Studies in Surface Science and Catalysis 30 CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL
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Studies in Surface Science and Catalysis Advisory Editors: B. Delman and J.T. Yates
Vol. 30
CATALYSIS AND AUTOMOTIVE POLLUTION CONTROL Proceedings of the First International Symposium (CAPOC I), Brussels, September 8-11, 1986
Ed itors
A. Crucq and A. Frennet Unite de Recherche sur la Catalyse, Universite libre de Bruxelles, Brussels, Belgium
ELSEVIER
Amsterdam - Oxford - New York - Tokyo 1987
ELSEVIERSCIENCEPUBLISHERS B. V Sara Burgerhartstraat 25 P.O Box 211, 1000 AE Amsterdam, The Netherlands Distriburors for the United States and Canada.
ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, NY 10017, U.S.A.
ISBN 0-444-42778-3 (Vol. 30) ISBN 0-444-41801-6 (Series)
© Elsevier Science Publishers B.V., 1987 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 B.V./ Science & Technology Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CeCL Salem, Massachusetts. Information can be obtained from the cee about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified. Printed in The Netherlands
CONTENTS
- Studies in Surface Science and Catalysis (other volumes in the series)
IX
- Foreword
Xl
- Acknowledgements
XII
- Financial Support
XIII
- List of Participants
XIV
- Scientific Papers General introduction to the problem of exhaust gas pollution - "Effect ofMotor Vehicle Pollutants on Health" , M. Chiron - "AutomotiveTraffic. Risksforthe Environment", R. Impens
11
- "Catalysis in Modern Petroleum Refining", J. Grootjans
31
- "The Point ofView ofthe AutomobileIndustry. Prevention is better than cure", C.Gerryn
39
- "Control ofDiesel Particulate Emissions in Europe", M.P. Walsh
51
- "The Problems involvedin Preparing and Upholding Uniform Exhaust-Gas Standards within the Common Market", H. Henssler
_
- "The Marketfor Car Exhaust Catalysts in Western Europe. A ReviewofTrends and Developments", W. Groenendaal
69
81
General introduction to the role of catalysis in exhaust gas control - "Automobile Catalytic Converters", K.C. Taylor (General lecture) - "Aspects ofAutomotiveCatalyst Preparation, Performance and Durability", BJ. Cooper, W.D.J. Evans and B. Harrison (General lecture)
.. ~ _
--~
97
117
VI
Reaction Mechanisms and Surface States - "Titrations ofCarbon Monoxide and Oxygen on a Platinum on Silica Catalyst", CO. Bennett, L.M. Laporta and M.B. Cutlip _ ~ ~ _ _ _
143
- "The AlF Window with Three-Way Catalyst. Kinetic and Surface Investigations", E.KobersteinandG. Wannemacher _ ~ _ . _ _ _ _ _ _ _ _
155
- "Elemental Steps during the Catalytic Decomposition ofNO over Stepped Single Crystal Surfaces ofPt and Ru", N. Kruse and J.H. Block ~______
173
- "Periodic Operation Effects on AutomotiveNoble Metal Catalysts. Reaction Analysis ofBinary Gas Systems", H. Shinjoh, H. Muraki and Y. Fujitani
187
- "The Role ofResearch in the Development ofNew Generation AutomotiveCatalysts", H.S. Gandhi and M. Shelef (Extended paper) - - - - - - - -
199
- "Mechanisms ofthe Carbon Monoxide Oxidation and Nitric Oxide Reduction Reactions over Single Crystal and Supported Rhodium Catalysts: High Pressure Rates Explained using Ultrahigh Vacuum Surface Science", G.B. Fischer, Se H. Oh, J.E. Carpenter, cr, DiMaggio, SJ. Schmieg, D.W. Goodman, T.W. Root, S.B. Schwartz and L.D. Schmidt (Extended paper) 215 - "Electronic State of Cerium-Based Catalysts Studied by Spectroscopic Methods (XPS, XAS)", F.Le Normand, P.Bemhardt, L.Hilaire, K.Kili, G.Krill and G.Maire - "An AESInvestigation ofthe Reactivity ofPt, Rh and Various Pt-Rh AlloySurfaces towards 02> NO, CO and H 2 " , F.e.M.J. M.Van Delft, G.H. Vurens, M.e. Angevaare-Gruter and B.E. Nieuwenhuys
__ 221
__ 229
- "Reactivity Studies ofAutomobileExhaust Catalysts in Presence ofOxidising or Reducing Conditions", G. Meunier, F. Garin, l.L. Schmitt, G. Maire and R. R o c h e - 243 - "The Effect ofWeight Loading and Reduction Temperature on Rh/Silica Catalysts for NO Reduction by CO", W.e. Hecker and R.B. Breneman
---- 257
- "Reactivation ofLead-Poisoned Pt/ Al20J Catalysts by Sulfur Dioxide", l.W.A. Sachtler, I. Onal and R.E. Marinangeli -- ---- 267
Support - "Alumina Carriers for AutomotivePollution Control", P. Nortier and M. Soustelle (General lecture) _ ~ ~ _
275
VII
- "Advances in AutomotiveCatalysts Supports", John S. Howitt - - -
30 I
- "Structural Consideration with respect to the Thermal Stability ofa New Platinum Supported Lanthanum-Alumina Catalyst", F. Oudet, E. Bordes, P. Courtine, G. Maxant, e. Lambert and J.P. Guerlet--
313
- "Influence ofthe Porous Structure ofAlumina Pellets and the Internal Convective Flow on the Effective DiffusivityofExhaust Gas Catalyst", S. Cheng, A. Zoulalian and J.P. Brunelle
323
- "The Effect ofthe Chemical Nature ofthe Wash-Coat on the Catalytic Performance of co Oxydation Catalysts ofMonolith type", L.B. Larsson, L.O. Lowendahl and J.E. Otterstedt
333
Metal-Support Interaction - "The Promotion of PtlSi02 Catalysts by W03 for the NO-CO Reaction", J.R. Regalbuto and E.E. Wolf
__ - 345
- "Surface Diffusion ofOxygen in RhlAl203 and PtlAl203 Catalysts", H. Abderrahim and D. Duprez ----~--- ---
359
- "Rhodium-Support Interactions in AutomotiveExhaust Catalysts", cz, Wan and J.e. Dettling
369
Base Metal Catalysts - "Development ofa Copper Chromite Catalyst for Carbon Monoxide AutomobileEmission Control", J. Laine, A. Albomoz, J. Brito, O. Carias, G. Castro, F. Severino and D. Valera
387
- "Development ofNon-Noble Metal Catalysts for the Purification of AutomotiveExhaust Gas", Lin Peiyan, Wang Min, Shan Shaochun, Huang Minmin, Rong Jingfang, Yu Shomin, Yang Heng Xiang and Wang Qiwu 395 - "Improving the S02 Resistance ofPerovskite Type Oxidation Catalyst", Li Wan, Huang Qing, Zhang Wan-Jing, Lin Bing-Xiung and Lu Guang-Lie - "Tungsten Carbide and Tungsten-MolybdenumCarbides as AutomobileExhaust Catalysts", L. Leclercq, M. Prigent, F. Daubrege, L. Gengembre and G. Leclercq
- 405
A17
VIII
Practical Studies - "Dynamic Behavior ofAutomotiveThree- Way Emission Control Systems", R. K. Herz (Extended paper) _ _ _ _ _ _ _ _ _ _ _ _ - "Effect ofLead on Vehicle Catalyst Systemsin the European Environment", M. Kilpin, A. Deakin and H.S. Gandhi
~
427
- - 445
- "ALaboratory Methodfor Determining the Activityof Diesel Particulate Combustion Catalysts", R.E. Marinangeli, E.H. Homeier and ES. Molinaro -
457
Fuels and Additives - "Synthesis ofHigher Alcohols on Low-Temperature Methanol Catalysts", G. Fomasari, S. Gusi, T.M.G. La Torretta, E Trifiro' and A. Vaccari - "An AlkeneIsomerization Catalyst for Motor Fuel Synthesis", E.G. Baker and N.J. Clark
469 .
483
IX STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Unlversite Catholique de Louvain, Louvain-Ia-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1
Volume 2
Volume 3
Volume 4
Volume 5
Volume 6
Volume 7
Volume 8 Volume 9
Volume 10
Volume 11
Volume 12
Volume 13 Volume 14
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 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 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 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photograph ic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon 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 Catalyst Deactivation. Proceedings of the International Symposium, Antwerp, October 13-15, 1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4,1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of the Symposium held in Bechyne, September 29-0ctober 3, 1980 edited by M. Laznicka Adsorption at the Gas-5olid and Liquid-5olid Interface. Proceedings of an International Symposium held in Alx-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metai·Additive Effects in Catalysis. 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 (Lvonl.Beprember 14-16, 1982 edited by B. Imelik, C. Naccache, G. Couduriar, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24,1982 edited by P.A. Jacobs, N.!. Jaeger, P. Jir(l and G. Schulz·Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
x Volume 15 Volume 16
Volume 17
Volume 18
Volume 19
Volume 20
Volume 21
Volume 22 Volume 23 Volume 24
Volume 25
Volume 26
Volume 27 Volume 28
Volume 29 Volume 30
Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets 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 Spillover of Adsorbed Species. Proceedings of the International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-0ctober 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. 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, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, V. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Brunei University, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Vu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of the International Symposium, Portoroz-Pcrtorose, September 3-8, 1984 edited by B. Drzaj, S. HoCevar and S. Pejovnik Catalytic Polymerization of OIefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6,1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalvtic Hvdrogenation edited by L. Cerveny New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by V. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis. edited by B.C. Gates, L. Guczi and H. Knozinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium (CAPaC I), Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet
XI
FOREWORD
In June 1984 the EEC Commission proposed new standards of permissible exhaust gas from motor vehicles to be introduced in Europe; these standards were approved by the Ministers of the Environment one year later. As the control of automotive pollution is at present mainly a catalytic problem, we thought this was a good opportunity to organize an International Symposium on the subject and an organizing committee composed of people engaged in catalytic research in the different Belgian Universities was constituted. As the symposium was the first one to be organized at international level in this otherwise very restricted scientific field, this decision may have initially appeared somewhat risky, but was justified by the success of the four-day symposium, with 177 people attending. Most participants came from the EEe countries, with large delegations from Belgium (33), France (32), West Germany (26), the United Kingdom (16) and the Netherlands (10) but we must note the size of the U.S. (20) and Swedish (10) delegations and the interest shown by people coming from Australia, China, Finland, Hungary, Japan, Switzerland and Venezuela. About 60% of the participants came from industry, mainly from the car and oil industries and catalyst manufacturers. The number of abstracts submitted was not very large (38) but as noted by the Paper Selection Committee and as the reader of the Proceedings will be able to judge for himself, the quality and the scientific interest of the papers presented are exceptional, and this was also true of the discussions following the presentation; unfortunately these discussions are not published. The introduction of the new EEC standards raised some controversy in the industries concerned as well as in public opinion. That is why the organizers chose to devote the first day of the conference to a general introduction to the problem of pollution by exhaust gas. Seven invited lectures were presented and are published in these Proceedings, dealing with the effects of exhaust gas on human health and the environment, with the economical and legislative problems associated with the new EEC standards, and with the points of view of the oil and motor industries. The first day ended with a round table, with the participation of W.D.J. Evans, C. Gerryn, W. Groenendaal, H.Henssler, K. Taylor and M. Walsh; the ensuing general discussion, which is unfortunately not published, was very stimulating. The topics to be dealt with during the catalytic sessions included not only the catalytic converters, but also such problems as specific pollution control of diesel engines, synthesis of adequate fuels, and additives adapted to catalytic converters. Surprisingly, very few papers (3) were submitted and presented on these subjects, whereas 24 papers were devoted to fundamental and applied studies on catalytic converters, support preparation and base metal catalysts. Finally the organizers have been strongly encouraged by many participants to hold a follow-up symposium in a not-too-short delay of 2 to 3 years. We hope the CAPOC II Conference will generate the same interest as CAPOC I, the Proceedings of which are contained in this volume.
XII
ACKNOWLEDGEMENTS The Organizing Committee is greatly indebted to Mr Ducarme, "Ministre de l'Environnement de l'Executif Regional Wallen", for his support and interest to this symposium and who accepted to give the opening address. The organizers also greatly appreciated the cooperation of the members of the organizing committee. In this respect, we are particularly grateful to W. Hecq, E. Cadron, M. Campinne and E. Derouane for the active part they have taken in the organization. The suggestions and advices of A. Derouane, G. Froment, A. Germain, G. Poncelet were very helpful. Special thanks are due to the members of the paper selection committee for their important contribution in selecting the proposed papers with conscientiousness (W.DJ. Evans, G. Leclercq, G. Maire, A. Pentenero, V. Ponec, M. Prigent). The Organizing Committee is indebted to all the authors of the lectures delivered during the introductory session who analyzed various points of view related to the general problem of pollution by motor vehicles exhaust gases : health, environment, economics. It is a pleasure to acknowledge the stimulating action of C. Gerryn as well in the organization of the symposium as in the introductory session. We also are grateful to K. Taylor for her outstanding general introductory lecture on the problem of exhaust catalysts. Special thanks to W.DJ. Evans for his active part in the paper selection committee and the scientific advisory board and who gave a remarkable general lecture on the exhaust catalyst. The Organizing Committee acknowledges the authors who presented papers, the Chairmen and all the participants who made the symposium fruitful. The Organizing Committee wants to associate with these acknowledgements the members of the "Unite de Recherche sur la Catalyse" of the "Universite Libre de Bruxelles" who contributed in various degrees to the success of this symposium: J.-M.Bastin, M.Cogniaux, L.Degols, J.-P.Demiddeleer, P.Moisin, B.Parmentier, G. Thiry, M.-N. Zauwen. We are indebted to the authorities of the "Universite Libre de Bruxelles'' who agreed that this meeting could be held in the facilities of the "Institut de Sociologie". The organizers,
AFRENNET Chairman of the Organizing Committee
ACRUCQ Secretary of the Organizing Committee
XIII
THE ORGANIZING COMMITTEE ACKNOWLEDGES THE FINANCIAL SUPPORT OF :
Minlstere de I'Environnement de l'Executif Regional Wallon Federation BeIge des Industries de l'AutomobiIe et du Cycle (FEBlAC)
Solvay & Cie S.A. Societe Chimique de Belgique Banque Bruxelles Lambert
XIV
LIST OF PARTICIPANTS
A.
FULL CONGRESS Andersson, Lennart
Univ. Chalmers Goteborg Sweden
Andersson, Soren
EKANobelAB Sweden
Ashworth, Richard
T.!. Cheswick Silencers United Kingdom
Baker, RG.
Univ. Flinders Australia
Baresel, D.
Rob. Bosch West Germany
Bauwens, Jean
Cockerill Materials Ind. Belgium
Bennett, C.O.
Univ. Connecticut
U.S.A. Berndt, Malte
Doduco K.G. West Germany
Blanchard, G.
Rhone- Poulenc France
Block, Jochen
Fritz Haber Inst. West Germany
Bordes, Elisabeth
Univ. Compiegne France
Boulhol, Olivier
Ag. Qual. Air France
Boulinguiez (Mrs)
Elf France
Bradt, Willy
Clayton Belgium
Brandt, Gerhard
Ethyl Mineral Additives West Germany
xv Cairns, J.
UKAEA Harwell United Kingdom
Campinne, M.
Ecole Royale Militaire, Brussels Belgium
Chapelet Letourneux, Gilbert
ElfSolaize France
Cheng San
Univ. Compiegne France
Chiron, Mireille
INRETS France
Colbourne, D.
Shell West Germany
Collette, Herve
FNDP, Namur Belgium
Cooper, Barry 1.
J ohoson Matthey USA
Courtine, Pierre
Univ. Compiegne France
Crucq, Andre
ULB, Brussels Belgium
Darville
FNDP, Namur Belgium
Davies, MJ.
UKAEA Harwell United Kingdom
Deakin, Alan
Ford United Kingdom
Degols,Luc
ULB, Brussels Belgium
Delmon, Bernard
UCL, Louvain La Neuve Belgium
Dettling,1.e.
Engelhard USA
XVI
Donnelly, Richard G.
W.R. Grace & Co USA
Douglas. J.M.K.
Johnson Matthey United Kingdom
Doziere, Richard
IFP France
Druart, Guy
Soc. Bel. Gaz Petrole Belgium
Dubas, Henri
Ciba-Geigy Switzerland
Duprez,D.
Univ. Poitiers France
Durand. Daniel
IFP France
Engler
Degussa West Germany
Evans, W.DJ.
Johnson Matthey United Kingdom
Finck, Francois
Univ. L. Pasteur, Strasbourg France
Fisher Galen B.
General Motors USA
Fitch, Frank
Laporte Inorganics United Kingdom
Fitoussi
Rhone Poulenc France
Foster, Al
BP United Kingdom
Fougere
UTAC France
Frennet, Alfred
ULB, Brussels Belgium
XVII
Frestad, Arne
EKANobelAB Sweden
Froment, G.
Univ. Gent Belgium
GandhiH.S.
Ford USA
Garin, F.
Univ. L. Pasteur, Strasbourg France
Garreau
Rhone-Poulenc France
GermainA.
Univ. Liege Belgium
Gerryn, Claude
Ford Belgium
Girard, Philippe
ElfSolaise France
Gonzalez-Velasco, Juan R.
Univ. Pais Vasco Bilbao Spain
Gottberg, Ingemar
Volvo Sweden
Gould David, G.
Ford United Kingdom
Groenendaal, Willem
Strategic Analysis Europe The Netherlands
Grootjans, J.
Labofina Belgium
Haas, Jurgen
Dornier West Germany
Hammer, Hans
Brennstoffchemie West Germany
Harrison, Brian
Johnson Matthey United Kingdom
XVIII
Havil
Univ. Paris 6 France
Hawker, P.N.
Johnson Matthey United Kingdom
Hecker, William C.
Univ. Brigham Young, Provo USA
Hecq, Walter
ULB, Brussels Belgium
Hegedus, L. Louis
W.R. Grace & Co USA
Held, Wolfgang
Volkswagen West Germany
Henssler, H.
EEC
Herz, Richard
Univ. California San Diego USA
Hickey, C. (Mrs)
Esso Petroleum United Kingdom
Howitt, John S.
Coming Glass Works USA
Imai, Tamotsu
Signal USA
Impens,R.
Fac. Agronomique, Gembloux Belgium
Ing,Hok
UTAC France
Jacobs, Peter
KUL,Leuven Belgium
Jagel, Kenneth I.
Engelhard USA
Johansen, Keld
Topsee Denmark
XIX
Jourde, Jean-Pierre
Renault France
Joustra, A.H.
Shell The Netherlands
Kaczmarec
Rhone Poulenc France
Kapsteyn, F.
Univ. Amsterdam, The Netherlands
Kilpin, Michael
Ford United Kingdom
Koberstein, E.
Degussa West Germany
Kruger
Hoechst West Germany
Kruse, Norbert
Fritz Haber Institute West Germany
Kuijpers, E.G.M.
VEG The Netherlands
Laine. J.
Inst. Ven. Invest. Cientificas Venezuela
Le Normand, F.
Univ. L. Pasteur, Strasbourg France
Leclercq, Ginette
Univ. Lille France
Leclercq, Lucien
Univ. Lille France
Lehmann, Ulrich
Condea Chemie West Germany
Lester, George R.
Signal USA
Li Wan (Mrs)
Univ. Beijing China
Lienard, Georges
ULB, Brussels Belgium
xx Lin Peyian (Mrs)
Univ. Hefei China
Lowendahl, L.
Univ. Chalmers Goteborg Sweden
Mabilon
IFP France
Maire, G.
Univ. L. Pasteur, Strasbourg France
Maret, Dominique
Peugeot France
Marinangeli, Richard E.
Signal USA
MarseII, Lars
Saab-Scania AB Sweden
Mathieu, Veronique
FNDP,Namur Belgium
Maxant, Genevieve (Mrs)
Comptoir Lyon Alemand Louyot France
Merian, Ernest
Journalist Chemosphere/IAEACISAGUF Switzerland
Mesters.C,
Shell The Netherlands
Meunier, Guillaume
Univ. L. Pasteur, Strasbourg France
Moles,P.J.
Magnesium Elektron United Kingdom
Mottier, Michel Henri
Consultant Switzerland
Murphy, Michael
General Motors Eur. Techn. Center G.D. Luxembourg
Naudin, Thierry
Peugeot France
XXI
Niemantsverdriet, J.W.
Fritz Haber Institute West Germany
Nieuwenhuys, B.E.
Univ, Leiden The Netherlands
Nortier, P.
Rhone-Poulenc France
Odenbrand, I.
Univ. Lund Sweden
Otterstedt, I.A.
Univ. Chalmers, Goteborg Sweden
Oudet, Francois
Univ, Compiegne France
Pentenero, Andre
Dniy. Nancy France
Pernicone, Nicolas
Institute G. Donegani Italy
Poncelet, G.
DCL, Louvain La Neuve Belgium
Ponec, V.
Dniy. Leiden The Netherlands
Praliaud, Helene (Mrs)
IRC, Villeurbanne France
Prigent, Michel
IFP France
Questiaux, Daniel
Labofina Belgium
Rinckel, Francis
Peugeot France
Roche.Rene
PSA-ER France
Salanne, Simo
KemiraOy Finland
Schay, Zoltan
Inst. Isotopes, Budapest Hungary
XXII
Schwaller
Univ. L. Pasteur, Strasbourg France
Seip, Ulrike (Mrs)
MAN West Germany
Senamaud, Jean Michel
Renault France
Shelef, Mordecai
Ford USA
Shinjoh, H.
Toyota Japan
Singoredjo, L.
Univ. Amsterdam, The Netherlands
Skoldheden, Per
Volvo Sweden
Slater, Hawes
AC Spark Plug USA
Smailes, R.
UKAEA Harwell United Kingdom
Soustelle, M.
Ecole des Mines, St Etienne France
Sposini, Mario
Ecofuel Italy
Stohr,H.
Grace GmbH West Germany
Tauzin
PSA-ER France
Taylor,
x.c
General Motors USA
Tsuchitani, Kazuo
Shokubai Kagaku Japan
Tuenter,G.
Neth. Energy Res. Found. The Netherlands
Umehara,K.
NGKEurope West Germany
XXIII
B.
Vaccari, Angelo
Univ. Bologna Italy
Van Delft, F.C.MJ.M.
Univ. Leiden The Netherlands
Vandervoort, Philippe
Toyota Motor Corp. Belgium
Virta Pirrko (Mrs)
KemiraOy Finland
Walsh Michael P.
Consultant USA
Wan, C.Z.
Engelhard USA
Weber, Kurt H.
Volvo Sweden
Wolf, Eduardo
Univ. Notre Dame USA
Wolsing, Wilhelm
Engelhard Kali Chemie Autocat. West Germany
Yamazaki Takayuki
Nissan Motor Co Ltd Belgium
Zhao, Jiusheng
Univ. Tianjin China
Zink, Uwe
Coming Keramik West Germany
1ST DAY INTRODUCTORY SESSION ONLY Crate
Volvo Car Corporation Belgium
De Nil, A.
Analis Belgium
Jensen, Bent
CEFIC Belgium
Luck, Lucien
General Motors Continental Belgium
XXIV
Machej
UCL, Louvain-La-Neuve Belgium
MacKinley
EEC
Norcross, Geoffrey
Intern. Prof. Assoc. Envir. Affairs Belgium
Rasson, Andre
Austin Rover Distribution Belgium
WiIlems,H.
Johnson Matthey Belgium
Evans,P.W.
Molycorp SARL France
Yonehara Kiyoshi
Nippon Shokubai Kagaku Co. Japan
Searles R.A
Johnson Matthey Chemicals, Div. Autocatalysts United Kingdom
Maegerlein
Degussa AG Dpt AC/GKA West Germany
Brunoli, Joseph A
Signal Automotive Products Norplex Europa West Germany
Hulsmann
Ford Werke AG. West Germany
Maegerlein
Degussa AG Dpt AC/GKA West Germany
Ogata,Hideo
Mitsubishi Motor Corp. West Germany
Schneider, Dietrich
Ford Werke AG. West Germany
von Salmuth, H.D.
Ford Werke A.G. West Germany
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
1
EFFECTS OF MOTOR VEHICLE POLLUTANTS ON HEALTH M.CHIRON INRETS,I09
Av.Salvadn~
Allende BP 75,69b72 BRON Cedex France
INTRODUCTION
The characteristic feature of pollution due to road traffic is its wide sp:eading such that the whole population is affected, including children,
invalids, old people and pregnant women. On the
other hand,the durations
within wide
li~its.
Thus
the
of exposure may
vary
traffic can be continuous
in
some areas and very intermittent in others while the displacement of people can vary to a great extent.
The pollutants can also be prevented from dispersing
because of local configurations or unfavourable weather conditions.
Further-
more it should be noted how certain pollutants can accumulate in the body in the absence of the long periods free from exposure that are required for them to be eliminated and how it is impossible to protect people suffering from some particular sensitivity or illness from the effects of pollution. All this must be borne in mind when considering the effects of motor vehicle pollutants on health. There is also the obvious difference between the evaluation of the effect of a pollutant dispersed in the environment as a whole and one that is dispersed in an industrial area where both the level of pollution and the duration of exposure are known, where the total duration of exposure cannot in any case exceed 45 years and where an individual can be withdrawn from the risk at any time. For pollutants in the form of a gas the dispersion is very rapid for the usual weather conditions and the exposure decreases with distance from the vehicle exhaust systems.
Thus the people exposed to the greatest levels of
pollution are first of all the drivers of the motor vehicles, then those making use of two-wheeled vehicles and finally the pedestrians. Pollutants in the form of particles on the other hand settle very quickly and the level of atmospheric pollution falls very rapidly on moving away from the vehicles.
However the particles land on the ground and in water and can
accordingly find their way into food, this giving rise to pollution at a distance which can even affect people living in country areas. CARBON MONOXIDE This is the pollutant for which the effects on the human organism are the most well understood.
2
The carbon monoxide in the atmosphere originates to a large extent from motor vehicles and is almost completely due to them in the vicinity of streets. In some
very polluted and poorly ventilated areas carbon monoxide
concentrations of 50 to 100 ppm can persist for several hours and the individuals that are obliged to remain in such areas because of their work are exposed to high levels of pollution solely because of motor vehicle traffic. It can be assumed that daily averages of 30 ppm apply for an individual travelling
by
car in town and exceptionally of 80 ppm for someone
standing
at a heavily polluted point (not taking _into account the inside of a tunnel). The action in the human organism is well understood:
the carbon monoxide
replaces the oxygen on attaching itself to the normal haemoglobin.
Thus it
inhibits the normal respiratory function of the haemoglobin which is to transport the oxygen contained in the air to the body tissues. The affinity of carbon monoxide for haemoglobin is 250 times greater than that of oxygen.
A permanent balance is established between the carbon monoxide
in the atmosphere and that in the blood;
there is no accumulation in the
organism and the carbon monoxide is completely rejected on expiring air when the atmospheric concentration is zero.
The speed of attachment or rejection
of the carbon monoxide depends in particular on the level of pulmonary ventilation.
Curves have been produced showing how the concentration of carbon
monoxide in the blood (in terms of the proportion of carboxyhaemoglobin) varies with that in the atmosphere, the duration of
eA~osure
and the pulmonary
ventilation (curves produced on referring to Coburn and Forster's equation). See Ref.l and figures
1
&2
3
The consequences of hypoxia (reduction in the transport of oxygen to the tissues) can be classified into three different categories: a)
For fairly high concentrations of carbon monoxide (greater than 50 ppm)
persisting for several hours, functional but unspecific disorders can be observed, mainly headaches, asthenia, giddiness and nausea. b)
For lower concentrations, of the order of those normally experienced by
town dwellers, the hypoxia can be sufficient to give rise to an hypoxia attack in the case of subjects already suffering from ischaemic arteriopathy.
These
subjects cannot compensate for the reduction in the carriage of oxygen by an increased flow of air. distal region.
Such attacks can occur in the coronary, cerebral or
A critical level of 2.5 per cent of carboxyhaemoglobin has been
established by the W.H.O. for this type of attack, corresponding to a long duration carbon monoxide concentration of about 13 ppm. c)
The third effect, again in the case of low carbon monoxide concentrations,
is to accelerate the formation of atheroma plaques corresponding to a premature ageing of the arteries.
It has not been possible to define a limiting concen-
tration for this effect since the accumulation of cholesterol in the arteries falls when the supply of oxygen is greater than normal.
Thus any increase in
the supply of oxygen is beneficial. NITROGEN OXIDES , OZONE AND OXIDIZING PHOTOCHEMICAL DERIVATIVES The nitrogen
oxides concentrations in towns can amount to about 1 ppm
during peak traffic hours.
Under the action of solar radiation the N02
dissociates into NO and atomic oxygen which gives rise to the formation of ozone 03'
The organic molecules react with the ozone
to form free
radicals which in turn act as a catalyst for the oxidation of the NO and the hydrocarbons.
Thus the irradiated exhaust gases are "biologically more active",
that is to say the total oxidising power is increased as well as the concentration of irritant aldehydes. The nitrogen
oxides
together with the photo-oxidising fog, the action
of which is similar to that of the ozone as the pulmonary aveola are concerned.
,act as irritating agents so far The active surface agent is oxidised
and there is an inflammatory reaction. A certain adaptation of the organism has been observed in the case of short duration exposures. The oxidising agents favour the onset of pulmonary infections and the induction of respiratory allergies. For people in good health, the results of epidemiological studies have indicated that the average concentration of N02 over a 24 hour period should not exceed 0.05 ppm.
.... HbC a
.s f t t Lng
b ,walking c ,working
50 ppn
- - - -0.08
~
~---===
0.06
-
'-------~
---
'- ...
~ "W"
0.04 10 pf'T1
10
2
FIG.l
HbeO-for a
male,versus
athmos~heric
pulmonary ventilation. (Ref. 2)
CO,duration
of exposure,
11
12
t
(hours)
co
HbCO
ppm
v..
tue
r 80
- -,
wed
- -
o thu
"ri
sat
I - - I - - I -- I
ambientCO
sun
mon
I
\: ~~~p
o.
smoker 70
60
50
40
FIG. 2 : HbCO for a saleswoman,frolll actual CO contents on her workplace (Ref.2)
0'
6
It should be noted however that in the case of more sensitive individuals, particularly those suffering of asUuna,this value is to high bu t there is a lack of data foY' the establishment of a more suitable value .
The peak concentrations, given the results of studies for this type of pollution, should amount to 0.25 ppm of ':02 two to three times a week for a period of one hour. HYIJROCARBONS
A large number of hydrocarbon compounds are emitted by the vehicles either as a result of a simple evaporation before combustion or of an incomplete combustion Some studies have been concerned with particular elements or a group of compounds and others with the petrol vapour as a whole. In all cases the studies have revealed evidence of mutagenic or carcinogenic action, eii::her on bacteria,on cell cultures or on living animals The responsible products are mainly benzene and its homologues and the aromatic polycyclic hydrocarbons. For the amounts encountered in the environment it is impossible to quantify the effects of the different carcinogenic agents that are present. The limiting exposure is often expressed in the f'o rm of a maximum amount that may be inhaled during a lifetime, as in the case of radiation.
This
amount is then converted to a maximum acceptable concentration. For example, the maximum amount of a-B.P. (a-Benzo Pyrene) that may be inhaled is 12 to 16 rug corresponding to a maximum acceptable concentration of O.1 5/, g/ m3.
Of the different aromatic hydrocarbons a-B.P. has been the subject of most
studies but is not the most carcinogenic. It should be noted however that the subject of chemical carcinogenesis is
still not well understood and there are multiple interactions between the different pollutants whether they
are of
alimentary, domestic or environ-
mental nature. Just as the combined effects of alcohol and tobacco are much greater than the sum of their individual effects,
it is likely that there are a number of
interactions between carcinogenic chemicals. Thus it does not make much sense to establish limiting values for each chemical given the fact that they have a combined effect. It should also be pointed out here that significant inhalations of hydrocarbons are possible in the vicinity of petrol filling stations.
7 DIESEL EXHAUST PARTICLES These particles when viewed under an electron microscope are in the form of clusters of smaller round sub-particles formed during combustion that subsequently have sticked together. The average diameter of the particles lies between 0.2 to 0.3 microns. They each have a nucleus of pratically pure carbon surrounded by adsorbed hydrocarbons. The particles, due to their small diameter, penetrate deep into the lungs as far as the alveoli. Some 80 per cent of the inhaled particles are retained in the lungs for long, almost indefinite, periods of time. Thus the lungs fill up with "dust". The diesel exhaust particles, as well as the hydrocarbons that are extracted from them, have a mutagenic effect in the laboratory but it has not been possible to quantify this effect as a result of epidemiological studies.
HEAVY METALS (excluding lead) Motor vehicles emit a number of metals: chromium, manganese, barium, vanadium,
iron,
aluminium,
cadmium,
nickel,aso.
However it is difficult to determine the contribution of the motor vehicles to this type of pollution. Many of these metals are toxic as it has been recognised in industrial medicine. In particular cadmium, nickel and chromium are carcinogenic while manganese is toxic so far as the nervous system is concerned. However it is
unlikely that any of these elements have any detec-
table effect when considered separately.
LEAD Lead pollution so far as man is concerned is of purely artificial origin. Lead additives pollute the atmosphere, the ground, water, vegetatim and finally animals and msn. In the vicinity of roads the pollutim, extends for sane hundred of meters. Beymd that distance, the levels are 10 to 30 times less than the levels in urban areas but are nevertheless still mainly due to the transfer over short or long distances of pollutants due to the motor vehicles. The fact that additives are responsible for most of the lead cmtent in the air, in dust and even in most of our food has allowed to estimate, as a result of a study of the intake by the mrren organism, that at least fJJ per cent of the lead in the body comes fran lead alkyls. Other food or food related sources (timed foods, capsules, filters, water pipes) playa much less important role than is generally believed. In areas where the traffic is important the contribution of the motor vehicle can account for
8
80 per cent of the lead in the human body. Lead, at the observed levels of exposure is acting on the proto-
porphyrin of the red corpuscles, whose increase in number is an indication of a restriction on the synthesis of haemoglobin.
Such an increase can be detect-
ed for lead concentrations in the blood as low as about 1 5 ~ g / d l ,
r g/dl or less is considered
a frequently
I
observed value ( a concentration of 35
as
normal)
However this effect, although detectable, cannot be regarded as a pathological one in the absence of any anemia. The most important effect, so far as public health is concerned, is the insidious one on the development of childrens' brains, with particular consequences for their intelligence (in terms of
I~'s)
and behaviour.
It is common for children to ingest lead in a particular way - on raising dirty hands and objects to their mouths likely to be contaminated with high lead content dust in areas where the
traffic is important.
100
90
80
~
70
~
i
'" ~ ~
a......
60
50
40
'" ... ;l:
!c ...
30
~
~
o
20
10
50
60
70
60
90
_
00
=
=
_
VERBAl LO.
fig.1.Cumulative frequency distributions of verbal 10 scores in high and low lead subjects(ref.3)
9
AIJ)EHYIJES These irritate the upper respiratory tracts and eyes.
The aldehyde
content in the exhaust of petrol engined vehicles give rise to concentrations in the atmosphere that are already at the limit established for irritant effects (0.1 ppm). Formaldehyde is classed as a mutagenic substance.
The limiting concen-
tration must accordingly be set very low and this is the emission which is of most concern to the public health specialists when considering the use of alcoholic fuels. ALCOHOLS:
ETHANOL AND METHANOL
Ethanol, when inhaled in the small concentrations in the atmosphere that could arise in the case of the use of partially alcoholised fuels, does not appear to constitute a public health risk. Nethanol on the other hand is very toxic as was recognised quite recently in connection with the adulteration of wines (the ingestion of only a few millilitres can be fatal). lung~
or skin.
Nethanol can penetrate into the organism via the
It accumulates in the body and the maximum acceptable con-
centrations in the absence of periods of non-exposure for the elimination of the poison, is very low (3ppm). The methanol is oxidised within the organism into formaldehyde and then into formic acid and these substances are the real poisons.
Ethanol is destroyed by
the same enzymes thai: a t t.ack the methanol.Thus the presence of ethanol can inhibit the formation of formaldehyde and formic acid and can therefore be regarded as an antidote. Nethanol (and its metabolic waste products) for low rates of exposure can cause irritation and damage to the eyes (optic nerve) while chronic exposure can lead to a permanent decrease in visual acuity. CONCLUSIONS On
considering the possibility of decreasing the emission of pollutants as
a result of catalytic action we can class the substances emitted by motor vehicles into three categories: a)
The concentrations of carbon monoxide, nitrogen monoxide and oxidizing
derivatives are, under normal conditions, at the limit of any detectable effects on health.
An appreciable reduction in the emission of these substances would
result in negligible concentrations for the general public (not counting professional exposures). b)
Lead is not eliminated from the enviTonment nor fTom the human
organism and its insidious action on the development of childrens' brains calls
10
for a cautious approach. Even if lead additives are eliminated, lead will remain in people's blood for a long time, to a large extent as a result of it being already present in the environment and in living beings as a result of previous motor vehicle emissions. c)
In the case of mutagenic or carcinogenic pollutants it is impossible to
establish a safe level of concentration" their combined action. pollution in general.
as we know almost nothing about
Some 80 per cent of cancers have been attributed to
There is probably some cell repairing activity for very
low concentrations but we have no precise knowledge of this.
The best that we
can do in these circumstances is to ensure that the total amount of carcinogenic pollutants in the environment, i.e. of benzene, aromatic polycyclic hydrocarbons, diesel exhaust particles and formaldehyde is kept as low as possible. Coburn R.F. ,Forster R.E.,Kane P.B. ,Considerations of the physiological variables that determine the blood carboxyhemoglobin concentration in man ,J. of clinical invest igat ions: vol 44,11, p , 1899-191 ('-; 1965 2Joumard R.,Chiron M.,Vidon R.,La fixation du monoxyde de carbone sur l'hemoglobine et ses effets sur l'homme,Institut de Recherche des Transports,Bron.France.Oct 1983 3 Needleman B.L.,Leviton L.A.,Bellinger D.,Lead associated intellectual deficit.,New England J.Med.,306:367 ,1982
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Puhl-shers B.V., Amsterdam - Printed in The Netherlands
AUTOMOTIVE TRAFFIC Risks for the Environment by R. IMPENS Departernent de Biologie vegetale, Faculte des Sciences Agronomiques de l'Etat, Gemboux (Belgique)
ABSTRACT Automotive traffic generates a lot of air pollutants, some metallic contaminants and causes troubles, not only for the roadside environment but also for the terrestrial and aquatic ecosystems. The exhaust gases of vehicle's engines contain mainly carbon monoxide and dioxide, nitrogen oxides, a few sulfur dioxide, a great number of hydrocarbons, or organic carbon derivates, and some heavy metals particulates. Some of these compounds are directly toxic for living organisms, when they occur in a closed environment such as inside the car, tunnels, subterranean car parks, or rooms; but they are harmless when emitted in open space, when natural diffusion conditions are sufficient to prevent high concentrations in the air. Other emitted gases will interact with oxidants (e.g. 03) to form new labile compounds, which have a high phytotoxic activity at low concentrations (p.A.N.,and photochemical smogs). These oxidants, obtained by photochemical reactions in the atmosphere, may be involved in the widespread dieback and decline of forests in both Europe and North America. The 03 and photooxidants theory, and its influence on acid deposition, will be shortly presented and discussed. Heavy metals contamination of soil, water and plant materials, near highways is well known, and there's a trend to accelerate the reduction of lead addition in the fuels. The vicinity of heavy traffic roads, is a source for important troubles to terrestrial and aquatic ecosystems. Some examples of these will be discussed for their direct or indirect effects on animal, microbiological or plant lifes. The regular use of deicing salts, essentially sodium and calcium chlorides, in winter period, affects the resistance to drought stress of trees and crops, and increases the sensitivity of plants to parasitic diseases. The compaction of soils near the road is involved in anaerobic conditions near the roots of trees, which will be followed by an important dieback. The risks for environment alterations could be prevented and reduced by clean motors, with a drastic reduction of gaseous pollutants. The lead problem will be progressively resolved by the new European standards of lead addition to fuels; but the lead already present in soils will remain a threat for some sensitive crops and forages. A passive protection of roadside contamination could be obtained by green
11
12
screens, containing resistant and rustic shrubs and trees, which will filter the air and act as efficient sinks for dust and heavy metals particles. Due to aerial long distance transport and photochemical reactions, prevention of damages to forests request more attention. The solution is reduced emissions of the precursors of lethal compounds: clean motors are wanted... Other risks for the roadside environment (chlorides, asphyxic conditions, etc.) are not directly involved with air pollutants emissions: disastrous landscape modifications by speedways construction are more fundamental.
1. INTRODUCTION Automotive traffic generates a lot of gaseous air pollutants,.some metallic contaminants, asbestos, and causes troubles not only to the roadside environment but also for the terrestrial and aquatic ecosystems. Three major pollutions emanate from the highway: smog, noise and dust. Effects of noise have ominous portent for the enjoyment of life by the human race, and are already affecting our health. The exhaust gases of vehicle's engines contain mainly carbon monoxide (CO) and dioxide (Cod, nitrogen oxides (NO,), a great number of hydrocarbons (HC), or organic carbonaceous derivates, a few sulphur dioxide, particles and soot (Table 1).
Table 1 Average exhaust gas composition of an Otto test engine Compound
co2 H20 02
NO,
% by Volume
12.8 10.5 1.o
0.5
Compound
co N2 H2 Hydrocarbons
% by Volume
2.3 76.0 0.4 0.1
(in V.D.I. Richtlinic 2282)
The emitted quantities are correlated to the traffic density. Estimations are made with different criteria: the total amount of emitted pollutants (Table 2) or the relative importance of traffic pollution in the global pollution pattern (Table 3).
13
I
Table 2 Estimation of the emissions due to automotive traffic in Belgium (year 1977) Type of fuel
Number of vehicles
Gasoline 3.0 x 106 Diesel 0.5 x 106
co 1 400 43
(CW, NO,
109 11
90 39
SO2
3.8 13.0
Pb++ Br-
1.8 0.9
CI-
0.7
Results given in Id T. (from Hecq and empoux 980)
Table 3 Estimation of the emissions of SO2 and NO, in France (year 1982) Pollutant
Industry 1157 KT (48.7%) 254 KT (19.0%)
Transport 57.5 KT (2.4%) 648.0 KT (52.0%)
Power plants 933.3 240.0
Domestic use
KT 230.1 KT (39.2%) (9.7%) KT 140.0 KT (18.0%) (11.0%)
Results given in 1@T. (or %) - (from CITEPA 1983)
The conditions of these emissions are well known, an important literature is devoted to correlate the pollutions with the type of engine, type of fuel, the speed of the car, the driving cycle, etc. (Sibenuler1972). Other parameters of the pollutions are :
- the type of traffic, and the emissions level of each vehicle - the traffic capacity - the wind velocity - the wind direction - the atmospheric stability - the type of site
- the distance from the source ( J o m r d and Vidon1970). 2. DESCRIPTION OF THE EMITTED POLLUTANTS
Carbon oxides (COX) Carbon monoxide is one of the three most common products of fuel combustion, carbon dioxide and water vapor are the other two. Most of the CO in the atmosphere results from incomplete combustion of carbonaceous materials.
2.1.
14
Carbon monoxide is quite stable in the atomosphere and is probably converted to C02, but the rate of this conversion (not known exactly) is low. Its a poisonous inhalent and no other toxic gaseous air pollutant is found at such relatively high concentrations in the urban atmosphere. Carbon monoxide is dangerous because it has a strong affinity for hemoglobin. The major risks for human or animal health are when CO is emitted in confined or enclosed spaces (inside the car, in tunnels or subterranean car-parks, etc.) where it will accumulate and reach the toxic levels. There are few data on eventual risks for plants. Fluckiger (1979) reports an increase of peroxydase activity and of ethylen synthesis by birches (Betula pendula) growing near highways. An early abscission ofleaves is observed too. Carbon dioxide is a normal component of air, it is an important material for plant life - emitted by all living organisms during the respiration and fixed in photosynthesis by green plants. Normal concentrations in the air are ranging from 300 to 380 ppm. Concentrations, which could be toxic are rarely observed (a volcanic emission, occurred recently in Cameroun, contradicts this optimistic opinion).
2.2.
Nitrogen oxides (NOx) Oxides of nitrogen are an important group of air contaminants, produced during the high temperature combustion of gasoline in the engine. The combustion fixes atmospheric nitrogen to produce first nitrogen monoxide (NO), which will be converted in nitrogen dioxide (N02)' This oxidation is rather rapid at high concentration, the rate is much slower at low concentrations. In sunlight, especially in presence of organic material (hydrocarbons), this conversion is greatly accelerated. By gasoline powered engines, NO x emissions increase with average speed (Pearce, 1986 -Joumard, 1986). The hazards associated with nitrogen oxides are: - a direct noxious effect on the health and well being of people; - a direct phytotoxic effect on plant communities. The measure NO x concentrations in the air, are generally always low, and don't cause plant damages, except when they are associated with other gaseous air pollutants as sulfur dioxide or ozone; - an indirect effect : due to photochemical oxidation of organic material, with an abundant production of toxic compounds.
2.3.
Hydrocarbons An analysis of hydrocarbons and other organic compounds emitted in exhaust gas of a four cylinder otto engine is listed in Table 4 (Becker KH. et al, 1985). The composition of car exhaust and of the organic fraction, is "in the road" condition quite variable and strongly dependant on the mode of driving. Among the substances responsible for photochemical air pollution are
15
insaturated hydrocarbons (faster reactors), saturated hydrocarbons (slower reactors), aromatics and aldehydes. Automobile exhaust is the major source; however hydrocarbons and other organic gases are also expelled during the production, refining and handling of gasoline.
2.4.
Oxidants The general terms "oxidants" and "photochemical air pollutants" include a large number of trace compounds, results of reactions between primary pollutants (NO, N02 and hydrocarbons) under the action of sunlight. Important reaction products (or secondary pollutants) are ozone (03), peroxyacetyl nitrate (p.A.N.), higher oxides of nitrogen, aldehydes and ketones, as well as several gaseous and/or particle-bound inorganic and organic acids. The effects of photochemical pollutants are mainly: - Plant damage: with a definite economic significance, because the damages to crops and forests. Some cultivated species are very susceptible to ozone and P.A.N (ex. tobacco and grape). There is considerable evidence that chronic exposure of a variety of plants to concentrations below these that cause irreversible damage, adversely affects plant growth, and decreases the resistance of plants to climatic stresses and parasitic diseases, and finally induces a progressive dieback. - deterioration of materials: ex. fast cracking of stretched rubber products. - eye irritation and health hazards. - decrease in visibility.
These oxidants could be involved in the forest dieback; this theory will be later discussed.
2.5.
Particles A large number of extremely fine particles are emitted from automobile exhaust systems, with approximately 70 percent in the size range of 0,02 to 0,06 micron. These particles consist of the both inorganic and organic compounds of high molecular weight. The quantity of solid and droplet material produced in the exhaust amounts to a few milligrams per gram of gasoline burned (Rose 1962).
16
Table 4 Volatile organic emissions of an Otto engine (Dulson 1981) Compound
% by m a s of total
Compound
Methane Ethine Ethene Ethane Propene Propane Acetaldehyde n-Butane Butenes Acetonitrilite Acetone Isopentane n-Pentane
7.0 10.9 15.7 1.6 0.2 1.1 0.7 1.8 0.7 1.3 0.9 5.2 1.4
% by mass of total
organic emissions
organic emissions
2-Methylpentane 3-Methylpentane n-Hexane Benzene 2-Methylhexane 3-Ethylpentane n-Heptane Toluene 1,l-Dimethylhexane Ethylbenzene m-, p-Xylene 0-Xylene Trimethylbenzenes
1.1 0.8 1.o 12.7 0.7 0.6 0.4 18.9 0.3 2.1 6.1
1.8 4.0
I
Most gasoline contain lead additives, which provide the antiknock characteristics that are required by present-day high compression engines. The most common additives contain tetra-ethyl lead or tetra-methyl lead together with organic chlorides and bromides. Lead as a pollutant in the air,on plants and in soils has elicited increasing attention during the last twenty years. The dispersion of this heavy metal in the terrestrial and aquatic ecosystems is well known, and the hazards, associated to increasing concentrations of lead in water, crops, forages and soils are well known. Legislative measures (quality standards of fuels) and regulations will progressively prohibit the use of alkyl-lead additions in fuels, and reduce the risks of lead contamination of the food-chain, but there will still remain an important problem of soil, sediments and water contamination by lead. Other heavy metals: Fe, Cu, Cd, Zn and Cr, are emitted by automotive traffic, due to panelbody alterations, tyres, brakes systems etc. Asbestos dusts could be released by brake-linings or clutch facings .
3.
EXAMPLES OF POLLUTIONS DUE TO AUTOMOTIVE TRAFFIC
Gaseous air compounds acting as primary pollutants. In 1974, a National Commission for Environment near Highways was created under leading of Dr E. MANNAERT. The first objectives were to measure air pollution, dust deposition and lead contamination, due to automotive traffic near motorways. The research was performed by our colleagues of the BECEWA (Rijks
3.1.
17
University Gent) in association with our laboratory (Gembloux). Six different sampling sites were choiced along the heavy loaded "OstendBrussels-Liege" highway. The sites differ by the traffic density and the road profile, all of them were in rural areas. Four gaseous air pollutants were measured at increasing distances from the motorway: CO, NO x, light and heavy hydrocarbons. Additional but sporadic measurements of 3-4 benzopyrene were made in only one sampling site (10 Km Wof Brussels). Deposited dusts, and soots were collected too. The results of these researches were published in a confidential report (1. Vandenbossche et al, 1976). As an example, we compare NO x distribution in the air, in flat country - near Gent with an average traffic density of ± 10 000 cars and ± 3 000 lorries during a 7h period (Fig. 1) and near Liege (traffic density ± 3 000 cars, ± 1 100 lorries during the same period) (Fig.2). The major influences on air pollutants dispersion are traffic capacity, wind direction, type of site and the distance from the source.
3.2.
Lead contamination.
A research collaboration between the "Green project" and the Plant Biology Department of Gembloux Faculty started in 1972. The aims of this research were to collect informations about lead emission by exhaust gases of cars, and to survey the fallout of lead particles near highways and prevent any contamination of the food chain. A survey of lead deposition on vegetation gives a lot of information on the level of contamination and on the various factors affecting the dust deposition patterns.
3.2.1.
Techniques
More than 20 sites were located near Belgian highways, in rural areas, some other sites were chosen in Brussels (parks and avenues). During five years, every month (every fortnight during the summer period), samples of soil, grass, tree leaves and vegetables were collected. Ten years ago, we started a programme of sampling (soil and grasses) to survey the efficiency of a windbreak. Vaselinated plates were placed: before, in- and behind windbreaks to follow the deposition of lead particles and dust. After being dried and extracted with a 1/1 HCI03 - HN03 solution, the samples are analysed for their heavy metals content. In all samples. Pb, Zn and Cd are determined by pulse polarography (Delcarte et aI1973) or by flame spectrometric atomic absorption. All the results, in the following tables and figures, are given in p.p.m. (mg/kg dry weight). Our sampling sites are located in a map (see Fig.3). A rural site, chosen far away from any road, serves as a control area, where samples are collected to measure the background levels of the studied heavy metals.
18
ppb
NO x
160
/'
....-
-.
"1\
\
/
140
/
I
120
"",,_
I
/
/
I
100
I
/
/
.......
~_-~\
''',,,
'"
1\ \, \ \
/
/ /.-'-'~.
..........
~.
<,
1
80
'\
Ii
~
il
\.
I'
j
60
!
;1
'.-,
\ -, "
"'
jf
40
~'
II
20
J''l 'i ";~I .)
4
2
3
1
DISTANCE FROM THE HIGHWAY: (!)&@ :16.5 M ; @&@
Caption
-----
------
FIG. 1
Date
Meteo
06.11.74
N.E - 5.5.0 (1.6m/s) 5.5.0 (6.6m/s) 5.5.0(3.1 rn/s)
13.11.74 17.04.75
Traffic density from 10 am to 17 om Cars Lorries 9.686 2.959
x
9.652 9.586 i
DISPERSION OF NOx - MOTORWAY OSTEND BRUSSELS LIEGE SITE NEAR GENT •
33 M .
x
3.180 2.959
x
19
ppb NOx
100 I
,"" ",,
\
i
I
80
,, \
\
I
!
, " »-r-, "
/'
5/
20
/
./ /
.>:
o
V
\ \
\
/
.I
~
\
~
//
2
-'-'-
11. 09.74
-------
18.12.74
9.09.74
1
~ & ~ : 18 M ; ~
DISTANCE FROM THE HIGHWAY
Date
\ .~
'<........... .....
.'
Caption
'.
<,
~./
4
FIG.2
\
/
I
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\.
V\
I
I
\
r;
I I
40
\
1\,/
I
60
".
/
\
I
MIHeo
'r-. .
3 : 41 M
;~:
37 M .
Traffic density from 10 am to 17 pm Cars Lorries 2994
x
1136 x
(2.7mJs)
2994
x
1136
W.SW (7.0m/s)
2984
E.S.E-SW(2.8m! ) E.
DISPERSION OF NOx - MOTORWAY OSTEND BRUSSELS LIEGE SITE NEAR LIEGE
1036
x
20
BELGIQUE
FIG.3
SAMPLING SITES .
3.2.2. Importance ofthe lead contamination The first aims of the study were to evaluate the levels of lead contamination in the roadside environment. To secure a good survey, we sample ubiquitous plant species e.g. : Tussilago farfara L., Plantago major L., Plantago lanceolata L., Ligustrum sp. and Trifolium repens L. Lead is present in a relative narrow zone along either side of the road. We compare lead presence in superficial soil, in plantain tissues (after or without washing of the samples in a 10% vol. RCI solution) (see Fig. 4) (Impens et af 1972)
21
" Total leG in SOl 1 o Total lead in plantain
Pb. concentration. ppm/DW.
IIashee! out lead (plantain
o
400
360
320
zeo 240
200
160
'20
eo 40
15
10
5
Road
-0
Distance from the road
15
20
25
30
35
IJ:)
Right
( in meters)
FIG.4
LEAD CONCENTRATION NEAR THE BRUXELLES-NAMUR HIGHWAY .
22
The distribution of lead is influenced by: - the distance to the road: on varying distances from highway, the lead content of soil and herbage can rise to 10 to 20 times the normal content and remains noticeable at a distance of at least 120 m from the highway. In centerstrips, concentrations of lead are very high, this is caused by washing away of lead containing dust to the center and the side-strips by rain. - the duration of exposition: there is a progressive increase of lead accumulation on leaves, from early spring time to the autumn days. In some needles of Pinus nigra L., we found about 1 500 to 2 300 ppm of lead. - traffic density: there is a clear correlation between the density of traffic and the amount oflead deposited. - climatic conditions: rain or snowfalls cause an important washout of the deposited dust on leaves, and a progressive contamination of soil, rivers and ponds. - profile of the road: highways are built in flat country, or as sunk roads or enbanked roads. One of our difficulties was to find, on the same highway, similar conditions of traffic density associated with different road profiles. The deposition of lead is influenced by the prevailing winds in the three different profiles. Lead gradients in the road transect are more perturbed in sunk roads (see Fig.S). - plant species: the anatomy of leaves (presence of cuticle, hairs, etc.), the habit of the plant, the pattern of growth and the ability for root absorption of heavy metals have to be considered. - the state of growth of the vegetation is very important. There is an increase in lead in the above ground portion of plants, when active growth shows a minimum.
3.2.3. Lead deposition in towns Streets, avenues and parks are heavily contaminated by the exhaust gases of cars. Grasses in lawns contain more than 150 ppm of lead. Shrubs and ornamental trees are important sinks for the aerial lead and may accumulate other heavy metals on their leaves (Impens et aI1972). In busy avenues, the street dust has a lead content ranging from 500 ppm to 2 500 ppm and plani screens are effective to reduce the lead burden in gardens. Some similar results were obtained in British cities (Davies 1978). 3.2.3. Lead deposition and food chain contamination Lead is transferred to the soil, plant or animal via sedimentation, impaction, precipitation or inhalation. The roadside environment receives metal particles of all sizes classes, the larger ones by sedimentation, and the smaller ones by the latter processes. The natural lead content of the soil always causes a certain concentration of lead in plants; but the absorption of this metal by roots being not very important, most of the contamination of leaves, stems, and fruits is due to impaction and precipitation and not to absorption and translocation from roots to epigeal organs (Ter Haar G.,
23
Pb. conc. mg/kg (DW)
Flat.
360
................ Excavated.
340
----- Enbanked.
320 300 260 260 240 220 200
leo 160 140 120 100 60 60 40 20
...... ....... 100m
LEFT
FIG.5
SCm
25m
5m" 5m
CENTER
25m
SCm
100m
RIGHT
LIEGE-BRUXELLES HIGHWAY.LEAD CONTAMINATION OF GRASSES.
24
1970; Impens et a1 1976; Davies, 1378). Commercial crops consumed by man (e.g. letttuce, spinach, cabbage, beans, peas and other vegetables) show a highly significant increase in surface coated lead, when growing near highways and therefore, are no more suitable for consumption, not even if 50 per cent of the deposited dust can be removed by an acid washing (see Table 5). Table 5 CONTAMINATION OF VEGETABLES NEAR BELGIAN HIGHWAYS (all samples were collected in end of September) Plant
Organ
Solanum tuberosum
leaves tuber leaves roots leaves roots leaves seeds leaves seeds leaves bean pods leaves
Beta vulgaris Cichorum intybus Pisum sativum Zea mays Phaseolus vulgaris Lactuca sativa
Distance from 10 m 10 m 16 m 16 m 16 m 16 m 16 m 16 m 15 m 15 m 15 m 15 m 15 m
Lead content (ppm/dm) 262 3 44 4 63 3 22 3 108 9 51 52 110
On the other hand, protected edible parts of plants like seeds, tubers (potatoes), bulbs (onions), roots (radish and carrots) show hardly any increase in lead. It's an evidence that lead salts are immobile in the soil and largely unavailable to plant's roots. The washout of lead, in artificial conditions, with solutions of HCI (10% vol) shows that some vegetables have an important retention ability for lead particles. The concentration of lead in grass is of special interest, because animals may eat it, and high concentrations of lead are found in grass, hay or silage from pastures near highways. Absorption by the alimentary tract of surface coated lead on forage seems not to be considerable, most of the lead being eliminated in the faeces (Delcarte et al, 1974). The Committee "Legislation of Foodstuff' of the E.E.C. has proposed a guideline value of 10 ppm lead in the dry matter of foods. This value seems difficult to be respected in some large areas.
25
3.3.
A complex pollution: acid rain Forest decline is now widely considered to be one of the most important environmental problems in the northern hemisphere. This forest decline is unprecedented in severity and geographical extent, and at the present time, it cannot be attributed to a known cause. Until the beginning of the 1980's, acidic precipitation was blamed as the major cause. The term "acid rain" has become so popular in recent years that it includes all types of pollution - induced injury occurring in ecosystems. In Belgium, the first damages were observed and recognized in our eastern forests of Hertogenwald and Eifel, near the German border. These forests contain mainly Norways spruce (Picea abies (L.) Karst) and some Silver fir (Abies alba, Mille) and Common beech (Fagus silvestris L.). These forests are of great economic value, the mean altitude of these regions is relatively high (from 450 to 700 m). The climatic conditions are severe, characterized by cold winters, snowing conditions hard frost, and long foggy periods. The soils are poor with acidic reaction. Damages to Norway spruces appear preferably on dominant trees. Symptoms are: - severe yellowing, especially of exposed needles occurs usually beginning with the oldest; - needle loss is observed, usually starting with oldest needles; - secondary shoots in upper crown are dropped by the tree; - adventitious buds develop in increased manner; - growth is inhibited, forming shorter needles; - fine roots are dying and the mycorrhiza regeneration capacity is reduced. However, the symptoms vary slightly from region to region (Krause, 1983, Laitat and Impens, 1985). There is still a great deal of argument about the main causes of this dieback, the main hypotheses are briefly discussed. a. Direct effects of sulphuric and nitric acid are not generally considered to be primarily responsible for the damage. b. The acid rain hypothesis was postulated by Ulrich in 1979 (Ulrich et al., 1979) and states that wet and dry disposition of acids leads to chemical reactions in the soil, destroying the buffering system and eventually leading to mobilization of toxic ions: aluminium and manganese. This process is enhanced by severe leaching of magnesium- and calcium ions due to increased H+ input and corresponding leaching processes. c. The stress hypothesis, formulated by Schutt in 1983 (Schutt et al., 1983) postulates that the total impact of air pollutants in the past decades and their combination of effects lead to a severe loss in vitality, and increases predisposition to climatic stress
26
of (frost, heat, water deficiency etc.) and plant pathogens. d. Ozone hypothesis, ozone and other photooxidants may be involved in the decline (Arndt et al, 1982). The following mode of action was proposed (Krause et al, 1983). Exposure to elevated ozone levels damages all membranes, resulting in an increased leaching of ions by acid fog and acid rain. Ozone also causes damage to chlorophyll, and thus reduction of the photosynthesis. Root growth is inhibited, reducing the ability to compensate for leached nutrients by increased uptake. Ozone levels in damaged forests in the FRG are often remarkably high of 80110 ug rrr-' (Black Forest) and 60-90 ug m-3 (Bavarian Forest). In Vielsalm (Belgian Ardennes) we have recorded during last July daily concentrations of Oj ranging from 40 to 80 ~ g m-3. Blank (1985) reports that there is only one data set which gives a clear idea of the trend in natural background ozone concentrations in Europe, over the past 20 years. On the Isle of Rugen (in the Baltic, off the German Democratic Republic) an area not affected by any local pollution, ozone levels increased by 60% between 1956 and 1977. A similar trend has been reported for two other stations in forest areas of East Germany. This coincides with the steady increase of NO x and hydrocarbon emissions from motor vehicles, which react with sunlight to produce ozone during transport over long distances from urban areas. The ozone and photochemical compounds then affect remote areas, particularly at higher altitudes. However, there are some gaps in the photochemical oxidant hypothesis. Probably, the decline is caused by ozone acting in combination of other stress as acid mist, drought, frost or pathogens. e. Climate and pathogens hypothesis. Climatic factors may be involved in the forest dieback, in association with chronic exposure to air pollutants. The series of dry and hot years experienced during the last decade has certainly affected tree vitality and reduced the resistance to pollutants and pathogens effects. Hot summers have often been followed by harsh winters, with early or late deep frost periods. These drastic and sudden changes in temperature and water availability may open the way for secondary cause of decline: fungal infections or some other noy yet identified diseases.
4. OTHER RISKS FOR THE ROADSIDE ENVIRONMENT Roadside environment is exposed not only to exhaust gases and metallic particles, but also to other stresses caused by the automotive traffic. Two of these environmental troubles will be briefly discussed:
27
1 chloride contamination of soil, stormwater runoff, and plants; 20 anaerobic conditions in the soil due to compaction and vibration. 0
In winter when it is freezing large quantities of de-icing salts are spread on the roads in order to make the traffic easier - NaCI and CaCl2 are principally concerned -. De-icing salts could be phytotoxic. We have made observations - in natural conditions near motorways, and performed experiences in a tree nursery, to compare the sensitivity of some tree species. In our trials, rates of salt application were comparable with those used for road de-icing in Belgium (30 or 60 g/m2 for a dose). There is an important increase of chloride concentrations in the upper soil (till 60 em depth), water collected near treated motorways is enriched in chloride ions. CaCl 2 induces phytotoxic effects, commonly expressed as foliar necroses, which appear at the leaf tips and margins (Paul et at. 1984). Sensitivity of trees to fungal diseases is enhanced. Both direct and indirect effects of de-icing salts must be considered when deciding on tree plantings along the margins of motorways, Poor aeration conditions in the soil may result from impeded exchange between the soil-gas-phase and the atmosphere (e.g. due to a sealed soil surface) soil compaction, and natural gas pipes leakages or from exceptionally high biological activity in the soil (Impens.Delcarte, 1979). Anaerobic soil conditions, whatever their causes may be, are especially disastrous for trees and shrubs. Root growth is inhibited while root respiration and uptake of water and nutrients are reduced. The oxidation-reduction status of the soil is closely related to oxygen presence. Under anaerobic conditions, nitrates, iron, manganese and sulfates can be reduced. Presence of sulfides, reduced manganese and nitrites is nearly always related with declining or dead trees. Another secondary effect of compacted soils is a severe waterstress responsible of an important dieback. This waterstress may be increased by chlorides accumulation in the soil.
5. CONCLUSION We should have the right to demand clean air conditions near motorways and a drastic reduction of all emissions of noxious exhaust gases and particles. The risks for environment alterations could be prevented and reduced by clean motors. Awaiting for this future clean engine, there is a request for new air quality standards, and the extension of legal control of vehicle emissions. The EEC has been the prime mover in the determination of standards - Member States have been required to frame their national laws within the EEC framework - Europe and
28
North America are both moving towards tighter emission control standards and unleaded gasoline. Some other lands, as Switzerland, where forest decline is important, are planning new legal dispositions with a drastic reduction of speed on highways with the hope of limited emissions of gaseous pollutants. If lead pollution will be progressively reduced by the nextcoming European standards of lead addition to fuels, the lead already present in soils will remain a threat for some sensitive crops and forages. A passive protection of roadside contamination could be obtained by green screens, implanted along the motorways. But some engineers have an irrational fear of trees, demanding they be kept far from the roadway for driver safety. These green screens must be designed before the building of the road, they must be planned with resistant and rustic shrubs and trees, which will filter the air and act as efficient sinks for dust and heavy metals particles. Some of these species may serve as bioindicators of air or soil pollution. Due to aerial long distance transport and photochemical reactions, prevention of damages to forests request more attention. The solution is reduced emissions of the precursors of toxic compounds: clean motors are wanted. It is a hard job, but it has to be done. 6. BIBLIOGRAPHY BECKER, K.H., W.FRICKE, J. LOBEL and U. SCHURATH. 1985: Formation, transport and control of photochemical oxidants in air pollution by photochemical oxidants, Ed. R. GUDERIAN, Ecological studies vol. 52. Springer Verlag, Heidelberg 1985. BLANK, L.W., 1985: A new type of forest decline in Germany. Nature, vol. 314 n? 6009: pp. 311-314. ClTEPA. 1983: in Anonyme: Agence pour la qualite de l'Air France. 1983. 32pp. DAVIES, B.E., 1978: Plant available lead and other metals in British garden soils. Science of total Environment, 9: 243-262. FLUCKIGER, A., 1979: Premature senescence in plants along a motorway. Env. Pollution 20 (3): 171. HECQ, W. et L. SEMPOUX. 1980: Aspects techniques et economiques de la lute contre la pollution atmospherique dans le secteur du transport routier. Poll. Atm.,juil.-sept. 1980: 299-312. IMPENS, R., Z. M'VUNZU and P. NANGNNT. 1972: Determination du plomb sur la vegetation Ie long des autoroutes. Analytical letters 6 (3): 253-264. IMPENS, R. and E. DELCARTE. 1979: Survey of urban trees in Brussels, Belgium. J. Arboriculture 5 (8): 169-176. JOUMARD, R. et VIDON. 1979: Niveaux de pollution en bordure des autoroutes et voies rapides urbaines. Poll. atm., avril-juin 1979: 149-152.
29
JOUMARD, R 1986: Influence of speed limits on road and motorways on pollutants emissions. in Highway pollution. Second Intern. Symp. 7-11 July 1986, London 58-67. KRAUSE, G.H.M., 1983: Forests effects in West Germany. Symp. Air pollution and the Productivity of the Forest. Washington D.C. Oct. 4-5, 1983,32 pp. KRAUSE, G.H.M., B. PRINZ and K.D. JUNG. 1983: Neuere Untersuchungen zur Aufklarung immissionsbedingter Waldschaden. VDI-Bericht W 500, 257-266, VDI-Verlag GmbH, Dusseldorf. LAITAT, E. et R. IMPENS. 1985: Surveillance du deperissement des forets en Belgique. in Poll. atmospherique n" 105: 16-23. PAUL, R, M. ROCHER and R IMPENS; 1984: Influence des epandages de CaCIZ sur le sorbier, l'erable, le tilleul et le platane. Bull. Soc. Roy. Bot. Belg. 117: 277-284. PEARCE, T.C., 1986: Vehicle emissions at high speed. in Highway pollution, Second Intern. Symp. 7-11 July 1986. London: 48-57. PRINZ, B., G.H.M. KRAUSE and H. STRATMANN. 1982: Vorlaufiger Bericht der Landesanstalt fur Immissionsschutz tiber Untersuchungen zur Aufklarung der Waldschaden in der Bundesrepublik Deutschland. LIS-Bericht Nr. 28: 154 p. Landesanstalt fur Immissionsschutz des Landes NW, Wallneyer Str. 6, 4300 Essen 1. ROSE, A.H., 1962: Automotive exhaust emissions. in Air Pollution A.C. Stem Ed. Vol. 2 pp. 40-80. Academic Press NY 1962. SCHUTT, 0., W. KOCH, H. BLASCHKE, KJ. LANG, H.J. SCHUCK and S. SIMMERER. 1983: So stirbt der Wald - Schadbilder und Krankenheitsverlauf. BLV-Verlagsgesellschaft Miinchen-WienZurich. SIBENALER, E., 1972: La pollution par les emissions des moteurs a combustion interne et allumage par etincelle. Symp. Intern "problemes sanitaires poses par le Pb dans l'environnement", 26 oct. 1972. Amsterdam. 808-72F: 159 pp + annexes. ULRICH, B., R. MAYER and P.K. KHANNA. 1979: Deposition von Luftverunreinigungen und ihre Auswirkungen in Waldokosysternen im Solling. Schriften a.d. Forstl. Versuchsanstalt, Band 58. J.D. Saverlander's Verlag, Frankfurt/M. VANDENBOSSCHE, 1. and R IMPENS. 1976: Verslage over leefmilieu langs autowegen 1974-1976. Ministere des Travaux Publics, Bruxelles, 342 pp.
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A. Crucq and .\. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Ne therl ands
CATALYSIS IN MODERN PETROLEUM REFINING by J. GROOTJANS
LABOFINA SA, Feluy, Belgium.
ABSTRACT The progressive lead phase out in motorspirits, deeper conversion schemes on crudes of poorer quality and more stringent regulations on polluant emissions determine to a large extent the ongoing research in catalysis related to petroleum refining. From the inspection of the gasoline pool of a conversion type refinery, it is clear that major contributions with respect to octane optimization, may be expected from the fluid catalytic cracker and the downstream upgrading of its products. The development of zeolites contributes very substantially to these goals, both by their introduction into FCC catalysts and their use in the upgrading of some of the side streams. The latter is illustrated by a new process developed at Labofina : low value C3-C4 olefinic streams are converted on a zeolitic catalyst to a light olefinic gasoline, particularly suited to be etherified with methanol. This combination process offers many advantages over the present commercial processes. INTRODUCTION The oil refining industry is obviously not the trendsetter with respect to regulations on polluant emissions. These regulations, whether they are related to automotive or heating applications are translated into some of the finished product specifications. Other specifications are dictated by the end use and the marketplace. The task of the oil refining industry is to use in the most profitable way the available resources (crude oil and refining processes) in order to satisfy the demands of finished specification products. During the last 15 years, the oil industry has been facing many challenging problems that are well known by the public. Many refineries extended and adapted their processing units in order to produce from less expensive heavy crude oils, more valuable and cleaner white products. However, the white products obtained by conversion processes generally require further upgrading in order to meet the final specifications. The progressive lead phase out in motorspirits makes it far more complicated. The refiner has to produce more octane barrils from components which are much poorer and more difficult to upgrade. In this paper we aim to illustrate how
:31
32
catalysis, and zeolitic catalysis in particular, allows for some major breakthroughs. FLUID CATALYTIC CRACKING
In a conversion type refining, the fluid catalytic cracker (FCC) produces directly and through downstream upgrading some 30 to 50% of the gasoline pool components. The normal feedstock of the FCC is straight run vacuum gasoil. Deeper conversion routes, metal passivators and the development of more performing hydrotreating catalysts, allow for the production of additional feedstocks: - atmospheric residues, - visbroken and coker vacuum gasoils - solvent deasphalted oils - hydrometallized residues FCC catalyst manufacturers are facing the challenge of developing materials that convert these more refractory feedstocks, while yielding cracked gasoline of improved octane numbers. Two concepts are being used in the design of an octane analyst: - Shape selective zeolite: A shape selective zeolite cracks the low octane paraffinic components out of the gasoline boiling range, and therefore enhances the octane numbers at the expense of decreased gasoline yield. The LPG olefinic fragments can however be converted to premium gasoline components in downstream upgrading units. - Zeolites with reduced hydrogen transfer activity: These zeolites of the faujasite type have as well good hydrogenation as dehydrogenation activity. Slowing down the hydrogen transfer versus the cracking activity is established via controlling the Si/Al ratio during their synthesis and the natural dealuminating process which takes place during the hydrothermal equilibration. The equilibrated unit cell size is well related to the Si/AI ratio, and is a convenient tool in selecting and controlling these octane catalysts. Slowing down the hydrogen transfer favors the production of olefins. Heavy naphtenic compounds are converted into aromatic gas oil components under fast hydrogen transfer, but into aromatic gasoline components when the hydrogen transfer is slowed down relative to the rate of cracking. These catalysts therefore contribute in two ways to increased gasoline octane. On the commercial scale we see effectively a gain of a few RON points when using these catalysts. The gain on the motor octane MaN is in general much less pronounced.
33
DOWNSTREAM UPGRADING More octane enhancement can be achieved through further processing of some of the FCC products. The traditional ones are : - iso-butane/butenes alkylation (HF and H2S04 catalyzed) - catalytic oligomerization of propylene and butenes (H 3P04 on Kieselghur) - dimerization of propylene and butenes (lFP's DIMERSOL) Alkylation is a mature process, and we see little incentive for new catalytic systems, unless they would allow to carry out the reaction without isobutane excess. The olefins condensation processes produce blending stocks of good RON's but somewhat low MaN's:
r - - - - - - - - - · - - - · - - · - - - - - - - - - - - - ---
,
RON
MON
---I
(RON + MONh
f-- -.--.-------.-.---- ---- . . !
C4 alkylate Oligomerisate (C4) Dimerisate cq)
93-95 96-97
92-94 81-82
93.5 89
97
79-82
89
More recently, new options have been made available: -MTBE: The FCC produces about 1.5 wt% on feed of isobutylene. Isobutylene is easily etherified with methanol into MTBE. MTBE has excellent octane numbers (RON == 117, MaN = 101), but obviously, even if all i-C 4 could be recovered from the FCC, the total MTBE product would be less than 2.5% of the gasoline blend. -TAME: The FCC produces roughly 2.5 wt% on feed of tertiary amylenes. They are also readily etherified on cationic resins into TAME. The octane numbers for TAME are somewhat lower than for MTBE : RON = 112, MaN = 99. In blends one finds that part of the MTBE may be substituted by TAME without penalty on the blend octane numbers. Again, full recovery and etherification of the tertiary amylenes would yield a total TAME product representing less than 3.5% of the gasoline pool. - Heavy ethers: Processes are being proposed that aim to etherify the total light catalytic gasoline. The tertiary olefins become however rapidly more difficult to convert with increasing carbon number.
,I
:34
Still other techniques are gaining interest in optimizing the octane barril from FCC derived products such as : - butene-1 to butene-2 isomerization for better alkylate - reforming of the low octane heart cut of the cat cracked gasoline. If the European gasoline demand for high MaN remains steady, esthers will increasingly contribute when lead anti-knocks progressively disappear.
A NEW COMBINA nON PROCESS Refineries that have access to isobutylene streams from steam cracking may face the problem that the existing alkylation and possibly catalytic condensation units cannot take the normal butenes which are contained in the pyrolysis stream. Skeletal isomerization of normal butenes is an active research domain, but has not yet found an industrial realization. Also in a conversion type refinery, there are several streams containing substantial amounts of olefins which are not upgraded: for instance, the propylene splitter bottom. Labofina developed a combination process that very effectively contributes to the octane barril : In the first step of the process, propylene and/or n-butenes are converted to species boiling in the gasoline range. The catalyst is a special shape selective zeolite, operating conditions are mild and the space velocity is exceptionally high. On a propylene feedstock, very substantial amounts of isobutylene are found in the reactor effluent. On a n-butene feedstock, attractive yields of polypropylene are obtained as well as iso-butylene. Material balances and product distributions are presented in Table 1. For comparison, the same analyses are given for the oligomerization on phosphoric acid. At a conversion of 90% on n-butenes, the differences in selectivity between both systems are striking. The ranges reflect the influences of the operating conditions. It is stressed that the shape selective zeolite is very slowly deactivated by coke lay-down. Cycle times of several months are readily obtained, regeneration is carried out by simple coke burning. On an octane basis, it is clear that the gasoline obtained on phosphoric acid is superior. The zeolite however produces essentially gasoline species boiling in the C4-C7 range, a substantial part being tertiary olefins. The phosphoric acid produces dominantly dimers and trimers.
35
Table 1 : Step 1 Material balances at 90% conversion
Feed Type:
C4 Effluent from a MTl3E process n-C, content : 50 wt%
Catalyst:
Shape Selective Zeolite
Yield on feed (wt%) 'ropylene so-butylene 3asoIine
Phosphoric Acid on Kieselguhr
Typical range 3.7 - 15.3 3.0- 6.3 38.3 - 23.4
0 0 45.0
Gasoline analvsis Vol % distillation)
36-98°C )8-150°C !50 - 195°C > 195°C
64.0 - 80.0 29.0 - 17.0 6.6. - 2.5 0.3 - 0.5
G 2 E 70 E 23 E 5
ipecific Gravity d c
0.71 - 0.74
0.74 - 0.75
[ON dON
89.0 - 94.0 78.0 - 80.0
96.3 - 97.3 81.0- 81.5
36
In a second processing step, the depropanized effluent of the zeolitic conversion is etherified with methanol on a cationic resin. Table 2 summarizes the global material balances, and again gives the comparison with the phosphoric acid process.
Table 2 : Overall material balances after etherification
Labofina combination process wt %
vol%
Phosphoric acid on Kieselguhr wt%
vol%
IN: I + N-Butane N-Bu tenes Methanol Total
50.0 50.0 4.3
50.0 50.0 3.2
50.0 50.0
50.0 50.0
104.3
103.2
100.0
100.0
OUT Light ends Propylene I + N-Butane N-Butenes MTBE TAME Heavy ethers Olefinic gasoline Total
0.9 8.3 50.0 5.0
50.0 5.0
50.0 5.0
2.0 2.3 30.3
9.6 50.0 5.0 4.4 1.6 1.9 24.4
45.0
36.1
104.3
96.9
100.0
91.1
5.5
Table 3 gives the analysis of the final etherified gasoline. Emphasis is given on the blend octane numbers since these reflect how this component will perform in the gasoline pool. For a complex refinery with alkylation and a phosphoric acid oligomerization process, the linear programming simulation selects the combination process while shutting down the phosphoric acid oligomerization unit. The alkylation remains at full capacity.
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.\. Cruce and A. Fren net (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.Y., Amsterdam - Printed in The Netherlands
THE POINT OF VIEW OF THE AUTOMOBILE INDUSTRY Prevention is better than cure by Claude GERRYN
FORD of Europe Inc, 2 Boulevard de la Woluwe, 1150 Brussels, Belgium,
ABSTRACT Emphasizing on the fact that prevention is better than cure, it is shown that the development of engines such as the lean bum engine, that produce only low levels of polluting gases and thus requires only simple oxidation catalytic converters or even no catalytic converter at all, appears much more promising, from an economical viewpoint - lower buying price, cheaper maintenance, lower fuel consumption-, than the complex technology of the 3-way catalytic converters. The new EEC standards are criticized because their introduction in a too short delay gives at best a half hearted support to- and at worse results in a slowing down of- the development, still under way, of the lean bum technology. Finally attention is also drawn on the fact that the use of 3-way catalytic converters may result in substituting some forms of pollution by others not necessarily less harmful: examples of such substitution are given.
THANK YOU and good afternoon Mr President, Ladies and Gentlemen! Let me first tell you how honoured and pleased I am to be with you today: - honoured to have the privilege of addressing such a select group of experts and decision-makers - pleased to be able to present our views on a topic of such pressing importance to us all. After reading the impressive list of outstanding papers and knowing most authors present, prepared to share their expertise with us, I am sure a better understanding of the differences and disagreements on this -international debate will evolve at the end of this symposium: and I wish, therefore, to thank and congratulate the Universite Libre de Bruxelles for taking this initiative.
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Let me quote our Vice-Chairman Walter Hayes in his key-note speech during the FIA Round Table in Bournemouth in May this year: and I quote "It would seem to me that anybody who does not want a strong economy, a safe and clean environment and the most efficient possible use of national and international resources is a very strange person indeed. Anybody who does not want sweeter engines, better roads, safer cars, more responsible drivers and happy owners is not really operating on all his mental cylinders." Unquote
However, there is no doubt that a large part of the increasing pressure for action to protect the environment has come about as a result of increasing public awareness of the issues, but unfortunately this is not always based on real facts or it clear understanding of the problem. Public opinion is being more effectively marshalled by pressure groups and this trend is not lost on the politicians. This results all too often in action whose primary function appears to be to satisfy the political need, to be seen to be "doing something". Realistic evaluation and assessment of the potential effectiveness of the "something" and of its benefit is rarely feasible before implementation, and resulting benefits are difficult to identify. It is accepted that governments have an obligation to serve broad national and international needs on complex environmental matters and act as clearing houses for ideas and programmes. Business, for its part, commands managerial and organisational abilities and can mobilise the scientific and technological resources required to solve these problems. It is imperative that these two great segments of society should rest on a firm foundation of knowledge and understanding, especially in the field of "public problem solving". Realism, without any sign of false sentimentality, should be the base for action and it is not so much what can be done which should determine the route to follow but what needs to be done, with the reasons why and when. We do recognise the international dimension of many environmental problems but these .have to be tackled by coordinated action - bilateral as well as multilateral - between industry, governments and their respective international representative bodies, organisations and the public concerned. We are indeed committed to conducting our operations in an environmentally sound manner and to reducing - as far as technically feasible - any undesirable effect of our products on the environment, but at the same time we have to fulfill the imperative of economic growth and have to produce commercially viable motor vehicles. We too are breathing in this world, want to optimise the use of scarce resources, want environmentally favourable energy options. Our professional burden does not immunise us from undesirable effects but... it does tend to sharpen our perceptions and make us more acutely aware if
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compared with our other social partners of the balance of advantages/disadvantages that must be weighed in order to be clear on the value to society of any given control measure. This concern is unfortunately often misinterpreted as obstructiveness. Allow me to remind you briefly of the six commanding ground rules which seem necessary to me to realise within our society sensibly-balanced environmental progress: * First, governments, in a national or international context, should carefully assess, before developing and implementing environmental policies, the need for such policies, the potential methods of achievement, and their impact on industry. * Second, due to the wide range and complexity of problems raised by environmental protection measures, the closest possible contact and consultation between industry and government should be sought. * Third, any environmental protection measures envisaged must be technically sound and economically acceptable, reviewed in a framework of global approach and where at least safety and energy-use are topics to be included in the global appreciation. * Fourth, care must also be taken to avoid substituting one form of pollution for another. * Fifth, the costs of control requirements, with the resulting benefit to the environment, must be part of the decision-making process. These costs, whether absorbed in the first place by the state or by industry itself, must ultimately be borne by the taxpayer or the consumer, i.e. the general public. * Sixth, especially in the case of motor vehicles, to avoid distortion of trade and to enable cost-effective solutions to be found, exhaust emission legislation in Europe should be, if not totally harmonised, at least accepted by all West European governments, including those who are not EC members. However, this cannot mean a global-worldwide-conformity of environmental pollution limits, since each case has to be judged on its own merits, in its own geographical context, within its road infrastructure, existing town-planning and layout, attitude of the population and their pattern of behaviour. Having said this, and to enable us to examine one of the key questions to the motor industry, namely the political dimension and commercial consideration given to the environmental pollution issue in Europe, we should review step by step in how far the basic ground rules have been respected: In view of the combined "political/emotional" dimension in this instance, it is superfluous to dwell on the assessment of need or the government/industry consultation aspects (the two first commanding ground rules). Whereas the EEC Commission for example can be praised for its effort in seeking through its ERGA Committee a common Europe-wide compromise which is technically and economically feasible, individual national governments have, unfortunately, for political considerations or other reasons - such as prospects for additional employment - diluted the effort available for the exploration of those needs and the means to meet them. The ongoing dialogue between industry and government
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is thereby diminished. It was only on June 27/28, 1985 - after about two years of uncoordinated tugof-war - that an agreement could be formulated by the EC Council of Environmental Ministers - with the exception of Denmark - on the basis of an EC Commission proposal specifying the exhaust emission levels and the introduction dates. Though tough, we were lad that a compromise appeared to have been reached, avoiding a division of the common automobile market. However, the continuing reservation more than one year later, of its position by Denmark and the apparent unwillingness so far of Sweden, Switzerland and Austria to recognise the EC proposal - be it only as an alternative to their national legislation - is a cause for concern. Now what compromise is in our view technically sound and economically acceptable? The third commanding rule: In general, requirements must be so framed that they do not prevent innovation; they must not present unrealistic or arbitrary standards. It must be borne in mind that however necessary the control of motor vehicle emissions and noise may be - and no one would deny the necessity - there are other considerations, such as safety, cost, reliability, which also have to be balanced. There are many major legislative requirements affecting the vehicle, and none of these can be handled in total isolation - all have some interaction on the others, there is a "knock-on" effect, so that a "solution" in one area raises a new task in another. Furthermore, there is a balance to be struck between what can be done and how soon it can be done. The greater the pressure on the technical resources, the higher the cost of meeting the requirements. Inevitably, if too much of a manufacturer's engineering capacity is applied to one objective, other perhaps equally desirable objectives may have to be abandoned, or at least postponed. The constraints defining vehicle design are three-fold: - legislative constraints, or what we must do - market demands, what we would like to do - resource constraints, what we are able to do. Regulation on health/safety, construction and use, trade and economic policy, taxation preferences, all come under the general heading of government policy, part of the legislative constraints. Economic factors, styling preferences, pricing, running and maintenance costs, performance, comfort, reliability, durability, quality at large, are key to customer satisfaction, the market demand. Enginnering and manufacturing resources, research and product development, technological development, skilled manpower, production cost and profitability, are some of the indispensable logistics to be looked at in the context of
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resource constraints. All in all, a tremendous job before a final decision can be reached on any new vehicle design. Due to the many different disciplines involved: design, testing, tooling, finance, manufacturing, homologation and marketing, the gestation period for a major new design - engine or vehicle - is some sixty months.
Please allow me for a few minutes to focus on engineering or, to be more precise, on the present technological developments available for immediate application: If we analyse the means to control exhaust emissions, there are two apparent basic ways in which pollution caused by motor vehicles can be further reduced: * by not generating emissions, developing more efficient cleaner-burning engines which control their emission levels inside the combustion chamber (prevention) or * by not letting them escape, uncleansed, into the environment, using "hang on" equipment to treat the exhaust gases after they leave the engine, and which I would call (cure). Prevention being better than cure, how far can one expect to go with the preventive measure and how good is today's cure? In the first case, the aim is to construct the engine and its combustion chamber, and control the combustion process, in such a way that the creation of undesirable components in the exhaust gases - CO, HC and NOx - is minimised, whilst at the same time its economy is improved and adequate power still developed. One of the techniques is known as "HCLB" - high compression lean bum. This development of engines much leaner than stoichiometric is already finding its way into production. Examples are Ford's 1100 Fiesta, 1.6 Emax Sierra, Volkswagen's new Golf, Jaguar's revised V12 and, of course, the recently introduced 1.8 I Sierra, and the 1.41 and 1.6 I Escort and Orion. Under the maxim "consume less, fume less" offering petrol economy improvements of up to 20% over current - 1983/1984 - engines, this route has the capability in the future of reducing NOx by 60% from the uncontrolled situation, and approximately 90% for CO. Thus, the "best of both worlds" appears possible. The "prove out" process has begun. It is to be hoped that the legislators will give us enough time to further work on it and enough confidence to further invest in it. In the second case, the "curing" or "clean up" approach, various techniques have been evolved to reduce the emission: closed crankcase ventilation systems, exhaust afterburning, catalytic converters, etc, whereby the control systems, and the technology, become more complex.
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There is no doubt that a three-way catalyst is extremely effective in reducing emissions: NOx emissions by up to 80% from the uncontrolled levels, and HC and CO by over 95%. However, such techniques inevitably affect the operation of the vehicle, its performance, its fuel consumption, its driveability. They may also demand specific engine operating conditions in order to be effective, such as : - rich mixtures to use after-burning techniques - stoichiometric air/fuel mixture ratios to ensure the functioning of a three-way catalyst. Catalyst systems are complex in themselves, requiring complex control technology and skilled use of expensive equipment to ensure their continued effectiveness, care in fuel and vehicle use, and regular skilled maintenance, which may be beyond the capability of the third and fourth owners. In addition, some further investigation on potential side-effects during its particular use seems needed : concerning potential odour problems caused by sulphurtrioxyde, or the alleged carcinogenic effect, perhaps falsely or unjustly attributed to the active platinum leaving some types of catalysts, fire hazards as recently reported in the German press where in Cologne a fire, caused by a vehicle equipped with a catalytic converter (Audi 100), caused damage to the extent of 70.000 ECUs or 3.150.000 BFr.
Anyhow, how should we now - more specifically - judge the compromise reached at EEC level from a technical and economic viewpoint? The proposed standards have to be regarded as very tough indeed. This is particularly so for large cars. On the basis of vehicle tests carried out by the German government/UBA/, 10% of cars presently available in the US would fail to comply with the new standards. By effectively requiring three-way catalysts, a duplication of effort and expense is imposed, especially on the manufacturers who are furthest advanced on lean-bum. This duplication of investment in product development and manufacturing will add to the industry's costs, at a time when it is least able to afford it. To have one so-called "green vehicle" in each of the car model ranges is costing Ford for even this limited programme some 250 million ECUs or dollars, which is equivalent to BFr 11 billion 250 million, and is involving up to 10% of European research and development manpower. For the "large" cars, over 2.0 litres, the fitting of three-way catalysts will be virtually necessary since this is the only certain way of ensuring compliance with the emission levels required. This means that all these cars will require considerable changes to the floor pan and exhaust system to accomodate the catalyst, plus additional heat protection - both for the occupants, the interior trim and for grass, which may catch fire as mentioned earlier, due to the heat given off by the catalyst - which works in the range of 350 to 800 degrees Celsius. Also required will be either multi-point or
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central, single-point fuel-injection or an electronically controlled carburettor, plus a microprocessor, and oxygen-lambda-sensor in the exhaust pipe, probably secondary air pumps and other equipment. Catalyst systems will add approximately 850 ECUs/dollars or BFr 40.000 to the pre-tax showroom "customer" price of these over 2.0 litre cars. Indications are that fuel consumption of the car will be up to 5-10% worse, and, although servicing intervals will remain unchanged, they will be very much more expensive. Looking at real world conditions, in Germany a Scorpio 2 I injection in its catalyst version costs 1.340 ECUs or BFr 60.500 more than the one without a catalytic converter. An Orion/Escort 1.080 ECUs or about BFr 50.000 more, and a Sierra 21 only 755 ECUs or BFr 34.000 more. If comparing a 1.6 injection catalyst engine of 66 kW power and with a manual five-speed gearbox with a 1.6 carburettor non-catalyst engine of the same 66 kW power and the same manual five-speed gearbox, the fuel economy for the noncatalyst versus the catalyst engine is just over 10%. It should, however, be mentioned that the catalyst engine runs on 91 RON fuel where the non-catalyst version is tuned to 96.5 RON fuel. The price difference between normal and super grade fuel should be brought into the final cost equation here to the advantage of the vehicle equipped with a catalytic converter. For "medium" cars of 1.4 to 2.0 litre capacity, the standards defined will most probably require the most complex and costly of the possible lean-burn solutions, greatly reducing the incentive to develop this new European technology. The lean-bum technology offers the prospect of a lasting substantially improved environmental impact with improved fuel economy and cost-of-ownership. Ford expects that an oxidation catalyst will be required, with lean-burn engines, either an "open-loop" - without microprocessor control - or "closed-loop" with microprocessor control. Similar floor pan and exhaust system changes as for three-way catalysts will be required, but the engine equipment will vary greatly. Some "medium" cars, which are of relatively light weight compared to the engine cubic capacity, may use a normal carburettor coupled with a simple computerised ignition system, plus "open-loop" oxidation catalyst. The showroom - or customer price effect of this lay-out could be about 350 ECUs or dollars, or BFr 16.000, before taxes. Other "medium" cars, where the power to weight ratio is less favourable, or where other constraints exist, may need similar fuel injection or electronically controlled carburettors and microprocessors to the three-way catalyst. Here the objective would be to maintain the engine in the lean bum air to fuel range of over 18:1, whereas the three-way system maintains it close to 14.7:1. The showroom price effect of such systems will be slightly lower than the three-way systems noted earlier for "large" cars. But for both systems, however, "open-loop" or "closed loop" with leanbum engines, Ford would expect to obtain an improvement in fuel consumption of
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between 12 and 20%. Service intervals may either remain as now or become even less frequent and, for the "open-loop" system, it will remain relatively simple and relatively cheap. "Closed-loop" systems or those involving injection equipment may become more expensive. "Small" cars, of under 1.4 litres capacity, will generally be able to use leanbum technology alone (although for some markets outside the EC, a catalyst solution may be necessary). Showroom price effects will be minimal, possibly in the order of 150 ECUs/dollars/(BFr 8.000) or less, depending on the type of electronic ignition equipment fitted. Servicing is unlikely to be different to today's cars, and fuel consumption may improve by 12% or so. This possible improvement is especially marked in comparison with the fuel consumption penalty associated with three-way catalysts. It is hoped that the second phase for small cars with introduction dates 1992/1993 and for which the emission levels still have to be decided will enable us to continue with the lean-bum technology. The manufacturing investment for one new lean-bum engine at Dagenham, approaching 250 million ECUs or dollars, or BFr 11 billion 250 million, plus a further 60 million ECUs or dollars or BFr 2 billion 700 million for design and development gives an indication of just how great the cost to Ford will be during a period when profitability, to put it mildly, is less than satisfactory.
The complexity of the matter is even more clearly demonstrated if one takes into account the need to avoid substituting one form of pollution for another. It gives further weight to the argument why industry cannot simply agree to any "quick shot" solutions: there are simply no easy answers to complicated questions. For example. it is recognised that an increasing concentration of C02 in the atmosphere causes a warming trend, leading to climatic changes in the next century, of sufficient magnitude to produce major physical, economic and social dislocations on a world-wide scale. Absorbing heat radiation from the earth's surface, trapping it, and preventing it from dissipating into space, plays a critical role in maintaining the earth's heat balance. Since it looks as if the global atmospheric C02 concentrations could double before the middle of next century, an average annual increase in global surface temperatures of about 2-3 degrees Celsius and possibly as much as 7-10 degrees Celsius could occur in the North Polar region during the winter. Changes in rainfall patterns, desertification, higher sea levels, and so forth ...could be expected. On June 23, 1986, the Energy, Research and Technology Committees of the European Parliament adopted Mr Fitzsimon's report on the measures to be taken against
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increasing C02 concentration in the atmosphere to avoid - as was said - an ecological catastrophe. A catalytic converter oxydising the harmful CO and HC to the so-called harmless C02 is in view of this problem not a benefactor but a producer of a potential hazard to world environment. Taking another example of a possible shift from one form of pollution to another is the major increase in the sales of diesel-powered passenger cars. Due to the uncertainty on the final national legislation and on sufficient availability Europe-wide of unleaded fuel, a vast number of our customers, in an understandable move to be independent of political bargaining, are giving preference to diesel engines. In this case, smoke and particulates which are characteristics of the diesel engine will increasingly need to be controlled in order to avoid another series of concerns. These concerns are recognised however and development of particulate and gaseous emission standards for diesel engined vehicles is already well advanced at the EC Commission, since for both a final proposal for a Council directive was submitted to the Council in the month of June. For the gaseous emissions for heavy commercial diesel vehicles over 3.5 t gross vehicle mass, the proposal is equivalent to the United Nations regulation R 49 but with the levels reduced by 20% for CO and NOx and 30% for HC, and this starting for new engine homologation on April 1, 1988 and for new registrations on October 1, 1990. Concerning the particulates emissions for diesel passenger cars, the proposal is based on the US measurement method, transposed into the European test procedure and intended to be introduced in two stages, first large cars: October 1, 1988 for new models and October 1, 1989 for new registrations, followed by medium and small cars: from October 1991 for new models and 1993 for new registrations. Legislating satisfactory and consistent diesel fuel quality will also enhance the environmental impact of diesel vehicles. Unlike vehicle legislation which would affect only new vehicle designs, attention to fuel quality would also benefit the environmental performance of the existing diesel vehicle park. Please allow me to interject here that changes in diesel or gasoline fuel quality in the recent past, pressures on refiners resulting in more secondary processing, reduction of lead in leaded gasoline, the introduction of unleaded gasoline and the introduction of three-way catalyst systems - all mean that a pan-European specification for both fuels is both timely and appropriate. Methanol. ethanol and other organic or oxygenate compounds added to gasoline to make up for a decrease in lead and thus to improve its anti-knock quality could also create health and environment problems. Aldehydes, polycyclic aromatic compounds, benzene, ethene, organic acids: some of them are known as potential respiratory, eye and skin irritants, others could mutate cells or cause cancer under specific conditions, dependent on concentration of dose... etc. - and, though there is considerable uncertainty as to the
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magnitude of the health risks, one should take them into account in any final appraisal. Replacement of asbestos by another product able to respond to equal heat and friction characteristics may well result in a similar atmospheric trace pollutant. It is essential that, before any alternative solution is enforced, one should as far as technically possible - be convinced about its advantages and health characteristics, in other words, its benefits to the environment as well as to its performance, durability, and resistance to deterioration which has to be equal or at least similar to the one being replaced, and - it has to be economically acceptable. All the costs for making this wide variety of vehicles and for testing/approving them are to be paid by "the consumer". His ability to pay for complex technology, including periodic replacement or maintenance, has to be taken into account. It is all part of the balance that needs to be kept between the desirable and the inappropriate, the necessary and the ideal. The estimates prepared by the EC Commission that the annual cost to the Community for the emission issue alone could exceed ten billion ECDs is believed to be accurate. Ford would support the view that all of the implications of the use of threeway catalysts - and to some extent oxidation catalysts - have NOT been fully explored. The concentration of the major, non-communist, supply sources for the raw materials in one country - South Africa - is also a cause for some concern, as are the recent reports of a fourfold increase in the price of rhodium, an essential constituent of catalysts. From less than $ 300 an ounce in 1984 to over $ 1.150 an ounce at the beginning of 1985, falling below $ 800 an ounce mid 1985, to climb again to $ 1.100 end 1985. This shows the instability and uncertainty which exist with the much-needed raw material for the present generation catalyst: the noble metal. The capability of meeting the potential automotive demand for rhodium from known reserves is also in doubt. Also platinum - about two grarnmes goes into each catalyst - stood in August at the highest levels since 1980: prices have more than doubled from a low of $ 237 an ounce last year to $ 545 an ounce, after reaching $ 560. New surges to $ 600-700 an ounce are not implausible. If the entire European auto market were to use catalysts, estimates of an additional annual demand of 500.000 ounces of platinum, 150.000 ounces of palladium and 30.000 ounces of rhodium have been put forward, depending on the number of automobiles sold in the EEC in a given year. The increases in total metal requirements from 1985 levels are expected then to be about 19% for platinum and 17% forrhodium. Not being an economist, I would nevertheless tend to predict in those circumstances some rather drastic price increases on the bullion market.
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And, remember, first cost is never the whole story: costs have a nasty habit of coming back and biting the purchaser again and again, by way of increased maintenance, replacement and general operating costs. Finally, allow me to underline the need to avoid any sort of unique "pioneer role" or "cavalier seul" behaviour of an individual government. European harmonisation of the environmental exhaust emission legislation is a basic demand from industry - to improve the general quality standard, through tess complexity and thus fewer line disturbances, and to avoid distortion of competition and trade between the European countries and to create - through excellence and economy of scale - the best possible product for the best possible price. It appears, however, that we are confronted with at least four different emission standards across Europe - 15.04, EC fifth amendment, Swedish A 10, US 1983/1987 - and this is, to say the least, to be deplored. Even those countries who have collectively agreed to go the 83 US route have different dates/procedures/conformity requirements, all of which increase the manufacturer's burden with no environmental benefit. We have had to undertake the radical re-engineering of no fewer than thirtyseven current and future engine applications. The cost of this enterprise is certainly not less than 200 million ECUs or dollars. No manufacturer, or administration for that matter, has a bottomless pot of gold, and the consumer, equally, has to operate within financial constraints. The cleanest, quietest, road vehicle in the world is of no use if few people can afford to buy it, run it, use it. In the end, this all adds up to compromise: The EC compromise might be assessed as giving at best half-hearted support for the new European technology of lean-burn. It is something - and in view of the circumstances - possibly the only political option left. We have an expression in Belgium: "Qui aime bien, chatie bien'V''Een goede vader spaart de roede niet"I"You always hit the child you love the most!" This may after all be the rationale behind it. Should this give us hope for the future? Thank you, Mr President.
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A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CONTROL OF DIESEL PARTICULATE EMISSIONS IN EUROPE by Michael P. Walsh
Formerly Director of the U.S, Environmental Agency's Office of Mobile Source Air Pollution Control, currently an environmental consultant in the motor vehicle pollution field.Address : 2800 N Dinwiddie Street, Arlington, Virginia 22207, U.S.A.
ABSTRACT The greater use of diesel equipped vehicles for private cars and all categories of commercial vehicles is the major trend observed worldwide over the last decade in the motor vehicle field. While the energy advantages of the diesel are unquestioned, concerns began to grow during the 1970's over the environmental consequences of increased dieselization. Although inherently cleaner than gasoline engines from the standpoint of carbon monoxide (CO) and evaporative hydrocarbons (HC), diesels produce more aldehydes, sulfur oxides (because of the higher sulfur content in diesel fuel than in gasoline) and nitrogen oxides. Offensive smoke and odor emissions are also a problem. Most importantly, however, uncontrolled diesels emit significant amounts of particulate. These particles are a direct health concern as well as a serious source of overall environmental degradation. The purpose of this presentation is to review the information regarding adverse health and environmental consequences associated with diesel particulate. In addition, possible control strategies will be summarized.
I. Background
The greater use of diesel equipped vehicles for private cars and all categories of commercial vehicles is the major trend observed worldwide over the last decade in the motor vehicle field. While, for the first three quarters of this century, the gasoline fueled internal combustion engine (ICE) powered the automobile industry to ever greater peaks of prosperity, the dramatic increase in fuel prices spurred by the OPEC oil embargo and reinforced by the later Iranian crisis, sent the world's automotive engineers searching for a more fuel efficient alternative. For the first time a potential market opportunity for an alternative engine was created - one which promised significantly better fuel efficiency than the conventional gasoline fueled, otto cycle powerplant.
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Not surprisingly, almost all eyes focused on the diesel, a powerplant which had been in common usage on trucks for almost as long as the otto cycle was used in cars. It was familiar, reliable, had the nucleus of a fuel distribution system in place, and most importantly had demonstrated advantages in fuel efficiency. Across the full range of driving conditions, diesel fuel economy was at least 25 percent better than gasoline cars of the same weight and size. In stop and go urban traffic, the efficiency advantage rose to 35 or 40 percent. Worldwide diesel car production increased from about 1.3% of the total passenger car market in 1976 to 5.4% in 1983. With commercial vehicles, worldwide penetration in both vehicle categories is continuing to grow. While the energy advantages of the diesel are unquestioned, concerns began to grow during the 1970's over the environmental consequences of increased dieselization. Although inherently cleaner than gasoline engines from the standpoint of carbon monoxide (CO) and evaporative hydrocarbons (He), diesels produce more aldehydes, sulfur oxides (because of the higher sulfur content in diesel fuel than in gasoline) and nitrogen oxides. Offensive smoke and odor emissions are also a problem. Most importantly, however, diesels emit substantial amounts of fine particulate. Because of this, during its 1985 deliberations regarding motor vehicle pollution issues, the European Community Environmental Ministers asked the Commission to develop a proposal to control diesel particulate emissions. Though originally intended by the end of 1985, it was not possible for the Commission to meet this deadline. However, during June, the Commission approved proposals for two new directives aimed at reducing air pollution caused by diesel powered vehicles. Unfortunately, only one, dealing with passenger cars, addressed particulate emissions and even this did little more than maintain the status quo. The purpose of this paper is to review the reasons why control of diesel particulate emissions is urgently needed, especially in Europe, to show that the technology is available to reduce these emissions and to illustrate the potential impact of introducing this technology in Europe.
II. Health and Environmental Concerns with Diesel Particulate Uncontrolled diesels emit approximately 30 to 70 times more particulate than gasoline-fueled engines equipped with catalytic converters and burning unleaded fuel. These particles are a concern from several standpoints:
1. Many areas already experience unhealthy air quality levels for total suspended particulate (TSP) matter. Most TSP comes from stationary sources but diesels
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contribute. These particles in urban air are of concern because a strong correlation between suspended particulate and variations in infant mortality and total mortality rates has been established. Further, clear evidence emerges from the body of epidemiological literature that implicates particles in aggravating disease among bronchitics, asthmatics, cardiovascular patients and people with influenza. Any significant increase in diesel particulate emissions would add to the difficulty of solving this problem. 2. Beyond the overall impact on TSP, diesel particles raise a special health concern because they are very small (averaging about 0.2 microns in size). SmalI particles, which are much more likely to be deposited in the deepest recesses of the lung (alveolar region) and which require much longer periods of time to be cleared from the respiratory tract, have a greater potential to adversely affect human health than larger particles. In addition, when emitted, they remain suspended in the air near the breathing zones of people for long periods of time. For these reasons, the Harvard University Health Effects Project recently concluded that "particulate pollution should be a public health concern because, even at current ambient concentrations, it may be contributing to excess mortality and morbidity. Furthermore, our recent analyses .... indicate that fine particles (FP) and sulfates (S04=) are among the most harmful particles to public health." 3. In addition, diesel particulate has also been singled out as especially hazardous and toxic because of its composition. The U.S. EPA has noted that up to 10,000 chemicals may be adsorbed on the surface of diesel particles and drawn deep into the lung with them. Many of these chemical compounds are known to be mutagenic in short term bioassays, and to be capable of causing cancer in laboratory animals. Based on an exhaustive multiyear program of in vitro and in vivo studies by EPA and others focusing on the comparative potency of diesel particulate with other known human carcinogens, EPA estimated the risk to range from 0.26 x 10-6 to 1.4 x 10-6 lung cancers per person per year due to a constant lifetime exposure to one microgram per cubic meter of diesel particulate. Since total national urban exposure to diesel particulate in the United States was estimated to range from 3 to 5 micrograms per cubic meter by 1995, it is easy to see why this has been a cause of great concern. Two new animal studies, one sponsored by General Motors and another underway at Lovelace Inhalation Toxicology Research Institute laboratories appear to add further evidence of the cancer risk. A recent study conducted by rno found similar problems in Europe. "As a tentative order of magnitude estimate for the mutagenicity of European exhaust, the following emission factors may be assumed:
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Because as noted in the Harvard project "most of the toxic trace metals, organics, or acidic materials emitted from automobiles or fossil fuel combustion arc highly concentrated in the fine particle fraction" and since diesel engine penetration in Europe is much greater than in the United States, the potential cancer risk is also substantially greater. Other epidemiologic studies have tended to reinforce these concerns. For example, a 1983 study of heavy construction workers found positive trends in lung cancer by length of union membership and a higher than expected rate among retirees. Further, a pilot study of U.S. railroad workers, conducted by researchers at Harvard, indicated that the risk ratio for respiratory cancer in diesel exposed subjects relative to unexposed subjects could be as great as 1.42, i.e., the possibility of developing cancer may be 42 percent greater in individuals exposed to diesels than in individuals which are not exposed. The follow up study which has now been completed appears to be equally alarming - "Using multiple logistic regression to adjust for smoking and asbestos exposure, workers age 64 or less at the time of death with lung cancer had increased relative odds (1.2 - 1.4, P less than 0.05) of having worked in diesel exhaust exposed jobs." Clearly, it is prudent to conclude that greatly increased numbers of diesels without substantial particulate controls could result in a significant increase in cancer risks in Europe as well as elsewhere. Further, since the diesel car population in the Community is projected to grow from today's 5.8 million to about 15 million by 1995, without substantial controls the risk will increase tremendously. 4. While health issues have been the cause of most concern, diesel and other particles can also become a nuisance, degrade aesthetics and material usage through soiling and may contribute directly, or in conjunction with other polluants, to structural damage by means of corrosion or erosion. 5. Impairment of visibility has been widely noted as an adverse effect of increased particulates. Diesel particles because of their composition (primarily carbon based) and size (in the size range of 0.2 microns) are very high light absorbers and scatterers and therefore have the potential to be especially harmful to visibility. During late 1985, the results of several new studies were presented which increased concerns regarding adverse health effects from diesel particulate emissions. In particular; 1. Stoeber (Fraunhofer Institute) reported on carcinogenicity in rodents after long term high dose diesel inhalation. On both mice and rats, malignant tumors increased with exposure to diesel exhaust. With the mice, however, gaseous phase
55
emissions seemed most important whereas with the rats the particles seemed to be the main cause. 2. Brightwell (Batelle-Geneva) reported that unfiltered diesel exhaust produced an increase in lung tumor incidence from 19% to 40%; gasoline emissions reportedly showed no effect. 3. In a summary presentation, McClellan (Lovelace) described the issue as no longer whether diesel exhaust is carcinogenic but rather under what conditions and how much.
III. Diesel Smoke and Particulate Control Outside Europe Because of these various problems associated with diesel smoke and particulate, control programs have been underway for many years. This next section will review the history of these programs to date. In general, one can note that the initial focus was on smoke control because it was clearly visible and a nuisance. As the evidence has grown in recent years regarding the serious health and environmental problems, more attention has focused on control of the particles themselves. Smoke is composed primarily of unburned carbon particles from the fuel and usually results when there is an excess amount of fuel available for combustion. This condition is most likely to occur under high engine load conditions such as acceleration and engine lugging when the engine needs additional fuel for power. Further, a common maintenance error, failure to clean or replace a dirty air cleaner, may produce high smoke emissions because it can choke off available air to the engine resulting in a lower than optimum air-fuel mixture. Vehicle operation can also be important since smoke emissions from diesel engines are minimized by selection of the proper transmission gear to keep the engine operating at the most efficient speeds. Moderate accelerations and lower highway cruising speed changes as well as reduced speed for hill climbing also minimize smoke emissions. United States U.S. emission control requirements for smoke from engines used in heavy duty trucks and buses were first implemented for the 1970 model year. These opacity standards were specified in terms of percent of light allowed to be blocked by the smoke in the diesel exhaust (as determined by a light extinction meter). Heavy duty diesel engines produced during model years 1970 through 1973 were allowed a light extinction of 40 percent during the acceleration phase of the certification test and 20 percent during the lugging portion; 1974 and later model years are subject to smoke opacity standards of 20 percent during acceleration, 15 percent during lugging, and 50 percent at maximum power. It appeared to the EPA during the early 1970's that before very significant
56
pollution controls on trucks and buses could actually be brought about a new test procedure encompassing truly representative modes of usage in urban areas was needed. A multiyear effort to develop such a test was therefore initiated. While this work was underway, EPA became alarmed by the sudden growth in diesel cars which started during the late 1970's. Even though trucks and buses were clearly more important sources of particulate than cars and light trucks at that time, EPA concluded that the latter vehicles were very significant and that it was possible to initiate controls on these vehicles more quickly than on trucks and buses. Accordingly, the first diesel exhaust particulate standards in the world were established for cars and light trucks in an EPA rulemaking published on March 5, 1980. Standards of 0.6 grams per mile (0.37 g/km) were set for all cars and light trucks starting with the 1982 model year dropping to 0.2 grams per mile (0.12 g/km) and 0.26 (0.16) for 1985 model year cars and light trucks, respectively. In early 1984, EPA delayed the second phase of the standards from 1985 to 1987 model year. Almost simultaneously, California decided to adopt its own diesel particulate standards - 0.4 grams per mile (0.25 g/km) in 1985,0.2 (0.12) in 1986 and 1987, and 0.08 (0.05) in 1989. Less than one year later, in January 1981, EPA formally proposed similar particulate standards for trucks and buses. A comprehensive urban truck and bus test procedure had been developed by that time and analysis clearly showed that smoke controls were inadequate to bring about truly significant particulate reductions. A four year delay ensued before final action was taken by EPA. During this time, a new Administration at EPA reevaluated the need for diesel particulate control as well as the newly developed truck test procedure. These reevaluations reached the same fundamental conclusions as the earlier work - truck and bus controls is extremely important because the pollutants involved endanger the public health and environment and trucks are a major contributor to those pollutants; as a result, the first particulate standards for heavy duty diesel engines were promulgated by the U.S. EPA earlier this year. Standards of 0.60 grams per Brake - Horsepower - Hour (g/bhph) (0.80 grams per kilowatt-hour) were adopted for 1988 through 1990 model years, 0.25 (0.34) for 1991 through 1993 model years and 0.10 (0.13) for 1994 and later model years. Because of the special need for bus control in urban areas, the 0.10 (0.13) standard for these vehicles will go into effect in 1991, three years earlier than for heavy duty trucks. These standards are required to be met over the full life of the vehicle or engine, rather than over half the life as is the case with cars. Also, EPA based the standard on the new "transient" test referenced above rather thanon the old "steadystate" test because the transient test is much more representative of the manner in which trucks are driven in cities.
57
Canaan In March of 1985, in parallel with a significant tightening of gaseous emissions standards, Canada adopted the V.S. particulate standards for cars and light trucks (0.2 and 0.26 grams per mile, respectively) to go into effect in the 1988 Model Year. Since then, Canada has initiated a review of truck controls and is considering adoption of V.S. standards for these vehicles as well.
Japan Japan does not currently regulate exhaust particulate emissions from diesel engines. However, smoke standards have applied to both new and in-use vehicles since 1972 and 1975, respectively. The maximum permissible limits for both are 50 percent opacity; however, the new vehicle standard is the more stringent because smoke is measured at full load, while in-use vehicles are required to meet standards under the less severe no-load acceleration test. Smoke standards versus particular standards While smoke standards provide a limited degree of emission control, by not focusing on particulate levels over an average driving cycle and because they are fairly lenient, their effect actually reducing particulate emitted is somewhat limited. It is safe to say that particulate emissions throughout the world outside the U.S. remain virtually uncontrolled at the present time.
IV. The European Response To Date A. Common Market Smoke limits similar to those described above in the United States and Japan have been in effect in Europe for many years. However, recognizing that these requirements are not adequate, during its 1985 deliberations regarding motor vehicle pollution issues, the European Community Environmental Ministers asked the Commission to develop a proposal to control diesel particulate emissions. Specifically, the proposed standards are:
Type approval 1.3 g/test
Conformity of production 1.7 g/test
These standards, which many European produced vehicles are already achieving, are intended to be introduced on the following timetable:
58
I
I Vehicle type New models over 2 000 cm3 2 000 cm3 or less D.I.
Introduction date New cars
October 1988 October 1991 October 1994
October 1989 October 1993 October 1996
Studies conducted by Germany have indicated that the approximate conversion rate between the US and ECE tests is about 3, i.e., 0.2 grams per mile on the US test is roughly equivalent to about 0.6 grams per test on the ECE test. Approximate conversions are summarized below: ~~
Test procedure
us
ECE Gramshest Gramdkilometer
Gramdmile 0.6 0.2 0.08
1.8 0.6 0.24
0.45 0.15 0.06
B. Non Common Market European Countries In stark contrast to the Common Market, several other European countries have been cooperating in moving toward more significant diesel particulate requirements. Sweden has already adopted the US passenger car standard to go into effect in 1989 and Switzerland and Austria are likely to do so in the near future. These countries are also looking hard at more stringent requirements for trucks and buses.
V. Impact of the Commission Proposal On Common Market Emissions Overall diesel car sales increased by 21.3% across Europe from 1984 to 1985.As illustrated below, the increase was even greater in some countries.
Country Belgium-Luxembourg Denmark France Ireland Italy Netherlands Spain United Kingdom West Germany
1985
95 000 10 400 264 800 8 400 438 600 71 300 124 900 66 200 530 800
9% Market 26.4 6.6 15.0 14.2 25.1 14.4 22.6 3.6 22.3
1984
% Change
96 900 10 300 240 400 6 000 425 300 61 000 126 700 45 100 321 800
1984-1985 -1.9 0.2 10.1 41.7 3.1 17.0 -1.4 46.8 64.9
59
It is especially ironic that West Germany has encouraged the growth of high particulate emitting diesel cars by allowing them to qualify for "low pollution" tax incentives without any requirement that they meet the same particulate levels as Gennan models exported to the US. In part as a result of these tax incentives, diesel car sales in Germany have accelerated in the last year, as is illustrated in Figure 1. Fortunately, in adopting its low pollution tax policies earlier this year, the Netherlands did not provide similar tax reductions for uncontrolled diesels. Because of the high growth rate for diesel vehicles in Europe and the modest particulate reductions proposed by the Commission, it appears that overall particulate emissions will grow tremendously throughout the next twenty five years. This is illustrated in Figure 2 which plots motor vehicle particulate during this period. This figure shows that emissions from all categories of vehicles will continue to grow under the Commission proposal. Even these projections may be understating the potential problem, however, as concerns have been growing that diesel fuel quality may deteriorate significantly in the future in Europe. Should this happen, particulate emissions will likely rise even further.
VI. What Is Possible Light duty vehicles Fortunately, it is possible to do something about these problems. Two major approaches exist for meeting stringent diesel particulate standards: engine modifications to lower engine out emission levels, and trap-oxidizers and their associated regeneration systems. Engine modifications include changes in combustion chamber design, fuel injection timing and spray pattern, turbocharging, and the use of exhaust gas recirculation. Further particulate controls appear possible through greater use of electronically controlled fuel injection which is currently under rapid development. Using such a system, signals proportional to fuel rate and piston advance position are measured by sensors and are electronically processed by the electronic control system to determine the optimum fuel rate and timing. Exhaust aftertreatment generally consists of a filter or trap to capture the particulate and a regeneration system to convert it to less harmful materials; Trap oxidizer prototype systems have shown themselves capable of 70 to 90 percent reductions from engine out particulate emissions rates and with proper regeneration the ability to achieve these rates for high mileage. Systems have now started to be introduced commercially. Figure 3 shows the distribution of emission results for 1986 model cars in the United States. Compared to an average emission rate of 0.6 grams per mile in 1980, it can be seen that current emissions now average about 0.2 grams per mile. (It
60
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FIG.2
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ESTIMATION OF PARTICULATE EMISSIONS FROM 1985 TO 2010 IN EUROPE FOLLOWING ECE COMMISSION PROPOSAL.
61
is important to note that this reduction occurred at that same time that NOx emissions were literally cut in half, from 2.0 grams per mile to 1.0 grams per mile.) The cleanest vehicles in this figure are equipped with the first generation trap oxidizer systems, which are designed to capture and burn the particles. Daimler Benz introduced two models equipped with these systems in California and neighboring Western states for 1985 Model Year. To date, over 20,000 vehicles equipped with these systems have entered commercial service without significant problems. As recently noted by Daimler Benz in full page advertisements in the Washington Post and the New York Times, "an ingenious trap oxidizer ... virtually eliminates visible diesel exhaust emissions." Further, field tests by Volkswagen of a trap system and an on board fuel additive regeneration system on vehicles operated up to 25,000 miles has proven potential to achieve a 0.08 gram per mile level. Some 1986 Mercedes models were certified in California below 0.08 (while simultaneously achieving less than 1.0 gram per mile NO x)' Truck and bus control technologies While not yet as far advanced, control technologies for trucks and buses are similar to those for light duty vehicles: engine modifications to lower engine out emission levels, and trap-oxidizers and their associated regeneration systems. As noted earlier, there was a four year delay between EPA's initial proposal for heavy duty truck particulate control and final EPA action. During this period, most manufacturers reduced their particulate control development work. However, two manufacturers which did continue their efforts, Daimler-Benz and Volvo White, made significant progress, with Daimler Benz predicting trap availability in 1990 and Volvo White in 1991. Daimler-Benz's position was based upon what appears to be the most advanced development and test program of any heavy duty manufacturer. Current applications of its traps on urban buses have already demonstrated a service life of 100,000 miles. When adopting standards in March of 1985, EPA emphasized its optimism that the particulate standards would be achieveable in spite of the limited work done to date by heavy truck manufacturers. First, the Agency noted that "review of new information submitted on the subject of trap oxidizer feasibility indicates for light duty diesels, continued progress has been made in solving the various technical difficulties associated with traps. Daimler-Benz has already introduced traps on light duty vehicles in California, and Volkswagen and other manufacturers will do so in the 1986 Model Year." Secondly, regarding heavy duty vehicles, EPA noted that "what little work has been done also indicates progress. Traps are not fully developed today, but they were not expected to be. The important issue is whether they can reasonably be expected to be available for future standards, and on this issue, EPA's position is unchanged. In fact, the new data which were included in manufacturers' comments were extremely promising, and EPA is confident in its projections of successful application of traps to heavy duty engines." Extensive cost-benefit analyses carried
62
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VI
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DISTRIBUTION OF EMISSIONS FOR 1986 MODEL CARS IN U.S.
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ESTIMATION OF PARTICULATE EMISSIONS FROM 1985 TO 2010 IN EUROPE FOLLOWING U.S. STANDARDS.
out on heavy duty vehicles has shown that truck and bus controls are even more costbeneficial than passenger car standards. One additional point is worth noting. For diesel buses especially, because of their long lifetimes and their intensive usage in densely populated urban areas, retrofit is a very attractive strategy and is being pursued by many areas around the world. VII. Tighter Standards Could Help Europe If the European Community were to require the introduction of standards similar to those being introduced in the United States, it could continue to enjoy the benefits of the diesel engine while reducing the environmental consequences. For example, Figure 4 shows what could happen if US type standards were introduced in Europe. In making these projections, the US standards for commercial vehicles were assumed to be introduced in 1995, the large car standards in 1989 and the small car standards in 1993. The contrast between the Commission proposal and US type standards is further illustrated in Figures 5, 6, 7 and 8. The figures show that not only can overall emissions be reduced, even with fairly large vehicle population growth, the pollution from individual categories can also be substantially lowered.
VIII. Conclusions Based on the information summarized above, it seems clear that the adverse health and environmental consequences of diesel particulate emissions are sufficiently serous to justify control to the limits of technological feasibility. In addition, technology has been sufficiently developed for cars and light trucks to achieve the 1987 U.S. standards (0.2 grams per mile and 0.26 grams per mile, respectively) and looks extremely promising for the 1989 California standard of 0.08 grams per mile. Finally, substantial progress has also occurred for truck and bus controls, both new and used. In view of this, and in view of the even greater need for particulate control in Europe than in the US because of the much greater proportion of the fleet powered by diesel engines, a much stronger proposal is warranted than that recently proffered by the Commission. It should include the following key elements: 1. The mandatory particulate standard should be 0.6 grams per test. 2. New truck and bus controls, similar to those recently adopted by the US EPA, should be adopted. 3. Because of the high public exposure to emissions from urban buses as well as the long lifetime of the existing fleet, the Community should do all possible to encourage the retrofit of urban buses with advanced particulate controls.
64
o 400
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STAIDARD
~ - - - - - - - - - - - - - - - - - - - - - - - - - : J l J
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CALENDAR
FIG.S
ESTIMATION OF PARTICULATE EMISSIONS IN EUROPE FROM 1985 TO 2010. COMPARISON BETWEEN ECE COMMISSION PROPOSAL AND U.S. STANDARDS. ( ALL DIESEL VEHICLES )
E22l
·
US
STAlDIIG
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FIG.6
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YEAR
ESTIMATION OF PARTICULATE EMISSIONS IN EUROPE FROM 1985 TO 2010. COMPARISON BETWEEN ECE COMMISSION PROPOSAL AND U.S. STANDARDS. (SMALL CARS )
~
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FIG.7
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ESTIMATION OF PARTICULATE EMISSIONS IN EUROPE FROM 1985 TO 2010. COMPARISON BETWEEN ECE COMMISSION PROPOSAL AND U.S. STANDARDS. ( LARGE CARS )
E2L1
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CALENDAR
FIG.8
2000
2005
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YEAR
ESTIMATION OF PARTICULATE EMISSIONS IN EUROPE FROM 1985 TO 2010. COMPARISON BETWEEN ECE COMMISSION PROPOSAL AND U.S. STANDARDS. (COMMERCIAL VEHICLES )
66
In addition, member countries which have adopted tax incentives to encourage consumers to purchase low pollution vehicles should either exclude diesels from any tax credits or require them at a minimum to achieve 0.6 grams per test (BCE test) or 0.2 grams per mile (US test) particulate to qualify for low pollution tax credits. IX. References
1. "The Benefits and Costs of Light Duty Diesel Particulate Controls," Michael P. Walsh, SAE #830179 2. "The Benefits and Costs of Light Duty Diesel Particulate Controls II," Michael P. Walsh, SAE, February 1984 3. "The Benefits and Costs of Light Duty Diesel Particulate Controls III - The Urban Bus," Michael P. Walsh, SAE, February 1985 4. "The Benefits and Costs of Light Duty Diesel Particulate Controls IV - The InUse Urban Bus," Michael P. Walsh, SAE, February 1986 5. "Cancer Incidence Among Members of A Heavy Construction Equipment Operators Union With Potential Exposure To Diesel Exhaust Emissions," Submitted to Coordinating Research Council by Environmental Health Associates, 18 April, 1983 6. Mortality Among Members of A Heavy Construction Equipment Operators Union With Potential Exposure To Diesel Exhaust Emissions," Submitted to Coordinating Research Council by Environmental Health Associates, 18 April, 1983 7. "Relation of Air Pollution To Mortality: an Exploration Using Daily Data for 14 London Winters, 1958-1972", Mazumdar, Schimmel, Higgins, Electric Power Research Institute, Palo Alto, 1980 8. "Diesel Cars, Benefits, Risks and Public Policy," National Academy of Sciences, December 1981 9. "Review of Recent Information Regarding Carcinogenicity of Diesel Engine Emissions", Pepelko to Gray, U.S. EPA, June 14, 1985 10. U.S. Environmental Protection Agency, Heavy Duty Diesel Particulate Regulations, Draft Regulatory Analysis, Approved by Michael P. Walsh, December 23, 1980 11. "Impact of Light Duty Diesels On Visibility in California," Trijonis, March 1982 12. "Diesel Exhaust Odor and Irritants: A Review," Nicholas P. Cernansky, Journal of the Air Pollution Control Association, February 1983 13. U.S. Environmental Protection Agency, Control of Air Pollution From New Motor Vehicles and New Motor Vehicle Engines; Gaseous Emission and Particulate Emission Regulations, Federal Register, March 15, 1985 14. "Trap-Oxidizer Technology For Light-Duty Diesel Vehicles: Feasibility, Costs and Present Status," Weaver and Miller, Report to EPA by Energy and Resource Consultants, March 1983 15. "Diesel Technology", National Research Council, Report of the Technology Panel of the Diesel Impacts Study Committee, National Academy of Sciences, 1982 16. "Draft Environmental Guidelines On The Diesel Vehicle," Clavel and Walsh, United Nations Environment Program, March 1983
67
t 7. "Benefits of Reducing Odors From Diesel Vehicles: Results Of A Contingent 18. 19. 20. 21. 22. 23.
24. 25. 26.
Valuation Survey," Prepared for Environmental Protection Agency by Charles River Associates, March 1983 "Diesel Exhaust and Air Pollution," TNO, Netherlands Organization For Applied Scientific Research, January 1986 "Begrenzung der Partikelemission von Dieselfahrzeugen im Rahmen der europaischen Abgasvorschriften", Umweltbundesarnt, October 1985 CCMC Manufacturers' Measurements of Particulates On Diesel Engined Passenger Cars, November 1985 "Health Effects of Airborne Particles," Ozkaynak and Spengler, Health Effects Project Staff, Harvard University, February 1986 OECD, "Road Research Programme, Impact of Heavy Freight Vehicles, Final Report," September 28, 1982 Shenker, Smith, Munoz, Woski, Speizer, "Lung Cancer Among Diesel Exposed Railroad Workers, Results of a Pilot Study," Harvard School of Public Health, 1982 Schenker, Oral Statement, American Lung Association Convention, May 1984 U.S. Environmental Protection Agency, Federal Register, March 5, 1980. Standard for Emission of Particulate, Regulation for Diesel Fueled Light Duty Vehicles and Light Duty Trucks U.S. Environmental Protection Agency, Regulatory Analysis of the Light Duty Diesel Vehicles, U.S. Environmental Protection Agency, February 20,1980
This Page Intentionally Left Blank
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE PROBLEMS INVOLVED IN PREPARING AND UPHOLDING UNIFORM EXHAUST-GAS STANDARDS WITHIN THE COMMON MARKET by H. Henssler
Commission of the European Communities, Directorate Internal Market and Industrial Affairs 200 rue de la Loi, B-1049 Brussels, Belgium.
ABSTRACT Uniform exhaust emission standards for passenger cars and light duty vehicles exist since 1970 in the Common Market. In June 1985 the Council of Ministers for the Environment agreed by majority a further decisive reduction of these standards. The future European emission standards are based on the principle of equivalence of their environmental effect with that of the current US standards, taking account of the European conditions in particular with regard to the composition of the car fleet and its operating characteristics. Like the preceding steps of the EEC exhaust emission regulations, the new standards refer to the present European test procedure which however at a later stage should be completed by a test cycle representing extraurban driving conditions. The new European emission standards will apply to passenger cars with a maximum mass up to 2,500 kg having not more than 6 seats. The limit values and the effective dates are differentiated according to 3 categories of engine capacity. The presentation describes the aims, the development and the rationale of the EEC exhaust emission regulations and also gives a summary of their legal bases.
THE EEC'S LEGAL BASES AND METHODS OF PROCEDURE 1. In March 1970 the Council of Ministers of the European Communities adopted, as the second separate directive of the EEC type-approval procedure, directive 70/220/EEC "on the approximation of the laws of the Member States relating to measures to be taken against air pollution by gases from positive-ignition engines of motor vehicles". Since this date, uniform exhaust emission standards exist throughout the whole Community for the concerned category of vehicles. 2. The present contribution is intended to describe the aims, development and significance of the European Communities' exhaust-gas standards. It appears suitable, by way of an introduction, first of all to describe the essential aspects of the EEC's legal bases and methods of procedure. 3. A fundamental aim of the 1957 Treaty of Rome establishing the European Economic Community is the creation of a common internal market among the
69
70
Member States! within which cinzens, goods and services may cross borders without let or hindrance. Naturally, if goods are to move freely, first of all customs barriers and then the so-called non-tariff barriers or technical barriers to trade must be removed. The major non-tariff barrier to the cross-frontier trade in motor vehicles is constituted by the Member States' type-approval procedures, together with their various technical requirements and administrative practices. It has for good reasons been impossible simply to do away with each Member State's procedures and to replace them with an EEC type approval procedure which, however, was needed if barriers to trade of this type were to be avoided. 4. Use was therefore made of "optional harmonization" whereby the EEC typeapproval is established in parallel to the Member State procedures, but does not replace them. From this results the possibility of a choice at two levels: - Member States may retain, alongside the EEC provisions which they are bound to introduce, divergent national provisions, and - Manufacturers may, where Member States decide to retain national provisions, choose whether they wish to manufacture their products in accordance with EEC or Member State provisions.
The relevant national authorities must approve vehicle types, permit vehicles to be sold on their territory and to enter service if they comply with EEC provisions. However, they may deliver such approval on the basis of other criteria, such as any national technical requirements already existing. 5. The European Communities issue their technical and administrative standards in the motor vehicle field in the form of "directives". A directive is one of the legal instruments made available to the executive of the Communities under the Treaty of Rome in order that it may perform its function. It is addressed to Member States and for them its aim is binding, while the choice of means of implementation is left open to them. Under normal circumstances four European institutions work together in preparing a directive: the Commission, the European Parliament, the Economic and Social Committee and the Council of Ministers. Their respective functions are laid down in Article 100 of the Treaty of Rome: the Commission has the right of initiative i.e. it proposes directives, while the Parliament and Economic and Social Committee deliver opinions on it and the Council adopts and issues the directive. IThe founder members of the EEC: Belgium, Federal Republic of Germany, France, Italy, Luxembourg and the Netherlands. Subsequent accession of: Denmark, Greece, Ireland, Portugal, Spain and the United Kingdom.
71
LINKS BETWEEN EEC AND ECE STANDARDS 6. Before we go into individual detail on the development of the Community exhaustgas standards, it would seem appropriate to make one or two basic remarks concerning the relationship between the Commission of the European Communities and the United Nations' Economic Commission for Europe (ECE), Geneva as regards vehicle regulations. Under the 1958 Geneva Convention the ECE has adopted a number of such regulations, which, however, are not embedded in a complete type-approval procedure. As signatories to this Convention the majority of the EEC Member States base their national type-approval systems more or less on these regulations. The Commission has therefore been forced to take them into account. The advantages of technically equivalent standards within the Community and in the much wider area covered by the Geneva requirements are obvious. The EEC has thus decided to transpose the technical requirements of the ECE regulations into the corresponding separate directives where these are relevant to its type-approval procedure. DEVELOPMENTS SO FAR AS REGARDS THE EEC'S EXHAUST GAS STANDARDS 7. The need for effective exhaust gas regulations arose on both sides of the Atlantic in the late 60's. This caused the European Community to adopt its Directive 70/220/EEC in 1970. In content, it is equivalent to ECE Regulation 15. Like the American approach the European provisions limited emissions only of carbon monoxide (CO) and unburnt hydrocarbons (HC) for health-policy reasons, or more precisely because of their effects on the quality of the air ingested in large conurbations. Dynamic methods of measurements which were representative of the type of operation which vehicles were subjected to in inner city areas were devised to back-up the relevant approval tests. 8. It is nowadays difficult to quantify the improvement of vehicle emission behaviour intended by this first generation of exhaust gas standards since there has been no statistically reliable study of the preceding period i.e, there has been no basis for reliable comparison. However, it can be taken that these standards correspond to the state of the art at the time and thus on average made a certain improvement to the relevant emissions by the overall vehicle fleet. 9. Later on these standards were tightened up several times in both the United States and Europe. The fact that in the US also the driving cycle and in Europe the sampling and analysis methods were altered makes it difficult to carry out a direct evaluation of the reduction in vehicle emissions thus achieved. It can be said overall
72
that nowadays, following the three reduction stages set out in the EEC Directives in line with amendments 01, 03 and 04 to ECE Regulation 15, the European approval tests require emissions that are 50% lower than the original limit values for CO and He. The corresponding reductions in the United States are about 90%. 10.Initially nitrogen oxide (NO) emissions by vehicles were not covered by European or American legislation. However, they were soon identified in the United States as contributors to the well-known Los Angeles "smog" and were limited from 1973 onwards. Europe reacted somewhat hesitantly, as public health hazards due to climatic conditions of this type were still largely unfamiliar. Admittedly here too, a considerable increase in NOx emissions was recorded, not least as a result of the motor industry's efforts to meet the legal requirements concerning CO and HC emissions, but also to reduce fuel consumption - since the first energy crisis was now in full swing. The relevant state of the art was therefore enshrined in the limit values set out in Directive 77/102/EEC (ECE Regulation 15.02). After two further reductions the European standards lay roughly 30% below the original limit values. In this instance the United States have achieved a cut of 65%. l1.Basically the European approach can be described as progress through continuous adaptation of its legislation to the state of the art achieved by the European motor industry. Conversely American Legislation had from the outset been shaped by political objectives such as Senator Musky's 1970 demand that vehicle emissions be reduced by 90% within ten years.
EEC REACTION TO THE GERMAN STEPS TOWARDS THE INTRODUCTION OF "LOW POLLUTION" VEHICLES - ORIGINS OF THE "LUXEMBOURG COMPROMISE" OF 27 JUNE 1985 12.The steps in Germany towards the introduction of low-pollution vehicles made exhaust gas regulations a political issue in Europe as well. Owing to the legal situation at the start in the EEC this initiative had of necessity to result in a discussion of an appropriate amendment to the exhaust gas directive. The Commission was at pains to place the proposals required of it on an objective basis. It convened the Working Party known as ERGA (Evolution of Regulations, Global Approach) which, as part of its global approach, was first of all made responsible for identifying the relationships between vehicle emissions and air quality, for describing possible technical ways of reducing those emissions and for examining their economic impact. Under a second mandate the Working Party then mapped out strategies for the EEC-wide introduction of unleaded petrol. The know-how which the, in this form certainly unique, Working Party acquired was used by the Commission to form the basis for its June 1984 proposals on the
73
introduction of unleaded gasoline and on the reduction of the permissible exhaust gas emissions from motor vehicles. The latter, moreover, was based on the principle of equivalence with the environmental effect of the current US exhaust gas standards while taking account of European conditions with regard to motor vehicle fleet and its operating characteristics. These proposals received the approval of the majority of the Member States at the meetings of their environment ministers of 20 March and 27 June 1985. However, since Denmark and Greece entered fundamental reservations regarding the future "European exhaust emission standards" agreed by the majority, it has not been possible even now to adopt this directive formally. UNDERLYING ASPECTS OF THE COMMUNITY'S FUTURE EXHAUST GAS STANDARDS 13.We will now go on to the underlying aspects of the exhaust gas standards agreed by the majority. These will apply to passenger cars having a maximum permissible mass of up to 2500 kg, while the limit values and dates of entry into force are divided up into three engine capacity classes. The pollutants covered are carbon monoxide (CO), unburnt hydrocarbons (HC) and nitrogen oxide (N0x)' 14.1t is assumed that vehicles with an engine capacity of more than 2 litres already exist in a US version, or else that the appropriate catalytic converter technology can be applied to them without raising technical and economic problems. The new European standards for this class are based on measurements taken from vehicles for which a US exhaust gas certificate has been issued and are such that basically vehicles of this type can also be issued with EEC type approval. For the approval of new vehicle types the following limit values have been established: CO:25g!test, for the combined emissions of HC and NO x: 6.5g1test, for NO x: 3.5g/test. For the control of production conformity a certain tolerance to these values is granted which results in the following limit values: for CO: 30g/test, for the combined HC and NO x emissions: 8.1g!test and for NO x: 4.4g!test. These limit values will apply to new vehicle types from October 1988 on and to all vehicles registered for the first time from October 1989 on. 15.Essentially the same technology can apply to vehicles having an engine capacity between 1.4 and 2 litres, but it should be possible to provide cheaper
74
alternatives and in particular lean-bum engines having a centralized fuel supply, plus oxidation type catalytic convertors or equivalent. Therefore, and because statistically speaking mid-range vehicles cover a shorter annual distance than topof-the-range vehicles, slightly higher limit values apply to CO and the combined emissions of HC and NO x ' while additional flexibility is gained by dispensing with a separate NOx limit value. For the approval of new vehicle types the following limit values apply: for CO: 30g/test, for the combined emissions of HC and NOx' 8g/test. The corresponding limit values for the control of production conformity are: for CO: 36g/test, for the combined emissions ofHC and NO x' 109/test. These limit values will be implemented as from October 1991 for new vehicle types and as from October 1993 for all vehicles registered for the first time. 16.0wing to a lack of experience by the European motor industry in meeting the US standards where vehicles have an engine capacity of less than 1.4 I, it is not considered possible to lay down equivalent European standards for the moment. For the time being limit values corresponding to the current state of the art will be applied to vehicles in this category. For the approval of new vehicle types these values are: For CO: 45g/test, for the combined emissions of HC and NO x: 15g/test and for NO x: 6g/test. For the control of production conformity the following limit values apply: For CO: 54g/test, for the combined emissions of HC and NOx: 199/test and for NO x: 7.5g/test. These limit values will be implemented as from October 1990 for new vehicle types and as from October 1991 for all vehicle registered for the first time. Before the end of 1987 a fmal European standard for this vehicle class will be laid down which then will apply, from October 1992, to new vehicle types and from October 1993, to all vehicles placed in service for the first time. 17.The limit values referred to are based on the current European urban driving cycle, which is considered by the majority of the Member States basically still to be representative of the traffic conditions in European conurbations. The expansion of the European test procedure by adding driving conditions representative of car operation outside built up areas -considered by most of those involved to be desirably in the longer term - is to be decided upon by the end of 1987.
75
I8.For private cars with an engine capacity of 1.4 litres and more the directive provides, as an alternative to the European test procedure limited in time, for transposition of the relevant parts of the American certification process - basically the FfP-75 driving cycle. The underlying notion here is that manufacturers with vehicle types meeting the US specifications will first of all have to adapt these to European test conditions before they may, on this basis, apply for type approval, while on the other hand it is desirable for environmental protection purposes to offer such vehicles on the European market in the near future. The limit values are those of the 1983 federal "'49 states") standards, i.e.: for CO: 2.llg/km, for HC: 0.25g/km and for NO x: 0.62g/km. These limit values will apply both to typeapproval and to control of production conformity according to the American sampling procedure. 19.As it is aware of the specific problems affecting diesel engines and more particularly the meeting of stringent NOx limit values, the majority of the Member States agreed to the Commission's proposal to apply the limit values for vehicles of between 1.4 and 2 litres engine capacity to all diesel cars larger than l.4litres. This is also intended to secure the necessary flexibility in the subsequent establishment of limit values for particulate emissions from diesel engines for which in the meantime the Commission has presented a proposal. For vehicles equipped with a direct injection diesel-engine of a capacity between 1.4 and 2litres the new European emission standards will only apply from October 1993 (new types) and October 1996 (first registrations) respectively. Thereby the Member States intend to grant an additional lead-time for those manufacturers who are developing such engine concepts in view of a further improvement of fuel economy as well as exhaust emissions of future diesel car generations. 20.Equally, for private cars equipped with automatic transmissions exceptional provisions have been agreed, provided that such cars are derived from models with manual transmissions for which an EEC approval has already been granted. In this case, the automatic transmission version will be approved against limit values which result from the multiplication of the above-mentioned limit values multiplied with a factor of 1.2 for the combined HC and NOx emissions and I.3 for the NOx emissions. 21.With a view to the rapid introduction of unleaded petrol it has also been agreed that all new car types subject to approval which have engines larger than 2 litres must be designed for the exclusive use of such fuels from 1 October 1988 and those with engines of less than 2 litres from 1 October 1989. Moreover, from 1 October 1990, Member States of the Community may in general terms prohibit the approval of new vehicles which are unable to run on unleaded fuel. Where a manufacturer can demonstrate considerable technical difficulty in converting
76
vehicles from leaded to unleaded furl those vehicles will be exempted and the dates decided for the implementation of the new emission standards will apply. 22.As mentioned before, the new European emission standards apply to private cars of not more than 6 seats and a total mass of not more than 2,500kg. It is this category of vehicles - except those cars having an engine capacity of less than 1.4 litres - with which European manufacturers have been able to gain experience of the technology required to meet the US standards. For all other vehicles covered by the scope of the present directive, notably those of category N 1 (Tight duty trucks") it has been agreed to follow an approach analogous to that chosen for the private cars below 1.4 litres i.e. to introduce interim emission standards. These consist of the limit values of Directive 83/351/EEC, the application of which to the concerned vehicles will imply a reduction of their HC and NO x emissions in the order of 25% compared to the present situation. These interim standards will apply from 1 October 1989 to new vehicle types and from 1 October 1990 to all new vehicles put into service. In 1987 the Council will decide, on proposal of the Commission, the definitive European standards which should apply to these vehicles in 1993/1994.
ASSESSMENT OF THE "LUXEMBOURG AGREEMENT" 23.The fact that the majority decisions of March and June 1985 aim at an intrinsic European solution to the environment problems caused by road traffic must be welcomed without reservation in Brussels. 24.For several reasons the - apparently so obvious - direct transposition of the US exhaust gas regulation would quite definitely not have been a feasible solution for the Community as a whole. One thing we can mention here is the completely different legal frameworks of the American and European regulatory systems. For example, options, such as that open to the US approval authority, of suspending the application of stringent exhaust gas standards either in general terms of in respect of individual manufacturers, or, as in the case of the commercial-vehicle exhaust gas standards now proposed of permitting manufacturers to "offset" these is not provided for either in the EEC's type approval procedure or in those of the individual Member States. Secondly, the passenger car market in the USA cannot be compared with that in the Community. For example, vehicles with engines smaller than l.4litres capture an insignificant part of the US market, whereas in Europe their market share is
77
somewhat over 50%. Similarly, diesel cars have virtually disappeared from the US market but currently account for 14% of new registrations in the Community, and the figure is rising. 25.The compromise effected via the "Luxembourg Agreement" takes overall account of European conditions. On one hand it enables motor manufacturers within the Community to back up existing know-how in Europe with proven US technology. On the other hand, it leaves them the option where the greatest turnover is, i.e. in mid-range and small cars, of developing cheaper alternative solutions which offer a considerable reduction in fuel consumption alongside lower emission values, thereby making them particularly attractive to a wide range of customers. The marketing of low-pollution vehicles on a voluntary basis on the German market can be taken a') confirmation that the industry has grasped this opportunity. 26.As a precautionary measure the "Luxembourg Agreement" seems to guarantee achievement of the environmental-protection target. The European CO and HC emission standards for top-of-the-range vehicles are 87% lower, and in the case of NO x emissions 70% lower than the original 1970 limit values. These values are roughly 80%n3% below the original values in the case of mid-range vehicles. In order of magnitude these figures are fully in line with the reduction aimed at by the standards laid down in the United States. In addition, calculations based on the available documents on the vehicle fleet, the mode of operation of the individual vehicle classes and the specific emissions from these show that the overall NOx emissions from the EEC fleet will reach the American level of roughly 1.5 million tonnes per year, when predominantly made up of vehicles meeting the new EEC standards i.e. roughly around the year 2000. EXTENSION OF THE EUROPEAN EMISSIONS REGULATIONS ON OTHER POLLUTANTS AND EACH VEHICLE CATEGORY 27.Pursuant its undertakings at the Environment Councils of 27 June an 28 November 1985 relating to the gaseous emissions of passenger cars and light commercial vehicles covered by Directive 70/220/EEC, the Commission presented on 23 June 1986 two further proposals on motor vehicle emissions to the Council. 28.The first proposal concerns the particulate emissions of diesel engines equipping the motor vehicles covered by the above-mentioned Directive. Particulate emissions are besides smoke - which, since 1972 has been controlled by a Community Directive - a specific problem of diesel engines. The limitation of these emissions appears today the more necessary as the diesel car pare is increasing rapidly and is forecasted to reach about 15 million units in the mid-90's which shall be compared with the present 6 million units.
78
The proposal is based on the presently available though limited knowledge of the performances of the best available diesel technology in the European motor industry as a whole. Furthermore, it takes into account the lack of accuracy and reproductibility which result from the simple transposal, into the European test procedure, of the sampling and analysis method of the current US legislation which is at present the only codified method available. The limit value proposed for the type-approval of new car models, independent of the weight and engine size, is 1.3g per European test as defined in Directive 70/220jEEC. The limit value proposed for conformity of production, or otherwise, that which every new car must meet on its first registration, is 1.7g/test. These limit values are proposed to be implemented at the dates agreed at the Luxembourg Council for the implementation of the new European standards for gaseous emissions. By that means the Commission intends to assure that the motor industry can concentrate its resources on adapting its production to the new Community requrements as a whole and that the administrative procedures related to the type-approval of modified car models will be limited to what is strictly necessary. 29.The second proposal concerns the emissions of gaseous pollutants (CO, HC, NO x) from the diesel engines of commercial vehicles and buses of all weight classes. The contribution of the emission of these vehicles - which until now in Europe are not controlled except for smoke - to the general air pollution becomes more important since the continuing regulatory efforts of the Community to reduce the emissions of light motor vehicles take effect. Here again, the absence of any control of the concerned emissions of heavy vehicles in the Member States results in very little data about the emission performance of big diesel engines and the possibilities of their improvement in this respect. The present proposal is based on a regulation ("R49") of the Economic Commission for Europe of the United Nations, from which it takes over the test procedure. The limit values of this regulation, however, no longer correspond to the state of the art in diesel technology and an across the board reduction for all three pollutants concerned appeared possible. Hence, the proposal contains limit values which for CO and NO x are 20% below those of R49 and for HC 30% below the R49 limit. In absolute figures, the limit values proposed are
11.2 g/kWh for CO 2.4 g/kWh for HC and 14.4 g/kWh for NO x
79
It is proposed that these limit values will have to be complied with both by new vehicle types, at the latest on 1.4.1988, and by all new vehicles from 1.10.1990 onwards.
30.FURTHER DEVELOPMENT This presentation aimed at the explanation of the past developments and the present situation of the Communities' exhaust emission regulations. It needs to be understood that these regulations are in a permanent process of evolution. On the political level, initiatives are being taken to allow the "Luxembourg agreements" to become operational, to adopt the proposed particulate standards and the standards for gaseous emissions of commercial vehicles. On the technical level expert talks are going on about the future test cycle representing extra-urban driving conditions, about the definitive emission standards for passenger cars below 1.4 litres engine capacity and for light duty vehicles as well as about possible European requirements relating to the durability of anti-pollution devices and evaporation losses. These latter discussions should allow the Commission to present, by the end of 1987, appropriate regulatory proposals for the concerned matters to the Member States.
This Page Intentionally Left Blank
"\. Crucq and A. Frennet (Editors), Catalysis and A utomot ioe Pollution Control 1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE MARKET FOR CAR EXHAUST CATALYSTS IN WESTERN EUROPE A Review of Trends and Developments by Willem GROENENDAAL
STRATEGIC ANALYSIS-EUROPE, Brussels, Belgium
ABSTRACT The application of catalysts to control the emission of harmful components in the exhaust gases of cars equipped with gasoline engines is an established technology in North America and Japan since the early seventies. In the last year alone, some 14 million catalyst equipped cars were sold. In Western Europe the development started much later and last year major decisions were taken which will determine the shape of the future European market. The paper will discuss the options in pollution abatement technology available under the EC regulations and the major role catalysts are expected to play. Estimates will be presented on the future catalyst demand both in the EC and other West European countries. Cost breakdowns for three way catalyst manufacture will be included. Apart from catalysts manufacturers with an established position in North America a number of new suppliers may well emerge in Western Europe. Announced catalyst and carrier manufacturing capacities will be compared with future market requirements. The demand for lead free gasoline in Western Europe is determined by the size of the catalysts equipped car population and government measures to stimulate the use. The expected demand developments will be discussed and the technology to produce lead free gasoline will be highlighted. INTRODUCTION Car exhaust catalyst systems have been installed in the majority of the new cars in the USA since model year 1975 and on all new gasoline fueld cars since 1981. The use of catalysts in the exhaust system of passenger cars and light duty commercial vehicles is considered the most practical way to comply with emission control standards in the USA and Japan. The technology is considered mature and proven (Ref. 1). In the last year alone some 14 million new catalyst equipped cars were sold and in total some 130 million catalyst cars are now in use in the world. Until 1985 catalyst equipped cars manufactured in Western Europe were exported to the USA and Japan. In 1985 legislation came in force in West Germany and Switzerland which encouraged the sales of cars with low levels of emissions of
81
82
pollutants. Also in that year lead free gasoline started to become available in most Western European countries. In 1985 fifty thousand new catalyst cars were sold in Western Europe. This paper will discuss the options in pollution abatement technology available to meet the EC regulations agreed upon in 1985 and the major role catalysts are expected to play. Estimates will be presented on the future catalyst demand both in the EC and other Western European countries. Cost breakdowns for catalysts will be included, which will show the major impact of the precious metal price and the metal content of the catalyst. Apart from catalyst manufacturers with an established position in North America new suppliers may well emerge in Western Europe. Announced catalyst and carrier manufacturing capacities will be compared with future market requirements. The demand for lead free gasoline in Western Europe is determined by government measures to stimulate its use and the size of the catalyst equipped car population. The expected development of demand will be discussed and the technology to produce lead free gasoline will be highlighted. POLLUTANT EMISSION REGULATIONS AND LEGISLATION The reduction of pollutants in automobile exhaust and in industrial waste and stack gases is of major concern in Western Europe. This was fueled in the last years by the extensive forest die-back caused by acid rain. Government policies are to achieve substantial reductions in the emissions of hydrocarbons (HC's), CO, S02, NO x and particulate matter. Road traffic is one of the major contributors to the man made emissions of HC's, CO and NO x' International and national regulations, legislation and incentives coming into force will substantially reduce these emissions. Since 1970 substantial reductions in emissions of pollutants from passenger cars already were achieved in the countries of the European Community (EC): HC emissions were reduced by 60%, CO by 70% and NO by 30% (2). These reductions were obtained by engine modifications. In the USA and Japan the reductions in emissions have been much higher. This was caused by the stringent legislation which made the use of exhaust catalysts unavoidable. In June 1985 EC regulations were announced which set a timetable for a drastic reduction of pollutants from automobile exhausts in the next ten years. These new standards differ from the US an Japanese standards. In the EC the passenger car fleet has been divided into three engine cylinder volume classes: below 1.4 liter (1), between 1.4 and 21 and above 21. The year in which the new emissions standard has to be met and the level of the emission differ for each class. The so-called Stockholm Group are the other countries in Western Europe,
8:J
that is the Nordic countries, Austria and Switzerland. Within the next years these countries will adopt the US '83 emission standards. However, the timetables are different. Denmark is a member of both groups of countries. The testing procedures used for measuring pollutant levels and the methods checking whether emission standards are achieved differ in the USA, EC and Japan. The EC test, R 15n5, lasts 13 minutes from cold start and the maximum speed is 50 kilometers (km) per hour. The US test, FrP 75, lasts 41 minutes and the maximum speed is 90 km per hour. In the US regulations pollutant levels should be below the legal limit during a period of 50,000 miles with the same catalyst installed. The forthcoming EC directive is summarised in Table 1.
Table 1 EC Car Exhaust Gas Emission Directive Decided on June 28, 1985 Passenger cars for less than 6 persons and a weight below 2,500 kg Engine size
Proposed date for implementation
Proposed emission limits gr/test maximum*
> 2 liter
1.10.88 new models 1.10.89 all new cars
CO NO x
:25.0 3.5 HC+NO x : 6.5
30.0 4.4 8.1
1.10.91 new models 1.10.93 all new cars
CO NO x
:30.0
36.0
HC +NO x : 8.0
10.0
CO NO x
54.0 7.5 19
(except Diesel) 1.4 - 21iter
< 1.4 liter
Stage 1:
1.10.90 new models 1.10.91 all new cars
:45.0 6.0 HC +NO x : 15
Stage 2 :
> 2 liter
* EC test R ISn3
1.10.92 new models 1.10.93 all new cars
Decision in 1987 on limits
for respectively new model approval and productionchecks.
Each member state is free to decide whether and when it will adopt these EC regulations and delays in implementation could occur in some countries. Standards for maximum levels of particulate emissions are being developed and will be decided upon in 1987. Also an EC test cycle with a high speed section is under development.
84
Tax incentives have been created in West Germany and the Netherlands to stimulate the early introduction of low emission cars before the EC regulations become effective. The announced legislation for the Stockholm Group of countries is summarized in Table 2.
Table 2 STOCKHOLM GROUP EMISSION LEGISLATION Country
Date of implementation
Standard
Austria
voluntary + new diesel all new cars> 1.51 all new cars < 1.5 I all new vehicles below 2500 kg 01.10.87 all new vehicles below 3500 kg 01.10.86 voluntary 01.10.88 all new cars similar to Sweden
USA'83 USA'83 USA'83
Switzerland
Sweden Finland, Norway
25.05.86 01.01.87 01.01.88 01.10.86
USA'77 USA'83 USA'83 USA'83
USA '83 regulations, valid in 49 States and from 1.09.87 in Canada, are as follows: Emission limits light duty vehicles; HC: 0.41, CO: 3.4, NOx: 1.0 and particulate 0.20 (in gr/mile).
Comparisons between the US limits and the existing and forthcoming EC limits have been made (Ref. 2). The relation between the two is strongly influenced by the type of exhaust clean up system used. In Table 3 the comparison between the US and EC limits has been expressed according to the EC test method, which results in a wide spread of the US emission limits.
85
Table 3 Comparison between US and EC Standards Engine size
US limits are expressed in EC terms as gr/test EC Pollutant US '83 Current Forthcoming
> 2 liter
CO HC+NO x
15 - 30 4- 9
80 - 100 26 - 29
30 8
104 - 2liter
CO HC+NO x
15 - 30 4- 9
70 - 80 24 - 26
36
CO HC+NO x
15 - 30 4- 9
70 24
54 19
< 104 liter
-
_
.
_
-
_
.
_
-
10
,
.
_
-
-
-
-
-
-
-
-
-
~
-
-
-
-
-
* *
I I I
-
* Stage 1 For the engine class above 21iter the forthcoming EC limits are close to the US standards. For the smaller two classes the EC standards are less stringent. EMISSION REDUCTION CONCEPTS The engine exhaust gas contains the products of the incomplete combustion of LPG, gasoline or diesel fuel. The typical composition of the exhaust gas of a gasoline engine is given in Figure 1. Figure 1 Typical exhaust gas composition from a gasoline engine
NO. HC THREEWAY LEAN BURN CATALYSIS I'l'l
4000 2DOO
3000
t5CO
ZOOO
1000
1 I I
I
U 1•• 7 IS
1,2 I II
I,.
I
20
I 22
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The current US emission limits for light duty vehicles are achieved for gasoline fueled cars by engines equipped with a controlled threeway catalyst system including an oxygen sensor and fuel injection. For the older engine types dual bed reduction/oxidation catalyst systems are often applied (Ref. 3). Cars with diesel engines meet the standards without exhaust catalysts. Except in California diesel cars meet the particulte emission standards. Soot traps or filters are generally installed in new diesel cars sold in California. In general the less stringent EC emission limits offer the possibility to apply a wider range of emission reduction techniques (Ref. 4). Threeway catalysis controlled and uncontrolled Dual bed reduction/oxidation catalysis Lean burn engine with oxidation catalysis Exhaust gas recycle and oxidation catalysis or thermal oxidation The effectiveness and the costs of the various methods vary greatly. Apart from the reduction in emissions, another advantage of the new lean burn engine technology is a reduction in fuel consumption. However only a small number of car manufacturers have opted for this route. For the three EC engine classes, the following techniques are considered likely to meet the forthcoming emission limits. Cars with above 2 liter gasoline engines The emission limits by model year 1989/1990 are close to the US standards and therefore the same systems can be applied. Controlled threeway monolith type catalysts, predominantly single bed, are the preferred choice (Ref. 5, 6, 7). The catalyst formulations are modified to cope with European driving conditions (Ref.8,9). Cars in the 1.4 - 2 liter engine class For this class of cars the forthcoming emission limits must be met by model year 1992/1994. This class can be divided in two groups:
- Heavy and high performance vehicles. Single bed controlled threeway monolity catalysts with fuel injection or a high performance carburator are the most likely choice. - Light vehicles.Uncontrolled single bed threeway monolith catalysts, internal or external exhaust gas recycle followed by an oxidation catalyst or even thermal oxidation, or a lean burn engine followed by an oxidation catalyst are being considered and/or applied.
87
Cars with engines below 1.4 liter A large number of car types already meet the Stage 1 requirements of the forthcoming EC standards for model year 1990/1991. The current types which do not meet Stage 1 standards are being adapted. Catalysts are expected to play an insignificant role. Stage 2 standards for model year 1993/1994, to be decided in 1987, are anticipated to be comparable or less stringent than those for the 1.4 - 2 liter engine class. Catalysts are expected to play a significant role. However, in view of the increasing costs of Platinum (pt) and particularly Rhodium (Rh), emission reduction systems are likely to be chosen which minimize the use of these precious metals. There are many car manufacturers in Europe and also many importers and each produces a number of engine families. In model year 1984, some 373 different engine families were sold in West Germany (Ref. 10), thus there is a a very wide variety of conditions with which to cope. The solutions selected will be engine specific and chosen from the range of options discussed above. WEST EUROPEAN CAR MARKET The total number of passenger cars manufactured in Western Europe in 1985 was 11.5 million, an increase of 4% compared with 1984; about 0.6 million were exported to the USA. Total worldwide production was 32.7 million cars. Total sales of new cars in the EC in 1985 was 9.5 million, an increase of 3% over the previous year, of which 0.9 million cars were imported from Japan. West Europe in total recorded new car sales of 10.5 million in 1985 of which 10.7% were Japanese imports. Diesel cars represented 17% of sales in the EC and 15.9% of the total cars sales in Western Europe. The distribution of the 1985 and 1990 car sales over the emission classes in the EC is estimated (forecasted) as follows:
I=n' t y~ Gasoline
l
Di,,,,l
Engine size, liter 1985
below 1.4 1.4 - 2 above 2
% of Sales
I
1990
50% 29 4
48% 28 4
17
20
I I
j
The distribution varies from country to country with a very high percentage of small cars in Italy and France and relatively high diesel car populations in West Germany and Italy. Total car sales by 1990 in the EC are expected to exceed 10 million units.
88
CATALYST TECHNOLOGY AND COSTS Catalyst technology Since the introduction of the exhaust emission controls in the US in the midseventies, catalyst technology has developed steadily and the European practice reflects the state of the art. The composition of a typical European catalyst, the operating conditions and performance are given in the following Table 4. ---_._. _..... _ - - - - - - - _ . _ - - - - - -
Table 4 Typical European Threeway Catalyst Composition
Carrier
Cordierite monolith with 400 passages per square inch and a waIl thickness of 0.15 mm.
Washcoat
20% wt. pseudo-boehmite promoted with a.o. lanthanides, to improve the high temperature stability and the adhesion to the carrier.
Metals
Pt + Rh: 35-40 gr/cu ft (1.24-1.41 gr/l), Pt/Rh wt. ration: 5.
Bulk density
0.45 gr/l,
OPERATING CONDITIONS
Temperature
300-900C
Space velocity
100,000 - 200,000 Ill. h
Catalyst to engine cylinder volume ratio:
0.8 - 1.5
PERFORMANCE
Controlled within; A = 0.99 +/- 0.06 Conversion in %: Fresh; HC: above 80%, CO and NO x: above 70% Uncontrolled within: A= 1.05 +/- 0.2 Conversion in %: Fresh; HC: min. 50%, avo 70%; CO min. 20%, avo 55%; NO x min.lO%, avo 50%
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The precious metal content of a typical threeway catalyst in the USA currently is 20gr/cu ft (Ref. 6). To maintain a catalyst life similar to the US standard of a minimum 50,000 miles at the anticipated much higher remaining lead content (max. 13 mg/l) of European lead free gasolines, the precious metal content will have to be higher. Instead of ceramic monoliths, high temperature Ni metal-based honeycomb carriers are available (Ref. 11, 12). In view of their overall weight and size advantages they are used as pre-catalysts and as retrofits. However on a price per volume basis, including canning, ceramic material currently is 2-3 times less expensive. The continuing price increases (see below) of Pt and particularly Rh, both indispensable components in a threeway catalyst, stimulated renewed R&D efforts to substitute at least in part, both products by less costly metals. To date these efforts have not resulted in a catalyst with equal peformance. Catalyst life in particular was impaired. Catalyst costs The sales price of threeway catalysts is for a major part determined by the precious metal costs, while for the less expensive oxidation catalyst, the impact is far less pronounced. This is illustrated in the following Figures 2 and 3.
Figure 2 Cost build-up for a Typical European Threeway Catalyst 1.3 liter ceramic carrier with 1.24 grlI Pt + Rh (ratio 5) Price: $ 47 per unit (large quantities)
~"""S STOCKS+LOSSES .,1
~3, & \
% OTHER
12,5
%
90
Figure 3 Cost build-up for a Typical European Oxidation Catalyst 1 liter ceramic carrier with 1.24 gr/l Pt + Pd (ratio 8) Price: $ 30 per unit (large quantities)
STDCKS+LDSSES
PLATINUM
3,9%
49,5% PALLADIUM
18, 2
0,7%
%
Because of the smaller size and the much lower price of Pd an oxidation catalyst is about 64% of the cost of a threeway catalyst. The prices for the relevant platinum group metals during the last three years are given in Figure 4, Figure 4 Monthly Average Prices of Platinum, Palladium and Rhodium 1985-1986.(source : Metals Bulletin)
$
per oz 1200....--------------:;-------, 1100 1000 900 m 800 ! ~ 700 ~ 600 1 I 500 II 400 I 300 200 100
o
2 4 1985 •
p l at inum
8 •
10 12
palladium
91
The increased prices of Pt and Rh increased the cost for a typical European threeway catalyst during the last half year by about 40% in US dollars. Worldwide consumption of precious metals in car exhaust catalyst ((Ref. 13) in 1985is given in Table 6.
Table 6 Precious Metals Consum tion in Auto Catalysts 1985 ~housank'~ r o yOunces
Region
-
pt
Pd
Rh
North America Japan Rest of Western World (incl.Europe)
650 175
190 100
96 30 9
Total
875
290
135
31%
11%
54%
Percentage of total demand
50
In 1985 close to 85% of the Pt supply of the Western World originated from South Africa. Based on the catalyst requirements for export models and the expected penetration of catalyst cars in Western Europe the precious metal requirements for auto catalysts is forecast to be 375,000 oz in Europe by 1994 (Ref. 13). CATALYST AND CARRIER MANUFACTURING CAPACITY AND DEMAND The US car industry mainly applies autocatalysts on ceramic monoliths; only GM uses pellets for 30% of its production. The two main manufacturers of ceramic monoliths are Coming, USA and NGK, Japan. The technology was developed by Coming in the early seventies. Four companies manufacture and supply monolith-based catalysts in the USA: Degussa, Engelhard, Johnson Matthey and Allied-Signal (UOP).In Japan and Europe the car industry applies only monolithic type catalysts. The total number of local catalysts manufacturers in Japan is seven; three are subsidiaries of car companies. The total local manufacturing capacity appears more than adequate to supply both existing and future requirements. In Western Europe the industry has been tooling up to meet the rising demand. Total planned catalyst manufacturing capacity is ten million units of which about seven million either exists or is under construction. Degussa and JMC
92
have existing facilities, Engelhard Kali-Chemie has a plant under construction and Allied-Signal has announced the construction of a plant in France. In West Germany Heraeus and Doduco supply catalysts for retrofitting existing cars. Three other firms have indicated their intention to build manufacturing facilities. Both Corning and NGK have cordierite monolith manufacturing facilities under construction in Europe with an initial total capacity of 6.5 million units. The total investment costs for these two plants are DM 140 million The current manufacturing capacity in Western Europe for metal monoliths is around 0.5 million. Future requirements for car exhaust catalysts in Western Europe for the next 5 to 10 years are extremely difficult to forecast as: - Emission limits for cars below 1.4 I stage 2 have not been decided - EC regulations are mandatory and the degree of compliance is not known. Therefore only ranges can be given for the expected future requirements. In Table 7 our forecasts are presented until 1994, the year the EC regulations are expected to become fully effective. Table 7 Demand Forecast for Car Exhaust Catalysts Western Europe: 1986-1994
Year
Minion units
1986 1990 1994
1.5 - 2 2.5 - 3 5.0 - 9
On the basis of this forecast the announced manufacturing capacity in Western Europe is adequate for the anticipated demand, including exports. With an average size of 1.3 liter and a precious metal load of 1.3g/l per unit, 380,000 oz of Pt, Rh and Pd are required for 7 million catalyst units. MOTOR GASOLINE QUALITY REQUIREMENTS The quality of the transport fuels, motor gasoline and diesel fuel in the next 10-15 years will be strongly influenced, apart from market demands, by: - Environmental legislation, directly and indirectly - Whitening of the barrel Critical aspects of gasoline quality for the performance of the engine are
93
octane and volatility. High octane is required for high compression engines with high performance and low fuel consumption. The current premium grade in Western Europe has an average Research Octane Number (RON) of about 97.5, regular is above 91 RON. From the 100 million tonnes of gasoline consumed annually, 25% is regular, but there are large differences in consumption patterns, ranging from 50% regular in West Germany to 95% premium in Italy. The estimated average Western European pool RON without lead additive is about 92.5. The average lead based octane increase is 3 to 3.5. The mounting concern in the last years on the negative health effects of lead in the environment has resulted in the reduction of the gasoline lead content and the promotion of the use of lead free gasoline. The lead content of motor gasolines by January 1, 1987, will be reduced to max. 0.15gr/1 in most European countries, the remaining countries are expected to follow in the early nineties. The EC has agreed on a timetable for the introduction of new cars fueled with lead free gasoline only: - by 1.10.89 all new models - by 1.10.91 all new cars Lead free gasoline has been available on a limited basis in all countries since 1985. The government policies in Western Europe are far from uniform: in some countries tax incentives have resulted in the replacement of regular by lead free regular; other countries promote by tax incentives the use of lead free super and still others only make lead free super and regular available. In the latter case market forces result in virtually no sales of lead free gasoline (the price is higher) which in tum reduces availability. There are doubts in the minds of consumers whether it is wise to buy a catalyst equipped car or even to use lead free gasoline. Given time a more uniform supply pattern will emerge. Two grades of lead free gasoline have been introduced; Euro Super with RON 95 and Regular with RON 90-91. Specifications for both are given below: Property
Regular
Super
Research Octane Number (RON) Motor Octane Number (MON) Density at 15C, g/ml Sulfur content, % wt Benzene content, % vol Lead content, gil
min.900r91 min. 80 or 82.5 0.70- 0.79 Below 0.1 Below 5.0 Below 0.013
min. 95 min. 85 0.70 - 0.79 Below 0.1 Below 5.0 Below 0.013
94
Within the next 10 to 15 years lead compounds will disappear as a gasoline octane improver and some 250 million octane tons per year (6 million octane barrels per day) are required to fill this gap. The current estimated average gasoline composition in Western Europe (Ref. 14) is illustrated in Figure 5 below. The two main blending components are reformate and cat cracked gasoline. By reforming paraffins and naphtenes present in naphtha into isoparaffins and aromatics the octane number is greatly improved. The main product of the conversion of heavy crude oil fractions in a catalytic cracker is gasoline. The light naphtha part in the average gasoline includes raffinate from aromatics extraction. Under the heading high octane light components: alkylate, polygasoline, isomerisate and oxygenates (alcohols and ethers) have been taken combined. Figure 5 Estimated Average Gasoline Composition Western Europe 1986
BUTANE 3.0 % UGHT NAPHTA 8.0 % CAT.CR.GASOLINE 33.0 %
REFORMATE 49.0 % HIGH OCTANE LIGHT COMP. The whitening of the barrel, we anticipate, will continue and will result in a lower percentage of straight run naphtha (derived) components and a higher percentage of cat cracker and hydrocracker derived components in the gasoline (Ref.15). The increase in octane number required to replace lead can be achieved in a number of ways (Ref. 16), all of which increase the manufacturing costs; some require substantial additional investment. At the current state of the art and prices the average cost of an octane-ton is about $ 1.8. The most attractive but limited octane enhancement is obtained by the use of octane boosting catalysts in the cat cracker. The technology to meet the octane level for total lead free gasoline is available and the time schedule leaves sufficient room for refiners to make the required adaptations and investments.
95
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
M.P. Walsh, Experience in the United States with Automobile Emission Control, Platinum Metals Review, July 1986, vol. 30, n03, 106-115. K.H. Neumann, Emission reduction by modern engine design, VDI-Bericht 531, Duesseldorf 1984. (In German). P. Oeser and W. Brandstetter, Fundamentals of Catalyst Systems for S.l. Engines, MTZ Motortechnische Zeitschrift, vol 45, 5/84, 1-6 (In German). Catalysts - Meeting new challenges in a $ 2.5 billion global business, Chemical Week, vol. 138, n026, June 251986,20-71. H.D. Schuster, J. Abthoff and C. Noller, Concepts of Catalyst Exhaust Emission Control for Europe, SAE paper 852095. W.B. Williamson et al, Durability of Automotive Catalysts for European Applications, SAE paper 852097. W.DJ. Evans and AJJ. Wilkins, Single Bed, Three Way Catalysts in the European Environment, SAE paper 852096. E. Koberstein, B.H. Engler and H. Voelker, Catalytic Automotive Exhaust Purification - The European Situation 1985, SAE paper 852094. BJ. Cooper and TJ. Truex, Operational criteria affecting the design of thermally stable single bed catalysts, SAE paper 850128. Eurosystem Vehicle Registration Report (Germany). November 29,1984. P. Oeser et al, Catalytic control of exhaust emissions by metal supported precious metal catalysts. M. Nonnemann,Metal Supports for Exhaust Gas Catalysts, SAE paper 850131. G.G. Robsom, Platinum 1986, Johnson Matthey pIc. W. Groenendaal, What is new for Fluid Cat Cracking - Outlook in Western Europe - Katalistiks 7th annual symposium, 1986. S. Bernstein, European Automotive Fuels for the 80's and 90's, SAE paper 845047. J.A. Weiszmann et al, Pick your option for higher octane, Hydrocarbon Processing, June 1986,41-45.
This Page Intentionally Left Blank
,\, Crucq and A, Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B,V" Amsterdam -- Printed in The Netherlands
97
AUTOMOBILE CATALYTIC CONVERTERS K. C. TAYLOR Physical Chemistry Dept., General Motors Research Labs, Warrer"
MI (USA)
INTRODUCTION The automobile is identified as one source of emissions of hydrocarbons, carbon monoxide, and nitrogen oxides to the atmosphere. such as conventional power plants are another.
Stationary sources
Concern for the danger of
these substances to public hea lth in urban areas has led to the development of motor vehicle emission regulations by industrialized nations, in general. Current and proposed regulations have been designed to improve air quality by reducing the impact of automobile exhaust on smog formation and on carbon monoxide levels, and more recently to reduce acid deposition. The relationship between automobile exhaust emission levels and stationary pollutant sources and air quality is not a direct one.
Complex mathematical
models have been developed for predicting trends in air quality.
These models
include as input information on vehicle populations, atmospheric chemistry, meteorological variables, and other variables which can impact on the air quality of an urban area.
Predicting the level of control needed to meet air
quality goals is complicated by the multiple inputs to the atmosphere in urban areas. EMISSION CONTROL REQUIREMENTS The emission control needs of countries differ and different emission limits for passenger cars and for trucks have been established throughout the world.
Exhaust emission standards for vehicles for countries where regula-
tions have been set cannot be compared directly because the tests on which the emissions are measured differ; however, the range of control for passenger cars regulated can be viewed by expressing the limits as the intended percent reduction from the uncontrolled level as shown in Table 1.
For example, the
U.S. Federal standards represent a reduction form uncontrolled levels of 96% for hydrocarbons and carbon monoxide and 76% for nitrogen oxides.
Standards
have also been established for gasoline fueled commercial vehicles and for diesel fueled vehicles.
98 TABLE Exhaust emission standards Passenger cars [1 J
--~~"---"-------
Country
Percent Reduction CO
2 U.S. Canada 2,3 Australia
2
1
HC
NO
96
96
76
70
80
24
82
86
24
Japan (10-mode and 11-mode)
95
93
92
Europe (ECE R15.04)4
70
Sweden,
x
50
c; Switzerland~
SWitzerland 5,6
67
72
24
87
88
51
1Percent reductions for U.S., Canada, Australia, Sweden and Switzerland based on 1960 U.S. uncontrolled models. 2Measured on 1975 U.S. FTP. 3Enforced in 1986. 4Standards vary with vehicle weight. 5Measured on 1972 U.S. FTP. 6Standards effective October 1, 1986.
The exhaust emission standards (limits) and the emission test procedures for passenger cars are listed in Tables 2 and 3, respectively.
The current
Federal U.S. standards for passenger cars are 0.41 g/mile hydrocarbons, 3.4 g/mile carbon monoxide, and 1.0 g/mile nitrogen oxides.
Different exhaust
emission control standards have been set for the State of California.
The
standards for nitrogen oxides for California are stricter than the Federal standards, optional 0.7 g/mile and a primary standard of 0.4 g/mile.
Start-
ing in 1989 passenger cars must be certified at the primary standard in California.
The current Federal U.S. HC/CO/NO
trucks are 0.80/10/2.3 g/mile, respectively.
requirements for light duty x The light duty truck diesel
eXhaust particulate standard is 0.6 g/mile, dropping to 0.26 g/mile effective in 1987.
Additional requirements apply to heavy duty vehicles.
99 ',ABLE 2 Exhaust
emissio~
Passe~ger
sta~dards
cars [ 1 ]
COU', try
Standard
co
1
U.S. Canada 1,2
(g/km)
HC
NO
2.11
0.62
0.25
15.5
1. 93
1.2
Australia 1
9.3
0.93
1.9
Japan (10-mode) (imported)
2.7
0.39
0.48
Japan (10-mode) (domestic)
2.1
0.25
0.25
x
(ll-mode) (imported)
85.0 g/test
9.50 g/test
6.00 g/test
Japan (11-mode) (domestic)
60.0 g/test
4.4 g/test
7.0 g/test
Europe (ECE R15.04)3
58-110 g/test
Japa~
.
Sweden, SWitzerland
4
Switzerland 1 ,5 8, canada 7, SWitzerland Sweden 9 Saudia Arabia, Israel, Singapore (ECE R15.03)3 6 Korea
x:
19-28 g/test
24.2
2.1
1.9
9.3
0.9
1.2
2.1
0.62
0.25
65-143 g/test
6-9.6 g/test
8.5-13.6 g/test
2.11
0.25
0.62
lMeasured on 1975 U.S. FIP. 2Enforced in 1986. 3Standards vary with vehicle weight. 4Measured on 1972 U.S. FIP. 5Standards effective October 1, 1986. 6Light duty vehicles, effective July, 1987. 7Standards effective September, 1987. 8Standards effective October, 1987 9Standards effective 1989 MY.
HC+NO
100 TABLE 3 Emissio~
test procedures
Passenger cars [1J
Cou'1try
Exhaust Emiss l or.s Evap. Emi ss ions Samplir;g Methods Dri v ir.g Cycle Mettl0ds
U.S.
1975 U.S. FTP
Ca'1ada
19'75 U.S. FTP
Australia
2
Japan
1975 U.S. FTP 10-Mode 11 - t ~ o d e 6-Mode
Europe (International)
6
ECE (R15. 04)
1 Test Fuels
CVS, FlO 3 CVS, FID 3
SHED~
UL,D
Trap
UL,D
CVS, FID 3 CVS, FID3
SHED~
UL,D
Trap 5
UL,D
CVS, FID
UL
CVS, HFID
D
CVS, FID
L,D
ECE (R15.03)
Big Bag, NDIR
L,D
1972 U.S. FTP
CVS, FlO
L,D
Sweden, Switzerland Switzer land 9
1975 U.S. FTP
CVS, FID
Saud i Arab ia
ECE (R15. 03)
Big Bag, NDIR
Israe 1, Singapore 8 Korea
ECE (R15.03)
Big Bag, NDIR CVS, FID 3
1975 U.S. FTP
1UL = unleaded gasoline; L
leaded gasoline; D
UL
diesel.
2Enforced in 1986. 3Heated FID used for diesel vehicles. 4SHED = Sealed Housing for Evaporative Determination. 5Gasoline vehicles only. 6 May b e chassis . or engine dy'1amometer procedure. 7Tr ap measurements accepted. 8Light duty vehicles, effective July, 1987. 9Effective October, 1986.
SHED 4,7
L L
SHED~
UL
101
Because exhaust emissions vary as a function of such factors as the driving mode and the ambient conditions, the exhaust emissions of a vehicle are compared with the standards using an established driving cycle and sampling method.
Like the emission limits these emission test procedures are a key
part of the emission regulations.
While test methods and instrumentation for
sampling the exhaust have been virtually standardized, significant differences exist in the driving cycle schedules required in Europe, Japan, and the United States.
These variations greatly dilute the complex emission control develop-
ment process and require costly triplication of compliance verification. Today's arguments for cycle differences are weak; good progress has been made during the last twenty years in the development of efficient road systems and improvement of traffic flow, both urban and rural.
The growing division with-
in Europe on this sUbject suggests that a fresh look at a harmonized driving cycle is entirely appropriate.
Vehicle speed vs time traces for the driving
cycles for the U.S., Japan, and Europe are shown in Figure 1.
Compared with
the European cycle, the U.S. cycle goes to higher top speeds, has a cold start and hot start, and is a longer test. The third key part of emission regulations is the protocol for evaluating vehicle compliance with the emission standards.
Common to emission regula-
tions world wide is the requirement to give evidence of compliance with emission regUlations and obtain government approval prior to vehicle production. This requirement generally takes the form of sUbmitting descriptive documents and test data on a prototype vehicle which indicates that the vehicle meets the emission regulations.
In the U.S., for example, prototype passenger cars
accumulate mileage using a standardized driving schedule in order to predict the emissions performance of vehicles during the 50,000 mile compliance period.
This procedure is called the AMA durability schedule.
A vehicle
representative of each engine family must be tested. The number of different engine families sold in the U.S. in 1984 was 192; an equivalent categorization showed 119 in Japan, 78 in Australia, and 373 in West Germany [1J.
In the
U.S., Sweden, and Switzerland the governments may run their own verification
tests on vehicles submitted for certification. nesses the testing.
In Japan, the government wit-
In saudi Arabia and Canada the manufacturer's test re-
sults are not required as part of the documentation. All emission regulations specify that the production vehicle must be bUilt to match the preprOduction prototype vehicle for which emission performance was extensively tested and shown to comply with the regulations.
Furthermore,
the emissions of the production vehicles must comply with the regulations. demonstrate compliance of production vehicles, some regulations require
To
102
UNITED STATES 1972 FTP, 1975 FTP
"'0 f------,.,.-----------------'--I
.J)lUS " ' ; ~POOI ) \
u S 1572 FTP o n l y - - - - - - -
~ ~
""'~~~:HO<) ~o
L:-:
2~O
"'~O
300
~ ~ O 6 ~ O 7 ~ O ll~O
9~O
10'00
ti oo
1200
101.1 , ~ "
,,,"
Tt,.".. ".,.
d·. I
d .. ".,pl'''\1 """
• •
. Jl! fli!! 1,\ A I
TJOO~~~o·
T I M ~ ISECONDS -
i\
.•
2S~~. . . i L. . L. C. - - ~. . LJ LJ .
I
I
- - - .....
I
,
I
<,. ,. '!>Q ... <, , ~ .... - -
\372 U C O " < l ' - - -
, • • 12.,.,
10 m ' ' ' ~ '
..
JAPAN " MODE 100
EUROPE EcE R1504. EEC 78/665
75
A
50 -
V,
\\\
VEHICLE SPEED
km/hr
25
/' ,
-
100
200
l dmup
tE j-
~I
300
400
500
600
TIMEfSECONOS
lot 25 second$
I
on. C'yc'- .. 120 second,
.nd----1
Tal.'I.1I umpling tilne .. 505 uconds
700
' ' ' :1nAAA _. 11= I
I
~An ~ &!A
,do 2~O ) ~O 460 s60 660 760 ebo m ",
1--- ..
",.~:::::';~:~:,o"
~
..,P''''9 1''''' • • 780UCO''d.
101,1 .. """,• • 82D 'HO"'"
JAPAN 10 MODE 100 75
so VEHICLE SPEED
kmlhr
'Of
w.rm up 15 minule",
one eyeN! '" 135 seconds
1-------------_1- Total samp1in9 -
Fig. 1.
lime ..
1005 s eccods
Total leSl lim • • 1140 s.conds
Driving cycles for various emission regulatious. (Reproduced with permission from reference [1J.)
103
testing of selected production vehicles just following production or testing of showroom vehicles or testing of vehicles at low mileage. Only the U.S. has a formal program for evaluating the emissions of vehicles in-use after extended mileage accumulation by vehicle owners.
The U.S.
Environmental Protection Agency or its contractors do emissions testing of privately owned vehicles.
The manufacturer, however, maintains responsibility
for the emissions performance of the vehicle for 50,000 miles or 5 years. Manufacturers may be forced to recall vehicle families with emissions significantly exceeding the standards. Numerous other details of world wide emission regulations and procedures will not be reviewed here.
An argument has been made for simplifying and
harmonizing the testing and compliance procedures which are used to demonstrate compliance with the emission standards in different countries [lJ. Harmonization could allow manufacturers which market vehicles worldwide to expend resources optimizing emissions control performance, rather than in duplication of testing and development activities. CATALYTIC CONVERTERS It is the responsibility of vehicle manufacturers to develop technologies for' meeting emission regulations.
The automobile catalytic converter is the
only technology available for meeting the most stringent standards.
This
technology has been extensively reviewed in the technical literature [2J and only a brief review of the historical developments will be given here.
Cur-
rently catalytic converters are needed to meet emission standards for passenger cars sold in the U.S., Canada, Australia, and Japan.
Also emission limits
for most light duty commercial vehicles sold in the U.S., Japan, and Canada have resulted in catalysts being used. Oxidation Catalysts Automobile catalytic converters have been used in the U.S. since 1974 (1975 model year vehicles) in order to meet regulated emission standards.
In the
early 1970's the use of catalysts for automobile exhaust emission control represented totally new catalyst technology. ogy is an environmental success story.
The development of this technol-
Environmental and political issues
came together to force technology development.
Use of the catalytic converter
for controlling emissions freed up constraints on engine parameter settings. Earlier less stringent emission standards had been met without the use of catalytic converters by changing engine calibrations including leaning the air/fuel ratio and retarding the spark.
General Motors felt that any further
compromise in engine calibration would lead to unacceptable fuel consumption
1M
ana driveability.
By electing to use catalysts on 1975 model year vehicles,
driveability was improved and fuel efficiency that had been lost during the years 1968-1974 was recovered. Initially catalytic converters were used to control just carbon monoxide and hydrocarbons.
The nitrogen oxide standard of 3.1 g/mile and later 2
g/mile could be met by using exhaust gas recirculation which leads to the formation of less nitrogen oxide in the engine.
The catalysts used through
1980 were oxidation catalysts containing the noble metals platinum and palladium.
A typical catalyst used by General Motors contained 0.05 oz t noble
metal per converter, with a 5/2 ratio of platinum to palladium.
New platinum
mines were opened in South Africa to supply noble metals for these catalysts. A development which paved the way for the catalytic converter was the removal of lead from gasoline.
General Motors had successfully argued for the
availability of unleaded gasoline which was necessary to prevent contamination of the catalyst. as well.
Actually removing the lead from gasoline had other benefits
First, lead emissions were eliminated.
Lead salts are a major
source of gasoline fueled automobile particulate emissions.
The presence of
lead salts in the enviromnent have posed a potential health concern.
Second,
emissions of unburned hydrocarbons were reduced because of additional oxidation which occurs in the exhaust system and lower production of unburned hydrocarbons in the combustion chamber.
Third, lead salt deposits which have
the potential to plug or alter the calibration of EGR systems were eliminated. Fourth, vehicle maintenance requirements for engine oil, spark plugs, and eXhaust systems were reduced through the elimination of both lead deposits and of the acids produced. Two basic catalyst structures were used, distinguished by the configuration of the catalyst support. The two support types are alumina pellets and alumina coated ceramic monoliths (Figure 2).
The pellets are approximately 1/8th inch
in diameter and are composed of thermally stable transitional alumina.
The
monoliths are made of a ceramic material such as cordierite (2Mg,2AI
20 3,5Si0 2)· A cross-sectional view of the pellet-type catalytic converter designed by
AC Spark Plug Division of General Motors is shown in Figure 3.
The catalytic
converter consists of an inlet plenum, a narrow louvered catalyst bed, and an exhaust plenum.
Exhaust gases flow in at the top of the converter through a
decreasing inlet plenum, pass through the catalyst bed, and exit through an increasing outlet plenum on the bottom.
This design ensures flow uniformity,
low restriction, and minimized catalyst movement.
105
Fig. 2.
Fig. 3.
Catalyst supports.
Cross-sectional view of pellet-type catalytic converter developed by AC Spark Plug Division of General Motors.
The two sizes of pelleted catalytic converters
us~d
by General Motors in
1975 were 160 cu in (2.6 L) and 260 cu in (4.3 L). These converters are shown in Figure 4.
The converter shell and internal parts were a stainless steel
which provided needed high temperature durability and corrosion resistance at low cost. The wide use of catalytic converters in 1975 was met with some concern and criticism.
Initial concerns were focused on the potential high exhaust system
surface temperatures to cause fires.
Extensive testing conducted by vehicle
manufacturers and by others indicated that exhaust system temperatures were no
106
Fig. 4.
Pellet type catalytic converters (A) 160 cu in, (B) 260 cu in.
more of a fire hazard than the temperatures produced in the exhaust systems of earlier automobiles without catalytic converters.
In fact, in 1977 the U.S.
National Highway Transportatior. Administration announced that catalytic converters did not present an unreasonable risk. A second concern was that under some conditions sulfur dioxide in exhaust could be emitted as sulfuric acid as a result of catalytic oxidation over the noble metal catalyst.
To answer this concern General Motors conducted a 350-
car test designed to simulate sulfate emissions on a busy expressway.
The
U.S. Environmental Protection Agency, other vehicle manufacturers, and several independent environmental monitoring organizations participated in the experiment.
This experiment showed conclusively that ambient levels of sulfuric
acid under this worse-case simulated exposure situation were far below threshold levels known to produce adverse health effects. Three-way Catalysts Since 1981, more complex emission control systems have been used in the U.S.
in order to satisfy the stricter 1 g/mile emission requirement for nitro-
gen oXides.
Exhaust gas recirculation alone was no longer sufficient to con-
trol nitrogen oxides.
Meeting this new nitrogen oxide emission standard to-
gether with the hydrocarbon and carbon monoxide standard required a new catalyst and a totally new approach to emission control. Now hydrocarbons, carbon monoxide, and nitrogen oXides are removed simultaneously over the same catalyst.
107
sc
The
+
02 - - - CO 2
+
02
H20
+
CO & H ----- N 2 2
+
CO 2 +
CO
2
& H20
and carbon mor,oxide are oxidized to CO
hydrocarbo~s
oxide is reduced to
nitroge~.
Simulta~eous
over a single catalyst has led to the
~ame
and H
20
2
while nitric
of all three pollutants
co~version
three-way catalyst.
The noble metal rhodium combined with p Lat inurn has the property to do both sets of reactions if the catalytic converter is operated at an air/fuel ratio close to the stoichiometrically balanced composition of 14.6.
A schematic
diagram illustrating the principle of operation of a rhodium containing threeway catalyst is shown in Figure 5.
Under
more
co~ditions
reduci~g
than the
14.6 air-fuel ratio, the conversion efficiency of the catalyst for reducing nitrogen oxides is high and the conversion efficiency of the catalyst for oxidizing hydrocarbons and carbon monoxide declines.
Under conditions more
oxidizing than 14.6, the efficiency of the catalyst for oxidizing carbon monoxide and hydrocarbons is high and conversion of
100
~itrogen
NOx
oxides declines.
CO ~HC
-dual
~ 50
3-woy
sVi ?; 1,0 co C>
'-'
ZO -
11,
15
Air I fuel rolio
Fig. 5.
Efficiency scan for a dual-bed catalyst and a three-way catalyst.
Rhodium is an essential ingredient in this catalyst and is found in all current exhaust cataysts which convert nitrogen oxides.
Many different cata-
lyst compositions are used as three-way catalysts and the noble metal content per converter varies widely. range 0.03-0.1
02
Noble metal usage in current catalysts is in the
t/converter platinum, 0.005-0.017
02
t rhodium, and 0-0.1
02
t/converter palladium. The rhodium to platinum ratio in all three-way catalysts exceeds the mine ratio of these metals. U.S. contain platinum and rhodium at Pt/Rh
=
Three-way catalysts used in the 10/1 if not higher rhodium.
The
108
mir.e ratio for these metals is approximately Pt/Rh lysts targeted for Europe metal per converter (e.g., are
co~sidered
present in
co~tai~
0."1
oz t per converter).
Three-way cata-
16."/1.
~
approximately Pt/Rh
~
5/1
a~d
high
~oble
High r.ob Io metal loadir:gs
"ecessary because of the high lead levels expected to be gasoline in Europe. The current high cost of the
u~leaded
metals and the demand that
expandi~g
~oble
world wide adoption of automobile exhaust
catalysts places on their availability requires that noble metal exhaust catalysts be prepared and used most effectively. Plati~um
is an effective oxidation catalyst for carbon monoxide and the
complete oxidation of hydrocarbons.
Palladium also promotes the oXidation of
carbon monoxide and hydrocarbons but is more sensitive to plati~um
in the exhaust
e~vironment.
poiso~ing
than
Both platinum and palladium promote the
reduction of nitric oxide but are less effective than rhodium.
In addition to
the noble metals, three-way catalysts contain the base metal cerium and possibly other additives such as lanthanum, nickel or iron.
These base metal
additives are believed to improve catalyst performance by extending
co~version
during the rapid air-fuel ratio perturbations and help to stabilize the alumina support against thermal degradation. In order to provide the proper stoichiometrically balanced exhaust gas composition reqUired for use of the three-way catalyst, an air/fuel ratio control system had to be developed for the vehicle.
Closed-loop electronic
air-fuel ratio control required the installation of an exhaust oxygen sensor and an on-board microprocessor to provide the necessary control capability. The continuous air-fuel ratio adjustments result in small 0.5-4 hertz perturbations of the exhaust composition with an amplitude of approximately +0.5 air-fuel ratio. A diagram of the control system components is shown in Figure 6. exhaust oxygen sensor is placed ahead of the catalyst.
The
The on-board
microprocessor receives signals from the oxygen sensor and a number of other sensors and generates output signals which are used to control engine airfuel, spark timing, transmission converter clutch, and a variety of other engine and drivetrain functions.
This system was first used primarily with
carburetors which over time are being replaced by fuel injection control. The three-way catalytic converter has to respond to a wide range of exhaust conditions because exhaust emissions vary as a function of the driving mode. Typical engine out exhaust emissions for a passenger car are in the range 0.04-0.4 vol% hydrocarbons, 0.03-2.5 vol% carbon monoxide, and 0.0-0.2 vol% nitrogen oxides.
Exhaust gas temperatures at the inlet to the catalytic
converter are typically 350-500 C for a warmed up catalytic converter.
At
ELECTRONIC CONTROL MOOULE THROTfLE BODY INJECTOH SYSTEM
MANIFOLD A8SOl U T[ PRESSURE SENSOH
~
\
J
\.-11
DISTRIBUTOR _ _ (,1 AIR CLEANER --
'==_
~ ~ ~ MASS AIR 'LOW SENSOR
\ ~---VAPOR
CANISTER
',----------",
TORQUE CONVERTER CLUTCH CONTRClL
EXHAUST OXYGEN SENSOR -
COOLANT SENSOR
Fig. 6.
Closed-loop emission control system on a three-way catalyst equipped vehicle.
start-up, however, the catalyst is cool and no reactions occur until the catalyst is heated to operating temperature by the hot exhaust gases. General Motors first marketed three-way catalytic converter systems in California during the 1978 model year and expanded their use in California during the 1979 and 1980 model years.
The California program allowed a
"phasing-in" of this new technology prior to introduction to the full U.S. market in 1981.
A simi lar "phase-in" opportun ity is proposed for Germany and
Austria by designating the strictest emission standards for only the largest passenger cars. Two types of catalytic converters are currently being used for meeting the passenger car emission standards in the U.S.: bed converters.
three-way converters and dual-
Both converters contain three-way catalysts, but with the
dual-bed converter the three-way catalyst is followed by an air injection/ oxidation catalyst system. of catalyst support are used:
As for the earlier oxidation catalysts two forms pellets (thermally stable transitional alumina)
and monoliths (cordierite honeycombs coated with a thin alumina washcoat). Figure 7 shows four catalytic converters currently being used by General Motors. CATALYST DURABILITY In the U.S. exhaust catalysts must have the durability to maintain high activity for 50,000 miles or 5 years.
The U.S. Federal regulations require
110
a
b
d
C
AC Spark Plug (A) 170 cu in (B) 170 cu in (C) 160 cu in ( D) 260 cu in
Fig. 7.
three-way catalytic converters. dual bed monolith (three-way + oxidizing). three-way monolith. three-way pellet. three-way pellet (trucks).
that the exhaust emissions of passenger cars not exceed the standards within this compliance period, and the automobile manufacturers maintain responsibility for meeting the emission standards.
Because catalysts do deactivate
with use, the ability to withstand mild deactivation is built into the design of the catalyst as well as the entire emission control system on a vehicle. This is done by setting up vehicles to operate well below the standards at low mileage, to select materials which are durable in the exhaust environment, and to prevent accessibility to vehicle adjustments which could alter emissions. All catalysts are not expected to experience the same deactivation in use because of the wide range of veh ic Ie operat i ng cond i tions.
Vehic Ie manufac-
turers have developed engine-dynamometer tests which are used for screening catalysts submitted from catalyst suppliers.
On these tests the catalysts are
exposed to a range of operating conditions and temperatures in order to assess activity and durability during a simulated aging schedule. Catalysts are selected for further testing on vehicles based on their performance on these initial durability tests. criteria.
Overall catalyst selection is based on performance
Vehicle manufacturers set noble metal loadings and the support
type, but the exact catalyst formulation including base metal additives is designed by the catalyst suppliers and this information is generally proprietary. The major mechanisms of deterioration of automobile exhaust catalysts are thermal damage due to exposure of the catalysts to very high temperatures, poisoning by contaminants in the exhaust, and mechanical damage of the catalyst support.
Research aimed at identifying and understanding the nature of
the deterioration and the impact on performance has included post-mortem
111
studies of used catalysts [3J a .o simulated aging studies ie; which catalyst performance is examined following exposure of the catalyst to high temperature and/or catalyst poisons
[~-12J.
In general, accelerated aging studies have
revealed that exposure of catalysts to high temperature oxidizing conditions damage CO conversion whereas catalyst poisoning damages hydrocarbon oxidation [12J.
Examination of used catalysts generallY reveals a number of changes and
except for severely damaged catalysts no single factor correlates clearly with per formance.
Exposure to high temperatures can damage catalysts by sintering the noble metal particles, resulting in a decrease in the fraction of the noble metal available for catalytic reactions.
Low temperature activity of the catalyst
is most impaired by noble metal sintering.
High temperatures can also promote
damaging interactions between the noble metals (alloy formation) and interact
ior.s between base metal (includ mg the catalyst support) and noble metal
components of the catalyst
Vehicle conditions which can produce high
[1~,15J.
catalyst temperatures are, for example, repeated misfire resulting in the oxidation of large amounts of unburned fuel over the catalyst.
High catalyst
temperatures are of concern for European catalyst applications since top driving speeds permitted in Germany are higher than in the U.S. Oxidizing conditions have been observed to damage three-way catalysts at lower temperatures than reducing conditions.
A platinum-rhodium three-way
catalyst (base metal additives present but not identified) aged on an engine dynamometer was deactivated more readily (at lower temperature) during a brief exposure to lean air-fuel ratios than to rich air-fuel ratios [13J.
Activity
loss as measured at 600 F at stoichiometry was appreciable fOllowing only 20 minutes exposure to lean exhaust at 1600 F [13J. Excessively high temperatures can damage the catalyst support.
The ceramic
monolith may melt forming channels for the exhaust to pass through the converter without contacting the catalyst.
High temperatures can potentially damage
the alumina support by promoting transition to alpha-alumina and loss of surface area.
Mechanical loss of catalyst support material can result from den-
sification and cracking of the monolith wash coat leading to poor adhesion of the catalyst layer to the ceramic monolith.
Other mechanisms of loss are
abrasion and breaking of catalyst pellets. Typical catalyst poisons are lead and phosphorus. low levels in unleaded gasoline.
Lead is present at very
Typical lead levels are 0.003 glgal although
0.05 glgal is the maximum allowed lead level in unleaded fuel. believed to be a major catalyst poison at the 0.003 glgal level.
Lead is not On the other
hand, use of leaded fuel will poison three-way catalysts, and catalyst activity is not fully recovered upon changing back to unleaded fuel.
Figure 8
112
100
80
60 40
20 0
I
0
5
10
i5
5
10
15
100
80
60
40
20 0 100
80 0
60
UNLEADFUEL LEAD FUEL
40
20
0
- ---
_ I _ _
1---5
I
10
-1-
15
0D0 METER -MI LES'100 0
F i g 8.
Cor,verter e f f i c i e n c y d u r i n g i n t e r m i t t e n t l e a d u s e . (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e [4l.)
113
snows how the activity of a typical three-way catalyst is impaired during and following intermittent operation with leaded fuel (1.2 g/gal) during 15,000 miles of vehicle operation [4J.
The converter efficiency of the control vehi-
cle was virtually unchanged at 94% for hydrocarbons, 95% for carbon monoxide, and 66% for nitrogen oxides [4J.
Following the misfueling shown here the
carbon monoxide emission level recovered to an acceptable level.
The hydro-
carbon and nitrogen oxide emissions did not recover to passing values [4J. Fuel switching can be a reason why some used vehicles fail to meet emission standards.
lei a 1984 survey conducted by the U.S. Environmental Protection
Agency 14% of the vehicles tested were found to have been misfueled by using leaded gasoline in catalyst-equipped vehicles.
The survey also showed that
fuel swi tch Ing was higher in areas wi th no inspect ion-rna intenance (I/M) program (19% fuel switching) compared with areas with liM programs (10% fuel switching) [16J.
These findings argue in favor of inspection programs which
check for proper maintenance of vehicle emission control systems and that all components are present.
This same survey showed that at least one emission
control component had been tampered with on 21% of the vehicles examined [16J. Tampering involved the catalytic converter itself, the EGR valve, altered !'iller neck inlets, disabled air pumps and evaporative systems, and tampering with PVC's [1 6J. Phase down of the amount of lead allowed in leaded gasoline which began in July, 1985 (from 1.10 glgal to 0.50 glgal and further to 0.10 glgal in January, 1986) will reduce lead emissions to the environment and fue I-sw itch ing. Phosphorus is recognized as a potential poison of automobile exhaust catalysts.
Phosphorus levels in gasoline are very low (0.2 mg/l), and fuel-de-
rived phosphorus at these levels does not damage three-way catalysts.
Phos-
phorus is present in high concentrations in engine oils (1.2 gil) and is the source of phosphorus contamination of catalysts [9,10J.
Phosphorus derived
from engine oil reacts strongly with the alumina support and tends to accumulate at the outer edge of the catalyst pellet in the same location as the noble metals (Figure 9).
Phosphorus can deposit on catalysts in more than one
chemical form and poisoning is not reversed by thermal treatments [6J.
Phos-
phorus poisoning of catalysts has been studied extensively in simulated poisoning studies (e.g., 6, 9-11). Fuel-derived sulfur does not interfere with the performance of noble metal exhaust catalysts as strongly as it does with base-metal catalysts.
Compati-
bility with SUlfur dioxide was one of the reasons for selecting noble metal catalysts.
Fuel contaminants such as organo-silicon compounds have been found
114
I
o
I
I
50
100
~ m
Fig. 9.
Scanning electron micrograph trace of phosphorus and aluminum profiles for a used (37 000 miles) three-way catalyst pellet. The vertical scale is concentration in arbitrary units.
to degrade both catalysts and oxygen sensors [7J. Manganese fuel additives have been shown to impair three-way catayst activity [17J. FUTURE ISSUES The regulatory agenda in future years which could result in new tions is likely to be driven by specific issues as in the past.
regula-
We might
imagine that relationships between health and air quality would be high on this list.
Attention to specific air toxics such as benzene has been of
concern recently to the California Air Resources Board.
Attainment of the air
quality standard for ozone has been difficult in many areas of the country and will likely continue to be so for several years. programs are intended to assist ozone attainment.
Inspection and maintenance The contribution of automo-
bile eXhaust emissions to acid deposition has been cited as a reason for propOSing more stringent emission controls for nitrogen oxides, in spite of the very small contribution of nitrogen oxide from passenger cars to the acidity (4.7% of the total acidity in the Eastern U.S.). In the U.S. regulatory emphasis at the present time is on in-use performance.
The pre-production accelerated durability tests cannot fully duplicate
the same distribution of performance as in-use vehicles.
Large numbers of
three-way catalysts introduced in 1981 and following years are now reaching 50.000 miles so that field performance can be evaluated.
115
The supply of noble metals for three-way catalysts and particularly the rhodium supply is of concern to manufacturers.
The rhodium use ie, platinum-
rhodium three-way catalysts exceeds the naturally occurring ratio of these metals.
Automobile catalytic converters are a large user of noble metals and
this imbalance in the use of platinum and rhodium can influence the price and availability of rhodium.
Noble metal recovery from spent automob ile exhaust
catalysts is currently a source of platinum and palladium and can be expected to be a source of rhodium after 1990.
ACKNOWLDEGMENTS The author wishes to thank David R. Monroe, Se H. Oh, and Michael J. D'Aniello, Jr. (General Motors Research Laboratories), Gerald J. Barnes and Mike C. Myal (General Motors Environmental Activities Staff), and Michael P. Murphry (General Motors Luxembourg Operations S.A.) for their assistance with the preparation of this manuscript.
REFERENCES
2 3 4
5 6
7
8
9
10
G.J. Barnes and R.J. Donohue, A Manufacturers's View of World Emissions Regulations and the Need for Harmonization of Procedures, Society of Automotive Engineers Paper No. 850391 (February, 1985). K.C. Taylor, Automobile catalytic Converters. Springer-Verlag, Berlin, 1984. R.K. Herz, E.J. Shinouskis, A. Datye and J. Schwank, Ind. Eng. Chem. Prod. Res. Dev , , 24, (1985) 6. B.R. McIntyre and L.J. Faix, Lead Detection in Catalytic Emission Systems and Effects on Emissions," Society of Automotive Engineers Paper No. 860488 (February, 1986). G. Kim, M.V. Ernest and S.R. Montgomery, Ind. Eng. Chern. Prod. Res. Dev , , 24, 525 (1984). G.C. Joy, F.S. Molinaro and E.H. Homeler, "Influence of Phosphorus on Three-Component Control Catalysts: Catalyst Durability and Characterization Studies," Society of Automotive Engineers Paper No. 852099 (October, 1985). H.S. Gandhi, W.B. Williamson, R.L. Coss, L.A. Marcotty and D. Lewis, "Silicon Contamination of Automotive Catalysts," Society of Automotive Engineers Paper No. 860565 (February, 1986). W.B. Williamson, H.S. Gandhi, M.E. Szpilka and A. Deakin, "Durability of Automoti ve catalysts for European Applications," Soc iety of Automoti ve Engineers Paper No. 852097 (October, 1985). F. Car-ace io 10 and J.A. Spearot, "Eng ine Oi 1 Phosphorus Effects of Catalytic Converter Performance in Federal Durability and High-Speed Vehicle Tests," Society of Automotive Engineers Paper No. 770637 (June, 1977) • F. Caracciolo and J.A. Spearot, "Engine Oil Additive Effects on the Deterioration of a Stoichiometric Emissions Control (C-4) System," Society of Automotive Engineers Paper No. 790941 (OCtober, 1979).
116
11
12
13
14 15 16 17
D.R. Monroe, "Phosphorus and Lead Poisioning of Pelle ted Three-Way Catalysts," Society of Automotive Engineers Paper No. 800859 (,Jur,e, 1980). B.J. Cooper and T.J. Truex, "Operational Criteria Affecting the Design of Thermally Stable Single-Bed Three-Way Catalysts," Society of Automotive Engineers Paper No. 850128. R.H. Hammerle and C.H. Wu, "Effect of High Temperatures on Three-Way Automotive Catalysts," Society of Automotive Engineers Paper No. 840549 (February, 1984). K. Otto, W.B. Williamson and H S. Gandhi, Ceramic Eng. and Sci. Proc., 2, (1981) 352. B.J. Cooper, Platinum Metals Rev., 27 (1983) 146. Helen Kahn, Automotive News, p. 50, November 4, 1985. J. Duncan and J. N. Braddock, "Combustor Study of the Deacti vat ion of a Three-Way Catalyst by Lead and Manganese," Society of Automotive Eng ineers Paper No. 841408 (oc tober, 1984).
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
117
©
ASPECTS OF AUTOMOTIVE CATALYST PREPARATION, PERFORMANCE AND DURABILITY B. J. COOPER, W. D. J. EVANS
2
and B. HARRISON
3
lJohnson Matthey PIc, Catalytic Systems Division, 456 Devon Park Drive, Wayne, PA 19087 (United States of America) 2Johnson Matthey PIc, Catalytic Systems Division, Orchard Road, Royston, Hertfordshire SG8 5HE (United Kingdom) 3Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG49NH (United Kingdom)
ABSTRACT The development of legislative controls on petrol engined passenger cars in the USA and Western Europe is reviewed. The application of catalytic control strategies to these requirements is discussed. The principle components of modern exhaust emission control catalysts are identified. They comprise (a) a ceramic substrate, (b) a high surface area wash coat, (c) base metal promoters and/or stabilisers and (d) platinum group metals either singly or in combination. The functional role of these components is discussed and their interaction reviewed from the materials technology standpoint. Aspects of catalyst performance and durability influenced by preparation factors are discussed with particular reference to factors (b), (c) and (d). LEGISLATIVE REVIEW The increasingly urban nature of industrialised society has resulted in deterioration of air quality and generated political pressure for control of atmospheric pollution.
Many states have introduced measures to reduce emissions
including latterly those from vehicle sources. During the early 1940's significant environmental problems were occurring with increasing frequencies in the Los Angeles basin.
In the early 1950's the smog
problem was related (ref. 1) to photochemical interaction of nitrogen oxides (NOx), hydrocarbons (HC) and oxygen.
Surveys established that a high proportion of man
made emissions in that locality were derived from the motor vehicle. These conclusions, supported by numerous studies, provoked intensive research into methods of emission control.
Notable contributors were Eugene Houdry who, in
1949, invented a form of the ceramic monolith now in almost universal use and the Inter Industries Emission Control Programme led by Ford and Mobil which, during the 1960' s, defined the emissions control system which would be required to meet severe regulations. Political
pressures
derived
from
an
increasingly
powerful
and
vocal
environmental lobby culminated in 1970 in the US Clean Air Act (ref. 2), which
118
included progressively more stringent regulations covering inter alia, emissions of CO, HC and NOx.
This targetted a reduction of approximately 90% in emissions
relative to an uncontrolled average late 1960 model year vehicle. Features of the legislation were introduction of lead free gasoline in 1974, a requirement for emissions control systems to be effective for at least 50,000 miles, and
the
definition
of
a
test
cycle
and
procedure
to
measure
emissions.
Intervention of international fuel crises during the 1970's caused some easing of the timetable and emissions limits, the historical development being summarised in Table 1. TABLE 1 Development of U.S. Federal Emissions Regulations Model Year 1970 1975 1980 1981 1983
CO
HC
NOx (g/mile)
34.0 15.0 7.0 7.0 3.4
4.1 1.5 0.41 0.41 0.41
4.0 3.1 2.1 1.0 1.0
The increasing stringency of the limits required progressive introduction of catalytic control strategies beginning in 1975. Subsequent to introduction of this legislation, standards of similar severity (involving a different test procedure) were introduced rapidly in Japan.
More
recently Australia (from January 1986) has adopted the US 1975 Federal limits. Universally, the solution to emissions control from motor vehicles for the US market has included a platinum group metal catalyst.
This has created, over a 12
year period, the largest single application for catalysts and certainly the largest application of platinum group metals (Fig. 1) (ref. 3). The complex political development of Europe relative to the US and Japan has resulted in a different and more fragmented approach to the problem of control of emissions from motor vehicles (refs. 4,5).
European nations under the auspices of
the United Nations Economic Commission for Europe (ECE) have developed a unique test cycle (ECE R15-04), sampling and measurement protocol.
Although the sampling and
measuring protocols are now similar to the US Federal Test Procedure (FTP-75) the driving cycle is radically different.
Thus, for the ECE-15 test, maximum and
average speeds are 50 and 18.7 km/hr respectively with approximately 31% at idle. This simulates city driving in congested conditions.
In contrast, the FTP-75
simulates urban driving, typical of that in the Los Angeles basin.
Maximum and
average speeds are 91 and 34 km/hr respectively, with 18.4% at idle.
119
RHODIUM DEMAND IN THE WESTERN WORLD 1985
Chemical 18%
Glass
Electrical
6%
PLATINUM DEMAND IN THE WESTERN WORLD 1985
Petroleum 1%
9%
Total Demand = 250,000 oz
Fig. 1
Glass
Electrical
5%
7%
Total Demand =2,810,000 oz
Rhodium and Platinum useage by major application.
As in the USA, limits were progressively lowered and refinements made to the test procedure (ref. 6).
However, in the USA a single standard applies to all
passenger vehicles whereas in Europe standards have traditionally been related to vehicle inertia weight. Currently regulation ECE R15-04 is in force (ref. 7) and has been adopted by the European Economic Community
(EEC) and
by most
other European States.
The
requirements of this regulation are lax relative to contemporary US and Japanese limits. By 1984, after several years of gradual reductions in emission levels, the political climate, notably in West Germany, favoured a much more rapid change.
The
West German proposals required introduction of three way catalysts and necessitated use of unleaded fuel.
After a lengthy period of debate, a compromise solution was
developed by the 'EEC' which substantially diluted the original proposals.
The
'final proposals' (ref. 8) were published in June 1985 and entail progressive introduction of standards (Table 2.) As a separate issue it had already been agreed that unleaded fuel should be made available throughout the community from 1989.
This date may be anticipated and the
fuel specification will be 95 RON, O.013g/litre lead (max.) The directive resulting from these proposals will be based upon the concept of optional harmonisation.
It will permit, but not require, Member States to adopt
national legislation in line with it. There remains considerable controversy surrounding the 'final proposals'. There is strong polarisation with respect to identification of Phase 2 standards for small vehicles targetted for January 1st 1987.
120 TABLE 2 Final Proposals for European Common Market Automobile Emission Control Standards Date of Introduction
Engine Capacity
Emissions, g/test
New Models
All New Cars
CO
(HC+NOx)
NOx
Oct. 1988 Oct. 1991
Oct. 1989 Oct. 1993
25 30
6.5 8
3.5
Oct. 1990 Oct. 1992
Oct. 1991 Oct. 1993
15 6 45 To be decided by 1987
Over 2 li tres 1.4 - 2 litres Less than 1. 4L Stage 1 Stage 2
Over one year after publication of the 'final proposals' there remains no immediate likelihood of ratification.
Nevertheless, West Germany has taken the
lead in promoting National Standards supported, during a voluntary introductory period, by significant fiscal measures.
In contrast, UK, France and Italy are not
expected to adopt or make the proposals mandatory for some time. The schism within the EEC is mirrored by further divisions reflecting the wide range of national interests of non-member states.
Thus, Sweden has announced a
proposal to adopt US 1983 standards from 1989. The dis pari t y between emission test procedures, allowable tail pipe emissions and local market conditions conspire to prevent a universal solution to world wide certification of any given vehicle.
Consequently, even though a basic vehicle may
be utilised in several markets, there are generally significant differences in subsystems to cope with, e.g. different emissions constraints.
In consequence,
vehicles of European manufacture may be produced in several specifications.
Thus,
models may be produced to Japanese specification involving an oxidation catalyst, to US specification involving TWC and to a range of European specification involving no catalyst at all.
This substantially magnifies the capital and human resources
required to maintain a broad market presence. CONTROL STRATEGIES The emissions from conventional spark ignition engines are strongly dependent on air:fuel (A/F) ratio.
No single operating regime exists within which levels of
emissions of all pollutants is low. In practical terms this has constrained the development of only three basic control strategies (refs 9,10) in the context of stringent legislation.
These are
all based upon application of supported platinum group metal catalysts.
The
strategies are: (1)
Removal of HC and CO by use of an oxidation catalyst (COC) generally containing
121 Pd or Pt/Pd with other means of reducing NOx emissions, e.g. exhaust gas recirculation.
This strategy normally entails a slightly lean tune and
secondary air injection.
The extent of NOx reduction is determined by
driveability considerations,
limiting
applicability
to
less
demanding
requirements. (2)
A combination of sequential reduction of NOx, over what is essentially a three way catalyst (TWC), followed by oxidation of residual CO and HC over a COC after injection of secondary air.
This procedure requires a rich tune to provide
the necessary net reducing atmosphere in the first catalyst, has an adverse impact on fuel economy and is not likely to be favoured in the European Context. (3)
Removal of pollutants by use of a TWC.
This can be achieved using a Pt/Rh
formulation but only if the engine management system controls the fuelling closely at the stoichiometric point (A/F : 14.7: 1;
A:
1).
Current European
practise for US models is unique in utilising only the single bed TWC and electronic multipoint fuel injection, under oxygen sensor control, for implementation of this strategy. These strategies as applied in the USA market, which can be implemented by a variety of routes, were recently reviewed by Duleep (ref. 10).
A strong trend
towards the single bed TWC operating under closed loop control of electronic fuel injection was noted. Strategies for the emerging European market have been reviewed recently by Evans et al (ref. 11). A significant benefit of a lean fuelling strategy is improved fuel economy. This has motivated intensive research into lean burn technology involving reliable operation at high air:fuel ratios typically in excess of 20:1 (refs 12,13).
A
corollary of such operation would be substantially reduced NOx emissions, (ref. 14) albeit at higher NOx levels than a comparable vehicle fitted with a TWC, but at the expense of an increase in HC.
Operation of conventional engines at high air/fuel
ratios is limited by onset of pre-ignition, rapid torque fluctuations, fast deterioration of the engine and poor driveability. Thus far it has not been demonstrated that acceptable driveability can be achieved for a vehicle operating at 20-22:1 A/F other than by a very high level of equipment, i.e. total electronic closed loop management with multipoint fuel injection.
However, even at that level, it is not possible to achieve severe
legislation limits without provision of a COC to remove hydrocarbon species (ref. 13).
Nevertheless, it is evident that substantial progress has been achieved and
that in the European context a fourth control strategy is potentially available for mid-range vehicles.
122 CATALYSTS FOR AUTOMOTIVE APPLICATION Catalyst technology was developed in the mid period of this century for chemical process operations.
In such applications the catalyst is generally sited
in a fixed bed reactor and after commissioning operates in a more or less steady state mode for a long period of time.
Furthermore, space considerations are
normally a minor factor in the design of the catalyst and reactor; space velocities are generally quite low with large catalyst volumes being employed.
Economic
considerations associated with selecti vi ty and yield generally dictate tight control of space velocity, temperatures and protection of the bed from poisons. Addi tionally complex reac tors,
often wi th recycle or interbed cooling, are
practical solutions to maintaining the required yields. The situation in a motor vehicle could not be more different.
The duty of the
automoti ve catalyst comprises a series of 'commissionings' followed by opera tion in a highly perturbed fashion.
In the USA, the mandatory cold start and 50,000 mile
durability requirement demands operation of the catalyst at low temperatures. During actual operation the catalyst would be subjected to extremes of gas flow and temperature with large variations in concentration of pollutants over the loadspeed envelope of the vehicle. In the emerging European market the situation is even more complex.
Thus,
vehicles are generally much smaller but average and maximum speeds are higher. However, the lower speed test cycle and consequent cooler exhaust gas temperature requires high catalyst activity at low temperatures.
Consequently the operating
temperature requirement is even broader than that for the US market (ref. 11). In addition to the highly non steady state operation, uncontrolled poisoning is a major threat to the catalyst. phosphorus and zinc (refs. 15-18).
The principal poisons are lead, sulphur, The latter two species are generally derived
from lubricating oil, principally from the anti-scuff agent ZDDP.
Very few
examples of significant catalyst deterioration in service have been reported due to Zn/P poisoning (ref. 19). Lead and sulphur are derived from the fuel and there is a complex equilibrium dependent
upon
temperatures
and
absorption/desorption of these poisons.
gas
composition
controlling
the
In the case of lead, extended trials have
demonstrated the feasi bili ty (ref. 20) of successful operation of oxidation catalysts on leaded fuel.
However, it has been noted that in the decade since
introduction of lead-free fuel in the USA, residual lead levels have fallen dramatically.
In that market, where leaded and unleaded fuels are both available,
incidents of poisoning reflect contamination of distribution deliberate misfuelling (refs. 21,22).
equipment
or
Sulphur may also be derived from lube oil
but its impact in the sense of poisoning is low on PGM catalysts.
Interaction with
catalyst components can, however, influence secondary/unregulated emissions of
sulphur bearing species such as sulphate (refs. 23-26). A further major difference with respect to chemical process reactors is the critical need to achieve low pressure drop to minimise power
losses.
This
requirement conflicts to a large extent with those for high activity, Le. good heat and mass transfer. In the early phases of the emerging market, the dominant technology for achieving the total requirement derived from conventional fixed bed pelleted catalyst technology, albeit with special high aspect ratio beds to minimise power losses.
However, widespread use was made of an alternative technology based on a
multicellular
ceramic
substrate or monolith
(ref.
27).
Due to persistent
durability problems with pellet bed reactors the monolithic support catalyst has become the dominant technology accounting for perhaps 95% of all new vehicle systems. The monolith has strong, porous, thin walls supporting an array of parallel channels presenting a high geometric surface area.
The high open area and
structure promote laminar flow, limiting pressure drop at high flow rate.
Use of a
low expansion body based upon Cordierite provides a high degree of thermal shock and strength while offering a high maximum operating temperature. Major advances in ceramic extrusion technology and processing have enabled substantial advances in product quality.
In consequence a wide range of shapes,
sizes and cell dimensions are available (ref. 27). Although ceramic monolith based catalysts dominate the global market, there has been significant interest in Europe latterly in metallic monoliths (refs. 2830).
The reduced wall thickness offers specific advantages in conversion in
applications where space is at a premium or ceramic based solutions are not possible.
Several major applications now exist (ref. 31) but presently cost
factors remain a major determinant in favour of ceramics. However, it is not possible to achieve the combination of strength and thermal shock resistance required for a ceramic monolith together with the high specific surface area required for catalysis.
This surface area is applied to the monolith,
generally in the form of an aqueous suspension of a highly porous material - the wash coat.
Its characteristics, along with those of the underlying support, have a key
role in determining the activity and durability of the catalyst system. Accordingly the key first stage of manufacturing a monolithic type catalyst is formulation of the wash coat and uniform application over the internal surface of the monolith.
Although commercial processes are proprietary with little detail
available, the coating is generally fixed, by calcination, at elevated temperature. The second key activity is application of precious metals and promoters, for economic reasons generally from solution or dispersion.
After drying, reduction
or calcination processes are used to fix the precious metal.
In principle. the
124 precious metal may be included with the wash coat. The catalytic species of current automoti ve catalysts are balanced mixtures of precious metals and promoters selected, as discussed previously, on the basis of application.
Precious metals are favoured due to high catalytic activity and
selectivity, particularly at low temperatures (as experienced with cold start tests).
Additionally their supported dispersions are relatively stable at high
temperatures and exhibit good resistance to poisoning. The idealised requirements of the three chemical constituents of the catalyst must be met in a manner which allows economic manufacture by routes compatible with mass production.
Subsequent sections are concerned with each of the three key
components (wash coat, base metal promoters and precious metals) and examine the influence of preparation on performance.
WASH COAT An autocatalyst wash coat must provide a high, stable surface area upon which crystallites of precious metals and promoters can be dispersed.
The overall
stability of the catalyst is to a large extent dependent upon that of the wash coat in terms of surface area and adhesion. Washcoats generally comprise mixtures of stabilisers, promoters and alumina. Alumina forms the bulk of the wash coat, frequently in excess of 90%, and accordingly its stability is crucial.
Preparation of wash coats is proprietary but generally
involves formation of a high solids
dispersion
of
activated alumina.
Such
dispersions are generally produced by milling or use of high shear mixers. Addi tions of dispersing agents, e t c , , are necessary to provide the surface tension and flow properties required to allow penetration of a 400 cpsi monolith and achieve uniform coating of cell walls. Choice of alumina precursor has a significant impact on stability of surface area (ref. 32).
This is illustrated in Fig. 2 for activated aluminas derived from
Boehmite and Gibbsite, the two major industrial raw materials commonly available. It is readily apparent that activated aluminas derived from boehmite are the most
thermally
stable
in
the
principle
temperature
ranges
of
interest.
Additionally •. transitional aluminas derived from gibbsite undergo major reordering of the lattice at lower temperatures than
¥' alumina with
significant implications
for shrinkage as well as the surface area changes noted above. The inherent stability of aluminas can be further improved by addition of other oxides (ref. 32).
Base metals can act as promoters and in an ideal si tua tion would
fulfill a dual role.
Fig. 3 shows the change in surface areas for boehmite derived
activated aluminas as a function of temperature.
It may be seen that addition of
barium retards phase transformation and consequent loss of surface area to well above 10000C.
125
Gibbsite X:
a
X ~
Boe'hmite
200 Surface Area m'g 100
o 400
600 800 1000 Temperature
c
1200
Fig. 2. Surface area thermal stability and phase transformations for transitional aluminas derived from Gibbsite and Boehmite.
100
40
20
o '--.,jL---,_ _,.--_r--r-.,.-' Fresh 750 1000 Temperature C
Fig. 3.
1200
Thermal stability of surface area of ~ alumina - metal oxide mixtures.
The benefits of such improved stability in terms of catalyst performance is illustrated in Fig. 4 for unstabilised and barium stabilised palladium and rhodium catalysts after ageing under the specified conditions. improved performance is achieved.
In each case significantly
126
Pretreatment
Test Hrs 300
950 C/ 1% 0,/10% H,O' lHour Pert 1.00 «\) 1.00 Hz
100,-----'--"'-'-....:...:'-'--;;::-::-'-"1
c 80' o 'iii Q; 60 > s:
o
JCO HC
___ -----,I--}~-
80
t:
'iii
Q; 60
> c
8 40
8 40
if!
if!
20'
20 (A)
0.96
0.98
1.00
1.02
1.04
0.96
Equivalence Ratio (r.)
0.98
1.00
1.02
1.04
Equivalence Ratio (Al
__ (1) Pd/AI ,0 3
-
NOx
(B)
__ (1) Rh/AI,03 (2) Rh/Ba/AI,03
(2) Pd/Ba/AI,03
Fig. 4. Static engine based selectivity test showing the. influence of barium stabiliser on catalyst performance (A) for palladium based catalysts after 300 hrs engine ageing (800 0C max, ) and (B) for Rh based catalysts after hydrothermal ageing for 1 hr at 950 0C. We must now consider a complex series of trade-offs that are involved in the application of the wash coat to the substrate. are as follows.
In simple terms the considerations
The wash coat provides the means for a highly dispersed catalytic
material to maintain a high surface area.
Therefore, for a given loading of
catalytic material, a higher quantity of wash coat will result in a more stable dispersion.
This is because, over the higher total surface area present, there
will be fewer next neighbour interactions between the precious metal components. Therefore, coalescence sintering will be reduced.
--
100 c:
80
0
'iii 60 i >
c: 0
U
40 20 0
/ / ~ ~ ~--I
.- / >..-
0.96
ICO NOx~
(Al HC ......
0.98
1.00
Lambda Value
1.02
In addition to this effect, the
100 c:
80
0
'iii
i
60
>
c: 0
40
20 ..:.--
o
0
0.96
'"
O
- -
~
NOx~
(6)
0.98
HC .....'
1.00
1.02
Lambda Value
Fig. 5. Static engine based selectivity test showing the influence of wash coat loading on the performance of a 5:1 Pt/Rh TWC after ageing for 200 hrs on an 8 mode cycle (peak temperature 850°C, 3mgL-llead) Catalyst A contains 68 percent by weight of the wash coat deposited on Catalyst B.
127 washcoat acts as a poison sink and the higher the surface area of wash coat present, the better the catalyst will resist the effects of poisons.
The effects of wash
coat loading on catalyst activity are illustrated in figure 5. Clearly the activity of the catalyst with a high wash coat loading and therefore higher surface area is better. Fig. 6 shows the activity pattern for a series of catalysts, differing solely in wash coat loading, after thermal pretreatment in a wet oxidising gas and subsequent 150 hours engine ageing in a perturbed ageing cycle.
CO and NOx
conversion shows a significant dependence upon wash coat loading in this test.
100 c
I ~ N O X I3 co
!mHC
I
90
o "iii
~
o () '"
80 70 60
50
1111 x
1.20X
1.42X
1.51X
Relative Washcoat loading
Fig. 6. Static engine test data showing the effect of wash coat loading on conversion at ~ = 0.995 in a selectivity test after extended ageing (150 hrs).
In addition to surface area stability, the wash coat must maintain good adhesion
to
the monolith at high loadings over the operating envelope.
In
principle, this can be achieved by increasing the solids content of the dispersion or repeated coatings.
However, close process control must be exerted over the
application process which otherwise becomes a source of adhesion problems.
Thus,
packing of solid particles during removal of occluded water by drying may provoke shrinkage cracks. loss of wash coat.
Thermal cycling during processing may provoke delamination and Prevention of premature failure due to these mechanisms
requires tight control over all aspects of wash coat preparation and application. Retention of high activity during service is critically dependent upon maintaining integrity of the wash coat/monolith bond.
However, even initially
well bonded coatings can be susceptible to deterioration due to frequent, rapid, high temperature cycling.
The
influence of
thermal ageing at
initially highly adherent coating is shown in Fig.
7.
such changes.
0C
on an
Severe shrinkage has
occurred due to major changes in surface area and the alumina phase. may be overcome by inclusion of phase stabilisers (Fig. 8)
1350
whi~h
This problem
defer and reduce
128
Fig. 7. Optical micrograph of wash coat after sintering at high temperature showing severe shrinkage.
Fig. 8. Optical micrograph of stable wash coat after high temperature exposure showing freedom from shrinkage cracking.
Benefits derived from these improvements may be seen from comparison of the hydrocarbon breakthrough for two otherwise identical catalyst systems after 350 hours operation at temperatures up to 800 fuel containing 3mgL
-1
0C
(for 80% of the time) when exposed to
lead (Table 3).
TABLE 3 Effect of Washcoat Type on the Durability of Pt/Pd Catalysts for Hydrocarbon Oxidation % Unconverted Hydrocarbon at 25 Hrs at 355 Hrs Coating A (Figure 7)
13
18
Coating B (Figure 8)
11
14
Coating A (Fig. 7) shows approximately twice the rate of deterioration of that for B (Figure 8). In addition to the specific features relating to activity and catalyst durability, it is critical that the wash coat does not adversely impact upon the
129 overall performance of the monolith. During normal service the monolith support is subjected to frequent thermal cycling.
Typically, exhaust gas temperature can reach several hundreds of degrees
celsius in less than a minute from a cold start.
In most converter designs the flow
distribution is non uniform with flow concentrated over the central region.
This,
coupled with highly exothermic reactions, results in development of strong axial and radial thermal gradients.
Radial gradients due to the relatively cool outer
skin are accentuated in the increasingly favoured non-cylindrical type converter. These rapidly fluctuating temperature gradients may induce a catastrophic failure of the ceramic as a result of thermal shock.
Low expansion bodies have demonstrated
ability to resist thermal shock during service life in the USA but such problems would be expected to be more severe in Europe due to different, more severe, driving patterns and a growing tendency to move the catalyst nearer to the exhaust manifold. Such problems can, however, be overcome by careful design of catalyst, converter and exhaust train (ref. 33). Fatigue type studies of thermally induced failures of ceramic monoliths have been
the
subject
of
intensive
investigation
(refs.
34,35).
However,
the
statistical nature of brittle fracture and the difficult nature of the property measurements has provoked development of a number of empirical tests.
The most
useful of these is the burner type test in which the unit is heated rapidly from room temperature to a predetermined high temperature and subsequently rapidly cooled by shutting off the fuel.
After a fixed number of cycles the unit is removed and
examined visually and accoustically for fracture. higher
temperature
until
failure
is
If unbroken, it is retested at a
experienced.
characteristic of the thermal shock resistance.
This
temperature
is
As with all strength tests of
brittle materials, it is essential that a statistically significant sample is taken as a measure of the mean property and dispersion. The thermal shock characteristics as determined by a burner type test for raw monolith and various types of coated catalyst are shown in Fig. 9.
It may be seen
that a coating of washcoat to early formulations resulted in a marked degradation in failure temperatures to a barely acceptable level.
This is attributed to the large
-6
differential in coefficient of thermal expansion of cordierite (10 x 10 alumina (60 x 10
-6
0
/ C) and
0
/ C) resulting in thermal stresses at the monolith/wash coat
interface. One method of preventing such interaction is precoating the monolith (ref. 36) with an organic material which is subsequently removed during calcination (to fix the wash coat).
The effectiveness of such processes, which have been widely
practised for several years, is shown in Fig. 9 where the differential is reduced to oC. 30/40
130
o en
CIJ
~ Min. Spec. Value
e Ol CIJ
o
1000
IIllI Pre-treat Cat.
D
Substrate
m
II
'74 Catalyst
New Catalyst
Q.
E CIJ
I-
800
...CIJ:;, C\J
LL.
c: C\J
CIJ
4 x 6 inch
:E
4.66 x 6 inch
SUBSTRATE ICATALYST SIZE
Fig. 9. Mean thermal shock failure temperature (burner test, minimum 15 units) for 400 cell ceramic monoliths and catalysts of various types. However, there are inherent disadvantages due to additional raw materials and extra process costs.
Furthermore process control is more difficult and the total
wash coat deposit feasible on a unit basis is much reduced.
In consequence this
provides an artificial and undesirable limitation on activity, durability and poison resistance.
In response to these limitations, a new process has been
developed which minimises surface interactions without resort to precoats.
The
data shown in Fig. 9 indicate that this technology enables the benefits of stabilised high wash coat levels to be achieved without adverse impact on thermal shock characteristics. BASE METAL PROMOTERS/STABILISERS The critical role of Rh in the performance of single-bed three-way catalysts and its extreme sensitivity to deactivation by exposure to high temperature lean operation, dictates that any new catalyst development must address the issues of Rh performance and stability.
Rh deactivation in three-way catalysts, after exposure
to high temperature lean ageing has been attributed to a strong Rh/Al (Ref. 38).
interaction Z03 Additional work (Refs. 39,40) has shown this interaction can be
eliminated, with substantial improvements in thermal stability, by supporting the Rh on zirconia.
Unfortunately,
the incorporation of Rh/ZrO
Z
into three-way
catalysts requires complex manufacturing methods which are not suitable for high speed production. Rh/Al
An alternative approach is suggested by work that indicates
interaction may occur preferentially at the grain boundaries of the
Z03 support (ref. 41).
We have thus chosen to incorporate a stabilizer into the alumina
support system designed to preferentially block this interaction. Although these results showed that substantial stabilization can be achieved they also demonstrated the major problem of utilization of single-bed three-way catalysts Extensive
CO and NOx performance around the stoichiometric air/fuel ratio. testing
of
base
metal
improvement in performance.
stabilisers
failed
to
secure
the
desired
However, incorporation of base metal promoters in
three-way catalysts has been shown to improve CO and NOx performance in the region of the stoichiometric air/fuel ratio.
The two mos t widely used and studied promoters
are nickel and cerium (refs. 42-46).
Their influence at equivalent total promoter
loading is shown in Fig. 10.
Conversion 0.02 wt% Rh Etf.ciency
-_.... Ce Promoted - - Unpromoted - - - NilCe Promoted
(%)
100
co
80 60 40
...~ NOx
20
.96
.98
1.00
1.02
1.04
Equivalence Ratio
Fig. 10. Performance of unpromoted, Ni/Ce and Ce only promoted 0.02 wt. % Rh 0C catalysts after ageing at 980 in 1% 02' 10% 02 atmosphere for 1 hr.
A substantial increase in performance, particularly in the stoichiometric region, is noted for both promoted systems. shows superior stability.
In that respect the ceria only system
This, at least in part, can be attributed to the reaction
of nickel and alumina to form nickel aluminate (ref. 42) at elevated temperatures. That effect, and increasing concern wi th regard to environmental impact of nickel, has resulted in a trend away from use of that element. The
activity/stability relationships of such catalysts has been further
explored by synthetic gas studies using a reactor system
cy~ling
between rich and
lean conditions as shown in Table 4. Under lean condi tions the promoter type and loading has very li ttle impact on performance or thermal stability.
Under rich conditions the promoter type and
loading affects both fresh performance and thermal stability.
Substitution of
cerium-only for nickel/cerium results in a dramatic improvement in fresh CO performance wi th a further more modest improvement seen from an increase in cerium
132 loading.
After thermal ageing the "arne performance trend" are obs er ved .
However.
only the high cerium ca t a Ly s t doe" not show a large drop in performance in comparison to the fre"h "tate.
TABLE 4 Tr ans I en t performance of f r esh and aged ca t a l ys r s (0.24% Pt/0.05% Rh) under lean and rich conditions. Temperature 400 oC. GHSV 100,000 hr- l• gas compo s Lt Lon s - base mix of 1200 ppm HC (C 3H6). 500 ppm NO, 14.0% COZ' 0.17% HZ and 10% HZO pI us either rich "pike 2.0% CO, 0.5% O2 for 4 s ec . or lean "pike 0.5% CO, 2.0% O2 for 10 sec. Balance
N2· Lean Spike (% conver"ion)
Rich Spike (% conver"ion)
HC
CO
NOx
HC
CO
NOx
Ce/Ni Promoter
Fresh Aged'"
98 96
89 86
34 30
88 72
51 24
50 46
Ce Promoter
Fresh Aged'"
100 95
89 87
39 32
86 84
71 46
54 48
ZX Ce Promoter
Fresh Aged'"
99 96
89 87
37 34
86 81
76 74
54 50
"'750 oC
I
10% H20
I
Air
I
5 hrs.
The origin of this large effect on CO performance has been explored by measuring the rich spike CO performance wi th and without H20 present. CO conversions under rich condi tions, after hydrothermal ageing at 900 0C in 1% oxygen for four hours are shown in Table 5. TABLE 5 Performance of hydrothermally aged 0.16 wt% Pt/0.03 wt% Rh catalysts containing ceria promoter in the presence and absence of water vapour. (Conditions otherwise as shown in Table 4). CO Conversion (%) with H2O
CO Conversion (%) without H2O
IX Ce Pr omot ar
54
49
2X Ce Promoter
64
49
6X Ce Promoter
70
49
133
°
Variation in cerium promoter level has no effect on CO performance when H is Z With HZO present in the feedgas CO performance is
absent from the feedgas stream.
higher and increases with increased cerium loading.
This is consistent with an
enhancement of the water-gas shift reaction upon addi tion of. cerium to Pt/Rh threeway catalysts.
This enhanced performance is at least partially transient in nature
with CO conversions dropping below 50% under steady state conditions. These results show that a Pt/Rh catalyst system, based upon a stabilized alumina wash coat designed to minimize the adverse effects of strong Rh/Al
Z03 interactions and a high cerium promoter level for enhanced CO performance and stability, should result in significantly improved three-way catalyst performance
and durability. This conclusion was confirmed by separate static engine ageing of replicate catalyst units under stoichiometric, lean and high temperature lean conditions. Data for the first two conditions (entailing a maximum temperature of 760 of the cycle) are similar;
that for lean ageing is shown in Fig. ll(A).
only catalyst shows enhanced stability in the stoichiometric region.
0C
for 17%
The ceriaData for the
much more severe high temperature lean cycle is shown in Fig. ll(B).
Conversion
Conversion Efficiency (%)
y (% Efficiencr-:....;. ....;. )---------------, 0.16 wt% PtlO.03 wt% Rh 100
0.16 wt% PtlO.03 wt% Rh
CO
100
HC
80
80
60
60
40
40
20
20 (B)
(A)
o
O ....... . . . . , . . . . - . - - . . , . . . . . - - r " - ~ . . . , . . . . - , . . . . _ . . , . . . . . _ . , . - - - - ' .96
.98 1.00 1.02 Equivalence Ratio
1.04
.96
.98 1.00 1.02 Equivalence Ratio
Fig. 11. Performance of high Ce promoted (solid lines) and mixed Ni/Ce promoted (broken lines) Pt/Rh Catalysts after lean ageing at (A) 760 0C and (B) 1050 0C peak temperatures.
This cycle, which involved lean excursions (0.3% excess oxygen), provokes much greater deterioration of the catalyst.
However, the high ceria system shows
superior stability relative to the mixed promoter system.
PRECIOUS METAL COMPONENT In the design of an automotive exhaust catalyst the method of precious metal incorporation plays an important role in the activity, selectivity, durability and cost effectiveness of the system.
In addition, the support material, together with
appropriate stabilisers and promoters, can playa significant role in determining
134 the precious metal location, dispersion and activity. these has been mentioned above.
The contribution of some of
This section examines the deposition of precious
metals with particular reference
to
those
presently most
commonly found
in
automotive catalysts namely platinum and rhodium. There are a number of possible methods of deposition of the metals onto support materials;
these
include
impregnation,
absorption
or
precipitation with the support and vapour deposition.
ion
exchange,
co-
Vapour deposition is not
practical on economic grounds and co-precipitation, often used for the preparation of base metal catalysts, cannot be used because of the problems of recycling and recovery. or
ion
Thus precious metal catalysts are usually prepared by the impregnation
exchange
of
metal
salts
onto
the
support
materials.
A schematic
representation of the ion exchange process is shown below.
Ion Exchange of Metal Salt onto Support I
I
Cationic exchange
S-OH+
+
S-OC+
C+
I
S
H+
+
(OH)
_
S-A
Anionic exchange C+
+
I
2+ 2+ ' Pd(NH 3)4 ' [Rh(NH 3)SClj 222PtCl 6 ' PdCl 4 ,RhCl 6 Pt(NH 3)4
2+
support surface
High pH promotes cation exchange, low pH promotes anion exchange.
As the pH is
lowered in a cation exchange regime, interaction between precious metal and the support decreases until the process can be considered a simple impregnation. same
process
occurs
as
the
pH is
raised
under
anion
exchange
The
condi t Lons .
Impregnation is considered a pore wetting process only, the salt being deposited on the support as the solvent is removed by drying.
This has the advantage that the
salt solution is not selectively depleted in precious metal during a continuous process.
If there are ion exchange processes,
depletion does occur and the
solution requires frequent monitoring and metal replenishment.
Ion exchange does,
however, have the advantage of the potential for selective metal placement whilst impregnation generally gives a uniform dispersion. The firing stage, following ion exchange or impregnation of the precious metal, is an important one in the catalyst preparation.
Depending upon temperature
and atmosphere the precursor salt decomposes to ei ther the metal or an oxide.
The
effects that can be achieved are illustrated in figures 12(A) and (B) where decomposition products, particle size and the light-off temperature (for carbon monoxide) are plotted against firing temperature for salts of platinum and rhodium.
135 The results shown in Figure 12(A) are for platinum deposited on alumina via the precursor platinum (II) tetrammine chloride.
Apparently some CO oxidation occurs
even on the undecomposed precursor, although this may be due to CO enhanced reduction of the salt.
As the firing temperature is increased the precursor goes
through several stages of decomposition, during which the CO oxidation light-off temperature also increases.
The most noticeable effect, however, is the sharp
increase in particle size and light-off temperature when the precursor is fully decomposed to the metal.
Hence, platinum, which does not have an oxide phase stable
o
above 400 C, sinters rapidly as the metal and the oxidation kinetics (which are negative order for CO over platinum) come into play.
TGA
TGA
r--t--?f--.:r"'-------,
Result I - - - - , ~ + " , . " . , . . . . . , ~ - : ; . . . . ; : ~ - - - {
Result ! - : L : + ~ ~ ; - - - - - - - - i 300
330
300~ o
tlIl
'0
250L
s:
Ol
:.J (Al
200
200 400 600 800 1000 Firing Temperature (OC)
L..-_,-----,..----._.....,._.....,._-:-/:200 200 400 600 300 1000 1200 Firing Temperature (OC)
Fig. 12. Curves showing correlation between metal crystallite size, light off temperature for CO oxidation and calcination temperature and composition for alumina supported catalysts prepared from (a) platinum tetrammine (chloride) and (b) Claus' salt (1%Rh/A1203) ) is used as the precursor for In contrast, when Claus' salt ( [Rh(NH 3\CljC1 2 rhodium, the initial decomposition product upon calcination is rhodium metal which retains a relatively low particle size (Fig. 12(B)).
As the temperature is
increased rhodium is converted to rhodium (III) oxide and particle growth increases markedly.
Thus, rhodium sinters as the oxide and a parallel, although not entirely
coincident, increase occurs in CO oxidation light-off temperature. Thus far, only one precursor of each of the precious metals has been discussed in the context of the calcination process.
In practice, a number of precursors are
available and these can play a major role in determining metal location and dispersion (ref. 47).
The effect of precursor on rhodium dispersion on alumina is
shown in Table 6 where the absorption of NO is used as a measure, of dispersion. The multiple absorption of NO on rhodium is characteristic of the highly
136 dispersed metal (refs. 37,48) and has also been observed for CO, 0z and HZ (refs. 49,50).
The ratio of NO to Rh would not normally be expected to be greater than Z.
TABLE 6 The effect of precursor on Rhodium Dispersion (1% Rh on alumina) Precursor
NO/Rh
[Rh(NH3)5CljClz
0.81
[Rh(Cl)6](NH 4)3
0.96
Rhodium nitrate
1. 54
Rhodium sulphate
1.78
The dispersion of a precious metal on a support material is also strongly dependent on the metal loading and the atmosphere in which the catalyst is fired. These effects are illustrated in figure 13 where NO uptake is plotted against rhodium loading on alumina for catalysts prepared from Claus' salt and rhodium chloride.
For
each
precursor,
three
firing
hydrogen/nitrogen and air, were investigated. precursors is immediately apparent.
atmospheres,
i.e.
nitrogen,
A major difference between the two
The catalyst prepared from Claus' salt does
not show a progressive increase in NO uptake above a critical rhodium loading.
This
can be related to the relatively low solubility of Claus' salt compared to rhodium chloride.
At higher concentrations, the former crystallises, or sinters as the
salt, during the drying process prior to firing.
,..:
w
~ ';"
1.0
E co
(;
..
E
0.1
~
.2 ~
.
:; E
0.01
"
s:
- - - N2
o
.... Air
oz 0.001 L0.01
- - ~ - - - - . J
0.1
1.0
10
Rh loading...u mole m- 2 (B.E.T.)
Fig. 13. Effect of concentration on rhodium dispersion using (A) [Rh(NH3)5CljCIZ and (B) Rh C13 as precursors.
137
A second difference between the two is the behaviour when the catalysts are fired in air.
Claus' salt initially decomposes to rhodium metal but in the presence
of air is converted to the oxide which sinters rapidly.
Thus a worse dispersion of
rhodium is observed when Claus' salt is fired in air than when it is fired in nitrogen or hydrogen/nitrogen.
In the case of rhodium chloride a superior overall rhodium
dispersion is achieved and air firing is not so detrimental to dispersion as it is for the ammine complex.
These observations can again be explained in terms of the
decomposition chemistry of the precursor.
Newkirk and McKee (ref. 51) have studied
the decomposition of rhodium chloride, both unsupported and supported on alumina, in a hydrogen atmosphere.
The salt is reduced to the metal at temperatures below
o
200 C and, in the case of the supported material, the hydrogen chloride evolved is strongly adsorbed by alumina and is not released until temperatures in excess of 600
0C.
The decomposition of rhodium chloride in air is slow and produces lower
chlorides or oxychlorides which retard the sintering process.
Nitrogen firing is
also likely to produce a lower chloride content. The role of the support material in determining the activity and selectivity of precious metal catalysts is critical and there is now a significant literature on metal support interactions.
The effect may be
considering alumina and ceria as support phases.
illustrated
for
rhodium by
In the case of alumina the metal
support interaction was investigated by firing 1%Rh/AI
Z03
samples in air over a
range of temperatures (table 7). TABLE 7 The effect of alumina phase and ageing (8 hr s in air at the specified temperature) on rhodium dispersion (1%Rh/A1 Z03 ex [Rh(NH3)5CljC1Z) ALUMINA PHASE Gamma
Delta
Theta
AGEING TEMP.
°c
NO/Rh
450
0.86
650
0.42
850
0.00
450
0.74
650
0.40
850
0.00
450
0.33
650
0.Z8
850
0.00
138
The rhodium dispersion becomes progressively worse on the higher temperature and, therefore, lower surface area alumina phases. the ageing temperature of each Rh/ Al
NO uptake also falls sharply as
The lower NO uptake can Z03 be explained partially by rhodium sintering (as the oxide) and also by a metal support interaction (Ref. 36).
phase is increased.
The interaction is less for the high temperature,
less reactive alumina phases but even here NO absorption is not measurable after ageing
at
850
0C.
The
rhodium/alumina
interaction
is
also
observed
when
temperature programmed reduction (TPR) is performed (Fig. l4(A) and (B).
2.8
r - - - . , . . - - - - - - - - - , 18.
;:-2.32
4.3
~
'"
B
::l
~
:e -:Cl.64 ". .:<
~1.36 e
'" e" .88 ,..
't>
:r
.4 L-_,....-_,....-_,....-_,....-----'
200
400
600
800
200 400 600 800 1000 Temperature Deg. Celsius
Temperature Deg. Celsius
Fig. 14. Temperature programmed reduction traces for (A) 1% Rh/AI Z03 and (B) 1% Rh/CeOZ catalysts. Rhodium begins to reduce at relatively low temperatures but the reduction peak o
shows a very long tail and reduction is not complete until 800 C.
In contrast, when
rhodium is supported on ceria the metal support interaction is weaker and reduction is complete by 250
0C,
the other peak in this system being assigned to the partial
reduction of ceria itself (Fig. 14(B)).
Thus,
in preparing precious metal
catalysts, careful attention must be paid to the choice of the support material since this strongly influences activity, selectivity and durability. In addi tion to individual precious metal/ support interactions, those between metals themselves must also be considered.
Thus, it has been established that Pt
and Rh can form alloys, surface enrichment of which, with oxidised Rh species, is adverse to high activity (ref. 52).
Thus, preparative methods must target
carefully the juxtaposition of all key components for optimum performance and durabili t y , CONCLUDING REMARKS High performance automotive emission control catalysts are a combination of the compromises required by the sometimes opposing requirements of their highly
139
dynamic operating environment. emission control.
In consequence there is no universal solution to
Choice of support, chemical componen t s and careful control over
interactions is crucial to activity and durability. Current generation systems achieve high activity and stability by combination of stabilisers/promoters, controlled dispersion and targetting of precious metal components to optimise metal support interactions.
Over the 12 years of vehicle
application thus far accumulated, substantial improvements have been achieved in performance, reflecting extensive investment in Research and Development.
Over
that relatively short period this has established automotive applications as the largest single application of heterogeneous catalysts and the principal consumer of platinum group metals. During that interval, the scientific basis of heterogeneous catalysis has advanced substantially.
New and improved techniques, e s g , temperature programmed
methods such as TPR and TPO, EXAFS, etc. have become more readily available and have been/are being applied more widely, together with metal-supported interactions.
establi~hed
tools to examine
Such techniques have proved of immense value in a
sector previously dominated by empirical techniques which nevertheless remain of great importance.
Although much has been achieved there remain major challenges
from established markets (USA, Japan), large emerging markets (Europe, Australia, 'Korea) and potential markets in developing countries such as Brazil.
Notable among
them are the economic and strategic requirements to reduce the absolute and relative proportions
of
precious metals
without compromising performance.
Although
significant progress has been achieved, it is evident that such increasingly demanding requirements can be met
only as a result of improved scientific
understanding of these complex interactions. ACKNOWLEDGEMENT The data reviewed in this paper is a selection from that of many workers in the Research and
Development Laboratories of
Johnson Matthey world wide.
The
particular contribution of Drs. T. Truex and P. N. Hawker in preparation of this review is gratefully acknowledged. Figures 4, 7, 9, 10 and 11 and Tables 4 and 5 are published by kind permission of SAE from paper SAE 850128 (ref. 46). Figure 13 and Tables 6 and 7 are reproduced by kind permission of Kodansha Lt d , , Tokyo, from Proceedings of 7th Int. Congo Cat. 1980 (ref. 47). REFERENCES 1. 2. 3.
A. J. Haagen-Smid, Ind.Eng.Chem., 44(1952) 1342. Anon., The Clean Air Act as Amended August 1977, U.S. Governmental Printing Office, 1977, Serial No. 95-11. G. G. Robson, Platinum 1986, Johnson Matthey PIc., May 1986, pp 26, 42 and 44.
140 4.
W. Berg, Evolution of Motor Vehicle Emission Control Legislation Leading to the Catalyst Car?, SAE 850384. 5. M. P. Walsh, Global Trends in Motor Vehicle Air Pollution Control, SAE 850383. 6. C. de Boer and J. A. Jeyes, The Interaction of Fuel Economy and Emission Control in Europe - A Literature Study, Paper G422/84, The Institution of Mechanical Engineers, 1984. 7. Anon., Addendum 14: Regulation No. 15 Geneva: United National Economic Commission for Europe, 1958, Revision No.3, 1981. 8. Anon , , Commission of the European Communi ties, Proposal for Amendment of ECE Directives in the Lead Content of Petrol and Motor Vehicle Emissions. Comm (85), 288 Final, 19th June 1985. 9. - G. J. K. Acres and B. J. Cooper, Automobile Emission Control Systems, Platinum Metals Review, 16(3) (1972) 74. 10. K. G. Duleep, Future Automotive Emission Control and Strategy, SAE 841244. 11. W. D. J. Evans and A. J. J. Wilkins, Catalytic Emission Control Strategies for Europe, Sci. Total Environ., In Press. 12. S. Matsushita, T. Inoue, K. Wakanishi, N. Kato and N. Kobayashi, Development of the Toyota Lean Combustion System, SAE 850044. 13. L. C. van Beckhoven, R. C. Rijkboer and P. van Slaten, Air Pollution by Road Traffic - Problems and Solutions in the European Context, SAE 850387. 14. Y. Kimbara, K. Shinoda, H. Koide and N. Kobayashi, NOx Reduction is Compatible with Fuel Economy Through Toyota's Lean Combustion System, SAE 851210. 15. W. B. Williamson, H. S. Gandhi, M. E. Heyde and G. A. Zawaki, Deactivation of Three Way Catalysts by Fuel Contaminants - Lead, Phosphorous and Sulphur, SAE 79094. 16. R. H. Hammerle and Y. B. Graves, Lead Accumulation on Automotive, SAE 830270. 17. B. Harrison, J. R. Taylor, A. F. Diwell and A. Salathiel, Lead Species in Vehicle Exhaust: A Thermodynamic Approach to Lead Tolerant Catalyst Design, SAE 830268. 18. B. J. Cooper, B. Harrison, E. Shutt and 1. Lichtenstein, The Role of Rhodium in Platinum/Rhodium Catalysts for Carbon Monoxide/Hydrocarbon/Nitrogen Oxides (NOx) and Sulphate Emission Control - The Influence of Oxygen on Catalyst Performance, SAE 770367. 19. W. B. Williamson, J. Perry, R. L. Gross, H. S. Gandhi and R. E. Beason. Catalyst Deactivation due to Glaze Formation from Oil Derived Phosphorous and Zinc, SAE 841406. 20. A. F. Diwell and B. Harrison, Car Exhaust Catalyst for Europe, Platinum Metals Review 25(4) (1981) pp 142-151. 21. B. D. McNutt, D. Elliot and R. Dalla, Patterns of Vehicle Misfuelling in 1981 and 1982, SAE 841345. 22. R. B. Michael, Misfuelling Emissions of Three Way Catalyst Vehicles, SAE 841354. 23. W. R. Pierson, R. H. Hammerle and J. T. Kummer, Sulfuric Acid Aerosol Emissions from Catalyst Equipped Cars, SAE 740287. 24. B. J. Cooper, E. Shutt and P. Oeser, Sulphate Emissions from Automobile Exhaust, Platinum Metals Review, 20 (2)(1976) 20. 25. C. M. Urban and R. J. Garbe, Exhaust Emissions from Malfunctioning Three Way Catalyst Equipped Automobiles, SAE 800511. 26. L. R. Smith and F. M. Black, Characterisation of Exhaust Emissions from Passenger Cars Equipped with Three Way Catalyst Systems, SAE 800822. 27. J. S. Howitt, Thin Wall Ceramics as Monolithic Catalyst Supports, SAE 800082. 28. C. A. Dulieu, W. D. J. Evans, R. J. Larbey, A. M. Verrall, A. J. J. Wilkins and J. H. Pavey, Metal Supported Catalysts for Automotive Applications, SAE 770299. 29. A. S. Pratt and J. A. Cairns, Noble Metal Catalysts on Metallic Substrates, Platinum Metals Review 21(3) (1977) pp 2-11. 30. M. Nonnenmann, Metal Supports for Exhaust Gas Catalysts, SAE 850131. 31. H. Schuster, J. Abthoff and C. Noller, Concept of Catalytic Control for Europe, SAE 852095.
141 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.
R. Gauguin, M. Graulier and D. Pap pee , Thermally Stable Carriers, Catalysts for Control of Automotive Pollutants, Ed. J. E. McEvoy, ACS Series 143, American Chemical Society, Washington D.C., 1975, pp 147-160. W. D. J. Evans and A. J. J. Wilkins, Single Bed, Three Way Catalysts in, the European Environment, SAE 852096. S. T. Gulati, Effects of Cell Geometry on Thermal Shock Resistance of Catalytic Monoliths, SAE 75071. J. D. Helfinstine and S. T. Gulati, High Temperature Fatigue in Ceramic Honeycomb Supports, SAE 852100. Corning Glass Works, U.S. Patent 4,532,228. H. C. Yao, S. .Japa r and M. Sheleef, Surface Interations in the System Rh/ A1203, J. Cat. 50 (1977) 407. H. C. Yao, H. K. Stepren and H. S. Gandhi, Metal Support Interactions in Automotive Exhaust Catalysts: Rh-Wash Coat Interaction, J. Cat. 61(1980)547. H. K. Stepren, W. B. Williamson and H. S. Gandhi, Development of Thermal Resistant Rhodium Catalysts, SAE 800843. J. V. Minkiewiez, B. J. Cooper and M. R. Baxter, Zirconia Supported Pt/Rh Three Way Catalysts for High Temperature Operation, AIChE Summer National Meeting, Detroit, Mich. 1981. T. Wang and L. D. Schmidt, Intraparticle Redispersion of Rh and Pt r-Rh Particles on Si0 2 and Al 203 by Oxidation Reduction Cycling, J. Cat. 70(1981)187. B. J. Cooper and L. Keck, NiO Incorporation in Three Way Catalyst Systems, SAE 800461. G. Kim, Ceria Promoted Three Way Catalysts for Auto Exhaust Emission Control, Ing.Eng.Chem.Prod.Res.Dev. 21(1982)267-274. E. C. Su, C. N. Montreuil and W. G. Rothschild, Oxygen Storage Capacity of Monolithic Three Way Catalysts, Applied Catalysis 17(1985)75. C. Z. Wan and J. C. Dettling, Effective Rhodium Utilisation in Automotive Exhaust Catalysts, SAE 860566. B. J. Cooper and T. J. Truex, Operational Criteria Affecting the Design of Thermally Stable Single Bed Three Way Catalysts, SAE 850128. B. Harrison, J. P. Heffer and F. King, Rhodium Containing Automobile Exhaust Catalysts, Proceedings of 7th Int.Cong.Cat.Tokyo 1980, pp 768-779. E. A. Hyde, R. Rudham and C. H. Rochester, .JvChem s Soc , , Faraday Trans. 1,80(1984)531. S. E. Wanke and N. A. Dougharty, J.Cat., 24(1872)367. E. Kibuchi, K. Ito, T. Ino and Y. Morita, J.Cat., 46(1977)382. A. E. Newkirk and D. W. McKee, J.Cat., 11(1968)370. G. J. K. Acres, The Characterisation of Catalysts. Platinum Metals Review, 24 (1)( 1980) pp 14-25.
This Page Intentionally Left Blank
143
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control !D 1987 Elsevier Science Publishers B.V .. Amsterdam -- Printed in The Netherlands
TITRATIONS OF CARBON MONOXIDE AND OXYGEN ON A PLATINUM ON SILICA CATALYST C. O. BENNETT, L. M. LAPORTA, and M. B. CUTLIP Department
of
Chemical
Engineering,
University
of
Connecticut,
Storrs,
Connecticut, 06268, USA.
ABSTRACT In the present work we study the reactions of CO with preadsorbed oxygen and a1so the subsequent react i on of oxygen with preadsorbed CO. The cat a1yst is 0.12% Pt/Si0 2 (Cabosil) which has a fraction exposed of 0.47. As the temperature is changed from 250C to 1920C the surface CO/Pt s ratio decreases from 0.85 to 0.68. However, at the same time the ratio of 0 atoms to Pt surface atoms changes from about 0.53 to 1.62. Thus at 1920C to ratio of 0 to total Pt atoms is 0.76. The increase of O/Pt s with increasing temperature is explained by the formation of subsurface PtOx' The oxide formation rate is structure sensitive; it forms at appreciable rates only for highly dispersed Pt such as that used in this study.
INTRODUCTION Thi s work has been undertaken as part of a program to add to our knowledge of the oxidation of CO over Pt via models based on elementary steps (refs.l,2,3,4). Here we measure the reaction of CO(g) with adsorbed oxygen, and 02(g) with adsorbed CO. These processes, although not elementary steps, are simpler to analyze than the full reaction. In the present study we are concerned only with
the amounts of
adsorbed CO and oxygen.
As such, the
measurements are equivalent to the titration of one adsorbed species by the other
gaseous
temperature.
species.
We
are
particlarly
interested
in
the
effect
of
The fraction exposed (FE) of the Pt particles supported on Si0
(Cabosil) may have an effect on the titrations also.
2
0C
At about 25 we have found that both titration reactions proceed slowly, so that it is convenient to work at higher temperatures. It is known that oxygen reacts re l uctantly with a Pt surface covered by CO (refs .5,6,2).
In addition,
when CO reacts wi th an oxygen-covered surface, onl y part of the oxygen may be removed at ambient temperature (refs.7 ,8,9). 60
0
Our titrations are performed at
a range in which each constituent appears to be completely removed
by the other. Titrations have long been used for the determination of the FE of supported metals, usually involving the couple H (refs.l0,11,12). 2-0 2
These methods are
144
based on an assumed stoichiometry of one adsorbed oxygen atom per exposed Pt atom (O/P\ = 1.0).
It is inconvenient to measure the H produced (some may 20 be adsorbed on the support), and the 02 disappearance has not usually been
measured.
Thus the amount of H consumed is the basis of the calculation of 2 fraction exposed, along with the assumed stoichiometry. The observed
the
departures
from O/Pt s investigated (refs.I3,I4).
1.0
for
Pt/
5i0
systems
2
has
been
extensively
For s i l i ca-supported metal s , for whi ch CO
is not adsorbed on the support, 2 CO-0 Then one can measure for the 2• oxygen-covered surface the CO disappearance and the CO production, and for the 2 CO-covered surfaces, the disappearance and the CO production. Now there is it
is
attractive to
no need
use
the
couple
°2
2
to assume a stoi chi ometry; the
amounts of
adsorbed
CO and
02 are
measured directly, and the carbon and oxygen balances can be verified.
st i l l
necessary to assume
non-dissociative adsorption of CO (true
It is
for most
metal s except iron and other typical methanation catalysts) and dissociative adsorption of
°2
(true
in
0C).
general
at T>25 Of course any chemisorption method requi res the use of a ratio O/Pt , H/Pt , or CO/Pt , supposedly obtained
s
s
s
from experiments on polycrystalline foils of pure metal. decreases, the di stribution
of exposed
crystal
geometic and electronic effects may come into play. (adsorbed
gas/surface
metal
atoms)
may
be
As the particle size
faces may change, and
other
In other words, the ratios
structure
sensitive.
calibration of chemisorption methods against physical methods
Thus
a
(EXAFS. electron
microscopy) is always desirable (ref.I5). Titrations by CO of oxygen-covered Pt powder, Pt/C, and Pt/Al 0 have been 2 3 0 performed at 25 C by Wentrcek et a l , (refs.I6,I7). However, they used the stoichiometry usual
at that time, i ,e , O/Pt
s
=CO/Pt
and Wanke (ref.I8) to criticize their methods.
s
= 1.0, which lead Flynn
This assumed stoichiometry is
not consistent with other results for polycrystalline Pt obtained by the same group (ref.I9,20). O/Pts = 0.5, at 25
In these articles it is proposed that CO/Pt
0C.
= 0.75 and s
It is well established that subsurface or even bulk platinum oxides can form at
temperatures
supported Pt. required
for
as
low
as
25
0C
(refs.I3,I4,21,22)
for
highly
dispersed
For bulk Pt, oxides also form, although higher temperatures are detectable
rates.
The
thermodynamics
of
the
formation
and
dissociation of s-PtO on a Pt wire have been studied by Berry (ref .23), who o 2 -1 0 -1 -1 finds ilH =-42 kcal mol and lIS = 49 cal mol K , from which Table 1 can be estab 1 i shed. Berry (ref.23) found that on a plot of rate (positive for oxidation) versus temperature, an isobar starts at a negligible rate at low temperature, rises as the kinetics become more favorable as the temperature increases,
pa~ses
through
a maximum and then falls to zero at the temperature given in Table 1 for the
145
particular isobar.
Further increase in temperature produces negative rates,
i.e., PtO dissociation. This is all completely normal. Thus in our z experiments (PO = 30 mbar) we can expect to form Pt 0z at temperatures below 474°C, if the kfnetics are favorable. Other oxides may form, for example Pt (ref.Z1) or Pt (ref.Z4). 304
°
TABLE 1 Equi] i br i um for Pt + Oxygen partial Pressure, mbar Z16 100 30 10
10- 1 10- 3 10- 5
Or
Pt 0z
Temperature,
°c
530 507 474 446 349 Z74 Z15
For bulk platinum there is some possibility that siliconor other impurities may segregate to the surface and serve as a getter for oxygen, as discussed in recent articles (refs.Z5,Z6,Z7), but this process occurs at temperatures above 0C, far above the range used in our work. In addition, for our 500 highly-dispersed Pt, segregation would not appreciably change the surface composition. EXPERIMENTAL METHODS The O.lZ wt % Pt/Cabosil M-5 catalyst was made by standard incipient wetness techniques, using H/tC1 HZO as the precursor. After drying the powder was 6 pressed into disks about O.lmm thick, as for infrared samples. These were then broken up and sieved so that the typical particles have a largest dimension of about 0.8mm. Thus a bed of such particles causes a low pressure drop, and intraparticle concentration and temperature gradients are also very low. The Pt loading is so low that X-ray diffraction did not lead to any useful information about the particle size of the Pt crystallites. Ultra-high purity HZ' He, 0z' and CO were used to make the necessary mixtures: 1 mol % CO, balance He, and 3 mol % 0z' balance He. Ar was sometimes used as a tracer. The inert gases were passed through suitable traps to reduce impurities especially HZO and 0z' to a minimum. However, it was found that passage of even the highly purified inert gases over a disk in the i r cell 0C
(typically 10mg) which had been saturated with CO at Z5 led to a continuous decrease in the ir band for adsorbed CO. Thus it was preferable to do the titration experiments in a large reactor with recycle (gradientless
146 with
the
catalyst during purges was small compared to the amount of adsorbed CO.
conditions),
so
that
the
The
titration experiments were
amount
of
impurities
which
might
react
done with 4.0g of catalyst at a flow rate of 34
mL/min to the reactor; recirculation was obtained by a metal bellows pump.
A
switch from Ar to He and back indicated a well mixed reactor of 32 mL volume. The fl ow system was arranged as al ready described
(ref .2) so that a step
function in the composition of the feed to the reactor could be produced with a rise time of about 2s.
The composition of the effluent from the reactor was
measured by a mass spectrometer with a continuous inlet system.
The infrared
system has also been described (ref.2). Before starting any CO/0 285
0C
cycles, the catalyst was reduced in flowing H at 2 2 for 15h, and then cooled in helium to the desired temperature.
RESULTS Some experiments were performed by direct switches between 1% CO and 3% 0 , 2 without any intervening inert-gas purge. After a few cycles, a reproducible result illustrated by Fig. 1 is obtained at 60
0C.
At zero time, at the left
edge of Fig. 1, the surface is saturated with oxygen at 60
0C.
At this time the
feed of 1% CO reaches the reactor, but the 02(g) concentration falls slowly because of the residence time in the reactor (t
r
=
32mL/34mL min -1
=
1 min).
Thus gas phase O and CO react during the initial period, and the CO 2 concentration rise is delayed. As the gaseous oxygen is depleted, the CO then consumes
the
surface
oxygen
saturation coverage at 60
and
then
occupies
the
Pt
surface
up
to
its
0C.
1.0 ...... - - ~ - - - - - - f M - - : - - ~ - - - - - , 3
2
d-J U
~ 05
0
0"
0
~
U
1
~
0
8
10
8
12
N
147
The right side of Fig. 1 shows the next switch, as 0 (g) replaces CO(g) over 2 the CO-covered surface. As al ready observed in other studies (refs .5,6,2) oxygen
finds
very few adjacent
sites
for dissociative adsorption until
the
gaseous CO concentration falls to a critical level, so that a small decrease in CO coverage can occur.
At this point there is a sudden production of CO
2
and
consumption of oxygen. The results of Fig. 1 permit the calculation, through material balances, of the
amounts
of
CO and
° adsorbed
based
various curves of Fig. l.
on the
However, the calculations involve the subtraction from each other of several integrated quantities.
The adsorbed quantities calculated in alternative ways
were not in satisfactory agreement because of their being based on relatively small differences of various pairs of measured quantities.
Rather than pursue
the
different
description
of
this
process
here,
we
turn
to
a
set
of
experiments, which have given satisfactory results. The interpretation of the data becomes
much more staightforward when an
inert gas purge is used between the two gas mixtures; the results at 60 shown
in
Fig.
2.
Although
no oxygen
desorbs
into
an
oxygen-covered surface, such may not be the case with CO.
inert
gas
0C
from
are an
We defer a detailed
discussion of this matter until after an analysis of the results of the type shown in Fig. 2 • . 1.0r--~=-""'------lf---:::;1111'--------r3
2
ON U ~
.
0.5
o o
o~
16 0 MIN Fig. 2.
Response curves with inert gas purge.
T
= 60 0C.
In Fig. 2, the CO peak on the left corresponds to the amount of O(ads) on 2 the surface after exposure to 02 in the previous cycle corresponding to the right
side
of
Fig.
2.
The mols
concentrations of the active gases by
of
°
ads,
N *' O
is
given,
for
small
148
( 1) -1
-1
where q is the flow rate, mL min and [C0 is mol mL During the same 2J period, the amount of CO which reacts plus that which adsorbs is given by NCO
= q
J'([Ref] - [COJ) dt
(2)
= B
o
Here the concentration [Ref], shown only in the left side response the CO would have if it did not react or adsorb. calculated from the res i dence time obtained from inert gas mentioned. In all these calculations the flow rate and measured at the same reference (ambient) temperature. From the right side of Fig. 2 we can calculate
of Fig. 2, is the Thi s curve can be swiches as already concentrations are
(3)
and (4 )
NO = q f([RefJ - [02J) dt = 0 o
where NO is the sum of the oxygen reacted with the preadsorbed CO* and the oxygen adsorbed. Thus there are two ways to calculate N CO*: C, CO
production
i)
N CO*
ii)
N = B - A, CO disappearance CO*
2
(5 )
(6 )
Similarly, there are two ways to calculate N O*: i )
N* O
ii)
N*
O
(7)
A, CO production 2
= 20
- C,
(8)
0 disappearance 0
0C,
The titration experiments were done at 60 C (Fig. 2), 100 (Fi g. 3).
0C
140°C and 192
149
1.0.,.....-----"""":::::::=--..."...---:::::-------~
o'"
u
.
~ 05 ~
oo
01\1 o~
o~
8 Fig. 3. Response curves with inert gas purge.
12
16
0C.
T = 192
We next consider some evidence as to whether any CO* desorbed during the inert gas purge at the various temperatures. These results indicate that there is no measurable desorption. 1. After saturation of the surface by CO(g), the feed is switched to inert gas, and [COJ is measured as a function of time (not shown in Figs. 2. and 3.). Then the amount of CO desorbed can be calculated by Eq. (2); a negative amount would indicate desorption. In all cases the quantity is zero to within experimental error. 2. By using Eqs. (1-6), the quantities of CO* have been calculated by the two methods, Eqs. (5) and (6), and the results are shown in Fig. 4. Since Eq , (5) gives the amount of C O ~ after the inert gas purge, and Eq. (6) gives the amount of CO adsorbed with CO in the gas phase, the good agreement of the two methods for CO* shown in Fig. 4 indicates that negligible CO desorbs during the purge. The same switches of feed concentration have been used in the 3. flow-through infrared cell, for the same 4 temperatures. After the switch to -1 CO, the absorbance of the single CO band observed at about 2070 cm has been measured, and corrected for emi ss i on. In Fig. 6 thi s infrared measurement of the quantity of CO* has been compared with that obtained from Fig. 4, and it is evident that the agreement is good.
150
ltf)
~
'"
S
~
~
-t( 2
8
0 - ~
0
x
U -l
0 L
~
II
40
0
120
2m
DEGREES C
Fig. 4. Quantity of CO adsorbed obtained by oxygen titration after exposure at CO, both at T,oC. x , calculated from CO disappearance; 0, calculated from CO 2 production.
~o
120
200
DEGREES C
Fig. 5. Quantity of 0 adsorbed obtained by CO titration after exposure to 02' both at TOC. x , calculated from 02 disappearance; 0, calculated from CO 2 production. By using Eqs , (1-4) and Eqs. (7-8), the quantities of 0* shown in have been found.
Fig. 5
Again the quantities of 0* calculated by Eqs. (7) and (8) are
in reasonable agreement, and we know that oxygen should not desorb.
151
f---
~ 2/r
028
--J
<:
f--<{
w o z
o ~
-r
.
018
()
O.aJ~
o
..J
o
If)
en
-r
~
:J..,
o
50
100
150
200
DEGREES C Fig. 6. Comparison of mass spectrometer(MS) data and infrared (IR) data for CO adsorption. The absorbance scale is made so that the two curves give the same ordinate at 600 C. Having given these arguments for the validity of the results of Figs. 4 and 5, we present in Table 2 the amounts adsorbed at all the temperatures. Included in Table 2 are the adsorbed quantities estimated at 25°C by extrapolation of the curves of Figs. 4 and 5. Satisfactory measurements were 0C not possible at 25 because the reaction rates were too low. TABLE 2 Adsorption of O and CO 2 Temperature,
°c
60 100 140 192 25*
Oxygen umol/g cat. 1.79 2.57 4.03 4.68 1.56
O/Pt 0.29 0.42 0.66 0.76 0.25
Carbon Monox i de umol/g cat. CO/Pt 2.40 2.28 2.16 1.95 2.47
0.39 0.37 0.35 0.32 0.40
* Extrapolated values. Oxygen partial pressure, 30 mbar CO part i a 1 pressure, 10 mbar
It is of interest to find also the values of CO/Pt and O/Pt and their s s variation with temperature. Pt refers to a surface (exposed) atom of s platinum. To compute these values we have chosen a reference value of CO/Pt s 0C, as in a previous study (2). This choice is based on the work of 0.85 at 25
152 Freel (ref.28) and of Nishiyama and Wise (ref.20).
It is then straightforward
to calculate the values in Table 3. TAI3LE 3 Surface concentrations Temgerature, C
O/Pt s
60 100 140 192* 25
CO/Pt s
0.62 0.89 1.40 1.62 0.53
0.83 0.79 0.74 0.68 0.85
* Extrapolated values In preparing Table 3 it has been assumed that at 25°C the measured COl Pt = 0.40 corresponds to CO/Pt with
0.85, so that the fraction exposed, FE
=
s
Thi s then requi res that at 25°C O/Pt
is 0.47.
that
generally
proposed
in
the
s
be 0.53.
=
Pt IPt,
s
This value agrees
literature,
O/Pt
(refs.l0,12,16,18,19,20).
0.5
s
DISCUSS ION As the temperature of O adsorption is raised, the CO peak produced starts 2 2 to show a shoulder, and in Fig. 3 a second peak is visible. As suggested by Herz and Shinouskis (ref.9), it is reasonable to assign the more reactive peak to surface 0*, and the later peak to oxygen from subsurface oxide.
Since the
formation of bulk oxide is thermodynamically favored over the temperature range of the present work, the increasing oxygen isotherm of Fig. 5 is explained by the kinetics of this activated process.
o
The reference values (25 C) of CO/Pt
s
=
based on studies for polycrystalline Pt. has been
found (refs.5,29).
0.85 and O/Pt
s
0
=
0.5 at 25 Care
For Pt (110) a ratio COlO of unity
The influence of other faces in
polycrystalline
samples apparently changes this ratio to about 1.7. The results of Table 3, support the
idea of the
in
which O/Pt
formation
goes
s
up to 1.62 at
of a subsurface oxide.
studies also support this interpretation.
192
Several
When supported Pt
0C,
also
infrared
is oxidized at
o
about 250 C and then cool ed to room temperature, CO can be adsorbed on the surface without oxidizing all experiment shows a band
the Pt,
as
already mentioned.
at about 2120 em-I,
interpreted as
This
type
of
arising
from
CO
adsorbed on oxidized platinum (ref.7,8,9). Salmeron et al , (ref .30) have exposed Pt single crystal surfaces to 02 at o 70U c and above. They propose that oxygen di sso1ves in pl at i num; .t nen when the
153
crystal
is cooled
to lower temperature, diffusion of the oxygen toward the
surface is slow but its virtual pressure is high, so that subsurface oxides are formed.
Yeates et al , (ref.31) have proposed that oscillations in the CO/0 2 reaction on Pt single crystals at about 300°C are driven by coupling between
the surface reaction and the oxidation and of Pt.
However at 300
0C
reduction of the subsurface layers
the oxidation and reduction seem too slow for bulk Pt
for thi s explanat ion to be val id (refs .30,32). dispersed,
supported
Pt ,
for
model might be appropriate.
which
oxidation
On the other hand, for highly and
reduction
are
rapid, this
As pointed out in the review by Razon and Schmitz
(ref.Z7), it is also difficult to evaluate the effects of traces of impurities 1 ike Si ,
In some cases rapid surface reconstruction may be the driving force
for oscillations (ref.32).
ACKNOWLEDGEMENTS
We are grateful to the National
Science Foundation for support under grant
No. CPE 8210100-01. We also thank Dr. J. S. Chung for assistance in the experiments.
REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
C.O.Bennett, Catal. Rev.-Sci. Eng., 13 (1976) 121. S.M. Dwyer and C.O. Bennett, J. Catal. 75, (1982) 275. M.G. Goodman, M.B. Cutlip, C.N. Kenney, W. Morton and D. Mukesh, Surface ser.. 120 (1982) L543. D. Mukesh, M.B. Cutlip, M. Goodman, C.N. Kenney and W. Morton, Chem. Eng. Sci. 37 (1982) 1807. H.P. Bonzel and R. Ku , Surface Sci. 33 (1982) 91. H.P. Bonzel and J.J. Burton, Surface Sci. 52, (1925) 223. H. Heyne and F.C. Tompkins, Trans. Faraday Soc. 63, (1967) 1274. E. Kikuchi, P.C. Flynn and S.E. Wanke, J. Catal. 34, (1974) 131. R.K. Herz and E.J. Shinouskis, Appl. Surface Sci., 19 (1984) 373. J.E. Benson and M. Boudart, J. Catal. 4, (1965) 704. D.E. Mears and R.C. Hansford, J. Cat a l , 9, (1967) 125. J .R. Wil son and W.K. Hall, J. Cat al , 17, (1970) 190. T. Uchijima, J.M. Herrmann, Y. Inoue, R.L. Burwell, Jr .; J.B. Butt and J.B. Cohen, J. Cat al , 50 (1977) 478. M. Kobayashi, Y. Inoue, N. Takahashi, R.L. Burwell, Jr., J.B. Butt and J.B. Cohen, J. Cat a l , 64, (1980) 74. B.J. Kip, F.B.M. Duivenvoorden, D.C. Konigsbergh and R. Prins, In preparation. P. Wentrcek, K. Kimoto and H. Wise, J. Catal. 33 (1973) 279. P Wentrcek and H. Wise, J. Catal. 36, (1975) 247. P.C. Flynn and S.E. Wanke, J. Cat a l , 36, (1975) 244. B.J. Wood, N. Endow and H. Wise, J. Cat a l , 18, (1970) 70. Y. Nishiyama and H. Wise, J. Catal. 32 (1974) 50. R.K. Nandi, F. Molinaro, C. Tang, J.B. Cohen, J.B. Butt and R.L. Burwell, Jr., J. Cat a l , 78, (1982) 289.
154 22 23 24 25 26 27 28 29 30 31 32
T. Fukishima, J.R. Katzer, D.E. Sayers and J. Cook, 7th Inter. Conqr , on Catalysis, Tokyo, 1980, paper Al. R.J. Berry, Surface Sci. 76, (1978) 415. C.G. Vayenos and J.N. Michaels, Surface Sci. 120 (1982) L405. H.P. Bonzel, A.M. Franken and G. Pireng, Surface Sci. 104, (1981) 625. H. Niehus and G. Comsa , Surface Sci. 102 (1981) Ll4. L.F. Razon, R.A. Schmitz, Cat a l , Rev.-Sci. Eng. 28(1), (1986) 89. J. Freel, J. Cat al , 25, (1972) 149. R. Dueres and R.P. Merrill, Surface Sci 55, (1976) 227. M. Salmeron, L. Brewer and G.A. Somorjai, Surface Sci. 112, (1981) 207. R.C. Yeates, J.E. Turner, A.J. Gellman and G.A. Somorjai, Surface Sci. 149, (1985) 175. M.P. Cox, G. Ertl, R. Imbihl and J. R"tistig, Surface Sci. 134, (1983) L517.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control ( i : ~' 1987 Elsevier Science Publishers B.V. Amsterdam - Printed in The Netherlands
155
THE A/F WINDOW WITH THREE-WAY CATALYSTS. KINETIC AND SURFACE INVESTIGA TlONS. E. KOBERSTEIN and G. WANNEMACHER Physico-Chemical Research Dept., Degussa AG, ZN Wolfgang, P.O.Box 1345, 6450 Hanau 1 (Federal Republic of Germany)
ABSTRACT Kinetic measurements, infrared surface investigations under running reaction conditions and model calculations for the system CO, NO and 02 on precious metal catalysts are reported. Sorption behavior of these compounds and moss transfer influences determine the width of A/F windows on the leon side. Larger increases of the leon A/F window width can only be attained in a compromise by reducing absolute reaction rates. INTRODUCTION Satisfactory conversion of all three pollutants (HC, CO, NO) by means of three-way catalysts is only possible if the oxygen partial pressure in exhaust gas - with three-way concepts determined by the A/F ratio - is kept within certain limits: the so-called A/F window (see Fig. 1).
['/,1 Conversion
In
on Integrol reoctor
I
100
I I I
- - - . - . W0...-""'1 -~
80
0/
0--"--
60
/~ i
• NO x
!\
'II
I I II II II
/'
o
I I II
14.1
co
I
o
20
o
o HC
·t
/
40
0-0-0-0
14.4
Fig. 1. A/F window with three-way catalysts.
14.7
'\
~
15.0 15.3
AIF
1fi6
Modern engine concepts (fuel cut, lean operation, etc.) require catalysts operating with leaner mixtures, i.e., with regard to NO x conversions an extension into the lean range is desirable. Experimentally, catalysts have been found which show considerable differences regarding this property (see Fig. 2). A more or less pronounced conversion maximum for NO x as function of temperature is found with all three-way catalysts.
1'1,) NO- Conversion in on Integrol reactor
100 GHSV ~ 10000 h-' T ~ 3300 [
80
60 40 20
o- L - - . - - - - , - - - r - - - - . - - - - , - - - , - - - , - _ 14.7
15.3
15.9
AF
Fig. 2. Catalysts with different A/F window width in the lean range.
To find the reasons for this behavior and to get hints on the principal parameters limiting A/F window enlargements, some basic investigations were initiated, part of which are described in this paper. As a first approach, the study was confined to the system: CO, 0Z' NO and HZ and precious metal model catalysts (Pt, Rh on ')'-alumina support) as well as technical automotive exhaust catalysts. The specific influence of non-precious metal oxide additives which definitely influence A/F windows will be reported in another study.
157 EXPERIMENT AL c::atalyst samples TABLE 1 Cat. No.1:
somples were cut out of 0 commercially used three-way catalyst type OM 2966 2 support: cordierite monolith 400 cells/inch washcoat: 100 gil -y-alumina + NPM-oxide additives precious metal: 1.3 gil Pt IRh = 5: 1
Cat. No.2:
-y-alumina pellets (spheres; d
3 rnrn) 0.3 % Pt
Cat. No.3:
-y-alumina pellets (spheres; d
3 mm) 0.3 % Rh
Cat. No.4:
model catalyst: Rh on non-porous a-A1 0 spheres (d 2 3
Cat. No.5 and 6:
samples for IR-measurement self-supporting -y-alumina cylindrical pellets (d = 13 rnrn, height approx. 0.1 mm) precious metal: 5 % Pt (Cat. No.5) 5 % Rh (Cat. No.6)
3 mm)
Kinetic measurements Integral reactar measurements. Far integral reactor measurements either engine test units or laboratory equipment with model gas mixtures were used. Both are described in (ref. 1). Differential reactor measurements. The apparatus used is indicated in Fig. 3. Defined gas mixtures from a dosing device could be switched either to a differential reactor with outer loop (reactor A) or to a reactor with internal loop (Berty reactor/reactor B) (ref. 2). Both reactors could be operated with high gas circulation (> 15: 1) guaranteeing differential reactor conditions. Reactor A consisted of a quartz tube (18 mm diameter) which could be heated up to 1100 °e by a Pt tube furnace. A correspondingly cut piece of monolith catalyst (length 5 mm) - with about 25 open channels - or a corresponding pellet layer were fixed within the quartz tube. A disadvantage of this device are remaining temperature gradients which can be avoided with the Berty reactor. On the other hand, reactor B could only be heated to a maximum temperature of 550°C. For monolithic catalysts a piece of 48 mm diameter and 5 mm length was fitted into the Berty reactar tube •.Pellets were pierced and stringed on a stainless steel wire. Both reactors could be switched alternatively to the analytical systems indicated for CO, CO
°
, NO analysis. The curves plotted in Fig. 5 - 12 represent reaction rates 2, 2 (moles converted divided by residence time and geometric surface area of catalyst). Infrared spectroscopy. The system for measuring surface infrared spectra (see Fig. 4) consists of a section for dosing model gases (in this case: CO,
°
, NO), two IR 2 measuring cells, the vacuum pumps, a quadrupol mass spectrometer for gas analysis and
an infrared spectrophotometer (here: IR Perkin Elmer 325). The measurement cells can
158
CO I Co, - IR-Analywr
01 - 4nalySfr 110 - o.m;hJllil1P.Ql'lP 4nolyst'
A (AU.
~ YS I
J '"
B
4l)(fh
100UIl
I I ~. : T: ' - ~
I
0
i
~r~ ~
r.-
660l/h
'f
, CATAlYSI ~ ~ ~
lURliINE
C")
~ ~ "G;
, ~I )
fl)
~'
Fig. 3. Differential reactor experimental set-up.
Fig. 4. Infrared equipment: 1: pressure gauge; 2: pressure gauge (high vacuum); 3: mass spectrometer; 4: oil trap; 5: adsorption trap; 6: pressure converter; 7: high vacuum valves; 8: gate valves; 9: saturator; 10: capillary tubes; Pl, P2: rotary pump; P3: turbo pump.
159
be brought into the IR spectrometer olternatively. Cell A is constructed according to (ref.
3) and allows measurements during running reactions on disk-shaped catalyst
sampels up to a temperature of 530°C. With cell B according to (ref. 4) samples can be
treated
up
to
1200 °C aut of the IR beam and then brought back into the
spectrometer without interrupting the' vacuum or gas atmosphere.
KINETIC INVESTIGATIONS While the A/F window on the "rich" side is limited by stoichiometry (oxygen concentrations approaching zero) and means to shift this side of the A/F window to "richer" conditions are well-known (e.g., surface oxygen storage; enhancement of the water-gas
shift
reaction, e tc.), the "leon" side limit is obviously defined by a compe-
tition of reaction rates for oxidizing carbon monoxide, hydrocarbons and hydrogen by nitrogen oxides or oxygen. Due to their importance in actual exhaust gas, reactions a) and b) and their combination were selected for this special study.
a)
CO
b)
CO + NO
+
(1)
1/2 02
(2)
Nearly identical kinetic results are obtained with pellet or monolith samples, Le.; there is practically no difference in this regard between the outer layer of a pellet and the coating of a monolith, if ')'-alumina and activation are the some.
Reaction a): CO + 1/2 02
CO 2 The results of kinetic measurements in the differential reactor described above are
shown in Fig. 5 and 6. Above temperatures of approx. 400°C on apparent first-order kinetic for
CO and for 02 under the reaction conditions indicated is found until
complete consumption of one reaction partner near the surface enforces zero-order kinetic. This is due to rate limitation by boundary layer diffusion. At comparatively low temperatures « approx. 250°C) self-poisoning occurs for CO, while on apparent first-
°
, 2 The corresponding Arrhenius plots (see Fig, 7) in the temperature range between
order kinetic is measured for
100°C and 800 °C - usual operating temperatures of automotive exhaust catalysts show four clearly separated sections which are interpreted as follows:
1) Ea - 100 kJ/mol: chemical reaction controlled
2) Ea 3) Ea
25 kJ/mol: alumina (wcshcoor ) pore diffusion controlled 6 kJ/mol: boundary loyer diffusion controlled
4) Homogenous gas reaction
160
r. [mol CDlm1s1 " PCG ~ GO)bar
1 ~ fOOoC
o POI
~
D.015bor
0.15 0.1 0.05 I
0.01
Fig. 5. Kinetics: reaction 0) (CO
+
1/2 02
I
0.02
-.
,
I
om CO ) T 2
.
0.04 POI[bar] P" [bar 1
> 400°C.
Cat. 1.
r,.lO' :mdCDIm's]
4.5
3
1.5
0.01
0.02
0.03
0.04 Po,[bil'!
Pco Ib..-J
Fig. 6. Kinetics: reaction c) (CO
+
1/2 02
-. CO ) T < 250°C. Cat. 1. 2
161
P (0: G. G' bar PO z : G.C:bor
-8 -10 900 0 [
JOGO': ,
1.'8 Fig. 7. Kinetics: Arrhenius plot for reoction a).
Reaction b): CO + NO
-
Ca t. 1.
1/2 N
+ CO 2 2 Fig. 8 to 10 present the data measured with reaction b). At high temperatures very
similar curves are found compared with reaction a), which is due to the controlling mass transfer influence (see Fig. 8). There is no difference whether the reaction rate is measured as function of carbon monoxide concentrotion or NO concentration. At low temperotures different kinetics result depending on whether NO or CO is varied, while the other component is kept constant (see Fig. 9).
r, (mol rO/m1sJ
o,ms
o P.o' 0,002 bar
1,500 0 C
" Pco' 0.002 bar
0,1
0.005 i
0,001
Fig. 8. Kinetics: reaction b) (CO
+
0,002
NO
-
0,003
l/? N
2
0,004
+
CO ) T 2
P co [bar]
P 10 Ibor)
> 400°C.
Cat. 1.
162
r, -10' [mol(Olm's] o
3
2 o p '0' 0,002 bar ., p [0 ' 0,G02 bar
T ,24GO(
0,001
Fig. 9. Kinetics: reaction b) (CO
+
0,002
NO
---+
0,003
1/2 N
2
0,004
+
CO
2)
T
p [0 lbcrl P ,o[bor]
< 250°C.
lnr, Prc' 0,005 bar
P MO' G.005 tnr
-8 -10 _12 900 0,8 I,D 0
(
6000C
1,2
5000 (
:OOO(
1,4
1,6
Fig. 10. Kinetics: Arrhenius plat for reaction b).
Cat. 1.
Cat. 1.
The Arrhenius plot (see Fig. 10) is olso cornpor oble and interpreted as for reaction a),
with
the
exception that no homogenous gas phase reaction (step 4) could be
detected.
Combined reactions a) and b) Fig. 11 to 12 show "Arrhenius diagrams" where reaction rates dnCO/dt resp. dnNO/dt under the reaction conditions indicated are plotted against the reciprocal temperature. Parameters are: fresh and aged technical catalyst 1 (Pt /Rh); high-surface (porous) and low-surface (non-porous) catalyst; single precious metals Pt and Rh. In all cases a similar pattern is obtained: When
CO conversion becames boundary layer diffusion
controlled, the reaction rate for NO x canversion begins to drop. The difference between the abso)ute reaction rates for reactian a) and b) is considerably larger for pure platinum cam pared with pure rhodium or Pt/Rh combinations. As could be expected, the curves for the aged catalyst are shifted to higher temperatures. The
pattern described
above
is
most
clearly
shown
with
high-surface (parous)
catalysts, while low-surface (non-porous) catalysts give nearly identical reaction rates on Rh over a large temperature range, resulting in relatively higher NO x conversions. The latter catalyst also gives higher NO x conversions in the lean range, increasing the A/F window width (see Fig. 2). It must be pointed out that the absolute reaction rates per
geometric catalyst surface are of course much greater with the high-surface
catalyst.
Inri/co
CoU
10(101,,"0)
fresh aged
a 0
CO - NO CO -NO
-3
-4 -5
-6
1.0
1.2
1.4
1.6
Fig. 11. Kinetics: Arrhenius plot for combined reactions a) and b); P = 0.01 bar; Po 0.0065 bar; P = 0.001 bar; CO NO 2 Cat. 1: fresh and aged.
164
In rs,co
[01.2 Cot 4
In(10r~oi
o
co • NO
o
CC
.. NO
-]
-4 -5
-6 -7
-8 L---r-----"--.--~-,___---"--.._--'--,_---
1,0
1,4
1.2
1,6
Fig. 12. Kinetics: Arrhenius plot for combined reactions a) and b); = 0.01 bar; Po = 0.0065 bar; P NO = 0.001 bar; P CO Cat. 2 and 4. 2
INFRARED SURFACE SPECTROSCOPY With the equipment described in chapter "infrared spectroscopy" the absorbance of the Pt-CO resp. Rh-CO bands on catalyst 5 re sp, 6 were measured as function of temperature and oxygen partial pressure under running reaction conditions. The OfF value (ratio: oxidant/fuel) was changed either by oxygen or nitrogen oxide variation. The results are shown in Fig. 13 to 17. For reaction a) similar patterns are obtained for Me-CO absorbance as functian of oxygen partial pressure and temperature with metallic (reduced) catalysts 5 and 6 (Pt resp. Rh). With Pt at low temperatures, CO coverage also in the lean range is found, while at higher temperatures and increasing oxygen partial pressures a step function indicating a sudden CO depletion close to stoichiometry was detected. In the case of rhodium the only difference are comparable CO coverages at lower temperatures and a higher density of the step function with regard to oxygen partial pressure. In case of reaction a) the CO absorbance, Le., the CO coverage, is completely reversible. Reaction b) shows a different behavior. While on catalyst 5 (Pt) CO coverage shews a similar pattern as with reaction a), it is not further reversible with increasing temperatures.
Measurements
at
indicate only small absorbances.
lower
temperatures after high temperature exposure
Obviously,
a large part of the surface is now blocked
16.5 by some reaction intermediate which still has to be characterized. On reduced rhodium (catalyst 3) rapid CO depletion is found at lower temperatures and at very low NO partial
pressures,
indicating
a displacement of CO
by
NO or by an
intermediate
product. With increasing temperatures the step function mentioned above is formed again. After heating of catalyst 6 for 4 hours at 800°C in air ("oxidized Rh"), hardly any Me-CO absorbance could be measured. This confirms the reversible poisoning effect of Rh by oxygen measured in integral reactors.
If 'half of the carbon monoxide is replaced by hydrogen in case of reaction a), a considerable shift of the Pt-CO absorbance "step" into the lean range is found (see Fig. 17).
ca- l/Z OZ- CO 2
Absorbonce PI- [·0
Pco ~ 0,02 bor
v~Z100[m"
ZOOO[ I
1.2
0,8
~\
26'lJO( JOOO[
1,0
+--..l-'
+~ ~
J4O"(
0,6 -&=-=--"T-oA ....L..o:::::::::--....
Cot. 5 (PI}
\
ll:I'[~cc
0,4
0,2 0,1
---'4"ii?C-:)\ '--'" 46lJO~
\
0,008
0,009
0,01
0,011
~ :-. \(0,012 0,013 pOllbal
Fig. 13. Infrared: Me-CO absorbance under running reaction conditions for Cat. 5 (Pt); reaction a).
166
Absorbarce
PI-e-o
CO· NO --C0 2 + 1/ 2N2
v,2100cm- 1
P [0
'
0,02 bar
3
150"C
/ <1 -
2
o - 300°C
0-----__ -
~
300°C (afte.-400°Cl
~--------.~_
"-350°C "
0.01
250°C
0.02
/400"C
0,03
0,04
P Holbar]
Fig. 14. Infrared: Me-CO absorbance under running reaction conditions for Cot. 5 (Pt); reaction b).
Fig. 15. Infrared: Me-CO absorbance under running reaction conditions for Cat. 6 (Rh); reaction a).
167
AOscrOonce
~h
I
co + NO-COz+'/zN)
;: ~ 'x
v,ZO?Ocm'
0.6
p :~'
ISGOC 2S00C
--'
0,1.
0,2
0.01
0.02
0.03
0.04
P.o 'bnrl
Fig. 16. Infrared: Me-CO absorbance under running reaction conditions for Cat. 6 (Rh), reaction b).
0,2
0,005
0.01
0,015
0,02
P 02 [bad
Fig. 17. Infrared: Me-CO absorbance under running reaction conditions for Cat.5 (Pt ), reaction a) with 50 % CO replaced by H 2.
168
MODEL CALCULA nONS To
illustrate
tendencies, not to calculate absolute data, some simplified model
calculations of reaction a) were made starting with a Langmuir-Hinshelwood kinetic (an Eley-Rideal kinetic would not change the basic conclusion regarding the width of A/F windows):
r'
(3)
The following data were determined from our kinetic measurements « 250 °C) by non-linear regression: 13 3 k 1 : 2.7 x 10m Imol . s
K K
1 2
0.36 m
3/mol
E a 1 : 90000 J/mol E
aZ
: 16000 J/mol
0
For calculating the concentration profile as function of channel length and radius the model of the isothermic tube reactor with catalytically coated wall and laminar flow (ref. 5) was used with the fallowing simplifications:
1. No entrance disturbance.
Z. Only radial, no axial diffusion. 3. No flow changes by mol changes.
4. Mass transfer only by gas pore diffusion within the catalytic layer. 5. Identical diffusion coefficients for CO and 0Z"
6. Deff - 1II 0 D (as determined on ),-AI 0 pellets). Z 3 Given these conditions the following equations were used:
1. Free gas volume:
(4)
w(r)
w
V!7rR Z c
169 2. Catalytic layer:
d c.
1 dc.
~ 2) r (cd--~
D e ff ( d )
+
r'
(5)
3. Boundary conditions:
l ( dC') dr D e ff
i) _ ~- dC -dr
(dC. )
T
~
r:O
(6)
Cat.
D
0
and ( _dc. _1_ ) 0 dr R +d' c
(7)
4. Langmuir-Hinshelwood kinetic (see equation 1)
The equation systems were solved numerically by a computer programm. Fig. 18 shows one example .f,or mass transfer determined conditions.
ReM""]
Fig. 18. Model calculation: CO concentration as function of monolith channel length and radius for reaction a); 3; 3. CCO: 0.4 mol/m Co 0.25 mol/m 2
170 To
characterize
carbon
monoxide and oxygen
coverages
during
the
stationary
reaction on the precious metal surface as well, reaction rate r' and the coverages for carbon monoxide 8
and oxygen 80 were calculated using literature data (ref. 7) for CO elementary reactions of the Langmuir Hinshelwood kinetic. The reaction rates obtained are comparable with reaction rates derived from kinetic measurements justifying the literature data used. CO
CO
°2
2 0ads CO 2
CO ads
+
°ads -
(8)
a ds
(9) (10)
The following equations are applied for the single reactions:
f27rR~CO
CO adsorption: r CO,ads
=
02 adsorption: r O,ads
= 2
. S'
{;~TMo
.S. 2
CJ
CO ' 8 v' CCO
(1° 2
8 . Co v
(11)
(12)
2
(13)
kCO,des . exp [-Eo CO,de/ R T] 8 C O
CO desorption: r CO,des
(14)
LH reaction:
(15)
Fig. 19 shows an example for a certain reaction condition. (T=250oC)
So~-(on[l.' fllfOIJOr>
[maflmJJ
0.3-r--_ _
eo O ) r - - - _ ~ ~
0.6 0.1
0.4 It-----f0.2 0.1
0.2
0.3 r lmml
0.4
XI
20
J()
40
50
it l ~ m I
Fig. 19. Model calculation: CO and 02 concentration gradients as function of channel radius (different scale for wcsficoct and gas volume) and CO resp. coverages.
°
171
DISCUSSION Kinetic measurements, infrared investigations and the model calculations give a consistent result, which allows one to understand the factors determining the width of A/F windows on the lean side. These factors are the sorption behavior of carbon monoxide, oxygen and nitrogen oxide as function of temperature and partial pressures and mass transfer influences controlled by the porous structures of the washcoat resp. the boundary layer gas diffusion. Looking upon the situation from the point of view of a precious metal cristallite down in the porous )'-alumina structure - or a differential catalyst element - at low temperatures its surface is blocked by CO on Pt and NO or a reaction intermediate on Rh. This explains the kinetics shown in Fig. 6 and 9 (c.q., self-poisoning by CO). With increasing temperature, reaction begins and quickly accelerates until mass transfer phenomena are rate-limiting. This leads to considerable differences between the local concentrations just above the precious metal surface and the concentration in the outer gas volume. This phenomenon causes a shift of the NO x conversion curve in the direction of stoichiometry - i.e., a reduction of A/F window width in the lean range - with integral reactors. As long as the local CO concentration is high enough - which is always the case under rich conditions - CO is adsorbed and reactions a) and b) proceed. A small local surplus of oxygen leads to a rapid depletion of CO (step function) which immediately stops the NO x conversion. The concentration gradients of the reducing agents caused by mass transfer can be flattened by adding a reducing gas with high diffusion coefficient such as hydrogen (Fig. 17). In a monolith or a pellet layer this consideration for a differential catalyst element has to be extended over the whole reactor, where temperatures and concentrations are changing considerably. Thus the influence of hydrogen is hardly to be detected with integral reactors, probably due to the fact that the very high reaction rate leads to a rapid hydrogen consumption at the entrance, leaving no more hydrogen in the following sections. Starting with a rich mixture in the system CO, NO, O residue
inside
the
catalyst,
enabling a
high
CO
2 coverage and
finally leaves a CO thus also an
NO
conversion. In the case of a lean starting mixture a surplus of oxygen remains, leading to an abrupt decrease in coverage around the stoichiometric point which stops NO conversion. This means that only a part of the catalyst is available for NO x conversion when starting with a lean mixture. By lowering the absolute reaction rate (e.q., low temperature) or by reducing the diffusion resistance (non-porous catalyst), the negative influence of mass transfer on the A/F window width can be counterbalanced. For the system studied here it has to be concluded that only a compromise between A/F window width in the lean range and absolute reaction rote can be attained.
172
LEGEND r'
: rate of reaction inside the catalyst
r
: rate of reaction referred to the geometric surface of the catalyst
[ mol/m
: rate of surface
[ s- 1 ]
S
C
R
c
react~on
(Langmuir-Hinshelwood
[ mol/m\ 2s
: radial coordinate in the tube reactor
[m ]
: axial coordinate in the tube reactor
[ m
: concentration
[ mol/m
: radius of the open channel
[m ]
]
w
: gas velocity
[ m/s ]
w
: average gas velocity
[ m/s ] 3/s
V
volume flow
[ m
D
diffusion coefficient
[ m
2/s
3
]
]
[ m2/s ]
D e ff d'
: thickness of the washcoat
[ m
R
: gas constant
[ J/mol . k ]
T
: temperature
[K ]
M
: molecular weight
[ kg/mol
S
: area of 1 mol surface metal atoms
[ assumed value: 4 2/mol 4 . 10 m
B Bv
: surface
a
: sticking probability
c
s
: effective diffusion coefficient in the catalyst
]
coverage
fraction of the vacant sites : surface metal atoms concentration
[ mol/m 3 ]
ACI
REFERENCES 1 2 3 4 5 6 7
E. Koberstein, Chemie in unserer Zeit, 18 (1984) 37-45. J.M. Berty, Chem. Eng. Progr., 70 (1974) 78. E. Gallei, E. Schadow, Rev. Sci. Instr., 45 (1974) 1504. H. Knoz inqer et aI., Chem. Ing. Techn., 42 (1970) 548. D. Boeker, E. Wicke, Ber. Bunsenges. Phys. Chem., 89 (1985) 629. G. Brauer, F. Fetting, Chern, Ing. Techn., 36 (1964) 921. CT. Campbell, S.K. Shi, J.M. White, Appl. Surf. Sci., 2 (1979) 382; CT. Campbell, J.M. White, J. CataI., 54 (1978) 289; P.A. Thiel, E.D. Williams, J.T. Jates, W.H. Weinberg, Surf. Sci., 84 (1979) 54; W.L. Winterbottem, Surf. Sci., 37 (1973) 195; W. Adlhoch, Dissertation, Kinetik der Reaktion zwischen CO und NO an polykristallinem Platin bei niedrigen DrUcken, TH Karlsruhe, 1978; S.H. Oh, G.B. Fisher, J.E. Carpenter, D.W. Goodman, to be published in J. Catal.
.\. Crucq and A. Frcnnet (Editors), Catalysis and Automotiue Pollution Control 1987 Elsevier Science Publishers B. V., Amsterdam -- Printed in The Netherlands
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ELEMENTAL STEPS DURING THE CATALYTIC DECOMPOSITION OF NO OVER STEPPED SINGLE CRYSTAL SURFACES OF PLATINUM AND RUTHENIUM N. KRUSE and J.H. BLOCK Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6 0-1000 Berlin 33
ABSTRACT The interaction of NO with stepped single crystal surfaces of Pt and Ru field emitters has been studied. Pulsed Field Desorption Mass Spectrometry (PFDMS) is employed to perform time resolved measurements. It is found that the NO adsorption (p = 1.3xl0 -5 to 6.7xl0 -4 Pa, T = 523 to 605 K) occurs molecularly on the high index planes of Pt with (111) orientation of the terraces, whereas molecular as well as dissociative adsorption is found to occur on the high index planes with (001) orientation of the terraces. By varying the repetition rate of the field pulses from 1 Hz up to 1 kHz (corresponding to a field free reaction time between 1 sand 1 ms) kinetic data of the adsorption, thermal desorption and decomposition of NO are obtained. The rate parameters for the first order -14 thermal desorption on stepped Pt(lll) are: Ed =139kJ/mol, ~ = 3xlO s. The initial stages of NO dissociation on stepped Pt(OOl) follow a complex kinetic mechanism which can be understood on the basis of structural autocatalysis. On stepped Ru(OOI) the dissociation proceeds according to first order kinetics. In addition, the oxygen deposition and accumulation leads to strong oxidation of Ru. This is evidenced by desorption of RUO~+ ions (x up to 3). A field induced decomposition of NO on Pt is observed and leads to high ionic rates of N20+ and N;. No marked face specifity has been found so far for this reaction. INTRODUCTION Adsorpti'on, thermal desorption and decomposition are elemental steps occuring during the catalytic reduction of NO over noble metals. Current interest in basic research work concerns the elucidation of the kinetics of these steps and their dependence on the surface structure of the catalyst. The platinum metal is one of the components in three-way automotive catalysts. It is mainly involved in oxidation reactions but may contribute to NO removal as well. Ruthenium has gathered interest as a candidate for NO decomposition. This metal is not used so far in practical applications since it is un-
174
stable and forms volatile oxides, Ru0 3 and Ru0 4· Various surface sensitive techniques have been employed under ultrahigh vacuum conditions to study the interaction of NO with Pt and Ru single crystal surfaces (for a review see ref.1). Recently, the Pt(410) plane has been found to dissociate NO (ref.2), whereas the flat (Ill) plane is inactive which is in accord with theoretical considerations (ref.3). The plane to plane variations in the NO interaction with Pt have prompted us to use field emitter tips with their well defined crystallography as model catalysts and to perform face specific measurements in a time resolved manner by employing pulsed field desorption mass spectrometry (PFDMS). This method has been shown in a number of previous papers (see e.g. ref.4-7) to provide valuable kinetic data on surface processes. EXPERIMENTAL A detailed description of the experimental set-up has been given by Block and Czanderna (ref.7). The basic principles are as follows. The catalyst is prepared in the form of a field emitter tip by electrochemically etching a thin wire (0 ~ 0.1 mm). At its apex this field emitter closely resembles a hemisphere and its surface can be imaged in real space with atomic lateral resolution by field ion microscopy (FIM). In PFDMS the emitter is located at a distance of ~ 0.1 mm in front of an electrode. High voltage pulses produce field pulses with heights up to 50Vjnm at half widths of ~ 100 ns and variable repetition rates ~ 100 kHz. The strong electrical field of the pulses causes molecules which are adsorbed at the emitter surface to desorb in the form of ions. Even metal atoms may be ruptured from their lattice site positions in this manner. The chemical identification of the desorbed ions is accomplished by time-of-flight mass spectrometry. A probe hole technique selects a small area of the emitter surface, containing a minimum of a few atomic sites up to several hundred of them. By tilting the emitter different crystallographic planes can be sampled. Between the field pulses, during the reaction time, tR,(see fig.1) no field is applied for measurements of kinetic parameters. An arbitrary steady electrical field can be applied in order to study the field dependence of a given chemical reaction. The surface temperatures are measured by a thermocouple spotwelded to the emitter base. The Pt field emitter is obtained by electrochemically etching a wire (99.99 % purity) of 0.1 mm 0 in dilute solutions of KCN at 2-4 V ac. Ru was cut by spark erosion from a boule and etched in dilute HC1. Cleaning of the surfaces was performed in situ by cycles of heat treatment and field evaporation. Kinetic data of surface processes can be obtained as sketched schematically in fig. 1. While the emitter surface is continuously dosed by nitric oxide at a
175
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Fig. 1. Time scheme of the field pulses leading to desorption at a field strength F which is the sum of FR (steady field, also called "reaction" field) and Fp D (pulsed field). Below: Schematic diagram illustrating the development of the surface coverage with field free reaction time; a steady surface coverage may result after a time T. steady gas pressure, adsorption takes place only in the field free reaction interval, t R, between the pulses. The next field pulse desorbs the adsorbed layer and analyses its composition. If desorption is complete, the measured ion intensity, i.e. the number of ions detected with each pulse (ions per pulse), directly represents the surface concentration of the species within the monitored area before the pulse. The new reaction period starts with zero coverage, and the longer t R is, the more the adsorption process proceeds. It may happen that thermal desorption occurs between the pulses at long t R and sufficiently high temperature. On the other hand the probability for chemical reactions increases with rising adsorbate concentrations. The kinetics of these processes can be studied by systematically varying t R. This is normally done in the range t R = 1 0 0 ~ s ... ls. Surface diffusion can disturb the measurements and mask true reaction kinetics. While the adsorbate at the apex of the emitter is desorbed by the pulses, the adsorbate at the shank is not. This may lead to influx of mobile species in-
176
to the monitored area during t R. Such an influence has been observed in particular cases (ref.6,S), and prevented by high enough field pulse amplitudes and repetition rates. RESULTS For all of our studies the emitter surfaces are continously dosed by NO at a steady gas pressure between 1.3xlO-5 Pa and 6.7xlO -4 Pa. In any case, under mere pulsed field conditions, the desorption of the adsorbed layer leads to the detection of high amounts of NO+. The substrate material is concomitantly field evaporated at lower rates in form of Pt n+ and Ru n+ (n=1,2). Since NO may chemically dissociate on surfaces with a certain geometrical arrangement of the atoms (details see below), various metal oxide species, MeO x' as well as mere 0 atoms can be desorbed as ions. The NO dissociation can be promoted by steady electrical fields giving high amounts of N20+ and N~. We first present results of probe hole measurements on the stepped surface in the vicinity of the (001) pole of a Ru emitter tip. Next we compare these results with those obtained for the stepped region close to the (001) pole of a Pt emitter. Results of the field induced decomposition of NO over stepped Pt(lll) will be reported. It has been pointed out in the experimental section of the paper that kinetic data of surface processes can only be obtained if all adsorbed molecules are completely desorbed by each field pulse. This requirement can be checked from field strength variation measurements. Fig. 2 displays the results of such an experiment performed with a Ru emitter. The measurements are performed at a pressure p = 6.7xlO- 4pa NO and a temperature T = 552 K. A repetition rate of the pulses f = 4 Hz, i.e. t R = 0.25 s, is applied. No steady electrical field is present during t R. Up to a field strength FO ~ 2 7 V/nm the different ionic species display an uniform trend of increasing intensities. The onset values for field desorption are different. At low field strengths the NO+ intensities dominate (for FO< 19 V/nm NO+ is the only species in the mass spectrum). Various Ru-oxides, RUO;+ (x=1 ... 3), appear subsequently. R U O ~ + is seen first, R U O ~ + and Ru0 2+ come up later. The occurrence of these species proves dissociation of NO ad to take place with subsequent build-up of an oxide layer. At low field strength the adsorbed layer remains nearly untouched by the field pulses. Only a small portion is desorbed but immediately refilled by continuous impingement of NO molecules. Thus, the adsorbed layer has a steady concentration. The oxidation state of the surface (and sub-surface region) is high and one of the characteristics under these conditions is the relatively high amount of RUO~+ ions. Nevertheless, there are still enough sites available for molecular adsorption of NO as evidenced by the occurrence of ~ i g h rates.
NO+ desorption
177
20
22
24
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28
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Fig. 2. Dependence of ion intensities on desorption field strength during reaction of NO with stepped Ru(OOl). NO pressure: 6.7xlO- 4pa; Ru tip temperature: 552 K; reaction time: 0.25 s, i.e. pulse repetition rate: 4 Hz. For rising field strength values the desorption probability of the adsorbed layer and, consequently, the ionic rates of the various species increase. However, for a field strength FD > 27 V/nm this trend only holds for the Ru0 2+ ion while the ionic rates of all other species decrease. The observed behaviour provides evidence for a strong structural change of the surface layer. Desorption at high field strengths lowers the surface oxide level considerably, and the high oxidation state cannot be restored during the reaction time, t R=0.25 s, at T = 552 K. Hence, the ionic rates of the high index R U O ~ + species decrease. This decrease at high FD is more pronounced for RUO~+ than for R U O ~ + The destruction of the oxide layer is associated with the creation of oxygen vacancy sites. N O ~ d may dissociate at these sites as evidenced by increasing intensities of Ru0 2 and decreasing intensities of NO+. At the temperature T = 552 K thermal desorption of NO ad also occurs during t R=O.25 K. This process competes with the dissociation reaction, thus only those NO molecules which are neither thermally desorbed nor yet decomposed can be detected.
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Interestingly, we find no mass spectrometric evidence for Nad emerging from the decomposition of NO This suggests either fast recombination with subsead. quent thermal desorption as N or diffusion out of the monitored area into the 2 neighbouring surface regions. Field strength variation measurements have also been performed for NO on Pt. The detailed results will be published elsewhere. It is found that the chemical dissociation of NO is strongly face dependent.While the stepped surface region ad with (001) orientation of the terraces (furtheron denoted as stepped (001)) is active in N-O bond breaking, the stepped (111) surface is not. 0ad formation on stepped Pt(OOl) leads to the detection of 0+ and PtO n+ (n=1,2) ions. PtO x species from high index planes are not present in the mass spectra. In order to obtain kinetic data of the NO decomposition reaction over stepped Ru(OOl) and stepped Pt(OOl) we performed reaction time variation measurements. The results of this comparative study are displayed in fig. 3. In the Pt case we set T = 543 K. At this temperature both the decomposition as well as the thermal desorption are competitive within the measurable time scale,and a kinetic analysis of these processes can be performed. The field strength amounts to FO=30V/nm for measurements on Ru and to FO = 28 V/nm for those on Pt. These high field strength values have been chosen in order to ensure high desorption probabilities of the adsorbed species. In fact, it has been found that the ion intensiies of Ru0 2+ and Pt0 2+ do not increase any further above these field strength values. Oesorption of NO ad from Pt leads to saturated NO+ intensities for FO> 20 V/nm (ref.g). Thus, under these conditions, the measured intensities reflect the surface concentrations of 0ad and NO ad within the monitored area, respectively. Since the high index RUO~+ species are only slightly above the detection limit at FO = 30 V/nm and T = 503 K, they are not plotted in fig. 3. The measured Ru0 2+ intensities and, consequently, the Dad concentrations follow a straight-line tR-dependence in fig. 3. Thus,the NO ad decomposition with subsequent oxygen deposition occurs at a constant rate. The time proportional increase of the Ru0 2+ intensity proves the dissociation to proceed independent on the concentration of 0ad' A completely different behaviour is found for pta+ (and a+ which is not plotted here). This species appears with a delay time of ~ a.1 s and we conclude that the Na decomposition is inhibited at shorter reaction times. The slope of the pta+ intensities is steep at t R ~ a.1 s but reduces with increasing t values, R This behaviour provides evidence that the dissociation in its initial stages follows a complicated kinetic mechanism. In fig, 4 we present the time dependence of both Na+ as well as pta+ intensities. Within the measured time scale the Na+ ions are always more abundant than the pta+ (and a+) ions. Thus, molecular adsorption prevails over dissociative adsorption. Moreover, for short t R, only molecular adsorption is observed. A suf-
179
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Fig. 3. Dependence of Ru0 2+ and PtO+ ion intensities on reaction time, t R· NO pressure: 1.3xlO- 5pa; impingement rate:0.19 molecules/s into the monitored area; tip temperatures: 503 K for Ru, 543 K for Pt; pulsed field strength: 30 V/nm for Ru, 28 V/nm for Pt. The ion intensities refer to the same size of the monitored area. ficient accumulation of NO ad is necessary for the dissociation to occur. Closer inspection of the molecular adsorption reveals some interesting kinetic details. The NO ad concentration increases linearly with rising t R values but levels off later on. This behaviour is characteristic for thermal desorption of NO ad during t R, If we assume adsorption to be associated with a constant sticking probability and thermal desorption to obey first order kinetics, dc/dt= -CiT, and, consequently, c = ~ (l_e- t / T), the mean lifetime T before thermal desorption occurs, is given by the time t R where the NO ad concentration reaches the (I-lie) level of its equilibrium value, at long times. At T = 543 K we find T = 0.2 s, i,e. k = 5 s-l for the first order rate constant. The NO+ intensity increase, i.e. the slope dc/dt, at short times defines
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180
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IsI Fig. 4. Dependence of NO+ and PtO+ intensities on reaction time, tR.NO pressure: 1.3x10- 5pa; T = 543 K, FD = 28 V/nm; T = mean lifetime of molecular adsorbed NO; impingement rate: 0.19 molecules/s into the monitored area. adsorption rate. Comparison with the impingement rate from the gas phase (checked by NO+ dc field ionization) yields sticking probabilities, s, between 0.6 and 0.85 which are in reasonable agreement with the values s ~ 0.6 found by Bonzel et al. (ref.10) on stepped Pt[4(100)x(111)J at the low coverage limit. Similarly, the dissociation probabilities,w, can be calculated by comparing the PtO+ intensities with the impingement rate. Within the measured time range, w is small. At a time t R =1 s, w is of the order of a few percent only. The dissociation probability of NO on stepped Ru(OOl) is higher and amounts to a constant value of more than 5 % (see Fig. 3). Note that the surface oxide coverage on both substrates, Ru as well as Pt, is far below a monolayer. Thus, the w values refer to the low coverage limit. As already mentioned,NO ad dissociation does not occur on the stepped Pt(lll) surface. Only molecular adsorption of NO is observed. A reaction time variation
181
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Fig. 5. Temperature dependences of the mean lifetime,T, of NO on pt; the values refer to measurements on a stepped surface region with (111) orientation of the terraces. measurement has been performed at various temperatures between 523 K and 602 K in order to probe the kinetics of adsorption and thermal desorption. The detailed results are published elsewhere (ref.9). By evaluating the temperature dependence of 'the mean lifetimes, T , according to Frenkel's equation, T=T exp(Ed/kT), the activation energy,E O d, for thermal desorption and the preexponential term, TO' have been determined (see fig.5). We find Ed = 139 kJ/mol and TO= 3x10 -14 s. These rate parameters are in good agreement with those obtained by other authors using different experimental techniques (ref.11,12). In PFDMS always the most stable adsorption state (with the longest lifetime) is sampled provided diffusion and conversion into this state are fast processes.We conclude that our
182
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Fig. 6. Field induced decomposition of NO on Pt, measured on a stepped surface with (Ill) orientation of the terraces;the steady field strength,F R, is varied and the pulsed field is adjusted so that the desorption field is constant at FD = 24 V/nm; NO pressure: 6.7xIO -5 Pa, T = 296K. rate data are largely determined by steps since these sites exert the strongest bonding to adspecies. The same conclusion has been drawn by other authors from their results (ref.II-I3). In fact, adsorption on perfectly flat Pt(III) is associated with a binding energy of 105 kJ/mol, the most probable value obtained by Serri et al. (ref.I4) from their theoretical model. The results presented so far have been obtained under mere pulsed field conditions where the various processes and their kinetics are not influenced by the probing pulses. However, steady electrical fields are frequently observed to change the chemical reactivity of the adsorbed species. This influence may become apparent in form of fragmentation, association, charge transfer processes, etc. We studied the field dependence of the NO adsorption by probing the stepped Pt(III) surface region where, by mere pulsed fields, only NO+ desorption occurred (besides some field evaporation of the substrate material). The results are displayed in fig. 6. The measurements are performed by increasing the steady field, FR, and adjusting the pulsed field, Fp' such that the total desorption field is
183
kept constant and amounts to FD = 24 V/nm. The reaction time is t R = 10 ms and the surface temperature is set equal to T = 296 K. In the lower steady field range only NO+ ions are detected. At moderate fields, however, various other species, N20+,N; and O+,appear additionally. The intensities of these species increase steeply with rising FR values. The onsets and 0+ desorption are coincident at FR ~ 4 V / n m , the N; ions come up for N 20+ somewhat later. The intensities of N20+ are observed to reach about the same level as those of NO+ for small FR' Further augmentation of the field strength is associated with decreasing NO+ rates. Both N; and 0+ intensities increase continuously within the measured range of FR values, however, the 0+ ions are much less abundant. The general result of fig. 6 is the field induced decomposition of NO. The chemical nature of the products and their high ionic intensities prove this process to occur in the adsorbed layer rather than in the gas phase near the surface where the field strengths are high too. We note that the ionic traces remain essentially the same for increasing surface temperatures as long as there is no loss of NO ad due to thermal desorption. The surface oxide level in the monitored area is low as evidenced by the small 0+ ionic rates. This is unexpected in view of the stoichiometry of the field induced decomposition. Further investigations are currently performed in order to clarify this point. DISCUSSION We studied the elemental steps of the interaction and reaction of NO over Pt and Ru. Results are presented for stepped surfaces with terrace orientations (100) and (111) on Pt and (001) on Ru. In fact, these planes, together with various others, form the surface of a field emitter tip which can be regarded as a catalyst particle with a diameter of about 20 to 200 nm. Although our probe hole measurements sample only a few atomic sites (up to about 200) the detailed crystallography of the probed area, i.e. terrace widths and step site symmetries, is not known because the concomitant removal of substrate atoms by field evaporation (from kink site positions) during the measurements causes continuous alterations of the morphology. The observed structure sensitivity of platinum in the decomposition reaction of NO is in accord with the known catalytic behaviour of the metal. While the stepped Pt(lll) surface adsorbs NO only in its molecular form, the stepped Pt(lOO) surface also decomposes NO. Although step sites are generally regarded as active in N-O bond breaking we must conclude that this does not apply always. Banholzer et al.(ref.3) developed a model in order to explain the plane to plane variations
184
of the catalytic decomposition activity. This model is based on symmetry conservation rules for chemical reactions. The flat Pt(IOO) clean surface is fairly unique in that it may exist in several phases (ref.15-17). The (lxl) phase (bulk lattice structure) is metastable and observed to transform above 400 K into a quasi hexagonal structure (5x20 or "hex"). This reconstruction is removed by adsorption of NO (ref.IO) or other gases (ref.15,18). In fact, the reversible phase transition may cause several interesting phenomena during heterogeneous catalysis. Ertl et al. (ref.19) have shown that oscillating rates in the CO 2 production during CO oxidation are driven by the CO-induced phase transition. In the present study on the stepped Pt(lOO) clean surface the ideal "hex" phase with its large unit cell has not been observed so far by field ion microscopy. LEED studies of the Pt[4(lOO)x(lll)]and Pt[9(lOO)x(lll)] (ref.lO) surfaces, however, give evidence for a reconstruction modulated by the steps. Our results on stepped Pt(lOO) show molecular adsorption and decomposition of NO to be sequential steps. The decomposition does not begin before the NO ad concentration has reached a certain level. In fig. 4 this level corresponds to caverages less than 1 % of a monolayer. The time dependence of the PtO+ intensities shows that not only the surface oxygen concentration, but also its production rate increases with time. The results on stepped Pt(lOO) can be understood on the basis of structural autocatalysis. We suggest that the reconstruction of the surface is removed by sufficiently high NO ad concentrations. This leads the decomposition to start immediately and to accelerate in an autocatalytic manner. Behm et al. (ref.20), in their detailed study of the CO induced h e x ~ (lxl) conversion of flat Pt(lOO), find CO ad concentrations of 5 % of a monolayer sufficient for this transition to occur in a patch-like manner. In our studies the steps play an important role as intermediate trapping sites for molecular adsorbed NO. Thus, the concentration of NO ad at step sites is much higher than at terrace sites. For an estimated step density of 10 % in the monitored area we expect, at t R = 0.1 s, several % of the step sites to be covered by NO ad. The role of steps in the reconstruction of the Pt(lOO) surface is not clear so far. Bonze1 et a1. (ref.lO) report on a stabilization effect of the (lxl) phase by steps. This finding calls for a rigorous analysis employing field ion microscopy as well as probe hole PFDMS with smaller selected areas than used so far in our work. Since the NO ad decomposition occurs at steps the continuous oxygen deposition at these sites prevents molecular adsorption. Under reaction conditions the surface oxide level is always small, and the molecular adsorption (and thermal desorption) is the prevailing process. The initial stages of oxide build-up have obviously not been studied so far by other authors. Auger (AES1, XPS or temperature programmed desorption (TPD) are not sensitive enough to sample this pro-
185
cess. Surface nitrogen has not been found in the present measurements. This is explained by rapid recombination and immediate thermal desorption of molecular nitrogen. The surface oxide level on stepped Pt(100) is always lower than on stepped Ru(OOl). When the pulsed field strength is lowered, the adsorbed layer is not completely removed any longer.This leads to Dad accumulation at the surface,but only on the Ru sample is the oxidation so strong that high index RUO~+ (x up to 3) can be desorbed. In a recent study of Ru oxidation by gaseous 0z (ref.Zl) we also found these ions. In addition, the corresponding neutral molecules are found to be mobile species on top of an oxide layer. It is likely that these molecules act as intermediates in the reaction towards the volatile Ru0 4 molecule. The intense oxidation of Ru is in accordance with the known behaviour of the metal under the "real" conditions of heterogeneous catalysis.It is remarkable that the NO decomposition reaction under our experimental conditions,i.e. at low pressures, also leads to a high oxidation state of the surface (and sub-surface) region. The field induced decomposition of NO over Pt is an interesting reaction phenomenon and presents another example for drastically changing chemical reactivities in high external electrostatic fields. It should be noted that we find no marked face specificity of the metal in this reaction. The presented data refer to the stepped Pt(lll) surface which shows no catalytic activity in N-O bond breaking under zero field conditions. Recently, Kiskinova et al. (ref.ZZ) reported the potassium promoted NO decomposition over macroscopic Pt(lll) single crystal surfaces to produce nitrogen and oxygen as well as NZO and NO Z due to secondary reactions. The potassium, adsorbed as K+, is considered as an electronic promoter which transfers electrons into the antibonding Zn molecular orbitals of the adsorbed NO molecule.This leads to weakening of the N-O bond and, finally, to its scisson. The field assisted NO decomposition cannot be explained by electronic effects only. The adsorption of NO is associated with an increase of the work function, thus there is a net electron transfer to the adsorbate (ref.Z3). The electrostatic interaction of the resulting dipole with the (positive) external field may be associated with a bending of the molecule and its ultimate dissociation. Adjacent NO ad species may associate during this process so that the detection of high amounts of NZO+ and N ~ becomes conceivable. ACKNOWLEDGEMENT This work was partially supported by the Sonderforschungsbereich (Sfb 6) at the Freie Universitat Berlin.
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REFERENCES 1 W. Egelhoff, Jr., in D.A. King and D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. IV, Elsevier, New York, 1982, p. 397. 2 Y.O. Park, W.F. Banholzer and R.J. Masel, Appl. Surf. Sci., 19 (1984) 145. 3 W.F. Banholzer, Y.O. Park, K.M. Mak and R.J. Masel, Surf. Sci., 28 (1983) 176. 4 D.B. Liang, G. Abend, J.H. Block and N. Kruse, Surf. Sci., 126 (1983) 392. 5 N. Kruse, G. Abend and J.H. Block, Z. Phys. Chem. N.F., 144 (1985) 1. 6 N. Kruse, Surf. Sci., in press. 7 J.H. Block and A.W. Czanderna, in A.W. Czanderna (Ed.), Methods and Phenomena, Vol. I, Elsevier Scientific Publ. Comp., 1975, p. 379. 8 D.L. Cocke, G. Abend and J.H. Block, Int. J. Mass Spectrom. Ion Phys., 24 (1977) 271. 9 N. Kruse, G. Abend and J.H. Block, Surf. Sci., submitted. 10 H.P. Bonzel, G. Broden and G. Pirug, J. Catal., 53 (1978) 96. 11 T.H. Lin and G.A. Somorjai, Surf. Sci., 102 (1981) 573. 12 C.T. Campbell, G. Ertl and J. Segner, Surf. Sci., 115 (1983) 309. 13 J.A. Serri, M.J. Cardillo and G.E. Becker, J. Chem. Phys., 77 (1982) 2175. 14 J.A. Serri, J.C. Tully and M.J. Cardillo, J. Chem. Phys., 79 (1983) 1530. 15 A.E. Morgan and G.A. Somorjai, J. Chem. Phys., 51 (1969) 3309. 16 P.R. Norton, J.A. Davies, O.K. Creber, C.W. Sitter and T.E. Jackman, Surf. Sci., 108 (1981) 205. 17 K. Heinz, E. Lang, K. Strauss and K. MUller, Surf. Sci., 120 (1982) L401. 18 M.A. Barteau, E.I. Ko and R.J. Madix, Surf. Sci., 102 (1981) 99. 19 G. Ertl, P.R. Norton and J. RUstig, Phys. Rev. Lett., 49 (1982) 177. 20 R.J. Behrn, P.A. Thiel, P.R. Norton and G. Ertl, J. Chern. Phys. 78 (1983) 7437. 21 G.K. Chuah, D.L. Cocke, N. Kruse, G. Abend, T. Kessler and J.H. Block, J. de Physique, C2,47 (1986) 359. 22 M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci., 140 (1984) 1. 23 M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci., 136 (1984) 285.
A. Crucq and A. Frennct (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V .. Amsterdam - Printed in The Netherlands
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PERIODIC OPERATION EFFECTS ON AUTOMOTIVE NOBLE METAL CATALYSTS --- REACTION ANALYSIS OF BINARY GAS SYSTEMS --H. Shinjoh, H. Muraki and Y. Fujitani Toyota Central Research and Development Laboratories, Inc., Nagakute-cho, Aichigun, Aichi-ken, 480-11, Japan.
ABSTRACT Catalytic activities and periodic operation effects in various binary gas systems (CO-0 2 , NO-CO, NO-H 2 , C3H 6-0 2 , and C3H a-0 2 ) over Pt, Pd, and Rh/ct-A1 20 3 were compared. In all reaction systems, periodic operation effects were found to some extent. That is, the conversion improved in the cycling feed compared to the static one. The periodic operation effects occurred most noticeably for catalysts having lower catalytic activity as a result of the difference of adsorption capability between the two reactants. INTRODUCTION Automotive three-way catalysts which simultaneously control NOx, CO, and HC emissions are designed to operate in the stoichiometric conditions of automotive exhaust gas. However, it is well-known that the stoichiometry of exhaust gas changes continuously between oxidizing and reducing atmospheres because of the step-like response characteristics of oxygen sensor equipment. Recently, this dynamic behaviour has been analyzed. Taylor et al (Ref. 1) reported the effects of both symmetric and asymmetric air/fuel ratio cycles during conversion by three-way catalysts. Schlatter (Ref. 2) and Herz et al (Ref. 3-6) studied the role of Ce02 added to three-way noble metal catalyst under dynamic conditions. The authors (Ref. 7) have investigated the behaviour of automotive noble metal catalysts in the cycled model feedstreams which simulated engine exhaust gas. We observed that the reaction using noble metal catalysts, particularly Pt and Pd, under cycling conditions was superior to that under static conditions, and that catalyst perfomance depends on the cycling period and feedstream composition. These phenomena may be used to improve the activity of three-way catalysts by selection of suitable periodic conditions. Further, it is very important to clarify the mechanism of the periodic operation effects over various noble metal catalysts. Our object was to investigate the performance of conversions over noble metal catalysts under periodic conditions. Accordingly, we have systematically investigated the various binary gas reaction systems, such as CO-0 2 (Ref. 8), CO-NO (Ref. 9), HC-0 2 (Ref. 10), over
188
noble metal catalysts. For example, the periodic operation effects were particularly observed in CO oxidation: the conversion depended on cycling period, and the existence of maximum conversion at a particular cycling period was confirmed. Following the investigation of binary gas systems under periodic conditions as mentioned above, we examined NO reduction with Hz over noble metal catalysts. While NO-Hz reaction may be considered as a main process of NO reduction in the reducing atmosphere of automotive exhaust gas, little is known so far concerning supported noble metal catalysts. Koblinski et al (Ref. 11) studied NO reduction with H2 and CO over noble metal catalysts. They demonstrated that H2 is a more effective reducing agent than CO. Yao et al (Ref. 12-13) showed the relationship between reactivities of catalysts in NO reduction with H2, and noble metal content of catalyst used. But, the dynamic behaviour of noble metal catalysts in NO-Hz systems has not studied at all. This paper introduces: (1) the detailed behaviour of the NO-Hz reaction under both static and dynamic conditions over three kinds of noble metal catalysts (Pt, Pd, and Rh/a-Al Z03), and (2) the relationships between catalytic activity and periodic operation effects obtained from the various binary gas systems over the same catalysts. Finally, in order to explore the role of periodic operation, the concept of self-poisoning of reactants contained in the binary gas systems is postulated, allowing reasonable interpretation of our results obtained under the cycling conditions. EXPERIMENTAL Catalyst All catalysts were prepared by a conventional impregnation method using a-Al 203 (pellet size: 2-3 mm¢, BET surface area: 10 mZ/g, bulk density: 0.79 g/cm3). Details of the catalyst preparation was reported elsewhere (Ref. 7). Loading amount of Pt, Pd, and Rh was 0.05 g/l (0.006 wt%) as noble metal. Experimental procedures The laboratory reactor system was a conventional flow system with a tubular fixed-bed reactor as shown in Figure 1. A characteristic feature of this reactor system was its ability to change the feedstreams to the catalyst bed quickly so that the feedstreams can be rapidly cycled between two different gas compositions. The cycling period was varied between 0 and 2.0 seconds. In this work, NO and Hz were fed periodically to the main stream of Nz (carrier gas). The time-average concentrations of NO and Hz were changed from 0.1 to 0.6 vol% and from 0.1 to 3.0 vol%, respectively. The ranges in these concentrations correspond to those in automotive exhaust gas. Chemically pure Hz (99.999%) from Nippon Sanso Co. and NO (99%) from Takachiho Chemical Co. were
189
used without further purification.
SiC CATALYST
MASS FLOW CONTROLLER
ANALYZERS NO
Figure 1. Schematic diagram of system for simulating catalyst to oscillating feedstream compositions. The relative activities of catalysts used were expressed in terms of percentage NO and H2 conversion as a function of catalyst bed temperature (100 600°C) at a constant flow rate (SV: 30,000/h) for both static and cycling experiments. RESULTS AND DISCUSSION NO-H 2 reaction under static conditions The reduction of NO with H2 was examined under static conditions (cycling period: 0 second). The feedstream composition of premixed gas was fixed at the stoichiometric 'ratio for converting NO to N2 • The concentrations of NO and H2 were 0.3 vol%. Figure 2 shows activity data obtained for reduction of NO with H2 over the three supported noble metal catalysts. By arbitrarily taking the temperature at 50% NO conversion as a measure of catalytic activity, the activity sequence was Pt > Pd > Rh. In contrast, Koblinski et al (Ref. 11) showed, in the same reaction system, that the activity sequence was Pd > Pt > Rh > Ru. The discrepancy between these results may be due to the different catalyst supports: the present study used
190
inactive a-Al Z03 while Koblinski et al used active Al Z03.
100
NO-H2
~
°
static.
c 0 (/\ L.
50
Q)
>
c u 0
0
z 0
400
0 Temperature
(OC )
Figure 2. NO conversion data of noble metal catalysts in NO-Hz reaction under static conditions. Otto et al (Ref. 13) studied the NO-Hz reaction over Pt and Rh catalysts and found that, at a given temperature, Pt exceeds Rh in the turnover frequency by two orders of magnitude. They considered this resulted from the different geometrical surface structure of Rh and Pt catalysts. Rh remains oxidized to a large degree under the conditions of these rate measurements and thus displays fewer active reaction sites. The higher affinity of Rh for oxygen has recently been shown (Ref. 12-15). Consistent with this concept is the fact that the amount of NO chemisorbed on an oxidized surface is smaller than that on a reduced one. This explains why Rh is less active than Pt in the NO-Hz reaction. It is also probable that Pd remains more oxidized than Pt, but to a lesser extent than Rh under present experimental conditions. The reaction products detected in the NO-Hz system, Nz, NzO, NH 3' and HzO, were similar to those in previous studies (Ref. 11). The reaction path could be estimated from the amount of consumed reactants ~ C ( N O ) and ~ C ( H z ) , or their ratio R, where R = [ ~ C ( H z ) / ~ C ( N O ) J . occur simultaneously:
That is, the following set of reactions may
191
NO + 0.5 H2 NO + H2 NO + 2.5 H2
-+ -+ -+
0.5 N20 + 0.5 H2O 0.5 N2 + H2O NH 3 + H2O
(A) ( B)
(C)
Thus for NO reduction processes, when reaction (A) occurs alone, the value of R is 0.5. Similarly, for reactions (B) and (C), the values are 1.0 and 2.5 respectively. Figure 3 shows the ratio of consumed reactants, R, as a function of reaction temperature. The behaviour of the three catalysts differed from one another. In the case of Pt and Pd catalysts, the value depended on catalyst bed temperature. However, the opposite was found in the case of Rh. Over Pt and Pd catalysts below 200°C, the value of R was smaller than 1.0. Therefore the main products of NO reduction with H2 might be N20 and N2• On the other hand, over 500°C, the main products might be NH 3 and N2• With increasing temperature, the main reaction paths may change gradually from (A) to (8) and further to (C) in present experimental conditions.
2
NO-H2
0
z
..........-. . . . . . Pt
u
-~'------
"<;J
~~9
<,
.......
Pd
Rh
/
/
N I
I
/
I
I
U "<;J
0 0
200 Temperature
400
600
(OC )
Figure 3. The relationship between R [ ~ C ( H 2 ) / ~ C ( N O ) J over noble metal catalysts in the NO-H 2 reaction.
and reaction temperature
NO reduction with H2 under dynamic conditions A similar activity examination of NO reduction with H2 was conducted under dynamic conditions. The feedstreams of NO and H2 to the catalyst bed were symmetrically cycled to give periods from 0.2 to 2.0 second.
192
Figure 4 illustrates the effect of cycling period on NO conversion at several temperatures over a Rh catalyst. At lower temperatures (below 300°C), the periodic operation effect was most noticeable.
,...
Rh
0
0-
NO-Hz C 0
...
Ul
50
> C 0
U
0 Z
0
1.0 Per i ad
2.0 (s e c.)
Figure 4. The effect of cycling period on NO conversion at several temperatures over a Rh catalyst. The maximum conversions are clearly observed for somewhat lower temperatures and the optimum period for the maximum conversion decreases with increasing temperature. Similar phenomena were observed in the systems of CO-O z (Ref. 8), CO-NO (Ref. 9), and HC-O z (HC: C3H 6 and C3H8 ; Ref. 10) over noble metal catalysts. Kinetic parameters The kinetics of NO reduction with Hz under the static conditions was sUbjected to empirical laws: rate of NO (or Hz) consumption, V, is generally expressed by the following formula, V = k x p(Hz)m x p(NO)n exp(-llE/RT), where P(H z) and P(NO) is the partial pressure of Hz and NO, respectively, and t.E is the activation energy. The partial reaction orders, m and n, are determined from the data obtained under the conditions of lower conversion, usually less than 30%. The result over a Rh catalyst is shown in Figure 5. With Rh catalysts, the reduction rate of NO with Hz was depended to the order of -1.4 with respect to NO partial pressure and the order of +6.0 with respect to Hz partial
193
pressure. Activities of Pt and Pd catalysts, on the contrary, were independent of NO partial pressure, and dependent to the order of +1.0 (Pt) and +0.7 (Pd) with respect to Hz pressure. It is known that the dissociation of Hz over noble metal catalysts is considerably faster than that of NO, and thus the rate determining step of NO reduction with Hz is the dissociation of NO (Ref. 13). Pirug et al (Ref. 16) have examined NO reduction with Hz over Pt catalysts and found that the reaction rate depended on the ratio P(Hz)/P(NO). For a high P(Hz)/P(NO) ratio, surface concentration of Hz will increase slightly due to the competitive adsorption between Hz and NO. So, the dissociation of NO and the NO-Hz reaction will commence at a lower temperature.
-2
Rh(250°C)
-o
u I
NO-H2
-3
01
c E
0
-4
-5
E
-6 >
P(HZ)var.
c
-7
-6.5
- 6.0
- 5.5
- 5.0
In P(Hz) or In P(NO) (rn o l e vs )
Figure 5. Partial pressure dependencies of reaction rate at 250°C for Rh catalyst [e.g., P(NO)var.: NO partial pressure is variable and Hz partial pressure is constant]. Information from some studies (Ref. 17) suggests an inverse dependence of the rate of NO decomposition with respect to the oxidizing atmosphere over noble metal catalyst. From the facts mentioned above, it is reasonable that the rate of NO-Hz reaction was positive order with respect to Hz partial pressure but negative order with respect to NO partial. pressure with Rh catalysts.
194
The negative order, that is NO inhibition, in NO-Hz reactions over noble metal catalysts is essentially concerned with the periodic operation effect. The periodic operation effect is perhaps due to the surface state of the Rh catalyst. That is, the catalyst surface under static conditions is easily oxidized by NO, so that NO chemisorbs to a smaller degree and reaction rates are suppressed. However, under the optimum cycling feeds, catalyst surface is sufficiently reduced and suitable surface compositions of reactants are maintained in order for reaction to proceed. As a result, the reaction rate reaches the maximum value. The periodic operation effect can be interpreted in terms of the strong affinity between Rh and oxygen and this concept can be extended to interpret the behaviour observed in the CO-O z (Ref. 8) and CO-NO (Ref. 9) systems. Catalytic activities and periodic operation effects in various binary gas systems over noble metal catalysts In order to clarify the mechanism of the periodic operation effect over noble metal catalysts, we studied several binary gas systems, which occur in automotive exhaust gas: CO-NO, CO-O z, C3H 6-O Z , C3H s-O z, and NO-Hz. The results obtained from the above-mentioned gas systems are summarized. At first, the order of catalytic activity, under static conditions, in each binary gas system was as follows: CO-O z CO-NO Hz-NO C3H 6-0 z C3H s-O z
Rh Rh Pt Pd Pt
> Pd > Pd > Pd > Pt > Rh
> Pt > Pt > Rh > Rh > 'Pd
It was noted that the activity sequence in NO reduction with CO was the reverse of that for the NO-Hz reaction. This inversion of activity order depending upon the reducing agent employed means that the reduction of NO with Hz over Pt, Pd, and Rh is faster than NO with CO. The order of catalytic activity in CO-O z and CO-NO is similar. This means that CO is concerned with the rate-determining step of respective reactions. It is well known that CO inhibits the reactions of CO-O z and CO-NO systems over noble metal catalysts. In the NO-CO-H z reaction, CO inhibits NO reduction (Ref. 11) and strong inhibition by CO is the rate-determining step over three-way catalysts. As can be seen from the above sequences of catalytic activities, Rh is a good catalyst for CO oxidation, Pt is good for NO reduction with Hz and C3H 6 oxidation, and Pd is good for C3H6 oxidation. Under dynamic conditions, periodic operation effects were observed in every binary gas system. The most efficient catalysts under periodic condition were:
195
Pt and Pd in CO oxidation with Oz or NO (particularly Pt), Pt and Rh in C3H s oxidation, and Pt in C3H 8 oxidation. In the NO-Hz reaction, periodic operation was particularly noticeable for the Rh catalyst. Comparing the results under static and dynamic conditions, it is clearly found that periodic operation effects occurred most noticeably for catalyst of lower catalytic activity and at lower reaction temperatures. These facts indicate that if catalysts have a poor catalytic activity or they are reacting at a lower temperature, their activities can be improved by periodic operation. Table 1 indicates whether or not catalytic activity was found to be positive order with respect to partial pressure in every binary gas system tests over noble metal catalysts. This table shows that the reactions are promoted or inhibited by each reactant. For example, in the CO-O z reaction, CO inhibits, and Oz promotes the reaction. These values of kinetic parameter were different for each of the three catalysts and in every binary gas system. TABLE 1 Partial reaction orders in the binary gas systems over noble metal catalysts [V=k x P(Reductant)m x P(Oxidant)n
exp(-~E/RT)J
CO-O z
CO-NO
m
n
m
n
m
n
m
n
m
n
Pt
-
+
- -
+
+
0
-
+
+ +
- -
Pd
-
+
-
+
+
0
- +
+
- +
+
Rh
-
+
0
-
+ +
-
+
-
+
0
C3H s-O z
Hz-NO
C3H 8- 0 Z
+ +: highly positive, +: positive, 0: independent,
-: negative, - -: highly negative, - +: partly negative(under higher partial pressure of HC).
In all cases where periodic operation effects were noticed, the product of two partial reaction orders, m x n, became negative. From these results, it seems that one of the reactants self-poisons a reaction occurring over noble metal catalysts and that the potential for self-poisoning corresponds to the degree of periodic operation effect.
196
The periodic operation effect depends on temperature and cycling period and each catalyst behaves differently. To improve catalytic activity, it is important that the optimum cycling period for each temperature is used. CONCLUSIONS Catalytic activity and periodic operation effects in binary gas systems (CO-O z, CO-NO, NO-Hz, C3H 6-O Z' and C3H g-O z) over three-way catalysts (Pt, Pd, and Rh/a-Al Z03), were compared in terms of conversions and kinetic parameters. The detailed behaviour of noble metal catalysts in NO reduction with Hz was investigated and the order of catalytic activity was found to be Pt > Pd > Rh. Periodic operation effects were found: that is, the conversion of NO is improved by the cycling feedstream compared to the static one. This \1aS most noticeable with Rh catalyst. For each binary gas system, our findings are as follows: (1) Comparative catalytic activities in binary gas systems under static conditions are CO-0 2 CO-NO NO-H 2 C3H 6-0 2 C3H g-O z
Rh Rh Pt Pd Pt
> Pd > Pd > Pd > Pt > Rh
> Pt, > Pt, > Rh, > Rh, > Pd.
(2) In binary gas systems, periodic operation effects are observed according to the catalyst used and the reaction temperature. (3) The optimum period for maximum conversion decreases with increasing temperature in all binary gas systems. (4) Periodic operation effects occurred most noticeably for catalysts having lower activity or when catalysts were operating at lower temperatures. (5) From the results of the partial reaction order, either reactant contained in binary gas systems can self-poison the surface of the catalyst and suppress the reaction. the potential for self-poisoning corresponds with the degree of the periodic operation effect. From these findings, we consider that periodic operation effects arise from a difference of adsorption capability between the two reactants on the catalyst surface, that is, the self-poisoning reactant is the one more strongly adsorbed on the catalyst surface. Accordingly, the catalyst surface under static conditions is almost covered by the stronger adspecies, and expected reactions are suppressed. Conversely, under optimum cycling conditions, these adspecies are eliminated and surface compositions are suitable for reaction to take place. Under these circumstances, the reaction rate reaches the maximum value. Periodic operation effects can be applied to improve the reactivity of
197
three-way catalysts and, particularly when catalyst bed temperature is lower, catalytic activity can be promoted by a selection of suitable cycling period.
REFERENCES
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
K.C. Taylor and R.M. Sinkevitch, Ind. Eng. Chem. Prod. Res. Dev., ~ , 45, 1983. J.C. Schlatter and F.J. Mitchell, Ibid., 19, 288, 1980. R.K. Herz, Ibid., 20, 451, 198!. R.K.Herz, J.B. Kie1e and J.A.Sell, Ib i d , , 22, 387, 1983. R.K. Herz and J.A. Sell, J. Catal., 94, 166;"" 1985. 385, R.K. Herz and LJ. Shinouskis, Ind. Eng. Chem. Prod. Res. D e v . , ~ , 1985. H. Muraki, H. Shinjoh, H. Sobukawa, K. Yokota and Y. Fujitani, I b i d . , ~ , 43, 1985. H. Muraki, H. Sobukawa and Y. Fujitani, Nippon Kagaku Kaishi, 176, 1985. H. Muraki and Y. Fujitani, Ind. Eng. Chem. Proc. Res. Dev., submitted 1985. H. Shinjoh, H. Muraki and Y. Fujitani, Appl. Catal. Submitted 1986. T.P. Koblinski and B.W. Taylor, J. Catal., 33, 376, 1974. H.C. Yao, Y.F. Yu Yao and K. Otto, Ib l d , , 56, 21, 1979. K. Otto and H.C. Yao, Ibid., 66, 229, 1980-:H.C. Yao, M. Sieg, H.K. Pl ummer,Jr., Ibid., 59, 365, 1979. M. Chen, T. Wang and L.D. Schmidt, Ibid., 6 0 ~ 3 5 6 , 1979. G. Pirug and H.P. Bonzel, Ibid., 50, 64, 1977. A. Amirnazmi and M. Boundari, Ibicr:-,11, 383, 1975.
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A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
199
1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
The Role of Research in the Development of New Generation Automotive Catalysts
H. S.
Gandhi and M. Shelef
Ford Motor Company Research Staff Dearborn, Michigan 48121
ABSTRACT
The development of new generation three-way catalysts (TWCs) is based on a broad, fundamental understanding acquired over a number of years. The efforts at Ford Motor Company have been mainly concerned with supported base metal and noble metal catalysts operating under oxidizing conditions at high-temperatures where the extent of the interactions or their absence does have a profound effect with an immediate bearing on the practical use. Use of Zr02, aA1 203 etc., as a support for Rh offers an opportunity to formulate a durable catalyst in which the thermal stability of Rh can be significantly enhanced and thereby it can remain active at temperatures >600°C in an oxidizing environment. Other work was aimed at the understanding of whether lead originating from the combustion of Pb-containing fuel associates preferentially with the noble metal sites supported on the much larger areas of inert-support materials. Direct association of lead compounds with noble metal sputtered onto Zr02, Ti02 and 11.12°3 wafer supports was noted for Pt, Pd and Rh. However, there are major differences in the interaction of Pb compounds with different noble metal surfaces which explain the long-known fact that Pd and Rh are more sensitive to Pb poisoning than Pt. Model reactions were employed as chemical probes to check whether the desired surface modifications have been achieved and also for determining mode r. of deactivation in used catalysts. For an optimum catalyst each precious metal has a specific function to perform and must be carried on a specific support material to maximize activity and durability.
INTRODUCTION Three-way catalysts (TWC) were conceptually put forward in 1968 [1] and the first experimental evidence of selective removal of nitric oxide in the presence of oxygen under conditions close to stoichometry was observed in 1971 [2].
In 1978 Volvo using an Engelhard supplied catalyst was the first automo-
tive company to actually implement a TWC catalyst in conjunction with an electronically-controlled fuolinjection equipped with closed-loop control. In the intervening eight years there has been an evolution of the system and, most importantly, great advances have been made in understanding the often
200 subtle but important chemical phenomena taking place on the catalyst surface. This understanding has led to the design of much improved catalysts.
Frinciples of the Operation of the TWC Fig. 1 demonstrates the principle of the operation of a three-way catalyst in a diagram of extent of conversion vs. the air/fuel ratio in the actual operating automotive engine.
The air/fuel ratio which is usually expressed in
the weight of air per weight of fuel is easily translatable into an equivalence ratio (>') whose value is unity at stoichiometry (when all the fuel is converted to water and C02).
Values less than unity represent the conditions of fuel
excess (rich mixtures) and values larger than unity represent excess air (lean' mixtures.
While complete conversion of the reducing species is favored under
the conditions of excess air, >.>1, the complete removal of the oxidizing species, nitric oxide, is favored under reducing conditions (>'<1).
It is only
in the region around >.-1, actually somewhat to the rich-side of >., where there exists a so-called "window" for efficient, simultaneous removal of all the three main regulated pollutants.
The task of the modern, computer-controlled
fuel-metering system is to mantain the A/F ratio as tightly as possible within this window over all possib:.e variations in driving conditions.
The task of
the catalyst designer, on the other hand, is to provide as wide a window of operation as possible without compromising the activity. The present embodiment of an automotive
100
catalyst consist of a 90
monolithic support made 80
of a high-melting
70
ceramic material,
80 CATO\LYST EFFICIENCY %
cordierite, typically
50
having 64
40
square cells
per square centimeter cross-sectional
area,
with the walls between
10
the cells being 150 lU
1~5
14.6
RICH
14.7
14.8
14.9
UAN
AIR/FUEL RAnD
Fig. 1 - Conversion of NO, CO, and hydrocarbons
thick.
~m
The walls are
coated with a high surface
"washcoat
'1
for a TWC as a function of the air-fuel
having a BET area of
ratio.
80-100 m2/g.
Since the
201 weight of the washcoat is -20-30% of the total weight of the catalyst body the specific BET area for the whole piece is between 16-25 m2/g.
The composition
of the washcoat can vary substantially depending on the desired performance, which will be discussed in the text to follow.
Nevertheless, it is known that
the most abundant ingredient of the washcoat is alumina either in its '"I-phase or in other transitional form such as 8
or
o.
The alumina may contain a
number of stabilizers usually chosen from the oxides of rare-earth metals and/or alkaline earth metals.
Into this "washcoat" there are incorporated
simultaneously either all three of the precious metals Pt, Pd and Rh or only P:· and Rh. It is the Rh that confers on the TWC the ability to selectively reduce nitric oxide in the presence of oxygen in a stoichiometric gas mixture (A-I). In this process the Rh-catalyzed reduction of nitric oxide is largely directed to molecular nitrogen.
One has to emphasize the scarcity of this metal, which
is mined at a ratio of 1/17 with respect to Pt with which it usually appears as a by-product.
This ratio in the present TWC is usually much higher, between
1/3 to 1/10.
This, associated with the much lesser degree of recovery of Rh
from used catalysts emphasizes the utmost desirahility of utilizing the Rh in an optimal fashion.
The Role of Metal-Support Interactions in TWC The interactions we are concerned with are not those usually classified SMSI (Strong-Metal Support Interactions) which are observed after treatment under reducing conditions and lead to oxygen-deficient forms of the insulator supports.
On the contrary, the interactions we refer to are associated with
oxidation of the active component and its interaction with the support by sharing oxygens that ultimately bridge the metal ions in the support and the metal ions of the active component.
An extreme example would be, the well-
known formation of a nickel or cobalt aluminate (spinel) if one would support Ni or Co on 1-A1203 and expose it to high temperatures under oxidizing conditions.
With noble metals more often than not such interactions are
limited to the surface or subsurface region of the insulator support, but not always.
As a rule, the more refractory the support and the more noble the
active metal the less pronounced is the interaction [3]. Of particular interest to the designer of the automotive catalysts are the interactions with supports of Rh on
o~e
hand and of Pt on the other, since they
may determine the availability of the active sites of these metals and the nature of these active sites which in turn determines reactivity. In general, one may expect that the interactions mediated ,by surface oxygen ions of the insulator support will be related to the reactivity of these
202 ions.
This in turn is related to the stability of the crystallographic form of
the supports.
It has been established that Rh begins to penetrate the subsur-
face of ,-A1203 at >600°C by the solid state reaction between Rh203 and ,A1
This is a temperature which is frequently encountered in an operating 203' catalyst. Minimizing the reactivity of the support will slow down this subsurface penetration and loss of active Rh.
Fig. 2 shows this behavior [4].
the surface Rh is measured by CO chemisorption.
Here
The initial dispersion is
quite similar on the different samples, the higher CO uptake on the Rh ,-A120 3 Treatment at high temperatures under
being due to geminal adsorption.
oxidizing conditions causes a large irreversible loss of site on ,-A1203 a small loss on
and virtually no loss on ZrC2 .
~-A1203
The consequence of the disappearance of Rh
e
E .03
....\
.........
"~bb \
from the surface is a
....
drastic loss of activity
8 : ~ ~ ~ , ~ ~ 800
1000
800
1000
800
as shown in Fig. 3a [5J. 1000
Using the data shown in
1200
CALCINATION IN AIR FOR 5 HOURS AT TEMPERATURE, oK
Fig. 2, one can design a
Fig. 2 - The effect of calcination in air on (A) 0.014 wt% Rh/,-A1203'
washcoat where the Rh is
(B) 0.017
protected from direct
wt% Rh/cr-A1203, and (C) 0.010 wt%
contact with ,-A1203'
Rh/Zr02' ---, Samples reduced at
This is shown in Fig 3b,
673°K; ---, samples reduced at 823°K.
where the Rh was
From Ref.
supported on zirconia
[4].
first and the resulting powder was incorporated into ,-A1203 washcoat on a monolithic body.
The
activity of this catalyst remains virtually intact after calcination in air at 1100°C for one hour [6]. A. 130 ppm Rh/y-AI.O. 100
80
i
60
en a:
40
z o
...> z
o
u
20
o
~~~~...l.-l....ad~~ 0.8
1.0
1.2
1.4
1.6
REDOX RATIO, R
1.8 OS
1.0
1.2
1.4
1.6
1.8
2.0
REDOX RATIO, R
Fig. 3 - The steady-state activities of (a) Rh/,-A1203 and (b) A1203 after thermal treatment at 1100°C.in air for Ih.
[Rh/Zr02]/,-
From Ref.
[5].
203 On the other hand in the consideration of interaction of Pt with insulator supports we are often faced with a completely opposite
tas~
i.e. that of trying
to maximize the surface interaction to enhance and maintain high dispersion. The reason for this is the relative instability of Pt oxide and its tendency to decompose at temperatures <550°C even under oxidizing conditions. Platinum oxide does not penetrate into the subsurface of any support material.
To
achieve a high dispersion additives are used in which the oxygen ions are more reactive than in 1-A1203 as for instance Ce02'
Thus the addition of 2.6% ceria 1-A1203 increases the surface concentration of pta from 2.2 ~ m o l e Pt/m 2 to 4.2 ~ m o l e Pt/m 2(BET) [3]. As will be shown below other more reactive metal to
oxide additives have a similar effect on Pt dispersion.
This, in turn affects
the behavior of the catalysts with respect to several structure sensitive reactions.
Based on the above examples it is quite apparent that the noble metal interactions with the support under oxidizing conditions playa significant role in the design of practical and durable catalysts. Another degree of modification of the catalysts can be achieved by introduction of components which on one hand affect the dispersion of the noble metal similarly to the ceria discussed earlier, but also possess catalytic activities of their own.
One example of such an additive explored in depth
at Ford Research is molybdenum oxide.
Molybdena, similar to ceria, forms a
two-dimensional phase on 1-A1203 and thereby also affects the Pt dispersion ane! its catalytic properties.
Platinum, in turn, affects strongly the reducibility
of molybdena, as shown in Fig. 4, using ESCA to characterize the oxidation state after reduction in the absence and presence of Pt [7]. A direct confirmation of this behavior is obtained by TPR [8].
One can
deduce that in the presence of Pt the average oxidation state of surface molybdenum ions will be lower in an operating catalyst.
It is possible to
postulate the existence of surface complexes of the type PtMoO x' where below 600"C x may range from a to 2 depending on the reaction temperature. Recent preliminary EXAFS results seems to corroborate such a picture [9J.
Conversely,
one can consider the surface Pt in such complexes as being more oxidized (electron deficient) than when dispersed in the absence of modifiers such as molybdena or ceria.
This is found to affect the catalytic properties of Pt.
similar behavior prevails in other systems as well. recently reported that addition of
cer~a
A
For instance, it was
to a Pd/A1203 catalyst results in a Pd
surface state which is more difficult to reduce [10]. One can illustrate the reactivity effect of an active modifier such as molybdenum by the change in behavior with respect to surface processes in which CO is an important reactant.
Table 1 [8] shows that the self-poisoning
204 behavior of Pt with respect to CO under reducing conditions is largely obviated by the presence of molybdenum. Thus, for the reduction of NO by hydrogen, the catalyst in which Mo is absent is much more reactive.
For reactions in
which CO is present the opposite is true.
Note that
the overall conditions are strongly reducing, causing the self-poisoning of Pt sites by CO.
The infrared examination
of another sample shown i.n Fig. 5 indicates the availability of chemisorption sites for CO associated with reduced surface Mo oxides [8].
Fig. 4 - Mo(3d) ESCA spectra after 3 h in H2 at 500'C (a) 3.8% Mo,
(b) 3.8% Mo/3.3% Pt,
(c) 3.8% Mo/24.0% Pt. 226.0
Ref.
230.0
From
[7].
B.E leV)
TABLE 1 Temperature ('C) for 50% and 80% Gross NO Conversion in NO-H2, NO-H2-CO and NOCO Reactions over Pt and Pt-Mo03 Catalysts supported on 1-A1203
Catalysts Pt (0.25%) Pt- Mo03 (Pt-0.25%, M o ~ 2 % ) Feedgas Concentrations
From Ref.
[3 ]
T50% 60 175 NO CO H2
NO-H2 T80% 100 185 0.1% 0 1%
NO-H2- CO
T5O% 180% 360 250 NO CO H2
500 350 0.1% 0.75% 0.25%
NO-CO T50% T80% NR NR 345 360 NO CO H2
0.1% 1%
0
205
c.
A. 2180 em-I
2120em- 1 E.
2120 crn" 2120 em-I
2080 em-I
2080 crn"
Fig.5 - IR data of CO chemisorbed on a Pt-Mo/(-A1 catalyst.reom Ref.(8). 20 3 (A) Fresh sample. (B)Pre-reduced at 300 C with 60 torr CO. (C) Exposed to air at 25 C.(D)Heated in air at 100 C for 20 min. (E) Heated in air at 300 C for 20 min. IR Peaks assignment: 2180 em-I_CO on PtO;2120 em-I_CO on Pt; 2080 cm-I_CO on M00 Mo+4 ) . 2(
100 0
N
Z
80
N
Z
t 0
z
60
>-
'= >
40
-<-"'/
I0
~O/
w 20 w
.....l ({)
~
~
0' 200
/ I 300
/
,CO
~ 7 ~
l
I,O-~
400
500
TEMPERATURE
600
700
r-c:
Fig.6 - Selectivity as a function of temperature for NO conversion over a P t / ~ - A I 2 0 3
and a Pt-Mo0
3/(-A1203
catalyst.From Ref.(ll).
The effect on selectivity in the reduction of NO by hydrogen, especially in the presence of CO,is still more dramatic.In this context the selectivity is defined as that leading to products in which the N-N bond has been formed such
206 as N2 or N20.
The formation of such a bond depends on the probability of
forming adjacent pairs of surface entities containing nitrogen.
Fig. 6 [11]
shows the enhanced selectivity to the desired product due to incorporation of Mo.
It was shown in another publication that partially reduced surface Mo iuns
adsorb geminal pairs of NO molecules (cis-dimers)
[12].
This, and the absence
of poisoning by CO shown in Table I, are the reason for the observed, enhanced formation of N-N bonds in the Pt-Mo/A1203 catalysts.
Dispersion of Noble Metals and Structure-Sensitive Reactions A number of structure-sensitive reactions take place in the catalytic converter which, as expected, will be influenced by the noble metal dispersion. These include first of all the very important oxidation of saturated hydrocarbons. It is an accepted view that whenever the surface reaction involves the scission of a C-C bond structure-sensitivity is to be expected. There is ample evidence that oxidation of saturated hydrocarbons, especially those of short chain length, does not proceed readily on Pt catalysts with very high dispersion. 100
Fig. 7 shows directly that the activity for the oxidation of
80 ;!.
propane of a catalyst containing
z
0.07%Pt/y-A1203 is strongly
0
(J)
0:: W
60
inhibited by the addition of 3.7%
>
Ce02.
Z
0
U
w
z
maintain the high initial dispersion
40
and to prevent the agglomeration of
0-
0:: 0-
The influence of ceria is to
Pt into discrete particles.
20
While
this would enhance the structure insensitive reactions such as the 200
300
400
500
TEMPERATURE (Oe)
Fig. 7 - Conversion of propane on a 0.07%
oxidation of
co
and nonsaturated
hydrocarbons it does strongly inhibit the oxidation of saturated
Pt catalyst supported on 7-A1203 with
hydrocarbons.
and without 3.7% ceria; 1000 ppm C3H8'
molybdena is the additive which
Similarly, when
2% 02.
stabilizes the dispersion of Pt, the oxidation of propane is strongly inhibited. (Table 2)
[8J.
207
TABLE 2 Propane Oxidation over Pt, Pt-Mo03 Catalysts
Temperature at Given Conversion
T5 0 %
T2 5 %
Catalyst
T 75 %
Pt (0.2%)
230
267
298
Pt- Mo03
458
640
NR
(Pt~0.25%,
Mo~2%)
Feedgas
C3 H8
0.05%
Concentrations:
02
2.25%
S02
20 ppm
N2
Balance
S.V.
60,000 hr- l
From Ref.
[8]
Other noble metals such as Pd or Rh which have more stable oxides than Pt and therefore tend to remain well-dispersed on ,-A1203 even in the absence of additives such as ceria or molybdena are usually poor catalysts for the oxidation of saturated hydrocarbons.
In fact,
in a mixture of hydrocarbons
containing 2/3 propylene and 1/3 propane the propylene will be easily oxidized <300'C while the propane remains untouched even at 600'C [13]. Another structure-sensitive reaction which is important in an indirect way to the behavior of the automotive catalyst is the oxidation of S02. average sulfur content of gasoline in the U.S. converts to -20 ppm in the exhaust stream. European countries.
The
is 300 ppm by weight which
It may vary in a wide range in the
Although sulfur compounds fall in the U.S. under the
category of unregulated emissions the oxidation of S02 to S03' or conversely its reduction under certain conditions to H2S, has environmental and customer acceptance implications and it might affect the durability of the catalytic converter by modifying the poisoning resistance, in particular by lead as will be shown below. Table 3 shows the suppression of S02 oxidation by stabilizing the Pt dispersion by ceria.
This behavior is almost completely analogous to that
observed in the case of the oxidation of propane by molybdena.
Indeed,
stabilization of Pt dispersion by Mo03 similarly inhibits S02 oxidation [8].
208
TABLE 3 S02 Oxidation Over Pt/l-A1203 and Pt-Ce02/1-A1203 % S02 Conversion at TOC 400
450
Pt (0.07 wt %)
80
88
92
82
Pt (0.07 wt %) + Ce (3.7 wt%)
52
54
55
57
Feedgas:
500
550
60,000h- 1
S02 0.02%, 02 1%, S.V.
Most importantly, the presence of sulfur oxides has a rather pronounced influence on the oxidation of saturated hydrocarbons and is intuitively unexpected.
Thus, the presence of 20 ppm of S02 in a stream of 500 ppm propane
in nitrogen carrier gas increases considerably the activity of a Pt/l-A1203 catalyst for propane oxidation. Fig. 8 [14] shows that in thepresence of S02 a catalyst
100 /
80 0
~ <3
60
-r
40
"
20
,
• V 70wt%P1 ... f:>.O.06.t%Pt .oO.03wt%Pf
,.j
/
/
,
,
I
I
/
r /
)7
/
4i
t!
t"
/ /
I
/
active as a catalyst containing 7% Pt, lowering the temperature of
P
J
50% conversion of the 0.03% Pt
c!
catalyst from 500'C to 250°C.
/ /
/
I
?
/
/
/
I
U'
r
/
Ii r
I
Y
I
containing 0.03% Pt becomes as
/
/
/'
P
, .d
This change of activity is
/0/
attributed to the formation of a
/£0_-0.-0--
00
400
600
surface sulfate group by S02
Temperofllre,'"C
adsorption and subsequent oxidaFig. S - Percentage conversion as a·func-
tion at 200°C.
tion of temperature for C3HS oxidation
shows that surface sulfates
The IR study [14]
over three Pt/l-A1203 catalysts of
promote the dissociative adsorp-
different Pt concentrations.
tion of C3HS on Pt leading to a
From Ref.
higher propane oxidation
[14].
activity. An important issue of structure sensitivity has to do with the oxidation of methane.
Although methane does not have a C-C bond to be cleaved, it is the
hydrocarbon most difficult to oxidize.
There are some indications that methane
oxidation may be structure-sensitive which will be studied further.
The
oxidation of this rather unreactive molecule is of practical importance, since there are proposals to lower the allowable hydrocarbon emissions that cannot be met without at least partial oxidation of methane.
209 Relation between Reactivity and Poison Resistance It turns out that the reactivity of the catalyst in structure sensitive reactions may have a large significance in determining the resistance to poisoning in particular by lead, and therefore can influence the catalyst durability and the ability to fulfill the regulatory requirements of useful lifetime. Although vehicles equipped by catalytic converters are fueled, by law, by lead-free gasoline the residual amount of lead can have quite a pronounced deactiving influence.
Also the effect of accidental misfueling can be
detrimental. It is well known that the three noble metals used in automotive converters have a widely disparate resistance to deactivation by lead.
The
most sensitive to deactivation is Pd. Fig. 9 [15] shows the extreme sensitivity of Pd catalyst to the trace lead levels in the fuel in the 0.22 "10 Pd CATALYST
range from 0 to 12 mgPb/gallon (equivalent to 3
S.V. -60,000 h·1
T .550·C
80 ill
0:
OJ
mgPb/l).
It is worth noting
that the present legal limit
z o
mq
Pb
,aI
60
in the
> Z o U o 40
u.s.
and West Germany
is 50 mgPb/gallon.
z
o
;!.
But it
should also be noted that the actual contaminant levels in
20
the
u.s
are considerably
lower, 2-3 mg Pb/gallon, that 1.0
1.2
1.4
1.6
is within the range shown for
REDOX RATIO, R
the data on Fig. 9. Fig. 9 - Effect of trace Pb levels on the
Fig 9
shows an extraordinary
steady-state NO activity of 0.22% Pd after
sensitivity of the catalytic
-15,000 simulated miles of pulsator aging at
activity to the lead levels
R - 1.3.
and the experiment resolves
From Ref.
[15].
clearly between minute increments of the lead in the fuel.
While the data in Fig. 9 refer only to the
loss of activity for NO reduction a similar trend is observed for hydrocarbon oxidation [15J.
The sensitivity of Pd to deactivation by traces of lead is thE
main reason why this relatively abundant and cheap noble metal is generally not used extensively in place of Pt, in particular in the first converter of a dual bed system.
210 The experience of automotive catalysis indicates that Rh is only somewhat less susceptible to poisoning by lead traces than Pd while Pt is by far thc most resistant. The use of model systems amenable to detailed surface analysis provides a means for the direct examination of the association of lead wih the surface of noble metals [16].
It immediately becomes apparent that in all the three
supported noble metals the lead is directly associated with the noble metal sites and not with the support material, which in actual catalyst constitutes over 95% of the exposed BET area.
This is shown 0:. Fig. 10 [16J, for Pt
supported on A1203, from the electron probe elemental maps.
The Pt and Pb maps
of samples exposed to simulated exhaust generated from combustion of iso-octane fuel containing 1.5 g Pb/gallon and 0.03 wt%S are exactly superimposed.
The
same obtains whether the support is 1-A1203, Ti02 or Zr02 on one hand or whether the metal is Pt, Rh or Pd.
Fig. 10 - Electron probe elemental map after Pb exposure for 24 h at 700'C for Pt supported on 1-A1203' From Ref.
[16].
211 Nevertheless, Pt is much more resistant than the other noble metals to lead poisoning and the reason for this is largely indirect. amount of sulfur acts as a scavenger for the lead.
Thus the small
To achieve this it is
necessary that the sulfur be in its hexavalent oxidation state to combine with lead oxide to form a stable lead sulfate which in itself is not a site-specific poison.
Only Pt, among the noble metals is a good catalyst for the oxidation
of S02 to S03 [17J and indeed on a Pt catalyst the lead is present as the sulfate as shown in Fig. 11.
It is clear that large amounts of lead sulfate
present in several overlayers will also act as a non site-specific poison by obstructing the access of the
reactants to the surface.
We have established
that in Rh-catalysts the lead is present as an oxide and in the case of Pd catalysts as an intermetallic compound with the Pd [16]. In all cases the association of the lead is
100
specific with the noble metal because the lead-carrying
... o N
:;;
molecules, most probably oxy-
80
halides, decompose on the noble metals sites leaving the lead on the surface.
<5 en .Q
60 >f--
...
Cf)
0
Z
a. --S: 0"
en
w
'"
f-Z
.Q
0.
40
Table 4 highlights the specificity of this association showing the relative lead counts in microprobe analysis when the same samples of model catalysts of
20
Pt, Pd, Rh supported on A1203' Ti02 or Zr02 are exposed to a combustion gas in which the lead was
28
originally present either as
Fig. 11 - X-ray diffraction pattern of
"motor mix" i.e. tetraethyl
Pt/1-A1203 after Pb exposure for 72 h at
lead with dibromide or
700·C.
dichloride scavengers or, in
From Ref.
[16].
one case, as Pb0 2 vapor in the exhaust. than two orders of magnitude difference in the
There is more
amount of lead deposited on the
noble metal as compared with that deposited on the bare support.
The
difference when the lead-carrying species is the lead oxide is much smaller and may be insignificant.
212
TABLE 4 Pb Affinity for Noble Metals (NM) and Various Supports Pb (counts s-l)a
a
NM
NM/ A1203
NM/Ti02
NM/ Zr02
Pt
758/6
1140/2
980/6
740/l(40/2l)b
895/7
989/1
246/8
Pd
344/10
Rh
896/6
Semiquantitative microprobe analysis: average over 10 areas of 100
~m
x
100Mm size; 20 KV beam energy; 20 s counting time; Pb present in isooctane as TEL Motor Mix (TEL+EDB+EDC scavengers). b
Pb present as Pb0 2 vapor in iso-octane exhaust (EDB and EDC scavengers absent).
The specificity of the association of lead which derives from the gasoline with noble metal sites on the surface of the catalyst is the reason that minute amounts are still quite detrimental as shown most clearly for Pd catalysts in Fig. 9.
CONCLUDING REMARKS The foregoing has made it abundantly clear that the automotive catalyst in itself is a very complex chemical system and becomes even more so when all the subtle interactions with the exhaust environment are taken into account. Relatively minor fuel constituents such as the always present sulfur or small amounts of halides may have a pronounced effect on its overall behavior.
By no
means has the preceding been a complete account of all the possible interactions.
Thus we have omitted the important effects of possible alloy formation
between the active metals [18, 19J and the various deactivating influences deriving from automotive lubricants, the most important being the effect of phosphorus [20].
Further, quite often unexpected contaminants may do severe
harm to the emission hardware [21]. The designer of the automotive catalyst has to take all these into account as well as the expected physical environment, the most important being the driving conditions which will determine the temperature of the device. In an optimal catalyst each precious metal has a specific function to perform, such as Rh for nitric oxide reduction, Pt for the oxidation of
213 salurated hydrocarbons, etc.
In choosing the proper support and its modifiers
for each of the noble metals one has to bear in mind what is the desired dispersion and one has to balance the utilization of the noble metal, that is the proportion available for the surface reaction, versus the probability of the irreversible interaction with the support which results in permanent loss during use.
Further, one has to consider the proper ratios of the noble metals
and the advisability of having them in close contact or separated. Although the development of modern automotive catalysts started about twenty years ago and they have been in use for more than 10 years, there still remains ample room for improvement and better utilization of the scarce noble metals.
This can only be achieved by acquiring more knowledge through well-
directed research. The driving force for this will be on the one hand more strict environmental regulations as now witnessed in California, and on the other, the ever widening environmental concerns in varying parts of the world.
REFERENCES 1 2 3
4 5 6 7 8
9 10 11
12
13 14 15 16
G.P. Gross, W.F. Biller, D.F. Greene and K.K. Kearby, U.S. Patent 3,370,914. J.H. Jones, J.T. Kummer, K. Otto, M. Shelef and E.E. Weaver, Env. Sci. & Tech., 2 (1971) 790-98. H.C. Yao, H.S. Gandhi and M. Shelef, "Metal Support and Metal Additive Effects in Catalysts", B. Imelik (Ed.), ElseVier, Amsterdam, 1982, pp. 159-169. H.C. Yao, H.K. Stepien and H.S. Gandhi, J. of Catalysis, 61 (1980), 54750. H.K. Stepien, W.B. Williamson and H.S. Gandhi, SAE Paper 800843, Dearborn, MI, 1980. H.S. Gandhi, J.T. Kummer, M. Shelef, H.K. Stepien, and H.C. Yao, U.S. Patent 4,233,189. J.E. deVries, H.C. Yao, R.J. Baird, and H.S. Gandhi, J. of Catalysis, 84 (1983), 8-14. H.S. Gandhi, H.C. Yao and H.K. Stepien, Am. Chern. Soc. Symp. Series, No. 178, "Catalysis Under Transient Conditions", A.T. Bell and L.L. Hegedus (Eds), 1982 pp. 143-162. S. Sakellson, G.L. Haller and H.S. Gandhi, personal communication. A.S. Sass, A.V. Kuznetsov, V.A. Shvets, G.A. Savel'eva, N.M. Popova and V.B. Kazanskii, Kinetika i Kataliz, 26 (1985) 1411-15. H.C. Yao, K.M. Adams and H.S. Gandhi in "Frontiers' in Chemical Reaction Engineering", L.K. Doraiswamy and R.A. Mashelkar (Eds.), Wiley Eastern, New Delhi, 1984, pp. 129-141. H.C. Yao and W.G. Rothschild, "Proc. 4th. Int. Conf. on the Chemistry of Molybdenum", H.F. Barry and P.C.H. Mitchell (Eds.), Golden, Colorado, 1982. W.B. Williamson, H.K. Stepien and H.S. Gandhi, Env. Sci. & Technology, 14 (1980), 319-25. H.C. Yao, H.K. Stepien and H.S. Gandhi, J. of Catalysis, 67 (1981), 23136. W.B. Williamson, D. Lewis, J. Perry and H.S. Gandhi, Ind. Eng. Chern., Product R&D, 23 (1984), 531-36. H.S. Gandhi, W.B. Williamson, E.M. Logothetis, J. Tabock, C. Peters, M.D. Hurley and M. Shelef, Surface and Interface Anal., Q (1984) 148-61.
214 17 18 19 20 21
H.S. Gandhi, H.C. Yao, H.K. Stepien and M. Shelef, SAE Paper 780606, Special Publication (SP43l), 1978. W.B. Williamson, H.S. Gandhi, P. Wynblatt, T.J. Truex and R.C. Ku, AICIIE Symposium Series, No. 201, (1980) p. 212. B.M. Joshi, H.S. Gandhi and M. Shelef, Surface Technology, in press, 1986. W.B. Williamson, J. Perry, R.L. Goss, H.S. Gandhi and R.E. Beason, SAE Paper 841406, Baltimore, MD, 1984. H.S. Gandhi, W.B. Williamson, R.L. Goss, L.A. Marcotty and D. Lewis, SAE Paper 860565, Detroit, MI, 1986.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
21.5
@)
MECHANISMS OF THE CARBON MONOXIDE OXIDATION AND NITRIC OXIDE REDUCTION REACTIONS OVER SINGLE CRYSTAL AND SUPPORTED RHODIUM CATALYSTS: HIGH PRESSURE RATES EXPLAINED USING ULTRAHIGH VACUUM SURFACE SCIENCE GALEN B. FISHER, SE H. OH,
~OYCE
+
E. CARPENTER, CRAIG L. DiMAGGIO, AND
STEVEN J. SCHMIEG Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090-9055 (U.S.A.) D. WAYNE GOODMAN Surface Science Division, Sandia National Laboratories, Al buquer que, New Mexico 87185 (U. s. A.) THATCHER W. ROOT*, SCOTT B. SCHWARTZ**, AND LANNY D. SCHMIDT Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (U.S.A.)
ABSTRACT The demonstration that surface parameters obtained in ultrahigh vacuum (UHV) experiments are applicable to high pressure catalytic reactions has long been a goal of catalytic surface science studies. This report summarizes a set of work which has successfully shown, for carbon monoxide oxidation and nitric oxide reduction over rhodium, that high pressure rates can be predicted quantitatively using parameters determined solely under ultrahigh vacuum conditions. One implication of this work is that, for this important class of reactions, the strongly-bound surface species present under the condi tions of UHV studies are the same species reacting at high pressures. INTRODUCTION An effort has been made in this work to evaluate the utility of surface parameters determined in UHV surface science experiments for understanding the high pressure kinetics of certain catalytic reactions.
We have chosen two
test reactions of considerable significance in automotive exhaust catalysis, CO oxidation (2CO rhodium.
+ O ~ 2C0 and NO reduction (2CO + 2 N O ~ 2C0 + N over 2 2) 2) 2 To accomplish this comparison, rate constants for the elementary
steps of both reactions were determined under ultrahigh vacuum conditions. +
Present Address: AC Spark Plug Division, General Motors Corporation, Flint, Michigan 48556. *Present Address: Chemical Engineering Department, University of Wisconsin, Madison, Wisconsin 53706. **Present Address: Sherwin-Williams Co. Research Center, 10909 South Cottage Grove, Chicago, Illinois 60628.
216
Then, steady state rates for each reaction were measured over both single crystal and supported catalysts at realistic, high pressures (1-300 Torr). The use of the UHV-determined parameters in kinetic models based on the surface chemistry studies is successful in predicting quantitatively the rate data taken at high pressures for both reactions. ULTRAHIGH VACUUM AND HIGH PRESSURE SURFACE CHEMISTRY STUDIES To begin wi th, the adsorption properties, activation energies for desorption and dissociation, the orientation, and the binding sites for chemisorbed nitric oxide, carbon monoxide, and oxygen were characterized on the single crystal Rh(111) surface with high resolution electron energy loss spectroscopy (EELS), UPS, XPS, LEED, and temperature programmed reaction spectroscopy (TPRS) [1-6J.
For example, we have found the useful results that the activa-
tion energy for NO dissociation on Rh(111) is 19 ± 1 kcal/mole Rh(100) is 18 ± 1 kcal/mole [6J.
[~J
and on
We've also observed that adsorbed NO and CO
form well-mixed surface layers near reaction temperatures [5J, and that the heat of adsorption for CO on Rh(111) is reduced by 8-10 kcal/mole in the presence of nitrogen atoms [3].
In addition, steady state kinetic studies of -5 -8 -10 Torr)
both reactions on Rh(111) were carried out at low pressures (10 [7,8J and high pressures (1-300iorr).
The high pressure results have been
compared with results over supported Rh catalysts for the same reactions which were measured for the same temperatures and pressures [9J.
Finally, we have
found that rate expressions based on UHV-determined elementary intermediate steps using UHV-determined rate constants quantitatively predict the rates at high pressures for both the CO-0 and supported Rh catalysts.
and NO-CO reactions over single crystal Rh 2 This is the first time we are aware that high
pressure catalytic reaction rates have been predicted solely from UHV-determined experimental parameters.
The success of these predictions based on UHV
work shows, for an important class of reactions, that the strongly-bound species present under the conditions of UHV studies are the same species reacting at high pressures. CARBON MONOXIDE OXIDATION More particularly for the eo-0 reaction, we have measured the reaction 2 rate over Rh(111) for a wide range of pressures around p(eO) ~ P(02) ~ 0.01 a t.m , , pressures similar to those found in automoti ve exhaust, and for temperatures between 450 K and 600 K.
These data are shown in Fig. 1.
is first order in oxygen and negative first order in CO.
The reaction
From 450 K to 600 K
the reaction rate increases by almost four orders of magnitude and is characterized by a single activation energy (29 kcal/mole).
We find excellent
agreement between the specific rates and acti vation energies measured for a
217
1000
Pco
P
:=
02
:=
0.01 atm
• Rh(lll} 1------1 Rh/AI 20 3 ............ Model Q)
'"
100
-C
a::
-,
'" Q)
::J
.. ~
U Q)
\\
o
E
10
\.
N
o
\.
U
'Co ~.
x. ~.
,.,.
i··..
.... Q)
>
o c....
::J
I-
0.1 L-_J--_...L-_..J-_-.L._--'-_--"'_ 1.8 2.0 2.2 1.6 1OOO/T (K- 1)
Fig. 1. Comparisons of the specific rates of the CO-0 reaction measured over 2 Rh(lll) and Rh/A1 at P(CO) = P(02) = 0.01 atm. from Ref. 9. The model 20 prediction fits qUarltitatively with the measured rate data for both catalysts. Rh(111) crystal and a 0.01 wt% Rh/A1
catalyst, an indication of a struc20 3 ture-insensitive reaction. The elementary steps which were used to model the CO oxidation reaction based on the rate constants measured in UHV surface chemistry studies are as follows: CO (g) ;::::' COra) °2 (g)
20(a)
CO(a) As is shown in Fig. 1. we are able to predict the measured absolute rates and activation energies using a kinetic model only employing parameters determined experimentally in UHV studies [9J.
In fact. the same rate expression used
successfully at high pressures predicts the CO-0 pressures (-10
-8
2
reaction rate
~t
much lower
Torr) and at lower temperatures «400 K) where the CO
218
coverage is approximately the same as at high pressures [7].
Because the
reaction rate essentially depends only on reactant surface coverages, our understanding of CO oxidation clearly bridges the "pressure gap". of the CO-0
The picture
reaction which is confirmed by this work is that the Rh surface
2 is predominantly covered by adsorbed CO and the reaction is limited by the
rate of CO desorption (Eq. 1) or, in other words, the rate of creation of a vacant site, where oxygen adsorption (Eq. 2) and subsequent reaction (Eq. 3) can occur. NITRIC OXIDE REDUCTION For the NO-CO reaction over Rh(111) at high pressures, we find that the reaction is positi ve order in NO and surprisingly is zero order in CO.
As is
shown in Fig. 2, from 500 K to 650 K the reaction has an activation energy close to 30 kcal/mole. with nitrogen atoms.
After reaction the Rh(111) surface is nearly covered (The nitrogen atom coverage is also high near the rate
maximum in low pressure studies [8J.)
The elementary steps which were used to
model the NO-CO reaction shown below were also chosen based on the UHV measurements of the rate constants of each step.
-. CO (a) CO(g) <-. NO(a) NO(g)
N(a)
COra)
+
Ora)
NO(a)
+
N(a)
(4) (5) +
Ora)
(6)
CO
(7)
2(g) N + Ora) 2(g)
(8)
2N(a) -. N 2(g)
(9)
Using a rate expression based on these steps for the NO-CO reaction, we were again able to predict the observed absolute rates and activation energies at high pressures over Rh(111) (see Fig. 2) using only UHV-determined elementary steps and rate parameters [9 J.
The explanation of the unexpe cted pr ess ure
dependence of CO at high pressures is the UHV result [3J that adsorbed nitrogen atoms reduce the CO heat of adsorption in Eq. 4 sufficiently to give a zero order CO pressure dependence.
In fact, the rate of the NO reduction
reaction over Rh(11 1) is controlled by nitrogen atom recombination and desorption (Eq. 9). In contrast with CO oxidation over the Rh/A1 over Rh/A1
20 3
catalyst, the NO-CO reaction 20 3 is structure-sensitive and has a lower rate and a higher
219
500 Pea
•
•
•
u:; 100
-
= PNO = 001 atm
Q)
Rh (111) Rh/AI 20 3 Model
( j)
L
a:
<, (j)
Q)
::J
o
Q)
\
10
0
\
I I
\
E
\ \ \
N
0
I
~
\
\
~
Q)
\ \
..Cl
E
I \
::J
•
I
Z
\
~
Q)
> 0 c ~
::J
l-
•
0.1
1.4
1.8 1) 1OOO/T (K-
1.6
2.0
Fig. 2. Comparison of the specific rates of the CO-NO reaction measured over Rh(ll 1) and Rh/AI 0 , at p(CO) = P(NO) = 0.01 atm. from Ref. 9. The prediction 2 of the surface chemrstry model follows closely the rate data for Rh(lll). activation energy at a 1:1 NO:CO ratio (45 kcal/mole) than over the single crystal catalyst. This is also clearly seen in Fig. 2. Over Rh/Al we 20 3 conclude that the NO-CO reaction is limited by the rate of nitric oxide dissociation, Which is about 2000 times slower on Rh/Al
203
than on Rh single crys-
tals [9J. CONCLUSIONS The knowledge from this work of the relative importance of the elementary steps of the CO-0 and NO-CO reactions clarifies which steps need modification 2 to increase overall reaction rates. Changes in the supported catalyst which increase the NO dissociation rate under NO-CO reaction conditions shOUld provide one path to a better overall reaction rate.
Studies which can clarify
the origin of the structure sensitivity of the NO-CO reaction over Rh will
220
also be helpful.
A fuller understanding of both the CO oxidation and NO re-
duction reactions will hopefully lead to the more effective use of rhodium in automoti ve exhaust catalysi s . REFERENCES 1 2 3 4 5 6 7 8 9
G. B. Fisher and S. J. Schmieg, J. Vac. Sci. Tech. A 1 (1983) 1064. T. W. Root, L. D. Schmidt and G. B. Fisher, Surface Sci. 134 (1983) 30. T. W. Root, L. D. Schmidt and G. B. Fisher, Surface Sci. 150 (1985) 173. T. W. Root, G. B. Fisher and L. D. Schmidt, J. Chem. Phys-.--(1986), in press. T. W. Root, G. B. Fisher and L. D. Schmidt, J. Chem. Phys. (1986), in press. G. B. Fisher, C. L. DiMaggio and S. J. Schmieg, in preparation. S. B. Schwartz, L. D. Schmidt and G. B. Fisher, J. Phys. Chem , (1986), in press. S. B. Schwartz, G. B. Fisher and L. D. Schmidt, in preparation. S. H. Oh, G. B. Fisher, J. E. Carpenter and D. W. Goodman, J. Catalysis 100 (1986) 360.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
221
ELECTRONIC STATE OF CERIUM-BASED CAT AL YSTS STUDIED BY SPECTROSCOPIC METHODS (XPS, XAS) 1 F. LE NORMAND 1,2, P. BERNHARDT 1, L. HILAIRE 1, K. KIll , G. KRILL 3 and 1 G. MAIRE 1Laboratoire de Catalyse et Chimie des Surfaces, Univers i t e Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France 2present address: L.U.R.E., Batiment 209D, Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France 3Laboratoire de Physique des Solides, Univer si t e de Nancy I, BP 239, 54506 Vandoeuvre-Les-Nancy Cedex, France
ABSTRACT X-Ray Photoelectron Spectroscopy (XPS) 0 f the 3d core level of cerium and X-Ray Absorption Spectroscopy (XAS) of the L absorption edge of cerium have been used to m study Pd/Ce0 Pd-Cel y AI and Ce/y Al catalysts. The oxidation state of ce2, Z03 3 rium was Fourid to decrease wlr h decreasing amounts of cerium on the surface. It was quite close to III for very low contents of cerium (Z-3%). For higher cerium contents the oxidation state was nearer to IV but differences between the two methods were found, owing to the fact that XAS is a volume sensitive probe. The oxidation state of cerium waiilalso lower for Pd-Cel y AI 20 than for Cel y AIZO y suggesting the forma3 tion of Ce OCI, chlorine coming from The precursor salt of palladium.
z0
INTRODUCTION It has been proved that cerium is a very efficient additive in catalysts for the con-
trol of automobile pollutants. According to literature data (ref. 1,Z), cerium in these catalysts acts as - a promotor of the catalytic activity for redox cycles; thus cerium can provide oxygen during the oxidative step of the cycle and remove it during the reductive step; this is the well- known oxygen storage capacity (O.S.C.) of cerium. - a stabilizer agent inhibiting the loss of y AI
Z0 3
surface area
- a dispersive agent for the transition metal. In view of the chemical properties of ceria, two questions arise. First, while the stable ceria CeO
is well known to be very difficult to reduce (ref. 3), the O.S.c. implies Z that it can be easily reduced and oxidized at moderate temperatures. Second, the nature of the interaction between the rare earth and the transition metal is not wellestablished : formation of an alloy (ref. 1,4), of an ionic or covalent chemical bond (ref.
5), decoration of a transition metal particle (ref. 6)... We have applied X-Ray photoelectron spectroscopy (X.P.S.) of the 3d core level of
222 cerium and X-Ray absorption spectroscopy (X.A.S.)
absorption edge of ceIII Pd-Ce/y AIZO) and Ce/y AlZO) catalysts. These probes can give us 0
f the L
rium to Pd/CeO Z' a description of the cerium electronic configuration, which can be linked to : - its chemical nature (metallic, ionic, ... ) . compoun d "OXI d crron . - f or an . IOniC , Its state (C e III or C e IV) The parameters investigated here are : - influence of the cerium content - influence of the presence of a transition metal.
EXPERIMENT AL Catalysts were prepared by coimpregnation of Pd(NH))4 CI
and Ce(NO))) on 2 support (42m'/g) was pre-
yAIZO) WOELM (15)m'/g). For the Pd/CeO sample, Ce0 Z 2 pared by precipitation of Ce (NO))) into hydroxide at pH = 9, followed by calcination
(5 hr s, 550°C). Each catalyst, after wetting, was calcined at ZOO°C, 4 hours, then reduced up to 400°C (4°C/mn, then 1 hour )OO°C), and passivated under nitrogen before spectroscopic investigations (Table 1).
TABLE 1 Characteristics of the different Pd-Ce/y AI
Catalysts n?
I - 1 I - 2 I - 3 I - 4 I - 5 I - 6 I - 7 I - 8 II - 1 II - 2 II - 3 II - 4 II - 5
(%Pd)wt
9.) 8.1 6.9 7.8 8.5 8.5 7.0 6.4
20)
with Pd(NH))4CI2 as precursor salt.
(%Ce)wt
BET surface area (m'/g)
0 0.02 0.3) 0.52 1.0
1.5 3.2 12.5 0.7
110 126 118 133 82 74 79
1.5 2.6 10.3 12.3
In series I, the cerium content was varied from 0.3 to 12.4 weight %. Series II includes Ce/ y AI
samples from 0.7 to 12.3 weight %. The palladium loading was main203 tained roughly constant at 8.0 ~ 1.5% weight %, for spectroscopic investigations. X.A.S. experiments were achieved using the synchrotron facility of the EXAFS III
spectrometer at LURE, Orsay, France with a Si(200) monochromator, or on an inlab spectrometer using a silver Rigaku anode and a Si(311 ) monochromator. Other details on XPS and XAS experiments have been reported elsewhere (ref. 7).
223 RESUL TS AND DISCUSSION 1) Influence of cerium content The 3d core level spectra of Pd-Ce/ 'l AI 0 catalysts are compared to -the CeO Z 3 Z spectrum (see fig. 1). We referred the binding energies to Al Zp at 119.8 eV. For CeO Z' some Al 0 was added mechanically. Apart from the spin-orbit coupling, the 3d 5/Z Z 3 transition of cerium exhibits three main contributions for ceria. We observed two transitions for the cerium based catalysts, as previously reported in the literature (ref. 8,9) III for Ce spectra. On the other hand we note l) A shift of the v' line of about Z.5 eV 94f 1 towards lower binding energies (line v"). This line is currently assigned to the 3d final state as the major contribution (ref. 10,11). ll ) The disappearance of the well cha94fO racterized u'" line, due to the 3d transition. Since there is no possible overlap for 94fO this transition, we could calculate the 3d spectral weight, which is an indication of IV the Ce oxidation state (see fig. 4). However this value must be considered with some caution since the probe introduces strong final state effects which results in a transfer of valence electrons to localized 4f states for the screening of the core hole (ref. 10,11). Finally we observe no shoulder at low binding energy (879-880 eV), characteristic of an alloy. Thus we must conclude that at the surface of the catalyst and under our treatment conditions, cerium is in an ionic form and preferentially in the +III oxidation state. edge spectra of a few Pd-Ce/ 'lAl 0 and Pd/CeO caz 3 Z III talysts. Intensity transitions are normalized to transitions in the continuum states. In
We report in fig. Z the XAS L
addition, we report in fig. 3 the deconvolution of CeO and Ce(OH)3 which exhibit the Z III IV difference between a Ce and a Ce compound. Ceria exhibits five satellites and III Ce(OH)3' like all Ce compounds (ref. lZ, 13, 14) only one which corresponds to B line. 1 1 III The B transitions are assigned to Zp5 4f 5d final states (Ce oxidation state) and the Z IV C transitions to Zp5 4fO (and 4f ?) 5d final states (Ce oxidation state). The doublets of the lines Band C are probably due to crystal field effects. As for XPS, we are able to calculate the 2p5 4fO 5d spectral weight from the C transitions. Note that in the case of X-Ray absorption, final state effects are of less importance due to the participation of the 5d photoelectron to the screening of the core hole. In good agreement with previously reported XPS and XAS spectra, the two methods give the same value of the spectral weights for pure CeO
(ref. 15). Z For the Pd-Ce/ y Al 0 catalysts, we observe a continuous evolution from the complex Z 3 spectrum of ceria to a strong single transition B as the cerium content decreases. Mo1 reover the narrow half width of this transition (co 3.0 eV) strongly suggests that cerium is in a ionic form. Hence, in agreement with our XPS results, cerium is in a strongly reduced state for low cerium content. However for cerium content higher than co Z-3%, discrepancies appear between the two methods, as evidenced by the respective spectral weights (see fig. 4).
224
!d
, , 4f
1.·
(ivnid
I A.U
II \
eQ13_o;;2)0
BE
i-ig. 1 : XPS 3d core level of Pd-Ce/y AI
\0
00
9
00
8
00
7
00
6
00
5
00
...
00
3
00
2
ee
\
00
006.
eV
In
Z0 3.
Influence of the cerium content.
ASS
Pd-Ce Pd-Ce Pd-Ce
0.3/ AI 20] 0.5/ AI20] 3.2/ A1 20]
Pd-Ce 12.5/ AI Pd 8.4 ICeD 2 Ce0
5733
fGIERGY
Figo Z
XAS L
m edge
of Pd-Ce/ y AI
Z0 3
00
5750
CEV)
0
Influence of the cerium content.
2
88
20]
225
5110
~730
5750
!OrgV)
Fig.3: Fit of XAS LI I I edge for the CeIV02
c 1 .~
•
%
•
Z v'"
and Ce
III
(OB)3.
l2
J0
Cerium
10
Content %
8.47. Pd/Ce0
2
15
Fig 4:Spectral weight,determined froJT,leXPS(3d 9 4f 0 land. XAS(,2p 5 4f 0 5d) final states,as a function of cerium loading.
226 As XAS is a volume sensitive probe, this suggests that ceria is growing up tridimensionnally on the support.
Z) Influence of the presence of a transition metal Some XAS results of Cel y Al are reported in Fig. 5. As for the Pd-Cel y AI Z0 3 Z0 3 catalysts, we note that for very low cerium content the spectra are indicative of a 3+ strongly +111 reduced state of cerium. Probably for these concentrations, Ce cations occupy vacant cationic sites of y A1 However for higher cerium content, a careful 203. examination (compare for example 6.4% Pd - 12.5% Cel y AI (N°I-8) and 1Z.3% Cel Z0 3 (nOII-5) suggests a difference between the two series. Cerium is in a more oxiy Al Z0 3 dized state for the Cel y AI series. This could be related to the formation of an Z0 3 oxychloride CeIIlOCI, as evidenced by XRD, in the case of the Pd-Ce/y AI cataZ0 3 lysts. The chloride anion comes from the palladium precursor salt. Probably also some interaction between the transition metal and the rare earth occurs, resulting in a partial electronic transfer from transition metal to the rare earth (ref. 7). UA
BE
Fig. 5 : XAS on the Ce L
In
eV
edge of Ce/y A1 20 3. IIl
CONCLUSION XAS and XPS have proved to be powerful probes for the study of the electronic configuration of cerium based catalysts. Thus they can give us useful informations on the nature of the cerium compounds. In the case of Pd-Cel y AI y AI
Z0 3
catalysts we have evidenced:
(chloride) and Cel
Z0 3 - a reduction of the cerium oxidation state as its content decrease on the surface of the catalyst. - a different behaviaur for the "surface" and the "volume" cerium.
227
We think that these properties can be related to an occupation of AI
3
+
vacant lattice
sites for low Ce loading. - a chemical reaction giving the oxychloride CeIIlOCI. - an interaction between the transition metal and the rare earth. on the surface, for higher Ce loading. 2 In order to understand better the role of ceria in such catalysts, further experiments
- the growing of tridimensionnal Ce0
are undertaken, especially in "in situ" conditions (ref. 7). ACKNOWEDGEMENTS We are very grateful to Dr. H.. Dexpert and P. Lagarde for their unvaluable help at LURE.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
J.e. Summers and A. Ausen, J. Catal., 58 (1979) 131. Y.F.Y. Yeo, J. Catal., 87 (1984) 152. G. Brauer and H. Gradinger, J. Inorg. Nucl. Chem., 17 (1955) 1792. P. Meriaudeau, J.F. Dutel, M. Dufaux and e. Naccache, Studies in Surface Sci. and Catal., 11 (1982) 95. J.A. Horsley, J. Amer. Chem. Soc., 101 (1979) 2870. T.H. Fleisch, R. Hicks and A.T. Bell, J. Catal., 87 (1984) 398 and references therein. F. Le Normand, L. Hilaire, K. Kili, G. Krill and G. Maire, in preparation. G. Praline, B.E. Koel, R.L. Hance, H.I. Lee and J.M. White, J. Electron Spectra. and ReI. Phen., 21 (1980) 17. P. Burroughs, H. Hammett, A.F. Orchard and G.T. Thornton, J. Chem. Soc., Dalton Trans. (1976) 1686. A. Kotani, H. Mizuta, T. Jo and J.e. Parlebas, Solid State Comm., 53 (1985) 805. E. Wuilloud, B. Delley, W.D. Schneider and Y. Baer, Phys. Rev. Letters, 53 (1984) 202. E. Beaurepaire, G. Krill and F. Le Normand, Proceedings of the 4th International Congress on EXAFS and Near Edge Structure, Fontevraud, France, 1986, J. de Physique, submitted. A. Bianconi, A. Marcelli, M. TomeIIini and I. Davoli, J. Magn. Mat., 47 (1985) 209. M. Gasgnier, L. Eyring, R.C. Karnatak, H. Dexpert, J.M. Esteva, P. Caro and L. Albert, Proceedings of the 17th Rare Earth Conference, Hamilton, Canada, 1986. E. Beaurepaire, These d'Etat, Universlte Louis Pasteur, Strasbourg, France, 1983.
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A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
AN AES
INVESTIGATIO~
229
OF THE REACTIVITY OF Pt, Rh AND VARIOUS Pt-Rh ALLOY
SURFACES TOWARDS O NO, CO AND H 2 2, f.C.M.J.M. VAN DELFT
G.H.VURENS*, M.C.ANGEVAARE-GRUTERandB.E.NIEUWENHUYS
Gorlaeus Laboratories, State University of Leiden P.O.Box 9502, 2300 RA
Leiden, The Netherlands
* Lawrence Berkeley Laboratory, University of California Berkeley, California 94720, USA
ABSTRACT The chemisorption and reactivity of oxygen, NO, CO and hydrogen on polycrystalline Pt, Rh, PtO.55-RhO.45 and PtO.12-RhO.88 alloy surfaces and on a PtO.25-RhO.75 (100) single crystal surface have been studied by AES. For Rh the results point to the presence of subsurface oxygen, following an exposure to 100 L 02 or NO at 290 K, under which conditions this species was not observed for Pt. The Pt-rich alloy behaved as pure Pt. The Rh-rich alloy showed occasionally a Rh-like or a Pt-like behaviour. The ambivalent behaviour of this Pt-Rh alloy appears to be determined by variations in the surface composition, which is strongly dependent on the equilibration temperature in vacuum. At low equilibration temperatures (below 1000 K) a Rh-rich surface is observed for Rh-rich alloys, whereas at high equilibration temperatures (above 1200 K) Pt surface segregation occurs. These effects are discussed in relation to surface segregation theories. The results show that the surface composition and the catalytic behaviour of Pt-Rh alloys are strongly dependent on the equilibration temperature, the presence of contaminants and the composition of the gas phase.
1.
INTRODUCTION The catalytic performance of alloys compared to those of the pure, constituent
metals is of both theoretical and practical importance. In the past few decades much research has been concentrated on the catalytic activity, selectivity and durability of binary alloys composed of an active and an inactive component (1). The effect of alloying on the selectivity can be understood mainly in terms of geometrical factors (ensemble size effect) (2,3). Much less information is available concerning the chemical properties of alloys composed of two catalytically active components, although bimetallic catalysts consisting of two active components are used for a number of very important reactions (1-3). In the present investigation we focus our attention onto the system
Pt-Rh for the following reasons:I) Many data are already available on the
adsorption properties and reactivity of pure Pt and pure Rh (both polycrystalline and single crystal) surfaces (4). II) Both Pt and Rh are components of the catalytic convertors, which are used for the purification of automotive
230
exhaust gases (removal of
NO~,
CO and hydrocarbons).
The mechanism of selective NO reduction (both reactant selective (NO vs.02) and product selective (N2 vs. NH)) has been the focus of many investigations (5). According to these data, Rh is the component which is most selective in NO reduction, whereas, Pt is more active in low temperature oxidation of CO and hydrocarbons. A synergetic effect has been reported by Oh and Carpenter for the combined Pt and Rh system compared to pure Pt, pure Rh and to a physical mixture of Pt and Rh as well (6). Su et al. observed enhanced oxygen storage capacity for the combined Pt and Rh system compared to pure Pt and pure Rh (7). Apart from selective NO reduction, another important quality of the Pt-Rh three-way catalysts is their stability during high temperature excursions in oxidative gas mixtures (5). It appears that alloying of Rh with Pt stabilizes the metallic phase of Rh, since Rh decomposes in air at 1100°C, 20 3 whereas in the presence of Pt it decomposes already at 800°C upon formation of a Pt-Rh alloy (8). According to the authors, the lower decomposition temperature is a result of the free energy of formation of the Pt-Rh alloy. Schmidt and Wang found by TEM, that on a silica support Pt and Rh form alloy particles, which decompose to Pt and Rh
during oxidation (9). It is likely, 20 3 therefore, that the synergetic effect reported by Oh and Carpenter (6) for the
combined system, is due to the presence of alloy particles. These alloy particles show both a higher activity in CO oxidation and a higher stability in oxidative gasmixtures than the pure metals (6,9). In the present investigation the chemisorption and reactivity of 02 and NO on several Pt-Rh alloy surfaces and pure Pt and pure Rh surfaces have been studied by AES in order to obtain a better understanding of the behaviour of Pt-Rh alloys compared to those of pure Pt and Rh. These results will also serve as central background for our continuing studies on various Pt-Rh alloy surfaces.
2.
EXPERIMENTAL Polycrystalline wires of Pt, Rh, PtO.S5-RhO.45 and PtO.12-RhO.88 alloys
were wrapped around the curves of a Ta multihairpin filament. The temperature of the samples was measured by means of a Pt/Pt-Rh thermocouple spotwelded onto the rear of the specimen. The filament could be heated up to 1500 K by Joule heating. The experiments were carried out in an all metal UHV system (base
pre~sure
- 1.10- 9Torr
consisting of 90% H2 and 10% CO approximately) equipped
with a single pass CMA with an integral electron
gun
(Physical Electronics).
Standard experimental conl:itions during Auger analysis were a primary beam of 2 keV electrons and a 5 eV peak-to-peak modulation amplitude. The samples were cleaned, depending on the nature of the contamination, by either Ar ion bombardment (S.lO-STorr Ar, 1 keV ions) or oxidation (5.10- 7Torr 02, 1100 K).
the major contaminants were S,
Ca (on Pt) and Fe (on Rh). Finally the samples
were heated around 1000 K for equilibration. The L of the desired gas
samples were exposed to 100
(S.10- 7Torr during 200 s) at room temperature. The adsor-
Date Auger signals were measured repeatedly either in vacuum or in a continuous flow of 1.10-sTorr of the desired gas during a temperature program as shown in fig. J .Tn the following figures a solid line connects the points measured at the given temperature and a dashed line connects the points measured in between, following a cooldown to room temperature after heating at the given temperature.
T(K)
t
1500 1000 500
o Fig. 1
10
20
30-.t (min)
Temperature program used for the experiments in the figs. 2-6.
In addition to the experiments on the polycrystalline surfaces, some AES experiments were performed on a PtO.2S-RhO.7S (100) single crystal surface. The cleaning procedures of this surface have been described in detail elsewhere (10).
3.
RESULTS AND DISCUSSION
3.1 CO exposure After a 100 L CO exposure on Pt, Rh and the Pt-Rh alloys, the C (272 eV) signal was monitored during the temperature program described above. The result for the PtO.12-RhO.ss sample is shown in figure 2. The intensity of the C (272 eV)
signal is normalized to the initial intensity at 290 K. The results
for the other samples are very similar to that shown for PtO.12-RhO.88' In all cases the 0 (510 eV) signal could not be detected, indicating that CO
232
I272 t 1.0
0.5
o
400
800
1200
-+
T (K)
Fig. 2 ~ o r m a l i z e d C signal intensity as a function of the temperature (programmed according to fig.l) for <1 100 L CO exposure at room temperature on the PtO.12RhO.88 alloy. The solid line connects the points measured at the given temperature, the dashed line c o n n e C L ~ the points measured in between, following room temperature cooldown after heating at the given temperature.
was molecularly adsorbed, since the cross section for Auger excitation of 0 in molecularly adsorbed CO is small (11). The maximum rate of desorption occurred around 550 K. Due to the relatively high CO residual pressure some readsorption of CO occurred during the intermediate cooldown, as shown by the higher level of the dashed line above 400 K.
~ 0 2
exposure
After a 100 L 02 exposure the 0 (510 eV)
signal was monitored during the
temperature program and the intensities, normalized to their initial values at 290 K, are shown in figures 3a and 3c for Pt and Rh respectively. As can be seen from the larger error bars, the initial amount of oxygen on Pt was much smaller than on Rh.
On Pt, the small 0 signal disappeared already at approxi-
mately 400 K. The following processes may contribute to the vanishing 0 signal intensity: a) desorption; b) reaction with the residual gas (CO and H2); c) diffusion of 0 to the bulk.
According to the literature data (4) any signi-
ficant desorption of oxygen should be unlikely. Hence, we disregard possibility a). Some contribution of c) cannot be completely ruled out, although no indications have been found of reappearance of oxygen on the Pt surface at higher temperatures. However, it is more likely that b) is the dominant process for 0
233 removal since it is known that.on Pt,O reacts very fast with H or CO in the relevant temperature range (4).lbe relatively low amount of adsorbed 0 following exposure at 290 K may also be caused by the rapid reaction with H or CO from the residual gas. The lower level of the dashed line is also consistent with b). On Rh the oxygen Auger signal intensity initially decreased up to 500 K, but then, it increased beyond the initial value in the temperature range 500 to 800 K. Finally, it decreased above 800 K. The initial decrease can be ascribed to a reaction with the residual gas or to absorption (see below). Desorption, is not probable for Rh at these temperatures (12-14). Here again, the dashed line is lower than the solid line, indicating a reaction with the residual gas during intermediate cooldown. The increase of the oxygen Auger signal intensity during heating in vacuum can only be explained by the presence of subsurface oxygen, which diffuses to the surface at elevated temperatures (above 500 K).
Since the initial Auger intensity is surpassed, this subsurface oxygen must
have been formed already at room temperature (during the exposure). The final decrease above 800 K is most likely due to desorption, since desorption has been reported to occur in this temperature range (14). The result for the Pt-rich alloy, PtO.55-RhO.45' is shown in figure 3b and displays a Pt like behaviour, which has been observed for all processes studied on this alloy in the present work. The Rh-rich alloy, PtO.12-RhO.88' showed three types of behaviour as shown in figures 3 d,e,f and indicated as types I, II and III respectively. Repeated experiments showed one of these three types of behaviour, although it could not be predicted a priori which type appeared under the conditions used. Type III (fig.3f) showed a constantly high oxygen level which did not disappear by heating up to 1300 K. The oxygen could only be removed by Ar ion bombardment. The positions of the vague maxima were not reproducible. In our opinion this type of behaviour is due to small amounts of Si and/or B subsurface impurities, which were not detectable in our AES analysis. Small amounts of these elements are known to form stable oxides at the surface (15-19). Type I and type II, however, were fully reproducible. Type I (fig.3d) is a Pt-like behaviour comparable to those of pure Pt (fig.3a) and the Pt-rich alloy (fig.3b). Type II (fig.3e), which shows a maximum for the oxygen intensity likewise at 800 K, is a Rh-like behaviour (compare fig.3c). The maximum relative intensity is lower than that observed for pure Rh. The figures also show that for the Rh-rich alloy the dashed line is much lower relative to the solid line than for pure Rh. This might indicate that the surface oxygen is more easily removed by the residual gas on the alloy than on the pure Rh. It was shown earlier that on a PtRh alloy surface oxygen preferentially occupies the Rh sites leaving the Pt sites initially free (10). If many free Pt sites are present at the surface
234
a
c
b
1.0
2 \
0.5
1
\
a 400 800 400 BOO
I
t 510
400 800
1200
L-.T(K)
f
e
d
1.0
0.5
a 400
800
400 800 1200
400
800 1200
Fig. 3 ~ 0 r m a l i z e d 0 signal intensity as a function of the temperature for a 100 L 02 exposure at room temperature on: a) Pt; b) PtO.SS-RhO.4S; c) Rh; d) PtO.12-RhO.88 type I; e) PtO.12-RhO.88 type II; f) PtO.12-RhO.88 type III.
adjacent to oxygen covered Rh sites, then
residual gas (CO, H2) can easily
chemisorb close to the oxygen ada toms resulting in a reaction. Dual selective chemisorption has been reported for a mixture of CO and NO on Pt-Ru alloys (20) and on Pt-Ni alloys (21), whereby CO selectively chemisorbs on Pt in both cases and NO selectively chemisorbs on Ru and on Ni respectively. These effects might form a basic line of thought for understanding the synergetic effects of alloying on the CO oxidation activity reported by Oh and Carpenter (6). We are still left, however, with the question, why the Rh-rich alloy shows reproducibily am-
235 bivalent behaviour. This problem will be discussed in section 3.5.
3.3
NO
exposure
The effect of an exposure of 100 L NO on the various samples was studied in tue S2me way as described for oxygen adsorption. The N (380 eV) signal intensity very Iowan Pt and on the Pt-rich alloy, but was easily observed on Rh and
W,l»
on the Rh-rich alloy,where it decreased around 440 K in both cases.The behaviour of the 0 (510 eV) signal was identical to that found after exposure to 02 for ai' samples (see figs.3a-f).This indicates that NO dissociated on all samples, l~aving
oxygen adatoms,which behaved further identically as those formed from
02.The NO dissociation is partly due to intrinsic chemical activity of the samples (4) and,most probably, partly induced by the primary electron beam.
3.4
Reactions on Rh and PtO.12-RhO.88 The reactions of NO with CO and H2 have been studied on pure Rh and on the
Rh-rich alloy sample. The experiments were carried out by running the temperature program in a flow of 1.10
-8
Torr of one reactant after a 100 L exposure of
the other reactant. The Rh results for preadsorbed NO reacting with CO are shown in figs. for the N (380 eV), C (272 eV) and 0 (510 eV)
4a,b,c
signals respectively. As can be
seen nitrogen disappeared at 400 K from the surface, whereas CO readily adsorbed on the sites left free by the nitrogen. The oxygen behaved similarly as in fig.3c, but during intermediate cooldown the oxygen at the surface shows a fast reaction with CO, as indicated by the low level of the dashed line in fig.4c. The Rh results for preadsorbed NO reacting with H2 are shown in figs. 5a,b for the N (380 eV) and the 0 (510 eV) signals respectively. Both the oxygen and the nitrogen disappeared at low temperatures from the surface. Apparently, hydrogen is capable of removing oxygen from the subsurface region whereas CO is not. The Rh results for preadsorbed CO reacting with NO are shown in figs. 6a,b,c for the N (380 eV), C (272 eV) and 0 (510 eV) signals respectively. CO reacted with NO, which adsorbed on the sites left free by CO but above 400 K nitrogen disappeared from the surface, although during intermediate cooldown NO was adsorbed again. The amount of oxygen on the surface was low but still some subsurface oxygen was observed to diffuse to the surface at elevated
temperatures. The ma-
ximum amount of near-surface oxygen is observed at 1000 K and, hence, at a significantly higher temperature than under the conditions of the experiments illustrated by figures 3 and 4. It appears
that this shift to higher tempera-
ture is linked up with the lower concentration of adsorbed oxygen. The ambivalent behaviour of the Rh-rich alloy was also manifested by the reactivity of preadsorbed NO with CO and hydrogen. The variation of the N, 0
236 and C AES peak intensities with the temperature showed either a Pt like behaviour (a small amount of 0 that disappeared upon exposure to CO or hydrogen) or the Rh like behaviour (with subsurface 0 that
reacted
easily with H but not
with CO) and, occasionaly, the third type of behaviour. In the last case the oxygen
could not be removed, not even by heating in a hydrogen atmosphere.
I 510 a
t
b
1.0
3
0.5
o
0.5
2
I~
400
400 800 T(K). 400
800
800
1200
Fig.4 Normalized N,C and 0 signal intensities (in a,b and c respectively) as a function of the temperature for Rh in a flow of 1.10- 8 Torr CO after previous room temperature exposure to 100 L NO.
1380
1510
t
1.0
1.0
b
0.5
0.5
a
t
a
400
BOO
400
800
-+
T(K)
Fig.S Normalized Nand 0 signal intensities (in a and b respectively) as a function of the temperature for Rh in a flow of 1.10- 8 Torr HZ after previuus room temperature exposure to 100 L NO.
1380
i I
1272
t
a
I
b
1.0
\
c
2 1
o
800 1200
400
L,OO
800
400
800--+T(K)
Fi~.6 Normalized N,C and 0 signal intensities (in a,b and c respectively) as a function of the temperature for Rh in a flow of 1.10- 8 Torr NO after previous room temperature exposure to 100 L CO.
~ 0 2
exposure of PtO.25-RhO.75 (100)
In our previous work (10) it was shown that the surface composition of a PtO.25-RhO.75 (100) face is strongly dependent on the equilibration temperature of the clean alloy in vacuum. At high equilibration temperatures Pt enrichment was observed. whereas below 1200K Rh enrichment could not be ruled out, as shown in figure 7. Surface segregation theories, taking into account the difference in sublimation enthalpy between Pt and Rh, predict a moderate Rh enrichment. It is very peculiar that the Pt surface enrichment increases with increasing temperature, whereas the existing segregation theories predict that any surface enrichment should decrease with increasing temperature. A similar temperature dependence was observed by Williams and Nelson for polycrystalline PtO.10-RhO.90 from ISS studies (22) and by Wolf et al. for the atomically rough surfaces of a PtO.12-RhO.88 tip with Field Emission Microscopy (23). A theoretical explanation for the Pt enrichment has recently been given by van Langeveld and Niemantsverdriet (24). These authors considered the lower Debye temperature for Pt atoms in the surface. The Debye temperature can be coupled to a vibrational entropy term see e.g. (25). Combination of the enthalpy and entropy effects yields the following equation for a one layer segregation model:
(l-x ) b x (l-x s) b
X
K
s
e
-!:£/RT
where K is the equilibration constant,
(eq. 1)
X
s is the surface molar fraction in Pt , xb is
the buLk molar fraction in Pt, L\G is the free energy 0:':: segregation, LH
is the en-
thalpy of segregation, R is the gas constant, T is the absolute temperature and LS v is the vIbrational entropy term. It can be seen from this equation that the enthalpy term dominates at low temperatures,yielding a small Rh enrichment. At higher temperatures the contribution of the enthalpy term diminishes, leaving the entropy term, which yields a strong Pt enrichmen[ as is shown in fig.8 with several literature data (10,22,24,26). The model by van Langeveld and Niemantsverdriet is qualitatively in good agreement with the experimental results for the temperature dependence of the surface composition. In a forthcoming publication we will further quantify the temperature dependence of the surface composition of Pt-Rh alloys (27).
1
Pt i X1
I
0.5
a
I -II -- ------500
1000
1500
-4
T(K)
Fig. 7 Temperature dependence of the surface composition of a PtO.25-RhO.75 (100) face. The dashed lines indicates the bulk composition of the alloy. In order to examine the possible effect of the annealing temperature on the chemical properties of Rh-rich Pt-Rh alloys, additional measurements were carried out on the PtO.25-RhO.75 (100) surface. On this alloy sample a uniform temperature could be adjusted, whereas the temperature of the polycrystalline samples was less uniform over the filament and less constant in time. After equilibration of the PtO.25-RhO.75 (100) surface (fig.7) during 5 minutes at 1425 K (Pt-rich surface) and subsequent 100 L 02 exposure at 290 K, temperature programmed AES without intermediate cooldown showed no significant amount of subsurface oxygen, as shown in figure 9a. However, after equilibration at 975 K during 45 minutes (Rh-rich surface) and subsequent 100 L 02 exposure at 290 K, substantial amounts of subsurface oxygen segregated to the surface at 1200 K, as shown in figure 9b. The point at 1240 K marked with a cross was measured after the measurement at 1300 K, indicating that above the desorption temperature diffusion to the surface is the rate limiting step for desorption.
1.0 x Pt
-:
1
<-:
/'
t
/
""
0.5
o
0.5 ----.
1.0
Fig. 8 The surface composition of clean Pt-Rh alloys as a function of the bulk composition with several literature data: U van Delft and Nieuwenhuys (10); 6 van Langeveld and Niemantsverdriet (24); • Williams and Nelson (22); x Holloway and Williams (26); The solid line shows the vibrational entropy contribution,the dotted line shows the enthalpy only contribution at 1000 K and the dashed line shows the theoretical curve for 1000 K according to the model by van Langeveld and Niemantsverdriet (24)
This subsurface oxygen was only observed on a low temperature equilibrated surface if the
subsurfac~
carbon contamination of the alloy was extremely low (see
fig.10), in accordance with the experiments by Salenov and Savchenko (28) for pure Rh (100). These authors observed an oxygen desorption state at 1230 K only if the Rh (100) had a very low carbon concentration in the near surface region. The Auger phenomenon at 1200 K in figure 9b and figure lOb is in our opinion due to subsurface oxygen and not to the decomposition of a
surface impurity
oxide, since this should yield a high oxygen Auger signal from room temperature up to the decomposition temperature (compare fig. 3f). The background oxygen signal however, might be due to the presence of small amounts of impurity oxides. Anyhow, the oxygen behaviour on the Pt-Rh(lOO) surface equilibrated at low temperatures resembles the behaviour of oxygen on Rh (100). Returning now to the polycrystalline samples, it may be expected from fig. 8,
240
10
975 K
b
o 3 2 1
500
1000 -. T(K)
500
1000
1425 K
a
o
-+
T(K)
Fig.9 Normalized 0 signal intensity versus the temperature (without intermediate cooldown) for PtO.25-RhO.75 (100) after a 100 L 02 exposure at room temperature preceeded by equilibration in vacuum a) during 5 minutes at 1425 K; b) during 45 minutes at 975 K.The point marked with a cross in b) was measured after the measurement at 1300 K.
that the surface of the PtO.55-RhO.45 alloy sample will be much enriched in Pt and, hence, exhibit a Pt-like behaviour after equilibration at all temperatures. For the PtO.12-RhO.88 alloy a Pt rich surface and hence. a Pt-like behaviour may be expected after equilibration at high temperatures (T
> 1200
K).
However, a Rh-rich surface and, hence, a Rh-like behaviour may be expected after equilibration at lower temperatures (T < 1000
K).
The observed ambivalent behaviour is, thus,understood. These surface composition variations together with the influence of small amounts of impurities have a profound influence on the adsorption behaviour and catalytic
performance of
the Rh-rich alloys as was shown in section 3.2.
4.
CONCLUSIONS The present results indicate that the chemical behaviour of Pt-Rh alloys is
strongly dependent on the bulk composition and the equilibration temperature. Depending on the bulk composition and the annealing temperature, a Pt-like or a Rh-like behaviour was observed. On Rh rich alloys both types of behaviour
241
10
1510
t
975 K
I
Ii
I
1~72 < 0.01 Rh
5
1 302 b
0
500
1000 -. T(K) 1C 272
3 900 K 2 1 a 0
=0.03
l Rh
302
500
1000 -+ T(K)
Fig.lO Normalized 0 signal intensity versus the temperature (without intermediate cooldown) for PtO.2S-RhO.7S (100) after a 100 L 02 exposure at room temperature preceeded by low temperature equilibration in vacuum a) with carbon contaminated bulk; b) with very low carbon concentration in the subsurface region.
NONO 290K...
290K +CO
7',),)77'
---=--~~
NNo8 In h " ,. 0
- C021290 K 177)} }}
o
<4
440 K NN };;'7; I; -N 0 2
Fig. 11 Scheme for the reaction between CO and NO on polycrystalline Rh and on a polycrystalline Rh-rich alloy with a Rh-rich surface.
242
could be realized by variation of the annealing temperature. A third type of behaviour could be attributed to the effect of a small level of impurities. Based on these results a scheme is given in figure II for the reaction between CO and NO
on Rh and on a Rh-rich alloy with a Rh-rich surface. The sub-
surface oxygen was readily observed on polycrystalline Rh and Rh-rich alloy surfaces, but was only detected on the (100) surface of such samples if the contamination level of the subsurface region was below a certain limit. Hence, the presence of even low concentrations of C prevents the diffusion of 0 into the bulk (oxide formation), 5.
ACKNOWLEDGEMENTS The authors are indebted to the Royal Shell Laboratories in Amsterdam for
the donation of the UHV AES apparatus used in this study. The stimulating discussions with dr.A.D.van Langeveld, drs.R.M.Wolf and M.J.Dees are gratefully acknowledged. 6. I 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
REFERENCES M.J.Kelley and V.Ponec, Progr.Surf.Sci., 11 (1981) 139 and refs. therein V.Ponec, "On the Chemistry and Alchymie of Metallic Catalysts" in: Proceedings IXth IntI. Vac.Congr.- Vth Intl.Conf.on Solid Surfaces, J.L.de Segovia, ed. (ass. Espanole del Vacio, Madrid, 1983) B.E.Nieuwenhuys, "Chemisorption of Gases on Metal Films", P.Wissmann,ed. Chapter 8, Elsevier, Amsterdam(1987) B.E.Nieuwenhuys, Surf.Sci., 126 (1983) 307 and refs. therein K.C.Taylor, "Automotive Catalytic Convertors" (Springer, Berlin 1984) and refs. therein S.H.Oh and J.E.Carpenter, J.Catal., 98 (1986) 178 E.C.Su, C.N.Montreuil and W.G.Rothschild, Appl.Catal., 17 (1985) 75 A.J.S.Chowdhury, A.K.Cheetham and J.A.Cairns, J.Catal., 95 (1985) 353 L.D.Schmidt and T.Wang, J.Vac.Sci.Technol., 18,2(1981) 520 F.C.M.J.M.v.Delft and B.E.Nieuwenhuys, Surf.Sci., 162 (1985) 538 B.E.Nieuwenhuys and G.A.Kok, Thin Sold Films, 106 (1983) L95 D.G.Castner, B.A.Sexton and G.A.Somorjai, Surf.Sci., 71 (1978) 519 P.A.Thiel, J.T.Yates and W.H.Weinberg, Surf.Sci., 82 (1979) 22 D.G.Castner and G.A.Somorjai, Appl. Surf. Sci., 6 (1980) 29 H.Niehus and G.Comsa, Surf.Sci., 93 (1980) L147 H.Niehus and G.Comsa, Surf.Sci., 102 (1981) 114 H.P.Bonzel, A.M.Franken and G.Pjrug, Sur f Sc i , , 104 (1981) 625 S.Akther, C.M.Greenlief, H.W.Chen, J.M.White, Appl.Surf.Sci., 25 (1986) 154 S.Semancik,G.L.Haller, J.T.Yates Jr., Appl.Surf.Sci., 10 (1982) 546 P.Ramamoorthy and R.D.Gonzalez, J.Catal., 58 (1979) 188 A.F.M.Wielers, C.J.G. v.d.Grift and J.W.Geus, Appl.Surf.Sci., 25 (1986) 249 F.L.Williams and G.C.Nelson, Appl.Surf.Sci., 3(1979) 409 R.M.Wolf, M.J.Dees and B.E.Nieuwenhuys, J.Physique (Paris) in press A.D.van Langeveld and J.W.Niemantsverdriet, Proceedings ECOSS-8 (Julich), Surf.Sci., in press K.Hoshino, J.Phys.S0c. Japan, 50, 2 (1981) 577 P.H.Holloway and F.L.Williams, Appl.Surf.Sci., 10 (1982) 1 F.C.M.J.M.v.Delft, A.D.v.Langeveld and B.E.Nieuwenhuys, to be published A.N.Salanov and V.I.Savchenko, React.Kinet.Catal.Lett., 29,,1 (1985) 101 s
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
243
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
REACTIVITY STUDIES OF AUTOMOBILE EXHAUST CAT ALYSTS IN PRESENCE OF OXIDIZING OR REDUCING CONDITIONS Guillaume MEUNIER*, Fr ar-cols GARIN*, Jean-Louis SCHMITT*, Gilbert MAIRE* and Rene ROCHE** *Laboratoire de Catalyse et Chimie des Surfaces, U.A. 423 du CNRS, Universite Louis Pasteur, 4 rue Blaise Pascal 67070 Strasbourg Cedex FRANCE **Peugeot S.A., Direction Technique, 18 rue des Fauvelles 92260 La GarenneColombes FRANCE
ABSTRACT In this study the catalytic behavior is determined under reducing (hydrogen atmosphere) or oxidizing (oxygen atmosphere) conditions. By using the correlations established between the mechanisms of the skeletal isomerization of hydrocarbons on metals and the structures (electronic, geometric or crystallographic) of the catalysts we could "characterize" the surface structure of the catalysts used for the CO oxidation reaction. The second time we studied I) the influence of the first impregnated salt on Al7 0 on the oxidation reactivity for Pt-Ni and Ni-Pt catalysts Ii) the influence of the 3 carcination step on the light-off for Pt-Co catalysts; Pt-Ce, Ni-Ce and Ce-Pt-Ni were also studied. We concluded that no segregation occurs and a random distribution of the small metallic particles occurs on AI7 and that the Pt-Co/AI 0 system without 2 3 calcination is more active; the iiqht-bff" for the CO oxidation being the lowest.
D.)
INTRODUCTION This paper deals with experimental studies of the reactivities of CO or alkanes with some platinum-based bimetallic catalysts under oxygen or hydrogen atmospheres, respecti vel y. Since the nature of the exhaust gases oscillates from oxidizing to reducing agents with a certain frequency, these catalysts have to be equally active under both environnements. From bibliographic studies several points have to be underlined : a) - Studies of bulk alloys have shown that chemisorption of atoms and molecules (H, 0 and CO) can change the surface composition drastically (1), thereby influencing the heterogeneous reaction rate. The clean Pt-Ni alloy surfaces in U.H. V. or under hydrogen atmosphere were found to be enriched in platinum in an amount increasing with increasing platinum concentration in the bulk. On the contrary oxidation treatments resulted in a nickel surface segregation which was only slightly offset after reduction (2). b) - For the CO oxidation reaction the whole elementary steps are not clearly explained.
244
A key question is whether the diatomic molecule in its interaction with metal surfaces remains molecular or dissociates into carbon and oxygen. Broden et a!. (3) predicted, by the perturbation of molecular orbitals for CO adsorbed, that only iron could dissociate CO. However, other metals in Group VIIl such as nickel (4) ruthenium (5) and rhodium (6) can dissociate CO. Recently Ichikawa et al.(7) observed that disproportionation of CO to CO
and carbon occurs on small particles of silica-supported pal2 ladium. These results show that carbon deposition phenomena may occur via either dissociation of CO on the metals used or disproportionation of CO to CO and carbon on 2 small platinum particles. Cant and Angove (8) studied the apparent deactivation of Pt/Si0 2 catalyst for the oxidation of carbon monoxide and they suggested that adsorbed CO forms patches and that oxygen atoms are gradually consumed. c) - The influences of the crystallographic orientations for the CO oxidation reaction may be important. From the work of Gland et a1.(9) with Pt (321) surface they clearly show that oxygen adsorbed on rougher step sites are less reactive than those on smooth terrace sites. A very important work was performed on the adsorption of CO on single crystals. For Pt(lll) Hayden and Bradshaw (10) studied this reaction by infrared reflection-absorption spectroscopy (IRAS) and they mentioned that bands due to the C-O stretch were found in both the linear and bridging regions and that there were distinct bands in the bridging region which could be assigned to CO in two fold bridge and three folcJ hollow sites. Crossley imrl Kinn
(J]) shJrliprl
CO on Pt(100) and Pt(I11) and
mentioned, from vibrational spectra that formation of islands at low coverages on Pt(100) occurs which dimension is about 10 molecules at 400 K for a coverage of 10 14 molecules cm -2 (8=0.1). On Pt(I11) at 300 K adsorption initially occurs into isolated singletons. d) - Finally for the CO oxidation reaction at low pressure on Pd(111), Engel and Ertl (12) have shown that this reaction is structure-insensitive as mentionned by Boudart (13). The reactions of CD labeled hydrocarbons on platinum catalysts under hydrogen atmosphere are structure-sensitive (14) and isomerization reactions are very sensitive to the crystallographic planes as observed on the platinum stepped surfaces where the bond shift mechanism is favored compared with the cyclic mechanism (15). The aim of this study is to get catalytic performances for two different reactions: one, the isomerization, performed under hydrogen; the other, the CO reaction, involving oxygen atmosphere. We hope to understand these reactions and the behavior of the catalysts under these opposite environments. EXPERIMENTAL A serIes of ten catalysts 0.2% Pt, 0.7% Pt, Ni-Pt, Pt-Ni, two Pt-Ce, Ni-Ce, Ce-PtNi and two Pt-Co were studied under O and/or H atmospheres. 2 2
245
Catalysts preparations All the catalysts were supported on Rhone Poulenc y-A1 pellets of 1.6-2mm dia20 3 r'"1eter (ref: CCO 64) with a surface area of 206m'g-\ a density of 1.19 and a pore diaD
meter of about 100 A; the impurities being (in ppm) : (Na
40, CaO 70, Fe 70, 20 20 3 SiOt 70, MgO £ 20, 50/- 150). Two impregnation techniques were used: - the fluidized bed technique (6) - the Ipatieff method. The first one is based on ionic exchange, the second one is a forced impregnation (17-18). The multimetallic catalysts were prepared by successive impregnations. All the catalysts were calcined at 400 DC during 2 h after each impregnation and finally reduced at 400°C during 2 h under a hydrogen flow of 40cm'min- 1. For one Pt-Co catalyst two reduction steps were performed: with Co in first. Two Pt-Ni were prepared with Ni or Pt impregnated in first. Catalysts characterization The atomic absorption technique was used to control, during the ionic exchange, the amount of metal removed from the metal ion species to the active sites on the support. Unfortunately we were not able to check the metallic particle size by T.E.M. so we used the test reaction of the methylcyclopentane hydrogenolysis (19). We have shown that the selectivity of the C
5
ring opening is correlated to the particle size (20).
Apparatus Catalytic reactions were performed in two differenUal reactor systems under 1 atm.. The laboratory reactor consisted of a glass tube heated by a furnace. A glass packing was used as preheater. The catalytic charge was always 200mg. For oxidation reaction the gas mixture was CO 1.5%, 02 1.5% and N 97% from 2 Air Liquide cylinders. Tylan Mass flow controllers permitted to vary gas flow between 20 to 100 em' min-I. Carbon monoxide analysis was done before and after the reactor by nondispersive IR gas analyzer Binos-Leybold Heraeus. The programmed heating rate was lODC min -1 until the first reaction temperature was reached and then 1°C min-I. Analysis measurements were made after an exposure time for each temperature to be sure that the catalyst was in a steady state. The catalytic reactions of hydrocarbons under hydrogen flow were performed in the classical system already described (19). Prior to hydrocarbon reactions the catalysts were reduced at 350 DC overnight. The reaction products were analyzed in a gas chromatographic apparatus.
.
N
0>
TABLE la Catalytic reactions of 2-methylpentane on various catalysts supported on IS' A1 0 Product distributions. 2 3.
Catalysts (weight%)
eOc
Ci
Conversion rate* T% moles mole'_ 1 u l.mih
Hydrocracking in moles % Extensive cracking
2C
3
C
4+C 2
C
5+C l
Isomers in moles % 3MP
nH
MCP
Selectivity in isomers
Observations
iB n-B
Pt 0.7
260
4
6.8
0
14
34
52
43
36
21
33%
Pt 0.2
300
12
21.6
0
14
36
50
38
45
17
60%
Ni-Pt 1.5 0.2
280
52
88.5
29
13
21
37
50
30
20
3% Pt first on the support
0.5
Pt-Ni 0.2 1.5
280
49
68.5
9
10
30
51
50
40
10
2% Ni first on the support
0.2
Pt-Ce 0.2 1
280
4
6.5
0
10
32
58
31
41
28
61%
Ni-Ce 1
280
37
59.2
17
9
21
52
57
20
22
4%
0.5
Ce-Pt-Ni 1 0.2 1.5
280
53.5
80
24
14
27
34
50
25
25
1.5%
0.3
Pt-Ce
280
B**
17
27.2
0
15
32
52
40
51
9
280
A'**
5
8
0
17
31
52
46
36
17
0.2
1
Pt-Co 0.2 0.4
300 280
B A
0.6 1.5
1 2.6
0 0
13 10
20 20
67 70
17 22
10 15
72 63
49% Pt and Ce were cal72% cined prior to the reduction at 400°C 69% Pt and Co were 76% reduced at 400°C
*rate = CiTF; F : hydrocarbon flow in u l.min- 1, w : catalyst weight 0.2g (constant value for all the experiments). w
**B : be Fore the CO oxidation reaction; A** : after the CO oxidation reaction. 3MP 3-methylpentane, n-H n-hexone, MCP methylcyclopentane, iB isobutane, nB n-butane.
14
27
6 14
TABLE 1b Methylcyclopentane hydrogenolysis.
Catalysts on yAl 0 2 3
Conversion T% moles
GOC
Ci
rate rnole u I.min- 1
% L: cracking
2MP 3MP
3MP n-H
Pt 0.7
220
6.5
2.6
6
1.4
0.8
Pt 0.2
240
1.6
1.3
0
1.7
0.75
Ni - Pt
240
7.5
28
1.4
3
Pt - Ni
240
35
62
1.4
4.3
Pt - Ce
240
2
1.8
3
2
0.7
Ni - Ce
240
12
9.6
54
1.2
4.7
240
25.5
18
56
1.4
4
Ce Pt Ni
10
Pt - Ce
240
B
14
11.2
5
1.9
0.7
Pt - Ce
240 A
4
2.8
2
2.8
0.7
Pt - Co
240
B
0.3
0.2
0
5.4
0.3
Pt - Co
240
A
1.4
1.1
2
2.4
0.6
"" """ _1
248
Results Experiments performed under reducing condition In table La, the distribution of the products obtained from 2-methylpentane are shown as percentages of reactants having disappeared according to reactions 1 to 6. C
-s-
iso C
C6
-+
IV1CP
(2)
C
-+
C
(3)
C C
C
6
6 6 6
-+
(1)
6
5+C 1 C +C 4 2
(4)
-+
2 C
-+
extensive cracking 6 C
(5)
3
+3 C
(6)
2 6 1 The cracking pattern on Pt-Ce is similar to the one observed on Platinum catalysts; on Pt-Co catalysts, the demethylation reaction is more important than on Pt catalyst. When Ni is added, repetitive cracking reactions occur; the ratio of isobutane over n-butane is lower than 1 on these latter catalysts which means that multiple processes occur on the catalytic surface because n-butane cannot be obtained from 2-methylpentane with only one carbon-carbon bond breaking. About the isomerization distribution
On Pt-Ce and on 0.2%Pt the n-hexane is the
major product, which means that the cyclic mechanism is the major process (14). The amount of methylcyclopentane is very high on Pt-Co catalyst because the conversion is very low, there is desorption of the cyclic product intermediate prior to be hydrogenolysed. On a Ni-containing catalyst, 3-methylpentane is the main isomerized product. The catalytic reaction of 2-methylpentane is not sensitive to the influence of the impregnation order.
9. f _tl:2e J ~ a s : t l 0 . r 2
~ 0 ~ 1 { 2 _O 2_ ~ _CQ2_: An oxidative reaction was performed between two isomerization reactions. The Pt-Ce catalyst has lost its activity .!.nD~e.r2c~
after the carbon monoxide reaction, indicated by the increase of the amount of methylcyclopentane and the selectivity to isomers. The same behavior is observed on Pt-Co catalyst, but its activity, which is always very low, is increased after the CO oxidation reaction; at 280°C the Pt-Co catalyst is inactive before the reaction CO
-+
CO
2
occurs. In both cases the cracking pattern is unchanged. In table Ib we have mentioned the activity r
a F, the reaction temperature and
=--w-
the ratio 2MP and 3MP obtained from methylcyclopentane hydrogenolysis. 3MP n-H The ratio 3MP has been correlated to the metallic dispersion (20) and on 0.7% Pt, n-H 0.2% Pt and on Pt-Ce and Pt-Co this ratio is equal to 0.7::0.1 which means that the methylcyclopentane hydrogenolysis is non selective (19) and that the catalysts are well dispersed. The values are equal "before" and "after" the carbon monoxide reaction on Pt-Ce and Pt-Co. At the opposite, on catalysts with Ni this ratio is higher (3.9::0.9).
249
The ratio 2MP is near the statistical values which is 2 whatever the catalysts 3MP used, but on Pt-Ce after the carbon monoxide oxidation and on the Pt-Co catalysts this ratio is very high which could mean that a memory effect occurs; in each case, before the hydrogenolysis reaction there is the 2-methylpentane isomerization. Repetitive processes occur on catalysts where nickel is incorporated, the amount of these reactions can be measured by the cracking contribution. For the methylcyclopentane hydrogenolysis, the experimental conditions are chosen so that no multiple carbon-carbon bond rupture will occur; which is not the case for Pt-Ni, Ni-Ce and Ce-Pt-Ni catalysts. Experiments performed under oxidizing conditions In figures 1 and 2 are plotted the carbon monoxide conversions (a ) as a function of temperature for various gas flows from 20cm' .min-1 to 100cm' .min -1. In figure 1 the Pt-Co catalyst used has been prepared following the classical method: cobalt was first impregnated on y -A1 and then calcined; the platinum was added later then 203 also calcined at 400°C and finally the catalyst was reduced at 400°C. In figure 2 the impregnation order was Co then Pt and at each step the salt was reduced prior to the second impregnation. In the last case the light- off is lower, but is a function of the gas flow. We may observe that: i)
on the calcined catalyst the flow rates have no influence
ii) on the reduced one, when the flow increases, a bending of the curve at high conversion occurs. In figure 3 we have plotted the curves a =f(T) for various catalysts mentioned previousl y. For supported nickel catalyst (3wt%), (where no isomers can be formed), the reaction rate is very low and it is necessary to increase the temperature to start the reaction. The bimetallic systems Ni-PtlA1
have the same activity for CO ...... C0 whatever 203 2 1. the impregnation order is. The light-off is about 210°C at 40cm'.min- The 0,2% PtlAl 203 catalyst has a half conversion temperature of 195°C; the trimetallic system Ce-pt-NilAI
where cerium was the last deposited metal did not significantly im203 prove the activity. The light-off was around 190°C. Furthermore, the Pt-Co where Co was first deposited on alumina gives T(a o ) -2-
IS0oC.
Discussion A vast amount of existing catalysis literature is devoted to the interaction of CO with 02 (21). This seemingly simple catalytic reaction includes the general problems of heterogeneous catalysis. On the other hand reforming reactions of hydrocarbons on metals and alloys are very well reviewed (22,23,14). But these two types of reaction
250 Convers ion <::0
110
100
50
Convers ion CO
50
oL_ _ . : : : : : : : : : : : : : : : : : ; : : : : : : : : = : : : : : : ~ : : : : : " " ' - - , _ 100
150
Temnerature (OC)
]50
Temperature (OC)
Conversion of CO to CO versus temperature and flow rate on 2 Fig. I. Pt-Co catalyst for which FiE. 2. Pt -Co catalyst for calcination steps
occurred.
which only reduction sten occurrea.
Flow rate
~ n
em
3
.
-I
a =
m~n
20, b
40,
c = 60,
d
80,
e
100.
Conversion (%)
100
50
° Fig.
ISO
Temperature (OC) versus temperature for different
200
250
3.
Conversion of CO to CO 2 -I 3 catalysts. Flow rate = 40 cm mn
3% in weight 20 30.2% (d) ce/Pt/Ni/A1 0 20 3 2 3 2 3 1%-0.2%-1.5% ; (e) Pt/Co/A1 0.2%-0.4% (f) Pt!Ni/A1 0 0.2%20 32 31.5% ; (g) Ni/Pt/A1 1.5%-0.2%. 20 3(b)
Ni/Ce!A1
-
1.5%-1%
:
(c)
(a) Ni/A1
300
Pt/A1 0 -
251
are not often studied in parallel and compared. The activity and selectivity of catalysts are determined by the properties of surface complexes formed by chemisorption. In this respect we may compare the thermodynamics of these two reactions and then analyse the influence of alloying on the chemisorption bond strength. *Oxidation of Carbon monoxide -e- CO Exothermic (6 HO298 = -282.6 kJ mole-I) and practically 2 2 irreversible up to 1 500 K (6 GO298 = -256.7 kJ mole-\ 6 So298 = -20.7 e.u.),
CO
+
112 O
On clean metal surfaces CO is adsorbed at a high rate with a sticking probability of 0.2 - 0.6, without activation energy. Adsorbed molecules of CO are located perpendicularly to the metal surface with carbon atoms facing the metal. The interaction involves an acceptor donor bond with electron transfer according to Engel and Ertl (24). *lnteraction of dioxygen with the surface of solid catalysts Oxygen chemisorption proceeds very rapidly on clean surfaces of most metals. At room temperature the sticking probability ranges 0.1 to 1 and is close to unity for many metals. This corresponds to a very small value of the activation energy of chemisorption. *Hydrocarbons Hydrocracking is exothermic 6 HO298 " -50 kJ mole
-1
(log K 500°C
=
4) and the
rate is slow. The adsorption is more confusing. Owing to C
l3
labeling experiments we obser-
ved that the rate determining step in the reforming reactions is the carbon-carbon bond rupture of a dehydrogenated species a or
TT
bonded to the surface (14) and not
adsorption or desorption steps. *Hydrogen It absorbs dissociatively on all platinum group metals without any appreciable activation energy. The initial sticking coefficient is typically of the order of about 0.1 but may also reach higher values (0.5 for Pd(lOO) (25)). The adsorption energy 1 ranges typically between 60 and lOOk J mor corresponding to strengths of the M-H 1. bond around 250 to 270 kJ mor The thermodynamics of these elements is quite different between the oxygenated and the hydrogenated compounds. Between CO and O a competitive adsorption could 2 occur as the sticking coefficients are similar. Furthermore, the catalytic surfaces could be very rapidly equilibrated under H or O as their activation energies of 2 2 chemisorption are small. Not only the thermodynamics controls the reaction, but also the structure of the catalyst. One question may arise concerning segregation of one metal preferentially at the surface of the catalyst.
252
With Pt-Ni catalysts, under reducing atmosphere, clean surfaces of bulk alloys are enriched with platinum when oxygen treatments show that the surface is then enriched with nickel (2). On Pt-t'li/Al 0 catalyst with only 0.2wt% Pt - l.Swt% Ni, the 2 3 crystallites may be small enough to prevent any segregation phenomenon. Arguments in favor of this suggestion are: i) For the CO oxidation reaction, whatever the temperature, the reaction rates are
the same for the two catalysts Ni-Pt or Pt-Ni supported on Al 0 y The Pt-Ni (or Ni2 Pt) catalvsts are better than Ni/Al 0 catalyst. The primary reaction pathway for CO 2 3 oxidation on noble metals is a surface reaction between adsorbed CO and adsorbed atomic oxygen (24). Over the Ni-only catalyst, oxides are readily formed under the net-oxidizing reaction conditions considered in this study and this oxide may suppress CO oxidation. A random distribution of Pt and Ni on the surface can explain the decrease in the light-off observed on Pt-Ni and Ni-Pt systems. ii) For the 2-methylpentane reaction, no great differences are observed in the distri-
butions of the catalytic products on these two catalysts. When Ni is the second metal to be impregnated the extensive cracking is increased slightly (table 1); at the opposite it is the C
S+C1,
which is favoured for the second catalyst. It is the only dif-
ference to be underlined. The activities, the isomers distributions and the selectivities are similar. iii)For the methylcyclopentane hydrogenolysis, the
3~P/nH
ratio is the same for the
two catalysts (3.6::0.6). The amount of cracked products and the total conversion are higher on Pt-NilAI 0 than on Ni-Pt/AI 0 2 3 2 3. At this stage we may suggest that no segregation occurs for Pt-NilAI 0 catalysts 2 3 and that the surface structure of the bimetallic catalyst can be visualized as a statistical mixture of Pt and Ni sites. The Pt-ColAl 0 catalyst may behave in a similar way 2 3 as its surface segregation is less important under hydrogen than Pt-Ni catalyst (3l). In these cases, the majority of the Pt metals would be deposited on to the alumina surface rather than on the top of nickel or cobalt oxide particles as mentioned by Oh and Carpenter (26). The results obtained under hydrogen show very clearly that the catalysts without nickel do not exhibit repetitive processes and that platinum crystallites are small. From previous works undertaken in the laboratory we were able to correlate, in the isomerization of 2-methylpentane and in the methylcyclopentane hydrogenolysis, that larger amounts of n-hexane compared with 3-methylpentane are due to the presence of o
small platinum crystallites, around 20 A or lower (19,27). The metallic particles of the catalysts used in the present study are well dispersed, which is the case for the following catalysts: (0.2% and 0.7% pt, 0.2wt%Pt - 1wt%Ce, 0.2wt%Pt - 0.4wt%Co supported on A1 0 2 3).
At the opposite for catalysts with nickel, the platinum particles could be well dispersed, but the interaction Pt
«»
Ni masks the platinum behavior.
,£'.,s the metallic particles are small, we could think that the surface structures will be modified, as previously mentioned at the beginning of the discussion, either they are working under oxidizing or reducing atmospheres. These experiments were undertaken with Pt-Ce and Pt-Co supported on Al y The CO oxidation reaction, in our 20 experimental conditions, is not stoechiometric as the gas mixture being 1.5% CO and 1.5% O in N As oxygen and carbon monoxide have similar sticking coefficient, we 2 2. can assume that the catalytic surface is in an oxidative state after a CO experiment. On the Pt-Ce catalyst, after this oxidation reaction, its activity is decreased for the hydrocarbon reaction. We may think that the oxidation state of cerium is reinforced and the oxide biocks platinum active sites. The activity of Pt-Co catalyst is increased after an oxidation reaction. We may suppose that this experiment has favoured either the reducibility of the cobalt or created Pt-Co interactions at the surroundings of the platinum crystallites (28). It is in agreement with the fact that cobalt oxides are reduced more easily than cerium oxides which may create "metal support interactions" with platinum. Nevertheless, there are no great differences in the product distribution occur after an oxidation-reduction cycle. On both catalysts the selectivity in isomers is increased after an oxidative reaction. The values of the ratio 3-MP/n-H obtained from the methylcyclopentane hydrogenolysis are not modified by this redox cycle. These results lead us to conclude that the nature of the catalytic sites is unchanged during the redox cycle. After analyzing the results obtained under hydrogen atmosphere we are going to underline the results concerning the CO
-+ CO reaction. 2 In the figures 1 and 2 we have mentioned the results obtained on the Pt-Co cata-
lysts and it is obvious that the light-off is 25°C lower on the catalyst where the two salts were reduced : i) the cobalt reduced at first, il) then reduction of platinum. Not only the double on reduction process is important, but also the CO+0
gas 2 flow mixture. When the flow is slow, 20cm'.min-1, we can notice differences between the two types of catalysts, but when it is equal to 80cm' .min -1 no large differences occur, only 5°C for the light-off, between the catalyst for which the two salts were reduced and the other one for which the two salts were previously calcined prior to reduction. To try to understand these differences, we may suggest that on the doubly reduced catalyst the cobalt is at the lower oxidation state CO(+II) and on the doubly oxidized catalyst the cobalt is only CO(+I1I). The first one dissociates oxygen more easily at slow flow rates so that the light off is lower. Boreskov (21) has mentioned that CoO shows higher catalytic activity in CO oxidation than co agreement with our results.
203
which is in
254 The comparison between the catalysts which were both calcined prior to reduction, except for the Pt-Co previously mentioned, shows that all the Pt-Co catalysts and the Pt-Ce-Ni system are better than the 0.2% Pt. The catalysts: Pt-Ni, Ni-Ce and 3% Ni have higher light-off temperatures (figure 3). It is remarkable that with Pt-Co catalysts under hydrogen, in isomerization reactions (28-29), cobalt behaves as a poison. with Pt-Ni catalysts, in the same conditions nickel plays the role of an additive except for the 50 atom% in Ni and Pt (30). In the oxidation reaction it is exactly in opposite behavior: Pt-Co is the best system and Ni is a poison for our studied systems. CONCLUSION Our laboratory reactor experiments have shown that the surface structure of the bimetallic used are composed with a random mixture of Pt and Ni or Co sites. This structural information has been obtained by using the chemical probes (isomerization and hydrogenolysis of 2-methylpentane and methylcyclopentane respectively and oxidation) which are more sensitive than the physical techniques. The cobalt which is a poison when it is added to platinum for the isomerization reactions is the best additive in our case for the CO oxidation reaction. The results of this study show the importance of understanding interactions between the metals and the metal support interactions. REFERENCES 1 - D. Tornmek, S. Mukherjee, V. Kumar and K.H. Bennemann, Surf. Sci., 114 (1982) lI. 2 - J. Sedlacek, L. Hilaire, P. Legare and G. Maire, Surf. Sci., 115 (1982) 54I. 3 - G. Broden, T.N. Rhodin, C. Brucker, R. Benbou and Z. Hurych, Surf. Sci., 59 (1976) 593. 4 - R.W. Joyner and M.W. Roberts, J. Chem. Soc., Faraday Trans. I 70 (1974) 1819. 5 - G.G. Low and A. T. Bell, J. Catal., 57 (1979) 397. 6 - F. Solymosi and A. Erdohelhi, Surf. Sci., 110 (1981) 663. 7 - S. Ichikawa, H. Poppa and M. Boudart, J. Catal., 91 (1985) I. 8 - N. W. Cant and D.E. Angove, J. Catal., 97 (1986) 36. 9 - J.L. Gland, M.R. Mc Clellan and F.R. Mc Feely, J. Vac. Sci. Technol. A., 1 (1983) 1070. 10 - B.E. Hayden and A.M. Bradshaw, Surf. Sci., 125 (1983) 787. 11 - A. Crossley and D.A. King, Surf. Sci., 95 (1980) 13I. 12 - T. Engel and G. Ertl, J. Chem. Phys., 69 (1978) 1267. 13 - M. Boudart and G. Djega-Mariadassou, in "La cinetique des reactions en catalyse heteroqene", p. 168 (Masson) 1982. 14 - G. Maire and F. Garin, Catalysis: Science and Technology, 6 (1984) 162 - Ed. J.R. Anderson and M. Boudart (Springer Verlag). 15 - F: Garin, S. Aeiyach, P. Legare and G. Maire, J. Catal., 77 (1982) 323. 16 - G. Meunier, B. Mocaer, S. Kasztelan, L.R. Le Coustumer, J. Grimblot and J.P. Bonnelle, Applied Catal., 21 (1986) 329. 17 - J.P. Brunelle, Pure Appl. Chem., 50 (1978) 121I. 18 - G.J.K. Acres, A,J. Bird, J. W. Jenkins and F. King, Catalysis: A specialist periodical reports, 4 (1981) 1 - Ed. Royal Society of Chemistry.
2SS
19 - J.M. Oartigues, A. Chambellan, S. Corolleur, F.G. Gault, A. Renouprez, B. Moraweck, P. Bosch-Giral and G. Oalmai-Imelik, Nouv. J. Chim., 3 (1979) 591. 20 - F. Garin, O. Zahraa, C. Crouzet, J.L. Schmitt and G. Maire, Surf. ScL, 106 (1981) 466. 21 - G.K. Boreskov, Catalysis: Science and Technology, 3 (1982) 39 - Ed. J.R. Anderson and M. Boudart (Springer Verlag). 22 - V. Ponec, Adv. Catal., 32 (1983) 149. 23 - F.G. Gault, Adv. Catal., 30 (1981) 1. 24 - T. Engel, G. Ertl, Adv. Catal., 28 (1979) 1. 25 - R.J. Behm, K. Christmann, G. Ertl, Surf. ScL, 99 (1980) 320. 26 - Se H. Oh and J.E. Carpenter, J. Catal., 98 (1986) 178. 27 - F.G. Gault, F. Garin, G. Maire, Growth and Properties of metal clusters, 451 (1980) - Ed. J. Bourdon. 28 - S. Zyade, F. Garin, L. Hilaire, M.F. Ravet, G. Maire, Bul. Soc. Chim. F., 3 (1985) 341. 29 - S. Zyade, These de speciallt.e 3eme cycle (1984) Strasbourg. 30 - S. Aeiyach, F. Garin, L. Hilaire, P. Legare and G. Maire, J. Mol. Catal., 25 (1984) 183. 31 - S. Zyade, F. Garin, L. Hilaire and G. Maire, unpublished results.
This Page Intentionally Left Blank
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
257
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE EFFECT OF WEIGHT LOADING AND REDUCTION TEMPERATURE ON Rh/SILICA CATALYSTS FOR NO REDUCTION BY CO
W.C. HECKER and R.B. BRENEMAN Chemical Engineering Department, Brigham Young University, Provo, UT 84602
ABSTRACT Rhodium catalysts with very low weight loadings (0.01 to 0.06%) are used to efficiently reduce the ni trogen oxides in automobil e exhaust. Many academic studies, however, are done using catalysts with high weight loadings (1 to 10%). The study reported herein explored differences in activity and surface properties between high and low weight-loaded Rh catalysts before and during NO reduction by CO. Ini ti al and steady-state turnover numbers were found to increase significantly (factor of 7) as weight loading was increased from 0.2% to 12%. At the same time the activation energy decreased from 36 to 24 kcal/mole. Power rate laws determined by varying NO and CO partial pressures were found to be fairly similar for the high and low weight-loaded catalysts. These results seem to indicate that NO reduction is a structure sensi tive reaction. The effect of varyi ng the reduction temperature of the catalysts between 200 and 450°C was also explored, but no significant effect was seen on hydrogen uptakes, infrared spectra, initial rates, or steady state rates.
INTRODUCTION The reduction of nitric oxide over supported rhodium is one of the most As such, it has been important reactions in automobile exhaust catalysis. studied quite extensively over the past ten to fifteen years. As one examines the literature on this subject, it is found that the studies can be classified into two categories according to rhodium weight loading. The first type are those done with weight loadings of around 0.01 to 0.06 weight percent, which are typical of actual weight loadings used in automobil e catalytic converters [1-2]. The rhodium in these catalysts is most probably 100% dispersed. The second type are those done on much higher weight loadings of approximately 1 to 10 percent rhodium. These higher loadings are often used for studies of an academic nature, and also so analytical tools such as infrared spectroscopy can be used with significant sensitivity [3-7]' The rhodium in these higher weight loading catalysts is often much less than 100% dispersed. The prime objective of the present study was to determine how the kinetics
258
of NO reduction by CO differ over high and low weight-loaded rhodium/silica catalysts.
A secondary objective was to determine if the temperature at which
these catalysts were reduced affects the kinetics or surface properties.
In
order to determine these effects, rhodi um/s i l ica catalysts of five di fferent weight
loadings
ranging
from
0.04%
to
12%
rhodium
were
prepared,
and
measurements of hydrogen uptake, IR absorption, and NO reduction activity were made.
EXPERIMENTAL The detail s of the experimental apparatus and procedures used can be found in
reference
8.
In
brief,
the
five
catalysts
were
made
by
aqueous
impregnation of RhC1 3.3H 20 onto cabosil silica. The catalysts were calcined at 450°C for two hours in air. Hydrogen uptake measurements were made us i ng standard volumetric methods [9].
One variation was that the catalysts were
saturated with hydrogen at 180°C instead of room temperature before measuring thei r isotherms. Reaction studies were carried out in a specially designed infrared cell which doubl ed as a flow reactor [10].
Before a given experiment, the catalyst
was pressed into the form of a thin wafer, placed into the reactor cell, and reduced at 200-450°C for twenty hours.
The catalyst was then run to steady-
state in 3.4% CO and 0.8% NO for sixteen hours before any steady-state data were obtained.
All data were obtained under differential reactor conditions
and analysis of the feed and product gases was accompl ished using an automated gas chromatographic system [11].
RESULTS Hydrogen Uptakes Table 1 shows hydrogen uptakes for four di fferent weight loaded catalysts and for three di fferent reduction temperatures.
As can be seen, the 4.3 and
1% catalysts show essentially no effect of reduction temperature for reduction temperatures between 200 and 450°C.
As weight loadi ng decreased, the absol ute
uptakes decreased also, and therefore it was more difficult to obtain accurate uptakes.
This
explains
the
variance
in
uptake measurements
for
the 0.2%
catalyst. Table 2 shows loaded catalysts.
average
rhodium
dispersions
for each of the five weight
These were calculated froo the hydrogen uptakes in Table 1
by a ssumi ng each hydrogen atom represented one rhodi um surface si te and from the definition of dispersion which is the fraction of total rhodium atoms that
259
TABLE 1 Hydrogen uptakes as a function of weight loading and reduction temperature.
Weight % Rh
12 4.3 1.0 0.2
HZ Uptake (Ilmoles/gram catalyst) Reduction Temperature (K)
473
573
723
111 91
88
33
34
88 32
9 -13
TABLE 2 Calculated average rhodium dispersions as a function of weight loading.
Wt% Rh
H/Rh
12 4.3 1.0 0.2 0.04
0.19 0.43 0.67 1.0 (1. O)a
aAssumed value based on extrapolation.
260
are surface atoms.
As can be seen, the dispersion increases significantly as
the weight loading decreases. The value of 100% dispersion for the 0.2% catalyst represents the average of its hydrogen uptake values. Therefore, by extrapolation, the 0.04% catalyst was also assumed to be 100% dispersed,
Ki netic data Fi gure 1 shows turnover numbers for NO reducti on by CO as a functi on of time for each of the five catalysts of this study.
Turnover number is defined
as rate per unit site and was determined using the di spersion data in Table 2.
As can be seen, the activity of each catalyst decreases f'rrm its initial
value
to
a
steady-state
value
after
approximately
10-15
hours.
Very
importantly, the steady-state and initial activities for the higher weight loading catalysts are significantly greater than those of the lOiler weight loading catalysts.
Also significant is the fact that the two lowest weight
loading catalysts, the 0.2% and 0.04% catalysts, both of which are apparently 100% dispersed, have essentially the same activity.
Thus, it would appear
that activity correlates with dispersion and as dispersion increases, activity decreases.
50
C'?
0 T"""
40
X
I
'(l)
.0
E
::J
Z
.... Q)
......,
T = 484 K p = .0071 atm 1'-0 p = .028atm CO Fresh Catalysts
12%Rh/Silica
0
4%Rh/Silica
X
l%Rh/Silica
0
.2%Rh/Silica
•
30
o
(l)
Cf)
•
.04%Rh/Silica
20
>
0
C
....
10
x
::J
r-
0 0
5
10
15
20
25
Time(hours) Figure 1. Rates of NO reduction by CO as a function of rhodium/silica catalysts of different weight loadings.
time for five
261
Figure 2 shows transient activi ty data for NO reduction by CO for two 4% rhodium sil ica/catalysts, one of which was reduced at 200°C and the other which was reduced at 300°C.
As can be seen, there is virtually no difference
at all at any point in time in the activity of the two catalysts.
Thus, once
again reduction temperature does not seem to effect the catalyst behavior at least
in this temperature range. 1.0
,----,----r----,----r-----, 4 % RhiSilica
0.8
T=191C P=.84 atm PNO=.0071 atrn PCO= ..028atm
0.6
Turnover Number 0.4
o
200C Reduction
•
300C Reduction
0.2
0.0 + - - - - - t - - - - - ' - - - - - + - - - - t - - - ~
o
2
4 6 Time (hours)
8
10
Figure 2. Effect of reduction temperature on rates of NO reduction by CO as a function of time for a 4% rhodium/silica catalyst.
Figure 3 shows steady-state activities as a function of temperature plotted in an Arrhenius form for the five different weight loadings of this study. Four
of
the
temperatures.
catalysts
are
al so
shown
at
two
di fferent
reduction
As can be seen, once again the 12% catalyst is more active than
the 4% catalyst which is more active than the 1% catalyst which is more active than the 0.2 and 0.04% catalysts. catalysts are coincidental
Once again also, the two low weight loading
in their steady-state activities.
Also, in each
case where two reduction temperatures were used, the data seen to foll ow the same line, except in the case of the 1% catalyst where there seems to be a sl ight variance.
Fran the slopes of the 1 ines in Figure 3, the apparent
activation energies for NO reduction by CO of each of the catalysts can be determined.
These activation energies are shown in Tabl e 3.
As can be seen,
the activation energies decrease from 36 to 24 l
from 100% to
262
P
_ .0071 atm
o
•. 028 atm
I!II 4%Rh 473 K Reduction o 4%Rh 573 K Reduction
f'O P
co
12%Rh 473 K Reduction
•
1%Rh 473 K Reduction
X
l%Rh 573 K Reduction
.....
ill
>
o
E
o
10. 3
:J
.2%Rh 473 K Reduction
• .2%Rh 573 K Reduction
I-
• . 04%Rh 473 K Reduction
o 10
.04%Rh 573 K Reduction
-4
1.8
2.3
1.9
2.4
Figure 3. Arrhenius plot for steady-state rates of NO reduction by CO for five rhodium silica/catalysts of different weight loadings and reduction temperatures. 19%.
This trend is consistent with the work of Oh, Fisher, et.al. [12J, who
showed that as they went from a low dispersion, single crystal catalyst to a highly dispersed
rhodium/alumina catalyst, the activation energy increased
from 30 to 45 kcal/mole. TABLE 3 Apparent activation energies of NO reduction by CO for rhodium/silica catalysts of fi ve di fferent we i ght 1oadi ngs and two different reduction temperatures. Rh loading
Reduction
Data
Apparent Activation
(wt.%)
Temperature (K)
Points
Energy (Kcal/mole)
12 4.3 4.3 1.0 1.0 0.2 0.2 0.04
473
4
24
473 573 473 573 473 573 573
4 5 6 4 4 4 3
33 29 31 30 35 35 36
Power rate 1aw expressi ons were determi ned for a hi gh and a low we ight loaded catalyst to detennine if there was any effect of weight loading on CO and NO partial pressure dependency. The partial pressure of CO (P e o) was varied between .0084 and .047 atmospheres and the partial pressure of NO (P NO) was varied
between
.0031
and
.013 atmospheres.
The resulting
power law
expression for a 0.2% rhodium catalyst shows NNO and for a 4.3% rhodium catalyst
= k 1 Pe8·2PNOO.4
NNO
= k2Pe8·1PNOO.5
NO reduction and k 1 and k 2 are rate positive order dependence on PCO and the moderate negative order dependence on PNO is quite consistent with previous where
NNO
is
coefficients. work [3-4J.
turnover
number
The very
However,
for
small
the slight difference in the expressions between the
0.2% and the 4% catalysts are probably statistically insignificant.
Thus, it
appears that there is little or no effect of weight loading (or dispersion) on the CO and NO partial pressure dependencies, at least in the range of partial pressures studied here. Infrared data Two types of infrared measurements were used in this study.
The first type
was made foll owing reduction of the catalys tin hydrogen and then decreasi ng the temperature of the catalyst to room temperature and exposing it to 3% CO.
The resulti ng CO bands all owed us to detennine information regardi ng the
structure of the surface [13J.
IR data of this type were obtained for 0.2%
and 4% rhodi um/sil ica catalysts. similar.
The resulti ng spectra [8J looked qui te Each had a large intense band between 2060 and 2080 cm-1 which is
characteristic of a site.
single CO molecule absorbed on a zero-valent rhodium
The spectra for the 0.2% catalyst did contain a slight bump at 2108
cm-1 which
is characteri stic of a Rh(I}
site.
Thus, there was a sl ight
difference between the low weight loaded catalyst and the high weight loaded catalyst. The second type of infrared spectra obtained under steady-state.
infonnation obtained consi sted of
in situ
reaction conditions after the catalyst had reached
Taking this type of spectra for the 4% catalyst, it was found
that the catalyst surface
is dominated
by adsorbed NO.
Thi s result is
consistent with what has been seen before [3,5J and it's also consistent with the observed negative order dependency on PNO' In situ spectra were much more difficult to obtain for the 0.2% catalyst because of the low amount of rhodium.
The signal
to
noise ratio became much greater and hence it was
difficult to make finn observations.
However, it did appear that the dominate
species on this catalyst was al so adsorbed NO.
264
DISCUSSION The
question
catalysts
are
of
more
why
the
active
high than
weight the
loaded,
low
weight
catalysts is an interesting one to consider.
low
dispersion
loaded,
high
rhodium
dispersion
One expl anation might be that
the Rh(I) sites seen fran the infrared spectra to be on the low weight loaded catalysts, sites.
are
somewhat less
However,
the very
active
small
(or
inactive)
number of
Rh (I)
canpared sites
to
the Rh(O)
seen on the 0.2%
catalyst is not nearly enough to explain the three to four fold difference in activity between the 0.2% catalyst and the 4% catalyst. A second
possible
explanation
for
the
greater
activity
of
the
lower
dispersed catalyst might be that the rhodium exists in two different phases, a crystalline phase and a dispersed phase, and that the crystalline phase, which more prevalent on the higher weight-loaded catalysts, is much more active than the dispersed phase. to
describe
This is similar to an argument used by Yao, et al. [14J
differences
rhodium/alumina catalyst.
they
saw in
activity
with
weight
loading
on
a
However, the infrared data tend to discount this
explanation as well, since according to Rice, et al. [13J, CO adsorbs on a dispersed phase in a dicarbonyl structure which has distinct bands at 2030 and 2100 cm-1. In thi s work, those bands were not observed. A third possible explanation for the observed behavior could be a metalsupport interaction.
Electronic metal-support interactions would result in an
oxidized rhodium surface which waul d be detectable in the infrared.
However,
since very little oxidized rhodium was detected, it appears that one can discount this possibility also. A final possible explanation is that of a traditional structure sensitivity as suggested by Boudart several years ago [15J.
In his work he showed that as
dispersion decreases, crystallite size increases and the average coordination number, or number of nearest neighbors for any site, increases.
In previous
work on rhodium/silica for NO reduction by CO [3], it has been shown that the rate determining step is probably NOa+S
+ Na+O a. Since this is a step that takes pl ace on two adjacent sites, it stands to reason that the more nearest
neighbors a catalyst had, the more readily thi s step, and thus the overall NO reduction reaction, would occur.
Also, as the rate detennining step, this
reaction is primarily responsible for the apparent activation energy. one can
Thus,
postul ate that as weight loading increases, the number of nearest
neighbor sites increase which decreases the activation energy of the ratedetermining step and increases its rate.
In the present work, of course, it
has been seen that the activation energy does decrease as the rate becomes greater and as
the weight loading increases.
Thus, there seems to be a
consi stency here that would suggest that NO reduction on rhodi um/sil ica is
265
structure sensitive and that this is the reason that activity increases as weight loading increases.
CONCLUSIONS In conclusion, rhodium/silica catalysts of low to intermediate dispersions and high weight loadings are significantly more active than catalysts of high di spersion and low weight loadings.
The high weight loaded catalysts have a
significantly lower activation energy than the low weight loaded catalysts. Power rate law expressions don't seem to be significantly different between high and low weight-loaded catalysts.
These observations along with infrared
observations tend to be consistent with the fact that NO reduction by CO on rhodium/silica is a structure sensitive reaction, and that as the coordination number rhodium crystallites increases, the activity also tends to increase. Further, it has been seen that reduction temperature has essentially no effect on rhodium dispersion or NO reduction activity.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
L.L. Hegedus and J .J. Gumbleton, Chemtech, 10 (1980) 630-642. K.C. Taylor, Automobil e Catalytic Converters, Berl in, 1984. W.C. Hecker and A.T. Bell, J. Catal., 84(1983) 200. W.C. Hecker and A.T. Bell, J. Catal., 85(1984) 389. W.C. Hecker and A.T. Bell, J. Catal., 92(1985) 247. H. Arai and H. Tominaga, J. Catal., 43(1976) 131. F. Solymosi and J. Sarkany, Appl. Surf. Sc i . , 3(1979) 68-82. R.B. Breneman, M.S. Thesis, Brigham Young University, 1986. C.H. Bartholomew and R.B. Pannell, J. Catal., 65(1980) 390. R.F. Hicks, C.S. Kellner, B.J. Savatsky, W.C. Hecker, and A.T. Bell, J. Catal., 71(1981) 216. W.C. Hecker and A.T. Bell, Analytical Chem., 53(1981) 817. S.H. Oh, G.B. Fisher, J.E. Carpenter and D.W. Goodman, presented at AIChE Meeting, Chicago (Nov 1985). See also the paper by Fisher et al. presented at this meeting. C. Rice, S. Worley, C. Curtis, J. Guin and A. Tarrer, J. Chem. Phys . , 74 (1981) 6487. H.C. Yao, Y.Y. Yao, and K. Otto, J. Catal., 56(1979) 21. M. Boudart, Adv. in Catalysis, 20(1969) 153.
This Page Intentionally Left Blank
.\. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
267
REACTIVATION OF LEAD-POISONED pt/A1 203 CATALYSTS BY SULFUR DIOXIDE
J. W. A. SACHTlER, I. ONAl, R. E. MARINANGElI Allied-Signal Engineered Materials Research Center, 50 East Algonquin Road, P.O. Box 5016, Des Plaines, Il 60017-5016, U.S.A.
ABSTRACT The effect of S02 on the oxidation of C3HS by Pb-poisoned and Pb-free Pt/Al?03 catalysts was investigated. Transient S02 injection experiments were performed on engine-aged catalysts and catalysts aged in the laboratory with PbBr2" In both cases, the increased activity for C3HS oxidation nue to S02 exposure is attributed to a reversible promotion of C3HS conversion by S02 (independent of Pb content) and a permanent reactivatlon. Catalyst characterization has shown that the permanent reactivation of the Pb-poisoned catalysts involves formation of large PbS0 4 crystals which effectively removes Pb from Pt crystallites. The C3HS oxidation activity of Pt catalysts is also permanently enhanced by S02 regardless of the presence of Pb. In this case, the mechanism may involve sulfate formed on the alumina. S02 poisoning of CO oxidation offsets the enhancement due to removal of Pb from Pt crystallites. Thus, the net effect of 502 on CO conversion by Pbpoisoned catalysts is small. I NTRODUCTI ON It is well known that lead and sulfur compounds can poison exhaust gas catalysts [Ref. 1J. Sulfur dioxide, however, can have beneficial effects. Michalko [Ref. 2] noted that exposure to anhydrous S03 was effective for regenerating exhaust gas catalysts poisoned by Pb. Hammerle and Graves [Ref. 3] observed that poisoning of a Pt/Al 203 catalyst by Pb is reversed by exposure to S02 below 650°C. On the other hand, Vao, et al. [Ref. 4] observed that S02 enhances the rate of C3HS oxidation over a fresh Pt/Al 203 catalyst. The purpose of this work was to study the effect of S02 on Pb-free and Pb-poisoned Pt/Al 203 catalysts and to gain insight into the mechanism of (re-)activation by S02' Such information is needed for the development of Pb-tolerant oxidation catalysts. This type of catalyst might be useful in countries where Pb-free gasoline is not readily available.
268
EXPERIMENTAL The catalyst aging and activity testing were done with a monolithic formulation consisting of 0.36 wt.% Pt (10 g/ft 3) on y-A1 203 supported on a 46.5 cells/cm 2 cordierite substrate. Engine-aging was conducted on a multi-mode cycle with a peak temperature of 760°C which has been described previously [Ref. 5J. The aging fuel contained 0.15 g Pb/l and 0.185 g SILo Aging was conducted for 300 hours at an average fuel rate of 6.9 kg/hour and an air/fuel ratio of 15.2. laboratory hydrothermal aging was conducted at 871°C or 927°C for 10 hours in 10% H20/90% air. laboratory Pb-poisoning consisted of exposure to PbBr 2 evaporated in N2 at 550°C. The flow rate during aging was 3 L/minute and the exposure time was two hours. Laboratory activity tests were conducted in a flow reactor on 2.2 cm diameter x 5.1 cm length cores taken from the inlet of aged monoliths. The synthetic exhaust gas composition is given in Table 1. The space velocity was 30,000 hr- 1 (STP) based on core volume. Catalyst response to 502 was examined by testing without 5° 2, continuing the test while injecting 20 ppm 5° 2, and testing again without 502' In order to study the effects of 502 and PbBr2 on CO adsorption and C3H8 oxidation in more detail, a series of experiments was conducted using infra-red (IR) spectrometry. The IR spectra were recorded on a Beckman IR-12 spectrometer; the samples for these experiments were pressed discs of 1 wt.% Pt/y-A1 203 with a Pt dispersion of 0.23 as measured by oxygen-hydrogen titration. A sample of the 1 wt.% Pt/y-A1 203 catalyst was poisoned with lead by exposure to PbBr2 vapor in N2, followed by hydrolysis with 3% water in He at 550°C. C3H8 oxidation was studied in a static mode, by injecting 1 mL C3H8 (STP) and 10 mL 02 (STP) into the 820 mL IR cell. C3H8 conversions were determined from the intensity of the gas phase C3H8 band at 2972 cm- 1 before and after heating 30 minutes at 260°C. Pb-poisoned and reactivated samples were characterized by Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-Ray Analysis (EDX), and X-Ray Diffraction (XRD). Results and Discussion Effect of 502 on
C~
Oxidation
Figure 1A shows the response of an engine-aged Pt/A1 203 catalyst to 502 injection at 300°C from 60 to 130 minutes. The conversion of C3HS increases from 15% to 53%. Following removal of the 5° 2, the C3H g conversion declines to
269
33%. The response to SOZ includes a permanent (long t e r ~ ) activation (15% to 33%) and a reversible activation (33% to 53%). Similar results were obtained at 400°C while at 600°C no effect of S02 can be observed (Figure lB). Figure Z shows the response of a PbBrZ-poisoned Pt/A1 Z03 catalyst to S02 injection at 500°C. This sample responds very much like an engine-aged catalyst. This result is consistent with reports that Pb-poisoning is due to transport of Pb to the catalyst in the form of halide species [Ref. 6J. The Pb halides may be hydrolyzed on the catalyst to form lead oxide species. However, catalyst exposure to PbO vapor in the same laboratory apparatus as used for PbBr Z aging did not cause significant deactivation. Since the PbBrZ-poisoned samples could also be reactivated with SOZ' we judged that the laboratory poisoning of catalysts with PbBr Z can be used to model the engine-aging of these samples with leaded fuel. The poisoning of the C3HS tion of a poisoned catalyst by IR experiments (see Table 2). wt.% Pt/A1 Z03 sample following of Pb species (Figure 3): i) ii) iii)
oxidation by PbBrZ' and the exposure to S02 and air was STEM and EDX examination of calcination in air at 500°C
permanent reactivaalso observed in the the PbBr Z poisoned 1 revealed three types
bimetallic Pt-Pb particles (50-l00A); Pb particles (probably PbO) (lOO-ZOOA); and amorphous Pb present everywhere on the A1 Z03 in a very thin layer.
Following reactivation by SOZ at 500°C, STEM examination (Figure 4) showed large Pb-containing particles with sizes up to 10,000A. Since XRD showed that the only crystalline Pb species present was PbS0 4, these large particles are concluded to be PbS0 4• In the IR spectrum of this reactivated disc, a large sulfate band was observed at 1390 cm- l• EDX consistently showed lower Pb levels in the Pt particles after reactivation. The results obtained with the engine-aged, laboratory PbBrz-aged, and model catalysts all clearly indicate that Pb-poisoned Pt/A1 Z03 catalysts can be reactivated by SOZ. The model experiments indicate that the reactivation mechanism involves the formation of PbS0 4, which removes Pb from the Pt. Additional evidence for this mechanism is found in the CO adsorption experiments, described in the next section. The PbS0 4 formed by S02 reactivation is not stable at high temperatures and its decomposition again causes poisoning of the Pt, as judged by the almost complete disappearance of the sulfate band in the IR spectrum and the low C3HS conversion observed after heating a reactivated disc in vacuum at 750°C.
270
TABLE 1 Synthetic Exhaust Gas Composition Component
Concentrat i on 2190 ppm (C Basis) 2% 1520 ppm 4.8% 10% 11.1% 71. 7%
TABLE 2 Reactivation of Model 1 Wt.% Pt/A1 203 Catalysts Poisoned by PbBr2 Range of C3H8 Conversions at 260°C Sample Fresh Pt/A1 203
53-64%
Pt/A1 203 with Sulfate Present (without Pb)
58-64% 0-7%
PbBr2 Poisoned Pt/A1 203 PbBr 2 Poisoned Pt/Al 0 Reactivated by S02 at 500°C
55-57%
TARLE 3 IR Absorption Band Positions of CO on Sample
Pt-Pb/A12~
Position of CO Band (cm-1)
Reduced Pt/A1 203
2096
PbBr2 Poisoned Pt/A1 203 After Reduction
2025
PbBr2 Poisoned Pt/A1 203 Calclned at 500°C
2086
PbBr2 Poisoned Pt/A1 203 After C3H8 Oxidation 250°C and Evacuation 500°C
2045
PbBr2 Poisoned Pt/Al 03 Reactivated by S02 at 500°C After C3H8 Oxidatlon 260°C and Evacuation 500°C
2093
271
1 0 0 , - - - - - - - - - - - - - - - - FIGURE 1 S020nl 90 r - - - ~ - - - - - - - - - Effect of S02 on C3HS T:600°C Conversion at Different g 80 Temperatures of an Enginee; Aged Pt/A1 203 Catalyst c 70 o "00 60 S020nl ~ S02 Off T:4000C <3 50 f 40 30 T=300°C S020nl 20
,
s
10 '----'------'-----'_-'------'---'-_"'----'------'-----'_-'--
o
~
@ 00 00 100
l~l@loo
l00~OnO~o
Time (Minutes)
FIGURE 2 Effect of S02 on C3HS Conversion at 500°C of a Pt/Al?03 Catalyst Poisoned by PbBr2
50 c
o
"00
~
o co
U I
C'?
U
50
100
150
200
250
300
350
Time (Minutes)
_. • FIGURE 3:
~
STEM Micrographs of a Pb-Poisoned 1% Pt on y-A1 203 Catalyst
.
FIGURE 3A 100 kX, Showing Pt and Pb(O) Particles
FIGURE 3B 10 kX, Area Scan
272
FIGURE 4: STEM Micrographs of a Reactivated Pt-Pb-y-A1 203 Catalyst FIGURE 4A FIGURE 48 20 kX, Area Scan Showing Abundance of 150 kX, Showing a Very Thin PbS0 4 PbS0 4 Crystals, Ranging from 0.05-0.5]J Crystal (a Rare Observation) FIGURE 5
/
80
Effect of S02 on C3HS Conversion at 350°C of a Pt/A1 203 Catalyst Hydrothermally Aged at S70°C
302011
§ 70 .~
~ c
o 060 co I
(3 50
45
~_--'--_~_---'-_----J.
o
50
100
_ _'--_....L------'
150 200 250 Time (Minutes)
300
350
9 0 , - - - - - - - - - - - - - - - - - - , FIGURE 6 Effect of S02 on CO Conversion at 350°C of a 80 Pt/A1 203 Catalyst ~ Hydrothermally Aged at 927°C e....70 c o "00
~60 o 50 o
8
40 30 '---_-'--_--'-_---'-_--'-_ _'--_-'--------' o 50 100 150 200 250 300 350 Time (Minutes)
As shown in Figure 5, a hydrothermally aged Pt/A1 203 catalyst which does not contain Pb also shows a response to 502 in the laboratory activity test. Again, a permanent (4S% to 60%) and a reversible activation (60% to S4%) can be distinguished. It was noted that in this case the responses to $02 are much faster than with the catalysts containing Pb. However, in IR experiments using a fresh I wt.% Pt/A1 203 sample, treatments with 502 and air that produced sulfate on the catalyst as detected by IR did not give a significant increase in the C3HS conversion (see Table 2). It thus appears that our IR experiments could not replicate the conditions that cause permanent activation of the hydrothermally agerl catalyst in the activity testing. It may be speculated that the sulfate which forms on the alumina support can playa role in the activity tests, in accordance with the report by Yao [Ref. 4J. However, the nature of the participation of the sulfate in the C3HS oxidation could not be determined in our experiments. Because of the above mentioned results for the Pb-free system, it is possible that the permanent reactivation of the Pb-poisoned samples observed in the activity test contains a contribution from both mechanisms. The reversible activation by 5° 2 was observed regardless of the presence of Pb. We have no data to explain this effect. However, it might be speculated that adsorbed 503 is more effective for C3HS oxidation than adsorbed oxygen. As the equilibrium of 5° 2 oxidation to 5° 3 becomes unfavorable at high temperatures, this could explain the decrease in the magnitude of this effect with increasing temperature. Effect of 502 on CO Oxidation The engine-aged catalysts were very active for CO oxidation (>99% conversion) during the laboratory activity tests. There was no change in activity when 5° 2 was introduced. Of course, small changes could not be measured at such high conversions. Figure 6 shows that 2 injection at 350°C causes a sharp drop in CO conversion for a hydrothermally aged catalyst but that the initial activity is recovered after 502 exposure ends. The C3HS conversion for this sample increased with exposure to 2,
5°
5°
5°
The effects of PbBr2 poisoning and reactivation by 2 on CO adsorption were also followed on the model I wt.% Pt/A1 203 sample. Table 3 shows that on a reduced sample, Pb shifts the IR absorption bands of CO on Pt/A1 203 from 2096 to 2025 em-I. Oxidation of the sample causes an increase in band position, as has been observed in the literature (Ref. 7). Reactivation by $02 at 500°C essent i ally restores the ori gi na1 CO absorption band.
274
SUMMARY Reactivation of Pb-poisoned Pt/AI 203 catalysts for C3HS oxidation by exposure to S02 and air has been demonstrated for engine-aged and laboratory PbBr2aged monolithic catalysts using a flow reactor activity test, and in model experiments using PbBr 2-poisoned Pt/A1 203 powder. It has been shown in the model experiments that the reactivation by S02 involves the conversion of Pt-Pb species to Pt and PbS0 4• In the flow reactor activity tests, the C3HS oxidation activity of a Pb-free Pt/A1 203 catalyst was also permanently enhanced by S02' This mechanism may involve sulfate formed on the alumina, but the mechanism of its participation in the C3HS oxidation could not be determined in our experiments. It is possible that the permanent C3H8 oxidation activity increase of the reactivated Pb-poisoned samples observed in the flow-reactor activity tests may have included a contribution from this latter mechanism. In addition to permanent effects, S02 also causes a reversible activation in the activity tests. At temperatures above 500°C, the effects of S02 are not observed. This may be due to the instability of the sulfates and the unfavorable equilibrium of S02 oxidation at high temperatures. S02 exposure reversibly poisons CO oxidation on Pt/A1 203• S02 exposure also converts Pt-Pb species to Pt and PbS0 4• Thus, the net effect is that exposure to S02 has little effect on the activity of Pb-poisoned Pt/A1 203 for CO oxidation. REFERENCES 1.
a) b) c) d)
G. J. M. E.
C. Joy, G. R. Lester and F. S. Molinaro, SAE Paper #790943 (1979). C. Summers and K. Baron, J. Catalysis, 57, 380 (1979). Shelef, K. Otto and N. L. Otto, Adv. C a ~ y s i s , 27, 311 (1978). C. Su, W. R. H. Watkins and H. S. Gandhi, Appl. <:.italysis, ~ ' 59 (1984).
2.
E. Michalko, U.S. Patent #3,121,694 (1964).
3.
R. H. Hammerle and Y. B. Graves, SAE Paper #830270 (1983).
4.
H. C. Yao, H. K. Stepien and H. S. Gandhi, J.
5.
M. G. Henk, J. J. White, J. F. Skowron and I. Onal, SAE Paper #830271 (1983).
6.
B. Harrison, J. R. Taylor, A. F. Diwell and A. Solathiel, SAE Paper #830268 (1983).
7.
A. G. T. M. Bastein, F. J. C. M. Toolenaar, and V. Ponec, J. Chern. Soc. Chem. Commun., (1982) 627.
Catalysis,~,
231 (1981).
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1 ~ l 8 7 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
275
ALUi-lINA CARRIERS FOR AUTOHOTIVE POLLUTION CONTROL Ii.
NORTIER 1 and i-1. SOUSTELLE 2
lRhone-Poulenc Recherches 14, rue des Gardinoux - 93308 AUBERVILLIERS (France) '}
de Chimie Physique des Processus industriels
-D~partement
Ecole Nationale
des Mines de Saint-Etienne
Sup~rieure
158, Cours Fauriel - 42023 Saint-Etienne (France)
ABSTRACT Transition aluminas help solving the problem of automotive pollution control. Their intrinsic advantages : chemical inertia, suitable porosity are found in the forms of pellets or washcoated monoliths, as well as one important drawback: thermal aging. This very important feature was studied, experimentally and theoretically; a model is proposed, and an answer found: s t ab i l iz a t i on ,
ALUMINA AS A CATALYST CARRIER General Since 1975, CATALYSIS has been the only practical way for automotive manufacturers to meet the severe regulation of exhaust gas emission in JAPAN and in the U.S. Similar measures will be applied in Europe in the near future. For
numerous
catalysts
are
dispersed on
economic
and
supported the
technical
catalysts.
surface
of
a
reasons, This
means
catalytically
automotive that
the
almost
emission control active
phase is
inert material. That
material is the subject of this investigation. The usual constraints for catalyst carriers are - chemical inertia
~n
the reacting medium
- ability to be impregnated and to carry the active phase. - to allow a good diffusion of reactants (resp. products) to (resp. from) the catalytic surface. This is particularly difficult in the case of exhaust gas control, where the catalyst is submitted to very harmful conditions i.e. - exposure to poisons (Pb, Zn, P, S from lubricants and gasoline) vibrations mainly due to the cyclic work of the engine - high temperature (>8OO°C) which is possible but not abnormal Furthermore, the conversion of the reactants (CO, CnHp, NOx) must be nearly total even though the residence time of gases in the converter is extremely low
276
o
G5
r
,
_r
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HYDRARG ILLI TE I.
r..
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r
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,
,
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1
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~
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r
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x
~
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hg. I - Different kinds of oxides and hydroxides of ahminum
ALUMINIIM
ALKOXIDE PROCESS
ALUMINA CARRIER
Fig. 2 - Industrial processes providing transition alumina carriers
277 'lost of these problems are sat isfac tori 1y resol ved by using t ransi t ion aluminas as catalyst carrier for the active phases (Pt, Pd, Rh) and promoters (CeO}, NiO, FeO..• ). Among the numerous transition aluminas (as illustrated in Fig.l),gamma, delta illld theta AI ~repared
are the most used. These disordered spinels can be Z03 by dehydration of boehmite. Fig. 2 shows the main ways of producing
such aluminas. The reasons for the choice of alumina are numerous : - Alumina is cheap, partly because the raw materials for special aluminas are GIBBSITE or ALUMINIUM, both of which are available in large amounts at low cost (especially GIBBSITE) derived from bauxite. - The Iso Electric Point of alumina is 9 ; its surface can be electrically charged either positively or negatively and therefore, can selectively adsorb ions. Alumina does not give rise to chemical reaction with the gas feed (except for some poisons). Moreover, since the diffusion of Platinum is very low upon alumina this active metal is stabilised as small clusters with a large surface area. Furthermore, alumina can be shaped with an accurate control of its porosity. This is very important because the catalytic processes of exhaust gas control are most often diffusion limited. From the outset, two kinds of shapes -pellets and monoliths- were developed. They are discussed separately below. Pellets At least four processes are known for making pellets from a powder - PAN PELLETIZING - OIL DROP - EXTRUSION - PILLING or TABLETTING Because of vibrations, edges have to be avoided and only the two first processes are still employed, since they provide spherical particles. The design of the carrier must include the necessity of achieving high efficiency at very low residence times. Consequently the lowest resistance to mass transfer from the gas flow to the catalytic material surface is required, leading to the following characteristics: - A small radius provides a large contact area between the beads and the gas, and lowers the intraparticle distance to the active site,
278
Fig. 3 - Cross section of a pellet type converter --------
-- - - - - - - - -
----------,
pore volume distribution: derivation of p.v.d. cumulat ive s.s.a.
L,
.. 200 ;::'8
'"
8
">e ...'e"
Q,
O.
10 100 PORE DIAMETER
1000 (nm)
fig. 4 - Illustration of bimodality
GOOD
Fig. 5 Shematic showing extremes of micro-macropore distribution
279 TIlis second aim is also the reason for peripheral impregnation of the precious metals, - A high level of macropores (diameter more than 0.1
~m)
facilitates the
intraparticular diffusion, as micropores (diameter less than 20 nm) are necessary to develop a high surface area. This double feature is know as bimodality, illustrated in fig. 4. - Furthermore, poisoning by Zn, Pb and P creates an amorphous, vitreous surface on the bead that clogs the micropores and only leaves the macropores open. Thus the distribution of microand macropores must be well designed as illustrated in Fig. 5. Photographs 6 to 9 show some details of a porosity distribution which is exceptionally well adapted to the automotive exhaust control application. To increase the porous volume by addition of macropores is also an advantage for a cold start efficiency, since this decreases the total heat capacity of the catalytic bed, and enables the catalyst to reach its lightoff temperature more quickly. Thus, the diffusional properties lead us to design very small and porous beads. However mechanical considerations limit this tendency since the crush strength of beads is proportional to the square of their radius and is a decreasing function of their porosity, as illustrated in Fig. 10. Practical considerations with respect to canning and pressure drop through the converter prohibit use of beads smaller than 2 mm diameter. These features as well as the industrial feasibility led to the use of carriers such as those shown in Fig. 11 (1975-1979) and Fig. 12 (since 1979). Monoliths Vibrations can put the beads in motion so that they collide each with other and their surface are abraded. This is the attrition phenomenon which is absent from monolithic structures within which no internal shock can occur. Two types of commercial monolithic substrates are available, made from CERAMICS (fig. 13) or refractory METALS (Fig. 14). The production methods are : ceramic monolith : mixing components, EXTRUSION and reactive calcination - metallic monoliths: wrapping two sheets of metal, one of them being corrugated, and the other flat. Neither of these two materials is suitable for direct impregnation with precious metals, their specific surface area being much too low (less than 2/g) 10 m to allow a good dispersion in a reasonable volume. This explains the need for an alumina coating on the monolithic substrate, this
c~ating
being
"" 00 o
Fig. 6,7,8,9 - S.E.M. photographs of particles exhibiting the chesnut-bur porosity
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Fig. 11-12 - Two commercial carriers
Fig. 13 - Ceramic Monoliths Photograph from Ceramiques et Composites, Bazet (France)
10
nIAf',f:T~E aE~
VPAIlENT
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Metallic Monoliths Photograph from SUddeutsche KUhlerfabrik Julius Fr. Behr GmbH & Co KG Fig. 14 -
Fig. 15 - Cross section of an alumina coated ceramic monolith
"" [JJ
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284
Fig. 16 - Macroporou5 alumina coating
Fig. 17 - S.E.M. photograph of a fi ller alumina
Fig. 18 - T.E.M. photograph of a binder alumina
285
itself the catalyst carrier. Although other processes have been developed, the most widely used method is wash-coating, i.e. dipping the substrate into an aqueous suspension of alumina, blowing out the excess suspension in order to unclog the substrate channels and calcining. An alumina coated ceramic monolith is shown on Fig. 15. A better diffusion of the reactants can be obtained by using a macroporous coating as shown on Fig. 16. The binder-filler process which involves two kinds of aluminas, one of which being dispersible (Fig. 17), and the other being made of coarse grains (Fig. 18), is then especially suitable. Pellets and monolith specific features Comparison of a classical washcoated monolithic carrier (20 W % Al
on 20 3 400 cell/inch2 cordierite monolith) with a standard pellet support (SCM 129 X, 2.4-4 mm), shows (Fig. 19) - similar densities and heat capacities - a greater external (or geometric) surface area for monolith. The intrinsic advantages of the beads are : - turbulent flow of the gas which facilitates mass transfer - mobility in the converter which lowers poisoning - as the pellets are small, thermal shocks are not harmful Their main drawback is : attrition Advantages of monoliths are : - no attrition - ease of assembly - lower pressure drop Their main drawback is a risk to melt or break under high temperature (especially in the presence of Pb). Characteristics of the alumina common to both pellets and monolithic substrates In each case, alumina must provide a suitable surface to precious metals. This requires both chemical purity for the nature of the surfaces and thermal stability of surface area. Chemical purity is not critical in automotive exhaust control, compared with ego Reforming; Fig.20 illustrates the required level of purity. Thermal stability is not so easy to obtain. Two mechanisms can account for the instability of porous transition aluminas: - On the one hand, they consist of small crystals and a decrease in specific surface area would mean a decrease in the free enthalpy, because of the surface energy. This is
illus~rated
in Fig. 21-22.
286
CARRIER
HEAT CAPACITY PER LITER
BULK DENSITY elg / i1ter)
MONOLITH 400 CELL / INCH
.
o.
( J 11.1
)
GEOMETRIC SURFACE AREA PER LITER (m2 /1)
39
343
2 8
O. 47
408
2. 8
o
360
1. 1
MONOLITH ALUMINA LAYER
BEADS
43
Fig. 19 - Comparison
pellets and monoliths
TOTAL
2
w%
1500
ppm
700
ppm
Si02
8000
ppm
S04~
6000
ppm
CaO
1500
ppm
MgO
1500
ppm
Na20 Fe203
Fig. 20 - Maximum impurity content in alumina for exhaust control
287
1/
Fig. 21 - T.E.M. photograph of a fresh carrier, exhibiting microporosity
Fig. 22 - T.E.M. photograph of an over calcined carrier (24 h at 982°C), showing collapse of the micropores
288
- On the other, the thermodynamically stable phase is corundum These two phenomena involve solid state transport and are very slow unless very high temperatures are reached (above 900°C). These conditions are abnormal in automotive exhaust systems, but can occur especially in the case of engine malfunctions, high load, and transient cond i tions. The consequences can be a decrease in the activity, because sintering of alumina leads to sintering of precious metals and moreover, mechanical damage of the carrier. This is the principal reason for Research and Developement into the design of thermally resistant catalyst carriers. One of the most interesting methods is stabilisation by adding to alumina a small amount of a foreign oxide. The most recent theoretical developments in this field are reviewed below. STABILISATION Alumina is the most commonly used carrier in catalysts for the automotive exhaust gas control. Aluminas exist in several forms, according to the temperature, the raw materials and the processing (especially thermal) conditions. This is illustrated in Fig. 1 and 2. Among all these forms, two kinds of aluminas can be distinguished besides hydrates : - transition aluminas - alpha alumina or corundum which is the most stable form at any
~mperaUFe.
The transformation of the transition aluminas into corundum is a serious drawbac% to the use of these solids as catalyst carriers,since this irreversible transformation has two effects: - a major decrease in the specific surface area, falling from 100 to 2/g, 10 m with a disastrous drop in the catalytic activity, as a consequence a 20% decrease in the specific volume, creating voids in the catalytic bed which leads to an increase in the attrition phenomenon. The transformation into corundum occurs above 900°C, which temperature can be reached in a catalytic converter. Consequently it is necessary to stabilize the transition aluminas to high temperature (1250-13OO°C) in order to improve the durability of the catalyst. This stabilization can generally be achieved by adding inorganic elements to the aluminas. These elements can be
alkaline earths (ref. 1, 2, 3, 8),
zirconium (ref. 3, 8, 11), thorium (ref. 4,7), rare earths (ref. 5, 6, 7, 11), titanium (ref. 8), boron (ref. 3) and silicon (ref. 10,12). According to these authors, the stabilizing elements are incorporated either
289 just mixing hydrated or transi tion alumina IViLh the corresponding oxides, or !,,' impregnaU.ng alumina with an aqueous so l ut i or; containing the s t a biI i zing I ern en t as a thermal 1 y unsta b I c sal t. The tr ca t.rne n t IVi t h a gas con t a i n ing the
stabilizing element has also been described in the case of silicon. In each case, a calcination step at temperatures betwen 600 and 950°C folloIV8 this addition or impregnation step. Amounts of additive in the range of 0.1 to ]S % bv
weight of oxides have been reported. In contrast, other elements destabilize the transition aluminas and increase
the transformation rate to corundum. Iron III (ref. 13, 14), i'langanese Ill, Vanadium V, Molydenum VI, Cobalt III, Zinc II, chromium III (ref. 13) exhibit such an effet. Contradictory results are described with magnesium II (r&f. 13, IS) Lanthanium III (r&f. 13, 16) and Zirconium IV (r&f. 13, 14).
TUCKER and HREN (ref. 17) reviewed the effects of several other additives. Other authors, including HARMER (ref. 18) have put f orward qualitative explanations concerning the mechanism. However none of the proposed models has a forecasting feature. Experimental study of the effect of alien cations on the transformation of transition aluminas into corundum We studied the transformation rate of transition alumina as a function of the added ion nature. RHONE POULENC provided us with the starting material, which is similar to some of the commercial products of that company. The diameter of these spherical macroporous beads was 2 to 4 mm, the pore 2jg. 3jg volume 0.92 cm and the specific surface area 118 m The crystallographic phases were assigned by X-ray diffraction analysis as gamma . Chemical analysis indicated a total impurity content 203 lower than 800 ppm.
and delta Al
Addition of foreign cations (doping) was achieved by the method of incipient wetness using an aqueous. solution of the nitrate salt of the cation. A drying step (24 hours at 110°C) and a calcination step (1 hour at 600°C) followed for each preparation. The
following criteria governed the choice of the doping elements :
- they were available as water soluble nitrate salts (in order to carry out all impregnations under the same conditions) - their ionic radius was either very close or very different from the one of A1 3+ - they have only one stable oxidation state. Any redox phenomenon in the solid state was thus avoided and the valency of the alien cation is well defined. Consequently we can forecast the nature of the point defects
290
1378K
% a-A 1203 100
8
60 40
Zr
20 • -.d __ . ! -~-"""---
o
20
10
.-.6-~-'=~Ca La 40 Tlme Ihours]
30
Fig. 23 - Transformation ratio as a function of time at 110SoC A3 is the undoped Alumina
120 1378K
100
80 60
120
10
20
40
40 Time (hour s)
30
20
60
80
Fig. 24 a - Specific surface area versus heating time Fig. 24 b - Specific surface area versus transformation ratio o : undoped alumina, 6: A (Mg)
291 created by the addition of this cations. Some features of the added elements are collected in table 1, which also includes the temperature of the top of the exothermic peak which correlates hith the transformation to corundum. This peak was determined by Differential Thermal Analysis (DTA). TABLE 1: Some characteristics of the added elements Element
Valency
Ionic
Cationic
Peak
Sample
radius
ratio
temperature
reference
Al
+3
0.50 A
0.01
l272°C
A(Al)
Mg
+2
0.65
0.025
1277
A(Mg)
Ca
+2
0.99
0.01
1350
A(Ca)
Ga
+3
0.62
0.01
1276
A(Ga)
In
+3
0.81
0.01
1260
A(In)
La
+3
1.15
0.01
1384
A(La)
Zr
+4
0.80
0.01
1345
A(Zr)
Th
+4
0.95
0.01
1386
A(Th)
A possible influence of the doping method on the behaviour of the different samples was eliminated by using an "alumina" doped sample as a reference. This sample was prepared by impregnation with an aluminium nitrate solution, drying and calcining in the described conditions. It was compared with a sample impregnated with aqueous nitric acid at the same nitrate concentrations. These two samples gave identical results, i.e. same temperature of the DTA exothermic peak and same rate of transformation into corundum. Transformation ratio as a function of time Isothermal transformation curves were measured as a function of time. Samples were heated in static air in an electric furnace at a controlled temperature and were withdrawn at different times (up to 45 hours). The transformation ratio (into alpha A1
203)
was obtained by XRD analysis with
an accuracy of 5 %. Fig. 23 shows curves obtained at 1105°C. Most of them show a sigmoidal appearance. Comparing to that for pure alumina, two families can be identified: one is the group of accelerator additives, with an increasing effect following the order In, Ga, AI, Mg, and the group of inhibiting additives Zr, Ca, Th, La. From the position of the A (AI) curve, we can assign an accelerator effect to
292 the nitrate ions. The inhibiting effect of thorium is substantial, since the transformation ratio is only 1,5 % after 45 hours at 1105°C and 4,5 % after 160 hours. In the case of lanthanum, it is still more significant since no alpha Al
Z03
was
detected after 45 hours and only 1.5 % after 150 hours After calcining new XRD peaks appear that cannot be assigned to corundum. In order to attribute them, we heated the sample to l450°C for two hours ; at this temperature the transformation rates are 100 % in all cases. Magnesium aluminate (MgA1 In 20 3 A(Th).
204) in A(In), Zr0
was detected in A(Mg), Ca Al 2
in A(Ca), 120 l 5 (tetragonal and monoclinic) in A(Zr), and Th0
2
in
No new phase was detected in A(Ga)( Gallium can substitute for aluminium to produce a solid solution in corundum), and obviously in A(Al), too. Thus transformation of doped transition alumina is accompagnied by precipitation of a new phase, either the oxyde of the doping element
or a
mixed oxide of this element and aluminium. Figures 24 a and 24 b respectively show the dependence of the specific surface area (according to the BET method) on the heating time and the transformation ratio for pure alumina and magnesium doped alumina. It is evident that the faster the transformation to alpha A1
the faster the 203 specific surface area decreases. Furthermore, there is a linear relationship
between the surface area and the extent of transformation provided the latter is more than 10 %. The lines for pure and doped alumina exhibit the same slope. Influence of the content of the doping element The influence of the content in doping element was studied in the case of thorium with different cationic ratios, namely: 0.001 0.005 and 0.01. DTA curves for the three compositions are shown in Fig. 25. Each exhibits a peak at l388°C which is assigned to the transformation of thoria-doped alumina. At the lowest content, a second peak appears at l283°C which corresponds to the transformation for undoped alumina. Therefore, this sample behaves like a mixture of thoria doped and undoped alumina. XRD analysis after treatment at temperatures between 1283 and l388°C showed no thorium oxide, unlike the sample treated above l388°C . Thus, only the doped part of the alumina is stabilised by solid solution with thoria. As expected the curves for the weakly doped products are intermediate between that of pure alumina and that of A(Th). This indicates only a minor influence of the content of doping element at least above the minimum amount which is necessary to provide an homogeneous solid solution and to achieve the full effect of stabilisation.
293
A3 0,01
-,
"
",
,
,
I
'-
- .. - - ""
I
1
,
°..O~'
",.
r. , ' , i
... .....
\, I
1173
\.
I
1373
1573
Fig. 25 - Effect of the amount of thorium on the D.T.A. curves (cationic ratio)
- l n ~ dt
5
J
a
+
+
/+
/
/
/ +
+
'428 '403 1378 1343 7 750
Fig. 26 - Effect of the temperature on the rate of transformation for a) pure alumina b) : Zr doped alumina
294
A similar result was reported by SCHAPER (ref. 15) for influence of lanthanum oxide on the stability of gamma alumina carriers. Influence of temperature The transformation rate of pure and zirconium doped alumina was studied at different temperatures (1070, 1105, 1130 and 1155°C). Transformation rates are plotted versus temperature in ARRHENIUS coordinates in Fig. 26. Two straight lines appear from which it can be assumed that the ARRHENIUS LAW is fulfilled and the apparent activation energies calculated from thei r slopes. This energy is not influenced by doping, and the values obtained (533 + 30 -1 -1 for A (Zr) are in good agreement for A(Al) and 560 ± 30 KJ mole 1 with the existing literature, where results between 450 and 650 KJ mole- are KJ mole
reported. Modelling of the transformation Several authors have already proposed kinetic laws to describe this transformation. Different reaction orders (0,1 and 2) have been suggested but none of these proposals specified the actual nature of the reacting species. The notion of order of an heterogeneous reaction, relative to the initial product is meaningless if this is considered as pure in its phase. TUCKER and HREN (ref.17) reviewed different attempts to build models of gamma to alpha transition. Most of them are summarized in table 2 Table 2
Main at temps at modelling the gamma to alpha transformation Mechanism
Nucleation-growth
"
"
(with necessary previous sintering)
Material gamma from alun gamma, theta gamma thin film " + La 203
Reference (6) (7) (8) (9)
(4,5)
Stacking faults growth (nucleation at surface and neck region of particles)
gamma powder and thin film
Synchro shear (diffusionless cooperative atom movement)
Fe and Cr doped gamma
(3)
Sintering/synchro shear
theta
(2)
Volume diffusion
gamma
(10)
295 As none of these models takes into account the chemical reacting species, they cannot explain the influence of ions added in the initial network of the alumina. TUCKER (ref. 17, 22, 24) demonstrated by transmission electron microscopy that the polycrystalline grains of gamma A1 are mainly transformed into 203 alpha A1 203 monocrystals by a nucleation/growth mechanism, located on the surface of isolated grains in the neck region of sintering particles. Our proposed model (ref. 26, 27) is based on the spinel structure of the transition aluminas, as discussed by WELLS (ref. 28) Furthermore, as proposed by SOLED (ref. 29), these aluminas contain hydroxyl (OH-) ions, substituting for some oxygen (0 2-) ions. Alumina does not contain any divalent cations and in order to comply with the requirement of the spinel structure, we must assume that all the cationic divalent positions of the network are empty (vacancies). Taking into account all these facts, the formula for the transition alumina (ref. 26) can be written as : A1 2 0 03-v/2 (OH)v< > (1-v/2) where the symbol 0 represents a cationic divalent vacancy and < >
an oxygen
(anionic) vacancy. Three levels of differences can exist between the different forms of transition aluminas : 3 - the distribution of the A1 + ions between the tetrahedral and octahedral positions of the spinel can be more or less complex the amount of OH- ions, the presence of which can slightly distort the network of oxygen ions, can be more or less great the OH- ions can be more or less gathered on the surface of the grains. These differences do not modify the following model : Considering the case of impurity or dopant-containing transition alumina, Z
the foreign cation identified as M + may be incorporated in the spinel 3 lattice either by substitution of A1 + in a trivalent site, or by insertion in a divalent cationic site. Insertion as interstitial cations can be excluded 3 since the A1 + ionic radius is small.It can be speculated that a cation 3 having its ionic radius similar to that of A1 + would be preferentially incorporated by substitution, though a larger one would occupy a divalent site, the size of which is expected to be larger than that of trivalent ones. Let N Z 3 be equal to the ratio of M + ions substituting A1 + to those incorporated in the vacant divalent sites, and x the ionic fraction of elements M as regards to the total amount of cations in the alumina. Then, the general formulation of doped transition aluminas similar to the
296 previously determined formula in the case of pure alumina is A1 2( l-xt\) ~ l x 0 ( l - 2 x ( l - ~ » 0 (4-v-v) (Oll\ < > v The condition of electroneutrality of the crystal leads to the relation +
y
x (3N-z) - v / 2
On heating a transition alumina, dehydration occurs which can be written as a quasi chemical reaction between structure elements, i.e. according to the KROGER notation H + (0 2-0)" + (V2(1) 20 0)" Although occuring in the homogeneous phase, this reaction creates some 2(Oll-0)"
•
oxygen vacancies in the vicinity of the surface. The higher the temperature, the greater is the concentration of anionic vacancies. These vacancies are active in the sintering phenomenon and they can react with the intrinsic cationic vacancies (i.e. structurally present in gamma A1
leading to 203), the destruction of the spinel structure and the transformation into the
corundum form, following the reaction : (V 2- ) .. + (V"
o
V
)'~O
(2)
As a consequence of the above mentionned scheme, alpha alumina formation would proceed by a nucleation and growth mechanism. We will make some basic assumptions in order to facilitate the quantitative treatment : - the particles of transition aluminas are spheres of initial radius roo - the transformation proceeds from the surface at the interface of radius r (the nucleation is supposed to be homogeneous on the particle surface). The mechanism is based on three steps : i) anionic vacancies formation by removal of water according to equation (1) ii) cationic vacancies diffusion towards the particle surface iii) reaction between the two kinds of vacancies according to equation (2) which is the rate determining step. According to this model, the transformation rate versus temperature will follow this equation A = l-(l-kS (l + x(SN-z-2»t)3 (3) o Where k is the rate constant of reaction (2) supposed to be simple : k depends only on the temperature (according to the Arrhenius law), and So is the initial specific surface area of the transition alumina. This equation is similar to that derived previously by Vereschagin (ref. 19), but emphasius the major influences of the initial surface area (So), of the dopant content (x) and of temperature (through k). It obviously includes the case of pure alumina (x
= 0).
Comparison with experimental derivation of equation (3) provides
297 /dt
(1-.\ )2/3
=
3k S
o
(l
+ x(51\-7~2»
l'ig. 27 shews a good agreement lues of d -rovt.l:
/dt versus
(4)
bct.wccn
the theoretical and experimental
(transformation rate), provided the svstr-n is in the
phase, i.e. the value of ,\ is greater than that at the inflexion point
in Fig. 23 At a given value of l, relation (4) becomes: d l/dt
=
A (1
+
x(SN-z-2»
"here A is a constant.
We can then compare the influence of the different dopants from the values of
:oJ
and
7.
Let N
"hen'
:oJ
depends on the cation radius and z is the cation charge.
1 for alumini.um ions and N =
° for the largest cation
(lanthanum).
Placing different cations in a radius versus charge space produces Fig. 28. The straight lines are the isospeed curves A (1 + nx) = C "ith n =
~6,
-5, -4,
-1,0, +1.
This provides a classification of these cations according to their influence on the transformation rate predicted by this model. The fit "ith experimental results is good, and some slight discrepancies 2 (e.g. Mg + ) can be explained by a difference between the actual ionic radius in the alumina and that reported in the literature. Thus using 0.62 A instead of 0.65 makes the (SN-z-2) term become positive (0.005 instead of -0.15). This model predicts that the ARRHENIUS law will be obeyed. A well known
= E1 + L Hi, where Ea is the apparent Activation Energy, El the actual Activation Energy(i.e. of the rate determining step, reaction (2»
relation is Ea
and L Hi the sum of the enthalpies of the reactions preceeding the rate limiting step. As dehydroxylation is complete before transformation into alpha A1
and diffusion of cationic vacancies is an athermal process at low 203, concentrations, the sum in L Hi is equal to zero. It then appears that, according to the model, the apparent activation energy is not modified by doping. This is experimentally verified in Fig. 5 (ref. 31). Furthermore, experiments on transition aluminas of different initial specific surface areas (So) verified that the transformation rate is proportional to So for a given dopant. This model is consistent with the variation of the D.T.A. exothermic temperature since calculations indicate (ref. 26) that (T - To)/T
= Bx
(z + I - 4N)
where B is a constant and To and T are the peak temperatures for pure alumina and doped alumina. The results for Zr, Ca, La and Th are plotted in Fig. 29 and compared with the model prediction straight line (the values of N are taken from Fig. 28). Relatively good agreement is achieved between actual and predicted values.
298
d' dt
J '00
'1g
<,
OR
<, , Al
CJ'l
Go
01.
1
<,
--
n
pure 02 0
x
8
6
0
Fig. 27 - Transformation rates for doped aluminas
,,
I +4
'\"'7
I
1
~,
I i Al
1\ 'I
+2
"\,Go
1'0
\ \In
\,
\
\
,
"
I "
'
" Hg "'- , - ''\
'.
,\\~" 1
j
,
L'.----\-.~T---~.~·· 0.50
I
La
'.
'\
i
,
\~
I
+3
, Zr,
,0 '- ',..>
\jI
Co,
.
\
-1---- - N
05 0.70
0 0.90
110
r (A)
130
Fig. 28 - Isospeed curves in a ionic charge-ionic radius diagram.
299
70
60
50 +
+
40 (Z+l-4N)
Fig. 29 - Changes in the relative temperature of transformation
300
CONCLLlSION From this study, we have defined a model of the transformation of transition aluminas into corundum, and the influence of the addition of alien cations into the alumina on it. Fig. 28 is efficient in the prediction of the effect of cations, as we verified, a posteriori, for In and La. It explains the influence of dopant nature, temperature and initial specific surface area. As the transition alumina stabilisation is more effective when the added cation is larger or more charged, it is recommanded that silicon, lanthanum and zirconium are used.
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 26 27 28 29 30 31
K.K.K. Kearby, N.J. Watchung (to Esso Research and Engineering Company) LIS Patent 291 564, December 1966 R.H. Whitman, L.L. Lento (to American Cyanamid Company), US Patent 3 907 964, september 1975 Norton Company, French patent 324 361, april 1977 S.E. Voltz, S.W. Weller (to Houdry Process Corporation), US Patent 2 810 698, October 1957 W.R. Grace et CO, French Patent 2 140 575, January 1973 A.L. Hausberger, E.K, Dienes (to United Catalysts Inc), US Patent 4 153 580, May 1979 M.Michel, R.Poisson (to Rhone Progil), French Patent 2257335, August 1975 Engelhard Minerals et Chemical Corporation,French Patent 2253560,July 1975 J. Burgin (to Shell Development Co), US Patent 2 422 884, June 1947 F. Buonomo, V. Fattore, B. Notari (to Snam Progetti S.P.A.), French Patent 2 249 852, May 1975 Tokyo Shibaura Denkikk, Japan Patent 58 183948 A, April 1982 P. Nortier, T. Dupin, B.Latourrette (to Rhone Poulenc Specialites Chimiques) European Patent 85 402353.8 June 1986 V.J. Vereschagin, V. Yu Zelinskii, T.A. Khabas, N.N. Kolova, Zh Prikl KHim. (Leningrad), 55 (1982) 1946. G.C. Bye, G.T. Simpkin, J. Am. Ceram. Soc.57 (1974) 367 H. Schaper, L.L. Van Reijen, Mat. Sci. Monogr. ,14 (1982) 173 H. Schaper, E.B.M. Doesburg, L.L. Van Reijen, Appl. Catal. 7 (1983) 211. D.S. Tucker, J.J. Hren, Mat. Res. Soc. Symp. Proc. 31 (1984) 337 M.H. Harmer, E.W. Roberts, R.J. Brook, Trans. J.Brit.Ceram.Soc. 78(1979) 22 F.W. Dynis, J.W. Halloran, J. Am. Ceram. Soc. 65 (1982) 442 H. Yanagida, G. Yamaguchi, J. Kubota, J. Cer. Soc. Jap. 74 (1966) 371 K.J. Morissey, K.K. Czanderna, C.B. Carter, R.P.Merril, J. Amer. ceram. Soc.(1984) C-88 D.S. Tucker, J. Amer. Ceram. Soc. 68 (1985) C-163 D.S. Tucker, E.J. Jenkins, J.J. Hren, J. Electr. Microsc.Tech., 2 (1985) 29 J.R. Wynnycktj, C.G. Morris, Met. Trans. B, 16 (1985) 345 J. Berekta, M.J. Ridge, J. chern. Soc. A 12 (1967) 2106 P. Burtin, These de Docteur Ingenieur, Saint-Etienne, November 1985. P. Burtin, M. Pijolat, M. Soustelle, J.P. Brunelle, to be published A.F. Wells, "Structural inorganic chemistry", Oxford Press London, 1962 S. Soled, J. Catal. 8(1983) 252 F.A. Kroger, The chemistry of imperfect crystals, North-Holland publishing Company, London, 1973 P. Burtin, M. Pijolat, M. Soustelle, J.P. Brunelle, to be published
.\. Crucq and A. Frennet (Editors}, Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ADVANCES IN AUTOMOTIVE CATALYSTS SUPPORTS JOHN S. HOWITT Technical Manager Corning Glass Works, MP·8·5·1, Corning, NY 14830
U.S.A. Introduction: 1985 is the eleventh year that catalytic converters have appeared on passenger cars as part of emissions control systems. Located between the engine and the muffler, the catalyst chemically transforms hydrocarbons (HC) and carbon monoxide (CO) with oxygen into carbon dioxide (C0 2 ) and water vapor (H 20). Since 1981 it also converts nitrous oxides (NO x) into oxygen (0 2 ) and nitrogen (N). The implementation of this exhaust clean up device on automobiles was actually forced by the Clean Air Act amendment of 1970 in the United States. After ten years of usage, nearly 150 million catalytic converters have been produced. It is broadly accepted as the most practical way for auto makers to comply with exhaust emissions regulations. Although the catalyst is the key ingredient, a vital element in converter design is the manner in which the catalyst (normally a combination of precious metals) is supported. The substrate must survive and perform in the very hostile environment of an automobile exhaust. An automotive catalyst support must provide: • • • • • • • •
High geometric surface area Good catalyst adhesion Low exhaust back pressure Resistance to high temperatures Thermal shock resistance Corrosion resistance Mechanical strength Low cost
When converter systems first appeared on vehicles, manufacturers were divided in their approach to catalyst supports. (FIGURE 1) But one commonality was the use of porous ceramics. One system utilizes a large number of small, highly porous alumina pellets on which the precious metal is irnpregnated. These are bulk packed into a compact container which is typically found under the floor boards of the vehicle. (FIGURE 2) A second catalyst support type is commonly called a monolith which is a thin walled multi channeled honeycomb. The ceramic walls between the channels are the base support surfaces for the catalyst. (FIGURE 3) Although they are porous, they are not the direct surface for the precious metal. An intermediate alumina coating called "washcoat" provides an ultra high surface for the catalyst sights (FIGURE 4 is an illustration of the washcoat precious metal relationship). The catalyzed monolith is likewise assembled into the metal container using a compressible interface material. Both substrate systems have served well, but in recent years emissions standards have become increasingly stringent and passenger car vehicle designs have become smaller and lighter. Monolithic converters are better adapted to these changing requirements and have become the more dominant design. At present, they account for nearly 80% of the world's new-car converters. However, pelleted units have distinct advantages and continue to play an important role. A third but less developed catalyst substrate offers some unique advantages. It is a high temperature metallic alloy normally produced also as a cellular honeycomb form. To date it has not found general production acceptance.
302
FIGURE 1
FIGURE 2
303
FIGURE 3
ONE CHANNEL Of MONOUTHIC SUBSTRATE
ALUMINA WASHCDAT
CERAMIC SUBSTRATE
FIGURE 4
:304 Ceramic Monolith The ceramic monolith is presently the substrate of choice in the world market because its cellular design provides the following benefits: 1 2. 3. 4. 5.
A high degree of geometric surface area. Fast catalyst light off Low exhaust gas back pressure. Compatibility with catalysts and coatings. Low cost.
Monolithic ceramic substrates have almost entirely been produced from cordierite which is a phase of the 2MgO-2AL 203"5S13 system. It has evolved as the industry standard because it combines the required properties, process capabilities and cost for this application. More specifically, the benefits of this material are: 1. Adaptability to the extrusion process which is uniquely suited for mass production. 2. Thermal shock fracture resistance through an inherently low thermal expansion coeficient (8-12 x 10-7°C). 3. A melting point of 1460oC. 4. Porosity and pore size distribution for catalyst coating application (30-35% open porosity and 4-15 micron median pore size). 5. Sufficient crush strength for assembly into the converter containers and to endure the rigours of automotive use. 6. Raw materials which are (a) economical, (b) readily available, (c) have acceptable firing properties. Ceramic monoliths have been produced in various cell densities and geometries. Table 1 is a comparison of the geometric properties of those utilized for automotive application. Automotive industry standard is mostly 400/6 sq. shaped cells and some 236/12 triangular shapes. The initial designs of 1975 were largely 200/12 but process developments over that period of time have allowed for increasing number of cells per square inch and thinner walls. These bring with the associated benefits of increased GSA, lower density and lower back pressure. Table I Cells Per Square Inch
200
300
400
236
Cell Shape
Square
Square
Square
Triangle
Web Thickness (ins)
0.0105
0.0105
0.0065
0.0115
Hole Size (ins)
0.060
0.0475
0.0435
0.042
GSA
(in 2/in 3 )
Density (GMS/ins 3 ) Open Area (%)
48
57
70
56
7.90
9.60
6.74
9.63
73
68
76
62
As the properties of ceramics monoliths are examined, it's important to note that changes or improvements in melting temperature, thermal shock resistance, strength, back pressure and/or catalyst surface area each have an effect on other properties. The present ceramic monolith design is the result of a compromise of these interrelated variables. Product Development Directions Auto makers believe present day monolith designs can still be improved upon. The directions of product development effort are:
~,05
Improved Thermal Shock Resistance Effective Use of Precious Metal High Temperature Resistance Design Flexibility Reduced Back Pressure Fast Catalyst Lightoff Mounting System Thermal Shock Resistance A key property of cordierite monolith is the ability to resist fracture from thermally induced stresses. In an automobile exhaust the temperature and rate of flow of the gases changes very rapidly. At engine start up the monolith is exposed to a very sharp rise in temperature. In most converter designs the gas flow is not evenly distributed across the cross section and, therefore, the honeycomb does not heat uniformly. The flow most often is concentrated in the center. It heats rapidly while the peripheral areas tend to remain relatively cool. Radial thermal gradients are created and the problem is accentuated in non round cross sections. Thermal gradients also occur during the warm up period in the longitudinal direction. The cordierite composition minimizes thermal fracture from these stresses by virtue of a low coefficient of thermal expansion. Resistance to thermal fracture has been noticeably improved in recent years largely the result of: • • • • • •
Low modulus of the cellular cross section. Better control of coeff of thermal expansion. Elimination of filleted corners. More uniformly straight cells. Control of the washcoat substrate adhesion relationship. The use of thicker more resilient mat mount.
Effective Use of Precious Metal Ceramic monoliths have proven themselves effective as substrates for catalyst washcoat and precious metal because they provide a relatively uniform porous surface. In the catalyst application process, the amount of alumina washcoat picked up depends upon the total porosity, as well as, the size distribution and shape of the pores within the wall. Likewise, the amount of precious metal picked up depends largely upon the amount of porous washcoat on the substrate. Catalyst coaters, therefore, have learned to optimize their process around typical properties of the substrate. However, through subtle variances in raw materials and process steps, variances in porosity occur piece to piece and lot to lot. The precious metal loading of the finished catalyst performs to strictly specified minimum requirements. As a result, variances in substrate porosity can be translated through the coating process steps to a variance in precious metal loading. Consequently, precious metal loading targets are set well above the minimums required to compensate for the fluctuation in actual loadings. To demonstrate the point (FIGURE 5) shows the relationship between water absorption (an industry standard measure of substrate porosity) vs. washcoat pick up and likewise washcoat pick up to precious metal. The history of this product has been a continuing effort to control raw material and process to provide a more consistent product, The progress to date in tightening this value has translated directly into a saving of expensive catalyst. High Temperature Resistance The resistance of the ceramic monolith to melting has been the object of research since it was first developed. The melting temperature of cordierite monolith (14600C) is well in excess of that reached in
:306
(a)
(b) /
/
0
0
z 0
~ C 5 ....
1( ....
~NO
'"
...J
~
a..
",
'" .... ....
x/ /
~NOM
'" " "" ",~C5
....J
",
u
/ /
":,6//
0
/ / /
~
NOM W/C LOADING
FIG.5 -
/
z -
'" '"
/
/ /
/":,6
"
NOM WATER ABSORPTION
WASHCOAT VS P.M. LOADING. (b) WATER ABSORPTION VS WASHCOAT LOADING .
(a)
FIG.6
307 normal operating modes in the exhaust of an automobile. However, engine ignition irregularities and other abnormalities in operation occasionally occur, and they produce a fuel rich exhaust. The catalytic reaction is exothermic, and the potential for dramatic increases in catalyst bed temperatures exists when the exhaust is heavy with unburned combustion products. The field results over a 10 year period show that monolith melting is not an unknown experience but is not at the level which is considered serious. However, improvement in this property has been the goal of emission systems designers for the following reasons: 1. To reduce the number of in-use melt incidents. 2 Converter applications which are close coupled to exhaust manifolds and, therefore, have a higher inlet temperature. 3. Truck catalyst applications with higher peak operating exhaust temperatures. 4 Certain European vehicles with higher speed operating conditions. Research results have determined some increase in high temperature resistance over a single phased cordierite by combining it with a refractory phase such as mullite. However, the cordierite portion still melts at temperatures approximately 14600C and only a marginal increase is achieved. The addition of substantially quantities of mullite in cordierite degrades the thermal shock resistance. The problem of developing higher temperature materials without sacrificing thermal shock resistance is difficult and has lead researchers on another track, that of examining the control of the micro structure, specifically a micro cracked body. Micro cracking is developed in a ceramic by causing a pattern of micro stresses to occur after heat treatment. This can be achieved in a number of ways. One is the result from a two phase system with substantially different expansion coefficients. Alumina titanate is an example of such a material. Its properties are shown in a Table II. The resistance to thermal stresses is achieved by a large number of micro cracks acting as minute stress relievers. These materials have been tested in laboratories simulating overtemperature conditions and the expected melt resistance has been demonstrated. They are now in the further evaluation stages to determine their adequacy for strength and long term structural integrity. Table II
Phase
Cordierite
Mullite + Alum. Titanate
Coeff of Ther. Expan. (25 0C-1000 DC) XlO- 7
10
21
% Open Porosity
33
30
Mean Pore Size
4
13
Axial Crushing Strength (PSI)
3000
2200
Melting Temp. °C
1450
1700
Design Flexibility The development of new automotive applications and markets has brought interest in extending the physical size and shape limitations of the extrusion honeycomb process. Previously limited to round, oval and race track configurations, (FIGURE 6) shows irregular cross section capability used to allow the positioning of the converter in unusual vehicular locations.
308 Back Pressure Back pressure produced by the presence of a catalytic converter in the vehicle exhaust system is important because it has a direct and negative effect upon engine volumetric efficiency and fuel economy Pressure change as a result of a monolithic converter is a function of, among other things, friction flow resistance of the substrate. The frictional loss across the monolith matrix is a fairly complex measurement but can be reduced by monolith designs having larger open frontal area and a larger hydraulic diameter of the cells. Both of these objectives can be accomplished by the use of thinner cell walls. Faster Lightoff The Federal test procedure requires that vehicles demonstrate their ability to pass the standards on assimilated driving cycle (CVS-CH) and important part of that cycle is the "cold start" when the engine is started after being at ambient temperature for a number of hours. The precious metal catalysts do not become active until they reach temperatures of 400-500 oF. Heat transfer from the incoming gases is the only energy source. Therefore, a key to effective performance is shortening the cold start interval because at this time the exhaust is particularly rich in pollutants as a result of carburetor choking. The contribution of the monolith to a shorter lightoff has been the object of much effort. In short, the thermal response of the monolith increases as its web thickness decreases. This is because substrates with thinner walls allow a faster heating of the front portion and have a lower front to rear thermal gradient. Both of these contribute to faster catalyst lightoff. EX-21
An extension of the present composition is being developed to address the solution to lower back pressure and faster lightoff namely thin walls. Designated as EX-21, it embodies a reduced total porosity of the ceramic body. The total pore volume as measured by the water absorption method shows a reduction of approximately 20%. The result is an increase in wall strength as measured by the industry standard abc axis crush test. The strategy for employment of EX-21 is to take advantage of that strengthen increase by producing the appropriately thinner wall cell walls without sacrificing the overall mechanical integrity of the ceramic monolith. Table 3 is a comparison of the new material with the existing composition. Physical Property Comparison Property
Thermal Expansion (inches/inch/oC X 10'7) Crush ABC-
Strength (PSI) Axis Axis Axis
Water Absorption (EM/inches 3 ) Softening Point % Open Porosity
EX-20
EX-21
76
5.8
4163 779 48
6138 1053 53
1.23
103
1440 0C
1440 0C
.34
.28
Monolith Mounting System
Improvements have been made in the support arrangement of the ceramic monolith within the metal container. A felt like blanket capable of withstanding high temperatures is now used to a great extent. It is composed of ceramic fiber in a vermiculite base. The material is capable of holding the monolith securely in place despite substantial differences in thermal expansion of the ceramic and the metal can. (FIGURE 7) It also acts as a gas seal to prevent exhaust gases from bypassing the catalyzed monolith.
809
FIG. 7
UTER WRAP CONVERTER SHEll
FILL PLUG
FIG. 8
:310 PELLETED CONVERTERS
Bulk material catalyst (small extrudates and pellets) have been effectively utilized as catalyst substrates in other applications such as the petroleum industry well before they were adapted to automotive use. They are especially well suited for a large volume, low speed engines with relatively low exhaust gas temperatures. This specific pelleted converter advantages are: 1. 2 3. 4.
High geometric surface area. High temperature resistance. Catalyst replacement ability. Thermal shock resistance.
Pellets are spherical or cylindrical in shape and vary between 1/8 to 1/10 of an inch in diameter. Since 1975, measures have been taken to improve the design and performance of pelleted converters. These were in response to tightened emissions standards and to smaller and lighter vehicle designs. 1. Improvements and performance durability which are largely the result of resistance to poisons have been achieved through a unique system of positioning various catalyst metal at different levels of subsurface. 2. Improvement in catalyst transient warm up performance through the use of smaller, high surface area substrates. 3. Both size and weight of the converter has been reduced through the use of lower density pallets which achieved be an increase in macro porosity. Unfortunately, certain models of dual bed pelleted converters have encountered a mechanical design problem after a period of time in customer use which has resulted in loss of power and drive ability of the vehicles. The upstream (three way catalyst) pellet bed has narrow crevasse areas at the junction of the retaining screen and container body. Pellets in these areas were being crushed by the relative motion caused by the expansion of the metal. The crushed fine particles filtered through the retaining screen eventually plugging the top layer of pellets in the second bed. Over a period of time a restriction in exhaust flow was built up. Approximately 1.265 million vehicles have been recalled by the manufacturer to correct this problem. New pelleted converter designs are being produced specifically for truck applications. They are designed for lower back pressure with larger plenums. They also incorporate higher temp. steels. Metal Monolith
Monolithic catalyst supports of metalilic alloys have been under development for a long period. Although they offer a number of potential advantages for various reasons these metallic units have not, to any large extent, succeeded in being applied to production vehicles. They are basically of the cellular design of ceramic monoliths but the honeycomb is formed normally by spirally wound alternate sheets of flat and corrugated metal. They offer a number of important advantages: 1. 2. 3. 4. 5.
An ultra thin wall (.04mm) structure for fast catalyst light off and high geometric surface area. Mechanical strength and thermal shock resistance. Design flexibility (size and shape). Potential for simplified canning assembly. Low back pressure.
311 The metal alloys for this purpose are available from a number of sources. Considerable research and development effort has been directed to the problem of application of the catalyst to the metal surface. The major problems to be overcome generally are considered to be those of cost to be competitive with existing materials, long term adhesion to catalyst washcoat to the metal surface and warping at high temps.
S~m' :Jl. ' l
__'L
The catalytic converter has been part of the automotive scene in America for nearly 11 years. Its use, as a result of government regulation, has spread to a number of other countries. Within the next decade, as the result of public concern over air quality, the majority of the free world automobiles will be so equipped. Its design is still evolving as engine technology and vehicle design changes. The catlyst support continues to demand research and development effort in the areas of (1) precious metal conversion through process consistency, (2) resistance to melting through advanced compositions, (3) cost reduction through process and improvements, (4) expanded applications into trucks, motorcycles and adaptation to varying emission certification cycles.
References 1. J. R. Adomaites, J. E. Smith, D. E. Achey, "Improved Pelleted Catalyst Substrates for Automotive Emissions Control", SAE Paper 800084, February 1980. 2. J. S. Howitt, "Thin Wall Ceramics as Monolithic Catalyst Supports", SAE Paper 800082, February 1980. 3. Spheralite Catalyst Carriers, Rhone-Poulenc. 4. I. M. Lachman, R. N. McNally, "High Temperature Monolithic Supports for Automobile Exhaust Catalyst", American Ceramics Society, May 1981. 5. V. D. Rao, "High Temperature Substrate and Catalyst System", SAE Paper 850553, February 1985. 6. M. P. Walsh, J. S. Howitt, "The United States Experience with Motor Vehicle Air Pollution Control, A Regulatory Success Story", 8th International Clean Air Conference, Melbourne, Australia, May 1984. 7. S.1. Gulati, "Long Term Durability of Ceramic Honeycombs for Automotive Emissions Control", SAE Paper 850130, February 1985. 8. M. Nonnenmann, "Metal Supports for Exhaust Gas Catalysis", SAE Paper 850131, February 1985
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A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control ~ ) 1987 Elsevier Science Publishers B.Y., Amsterdam -" Printed in The Netherlands
STRUCTURAL CONSIDERATION WITH RESPECT TO THE THERMAL STABILITY OF A NEW PLATINUM SUPPPORTED LANTHANUM-ALUMINA CATALYST by F. OUDETl, E. BORDESl, P. COURTINEl, G. MAXANT2,
C. LAMBERT2, J.P. GUERLET2 1Universite de Technologie de Compiegne, Dept Genie Chimique, B.P. 233, 60206 Compiegne Cedex ZComptoirLyon-Alemand Louyot, 75139 Paris Cedex 03.
ABSTRACT The influence of lanthanum aluminate, LaAI03, on the thermal stability of both alumina and platinum supported alumina catalysts is investigated. In the case of alumina, the stabilization is interpreted in terms of structural coherence between &-A1 20 3 and a three-fold superstructure of LaAI0 3. The addition of LaAI0 3, is shown to increase both the dispersion and the resistance to sintering of the platinum supported alumina catalyst. Moreover, lanthanum hexaaluminate (La-j3-AIz03) is present in the platinum catalyst fired at 1150°C. These observations are assumed to result for the epitaxial relations between platinum and the lanthanum-alumina support.
1. INTRODUCTION The catalytic layer of monolithic automotive reactors usually consist of active metals (Pt, Pd, Rh) supported on alumina. One of the most important problems set by these catalysts is the decrease in their activity after thermal exposure to the exhaust gas itself (Ref. 1). It is well known that this thermal deactivation is directly related to the sintering of the active components. Moreover, this modification of the supported metal is drastically enhanced by structural changes of the support. Thus using TEM experiments, Chu et al (Ref. 2) have reported rapid sintering of platinum during the structural transition y-AIZ03 to a-AIZ03' It has been clearly established that the addition of lanthanum improves the thermal stability of both active alumina and platinum-supported alumina catalysts (Ref. 3-7). However, the actual cause of the lanthanum effect is still not well understood. We have developed a method of preparation and carried out new investigations on the thermal stability of these catalysts. Lanthanum is present in two
31:,
:314
different complex oxides with aluminum, i.e. lanthanum aluminate (LaAI0 3) and lanthanum hexa-aluminate (La-J3-AI 203 ) . The stabilization of the catalyst is discussed in terms of structural coherence between: i) LaAl0 3 and o-A1 203 and ii) the platinum and the lanthanum-alumina support.
2. EXPERIMENTAL The lanthanum doped alumina support was prepared by addition of lanthanum and aluminum hydroxides to Boehmite. The resulting slurry was then dried and fired at SOO°C in order to form the perovskite-type compound LaAI0 3. The crystallization of this compound occurs simultaneously with the topotactic dehydration of Boehmite to y-A1203 (ref. 8). Pure LaAl 203 is prepared by coprecipitation of La(OHh and Al(OH)3, followed by calcination of the precipitate at 600°C. The platinum catalyst was prepared by the wet impregnation technique, using H2PtCI6, and reduced at SOO°C for two hours in flowing pure hydrogen. The thermal stability of the support was determined by X-ray diffraction (XRD), BET surface area measurements and Transmission Electron Microscopy (TEM) (Jeol 1200 ex, 120 KV, L=80cm). Platinum dispersion was measured by the pulse chromatography method (H2 adsorption at ambiant temperature) (Ref. 9, 10). Laser diffraction patterns were recorded on Polaroid film directly from the TEM negative film (A = 6328 u, L = 3,77 m).
3. RESULTS 3.1. Thermal stability of the support The surface area of the La-containing support, measured for different molar concentrations of LaAI03 in alumina is shown (Fig. 1). An optimum concentration is observed at 1% molar lanthanum. The crystalline phases detected by XRD are reported in Table I for each concentration. LaAI03 peaks are absent from the XRD patterns at concentrations lower than 2%. Nevertheless, according to Schaper et al (Ref.S), it can be assumed that this phase is present in the product. Examination of the XRD patterns of y-A1203 and 5% La-A1203 both fired in air at 1150°C (12 hours) shows the transformation of pure y-A1203 to corundum. However, in the latter case y-Al203 is transformed to o-Al 203 (Ref.11) and well crystallized lanthanum aluminate is also detected (Ref.12).
:315
Table 1 Surface area of the support and crystalline phases detected by XRD for different molar concentrations of LaAI03 S m 2g-1 a
LaALO (mol %)
°
XRD
b
a
3
3 56 63 57 45 29 19
0.5 1 1.5 2 5 10
S m2g-1
25 31 24 19
XRD b
a-A 120 3 1'}-AI203 &-A1203 &-A1 203 &-A1203 &-A1 203 + LaAI03 &-A1203 + LaAI03
a-A1203 a-A1203 1'}-AI203 1'}-AI203 1'}-AI203 + LaAI03 1'}-AI203 + LaA103 1'}- AI203 + LaAI03
aafter firing 12h at 1150°C bafter firing 1h at 1300°C
Figure 1: Evolution of the surface area of the La-AlzO z support for different molar concentrations of LaAI0 3
70
...
.-.... 'Cl
N
E ....... oCt
w
40
0: oCt W
o
Lf0:
=> (J)
10
o
5 LANTHANUM MOL. %
10
3.2. Thermal stability of the platinum catalyst Table 2 summarizes the characteristics of the catalysts investigated and Fig.2 shows the evolution of the dispertion (DIDo) versus time, at 650°C in pure nitrogen.
Table 2 Characteristics of the catalysts and their final dispersion Sample
La(mol%)
C1 C2 C3 C4
0 1 0 1 -_..._ - . __ .. -
Pt (wt%)
1 0.8 0.98 1.1
Ta 0
600aC 600aC 800aC 800aC
D}}
60%
98% 60%
98%
-------_.
'ITo is the calcinationtemperature of the support prior to impregnation bDois the initial dispersion
Figure 2: Evolution of the dispersion DIDo versus time at 6S0aC in pure N2
- - - - - - - - -__ C2
L J - - - - - - - - _ - - i " ] C4 lk:::===-----------b. C 1 C3
After firing in air at 1150°C, La-j3-Alz0 3 peaks are observed in the XRD patterns of the catalyst (for lanthanum contents greater than 2%). Therefore, the support is a mixture of two complex oxides (LaAI0 3 and La-p-Al z03) in alumina. 4. DISCUSSION The experimental results show that the thermal stability of both alumina and platinum catalysts is improved by the presence of lanthanum. In the case of the support, this improvement can be related to the formation of LaAl03 with 8-Alz03. For the platinum catalyst, lanthanum has additional roles: promoting a better dispersion of the metal and improving its resistance to sintering. 4.1. Stability of the support Courtine (Ref. 13, 14) developed a model for solid-solid interactions
:317
between oxides, which takes into account the crystallographic relations between the two compounds. These relations can be compared with epitaxial interactions (Ref.15). Such considerations imply that the LaAI03/o-AI203 interface is of major interest. Fig.3 shows an electron diffraction patterns of o-Al203.Three reference axis are well defined: [110], [001], [111], according to previous observations (Ref. 11, 16, 17). A typical diffraction pattern of LaAI03 is sh<;:wn in Fig. 4 and is indexed in the cubic structure. The two main axis are [110] and [110]. The exposed plane of this perovskitetyEe compound is then statistically a (001) plane. Since o-A1203 preferentially exposes (110) planes, it can be assumed to a first approximation, that the LaAI03/o-A1203 interface is composed of these planes. In order to examine the structural analogies which could exist between the two oxides, pure LaAl03 was impregnated directly on o-A1203 and fired at 800°C. Fig. 5 shows a typical image of the resulting product. Comparison of Fig. 6 with the diffraction pattern of pure LaAI03 (FigA) shows that the framework obtained could represent a three-fold superstructure of lanthanum aluminate (3X-LaAl03)' The formation of such a superstructure normally requires reaction at high temperature because of the high activation energy of these Figure 3: Electron
Figure 4: Electron diffraction pattern of the [001] zone axis of
8-A1203
(scale expansion: 1)
(scale expansion: 1)
LaAI0:l
:U8
Figure 5: a) TEM image of the La-AI20 3 support b) resulting laser diffraction pattern.
Figure 6: Calculated electron diffraction from Figure 5.
iI
._-
II
--
I
020
-1I
/
220
I
T/ /1
/
/
'/
/
110 f--.~
I
The comparison with Figure 4 show that this structure could be a three-fold superstructure of LaAI03 (scale expansion: 2)
reactions (Ref. 18, 19).In this investigation a superstructure was obtained at quite low temperature (800°C). Thus the activation energy has been lowered by a favourable situation between the two reactive compounds. Such a situation may occur when two solids are in structural coherence at least at their interface (Ref. 13, 14). Fig.7 demonstrates the possibility of a crystallographic fit between 3 X-LaAI03, and &-AI20 3· On this simulated electron diffraction pattern, there is indeed quite a lot of corresponding vectors from the two reciprocal lattices as in the case of epitaxial interaction between two compounds (Ref. 20). These results are in good agreement with those of Schaper et a!. who invoked surface diffusion to account for the sintering mechanism of active alumina, the effect of lanthanum being interpreted as a surface interaction (Ref. 5, 6). Our work confirms that lanthanum aluminate LaAI03 is the lanthanum active species, and suggests a model for the thermal stabilization of alumina which takes into account the structural interface between lanthanum aluminate and &-alumina.
319
Figure 7 : Superposition of the electron diffraction pattern of o..AI203 and 3X-LaAI03' The good matching between the two patterns shows the possibility of a crystallographic fit between the two structures. (for indexation, see Fig. 3 and 4). (scale expansion: 2)
0-
Figure 8: The superposition of the diffraction patterns of Pt[OlI] zone axis and La-A1203 simulated diffraction pattern Shows that the relations between platinum and the support could be of epitaxial type. (scale expansion : 2)
411 .
all 0
• • • ·O~O~O~O
• • 01110
• • • i
.. ..... I
OIilOIjJOIilO·
.
~
i
I
I
0
0
o
/8'11
01110'
• 1•a • • •
• • OIilO
~11
0
O~O
OIilOIilOfjlO·
• •
•
0
0
all
• • t>lif
4.2. Stability of the platinum catalyst The results presented in Table 2 and Fig. 2 show that the presence of lanthanum aluminate increases the initial dispersion and the resistance to sintering of supported platinum. It thus confirms that an interfacial interaction proceeds between lanthanum aluminate and &-AI Z03 . The formation of L a - ~ - A l z 0 3 besides LaAI03, &-Al z03 and platinum must be carefully examined. The classical synthesis of this compound from LaAI0 3 and AlZ03 requires very high temperature (1650°C) (Ref. 21). The mechanism of this reaction, suggested by Ropp and al. (Ref. 22) involves electron migration and diffusion of lanthanum or aluminium ions between the two solid phases. In our case, the presence of free electrons is undoubtedly connected with the presence of platinum. To confirm this assumption, we have used another compound known to provide free electrons: thus introduction of graphitized carbon black in the support yields identically L a - ~ - A l z 0 3 after calcination at 1150°C. Moreover, at temperature higher than 600°C, the carbon black is oxidized to COz. This implies that the nucleation of the ~ - p h a s e begins at a temperature lower than 600°C but that La-f3-Al z03 is not
320
sufficiently crystallized to be detected in the XRD patterns. On the other hand, the two solid phases LaAI03 and &-A1 203 must be in a highly favourable relative position at their interface in order to allow the diffusion of La3+ and A13+ species. This consideration strengthens the hypothesis of coherent structural interface between the two oxides which is the only method for transfer of electron or species between two solid phases at low temperature (Ref. 13, 14). The easy electron migration from platinum crystallites to the biphasic support clearly indicates that a coherent interface between the metal and the substrate must also be consid.:red. Fig.8 shows the superposition of the electron diffraction pattern from the [OIl] zone axis of platinum with a simulated diffraction pattern for the support. The good matching between several reciprocal vectors of the two structures indicates that the relationship between the platinum and the support is similar to that noted in previous observations on the epitaxial relationship between Pd[Ol I] zone axis and y-A1 203 [110] zone axis (Ref. 23). According to Dexpert et a!. (Ref. 23), the resistance to sintering can, therefore, be interpreted as the result of the strength of the epitaxial interactions which is intermediate between chemical bonds and Van der Waals forces.
s. CONCLUSION The present work has shown that our method of preparation leads to: i) the thermal stabilisation of the alumina support, ii) a better dispersion of the supported platinum, iii) an improvement of the resistance to sintering of the platinum catalyst. Structural characterization of the support indicates that the lanthanum active component is the perovskite-type compound LaAI03. The experimental results are interpreted as the formation of a three-fold superstructure of LaAI03 which is in a good interfacial crystallographic fit with &-A1203 and by the epitaxial relationship between the platinum and the support. The model of coherent interfaces, already suggested for other catalytic systems (Ref. 13, 14,23) is considered here as a good working assumption and should be confirmed by further investigations.
Acknowledgements: Thanks are due to Dr A. Vejuxforthe TEM experiments and helpful discussions.
321
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
KA. Dalla-Betta, KC. McCune, J.W. Sprys, Ind. Eng. Chern., Prod. Res. Dev., 15-3 (1976) 169-172. Y.F. Chu, E. Ruckenstein, J. Catal, 55 (1978), 281-298. S. Matsuda, A. Kato, M. Mizumoto, H. Yamashita, Proc. 8th ICC; Berlin 1984, pp.879-889. YJ. Ying, W.E. Swartz Jr., Spectroc. Lett., 16(6-7) (1984), 331-343. H. Schaper, E.B.M. Doesburg, L.L. Van Reijen, Appl. Catal, 7 (1983), 211-220. H. Schaper, OJ. Amesz, E.B.M. Doesburg, L.L. Van Reijen, Appl. Catal. 9 (1984),129-132. P. Burtin, These 1985, Ecole des Mines de Saint Etienne, France. P.Y. Klevstov, L.P. Sheina, Inorg. Mater., 1 (1965),2006-2012. J. Freel, J. Catal., 25 (1972),139-148. P.A. Compagnon, C. Hoang-Van, SJ. Teichner, Bull. Soc. Fr. Chirnie, 11 (1974),2311-2316. B.C. Lippens, Thesis 1961, Delft, The Netherlands. S. Geller, V.B. Bala, ActaCryst., 9 (1956),1019-1025. A. Vejux, E. Bordes, P. Courtine, IX Eur. Chern. of Interface Conf. Zakopane, Poland, May 19-25,1986. P. Courtine, ACS Symp. Series, 279, R.K. Grasselli, J.F. Bradzil Eds. (1985), pp.37-56. J.W. Matthews, in Epitaxial Growth, Academic Press, 1975. J.P. Beaufils, Y. Barbaux, J. Chim. Phys., 78 (1981), 347-352. H. Dexpert, J.F. Larue, I. Mutin, B. Moraweck, Y. Bertaud, A. Renouprez, J. Metals, Nov. 1985, 17-21. M.A. Alario-Franco, MJ. Rodriguez-Henche, 3rd Eur. Conf. Sol. State Chern., May 29-31,1986, Regensburg, Vol. 1, pp. 201-202. M. Vallet-Regi, J.M. Alonso, J.M. Gonzalez-Calbet, 3rd Eur. Conf. Sol. State Chern., May 29-31, 1986, Regensburg, Vol. 3, pp. 499-500. R. Bonnet, Mat. Res. Bull., 7 (1972), 1045. R.C. Ropp, G.c. Libowitz, J. Am. Ceram. Soc., 61 (11-12) (1978), 473-475. KC. Ropp, B. Carroll, J. Am. Ceram. Soc., 63(7-8), (1980),416-419. H. Dexpert, E. Freund, E. Lesage, J.P. Lynch, in B. Imelik et al. (Eds), MetalSupport and Metal-Additive Effects in Catalysis, Elsevier (1982), pp. 53-61.
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323
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control © 1987 Elsevier Science Publishers B.V .. Amsterdam - Printed in The Netherlands
INFLGENCE OF
THE
POROUS
STRUCTURE
OF
ALUMINA
PELLETS
AND
THE
INTERNAL
CONVECTIVE FLOW ON THE EFFECTIVE DIFFUSIVITY OF EXHAUST GAS CATALYST
S. CHENG
1,
A. ZOULALIAN
,
"Department
of
l
and J.P. BRUNELLE
Chemical
Engineering
-
2
Uni versi ty
of
Technology,
BP
233,
60206 Compiegne Cedex (France) 2Hesearch
Center
of
Aubervi lliers
Rhone
Poulenc Recherches,
14
Hue
des
Gardinoux, 93308 Aubervilliers Cedex (France)
ABSTRACT To analyse the performance of exhaust gas catalyst (actiVity and poisoning) the effective diffusivi ty of six Rhone Poulenc alumina supports is measured by a physical dynamic method in a single pellet string reactor. Informa tions on the porous structure of supports give an idea of the effective diffusivity values, but specifie neither their absolute values nor the direction of their variations. Moreover, the experiments demonstrate the necessity of taking the measurements in an external fluid flow so as to determine the influence of internal convective flux on the value of effective diffusivity.
INTRODUCTION One
of the
causes of deactivation of the exhaust gas catalyst,
pellet and monolith form,
is
clogging by
lead,
in both
phosphorus and zinc traces
contained in exhaust gases. The flow
impurities
into
considerable process.
One
the
stick to
the periphery of particle pores making the gas
catalyst difficult or impossible.
increase way
of
in
the
fighting
diffusion this
This
resistances
phenomenon
is
in turn leads to a
during to
use
the
catalytic
double-porosity
alumina. Micropores of about 20 nm are always useful to develop the specific surface area necessary for a good dispersion and stability of the catalytic phase. Macropores over 100 nm in diameter help to diffuse the reagents within the particles. However, as
the proportion of macropores must not be too great,
that would diminish the mechanical properties of the support correspon-
dingly. For this reason,
Rhone Poulenc has,
since 1974, developed
and mar-
keted various eXhaust. gas catalyst supports with specific surface areas of 2/g around 100 m whose porosity and porous distribution are extremely variable. To analyse the performance of a catalyst during a chemical transformation,
324 it
is absolutely
necessary
to know the e f'f e c t i v e diffusivities of the rea-
gents in the catalytic support system. Determining these values and observing their evolutions as a funcion of certain operating parameters often reflect their
qualities
in post combustion reactions.
determining pore diameter distribution (using, simeter)
proves
to
be
insufficient
to
As
a matter of fact,
for example,
calculate
this
merely
a mercury poro-
parameter
a
priori
(ref. 1). In this study, we shall deal with the values of e f f e c t i ve d i f fu s iv i ty 2/g, of six different transition alumina supports of about 100 m in the form 3 of spheres with an average diameter or 3,2 x 10- m. The efrective dirfusivities are
found by using a
physical dynamic method.
A stimulus of tracer
is introduced in the carrier gas crossing with a constant flow rate an open system
containing
the
alumina
particles.
The
concentration
or
tracer
is
registred at the entrance and at the exit of the system. From these experimental
inrormations,
an
experimental
transfer
runction
is
obtained.
The
theoretical transfer function can be derived by modeling the system and the effective d i f f'us i vi ty
is one of the parameters or this
Identification of experimental and
theoretical
transfer
transfer function. functions makes it
possible to estimate the effective diffusivity.
EQUIPMENT AND EXPERIMENTAL PROCEDURE The
schematic
diagram
of the overall experimental set up
is given
in
Fig. 1. A fixed bed reactor of small diameter and large length (internal diameter
; 8,5 x 10-3m; total length; 1,30 m) is filled with alumina particles.
A 6-way chromatographic valve is used to mix a tracer gas with the carrier (nitrogen) before introduction into the fixed bed. The concentration of the tracer (length
in
the
carrier
is
measured
at both extremities of the study zone
1 m) wi th a mass spectrometer (Micromass MM 601). The "response"
curves are recorded and stored in a flexy disk of an Apple II microcomputer. For each flowrate of the carrier gas, at least 10 pairs of response curves entrance/exit are stored. curve is defined. from
From these curves, an average entrance (or exit)
Only the curves whose area and first moment do not deviate
the mean values by more
than
± 5% are considered
curves are retained in calculating the average curves).
(generally, all the
32.5
REACTOR
8
7
1
MASS MOLECULAR SIEVE
SPECTROMETER
TRACER GAS CARRIER GAS ELECTRO VALVE
APPLE II
-Legend: I : spherical float flow meter, 2 : needle valve, 3 : chromatographic entrance sampling, valv~,4 : sample loop, 5 : gas mixer, 6 : port selector, 7 8 : exit sampling, 9 : gas meter Fig. I . Diagram of the experimental equipmenT. Let x(t)
and y(t)
be
the
average
curves at the entrance and the exit
of the study zone. The experimental transfer function can be expressed by GE(s)
y(s) / XIs)
~
(1)
with
roo
xIs)
(
0 x(t) exp(-st/\ll)dt) / S x
(2)
Y(s)
( J:OOy(t) exp(-st/\ll)dt) / S y
(3)
In relations (2) and (3), Sand S represent the respective area unx y der the average entrance and exit curves, that is :
]:=
x(t)dt
(I))
(+=
Y(t) dt
(5)
J0
WI is the rirst moment or the study zone, that is
J
O + OO
WI
ty(t)dt (6 )
sx
The experimental
transfer
function GE(s)
is calculated ror twenty real
values or the reduced Laplace parameter situated between 0 and 4. If, neous
however, model,
the alumina particles are represented by
assuming
the
f i Irn mass
the
e f f'e c t.s
slab
t r-ans rer-
or
these
geometry,
resistance
neglecting
the
the pseudohomoge-
axial
(it has been demonstrated
two variables are negligible
dispersion and (r-e f ,
2)
in comparison with
that the
errect or the internal dirfusion resistance), the theoretical transrer runction ror the study zone is given by os
exp [ -
where
0
(7)
x
y
is the external porosity or the bed, B is the porosity or the alumina
support, and y is the overall porosity: y internal d i f'Fue i cn
time,
which
is related
0
+ (I-dB.
CD designates the
to the ef'f'ec t i ve d.i f'f'us i vi ty De
by the expression
B d 2 p
!
(8)
36 De
Lden t.i f i c a t i on
of
the
theoretical
and
experimental
transfer functions
in order to estimate the effective diffusivity De is obtained by minimizing a
relative
error
function
taken
between
the
two
Rosenbrock method of optimization has been used. been
made
at
room
temperature and
something
transfer
functions.
The
All the measurements have
close
to
normal
atmospheric
pressure. The only parameter that changes is the carrier gas flow rate.
EXPERIMENTAL RESULTS The main characteristics of the six Rhone Poulenc alumina supports are given in Table 1. In Figure 2, are presented the porosity distribution obtained with a mercury porosimeter at the moment the pressure was increased (the mercury penetration curve).
327 TABLE 1 Characteristics of the Rh6ne Poulenc support systems and of the fixed bed
Reference
Apparent volunic mass of the support 3) (kglm
A1 A2 A3 A4 A5
920 1170 810 795
A 6
673
62JJ
3/g) Porous volume (em total
1,160 0,790 0,5SO 0,800 0,960 1,190
I porosity Particle
I > IIJlTl[> o,llJlTlf
I
0,195 O,OSO 0,010 0,060 0,320 0,100
0,395 0,100 0,045 O,lSO 0,410 0,420
Fixed bed porosity
!J
E
0,788 0,727 0,644 0,713 0,763 0,801
O,S05 0,538 0,528
0,519 0,531 0,524
<110,8
E :J
o >04 ,
'"
:J
o L
o
Q.
100 1000 pore diameter
(nrn)
Fig. 2. Porosity distribution of the different supports. For values
the
various
of the
carrier gas
flow
rates,
the
experimentally obtained
effective diffusi vi ty are given in Table 2 and represented
graphically in Figure 3. Table 2 contains also the values of the effective diffusivity that were theoretically deduced from the pore diameter distribution by applying either the Johnson-Stewart model (ref. 3) or the Wakao-Smith model (ref. 4).
328
TABLE 2 Effective diffusivities of the different supports Effective Mean effective diffusivity slab diffusivity pseudcharogeneous pud rrode1 __.2. p 6 2/s) 6 2 De x 10 (m De x 10 (rn Is)
Reynolds nunber Reference Re p
A1
=
20 35
A3
A4
A5
A6
~
N'E ,0
-0
Johnson~tewart
Waka~mith
rrode1
model 6 2/s) De x 10 (m
6 De x 10 (l/s)
1,242 1,437
1,202
7,86
12,01
20 35
0,655
CO
0,891
20 35
0,497 0,622
CO
0,692
20 35
0,655 0,855
CO
0,923
20 35
0,817 1,187
CO
r.zss
20 35
0,673
CO
1,043
o.rss
0,781
5,58
6,16
0,604
3,32
5,59
0,811
5,27
6,28
1,101
9,81
9,17
0,923
0,880
9,33
12,9
1.6r------------------------,
1,2
X QI
a
Effective diffusivity
0,928
CO
A2
Effective diffusivity
0,8
20
30
40
Fig. 3. Effective diffusivities of the different supports.
:J29 For
the
Johnson-Stewart
rnodc l ,
the
effecti vc
diffusi vi ty
is
evaluated
with the relation fir)dr 1
j'~ro
Jl
De
1
P
(9 )
1
DK(r)
DAB
the molecular diffusivity of the nitrogen-helium binary system (in -6 2 can be evaluated hy rn / s , but, in general, D c3se, 68 x 10 j
DAB the
s
I"1B
the kinetic theory of gases, with the relation
J -,
0,001858 T3/2
1 ----
r"B
(10 )
2
P (JAB
(JAB
OK is the Knudsen diffusivity of helium in the cylindrical pore of radius r, DK(r) is given by the relation
Is)
f(r)dr
is
j
9700 r
the
fraction
(11 )
T
["A
of internal
porous
volume of the cylindrical pores
incompassed in the interval from r to r+dr. In numerical 3Jl,
calculations,
the
tortuosity
factor
T
p has bcen taken a';
the value recommfnded by the authors for isotropic porous media. The Wakao-Smi th model has been
support
where
the
effective
found
diffusi vi ty
app r-op ri a t e- for bidisperse porous can be predicted
structure of the particles. According to this model,
from
the porous
the effective diffusi-
vity can be evaluated using the relation:
13 2 D
De
a
where D a
a
+ (1
(_1_
DAB
D and D are Ka Ki the micropores.
- 13 a i
?
D. + 4 13 (l 1 a
_1_)-1
D Ka
1 Ga)(-D- + a
1 -1 --u:-)
(12 )
1
13.
and D.
1
(_1_ _1_)-1( _ _"_)2 + D 1 - G DAB Ki a
(13)
the respective Knudsen diffusivi ties of the macropores and
G and G are the respective internal porosities of the macropores and the a i micropores
DISCUSSION In many problems of mass transfer in a solid porous medium with a large specific surface area (as with catalysts), tion,
with or without a chemical reac-
the solutes are considered to be carried only by diffusion (molecular,
superficial or Knudsen diffusion),
the molecular barycentric velocity being
330 nul. Therefore,
the parameter that expresses the diffusive transport (effec-
tive diffusivity) must be independent of the flow rate of the external fluid. Our experimental results, si vi ty
as
a
function
however,
of the
show a clear increase in effective diffu-
carrier gas
to experimental measurement error alone.
flow,
which cannot be attributed
It is also worthy of noting that the
effect of the flow grows greater as the relative volume of macropores increases.
This
evolution can
only be explained by adding an internal convection
flux to the diffusion flux. Pismen
(ref.
5),
order reaction reaction
who defined the effects of internal convection on
(ref.
5)
6).
The
(ref.
The hypothesis was first put forward by Nir and
and
a
first
on the selecti vi ty of a concurrent-consecuti ve
internal
convection
flux
internal velocity evaluated by Rodrigues and al.
can
be
described
by
an
(ref. 7) using the pressure
drop at the extremities of the particles and their permeability coefficient. If
the
phenomenon
diffusivity fluid
flow,
It
thus
is
of
internal
convection
flow
("apparent" effective diffusivity)
is
ignored,
the
effective
increases along with external
and even more so as the permeability coefficient grows larger. observed that in support systems such as AI,
A 5 and A 6,
the
increase in the "apparent" effective diffusi vi ty is greater than in supports A 2, A 3 and A 4. In the or
principle,
preceding zero.
ments,
In
the
the "true" effective diffusi vi ty should be calculable for
measurements when
fact, range
the external carrier gas flow is very low
because of the of
the
flow
limitations of the experimental measure-
rates
studied
is insufficient to
unambiguous result for the "true" effective diffusi vi ty,
achieve
an
or for the permea-
bility coefficient, which is closely linked to it. It must be conclued, then, that it the
will
"true"
be exceedingly
effective
"apparent"
difficult
to use
effective diffusivity found,
in actual practice,
the method chosen to measure
diffusi vi ty of a porous particle wi th macropores. however,
is surely the best,
The
since,
the particles are to function essentially with an exter-
nal flow (fixed bed, fluidized bed, ... ). When compared
the experimental values obtained for the effective diffusivity are with
the
theoretical
values
deduced from
the
Johnson-Stewart and
Wakao-Smith models, two points stand out: The
experimental
values
of
the
effective
diffusi vi ties
lower than the values deduced from the theoretical models, consideration lues
depend
particle, (1,8
models
on the
but
times
variance)
the internal convective flow.
even
those
would be
Of course,
are
clearly
even taking into
the experimental va-
pseudohomogeneous model chosen to represent the alumina if the spherical model obtained less
with
than
based on the porous
the
slab
were model
used, by
the theoretical values.
the
values obtained
identification Thus,
structure of the particles
of
the
the theoretical
cannot be
used for
331 an a priori
calculation of the effective diffusi vi ty of a particle placed
in a flow. -
The
variations
in the effective diffusivities as a
function of the
porous structure of the supports coincide neither for the theoretical values nor for the experimental values. As the previous discussion has already shown, the effective diffusivity cannot be estimated from the pore diameter distribution. it
is
The
arrangement of micropores and macropores must be known.
true
forward
by
that Mann
stochastic and
model
Golshan
(ref.
of
porous
8)
structure
such
as
While
those put
might allow the arrangement of the
different pores to be represented, the resolution of these models in a chemitoo complicated.
cal reaction appears At
the
present
time,
effective diffusivity rentiate
porous
in our opinion,
in a
supports
only direct measurements of the
device with a flowing external fluid can diffethe
porous
structure
of which
is
known.
Thus,
among the six alumina supports, A 1 must be used when the greatest diffusivity is required,
in preference to A 6,
the theoretical values of which are
nevertheless greater. The difference might be due to the degree of homogeneity of the macropore and micropore distribution inside the beads of the two alumina
supports.
In the theoretical model,
the porosi ty distribution was
considered to be homogeneous wi thin each alumina bead. has
If the A 6 support
more micropores on
the periphery of the beads than the mean porosi ty
distribution indicates,
it would not be surprising to find that it has less
effective diffusivity than the A 1 support.
CONCLUSION The whole set of measurements carried out has shown that using a mercury penetration curve to get
information on
the porous
structure of a support
gives an idea of the probable effective diffusivity, but specifies neither its absolute value nor the direction of its variation. Moreover, our experiments
have
effecti ve
demonstrated
diffusi vi ty
the
necessity
in a device with a
of
taking
the
flowing external
measurements
of
fluid so as
to
determine the influence of the internal convection flux on the value of the effective diffusivity.
REFERENCES 1 2 3
G. Antonini, A.E. Rodrigues and A. Zoulalian, International Chemical Reaction Engineering Conference, Pune, 1984 S. Cheng, A.E. Rodrigues and A. Zoulalian, Proceedings of the IX Ibero America Symposium, 1984, pp. 301-309 M.F.L. Johnson and W.E. Stewart, Journal of Catalysis, 4, 1965, pp. 248252
332 4 5 6 7 8
N. Wakao and J.M. Smith, Chern. Eng. Sci., 17, 1962, pp. 825-834 A. Nir and L.M. Pismen, Chern. Eng. Sci., 32, 1977, pp. 35-41 A. Nir, Chern. Eng. Sci., 32, 1977, pp. 925-930 A.E. Rodrigues, B.J. Ahn and A. Zou1a1ian, A.l.Ch.E. Journal, 28, pp. 541-546 R. Mann and H. Golshan, Chern. Eng. Comm., 12, 1981, pp. 377-391
1982,
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
333
© 1987 Elsevier Science Publishers B.Y., Amsterdam - Printed in The Netherlands
THE EFFECT OF THE CHEMICAL NATURE OF THE WASH-COAT ON THE CAT AL YTlC PERFORMANCE OF CO OXIDA TlON CA TAL YSTS OF MONOLITH TYPE. Lennart B. Larsson, Lars O. Lowendahl and Jan-Erik Otterstedt Department of Chemical Engineering 1, Chalmers University of Technology, 5-412 96 Gothenburg (Sweden) ABSTRACT Light off temperatures and efficiency for Pt on wash-coats of alumina, silica, aluminosilicate, posivitely charged silica and positively charged aluminosilicate were measured. 0.1 wt% Pt, based on the weight of the wash-coat, which is only 10% of the usual amount of Pt in commercial catalysts, on silica and alumina showed as low light off temperatures and as high efficiency as commercial catalysts. INTRODUCTION In the US and Japan automobile exhaust catalysts containing the noble metals platinum, palladium and rhodium are being used for the control of carbon monoxide, hydrocarbons, and nitrogen oxides in order to satisfy regulatory emission control requirements and such catalysts will be introduced in Europe in the near future. The concentrations of hydrocarbons, CO and NO x can be reduced to the desired level in a single catalyst unit, a so called three way catalyst, operated in a narrow range around the stochiometric air/fuel ratio (ref. 1). A typical three way catalyst consists of a honeycomb monolith structure of a ceramic material such as cordierite, A14Mg2Si5018 (ref. 2). The ceramic surface is provided with a layer of high surface area alumina as a washcoat which then will act as a substrate for the active ingredients. The thickness of the washcoat is usually not uniform but varies in the range 10-150 Jim (ref. 3). The washcoat may amount to 5-15 wt% of the monolith, and may provide 15-30 m 2/g of surface area (ref. 4). The converter typically contains 0.15-0.30 g rhodium, which reduces NO x to Nb and 1-2 g platinum, which oxidizes CO and hydrocarbons to C02 (ref. 5). Palladium is sometimes used in combination with platinum as oxidation catalyst but possible detrimental interactions between Pd and Pt or Rh when they are used together have been reported (ref. 6). The reactions taking place on the three way catalyst, that is oxidation of CO and hydrocarbons to C02 and water and reduction of NO x to N2 interfere with each other (refs. 5,7). Under reaction conditions strongly chemisorbed CO thus inhibits the oxidation reactions and chemisorption of NO x also negatively affects the rates of these reactions (ref. 8). In this investigation the effects of the chemical nature of the substrate and the method of depositing platinum on the substrate on the efficiency of Pt as an oxidation catalyst were studied. In subsequent studies the effects of substrate and deposition method
on the efficiency of rhodium as a reduction catalyst and of a complete three way catalyst will be investigated.
EXPERIMENT AL 1. Materials
Ludox T M: 22 nm silica sol containing 49.5% Si02 from du Pont. Ludox SM: 7 nrn silica sol containing 30% Si02 from du Pont. Disperal: Dispersible powder of boehmite (AIO(OH» from Condea Chemie GmbH, Brunsbuettel, West Germany. Hydrazine hydrate: N2H50H, 100% "zur Synthese" from Merck, Schuchardt, Hohenbrunn, West Germany. Ammonia solution: 25% aqueous NH 3, AnalaR, from BDH Chemicals Ltd., Poole, England. Sodium aluminate: NaAI02 powder technical grade from Kebo Lab AB, Gothenburg, Sweden. Hydrochloric acid: 37% aqueous HCl"pro analysi" from Merck, Darmstadt, West Germany. Chlorhydrol Micro-Dry: Aluminum chlorohydrate, from Reheis Chemical Co., New Jersey, USA. Calcium chloride: CaCI2'2H20, "pro analysi", from Merck, Darmstadt, West Germany. Chloroplatinic acid: Prepared by dissolving platinum metal in aqua regia (ref. 9). Monolith: Honey comb structure of cordierite containing 64' square channels per square centimeter from Corning Glass GmbH, Wiesbaden-Biebrich, West Germany. Commercial catalyst: Honey comb structure made by Degussa AG., West Germany, and obtained from Volvo AB, Gothenburg, Sweden. Ion exchange resins: Dowex 50 W-X8 from Dow Chemical Co., Midland, Michigan, USA and Amberlite IRC-50 from BDH Chemicals Ltd., Poole, England. 2. Preparation of colloidal particles Ludox TM and SM solutions of colloidal silica were decationized using a strong acid resin, Dowex 50W, in order to reduce the sodium content before they were used as starting materials for making other colloidal particles. The modification of the surface of silica sol particles by reacting with aluminum to form strongly acidic aluminosilicate sites have been described by Alexander (ref. 10) and Her (ref. 11). The surface of colloidal silica contains 8 Si atoms nm- 2 (ref. 12). About 2 of the Si atoms can be replaced by Al atoms to form negatively charged aluminosilicate sites, corresponding to a surface coverage of 25% Al atoms. Aluminum in the form of freshly prepared Na-aluminate solutions and in an amount corresponding to the desired surface coverage was added in a fine stream to the vortex of a vigorously stirred and decationized sol of Ludox TM at 25 0C. The mixture was centrifuged for I hr at 3000 rpm and the supernatant liquid was aged for 25 hrs at 95 0C and again centrifuged for I hr at
335
3000 rpm. During the two centrifuging steps a small amount of solids, corresponding to less than 196 of the solids content of the solutions, settled to the bottom of the tube, whereas Ludox TM centrifuged at the same conditions did not settle, indicating that a small amount of the sol coagulated during the formation of the aluminosilicate particles. In this work aluminosilicate sols with a surface coverage of 19.3 96 Al atoms were prepared. The preparation of positively charged silica sols by treating them with basic aluminum chloride, Chlorhydrol, has been described by Alexander (ref. 13). Basic aluminum chloride consists of extremely small positively charged particles, about 1 nm, with the composition [A113 0 4 ( O H ) 2 4 ( H 2 0 ) 1 ~
7+ (ref 14). Assuming that the particles have the shape of
hexagonal prisms with I Al atom at each corner and I Al atom at the center of the prism, 4.5 g Chlorhydrol Micro Dry (containing 46.8 wt 96 Al203) per 25 gram Ludox TM or aluminosilicate-modified Ludox TM particles will correspond to a I: I ratio of 5i-surface atoms to Al atoms from ChlorhydroJ. 2396 by weight solution of Ludox T M or aluminosilicate modified Ludox TM (j 9.396 surface coverage by AI) were run into the vortex of vigorously stirred solutions of Chlorhydrol, containing 3.0 wt96 A1203, at a rate of 0.13 g Ludox TM particles per minute. The mixtures were centrifuged for I hr at 4000 rpm and the supernatant liquid contained non-coagulated, non-associated positively charged particles of silica (+T M), or of aluminosilicate modified silica (+AL5I). About 296 by weight of +TM and about 896 by weight of +AL51 sedimented during centrifugation. In the case of +AL51, 8596 of the aluminum (AI) from the Chlorhydrol was adsorbed on the surface of the AL51 particles. Colloidal solutions of alumina were prepared by adding 300 g Disperal powder to a solution of 9.5 g 3796 HCl in 690 g H20 under vigorous stirring. The alumina slurry was stirred for 10 minutes and centrifuged for 1 hr at 2500 rpm. The supernatant liquid, containing about 30 nm aggregates of about 4 nm primary particles of boehmite, was used for catalyst preparation. 3. Coating of colloidal particles with Pt Colloidal particles were coated with Pt by reducing Pt 4+ with hydrazine in the same solution as the colloidal particles. Excess of hydrazine hydrate was added to vigorously stirred solutions of Ludox TM or aluminosilicate modified TM, containing about 2096 by weight of 5i02' By using a metering pump a solution of chloroplatinic acid was slowly added to the sol solution; typical addition rates were 2'10- 5-5'10-6 g Pt min-I. The pH was maintained at 8.5 for TM and 9.5 for AL51 by adding NH3 solution (2M). The concentration of the H2PtCI6 solution was adjusted so that the Pt-coated sol contained 14-15 wt 96 5i02' After completed addition of H2PtCl6 the solutions were centrifuged for I hr at 4000 rpm and the supernatant liquid was used within 18 hrs in catalyst preparation. Colloidal particles of alumina were coated with Pt in a similar manner except that the pH was maintained at 4 by adding solutions of HCI (2M) and NH3 (2M).
336
4. Catalyst preparation Catalyst preparation consisted of the following steps: a. Preparation of monolith b. Deposition of wash-coat on monolith c. Deposition of Pt on the surface of the wash-coat a. Samples of monolith (length 15 mrn) with a square cross-section containing 81 square channels were cut from a commercial honeycomb structure of cordierite. The corners were trimmed off, resulting in a cross-section with 69 channels. In order to ensure that Pt was deposited on the surface of the wash-coat only, when Pt was applied by direct impregnation, the coarse porosity of the cordierite samples was eliminated by repeatedly impregnating them with Ludox SM, containing 30% Si0z, for a total uptake of 17-19% Si02' Excess Ludox SM was drained from the samples and they were dried at 1l00C for hr after each impregnation. After the final impregnation the samples were first calcined for I hr at 1050 0C and then at 550 0C in 100% steam for 3 hrs in order to sinter the 7 nm Ludox SM particles to density. b. Wash-coat was deposited on samples of monolith from a. above by repeatedly immersing them in colloidal solutions containing about 14% by weight of Si02 or A1203' The immersion time in Ludox TM and aluminosilicate modified TM was 120 seconds whereas it was only I second for solutions of colloidal alumina or positively charged TM and aluminosilicate modified TM in order to prevent dissolution of already deposited alumina. Excess colloidal solution was drained from the samples and they were dried at 120 0C for I hr. Samples with wash-coats of alumina or positively charged T M/ alurninosilicate modified T M were heated at 550 0C for 3 hrs after the final application of wash-coat. In this manner the wash-coat was built up layer by layer to give a final surface area in the range of 16-26 m 2 per gram of monolith + wash-coat. Depending on the colloidal solution and the preparation of the monolith, it required from 5 to 20 applications to obtain the desired surface area. This corresponds depending on the sample, to a washcoat weight of 8-25 wt% of the total weight. c. Deposition of Pt on the surface of the wash-coat was done by I) using Ptcoated colloidal particles to build up the wash-coat in b. above, 2) directly impregnating the wash-coat with a solution of chloroplatinic acid and driving off the solvent (water), or 3) using an adsorption procedure. In the second method the pore volume of the wash-coat and the volume of the channels were filled by immersing the wash-coated samples in solutions of chloroplatinic acid. The samples were dried at 80°C for 4 hrs, In this method monolith samples prepared as in a. above were used. The method of depositing Pt by adsorption has been described by van den Berg et al (ref. 15) and can be applied to positively charged surfaces; i.e, wash-coats of alumina and positively charged TM/aluminosilicate modified TM. In this method solutions of chloroplatinic acid were circulated through the monolith
:3.37 channels and PtC162- ions were adsorbed on the positively charged wash-coat surface. However, the adsorption procedure could only be successfully applied to wash-coats of alumina. 5. Catalyst testing The apparatus for catalyst testing used in this investigation has been described by Gandhi et al (ref. 16). The reactor consisted of a vertical stainless steel tube, 900 mm long and with an inner diameter of 16 mm encased in a tubular furnace. The catalyst was sealed in the middle of the heated zone with quartz wool. A downflow of the reactant gas mixture in N2 as carrier gas was led through the reactor and the gas temperature was measured with a movable vertical thermocouple at the inlet of the catalyst. Reactant and product gases were analyzed on line using a Beckman OM-14 02 analyzer and two Maihak Unor 6N lR analyzers for CO and C02' Catalysts containing PtC162- were first oxidized in an air flow of 500 cm 3 min- l at 500 0C for 40 minutes and then reduced in a hydrogen flow of 200 cm 3 rrurr ! at 450 0C for 120 minutes. Prior to testing the catalysts were exposed to a gas flow with a space
velocity of 49000 h- l and containing 3.4 % 02 and 0.6 % by volume of CO in N2 (the composition of the reactant gas mixture) at 400 0C for 2 hours. In order to determine the light off temperature the temperature of the catalyst was raised from J500C at a rate of 4 0C per minute and the increase in CO conversion was recorded. T50 in Table 1 is defined as the temperature at which the CO conversion is 50%. The efficiency of the catalysts was determined by measuring the CO conversion of the reactant gas mixture at space velocities 196000,245000,291000,317000 and 336000 h- l at 400 0C and at 500 0C for catalysts with low and high light off temperatures respectively.
RESULTS AND DISCUSSION The platinum content, BET specific surface area, light off temperature (T50), type of sol and deposition method of Pt for the catalyst samples studied in this investigation are shown in Table 1. The Pt contents faU in three groups: a high content in the range 0.4-0.6, a medium content in the range 0.1-0.2, and a low content in the range 0.02-0.05 mg Pt per gram of catalyst sample. The commercial sample (monolith 26) contained 1.77 mg Pt per gram of catalyst. The surface area, measured by a Digisorb 2600 from Micromeritics, varied in the range 16-31 m 2g- 1.This corresponds to approximately 100, 20 and 5 Pt atoms per 1000 nm 2 for the three ranges of Pt contents respectively and to 200 Pt atoms per 1000 nm 2 the commercial sample. ALSl, +TM, and +ALSl in column 5 stands for aluminosilicate modified TM, positively charged TM and positively charged aluminosilicate modified TM respectively. The numbers
338
TABLE 1 Pt content, BET surface area and light off temperature (T50) for catalysts with different wash-coats and deposition methods of Pt.
Monolith
Pt content
BET area
No.
Type of
°C
sol
Deposition of Pt Method No.a
[- --------6.44-- - -- - - --i6".5---------itl6----fM ------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
0.40 0.56 0.63 0.53 0.48 0.53 0.61 0.093 0.16 0.097 0.13 0.11 0.11 0.1 I 0.10 0.023 0.033 0.053 0.023 0.032 0.024 0.024 0.027 0.027 1.77
22.3 19.6 16.2 19.0 18.7 18.5 16.8 19.3 21.9 18.2 16.2 19.7 17.3 19.3 19.2 19.0 18.4 31.1 18.5 17.5 20.6 17.1 19.0 18.7 29.0
275 253 253 257 256 272 255 290 289 308 304 289 288 287 299 382 333 418 340 322 351 324 338 331 274
TM TM Disperal ALSI +TM +ALSI Disperal ALSI TM TM Disperal ALSI +TM +ALSI Disperal ALSI TM Disperal TM Disperal ALSI +TM +ALSI Disperal Commercial
1 2 2 2 2 2 3 1 1 2 2 2 2 2 3 I 1 I 2 2 2 2 2 3 catalyst
1,2 and 3 in column 6 refer to deposition of Pt by using Pt-coated sol particles for the wash-coat, direct impregnation with H2PtCI6, and adsorption of PtCI62-respectively. I. Effect of Pt concentration and sol type on catalytic performance Figure 1 shows that the light off temperature increases with decreasing Pt concentration but is lower than that of the commercial sample for catalyst samples containing the highest concentrations of Pt. Pt was deposited by direct impregnation for all the samples in Figure 1 (and in Figure 2). TM gives the lowest, +ALSI the highest light
339
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Flg.L The effect of Pt concentration and sol type on light off temperature. (Pt applied by direct impregnation.) - - - High: 0.4-0.6, _. _. - Medium: 0.1-0-2,' ..... Low Pt range: 0.02-0.05 mg Pt per gram of catalyst. - - Commercial catalyst: 1.77 mg Pt per gram of catalyst.
100
r. .
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300 -3
-1
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Fig. 2. The effect of Pt concentration and sol type on catalyst efficiency (Pt applied by direct impregnation).
340
off temperature and Disperal, +TM and ALSI give intermediate values for catalysts with the highest concentrations of Pt. For intermediate concentrations of Pt, ALSI, +TM and +ALSI give the lowest, TM the highest and Disperal intermediate values of the light off temperature. For the lowest concentrations of Pt, Disperal and +TM give the lowest whereas ALSI gives the highest and +ALSI and TM intermediate values of the light off temperature. The increase in CO conversion with temperature becomes more gradual as the Pt content decreases. This is particularly notable for T M at the intermediate and lowest concentrations of Pt (direct impregnation). Figure 2 shows that there is no clear cut dependence of catalyst efficiency, expressed as CO conversion as function of space velocity, on Pt concentration. Thus, intermediate concentrations of Pt on TM and Disperal are as effective as high concentrations of Pt and
100 ;;'- 80
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........
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go
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c)
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85
o
u
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c
··.19(500·C)
300
250 -3
-1
Space velocily x \0. H
Fig. 3. The effect of the method of depositing Pt on catalyst performance. (Silica and alumina as wash-coats.)
341
both concentrations of Pt on these substrates are more effective than the commercial catalyst. It appears, however, that the Pt concentration cannot be lower than a critical value without rapid loss of catalyst efficiency. Low concentrations of Pt on Disperal (No. 21) appears to be an exception and gives remarkably high catalytic performance with only 6 x 10- 5 Pt atom per A2. 2. Effect of the method of depositing Pt on catalytic performance Catalysts with low concentrations of Pt were studied in order to bring out differences in the effect of different methods of application of Pt on catalytic performance. Figure 3 a shows that wash-coat of Pt-coated TM gives a much steeper increase in CO conversion with temperature than wash-coat of TM directly impregnated with Pt. Figure 3 b shows that for wash-coats of Disperal the situation is the reverse; namely that direct impregnation and adsorption results in lower light off temperature and faster response to increase in temperature than wash-coat of Pt coated alumina. Figure 3 c also demonstrates dramatic differences in efficiency of Pt applied by different methods. Direct impregnation of silica with Pt and wash-coat of Pt-coated alumina result in low efficiencies whereas Pt-coated silica and direct impregnation of alumina with Pt give catalysts with high efficiencies. 3. Effect of rate of deposition of Pt on catalytic performance Figures 4 a and b shows that a faster rate of deposition of Pt on sol particles (0.8 mg Pt
100
(
I I I 2
I 80 ;;' c
~ 60
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a
250 300 Iernpercturet C
-,
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u
a
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30G
10-~
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Fig. 4. The effect of rate of deposition of Pt on catalytic performance. (Pt-coated silica particles as wash-coat.), min-I) gives a lower light off temperature but lower efficiency than a slower deposition rate (0.02 mg Pt min-I). This probably reflects differences in the degree of dispersion of Pt caused by differences in the addition rate of Pt to the silica sol in the coating procedure.
342
Obviously the chemical nature of the substrate and the method of depositing Pt on the substrate have a profound effect on the properties of Pt as an oxidation catalyst for CO. In an effort to explain the observed effects, the degree of dispersion of Pt on a number of catalyst samples were determined by chemisorption of H2 and the results are shown in Table 2. Samples I through 8 refer to catalysts with Pt amounts in the high range; see Table 1 for sample identification. Sample 16 refers to a medium range content of Pt on alumina and has a degree of dispersion of 88% which agrees well with that of sample 8, 90%. The degrees of dispersion of Pt on the catalysts with low range contents of Pt could not be accurately determined but it is assumed that they show the same trends as those of the catalysts with high range contents of Pt Fi.gures 3 a and 3 c show that sample 18, Ptcoated silica sol, has a faster light off response and much more efficient CO conversion than sample 20, silica wash-coat directly impregnated with Pt, and yet the degrees of TABLE 2
The degree of dispersions of Pt on selected catalysts. Monolith No.
Degree of dispersion %
- - - - 1 - - - - - - - - . - - - ~ f 7 - · - - - - - - - - - - - - - - - - - - - · - - - - - -
2 3 4
18(22) 14(30) 50
5
11
6
21 15 90 43(66) 88
7 8 10 16
a The degree of dispersion was measuredby a Chemisorb 2800frOm Micromeriticsat 35 0C and at 400 0C (figures within parenthesis). dispersion of Pt on the two catalysts are about the same; d. samples 1 and 6 in Table 2. On the other hand, the catalytic behaviour of Pt applied by direct impregnation and adsorption on alumina, ct. samples 21 and 25 in Figures 3 band 3 c, is almost the same although the degrees of dispersion of Pt by the two methods of application are quite different, 50% and 90% respectively for the two samples. Clearly, chemisorption of H2 on the catalysts of this study cannot be used to explain or predict their catalytic behaviour. Pt is considered not to disperse well on silica and difficulties have been encountered in using hydrogen chemisorption to determine the degree of dispersion of Pt on silica (ref. 17). The catalytic behaviour of Pt on a substrate must depend on the interaction between Pt and substrate, as expressed by dispersion and the structure of the dispersed particles. Kummer (ref. 4) has proposed that large Pt particles on alumina is more active in the
sense of causing a low light off temperature than highly dispersed, i.e, very small, particles of Pt, but more sensitive to the inhibiting effect of CO on the oxidation rate. Comparing samples I and 2 in Figure It then suggests that for Pt-coated silica sols a high rate of deposition of Pt on the silica may result in larger Pt particles on the silica surface than a low rate of deposition although the degree of dispersion does not indicate a difference in particle size. Chemisorption of 02 and CO on samples I (Pt on silica) and 4 (Pt on alumina) were also measured at 1t00oC. In the pressure range 1-10 torr Pt on silica and Pt on alumina adsorbed about 0.02 cc (STP) of 02 or CO (corrected for adsorption on support) which corresponds to about 2 x 10 17 molecules of 02 or CO per gram of catalyst sample. The Pt content of the two samples, about 0.5 mg Pt corresponds to about 2 x 10 18 Pt atoms per gram of catalyst. If Pt is not completely dispersed but forms clusters containing 100 or 1000 Pt atoms these clusters would have diameters of 1.4 and 3.1 nm respectively, and contain 70 and 40% respectively, of the Pt atoms as surface atoms. The number of surface Pt atoms per gram of catalyst would thus be of the order 10 18• 100% conversion of the reactant mixture (0.6 and 3.1t % by volume of CO and 02 respectively in N2) at a space velocity of 200000 h- I corresponds to 6 x 10 18 molecules of CO reacting per second and gram of catalyst. Assuming the time of adsorption of CO and 02 on Pt is less than I second, say 0.1 second, there are enough surface Pt atoms available to account for the surprising similarity in catalytic performance of Pt on silica and alumina for some of the samples studied. CONCLUSIONS The catalytic properties of Pt as oxidation catalyst for CO depend strongly on the substrate and the method of depositing Pt on the substrate. The catalytic performance of Pt on silica, applied by direct impregnation or using Ptcoated silica sol, is as high as that of Pt on alumina, applied by adsorption or direct impregnation. The Pt content can be reduced by a factor of at about 15 while maintaining the catalytic performance at the same level as that of a commercial auto exhaust catalyst. The catalytic performance of Pt on alumina or silica does not correlate with the degree of dispersion of Pt determined by chemisorption of H2. Future work will include studying the effects of substrate and method of deposition on rhodium as a reduction catalyst for NO x and on platinum + rhodium as a three way catalyst. ACKNOWLEDGEMENT We are indebted to the Swedish Board of Technical Development for their support of this project. Du Pont, Condea, Corning Glass and Volvo kindly supplied samples of their products used in this investigation.
344 REFERENCES
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
C.N. Satterfield, Heterogeneous catalysis in practice, McGraw-Hill, New York, 1980, 416 pp. J. Wei, Advances in catalysis, (1975), 57-125. Unpublished results. J.T. Kummer, Prog. Energy Combust. Sci., 6 (1980) 177-199. K.e. Taylor, Catalysis, Science and Technology, 5 (1984) 119-170. J.e. Summers and K. Baron, J. Catal., 50 (1977), 407. L.L. Hegedus and J.J. Gumbleton, Chem Tech., October (1980) 630-642. S.H. Oh, Accepted for publication in J. Catal. S. Eo Livingstone, Comprehensive inorganic Chemistry, 3(1975) 1330. G. B. Alexander, U.S. Patent No. 2,892,797 (1959). R. K. Iler, J. Colloid Interface Scl., 55 (1976) 25-34. R. K. Iler, J. Colloid Interface Sci., 43 (1973) 399-408. G. B. Alexander, U.S. Patent No. 3,007,878 (1956). G. Johansson, Acta Chern. Scand., 14 (1960) 771-773. G. H. van den Berg and H. T. Rijnten, Preparation of Catalysts 2, Elsevier, Amsterdam (1979) 265-277. H. S. Gandhi, A. G. Piken, M. Shelef and R. G. Delosh, SAE paper No. 760201. G.C. Bond and P.B. Wells, Preparation of Catalysts 4, Louvain-la-Neuve (Belgium) September, 1986.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control 1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE PROMOTION OF Pt/Si02 CATALYSTS BY W03 FOR THE NO-CO REACTION J.R. REGALBUT0 2 and E.E.WOLF 1 lChemical Engineering Department University of Notre Dame Notre Dame, Indiana, 46556, USA
ABSTRACT The activity of a series of Pt/W03/Si02 for the NO-CO reaction has been studied using in-situ Fourier Transform Infrared Spectroscopy. It was found that high loadings of W03 promote the activity of Pt for this reaction. Catalyst characterization studies, conducted by x-ray diffraction, x-ray photoelectron spectroscopy, CO chemisorption, and transmission electron microscopy, indicate that Pt was decorated by an overlayer of partially reduced WO x• The increased activity can be quantitatively related via a two site mechanism involving Pt sites and adlineation sites formed at the interface between the overlayers and Pt. I NTRODUCTI ON Elemental tungsten has high NO dissociation activity, but adsorbs strongly Nand adatoms and furthermore, under oxidizing conditions, forms W0 3• However if W0 3 is combined with Pt, the noble metal can help to partially reduce the oxide via hydrogen spillover to yield hydrogen tungsten bronzes or HTB [1,2]. The combination of a partially reducible oxide with a noble metal can also result in an interaction between the metal and the support or SMSI effect, as in the case of Pt, Rh,and Ni supported on Ti02 [3] and Pd/La203 [4]. The purpose of the investigations presented in this paper was to study the promotional mechanism that can arise when a Pt/Si0 2 catalyst is promoted with W0 3 for the NO reduction reaction. This paper summarizes the main results of characterization studies, TEM studies, and FTIR studies which has been described in detail elsewhere [5-7].
°
To whom correspondence should be addressed 2 Present address Department of Chemical Engineering University of Illinois at Chicago Chicago, IL 60680, USA
:345
346
EXPERIMENTAL Catalysts The compositions of the three c a t a l y s ~ s series used in this study are shown in Table 1. The catalystswere supported on a high surface area silica 6el (Harshaw, 600m 2/g), and were prepared by impregnation of the silica support by solutions of chloroplatinic acid, anhydrous ammonium tungstate (AAT) for the low tungsta containing catalysts series, and ammonium metatungstate (AMT) for the high tungsta series. The catalysts contained variable loadings of Pt ranging from 1.2, 2.5, 3.8, and 5.0 wt%. The low tungsta series contained W0 3 loadings varying from 6.2 wt.X to 1.7 wt. X, so that the total metal loading adds to 5.0%. In the high tungsta loading series, only the Pt loading was varied, the tungsta loading was kept constant at 25 wt X. The Pt/Si0 2 catalysts were prepared by impregnation of the silica support with chloroplatinic acid to incipient wetness, followed by vacuum drying, calcination in air to 300'C, and reduction in pure H2 at 425'C. The low tungsta series was prepared by first impregnating the support with AAT, followed by a calcination at 700'C and then subsequent Pt impregnation using the same procedure as for the Pt/Si0 2 catalysts. The same sequential impregnation was used for the high tungsta loading catalysts using instead an AMT solution to produce 25 wtX W0 3• Catalyst characterization X-ray Diffraction(XRD). Diffraction patterns were obtained in a Diano XPG diffractometer equipped with a Cu-Ka source and a graphite monochromatOr. The powdered catalysts samples were pressed into wafers and affixed to a glass holder. Instrumental broadening and broadening due to sample depth, was measured by using a standard consisting of annealed Pt fillings mixed with Si0 2• Diffraction patterns were obtained after each preparation step to identify the crystallographic phase of the catalytic component and to estimate crystallite size using Scherrer's equation.
347
TABLE 1 Catalysts compositions and designations a
Tungsta Free Pt/Si0 2 b
1.2 Pt/Si0 2 2.5 Pt/Si0 2 3.8 Pt/Si0 2 5.0 Pt/Si0 2
Low Tungsta Pt/W03/Si02b 6.2 W0 3/Si02 1.2 Pt/4.7 W0 3/Si02 2.5 Pt/3.2 W0 3/Si02 3.8 Pt/1.7 W0 3/Si02
High Tungsta Pt/W0 3-Si02 b 25W0 3/Si02 1.2 Pt/25W0 3-Si0 2 2.5 Pt/25W0 3-Si02 3.8 Pt/25W0 3-Si0 2 5.0 Pt/25W0 3-Si0 2
a The numbers preceding the catalytic component represents its composition in wt%. D generic designation. Chemisorption. CO chemisorption measurements were carried out by injecting pulses of 10% CO in He. into an ultrapure He stream flowing through a quartz tube containing the catalysts. Prior to chemisorption measurements. the catalysts were pretreated with oxygen at 300·C.reduced in ultrahigh purity H2 at 425·C. and then degassed and cooled to room temperature. Infrared results indicated that CO adsorbed preferentially on Pt. furthermore. no CO adsorption was detectable on W0 3• Consequently. dispersion was calculated using a 1:1 adsorption stoichiometry between CO and Pt. and crystallites sizes were estimated assuming hemispherical geometry. X-ray photoelectron spectroscopy (XPS). These studies were performed at the Amoco research center at Naperville. Illinois. by Or. Theo Fleisch. A Hewlett Packard 5950B ESCA spectrometer was employed using a monochromated A1 source. Sample wafers were pressed from approximately 50 mg of catalyst powder. and placed in a pretreatment chamber attached to the spectrometer. A detailed description of the apparatus is given elsewhere [8]. Catalysts were scanned for the Pt 4f. (73-71 eV). W4f (33-36 eV). Cl 2p (198-200 eV). Si 2p(103-105 eV) and 1s(532-533 eV) transitions. The relative amounts of Win its various oxidation states were estimated by deconvolution using a Gaussian peak fitting routine. Surface compositions were estimated in a conventional manner described elsewhere [ref. 9].
°
348
Transmission electron microscopy (TEM). Conclusions drawn from the above characterization studies were further corroborated by direct observation of model catalysts. These model catalysts were prepared by depositing the active components directly on gold microscope grids coated with a planar silica substrate. After deposition of the precursor salts, the grids were placed in a quartz flow reactor where in order to mimic the preparation of the real catalysts, they were subjected to the same pretreatments, as described in the catalyst preparation section. A JEOL JEM 100C scanning transmission microscope (STEM) was used in these studies. Most imaging was done in the bright field mode, electron diffraction patterns were also obtained to differentiate the Pt and W0 3 phases. Details of the procedures used in the TEM studies are described elsewhere [6,10]. Activity and infrared studies. All activity measurements were conducted in an in-situ infrared reactor cell placed in the sample compartment of a DIGILAB 15C Fourier Transform Infrared (FTIR) Spectrometer. The reactor, described in detail elsewhere [11], consisted of two aluminum flanges with CaF 2 IR transparent windows, a gas inlet and outlet, and two foil fast response thermocouples which were placed in direct contat with the catalyst. The reactor temperature was maintained constant by external heaters controlled by a temperature programmed controller. A Teflon coated recycle pump permitted to maintain near isothermal conditions and improve the mixing in the reactor. The reactor and associated lines were tested for activity at the highest temperature used, and it was found to have negligible activity. The flow rates of all the gases used were metered by a four channel mass flow controller. The electronic flow controller was equipped with a specially designed circuitry which permitted to increase and/or decrease linearly the flowrates of a specific reactant to perform "concentration programming reaction" or CPR experiments. The concentration of CO, CO 2 and NO were measured at the reactor outlet via infrared analyzers. The catalyst, in the form of a thin (100v) wafer held by the aluminum gasket sealing the reactor, was placed perpendicular to the IR beam. The reactants flowed along both sides of the catalyst wafer. The wafers were prepared by pressing approximately 30 mg of the prereduced catalyst powder at a pressure of about 7,000 psi. The wafers, approximately 2.5 cm dia., were reduced again in the reactor for 12 hours at 200·C.
~49
RESULTS Catalyst characterization. X-ray diffraction. Fig. 1 shows the XRD patterns of the 2SW0 3/Si02 (pt free) catalyst after calcination (yellow), and reduction at 42S·C (blue), and of the 1.2Pt/4.7W0 3/Si02 low tungsta catalyst after reduction at 400·C (black). The chromatic effect has been associated with the formation of hydrogen tungsten bronzes [12] which agrees well with the XRD patterns. For the 2SW0 3/Si02, the dissappearence of the (111) W0 3 peak at 28=27.4 degrees, and the conglomeration of the peaks centered at 28=24.S degrees into the (110) HTB peak, further corrroborates the phase transformation. from the fully oxidized W0 3• to either the rombic (H.33W03) form or cubic (H o•SW03) form of the HTB. The diffraction patterns of these two forms are very similar and their differentiation was hindered by peak broadening. The XRD pattern of the 1.2Pt/W03/Si02, low tungsta catalyst (black color), is practically identical to the blue HTB plus additional small and broad lines corresponding to Pt. This catalyst was reduced at a temperature of 400·C, wherein the high tungsta catalyst showed no chromatic effect. This indicates that Pt is indeed helping to reduce W0 3 to a HTB. The crystallographic composition of the Pt phase was also studied as a function of pretreatment on the various catalysts. It was found that after calcination the only crystalline form present in the Pt/Si0 2 and Pt/W0 3/Si02 catalysts series was PtC1 2• In addition, W0 3 lines were also observed in the high tungsta series [6]. In each series, PtC1 2 peaks broadened with increasing W0 3 loading, i.e. W0 3 seemed to reduce the size of PtC1 2 crystallites. Upon reduction, peaks corresponding to metallic Pt formed, eXhibiting broadening proportional to the amount of W0 3 present. HTBs were detected only in the high tungsta series. Average crystallite sizes, calculated for each catalysts from the broadening of the Pt(lll) lines, are listed in Table 2 and seem to decrease with W0 3 loading. CO Chemisorption. CO uptake was the highest for the Pt/Si0 2 catalyst and decreased with increasing Pt loading. A significant reduction in CO uptake was obtained with increasing W03 loading. Average crystallite size calculations based on the chemisorption results, listed in Table 2, show that according to these estimates, crystallite size increases with W0 3 loading. This trend is opposite to the one shown by the XRD results.
cc V,
o
HT8(IOOI
HTB(200l Pt(200l
I
>Ien
z
w
I-
Z
I
.Il
t
25W0 3-SiO Z W0 Olidized
70
65
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3
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24T 241 42T
041 202 240 40T
60
55
222
I (YELLOW) •I
1
W03
W0 3
WO,
400 140
131 13T
22T 221
50
45
40
001 020 200
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35
30
DEGREES 28
Fig. 1 XRD diffraction patterns of oxidized (yellow), and partially reduced 25 W03/SiOZ catalysts, and a reduced 1.ZPt/4.7W03/SiOZ catalyst [5).
25
20
TABLE 2. Average crystallite sizes estimated by XRO and CO chemisorption Ca ta lyst
Crysta 11 i te size
Pt loading
Pt/Si0 2
Pt/W0 3-Si02
Pt/WO/Si0 2
XRO
CHEM
1.2P t
600
47.
2.5Pt
500
58.
3.8Pt
385
79.
5.0Pt
260
133.
[A]
CHEM
XRO
CHEM
107
95
394
315
125
95
294
255
154
75
336
115
284
XRO
X-ray photoelectron spectroscopy. The XPS spectrum of three catalysts are shown in Fig.2 [5]. The lower spectrum corresponds to the oxidized 25W0 3/Si0 2 catalysts showing the 14 4f transition corresponding to the +6 state. The upper two spectra corresponding to a reduced low tungsta and high tungsta Pt containing catalyts, show the appearence of shoulders on the low energy binding side which corresponded to the 14+ 5 and 14+4 state respectively. The relative amounts of 14+ 6, 14+ 5, and 14+4 present on each catalyst listed in Table 3, were obtained from spectra like the ones shown in Fig.2. TABLE 3. 6 Extent of reduction of 14+ obtained from deconvolution of the 14 4f spectra. Pt loading
Pt/W0 3/Si02 14+ 6 14+ 5 14+ 4
100
0 Pt 1.2 Pt
70
30
NO
63
37
NO
2.5 Pt
75
25
NO
67
33
NO
3.8 Pt
85
15
NO
42
33
25
40
39
21
5.0 Pt NO
Pt/W0 3-Si02 14+ 6 14+ 5 14+4
not detected
352
>
tCI.)
Z
W
t-
Z
calcined, reduced T< 4O
46
42
38
34
30
26
ELECTRON BINDING ENERGY CeV) Fig. 2 XPS spectrum of W4f transitions of calcined and partially reduced low and high tungsta catalysts [5].
Transmission electron microscopy. A representative micrograph of an area of a calcined model catalyst showing the formation of overlayers is displayed in Fig. 3. Such overlayers were observed in 4 of 7 areas studied, and were formed after calcination on both the W0 3 crystallites (marked l'l and on the Pt crystallites (marked 2' l. Other results described in detail elsewhere [6], indicate that after reduction overlayers spread, and HTBs formed. Overlayers do not form on Pt in the absence of W0 3, and they disappeared upon exposure to the electron beam. Possible carbon and silica contamination did not affect overlayer formation. The overlayers observed on the model catalysts were not detected on the real catalyst. in part due to lack of constrast. and in part because diffusional conditions that facilitate overlayer formation. are greatly diminished in the real catalysts. Consequently the model catalysts represent a highly exaggerated view of events that occur at a much smaller scale in the real catalyst.
Fig. 3 TEM micrograph of a region of a model catalyst after calcination, magnification 750,000. Activity and FTIR results. The infrared spectrum of CO and NO on the Pt/Si0 2 catalyst shows bands at 2100 and and at 1400 cm- 1 corresponding to linearly adsorbed CO and NO on Pt. At 220·C, CO-Pt adsorbates quickly displaced NO-Pt adsorbates without reaction, and the surface become predominantly covered by CO. In tungsta containing catalysts, a small shoulder appeared on the left hand of the 2100 cm- 1 CO-Pt band, and another small broad band at 1400 cm- 1• These bands were detected only when both NO and CO were present. Results of a CO-CPR experiment, in which the CO concentration was increased linearly from 0 to 0.7% and then decreased likewise, are displayed in Fig. 4 [7]. At this temperature CO 2 production became appreciable for four representative catalysts shown in Fig.4a in terms of mole % CO 2 produced versus time or its equivalent, CO inlet concentration. Fig. 4b displays the intensity of the 2100 cm- 1 band during the experiment.
354
(\
0
o 0
0' o l-
z w c
a:
.7
\
I
\
/
\
I
.5
..
COfecd
\""
/
.6
\
I
\
/
\
I
w
.3
W
.2
\
Q.
...J
\
0 ~
.I
PI
0 0.0
0.5
1.0
I.S
2.0
TIME (hrs)
1.0
W
.75
"w
0
o W o
.50
< u.
a:
::>
en
.25
TIME (hr s)
Fig 4 CO-CPR (0.-0.7-0 %CO) into 2% NO at 280·C over representative catalysts [7].
355
The results show that as CO concentration increases, the rate of CO 2 production at first increases, and then decreases. The decrease in rate correlates with an increase in CO adsorbed during the ramp, i.e. is due to CO inhibition of adsorbed NO species. Furthermore, when the various catalysts are compared at a given time, the high tungsta loading catalysts, which exhibit the highest rate of CO 2 production, also exhibits the smallest amount of adsorbed CO. Thus, it is seen that the presence of tungsta removes CO inhibition by decreasing Pt adsorption capacity. The promotional effects of W0 3 are also seen in Table 4, which summarizes overall and specific rates of CO 2 production on all the catalysts studied. IR absorbance of the W0 3 promoted catalyst remained higher than that of Pt/Si0 2 catalysts indicating that the promoter also has an effect on stabilizing Pt. TABLE 4. C02 Production (rgxl0 17 mo1ec/sec) and rates (rsxl0 18 mo1ec/sec/g Pt) of CO 2 Production Pt
loading
Pt/Si0 2 rg
rs
1.2 Pt
Pt/W0 3/Si02 rg
Pt/W03/Si02
rs
rg
rs
0.789
1.44
0.789
2.13
1.83
5.06
2.82
7.81
2.5 Pt
0.747
2.55
0.913
2.27
3.8 Pt
1.25
3.33
0.664
1.31
5.0 Pt
1.33
3.84
DISCUSSION Catalyst characterization. The XRD results indicate that the W0 3 phase is present in the reduced catalysts in the form of a HTB. According to XPS data, the oxidation state of reduced tungsta varies from w+ 6 to w+ 4• If the composition of the HTB are estimated from the relative % of the various oxidation states obtained from the XPS data, it agrees with the XRD only if the HTBs contain only It follows that the W+4 state must form a tungsta in W+6 and r ~ 5 state.
356
separate phase in order to reconcile the XRD and XPS results. This state can be attributed to WO Z which has a dark brown color which would result in the black coloration observed in the reduced high tungsta catalysts. Pt crystallite sizes shows opposite trends with tungsta loading depending if they are estimated from XRD or CO chemisorption data. For the Pt/SiO Z catalysts. XRD estimates give larger sizes than chemisorption. This case is commonly encountered when a fraction of the crystallites has sizes below the detection limit of XRD. but they are detectable by chemisorption. However with the high tungsta loading catalysts. this trend reverses, with chemisorption estimates being higher than the more conservative XRD estimates. This indicates clearly that chemisorption is insufficient to account for the Pt area detected by XRD, and implicates that chemisorption suppresion occurs with the addition of W0 3• Chemisorption suppresion can be accounted for if a partially reduced oxide is decorating the surface of the active metal. Such a model is now well established in the literature for several SMSI systems. In this case, WO z• which was shown to form a separate phase than the HTB, is likely to be the species that is decorating the Pt surface. The TEM results obtained in a model catalyst, clearly show that overlayers can form in some areas of these ideal surfaces. Over1ayers could not be detected on the real cata1yts but the behaviour of the model catalyst is interpreted as an exaggerated view of what occurs in the real catalyst. On the basis of the above information, a model has been proposed [5,6] which consists of small and large crystallites located on the silica support and on the tungsta phase. Pt on the tungsta phase increased the reducibility of W0 3 and lead to the formation of HTBs and W0 2• The tungsta phase in turn reduced the Pt crystallite size by decreasing the amount of PtC1 2 formed after calcination, whereas tungsta suboxide forms a separate phase and decorates the Pt surface thus decreasing CO chemisorption. The FTIR studies indicate that the presence of W0 3 inhibits CO chemisorption thus decreasing in part CO 2 production rates by removing CO inhibition. Furthermore the NO-Pt coverage was inversely proportional to CO-Pt coverage.Additiona1bands at 2100 and 1400 cm- 1 accumulated only during the transition to the CO inhibited regime occurred. Experiments conducted to measure the rate of NO dissociation indicate that the high tungsta catalyst eXhibited the lowest rate of NO dissociation.
w+ 4
357
While no information is available in the literature on the nature of 2100 and 1400 cm- 1 bands, they can be attributed to CO and NO adsorbed on WO x type sites formed at the interface between the Pt and the decorating patches of W0 2 which were determined by the characterization studies. This type of sites, termed adlineation sites, have been associated with the increased activity of several SMSI catalysts [12J. Using the concept that two sites are responsible for the promotional effects observed, it is possible to correlate quantitatively CO 2 production activity with the surface concentration of both metals provided by the characterization results. The contribution of the Pt sites can be calculated by multipling the number of Pt sites Npt, measured by CO chemisorption on each catalyst, with the turnover number of the Pt sites, TON pt' The contribution of the adlineation sites can be assumed to be proportional to the product of its concentration Npt W' times its turnover number TON pt W• The concentration of the adlineation sites can not be measured, but it can be estimated to be proportional to the product of NptXW s' where Ws is the atomic surface concentration of W, measured by XPS. One can then write:
A plot of rC0 2 - TONptN pt versus Nptw=NptxWs' all measured quantities, yielded a straight line with a slope proportional to TON pt W[7J. The turnover number for the adlineation sites was found to be about 360 Npt' which confirms the expectation that the adlineation sites are fewer, but much more active than the Pt sites. Kinetic analysis of the reaction has provided further support for the two site mechanism [13J. In conclusion, this paper summarizes studies on the mechanism of promotion of Pt by W0 3• It is shown via detailed surface characterization that a surface suboxide WO x' decorates the Pt surface. The decorating WO x species form special adlineation sites that are assumed to be responsible for the promoted activity. A correlation based on a two site mechanism, one for the Pt sites, and the other for the adlineation sites,explains quantitatively the results obtained •
358
REFERENCES
2..
[1]
Benson. J. E•• Kohn. H. W. and Boudar t , M•• J. Ca ta 1••
[2]
Sermon. P. A. and Bond. G. C•• J. Chern. Soc. Farad. Trans •• (1976) •
[3]
raus ter , S. J •• ACS Syrnp , Ser , 298. 1 (1986) •
[4]
Fleisch. T. H. , Hi cks , R. T. and Bell, A. T., J. Catal §l., 398 (1987).
[5]
Regal buto, J. R. and Wolf, E. E., submitted
to J.Catal.
[6]
Regalbuto, J. R. and Wolf, E. E., submi tted
to J.Catal.
[7]
Rega 1buto , J. R. and Wolf, E. E. , submi tted
to J.Catal.
[8]
Fleisch, T. H. and Mains, G. J., J. Chern. Phys., l!(2) , 780 (1982).
[9]
Hercules, D. M., Trends in Analytical Chemistry, l(5), 125 (1984).
[10]
Sing, A. K., Pande, N. K. and Bell, A. T., J. Catal., 94, 422 (1985).
[11]
Kaul, D. K. and Wolf, E. E., J. Catal., 89, 348 (1984).
[12]
Boudart, M., Vannice, M. A. and Benson, J. E., Zeitschrift fur physikaliche Chemie Neue Folge, ii, 171 (1969).
[13]
Regalbuto, J. R., Ph.D. Thesis University of Notre Dame, 1986.
307 (1966) •
!.
72
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
359
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SURFACE DIFFUSION OF OXYGEN IN Rh/A1203 AND Pt/A1203 CATALYSTS.
H. ABDERRAHIMI and D. DUPREZ Laboratoire de Catalyse en Chimie Organique UA CNRS 350 Universite de Poitiers, 40, Avenue du Recteur Pineau 86022 Poi tiers Cedex, France. 1 On leave from the Ecole Normale Superieure, Algiers.
ABSTRACT Exchange of gaseous 1802 with the 160 of the support was studied at 300500°C On precious metal (PM)/alumina catalysts. Rhodium is approximately four times more active than platinum in the exchange reaction. On the other hand, palladium is virtually incapable of promoting oxygen exchange. As regards Pt, the rate of exchange is determined by the rate of adsorption-desorption of oxygen On the PM particles. The true rate of migration can be measured only On rhodium catalysts. The coefficient of diffusion appears to have little to do with the nature of the alumina used as a support. Structural parameters such as the metal area, the perimeter of the metal/support interface, the degree of reduction playa determining role in the overall process of exchange. These various factors are analyzed in the present report.
INTRODUCTI ON Oxygen plays an important part in exhaust gas catalytic purification. The ability of the catalyst to store oxygen for smoothing rapid large oscillations of oxygen pressure in the gas phase, is generally promoted by the addition of rare earth oxides, especially cerium oxide [1,2] . Nevertheless. the presence of precious metals (PM) has been shown to enhance the oxygen storage capacity of three-way catalysts [3,4] . It may thus be inferred that the precious metals playa critical role in the transfer of 02 from the gas phase to the promoter, via surface diffusion on the support (alumina). We shall nOW report rate measurements of this transfer process, deduced from 180- 160 isotopic exchange On PM/A1203 catalysts. EXPERIMENTAL Exchange of gaseous 1802(> 99%, CEA France) with alumina-supported metal catalysts was carried Out in a recirculatory reactor (ca 50 cm 3) coupled with a mass spectrometer allowing the masses 32, 34 and 36 to be mOnitored versus time (Fig 1). The vacuum leak to the mass spectrometer (AEI MS-20)is calibrated
360
vacuum
-
to MS
o
Vacuum
valves
PG Pressure gauge RVP Recirculatory vacuum 170 cm 3
pump
S_1
Te Temperature
controller
:to.SoC
Fig 1
Recirculatory reactor adapted for exchange experiments
so as to ensure a decrease of less than Z mbar in the reactor within 1 hour (initial pressure: 60 mbar). Preliminary investigations [5] have shown that oxygen can exchange only via the metal particles : rates of exchange are negligible on the bare alumina support. Moreover, exchange was found to be severely inhibited by certain impurities and, particularly, by chloride ions. Accordingly, the catalysts used in this study were prepared and dechlorinated by means of the following procedure. The support (Rhone-Poulenc GFS C palumina, ZOO m2g- 1, mean pore radius 45 A, Na, Fe, and Si impurities < 400 ppm) was pretreated either under air flow at 500°C or under a flow of hydrogen at 850°C. The resulting materials are referred to as Ao and A, respectively. The catalysts were prepared by ion exchange of the supports in aqueous solutions of rhodium chloride hydrate or chloroplatinic acid, using the low acidity medium preparation described in Ref 6. The catalysts were subsequently dried at 1Z00C and calcined at 450°C. Dechlorination was performed by treatment under a steam/hydrogen gaseous flow at 450°C for 5h (HZ/HZO molar ratio of 1.35, weight hourly space velocity of steam: Zh-1). The choice of HZ/HZO mixtures arose from previous findings showing that the presence of HZ in steam inhibited the formation of the diffuse oxide phase of rhodium in alumina (Ref 7). The influence of this diffuse oxide phase on the rate of exchange was ascertained on catalyst samples calcined at high temperatures. For the sake of comparison, Rhone Poulenc SCS 79 and Degussa Oxid C aluminas were likewise used for the purposes of this study. They are referred to as RPA and DA, respecti-
361 vely. The dispersion of the catalysts was measured by hydrogen chemisorption and oxygen titration in a pulse flow chromatographic system, as described elsewhere (Ref 8). The catalyst sample (0.02 to 0.2g) was reduced in situ in a flow of hydrogen, and subsequently outgassed at 400°C. A dose of 1802 (0.1 to 0.2 mmole) was then introduced in the recirculatory reactor. After a rapid decrease of pressure, corresponding to oxygen adsorption (Pt catalysts) or absorption (Rh catalysts), the partial pressures P32 (1602), P34 ( 160180) and P36 (1802) were recorded as a function of elapsed time, whereas total pressure remained virtually constant. The rate of exchange rE (in atom of 160 exchanged per min and per g of catalyst) is calculated on the basis of the mass balance of 180 in the PM particles. NV d dx r E = RT dt (-2 P36 - P34) - NM dt (1) where N is the Avogadro number V the gaseous volume of the reactor R the gas constant T the gas phase temperature (K) t the time (min) NM the number of oxygen atoms in the PM particles per g of catalyst x the fraction of 180 atoms to be found in these particles. Making abstraction of the pool of oxygen atoms in the PM particles, becomes (2) 40 - - - - - - - - - - - - - - - ,
p (mbar) 34
32
o Fig 2
t
(minI
50
A typical curve of exchange (0.52 % Rh/A1 203, 329°C)
eq
362
A typical curve of exchange is shown in Figure 2. Generally, P32 t=O=O, so that the initial rate of exchange may be computed, in this instance, by a simple equation: r E = NV RT
l ] P34
(3 )
When equilibrium is attained, the fractions of 180 are equal (,,*) gaseous phase and the support; hence
in
the
(4 )
where Ns is the number of exchangeable oxygen atoms in the support (NG + NM) the initial number of oxygen atoms in the gaseous phase and in the PM particles Generally, NM « NG and (4) becomes N S
=
N
G
(1 - a*) ~
(5 )
The value of Ns computed from Eq (5) may be compared to No, the number of surface oxygen atoms having been calculated from the saturation coverage of the hydroxyl group of aluminas : 6.2 oxygen/100 A2 [9] The equilibration reaction (eq 6) was performed on certain catalysts. 1602 (g) + 1802 (g)-2 160180 (g) (6) Mixtures of gaseous 1602 and 1802 containing approximately 50% of each constituent were prepared and contacted with the catalyst, which was activated in the same conditions as for exchange reactions. The rates of equilibration were calculated on the basis of the rate of appearance of the mass 34 in gaseous phase.
RESULTS Rhodium The The
AND DISCUSSION Catalysts samples used in this study are listed in Table 1. metal area Am (m 2Rhg-1) was calculated using equation 7
(7) Am = 0.0462 Dox m where Do is the dispersion (%) and xm the metal loading (wt %). Eq 7 is on the assumpti on that a Rh atom occupi es 7. 9x10- 20 m2.
based
363
TABLE 1 Rhodium catalysts Support Sample wt% Rh
0.017 0.063 0.56 0.52 1. 76 0.6 0.6
1 2 3 4 5 6 7
Ao Ao Ao A A RPA DA
Dispersion
Grain size (mm)
(%)
Am m2 Rh g-l
100 100 75 80 58 55 50
1.2 1.2 1.2 0.15 0.15 0.15 0.15
0.078 0.29 1.94 1. 92 4.71 1. 52 1.39
Effect of metal loading (samples 1-5). In this series of cata l ys ts jx., was varied by two orders of magnitude; our findings allowed us to determine the initial rates of exchange rE over a wide range of metal areas. The results are plotted in Arrhenius coordinates in Fig.3. For this series of catalysts, .. 1.76 % • 0.56 % 00.52 % D 0.063
%
... 0.017 % '0>
"";.= 46 E
-'" ...
o
UJ
Cl
o
...J
45
300°C
1.4
1.6 10
Fig 3
3
/
1.8
T
Arrhenius plot of the initial rates of exchange on Rh/A1
203
catalyst.
364
the curves show a break point in the region To = 300-380 0C. Taking account of previous results obtained with the 0.52 % Rh/A1203 catalyst in the equilibrat i on reaction (1602 + 1802 - 2 160180) , the break poi nt at To can be exp1a ined as follows: (i) at T < To, the limiting step of exchange is the adsorption-desorption process on the rhodium particles; the apparent activation energy of this step is in the range 70-80 kJ mol- 1 throughout this series of catalysts; (ii) above To, the rate of exchange is determined by oxygen migration upon the support; in this last instance, the apparent activation energy is relatively low (19-22 kJ mol-I) in accordance with the very nature of the determining step. If this hypothesis holds, the rate of exchange should be proportional to the metal area in the region in which exchange is controlled by adsorption-desorption of 02 on rhodium particles (T
% Rh
o (pure alumina) 0.017 0.063 0.56 0.52 1. 76
° 300°C rE 19 at min- 1g x1019 at min- 1 m- 2 x10 0.08 0.37 1. 35 6.23 6.B3 7.80
4.7 4.7 3.2 3.6 1.7
It is clear that, with the exception of the higly loaded sample, the rate of exchange is linearly dependent upon metal area. The behaviour of sample 5 (1.76 % Rh) could be due to either the presence to residual chlorine, which proved to severely inhibits oxygen exchange (Ref 5), or to the fact that the break·point at To is ill-defined in the case of this catalyst. In the field of low activation energy, in which the rate determining step is oxygen migration on the support, exchange contributes to complex kinetics which depend upon both the coefficient of surface diffusion D, and the specific perimeter of the particles. According to Kramer and Andre (Ref 10)the quantity of atoms diffused in the early period exchange would amount to : (8) q = IoCe ( ~ D t ) 1 / 2 where Ce is the concentration of 180 in the metal particles, considered as circular sources. As concerns rhodium, c a t a l y ~ t s , assuming a homogeneous distribution of hemispheric particles (Ref 11), 10 (in m of perimeter per g of catalyst) is given by
:365
10 = 4.13x10 8 (A2/x ) (9) m m which, combined with (7), yields: 10 = 8.81x10 5 D;.X (10) m 0C 1/ Initial values of dq/dt 2 at 400 are recorded in Table 3. Also reported are the values of the coefficients of diffusion calculated from eq.7, assuming that Ce is virtually constant and equal to the overall concentration of oxygen on rhodium particles (1.9x10 19 at. 0 m- 2) TABLE 3 Determination of the coefficients of diffusion at 4000C Sample
1 2 3 4 5
Rh (wt %) 0.017 0.063 0.56 0.52 1. 76
10
(l08 m g-l) 1.5 5.5 28 29 52
dq/dtl/ 2 D (10 19 at.g- 1s-1/2) (l0-18 m2 s-l) 1.48 2.47 3.98 3.87 3.90
8.6 1.8 0.19 0.16 0.05
The samples with the lower rhodium loadings appear to be relatively more capable of exchanging oxygen of the support. This could be indicative of marked heterogeneity, as the number of particles increases. The mean interparticle distances are 57, 29, 15, 13 and 11 nm in samples 1-5, respectively; rhodium particles act as individual sources interfering much more rapidly in highly loaded catalysts, particularly if the rhodium particles are not homogeneously distributed at the alumina surface. Exchange on various alumina-supported catalysts (samples 3-4-6-7) Exchange was performed on 0.6% Rh catalysts supported on various types of alumina.The results are recorded in Table 4. For the sake of comparison, the rates of equilibration are also given. It is clear that equilibration, which is exclusively TABLE 4 Exchange and equilibration on 0.6% Rh catalysts supported on various aluminas Sample
Support
BET area
m2g- 1 3 4 6 7
Pretreatment
Exchange Equilibration x 1019 at min- 1g-1
Air 450 Ao 210 20.8 36 A 180 H2 850 18.8 36 RPA 80 Air 450 13.3 33 Air 450 DA 100 14.0 29 nature of the of the independent area, is a function of the metal support; on the other hand, exchange would appear to be slightly ~ e n s i t l v e
366
to the alumina used. The coefficients of diffusion calculated from eq.7 are 0.12 and 0 . 3 4 ~ 0 - 1 8 m2s- 1 for aluminas RPA and OA, respectively. These values are similar to the coefficient of diffusion for Ao' measured at equivalent metal loading, which indicates that the rate of oxygen migration is virtually independent of the nature of the alumina used. Influence of the degree of reduction of rhodium. When rhodium-alumina catalysts are heated in an oxidizing atmosphere, at elevated temperatur($ (>6000C) there appears, in the alumina matrix, a diffuse oxide phase (OOP) of rhodium which is difficult to reduce at 5000C (Refs 7,12). Given that threeway catalysts are exposed to extreme of temperature, it was of significant interest to study the influence of the OOP on the rates of exchange and equilibration. The results, reported in Table 5, show that rE decreases in parallel with the degree of reduction at 5000C, whereas the rate of equilibration remains unchanged. This result suggests that oxygen included in the DOP TABLE 5 Influence of the temperature of air calcination on the rates of equilibration at 4000C (sample 5, 1.76 % Rh). TOC calcination
Rhodium reducible at 5000C, %
450 700 900
100 80 50
exchange
and
Exchange Equil i brati on x 1019 at min- 1g- 1 23 19 11
37 36 36
can contribute to the reaction of equilibration. The reason why exchange is relatively adversely affected by the presence of OOP has yet to be clearly elucidated. This could be due either to a decrease of the specific perimeter 10 of the rhodium particles or to a qualitative modification of the support for instance, the coverage of residual hydroxyl groups, which exerts a slight influence on the rate of exchange (Ref 5). Other PM catalysts Exchange was performed on platinum and palladium alumina as with rhodium catalysts.
supported
on the
same
The results, recorded in Table 6, demonstrate that rhodium remains the most active metal in the promotion of oxygen migration on the support. Platinum is approximately four times less active than rhodium, and palladium cannot promote, at a measurable rate, the reaction of exchange at 4000C.
TABLE 6 Comparison of the rates of exchange at 400°C on various PM catalysts. PM
Meta1 loading wt %
Pt Pd Rh
1. 06 0.62 0.52
Dispersion Do %
Rate of exchange 1019 at.min- 1g- 1
65 33 80
4.3 0 18.8
The effect of metal loading is shown on Fig 4. For purposes of comparison, 46 <, <, 'O.063%Rh
<,
<,
"1.06%Pt
"- <, <,
D
45 c: E
ttl
o
...
UJ
0144
o
..J
400
1.5
350"C
1.6
Fig 4 : Rates of exchange on Pt/A1 203 catalysts. the curve of the 0.063 % Rh sample is drawn as a dotted line in the figure. The relatively high energy of activation (67 and 59 kJ mol- 1 for the 0.17 % Pt and the 0.46 % Pt samples, respectively) implies that the rate determining step is the adsorption-desorption of 02 on the platinum particles. This is compatible with the fact that the rates of exchange on Pt remain low woth respect to the rate of migration, which can be measured on Rh catalysts. Surprisingly, rE no longer increases in the most loaded sample (1.06 % Pt). And yet, the degree of dispersion of this catalyst is very close to that of the less
368
loaded samples. This result could be due to residual chlorine, which has proven to remain strongly held on platinum catalysts. Nevertheless, this question requires further investigation.
CONCLUSIONS On the basis of the results on the isotopic exchange between gaseous 180Z and 160 of the support in precious metals/alumina catalysts, it can be concluded that : (i) rhodium is the most efficient metal for promoting this exchange. Platinum is approximately four times less active and, palladium is virtually unable to promote the reaction. (ii) for rhodium, the rate determining step at low temperature is the adsorpton/desorption process of Oz on the metal. The coefficient of surface diffusion can be estimated only at high temperature (> 3500C) in the region in which the rate determining step becomes the migration of oxygen at the alumina surface. (iii) the lowest loaded Rh/A1Z03 catalysts « 0.1% Rh) appear to be relatively more active in exchange, probably because the particles are homogeneously distributed at the surface. As a rule, the rate of migration depends only to a small extent on the nature of the alumina used as a support.
REFERENCES 1 J.e. Schlatter and P.J. M i ~ c h e l l , Ind. Eng. Chem. Prod. Res. Develop.,19 (1980) 288. Z J.C. Summers and S.A. Ausen, J. Catal., 58 (1979) 131. 3 H.C. Yao and Y.F. Yu Yao, J. Catal., 86 (1984) Z54. 4 E.C. Su, C.N. Montreuil and W.G. Rothschild, Appl. Catal., 17 (1985) 75. 5 H. Abderrahim and D. Duprez, in preparation 6 D. Duprez, A. Miloudi. J. Little and J. Bousquet Appl. Catal. ,5 (1983) Z19. 7 D. Duprez, G. Delahay, H.Abderrahim and J. Grimblot, J. Chim. Phys., in press 6 D. Duprez, J. Chim. Phys., 80 (1983) 487. 9 B.C. Lippens and J.J. Steggerda, "Physical and Chemical Aspects of Adsorbents and Catalysts", B.G. Linsen, Ed., Academic Press, London and New York, 1970 p 171. 10 R. Kramer and M. Andre, J. Catal. 58 (1979) 287. 11 D. Duprez, P. Pereira, A. Miloudi and R. Maurel, J. Catal. 75 (1982) 151. 12 H.C. Yao, S. Japar and M. Shelef, J. Catal. 50 (1977) 407.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control © 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
RHODIUM -
SUPPORT INTERACTIONS
C.z. Van and J.C. Research Engelhard
EXHAUST CATALYSTS
Dettling
& Develop.ent Dept., Corp.,
Ii AUTOMOTIVE
369
Menlo Park,
Specialty Che.icals Division New Jersey
08818-2900 (U.S.A.)
ABSTRACT Within the current TWC catalyst washcoats, rhodium is susceptible to deleterious interactions with various components during a prolonged lean high temperature excursion. To elucidate the potentially detrimental rhodium compounds formed under such circumstances, unsupported rhodium oxides, rare earth metal rhodates, and aluminum rhodate are characterized and measured for catalytic activity. The intrinsic activities at 673K of NO, CO and C3H6 conversions over various unsupported rhodium oxides species are basically structure insensitive. However, the intrinsic activities at the same temperature of both the rare earth metal rhodates and aluminum rhodate appear to be sensitive to their structure. The interaction between rhodium and the rare earths especially cerium, is found to be much stronger than that between rhodium and aluminum. INTRODUCTION The current catalysts controlling automobile exhaust emission contain noble metals such as Pt, metals being used,
Pd and Rh.
Of the three precious
rhodium is the most effective for reducing the
oxides of nitrogen to harmless nitrogen. other precious metals,
However,
compared to the
rhodium usage in present three-way
catalysts (TWC) technology far exceeds the naturally occurring ratio in the mines.
Motivated by the high cost and limited
availability of rhodium,
it is imperative that methods be
developed for its more effective utilization.
Conventionally,
rhodium is dispersed on a support such as alumina having sufficiently high surface area to enhance its durability. Alternative support compositions have been used recently to promote Rh activity, e.g.
Rh on Ti02 (ref.
1,2) •
However,
preliminary results indicate that the benefits can only be substantiated in a net reducing exhaust and no information on the
370
effect of lean exposure and durability is reported. The advent of more fuel efficient vehicles and engine control strategies in some cases increase the exposure of TWC catalysts to lean conditions.
Consequently, it is important to understand how
rhodium interacts with components within the TWC catalyst wash coat under these conditions.
It has been found that rhodium dispersed
on a relatively high surface area gamma alumina support interacts with the support alumina when exposed to a temperature in excess of 873K in an oxidizing atmosphere (ref.
3,4).
We have recently
demonstrated that rhodium can be rendered less active when it is in intimate contact with the rare earth oxides or the gamma alumina support components of the wash coat during a lean high temperature excursion (ref. 5).
Since cerium oxide and other rare
earth oxides are extensively used in current "High Tech"
TWC
catalysts to achieve a wide operating window lean and rich of stoichiometric conditions,
an in-depth assessment of the Rh
interaction with rare earth components and alumina was undertaken in this study. The present investigation was conducted to identify and determine the degree of Rh-base metal oxide interaction, using unsupported rhodium oxides and bulk aluminum and rare earth metal rhodates.
Catalytic activities were determined using monolithic
catalysts containing various bulk rhodium species exposed to a simulated stoichiometric auto exhaust composition.
The activities
were correlated with information obtained from CO chemisorption measurements,
temperature-programmed reduction,
X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy.
EXPERIMEIiTAL Materials Unsupported rhodium (III) oxides were prepared by precipitating the rhodium ion from a rhodium nitrate solution (5 wt
% Rh)
concentrated NH40H (6N) at pH equal or slightly above 7.0.
with The
precipitate was washed with deionized water until the conductiVity of the washing solution was less than x 10- 5 n -1 cm- 1 and then dried at 398K for a period of 16 hours.
The dried precipitate was
heated in air separately at 973K for 24 hours and 1173K for 3 hours to form
of.
-Rh203 and
fJ
-Rh203 respectively.
Unsupported
Rh02, a brownish powder, was obtained from Alfa Products and used without further treatment.
Aluminum rhodate was prepared by co-
371 precipitating a rhodium nitrate solution (2.5 wt % Rh) with an equimolar sodium aluminate solution.
Rare earth metal rhodates
were prepared by precipitating the metallic ions from an equimolar solution of a rare earth metal nitrate (i.e. lanthanum, or cerium) and rhodium nitrate (in case of cerium, cerium was used)
with concentrated NH40H (6N)
slightly above 7.0.
neodymium
half molar
at pH equal or
The various rhodium containing precipitates
were washed with deionized water until the conductivity of the washing solution was less than 1 x 10-5 h
-1
~m-1
and then dried
at 398K for a period of 16 hours and further heated in air 873K for 70 hours to form corresponding rhodates.
proprietary rhodium nitrate solution (chloride content rhodium purity> 99.5%)
at
Engelhard
<
0.4%,
and Fisher certified grade chemicals
(except reagent grade ammonium hydroxide and sodium aluminate) were used for the sample preparation. Characterization Surface areas were determined by the BET technique (0.162 nm2 for the cross-sectional area of N2) (2500 series) instrument.
with a Micromeritics Digisorb
XRD analyses were obtained with a
computerized Phillips diffractometer using Cu photoelectron spectroscopy (XPS)
K~
radiation.
X-ray
measurements were carried out
with a PHI 5100 series ESCA spectrometer using Mg 300W X-ray excitation.
Binding energies were referenced to C(1s) at 284.6 eV
for all the measurements.
TGA and TPR were studied with a Dupont
(1090 series) thermal analyzer.
CO chemisorption was determined
by a dynamic pulse technique according to Freel (ref. 6) using helium as a carrier gas with automated pulse injection. Catalvst Preparation and Evaluation The catalyst samples were prepared on cordierite Corning monoliths with 62 passageways per square centimeter (400 cells/in 2). This ceramic substrate was coated with a thin layer of a homogeneous mixture of unimpregnated gamma alumina particles and particles of an unsupported bulk rhodium species of interest (including various bulk rhodium oxides and various bulk metal rhodates) and dried at 398K to provide a desired Rh metal loading. The catalysts were placed in a laboratory flow reactor and evaluated at 673K inlet temperature at a space velocity of 112,000 hr- 1 under steady state conditions. The reactive gas composition, simulating an engine exhaust gas near stoichiometry, was 1.54% CO,
372
0.51
% H2,
300 ppm C3H6,
and balance N2'
2000 ppm NO,
1.0% 02,10% C02,
The analytical section of the reactor system
consisted of detectors for NO (Beckman Model 951A), Model 864),
02
10% H20
(Beckman OM-11EA)
CO (Beckman
and hydrocarbon (Beckman Model
400) with computerized data acquisition.
RESULTS AND DISCUSSION Unsupported Rhodium Oxides The physical properties of the three rhodium oxides as prepared and after reduction at 673K are shown in Table 1. the
As anticipated,
d. -Rh203 as prepared shows exclusively hexagonal Rh203
structure and
p -Rh203
as prepared demonstrates virtually
orthorhombic Rh203 structure.
It is quite surprising that the
Rh02 sample which yields a composition very close to Rh02.0 as determined by TGA does not fall in the rutile structure as described by Muller et a L,
(ref.
7).
Chemical and XPS analyses
confirm that the Rh02 sample was contaminated with about 2.5 wt% K20 which apparently affected the Rh02 crystal structure. Examination of the three oxides by SEM at a magnification of 20,000X indicates that the crystals of
0< -Rh203 and
fJ
-Rh203 are
more or less spheroidal while the Rh02 is polygonal plate-like with particles having roughly thickness.
in diameter and 300A in
0.4~
After 673K reduction,
all three oxides show
exclusively Rh metallic structure as determined by XRD analyses and demonstrate little change in crystal morphologies. 1,
In Table
BET surface areas of the reduced oxide samples are found to be
TABLE 1
Properties of Unsupported Rhodium Oxides Species
BET
S.A. m2;g
01. - Rh 20 3
(3 - Rh 20 3 Rh02
a) b) c)
Rh CO Chern Content jlmole/g Rh % (c)
CO(S); XRD Rh(s) Structure
EM
a) 18.8 b) 11.9
81 100
121.8
0.46
Hex Rh203 Rh metal
-O.l~ spheroidal -0.1)' spheroidal
a) b)
81 100
59.0
0.51
Orth Rh203 Rh metal
-0.2/, spheroidal -0.2,.'l spheroidal
215.5
0.46
Unknown Rh metal
6.0 5.2
a) 31.4 b) 21.3
74.5 96.5
As prepared After reduction treatment in h y d r o g ~ n Measured at 298K
0.03 - 0.4;" plate 0.03 - 0 . 4 ~ plate
at 673K for one hour
:373
proportional to CO chemisorption capacities of the samples. Assuming an area per surface rhodium atom of 7. 6A2 (ref.
8),
it is
estimated the reduced unsupported rhodium oxides have a surface CO(s)/Rh(s) stoichiometry approximating 0.5 which corresponds well to a dispersion of a typical heavily-loaded supported Rh species determined by Yao (ref. 3).
The crystallite sizes of the reduced
rhodium oxide samples estimated from CO chemisorption capacities also qualitatively agree with that from SEM analyses.
Thus,
the
CO chemisorption capacities appear to adequately approximate the true available adsorption site densities of the three unsupported rhodium oxides of interest. The results of X-ray photoelectron spectroscopic studies of three oxides as prepared and their reduced states after exposure in 7% hydrogen (balance N2)
at 673K are summarized in Table 2.
It
is interesting to note that similar binding energies of both the Rh (3d) and 0(1s) electrons for all three oxides are observed irrespective of their distinctive differences in crystallite structures.
The surface Rh of the three unsupported rhodium
oxides apparently exhibit a +3 oxidation state.
The undetectable
+4 Rh oxidation state in the Rh02 sample may result from either thermodynamic equilibration between Rh02 and
~
-Rh203 in the
surface structure as a function of oxygen partial pressure (ref. 7,9), or from the possible potassium impurity. hydrogen at 673K,
After reduction in
all three unsupported rhodium oxides show not
only an exclusively metallic Rh surface but also metallic Rh structure in the bulk.
Additional TPR and XRD experiments
TABLE 2 XPS Analyses of Unsupported Rhodium Oxides Species
Binding Energy, eV Rh Od) 3d 5/2 3d 3/2
0(1s)
1-
lI. - Rh203
308.6
313.6
530.2
2.
(J - Rh203
308.6
313.6
530.2
3.
Rh02
308.6
313.6
530.2
4.
Reduced Oxides·
307.0
311.8
• After reduction in hydrogen at 673K for one hour
374 demonstrate that the three rhodium oxides can be reduced to metallic Rh even at 523K in a hydrogen atmosphere. Catalytic activities of monolithic catalysts containing various unsupported rhodium oxide species are shown in Figure 1.
Plotting
total conversions of NO, CO and C3H6 respectively of a simulated stoichiometric gas composition against the total CO chemisorption capacities per liter volume of Rh containing catalysts, all three rhodium species of interest at varying Rh loadings demonstrated a one-to-one correlation.
This suggests that conversions of NO, CO
and C3H6 respectively over the catalysts containing various unsupported rhodium oxide particles are structure insensitive. observed in our own studies and as seen in reference (ref.
As
10),
non-interactive Rh species can be readily reduced to metallic rhodium in a stoichiometric gas mixture under the reaction Thus, it
conditions (673K) as well as in a hydrogen environment.
100
D
100
~
o ,(- Rh203 A (3-
NO
Rh203
50
r:
o Rh0 2
0 100
c
c .~
0 'Vi
Q; >
c
CO
0
50
Q; > c 0
U
U
*
* 50
C3Hb 20
40
"'" mol CO/liter Catalyst
Fig.
1 (Left).
r
H6
"'" mol CO/liter Catalyst
Activity of monolithic catalysts containing various bulk rhodium oxides.
Fig. 2 (Right). Activity of monolithic catalysts containing bulk aluminum rhodate.
375
is understandable that the Rh activities of these catalysts containing various unsupported rhodium oxide particles are independent of their distinctively different crystalline structures.
UNSUPPORTED
METAL RHODATES
Physical Properties of Unsupported Metal Rhodates Four metal rhodates were studied,
i.e. La and Nd rhodates which
may exist in perovskite structure, and Al and Ce rhodates which do not exist in crystalline composite structure according to the literature (ref.
11).
The rhodium contents in the rhodates as
prepared and after reduction at 723K as determined by chemical analyses and TGA studies are shown in Table 3.
The rhodium
contents in aluminum rhodate, lanthanum rhodate and neodymium rhodate are about 15% higher than the theoretical stoichiometric amount while that in cerium rhodate is very close to the theoretical value. TABLE 3 Properties of Various Unsupported Metal Rhodates Species
1. Aluminum rhodate (Al/Rh=1/1)
Rh Content %
BET S.A. (m 2/g)
CO Chem )lmole /g Rh (c)
~
Rh(s)
a) 110.0 66.0 b)
55 73.5
442
0.42
Lanthanum rhodate (La/Rh=1/1)
a) b)
0.7 11.0
37.8 43
89
0.85
3. Neodymium rhodate (Nd/Rh=1/1)
a) b)
0.6 11.5
36.2 41.1
87.5
0.84
4. Cerium rhodate (Ce/Rh=1/2)
a) b)
0.8 21.2
44.5 54.3
11.2
0.04
2.
a) As prepared (873K/70 hours/air) b) After reduction treatment at 723K for one hour c) Measured at 298K BET surface area and CO chemisorption capacity of aluminum rhodate are about a factor of 4 to 6 higher than that of the corresponding unsupported
~
-Rh203.
Assuming A1203 and Rh species
:176
in the aluminum rhodate after reduction at 723K are contributing BET surface area in proportion to their measured contents,
it is
estimated that the reduced aluminum rhodate has a surface CO(s)/Rh(s)
stoichiometry of about 0.42 which is similar to the
value obtained with the corresponding reduced ..,( -Rh203'
The
aluminum rhodate and the 0< -Rh203 appear to have similar CO adsorption properties on the basis of unit surface rhodium density. In contrast to aluminum rhodate, demonstrate peculiar properties.
rare earth metal rhodates
They exhibit very small BET
surface areas as prepared and appear initially non-porous in structure.
However,
twenty fold increase in surface areas are
observed when the rhodates have followed a reduction treatment at 723K.
This suggests,
that the Rh species present in the rare
earth metal rhodates as prepared, are quite different from that in the aluminum rhodate.
The rare earth metal ions appear to
strongly associate with the rhodium oxide resulting in a nonporous interactive structure.
Upon hydrogen exposure at 723K,
the
rhodium species in the rhodates migrate out of the structure to form clusters and leave behind a porous structure.
The CO
chemisorption capacities of lanthanum rhodate and neodymium rhodate after reduction are similar,
and correspond to an
estimated surface CO(s)/Rh(s) stoichiometry of about 0.85 assuming BET surface area partitions according to the Rh species and rare earth oxide contents in the rhodates. CO (about 15%)
Although small amounts of
may have been irreversibly adsorbed on the
lanthanum oxide as determined separately,
this number is
significantly higher than that obtained with the aluminum rhodate or
d -Rh203'
Unexpectedly,
cerium rhodate demonstrates extremely
low CO chemisorption capacity after the reduction treatment even though its BET surface area is about twice the value seen on the other two rare earth metal rhodates.
The behavior of cerium
rhodate is similar to an effect of SMSI (strong metal-support interaction)
phenomenon (ref.
12, 13).
Activities of Metal Rhodates The results of the catalytic activity of monolithic catalysts containing various amounts of unsupported aluminum rhodate particles in a simulated stoichiometric gas mixture are shown in Figure 2.
The activity increases with an increase in Rh loading
and levels off in the mass transfer controlled region.
Comparing
the non-mass transfer controlled reaction regime results in Figure
2 with those in Figure 1,
the aluminum rhodate appears to have
only about 1/3 the activity of the d -Rh203 for NO, CO and C3H6 conversions on the basis of equal accessible surface Rh density, The less active aluminum rhodate species apparently results from the interaction of the Rh species with the associated aluminum oxide in the structure, Table 4 summarizes the actiVity evaluation results of four monolithic catalysts containing various metal rhodates of the same Rh metal loading before and after hydrogen reduction at 723K.
It
clearly shows that the hydrogen reduction treatment of the catalysts does not improve catalytic activity.
This is not
surprising since it was previously demonstrated that the active components in the catalysts can be reduced in a simulated stoichiometric gas mixture as effectively as in a hydrogen environment.
The lanthanum
and neodymium rhodates which exhibit
similar physical properties demonstrate similarly low activities. It is estimated that the lanthanum or neodymium rhodate has about 1/10 the activity of the
~
-Rh203 on the basis of equally
accessible surface Rh density.
The cerium rhodate is practically
inactive and does not show appreciable CO chemisorption capacity, The results of Table 4 suggest that the reactions of NO, CO and C3H6 conversion over metal rhodate containing catalysts are sensitive to their structures.
It also reveals that different
TABLE 4 Activities of Various Rhodates Containing Monolithic Catalysts Catalysts:
8.5 x 10- 2 gIl Rhj 2.5 c m(D) x 7.6 cm(L)
Catalyst/Species C3 H6
Conversion % CO
NO x
1-
Aluminum Rhodate
(a) (b)
70 71
74 74
89 90
2,
Lanthanum Rhodate
(a) (b)
5 4
8 7
11 9
3.
Neodymium Rhodate
(a) (b)
4 3
7 5
10 6
4,
Cerium Rhodate
(a) (b)
2 0
4 1
5 2
a) b)
As prepared After reduction in hydrogen at 723K for one hour.
378
activities of aluminum rhodate, lanthanum rhodate (including neodymium rhodate)
and cerium rhodate may result from three In order to elucidate the
distinctive interaction mechanisms. origins of the interactions, aluminum rhodate,
lanthanum rhodate and cerium rhodate samples in
rJ. -Rh203 was carried out.
comparison with Temperature -
thorough characterization of the
Programmed Reduction
Figure 3 shows the TPR of three rhodates in comparison with that of
rJ. -Rh203'
difficul t
The three r-h o d a t e s were found to be more
to reduce than the
rJ. -Rh203 indicating that the rhodium
species in the rhodates strongly interact with the added metal cations.
In the three rhodates studied,
the difficulty of
reduction follows the order lanthanum rhodate
>
cerium rhodate
2 aluminum rhodate
In the lanthanum rhodate sample, it appears there is an intermediate reduction state around 873K.
This TPR result is not
totally consistent with that in a recent publication (ref.
14).
Above 1173K all the metal rhodates are completely reduced to metallic rhodium and refractive oxides as indicated by TGA and XRD analyses.
Thus,
after reduction treatment at 723K for one hour in
7% hydrogen (balance nitrogen), determined to be about 93%,
the degree of reduction was
67%,
90% and 100% completion for
aluminum rhodate, lanthanum rhodate, respectively.
cerium rhodate and
~-Rh203'
It confirms that a significant portion of the
A
--------------------
'"c
.~
C
en
1J
273
Fig.
3.
473
D
673
873
1073
K
TRP spectra of various bulk rhodium species in hydrogen (20K min- 1)
(A) aluminum rhodate (B) lanthanum rhodate (C) cerium rhodate (D)
,j.
-Rh203
379
lanthanum rhodate sample is not totally reduced at 723K. X-ray Diffraction The rhodates as prepared yield only amorphous structures as shown in Figure 4.
SEM analyses confirm the homogeneous nonThe XRD amorphous
crystalline character of the rhodates.
structures of the rho dates strongly suggest the rhodium oxide lattices in the rhodates have been totally obscured by the presence of the metal cations.
It is interesting all the rare
earth metal rhodates as prepared demonstrate nearly identical XRD amorphous structures which are slightly different from that of the aluminum rhodate.
The extremely low BET surface areas of the rare
earth metal rhodates may be correlated to the XRD results.
A
LaRh03 perovskite synthesized at 973K and exhibiting quite different XRD patterns was reported recently by Tascon et al. (ref.
14).
Since the preparation method of the lanthanum rhodate
is different between the two laboratories, it is difficult to compare the experimental results.
A A
C '§ B
OJ
B
.£
c 20
40
60
80
20
2 P (degree)
Fig.
4 (Left).
40
60
2
80
e (degree)
XRD patterns of various rhodates as prepared.
(A) aluminum rhodate (B) lanthanum rhodate (C) cerium rhodate Fig. 5 (Right).
XRD patterns of the same rhodates after reduction treatment in hydrogen at 723K. 1. Rh; 2. La203; 3. Ce02
After reduction treatment at 723 0K for one hour in 7% hydrogen (balance nitrogen),
X-ray diffraction patterns of the three
380
rhodates are presented in Figure 5.
The reduced aluminum rhodate
shows only peaks of metallic Rh having a particle size about 39A in diameter.
The reduced lanthanum rhodate yields a pattern which
can be identified as a mixture of La203 and metallic rhodium having a Rh particle size about 23A.
However,
the broad spectrum
indicates that the reduced lanthanum rhodate is somewhat amorphous in nature.
The reduced cerium rhodate gives a clear pattern of a
mixture of Ce02 and metallic rhodium having crystallite sizes about 50A in diameter for both the Ce02 and Rh particles.
The XRD
as well as TPR results confirm that lanthanum rho date is more difficult to reduce in the bulk structure. X-ray Photoelectron Spectroscopic Studies Photoelectron spectroscopic studies of the three rhodates in comparison with
~
-Rh203 were performed in order to determine
the Rh surface structures.
Figure 6 shows XPS spectra of Rh 3d
c1. -Rh203.
electrons of the r ho d a t e e as prepared and
The three
rhodates exhibit identical Rh XPS spectra with a 3d 5/2 peak at 308.2 eV and a 3d 3/2 peak at 313.0 eV.
These spectra indicate
the binding energies of the Rh 3d electrons of the rhodates shift
313.0
316
312
308.2
Rh[3dl
308
304
Binding Energy, eV
Fig.
6.
XPS Rh(3d)
spectra of various r ho d a t e s as prepared a n d
~ - Rh203
(A) aluminum rhodate (B) lanthanum rhodate (C) cerium rhodate (D)
c1.-Rh203
381
to a lower number by 0.4 eV for the 3d 5/2 peak and 0.6 eV for the 3d 3/2 peak which results in a smaller energy spacing between the two 3d peaks in comparison to the Rh +3 species of
~
This
-Rh203'
suggests the bonding between oxygen and rhodium species in the base metal rhodate being more covalent in nature with respect to those of
~
-Rh203'
Figure 7(a),
7(b) and 7(c) show 0(1s) XPS spectra for the
aluminum rhodate, respectively.
lanthanum rhodate and cerium rhodate
The spectrum of aluminum rhodate indicates the
binding energies broaden toward the lower side while a peak shifts to a higher number in comparison with a reference alumina.
The
spectra of lanthanum rhodate and cerium rhodate appear to be more complicated and deviate appreciably from those of 01.. -Rh203 (only single peak at 530.2 eV) and the corresponding base metal oxides. Although contribution of each oxygen species in the rhodate to the O[ls]
AI [2pl
70
La[3dJ
c
c "'"" 1;
"§ OJ
OJ
1;
Binding Energy, eV
858
850
842
834
826
Binding Energy, eV
Fig. 7 (Left). XPS 0{1s) spectra: base metal rhodates as prepared (solid line) and reference base metal oxides (dashed line). Fig. 8 (Right).
XPS spectra of base metal: base metal rhodates as prepared (solid line) and reference base metal oxides (dashed line).
382
oxygen XPS spectra is difficult to measure,
there are at least two
distinguishable peaks of the oxygen binding energies that can be resolved and related to the interactive structures.
One peak is
at 531 eV and the other is around 529 eV which appears on the shoulder of the spectrum in the case of lanthanum rhodate and aluminum rhodate.
The higher energy peak results from the oxygen
associated with the rhodium.
The bonding between oxygen,
rhodium
and base metal in the rhodate has more covalent character than that found in d-Rh203'
The lower energy peak may be attributed
to the oxygen associated with the base metals; and it suggests that the bonding between oxygen and base metals in the rhodates is more ionic in character with respect to the corresponding base metal oxides.
Of the three rhodates studied, one can see the
interaction most dominant in the cerium rhodate. Figure 8(a) shows the Al(2p) XPS spectrum of the aluminum rhodate in comparison with a reference alumina. energies of the Al(2p) However,
The binding
peak of the two samples are identical.
the spectrum of the aluminum rhodate appears to broaden
toward the higher binding energy side indicating a disturbance of electron distribution around the nearby oxygen neighbors which correlates with the broadening of 0{1s) XPS spectrum observed. Figure B(b) and 8(c) present the La(3d) and Ce(3d) XPS spectra for lanthanum rhodate and cerium rhodate respectively.
They
demonstrate 3d electron binding energies of the rare earth metal ions shift to a higher number in comparison with reference La203 and Ce02'
The binding energy shift of Ce 3d electrons appear to
be most pronounced.
The raising of the binding energy shift of La
or Ce 3d electrons indicate a partial electron transfer from the La or Ce cations toward nearby oxygen neighbors whioh are not bridging between the rhodium and the rare earth metals.
These
behaviors are consistent with the conclusion learned from 0(1s) XPS results.
It also suggests the interaction characteristics in
the rare earth metal rhodates are different from those in aluminum rhodate.
TheJ also correlate with the different physical
properties previously seen.
Although a complete picture as to how
the interactive structures are formed in the rhodates is not certain, it is speculated that microsphere particles of rhodium hydroxide may have been produced during the sample preparation. These particles then were layers.
surr~unded
Upon drying and calcining,
by base metal hydroxide interactive oxygen bridging
between the rhodium and base metals over the surface of
;J83
microsphere results in the formation of interactive structures. Since the interactive structures alter the lattice parameters of the rhodates,
it is understandable why the rhodates exhibit no XRD
crystalline structures. After reduction at 723K for one hour in 7% hydrogen (balance nitrogen) XPS spectra of Rh (3d) electrons of the rhodates are given in Figure 9. peak.
However,
All rhodate samples show a major metallic Rh
significant portions of surface Rh in aluminum
rhodate and cerium rhodate apparently remain in the interactive state which the 308.2 eV peak on the shoulder of 3d 5/2 spectrum identifies.
The behavior of surface Rh in lanthanum rhodate
is comparatively close to that of metallic Rh.
XPS spectra of Al
0(15)
311.8
307.0
Rh[3d]
316
312
308
304
Binding Energy, eV
Fig.
Binding Energy, e V
9 (Left). XPS Rh(3d) spectra of rhodium species after reduction treatment in hydrogen at 723K.
(A) aluminum rhodate (B) lanthanum rhodate (C) cerium rhodate (D)
Fig.
rJ. - Rh 2 0 3 10 (Right).
XPS O(1s} spectra: base metal rhodates after reduction treatment in hydrogen at 723K (solid line) and reference base metal oxides (dashed line)
384
(2p),
La
(3d) and Ce (3d) of the rhodates after the reduction
treatment are similar to patterns from A1203' respectively.
Thus,
La203 and Ce02
it is quite likely the interactive phase in
the aluminum rhodate and cerium rhodate after the reduction treatment exist in the vicinity of the Rh particles. Figure 10 shows the 0(1s) XPS spectra of the rhodates after reduction.
In the reduced aluminum rhodate,
only small amounts of
interactive oxygen species can be identified from the spectra. These oxygen species are apparently located at the interface between the rhodium particles and the alumina particles.
Rh
particles under the influence of such interaction naturally produce inferior activity when compared to non-interactive Rh particles of similar metal surface area.
0(1s)
spectra of the
reduced cerium rhodate sample reveal even higher concentrations of interactive oxygen species remain at the interface between rhodium and cerium oxide.
It is very likely that significant portions of
the Rh particle surfaces in the reduced cerium rhodate are in contact with interactive oxygen species preventing adsorption of reactive species by Rh.
Under such circumstances, most of the Rh
surfaces in the cerium rhodate are not accessible for catalyzing the redox reactions resulting in extremely low activity.
The
reduced lanthanum rhodate gives similar 0(1s) XPS spectra as La203 indicating very little interaction between Rh and La203 on the reduced rhodate surface.
However, examining the ratio of base
metal to rhOdium in the rhodate by XPS intensity measurement before and after reduction treatment as indicated in Table 5,
the
TABLE 5 XPS Estimated Metal Concentration Ratio Species
Atomic Ratio
As Prepared
After Reduction at 723K for one hour
Aluminum Rhodate
Al/Rh
1.2
1. 25
Cerium Rhodate
Ce/Rh
0.6
0.5
Lanthanum Rhodate
La/Rh
1.3
1.9
385
surface of reduced lanthanum rhodate appears to be enriched in lanthanum while the surface composition in aluminum rhodate and cerium rhodate remain virtually unchanged.
Apparently,
t he
observed lanthanum enrichment on the surface of the reduced lanthanum rhodate and the incomplete rhodium reduction in the bulk are both effects rendering it less active.
CONCLUSION Conversions of NO,
CO and C3H6 are structure insensitive over a
catalyst containing various unsupported rhodium oxide particles. This is understandable because the original crystalline structures of the oxides are short-lived under reaction conditions in a gas composition near stoichiometry at 673K where the oxides are easily reduced to metallic Rh. Conversely over the metal rhodate containing catalysts, CO and C3H6 structures.
the NO,
conversions appear to be sensitive to their Unlike the easily reducible rhodium oxides,
the
rhodates are harder to reduce under reaction conditions as well as in a hydrogen atmosphere.
XPS spectra reveal that interactive
structures exist between the rhodium ion and the oxygen anion associated with the metal cations of the rhodates. reduction at 723K,
Even upon
a significant portion of surface Rh in aluminum
and cerium rhodates remains in the interactive state while the interactive structure is apparently destroyed at the surface of lanthanum rhodate.
In the reduced cerium r ho d a t e sample,
the
interactive structure dominates the adsorption properties of surface rhodium resulting in extremely low activity. reduced aluminum rhodate containing catalyst,
In the
it is possible the
interactive species present at the interface between the rhodium and aluminum oxide reduces the catalytic activity. lanthanum rhodate species,
In reduced
it is believed that lanthanum
enrichment on the surface and incomplete reduction in the bulk render the rhodium less active.
REFERENCES 1.
2. 3. 4. 5.
Rives-Arnau, V., and Munuera, G., Appl. Surf. ScL, 6(1980), 122. Pande, N.K., and Bell, A.T., J. c a t a l , , 98(1986), 7. Yao, H.C., Japar, S. and Shelef, M., J. Catal., 50 (1977), 407. Yao, H.C., Stepien, H.K., and Gandhi, H.S., J. Catal., 61 (1980), 547. Wan, C.Z., and Dettling, J.C., SAE Paper No. 860566 (1986).
386 6. 7. 8. 9. 10. 11. 12. 13.
14.
Freel, J., J. Catal., 25 (1972), 139. Muller, 0., and Roy, R., J. Less - Common Metals, 16 (1968), 129. Yates, D.J.C., and Sinfelt, J.H., J. Catal., 8 (1967),348. Bayer, G., and Wiedemann, H.G., Thermochim. Acta, 15 (1976), 213. Oh, S.H., and Carpenter, J.E., J. Catal., 80 (1983), 472. Wold, A., Arnott, R.J. and Croft, W.J., Inorg. Chem. 2 (1963), 972. Tauster, S.J. and Fung, S.C., J. Catal., 55 (1978), 29. Meriaudeau, P., Dutel, J.F., Dufaux, M., and Naccache, C., "Metal-Support and Metal-Additive Effects in Catalysis", Elsevier Scientific Publishing Co., Amsterdam, 1982, P. 95. Tascon, J.M.D., Olivan, A.M.O., Gonzalez Tejuca, L. and Bell, A.T., J. Phys. Chem., 90 (1986), 791.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
387
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
DEVELOPMENT OF A COPPER CHROMITE CATALYST FOR CARBON MONOXIDE AUTOMOBILE EMISSION CONTROL J. LAINE, A. ALBORNOZ, J. BRITO, O. CARIAS, G. CASTRO, F. SEVERINO
and D. VALERA Laboratorio de Catalisis Heterogenea, Centro de QUlmica, Instituto Venezolano de Investigaciones Cientlficas, I.V.I.C., Apdo. 21827, Caracas 1020-A, Venezuela
ABSTRACT Copper-chromium catalysts employed for CO oxidation were found to be affected by composition and pretreatment parameters. CuCr204 was more active than CuO only if prereduction was carried out and if metal concentration on alumina support was larger than 12 wt%. The presence of Cr with Cu in the oxide limited the extent of catalyst reduction leading also to less deactivation as compared to Cu on alumina. The presence of Cr also decreased an activity inhibition effected by water. A supported Cu-Cr catalyst used in an automobile ran with leaded petrol was deactivated by lead deposition. Deposits were mainly lead sulphate located on pellet periphery. Also, lead was preferentially distributed on the alumina instead of on the active metal-rich zones of the catalysts. INTRODUCTION Urban atmospheric pollution in countries without strict regulations for automobile exhaust control is currently an important consideration in relation to possible threats to health. Many growing cities like Caracas, have increasingly high air pollution levels, CO being one of the most important contaminants. Among non-precious metals, copper-chromium combinations seem to be the most effective catalysts for CO exhaust elimination. For example, monoliths and pellets impregnated with copper-chromite have been reported to have activities near those of precious metal-based auto-emission control catalysts (ref. 1,2). This comparison has also been extended to the oxidation of CO with NO (ref. 3,4), another important auto-emission pollutant. The present communication reports results regarding CO oxidation over copper-chromium catalysts. The work was started studying unsupported catalysts and then supporting on alumina, looking for optima catalyst composition and pretreatment. Afterwards, the
388
changes arlslng in a copper-chromium catalyst as a result of using in an automobile exhaust were examined. EXPERIMENTAL The catalysts were prepared as described in detail elsewhere (ref. 5,6). Briefly, unsupported catalysts of various Cu/Cr ratios were made by re-crystallization of mixed copper and chromium nitrate hot solutions followed by drying and calcining, and supported catalysts by impregnating y-A1 Z0 3 powder (100 mZ/g) with similar solutions also followed by drying and calcining. Three series of supported catalysts were prepared: CuO, CuCr Z04 and Cr Z0 3 with metal concentrations ranging between Z and 30wt%. The series of chromia catalysts did not present any detectable activity under the experimental conditions employed in this work, thus, we report here results concerning only the Cu and Cu-Cr catalysts. For the automobile test, a copper-chromite catalyst was prepared similarly by impregnation of y-A1 Z0 3 tablets (4.6 mm diameter x 4.6 mm length). Catalyst activities for CO oxidation were measured in a continuous flow system. Other characterization techniques employed were: X-ray diffraction, Temperature Programed Reduction, Auger spectroscopy, and Scanning Electron Microscopy. The automobile test was carried out using a 3.8 liter-V6 engine with a Z liter exhaust catalytic converter. A non-standard Z,150 Km urban driving was carried out in order to compare fresh and used catalysts. RESULTS AND DISCUSSION The first part of this work was devoted to an optimization study of the composition and pretreatment parameters of unsupported and supported copper and copper-chromium catalysts for the reaction of oxidation of CO. The activity of unsupported CuCr Z04 was found to be increased by pretreating the catalyst with pure CO (ref. 5). Also confirming earlier findings (ref. 7,8) CuCr Z04 was found to be more active than CuO for CO oxidation. However, we found (ref. 5) that this was the case only if catalyst prereduction was carried out. Optimum Cu/Cr ratio found in unsupported catalysts was that corresponding to the stoichiometry of CuCr Z04 (ref. 5). Auger spectra (ref. 9) demonstrated that the activity enhancement achieved by prereducing
389
1 0 0 - - · ·....- . -
-----
80
~ c: 0
...
,,;~---
lJ)
,
Q)
>
c: 0
u
40
,r
0
U r
r
r
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,~ '
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/
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-. 10
-_ ... -- ---_. 20
30
Metallic Concentration (%)
Figure 1. Effect of metal concentration of supported catalysts on activity at 400°C .• , CuO/A120s; 0, CuCr 204/A1 20 s . Catalyst pretreatment: CO reduction at 400°C for 3h. Sample: 5 mg. Reaction atmosphere: 5 vol% CO in 540 ems/min air;---, dry air;---,15% H20. CuCr Z04 was due to an enrichment in copper concentration at the catalyst surface, confirming that copper is the active species. Supporting CuO or CUCr Z04 on alumina produced optima for higher activities located at different metal concentrations (Fig. 1). CuOsupported catalysts were found to be more active than CUCr Z04-supported catalysts when metal concentration was smaller than approximately 1Zwt%. At larger concentrations supported CuCr Z04 was more active. Figure 1 also shows that the oxidation of CO is inhibited by the presence of water in the gaseous reaction stream, that inhibition being significantly more pronounced for CuO than for CUCr Z04. Figure Z shows the behavior of the activity with time on stream of low (5wt%) and high (30wt%) metal concentration Cu and Cu-Cr catalysts. At low metal concentrations, it can be seen that copper catalyst is affected by a pronounced deactivation whereas the Cu-Cr catalyst is not. Accordingly, it can be suggested that active copper sites in CuCr Z04 are less prone to deactivation, i.e., more stable, than active copper on alumina. In the case of the high con-
:390
80 0~
c 0
...
.........
60
•
.'"
•
• *
....
40
(J')
Q)
>
c
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u
0
u Cu C'20.1 AI2O,
60
~
40
.... - ..
20 _...-__...----............... eM .....
o
2
3
4
5
6
Time (h)
Figure 2. Activity behavior at 200°C of supported catalysts. Metal concentration: o e ; 30wt%. 6 .. ; 5wt%. Catalyst pretreatment: open symbols, CO prereduction at 300°C for 3h. Filled symbols, no prereduction. Sample: 10 mg of supported metals. Reaction atmosphere: 15% CO in 150 cm 3/min air. centration Cu samples (i.e. 30wt%), the activity overcame an induction period (Fig. 2). This is probably due to excessive reduction to metallic copper by the CO pretreatment, as confirmed by XRD (ref. 6). Accordingly, copper reoxidation is not easy under the experimental conditions employed in this work. Figure 3 shows the temperature programmed reduction of supported samples, weighted to constant Cu content. It is seen that chromium significantly affects copper reducibility, i.e., Cu supported catalysts were more reduced as metal concentration increased, while in supported Cu-Cr reducibility decreased as metal concentration increased. Also, the first peak that appears in C u ~ C r samples (about 250°C) has an intensity which almost remains constant with varying metal concentration. Therefore, that peak could be attributed to a surface CUA1 Z04 phase, as the amount of this phase is probably a saturation value that mainly depends on the alumina surface area available (ref. 10). The second peak can be assigned to CUCr 204 phase which is more difficult to reduce as its concentration in-
391
30%
20
10 :::J
0
c
Q
0.. E
Cu 0 1 AlzO,
Cu Crz 0 41 AlzO,
:::J
(f)
C
0
U
0 U
5
100 200 300 400 100 200 300 400 Temperature (OC)
Figure 3. Temperature programmed reduction of supported CuO and CuCr Z04 of various concentrations. Reducing atmosphere: 25 vol% CO in He. Sample: enough catalyst to provide 25 mg of Cu. creases, contrary to what was observed above for the Cu catalysts. Accordingly, these results confirm that the catalyst is protected by Cr against excessive reduction. The second part of this work consisted in testing a copper-chromite catalytic converter in an automobile ran with leaded gasoline (about 0.6 g Pb/l ). Table 1 shows catalyst composition before and after using for 2,150 Km. The amount of lead deposited in the catalytic converter accounted for about 30% of total lead emitted during driving. XRD analyses demonstrated that lead on catalyst is mainly as lead sulphate (ref.11). Also, microscopy examination showed (ref.11) that lead deposits were almost entirely located at the catalyst pellet periphery penetrating only about 0.3 mm on average of the total pellet radius (2.3mm) Comparing activities of fresh and used catalysts (Table 2), it can be noticed that the catalyst was deactivated during the run,
392
TABLE Characteristics of catalysts before and after use in automobile exhaust for 2,150 Km.
Catalyst Fresh Used*
Composition (wt%) Cr Cu Pb Br 3.0 3.4
2.7 2.9
o 7.3
Surface Area m2 /g
o
210 170
0.5
* Used catalyst also contained certain amounts of: S, Fe and Cl. Used catalyst pellets weighted about 8% more than fresh.
TABLE 2 Relative activities for CO oxidation of powdered catalyst. Central and periphery zones were knife-separated before grinding for activity measurement. Catalyst Fresh (whole) Used (whole) Used (centre) Used (periphery)
Activity (%) 100 78
88 52
that deactivation being significantly more pronounced in the leadedperiphery zone than in the non-leaded-central zone. This indicates that lead is a major catalyst poison, as compared with other possible catalyst deactivating elements as for example: sulfur, whose concentration is about 0.1% in the gasoline employed. Energy Dispersion Analysis of X-rays performed during scanning microscopy examination of used catalysts, suggested that lead was preferentially deposited on the alumina support and to a significant lesser extent on the copper-chromium-rich zones of the catalyst (Fig. 4). This suggests that non-alumina-supported copperchromium (e.g. all-metal catalysts) might be a better lead-tolerant catalyst. Work has also been undertaken with lead filtration devices (ref. 12), as a part of the present development that looks for better performance of the catalyst.
393
6
• •
.0
•
c,
• • •
2
• 0
02
• 0.4
• 0.6
Cu tCr IAI
Figure 4. Relative point concentration of Pb and Cu+Cr by microscopy analysis of catalyst used in automobile exhaust ran with leaded gasoline. ACKNOWLEDGEMENTS The authors thank the Venezuelan Consejo Nacional de Investigaciones Cientfficas y Tecno16gicas (Grant Sl-1184), Ford Motor Co. of Venezuela, Octel Co. and Degussa AG for their valuable assistances. REFERENCES 1. G.J. Barnes, Adv. Chem. Series, 143 (1975) 72. 2. J.T. Kummer, Adv. Chem. Series, 143 (1975) 178. 3. T. Ohara, The Catalytic Chemistry of Nitrogen Oxides, R.L. Klimisch, J.G. Larson (Ed s . ) , Plenum Press, N.Y., 1975, p. 191. 4. R. Hierl, H. Knozinger and H.P. Urbach, J. Catal., 69 (1981) 475. 5. F. Severino and J. Laine, Ind. Eng. Chem. Prod. Res. & Dev., 22 (1983) 396. 6. F. Severino, J. Brito, 0 Carfas and J. Laine, J. Catal, in press. 7. M. Shelef, K. Otto and H. Gandhi, J. Catal., 12 (1968) 361. 8. J.F. Roth and R.C. Doerr, Ind. Eng. Chem., 53 (1961) 293. 9. G. Castro, F. Severino, J. Laine, in preparation. 10. R.M. Friedman, J.J. Freeman, F.W. Lytle, J. Catal., 55 (1978) 10. 11. J. Laine, F. Severino, A. Albornoz, O. Ca r i a s , B. Griffe, E. Marcano and F. Martf, Acta Cientif. Venezolana, in press. 12. J. Laine, F. Severino and A. Albornoz, Acta Cientif. Venezolana, in press.
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A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
395
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
DEVELOPMENT OF NON-NOBLE METAL CATALYSTS FOR PURIFICATION OF AUTOMOTIVE EXHAUST GAS
THE
by Lin Peiyan, Wang Min, Shan Shaochun, Huang Minmin, Rong Jingfang, Yu Shomin, Yang Hengxiang and Wang Qiwu Modem Chemistry Dept., Science and Technology of China Univ., Hefei (The People's Republic of China)
ABSTRACT Non-noble metal catalysts A and B were developed in view of the rich resources of transition metal oxide and rare earth oxides as well as low cost in China. The catalysts were characterized by high effectiveness for the conversion of CO and HC, high crush strength and high stability to prevent poisoning by S02 and Pb. The catalytic converter can also be used in place of exhaust muffler while the consumption of fuel does not increase.
INTRODUCTION Since the pollutants from automobiles have greatly increased with the rapid increase of automobiles in recent years in China, the Ministry of Environmental Protection of China decreed the first regulation controlling pollutants from automobiles in 1983. Successes have been scored in some large cities in the implementation of the regulation in their battle against pollution. The catalysts currently used for this purpose are mainly those containing noble metals of Pt, Pd, Rh etc. But it would be valuable to develop non-noble metal oxide catalysts in view of the rich resources and low cost in China. The perovskite-type catalysts (ref.1), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref A and 5), the adsorption and thermal desorption of CO, C2H6 and 02 (ref6 and 7), the reactivity of lattice oxygen (ref.S), the electric conductance of catalysts (ref.9), the pattern of poisoning by S02 (ref.10 and 11), the improvement of crushing strength of support (ref.12) and determination of the activated surface of complex metal oxides (ref13) have also been reported. Based on the above-mentioned work, two kinds of mixed metal oxide catalysts A and B mainly containing copper oxide as a major compound on y-A1203 have been developed. They were made by a special impregnation procedure, dried, calcined and activated. The catalysts were made into a spherical shape (05-7) for
396
practical use. The physical characters are shown in Table 1.
--
i - ~ _ · · _ - - - - - - - - - - - - ~ ~ b l e ~ - - - - - _ · _ - - - - -
Physical Characters of Catalysts
I
Crushing strength (kg/gran) BET surface area (m2/g) Bulk density (glml) Pore volume* (ml/g)
*
Support
Catalyst A
Catalyst B
19.0
20.0
25.0
152.0
90.7
82.0
0.73
0.83
0.90
0.45
0.39
0.35
_J
Benzene replacement method.
EVALUATION OF CATALYTIC ACTIVITY
Micro-reactor tests Catalytic activity experiments were carried out by a micro-reactor. Reactants and products were analyzed by chromatography. Some of the results are given in Table 2 and Table 3.
Table 2 Percent Conversion of CO on Catalyst A (0.2 g of sample, 40-60 mesh, SV ::: 4500h-1,
reactants: CO 0.5%,023-5% and balance with
No activation treatment
lI
NV
Reaction Temperature (0C)
250
307
380
Percent CO oxidized (%)
37 Activation treatment
83
96
Reaction Temperature (0C)
210
250
300
Percent CO oxidized (%)
56
88
95
J
397 ---------------_._--------_._-----
I
Table 3 Effect of La203 or Ce02 in support of Catalyst B
I
I I I
(Reaction conditions are the same as those in Table 2) 20 248.5
233.0 168.0 Table 3 indicates that the improve catalytic activity.
Percent CO oxidized (%) 50 Reaction temperature (OC)
90
292.0 278.0 242.0
340.5 318.0 300.5
iI I
I
I
y-A1203 containing La203 or CeOZ could
75 ml reactor tests Catalytic activity experiments were carried out by a 75 ml flow reactor system, the results are shown in Table 4.
Table 4 Conversion of CO for Granular Catalyst (75 ml of granular catalysts, 0 5-7, SV=4500h-1, reactants: CO 1.1%, O2 6-8% and balance with N2 ).
233.5
Reaction Temperature (0C) 288.5 343.0 384.0
45.1
Percent CO oxidized (%) 67.8 85.9 95.1
228.0
Reaction Temperature (0C) 280.0 323.0 372.0
49.1
Percent CO oxidized (%) 64.0 75.3 85.4
Catalyst A
Catalyst B
Table 5 indicates that the conversion of CO remains as high as 69.3% at 436°C after Z hours on catalyst B containing Ce02 when the reactant is CO only (1.1%) diluted in N2. The results suggest the mobility of lattice oxygen was rather high. The lattice oxygen took part in oxidation even though there was no oxygen in the gaseous
398
phase during this period. If 6-8% 02 was again added into the mixed reactants, the conversion of CO could be restored to 92% after adding 02 for 0.5 hr at 400°C. This property is important for practical purposes.
Table 5 Conversion of CO in the absence of O2 on Catalyst B (reaction conditions are the same as those in Table 4) 0.0 Reaction Time (min) 72.3 Percent CO oxidized (%) Reaction Temperature (°C)416.0
5.0 15.0 84.4 80.7 406.0 412.0
25.0 78.6 426.0
45.0 75.1 431.0
70.0 72.6 434.0
115.0 125.0 69.9 69.3 436.0 436.0
Purification ofexhaust from gasoline engine Purification of exhaust from HFG 427A gasoline engine without additional secondary air was tested with 6.25 kg of catalyst A. The catalyst was packed in a stainless steel converter which was connected to the exhaust outlet. The contents of CO and HC were measured by an RI503 TH-S CO/HC infrared gas analyzer (Japan). Simulated tests of different rotational speeds of the engine were carried out as indicated in Table 6.
Table 6 Purification of CO and HC in exhaust Rotational speed of engine (twins/min)
600 820 1230 1640 2050 2460 2850
He
CO Before reaction
Percent CO removed
(%)
(%)
5.8 1.9 0.6 0.23 0.33 0.41 0.44
95.5 83.7 88.4 78.3 85.1 88.4 83.0
Before reaction (ppm)
3800 500 510 740 320 350 530
Percent He removed (%)
93.7 84.0 80.4 89.2 97.0 94.3 98.1
In addition. the bifunction of the catalytic converter was found. The catalytic converter could better decrease noise pollution compared with the exhaust muffler. Therefore. it would be used to replace the exhaust muffler.
399
STRENGTH AND THERMAL STABILITY The support was formed by extrusion, cold roll forming, drying and calcination. The formulation (containing binder, material of pore making, material for improving strength and structure stabilizator etc.) and the preparation procedure of support were studied by conducting about one hundred experiments. The X-ray diffraction data of both supports, adding rare earth oxide and pure y-A1203 after calcination are given in Table 7. ~ ...
__ .
------
---
. ~ - - _
.... - - -...
----------c
Table 7 X-ray diffraction data of support and pure y-A1 20 3 Support (calcined at 900°C d
IJI
4.60 20 2.39 80 2.28 70 1.98 100 1.51 30 1.40 100
Support (calcined at 1 050°C d
IJI
4.42 20 2.82 80 2.72 90 2.42 60 2.33 40 2.27 60 2.00 70 1.56 50 1.41 60 1.39 100 3.79 50 2.675 100 2.196 30 1.897 60 1.698 20 1.598 40 1.360 30
y-Alz0 3 (calcined at 900°C d
IJI
4.51 30 2.83 70 2.72 80 2.44 70 2.29 30 2.20 30 2.00 60 1.53 20 1.40 80 1.39 100
d : Spacings ofthe plane nets
y-Alz0 3
d
IJI
4.56 40 2.39 80 2.28 50 1.977 100 1.520 30 1.390 100
e-Al z0 3 LaAl z0 3 (from JCPOS card) d 4.50 2.85 2.72 2.43 2.31 2.22 2.01 1.54 1.40 1.39
IJI
d
1II
60 80 80 80 60 60 80 60 60 100
i
3.797 80 2.657 2.188 1.896 80 1.696 60 1.542 80 1.342 50
III: Relative intensity ofdiffraction.
I
lJ
It was found that the temperature of the crystalline phase change from y-Al203 to e-A1203 increased by about lOO°C-I50°C when adding rare earth oxides as stabilizator. Even calcined to 1050°C, the a-A1203 phase did not appear. The effects of rare earth oxides could be concluded from Table 3, 5 and 7 : (a) to improve catalytic activity; (b) to improve mobility of lattice oxygen in catalyst B and its activity for CO and HC conversion in a lower ratio of air/fuel; (c) to improve thermal stability of support to resist crystalline phase change to a phase and maintain crushing strength and surface area.
400
HIGH STABILITY TO RESIST S02 POISONING In general, the gasoline in China contains HZS, SOz and other organic sulphides. There is a minimum of 30-40 ppm S02 and other sulphides in the exhaust. It is well known that the sulphides possess strong toxicity for basic metal oxide catalysts (ref. 14). Therefore, a study on catalytic resistance to S02 poisoning is important. The relation between catalytic activity and reaction time when the reactant contains S02 (50 ppm) was determined. It was found that the conversion of CO remained 81.6% for catalyst A after 112 hours and 76.1 % for catalyst B after 64 hours in micro-reactor conditions. However S02 poison was reversible for catalyst A. When S02 concentration decreased and temperature increased, the activity of the poisoned catalyst could be restored. The data are shown in Table 8 and Table 9.
------------1
-----------------------
I
Table 8 Relation between activity of poisoned catalyst A and change of reaction temperature (In micro-reactor, 80250 ppm) Time (min)
o
30 60
90 -120
540 660
840 1800
Reaction temperature (oq
400 450 450 450 450 450 400
Reaction of CO
I
(%)
82.6 99.8 99.8
100.0 100.0
100.0
400
96.4 89.4
400
83.2
I
401
Table 9 Relation between activity of catalyst A and content of S02 in reactants (Reaction temperature: 400°C in micro-reactor) Initial percentage
I. _<0"""'00
Percentage conversion of CO 50 ppm 25 ppm 10 ppm (S02)
of CO
80
100
(S02)
(S02)
85
88
APPLIED FJELD TEST OF CATALYST A A field test of 70000 km was made on an EQ-140 type bus with a 6cylinder gasoline engine (using N° 75 leaded gasoline which contained Pb 0.81.0 g/l.) The conversions of CO and HC were measured continually during running or idle states. The catalytic converter packing (8 kg of catalyst A) was placed in place of the exhaust muffler. The data obtained are listed in Table 10.
Table 10 The change of activity of catalyst A in the 70 000 km field test Accumulative total km
o
1047 5112 7804 15081 17290 25175 37033 51050 70604
Working state idle idle idle idle
mnning* idle idle idle idle
mnning*
Average percentage CO 90.7 92.7 80.1 90.1 81.8 81.9 82.0 84.8 88.5 75.6
~ Remove of
HC
84.6 88.6 88.8 77.0 81.8 81.2 85.6 77.5 76.0 66.8
* Averagevelocityofvarious vehiclespeedsat 30 km, 40 km, 50 km and 60 kmper hour. The average consumption of fuel in 70 000 km process is indicated in Table 11. It was proved that the consumption of fuel did not increase while the catalytic converter was used in place of the exhaust muffler. The compositions of fresh catalyst A and of catalyst A used after 70 000 km were analysed by means of an X-ray fluorescence spectrometer. It was identified that the relative content of main components did not change after 70 000 km. Catalyst A after the 70000 km test was found to contain 0.4% by weight of Pb. It should have been introduced by leaded gasoline.
I
402
The evaluation of catalytic activity for fresh catalyst A and catalyst A used for the 70000 km test were also measured in Table 12.
Table 11 Average consumption of fuel in field test (EQ-140 type bus, 660 type gasoline engine)
Exhaust muffler
Bus Number
Consumption of fuel for 100 km (liters)
1
7 8 9 10
20.31 22.65 23.59 24.62 22.08 20.13 23.63 23.04 21.33 20.87
5.04 5.55 5.80 6.28 5.68 5.04 5.78 5.94 5.15 5.77
11
20.90
5.44
2 3 4 5 6
Catalytic Converter
Consumption of fuel for 1,000 persons per km (liters)
Table 12 The activity of fresh catalyst A and used catalyst A after 70 000 km (75 ml reactor, reaction conditions are the same as those in Table 4) Sample
Reaction temperature
Percent CO oxidized
(OC)
(%)
Fresh catalyst A
246.0 330.0 426.0
61.2 89.5 96.3
Catalyst A used
271.5 331.5 421.5
36.6 63.6 85.0
Table 12 indicates that the conversion of CO could remain 85% at 421,SOC when catalyst A contained 0.4% Ph on the surface after the 70000 km field test. But the conversion of CO decreased considerably at low temperatures.
403
REFERENCES 1. Wang Qiwu, Rong Jingfang, Lin Peiyan and Shan Shaochun, KEXUE TONGBAO (Science), 25 (1980), 495-497. 2. Yang Hengxiang, Huang Minmin, Wang Qiwu, Lin Peiyan and Rong Jingfang, HUAN JING HUA XUE (Environmental Chemistry), 2 (1983), 17-22. 3. Wang Min, Yu Shomin, Huang Minmin, Shan Shaochun, Rong Jingfang, Yang Hengxiang and Lin Peiyan, CUlHUA XUEBAO (1. Catal), 5 (1984), 300-301. 4. Wang Qiwu, Shan Shaochun and Xiao Yujie, ZIRANZA ZHI (Nature), 2 (1979), 659. 5. RongJinfang, SHIYOUHUAGONG (Petrochemical Technology), 10 (1981), 310313. 6. Lin Peiyan and Fu Yilu, SHIYOU HUAGONG (petrochemical Technology), 8 (1980),822-828. 7. Fu Yilu and Lin Peiyan, CUlHUA XUEBAO (J. Catal), 3 (1982),205-211. 8. Lin Peiyan and Yu Min, HUAXUE TONGBAO (Chemistry), 3 (1985),13-14. 9. Lin Peiyan, Yu Min, Shi Wenjun and Wei Zhenyu, J. China Univ. of Sci. & Tech., 15 (1985) 426-433. 10. Huang Minmin, Yang Hengxiang, Wang Qiwu, Lin Peiyan and Rong Jinfang, CUlHUA XUEBAO (1. Catal), 3 (1982) 277-282. 11. Wang Minmin, Yang Hengxiang, Wang Qiwu, Lin Peiyan and Rong Jinfang, CUlHUA XUEBAO (1. Catal), 4 (1983), 312-314. 12. Shan Shaochun and Huang Minmin, Xl'Tl.I (Rare earth), 1 (1984),64-67. 13. Lin Peiyan and Fu Yilu, J. China Univ. of Sci. & Tech., 13 (1983),68-73. 14. Y.f. Yu Yao, 1. Catal, 28 (1973),124; 28 (1973), 139; 33 (1974),108; 36 (1975), 266; 39 (1975), 104.
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A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control © 1987 Elsevier Science Publishers B.V., Amsterdam _. Printed in The Netherlands
IMPROVING THE S02 RESIST ANCE OF PEROVSKITE TYPE OXIDATION CATALYST by LI W AN* and HUANG QINGl, ZHANG W AN-JING2, LIN BING-XIUNG2, and LU GUANG-LIE2. 1Dept. Chemistry & Environmental Eng., Beijing Polytechnic University, Beijing. zDept. Chemistry, Peking University, Beijing, P.R. China.
ABSTRACT A properly designed perovskite-type base metal catalyst Lao.6SrOACoi_xMx03 was found to have certain SOz resistant properties. Over this catalyst at 56000/hr CO could be oxidized at 100% in the presence of 20, 200, and 400 ppm of SOZ at 200 0 , 300°, and 400°C, respectively. Its deactivation by 50 and 400 ppm of SOZ at 200° and 300°C respectively was reversible. As revealed by IR spectroscopy, SOZ adsorbed on B site ions deactivate the catalyst by blocking the surface sites that are necessary for CO adsorption and lattice oxygen replenishment. A comparison was made with another Lao.6SrOACo03, which was found to be more susceptible to SOz poisoning.
INTRODUCTION To make a perovskite-type base metal oxide capable of being an alternative to noble metals catalyst for automotive pollution control, its SOZ resistance must be improved. Some papers reported that perovskite type oxides such as LaCo03' LaMn03, Lao.sSrO.SMn03' and Lao.7Pbo.3Mn03 deactivated irreversibly by several tens ppm of SOZ at 500°C or lower, and that they become sulfur resistant only when the sample contains several tens ppm of Pt (ref. 1-5). One of the authors of this paper has prepared a Pd-containing perovskite which is highly resistant to SOz (ref.6). In this paper studies on a perovskite-type base metal catalyst without any noble metal that can stand a certain level of SOZ are presented. Infrared spectroscopy has been used to identify the nature of the adsorbed species. A discussion on the adverse effect of SOZ has been included. EXPERIMENTAL
The Catalyst Catalyst A, Lao.6SrOACol-xMx03' and catalyst B, Lao.6SrOACo03, were
* To whomcorrespondence shouldbe addressed.
405
406
prepared by pyrolysis of their amorphous citrate precursers at 850°C (ref.7). Their structures were both single phase perovskite type as determined by Rigaku D/Max r.a. X-ray diffractometer. The surface area was determined by gas chromatographic method and calculated by BET equation. Reduction by CO About 20 mg of the catalyst were placed in a Rigaku TG-DTA thennobalance. The temperature was raised at a rate of 10°C/min., while highly pure N2 was introduced through the sample at a rate of 60 ml/min. When the desired temperature was reached, pulses each containing 3.6 ml CO were introduced and TG and DTA curves were recorded. No air was admitted after each pulse. The continuous weight loss observed was far greater than the amount of oxygen which could be present on the surface. Test ofS02 effect The susceptibility of the catalyst to poisoning by S02 was tested using 0.5 ml of catalyst placed in a microreactor connected with a nondispersive infrared CO analyzer and a S02 gas analyzer. Firstly, a stream of air containing various amounts of S02 (from 20 to 400 ppm) was allowed to pass through the catalyst for one hour, then the reactant was introduced and the activity expressed as % conversion of CO was recorded. Another series of experiments were carried out with the reactant blended with S02' The composition of the gas stream was analyzed at the inlet and outlet of the reactor. IR spectroscopy
A Nicolet NIC-7199 F.T. infrared spectrometer was used to determine the nature of the adsorbed species. For CO adsorption, the sample disc was mounted in an lR cell connected to a vacuum rack. It was evacuated at 200°C and 1O-4mm Hg for 4.5 hours before 100 Torr of CO was introduced at 120°e. It was allowed to stand for 20 minutes and without outgassing further, the IR spectra were recorded. For S02 adsorption, disc made by pressing the sample powder with KEr, was mounted in the spectrometer directly.
RESULTS AND DISCUSSION Activityofthe catalyst The activities of the catalysts were tested at 56000/hr, with air stream containing 2 vol% of CO, and at 38000/hr with 1800 ppm of CH4 in air. The results are shown in Table 1.
407
- -!
Table 1 : Activity of the catalyst Catalyst
Surface area (m 2jg)
Bulk density (g/ml)
13.2 2.9
A B
1.7 1.8
% conversion of CO
180°C
99 27
% conversion of
200°C
100.0 97.0
CH4 at 460°C
I I
i
76.6
S02 tolerance The effect of S02 on catalyst activity has been much less pronounced for eH4 oxidation than for eo, therefore, only results for eo are presented here. Table 2 gives data of S02 tolerance of catalyst A for eo oxidation with a reactant of 2 vol% of eo in air, at 56000/h. In this Table, "poisoned" means the activity dropped to less than 5% of the original value, "reversible" means the activity did not recover after shutting off SOz for 30 minutes at otherwise the same condition, resumed some of its activity at 250 o e, recovered completely at 280 o e, and remained so when the temperature was lowered to 200 0 e again. At 200 0 e S02 poisoning was very quick and the fatal dose was 0.02 ml/m2 or 0.14 monolayer on the basis of 30 A2 for the area occupied by one S02 molecule from Yao. (ref. 3)), but it took 40 minutes at 300 o e, and 5.2 monolayers.
Table 2 : Effect of S02 on catalyst A T eC)
S02 (ppm)
Exposure to S02 1 h
300 400
Note
S~~ed
Poisoned (yes/no)
Poisoning time
SO~ p~sed (m m)
0.05 0.12
no yes
not in 1 h 10 min.
0.05 0.02
reversible
20 - 200 400
0.05 - 0.49 0.99
no yes
not in 1 h 40 min.
0.05 - 0.49 0.66
reversible
20 - 400 170 400 80
0.05 - 0.99
no
not in 1 h 5h 2h not in 72 h
0.05 - 0.99 2.09 1.97 0.90
reversible reversible s.v.36001h
( 200
S02 admixed in reactant
20 50
m)
One point is worth notice; i.e. before the catalyst gets poisoned, S02 was uptaken completely from the gas phase, and there was no SOz detectable in the outlet gas stream. For example, at 300 0 e the catalyst was not poisoned in the presence of 200
408
ppm of S02 within one hour, so no S02 was detectable in the product, but altogether 0.49 ml/m 2 of S02 were passed over which amounted to four monolayers. We do not know what kind of transformation has been undergone by the retained S02; we failed in looking for a second phase by X-ray diffractometer when a sample poisoned but recovered and again poisoned by 400 ppm of S02 at 400°C for two hours was examined. The result was the same when it was exposed to pure S02 gas at 700°C. Perhaps most of the S02 has reacted by another route over this catalyst. It was also found that when the catalyst was recovered once from poisoning and was used again, it will get poisoned more readily than the fresh one. The effect of S02 on catalyst B was shown in Table 3. Catalyst B was much less resistant to S02 and its poisoning was not reversible. Two types of irreversibility were observed and mentioned in Table 3 as IRR I and IRR 2. IRR I means that the catalyst could recover its activity at high temperature, e.g. at 400°C (300°C is not high enough), but became inactive when the temperature was lowered to 200°C again. IRR 2 means the activity was recovered when S02 was shutted off at otherwise the same condition, but was lost again when returning to 200° or 300°C.
Mobility of lattice oxygen Reducibility of the catalyst by CO or the mobility of lattice oxygen of catalyst A and B are quite different. The weight loss of the catalysts at different temperatures given in Table 4 could be used as a measure of these properties.
,-
409
Table 4 : Mobility of lattice oxygen
I I
;
Catalyst
fresh A poisoned
I
fresh
Weight loss temperature (O°C) 115
140
+
++
208
216
266
340 360
400
+
++
+++ +
+
+++ ++
B poisoned In this Table one plus sign "+" means "noticeable", three plus "+++" means "very significant", and minus sign "-" means there is no reduction at all. The lattice oxygen of catalyst A was active and mobile at temperatures below 150°C, and became very significant at 200°C and higher. After it was poisoned at 200°C by SOz the weight loss began at 208°C and became significant at 266°C. These results are consistant with the regeneration test described in the previous text. The activity of samples poisoned at 200°C could be 100% recovered at 280°C. These facts suggested that the activity of the catalyst is closely related with the mobility of its lattice oxygen. Catalyst B was not so easily reduced. Its weight loss began at 340°C and was significant only at 360-400°C. Over this catalyst the redox mechanism of CO oxidation is only possible at 400°C or higher, below 400°C reaction probably proceeds by adsorption mechanism. Over catalyst A redox mechanism is possible at 180°C and lower. Nature 0/ adsorbed CO and S02 Adsorption o/CO Because of the dark color and the low surface area of the sample, the IR spectra, especially that of catalyst B, are not very satisfactory, but it gave some information as follows. Catalyst A: After the pretreatment described in the experimental part, water and CO2, free or weakly adsorbed, should be expelled from the IR cell. Since the lattice oxygen of this catalyst is mobile at 115°C, there might be some C02 produced when the sample was exposed to 100 Torr of CO at 120°C for 20 minutes. The spectra were taken without outgassing after CO was introduced, some CO remained. As shown in Figure 1, besides CO and C02, there are also bands of the following species: Unidendate carbonate: 1463, 1451, 1289, and 1073 crrr! Bidentate: 1627, 1289, and 1043 crrr! Bridging: 1764, 1738, and 1198 cm-I Carbonyl: 1938, 1985,2005, and 1859 crrr l. The bands assigned as carbonyl are similar to that of Caz(CO)g. The assignment of bands are made by reference to ref. 8-9.
410
I-bae. line 2-po1eoa.d at 200· C
a:J
3-calcied at 400" "; in air
2'i-OO
2000
~
1600
1200
800
~AVENUMBERS
Figure 1: Adsorption of CO, I-Catalyst A 2-Catalyst B
Fig. 2 : Adsorption of S02 on catalyst A
The spectrum for catalyst B was not good (line 2), but bands can be recognized by enlarging the spectrum and comparing with the data given by the computer attached. There existed various types of adsorbed carbonate species such as unidentate at 1483-1411 and 1384 crrr l, bidentate at 1690-1551 crrr l, and bridging at 1900-1700 and 1163 cnr l, but no carbonyl band was clear enough.
Adsorptiono!SOZ IR spectra for catalyst A after poisoning by 50 ppm of SOz admixed in the reactant at 200°C was shown in Figure 2. Bands at 1133 and 994 crrr! could be assigned to adsorbed SOZ. They are very close to a coordinated SOz species with a bridging structure I, at 1135 and 993 crrr! (ref. 8) :
o
II
M-S-M
(I)
II
o
There were also some carbonate species such as unidentate at 1512, 1202, 1098 and 858 em-I, uncoordinated at 1417, 1455, and 875 crrr l , and bridging type at 17101760 em-I. Bands at 2857 and 2926 em-I could be assigned to a formate species. After calcining this poisoned sample at 400°C for 2 hours in the air, the bands for SOz did not vanish (line 3), but more bands due to carbonate species appeared, indicating that the amount of SOZ adsorbed was decreased. Some surface sites were set free which in turn benefited the formation of carbonates that would be limited otherwise by lattice oxygen deficiency.
411
IR spectrum for catalyst B after poisoning by S02 at 200°C was shown in Figure 3, line 2. Bands at 1294 and 1128 cm! could be ascribed to adsorbed S02, similar to a coordinated species II
s:r
.. ··0
M -
(II)
·~o
with bands at 1301-1278 and 1100 em-I. There were also unidentate, bidentate, and uncoordinated carbonate. Bands due to carbonate as a whole were noticeably less than in the same sample not being exposed to S02' After calcination at 400°C for 2 hours in the air, bands of adsorbed S02 shifted to 1146,1124, and 98cm- I, which are closer to species (I) as on catalyst A. There could be a decrease of the concentration of the adsorbed S02, but the vacated sites did not seem to be occupied by C02 at lower temperature. La203 : Samples (commercial, 99.9% pure) were exposed to undiluted pure S02 gas at room temperature for IS hours, and then mixed and pressed with KBr to form discs. The discs were mounted directly to the spectrometer and the IR spectra were taken. No band here could be assigned to S02 adsorption, but strong and sharp bands due to carbonate species were clearly shown as in Figure 4 line 2. MOx : IR spectra of Max pretreated in the same way as for La203 were made. There are only very weak bands due to S02 adsorption, such as 1199, 1137, 1125, 1099, and 1089 em-I, which are similar to those for S02 coordinated to metal ions in the shape as in II at 1198-1185 and 1048 crrr l. There are also bands due to carbonate species.
2
<Xl N
:3
1J.J U
Z 4:
~
c
Ul ~
l-base line 2-po1soned at 200" C :3-calcined at 400· C
~~oo
2000
1600
1200
~oo
W~"e:NUMBERS
Figure 3: Adsorption of S02 on catalyst B
412
N ~
~
!~ l-base
line
2-after exposed
to
: : : (i ~
t.J u
Z
a: o If)
Q)
I I
I
i
~-
2'±OO
2000
1600
1200
WAVENUMBERS
aoo
Figure 4: Adsorption of S02 on La203
C0304 : IR spectra for commercial C0304 (AR reagent), treated in the same way with pure S02 gas were shown in Figure 5. Besides adsorbed carbonates, there are bands due to sulfate species which are more obvious for sample calcined in the air at 240°C for 2 hours, e.g. 1143, 1098, 1018, and 871 em-Ion line 2 and 1136, 1109,1017,983, and 874 em-Ion line 3. Each number 2 line was taken from Figure 2, 3 and 6 respectively, and placed together in Figure 6. It is interesting to notice the similarities of the bands due to S02 adsorption on these three compounds, especially when .they were compared with that on La203 (Fig. 4). It is reasonable to regard Co ions as the seats for S02, and La3+ as the metal coordinated with most of the carbonate species. Sr2+ should behave like La 3+ because of its basicity, but we do not have the experimental data. From the data given above, it is reasonable to attribute the poisoning effect of S02 on these catalysts to the scramble of surface sites on B site ions. There are two types of adsorbed CO species on the surface of the catalyst. One is the carbonyl species which is responsible for lower temperature oxidation of CO, as suggested for CuC0204 by Hertl et al (ref.lO). It is adsorbed on can+. Another is the carbonates formed by adsorption of CO on lattice oxygen, as suggested for LaCo03 by Tascon et al (ref.l1). It will decompose at 200°C and higher to give C02. For each CO2 desorbed one anionic vacancy would be left, which should be filled up by 02 adsorption, and this would happen again on the B site ions. S02 is in competition with both carbonyl and 02 for the same surface sites. The fast and strong adsorption of S02 is therefore detrimental for these two reaction intermediates to form, so it deactivates
413
l-bS.bl8 line 2- exposed to S O ~ , r . t .
3-heated in air,240'C
~
2 ~
CO
~~\
6
~
V)
fl)
---,
elf00
2000
1600
1200
800
WAvENUMBEF\S
Figure 5 : Adsorption of 802 on C0 304
l-CO j 0+ exposed to SOl t r , t. 2-Cat.A poisoned at 20cfc Q 3-Cat.E poisoned at 200 C
w u ~
ID €X:
o(J)
2
co
zsuo
2000 1600 1200 wnVENUr1B£F\S
800
Fig.6: Adsorption of 802 on COJ04 (1) Catalyst A (2), Catalyst B (3)
414
the catalyst quickly. It was noticed that there were changes in number and strength of bands due to carbonate species before and after the catalyst was poisoned. This arose from the changes in B site ions available for 0z adsorption thus promoting or dampening the carbonate formation. At 400°C, although there are still some SOZ adsorbed, the reaction goes freely if there are enough sites left for lattice oxygen replenishment. As long as the reactants containing SOZ are flowing through, the amount of SOZ adsorbed will accumulate gradually with time. Whenever an end limit is reached, the catalyst has to unload its SOz burden at 400°C with fresh reactant or at higher temperature. That is the case for catalyst A, 400 ppm of SOZdid not deactivate it till the end of two hours. Another feature of catalyst A is when it was poisoned at 200°C, 30 minutes was too short, or 200°C was too low to remove the adsorbed SOz. But when some of its active sites were set free from the adsorbed SOZ at 280°C, it could react according to the redox mechanism by its mobile lattice oxygen when the temperature was lowered to 200°C again. By reduction with CO in NZ the reactivation of catalyst A poisoned at 200°C started at 208°C (TG-DTA data), which is 40°C lower than that needed in the air stream containing CO (at 250°C). This fact indicates that the surface could also be set free of SOz by reducing it with CO in NZ' By passing a stream of 2% CO in NZ through the poisoned catalyst, a 24% conversion of CO was observed and it remained so for 1/2 hour. Deactivation of catalyst B is not reversible, although the concentration of adsorbed SOZ could decrease upon heating at 400°C, It becomes active at this temperature. That is a consequence of thermomobilization of its lattice oxygen. The reaction could run by decomposing the surface carbonate species, a redox reaction, although there is still some SOz on the surface. Lowering the temperature to 200°C, its lattice oxygen are not mobile anymore. Oxidation run by adsorption mechanism could not proceed without enough surface sites for the reactant, so the reaction stopped.
CONCLUSION SOz gas deactivates the cobaltate catalyst for CO oxidation by blocking the surface sites on the B site ions and inhibiting the formation of reaction intermediates. The concentration of the adsorbed SOz species could be partially diminished by continuing the reaction with fresh reactant, or by heating to higher temperatures, but could not be removed completely at temperatures ~ 400°C, At 400°C lattice oxygen of both catalyst A and B are mobile enough to carry out the reaction by decomposing the surface carbonate species. Coming back to 200°C from 400°C, only the lattice oxygen of catalyst A are active, that of catalyst B are not mobile anymore, therefore, catalyst B could not recover its activity at 200°C, The mobility of lattice oxygen and the ability of forming weaker bonds with sulfur seems to be essential for improving
416
the S02 resistance of a perovskite type oxidation catalyst. As shown above the adsorptive poisoning of these catalysts by S02 was highly temperature dependent. Since the warm up period is usually very short, a catalyst which uptakes S02 at that period, but rapidly recovers its high activity when the automobile runs normally would be a promising candidate for use in autoexhaust purification. REFERENCES 1. R.J.H. Voorhoeve, L.E. Trimble, AND c.r. Khattak, Res. Bull., 9 (1974)655 2. P.K. Gallagher, D.W. Johnson, Jr., E.M. Vogel, and F. Schrey, Mater. Res. Bull., 10 (1975) 623-628. 3. Y.F.Y. Yao, 1. Catal., 36 (1975) 266-275. 4. Y.F.Y. Yao, J. Catal., 39 (1975) 104-114. 5. S. Katz, 1.1. Croat, and J.V. Laukonis, I.E.C. Prod. Res. Dev., 14 (1975) 274. 6. Xi Zhen-Sheng and Li Wan, submitted for publication. 7. C. Marcilly, P. Couty, and B. Delmon, 1. Amer. Cer. Soc., 53 (1970) 56. 8. K. Nakamoto, Infrared and Raman Spectra Of Inorganic and Coordination Compounds, 3rd Ed. (1978), N.Y. Wiley-Interscience. 9. G. Busca and V. Lorenzelli, Mater. Chern. 7 (1982) 89-126. 10. W. HertI and R.J. Farranto, 1. Catal., 29 (1973) 352-360. 11. 1.M.D. Tascon and L.G. Tejuca, Zeit. Phys. Chern. N.F., 121 (1980) 63-78.
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A. Crucq and A. Fronnet (Editors), Catalysis and AutomotivePollution Control
417
© 1987 Elsevier Science Publishers B.V .. Amsterdam - Printed in The Netherlands
TUNGSTEN CARBIDE CATALYSTS
AND
TUNGSTEN-MOLYBDENUM
CARBIDES AS AUTOMOBILE
EXHAUST
L. LECLERCQ1, M. PRIGENT 2, F. DAUBREGE 1, L. GENGEMBRE 1 and G. LECLERCQ1 lLaboratoire de Catalyse Heterogene et Homogene, U.A. C.N.R.S. n° 402, Universite des Sciences et Techniques de Lille Flandres-Art-u s , 59655 Villeneuve d'Ascq Cedex (France). 2lnstitut F r a n ~ a i s du Petrole, B.P. 311, 92506 Rueil-Malmaison Cedex (France).
ABSTRACT Several catalyst samples of tungsten carbide and W,Mo mixed carbides with different Mo/W atom ratios, have been prepared to test their ability to remove carbon monoxide, nitric oxide and propane from a synthetic exhaust gas simulating automobile emissions. Surface characterization of the catalysts has been performed by X-ray photoelectron spectroscopy (XPS) and selective chemisorption of hydrogen and carbon monoxide. Tungsten carbide exhibits good activity for CO and NO conversion, compared to a standard three-way catalyst based on Pt and Rh. However, this Wcarbide is ineffective in the oxidation of propane. The Mo,W mixed carbides are markedly different having only a very low activity. INTRODUCTION Transition metal carbides (mainly of Wand Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been stUdied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. EXPERIMENTAL Catalysts The tungsten and molybdenum carbides supported on alumina were prepared by UGICARB MORGON (Grenoble, France). The alumina support (grain size < 80 11m, BET surface area: 100 m2.g- 1) is impregnated with ammonium heptamolybdate and
418
paratungstate solutions followed by evaporation of water and further drying at 127°e during 10 hours. Then the sample is calcined for 16 hours under flowing for the catalyst conts iru no n t t roqen to decompose ammonium ions : at 550 0e only Wand at 280 for mixed W. Mo catalysts owing to the possible sublimation of Mo0 The reduction of the supported oxides is then carried out under 3. flowing hydrogen for 6 hours up to 600 (heating rate 1°e/mn) and the temperature is increased from 6000 e to 9000e for 6 hours. At this last temperature. the carburization occurs under flowing carbon monoxide during 40 hrs. The catalyst is then cooled down under eo and passivated at room temperature with a gas mixture of 1% oxygen in argon. A commercial catalyst based on platinum and rhodium from PROeATALYSE was used as a reference catalyst (about 0.3 wt % of Pt and Rh. Pt/Rh = 5). 0e
0e
XPS Experiments XPS spectra were obta i ned us i ng an AE I ES 200 B spectrometer WI th an Al cathode (hv = 1487 eV. 300 W). The pressure inside the spectrometer chamber -9 was kept lower than 1x10 torr. Severa I references were taken for the calculations of binding energies (BE) : AU 4f7/ 2 = 84 eV. e 1s = 285 eV. AI 2p 74.8 eV). Intensity ratios IMo/lW were corrected by taking into account the dIfference of kinetic energies according to EBEL (ref .2). Adsorption Measurements The BET surface areas and the hydrogen and carbon monoxide adsorption isotherms were determined by volumetric adsorpti on performed with a Texas Instrument quartz spiral BOURDON gauge in a system already described elsewhere (ref .3) . Laboratory Bench T e ~ t A laboratory dynamic flow reactor system has been specially designed for powder samples at the Institut F r a n ~ a i s du Petrole which allows to observe the catalyst performances as a function of air to fuel ratio (A/F). For activity testing, the A/F was set a; different values which involved the flow rates control of NO, CO and propane at a selected space velocity and operating temperature. The reactor inlet and outlet 02 concentrations were also measured with an oxygen sensor. The light-off performance of the catalyst was evaluated by varying the temperature for a stoichiometric gas mixture. Three analyzers were used to determine CO, NO and propane conversions : a flame ionization detector (BECKMAN) for detection of remaining propane, an IR detector (COSMA) for CO and a COSMA apparatus for the detection of nitrogen oxides by chemiluminescence. 10% of CO 2 and 10% of H were added to the feed stream. and CO was 20 mixed with hydrogen in such amount that its concentration by volume is about
419
0.5% of the initial gas mi xture (CO/H 2=3). The gas flow rate at the reactor outlet was about 80 l/h wi th a correspondIng space velocity of 25000 h- 1. A sample of about 0.5g of carbide catalyst was added to 2g of ground cordierite (100 < 0< 160 um) to increase the heat transfer. A same amount of reference catalyst, about 2.5g, of a ground conmer-c i al catalyst based on Pt and Rh was used to compared the performance of the catalysts in the same experimental conditions. RESULTS AND DISCUSSION Catalyst compositIons The composition of the catalysts are reported in Tables 1 and 2. TABLE 1 Weight composition of catalysts from chemical analysis (wt%) Catalysts Ko K1 K2 K3 K 3_o
WC/Al 203 (W,Mo)ClAI 203 (W,Mo/ClAl 203 (W,Mo)ClAI 203 (W,Mo)C/AI 203
CT 0.80 0.34 0.36 1.07 1.99
Cd
C
0.64 0.06 0.10 0.81 1.55
0.16 0.28 0.26 0.26 0.44
Mo
W
Mo+W
0 4.09 0.73 4.32 1.21 3.08 1.82 2.70 4.36 1. 11
4.09 5.05 4.27 4.52 5.47
TABLE 2 Atom composition of catalysts and their specific surface areas Catalysts
Ko K1 K2 K3 K 3_o
WC/AI 203 (W,Mo)C/Al 203 (W/Mo)C/Al 203 (W,MO)C/Al 203 (W,Mo)C/Al 203
Mo/Mo+W (atom %)
°
24.5 42.9 56.4 88.3
C/Mo+W (Atom ratio) 0.60 0.75 0.74 0.77 0.71
Cd/Mo+W (atom.%) 2.4 0.16 0.28 2.0 2.5
BET Surface area m2 .g -1) 87.7 95 44.6 85 99.2
The molybdenum and tungsten contents were determined by atomic absorption spectrometry in areducing flame of acetylene-nitrogen protoxid at the Service Central d'Analyse du C.N.R.S. (Lyon). The metal carbides are always loaded with free carbon the amount of which depends on the preparation method. The total amount of carbon was obtained
420
from the combustion of the sample in pure oxygen in a high frequency oven (LECO apparatus). The CO 2 formed is then quantitatively detected by a thermal conductivity cell with an oxygen reference flow which directly gives the carbon percentage. The free carbon content is obta ined by a couIomet ri c method. The sample is attacked by a hot mixture of nitric and hydrofluoric acids which dissolve all the components except the non combined carbon. This carbon is then transformed into CO 2 in flowing oxygen at 1300°C before the titration in an electrochemical cell. The determination of the combined carbon (carbidic C) is obtained by difference between the total carbon content (C T) and the free carbon one (carbon deposit Cd)' From these values, we can calculate : - the stoichiometry of tungsten and molybdenum carbides (C/Mo+W) i.e. the number of combined carbon atom per metal element ; - the atom ratio of free carbon deposit Cd/Mo+W ; - the atom percentage of molybdenum in the mixed carbide compounds (Mo/Mo+W%). The results are summarized in Table 2. XPS Experiments XPS spectra of W4f and M0 3d for alumina supported carbides are very different from those of bulk carbides (ref .4). Besides Wand Mo carbides, W or Mo oxidized are present in large amounts, probably as W(Mo)+4, Mo+ 5 and W(Mo)+6 (Fig.1 and 2). Such oxides are probably partly formed during the passivation treatment. However, while with bulk carbides these oxides are easily reduced by a treatment under hydrogen at 300°C or 500°C (ref .4,5), with supported carbides similar reducing treatments do not seem to markedly influence the proportion of oxide phases. Such a stability of these oxides could be related to the formation of combinations with the alumina support (Ref.6). The presence of carbidic surface phase was also checked by the C1s peak at a binding energy of 282.5 eV assigned to carbidic carbon (Fig. 3a). It can be mentioned that carburization can be increased by a treatment in CO and H2 at 300°C as can be seen in figure 3b. Using the intensities of Mo 3d, W4f photopeaks corrected according to EBEL (ref.2) we have ~ s t i m a t e d the surface composition of the various catalysts as reported in table 3. From table 3, it can be seen that most of Wand Mo are in an oxidized state in the layer analyzed by XPS. In this respect K2 seems to be an exception since the proportion of carbide is much higher than in the other catalysts. Compared to the bulk composition from chemical analysis, the surface composition from XPS data seems to indicate a slight molybdenum surface enrichment which can explain that molybdenum is more affected by oxidation as shown in table 3.
421
Mo
23' Binding
En."SI)'
leV]
Binding Energy
3d
tI I I I
eV]
I
230
Fig. 1 and Fig. 2. Spectra of W4f (Fig.1) and M03d (Fig.2) from the mixed carbide catalyst (W,Mo)C/A1 (K 203 2). TABLE 3 Surface compositions from XPS data Catalysts Mo/Mo+W bulk Mo/Mo+W XPS WOxide WCarbide Mo Oxide (atom %) (atom %) (%) (%) (%)
Ko Kl K2 K3 K3-0
o 24.5 42.9 56.4 88.3
o 30.5 49.6 57.6 100
95 88 45 96
5
12 55 4
Mo Carbide (%)
o o
100 59 100
41
89
11
Adsorption Measurements Before adsorption measurements, the catalysts were pretreated at 400°C under flowing hydrogen for 7 hours and then outgassed at 400°C for 10 hours at a pressure of 10-6 torr. Hydrogen chemisorQtion. The amount of adsorbed hydrogen, derived from the adsorption isotherms at room temperature is zero. But if the adsorption temperature is increased the hydrogen uptake also increases as seen in table 4. Since hydrogen adsorption is generally exothermic, the hydrogen uptake at equilibrium must decrease when the temperature increases (ref .7). The opposite result obtained leads to the conclusion that the hydrogen adsorption is an activated process. A similar result was reported by BENZIGER et a1. (ref.8) for a tungsten single crystal carburized by ethylene. In order to compare the various catalysts, the hydrogen adsorption isotherms have been determined at 400°C (Table 5).
422
Fig.3. C( 1s) electron spectrum from WC/AI Z03 (Ko)' (a) as obtained after catalyst preparatIon (b) after further carburlzatlon wIth CO and HZ at 300°C. TABLE 4 Hydrogen uptake on the sample K3(W,Mo)C/ AI Z03 as a function of the adsorption temperature. Values obtained by extrapolation at zero pressure. Adsorption Temperature (OC)
Hydrogen Uptake (]J mol.g- 1)
23 300 400 500
13.2 18.3 34.0
°
The relative number of potential hydrogen adsorption sites (H/Mo+W) increases with the atom percent of molybdenum, however it is rather low and significantly lower than 1. When the ratio H/Mo+W is plotted versus the bulk %Mo/ Mo+Wa straight line is obtained ( F ~ 9 . 4) which could indicate that the slight surface enrichment in molybdenum suggested by the XPS results after the passivation treatment, disappears in hydrogen at 400°C leading to Mo and Wsurface compositions very close to the bulk compositions. In addition, such a linear function clearly shows that the hydrogen uptake is not significantly influenced by the amount of free carbon which is the lowest for the K1 sample and maximum for K their corresponding experimental points fitting very well on 3_o, the I inear plot. The only exception is for the K2 sample which adsorbs a larger amount of hydrogen. This result can neither be explained by the amount of free carbon (higher than in K nor by the surface enrichment in molybdenum 1) (which is the lowest for K2). The larger hydrogen uptake for this K2 sample
423
could be a consequence of the largest extent of the carbided phase as seen in table 3. TABLE 5 Chemisorption of H2 and CO on Wand Mo carbides Catalysts
Mo/Mo+W (atom%)
KO K1 K2 K3 K 3_o
0 24.5 42.9 56.4 88.3
a) H2 uptake H/Mo+W (IlmOl.g-1) 4.1 10.0 18.0 18.3 39.6
aAdsorotion temoerature
400°C
3.7 6.4 12.3 10.9 15.4
10-2 10-2 10-2 10-2 10-2
b) CO uptake (Il mol.g- 1) 8.6 19.2 28.2 31.2 30.5
Co/Mo+W
3.9 6.2 9.6 9.3 5.9
10-2 10-2 10-2 10-2 10-2
H/CO
0.95 1.04 1.28 1. 19 2.60
bAt room temperature
Carbon Monoxide Chemisorption. As for hydrogen adsorption, the number of active sites for CO adsorption is obtained by extrapolation at zero pressure of the isotherms recorded at room temperature (Table 5). The pretreatment of the catalysts was exactly identical.
" 10
0113·0
,,
j o
L-
- - ~ - ~ ~
Mo/Mo ..W [At.";I
.0
100
Mo/Mo. w [At', ]
Fig.4 and Fig.5. Variations of the number of hydrogen adsorption sites (H/Mo+W) (Fig.4) and of the number of CO adsorption sites (CO/Mo+W) (Fig.5) .as a function of W, Mo bulk composition.
424
The variation of the number of CO adsorption sites as a function of W, Mo bulk composition is similar to that found in hydrogen adsorption including the K exception (Fig .5). For all the samples the atom ratio of the adsorbed 2 amounts H/eO is close to 1 except for the K3_ 0 sample H/eO is 2.6. This ratio H/eO equal to 1 for tungsten and mixed carbides seems to indicate that on these catalysts CO is adsorbed non dissociatively and linearly. This is in agreement with the results of BENZIGER et al. (ref.8) which, by comparing the CO adsorption on W metal and carbide, have shown that the di ssociati ve adsorption is strongly inhibited by the formation of surface carbide leaving CO molecules weakly bound in the linear form. Bui: for molybdenum carbide, the adsorption of one molecule of CO needs two (or more) potential sites, either as dissociative or bridged species. eata lyti c Tests Two types of experiments were carried out in this study : - The steady state conversion of CO, NO and propane was determined as a function of the equivalence ratio R which is related to the air to fuel ratio (A/F) and to the stoichiometry of the reaction: R = (A/F) stoichiometric/(A/F) actual. R = corresponds to a stoichiometric gas mixture where A/F = 14.68. R> 1 represents an overall reducing gas mixture (rich of stoichiometry) R < 1 represents an overall oxidi z i nq gas mixture (lean of stoichiometry). R was varied from 0.95 to 1.05 to reproduce the narrow operating window of a three-way catalyst. - The conversion was studied as a function of the temperature at R=l to determi ne the 1i ght-off temperatures where at 1east 50% convers ion of the reactants occurs, but a1so to test the res i stance of the cata lysts to high temperatures. The activity of the carbide catalysts was compared to that of a reference catalyst based on platinum and rhodium strictly in the same experimental conditions. Typical curves of the steady state CO, NO, propane conversions as a function of temperature or as a function of the equivalence ratio are represented respectively in Figures 6 and 7. The light-off temperatures of the CO, NO, propane are respectively 160°C, 180°C and 250°C. They are indicative of usual commercial catalysts. By comparison, for the tungsten carbide Ko the light-off temperatures of the CO and NO conversions are respectively 300°C and 360°C (Fig.8). They are higher than for the usual Pt-Rh catalysts but they are still low enough to be interesting (ref.9). Concerning the variation of the conversion with the equivalence ratio at 465°C, the tungsten carbide catalyst seems very stable with a good conversion for CO and NO (Fig.9). However in no case the conversion of propane is significant on carbides.
425
VYH; 25000 /HOUR
REFeRrNcr C.ATALYST 450 d.g C
20
Yl'H
25hDO /HOUR
:tn;rgMC.kllON -r-- NO,
""
Q"
D'n
'.DD
'Dt
'''2
'vJ
fUI11)
faJl~AHIO
Fig.6 and Fig.? Diagrams of catalyst conversion efficiency in CO, NO and propane removal plotted against temperature (R=1) (Fig.6) or againts equivalence ratio R at 450°C (Fig.?) on the reference catalyst (Pt-Rh PROCATALYSE, France) (- CO, 0 NO, * C3HS)
¥YB: 25000 /HOUR K o
465 dl!g C
Ko
YVH
25000 /HOUR
->,
~
loi+- ----,--------+-------i--
,-{,
~ 7~
;;; ~
60
~
S/1
'"
«o
'-'
~_;~~H(,
o
~,~~~#~~~~#~~#~,~~~,~~~
TFMPFRAT1!RF
lot
fin
HkllVAlflU
'."4 ~AfIO
Fig.S and Fig.9. Diagrams of catalyst conversion efficiency in CO, NO and propane removal plotted against temperature (R=1) (Fig.S) or against equivalence ratio at 465°C (Fig.9) on the tungsten carbide catalyst (WC/AI 203, K ). 0 (_CO,O NO, * C3HS) For the mixed carbides (K 1, K2 samples) the conversions are surprisingly very low as shown in figure 10 for K1. K2 is almost inactive at any temperature. Hence tungsten carbide seems to be a much better catalyst for postcombustion catalysts than molybdenum carbide. The unexpectedly low activity of K1 and K2 which contains rather high tungsten contents could be related to a molybdenum surface enrichment in an oxidizing atmosphere, which seems to be revealed by XPS analysis.
426
VYR: 25000 /HOUfl
i
?
i IJg
•
TTl T.-T I
. I
l.I.I... ;... L_~ i
I
!
i
ii
j I_
t
! I i I
'
1
I
~ j l j
i
i
1
I !
Fig.10. Diagram of catalyst conversion efficiency in CO, NO and propane removal plotted against temperature (R=1) on the mixed carbides catalyst (W,Mo)C/AI 203, K1. (.CO, o NO, *C 3H8). After a catalytic test at 600°C, the XPS spectra show that the carbidic species of Wand Mo have totally disappeared. Only the oxide surface phases are present. CONCLUSION Of course, the oxidability of these carbides is a handicap for the moment. However one might envisage that some support effects, or (and) the addition of elements exhibiting oxygen storage capacity could improve their resistance to oxidation. In that case, owing to its reasonably good activity for CO and NO conversions, tungsten carbide might be considered as a possible component of the catalysts for automobile emission control, probably in association with some other elements active for hydrocarbon oxidation.
REFERENCES 1 R.B. Levy and M. Boudart, Science, 181 (1973) 547. 2 M.F. Ebel, Surf. Interface Anal., 2 (1980) 173. 3 G. Leclercq and M. Boudart, J. Catal., 71 (1981) 21. 4 M. Provost, Thesis: "Etude des propriAtAs catalytiques d1alliages du molybdene et du tungstane avec Ie carbone"; University of Poitiers (France) 1984. 5 B. Vidick, J. Lemaftre and B. Delmon, J. Catal., 99 (1986) 428. 6 I.E. Wachs, C.C. Chersich and J.H. Hardenbergh, Applied Catal., 13 (1985) 335. 7 S.E. Wanke and N.A. Dougharty, J. Catal., 24 (1972) 367. 8 J.B. Benziger, E.I. Ko and R.J. Madix, J. Catal., 54 (1978) 414. 9 J.T. Kummer, Progress in Energy and Combustion Sci., 6 (1980) 177.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
427
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
DYNAMIC
BEHAVIOR
OF AUTOMOTIVE
THREE-IvAY
EMISSION
CONTROL
SYSTEMS
RICHARD K. HERZ Dept. A.M.E.S./Chemical Engineering, University of California at San Diego, La Jolla, CA
92093, U.S.A.
ABSTRACT The operation of warmed-up automotive three-way catalysts is considered. Special emphasis is given to the observation that significant fractions of CO, hydrocarbon, and NO emissions in urban driving tests occur during vehicle acceleration. The increased emissions during acceleration occur as a result of increased exhaust flow rates and rich air-fuel ratio excursions of the air-fuel ratio control system. A method is presented for displaying and analyzing catalyst response to dynamic changes in operating conditions. The inclusion of a rich air-fuel ratio excursion test in catalyst evaluation procedures is recommended. INTRODUCTION Three-way automotive catalysts never operate under steady-state conditions: catalyst temperature increases rapidly after engine starting, and the exhaust flow
rate
Numerous
and
composition
studies have shown
fluctuate
rapidly
under
all modes
that the performance of catalysts
of
operation.
under dynamic
conditions differs greatly from their performance under steady-state conditions (e.g., ref.1-4).
Thus, it is manditory to evaluate and compare the performance
of three-way catalysts on the basis of tests that involve dynamic conditions. The dynamic behavior of three-way emission control systems involves several catalyst behavior during warm-up
aspects: control
following
catalyst warm-up,
and
from a cold start, air-fuel ratio
catalyst behavior following warm-up.
This discussion considers only the last aspect. results and discussion sections.
The report consists of two main
In the next section, an analysis of dynamic
conditions and catalyst performance during variable speed driving is presented. In the following section, a method analysis of dynamic response experiments is presented.
All of the experimental work described in the report was performed
under the direction of the author at the General Motors Research Laboratories. The mathematical modeling work that is presented was performed since the author has been a member of the faculty of the University of California at San Diego.
428
DYNAMIC
CONDITIONS
A good
DURING
way to get
a
DRIVING
feel
for
the
dynamic nature
of automotive catalyst
operation is to look at a plot of speed versus time during the U.S. Federal Test Procedure (FTP)
as shown in Fig.
1.
Clearly, steady-state conditions do not
exist in this test, which simulates the acceleration and deceleration cycles of urban driving and which is similar to the European test procedure. The first
step in analyzing the performance
of a catalyst in an emission
control system is to determine "what the catalyst sees" in terms of temperature, exhaust composition, and exhaust flow rate variations during the driving cycle. The
nature
of
the conditions
that a
catalyst
is exposed
to
is
not
only a
function of the driving cycle and the vehicle type, but also is dependent upon the air-fuel control system.
Tests which record the dynamic conditions have to
be repeated and evaluated statistically since the detailed results of each test will vary as a result of random test-to-test variations. The second step in the analysis is to classify the different conditions of catalyst
operation and
determine
the
distribution of
to
identify
performance
the can
most be
significant
improved.
conditions
In
emissions
between
the
This step in the analysis allows one
different types of operating conditions.
catalyst
of
operation
aging
tests,
where
catalyst
can
identify
one
condi tions of operation where catalyst performance has deteriorated most and, thus, were stability should be improved. The changes in operating conditions that a catalyst sees during driving can be separated into two time scales.
Fast oscillations (ca. 0.5 to 4 Hz) in air-
fuel ratio (A/F) about the A/F control point occur as a result of the response characteristics
of
the
100
~
Nt
80
:2
Co
"0 Q) Q)
Ul
40
20
control
system.
Slower
changes
in
exhaust
Warmed.UP-... Operation
,
60
~ Co
A/F
~ ! ~
~I ~
a
a
500
1000
1500
2000
Time(s)
Fig. 1. Plot of vehicle speed during the urban driving cycle of the U.S. Federal Test Procedure (FTP).
429 composition,
flow rate,
deceleration. less.
and temperature occur as a result of acceleration and
These slower changes have characteristic frequencies of 1 Hz and
In this
section, we focus on the
relatively slow time-scale
transients
associated with acceleration and deceleration. The results presented in this section were obtained with a carbureted vehicle that was mounted on a chassis dynamometer and that was driven through the U.S. PTP.
All
of
the data
presented are
for
operation following
catalyst (i .e., accel-dece1 cycles number four and greater).
warm-up of
the
Two complete sets
of emission analyzers allowed simultaneous measurement of exhaust composition at the inlet and outlet of the pellet-type catalytic converter. brief review of
the results of
these
tests.
Here we present a
A more detailed description is
given in (ref.4). Concentrations of exhaust components were digitized and recorded by computer every 0.5 seconds during each driving test. vehicle speed, every
0.5
In addition,
seconds.
All
data
recorded
were
computerized relational data base program. of AlP and
(e. g., data
(e.g.,
other data such as
throt tIe position, and fuel consumption rate also were recorded
exhaust flow rate),
loaded
into
a
logical searches and classification of the Alp and
the acceleration were
and plotting of results to be
performed qu i ck l y and
selection of all periods where
within specified limits),
electronically
This program allowed computations
easily. The
impact of accelerations on
emissions can be
seen in
Pig. 2.
The
left
column of plots were obtained when the automobile was operated at constant speed (not
during
the
PTP urban
driving
cycle).
The
right
column
of
plots
show
results obtained during driving through several of the accel-decel cycles of the U.S. PTP.
The top row of plots is driving speed.
the AlP during.
The bottom two rows of
The second row of plots shows
plots show the rate of emission of CO
and NO in terms of grams per second at the outlet of the converter.
Emissions
of these species are low during constant speed operation but significant burst or
spikes of emissions occur during
variable speed dri ving.
The emission of
increased amounts of pollutants during variable speed driving results
from (a)
perturbations to the AlP control system during acceleration (see third row of plots),
(b) perturbations to the exhaust gas recirculation (EGR) control system
during
acceleration
acceleration emission
(see
rates
(not
fourth
(even at
shown), row
of
and
(c)
plots),
constant exhaust
increased
which
causes
exhaust
flow
increased
during
engine-out
composition) and decreased
reactant
residence times in the converter. The
A/F control
system on
control at a mean Alp of 14.7. used was
14.6.
the
automobile used
in these tests
was
set
to
The stoichiometrically balanced AlP for the fuel
We arbitrarily divided
the A/F
scale into
three regions
for
430 analysis of these tests:
"lean excursions" where the A/F was greater than 14.9
(i.e., the control ratio of 14.7 + 0.2 A/F units, a control region where the A/F was between 14.9 and
14.5, and "rich excursions" where the A/F was less than
14.5. summarizes the performance of the vehicle and emission control system
Table
during the U.S. urban driving cycle following warm-up.
The distribution of the
results between rich and lean excursions and the control region will of course be different for different engines and, especially,
CONSTANT SPEED
FTP DRIVING
50
50
40
40
~
30
30
""
20
20
10
10
0
o
15
15
14
14
13
13 20
Z "tl
for different A/F control
c,
C/)
2
;;;
a: 0; :J
u,
';::
...
<;)
-,
;;;"
a:
10
s0
u:
<;) '<,
rn
0 u
<;)
-,
C>
0 Z
"~
10
0
"L
o 0.4
02
02
0
o
O"'L 0000
0
10
O " ' ~ 0 . 0 0 0 , 500 550 600 650 700
Time(s) lEe Prod.Res. s Dev.
Fig. 2. Comparision of air-fuel ratio control, exhaust flow rate, and CO and NO emission rates during constant speed driving and variable speed driving through a selected segment of a U.S. FTP test (ref.4).
431 systems.
We
expect
that
the results apply
qualitatively
to
most
vehicles,
however. that whi Le the A/F was in a rich excursion only 14% of
Note
disproportionate
share of the
total exhaust
flow,
the time,
20%, occurred
during
a
rich
excursions.
This is because rich excursions occur as a result of acceleration
and
flow
exhaust
significantly,
rate
increases
disproportionate
with
rate
shares
of
hydrocarbons occur during rich excursions.
of acceleration the
emissions
(ref.4).
Most
CO,
and
of
NO,
This result is significant but not
surprising for CO and hydrocarbon emissions. The
resul t
that
negligible during basis
NO emissions are significant
during
lean excursions may be surprising
of measurements
of NO
conversion
during
rich
excursions and
to many people.
steady-state
or
On the
"cycled-A/F"
tests, we expect NO conversions to be high under rich conditions and low under lean
conditions
(in cycled-A/F tests,
average
conversion
is measured
cycling of the A/F, typically at a frequency of 1 Hz and an amplitude of
A/F units).
during
± 0.5
This is an excellent example of how steady-state and even cycled-
A/F tests can be misleading. There
are
several
reasons why one
conversion measurements to directly driving.
cannot
use
steady-state
or
cycled-A/F
predict emission rates versus A/F during
The tail-pipe emission rate, for one species at a given A/F, is equal
to the mutual product of (a) inlet concentration, (b) exhaust flow rate, and (c) fractional
conversion.
Each
cond i tions of opera t ion.
of
these
factors
differ
between
the
Relati vely high NO concentrations can
different
occur during
initial periods of acceleration as a result of the response characteristics of the EGR control system.
High exhaust flow rates during acceleration contribute
to high engine-out emission rates, even at constant composition, and result in decreased
reactant
residence
time
in
the
converter.
Finally,
different
fractional conversions are obtained during A/F excursions while driving because slowly responding transient chemical processes cause the catalyst to be in a
Table 1. A/F CONTROL AND EMISSIONS DURING WARMED-UP PORTION OF FTP "Rich Excursion"
"Control"
"Lean Excursion"
A/F<14.5
145< A/F< 149
A/F> 149
10
% of Total Time
14
76
% of Total Flow
20
73
7
% of Total CO
48
39
2
% of Total He
56
41
4
% of Total NO
34
64
2
lEe Prod. Res.& Dey.(rol.4)
432
different
chemical state
than
it is at
AIF during steady-state or
the same
cycled-A/F tests. Fig. 3 shows the average conversion obtained versus A/F during the FTP.
The
A/F was always fluctuating during these tests and stayed only briefly at each value of A/F.
The total mass of component (e.g., CO) entering the converter and
the total mass of component exiting the converter, whenever the A/F was at the given value, were used to calculate the conversion. Fig.
3 looks very different from
a plot of conversion versus A/F obtained
during steady-state tests or cycled-A/F tests.
First, note that NO conversion
is high at lean A/F's, whereas steady-state and cycled-A/F tests would show low conversion. Transient chemical processes, "oxygen
storage"
components
in
the
such as reaction of NO with reduced
catalyst,
serve
to
maintain
high
NO
conversions during transient lean AIF excursions. Second, note that CO and hydrocarbon conversions remain relatively high even at
very rich
oxidize
CO
A/F's.
and
The oxygen storage function of the catalyst serves
hydrocarbons
during
rich
excursions
(through
to
stoichiometric
reactions with a limited capacity and, thus, limited duration) in the absence of sufficient gaseous oxygen. The fact that NO conversions drop-off at rich A/F's in Fig. 3 may be due to inhibition of NO conversion by high CO concentrations.
On the other hand, the
fact that the conversions of all three components are about the same over the
100
90
c 0 ·Vi
Q;
80
---co
>
C
----NO
0
U
C Ql
-'-'-HHC
70
~ Ql
0..
60
5 0 + - - - - -....- - - -....- - - - - , - - - - - - - , 135
14
145
15
155
Air/Fuel Ratio lEe Prod. _ . & Dev.
Fig. 3. Average conversions obtained over a pelleted catalyst during the warmed-up portion of the U.S. FTP (ref.4).
433
entire A/F range suggests that the conversion of each reactant may have been limited by mass transfer rates during this test.
The inverse correlation of
exhaust flow rate and A/F that is obtained during driving,
and the resulting
positive correlation of reactant residence time in the converter with A/F, may explain why lower conversions are obtained for all
three components at
rich
A/F's. At this stage, we have discovered some of the conditions that a catalyst can see
during
driving.
acceleration are
hydrocarbons, and NO. A/F
tests
driving.
do
We
have
found
that
rich
excursions
associated
responsible for a disproportionate share of emission
not
with
of CO,
And finally, we have seen that steady-state and cycled-
accurately
reflect
the
performance of
a
catalyst
during
In the next section, we will discuss studies that try to identify the
transient chemical processes in a catalyst that determine reactant conversions under dynamic conditions. ANALYSIS OF DYNAMIC TESTS Introduction The
goal
of
the analyses
determine the mechanism
discussed
in
this
section is
to
identify
and
and kinetics of transient chemical processes that can
affect the dynamic performance of an automotive catalyst.
By "transient II we
mean that the conversion due to the process changes at a rate that is somewhat slower
than the
rate of change in conditions during driving.
That is,
the
conversion due to the process does not change either instantaneously or very slowly.
Some of
the
the
transient
processes
that
have
been
identified in
previous work are listed below: 1. Adsorption and accumulation of CO on the surface of the precious metals in the catalyst.
Stoichiometric reaction of the accumulated CO during
rich-to-lean transients (ref.S). Z. Accumulation of reactive oxygen atoms by adsorption and/or reaction of 0z
and
NO
with
the
precious
metals
and
base
metal
oxides.
Stoichiometric reaction of the accumulated reactive oxygen atoms with CO, HZ' and hydrocarbons during lean-to-rich transients (ref.Z,6,7). 3. Transient catalytic reaction of H with CO (water-gas shift) during 20 rich conditions over Rh oxidized under lean conditions (ref.8,9). 4. Oxidation and partial deactivation of the catalytic activity of one or more
of
the precious metals under
lean conditions.
reactivation under rich conditions (ref.lO).
Reduction and
434 [n addition to the transient processes listed above, there may be others that are important and that have not been identified yet.
Although the processes
listed have been identified, there is much to be learned about their mechanisms and kinetics. In addition to transient chemical processes, transient thermal processes may also
be
important
to determining catalyst
response
to changes
in operating
conditions, even following warm-up.
We do not consider the participation of
transient
report.
thermal
processes in
this
These processes should
not
be
neglected in experimental work, and converters and laboratory reactors should be instrumented with thermocouples at several different locations in the catalyst pellet bed or monolith. We can introduce a simplification in our analysis of catalyst response by classifying each of the
possible transient chemical processes as one of
two
types. The first two processes listed above involve accumulation of reactive species during some periods of operation, followed by reaction of these species during subsequent
periods of
processes.
The presence of this type of transient process can be identified by
the
operation.
I
will
call
these
presence of a transient discrepancy in the mass
chemical elements across the converter.
"accumulation-reaction" balance of one or more
For example, more oxygen atoms might be
coming out of the converter in the exhaust at a particular instant in time than are entering in the exhaust (when correction is made for the residence time of exhaust in the plug-flow converter). The
last
two processes
listed above
involve
only minor
accumulation
reacti ve oxygen atoms during oxidation of the precious metals. involve primarily a change in the catalytic activity of
Instead,
the catalyst.
of they The
change in catalytic activity occurs sufficiently slowly such that the dynamic response
of
conversion
over
the
catalyst
is affected.
I
will
call
these
"activity change" processes. The presence of this type of transient process can be
identified
by
the
presence
of a
complex
dynamic
response
that
is
not
accompanied by a discrepancy in an elemental mass balance across the converter. One type of experiment that can be performed to study the dynamic response of a catalytic converter is to make rapid changes in conditions - composition, flow rate,
temperature
while
continuously
measuring
exhaust
simultaneously at the inlet and outlet of the converter.
composition
Ideally,
like to make fast measurements of all important exhaust species.
one would
Although this
is not possible for all species at present, we have made fast measurements of CO in exhaust at
the inlet and
outlet of a converter.
The apparatus used is
described in detail elsewhere (ref.ll,12,13). Fig. 4 and 5 show the results of measurements made during A/F cycling and following a step-change in A/F setting.
How does one go about analyzing the
435
2 0 r - - - - - - - - - - - - - - - - - - - - - - - ~
10Hz
0.5 Hz
167 Hz
~
c
.9
~
C 1.0 ~ c o
u
a u
O'--.,-
-,_-,-
t - - - - 4 5 - -........-1
,--,--
1-----45-----<""
---,_.....J
1 - - - - 45
--~..,
lEG Prod.Res. & Dev.
Fig. 4. CO concentrations measured by infrared absorption spectroscopy at the inlet of a converter as the A/F setting was switched in a squarewave between a rich setting of 14.1 and a lean setting of 15.1. Equal periods of time were spent at each A/F setting (ref. 11).
1.5
~ z a
1.0
>=~
a: >Z
w
U
z
a u
05
0 u
10
20 TIME (5)
30
Fig. 5. CO concentrations measured by infrared absorption spectroscopy at the inlet (top) and outlet (bottom) of a catalytic converter during a lean-to-rich transient. The A/F setting was switched from a lean setting of 15.1 to a rich setting of 14.1.
436 results of such experiments? to
A common procedure is to compare experimental data
the predictions of a detailed mathematical model
of the physical
system.
Such a model is not currently available. We found a way out of this difficulty by going back and asking what it was that we wanted to find out.
First, we want to know whether the dynamic response
of a catalyst is "complex," that is, affected by changes in a transient chemical process, as defined above.
Second, If the response is complex, we would like to
be able to determine whether "accumulation-reaction" processes are present and Only after these first
whether "activity change" processes are present.
two
questions are answered do we need more detailed information that would require a detailed mathematical model. In order to determine whether the response of a catalyst is complex, we only need to compare the measured response to that predicted for a model catalyst Such
that has a "simple" response.
a model catalyst with a simple response
would have (a) the same steady-state performance as the real catalyst, (b) no of reactive
accumulation transients,
and
transients.
only
(c)
We
call
species or
the
in
changes model
response of a
response. "
"instantaneous
accumulated
reaction of
instantaneous
Essentially,
species
during
catalyst activity
during
catalyst model
the
with this
behavior
catalyst
exhibits
instantaneous response to steady-state conditions. Calculation of
the instantaneous response
that
corresponds to
measured response of a real catalyst is simple and At
(reLll).
each instant
in
time,
one
takes
the actual
is described in detail in
the measured composition
of
exhaust entering the converter, goes to a table of steady-state measurements and finds the corresponding outlet composition, accounts for the residence time of exhaust in the converter, and plots the "instantaneous" outlet concentrations determined in this way along with the measured concentrations. a
discrepancy
between
clearly indicates
the
instantaneous
response
that the dynamic response of
and
The presence of
the measured
response
the catalyst is complex.
In
addition, the discrepancy between the two response curves for an exhaust species can be integrated to give a quantitative measurement of the discrepancy. This procedure can also be applied to "cycled-A IF" experiments in which only time-averaged concentration measurements are recorded. average
the
appropriate
inlet
concentrations over
steady-state
outlet
the
Note that one must first
A/F cycle and
concentrations
over
the
then
average
A/F cycle
the
before
calculating an average "instantaneous response" conversion.
One can not average
the
to
steady-state
conversion
levels
themselves
in
order
get
an
average
conversion. Fig. for
a
6 shows the instantaneous and measured CO response curves determined catalyst
following
a
step-change in
A/F setting from lean to rich
437
conditions.
The notation next to the curve indicates that the action of
the
transient chemical processes in the catalyst resul ted in the "extra" conversion of 44 micro-mol of CO per gram of catalyst following the step-change .i n A/F setting. The power of this method of analysis is shown, for example, in (ref.S) where the
presence of
the
transient
enhancement of water gas shift,
change" process, was demonstrated. types of transient processes
an
"activity
Although we were able to separate the two
to some extent using CO measurements alone,
general, one requires measurement of more than just CO.
in
Especially critical is
measurement of 02' Numerical Simulation of CO Oxidation Response In order to more fully explore the introduce
a
simple mathematical
catalytic reactor.
powers of this method of analysis,
simulation
of
CO
oxidation
in
a
we
plug-flow
The purpose of the simulation is to demonstrate the method
of analysis, not to accurately simulate an automotive catalytic converter.
The
advantage of using the simulation here is that we can look at the 02 and CO
2
response as well as the CO response. The
equations
used
in
the mathematical
simulation
are
described
in
the
Appendix.
The only species considered are CO, 02' and CO For convenience, we 2' refer to the calculated outlet concentrations as the "measured" concentrations
or
responses.
The
"instantaneous" responses
shown were determined
from
the
inlet signals and the steady-state performance of the simulated converter by the procedure described above. First, consider the response of the simulated converter to a lean-to-rich A/F transition.
The inlet 02 and CO signals are shown in Fig. 7, where the unit of
LEAN---...----RJCH_
10
~ I-
w
.... ~
05
0 0
o
44,.,mol/g
J 0
"
10
20
TIME (5)
30
J. Catalysis
°
Fig. 6. Solid line: CO response measured by infrared absorption spectroscopy at the outlet of a converter containing a Pt/Rh/Al pelleted catalyst. Dashed line: computed instantaneous r e s p o n ~ e ~ The area between the two curves shows that transient chemical processes in the catalyst resulted in enhanced conversion of 44 micro-mol of CO on average, per gram of catalyst (ref.S). '
438 time is reactor residence time.
The measured and
responses are shown in Fig. 8 through 10.
instantaneous 02' CO, and CO 2 The reasonableness of the shape of
the CO response can be seen by comparing Fig. 9 to Fig. 6. complex dynamic behavior is clearly seen in the CO and CO
2
The existance
of
responses, but is not
readily noticable in the 02 response. Performance of a mass balance on oxygen results in the top curve in Fig. 11 which shows that there is a discrepancy in the direction of excess oxygen atoms appearing
in
the
outlet
of
the
converter
during
and
following
the
AIF
Performance of a complete mass balance in the real situation of
transition.
exhaust gas is difficult, of course, since several other reactant and
product
species are involved. We now use the mass balance discrepancy to subtract the contribution of any "accumulation-reaction"
processes
present
from
the
measured
responses.
example, the corrected response curve for CO, shown in Fig. 12, each
instant in
time
by adding
to
the measured
amount of CO that was converted to CO the
catalyst,
as
determined
from
For
was obtained at
outlet CO concentration
the
by reaction with oxygen atoms stored in
2 the oxygen
balance.
The
fact
that
the
corrected CO response and the corrected CO response, shown in Fig. 13, do not 2 match the instantaneous response demonstrates the action of an "activity change" type of transient chemical process.
The process resulted in lower-than-expected
CO conversion, since the activity change process included in the simulation was partial deactivation of the catalyst in lean exhaust (e.g., by oxidation of the precious metal). Note
that
the
particular
activity change process
included here
tends
to
lessen the difference between the measured and instantaneous response curves. Theoretically,
one
curves
unlikely event
in
the
could
get
offset each other.
However,
mass
any
balances
discrepancies
if
between
the
identical
measured
and
that
transient
chemical
two
instantaneous
response
processes exactly
discrepancies would probably still appear in the
accumulation-reaction measured
and
process
instantaneous
were
present,
response
curves
and would
probably appear in other types of transient response experiments on the same catalyst. Next we consider the response of the simulated converter to a rich-to-lean transition. 7.
The inlet signals are not shown but are just an inversion of Fig.
In contrast to the case of the lean-to-rich transition, we find that only
minimal differences exist between the measured and instantaneous response curves for CO and CO
(Fig. 14 and 15). However, now there is a substantial difference 2 between the two responses for 02' as seen in Fig. 16. The discrepancy in the oxygen balance is shown by the lower curve in Fig. 11.
The integrated area below the positive-going lean-to-rich curve is equal to the
439 1.0
1.0 0.8
0.8
~
Z
0.6
'"
0 c:
8
0
0.4
0.6 0.4 0.2
0.2
0.0
0.0 0
2
4
10
8
6
0
14
12
4
2
6
8
10
12
14
Time
Time
Fig. 8. Oxygen response at converter outlet, lean-to-rich transient.
Fig. 7. Inlet signals to converter in simulated lean-to-rich transition from AlF=15.1 to AlF=14.1. Time is in units of converter residence time in this and all successive plots.
0.8,.---------------,
11.0 ~ - - - - - - - - - - - - - - . ,
lit instantaneous 10.8
0.6
~ 0
0.4
0
z
10.6
8'"
10.4
0.2
10.2
J0:====--t "\. instantaneous
10.0 L.-_.l-_....L._-'-_.......l_ _L - _ . l - _ . J 12 14 6 8 10 4 o 2
0.0 l . . - _.....""--'-_...L..._...L-_....L._........_...J 12 2 8 10 14 4 o 6
Time
Time
Fig. 10. Carbon dioxide response at converter outlet, lean-to-rich transient.
Fig. 9. CO response at converter outlet, lean-to-rich transient.
0.4 0.2
~
t1
0.0
Q)
1il > '5
~
-0.2
0<'<
-0.4 -0.6 0
2
4
8
6
10
12
14
Time
Fig. 11. Oxygen atom discrepancy resulting from mass balance over converter during transients.
440
~
"-0 o
08
10.8.----------------...,
0.6
10.6
0.4
,".~'-~'
oo'"
0.2
,
102
0.0
10.0 L-_.L-_-'-_-'-_--'_---''-_-'-_-' 4 10 6 8 12 14 o 2
L--i~_1.--_L---JL---JL---JL---'
o
2
4
10
8
6
14
12
TIme
TIme
Fig. 12. CO response corrected to show contribution of "activity change" process and compared to instantaneous response, lean-to-rich transient.
Fig. 13. Carbon dioxide response corrected to show contribution from "activity change" process and compared to instantaneous response, leanto-rich transient.
0.8
11.0 10.8
0.6
e:.0
measured· 0 discrap
l
0.4
lit'"
measured slighijy higher
10.6 N
o
8
10.4
0.2
10.2 10.0 2
4
8
6
10
14
12
0
4
2
Fig. 14. Measured and instantaneous CO response at converter outlet, simulated richto-lean transient from AJF = 14.1 to AJF = 15.1.
0.8
d"
10
12
Fig. 15. Measured and instantaneous carbon dioxide response, rich-to-Iean transient.
1.0
e
8
6 TIme
TIme
0.6
measured
0.4 area closely approximates oxygen uptake by catalyst
0.2 0.0 0
2
4
8
6
10
12
14
TIme
Fig. 16. Measured and instantaneous oxygen response, rich-ta-Iean transient:
14
441 area above the negative-going rich-to-1ean curve. tail
of
the
ca ta1ysts,
rich-to-lean
where
curve
reflects
the
The earlier peak and lesser behavior
of
real
re-oxidation of the oxygen storage component is
three-way
fas ter than
reduction (ref.6). When the measured 02 response curve is corrected for the loss of oxygen from the outlet gas by re-oxidation of the oxygen storage component, we find that the corrected curve and the instantaneous response curves are almost identical (not shown).
Thus,
between
the
even in the presence measured
and
of an acti vi t y change process,
instantaneous
response
curves
for
the area
02
closely
approximates the oxygen storage capacity of the oxygen storage agent. This result suggests that a good estimate of the oxygen storage capacity of a three-way catalyst can be obtained from dynamic measurements of 02 during richto-lean transitions, even in the presence of other transient chemical processes. Of course,
measurement of NO and other exhaust constituents would add to the
reliability of reactive
the estimate.
oxygen
content,
In
(ref. 6)
however,
those
we made measurements of changes in measurements
involved
flushing
the
catalyst with dry N between exhaust exposures. Dynamic measurement of 02 would 2 provide this information without such drastic perturbation of the catalyst.
RECOM!1ENDATrONS Current
test
methods do
not
performance following warm-up.
provide
an
accurate
indication
of
catalyst
Whereas steady-state conversion measurements are
valuable for research purposes, they have no relevance to catalyst performance during driving.
Cycled-A/F tests are of limited value since they only provide
an accurate measure of performance during constant speed operation. defense of cycled-A/F tests,
we have found
that they usually give
qualitative ranking of CO conversion performance during driving. are not very sensitive:
(In partial the right
However, they
differences measured between catalysts in cycled-A/F
tests are much smaller than differences measured during driving.) Clearly,
there is a need for development of additional tests that provide
measurements of catalyst response to the types of transients that occur during variable
~peed
driving.
Our work indicates that a reasonable test would be one
in which a catalyst is stabilized at constant speed conditions (i.e., about
the
A/F
control
point)
and
then
is
subjected
to
a
rich
cycling
transient,
preferably with an increase in gas flow rate. A reliable estimate of catalyst performance during driving might be obtained by averaging emissions measured during A/F cycling about the control point with emissions measured in a rich transient test, using weighting factors appropriate to the A/F control system under consideration.
The weighting factors could be
determined by (a) taking a variety of catalysts of different formulation,
(b)
measuring
the
their
performance in
the
two
simplified
tests as well
as
in
442
complete
driving
cycle,
and
(c)
determining
weighting
factors
for
the
two
simplified tests that give the best correlation with the driving cycle results. The rich transient test would receive lower weighting with A/F control systems that maintain tighter control of A/F during acceleration. Even with perfect A/F control during acceleration, however, there will still be a large increase in exhaust flow rate.
Catalyst response to this increase in
flow rate will be affected by the transient chemical processes in the catalyst, resulting in a complex change in emissions during acceleration that should be measured in the process of catalyst testing. In conclusion, we hope studies of
catalyst
that our work has demonstrated the need for further
response
during
variable speed driving and
for
further
studies of the transient chemical processes that affect the dynamic behavior of automotive catalysts.
REFERENCES
2 3 4 5 6 7 8 9 10 11 12 13
L. L. Hegedus, J. C. Summers, J. C. Schlatter and K. Baron, J. Catal. 54 (1979) 321. H. S. Gandhi, A. G. Piken, M. Shelef and R. G. Delosh, Soc. Auto. Eng. Paper No. 760201 (1976). Y. Kaneko, , H. Kobayashi, R. Komagome, O. Hirako and O. Nakayama, Soc. Auto. Eng. Paper No. 780607, SAE Trans. 87 (1978) 225. R. K. Herz and E. J. Shinouskis, Ind. Eng. Chern. Prod. Res. Dev. , 24 (1985) 385. Y. Barshad and E. Gulari, Am. Inst. Chern. Eng. J., 31 (1985) 649. R. K. Herz, Ind. Eng. Chern. Prod. Res. Dev., 20 (1981) 451-457. M. A. Shulman, D. R. Hamburg and M. J. Throop, Soc. Auto. Eng. Paper No. 820276 (1982). R. K. Herz and J. A. Sell, J. Catal., 94 (1985) 166-174. J. C. Schlatter and P. J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev . , 19 (1980) 288. E. Koberstein, Soc. Auto. Eng. Paper No. 770366 (1977). R. K. Herz, J. B. Kiela and J. A. Sell, Ind. Eng. Chern. Prod. Res. Dev., 22 (1983) 387-396. J. A. Sell, R. K. Herz and D. R. Monroe, Soc. Auto. Eng. Paper No. 800463, SAE Trans. 89 (1980) 1833. J. A. Sell, R. K. Herz and E. C. Perry, Soc. Auto. Eng. Paper No. 820388 (1982) .
APPENDIX The mathematical simulation of CO oxidation in a catalytic converter that is discussed above includes the following processes: Reaction of CO with O with inhibition by CO. 2 Oxygen storage and reaction of stored oxygen. - Partial deactivation of CO oxidation activity by metal oxidation under lean conditions.
443
The differential equations describing reaction over the simulated catalyst are:
k l (1- 8 )[02 ] [CO]
d[CO] dt
(1 + K [CO])2
d[02] dt
kred (\jI-\jIe) I zero when \jIe >
I \jI
-0.5 k l (l-8 )[02 ] [CO] () (1 + K [CO])2 - 0.5 k ox \jIe - \jI I I zero when \jIe < \jI
d8 dt
\jI
e
0.0165 k Oll 2 [02 ] k red (0.01 + [CO] )
d\jl
dt
-
k red
-
'I'cap
(\jI-'I':)
I zero when
e
\jIe >
I 'I'
k
--.2! ('If, - '1') 'I'cap e
+
I
I
zero when \jIe < \jI
Where, [CO]
[Oz]
CO concentration (%) Oz concentration (%)
8
fractional coverage, by deactivating oxide, of surface active for CO oxidation
8m
maximum value allowed for 8 (= 0.65)
'I'
fractional extent of oxidation of "oxygen storage component"
'l'e
"equilibrium" or steady-state extent of oxidation of oxygen storage component
'I' cap
capacity of oxygen storage component (= 2 %)
kl
rate constant for CO oxidation reaction (= 15 %-1 time-I)
K kax
CO inhibition parameter (= 1.7 %-1) rate constant for oxidation of oxygen storage component (= 3 % time-I)
lcred
rate constant for reduction of oxygen storage component (= 0.9 % time-l)
kp-on
rate constant for oxidation and deactivation ofCa oxidation activity (= 1 %-1 time-I)
kp-off
rate constant for reduction of CO oxidation activity (= 0.8 %-1 time-I)
The values given in parentheses are the values of the parameters used in the solution of the equations for the results presented in this paper. The equations
describing
the action of the oxygen storage
component are
written so that they show the experimentally observed behavior that the oxygen storage
component
does
not
contribute
to
CO conversion
after
steady-state
444 conditions are reached.
The mechanism and kinetics of oxygen storage component
action in automotive catalysts is not well understood at the present time. The
local
rate
equations
given
above
were
incorporated
in
conservation
equations for a plug-flow reactor and integrated to give the results plotted in the text.
The units of time on the abscissa of all of
the plots is reactor
residence time. Fig. 17 below shows the predic ted steady-state CO conversion versus simulated
A/F.
Fig.
18
shows
the
predicted variation
in
content of the simulated converter with simulated
0
steady-state reactive
A/F.
1.0
o 0 c; 0
.§
0.8
Ql
> c 0
o
0.6
"iii c 0
~
0.4
LL
0.2 L_...L...._....L._--L_ _L...-_..I.-_....L.....l 14.0 14.2 14.4 14.6 14.8 15.0 15.2
AJF
Fig. 17. Steady-state fractional conversion of CO versus simulated NF predicted by numerical simulation.
100 . - - - - - - - - - - - - - - , 80
60 Oxygen Content (% of max.)
40
20
14.15 14.35 14.55 14.75 14.95 15.15 NF
Fig. 18. Reactive oxygen content of simulated converter at steady-state conditions.
oxygen
445
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control © 1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
EFFECT OF LEAD ON VEHICLE CATALYST SYSTEMS IN THE EUROPEAN ENVIRONMENT
A Deakin'
M A Kilpin'
H S Gandhi'
'Engine Engineering, Product Development, Ford of Europe 'Research Staff, Ford of USA
ABSTRACT
There
are
two
catalyst
operating
different in Europe to the USA.
parameters
These are the
that could be significantly
average
operating
temperature
and the lead levels in fuel.
A
test
programme
Way Catalyst (TWC) running.
was with
initiated to investigate the effect of lead on Three high
programme
The
temperature was
excursions
completed
in
to
three
simulate stages:
autobahn
Laboratory,
Dynamometer and Vehicle tests.
Testing showed that, depending on owner levels
of
lead,
according
to
usage,
the
effect
of
permissible
DIN standard, in the fuel could significantly
affect the efficiency of the catalyst with extended usage.
INTRODUCTION
Background:The maximum lead level in unleaded fuel has been set at 13 DIN
standard
applicable
applicable
for
6
in
EEC
territories,
months after introduction.
with
a
mg/l
waiver
Pb to
in 20
These levels are anticipated to
give a concern of catalyst poisoning if they appear in the
field.
TWC's
particularly affected by lead oxide compound covering the Rhodium sites,
Typical
Pb
level
in
without
European
as
results in Pb
such
levels
are
(2).
fuel generally available in U.S.A. is 0,8 mg/l. TWC's
can contain this level market
the mg!l
concern.
non-dedicated
reaching
the
legal
However
if
unique
conditions
in
tankage, or octane boosting using Pb maximum,
then
following data, there will be a high risk of contamination.
as
shown
by
the
446 It
was
the
possibility
of high lead levels in the pump fuels which led to
the initiation of the extensive test programme described in this paper.
TEST PROGRAM
The test stages were:-.
1.
Laboratory
Pulsator Tests
2.
Dynamometer
Simulated 80K km Ageing
3.
Vehicles
80K km ageing on AMA City Driving Schedule
Two lead levels were used during the test programme. Trace lead (up to 3mg/l) similar to
that
found
currently
in
U.S.A.
pump
fuel. 10mg/1
was
chosen
as
it
was
anticipated that early supplies of unleaded
fuel in Europe could be close to the legal limit.
Each
stage
contributed
data
from
a
aspect
different
advantage of Laboratory and Dynamometer data was that
it
of
ageing.
could
be
The
generated
much quicker than by using 80K km vehicle tests.
Laboratory Pulsator Tests
Catalyst samples were aged in a pulse flame reactor (1).
The
were
as
shown in Fig. 1.
It
included a high temperature mode (1000 deg C) for 25%
of
the
to
take
test
account
simulate
the
modification
cycle
of
and
activity
autobahn
conditions for
driving. found
Europe,
and
duplicates 48km/h steady state temperature
effects
on
measurements
Pb
on
The the
test AMA
procedure City
cycle was
Driving
Cycle,
with
has a nominal space velocity of 40000/hr which vehicle
operation.
To
be
able
to
evaluate
retention another catalyst was aged on a modified
cycle that used only 730 deg C for 6% of the cycle instead of 1000 25% of the cycle.
time
developed to
deg
C
for
447
Catalyst Temperature Cycle:
25% Time: 1000°C
max. with 3% CO excess
75% Time: 500°C
14.45:1 AFR
Activity Measurements:
Pulsator Modulation: 500°C; 40000/hr (Nominal); + 1 AFR at 0.5 Hz;
Final Steady-State:
Fig. 1
550°C; 60000/hr
Pulsator Test Cycle and Activity Measurement Conditions
Reprinted with permission c 1985 Society of Automotive Engineers, Inc
The
ageing
mg/l.Pb
fuels
added.
tetraethyllead
consisted of isooctane with 0.2 mg/l P and either 3 or 10
The (TEL),
source
of
ethylene
Pb
was
in an atomic ratio of Pb:Cl:Br of 1:2:1 injected furnace
with for
catalysts
a
nebulizer
combustion.
were
measured
directly Steady
at
"TEL
Motor
Mix"
containing
dichloride (EDC) and ethylene dibromide (EDB)
550
The into
state deg
Pb
the
containing hot
activities C
and
isooctane
was
portion of the pulsator of
the
40000/hr.
pulsator
A
diagram
aged of the
apparatus and the synthetic gas mixture used is described in reference (3).
Dynamometer Tests
To maximise lead deposition, and to simulate life
doing
Fig. 2, the ageing duration is 300 hrs.
two
vehicle
spends
its
The
cycle
is
summarized
in
This represents 80K km on the road.
catalysts, one aged with 3mg/l fuel and the other with 10mg/l were,
in turn, fitted to an emission test data vehicle, that performance
that
city driving, two catalysts were aged on a dynamometer engine to a
predominantly low temperature, low load cycle.
The
the
using
a
6,4K
km aged catalyst.
were undertaken with both catalysts.
had
a
known
emission
A series of 83US emission tests
448
TWC Ageing
Condition 1
8%
Time Inlet Temp
815 -c
885°C
RPM
84%
8%
14,65 + 0.10
AIF Ratio
Condition III
Condi tion II
465°C 14,65 + 0.10
14,95 + 0.10
3000 - 3500 RPM
Fuel
Ageing Time
Lead:
0.003
or 0.010
gil
Phosphorus:
0.001
gil
Sulphur:
0.225
gil
Fig 2
300 hrs
80,000 km
Ageing Cycle for Dynamometer Tests
Reprinted with permission c 1985 Society of Automotive Engineers, Inc
Vehicle Durability
A fleet of 5 vehicles were prepared to each complete 80K km to the Driving
Schedule.
Two
vehicle
widen the database generated. from
a
european
types
and
Vehicles 4 and 5 were 49
competitor.
They
AMA
State
Federal
models
were 1985 model year production vehicles
purchased from a franchised dealer in the USA. Vehicles 1 and 2, 4 and paired,
one
running
on
trace
lead
fuel
the
other on 10 mg/l.
assigned as shown in Fig. 3. Vehicle 3 was tested
at
then
mg/1.
run
straight
through
to
80K
km
on
10
0
mile,
the
test
5
were
They were
6,4K
km
and
This was to generate
information as quickly as possible. Knowing data from this to
City
engine capacities were chosen to
car,
modifications
method, and emission test interval, for the other vehicles could
be incorporated if desired. Vehicles 1,2,4, and 5
have
been
emission
to the 83 U.S. test procedure according to the schedule shown on Fig. 4.
tested
449
1
2.0L
10 mg/l
2
2.0L
Trace Lead
3
2.0L
10 mg/l
4
1.8L
5
1.8L
10 mg/l Trace Lead
Engine Size & Lead Levels for 80K km Vehicles
Fig 3
o
6.4
10
30
50
80
1
X
X
x
X
X
X
X
X
X
Vehicle Ident
Fig 4
2
X
X
3
X
X
4
X
X
X
X
X
5
X
X
X
X
X
K km
X
Test Schedule for 80K km Vehicles
The
vehicles
were
all
multi point EFI equipped with HEGO control and full
engine management suitable for 83 U.S. markets. the
Fuel Lead Level
Engine Size
Vehicle Identification
routine
specified
for
the
vehicle
Servicing was carried
plus
out
to
any non scheduled maintenance
required.
DISCUSSION
& RESULTS
Laboratory Pulsator Tests
Increasing residual Pb levels in the fuel from 3 ageing
at
a
maximum
temperature
of
to
10
mg/l
for
pulsator
1000 deg C substantially decreased TWC
performance during pulsator modulation and steady state
conditions.
See Fig 5.
450
% Conversion
Steady State (550°C)
Pulsator (500°C) 14.5 AFR
Simulated Fuel
Mileage Km
mg Pb/l
(OOO'sl
14.3 AFR
14.6 AFR
HC
CO
NOx
HC
CO
NOx
HC
CO
NOx
3
24
63
67
67
95
98
98
66
41
82
10
24
37
33
22
92
95
92
52
45
67
Fig 5
Effect of Fuel Pb levels on Activity of Pulsator-Aged Catalysts
Reprinted with permission c 1985 Society of Automotive Engineers, Inc
Evaluated
at
500
deg C at an air fuel ratio (AFR) of 14.5:1 + 1 A/F at 0.5
Hz the Nox performance was the most affected dropping from 24K
for
3
mg/l to 22% conversion for 10 mg/l.
efficiency and HC was least affected with rate.
Analysis
of
the
retention on the catalyst
a
26%
catalysts
after
surface.
Therefore
67%
conversion
at
CO suffered a 34% decrease in drop
ageing the
to
with
a 3
37%
conversion
mg/l showed no Pb
threshold
for
retention
occurs above 3 mg/1 but is already highly deleterious by 10 mg/l.
Steady
state
conditions
measured
at
550
deg
respectively shows that at stoichiometry the HC,
CO
and
Nox
is
mg/1 aged catalysts.
3%,
C at AFR 14.5:1 and 14.3:1
conversion
efficiency
loss
for
3% and 6% respectively when comparing 10 mg/1 and 3
However at AFR
rich
of
stoichiometry
the
performance
deterioration is very significant for HC and Nox at 14%, and 15% respectively.
% Conversion (550°C) Max Temp C
Fuel
Simulated
mg/l
km (OOO's)
14.6 AFR
14.3 AFR
HC
CO
Nox
HC
CO
Nox
1000
3
24
95
98
98
66
41
82
730
3
24
96
98
98
32
60
69
Fig 6
Steady State Activity for Catalysts Aged with 730
& 1000 deg C Maxima
451 The cycle was modified to include 6% of the time at 25%
at
1000 deg C.
3 mg/l, at 550 deg C instead
of
66%.
at 1000 deg C. with
3
730
deg
C
instead
of
The results are shown in Fig. 6
for the same Pb level of
14.3
HC
AFR
steady
state,
the
conversion
was
32%
The surface area of the catalyst at 730 deg C was twice that
However as stated previously the Pb retention
mg/l was zero.
and 730 deg C is more
at
1000
deg
C
Therefore the poisoning effect of Pb deposition at 500 significant
than
the
loss
of
50% surface
area
to
catalyst efficiency.
Dynamometer Ageing
A
pair
of catalysts, one dynamometer aged to 80K km on 3 mg/l and the other
10 mg/l Pb was tested in turn on a 1.6L Ford Escort with
~n
history.
83
U.S.
tests
were
conducted.
a
known
emission
The results obtained are shown in
Fig 7.
HC
co
km
0.18
1.18
0.11
80K km
0.26
1.85
0.12
3 mg Pb/l
80K km
0.80
4.52
0.26
10 mg Pb/l
Legal Limit
0.32
2.62
0.77
Assumes 1.3 D.F.
o
Fig 7
NOx
Emission Results with Catalyst Dynamometer Aged (Values in grams/mile)
The maximum temperature reached during This
temperature
the
ageing
cycle
was achieved for only 8% of the cycle.
lead
indicates
deposition that
a
was
typical
high. vehicle
This is
accounts able
to
Pb
level
is
significantly
and
Interpolating
between
3
mg/l.
produces
HC
whereas
10
and
figures
CO
deg
C.
area
but
it
also
for the deactivation, but travel
relatively low temperature, driving and still remain the
885
84% of the cycle was
at 475 deg C which was low enough to maintain high surface meant
was
80K
inside
mg/l
deactivates above
km
legal
the
of
urban,
levels
the legal
if
catalyst level.
these points, assuming linear deactivation against lead
level, up to 5 mg/l could
be
tolerated
before
deactivated to remain inside the legal limits.
the
catalyst
would
be
too
452 To
demonstrate
this,
catalysts were tested on the pulsator rig and results
showed that efficiencies had decreased to 50%, 61% and 47% for These
respectively.
results
compare
and broadly substantiate the assumption
HC,CO
and
Nox
with those at 14.5 AFR shown in Fig 5. that
increasing
lead
levels
reduce
catalyst activity linearly in this range.
sequence
test
This
clearly
indicates
that
conformity at zero mile and 80K km with 3mg/l fuel
a
vehicle
that
deteriorates
has
good
significantly
with lOmg/l Pb fuel.
Vehicle Durability
The
vehicles
used
during this stage of testing are shown in Fig 3. and the
emission test schedule undertaken is shown in Fig. 4.
A summary of the 83 U.S. emission test data, and the
corresponding
catalyst
conversion efficiencies is shown in Fig 8.
Vehicle No
,000 km
Emissions (gms/mile) CO Nox HC
% Conversion HC
CO
Remarks Nox
0.32
2.26
0.77
1
0 6.5 50 80 80
0.285 0.509 1.012 1.260 0.748
2.24 4.32 7.66 6.83 4.14
0.26 0.38 0.41 0.56 0.87
86.4 79.9 71.4 68.6 75.1
80.9 67.7 55.6 54.1 66.3
91.3 89.3 86.3 81.8 73.8
Aged Hego 10 mg Pb/l Aged Hego Fresh Hego
2
0 6.5 50
0.248 0.418 0.479
1.07 2.52 3.92
0.61 0.58 0.45
89.4 83.8 83.03
89.3 77.5 71.93
80.7 84.6 84.8
Aged Hego
3
6.5 80 80
0.152 0.607 0.358
1.36 6.00 2.86
0.62 0.70 1.03
89.2 65.0 76.7
88.7 69.4 79.1
85.4 83.6 76.2
Aged Hego 10 mg Pb/1 Fresh Hego
4
0 6.5 50
0.156 0.358 0.675
1.01 2.24 3.32
0.26 0.63 1.18
90.5 78.3 71.8
89.6 82.0 69.0
88.6 84.2 59.7
Aged Hego
0 6.5 50
0.175 0.184 0.216
0.85 1.16 1.47
0.44 0.70 1.37
88.8 89.8 90.0
86.9 80.4
77.0 52.5
Aged Hego
5
Fig 8
Legal level assuming 1.3 D.F
Trace Pb
10 mg Pb/1
Trace Pb
Summary of Emission Results for 80K km Durability Vehicles
453 Vehicles
1
and
2
were
fitted
higher than ideal emission levels whilst
vehicle
with an early, partly developed, hence the at
2 ran trace Pb fuel.
zero
mile.
Vehicle
the two vehicles and the catalyst efficiency throughout the damaged
was
Sufficient distance had been covered to
performance
characteristic.
catalyst
demonstrates
a
significant
loss
which HC and CO conversions were never above vehicle
2
with
trace
mg/l
test.
Vehicle
2
from
demonstrate
the
progressed.
for
72%
The
vehicle
HC and CO by 10K km after
and
65%
respectively.
On
lead however the HC performance remained constant over
50K km with conversions always occur
10
Fig 9 illustrates the large differences
in catalyst efficiencies that developed as the test 1
used
before 80K km had been reached resulting in the 50K km test being
the last data point. catalyst
1
Fig. 8 shows the emission performance of
above
80%.
For
CO
some
deterioration
did
90% at start of test to 72% at completion, but its performance was
superior to the 10 mg/l catalyst.
':r
-------------2
'-.)
J:
1
60
><
c:
100
2;
w
H
'-.)
H (,.. (,..
w 2;
2
0
'-.)
0
6
H [fJ
~ - - - - - - - l
0:: W
> 2;
100
0
'-.)
x
0
80
1
2;
60 0 Fig 9.
10
50
Catalyst Efficiencies for Vehicles 1 and 2
The Nox conversion performance of both catalysts deterioration
factor
generated
that of the 10 mg/l catalyst. km
the
80
was
satisfactory
but
the
by the trace lead catalyst is 32% better than
Although vehicle 2 had to be stopped
after
50K
superior performance of the catalyst at this point relative to vehicle
1 is demonstrated by the HC figures of 0.48 g/m against 1.01 of 3.92 g/m against 7.66 g/m.
g/m
and
the
CO
454 The
catalyst
that
had
been
subjected
to
the
suffered 10% to 15% performance loss due to lead. generated
a
fresh
The results
show
improvement
for
10
After
had
a
7%
conversion
efficiency
data
improvement
CO and a 8% deterioration of Nox.
been
this
had
been
HEGO sensor was fitted to vehicle 1 and the test repeated.
a
for
HC,
a
12%
This indicates that it was
controlling the engine leaner than the 80K aged HEGO. there
mg!l fuel has clearly
Therefore
rich drift, and maximum catalyst
with
ageing
conversion potential was
not being used.
The result of vehicle significantly
inside
3 the
at
6,4K
legal
from 85% to 89% on the three gases. HC
and
km
shows
limit,
the
HC,
CO
and
Nox
levels
with conversion efficiencies ranging
At 80K km the
conversion
efficiency
for
CO had dropped to 65% and 69.4% respectively which results in tailpipe
levels of 0.61 g/m and 6.0 g/m.
Both these are
above
the
legal
level.
Nox
conversion however was retained at 84% giving a 0.7 g/m result.
The
results
sufficient to
from
vehicle
3
show
that
the
to achieve legal levels at 80K km.
vehicle
3
showed
the
same
trend
as
rich
during
ageing.
HC 10% for CO and 7% for Nox.
Fitting
activity is almost
a
fresh
HEGO
sensor
vehicle 1. HC and CO efficiencies
increased whilst Nox efficiencies decreased drifted
catalyst
indicating
the
HEGO
sensor
had
The changes observed for vehicle 3 were 10% for This is of
similar
order
to
the
changes
on
vehicle 1.
Vehicles
4
and
5
were
the
competitor
vehicles
as described in Fig. 3.
Vehicle 4 was fuelled with 10 mg/l, vehicle 5 with trace Pb. two
vehicles
is available to 50K km.
the engine settings were found to be and
so
away
from
specification
emission data generated at 0 mile was discarded.
to specification and retested.
Data
for
these
At the 6,4K km test point for vehicle 5
The subsequent poor
significantly,
The engine was reset
Nox
performance
of
this
vehicle has not been explained but is subject to further investigation.
Fig
10
shows
the
catalyst
efficiencies over 50K km and comparing the two
vehicles for HC and CO only, it can retains
a
constant
deterioration. the
test.
conversion
The
performance
be
seen for
that HC,
the and
Catalyst conversion remained between 10mg/l
efficiency
catalyst and
a
efficiency between 70% and 80%.
13%
however loss
has for
trace only
80%
suffered CO,
and
lead
catalyst
exhibits 90%
a
12%
bringing
the
7%
CO
throughout loss
in HC
conversion
455 The
Nox
conversion
efficiencies
of
deterioration over 50K km, indicating a This
fuel.
deterioration
almost 19/m to 1.18g/m. limits
for
HC
and
The
earlier.
CO
mileage
Consequently
progress.
results
the catalyst on vehicle 4 shows a 29% severe
in
effect
from
50K
is
in
the
be
within
legal
km, but Nox must be disregarded as explained
accumulation data
Pb
the tailpipe Nox levels increasing by
Vehicle 5 emission data shows it to at
the
for
not
vehicles
4
and
5
is
still
in
yet available for the BOK km stage, or
for the fitting of a fresh HEGO sensor.
o><
100
0 ==---0
Z
r::I
90
H
t.l
x-x
~
~
r::I
80
Z 0
70
H
o ~ x oX' x _ ___ :
X
x 5
x
HC CO
4
X
t/.I ~
0
o 5
0_0
H
60
~
0
t.l
50
10
30
50
,000km
HC & CO Catalyst Efficiencies for Vehicles 4 and 5
Fig 10
CONCLUSIONS
The programme described was intended to be wide ranging in
the
simulation
of
service conditions.
The
laboratory
pulsator test simulated mixed urban and autobahn driving.
results indicate that for any vehicle subjected to this to
3
mg/l
will
surface
lead
levels
The up
not cause concern, due to lead being returned to metallic Pb
and removed from the catalyst. catalyst
mix,
reduction
However, 10 mg/l
fuelling
together
with
the
caused by, high temperature excursions will result
in unacceptable catalyst efficiency deterioration.
The dynamometer ageing test results demonstrated that unacceptable.
However,
if
the
effect
of
linear then maximum Pb levels of appro x 5 mg/l can levels
being
fuel
with
10
mg/l
is
lead deposition is assumed to be result
in
legal
emission
achieved at BOK km when catalysts experience a modest duty cycle
as described in this paper.
456 The
vehicle
durability
tests
have
consistently
shown
catalyst deactivation takes place with fuel at 10 mg/l Pb. mg/l km
and of
below
AMA
continually
allow cycle
drives
at
that
substantial levels
of
3
catalyst systems to function satisfactorily during 80K
drive
which low
would
indicate
that
even
if
customer
a
speed (which gives max lead deposition condition)
then catalyst deterioration due to lead will indicate
that Lead
be
The
minimal.
test
results
the effect of lead on the HEGO sensor is more critical than its
effect on the catalyst.
Since the test schedule started the lead levels in at
the
pumps
in
Europe
unleaded
Against expectation lead levels have dropped rapidly to an This
level,
if
fuel
available
(Germany, Switzerland, Austria) has been monitored.
maintained,
will
average
of
2mg/l.
ensure that the effect of lead on catalyst
systems will be negligible up to 80K km.
This test programme has also illustrated that future
to
10
mg/l
or
above,
catalysts
if
and
lead
HEGO
levels
deactivated such that compliance with 83 US legal levels at 80K be
possible.
This
may
arise
do
rise
in
sensor systems would be km
would
not
in a territory that introduces lead free fuel
with less control than has been exercised in
Germany/Switzerland
and
Austria
to date.
REFERENCES
1.
K
Otto,
R
A
Dalla
Betta,
and
H
C
Yao,
"Laboratory Method for the
Simulation of Automobile Exhaust and Studies of Catalyst
Poisoning"
APeA J 1974 24, 596 2.
H
S
Gandhi,
W B Williamson et al "Affinity of Lead for Noble Metals on
Different Supports".
3.
H S Gandhi, A G Piken,
M Shelef,
R
Delosh
"Laboratory
Evaluation
of
Three Way Catalysts" SAE Transactions 1976.
4.
W B
Williamson,
H
S
Gandhi,
M E
Szpilka,
A
Deakin "Durability of
Automotive Catalysts for European Applications". SAE paper 852097.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
457
(D 1987 Elsevier Science Publishers B.Y. Amsterdam - Printed in The Netherlands
A LABORATORY METHOD FOR DETERMINING THE ACTIVITY OF DIESEL PARTICULATE COMBUSTION CATALYSTS
R. E. MARINANGELI, E. H. HOMEIER and F. S. MOLINARO Allied-Signal Engineered Materials Research Center, 50 East Algonquin Road, P.O. Box 5016, Des Plaines, Illinois 60017-5016
ABSTRACT Diesel particulates are a health hazard and legislation has been established (in the U.S.A.) to reduce diesel particulate emissions. Particulate traps have been developed which can filter (up to 90%) of these particulates [Ref. IJ, but require some external means to burn the collected particulates. One way to ignite these particulates effectively at low temperatures is to use traps which initiate soot combustion catalytically. In order to determine the relative activity of various catalytic compositions, a two stage method to collect diesel particulates and accurately determine the activity of the catalytic materials has been developed. In addition to the description of the two-stage method, the activity of selected base metal and noble metal catalysts are compared. The mechanism for soot combustion is also discussed in light of the combustion rates found.
INTRODUCTION Particulate emissions from diesel engines are implicated in health problems (e.g., cancer, respiratory stress, etc.) and contribute to lowered visibility in densely populated urban areas [Ref. 2J. Owing to these factors, the U.S. Environmental Protection Agency plans to implement strict standards aimed at controlling diesel emissions. Since the most severe standards cannot be readily met by engine modifications, work has focused on trapping the particulates. Numerous trap designs have been tested including fibrous filters [Ref. 3J, woven filters [Ref. 4J, metal mesh filters [Ref. 5J, ceramic foam filters [Ref. 6J, and wall-flow monolith filters [Ref. IJ. Of these systems, only ceramic foam and wall-flow monolith filters have shown promise as effective diesel particulate traps.
158
Once the particulates are trapped, the next problem is conversion of these particulates into innocuous substances. Since the combustion temperature of diesel particulates is about 650°C in the absence of catalysts and since the exhaust temperature in diesel passenger vehicles is often no higher than 300450°C, the particulates will not spontaneously ignite. Alternate ways are needed to burn the particulates so that excessive back-pressure due to trap plugging does not occur. Two ways have been proposed to burn the particulates: 1) external means of heat generation to increase the exhaust temperature, and 2) use of catalytic materials which will lower the combustion temperature of the particulates. This paper will focus on the catalytic combustion of diesel particulates. The majority of the work on catalyzed diesel traps has focused on engine or vehicle measurements. However, some work has been done to quantify the catalytic combustion of diesel particulates. For example Otto, et al. [Ref. 7J collected particulates and then burned them in the laboratory. They determined the effect of temperature, oxygen pressure, and step-wise combustion on reactivity. No catalyst was involved in this study. Hillenbrand and Trayser [Ref. 8J took soot collected from an engine, mixed it with metal salts (Cu, Na, Co, and Mn), and burned it in a laboratory reactor. A substantial lowering of the combustion temperature was observed with the use of such salts. McCabe and Sinkevitch [Ref. 9J also looked at mixing base metal additives either with the soot or the fuel and then determined the effect on soot combustion temperature. Finally, Goldenberg, et al. [Ref. 10J looked at soot oxidation either alone or on a catalytic material. Most of the work cited above has dealt with treating the soot in some way before doing the combustion experiments. We wish to report experiments conducted on soot from a diesel vehicle which has been deposited onto catalytic monolithic substrates. This sooted substrate is then placed in a laboratory apparatus where a synthetic gas mixture flows over the sample, and the soot combustion is monitored as a function of temperature. The laboratory set up simulates regeneration conditions on a vehicle. Using this technique we have been able to obtain kinetic information about the oxidation of soot and gaseous products. Comparisons of base metal and noble metal catalysts were also conduct~d and are reported. It is intended that this work will help elucidate the mechanism involved in the catalytic combustion of soot which should help in developing improved catalytic materials.
459
EXPERIMENTAL Catalyst Preparation Catalysts were prepared on Corning EX20 cordierite, open channel monolithic substrates (nominally 62 square channels per square centimeter). High surface area supports were activated with base or noble metal components. The final composition of the fresh catalysts are shown in Table 1, where the metal content is expressed as grams of metal per liter of catalyst (including substrate). The monolithic substrates were cut lengthwise into quarter sections prior to preparation of the catalyst. Once the four catalytic samples were prepared, they were combined to yield a complete monolith by cementing the quartered sec· tions together with Sauereisen Number 8, a ceramic adhesive, as indicated in Figure 1. Deposition of Diesel Soot Once an open channel monolith was reassembled it was sealed into a demountable catalyst holder and placed in the exhaust of a diesel vehicle which was driven over a prescribed cycle on a chassis dynamometer. The vehicle was a 1977 International Harvester diesel Scout equipped with an indirect injected 3.2L, six cylinder engine. Commercial number two diesel fuel was used for all the vehicle experiments. The diesel soot deposition cycle which was used is described in Table 2. The maximum temperature at the inlet of the catalyst was maintained at 288°C (550°F) by adjustment of the load. Generally, 48 hours of soot collection was sufficient to permit evaluation of the catalysts. No catalyst durability experiments will be reported here. However, for some catalysts an accelerated aging was used which involved eight consecutive sootings at an inlet temperature of 370°C for three hours each followed by a regeneration during which the inlet temperature was increased to 650°C for 15 minutes. The regeneration and sootings were performed at constant engine speed and load. Following the accelerated aging, the diesel soot was applied for six hours using the previously described soot deposition cycle. Laboratory Activity Test The soot containing cores were tested for conversion of C3H g, a model hydrocarbon, and the retained carbonaceous soot using an automated laboratory
460
TABLE 1 Composition of Experimental Diesel Catalysts Catalytic Metal Content, Support
Grams/Lite.~r
_
Pt , 0.53 Pt, 0.53 None NS/ A1 20 3 Al 20 3 Al 203
None Pt/Pd/Cu/Cr, 0.53/0.53/3.53/1.77 Pt/Pd, 0.53/0.53 Cu/Cr,3.53/1.77 aNovel support.
TABLE 2 Diesel Soot Deposition Cycle Speed (MPH)
Time (Seconds)
Inlet Temperature (OC)
Idl e Idle-24 24 Cruise
15 14
149 Average 193 Peak 182 Average
11
6
24-20 20-35 35
21 44
171 Minimum 254 Peak 240 Average
7 8 9
35-20 20 20-Idle
17 10 8
177 Minimum 177 149 Minimum
10
Idle Idle-40 40 4O-Idle
10
149 288 Peak 288 149 Minimum
Mode 1 2
3 4 5
11
12 13
13
17 40 20
461
FIGURE I
Cemented
.__----7"-----.. CatalystB
CatalystA
00: 0 0
Cemented -
- - - -
o 0- T -6-0- I
Catalyst0
Schematic Design of the Reassembled Monolith
- Cemented CatalystC
I I
I I
I I
l~j I
I
I
I I
Cemented
FIGURE 2 Experimental Apparatus
METERED GASES SELECTOR VALVE
E
Z400~,
-~---~~
~--
FIGURE 3
· , - - c .-
§:ZZOOf
fJ Zooof
o PtlNS
z 1600f
o
C? 1800f
81400f
~ 1zoof
Effects of Support and Noble Metal on CO 2 Prorluction During Diesel Particulate Oxidation
{:, AIZ030n1y PtlAl203
o NSonly
aJ
l000[ o 800 600
5 ~
400f ~ ZOO
~
o
0l ' o' \ on~~>' --
FIGURE 4 Effects of Support and Noble Metal on CO Production During Diesel Particulate Oxidation
o PtlNS {:, AI2030nly
o PtlAI203
o NSonly
200
300
400
500
600
INLETTEMPERATURE(DEG.C)
700
800
462
test as follows. Pieces of the cylindrical cores (2.22 cm in diameter by 1.27 cm high) were subjected to a temperature programmed oxidation in the apparatus shown schematically in Figure Z. The activity testing equipment has been described previously [Ref. 11], and was modified for these experiments by addition of a low level (0-Z500 ppm) CO Z analyzer. The feed gas composition was selected to simulate a highly oxidizing (lean) diesel exhaust gas except that CO Z was absent. This gas composition is summarized in Table 3. With this simulated exhaust flowing over the catalyst, the temperature at the catalyst inlet position was increased from 120°C to 750°C at 15°C/minute with 15 minute holds at 300, 350, and 400°C. The analysis of the product gas for CO, CO Z' C3HS and 02 permitted determination of propane and soot-carbon (i.e., carbon and adsorbed hydrocarbon) burning rates versus the inlet temperature. The CO 2 formation rate minus the C3HS and CO disappearance rates equals the soot combustion rate. When a temperature of 750°C was reached the catalyst was immediately cooled to 1Z0°C to determine if residual soot remained and to permit comparison of C3HS burning rates without the soot present.
RESULTS Effects of Noble Metal Addition Table 4 shows the average carbon content after soot deposition for monoliths wash coated with A1 Z03 and monoliths wash coated with Pt/A1 Z0 3• The average carbon content after soot deposition is the same for both types of monolith. The Pt/A1 Z03 catalyst does not initiate soot combustion during the standard soot deposition cycle. Figure 3 compares the CO Z production, a measure of soot combustion rate, as a function of temperature for A1 Z03 and Pt/A1 Z03• The Pt/A1 Z03 catalyst not only initiates soot combustion at a lower temperature «300°C) but reaches a maximum rate at a lower temperature (500°C versus 550°C). Since soot oxidation occurs over a wide temperature range, local hot spots may be minimized. Figure 4 shows that no CO is produced during soot oxidation by Pt/A1 Z03, while A1 Z03 produces a substantial amount of CO. Figure 5 shows that the rate of hydrocarbon oxidation, as measured by C3HS conversion, is substantially higher for Pt/A1 Z0 3• We believe that Pt initiates oxidation of the easily combusted, adsorbed hydrocarbons on the particulate (T <300°C). This provides a local exotherm
463
TABLE 3 Simulated Laboratory Exhaust Gas a Component
Concentration 300 ppm
o o o 10% 100 ppm
ppm Balance 50
aDry basis.
Added 10% steam at the reactor.
TABLE 4 Average Carbon Content on Catalysts After Sooting (Estimated Standard Deviation = 0.4%) Catalyst A1 Z03* Pt/A1 Z03* Novel Support* Pt/Novel Support* Pt/Pd/Cu/Cr/A1 Z0 3** Pt/Pd/A1 Z03** Cu/Cr/A1 Z03**
Carbon Content (Weight Percent) 3.5 3.5
Z.6 Z.O Z.6 3.6 5.8
*Six measurements; six hours sooting after eight sootings and regenerations. **Three measurements only; 48 hours sooting.
464
which lights-off oxidation of the relatively less reactive carbonaceous particulate. Pt activates oxygen effectively. This enhances the soot oxidation rate at all temperatures and minimizes formation of partially oxidized particles such as CO. Effects of the Support Material The influence of the support material was determined by examining monoliths coated with a novel support. Table 4 shows that this support was less effective than A1 203 for particulate trapping during the standard sooting cycle. This support material may also catalyze some particulate burning during the soot trapping cycle. The Pt/novel support (NS) catalyst did catalyze particulate oxidation during the sooting cycle as determined by the lower carbon content after sooting. Figure 3 shows that the novel support catalyzes particulate oxidation at a lower temperature than A1 203 and that Pt/NS is more effective at initiating and completing particulate oxidation than Pt/A1 203• Figure 4 shows that Pt/NS is effective at minimizing CO emissions. Figure 5 shows that Pt/NS may be less effective than Pt/A1 203 for C3H8 oxidation. The Pt/ A1 203 results, however, may be influenced by a substantial local exotherm due to the high rate of soot oxidation from 400°C to 500°C. The plot of C3H8 outlet concentration shows a deviation at an inlet temperature of 450°C which may be explained by the temperature rise in the catalyst bed. The support obviously influences the performance of particulate trapoxidizers. First, the novel support is apparently less effective at trapping particulates. This support obviously has some catalytic activity itself as the comparison of the novel support to A1 203 shows. The activity of Pt/NS is still higher than can be explained by the activity of the novel support itself. We can speculate that the support affects the overall activity of the system by influencing the activity of the Pt itself or by influencing the deactivation rate of the catalyst. Effects of Base Metals Both CuO and Cr203 are known to catalyze graphite oxidation [Ref. 12J at relatively low temperatures. Particulate oxidation catalysts were prepared with Cu/Cr/A1 203, Pt/Pd/A1 203 and Pt/Pd/Cu/Cr/A1 203 to determine the effects of these base metals. Table 4 shows that both Pt/Pd/A1 203 and Pt/Pd/Cu/Cr/A1 203 accumulated significantly less carbon than Cu/Cr/A1 203 during sooting indicating that these catalysts are active for particulate oxidation during the sooting cycle. Pt/Pd/Cu/Cr/A1 203 may be more active than Pt/Pd/A1 203 during
465
FIGURE 5 Effects of Support and Noble Metal on C3H g Oxidation During Diesel Particulate Combustion
200
300
400
500
600
700
INLETTEMPERATURE(DEG C.)
FIGURE 6
10000~-~---
'[ Q.
N 9000 8000 7000 ~ 6000 a: 5000
!2:ui
Comparison of Catalysts Containing CulCr for CO 2 Production During Diesel Particulate Oxidation
o PtJPd/Cu/Cr
8 is
6 PtJPd OCu/Cr
4000
o
is
3000 2000
u
~
1000 O~~=c;;:c;;---:;;~-~cc--~~=-=:':::J 200
~
o
FIGURE 7 Comparison of Catalysts Including CulCr for CO Production During Diesel Particulate Oxidation
o PtJPd/Cu/Cr 6 PtJPd OCu/Cr
In
300
400
500
600
700
INLETTEMPERATURE- DEG. C
FIGURE g
E350,---~---,--~--~-~ Q.
g U
Comparison of Catalysts Including CulCr for C3HS Oxidation During Diesel Particulate Oxidation
o PtJPdlCu/Cr
300h-""""~-&o-XD--~
I
6 PtJPd OCu/Cr
Z 250 o ~ 200
a: e-
m150
o
is 100 o tiJ --' o~
50
02 0 S C O - - - - - - c ~ - - - - - . 7 v c - - ~ ~ - ~ o c - - - d 300
400
500
600
INLETTEMPERATURE- DEG. C
700
466
sooting, but this conclusion is not statistically significant at the 90% confidence level. Figure 6 shows that during the laboratory soot oxidation test these two catalysts have the same activity. The only difference between the CO 2 evolution curves is due to the higher carbon content on Pt/Pd/A1 203• The Cu/Cr/A1 203 "lights-off" for particulate oxidation at about 400°C. Figure 7 shows that this catalyst produces a significant amount of CO during particulate oxidation. Figure S shows that the C3HS oxidation rate is enhanced by the exotherm produced by soot oxidation. The local temperature increase is about 100°C. Base metals such as Cu/Cr/A1 203 may initially catalyze soot oxidation, but they are probably rapidly poisoned by sulfate formation. Fishel et al. [Ref. 13J have shown that sulfate poisoning deactivates Cu/Cr/A1 203 base metal automotive catalysts. These sulfates decompose at temperatures of ~ 4 6 0 ° C [Cr2(S04)3J [Ref. 14J to 5S0°C (CuS04) [Ref. 13J thereby reactivating the catalyst. The light-off of the Cu/Cr/A1 203 catalyst at about 400°C in the lab test is consistent with a mechanism involving generation of local hot spots leading to sulfate decomposition and a rapid increase in catalyst activity. Base metal oxides such as CuO and Cr203 are, therefore, not effective for initiating low temperature particulate oxidation compared with noble metals such as Pt or Pd. Base metal catalysts, however, could effectively propagate the soot oxidation reaction after initiation by noble metals if the local catalyst temperature exceeds the sulfate decomposition temperature. The data for carbon content during sooting are consistent with some enhanced performance for Pt/Pd/Cu/Cr/A1 203• This catalyst, however, would be in the less active sulfated state at the beginning of lab testing.
CONCLUSIONS The study which we have conducted shows that the support material has an effect on the soot combustion characteristics of a supported catalyst, with the best support being a novel support. Comparing base metal to noble metal catalysts ~ e have found that certain base metal catalysts (i.e, CuO or Cr203) are poisoned by sulfur, do not promote complete combustion of the soot, and initiate soot combustion at a higher temperature than noble metal catalysts. These results are consistent with a soot combustion mechanism in which the metal (especially the noble metals) initiates oxidation of easily combusted adsorbed hydrocarbons and thereby provides local exotherms which initiate the oxidation of the soot.
467
REFERENCES 1.
N. Higuchi, S. Mochida, and M. Kojima, Society of Automotive Engineers, Paper No. 830073 (1983).
2.
M. P. Walsh, Society of Automotive Engineers, Paper No. 840177 (1984).
3.
J. S. MacDonald and G. L. Vaneman, Society of Automotive Engineers, Paper No. 810956 (1981).
4.
H. F. Sullivan, G. M. Bragg and C. E. Hermance, Society of Automotive Engineers, Paper No. 800338 (1980).
5.
B. E. Enga, M. F. Buchman and I. E. Lichtenstein, Society of Automotive Engineers, Paper No. 820184 (1982).
6.
Y. Watable, K. Irako, T. Miyajimor, T. Yoshimoto and Y. Murakami, Society of Automotive Engineers, Paper No. 830082 (1983).
7.
K. Otto, M. H. Sieg, M. Zinbo and L. Engineers, Paper No. 800336 (1980).
8.
L. J. Hillenbrand and D. A. Trayser, Society of Automotive Engineers, Paper No. 811236 (1981).
9.
R. W. McCabe and R. M. Sinkevitch, Society of Automotive Engineering, Paper No. 860011 (1986).
10.
E. Goldenberg, M. Prigent and J. Caillod, Revue De 1'Institute Francais du PHrole, ~ ( 6 ) , pp, 793-805 (1983).
11.
G. C. .loy , G. R. Lester and F. S. Molinaro, Society of Automotive Engineers, Paper No. 790943.
12.
D. W. McKee, Carbon !, 623 (1970).
13.
N. A. Fishel, R. K. Lee and F. C. Wilhelm, Environmental Science and Technology !(3), 260 (1974).
14.
P. 5. Lowell, K. Schwitzgebel, T. B. Parsons and K. J. Sladek, Ind. Eng. Chern. Process Des Develop. ~ ( 3 ) , 384 (1971).
~artosiewicz,
Society of Automotive
This Page Intentionally Left Blank
469
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control © 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SYNTHESIS OF HIGHER ALCOHOLS ON LOW-TEMPERATURE METHANOL CATALYSTS G. FORNASARI, S. GUSI, T.M.G. LA TORRETTA, F. TRIFIRO' and A. VACCARI Istituto di Tecnologie Chimiche Speciali, Facolta di Chimica Industriale, Viale del Risorgimento 4, 40136 BOLOGNA (Italy).
ABSTRACT The synthesis of higher molecular weight alcohols together with methanol from synthesis gas was investigated using low temperature methanol catalysts obtained from homogeneous precursors with hydrotalcite-like structures. The addition of potassium up to about 0.4% by weight favoured the synthesis of higher molecular weight alcohols, a formation which, however, was observed also with the undoped catalysts, as a function of the catalyst composition and the reduction condition adopted. The catalyst composition influenced also the selectivity in the different alcohols. The formation of higher molecular weight alcohols was favoured by the pressure and low values of the HZ/CO ratio in the gas mixture and of the inlet space velocity, with a range of operating temperature limited to 5Z3-573K. INTRODUCTION Despite the actual low price of oil, the development of new syngas-based processes is one of the objectives of the near future, especially in light of the fact that syngas can be produced from various carbonaceous sources (natural or associated gas, coal, heavy residual, a.s.o.) (1-3). One of these short-term objectives is the production of motor-fuel substitutes from non-petroleum sources. Furthermore, there is a tendency above all in the Western world towards the elimination of lead alkyl additives from gasoline (4). To compensate for the considerable loss in fuel octane rating caused by the elimination of lead, it is possible to modify the refinery process and/or to add non-hydrocarbons having themselves a high octane rating. There is general agreement that methanol blends in gasoline will be a suitable way to improve the octane number.Methanol is facing serious overcapacity and is available at low cost(5-9);however,it needsa cosolvent in qasrl tne.ma i nly CZ-C 6 al cohols,to avoid problems such as phase separation,high
volatility
(lO-lZ).
The higher the molecular weight of the alcohol, the more effective is its
470
influence as a cosolvent. Higher molecular weight alcohols (H.M.A.) can be produced at a high cost from olefins, or in a cheaper way together with methanol from syngas. It was already known that it was possible to produce H.M.A. together with methanol by addition of potassium to the classical Zn/Cr catalysts operating at high temperature and pressure (13). However, the last few years have seen a great development of studies on catalysts operating at low temperature and pressure (14-19), and many processes have been claimed by different companies (2025). The main features arising from the patent and scientific literature are summarized in Table I. TABLE 1 Range of operating conditions for the synthesis of methanol and higher molecular weight alcohols.
CAT~lYST
)/AcKAL I
Cu/Co/CR(~L
TEMPER~TURE
PRESSURE
( K )
( MPA )
493-623
H/CO R ~ T I O
I NlE T
sr~CE
VELOC I TY
RFF F REflCF
( wI )
5-15
0.5-4. a
3000-6000
20
553-603
5.3-7
1.0-2.0
27- laO
21
CU/ZN/~L!~LKAll
523-673
8-15
1.7-3.0
1000-10000
22
CullNICR/AcKAL I
603-703
9-18
0.5- 3. a
3000-15000
23
513-598
10-27
0.7- 3. a
300-5000
2/IA
0.3-1.0
> 3000
211B
Cu/TH/CR/~LKAL
MOS/~LKAL
r
r
523-673 473-593
5-15
ZNICR/K
710
25.3
5.0
20000
13
CU/ZN/K
560
7.5
0.45
2500-5000
16
Cu/TI/N~
620
6.0
2.0
11000
18
CUIlN/~L!K
555
13.0
0.5
3200
33
Cu/b';~L!~lKAll
0.3-1.9
3000-15000
25
In our recent studies, a characterization of title propert i es and of the cat alyt i c behaviour in the low temperature methanol synthesis of Cu:Zn:Me (Me= Al and/or Cr) catalysts have been reported as a function of the composition (26-28). The aim of this paper was to investigate the possible parameters which influence the selectivity of these catalysts towards the synthesis of H.M.A., with a particular emphasis on reaction conditions. Thus we tested catalysts chosen among the
471
most active and selective in the methanol synthesis, focusing our attention on those obtained from homogeneous hydrotalcite-like precursors (26-28). As previously reported, these phases are characterized by the presence of all the cations in positively charged brucite-like layers (29), thus favouring the interactions among the elements. EXPERIMENTAL The precursors with different composition (see below, Table 2) were obtained by coprecipitation from an aqueous solution of the nitrates of the elements with sodium bicarbonate at constant pH and 333K, under continuous stirring. The resulting precipitates were filtered and washed in vacuo until the complete elimination of the nitrates and until the residual amount of sodium, determined with a Mark II EEL photometer, was less than 0.05% (as Na dried at 363K for 12h, calcined at 623K for 24h
The precipitates were 20). and crushed
to a particle size of 0.250-0.420 mm. The catalysts were impregnated with different percentages of potassium (w/w) using solutions of CH and calcined at 3COOK 623K. K-doped alumina was prepared in the same way using a Y-A1 (Akzo-Chemie, 203 2 grade E) with a surface area of 125 m / g, and the absence of surface acid centers was verified by titration (30). XRO powder patterns were collected with Ni-filtered CUK u radiation (A= 0.15418 nm) using a Philips goniometer equipped with stepping motor and automated by means of a General Automation 16/240 computer. The phase compositions and crystal sizes were determined by a profile fitting method, comparing the observed profiles with the computed ones, calculated according to Allegra and Ronca (31). A Carlo Erba Sorptomatic 1826 apparatus with N adsorption was used to 2 measure the surface area and pore volume. The calcined precursors were reduced in the reactor by hydrogen diluted in nitrogen, with the hydrogen content and temperature being progressively increased (14,23,32). The catalytic tests were performed in a copper
plug flow reac-
tor, operating up to 2.0 MPa and 623K, using 0.3-0.5 g of catalyst, different space velocities and reaction gas mixtures. The reaction products were analyzed on-line without condensation using a Carlo Erba 4300 gas chromatograph equipped with FlO and two columns (1/8-in. diam. x 2.0-m long) fitted with 80-120 Poropack OS. After cooling at 263K, the gases were analyzed by a Carlo Erba 4300 gas chromatograph equipped with TCO and two
472
columns (1/8-in. diam. x 2.0-m long) fitted with Carbosieve 100-120. The chromatographic data were collected and processed by a Perkin-Elmer Sigma 15 Data Station. RESULTS AND DISCUSSION In Table 2, the compositions and the characteristic data of the catalysts examined, after both drying at 363K and calcination at 623K, are summarized, while the XRD powder patterns are reported in Figures la and b, respectively. TABLE 2 Catalyst compositions and characteristic data after drying at 363K and calcination at 623K for 24h.
SAnPLE
Co,~pos
I T I ON
ArOMIC RAT ros
SURFACE ARF.A*
SURFACE AREA:t::t:
CAT
1
CU:ZN:CR
CAT
2
Cu,ZN,AL,CR
CAT
3
Cu:ZN:AL
38,0:38,0:24,0 38,0,38,0,12,0,12,0 38,0,38,0:24,0
106
(RYST III
CuO
ZNO
119
6,5
5,0
138
5,0
~.
72
3,0
11,5
(%)
a
SIZE (rm) SPlflEl-lTKE PHASE
3,0 QlJEoi?~
-AlIORPHOUS
2. . !: !~5!
MlORPHOUS
* AFTER DRYING AT 363K FOR 12H, ** AFTER CALCINATION AT 623K FOR 24H,
In all the precipitates only a hydrotalcite-like phase was present, with lower crystal size for the chromium containing compounds. After calcination, a strong increase of the surface area was observed for all the samples. They also showed pore size distribution curves with a narrow peak centered around the most frequently occurring pore radius (28) and low crystal sizes. Role of the potassium concentration and of catalyst composition The relationship between the catalyst characteristics and the amount of potassium added are shown in Figures 2 and 3. It is possible to observe a decrease of surface area by increasing the amount of promoter added, with this effect being more marked for the chromium containing sample. However, the decrease of surface area did not exceed the 40% of the original values even for the highest amounts of potassium examined.
473
b
Cat 2 Cat 2 C")
0 0
Cat3
<0 0 0
10
20
Cat 3
40
50
60
70
70
28-
Fig. 1. XRD powder patterns of the samples after drying at 363K (a) and after calcination at 623K for 24h (b).
"" Cl N......
140
....E (il
.--.--.-.-.------. .-
100
.~
Ql
...
(il
Ql
60
0
.......(il
::J
(J)
20
0
2
0
3
K percentage (w/w)
Fig. 2. Potassium concentration effect on BET surface area for Cat 1 (4t) and Cat 3 (.). The doping gave also rise to an increase of crystal size, particularly in the 0-2% range of potassium added, that was more evident for the aluminum-containing sample (Cat 3) which, without
potassiu~
showed a quasi-amorphous pattern (Fig. 1).
474
Furthermore, as reported in Figure 3, the chromium containing catalysts show the presence of a new phase (marked with an asterisk) that may be identified as a compound, even though its diffraction pattern did not correspond K 2Cr 207-type exactly to that reported for Lopezite (NBS 12-300). Figure 4 reports the catalytic activity for the Cat 1. It is worth noting that H.M.A. were obtained also with the undoped catalyst, even if an increase of activity both in methanol and H.M.A. synthesis was observed by doping with a low amount of potassium (up to 0.4% ca.). These data are in good agreement with those reported in the literature (16,33), taking into account the different ways to express the amount of potassium added. Furthermore, a decrease of the hydrocarbon formation was observed in this range.
10
20
30
40
50 29-
60
70
Fig. 3. XRD powder patterns of Cat 1, undoped and doped with different potassium percentages (w/w). ( . KiCrzOrtype phase )
475 C'l
0
-
x .....
co
U Cl
5
<'J
10
U
::s:::
Cl
.J:
-,
en
...
4
::s:::
~
8
.J: <,
en
Q)
Q)
0
E
3
...>...>
2
U
:::l "0 0
6
->:
i
"-
a.
4 2
e, 0
0 0
0.2 K
0.6
0.4
0.8
0
E
1.0
...>...o>
:::l "0 0 "-
a.
percentage (w/w)
Fig. 4. Productivity in methanol (II), H,M.A. ( ~ ) and hydrocarbons (.-) for Cat 1 as a function of the amount of potassium added (T= 553K; P= 1.5 MPa; H 2; GHSV= 1700 h- l).
2/CO=
.....
.....
:J
I
U
:J
I
I
I
U
Cl <, N
E 125 al
Q)
100
I-
!~
+
'-
al
Q)
c
...
75
al
"-
:::l
en 50 t.. Q)
a. 25 a.
'-/"'. ~
-
<,
Cl <,
-
30
-
24
E al
Q)
'-
t'O
.~-
18
Q)
U
...
al
""-,:: 12 en:::l
""•
"-
- 6
0
U
N
Q)
a. a.
0
0 0
I
I
0.2
0.4
\.
0.6
I
I
0.8
1.0
0
U
K percentage (w/w)
Fig. 5. Copper surface area for Cat 1 as a function of the amount of potassium added: (.-) after reduction; (II) after reaction.
476
By increasing the amount of potassium, we observed a deactivation of the catalyst which was practically complete for percentages higher than 1%. The decrease in the activity is more significant than that of BET surface area, and may be attributed to a specific interaction with the active phase (16,33). Furthermore, its trend is similar to that observed for the copper surface area after both reduction and reaction ( Fig. 5) , even if this parameter did not alone justify the differences observed in the catalytic activity. Worthy of note are the lower values from all the samples after reaction; this fact may be attributed to the surface adsorption of higher molecular weight compounds (34). However, the XRD powder patterns evidence a strong interaction of the potassium with the spinellike phase present after calcination (28,35), as was observed for the Zn/Cr catalysts (34,36). The behaviour of the catalyst containing aluminum was similar (Fig. 6): however, in this case the potassium did not show any activating role on the methanol synthesis and the H.M.A. were not obtained with the undoped catalyst, with this latter characteristic being strictly related to the reduction condition adopted. fraction Furthermore, this catalysts showed a selectivity towards the C 4-alcohol
.......
N
t'il
~
U OJ ~
1.2
J: <,
~.
6
(J)
0
E
0.8
...>-
">...
--. 0.4
o
:::l "0
...0
a.
------. ---------. ~
&--- + ~ &
QJ
---
+---
0.2
0.4
0.6
0.8
K percentage (w/W)
t'il
U
OJ ~
J: <,
4
(J)
QJ
0
E
.'
0 0
....»c...
1.0
2
...>...U>
0
:::l "0
0
...
a.
Fig. 6. Productivity in methanol (.), H.M.A. (.) and hydrocarbons (e) for Cat 3 as a function of the amount of the potassium added (T= 553K; P= 1.5 MPa; H 2; GHSV= 1700 h- 1).
2/CO=
477
(mainly isobutyl alcohol), in agreement with that reported in literature (15,16), which was higher than that observed for the chromium containing catalyst. Cat Z, containing both elements, presented an activity similar to that of Cat 1, while the H.M.A. distribution was more similar to that observed for Cat 3. On the other hand, the tests with K-A1
3 mixtures showed that the acZ0 3/Cat tivity depends only on the amount of methanol catalyst present and not on that of the alumina-supported potassium (Fig. 7), in disagreement with the Morgan et al. hypothesis of an aldolic condensation (37). Effect of the gas mixture composition With the hydrogen-rich feed typical of the recycling loop in an industrial plant for the low temperature methanol synthesis, only methanol was observed. Appreciable productivities of H.M.A. were obtained for HZ/CO ratios
~
Z, with
the maximum for every alcohol progressively displacec towards the lower values of the H /CO ratio when che chain length lncreases. Z
At the same time a li-
near decrease of the methanol productivity was observed. Therefore, by increasing the CO concentration, the relative rate of hydrogenation of the methanol precursor decreases. Furthermore, H.M.A. formation appears to be a slow step in comparison to the rate of,hydrogenation of the methanol precursor on the surface. It is worth noting that at HZ/CO ratios < 1, a strong increase of hydrocarbon formation, mainly methane, was observed especially for the undoped catalysts. On the other hand, it is also necessary to consider the water gas shift reaction (in particular when the synthesis is performed continuously with recycle of the ratio. Therefore a HZ/CO ratio = 2 was employed gas) that increases the H 2/CO in the following tests, in line also with the data reported by some authors (14, 20, 38). Role of the reaction conditions The influence of the reaction conditions (pressure, temperature and inlet space velocity) are illustrated in Figures 8, 9 and 10, respectively. The pressure favours the synthesis of alcohols (methanol and H.M.A.) much more than that of hydrocarbons, irrespectively of reaction temperature. The H.M.A. synthesis was limited to the 523-573K range, with similar trends for all the catalysts and the different potassium percentages being examined. At
478
-
,....
1
m
I
1
I
U Ol ~
2.0 l-
/~
.e <,
(Jl
/:
1.5 I-
Q)
a
E
~
1.0
-
-
•
./e
;- 0.5-
....
-
e
U
:J
0
l::l
aLa.
1/
1
I
I
I
o
0.2
0.4
0.6
0.8
1.0
Cat 3/Cat3+K.AI 203 (w/wJ
Fig. 7. Total productivity for Cat 3 mixed with different amounts of 3%K-A1 203 (4t) and 6%K-A1 (II) (T= 568K; P= 1.5 MPa; H 2; GHSV= 2800 h- l). 203 2/CO=
C'l
52x
I
....
-
012.5 Ol ~
'E.
10.0
f-
aE
7.5
t-'
~
5.0
I-
(Jl
Ql
>
....
g
-
2.5 I-
l::l
a:a
I I I I Ol.--....L--...l-----'----'----'---'
1.4 Pressure
1.6
1.8
2.0
(MPaJ
Fig. 8. Pressure effect on productivity in H.M.A. ( ~ , L 1 ) and hydrocarbons (4t,()) for Cat 1 doped with 0.2% of potassium (temperature: 543K (closed symbols), 563K (open symbols); P= 1.5 MPa; H 2; GHSV= 1700 h- l). 2/CO=
479
-
,...
N
0
u
Ol
\
~
..c 5.00
-.
ril
e/
\
<, (j)
-...
/
">
0 :J "'0 0
1.25
_./
0
"-
~
+
9
~ /'"
>- 2.50
a.
'.~
~.,"-.. '"\
(j)
---
6
E
3
...>. ...0>
0 550
<,
<1l 0
e
530
Ol
..c
,\e
E
U
12
/
\ II
<1l 0 3.75
-
x ,...
ril
590
570
:J "'0 0 "-
Temperature
a.
(K)
Fig. 9. Temperature effect on productivity in methanol (.), H.M.A. (A) and hydrocarbons ("J for Cat 1 doped with 0.2% of potassium (P= 1.5 MPa; H 2; 2/CO= GHSV= 1700 h- l).
,...
-
I
I
I
/-
ril
U Ol
10
f-
8
f-
/
~
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I
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E
/
6
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-
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/.
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./
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-
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4
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,.0
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- 12.5
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velocity (h-1 ) x 10 -3
Fig. 10. Inlet space velocity effect on productivity in methanol (.,[]), H.M.A. (~,L» and hydrocarbons (",()) for Cat 1 doped with 0.2% of potassium (temperature: 543K (closed symbols), 563K (open symbols); P= 1.5 MPa; H 2 ) 2/CO=
480
low
temperatures only methanol was observed (selectivity
~
99.5%), while at
the higher values the methanation reaction became marked, together with a deactivation of the catalyst. It must be noted that, in the temperature range reported above, the methanol synthesis reaction was at the thermodynamic equilibrium, if we take into account the pressure employed (39,40). The decrease of the inlet space velocity, strongly increased the selectivity to H.M.A. while decreasing that in methanol. However, it must be taken into account that in lowering the inlet space velocities an increase of the hydrocarbon formation was also observed, particularly at the highest temperatures examined. Our data confirm the Smith-Anderson pattern for alcohols (15,33) and agree also with Klier (41). On the other hand, it must be noted that both the industrial and the scientific papers (see Table 1) report inlet space velocities lower than about 5000 -1
h
for the system operating at low temperature.
CONCLUSION H.M.A. together with methanol was obtained with low temperature methanol catalysts, without and with the addition of potassium. In this latter case the productivity in H.M.A. increased up to about 0.4% of the added potassium, after which a deactivation was observed with a trend similar to that observed for the copper surface area. It is noteworthy that all the catalysts showed lowest values after reaction, attributable to the presence of high molecular weight compounds adsorbed on the surface. In all cases the deactivation must be attributed to an interaction of the potassium with the active phase. The reaction parameter played an important role: the H.M.A. synthesis was favoured by low HZ/CO ratios in the feed and low inlet space velocities, with a limited range of operating temperature and a positive effect from pressure increase. The reported data are in agreement with the mechanism of H.M.A. formation reported in the literature (15,16,19) and can be understood on the basis of a slow initial growth step and a rate of growth favoured by the increase of the CO partial pressure.
481
ACKNOWLEDGMENT The authors wish to thank SNAMPROGETTI (S. Donato Milanese, Italy) for financial and scientific support. REFERENCES 1 B.M. Harney and G.A. Mills, Hydroc. Process., 59, February 1980, 67-71. 2 A. Aquila, J.S. Alder, D.N. Freeman and R.J.H. Voorhoeve, Hydroc. Process., 62, March 1983, 57-65. 3 J. Haggin, Chem. Eng. News, June 1982, 31-32. 4 Anonymous, Chemical Business, February 1985, 42-46. 5 W.H. Calkins, Cata1. Rev.-Sci. Eng., 26 (1984) 347-358. 6 A.B. Stiles, AIChE J., 23 (1977) 362-375. 7 S. Stre1zoff, Chem. Eng. Progr., Symp. Ser., 66 (1970) 54-68. 8 M.J. Royal and N.M. Nimmo, Hydroc. Process., 48, March 1969, 147-153. 9 R. Nageswaran and D. Amstrong, AIChE 1986 Spring National Meeting, Fuels and Petro1chemica1 Division, New Orleans, U.S.A., April 6-10, 1986, paper 29A. 10 F. Anci10tti, P.P. Garibaldi, N. Passarini, G. Pecci and M. Sposini, Chim. Ind. (Milan), 60 (1978) 931-936. 11 J.L. Keller, Hydroc. Process., 58, May 1979, 127-138. 12 J.A. Valencia-Chavez and R.G. Donnelly, AIChE Symp. Ser., 73 (1977) 312-318. 13 G. Natta, V. Colombo and I. Pasquon, in P.H. Emmet (Ed.), Catalysis, Vol. V, Reinhold Pub1. Corp., New York, 1957, Ch. 3. 14 P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Cata1., 17 (1982) 241-254. 15 K.J. Smith and R.B. Anderson, J. Catal., 85 (1984) 428-436. 16 G.A. Vedage, P.B. Himelfarb, G.W. Simmons and K. Klier, in R.K. Grassel1i and J.F. Brazdil (Eds.), Solid State Chemistry in Catalysis, American Chemical Society, Washington, 1985, pp. 295-312. 17 C.E. Hofstadt, M. Schneider, O. Bock. and K. Koch10efl, in G. Poncelet, P. Grange and P.A. Jacobs (Eds.), Preparation of Catalysts III, Elsevier, Amsterdam, 1983, pp. 709-718. 18 S. Uchiyama, Y. Obayashi, M. Shibata, T. Uchiyama, N. Kewata and T. Konishi, J. Chem. Soc., Chem. Commun., (1985) 1071-1072. 19 T.J. Mazanec, J. Catal., 98 (1986) 115-125. 20 A. Sugier and E. Freund (I.F.P.), French Pat. 33046 (1976). 21 H.F. Hardman and R.I. Beach (Standard Oil), European Pat. 005492 (1979). 22 C.E. Hofstadt, K. Kochloef1 and O. Bock (SUd-Chemie), European Pat. 0034338 ( 1981). 23 V. Fattore, B. Notari, A. Paggini and V. Lagana (Snamprogetti), Italian Pat. 25390 (1981). 24 G. Quarderer and G.A. Cochran (Dow Chemical), European Pat. 0119609 (1984). (b) N.E. Kinkade (Union Carbide), European Pat. 0149255 (1985); C.A. 104 (1986) 151250y. 25 G. Cornelius, W. Hilsebein, P. Konig, F. Moller and M. Supp (Meta11gesel1schaft), European Pat. 0152648 (1985). 26 P. Gherardi, O. Ruggeri, F. Trifiro, A. Vaccari, G. Del Piero, G. Manara and B. Notari, in G. Poncelet, P. Grange and P.A. Jacobs (Eds.), Preparation of Catalysts III, Elsevier, Amsterdam, 1983, pp. 723-731. 27 S. Gusi, F. Trifiro, A. Vaccari and G. Del Piero, J. Catal., 94 (1985) 120-127.
482
28 S. Gusi, F. Pizzoli, F. Trifir6, A. Vaccari and G. Del Piero, IV Intern. Symp. Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-laNeuve (Belgium), September 1-4 1986. 29 C. Busetto, G. Del Piero, G. Manara, F. Trifir6 and A. Vaccari, J. Catal., 85 (1984) 260-266. 30 M. Ballkova and L. Beranek, Collect. Czech. Chem. Commun., 40 (1975) 3108-3113. 31 G. Allegra and G. Ronca, Acta Crystallogr., A34 (1978) 1006-1013. 32 P. Courty, D. Durand, A. Sugier and E. Freund (I.F.P.), U.K. Pat. 2118061A (1983). 33 K.J. Smith and R.B. Anderson, Can. J. Chem. Eng., 61 (1983) 40-45. 34 M. Di Conca, A. Riva, F. Trifir6, A. Vaccari, G. Del Piero, V. Fattore and F. Pincolini, Proceedings 8th Intern. Congress on Catalysis, Vol. II, DECHEMA, Frankfurt am Main, 1984, pp. 173-183. 35 S. Gusi, F. Trifir6 and A. Vaccari, Reactivity of Solids, in press. 36 A. Riva, F. Trifir6, A. Vaccari, G. Busca, L. Mintchev, D. Sanfilippo and W. Manzatti, Faraday Symposium 21 Promotion in Heterogeneous Catalysis, Bath (U.K.), September 23-25 1986. 37 G.T. Morgan, D.V.N. Hardy and R.H. Procter, J. Soc. Chem. Ind., 51 (1932) 1-7T. 38 M.E. Frank and A. Hernandez-Robinson, AIChE 1986 Spring National Meeting, Fuels and Petrolchemical Division, New Drleans (USA), April 6 - 1 ~ 1986. 39 R.H. Ewell, Ind. Eng. Chern., 32 (1940) 149-152. 40 W.J. Thomas and S. Portalski, Ind. Eng. Chern., 50 (1958) 967-970. 41 K. Klier, in S. Kaliaguine and A. Mahay (Eds.), Catalysis on the Energy Scene, Elsevier, Amsterdam, 1984, pp. 439-454.
A. Crucq and A. Frennet (Editors), Catalysis and AutomotivePollution Control
483
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
AN ALKENE ISOMERIZATION CATALYST FOR MOTOR FUEL SYNTHESIS B.G. BAKER and N.J. CLARK School of Physical Sciences, Flinders University, South Australia 5042 SUMMARY A method for the preparation, conditioning and operation of a catalyst containing tungsten oxide is described. The active catalyst contains tungsten in an intermediate valence state maintained by the alkene in a carrier gas containing hydrogen. Results for the isomerization of 1-butene, 1-pentene and 1-hexene show that the ratio of branched to straight chain alkenes in the product approach the equilibrium ratios, that double bond shift and single chain branching adjacent to the double bond are the major reactions and that hydrogenation of the alkene is negligible. The application of the method for the synthesis of MTBE, TAME and other octane improvers for gasoline is proposed. An application of the method to branch alkenes synthesised by a selective Fischer-Tropsch catalyst is demonstrated. INTRODUCTI ON The widespread introduction of unleaded gasoline to fuel engines fitted with catalytic converters has generated problems for the petroleum refining industry. Octane ratings and octane distribution requirements now have to be met by high octane blend stocks of appropriate boiling range. Generally these are not available in sufficient quantity without major refinery changes and the introduction of new processes. The light alkenes (propene, butene and pentene) are important feedstocks for alkylation, oligomerization and the synthesis of ethers (refs. 1,2). MTBE (methyl tert-butyl ether) and TAME (tert-amyl methyl ether) have research octane numbers of 118 and 112 respectively. These premium blend stocks are synthesised by reaction of methanol with isobutene or isopentene (refs. 3,4). The reaction with methanol is selective towards the branched alkenes so that a mixture may be treated and the straight chain alkenes recovered for other processing such as alkylation. The supply of branched alkenes from cracking processes is limited and there is a need for catalytic isomerization processes to increase the supply. One method is to isomerize butane and then to dehydrogenate the isobutane (ref. 5). An alternative strategy is to catalyse the skeletal isomerization of an alkene without destroying the double bond. The present work describes the procedure for preparing, conditioning and operating a catalyst containing tungsten oxide to achieve this objective. Other catalysts containing W0 3 have been shown to have activity in double bond shift and in metathesis (ref. 6,7,8). The conditions for skeletal isomerization developed in the present work do not favour the metathesis reaction and it makes no apparent contribution to the product. However double bond shift does accompany skeletal isomerization.
484
CATALYST PREPARATION The active constituent of the catalyst is an oxide of tungsten prepared by partial reduction of W0 3. Unsupported W0 3 was found to have activity but better specific activity is obtained by depositing the oxide on a support. A number of grades of alumina and silica were tested. Gamma alumina was found to react with W0 3 at 400°C to a considerable extent to form aluminium tungstate. This was identified by XPS and tests on A1 2(W04)3 showed it to be inactive as a catalyst for the isomerization reaction. A heat treated y-alumina, partially converted to a-alumina by heating to 1200°C for 30 min. and having a surface area of -20 m2g-1 was preferred. The preparation and structure of this support is described elsewhere (ref. 9). This HT-alumina is relatively unreactive to W0 3 and satisfactory catalysts have been prepared with loadings of 1% by weight. Better catalyst life is achieved with 6% W0 3/HT-alumina and the tests described here were made on such catalysts. A catalyst was prepared by impregnating a sample of alumina with a solution of sodium tungstate and mixing with the aid of ultrasonic agitation. The sample was dried in a vacuum dessicator (-4 hour) and in air at 90°C (1 hour). Concentrated nitric acid (10 mL) was added and the beaker warmed on a hot plate for 5 minutes and the solid was then washed with -1M nitric acid by decantation. To remove sodium nitrate, 1M nitric acid (250 mL) was added, the catalyst digested for 1 hour on a hot plate and the nitric acid decanted. This washing was repeated three times and the catalyst dried at 150-160°C. CATALYST CONDITIONING AND TESTING The activity for skeletal isomerization of alkenes was only achieved after a partial reduction of the yellow, W0 component of the catalyst. This reduction 3, was made by flowing a mixture of hydrogen and water vapour in a ratio about 40:1 over the catalyst at -400°C for 16 hours. Under these conditions W0 3 (yellow) is partially reduced to a dark blue oxide, suggesting that W (blue) and 20058 W18049 are formed. The reactivity of the catalyst was tested by flowing alkene vapour in a suitable carrier gas at temperatures 260-360°C. Product was sampled by a gas sampling valve and analysed by gas chromatography. The column, n-octane on porosil C, was operated at 42°C to analyse butenes and at 92°C for the pentenes. Product distributions are reported in weight percent. The following reaction parameters apply to the tests unless otherwise specified. Catalyst composition 6% W03/HT-alumina Catalyst sample Ig 10 mL min- l (i.e. 600 hr- l) Flow Reaction pressure 1 atmos.
485
-1
I-butene 25 mg hr -1 I-pentene 25 mg hr Jater content of reactant stream -0.025 atmos. ~drocarbon
reactant
The composition of the reactant gas stream needed to optimize branching lctivity was determined as follows. The catalyst were preconditioned in wet lydrogen for 16 hours at 380·C. These samples were then cooled to operating temperature and hydrocarbon admitted along with a variety of different carrier Jases. The results for I-butene and I-pentene are in figures 1 and 2. Isomerization for I-pentene is best with a H2 carrier stream. For I-butene HZ/H 20 promotes the longest useful isomerization life. It appears that in order to sustain isomerization the hydrocarbon must be accompanied by some hydrogen as running in argon resulted in rapid deactivation for both I-pentene and I-butene isomerization. The product distributions (tables 1 and Z) show that double bond shift and single chain branching adjacent to the double bond are the major reactions. The maximum yields are limited mainly by equilibrium rather than kinetic factors. Irrespective of the carrier gas the catalyst initially displays a wider product spectrum including some cracked and hydrogenated hydrocarbons. However with time on stream these reactions diminish. The product distribution for I-pentene under the various carrier gases after extended reaction times are in table 3. In all cases hydrogenated product is minor. Throughout these tests no substantial occurrence of metathesis was observed. Only minor disproportionation activity is evident upon initial contact of hydrocarbon with the catalyst and this behaviour is short lived. Other preconditioning treatments were investigated. Samples of catalysts were heated at 380'C for 16 hours in high purity dry HZ (10 mL min-I). After treatment the catalysts were cooled to operating temperature and isomerization activity for both I-pentene and I-butene investigated. The results in figures 3 and 4 are compared with Hz/HzO treatments. It appears that I-pentene is more sensitive than I-butene to this treatment and low isomerization activity r e s u l t ~ However hydrogenated product represents only Z.6% of the product after 75 min even though the catalyst has been exposed to quite severe reduction treatment. The effects of argon and pentene on the catalyst in the absence of hydrogen were tested. The yellow catalyst containing W0 3 treated in a glass reactor in argon at 450'C and cooled to 300·C. No visible change in colour or morphology of the catalyst was noticed during this preconditioning. However immediately I-pentene vapour came in contact with the catalyst a colour change to grey occurred indicating some reduction. Reaction in the absence of hydrogen was investigated. Two fresh samples of catalyst were treated in the following ways then tested for I-pentene reaction
486
TABLE 1
TABLE 2
Reaction of I-pentene/H 2/H 20 (300°C) at stated reaction times Product distribution (%) 10 mins 75 mins propene 2-methyl propane I-butene 2-methyl propene 2-methyl butane pentane 2-methyl I-butene I-pentene 2-pentene 2-methyl 2-butene
0.7 0.5 trace 9.6 8.7 trace 1.3 1.4 31. 4 46.3
Reaction of I-butene/H
2/H 20
3.7 1.2 1.6 1.9 37.1 54.5
(360°C) at stated reaction times
Product distribution (%) 10 mins 75 mins propene 2-methyl propane butane I-butene 2-butene 2-methyl propene
45.-------------,
11. 8 5.8 trace 8.6 37.6 36.1
4.2 1.6 0.4
12.1 45.2 36.4
70.--------------------,
-o
-5c'"
25
o
as 20
15 L -_ _-'----_ _--:-_ _-'------'
o
1
2
Time (hours)
Fig. 1. Catalytic isomerization of I-butene at 360°C in various carrier gases. Catalyst preconditioned in H2/H20 at 380aC
4
Time
Fig. 2. Catalytic isomerization of I-pentene at 380°C in various carrier gases. Catalyst 380°C.
487
TABLE 3
Reaction of I-pentene at 300°C after extended operation in various carrier gases Product distribution ( ) HZ/HZO (Z3 hr) HZ (Z3 hr) Argon (4 hr)
Z-methyl propene Z-methyl butane pentane Z-methyl I-butene I-pentene Z-pentene Z-methyl Z-butene
0.3 O.Z
0.4
1.7 3.6 46.6 47.6
0.6 1.4 38.0 57.4
Z.1
70.------------,
50,--------------,
•
65
~ ~\ . 45
~
40
~ 0....
o -o ::.. 35
0.7 4.6 54.4 40.3
.~
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u
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OJ 25
45
l-
o
I
I
1
2
I
Time (hours)
Fig. 3. Isomerization of I-butene/H? at 360°C. Catalyst preconditioned.at 380°C in HZ only, • ; and ln HZ/HZO, o.
e_e_
o
4
(hours I
Fig. 4. Isomerization of I-pentene/H at 300°C. Catalyst preconditio~ed at 380°C in HZ only, • ; and in HZ/HZO, o.
in an argon carrier at 300°C. Low temperature treatment; heated at 300°C in argon (5 min) then I-pentene/argon admitted. High temperature treatment; heated at 450°C in argon, (10 min) cooled to 300°C in argon then I-pentene/argon admitted. The results in table 4 show the initial distributions. In both catalysts activity fell rapidly with time. It is important to note that neither catalyst has been exposed to HZ or HZO in its pretreatment or during the reaction test. The alternative procedure of conditioning in argon then running in HZ/HZO was investigated. In this experiment I-butene was used as the test hydrocarbon and the following pretreatment undertaken on fresh sample of catalyst. Low temperature; air at room temperature then argon at 360°C then I-butene/H Z/H ZO admitted. High temperature; argon heated to 450°C, 15 mins in air at 450°C then
488
cooling to 360°C in argon then I-butene/H admitted. Results are in table Z/H ZO 5. After low temperature treatment the major activity is double bond shift, while after high temperature treatment hydrogenation activity predominates. The effect of the temperature of preconditioning was investigated to test whether shorter times at higher temperatures are effective. Fresh catalyst was heated in HZ/HZO to 450°C for 30 mins and cooled to 300°C then tested for reaction of I-pentene in HZ/HZO at 300°C. The results (table 6) are compared with the previous results obtained after catalyst conditioning at 380°C for Z8 hours. This high temperature activation treatment results initially in rather more disproportionation and hydrogenation. Catalyst life and ultimate product distributions were not adversely affected. Exposure of a conditioned catalyst to air was found to be detrimental, particularly for the butene isomerization. Fresh catalyst was heated in a glass tube at 400°C for 4 hours in HZ/HZO. Upon cooling the catalyst was transferred in air to a reactor tube, heated under HZ/HZO to 360°C and I-butene admitted. Results (table 7) indicate that decreased skeletal isomer is formed and that increased hydrogenation occurs. ISOMERIZATION OF HIGHER ALKENES Reaction of I-hexene (table 8) occurs at lower temperatures and yields higher ratio of branched than pentene. Very high activity for the isomerization of I-hexene was observed at higher temperatures. At 400°C the loading of hexene was increased to 670 mg/g of catalyst with conversion to 50% branched product. With higher molecular weight alkenes a competitive reaction occurs which becomes more dominant as the molecular weight of the alkene increases. This reaction involves cracking the alkene to produce mainly propene, Z-methylpropene or Z-methyl 2-butene. Table 9 shows the product distribution, by carbon number, from the cracking of l-octene: greater than 95% of the products were branched. At Z80°C only 35% of the octene was cracked, mainly to Z-methylpropene and propene. All of the alkenes produced by cracking show very high branched/ straight chain ratios e.g. Z-methylpropene/Z-butene = 4.4 and Z-methyl Z-butene/ Z-pentene = 2.0. At Z80°C the l-octene which was not cracked was highly isomerized but identification of the isomers was not made. When I-dodecene was passed over the catalyst at 300°C the product distribution shown in Fig. 5 was obtained. Within anyone carbon number the ratio of branched chain/straight chain molecules was very high, being about 4:1 for C 4's and 3:1 for C5's. The lifetime of catalysts was substantially reduced by cracking but as the cracking activity decreased the ability of the catalyst to skeletally isomerize without cracking became apparent. Thus, after 21 hours cracking of I-dodecene at 300°C the products from the catalyst consisted almost entirely of branched (but unidentified) dodecenes.
489
TABLE 4
Effect of preconditioning in argon; reaction of 1-pentene/argon at 300°C Product distribution (%) preconditioned preconditioned 450°C, 10 min 300°C, 5 min 2-methyl propene 2-methyl butane pentane 2-methyl I-butene 1-pentene 2-pentene 2-methyl 2-butene
TABLE 5
1.9 4.5 39.7 50.3
10.0 77 .0
13.0
Effect of preconditioning in air/argon; reaction of 1-butene/H 2/H20 at 360°C Product distribution (%) preconditioned preconditioned air at 25°C air at 450°C then Ar at 360°C then Ar at 360°C propene 2-methyl propane butane I-butene 2-butene 2-methyl propene
TABLE 6
2.3 1.3
0.35 0.35 16.8 70.5 11. 9
1.3 0.3 42.0 6.4 28.8 22.0
Effect of precondition conditions with H 2/H 20; reaction of 1-pentene/H 2/H 20 Product distribution (%) preconditioned preconditioned 450°C, 30 min. 380°C, 28 hr. 2-methyl propene 2-methyl butane pentane 2-methyl I-butene 1-pentene 2-pentene 2-methyl 2-butene
6.8 3.9 3.1 1.6 1.6 30.4 52.4
1.6 1.9 37.1 54.5
Total branched isomers
64.7
61.0
3.7 1.2
ISOMERIZATION OF FISCHER-TROPSCH PRODUCT The conditions for operating the tungsten isomerization catalyst are compatible with the composition of the exit streGm from a Fischer-Tropsch reactor. The presence of unreacted hydrogen and water vapour together with CO and CO 2 provides an effective oxygen partial pressure equivalent to that required by the isomerization catalyst.
490
Effect of exposure of conditioned catalyst to air; reaction of 1-butene/H 2/H 20 at 360°C Product distribution ( ) catalyst reduced catalyst reduced then exposed to air on line
TABLE 7
6.1 1.4 19.7 8.1 32.7 31.9
propene 2-methyl propane butane I-butene 2-butene 2-methyl propene
6.8 2.6 0.3 10.0 43.7 36.6
Isomerization of 1-hexene/H /H20, Catalyst preconditioned H 2 / ~ 2 0 at 380°C Product distribution (%) 250°C 320°C
TABLE 8
2-methyl pentane hexane 2-methyl 1-pentene 1-hexene 2-hexene 3-methyl 2-pentene 2,3-dimethyl 2-butene
TABLE 9
3.3 2.3 1.6 7.5 20.6 58.3 6.3
5.6 3.6 2.0 8.2 15.7 58.2 6.8
Products from the cracking and isomerization of at 300°C 1-octene/H 2/H 20 Products numbers C1
C2
C3 4.1
Weight %
C4 39.8
C5 8.0
C6 3.6
C7 2.9
25
-
20 ftZ
W
u 15 0::
w
0..
t:I:
-
10 f-
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W ~
5
o
2
~
3
4
5
PRODUCT
FIG. 5.
6
7
CARBON
8
9
10
11
12
NUMBER
Product distribution for reaction of 1-octene/H Z/HZO on 6% WOx/HT-alumina, 300°C.
C8 41.6
C9
491
o Product 25
from F-T catalyst
[1i:l Straight chain} Products from •
Branched
WOx catalyst
!z 20 LlJ '-'
a:
~ '5
5
23456 7 PRODUCT CARBON
FIG. 6.
"
12
Product distribution from an alkene selective Fischer-Tropsch catalyst before and after passage over 6% WOx/HT-alumina isomerization catalyst.
A Fischer-Tropsch catalyst with high selectivity to alkenes has been developed (ref. 9). Product from this reaction was passed over the 6% W0 3/HT-alumina catalyst contained in a separate reactor tube and preconditioned in H2/H The 20. F-T product before and after the isomerization catalyst shown in fig. 6. Alkenes above C6 were cracked to branched alkenes and C4-C 6 alkenes were branched. The resulting ratios of branched/straight chain alkenes were close to equilibrium values. Over a period of Z hours the tungsten catalyst lost its branching activity and produced mainly straight chain Z-alkenes. However it was regenerated when treated with air at 450°C for 5 minutes and resumed its initial activity. The apparatus was later modified so that the products from reactor 1 passed through a cooling coil to trap hydrocarbons greater than CS' The remaining products were passed over the tungsten oxide catalyst in reactor Z. The product distribution was that which would be expected from isomerization alone, with little cracking, and the lifetime of the catalyst was much greater. CATAL1ST REGENERATION The specific conditions for catalyst activity depended in the al kene and on the operating temperature. Diminished activity was observed after various reaction times (see figs. I,Z). Treatments to regenerate isomerization activity were investigated. These involved oxidation followed by reduction. The reaction of I-pentene/H Z/H 20 at 300°C was followed after each of the following treatments on catalysts which had lost activity. A. Heat 450°C in air, 15 min; cool in argon B. Heat 450°C in air, 15 min; cool in HZ/HZO
492
C. D.
Heat 380°C in air, 60 min; cool in argon Heat 380°C in air, 60 min; cool in HZ/HZO The results in table 10 refer to initial catalytic activity 10 mins after addition of the hydrocarbon to the stream. The trend in all cases is for hydrogenation activity to subside with time. The final column contains data from an optimally conditioned catalyst for comparison. It is concluded that in order to minimize pentane production and maximize branched product the catalyst should be exposed to HZ/HZO prior to admission of 1-pentene. at 360°C was tested after the following Reaction of 1-butene/H Z/H ZO regenerations. (a) Increase temperature to 450°C; 15 mins air; decrease to 380°C in argon then Z8 hours in HZ/HZO. (b) Increase temperature to 450°C; 15 mins air; decrease to 360°C in argon and immediately introduce 1-butene/H Z/H ZO. (c) Increase temperature to 450°C; 15 mins air then 30 mins HZ/HZO at 450°C prior to cooling 360°C. The results are shown in figure 7. In case (b) significant quantitites of butane are present in the product stream. It appears that the most effective regeneration is (c) as it is quick and returns the catalyst to excellent isomerization activity. This activity is prolonged as 33% total branched product was observed after ZZ hours. The data in figures 1 and Z indicate that the effective catalyst lifetime for I-butene isomerization is ~ u b s t a n t i a l l y shorter than that of I-pentene. Experiments were designed to investigate whether a catalyst inactive for I-butene isomerization could effectively isomerize 1-pentene and also to determine the level of I-butene isomerization for a catalyst that has been
~ 5 r-
·,,"0
-
Fiq.7. Reaction of at 360°C. I-butene/H z/HzO Comparison of three regeneration procedures (see text).
o~
;l.
- ~o
r-
lc~·-
. ' \ . .___________
~
\
o
101
0-
0_
5. 35 _ _0 -
"0
0"'1 bl
1
g 30-/
(IJ
25
o
I
I
I
I
2
3
Time afler regeneration
I hr s I
493
TABLE 10
Reaction of I-pentene/H Z/H ZO at 300°C on regenerated catalyst Treatment as in text A B C D Fresh catalyst conditioned Hz/HZO 380°C
ZZ 1.0 1.9 3Z.7 4Z.0
3.1 11.6 9.Z 10.Z 0.9 1.Z Z4.0 39.8
1.0 0.3 15. Z 1.3 3.3 38.0 40.9
3.0 0.8 lZ.8 1.9 Z.1 30.8 48.7
1.3 1.4 31.4 46.3
43.3
61.5
43.5
54.4
65.9
0.3
1.Z 9.6 8.7
isomerizing I-pentene efficiently for an extended period. at 360°C for -4 In the first instance a catalyst running in I-butene/H Z/H ZO hrs and giving <30% branched product was exposed to I-pentene/H at 360°C. Z/H ZO The resultant branching of I-pentene was excellent -63%. Other runs in which the reaction temperature was lowered to 300°C prior to admission of I-pentene agreed with the above result. It is concluded that a catalyst inactive for I-butene isomerization may still have activity for I-pentene isomerization. In the second case, a catalyst that had been effectively isomerizing I-pentene at 300°C in HZ for -Z3 hours was heated to 360°C and I-butene/HZ admitted. The level of I-butene isomerization to branched product under these circumstances was very poor <16%. DISCUSSIONS AND CONCLUSIONS The activity for skeletal isomerization exhibited by these tungsten catalysts is developed only under specific conditions of treatment and operation. The source of tungsten oxide and the method of support are not critical. The HTalumina favoured in these tests had the advantage of good dispersion of W0 3 and limited loss by reaction with the support to form aluminium tungstate. To summarize the best conditions for isomerization activity, it must be noted that the differences between pentene and butene is also a difference in reaction temperature; probably the determining parameter. For I-pentene, 300°C; best carrier gas hydrogen. For I-butene, 360°C; best carrier gas HZ/HZO in the ratio 40:1. The favoured preconditioning treatment of the catalyst is 380°C in HZ/HZO for 16 hours. Also 450°C in HZ/HZO 30 min is effective but the initial product distribution is not optimum. The best regeneration technique is to heat to 450°C in air for 15 min then to convert to HZ/HZO before cooling and admitting the alkene.
494
The skeletal isomerization was associated with double bond shift l-ene to 2-ene. The equilibrium favours the 2-ene at these reaction temperatures so that it is not obvious that a shift of the double bond is an essential step in the branching reaction. The 2-enes undergo the same branching reaction. The nature of the active catalyst is not easily defined. The conditions employed, the blue colour and XPS analysis all indicate the presence of mixed valence states of tungsten, wax where 2.65< x >2.95 approximately. Within this range W20058 and W18049 have defined structures (fig. S). The structure W0 20058 (light blue) has W(V) at sites along the shear planes: WlS049 (dark bluepurple) has more W(V) ions. It is possible that these are reaction sites. Excessive reduction, particularly after oxidation treatments favours hydrogenation probably due to the production of zero valent tungsten. The oxidized catalyst on the other hand lacks skeletal isomerization activity but shifts the double bond. This is a reaction characteristic of acidic catalysts. The role of water in the hydrogen carrier gas is likely to be that of establishing a controlled oxygen partial pressure to prevent excessive reduction. It might also produce a surface concentration of OH groups which participate in the reaction. The practical application of a skeletal isomerization catalyst for alkenes are numerous. There is an increasing interest in conventional petroleum refining in optimizing the use of light alkenes both to increase liquid yields and at the same time improve octane quality.
a FIG. 8.
Structures of (a) W20058 and (b) W18049 projected on to the (010) plane.
495
A particularly favourable application would be in the synthesis of MTBE and TAME. The selective reaction of methanol with the branched alkene would enable the straight chain alkenes to be recycled through the isomerization catalyst. Since the methanol for such a process would likely be synthesised from CO and HZ it would be possible to run this process in parallel with an alkene selective Fischer-Tropsch process to achieve a self contained conversion of CO and H to a 2 high octane fuel blend stock. The isomerization catalyst described here is the subject of patent applicatlon (ref. 10). ACKNOWLEDGEMENTS This work was supported by the Australian National Energy Research Development and Demonstration Council. The authors wish to thank H. McArthur for assistance with the experiments. REFERENCES 1. V.E. Pierce and A.K. Logwinuk, Hydrocarbon Processing, 64 Sept (1985) 75-79. R.M. Heck, R.G. McClung, M.P. Witt and O. Webb, ibid, 59, April (1980) 2. 185-191. 3. J.D. Chase and B.B. Galvez, ibid, 60, March (1981) 89-94. 4. L.S. Bitar, E.A. Hazbun and W.J. Piel, ibid, 63, Oct. (1984) 63-66. G.R. Muddaris and M.J. Pettman, ibid, 59, Oct. (1980) 91-95. 5. 6. F.P.J.M. Kerkhof, R. Thomas and J.A. Moulijn, Rec.Trav.Chim.Pay-Bas 96 (11) (1977) M 121-126. 7. A.J. Van Roosemalen and J.C. Mol, J. Catalysis 78 (1982) 17-23. J.J. Rooney and A. Stewart, in "Catalysis" (special ist periodical reports) 8. (C. Kemball, ed.) The Chemical Society, London, Vol. 1 (1977) 277. B.G. Baker and N.J. Clark, IV International Symposium on the Scientific 9. Bases for the Preparation of Heterogeneous Catalysts, Elsevier (1986). 10. B.G. Baker, N.J. Clark, H. McArthur and E. Summerville, International Patent Application PCT/AU83/00110.
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