminate (4) and of the CU solid solution 20-ZnO (5, 6). In a general manner, the superficial composition of the "ready for use" catalyst, which can be deduced from ESCA studies for instance, is different from the bulk composition.
This is often emphasized by the interaction bet-
ween the catalyst and the reaction medium. In all cases, the mixed oxide catalyst is the result of a complex synthesis method comprising
many elementary steps.
These steps must lead to the
desired state of the following characteristics : bulk structure, texture, superficial and bulk composition and metal oxidation state.
This implies the
fixing and the controlling of the main parameters of each elementary step and a systematic characterization of all the intermediary precursors.
In the
first part of this paper, some of the main steps will be studied. The mixed oxide coming from the last thermal activation step is usually different from the catalyst which has a steady-state interaction with the
486 reaction medium.
In the second part, transformations Qf the catalyst in contact
with the reaction medium
will be illustrated by a few examples.
II. THE VARIOUS METHODS OF SYNTHESIS OF BULK MIXED OXIDE CATALYSTS
Among the numerous methods reviewed in the literature (2), we will focus more particularly on those described in Fig. 1. only in specific cases.
Procedures b, c and d are used
Procedure a, which is by far the most common method,
will be illustrated by most of the examples given here. SOLUTION CONTAINING SOME PRECURSOR METALLIC IONS
~
__
Ib~_d
COPRECIPITATION
I
COMPLEXATION lacid_alcohols addn.)
I
GEL FORMATION
I
CAGINGJ
WA~~NG
DR~NG
OR~NG
I
CASlNGJ
MALAX;NG WITH OXIOES PRECURSORS
I
I
CAGING'
THERMAL ACTIVATION
THERMAL ACTIVATION (solid.stale reaction)
I
REMOVAL OF VOLATILE COMPOUNDS EXTRUSION OR I BALLS AGGLOMERATION Jl CAG'r
I
I
DRYING
I THERMAL ACTIVATION
THERMAL ACTIVATION decomposilion ) ( of the complex.
\
~
ADDITION OF OTHER REMENTS lmpregnalioft
malaxrn; balls ogglomerotian
I CAGING' I DRYING
I
THERMAL ACTIVATION '"
---(OPTIONNAll
FORMING PROCESS
, extrUSion,) (tabIet'i"' balls agglamerallon I . (AGING,
I DRYING
I THERMAL ACTIVATION
I CATALYST - READY FOR USEFig. 1
Different methods of synthesis of bulk mixed oxide catalysts.
487 The complex sequences shown in Fig. 1 for procedures a and, to a lesser extent, c and d, include the following main elementary steps : - separation of the hydrated precursor (Liquid to Solid reaction) - washing - hydrothermal transformations of the hydrated precursor (aging) - drying - thermal activation forming
- possible addition of other elements to the precalcined mixed oxide hydrothermal transformations.
During these elementary steps, important modifications of the solid occur.
In
the case of supported metallic catalysts, complex phenomena occurring during impregnation, drying and thermal activation are not perfectly known ; as well as the bulk and superficial composition of metallic association, the homogeneity and the dispersion may change, but the texture and the structure of the carrier are,in general,rarely modified.
In the case of bulk mixed oxide catalysts ho-
wever, the eVOlution is even more complex, as all these properties can change simultaneously.
This is one of the reasons why, in general, few publications
deal with the synthesis of mixed oxide catalysts and why the know-how is very rarely given. The fundamental study of the evolution of the catalyst precursor during each elementary step (the product obtained at each step becoming in turn the
precur~
sor for the next) has been,and still is,the subject of a great deal of research which should lead to a better understanding and, in many cases, to a better control of its transformation.
We will limit ourselves to the known general
principles of its production and illustrate them by a few examples.
11.1. Obtaining the hydrated precursor The aim of this step is to obtain a hydrated precursor which is as close
as
possible to the ideal precursor having the following qualities - relative proportions of active agents similar to those of the final catalyst - homogeneity - during decomposition, it should keep its homogeneity and permit the production of the active agents in the optimal states of oxydo-reduction and dispersion. The researcher has two main general ways to reach these targets : - separation of a solid phase within a liquid phase (discontinuous heterogeneous reaction)
: coprecipitation
- continuous transformation of a solution in a hydrated solid precursor (continuous homogeneous reaction) by gelification or complexation. Coprecipitation, the most common method, does not always allow the production
488 of a hydrated precursor with the desired composition.
In this case, one can
either make the coprecipitate change to this desired composition,by hydrothermal transformation for instance,(but such a transformation is not always easy to control) or use two other methods : gelification or complexation.
11.1.1. Coprecipitation 11.1.1.1. General principles Coprecipitation consists of precipitating simultaneously at least two metallic compounds within a solution, u5ually an aqueous one.
The supersaturation which
is necessary for the formation of the coprecipitate can be obtained : - physically:
by a change of the solution temperature by solvent evaporation
- chemically
by the addition of a compound which decreases the solubility (common ions, pH modifiers, etc •.. ) by mixing solutions, the salts of which react to give an insoluble compound (double decomposition) •
Precipitates can be very different from each other in terms of their morphological, textural and structural characteristics, but they can all be described in relation to two extreme cases - crystallized precipitates which are more or less hydrophobic - amorphous precipitates (gels, coagulates,flocculates) with an hydrophilic character. crystallized precipitates (chromates, molybdates, tungstates, phosphates, etc ..• ) have often a rigidly defined stoichiometry.
Examples of solid solutions
the composition of which can vary continuously and extensively, are not numerous. The
existence of such solid solutions which implies at least a partial misci-
bility
of the various compounds, is difficult to foresee.
MITSCHERLICH's
(1818) and GRIMM's (1924) rules, which express respectively the necessity for chemical similarity and for cases,
structural isomorphism, are obeyed in a few
but are far from being general.
Amorphous precipitates are not easily characterized by X-ray diffraction or by electron diffraction.
They usually have a flexible composition and are of-
ten unstable or metastable under their own conditions of preparation.
Many
precipitates are indeed intermediate between the two extreme categories defined above : they can move more or less rapidly from the amorphous state to a more organized one (7). Morphology, texture and structure depend on many parameters.
One of the most
important of which is supersaturation,which is crucial for the two main steps of the formation of precipitates : - nucleation (8) which is essentially homogeneous at high supersaturation and
489 heterogeneous at low or medium supersaturation - particle growth which occurs more or less with agglomeration and/or coalescence of particles. Among other factors which playa role in the quality of the precipitate, but to a lesser extent, we can mention the following : pH, temperature, nature of reagents, presence of impurities, method of precipitation, etc ... Supersaturation is a critical parameter for particle size, i.e. for texture. If it is high, the consumption rate of reagents by nucleation is much greater than that of particle growth : particles are numerous but small.
On the contra-
ry, if it is low and if solid impurities are absent (i.e. no heterogeneous a few but large particles are obtained.
nucleati~n),
Supersaturation is also very important for the structure of precipitates. During precipitation, many compounds can be obtained with several possible structures which are more or less metastable.
OSTWALD-LUSSAC's rule (9)
and
GOLDSMITH's principle of simplexity (10, 11) foresee that the higher the supersaturation the less stable and organized the structure is (higher simplexity).
As a matter of fact, when supersaturation is very high, dimensions of the
critical nucleus are very small and can be incompatible with the minimum level of information that must be included in this nucleus to form an organized structure (12).
In such conditions, only a metastable and poorly organized
phase can develop.
This phase can then change to another with less entropy
by hydrothermal transformation : this is the case of many metallic hydroxides and of the hydrothermal synthesis of zeolites which usually starts with a rapid gel formation.
Moreover, an intermediary metastable phase can favor
and orientate the heterogeneous nucleation of a more stable phase. In addition to the three characteristics mentionen above,
those of composi-
tion and, above all, of homogeneity must be added for coprecipitates. pitation rarely allows us to obtain a good macroscopic homogeneity.
CopreciIn a sys-
tem with two components A and B, the dAZ/dB z ratio of A and B within the elementary coprecipitate Z (which can be formed at any moment), and the A+S/B+S
ratio in the corresponding mother liquor S, are related to each other
by the derivative of the DOERNER-HOSKINS equation (13, 14) A+S
, where
:
A is the heterogeneous distribution coefficient.
B+S If the precipitation rate is low,
A~
1 : the coprecipitation is selective
and the coprecipitate is heterogeneous because the first crystals that appear do not have the same composition as the rest that follows.
If the precipita-
tion rate is high, the reaction· is limited by diffusion and A small
1 for ions with
differences in charges and sizes : the coprecipitation is unselective
490 and the proportions of components A and B in particles, as well as in the mother liquor, do not vary from the beginning to the end of the operation. Heterogeneities of the coprecipitate can also result from the method of coprecipitation : for instance, local heterogeneities of composition
may occur
in the system when reagents are mixed.
11.1.1.2. Pratical realization In a practical point of view, the main parameters are : nature and relative proportions of the metallic salts, concentrations, pH, temperature and the duration of the operation.
Other less obvious parameters may also be important
for instance, type and duration of stirring as mentioned mixed oxides precursors by JENSEN et al.
(15).
for iron molybdenum
This is consistent with our
own observations (not published).
Fig. 2 presents three REACTION PROCESS FLOW _ SCHEME
FIXED PARAMETERS
VARIABLE PARAMETERS
practical approaches to coprecipitation and indicates the
Tempero/ure
pH
concen/rolions resid,nc, liru AlB rolio in s/urr,
o
fixed and variable parameters.
Devices
A and B are particularly well adapted
BATCH (,ariable pH) COPRECIPITATION
for producing crystallized hydrated pre-
pH
T,mperolure AlB rolio
concenlrolions resid,nce lime
cursors which are usually obtained at low supersaturation
G)
(high dilution, high
BATCH (fixed pHl COPRECIPITATION
temperature). Device A is used rather for
pH
Temperolure conCflnlrolion residence lime AlB rati«
CONTINUOUS (fixed pH) COPRECIPITATION
double decomposition at variable pH between soluble salts of metals (Fe, Co, Ni, Cu, etc .•. ) and heteropolyanionic complexes and,more particularly,
Fig. 2 : Scheme and main parameters of the coprecipitation processes
for the synthesis of multicomponent metal catalysts, during which the feed compo-
491 sition can be modified.
For both procedures, concentrations and residence time
are variable ; therefore, differences of crystalline organization between the start and the end of the precipita{ion are difficult to avoid as a result of precipitate aging in the presence of its mother liquor. Device C, which corresponds to a perfectly stirred chemical reactor, is the most satisfactory.
It operates under steady-state conditions (residence time,
concentration, pH, temperature) and allows continuous production and better control of precipitate aging in the mother liquor.
It is used rather for pro-
duction of amorphous or poorly organized precursors.
These precursors which
have a flexible composition are obtained at high supersaturation and low temperature. Sequencial precipitation, where a first precipitated precursor is used to initiate the nucleation of be
a
second precursor with a different composition, can
performed in devices B or C i.n controlled pH conditions.
This method is
described in several patents (4, 16).
11.1.1.3. Examples of coprecipitation a) introduction Catalysts for the synthesis of methanol or for the
low temperature
shift
conversion usually contain a large excess of copper oxide associated to zinc oxide and to trivalent metal oxides like those of aluminium and/or chromium. They can also contain various other additives. be written
Their bulk atomic formula can
as (5, 6)
(cuO) 1 (znO)0.05_0.7
~
A I 20 3 cr
20 3
j 0.05-0.4
They are usually obtained by precipitation at constant pH. Publications H4 (GHERARDI et al.), H5 (PETRINI et al.) and H7 (SHISHKOV et al.) of the present congress deal with the coprecipitation of various crystallized precursors, their characterization and the study of their decomposition products (17, 18, 19). Towards 1977, KLIER et al. 'mentioned.
(5, 6) characterized some of the precursors above
Table I, which is non-exhaustive, sums up their results and those,
more recently obtained by many other researchers (5, 6, 16 to 24).
Only one of
the seven products identified in Table 1 contains three metals : CU, Zn and Al or Cr.
It is probably difficult to obtain this particular one in a selectiveWly.
This precursor, partly described by KLIER et al.
(5) for the Cu-Zn-AI system,
has a lamellar structure, isomorphous to HYDROTALCITE Mg A1 C0 4 H 2(OH)16 6 20 3, which can be indexed in the hexagonal system (space group R3m). This phase, which is also described by TRIFIRO et al.
(17) in the present congress for the
CU-Zn-AI system, will be called (HC) in our text.
GERHARDITE and Na-Zn hydro-
492 xycarbonate (table I) are not suited as precursors because the NO; and Na+ ions favor the sintering of the active phase during the successive steps, especially during thermal activation.
(see Fig. 4).
TABLE I Crystallized phase formed by coprecipitation of Cu(Co) Cr AI(Zn) based hydroxycarbonates References
Formula
Name
3+ H M (OH) 16 C0 3-4 20 2 ++ ++ ++ ++ Cu ,Co ,Zn ,Mg 3+ 3+ 3+ A1 cr Fe
HYDROTALCITE TYPE PHASE (HC)
Cu 2+
MALACHITE TYPE PHASE (ROSASITE)
2-x
COPPER-ZINC HYDROXYCARBONATE (AURICHALCITE)
Zn 2+ (OH)2 C0
x
2+) (Cu 2+ zn 5-x x
(5, 16, 17, 20, 21)
(6, 22) 3
(OH) 6 (C0
3)
(18, 20')
2
BASIC COPPER NITRATE (GERHARDITE)
(6, 23)
, ZINC HYDROXYCARBONATE (HYDROZINCITE)
(6, 24) (17)
i SODIUM-ZINC HYDROXYCARNATE
(19)
ALUMINIUM HYDROXYCARBONATE (SCARBROITE)
b)
preparation of hydroxycarbonates from nitrates at variable pH
Precipitation curves of Cu, Zn or Al hydroxycarbonates are so different that it is very difficult to obtain a tion at constant pH.
homogeneous precursor other than by precipita-
Fig. 3 presents the neutralization curve of a solution
containing the three metal nitrates by disodic carbonate at 70°C : the three neutralization waves show that precipitation is heterogeneous and that each hydroxycarbonate precipitates essentially on its own.
However, it can be obser-
ved that the precipitation pH for Cu and Al (4.4 and 2.9) are much lower than those obtained when hydroxycarbonates are precipitated alone from a solution containing only one of these two metals (respectively 6.7 and 5.5). carbonate
precipitates at a normal pH (7 to 7.5).
Zn hydroxy-
A study by X-ray diffraction
of the hydrated precursor (Fig. 4.a) which looks homogeneous on a visual scale, shows that it contains essentially GERHARDITE and a small proportion of the (HC) phase described in Table I.
Analysis confirms the presence of large quantities
of nitrates. A mild calcination (350°C, 3 hrs) of this heterogeneous precursor leads to a well crystallized mixture of sintered CuO (TENORITE) and ZnO (ZINCITE)
(Fig. 4.b).
493
••
pH
T.70·C N03-/CO"z-
8
• CuO • ZnO
Ij
(Mn+. AI"· ...Znt' ...Cu z·)
7
Calc.oxides mix!.
i \:~i
2 C03 2- II;
0.2
0.4
0.6
08
nh M O+
",--,---,-".:",
1.0
Fig. 3 : Neutralization with sodium carbonate (0.4 M) of Cu 2+, AI3+, Zn 2+ solution (0.35 M).
Fig. 4 : X-ray diagrams of the hydrated precursor (a) and oxides mixture (b) from neutralization sample (Fig. 3).
Cu ColnAI
Cu AI In
f
Calc oxide
(31IJ
Calc. spinel
:'
f)recursor .
•
,,-_',....- ;_.~.l ;\;.' ~il(;i.\t~; ,~;~--:,
~------:-:-:.~,:........,~.~.~r~_~----:-:
• {He] ternary phase malachite and lor rosasite phases
:\".~ ~\:;!. ~;:J~_ . _'.-~ __ y-;-_~.~- - :;:;
Fig. 5 : x-ray diagrams of Cu Zn Al precursor (a) and mixed oxide (b).
Fig. 6 : x-ray diagrams of Cu Co Zn Al precursor (a) and mixed oxide (b).
494 Alumina is not visible.
The relatively high value of the specific area of the 2.g- 1) calcinated product (50 m suggests that alumina has not sintered. Study of the precursor rich in GERHARDITE by electron microscopy shows
presence of large crystallites (up to 1 of crystals of TENORITE
the
Calcination leads to the formation
~m).
(CuO) of similar morphology.
KLIER et al.
(6) who pre-
pared his Cu-Zn mixed oxides according to the same method (slow addition in 90 mn of a 1 M carbonate solution in a 1 M nitrate solution at 80-85°C, with an increase of pH from 3 to 6.8-7), also obtained major proportions of GERHARDITE when the atomic ratio CU/Cu + Zn
~
0.5.
CAMPBELL had already pointed out, as
early as 1970 (25) that such a method leads to precursors in large crystallites and that it is preferable to operate in the opposite way (addition of a nitrate solution in a carbonate solution) in order to obtain finely divided precursors. c) batch precipitation of Cu-Zn-Al precursors Study of the batch precipitation of Cu-Zn-Al hydrated precursor shows that the crystallized ternary (HC) phase selectively appears onlyin a narrow range of compositions(l~.
If the atomic ratio Cu/Zn
<
1 and if the pH is
cons-
main~ed
tant (Fig. 5.a), a complex biphasic precursor is obtained, containing the (HC) phase and a Cu-Zn binary phase, ROSASITE as previously described by KLIER et al. (5, 6). (table I)
This phase is isomorphous and difficult to distinguish from malachite ; copper hydroxynitrate is
no longer formed.
After washing,
drying and mild thermal activation (300-350°C), a divided phase is obtained 2.g- 1 which has a specific area between 80 and 120 m and contains the three oxides in which Cu and Zn are partly combined (5, 6).
Only TENORITE (CuO) is
visible in the X-ray diagram (Fig. 5.b). d)
cobalt introduction in ternary precursors
Cobalt introduction in Cu-Zn-Al precursors results in a selective and highly homogeneous (HC) phase (16, 21). Fig. 6.a presents the X-ray diagram of this hexagonal structure (a = 0.305 nm, c = 2.24 nm, space group R3m) isomorphous with
the ternary structure (Cu-Zn-Al) mentioned above. The strong intensities
of (0, 0, 1) reflections and
scanning electron microscopy (STEM) (Fig. 7)
that this precursor has a lamellar structure. on STEM reveals it is highly homogeneous.
show
X-ray fluorescence microanalysis
After washing, drying and
moderate
thermal activation (350-450°C) a homogeneous and well divided spinel-type phase (a = 0.810 nm) is obtained (Fig. 6.b) its specific area is between 150 and 2.g- 1 200 m ; TENORITE is no longer visible. e) influence of operating conditions of coprecipitation on the crystallinity of the Cu-Zn-Co-Al hydrated precursor. The level of organization of coprecipitates depends on the nature and on the relative proportions of the various metallic ions.
For a fixed composition,
495
Fig. 7 : Electron micrograph (STEM) of (HC) phase and related microanalysis
OPEIWING COMJITKlNS
':[003J
[006J
1[012J i i
(015)
:[018J
SODIUM
SATU- (%wtl RATION
~Ii:'
I I
~R
Cu_Co_ALZn based TYPE OF precursor; [He] phose
IN
~~D
T"C (Fig.21 RANGE
OXIDE
[1019) [OTIJ] B 60-90
lOW
0.0050.015
CIS 50·80 MEDIUM 0.02' 0.05 10-40
HIGH
0.1' 0.3
Fig. 8 : Influence of the operating conditions of coprecipitation (supersaturation level, duration and temperature of reaction)on the crystallinity and the residual alkali content of Cu Co Ai Zn (CH) precursors.
496 the structural organization will depend, as indicated in the general principles
(§ II.l.l.l.), on the operating conditions and especially on supersaturation and on the length of the reaction.
Fig. 8 (26) which concerns hydrated precur-
sors of Cu-Zn-Co-Al mixed oxides (catalysts for the synthesis of alcohols described in ref. 16 and 21), shows that changes in operating conditions allow us to control the crystallinity of the precursor precisely without modifying its degree of homogeneity.
These precursors have the lamellar (HC) structure
previously described.
11.1.2. Complexation To avoid the usual imperfections of coprecipitation methods, a more general method was
developed a few years ago (27, 28, 29).
It permits the production
of an amorphous solid compound with a vitrous structure and of homogeneous composition, without physical discontinuity (phase separation) from the starting sOlution.
This is achieved by evaporating a solution containing various metal-
lic salts in any proportion and a complexing acid.oc
alcohol.
This method was
used for the preparation of many mixed oxides (29) and many bulk and supported catalysts (30, 31, 32). The metallic elements to be combined are added as soluble salts (or as reactive compounds) to an aqueous solution containing 0.5 to 2 gram equivalent of acid per gram equivalent of metal.
The following complexing acids are mainly
used: citric, maleic, tartaric, glycollic and lactic acids. complexing,
With citric acid
the evaporation under vacuum controlled conditions of a solution
results in a vitrous, transparent, amorphous solid precursor which is a mixed hydroxycitrate of the various metals. very homogeneous and isotropic.
Its composition and its structure are
Its thermal activation leads to the desired
mixed oxide. Table II presents the composition of such a citrate-type precursor containing Al and Y as a function of treatment conditions (29).
Such
decomposition is
progressive if metals active for oxidation (Cu, Ag, Fe, Co, Ni, etc ... ) are absent from the precursor.
In presence of such metals, thermograms of deeompo-
sition of the vitrous precursor usually show one or several plateaus; the semi-decomposed precursor does not contain any clearly illustrated by Fig. 9 (33, 34).
more
nitrate ions.
This is
In the presence of metals with oxydo-
reduction properties, it is possible to control thermal decomposition only by decreasing the concentration of nitrate ions or by dispersing the acid OC alcohol complex on a porous carrier (32).
497 TABLE 2 Composition of a citrate-type precursor of the Y AI0
3
perowskite as a function
of treatment conditions
Conditions of treatment
Product
In1 tial solution
Formula (calculated after analysis)
T '" 40°C
(COO'Iplete dissolution) Amorphous ,"l transparent
T :: 60°C - 20 brs (drying in vacuum)
Al~~132
Y~~125
(00;)0.426
(C6H507)~~115(C6H807)O.126{H20)O.658
T = 135°C -
opaque precursor
10 hrs (partial decomposition in vacuum)
Al~~132
Y~~125
(t«>~)O.OO3
(C6H507)~~154
Mixed oxide
T = 500°C, 4 hrs
rv
vi trous precursor Semi -deccepcsec , amorphous
(OB-l O. 30 6
A10 - 0.0028 A1 3 20 3)O.125 (perowskite substoechiometric phase)
(air calcination)
11.1.3. Gelification by polymerization in solution In a few cases, it is possible to prepare an amorphous hydrated precipitate having a strong hydrophylic character that favors its interaction with the mother liquor and its transformation by tridimensional
reticulation in a
homogeneous hydrogel retaining the majority of the solution in its net. Such is the case for iron molybdate gels (pure or with additives) which were developped at I.F.P. methanol to formol
(35) for the production of catalysts for the
(36). Fig. 10
oxidation of
(36, 37) shows that gel formation only occurs
in a narrow range of composition and operating conditions.
Main parameters
are the Mo/Fe ratio, the nature of salts, the possible additives and their relative proportions, the temperature, the concentrations and the type of stirring.
Aging increases reticulation and homogeneity; the hydrogel becomes
hard and brittle
(conchoidal break characteristic of hydrogels).
Depending
on the concentrations and Mo/Fe ratios, the precipitate will partly or totally dissolve to give a gel. atomic ratios Mo/Fe
~
A true metastable solution is transiently formed for 1.5 (37).
The aged gel is then dried to give a bi own
transparent, homogeneous xerogel containing less than
wt % water and which
produces the activated catalyst after a succession of other elementary steps (Fig. 1).
11.2. Hydrothermal transformation of the hydrated precursor 11.2.1. General principles on hydrothermal transformations These transformations cover all spontaneous reactions between a solid and an aqueous solution (0-300°C) or water vapor (200-500 0C) at atmospheric pressure or under pressure. calcinated solid.
They concern the hydrated precursor as well as the
498
weigh!
loso% 0
M~
~'.,
0
.~
~<,
\
50
~O
~f~'
0
100
®
Fe (at)
Ik
COLLOiDAL PRECIPITATE .... solution .... elastic,transparent, homogeneous gel
120.
\ Ni
--
~
(T.IO_20·cI
'.
roc 100
200
300
40Q
©
500
COLLOiDAL PRECIPITATE .... opaque, hard,britfle gel
2
(T.IO _ 20'C)
Fig. 9 : Effect of oxidation active metals (Fe, Ni) on the thermal decomposition of amorphous citric complexes (33, 34)
.... transparent.homogeneous gel (T.2O _ 400CI
@
0
COllOiDAL PRECIPITATE .... heterogeneous gel+ precipitate
3
II
AGING
II
T.SO_BO·C
PHzOllfP sot.
4
mechanical stirring
"
II
II
[M004]2[Fe]3+ion.g.L-'
15
0.5
2
25
Fig. 10 : Effect of the concentrations and Mo/Fe ratios on gel formation for iron molybdate precursors.
Inthe
presence of aqueous solution or water vapor, a solid spontaneously
modifies with a decrease in free energy.
This results in changes of one or
more of the following characteristics : bulk or superficial composition, precursor homogeneity, texture, structure.
Hydrothermal transformations aim at
acting as selectively as possible on these characteristics. Any change in the free energy, sum of two terms ; tne first one,
6 6
of the solid can be considered as the
G~,
v'
G
is related to the number and to the
strength of bonds within the solid; the second one, 6G , is assumed to be s proportional to the change of specific area 6 (6Gs = k , 6:f)
'I
0
When the hydrothermal transformation is essentially a textural one (the structure does not change : 6GVN 0) , 6 solid
increas$~and
f
is negative : the particle size
of the
the specific area decreases. This happens when autoclaving
a pseudoboehmite hydrogel in the presence of liquid water between 200 and 300·C, the specific area decreases from 310 to 72 mL.g- 1 ; if a mineralizing agent
499 (NaOH
0.01 M) is added, the specific area can decreases to 13 m2.g- 1 after
42 hrs (3B). In structural hydrothermal transformations, there is either a solvation
(Ii G :;II!: 0). If Ii G v v be positive: this results
change or a modification of the structural organization
IliGvl
is negative and if
is high enough,
.A~could
in an increase in the specific area as in the case of zeolite synthesis from gels. In general, hydrothermal transformations proceed in three steps ; -"breaking of the bonds at the solid surface,solvation - diffusion of
and/or dissolution
more or less solvated species
- integration of the species into a new particle or a new structure. In the liquid phase, dissolution or diffusion are the limiting steps. Adding mineralizing compounds (acid, base, salt) and/or increasing the concentration gradient between both solid phases are ways of increasing the rate of these steps.
11.2.2. Examples 11.2.2.1. Hydrothermal transformation in the presence of aqueous solutions As early as 1939, H. FORESTIER and J. LONGUET (39) found out that hydroxides coprecipitated or in physical mixture, kept in boiling water can react with each other and transform into crystallized mixed oxides.
Ferrites with spinel-
type structure are thus obtained. by aging a mixed hydroxyde of
Fe and Zn, Co
or Ni between 60 and 220°C. Ni cr
can be obtained by the same method even 204 at room temperature in about 13-15 days (40). The aging of mixed hydroxides
of Al and Ni at 100°C results in the formation of a crystallized hydrated precursor having a lamellar structure isomorphous to ANTIGORITE Mg Si 0S(OH)4 2 3 and close to that of the (HC) hexagonal phase. Calcining this precursor at 1000 0C leads to the Ni Al
spinel. Binary couples Zn-AI and Mg-AI 204 give a similar precursor (HYDROTALCITE for Mg-Al). Fig. 11 and 12
(41) present the X-ray diagrams of the products originating
from coprecipitating Zn-AI hydroxy carbonates (70°C, constant PH) and by aging them. If aging is short, a mixture of a spinel is obtained.
(HC) - like phase and of the Zn Al
204 A B houmaging at BOoC in pure water favors a selective
growth of this (HC)- like phase, without noticeably changing the proportion of spinel.
After washing, drying and calcining (450°C, 3 hrs) , the desired 2.g- 1). mixed oxide (Zn Al spinel) is obtained in a divided form (260 m 204 Nuclei of Zn Al spinel formed during coprecipitation certainly favor the 204 transformation of the precipitate into the spinel phase. Crystallized hydrated precursors containing copper are metastable.
Their
drying must be performed in particular conditions. Aging them at GO-BO°C in the presence of water, causes their hydrolysis (Fig. 13) and leads to a recrystal-
500
• [HC] precursor spinel
[311J
aging. B hrs.lBO"C
[2201
[400]
b
[4~21~
[511] [440] ,
b
• [HC] precursor
ZnAI204,[CHj ~rec.
spinel
aging, 0.2 hr.l20"C
aged precursor
Fig. 12 : X-ray diagrams of (HC) precursor (a) and activated Zn A1 20 4 mixed oxide (b)
Fig. 11 : Effect of aging on crystal(HC) type prelization of Zn A1 204 cursor
..
•
CuO
• [He] malachite and/or rasasite
a
Fig. 13 : X-ray diagrams of Cu Al Zn, (CH) type dried precursors : a properly dried b : dried under high steam partial pressure
501 lization of black cupric oxide which is easily identified by X-ray diffraction. The crystallization of the (HC) phase is improved by this treatment. Another type of undesirable aging can occur "if the desired mixed oxide has a high rate of sintering in the condition~" by TRIFIRO et al.
of its formation.
As shown
(42), such a reaction occurs with the hydrated precursor of
the Fe-Mo catalyst and leads to a stoichiometric Fe (M00 mixed oxide in a 2 4)3 2.g- 1) poorly divided form (3.5 m and with poor catalytic properties.
11.2.2.2.
Hydrothermal transfornlation in the presence of water vapor
Examples of hydrothermal transformations of solids in the presence of water vapor are numerous.
In a lot of cases, they result in
sintering characterized
by a decrease of the specific area at nearly constant pore volume. A specific hydrothermal effect has been observed by with a low sodium content (~O.7 water vapor (43).
calcining NH zeolite 4Y wt %) under a controlled partial pressure of
As shown in Fig. 14, calcination above 450°C using very dry
air leads to a tremendous decrease in the specific area due to the collapse of the structure.
A metastable and very disorganized HY form is obtained after
calcination at 500°C under a small partial pressure of" water vapor (about 30 Torr)
; the
porosity of such a metastable product is mostly intact
2.g- 1). sorption = 21.7 wt % at 25 Torr and 2SoC, S = 712 m The evolution 6H6 of this metastable form can be directed in two different ways depending on the (C
treatment : - complete collapse of the structure if calcination is performed above 500°C under very dry air - stabilization of the structure if calcination is continued at 500°C with a high partial pressure of water (about> 150 Torr).
The zeolite is thus cha-
racterized by a well organized structure, an important microporosity (C 6H6 2.g- 1) sorption = IS.4 wt %, S = 672 m and a high thermal stability (up to about SOO°C) . If NH is calcined from 150 to 500°C under high water partial pressure 4Y ( > 500 Torr) a limited collapse of the structure is observed (C sorption 6H6 2.g- 1) 12.2 wt %, S = 406 m (43, 44). All these results can be explained by the intervention of two competitive modification processes : one is aluminum extraction from the framework, the other is migration of silicon created by the removal of aluminum. on the pperating conditions. is much faster than silicon
toward holes
Relative rates of both processes depend
The structure collapses if aluminum extraction migration.
crystallinity is maintained if the
rates of both processes are of the same order of magnitude.
502
SAMPLE
nr
CALCINATION wt%
final
lTemp atmosphere adsorbed C6H6 °C
SAMPLE CHARACTERISTICS S m~g_l
X .ray Diffraction
. 1
450 very dry air
< 1 <10
2
500 slightly wet air
21.7 712
« 30torr H2O)
J ~
product 2
3
500 reea/eined 18.4 672 under wet air (>200torr H2O
4
1Jld II
air... water
500 vapor from 12.2 406 150°C >500tarr H2O)
.1.
".~J.
I
"J
Fig. 14 : X-ray diagrams of NH zeolites after air calcination under various 4Y conditions
11.3. Washing of the hydrated precursor This elementary step aims at removing compounds (by-products of coprecipitation) retained in the hydrated precursor and which are undesirable because of the various following risks : - inhibition or modification of activity and/or selectiVity of the final catalyst - bad evolution of the precursor during the next elementary steps. Compounds to be removed can be divided into two groups : - those dissolved in the mother liquor still present within the porosity ions, mineral or organic molecules. - those fixed at the surface of the hydrated precursor
mainly ions.
503 The first ones can easily be removed by simple washing with distilled water. But the ease of washing will depend
·on the nature of the precipitate.
Amorphous precipitates like hydrogels or coagulates are in general hydrophilic, voluminous and their sedimentation is slow.
They are difficult to wash out
because of diffusional limitations in their particular porous texture.
On the
contrary, well crystallized precipitates decant more easily and can be washed out without difficulty.
This case is well illustrated by Fig. 8 which shows
that the sodium content of washed Cu-Zn-Co-Al
mi~ed
oxides decreases when the
crystallinity of the precursors increases. Removal of ions of the second group requires special washing conditions which will be briefly described.
An amphoteric oxide placed in water acts either as an
anionic or cationic exchanger depending on pH and ionic strength conditions. Its surface, which is indeed essentially
positively or negatively charged,
is neutralized by ions from the solution (45 to 48).
The isoelectric point
(which corresponds to an overall neutral surface containing an equal number of positive and negative charges) is reached when the pH value is equal to pHi. If pH
>
pHi' the solid surface is negative and the solid acts as a cationic
exchanger.
If pH
<
pHi' the solid surface is positive and the solid becomes
an anionic exchanger.
Two methods can
theoretically be used to remove unde-
sirable ions : - replacement of these ions by ions of the same kind, which are not inconvenient or which decompose easily during calcination : this is ionic exchange - washing under pH conditions where undesirable ions are not fixed. anion can be eliminated by washing with water at pH
>
Thus an
pH., because in this ~
condition, the solid is negatively charged and does not fix anions.
For ins-
tance, nitrate, sulfate and chloride anions can be eliminated by ammoniacal washing.
Sodium and calcium can be eliminated by acidic washing.
Suspensions of amphoteric oxides are washed in most cases by the second method which makes the removal of the strongly adsorbed ions easier.
Suspen-
sions of non-amphoteric ion exchangers like silica rich silica-alumina and zeolites, are washed according to the first method.
Their framework is indead
negatively charged whatever the pH may be, due to the valence 3 and the coordination 4 of aluminum.
In both kinds of washing, particular attention must be
paid to some secondary reactions that might occur during the contact between the hydrated precursor and the washing solution (hydrolysis, selective tions)
dissolu~
(49).
11.4. Drying Drying is aimed at removing water filling the pores as well as adsorbed water.
Obviously, it is rather difficult to dissociate this elementary step
and its secondary effects, especially textural and structural hydrothermal
504 effects. Classical drying is performed under atmospheric pressure with a high air flow rate and for long periods.
Particular techniques such as spray drying
or vacuum drying have also been developed. Drying of well crystallized compounds with low or medium. porosity is usually an easy operation ; texture and structure are not affected if drying is fast enough to limit hydrothermal effects.
On the other hand, drying of amorphous
or poorly crystallized precipitates and of the organic complexes previously mentioned can be responsible for considerable morphological, textural and structural evolutions.
In this case, this elementary step is important because of
its secondary effects. Specific phenomena which occur during drying of a hydrogel are due to interfacial tensions which develop within the porosity.
Below the critical
temperature of water, drying is characterized by a contraction of the solid and therefore usually leads to a xerogel of high density and low porous volume. Above the critical temperature, capillary tensions no longer exist and an aerogel of low density and high porous volume is obtained (50).
Effects
of interfacial tensions can be limited in several ways : - hydrothermal pretreatment resulting in an increase of pore diameter (at constant structure) - addition of a substance which lowers capillary forces (51). Secondary effects can be limited by drying at low temperature under vacuum or low pressure, or by fast drying like spray drying.
11.5. Thermal treatments
The objective of thermal treatments is to transform the previously dried precursor into a mixed oxide (or an association of single and mixed oxides) having the desired characteristics of texture and structure.
During these
treatments, volatile or decomposable compounds still present after the preceding step, are eliminated. It seems that there are no general rules concerning thermal treatments. It is only to be noted that some interferences often appear between the following reactions : - thermal decomposition of the precursor - formation of the mixed oxide - thermal sintering of the mixed oxide. Many parameters among the selected operating conditions (temperature, pressure, atmosphere. procedure of activation, solid-gas contact technology, etc .•. ) can playa decisive role on these three reactions. In other respects, in severe thermal conditions (T
~
400°C for instance), solid state reactions
505 or composition modifications by sublimation of volatile compounds (such as M00
v etc •.• ) can occur. Suitable programming of the operating conditions 3, 205' could be established with the aid of a study of the thermal decomposition of the precursor (TGA, DTA) and of correlations between temperature and sintering, like those presented in Fig. 15 for Fe-Cr-K mixed oxides which catalyse ethyl-
benzene to styrene dehydrogenation (52). As
in supported metallic catalysts, the kinetics of sintering depends
largely on the partial pressure of water vapor and also on the presence of various compounds (alkaline ions, nitrate ions, etc ... ) in the solid and/or gaseous phases.
This is the case in thermal decomposition of organic complexes
and of the Fe-Mo xerogels previously mentioned,
which may have retained in
their porosity some of the by-product ions of the precipitation.
These cases
require particular thermal and/or hydrothermal treatments (36). Another example presented in Fig. 16 (26) concerns the influence of thermal sintering on the degree of the crystalline organization of a divided AB
20 4,
-£ AO spinel phase containing an excess of the bivalent ion A (A B = Al).
= Co, Cu Zn
In spite of this excess above stoichiometry, these phases remain
remarkably homogeneous and highly dispersed when the temperature and the length of calcination increase.
If the same hydrated crystallized precursor
contains significant quantities of alkaline ions, even in the absence of nitrate ions, a similar preparation concluded with a mild calcination (350·C, 3 hrs) leads to two distinct phenomena : - demixing of cupric oxide having a TENORITE structure easily identified by X-ray diffraction - sensible sintering of the spinel phase 2.g- 1 from 190 to about 110 m A study
the specific area decreases
of this product in electron microscopy coupled with X-ray fluorescence
microanalysis reveals the CuO demixing areas and shows that the composition of the spinel phase remains homogeneous (Fig. 17)
(21, 53).
These phenomena
of demixing and sintering will be emphasized if the mixed oxide is calcined at higher temperature. To conclude on thermal treatments, let us remember that the solid state reactions mentioned
in Fig. 1 can be accelerated by artificial nucleation.
In the case of the Ni cr et al.
2
04 sYnthesis, this has been illustrated by CHARCOSSET
(54).
11.6. Possible addition of other elements to the precalcined mixed oxide. Forming. Hydrothermal transformations. Usually, the thermally activated mixed oxide still has to go through a few operations before obtaining the "ready for use" catalyst.
These operations
506
O.
average pore diameter l¢ pAl
cumulated(Vp cm 3g- l ) porosity
( 0.2000 AIm)
Vp
5000
025 4000
3000
0.20
specific area
4
2000
(5 m2g " )
3
s 1000
hrs. oir calcinotion at 900
.r-c
950
1000·C
Fig. 15 : Fe Cr K oxides based catalyst for dehydrogenation of ethylbenzene to styrene. Influence of the calcination temperature on the textural properties.
, ~[hkIJ spinel
[311]
"I
;]\1 phase
,j 1\\ ;1'\\\
.; !\\Vt~~~
"'\\~l ;; I :°11
i:j ,
I
\
:q
:1~'~:
;
[2~0]
11\
::J
[440]
I \
I
I
i ~;~I
'1\I \
II~'I
I I
I I
OF CALCINATION (M 2G-')
[4001
)1I :~\.
II
[51ll I
:I
II
Iii
TEMPERATURE SPEC.
AND DURATION AREA
i II I
I
450OC/20hrs.
158
4500C/3hrs.
170
350OC/3hrs.
190
I
::
L',.. . . ..... :
I
Fig. 16 : Cu Co Al Zn mixed oxide (spinel structure). mal sintering on the degree of crystallinity.
Effect of ther-
507 are mainly : - addition of other elements to the calcined
mixed oxide by means of
impregnation or malaxing with a solution - the forming of the catalyst after a controlled moistening of the mixed oxide (balls agglomeration, extrusion, etc .•. ). During both operations (the two can be combined in some cases (55»
or during
subsequent drying, hydrothermal transformations of the activated mixed oxide may occur.
11.6.1: Deposition of metals on the precalcined mixed oxide Rules which govern impregnation and malaxing have been extensively described elsewhere (48, 55).
A fundamental study of ion exchange properties of mixed
oxides must be undertaken for each particular case.
11.6.2. Forming by controlled moistening Many pUblications deal partly with this elementary step (55, 56).
The mixed
oxide must have a hydrophilic character and its moistening must lead to a phenomenon of setting or allow the formation of a paste.
homogeneous and thixotropic
Both properties are characteristics of an imperfect state of crystalli-
zation.
If the mixed oxide does not present at least one of these properties,
it must be formed by tabletting.
11.6.3. Hydrothermal transformation of the calcined oxide A genuine example of hydrothermal transformation of the calcined
mixed
oxide is that of an industrial catalyst for the synthesis of methanol (Fig .18)(41). In most cases, the crystalline structure of this oxide is deeply modified by a simple moistening followed by a mild drying.
A partial reversal is observed
as well as recrystallization of the (HC) phase isomorphous to HYDROTALCITE. This is the reason why it is advisable to stock this catalyst in a dry atmosphere. As early as 1953, TERTIAN, PAPEE et al.
P and ~
for alumina.
(38, 57) described a similar reaction
aluminas, obtained by dehydration
hydrated aluminas at 200-400°C
un~rhigh
of crystallized
vacuum, can be transformed by rehydra-
tion and suitable aging, either into BAYERITE (T = 25°C, 14 days, liquid water) or into BOEHMITE (T
=
200-300 oC, liquid water)
(38).
To our knowledge, such a
reaction, well known for alumina, does not seem to have been described for mixed oxides. The few following examples concern again the mixed oxides used as catalysts for the synthesis of methanol. Zn Al
204
spinel phase, the preparation of which has been described in the
508 second part (calcination of the crystallized hydrated precursor 3 hrs at 450°C) is a well divided 2.g- 1) (260 m and homogeneous solid.
Its X-ray diagram is presen-
ted in Fig. 19.a (41).
Simple
moistening of this oxide, followed by aging at 80°C sence of
in the pre-
liquid water, resul ts in a'
recrystallization of the (HC) phase,whereas (Fig. 19.b) the degree of crystalline organization of the spinel phase does not change ; calcination must be
per~
formed at temperatures higher than 450°C and for a long period so as to avoid recrystallization by rehydratation.
Fig. 17 : Electron micrograph (STEM) of Cu Co Al Zn alkalinized mixed oxides and related microanalysis •
• CuO [HC) phase
Fig. 18 : Effect of moistening, aging and drying on the crystalline structure of an industrial catalyst for methanol synthesis.
Fig. 19 : x-ray diagrams of air calcined Zn A1 before (a) and after 204, (b) moisten1ng, aging and drying.
509
Fig. 20 : X-ray diagrams of air-calcined Cu Al Zn mixed oxide (Fig. 5) before (a) and after (b) moistening, aging and drying .
• CuO
• [Hqternary phase
.
. .a
e .
.....
• •\:\.. . . . . __"'_./.-.<'.., ........>:
.......b
-'
CuZnAI
~
Calc.oxide
.
...
.'
With Cu-Zn-Al mixed oxide, the preparation of which has been previously described (Fig. 5), a similar phenomenon is observed (Fig. 20) case, i t might be due to
(41).
In this
selective recrystallization of hydrated zinc alumi-
nate, because copper aluminate neither recrystallizes in a hydrated state nor in an anhydrous state when calcination temperatures are lower than 450°C, and its rehydration causes a partial hydrolysis in CuO + Al(OH)3 (58, 41) without any formation of the (HC) phase. tallizations which
a~e
Water is essential for these recrys-
not observed in non aqueous medium.
Mixed oxide catalysts are usually thermally activated at temperatureS
which
are not too high in order to obtain a high state of division, that is to say a poor state of crystallization.
So it is not very surprising that such solids
are reactive enough to be hydrothermally transformed.
III. TRANSFORMATIONS OF MIXED OXIDES IN THE REACTION MEDIUM 111.1. Introduction The 'ready for use'catalyst made of the thermally activated mixed oxide is, in most cases, very different from the steady-state catalyst which is in equilibrium with the reacting medium under catalytic operating conditions. In mild oxidation, reaching the steady-state does not cause an extensive modification of the bulk or superficial composition of the catalysts. But in many other catalytic operations, the composition, the texture and the structure undergo important changes owing to interactions between catalyst and reactants, products and by-products.
Among these modifications, the following can be
emphasized : - formation of divided alloys or metals (hydrogenation, CO + H catalysis) 2 - formation of single or mixed sulfides and/or sulfur-metal association (hydrotreating) - superficial coking and structural modifications (dehydrogenation)
510 - carburization (CO + H catalysis) 2 - nitridation (NH synthesis or decomposition) 3 - modification of bulk and/or superficial composition (thermal effects on volatile oxides, chemical or physical migration, attack by reactants or products). It is not the purpose of this communication to deal with catalyst deactivation in the reaction medium.
This has been treated in
in various specific congresses (59, 60).
many publications
We will restrict ourselves to modi-
ficat.ions which lE:,ad to the ini.tial steady-state.
Study of these modifications
implies a precise characterization of the steady state catalyst (bulk and superficial composition, texture, structure, oxidation degree of metallic elements, mechanical properties, etc ... ). This leads us to two remarks : - the steady-state depends, in most cases, on the operating conditions. It is sometimes reached after a long period (up to 1000 hrs) and is very difficult to characterize lIin vitro" - there are no general rules which permit foresight into and control of the catalyst evolution toward the steady-state.
This evolution is often even
much more complex than the preparation of the catalyst. The following
three examples will illustrate the main problems which can occur.
111.2. Molybdenum or vanadium based
catalys~for
mild oxidation processes
Under reaction conditions, many mixed oxide catalysts for mild oxidation undergo significant modifications of their superficial composition and very often oxide sublimation (M00
V which deteriorates the homogeneity and 3, 205) the textural properties of these catalysts. For instance, migration of M00
has been observed in the Fe - Mo0 2(M004)3 3 3 oxide combination used as a catalyst for methanol to formol oxidation (61, 62, 63), in Mo0 - V supported mixed oxides used as catalyst for benzene to 20 S 3 maleic anhydrid oxidation (64) and also in the M.C.M. catalysts (Bi, Co, Fe, Ni, molybdates) for ammoxidation of propene (65).
In the latter case, an
enrichment of the surface in Mo, Bi and Fe is observed.
These composition
modifications are due to the thermal instabilit:y of molybdate phases in the operating conditions.
At least two parameters are decisive:
- the average temperature of operation and the temperature of the hottest point of the catalytic bed - the partial pressure of oxygen. The partial pressure of oxygen is often decreased when recycling a part of the used air after the water absorption of reaction products. raising
This allows the
of the lower limit of ignition of the reaction medium and the use
511 of higher concentrations of reactants, but has two consequences - an increase in the heat of the reaction per unit volume of catalytic bed - an increase in the rate of the reduction of the catalyst, in the MARS and VAN KREVELEN - type equilibrium (66) oxidized Cat. + reduced Cat. +
Cat. + products
reactants~reduced
oxidized Cat.
02------------~.~
(M00 - Mo0 oxide mixture, the reduction 2 4)3 3 species (62, 63) is accelerated and results in a
For instance, in the case of Fe toward the
IX - or
demixing of Mo0
3
The c~talyst
p-
Fe MoO4
which mOVes inside the solid. stabilization under process conditions can be improved by
formula modifications and by using milder operating conditions (lower concentration of organic reactant, dilution of the catalyst, etc •.. ).
In all cases, the
best economical compromise must be sought.
111.3. Catalyst for ethylbenzene to styrene dehydrogenation This bulk catalyst is composed of ferric oxide promoted with chromium oxide and stabilized by potassium.
Moreover, it
can contain various other textural
and/or structural promoters (67, 68, 69). The "ready for use" catalyst is in general obtained by malaxing iron hydroxide or oxide with a solution containing promoters. ded, aged and then dried.
A
The paste obtained is extru-
solid-state reaction (procedure d of Fig. 1) is
performed in severe thermal conditions (7OO-1200°C). Fig. 15 (paragraph 11.5) presents correlations between temperature and sintering for one of our catalysts. An X-ray diffraction study of the activated catalyst (Fig. 21) shows that it contains essentially the IX -Fe
HEMATITE phase (52). 20 3 Ethylbenzene to styrene dehydrogenation is performed with this catalyst
under the following operating conditions LHSV
= 0.3-0.7
h- 1, Steam/oil ratio (wt)
water vapor improves the performance is reached after about 1000 hrs.
T = 580-590°C, P = 2 bar, 2.
Dilution of ethylbenzene by
and limits coking.
The true steady-state
During the first 300 hrs of this transitory
period, a slow modification of the
crystalline structure occurs
catalys~s
whereas its texture is not significantly affected (Fig. 21). is reduced
to the ferrimagnetic Fe
interact and lead to
IX K
Cr
04'
2 2 because the molar ratio PH2/PH~
phase.
The IX-Fe phase 20 3 Chromium and potassium oxides
30 4 Reduction of Fe
does not go beyond 20 3 is always less than 0.1 (52). Simulta-
Fe 0 3 4 neously, secondary reactions of dealkylation under water, which lead to benzene and toluene
decreases,and the molar selectivity in styrene increases in a
significant way. more selective (Fig. 22)
(70).
After 1000 other hours of operation, the catalyst is much although the structure obtained after 300 hrs is not modified
512 x
X [104J ./00
[1I0J .50
CAT.AFTER
300 HRS. PILOT TEST
X (116]. 60
[021].25 X [02.]40
X mognelile(Fe,O.) .. promotors
X
(300]35 X X (214]35
o K2Cr040(
X[113] 30
60
40
50
20
e[311]100 30
NEW CAT. e [440]. 40 e(120J30
hemotile( Fe20,c<) . promotors e
e(511] 30
e (400].10
.. • (422]10
60
50
40
30
Fig. 21 : Fe Cr K oxides based catalyst for dehydrogenation of ethylbenzene to styrene. Effect of the stabilization (in reaction medium) on the crystalline structure.
Benz...
'!o(mol.)
90
0_._ 0_._
0_.-.-.-
0 - . - _ . _ 0 .......
i
T.59cJot 80 70
I
p. 2.bar, TaSeo-c STUNJOIL= 2(wI)
60 50
___.. . . . _. °'-_0 __ 0 __ 0 _ _ 0 __
40
/ I
P.2Dor.
T.aeooc
LH$V. O.lth-1 STPIl/OILdtwt)
I
I
li~1O i i Ethytbenzone 1::----', i'--'-'",,__,
°j
1
•i I i
TEAMIOIL..2(wl)
j~
rr.6OO"C I
I
, I
"'o__ o__ oj
I
0----
i I I i i i~.
0
__
conversiCJfl 0
__
0
__
__• 0
Sw- yield
i , l ' /
!
I I
I I
!..
··.411
1300
5
'~\,
I i i ' ,
~ __-------: 200
15
P.z bart
f ~ LHSV.o..,tr-1 j STfAN I
1
i
100
I
I
I
I
,
20
j 0 .... 1.,//
I I
..o_. __ .-oyield
I .".""",
I
30
i I
I
Toluene
•
I-
/i
:=.. .
I
i r.s'O"C
I/ OOO hru
LHS\I'=O..l4.-I
I _'-0-'_0_.-01
i
I
S ...!!.~".!_~r!c!!!~!_._o_
i
T _ yiold "°"_0'"
0
.... 0
14
Fig. 22 : Dehydrogenation of ethylbenzene to styrene. and steam-treatment on performance .
.~.o
__-r~,-
i~,--___o__--!~~~~ 1600
17
hours of operation
Effect of aging
513 At the end of this transitory period, injection of ethylbenzene is stopped and the catalyst is kept under water vapor at 600°C for
48 hrs.
After such
steaming, the catalyst presents again a relatively high dealkylating activity and a new 300 hrs period is necessary to reach the previous steady-state. Analysis of gas collected during steaming reveals the presence of a significant proportion of CO which probably results from superficial decoking of the 2 catalyst according to the water-gas and shift reactions; moreover, these reactions are catalyzed by alkaline metals. To a lower extent, one other reaction produces CO 2
but, under steaming and operating conditions, the amount of K transformed 2C0 3 is quite low (71). Therefore, the steady state of the catalyst probably results from two consecutive phenomena: (70). - a slow reduction of Fe
into Fe 20 3 304 - a selective coking under water of sites active for steam-dealkylation. This steady-state is metastable.
A slight decrease of the steam/oil ratio
is sufficient to increase drastically the coking of the centers which are active for dehydrogenation and therefore to reduce heavily the production of styrene. The catalyst activity can be partly restored by steaming. The optimization of such a catalyst implies a setting up of correlations between the physicochemical properties and the stability of dehydrogenating activity under the most economical operating conditions.
The proportion, the
distribution and the degree of combination of the alkaline elements are some of the main parameters of this optimization (67).
111.4. Catalysts for the synthesis of C
to C alcohols 1 6 We will summarize here some recent results (21) obtained in the study of
the transitory phase of some catalysts in the synthesis of C
to C alcohols. 1 6 These catalysts are composed of Cu-Co-Al and possibly Zn mixed oxides promoted by alkaline metals.
In paragraph 11.5., an
electron microscopy study of the
activated catalyst was presented: except for some copper oxide demixing, this catalyst is very homogeneous.
A T.P.R.
(Temperature Programmed Reduction)
study shows that the reduction of cupric oxide to copper is complete at 240°C ; the reduction of CoO to cobalt about 700°C.
= 6-10
MPa, H = 2, GHSV 2/CO gen is never complete. P
starts at 380°C and is complete at
So, i t appears that under the
=
process conditions (T = 260-300°C,
4000 hr- 1 NTP), the reduction of CoO by hydro-
When the catalyst, prereduced by hydrogen, comes into contact with
~he
514 synthesis gas under the process conditions, the following phenomena are observed : 2 - reduction of co + to metallic cobalt - exothermic chemisorption of CO on metallic cobalt formation of cobalt carbonyl for certain compositions, which results in a cobalt migration - transitory reaction of the very exothermic methanation. Study of the catalyst stabilized in the reaction medium reveals a decrease of the cobalt content in the spinel phase, which is not easily detected by X-ray diffraction.
~Very
small crystals 1 to 3 run in size, containing Cu and
Co and possibly Zn, are formed (Fig. 23).
Under special operating conditions,
formation of large crystallites of cobalt carbide can be revealed by X-ray diffraction.
Fig. 23 : Electron micrograph (STEM) and related microanalysis of Cu Co Al Zn mixed oxides catalyst after H reduction and 2 aging in (CO + H medium. 2)
In this last example, transformation of the mixed oxide is extremely complex and
characterization of each intermediate of the solid transformation
is indispensable for the understanding and the control of its evolution.
515 IV. GENERAL CONCLUSION The preparation of mixed oxide catalysts, which is usually very complex, can be described as a succession of many elementary steps which are governed by more or less empirical rules and where the evolution of composition, structure and texture often occurs simultaneously.
A systematic and rigorous characteri-
zation of each intermediate is necessary to set up a continuous relationship between the first hydrated precursor and the final activated mixed oxide. No results have been presented on mixed oxides, the surfaces of which have properties (composition, degrees of oxidation) different from those of the bulk.
In general, such modifications occur during drying or thermal activa-
tion and sometimes during reduction of the oxide (72). The "ready for use" catalyst can undergo many transformations when put into contact with the reaction medium.
The examples given in the third part show
the diversity, the complexity and the specificity of these transformations. In some cases, the steady state catalyst is so different from the activated catalyst (Cu-Co crystals of example 111.4) that other ways of synthesis can be considered where the last precursor (the activated mixed oxide) is not always indispensable.
As a matter of fact, products thus obtained can give the opti-
mal steady-state catalyst more easily than
the activated mixed oxide.
Such
an approach, which is relatively new, could initiate in some specific cases the perfecting of new preparation processes. Mixed oxides occupy an important place in heterogeneous catalysis.
If alu-
minosilicates are excluded, mixed oxides catalyse the main following reactions synthesis of methanol and of C to C alcohols, FISCHER TROPSCH synthesis, the 2 6 hydrogen chain and some mild oxidations. They are also carriers and cocatalysts of many metals.
More recent works show that fields of application of mixed
oxides in catalysis are more and more diversified.
Important developments
in
research into the synthesis of mixed oxides are expected, more especially as many other fields (electronics ,microprocessors , magnetism, nuclear, etc ... ) initiate new research into this subject. Whatever the method of preparation, there must be a good compromise between scientific, ,technical and economical imperatives as shown in the following diagram : catalytic performances
process conditions
-
tt
final catalyst optimization
1!
~
preparation procedure (costprice of catalyst)
process ec cnomy
516 REFERENCES Intern. Symp. on Scientific Bases for the Preparation of Heterogeneous catalysts (editors B. Delman, P.A. Jacobs and G. Poncelet) Elsevier, Amsterdam 1976. 2 Ph. Courty, Ch. Marcilly, General Synthesis method for mixed oxide catalysts, in ref. 1, 1st Symp., 1976, pp. 119-145. 3 M.W.J. Wolfs, J.B.C. Van Hooff, in ref. 1, 1st Symp., 1976, pp. 161-171. 4 I.C.I. Patent U.S. 3.850.850, 1972. 5 S. Melta, G.W. Simmons, K. Klier, Proceed. 7th Congr. on Catalysis, Tokio, 1980, pp. 475-489. 6 R.G. Herman, K. Klier, G.W. Simmons, B.P. Finn, J.B. Bulka, T.P. Kobylinski, J. Cat., 56, 19708, pp. 407-J2~. A.C.S. ANAHEIM Meeting, March 12-17, 1978, pp. 595-615. 7 G.K. Boreskov, Scientific Basis of Catalyst Preparation, in ref. 1, 1st Symp. 1976, pp. 223-250. 8 A.C. Zettlemoyer, "Nucleation", Edt. Marcel Dekker Inc., N.Y., 1969. 9 H. Furedi-Milhofer, 4th Intern. Conf. on .Surface and Colloid Science, Jerusalem, Israel 5-10 jUly 1981, in IUPAC - Pure and Applied Chemistry, Pergamon Press, 53, 1981, pp. 2041-2055. 10 J.R. Goldsmith, J. Geol., 61, 1953, 439. 11 D.W. Breck, "Zeolite Molecular Sieves, Structure Chemistry and Use", John Wiley and Sons, London, 1974. 12 Ph. Caullet, J.L. Guth, G. Hurtrez, R. Wey, Bull. Soc. Chim. Fr., 7-8, 1981, 1.253-257. 13 B.A. Doerner, W.M. Boskins, J. Am. Chern. Soc., 47, 1925, 662. 14 A.G. Walton, The Formation and Properties of Precipitates, Intersciences Publishers div. of.J. Wiley and Sons, 1967. 15 E.J. Jensen, K. Johansen, Man-Yu Topso~ J. Villadsen, This symposium (publ. F6 - Communication withdrawn) . 16 I.F.P. Patent NE 82.05368, 1982. 17 P. Gherardi, O. Ruggeri, F. Trifiro, A. Vaccari, G. Del Piero, B. Notari, G. Manara, This symposium (publ. H4). 18 G. Petrini, F. Montino, A. Bossi, F. Garbassi, This symposium (publ. H5). 19 D.S. Shishkov, N.A. Kassabova, K.N. Petkov, This symposium (publ. H7). 20 A.S.T.M. 14-191, 22-700, 25-521. 20'A.S.T.M.17.743. 21 Ph. courty, D. Durand, E. Freund, A. Sugier, Catalytic reactions on one carbon molecules, Symp. Bruges 06/82 (to be pUblished in J. of molecular Cat.' 22 S.D.H. Donnay, H.M. Ondik, Crystals datas, Determination tables, N.B.S. WASHINGTON D.C., 2, 1973. 23 I.M. Vasserman, N.I. Silant'Eva, Russ. J. Inorg. Chern. 13, 1968, 1041. 24 S. Ghose, Acta Crystall. 17, 1964, 1051. 25 J.S. Campbell, I.E.C. Process Res. Develop. 9(4), 1970, pp. 588-95. 26 Ph. Courty, D. Durand, E. Freund, B. Rebours, A. Sugier, Unpublished results, 1981. - C. Durand, B. Rebours, Ph, courty, Unpublished results 1982. 27 I.F.P.-C.E.A. Patents, F.l.604.707 (1968), F.2.045.612 (1969). 28 Ch. Marcilly, Ph.D. Thesis, Grenoble, 1968. 29 Ph. Courty, H. Ajot, Ch. Marcilly, B. Delmon, Powder Technology, 7, 1973, pp.21-38. 30 I.F.P. Patents, F.2.086.903 (1972), U.S.4.122.110 (19~8). 31 Procatalyse Patents, NE.71-13858, (1971), NE.72-12020, (1972). 32 Ph. Courty, B. Raynal, B. Rebours, M. Prigent, A. Sugier, I.E.C. Prod. Res. Dev. 19, 1980, pp.226-231. 33 J. Droguest, Memoire de licence en sciences chimiques, LOUVAIN (1972). 34 B. Delmon, J. Droguest, 2d Intern. Conf. on "Fine Particles". 35 Ph. Courty, H. Ajot, B. Delman, C.R. Acad. Sc., 276C, 1973, pp.1147-49. 36 I.F.P. Patents, F.1.600.128 (1968), F.2.060.171 (1969), F.2.082.444 (1970), U.S.3.716.497 (1973), u.S.3.846.341 (1974), U.S.3.975.302 (1976), U.S.4.000.085 (1976), u.S.4.141.861 (1979).
517 37 38 39 40 41 42 43 44 45 46 47 48
49
50 51
52
53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70 71 72
Ph. Courty, H. Ajot, B. Delmon, Unpublished results (1967). R. Tertian, D. Papee, J. Chim. Phys., 1958, p.341. H. Forestier, J. Longuet, C.R. Acad. Sc., 208, 1939, p.1729. J. Longuet-Escart, Bull. Soc. Chim. Fr., 1949, p.153. Ph. Courty, Unpublished results, 1981. F. Trifiro, P. Forzatti, P.L. Villa, in ref. 1, 1st symp., 1976, p.148. F. Ribeiro, Ch. Marcilly, 5th Intern. Conf. on zeolites, Recent progress reports, Naples june 2-6 1980, pp.135-138. UNION OIL of Calif. Patent (HANSFORD), u.S.3.506.400, 1970. A.W. Adamson, Physical Chemistry of Surface, Intersciences Publishers Inc., N.Y., 1960. D.J. Shaw, Introduction to Colloid and Surface Chemistry, 2nd edition Butterworth and Co, LONDON 1970. S.J. Gregg, The Surface Chemistry of Solids, Chapman and Hall, LONDON 1961. J.P. Srunelle, Preparation of catalysts by adsorption of metal complexes on mineral oxides, in 2d Int. Symp. on Scientific Bases for the preparation of Heterogeneous Catalysts - Elsevier 1979, pp.211-232. H. Boerma, Preparation of Copper and Zinc Chromium oxide catalysts for the reduction of fatty acid esters to alcohols, in ref. 1, 1st symposium, 1976, p.105-118. D.R.M.E. (French State), F.2.050.725, 1969. E.J. Newson, J.V. Jensen, The effects of preparation parameters on the oxidation activity of catalysts made by coprecipitation, in ref. 1, 1st symposium, 1976, pp.91-103. Ph. Courty, J.F. Le Page, Relationship between average pore diameter and selectivity in iron - chromium - potassium dehydrogenation catalysts, in 2d Int. Symposium on Scientific bases for the preparation of heterogeneous Catalysts - Elsevier 1979, pp.293-305. M. Bisiaux, Ph. Courty, H~ Dexpert, E. Freund, Unpublished results, 1981. H. Charcosset, P. Turlier, Y. Trambouze, J. Chim. Phys., 107, 1964, pp.1249-1256, 108, 1964, pp.1258-1261. J.F. Le Page et al., "Catalyse de contact", Ed. Technip, 1978. Ph. Courty, P. Duhaut, Rev. I.F.P., 29(6), 1974, pp.861-877. D. Papee, J. charrier, R. Tertian, R. Houssemaine, Congres de l'aluminium, Paris, juin 1954. G. Barrera, Ph. Courty, B. Rebours, A. Sugier, Unpublished results, 1981. Conference on catalyst deactivation and poisoning - May 24-26 1978, Lawrence Berkeley Laboratory - BERKELEY, California 94720. International symposium on catalyst deactivation. Antwerp oct. 13-15 1980, (Studies in surface science and catalysis 6. B. Delmon and G.F. Froment editors) Elsevier 1980. S. Peirs, Ph.D. Thesis, Lille, 1970. Ph. Courty, J.F. Le Page, unpublished Results, 1976. N. Burriesci, F. Garbassi, M. Petrera, G. Petrini, N. Pernicone, in ref. 60, pp.115-126. A. Bielanski, M. Najbar, J. Chrzaszcz, W. Wal, in ref. 60, pp.127-140. T.S.R. Prasada Rao, P.G. Menon, J. Cat. 51, 1978, pp.64-71. P. Mars, D.W. Van Krevelen, Chern. Eng. Sci., 3, 1954, p.41. I.F.P. Patent, U.S.4.134.858, 1979. SHELL Patent, U.S. 4.052.338, 1977 - 4.098.723, 1978. GIRDLER Patent, D.T.2.406.280, 1977. Ph. Courty, Ph. Varin, J.F. Le Page, Unpublished results, 1980. GMELIN, 22(4), 1937, p.841 (potassium). E.M. Thornsteinson, T.P. Wilson, F.G. Young, P.H. Kasai, J. Cat., 52, 1978, pp.116-132.
ACKNOWLEDGEMENTS The authors wish to thank all these who contributed to the realization of this paper as well as Dr. E. Freund and Dr. G. Martino for helpful discussions.
518 DISCUSSION A. BOSSI: In your lecture you presented much structural characterization data (mainly by XRD) of Some mixed oxide systems. It seems that you neglected to take into account the surface characterization data (obtained by AES, XPS, etc .. ) which appear more suitable for correlations with the catalytic activity. Have you any comments on this? P. COURTY: The correlations we have presented deal mostly with either preparation or stabilization procedures and XRD or STEM and XRD microdiffraction characterization. For mixed oxides based catalysts, some doubts exist on the results obtained with surface characterization (AES, xPS, etc .• ) as high vacuum conditions are required to get these results; therefore, partial reduction or modifications of the surface composition cannot be neglected. On the other hand, XRD under controlled atmosphere gives an inclusive but accurate answer. L. RIEKERT: Can we differentiate between those variables of the preparation procedure which are important with respect to the steady state, in contact with reactants, and those which are not? P. COURTY: No general rules allow us to select between the numerous parameters of the preparation procedure as well as between the numerous ones of the procedure which permit to reach the steady-state of the catalyst in the reaction medium. The same catalyst, for example, can be used for very different reactions which can be "homogeneity demanding or homogeneity non-demanding" ones. In iron molybdate based catalysts, homogeneity is very important when used in methanol oxidation to formaldehyde. Homogeneity is much less important for uses in reducing conditions (NOx reduction to N2 or NH3 decomposition). ZHAO JIUSHENG pH of CU, Zn, cipitation pH control it in
: When you make the Cu-Zn-AI catalyst precursor, the precipitation Al with CO~are not the same. How do you determine the prein order to obtain the homogeneous composition and how do you the coprecipitation reactor ?
P. COURTY: Your question deals mostly with the know-how for making such catalysts. Therefore, one can say that coprecipitation pH, (pH)M is a "good compromise" between the extreme values of individual precipitation pH. (pH)M allOWS the best homogeneity and results in minimizing metal losses in the mother liquor. (pH)M can be controlled through a precise adjusting (or monitoring) of the flow rate ratios during the coprecipitation reaction. F. TRIFIRO: What do you think is the importance to start from the hydrotalcite structure? Does it stabilize CuO or metallic Cu? Does it determine the formation of CuO with small crystallite size ? P. COURTY According to your own results (1) binary copper-zinc based precursors and ternary copper-zinc aluminum ones produce CuO crystals of equivalent sizes (about 7.5 nm) after thermal treatment. The comparison between these methanol synthesis based catalyst precursors has to be done in terms of stability. The stability of the performances of Cu-Zn-AI based catalysts is much higher than that of Cu-Zn based catalysts. Alumina is supposed to reduce the rate of sintering of CuI-Zn I I based active species. Therefore, the use of the ternary precursor allows the best dispersion of the Cu-Zn active sites inside the alumina matrix and reduces the sintering rate during the operation. (1) O. Ruggeri, F. Trifiro and A. Vaccari, J. of Solid State Chern. 42, 120 (1982). J. SCHEVE: I cannot completely agree with your statement, that there is no general rule for controlling the sintering process. According to our experience, the old rule established by Huttig operates well i.e. using the ratio between heating temperature and the melting point of the substances in question. You can
519 gain a lot of insight in what would happen during firing your catalyst. you comment on this ?
Would
P. COURTY: I have only mentioned that "three are no general rules which allow foresight into and control on the effects of thermal treatments". Taking into account the ratio between heating temperature and melting point allows a partial foresight of the level of sintering; but, on the one hand, sintering level depends also on many other parameters (size and shape of elementary particles, crystallinity degree, preferential orientation, effect of impurities on crystal growth .•. ) and, on the other hand, at least two other reactions (thermal decomposition of the precursor and formation of the mixed oxide) occur simultaneously. The effect of these two latter ones seems more difficult to predict.
J. GEUS: Your pH curve for AI3+, cu 2+, Zn 2+ indicates three plateaus before the increase to the Zn 2+ precipitation level proceeds. We have evidence that copper hydroxy-nitrate is precipitating already at pH level below 4 and that the hydroxy-nitrate becomes unstable at pH level of about 8 when no carbonate is present. With carbonate ions present the hydroxy-nitrate becomes unstable already at a pH level of about 5. I therefore should like to suggest that the second plateau in your curve is due to precipitation of the hydroxy-nitrate and the third plateau is either due to a partial reaction of the hydro~-nitrate to the hydroxy-carbonate or to a combined precipitation of Zn 2+ and Cu + evolving from the hydroxy-nitrate. P. COURTY: We believe that the three plateaus correspond to the successive precipitation of alumina, copper and zinc based hydroxy-carbonates. Therefore, we cannot exclude that some copper is simultaneously precipitated with alumina during the first precipitation step. This could explain the very high stability of such "alumina doped" copper hydroxy-nitrates which resist either acidic or alkaline washing, without any decomposition to hydroxy-carbonates. These properties are very different from those you have mentioned in your comment for pure copper hydroxy-nitrate. B. GRIFFE DE MARTINEZ: We have been working on copper oxide-chromium oxide catalysts in the oxidation of CO to CO2, We have prepared copper oxide (CuO) , copper oxide-chromium oxide (CuO-Cr203) and chromium oxide alumina supported catalysts in different weight percents. We have found that the last ones are not active, whereas the former ones (CUO and Cuo-cr203/A1203 are very active at a reaction temperature of 200°C, especially those in the range of 10-20 wt %. Unfortunately, the CUO/A1 20 3 catalysts become almost deactivated after they are submitted to a water vapour treatment at 400"c. On the contrary, the CuO-cr203/ A1203 catalysts do not lose much activity. It must be said that the CUO/A1203 catalysts are reactivated after being submitted to the reaction conditions. We are at the moment carrying on research to try to give a scientific explanation for these differences. Would you please give a comment on this ? P. COURTY : The interactions between copper and chromium oxides (CuCr204 formation) are much stronger than those between CuO and A1203' Thus a stabilization of Cu-Cr species (bulk or alumina supported ones) can be invoked to explain these discrepancies.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation or Catalysts III 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherland.
521
©
DEPOSITION OF TERNARY OXIDES AS ACTIVE COMPONENTS BY IMPREGNATION OF POROUS CARRIERS M. KOTTER, L. RIEKERT, F. WEYLAND Institut fUr Chemische Verfahrenstechnik der Universitat Karlsruhe, Postfach 6380, Karlsruhe, FRG
ABSTRACT The ternary oxide coppermanganate is generated in highly dispersed form in the pores of inert a-A1 203 carriers by impregnation with a viscous solution containin9 both metals, drying and calcination. Kinetic data for this catalyst in the total oxidation of CO are compared with similar catalysts containing only CuO, MnO or CuO together with MnO as separate crystallites. In the case of ironmolybdate it is also possible to obtain crystallites of the ternary phase in the pores of a-alumina by way of impregnation, either with solutions containing ammoniummolybdateand ironnitrate or with the solution of ironmolybdate in a mixture of monoethanolamine and water. Properties of both types of catalysts and of catalysts consisting of pure ironmolybdate are compared, focusing on the selectivity in the partial oxidation of methanol to formaldehyde.
INTRODUCTION It has been shown in a contribution to the preceding symposium how the distribution of an active component in impregnation-type catalysts on inert porous carriers can be controlled through the viscosity of the impregnating solution and through the rate of drying [1]. The results to be described here concern the preparation of impregnation catalysts containing ternary oxides as active components. In this case salts of two metals must be deposited at the same locations in the pores of the carrier through impregnation and drying, in order to obtain an active component consisting of the desired ternary phase by subsequent calcination.
RESULTS Formation of copper-manganate in porous a-A1 2Q3_ The migration of soluble precursors of the active component during drying of the impregnated carrier can be prevented by the use of an impregnating solution of high viscosity, obtained through small additions of water soluble polymers such as hydroxyethylcellulose [1]. It was expected that it is also possible to deposit an intimate
522
mixture or mixed crystals of the nitrates of two different metals in the pores of the carrier in this way, preventing an otherwise possible segregation due to migration and different solubilities of the nitrates. In order to test this hypothesis we investigated the formation of CuMn 204 in the pores of a-A1 203• CuMn 204 is known to be catalytically much more active than the oxides of either copper or manganese in the oxidation of CO [2,3]. Five catalysts were prepared through impregnation of an a-alumina with a BET-surface of 11 m2/g (SCS 9 from RhBne-Poulenc) with the following solutions d 2 m Mn(N0 3)2 2 m Mn(N0 3)2' 1 m Cu(N0 3)2 e 1 m Cu(N0 3)2 2 m Mn(N0 3)2' 1.2 m Cu(N0 3)2 c 2 m Mn(N0 3)2' 0.8 m Cu(N0 3)2 2 wt% hydroxyethylcellulose were added to all five solutions which increased their viscosity about 70-fold. The pellets were then dried 2 h at 250°C and calcined by heating within 1 hour to 500°C. Figure 1 shows the rate coefficients of oxidation of CO (0.1 % in air) obtained in a gradientless recirculation reactor for the five resulting catalysts which were prepared in identical ways except for the composition of the impregnating solutions.
a
b
60
40 30 k
20
cm3g01.S1
10
8
6 4
3 2 2.0
2.1
2.2
2.3
2.4
103K -1-
2.5
Fig. 1. Rate coefficients per unit mass of catalyst in the oxidation of CO; catalysts prepared with impregnating solutions a-e. Catalyst (a), where this composition amounted to a molar ratio Mn : Cu of exactly 2.00, exhibits the highest activity and lowest activation energy as would be expected as active component. We can therefore conclude that it is possible in for CuMn 204 this way, to control the composition of the mixed oxide which is being formed in the pores of the carrier through the composition of the impregnating solution.
523
Iron-molybdate on a-A1 203 - partial oxidation of methanol to formaldehyde component in many commercial catalysts for the partial Iron-molybdate is the ac~ive oxidation of methanol witn 'air to formaldehyde [4]; the Fe/Mo-ratio in the mixed oxide is believed to be critical with respect to the activity and selectivity of the system [5]. The preparation of a selective commercial catalyst by precipitation of ironmolybdate and tabletting has been described by P. Courty et al. [6]. A procedure for introducing the same compound into a porous structure by dissolving a complex of the active phase e]ements in a mixture of monoethanolamine and water and impregnating a carrier was later found by P. Courty et al. [7]; both patents originated in the Institut Fran~ais du petrole. We have investigated if the same selectivities and activities can be obtained with catalysts, obtained by impregnation of porous a-alumina with solutions containing Fe(N0 3 ) 3 and (NH4)6Mo7024 in aqueous 2m nitric acid, the viscosity of the impregnating solution being increased to about 100 cSt through addition of 4 wt% hydroxyethylcellulose. Furthermore the relative influence of intrinsic properties of the active component on the one hand and of its accessibility (mass transfer limitations) in the porous structure on the other hand on the selectivity of impregnation catalysts for the oxidation of methanol has been studied. The reaction network under consideration is triangular:
(1)
the rate of reaction of methanol being approximately first order within the range of temperature (250 - 40QoC) and composition (PCH OH/PO $0.1) investigated [8]. Three h 'In3 2 . parameters are needed to characterize cata 1yst-performance Slnce t here are tree dependent reactions, e. g. the three first order rate coefficients r
k 2
2 cCH 2
=--
°
r3 k =-3 c CH OH 3
Here r 1, r 2, r 3 are the rates of reactions 1, 2 and 3 in the above scheme (amount CH 30H or CH 20 converted per unit of time and unit mass of catalyst in the resp. reaction), the concentrations c i designating the comoosition of the gas-phase adjacent to the porous catalyst grain. In order to characterize a catalyst by activity and selectivity we use the alternate set of parameters (1) k
SK -- k
1
1
+ k
(selectivity)
3
k
AK = ~ 1 2
(product-stability)
( 2)
524
The subscript K indicates that these quantities characterize the porous catalyst grain, depending on the intrinsic properties of the active component as well as on the extent and accessibility of its surface. This point will be discussed later. An integral fixed-bed reactor was used for kinetic experiments, the gaseous feed contained 2 vol% of methanol in air, the products were analyzed by gas-chromatography with thermal-conductivity-detector. The observables in this system are volumetric flow rate v of feed-stream, mass mK of catalyst, conversion U of reactant CH 30H and selectivity SR of the reaction-system (riCH O)out 2
•
SR = - - - - - - - - - - -
(4)
U and SR describe reactor performance under given operating conditions (v/mK) and are related to the quantities characterizing the catalyst (kK, SK' AK). For first order reactions we have kK (5) mK U = 1 - exp v SK/AK SK 1-( 1-U) ) (6) (1 SR = 1 - SKIA'K U
1-
I
where kK is the mass-specific activity as defined in equ.(1). The selectivity-parameters SK and AK of the catalyst can be obtained from the observed dependence of reactor-selectivity SR on conversion U, see equ. (6). Selectivity-behaviour can be evaluated in a somewhat summary way on the basis of reactor-selectivity SR' provided values of SR observed at the same level of conversion U are being compared. Impregnation-type catalysts were compared with a catalyst prepared according to P. Courty et al. 16] from a gel of FeZ0 3,(M003)3.Z' which was pulverized and tabletted (cylinders of 3 mm diameter and 3 mm length, BET surface area 6 mZ/g). Two porous carriers consisting of a-alumina were used for impregnation: T 1: Spheres of 3 mm diameter, BET surface area 11 mZ/g, porosity 66% (SCS 9 from Rhone Poulenc); 2 T 2: Cylinders of 3 mm diameter and 3 mm length, BET surface area 2.9 m /g, porosity 54% (from Dr. Otto GmbH, Bochum). Pore-volume distribution curves from mercury porosimetry for these carriers are shown in figure 2. The following solutions were used to impregnate these carriers. A: (NH4)6M07024 (p.a., Merck) and Fe(N0 3)3 (p.a., Merck) dissolved in varying proportions in 2 m aqueous HN0 with the addition of 4 wt% hydroxyethylcellulose. 3 B: A gel of FeZ03·(Mo03)4 prepared by combining aQueous solutions of Fe(N0 3)3 and (NH4)6Mo7024 at 13°C is ripened at 20°C for 1 h, then 30 min at 45°C and dried at
525 0.5 r - - - - - - - - - - - - - - - - : : : : : : - - - - .
0.375
0.25
0.125
o _f_
nm
Fig. Z. Pore volume - distribution in carriers T1 and TZ as obtained from mercury porosimetry. 65°C. 110 g of the product are dissolved in a mixture of 57 wt% monoethanolamine and 43 wt% water [ 7]. Dry carriers were soaked for 20 h in the impregnating solution. Catalysts prepared with solutions of type A were dried for 5 min in an airstream of 250°C, cooled, then heated within 1 h from Z5°C to 450°C and left at 450°C for another 10 min. Catalysts prepared with solution B were dried by heating within Z h to ZOO°C, temperature was then held at 200°C for 2 h, thereafter increased within 2 h to 400°C and held at 400°C for 1 h. The following catalysts were thus obtained: Designation
A 1 to A 10 B1 BZ p
content of (Fe 20 3 + 10100 3) wt% 0, 0.25, 0.4, 0.5, 0.6 3.5 T1 A 0.62,0.63,0.67,0.75,1 16.8 T1 0.67 B 11.0 0.67 TZ B 100 compacted powder from gel [ 6] 0.62 Carri er
Impregnating sol ution
nMo nMo + nFe
The X-ray diffraction pattern of powdered catalyst P is identical to that of Fe as described in the ASTM-index. Only X-ray reflections characteristic of 2(Mo0 4)3 Fez(Mo0 4)3 and of a-Al z03 could be identified in powder diffraction-patterns for catalysts B 1, B Z and A 5 (Mo/Fe = 1.5). The Fez(Mo04)3-phase can thus be generated in the pores of a-alumina either from solutions of type A or from solution B. NO was
526
chemisorbed on catalysts of type A at 363 K[9] only if the ratio nM/nMo+ nFe was smaller than 0.6 (Fe/Mo>2:3), indicating that Fe was present only in catalysts 203 with an excess of iron above the stoichiometry of Fe203·(Mo03)3' Figure 3 shows the activity kK (rate-co~fficient of aggregate reaction of CH 30H) and the rate of formation of formaldehyde nCH20/mK per unit mass of catalyst, figure 4 the selectivities SR obtained in the integral reactor with different catalysts.
•
n~20
v
106
K mol· g-1.5 -1 8
60 0.4
40 20
0.2
0.8
0.2
0.4
0.6
0.8
nMO Fig. 3. Rate coefficients kK for converFig. 4. Integral reactor selectivity SR sion of methanol and integral rates of of formation of formaldehyde at formation of formaldehyde at UCH30H = 0.80 UCH OH = 0.80 and {} = 380°C for differ3 ent catalysts: and 380°C for different catalysts: o catalysts A 1 to A 10; 6 B 1; sr B 2; • P (pure Fe 203" (Mo0 3)3.2) All data shown in figures 3 and 4 have been obtained at constant methanol-conversion of 80% at 380°C by varying the flowrate of reactant as required by catalyst activity. The integral reactor-selectivity SR increases monotonously with the molybdenum: iron ratio until the stoichiometric ratio Mo:Fe = 3:2 is reached and remains constant thereafter in catalysts of type A. No discontinuity in SR was observed near the stoichiometric composition Fe 203" (Mo0 3)3' Boreskov [5] has found that the activity of catalysts containing only the oxides of molybdenum and iron falls off rapidly if the ratio of Fe:Mo is in excess of the stoichiometric ratio. It seems possible that the formation of the active phase is rather less prohibited by an excess of iron in a porous carrier. Catalyst B 1 prepared with an impregnating solution of ironmolybdate in MEA and water as solvent shows the same selectivity SR as catalysts of series A, made with the same carrier. Higher selectivities SR are observed with compacted iron molybdate (p) and with catalyst B 2, made with solution B on a different carrier. A significantly lower actiVity was observed for catalysts of type A near the stoichiometric
527
composition Fe203'(Mo03)3 of the active component; the phenomenon was found to be reproducible. However, rates can only be obtained per unit of mass of catalyst (as in figure 3) or per unit of mass of the active component, since we did not find a reliable way to measure the surface area of ironmolybdate by chemisorption. The selectivity SR of the integral conversion in a fixed bed results from the partial and total oxidation taking place in the catalyst grain as well as from the subsequent oxidation of formaldehyde on the catalyst, under the specific conditions of react~r operation. A more general characterization of the selectivity of different catalysts and a deeper understanding of their behaviour can be obtained if catalystspecific selectifity parameters SK and AK are evaluated from the variation of SR with conversion U by means of equ. (6). Selectivity-parameters characterizing the active component can then be obtained from SK and AK by taking into account concentration gradients (mass-transfer resistance) in the porous grain. This rather laborious analysis has been completed for three of the above catalysts. The dependence of SR on conversion U at 380°C is shown in figure 5 for catalysts B 1, B 2 and A 8 with Mo:Fe - ratios of 2.0.
1.0.------------,
0.7 B1
0.6
6
82
v
A8
0
0.5 -'---,-_...,...._-,--_-,....-_--l
0.2
0.4
0.6
U
0.8
Fig. 5. Integral selectivities SR at 380°C as function of conversion U of methanol for catalysts B 1, B 2 and A 8. Curves represent equ. (6) with parameters given in text. SR foll ows exactly the prediction from equ. (6) for the sets of parameters Catalyst: SK = AK =
B1
0.89 9
B2 0.97 30
A8 0.91 12
528
Parameters SK and AK characterize the selectivity-behaviour of the porous grains completely at a given temperature. However, they are not necessarily characteristic of the selectivity of the active component in the grain, since the relative importance of parallel and consecutive reactions can be influenced by concentration-gradients in the porous material. This subject has been treated for the case of a consecutive reaction A->B-+C by Wheeler [10], general results for the more realistic case of consecutive and parallel reactions taking place simultaneously (scheme I) at the active component have been presented elsewhere [11]. If S and Arepresent selectivity parameters of the active component in terms of local concentrations of CH 30H and CH 20 (analogous to SK and AK) and if ~ and p are effectiveness and Thiele-number, resp., for the conversion of methanol in the porous structure, then we have for first order reactions
(with QK = density of the porous grain) and
- S!A
(1 _VS!X tgh(pVS!X) tghp
S
(
S
A = K
1 - S!A
tghp vsn. tgh(PM)
(8)
- 1 )
(9)
The effectiveness factors ~ and Thiele-Moduli p of catalysts B 1, B 2 and A 8 at 380°C were obtained through the observation of kK over a wide temperature range, assuming that the intrinsic rate constant k follows Arrhenius'law (figure 6). With ~ and p known one obtains k from kK through equ. (7); the intrinsic selectivity parameters S and A can then be found by solving equ. (8) and (9) simultaneously through an iterative computation. For the three catalysts under consideration we obtain the following numerical values at 380°C: Catalyst ~
P S
A
B1 0.30 3.3 0.96 29
B2 0.97 0.3 0.97 31
A 8
0.43 2.3 0.96 28
Intrinsic properties of the active component in B 1, B 2 and A 8 are identical j different selectivities of these catalysts are caused only by diffusion - disguise in Bland A 8 where the pore-diameters in the carrier are smaller than in B 2.
529
\
\
\
100
\
,
\
\
\ ,\ \
\
\
\
~
\ '0....
"\
\
\
\
<, \
\
66\
0
\
~
\
~\,",\~
V\
0
'\
10
V,
\
\
\ 1.3 1.4 1.5
1.6
~
'\
'.to
0\
\
\
\
1.7 1.8 1.9 2.0 103K
-r-
Fig. 6. Rate coefficients kK for conversion of methanol on catalysts B 1, R 2 and A 8. Efficiency factor at 380°C is obtained as ratio between kK extrapolated from Arrhenius-line at low temperatures and observed value. Symbols as in fig. 5. The bewildering variety of catalyst-properties in this system can thus be quantitatively understood as resulting from two distinct sets of physical causes: intrinsic properties of the active component and its accessibility in the porous structure.
REFERENCES 1 M. Kotter, L. Riekert, in B. Delmon et al. (Eds.), Proc. 2nd Int~ Symp. Scientific Bases for the Preparation of Heterogeneous Catalysts, LouvainLa-Neuve, September 4-7, 1978, Elsevier, Amsterdam, 1979, pp. 51-63. 2 M. Katz, Adv. Catal., 5 (1953) 177. 3 G.M.Schwab, S.B. Kanungo, Z. Phys. Chern. N.F., 107 (1977) 109. 4 H. Adkins, W.R. Peterson, J. Am.Chem. Soc., 53 (1931) 1512. 5 G.K. Boreskov, Kinet. Katal., 6 (1965) 1052. 6 P. Courty, H. Ajot, B. Delmon, Brit. Patent 1.282.949 (1972). 7 P. Courty, A. Suqier. J.F. Le Page, US Patent 3.975.302 (1976). J. Tichy, J. Catal., 21 (1971) 143. 8 P. Jiru, B. l~ichterbra, 9 K. Otto, M. Shelef, J. Catal., 18 (1970)184. 10 A. Wheeler, Adv. Catal., 3 (1951) 250. 11 M. Kotter, L. Riekert, Chern. Eng. Fundam., in print.
530 DISCUSSION Have you studied the effect of adding water on activity R. FARINHA PORTELA and selectivity of your iron molybdate-catalysts for methanol oxidation ? L. RIEKERT The feed of our fixed bed reactor consisted of and excess air; water vapour was not added.
methanol~vapour
J.W. GEUS: Working with iron molybdate catalysts in selective reactions involving propene, it turned out to be particularly difficult to avoid formation of iron(III) oxide, which exhibits a very low selectivity. Does your catalyst contain any unreacted iron oxide and what is the selectivity of pure iron oxide ? With our catalysts it was easy-to discern the presence of pure iron(III) oxide from the color (brown to ~ed-brown in contrast to the yellow color of iron molybdate) . L. RIEKERT The selectivity of pure iron oxide on a-alumine for the production of formaldehyde is indeed nearly nil, as shown in Fig. 4 of the paper. The for·mation of iron oxide in the pores of an inert carrier can be avoided if the composition of the impregnating solution reaches or exceedes the stoichiometric ratio (Mo : Fe ~ 1.5) and if segregation of the precursors of the active component (iron nitrate and ammonium molybdate) during drying is prevented by using an impregnating solution of SUfficiently high viscosity. The catalysts obtained in this way exhibit only a bright yellow-green color throughout the grain. The high intrinsic selectivity for formaldehyde production (97%) as well as the fact that no NO is chemisorbed demonstrate that no free Fe203 is present in these catalysts. Catalysts of low selectivity exhibiting a non-homogeneous brownish color are obtained if an impregnating solution of low viscosity (without additives) is used. F. TRIFIRO: I have investigated the oxidation of methanol on Fe-Ho oxides supported on a-Al203 and Si02 in a fluidized bed reactor. I found that by increasing the surface area of the support a decrease of selectivities occurs accompanied by a decomposition of the catalyst. Do you exclude any effect of destruction of Fe2(Mo04)3 by supporting it on the surface of Al203 ? G. PETRINI: I agree with Prof. Trifiro's comments and also add that some chemical interaction between the active phase and the suport can occur without any apparent colour change and this may influence the selectivity. Another point is that your correlations take into account only the influence of the porous structure of the carrier and not the pore structure of the active component (iron molybdate), which is very important in determining selectivity. L. RIEKERT: (to F. Trifiro and G. Petrini). The influence of secondary reactions in the catalyst pores on observed product distribution has to be taken into account, if conclusions about the chemistry at the active component are to be derived from selectivity data. We can conclude from our data that the intrinsic selectivity of the iron molybdate on porous a-Al203 is independent of the method of preparation and independent of the porosity of the carrier. This result does by itself not exclude any kind of interaction between active component and carrier. However, any such interaction can hardly be crucial with respect to the selectivity in the case of our catalysts, since the intrinsic selectiVity of the iron molybdate was invariably found to be as high as 97%.
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III
.
531
© 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
EFFECT OF SUPPORT AND PRFPArtATION ON STRUCTURE OF VANADIUM OXIDE CATALYSTS
Y. MURAKAMI, M. INOt4ATA+, K. MORI+, T. UI, K. SUZUKI, A. MIYAMOTO and T. HATTORI Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo~cho, Chikusa-ku, Nagoya 464 (Japan)
ABSTRACT Supported vanadium oxides were prepared by both the conventional impregnation and vapour-phase-supporting methods; Ti02(anatase), Ti02(rutile), Ti02(mixture of anatase with rutile), A1 203, Zr02, Si0 2, and MgO were used as supports. The structure of the catalyst was characterized by using the rectangular pulse technique coupled with various physico-chemical measurements. It was found that the structure of the vapour-phase-supported V20S/A1 203 catalyst does not change significantly from that of the V20S/A1 203 prepared by the conventional impregnation method. The (010) face of V205 was found to grow epitaxially on the Ti02 support, irrespective of the modification of Ti02. It was also found that the structures of V20S/Ti02, V20S/A1 203, V20S/Zr02, V20S/Si02, and V20S/MgO differ greatly from one another. From these results, it was concluded that the structure of vanadium oxide on support is mainly determined by the kind of support, while neither the method of preparation nor the modification of the support affects the structure significantly.
INTRODUCTION Supported vanadium oxide catalyst has been widely used as a catalyst in the selective oxidations of hydrocarbons(refs. 1-8). From the environmental point of view, the catalyst has recently received attention as one of the best catalysts for the reduction of nitric oxide with ammonia(refs. 9-11). It is therefore highly desirable to investigate the effect of support and preparation on the structure of vanadium oxide catalyst. Although valuable information has been obtained for the structure of supported vanadium oxide catalysts by using various physico-chemical measurements(refs ..5-24), further investigations are necessary to reveal the effect of support and preparation. We have previously established the rectangular pulse tpresent address; Kinu-ura Research Department, JGC Co., Sunosaki-cho, Handa, Aichi 47S (Japan)
532
technique which allows the determination of the number of surface V=O species and the number of layers of VZOS lamellae on the support (refs. ZS,Z6);the surface V=O species is located in the (010) face of VZOS crystal. This technique is based on the following reactions; (1) V=O + NO + NH 3 ---+1 V-OH + NZ + HZO, V-OH
bulk V=O
V=o + l/Z HZO,
(Z)
together with the introduction of the mixture of NO and NH 3 in rectangular pulse shape onto the preoxidized catalyst and the detection of the concentration profile of NZ produced by Eq. 1. The separation of NZ due to the initial surface V=O species from that due to the reproduced V=O species by Eq. Z leads to the determination of the number of surface V=O species whereas the detailed analysis of the concentration profile of N2 due to the reproduced V=O species leads to tne determination of the number of layers of VZOS lamellae on support. In this study, effects of support and preparation on structure of vanadium oxide catalyst were investigated by using the rectangular pulse technique coupled with various physico-chemical measurements. EXPERIMENTAL Catalysts Ti02, A1 203, Zr02, 5iOZ' and MgO were used as supports. For the TiO Z' three kinds of Ti0 2, denoted by TiOZ(a), Ti0 2 (r ), and TiOZ(a-r), were used; they consisted of anatase, rutile, and mixture of anatase with rutile, respectively. Ti0 2 (a) and Ti02(r) were prepared by hydrolysis of Ti(50 4)Z and TiC1 4, respectively, followed by calcination in air at 600°C for TiOZ(a) and in 0z at SOD °c for TiOZ(r). TiOZ(a-r) was prepared by calcination of TiOZ(Nippon Aerosil) in 0z at SOU °c. A1 Z03(y) and 5iOZ were commercially available(5umitomo and Fuji-Davison, respectively). ZrO Z was prepared by hydrolysis of ZrOC1 Z with ammoniacal solution, followed by calcination in air at SOO °C for 3 hr. MgO was prepared by calcination of Mg(OH)Z in air at SOO °C for 3 hr. Respective BET surface areas of TiOZ(a), Ti02(r), TiOz(azr), A1 Z03, ZrO Z' 5i0 2, and MgO were 48.2, 16.8, 40.0, 230.0, 93.6, Z04.2, and 181.S m /g. Vanadium oxide supported on the carrier was prepared by impregnation of the carrier with an oxal ic acid solution of ammonium metavanadate, f'o l l owed by calcination at SOO °C in a stream of 0z for 3 hr. Vapour-phase-supported VzO s/Al z03 catalyst was prepared by treating the A1 Z03 with the stream of NZ containin~ VOC1 3 followed by its hydrolysis with steam diluted with NZ at ZOO °C; this cycle was repeated various times to increase the content of VZO S in the catalyst(refs. 19,27,Z8). This catalyst is hereafter denoted as VZOS/A1 Z03(VP5) to discriminate it from the V20S/Alz03 catalyst prepared by the conventional impregnation method. Characterizations of catalysts The content of vanadium ions(V S+ and V4+) in the catalyst was determined by the
533
TABLE 1 Methods used for the characterization of the catalyst Method
Properties that can be measured by the method
Rectangular pulse technique
Area of the (010) f'aceof V205 on the catalyst, S(OlO)
BET
Area of the whole catalyst surface, SBET
In situ IR spectra of Exposure of the support and V 0 surfaces NH 3 or benzaldehyde Nature of surface vanadium io~s~ inactive vanadium ion on the catalyst or vanadium ion on V205 layers X-ray diffraction Crysta 11 ine phase of V;I0~ and support Formation of new compoanas Infrared spectroscopy Coordination of oxygens around vanadium ion; the V=O stretching vibration at 1020 cm- l Electron spin resoAmount of V4+ ions nance Dispersion or aggregation of vanadium ions Coordination of oxygens around V5+ ion; 02-_V5+ charge UV-Visible spectroscopy transfer band 4+ Amount of V4+ ions; d-d transition in V X-ray photoelectron Electronic state of vanadium ion(V 5+ or V4+) spectroscopy Exposure of support surface Thickness of V205 layers on support Transmission electron Particle size of V205 on support microscope . the catalyst Chemical analysis Amounts of V5+ and V4+ 1n
reduction-oxidation titration using KMn04 and Mohr's salt(ref 29). X-ray diffraction diagrams of the catalysts were obtained by Rigaku GF 2035 X-ray diffractometer with Cu target. IR spectra of catalysts were recorded on JASCO EDR-31 emissionless diffuse refl ectance IR spectrometer. ESR absorpti on measurements were made at X-band on JEOL ME lX spectrometer at room temperature. The number of surface V=O species and the number of layers of V205 lamellae on the support were determined by using the rectangular pulse technique. The surface area of the (010) face of V205, S(OlO)' was calculated from the number of surface V=O species by dividing with the surface density of the V=O species in the (010) face, i.e. 4.872 nm- 2(ref. 30). The BET surface area of the catalyst was determined by using a conventional flow-type apparatus with N2 as adsorbate. The infrared measurements of the adsorption of NH 3 or benzaldehyde on the catalyst was carried out in situ on JASCO IR-G spectrometer; detailed procedures have been described elsewhere(ref. 8). XPS spectra of catalysts were measured on SHIMAZU ESCA 750 electron spectrometer. Electron micrographs of
534
catalysts were observed using HITACHI H-700H. These methods characterized the supported vanadium oxides as described in Table 1. RE5ULT5 AND DI5CU55ION Characterizations of catalysts Because of the limited length of the manuscript, only results obtained by using the rectangular pulse technique are shown in Figs. 1 and Z. Figure la shows results of 5(010)/5 BET - fraction of the (010) face of VZ05 in the whole catalyst surface and VZ05/TiOZ(a-r). When the content of VZ0 5 for the VZ05/TiOZ(a), ~Z05/TiOZ(r), was 0, 5(010)/5 BET was equal to 0, indicating the surface of an uncovered TiO Z' As the content of VZ05 increased up to 5 mol%, 5(010)/5 BET increased almost linearly. This means that the VZ05 spreads over the TiO Z surface with increasing VZ05 content. When the content of VZ05 was 5 or 10 mol%, the maximum fraction of the (010) face of VZ05(ca. 90 %) was attained. When the content of VZ05 increased further, the fraction of the (010) face decreased to the value of the unsupported VZ05(50 % at 100 % of VZ05 content) where various crystal faces are considered to be exposed in addition to the (010) face(ref. Z5). It is evident from Fig. la that the relationship between 5(010)/5 BET and the content of VZ05 does not change markedly with the kind of TiO Z support, TiOZ(a), TiOZ(r), or TiOZ(a-r). Figure Za shows the dispersion of VZO S' D, which is defined as the ratio of the number of surface V=O species divided by the number of VZO S in the catalyst. As shown in Fig. Za, the dispersion was SO-60 %when the content of VZ0 was low(l or Z mol% VZ05). This means that almost half of 5 the VZ05 in the catalyst forms the surface V=O species. As the content of VZ0 5 further increased, the dispersion decreased monotonically. This is due to the increase in the number of VZ05 layers on support. It should be noted from Fig. Za that the value of dispersion does not change markedly with the modification of TiO Z at any content of VZ05 in the catalyst. As for the VZOS/A1Z0 3 catalyst, 5(010)/5 BET and D were negligibly small when the content of VZOs was 1 or Z mol%[Fig. lb and Fig. ZbJ. This means that the loaded VZ05 barely forms the (010) face of VZ05' but does form inactive vanadium ions. As the content of VZ05 in the VZ05/Al z0 3 increased 5 to Z5 mol%, 5(010)/5 8ET increased abruptly and attained almost a constant value above Z5 mol% VZ0 5' However, the constant value did not exceed the value of the unsupported VZ05 catalyst(50 %). Correspondingly, the dispersion increased with increasing content of VZOS up to 10 mol%, attained its maximum at 10 mol% VZ05, and then decreased to a very small value for the unsupported VZ05 catalyst. It should also be noted from Figs. lb and Zb that the VZ05/A1 Z03(VP5) catalyst exhibits almost the same behavior as the VZ05/A1 Z03 prepared by the conventional impregnation method except for the dispersion at high VZ05 contents. At about 10 mol% VZ05' the dispersion for the VZ0 5/Al z0 3(VP5) was considerably larger than that for the VZ0 5/A1 Z0 3. As shown in Fig. 1, the behavior of 5(010)/5 BET at various contents of VZ05 for the VZ0 5/ZrOZ was similar to that for
535
100.------------------,
100 r - - - - - - - - - - - - - - ,
(a)
(b)
80
VZOS/A1Z0 3(VPS)
I-
w
co
z·
10
Vl
"::-.. 40 o
5.
20 I
I
20 40 80 60 100 100,---------------,
°
°
20 40 60 80 Content of VZOs(mol%)
100
(c)
80 ..".
60
I-
w
co Vl -<;
0
40
~
0 Vl
ZO
°
ZO 40 60 80 Content of V20S(mol%)
100
Fig. 1. Fraction of the (010) face of V20S in the whole catalyst surface, i.e. S(OlO/SBET" (a): 0, VzOs/Ti0 2(a) ;Ll, V20s/Ti02(r) ;0, VZOs/Ti0 2(a-r). (b): 0, V20s/A1Z03 prepared by the conventional impregnation method;4t, VzOs/Alz0 3(VPS) (the number near the closed circle represents the number of VPS cycles). (c):O, VZOs/ZrOZ;Ll, VZOs/SiOZ;O, VzOs/MgO. the VZOs/A1Z0 3. However, the was much larger than that for loaded on ZrO Z slightly forms produces surface V=O species. than those for the VZOS/TiOZ' results of XRD, the number of was found that this is due to
dispersion at low content of VZOs for the VZOS/ZrOZ the VZOs/A1Z0 3(Fig. Z). This means that the VZOs inactive vanadium ions and therefore effectively S(OlO)/SBET and D for the VZOs/SiOZ were much smaller VZOs/A1Z0 3 or VZOs/ZrOZ(Figs. 1 and Z). From the layers of VZOS lamellae, IR, ESR, UV-VIS, and TEM, it the formation of large VZOs particles on SiO Z. Large
536
100
100 (a)
(b)
80
"'"
~
80
60
60
"'c:"
c: 0
V2OS/A1 203(VPS) 5
0
III
s, Q)
0-
III
40
.~
<:)
40
s, Q)
V2OS/Ti02
III
0III .~
<:)
20 0
6--°_
°
20
40
60
80
100
10O
°
20 40 60 80 Content of V2OS(mol%)
100
(c) 80 ~
"'"
~
c: 0
III ~
Q)
0III <:)
V2OS/MgO
°
20 40 80 60 Content of V2OS(mol%)
100
Fig. 2. Dispersion of V20S in the catalysts with various contents of V20S' (a):(), V20S/Ti02(a);~, V20S/TiOZ(r);[], VZOS/TiOZ(a-r). (b):(), V20S/AlZ03 prepared by the conventional impregnation method;", VZOS/A1Z0 3(VPS)( the number near the closed circle represents the number of VPS cycles), (c);(), VZOS/ZrOZ; ~, VZOS/SiOZ;[]' VZOS/MgO. V20S particles were found to be also formed even for the VZOS/Si0 2(VPS) catalyst. As for the VZOS/MgO, S(OlO)/SSET and 0 were at the content of V20S lower than 10 mol%. According to the XRD data, this is due to the formation of inactive compound between V20S and MgO(M9 l. SV04). As the content of VZOS further increased, S(OlO)/ SSET and D increased, However, their values for the V20S/MgO were much lower than those for the VZOS/TiOZ' VZOS/A1Z0 3, or VZOS/ZrO Z' From the results of XRD, the number of layers of VZOs lamellae, IR, ESR, and UV-VIS, this was found to be due
°
537
partly to the formation of M9 l .SV04 and partly to the formation of large VZOS particles. Structures of the catalysts From the results of characterizations, structures of the supported vanadium oxide catalysts were determined. Examples of the structures are shown in Fig. 3. Figure 4 shows the structures of VZOS/A1Z0 3(10 mol%) and VzOS/Alz0 3(VPS-S; 11.4 mol%) to exhibit the effect of preparation on the structure of the catalyst. Here, bold lines refer to the (010) face of VZOS exposed to the surface, while small closed circles represent inactive vanadium ions interacting strongly with the support. Since the structure of the VZOS/TiOZ(r) or VzOs/TiOz(a-r) does not differ significantly from that of the VZOS/TiOZ(a), the structure of the VZOS/TiOZ(a) shown in Fig. 3 can also be applied to the VzOS/TiOz(r) and VzOs/TiOz(a-r) with minor corrections of the number of VZOS layers, coverage of VZOS on support, and the amount of inactive vanadium ions. As shown in Fig. 3, the (010) face of VZO s is selectively exposed to the surface of VzOs/TiOz(a)(Z mol% V20S); the number of V20S layers is 1-2. In the VZOS/ TiOZ(a)(ZS mol% V20S)' the V20S covers completely the Ti02 surface and the number of V20S layers increases significantly. It is interesting to note that the (010) face is selectively exposed{ca. 80 %) even for the catalyst with 30-40 layers of V20S lamellae. In contrast to the structure of V20s/Ti0 2(a)(2 mol% V20S), a considerable amount of inactive vanadium ions are formed for the V20s/A1 203(S mol% V20S) and only small V20S particles spread over the support. As for the V20S/A1 203(3S mol% V20S), V20S particles with 3-7 layers cover the A1 203 surface. In contrast to the V20S/Ti02{a){2S mol% V20S), however, various crystal faces of V20S are exposed to the surface in addition to the (010) face. As shown in Fig. 3, the structure of V20S/Zr02 is similar to that of the V20S/A1 203 except for the inactive vanadium ion; its amount in the V20S/Zr02 is much smaller than that in the V20S/A1 203. As for the V20S/Si02, large V20S particles with 10-30 layers of V20S lamellae are formed even for the V20S/Si0 2{2 mol% V20S) and the size of the V20S particles further increases for the V20S/Si0 2(2S mol% V20S). Even for the V20S/Si02(2S or SO mol% VZOS), the Si0 2 surface is considerably exposed to the catalyst surface. In the V20S/MgO(2 mol% VzOS), all of the supported V20S forms inactive compound between V20S and MgO while no surface V=O species is formed. As for the VZOS/MgO(ZS mol% VzO S), large V20S particles are produced on the compound. As can be seen from Figs. lb and 2b, the structure of the VzOS/Alz0 3(VPS) is almost the same as that of the VZOS/A1 203 prepared by the conventional impregnation method when the content of V20S is S mol% or lower. When the content of VZO is 10 mol% or higher, the structure of the V20S/A1 203(VPS) S is somewhat different from that of the VZOS/A1 Z03. As shown in Fig. 4, the VZO S layer in the V20S/A1 Z03(VPS-S) is thinner than that in the VzOs/A1 203(10 mol% VZO S); correspondingly, the (010) face of VZO S is exposed more in the VzOS/Al z03{VPS) than in the V20S/A1 Z0 3 prepared by the conventional impregnation method.
538
High Content
Low Content
25 mol%
...J 2 mol%
30-40 layers
LJ
1-2 layers
V205/A1 Z03 5 mol%
35 mol% 1-2 layers
':'. . . .'"7
3-7 layers
~ ~ ..................
5-10 layers
25 mol% 2 mol%
2 mol%
1-2 1ayers po,
25 mol% 10-30 layers m~
___
__
30-50 layers
_u'----_EIL
25 mol%
==============~./===M9l.5V05~ MgO
10-30 layers
ED,--
_
MgO
Fig. 3. Structures of vanadium oxide supported on various supports. Bold line; the (010) face of V205 exposed to the catalyst surface. Small closed circle; inactive vanadium ion interacting strongly with the support.
539
1-2 1ayers
1-3 1ayers
~ •. • r73••• A
n . r-:-1
.e.
Fig, 4. Structures of V20S/A1 203 catalysts prepared by different methods. mo1% V prepared by the conventional impregnation method, (a): V 20S) 20S/A1 203(10 (b): V20s/A1203(VPS-S; 11.4 mol% V20S)' Bold line; the (010) face of V20S exposed to the catalyst surface. Small closed circle; inactive vanadium ion. Effects of support and preparation on the structure of supported vanadium oxide catalyst The course of preparation of the V20s/A1 203(VPS) catalyst is significantly different from that of the V20S/A1 203, As shown in Figs. lb, 2b, and 4, the structure of the V20S/A1 203(VPS) does not differ significantly from that of the V20S/A1 203. At high content of V20S' the V20S/A1 203(VPS) exposes the (010) face of V20S more than the V20S/A1 203, However, this difference is not significant compared to the effect of tre support on the structure of V20S' This means that the method of preparation does not much affect the structure of V20S/A1 203 catalyst. As can be seen from Figs. 1a and 2a, the structure of the V20s/Ti02 catalyst does not change significantly with the modification of Ti02, i.e, anatase, rutile, and mixture of anatase with rutile. This means that the difference in the modification the suppo~.On the other hand, of Ti02 is too small to affect the structure of V 20Son the structure of supported vanadium oxide catalyst is greatly changed with the kind of support, i.e. Ti00 , A1 203, Zr02, Si0 2, and MgO(Fig, 3), These results lead to a conclusion that the structure of supported vanadium oxide is mainly determined by the kind of support, while neither the method of preparation nor the modification of support affects the structure significantly. It is generally accepted that the effect of Ti02(anatase) support on V20S is brought about by a remarkable fit of the crystallographic patterns in contact at V20S-Ti02(anatase) interface. According to the model proposed by Vejux and Courtine (ref. 7), such a fit of the crystallographic patterns leads to the selective exposure of the (010) face of V20S on Ti0 2(anatase) support. This agrees with the experimental results shown in Fig. I a and the proposed structure of the V20S/Ti02(a). As shown in Fig. 3, the (010) face of V20S is not selectively exposed to the surface of the V20s/A1 203, V20S/Zr02, V20S/Si0 2 or V20S/MgO, This is reasonable, since the remarkable fit of the crystallographic patterns cannot be expected for the latter catalysts. It can also be noted from Fig. 3 that particle size of V20S on support for V20S/A1 203 Since large V20S particles and V20S/Zr02 is much smaller than that for V20S/Si0 2, are also formed for the V20S/Si0 2(VPS) catalyst, the difference suggests that the
540
interaction between V and support for the V205/Si02 is significantly weaker than 205 that for the V205/A1 203 or V205/Zr0 2. In conclusion, following points can be noted for the effect of the support on the structure of V205. When a remarkable fit of the crystallographic patterns at the V205-support interface can be expected, the interaction between V205 and support leads to the formation of thin layers of V205 lamellae on the support where the (010) face of V205 is selectively exposed. When such a fit of the crystallographic patterns cannot be expected, the interaction leads to the formation of small V205 particles on absence ~f a significant interaction between V205 and the support results the suppo~The in the agglomeration of the supported V205 to form large V205 particles. REFERENCES 1 D.J. Hucknall ,"Selective Oxidation of Hydrocarbons" Academic Press, London (1974). 2 R. Higgins and P. Hayden,"Cata1ysis" Vol. 1, 1977, p. 168. 3 A. Bie1anski and J. Haber, Cata1. Rev., 19(1979) 1. 4 D. Vanhove and M. Blanchard, Bull. Soc. Chim. Fr., 9(1971) 3291. 5 D.J. Cole, C.F. Cu11is, and D.J. Huckna11, J. Chern. Soc. Faraday Trans. 1, 72 (1976) 2185. 6 G.C. Bond, A.J. Sarkany, and G.D. Parfitt, J. Cata1., 57(1979) 476. 7 A. Vejux and P. Cortine, J. Solid State Chern., 23(1978) 93. 8 Y. Murakami, M. Inomata, A. Miyamoto, and K. Mori, Proc. 7th Internat. Congr. Catal. (Tokyo), 1981, p. 1344. 9 M. Inomata, A. Miyamoto, and Y. Murakami, J. Cata1. 62(1980) 140. 10 G.L. Bauerle, S.C. Wu, and K. Nobe, Ind. Eng. Chern. Prod. Res. Dev., 14(1975) 268. 11 T. Shikada, K. Fujimoto, T. Kunugi, and H. Tominaga, ibid., 20(1981) 91. 12 K. Tarama, S. Yoshida, S. Ishida, and H. Kakioka, Bull. Chern. Soc. Jpn, 41 (1968) 2840. 13 H. Takahashi, M. Shiotani, H. Kobayashi, and J. Sohma, J. Cata1. 14(1969) 134. 14 V.B. Kazansky, V.A. Shvets, M.Ya. Kon, V.V. Nikisha, and B.N. She1imov, Proc. 5th Internat. Congr. Cata1.(Miami Beach), 1972, p. 1423. 15 A.M. Gritskov, V.A. Shvets, and V.B. Kazansky, Kinet. Kata1. 14(1973) 1062. 16 W. Hanke, R. Bienert, and H.G. Jerschkewitz, Z. anorg. a11g. Chern., 414(1975) 109. 17 V.A. Fenin, V.A. Shvets, and V.B. Kazansky, Kinet. Kata1. 16(1975) 1046. 18 B.N. She1imov, C. Naccache, and M. Che, J. Cata1. 37(1975) 279. 19 V.A. Kha1if, E.L. Aptekar' , O.V. Kry1ov, and G. ~h1mann, Kinet. Kata1. 18(1977) 1055. 20 D.J. Huckna11 and C.F. Cu11is, J. Thermal Anal. 13(1978) 15. 21 M.R. Goldwasser and D.L. Trimm, Ind. Eng. Chern. Prod. Res. Dev. 18(1979) 27. 22 F. Roozeboom, T. Frasen, P. Mars, and P.J. Ge11ings, Z. anorg. a11g. Chern., 449 (1979) 25. 23 F. Roozeboom, J. Medema, and P.J. Ge11ings, Zeit. Phys. Chern. N.F., 111(1978) 215. 24 F. Roozeboom, M.C. Mitte1meijer-Haze1eger, J.A. Mou1ijn, J. Medema, V.H.J. de Beer, and P.J. Ge11ings, J .. Phys. Chern., 84(1980) 2783. 25 A. r1iyamoto, Y. Yamazaki, M. Inomata, and Y. Murakami, J. Phys. Chern., 85(1981) 2366. 26 M. Inomata, A. Miyamoto, and Y. Murakami, ibid., 85(1981) 2372. 27 S.I. Ko1 'tsov and V.B. A1eskovsky, Zh. Prikh. Khim., 40(1967) 907. 28 S.I. Ko1 'tsov and V.B. A1eskovsky, ibid., 42(1969) 1950. 29 M. Blanchard, G. Louguet, J. Rivasseau, and J.-C. De1grange, Bull. Soc. Chim. Fr. 8(1972) 307l. 30 A. Bystrom, K.A. Wilhelmi, and O. Brotzen, Acta Chern. Scand., 4(1950) 1119.
541 DISCUSSION
J. SCHEVE: I cannot confirm your results on Ti02, Si02 and A1203 supports at all. I feel that you should control the influence of the pulses of NH3 and NO qnto the structure of V20S because it is known from literature that already four adsorption cycles of water at nearly 200°c change the structure of V20S drastically. We found V6013 and V307 crystallites only at the surface of anatase between 2 and 20 mole % V20S' Coat-like layers of amorphous "heaps" in the other cases are found; i.e. anatase has a very different influence on the formation of V20S particles than rutile, Cab-O-Sil and alumina p2S. This agrees with catalytic data and confirms my disagreement with your model. Y. MURAKAMI' We have characterized catalysts in the oxidized state; therefore, vanadium ions were mostly in V+S state and neither V6013 nor V307 was detected in the XRD diagrams of our catalyst. I guess that you have characterized reduced catalysts. It has been well established that Ti02 (anatase) promotes the reduction of V20S whereas Ti02 (rutile) does not. As described in the text, the structure of vanadium oxide changes greatly with the kind of support, i.e. Ti0 2, A1203' Si02, or MgO while the effect of modification of Ti02 is smaller than the effect of the kind of support. We have done some catalytic experiments on our catalysts and have found that there are both structure sensitive reaction and structure insensitive reaction. As for the structure sensitive reaction, a slight difference in the surface structure can lead to a great difference in the activity and/or selectivity. Consequently, when one discusses the catalytic data in relation to the structure of the catalyst, one should keep in mind the structure sensitivity of the reaction. G.C. BOND I wonder if it is really proper to speak of V20S when one has only one or two layers. The structure of bulk V20S is characterized by layers which are weakly linked through V=O... V bonds, and a minimum of two layers is therefore needed for its development. We tend to believe that the first layer of vanadium oxide is disordered, perhaps comprising dimers or trimers of VO) ions adsorbed on the Ti0 2 surface. For example, the 1020 cm- 1 band which is characteristic of v=o stretching in bulk V20S is not observed at low vanadium concentrations. In support of the remarks of Dr. Scheve, the good catalytic properties for O-xylene oxidation exhibited by V20S/Ti02 are not shown when other support are employed. Nevertheless,you claim that the surface structure does not depend on the support, and that V20S crystals are always formed. If this is so, the effect of the support on the catalytic properties remains to be explained. Y. MURAKAMI I do not think that the structure of "V20S" for supported catalyst is exactly the same as that for a large V20S crystal but do think that it can be deformed by the interaction of the support to minimize the free energy of the system. However, it should be noted that as far as our catalysts are concerned, the coordination of oxygens around VS+ for V20S (i.e. deformed octahedron) is not changed significantly by supporting on the basis of the unchanged red edge of 02--V S+ charge transfer transition and i.r. spectra of NH3 adsorbed on the catalyst. This leads to the structure of one layer "V20S" as oligomer of such octahedral units. This view is similar to yours except for the coordination number of oxygen atoms around VS+ ion. Vejux and Courtine have proposed a detailed structure of V20S-Ti02 interface (Fig. 6 in Ref. 7). As you pointed out, the 1020 cm- l band is not observed .at low vanadium concentrations. I think that this is due partly to the low sensitivity of conventional infrared spectroscopy. The difference in lattice parameter or metal-oxygen bond length between support and V20S leads to the deformation of "V20S" on support. Furthermore, the difference in the electronic property between support and V20S leads to a modified electron distribution in "V20S on support. These effects can bring about the effect of support on catalytic properties.
542 F. TRIFIRO: The reduction of V(V) containing species with NO-NH3 is surely a measure of the amount of V(V). How can you discriminate between the reduction of v=o and V-O-V species ? Y. MURAKAMI The selective reduction of v 5+=O species was confirmed by IR spectroscopy coupled with pulse and continuous flow techniques (Refs. 9, 25 and 26 in the text). The stoichiometry (V=O + NO + NH3 ~ V-OH + N2 + H20) was also confirmed by 15N-tracer experiments (A. Miyamoto, K. Kobayashi, M. Inomata, and Y. Murakami, J. Phys. Chem. 86, 2945 (1982). The surface area of the (alar-face unsupported V205 is 2.7 m2/g which is calculated on the assumption that NO and NH3 react only with v=o on the (010) face. This value seems quite reasonable because it is a half area of the total surface measured by the BET method which contains the (010) face and the other faces.
543
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
©
STRUCTURAL MODIFICATIONS OF V-P MIXED OXIDES DURING CALCINATION IN AIR OR IN A MIXTURE OF BUTENES-AIR G. GENTI , C. GALASSI, T. MANENTI, A. RIVA ano F. TRIFIRO'x Facolta di Chimica Industriale, Viale Risorgimento 4, 40136 Bologna (Italy)
ABSTRACT The chemistry of the preparation of vanadium-phosphorous mixed oxides catalysts active in the oxidation of the C
4
fraction to maleic anhydride was inves-
tigated. Different compounds can be obtained according to (i) P:V ratio; nation temperature;
(iii) calcination atmosphere;
(ii) calci-
(iv) calcination rate.
The presence of a small excess of phosphorous with respect
to P:V ratio
1.0 and a slow calcination were suitable to form the active and selective phase, a polyphosphate of V(IV) and to prevent its oxidation to a,8-voP0
4. The active phase can be formed under the reaction conditions either from
the precursor or from VOP0
4
and its formation does not depend on the ViP ratio.
INTRODUCTION Vanadium and phosphorous mixed oxides have been widely used as catalysts for the oxidation of the C
fraction to maleic anhydride (ref.1). In the patents 4 literature it is pointed out that optimum activity is obtained for a P:V ratio
of about 1.1, where a crystalline compound, called the 8-phase (ref.2) , is present. This phase is formed by calcination at high temperature, and both vanadium (IV) and (V) are present. The aim of this study was to investigate the chemical transformation occurring during the calcination and the nature of the compound obtained in order to determine the influence of different calcination procedures on the formation of the active species.
EXPERIMENTAL Preparation of catalysts The catalysts were prepared by dissolving 40 g of V in 500 ml of 37 % HCl 205 at 100 DC. After complete reduction to V(IV), a solution of 85 % H was added 3P04 in sufficient quantity as to obtain the expected P:V ratio. The mixture was boiled under reflux for 1 hour. After evaporation of the water and drying at 125 DC, the precursor was calcined following two procedures: slOWly from
(i) it was heated
2C DC to 380°C (in two hours), kept at 380 DC for 1 hour, and then
calcined at 500 DC;
(ii) it was let directly into the muffle furnace at 400°C,
544 kept for 2 hours and then calcined at 500°C. Catalysts were prepared with P:V ratios between 0.90 to 1.40. Characterization Chemical
of vanadium (IV) and (V) content was effected volumetri-
a~alysis
cally. Infrared (IR) spectra were reCorded with a JASCO A 202 spectrophotometer using the KBr disc technique; Raman spectra were measured on a Cary 83 laser Raman spectrophotometer (4880
A excitation)
.
X-ray diffraction (XRD) measurements were taken with a Rich-Seiffert diffractometer using CUK
a
radiation; diffuse reflectance spectra were recorded on a
Perkin Elmer 124 spectrophotometer, using MgO as a standard.
RESULTS AND DISCUSSION The nature of the precursor The chemical analysis of vanadium in the catalyst dried at 125°C showed that the vanadium was present as vanadium (IV) with the exception of very small amounts of vanadium (V) «5 %). The XRD patterns of catalysts with P:V in the range 0.9-1.4 were similar; only additional weak lines indicated with asterisks were present in catalysts containing vanadium (V) (Table 1). The XRD patterns corresponded to that reported in a patent (ref.3) as characteristic of an optimum precursor but with modification of the relative intensity of lines due to imperfect crystallization. The IR spectra were also similar for all the P:V ratios examined (Fig. la); 1 the bands at 1200, 1130, 920 cm- were attributed to the stretching modes; those -1 at 630, 530, 480 cm to the bending modes of the pyrophosphate ion and the band -1 at 975 cm to the stretching mode of the V=O group, in agreement with those re1 ported by Hezel and Ross (ref.4). The two bands at 3390 and 1630 cm- were assigned to the stretching and bending modes of water (ref.5). No band of P-O-H group was detected. The Raman spectra of
precurso~s
with two different P:V ratios are shown in
Fig. 2a and b. The spectra confirm the structure of vanadyl pyrophosphate, but in the case of catalysts with P:V ratios 1.16,two additional weak bands,indica1. ted by asterisks, are present at 940 and 626 cmThey are attributed to the stretching of orthophosphate group (ref. 6). By washing the catalysts with water at 25 ·C, all the vanadium (V) was removed and the P:V ratio became 1.0. After washing, the weak additional lines in 1 XRD patterns and the two bands at 940 and 626 cm- in the Raman spectra disappeared. The diffuse
refl~ctance
spectra are shown in Fig. 3 for two P:V ratios (Fig.
3 a and b) and after washing (Fig. 3 c)
. The bands at 775 and 640 nm were pre-
calc. temp.,·C
p:v
a
b
c
d
e
125 1.25
380 l25
380 l02
500 1.02
380 1.02
note
after catalytic test
f
calcination temp.:C
500 1.13 rapid calc. a b
c d e
125 125 380 500 500
p:v note l02 1.16 l02 1.16 e 1.16 washed
d
d
c
a b
b
a
,
3600
I
L
I
~
I
--.l_
2000 ----'600------1200
1
•
I
800
IR SPECTRA, cm- 1 Fig. 1 Infrared spectra of V-P mixed oxides.
c 1200
'
800
'
,DO
I
RAMAN SPECTRA,6cm~ Fig. 2 Raman spectra of V-P mixed oxides.
... 01 01
546
e
d
a
b c
i Ql
u
c
0 .D ~
0 III
calcination
p:v
a b
temp .. ·C 125 1'25
1.02 1.16
c d e
125 380 380
1.02 1.16 1.16
note
washed washed
.D 0
DIFFUSE REFLECTANCE SPECTRA, nm
Fig. 3 Diffuse reflectance spectra of V-P mixed oxides. sent in all the catalysts and were assigned to the d-d transitions of V(IV) (ref. 7), but for the catalysts containing vanadium (V) an additional band at 400 nm due to the charge transfer of coupled vanadium (IV) and (V) is noted. This band disappeared after washing. From previous thermogravimetric data (ref.B) we could draw the water of crystallization and so we assigned the composition (VO)2P207' 2H
to the pre20 cursor of all the catalysts. The excess of phosphorous with respect to the P:V ratio 1.0 was present as mixed vanadium (IV) and (V) on the surface, and the washing procedure removed it. The formation of a vanadyl pyrophosphate also explained the fact that a solid could be obtained only from evaporation and therefore in a concentrated solution. Chemistry of calcination Slow calcination in air. In the range of calcination temperature 125-350 ·C no significant variation in the IR and Raman spectra for all the catalysts could be noted, but the XRD lines of the precursor disappeared and formed an amorphous phase (Table 1). This shows that in the amorphous phase vanadyl and pyrophosphate group were still present, but in random arrays. The temperature of amorphous phase formation coincided with the beginning of the loss of crystallization water and with the oxidation of V(IV) into V(V).
547 The increase of P:V ratios from 0.9 to 1.4 shifted the formation of the amorphous phase toward higher temperatures, but always below 380°C. At the calcination temperature of 380 °c, the IR (Fig. 1 b and c) and Raman (Fig. 2 c) spectra, and the XRD patterns (Table 1) showed the formation of a new compound for all catalysts, but with:
(i) P:V ratios
<
with a different structure for the catalysts
1.0 and (ii) P:V ratio> 1.0.
TABLE X-ray patterns of v-p-o Catalysts. Lattice spacing, d(~)and (vs ) very strong,
(s) strong,
(m) medium,
lines intensity:
(w) weak.
T calc. °c
P:V ratio
125
370
380
380
500
380
500
500
1. 02
1. 25
1. 02
1. 25
1. 02
1. 02 (a)
1.13 (b)
1. 30 (b)
5.21 m 4.62 m 3.90 w 3.49 m 3.40vs 3.18 m 3.06 s 2.98 w 2.82 w 2.21 w 2.18 w
5.70 w 4.80 w 3.87vs 3.14 s 2.ge m 2.66 w 2.44 w
5.64vs 4.51 s 4.07 W x 3. 9 8 w 3.66 s 3.29 m 3.16 s 3.10 m 2.93vs x 2. 8 3 w 2.79 w x 2. 6 5 w x 2. 6 1 w
5.64 4.51 3.98 3.66 3.29 3.10 2.93 2.79 2.65 2.41
s m w w m w s w w w
4.90 w 4.24 w 4.15 w 4.02 w 3.57 s 3.07 s 3.01vs 2.94 w 2.22 w 2.13 w
3.89 3.87 3.66 3.58 3.30 3.14 3.09 3.00 2.12 2.09
s m w s w m w s w w
l3-phase
two very broad bands as background
a after catalytic test b rapid calcination x ; spurious lines (see text) xx; l3-VOP0 4 In the first case the IR (Fig. 1 c) and Raman (Fig. 2 c) lar to
a-voPo
5.64 m 5.41 m xX 5. 2 2 w xX 4. 6 2 W 4.17 w 4.00 m 3.70 s 3.56 s xx 3. 4 8 m xX3.42vs 3.37 s 3.30 s xx 3. 17 m xX 3. 0 8 m 3.02
5.76 w 4.81 w 4.76 m 4.15 m 3.89vs 3.16 s 3.10 m 2.96 m 2.60 w 2.45 w
spectra were simi-
(ref. 9), but XRD patterns (Table 1) were different from those
4 reported in the literature (ref. 10).
Jordan and Calvo (ref. 11) have noted a disorder in the crystal structure of a-voPo
having a layered structure. Besides, 14 % vanadium (IV) was still 4 present in these samples calcined at 380°C, the presence of V between the layers could alter the XRD patterns of a-vopo
4,
a similar effect was observed for
hydrated a-voPo
(ref. 9). 4 In the case of catalysts with P:V ratios> 1.0 calcined at 380°C, the a-
mount of vanadium (V) was lower (60 % and 20 % V(V)
for
P:V 1.16 and 1.25, res-
pectively). The XRD patterns (Table 1) showed the same lines of altered a-vopo
4,
548 but also those of the S-phase that in the patent literature (ref.2) was pointed out as the active and selective phase. The IR spectrum (Fig. 1 b) was also different from those of catalysts with a P:V ratio of band at 1240 cm
-1
~
1.0 and, in particular, a
was present that could be attributed to more condensed phos-
phates (ref. 5). Unlike the catalysts dried at 125°C, for those calcined at 380 °c, the P:V ratio slightly increased after washing; this indicated that the excess of phosphorous was not free on the surface, but irreversibly adsorbed. The washing removed a superficial
phase of vanadium (V) with a P:V ratio of about
1.0; in
fact, only after washing the d-d transition of vanadium (IV) at 775 and 640 nm was present in the diffuse reflectance (Fig. 3 d and e)spectra. The calcination at 500°C did not alter substantially the IR spectra of the catalysts with P:V >1. An increase in intensity of the e-phase lines was noted in the XRD patterns. The amounts of vanadium (V) rose from 50 to 60 % in the case of catalysts with P:V = 1.16 and from 20 to 50 % for P:V = 1.25. The washing procedure removed all the vanadium (V), but did not alter the XRD patterns. This showed that the vanadium (V) phase was amorphous. The Raman spectra (Fig. 2 d)
corresponded to a vanadyl phosphate; when the amorphous phase was removed
(Fig. 2 e) only the two bands of the vanadyl group were present. This indicated that in the amorphous phase, phosphorous was present as P0
group, 4 Bourdes and Courtine (ref. 10) reported that the XRD pattern of the S-pha-
se corresponds to (VO)2P207' but the IR spectrum is different from the one of pyrophosphate. Besides the pyrophosphate group is
active in Raman (ref. 4), un-
like those we found and we observed an endothermic reaction during the transformation of the amorphous intermediate pyrophosphate into the S-phase, while the transformation from the amorphous to the crystalline compound is
exothermic.
Therefore, we think that the S-phase is a more condensed phosphate of vanadium (IV) or a superficially modified pyrophosphate of vanadium (IV). It is also 1 interesting to note that the V = 0 stretching at 940 cm- was the same for vanadium (V) phosphate (Fig. 4 a) and vanadium (IV) phosphate (Fig; 4 b), and therefore the reactivity of the double bond is similar. In fact in the active component, as claimed in the patent literature (ref. 2), these two components were present. The V =
°
stretching in S-voPo is at higher Raman shift (ref. 9). 4 This shows that Raman spectra are very important in the study of these
catalysts, while due to the low polarizability of the constitutive atoms of phosphate (ref. 12) the bands of vanadium are more noticeable. In the case of the catalysts with P:V spectra (Fig. 1 d)
~
1.0, XRD patterns (Table 1) and IR
showed the formation of S-voPo
and chemical analysis showed 4 the complete oxidation to vanadium (V). This phase was not soluble in water. Rapid calcination in air. When the precursor dried at 125°C was directly
549 introduced into the muffle furnace at high temperature, the XRD patterns (Table 1) and IR spectra (Fig. 1 f) showed the presence of B-VoP0 but not of a-vopo 4, 4, for P:V ratios ranging between 0.9 and 1.25, and for a crystalline vanadium (IV) phosphate not reported in literature. The washing procedure reduced only partially the amounts of vanadium (IV), but did not alter the XRD patterns. At P:V ratios
the XRD patterns (Table 1) showed the presence of B-
~1.25,
phase and of other compounds not identified. The IR spectra corresponded to that of B-phase. The chemical analysis showed that 60 % of vanadium (V) could be removed
completely by washing. In this case too no substantial change in XRD pat-
terns
was noted after the treatment. Modification in the reaction conditions. When the calcination of the pre-
cursor was carried out in vacuum or in air plus 1 % butene, only the B-phase was detectable from the IR spectra and XRD patterns (Table 1) for all the P:V
ratios~
the catalysts were completely reduced to vanadium (IV).
va nad ium(IV) phases
(voh
P207 2H 20
pyrophosphate amorphous fi-phase non Identified phosphate
~
CALCINATION
vanadium(V)phases o(-VOPO" modified
GJ 0
Q
slow
non cr ystaillne8 o(-VOPO"
r:==t>
rapidO
0
ct>
ATMOSPHERE OF CALCINATION
GJ c:::::J
ft-VOPO"
0
AIR
1i::;:I:iJIA 1% BUTENE IN AIR
P:V about 1.0
o DODOr:>[±EJ P:V>l
.
"
&?;.; 0,. 01 t.'io;~:.
C
I.. ._a---'p 0 0 0 0 ~ 500·C
P: V> 1.25
•
~
Fig. 4 Block diagram of the chemical transformations which take place during calcination.
550 Even the modified a-voPo 4 and S-VOP0 could be completely reduced to S-pha4 se at 380°C under an atmosphere of 1 % butene in air (Fig. 1 e), but in the second case the reduction was very slow. When both S-phase and amophous phosphate of vanadium (V) were present, the latter partially reduced to B-phase. In the case of the catalysts prepared by rapid calcination, the treatment with air and butene did not alter very substantially the XRD patterns, but only the line intensity.
CONCLUSIONS All the results can be summarized in the block diagram of Fig. 4. The catalysts with P:V ratios between 0.9 - 1.4 showed the same hydrated vanadyl-phosphate after drying at 125°C; the excess of phosphorous with respect to P:V ratio
= 1.0 was present as mixed vanadium
(V) and (IV) phosphate on the
surface of the catalyst. During the calcination at high temperature new compounds were formed through an intermediate amorphous phase of vanadyl-pyrophosphate. The different compounds, that could be obtained varied according to : P:V ratio;
(ii) calcination temperature;
(iii) calcination atmosphere:;
(i)
(iv)
calcination rate. The presence of an excess of phosphoric acid prevented the total reoxidation of vanadium (IV) and permitted the formation of the active S-phase, as during the calcination in air. The calcination in vacuum or in the presence of butene formed for all the P:V ratios only the B-phase, while in air an additional amorphous phase of vanadium (V), similar to a-vopo
was formed. 4, For the catalysts with a P:V ratio of about 1.0, the calcination in air
caused the total oxidation of vanadium and the formation of B-vOP0
4
through an
intermediate phase of modified a-vopo
4• The treatment with air and butenes formed the B-phase in both cases, but at
a different rate. Rapid calcination did not permit the reaction between amorphous pyrophosphate and the excess of phosphorous and so formed a new compound of vanadium (IV) associated with two phases of vanadium (V). The reduction to the B-phase could not be obtained. The B-phase was probably a condensed phosphate of vanadium (IV) that in the reaction condition is in equilibrium with an amorphous phosphate of vanadium (V).
ACKNOWLEDGEMENTS The present work was carried out with the contribution of the research program "Progetto Finalizzato per la Chimica Fine e Secondaria" of the National
551 Research Council,Rome (Italy).
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
R.L. Varma and D.N. Saraf, Ind. Eng. Chern. ,Prod. Res. Dev., 18 (1979) 7-13. R.A. Mount and H. Rafe1son, u.S. Patent No. 3,330,354 (1975). G. Stefani and P. Fontana, U.S. Patent No 4,100,106 (1978). A. Hezel and S. Ross, Spectrochimica Acta, 23 (1967) 1583-89. D.E. Corbridge and E.J. Lowe, J. Chern. Soc., (1954) 493-502. A.C. Chapman and L.E. Thirlwe11, Spectrochirnica Acta, 20 (1964) 937-947. G. Martini, L. Morselli, A. Riva and F. Trifiro, Reac. Kinet. Catal. Lett., 8 (';'978) 431-435. G. Poli, I. Resta, o. Ruggeri and F. Trifiro, Appl. Catal., 1 (1981) 395404. R.N. Bhargava and R.A. Condrate, Applied Spectroscopy, 31 (1977) 230-235. E. Bordes and P. Courtine, J. Catal., 57 (1979) 236-252. B. Jordan and C. Calvo, Can. J. Chern.,51 (1973) 2621-25. M.T. Pasques-Ledent and P. Tarte, Spectrochirnica Acta, 30A (1974) 673-689.
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O. Poncelet, P. Orange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
METHOD OF IMPREGNATION WITH TRANSITION METAL
553
ALKOXIDES.
VANADIUM-ALUMINA AND VANADIUM-SILICA SYSTEMS
M. GLINSKI
and J. KIJENSKI
x
Institute of Organic Chemistry and Technology, Warsaw Technical University (Politechnika), Warsaw (Poland)
ABSTRACT The method of preparation of vanadium-alumina and vanadium-silica catalytic systems is described. The basis of the proposed saturation procedure is the reaction of vanadyl triisobutoxide with OH groups on the surfaces of oxide carriers. The physicochemical properties and catalytic activity of the systems prepared have been investigated. INTRODUCTION The present work aims to describe the method of preparation of transition metal oxides systems deposited on oxide carriers. The base of the proposed method is the interaction of alkoxides molecules with the hydroxyl groups on the oxide surfaces. The results presented concern the preparation of vanadium oxide-alumina and vanadium oxide-silica systems, although the method described is useful for oxides of all transition metals, which form soluble alcoholates. In one of our previous papers (ref.l) we have presented a method for the modification of the alumina surface with alkali metal ions using alkali metal alkoxides as donors of the corresponding cations. This method gave the possibility of obtaining A1
modified by alkali ions of defined properties and known 203 alkali metal contents, which can be controlled by the precalcination temperature of the oxide and the thermal treatment of the freshly
prepared catalyst.
As in case of impregnation with alkali metal alkoxides, the application of transition metal alcoholates allows the catalytic systems of predicted metal content and properties to be designed. The impregnation procedures carried out in waterfree conditions permits the secondary effects caused by the interaction of water with the dehydrated surface and the formed catalyst to be avoided. The reaction of alkoxides with surface hydroxyls is very selective and the distribution of the deposited metal ions corresponds to the distribution of OH groups on the starting gel surface even at relatively high temperatures of calcination. The composition of the vanadium-alumina and vanadium-silica catalysts obtained by impregnation with vanadyl triisobutoxide confirms the stoichiometry of the principal reaction. unexpectedly, the change of carrier causes dramatic changes of the acidity while maintaining the same structure of surface
554 vanadyl hydroxides. EXPERlMENT~L
A1 used in the present work was obtained by hydrolysis of aluminium iso203 propoxide, previously purified by distillation under vacuum - B.p. 413 K/1.07 2 kN mThe hydrolysis procedure has been described elsewhere (ref.1). Before calcination,Al(OH)3 was dried at a temperature of 313 K for 24 hrs., at 353 K for 24 hrs. and at Si0
39~
K for 24 hrs.
was obtained by hydrolysis of ethyl orthosilicate, previously purified
2 by distillation at normal pressure - B.p. 441-2 K. 500 g of freshly distilled
ester and 1000g of bidistilled water were stirred and heated at a temperature of 318 K for several hours. The water layer was separated and dried at 333 K during 24 hrs. and then at 393 K for 24 hrs. Before impregnation, both alumina and silica were calcined in nitrogen at temperatures of 573, 773 and 873 K for 5 hrs. Grains of diameters within 0.5 1.02 mm have been used for the impregnation. Vanadium ions were introduced onto the surface in the form of vanadyl isobutoxide (prepared according to ref. 2) and distilled twice under vacuum - B.p. 2) 414-5 K/l.07 kN mfrom water-free n-hexane. After impregnation, the catalysts were calcined at a temperature of 573 K in a stream of dry air for 3 hrs. Some of these catalysts were reduced ina stream of dry hydrogen for 2 hrs. at temperatures
of 573 and 723 K.
The number of OH groups on the A1
and Si0 surfaces, as well as on vana20 3 2 dium doped catalysts, was established using the sodium naphthenide titration method (ref.3). The total amount of vanadium ions introduced was determined v5 + ions, by iodine titration of vanadium
gravimetrically and the number of
extracted with HCl from the catalysts surface. The ESR spectra of the catalysts were obtained at room temperature using a Jeol JMX spectrometer. I.r. spectroscopic investigations of the catalysts' acidity have been carried out using a Specord 75 apparatus. For the i.r. measurements, the catalysts samples were 2). 3 2 10 kN m- into thin wafers (10 mg cmA
pressed under a pressure of 2.02
wafer was placed into the vacuum cell and treated in the same way as a normal catalyst. pyridine was adsorbed at room temperature at a pressure of 1.33 kN
m~
After 10 min. exposure the cell was evacuated. The spectra were recorded after evacuation at temperatures of 298, 393, 443, and 573 K. The catalytic activity of the systems under study has been investigated in n-heptanol transformations in the absence and presence of dry air. The
reactio~s
were carried out in a typical fixed-bed flow reactor at a temperature of 573 K, HSLV being 2 (g of n-heptanol/g of catalyst . 1 hr), the flow rate of air being 1 3 5 dm hrRESULTS The amount of vanadium ions introduced onto alumina and silica and the
555 amounts of OH groups on the carriers and on the vanadium-doped catalysts are given in Tables 1 and 2. TABLE Amounts of vanadium ions deposited on A1
203
and Si0
2
and numbers of OH groups
on the precalcined oxides.
Oxide
Temperature of calcination (K)
A1
20 3
Si0
2
Concentration of surface OH groups -1 (mmole.g )
Amount of V5+ ions
Total amount of vanadium ions -1 (mval.g )
(mval.g
573 773 873
0.46 0.44 0.34
0.41 0.46 0.36
0.40 0.41 0.34
573 773 873
0.46 0.31 0.28
0.50 0.33 0.30
0.48 0.34 0.30
In the reaction of vanadyl isobutoxide with the OH groups on A1 surfaces,isobutanol is produced
/
and Si0
2
o-i-C H I 4 9
-Al-O-V=O
V=o
+
)
according to the reaction
-,
"
-Al-OH
203
-1
/
(1 )
+
I
O-i-C
4H g
During the calcination of the impregnated oxides in a stream of air the formation of butenes and 1-butanal was detected. O-i-C H \ I 4 9 -Al-O-V=O
I
I
O-i-C
OH
" I -Al-O-V=O /
(2)
J
OH
4H g
and plausibly OH
?-i-C Hg
4
/
°"
/
4Hg O-i-C H I
-Al-O-V=O
/
" I -Al-O-V=O
Al-O-V=O I O-i-C
4 9
J
°\ °I - Al-O-V=O /
(3)
I OH
J
O-i-C
4Hg 1-butanal being probably the product of oxidation of the isobutanol residue. According to Table 1 the amounts of vanadium ions deposited on A1 Si0
and 203 surfaces are stoichiometric to the number of hydroxyls determined by the
2 naphthenide titration method . The results of the determinations of the OH group
concentrations on vanadium doped catalysts give support to the supposition that
556 TABLE 2 Concentration of OH groups on surfaces of vanadyl alkoxide doped alumina and silica precalcined at 773 K.
Catalyst
Temperature of reduction with H 2
Concentration of the surface OH groups (mmole g-1)
(K)
Alumina vanadyl triisobuto~de
Silica vanadyl triisobutoxide
a
a 573 723
0.85 0.80 0.70
573 723
0.63 0.60 0.62
catalysts calcined in dry air at 573 K.
on surfaces of silica and alumina calcined at higher temperatures, the reaction (2) occurs dominantly - the supported catalysts exhibit concentration of hydroxyls two times higher than the starting oxides and than the number of deposited vanadium ions. Vanadium is present on the catalyst surfaces in the form of
v5 +
and
v4 +
cations. The ESR spectra of the systems investigated exhibit the presence of a characteristic signal' (ref.4) derived from 51 V(IV) ions (Fig. 1). TABLE 3 Changes in the content of 51 V(IV) ions on the surface of vanadium doped alumina
Form of catalyst
after after after after
impregnation calcination in air at 573 K reduction at 573 K reduction at 723 K
4 Number of v + i o n s (a.u.)
5.4 5.6 8.4 29.8
4 The amount of v + ions present on the surface of the freshly
prepared cata-
lyst increases after calcination and remarkably during reduction with hydrogen (Table 3). The catalytic activities of the systems under study are compared in Tables 4 and 5. Pure alumina and silica are practically inactive in n-heptanol transformations and oxidation: the oxidative conversion in the absence or presence of
557 air does not exceed 1 mol %. In the absence of air vanadium-doped Al exhibits 203 activity only in the dehydration of n-heptanol: under the same conditions the silica-vanadium system possesses a remarkable activity in both the dehydrogenation and the dehydration processes. The oxidation activity is much higher for the silica-containing catalysts: the selectivity of the aldehyde formation is also higher for this system. It should be underlined that in the case of the alumina-vanadium system, di n-heptyl
ether~
a favoured dehydration product as
opposed-to pure carriers and silica-vanadium systems which produce selectively n-heptene. For the most active silica-vanadium and alumina-vanadium catalysts, as well as for pure silica and alumina, acidity investigations using pyridine adsorption have been done. The results of pyridine adsorption on the catalysts studied are presented in Fig. 2. The adsorption of pyridine on A1 calcined at 773 K results in the 20 3 appearance of a series of new bands corresponding to the interaction of basic molecules with surface acidic centres (ref.5). The bands at 1440, 1448, 1573, 1 1615 cm- should be ascribed to the adsorption on Lewis type sites and the band 1 at 1487 cm- to the adsorption on Br¢nsted sites. All these bands remain after pyridine desorption even at 573 K.
-400G-05
Fig. 1 ESR spectrum of 51 V(IVl ions on the surface of vanadium triisobutoxide doped alumina; gl
=
1.938, g,
=
1.997 .
558 2000 I
4800
,4600 I
41100 I
Fig. 2 l.r. spectra of pyridine adsorbed on vanadium-silica catalyst; 1- sample of catalyst before adsorption, 2- after evacuation at 298 K, 3- after evacuation at 373 K, 4- after evacuation at 423 K.
TABLE 4 The products of n-heptanol reactions carried out over vanadium doped alumina and silica in the absence of air.
Carrier
Temperature of carrier calcination (K)
573 773 873 573 773 873 573 773 873 573 773 873 573 773 873 573 773 873
Temperature of catalyst reduction (K)
573 573 573 723 723 723
573 573 573 723 723 723
Reaction products (moles from 1 mole of substrate related to 1 mmole of vanadium) di-n-heptyl n-heptene ether n-heptanal 0.10 0.16 0.34 0.03 0.08 0.37 0.20 0.21 0.32 0.08 0.04 0.02 0.03 0.02 0.02 0.03 0.04 0.03
0.18 0.17 0.11 0.12 0.22 0.08 0.15 0.15 0.17 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01
o o o o o o o o o 0.05 0.06 0.05 0.03 0.05 0.05 0.03 0.03 0.05
559 TABLE 5 The products of n-heptanol reactions with air over vanadium doped alumina and silica.
Carrier
Temperature of carrier calcination
Temperature of catalyst reduction
(K)
(K)
573 773 873 573 773 873 573 773 873 573 773 873 573 773 873 573 773 873
Reactions products (moles from 1 mole of substrate related to 1 mmole of vanadium) di-n-heptyl n-heptene ether n-heptanal
573 573 573 723 723 723
0.17 0.31 0.35 0.12 0.18 0.23 0.10 0.12 0.12
traces 0.02 0.03 0.02 0 0.01 0 0 0
573 573 573 723 723 723
0.24 0.26 0.25 0.28 0.27 0.30 0.29 0.26 0.27
0.01 0.02 0.02 0.01 0.02 0.02 0.01 0.03 0.03
upon adsorption of pyridine on
Si~t
0.09 0.05 0.05 0.08 0.04 0.03 0.03 0.04 0.08 0.09 0.14 0.16 0.08 0.16 0.19 0.08 0.14 0.17
calcined at 773 K intense i.r. absorp-
tion bands appear at 1443 and 1590 em
,corresponding to the interaction with 1, Lewis type acidic centres, and at 1485 cmdue to pyridine in interaction with Br¢nsted centres: these bands disappear practically after evacuation at 373 K. On adsorption of pyridine on vanadium-doped alumina (precalcination at 773 K) and on the same catalyst reduced with hydrogen at 573 K, only very weak bands at 1 1 1485 cm- and 1543 cm(both connected with Br¢nsted sites) appear in the i.r. spectrum, which disappear completely after evacuation at 373 K. On the contrary, the i.r. spectrum of pyridine adsorbed on the silica-vanadium system (precalci-1 ned at 773 K) indicated the presence of intense bands at 1445, 1450, 1487 em 1 (interaction with Lewis centres) and at 1540 cm(adsorption on Br¢nsted sites), from which only the last one disappears after outgassing at 423 K. Taking into consideration the above results, it should be emphasized that the catalysts studied possess quite different acidic properties: silica and vanadyl alkoxide doped alumina are very weak acids, the first being rather of a Lewis character and the second of Br¢nsted, although alumina and silica-vanadium catalysts have strongly acidic Lewis-type properties. CONCLUSION i
The interaction of vanadyl triisobutoxide with surface hydroxyls on alumina
560 and silica leads to the formation of -O-VO(OH)2 groups in amounts corresponding to the number of hydroxyls on the starting carrier. ii
The negligible amount of 51 V(IV) ions present on the surface directly after
impregnation increases after calcination of the catalysts and after reduction with H ( 5-fold increase). 2 iii Vanadyl triisobutoxide supported Si0
and A1 differ considerably in sur2 20 3 face acidity: catalysts obtained by reacting strongly Lewis acidic alumina with
vanadium alkoxide possess very weak acidic properties: on the other hand the catalytic system formed by reacting weakly acidic Si0
2
with vanadium alkoxide
shows high acidity. iiii Differentiation in acidic properties is reflected in the catalytic activity of the catalysts investigated - the more acidic silica
vanadium system
being more active and unexpectedly more selective in n-heptanol oxidation than the alumina
containing catalyst.
Di-n-heptyl-ether seems to be the product of n-heptanol dehydration via a nonacidic pathway. REFERENCES 1. 2. 3. 4. 5.
R. Hombek, J. Kijenski and S. Malinowski, Preparation of Heterogeneous Catalysts, ed. B. Delman, G. Poncelet, p. 595, Elsevier, Amsterdam, 1978. H. Funk, W. Weiss and M. Zeising, Z. Anorg. allgem. Chern., 296 (1958) 36. J. Kijenski, R. Hombek and S. Malinowski, J. Catalysis, 50 (1977) 186. H. Takahashi, M. Shiotani, H. Kobayashi and J. Sohma, J. Catalysis, 14 (1969) 134. E.P. Parry, J. Catalysis, 2 (1963) 371.
561 DISCUSSION A. MIYN10TO : -Is it possible to prepare catalysts with higher V20S contents by repeating reactions (1)-(3) ? -Is the catalyst stable in the presence of, say, H20 at 500°C? -What is the percentage of V4 + in the total V ions supported ? -It may be interesting to investigate various catalytic reactions in addition to the reactions of toluene and n-heptanol. J. KIJENSKI
: -The introduction of the first portion of vanadium alkoxide onto the carrier surface is the starting point of the work presented. The repetition of the procedure adopted is possible, although it should be underlined that the next portion of vanadium ions fixed with their first layer should strongly depend on the acidity of the OH groups in the vanadyl hydroxides previously formed. -The catalysts examined keep their catalytic activity as well as their yellow coloration and paramagnetic properties even at 823 K and in the presence of water. -The amount of V4 + ions supported onto the carrier surface is in the range of 0.8% with respect to the total amount of vanadium. -The reactions reported have been chosen as catalytic tests; further oxidation reactions are under study.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publisbers B.V., Amsterdam - Printed in The Netherlands
563
THE SIGNIFICANCE OF THE MULLITE PHASE IN A SILVER CATALYST FOR THE OXIDATION OF ETHYLENE INTO ETHYLENE OXIDE LIN BING-XIONG, ZHANG WAN-JING, YAN QING-XIN, PAN ZUO-HUA, GUI LIN-LIN and TANG YOU-CHI Laboratory for structure of Matter, Institute of Physical Chemistry, Peking University, Beijing (P.R.C.)
ABSTRACT An a -alumina support with suitable pore structure and good heat transfer properties has been prepared. Silver dispersed in uniform grains of suitable size is important for the selectivity of the silver catalyst. IVe tried various preparative procedures to disperse silver on the support, and found out that the master key to success was the addition of a muL'l i, te phase to the
a -alumina
support. On recognizing the significant role played by the mullite phase, we prepared the support for the silver catalyst from corundum and alumina-silica gel. Satisfactory results have been obtained.
INTRODUCTION Silver has a unique activity in this industrially important reaction, and is normally used on an a-alumina support. The complete oxidation of ethylene to carbon dioxide and water produces more than ten times as much heat as the desired reaction. The highly exothermic reaction leading to complete combustion has a higher activation energy than that leading to ethylene oxide, and the selectivity therefore falls abruptly with increasing temperature. An increase in temperature leads to a fall in selectivity and lower selectivity causes the temperature to rise further. Thus, the reaction has a pronounced tendency to be involved in a vicious circle. Very careful attention to temperature control is required in the design of the reactor. On the other hand,we have been very attentive toward the intrinsic selectivity of the silver catalyst. This paper describes the preparation of a suitable support for the silver catalyst.
GUIDELINES FOR A GOOD
a -ALUMINA SUPPORT
Because heat transfer as well as selectivity are of vital importance for such a silver catalyst, we had to choose in the first place a supporting framework with good heat transfer properties, small specific surface and relatively coarse pores. Secondly, high selectivity requires silver dispersed in uniform grains of suitable size. For framework material we chose corundum powder. How-
564 ever, we observed that
a-alumina itself is not good at dispersing and carrying
silver. Investigations have enabled us to establish that mullite crystallites can provide the right surface for dispersing and carrying silver.
MULLITE
SU~ACE
AND DISPERSION OF SILVER
Observations Silver is poorly deposited on the naked a-alumina surface (Fig. 1a). Silver is obtained in the usual way, i.e., by reducing silver lactate soaked into an a
-alumina support. However, silver can be dispersed into fine grains on the
mullite surface
(Fig.
1b). The mullite phase consists of prismatic crystallites
(Fig. 1c). Mullite prisms of about 1
~m
in width carryon them silver grains of
comparable size. Figure 1d shows silver deposit on an a-alumina surface only partially covered by mullite prisms, and the uncovered part is seen to be barren. Our findings strongly suggest that the mullite surface may have some peculiarity which facilitates the reduction of the silver salt.
Figure 1 (a) Deposit of silver on naked a-alumina; Figure l(b) Dispersion of silver on mullite prisms of about 1 ym in width; Figure l(c) Mullite crystallites; Figure l(d) Silver deposit on an a-alumina surface only partially covered by mullite prisms
565 The structure of the mullite phase The structure of the mullite phase has been determined
(Ref. 1). It is
closely related to the structure of sillimanite and andalusite. Both of them have the composition AI
The mullite phase is richer in aluminium, and 4Si Z0 10. contains less oxygen. Its composition ranges approximately from AI4.5Sil.509.75 to AI4.SSil.Z09.6' The structure of sillimanite may
well be depicted by the
structure formula [AI~
. ,+10 [ ] -5 ] -') [ AIZSlZOZJ AI0 4
-5 chains formed by AI0 octahedrons through sharing edges run 6 4] parallel to the c-axis at each corner and the center of the unit cell. They are
where the l AI0
bound together by tetrahedral silicon and aluminium atoms with additional oxygen . +10 in the formula. Tetrahedrons are paired ZSl Z0 2] by sharing vertices. The mullite phase has a similar structure except that the atoms, as represented by [ AI
arrangement of tetrahedrons is involved in disorder. For a mullite phase of the composition AI4.SSil.Z09.6' the stucture formula is as follows: A10
-5
4
]
(AIO.ZSil.Z)AI0.S01.J +10 [ AI0 4] -5
The tetrahedral atoms which bind the
[AI0
two sets. Each of them is allocated four
-5 chains together are grouped into 4] equivalent positions. The first set is
very similar to the one occupied by the tetrahedral atoms in sillimanite. Were it not for the higher content of aluminium, all the tetrahedral atoms in the mullite phase should have occupied the four positions in the first set, though in a random distribution. In fact, a part of the tetrahedral aluminium atoms have now moved to the neighbouring positions of the' second set and each of such aluminium atoms requires that an oxygen atom moves to a new position which surrounds not only this aluminium atom but also two tetrahedral aluminium atoms occupying the first
set. Two seventh of the tetrahedral aluminium atoms occupy
randomly in this manner the positions of the second set. The structural feature described above for the mullite phase is conducive to the better distribution of charges on oxygen atoms and to the stabilization
oft~
structure as a whole. However, the mullite structure is a highly disordered one. We may reasonably imagine that structural interruptions, disturbances and distortions should occur on the surface as well as in the bulk of the mullite crystals. DISCUSSION Two observations emerge from Fig.l. First of all, concerning the dispersion of silver, the surface of mullite is preferable to that of a-alumina. And the preference is rather overwhelming. The second fact is that the mullite prisms
566 of about 1
in width carryon them silver
~m
grains of comparable size. We may
thus conclude that mullite crystals can provide a more active or satisfactory surfac~
than
a-alumina for the reduction of silver salt into silver and that on
mullite prisms, the size of silver grains can be controlled by the width of the former. In the light of the structure of the mullite phase, we may attribute the peculiar actiVity of mullite surface to the structural disturbances and distortions due to the disorder in the crystal. PREPARATION OF
TH~
MULLITE PHASE FOR THE CATALYST
Choice of starting materials As raw materials we have used kaolin and alumina-silica gel. Kaolin, A1 (OH)4' has a layer structure. By heating up to 980°C, 2(Si 205) kaolin is converted into an oxide (AIO.7SiO.3)203.3 and silica. This oxide has a defect spinel structure similar to that of
Y-AI
and is transformed into 203 fine mullite prisms when heated to 1240 DC. This'primary mullite'is the desired one. However, excess silica becomes silica glass at high temperature and, at 1580 DC, silica glass reacts with corundum in the catalyst support to form the 'secondary mullite', which causes primary prisms to grow into undesirable coarse crystallites. In order to get rid of the excess of silica in the starting material, we chose an alumina-silica gel with the A1
ratio of about 7 to 3. Differen20 3/si02 tial thermal and thermogravimetric diagrams indicate that the sample loses adsor-
bed water at 250°C and is further dehydrated at 490 °C to yield the oxide (AIO.7SiO.3)203.3 mentioned above. At 1270 DC, this oxide is completely converted into mullite crystallites. They are desirable prisms as shown in Figure 2a. For comparison, Figure 2b shows mullite crystallites nurtured by the secondary mullite, as prepared from kaolin and
a-alumina at 1580 DC.
Figure 2(a) Primary mullite from alumina-silica gel which approximates the chemical composition of the mullite; Figure 2(b) Mullite crystallites nurtured by the secondary mullite as obtained from Kaolin and
a-Alumina at 1580 DC.
567 Size control of mullite crystallites First of all, we have to realize that we are preparing a mullite phase for the catalyst. And in the preparation of the catalyst support, alumina-silica gel has to be calcined in the presence of a-alumina or corundum. In this case the basic factor which controlli the size of the mullite crystallites is obviously the A1
ratio of the alumina-silica gel. Excessive silica leads to the forma20 3/Si02 tion of s~condary mullite at high temperature and spoils the preparation. Besi-
des, we
have found two other factors which influences
the crystallites size.
Whether the calcination takes place in a closed atmosphere or under a gas flow also makes remarkable difference. Q'1antitative experiments reveal that calcination under a gas flow always gives better results. Closed atmosphere favours the formation of silica glass and gives less primary mullite.And the more so, the more the gel contains alumina. In order to enhance the strength of the catalyst support we add a small amount (about 1 %) of MgO as flux. This additive also favours the formation of silica and reduces the yield of the primary mullite. Figure 3 summarizes the influence of various factors on the formation of silica glass versus the A1
ratio of the alumina-silica gel. With this 20 3/Si02 diagram as a guide, we used a gel with A1 ratio between 61/39 and 64/36. 20 3/Si02
'< ,",. (]) I-~
O
---t I
H,
tI1
,",. ~
'0"',
J'l
64
61 39
P-
I
20
36
toI I I
()'q ~
~
tI1 CD
10
closed atmosphere
u 1-.
1•
under gas flow
0
ro
ci~
0
1.0
2.0 '.5 ratio A1 2O/Si0 2 of Gel
2.5
Figure 3. The influence of MgO, closed atmosphere and gas flow on the yield of silica glass versus the A1203/Si02 ratio of the gel.
568 Results Good catalyst supports are prepared by mixing the following constituents: Corundum Alumina-silica gel !lgO
Activated carbon Dextrin and then calcining the mixture at 1580 °C in a tunnel kiln. The support contains about 25 % of
mulli~e.
Scanning electron micrographs of the support and the
silver catalyst, respectively, are shown in Fig. 4 a and b. The catalyst contains about 15 % of silver. The silver grains have a rather uniform diameter of about 0.4
~m.
Figure 4 (a) SEM of the catalyst support;
(b) SEM of the silver catalyst.
The silver catalyst prepared in our Laboratory has been tested in industrial reactors as well as in reactors of laboratory scale, and the performance is very satisfactory. REFERENCE 1. R. Sadanaga, M. Tokonami and Y. Takeuchi, Acta Cryst., 15
(1962) 65-68
569 DISCUSSION M. FARINHA PORTELA: How did you measure the contents of the mullite phase of the
supports ? GUI LIN-LIN : The contents of the mullite phase were determined by X-ray quantitative analysis. A. KORTBEEK: Can you lndicate the activity and selectivity of your catalyst? GUI LIN-LIN: Usually we can get selectivity of 76 % and conversion of 16 %.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
571
PREPARATION OF ACTIVE CARBON SUPPORTED OXIDATION CATALYSTS J.L. FIGUEIREDO, M.C.A. FERRAZ and J.J.M. ORFAO Faculdade de Engenharia, 4099 Porto Codex, Portugal
ABSTRACT A cobalt oxide catalyst supported on activated carbon was prepared by impregnation of sawdust with cobalt nitrate, followed by carbonization and partial gasi fication. Key variables in the preparation procedure were identified and their effects upon the porous structure and activity of the resulting catalyst were ascertained.
INTRODUCTION Active carbons are finding increasing application as catalyst supports in the treatment of gaseous or liquid effluents, where advantage is taken of the enhanced retention of organic solutes in the pore system of the carbon. In this way, adsorption is combined with catalytic oxidation in order to achieve the complete destruction of the pollutant (ref. 1). In the present communication, we report the preparation of a cobalt oxide catalyst supported on active carbon to be used in the oxidative destruction of organic compounds in air. Very high activity at low temperatures is required in order to promote nearly complete conversion of the organic compound without significant loss of carbon by gasification. Therefore, a good distribution of the active phase must be combined with a suitable pore structure in the support. This was obtained by impregnating a carbon precursor rather than the carbon itself. The method of preparation involves impregnation of a suitable carbon precursor (e.g. sawdust) with a metaJ salt (e.g. cobalt nitrate), followed by carbonization in inert atmosphere and activation by controlled gasification with a reac tive gas (e.g. C02)' The final product is a porous carbon containing cobalt oxide which shows high activity for the deep oxidation of organic compounds such as hydrocarbons, alcohols and acids. The role of the impregnant during the carbonization and activation steps was monitored by thermogravimetry. The kinetics of these steps were determined and correlated with the porous structure and activity of the resulting catalysts.
572
EXPERIMENTAL The impregnated active carbon catalyst (SA/Co) was prepared from pinewood sawdust. After acid washing (15 hours with a 10% H2S04 solution) and drying (15 hours at 1100C) it was impregnated at room temperature and under vacuum with 0,1
M
cobalt nitrate solution (20 cm3 solution/g dry sawdust). After drying, the
impregnated sawdust was carbonized in a tubular furnace under nitrogen flow at 100C/min up to 850 0C and held there for 1 hour. Activation was carried out by controlled gasification with carbon dioxide at 825 0C for 15 minutes. For comparison, an active step
carb~n
was prepared in the same way but without the impregnation
(SA). The activation time was
extended in order to obtain the same degree
of burnoff. Nitrogen adsorption isotherms were obtained by conventional methods and used for textural analysis. Thermogravimetric studies of the carbonization and activation steps were carried out by means of a C.I. Electronics micro force balance with a suitable flow attachment, electric furnace and a Stanton Redcroft linear temperature programmer. Catalyst activity towards the complete oxidation of organic compounds was determined in a chromatographic pulse reactor.
RESULTS Thermogravimetric studies In order to investigate the effect of the impregnant, the preparation procedure was duplicated in the thermobalance. Thus, Figure 1 shows thermograms of sawdust and impregnated sawdust under inert atmosphere, the weight loss being referred to the dry materials.
o
10K/min
20 ~'" 40 ~
.c:
~"" 60
80 ~
~
~
~O
Temperature (KI
Fig. 1. Thermograms of sawdust (curve 1) and impregnated sawdust (curve 2) under nitrogen flow (5 cm3/s).
573
It is apparent that the pyrolysis kinetics (between 520 and 650 K) is not affected by the presence of the impregnant. However, further
volatilization
occurs with SA/Co (curve 2), as compared with SA (curve 1). On the other hand, the gasification kinetics is quite different (Figure 2): the impregnated carbon is gasified at much higher rate initially, although this tends to level off at about twice the rate for the non-impregnated carbon.
0.6..------------------,
oil oil
0.4
o
oL==:i===~==~=:J. 10
30
40
Fig. 2. Gasification of carbonized sawdust (curve 1) and carbonized impregnated sawdust (curve 2) at 1099 K and 1 atm CO 2 , Carbonization temperature = 1123 K.
A detailed kinetic study of the gasification step was carried out with SA in order to establish the maximum temperature for reaction in the absence of diffusion limitations. In fact, activation can only result from an even removal of carbon atoms throughout the structure. If the reaction is diffusion limited, there is no development of the porous structure, and gasification occurs only at the external surface, leading to the shrinking of the carbon particles. Thus, Figure 3 shows the rates of gasification of carbonized sawdust in the form of an Arrhenius plot. The transition from the chemical regime (activation energy kJ/mole) to the diffusional regime (activation energy
=
238 kJ/mole)
is
=
414
well
defined, showing that the activation temperature should not exceed 1153 K for the particle size considered. A different situation occurs with the impregnated carbons, as the rates of gasification change with burnoff. Thus, the initial rates of gasification of SA/Co are two orders of magnitude higher than those observed with SA, while the final rates are only about twice the latter. Gasification of SA/Co was studied in the range of temperatures from 1062 to 1128 K and activation energies of 264 kJ/mole (for the final rates) and 58 kJ/mole (for the initial rates) were derived.
574
Fig. 3. Rates of gasification of carbonized sawdust (sawdust average particle 0.9 mm; carbonization temperature = 1273 K).
si~e
Textural studies The texture of the resulting activated carbons is quite different, as shown by the adsorption isotherms in Figure 4 and by the textural parameters collected in Table 1. Thus, while SA is essentially microporous, SA/Co exhibits considerable mesoporosity. Note also the presence of low pressure hysteresis in both carbons, which has been discussed elsewhere (ref. 2). The surface area of the impregnated carbon was found to be 210 m2/g before activation.
~
ii: ....
SA
V!
"'e ~ 'a
1/200 "Ii "Cl
..'"
e
.3 ~1dl
o' - - - - " " - - - - ' - - - - - ' - - - " " - - - - - - rtol Al!lative pmsII'e PI ~
Fig. 4. Nitrogen adsorption isotherms at 77 K on SA and SA/Co.
575 Table 1 - Textural parameters Surface areas (m2/g)
Metal load Carbon
(%)
2.31
Vme s o Vmicro Vmacro
Smeso
SBET
SA SA/Co
Pore volumes (cm3/g)
1084
17
0.03
0.44
0.51
287
114
0.34
0.14
1.87
Activity studies The impregnated carbon SA/Co was tested for the deep oxidation of several organic compounds in air. The model of Langer et a1. (ref. 3) was used to derive kinetic parameters from the data. The reaction was found to be of 1st order in the organic compound, and the activation energies were determined. Table 2 summarizes the results obtained. Table 2 - Kinetic parameters for the oxidation of organic compounds cata1ysed by SA/Co Compound
E(kJ/mo1e)
tnk o
X250
Benzene
147
29.2
0.98
Propene
92
18.9
0.98
Butanol
65
13.0
0.98
Toluene
84
14.4
0.63
Butanoic ac.
69
13.7
0.98
E, k O = parameters of the Arrhenius equation, k=kO exp(-E/RT) X250
conversion obtained at 2500C in the chromatographic pulse reactor (column: length 284mm, i.d. 4.8mm; catalyst: 89Omg; air flow rate: 5 4.1x10- mo1e/s)
DISCUSSION The results presented in the previous section are consistent with the following role of the inorganic material during the preparation of the catalyst: -In the carbonization stage, the impregnant acts merely as a spacer between the wood the
fibres, preventing their shrinkage and originating an open structure in carbon. This allows further
volatilization
to occur and, moreover, it fa-
cilitates the access of the reactant in the next stage of the preparation. -In the activation stage, the inorganic material acts as a catalyst for carbon
576 gasification, but becomes quickly deactivated as reaction proceeds. We believe that the catalyst may be originally present in the metallic state, but is then converted to the oxide CoO, which is the form actually identified in the final product by X-ray diffraction: Co
+
CO 2
=
CoO + CO. Similar results were reported
by several authors (ref. 4-6). Thus, the initial rates in the activation of SA/Co reflect the catalytic gasification of carbon, while the final rates are closer to those obtained for SA. The activation energies determined support this 238 kJ/moie for SA whe
vie~,
(264 kJ/mole for SA/Co and
:ification is diffusion limited). Gasification of SA/CO
is still faster, probauLy as a result of higher gas diffusivities associated with the mesoporous structure. Marsh et al. (ref. 6) studied the gasification of pure and doped polyfurfuryl alcohol carbons, and reported activation energies of 210 kJ/mole (Co doped) and 370 kJ/mole (pure) in the kinetic regime. It seems, therefore, that the catalytic gasification of SA/Co is also diffusion limited at the temperatures considered in the present work. Nevertheless, most of the carbon is removed while the
CR-
talyst is still active, i.e., gasification occurs at the interfaces carbon/catalyst. The degree of activation achieved is therefore quite satisfactory and the catalysts obtained exhibit an adequate texture and perform reasonably as oxidation catalysts for the destruction of small amounts of organic compounds in air.
ACKNOWLEDGEMENTS This work was supported by Instituto Nacional de Investiga~ao (INIC) and by Junta Nacional de
Investiga~ao
Cientifica
Cientifica e Tecno1ogica (JNICT,
research contract n9 45.78.05).
REFERENCES 1 M. Beltran, Chern. Eng. Progr., 70(1974)57. 2 J.L. Figueiredo and t!.C.A. Ferraz, in "Adsorption at the gas-solid and liquid-solid interfaces", ed. J. Rouquerol, K.S.W. Sing, Elsevier, in press. 3 S.H. Langer, J.Y. Yurchak and J.E. Patton, Ind. Eng. Chem., 61(1969)11. 4 S. Kasaoka, Y. Sakata, H. Yamashita and T. Nishino, Int. Chem. Eng., 21(1981) 419. 5 E.T. Turkdogan and J.V. Vinters, Carbon, 10(1972)97. 6 H. Marsh and R.R. Adair, Carbon, 13(1975)327.
577 DISCUSSION S.P.S. ANDREW There have been movie photographs of the catalytic action of metal particles on the gasification of carbon. These show that the metal particles move around "burning" a path in the graphite. If this is the phenomenon why does the gasification catalysis by the cobalt erase in your experiments? Could it be either that the cobalt particles coalesce and lose mobility or that they collide with an inorganic impurity in the carbon which they cannot burn their way through ?
J.L. FIGUEIREDO:
The acid washing treatment completely eliminates the ash content of the starting material, so there are no inorganic impurities at all in the char. We do not think that the loss of catalytic activity seen in Fig.2 (curve 2) may result from the sintering of cobalt particles, as we observed a much faster deactivation at lower temperatures (e.g. 775°C). We believe that the effect is due to the inability of cobalt to remain in the metallic state in the presence of carbon dioxide. At the end of the process, we indeed detected cobalt oxide.
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579
G. Poneelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.Y., Amsterdam -Printed in The Netherlands
INFLUENCE OF PREPARATION VARIABLES ON THE ACTIVITY AND ON THE MECHANICAL PROPERTIES OF AN INDUSTRIAL CATALYST FOR THE PROPYLENE OXIDATION TO ACRYLIC ACID R. COVINI (I), C. D'ANGELI (2)and G. PETRINI (3) (1) Montedipe S.p.A. - Research Center of Bollate (Italy) (2) Montefibre S.p.A. - Via Pola 14, Milano (Italy) (3) Ausind S.p.A.- Attivita Catalizzatori - Via Fauser 4, Novara (italy)
ABSTRACT Preparation of a (Ni-Mo-Te)O catalyst for the production of acrylic acid from propylene was investigated on a statistical basis: It was found that catalytic performances and mechanical properties can be independently optimized, by a proper selection of experimental conditions.
INTRODUCTION In a research for the one step oxidation of propylene to acrylic acid, a preliminary work (ref. I) had resulted in the choice of a (Ni-Mo-Te)O catalyst which, on a microreactor scale, afforded sufficiently high selectivities, under conditions providing almost complete conversion. The commercial application of the process implies the use of a tubular reactor with high enough heat transfer efficiency and requires that the catalyst is shaped in proper way, so that the pressure drop through the tubes be acceptable and that the mass transfer phenomena through the pellets be kept under control. Altogether, SxS mm hollow cylinders, with a 2 mm hole, appeared to be convenient: the aim of this work was to find preparation conditions affording proper morphology and sufficiently good mechanical properties to such pellets.
EXPERIMENTAL Catalyst preparation I To a solution of (NH ) Mo 0 (Climax) containing 4.7 moles.l4 7 MoO ,the s«achiometric a~ou~t of a Ni(NO ) .6H (C. Erba, RPE) 20 solation was added, under thorough mixingj ~he operation was carried out at 6S GC, while pH was kept constant at 6.S through addition of aqueous ammonia. After filtration, the precipitate was washed with cold distilled water, dried overnight at 110 GC and calcined 8 hours at the selected temperature in a furnace controlled by a linear temperature time programmer, to obtain a powder with a slight excess of M00
3
580
(chemical analysis: MO/Ni=I.1 atoms) The powder was thoroughly dry mixed with an amount of TezMoO 7 (ref. 2) corresponding to 5% TeO by weight, with the selec'ted amount of lubricant (stearic acia) and with 3% binder (Hydroxy propylcellulose: Klucel 250L, Degussa) By addition of distilled water, a paste was obtained which, after drying overnight at 100°C, and milling through a I mm grid, was tabletted in a rotatory type machine, equipped with forced feed and two stages compression, and fitted with tungsten carbide dies. The table~ng pressure was controlled by checking the weight of samples of 30 tablets: 10 s~ch samples were controlled for each preparation condition, and tablets were discarded when the deviation exceeded ± 0.1 gr. Activation was performed in air containing about 20% steam: a tubular isothermal reactor was used, heated at a rate of about 1°C/min and kept 8 hours at the desired temperature. All other conditions being maintained constant, the following were chosen as independent variables: - Calcination temperature of (Ni-Mo)O , from now on: Tc - Lubricant concentration (Lc) - TablettifB pressure, expressed as weight of 30 pellets (Pt) - Activation temperature of the tablets (Ta) The experimental ranges were: 610°C ~ Tc ). 690°C 5% ~ Lc ~ 9% 7. 7gr ~ Pt ~ 8. 3gr 480°C ~ Ta ~ 520°C The heat treatment had to be split into two steps (cal cination of the powder activation of the tablets) as: on the one hand it must be sufficiently severe, as a whole, to bring the surface area to the desired values; on the other, the temperature of the second step is limited by the occurrence of a phase transition in the system, because of which a heating at 580°C followed by cooling below 200°C results in a complete collapse of the tablet; and even more it is limited by the dispersion of surface area values, which is obtained at any temperatures above 530°C. Catalyst characterization Conventional methods were used for the determination of morphological properties: N adsorption for surface area (from now on: As) and Hg porosimetr~ for pore volume (Vp) and pore size distribution (Rp=average pore radius). Axial crush strength (CSa) and radial crush strength (CSr) were determined by an apparatus assembled in our laboratory and equipped with a strain gage cell and a recorder. Catalytic tests were performed in a tubular reactor 3 cm wide and 80 cm long, immersed in a melted salts bath; the feed was propylene 4%, air 56% and water 40% by volume. Propylene conversion (Cv) and selectivity to each product are given by: Cv = propylene fed - propylene recovered • 100 propylene fed
581
S
C atoms converted to each product • 100 C atoms converted
In Tab. 2 and Fig. 5, Saa, Sa and Scox stand for selectivity to acrylic acid, acrolein and carbon oxides, respectively; balance to 100% is accounted for by small amounts of by-products, mainly acetic acid.
RESULTS To reduce the number of experiments and according to reported experiences (ref. 3l,a composite design was used, made up by 8 tests selected out of a 2 factorial with a proper confusion technique; by 8 additional tests for the evaluation of the quadratic effects; and by the central point of the program, repeated 4 times, for an independent estimation of the ex?~rimental error. A drawback of this program is that, in the presence of a second degree interaction it is not possible to point out the couple of variables, which are involved; this problem was partially solved, however, by performing some extra tests on pellets a~vated at somewhat different temperatures. In the absence of indications pointing out equations with theoretical significance, an empirical quadratic polynomial has been chosen. The coefficien~were evaluated by a stepwise regression (ref. 4), which is a modification of the least squares method, and only those were retained, having a probability higher than 95% to be different from zero. Catalytic performances and surface areas were found to show a quite similar dependence on preparation variables, so that it is possible (and it was deemed more convenient) to express the first as a function of the latter. All resulting correlations are reported in Tab. 1 - 2, together with the related multiple correlation coefficient (Cc). For the same correlations, the ~isher's F test (ref. 5) shows that the regression standard error does not differ significantly from the reproducibility standard error.
DISCUSSION Morphological properties depend only on calcination and activation temperatures. Among them, specific surface area depends much more on the second (Fig. 1), as activation is made on a mixture containing Te molybdate which, with the small M00 excess, originates a 3 low melting (526°C) highly mobile eutectic composition (ref. 6). A comparison between Vp and Rp equations (Tab. 1) shows that in the absence of tellurium compound (calcination), the pore size increases at constant pore volume, as in a plain sintering process; while in the presence of tellurium (activation), the increase of pore size, beside being more pronounced, is also accompanied by a decrease of pore volume, pointing out an occlusion of smaller pores
582
TABLE 1 Dependence of morphological and mechanical properties on preparation variables. Variables normalization: Tc - 650 40
Ta - 500 20
Lc - 7
2
As
8.122 - 1.939x1 - 3.599x2 - 2.869x2 + 1.361x
Vp
0.1270 - 0.6278x2
Rp
380 + 181x
1
+ 218x
0.0148x 2
+114x~
2
- 17. 125 x
2
3
- 12.312x
2.37 - 0.50 x
1
+ 2.55 x
2
219X~
+
=
Pt - 8
--=--=0.3
(Cc=0.992)
2
+
(Cc=O.977) 2
16.420X + 30.125x2 4
+ 17.037x1x3
1x2
1x2
4
(Cc=0.899)
2
CSa = 58.875 - 13.410x1 + 49.580x
CSr
x
2
2 + 1.32x 2
(Cc=O.976) (Cc=0.953)
TABLE 2 Dependence of catalytic performances on specific surface areas. Variable normalization: As - 6.625 3.481 A) Constant salt bath temperature 315°C Cv = 41.050 + 12.880x
(Cc=0.963)
5
Saa = 11.63S + S.834xS
(Cc=0.95S)
Sa = 75.026 - 8.885xS +1.S20x Scox = 9.105 - 2.095x5
2
(Cc=0.9S6)
5
(Cc=0.831)
B) Constant conversion 80% Saa = 39.397 - 1.810x Sa = 46.520 Scox = 11.20S
2
S
- 0.913xS
(Cc=0.782)
583
C~a.(k~)
A~ (m2/gy)
-\SOI.-------r------.---..-----/-~
-i4,----,----,----,-------,
iOOI------+---+--/+~~--j
SOI------b~~~C---+_----l
21----+---+----!---'04&0
500
Ta. (O()
°460
520
Fig. 1. Specific surface area vs. activation temperature, at different Tc.
CSa. Clq~)
-\SO r - - - - , - - - - r - - - - r - - - - - - ,
500
Ta. {Oe}
520
Fig. 2. Axial crush strength vs. activation temperature at different Tc (Lc~7%; Pt~8g).
cs-
(~9)
7 .-----,-------r--~--~
-\00 1----+----+---+-----1
51-----I----+---~
31-----I---~~'#-+-----I
50 ~".e::-+_-_+--+-='.....,_--j
"''=--''''''''~--+---+-----I
Lc. (%)
9
Fig. 3. Axial crush strength vs. lubricant concentration at different Ta (Tc~650oC; Pt~8g).
04&0
500
Ta.(oC)
520
Fig. 4. Radial crush strength vs. activation temperature, at different Tc.
584
G/o 90-.-------------,
50
20
"'0
5aa:
0
0 0
'Ii
5
As (m2.fgr)
Fig. 5. Catalytic performances vs. specific surface area, at bath temperature 315°C.
by the mobile Te containing system. As for mechanical properties, axial crush strength increases with tablettingpressure at a linear rate (Tab. 1), while its dependence on activation temperature suggests that the tellurium compound acts as a strong binder, during its migration towards the grain boundary. The influence of lubricant concentration on axial crush strength is more complex: apparently, at low concentration the lubricant is not sufficient for a good compacting and at too high concentration it acts asdiluting agent, affecting particle boundary bonding. Radial crush strength shows a somewhat similar relationship with activation temperature (Fig. 4), but does not depend on the two other variables, at least in the limits of our investigation. The negative effect of calcination temperature on both crush strengths suggests that it strongly affects the surface roughness of (Ni-Mo)O particles, which is responsible for bonding in subsequent activation. An interesting result of this work is that catalytic performances can be expressed as function of surface area and mechanical properties can not. We believe that the morphology of the particles, which build up the pellets, beside being responsible for the surface area of the catalyst, has also an influence on its mechanical properties; these latter, however, strongly depend also on how the particles have interacted during the shaping process and the subsequent heat treatment. As a conclusion, catalytic performances and mechanical properties can be independently optimized, by a proper selection of experimental conditions.
585
REFERENCES 1 J.C.J. Bart, A. Bossi, G. Petrini, G. Battiston, A. Castellan and R. Covini, Appl. Cat. (in press). 2 J.C.J. Bart, G. Petrini, N. Giordano, Z. Anorg. Allg. Chem. 412 (1975) 258 - 270. 3 O.L. Davies, Design and Analysis of Industrial Experiments, Oliver and Boyd, London 1956, pp 532 - 537. 4 M.G. Kendall, Advanced Theory of Statistics, Vol. I, Griffin, London, 1948, pp 167 - 174. 5 J. Mandell, Statistical Analysis of Experimental Data, Interscience Publishers, New York, 1964, pp 272 - 285. 6 G. Petrini, J.C.J. Bart, Z. Anorg. Allg. Chem. 474 (1981) 229 232.
586 DISCUSSION D.D. SURESH
What is your best yield of acrylic acid in a "single" pass ?
R. COVINI: At almost complete conversion, our best selectivity in a "single" pass was 45% acrylic acid; 40% acrolein was co-produced, which was further converted to the acid when recycled through the reactor. M. FARINHA PORTELA May you provide information concerning the stability of the catalyst with respect to the tellurium content ? R. COVINI: The catalyst instability, with respect to tellurium content, is bound to the formation of metallic tellurium, which has a relatively high vapour pres~ sure at the reaction temperature. Confirming the results of a previous investigation (J.C.J. Bart, G. Petrini and N. Giordano: Z. Anorg. Allg. Chern. 413,180 (1975), we have found that at this same temperature and in the presence of an excess of Mo oxide, metallic Te does not form, or tends to be oxidized even in the presence of a low partial pressure of oxygen. Accordingly, our catalyst gave constant performances in duration tests of more than 2,000 h, provided an excess of oxygen was present in the reacted mixture. ZHAO JIUSHENG: The crush strength of catalysts is a very important character for industrial catalysts. 1. Could you tell us how do you determine the axial and radial crush strengths correctly ? What apparatus do you use ? 2. What is the relationship between the crush strength and the factors you mentioned such as Tc' P t, Lc and Ta ? How do they influence ? R. COVINI: 1. For the determination of the crush strength we use an apparatus built up in our workshop. It consists of : - a "crushing" device formed by two horizontal, parallel plates having smooth faces between which the pellet is located; the plates have 15 mm diameter and 5 mm thickness and are made of stainless steel with hardness) 80 roques; - a device which moves one plate towards the other at a constant speed ( 2 mm/min; - a device (as a strain gage load cell) which measures the resistance opposed by the pellet; - a device (as a recorder and/or a peak detector) which records the force value corresponding to the pellet crushing. 2. The exial crush strength depends on all considered variables, with sOme extent of second degree interaction (Tab. 1). As a consequence, simple two-dimension diagrams (Figs.2 and 3) do not give an exact description of the existing correlations; however, giving a Simplified picture, they help to visualize the effects of each single independent variable. The radial crush strength probably depends again on all the variables; however, their values being lower and, as a consequence, the percentage error in their determination higher, only the effect of the most relevant ones (temperatures of calcination and of activation) are clearly evident (Fig. 4). J.M.D. TASCON: You said that the interaction between Te2Mo07 and excess Mo03 in your catalyst gave an eutectic mixture. Did you detect the formation of any new phase, including Ni and Te, after thermal treatment of your NiMo04-Te2Moo7 intimate mixture ? R. COVINI: We have quite deeply investigated this point, and we came to the conclusion that no new phases including both Ni and Te (neither with, nor without Mo) are formed under our pretreatment or reaction conditions. Accordingly, the starting compounds (Ni molybdate and Te molybdate) have always been found after such treatments. We only observed migration of the Te-containing phase, diffusing through the mass of Ni molybdate.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
587
©
DESIGN AND PREPARATION OF HYDROCRACKING CATALYSTS ~J. WARD Union Oil Company of California, Science & Technology Division P.O, Box 76, Brea, California 92621 (U.S.A.)
J.
ABSTRACT The catalyst features are the key to most modern hydrocracking processes. The inter-relationship between catalyst, process, feedstock and products is discussed. Factors determining the selection of the hydrogenation and cracking components along with methods of catalyst preparation are considered. The influence of the process condition on the catalyst design parameters are evalu~ ated. Examples are given illustrating the impact of the catalyst properties on the types of product produced. Consideration is given to the impact of activation procedures and reactivation techniques on catalyst activity. INTRODUCTI ON In the modern refining industry, there are three basic processes for the conversion of heavy oils into useful products, namely: coking, fluid catalytic cracking, and hydrocracking. While the first two processes function basically by rejecting carbon from the feed molecules, the hydrocracking process converts large high-boiling point molecules into desired lower-boiling products by simultaneously hydrogenating and breaking carbon-carbon bonds. Hydrocracking appears to have been first practiced on a commercial scale for hydrogenating coal in Germany in about 1927. During World War II, hydrocracking, using unsupported metal sulfides such as those of molybdenum and tungsten, was used at very high pressures. In contrast, the modern processes which started to emerge in the late 1950's employ highly dispersed metals or metal sulfides supported on high-area acidic materials. Hydrocracking, as the name implies, is a dual-functional process embodying both hydrogenation and cracking reactions either simultaneously or sequentially. Typical catalysts incorporate both hydrogenation and cracking components. Modern hydrocracking was initially designed and used to upgrade low-value distillate feedstocks such as cycle oils (highly aromatic products from catalytic crackers), thermal and coker gas oils and other heavy gas oils. Initially, the product was mainly gasoline boiling range material. However, more recently, a broader range of products rang.ing from high yields of liquid petroleum gas to middle distillate have been produced. This inherent flexibility makes hydrocracking one of the most versatile modern petroleum refining
588
processes. However, because of the abil ity of the process to trea t a wi de range of feedstocks to a wide range of products, for maximum efficiency, it is desirable to have a wide range of catalysts broadly optimized for the desired use with the right balance between hydrogenation and cracking functions. Furthermore, it has been found that different process concepts and designs are preferred for different types of feedstocks and products. In general, the type of feedstock and desired range of products determine to a large extent the type of processing and catalyst used. A number of puBlications have appeared in the last few years which discuss the reactions and mechanisms involved in hydrocracking (ref.l,2). These studies have been carried out mainly from the reaction mechanism viewpoint and nature of the reactive intermediates. Little attention has been paid to the design parameters involved in or methods of catalyst preparation. Furthermore, little work has been reported on the influence of process parameters. REACTION MECHANISMS Several hydrocracking studies of pure hydrocarbons have been reported and can be typified by the excellent work of Weitkamp and coworkers (ref.l,2). Using platinum and palladium on V-zeolite, they confirmed earlier proposals for paraffin hydrocracking. The mechanism is basically that of catalytic cracking with the addition of hydrogenation and isomerization. The initial step is dehydrogenation to an olefin followed by adsorption on an acid site and conversion to a carbonium ion. The ion rearranges to a more stable form which can either rehydrogenate to give back an isomerized paraffin or it can crack into lighter fragments of an olefin and anion which are hydrogenated up to the paraffins. The proposed mechanism is shown in Figure 1 (ref.2).
N-DODECANE
'~'
')_2H
H
N-DODECENE
~
f l-H® SEC- C12_ _ CARBONIUM IONS -
.
Jt
! ,- - ~ -
H .H&-2H METHYL UN DE CANES
Fig. 1.
Mechanism of primary n-dodecane hydrocracking.
589
The similarity of the carbon numbers in the product distribution from cracking and hydrocracking shown for n-dodecane supports these concepts (ref.3). The major difference between the two mechanisms is that the products from hydrocracking are derived mainly from pure primary cracking. Weitkamp and Schultz (ref.2), in subsequent work, provided additional evidence for the reaction path proceeding via olefinic intermediates. Isoparaffins and cycloparaffins are hydrocracked by similar mechanisms. Polycyclic naphthenes, such as decal in, characteristically crack by opening This breaking of naphthenic rings of a s;"ngle ring in the molecule (ref.4). with the retention of a single ring is characteristic of hydrocracking reactions. Hydrocracking of aromatics usually occurs by first hydrogenation of the aromatic rings followed by ring opening. Aromatics containing multi carbon atom side chains are dealkylated resulting in paraffin formation. Polycyclic aromatics are generally hydrogenated and undergo progressive ring opening. However, some of the large multiring aromatics such as coronene and ovalene, once hydrogenated, dehydrogenate rather than crack. Most of the studies leading to these conclusions have been carried out under simple idealized process conditions. With industrial feedstocks, the basic mechanisms apply but are complicated by the presence of many hydrocarbon types along with sulfur, nitrogen and oxygen compounds. High boiling compounds such as asphaltenes tend to cause problems in that, instead of being converted, they deposit on the catalyst sites and result in deactivation. Overall, typical reactions occurring can be summarized as in Figure 2 (ref. 5). HYnROOESULFURIZATION
HYOROOENITROGENATION
HYOROGENATlONS OF AROMATICS lAND OLEFlNSJ HYOROOECYClIS~ ISOMERIS~TION
TlON OF
PARAFFINS AND N~PHTENS
CRACKING OF ISOPARAF - ISOPARAFFINS H2 PRIMARY H2 SECONDARY FINS, ALKYL NAPHTENS ALKYlNAPHTENS--CRACKEO--CRACKEO AND ALKYL AROMATICS ALKYL AROMATICS PRODUCTS PRODUCTS COKING
Fig. 2.
MULTI-RING AROMATICS RESINS, ASPHALT,ENS ~ALr.YLATlOJN COKE OL N PRECURSORS +-. ".H2 +--CYCLISATION
Hydrocracking Reactions
H2
590
As will be discussed later, the presence or absence of ammonia and hydrogen sulfide in the reaction system can influence the product distribution, in particular the degree of hydrogenation of aromatics. HYDROCRACKING PROCESSES Various hydrocracking process schemes have been reported in the literature designed to optimize the conversion of various feedstocks to particular products. In general, operating conditions are 500 to 850°F catalyst temperature, hydrogen pressure of BOO to 3000 psig, liquid hourly space velocities of 0.3 to 2.5 h- 1 and a hydrogen to oil ratio of about 4000 scf per barrel. The use of high hydrogen pressures and dual functional catalysts results, in contrast to catalytic cracking, in a very low rate of coke deposition and onstream cycle lengths of several years. The typical hydrocracking process usually utilizes fixed catalyst beds in the reactors. Because of the hydrogen consumption and because the cracking process is exothermic, multiple beds are usually used in the fixed bed reactors with cooling by the introduction of cool hydrogen between the catalyst beds. Products are usually free of olefins but depending upon the process conditions, they mayor may not contain aromatic compounds. The processes can be divided into two major groups, single-stage and twostage. The simplest of all schemes is that employing a single-stage process utilizing a single catalyst in a single reactor. Such a scheme is shown in Figure 3. The fresh feed and unconverted oil are passed downward through the
REACTOR
RECYCLE GAS COMPRESSOR
FRESH FEED
H~~~~~~N
~--------
~------M-A-K-EU-P
1 COMPRESSOR
WASH WATER
-' TO GAS PLANT
~
'Qb--1---..1.-
¢--------I.. .f----==-:-:-..-I-=- :~_:_:_:_:_:_
SEPiFiAT~
RECYCLE OIL (FRACTIONATOR BOTTOMS)
Fig. 3.
Single Catalyst Unicracking Unit
:
R TO FRACTIONATION -------
591
catalyst bed with hydrogen. The products are passed through a high-pressure separator to remove excess hydrogen and other light gases which are recycled together with makeup hydrogen back to the reactor. The product is passed on through a low-pressure separator to a fractionator system for separation into desired products and unconverted oil. The unconverted oil is recycled back to the reactor. Commercial hydrocrackers rarely convert 100% of the feedstock to ,. the desired product in one pass; typically, 40 to 70% of the feed is converted. Due to the gas separation system, the recycle gas will contain various amounts of light hy~rocarbons, ammonia and hydrogen sulfide depending on the nature of the feedstock and the extent of scrubbing. Thus the hydrocracking does not usually take place in the presence of pure hydrogen. This type of processing is rather limited to feedstocks containing low levels of organic nitrogen and sulfur. The next alternative scheme is to employ two catalysts either in the same reactor or in different reactors in series, as shown in Figure 4. In this
SINGLE-STAGE: TWO REACTORS, RECYCLE TO R-2 TWO-STAGE: THREE REACTORS, RECYCLE TO R-3 R-1 REACTOR
R-2 REACTOR
R-3
_________ ~-;~REACT~~----1
RECYCLE GAS COMPRESSOR
I
I
I
I
I-
I
I
I
I
t
I I
I
H.P. SEPARATOR
I
I
tL
I
i-
I
I
t.h - - t - ---E:34"""",¥""~
FRESH
'I
FEED~
HYDROGEN MAKEUP
""-----1
-.
r"S'
JI
I
I I
:I I I
-' ~ MAKEUP......
I I
;..:
c>c>
1
I
TO GAS PLANT
:
I
~
I
COMPRESSOR~---I·--~ WASH WATER ~-----·I--""'--------'
TO FRACTIONATION
I I I
RECYCLE OIL (FRACTIONATOR BOTTOMS)
Fig. 4.
Typical Unicracking Process Configuration.
case, the first catalyst converts the organic nitrogen and sulfur compounds to ammonia and hydrogen sulfide. These molecules, although they can act as catalyst poisons for the cracking catalyst, are much less inhibiting than the organic heteromolecules. The use of two reactors allows the choice of recycling unconverted feed and hydrogen to either of the two reactors. Recyc 1i ng to the
592
second reactor containing the cracking catalyst is most common. This process scheme allows the conversion of organic heterocompound~ optimally in the first reactor into less objectionable compounds such as ammonia and hydrogen sulfide and an independent control of the levels of unconverted heterocompounds over catalysts specially tailored for the purpose. Such catalysts are typically the well-known hydrotreating catalysts comprising sulfided molybdenum or tungsten and cobalt or nickel supported on alumina. Since in hydrocracking, organonitrogen compounds are the most objectionable, nickel-molybdenum on alumina catalysts are the usual catalysts of choice. Usually only a nominal conversion of the hydrocarbon feedstock occurs (i.e., change in boiling point range), although there may be substantial hydrogenation of unsaturated hydrocarbons. Most of the hydrocracking occurs in the second reactor. The feedstock is cracked to lower its boiling point to an extent of between 40 and 60 volume percent per pass. After fractionation of the product, the unconverted oil is passed back to the second reactor inlet for further processing. The catalyst in the second reactor is usually optimized for the hydrocracking operation and is the major subject of this discussion. Only in a few specialized cases, such as when the two catalysts are the same, or when the organonitrogen compounds are very difficult to remove, is recycle to the first reactor employed. Whichever configuration is used, the important operating factor in a single-stage operation is that the hydrocracking takes place in the presence of unconverted organo-nitrogen and sulfur compounds or in the presence of ammonia or hydrogen sulfide and thus the catalyst hydrogenation components are metal sulfides. The most flexible and versatile hydrocracking process is the so-called twostage process. Basically in this process, the hydrotreating, and in some processes, part of the cracking, are carried out as in the single-stage process. Next, however, the product is fractionated and the unconverted oil is passed to an additional reactor containing hydrocracking catalyst. The recycle gases are stripped, or washed free, of most (usually all) of the contained ammonia and, depending on the process, most or part of the contained hydrogen sulfide. Thus in this process, the hydrocracking catalyst in the second stage is operating in the absence of ammonia and in a sulfur-free or sulfur-containing atmosphere. Although considerably more expensive to construct and operate, the two-stage system results in all or part of the cracking being conducted at much lower temperatures because of the absence of ammonia. For instance, first-stage cracking temperatures are usually in the range of 650-800°F whereas secondstage temperatures are usually in the range of 500-700DF.
593
A two-stage system is generally used when the nitrogen content of the feedstock is such as to require very high operating temperatures at conventional space velocities or when reactor volumes be~ome uneconomically large. A two-stage system may have two recycle gas systems and, depending on the design, one or two separator systems. As well as controlling the ammonia atmosphere, the level of hydrogen sulfide can be controlled over a relatively large range so that it is possible to hydrocrack over metal catalysts or metal sulfide catalysts. The metal sulfide catalysts are most common and only in cases where a very hydrogenated product is desired are sulfur-free systems used.
FEEDSTOCKS AND DESIRED PRODUCTS In principle, suitable feedstocks can be converted into a range of products from liquid petroleum gas to catalytic cracker feedstocks. Variations in the process configuration, catalyst and process conditions permit highly selective conversion to: gasoline kerosine fuel oil middle distillate fuels (turbine & diesel)
lubricating oils liquified petroleum gas catalytic cracker feedstocks petrochemical feedstocks
Changes in optimum production of the desired product can often be achieved by small changes in operational conditions although for some product changes, major revamping is necessary. Another important feature of hydrocracking is that the products contain low concentrations of photochemically active hydrocarbons, such as olefins, and very low, if any, concentrations of sulfur and nitrogen compounds. Changes in operating conditions of modern catalysts such as distillation column cutpoints, small reactor temperature changes, and crack per pass have been shown to produce substantial product changes as in Table 1. It is seen that the product objective for processing a Kuwait virgin heavy gas oil can be varied from a high yield of gasoline over to a high yield of heating oil simply by a change in fractionation conditions and a ten degree change in reactor temperature.
594
TABLE Hydrocracking of Heavy Virgin Gas Oil Feedstock properties Gravity, °APl Sulfur, wt% Nitrogen, ppm Boiling range, of
22.3 2.9 820 600-1000 Maximum product objective Turbine Diesel Heating Gasoline Fuel Fuel Oil
Yields on feed, vol% Cl-Lp scf/bb 1 Butane Light gasoline Heavy Gasoline Jet fuel Diesel fuel Heating fuel Chemical H? consumption, scf/bol When more drastic changes are required in the product slate, usually a change to specifically designed catalysts with the appropriate hydrogenation and cracking components is necessary. An example of the influence of catalyst and process is given in Table 2 (ref.6). TABLE 2 Catalyst and Process Variations in Unicracking Heavy Gas Oil for Gasoline and Turbine Fuel Feedstock Properties: Gravity, °APl Distillation, D 1160, of lBP 10 50 90 EP Sulfur, wt% Nitrogen, ppm
20.3 520 641 728 820 890 1.33 2770
Product Yields and Properties: Yields: Cl-C3, scf/bbl light gasoline C3-C6 C7-plus gasoline Turbine fuel Total C -plus H2 consumetion, scf/bbl Turbine Fuel Properties: Aromatics. % vol Smoke point, mm
One-Stage Cat A
One-Stage Cat B
Two-Stage Cat C
146 32.5 40.6 45.0 121.2 1750
50 18.2 34.1 61. 1 121.7 1950
110 16.5 34.4 61.3 120.8 2110
34 13.6
19 20.1
2.0 29.7
595
This table shows three situations for hydrocracking the same feed resulting in significant differences in products. The data show that Catalyst A, developed for primarily gasoline production can produce substantial amounts of turbine fuel. However, at the same operating conditions, Catalyst B, which was designed to produce larger yields of turbine fuel, produces about 35% more turbine fuel. The use of another Catalyst C with two-stage processing conditions r~sults in the same production of turbine fuel but with much reduced aromatic content and a considerable change in the hydrogen consumption in the process. Furthermore, alteration of fractionation conditions and minor changes in catalyst temperature would permit production of 100% gasoline from the feeds tock. CATALYST DESIGN AND PREPARATION The hydrocracking catalyst is a carefully formulated combination of hydrogenation components and cracking components. The catalyst can initially be considered in terms of these two basic groups of components relatively independently. Later their joint properties will be considered. The Cracking Component The cracking component provides generally two basic functions: 1) the acidic function and 2) the high surface area porous support which allows ready diffusion of reactive molecules and provides a high surface area onto which the hydrogenation metals can be dispersed. Typical support materials are amorphous metal oxides having surface areas greater than about 150 m2g-1. Pore size distributions in hydrocracking supports, in contrast to hydrotreating supports, for example, have not been shown to be of great significance except in some special applications. Additional requirements are that the supports should be stable under thermal and hydrothermal conditions for several years. The second criterion of acidity largely controls the activity of the catalyst, at least for cracking. It is generally accepted that cracking reactions are proton or Bronsted acid catalyzed. Hence, it is desirable for the support to be a proton acid or to be capable of modification into a proton acid. Acidity can have two major distinguishing properties, strength and quantity. For hydrocracking, the greater the number of sites, the more active the catalyst will be; all other properties being equal. The ro1e of streng th is more complex and is i nter- re 1ated with the type of products desired. These phenomena will be discussed later. In initial designs of hydrocracking catalysts, cracking components used were similar to those already being used in catalytic cracking, namely: alumina, silica-alumina, silica-magnesia and the like. Some supports appear to be specially developed for hydrocracking applications such as
596
silica-zirconia-titania (ref.?) and alumina-titania (ref. B). These supports are generally prepared by coprecipitation of appropriate salts followed by washing and drying. More recently, just as in catalytic cracking, molecular sieve zeol ite supports have become important and probably dominate most areas of hydrocracking. Four zeolites appear to have most commonly been used up to now, V, ZSM-5 types, erionite and mordenite, with the V-zeolite being by far the most common. The lSM-5, erionite and mordenite appear to have only been used in special applications which req~ire shape-selective reactions controlled by pore geometry. There is also a possibility that X-zeolite was a component of some early catalysts. Currently, hydrocracking is the second largest catalytic use of zeo1ites. The princlpal requirement for a commercial catalyst is that the zeolite exhibits a high cracking activity (i.e., acidity). Since catalysts are used for long periods, the activity and stability of the structure to thermal and hydrothermal conditions as well as ammonia and hydrogen sulfide at operating conditions is vital. The ability to reactivate the catalyst after use is also desi rab1e. Examination of the patent literature and various publications show that it was soon realized that the development of the necessary properties required modifications of the zeolite and this has occupied much work and skill. The as synthesized sodium zeolite has essentially no catalytic activity. However, it was known that by ion exchange with ammonium ions, followed by thermal decomposition, acidic hydroxyl groups could be generated in the zeo1ite. Na+
NH
+
4
Exchange to about 2 weight percent sodium from about 11 weight percent was readily achieved. It was found that such materials, although catulytically active, were usually unstable and would lose crystallinity and surface area and hence activity relatively rapidly. However, partial re-exchange of the ammonium zeolite with multivalent cations such as magnesium (ref.g) or rare earths (ref.10) resulted in a stable zeolite which was still highly acidic. Such zeolite supports maintain high surface areas (over 500 m2g1) and high zeolite crystallinity.
597
Subsequently, it was found that by exchanging the residual sodium to a very low level the zeolite acidity and catalytic activity could be increased. For example, in Figure 5, the catalytic activity for ortho-xylene conversion is
50
2
Fig. 5.
4 6 8 PERCENT SODIUM
10
Conversion of a-xylene as a function of sodium content.
shown as a function of the residual sodium content (ref.ll). Similar observations have become available for paraffin isomerization over a platinumhydrogen-Y catalyst (ref.12) as revealed by Table 3. TABLE 3 Influence of Sodium Content on Isomerization of n-Pentane over Pd on Hydrogen-Y (ref.12) Crystallinity %
Na 2%0 Wt
Temperature (OC) for 30% Conversion
90 80 80
2.02 0.27 0.02
305 300 250
A secondary benefit of removing sodium ions at the synthesis stage is that any residual sodium, during use of the catalyst, will migrate from its initial position into sites probably in the supercages in which the sodium ions inhibit the catalytic activity. As will be shown later, these residual ions can be removed by reactivation techniques. The removal of residual sodium ions to a very low level is possible by using a large number (10-20) of exchanges. However, such a method would be impractical on a commercial scale. It has been found that an intermediate calcination can produce a rearrangement of the ions in the structure with ions such as
598
hydrogen, magnesium and rare earths migrating into the zeolite structure and displacing sodium ions. These sodium ions can readily be removed by further ion exchange with ammonium or other cations. Thus a sodium V-zeolite can be partially exchanged with rare earth ions and then calcined at elevated temperature. The displaced sodium ions can then be removed readily by further exchange. A second type of zeolite whi ch has been frequently reported in the patent literature for hydrocrading is the stabilized V-zeolite. These zeolites are characterized by being metal cation free and thermally stable to temperatures higher than that of the parent V. A typical preparative procedure (ref.13) involves exchanging a sodium V-zeolite with an ammonium salt until it contains less than about 3 weight percent sodium. The zeolite is then calcined at about 5400e for three hours and subsequently re-exchanged with an ammonium salt. When the sodium content is sufficiently low (below about 0.2 wt%) the zeolite is washed, filtered and stabilized by calcination at 815°e for three hours. This product is thermally stable up to 10000e. The zeolite produced has a high surface area and crystallinity. It is characterized by a decrease in the unit cell constant from about 24.65 to 24.4A in the finished product. A typical catalyst base can be produced from such a zeolite by extrusion with about 20 weight percent alumina. Another method of preparing a stabilized zeolite is to heat an ammonium Vzeolite (containing about 2 wt% sodium) to between 550 and 8000e in flowing steam for up to four hours. The product can be readily ammonium ion exchanged to produce a stable low sodium zeolite suitable for a hydrocracking catalyst base (ref.14). The characteristics of a support for hydrocracking can be summarized in terms of the following analysis: alkali metal surface area stability, thermal
- minimum >150 m2g-1 hi gh
aci dity, amount acidity, strength crystall inity (zeal t te )
high variable high
Recently, selective hydrocracking of normal or slightly branched hydrocarbons has become of interest. There are at least three processes designed using this phenomenon: 1) Selectoforming, or selectively cracking linear paraffins from reformer feedstocks. These molecules are low octane molecules and thus, their removal results in increased product octane.
599
The catalyst is tailored around the small pore diameter of zeolites such as erionite and lSM-5 which do not readily admit desirable highly branched molecules but selectively crack normal and slightly branched paraffins (ref.15). 2)
Lube oil and middle distillate dewaxing processes also operate on the same principle of removing normal and slightly branched paraffins by preferential cracking. Catalysts-based on mordenite and lSM-5 have been successfully used for dewaxing (ref.16,17).
The Hydrogenation Component Many hydrogenation components have been evaluated for hydrocracking. These have generally been those well known for hydrocarbon hydrogenation and constitute the noble metals, particularly platinum and palladium and the non-noble metals of Group VIb and VIII, especially nickel, cobalt, molybdenum and tungsten. Noble metals are reported to be used in amounts less than 1 weight percent whereas the non-noble metals are used in levels similar to those in hydrotreating catalysts (i.e., 2-8 wt% nickel and cobalt and 12-30 wt% molybdenum and tungsten as oxides). Noble Metal Components Noble metals may be introduced into the catalyst support by several methods. Impregnation by pore saturation, adsorption from solution or the gas phase, comulling and ion exchange are some of the more common methods. All of these methods except ion exchange are applicable to the amorphous supports such as silica-alumina. A typical method involves the measurement of the pore volume of the support, dissolving the desired amount of the noble metal salt, e.g., palladium chloride, in the measured pore volume of water contacting the solution and support for fifteen to sixty minutes, filtering off any excess liquid, drying the impregnated support and calcining in air at around 900°F. Palladium and platinum can be impregnated from acidic, near neutral or basic, e.g., ammoniacal, solutions. Noble metals can be incorporated into molecular sieve zeolites by the above methods. Since most zeolites have an exchange capacity far greater than that necessary to accommodate the amount of noble metal, exchange is one of the preferred methods. It is necessary that the metal be present in the exchange solution as a positive cation which usually requires formation of a complex cation which is easily decomposable. In general, up to about 2 weight percent of metal can readily be exchanged into the zeolite quantitatively. Platinum and palladium are readily introduced via the tetra-amine salts in neutral or
600
ammoniacal solution. Pyridine complexes can also be used. The introduction of the noble metal is usually done after all other modifications and cation exchanges to the zeolite powder have been carried out. The major reason for this is to avoid waste of expensive metal. In a typical preparation, 250 grams of an ultrastable zeolite were stirred in 500 ml of water containing 25 ml of concentrated ammonium hydroxide solution. To the zeolite slurry was added dropwise over a period of several hours a solution of 2.88 gm o~ palladium chloride dissolved in 90 ml of water plus 20 ml of concentrated ammonium hydroxide. After standing overnight, the zeolite was filtered, washed free of chloride and dried. For activity evaluation, the zeolite would then be pelleted with about 15-25 weight percent of a binder, such as alumina. The catalyst would contain about 0.5 weight percent palladium. It is also possible to incorporate the noble metal after pelleting. With powders the metal ions will be uniformly distributed, whereas there is a strong possiblity with pellets that the metal will be located preferentially near the pellet edge. Recently, however, it has been shown that high activity catalysts can be made by impregnating pellets with a Pd(NH 3)4(N03)2 solution using the pore saturation method (ref.18). The catalyst made by this method had similar activity to that of a more conventionally made catalyst. It has been shown that catalysts made by incorporation of the noble metal as an anion are less effective. It has been found desirable to decompose the tetramine complexes in flowing air rather than by direct reduction of the cation to the metal in hydrogen. Direct reduction appears to favor agglomeration of the metal (ref.19) whereas calcination in air above 800°F will decompose the tetramine complex into the oxide and remove most of the physically absorbed ammonia and water. Non-Noble Metals Many non-noble metals have been investigated for hydrogenation components both singularly and in combinations. The metals used have mirrored the pattern used in hydrotreating catalysts. In general, the most frequently used components are selected from the non-noble metals of Group VIII and the metals of Group VIb; nickel, cobalt, molybdenum and tungsten being the most frequently chosen. Nickel and cobalt are usually used at the 2-8 weight percent and molybdenum and tungsten at the 12-30 weight percent levels. Because of the many possible combinations, numerous methods are available for incorporating the non-noble metals. If single component nickel or cobalt is used, the methods discussed under noble metals such as impregnation, poresaturation and ion exchange are applicable.
601
Since most catalysts usually contain a Group VIb and VIII component, the most frequent methods used are: 1) comull ing 2) impregnation 3) exchange and impregnation Comulling is a common method of incorporating the hydrogenation components. In a typical preparation of say a nickel molybdate catalyst, the appropriate amounts of: l~
a nickel compound a molybdenum compound the catalyst base a binder and possibly an amorphous diluent such as alumina are mulled together as solids plus a suitable amount of water until an extrudable paste is formed. The paste is then extruded and chopped into suitable lengths followed by drying and calcination in air at about 900°F. Typical nickel compounds are nitrates, carbonates, oxides and the like or their combinations. Molybdenum compounds can be ammonium heptamolybdate, molybdenum oxide, ammonium dimolybdate, etc. A suitable binder is an acid peptized alumina. Thus a typical catalyst may have the composition of:
3% NiO Mo0 3 Zeolite Alumina Binder
15% 20% 42% 20%
For pH control, if desired, it is possible to use mixtures of salts or to add small amounts of acid or ammonia. This is probably the simplest and most economical method of preparation although scientifically, the least satisfactory. A typical example of a comulling process is given in ref. 20. A modification to the above comulling method is to mull together the catalyst base and binder and add to it a solution of the hydrogenation components in water or other media. In this' process, it is often necessary to add a solubilizing agent to the metal components. Thus nickel nitrate and ammonium heptamolybdate are insufficiently soluble in water to meet the levels needed on the catalyst. However, addition of ammonia or phosphoric acid can markedly increase the solubility and form very stable solutions for impregnation purposes (ref.21)
602
2) The impregnation route basically is the addition of the metal compounds in solution to the already pelleted catalyst base. Two possible routes are available. Firstly, the catalyst pellets are contacted with a solution of the metals equivalent in volume to the pore volume of the support, followed by drying and calcination. Secondly, the catalyst pellets are contacted with an excess solution of the metals, then separated, dried and calcined. Both of these two methods can be carried out in one or two stages, that is, all the metals added at the same time or each metal added as a separate solution followed by drying and then reimpregnation. Thus a typical catalyst could be prepared by pelleting about 20% zeolite, 60% alumina and 20% binder, drying and calcining at 900°F. The pellets would then be impregnated by pore saturation with a solution containing 6 weight percent nickel as NiO, 18 weight percent molybdenum as Mo0 3 and 3 weight percent phosphorus. This solution has a pH of about 1.1 before contacting the pellets. After drying, the pellets are calcined at 900-1000°F. If nickel or cobalt and tungsten are the desired components, the solubility is such that no additional stabilizer is necessary although naturally other components could be added, for example, for pH control. The third potential method of exchange followed by impregnation simply implies that one hydrogenation component could be introduced at an early stage by ion exchange, e.g., into a zeolite during the early modification stages and the second metal at a later stage. Whereas by comulling, if carried out properly, the metal components will be uniformly distributed throughout the catalyst pellet, by impregnation, possibilities exist for non uniform distributions with metal concentrations being greater near the exterior of the pellets resulting in a rind formation. It is also possible for the metals to diffuse through the pellets at different rates which could result in uniform distribution for some components and rind effects for other components. Thus it is necessary to control the metal addition steps very carefully and to verify the disposition of the metal by some physical or chemical technique. X-ray diffraction, electron microscopy and chemisorption techniques can supply data on the state of dispersion and particle size of the supported metals while scanning electron microscopy can supply information on the uniformity of distribution. A selection of techniques for preparing nonnoble metal catalysts can be found in the patent literature. In most cases, catalysts are prepared in such a manner as to maximize the interaction and proximity of hydrogenation sites and cracking sites. Although not certain, it is possible that a close proximity of the basic components may aid in enhancing the rate of the overall reactions and in reducing the rate of carbon deposition at the acidic cracking sites. However, it has been found in
603
certain cases that excellent catalysts can be prepared in which the hydrogenation components and the cracking components are kept isolated, at least in the preparation (ref.22). Catalyst Shape Two basic types of catalyst have been used in hydrocracking: pellets (or tablets) and extrudates. The former are made by mechanically compacting the catalyst base with about 20% of a binder such as alumina in the presence of about 270 carbon as a lubricant. Pellets are usually very regular in shape and are usually in the form of 1/8" cylinders. More frequently used nowadays are extrudates. In the simplest case, the catalyst base, with or without a binder and with or without hydrogenation components, is forced under pressure through a circular orifice. The bead formed in 1/8, 1/16 or 1/32" diameter is broken into short lengths «1/2"), dried and calcined at about 900-1200°F. Amorphous catalyst bases can often be extruded without added binder, but zeolites require dilution with about 20 or more weight percent binder such as peptized 0 alumina. For many years, catalysts were universally cylindrical in shape. Recently, it has been shown that improved activity can be obtained by increasing the surface/volume ratio of the extrudate by using shaped extrudates (ref.23). Tri1obe(R), tetra10be and in general po1ylobe catalyst bases have been prepared and used commercially (ref.24,25). Although Trilobe(R) and tetralobe catalysts have been used in hydrotreating and reforming, they appear not to have yet gained great util ity in hydrocracking. In general, 1/16" and 1/8" cy1 inder extrudates are the most common shapes for hydrocracking. CATALYST ACTIVATION The next critical operation in determining the performance of a hydrocracking catalyst is the technique of activation. Noble metal catalysts are activated by reduction, usually with hydrogen. The key to success in this case is remembering that the small amount of noble metal (about 0.5 wt%) is very labile and by malhandling is easily agglomerated or poisoned. The reduction of the metal oxide can be carried out at pressures from atmospheric up to about 50 atmospheres in flowing hydrogen. In order to avoid metal agglomeration from water liberated during the reduction and physically adsorbed on the zeolite or catalyst support, the temperature should be increased slowly to around 700°F and then maintained constant until reduction is completed. The catalyst temperature is lowered in preparation for feed introduction. The final conditionsof reduction can markedly effect the activity of the catalyst. In Figure 6, the influence of temperature and time in the final stages of the reduction are illustrated for second-stage activity.
604 +60.----------------------::1
eel
la) Z HRS 700°F Ibl 6 HRS 650°F lei 6 HRS 850°F Id)6 HRS 700°F Ie) 6 HRS BOOoF I~ Z HRS BOOoF BASE
L-
-±:50
~:_;;_---~---~
100
150
200
CATALYST AGE, HR.
Fig. 6.
Influence of reduction conditions on hydrocracking activity.
Variations from 2 to 6 hours and 650 to 850°F in calcination temperature can make over 20°F difference in activity for second-stage conditions which is equal to doubling of the activity. At first-stage conditions, no changes outside experimental error are observed. Although the exact nature of the physico-chemical phenomena occurring is not known, since the activity differences occur in the second-stage process which takes place in the absence of ammonia, it would appear that some changes involving the state or location of the palladium is involved which influences the hydrogenation activity of the catalyst. Non-noble metal catalysts are usually activated by sulfiding rather than by direct reduction. Sulfiding media are generally introduced into the catalyst system at low temperatures and elevated pressures. Typical sulfiding systems are very dilute hydrogen sulfide in hydrogen, carbon disulfide in hydrogen, butyl mercaptan in kerosene and the like. The temperature is slowly raised until sulfiding starts, as indicated by an exothermic wave in the catalyst bed and by water evolution. The temperature is held until hydrogen sulfide is evolved and then it is slowly raised to about 700°F, maintaining hydrogen sulfide evolution. When sulfiding is completed, the catalyst is cooled and readied for feed introduction. Relatively minor empirical changes in conditions of activation can result in optimization of activity for different catalysts. Nickel-only containing catalysts can be activated by either direct reduction or sulfidation depending on the type of service.
605
FACTORS INFLUENCING THE BALANCE OF CATALYTIC ACTIVITY The previous sections have discussed some of the factors involved in selecting hydrogenation and cracking components. It now remains to show how process conditions and catalyst formulation can affect operations. The ability of molecular sieve catalysts to operate in the presence of a substantial concentration of ammonia is a marked contrast to that of silicaalumina based catalysts which are strongly poisoned by ammonia. This difference is illustra~ed in Figure 7. The silica-alumina catalysts require severe
....I
o ~
z
o
iii a: w >
50
Z
o
o
1·15% HYIO.4% PI) IN SIOZ Z-SiOZ-AI Z03 (0.4% PI)
00 1000 2000 NITROGEN WPPM (FROM QUINOLINE)
Fig. 7.
Influence of ammonia on hydrocracking of heptane.
hydrotreating of the feedstock and essentially complete removal of nitrogen compounds from the feed to the hydrocracking reactor. The advent of the ammonia tolerant molecular sieve catalysts led to the birth of the two catalyst singlestage operation with the resulting economics and improved gasoline product slate. It is believed that the greater ability of molecular sieves to tolerate ammonia is due to their greater acidity. Presence or absence of ammonia should alter the role of the cracking component and it should be possible to change the catalyst from one which is balanced to one which is hydrogenation limited to one which is cracking limited. Such effects can readily be demonstrated using a hydrofined gas oil as a feedstock. Table 4 shows that in the absence of ammonia, two catalysts with different zeolite levels, are of approximately the same activity.
606
TABLE 4 Effects of Ammonia on Hydrocracking Activity Ammonia Present 40% Conversion* Cracking Component Level Effects High Zeolite Low Zeolite Hydrogenation Component Level Effects High hydrogenation level Low hydrogenation level *~
Base Base + 18°F
Base Base + 2
Ammonia Absent 60% Conversion* Base Base _2°F
Base Base + 20°F
temperatures for given conversion.
However, in the presence of 2000 ppm of ammonia, the higher zeolite content catalyst is 18°F more active, which kinetically translates into double the activity. This example shows that catalysts which have the same hydrogenation activity, can be activity limited by the cracking component in the presence of ammonia. In the absence of ammonia, the activity is independent of the cracking component suggesting hydrogenation limited reactivity. This is confirmed by the second example in Table 4 in which catalysts with the same molecular sieve contents but different hydrogenation component contents. Although the catalysts are of similar activity in the presence of ammonia, in the absence of ammonia the high hydrogenation level catalyst is twice as active. This shows that the high hydrogenation component catalyst is cracking limited whereas the low level hydrogenation component is hydrogenation limited. Clearly, optimum catalyst formulation may depend strongly on the process design, as well as on the feedstock and process objectives. The influence of palladium level on catalyst activity is demonstrated in Figure 8. This figure illustrates the change in activity for a catalyst as the noble metal component is varied by a factor of almost four in an ammoniafree cracking environment. In th€ absence of ammonia, virtually no change of activity was observed.
607
+80 0 +60
1!-
..r a:
0
:::l
l-
ea:
+40
lU
A-
~ lU
I-
0
+20
BASE~---..'.--------..I;.-_.....,-',,-----_-..'.
o
0.5 1.0 1.5 2.0 PALLADIUM CONCENTRATION, WT. %
Fig. 8.
Influence of palladium level on hydrocracking activity.
In the presence of ammonia, the concentration of the cracking base not only influences the activity of the catalyst but can also have a marked effect on the selectivity. Figure 9 illustrates the change in efficiency for diesel fuel
~------------,80
70
1!- +40
~
..r a:
,:
IC
Z lU ij
U
:::l
a:
u::
lU
A-
60
~ +20
I&.
lU
I-
50
BASE
o
5
10
15
20
% ZEOLITE
Fig. 9.
Variation of activity
(~)
and efficiency (a) with zeolite content.
608
production and activity of a series of non-noble metal catalysts as the percent of zeolite in the support is changed from zero to 20 weight percent. With the constant hydrogenation activity, as level of support is increased and hence the level of cracking activity is increased, the overall catalyst activity is increased by about 25°F, roughly equivalent to doubling of the activity while the efficiency for turbine fuel production drops from about 80 to 54 volume percent. The selectivity of the hydrocracking catalyst can be changed considerably by varying the type ?f cracking components. A broad classification of catalyst characteristics has been given by Scott et al. (ref. 26) and is illustrated in Table 5. TABLE 5 Hydrocracking Catalyst Types
Desired Reaction Hydrocracking Conversion A. Naphthas to LPG (ref.27,28) Gas oils to gasoline (ref. 29,30) B. Gas oils to jet and middle distillate (ref. 31,32,33)
Catalyst Characteristics Hydrogenation Surface Area Acidity Activity Strong
Moderate
Moderate Strong
Porosity
High
Low to Moderate
High
Moderate to High
Moderate
High
Gas oils to high V.I. lubricating oils (ref.34,35) Solvent deasphalted oils and residua to lighter products (ref.36,37) Hydroconversion of Nonhydrocarbon Constitutents Sulfur and nitrogen in gas oils (ref.38,39)
Weak
Strong
Basically, for maximum extent of cracking, e.g., to make gasoline or lighter products, high cracking activity is required and can be furnished by highly acidic materials such as molecular sieve zeolites and some amorphous oxides. On the contrary, to produce high yields of middle distillate, lower acidity but high hydrogenation activity is required so that secondary cracking is minimized and aromatics are saturated. Hence, a high surface area alumina or silica-alumina with large pores· and loaded with a high level of metals constitutes a preferred middle distillate catalyst. A typical example of the effect of catalyst base is given in Table 6. This table compares
609
two catalysts containing the same hydrogenation components and the same amount of two different molecular sieves. It is seen that Catalyst A, despite being more active, also produces a substantially larger amount of turbine fuel. By changing the catalyst support to an amorphous alumina or silica-alumina, a higher turbine fuel efficiency would be expected. TABLE 6 Hydrocracking of a Gas Oil for Turbine Fuel Catalyst A Temperature for 65% Conversion to 675°F-product Turbine Fuel Efficiency, %
Catalyst B
Base -7
Base
71
55
Within the series of potential hydrogenation components, the ranking Ni-W > Ni-Mo > Co-Mo > Co-W has been found (ref.5). Platinum, when not sulfided, is the most active hydrogenation component. From a study of toluene hydrocracking (ref.5), the atomic ratio Group VIII Metal ~ Group VIII Metal + Group VIb Metal ~0.25 has been found to be an optimum.
50.0',--------=-=-----, oNi
.«
oNi - Mo .Co - Mo oCo- W
z
37.5 SUPPORT ALUMINA
o
iii II:
W
> Z
o
U
25
50 % ATOMIC
CO Co + Mo
Fig. 10.
Co Co + W:
NI
Ni""+iiO:
Influence of hydrogenation component.
NI HI + W
610
Data on which this correlation is based are plotted in Figure 10. It is seen that for the four different metal combinations supported on alumina. the maximum conversion occurs when the atomic ratio is about 0.25. Progressing a step further. the relative isomerization to hydrocracking of heptane is illustrated in Figure 11 for a series of hydrogenation components on silica-alumina in the
HYDRO ISOMERIZATION
100 I- ClI-Mo
Z- Ni-Mo 3 - NiW 4 - 0.15% PI WITHOUT S 5 - 0.3,0.5% PI WITHOUT S
20
40
60
80
HYDROCRACKING
Fig. 11.
Effect of hydrogenation component on hydrocracking activity.
presence of ammonia and hydrogen sulfide. In contrast. Figure 12 illustrates the influence of changing the silica-alumina ratio. As the silica-alumina ratio increases. with resulting increase in acidity. the conversion and cracking activity increases (ref.5). HYDROISOMERIZATION
100....---------------, Ni-Mo/SiOZ-AI Z03 1 70% 30% Z 50 50 3 30 70 +R-S-S-R + n BUTYL-AMINE
20
40
60
HYDROCRACKING C3 + C4
Fig. 12.
Effect of silica-alumina content on hydrocracking.
611
For most of the hydrocracklng processes and catalysts described above, the most desirable supports have been amorphous metal oxide mixtures or large pore molecular sieve zeolites, the smallest pore size used being that of V-zeolite. However, it waS indicated initially that for certain types of selective hydrocracking, small pore molecular sieves are highly desired since they are able to sieve the molecules and limit the accessibility of hydrocarbon molecules into the pore structure and hence to the vicinity of the active sites. Selective hydrocracking has been applied in several ways to remove normal or slightly branched paraffins. Advantages of small pore zeo1ites for the 1owering of the pour point of di s t i l 1ate fuels have been utilized by British Petroleum, using a noble metal impregnated mordenite and by Mobil Corporation using a ZSM-5 zeolite. Both processes operate by removing long-chain normal paraffins. Figure 13 illustrates the change
AFTER PROCESSING
BEFORE PROCESSING
PROGRAMMED TEMPERATURE, DC
Fig. 13.
Chromatograms of a gas oil before and after dewaxing.
from feed to product for the ZSM-5 processing showing the dramatic removal of normal paraffins (ref. 40). The change in pour point accompani ed by the slight change in boiling point distribution is shown in Table 7 (ref.40).
612
TABLE 7 Properties of a Gas Oil Before and After Hydrodewaxing Fraction
Virgin Heavy Gas Oil
MMDW Processed Heavy Gas Oi 1
TBP cut, of Yield on Crude, Vol%
650-750 7.5
Properties: Gravity, °API Pour Point, of Cloud Point, of Sulfur, wt% Diesel Index
27.8 60 66 2.3 46
25.4 -10 +22 2.5 38
658 685 732
653 682 731
ASTM 10 50 90
Dist., of vol% vol% vol%
650-750 6.3
Finally, it is of interest to compare a narrow pore shape-selective catalyst with a large pore catalyst for the hydrocracking of ~-paraffins. Jacobs et al. (ref.41) have shown that platinum supported on HISM-5 was more active than platinum supported on ultrastab1e V-zeolite for the hydrocracking of n-decane. CATALYST REACTIVATION In commercial use over several years, hydrocracking catalysts slowly lose activity. The desired conversion of feedstock to product is generally kept constant by gradually increasing the catalyst bed temperature until limiting factors such as reactor metallurgy or product distribution dictate reactivation or replacement of the catalyst. The cycle length can typically vary from about 1 year to 5 years. The activity of most hydrocracking catalysts can be restored to close to that of fresh activity by oxidative combustion of the carbon deposited on the catalyst. This can be carried out in-situ in the reactor or ex-situ in suitable equipment (ref.42). Reactivation by carbon burning has been reviewed recently (ref.43). Other causes of deactivation can be: 1) Decay of the hydrogenation function. 2) Inhibition of the cracking function. For non-noble metal hydrocracking catalysts currently used, carbon removal is usually sufficient to substantially restore the activity of the catalyst.
613
Noble metal hydrocracking catalysts are much more sensitive than nonnoble metal catalysts. After regeneration to remove carbon, although they genera lly exhi bit good recovery of fi rst- stage activity, they often exhi bit a considerable loss of second-stage activity. This activity loss is related to loss of hydrogenation activity due to metal agglomeration or redistribution. X-ray diffraction, electron microscopy and hydrogen chemisorption studies confirm the metal agglomeration. The metal agglomeration is believed to be due to the prolonged exposure of the catalyst to elevated temperatures in the presence of water vapor, ammonia and hydrogen sulfides. As discussed previously, the noble metal hydrogenation component in molecular sieve catalysts has been incorporated by ion exchange rather than by impregnation. This results in a uniform, extremely fine dispersion of the metal throughout the catalyst and hence has greater potential for agglomeration than poorly dispersed metals prepared by impregnation or other methods. Techniques have been developed which permit the redispersion of the noble metals to that approaching fresh catalyst. The earliest technique used (ref.44) was the treatment of the molecular sieve catalyst after hydration with ammonia. Basically, the catalyst is treated with a moist air stream until saturated with water and then with a gaseous ammonia stream until saturated with am~onia. It is believed that the noble metal dissolves in the strong ammonia solution in the molecular sieve pores, reforms the tetramine complex, and then is redistributed at the ion exchange sites in the molecular sieve. The rejuvenation is finished by removing the excess of ammonia with a nitrogen purge followed by calcination at a suitable temperature. Typical catalyst comparisons of fresh and rejuvenated catalyst are shown in Table 8 (ref.45). It is seen that the activity of the catalyst is improved dramatically. Later the process was modified such that the molecular sieve catalyst could be treated with aqueous ammonia rather than gaseous ammonia (ref.46). TABLE 8 Properties of Regenerated and Rejuvenated Catalyst After Regeneration Surface Area, m2g-1 Crystallinity, % (relative to fresh catalyst) Relative Activity, OF Second Stage (compared to fresh catalyst)
After Rejuvenation
100
535 100
+61
-9
550
614 It was later realized that the acidic zeolite portion of the catalyst could
have lost a substantial part of its contribution to the catalyst activity. Early molecular sieve catalysts comprised zeolites which had been partially exchanged with ammonium ions to remove sodium down to an easily achieved level, e.g., about 1.5-2.0 percent. Thus the sodium ions which are difficult to remove by straight ion exchange were left in the zeolite. It is feasible that either the initial preparati'on of the catalyst or the hydrothennal exposure the catalyst receives in use causes migration of the sodium ions from inaccessible portions of the structure to the vicinity of the acidic center resulting in a loss of cracking act ivfty , Treatment of the deactivated catalyst with an aqueous ammonium salt solution removed substantial amounts of the residual sodium from the catalyst. On evaluation, the activity was found to have been improved at first-stage conditions but little effect was found at second stage suggesting that the sodium had been removed from the zeolite with a resulting boost in acidity and cracking activity. A desirable extension of these two processes was to sequentially treat the catalyst with aqueous ammonia and then with an aqueous solution of an ammonium salt (ref.47). It was found that the catalyst could be rejuvenated with an ammoniacal ammonium salt solution (ref.48). Furthennore, it is possible to rejuvenate the catalyst before carbon burn off. Typical properties of two commercially used catalysts before and after rejuvenation are given in Table 9. TABLE 9 After Regeneration Catalyst Surface Area, m2g-1 Pellet Crush Strength, lb Attrition Loss, Wt% Relative Activity of, First Stage, compared to fresh catalyst
After Rejuvenation
A
B
A
B
513
420
494
433
22 1.7
16
18
15
1.0
+35
+45
-7
0.9 -2
The data shows that the activity of the catalyst can be substantially improved by chemical treatment, based on a knowledge of the catalyst composition and potential deactivation mechanisms. Catalysts reactivated by the above methods have been used in typical commercial operations with outstanding success. Performance at least equal to, and in several cases, superior to fresh catalyst has been obtained.
615
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
H. Schulz and J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev. 11 (1972) 46. J. Weitkamp and H. Schulz, J. Cata1. 29 (1973) 361. G. E. Langlois and R. F. Sullivan, Adv. Chern. Ser. 97 (1970) 38. R. Beecher, A Voorhies, P. Eberly, Amer. Chern. Soc. Div. Petrol. Chern. Preprints 1967, B5. J. P. Franck and J. F. LePage, Proc. 7th Inter. Congr. Catalysis 1981, 792. W. J. Bara1 and H. C. Huffman, Proc. World Petrol. Congr., 8th 1972, PD12 (1). R. C. Hansford, U.S. Patent 3,159,588 (1964). J: Jaffe, U.S. Patent 3,401,125 (1968). R. C. Hansford, U.S. Patent 3,173,853 (1965). C. J. Plank and E. J. Rosinski, U.S. Patent 3,140,249 (1964). J. W. Ward and R. C. Hansford, J. Catalysis 22 (1971) 9. H. W. Kouwenhoven, Advan. Chern. Ser. 121 (1973) 529. C. V. McDaniel and P. K. Maher, Conf. Mol. Sieves, Chern. Ind., London 1967, 186. J. W. Ward, J. Cata1. 27 (1972) 157. A. P. Bolton in J. A. Rabo (Ed), Amer. Chern. Soc. Mon. 171 (1976) 714. R. N. Bennett, G. J. Elkes and G. J. Wanless, Oil Gas J. 73 (1) (l975) 69. N. Y. Chen, R. L. Gorring, H. R. Ireland and T. R. Stein, Oil Gas J. 75 (23) (1977) 165. E. Gallei, L. Marosi, M. Schwarzmann and E. Lorenz, U. S. Patent 4,252,688 (1981). R. A. Dalla Beta and M. Boudart, Proc. Int. Congr. Catal., 5th Miami Beach 2 (1972) 96-1329. D. A. Young, U.S. Patent 3,890,247 (1975). J. W. Ward, U.S. Patent 3,835,027 (1974). J. A. Meyer, U.S. Patent 3,764,519 (1973). W. R. Gustafson, U.S. Patent 3,966,644 (1976). Akzo Chemie Catalyst Symposium, Amsterdam, May 1982. R. L. Richardson, F. C. Riddick &M. Ishikawa, Oil Gas J. 77 (22) (1979) 80. J. W. Scott and A. G. Bridge, Adv. in Chern. Ser. 103, (1971) 113. J. W. Scott and N. J. Patterson, World Petrol. Congr., Mexico City (1967). Oil Gas J. 63 (52) (1965) 130. J. R. Kitrell, G. E. Langlois, J. W. Scott, Oil Gas J. 67 (20) (1969) 118. Oil Gas J. 67 (24) (1969) 76. J. W. Scott, A. J. Robbers, H. F. Mason, N. J. Patterson, R. H. Kozlowski, World Petrol. Congr. Frankfurt (1963). R. H. Kozlowski, H. F. Mason, J. W. Scott, Ind. Eng. Chern., Process Design Develop. 1 (4) (1962) 276. C. H. Watkins, 'W. L. Jacobs, Oil Gas J., 67 (47) (1969) 94. Oil Gas J., Newsletter, 67 (20) (1969) 1. H. Beuther, R. E. Donaldson, A. M. Henke, Ind. Eng. Chern., Prod. Res. Develop. 3 (3) (1969) 174. D. H. Stormont, C. Hoot, Oil Gas J. 64 (17) (1966) 145. E. M. Blue, P.O. Harvey, R. P. Lance, W. J. Rossi, API, Div. Ref., 33rd Midyear Meeting, Philadelphia (May 196B). R. A. Flinn, O. A. Larson, H. Beuther, Hydrocarbon Processing Petrol. Refiner 42 (9) (1963) 129. J. J. VanDeemter, European Symp. Chern. Reaction Eng., 3rd, Amsterdam (Sept. 1964).
616
40 41 42 43 44 45 46 47 48
H. Heinemann, Cat. Rev. - Sci. Eng. 23 (1981) 315. P. A. Jacobs, J. B. Uytterloeven, M. Steyns, G. Froment and J. Weitkamp, Proc. 5th Intn. Conf. Zeolites (1980) 607. Oil Week, Jan. 25, 1982, 12. P. K. Maher, A. J. Garrett, Oil Gas J. 75 (15) (1977) 51. R. C. Hansford, U.S. Patent 3,899,441 (1975). A. D. Reichle, L. A. Pine, J. W. Ward, R. C. Hansford, Oil Gas J. 72 (30) (1974) 137. J. W. Ward, U.S. Patent 4,107,031 (1978). D. E. Clark and J. W. Ward, U.S. Patent 3,692,692 (1972). J. W. Ward, U.S. Patent 3,849,293 (1974).
617 DISCUSSION J. GUEGUEN How many rejuvenations is it possible to apply before a regeneration? How does it affect the acidity of the catalyst? J. WARD: Reactivation of some types of hydrocracking catalysts comprises i) regeneration and then ii) rejuvenation. Rejuvenation is always accompanied by regeneration (coke burn off), allthough regeneration is not always accompanied by rejuvenation. Rejuvenation remOves alkali and other metal ions from the catalyst. Since these ions are poisoning metal sites, rejuvenation increases the catalyst proton acidity. P.A. JACOBS: In Fig. 6 you clearly showed the effect of changing the reduction temperature on the hydrocracking behaviour. In the scientific literature, a similar effect is known for noble metal zeolites when the oxygen or air treatment temperature prior to reduction is changed. Does a similar effect exist for real hydrocracking catalysts containing either noble metals or group VI B elements ? J. WARD: The conditions of heat treatment prior to reduction are very important for noble metal containing zeolites for real catalysts as for noble metal zeolites. Conditions have to be chosen carefully in order to avoid agglomeration and maldistribution of the noble metal before reduction. Catalysts containing group VI B elements appear to be less sensitive. It is routine practice to treat all the catalysts discussed in dry air pi or to reduction. The data in Fig. 6 attempt to show the influence of time and temperature of exposure to hydrogen, even though the catalyst must be seentially totally reduced prior to these changes. R.J. BERTOLACJNI: During regeneration-rejuvenation with ammonia, do you see any changes in the zeolite or zeolite type? J. WARD: No significant change in the zeolite is seen during regeneration: there is a slight reduction in the unit cell constant due to the high temperature exposure during coke burn off. During rejuvenation, the only significant change is a lowering of the sodium content due to the ammonium ion exchange. D. CHADWICK: You illustrated the stability of the activity of zeolites operating under a partial pressure of ammonia. Do you see any changes in the product spectrum as a function of ammonia partial pressure ? J. WARD: Depending upon the ammonia partial pressure and operating condition changes, there mayor may not be a change in the product spectrum. The product spectrum depends upon a complex interaction of catalyst, process conditions and whether conversion is allowed to remain constant or change on ammonia addition. M.M. BHASIN What is the difference in the ammonium ion treatment rejuvenation of the used catalyst and that of the fresh catalyst ? J. WARD: Essentially none. The ammonium ion exchange in the fresh catalyst is used to remove the zeolite sodium content from that of sodium Y to that of an ammonium Y containing about 1.5-2.5 wt % Na. This residual sodium is difficult to exchange. During use, the hydrothermal conditions result in migration of the sodium ions to easily exchangeable positions. It is this sodium which is removed during rejuvenation. C. MARCILLY: 1.Usually zeolite-based catalysts are used for producing a maximum of gasoline. Recently Union Oil has claimed the use of a zeolite-based catalyst which allows to maximize either gasoil or jet fuel. Could you make any comment on this ? 2. Is ZSM-5 used in industrial hydrocracking catalysts?
618 J. WARD: 1. You are correct in that zeolite-based hydrocracking catalysts are used for producing maximum gasoline. A new zeolite-based hydrocracking catalyst for the production of jet fuel or diesel was announced (see Hydrocarbon Processing, 1981). We believe that this is a significant breakthrough. 2. To my knowledge, at least two industrial hydrocracking processes use ZSM-5 type zeolite containing catalysts. These are lubeoil dewaxing and middle distillate dewaxing. J.A. MARTENS: Does the mechanism for hydrocracking of n-dodecane account for all the hydrocracking catalysts mentioned in your paper? In shape selective zeolites (i.e. ZSM-5), the mechanism may proceed via dimethyldecanes. J. WARD: We have not examined n-dodecane hydrocracking over the several types of catalyst discussed. The mechanism cited is that of weitkamp and coworkers (Ref. 2,3) and applies to wide pore zeolite such as Y. Recently Jacobs and coworkers (Ref. 41) examined platinum supported on Y and ZSM-5 zeolites. ZHAO JIUSHENG The surface acidity may influence a lot the cracking ability and coking. When you design the catalyst, how to select the acidity and acid amount and how to control them , by ion exchange or by additives? J. WARD The surface acidity is very closely related to the cracking activity (see Figs. 5 and 9, and Table 3). Although the tendency to coke increases with acidity, the coking rate is minimized by the hydrogenetaion activity. The concentration of active sites can be controlled by any means of controlling the popUlation of acidic hydroxyl groups. This the zeolite content, extent, of ion exchange, type of cation exchanged into the zeolite (e.g. monovalent vs. divalent), thermal treatment to change the hydroxyl group content, etc.. Few criteria are available for controlling acid strength. However, it is generally believed that amorphous oxides such as silica-alumina or silica magnesia are weaker acids than zeolites.
619
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
INFLUENCE OF SODIUM CHLORIDE ON THE CAT ALYTIC PROPERTIES OF TELLURIUM-LOADED Y-ZEOLITES B.E. LANGNER+ and J.H. KAGON Institut fUr Technlsche Chemie, Universitat Essen (FB 8) Universrtatsstr s J , 43 Essen (FRG)
ABSTRACT The catalytic properties of tellurium-loaded Na- Y zeolites which have been modified prior to the tellurium loading by a thermal treatment in the presence of 0-15 wt.-% sodium chloride at 670 0C have been studied by physical methods and by dehydrogenation reactions.
The results show
that both the crystallinity and
the free pore volume of the zeolite decrease with increasing amounts of added sodium chloride. The retainment of tellurium in the zeolite after a hydrogen treatment at 540
0C
exhibits a maximum at a sodium chloride content of about 4-8 wt-%. In the same range of salt addition the catalysts exhibit maximum activity for the dehydrogenation reactions of ethylbenzene to styrene, of isobutane to isobutene , and n-hexane or cyclohexane to benzene.
INTRODUCTION Tellurium-loaded zeolites have been proved to be excellent catalysts for aromatization reactions (ref. 1-4 ,5,6) leading to a yield of more than 80% in the conversion of n-hexane to benzene at 530 0C on a Na-X/Te-catalyst. Furthermore, tellurium-loaded Na- Y zeolites catalyse the dehydrocyclodimerization of
butenes
to aromatics at selectivities of more than 60% at conversions below 10% (ref.?). The nature of the active sites of these catalysts is discussed in different ways. Whereas Olson et al ,
(ref. 3) conclude from X-ray powder diffraction patterns
that telluride ions are responsible for the catalytic activity,
Hightower et al ,
(ref. 5 ,6) concluded from surface measurements that tellurium atoms in a special zeolitic environment provide the high dehydrogenation activity. Although Hightower et al , (ref. 6)
did not
find any acidic activity for the
catalysts, we could detect cracking products in the dehydrocyclodimerization of
"re
whom correspondence should be sent.
620 butene which point to the existence of residual acidic sites on a Na- Y/Te catalyst. These acidic sites could have been formed during the hydrogen treatment of the physical mixture of tellurium and Na- Y at 530 0C according to the equation:
in which telluride ions are assumed to be responsible for the catalytic activity. Furthermore, the commercial Na- Y zeolites usually contain impurities of calcium ions which are known to possess acidic activity. In any case acidic activity is not required if one looks for a pure dehydrogenation catalyst. To destroy the residual acidic activity of the catalysts we used a method first proposed by R abo (ref. 8,9). Rabo has demonstrated that double bond isomerisation of I-butene on a commercial Na- Y zeolite could be suppressed by mixing solid sodium chloride to the zeolite followed by a thermal treatment at about 500 0C. By this procedure sodium chloride migrates into the zeolite structure in the solid state and destroys acidic sites of the zeolite by ion exchange:
where Z-O-H represents the zeolitic hydroxyxl groups and ZO- stands for the zeolitic framework. From these results we expected that tellurlum-Ioaded zeolites which have been modified by a thermal treatment in the presence of sodium chloride could lead to pure dehydrogenation catalysts as residual acidic activities have been destroyed by the salt addition.
EXPERIMENTAL For each experiment and each analysis 1.00 g catalyst was prepared by a strictly kept procedure. The catalyst bases were dried commercial Na- Y zeolites in the powder form (Linde SK 40, ratio Si0 which have been mixed 2/A1203=4.8) with 0-15 wt.-% solid sodium chloride in a ball-mill for 10 min. (too long ballmilling partially destroys the zeolite structure). The mixture was held in a muffle furnace at 670 0C for 17 hours. After the thermal treatment about 25 wt. -% tellurium powder was added and mixed in a mortar. The mixture was placed into the reactor which consisted of a quartz tube (d= 15 mrn) with an internal quartz frit to uptake the catalyst. The reactor was mounted in a tube furnace (Heraeus Type B/ A 1. 7/2.5) the temperature of which could be regulated at ±3K. Prior to the reaction, the Na-Y/NaC1!Te mixture was heated in a stream of dry hydrogen (2.5 Iiter/hr ,') at 530 0C for 17 hours. By this treatment a fraction of the tellurium
621
is evaporated and condenses at cooler parts of the apparatus from where the tellurium needles are removed before the reaction starts. The partial pressures of the hydrocarbons (ethylbenzene, n-hexane, cyclohexane) were adjusted by the saturation of a stream of dry nitrogen (2.5 liters/hr.) at lOoC. The dehydrogenation of isobutane to isobutene was carried out at atmospheric pressure without any dilution. Analyses of the reaction products were made gaschromatographically on a 6mx 1/4" column filled with 17% Sebaconitril on Chromosorb for the separation of the at 250C and a 3 mx1/4" column filled with 10% Benton-34 5-hydrocarbons on Chromosorb for the separation of the monoaraomatics at 900C. To follow the
C l-C
deactivation of the catalysts product samples were taken every 20 min.
The d-
spacings of the powder diffraction patterns (apparatus:Siemens spectrometer Type M34) were calculated according to the tables of Strunz (reLIO). Adsorption isotherms were determined with nitrogen at -196 0C in a conventional BET-apparatus , The adsorption capacities of the catalysts are expresssed in x/m = gN 2,adsorbed/ g catalyst at a partial pressure of 150 torr.
The analyses of the ultimate tellurium content of the catalysts after the hydrogen treatment were made (0 by weighing out the tellurium which has been crystallized on the walls of the reactor and (li) by a potentiometric titration procedure after extracting the tellurium from the catalyst with nitric acid.
RESULTS AND DISCUSSION Influence of sodium chloride on the physical properties.
During the thermal treat-
ment of the Na- Y/NaCI mixture the physical properties of the system drastically change. Although the calcination temperature is below the melting point of
= 800 0C), sodium chloride loses its crystal structure and rnis grates into the zeolite. This is confirmed by X-ray powder diffraction patterns
sodium chloride (T
shown in Fig.I. Whereas in the physical mixture of sodium chloride and Na-Y both the strong line of the sodium chloride crystals and the zeolite lines can be observed, after 89 hours at 590 0C the sodium chloride peak has disappeared. This shows in accordance with the results of Rabo (reL8) that sodium chloride is able to migrate into the zeolite structure below its melting point. (The migration process of the salt can be accelerated by elevated temperatures; therefore we have carried out most of the experiments at a calcination temperature of 6700C for 17 hours). Also the adsorption capacities of the zeolite for nitrogen at -196 0C and 150 Torr change during the thermal treatment in the presence of sodium chloride. Fig.2 demonstrates that the addition of sodium chloride results in a decrease of the adsorption capacities of the zeolites leading to a pore volume for a Na-Y zeolite + 8%NaCI catalyst which is less than one third of a catalyst without the
622
Oh
89 h - 550°C
-,
"No CI
No- Y
<,
No-Y
28 33
32
31
30
33
32
31
30
Flgv l , X-ray diffraction pattern of a mixture of 6% sodium chloride and Na- Y
0.3
01
0.2
01
E
x
0.1
O-+-------r-------r-------r-----..........- -
o
5
15
10 Noel
[wt -
20
%1
Fig.2. Adsorption capacity of Na- Y as a function of the sodium chloride conterr
623
addition of sodium chloride. One reason for the decrease of the pore volume may be the migration of sodium chloride into the super cages of the zeolite where it blocks adsorption sites. But, on the other hand not onl y the structure of the sodium
chloride changes during this treatment but also the crystallinity of the
zeolite. In Fig. 3 the peak heights of three strong lines in the powder diffraction pattern of the Na- Y are plotted against the sodium chloride content. It can be seen that the crystallinity decreases with increasing amounts of sodium chloride. The destruction of the structure could be caused by the action of the basic chloride ion -at high temperatures which may have similar effects on aluminosilicates like the carbonate ion in a soda/potash smelt.
100 QI
.~
"0
80
w cr 1: 60
.2'
dA:3.78
QI
I
~
0
40 dA: 5.71
0..
20 0+---,-----.--,---,--,-----,----.-_ _
o
2
4
6 Noel
10
8 [wt-%
12
14
I
Fig.3. Relative peak intensities of main peaks of the X-ray powder diffraction as a function of the sodium chloride content These partly destroyed "zeolites" have been mixed with 20-30% tellurium and treated in hydrogen at 530 0C. By this treatment most of the tellurium is removed from the catalyst by evaporation. But, after about 20 hours a constant tellurium level is achieved which does not change at longer times. This ultimate tellurium content is not dependent on the amount of tellurium which has been added to the "zeolite" before the hydrogen treatment. But it was a surprising result to us that the tellurium level which is reached after the hydrogen treatment is strongly dependent on the sodium chloride content of the zeolite. Fig. 4 clearly demonstrates that the tellurium content increases
624
from less than 1 % for a pure Na- Y without the addition of sodium chloride to nearly 4 % for zeolites with about 5-8 % sodium chloride. At higher salt levels the ultimate tellurium content slightly decreases.
A comparison of Fig.4 with
Fig.2 and Fig.3 shows that the tellurium retention after the hydrogen treatment increases with increasing amounts of sodium chloride although both the pore volume (Fig.2) and the crystallinity (Fig.3) continuously decrease •
/~o
o
3
o
~
0
o
0~
I
2
~
{E
0
_0__ 0
)
0 0
5
10 NaGI
15 [ wt - %
20
1
Fig.4. Tellurium content after a hydrogen treatment of 17 hours as a function of the sodium chloride content These results indicate that the addition of sodium chloride not only lead to the destruction of acidic sites. as shown earlier by Rabo (ref.8) but also to a higher retention of tellurium after the hydrogen treatment of tellurium loaded zeolites. Although the state of (ref. 6) ,
tellurium in these catalysts is not ambi guous 1)' identified
we speculate that the basicity of the chloride ion may play an import-
ant role for a heterolytic dissociation of the acid hydrogen telluride which may be formed during the hydrogen treatment. Dehydrogenation reactions on NaCl/Te-Na- Y catalysts. As it could be expected that the teHurium content strongly i nfl uences the catalytic activity of NaCl/Tel Na- Y catalysts,
different dehydrogenation reactions of
hydrocarbons have been
625 investigated as a function of
the sodium chloride content. Fig. 5 shows that
the conversion of ethylbenzene at 418 0C and 14.1 Torr increases from less than 3% on a catalyst with 2% sodium chloride to about 10% on a catalyst with 6-8% sodium chloride (without the addition of NaCl only poorly reproducible catalysts could be produced. Therefore only catalysts with at least 2% NaCI are considered in Fig.5). At higher salt levels the conversion decrease until the catalyst is inactive for dehydrogenation reactions, if it contains more than 15% sodium chloride. Remarkable is the high selctivi ty of more than 95% for the formation of styrene •• The only side reaction is minor cracking to stoichiometrical amounts of methane and toluene which is probable a thermal reaction. The conversions can be easily increased by an increase of the reaction temperature. At 530 0C more than 60% styrene can be achieved on a NaCl/Te-Na-Y with 6 % sodium chloride.
16
100
0
o~o~O 14 12 ~ I
90
~ QI
8
j
C
~
>-.
Ln
T = 418°C W/F = 0.88 9 h/mmol
10
6 4
.~.~~
.
p = 14.1 Torr
~
0
80
>,
>
U C!J
Qi
70
Vl
x
2
a
x
a
2
4
6
8
10
12
14
16
18
NaGI [%]
Fig.5. Reaction of ethylbenzene as a function of the sodium chloride content Another important dehydrogenation reaction is the conversion of isobutane to isobutene which is an intermediate for the production of the gasoline blending compound MTBE.
Fig. 6 shows that similar to the reaction of ethylbenzene the
conversion reaches a maximum at about 6% sodium chloride. Also the selectivity for the formation of isobutene increases from 80% to 90%. The main side reaction is thermal cracking to propylene and methane.
626 Fig.7 demonstrates that also in the aromatization of n-hexane improvements both in activity and selectivity can be achieved by the addition of sodium chloride to the tellurium zeolites leading to a yield of benzene of about 6% on a catalyst with 2% NaCI, but to yield of about 20% on catalysts which contain 6-8% sodium chloride.
20
/
~0-4. > -0",0
9"
L,"" o
15
~ 0
"i Cl>
c
10
Cl> ~
::l
0', "
,,
to
I
~,
,'\,'i1 .... 0
T
5
85
'l\
0~
80 ~
\\
..c 0 ~
90
~,
=
570°C
W/F = 0.018 9 h Immol 760 Torr p
o~
~.
,0 ~
ti Cl> 75
jj
70
o+---..-.----,--,--.----,--,----,,....-..--,---,-..,--_+_ 12 10 o 4 6 8 2 NoCI [wt
-%J
Fig.6. Dehydrogenation of isobutane as a function of NaCI content
627 The yield of benzene can be doubled if cyclohexane is used instead of n-hexane, as ring closure is a slow step in the reaction sequence from paraffins to aromatics. Nevertheless, even for the aromatization of cyclohexane a strong influence of sodium chloride can be stated (Fig.8). On the other hand, methylcyclopentane is no appropriate feedstock for the formation of benzene with this catalyst as only cracking products and coke formation could be observed leading to a rapid deactivation of
the catalyst. This can be explained by the lack of acidic sites
which are necessary for ring expansion reactions from a five-membered ring to a six-membered ring.
50
100
/~-g
0
o
40 ~ I
~
30
0
/
/'
x
90 x x
C
T = 530°C W/F= 0.22 9 h/mmol 60 Torr p =
(II
c
0
x
QJ
N
-0
20
QJ
co
-,
~
0
80
x
1;>
ti QJ
70
10
Qi
V1
0 0
2
3
4
NoCI [%
5
6
7
8
1
Fig.8. Aromatization of cyclohexane as a function of NaCl content
CONCLUSIONS A novel catalyst system which exhibits excellent dehydrogenation activity and selectivity can be made by modifying tellurium-loaded Na- Y zeolites with about 4-8% sodium chloride. Although the crystallinity and the pore volume of the zeolites decrease during the thermal treatment with sodium chloride at 670o C , both the catalytic activity and the amount of retained tellurium in the catalyst are increased by the addition of sodium chloride. The results indicate that the salt not only destroys residual acidic activity of
the zeolite but also stabilizes the
active form of tellurium. Nevertheless, the nature of the active tellurium site remains obscure.
628
ACKNOWLEDGEMENTS The authors want to thank Mrs.A.SchrOder for valuable technical assistance.
REFERENCES
1 W.H.Lang,R.J.Mikovsky,A.J.Silvestri, J.Catal. 20 (1971) 293 2 R.J.Mikovsky,A.J.Silvestri,E.Dempsey,D.H.Olson, J.Catal. 22 (1971) 371 3 D.H.Olson,R.J.Mikovsky, G.F.Shipman, E.Dempsey, J.Catal. 24 (1972) 161 4 A.J.Silvestri,R.L.Smith, J.Catal. 29 (1973) 316 5 G.L.Priee,Z.R.lzmagilov,J.W.Hightower J.Catal. 73 (1982) 361 6 G.L.Priee,Z:R.Ismagilov,J.W.Hightower in Proe. 7th Int.Congress Catal., Elsevier Tokyo p.708 (1980) 7 K.D.Hungenberg,J.H.Kagon,B.E.Langner, J.Catal. 68 (1981) 200 8 J.A.Rabo ACS Monograph 171 (1976) 335 9 J.A.Rabo,M.L.Poutsma,G.W.Skeels, Proe.lnt.Congr.Catal. 5th, North Holland Publ.Co. 98 (1977) 1353 10 H.Strunz "Mineralogisehe Tabellen" Akad.Verlagsgesellsehaft, Leipzig 1966
629 DISCUSSION D.D. SURESH Have you studied the effect of stronger bases such as Rb+ and Cs+ to further improve the retainment of Te and increase the activity ? B.E. LANGNER: We did not. But there may be an even greater effect on the activity of tellurium-loaded zeolites according to the greater effect of potassium salts in comparison with sodium and lithium salts. But as I mentioned before,it is very difficult to define equal calcination conditions for the zeolite/saltmixtures, as the salts have different melting points. LIN LIWU: 1) You used N2 as carrier gas to maintain lower partial pressure of reactants. If nitrogen is circulated, the H2 partial pressure of the reacted gas should be increased. Did you observe the effect of H2 partial pressure on the reaction performance for this kind of catalysts ? 2) Is it possible to maintain the concentration of tellurium and sodium chloride in the catalyst composite without any loss during the high temperature operation? B.E. LANGNER : 1) We have carried out all the experiments in afixed-bed reactor without recycling the carrier gas. But it may be probable that there is a kinetic effect of hydrogen according to a Hougen-Watson equation for dehydrogenation reactions. Furthermore, in some experiments the conversions were close to the thermodynamic equilibrium, and there should be an effect of hydrogen too. 2) If you have hydrogen or hydrocarbons in the carrier gas, then there is only minor loss of tellurium. But in the absence of hydrogen or hydrocarbons, tellurium is evaporated. A loss of sodium chloride during the reaction or during the hydrogen treatment was not examined. R. CAHEN 1) Have tou examined the stability of this very interesting catalyst? 2) Can the catalyst be regenerated ? B.E. LANGNER: l)There is deactivation of the catalyst by coke formation in all reactions, but the rate of the deactivation is strongly dependent on the kind of feed and on the reaction conditions (partial pressure, residence time, temperature). Thus the catalysts lose 50% of their initial activity in the DHCD of butene within a few hours, but in the dehydrogenation of ethylbenzene, within about two weeks. 2) We only made preliminary experiments on the regeneration of coked catalysts. But it could be shown that about 70% of the initial activity is restored, if the coke is burnt off in air followed by a new hydrogen treatment of the catalyst. T. BEIN You observe volcano curves for the catalytic activity is several reactions on NaCl/Te-Na-Y catalysts with a maximum at a NaCl content of around 6%. Could an explanantion for this effect be the competition between the following influences: on the one hand, the tellurium retention increases strongly with NaCl content up to 5% and on the other hand, crystallinity and adsorption capacity (Fig. 4) of the samples decrease with increasing NaCl content (Figs. 2,3). Could the catalytic activity, due to these effects, be a function of available tellurium sites ? B.E. ~NGNER : I agree with your hypothesis that there are two effects for the catalytic activity: salt content and crystallinity of the zeolite or at least of a part of the zeolite· structure. But we do not know which are the active sites. And we even do not know in which form the tellurium exists in the zeolite structure. Is there elemental tellurium, are there telluride ions or are there even poly-telluride ions? Therefore, we can only speculate about the role of the salt in tellurium-loaded zeolites.
This page intentionally left blank
631
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CONTROL OF THE PORE STRUCTURE OF POROUS ALUMINA
T. ONO, Y. OHGUCHI and 0, TOGARI Chiyoda Chemical Engineering and Construction Co., Ltd. Yokohama, Japan
ABSTRACT A new and simple method to prepare porous alumina is described.
This
alumina has a large pore volume ranging from about 0.5 to 1.5 ml/g and a narrow pore distribution even at a large pore diameter (10 - 100 nm).
In the
present method, both aluminum salts of acid and alkali are admixed alternately in the gelation process resulting in a swinging pH value.
Experiments were
carried out to determine the possible effects of the principal influencing factors, such as the frequency of pH swing, the pH value and reaction time On the pore structure of alumina.
INTRODUCTION Alumina is used as a carrier in various fields, owing to its superior pore structure.
The pore structure of the alumina has a close bearing on the
catalyst activity, selectivity and life.
It is very important to produce an
alumina having a pore structure suited for the intended reaction. alumina can be obtained by the dehydration of pseudo-boehmite.
Porous
The pore
structure of the resulting alumina is greatly influenced by the size, shape and further by the aggregation and disposition of pseudo-boehmite particles. The preparation of an alumina carrier includes controlling its pore structure during gelation, drying and calcining in order to provide a carrier suited for the intended reaction.
A lot of methods have been proposed in this connection.
For example, raw materials selection [1] and gelation conditions [2] are applied to controlling the size of pseudo-boehmite.
Aging of pseudo-boehmite
[3] is applied to controlling size distribution and growing.
Other methods
include replacing water among particles in a gel with an organic solvent having a lower surface tension, such as alcohol
[4],
to control the pattern in which
they are aggregated, or utilizing the interstices formed by calcining a pseudoboehmite gel in which a water-soluble organic polymer has been mixed [5].
632 The calcining temperature is also controlled to provide a desired pore structure [6].
But these methods are not necessarily effective in preparation
an alumina having a large pore volume and a large pore diameter, and a narrow pore distribution.
The advantage of this new method is that the alumina having
a desired pore diameter and a narrow pore distribution can be prepared easily.
EXPERIMENTS Apparatus Most important for carrying out the present method is the pseudo-boehmite preparation apparatus shown in Fig. 1.
A gelation vessel has a capacity of 100
liters, and is provided with a steam jacket, a stirrer and a condenser.
A
vessel is provided over the gelation vessel for introducing prescribed quantities of sodium aluminate and aluminum nitrate. The temperature of the gelation vessel was controlled by a TRC unit.
The pH
of the pseudo-boehmite slurry was measured while it was being circulated.
A
vacuum filter was used to clean and filter the gel, and a hydraulic piston extruder to
form
it.
A hot air circulation type drier was used to dry the
product, and a muffle furnace to calcine it.
water steam gelation
Fig. 1.
~~~~~to
filter
Apparatus for alternating pH swing gelation.
Raw materials An aqueous solution of sodium aluminate (20wt% as A1 Na/Al=1.6) and 20 3, ) were used as raw materials for the alumina.
aluminum nitrate (5.4wt% as A1 0, 2
633
Experimental procedures and conditions First, 40 to 60 liters of water were placed in the gelation vessel and usually heated to a gelation temperature of 100°C.
The quantity of both sodium
aluminate and aluminum nitrate was determined in the desired pH swing range from the titration curve.
The first pH swing operation started with the
addition of an aluminum nitrate in the gelation vessel and ended with the addition of a sodium aluminate.
The second and subsequent substantial pH swing
operations consisted of alternately adding an aluminum nitrate and sodium aluminate after a prescribed reaction time between each addition. In the present method, if these operations are repeated as required, the gelation procedure is completed.
The pH swing operation referred to include
a set of operations for rendering the slurry acidic and then alkaline.
The
pseudo-boehmite prepared as described above was washed until it contained not more than 0.02% by weight of Na
20
and a cake containing 20 3, was formed by filtration. This cake was
relative to A1
about 20 to 30% by weight of A1 20 3 then formed into extrudates having a diameter of Imm¢.
The extrudates were
dried at 120 DC, and calcined at 500 DC for three hours, whereby alumina was produced.
Analyses The pore volume, pore distribution and modal pore diameter of alumina were determined by a 2,000 kg/cm 2 mercury porosimeter (Model 70, Carlo Erba). The specific surface area of alumina was measured by BET method using a surface area measurement instrument (SA-2000, Shibata Scientific Instrument Co.).
The
Scherrer's equation according to the line broadening method by X-ray diffractometer (Geiger-Flex KG-X, Rigaku Electric Co.) was used to determine the crystal size of pseudo-boehmite on the (020) plane, and the crystal size of alumina on the y-alumina (440) plane.
The size and shape of pseudo-boehmite
particles were observed by an electron microscope (H-600, Hitachi Ltd.).
RESULTS Fig. 2 shows the pore distributions of aluminas prepared by the present method, and Table 1 shows their physical properties and the preparation conditions.
Fig. 2 and Table 1 are believed to indicate clearly the features
of this method.
Aluroinas A to I were prepared under substantially the same
conditions except for the frequency of pH swinging, and showed an increase in pore volume from 0.54 to 1.49 ml/g with an increase in modal pore diameter. It is clear that the present method makes it possible to control the pore diameter in a wide range. In the following description, the pore size of alumina is expressed in modal
634 pore diameter, since it is believed to represent most clearly the pore
distributions as shown in Fig. 2.
B
20
c 15
o
Ci
E F
e
OJ
o 10
G H
<,
>
5
5
10
5
100
Pore diameter (nm) Fig. 2. Pore size distribution of aluminas. * Measured by 60000 psi mercury porosimeter.
Table 1 Preparation conditions* and physical properties of aluminas Frequency of pI! swinging (n)
Crystaline size Pseudo-boehmite y-Alumina D(440) D(020) (nm ) (nm ) (nm) (ml!g) (m2 ! g) A 1 0.54** 318 5.9** 3.2 2·9 B 3 0.59 295 9·2 3.4 4.9 C 0.80 13.0 285 5 3.8 5.9 D 1.02 21. 4 9 239 4.5 7.6 e*** 10.5 232 0.75 17·0 8.3 4·7 11 E 1.08 230 28.0 4.8 8.5 F 13 1.19 177 38.5 5.3 9·9 G 15 1.38 154 51.8 11.8 5·9 H 1.43 65.3 133 17 6.4 13.6 I 120 19 1.49 14.8 79·2 6.9 The pH value was swung between 2 and 10, the reaction time was five minutes both on the acidic and alkaline sides. ** Measured by 60000 psi mercury porosimeter. *** Sample on the acidic side pH.
Sample
*
Modal pore diameter
Pore volume
Surface area
635 Effects of the pH value on the acidic side The modal pore diameter increases with an increase in the frequency of pH swing when the pH value on the acidic side is 2, 3 and
4
(Fig. 3).
It is,
however, noted that the increase in modal pore diameter shows a greatly different tendency when the pH value is 2 than when it is 3 or clarification on this difference, reference is made to Fig.
4
4.
For
showing the pore
distribution of aluminas prepared with a pH swing frequency of six times.
150
15
E
c
pH 2
"-
OJ
-i-'
OJ
10
100
E
i=l
"0
OJ 0
0
OJ
"0
50
>
0.
0 "0 0 ::;:
0
0 0
5
10
pH 4
5
15
10
Frequency of pH swing (n) Fig. 3. Effects of pH on acidic side. The pH value on the alkaline side was kept at 10, and reaction time was five minutes both on the acidic and alkaline sides. Fig. value.
4
100
Fig. 4. Pore distribution of aluminas prepared with a pH swing frequency of six times.
shows a broad pore distribution for alumina prepared at a large pH
Although this alumina has a large modal pore diameter, it also has
many small pores. pores.
50
Pore diameter (nm)
The figure indicates that fine particles result in fine
In the case of amorphous aluminum hydroxide, it is dissolved well on
the acidic side where the pH value is as low as 2. discussed
This point will be
later.
Effects of the pH values on the alkaline side It is noted that the pH value on tlle alkaline side also affected the modal pore diameter of alumina greatly (Fig. 5).
When the pH value on tlle alkaline
side was as low as about 8, the modal pore diameter ceased to increase sharply. The aluminas obtained witll tlle different pH value showed almost the same pore distribution at the same modal pore diameter.
It is thought tllat the pH value
on the alkaline side is only related to the growth rate of pseudo-boehmite
636
particles.
This is an aspect which differs from the effects of the pH in the
acidic range.
150 r----......----r---....... E
c
'-
Q)
+-'
Q)
100
E
o
"0
pH 9
Q)
'-
o
50
0.
o
"0
o
0
::E
0
5
15
Frequency of pH swing (n) Fig. 5. Effects of pH on alkaline side The pH value on the acidic side was kept at 3, and the reaction time was five minutes both on the acidic and alkaline sides.
Effects of the reaction time on the acidic side With an increase in the reaction time on the acidic side, the modal pore diameter showed a lower tendency to increase (Fig.
6).
The alumina obtained
with the shorter reaction time showed a broad pore distribution with a lot of small pores.
It is concluded that the reaction time on the acidic side is also
related to the dissolution of fine particles as the pH on the acidic side. Therefore, preparation of alumina requires both an appropriate pH value and an appropriate reaction time.
Effects of the reaction time on the alkaline side The variation of the reaction time on the alkaline side between 5 and 15 minutes did not have a clear effect on the modal pore diameter of alumina (Fig.
7).
It is seemed that the reaction time does not have any effect on the
growth of pseudo-boehmite particles.
Although the aging of pseudo-boehmite is
likely to have certain effects when it requires from several to several tens of hours,no such
effect is clearly indicated in the present process which
involves a much shorter aging time, namely five and fifteen minutes. Effects of frequency of pH on the modal pore diameter The effects which the principal working factors of the gelation process according to the present method, i.e., pH and reaction time, had on the modal
637
E
c
c
L-
ClJ +-' ClJ
E
.-
L-
ClJ +-' ClJ
100
-15 min.
.-
u
u
ClJ
ClJ
L-
0.
o 5 min.
100
E
o
0
a
150 , . . - - - - r - - - - , - - - - ,
E
150
L-
g
50
0
50
o a
U
u
a
~
o'--__ o 5
.l..-_ _......... _ _~
~
0 0
5
10
15
Frequency of pH swing
10 15 Frequency of pH swing (n)
(n)
Fig. 6. Effects on reaction time on acidic side. The pH value was swung between 2 and 10, and the reaction time on the alkaline side was five minutes.
Fig. 7. Effects on reaction time on alkaline side. The pH value was swung between 3 and 10, and the reaction time On the acidic side was five minutes.
pore diameter of alumina have been described with reference to Figs. 3, 5, 6 and 7.
All of them indicate that the modal pore diameter increases when the
frequency of pH
swing
is increased.
frequency of pH
swing
is the most important factor of the present method
It is, therefore, obvious that the
for controlling the pore diameter of alumina.
DISCUSSION Effect of alternating pH swing gelation on pore structure It is generally desirable to use alumina having limited pore distribution as a carrier for a catalyst.
The pore structure of alumina depends on the
size, shape, and aggregation of pseudo-boelunite particles. In order to obtain a narrow pore distribution, it is essential to have pseudo-boehmite particles of about the same size.When pseudo-boehmite is made by the hydrolysis of an aluminum sa.l t in an alkaline aqueous solution having a pH value of 8 to 10, it is usually impossible to avoid the formation of amorphous aluminum hydroxide to which an Uneven pore distribution is attributable.
Accordingly, pseudo-boehmite is aged on the alkaline side so
that amorphous aluminum hydroxide may be removed by occlusion into pseudoboehmite, and the pseudo-boehmite particles may be unified.
On the other hand,
the present method is characterized by swinging the pH value of a pseudoboehmite slurry alternately between the acidic and alkaline ranges during the
638
Fig. 8-1.
Pseudo-boehmite of sample A
Fig. 8-2. Pseudo-boehmite of sample E
gelation process in which pseudo-boehmite is formed.
Pseudo-boehmite is
usually far more difficult to dissolve in an acid than amorphous aluminum hydroxide.
The present method makes it possible to dissolve fine particles,
such as amorphous aluminum hydroxide by swinging the pH of the slurry to the acidic side.
The dissolved aluminum hydroxide is deposited on pseudo-boehmite
particles and contributes to their growth when sodium aluminate is added to swing the pH of the slurry to the alkaline side. If the pH of the slurry is swung repeatedly in accordance with the present method, uniformly sized pseUdo-boehmite particles are possible, thereby obtaining alumina having limited pore distribution.
Figs. 8-1 and 8-2 are
electron microphotographs of pseudO-boehmite, taken to show the effect of alternate pH swing gelation.
Fig. 8-1 shows pseudo-boehmite particles
surrounded by amorphous aluminum hydroxide.
The pseudo-boehmite in alumina A
is the product formed when aluminum nitrate was neutralized with sodium aluminate only once, while the pH of the slurry was not swung to the acidic side.
Thus, the photograph confirms that the mere hydrolysis of an aluminum
salt is insufficient in preventing the ,formation of amorphous aluminum hydroxide.
The photograph in Fig. 8-2 shows relatively uniform
pseudo-boehmite particles. hydroxide.
There is no evidence of amorphous aluminum
The pseudo-boehmite in alumina E was formed when the pH of the
slurry was swung. Pore structure of alumina Table I shows the crystal sizes of aluminas A to I, based on the y-alumina (440) plane and the pseUdo-boehmite (020) plane.
The data shown in Table 1
clearly indicate that the modal pore diameter and the pore volume increase with
639
an increase in the crystal size of pseudo-boehmite and y-alumina. Alumina e has a modal pore diameter of 17 nm, while Alumina E has a large diameter of 28 nm.
Alumina e has a pore volume of 0.75 ml/g, while the pore
volume of alumina E is 1.08 ml/g.
These differences are, however, not thought
to have resulted from the reduction in size of pseudo-boehmite dissolution on the acidic side, since they have approximately size and surface area.
particles by e~ual
crystal
Rather it is suggested that the differences are due to
the aggregation of pseudo-boehmite particles between the acidic and alkaline Taking the neutrality of the filtration cakes of aluminas E and e into
sides~
consideration, it is thought that the atmospheres employed on the acidic and alkaline sides during the gelation process have a decisive bearing on the aggregation of particles and that these characteristics are carried forward to alumina. If it is assumed that alumina pores are formed by the primary particles and the aggregated secondary particles of the primary particles, it can be said that alumina has a bimodal pore distribution having two peaks.
As shown in
Fig. 2, however, the alumina prepared by the present method has a narrow pore distribution which cannot be explained merely by the aggregation of the primary particles. If all of these facts are taken into consideration, it can reasonably be concluded that the alumina prepared by the present method retains the threedimensional network structure which is formed in the alkaline side during the gelation process.
We also know its narrow pore distribution leads to
uniformity of pseudo-boehmite particles, and that it is clear that the pH swinging operation plays an important role in the uniform growth of pseudoboehmite particles.
CONCLUSION The present alternating pH swing method was found to produce various types of alumina having narrow pore distributions.
The principal influencing
factors of its gelation process were found to have the following effects on the physical properties of alumina. (i)
The pH value and reaction time on the acidic side are important factors
for providing a narrow pore distribution, as they are related to the dissolution of fine particles, such as amorphous aluminum hydroxide. (ii)
The pH value on the alkaline side is related to the growth rate of
pseudo-boehmite crystals to the extent that the higher the pH, the faster the crystals grow, while the formation of pseudo-boehmite requires only a short reaction time, and no aging effect appears. (iii)
An increase in the frequency of pH swinging brings about an increase
modal pore diameter and pore volume.
640
REFERENCES 1
T. Kotanigawa, M. Yamamoto, M. Utiyama, H. Hattori and K. Tanabe, Applied Catalysis, 1(1981) 185-200. 2 G.P. Vishnyakova, V.A. Dzis'ko, 1.M. Kefeli, 1.F. 1okotko, I.P. 01en'kova, 1.M. Plyasova, I.A. Ryzhak and A.S. Tikohova, Kinetika i Kataliz, 11(1970) 1545-1551. 3 Ya.R. Katsobashvili, N.S. Kurkova, O.A. Bukhtenko, N.A. Akchurina and V.F. Safonova, Zhurnal Prikladnoi Khimii, 48(1975)2357-2361. 4 M.F.L. Johnson and J. Mooi, Journal of Catalysis, 10(1968)342-354. 5 D. Basmadjian, G.N. Fulford, B.l. Parsons and D.S. Montgomery, Journal of Catalysis, 1(1962)547-563. 6 H. Kanoh, T. Nishimura and A. Ayame, Journal of Catalysis, 57(1979)372-379.
641 DISCUSSION K.S.W. SING I do not believe that you are strictly correct in claiming that you are able to use your method to obtain a narrow pore distribution, even at a large pore diameter. The magnitude of the surface area values in Table 1 indicate the presence of a micropore structure in the case of the alumina samples having large modal pore diameters. M.K.L. JOHNSON: With respect to Prof. Sing's comment, I want to point out that if one calcines boehmites having crystallite sizes greater than about 10 nm, under sufficiently dry conditions, one observes micropores by nitrogen adsorption. Therefore, surface areas are greater than what one calculates from pore volume and modal pore diameter. G. MARCELIN: Did you measure the micropore distribution of particularly the large pore materials, and if so what is it like? T. aNa: (To K.S.W. Sing, M.K.L. Johnson and G. Marcelin) We have measured the micropore of under 7.5 nm for some samples by the mercury porosimeter in 60000 psi. The samples show a tailed pore distribution at the micropore region. That is, the magnitude of the micropore can be extrapolated from each pore distribution shown in Fig. 2. G. MARCELIN: Is there a difference in the observed porosity of the material before and after extrusion ? T. aNa: There is no difference in the porosity. N.P. MARTINEZ I understand that some of your interest in forming extrudates of 1 mm diameter is related to a probable use as a support for catalysts. If so, you must have a significantly high mechanical resistance, according to the method you are using. If you dried your material in an air atmosphere, because you have such a large content of water,you will get weak bodies with very low mechanical resistance and high attrition. Did you measure the mechanical resistance of your extrudates? Would you please, comment on this subject? T. aNa: We prepared the aluminas having various modal pore diameters and pore vOlumes as shown in Table 1 and measured these side-crushing-strengths. The side crushing-strength of the extrudates decreases with the increase in mOdal pore diameter and pore volume. Our extrudates have approximately the same side-crushing-strength as that of commercial hydrotreating catalysts at model pore diameter under 30 nm. A.H. JOUSTRA: Could you give an indication of the range of compacted bulk density of the materials A ~ I in the form of a defined diameter (e.g. 0.8 mm or 1. 5 mm) • T. aNa: The range of this in a diameter of 0.8 mrn. T.DES COURRIERES alumina ?
compacted bulk density is from about 0.3 to 0.7 glml
Can you adapt your method for the preparation of bimodal
T. aNa : Unfortunately we have no experience to prepare the bimodal alumina with our method. P.J. NAT: THe high frequency of pH-swing might give problems (mixing) when scaling up your process to industrial scale. Can you give a comment on this ? T. aNa: We investigated many cases of our method from a viewpoint of commercialization. The problems due to mixing in a scaling up have been solved by applying the method of the mUltipoint injection or the line mixing in a supplying of raw materials for an alternating pH-swing operation.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
643
THE PROPERTIES OF COt114ERCIAL ALU~IINA BASE f1ATERIALS AND THEIR EFFECT ON THE r4ANUFACTURE OF ACTIVE POROUS ALUmNA SUPPORTS BY l'IEANS OF EXTRUSION W. STOEPLER and K.K. UNGER Institut fUr Anorganische Chemie und Analytische Chemie der Johannes GutenbergUniversit~t, 6500 Mainz (G.F.R.)
ABSTRACT The behaviour of 60 commercial base aluminas (macrocrystalline non-porous gibbsites, microcrystalline non-porous and porous boehmites, thermally activated porous boehmites and gibbsites) was studied via paste processing and extrusion in order to produce active porous alumina pellets. The most decisive properties of the base materials with respect to extrusion were: the degree of dehyd~ation, the mean of aggregate size, the shape of aggregate size distribution, the shape of aggregates, the size of primary crystallites and the specific surface area. The relevance of these properties to those of the final pellets, i.e, crushing strength, porosity, pore volume distribution and specific surface area, were exami ned.
INTRODUCTI ON In catalyst preparation the forming operation is a necessary step to obtain pellets of the desired size and strength. Little attention is paid, however, to the effect shaping has on the porosity, pore size and specific surface area as well as the bulk and surface composition of the pellets thus influencing strongly the catalyst's activity and selectivity (ref. 1,2). This is particularly important in the design of bifunctional catalysts composed of aluminas, silicas or alumosilicates, and metals and metal oxides as deposits in which the type of starting support material, the type of agglomeration process and the operating conditions will control the final properties of the catalyst. In the majority of cases the desired properties are achieved by empirical means rather than by a systematic approach (ref. 3,4,5). The present work is concerned with the shaping of commercial alumina base materials into active porous alumina pellets by means of extrusion. The aim is (i) to establish the agglomeration behaviour of base aluminas as a function of their origin and properties and
644
(ii) to evaluate the effect of binders and additives in paste processing and ,"xtrusion on the crushing strength, porosity, pore size, specific surface area and acidity of pellets. This paper represents the initial resul ts of the investigation. EXPERH1ENTAL I·lore than 60 different grades of commercial alumina base materials were examined supplied by Condea Chemie GmbH, Hamburg, GFR, Giulini Chemie GmbH, Ludwigshafen, GFR, f1artinswerk GmbH, Bergheim, GFR, Rhone-Poulenc Industries, Salindres, Fran~e and Vereinigte Aluminiumwerke (VAW) AG, Bonn, GFR. The additives and binders employed were charcoal, cellulose i.e. Avicel-types (Lehmann & Voss & Co, Hamburg, GFR), Abrocel-types (Rettenmaier & Sohne, Holzmuhle, GFR), type 402-2b (Mikro-Technik GmbH, Miltenberg a.M., GFR), diluted solutions of nitric acid, ammonium salts and gels made by peptisation of Condea boehmites with nitric acid, as well as stabilised alumina sols. Solid components were homogenized by mixing before the binder solution was added. In paste processing the kneading was continued until the consistency of the paste remained steady. The pastes prepared were subjected to extrusion under constant operating conditions, employing a home-made single screw extruder fitted with a disc containing holes of 2.6 mm in diameter. A reduction in diameter of the wet pellets during drying at 383 K was observed to be dependent on the size of the primary crystall ites, the type and amount of additives and the extent of dehydration of base aluminas. On average, pellet size diminished to 2.3 mm in diameter, in some cases to 1.9 mm. Activation of pellets was carried out for 5 hours at 823 K for gibbsite base materials and at 923 K for boehmite types as well as for activated boehmites and gibbsites. Volatile pore-forming additives were removed during a pre-treatment lasting 3 hours by annealing at 673 K for microcrystalline cellulose and at 773 K for charcoal. Crushing strength (CS) was measured by means of an Erweka apparatus of type Ti3 24 from Erweka, Heusenstamm, GFR. On average 30 to 35 pell ets from one batch were employed to assess the mean CS. Total pore volume and porosity of the extrudates were calculated on the basis of apparent mercury and helium densities by applying a helium and mercury pycnometer (ref. 6). Pore volume distribution was measured by means of a horne-made porosimeter applying pressures up to 4.5 kbar and using a contact angle of mercury at 293 K of 1400 and a surface tension of mercury at 293 K of 480 x 10-3 N/m.
645
RESULTS AND DISCUSSION Assessment of the extrusion behaviour of base aluminas The commercial base aluminas employed in these examinations differed widely in their phase and surface composition, the size of primary crystallites, the shape and size distribution of aggregates, the specific surface area, pore size and content of inorganic impurities. The degree of crystallinity, the mean aggregate size, shape of aggregate size distribution and the water content i.e. the degree of dehydration of the starting material, were found to be the dominant properties in the paste processing and extrusion. Accordingly the base aluminas were grouped into 3 categories: group a: macrocrystalline non-porous materials of gibbsite type, Al(OH)3 conta i ni ng 35 % (w/w) of wa ter group b: macrocrystalline non-porous materials of boehmite type, A10(OH) containing 15 % (w/w) of water microcrystalline materials of boehmite type, containing 22 % to 26 % (w/w) of water group c: macrocrystalline porous dehydrated gibbsite of group a and thermally activated boehmite of group b containing 1 % to 13 % (w/w) of water· Aluminas of group b were converted into extrudable pastes simply by peptisation with dilute acid solution~, while group a and group c materials achieved sufficient paste plasticity on addition of alumina gels or sols, charcoal and organic polymers, i.e. starch, cellulose, PVA, CMC. The additives in this particular case acted as a lubricant, thus reducing the tensile strength forces between the original particles when compacted during kneading. The superior shaping behaviour of group b materials may be attributed to their aggregate structure consisting of extremely small primary particles of about 10 nm size. The plasticity of pastes obtained from microcrystalline boehmites improved with increasing binder acid concentration. However, when a certain concentration of peptisizing acid was exceeded, without pore-forming additives, the paste im~ediately became sticky and could not be extruded. Another distinct difference in the behaviour of group b compared to group a and c materials was the pronounced shrinkage of the wet pellets during drying, at 383 K which may be attributed to the relatively high water content and microcrystallinity of the Condea boehmite. While boehmites usually contain 15 % (w/w) of chemically bound water as hydroxyl groups, the additional amount of 10 % (w/w) consists of physically adsorbed water held between the octahedral boehmite layers (ref. 7). In comparison, pellets of group a and c were subject to only 6 % shrinkage in diameter. Table 1 shows the data of pellets made from various base aluminas under comparable operating conditions. It is interesting to note that the crushing strength of dried, non-activated
TABLE 1 Data of non-activated and activated pellets made from various base aluminas under comparable operating conditi ons (% (w/w) of binder is related to mass of alumina) base aluminas
group of classification
additive and binder
designation of pe11 et
CS of dried nonactivated pe11 ets (N/mm 2)
CS of dried and activated pellets (N/rrrn 2 )
tota 1 spec ifi c pore vo1ume V (t)
(~l/g)
pellet porosity
(%)
1
61
±
5.4
14
±
2.3
0.67
69.9
a
10 % (w/w) of Avicel 15 % (w/w) of Pural NG 1.27 9 HNOf cone. per 100 9 of a umina same as in 1)
2
78
±
7.4
24
±
2.3
0.63
68.1
a
same as in 1)
3
57
±
8.2
11 ±
0.5
0.66
68.6
b
1.27 9 HNOf cone. per 100 9 of a umina same as in 4)
4
37
±
5.3
73
±
10.1
0.59
66.1
5
19
±
2.7
49
±
6.2
0.51
63.1
6
44
±
6.1
25
±
3.8
0.74
71.0
c
1.27 9 HNOf cone. per 100 9 of a umina 10 % (w/w) of Avicel 15 % (w/w) of Pural NG same as in 6)
7
40
±
3.9
17
±
2.1
0.86
75.1
c
same as in 6)
8
49
±
6.1
18
±
2.7
0.75
71.9
Hydrargi 11 it LH 20-15 VAW
a
Hydrargillit LH 20-35, VAIl Apyral 120 VAW rural SCF Condea Pural NG Condea Apyra 1 120 VAW pre-activated at 583 K Pural fJG Condea pre-activated at 823 K Pura1 SCF Condea pre-activated at 823 K
b c
0>
....
0>
647
pellets of group a and c diminished on activation, while for the pellets of group b the reverse occurred. The reinforcement of dried non-activated group b pellets during calcination is again caused by the high content of physically bound water still present after drying. It must be emphasized that the crushing strength is not an independent variable but is closely related to the specific pore volume and pore volume distribution, particularly when Vp exceeds 0.5 ml/g. Recognizing this relationship the low crushing strength of group c aluminas is explainable in terms of the specific pore volume which ranges from 0.74 to 0.86 ml/g. I~ view of the wide divergence in their properties the feasibility of processing the various commercial base aluminas into extrudable pastes of the same composition is slight. Effect of pore forming additives on the extrusion properties of alumina pellets Examinations were carried out on group b-aluminas because these could be pelletised without as well as with pore forming additives. Some of the obtained results are listed in Table 2. In the first three experiments, referring to pellets No 9 to 11, the influence of the binder acid concentration on pellet properties in paste processing is illustrated for Pural SCF. No additives were employed and all other conditions were constant. With increasing nitric acid concentration the total specific pore volume decreases, whereas the crushing strength changes in the reverse order. The binder acid concentration also significantly controls the pore volume distribution curve as shown in Fig. 1. While the first two preparations of pellets No 9 and 10 showed distinct bimodal distributions, the large pores of 0 > 15 nm completely disappeared in pellet No 11. In this latter case the binder acid concentration appears to be so high that the aggregate structure collapses. Below this limiting value slight but significant effects of the acid concentration on pore size formation are observed in the macroporous and mesoporous size range. The results obtained for Pural SCF can be generalized to indicate that a critical maximum binder acid concentration exists for each microcrystalline boehmite type at which the macroporosity and a large proportion of the mesoporosity vanishes. This critical acid concentration is a direct function mainly of the mean of aggregate size, of the shape of aggregate size distribution and to a lesser extent of the shape of aggregates and size of microcrystallites. Working with nitric acid as binder, concentrations of 0.005 mol HN03/mol microcrys ta 11 i ne boehmi te up to 0.07 mol HfW 3/mo 1 mi crocrys ta 11 i ne boehmite were used. The concentration of nitric acid employed in the experiments was dependent on the boehmite base material and the desired pore volume distribution. The different peptizing effect of various organic and inorganic acids on microcrystalline boehmites base materials depen~on the strength of the acid and
...
IS>
TABLE 2 Data of activated pellets made fro~ boehmites of group b with different paste conditions pellet diameter: 2.0 mm; pellet length: 5 - 6 mm D = most frequent pore diameter of the relative pore volume distribution curve max
base aluminas
additive and binder
designation of pe11 ets
tota 1 specifi c pore volume Vp( t) (ml/g)
proportion of V (t) for a given pore diamgter range in (%) 15 nm 15-500 nm 500 nm
00
pore size maxima ( 1) (2) Dmax Dmax (nm) (nm')
CS (N/mm
2)
Pural 5CF Condea
1.27 9 HNO conc. per 100 9 of a umina
r
9
0.59
56
18
26
8.0
1960
73
±
10.1
Pural 5CF Condea
1.92 9 HNO conc. per 100 9 of a umina
r
10
0.45
72
5
23
8.2
1060
81
±
12.6
Pural 5CF Condea
2.54 9 HtlO conc • per 100 9 of a1umina
11
0.39
97
3
0
9.1
-
120
±
21.1
Pural 5B Condea
2.54 9 HN03 conc. per 100 9 of alumina
12
0.52
75
11
14
8.7
-
66
±
10.9
Pura1 SCF 2.54 9 HNOf conc. per Condea 100 9 of a umina 10 % (w/w) of 'charcoal
13
0.69
65
3
32
10.8
1160
20
±
3.7
Pura1 SCF Condea
14
0.48
95
4
1
9.7
-
94
±
20.6
2.54 9 HN03 conc. per 100 9 of alumina 10 % (w/w) of Avicel
649
the tendency of the anion of the acid to form complexes with aluminum. Preferable are strong acids whose anions are non-complexing or very weakly complexing with aluminium. Equivalent acidities are also produced via aqueous solutions of acidic salts (ref. 8).
0~
0
~
E
~x o·
°I;l
:::J
O.
~ Q..
0
>
0
o.
o o
o
o~
o
LI'l
x
---~-----------------------------------0 0
DO
.
0 0
O. x x
0
.
0
DO
x •
0
0
0
..• .
0
0
0 0
0 0 0
101
...
•
0
~ 0
0
0
10 2
0
o
0
0
10 3
0
o.
o •
O(nm) 10 4
Fig. 1. Cumulative pore volume distribution of pellets No 9 (0), 10 (x) and 11 (0) determine:J by means of mercury i ntrus i on The next two preparations, No 11 and 12, are obtained at the same binder acid concentration; however, the boehmite types differ from each other in their physical properties. Both aluminas have a similar specific surface area but vary in the shape of the size distribution curve of their aggregates: Pural SCF possesses a homogeneous, even distribution with a mean size of 21 u m while Pural 5B exhibits an uneven distribution with a mean size of 60 fJm. Compacting the peptisized Pural SB aggregates yields a much more open pellet structure than Pural SCF. Accordingly the pore volume distribution of Pural SB pellets resulted in larger pores compared to Pural SCF (s. colu~n 5 of Table 2). The last two examples (13 and 14) demonstrate the effect of the type of poreforming additive on the pellet porosity. The charcoal in No 13 was substituted by microcrystalline cellulose in [10 14 at otherwise constant condi t iuns . In contrast to charcoal, cellulose has a less pronounced effect on large pore formation.
650
Compared to the pellets made only with a binder acid without an additive, (pellet ~jo 11), the specific pore volume increases slightly from 0.39 ml/g to 0.48 ml/g in the presence of cellulose. The most frequent pore dia~eter remains practically unchanged. The cumulative pore volume distribution of all three pellets (No 11, 13 and 14) is shown in Fig. 2.
0~
0
~
~
;
E
o
:::l ~
.0
0 0 0
0-
>
c! 0
o
0
n
---------------------------------------x o
0
o o o
000
o 0
0 0
o
o
o
x
c
o x
ox
~
o
Ox
oliCOX' OX xo 0 x OX)( C>'-1-_ _.,.-_~C_.!<..:.;...~..;_:~---:y......l~:.......~:.......~~~~......l~~~~~.::::...>~~::.. J(
)(
o
x
)( 0
J(
0 xO
OxO 0 "0 ox 0011
D(nm) 10 4
Fig. 2. Cumulative pore volume distribution of pellets No 11 (0), 13 (0) and 14 (x) determined by means of mercury intrusion The experiments indicate that for a given boehmite type the binder acid concentration and the type and concentration of the pore forming additives resul t in a specific pore structure and porosity of the pellets. For each boehmite studied, a maximum crushing strength of the pellets was achieved only by altering the binder acid concentration without adding a poreforming additive. The crushing strength was observed to decrease steadily in the sequence Pural SCF > Pural SB > Pural NF > Pural NG > Dispural. This behaviour appears to be caused by the shape, composition and size distribution of alumina aggregates, as mentioned before.
651
CONCLUS IONS Although a quantitative relation cannot yet be established between structure properties and extrusion behaviour of aluminas. There are strong indications that paste formation and extrusion are controlled principally by the size of the primary particles and their aggregations. Illustrating the two extre~sare gibbsite, obtained by the Bayer process and consisting of aggregates of 1 u m crystallite" size yielding paste of barely extrudable consistency and boehmites of the Condea type, composed of aggregates of about 10 nm primary particle size. The above supposition is strongly supported by the fact that Alu C of Degussa, Hanau, GFR, being a pyrolytic alumina of about 10 nm particle size containing 0.5 % (w/w) of chloride can be pelletized easily only when water is employed as pelletizing liquid. The excess of water of the Condea boehmites and their microcrystalline size additionally favour the paste formation process. Activated boehmites and gibbsites behave differently in paste formation which may be attributed to their low water content. It was demonstrated that unfavourable extrusion properties can be overcome by adding boehmite, alumina gels and sols as binder and poreforming additives. REFERENCES 1 G.Berrebi and Ph. Bernusset in B. Delmon, P.A. Jacobs and G. Poncelet (Eds.), Preparation of catalysts, Elsevier, Amsterdam, 1976, PP 13-38 2 D.L. Trimm (Ed.), Design of industrial catalysts, Chemical Engineering f'1onographs, Vol. II, Elsevier, Amsterdam, 1980, pp 3-36 3 DPS 23 49 773 (18.04.1974) 4 DAS 25 11 967 (09.10.1975) 5 DOS 26 39 285 (02.03.1978) 6 K.K. Unger, St. Doeller and K.F. Krebs in S.J. Gregg et al. (EdsJ, Character. Porous Solids, Proc. Symp., 1978, pp 291-299 7 R. Tettenhorst and D.A. Hofmann, Clays Clay Miner., 28(5) 1980, pp 373-380 8 B.E. Yoldas, Ceramic Bulletin, 54(3), 1975, pp 289-290
This page intentionally left blank
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
653
INFLUENCE OF ALUMINIUM HYDROXIDE PEPTIZATION ON PHYSICAL PROPERTIES OF ALUMINA EXTRUDATES K. JIRATOVA, L. JANACEK+ and P. SCHNEIDER Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6, Czechoslovakia
ABSTRACT Physical properties of alumina extrudates prepared from differently peptized aluminium hydroxide were studied. The effect of different peptizing acids on the physical properties of extrudates can be generalized by using the Hammett acidity function, Ho' of the peptization solution. In the range of 0 < Ho < 1, the physical properties of extrudates change most significantly. The mean radii of transport pores were determined by combining the permeation and countercurrent diffusion measurements. Correlation was proved between the mean radius of transport pores, calculated from diffusion measurements, and the volume of macropores.
INTRODUCTION Heterogeneous catalysts used in chemical industry must possessa well-defined geometric shape and convenient physical and chemical properties. As the most important are considered sufficient surface area, mechanical strength, and low resistance to internal diffusion which depends on porous structure. Recently, catalysts are manufactured very often by extrusion. This procedure consists in mixing dry aluminium hydroxide with a small amount of water and peptizing agent. By kneading, a paste is formed which is extruded through nozzles and the extrudates are dried and calcined. Extrusion permits production of catalysts and supports with smaller dimensions and at lower expenses than pelleting. For extrusion, it is necessary to prepare the mass of pasty consistence which is sufficiently plastic. The plasticity is reached by different ways (ref. 1); in case of aluminium hydroxide, the +Chemical Works, Research Institute of Hydrocarbon Utilization, 436 70 litvinov, Czechoslovakia.
654
'chemical dispersing (peptization) by inorganic or organic acids or hydroxides (refs. 2-S) is used most frequently. In the course of drying and calcining, the extrudates shrink, which leads to the formation of macropores; however, simultaneously also the mesopores in the primary aluminium hydroxide particles can be influenced. Consequently, by changing the amount and type of peptizing agent, the porous structure and therefore also the mechanical strength o~ extrudates can be influenced. This work continues our recent study (ref. 6), and its aim is to find how the amount and type of peptizing agent and the size of aluminium hydroxide particles used influence the physical properties of extrudates, namely their porous structure. EXPERIMENTAL Preparation of alumina extrudates Commercial aluminium hydroxide Pural (Condea Chemie, FRG) of boehmitic structure was used. The characteristic dimension of particles d SO was 18, 32, 47, and 19S ~m (d SO is that particle diameter for which SO % wt. of particles are smaller than d SO)' The aluminium hydroxide paste was prepared at laboratory temperature by kneading for 1 hour 2S0 g dry aluminium hydroxide with a chosen amount of peptizing agent and such an amount of water that it should be possible to extrude the resulting paste by a piston extruder at pressure 4 MPa through 2 mm nozzles. The amount of water added varied between 80 + 20 cm 3/100 g aluminium hydroxide. The kneading procedure, i.e. the kneading machine (Becken, FRG), the rate of mixing, and order of adding the components was always the same. The extrudates were dried at l20 0C and calcined for 4 hours at 600 0C. Evaluation of physical and mechanical properties of extrudates The BET surface area was determined from the low-temperature nitrogen sorption by measuring several points of adsorption isotherm in the range of relative pressures 0.05 to 0.3 at liquid nitrogen temperature using modified method of De Baun and Fink (ref. 7). The volume and distribution of mesopores (pores with radii 2 - 15.8 nm) were determined from the adsorption isotherm of benzene. The volume and distribution of macropores (pores with radii lS.8 - 6 500 nm) were determined by mercury porosimetry (porosimeter Carlo Erba, model 6S A). The mechanical strength was determined by gradual loading the pellet with an edge in normal direction to the cylindrical surface of extrudates and is given by the force which breaks a pel-
655
let of unit diameter. It is expressed in N.m- l. Determination of transport pores The mean radii of transport pores were determined by combining the permeation. and countercurrent diffusion measurements (refs. 8-10). Fifty pieces of extrudates were vertically mounted in a horizontal silicon rubber plate 4 mm thick in which holes with diameter about 0.2 mm smaller than measured particles were cut out. In th}s way, the needed tightness between measured particles and the silicone plate was ensured. RESULTS AND DISCUSSION Peptization of aluminium hydroxide particles of various size by acetic acid The samples of aluminium hydroxide with particle sizes d 50 = 17; 32; 47; and 195 ~m were used for measurements. The dependence of mechanical strength of extrudates on the amount of acetic acid added is plotted in Fig. 1.
........,
8
'E E
b
CL
4
6
(CH 3COOH) [% wt.]
Fig. 1. Dependence of the extrudate mechanical strength P on the amount of acetic acid (% wt.) for different sizes of aluminium hydroxide d 50: ~ - 17 ~m; ~ - 32 ~m; • - 47 ~m; 0 - 195 ~m The mechanical strength reached on using the same amount of acetic acid increased with decreasing aluminium hydroxide particle size. In the investigated region, a linear dependence holds approximately between the particle size and the amount of acetic acid required to reach a chosen mechanical strength (Fig. 2).
656
s:o a
u
'"
I: U
[,um]
Fig. 2. Dependence of peptizing agrnt amount required to reach mechanical strength 1; 2; 4; 6 N mm- on the aluminium hydroxide particle-size d SO Because of the strong dependence of mechanical strength of extrudates on the size of primary particles of aluminium hydroxide constant particle size was used in the following measurements. Comparison of different peptizing agents The effect of various acids on the aluminium hydroxide peptization was evaluated from the properties of calcined extrudates. The aluminium hydroxide used had the particle size d SO = 32 wm and the peptizing agents were sulphuric, nitric, hydrochloric, trichloroacetic, phosphoric, oxalic, lactic, and formic acids. The dissociation constants of the acids used varied from -6.1 to +3.8 (ref. 11). The amount of the peptizing agent used during kneading varied from O.OS to S wt. % of dry aluminium hydroxide. It is evident from the left-hand part of Fig. 3 that with increasing amount of acids in the paste, the mechanical strength increases and the macropore volume decreases. The mesopore volume and surface area change only slightly, except at high concentrations of the peptizing agent. The results obtained are in agreement with following concept of formation of porous structure of solids. The irregularly shaped primary particles of aluminium hydroxide contain certain amount of mesopores. By compacting the primary particles, the macropores are formed between them. Aluminium hydroxide peptization makes possible a closer arrangement of primary particles so that the free vo-
657
lume among particles in the paste decreases. Therefore after drying and calcination the macropore volume in extrudates decreases. On using a larger amount of peptizing agent, deeper layers of primary particles react and thus the mesopore volume decreases.
15r----,------,------,;:Jrr-:rr--,--,---,-ff-,--, p N/mm 10·
S m2.jg
@
@
Vme
V cm3/g
o
4
m [Ofo wt']
Fig. 3. Dependence of mechanical strength P, surface area S, and pore volume V , V on the amount of peptizing agent in paste m and on the va~~e o~aH : (J - HC1, () - HF, 'l) - HCOOH, • - HNO , @ (COOH)2' e - H3P0 4, 0 0 _ H2S04, ~ - CC1 300H, ~ - CH 3CH(OH)COeH It was found experimentally that different acids have different effects especially on the macropore volume. This is probably connected with their ability to react with aluminium hydroxide which depends,ingeneral,on the co nc e n t r a t i on of H+ ions. Since concentrated solutions of acids (up to 3 mol 1-1) are used for peptization, it is not possible to express the acidity of peptization solutions by a quantity appropriate for diluted electrolytes. Therefore, we have characterized the concentration of hydrogen ions in the peptization solutions with concentrations higher than 0.1 mol 1-1 by the Hammett acidity function Ho (ref. 12). H for hydrochloric, o sulphuric, and nitric acids were taken from Paul and Long (ref. 13), for phosphoric, hydrofluoric, and trichloroacetic acids from Rochester (ref. 14) and for formic acid from Milyaeva (ref. 15). Ho for mildly concentrated solutions of oxalic and formic acids were calculated from the relation
658
Ho - 1/2 pK a - 1/2 log c a proposed by Randles and Tedder (ref. 16). On the right-hand side of Fig. 3, the physical properties of extrudates in dependence on Ho of peptizing agent are plotted. When peptizing by strong inorganic acids and trichloroacetic acid, the physical properties of extrudates in the region Ho > 1 do not change and approach the values of aluminium hydroxide kneaded with water (H o = 7). For 0 < < H < 1, significant changes in physical properties of extrudates o take place. With ~ecreasing Ho' the macropores vanish and the mechanical strength increases, which proves that both these properties are mutually connected. The mesopore volume changes only slightly. The effect of Ho on the specific surface area depends on the anion of peptizing acid. Sulphuric and phosphoric acids, which form non-volatile compounds with aluminium hydroxide, increased the surface area unlike the acids whose anions are almost completely removed by calcining. Since the dependences for the surface area and the mesopore volume are not similar, we assume that by the effect of sulphuric and phosphoric acids, a change in the micropore volume (i.e. pores with radii < 1.7 nm) takes place. With a further increase in the acidity strength of peptizing agent below Ho < 0, the macropores dissappear, and the mesopore volume started to decrease more significantly. However, the mechanical strength did not increase any more, as it might be expected, and, on the contrary, after exceeding Ho < -0.25, it strongly decreased. On extruding such a peptized paste, a high internal stress arises after calcining which manifests itself by longitudial splitting of extrudates after their loading by an edge, and consequently, in a striking decrease of the mechanical strength. The aluminium hydroxide peptization by hydrofluoric acid and weak organi c aci ds takes pl ace in a somewhat di fferent way than wi th strong inorganic acids. The decrease of macropore volume and the corresponding increase of mechanical strength appears at higher values of Ho' The region of significant changes in physical properties of extrudates, localized to Ho equal to 0 - 1 in case of peptizing by strong acids, extends to H equal to 0 - 3 for weak orgao nic acids. Thus, it follows that besides the H+ ion concentration, also the nature of the acid anion and of the undissociated acid plays a role in peptizing aluminium hy d r ox i de Oreoof the possible explanations is based on the physical adsorption of anions and undissociated molecules on the aluminium hydroxide particles. It is well-known i
659
that adsorption of polar particles and ions on solids proceeds more easily and to a greater extent than adsorption of nonpolar molecules. Anions of strong inorganic acids and their undissociated molecules are more polar than organic anions, and apparently are adsorbed in larger amounts than organic anions and undissociated acids. This accounts for a less dense packing of primary particles in the dispersion system with strong inorganic acids so that the macropore volume is larger than with weak organic acids. This is supported also·by similar peptizing effect of strong trichloroacetic acid and strong inorganic acids. Effect of peptization on porous structure We have shown that peptization influences considerably the macropore volume and the mechanical strength. It was therefore interesting to investigate how the distribution of pore sizes was changed by peptization. As an example, the pore size distributions of extrudates peptized by various amounts of sulphuric acid (0 < Ho < < 2.2) are illustrated in Fig. 4.
nm
Fig. 4. Pore size distributions of extrudates peptized by various amounts of sulphuric acid m (wt. %) It is evident that the size.of the mostfrequent mesopores (radius -5 nm) is nearly independent of the acid amount. Certain changes are
660
apparent in the macropore region, however, it is not possible to quantify them accurately. Tischer in his work (ref. 17) made similar observations. For a deeper characterization of porous structure of differently peptized aluminas, we have determined the m~an radii of pores (refs. 8-10) through which the transport of gases takes place (transport pores). The resulting parameters r~, ~ characterize the transport of ~ases through the porous medium. On the left-hand part of Fig. 5 it is shown how these parameters vary owing to the acidity of peptization solution Ho' 3000
.!:..!f
v
2000 1000 0
v 0.05 0 200 rip
100 0
-1
Fig. 5. Dependence of parameters r~, ~, and r~/~ ~n H of peptization solution and on the macropore volume V a (cm g-lo) for various peptizing agents: ~ - HF, ~ - HCOOH, • - HN~3' ~ - H20, 0 - H2S04, o - CC1 3COOH, 0 - (CH3COO)2Zn It can be seen that parameter r~ decreases on deeper peptizing, the decrease being steeper for peptization by weak acids. For non-peptized aluminium hydroxide r~ = 270 nm. The lowest measured value of r~ of peptized aluminium hydroxide was 3.7 nm. Analogously to the dependence of macropore volume on Ho' higher values of r~ were observed for extrudates peptized by trichloroacetic acid. The geometric transport parameter of extrudates, ~, decreased
661
due to peptization from original value 0.095 to 0.002 and exhibited a similar S-shaped dependence on Ho as the parameter r~. By combining parameters r~ and ~, the mean radius of transport pores, (r~/~), was evaluated. Peptization decreased its value from 2 900 nm to 300 nm. We estimate that the radius of transport pores of this type of alumina can range from 600 to 1 200 nm for the usual regions of peptization. It follows from the similarity of dependences r~ vs. Ho and Vma vs. Ho khat the macropore volume is decisive for the size of transport pores. This assumption is confirmed by the right-hand side of Fig. 5: parameters r~ and (r~/~) show an exponential increase with the macropore volume Vma' whereas the geometric factor, ~, increases with Vma linearly. CONCLUSION It has been shown, using aluminium hydroxide Pural, that the size of primary aluminium hydroxide particles influences substantially the physical properties of extrudates prepared from it. Physical properties of differently peptized extrudates are dependent on the Hammett acidity function of peptized solution, Ho' The de~~n dence of the mechanical strength of the extrudates 'In the macropore vol ume has been proved. Although the conventional pore si ze di stri bution does not show a significant dependence on peptization, the transport parameters r~, ~, and (r~/~) allow to prove that a decrease of mean radii of transport pores takes place with decreasing Ho of peptization solution. A simple dependence of parameters r~, W, and (r~/~) on the macropore volume was found. It is necessary to state that the dependences obtained hold for the type of aluminium hydroxide used. Due to large variability of properties of aluminium hydroxides, it is possible to expect different values of physical properties o~ extrudates originating from aluminium hydroxides prepared by different methods. ACKNOHLEDGEtlENT The authors thank Mrs. J. Aunicka and Mrs. V. experimental assistance.
Ne~kodna
for their
SYMBOLS concentration of peptization solution (mol 1-1) mean diameter of primary particles of aluminium hydroxide (Il m)
662
Hammett acidity function dissociation constant of acid quantity of peptizing agent related to dry aluminium hydroxide (wt. %) mechanical strength of extrudates (N mm- l) P radius of most frequent pores (nm) r transport parameters for Knudsen and bulk diffusion, resp. r
663 DISCUSSION K.S.W. SING It seems likely that strong specific retention of anions will occur under the peptization conditions studied. If this is the case, the presenCe of the anionic impurities will affect the changes in crystallinity and pore structure which occur during drying and calcination. I suggest that to explore further the direct effect of peptization, the calcination stage should be omitted. K. JlRATOVA I agree with this suggestion. However, we are interested in the integral effect of impurities on the properties of the finished catalyst carrier.
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G. Poneelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Fom\~TION
665
OF SILICA GEL POROUS STRUCTURE
V.A. FENELONOV, V. Yu. GAVRILOV and L.G. SIMONOVA
Institute of Catalysis, Novosibirsk (USSR)
ABSTRACT. Silica gel pore structure formation has been analyzed on the basis of the general physico-chemical and structural geometric description of the characteristic steps, of processes and mechanisms. 1any of these mechanisms are typical for the genesis of various catalysts and supports obtained by precipitation and so can be used for their stUdy.
INTRODUCTION Silica gel - a well-known adsorbent and catalyst support (ref. 1-4) - is at the same time a relatively simple system which can be used as a simplified model for the analysis of the porous structure genesis for more complex disperse materials. However, mechanisms of the silica gel structure formation at different steps of synthesis are not yet definitely established (refs. 3,4l Ou~ objective is to describe the general physico-chemical properties of such mechanisms based on the analysis of two limiting regimes of synthesis: 1) consecutive realization of the main steps: a) hydrolysis of alkaline silicates to produce low-molecular forms of Si0 2 (LMS) , b) formation and ageing of sols, c) formation and ageing of gel, d) washing and drying of gel, e) thermal treatment of xerogel; 2) simultaneous realization of the steps "a-c" at a continuous introduction of initial reagents and removal of hydrogel, followed by consecutive steps "d-e". The peculiarities of the silica gel structure formation can be explained in terms of the structural-geometric and surface-capillary concepts (refs. 5-8). Let's distinguish between the two main groups of processes: i) transfer of low-molecular weight states (LMS) of Si0 in the form of silicate ions, molecules, 2
666
incomplete hydrolysis products, low condensation polymers etc., and ii) transfer of 8i0 2 globules and aggregates. Transfer processes (il~S) determine formation and growth of g Lobu.Lee and are due to the dependence of the chemical potential ...26v ,. on curvature of the surface area ,of the solid phase f42 :(H-.- T where ttl" and jli., are chemical potentials of a flat and curved surface; eI and" are respectively the interfacial surface energy and molar volume of the Si0 2 phase; 'l. is the radius of curvature (negative and positive for concave and convex surfaces, respectively). Between the surface sites with different curvatures there grad.ient dfM that determines the direction of LJJiIS transfer (ref. 1). The rate-determining steps of il~S transfer may be formation, diffusion or condensation of these states. The correlation observed (refs. 1-9) between the surface area and solubility of Si0 2 suggests that the rate-determining step is usually that of the formation of mobile groups of il~. Such a formation is usually due to the weakening of siloxane bond upon temperature rise or interaction of Si0 with the medium components, impuri2 ties of alkaline metals, for example. In the syster,l of isolated globules ,the gradient d(H leads to the transfer of LI.IS from small to large globules (recondensation or Ostwald ripening mechanism, abbreviated MRC). In the system with directly contacting globules, LMS must initially transfer onto the sites with the negative radius of curvature (the crevice near points of contacts between particles) and then from loose to close-packed sites (i.e. from large to small cavities between particles)(refs. 5-7). Wetll call this LMS transfer mechanism the globules-grow together mechanism or cementing mechanism (MC). The precipita tion of large amounts of LJVlS in a particular geometrically restricted space between globules may result in the appearance of appreciable volume of ultramicropores showing molecularsieve properties (refs. 6-9). The appearance of these micropores is due to a loose packing of LMS in cavities between globules (mechanism of globular cementing with micropores formation, (MC-
MPF». Migration of particles_caused by electrostatic, capillary, gravitation and other effects, occurs upon variation of the equilibrium conditions and determines the porosity of the particles packing (8 ). The minimum porosity of a dense random packing of particles of any size (e min) is 0. ,36-0. 40. For the aggregates of globules cemented together, £min >0.4 and tends to increase with
667
the size and degree of branching of aggregates (e.g., for chains composed from 5-6 globules £min = 0.5-0.6). In turn the average pore (pore throat) size d is related to the average globule size 'l) and porosity e through d .... O. 6.0 l-£e • The specific geometric surface S (m2 j g) is independent of e and is determined by size 2) through relation S = "R~ where R is the density of a solid. Therefore, the recondensation processes have no effect on mi n , while MC-processes leading to the formation and strengthening of aggregates are accompanied by the increase of c mi n ' Note also that the vol~me of the space between glob~les, V, expressed in cm3jg Si0 2 is V= ~(j~) • Now we'll use these general ideas to analyze the peculiarities of the formation of silica structures in various regimes.
e
OF THE SILICA GEL STRUCTURE IN REGI1lli I Formation of the silica structure at consecutive realization of the main steps of synthesis is represented by Scheme I.(Fig.1). Step of sol determines the maximum surface area So' At this step sol globules are surrounded by ion-solvate shells of thickness t that prevent the direct contact of globules. "Protective" properties of the shells are determined by the type and composition of electrolyte and by other components of intermiscellaneous medium. The main processes at this step are: formation of sol globules and their growth via the mechanism of recondensation (I.1RC) or IJ.rs deposition from intermiscellaneous solution or Lr.rs introduced from the external source (refs. 1,5). The size of g LobuLes (D ) may vary from 2-4 nm to 100-200 nm, i.e. up to the limit of sedi.rrentary s ta bility • At the step of coagulation a gel is produced with the volume of the space1:etween the globules being V0 = ~~c cm3/e Si0 2, where C is the concentration of Si0 gjg gel, ~ 4 is the density of the 2 intermiscellaneous solution. Vo determines the upper limit of the xerogel porosity (Fig. 1) and depends on the conditions of synthesis. Step of gel ageing includes the processes of syneresis, leading to the decrease of V , and formation of immediate contacts a between globules (irreversible coagulation) and transfer of Llffi, accompanied by the decrease of S and increase of mi n (Vmin)(ThffiC and MC mechanisms). Such processes are more intense in hydrothermal conditions. Fig. 2 illustrates the interrelation between S and pore volume of xerogel, VJ; , obtained from hydrogels subjectFOR1~TION
e
668
%.
n I " I \ \
\
-1-, I
\
I
\
I
I
\
\ \
\
\ \ Sof
Fig. 1. Variations in surface area (S/$o ) and porosity (V/V ) at different steps of synthesis of silicas; 1,2 - variations in°surface area; 3,4 - upper, 5,6 - lower limits of porosity; 2-5 variations in structure under conditions favoring LMS transfer; 1,6 - in the absence of transfer; 3 - change of the upper limit of porosity in the absence of volume contraction, 4 - in the presence of contraction. ed to hydrothermal ageing. Ageing in acid medium leads to the formation of contacts and cementing together of globules to aggregates (Fig. 2), c,d, MC mechanism), and decreasing of the surface area. Increase in the size of such aggregates leads to the increase of e. and Vz upto V0 • Ageing in the alkaline medium m~n or water layer occurs initially via the same scheme but more intensely. At higher temperatures a partial peptization of the 0e1 yields branched aggregates of globules,. which is accompanied by a decrease of VEo • This phenomenon is due to the intense transfer of LMS between aggregates (MRC) and in their volume resulting in the transformation of branched aggregates into macroglobules with low values of Emin (Fig. 2,e). Such gels may have an appreciable volume of ultramicropores caused by the loose LMS packing (MC-MPF transfer). Ageing at 2()-25°C occurs at a far slower rate, which is
669
v. ern; "
E.T
g
a
e.e- • •
20
"
~-\-.
" "
,
C
<.J
e
I
'0
0' Ol
Fig. 2. Variations in structure of xerogels at hydrothermal ageing of hydrogels in 1) acid, 2) neutral, and J) alkaline media (according to ref. 9). a-b-scheme of recondensation (1ffiC process~ c-e - scheme of cementing globules mechanism (1~ process). especially slow with the globules having "protective" shells or in the media where Si0 2 solubility is negligible. Step of gel washing prior to drying affects the formation of the structure essentially because of the change in medium where the transfer processes take place during the step of drying. Ageing processes continue to occur also. Step of gel drying. Formation of the structure at this step is determined by the combined action of capillary forces leading to a more dense packing of particles (particles transfer) and LMS transfer. For the sake of convenience let's outline three characteristic steps of the step of drying. Step of drying I detennines the total porosity of xerogel Vt and continues until the evaporation front passes into the grain volume (Fig. Ja,b). Forn~tion of menisci of liquids at the evaporation boundary is accompanied by the appearance of capillary pressure P which reaches the maximum value P = 2o:.CasG ~ c
c
LL
670
I
il
'l(f"oveJ
1--~i------t-7~
If"
'1.11.0
I I
I \
Fig. 3. Scheme of xerogel structure formation during drying of hydrogel: step of drying I (a-b); step I I (c-d), step I I I (e-f); ~~ is the change of the rate of drying; F5 is the value of contraction capillary forces; ~~ is the extent of deformation; PIPs is the relative pressure of vapor. = 3.3
6'di - c:) Cos e
2> Co at the moment of passing of menisci through the pore "throat" (r and 6 L are respectively the radius of curt vature and surface tension of the liquid phase; e is the wetting angle). The volume of gel, Vg, is fl-1+VL ' where VL is the volume of the liquid phase (cm 3/g SiO ). The decrease in V is par2 L alleled by volume contraction which decreases for aggregated (aged) hydrogels, and also when the rate of drying, f) and 1) increase or ~ decreases. Step of drying I I determines the pore size distribution and lasts until the residual moisture content Vn is attained. At this moment the liquid phase remains solely at contact points and on the surface of globules (V approx. corresponds to the lower n point of the hysteresis loop of adsorption isotherm on xerogel). As the evaporation front moves in the grain volume, individual
zones-domains filled with liquid are formed (Fig. 3, c,e). The
671
capillary forces are directed to the surface of these domains, and therefore, at this step the local contraction in some zones is possible at minimum changes in the total pore volume Vr. • Special regimes of drying (rapid drying of gel at step I and slow drying at step II), in principle, allow preparation of silicas having the bidisperse structure (ref. 6). Step of drying III affects predominantly the surface area of the xerogel.The- destruction of the shells has been accomplished, the contact points between globules are formed, and the transfer MC processes are intensified. The maximum decrease of the surface area is observed during drying of alkaline hydrogels, i.e. when the volume filling of part of the cavities may be accompanied by the appearance of "molecular-sieve" effects (MC-UD?F mechanism) (ref. 7). Thus, the transfer of LlIlS determines rhe formation of the surface and corosf.ty of 'the xerogel : at the steps that follow sol formation the u~c transfer leads to the decrease of S, while at the steps that follow the formation of hydrogel - to the growth of V~ • The final formation of the structure occurs at the step of drying. The experimental data reported in refs. 1-9 can be satisfactorily described in terms of such mechanisms. Regime II is accomplished in the situation where the steps of a) hydrolysis of an alkaline metal silicate, b) sol formation, and c) rapid coagulation occur simultaneously (for example, interaction of silicates with the easily hydrolyzable salts or in the presence of earlier formed gel (refs. 1,2,8). The details of the formation of such silicas were described in (ref. 8) for the continuous method of silica precipitation at constant pH, temperature and the rate at which the reagents are brought together with a simultaneous removal of the formed coagel from the reaction zone. Then the precipitates were filtered, washed, plasticized and extruded. The structure of thus prepared xerogels is determined essentially by the precipitation conditions employed, being almost independent of the drying regime. One can outline two limit regimes of precipitation that allow drastically different structures to be formed: regime IIa - high value of pH, temperature, rate of stirring n, and low rate of bringing initial reagents together q; regime lIb - low value of pH, temperature, n and high q. Hydrogels prepared by operation in regime IIa consist of aggregates made from close-packed globules different in
672
size, which are pr ac t.Lce Ll.y not destroyed by mechanical treaiment in the plastifying apparatus. After drying the xerogels have the structure of nonuniform porosity and appreciable volume of ultramicropores accessible to water molecules and inaccessible to larger molecules of argon, nitrogen, etc. In typical examples the size of aggregates is 10 2_10 3 run, SAr", 20-40 m2/g, SI-/2 0 - 300 400 m2/g, the volume of ultramicropores is ~ 0.05 cm3 / g. Hydrogels prepared by operation in regime lIb, consist of loose aggregates which are readily destructible by mechanical treatment. Xerogels possess no molecular-sieve properties, their characteristic: S '" 300-700 m2/g "and V~ ~ 2-0.5 cm 3 / g. The peculiarities of silica formation in regime II are caused by that LMS, sol, gel are continuously formed and co-exist in the reaction zone. The Ll-1S groups formed can be consumed for 1) the formation of sol globules, 2) growth of available globules, and 3) cementing together of aggregates of contacting globules. It can be ShOVill that gradients ~~t , that determine the motive forces of the corresponding transfer processes of Si0 change in the order "A": 2, !J~3> A(IA 2 > 1If'11. Moreover, sol globules can be consumed to form new aggregates route ("B") or growth of already aV9.ilable aggregates route ("C"). The formation of new aggregates demands some oversaturation. The conditions of regime lIn favor the intense mass-exchange, absence of oversaturation, i.e. process occurrence via routes "A" and "B". In regime lIb the mass-exchange processes are restricted to individual zones and include consecutive steps "a-c" and processes proceeding via route "B" in each zone. For these reasons, conditions of regime IIa favor the formation of large and stable aggregates in which the space between globules is filled with loosely packed groups of LIVIS. SINTERING OF SILICA In general one may outline four types of structural variations: 1) decrease in S with increasing the mean pore size d at c.=const; 2) simultaneous decrease in Sand C at small variations of d, 3) simultaneous decrease in S, G , and d, and 4) decrease in Sand E. with increasing d. The type I variations are typical for sintering in hydrothermal conditions (ref. 10), low-temperature sintering of silicas in the presence of alkaline metal impurities (ref. 10) or water vapor (ref. 11). These variations are attributed to the LMS
673
transfer along the globule surface (or through the volume of intermiscellaneous medium in hydrothermal conditions) via the MC mechanism. Sintering occurs without coming together of globular centers ( C-const). UmJ transfer from loosely to densily packed sites is accompanied by "filling" of these latter, with the mean pore size being increased (ref. 6). The high-temperature sintering is usually followed by variations of type 2 and then of type 3. At intermediate steps the type 4 variations are also possible. These transformationp ~y be described in terms of the LM8 transfer mechanism at the expense of volume diffusion or viscous flow of 8i0 2 phase accompanied by coming together of globular centers (ref. 6). The type 2 variations are typical for silicas thorougly purified from impurities (ref. 10) and can be associated with the coalescence of globules in the domains having increased dispersion or density of packing, and also high residual content of impuritieo. Finally, the sites are formed that possess viscous-fluid properties. At the same time the domains with rigid structure are still retained. Under the action of capillary forces, the "rigid" domains become more densely packed (contraction of the type shown in Fig. 3e), while Sand c- proportionally decrease. with no essential changes in pore distribution in "rigid" domains. \fIith the further temperature rise the fraction of coalescent sites increases. In these conditions it is possible that the viscous-fluid phase would move into the porous space of "rigid" domains. If the melt fills the domains solely (completely or partially), variations of type 4 are possible leading to the increase of mean pore dimensions. Once the melt fills both the "domains" and the porous space between them, S, e, and d start to decrea se simultaneously (type 3 variations). On further increasing the temperature, the whole grain gradually transforms in the viscous-fluid state. Hence, the type 3 variations are always typical for the final step of sint e r Lng , CONCLUSION The above mechanisms of the formation and variation of the structure of silicas at all steps of synthesis may be considered to be a basis for the analysis of genesis of more complex systems. For example, the role of structural-geometrical factors determining Vo, Cmin and VL at the step of drying remains the same when
674
passing to the mUlti-component and crystallizable systems (ref.12). Variations in S with the thermal treatment can be complicated by phase transformations. but still the general scheme of sintering processes remains valid. REFERBNCBS 1 R.K. Iler, The Chemistry of Silica, Wiley Interscience Publ. Jolm Wiley Sons, New York, Chichester, Brisbane, Toronto, 1979. 2 Adsorption and Adsorbents, Vol. 1, Naukova Dumka , Kiev, 1972. 3 C. Okkerse, in B.G. Linsen (ED.), Physical and Chemical Aspects of Adsorbents and Catalysis, Aca d, .Pr-esa , London, 1970. 4 I.E. Neimark and R.Yu. She infa in , Silikagel, Ego Poluchenie, Svoistva i Primenenie, Naukova Dumka Kiev, '1973. 5 D.V. Tarasova, V.B. Fenelonov, V.A. Dzis'kO, V.Yu. Gavrilov and A.U. Khutoryanskaya, Kolloidnyi Zhurnal, 37(1977)207-212. 6 V.B. Fenelonov, D.V. Tarasova and V.Yu. Gavrilov, Kinetika i Illitaliz, 18(1977)480-487; 19(1978)222-227; Izvestiya SO AN SSSR, Ser. Khdrn, Nauk , 4(1978)116-129. 7 V.Yu. Gavrilov, A.P. Karnaukhov and V.B. Fenelonov, Kinetika i Kataliz, 19(1978)1549-1556. 8 V.B. Fenelonov, L.G. Simonova, V.Yu. Gavrilov and V.A. Dzis'ko, Kinetika i Kataliz, 23(1982)444-450. 9 V.M. Chertov, D.B. Dzhambaeva and I.E. Neimark, Kolloidnyi Zhurnal. 27(1965)279; Ukrainskii Khim. Zhurnal, 31(1965)1149,
1253-1258 10 N.B. Akshinskaya, A.V. Kiselev and Yu.S. Nikitin, Zhurnal Fiz. Khim., 36(1962)2277; 37(1963)921; 38(1964)488. 11 VI.G. Shalaffer, C.R. Adams and J.N. Wilson, J. Phys. Chem., 69(1965)1530. 12 V.A. Dzis'ko, A.P. Karnaukhov and D.V. Tarasova, in: Phisikokhemitscheskie Osnovy Sinteza Okisnykh Katalizatorov, Nauka, Novosibirsk, 1978.
675
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catolysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STUDY OF THE PREPARATION OF IRON CATALYSTS FOR LIQUEFACTION OF COAL BY HYDROGENATION UNDER PRESSURE
M. ANDRES (I) , H. CHARCOSSET(I) , P. CHICHE(Z) , G. rJEGA-MARIADASSOu(3) , J.P. JOLY (4)
and S. PREGERMAIN(Z)
(I) Institut de Recherches sur la Catalyse, Villeurbanne (France) (2)
CERCHAR, Verneuil en Halatte (France)
(3) Laboratoire de Cinetique Chimique, Paris (France) (4)
ESCIL, Lyon (France)
ABSTRACT The catalytic properties of various iron compounds were investigated in the high pressure hydroliquefaction of coal. In situ sulfidation of iron was carried out by addition of CS
to the coal/catalyst/tetralin charge. Iron acetylacetonaZ te soluble in tetralin, FeS0 deposited on the coal by aqueous impregnation, red 4 mud, as well as various iron supported catalysts were not found to decrease the percent residue of distillation very significantly. Precipitated iron oxide, particle size 1_3 wm was found more active. The best results were obtained with highly dispersed, particle size ~
0,05
m, iron oxide prepared by a flame me-
thod.
INTRODUCTION It has long been recognized that iron compounds, in particular pyrrhotite Fe1_xS' have a catalytic influence on hydroliquefaction of coal (refs.
I). In
this reaction, coal is in suspension in a recycle oil and is treated at a temperature of about 450°C under a hydrogen pressure of roughly 200 atm. But very rare are the studies concerning the relationships between ; - the mode of preparation-the physicochemical properties - and the catalytic activity of the catalysts (refs. Z). The present work deals. with a preselection of iron based catalysts which are sulfided in situ during liquefaction by the addition of CS foil suspension.
Z
to the catalyst/coal
676 IU:SULTS Coal The coal used was a highly volatile bituminous coal (Freyming,France) , which was pulverized to less than 80jUm prior to use. The analytical and petrographic data are listed in Table 1. TABLE 1. Proximate, petrographic and ultimate analyses of Freyming coal
~~~~~~~~~_~~~~l~~~· (wt % air dried basis)
(O_80~m)
~~~~~~~~_~~~~l~~~ (wt % MAF* basis)
Moisture
1.9
C
Ash
7.4
H
5.4
S**
0.5
Volatile matter Fixed carbon
35 55.7
Vitrinite
76
Exinite :
9
Inertinite
10
Mineral matter : 4
83.3
N
1.1
0
9.7
MAP
moisture and ash-free
Pyritic sulfur
0.3
%
Catalytic hydroliquefaction of coal Batch experiments were performed using a 830 ml rocking autoclave. In a typical experiment the autoclave was charged with tetralin (200 g), coal (100 g) , CS
(1 g) and the iron oxide catalyst. CS was used to sulfide the catalyst du2 2 ring the experiment. The autoclave was filled with cold hydrogen to an initial
pressure of 150 atm at room temperature, heated to the reaction temperature (450°C) at a heating rate of 200°C/h, held at 450°C for 3 hours and allowed to cool to room temperature. The gaseous products were then removed for analysis and the autoclave content fractionated, following the conventional scheme described in refs. 3-4. A preliminary vacuum distillation allows the removal of the volatile products in order that the bulk of the liquid product may be filtered without uncontrolled loss of volatile substances. Filtration of the stripped solution was carried out under vacuum. Recovery of solvent and distillate products arising from coal was performed by a further vacuum distillation of the filtrate. The amount of vacuum distillation residue was further determined (hard pitch with a softening
poin~
150°C),
The wet filter cake was treated with pyridine in order to extract all of the soluble material. The extraction residue was assumed to represent the part of the coal which is insoluble in tetralin after reaction (mineral matter and insoluble
677 organic matter) . The activity of any added catalyst is evaluated mainly by the value of the percent residue of distillation, which should be decreased as much as possible compared to the blank experiment. Note that the percent residue of distillation does note include the inertinite plus mineral matter content of the coal. Iron catalysts precursors, soluble in tetra lin Iron acetylacetonate was studied as a model compound (10 g were included in the
char~e,
all of the other conditions being as stated above). The percent re-
sidue of distillation (24.5 %, Table 2) was somewhat less than in the blank experiment (26.0 %). The relatively low catalytic activity may be related to the low dispersion of the iron sulfide arising from the in situ sulfidation of iron acetylacetonate. It was verified in fact that iron acetylacetonate became sulfided under the above conditions of coal hydroliquefaction. X-ray Diffraction Analysis (XRD) showed Fe as the sulfidation product with a mean crystallite size of O. 95S about 70.110 nm. TABLE 2 Hydroliquefaction of coal in the presence of iron acetylacetonate.
Catalyst wt % H2 % residue of % ~xcess consumed distillation l~quid
% insoluble %C02 +co %C 1-C 4 %H 20
Total yield
No added catalyst
4.15
26.0
46.5
16.1
1. 35
8.5
5.7
104.15
Fe (acac)3
4.5
24.5
44.0
17.3
1.8
9.1
7.8
104.5
Impregnation of coal by an aqueous solution of
Fes04'~2~(10
wt % of sulfate
Icoal). The hydroliquefaction results showed a poor reproducibility of the impregnation technique and a relatively low catalytic effect. Iron sulfide (Fe
was identified by XRD in the solid residue of the coal O. 9 5S) hydroliquefaction. Its crystallite size was too large to be evaluated from the XRD lines broadening. Here again the poor dispersion of the iron sulfide accounts for its low catalytic' activity. Natural iron containing solid precursors (i.e. red mud, possibly modified) 5 grams of red mud 4.0 % Ti0
(Composition: 42.1 % Fe203 ; 16.7 % A1
11.7 % Si0 2 20 3 ; 5.9 % CaO ; weight loss under heating up to 1000 0C :
7.5 % Na 2 20 10.3 %) were added to the charge, all other conditions being as stated above.
The results (Table 3)
showed no evidence of a significant catalytic activity (%
residue of distillation: 26.6).
678 TABLE 3 Coal hydroliquefaction in the presence of red mud, or of supported iron catalysts
Catalyst
%H %C %H % Residue of % Excess % insolu- %c0 2 2+co 1-C4 2O consumed distillation liquid ble
Total yield
No added catalyst K"d mud
4.15 4.40
26.0 26.6
46.5 46.3
16.1 17.8
1.35 1. 45
8.5 5.7 6.15 6 .10
104.15 104.4
Fe
4.1
26.6
44.8
17.9
1.4
7.1
6.3
104.1
4.1
29.0
42.2
17.6
1.5
7.2
6.6
104.1
s i o:r 4.5
26.5
43.4
18.2
1.5
7.4
7.5
104.5
4.1
29.4
42.1
17.9
1.4
7.1
6.2
104.1
= 6.5
2/g). m
Al Fe
20/ 20 3 203
Si0
1
2
Fe 20 3 /
Al Fe
20 3 20/c
This likely arises from the low dispersion of the red mud (S
BET
It was further observed that i) The catalytic actiVity of red mud or other disposable solids like fly ash is highly variable from one sample to another one. ii) We tried to increase the dispersion of iron in the red mud by acid leaching (HN0 10 %, at 60°C) followed by ammonia reprecipitation. This was unsuccesful 3 since only the alumina and not the iron oxide in the red mud was dissolved and reprecipitated under a much higher surface specific area form of A1
20 3.
Iron supported catalysts The following supports were used : 2/g SCS 59 (granulates S 100 m ; Vp = 610 mm 3/g ; ¢p 1\120 nm) 203 2/g - Si0 MAS 100 (powder; S 112 m ; Vp = 550 mm 3/g ; ¢p ~ 15 nm) 2 (75 %) HTH (powder; S 680 m2/g) - Si0 2-A1 203 - carbon black VULCAN 6 (granulates S = 113 m2/g) - Al
6 wt % Fe were deposited on each of these supports by impregnation with an aqueous solution of Fe(N0
9H followed by 1N ammonia precipitation. Water 3)3' 20, washing and air drying at 110°C were finally performed. These catalysts were tested as mentioned above (5 g of catalyst and 19 of CS
2 in the charge). The results, reported in Table 3 show no example of any decrease of the percent residue of distillation, which is on the other hand significantly increased in the presence of Fe
or of Fe 20 3/Si02 203/c. The inactiVity of those iron supported catalysts may be at least partly rela-
ted to the low % of iron versus coal (0.6 %). Low accessibility of the sulfided iron to the tetralin, and to its dehydrogenation product naphtalene, may further
679 be involved. Precipitated iron oxide The starting iron salt was Fe(N0
9H
3)3'
20
diluted in water. Two precipitating
agents were employed i) the buffer NH (pH 9.2) solution 40H-NH4CI ii) diluted NH alone (5 %) 40H and also two modes of drying of the precipitates subsequent to their water washing i) air oven drying at 110°C ii) spray-drying (Minispray dryer Buchi) Typical results are reported in Table 4. TABLE 4 Coal hydroliquefaction in the presence of precipitated iron oxide
Precipitated by
No added catalyst
Drying
NH 4OH-NH4CI Air-oven
%H consumed 2 %residue of distillation
4.15
NH 40H Air-oven 4.55
4.55
NH 40H Spray-dried 4.8
26.0
38.1
26.2
20.9
% excess liquid
46.5
34.8
46.8
52.0
% insoluble
16.1
17.45
% CO + CO 2 %C - C 4 1 %H 2O Total yield
15.3
16.75
1. 35
1.4
1.1
1. 25
8.5
6.0
6.85
7.0
5.7
7.2
8.3
6.9
104.15
104.55
104.55
104.8
The air-oven dried Fe precipitated by NH + NH gave rise to a percent 40H 20 3, 4CI, residue of distillation value (38.1 %) which is much more than the blank experiment value (26.0 %). This was proved to be related to the retention of CI
ions
by the precipitated iron oxide prepared inside a single precipitation operation, and the percent residue of distillation of the coal hydroliquefaction product also increased accordingly to (ref. 5). Iron oxide precipitated by water diluted ammonia differed very much in catalytic activity according to its mode of drying (Table 4). In fact the oven-dried solid gave results highly comparable to those of the blank experiment. On the other hand the spray-dried solid showed an important decrease in the percent residue of distillation (20.9 instead of 26.0 %). This is due to the smaller particle size of the spray-dried iron oxide ( ~ 5
rm)
(1-3~m)
compared to the oven dried one
as shown in more detail elsewhere (ref. 6).
680 ]ron oxide prepared by a flame method As a small particle size of the oxidized precursor of the iron sulfide was found to be important for a good catalytic activity (comparison of spray-dried and of oven-dried iron oxide precipitates) it was thought of interest to test a very finely dispersed (particle size
~
0.05~m)
iron oxide. Such a sample was
prepared by combustion of FeCl
vapor in a H flame (ref. 7). The results 2-02 3 obtained with only 0.5 wt % Fe in the charge (the quantity of CS was 2 203/coal maintained equal to 19) are reported in table 5. For comparison, the results obtained with V 0 and M00 also prepared by the flame method, granulometry and 2 5 3, wt %/coal being nearly the same as for Fe203, are also mentioned. TABLE 5 Coal hydroliquefaction in the presence of added (0.5 wt%/coal) Fe
Catalyst
203,
V Mo0 3 205, Total yield
%H con- %residue of %excess % inso- %C0 2 2-CO sumed distillation liquid luble
NO
4.15
Fe
20 3 V 20 3 M00 3
26.0
46.5
16.1
1. 35
8.5
5.7
104.15
4.95
20.75
54.1
16.4
0.7
6.4
6.6
104.95
4.90
28.4
45.0
17.7
0.8
6.65
6.35
104.9
4.75
24.4
50.2
16.4
0.8
6.6
6.35
104.75
The two main points relevant to this Table are i) the relatively high efficiency of that iron oxide preparation, even if used at a low wt %/coal ii) the higher activity of the Fe203 precursor, compared to the V and M00 3 20 5 ones.
DISCUSSION Certainly no definite conclusion may be drawn from the present work since the efficiency of any catalyst in the coal hydro liquefaction may be highly dependent on the coal investigated. All of our experiments were performed over a single coal and obviously,it is not
possible to extrapolate to other coals. Our pur-
pose is to find some improved disposable iron catalysts, by looking at the correlations between the (in) activity of these catalysts and their physicochemical properties. 1. Oil soluble iron organometallics are attractive catalyst precursors. They should be able to penetrate the internal texture of the coal and to give rise to a highly dispersed sulfide, located in the vicinity of the free radicals arising from the thermolysis of coal. The most probable mechanism of the hydroliquefaction is actually (ref. 8)
considered to be :
681 thermolysis ______________________
Coal
~.
Free radicals (RO)
hydroaromatics in
RO
-I- - - - - - - - - . ,~.
~
RH + Aromatics
the solvent RR (repolymerisation, to be prevented)
The role of the sulfided iron catalyst should be to improve the rehydrogenation of the aromatics (model compound : naphtalene) to H donor hydroaromatics (model compound : tetralinel. Nevertheless, at least the present Fe organometallic
(acetylacetonate) in pre-
sence of the actual coal, gives rise to coarse instead of highly dispersed iron sulfide. 2. Iron sulfate is a most interesting precursor since it is available very cheaply in very large amounts. But preparing highly dispersed iron sulfide "supported" by the coal, was not achieved via the very simple present procedure (simple impregnation of the coal by an aqueous solution of iron sulfate) . 3. Red muds and other disposable iron containing solids have the main drawback of being highly variable in catalytic activity, according to their actual compositions and physicochemical properties in general. 4. Unsuccessful results were presently obtained with various iron supported catalysts. The support has a double role : i) presumably to increase the dispersion of iron ii) to decrease the percentage of iron,
con~ared
to unsupported iron catalysts.
This second negative effect appears as dominating, in the present study. S. Active synthetic iron catalysts precursors may be prepared by
precipitatio~
provided i) the presence of chloride ions should be avoided in the precipitating medium ii) the drying of these precipitates should prevent agglomeration processes, hence the sUitability of spray-drying. 6. A still more appropriate iron oxide, as a precursor for the iron sulfide catalytic active phase, arises from the combustion of FeC1 flame. Xhis method results in small (particle size
~
vapours in a H 2-02 3 O.OSJ'ml non porous iron
oxide particles. It should be outlined that similarly prepared V 0 M00 do not 3 2 S' appear as good hydroliquefaction catalyst precursors, as opposed to Fe 3• 20 ACKNOWLEDGEMENTS The GECH (Groupe d'Etude de la Conversion du Charbon par Hydrogenation) is acknowledged for helpful discussions and financial support'.
682 REFERENCES
2
3
4
5 6 7 8
For review articles see a) W. KAWA, R.W. HITESHUE, R.B. ANDERSON and H. GREENFIELD, U.S. Bur. of Mines, Rept. Invest n° 5690 (1960), 16 pp. b) P.A. MONTANO and B. GRANOFF Fuel 59 (1980) 214-216. c) M. ANDRES and H. CHARCOSSET J. Chim. Phys. 76 (10) (1979) 887-901. a) D.K. MUKHERJEE, J.K. SAMA, P.B. CHOUDHURY and A. LAHIRI Proceeding~ of the Symposium on Chemicals and Oil from Coal (1969, (Pub. 1972», p. 116. b) R.P. ANDERSON 15th Intersociety Energy Conversion Engineering Conference, August 18-22, Seattle, Washington, 1980, p. 1557. c) J.A. GUIN, A.T. TARRER, J.W. PRATHER, D.R. JOHNSON and J.M. LEE Ind. Eng. Chern. Process Des. Dev. 17 (1978) 118-126. C.H. WRIGHT, R.E. PERRUSSELL and G.R. PASTOR (PAMCO), Development of a process for producing an ashless, low-sulfur fuel from coal. Autoclave experiments. R. and D. Report nO 53 Interim Report n° 6 prepared for Office of Coal Research (february 1975). C.H. WRIGHT, G.R. PASTOR and R.E. PERRUSSEL (PAMCO), Development of a process for producing an ashless, low-sulfur fuel from coal. Continuous reactor experiments using anthracene oil solvent. Rand D Report nO 53. Interim Report nO 7 prepared for Energy Research and Development Administration (september 1975). FE-496-T4. M. ANDRES, Thesis to be published M. ANDRES, H. CHARCOSSET, P. CHICHE, L. DAVIGNON, G. DJEGA-MARIADASSOU, J.P. JOLY and S. PREGERMAIN, Submitted to publication, Fuel. P.G. VERGNON and H. BATIS LANDOULSI, Ind. & Eng. Chern., Prod. Res. Dev., 19 (1980) 147-151. R.C. NEAVEL Fuel 55 (1976) 237-242.
683 DISCUSSION L. GUCZI: You gave many characteristics about goof iron catalysts prepared for coal liquefaction. I wonder if one could hear anything about the mechanism, in particular, about the hydrogen ~ctivation on FeS ? H. CHARCOSSET: In order to simulate the rehydrogenation of the recycle oil during the liquefaction of coal, we used the model reaction of the inverse reaction of the naphtalene hydrogenation, that is the tetra I in dehydrogenation. We have found (H. Zimmer, M. Andres, H. Charcosset, G. Djega-Mariadassou, to be published) that the specific catalytic activity is highly dependent on the S/Fe ratio. The activity of Feo is suppressed by a very small degree of sulphidation and is restored when entering into the pyrrhotite Fel-x compositions range. K.S.W. SING: It is interesting to see that the flame hydrolysed iron oxide showed the best catalytic activity of all the samples so far studied. It is possible that the high activity of this material is due in part to its surface structure, i.e. not only to its particle size distribution. H. CHARCOSSET : This hypothesis is not unlikely, provided one considers that the surface structure of the oxide influences the surface properties of the resulting sulphide. The active phase should be in fact Fel-xS.
This page intentionally left blank
685
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
IMPREGNATION OF Y-ALUMINA WITH COPPER CHLORIDE. EQUILIBRIUM BEHAVIOUR, IMPREGNATION PROFILES AND IMMOBILIZATION KINETICS R.J. OTT + and A. BAlKER Swiss Federal Institute of Technology (ETH), Department of Industrial and Engineering
Ch~mistry,
CH-8092 Zurich, Switzerland
ABSTRACT Different aspects of the impregnation of y-alumina pellets with aqueous solution of copper (II) chloride are considered. These are the equilibrium behaviour and the effect of time of impregnation, previous state of the alumina and concentration of the impregnation solution on the resulting internal distribution of copper. The measured copper profiles indicate a shell progressive immobilization. Quantitative studies of the ion concentration changes in the impregnation solution show that the main immobilization process is equivalent adsorption of CuC1 2, whereas Cu 2+ immobilization due to ion exchange becomes only important with larger impregnation times.
INTRODUCTION Supported catalysts are frequently prepared by impregnation of porous supports with a solution containing the active component. In its simplest form this method of catalyst preparation involves three steps: the contacting of the porous support with the impregnation solution, drying and calcination. Several aspects of the processes taking place during these steps of preparation have been studied in the past and were recently reviewed (ref. 1). The present work is centered on the preparation of alumina-supported copper processes (ref. 2). Liquid-phase chloride as used for gas phase oxychlorin~tion impregnation is a widely used technique for preparing such catalysts. The following aspects of the impregnation of the alumina support with an aqueous solution of copper (II) choride are considered: the equilibrium behaviour and the effect of time of impregnation, previous state of the alumina and concentration of the t Present address:Schweiz. Unfallversicherungsanstalt, CH-6002
Luzern.
8M
impregnation solution on the internal distribution of the active component within the porous support. In addition, some characteristic features of the adsorption kinetics are presented which should help to define impregnation conditions for tailoring internal profiles. EXPERIMENTAL Support Commercial y-alumina pellets (Girdler-SUdchemie,T-126) were employed as support material. The physical properties of the cylindrical alumina pellets of 4.3 x4.3 mm size were: surface area (BET) 197 mZ/g, solid density 3.43 g/cm3, apparent density 1.37 g/cm3, specific pore volume measured by mercury intrusion 0.44 cm 3/g. Impregnating compound Copper chloride (CuCli ZHZO) from Fluka AG., A.R. Grade was used in an aqueous solution. Analysis Solution concentrations of CUC1 Z were measured by visible absorption spectrophotometry, using an SP 700 A UNICAM recording spectrophotometer. The CUC1 Z immobilized on the support was extracted with 60% nitric acid at 600C and then measured spectrophotometrically. For the spectrophotometric measurements the total extract was diluted with water to give a nitric acid content of 8%. The amount of CUC1 Z measured in the extract was checked by the CUC1 uptake calculated from Z spectrophotometric measurements of the CUC1Z-solution concentration before and after impregnation. Z The internal Cu + distribution along the catalyst pellets was measured by means of an X-ray fluorescence microprobe analyzer (Novelco AMR-3) using the Cu Ka line. The samples were imbedded in plastic and cut to reveal their cross section. The analysis was carried out along the diameter of the polished cross section, in points approximately 0.05 mm apart. The copper concentration of each point was calculated relating the number of impulses measured with a calibration line obtained from uniform samples of known copper content. The chloride ion concentration in the impregnation solution was determined employing the titration method described by Volhard (ref. 3). 3+ concentration was determined by complexometric zinc back titration using The A1 ethylene-diaminetetraacetic acid (EDTA) as complexing agent and dithizon as indicater (ref. 4) after removing the Cu Z+ ions by precipitation with NaZS. o
The determination of the H+/OH- concentrations were performed with a digital pH-meter using a single rod assembly electrode.
687
Impregnation procedure Impregnations were carried out with constant bath concentration as well as with finite bath concentration of solute. For impregnation with constant bath concentration, typically, five grams of the alumina support, either dried or previously wetted, were immersed in 1 liter of aqueous CuC1 2-so1ution of a given concentration. The impregnation bath was thermostated at 250C and vigorously stirred during the sorption experiments. After the desired time has been elapsed the solution was drained off and the pellets quickly washed with distilled water and then dried at 1100C for 24 hours. The alumina support was pretreated in two different ways previous to the impregnation experiments. For the experiments with a dry support, the alumina was dried for 12 hours at 4000C before use, whereas in the case of a wet support, the alumina was preconditioned for 12 hours in distilled water at room temperature. RESULTS AND DISCUSSION Equilibrium behaviour In order to determine the equilibrium behaviour of the impregnation system, the amount of CUC1 2 taken up by the alumina was measured after different equilibration times. The uptake curves plotted in Fig. 1 indicate that the equilibration is very slow and requires more than 17 days. Equilibration times higher than
3.0
1.0
o
0.5
1.0
1.5
c, [Mol/I] Fig. 1
Dependence of CuC1 2-uptake (C a) on bath concentration (C ) for different equilibration times e (wet support, equilibration times: 05, L:.1l, 017, .23 days)
688
23 days did not lead to significantly different uptake values, indicating that the uptake values measured after 23 days are close to equilibrium. The adsorption equilibrium constant K and the total concentration of adsorption sites S on the alumina were calculated applying the linearized form of the Langmuir isotherm (Eq. 1) to the measured data. KC
(1 )
a + SK
The equilibrium parameters found for the 23 days isotherm were: K
3.4 + 0.7 l/Mol,
and S = 5.0 ~ 1.2 Mol/kg. Impregnation profiles Effect of time of impregnation The effect of time of impregnation on the internal concentration profiles of copper is shown in Fig. 2 for the wet (2.a) and dry support (2.b), respectively.
2.0
a)
~=
b)
20 min
t
~=30
1:3= 40
t=40 3 t 4=60
t 4=60
'* .... s
1=15min
t 2= 30
" " " t4
~
U
....:J
U
t4
0
1
0.5
RfR o
t1
o1
t2 t 3
0.5
0
RfR o
2 Fig. 2 Effect of time of impregnation on internal Cu + concentration profiles. a) wet support, b) dry support (constant bath concentration C = 0.5 M) e The concentration profiles determined for the wet and dry support differ, in particular, in the penetration depth. With the dry support the impregnation front moves markedly faster into the center of the pellet and more solute is adsorbed. This behaviour is due to the different way of intake of the impregnation solution into the pores: for a wet support the transport of impregnation solution into the pores takes place by diffusion only, whereas for a dry support penetration (capillary
689
flow) is dominant. In both cases, a progressive shell immobilization (ref. 5) is observed with a relatively small concentration gradient between the exterior of the pellet and the impregnation front. Figure 3 presents a quantitative comparison of the progress of the impregnation front for the wet and dry support, respectively.
1 • WET SUPPORT
o DRY SUPPORT
o ~
0.5
a:
o o
1
2
TIME (hours]
Fig. 3 Progress of the impregnation front. (constant bath concentration Ce = 0.5 M) Effect of solution concentration In order to study the concentration effects, several impregnations with different solution concentrations were performed with a short impregnation time of 15 minutes. Some of the internal profiles obtained with these impregnations are presented in Fig. 4. For both the wet and dry support, the solution concentration mainly influences the penetration depth of the impregnation front, whereas the shape of the profiles does not depend markedly on concentration. With all impregnations a sharp impregnation front is observed ,indicating a shell progressive behaviour. The internal concentration increases as the solution concentration increases; however, no great effect is observed on the internal distribution of the immobilized copper ions. For all impregnation conditions studied,more solute is immobilized using a dry support and the impregnation front penetrates deeper into the pellet.
690
Effect of subsequent precipitation step To study the effect of a subsequent precipitation step on the internal concentration profiles, the impregnated alumina pellets were immersed in a 0.2 MNaOHsolution for 12 hours after the impregnation and then dried at 1100C for 24 hours. Some profiles obtained with this impregnation method are presented in Fig. 5. 2.0~-'--"--'--"--'--"--'--"--'--
4.a)
5.a)
4.b)
5.b)
c
?P: ... ~
1.0
~c
....::I
U
C 1 C2
0
1
C3
C4
0.5
R/R o Fig. 4 Effect of concentration of impregnation solution. a) wet support, b) dry support constant bath concentrations C = 0.1, C = 0.25, C = 0.5 2 l 3 C4= 0.75 and C = 1.0 MCUC1 2 5 impregnation time = 15 minutes
o1
0.5
R/R o Fig. 5 Effect of concentration for impregnation with subsequent precipitation step. a) wet support, b) dry support conditions see Fig. 4 precipitation: 12 hours in 0.2 MNaOH
o
691
Ce - 0.15 [Mol/I)
0
8
0.5
0.25
0.75
a)
~
::l 0
.£ 6
w ~
i= 4 z
0
fi
a: 2 w
I-
Z
W
e,
0 0.9
o
0.25
0.5
0.75
1.0
c, [Mol/I] Fig. 6 Penetration times and shell progressive uptake of CUC1 Z on the alumina support a) Penetration time after which the impregnation front reached the center of the alumina pellet as a function of the impregnation bath concentration. b) Dependence of CuC1Z-uptake on the impregnation bath concentration, measured when penetration front reached the center of the pellet (shell progressive uptake). ------ pore volume contribution (wet support, constant bath concentrations)
692
For both the wet and dry support, the precipitation step leads to considerably steeper profiles. The precipitation step causes a concentration depletion in the liquid filling the pore volume, and this may lead to a reverse concentration gradient which forces the solute to migrate from the inner pore volume to the external shell, where it precipitates. This effect of the precipitation step has also been observed with the impregnation of alumina pellets with Ni(N0
3)2
solution (ref. 6).
Immobilization kinetics The progress of the impregnation front towards the center of the pellet was studied in a series- of experiments with different impregnation times and impregnation solution concentrations. The penetration depth of the impregnation front in the pellets were determined from the internal concentration profiles measured with the microprobe analyzer and from optical micrographs taken from the pellet cross section. The penetration depth obtained with both methods were in good accordance. The penetration time the impregnation front requires to reach the center of the pellet is dependent on the solution concentration as shown in Fig. 6.a) The corresponding total uptake of CUC1 after the impregnation front reached the 2 center of the pellet is plotted in Fig. 6.b). The uptake curve obeys a linear relationship between the impregnation bath concentration,C and the amount of CUC1 2 e, taken up by the support. The dashed line corresponds to the uptake which is due to solute intrapped in the pore volume. From the intercept of the uptake line we estimate a total of 0.32 mol sites per kg support which are occupied during the strong shell progressive adsorption process. Thus, two types of immobilization sites with different affinity for Cu 2+ adsorption may be distinguished based on kinetic grounds. One type is characterized by a fast, the other type by a slow, adsorption. The former leads to a shell progressive adsorption behaviour. More information about the different immobilization processes emerges from a quantitative study of the concentration changes of the ions present in the impregnation solution. The measured decrease of the Cu 2+ and Cl concentrations during impregnation are presented in Fig. 7 along with the change of the pH-value of the impregnation solution. The results plotted in Fig. 7 indicate an increase in nonequivalent adsorption of the Cu 2+ and Cl- ions with time of impregnation. The concentration of the Cu 2+ ions in the bulk solution decreases faster than the equivalent concentration of the Cl ions. After 12 hours an excess of 5.3% Cl ions was measured which increased to 11.3% after 10 days. The nonequivalent adsorption is due to ion exchange between A1 3+ from the support and Cu 2+ from the solution as is substantiated by the results plotted in Fig. 8. The amount of positive charges due to A1 3+ fits very well with the excess negative charge of Cl expected for electroneutrality reasons.
693
TIME [hours!
o
1.0
2
1.0 • Cc/-(t) 4
CCU2+
4.0
(t)
0.9
~~
J:
o
Q.
........ ~
3.75
~
o 0.8 ------L-
o
---!.....J 3.5
..J.-
4
8
12
TI MEl hours] 2+ Change of Cu , Cl and impregnation (wet support, weight of pregnation solution 50 0.25 MCuC1 2)
Fi g. 7
+ H concentrations during alumina 5g, volume of imml , initial concentration
4
";""'
3
:J
tT
QI
N
~
2
.... e
)(
~
~
o
1
• C(t)=IZcr' Ccr(t)-IZCU2+ICcU2+(t) A C(t)=IZA1,+1 CA1,+(t)
0 0
6
12
18
24
TIME [hours! Fig. 8
Kinetics of the apparent excess concentration of Cl ion~ relative to Cu 2+ due to ion exchange between A13+ and Cu : (wet support, weight of alumina 100g, volume of impregnation solution 100 ml, initial concentration 0.25 MCuC1 2)
694
The increase of the pH-value in the initial period of impregnation (Fig. 7) is to be ascribed to the adsorption of H+ on the alumina, whereas the subsequent decrease is caused by the commencing hydrolysis of the A1 3+ ions which form easily stable hydroxocomplexes under the conditions given (ref. 7). For short impregnation times, the ion exchange appears to be of minor importance; however, it becomes very significant when longer impregnation times are used, as indicated by the results shown in Figures 7 and 8, respectively. Thus, an attempt to describe the kinetics of the impregnation process for short impregnation times can proceed from the premise of only one important immobilization process, namely, an equivalent adsorption of the CUC1 2 on the alumina, whereas with longer impregnation times both immobilization processes have to be taken into account. CONCLUSIONS The impregnation of y-alumina with an aqueous solution of CUC1 2 shows a shell progressive immobilization. The previous state of the alumina support (wet or dry) mainly influences the penetration rate of the immobilization front, whereas the shape of the internal Cu 2+ concentration profiles remains about the same. Subsequent precipitation after the impregnation with 0.2 MNaOH-solution leads to a larger amount of immobilized Cu 2+and the internal profiles become steeper. Two different immobilization processes can be distinguished: equivalent adsorption of CUC1 2 and ion exchange between A1 3+ and Cu 2+. The ion exchange is comparatively slow and becomes only significant with larger impregnation times. ACKNOWLEDGEMENTS The experiments presented in this paper were carried out by one of us (R.J.O.) while spending a postdoctoral year at the Consejo Superior de Investigaciones Cientificas, Madrid. Partial financial support of the fellowship by "Stiftung fUr Stipendien auf dem Gebiet der Chemie", Basel, is kindly acknowledged. Thanks are also due to Dr. C. Martinez Perez for valuable help with the microprobe analysis. REFERENCES 1 A.V. Neimark, L.I. Kheifez and V.B. Fenelov, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 439. 2 J. Blanco, J. Fayos, J.F. Garcia de la Banda and J. Soria, J. Catal., 31 (1973) 2~, 3 W. Fresenius and G. Jander, Handbuch der analytischen Chemie, Vol .11, Springer, Berlin, 1967, p. 98. 4 E. Wanninen and A. Ringboom, Anal. Chim. Acta, 12 (1955) 308. 5 P.B. Weisz and R.D. Goodwin, J. Catal., 2 (1963) 397. 6 J. Cervello, E. Hermana, J. F. Jimenez and F. Melo, in B. Delmon, P.A. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 251. 7 C.F. Baes and R.E. Mesmer, The Hydrolysis of Cations, Wiley, London, 1976, p. 112.
695 DISCUSSION L. RIEKERT From your observations one has to conclude that in a given preparation of Cu 2 + on y-A1203 obtained by impregnation, there must be at least two species of Cu 2 + which will also differ with respect to radial distribution. After calcination we may then have two different active components. The catalytic properties of such pellets do not follow from the distribution of Cu alone, since the variation of the distribution of copper between different types of solid compounds with radial position may be important. A. BAlKER: I completely agree with your valuable comment. As you pointed out, under certain conditions, we will have at least two different types of immobilized copper species with a different radial distribution. Evidence for this emerges from" TPR-studies we have carried out with alumina samples which have been impregnated employing different impregnation times. For pellets impregnated with short impregnation times (( 15 minutes) we obtained predominantly copper species which are reduced at about 370°C, whereas with larger impregnation times (up to 24 h) copper is preferentially immobilized in a form which is reducible at about 200°C. Hence, it is to be expected that the catalytic properties of pellets which show more than one maximum in the TPR-profiles do not follow from the copper distribution alone. S.P.S. ANDREW: Many times in this symposium during impregnation it has been pointed out that the support, particularly when it is high area and mark of imperfect and therefore reactive solid, reacts and to some extents dissolves in the impregnating solution especially when using aqueous and somewhat acidic solutions. The corrodability of the support material during impregnation is thus an important variable. A. BAlKER: I think the "corrodability" of the support material becomes an important variable in particular when long impregnation times are employed. with short impregnation times the "corrodability" may be neglected due to the fact that the corrosion process is usually considerably slower than the immobilization by adsorption. NG CHING FAI: Did you carry out XRD measurements on your resulting catalysts ? If so, did you detect bulk phase CUCI2? If so, could you suggest how y-A1203 stabilize the otherwise highly hygroscopic CuCl2 ? A. BAlKER : Preliminary XRD measurements carried out on the impregnated alumina gave evidence for the existence of crystalline CuCI2.2H20, however, only for samples with a higher copper concentration (> 2 wt %). This result seems to be in agreement with earlier findings by J. Blanco et al., J. Catal. 1l,257 (1973). In the present state of the investigation we are not able to make a suggestion in which way the y-alumina stabilizes thehighly hygroscopic CuCI2.2H20. M.V. TWIGG: Over what period is it necessary to treat the alumina support with water to observe the effects you have described ? Are the effects due to more than just surface hydration ? A. BAlKER The duration of the pretreatment of the alumina support with water is not critical, because the effects on the copper distribution observed are to be ascribed to the different way of intake of the impregnation solution. with the wet support (immersed to water) the pores are filled with water and the transport of the impregnation solution into the pores takes place by diffusion only, whereas with the dry support the pores are empty and the diffusion flux is superimposed with a capillary flow. The degree of surface hydration has, however, an effect on the type of copper immobilization as some preliminary TPR studies carried out in our laboratory indicate.
696 S. KALIAGUINE: I am interested by the technique of X-ray microprobe. Would you care for example to tell us about spatial resolution, possibility of measuring the chloride concentration,etc ... ? A. BAlKER: The X-ray microprobe or better termed electron probe microanalyzer (EPMA) has a very limited spatial resolution and is therefore only useful for purposes where examination on the micron scale is pertinent, as e.g. is mostly the case with the determination of concentration profiles in a catalyst precursor. Quantitative microanalysis of volumes 1-10 ~m3 is regarded as routine, provided that the specimen is homogeneous over the emitting volume. The spatial resolution is depending on different factors, such as chemical composition of the specimen, acceleratiQn voltage and density of the electron beam, and characteristic X-ray line. Depending on the type of detector employed (wave length-dispersive or energydispersive) the concentration of elements with atomic numbers greater than five can be determined. A review which covers your questions has been written by G. R. Purdy, R.B. Anderson in R.B. Anderson and P.T. Dawson (Editors), Experimental Methods in Catalytic Research, Vol. II, Academic Press; New York, 1976, p. 95137.
G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
697
PREPARATION OF COPPER SUPPORTED ON METAL OXIDES AND METHANOL STEAM REFORMING REACTION H.KOBAYASHI, N.TAKEZAWA, M.SHIMOKAWABE and K.TAKAHASHI Department of Chemical Process Engineering, Hokkaido University, Sapporo 060 Japan
ABSTRACT The effects of the catalyst preparation on the title reaction were studied over various copper-containing catalysts. It was concluded that highly dispersed CuO clusters formed preferentially on metal oxides at high copper loading or high calcination temperature. These precursors were readily reduced to metallic copper under reaction conditions and provided highly active and selective catalysts. The support effect on the reaction emerged when the metallic copper surface area exceeded 100 mZjg Cu. The precursors on A1 Z0 3, ZrO Z and MnO Z were highly susceptible to the reduction and fine particles of metallic copper were formed on these oxides.
INTRODUCTION Steam reforming of methanol CH 30H + HZO = 3H Z + CO Z is thermodynamically favorable and proceeds with high selectivity and high activity over copper-containing catalysts (refs. 1-4). Hydrogen atoms in methanol as well as in water are, therefore, effectively converted into gaseous hydrogen. In the present work, the title reaction is carried out over various copper-containing catalysts which are supported on a variety of metal oxides and the effects of the preparation of the catalysts upon the reaction are studied.
EXPERIMENTAL The catalysts were prepared by the ion exchange between hydroxyl protons on metal oxide surfaces (SiO Z' A1 Z0 3, TiO Z' CeO Z' MnO Z and ZrO Z) and tetrammine copper (II) cations at pH = ll-1Z. The ion exchanged metal oxides were thoroughly washed with distilled water, dried at 110°C overnight and calcined in air at a given temperature in a range from 400° to 900°C for 3 hrs. (A10-4, ZOO mZjg), TiO Z (7.8 mZjg) and MnO Z (31 mZjg) were SiO (413 mZjg), A1 Z Z03 available from Japan Chromato Co., Catalysis Society of Japan, Wako Pure Chemicals, and Diichi Carbon Co., respectively. ZrO Z was prepared by decomposition of zirconium nitrate (Wako Pure Chemicals) at 500°C in air for 3 hrs. CeO Z was formed
698
by calcination at 500°C in air for 3 hrs after the ion exchange had been carried out over cerium hydroxide (Wako Pure Chemicals). Steam reforming of methanol was carried out in a flow system at atmospheric pressure. An equimolar mixture of methanol and water was fed with a micropump and rapidly vaporized in a nitrogen stream before entering the catalyst bed. The total inflow was always kept at 96 cc STP/min and the partial pressures of methanol and water were both kept at 0.Z4 atm. The reactants and the products were determined by gas chromatography. The UV diffuse reflectance spectra were obtained by means of a Hitachi Model 330 spectrophotometer to which an integrating sphere was attached. The presence of bulk CuO'was confirmed by XRD (X-ray diffraction method, Rigaku Denki Zl14). TPR (temperature programmed reduction) experiments were carried out in a hydrogen stream (0.04 atm.) which was diluted with nitrogen at a total flow of 50 cc STP/min. The temperature was raised at a programmed rate of 10°C/min and the hydrogen consumption was determined by gas chromatography (Ohkura Model 701). The surface area of metallic copper was determined by titration with nitrous oxide according to the method proposed by Scholten and Konvalinka (ref. 6). Correction was made for the reaction with mangania when the surface area of metallic copper was determined on copper/mangania catalysts. The turnover frequency of the reaction was estimated from the rate of hydrogen production (HZ molecules/sec) and the number of metallic copper exposed on the surface.
RESULTS AND DISCUSSION Steam reforming reaction Effect of copper loading upon the reaction Table 1 lists the kinetic parameters obtained at steady states of the reaction over a variety of copper/silica catalysts. The results previously obtained over a support-free Cu or the catalysts prepared by kneading method (ref.4) are also listed for comparison. The parameter HZ/CO Z which is related to the selectivity of the reaction is estimated from the outlet partial pressures of hydrogen and carbon dioxide. It is seen that at higher copper loadings the parameter Hz/CO z as well as the activation energy is practically kept constant irrespective of the loading. However, at lower loading these kinetic parameters vary sensitively with the loading. Methyl formate was formed to a considerable extent together with hydrogen and carbon dioxide at lower loadings while at higher loadings the latter two products predominantly formed. The reaction proceeded selectively over the latter catalyst. The rate of hydrogen production (alloted for the weight of copper used) increased with the decrease in copper loading. However, when the loading was decreased to 0.5 wt.% copper, the rate decreased markedly. Table Z shows the results obtained over the catalysts which were supported on
699
I Kinetic parameters obtained over Cu/Si0 2 catalysts with various copper loadings a) Cu loading Rate of H b) H /CO b) activation energyC) Catalyst . 2 2 2 pro ductlon (-) (kcal/mole) (wt.%) (cc STP/min.~.Cu)
TABL~
3e) Cu Cu/Si0 2 l3 d) Cu/Si02 14d) Cu/Si0 21-l Cu/Si0 2 1-2 Cu/Si0 2 1-3 Cu/Si02 1-4 Cu/Si02 1-5
100 34.6 17.4 11. 67 9.97 1.89 0.94 0.52
5.3 48.3 74.7 101 85.6 279 89.9 8.7
20.3
3.3 3.7 3.7 3.7 4.2 4.8 f)
7.7
21.8 21. 7 18.7 20.5 24.1 27.1 31. 7
a) calcined at 500aC b) obtained at 2Z0 aC c) determined from H2 formation d) support-free e) kneaded catalyst f) not determined. TABLE 2 Kinetic parameters obtained over copper supported on various metal oxides Catalys t a)
Cu/A1 203 5d) 6 Cu/Mn0 2 4d) H-l Cu/Zr02 H- 3 H-l Cu/Ti02 ld) H-l Cu/Ce0 2 H-l a) calcined at
Cu 1oadi ng (wt.%) 13.4 0.5 26.7 0.61 2.99 0.52 28.4 2.4 0.52
sooac
Rate of H2 production b) H2/C020) activation energyc) (cc STP/min.g.Cu) (--) (kcal/mole) 57.4 564 21.8 162 331 545 1.9 0.78 144
b) obtained at 220°C
4.4 4.8 4.3 4.Z 3.1 5.0 3.4 6.0 4.2 c) HZ production
16.8
°
21. 16.1 24.1 22.8 30.8 18.5 41.6 27.0
d) kneaded catalyst
various metal oxides. As obtained over copper/silica catalysts, the selectivity as well as the activation energy of the reaction was strongly affected by the copper loadings. Theactivities obtained over copper/zirconia and copper/alumina with 0.5 wt.% copper were found to be about 60 times as high as those obtained on the corresponding copper/silica catalysts.
700
N
0
150
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0 200 400 600 800 1000 Calcination temperature (OC)
u
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Rate of H2 formation versus calcination temperature and selectivity of steam reforming of methanol
Effect of calcination In Figure 1, the rate of hydrogen production (alloted for the weight of copper used) and the selectivity of the reaction obtained at 220°C are plotted against the calcination temperature of copper/silica catalysts with 0.5 wt.% copper. The rate obtained remains practically constant for the calcination temperatures below 700°C. However, it increases markedly after the catalyst is precalcined at 900°C and the activity thus attained is estimated to be about 10 times larger than those obtained over the catalysts precalcined at lower temperatures. In conformity with these results, the selectivity is markedly increased by precalcination at 900°C. Precursor and working states of the catalyst States of the catalyst and copper loading In the previous work, various copper/silica catalysts which had been calcined at various temperatures were characterized with the help of a variety of analytical methods such as UV, XPS, AES, XRD TG, DTA and chemical analyses (ref. 7). It was found that the states of the catalyst precursor were greatly influenced by the copper loading and the calcination temperature. These results were all confirmed by the present observations with XPS, UV and TPR. The UV spectra of copper/silica catalysts were obtained after precalcination was carried out at 500°C in air. As previously observed (ref. 7), the strong absorptions occurred near 750, 270 and 300-350 nm. The absorption at ~ 750 nm was assigned to d-d transition of cupric ion in octahedral environment (ref. 8), suggesting that cupric ion was abundantly present on the surface. The absorption at ~ 270 nm was assigned to the charge transfer between surface oxygen and isolated cupric ion whereas that at ~300-350 nm was ascribed to the charge transfer between surface oxygen and clustered curpic ion (ref. 9). The former absorption appeared preferentially at lower copper loading while the
701
latter grew as copper loading was increased (ref. 7). In accord with these results, ESR intensity per cupric ion loaded was found to decrease with the increase in copper loading (ref. 10). The intensity obtained on the catalyst with 10 wt.% copper was about one-fortieth of that obtained on the catalyst with 0.5 wt.% copper. Clustering of cupric ion held occurrred as the loading was increased. In our previous work, TPR experiments were also briefly carried out on the catalysts containing different amounts of copper. Two TPR peaks occurred around 265° and 65QoC on the catalyst with 0.5 and 1.0 wt.% copper while the catalyst with 10 wt.% copper gave one strong peak around 290°C. Based upon these results together with those obtained by other analytical methods (ref. 7), it has been concluded that the catalyst precursor exists primarily as isolated cupric ion at lower copper loading while it exists as clustered CuO at higher copper loading. On the other hand, the crystallized bulk CuO was found to exist in support-free and kneaded catalysts when the catalysts were calcined at 500°C in air. This was onfirmed by the present observations with XRD and UV spectroscopies. Figure 2 illustrates typical UV spectra of the copper/silica catalyst which had
0.5wt.%Cu
OJ
v
<:
~
v
10 wt.%
OJ
~
eu
Support-free
ex:
300
700 500 Wave length (nm)
Fig. 2 UV diffuse reflectance spectra of the catalysts obtained after the reaction was carried out. been employed in the reaction. The absorption edge which is a characteristic of metallic copper (refs. 11,12) is evidently discernible around 560 nm on the catalyst with higher copper loading or support-free catalyst. This indicates that CuO clusters or bulk CuO was reduced to metallic copper during the course of the reaction. On the other hand, no absorption edge which is ascribed to metallic copper is perceptible in the spectrum of the catalyst with lower copper loading. The absorptions occur around 600 and 400 nm, suggesting that a large portion of the
702
catalyst precursor is not completely reduced to metallic copper. This is consistent with the previous observations with AES, UV and phosphorescence spectroscopies, and titrations with nitrous oxide and carbon monoxide (ref. 13). It was shown that when the catalyst with higher copper loading was reduced by hydrogen, it readily transformed to metallic copper to a considerable extent. On the other hand, the one with lower copper loading transformed to monovalent copper. On the basis of these results, it was concluded that isolated cupric ion present on silica support was difficult to be reduced to metallic copper under the reaction conditions while CuO clusters or bulk CuO were rapidly reduced into metallic copper. In conformity with these results, the surface area of metallic copper (alloted for the amount of copper loading) on the catalysts which had been used in the reaction was found to increase with the decrease in copper loading but markedly decreased at the loading belowO.~1.0 wt.% copper. Metallic copper was less exposed on the catalysts with lower copper loadings possibly because of their irreducibility. These catalysts were less active and less selective for the title reaction. On the other hand, the catalysts with higher copper loadings were highly active and selective in the reaction since metallic copper was preferentially exposed on the surface. State of the catalyst and the calcination temperature Figure 3 illustrates how the UV spectra of copper/silica catalysts with 0.5 wt.% copper vary with the calcination temperatures. The absorptions occur around 250-350 nm with the one around 750 nm. The former absorptions are sensitively varied with the calcination temperatures. When the catalyst is calcined at 4000 or 500°C, the absorption due to the charge transfer occurs predominantly at 270 nm. When the catalyst is calcined at 700°C, the absorption is discernible around 300 nm together with that around 270 nm. When the catalyst is further calcined at 900°C, the former absorption grows considerably and exceeds that of the latter in its intensity. Isolated cupric ion is suggested to be in part clustered by calcination. When the title reaction is carried out over the catalyst calcined at 900°C, the characteristic absorptions either due to isolated Cu++ or clustered CuO diminish and the absorption edge is newly observed at 560 nm. In contrast to the results obtained on the catalysts calcined at lower temperatures, metallic copper is readily formed in the course of the reaction. In accord with the results obtained on the catalyst with higher copper loadings, clustered CuO should be rapidly reduced to metallic copper under the reaction conditions. The metallic copper exposed on the surface was titrated with nitrous oxide after the catalyst was subjected to the reaction. As seen from Figure 4,it markedly increases over the catalyst calcined at 900°C. This, therefore, resulted in the marked increase in the catalytic activity when the catalyst was calcined at 900°C.
703
QJ
U c;
'"
-f-' U QJ
4QJ
2.5% 200 300 400 Wave length (nm)
800
400 600 Wave length (nm)
200
Fig. 3 UV diffuse reflectance spectra of Cu/Si02 catalyst calcined at various temperatures. Cu loading: 0.5 wt.%. Absorption around 215-220 nm is ascribed to silica. The ordinates in Figures are displaced to avoid overlapping traces. The ordinate of spectra in Figure 3 (8) are expanded for those in Figure 3 (A) in wavelength 200-400 nm.
30
"'0
QJ
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so
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~
N2
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o
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20
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zNtJ:i 10 U
u
200 400 600 800 Calcination Temp. (OC)
1000
Fig. 4 Nitrogen evolution by titration of surface metallic copper on Cu/Si0 2 (0.5 wt.% copper) with nitrous oxide States of the catalysts on other metal oxides The TPR experiments were carried out over copper/alumina and copper/zirconia*) *) As for copper/mangania, the support was found to be reduced at 23QoC together with the precursor species. The peak due to the precursors would not be separated from that of mangania.
704
-0-
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w
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20
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10 100 1000 Surface area of metallic copper (m 2/gr Cu) Fig. 5 Activation energy and the parameter H2/C02 versus surface area of metallic copper
1.0 -<>-Cu/Si0 2 -&-Cu/Zr0 2
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-O-Cu/Mn0 2
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<=
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O. 01 1 10 1000 100 Surface area of metallic copper (m 2/gr Cu )
Fig. 6 Turnover frequency versus surface area of metallic copper which exhibited high activity toward steam reforming of methanol. The TPR peaks were observed at 220° and 330°C on copper/zirconia and copper/alumina with 0.5 wt. copper, respectively. In marked contrast to the results obtained on the corresponding copper/silica catalyst, no TPR peakofthe·precursor species was observed at higher temperatures. The precursor species would be weakly held on these more basic metal oxides and reduced to metallic copper even at lower copper loadings. In particular, copper/zirconia was reduced at temperature much lower than that of the
705
reduction of CuO clusters. The observations with UV reflectance spectroscopy revealed that metallic copper was formed by the reduction with hydrogen. However, no appreciable amount of monovalent copper was detected by phosphorescence spectroscopy. Zirconia facilitates the reduction of the precursors species pronouncedly to metallic copper. When the loading was increased, the peaks appeared at 2200 and 300°C on copper/zirconia. One peak was observed at 290°C on copper/ alumina when the loading was increased. In comparison with the TPR spectra obtained on copper/silica catalyst, the peak around 300°C would be ascribed to the reduction of small clusters of CuO dispersed on metal oxides. The clustering also occurred on these oxides when the loading was increased. The surface metallic copper formed on the supports was titrated with nitrous oxide after the catalysts had been employed for the reaction. The surface areas of metallic copper were estimated to be 150-500 m2/gr Cu over copper/zirconia, copper/alumina and copper/mangania with 0.5 wt.% copper. This was extremely large compared to that obtained on the corresponding copper/silica catalyst of which the surface area was estimated to be below 10 m2/gr Cu. Basic metal oxide supports greatly affected the reducibility of the precursor and stabilized metallic copper as very fine particles, in particular at low copper loading. Effects of dispersion of metallic copper and the support upon the reaction In Figure 5 and 6, the kinetic parameters obtained were plotted against the surface area of metallic copper. On the catalyst having the surface area of metallic copper below 100 m2/gr Cu , the turnover frequency, selectivity and the activation energy are practically the same as those obtained on support-free catalyst and remain practically constant irrespective of the support used. This has been also found in our previous work (ref. 4) on the kneaded catalysts (Cu/Si0 2, Cu/A1 203, Cu/TiO Z' Cu/ZnO, Cu/CrZ0 3, Cu/CaO, Cu/FeZ0 3 and cu/sno~) of which the surface area copper formed are seemingly below 100 m /gr Cu. In marked contrast to of metallic these results, the kinetic parameters obtained were affected by the surface area at the surface areas larger than 100 m2/gr Cu. However, under this condition the kinetic parameters are also found to depend upon the support used at a given surface area of metallic copper. The effect of the support was, therefore, remarkable when the particle size of metallic copper was kept below 10 nm. The effect of the surface area of metallic copper observed in the present work could, therefore, be attributed to the support effect rather than so-called crystal size effect (refs. 14,15). SUMMARy Steam reforming of methanol was carried out over copper-containing catalysts which were supported on various metal oxides. Based upon the characterization of the cata lys ts , it was cone1uded that the c1ustered CuO is a proper precursor of the reaction. This can be produced when the loading was increased or the calcination
706
temperature was increased. On basic metal oxides, metallic copper was readily formed and allowed to be stabilized. Some support effects upon the reaction were observed at high metallic copper surface area. REFERENCES 1 A.Sugier and a.Bloch, Proc. Fourth Internat.Cong.Catal., Akademiai, Tiado. Budapest, 1971 p.238. 2 H.Kobayashi, N.Takezawa and C.Minochi, Chem.Letters, 1347(1976). 3 C.Minochi, H.Kobayashi and N.Takezawa, Chem.Letters, 705(1979). 4 H.Kobayashi, N.Takezawa and C.Minochi, J.Catal. 69 (1981) 487. 5 K.Takahashi, N.Takezawa and H.Kobayashi, Appl .Catal., in press. 6 J.J.F.Scholten ind J.A.Konvalinka, Trans.Faraday Soc., 65 (1969) 2465. 7 M.Shimokawabe, N.Takezawa and H.Kobayashi, Appl.Catal., in press 8 LS.Sventsitskii, V.N.VorobevandG.Sh.Talipov, Kinet. i Katal., 18 (1977) 201. 9 G.K.Boreskov, Proc. Sixth Internt.Cong.Cong.Catal. The Chemical Soc., London 1977 p.204. 10 M.Shimokawabe, N.Takezawa, M.Shiotani, H.Kobayashi and J.Sohma, unpublished results. 11 S.Roberts, Phys.Rev., 118 (1960) 1506. 12 D.Beaglehole, Proc.Phys.Soc., London,85 (1965) 1007. 13 Y.Kamegai, N.Takezawa, M.Shimokawabe and H.Kobayashi, to be published. 14 M.Boudart, Adv. in Catal., (D.D.Eley, H.Pines and P.B.Weisz ed.) vol.20, Academic Press, New York, 1959 p.153. 15 R.Van Hardeveld and R.Van Montfoort, Surf.Sci., 17 (1969) 90.
707 DISCUSSION C. BROOKS: Attention is called to the possibility of obtaining misleading information on copper dispersion on systems with MnOZ present where there is a strong possibility of nitrous oxide reactivity with non stoichiometric manganese dioxide present. N. TAKEZAWA Titration of metallic copper was carried out at 50°C over Cu/MnOZ' MnOZ support reacted with NZO under this condition but the amount of nitrogen evolved over Mn02 was found to be about 10% of that evolved over Cu/Mn02' For determining the surface area of metallic copper, the correction was, therefore, made for cu/Mn02 (see the experimental section described in p. Z of the paper. B. GRIFFE DE MARTINEZ: 1) Have you identified copper aluminate (CuA1204) by XRD ? 2) How can you be sure about the metallic state of the copper ? N. TAKEZAWA: 1) No, we have not. But we examined the presence of copper aluminate by UV diffuse reflectance spectroscopy as Friedman et al. did (J. Catal. ~, 10 (1978). No CUAIZ04 was detected under the present preparation conditions. Z) The presence of metallic copper was confirmed by UV diffuse reflectance spectroscopy (see Fig. 2 and discussion in p. 5 of the paper) . H. CHARCOSSET: You use N20 decomposition to measure the dispersion of Cuo. Could you compare the values with data obtained by other techniques (electron microscopy, for example) in some cases ? N. TAKEZAWA: The crystal size of metallic copper formed on a Cu/SiOZ catalyst with 10 wt % copper was examined by electron microscopic method. The size obtained was practically the same as that estimated by N20 titration. A. KORTBEEK: You have been showing apparent activation energies for the overall reaction which are varying by more than a factor of two; could it be that this is caused by diffusional limitations or by variations in intrinsic activities per active site of your different catalysts? N. TAKEZAWA: On the basis of the equation proposed by Weisz (for example, Science 179, 433 (1973)), the effectiveness factor was estimated to be 1.0. Under the present experimental conditions, we, therefore, conclude that the reaction w-s limited by chemical process. The effectiveness factor will decrease to 0.9 when we can fortunately prepare the catalyst of which the activity becomes 10 times higher than that obtained on the present catalyst. S.P.S. ANDREW: A comment: you have two catalysts, the copper which gives you the desired product and the support which gives the undesired product. With a small copper area the support wins, with a high copper area the copper wins. You need a less catalytically active support. N. TAKEZAWA: We previously found that no reaction occurred over SiOZ and Mn02 supports (Chemistry Letters, 1347 (1976) and J. Catal., 69, 487 (1981)). We, therefore, concluded that at least over Cu/Si0 2 and Cu/MnOz the metal oxide supports were inactive for methanol steam reforming reaction even when the catalysts with lower copper loadings were employed for the reaction. R. VAN HARDEVELD: In Tables 1 and Z,H2/COZ ratios as high as 7.1 are indicated, whereas the stoichiometry of your reaction only allows a ratio HZ/COZ = 3. So the mass-balance does not seem to be right unless there are by-products which you did not list in your paper, or detect in your analyses. Can you give some more details on the conversion and selectivity of the reaction, and of the by-products? N. TAKEZAWA: As by-products, HCooCH3 was formed in particular over the catalysts with lower copper loadings. This is why the ratio HZ/C02 deviates from 3.
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709
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
EFFECT OF PREPARATION METHODS AND PROMOTERS ON ACTIVITY AND SELECTIVITY OF Cu-ZnO-A1 Z03-K CATALYSTS IN ALIPHATIC ALCOHOLS SYNTHESIS FROM CO AND HZ C.E. HOFSTADT, M. SCHNEIDER. O. BOCK, K. KOCHLOEFL SUd-Chemie A.G., MUnchen (G.F.R.) ABSTRACT Four different methods have been described to prepare Cu-ZnO-A1 Z03-K catalysts promoted by Mn, Cr or Th, which were applied in the synthesis of methanol and higher alcohols as a substitute for gasoline. from CO and HZ at 3S0 0C and 100 bar. The most active and thermoresistent catalysts were prepared by coprecipitation of corresponding metal nitrates with a KZC03-water solution. Promoters like MnO favour the formation of CZHSOH; CrZ03, on the and ThO Z' predominantly of C4H gOH. The hypotheses other hand, that of C 3H70H elucidating the effect of promoters and other catalyst components are discussed.
INTRODUCTI ON The steep increase in the price of oil during the past decade prompted a feverish search for alternative fuel sources. The finding that methanol, containing about 10% of higher aliphatic alcohols, could substitute for gasoline ~. 1-3) initiated the reinvestigation (ref. 3-6) of the already well-known direct synthesis of alcohols from CO and HZ (ref. 7,8). (1)
a Cu-ZnO-Al z0 3-system used in low temThe base for our investigation was perature methanol synthesis (e.g. ref. 9-13) which we modified for the formation of aliphatic alcohols from CO and HZ' EXPERIMENTAL The investigated catalysts were prepared by the following methods: PM I. Coprecipitation of an aqueous metal-nitrates-solution at 700C with a KZC03-water-solution, during vigorous agitation for 30 min. When the pH of 6.8 was reached. the agitation was continued for about 1 h. After that the precipitate was filtered and washed with d.i.HZO at 700C (K-content <0.1%) and then dried at lZOoC for 16 hrs. The calcination was carried-out at Z80 0C for 8 hrs. Subsequently, the material was impregnated with aqueous KZC03, dried
710
for 16 hrs at 120 0C and formed into 6x3 mm pellets after 1% graphite had been added. The pelletizing pressure was kept at such a level as to produce pellets with a crush strength of 12-15 kg's. PM II. Thermal decomposition of a mixture of aqueous Cu(NH3)4C03 and 2 Zn(NH3)4C03 at 100°C in the presence of A1 203 (90m .g-l) during 2 1/2 hrs. After the filtration, the material was dried and calcined, impregnated with K2C03 and formed into pellets in the same manner as described under PM I .. PM III. Coprecipitation of Cu-, Zn-, Al-nitrates with K2C03 analogously to PM I. and additional impregnation with promoters in the form of metallic nitrates. The decomposition of the metallic nitrates was effected at 400°C during 3 hrs. Further procedures were identical to those described under PM I .• PM IV. Thermal decomposition of Cu-, Zn-amine carbonates and additional Other preparaimpregnation with promoters in the form of metallic nitrates. tional steps were analogous to PM III .. The chemical composition and physical properties of prepared catalyst precursors are summarized in Tables and 2. The catalyst precursors were activated in the test unit with an H2(2%)-N2mixture for 16 hrs from 145°C to 250°C. The activation was then continued with pure H2 for another 4 hrs. The activity measurements were conducted in the tubular flow-reactor (18 mm i.d.) at 250, 300 or 350°C; total pressure applied was 100 bar at different space velocities (1250,2500 and 5000 1 h-1.l-1cat' using 30 ml of halved (1.5 x 3.0 mm) catalyst pellets. Feed composition (vol.%): CO 29.5, CO 2 1.5, H2 62.5, NZ 6.5. The liquid reaction products taken in an 8-hour period were weighed and analysed by GC using Carbowax (0.2%) on Carbopack C (3 m column) at 65-1400C (4°C min- 1) for methanol and higher aliphatic alcohols; Porapak QS (2.5 m column) at 120°C for H and Carbowax (10%) on Chromosorb (3 m column) at 5020 130°C (2°C min- 1) for hydrocarbon determination. The gaseous reaction products and the feed were analysed by GC applying the standard technique. Chromatograms were evaluated by electronic integrators and the results treated using an on-line computer. The reduced and spent catalysts (after 72 hrs on stream) were investigated by XRD • Surface investigations (without 02 contamination)were conducted by JEOL (Mod. JSM - 35 C) scanning electron-microscope. The Cu-surface area of the fresh catalyst was determined by O-chemisorption (NZO-decomposition) using the pulse chromatographic technique (ref. 14). The obtained data are given in Table 2 and Figures 1-4.
TABLE 1 Chemical Composition and Some Physical Properties of Cu-ZnO-A1 203-K Catalysts
Catalyst
A B C 0 B1 B2 B3 B7 02
PM
I I I II I I I III IV
Nominal Content a) (wt %) CuO ZnO A1 203 30.3 43.8 61.5 43.8
43.8
31.3 33.4 21.8 33.4
33.4
33.1 17.5 11.4 17.5
9.5
Promoterb) (wt %)
-
(MnO) (Cr203) (Th0 2) (Cr203) (Cr 203)
8.0 8.0 8.0 8.0 8.0
BO (g/l)
955 989 1018 1509 895 916 1109 1393 1547
PV (cm 3/g)
0.29 0.31 0.32 0.16 0.38 0.30 0.27 0.16 0.15
PSO % Pore diameter (nm) 150001750801750 80 14 0 0.8 1.0 0.5 1.7 0.2 0.6 0.4 0.5
32.4 16.0 3.1 2.6 15.4 6.4 0.3 17.9 0.5
60.0 73.2 82.1 77.0 72.0 80.1 81.2 70.2 88.6
147.5 7.6 10.0 13.8 19.9 11.0 13.3 17.9 11.5 10.4
PM = preparation method, a) calculated on water-free material, b) all catalyst precursors contain 5.3% of K2C03 BO = bulk density, PV = pore volume, PSO = pore size distribution - measured by Hg-porosimeter of Carlo Erba (Mod. 225, 2000 bar pressure) -J .... ....
712
TABLE 2 Physical Properties of Cu-ZnO-A1 203-K Catalysts Catalyst
A B C 0 Bl B2 B3 B7 02 a) d)
Cu-Contenta) (wt %)
22.9 34.4 48.0 34.6 33.8 35.1 31.7 32.5 33.5
by chemical analysis; from XRD-spectra; e)
Mean Crystallite Size (nm) CuO d) Cue) Cu f)
80 75 70 40 77
72
54 40 15
32.3 30.5 28.1 12.9 36.7 26.8 32.8 11.4 10.0
4.0 4.2 5.6 16.0 6.4 6.9 6.3 22.2 20.6
17.4 18.4 20.0 43.5 15.3 21.0 17.1 49.3 56.2
17.9 22.1 32.3 73.9 23.8 21.5 18.2 51.2 78.7
b) NZ-BET; c) measured by O-chemisorption; calculated from Cu-SA; f) from XRD after use
RESULTS The effect of the reaction temperature and space velocity on CO-conversion as well as the yield of 1iquid products is presented in Tables 3 and 4. The final test conditions were chosen on the basis of those data and thermodynamic considerations (ref. 7 and 8). CO-conversion and the yield of liquid reaction products, especially of higher alcohols, were taken as a measure for the catalysts' activity. The selectivity was defined by distribution of higher aliphatic alcohols. The results are shown in Tables 5 and 6. DISCUSSION a) Preparation Methods The data compiled in Tables 1 and Z reveal that only PM I. provides catalysts A, B, C, B1, B2 and B3 with BET-surface areas of >50 m2/g and pore volumes of >0.25 cm 3/g and - with CuD - mean crystallite sizes of <7 nm. 2/g Aforementioned catalysts possess a Cu-surface area higher than 25 m cu' On the other hand, catalyst 0 prepared according to PM II. has an evidently lower BET-SA, lower PV and larger CuD and Cu crystallites than the catalysts mentioned above. The additional impregnation of CuO-ZnO-A1 203 precursors with metal nitrates (PM III. and IV.) leads to catalysts with a relatively low BET-SA,
713
TABLE 3 Effect of Reaction Temperature on CO-Conversion (X CO) and Yield (Y) of Liquid Products (Catalyst B, P = 100 bar, SV = 2500 1 h- 11 t- 1 corresponds to 737.5 lCO -1 -1 sg ca h \at ) XCO (%) 250 300 350
Y (g) a
55.0 33.8 21.0
CO-Converted to (%) M HA
b 185.6 115.0 50.8
464.0 287.5 127.0
78.5 72.1 53.6
0.5 7.2 23.7
-1 -1 -1 -1 3 -1 a) 9 h \at b) 9 h \at tm sg) ; M = methanol; HA = higher aliphatic alcohols; sg = synthesis gas (29.5% CO)
TABLE 4 Effect of Space Velocity (SV) on CO-Conversion (X CO) and Yield (Y) of Liquid Products (Catalyst B, P = lob bar, T = 350oC) SV ) (1 h-1 1-1 cat sg CO 1250 2500 5000
368 737 1475
XCO (%) 21.2 21.0 10.5
y
a 57.6 127.0 183.4
(g)
CO-Conversion to b
46.1 50.8 36.7
M
43.8 53.6 60.5
HA 27.5 23.7 19.5
For abbreviations see tab. 3 low PV and large CuO and Cu crystallites. Responsible for this effect is the fairly high calcination temperature (400oC) which is necessary for the decomposition of the corresponding metallic nitrates. b)
Chemical Composition With the increasing Cu/Zn atom. ratio tcat. A = 0.99, B the CuO and Cu crystallites increase.
1.34, C = 2.89)
714
TABLE 5 Effect of Preparation Method, Chemical Composition and Various Promoters on CO-Conversion (X CO) and Yield (Y) of Liquid Products ( T = 3500C, P = 100 bar, SV 2600 1 h- ll t- l) sg ca Catalyst
A B C D B1 B2 B3 B7 D2
PM
I I I II I I I III IV
XCO (%) 17.6 20.2 30.0 12.2 28.8 27.4 27.6 11.6 4.3
Y -1 -1 (g h lcat ) 110.7 132.0 188.8 76.7 115.0 158.3 156.7 67.0 27.0
CO-Conversion (%) to BP HA CO 2 M 46.8 55.7 69.6 65.6 40.5 39.3 40.3 39.1 55.6
18.0 22.8 15.0 21.2 26.2 29.8 29.1 32.9 28.5
26.2 17.4 12.2 10.8 24.9 24.5 23.0 24.0 14.9
9.0 4.1 3.2 2.4 8.4 6.4 7.6 4.0 1.0
BP = by-products, mainly unsaturated hydrocarbons; other abbreviations see tab. 3
c)
Promoters The impregnation of the catalyst precursor with K2C03-water solution and subsequent drying does not change the physical properties of the CUO-ZnO-A1 203 system too much. The introduction of promoting metals like Mn, Cr or Th to the Cu-ZnO-A1 203 system by PM I. results mainly in the increase of the CuO and Cu crystallites and the share of pores <80 nm, especially 14 nm. A1 203, which improves the thermostability of the CuO-ZnO-system,is always present in the higher alcohol synthesis catalyst (ref. 13). Effect on Activity and Selectivity As has been found by many authors (ref. 4,7), the presence of K in the Cu-ZnO-A1 203 catalysts has a pronounced effect on the formation of higher aliphatic alcohols. K could be introduced in the form of different salts (ref.7), especially like K2C03 or CH The optimal K-concentration is in the range 3COOK. of 1.4 to 3.5% (ref. 4). In the catalysts described in this study the K-concentration was held constant at 3%. d)
The data in Table 3 indicate the increase of the CO-conversion and yield of liquid products along with the increasing Cu/Zn atom. ratio (cat. A,B,C);
715
however, the maximum on higher alcohols was reached by catalyst B. The gradual increase of the CO converted to methanol in the catalysts A, Band C concurs with data given in ref. 13. Unsuitable physical properties (SA, PV, PSO) and quite large CuO and Cucrystallites in catalysts D or B7 are evidently responsible for the low COconversion and t~e low yield of liquid products. As can be seen from Table 5, the applied promoters in catalysts B1, 82 and 83 cause a substantial increase in the CO-conversion to higher aliphatic alcohols. The data in Table 6 demonstrate the effect of the incorporated promoters on the selectivity of Cu-znO-A1 203 catalysts. E.G., the 81 catalyst containing MnO provides higher aliphatic alcohols with a predominant content of ethanol. On the other hand, Cr203-promoted catalysts (B2, B7 or D2) form mainly propanols. In contrast, the Th0 2 containing catalyst (83) is resulting in a formation of butanols. The described effects can also be observed on promoted catalysts prepared by the impregnation technique (PM III. and IV.; cat. B7 and D2) . e)
Active Phases and Reaction Mechanism According to the theory of methanol formation on Cu-ZnO, presented recently by Klier (ref. 15,16), one can assume that Cu+ solved in ZnO and metallic
TABLE 6 Effect of Promoters on Selectivity of Cu-ZnO-A1 203-K Catalysts Cat.
Promotera)
B B1 B2 B3 B7 02
MnO Cr203 Th0 2 Cr203 Cr203
Distribution of Higher Aliphatic Alcohols (%) C4 C2 C3 C5 1i21223.1 38.4 18.3 11.9 18.2 18.0
2.5 2.3 2.4 2.2 2.5 2.5
33.1 36.1 36.5 25.9 37.4 38.1
2.0 2.0 1.6 4.0 1.5 1.4
31.4 14.5 34.6 42.9 34.1 34.0
1.9 2.0 1.5 4.3 1.4 1.3
4.0 2.7 2.3 5.6 2.2 2.1
>C 5 2.5 2.0 2.8 3.2 2.7 2.6
C4,2-= butan-2-01, C2 ethanol; C3,2-= propan-2-o1; 1-= propan~1-o1; i = 2-methylbutan-1-ol, 1-= butan-1-ol; C5 = pentanols, )C5 = higher aliphatic alcohols, a) all catalysts contain 3% K ard A1 203
716
copper could be active phases in the synthesis of higher aliphatic alcohols. As was proven by other authors (ref. 17), the Cu+ -ZnO-phase favours the formation of oxygen-containing species (2) in the reaction of H2 and CO. On the other hand, species with a methylene structure (3) will be formed mostly on metallic copper. OH
I
H-C-H
(2)
(3)
I
*
It can be assumed that promoters like Mn, Cr or Th affect the Cu+/Cuo-ratio and also the concentration of the surface species (2 and 3), allowing the formation of alcohols with C-chains of different lengths. However, a further hypothesis can be established on the basis of the pore size distribution data of differently promoted catalysts (B1, B2, B3) as is demonstrated in Table 1. The gradual increase in the share of pores with a diameter of <.80 nm is accompanied by a growing diffusion limitation and so the surface intermediate, formed from species (2) and (3), will not react only with hydrogen to provide ethanol after desorption but more likely with another species (3) to result in propanol - or by subsequent addition of species (3) - in butanol-intermediate. The interreaction of surface species is evidently being accelerated by Kcontaining active sites, as has already been mentioned by some authors (ref. 17, 18). As can be seen from Table 5, a consecutive competitive reaction to the methanol and higher alcohols synthesis is the CO-shift reaction (4) proceeding on account of water formed by reaction (1). CO + H20 = H2 + CO 2 (4) Another competitive reaction is the formation of hydrocarbons - mostly olefines - by dehydration of produced alcohols or corresponding surface intermediates. This reaction is favoured by A1 203 or promoting oxides like MnO or Th0 2" Its extent is strongly limited by potassium, which poisons the acidic active sites. f)
Catalyst Deactivation Activity measurements conducted during 72 hrs revealed that catalysts of the B-series, especially B2 and B3, maintained their original activity. This property can be attributed to the thermo-stabilizing effect of Cr203 or Th0 2. Catalysts C, or 02, on the other hand, lost their activity quite rapidly. As a measure of the investigated catalysts' thermal stability can serve the size of the eu-crystallites after use (see Table 2) which exhibits a parallel trend
°
717
2
3
4
Figures 1-4 represent SEM photographs (mag. 10 000 x); 1 of catalyst B2 after reduction, 2 after use, 3 catalyst C after reduction, 4 after use. to their activity loss. The Figures 1 and 2 show the SEM representation of the surface of catalyst B2 after reduction and after 72 hrs on stream. As is evident, there is no substantial difference in the morphology of both samples. However, catalyst C (Figures 3 and 4) shows marked changes in the surface of the spent catalyst which are caused by recrystallization.
718
ACKNOWLEDGEMENTS We are indebted to Dr. J. Ladebeck for the evaluation of the XRD spectra, valuable discussions and for a reading of the manuscript. REFERENCES 1 J.L. Keller, Hydrocarbon Proc., 58(1979)127. 2 M. Schaffrath, Erdoel und Kohle, 29(1976)64. 3 M.I. Greene, Symp. on Indirect Liquefaction of Coal For Fuel Production, Detroit National Meeting 1981, paper No. 396. 4 C.E. Hofstadt, K. Kochloefl and O. Bock, DOS 3 005 551 (1981). 5 A. Sugier and E. Freund, DOS 29 49 592 (1980). 6 H.F. Hardman and R.I. Beach, EPA 0 005 492 (1979). 7 G. Natta, V. Colombo and I. Pasquon in P.H. Emmett (Ed.) Catalysis Vol. 5, Reinhold, New York, 1957, Ch. 3,p.131-174. 8 B. Cornils and W. Rottig, in J. Falbe (Ed.) Chemierohstoffe aus Kohle, G. Thieme, Stuttgart, 1977, Ch.10,p.323-333. 9 Ch.L. Thomas, Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970, Ch.14,p.151. 10 F.C. Broecker, DBP 2 056 612 (1979). 11 B.M. Collins, DOS 2 302 658 (1973). 12 W. Kotowski and coworkers, DOS 2 928 435 (1980). 13 E.K. Dienes, R.L. Coleman and A.L. Hausberger, US Pat. 4 279 781 (1981). 14 B. Dvorak and I. Pasek, J. Catal., 18(1970)108. 15 R.G. Hermann and coworkers, J. Catal., 56(1979)407. 16 R.G. Hermann, G.W. Simons and K. Klier, Proc. of the 7th Intern. Congress on Catal., Tokyo, 3o.June - 4.July, 1980, Elsevier, Amsterdam,1981 ,Vol .A, p.475. 17 P.J. Denny and D.A. Whan, in I.e. Kemball, a.A. Dowden (Eds.),Catalysis, Vol. 2, The Chemical Soc. London, 1977, p.46. 18 R. Nagishi, Rev. Phys. Chem.,Japan, 18(1944)47.
719 DISCUSSION P. COURTY: Have you characterized the crystalline structure of the coprecipitates produced by the first method? Do these structures (may be similar to the hydrotalcite and/or malachite-like phases) vary with the nature of the promoter? Have you compared the effect of the different alkali elements on the selectivity towards higher alcohols ? K. KOCHLOEFL We measured the XRD-spectra of dryed catalyst precursors (before calcination) and identified different phases like copper hydroxide/carbonate (malachiteT, mixed copper, zinc hydroxide carbonates or zinc hydroxide carbonate. The ratio of these was mainly affected by the copper- and zinc content in the material investigated; however, we did not observe the effect of promoters on the composition of the phases mentioned. Sodium promoted CU/Zn-catalysts show higher activity with respect to higher alcohol formation than those conatining potassium; however, their selectivity (hydrocarbon formation) is lower. Rubidium promoted catalysts seem to be very active, but too expensive. E. FREUND: With a basic promoter like thoria you tend to get an alcohol distribution of the isobutyl oil synthesis type. Don't you think that for such promoter the mechanism proposed by Natta : (formation and then reduction of carbocylates) may apply ? K. KOCHLOEFL: In our paper we mentioned that oxygen-containing species (I) will be formed from CO and H2 on a Cu+/ZnO phase. OH CO +
I
+
2H2~H-C-H
I Cu+/znO
H
(I)
Cu+/ZnO
On the other hand, metallic copper will favour the formation of species with a methylene structure (II).
The synthesis of methanol and higher alcohols can be assumed as represented in scheme I. However, this scheme does not involve the effect of potassium (its function could be assumed to be analogous as in the Fischer-Tropsch synthesis). It cannot be excluded that oxygen-containing species react with CO adsorbed on metallic copper according to scheme II, which involves the function of potassium and is close to the original Natta concept. OH
I
H-C-H
+ __ -CH2
~--------
!-
Cu+/ZnO
CuO
(1 )
Cu+/ZnO
CuO
(2)
CuO
Cu+/ZnO
Cu+/ZnO
CuO
Al low Cu+/Cuo ratio (effect of promoters), then reactions (1) and (2) will be more favoured.
720 Scheme I OH
(cont'd)
I
H-c--H
+
.L
(3)
H
I Cu+/ZnO
+
(4)
H
I Cu+/zno At high CU+/Cu o ratio, then reaction (3) will be more favoured. Scheme II K
H
I
H-C-OH
+
..
0
I
I
H
H
I
>
I H- C-OK
+
0
/
(1)
I
I
Cu+/ZnO
H
0
I
II C
H-C-OK
+
H
I ° II
I
I
Cuo
Cu+/ZnO
(2)
H-C-C-OK
Cuo
Cu+/znO
At low Cu+/cu o ratio (effect of promoters) then reaction (2) will be more favoured. H
H
0
I
H-C-C
I
"\OK
H
I HO-C-H
+
')
I
I
0
K
I II H-C-C
+
\
°
I
bCH 3
H
(3)
I
Cu+/zno H
H
I ~ H-C-C
_I
+
\
.,
2H2
I H-C-CH -OH 2
OCH}
I
+
CH3OH(g)
(4)
Cu+/ZnO H
I
H-C-CH-OH _1_ 2
+
I I
-'
)
C 2HSOH(g)
(S)
Cu+/ZnO
OH H-C-H
H
+
H
I cu+/zno
~
CH}OH(g)
(6)
721 A. KORTBEEK: I have problems understanding'~he mechanism that you are proposing for the formation of higher alcohols from syngas: on the one hand, you are suggesting that a hydroxy-methyl species is formed on a Cu+ species via a non-dissociative adsorption of CO, whereas on the other hand you propose that methylene species are formed on metallic copper; what evidence do you have to support a dissociative adsorption of CO on metallic CU and how would you compare your mechanism with that proposed recently by Katzer et al. (Transactions of the Faraday Society, 1982) ? K. KOCHLOEFL Unfortunately, we do not have any direct proofs of the prposed mechanism. The formation of species with a methylene structure on metallic copper was obs~rved by different authors (e.g. V.M. VIa senko and coworkers, React. Kinet. Catalysis Letters, ~, 195 (1977)). Thus, on the basis of these facts, we speculate that hydroxy-methyl species could react with methylene ones as represented in Scheme I. At present we are conducting experiments with feeds containing alkylformates (C2-C4) or carboxylic acids (C2-C4) to prove a mechanism analogous to the Natta's concept. It seems that such a mechanism (Scheme II) is much more probable. Unfortunately, I did not find the Katzer publication in Trans. Faraday Soc. 1982; however, a paper entitled "Ethanol Formation Mechanism from CO and H2" by Takeuchi and Katzer appeared in J. Phys. Chem., 86, 2438 (1982). A carbene theory presented there seems to be valid for Rh supported on Ti02, but I am not sure whether analogous presumptions could be made for K-containing Cu/Zn-catalysts. D. CHADWICK: To add to the comment of the previous questioner, studies of CO chemisorption on a range of copper crystal faces by surface science techniques have produced no evidence for CO dissociation. I think it unlikely, therefore, that the methylene structure will be formed on metallic copper. M.V. TWIGG: What is the effect of pressure on conversion and selectivity to higher alcohols with the catalysts you have described? K. KOCHLOEFL: With increasing pressure of the syngas the CO-conversion as well as the yield of higher aliphatic alcohols and methanol increases. Thermodynamicaly speaking, higher CO-conversions have to be reached for ethanol than for methanol, for propanol than for ethanol etc ... This can be demonstrated e.g. by ~-values (350°C) for methanol (3,15 10- 5), ethanol (3,4 . 10- 3) and butanol (4,10 . 10- 2) and by Kp-definition : n PA/C· PH20 CH30H : n = 0, m 1, s = 2 Kp m s 4, s = 8 PCO .PH C4HgOH: n = 3, m 2 However, we did not observe a pronounced effect of pressure on the formation of individual alcohols in the studied pressure range (50-100 bar), because our conversions have not reached a thermodynamic equilibrium.
F. TRIFIRO: It is known that K is a promoter of selectivity either for the ZnO-Cr203 system or for CUO-ZnO-AI203 as you have shown in your paper. Could you explain the strong poisoning effect of K on the activity of the system that you investigated ? K. KOCHLOEFL: We observed that copper recrystallizes faster in the Cu/ZnO catalyst containing potassium than in an analogous system free of this element. However, promoters like A1203' Cr203 etc. exhibit a stabilizing effect. The lower activity of a Cu/ZnO catalyst, containing K, with respect to methanol formation and in comparison with a potassium-free catalyst, could be explained by the different mechanisms of methanol synthesis. One can postulate that the methanol synthesis proceeds on a potassium-free Cu/ZnO catalyst according to reaction (3) in Scheme I. On the other hand, on the Cu/ZnO catalyst containing potassium, methanol will be formed partially by hydrogenolytic splitting of the surface formate (Scheme II, e.g. 4).
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
723
PREPARATION OF Cu-Zn-Al MIXED HYDROXYCARBONATES PRECURSORS OF CATALYSTS FOR THE SYNTHESIS OF METHANOL AT LOW PRESSURE P. GHERARDI, O. RUGGERI, F. TRIFIRO'* and A. VACCARI Facolta di Chimica Industriale, Viale del Risorgimento 4, 40136 Bologna (Italy) G. DEL PIERO, G. MANARA and B. NOTARI ASSORENI Co, Via Fabiani, 20079 S. Donato Milanese (MI)(Italy)
ABSTRACT The aim of this work was to study the preparation chemistry of Cu-Zn-Al mixed oxides active in low pressure methanol synthesis. A ternary precursor with hydrotalcite-like structure was successfully prepared by precipitation: it was obtained pure when the Cu+Zn/Al ratio was nearly 3 and the Cu/Zn ratio 1. At different ratios, it was obtained together with other phases, Cu-Zn hydroxycarbonates or Al(OH)3' The ternary phase decomposes at lower temperatures than the Cu-Zn hydroxycarbonates and forms, by reduction, very small copper crystallites (nearly 3.0 nm). The surface area, the CuO crystal size and the pore volume have been correlated with the nature of the precursors and the relative percentage of the three ions,
INTRODUCTION Cu-Zn-Al mixed oxides are well known as catalysts used in methanol synthesis and CO shift conversion reaction at low temperatures. These catalysts have unjergone extensive research both from the scientific point of view and from the point of view of patenting [1-7J. In previous papers, we have reported the results of our studies on the influence of the preparation method [8J and on the activation by reduction [9J. The aim of this research was to study the influence of the chemical composition on the nature of the precursors and on thp properties of the species formed
724
by calcination and reduction.
EXPERIMENTAL Precursors with different atomic ratios (see below, Table 1) were obtained by precipitation from a solution containing the dissolved nitrate salts of the elements with a slight
exces~
of NaHCO
l
at 60°C, with energetic agitation. The so-
lution was sUbsequently filtered and then washed with distilled water at about 60 °C until the sodium concentration, determined with a Mark II E.E.L. flame photometer, was less than 0.05% (as NaZO). The drying temperature was 90°C. The precursors were calcined in air for 24 hours at 360°C, reduced to a powder and crushed to a particle size of 0.125-0.250 mm. The reduction reactions were performed in a Cahn RG thermoba1ance using a flow, as previously described [9]. X-ray data were collected by means of a H 2-NZ standard Philips diffractometer. A special camera was applied to the diffractometer in order to keep the reduced samples under a dry nitrogen atmosphere throughout data collection. A C. Erba Sorptomatic model 1826 apparatus with N adsorption was used to 2 measure the surface area, the volume and the radius distribution of pores. The radius distribution of pores was calculated by means of the Pierce's method[lO]. The DSC - curves were obtained by means of a differential scanning calorimeter Perkin Elmer model 2C in the 50-500°C range. E.S.R. spectra were obtained at the liquid Nitrogen temperature using a Varian E12 model spectrometer operating in X band and equipped with the conventional accessories.
KESULTS AND DISCUSSION Nature of the precursors From Table 1, it is possible to note that changing the ratios ~=
a=
Cu/Zn , it has been possible to obtain a single ternary phase for
prox. 3 and~~l
zn cU;l and
a=
ap-
(Cat 8 and Cat 9 of Table 1), whereas in the case ofa)3 or~>l
this phase is accompanied with Cu-Zn hydroxycarbonates, which are generally of the malachite kind. The X-ray pattern of the sample Cat 7, in which both the described phases can
725
be seen, is reported in Figure 1. TABLE 1
Atomic ratios of the elements and compounds identified by X-ray diffraction in the different
precursors
prep~red
Samples
Cu:Zn:Al
Identified compounds
(as atomic ratio %) Cat 1 " 2 " 3 " 4 " 5
60.0:30.0:10.0 45.0:45.0:10.0 30.0:60.0:10.0 55.3:27.7: 17.0 41.5:41.5: 17.0 27.7:55.3:17.0 60.8:15.2:24.0 50.7:25.3:24.0 38.0:38.0:24.0 25.3:50.7:24.0 34.5: 34. 5: 31 .0
" 6 7 u 8 " 9 10
n
11
(after drying at 90°C) HY; M M; HY HY; quasi-amorphous phases HY; M HY; i'1 HY; quasi-amorphous phases HY; M HY; jVj HY HY HY; Al (OH) 3
HY
hydrotalcite-like phase (Cu,Zn)sA12C03(OH)16'4H20
M
malachite-like
(Cu,Zn)2C03(OH)2
*
*
(Cu,Zn)6AI2C03(OHl1604H20
• (Cu,Zn)2 C03 (OHh
t
>-
en
e
•
GI
•
*
*
c
10
30
70
50 2e~
Fig. 1. X-ray pattern of the sample Cat 7 dried at 90°C.
726
The samples Cat 3 and S, in which the hydroxycarbonates are quasi-amorphous and may be interpreted as auricalcite- and/or hydrozincite-like phases, are exceptions. In the case ofa(3, the aluminum in excess of the ternary phase stoichiometry is precipitated as hydroxide. The X-ray pattern and crystallographic parameters of the ternary phase are similar to those reported for the mineral hydrotalcite M96A12(OH)16C03'4H20[11]. Rhombohedral hydrotalcite cgnsists of positively charged brucite-like layers [M9
alternating with disordered, negatively charged interlayers 2+ In our compound Cu 2+ and Zn 2+ ions substitute the Mg ions. More
sA1 2(OH)lS]2+
[C0 3·4H20]2-. detailed information concerning the structure and properties of this phase will be published in a later paper[12]. Study of the calcination process
The calcination process of the precursors has been investigated by differential scanning calorimetry, thermogravimetry, X-ray diffraction and E.S.R. spectroscopy. The DSC-curves of several precursors are shown in Figure 2. The various endothermic transitions, all of which were accompained by weight loss, were interpreted on the basis of X-ray and thermogravimetric analyses. The transition in the temperature range 170-190°C may be attributed to the crystallization water-loss of the hydrotalcite-like phase, with the formation of quasi-amorphous phases (hydroxycarbonates in all probability), while the peaks at 230-240°C and at 280-290 °C may be attributed to the decomposition to oxides of those phases. In the samples in which the hydrotalcite-like phase is accompanied by Cu-Zn hydroxycarbonates, there are also peaks at 340-360°C and at 380°C, attributable respectively to hydrozincite- and malachite-like compounds[13]. The calcination at temperatures increasing up to 3S0 °C showed a gradual de2 crease in E.S.R. total intensity, suggesting that the Cu + ions are progressively grouped into clusters on the supports. This effect was less marked in the case of samples containing only the ternary phase (for example Cat 9 of Fig. 3). It is therefore possible, given the hydrotalcite-like phase as a starting point, to obtain the related oxides at temperatures lower than in the case of 2+ binary hydroxycarbonates and with a greater dispersion of the Cu ions inside the diamagnetic matrix.
727
100
500
300 Temperature (OCI
Fig. 2. Differential scanning calorimetric curves of some precursorSl a) catalyst 2; b) catalyst 5; c) catalyst 7; d) catalyst 9. I
I
10
\
• Catalyst 9
l-
."
"
7
~
~
·~5
I-
....c: Q)
o
o
-
\ ~~
-
\....
'.-J
I
200
-
I
-
:A"
I
400
Temperature (OCI
Fig. 3. E.S.R. total intensity as a function of the calcination temperature.
728
The X-ray patterns of the sample Cat 9 are shown in Figure 4: the precursor dried at 90°C is a well crystallized hydrotalcite-like phase (Fig. 4-A), whereas the compound calcined at 360°C shows a quasi-amorphous pattern (Fig. 4-B), from which the extremely low dimensions of the crystallites of the obtained oxides are clearly apparent.
A
-
I
I
I
10
30
50
I
70
2tJo -
> "00
B
c
Q)
I
of: 15
I
I
25
35
I
55
45
2tJo Cu Cu
~ I
I
15
25
~
C
I
35
45
2tJo -
I 55
Fig. 4. X-ray patterns of Cat 9: A) after drying at 90°C; B) after calcination for 24 hr at 360°C; C) after reduction in H flow (Cu-Karadiation; 2-N2 ). = 0.15418 nm) . In the case of some calcined samples showing a relatively greater sharpness of the peaks, X-ray identification of the involved phases was possible, namely CuO, ZnO and ZnA1
204. ternary precursors.
The smallest oxide crystal sizes were obtained from pure
The reduction of the oxides in HZ-N
showed that binary phases are more easiZ ly reducible than the pure ternary phase. This difficulty in reduction might be ascri bed to greater Cu-Zn i nteracti on[4]. However, the dimensions of the copper crystallites obtained from the pure hydrotalcite-like phase (Fig. 4-C) were lower (nearly 3.0 nm) than those obtained by binary precursors (5.0 - 7.0 nm)[9].
729 I
100
~
I
•
Cu -1 Zn -
•
l-
~
~
... -c
50
-
•
Q)
0
I-
...
~
•
I
0 0
10
E
-
c::
t
-
5
Q)
N
C/)
•
•
•
t
:::3 C/)
-
•
E Q)
I
I
20
-
~ .....
en
...::>-
o
I
0
•
40
AI Content (atomic%) Fig. 5. Surface area and CuD crystal size of the calcined precursors as a function of the aluminum content (the arrow shows the samples obtained from precursors in which only the hydrotalcite-like phase was identified). Figure 5 shows the surface area and CuD crystal size values, determined by profile-fitting methods, in the calcined samples in relation to the aluminum content (for the ratio Cu/Zn = 1). It may be noted that the CuD crystal sizes decrease with the increase in the aluminum content, together with the passage from samples obtained from biphase precursors and others obtained from precursors in which the ternary phase was pure or accompanied by Al(DH)3. The net increase in the surface area of the Cat 11 sample may be ascribed to the microporosity of the alumina obtained by calcination of Al(DH)3 (Fig. 6). The Cat 9 sample obtained by calcination of the pure hydrotalcite-like phase presents CuO crystallite dimension, surface area and cumulative pore volume values lower than those of the samples obtained from biphase precursors. Likewise to Figure 5, Figure 7 shows the surface area and CuO crystal size values in the case of calcined samples in relation to the ratio Cu/Zn (for an aluminum content of 24%). It may be noted that the CuO crystal sizes decrease with the increase in the zinc content. The high surface area of sample Cat 10 may also be interpreted in terms of different porosity: this sample in fact presents a pore radius distribution characterized by a large number of small
730
and large pores, therefore with a cumulative pore volume value similar to that 3
of other samples (for example Cat 10 = 0.536 and Cat 7 2/g respectively). a greater surface area (113 and 57 m
0.528 cm /g), but with
,Ol
ME
Cu/Z n= 1:1
c Q;"OA
&
E ::;,
0
"0
..
...
•
>
Q)
Catalyst 2 5 " 9 "
11
&0.2
...
.~
ell
"S
E
Bo 1.0
10 average pore radius (nm)
2
10
Fig. 6. Cumulative pore volume distribution of some calcined precursors with different aluminum content. I
• 100 _
t
I
I
I
-
AI=24%
10
OJ ~
§.
E
-
...
•
CI)
-c
50
CI)
CJ
&
•
t
I.....
•
...::J ~
t
0 0
I
CI)
N
- 5~ - o
(J)
• • t
Cf)
E.
•
I
2
I
I 4
0
Cu/Zn ratio
Fig. 7. Surface area and CuD crystal size of the calcined precursors as a function of the Cu/Zn ratio (the arrow shows the samples obtained from precursors in which only the hydrotalcite-like phase was identified).
731
This may be attributed to the formation in Cat 10 of a greater quantity of spinel Zn A1
204
as compared with other samples, associated with its chemical
composition.
CONCLUSIONS We were ab1e to prepare a Cu-Zn-A1 ternary phase having a hydrota1cite-like structure. . Cu + Zn This phase was obtai ned in an almost pure form when the r at i o -A-1- was fixed at approximately 3 (which corresponds to the hydrota1cite mineral ratio Mg/A1). With lower ratios, free alumina is present, whereas with an increase in the ratio, considerable quantities of Cu- Zn containing compounds are formed, generally malachite-like and occasionally auricalcite and/or hydrozincite-like phases. The ternary phase decomposes to oxides at a lower temperature than hydroxycarbonate phases and leads to the smallest crystal size oxides which give rise, through the reduction in the H - N flow, to copper crystallites of extremely 2 2 small size (nearly 3.0 nm). The production of these very small crystallites is doubtless due to the perfect distribution of the elements obtained inside the ternary structure. In this way there are therefore
premises
for the development of indu-
stria11y valid catalytic systems for the low-temperature synthesis of methanol. REFERENCES 1 G. Natta, in P.H. Emmet (Ed.), Catalysis, Vol. III, Reinhold Publ. Corp., New York, 1953, Ch. 8. 2 E. B1asiak, Polish Patent n. 52,572(1967). 3 J.T. Gallagher and J.M.Kidd, British Pa~Qnt n. 1,159,035(1969). 4 R.G. Herman, K. Klier, G.W. Simmons, B.P. Finn, J.B. Bulka and T.P. Kobylinski, J. Cata1., 56(1979)407-429. 5 O.Yu. Prudnikova~O.V. Makarova and T.M. Yurieva, React. Kinet. Cata1. Lett., 14(1980)413-416. 6 P. Davies and A.J. Hall, U.S. Patent n. 3,961,037(1976). 7 J. Voe1ter, H. Berndt and G. Lietz, Chem. Tech. (Leipzig), 28(1976)606-610. Ibero8 O. Ruggeri, A. Tredici, F. Trifiro and A. Vaccari, Proc. 8t~Simposio americana de Cata1isis, La Rabida (Spain), July 12-17, 1982. 9 O. Ruggeri, F. Trifiro and A. Vaccari, J. Solid State Chem., in press. 10 C. Pierce, J. Phys. Chem., 57(1953)149-152. 11 R. A11mann and H.P. Jepsen,--Neues Jatrb. Mineral. Monatsh., (1966)544-551. 12 O. Ruggeri, F. Trifiro, A. Vaccari, G. Manara and G. Del Piero, in preparation. 13 C.W. Beck, Am. Mineralogist, 35(1950)985-1013.
732 DISCUSSION S.P.S. ANDREW : I have theory that the more different types of atom there are in a crystal the more unlikely is it to be immediate precipitate and more likely to form as a result of aging the initial precipitate. Did hydrotalcite only form after aging the initial precipitate? F. TRIFIRO: Our catalysts have been prepared at constant pH adding the solution of the Cu, Zn and Al nitrates to the solution of NaHC03 and stirring for 30 min. We think therefore that an aging process does not appear to be necessary in order to obtain the hydrotalcite-like phase with our method. In preparation methods which consider the separate precipitation of the three elements, an aging step would be essential in order to obtain the hydrotalcitelike phase. P. COURTY: With respect to your results, I will mention that pure hydrotalcite phase can also be selectively produced by coprecipitation with Na2C03 (cfr paper by P. Courty and C. Marcilly, this Symposium). Cuo formation, already described by J. Longuet-Escard (ref. 40 in our paper) is only due to aging (T ) 80°C, pH ) 8) in mother liquor which produces dehydration of the copper-containing phase into black and sintered copper oxide. F. TRIFIRO We agree with your observations. The real problem to obtain the pure hydrotalcite-like phase is that of the precipitation at constant pH and of the ratios between the components. Our method of "inverse precipitation" with NaHC03 allows us to operate in a very simple way without any particular precaution (as pH and flow controls, etc .. ) and still obtain the desired phase in a pure state. The simplicity of the method makes it suitable for the industrial production of the catalyst. About the second statement, we should like to report that in some of our preparations, we obtained very stable copper-containing phases that did not give rise to CuO formation. W. GLASZ: Are the catalysts based on hydrotalcite more or less active than those prepared from malachite-like precursors ? F. TRIFIRO: Our data show that the catalysts obtained from the hydrotalcitelike phase present a catalytic activity slightly higher than that of catalysts obtained from malachite-like phases. In every case, the reduction step appears to be particularly critical: during the step, copper metal particles are formed and the catalytic activity is strictly connected with the dimensions of the particles, as already discussed by Kochloefl and Notari (Proccedings of the 7th International Congress on Catalysis, Tokyo, 1980, Elsevier Scientific Publishing Co, p. 486. K. KOCHLOEFL Your data summarized in Fig. 7 are in good agreement with ours. However, there are Some discrepancies in the pore distribution. Could you comment the curves in your Fig. 6. F. TRIFIRO: The curves of Fig. 6 allOW us to attribute the high surface area of the sample with 31% of Al to the microporosity of the alumina, which is formed by calcination of the Al(OH)3 precipitated as a separate phase. From this figur~ it is also possible to see that catalyst 9, obtained from a pure hydrotalcitelike phase is characterized by a smaller pore volume. Concerning the difference with your data, it is necessary to pay attention to the fact that your catalysts were prepared in a different way and further im~ pregnated with K2C03' We have observed that after impregnation with about the same amount of K, there is a decrease in the surface area value and in the pore volume that are related to a strong modification of the pore size distribution. We believe that the exact method followed to introduce the K salt might have an influence on the surface area and pore volume.
733 D.C. PUXLEY: Haveyou investigated the phase changes and ordering of the oxide form of the catalyst as a function of calcination temperature? One might expect significant changes in X-ray diffraction patterns and ESR spectra at high calcination temperatures, say aOO-1000°C. F. TRIFIRO: We have investigated the behaviour of some catalysts up to a calcination temperature of 600°C. The X-ray pattern of the hydrotalcite-like phase disappears at 150°C when a pattern similar to that of hydrozincite and/or aurichalcite appears. In the range l50-350 oC, we observe the gradual transformation of hydroxy-carbonate phases into oxide phases; the transformation is complete at 350°C .• At higher temperatures, sintering phenomena take place and the phases are now CuO, ZnO and ZnA1204 (possibly also CuA1204). The ESR total intensity decreases in the range up to 350°C, after that for higher temperatures, we observe an anomalous increase of the signal, probably associable to the sintering phenomena.
This page intentionally left blank
735
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of CatalystsUl 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
©
PREPAP~TION
AND CHARACTERIZATION OF VERY ACTIVE Cu/ZnO AND Cu/ZnO/A1
203
LTS
CATALYSTS USING A SINGLE PHASE Cu-Zn PRECURSOR COtTOUND :!: G. PETRINI, F. 1~~TINO, A. BOSSI and F. GARBASSI Istituto Guido Donegani S.p.A., Centro Ric~rche Novara, Via Fauser, 4 28100 Novara (Italy)
ABSTRACT Cu-Zn and Cu-Zn-Al catalysts for the low temperature CO shift reaction are investigated. It is shown that very active catalysts are obtained starting from a single phase precursor having the hydrozincite structure. From chemico-physical characterization and activity results, it is inferred that in reaction conditions a free Cul + containing phase is the active phase. The activity of the binary catalysts is maintained when the Al component is added successively, while by coprecipitation a less active catalyst is obtained.
INTRODUCTION The role and chemical state of copper in ZnO-CuO catalysts, with or without Al 0 , for the low temperature CO shift (LTS) reaction is till now controversial 2 .. f) Th e 3act~ve . phase was ~n. d i~cated as f~nely d~spersed meta 1 l~c. Cu ( re. 1, b ut evolution of the copper crystallites towards a different phase was observed by Semenova and cow. (ref. 2) in reaction conditions. More recently, Herman et al. (refs. 3-4), in an extensive investigation on a similar catalytic system for the synthesis of methanol, have brought good evidence of the existence of a solid solution of Cul + in ZnO, which was indicated as the active phase. The aim of this work is to bring a contribution to the knowledge of the Cu-Zn-(Al) catalytic system, starting from a precursor constituted by a single phase Cu-Zn mixed basic carbonate, rather than from a mixture of phases. EXPERIMENTAL Preparation of catalysts Precursors were prepared by adding an aqueous solution of Na C0 and NaOH to 3 a solution of Cu and Zn nitrates in suitable proportions. The p~ec~pitate was filtered, washed with hot distilled water until t~ the disappearance of Nal + ions and dried at 383 K overnight. After powdering, samples were calcined at 588 K in air for 6 hours, mixed with 2% graphite and tableted. Tablets were subsequently broken and the 35-50 mesh fraction used for the activity tests. :!:Present address: Ausind S.p.A., Attivita Catalizzatori , Via Fauser, 4 Novara (Italy).
736 AI-containing samples were prepared by coprecipitation, following the above described proced~re, or alternatively by adding Al(OH) to the preformed Cu-Zn . . 3 . prec1p1tate. Physico -chemical characterization Chemical analyses for Zn, Cu, CO
2-
and N0 3
were carried out on dried
sample~;
3 . ter l ca ' C1nat10n. Na+ analyses were made a f
X-ray diffraction (XRD) patterns were obtained with a Philips PW 1050 diffractometer with Ni-filtered CuK~ radiation. Total specific surface areas were measured by N adsorption at 77 K (B.E.T. 2 method) with ~ Carlo Erba Sorptomatic System. Copper ~urface areas were determined by oxygen chemisorption at 123 K, assuming an area of 0.186 nm2 per Cu atom, as measured by N adsorption on pure Cu. The Cu/O stoichiometry was assu2 med to be 1:1. Electron micrographs were obtained using a Philips EM300 electron microscope. The thermal stability of precursors was studied by a Du Pont 900 Differennm Scanning Calorimetry (DSC) cell. Temperature Programmed Reduction (TPR) diagrams were obtained by an apparatus described elsewhere (ref. 5). X-ray Photoelectron Spectroscopy (XPS) was performed with a Physical Electronics Mod. 548 system connected to a PDP 11/50 computer, using ~lgK~ radiation Quantitative surface analysis was obtained from the digitally integrated peak intensities with the elemental sensitivity factor method (ref. 6). Activity measurements 3) Catalysts (3 cm were placed into a standard flow reactor and reduced with a 0.5-3% H in N gas mixture up to 503 K. Activity tests were performed on re2 2 duced samples at 1 x 105 Pa in the temperature range 453-503 K, with a molar ratio H mixture = 0.5 and 6.9 sec- l space velocity V. The gas mixture 20/gas composition was 3% CO, 15% CO 58% H and 24% N • 2 2, Rate constants were calculated according to tfie 1st order equation: K = -v log (I-F) where F is the CO conversion. Catalyst performances were compared by interpolation at 473 K and normalized to Cu surface area values. TABLE 1 Chemical analysis of Cu-Zn samples after drying Sample ZnO C3 C5 ClO C16 C31 C54
* After
Zn (wt%) 59.1 57.1 56.1 52.7 49.7 40.2 26.7
Cu (wt%)
1.7 2.9 6.0 9.0 18.0 30.5
heating at 588 K.
CO (wt%) 2
N0 (wt%) 3
14.8 15.0 14.3 14.4 15.0 15.1 10.2
0.3 0.6 0.6 0.6 0.6 0.5 7.3
Na (wt%)* 0.002 0.004 0.005 0.004 0.004 0.003 0.003
Cu/Zn (Zn+Cu)/C0 2 (atomic) (molar) 0.034 0.058 0.111 0.184 0.459 0.854
2.6 2.6 2.7 2.7 2.6 2.6 3.9
•
737 RESULTS Binary samples Zn-Cu precipitates. Chemical analyses of binary samples after drying are reported in Tab. 1. Samples are identified by symbols Cx, where x represents the Cu(Cu+Zn) atomic percentage. All samples are characterized by a very low content of Na and nitrates (except for sample C54). The XRD powder patterns, reported in Fig. 1, show the presence of one phase only, having the hydrozincite Zn (CO) (OR) structure (ASTI! 19-1458). An ex. .' . samp 1 e C5, 4 were h 5 alSO ) 2 the d1ffract1on 6. .. cept10n 1S aga1n llnes of copper h y d roxide nitrate Cu2(OR)3N03 were identified (ASTM 15-14). The presence in the samples of (Cu,Zn)2(OR)ZC0 (ASTM 18-1095) and Zn4C03(OR)6·R20 (ASTM 11-:87) can be 3 excluded, on the bas1s of the X-ray patterns. XRD photographs taken w1th the Guinier camera confirm the above picture. DSC data are consistent with the presence of a single-phase in the precipitates with low or medium Cu loadings. A single narrow decomposition peak was obtained in the DSC diagrams of all samples but C54. A shift of the maximum peak temperature by changing the Cu content was observed (Fig. 2). Such displacement assumes the highest value of 603 K for sample C3l, while both the peak maxima observed at the highest Cu content are lower than 533 K and very similar to those obtained in the absence of Zn.
65
60
55
50
45
40
35
30
25
20
15
29
Fig. 1. X-ray diffraction spectra of Cu-Zn samples: a) CO; b) C3; c) C5; d) ClO; e) C16; f) C3l; g) C54; h) C3 calc. 588 K; i ) C54 calc. 588 K.
738
TABLE 2 Surface concentrations from XPS data on calcined samples Sample CO(ZnO) C3 C5 ClO C16 C3l C54 ClOO(CuO)
0 62.0 70.8 65.3 61.5 52.1 60.0 57.0 55.9
Zn Cu (atomic %) 38.0 28.0 32.9 35.4 42.6 28.5 26.1
1.2 1.8 3.1 5.3 U.5 16.9 44.1
It can be inferred, from XRD and DSC data, that the single phase corresponds to a solid solution of copper in the hydrozincite structure, i.e. a compound having the formula (Zn,Cu) (CO) (OH)6' i n tYhe XRD 1,5 3 2 observe d ' , '1 ' No s h 1i f ts 1n 1nes were , oW1ng to the S1m1 ar1ty 0 f t h e Cu 2+ 2+ and Zn ionic radii. (Zn+Cu)/CO molar ratios from chemical analysis agree very well the expected value of ~.5. The presence of two phases in sample C54, the solid solution and copper hydroxide nitrate explains the high content of NO- and the high (Zn+Cu)/CO ratio , t h 1S i , 1n samp 1 e. DSC data also suggest that the 31ntroduct10n of Cu2+,10ns 21n t h e hydrozincite structure favours its thermal stabilization. Calcined samples. After calcination at 588 K, all samples show the XRD patterns of ZnO and CuO, with the respective intensities directly proportional to the cation concentrations. The examination of the peak broadenings indicate that quite small crystallite dimensions are obtained in all cases, but for sample C54. Crystallite sizes of 15-20 nm and 6-8 nm were determined for ZnO and CuO respectively, using the Scherrer formula (ref. 7). In sample C54, sizes were respectively 90 and 40 nm. pure CuO 0.8
~
,
u
0.6
0.4
Fig. 2. Relationship between the peak maximum temperature in the DSC diagrams and the atomic Cu/Zn ratio.
0.2
560
550
600
TIK)
739
TABLE 3 Activity data of Zn-Cu catalysts per Catalyst
K
C3 C5 ClO C16 C3l C54
0.0143 0.0287 0.0260 0.0324 0.0326 0.0266
unit area of Cu 1n the reactor
spec
The substantial difference among sample C54 and the others is put in evidence also by comparing TE¥. observations of samples before and after calcination. A unimodal distribution of particles is observed in single phase samples. The calcination step increases the dispersion by a factor of 5 (Fig. 3a,b). In sample C54 two types of particles were observed before calcination, small for carbonate and large for nitrate. Also in this case the thermal treatment gives rise to a redispersion phenomenon, forming however particles larger than in the previous cases (Fig. 3c,d). The composition influences also the specific surface area of catalysts, as shown in Fig. 4. A fall of the total and Cu surface area values between samples C3l and C54 was observed, confirming the peculiar characteristics of the latter. Surface concentrations from XPS data are shown in Tab. 2, and measured Cu/Zn atomic ratios are compared with those expected in Fig. 5. A straight line relationship suggests that there are no remarkable differences between bulk and surface concentration. Measurements made on precursor samples and at intermediate calcination temperatures show that no changes of the surface Cu/Zn ratio are caused by calcination. A particular attention was spent in the measurement of the intensity ratio between satellite and main peaks which characterize the Cu 2p / photoelectron " . Cu 2+ conta1n1ng . . . f3act2 d emonstrate d transLt10n 1n compoun d s (F'19. 6) • It was 1n 2+, that such ratio is dependent on the nearest neighbours of Cu so that Cu2+ ions isolated in other oxides as MgO have a weaker satellite intensity than cupric ions in CuO (ref. 8). Results shown in Fig. 7 indicate that the systems CuO-MgO and CuO-ZnO behave very similarly from this point of view. Reduction. Weight losses by thermogravimetry and H consumptions by TPR indicate the complete reduction of cupric species to metallic Cu. Experimental profiles suggest a single species reduction. Catalytic behaviour. Activity results are reported in Tab. 3. The rate constants K per unit area of Cu are quite constant in all samples, apart C3, which shows a significantly lower specific activity. The relationship between K and total Cu area in the reactor is well fitted by a straight line which does not pass through the origin. An induction period was observed when catalytic experiments were performed at 483 K with a gas mixture containing 3% CO, 40% H and 57% N (Fig. 9). In indu2 20
740
Fig. 3. Electron micrographs of Cu-Zn samples: a) C16, dried; b) C16, calcined; c) C54, dried; d} C54, calcined. strial conditions such phenomenon is much less evident. Ternary samples Ternary samples conta~n~ng Al were prepared only at a Cu/Zn atomic ratio near 0.45, with aluminum content of about 10% (atomic). Coprecipitation gave rise to a single phase XRD pattern of unknown structur~ while the addition of Al(OH) to the preformed hydrozincite phase did not substantially change its Physicg-chemical characteristics. Calcination in both cases produced a mixture of oxides. EXAFS and XANES experiments excluded the formation of ZnAl 04 or CuAl 04 (ref. 9). Activities of Ehe copreci~itated and mixed ternary catalysts are compared in Tab. 4, where the activity of C3l is also reported. It is apparent that in the first case a catalyst of low activity is obtained, while in the second case the activity is comparable to that of the binary catalyst. DISCUSSION XRD, DSC and chemical analysis data are all consistent with the presence of a single phase precursor having the hydrozincite structure up to a Cu/Zn atomic
741 TABLE 4 Catalytic activity of ternary samples Spec. Surf. Cu 2 -1 (m.g )
Catalyst
94 128 128
Coprecipitated Mixed C3l
K
K v
spec
(473 K) 3 8.9 x 10-
1. 55 3.63 3.56
3.9 x 10-2 3.3 x 10-2
ratio of about 0.45. Near (Zn,Cu) (OH)6(CO )2' the malachite structure (Cu,Zn)2 53), (OH) C0 was never observed (ref. whil~ copper hydroxide nitrate Cu2(OH)3N03 . 2 3 1S present only for large eu contents. The obtention of a single-phase precursor favours the formation of a highly dispersed CuO-2nO mixture during calcination, as it is demonstrated by comparing TEM and specific surface area results of samples C31 and C54. In the second cas~ quite lower area values and dispersion are obtained. On calcined samples, starting from different precursors, the dissolution of a small amount (2-4%) of CuO in ZnO has been observed by XRD and STEM (ref. 3). Our XRD results do not permit to confirm the above conclusion. However, XPS data on the satellite/main peak intensity ratio of the Cu 2p L transition suggests 2 that, at least in the first surface layers, Cu + is parfi~lly surrounded by
.,<>
1
·· ··
'50
u
·,
'QQ-
,!!
u
~
'"
~\\ \
0.7
e ~ ~
~ \
50
\-0--0---0 . ............
o
\\'"
0,4
02
\ \
-----\,
Q.
0.5
0.3
<,
0.2
0.6
06 Cu/Zn
U8
01
0/0 Qt
Q2
OJ
O'
0.5 0.6 0.7 CCu/Zn)buUc
08
Fig. 4. Relationship between atomic Cu/Zn ratio, total specific surface area (0) and Cu specific surface area (.). Fig. 5. Relationship between bulk atomic Cu/Zn ratio and surface Cu/Zn ratio.
742
04
0.2
930
932
934
936
938
940 942 944 946 Binding f'n.rgy (.V)
0.2
0.4
0.6
0.8 Culln
Fig. 6. X-ray photoelectron spectrum of Cu 2P3/2' Fig. 7. Relationship between atomic Cu/Zn ratio and satellite/main peak intensity ratio of Cu 2 P 3/ 2. 2+ Zn . Thermogravimetric and TPR data show that the reduction treatment is sufficient to bring copper completely to the metallic state. Copper specific surface area values are strictly connected to the total area of the starting oxide mixture, and not depending on the Cu content. This suggests that a dispersed CuO phase is reduced rather than a solid solution in the ZnO matrix. In a first approximation and in agreement with literature (ref. 1), catalytic activities are linearly dependent on the total area of Cu, stressing the importance of the precursor morphology and structure for obtaining very active catalysts. There are ,however ,some discrepancies among this conclusion and some aspects of the catalytic behaviour. Namely, if Cu(O) crystallites are the active phase, the existence of the induction time shown in Fig. 9 and the disappearance of most of the copper observed under catalytic conditions without formation of a new phase (ref. 2) are difficult to explain. Another anomaly seems to be the low specific activity of sample C3, in spite of its very high Cu dispersion. A way to manage these contradictory results is to assume that in the reaction conditions Cu(O) particles oxidize to a new phase, presumably Cu 20. A small part of it, arising from the smallest crystallites, enter in solid solution with ZnO, as already observed (ref. 4). In the present+case, contrariwise to that proposed for the methanol synthesis (ref. 4), such Cu species should be less or not active for the CO shift reaction. Its prevalence in sample C3 would explain its low specific activity. This interpretation accounts well also for the fact that the correlation line fore~gn
..
ne~ghbours,
.
~.e.
743 ---------------,
,
!
>
'"
>-
s
.
U 3
O-+-L--~-_r--_r_--.---....,...---J
o
50 Tim" (rt)
Fig. 8. Relationship between total Cu area and volume rate constant at 473 K. Fig. 9. Activity as a function of time of C31 catalyst. of Fig. 9 does not pass through the origin. As the equilibrium partial pressure of oxygen is very low in the reaction gas mixture, the reoxidation process should be very slow and the metal particles morphology substantially maintained. It can be pointed out that in our experimental conditions (temper~ture near 473 K and Cu previously completely reduced) the bulk dissolution of Cu in ZnO is scarcely probable. It is likely that the lack of its observation by XRD is due to the formation of amorphous Cu like 20, amorphous CuO is formed by deep re-oxidation of samples after the catalytic tests. The role of aluminum in ternary catalysts can be also explained on the light of the above picture. The presence of Al can favour the dissolution of Cu+ in ZnO in coprecipitated catalyst, decreasing their activity (ref. 4), while the successively added. Al phase can act as anti-sintering agent (ref. 2). REFERENCES 1 J.S. Campbell, Ind. Eng. Chem., Process Des. Develop., 9(1970)588-595. 2 T.A. Semenova, B.G. Lyudkovskaya, 11.1. Markina, A.Ya. Volynkina, G.P.Cherkasav, V.I. Sharkina, N.F. Khitrova and G.P. Shpiro, Kinet. Catal. (Engl. Transl.), 18(1977)834-838. 3 R.G. Herman, K. Klier, G.H. Simmons, B.P. Finn, J.B. Bulko and T.P.Kobylinski, J. Catal., 56(1979)407-429. 4 S. Mehta, G.W. Simmons, K. Klier and R.G. Herman, J. Catal., 57(1979)339-360.
744 5 A. Bossi, A. Cattalani and N. pernicone, Proc. 7th Int. Conf. Thermal Analysis, Kingston, August 22-28, 1982, Heyden, London. 6 C.D. lvagner, W.M. Riggs, L.E. Davis, J.F. Molder and G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corp., Eden Prairie, 1979. 7 H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures, John Wiley & Sons, New York, 1959, p.5l2. 8 M. Oku and K. Hirokawa, J. Electron Spectrosc. Relat Phenom., 10(1977)103-110. 9 G. Vlaic, J.C.J. Bart, W. Cavigialo and S. Mobilia, Chern. Phys. Lett., 76(1980) 453-459.
745 DISCUSSION J.A. PAJARES I have a general comment. You know that we are far away to dispose of a universally accepted method for the determination of specific surface copper areas; the method launched by Scholten and Konvalinka is more a well accepted recipe, and much more work is needed. But I ~ nt to try to verify a situation : Professor Scholten, using the Lundquist criterium (75:20:5) for the principal crystallographic planes, calculates 1.7 x 10 14 Cu atoms/m 2, that is a ~ 5.7 A2 for the surface Cu atom eros-section. Anderson, in his "Structure of metallic Catalysts" with the simple 33/33/33% criterium calculates 1.47 Cu atoms/m 2 that is ~ 6.8 A. Now you gave 18.6 ~2, that is about three times a greater area. If now you use a stoichiometry O:Cu = 1.1, the discrepancy with Scholten's method (O:Cu = 1:2) comes larger. So I an now more confused that before to read your paper. Did you realize this and did you have serious reasons to use these new numbers ? G. PETRINI We agree about the difficulty to have a general method for Cu area determinations. We have in progress some studies by chemisorption and calorimetry with the attempt to improve this situation. We measured the total area of pure Cu by N2 adsorption at 77 K, choose a temperature range for the 02 adsorption measurements where the coverage is independent of T and put in relation the oxygen uptake with the total surface area value. In this way we obtained the value of 18.6 A2. The assumption of the O:Cu = 1:1 stoichiometry was made arbitrarily, in the absence of experimental evidences for a different situation. T~e above method is obviously empirical and the obtained values can be arbitrary. However, it is sufficient for the aim of this workltallows to compare different samples,and Cu values obtained in this way are very well correlated to the activity data. F. TRIFIRO: I am surprised of the high surface area of your binary oxides,much higher than the ternary oxides. My questions are: 1) are your catalysts stable in the time ? 2) is the high surface area due to some special procedure of calcination, or to Some impurities added, or to the special phase you believe to have formed ? G. PETRINI In laboratory tests our catalysts seem rather stable with time An influence of the precursor phase on the total surface area values is suggested by the strong decrease observed in the biphasic C54 sample. Ph. COURTY: I would mention that the binary copper-zinc hydroxycarbonate described in your publication as similar to the hydrozincite structure seems having already been indexed in ASTM (ASTM 17-743) as Aurichalcite CU5-xZnx(C03)2(OH)6. Could you comment please ? G. PETRINI : Hydrozincite and aurichalcite show quite similar X-ray diffraction patterns, with some difference in low intensity lines. Starting from the sample not containing copper, and increasing the concentration of this element, we obtained always XRD patterns with similar characteristics and well inteIpreted as due to the hydrozincite structure. In addition, from ASTM it results that aurichalcite is a mineral sample, at our knowledge never prepared in laboratory. E.B.M. DOESBURG Did you find any increase in the reduction temperature of the copper oxide by the alumina in the CuO/ZnO/AI203 compound made by coprecipitation?
746 G. PETRINI: An increase of the reduction temperature in samples containing alumina was actually observed (see A. Bossi, A. Cattalani and N. Pernicone, Proc. 7th ICTA, Kingston, 1982, paper V-B-1). D. CHADWICK: with reference to your XPS results in Fig. 5, I do not accept these data as a sufficient evidence that there is no difference between surface and bulk compositions. May I suggest that a study of the cu/Zn ratio using both the 3p or 3s peaks and the 2p peaks would resolve the issue beyond doubt. G. PETRINI Measurements with the 3p peak intensities were made, confirming the present results. In the general economy of this paper, this was not considered as essential.
747
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Cui KIESELGUHR CATALYSTS FOR HYDRATION OF ACRYLONITRILE E. NINO, A. LAPENA, J. MARTINEZ, J.M. GUTIERREZ, S.
MENDIOROZ~,
J.L.G.FIERRO~
and J.A. PAJARES~ Explosivos Rio Tinto, Division de Quimica Organica,Castellana 20, Madrid (1) ~
Instituto de Catalisis y Petroleoquimica, CSIC, Serrano 119, Madrid (6)
ABSTRACT Cu/kieselgbhr catalysts (approximately 50 wt%. Cu)
were prepared from cupric
hydroxide precipitated on a suspension of the support. Reduction is a critical operation and a recipe is given to carry out this process in a dilute H 2 stream and temperatures~225 °c. TG studies show that CUO/kieselguhr partially decomposes in vacuum to cu
20;
reduction in H follows kinetics with surface 2
formation of cu
germs. 20 The specific metal areas of the catalysts after different thermal treatments
were determined from the chemisorption of oxygen from catalytically decomposed N eu/kieselguhr showed good activity and selectivity (~ 100 %) for the hy20. dration of acrylonitrile to acrylamide between 78-104 °c, its behaviour being comparable with that of more conventional Raney-Cu commercial catalysts. INTRODUCTION Undoubtedly, on economical grounds and because of its chemical simplicity, the heterogeneous selective hydration of acrylonitrile to acrylamide is becoming the most attractive way for the fabrication of this last product. Initially developed by Matsuda et al. [1]
from Mitsui, the first patents for this
process date from 1973. While in the presence of Ni-based catalysts the hydrolysis of acrylonitrile takes place preferentially on the C=C bond, and 75 % of the product of the reaction is ethylenecyanohydrin, the hydration of the C=N group to acrylamide is carried out with a selectivity near 100 % when Cu-based catalysts are used [ 2] . other metallic oxides and metals -Mn0 Rh(American 2, Cyanamid); Fe, Cr, Co (Mitsubishi}; CoO, Cr(OH)3 (Sumitomo)- and lately supported microorganisms (Nitto, [3]) appear among the different catalysts that in the patent literature have been proposed. Copper catalysts are specifically referred to in near one hundred patents since 1975. Raney-Cu seems to be preferred, but Ullmann copper, Adkins catalysts or copper catalysts prepared after different procedures -formiate decomposition, reduction of copper salts with NaHB
or hydrazine, decomposition of 4 the hydroxide- have also been used. The academic literature on this subject
is scarce [4-9] . A fixed bed reactor, the catalyst being a copper-chromium combination, has been used by Dow [10] • eu/kieselguhr catalysts have shown
748 also a good activity, comparable on a metal basis with Raney copper catalysts, with a selectivity near 100 %, and without some of the inconvenience of handling, pressure drop, etc., that Raney-Cu use implies [11]. The reduction of supported Cuo is a fairly delicate process [12,13]
. The
operation must be carried out in a dilute hydrogen atmosphere, with a very careful temperature control. Strong sintering with a parallel deactivation takes place easily at temperatures slightly higher than
250°C.
In this pape; the preparation method of a Cu/kieselguhr catalyst precursor with the subsequent reduction procedure is given. Results on the reduction in vacuum, its behaviour after successive oxidation-reduction cycles and reduction kinetics are also discussed. Finally, data on its catalytic activity for the direct hydration of acrylonitrile to acrylamide are reported.
EXPERIMENTAL Catalyst preparation A diatomaceous earth, Diatosil P, supplied by CECA (Compagnie Espagnole des Charbons Actifs, S.A.)-mean particle size, d = 14 2 -1
SBET=29.1 m g
(C=178); apparent density, d
L
~m:
specific surface area, 3
g cm- - was used as support. a=0.35 Its chemical analysis, as stated, was: silica, 74.1 %: CaO,6.60 %: Fe and Alo-
xides, 6.85 %: MgO, 1.20 %: alkalis and unspecified matter, 3.75 %. weight loss upon ignition, 7.5 %. The nitrogen adsorption isotherm presents a type A hyste3g- 1. resis, with a low volume of mesopores: 0.05 cm Mercury penetration gave a 3 pore volume of 1.54 cm g-l,mainly macropores. The kieselguhr was added to a dilute solution of copper nitrate, the suspension heated to 90-100 °c and then precipitated as cupric hydroxide by sudden addition of NaOH (final
pH~
9). After filtering and washing, the CU(OH)2/kiesel-
guhr cake was dried (100-105°C), ground and pelletized (11 x 6 rom pills). In a representative batch (40 Kg) this catalyst precursor had a specific surface 2g- 1 area, S = 44.3 m (after pumping at 140°C overnight), apparent density BE~3 3-1 1.35 g em and a pore volume, as measured by mercury porosimetry, of 0.42 em g, 3 -1 of which 0.22 em g corresponded to mesoporosity. Reduction
is a critical operation. It was carried out in an especially desig-
ned reactor, provided with an inner circular ring for the supplementary entry of diluting inert gas. The catalyst was purged in N flow at room temperature du2 ring 1 h, and slowly heated in this atmosphere to 120°c (2.5 h). A large amount of water was eliminated at this stage, probably as a product of the decomposition of the cupric hydroxide to CuO. Thereafter, hydrogen was progressively incorporated up to a H 2/N2=0.5/100 -1 ratio after 0.5 h. The total space velocity was 420 h . The temperature of the catalyst bed was then raised to 180°C in 3 h. water was formed and a gradient of about 5 °c was found between the higher and lower zones of the reactor. In some reductions, for higher values of this gradient, diluting nitrogen was added
749 by the inner entry [13} The temperature was then raised to 200°C (1.5 h). Now the hydrogen/nitrogen ratio was increased to 1.0/100 and 1.5/100 in two steps (2 h). Water was mainly eliminated in the first minutes after the relative hydrogen increase. The temperature was raised to 210°C (4h) and the H ratio to 2/N2 3/100 (3h) and to 6/100 (3h). Some water production was observed at this stage. The reduction treatment ended with the temperature at 225°C and a 10/100:H / N 2 2 ratio (no water production observed). The catalyst was then cooled at room temperature in bitrogen atmosphere. The reduced CU/kieselguhr pellets, whose textural characteristics are given below, have a black colour and a homogeneous appearance. The copper content varies between 45-58 wt % for different formulations prepared. The catalyst must be maintained under inert or reducing atmosphere. In contact with air, it easily oxidizes at room temperature with heat evolution that warms the container. The copper oxide percentage can reach 2-6 wt %. Catalytic activity and characterization techniques The catalytic activity was measured upon a homogeneous acrylonitrile aqueous solution (6.5 %) in three tests: a)quick tests at atmospheric pressure 3 in a glass apparatus with a 180 cm reactor; b) a laboratory scale apparatus, 2 for testing at 7.5 kg cm- and 60-120 °C, in a 2-m long, 20-mm inner diameter tubular reactor; c) a (AAM)
small pilot plant with a 100 kg/day of acrylamide monomer
production. Chemical analysis (acrylonitrile, acrylamide, ethylenecyanohy-
drin and nitrile-tris propylamide) was effected with a Hewlett-Packard 5830 gas chromatograph, with a Carbowax 20 M-Chromosorb 80/100 4-m column, at lOS-140°C (programmed at 10°C/min) working temperature. Acrylamide was more carefully analyzed by double-bond formation and water by conventional Karl-Fisher analysis. °2 BET surface areas were measured by nitrogen adsorption (A ~16.2 A ) in a N2 Micromeritics 2100 D apparatus; the same system was used for the determination of the mesopore distribution. Mercury porosimetry was made in an Aminco 60 000 machine. Thermal decomposition, oxidation-reduction cycles and decomposition kinetics were followed in a Sartorius 4102 electrobalance connected to a HV system. The kinetics of reduction of the precursor was carried out on 100-mg samples dried under vacuum and kept at reduction temperature up to constant weight. A 300 Torr H pressure was used and a liquid N trap was placed near 2 2 to the samples in order to remove water from the gas-phase. XR diffractograms were recorded with a Philips 1010 apparatus.
SEM micrographs were made in a
Jeol JMS - 50 A microscope. RESULTS AND DISCUSSION Thermogravimetric studies Thermogravimetric analyses of both CuO/kieselguhr precursor and kieselguhr (support) samples are shown in Fig. 1. The highest temperature was achieved by
750 heating at a constant rate (2°C/min) under dynamic vacuum (
~ 10~3
Torr) to
avoid readsorption of water on the support. The importance of this process can be emphasized by the TG curve of the support kept in contact with the atmosphere (dotted line in Fig. 1). The thermograms in vacuum for both precursor and support are
qualitatively similar, showing two well separated regions: one below 100 °C,
mainly
due to the removalof H from cupric hydroxide and/or adsorbed water, 20 and a second one at higher temperatures, associated to strongly adsorbed water in the channel structure of the support and also to oxygen elimination from the lattice of CuO, small cu
catalyst (containing
~50
domaliE appearing probably. The weight loss of the 20 % of support) between 100 and 400°C is nearly 2.3
times as high as that found for the support, a value of 6 % of the theoretical one calculated on the basis of a quantitative reduction CuO
being attained
~~u20
at the highest temperature (400°C). Additional information on the chemical composition of the catalyst and the active phase can be obtained by consecutive oxidation-reduction cycles [14J . Reduction in H at 300 °c of a fresh CuO/kieselguhr sample followed by oxidation 2 at the same temperature and by a second reduction shows: i) The water content in the outgassed sample is nearly 2.6 wt %. ii) The oxidation rate of the reduced catalyst was found to be considerably higher than that of the reduction of CuO to Cu metal. iii) For each oxidation-reduction cycle the CuO or Cu content calculated on the basis of a dry substrate, coincide fairly well with the chemical analysis (within 1 %). Typical reduction kinetic curves at temperatures between 177 and 211°C are shown in Fig. 2. Gravimetric reduction data are presented as CuO reduction degree ( a) versus time. The parameterais the ratio of experimental to theoretically calculated weight loss of the catalyst upon reduction(cuo
~
CUO). S-sha-
ped curves appeared in all cases,i.e. there is an induction period. Both this period and the time required to achieve a quantitative reduction of CuO become shorter at increasingly higher reduction temperatures. Obviously, in the primary outgassing a small concentration of domains of CU could appear on CuO 20 crystallites, which should define a fast progress of the reaction interface [15J . From the slope of the quasi-straight line of these curves and/or the reciprocal time required to attain a definite reduction degree, an apparent has been calculated. Its value, E = 19.7 + 1.0 kcal mol; a a) agrees satisfactorily with that reported beforety Puchot et al. [16] and by
activation energy (E
Schoepp and Hajal [17J who studied this reduction on powdered bulk CuO. However discrepancies emerge when compared with E in the absence of some cu
20
values calculated from data obtained a domains preformed on CuO crystallites [ 18J
Metal dispersion The active surface of the metallic component in Cu/kieselguhr catalysts was
751 measured following the method of Scholten and Konvalinka [ 1~
.Oxygen chemisorp-
tion from the catalytic decomposition of N was gravimetrically determined un20 til p ~250 Torr. Two or three experiments under 70°C have been made for N20 each sample. The adsorption results fit well a Freundlich isotherm, the monolayer being obtained from their crosspoint [20] • The laboratory has previous experience in the use of the experimental conditions of scholten[19]
and Osinga
et al. (21] over copper supported catalysts (22] .
200
300
40
TC-C>
Sao
.............................................
80 100L.-_---L_ _--'-_ _..L...-_--J._ _--L-...J
a
100
1.0
200
:;::;:::::.»&I"--'""::::.::.:=~::::::==----_
300
..... -=.......
.8 ~
.6
10
20
30 t(min) 60
Fig. 2. Kinetics of reduction of CuO/kieselguhr in 300 Torr H 2•
752
2 7'10 0\8
E'6
II
-4
e
_~~
~'-V~:;'
... -.
III
2
10
Fig. 3. Freundlich's linear plots for oxygen adsorption (from N
20
decomposition)
on CU/kieselguhr catalysts. Experimental data are given in Fig. 3. Plot I corresponds to oxygen chemisorbed on a Cu/kieselguhr sample reduced according to the conventional procedure described. Plot II has been obtained with a sample from the same batch, after stabilization through sintering in N flow at 400 °c for 4 h. Results for a sam2 ple of the same batch sintered in N for 2 h at 500 °c are also shown (Plott III) 2 The copper content of the starting material was 46.2 wt %. Specific metal areas 19 obtained assuming an adsorption stoichiometry O/Cu: 1/2 and a factor of 1.7x10 2 Cu atoms per m (according to the criterium of Sundquist [23] : a composition 25 % (100), 5 % (110), 70 % (111) for the low index planes in the equilibrium) (d = 6/p S, PCu= 8.95 g cm- 3)
and the particle sizes obtained from these values are given in Table 1.
The conventionally reduced Cu/kieselguhr catalyst shows a reasonable catalytic active surface. Heating in nitrogen at 500°C introduces a severe sintering. Still higher sintering is produced by an ill-controlled reduction treatment at much lower temperatures. SEM micrographs show a well dispersed, uniform aspect for the reduced catalyst. On the contrary, other samples from severe reductions presented red zones, more marked in the center of the pellets (probably due to difficulties in eliminating the heat of reduction) with the appearance of large prismatic copper crystals. Also large cylindrical pores and cracks appear due to the uncontrolled evolution of H during the reduction process [ 14] 20 Some results for the mean particle size of these catalysts obtained from
753 X-ray diffraction patterns are given in the last column of Table 1. Actually, we cannot state if the discrepancy between these values and those obtained from oxygen chemisorption is due to error in the experimental XRD procedure or to the use of too high values for the monolayer in the calculation of the metal surface. Catalytic activity Data of conversion percentage at 78°C and one atmosphere on the catalytic activity for the hydration of acrylonitrile to acrylamide, obtained with the three catalyst samples discussed above, are given in Table 2. Samples I and II present a remarkable catalytic activity for residence times 1-2 h, comparable with that claimed for the best catalysts in the patent literature. The pressure has little influence on the specific catalytic activity, but temperature has a drastic effect: while the three samples only show a small catalytic activity at 65°C and 1 atmosphere, sample II reaches a 93 % conversion at 90°C and 7.5 kg pressure for a time of residence of 6 h. Sample III shows a lower catalytic activity, probably related to the lower specific metal area produced by the sintering treatment. Sample II (Smet~75 2/g m Cu, d ~ 89 showed always the highest activity, above that of sample I.
A)
The same phenomenon was found with samples of similar textural characteristics. Two explanations are possible: i) a minimum size is neccessary for the right work of the copper particle, as it was found for Ni hydrogenation catalysts[24] ii) the higher activity of sample II is due to its higher ability to activate water on Lewis acid centers on the uncovered support surface, as suggested by Elsemongy and Onsager
[5].
TABLE 2 Hydration of acrylonitrile to acrylamide on Cu/kieselguhr Sample
Time of residence
Conversion
h
%
I
1.55 2.33
32.5 50.3
II
0.99 2.25 3.53
25.4 63.3 68.1
III
1.42 1.97
8.8 16.8
Feed, 6.5 % in water acrylonitrile. T, 78°c.
P, 1 atm.
A comparative test with two commercial Raney-Cu catalysts -Degussa B3113w and Davison CUR29- at 78°C and high conversions gave the catalytic activities per hour and gram of catalyst summarized in Table 3. Although the numbers
754 have only a relative importance because the acrylamide retards the
reaction [5],
TABLE 3 Catalytic activity of copper catalysts Sample
g (AAM)/h,g Cu
Degussa B3113W
0.058
Davison CuR29
0.013
CU/kieselguhr
0.034
the efficiency of the Cu/kieselguhr in comparison with both conventional commercial catalysts can be seen. Sample II showed also the best selectivity to acrylamide other samples being near this value (Sample
II,~
(~
100 %), the two
99 %). Ethylenecyanohydrin is
present as traces and only sample I shows a small peak
(~
1 %) of an unidenti-
fied product in the tail of the acrylamide peak. Contrarily to the behaviour of sample II, the product from the reaction over sample I is slightly coloured. A similar result is found with partially oxidized catalysts. It seems that heating at 400°C in N
introduces a double effect on surface stabilization and particle 2 size of the Cu/kieselguhr contact which is particularly favourable for its use in the hydration of acrylonitrile to acrylamide. The temperature has no apparent effect on the selectivity, at least up to 104°C, the highest temperature used. The catalysts showed a practically cons-
tant activity in 1200 h tests. Some tarry product appeared on the catalyst surface after one of these tests, with little or no influence on its performance: after some time the catalysts could be reused with an activity practically equal to the initial one. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
E. Otsuka, T. Takahashi, N. Hashimoto and F. Matsuda, Chern. Econ. & Eng. Rev. 7, No.4 (1975) 29-32. F. Matsuda and N. Hashimoto,Ihformations Chimie 160 (1976) 223-27. European Chern. News 37, No 990 (1981) 20. F. Matsuda, Chern. Tech. 7 (1977) 306-8. M.M. Elsemongy and O.T. Onsager, Acta Chern. Scand. B32 (1978) 167-70. H. Hayashi, H. Nishi, Y. Watanabe and T. Okozaki, J. Catal. 69 (1981) 44-50. F. Matsuda and N. Hashimoto, Shokubai 17 (1975) 192-6. M.S.Wainwright, N.I. Onuhoa and R.P. Chaplin, Natl. Conf. Publ. Inst.Eng. (Austr.) 79-8 (1979) 109-13. E.S. Pugach, A.I. Sich and T.M. Mokrivskii, Visn. L'viv. Politekh. Inst. 96 (1975) 71-4. Chern. Eng. 80 Nov. 26 (1973) 68~9. E. Nino, A. Lapena, J.M. Gutierrez and J. Martinez, Unpublished Results. D.R. Goodman and B.A. Oxon in "Catalyst Handbook", Wolfe, London, 1970, p.161. N. Pernicone and F. Traina in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Eds.) ,"Preparation of Catalysts II", Elsevier, Amsterdam, 1979, p.321. S. Mendi6roz, J.L.G. Fierro and J.A. Pajares, Industrial Report, 1981.
755 15. W. Verhoeven and B. Delmon, Bull. Soc. Chim. France (1966) 3065-73. 16. M.T. Puchot, W. Verhoeven and B. Delmon, Bull. SOc. Chim. France (1966) 91117. 17. R. Schoepp and I. Hajal, Bull. Soc. Chim. France (1975) 1965-69. 18. W.D.Bonct, J. Phys. Chern. 66 (1962) 1573-77. 19. J.J.F. Scholten and J.A. Konvalinka, Trans. Faraday Soc. 65 (1969) 2465-73. 20. M.A. MartIn, J.M.D. Tasc6n, J.L.G.Fierro, J.A.pajares and L. Gonzalez Tejuca J. Catal. 71 (1981) 201-4. 21. Th. J. Osinga, B.C. Linsen and W.P. van Beek, J. Catal. 7 (1967) 277-303. 22. J.A. L6pez de Guerenu, Graduate Thesis, University of Bilbao, 1978. 23. B.E. Sundquist, Acta Metallurgica 12 (1964) 67-75. 24. G. Cocco, S. Enzo, L. Schiffini and G. Carturan, J. Mol. Catal. 11 (1981) 161-66:
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757
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PHASE STRUCTURAL CHARACTERISTICS OF THE OXIDE SYSTEMS IN THEIR FIRST STAGE OF PREPARATION USING ONE OF THE INGREDIENTS OF THE CATALYST COMPONENTS AS A PRECIPITATING REAGENT D.S. SHISHKOV, N.A. KASSABOVA and K.N. PETKOV Higher Institute of Chemical Technology, Sofia (BULGARIA)
ABSTRACT The phase structural characteristics have been investigated for the systems CuO - ZnO - Cr
and CuO - ZnO - A1 obtained by different methods, 20 3 20 3 using one of the catalyst ingredients. It has been found that the conditions for obtaining the system predetermine at the first stage the subsequent processes involved in the production technology of the catalyst itself. Optical
~icroscopy
and other methods show the possibility for a preliminary estimation of contact composition properties. There is a correlation between the phase structural characteristics of the systems at the first stage of their preparation and catalytic properties for the two most widely used catalysts for the low temperatures conversion of CO to CO and H the synthesis of CH from CO and H 2. 2 30H 2,and INTRODUCTION The scientifically founded technology for catalyst production includes parameters determining the catalytic and performance properties of contact masses. These parameters are: chemical composition, structure and texture, shape and size of the granules, mechanical strength and resistance extent in the
reaction~e
dium. It is known that industrial catalysts are evaluated according to their catalytic activity per unit volume. The volume output, according to Boreskov (ref. I) , depends on the specific catalytic activity, specific surface area and extent of its use. (I) K = K K s T1 The above equation shows that a correct selection of the chemical composi-
tion of the catalyst group is necessary, on which the specific catalyst activity -
~s
is dependent. This condition is necessary, but not sufficient for indus
trial catalyst production: maximum output per a unit volume is reached by creating a developed and accessible surface(S and
n ).
Consequently, the phase struc-
ture characteristics of the catalyst composition predetermine
its volume out-
put. The purpose of this research consists in
studying the influence of the out-
let components on the chemical nature and preparation conditions on the phase structural characteristics of CuO - ZnO - cr
and CuO - ZnO - A1 In addi20 3. 203 tion an indirect evaluation of the catalyst activity of a given contact mass
758 was found possible after the first step of its preparation. RESULTS On the basis of the above-mentioned catalyst compositions, the technology has been developed for producing low temperature catalysts (LTC) for steam conversion of CO in CO and HZ and CH synthesis from CO and HZ at low temperatuZ 30H re and pressure. Since many research workers (ref.Z,3) consider that the active component of these catalysts is Cu, the form and dispersion of Cu in the contact mass have a decisive
eff~ct
on the catalytic properties. Detailed physico-chemi-
cal research on the copper salt-alkali-water system has always shown that in the resulting precipitate, there is an alkali salt - e.g. CU(N03)2.3 Cu (OH)2 (ref. 4)- whose quantity depends on the method of preparation. The catalytic properties of the Cu catalysts depend to a great extent on that quantity. In most cases, while metal hydroxides are deposited, alkaline solutions are used ( Na2co3,(NH4)2C03' NH NaOH, etc.). This method of preparation has one 40H, disadvantage.During the preparation some non-uniform sections are created in the systems,which lead to velocity changes in embryo-forming during the whole process. This determines rather vast uncontrolled repartition of particle sizes. For obtaining contact masses with rigidly defined properties, i.e. to eliminate the above-mentioned disadvantage, some methods can be used in which the precipitating agent is added in a hidden form, which means that no deposit with the dis solved salt is formed but under certain conditions, this reagent appears as a OH donor, or as other radicals
(e.g. rising temperature when urea is used as a do-
nor) which are to be released under controlled velocity. Another way for replacing the precipitating reagent with alkali properties is using compounds including one of the catalyst composition component. For the systems CuO - ZnO - cr
CuD - ZnO, CuO - 2nO - A1 and CuO -ZnO - Cr 203, 203 Z0 3 A1 ' the complex of bivalent (one-valent) copper with ammonia-copper-ammonia20 3 2+(cu+) carbonate solution (CAS) can be used and it appears simultaneously as cu
-
donor and as OH
and C0
2-
ions donor. Both cases create conditions for obtaining 3 a homogeneous precipitate by all known methods. Chemical precipitation conditions from homogeneous solution are available. The method is characterized by including the embryo-forming stage with a subsequent uniform embryo growth. The combination of these conditions when preparing poly-component systems, and especially when one of the ingredients is used as a precipitating ion donor, favours the obtention of homogeneous precipitates in which topo-chemical reactions take place. In many cases the extent of their development determines the specific catalytic activity. For example, when preparing the CuO - ZnO - A1 system, the spinel-forming reaction (ZnO - A1
20 3)
Z0 3 would have occurred with
oxide
greater velocity at lower temperature. In order to illustrate this method, several oxide systems are discussed. The major process which occurs between Cro
3
and Zn(N0
3)Z
water solutions and CAS
759 can be reduced to the following reactions 2+ = 2+ CU(NH + Cr + Cu + 2 cro~ + 2 NH: 3)4 207 cr + 2 NH + H + 2 cro~ + 2 NH~ 3 2o; 20 2+ Cu + Cr0 4 2+ Zn + cro 4 The following reaction takes place during the precipitation of the CuO - ZnO - Al;203 system: 3 zn 2+ + A1 + + Cu(NH
3)2+
+ c0
3
+ OH + x Zn(OH)2 y znC0
3
+ pCu(OH)2 qCUC0
3
Consequently by using CAS as a precipitating agent the conditions for simultaneous precipitation of all ingredients are created,i.e. conditions providing homogeneous contact mass
(ref.4).
The catalysts were obtained using CAS as a reagent and were compared with the samples precipitated with NH at constant pH, temperature, time of precipita40H tion and ageing. The results obtained on two types of Low Temperature Shift Catalyst (LTC) are given in Table I. LTC - A is obtained from Cu and Zn nitrate solutions and water solution of cro
with NH as precipitating agent, and LTC - B by CAS. 40H 3 Activity and dispersion data show that LTC -B has higher catalytic activity
and stability in comparison to LTC - A. TABLE I Activity(percent conversion of CO ) and crystallite sizes (D,A) before and after thermal treatment, at t
=
200 DC, P
-= 2 MPa, steam-to-dry gas ratio = 0.7
and CO inlet concentration = 3.6 % (Percent equilibrium conversion of CO -98.89)
LTC - A
W
h- 1
LTC - B 2
2 98.89
98.33
3000
98.89
86.67
6000
98.33
86.11
98.89
96.67
10000
96.11
98.89
89.44
20000
92.22
96.67
86.11
D Cu D zncr
20 4
170
242
170
218
120
300
90
120
- before thermal treatment 2 -after thermal treatment These results show a correlation between the phases 'dispersion of the two cataysts and their activity. For industrial
practice, it is important to find such a technology which
760 will provide the production of a given catalyst with high technologi-economical qualities. A method which enables us to solve this problem with less time and labour, is the method of experimental design. It can investigate the complex influence on all parameters which determine the physico-chemical properties of the contact mass obtained. This method was used to study in detail the systems CuO - ZnO- A1 CuO - ZnO - cr
and 203 at the first stage of their production, using CAS as a preci-
20 3 pitating agent(ref, 6). The parameters pH, temperature, precipitating and aging time are studied by multi-factor experiment according to a centrally composed orthogonal design. The objective function of the experiments carried out is the catalytic activity with respect to CO-water conversion, expressed by a conversion degree stage. The selection of the varying intervals is made on the basis of theoretical considerations and prior information. While studying the systems CuO - ZnO - cr
and CuO - 2nO - A1 the second order regressional equa203 203, tions are obtained, which describe adequately the precipitation processes. The
optimal values at maXimizing the regressional function y = f(pH,T T aging)
are pH = 203 14 min, and aging time
for the system CuO - 2nO - cr
rature: 57 °C; precipitation time system CuO -ZnO - A1
T prec. p r e c. 6.0, precipitation tempe-
' these values are as follows : pH
44 min. For the 6.2. precipitation
203 temperature:96 °C; precipitation time: 22 min; aging time: 117 min.
The more detailed theoretical interpretation of the experimental data on the catalyst compositions and the catalytic properties depends on their chemical nature, and the preparation conditions predetermine
the requirements for: pha-
se-structural characteristics and ESR and electron-spectroscopy relations of a contact mass taken at different steps of its preparation. Phase-structural Characteristics of precipitates A more complete study on the phase-structural characteristics was made for precipitates developed according to matrices on the system CuO -ZnO - A1
203. The following techniques were applied for characterizing the precipitates
formed: optical microscopy, X-ray diffraction and DTA and, for some samples, infra-red spectroscopy and electron-microscopy. The infra red analysis results show that according to the chemical composition~he
precipitates are mainly hydroxycarbonates,aluminohydroxycarbonatesand
hydroxynitrates.several precipitated phases are defined by the methods applied for phase diagnosis of samples. Copper is mainly precipitated as malachite, gerhardite - CU2(OH)3N03 and probably in the form of isomorphous impurity in hydrozincite
(Zn, CU)5(OH)6(C0
The zinc is as hydroxYcarbonate with a composi3)2. tion close to hydrozincite 2n In the hydrozincite, the zinc can S(OH)6(C03)2. be replaced isomorphically by copper over a large range to form aurihalzite. The aluminium in the precipitates obtained is most probably in the form of
761 AI(OH)3 and
scarbroite, with a theoretical composition: 12 AI(OH)3AI2(C03)3'
This phenomenon, however, is typical only of non-active samples that are very characteristic and completely different. In the non-active samples, there is an absorption zone at 1340 cm-
1
which
is typical of nitrates (there is no such a zone in the active samples). In the 1, active catalysts, there are two absorptions in the range of 1515- 1380 cm1
typical of malachite (1410 cm- ) and of hydrozincite (1515 cm
-1
).
The phase structural characteristics of the precipitates allows to
distin-
guish the active samples from the non-active ones . Indeed: a/Precipitates of highly active samples are characterized
by sub-microscopic, crypto-crystals
with an almost amorphous structure. Due to this, the separated phases in these samples cannot
be detected by a microscope because copper-zinc phases are for-
med. This structure is reflected on the X-ray-diffraction film which is not as clear as the one of non-active samples. The microstructure of the precipitate appears as spherically-united acicular crystals with
good rheological proper-
tiesb/Precipitates of non-active samples are characterized
by a clearly defi-
ned macrograined structure made of acicular lamellae. c/ln highly active
sam~
pIes, according to X-ray phase analyses data, the major copper-containing phase is malachite, and the zinc is present as hydrozincite. d/ln low activity samples, the copper is found as gerhardite (which is also proved by N0 analy3 ses). In some of the samples, large blocks are observed. The aluminium here is in the form of scarbroite. Thus, the two groups of samples differ
microscopica~
ly. The active samples are coloured in light green, reseda nuance. This colour is due to the fine uniform phase distribution. Non-active samples are coloured in blue, peacock-blue due to gerhardite. The macrostructural difference of the samples leads to an essential difference in their structural characteristics. The samples with spherical macrostructure
have greater porosity and more uni-
form structure while the samples with acicular-Iamellate structure
are charac-
terized by bidispersed structure. The results obtained show that with a precipitation pH above 5.5,
(6.5-7 in the filtrate)
copper is immobilized in the
form of malachite, at simultaneous and uniform coprecipitation of copper and zinc. At a pH of precipitation below 5.5 (6.5 to 5.5 in the filtrate), it is found in the form of gerhardite and azurite. The pseudomorphism phenomenon (increase of
surfacearea~s
observed at the drying and quenching steps. The surfa-
ce properties have a major influence on the breakdown velocity. The quantity of germs in the new phases is most important for the breakdown velocities; at low velocities their number increases.The macrostructure exerts a decisive influence in the thermal breakdown of contact mass on the rheological and catalytic properties (ref.7).
762
\
ESR and electron-spectroscopic' investigations on the systems
CuD - ZnO,
The purpose was to compare the copper-ion distribution in systems containing zinc, chrome and aluminium oxides under contitions similar to those at the genesis of commercial LTC with the same composition and to make some conclusions concerning the role of aluminium oxide. Four samples of zinc oxides were studied in mixture with 0,1, 1,5
and
10 % CuO, symbolized as E-O.l; E-l; E-5 and E-l0. Sample F with a composition: 35 % CuO, 40 % ZnO and 20 % Al was also examined. All samples dried at 100°C 203 2 give the typical anisotropic ESR signal. The signal is due to cu + ions in the zone with axial symmetry and is better resolved at lower copper concentrations. 2 The ESR signal are not due to Cu + ions in the ZnO lattice, because the copper ions do not emit an ESR signal in this matrix at room temperature. In the reflection spectrum of sample E-l0, quenched at 300°c two intensive overlapping absorptions are observable in the range of 300-400 nm. The first 2 band at about 360 nm is typical of cu + in a zone with tetrahedral symmetry (Td). Most probably these are copper ions inserted in the tetrahedral lattice eu2 + ions in flat
of zinc oxide. The component at 320 nm can be attributed to square (D
coordination. Having this in mind, conclusion can be made that 4h) adding ammonia to copper and zinc nitrates yields a mixture of hydroxides in
which the copperions me distributed quite uniformly. The behaviour of sample F is different when heated up to 100
and 300°C.
In both cases the ESR signal is anisotropic, which is due to copper ions in a zone with an axial symmetry. Most probably the crystal zone with an axial symmetry is modelled by octahedral cavities with a znO.Al?03 spinel type structure or the corresponding hydroxides. In the reflection spectra, a wide band in the range of 320 -360 nm is observable which characterizes the tetrahedral and flat square coordinations. The main conclusion which can be drawn from our results is that with the available aluminium oxide in the system of coprecipitated copper and zinc oxi2 des, spinel structures are produced in which cu + ions are stabilized in an ax'tally distorted octahedral zone. When quenching at 300°C, the zone character is preserved and the copper ions in the system CuO - ZnO - Al
203
remain distri-
buted uniformly, without aggregation. ESR and electron system
microscopic observations were made also on samples of the
35 % CuO, 55 % ZnO, 5 % Al
. It was found out that in the quenched 203 2 samples, the active catalysts have finely distributed cu + in the ZnAl spi204 nel lattice. In the reflection spectrum, this is confirmed by the presence of absorptions at 700, 800 and 860 nm. These bands are due to the passage B?,,+E N
763 typical of the configuration in the octahedral zone, e.g. in the spinel CUxzn1_xAI204' It is characteristic for these samples that the band with less intensity is at ~380 rum, part of which is due to the passage of tetrahedrally 2 coordinated cu + ion into the ZnO lattice. ESR results confirm the fact that 2 the active LTC contain Cu + ions, finely distributed in the spinel phase. 2 In the spectrum of non-active catalyst only the band typical for cu + ion replaced in Al is observable (g" '" 2.30 with surperfine structure and gl = 20 3 (2.05). In the ESR spectrum of the most active sample this weak signal, typi2 cal of.cu + in A1 is superimposed over an intense signal at g = 2.00, typi203, 2 cal of Cu + in octahedral sections in ZnA1 widened by the volume interac20 4, tion. For middle-activity sample the ESR spectrum shows two signals with al2+ 2+ most the same relative intensity, typical of Cu /A1 and Cu /ZnAI 20 4. 203 The results obtained on the systems CuO -ZnO, CuO - ZnO - cr and 20 3 CuD - ZnO - Al and their interpretation do not differ from Boreskov's (ref. 203 8), who carried out ESR and electron- microcopic analysis on the CuO -MgO systern. The general conclusion of this work is that the active samples have a good aluminium distribution, which gives a spinel structure CUxZn1_xAI204' whereas in the non-active samples, the aluminium remains as Al aggregates, in which 203 2 there are substitutions by cu + ions. REFERENCES 1. G.K. Boreskov, and M.G. Slinko, Newspaper, Ac. Sc. 10 (1961) 29. 2. Y. Morita, K.Tsuchimoto and K. Yamamoto, J. Chern. Soc. Japan, Ind. Chern. Sect., 70 (1967) 665. 3. I. Escarol, I. Martin and R. Sibut-Pinotte, Bull. Soc. Chern. ,France, 10 (1970) 3403. 4. L. Ilcheva and J. Bjerrum, Acta Chern. Scandinavica, A 30 (1976)343-350. 5. Pat, BUlgarian, N° 25263, 31.03.1977 and N° 19638, 02.04.1974. 6. D.S.Shishkov and N.A. Kassabova, Comptes Rend. Ac. Sc. Bulg. 28, (1975)16571660. 7. V.I. Sharkina, L.N. Mihalina, N.N. Aksenov, V.L. Gatman and T.A. Semenova J. Inorg. Chern., 26 (1981), 2346-2349. 8. G.K. Boreskov, The 6 Intern. Congress on Catalysis, London, A12, (1976)1-6.
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MINISYMPOSIUMON CATALYST NORMALIZATION Chairman: N. Pernicone
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767
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STANDARDIZATION OF CATALYST TEST METHODS
R. J. BERTOLACINIl and ARTHUR NEAL2 lAmoco Oil Company, P. O. Box 400, Naperville, Illinois
60566, U.S.A.
2Exxon Corporation, P. O. Box 2226, Baton Rouge, Louisiana
70821, U.S.A.
ABSTRACT This is a report on the progress made by ASTM Committee D-32 on Catalysts since the last paper given at this meeting in 1978. The main committee has seven subcommittees of people with interest and expertise in specific areas related to the main committee. subcommittees is reviewed.
Progress of the
These include catalytic properties, chemical
analysis, physical-chemical properties, physical-mechanical properties, and nomenclature.
Eight standards have been published and approximately 23 others
are at various stages of development in the task groups.
INTRODUCTION Previous speakers at this symposium (ref. 1-2) reviewed the early history of the American Society for Testing Materials, Committee D-32 on Catalysts. The scope of ASTM D-32 is to develop test methods, classifications, recommended practices, and definitions pertaining to catalysts and materials used in their manufacture, and stimulation of research related to catalysts.
For the present,
the catalysts considered have been heterogeneous, but if an interest or demand existed in standards for homogeneous or heterogenized catalysts it would be appropriate for D-32 to consider these system? Since its organization on January 14, 1975, ·the membership has steadily increased and today is 121.
Each member must be classified as having a voting
interest or not and identified as representatives of producers, users, or general interest.
Users represent organizations which either purchase or use
catalysts, and who could not be classified as producers. companies which make and sell catalysts. neither user nor producer.
Producers represent
The general interest category
~s
This category includes private consultants,
instrument and equipment companies, independent laboratories, consumer groups, and university researchers. of D-32.
Table 1 shows the classification for the membership
Only a single vote is allowed to representatives of one institution
or major separate functional division of it. dominate.
We are in good balance.
No interest can be allowed to
768 TABLE 1 Membership classification Users Producers General Interest Unclassified Total
45 52 23 1 121
Fourteen of the 121 members represent eleven foreign countries; n1ne are Europeans, two South Americans, two Canadians, and one Australian.
In addition
to representatives from these countries, we have had contacts with interested researchers in the USSR, Japan, and India. Table 2 shows the current officers of D-32. balance of interest.
Even here, we must maintain a
The chairman cannot be a producer of catalysts.
vice chairmen represent both users and producers.
The two
Committee D-32 has a liaison
chairman who has contact with other organizations and researchers outside the United States who are interested in standardization.
Each committee has an
ASTM-assigned staff manager to assist the committee and provide liaison to the parent organization.
TABLE 2 D-32 Officers Chairman - R. J. Bertolacini, Amoco oil R&D Producer Vice Chairman - K. I. Jagel, Engelhard User Vice Chairman - R. M. Koros, Exxon R&D General Secretary - D. E. Gross, Monsanto Membership Secretary - F. G. Young, Union Carbide Liaison Chairman - A. H. Neal, Exxon R&D ASTM Staff Manager - A. Cavallero ASTM Committee D-32 is subdivided into four technical subcommittees and three support subcommittees.
These are shown in Table 3.
TABLE 3 Subcommittees Technical Physical-Chemical Properties Physical-Mechanical Properties Chemical Analyses Catalytic Properties Support Nomenclature Editorial Statistics and Data Handling Subcommittee officers are either appointed or elected for each group. The officers include a chairman and vice chairman. subdivided into task groups.
Each subcommittee is fUrther
Their fUnction is to initiate draft standards
769 and revisions.
This is where the work begins.
elected, appointed, or volunteer.
The task group leader can be
As a member of D-32, one chooses the
subcommittee and task groups of interest, and then becomes involved in the standardization process. Previous papers (ref. 1-3) have described the standardization procedure, so we will only briefly review the process. works is illustrated in Figure 1.
How the standardization process
The members of a task group, let's say 1n
the subcommittee on Physical-Chemical Properties, decide that the determination of surface area by a static single-point procedure is an important catalyst characterization method.
Each member is then solicited for a possible proce-
dure to be considered by the task group.
After tentative selection of a method,
reference samples are circulated and each active member tests the samples using the selected procedure.
The resulting data are analyzed and a draft
method written; the method 1S reviewed by the subcommittee through a balloting procedure.
If approved in two-thirds of the ballots returned (a minimum of
60% of the eligible subcommittee voters must return ballots), the document moves on to the full D-32 Committee.
At the committee level, the proposed
document must be approved by 90% of those balloting and again a 60% return is required.
If approved, it moves on to the full ASTM Society ballot.
each member of ASTM has an opportunity to vote on the document.
Now,
Of those
voting, 90% must approve; but before final, official sanction as an ASTM method, the document must be submitted to the Committee on Standards of ASTM.
If the
Committee on Standards of ASTM determines all the Society requirements have been met,
the document is now an approved ASTM standard.
At each step,
from subcommittee through ASTM Society ballot, any negative
ballot must be rigorously considered and reconciled before proceeding.
The
minority opinion must be heard, and only after concensus is reached is the document allowed to proceed to the next step.
The process is deliberate, but
has resulted in over 6,000 ASTM standards.
Fig. 1. Standardization process Now we will concentrate on the methods which have been published and others
770 which are now being evaluated by each subcommittee.
Eight methods have been
standardized and these are conveniently published in a brochure (ref. 4). This is available at nominal cost from ASTM.
In addition, the eight standard
methods are published in Part 25 of the 1982 annual book of standards (ref. 5). This volume includes standards on petroleum and lubricants as well as aerospace materials, so it is considerably more expensive than the brochure; however, the volume 1S available free to ASTM members.
SUBCOMMITTEE ACTIVITIES Physical-Chemical Properties This subcommi ttee has publ i sh ed four standards:
D3663-78, Test for Sur face
Area of Catalysts; D3906-80, Test for Relative Zeolite Diffraction Intensities; D3908-80, Test for Hydrogen Chemisorption on Supported Platinum on Alumina Catalyst by Volumetric Vacuum Method; and D3942-80, Test for Determination of the Unit Cell Dimension of a Faujasite-Type Zeolite.
Currently, the subcom-
mittee is working to improve the precision and accuracy method.
0
f the sur face area
A revision has also been made to the precision and accuracy statement
on the procedure for hydrogen-chemisorption by platinum on alumina catalysts. Several test procedures are in various stages of development.
A test method
for pore-size distribution by mercury intrusion is in the final approval stage. A second test method for measuring pore-size distribution by nitrogen adsorption desorption isotherms is undergoing revision for committee approval. Five procedures are in the early development stage and the subcommittee members are evaluating test procedures and testing reference samples.
These
methods are: (1) Procedure for oxygen chemisorption of silver catalysts on alpha alumina. (2) A dynamic (pulse) procedure for carbon monoxide on platinum-alumina. (3) Nitrogen adsorption for determining effective zeolite content
0
f a catalyst.
(4) X-ray diffraction to estimate gamma-alumina in cracking catalysts. (5) Methods for determining surface acidity of catalysts.
Physical-Mechanical Properties This committee has just completed development of a rotating-drum method for measuring attrition-abrasion of formed catalysts. Final revisions are being made on a method for determining the crushing strength of single pellets and spheres. for extruded catalysts.
The method is also being evaluated
Reference samples are being sent to participating
laboratories. Problems in obtaining reproducible results for a test of bulk crushing strength are being resolved by modifications to the procedure.
Interlaboratory
samples incorporating the new modifications are now being tested.
771 The committee 1S also revising methods for vibratory and tapped bulk density for formed catalysts and introducing a project to extend the method to powders. Methods for measuring particle-size distribution using two commercially available automated counting units are now being drafted.
A third method for
evaluating a sonic sifter technique will be circulated next.
The committee is
also reactivating a study for methods to measure the attrition-abrasion of powdered catalyst by jet stream attrition techniques.
Chemical Analysis This subcommittee has issued two standards:
D36l0-77, Test for Total Cobalt
1n Alumina-Base Cobalt-Molybdenum Catalysts; and D3943-80, Test for Molybdenum in Fresh Alumina-Base Catalysts.
The group is redrafting a procedure for
determining high nickel concentrations 1n alumina-base catalysts and a method based on atomic absorption for low (up to 6%) concentrations of nickel.
Two
reforming catalysts are being used in interlaboratory tests to develop a procedure for analyzing platinum in fresh alumina-base catalysts.
Catalytic Properties This group has issued one standard method, D3907-80, Testing Fluid Cracking Catalysts by Microactivity Test.
They are now working to develop a procedure
for measuring individual component yields (gas, liquid, and coke) microactivity test to determine catalyst selectivity. a companion to the microactivity test.
from the
This procedure would be
Because steaming is a normal pretreat-
ment before measuring fluid cracking catalyst activity, the committee is in the initial stages of developing a procedure to measure steam deactivation. Interlaboratory tests are defining important variables such as temperature, fixed versus fluid bed, and the advisability of using thermal shock.
Several
zeolitic cracking catalysts have been tested and the evaluation is continuing.
Nomenclature This group has issued one standard, D3766-80, Definitions of Terms Relating to Catalysts and Catalysis. In addition to the four technical standards subcommittees, D-32 has two special support subcommittees which serve as advisors to the others.
An
Editorial subcommittee holds periodic workshops to diacuss writing formats, research reports, metrication, and other timely topics to assist the other subcommittees in publishing their reports.
A subcommittee on Statistics and
Data Handling works with task groups to advise them on data processing, precision and accuracy statements, and computer programs for data handling. The statistics subcommittee has also developed a computer program to design experiments for interlaboratory testing.
772 Committee D-32 also has an ad hoc committee on Liaison with other ASTM committees and with foreign groups with similar interest in standardization and characterization methods for catalyst testing.
SUMMARY since its organization 1n 1975, ASTM Committee D-32 on Catalysts has expanded its membership from about 70 to 121.
Liaison has been established with others
interested in standardization, including groups in Canada, USSR, Europe, South America, Japan,
Au~tralia
and India.
ASTM encourages this participation by
either active membership in ASTM or cooperation through the liaison chairman. Eight methods have been approved and published as standards. methods are in either the evaluation or development stage.
About 23 test
If the usual time-
table for acceptance is followed, about six of these will be approved as standards during 1982.
Thus far, Committee D-32 has concentrated its efforts
in the area of heterogeneous catalysts closely related to petroleum refining and petrochemicals processing.
However, if there is interest in either other
heterogeneous systems or homogeneous catalysts, the committee 1S willing to develop test procedures to fit these needs. consider any catalyst test procedure.
The committee is willing to
We welcome participation in this work
as active members, cooperating in the tests, or as interested catalytic scientists.
REFERENCES 1 J.R. Kiovsky, Oral communications, Proceedings of the International Symposium on the Relations Between Heterogeneous and Homogeneous Catalytic Phenomena, Brussels, October 23, 1974. 2 A.H. Neal, "Preparation of Catalysts II," B. Delmon, et al., Ed., p.719, Elsevier, Amsterdam, 1979. 3 R.J. Bertolacini, et al., Papers 8a-8e, American Institute of Chemical Engineers, Chicago, November 18, 1980. 4 "ASTM Standard on Catalysts," PCN 06-432080-12, ASTM, 1916 Race Street, Philadelphia, Pennsylvania, 19103. 5 American Society for Testing Materials, 1981 Annual Book of Standards, Part 25, p.1103-1143.
773 DISCUSSION W. RODER: Are there any intentions up to now, to standardize catalytic properties of other catalysts than fluid-cracking-catalysts (for instance hydrocarbon oxidation or methanation ? R.J. BERTOLACINI: There is at present no interest in the USA for developing standard test procedures for hydrocarbon oxidation or methanation. We have discussed the possibility of developing standardized tests for HDS and catalytic reforming but to date, there has been no interest. ASTM Committee D-32 is currently working on a method for measuring selectivity for cracking catalysts, an adaptation of the MAT test. The selectivity testing is in progress and I estimate a standardized procedure should be published in the latter part of 1983 or early 1984. M.F.L. JOHNSON ASTM consciously attempts to write procedures in such a way that it will encompass commercially available equipment as well as home-constructed equipment. R.J. BERTOLACINI Yes, ASTM doe not specify a single commercial unit but rather writes broad apparatus specifications so as to eliminate any conflict of interest M.M. BHASIN: In standardization of catalyst/carrier test methods, are you developing a library of primary standard test samples? Also would you elaborate on how they are stored to ensure they remain primary standards? R.J. BERTOLACINI: Yes, we have a number of reference samples for all of our procedures. These are stored by the supplier under proper conditions to assure their quality. We are attempting to interest commercial specialty chemical supply firms to catalog and sell these items. If this approach does not work out, we have indications that the USA-National Bureau of Standards would be interested in storing and cataloging the samples as reference materials. If the Bureau catalogues the materials, the samples would be available at a nominal cost.
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G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
775
PROGRESS REPORT OF THE COr1tlITTEE ON REFERENCE CATALYST, CATALYS IS SOCI ETV OF JAPAN Yu i chi ~1URAKAMI Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan The committee on Reference Catalyst, Catalysis Society of Japan,was founded in 1978, after the conclusion of a symposiur:l on "Research by Using Reference Catalysts" in November, 1977. The objectives of the committee are to distribute reference catalysts, to collect and report the data on the reference catalysts, and to develop standard methods for catalyst characterization. Using reference catalysts generates solidarity and cooperation among individuals and laboratories. A wide variety of methods are used to characterize the catalysts and to evaluate catalytic properties, and each of the methods has SOr:le advantages over the others. The differences in the methods and/or catalysts result in the lack of common background for precise discussions. The reference catalysts make it possible to correlate wide variety of methods with each other and to develop discussions. This promotes cooperation among researchers and leads to a better understanding of catalysis. An important application, especially for the beginners, is the examination of methods for the characterization and the evaluation of catalytic performance by using reference catalysts.
ORGANIZATION Organization of the committee is as follows. Chairman: Prof. Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa, Nagoya 464, Japan Secretary: Dr. Hideyuki Matsumoto Research and Development Division, JGC Corporation, Bessho 1-14-1, Minami-ku, Yokohama 232, Japan Liaison: Prof. Tadashi Hattori Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa, Nagoya 464, Japan The other committee members are Prof. H. Hattori (Hokkaido Univ.), Prof.
776
S. Okazaki (Ibaragi Univ.), Dr. J. Take (Univ. of Tokyo), Prof H. Niiyama (Tokyo Inst. Tech.), Prof. S. Yoshida (Kyoto Univ.), Prof. T. Imanaka (Osaka Univ.), Prof. H. Arai (Kyushu Univ.), and Dr. I. Furuoya (Takeda Chern. Ind., Ltd.). Anyone can be given the catalysts through the committee members without any obligations and can take part in the activity of the committee. So~e projects and symposia have been organized by the users. ACTIVITY Ten commercial £atalysts and/or supports, shown in Table 1, were selected as Table 1 List of reference catalysts Catalyst Alumina« Alumina b Mark JRC-ALO-l JRC-ALO-2 Composition (wt%) Ig. loss 4.1 0.03 0.03 Fe203 0.03 0.22 Si02 0.03 0.04 Na20 Ti02 Sf 0.05 2.00 Pore volume (cm 3/g) 0.67 0.72 Surface area (m2/g)g 176 298 Remarks n,Y-A12 03h n- A1203 h SilicaCatalyst Silica gel alumina Mark JRC-SIO-l JRC-SAH-l Composition (wt%) 3.05 Ig. loss 10.8 1. 31 28.61 A1203 CaD 1.73 G.05 0.02 Fe203 0.03 0.013 Na20 Sf 0.01 0.26 3/g) Pore volume (cm 0.58 0.93 Surface area (m 2/g) 166 511 granular Remarks FCC type type
Alumina c JRC-ALO-3
Alumina d JRC-ALO-4
Alumina e JRC-ALO-5
0.01 0.01 0.3 0.01 N. D. 0.51 128 Y-A1203 h Silicaalumina JRC-SAL-2
0.01 0.01 0.01
0.68
11.0 13.75 0.02 0.012 0.43 0.73 560 FCC type
N.D. 0.66 174 Y-A1203 h Zeolite JRC-Z-l
0.02 0.57 0.41 253 n,Y-A1203 n Titanium oxide JRC- TIO-l
2.8
H20 1.66
N.D. 0.35 670 NaY type
4.73 72.6 anatase
a: prepared by tableting gibbsite produced by Bayer's Process and by calcinating
at 973 K. b: prepared by washing precipitate from sodium aluminate and aluminum sulfate and by calcinating at 723 K. c: prepared by tableting and calcinating powders of X,p-aluminas and gibbsite to form y-alumina and by calcinating again at 973 K. d: prepared by tableting and calcinating boemite powders to form y-alumina and by calcinating again at 973 K. e: prepared by drying desulfated aluminum hydrogel produced from aluminum sulfate and by calcinating at 82Jv873K. f: measured by N. Nojiri, M. Nakashima and N. Ii (t,litsubishi Petrochem., ref. 3). g: Surface areas of ALO-l 'VALO-5 were determined by the project mentioned below. h: determined from XRD patterns by K. Mukaida (Muroran Inst. Tech., ref. 1). The other data were given by the makers.
777
Table 2 List of reference supported metal catalysts Number No.1 NO.2 No.3 No.4 No.5 No.5 No.7 No.8 No.9 JRC-Al JRC-A4 JRC-A4 JRC-S2 JRC-SAH JRC-SAL JRC-Zl JRC-A4 JRC-A4 ~1ark -0.5Pt -0.5Pt -5.0Pt -0.5Pt -0.5Pt -0.5Pt -0.5Pt -0.5Pd -0.5Rh ALO-l ALO-4 ALO-4 SIO-2a SAH-l SAL-2 Z-l ALO-4 ALO-4 Support Material PtClb PtClb PtClb PtClb PtNHC PtNHC PtNHC PdCl z RhC1 3 M~~~~ent(wt%) 0.50 0.50 5.1 0.50 0.54 0.72 0.5 0.50 0.50 Preparatian IMpd IMpd H1P d IW e IMPd IMPd IEf IMPd IMPd a: Bead type; Composition(wt%), Na zO(0.03i, FeZ03(0.01), Al z03(0.07), CaO(0.09); Pore volume, 1.lcm 3/g; Surface area, 280m /g. b: HzPtC16·6HzO. c: [Pt(NH3)4]C1Z' d: Impregnation. e: Incipient wetness. f: Ion exchange. No.1 '\,4 were prepared in Univ. of Tsukuba, No.5,6,8 and 9 in Nippon Engelhard, and No.7 in Kyushu Univ. the reference catalysts, and they have been distributed to more than one hundred laboratories of industries and academic institutions. For the projects of metal surface area, nine supported precious metal catalysts, listed in table 2, were prepared and distributed to more than twenty laboratories including Argentina. Several supported precious and non-precious metal catalysts are going to be added to the list. The following three projects were organized and are still continuing; the determination of BET surface area of reference alumina catalysts, the determination of metal surface area of supported precious metal catalysts, and the standardization of rapid determination of metal surface area by the pulse method of CO chemisorption. The following four symposia have been held and a fifth one is scheduled in October, 1982. (1) 1st annual symposium: General Properties of Reference Alumina Catalysts (Fukuoka, October, 1979). (2) 2nd annual symposium: Metal Surface Area (Sendai, September, 1980). (3) Special symposium: Metal Surface Area II (Nagoya, June, 1981). (4) 3rd annual symposium: Support Effect (Kyoto, October, 1981). Data reported in the first annual symposium were published in SHOKUBAI (CATALYST), a bulletin of Catalysis Society of Japan (ref. 1), and those reported in the special and the third symposia were published as preprints (ref.2,3). A part of data reported in the second symposium was briefly summarized in a report (ref. 4). RESULTS A wide variety of results have been reported, but only the subjects conducted cooperatively were briefly shown below. One would notice that the solidarity and collaboration are growing.
778
1. BET Surface Area of Reference Alumina Catalysts. (T. Hattori, ref. 1,3) The project of the entitled subject was organized in 1979 after the first symposium. The objective of the project includes the re-examination of the procedures employed in individual laboratories as well as the determination of the BET surface areas. By reason of the former, it was decided that the measurement was done by the procedures which had been employed in individual laboratories. Only the pretreatment was fixed as follows: (1) drying catalysts at 383 K overnight, (2) allowing to cool and to stand for longer than 24 h in a desiccator, (3) weighing the sample,-and (4) evacuating at 573 K for 2 h. Eleven laboratories, listed in the appendix, participated in the project. Three laboratories employed the flow method and eight laboratories employed the static volumetric method. The surface area was calculated by the single point method in three laboratories employing the flow method, by the BET plot in seven laboratories, and by a new plot (Takagi plot), shown below, in one laboratory. l/v(l-x) = l/v m + (l-x)/xvmC The average surface areas thus determined per unit weight of evacuated catalysts were shown in Table 1. The scatters of the surface areas per unit weight of unevacuated catalysts were larger than those of evacuated catalysts. The latters were shown in Fig. 1. The scatters in the static method, shown by the empty in Fig. 1, were almost equal to those obtained in the SCI/IUPAC/NPL project(ref. 5). The scatters in the flow method, shown by the solid, were larger than those in the static method. The difference is larger on ALO-2 and 5 whose surface areas also are larger than the others. One of the reasons may be the non-linear relation between the response of thermal conductivity detector and the composition of He-N 2 mixture in the flow method. Fig. 2 shows another possible reason. The surface area increased with the evacuation temperature on all aluminas, but the effect was larger on ALO-2 and 5. This suggests that the effect of evacuation may be more significant in the flow method. Further examination will continue for the establishment of the flow method for the rapid determination of BET surface area. The other physical textures, such as pore volume, pore size distribution and XRD patterns, were reported by K. Mukaida (Muroran Inst. Tech., ref. 1). Surface areas, pore volumes, and pore size distributions were also measured by the adsorption of benzene (J. Kobayashi, Shizuoka Univ., ref. 1). The change of physical textures with calcination was reported by T. Inui, T. Miyake and Y. Takegami (Kyoto Univ., ref. 1,3). TEM microphotographs were reported by M. Yamada and H. Matsumoto (JGC Corp., ref. 3). 2. Acid Properties of Reference Alumina Catalysts. The variety of methods are used to characterize the acid properties of cata-
779
&---...L-_--'
ALO-l
1.0 o
(l)
L
c::I: (l)
u
o
4L
e55 0.9
o ALO-l
(l)
>
o ALO-5
W.
L . . . - _.......
0.8
!
1.0 1.2 S,A, / S,A, (average)
Fig. 1. Frequency distribution of surface areas of reference aluminas. Open square, by static method; solid square, by flow method (ref. 1,3).
• ALO-2 A ALO-3 • ALO-4 o ALO-S
0,8'----'-----'----'-----1 400 500 600 700 Evac. Temp. (K) Fig. 2. Effect of evacuation temperature on surface area. Evacuation period, 15 min. (N. Nojiri and M. Kurashige, Mitsubishi Petrochem., ref. 3).
lysts. Three methods have been applied to the reference alumina catalysts: microcalorimetric measurement of the differential heat of adsorption of ammonia (H. Yamaguchi, K. Tsutsumi and H. Takahashi, Univ. of Tokyo, ref. 1); temperature programmed desorption of n-butylamine (S. Ogasawara and S. Kanamaki, Yokohama Natl. Univ., ref. 1); and infrared spectra of adsorbed pyridine (J. Take and Y. Yoneda, Univ. of Tokyo, ref. 1). Microcalorimetry tells both the acid amount and strength, and is superior to others in the measure of acid strength. The acid strength is given by the immediate interaction energy of acid sites with the probe bases. TPD method also gives information on the amount and strength of acid sites. The acid strength is given in terms of desorption temperature. The product distribution gives information on the nature of acid sites (ref. 6). IR method is distinguished by the fact that it can tell the type (Bronsted or Lewis) of acid sites. It also gives the amount and strength of acid sites. The strength is given in terms of evacuation temperature of catalysts preadsorbing probe bases (ref. 7). These three methods are different from each other in the probe bases and in the measure of acid strength. In spite of these differences, good correlations were obtained among the acid amoun~measured by
780 1.0r------------~
0.8fc-, .0
0.
u
+-J
E
(l)
o
0.6-·
~NU I
-
0
:z:CJ)(l) > I
~
.::t
U I
C
I.-
>.
I - _ . LI- _ . . L -I - _ . . LI- - - - - - I 0 . 4 L . . - _ I. L - _ . L
0.2
0.4
0.6
o
o;
NH 3 (ad) by Calorimetric (mmol/g) (heat of ads.
>
80 kJ/mol)
Fig. 3. Comparison of acid amounts of reference aluminas measured by three methods (ref. 1).
0/0
three methods, as shown in Fig. 3. / Q <, The difference in absolute values 0.2 0 E may be due to the difference in E the strength of acid sites measI.ured. These correlations led to 0 the further effort cooperatively 0 u 0.1 made to obtain quantitative agree>. .0 ment between microcalorimetric o ALO-l TI and IR methods (J. Take et al, 0' A ALO-3 0 ref. 3,8). The evacuation tem>. c, perature was correlated with the 0 0.1 0.2 heat of adsorption by microcaloPy(ad) by IR (mmol/g) rimetry, and a linear relation Fig. 4. Comparison of acid strength was obtained between the heat of distribution measured by IR and microcalorimetric methods (ref. 3,9~ adsorption and the reciprocal of The evacuation temperature. amount of adsorbed pyridine with the heat of adsorption larger than various level was calculated from the infrared spectra by the aid of the above-mentioned linear relation, and was compared with those of adsorbed pyridine and ammonia measured by microcalorimetry. As shown in Fig. 4, quantitative agreement was obtained between microcalorimetry and IR method.
/a
s
O}{{
781
3. Metal Surface Area of Reference Supported Metal Catalysts. (H. Matsumoto, ref. 2,4; and N. Nojiri, ref. 11). The project of entitled subject was organized to give a chance for comparing the metal surface area measured by different researchers with different methods. Nine supported precious metal catalysts, shown in Table 2, were provided for the project and two symposia were held. As one of the objectives was the examination of the methods which had been employed, any of the experimental conditions were not fixed. The res~lts were summarized in Table 3. Participants in the project are listed in the appendix. It should be noted that the researchers in industry employed the dynamic (pulse) method of CO chemisorption, although the variety of methods were listed in Table 3. It was interesting that the results were rated high by industry researchers but low by university researchers. Most of the former felt that the results agreed pretty well with each other,except sample No.1, considering the difference in the experimental conditions and method, Table 3 Metal surface area of reference supported metal catalysts a Method
NO.1
Catalyst No.2 No.3 No.4 No.5
XRD N.D. N.D. XRD N.D. N.D. TEM 1.0 1.0 small TEM &5'1,10 7.0 Chemisorption methods CO pulse 49 78 CO pulse CO pulse 17 66 CO pulse 53 92 CO pulse 92 116 CO pulse 25 87 CO pulse 64 108 CO static 53 83 CO static 140 133 02 pulse 58 57 02 pul se 25 36 02 stati c 31 33 Hr0 2 pulse 76 124 46 02- H2 {02 44 titratn H2 130 144 H2 static 72 77 H2 TPR 56 60 CS2 7.6 34 poisoning
No.6
No.7
No.8 No.9
Reduction condition
22 22 N.D. N. D. N.D. 19 19 20 N.D. 2.2 6.8 1.0 0.9 1.8 1.7 1 & 2'1,3 & h6 2.5'1,4.5 - 0.6'1,1.5 6'1,9 1arge 53
26
53 66 78 62 67 56 84 44 28 28 94 36 108 67 45 23
28 29 21 34 22 11 6.3 7.6 25 13 38 16 19 8.9
44 39 45 47 56 55 59 66 95
38 1.7 0.4 31 43 0 8.5 0.7 55 0.6 48 5.0 51 0 58 101 84 33
15 21 47 12 61 44 39 43
16 13 20 4.7 55 1.8 9.3 1.8 52 5.6 7.5 35 2.2 32 27 2.8
60 79 63 73 138 95 110 32 172
198 147 50 158 160 33 228 112 281
42
99 94 271 86 146 90
21 95 96 94 66
-
473'U673K, lh 473'U673K 313K, 10min 453'U623K, 30mi n 723K, 10min 723K, 10min 723K, 10min 673K, 2.5h 573K, 1h 723K, 45min 823K, 30min 573K, 1h 823K, 30min 823K 823K 573K, 1h 773'U823K 423'U573K
aXRD and TEM, average diameter (nm) from XRD patterns and from TEM microphotograph. Chemisorption methods, cm 3-adsorbed gas/g-meta1.
782
and they seemed to gain confidence in their methods. On the other hand, some of the latter held the view that the re-examination under standard conditions is necessary. These led to the project of standardization of rapid measurement of metal surface area by the pulse method of CO chemisorption for industrial purpose and a joint research on the dispersion of these catalysts. In the latter, good agreement was obtained among the chemisorption data by static and TPR methods and TEM data, when reversibly adsorbed species are taken into consideration (ref 9). Further, tbe remarkable change in HZ chemisorption by the treatment with HZ and Oz was observed, and the effect of the supports, especially the effect of sulphur content, on the change was examined by K. Kunimori et al. (Univ. of Tsukuba and Mitsubishi Petrochem., ref. 3, 10). 4. Standardization of Rapid Measurement of Metal Surface Area by Pulse Method of CO Chemisorption. (N. Nojiri, ref. 3, 11). In this project, the standard pretreatment condition was fixed as follows: (1) increasing temperature from room temperature to 673 K in an inert gas flow, (2) increasing temperature from 673 K to 723 K in HZ flow, (3) holding temperature at 7Z3 K for 0.5 h in HZ flow, (4) holding temperature at 7Z3 K for 0.5 h and then cooling to room temperature in an inert gas flow, and (5) measurement of CO chemisorption by the pulse method. But the other experimental conditions were not fixed. The results are shown in Table 4. Participants in the project are listed in It also was the appendix. Again a large scatter was observed on sanp l e No 1. reported that the data on sample No 1 was remarkably affected by the pretreatment condition, and a view that milder pretreatment may be favorable, especially for sample No 1 , was presented. On the other hand, the results on samples No 5 and 6 were reproducible. The results were devided into two groups: the first group is A and B, and the second, C, D, E, F and G. The Table 4 agreement in each group is excellent. In A, the CO Chemisorption a by pulse method after b effluents were collected in a Porapak trap at standard pretreatment dry-ice temperature and then analysed. In B, Catalyst the effluents were analysed with a 70cm column No.5 No.6 No.1 of activated carbon at 330 K. In the others, A 54 40 38 effluents flowed immediately to a thermal con44 38 B 51 60 43 C 38 ductivity detector. This difference would 46 56 D 38 55 44 cause the difference between two groups. The E 43 56 53 F 40 average experimental conditions were as follows. 44 53 G 49 The purification of carrier gas is not necestext. sary. r1easurement was done around room tempera-
783
ture. The ratio of CO pulse size to catalyst weight vias about 1/1 (mm 3/mg). The interval of CO pulse was 2~3 min. The long interval may allow the desorption of reversibly adsorbed CO, and, therefore, it leads to the evaluation of only irreversibly adsorbed CO. The interval of 2~3 min may be sufficient to measure both reversibly and irreversibly adsorbed CO. The above mentioned joint research (ref. 9) concluded that the reversibly adsorbed species also should be taken into consideration to evaluate the degree of dispersion (percentage exposed). This project is stilt in progress and will be a subject of the fourth annual symposium scheduled~in October 1982. 5. Miscellaneous. In addition to the subjects mentioned above, several reports and comments on the reference catalysts have been presented in the symposia. Only the titles are shown below. (1) CH 4-D 2 exchange on reference alumina catalysts (H. Hattori, M. Uchiyama and K. Tanabe, Hokkaido Univ., ref. 1). (2) Surface reaction rate of dehydration of alcohols by pulse surface reaction rate analysis (PSRA) and an emmisionless infrared diffuse reflectance spectrometer (EDR) (T. Hattori, K. Shirai and V. Murakami, Nagoya Univ., ref. 1 and 12). (3) Turnover frequency of dehydration of sec-butylalcohol as a function of strength of Lewis acid sites (K. Nakacho, J. Take and V. Voneda, Univ. of Tokyo, comment in 3rd symposium). (4) Optimum reaction condition for the formation of ethyl ether in the dehydration of ethanol by a full automatic computer-operated reaction system in laboratory (V. Tsuchida and H. Niiyama, Tokyo Inst. Tech., comment in 3rd symposium). (5) Adsorption of Ni, Cu and Pt ions on reference aluminas (H. Niiyama, T. Ogiwara, N. Suyama and E. Echigoya, Tokyo Inst. Tech., ref. 1 and 2nd symposium). (6) IR spectra of dissociatively adsorbed hydrogen on reference supported metal catalysts (V. Soma, Univ. of Tokyo, 2nd symposium). (7) Hydrogenation of 1,3-butadiene on reference supported metal catalysts suspended in acid solution (H. Kita and K. Shimazu, Hokkaido Univ., 2nd symposium and ref. 3). (8) Methanation of CO over Ni-La203 supported on reference aluminas and over reference supported metal catalysts (T. Inui, T. Miyake and Y. Takegami, Kyoto Univ., ref. 1 and 3). (9) Support effect in the hydrogenation of cyclopentadiene on supported nickel catalysts (A. Sannomiya, M. Yano and Y. Harano, Osaka City Univ., ref. 3 and 13).
784
(10) Temperature programmed reduction of nickel catalysts supported on reference a1uminas and thier catalytic behavior in the methanation of CO and CO 2 (Y. Nakagawa and S. Ogasawara, Yokohama Nat1. Univ., comment in 3rd symposi um). REFERENCES 1 Data on Reference Catalysts, Shokubai, 22(1980)115, H. Matsumoto, Shokubai, 22(1980)107. 2 Preprints of Symposium on Metal Surface Area II, Nagoya, June 19, 1981. 3 Preprints of 3rd Imnual Symposium on Reference Catalysts, Kyoto, Oct. 11,1981. 4 H. Matsumoto, Shokubai, 22(1980)410. 5 D.H. Everett, G.D. Parfitt, K.S.W. Sing and R. Wilson, J. Appl. Chem. Biotechno1., 24(1974)199. 6 M. Takahashi, Y. Iwasawa and S. Ogasawara, J. Cata1., 45(1976)15. 7 J. Take, T. Ueda and Y. Yoneda, Bull. Chem. Soc. Japan, 51(1978)1581. 8 J. Take, H. Matsumoto, S. Okada, H. Yamaguchi, K. Tsutsumi, H. Takahashi and Y. Yoneda, Shokubai, 23(1981 )344. Further details will be published. 9 K. Kunimori, T. Uchijima, M. Yamada, H. Matsumoto, T. Hattori and Y. ~1urakami. Appl. Catal., submitted. 10 K. Kunimori, T. Okouchi and T. Uchijima, Chem. Lett., (1980)1513. K. Kunimori, Y. Ikeda, T. Okouchi, N. Nojiri, M. Soma and T. Uchijima, Shokubai, 23(1981) 365. 11 N. Nojiri, Shokubai, 23(1981 )488. 12 T. Hattori, K. Shirai, M. Niwa and Y. Murakami, React. Kinet. Catal. Lett., 15(1980)193. 13 A. Sannomiya, T. Tashiro, M. Yano and Y. Harano, Shokubai, 24(1982)112. APPENDIX Institutions participating in the project of BET surface area of aluminas Tanabe's lab., Hokkaido Univ.; Okazaki's lab., Ibaragi Univ.; Central Res. Lab., Idemitsu Kosan Co.; Mr. Takagi, Tokyo Inst. Tech.; Ogasawara's lab., Yokohama Natl. Univ.; Kinuura lab., JGC Corporation; Murakami's lab., Nagoya Univ.; Inui's l ab. , Kyoto Univ.; Yoshida's lab., Kyoto Univ.; Tsuchiya's lab., Yamaguchi Univ.; and Kagawa's lab., Nagasaki Univ .. Participants in the project of metal surface area S. Yoshida (Kyoto Univ.), T. Murata (JGC Corp.), A. Furuta and M. Yamada (JGC Corp.), H. Arai and Y. Kibe (Kyushu Univ.), Y. Akai (Idemitsu Kosan Co.), T. Hattori and Y. Murakami (Nagoya Univ.), E. Kikuchi (Waseda Univ.), K. Aika and O. Kato (Tokyo Inst. Tech.), K. Kunimori, S. Matsui, Y. Ikeda, E. Yamaguchi and T. Uchijima (Univ. of Tsukuba), K. Iida and T. Imai U~itsubishi Heavy Ind. Co.), and those participating in the following project. Participants in the project of standardization of rapid measurement of metal surface area by CO-pulse method S. Nishiyama and H. Niiyama (Tokyo Inst. Tech.), T. Mori (Gov. Ind. Res. Inst., Nagoya), E. Yasui and F. Haga (Nippon Oil Co.), T. Nakata and T. Sakurai (Nippon Engelhard), N. Nojiri and K. Kurashige (Mitsubishi Petrochem. Co.), T. Inui, T. Miyake and Y. Takegami (Kyoto Univ.), and T. Suzuki (Asia Oil Co.).
785 DISCUSSION ZHAO JIUSHENG Could you tell something about the measurement of the acid amount of Y-AI203: 1) Did you use the titration method, and if so, why? 2) What carrier gas have you used for the TPD study. Is there any difference when you use another carrier gas? 3) In the infrared measurements, have you distinguished the Lewis acid sites from the Bronsted ones? Y. MURAKAMI: 1) I do not think that the titration method is suitable for measuring the acidity of alumina catalysts, because the indicator turns into acidic color only after the alumina is treated in vacuum at high temperature and the amount of n-butylamine is too sensitive to the pretreatment. Further, the change of color of the indicator can hardly be controlled. 2) In the TPD method, nitrogen was used as a carrier gas with an FID detector. Although no attempt has been made to examine the effect of the carrier gas, I believe that any inert gas will give the same results. 3) Of course,Lewis and Bronsted acid sites were distinguished. On alumina catalysts, only coordinatively adsorbed pyridine on Lewis acid sites was observed on the IR spectra. Therefore, Fig. 4 in the text shows the relationship for the Lewis acid sites. The relationship for the Bronsted acid sites has also been examined by using H-ZSM 5, on which only pyridinium ion (Bronsted acid sites) was observed. A linear relationship has been obtained between the amount of adsorbed ammonia by microcalorimetry and the amount of adsorbed pyridine (pyridinium ion) by IR method. (Ref. 8 in the text) . J.J.F. SCHOLTEN The CO-pulse method may also be used in hydrogen as a carrier gas (McKee's method). The advantage is that: a. strongly chemisorbed hydrogen, still present from the reduction pretreatment, has not to be desorbed at temperatures as high as 500°C. Many metals sinter very strongly at this temperature. b. Pulsing is performed in the reduction hydrogen stream, and for many noble metals,the reduction temperature may be chosen very low (100°C,for instance). Sintering will not disturb the results. The hypothesis on which the method is based is that CO kicks off the hydrogen from the surface. This asks for further fundamental studies for various metals. G.C. BOND I can confirm that the method of pulsing CO in a H2 stream works perfectly well for the routine determination of Pd dispersion. The method was developed by myself and my colleagues in the Johnson Matthey laboratories in the mid-1960's. The problem of using CO in an inert carrier gas is not only that of removing adsorbed H remaining from the reduction, but also that of using an extremely pure (especially, 02-free) gas stream: if this is not done, consistent results cannot be expected. Y. MURAKAMI: (To Prof. Scholten and Prof. Bond). The project of metal surface area possesses two subjects: the research for true metal surface area and the standardization of rapid determination of metal surface area for pratical purpose. One might prefer to start the second subject after the first one, but industrial researchers are not so patient. They must determine the metal surface area of catalysts which are presently used and are going to be used. Actually, they determine it by their own methods. As the initial step of the second subject, the standardization of the most popular method has been undertaken. The next step is the examination of the standardized method for the final goal. For such purpose, it is necessary to solve the problems which have been raised, to compare the method with others, and to examine the applicability of the method to various catalysts. At present, these are in progress, and the first subject (basic research) will be of a great help in this step. The effect of the purity of the carrier gas is one of the ~roblems raised. It has been reported that the purity of the carrier gas has o"ly a little effect, as shown in Table 1, although a few contradictory results have also been privately communicated. The effect seems to depend on the metal and support. This
786 will be a topic of the next meeting of the committee. The CO-pulse method in hydrogen as a carrier gas has also been examined. The results, shown in Table 1, are in good agreement with those in He as carrier gas. Kicked off hydrogen also was measured by using nitrogen as a carrier gas, but the amount did not coincide with that of adsorbed CO, as shown in Table 1. This result may be examined further in relation to the comparison with other methods. It has been pointed out in the committee that the reduction temperature appears too high. Some contradictory results have been reported on the effect of the reduction temperature. This also will be a topic of the next meeting, and lower reduction temperatures will be employed for a new series of catalysts. A new series of catalysts listed in Table 2 has been prepared and distributed for the comparison with other methods and for the examination of the applicability of the method. The committee is ready to supply them to the foreign countries, although the total amount prepared is limited. TABLE 1.
Effect of the carrier gas on CO chemisorption CO chemisorbed and H2 kicked off
Carrier gas
He (purified
Catalyst
a)
He (unpurified) a) H2 (purified b) N2 (purified
CO (ads) CO(ads) CO (ads) H2 (des)
c c c d
No 1
No 5
No 6
48
44
38
52
44
38
48
44
37
35
31
29
a, by silica gel trap at 77K; b, by oxytrap; c, in cm d, kicked off H2 in cm 3 H2/9-P t. TABLE 2. Number 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
3
CO/g-Pt;
List of the new series of catalysts Mark JRC-A4-0.5Pt JRC-A4-0.5Pt JRC-A4-0.5Pt JRC-A4-5.0Pt JRC-A4-5.0Pt JRC-A4-0.5Rh JRC-S3-0.5Rh JRC-A4-0.5Ru JRC-S3-0.5Ru JRC-A4-0.5Pd JRC-S3-0.5Pd JRC-A4-30Ni JRC-A4-50Ni JRC-S3-30Ni JRC-S3-50Ni JRC-A4-5.0Ni JRC-S3-5.0Ni
(1.0) a (0.5)a (0.1) a (1.0) a (0.2)a (2.0)b
(2.0)b
Metal
Metal content
Pt Pt Pt Pt Pt Rh Rh Ru Ru Pd Pd Ni Ni Ni Ni Ni Ni
0.5 wt % 0.5 0.5 5.0 5.0 0.5 0.5 0.5 0.5 0.5 0.5 30 50 30 50 5.0 5.0
a, expected dispersion; b, second batch.
Support A1203 A1203 A1203 A1203 A1203 A1203 Si02 A1203 Si02 A1203 Si02 A1203 A1203 Si02 Si02 A1203 Si02
787
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE SCljIUPAC/NPL STANDARD NICKEL-SILICA CATALYST
R. BURCH and A.R. FIAMBARD, the University of Reading, England .M.A. DAY, Imperial Chemical Industries Ltd. R.L. MJSS, Warren Spring Laboratory, Department of Industry N.D. PARKYNS
and
A. WILLIAMS, British Gas Corporation
J .M. WINI'ERBClI'I'a1 and A. IVHITE, The University of Birmingham
ABSTRACT
The intention of this work is to devise clearly defined pretreatment and reduction procedures for =nverting the pre=sor to a standard nickel-silica catalyst which will exhibit a nickel surface area and activity for benzene hydrogenation falling within specific limits when agreed test methods are adopted.
This paper describes the methods which have been used to investigate
this catalyst, and reports sane of the results obtained.
It is hoped that
improvements in procedure can be suggested and then introduced into future experiments.
INTIDDlJCrICN The task of producing a standard catalyst is a formidable one and it is important at the outset to define the objectives.
For example, these might
include the use of the standard catalyst (L) to confirm the validity of catalyst test equiprent (ii) as a reference material for establishing catalyst characterization techniques (iii) for the accumulation of a more =herent lxxiy of catalytic data and (iv) for canparison with new catalysts under investigation. For such purposes where a stable supported metal catalyst is involved, then the standard catalyst might 1I\ell be supplied in its final reduced state, thereby retaining cont.ro.l of more of the experimental procedure within a single organisation.
Often, however, catalysts with a range of properties, e.g., metal
crystallite size, are required and then it seems more convenient to supply the catalyst pre=sor and to carefully specify conditions for generating the required range of reduced catalysts.
Again it needs to be recognised that the
catalyst pre=sor may change during storage. The Working Party on Catalyst Reference materials set up by the Society of Chemical Industry (SCI), following a proposal by the IUPAC carmission on Colloid
788 and Surface ChEmistry, has available a standard nickel-silica catalyst precursor
vkiich is distributed by the National Physical Laboratory (NPL).
The material
was obtained as a single batch (100 kg), prepared by in'pregnating the silica support with a solution of nickel nitrate, follO'l.led by drying at 393 K.
Tests
for hanogeneity on different particle size fractions indicated that the +150 pm material should be ranoved., the re:nainder of the batch was then subdivided into 80 g representative sarrples.
Initially
'We
have concentrated on the changes which occur during the pre-
treabnent and reduction of the catalyst precursor as shosn by the extent of hydrogen adsorption and activity for benzene hydrogenation.
We hope to
establish a very clearly defined procedure vkiich will allow other workers, starting fran the
I
as-received' material to produce a reduced catalyst having a
nickel surface area and (benzene) hydrogenation activity falling within specj.f.ied limits.
Additionally other nickel-silica catalysts with a range of propertaes
may be generated fran the precursor using information accumulated by the participants. EXPERIMENTAL ME:I'HODS
Table 1 surrrnarises the properties of the catalyst precursor. TABLE 1
Properties of the catalyst precursor
toss on ignition (1 h at 1273 K) Ni content 004 content N~P content s~I~ca
21.6%wb 11.1%db 0.2%db 0.07%db balance
apparent bulk density cempacted bulk density surface area
-3
0.55 g em_
0.64 9 em 197 m2 g-l
3
l'iisorption measurenents Three methods 'Were used to detennine the amount of hydrogen adsorbed by the nickel, narrely static (gravimetric or volumetric), pulse, and temperatureprogranmed adsorption or desorption.
The latter twJ methods offer the advantage
of speed and simplicity, but, can only give information on the total amount of hydrogen adsorbed under dynamic conditions.
For pulse experiments, the choice of
temperature and the cemposition of the carrier gas is important, for TPD experiments, the choice of heating rate, initial temperature, final temperature, carrier gas cemposition, are all important pararreters.
Static volumetric
adsorption measurements, although more time consuming, offer greater control over the experimental conditions, and allow an isothenn to be determined.
The shape
789 of the isotherm can be a guide to the cleanliness of the metal surface. the static method it is necessary to chose a prefe=ed procedure.
Even for
The temperature
must be specified, the range of hydrogen pressures to be used must be specified, the time required for equilibrium must be established, and t.be method of determining the rronolayer coverage must be decided upon.
The experimental
conditions used in the recarrnended procedure :liar static hydrogen adsorption measurements are as follows.
The adsorption tffilJJerature, 273 K, was chosen
partly because of its convenience, and partly because at higher temperatures the pressures required to attain rronolayer coverage with supported metals increases beyond the preferred range.
The preferred pressure range chosen (0-50 torr) is
convenient and easily measured with manareters, pressure transducers, etc.
The
attainment of equilibrium was considered camplete after 1 h. Having determined the adsorption isotherm, it is necessary to extract a value for the hydrogen uptake corresponding to rronolayer coverage.
'I'No methods
are canmonly used, namely, extrapolation of the linear portion of the isotherm (at higher pressures) to zero pressure, or fran a Langmuir plot (using the equation for dissociative adsorption).
In the context of a standard catalyst
t.he best; way of presenting the adsorption data is the quantity of hydrogen
adsorbed (as moles or molecules) per unit weight of catalyst or per unit weight of nickel.
However/to allow canparisons with published data it is helpful to
recarmend a factor for calculating the metal surface area.
Hence we have
assumed that each surface Ni atan adsorbs 1 H atan, and that each H atan 2. occupies a constant area of 0.065 nm Activity for the hydrogenation of benzene The choice of benzene as a reactant for the determination of the hydrogenation activity of the reference catalyst (based on a survey of catalyst users and researchers) conveniently allowed a temperature above ambient to be used (323 K). Only a very limited number of experiments have :teen performed by one research group and it is premature to offer a recx:mrended procedure.
However, it is
relevant to indicate the reproducibility which has been obtained for the procedure used.
The conditions chosen on the basis of preliminary experiments
to give an activity (defined as % conversion of :tenzene to cyclohexane) of <:10%, 3 3 v.ere, benzene flow 0.4 em /h, hydrogen flow 1.2 dm /h, temperature 323 K, pressure 1 bar, sample size 0.25 g.
The benzene was injected into the hydrogen
stream using a motor driven syringe, and hanogenised by passing the mixture through a container filled with glass beads.
This is not an ideal method of
preparing a benzene/hydrogen mixture, but it is rapid and s:iJrg:>le, and experience has shown that reproducible results can be obtained.
Similarly,
there is little doubt that t.he other experimental variables could be optimised, and the purity of the benzene should be specified for future work.
790 It is also necessary to decide whether initial activities should be rreasured or, as v.e have done,
I
steady state I activities determined.
The
products were sampled every 10 minutes for 1.5 h , and analysed using a Perkin Elmer F33 gas chromatograph fitted with a FID, a silicone fluid column, and coupled to an Infotronics CRS 308 electronic integrator.
The activity reported
was determined by extrapolating the linear portion of the activity/time curve to zero time. RESULTS AND DISCUSSION Part A. (a)
Evolution of a preferred procedure Storage of 'as received' material.
Changes in the structure of the
material are most probably associated with the water content.
Dry sarrples
should be more stable, so it is recarmended that the 'as received' material be dried at 393 K for 16 h and then stored in a sealed container. (b)
Pretreatment of starting material - re-drying, cooling and weighing.
The outline procedure is given in Table 2. TABLE 2
Proposed 'standard' procedure for pretreatment of the reference catalyst 1.
2. 3. 4.
Dry 0.5 g
I as received I catalyst for 16 h at 393 K. Place catalyst in a desiccator and cool for 20 minutes. Transfer about 0.350 g to v.eighing bottle and v.eigh accurately. Transfer the accurately v.eighed sarrple to a glass reactor.
Sarrples (0.5 g) of the starting material are removed frem the sealed container, placed in an open crucible and redried for 16 h at 393 K. important for t:Y.o reasons.
This redrying step is
First, it gives a reproducible starting material,
the v.eight of which can be related accurately to the nickel content.
Second,
it establishes a reproducible water content, which is important because the degree of v.etness of catalyst samples can seriously affect their characteristics during, and after reduction.
After drying, the crucible is placed in a small
desiccator for 20 minutes to cool.
About 0.350 g of the material is transferred
to a ground glass stoppered v.eighing bottle, and v.eighed accurately.
The
accurately v.eighed sarrple is transferred to the reactor (a U-tube fitted with a 1 an diameter No. 2 grade Pyrex sinter) which is then connected to the gas handling system. (c)
Thermal treatment.
The starting material can be decanposed in three
fundamentally different ways, namely, calcination in air, decanposition in nitrogen, or reduction in hydrogen.
The characteristics of the final material
will depend on the choice of decanposition procedure.
Table 3 surrroarises sane
of the experimental parameters which can affect the properties of the final material.
791 TABLE 3
Experimental parameters which can affect the catalyst 1. 2. 3. 4. 5. 6. 7. 8. Gas
gas flow rate gas purity gas cauposition (air, nitrogen, hydrogen) sample size catalyst bed depth heating rate final temperature time at temperature flow rates and gas purity (especially concentrations of oxygen-containing
gases) can affect the properties of the final material because the higher the flow rate, the lower the residual water vapour pressure in the catalyst bed. The sample size and l::ed depth can similarly affect the water vapour pressure a deep bed, for example, during dehydration and/or reduction will produce water vapour at the entrance to the bed, and this will be carried down with the gas stream through the remainder of the bed.
Heating rates again detennine the
instantaneous water vapour pressure above the samples.
Heating rate also
affects the decauposition and reduction directly because of changes in the number of nuclei on the surface of the particles.
Water vapour levels and
heating rates canbine to detennine the decauposition and reduction characteristics of the material.
The final temperature and the time at this
temperature canbine to detennine the reactivity of the oxide (if air or nitrogen are used for the decanposition), and the surface area of the metal (if hydrogen is used during decanposition). The reactivity of the oxide can l::e affected in many ways.
If the decanpos-
ition temperature is too low, residual nitrate will be present, if the temperature is too high the oxide may anneal, sinter, or even canbine with the support.
Each of these possibilities decreases by varying amounts the
reducibility of the oxide, and affects the metal surface area which can be obtained.
If hydrogen is used during decauposition the maximum temperature and
time will affect the metal particle size and size distribution.
lJ:M
temper'atures favour high dispersions, but may not give cx:rnplete reduction. tE!llperatures give total reduction, but at the cost of sintering.
High
The optimum
metal surface area needs to be defined because this will determine the choice of decanposition procedure.
High surface areas, although valuable in principle,
may not be ideal in practice because of their inherently lower stability and problems of reproducibility. (d)
Decarposition versus direct reduction.
When this work cannenced there
was no clear consensus in the literature as to whether decarposition should be perfo:nred prior to, or simultaneous with, reduction.
One of our first
792
objectives was to investigate these alternatives. quite clear.
The results (see later) were
Dece:rnposition prior to reduction invariably gave lower surface
areas (by a factor of about three).
How=ver, direct reduction gave high surface
area materials whose surface area decreased rapidly with tine and tanperature. Although high surface areas are advantageous in sane respects, it was decided that stability and reproducibility were more important at this stage.
The choice
was made to perfonn the dece:rnposition and reduction reactions separate1y. (e)
Decanposition and reduction procedures.
No advantage appeared to be
gained by deccmpositioh in nitrogen rather than in air, (indeed preliminary experinents indicated that the surface area was significantly 10¥Jer if nitrogen was used), so it was decided to perfonn decompositions in air.
The recarraended
procedure for the deccmposition and reduction is sunmarized in Table 4. TABLE 4
Recarmended procedure for deccmposition/reduction of catalyst 1. 2. 3. 4. 5.
Flush reactor with dry air. Raise tanperature at 7 K/minute to 623 K. Calcine at 623 K for 2 h. Flush with nitrogen for 5 minutes at 623 K. Reduce in hydrogen for 2 h at 623 K.
I 3 The sample in the reactor is flushed with dry air (flow rate 80 an gcatminute-I) for 5 minutes, and then heated in flowing air at 7 K minute-1 to 623 K. The ternr:;erature is held at 623 K for 2 h, after which the air is turned off, the reactor flushed with nitrogen (80 em3 gcat- 1 minute-I) for 5 minutes, with the semple still at 623 K.
The nitrogen supply is turned off, and the samp.Ie reduced in flowing hydrogen (80 em3 gcat-1 minute-1) for 2 h at 623 K. (f)
O1tgassing and cooling procedure for chemisorption experiments.
To prepare the reduced catalyst for the chemisorption experiment the sample was
evacuated at the final reduction tEmperature for 0.5 h, cooled to roan tanperature, and evacuated for a further 16 h. Part B.
Preliminary results
(a) TGA.
Figure 1 shows a representative thennogravimetric analysis
profile for the starting material.
The results underline the necessity of
predrying the stored sempl.es to obtain a reproducible starting state, and show that constant weight is obtained quickly.
(Our choice of 16 h for drying is
longer than required, rot is convenient.)
On raising the tanperature to 623 K
the samp'le dece:rnposes and rapidly attains a constant weight.
Addition of
hydrogen at 623 K leads to rapid reduction and constant weight is produced within about 30 minutes.
The weight change is consistent with almost canplete
reduction of Ni(II) to Ni(O).
793
d
~
I
:E
en
·w ~
OJ
Ci.
E
o
If)
2
4
6
Time I h Fig. 1. TGA profile for the catalyst precursor. Letters refer to different experimental regiIres. a, heating in air at 10 K/ minute to 393 K; b, air, 393 K; c, heating in air at 10 K/minute to 623 K; d, air, 623 K; e, nitrogen, 623 K; f, hydrogen, 623 K.
(b) Temperature-prograrrmed reduction (TPR). obtained by various research groups.
Figure 2 shows TPR profiles
This figure is included partly to indicate
the extent to which the reducibility is affected by the reduction conditions, and partly to underline the need to establish acceptable standard procedures. * As can be seen, the conditions typicailly used by various groups range in gas
canposition fran 5 to 25 % hydrogen, and in heating rates fran 5 to 30 K/minute. Although for each set of exper:iInental parameters this technique gives very reproducible data, reliable canparisons of data fran different laboratories is virtually impossible. (c) Metal surface area rreasurements. conditions.
(i) influence of deCOItlj:X?sition
Table 5 canpares the surface area determined on samples decanposed
in air, nitrogen or hydrogen and emphasises the great variation in surface area
which can be produced. each procedure is good.
It should be emphasised that the reproducibility for As noted earlier the highest surface area was not
*It should be recognised that many of the data presented in this paper were obtained while the preferred experimental procedures were being developed. For this reason, the experimental parameters used sanet:iInes differ fran those rec:x:mtended .
794
Q e 0 ....... 0E
,,
~
Ul
e 0
....
....
U
e
d-
~
OJ
(J)
0
L..
"0 ~
:r:
750 Fig. 2. TPR profiles for the uncalcined catalyst precursor. Ex:perinEntal pararooters: a, 5 % hydrogen, 5 K/rninute: b, 10 % hydrogen, 30 minute: c, 25 % hydrogen, 7 K;1ninute: d , 25 % hydrogen, 27 Kzmimrt.e,
KI
TABLE 5
SUrface areas of samples decanposed in different atmospheres. Atmosphere
SUrface area
air nitrogen hydrogen
25 13 118
1m2
necessarily the most desirable.
gNi
-1
Figure 3 shows how the surface area can change
rapidly during the early stages of reduction.
Clearly, it would be difficult to
stop this initial reduction at a reproducible point. (ii) adsorption isotherms. ,Figure 4 shows the hydrogen adsorption isotherms
obtained in different laboratories for samples prepared in canparable ways (Le. precalcined and reduced at 623-723 K).
We note again the disparity in the
methods adopted in this preliminary VJOrk, e.g. choice of temperature for adsorption, range of pressures, sample size (fran 0.2 to 3.0 g).
795
-
100
a
OJ
L...
a
OJY
-
a z0)
U
L.
:J
N
50
E
If)
2
[.
Time I h Fig. 3. Change in surface area with time of reduction . • , uncalcined precursor : 0 , precursor calcined for 0.5 h at 673 K.
-
\J OJI
.c .Z
L...
0
0)
Vl
\J
a~
C
6
-
0 c ..-- [.
OJ
0)
0
L...
\J
X Vl OJ
-0
~ :r: E
A
2
0-
0
0
=0-
S
150
75
PI mbar Fig. 4. Hydrogen adsorption isothenns measured in different laboratories. A. laboratory A (293 K): 8, laboratory B (307 K): C, laboratory C (273 K). The variation in the shapes of theisothenns is notable.
For non-linear
isothenns such as these there are real problems in accurately detennining the rronol.ayer coverage fran an extrapolation to zero pressure.
A more accurat.e value
for the monolayer coverage can be obtained fran a Langmuir plot, using the equation for dissociative adsorption. (iii) repeatability.
This work is still at an early stage and much effort has
been concentrated on identifying generally acceptable standard procedures. Therefore, only a few data are presently available to indicate the probable reproducibility of the chemisorption experiments. relevant data.
Table 6 simnar.Lses sane
The repeatability for catalysts prepared under identical
796 TABLE 6
Repeatability of hydrogen chEmisorption data Laboratory A sample
a
l
4
f
Sample 15
25 26 26 22 19 19
2 3 5 6
SUrface area m2 gNi- 1
Laboratory C
Laboratory B
2 d 1
Surface area m2 gNi- 1
Sample
33.1 32.1 23.3 32.6 37.2 41. 9 50.3 50.3 50.3
1c 2 Ie 2
Surface area
m2 gNi- 1 44.7 47.2 124.5 112.8
asamples calcined at 673 K and reduced at 673 K, bSamples reduced at 673 K in static hydrogen, cSamples calcined at 573 K and reduced at 573 K, dsamples reduced at temperatures rising fran 303 to 643 K, and hydrogen adsorption determined by pulse method using the same sample, ~calcined
samples reduced at
723 K, fsamples calcined at 673 K and reduced at 773 K. conditions is encouraging.
(Using a procedure similar to that recanmended 1 2 earlier, a surface area of 38 m gNi- has been obtained.) The importance of agreeing on a suitable method of extracting the monolayer coverage fran the adsorption isotherm is illustrated by the data in Table 7. TABLE 7 ~nolayer
coverages fran Langmuir and zero pressure calculations.
Experiment
V (Lanqmzf.r) m
V (zero pressure) m
V (Lang.) /V (zero) m m
1 2 3 4 5 6
56.9 51.5 51.4 153.6 140.1 38.4
44.7 47.2 39.0 124.5 112.8 32.9
1.272 1.091 1.318 1.234 1.242 1.167
These data show that the ratio of the monolayer coverage fran the Lanqrnri.r plot to that fran the zero pressure extrapolation is not constant.
Indeed, the
scatter is of the same order of magnitude as the total error in the experiment. (d) Activity for the hydrogenation of benzene. been performed on the hydrogenation of benzene.
Only a very few experiments have After calcining and reducing the
samples using a procedure similar to that outlined above (see Table 4) the activity was determined.
{The main difference in procedure was that the final
797
15
-
'::!?
0
c
0 VI
10
L..
OJ
>
C 0
0
5 25 50 75 Time I minutes
Fig.
5. Change in benzene hydrogenation activity with time.
temperature for reduction was 673 K rather than 623 K.)
Figure 5 shows how the
activity decreases with time, requiring the extrapolation to zero time previously mentioned.
The recorded activities in the three determinations shown were 15. 0 ,
15.6, and 14.7 prrol/g cat.;1ninute.
Allowing for the fact that these results were
obtained on separate samples which had undergone drying, calcination, and reduction stages, the reproducibility can be considered satisfactory. CCNCLUSIONS This work has demonstrated that in addition to providing sources of catalyst reference materials, there is a clear need to establish standard techniques of pretreatment and measurement.
In
the present work, where there has been
consistency of technique, the repeatibility of the experiments has been rrost encouraging.
It is reasonable to expect that a reproducibility within
could be obtained in both nickel surface area and benzene hydrogenation activity measurements. ACKNCWLEI:X;EMENT
The Society of Chemical Industry Working Party on Catalyst Reference Materials wishes to thank Akzo Chemie Nederland bv for provision of the catalyst precursor.
±5
%
798 DISCUSSION J.W.E. COENEN Did you verify that the hydtogen adsorption isotherms really represent equilibrium states. Our experience is that adsorption and desorption "isotherms" never coincide. Deviations are worse when reduction prior measurement is poorer. A.R. FLAMBARD: The problem of deciding whether or not an isotherm truly represents an equilibrium state is a difficult one. For the standard catalyst we did not measure any desorption isotherms. I can only report the observations that we have made in our laboratories. For each point o~ the isotherm the hydrogen uptake followed an often reported scheme; a rapid adsorption followed by a much slower (activated) adsorption. In the time allowed for the measurement of each point, this second adsorption process did not in fact come to equilibrium. However, we have alsi determined that after much longer eqUilibrium times no significant change in either isotherm shape or amount of hydrogen adsorbed were observed. We therefore believe that even though the isotherms do not represent true kinetic equilibria they do in fact come quite close to it. We have also observed that with poorly reduced Ni/Si02 catalysts the contribution of the slow uptake to the total hydrogen adsorption increases quite dramatically and the measured isotherms appear more "rounded". As you have said, deviations from eqUilibrium become worse. The question must therfore be asked as to if this slower adsorption is not in fact due to some contamination effect and if it should be considered when evaluating free metal surface areas. A. FRENNET: I feel some difficulties in the use determine the monolayer coverage as this isotherm of the heat of adsorption with coverage, which is heat of adsorption of hydrogen on nickel is known with coverage.
of the Langmuir isotherm to model is based on a constancy generally not the case. The to vary in a important way
A.R. FLAMBARD: As noted in Prof. Bond's boof "Catalysis by Metals", there is no definition of surface coverage which is either generally or satisfactorily applicable. At the last Minisymposium on catalyst normalization, Prof. Scholten recommended the use of extrapolation to zero pressures in order to evaluate the monolayer coverage. Faces with an isotherm such as C in Fig. 4 and from our experience with poorly reduced catalysts, as I outlined in reply to Prof. Coenen's question, we decided that this method was perhaps not the best. We have tried to fit our data to the Freundlich and Tempkin (Slygin-Frumkin) models of adsorption as well as the Langmuir and found that the best approximations to linear plots was obtained with the latter. Of course, the heat of adsorption of hydrogen on nickel does not remain constant with coverage. However, in respect to the standard catalyst and catalyst normalization we are at present more interested in determining a standard procedure of activation and measurement that will give repeatable results rather than investigating suitable adsorption models. It is for this reason that the question of tree equilibrium raised by Prof. Coenen has not been thoroughly investigated with this catalyst. Under the conditions specified and with the model used, a repeatable figure for the free metal surface area will be obtained, which was after all our first objective. B. SMEDLER: A main topic of catalytic hydrogenation on Ni/silica catalysts is that of enhancing the selectivity in consecutive reactions. Therefore, wouldn't it be recommendable to complete your activity test (benzene hydrogenation) with a few other hydrogenations (for exampl~ olefin and aldehyde hydrogenation) in order to get some information of the sleectivity properties of the catalyst? A.R. FLAMBARD: Benzene hydrogenation was chosen as the test reaction for the standard catalyst after a survey amongst catalyst users and researchers. We have also considered the selectivity properties of this catalyst and have determined
799 its activity and selectivity for both n-hexane hydrogenolysis and carbon monoxide hydrogenation (Fischer-Tropsch) reactions. As with benzene hydrogenation, the results are encouraging with good reproducibility for a specified activation procedure. We have not considered using alkene or aldehyde hydrogenation but I thank you for your comments.
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G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
RESEARCH GROUP ON CATALYSIS, COUNCIL OF EUROPE.
801
STANDARD CATALYST PROJECTS
J.W.E. COENEN l and P.B. WELLS 2 lCatholic University, Nijmegen, The Netherlands 2Univers-ity of Hull, United Kingdom
ABSTRACT In 1975 a research group on catalysis was started in which 22 research groups from 9 European countries participate. Aim of the group is joint discussions and collaborative work in the field of metal catalysis. Two projects were started, each centred on a standard catalyst. EURONI-l is a nickel/silica catalyst which in the unreduced state contains 25% Ni and has a total surface area of 270±20 m2 g-l. Its structure in the active reduced state depends strongly on the reduction conditions appl ied. Reduced at 6500C the nickel crystallite size ranges from 1 to 10 nm, average about 4.5 nm. At lower reduction temperature, where reduction remains incomplete smaller crystallites are formed. The catalyst was subjected to a number of physical examination methods from which a reasonably consistent picture of its structure was obtained. EUROPT-l is a platinum/silica catalyst of total surface area 185±10 m2 g-l containing 6.3 wt% Pt. The platinum crystallite size distribution extends from 1 to 3.8 nm, the maximum being just below 2.0 nm. 80% of the crystallites have a diameter less than 2.2 nm. The chemisorption of hydrogen, of oxygen and of carbon monoxide has been studied in detail and the extents of adsorption at saturation defined. Concordance of results was better for the platinum catalyst than for the nickel catalyst, which is undoubtedly due mainly to the difficulty of reducing the later catalyst in a reproducible manner. INTRODUCTION At the initiative of E. Derouane of the University of Namur, Belgium, a Research Group on Catalysis was started early in 1975 under the aegis of the Committee on Science and Technology of the Council of Europe. In this group 22 catalysis research groups located at universities and research institutes in 9 European countries are represented. The group decided to start collaborative work on two projects, each centred on a standard catalyst, a quantity of which was distributed among the participating laboratories. As standard catalysts were selected a 25% nickel on silica catalyst and a 6% platinum on silica catalyst. Aim of the projects was twofold. In the first instance it was hoped to obtain insight into the possibility of obtaining concordant results in different laboratories with various methods of catalyst characterisation. Secondly it was expected that the combined measuring results would yield for the two catalysts a
802
more complete characterisation than would ever be possible in any single laboratory, so that the catalysts would be highly valuable bases for subsequent catalytic studies. In the following the results of the studies on the two catalysts are presented in compact form. The authors of this paper earlier prepared more comprehensive reports on the nickel and the platinum catalyst respectively. In the acknowledgement at the end of this paper the participating teams are identified by their leader or one member. THE EURO NICKEL CATALYST, EURONI-l Catalyst Preparation For a loading of about 25% nickel a precipitation method is more suitable than impregnation. A wide choice of precipitation methods has been described but most of them have the risk of local excess of ingredients causing precipitation of part of the nickel not attached to the support. The method of homogeneous precipitation, developed by Geus, provides an elegant method to avoid this difficulty. In this process all ingredients for the catalyst are present in the reaction vessel at the start of the precipitation: nickel nitrate and urea, together in a suitable volume of water and the silica support, aerosil 180, suspended in the aqueous solution. The essence of the method is that the precipitating hydroxyl ions are generated very gradually throughout the solution by hydrolysis of the urea upon heating of the reaction mixture to 900C. Under these conditions the support nucleates precipitation of nickel compounds so that all nickel precipitate is associated with the support. Precipitation was done at Nijmegen in a 75 litre glass reactor with stirrer and heating. To prepare the required amount two batches had to be made. Per batch 3 kg Ni(N03)2,6aq (p.a.), 1.4 kg Aerosil 180 silica ex Degussa, 1.8 kg urea (p.a.) and 50 litres demineralised water were used. The charge was kept at 900C for 20 hrs and then filtered hot. The precipitate was twice resl urried in hot water and filtered. The combined precipitates were then reslurried and spray dri ed. Catalyst Reduction To obtain the catalyst in active form the catalyst must be reduced with hydrogen at elevated temperature. It is known from the literature that this reduction for silica supported catalysts is a complex and difficult process. Water formed in the reduction acts as an inhibitor for the reduction. Hydrothermal conditions can prevail by the combination of high temperature. and significant water vapour pressure unless measures are taken for its fast elimination and this can induce formation of more difficultly reducible nickel-silicate structures. The combined result of these effects makes the result of the reduction highly sensitive to the detailed conditions temperature, purity and flow
803
rate of hydrogen, geometry of the catalyst vessel and conditions of hydrogen flow, through or over the catalyst bed. Also the quantity of catalyst reduced in one batch has significant influence on the result. Reproducibility in one laboratory already proves difficult, between different laboratories seems almost impossible. The cause of not very good concordance of results obtained in different locations will be mainly located in the reduction. Two laboratories exposed the unreduced catalyst to a hydrogen atmosphere with temperature linearly increasing with time. In one case heating at SoC min-l up to 635 0C was applied and the weight loss followed of a precalcined sample. A weight loss of about 1% occurs gradually up to 4300C, followed by an abrupt loss of 6% at this temperature. A further equally abrupt weight loss of about 2% occurs at about 5300C. Since nickel oxide formed by calcination of basic carbonate or hydroxide is easily reduced at or below 300°C, at most 10% of the nickel is present in the unreduced catalyst in these easily reducible forms, the remainder presumably as more difficult reducible nickel silicates. In the second case the temperature rise was 10°C min-l and reduction progress was followed by hydrogen consumption. Again about 10% of the nickel is reduced at low temperature, the remainder between 400 and 700 0 . Comparison with curves obtained with two model silicates indicates that this nickel is likely to be present as badly ordered nickel antigorite. Isothermal reduction with hydrogen flowing through the catalyst bed, performed in several laboratories, showed that complete reduction of nickel in the catalyst is only attained with reduction times of 24 h or more at 630°C or above. Reduction at lower temperatures, e.g. at or near 450°C produces significantly lower degrees of reduction and significant deviations between laboratories. Values as low as 50% and as high as 90% were obtained. Chemical Characterisation The nickel content of the unreduced catalyst was determined by titration after acid extraction, atomic absorption spectroscopy, X-ray fluorescence and proton induced X-ray emission. Values obtained ranged from 23% to 25.5%. Sources for variation were the hygrqscopic character of the catalyst and difficulties met in the complete extraction of nickel with acid. These causes make the higher values to be more likely to be correct. A silicon content of 25.5% was obtained. The unreduced catalyst further contains 1.0% nitrogen, 0.45% CO 2 as memory of the urea precipitation. Trace impurities were Fe, Ca, Cl and S all in the ppm range. Physical Characterisation Two laboratories obtained X-ray diffraction patterns of X-ray diffractio~. the catalyst in the unreduced state and after calcination in one case at 450°C, in the other at 5000C. Conclusions were closely concordant. The pattern of the unreduced catalyst gave no evidence for the presence of nickelhydroxide and showed most resemblance to that of badly crystallised nickelantigorite. The cal-
804
cination treatment produced only negligible change in the pattern and hardly if any evidence was found for presence of nickel oxide in the calcined sample. This confirms the earlier conclusion that at most only a small amount of easily reduceable nickel compounds can be present. One laboratory also obtained patterns of the catalyst after reduction at 45cPC and passivation and after reoxidation of the latter sample in air at 4500C. In the reoxidised sample there was clear evidence of finely divided nickel oxide, with a crystallite size of 2.4 nm. Assuming that each oxide crystal was formed from one nickel crystal in-the reoxidation the average crystallite size in the reduced sample would be 2.2 nm. However the pattern of the reduced/passivated sample showed strong evidence for the presence of nickel oxide and only weak indication for nickel metal. Clearly the passivation treatment must have oxidised most of the nickel metal produced in the reduction treatment. It is known that the type of passivation treatment applied in general oxidises about 2-3 atom layers deep and with the fine state of subdivision this would involve most of the nickel as was indeed found. The crystallite size was confirmed in a third laboratory where from the X-ray pattern of the catalyst reduced at 4500C for 70 h a crystallite size of 2.5 nm was derived. The conclusion from this investigation is that most of the nickel in the unreduced catalyst is present in a structure which is thermally stable at 4500C, e.g. sheets of nickel antigorite, which have no ordered three-dimensional stacking. The nickel atoms in this structure can however be mobilised at 4500C when they are reduced to the zerovalent state. They then cluster to crystallites of 2.2 to 2.5 nm. Electron spectroscopy. (XPS). Three laboratories did ESCA studies on several forms of the catalyst. Electron binding energies differed marginally between the laboratories and they yielded no enlightening information. The intensities were more informative. Two laboratories observed that the Ni(2p)3/2/Si(2p) ratio decreased marginally on calcination of the unreduced catalyst. This confirms the picture already obtained from the X-ray data: there is very little structural change, but some dehydration/dehydroxylation. Therefore the deeper-lying silicon in the antigorite structure is less attenuated. Two laboratories observed that upon reduction the intensity ratio dropped significantly. This again is in line with X-ray findings: a significant proportion of the nickel is now shielded by nickel - in the crystallites - whereas the silicon signal for that part of the surface from which nickel has migrated is less shielded. One laboratory also took the ESCA-pattern of the reoxidised catalyst and observed hardly any change, as compared to the passivated reduced sample, again in line with X-ray findings. Electron microscopy. Two laboratories did scanning electron microscopy at moderate magnification on the unreduced catalyst wi th- identical results: a spherical particle shape typical for spray drying. The size distribution is wide, from
805
4 to 70 ~m. The same labs did transmission electronmicroscopy on the unreduced catalyst at magnifications up to 450,000 again with closely concordant results. The typical microspheroidal structure of the Aerosil has virtually disappeared and is replaced by a loose network of thin randomly oriented wrinkled sheets. In view of ESeA and X-ray results we may identify these as antigorite sheets. We may expect these to retain their structure on heat treatment at 4500e and they are found to do so. On reduction the sheetlike structure persists, but minute speckles, th~ nickel crystallites can now be seen, which persist on reoxidation. Four laboratories did electron microscopy to obtain the crystallite size distribution in the reduced catalyst of the nickel crystals. Three laboratories us630-6500e for periods ranging from 15 to 45 h. ed similar reduction treatmen~at The results are closely concordant: the surface average crystallite size is found to be 4.6±0.3 nm and thehalfwidth of the distribution curve is about 0.4 nm. Longer reduction times increase the average crystallite size only marginally. Magnetic measurements. One laboratory did magnetic measurements on the catalyst after its reduction at 6300e for 26 h at magnetic fields up to 21 kOe. From the measurements a surface average crystallite size of 5.0 nm is derived in good agreement with the electronmicroscopic results. Another laboratory provided evidence with respect to the monolayer definition in hydrogen adsorption on the reduced catalyst. On increasing hydrogen pressure from e.g. 1 Torr to 760 Torr not only additional hydrogen adsorption is observed but also additional decrease of the magnetization. This observation indicates that this additional adsorption takes place on nickel metal and should therefore be counted in the monolayer coverage. Adsorption studies Nitrogen adsorption. On the unreduced catalyst total surface areas by the BET-method from nitrogen adsorption isotherms were measured in six laboratories. The concordance in the results is rather disappointing. Also several laboratories report that the reproducibility of the measurement is poorer than usually obtained on samples of similar surface area. The deviations bet~en laboratories are largely due to different degassing treatments, for which temperatures ranging from 200e to 1200e were used. Total surface areas ranged from about 220 to 270 m2 g-l and it appears likely that the true value is close to the latter, which is also the value obtained in one laboratory in sevenfold ±l m2 g-l. It is highly likely that poor degassing is the cause of lower values. We recall from the scanning electron microscopical data that we are dealing with relatively large particles, so that the real attainment of a suitable low pressure may well take considerable time, especially in apparatus with narrow tubing. Two laboratories did measurements on the catalyst after various reduction treatments. Reduction at 4500e reduced the total surface area to 252 m2 g-l, at
806
5000 e reduction to an area of 234 m2 g-l was obtained whereas at 700°C the surface area had shrunk to 215 m2 g-l All these values were obtained in the same laboratory. In another laboratory surface areas of 212 and 192 m2 g-1 were obtained after reductions at 650°C during 15 and 45 h respectively, which is in reasonable agreement with the figures quoted above. Hydrogen adsorption. This measurement, used to quantify the available nickel surface area in the reduced catalyst, is obviously of crucial importance in this cooperative project. The problem is that on the one hand the individual reductions done in the differe~t laboratories may well produce significantly different results, as explained earlier, but also that the methods for measuring hydrogen adsorption and their interpretation differs for different locations. For reduction in three laboratories at 430-4500C during times from 4 to 24 h hydrogen adsorption at 200C yielded values of 53, 53, 19, 46, 46 and 44 ml STP (gNi)-l. The agreement is thus quite reasonable with one exception. Ignoring this low value we find an average monolayer capacity of 48.4 ml STP (g Ni)-l. For reductions in four laboratories at 630-6500C during times from 3 to 26 h hydrogen adsorption at 20°C yielded values of 37, 32, 28, 26, 25, and 28 ml STP (g Ni)-l. Again agreement is not unreasonable, we find an average capacity of 29.3 ml STP (g Ni f l . From the average va1ues degrees of di spers i on of 25 and 15% can be calculated. Summary and conclusions From X-ray, EM, ESCA and TPR data we can conclude that in the unreduced catalyst most of the nickel is present in disordered sheets of a basic nickel silicate with possibly less than 10% of a more easily reducible nickel compound, hydroxide or basic carbonate. The total surface area of the unreduced catalyst is about 270 m2 g-l. Ignition at 450°C hardly changes the structure, apart from some dehydration. Reduction at progressively higher temperatures reduces the total area gradually to 215 m2 g-l at 700°C, referred to as unit weight of the original material. In the reduction in hydrogen th.e nickel atoms in the antigorite sheets are mobilised and cluster to small crystallites. With a hemispherical model for the crystallites used by one of the authors (JWEC) an average crystallite size of 2.1 nm can be calculated for the catalyst reduced near 4500C, which agrees excellently with the value of 2.2 and 2.5 nm obtained by X-rays. Similarly from the hydrogen adsorption value of the catalyst reduced near 650°C an average crystallite size of 4.3 nm can be calculated, again in good agreement with the electronmicroscopic observations. THE EURO PLATINUM CATALYST, EUROPT-l Preparation EUROPT-l was prepared by Johnson Matthey Chemicals of the UK. 6 kg silica
807
(Sorbsil grade AQ U30, Crossfield Chemicals) was impregnated with Pt(NH 3)4++ at pH 8.9, filtered, washed, dried, and reduced in hydrogen at 400oC. The material was granular (size range 62 to ~750 microns). Physical Characterisation Random samples contained 6.3 wt%Pt by spectrophotometry (ptSn4C11+), 6.2% by atomic absorption, and 6.2% by proton induced X-ray emission (values ±0.3%). The agreed value is 6.3%. The largest granules exhibited a somewhat lower platinum content ~s expected. Trace elements present included: Al 500 ppm, Ca 500, Na 400, Ti 400, Mg 200, K 150, Fe 90, Cl <50, Cr 10 ppm (total 0.22%). ESCA also revealed the presence of N. Weight loss on heating in air to 10000C was 4.2%. Total surface area, measured by the B.E.T. method of nitrogen adsorption at 250C (7 laboratories) was 185 ± 10 m2 g-l. Values measured over the range 60 to 2000C were all within these limits of uncertainty. The pore size distribution ranged from 5 to 30 nm with a maximum at 15 nm. Platinum particle size distributions were measured by transmission electron microscopy (4 laboratories). The platinum-containing particles gave good images, distinguishable from the silica background, for diameters down to 1 nm. The maximum in the particle size distribution in the as-received material occurs at about 1.8 nm and no particles larger than 3.8 nm have been observed. Two laboratories record 74% of particles <2.0 nm in diameter, one records 78% of particles <2.1 nm in diameter, and the fourth records 80% of particles <2.2 nm in diameter. Mean particle sizes by X-ray line broadening (using the (111) diffraction line of platinum metal at 2 e = 39.6°) agreed with the values from electron microscopy. Curiously, (in view of the X-ray measurement) an EXAFS study of the as-received material gave evidence of platinum-oxygen bonding, closely similar to that in B-Pt02, but not of platinum-platinum bonding. Particle size distributions were determined for samples reduced in hydrogen at elevated temperatures. Sintering was mild at temperatures up to 800°C, but extensive at 10000C. Volume average particle sizes by electron microscopy were 2.1 nm after heating at 600°C for 5 h, 2.3 nm after heating at 800°C for 6 h, and 5.7 nm after heating at 10000C for 4 h. Values by X-ray line broadening were 1.6, 2.1, and 4.6 nm respectively. Hydrogen Chemisorption Hydrogen chemisorption has been studied by the volumetric method (8 laboratories), by frontal analysis (2 laboratories), and by temperature programmed desorption (2 laboratories). Hydrogen adsorption by the support was negligible except at -196°C. The process of hydrogen adsorption is clearly complex, and will be the subject of further discussions within the Group, and of detailed report in the
808
literature in due course. Isotherms at room temperature showed extensive adsorption at equilibrium pressures below 1 Torr and a plateau at pressures beyond 10 or ZO Torr. The as-received material was re-reduced in flowing hydrogen at an elevated temperature and evacuated before measurements of isotherms were carried out. Measurements of the extent of hydrogen adsorption at room temperature, n, show fair agreement. Values for n/~mol HZ (g sample)-l were (i) by the volumetric method at an equilibrium pressure of 40 Torr: 150, 171, 174, 177, 186,190, and 194; (ii) by frontal analysis: 146 and 18Z; and (iii) by temperature programmed desorption, 194 (Z laboratories concurred). Although a spread of values was obtained for method (i), any given laboratory reported that its contributed-value was reproducible, given the apparatus at their disposal. This variation in the recorded values by method (i) is likely to have its origin in the quality of the vacua achieved in our several laboratories before the adsorption measurements. Desorption studies show the presence of a strongly adsorbed state of hydrogen (see below) and it is possible that, following the necessary re-reduction of the as-received material, surfaces substantially free of adsorbed hydrogen were obtained only after pumping to near-u.h.v. conditions. We thus accept the highest values recorded, say 190 ± 4 ~mo1 HZ (g sample) -1 , as those most likely to represent the hydrogen adsorption capacity of EUROPT-1 at room temperature. Hydrogen adsorption has been measured as a function of temperature for samples which were pretreated in flowing oxygen before reduction and evacuation. Pretreatment conditions were: (1) no exposure to OZ' reduction in flowing hydrogen at 300 0e for 4 h, evacuation at 3000e at 3 x 10- 7 Torr for 15 h; (2) exposure to Oz at zoooe for 4 h, reduction and evacuation as above; (3) exposure to Oz at 300 0C for 4 h, reduction and evacuation as above; (4) exposure to 0z at 4000C for 6 h, reduction and evacuation as above. Extents of hydrogen adsorption, n, at equilibrium pressures, P were as follows: e, T/oC Pe/Torr n for pretreatments (1) (Z) (3) (4) ZO 41 186 191 189 191 88 44 173 174 169 178 Z87 5Z 106 114 106 116 Clearly, the extent of adsorption decreased with increasing temperature. Hydrogen desorption isotherms were measured at seven temperatures (-80 to Z89 0C) over the range 10Z to 5 x 10- 5 Torr. The isosteric heat of adsorption fell from about 95 kJ mol-1 at e = 0.38 to about 35 kJ mo1- 1 at e = 0.97. The estimated number of hydrogen molecules adsorbed at full coverage corresponded to a value of n of 194 ~mol HZ (g sample) -1 . The technique demonstrated that, at 1770C, 1019 hydrogen molecules (about 8%) had entered a particularly strongly adsorbed state so that it was not desorbed as expected at 10- 5 Torr and was
809
removable only slowly by evacuation at 10- 7 Torr over some hours. Temperature programmed desorption of hydrogen into a nitrogen stream was investigated. The as-received material was first heated in flowing hydrogen to 4000C and maintained at 4000C for various times, t, before being cooled to room temperature in hydrogen, purged in nitrogen for 5 minutes, and then heated to 5500C at a rate of 25 deg min-l. The desorptograms showed two resolved peaks; the first (possibly comprised of two components) was large, having a maximum at about 2000C and.an area independent of t in the range 0.1 to 26 h, whereas the second was small, having a maximum at 4500C and an area that doubled over the studied span of time, t. Another laboratory investigated temperature programmed desorption into a vacuum over the range -196 to 5000C using heating rates not greater than 40 deg min- l. Desorption of hydrogen from a highly covered surface gave three desorption peaks, the first at -60 to -lOoC, the second at about 950C and the third at 2230C. As the initial hydrogen coverage was reduced so the second and then the third peaks became the more important in the spectrum. The maximum population of the most strongly adsorbed state was estimated as 13 ~mol H2 (g sample)-l. These two TPD investigations concur, in that the amount of hydrogen not removed by desorption at 4000C in the second study is similar to the amount desorbed between 400 and 6000C in the first study. This increment of hydrogen may be located on the support by spill-over from the metal. Evidence for two states of adsorbed hydrogen on EUROPT-l was also obtained by lH n.m.r. (Brucker CXP, 96.68 MHz). Each spectrum contained two signals, one shifted weakly and one strongly to higher field with respect to gaseous hydrogen. The position of the weakly shifted signal (01 = -1 ppm) was almost independent of the amount of hydrogen adsorbed, whereas the shift of the other signal decreased almost linearly with increasing coverage (02 = 30 ppm at n = 100 ~mol -1 H2 (g sample) , 02 = -11 ppm at n = 203). Summary. Hydrogen chemisorbs on EUROPT-l giving an isotherm of conventional appearance. Desorption experiments reveal the presence of at least three adsorbed states. The establishment of hydrogen in a particularly strongly adsorbed state during heating to elevated temperatures renders desorption by evacuation very difficult, and measurements of adsorption capacity as between one laboratory and another have led to the accumulation of a spread of values. Experience would seem to suggest that pre-reduction at, say, 2500C and subsequent evacuation at 10- 8 Torr for some hours provides a surface substantially free of adsorbed hydrogen on which subsequent adsorptions can be measured with reasonable confidence. Oxygen Chemisorption Oxygen chemisorption at room temperature has been measured by the volumetric method (5 laboratories) and by gravimetry, frontal chromatography, and pulse
810
chromatography (1 laboratory). The general appearance of the oxygen adsorption isotherms was similar to those obtained for hydrogen adsorption. Re-reduction of as-received material in hydrogen at 3000C or above, and evacuation at the reduction temperature provided an acceptable pretreatment. Two laboratories recorded limiting values of the extent of adsorption as P + 0, nO/~mol Oz (g sample)-l, after various pretreatment temperatures TloC as follows: 500 500 600 400 400 400 155 300 8Z 86 81 84 86 100 84 84 Other laboratories recorded n = 72, 76, and 78 umo l Oz (g sample)-l at various low equilibrium pressures of oxygen. Results of other methods were as fo11 ows: (i) gra vimetry, pretreatment at Pe = 100 Torr; (ii) frontal temperature 4000e, n = 100 umo l 0z (g ~amplefl chromatography, (a) pretreatment temperature 4000 e, n = 100 ~mol Oz (g sample)-l at Pe = 10 Torr, (b) pretreatment temperature 500°C, n = 8Z ~mol Oz (g sample)-l at Pe = 10 Torr; (iii) pulse chromatography, pretreatment temperature 40ooe, n = 94 ~mol O2 (g sample) -1 . The extent of oxygen adsorption at an equilibrium pressure of 1.3 Torr increased as the temperature at which the adsorption was measured, T/oe, was increased: ZO 53 157 T -80 n 58 76 91 106 Summary. The concensus appears to be that EUROPT-l chemisorbs 76 - 86 ~mol Oz (g sample)-l at room temperature. Failure to remove adsorbed hydrogen during the pretreatment would result in the titration of that hydrogen by oxygen and we speculate that this may be the origin of values of n higher than about 86 -1 ~mol Oz (g sample) . Carbon Monoxide Chemisorption CO chemisorption at room temperature has been measured by the volumetric method (4 laboratories). The general appearance of the isotherms was similar to those obtained for the adsorption of HZ and of OZ' Val ues of n varied as between one laboratory and another because pretreatment conditions and quoted However, it was clear that the extent of CO equilibrium pressures differed. adsorption under given conditions in a given laboratory was closely similar to the extent of HZ adsorption under the same conditions in the same laboratory. CO adsorption on EURoPT-l was investigated by infrared spectroscopy (1 laboratory). The as-received material was re-reduced in flowing hydrogen (1 Torr) at 3Z70C for 1.5 h and in 30 Torr hydrogen at the same temperature for a further 1.5 h. To obtain 'a hydrogen-covered surface', the sample was cooled in HZ and evacuated at room temperature; to obtain 'a hydrogen-free surface' the
811
sample at 3270C was purged with helium before cooling; to obtain 'an oxygencovered surface' the hydrogen-free surface was exposed to 1 Torr O2 at 250C. Adsorption on the hydrogen-free surface gave a very strong band at 2070 cm -1 and a rather weak broad band at 1850 cm- l which correspond to the formation of or multi-centre-bonded-CO respectively. About 10% linear-bonded-CO and bridge'd~ of the total integrated intensity was due to bridged-CO. The high frequency band appeared first at 2060 and shifted at 2070 cm- l as saturation coverage was approached. Adsorption on the hydrogen-covered surface gave a similar spectrum except that the high frequency band was shifted by about 10 cm- l to lower wave number as compared with that given by the hydrogen-free surface. A shift in the opposite direction was observed in the spectrum of CO on the oxygen-covered surface. Discussion The chief object of the Group, when establishing this joint programme, was to characterise EUROPT-l by all methods available to us so that it would become a useful standard for the wider scientific community. In this we have succeeded to a fair degree. We have a material which will be valuable to laboratories who wish to compare their procedures for measurement of total surface area, metal content, or particle size distribution against a standard. For the calibration of procedures for metal surface area or metal dispersion by selective chemisorption, EUROPT-l again provides a valuable standard with the reservation that, although the measurements may be simple, meaningful results require very precise control of conditions throughout the course of every measurement. It is not the purpose of this paper to discuss the chemisorption results in a comparative fashion. However, we note that g EUROPT-l contains 1.95 x 1020 Pt atoms and that, as a likely value for the dispersion from the electron micrographs is 0.6, about 1.1 x 1020 Pt atoms are in principle available as sites for chemisorption. The number of CO molecules and O-atoms adsorbed at saturation is close to this value (1.1 x 1020 and 1.0 x 1020 respectively) whereas the number of H-atoms adsorbed at saturation, 2.2 x 1020, exceeds it. However, with hydrogen occupying four adsorbed states a 1:1 correlation of (H-atoms adsorbed):(Ptatom exposed) is hardly to be expected. ACKNOWLEDGEMENT The present authors participated in the work described, but many others did as well, either by measurements in their laboratories or by participating in discussion. In the present compact account it was hardly possible to identify each individual contribution. To give credit for the contributions in toto we list below the composition of the group as it was at the time of the investigations.
812
AUSTRIA BELGIUM
IRELAND FRANCE
GERMANY
Professor H. L. Gruber NETHERLAN DS INNSBRUCK Professor B. Delmon Dr. M. Houalla LOUVAIN-LA-NEUVE Professor E. G. Derouane NAMUR Dr. A. Frennet BRUSsE'Ls Dr. J.K.A. Clarke NORWAY DUB LIN Professor J. J. Fripiat SWEDEN ORLEANS Professor B. Ime 1i k Dr. C. Naccache Dr. J. Vedrine UNITED VILLEURBANNE KINGDOM Dr. G. Ma ire STRASBOURG Professor R. Maurel Dr. G. Leclercq POITIERS Professor H. Knozinger MUNICH
Professor NIJMEGEN Professor UTRECHT Professor LE IDEN Professor Professor EINDHOVEN Professor
J.W.E. Coenen J. W. Geus V. Ponec R. Prins J.H.C. van Hooff D. L. Trimm
(resi qne d 1978)
TRQNDHEIM Dr. R. Larsson LUND Dr. P. Stenius STOCKHOLM Professor G. C. Bond UXBRIDGE, LONDON Dr. R. Joyner BRADFORD Professor M.W. Roberts CARDIFF Dr. P. B. Wells HULL Dr. D. A. Whan EDINBURGH
813 DISCUSSION J.J.F. SCHOLTEN On page 5 of their communication, Coenen and Wells rightly state that, in measuring hydrogen chemisorption isotherms on nickel around room temperature, further hydrogen chemisorption takes place in the range of pressures from e.g. 1 Torr to 760 Torrs. Besides the fact that additional decrease of magnetization is observed by Coenen, the value of the adsorption enthalpies of hydrogen measured for the weak type of chemisorption in this range of pressures, viz. -33 kJ.mol- 1 and -53 kJ.mol- 1 (see ref. 1), also points to this weak adsorption being chemisorption. The question if we should include this weak'~ype C" hydrogen adsorption in the determination of free-metal surface areas or not, is, however, a more general one. Weak type C hydrogen, also called hydrogen chemisorption in excess of the strongly held monolayer (Bond, ref. 2), is not only found on nickel but on various group VIII metals like platinum, iridium, palladium, ruthenium etc .. (see refs. 1 and 3). In all these cases previous research workers, including R.Burch, A.R. Flambard et al. (this Symposium) who studied the SCI/IUPAC/NPL nickel catalyst, have decided to take the extent of the strongly held chemisorbed monolayer as a mesure of the free-metal surface area. I therefore think that, for purely practical reasons, it is better to follow this procedure also in the case of nickel: there are no special reasons to make an exception for this metal. Most investigators think (see also ref. 1) that strong hydrogen chemisorption takes place on the so-called C8 sites, the number of which is about equal to the number of exposed metal atoms in the (111) plane, and hence corresponds with a surface stoichiometry of about one. Weak hydrogen adsorption is supposed to occur on bridged (B) and atop (A) sites, or on C4 sites, which justifies Bond's term: adsorption in excess of the strongly held monolayer. Ref. 1. J.A. Konvalinka, P.H. van Oeffelt and J.J.F. Scholten, Applied Catal., .!c (1981), 141. Ref. 2. G.C. Bond, in "Catalysis by Metals", Academic Press, London and New York, 1962. Ref. 3. J.R. Anderson, in "Structure of Metallic Catalysts", Academic Press, London and New York, (1975), p. 295-312. J.W.E. COENEN: The group is well aware that monolayer defibition in hydrogen adsorption is a controversial subject. In fact, all extremes of opinion are represented in the group, which devoted a special discussion session to the subject. With respect to the Pt-catalyst the following may be considered relevant: in one laboratory (Frennet, Brussels) very extensive adsorption data over a wide range of temperatures and partial pressures were measured. All data could be correlated by a Temkin adsorption isotherm equation. This then,provided one believes that this is a "true" description, affords a measure of the available metal surface area and Frennet remarked that adsorption of HZ at 1 bar, 20°C,was still a few % short of a monolayer and all other adsorption values obtained at lower pressures, including of course the extrapolation to zero pressure, were too low to a greater extent. He also pointed out that the extrapolated value was only to be considered as a good standardizable one if one agrees on the range of pressures used in the isotherm measurement. Both an isotherm measured up to 1 Torr and one measred up to 1 bar yield an extrapolation result, but they differ materially. with respect to the nickel catalyst, the very satisfactory agreement between crystallite sizes obtained from EM, XRD and HZ-adsorption led the speaker to a preference for the higher adsorption values. Monolayer definition remains a very difficult problem about which it will be very difficult to obtain certainty. For nickel also the observation that hydrogen additionally adsorbed between 100 and 760 Torrs still decreases the magnetization led the speaker to use the higher adsorption values. Finally, it should be mentioned that the choice is a personal one of the speakeL Within the group differences of opinion remain.
814 S. VASUDEVAN:1) What is the idea of preparing such high Pt loaded catalysts and on Si02 supports, whereas industrial catalysts have Pt loadings < 1.0 % and are usually supported on yc-aluminas ? 2) In the IR absorption studies of CO adsorption, the LF band at 1870 cm- 1 has been attributed in your paper to bridged or multi-centre bonded CO, and it is suggested that 10% of CO is adsorbed as bridged species. But your stoichiometry of CO molecules to pd atoms is (1.1 x 10 20/1.1 x 10 20) ,which is exactly equal to 1.0. J.W.E. COENEN: l)We were well aware that we did not imitate the most usual Ptcontaining industrial catalyst,but with respect to the level of metal loading and in the choice of the support. The relatively high metal loading was chosen to make the catalyst accessible to a wide variety of investigation techniques (e.g. EM particle size distribution) . The Si02 support was chosen to provide a parallel with Si02-supported nickel catalysts. Other catalysts are planned for the future, among them also a Pt/AI203 catalyst. 2) We do not pretend that the surface site count is accurate to within 10% in absolute terms, so that accomodation of 10% of a bridged form cannot be excluded from the surface stoichiometries. It may also be advisable to recall that for the very small crystallites where surface atoms of low coordination must be numerous in the surface the possibility of a surface atom bearing more than one CO ligand cannot be excluded. If this occurs, this phenomenon would provide "space" for the bridged form as well.
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
815
PROGRESS REPORT OF BCR ACTIVITY IN SURFACE AREA AND PORE SIZE REFERENCE HATERIALS N. PERNICONE G.Donegani Research Institute, Montedison Croup, Novara (Italy)
ABSTRACT A brief aCCBunt is given of the present status of the research work done by the BCR Working Group "Particulate Haterials". The work devoted to the development of Reference }hterials for low surface area and nleso-macro pore size is discussed in some detail. Suitable R}ls in these areas are expected to become available in the course of 1983.
INTRODUCTION The Community Bureau of Reference (BCR) is an organization within the framework of the Commission of European Community with the objective of studying, developing and supplying reference materials (ID1's) for which a definite need has been recognized by technical institutions and industries within the community. I t was set up following a decision of the Council of Hinisters in 1970; about 70 R}1's have since been certified and many others are currently under development. Several working gr oups (HG' s ) have been established by BCR to deal with specified areas of need; qualified experts from various countries within the community are invited to become members of the WG's. WG 4.13 on Particulate R}1's, chaired by Professor D.H. Everett FRS, is of special interest to workers on catalysis. This group has established three subsidiary Expert Groups, namely: - EG 4.13.1 - Particle Size R}1' s - EG 4.13.2 - Surface Area R}1's - EG 4.13.3 Pore Size R}1' s Experimental work is done, partly under contract to BCR, in academic, governmental and industrial laboratories in community countries. Samples to be studied are statistically abstracted from relatively large bulk quantities by qualified institutions such as the National Physicm Laboratory, U.K., and the Laboratoire National d'Essais, France. A brief account will be given of the present status of work done by members of EG 4.13.2 and EG 4.13.3 on surface area and pore size R}1's which could be useful to workers on catalysts. SURFACE AREA R}1's A definite requirement for ~'s with low surface areas in the range 0.1 to 10 m2g-1 was established. R}1's in the range 10-300 m2g- 1 were already available from the National Physical Laboratory. Many catalysts have low surface areas, less that 10 m2g- 1, especially those used for selective oxidation, and the influence of surface area on catalytic performance is often understimated; this
816 is partly because of difficulties encountered in obtaining reliable measurements of low surface areas. The availability of good mr's for the checking and calibration of equipment used for the measurement of low surface area could well encour~ap reconsideration of the problem. The following ten materials were chosen for preliminary investigation: five aluminas, two quartzes, rutile titania, spheroid;7P~ bronze and tungsten. The measurement techniques employed were: - multipoint BET nitrogen adsorption (volumetric and grav~metric) multipoint BET krypton adsorption (volumetric). Experimental work was done at the following laboratories: Battelle Institute, Frankfurt, FRG - CNRS Research Centre for Microcalorimetry and Thermochemistry, 11arseille, France - Ausind Research Centre, ~ontedison Croup, Novara, Italy National Physical Laboratory, Teddington, U.K. Raw adsorption data were sent to the National Physical Laboratory where results were calculated using the methods described in referenc~ I and 2, and a full report has been prepared by the National Physical Laboratory describing the results from the initial feasibility study in detail; sources of error are also indicated and recommendations for the subsequent certification study are made. The following is a brief summary of the finding and recommendations: a) Considering thp nroblems of measuring precisely values of low surface area,
b) c) d) e) f) g) h) i)
the resul ts were most encouraging and no gross systematic errors in the techniques employed by the respective laboratories could be detected. Three of the reference materials, that is two of the aluminas and one of the quartzes,should be eliminated from further study. Krypton BET plots for titania were inherently curved and certification of titania via krypton BET was therefore queried. More care should be exercized in the selection of analytical portions and in the weighing of these portions. More care should be given to ensure that a minimum number of six data points were within the recommended p/po limits. The croystat temperature should be maintained and monitored more carefully to enable po to be calculated more accurately. Calibrations should be checked and more care given to the calculation of "dead space", particularly for the krypton BET measurements. Buoyancy corrections for the gravimetric determinations should be checked more carefully. Liquid values for krypton po were favoured and Jaycock's procedure (ref. 2) for the calculation of krypton BET values was recommended.
PORE SIZE RlI' s Porosity characterized by total pore volume, mean pore size, and pore size distribution is an important physical property for many industrial materials. In the case of catalysts, vity and selectivity; the ject to much experimental size RM's were available,
it is well known that it can strongly influence actimeasurement of so called "textural properties'is subwork by laboratories involved in catalysis. As no pore WG 4.13 set up a programme for the certification of
a series of pore size ml's covering the mesoporous range,
2-2~
nm and the macro-
817 porous range,~20 nm. Four commercially available silica gels, with nominal pore diameters of 17, 11.5, 8.0 and 6.0 nm, were selected for preliminary investigation as candidate mesopore RM's. The experimental techniques chosen were mercury porosimetry and nitrogen gas adsorption, and work was done at the following laboratories: - Battelle Institute, Frankfurt, FRG - Bristol University, U.K. - CNRS Research Centre for Microcalorimetry and Thermochemistry, Marseille, France - Ausind Research Centre, Montedison Group, Novara, Italy - National Physical Laboratory, Teddington, U.K. After only a few preliminary experiments, it was evident that the silica gel materials were not suitable as pore size RH's for mercury porosimetry, as the pore structure is readily distorted under the pressures needed for such measurements. Thermogravimetric analysis indicated that for the gas adsorption studies an outgassing temperature of 100-150°C was optimum. A temperature of 140°C was actually chosen, to minimize small errors caused by small differences in temperature between the equipments used. Complete adsorption/desorption isotherms for the four silica gels were obtained by the five participating laboratories, and surface areas were calculated as described in reference 1; results are given in Table 1. It is clear that, with the possible exception of sample 4, the discrepancies are exceSS1ve. TABLE 1 2/g) Surface area results on silica gels (m SILICA
LABORATORY Battelle Bristol CNRS Ausind NPL
354.2+2.1 326.1+3.8 328.5+ 1. 9 327.4+1.6 358.4+1.7
GELS 2
3
4
421.2+4.5 399.2+1.1 376.3+1.7 400.1+0.5 430.6+0.8
581.1+4.3 503.2+3.8 483.3+1.6 516.4+2.1 533.3+1.0
587.5+7.3 574.0+1.1 544.1+0.9 556.1+0.6 577.6+1.9
The isotherms from the five laboratories were compared at Bristol University and it was considered that these too differed excessively, considering the experience of the laboratories involved. llodal values of pore diameter determined from pore size distribution plots calculated as described in reference 3 are given in Table 2. The most likely source of difference between the results were considered to be: - sampling errors - outgassing conditions including time, temperature and ultimate vacuum - equilibration conditions; that is rate of pressure change before starting a new point. From information supplied by the participating laboratories, the latter was eliminated as a possible source of difference; the difference was opposite to that expected from the different equilibration conditions. Therefore, assuming
818 TABLE 2 Most frequent pore size of silica gels (nm)
LABORATORY Battelle Bristol
cms Ausind NPL
1 13.18 11.94 12.96 13.26 12.78
SILICA GELS 2 3 9.66 11.12 10.84 11.02 10.48
7.46 8.40 8.24 7.28 7.46
4 6.02 6.66 6.48 6.56 6.46
that there were no unsuspected systematic differences, it was concluded that the samples, as prepared for measurement, had different adsorption properties. The cause of this was most likely to be a consequence of the sensitivity of the silica gels to slightly different outgassing conditions or, less likely, a consequence of random sampling differences. However, it was considered that both reasons were sufficient to render the silica gels as unacceptable as potential RM's. A series of mesoporous (and macroporous) controlled pore glasses have been considered as an alternative. Preliminary gas adsorption and mercury porosimetry measurements at NPL are most encouraging; seven materials in the pore diameter range 7-300 nm have been examined and three materials, in the range 10-25 nm, have been found suitable for both techniques. The most severe problem is the high cost of bulk quantities of the ma~erials which will be required for a viable certification campaign and this is likely to restrict severely the number of RM's eventually certified.
REFERENCES 1 D.H. Everett, G.D. Parfitt, K.S.W. Sing and R. Wilson, J. Appl. Chern. Biotechnol., 24(1974)199. 2 M.J. Jaycock, in M.S. Groves Ed., Proc. Conf. on Particle Size Analysis, Salford, U.K., September 1977, Heyden Publ., 1978, 3 D.C. Havard and R. Wilson, J. ColI. Interf. Sci., 57(1976)276.
819 DISCUSSION N. PERNICONE : In order to promote in some way this discussion, I would like to recall some general problems, derived from the present contributions, which could be debated in some detail. First, there is really a need for both standard methods and reference materials in catalysis. If the answer is affirmative, as it seems from the previous lectures, should the two activities of methods standardization and RMs preparation be considered independent or strictly connected? In the latter case, why most organizations are involved with only one of them? Moreover, is it preferable that such activities are carried out on a voluntary basis or with research contracts from national and international institutions, by acaoemic or industrial laboratories? Probably there is a need for worldwide coordination of the various initiatives in this area, how could this be accomplished? Which is the importance of the "fallout" towards the participating laboratories in terms of accuracy improvement and better knowledge of the subject in general ? This is only to suggest some possible arguments; other important problems will be probably raised during the discussion. K.S.W. SING You have recommended that P/p o limits should be stated for the applictaion of the BET plot. In fact, the location of point B and hence the position of the BET monolayer capacity is dependent on the particular gas-solid system and therefore, no general recommendation should be made for the appropriate p/p o range. In my view, the lack of good agreement between the BET areas of silica gels as determined in the different laboratories is likely to be due to the lack of strict control over the outgassing conditions (especially the outgassing temperature). The silica gels specified in Table 1 are said to be mesoporous, but it is quite likely that they are to some extent microporous. If this is the case, the outgassing temperature of 140°C may not be high enough to ensure complete removal of physisorbed water from the gel structure. L. PERNICONE: I agree with you that the linearity range of the BET plot is not the same for all the inorganic materials. However, in the case of RMs , it is probably preferable for the user to have to do with a set of materials having the same linearity range. Concerning the surface area of our silica gels, I am pleased to note that your explanation of the lack of agreement coincides with that we have proposed as the most probable. K.S.W. SING: A Subcommittee of IUPAC Commission 1.6 was established in 1979 to consider the reporting of gas adsorption data. The IUPAC has now approved the publication of a Provisional Report on "Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity". This will appear in Pure and Applied Chemistry and will deal with the terminology and symbols to be used in reporting gas adsorption data, with experimental procedure and with the calculation of surface area and pore size distribution. Comments on such recommendations are welcome and should be sent to the Chairman (K.S.W. Sing), so that they can be discussed by the Subcommitte in August 1983. R.J. BERTOLACINI: I have noted that there is a difference of philosophy in standardization between the USA-ASTM D-32 and the Japanese and European Groups. The US Committee is composed of industrial researchers, while that of Japan is academic, and of the European Groups only that of BCR is partially industrial. ASTM policy is to standardize a method with a reference material, with due attention to the detailed procedure for sampling, handling, pre-treatment, recommended apparatus and experimental methods. The standard procedure will result in inter and intra laboratory agreement within the specified precision and accuracy only when the researchers use the identical method with the same reference. Once having established the accuracy and
820 precision for the reference material, the researcher can be finally confident that inter-laboratory samples of non-reference samples will also be in good agreement as specified by the accuracy and precision statement in the published standard. It could be useful to all those interested in standardization (Japanese, European and American), to meet and develop a joint set of objectives, make reference materials, and discuss common goals. J.J.F. SCHOLTEN: Prof. P. Emmett, one of the pioneers in measuring the metallic part of the surface area of catalysts, introduced the term (1938) freemetal surface area. I think this terminology has to be preferred above "metal surface area", because it makes clear that only that part of the metal surface is measured which is not covered by irreversible poisons and/or by the support. The quantity should be noted as m2 of metal per gram of metal. Hence the total amount of metal atoms present in the adsorption vessel should be determined separately. This is a better basis than expressing this quantity per g of catalyst as the catalyst weight may change strongly depending on the H20 and OH-group content of the support (~ 10%) . J.J.F. SCHOLTEN: When normalizing the measurement of pore volumes per gram of catalyst (Vp in cm 3 /g of catalyst), it is very important to indicate the lower level and especially the upper level of pore radii for which the Vp value holds. In measuring Vp from the total amount of liquid nitrogen taken up at 78 K and 0.99 Atm, the upper limit of the radii of the pores is about 300 ~ (30 nm), and the lower limit is about 7 A (0.7 nm). However, applying mercury penetration and working in the pressure range from 0.1 Atm up to 4000 Atm, the lower limit is 17 (1.7 nm) and the upper limit is 75 x 10 4 ~ (75.000 nm). For many samples Vp values 1.5 to 2 times as high as found from the N2 method are arrived at in such case. Hence Vp should be noted as :
A
x
cm g
3
G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
821
STANDARDIZATION OF PROCEDURES FOR DETERMINATION OF ACTIVITY AND SELECTIVITY OF COMMERCIAL CATALYSTS BY COMPARATIVE KINETIC INVESTIGATIONS IN DIFFERENT LABORATORY REACTORS
M. BAERNS anc! H. HOFMANN Lehrstuhl fur Technische Chemie, Ruhr-UniversiUit Bochurn, Bochum (GFR) Institut fUr Technische Chemie, UniversiUit Erlangen/Nurnberg, Erlangen (GFR)
ABSTRACT Suitable measures for the characterization of catalyst activity and selectivity are discussed. To evaluate experimental methods which might be applicable for standardized catalyst testing, different reactions and laboratory reactors are being considered. As a single reaction the methanation of carbon monoxide and as a multiple reaction the oxidation of butene-1 to maleic anhydride have been chosen. Specific results obtained by using gradientless recycle reactors and differential or integral catalytic fixed bed reactors are communicated. INTRODUCTION Important criteria for the characterization of commercial catalysts are activity and selectivity besides other properties like deactivation and mechanical as well as thermal strength. To oompare objectively
activity and selectivity of various catalysts
or of one catalyst studied by various investigators (for instance user and supplier) standardized experimental procedures are required. Such procedures depend on the type of catalyst and on the type of reaction being considered. Catalysts may be finely divided, i.e. for fluidized and entrained bed reactors, or they may exist in the form of pellets as they are commonly used in fixed bed reactors. Furthermore, catalyst pellets may have a homogeneous or heterogeneous structure; co-precipitation and subsequent pelletization of catalyst mass generally leads to homogeneity while impregnation of a support may result in a concentration profile of the catalytic active compound within the pellet.
822 To study the bases of standardizing the experimental determination of activity and selectivity and its application to industrial catalysts a joint research project is presently carried out by the Institut fur Technische Chemie, Universitat Erlangen/Nlirnberg and by the Lehrstuhl flir Technische Chemie, Ruhr-Universitat Bochum. The project is funded by the Ministry of Research and Technology of the Federal Republic of Germany. This paper intends to outline the approach and the scope of the project and to communicate selected results obtained up to now. GENERAL APPROACH Activity and selectivity of a catalyst are being defined in various ways depending on the level of information needed. The ultimate goal would certainly be to predict the performance of a catalytic reactor in which the catalyst is applied. Several quantities based on the recommendations of the European Federation of Chemical Engineering by which activity and selectivity can be described are put forward and discussed below: The fractional conversion XA of a key reactant (1)
can be used as an integral measure of activity. However, for comparison of various catalysts or experimental results obtained by various investigators for one catalyst all process conditions have to be kept constant; these are particularly temperature, initial concentration of the reaction mixture, catalyst history, space velocity, mass of catalyst and reactor geometry. A typical example of such a standardized method is the ASTM microactivity test for fluid cracking catalysts (ASTM-D 3907-80) for which all the variables are fixed. The selectivity of a catalyst characterizes the specificity of a complex reaction with respect to a product P. This specificity can be described a) by integral and differential yields.
~p
~p
=
v
n p vA p (nAO - n A)
VA Rp RA
Vp
overall (integral) relative yield
(2)
point (differential) relative yield (3)
823 or b) by the integral and differential selectivities
8 1,2
vp 2 np 1 vp 1 np 2
overall (integral) selectivity
(4 )
81,2
v p 2 Rp 1 v p 1 Rp 2
point (differential) selectivity
(5)
(It should be mentioned that the above yields are in some instances also termed selectivities.) Using overall yields or selectivities, which are obtained from integral reactor data, for catalyst characterization requires a standardization of all process conditions in the same manner as outlined for the conversion. Differential yields or selectivities can be associated to defined conditions of concentration and temperature; they are independent of reactor geometry and other conditions as long as no intraphase transport limitations exist. Their experimental determination requires a differential or gradientless recycle reactor. For predicting the performance of commercial scale catalytic reactors the differential values are more suited than the integral ones since reaction rates and their dependence on concentration and temperature needed for reactor simulation can be directly derived while otherwise differentiation of the integral data is necessary. The evaluation of
~ntegral
data may be further complicated if
there are temperature gradients in an integral reactor. A comprehensive characterization of a catalyst appears, however, only possible on the basis of a complete kinetic model of the reaction or the reaction network respectively. It is obvious that such an approach is quite time and hence, money consuming and will be only justified from an economic point of view in special situations; of course,considering basic scientific aspects it is the only way possible. When evaluating catalysts it is also required to account for intra- and interphase tranport phenomena which may differ under conditions of laboratory and industrial scale reactor operation. The availability of the kinetics and the respective parameters of a reaction or a reaction network and, if necessary, the knowledge of all relevant transport parameters makes it possible not only to characterize the catalyst but also to predict the performance of a catalytic reactor over a wide range of conditions.
824
The objective of the present investigation is manifold with respect to catalyst characterization: (a)
the various measures of activity and selectivity are compara-
tively evaluated; (b)
the accuracy and reproducibility of experimental data obtained
in the two laboratories are compared in order to establish the crucial conditions of experimentation for standardized procedures; (c)
the applicability of different laboratory reactors for deriv-
ing kinetic data L$ investigated. In order to evaluate the significance of these data they shall be used for predicting reactor performance which will be compared with actual operation of industrial reactors. SCOPE OF EXPERIMENTAL INVESTIGATION Two reactions are being kineticly investigated: the methanation of carbon monoxide at low concentrations of CO was chosen as a single reaction while the oxidation of butene-1 to maleic anhydride (MA) was selected as an example for a multiple reaction i.e. complex network. The methanation is carried out over a wide range of experimental conditions to include the rate limiting effect of pore diffusion. For each reaction different catalysts are applied. The following types of laboratory reactors are used: differential and integral catalytic fixed bed tubular reactor (DFBR and IFBR), gradientless recycle reactor (GRR), single pellet diffusion reactor (SPDR), micro pulse reactor (MPR), and differential thermal analysis reactor (DTAR). The experimental conditions applied are summarized in Table 1. TABLE 1 Laboratory reactors used and experimental conditions Reaction Reactor type
Methanation
Butene Oxidation
GRR, DFBR, SPDR, MPR, DTAR, IFBR
GRR, DFBR, IFBR, MPR
Catalyst
V
No of catalysts Temperature K Pressure bar
20 5/P 20 5
(A1
3
453 to 557 1 to 25
350 to 440
20 3)
825
RESULTS Experimental results on the kinetic characterization of catalysts which will serve as bases for standardization of test methods for activity and selectivity of
ca~alysts
are described be-
low. for oxidation of butene-1 to MA
~2Q5/P2Q5-catalysts
The oxidation of butene-1 to MA proceeds mainly via 1,4-butadiene and
fura~
as intermediates. Simultaneously undesired side reac-
tions yield oxygen containing derivatives such as carboxylic acids, aldehydes, ketones, high boiling compounds, the latter are probably formed homogeneously, and finally carbon mon- and dioxide. For a suitable kinetic description of the system a simplified reaction scheme has been used (ref. 1): Side Products homo g . \
/catal.
Butene-1 --... Butadiene ----.Furan
~
Maleic Anhydride
~ CO CO
2
Other schemes of more and less sophistication are presently tested in order to characterize the catalyst in the simplest but sufficient way for industrial purposes. It is intended to derive from the final results some generalization for catalyst testing with respect to complex reaction networks. Some measures of catalyst characterization which can be derived from kinetic experiments based upon the above reaction scheme and carried out in both, a GRR and DFBR, are mentioned below. The overall relative yield
~MA
of oxidation of butene-1 or 1,4-
butadiene respectively can be derived for various process conditions from the kinetics which have been reported in detail elsewhere (ref. 1). In Figure 1
~MA
is plotted versus reaction tempera-
ture assuming isothermal reactor operation and negligible oxidation of MA (ref. 2). Such a diagram can serve as a basis for comparison of various catalysts; moreover, it also gives indication of how to operate the process: in the present case a two-stage process, i.e. first dehydrogenation of butene to butadiene with high selectivity and then secondly oxidation of butadiene to MA, might be preferable because of the different overall yields resul-
826
ting from the two educts. Applying a suitable reactor model, profiles of temperature and of the concentrations of MA, the intermediates and side products
MA
C-atom%
60
40
~2
30 450
400
350
T
°c
Fig. 1. Dependence of overall relative yield of MA on reaction temperature for oxidation of butadiene ("1") and of butene ("2"). can be predicated (ref. 2). The results of such a procedure give an even more refined measure to compare various catalysts. A comparison of activity as obtained in two different reactors, i.e. GRR and DFBR, is shown in Table 2; as a measure of activity the kinetic constants of the Mars/van Krevelen equation which was applied for the butene consumption were used (ref. 3): kBPB (6)
Deviations between the results of the two reactors are in the range of reproducibility and accuracy. Table 2 also shows the importance of the same catalyst pretreatment in order to get comparable data. TABLE 2 Rate constants k
and k obtained from different laboratory reB Re o x actors and from applying different temperatures T t during capre r. talyst pretreatment with a butene/air mixture.
rate constants mole/g-cat. h T
k
B
(396°C)
k Re o x (396°C)
Reactor
pretr.
400°C
0.14 0.13
0.014 0.008
DFBR GRR
Tpretr.
420°C
0.27 0.22
0.011 0.011
DFBR GRR
827
Alumina-supported nickel methanation catalysts Results on activity are described for two commercial methanatioD catalysts (see Table 3). The activities of the two catalyst pellets show significant differences (ref. 4) as exemplified in TABLE 3 Properties of methanation catalysts G and R Catalyst
G
R
Nickel content wt-%
23
18
Pellet geometry D mm Lmm
6,2 6,2
BET-surface area m2 g -1
64
169
Av. radius of mesopores nm Ni-surface area m2 g -1
4,3
4,1
approx. 10 to 15
6 5
Figure 2. Analysing the data proves that the reaction is strongly affected by pore diffusion of carbon monoxide. To compare the activity of the two catalysts on a more fundamental basis it would be necessary to describe quantitatively the intrinsic activity and the pore diffusion process. For catalyst R, the size of which was reduced to ca 1 mm, the intrinsic kinetics were investigated. The r
CH4
. 10 3
mole/g h
50
40
30
20
10
0 0
0.10
0.20
0.30
0.40
Pco bar Fig. 2. Dependence of methanation rate on partial pressure of CO for catalyst pellets R (p 2 = 4.4-4.7 bar) and G (p = 3.3-3.8 bar at T = 554::2 K. H H2
828 following kinetic equation was obtained:
(1
+K~
exp (-L'>He/RT)
The underlying
yP co'
(7) +
mechanism is discussed elsewhere (ref. 5). The
parameters of eq.
(7) which have been obtained independently in the
two laboratories and which show reasonable agreement are listed in Table 4. If only a small range of hydrogen partial pressures, which TABLE 4 Parameters of eq.
k
(8);
(BO: Bochum (ref. 5)
O
mole/g h BO
4.8 10 9
ER
9
5.2 10
L'>H
K e
E 0
kJ/mole
bar
-0.5
ER: Erlangen (ref. 6)).
O
C
kJ/mole
K
L'>HH
H
bar
-0.5
kJ/mole
103
4 5.8 10-
-42.1
0.016
-16.0
101
10- 4
-40.0
0.016*
-16.0*
6.6
*fixed value for optimization of the other 4 parameters exists under industrial conditions, is considered a simpler kinetic equation may be used (ref. 6) k
to characterize the rate of reaction
~l{P;;;
(8)
1 + K Peo
For a comparison of catalysts G and R the knowledge of the kinetic parameters of G are required. Since for catalyst G only limited kinetic data, not disguised by diffusion, are presently available the following procedure was used: assuming that the adsorption constants K and K are equal for both catalysts the rate C H constant k for catalyst G is estimated by eq. (7) from the descending parts of the two curves in Figure 3 not affected by pore diffusion. Surprisingly, the intrinsic rate constants are very similar as shown in Table 5. This result means that the difference TABLE 5 Intrinsic rate constants k/mole g-l h- 1 of catalysts Rand G T /K/ 483 503
R
G
9.2 10- 2 27.5 10- 2
829
r
CH
10- 3
14
4
mole/ g h
12 10 8 6 o~
o
4
----===-=1....-.-...-:..-.
2
_
()-
0
0.10
0.05
0.15
0.20
0.25
Pco bar Fig. 3. Dependence of methanation rate on partial pressure of CO for catalyst G (particle size: 1 to 1.4 mm); effective rate: -0----0--, rate without limitation by pore diffusion: ----in activity of catalyst pellets Rand G are mainly due to their effective pore diffusion coefficients D which can be derived from e the effective Knudsen- and effective binary diffusion coefficients: De
K, i
= K
0
V
8 RT i
..f-D
11 m
T
( 9)
1,2
The required porosity data are listed in Table 6. TABLE 6 Porosity data of catalyst pellets Rand G (ref. 7) catalyst pellet G
R
K
o
0.05 10- 8 0.26 10- 8
0.014 0.086
As a first approximation the ratio of the activities of the two catalyst pellets expressed by the effective reaction rates r e at equal temperature and concentrations which corresponds to the effective rate constants k
e
can be described by eq.
(10):
(10)
830 and L being the characteristic dimensions of the two pellets. G R Inserting the relevant data into the right hand side of eq. (10)
L
results into a value of 0.4 which is very close to the ratio of the experimentally determined effective reaction rates, i.e. within less than 10 % deviation. Summarizing, it can be concluded that a more fundamental measure of activity, when pore diffusion prevails, comprises the intrinsic rate constant and the effective pore diffusion coefficient. CONCLUSION ANB OUTLOOK The present results have principally shown that catalysts should be characterized with respect to their actual performance by kinetic parameters for the chemical and transport processes. On the basis of these results further work will deal with the standardization of the experimental procedures and conditions as required for catalyst testing in the most general way possible. REFERENCES 1 B. Muller and M. Baerns, Chern. Ing. Techn. 52(1980) 826-830. 2 B. Muller, Dissertation, Ruhr-Universitat Bochum, Bochum 1981. 3 Le Duy Duc, unpublished results. 4 D. Kreuzer,
Diploma-Thesis, Ruhr-Universitat Bochum, Bochum
1981 . 5 J. Klose, Dissertation, Ruhr-Universitat Bochum, Bochum 1982. 6 Zhang Ji-Yan, unpublished results. 7 Tran Vin Loc, unpublished results.
831 DISCUSSION S.P.S. ANDREW My question is entirely of a philosophical nature concerned with the knowledge and the intellectual ability of the people required to effect a satisfactory scale-up of design from data from microreactors and gradientless reactors. The old method of using pilot plants for scale-up required less intellectual ability and less chemical and physico-chemical knowledge than scaleup from gradientless reactors and the use of Thile moduli etc.. Take for instance ammonia synthesis catalysts: it is now well known that small particles of catalyst have a much higher specific surface than do large particles due to reduction phenomena and therefore have a higher intrinsic activity. If the person engaged in a scale-up operation involving microreactor measurements, on ammonia synthesis, is unaware of this phenomenon than he will err greatly. Can we be sure that the practical user of these techniques will have the necessary enchanced intellectual capacity to be able to use them correctly ? M. BAERNS Reactor modeling based on kinetic data obtained in laboratory reactors and on transport parameters, if needed, facilitates (1) to study the effect of process variables on reactor performance by simulation and (2) to design a chemical reactor. Both possibilities may support the industrial chemist and the chemical engineer in his scientific and technical work. Having a suitable eductaion in the field of chelical reaction engineering there should be no intellectual obstacles to apply and work with reactor models. J.W GEUS: You are studying considerably exothermic reactions and consequently the problem arises whether you can determine correctly the temperature at the catalytic active site. Is it required to do measurements in a gradientless reactor at low conversions per pass or is it possible to work at higher conversions per pass and derive the temperature at the catalytic sites and possibly the distribution by calculation ? M. BAERNS The gradientless reactor requires by definition low conversions per pass through the layer of the catalyst pellets; the overall conversion of the feed to the reactor may, however, vary between very low and very high values. The temperature gradient from the gas phase to the catalytic surface depends on the heat transfer coefficient "pellet surface/gas" which can be influenced by the linear gas velocity through the catalytic bed, i.e. recycle rate. This temperature gradient and hence, the temperature of the catalytic surface, can be calculated; intraparticle temperature gradient rarely occurs. R.A. VAN NORDSTRAND: In answer to the Chairman's general question (see preceeding paper), I propose that we consider objectives. The ASTM in USA is a group of catalyst scientists trying to expedite commerce in catalysts, providing detailed and practical tests. The European group represented here seem to be academic catalyst scientists whose objectives are to promote the science of catalysis. Perhaps it is expecting too much to bring closely together groups with objectives so different. M. BAERNS: To my opinion the objectives of both groups as defined by Dr. van Nordstrand are not detrimental to each other but supplementary. The European Groups are (a) developing basic methods and respective reference materials for testing physical and chemical properties of the solid material and (b) evaluating the bases for, the experimental determination of the catalytic activity and selectivity which can be extrapolated to process conditions. Both these objectives will not only contribute to a better understanding of various scientific aspects of catalyst testing but they will eventually also improve the experimental methods and techniques applied in the characterization of industrial catalysts.
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833
LIST OF PARTICIPANTS
ADM1S C., Dr.
ALTHAUS H., Dr.
ANDERSSON A.S., Dr.
ANDRES M.
ANDREW S.P.S., Dr.
A.."
MlTOS G.J.
ANUNDSKAS A., Dr.
AOMURA K., Prof.
ARGENTON A.
ARIS R., Prof.
ARNTZ D., Dr.
ARPE H., Dr.
Unilever Research Lab. Port Sunlight Wirral, Merseyside
U.K.
Lonza AG Holzackerweg 6 CH-3902 Glis/VS
SWITZERLAND
Lund Institute of Technology Dept. of Chem. Technology P.O.B. 740 S-220 07 Lund
SWEDEN
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FRANCE
leI-Agricultural Division P.O.B. 6 Billingham, Cleveland Ts23 1LB
U.K.
Redco SA B-2920 Kapelle-op-den BOS
BELGIUM
UOP Inc. Drawer C, Riverside, II. 60546
U.S.A.
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NORWAY
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JAPAN
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BRAZIL
University of Minnesota Dept. of Chem. Eng. Minneapolis, Minn. 55455
U.S.A.
Degussa AG Abt. FC-O Postfach 1345 D-6450 Hanau 1
W. GERMANY
Hoechst AG (D 569) Postfach 800320 D-6230 Franckfurt (M) 80
W.
GE~"
834 BAERNS M., Prof.
BAlKER A., Dr.
BALINGIT G.F.
BERETS D.J.
BERREBI G., Dr.
BERTOLACINI R.J.
BHASIN M.M., Dr.
BICKER R., Dr.
BLINDHEIM U., Dr.
BOND G.C., Prof.
BOOTH J.
BORG F.
BOSSI A., Dr.
BOURNONVILLE J.P., Dr.
Ruhr-Universitat Bochurn Postfach 102148 D-4630 Bochurn
W. GERMANY
Swiss Federal Institute of Technology Dept. Chern. Eng. ETH-Zentrurn CH-8092 Zurich
SWITZERLAND
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W. GERMANY
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NORWAY
BruneI University Uxbridge, UB8 3PH
U.K.
ICI PLC P.O. Box 11 The Heath, Runcorn, WA7 4QE
U.K.
Compagnie Fran~aise de Raffinage Centre de Recherches Total B.P. 27 F-76700 Harfleur
FRANCE
Istituto G. Donegani S.p.A. Via Fauser 4 1-28100 Novara
ITALY
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FRANCE
835 BRITSCH R., Dr.
BROOKS C.S.
BRUNELLE J.P.
BUTLER G., Dr•.
BUTLER J.P., Dr.
BUTT J.B., Prof.
CAHEN R., Dr.
CANDIA R.
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Northwestern University Dept. of Chern. Eng. Evanston, II. 60201
U.S.A.
Labofina SA Siege de Feluy, Zone Industrielle B-6520 Feluy
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Haldor
Tops~e
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DENMARK
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P.R. CHINA
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FRANCE
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FAA-NCE
Unilever Research Lab. P.O. Box 114 NL-3130 AC, Vlaardingen
THE NETHERLANDS
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FRANCE
Nym~llevej
CARDEW P.T., Dr.
CHADWICK D., Dr.
CHANG SHI
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COENEN J. W.E., Pro'f.
COGNION J.M.
836 CORDIER G., Dr.
COSYNS J., Dr.
COURTY Ph.
COVINI R., Dr.
COX D.
CROSS M.
DE BEER V.H.J., Dr.
DE BOKX P.K., Drs.
DE CLIPPELEIR G., Dr.
DE KOK A.
DELMON B., Prof.
DENAIS R.
DE RADZITSKY P., Dr.
DE ROSSI S., Dr.
Rh6ne-Poulenc Recherches 85 rue des Freres Peret F-69190 St. Fons
FRANCE
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DETTMEIER U., Dr.
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ELM R., Dr.
FIGUEIREDO J.L., Prof.
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GALVAGNO S., Dr.
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PORTUGAL
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FRANCE
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CANADA
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ITALY
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GERMAINE A.
GEUS J.W., Prof.
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GLAIZE Y.
GLASZ W.
GOOD M.L.
GRA-T1CE P., Dr.
GRANGEON P.M.
GREMMELMAIER C.
GRIFFIN K.G.
GUACCI A.
GUCZI L., Prof.
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University of Bradford Richmond Road Bradford, W. Yorks. BD7 1DP
U.K.
Shell Recherche Centre de Recherche F-76530 Grand-Couronne
FRANCE
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IlAEGH G.S., Dr.
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HA.~SEN
J .H.B.
HEINRICH L., Dr.
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HENTHORN R. S.
HESSELINK W.H., Dr.
HEYEZ S.
HOCKELE S., Dr.
HOEK A., Dr.
HOFLUND G.B., Prof.
HOUALLA M., Dr.
Peking University Chemistry Dept. Beijing
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NORWAY
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NORWAY
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University of Pittsburgh Dept. of Chemistry Pittsburgh, Pa. 15260
U.S.A.
GERMANY
GERMANY
840 HYDE E.K.
HU
~~EI
IIDA H.
JACOBS P., Dr.
JANSSEN F., Dr.
JARAS S.
JENKINS J.W.
JENSEN E.J.
JENSEN P.E., Dr.
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U.K.
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R.P. CHINA
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JAPAN
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DENMARK
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DEN/1ARK
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DEN/lARK
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THE NETHERLANDS
Girondelle 86 463 Bochum
W. GERMANY
JOHNSON M.
JOUSTRA A.H., Dr.
KAGON J.
841 KALIAGUINE S., Prof.
KIESER D.C.
KIJENSKI J., Dr.
KIMURA '"
KIWI J.
KOCHLOEFL K., Dr.
KOCK A.J.H.M., Drs.
KORTBEEK A.
KOSTKA H.
KREMENIC G., Dr.
KRICKFALUSSY Z., Dr.
KRYLOVA A. V., Dr.
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KRUISSINK E.C., Drs.
Universite Laval Dept. de Genie Chimique Quebec GIK 7p4
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D.D.R.
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POL~ND
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SWITZERLAND
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SPAIN
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USSR
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W. GERMANY
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THE NETHERLANDS
GER~NY
842 LANGNER B.E., Dr.
LAWSON R.J.
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LEROT L., Dr.
LI DADONG
LIN LI WU, Prof.
LITTERER H., Dr.
LO JACONO M., Prof.
LOK C.M., Dr.
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LYCOURGHIOTIS A., Prof.
MARCELIN G.
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FRA"ICE
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FRANCE
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Petroleum Corporation Beijing
P.R. CHI1'JA
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P.R. CHINA
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\'1.
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GERMANY
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GREECE
Gulf Res. & Dev. Co. P.O. Drawer 2038 Pittsburgh, Pa. 15230
U.S.A.
843 MARCILLY C., Dr.
MARENGO S., Dr.
MARGITFALVI J., Dr.
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MARTINEZ N.
MARTOS J.
MAS C., Dr.
MElDER H., Dr.
MERCIER M.
MEURIS T.
MILBERGER E., Dr.
MILLER A.W.
MILLS K.J., Dr.
MINELLI G.
Institut Fran~ais du Petrole 1 et 4 avo de Bois-Preau F-92500 Rueil-Malmaison Cedex
FRANCE
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ITALY
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HUNGARY
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VENEZUELA
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SPAIN
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W. GERMANY
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YUGOSLAVIA
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FRANCE
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BELGIUM
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U.K.
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U.K.
Universita di Roma Centro studi "S a c s o ll 1st. Chim. Gener. e Inorg. 1-00100 Roma
ITALY
844 MINGELS W.
MIYAMOTO A., Dr.
MOY K.B.
MORIMOTO T., Dr.
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MROSS W., Dr.
MULLER D.J., Dr.
MURAKAMI Y., Prof.
NAGY B., Prof.
NAUMANN A.W.
NEUKERMANS H., Dr.
NG CHING FAI, Dr.
Sopar Chemie 18 Vredekaal B-9060 Zelzate
BELGIUM
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JAPA-l\l
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JAPAN
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ITALY
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W. GERMANY
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JAPA"J
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U.S.A.
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BELGIUM
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HONG KONG
845 NGUYEN T.T., Dr.
NIELSEN B.
NIKLASSON C.
NONNON- R.
NOWECK K., Dr.
ODENBRAND I., Dr.
ONO T.
OSTERWALDER H.
OTTERSTEDT J.E.
PAJARES J., Dr.
PANG LI, Prof.
PAYNE P.
PEPE F.
PERNICONE N., Dr.
PESSIMISIS G.N.
Natl. University of Singapore Dept. of Chemistry Kent Ridge, S'pore 0511
SINGAPORE
Haldor Tops~e A/s Nym¢llevej 55 DK-2800 Lyngby
DEN/1ARK
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SWEDEN
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BELGIUM
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1>1. GERMANY
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SWEDEN
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JAPAN
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SWITZERLAND
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SI>lEDEN
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SPAIN
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P.R. CHINA
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U.K.
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ITALY
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ITALY
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U.S.A.
846 PETERS U.
PETRINI G., Dr.
PETRO J., Prof.
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PITANCE Th.
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ITALY
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HUNGARY
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BELGIUM
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Universite Cath. de Louvain Groupe de Physico-chimie Min. et de Catalyse Place Croix du Sud, 1 B-1348 Louvain-la-Neuve
BELGIUM
PORTELA FARINHA M., Prof. Instituto Superior Tecnico Av. Almirante Reis 127 5° C 1100 Lisboa PRIGENT M.
PUXLEY D.C., Dr.
REI MIN-HON
REIMERINK W., Dr.
REMPEL G.L., Prof.
REUTER P., Dr.
REYMOND J.P., Dr.
~ICHARDSON
J.T., Prof.
PORTUGAL
Institut Fran~ais du Petrole 1 et 4 av de Bois-Preau F-92500 Rueil-Malmaison
FRANCE
British Gas London Research Station London
U.K.
Natl. Taiwan University Dept. Chern. Eng. 1, Sec. 4 Roosevelt Road Taipei, Taiwan 107
R.O. CHINA
Dow Chemical (R & D) Herbert Dow Weg. 5, BBB Terneuzen
THE NETHERLANDS
University of Waterloo Dept. Chern. Engineering Waterloo, Ontario N2L 3GI
CANADA
BASF AG Neuwiesenstr. 22 D-67 Ludwigshafen
\'1. GERMklllY
Universite Cl. Bernard CNRS L.A. 231 F-69622 Villeurbanne Cedex
FRANCE
University of Houston Dept. of Chern. Eng. Houston, Texas 77004
U.S.A.
847 RIECK H.P., Dr.
RIEKERT L., Prof.
RIIS T.
RIVA A., Prof.
RODER W.
ROELOFSEN J.W., Dr.
ROESSLER F., Dr.
ROHM W., Dr.
ROSS J., Prof.
ROUCO A., Dr.
RUSEK M.
SAMBROOK R.M., Dr.
SANDSTROM J.
SCHANKE D.
SCHAPER H.
Hoechst AG (Geb. C 315) Postfach 800320 D-6230 Frankfurt (Main) 80
W. GERMANY
Universitat Karlsruhe Box 6380 D-7500 Karlsruhe 1
\'i.
Central Inst. for Ind. Research P.O. Box 350 Blindern N-Oslo 3
NORWAY
Faculty of Ind. Chern. Viale Risorgirnento 4 1-40136 Bologna
ITALY
universitat Erlangen Inst. fur Techn. Chernie I Egerlandstr. 3 D-Erlangen-Ntg.
\'i.
Akzo~Chernie,
GERMlLNY
GERMANY
Ketjen Catalysts
P.O. Box 15 Amsterdam
THE NETHERLANDS
F. Hoffman La Roche & Co. AG Grenzacherstrasse CH-4000 Basel
SWITZERLAND
Sud-Chernie AG Postfach 202 240 D-8000 Munchen 2
W. GERMANY
Twente University of Technology Dept. of Chern. Techn. NL-7500 AE Enschede
THE NETHERLANDS
Plapiqui 12 de Octubre 1842 8000-Bahia Blanca
ARGENTINA
Ciba-Geigy AG R-1055.608 CH-4002 Basel
SWITZERLAND
Dyson Refractories Ltd. 381 Fulwood Road Sheffield S6 6GB
U.K.
Outokurnpu Consulting Engineers P.O.Box 26 F-67101 Kokkola 10
FINIAND
university of Trondheirn Norwegian Inst. of Technology Lab. of Industrial Chern. N-7034 Trondheirn-Nth.
NORWAY
Technical University of Delft Julianalaan 136 Delft
THE NETHERLANDS
848 SCHEVE J., Dr.
SCHIEFLER J., Dr.
SCHLIMPER H.D., Dr.
SCHMIDT F., Dr:
SCHOLTEN J.J.F., Prof.
SCURREL M.S., Dr.
SEFERIADIS N.
SERVAIS D.J.H.
SIGG R., Dr.
SIMONSSON D., Dr.
SING K.S.W., Prof.
SMEDLER B.
SMIT C.H., Dr.
SONNEMANS J.W.M., Dr.
SPEK Th., Dr.
Zentralinstitlit physik. Chemie der AdW de DDR DDR-1199 Berlin, Adlershof Rlidower Chaussee 5
D.D.R.
Condea Chemie GmBH Zentralverkauf Hamburg Mi ttelweg 13 D-2000 Hamburg 13
W. GERMANY
BASF AG c/o D-ZH/Tagungen D-6700 LUdwigshafen
W. GERMANY
Degussa AG Postfach D-7888 Rheinfelden
W. GERMANY
DSM Central Labs. P.O. Box 18 NL-6160 MD Geleen
THE NETHERLANDS
Technical University of Denmark Instituttet for Kemiindustri DK-2800 Lyngby
DENMARK
University of Zurich Winterhurestrasse 190 CH-8057 Zurich
SWITZERLAND
Gulf Technology Europe B.V. Moezelweg 251 NL-3181 LS Rozenburg
THE NETHERLANDS
Chemische Werke Hills D-4370 Marl
W. GERMA-l\lY
Swedish Natl. Dev. Co. Box 34 S-18400 Akersberga
SWEDEN
BruneI University Dept. of Applied Chern. Uxbridge, Middlesex, DBB EPH
U.K.
vept. of Chern. Reaction Eng. CTH, 41296 Gothenburg
SWEDEN
Kon. Shell Res. Lab. Badhuisweg 3, aid HCP NL-1031 CM Amsterdam
THE NETHERLANDS
Akzo-Chemie, Ketjen Catalysts P.O. Box 15 Amsterdam
THE NETHERLANDS
Shell Int. Chemical Co. York Road London SE1 7PG
U.K.
849 SPIERS A.I., Dr
STAAL L.H., Dr.
STEFANI G.
STOEPLER W.
STONE F.S., Prof.
STRINGARO J.P.
SURESH D.D., Dr.
SUVKAMP A.
TAKEZAWA N., Dr.
TENG H., Dr.
THIBAULT C., Dr.
TOWNEND J., Dr.
TRIFIRO F., Prof.
TRUONG T.B.
TUNGLER A., Dr.
Kon. Shell Res. Lab. Badhuisweg 3 NL- 1031 CM Amsterdam
THE NETHERLANDS
unilever Research P.O. Box 114 NL-3130 AC Vlaardingen
THE NETHERLANDS
Alusuisse Italia Div. Ftalital PCS SpA 24020 Scanzorosciate Bergamo
ITALY
Anglestr. 68 D-6112 Gr-Zimmern
W. GERMANY
University of Bath School of Chemistry Bath BA2 7AY
U.K.
Alusuisse R &D CH-8212 Neuhausen
SWITZERLAND
The Standard Oil Co. 4440 Warrensville Ctr. Rd. Warrensville Hts., Ohio 44128
U.S.A.
Akzo-Chemie, Ketjen Catalysts P.O. Box 15 Amsterdam
THE NETHERLANDS
Hokkaido University Dept. of Chern. Proc. Eng. Sapporo 060
JAPAN
Exxon Chern. Co. 4 Pearl Court Allendale, N. J. 07401
U.S.A.
SNEA (P) Centre de Recherches de Lacq BP 34 Lacq 64170 Artix
FRANCE
Fisons Holmes 'London Crewe,
U.K.
Pharmaceutical Div. Chapel Road Cheshire CW4 8BE
Faculty of Indust. Chern. Viale Risorgimento 4 1-40136 Bologna
ITALY
Alusuisse R &D CH-Neuhausen 8212
SWITZERLAND
Hungarian Academy of Sciences Javorka Adamzu 61 H 1147 H-Budapest XIV
HUNGARY
850 TURLIER P., Dr.
TWIGG M.G., Dr.
ULRICH B., Dr.
UNGER K., Prof.
VACCARI A., Prof.
VANDEN EYNDE I.
CNRS-Institut de Catalyse 2 avo A. Einstein F-69626 Villeurbanne Cedex
FRANCE
ICI-Agricultural Div. P.O.Box 1 Billingham, Cleveland T23 1LB
U.K.
BASF AG D-Zakir, 11 301 D-6700 LUdwigshafen
W. GERMANY
Johannes Gutenberg Universitat Institut fur Anorg. Chern. und Anal. Chern. D-6500 Mainz
W. GERMA.l\jY
Faculty of Indust. Chern. Viale Risorgirnento 4 1-40136 Bologna
ITALY
UCB SA rue d'Anderlecht 33 B-1620 Drogenbos
BELGIUM
VAN DER VLEUGEL D.G., Dr. Esso Chemie BV Botlekweg NL-3197 KA Rotterdam-Botlek. VAN DER WAL W.J.J.
VAN DER KRUK A.
VAN HARDEVELD R., Dr.
VAN HENGSTUM T.
VAN NORDSTRAND R.
VAN OMMEN J. G., Dr.
VAN REIJEN L.L., Prof.
THE NETHERLANDS
State University of Utrecht Croesestraat 77A NL-3522 AD Utrecht
THE NETHERLANDS
Catalyst Recovery 420 Route de Longwy Rodange
LUXEMBURG
DSM Central Res. Lab. P.O. Box 18 NL-6160 MD Geleen
THE NETHERLANDS
Twente Univ. of Technology Dept. of Chern. Techn. P.O. Box 217 NL-Enschede
THE NETHERLANDS
Chevron Research Co. 576 Standard Avenue Richmond, Ca. 94802
U.S.A.
Twente University of Technology Dept. of Chern. Techn. P.O.Box 217 NL-Enschede
THE NETHERLANDS
T.H. Delft Lab. voor Anorg. en Fys. Chemie 136 Julianalaan Delft
THE NETHERLANDS
851 VAN REISEN C., Dr.
VAN SINT FIET T., Dr.
VASUDEVAN S.
VERSLUIS F.
VIC BELLON S., Dr.
VILJANEN J.
VIVILLE L.
VOLMER A.
VOLPE L.
WALL B.R., Dr.
WALSH P.T., Dr.
WANG HONGLI, Prof.
WARD J. W., Dr.
WEISS M., Dr.
WEYLAND F.
WIELERS A.F.H.
Akzo Zout Chemie Nederland BV P.O. Box 25 NL-7550 GC Hengelo (0)
THE NETHERLlINDS
Dow Chemical Herbert H. Dowweg Terneuzen
THE NETHERLANDS
c/o. Dr. S. Parthasarathy L-1 Green Park Extn. New Delhi- 110016
INDIA
Harshaw Chemie BV Strijkviertel 67 NL-3454 PK De Meern
THE NETHERLANDS
Centro Inv. de Enpetrol C. Embajadores 183 Madrid 5
SPAIN
Neste Oy Research Center SF-06850 Kulloo
FINLAND
BP Chemicals Belgium Postbus 30 B-2050 Antwerpen
BELGIUM
Akzo-Chemie, Ketjen Catalysts P.O. Box 15 Amsterdam
THE NETHERLANDS
Stanford University Dept. of Chem. Eng. Stanford, Ca. 94305
U.S.A.
Crosfield Chemicals warrington WA5 lAB
U.K.
Health & Safety Executive Red Hill Sheffield s3 7HQ
U.K.
Dalian Institute of Chem. Phys. Dalian
P.R. CHINA
Union Oil Co. of California P.O. Box 76 Brea, Ca. 92621
U.S.A.
57th Mivza Nahshon Beer-Sheva
ISRAEL
Universitat Yarlsruhe Gottesauerstr. 12 D-75 Karlsruhe
W. GERMANY
State University Utrecht Croesestraat 77A NL-3522 AD Utrecht
THE NETHERLANDS
852 WIESENHAAN H., Dr.
WILHELM F.
WILKINSON N.P., Dr.
WRIGHT C.J., Dr.
WUNDER F.A., Dr.
XU HUI-ZHEN, Prof.
YOUNG D.A.
YU ZHIYIN
ZABALA J.M., Dr.
ZHAO JIUSHENG
Unilever NV, Patent Div. P.O. Box 137 NL-3130 AC Vlaardinger
THE NETHERLANDS
Air Products P.O. Box 427 Marcus Hook, Pa 19061
U.S.A.
B.P. Research Centre Chertsey Road Sunbury, Middlesex
U.K.
Aere Harwell Bldg. 521 Didcot, oxon
U.K.
Hoechst AG (D 569) Postfach 800320 D-6230 Frankfurt (M) 80
W. GERMANY
Lanchow Inst. of Chern. Phys. Academia Sinica Lanchow
P.R. CHINA
Union Oil Co. of California P.O. Box 76 Brea, Ca. 92621
U.S.A.
Petroleum Corporation Beijing
P.R. CHINA
Diputaci6n Foral de Navarra Dir. de Industria San Ignacio 1, Pamplona
SPAIN
Tianjin University China Chemical Engineering Dept. Tianjin
P.R. CHINA
853 AUTHOR INDEX Aitchison, D.W. Andres, M. 675 Aris, R. 35
323
323 Badilla-Ohlbaum, R. Baerns, M. 821 Baiker, A. 685 Baronetti, G.T. 47 Benvenuto, E.R. 47 767 Bertolacini, R.J. Berton, A. • 431 Blanchard, F. 395 Blanco, M.N. 333 Bock, O. 709 Bogyay, I. 451 Boitiaux, J.P. 123 Borgarello, E. 135 Bossi, A. 181, 735 Boudart, M. 147 Bournonville, J.P. 81 Burch, R. 311, 787 Butler, G. 159 Caceres, C.V. 333 Camia, M. 431 Castro, A.A. 47 Chadwick, D. 323 Charcosset, H. 675 Chiche, P. 675 Clausen, B. S. 385 Coenen, J.W.E. 801 Cosyns, J. 123, 463 Courty, Ph. 485 Covini, R. 579 Damyanov, D.P. 101 D'Angeli, C. 579 Day, M.A. 787 De Jong, K.P. 111 Del Piero, G. 723 De Miguel, S.R. 47 Derouane, E.G. 193, 385 Dexpert, H. 463 Djega-Mariadassou, G. 675 Doesburg, E.B.M. 301 Duonghong, D. 135 Elofson, R.M.
409
Fenelonov, V.A. 665 Ferraz, M.C.A. 571 Fierro, J.L.G. 747 Figueiredo, J.L. 571 Flambard, A. R. 311, 787 Franck, J.P. 81 Freund, E. 463
Gadallah, F.F. 409 Galassi, C. 543 Garbassi, F. 735 Gavrilov, V.Yu. 665 Genti, G. 543 Gentry, S.J. 203 Gerritsen, L.A. 369 Geus, J. W. 1, 111 Gherardi, P. 72 3 Girelli, A. 359 Glinski, M. 553 Gob616s, S. 473 Gourgue, A. 193 Gratzel, M. 135 Gubitosa, G. 431 Guczi, L. 451 Gui Lin-Lin 563 Gutierrez, J.M. 747 Haase, R. 213 Hanika, J. 69 Haruta, M. 225 Hattori, T. 531 Hegedus, M. 473 Highfield, J. G. 181 Hoflund, G.B. 91 Hofmann, H. 821 Hofstadt, C.E. 709 Houalla, M. 273 Iannibello, A. 359 213 Illgen, U. 531 Inomata, M. 69, 653 Janacek, L. 653 Jiratova, K. 675 Joly, J.P. 323 Josefsson, L. Kagon, J.H. 619 Kassabova, N.A. 757 Kehl, W.L. 169 Kijenski, J. 553 Kitchener, I.J. 237 Kiwi, J. 135 Klissurski, D.G. 421 Kobayashi, H. 697 Kochloefl, K. 709 Komodromos, C. 237 Kotter, t~. 521 Krylova, A.V. 441 Kunz, J. 69 Langner, H.E. 619 Lapena, A. 747 Lee, S. Y. 35
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