ADVANCES IN CATALYSIS
VOLUME 24
Advisory Board G . K. BORESKOV Novosibirsk, U.S.S.R.
M. BOUDART Stanford, Calijmnia...
31 downloads
2200 Views
21MB Size
Report
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form
ADVANCES IN CATALYSIS
VOLUME 24
Advisory Board G . K. BORESKOV Novosibirsk, U.S.S.R.
M. BOUDART Stanford, Calijmnia
P. H. EMMETT Baltimore, Maryland
W. JOST
J. HORIUTI Sapporo, Japan
G. NATTA Milan, Italy
M. CALVIN Berkeley, Calijmiu GGttingen, Germany
P. W. SELWOOD Santa Barbara, Calijarnia
ADVANCES IN CATALYSIS VOLUME 24
Edited by D. D. ELEY The University Nottingham, England
HERMAN PINES Northwestern University Euanston, Illinois
PAULB. WEISZ Mobil Research and Devebpment Corporation Princeton, New Jersey
1975
ACADEMIC PRESS NEW YORK SAN FRANCISCO LONDON A Subsidiary of Hareart Brace Jouawich, Publishers
COPYRIGHT 0 1975, BY ACADEMIC PRESS,INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.
ACADEMIC PRESS, INC. 111 Fifth Avenue, New
York, New York 10003
United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road. London NWI
LIBRARY OF CONGRESS CATALOG CARD NUMBER:49-7755 ISBN 0-12-007824-4 PRINTED IN THE UNITED STATES OF AMERICA
Contents CONTRIBUTORS PREFACE-CATALYSIS AND RELEVANCE, . .. . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . SIRHUGHS. TAYLOR (1890-1974). . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . .
ix xi
xv
Kinetics of Coupled Heterogeneous Catalytic Reactions L. B E R ~ N E K I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 11. Principles of Some Methods of Kinetic Analysis.. , . . . . . . . . . . . . . . . . . . . . 111. Specific Features of the Kinetics of Coupled Heterogeneous Catalytic Reactions . . . . . . . . , . . . . , . , . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . 9 IV. Experimental Kinetic Studies of Some Systems of Coupled Reactions. . . . . 22 List of Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Referenc......................................................... 51
Catalysis for Motor Vehicle Emissions JAMES
I. 11. 111. IV. V. VI. VII. VIII.
WEI
Introduction. . . . . . . . . . . . . . . . . .................. Properties of Automotive Exha _................... Catalysts and Reactors. . . . . . . Kinetics and Mechanisms. . . . . . . . . . . . . . . . . . . . . . . . Physical Transport Processes. . . ...................... Durability of Catalytic Converters. . . . . . . . . . . . . . . . ...................... Reactor Engineering. . . . . . . . . . Future Prospects.. . . ............................ List of Symbols. . . . . . . . . . . . . . _._.......... References. . . . . . . . . . . .. . . . . . . , . . . . . . . . . .
-57 63 71 86 97 109 114 122 124 125
The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. MOLAND J. A. MOULIJN I. 11. 111. IV.
Introduction. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . Reactants and Catalyst Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Mechanism... . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermodynamics and Kinetics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
V
131.132 141 155 168
CONTENTS
One-Component Catalysts for Polymerizationof Olefins Yu. YERMAKOV AND V. ZAKHAROV I. Introduction . . . . . . . . . ......................................... Metals. . , . . . . . . . . . . . . . . . . IV. Subhalides of Transition Metals. . . . . . . . . . , . . . . . . . . . . . . . . . .
173 175 184 192 194
VI. Some General Features of Propa
......................... VII. Conclusion
202 213 213
The Economics of Catalytic Processes J. DEWINGAND D. s. DAVIES I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Two Classes of Useful Chemical Transformations.. . . . . . . . . . , . . . . . . . . . 111. The Network of Technological Factors and Constraints Affecting Catalyst Choice.. . . . . . . , . . . . . . . . . . . . , . , . . . . . , , . . . . . . . . . . . . . . . . . . . . IV. The Economic Factors and Constraints Affecting Catalyst Choice. . . . . . . V. Factors Involved in Economic Improvement t.0 Typical Processes and Guidelines for Assessment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . References. . . . , . . . . . . , . . . . . . . . . . . . . . , . , . . . . . . . . . . . . . . . . . . . . . . . . . .
.
221 222 225 231 241 243
Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA I. Introduction .... . , . , . , . , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . 11. Formation, Structure, and Properties of Palladium and Nickel Hydrides. . 111. Catalytic Activity of Hydride Phases of Palladium and Its Alloys with Gold or Silver. . . . . , , . . . . . . . . . . . . . . . , , . , . . . . . . . , . . . . . . . . . . . . . . . . . . IV. The Effect of Transformation into Hydride on the Catalytic Activity of Nickel and Its Alloys with Copper. . . . . . . . . . , . . . , . , . . . . . . . . . . . . . . . . . V. Catalytic Activity of Other Metal Hydrides in Test Reaction of Hydrogen. VI. General Remarks and Conclusions.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
245 247 253 268 283 285 289
Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species R. P. COONEY,G. CURTHOYB, AND NGUYBN THETAM I. Introduction .... . . . . . . . . . , . . . . . , . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . 293 11. The Origin of the Raman Effects.. . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
vii
CONTENTS
111. Instrumentation. . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Recording Spectra of Adsorption Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Raman Spectra of Adsorbed Molecules.. . . . . . . . . . . . . . . . . , . . . . . . . . . . . . VI. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
306 320 333 339
Appendix : Normal Coordinates, Vibrational Wavefunctions, and Spectral Activiti .......................................................... 339 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1
Analysis of Thermal Desorption Data for Adsorption Studies MILO&SYUTEK, SLAVOJCERNP,AND FRANTI~EK BUZEK
I. Introduction. ............... .. . ... . . 11. Fundamental tions and Re1 .. . . . . . . 111. Temperature Schedules in Therm ...... . . IV. Fundamental Relationships for ctivation Energy of Desorption, of the Order of Desorption and of the Preexponential Factor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Processing of the Experimental Data to Estimate the Kinetic Parameters of Desorption.. . . . . . . . . . . . . . . , . , . . . . . . . . . . . VI. Effects of the Surface Hete VII. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of Symbols. . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
343 347 38 1 365 372 380 388 390 39 1
397 AUTHORINDEX .......................................................... SUBJECTINDEX. . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . , , . . . . . . . . . . . . . . . . . . . 415 CONTENTS QF PREVIOUS VOLUMES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
.
This Page Intentionally Left Blank
Contri butors Numbers in parentheses indicate the pages on which the authors’ contributions begin.
L. B E R ~ N E K Institute , of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, Prague-Suchdol, Czechoslovakia (1) FRANTI~EK BUZEK,The J . Heyrovskg Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Mdchova, Prague, Czechoslovakia (343) SLAVOJ~ E R N PThe , J . Heyrovskg Institute of Physical Chemistry and Electrochemistry, Czechoslovak A cademy of Sciences, Mdchova, Prague, Czechoslovakia (343) R. P. COONEY,Department of Chemistry, University of Newcastle, New South Wales, Australia (293) G. CURTHOYS, Department of Chemistry, University of Newcastle, New South Wales, Australia (293) D. S . DAVIES, Imperial Chemical Industries, Milbank, London, England (221) J. DEWING,Imperial Chemical Industries, Corporate Laboratory, Runcorn, Cheshire, England (221) J. C. MOL, Institute of Chemical Technology, University of Amsterdam, Amsterdam, The Netherlands (131) J. A. MOULIJN, Institute of Chemical Technology, University of Amsterdam, Amsterdam, The Netherlands (131) W. PALCZEWSKA, Institute of Physical Chemistry, Polish Academy of Sciences, Warszawa, Poland (245) MILO; SMUTEK, The J . HeyrovskQ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Mdchova, Prague, Czechoslovakia (343) NGUYENTHE TAM, Department of Chemistry, Uniuersity of Newcastle, New South Wales, Australia (293) JAMESWEI, Department of Chemical Engineering, University of Delaware, Newark, Delaware (57) Yu. YERMAKOV, Institute of Catalysis, Siberian Branch of the U S S R Academy of Sciences, Novosibirsk, U S S R (173) V. ZAKHAROV,Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences, Novosibirsk, USSR (173) ix
This Page Intentionally Left Blank
Preface CATALYSIS AND RELEVANCE There have been many challenges in our decade to institutions, customs, ideas, to nearly everything that has grown and accumulated over so many human generations. This includes the practices of the scientific community, of teaching, and of research. In the arguments concerning the purposes and benefits of scientific endeavors, those in catalysis have not been overlooked. Indeed, they shouldn’t be, for catalysis is inevitably a phenomenon of the utmost importance to society. It may provide, in fact, an ideal arena for all sorts of exercises concerning “relevance.” We have witnessed discussions and participated in panels formed to determine what catalysis research is most relevant. It occurred to us that never once did anyone ask (let alone answer) the question “relevant lo what?” Surely, how could we expect meaningful conclusions without agreeing on a common premise? Undoubtedly, the concern for relevance springs from a sense that not all is well with society, and that a shift of priorities in human efforts may be indicated. What kind of priorities? There is within us a biologically intrinsic order of priorities, which reads something like: survival > health > comfort > pleasure. As individual organisms we tend to progress from left to right to the extent that we satisfy the previous priority. Societies do the same, although there the process of self-analysis and appraisal is more complex, more likely to be delayed or faulty. But that is probably what this discussion is all about. Humanity has created so many sites of such great activity on the earth’s surface that the gas phase above it is no longer acting as a rapid and infinite sink for the ultimate reaction products or side products. Their concentrations are no longer perceived as zero a t all times. Thus, in society’s kinetic system, products’ poisoning rates have become noticeable in some places; pollution has evidenced itself as affecting pleasure, at times comfort, and, in some cases, even health. The priority bell is getting louder, moving higher. Now it is realized that there are developing constraints on the utilizable sources of fuel and energy that feed the entire kinetic complex of human society. The prospect of the primary rate constants becoming limiting, diminishing, or even vanishing, places the associated problems high on the xi
xii
AN EDITORIAL PREFACE
ladder of evolutionary priorities, rapidly advancing to the rung marked “health,” and then to “survival.” Might we assume then that the search for relevance is a search for ways to be reoriented toward the higher or highest evolutionary priorities? Even discarding the (controversial) need for growth in humanity’s kinetic machinery, there is a clear need for effort to just sustain the feeder reactions and to control the undesirable end products from the total system. The catalytic scientist surely feels that he must already be-and is-a key participant in that very play (or drama) of human survival or evolution. There are always some who insist that scientists must not keep goals in mind, other than the goal of knowledge (any knowledge?!). We, too, believe that humanity can benefit from a number of brilliant scientists who think their thoughts without any constraint,s (or who think that they think that way); but if such were to be the universal goal that all good scientists were to follow, then the lemmings’ parade of self-destruction would be nicely duplicated by Homo sapiens. So the question should never be (nor has it ever been) one of choosing between all catalytic chemists studying ortho-para hydrogen conversion, molecular orbitals and the like, or all catalytic chemists studying fuel synthesis and exhaust catalysts; a healthy society is a judiciously balanced society, and the concern for relevance is one for a shift toward greater dedication in the direction of the most vital needs for the survival and health of the kinetic system of human society. Surely, it is when basic science and basic human goals can and do interact that humanity progresses. We have included in this volume two chapters specifically related to society’s kinetic system. We have asked James Wei of the University of Delaware, recent Chairman of the consultant panel on Catalyst Systems for the National Academy of Sciences Committee on Motor Vehicle Emissions, to illustrate key problems and bridges between the catalytic science and the practical objectives of minimizing automobile exhaust emissions. We have also asked for a portrayal of the hard economic facts that constrain and guide what properties in a catalyst are useful to the catalytic practitioner. For this we have turned to Duncan S. Davies, General Manager of Research and Development, and John Dewing, Research Specialist in Heterogeneous Catalysts, both from Imperial Chemical Industries Limited. The behavior of kinetic systems with even a few interacting species can become very complex. L. Berhek treats a few key principles and accompanies them with experimental observations in “Kinetics of Coupled Heterogeneous Catalytic Reactions.” In “One-Component Catalysts for Polymerization of Olehs,” Yu. Yermakov and V. Zakharov review results
AN EDITORIAL PREFACE
xiii
and mechanisms concerning one class of olefin polymerization catalysts, the one-component catalysts, in contrast to the Ziegler-Natta types that have been the subject of several reviews. J. C. Mol and J. A. Moulijn, in “The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds,” are describing catalysts, reactions, and mechanisms for one of the most recent, fascinating, and useful reactions which many of us still know best as olefin disproportionation. In view of the dominant role played by the group VIII metals in catalytic reactions involving hydrogen, we are glad to have an in-depth discussion by W. Palczewska on the “Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases.” Then we have two chapters devoted to new research techniques for adsorption studies : “Analysis of Thermal Desorption Data for Adsorption Studies” by M. Smutek, S. Cernf, and F. Buzek; and “Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species” by R. P. Cooney, G. Curthoys, and N. T. Tam.
* ‘* * We are saddened t o have lost one of the first and foremost pioneers of catalysis, and a member of our Advisory Board, Sir Hugh S. Taylor. The present volume includes an obituary of Sir Hugh written by one who knew him well, John Turkevich. We are further saddened by the untimely passing on July 27,1973, of a young pioneer, Richard J. Kokes.
PAUL B. WEISZ
This Page Intentionally Left Blank
Sir Hugh S. Taylor (1 890-1 974) The catalytic community of the world records with sorrow the death of Sir Hugh Taylor, an active practitioner of the art of catalysis, an imaginative investigator of its scientific basis, an inspiring teacher of many, many catalytic chemists and physicists, and a leader in the field of catalysis for fifty years. Born in Liverpool in 1890, he received his education at the University of Liverpool under the guidance of Sir Frederick Donnan. He did postdoctoral work at the Nobel Institute in Stockholm under Svant6 Arrhenius and at the Technische Hochschule in Hannover under Max Bodenstein. He first came to Princeton in 1914 bearing the intellectual inheritance of three illustrious scholarly lines of succession-British, Swedish, and German. He left Princeton during World War I1 to work with Eric K. Rideal in England on a war project for the Ministry of Munitions. This was his first contact with heterogeneous catalysis and the basis of his long friendship with Sir Eric Rideal. One outcome of this association was the book by Rideal and Taylor, “Catalysis in Theory and Practice” (1926). On his return to Princeton after the war, Hugh Taylor organized catalytic research at the Frick Chemical Laboratory. He applied high vacuum technique, liquid air cryoscopy to the study of adsorptive characteristics of catalysts, correlating rates of catalytic reactions and rates of adsorption. He introduced the concept of “activated adsorption” and defended it against “all comers.” His researches and those of his pupils led to his formulation in the twenties of the concept of “active catalytic centers” and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To exxv
xvi
SIR HUGH S. TAYLOR
amine theoretical aspects of surface reactions, Hugh Taylor attracted Henry Eyring to Princeton. With World War I1 threatening, Hugh Taylor embarked on catalytic processes for hydrocarbon transformations. The first large catalytic petrochemical and gasoline reforming process was developed for converting heptane into toluene. This was followed by work on butene dehydrogenation to butadiene. Hugh Taylor was one of the first score of scientists engaged in the atomic energy projects, working with the British and Canadian group before the Manhattan Project was organized. At Princeton, work was done on developing catalysts for the exchange of hydrogen with heavy water for production of deuterium moderator. With the organization of the Manhattan Project, he collaborated with the Columbia University group not only on this phase of the atomic energy work but also on production and stabilization of the barrier for diffusion of uranium hexafluoride. He had a group working a t Princeton but he also staffed and directed a large section of the SAM Laboratory at Columbia University. After the war, Sir Hugh became Dean of the Graduate School a t Princeton but continued to lead a group of graduate students and visiting scholars at Frick Chemical Laboratory in applying solid state theory to catalysis. He retired from the Princeton faculty in 1958, but continued his interest in catalysis through writings, consultations, and attending conferences. The last Gordon Conference on Catalysis, whieh series he attended since its inception, was four years ago. During his long association with catalysis, Sir Hugh Taylor was a vigorous personality. Prominent young scientists came from all over the world to work with him, to receive training in principles of catalysis, and to contact his contagious enthusiasm for new ideas and new techniques. They subsequently became leaders in many countries around the world. The ideas that he developed and the enthusiasm he generated are continued in many centers of catalytic research throughout the world.
JOHNTURKEVICH
Kinetics of Coupled Heterogeneous Catalytic Reactions L. BERANEK Institute of Chemical Process Fundamentals Czechoslovak Academy of Sciences Prague-Suchdol, C2echoslovakia
I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Principles of Some Methods of Kinetic Analysis.. . . . . . . . . . . . . . . A. Simultaneous Solving. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Elimination of Time Variable. . . . . . . . . . C. Isolation of Individual React ions. . . . . . . . . . 111. Specific Features of the Kinetics of Coupled Reactions . . . . . . . . . . . . ................... A. Coupling through the Catalytic Surface.. . . . . . . . . . . . . . . . . . . . . . . B. Effect of the Arrangement of the Slow Steps. . . . . C . Selectivity and Relative Reactivity.. . . . . . . . . . . . . . . D. Nonsuitability of the Power-Law Type Equations.. . IV. Experimental Kinetic Studies of Some Systems of Couple A. Experimental Method and Treatment of Data.. . . . . B. Consecutive Hydrodemethylation of Xylenes . . . . . . . . . . . . . . . . . . . C. Consecutive Hydrogenation of Phenol. D . Parallel Ketoniration of Carboxylic Acids. . .......... E. Competitive Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Parallel-Consecu G. Discussion. . . . . List of Symbols.. . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9 9 13 18 21 22 25 28
31 35 37 43 48 49 51
1. Introduction The development of methods for the kinetic measurement of heterogeneous catalytic reactions has enabled workers to obtain rate data of a great number of reactions [for a review, see ( I , . @ ] . The use of a statistical treatment of kinetic data and of computers [cf. (3-7')] renders it possible to estimate objectively the suitability of kinetic models as well as to determine relatively accurate values of the constants of rate equations. Nevertheless, even these improvements allow the interpretation of kinetic results from the point of view of reaction mechanisms only within certain limits; 1
2
L. B E R ~ N E K
the reasons are the great complexity of surface processes and lack of reliable knowledge of the nature of their individual steps. A study of the kinetics of an isolated, stoichiometrically simple catalytic reaction solely cannot thus provide much information. However, one of the means of extending the contribution of kinetic measurements to the knowledge of the mechanism of catalysis is the study of coupled systems, i.e. systems where several simple reactions proceed simultaneously on the same catalyst. Coupled systems are defined here as catalytic transformations stoichiometrically not simple, the course of which should be characterized by more than one conversion parameter. The examples are consecutive and parallel reactions, and various combinations of these. When the term “c~mplicated’~ or “complex” reaction is used in this article it does not refer to the complexity of its mechanism. Heterogeneous catalytic reactions are always multistage processes and consist of a series of elementary surface steps ( e g adsorption, surface reaction, and desorption). If intermediates of these steps are unstable or cannot desorb, the reaction appears to be stoichiometrically simple with respect to the composition of the bulk phase. If, however, the reaction intermediates are stable and capable of desorption into the bulk phase, the catalytic transformation is stoichiometrically not simple and the reaction appears as consecutive or parallel also with respect to the bulk phase. The study of the kinetics of transformations of these types is also of practical importance, since most industrial heterogeneous catalytic processes are stoichiometrically complex. The examples of these may be the oxidation of naphthalene to phthalanhydride, or its hydrogenation to decaline via tetraline, the isomerization or the oxidation of xylenes, the hydrogenation of carbon monoxide, the production of butadiene, both by the old process from ethanol and by the dehydrogenation of butane, and catalytic hydrocarbon processing in general, which always involves cracking, isomerization, dehydrogenation, and other reactions. From a theoretical point of view the study of the kinetics of coupled catalytic reactions makes it possible to investigate mutual influencing of single reactions and the occurrence of some phenomena unknown in the kinetics of complex reactions in the homogeneous phase. This approach can yield additional information about interactions between the reactants and the surface of the solid catalyst. This article will be devoted to analysis of some specific features of the kinetics of coupled heterogeneous catalytic reactions and to experimental results and conclusions derived from them, which were obtained by the present author and his coworkers. The general discussion of the kinetics of complicated reaction systems will be restricted to a brief characterization of fundamental approaches; the survey of experimental works of other
KINETICS OF COUPLED CATALYTIC REACTIONS
3
authors dealing with stoichiometrically not simple catalytic reactions will be presented only when the results closely relate to the questions discussed.
II. Principles of Some Methods of Kinetic Analysis The kinetics of a coupled reacting system consisting of n stoichiometrically simple reactions is described generally by a set of n differential equations d 2 1 / d ~ = f 1 (CjO, ~1
d t i / d r = fi
(cj',
dx,/dT
(cjo, ~1
= fn
~1
*
.
*
--
.
K ~ 2, 1
.
K ~ 2, 1
.
K ~ 21 ,
* *
zn),
*
zn),
(1)
2,).
These equations can be derived on the basis of a presumed reaction mechanism or they can be only formal kinetic equations. In the most general case, which does frequently occur in heterogeneous catalysis, the rate of each reaction is a function of the conversions xi of all the reactions taking place in the system and of the corresponding number of constants K . A kinetic description of the coupled reaction system can be obtained in different ways; we will show the principles of some frequently employed approaches.
A. SIMULTANEOUS SOLVING One of the possibilities is to study experimentally the coupled system as a whole, at a time when all the reactions concerned are taking place. On the basis of the data obtained it is possible to solve the system of differential equations (1) simultaneously and to determine numerical values of all the parameters unknown (constants). This approach can be refined in that the equations for the stoichiometrically simple reactions can be specified in view of the presumed mechanism and the elementary steps SO that one obtains a very complex set of different reaction paths with many unidentifiable intermediates. A number of procedures have been suggested to solve such complicated systems. Some of them start from the assumption of steady-state rates of the individual steps and they were worked out also for stoichiometrically not simple reactions [see, e.g. (8, 9, 9u)l. A concise treatment of the properties of the systems of consecutive processes has been written by Noyes (10). The simplification of the treatment of some complex systems can be achieved by using isotopically labeled compounds (8, 11, 12, Ida, 12b). Even very complicated systems which involve non-
4
L.
BERANEK
lO#lO
-
201125-10If10
1.8
5
FIG.1. Dependence of relative concentrations cj on time variable T (arbitrary units) for consecutive catalytic reaction
A(ads)
-+ B(ads) --+
C(ads)
for various combinations of rate constants of adsorption and desorption and of surface reactions. From G. Thomas, R. Montarnal, and P. Boutry, C.R. Acad. Sci., Ser. C 269, 283 (1969).
linear mechanisms, and the solution of which cannot be obtained explicitly, can be solved by the use of computers. The number of constants being determined may amount to several tens [see, e.g. ( I S ) ] . The simultaneous determination of a great number of constants is a serious disadvantage of this procedure, since it considerably reduces the reliability of the solution. Experimental results can in some, not too complex cases be described well by means of several different sets of equations or of constants. An example would be the study of Wajc et ul. (14) who worked up the data of Germain and Blanchard (16) on the isomerization of cyclohexeneto methylcyclopentenes under the assumption of a very simple mechanism, or the simulation of the course of the simplest consecutive catalytic reaction A + B + C, performed by Thomas el ul. (16) (Fig. 1). If one studies the kinetics of the coupled system as a whole, one cannot, as a rule, follow and express quantitatively mutually influencing single reactions. Furthermore, a reaction path which a t first sight is less probable and has not been therefore considered in the original reaction network can be easily overlooked.
B. ELIMINATION OF TIME VARIABLE Sometimes the time variable is eliminated from the set of differential equations describing the kinetics of the coupled system, e.g. by dividing
KINETICS OF COUPLED CATALYTIC REACTIONS
5
all the equations of the system (1) by the first of them. In the rate equations of heterogeneous catalytic reactions the functional relations fi (Cj', KI K ~ XI , x,) contain frequently a rational function of the C K j ~ j )If~ .this term is the same for all the equations (they type 1/(1 have a common denominator), which is frequently the case in heterogeneous catalysis, by elimination of the time variable it disappears. If, for example, the rate equations of the individual reactions have the form '
+ 0
.
dxi/dr = ki K A ,C~A , i / ( l and dxl/dr
=
kl
KAJc A , l / ( l
+ c KjcJa +c
KjCj)a
for ith reaction,
(2)
for 1st reaction,
(3)
on dividing (2) by (3) one obtains dxi/dxl
=
(4)
ki KA,i C A , < / ~ I K A ,cA.1. ~
The resulting relation does not contain a time variable and has a lesser number of conversions xi. Concentrations C A . ~in equations of the type (4) are functions of the lesser number of xi than are the denominators (1 C Kjcj). in equations of the types (2) and (3). So, for example, for a consecutive, irreversible reaction the following holds :
+
fi'fxi, Xi-1) and so that from Eq. (4) it follows that C A , ~=
dxi/dxl
=
CA.1
= fl'(Xi)
Krelfi'(zi, ~ i - i ) / f i ' ( ~ ~ ) -
(5)
(6)
Simultaneously with the decrease of the number of variables, the number of the parameters to be determined also decreases; the remaining parameters are, of course, only relative values of, in our case, the rate constants and adsorption coefficients Kmi
= ki Ka,i/ki KAJ.
(7)
The resulting system of equations of type (6) is thus simpler than the system of equations (1). Its solution yields as the final result the relations between the concentrations of reaction components. The procedure for solving the relations between concentrations has been used in kinetic studies of complex catalytic reactions by many authors, among the first of them being Jungen and his co-workers (17-20), Weiss (21,22),and others [see, e.g. (23-26a)l. In many papers this approach has been combined with the solution of time dependencies, a t least for some of the single reactions. Also solved were some complicated cases [e.g. six-step consecutive reaction (26,.%a)] and some improvements of tbis time-elimination procedure were set forth (25").The elimination of time is
6
L.
BERANEK
also the basis of the method of competitive reactions which will be discussed in Section IV.E.2. The relations between concentrations of reaction components obtained by the time-elimination procedure can be employed with success, e.g. in a study of the selectivity of catalysts, the effect of some reaction conditions on the selectivity of reactions, or in the determination of relative reactivities of a series of compounds. The elimination of the time variable can also be an important step in solving the complete kinetics of a complex system. The use of this procedure only, however, is not suitable for obtaining data for the design of catalytic reactors or for a study of mutually influencing single reactions of the coupled system. An interesting method, which also makes use of the concentration data of reaction components measured in the course of a complex reaction and which yields the values of relative rate constants, was worked out by Wei and Prater (28). It is an elegant procedure for solving the kinetics of systems with an arbitrary number of reversible first-order reactions; the cases with some irreversible steps can be solved as well (28-30). Despite its sophisticated mathematical procedure, it does not require excessive experimental measurements. The use of this method in heterogeneous catalysis is restricted to the cases which can be transformed to a system of firstorder reactions, e.g. when from the rate equations it is possible to factor out a function 4 which is common to all the equations, so that first-order kinetics results. dCA,i/dT
=
4[-
C E C A , ~+ C Fi
CA,j].
(8)
This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silicaalumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/A1203 catalyst, including adsorption-desorption steps. As the presumption of the identity of the function 9 for all the reactions of the system may not be always fulfilled, this method has not met with wide application by catalytic chemists. Rather, it attracts theoretical interest (29, 30, 33-36), even though, for example, the authors of the last mentioned paper (36) used their own experimental data on the isomeriza-
KINETICS OF COUPLED CATALYTIC REACTIONS
7
tion of butenes on zeolite. In the case of a nonlinear reaction network, the use of isotopically labelled reactants allows us to simplify it to an equivalent network of linear reactions which can be analyzed by linear algebra, as demonstrated for 1,3-butadiene hydrogenation on supported palladium (12%).The approach of Wei and Prater was also used for solving the kinetics of a three-component system with some irreversible steps, namely parallel hydroisomerization and hydrocracking of cyclohexane in the presence of a zeolite catalyst (36a).
C. ISOLATION OF INDIVIDUAL REACTIONS A description of the kinetics of a coupled system of heterogeneous catalytic reactions can also be obtained using a procedure consisting of isolating single, stoichiometrically simple reactions and in separate study of the kinetics of each. In this way one can obtain more reliable data necessary for determining the form of individual rate equations and more accurate values of their constants, since the number of simultaneously determined parameters is much less than in the case of simultaneous solution. However, for by far the greater amount of experimental work, this procedure is not so frequently employed; at the same time, interpretation of the kinetics of single reactions is frequently greatly simplified, the solution being sometimes made easier by using concentration-concentration relations or combined with the treatment of integral data. Of the earlier papers, the kinetic study by Coussemant and Jungers on consecutive hydrogenation of phenol via cyclohexanone ty cyclohexanol on Raney nickel (18) or that by Fognani and Montarnal (37) on the kinetics of parallel-consecutive oxidation of ethylene to ethylene oxide and carbon dioxide on a silver catalyst can serve as examples of this approach. Other examples are ammoxidation of m-xylene and other isomers via toluonitrile to phthalonitrile on a vanadium catalyst (38,38a), hydrogenation of mesityl oxide via methylisobutylketone to methylisobutylcarbinol on a copper-chromium catalyst (S9), two-step dehydrogenation of diethylbenzene to divinylbenzene on commercial FezOs-ZnO catalyst (40), or the system of three parallel reactions of diacetone alcohol, catalyzed by an acidic ion exchanger and leading to mesityl oxide, acetone, and phorone (41). We might also mention the oxidation of o-xylene on vanadium oxide catalyst leading to o-tolualdehyde, phthalic and maleic anhydrides and carbon monoxide and dioxide (Qla) and the dehydration of 2-butene-l,4-diol on acid type catalysts ( d l 6 ) with preferential formation of 2,6dihydrofuran. If the kinetics of single reactions is reliably determined by a separate study, it is possible on this basis to ascertain how single reactions influence
8
L. BERANEK
one another when they are not isolated but are run simultaneously, i.e. are coupled. Isolation of single, stoichiometrically simple reactions (not, the individual steps of a detailed mechanism, however) is in most cases possible. One of the most suitable techniques for this purpose is the method of initial reaction rates, which eliminates the effects of products and their further transformations on the reaction rate measured. In some cases it requires a very sensitive analytical method, in order that the degree of conversion be as low as possible. By stepwise isolation of individual reactions also some reaction paths which were not considered in the original network can be detected. Problems of isolation of reactions in a complex reaction system have been treated in detail, e.g. in the monograph by Jungers and co-workers (42). The measurements of the rates of single reactions in a complex reaction system can be made less tedious by using isotopically labeled reactants [see, e.g. (11,12, 12b, 49)]. The equations obtained for the initial region should be extended by the terms which express the effect of the products, so that they enable one to describe the course of the reaction to the highest degree of conversion. As the individual reactions may be influenced through all the reaction components which adsorb on the catalyst, for each reaction it is necessary to express quantitatively its influencing by all the substances present in the coupled system (even though they do not participate otherwise in the reaction concerned). This can be experimentally performed by measuring the reaction rate of each of the single reactions in the presence of varying amounts of the compound whose effect is to be ascertained. These data are treated mathematically under certain assumptions concerning adsorption or another influence of the given compound, and the original rate equations are then extended by the corresponding term. The system of differential equations obtained describes the kinetics of the coupled reaction system as a whole. The verification of the validity of this equation system can then be made by numerical integration on a computer and by comparison of the calculated data with experimental integral dependences, i.e. concentration-time dependences for all the components present in the reaction system. In a recent paper, Dalla Lana et al. (43a) suggest an interesting approach to the analysis of kinetics of a complex reaction network. Rate data for each single reaction can be obtained from the overall kinetics by its decomposition using linear algebra and can then be used for modeling the kinetics for each reaction separately. The necessity of estimating a large number of parameters simultaneously is thus eliminated. Very accurate integral data, however, are needed in order that this method be reliable for discrimination between different kinetic models and overall reaction networks.
KINETICS OF COUPLED CATALYTIC REACTfONS
9
Ill. Specific Features of the Kinetics of Coupled Heterogeneous Catalytic Reactions
In kinetic analysis of coupled catalytic reactions it is necessary to consider some specific features of their kinetic behavior. These specific features of the kinetics of coupled catalytic reactions will be discussed here from a phenomenological point of view, i s . we will show which phenomena occur or may occur, and what formal kinetic description they have if the coupling of reactions is taking place. No attention will be paid to details of mechanisms of the processes occurring on the catalyst surface from a molecular point of view. A. COUPLING THROUGH THE CATALYTIC SURFACE
A kinetic description of a heterogeneous catalytic reaction will in most cases be different when the reaction proceeds simultaneously with other reactions in a complex system, compared with the case where its kinetics was studied separately. The most important is the effect in the case where the reactions concerned take place on the same sites of the surface of a catalyst. Let us take, for example, the system of competitive reactions A+C+D (18) B+C+E (Ib)
In the case of noncatalytic reactions, the rate of each single reaction depends, as a rule (except for too concentrated systems), only on the concentrations of the compounds undergoing chemical conversion in this reaction, and sometimes also on the concentrations of the compounds formed by the reaction T1
= .fl(cA,
CC, CD),
(94
(9b) This holds for noncatalytic reactions both isolated and in competitive system, as well as for isolated catalytic reactions. The rate of catalytic reaction in competitive (and generally in any coupled) system depends, however, on the concentrations of all the compounds present in the system, insofar as they are adsorbed on the same active centers on which the given reaction is taking place. T2
= f2(cB1
CC, CE).
rl = .fi(cA,
CC, CD, CB, CE),
(104
r2 = fZ(CBt
CC, CE, CA, CD).
(lob)
10
L.
I
I
BERANEK I
I
I
-A
t
FIQ.2. Pressure fall -AP (Torr) against time t (arbitrary units) in hydrogenation of acetylene on Pt/A1203 catalyst a t 110°C and p&/p& 2. In the initial slow period of the reaction the main product is ethylene, and after the acceleration, further hydrogenation of ethylene to ethane predominates. From G. C. Bond and P. B. Wells, J. Catal. 4, 211 (1965).
>
This means that the individual reactions are kinetically coupled through active centers on the surface of a solid catalyst (44). This phenomenon has two consequences. First, selectivity of coupled reactions can be quite different from the selectivity estimated from the kinetics of separately studied single reactions; these problems will be discussed in Section 1II.C. Second, the coupling can influence absolute values of the reaction rates, which may sometimes result in unexpected time dependencies. Although the reaction rate generally decreases in the course of the reaction with decreasing concentration of reactants, rate acceleration with increasing total conversion may sometimes be also observed. Such a case occurs, for instance, if the less reactive reactant is adsorbed more strongly than the more reactive one. As a consequence of preferential occupation of the surface by the more strongly adsorbed reactant, this reacts in preference to the other and only after its concentration falls below a certain limit, the reaction of the other, more reactive reactant begins to proceed, indeed at the higher rate. Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive; alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of
KINETICS OF COUPLED CATALYTIC REACTIONS
11
the pressure of the reaction mixture, which is a measure of the extent of the overall reaction, and its dependence on time. The two stages, differing markedly in their rates, can be clearly distinguished. A similar phenomenon occurs, although to a lesser extent, in the hydrogenation of dienes via alkenes to alkanes (47). In order to demonstrate the above effect of the coupling in a quantitative kinetic description, Fig. 3 shows a calculated, hypothetical case of consecutive reaction of the type A 7ii+ B 7iT C which follows the simplest kinetics of the Langmuir-Hinshelwood type Ti = r2
=
d- K A ~-kA KBPB-t K c p c ) ,
(1W
~ K B P B / ( ~KBPB KAPA Kcpc).
(1lb)
kiKapA/(l
+
+
+
Figure 3a corresponds to the following values of the constants: kl = 1, = 50, and K a / K B = 100. It is seen that the later stage of the process (B + C) proceeds a t a higher rate than the initial one (A 3 B), indeed only after the major part of the strongly adsorbed reactant A has been consumed. If the strength of adsorption of both reactants A and B is the same (KA/KB = 1) or if we are not dealing with a heterogeneous catalytic reaction (both of which cases are represented in Fig. 3b), the above discussed phenomenon does not occur. That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was
kz
CI
b
t
z
FIG.3. Dependence of relative concentrations c i on time variable T (arbitrary untis) in consecutive catalytic reaction A -+ B -+ C following the rate equations (lla) and (llb). a: k , = 1 ; kz = 50; K A = 100;K g = 1; K O = 1 (arbitrary units). b: kl = 10; kz = 5 ; K A = 1; K B = 1; Kc = 1 (arbitrary units).
L.
12
BERANEX
TABLE I Competitive Hydrogenation of Tetraline ( A ) and p-Xylene (B)
Overall degree of conversion
Overall reaction rate r (mmole min-1g-1)
(%I
rexp
Toslo
0 10 32 56 81
8.5 8.8 9.4 10.4 11.4
8.5 8.9 9.4 10.4 11.3
Temperature 170°C, catalyst RaneyNi, elevated pressure. /CA = 6.7 mmole min-lg-l; kg = 12.9 mmole min-lg-1; K A / K B= 5.4. Reproduced by permission from J. P. Wauquier and J. C. Jungers, Bull. SOC.Chim. Fr. p. 1280 (1957). (I
confirmed by calculation based on a Langmuir-Hinshelwood kinetic model. I n an extreme case, parallel reactions may proceed consecutively : only after the total consumption of strongly adsorbed reactant does the other competing reagent react. This was observed, e.g. in competitive hydrogenation of but-3-ynoic acid and buta-2,3-dienoic acid on Pd/BaSOa in n-pentane (49) or in competitive hydrogenation of naphthalene and 1,4di-tertbutylnaphthalene on Pd/C in acetic acid (60). The coupling of reactions may operate also via a common reagent X such as e.g. hydrogen in consecutive hydrogenations of the type A -+ B %C. If the common reagent is used in excess, which is frequently the case, it may occupy a greater part of the surface and consequently the reactions are slowed down. The consumption of a certain amount of the common reagent X in the first reaction may release a part of the surface for the reactant B, which is transformed in the second reaction. As a result, the second reaction then proceeds faster than it would have proceeded with the initial concentration of reagent X. Such a case has been experimentally observed in the hydrogenation of phenol (61)and will be demonstrated in Section IV. The examples just discussed clearly show that already the qualitative study of the kinetics of coupled catalytic reactions provides us with information, e.g. about relative adsorptivity of reaction components or whether ,the reaction components are adsorbed on the same active centers.
KINETICS OF COUPLED CATALYTIC REACTIONS
13
By a quantitative study of mutual influencing of reactions in a coupled system, i.e. by a determination of the form of functional dependences (10) from the values of corresponding parameters, one can then draw some conclusions about whether a certain substance affects all the reactions in the same or a different way, which might provide further information about the processes occurring on the surface of the catalyst. B. EFFECT OF
THE ARRANGEMENT OF THE
SLOWSTEPS
In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorptiondesorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions (52, 53) were calculated, corresponding to schemes (IIa) and (IIb), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. +X ASB+Y A-B +Y (1)
(4)
11
+x C+Y
(114
(2)
11c + :; + ZY
(IIb)
14
L.
BERANEK
Ads X or Des Y
t FIG.4. Dependence of relative concentrations nj/nAOof reaction components A, B, and C on time variable T (arbitrary units) in the case of consecutive (4 3 )reactions according to scheme (IIa) or parallel (<)reactions according to scheme (IIb). Ads X, Ads A, Des Y denotes that the rate determining step in the overall transformation is adsorption or desorption of the respective substance; Des (B C) denotes that the overall rate is determined by simultaneous desorption of the substance B and C. a : K J K 2 = 0.5 for consecutive, and K 1 ¶ / K a= 0.5 for parallel reactions. b: nxO/nAO= 2.5; for consecutive reactions K Z = 0.5, and for parallel reactions K z / K I = 0.5. c: nxO/nnO= 2.5; kdeaBKI'KY/kdesCK1'KX= 10 (cf. (SS)].d : Kz = 1.75 for consecutive, and K2/K1 = 1.75 for parallel reactions.
+
I n Fig. 4 are illustrated the changes of relative concentrations of substances A, B, and C in the course of the transformation for some typical cases. The graphs are drawn in the form of time dependences, the consumption of the starting compound A being represented by the simplest dependence of the first order, although it is controlled in general by more complicated functions. It is important, however, to follow the change in concentrations of the other two compounds B and C during the transformation of the starting reactant A. Figure 4a shows the course of consecutive reactions proceeding according to scheme (IIa), provided that the rate of the overall transformation is controlled either by adsorption of the common reagent X, or by desorption of the common product Y; thc composition is governed by the ratio of the equilibrium constants of the first and second reactions. An analogous course is however observed with branched parallel reactions of the type represented by scheme (IIb) if the stoichiometry of both branches is not the same (z = 2). Figure 4b graph-
KINETICS OF COUPLED CATALYTIC REACTIONS
15
cially represents examples of consecutive reactions proceeding according to scheme (IIa), or of branched reactions described by scheme (IIb) (z = 2), if the rate of overall transformation is controlled by adsorption of reactant A. Figure 4c shows the course of the same reactions, if the rate-determining step is simultaneous desorption of substances B and C. If the stoichiometry of both branches of the parallel reactions is identical (z = l), the reactions have the course illustrated in Fig. 4d, irrespective of whether they are controlled by adsorption of substances A or X, desorption of compound Y, or by simultaneous desorption of compounds B and C. The curves shown in Fig. 4d are typical for branched reactions, but the ratio of compounds B and C is determined by the ratio of the equilibrium constants of both reactions, and not by the kinetic constants. The same course, typical for parallel reactions (Fig. 4d), can also be found with the consecutive reactions controlled either by adsorption of the starting substance A or by simultaneous desorption of compounds B and C, if they proceed without participation of the common reagent (A -+B -+ C). It is immediately clear that in these cases of thermodynamic control of the composition of the reaction mixture one sometimes may find, with parallel reactions, the curves whose forms are typical for consecutive reactions and vice versa. In other cases, consecutive reaction with the sequence A 4 B + C may simulate the sequence A + C +B (Figs. 4b and 4c). The example of consecutive, irreversible heterogeneous catalytic reaction of the type A -+B 4 C has been solved in a more general way by Thomas et al. (16). The authors considered scheme (111) with the listed values of the rate constants of surface reactions along with the constants of adsorption and desorption of the reactant A and of the product C.
(111)
They varied only the values of the adsorption and desorption rate constants of the reaction intermediate B, and by using the simplest Langmuir kinetics, they calculated time-concentration curves of compounds A, B, and C shown in Fig. 5. Also from this example, which does not consider any step as clearly rate determining, it is evident how very different concentration versus time plots can be obtained for the same sequence of surface reactions if adsorption and desorption of the intermediate B proceed by different rates, which are, however, comparable with the rate of surface reactions. In particular, the curves in the first and second columns of Fig. 5 simulate the parallel formation of substances B and C, at least
16
L.
k,dsB/kdesB
BERANEK
= 10 (strong adsorption of intermtdiatte 6)
kadsB/ kdesB = 1 (medium adsorption of intermediate B) 1
kadsa/ kd&
I
= 0.1 (weak adsorption of intermediate B)
FIG.5. Dependences of relative concentrations c j on time variable T (arbitrary units) for consecutive catalytic reactions according to scheme (111) for various values of rate constants of the adsorption kad& and desorption kd& of the intermediate B. Lefthand column (kdesLI/k2 = 0.1) : desorption of B is slower than its surface transformation. Middle column (kde&/k) = 1 ) : equal rates of desorption of B and of its surface transformation. Right-hand column (kdesB/kl = 10): desorption of B is faster than its surface transformation, From G. Thomas, R. Montarnal, and P. Boutry, C.R. Acad. Sci., Ser. C 269, 283 (1969).
at the beginning of the reaction, although the sequence of surface steps is consecutive; this is associated with the fact that the slow steps [B(ads) -+ B(g) and B(ads) + C(ads)J are arranged in parallel. The possibility of such a distortion of the course of consecutive catalytic reaction by slow desorption of the intermediate product has been pointed out by de Boer and van der Borg (64). However, the authors were able to solve quantitatively only some special cases.
KINETICS OF COUPLED CATALYTIC REACTIONS
17
The phenomena shown in Figs. 4 and 5 illustrate what Wei in his article on “disguised kinetics” (55) calls “adsorption-desorption disguises.” These phenomena do not occur in stoichiometrically simple reactions; a certain complexity in the reaction scheme is a necessary condition. In the case of single reactions the basic form of the concentration curves does not depend on which step is rate determining: the concentration of starting reactants always decreases monotonously and that of products increases monotonously. Some of the phenomena discussed for complex reactions are bound by the condition that the reactions involved take place on the same sites of the catalyst. This is, for example, the case treated by Thomas and coworkers [scheme (111) and Fig. 51 or the case of parallel reactions controlled by adsorption of substance A (Fig. 4b), or of consecutive reactions controlled by simultaneous desorption of product C and intermediate product B (Fig. 4c). If single reactions took place on different centers [e.g. in so called polyfunctional catalysts ( S S ) ] , these cases of “adsorptiondesorption disguises” would not occur. Other cases, particularly those of purely thermodynamic control of the composition of the reaction mixture, may occur also on catalysts with different centers for individual reactions. The study of the phenomena which may be caused by slow adsorption or desorption and the analysis of concentration dependences a t least qualitative, is important in the investigation of coupled heterogeneous catalytic reactions. If the structure of the reaction network is not known, one should be careful not to be misled by the apparent kinetics measured in the bulk phase and not to suggest incorrectly the reaction scheme and sequence of individual reactions. On the other hand, if the arrangment of the individual reactions in the reaction network is safely ascertained one can judge from the form of the concentration curves the nature of the slow, i.e. ratedetermining steps; in contrast to stoichiometrically simple reactions, in the case of coupled reactions these curves provide further independent information. This is important, since the usual estimation of the nature of the rate-determining step from the form of the rate equation r = f(cj) of a single reaction is often ambiguous. The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater (32) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment (28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole.
18
L.
BERLNEK
C. SELECTIVITY AND RELATIVE REACTIVITY In stoichiometrically not simple reactions, the important problem from a practical point of view is that of selectivity, i.e. to what extent, along with the desired transformation, there proceed other reactions, either consecutive or parallel. This is, in general, a very complex problem, since it is connected with the properties of catalysts, reactants, reaction conditions, etc. In this paragraph we wish only to show which specific features in the study of selectivity and in its definition arise from the fact that single reactions in a complex system may be coupled through a solid catalyst. We will pay attention also to competitive reactions, where two or more substances are purposefully allowed to react simultaneously and where, instead of selectivity, we speak rather about relative reactivity, Irrespective of how selectivity (or relative reactivity) is defined, it holds that the selectivity found by a study of coupled catalytic systems differs from the selectivity which would result from the kinetics of separately proceeding reactions. This specific feature does not exist in noncatalytic systems and it manifests itself in different ways, depending on whether purely thermodynamic control of the composition of the reaction mixture (adsorption or desorption of a reaction component is the limiting step) or kinetic control (surface reactions are the limiting steps) is the determining factor. Let us take an example of thermodynamic control of consecutive transformation according to scheme (IIa), in which adsorption of the common reagent X (Fig. 4a) or simultaneous desorptions of products B and C (Fig. 4c) are the limiting steps. I n these cases the rates of both independently proceeding single reactions are either the same (the rate of adsorption of X is rate determining) or are commensurable (similar rates of desorption of B and C). The form of the concentration curves of coupled reactions should therefore be similar to that shown in Fig. 3b, which illustrates a typical course of noncatalytic consecutive reaction (or of catalytic, kinetically controlled reaction with comparable rate and adsorptivity in both steps). The form of the curves in Figs. 4a and 4c, however, is different. Coupling of the reactions results in the selectivity which is determined by thermodynamic constants, e.g. in the case demonstrated in Fig. 4a by the ratio of the equilibrium constants of the first and the second reaction K 1 / K 2 (= 0.5) (62), instead of the ratio of the kinetic constants. In a general case, the concentration curves of substances B and C, important for a definition of selectivity, may have in Fig. 4a any form, depending on the K 1 / K 2ratio, regardless of the rates of separately studied reactions. Also from the examples shown in Fig. 5 (the transient case where no step is clearly rate determining) it is evident that the selectivity of the consecutive reaction A -i B -iC, as estimated from the curves, will be in
KINETICS OF COUPLED CATALYTIC REACTIONS
19
many cases quite different from the selectivity which would be obtained if all the adsorption-desorption steps were fast enough and the transformation were controlled only by the kinetics of the surface reactions A --$ B and B + C (see Fig. 3b). The case of transformations controlled by the rate of the surface reactions can be demonstrated on a system of two reactions with a common reagent, which may proceed either as consecutive or parallel processes A
4X + products UVa)
B 4X -+ products UVb)
If the rate of both single reactions is expressed separately, e.g. by means of the equations of Langmuir-Hinshelwood type (when written in a more general way and if the mechanism of both reactions with common reagent X will be the same), we obtain T1,isol
Tz,isol
+ (KAcA)"+ ( K X ~ X ) ~ ] ~ , = ~(KBCB)@(KXCX) ?/[I + (KBcB)"+ (KX~X)~]'. (12b)
=
ki(KAca)a(Kxcx)f/{l
For simplicity, the adsorption of the products is not considered. If the ratio of the rates of both single reactions were used to express the selectivity, we would obtain
In a system coupled through active centers the rates of the individual reactions are, however, T1,ooupl
=
h(KACA)"(KXCX)'/[1
+
(KACA)'
+
(KXcXlw
f
(KBCB)']',
(144 ~ 2 , c 0 u p 1=
+
~(KBcB)S(KXCX) E/[I
(KBcB)'
+ (KxcxIw+ ( K A c A ) ~ ~ : (14b)
and their ratio is given by the expression rl .coupl/r2
,coup1
=
kl(KACA)OL/k2(KBCB)'
scAa/cB'.
(15)
The relation (13) for isolated reactions becomes identical with the expression (15) for coupled reactions when (KAcA)' = (KBcB)",while, on the other hand, the difference in the values of the expressions (13) and (15) will be the greater, the greater is the difference in the adsorptivity of sub-
20
L.
BERANEK
TABLE I1 9
Rate Constants k (mmole min-1g-1) of Isolated Reactions, and Relative Reactivities S from Competitive Reactions Obtained in the Hydrogenation of Aromatic Hydrocarbons Isopropyl- EthylQuantity Benzene Toluene benzene benzene k
S
49 18
38 4.2
27.5 1.8
22.4 3.4
pm0TetraXylene Xylene Xylene line
12.9 1
10.4 0.8
7.8 1.2
6.7 2.8
Temperature 170"C, catalyst Raney-Ni, elevated pressure. Reproduced by permission from J. P. Wauquier and J. C. Jungers, Bull. Sac. Chim. Fr. p. 1280 (1957). (1
stances A and B. In any case the selectivity factor S differs, of course, from the factor for noncatalytic competitive reactions (S = kl/k2) in that it includes also the adsorption coefficients of competing substances. If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S , determined by the method of competitive reactions (Table 11). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 60) have already been discussed (see p. 12). With consecutive reactions, this difference in expressing the selectivity may be demonstrated on a model of the simplest consecutive catalytic system A 3 B -+ C with the kinetics described by Eqs. (lla) and (llb) and with the constants kl = 1, k z = 50, K A = 100, KB = 1,and K c = 0. By putting these values into Eq. (15) (a = @ = l),the selectivity in the coupled system is equal to 2; this case is presented in Fig. 3a. The ratio of
KINETICS OF COUPLED CATALYTIC REACTIONS
21
the rates of separately studied reactions is
from which, by substituting the above values of the constants, e.g. for CA = CB = 1, it follows that ~ l , i ~ ~ l / ~ p ,= i ~0.04; ~ l the selectivity in the coupled system is, however, fifty times higher. The estimation of the selectivity of the formation of the intermediate product B on the basis of the ratio of the rates of single reactions determined separately (which is possible with noncatalytic reactions) has met with failure. The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. It should be noted that many practically important catalytic transformations (such as isomerization of or hydrocracking of paraffins), which are presumed to proceed via consecutive mechanisms, are performed on multifunctional catalysts, with which the coupling of reactions in the sense just discussed may not necessarily occur. The problem of the selectivity of some models of polystep reactions on these catalysts has been discussed in detail by Weisz (56).
D. NONSUITABILITY OF THE POWER-LAW TYPEEQUATIONS As follows from our experimental studies to be discussed in Section IV, the kinetics of single reactions in the initial region could in most cases be better described by equations of the Langmuir-Hinshelwood type than by power-law type equations, Similarly, for describing the effect of products and of mutual influencing of the reactions, and thus for describing the kinetics of coupled catalytic systems in general, the equations of the first type are particularly suitable. The use of the power-law equations meets with difficulties already in the description of a single reaction, e.g. of the type A B + C (T = k p p B~b pee), ~ since these relations are unable to express the initial reaction rate. If the partial pressure of product C equals zero ( p c = 0), one obtains for the initial reaction rate either zero value (TO = 0 ) (for positive values of the exponent c; c > 0), or an infinite value (To = a) (for negative values of the exponent c; c < 0). This makes the usual nu-
+
22
L.
BERXNEK
merical integration of the equation starting from initial conditions impossible. Another disadvantage arises in the description of the kinetics of a consecutive reaction. In order to express an inhibiting effect of products, which was found experimentally in most cases, the exponent at the partial pressure of the product in the power-law equation should be negative. If, however, this substance undergoes further transformation in a subsequent reaction, this exponent should be positive, since the substance becomes the starting compound in this reaction. Since for each substance present in the coupled reaction system we must have two different parameters, the number of the constants to be determined in the equation system describing the kinetics of the overall transformation will markedly increase. By contrast, equations of the Langmuir-Hinshelwood type can describe the course of the coupled reaction from its very beginning and require many fewer constants.
IV. Experimental Kinetic Studies of Some Systems of Coupled Reactions
In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations a t all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material (27-90,37, 48, 5962). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain a t least some brief information on the relevant literature. Some of these studies were already discussed in Section I1 from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. These are above all consecutive transformations, in particular two-stage
KINETICS OF COUPLED CATALYTIC REACTIONS
23
ones. Of the hydrogenation reactions, it is worth mentioning, for example, the hydrogenation of phenol via cyclohexanone to cyclohexanol on Raney nickel (18) or on a palladium catalyst (62, 62u), the analogous reaction of cresols on platinum group metals (23, 63), the hydrogenation of mesityl oxide via methylisobutylketone to methylisobutylcarbinol on copper chromite catalyst (39),the stepwise hydrogenation of 2-methyl-3-butyne2-01 to the saturated alcohol on a palladium catalyst (64), or the stepwise hydrogenation of nitrile groups in isophthalonitrile on a nickel catalyst (65). The examples of alkylation reactions may be the stepwise alkylation of benzene or isopropylbenzene by propylene on silica-alumina (66, 67) and reductive alkylation of 4-nitrodiphenylamine by acetone (68);there should be also mentioned the hydrodealkylation of alkyl-substituted aromatic hydrocarbons on a commerical Detol catalyst (22), or the stepwise dehydrogenation of diethylbeneene to divinylbenzene on a commercial FezOsZnO catalyst (40).Also studied was the stepwise alkylation of amines (19) or of aniline (69) on alumina or thoria, which may be complicated by other reactions (70). In the case of consecutive reactions the formation of the final product may sometimes appear as a parallel reaction to the formation of the intermediate product, so that some authors consider the scheme
A-
\*/,
as, for example, in the hydrogenation of linoleic acid ester on a platinum or a nickel catalyst (54), the hydrogenation of cresols on platinum metal catalysts (23, 63), or in the cyclohexane-cyclohexene-benzene interconversion on Pt/A1203 catalyst (32). In amooxidation of xylenes on a vanadium catalyst (38,38u) this scheme is complicated by reactions which lead to the formation of carbon dioxide, carbon monoxide, and hydrogen cyanide. Also complex is the scheme of amooxidation of propylene (71),or of hydrogenation of 4-vinylcyclohexene on a palladium catalyst (72),which is accompanied by isomerieation of transiently formed mono-olefins. The above-mentioned parallel-consecutive scheme was also employed to interpret the kinetics of some oxidation reactions, in which the product C, formed both in parallel and in consecutive reactions, is carbon dioxide. This was the case, for example, in the oxidation of benzene on a vanadiamolybdena catalyst (73,74)and of furfural on a vanadium catalyst (76) to maleic anhydride, or of ethylene to ethylene oxide on a silver catalyst [cf. 37, 75a, 61.An analogous scheme, however, with more stages was also suggested in a study of the oxidation of anthracene (76, 77). One of the widely studied cases of a complex catalytic transformation is the triangular
24
L.
BERANEK
network of reversible isomerization reactions of n-butenes, which was kinetically treated first by Haag and Pines, who used the data obtained on sodium on support and on alumina (78);the same scheme and data for an alumina catalyst were used to demonstrate the already mentioned method of Wei and Prater (28). This system was studied on alumina and other acid catalysts by other authors (36, 79-84, 84a, 84b) also, along with the kinetics of an analogous scheme of isomerization of xylenes (31,84c).If reactions of butenes (Idb, 85,86) or other olefins (87) susceptible to isomerization are accompanied by another reaction, such as dehydrogenation (over chromia-alumina) or hydrogenation (on a palladium or a rhodium catalyst), the kinetic description is more complicated. The reaction scheme is rather complex also in the case of the oxidation of o-xylene (Qla, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Nikieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-A1203 catalyst (87f)or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of nand sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87%). Purely parallel reactions are e.g. competitive reactions which are frcquently carried out purposefully, with the aim of estimating relative reactivities of reactants; these will be discussed elsewhere (Section 1V.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclopentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (@), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). The studies mentioned in this brief account did not concern the kinetics of complex reactions taking place on the so-called polyfunctional catalysts, which were treated by Weisz (66);the theory of the use of these catalysts has been further worked out for some consecutive or parallel reactions carried out in the reactors with a varying ratio of catalyst components along the catalyst bed [e.g. (90, 91, 91a)]. Although the description of these reactions, not coupled in the sense defined in Section 111, is beyond the scope of this treatment, we mention here at least an experimental
KINETICS OF COUPLED CATALYTIC REACTIONS
25
kinetic study of the isomerization of n-pentane on Pt/A1203 (92) or the simultaneous steam reforming reaction of methane and the hydrogenation of nitroaromatic compounds on an intimate mixture of two commercial catalysts (93). The aims of the kinetic studies of complex heterogeneous catalytic reactions performed so far were very different; the same holds also for the methods used and for the quality of the kinetic analysis. Some of these studies aimed at obtaining a formal mathematical description of the kinetics of the system, frequently for purposes of industrial application; others focused on a kinetic description for expressing the selectivity of a catalyst or the relative reactivity of reactants. In some studies an attempt was made to obtain from the kinetic analysis some information on the mechanism of the catalytic reaction. The interpretation of the kinetics was frequently oversimplified, e.g. by the use of first-order equations or of equations with fractional exponents, and in many cases the kinetics of some single reactions were postulated without experimental evidence. There are still a few studies in which the kinetics of single reactions were studied separately and reliably and their aim was not the investigation of mutual influencing of single reactions in the coupled system. The aim of our investigations, whose results will be discussed in SUCceeding sections, was to obtain, by the isolation and separate study of single reactions, the kinetic equations which would best describe these reactions. By a systematic study of the effect of products and other reaction components on single reactions and by the study of a given transformation as a whole we wished to find whether and how the form of the rate equations of single reactions and the values of their constants obtained by the study of isolated reactions would change in the coupled system. This might show to what extent they have physical meaning. I n this way we wished to investigate and to describe mutual influencing of single reactions in a coupled system and to ascertain from its analysis whether further information can be obtained concerning the interactions of reactants with catalyst surface.
METHOD AND TREATMENT OF DATA A. EXPERIMENTAL 1. Apparatus and Procedure Kinetic measurements were made with a glass tubular one-pass fixed bed reactor. The internal diameter of the reactor was 9-12 mm, and the thermocouple well of external diameter 5-6 mm reached into the catalyst bed. The amount of the catalyst varied within the range of 0.01 to 1 g for pseudodifferential measurements (depending upon the activity of the catalyst
26
L. B E R ~ N E K
and the rate of the reaction) and from 0.1 to 6.0 g for integral measurements. The catalyst was mostly mixed with fine glass balls for the purpose of lowering the effect of the exothermicity or endothermicity of the reactions. The diameter of the catalyst particles was 0.16-0.4 mm. The reactor was placed either in a vertical glass tubular furnace equipped with electric resistance heating, or in a circulating silicone oil bath. The latter method was used in all hydrogenation reactions, which are exothermic, and in other reactions carried out a t temperatures below 200°C. The temperature was controlled automatically in such a way that inside the catalyst bed it was kept constant within f0.5"C. Liquid reactants were fed by a hypodermic syringe controlled by a synchronous motor with changeable gear. The feed rate was 2-10 ml/hr; the space velocity F/W was further changed by varying the amount of catalyst in the reactor. Liquid reactants were introduced from a feeding device into a glass coil evaporator equipped with an electric heating mantel in which their vapors were mixed with gaseous components (with hydrogen and/or nitrogen). The flow rate of these gases was adjusted with the aid of a manostat and a fine needle valve and measured with a differential capillary flow meter. The ratio of the feed rate of liquid reactants to the feed rate of gases determined the partial pressures of the reactants. Nitrogen was used to lower the total pressure of the reactants. The total pressure in the apparatus, inclusive of nitrogen, was atmospheric. Partial pressures of individual reactants were changed within the range of 0.015-0.95 atm, except for phenol and alkylphenols where the limit was 0.5 atm, due to the low tension of saturated vapors a t the reaction temperature. Liquid reaction products were condensed by cooling with dry ice. The steady state in the reactor was checked by following the changes in conversion. After the steady state had been established the samples (as a rule, three 1 ml samples) were taken at a fixed time interval and subjected to analysis. The conversion for evaluating the reaction rate was determined as the mean value of the analysis of these three samples. Liquid products were analyzed by gas-liquid chromatography. The stationary phases and conditions of analysis for individual reaction systems were reported in original papers (61, 94-98). For each of the reactions studied, the conditions were first found (size of catalyst particles, mass feed rate) which ensured that the measured reaction rate was not influenced either by internal or external diffusion. 2. The Catalysts
The kinetic study of a given reaction system was always performed with samples of the catalyst taken from the same batch; in most cases each
KINETICS OF COUPLED CATALYTIC REACTIONS
27
determination of the reaction rate was made with a fresh sample. The stability of the activity of each catalyst was followed during its use. Nickel-aluminum oxide catalysts [%yoA1203 and 8.40/, (wt.) A12031 were prepared by coprecipitation of nickel and aluminum nitrates (5y0 solution) with 5Y0 ammonium hydroxide solution (the catalyst with 55% A1203) or with 5% potassium hydroxide solution (the catalyst with 8.4y0 A1203). After repeated washing with distilled water (until a negative test on NO; ions), the obtained gel was dried at' 105°C. Each sample was reduced directly in the reactor at 350-400°C for 1-2 hr. The reduced catalyst with 55% A1203 had a BET surface area of 278 mz/g, that with 8.4% A1203 had a surface area of 88 m"g. A platinum on silica gel catalyst was prepared by impregnation of silica gel (BDH, for chromatographic adsorption) by a solution containing 0.5% (wt.) of sodium hydroxide and 0.5% (wt.) of chloroplatinic acid (both of analytical grade). The dried catalyst contained 1% (wt.) of platinum and a corresponding amount of the alkaline component. The BFT surface area of the catalyst was 40 m2/g, the mean pore radius 150 A. The catalyst was always reduced directly in the reactor in a stream of hydrogen a t 200°C for 2 hr. Thorium oxide on activated carbon was prepared by absorption of thorium nitrate from its solution in anhydrous acetone on the activated carbon Supersorbon. The excess solution was decanted, the catalyst was dried a t 8O"C, and the adsorbed thorium oxide was decomposed by excess 5% ammonium hydroxide solution. After repeated washing and decantanation with distilled water and acetone, the catalyst was dried at 180°C. It was then stabilized by heating to 360°C for 5 hr in a stream of nitrogen. The content of thorium oxide was 2.9% (wt.). The BET surface area was 870 m2/g. Prior to kinetic measurements, the catalyst was modified by passing over acetic acid vapors (100 g acid/l g catalyst). A sulfonated ion exchanger catalyst (Research Institute of Synthetic Resins and Varnishes, Pardubice, Czechoslovakia) was a macroreticular styrene-divinylbenzene copolymer containing 25% divinylbenzene and 2.4 meq/g of -SOsH groups. It was dried prior to using a t 90°C/14 Torr. The BET surface area, determined in a dry state, was 49 m2/g, and the mean pore size was around 100 A. A platinum-iron on silica gel catalyst was prepared by impregnating silica gel (BDH, for chromatographic adsorption) with an aqueous solution of chloroplatinic acid (analytical grade) and sodium hydroxide (analytical grade). The dry product was then impregnated by a ferrous sulfate solution (C.P. grade) and the water was removed in a rotating evaporator. The prepared catalyst contained 1% Pt, o.7y0Fe, and 2% NaOH (by
28
L. B E R ~ N E K
weight) and its BET surface was 370 m2/g. The catalyst was reduced directly in the reactor in a stream of hydrogen at 200°C for 2 hr. 3. Treatment of Data
By plotting the found conversions x of the single reaction under study against the reciprocal space velocity W / F in the region of very low conversions, we obtained the values of the initial reaction rates as slopes of the tangent a t the origin. Up to z = 0.1, the experimental dependences z = f ( W / P )were linear in the investigated reactions. For a description of experimental data we derived (according to the type of the reaction) twenty to thirty equations on the basis of Langmuir’s presumption of the identity of active sites, and that for different rate-determining steps and different modes of adsorption of the reaction components. The power-law equation was also used. For each single reaction we determined 12-30 values of the initial reaction rates (according to the complexity of the presumed mechanisms), using several different combinations of partial pressures of the reactants. The data obtained were treated by a combined method of linear and nonlinear regression (‘7) using a computer. The relations which did not satisfy even the intentionally chosen very low regression coefficient as a criterion of linear regression were rejected; the remaining ones were then treated by nonlinear regression, the values obtained in linear regression being used as the first estimates. The selection of suitable equations by nonlinear regression was based on the so-called critical value of the sum of squared deviations (7,99). An analysis of the effect of products and other reaction components, carried out experimentally in the same apparatus, however, with varying feed composition, was performed on the basis of the best equation from nonlinear regression, after its extension by the corresponding term. For this purpose we frequently tested the assumption that the added component is adsorbed on the same centers (either on one or on two) on which the studied reaction is taking place. Experimental data were treated either graphically after linearization of the extended equation (e.g., 61, 94, 100) or by one-parameter nonlinear regression (95). The set of obtained rate equations with optimum values of the constants was integrated on a computer; by this integration we finally obtained the curves of the concentration dependence of individual reaction components on reciprocal space velocity (“contact time”). B. CONSECUTIVE HYDRODEMETHYLATION OF XYLENES Using this reaction as an example, we would like to illustrate in detail the procedure used in our studies. Hydrodemethylation of xylenes was investi-
KINETICS OF COUPLED CATALYTIC REACTIONS
29
gated in the gaseous phase on a Ni-Alz03 catalyst (55% wt. A l ~ 0 3 see , p. 27) at 330°C. The reaction proceeds stepwise: at first toluene is formed by cleavage of one of the methyl groups and subsequently toluene is transformed to benzene. In both steps one molecule of methane is formed [scheme (V)]. The reactions are irreversible under the conditions used and isomerization of xylenes was not observed to occur to a significant extent.
First, the kinetics of the reactions of 0-, m-,and p-xylene as well as of toluene were studied separately (96)at various combinations of initial partial pressures of the hydrocarbon and hydrogen. From a broader set of 23 rate equations, using statistical methods, we selected the best equations for the initial rate and determined the values of their constants. With xylenes and toluenes, these were Eqs. (17a) and (17b).
+ = kzKcpcOp&,/(l + Kcpc”2.
ri0 = k i K ~ p ~ ~ p & 1 , / (K A ~ A ’ ) ~ ,
(17d
r20
(17b)
A further procedure will be described only for m-xylene, for which we obtained the following values of the constants: kl = 173.7, k~ = 84.2 mole hr-l kg-1 atm-I; K A = 20.6, Kc = 25.8 atm-l. The conclusions drawn from the study of consecutive hydrodemethylation were similar for all the three xylenes studied (100). The influencing of individual reactions by products and by the intermediate product was determined experimentally, by measuring their effect on the reaction of m-xylene and toluene. The adsorption coefficients, which express this effect, are listed in Table 111. TABLE 111
Values of Adsorption Coeficients Ki (atm-1) of Products Obtained in the Study of Demethylatwn of m - X y l e m and Toluene Demethylation of Substance j m-Xylene Toluene Toluene (C) Benzene (D) Methane (E)
20.9 13.5 4.6
25.8 15.7 4.2
30
.
L. B E R ~ N E K
As follows from the table, each of these compounds affects both reactions in practically the same way. This may indicate that both reactions proceed on the same sites of the surface of the catalyst and that each of the compounds involved is adsorbed on these centers in only one way. The experimentally found effect of the products and intermediate product could be best expressed by extending the equations (17) to the forms: Ti
=
+
+
dzi/d(W/F) = kiKapapH,/(1 -b KAPA Kcpc -b KDPD KEPE)~, ( W
r2
=
d x ~ / d ( W / F )= kaKcpcpH,/(l
+ Kcpc + K A ~+AKDPD+ KEPE)~. (18b)
The verification of the ability of this set of differential equations (Ma) and (18b) to describe the behavior of the overall consecutive process of the demethylation of xylene via toluene to benzene was made by its numerical integration on the computer, after actual partial pressures of the reactants had been expressed as functions of the conversions x1 and x2 and of the initial partial pressures p A 0 and pkI. In the integration we used the values of the constants obtained for isolated reactions; for each compound a single value of the adsorption coefficient (for the products these were the mean values from Table 111) was used in both Eqs. (Ha) and (18b) [i.e. for both reactions (1) and (2)]. The curves obtained can be compared with the experimental dependences of relative concentrations of xylene, toluene, and benzene on reciprocal space velocity, which we measured to high total conversions (100). Figure 6 presents the result for m-xylene, obtained a t the initial ratio of the reactants pkJpA" = 5 ; similar results were obtained also with other xylenes and with other ratios of the reactants. From the agreement between the calculated curves and experimental points it follows that the form of the rate equations and the values of their constants, obtained by the study of isolated reactions, are valid also in the case of the coupled system, where more substances are simultaneously adsorbed on the surface of the catalyst and where several reactions are taking place simultaneously on it. The equations express the fact that mutual influencing of single reactions consists in the competitive adsorption of reaction components on identical sites of the catalytic surface. The form of experimental dependences shown in Fig. 6 indicates that slow steps are arranged consecutively in the coupled system and that desorption of the intermediate product, toluene, is fast compared with its further transformation to benzene. Otherwise, the concentrations of these two compounds (at least a t the beginning of the reaction) would have shown a parallel increase [compare with (16)and with Fig. 51. Similarly,
KINETICS OF COUPLED CATALYTIC REACTIONS 1.01
I
I
I
31
I
0.5
0
0.2
0.1 W/ F
FIG.6. Dependence of relative molar concentrationsnj/nAoof reaction components on reciprocal space velocity W / F (hr kg mole-') in the consecutive demethylation of mxylene. Temperature 330"C, catalyst Ni-AlZOz (55% wt. AISOS),initial molar ratio of reactants G = 5. The curves were calculated (I-xylene, 2-toluene, 3-benzene); the points are experimental values.
also the adsorption of xylene or of hydrogen, or the desorption of benzene or of methane cannot be rate-determining steps, since in such a case the composition of the reaction mixture would have been determined by thermodynamic factors [(52, 53); see also Fig. 4). In the case of these irreversible demethylation reactions, the form of the curves in Fig. 6 should be different. From the study of coupled reactions it may be therefore assumed that adsorption-desorption steps in the studied transformation of xylenes are not, very likely, rate determining, which is, after all, in agreement with the form of the rate equations (17) obtained by a statistical treatment of initial rate data on isolated reactions. C. CONSECUTIVE HYDROGENATION OF PHENOL
A procedure similar to that used in the investigation of the hydrodemethylation of xylenes was also employed in a study of the consecutive hydrogenation of phenol via cyclohexanone to cyclohexanol in gaseous phase on a platinum on silica gel catalyst (p. 27) at 150°C [scheme (VI)]; at this temperature both reactions were irreversible under the excess hydrogen used. 0
4
(A)
H
+an2
__f
0=o 2 5 0
-OH
(1)
(2)
(C) (VI)
(D)
32
L.
BERANEK
The kinetics of hydrogenation of phenol has already been studied in the liquid phase on Raney nickel (18). Cyclohexanone was proved to be the reaction intermediate, and the kinetics of single reactions were determined, however, by a somewhat simplified method. The description of the kinetics of the hydrogenation of phenol in gaseous phase on a supported palladium catalyst (62)was obtained by simultaneously solving a set of rate equations for the complicated reaction schemes containing six to seven constants. The same catalyst was used for a kinetic study also in the liquid phase (6%). In our study we first investigated separately the kinetics of the hydrogenation of phenol and of the hydrogenation of cyclohexanone (7), and from twenty-six different equations, using statistical treatment of the data, we found the best equations for the initial reaction rates to be
+
no = ~ K A P A ~ ( K R , P R , ) ~K/APA" (~
+
+
+KH~PRJ~,
rZo = ~ ~ K C ~ C ~ K H , P KcpcO R , / ( ~ KH&,)~.
(194 (19b)
Experimental measurement of the effect of the products and other components added in the first and second reactions revealed that the adsorption of all the substances present is competitive, and that this effect may be best expressed by an extension of Eqs. (19)to Eqs. (20). T1
=
dn/d(W/F) =
~XAPA(KH~PH $.JKAPA ~ / ( ~-k K H ~ P-kHKcPc ~ -k K D P D ) ~ ,
=
dx2/d(W/F)
=
h K c p c K ~ , p ~ , / (-k1 Kcpc -f K H ~ P H KAPA ~ -k KDPD)'.
(204 r2
+
(20b) The integration of this equation system was carried out numerically on a computer, after the partial pressures had been expressed as functions of the conversions :
- Zi)P/(1 -k G - 221 - 22), = (G - 221 - ~ z ) P / ( + 1 G - 221 - ZZ), = (21 - ~ z ) P / ( + 1 G - 2x1 - ~z), = Zzp/(1 + G - 221 - 22).
pn = ( 1
(2W
(21b) pc (21d PD (21d) I n the integration we used the following values of the constants, which were obtained by the study of single reactions and of the effect of the products: kl = 877, Ic2 = 28.7 mole hr-' kg-l; K A = 9.3, K H * = 1.01, K C = 7.7, and K D = 5.3 atm-l; total pressure P = 1atm. The agreement between the calculated curves and the experimentally measured integral PH,
33
KINETICS OF COUPLED CATALYTIC REACTIONS
0
0.5
1.0 WIF
FIQ.7. Dependence of relative molar concentrations nj/nAo of reaction components on reciprocal space velocity W / F (hr kg mole-’) in the consecutive hydrogenation of phenol. Temperature 150°C,catalyst Pt-SiOz (l’%wt. Pt), initial molar ratio of reactants G = 9. The curves were calculated (1-phenol, 2-cyclohexanone, 3-cyclohexanol) ; the points are experimental values. From Ref. (61). Reproduced by permission of the copyright owner.
dependences of relative concentrations on the reciprocal space velocity (51) is evident from Fig. 7. Also here, as in the demethylation of xylenes, the behavior of each reaction component could be expressed by a single value of the adsorption coefficient; from mutual influencing of the reactions (competive adsorption) it became clear that all the substances participating in the reactions are adsorbed on identical sites of the surface. We also performed the integration of the rate equation for the hydrogenation of cyclohexanone itself [Eq. (20b) with pa = 01 and measured experimentally the dependence of its conversion on the reciprocal space velocity W/F. The comparison with the data obtained starting from phenol revealed that at the same “contact time” W / F and at the same hydrogen to organic substance initial ratio (G = 19) the conversion to cyclohexanol (z2) is higher if we start from phenol than when we start from cyclohexanone, although in the first case cyclohexanone must be first formed from phenol (Table IV). This result, obtained both by calculation and by experiment (51), could be explained by coupling both reactions through the common reagent, hydrogen. If a large excess of hydrogen (G = 19) is used at the beginning of the reaction, the surface is occupied predominantly by this reagent. If we start from phenol, two moles of hydrogen are consumed in its hydrogenation to cyclohexanone and then a larger part of the surface becomes available for the organic compound and the hydrogenation of cyclohexanone proceeds faster, compared with the case where we started directly from this compound at the same hydrogen to organic substance
34
L. B E R ~ N E K
TABLE IV A Comparison of the Degrees of Crmversion to Cyclohexanol (52) in the Hydrogenation of Phenol and of Cyclohexanone, respective& 5 2 found for 5 2 computed for hydrogenation starting hydrogenation starting from from
W / F (hr kg mole-')
Phenol
Cyclohexanone
Phenol
Cyclohexanone
0.48 0.85
0.66 0.96
0.51 0.78
0.62 0.84
0.55 0.82
4 Initial molar ratio of hydrogen to organic substance G = 19. From Ref. (61). Reproduced by permission of the copyright owner.
initial ratio (G = 19). This assumption may be supported by the analysis of the rate equation (19b) for the hydrogenation of cyclohexanone. In agreement with the above hypothesis, experimental and calculated results showed that this increase in the conversion x2 did not occur on using a smaller initial excess of hydrogen (G = 9). The phenomenon discussed thus means that hydrogen is adsorbed on the same centers as is the organic substance; this fact is, after all, expressed by Eqs. (19) and (20). It is worth noting that the ratio of rate constants kl/kz = 30.55 of the first and the second reaction, as well as the ratio klKA/k2Kc = 36.95,i.e. the expression by which the selectivity of catalytic reactions is usually defined, is quite high and it would correspond to the maximum relative concentration of the intermediate product around 90%, which contrasts with Fig. 7. However, it is necessary to bear in mind that the rate equations (20a) and (20b) for the first and the second reaction are of different form and that the expression (22) results for the selectivity from their ratio. S ~ X A & X , P K , / ~ Z KKAPA C ( ~ K H ~ P H PKcpc KDPD). (22)
+
+
+
+
By an approximate evaluation of this expression, starting from e.g. the initial partial pressures PAO = 0.1 atm and p R , = 0.9 atm and the above mentioned values of the constants, we obtain S 5 14; the maximum concentration of the intermediate product corresponding to this value is about SO%, in agreement with the experimental results in Fig. 7. We attempted to describe the integral dependences in Fig. 7 also by the power-law equations TI = kip~~pk,pc 'PD~, (23a) c' b' a' d' (23b) r2 = k2PCPH,PAPD-
KINETICS OF COUPLED CATALYTIC REACTIONS
35
By an approximate analysis of the experimental dependences in Fig. 7 using Eqs. (23) (data for the initial rates cannot be described by these equations) we obtained the following values of the constants kl = 4 mole hr-1 kg-1 atm-(a+b+++d),kz = 0.52 mole hr-1 kg-1 atm-(c’+b’+a’+d’), a = 0.25, U’ = -0.56, b = 1.9, b’ = 1.0, c = -0.28, C’ = 1.09, d = -0.18, and d‘ = -0.43. The values of the exponents at the partial pressures of the reaction components for the first and the second reaction differ markedly from one another and the description of the kinetics of the overall transformation by means of power-law equations requires the use of ten constants, while when using the equations (20a) and (20b), only six constants are necessary. Using similar arguments as with the demethylation of xylenes (p. 31) we can assume from the form of the integral dependences shown in Fig. 7 that here also neither adsorption nor desorption is a rate-determining step. This is, after all, in agreement with the form of the best equations (19a) and (19b) found for the initial reaction rates of single reactions.
D. PARALLEL KETONIZATION OF CARBOXYLIC ACIDS In contrast to consecutive reactions, with parallel competitive reactions it is possible to measure not only the initial rate of isolated reactions, but also the initial rate of reactions in a coupled system. This makes it possible to obtain not only the form of the rate equations and the values of the adsorption coefficients, but also the values of the rate constants in two independent ways. For this reason, the study of mutual influencing of the reactions of this type is centered on the analysis of initial rate data of the single and coupled reactions, rather than on the confrontation of data on single reactions with intergal curves, as is usual with consecutive reactions. Parallel ketonization of acetic acid and propionic acid was one of the transformations of this type studied in our Laboratory. RybAEek and Setinek (94) investigated the kinetics of these reactions in the gaseous phase at 316°C using thorium oxide on activated carbon (p. 27) as the catalyst. This model system allowed the study of each reaction separately as well as of the simultaneous conversion of both acids. 2 CH&OOH + CHsCOCH3
+ CO, + HzO
(VIIa) 2 CzHaCOOH + CzHsCOCzHa
+ COz + Hz0
(VIIb)
In the latter case, along with the reactions (VIIa) and (VIIb) the cross reaction (VIIc) of both acids also takes place yielding the unsymmetrical
36
L.
BERANEK
ketone, methylethylketone CHiCOOH
+ CzHaCOOH
---t
CHsCOCzH:,
+ COz + Hz0
(VIIIC)
On the basis of regression analysis of the initial rate data obtained for both isolated reactions (VIIa) and (VIIb) and for each of the three reactions proceeding in the coupled system [reactions (V1Ia)-(VIIc)], of the set of twenty-five equations, the best equation was always found to be of the same type. For an isolated reaction the best one was Tio
=
ki(KACpic)'/(1
+ KAcpi~)~,
(24)
and for parallel reactions they were
+ KA h(KAc,picJ2/(1+ KA
rl0 = kl(KAcipic1)2/(1 r2O
=
c$Acl
+ KAenPicr)'~
(254
KAczPid3,
(25b)
CIPACI O $.
k ~ ~ ~ ~ , K ~ ~ ~-k pK i A, g~ i~-k~PK ~~ /,A(~ ~~ ~ P ~(254 ~ . The values of the rate constants and adsorption coefficients obtained by the study of isolated reactions agreed well with those obtained by the study of parallel reactions (Table V). The three values of the adsorption coefficient of each acid were obtained independently. In addition to one value from the study of isolated reactions, two additional values were determined by the study of the parallel system: one from the kinetics of the consumption of the given acid by reaction (VIIa) or (VIIb), and one from the kinetics of reaction (VIIc). =
TABLE V Values of Rate Constants ki (mole hr-lkg-l) and Adsorption Coeficients K j (atm-l) Obtained in the Study of Ketonization
Isolated reactions CHsCOOH (Aci) 36.9 CZHGCOOH ( A c ~ ) - 16.5
-
25.4
-
- 12.6 41.1 12.7
1.4 0.6
Parallel reactions CHtCOOH C~HGCOOH CHsCOOH CZHGCOOH
+
37.5
-
19.0
-
-
49.5 38.1 21.6 48.5
25.0
-
-
-
-
KINETICS OF COUPLED CATALYTIC REACTIONS
37
The results obtained showed, again, that the form of the rate equations and the values of their constants, 'obtained by the study of isolated reactions, are valid also in the coupled system. This was also confirmed by the observed agreement between the calculated and the experimental integral data (94). Kinetic results and the analysis of the effect of reaction products revealed that adsorption of the reaction components was competitive and that all the compounds involved in the three reactions were adsorbed on the same sites of the catalytic surface. Using the results of the study of the ketonization reaction it may also be demonstrated how selectivity in the coupled system differs from the ratio of the reactivities of the separately reacting acids. On dividing the equations (25a) and (25b), and using the following constants Icl = 36.9, kz = 16.5, = 25.4, and K A =~41.1 ~ (Table V), we obtain the ratio = 0.85. of the rates in the coupled system (if p i c , = p i o ) r~,ooup~/r2,coupl 0 By dividing equations of the type (24) for acetic and propionic acids in the isolated system and using the same values of the constants, we obtain 0 Ti,isol/Tz,isol = 2.55 (for p i c l = p i c 2 = 0.1 atm). Therefore, in the coupled system propionic acid is more reactive, while from the results of the study of isolated reactions it follows that acetic acid is more reactive, both of which were observed experimentally (94).
E. COMPETITIVE REACTIONS 1. Competitive ReesteriJicationof Organic Esters by Alcohols
This reaction was studied in the gaseous phase at 120°C with a sulfonated organic ion exchanger as catalyst (p. 27), under both the competitive 1 alcohol) and the system (VIIIb) (1 ester system (VIIIa) (2 esters 2 alcohols)
+
+
CH3COOC2Hs
+ CaHiOH
(Ad (CHs)&HCOOCzHs
+ CjHrOH
(A21
+ CaHsOH
4
CHsCOOCjH7
4
(CHa)zCHCOOCaHr
(Bd
(Bi)
(VIIIa)
+ CZHSOH
38
L.
BERANEK
The study of the initial reaction rates of isolated (97) and of competitive reactions (98) led to the best equations of the same type: single reactions TO
f
k' p A o p B o / [ l
=
KAPAo
+
(26)
(KBpBo)'12]3,
competitive systems: (VIIIa) To
= k'pAopBo/[l
+
K@A0
+
(KBpB0)'/2
+
KAjPIj13i
(27)
=
+
KAPAo
+
(KBPB0)'I2
+
(KBj&i)1/2]3,
(28)
(VIIIb) To
k'pAopBo/[l
where k' = kKAKB.These equations express the assumption that all the reactants are adsorbed on identical active centers. As is evident from Table VI, the rate constants and adsorption coefficients obtained from the study of isolated reactions are in satisfactory agreement with the values found for the competitive reactions. The rate constant k;,B1 for the reaction of ethyl acetate with propanol was determined in both competitive systems. For each reactant, the two values of the adsorption coefficient were deterTABLE VI Values of Kinetic Constants in the Reesteri3cation of Esters by Alcohols [Schemes (VllZa) and ( V l l l b ) ] ~
Constant" khl
~
~~
Competitive reactions 6.0b
kklBl
KBi
0.93d 0.98" 2.76*
KBZ
0. 16d
KA,
7.4s 0.84 12.5
kkZB1
KA1
~
1 .05c 0.8d 2.6d O.OOO3"
~
~
Single reaction 8.0 0.98 12.1 1.0 0.6 2.8 0.3
a Dimension of constants k' is (mole X 102 hr-lkg-'atm-*) and of adsorption coefficients K j is (atm-1). b From the study of competitive system (VIIIa). c From the study of competitive system (VIIIb). dFrom the rate data of the transformation of the given substance. From the rate-retarding effect on the transformation of the competing substance.
KINETICS OF COUPLED CATALYTIC REACTIONS
39
mined independently in the competitive system: one value from the rate data on transformation of the given substance and one value from its rateretarding effect on the rate of transformation of the competing substance. Both values were always quite comparable, except for the adsorption coefficient,of methanol, where the value determined from the analysis of its inhibiting effect on the competitive reaction of propanol with ethyl acetate was extraordinarily low; a t present, no explanation can be offered for this deviation. 2. Determination of Relative Reactivity bg the Method of Competitive Reactions
The reactivity of a series of substances in a given reaction is frequently determined by the method of competitive reactions, particularly in a homogeneous medium, where its kinetic interpretation is very simple ( S = kl/k2). This method was employed for relative reactivity determinations also in heterogeneous catalysis [see, e.g. (48, 61, lOl-ll4)I. The competitive method was used in the study of heterogeneous catalytic reactions for other purposes (1'7, 60, 111, 115-119). Relative reactivities were successfully determined also in the case where the competing substances (olefins) may isomerize to one another and where the reaction network is more complicated (25, 119a). In studies (111, 11.4, 117, 119a), it was also shown how the method of competitive reactions can be applied to solving some problems of formal kinetics. In the case of heterogeneous catalytic reactions the meaning of the quantity termed relative reactivity or selectivity is not, however, quite unambiguous. It is often considered as the ratio of the products of the rate constant and adsorption coefficient of competing substances [S = k i K ~ / kZKB, see also Eq. (15)]; this definition corresponds to the assumption that the competing reactions obey the same kinetics with surface reactions as rate-determining steps. There is often a need to separate the kinetic and adsorption equilibrium terms in the expression for S. If single reactions are described by simple kinetics (e.g. of zeroth order) kl and kz can be easily determined separately, which enables the relative adsorption coefficients K A / K Bto be obtained from the values of S (48, 61, 101-106, 108, 109). Somewhat more general methods for determining relative adsorption coefficients were suggested by Smith and Rader (101,202)andby Moro-oka and co-workers (118, 119). If the rate-determining step were not a surface reaction but adsorption of competing reactants, we should obtain from the corresponding equations the expression which is formally identical with Eq. (15), in which, however, the relative reactivity is given by the expression S = k s d s A / k s d s B . On the basis of data on competitive reactions only, these two cases cannot there
40
L.
BERANEK
TABLE VII Comparison of the Relative Reactivity Values Obtained b y Different Methods in the Reesterification of Esters
Method of calculation
Reaction Reaction system system (VIIIb) (VIIIa) SA,IA~ S B p m
From competitive integral data [Eq. (29)]
7.60
1.20
From competitive initial data [Eq. (15)] From kinetic constants (see Table VI) of competitive reactions of single reactions
6.85
1.31
7.14 8.17
1.69 1.50
fore be distinguished. Some authors assume that the relative reactivities obtained by them express rather the ratio of adsorption rates (108, 11%'). Maurel and co-workers (107) believe that in the hydrogenation of olefins on Pt-SiO,, a t higher hydrogen pressures, instead of surface reaction, adsorption of hydrocarbons becomes rate determining, as adjudged from the observed dependence of relative reactivity on hydrogen pressure. The above mentioned studies were in most cases performed with the aim of obtaining relative reactivities or relative adsorption coefficients from competitive data, sometimes also from the combination of these with the data obtained for single reactions. In our investigation of reesterification (97,98),however, a separate analysis of rate data on several reactions provided us with absolute values of rate constants and adsorption coefficients (Table VI). This enabled us to compare the relative reactivities evaluated by means of separately obtained constants with the relative reactivities measured by the method of competitive reactions. The latter were obtained both from integral data by means of the known relation
s = log(1 - zl)/log(l -
22)
(29)
and from initial rate data on competitive reactions according to Eq. (15). The values of relative reactivities obtained by different methods for reaction systems (VIIIa) and (VIIIb) are presented in Table VII. In spite of certain differences between individual values, it becomes immediately clear that the method of competitive reactions can yield even in a heterogeneous catalytic reaction the data which are reliable enough for reactivity studies and, furthermore, are in harmony with the physical meaning of the
41
KINETICS OF COUPLED CATALYTIC REACTIONS
quantity S such as follows from the detailed kinetic analysis of single reactions. We have further attempted to suggest a procedure which would make use of the advantages of the method of competitive reactions, i.e. its simplicity and little time demand, and at the same time would yield separately the absolute values of rate constants and adsorption coefficients also for reactions with a more complicated kinetics. Using the values of relative reactivities S from the method of competitive reactions, the adsorption coefficients, for example, of the alcohols (KB)in the reesterification reaction described by Eq. (26) can be evaluated from the relation
KB =
[(Slcr,rKB,r~fKAPAoPBo/r0)”3
- KApAo - l]*/pBO
(30)
In addition to the competitive measurements, i t is necessary to perform the kinetic analysis of only one, arbitrarily chosen reference substance, K B , ~and ~ ~K,a (A here is the common which will yield the values of her, reagent), and to determine the initial reaction rate for the alcohols to be compared (at least at a few different values of partial pressures PBO in order to get reliable values of K B ) .For propanol and methanol (and in a similar way also for ethyl acetate and ethyl isobutyrate) reacting in reactions (VIIIa) and (VIIIb), using this simplified method and the data from our studies (97, 98) we calculated the values of adsorption coefficients which are compared in Table VIII with the values obtained by a detailed kinetic analysis of each single reaction. The agreement is satisfactory and demonstrates the applicability of the suggested procedure, which might be advantageous particularly in studying the reactivity of a broad series of substances. We have also used this method in an investigation of the gasphase hydrogenation of phenol and alkylphenols on a nickel catalyst containing 8.4% A1203 (120). The results of the study of the hydrogenation of phenol and alkylphenols TABLE VIII Comparison of the Absolute Values of Adsorption Coeficients (atm-1) Obtained by Two Methods in Reesterification Reactions Method of calculation From the simplified method using relative reactivities and initial reaction rates From detailed kinetic analysis (cf. Table VI)
KA2
KBI
K%
0.87
0.74
3.4
0.28
1.0
0.57
2.8
0.34
KAI
42
L.
BERANEK
Fro. 8. Relationship between relative reactivities S and the ratios of the initial reaction rates qo/r*0 of alkylphenols to phenol in the hydrogenation on Ni-catalyst containing 8.4% (wt.) A1203 at 160°C and initial molar ratio of hydrogen to organic substances G = 19. Alkyl substituents in phenols: Me-methyl, E t - e t h y l , Pr+propyl, i-Prisopropyl, s-Bu--sec-but,yl, t-Bu-terc-butyl.
(190) may also be used to demonstrate how the relative reactivity S from competitive reactions differs from the ratio of the initial reaction rates of isolated reactions (Fig. 8); the question which of these quantities is more suitable for comparing reactivities in heterogeneous catalytic reactions has already been discussed (Section 1II.C). As follows from Fig. 8, for all the alkylphenols studied the values of the reactivities (with respect to nonsubstituted phenol) from competitive reactions are lower than the ratios of the rates of isolated reactions. This is obviously due to the fact that alkylphenols are adsorbed more weakly than phenol [cf. (190)l and that on coupling the reactions, phenol retards the reaction of the alkylphenol more strongly than the alkylphenol does the reaction of phenol. A similar effect was also observed in the reesterification of ethyl acetate by methanol (KB, = 0.3 atm-l) and by propanol ( K B ,= 2.8 atm-l). The relative reactivity of methanol with respect to propanol obtained by dividing equations of the type (28) for competitive reactions, after substituting for the constants from Table VI, equaled 1.7. The ratio of the initial reaction rates of single reactions, calculated from the equations of type (26) with the use of the values of the constants from Table VI, was found, however (e.g. for PAO = PBO = 0.5 atm), to be 4.3.Both results agree also with the experimental measurement.
KINETICS OF COUPLED CATALYTIC REACTIONS
43
From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (lit?), where the retardation of the reaction of l-butene by cis-2-butene was greater than the effect of l-butene on the reaction of cis-2-butene; the ratio of the adsorption coefficients K , i , - s - b / K 1 - b was 1.45. In a study on hydrogenation over C0304 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (1Sou).
F. PARALLEL-CONSECUTIVE HYDROGENATION OF CROTONALDEHYDE 1. Kinetic Study As an example of the system in which parallel and consecutive reactions 'occur simultaneously, we have chosen the hydrogenation of crotonaldehyde, which may lead through two two-stage paths (via butyraldehyde and via crotyl alcohol) to the same final product, butanol
In general, this scheme is frequently observed in catalytic transformations of bifunctional compounds containing nonequivalent functional groups. We used the catalyst containing 1% platinum and 0.7% iron on the silica gel alkalized by sodium hydroxide (p. 27), which gave a suitable ratio of the rates in both pathways. The reaction was studied in the gaseous phase phase at 160°C under the conditions which ensured that all the reactions mentioned were irreversible (95). We measured the initial reaction rates of the hydrogenation of crotonaldehyde simultaneously to butyraldehyde [reaction (l)] and to crotyl alcohol [reaction (2)] and further the rates of the hydrogenations of butyraldehyde [reaction (4)] and of crotyl alcohol [reaction (5)]. From the set of 32 kinetic models we selected the best equation for each single reaction, using combined linear and nonlinear
44
L.
BERANEK
WIF
FIG.9. Dependence of relative molar concentrations njln.40 of reaction components on reciprocal space velocity W / F (hr kg mole-’) in the parallel-consecutive hydrogenation of crotonaldehyde. Temperature 160°C, catalyst Pt-Fe/SiOt (1% wt. Pt, 0.7% Fe), initial molar ratio of reactants G = 10. The curves were calculated (1-crotonaldehyde, 2-butyraldehyde, 3-crotyl alcohol, 4-butanol); the points are experimental values.
regression. Both reactions (1) and (2) of crotonaldehyde and the reaction (4) of butyraldehyde were best described by the same equation, while the kinetics of the hydrogenation of crotyl alcohol (5) corresponded to another kinetic model. We had to further study the kinetics of the isomerieation of crotyl alcohol t o butyraldehyde [reaction (3)]) which was not originally considered in the reaction network. This reaction was found as a parallel reaction in the separate study of the hydrogenation of crotyl alcohol, and i t would not likely have been discovered without isolation of the reactions. This reaction proceeds by a combined mono- and bimolecular mechanism (121)on the catalyst used. By a systematic study of the effect of products and other substances on individual reactions we obtained from the equations for initial rate the following relations: For hydrogenation reactions (1)) (2), and (4) rj
=
+
+
+
ki’p~p~JC1 ( K A P A ) ~ ~ (‘ K H ~ P H ~ ) (~K~ ~’ p d ’ ’ ~
+
f KNPN KRPR]~,
(31)
for reaction (5) (hydrogenation of crotyl alcohol) T6
= kS’PNpHn/[1
+ (KApA)’” + KHZPHS + ( K M P M ) ’ ~ ~
+ KNPN+ KRPRI2,
(32)
45
KINETICS OF COUPLED CATALYTIC REACTIONS
for reaction (3) (isomerization of crotyl alcohol) r3
=
+ (KAPA)'"+ K H ~ P+H (Ki~paa)"~ ~ + + k:'pN/(1 + + KfI.$€Ih.
~'PNPH~/[~
+
KNPN
KRPR]'
KN'PN
(33)
The corresponding values of the constants are listed in Table IX.Using these values and substituting the conversions for partial pressures as in the hydrogenation of phenol (see p. 32), by numerically solving the system of five differential equations we obtained the curves presented in Fig. 9, which agreed well with experimental points. From the results of this kinetic study and from the values of the adsorption coefficients listed in Table IX, it can be judged that both reactions of crotonaldehyde as well as the reaction of butyraldehyde proceed on identical sites of the catalytic surface. The hydrogenation of crotyl alcohol and its isomerization, which follow different kinetics, most likely proceed on other sites of the surface. From the form of the integral experimental dependences in Fig. 9 it may be assumed, for similar reasons as in the hydrodemethylation of xylenes (p. 31) or in the hydrogenation of phenol, that the adsorption or desorption of the reaction components are most likely faster processes than surface reactions. It should be noted that the kinetic analysis of this system consisting of five reactions represents the limiting case which can be reliably solved by the current experimental technique, if we wish its kinetic description to be in agreement with the kinetics of single reactions and the corresponding TABLE IX Values of Constants ki' (mole hr-lkg-latma) and of Adsorption Coefficients Kj (atm-l) in Parallel-Consecutiue Hydrogenation of Crotonaldehyde Reaction in scheme IX
Kinetic equation
(1 1 (2) (4) (5) (3)
(31) (31) (31) (32) (33)
ki'
Kc
4329 1758 8877 1060 802" 13b
18 18 18 8.7 18
-
KH*
Kn
KN
KR
0.55 0.55 0.55 0.38 0.042O 3.P
25 25 25 16 25
14 14 14 14 29.5" 4.2
0.9 0.9 0.9 7.8 0.9
-
-
For bimolecular isomeriration. For monomolecular isomeriration: kg" (mole hr-1kg-latm-l) and KfI, and KN' (atm-1) in the second term of Eq. (33). b
46
L.
BERANEK
equations to be more than mere empirical relations. An empirical kinetic description may in fact be always obtained even for much more complicated systems, e.g. by simultaneous treatment of the data on overall transformation (see Section 1I.A). 2. Adsorption Coeficients of Reactants in Branched Reactions
It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and ( 2 ) (hydrogenation of crotonaldehyde to butyraldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-I). This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. Nevertheless, in another branched reaction, the hydrogenolysis of methylcyclopentane on Pt-A120s (10% Pt) at 230°C, leading to 2- and 3-methylpentane (n-hexane is not practically formed under the conditions used)
(XI
different values of the adsorption coefficient of methylcyclopentane were A ,l.S6), ~ although the found (222) for both reaction branches ( K A , ~ / K = form of the rate equation was identical To
= kKAKH#Aop&z/[l
+ (KApA0)1'2+ KHaP!I,I3
(34)
The values of the adsorption coefficient of hydrogen for both reactions were practically identical (1.9 and 2.1 atm-l). Here, the selectivity of the branched reactions depends on the partial pressure of methylcyclopentane. This difference may be accounted for by assuming that either the cleavage of the C-C bond of methylcyclopentane in the @-positionand in the y-position with respect to the methyl group does not take place on the same sites of the surface of platinum (or on the sites of the same activity), or that the mechanism of hydrogenolysis is more complex than that ex-
KINETICS OF COUPLED CATALYTIC REACTIONS
47
pressed by Eq. (34) and, hence, the coefficient K Aincludes more processes, not identical in both reaction branches. A similar difference in the adsorption coefficients of the starting reactant of branched reactions was also found in the parallel dehydration and dehydrogenation of isopropyl alcohol on some oxide catalyst (125);here, of course, the chemical nature of both branches is clearly different. It is of interest, however, to note that for the series of catalysts with varying
(XI)
selectivity (the ratio of dehydrogenation to dehydration), above all the ratio of the rate constants of these two branches undergoes marked changes (within the range of several orders of magnitude) whereas the selectivity does not depend very much on the adsorption coefficients (Table X). It seems likely therefore that, in spite of a certain difference in the values of the adsorption coefficient of isopropyl alcohol for dehydration and dehydrogenation, these quantities depend only to a small extent on the chemical nature of the catalyst and of the reactions occurring on its surface, and they express rather a nonspecific interaction of the substrate with the catalytic surface. This hypothesis would agree also with the results of the study of the influence of added substances on some other reactions (id4). As an example, dehydration of cyclohexanol on alumina a t 220°C is retarded by cyclohexanone, the dehydrogenation of cyclohexanol to cyclohexanone (the second reaction branch) not occurring with this catalyst at all. Hence, cyclohexanone is adsorbed on dehydration centers, on which the reaction which would lead to its formation does not take place a t all. A similar result was obtained also for the second reaction branch, the dehydrogenaTABLE X Ratios of Rate Constants and of Adsorption Coeficients in Parallel Dehydrogenation ( 1 ) and Dehydration (2)of Isopropyl Alcohol on Some Oxide Catalysts (183) Relative constant
ThOz
Ti02
ZrOz
MgO
0.035 h/h K A , I / K A ~ ~0.46
0.062
3.2
25
1.5
1.6
1.55
48
L.
BERANEK
tion of cyclohexanol to cyclohexanonc, when we catalyzed this reaction with palladium (11) oxide, which is inactive for dehydration. Water and cyclohexene exhibit a rate-retarding effect in the dehydrogenation, even though they cannot be formed on the active centers of this catalyst. With the same catalysts we studied also two independent reactions, the dehydration of 1-methyl-1-cyclohexanol to the corresponding olefin on A1203 and the decarbonylation of benzaldehyde t o benzcne and carbon monoxide on PdO. The first catalyst is not active in the second reaction, and the second one is inactive in the first reaction. Nevertheless, 1-methyl-1-cyclohexanol (and also methylcyclohexene and water) retards the reaction of benzaldehyde on PdO, and benzaldehyde and benzene retard the reaction of 1-methyl-1-cyclohexanol on alumina, although these substances are not able to react or to be formed on the respective active centers. It seems therefore that these cases of inhibiting effect, which is obviously adsorption in character, are due rather to nonspecific interactions of substances with catalytic surfaces.
G. DISCUSSION The main results following from the investigation of stoichiometrically not simple heterogeneous catalytic reactions carried out in our laboratory can be summarized in this way: 1. The kinetics of a complex catalytic reaction can be derived from the results obtained by a separate study of single reactions. This is important in modeling the course of a catalytic process starting from laboratory data and in obtaining parameters for catalytic reactor design. The method of isolation of reactions renders it possible to discover also some other reaction paths which were not originally considered in the reaction network. 2. The kinetics of a coupled catalytic reaction can be well described by equations of the Langmuir-Hinshelwood type, since these are able to express mutual influencing of single reactions. Power-law equations are not suitable for this purpose. 3. The form of equations of the Langmuir-Hinshelwood type and the values of their constants, obtained by the study of isolated reactions, are valid also in the case of the coupled system, where more substances are present and a greater number of interactions is taking place simultaneously on the surface of the catalyst. 4. In the cases under study the behavior of each of the compounds present could be expressed by a single value of the adsorption coefficient in all the reactions occurring on the given catalyst. This indicates that this coefficient has a more general meaning, since it is able to characterize a certain substance in different reacting systems [cf. (97')l. It is reasonable to
KINETICS OF COUPLED CATALYTIC REACTIONS
49
assume that the meaning of this quantity is close to the physical meaning of the equilibrium constant of adsorption; here, of course, the adsorption on a catalytically (not adsorptionally) active surface. 5. The relative reactivities obtained by the method of competitive reactions corresponded to the values of the separately obtained rate and adsorption constants. The reactivities obtained by the competitive method differ, of course, from the ratio of the rates of the separately studied single reactions; this difference increases with the difference in the values of the adsorption coefficients of competing substances. From the study of the influencing of single reactions by products and by other added substances and from the analysis of mutual influencing of reactions in coupled systems, the following conclusions can be drawn concerning adsorption of the reaction components. (1) With the exception of crotyl alcohol on the platinum-iron-silica gel catalyst, all the substances present in the coupled system, i.e. reactants, intermediate products, and final products, always adsorbed on the same sites of the catalytic surface (competitive adsorption). This nonspecificity was established also in our other studies (see Section IV.F.2) and was stated also by, for example, Smith and Prater (32).(2) The adsorption of starting reactants and the desorption of the intermediate and final products appeared in our studies always as faster, relative to the rate of chemical transformations of adsorbed substances on the surface of the catalyst. With regard to a comparatively high rate of adsorption-desorption steps and nonspecificity of the adsorption of reaction components, it could be assumed that this adsorption, intervening in the kinetic description of the reactions studied, was probably a weak, nonspecific interaction of the reaction components with the catalytic surface. The rather physical character of the adsorption might be indicated also by the fact that the adsorption coefficients of low-boiling compounds (gases such as methane, carbon dioxide, hydrogen) were found to be markedly lower, compared with those of higher-boiling organic compounds. At present, however, the above view is only a hypothesis, and further experimental material would be needed before more general conclusions could be drawn. LISTOF SYMBOLS (ads) Ci
F F lW (9)
G
adsorbed state concentration of the substance j feed rate of reactant space velocity gaseous state initial molar ratio of hy-
k
I, k’ kdp, khbs
drogen to organic substance rate constant rate constant of the forward and the reverse reaction, respectively rate constant of adsorp-
L. B E R ~ N E K
50
k’
ka”
KP
Exponents a, b, c, d, a’, b‘, c’, d’ 8
tion and desorption, respectively overall constant in the numerator of rate equations which is the product of the rate constant and adsorption coefficients of reactants overall constant in the numerator of the rate equation in the monomolecular isomerization of crotyl alcohol equilibrium constant of the ith reaction equilibrium constant of the ith surface reaction adsorption coefficient of the substance j adsorption coefficient in the monomolecular isomerization of crotyl alcohol relative constant defined by Eq. (7) number of moles of substance j partial pressure of substance j total pressure reaction rate selectivity or relative reactivity time weight of catalyst degree of conversion stoichiometric coefficient common function for all rate equations of the system constants of kinetic equations in general time variable (t or W / F )
u, v, w
8, E
Hinshelwood type equations exponents of the adsorption terms Kici in the denominator of the Langmuir-Hinshelwood t
Indices and notation of reaction components
(upper index) initial value 1,2,, . , i, . . .n designation of reactions in reaction network general designation of rej action component (in competitive reactions j denotes competing substance) calculated value calc value in coupled system coup1 experimental value exP value for isolated reaction isol value for reference subref stance reaction components in A, B, c general common reagent X common product Y starting reactant in genA eral, and xylene, phenol, ester, crotonaldehyde, and methylcyclopentane, in particular alcohol B ethyl acetate A1 ethyl isobutyrate A2 propanol Bi methanol B2 carboxylic acid Ac acetic acid Aci propionic acid Ac2 toluene or cyclohexanone C benzene or cyclohexanol exponents of partial pres- D methane sures in power-law-type E but yraldehyde equations M crotyl alcohol exponent in the denomi- N butanol nator of the Langmuir- R 0
KINETICS OF COUPLED CATALYTIC REACTIONS
51
ACKNOWLEDGMENTS Thanks are due to Dr. Setlnek for his cooperation in this field, to thelate Prof. Baiant for his interest and encouragement in the work, and to Dr. Kraus for stimulating discussions in the course of these studies and during the preparation of the article as well as for critical reading of the manuscript.
REFERENCES 1. Kiperman, S. L., “Vvedeniye v kinetiku geterogennykh kataliticheskikh reaktsij,” Nauka, Moscow, 1964. 2. Schneider, P., and Kraus, M., Chem. Listy 55, 1358 (1961). 3. Kittrell, J. R., Hunter, W. G., and Watson, C. C., AZCHE J. 11, 1051 (1965). 4. Lapidus, L., and Peterson, T. I., AZCHE J . 11,891 (1965). 6 . Hunter, W. G., and Mesaki, R., Can. J. Chem. Eng. 45, 247 (1967). 6. Reilly, P. M., Can. J . Chem. Eng. 48, 168 (1970). 7. Han6i1, V., Mitschka, P., and BerAnek, L., J . Catal. 13,435 (1969). 8. Temkin, M. I., in Mechanism and Kinetics of Complex Catalytic Reactions (S. 2. Roginskij, G. V. Isagulyants, and I. I. Tretjyakov, eds.), p. 57. Nauka, Moscow, 1970. 9. Miyahara, K., and Yokoyama, S., J . Res. Inst. Catal. Hokkaido Univ. 19, 127 (1971). 9a. Miyahara, K., J . Res. Znat. Catal., Hokkaido Univ. 20, 71 (1972). 10. Noyes, R. M.,Progr. React. Kinet. 2, 337. (1964). 11. Neiman, M. B., Zh. Fiz. Khim. 28, 1235 (1954). f 2 . Neiman, M. B., and G&l,D., “The Kinetic Isotope Method and its Application.” Akadhmiai Kiad6, Budapest, 1971. 12a. Happel, J., Caatal. Rev. 6, 221 (1972). 12b. Bindo, J. S., Ph.D. Thesis, Tulane University, 1969; Dissert. Abstr. Znt. B 30, 4598 (1970). 13. Malkin, I. I., Ostrovskij, G. M., and Snagovskij, Yu.S., Preprint No. 6, Symposium “Mechanism and Kinetics of Complex Catalytic Reactions”. Moscow, 1968. 14. Wajc, S. L., Mansour, A. H., and Jottrand, R., Rev. Inst. FT. Petrole Ann. Combust. Liquid@ 20, 849 (1965). 15. Germain, J. E., and Blanchard, M., Bull. SOC.Chim. Fr. p. lo00 (1958). 16. Thomas, G., Montarnal, R., and Boutry, P., C. R . Acad. Sci., Ser. C 269,283 (1969). 17. Thonon, C., and Jungers, J. C., Bull. SOC.Chim. Belg. 59,604 (1950). 18. Coussemant, F., and Jungers, J. C., Bull. SOC.Chim. Belg. 59, 295 (1950). 19. Derrien, M., and Jungers, J. C., Bull. SOC.Chim. Fr. p. 2164 (1962). 90. Catry, J. P., and Jungers, J. C., Bull. SOC.Chim. FT.p. 2317 (1964). bf. Weiss, A. H., Chem. Eng. 70, [7], 89 (1963). 22. Doelp, L. C., Brenner, W., and Weiss, A. H., Znd. Eng. Chem., Process Des. Develop. 4,92 (1965). 23. Takagi, Y., Nishimura, S., and Hirota, K., J. Catal. 12, 214 (1968). 24. Lavrovskij, K. P., Rosental, A. L., and Khabihulaeva, 0. K., Kinet. Katal. 10, 129 (1969). 25. Pecque, M., and Maurel, R., Bull. SOC.Chim. FT. [6] pp. 1878 and 1882 (1969). 25a. Kolesnikov, I. M., Kovalenko, V. I., Kovalenko, T. I., and Malook, G. P., Zh. Fiz. Khim. 46,527 (1972). 26. Kosorezov, Yu.I., Kinet. Katal. 12, 520 (1971).
52
L.
BERANEK
26a. Kozorezov, Yu.I., and Alekseev, Yu.A., Zh. Prikl. Khim. (Leningrad) 45, 1522 (1972). 27. Draper, N. R., Kanemasu, H., and Mezaki, R., Znd. Eng. Chem., Fundam. 8, 423 (1969). 88. Wei, J., and Prater, C. D., Advan. Catal. 13,203 (1962). 89. Prater, C. D., Silvestri, A. J., and Wei, J., Chem. Eng. Sci. 22, 1587 (1967). SO. Silvestri, A. J., Prater, C. D., and Wei, J., Chem. Eng. Sci. 23, 1191 (1968). 31. Silvestri, A. J . , and Prater, C. D., J . Phys. Chem. 68, 3268 (1964). 38. Smith, R. L., and Prater, C. D., Chem. Eng. Progr. 63, Symp. Ser. No. 73, 105 (1967). 33. Wei, J., Ind. Eng. Chem., Fundam. 4, 161 (1965). 34. Faith, L. E., and Vermeulen, T., AZCHE J . 13,936 (1967). 36. Ozawa, Y., and Gillespie, B., Chem. Eng. Sci. 27, 165 (1972). 36. Lombardo, E. A., and Hall, W. K., AICHE J . 17, 1229 (1971). 36a. Luzarraga, M. G., and Voorhies, A., Jr., Znd. Eng. Chem., Prod. Res. Develop. 12, 194 (1973). 37. Fognani, F., and Montarnal, R., Rev. Inst. Fr. Petrole Ann. Combust. Liquides 14, 191 (1959). 38. Ito, M., and Sano, K., Bull. Chem. SOC.Jap. 40, 1307, 1315, and 1321 (1967). 38a. Rizaev, R. G., Sheinin, V. E., and Mekhtiev, S. D., Kinet. Katal. 13, 1496 (1972). 39. Hashimoto, K., Tsuta, K., Miyamoto, K., Hashimoto, N., Got6, N., Tada, T., and Nagata, S., f.Chem. Eng. Jap. 2, 158 (1969). 40. Forni, L., and Valerio, A., Ind. Eng. Chem., Process Des. Develop. 10, 552 (1971). 41. Lemcoff, N. O., and Cunningham, R. E., J . Catal. 23,81 (1971). 4la. Lyubarskij, A. G., Gorelik, A. G., Petoyan, V. P., Lyapin, E. V., and Beskov, V. S., Kinet. Katal. 14, 410, 956 (1973). 41b. Singh, H. B., Klinzing, G. E., and Coull, J., Ind. Eng. Chem., Prod. Res. Develop. 12, 184 (1973). 42. Jungers, J . C., Sajus, L., de Aguirre, I., and Decroocq, D., “L’analyse cinbtique de la transformation chimique,” Vol. I, p. 1-65. Technip, Paris, 1967. 43. Isagulyants, G. V., Balandin, A. A., Popov, E. I., and Derbentsev, Yu.I., Zh. Fiz. Khim. 38,20 (1964). 43a. Dalla Lana, I. G., Myint, A., and Wanke, S. E., Can. J . Chem. Eng. 51,578 (1973). 44. Boudart, M., “Kinetics of Chemical Processes,” p. 107. Prentice-Hall, Englewood Cliffs, New Jersey, 1968. 46. Bond, G. C., and Wells, P. B., J . Catal. 4, 211 (1965). 46. Bond, G. C . , and Wells, P. B., J . Catal. 5, 65 (1966). 47. Bond, G. C., Webb, G., Wells, P. B., and Winterbottom, J . M., J . Chem. SOC., London p. 3218 (1965). @. Wauquier, J. P., and Jungers, J. C., Bull. SOC.Chim. Fr. [5] p. 1280 (1957). 49. Crombie, L., and Jenkins, P. A., Chem. Commun. p. 394 (1969). 60. van Bekkum, H., Nieuwstad, T. J., van Barneveld, J., and Wepster, B. M., Tetrahedron Lett. p. 2269 (1967). 6 1 . HanEil, V., and Berhek, L., Chem. Eng. Sci. 25, 1121 (1970). 62. Berhek, L., Collect. Czech. Chem. Commun. 32, 489 (1967). 63. Bertinek, L., Collect. Czech. Chem. Commun. 33, 3541 (1968). 64. de Boer, J . H., and van der Borg, R. J. A. M., Proc. Int. Congr. Catal., Bnd, 1960 Paper No. 40, p. 919 (1961). 66. Wei, J., Ind. Eng. Chem. 58, 38 (1966). 66. Weisz, P. B., Advan. Catal. 13, 137 (1962).
KINETICS OF COUPLED CATALYTIC REACTIONS
53
57. Hamilton, W. M., and Burwell, R. L., Jr., PTOC.Znt. Congr. Catal., dnd, 1960 Paper No. 44, p. 987 (1961). 58. Meyer, E. F., and Burwell, R. L., Jr., J. Amer. Chem. SOC.85, 2877 (1963). 69. de Ruiter, E., and Jungers, J. C., Bull. SOC.Chim. Belg. 58, 210 (1949). 60. Kuriacose, J. C., and Jungers, J. C., Bull. SOC.Chim. Belg. 64, 502 (1955). 61. Claes, F., and Jungers, J. C., Bull. SOC.Chim. FT. [5] p. 1167 (1958). 68. Strelets, M. M., Snagovskij, Yu.S., Borisov, V. V., and Lyubarskij, G. D., Khim. Prom. (Moscow) 44, 567 (1968). 6da. Sakai, T., Nukui, K., and Ohi, N., Nippon Kagaku Kaishi p. 821 (1972); Chem. Abstr. 77, 47669e (1972). 63. Takagi, Y., Nishimura, S., and Hirota, K., Bull. Chem. SOC.Jap. 43, 1846 (1970). 64. Sporka, K., Hanika, J., RfiiiEka, V., and Vast@, B., Collect. Czech. Chem. Commun. 37, 52 (1972). 65. Tashiro, M., Yuki Gosei Kagaku Kyokai Shi 25, 571 (1967); Chem. Abstr. 67, 90254w (1967). 66. Ogino, A., and Kawakami, T., J. Chem. SOC.Jap., Znd. Chem. Sect. 68, 45 (1965). 6’7. Kolesnikov, I. M., Panchenkov, G. M., and Morozov, E. A,, Zh. Prikl. Khim. (Leningrad) 39,415 (1966). 68. Ioffe, I. I., Nikolaev, Yu.T., Sukhareva, G. A,, Balashov, V. M., and Kamneva, L. S., Kinet. Katal. 7 , 898 (1966). 69. Dobrovolskij, S. V., and Polotnyuk, V. Ya., in “Organicheskie poluprodukty i krasiteli” (Organic Intermediate Products and Dye-stuffs) (A. I. Korolev, ed.), Vol. 1, p. 184. Gos. Nauchnotekh. Izd. Khim. Lit., Moscow, 1959. ‘70.Prakash, G., and Doraiswamy, L. K., Znd. Eng. Chem., Process Des. Develop. 9, 26 (1970). ‘71. Ostrovskij, G. M., Volkova, A. N., Sadovskij, A. S., and Gelbshtejn, A. I., Khim. Prom. (Moscow) 41, 31 (1965). ‘79.Sporka, K., Hanika, J., RiiitiEka, V., and PouEek, J., Collect. Czech. Chem. Commun. 38, 166 (1973). 73. Ioffe, I. I., and Lyubarskij, A. G., Kinet. Katal. 3, 261 (1962). ‘74. Dmuchovsky, B., Freerks, M. C., Pierron, E. D., Munch, R. H., and Zienty, F. B., J . Catal. 4, 291 (1965). 75. Slavinskaya, V. A,, Gulevskij, E. K., Shimanskaya, M. V., Giller, S. A., and Ioffe, I. I., Kinet. Katal. 3, 276 (1962). ‘75a. Aliev, V. S., Guseinov, N. M., Abilov, A. G., and Bagmanov, Z. A., Azerb. Khim. Zh., No. 1, p. 13 (1972). 75b. Greco, G., Jr., Gioia, F., and Alfani, F., Chim. Znd. (Milano) 54, 990 (1972). ‘76. Gut, G., and Klopfenstein, E., Chimia 21, 377 (1967). 7‘7. Nair, C. S. B., Ghosh, A. K., Basu, A. N., and Lahiri, A., Preprint No. 21, Symposium “Mechanism and Kinetics of Complex Catalytic Reactions”. Moscow, 1968. ‘78. Haag, W. O., and Pines, H., J. Amer. Chem. SOC.82,387 and 2488 (1960). ‘79. Ka116, D., and Schay, G., A d a Chim. (Budapest) 39, 183 (1963). 80. Ogasawara, S., and CvetanoviE, R. J., J. Catal. 2, 45 (1963). 81. Hightower, J. W., and Hall, W. K., J . Phys. Chem. 71, 1014 (1967). 82. Forni, L., Zanderighi, L., and Car&, S., J . Catal. 12,298 (1968). 83. Ka116, D., Preszler, I., and Schay, G., Acta Chim. (Budapest) 64, 211 (1970). 84. Misono, M., and Yoneda, Y., J. Phys. Chem. 76,44 (1972). 84a. Chang, C. C., Conner, W. C., and Kokes, R. J., J. Phys. Chem. 77, 1957 (1973). 84b. Uematsu, T., Bull. Chem. SOC.Jap. 45, 3329 (1972).
54
L. B E R ~ N E K
84c. Hopper, J . R., and Shigemura, D. S., AICHE J . 19, 1025 (1973). 85. Forni, L., Zanderighi, L., Cavenaghi, C., and Carrh, S., J . Catal. 15, 153 (1969). 86. Ragaini, V., Somensi, G., and Card, S., J . Calal. 13, 20 (1969). 87. Cervenf, L., and R%i&ka, V., Collect. Czech. Chem. Commun. 34, 1560 (1969). 87a. Hertwig, K., Lucas, K., Flock, W., and Bucka, H., Chem. Techn. 24, 393 (1972). 87b. Komarovskij, N. A., Cailingold, A. L., Basner, M. E., Filipenko, F. S., and Slinko, M. G., Kinet. Katal. 14, 1483 (1973). 87c. Alkhasov, T. G., Lisovskij, A. E., Safarov, M. G., Lapin, V. B., and Kurbanov, N. A,, Kinet. Katal. 14, 1182 (1973). 87d. Stefoglo, E. F., Yermakova, A., Vasyunina, N. A., and Klabunovskij, E. I., Khim. Prom. (Moscow) 49, 576 (1973). 87e. Orlickas, A., Hoffman, T. W., Shaw, I. D., and Reilly, P. M., Can. J. Chem. Eng. 50, 628 (1972). 87f. Evgrashin, V. M., Ioffe, I. I., and Polyakov, A. A., Kinet. Katal. 14, 440 (1973). 87g. Barinov, N. S., and Mushenko, D. V., Zh. Prikl. Khim. (Leningrad) 46,940 (1973). 87h. Imai, T., and Anderson, R. B., Ind. Ens. Chem. Prod. Res. Develop. 12,232 (1973). 88. Ryashentseva, M. A,: Minachev, Kh.M., Kolesnikov, I. M., and Panchenkov, G. M., Kinet. Katal. 8, 917 (1967). 89. Allan, D. E., and Voorhies, A., Jr., Ind. Eng. Chem., Prod. Res. Develop. 11, 159 (1972). 89a. Forni, L., and Carrh, S., J. Catal. 26, 153 (1972). 90. Gunn, D. J., Chem. Eng. Sci. 66, 963 (1967). 91. Mendiratta, A. K., Prabhu, A. V., and Davidson, B., Chem. Eng. Sci. 26, 885 (1971). 91a. Thomas, W . J., Trans. Znst. Chem. Eng. 49, 204 (1971). 92. Hosten, L. H., and Froment, G. F., Ind. Eng. Chem., Process Des. Develop. 10, 280 (1971). 9s. Polinski, L. M., and Harvey, E. A., Znd. Eng. Chem., Prod. Res. Develop. 10, 365 (1971). 94. RybhEek, L. and Setinek, K., Collect. Czech. Chem. Commun. 33, 3528 (1968). 95. bimonik, J., and Berhek, L., Collect. Czech. Chem. Commun. 37, 353 (1972). 96. Setfnek, K., Pecev, N., and Bagant, V., Collect. Czech. Chem. Commun. 33, 1451 (1968). 97. Setinek, K., and Berhnek, L., J . Catal. 17, 306 (1970). 98. Zanderighi, L., Setlnek, K., and Beriinek, L., Collect. Czech. Chem. Commun. 35, 2367 (1970). 99. Beale, E. M. L., J. Roy. Statist. Soc., Ser. B 22, 41 (1960). 100. Setlnek, K., Berhek, L., and Bagant, V., Collect. Czech. Chem. Commun. 35, 2158 (1970). 101. Smith, H. A,, and Rader, C. P., Proc. Int. Congr. Catal., 6nd, 1960 Paper NO.58, p. 1213 (1961). 106. Rader, C. P., and Smith, H. A., J . Amer. Chem. SOC.84, 1443 (1962). 103. Smith, H. A,, and Campbell, W. E., Proc. Int. Congr. Catal. Srd, 1964 Paper 11-14, p. 1373 (1965). 104. Freidlin, L.Kh., Nasamva, N. M., and Roshdestvenskaya, I. G., Dokl. Akad. Nauk SSSR 176, 1323 (1967). 106. Freidlin, L. Kh., Nasarova, N. M., and Abduraimova, M. A,, Zzv. Akad. Nauk SSSR, Ser. Khim. pp. 1223 and 2510 (1968). 106. Maurel, R., and Tellier, J., BuZl. SOC.Chim. Fr. [5] pp. 4191 and 4650 (1968).
KINETICS OF COUPLED CATALYTIC REACTIONS
55
107. Maurel, R., Mariotti, J. F., and Marquois, J. C., C.R. Acad. Sci., Ser. C 269, 80 (1969). 108. Hussey, A. S., Baker, R. H., and Keulks, G. W., J. Catal. 10, 258 (1968). 109. Nihira, H., Fukushima, T., Tanaka, K., and Ozaki, A., J . Catal. 23, 281 (1971). 110. van Bekkum, H., Kieboom, A. P. G., and van de Putte, K. J. G., Rec. Trav. Chim. Pays-Bas 88, 52 (1969). 111. Belousov, V. M., and Rubanik, M. Ya., Kinet. Katal. 4, 892 (1963). 118. Phillips, T. R., Mulhall, J., and Turner, G. E., J . Catal. 15, 233 (1969). 113. Davis, B. H., J. Catal. 23, 365 (1971). f14. Zdragil, M., J . Catal. 31, 313 (1973). 115. Kolesnikov, I. M., and Panchenkov, G. M., Izu. Vyssh. Ueheb. Zaved. Neft Gat 10, 61 (1962). 116. Kalechits, I. V., and In Yuen-ken, Zh. Fiz. Khim. 35, 501 (1961). 117. Kuriacose, J. C., J. Sci. Znd. Res., Sect. B 20, 82 (1961). 118. Moro-oka, Y., and Ozaki, A., J. Amer. Chen. Soc. 89, 5124 (1967). 119. Moro-oka, Y., Kitamura, T., and Oraki, A., J. Catal. 13, 53 (1969).
119a. Rao, M. S., Hudgins, R. R., Reilly, P. M., and Silveston, P. L., Can. J. Chem. Eng. 49, 354 (1971). 1.90. Vejrosta, J., KleGha, V., and Ber&nek,L., Collect. Czech. Chem. Commun. 37, 1097 (1972). 1.9Oa. Quinlan, C. W., Okay, V. C., and Kittrell, J . R., Ind. Eng. Chem., Process D ~ s . Develop. 12, 359 (1973). 121. hnonlk, J., and Berhek, L., J. Calal. 24, 348 (1972). 122. Corolleur, C., Gault, F. G., and Berhnek, L., to be published. 183. Jambor, J., and BerBnek, L., Collect. Czech. Chem.Commun. (in press). 1.94.BerBnek, L., Collect. Czech. Chem. Commun. (in press).
This Page Intentionally Left Blank
Catalysis for Motor Vehicle Emissions JAMES WE1 Department of Chemical Engineering University of Delaware Newark, Delaware
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Properties of Automotive Exhaust Gases. ....................... A. Transience in the Urban Car.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Composition of the Exhaust Gases.. ......................... C. Thermodynamic Equilibrium. . . .......... D. Thermal Properties. . . . . 111. Catalysts and Reactors.. . . . A. Reaction Path Strategy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Active Ingredients and Support Materials.
.
..............
57 63 63 65 68 71 77
IV. Kinetics and Mechanisms. . . . . . B. Oxidation over Noble Metals. C. Decomposition and Reduction of NO. ....................... 94 V. Physical Transport Processes., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 A. Fluid Mechanics.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 B. Heat and Mass Transport in the Porous Catalyst. . . . . . . . . . . . . 100 C . Heat and Mass Transport from the Gases to the Solid Surfaces. . 101 D. Heat and Mass Transport within the Catalyst Bed. . . . . . . . . . . . 106 VI. Durability of Catalytic Converters, . . .
.............
VII. Reactor Engineering. A. Mathematical Mo B. Analysis of Altern
.............
............................ ............... List of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
118 122 124 125
I. Introduction Millions of cars that go on sale in September, 1974 will be equipped with catalytic converters to reduce the emissions of carbon monoxide and 57
58
JAMES WE1
hydrocarbons ( 1 ) . There are only two routes for an automotive power plant to pass the strict emission standards for the 1975 model year in California: the installation of conventional engines equipped with catalytic converters, or a major change of engine design to the stratified charge, rotary, or Diesel engines. It has been estimated that more than 50% of the automobiles, built to satisfy the less strict standards for the other 49 states, would also have catalytic converters. Catalytic reactors have worked to the benefit of the chemical and petroleum industries for many decades, under the watchful eyes of plant engineers and batteries of monitor and control instruments. The automotive catalytic converter will be the first mass produced catalytic reactor placed directly in the hands of the public, who can provide little more than benign neglect. A catalytic oxidation system may cost $150 per car, but the catalyst cost is estimated to be $30, less than 1% of the cost of an automobile ( 2 ) . In a few years, the gross sale of automotive catalysts in dollars may exceed the combined sale of catalysts to the chemical and petroleum industries (3). On the other hand, if the emission laws are relaxed or if the automotive engineers succeed in developing a more economical and reliable noncatalytic solution to emission control, automotive catalysis may turn out to be a short boom. Automotive catalysis is still in its infancy, with tremendous potential for improvement. The innovations of catalytic scientists and engineers in the future will determine whether catalysis is the long term solution to automotive emissions. Air pollution is principally a problem in urban and heavily industrialized areas, where the flow of clean air from surrounding areas is insufficient to disperse the accumulation. Motor vehicles account for more than 50% of the man-made emissions of carbon monoxide, hydrocarbons, and nitrogen oxides ( 4 ) . More than half of the U.S.annual trillion vehicle miles are driven in urban areas (6). Nature produces much more pollutants than all man-made sources, but natural emissions are widely dispersed and do not contribute heavily to urban pollution problems (6, 7). Carbon monoxide is a noted poison, which has an affinity for hemoglobin in the blood 210 times greater than the oxygen affinity. Prolonged exposure to levels above 9 ppm can lead to reduced mental acuity for some individuals. Hydrocarbons and oxides of nitrogen lead to photochemical smog in sunlight, giving rise to the photochemical oxidants: ozone, nitrogen dioxide, and peroxyacyl nitrates (PAN’S). Exposure to these oxidants above 0.08 ppm can cause eye irritation and impairment of lung function in persons with chronic pulmonary disease. These oxidants also cause damage to vegetation and rubber tires (8, 9). The abundant sunshine and automobiles in California gave rise to the
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
59
most serious problem of photochemical smog, and the need to control emissions from automobiles since the early 1960’s. The first Federal regulation on automotive emission of carbon monoxide and hydrocarbons affected cars built after 1968. The Clean Air Act of 1970 called for a 90% reduction of emissions of carbon monoxide and hydrocarbons for automobiles manufactured during and after model year 1975 in comparison to model year 1970, and called for a 90% reduction of oxides of nitrogen for model year 1976 compared to model year 1970. These standards are to apply for the useful life of an automobile, which is five years or fifty thousand miles, whichever occurs first. As long as there is no emission standard on older cars, the ambient air quality cannot make immediate and dramatic improvements by merely regulating new cars. The full effect of a new emission standard on new cars will be felt after ten years, when the older polluting cars are all scrapped. For a more rapid improvement of ambient air quality, which is mandated in the Clean Air Act of 1970, one must either “retrofit” older cars with antipollution devices or reduce vehicle miles traveled in metropolitan areas. The Federal reduction schedules are given in Table I (10).The standards are given in grams of emission per mile traveled, regardless of engine displacement size or automobile weight. The test procedures have changed twice in the meantime, but the test object remains the simulation of an urban car starting with a cold engine in the morning, and traveling in heavy traffic in a stop-and-go fashion. Prior to placing the automobile on a chassis dynamometer, the engine must be turned off for at least 12 hr, and allowed to stand in an ambient temperature of 68 to 86°F (11). The presently accepted CVS-CH procedure calls for a rigidly specified driving schedule for 22.9 min from a cold start, covering 7.5 miles at speeds up to 56.7 mph, and bringing the car to a stop 18 times. This is followed by a shutdown for 10 min, and a restart for 8.4 min. The gases from the tailpipe are diluted and collected in bags for measurement of hydrocarbons over a flame ionization detector which detects C ions, of carbon monoxide over a nondispersive infrared instrument at 4.5 to 5p wavelength, and of oxides of nitrogen over a chemiluminescence instrument (8).The CVS-CH driving schedule is shown in Fig. 1. The durability test for mileage accumulation also involves a Durability Driving Schedule of 11 laps over a 3.7 mile course with speeds up to 70 mph. After each 4000 miles of driving, the car is cooled down for an emission measurement. The history of catalytic converters was reviewed recently by Ebel ( I d ) . Some of the early patents and publications on catalytic treatment of exhaust gases date from 1925. One of the earliest uses of an oxidation catalyst was in chemical plant exhaust control beginning in 1949, when the main gases to be removed were carbon monoxide and hydrocarbons.
cn 0
TABLE I Federal Emission Control Requirements for Light-Duty Vehicles
Model year
Pre-1968
1968
1970
1972
1975
1976
Test procedure
FTP.
FTP
FTP
CVSC
CVSCHb
CVSCH
3.4
0 . 9 Calif. 1 . 5 other states
0.41
39.0
9 . 0 Calif. 15 .O other states
-
3.0
Hydrocarbons &/mile)
10 (eq 900 ppm by volume as hexane)
3.4 (eq 275 pprn)
2.2 (eq CVSC 4.6) (eq CVSCH 4.1)
CO @/mile)
77 (eq 3.2% by volume)c
35.0 (eq 1.5%)
23 .O (eq CVSC 47.0) (eq CVSCH 34.0)
4-6 (eq 1500 ppm by volume)c
-
-
FTP is Federal Test Procedure. CVSCH is Contant Volume Sampling-Cold and Hot. Interim standard announced April 11, 1973 by EPA. c For 4OOO found vehicles. Enforcement delayed to 1977, interim standard a t 2.0 g/mile announced July 30, 1973 by EPA.
a
62
JAMES WE1
The advantage is an oxidation temperature of 500°F compared to noncatalytic combustion of 1500°F.The active ingredients used were platinum, as well as the base metal oxides of cobalt, nickel, manganese, chromium, and iron. The support material included nickel-chromium ribbons, ceramics rods, beads, and pellets (Zs-l7>. Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith-porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 : American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. The catalyst companies were encouraged to resume their research activities in automotive catalysis in the late 1960’s as further tightening of automotive emissions standards became imminent, and it appeared that mere engine modifications might be inadequate to meet the new standards. A systems approach was first used upon the formation of the Inter-Industry Emission Control Program by the Ford Motor Company and the Mobil Oil Corporation in 1967, which was joined by a number of oil companies in the U.S. and a number of automobile companies in Italy, Japan, and Western Germany. The Clean Air Act of 1970 set new standards that went beyond the capabilities of the then existing technology, and spurred a very intensive research effort. The Clean Air Act also called for a study by the National Academy of Sciences of the technological feasibility of meeting the emission standards. On April 11, 1973 William D. Ruckelshaus, administrator of the Environmental Protection Agency, announced a delay in enforcing the 1975 standard by one year, to be replaced by an interim standard for California and a more relaxed interim standard for the rest of the forty-nine states. Critics of catalytic converters have complained about their cost and the adverse effect on the fuel economy, about the need to remove lead from
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
63
gasoline, about their unproven reliability in everyday use, and about the need to import precious metals from South Africa and the Soviet Union for their manufacture. Most of these criticisms are exaggerated, and cannot obscure the fact that there is as yet no solution better than the catalytic converter. At least one automobile company has moved from a position of skepticism to a position of enthusiastic endorsement of catalytic converters (20). To improve fuel economy and drivability, General Motors is considering plans to put catalysts in all or almost all of its 1975 vehicles, even where the standards could be met without them.
II. Properties of Automotive Exhaust Gases A. TRANSIENCE IN THE URBANCAR In the major catalytic processes of the petroleum and chemical industries, continuous and steady state conditions are the rule where the temperature, pressure, composition, and fiow rate of the feed streams do not vary significantly. Transient operations occur during the start-up of a unit, usually occupying a small fraction of the time of a cycle from start-up to shut-down for maintenance or catalyst regeneration. In an urban car, start-up and transient conditions prevail throughout a cycle from starting a cold engine in the morning, driving in a stop-and-go fashion through the city, and stopping at a destination not too distant from the origin. These transiences are shown in Fig. 2 from a test cycle designed to simulate a California urban car, where the gas flow rate and the concentration of the three principal pollutants vary by a factor of 10 to 20 from the lowest to the highest values (21-28).The quantity of 4200
60
SPEED ( MPH )
1900
40
NOx (PPM)
20
1995 1190
510
OI
co
/ I I l l
I
I1
IM
li7
HC (PPM)
(MOLE%)
AIR FLOW CFM ) I
0
TlME(SEC)
.
34
60
TIME(SEC)
FIG.2. Transience in the California Cycle.
64
JAMES WE1
J
I 0
20
I 40
/
I
I
60
80
MPH
FIG.3. Exhaust gas flow rates and temperatures for a 4000 lb car.
pollutants emitted by a car during a cycle is given by the integral of the exhaust gas flow rate multiplied by the pollutant concentration:
where p is the density of the pollutant. Periods of high flow make heavy contributions to the total quantity emitted. Automobiles vary enormously in their weight from 2500 to 5500 lb, which requires an engine displacement from below 100 to above 500 cu. in. In a similar driving schedule, a 5000 Ib car will emit nearly twice the
65
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
quantity of exhaust gas and is permitted half the pollutant concentrations of a 2500 lb car. For a given 4000 lb car with an engine of 350 cu. in. displacement, the exhaust gas flow rate and temperature delivered to a catalytic converter below the passenger compartment are shown in Fig. 3 (24). Both exhaust gas quantity and temperature increase rapidly with car speed, and are especially high during maximum acceleration with wide open throttle. Gas flow rates are usually measured in standard cubic feet per minute, SCFM. B. COMPOSITION OF
THE
EXHAUST GASES
Ideally, an engine receives a stoichiometric mixture of fuel and air, and completes the combustion process to carbon dioxide, water vapor, and nitrogen. The stoichiometric mixture for a typical fuel consists of 14.7 parts by weight of air to one part of gasoline. In actual practice, an engine is designed to produce lean mixtures (such as 16:l) for best fuel economy while the car is cruising at medium speed. The engine is also designed to produce rich mixtures (such as 12:l) in order to prevent stalling during idling and cold start, and to produce maximum power during acceleration and very high speed cruising. The combustion process is never complete, so that the exhaust gas will contain carbon monoxide, hydrocarbons, hydrogen, and a number of oxygenated compounds such as aldehydes. Some of the nitrogen in the air will combine with oxygen to form oxides of nitrogen during high temperature oxidizing conditions in TABLE I1 Major Components in Exhaust Gases, Equilibrium at .looo"R
Air/fuel Ratio
Deficiency of oxidant" Minimum volume of secondary air per volume of exhaust gas
Lean 16.5
Stoichiometric 14.7
Rich 12.5
12.4 0.5 0.1 2.6 13.5 70.9
13.4 1.2 0.3 0.7 15.1 69.3
10.3 6.4 1.9 0 15.8 65.6
100.0 -2.3
100 0
100 +4.2 0.20
66
JAMES WE1
AUTO EXHAUST I
I
I
I
I
4
-
3000
E
m n
I4 18 AIR FUEL RATIO BY WT
FIQ.4. Pollutant concentrations and air-to-fuel ratio.
the cylinders. Exhaust gas pollutant concentrations depend mainly on air-to-fuel ratios, as shown in Fig. 4. The ideal exhaust gas composition is given in Table 11, based on a fuel with an H to C molar ratio of 2.103, and assuming that the equilibrium is established and frozen a t 4000"R (26).To complete the combustion in a rich exhaust, secondary air must be supplied to cover the deficiency in oxidants. A large variety of hydrocarbons is found in the exhaust gases. The molecules with six or more carbon atoms represent unreacted gasoline. For instance, engines operating with platforming gasoline give the highest percentage of aromatics in the exhaust gas, and engines operating with catalytically cracked gasoline give the highest percentage of olefins (86). The very large quantity of olefins and paraffins with three or fewer carbon atoms indicates considerable cracking. The composition depends somewhat on the driving mode: idling and deceleration favor the formation of acetylenes, while cruising and acceleration favor the formation of olefins.
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
67
An example of the hydrocarbon analysis is shown in Table 111. The photochemical reactivity of hydrocarbons in smog formation listed in declining order are: internal olefins, dialkyl and trialkyl benzenes, diolefins and terminal olefins, ethylene, toluene and other monoalkyl benzenes, paraffins with six or more carbon atoms, propane and benzene, ethane, and finally methane ( 2 7 , 2 8 ) .This ranking of reactivities agrees well with the ranking of catalytic oxidation reactivities, so that the worst smog formers are preferentially removed. The olefins and aromatics are easier to oxidize, and the paraffins are more difficult. Within a group, the higher the carbon number the faster the oxidation rate. Methane does not participate in photochemical smog formation, so that Federal regulations on the ambient quality of air are only concerned with nonmethane hydrocarbons. Various aldehydes, alcohols, esters, and ketones are also found in exhaust gases, at concentrations up to several hundred parts per million. Formaldehyde and acrolein are outstanding eye irritants in the raw exhaust gases TABLE I11 Catalytic Conversion of Exhaust Gas Hydrocarbons at 760°Fa
HC Percent ppm inlet ppm outlet conversion Saturates Methane Ethane Propane Other Total saturates
147.5 21.4 2.2 141.8 312.9
116.7 12.9 0.5 27.9 158.0
20.9 39.7 77.2 80.3 49.5
Olefins Ethylene Propylene Acetylene Other Total olefins
160.4 58.4 88.0 67.5 374.3
51.5 1.5 0.0 4.0 57.0
67.9 97.4 100.0 94.1 84.8
Aromatics Benzene Toluene Other Total aromatics
21.3 49.3 30.7 101.3
9.2 4.1 1.4 14.7
56.8 91.7 95.4 85.5
Total hydrocarbons
788.6
288.9
71.0
a
All data have been corrected to 15% CO
+ COZ.
68
JAMES WE1
(29, SO). Particulates from 0.02 to 10 diam are found in the exhaust gas at a rate of 0.22 to 3.2 mg/g of gasoline burned. The largest fraction of these are lead compounds from the use of tetraethyl lead.
C. THERMODYNAMIC EQUILIBRIUM The oxidation reactions of hydrogen, carbon monoxide, and hydrocarbons in the temperature range of 500 to 1500"F, where the catalytic reactors can be expected to operate, are all favored thermodynamically to completion (31, 3 2 ) .
+ to, = HzO, co + to, = con, Hz
+ 502 = 3coz + 4Hz0, CaHs + 7402 = 6C02 f 3Hz0, C8H18 + 12302 = 8COz + 9Hz0. C3Hs
(2)
(3)
(6)
(7)
(81 The two reactions between NO and hydrogen are favored to completion (33)
+ gHz = NH3 + HzO, reduction without defixation, NO + Hz = fN2 + HzO, reduction with defixation.
NO
(9)
(10)
The decomposition reaction of NO is also favorable.
+
(11) Even at 1340"F, the equilibrium constant of this reaction is K = lo4, so that the NO concentration in equilibrium with 70% Nz and 1% O2 is only 8 ppm, considerably below the target of about 100 ppm. Several equilibrium constants are given in Fig. 5. Three reactions play a role, but are not favored to completion: NO = )Nz
CO
+ H20 = C02 + H2, water gas shift reaction,
+ 302 = NO,, tNz + tHz = NHs. NO
302.
12) (13'1
(14) All three reactions become more favorable at lower temperatures. Since the water gas shift equilibrium constant is between 1 and 10, the equilibrium ratio between Hz and CO will vary from 1 to 7 which is many
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
3
2
1.5
L2
1.0 *F ( x 107
B .7
.6
69
5
FIG.5. Equilibrium constants in exhaust gases.
times higher than the ratio of 1/2 to 1/4 at engine temperatures. The equilibrium constant between NO and NOz varies between 0.1 and 10, so that the equilibrium ratio N02/N0 in equilibrium with 70% Nz and 0.1% 0 2 would vary between 0.003 and 0.3. The formation of ammonia is not favorable under these temperatures. Shelef and Gandhi have calculated that the concentration of ammonia in equilibrium with 75% Nz and 0.5% Ha is less than 13 ppm at temperatures above 620°F. Thus, if reactions (12)-( 14) proceed to equilibrium with sufficiently active catalysts, water may be an effective agent to oxidize up to 85% of the carbon monoxide and to produce hydrogen, the concentration of nitrogen dioxide should remain small in relation to the concentration of nitric oxide, and the concentration of ammonia formed should be negligible. See Table IV.
D. THERMAL PROPERTIES The reactions that have a strong heat effect are the oxidation reactions, which may increase the exhaust gas temperature by hundreds of degrees.
70
JAMES WE1
TABLE IV Equilibrium NHs Concentration in Typical Rich Exhaust, Y6% Na and 0.6% HZ
300 400 500 600 700 800 900 1000
22,044 721 1,933 6.3 99.6 0.33 12.8 0.042 2.8 0.0092 0.0029 0.9 0.35 0.00116 0.17 0.00055
The important ones are listed in Table V (26).The major constituents of exhaust gas are mostly biatomic, such as hydrogen, oxygen, nitrogen, and carbon monoxide, with a heat capacity ranging between 7.0 to 8.4 Btu/lb moleOF ( 3 4 ) . The triatomic gases water and carbon dioxide have a somewhat higher heat capacity, water ranging from 8.6 to 10.0 and carbon dioxide from 11.2 to 13.2. A weighted average heat capacity of stoichiometric exhaust gas would be about 7.9 to 8.9. Using a medium figure of 8.6 Btu/lb moleOF, the computed adiabatic temperature rise in TABLE V
Heats of Combustion Low heat of Heat of combustion combustion Reactants (Btu/mole) (Btu/mole C)
Hz
co Methane Ethylene Ethane Propane n-Octane Benzene Toluene
CHI CzH4 CtHe CsH8 CsHls CaH6 C,Ha
103,967 121,665 344,942 568,769 613,865 872,289 2,177,670 1,348,073 1,605,304
344,942 284,385 306,932 290,763 272,200 224,680 229,330
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
71
the exhaust gas would be 1% co
141O F ,
1%H z
121°F,
100 ppm hexane
20°F.
The amount of chemical heat contained in exhaust gases can be enormous compared to the heat capacity of a catalytic bed. Ten pounds of catalysts at a heat capacity of 0.28 Btu/lb"F would amount to a thermal inertia of 2.8 Btu/"F. But a flow rate of 100 SCFM containing 5% GO would deliver 28 Btu/sec, enough to drive the catalytic bed temperature up at the rate of lO"F/sec. If one of the eight cylinders in the engine is misfiring, the exhaust gas may contain 2% gasoline by volume, and deliver chemical heat a t the rate of 212 Btu/sec. This may give rise to a heating rate of 75"F/sec, which may lead to catastrophic failure of the solid. If two pounds of monolith catalysts are used instead, the rate of the temperature rise could be increased fivefold. It is also useful here to compare the energy stored in the lead-acid battery. A standard 12 V battery may be able to deliver 10 A of current for 4 hr, for a total of 480 W-hr. This amounts to 1632 Btu, which is sufficient to raise 10 lb of catalysts by only 580"F, over the course of four hours.
111. Catalysts and Reactors A. REACTION PATHSTRATEGY The strict control of hydrocarbons and carbon monoxide required for 1975 cars can be satisfied by operating the engine with lean mixtures, and using secondary air to complete the oxidation with a catalytic converter. A proposed 1975 model car with these devices is shown in Fig. 6 (35). This system does not exact a penalty in fuel consumption per mile traveled, and does not harm the driveability of the car (such as difficulty in starting, rough idling, stalling, backfire, and hesitation). The oxidation catalysts are poisoned by lead, sulfur, and phosphorus-compounds that must be minimized or excluded in the gasoline and lubricating oil. The technology of these catalytic systems lacks only proven durability, and is heading towards mass production. The strict control of oxides of nitrogen required for 1976 cars can be partially met by operating the engine with rich mixtures, and by using spark retardation and exhaust gas recirculation (EGR) to reduce the peak
72
JAMES WE1
Conventional Resonator
FIG.6. Single bed oxidizing catalyst system.
temperatures in the cylinders. These measures will exact a heavy penalty in fuel consumption and will harm the drivability of the automobile. These measures are not expected to reduce NO, below 1.0 to 1.5 g/mile, so that a NO, destroying catalyst would be needed. The decomposition of NO to nitrogen and oxygen is thermodynamically favored to near completion, as the equilibrium composition of NO is no more than 8 ppm. However, this is an exceedingly slow catalytic reaction. Boreskov has compared a number of reactions under identical experimental conditions, and concluded that the rate of nitric oxide decomposition is one or two orders of magnitude slower than the exceedingly slow oxidation of methane (36, 3 7 ) . The viability of this route would depend on the discovery of catalysts with very novel properties. The catalytic reduction of NO by a reducing agent such as CO and hydrogen proceeds at a sufficiently attractive rate over a number of catalysts. When the gas contains both NO and oxygen, the reducing agents CO and hydrogen will preferentially attack oxygen (38, 39). Some noble metal catalysts will direct the preferential attack of hydrogen on NO, but this preference is exhibited only a t temperatures below 400°F at a rate that is unattractive. It is thus necessary to provide a net reducing atmosphere in order to reduce NO. The need of hydrocarbons and CO for an oxidizing condition is contradictory to the need of NO for a reducing condition. The favored dual-bed approach today consists of operating the engine with a rich mixture to produce a net reducing exhaust gas,
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
73
Improved carburetion and choke altitude and temperature compensation,
fp-Mod
if ied spark
I
u’“
Carbon canister
FIG.7.Dual catalytic converter system.
which is fed into a bed of catalysts to reduce NO by hydrogen and CO; secondary air is added to the exit from the first bed to produce a net oxidizing gas, which is fed to a second catalytic bed to oxidize hydrocarbons and CO. A typical arrangement for the dual-bed approach for 1976 is shown in Fig. 7 (40-43). It is not possible to reverse the order of the two reactions, as oxidation in the first bed to completion will leave no hydrogen and CO to reduce NO in the second bed. Ammonia is often found as a product of NO reduction in the first bed, at a quantity that is far in excess of the thermodynamic equilibrium concentration of 10 ppm or less. This ammonia produced in the first bed would be reoxidized back to NO upon entering the second catalytic bed (44). Other variations of the dual-bed scheme exist as a combination of thermal oxidizing reactors and catalytic reducing reactors. The Questor company has developed a reactor with three zones: the first zone is a thermal reactor with limited air to raise the temperature of the exhaust gas, the second zone is a catalytic bed of metallic screens to reduce NO, and the third zone is another thermal reactor where secondary air is injected to complete the oxidation of CO and hydrocarbons (46). Simultaneous oxidation and reduction can take place in a single catalytic bed, provided that the air-to-fuel ratio is adjusted precisely at the stoichiometric 14.7 =t 0.1. This precise metering is required for the redox or “three-way’’ catalyst as shown in Fig. 8. A narrow “window” exists for some catalysts where more than 80% conversion efficiency can be obtained on all three pollutants ( 4 6 ) . This precise metering cannot be attained by
74
JAMES WE1 Window
15:l
14: I
13:l
Air-fuel ratio
FIG.8. Principle of operation. "Three-way" redox catalyst conversion characteristics.
I
Engine
*
I I
-
I Single Bed Converter
Oxygen Sensor
-
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
75
conventional carburetors, where the air-to-fuel ratio often varies by more than f l from the set point; a change of fuel type, a change in atmosphere humidity, and a change in altitude will also affect this ratio. A feedback control mechanism, equipped with an oxygen sensor to measure the oxygen pressure in the exhaust gas, has been developed that can achieve this precision in the air/fuel ratio. This strategy also leads to good fuel economy and drivability. The suggested oxygen sensor is a solid electrolyte of zirconium oxide doped with other elements. A scheme of this process is shown in Fig. 9. The durability of the oxygen sensor must be sufficient to survive several hundred hours in operation, and this has not yet been achieved.
B. OPERATIONS IN
THE
FOURWINDOWS P
There are many constraints in the design of a catalytic converter for an automobile. The desired converter volume is no more than the engine displacement and preferably half of that. The shape and location of converters are largely governed by whatever convenient space can be allocated in an existing automobile design, which means either a cylindrical object under the hood or a flat pancake or oval shaped object under the passenger seat. The converter is asked to remove more than 90% of the inlet pollutants, at a space velocity of up to 200,000 hr-’, or a contact time of 5 msec after accounting for the volumetric expansion of gases with temperature. This hourly gas space velocity requires an extraordinarily fast catalyst, approaching in its activity that of the ammonia oxidation catalyst. On account of the dilute concentration of pollutants, the turnover number is seldom more than ten molecules per atom of platinum contained per second. Another important constraint comes from the pressure drop across the catalytic bed, which must be kept to a minimum to avoid a loss in engine power and performance. This requirement is satisfied by a very shallow pellet bed of no more than ten pellets deep, a monolithic structure with many open parallel channels, or a pile of metallic screens and saddles. A given catalyst in a given reactor has a fairly narrow set of operating ranges where the performance is optimal. This does not cause a problem in industrial reactors that are subject to steady state conditions for long periods of time. Similarly, an automotive catalytic converter operates well within four narrow operating ranges (or “windows”) in each and every one of four variables: temperature, gas composition, gas flow rate, and poison concentration. The catalytic system can tolerate occasional excursions from these windows, but prolonged excursions invariably lead to slow chronic aging or quick failures as shown in Fig. 10 ( 4 7 ) .
76
JAMES WE1 air-to-fuel ratia
uitaMe for witabte for NO, reduction oxidation of CO and HC (a)
Cotalyh too cold
best &ration
CatalyA thermal ageing and destruction
Ib)
gaa hourly space velacity hr.-'
time, channrlling (C)
h
but durability
rapid again9 (d)
FIG.10. Four windows of operations. (a) Exhaust gas composition. (b) Catalyst temperature. (c) Gas flow rate. (d) Poison concentrations.
In actual practice, an automobile is always in transient conditions: the catalyst is too cold during start-up, and too hot during a long downhill cruise; the air/fuel ratio is too rich on idle, and too lean while cruising; the exhaust gas %owis slow during idle, and fast during an upgrade cruise. The catalysts are also exposed to repeated cycles of heating and cooling, evaporation and condensation of water, pulsating %owfrom exhaust gases, vigorous shaking on the road, and a variety of poisons including lead and sulfur. The excursions from the optimal operating conditions cause the catalysts in automobiles to deteriorate prematurely. Much progress has been made in widening the operating windows. Stabilized supports have been developed which can tolerate occasional temperature excursions to 2000"F, but even this temperature may be
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
77
exceeded in long downhill cruises. The alternative is to design hardware to ensure that the operation will deviate from these windows as little as possible.
C. ACTIVE INGREDIENTS AND SUPPORT MATERIALS The catalytic materials under consideration were recently reviewed by Yolles and Wise (48),by Dwyer (49), and by a panel report of the National Academy of Sciences ( 4 7 ) .We may divide the active ingredients into three classes: noble metals, base metal oxides, and alloys. Although many materials will oxidize CO and hydrocarbons a t “high conversion” to C02, the requirements for automotive catalysts are severe enough to rule out many of them from consideration. The catalyst must be able to remove more than 90% of CO and hydrocarbons at a space velocity of up to 200,000 hr-l, a t a temperature of 500-1500°F, in an atmosphere of 15% water vapor and barely enough oxygen to complete the oxidation. It is a very expensive and time-consuming jwrney from the formulation of a few grams of catalysts that may have potential in automotive use to the large-scale commercial adoption by an automobile manufacturer. The ability to oxidize CO in a laboratory with synthetic gas streams is only one of the many necessary conditions for success. Thousands of catalysts have been formulated and tested in the leading industrialized nations and discarded, for lack of one or more of the other necessary conditions. In the first screening stage, a hundred grams of catalyst and standard laboratory equipment will suffice (60, 6 1 ) . A tube of catalyst is placed in a furnace, or a fluidized sand bath, or a molten salt bath to maintain constant temperature. A synthetic gas mixture containing about 400 ppm of propylene, 1% CO, 4y002,10% COz,75% nitrogen, and saturated with water vapor at 125°F would be an adequate simulation of engine exhaust gases. An ingenious pulse-flame burner was designed by Meguerian, which operates with gasoline and generates a burnt gas with 600 ppm hydrocarbon, 1.0% CO, 3.5% 02, and 50-100 ppm NO (44).The catalyst efficiency in converting CO and propylene is then measured at a variety of temperatures from 400 to 800”F, of space velocities up to 40,000 h r l , and CO concentrations up to 5y0. The catalysts with better than 90% conversion efficiencies are then given brief thermal stability tests at temperatures up to 1300°F. The dynamic rapid warmup test is used by some laboratories to measure the catalyst activity and heat capacity simultaneously ( 5 2 ) . The catalyst bed is maintained at 100°F initially. Simulated exhaust gas at 100°F is passed over the bed, and the temperature of the gas is increased linearly to 750°F in 2 min. The inlet and outlet
78
JAMES WE1
concentrations of CO and hydrocarbons, BS well as the bed temperatures, are continuously measured to determine the length of time it takes to reach a high conversion efficiency. Laboratory catalyst testing is sometimes done under conditions that are far removed from exhaust gas conditions, and can be a very unreliable guide to the utility of a catalyst. For instance, noble metals may rank below base metal oxides in oxidation activity at low temperatures, but the ranking reverses at high temperatures. These and other hazards were pointed out by Schlatter et al. (63).Laboratory catalyst testing is usually done by the catalyst manufacturers, resulting in the rejection of a vast majority of formulations. In the second stage of testing with an engine stand, ten pounds of catalyst and a test engine with dynamometer monitoring would be required. The engine should run at a steady number of revolutions per minute or be controlled by a computer tape to simulate an urban driving schedule-running for five hours, cooling down for one hour, and repeating. The catalyst is placed in a suitably designed converter, and positioned to receive the exhaust gases. Conversion efficiencies are measured a t different engine speeds (which changes gas flow velocities and inlet temperatures) and carburetor settings (which changes air-to-fuel ratio, affecting the composition of the exhaust gases). Physical durability of the catalyst is tested by blowing the pulsating exhaust gases upward through a pellet bed that is not fully packed to measure attrition resistance. The weight loss of catalysts with mechanical shaking is measured by the loss-onshaker test, where a cylinder is filled with pellets to 80% capacity and rotated end to end a t 800 rpm for 10 min. The weight loss is measured. Engine testing of catalysts is performed by both the catalyst manufacturers and by the automobile manufacturers. Another large fraction of catalysts fail this series of tests. Vehicles and many more pounds of catalysts are required in the third stage of catalyst testing (64). There are three main approaches, differing in cost and controllability: “customer type service” consists of asking company employees or taxicab drivers to drive cars equipped with catalytic converters in their daily routine; test track aging consists of employing full-time drivers to take cars around a prescribed schedule, requiring far more elaborate facilities, but giving more controlled and reproducible results; programmed aging ties a vehicle to a chassis dynamometer, going through the Federally approved CVS-CH test procedure and the Durability Driving Schedule, and giving results that are controlled and recognized (11). Many more catalysts will be found to lose activity too rapidly to meet the standards at 25,000 miles, or to melt and to increase the pressure drop across the converter. The surviving ones are given the ultimate fleet
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
79
test with many dozens of vehicles to accumulate experience and statistics. It takes about 3 4 months to accumulate 50,000 miles. While some catalyst makers have a few cars for customer type service testing, only the automobile manufacturers can afford the fleet tests. Vehicle data represents the complex interaction of many variables, including vehicle performance, reactor design and location, as well as catalyst properties. For 2% catalyst that has failed, one must ponder whether the converter design used in the vehicle is suitable for the catalyst, the quantity of the catalyst used is correct, the distance of the converter from the exhaust manifold is suitable, the catalyst has been inadvertantly overheated to above 2000°F or poisoned by lead and sulfur, or whether the catalyst can be modified to produce a far superior product. Most catalysts consist of active components dispersed as small crystalIites on a thermally stable, chemically inactive support such as alumina, ceramics, or metallic wires and screens. The supports are shaped into spheroids, cylinders, monolithic honeycombs, and metallic mesh or saddles. The most successful class of active ingredient for both oxidation and reduction is that of the noble metals: silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to Nz and to NH,. Platinum and palladium are used in small quantities as “promoters” for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/reductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH, concentration in NO, reduction in comparison with other catalysts, but it forms volatile toxic oxides. The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO, in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-68).Hopcalities are out-
80
JAMES WE1
standing low temperature oxidation catalysts, but lack durability toward water vapor and heat. Several all-metal or alloy catalysts have been developed for the reduction of NO, (42, 4 5 ) . One system uses stainless steel 310 in the form of open metallic mesh or saddles as a substrate clad with monel metal alloy (60-70 Nil 25-35 Cu) . The Questor system uses a variety of catalysts including woven screens of RA 330 steel at 0.029 in. strand thickness, or Inconel 601 material plated with copper. I t was found that the activity of the monel surface increases with usage, possibly due to surface roughening, which exposes more useful surface areas. Pretreatment with etching agents such as aqua regia, oxalic acid, potassium cyanide, electrodeless deposition of a thin copper coating, and calcination in dry or moist air may speed up this aging to achieve maximum catalytic activity. The Could GEM catalyst employs foils of 0.004 in. thickness, which are perforated and expanded into screens. International Nickel has developed a promising alloy N-155, which contains cobalt, molybdenum, nickel, chromium, and aluminum. The support materials for the active ingredients must have high thermal stability, high resistance to attrition and crushing, low volumetric shrinkage with temperature, high surface area, and relatively low density and heat capacity. The most important material is probably gamma alumina. The demand for rapid warm-up for automobile emission control has led to the development of high surface area materials of much lower densities, such as alumina substrates at 0.3 g/cm3 bulk density. The BET surface area of a pellet should be in excess of 100 mz/g ( 6 4 ) .While most substrates experience loss of surface area at temperatures exceeding 1500”F,some stabilized aluminas have been developed that can tolerate occasional excursions of temperature exceeding 2000°F. The monolithic supports are made of alumina and related materials such as cordierite ( Al4Mg2Si6OI8), mullite (3Al2o3.2Si02)2t spodumene ( 6 5 ) . They are chosen [LiA1(SiOs)z],and asbestos [Mg3(Si206)2(OH)2] for their resistance to high temperatures and thermal stresses from repeated thermal cycling, and for their good mechanical strength in withstanding physical shock and vibrations. A complex zirconium oxide-titanium metal has been developed that can stand temperatures up to 4000°F. Such ceramic materials have little surface area and are not suitable for dispersion of active ingredients. A high surface area alumina is usually placed over the ceramic surface as a “washcoat” with a thickness of about 0.001 in., which then acts as a substrate for the active ingredients. This washcoat may amount to 15 wt. % of the monolith, and may provide 10-17 mz/g of surface area (66-71 ) .
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
81
The advanced all-metallic catalysts are believed to be formed by bonding active copper-nickel alloys onto stainless steel wires. Under the scanning electron microscope, it appears that the surface area may be more than twenty times the geometric surface area ( 4 2 ) . The raw materials needed to supply about ten million new automobiles a year do not impose a difficult problem except in the case of the noble metals. Present technology indicates that each car may need up to ten pounds of pellets, two pounds of monoliths, or two pounds of metal alloys. The refractory oxide support materials are usually a mixture of silica, alumina, magnesia, lithium oxide, and zirconium oxide. Fifty thousand tons of such materials a year do not raise serious problems ( 4 7 ) . The base metal oxides requirement per car may be 0.1 to 1 lb per car, or up to five thousand tons a year. The current U.S. annual consumption of copper, manganese, and chromium is above a million tons per year, and the consumption of nickel and tungsten above a hundred thousand tons per year. The only important metals used at the low rate of five thousand tons per year are cobalt, vanadium, and the rare earths. For the noble metals used in oxidation, the loading is about 0.1 oz per car, with calls for a million ounces per year. The current world production rates of platinum, palladium, and rhodium are 1.9, 1.6, and 0.076 million ounces respectively; the current U.S.demand for platinum, palladium, rhodium, and ruthenium are 0.52, 0.72, 0.045, and 0.017 million ounces respectively (72, 7 3 ) . The supply problem would double if NO, reduction requires an equal amount of noble metal. Pollution conscious Japan has adopted a set of automobile emission rules that are the same as the U.S., and Western Europe may follow; this creates a demand for new car catalysts approaching the U.S. total. The bulk of world production and potential new mines are in the Soviet Union and South Africa. The importation of these metals, assuming the current price of platinum at $155/oz and palladium at $78/0z, would pose a balance of payment problem. The recovery of platinum contained in spent catalysts delivered to the door of precious metal refiners should be above 95%; the value of platinum in spent catalysts is greater than the value of lead in old batteries, and should provide a sufficient incentive for scavengers. Almost all the materials which are being considered as components in automobile exhaust catalyst are somewhat toxic (7'4). Most of the compounds considered are low vapor pressure solids which can only escape from the exhaust system as very fine airbone dust particles formed by catalyst attrition. A few compounds, such as the highly toxic metal carbonyls and ruthenium tetroxides, are liquid under ambient conditions and have boiling points less than 100°C. These compounds are not present in
82
JAMES WE1
the original catalyst, but may be formed by reaction with the exhaust gases and emitted into the atmosphere in the vapor state. These compounds decompose at relatively low temperatures, so they may escape decomposition after formation only during warm-up of the catalyst bed from a cold start. Fine particles in the form of “condensation nuclei” have been found in gases from automotive emission control catalysts (‘76). They appear especially during high CO concentrations, suggesting carbonyl formation. Ruthenium has been found to vanish completely from the catalyst after a period of operation. Gasoline normally contains 0.0401,by weight of sulfur, which is oxidized into sulfur dioxide in the engine. Automobiles contribute about 2% of manmade sources of sulfur in the air. It has been reported recently that oxidation catalysts may accelerate the oxidation of sulfur dioxide to sulfates, which is a more serious respiratory hazard than sulfur dioxide. It may be necessary to reduce the sulfur in gasoline. With the exception of ruthenium and chromium, all the materials mentioned for automotive catalysts are lower than lead in toxicity ( 7 4 ) . The danger to human use is minimal, except during manufacturing and installation of the catalysts where safety precautions can be adequately controlled.
D. CATALYST GEOMETRY AND CONVERTER CONFIGURATION The three principal catalyst bed configurations are the pellet bed, the monolith, and the metallic wire meshes. An open structure with large openings is needed to fulfill the requirement of a low pressure drop even at the very high space velocities of 200,000 hr-’. On the other hand, packings with small diameters would provide more external surface area to fulfill the requirement for rapid mass transfer from the grs stream to the solid surface. The compromise between these two ideals results in a rather narrow range of dimensions: pellets are from to $ in. in diameter, monoliths have 6 to 20 channels/in., and metallic meshes have diameters of about 0.004 to 0.03 in. The ceramic monoliths are usually cylinders from 3 to 6 in. in diameter, and 2 to 9 in. in length. They contain many parallel channels extending in a longitudinal direction along the entire length of the cylinder, and are sometimes surrounded by an integral outer skin. A Corning monolith is extruded and then fired, forming channels with square cross sections. An American Lava monolith is made with a corrugated paper processing machine, using a mixture of fiber and ceramics, and laying down horizontal and sinusoidal layers alternately, These two types of monoliths have walls ranging from 0.008 to 0.010 in. in thickness, so that the open space in the
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
83
FIG. 11. Monolithic supports. (a) American Lava Thermacomb. (b) Corning W-1. (c) DuPont Torvex. (d) Kalichemie.
monolith amounts to 60 to 80%. The DuPont Torvex has hexagonal cells with diameters Q to in., with wall thicknesses from 0.03 t o 0.05 in. There are other designs which contain much less open space, where the cross section is a plane penetrated with numerous circular holes arranged in geometric order. The designs with thick walls have the disadvantage of less open area and higher pressure drop, as well as a longer diffusion path for reactant molecules and a higher heat capacity. See Fig. 11. The metallic catalyst support can be in form of chips, open-mesh and reinforced wire structures, and staggered layers of metal screens or saddles. In one design, screens woven from metallic wires 0.01 to 0.03 in. diam are placed in a deep stack. In another design, metal foils 0.004 in. thick are perforated and expanded to form a screen, which is then rolled into a cylinder. See Fig. 12. The converters are mostly designed by muffler manufacturers or by
+
84
JAMES WE1
FIQ.12. Rolled screen catalyst.
muffler divisions of automobile manufacturers, whose experience in chemical reactor engineering is limited. The optimal quantity of catalyst used should be large enough to provide sufficient residence time for the exhaust gas even during high flow, and to provide sufficient thermal mass to prevent surges in high temperature; the quantity should also be low enough so that the bed can be quickly warmed up from a cold start. It would seem that the ideal material for catalyst support should have a very low heat capacity from 0 to 600"F, but a very high heat capacity from 1500°F on. A pellet bed must be shallow to avoid a high pressure drop. Most designs have a depth of 1 to 2 in., representing 5 to 15 layers of pellets. This shallow bed differs considerably from industrial practices in petroleum and chemical plants where a depth of several hundred layers is the rule. The more open monolith and metallic screens offer a lower pressure drop per inch, so that a bed 6 in. deep is still acceptable. Two pellet beds in series would create very high pressure drops. The direction of gas flow through the pellet bed could be important. A pulsating high speed flow of exhaust gases can cause rapid attrition of catalysts, especially if the converter has empty spaces due to catalyst loss or shrinkage, which would promote the internal circulation of catalysts in the converter. The design of a sideflow or an upflow bed must include provisions to avoid empty spaces. A downflow design would minimize these attrition losses. Some pellet beds are housed in circular or elliptical cylinders, where the exhaust gas flows in the axial direction. The radial flow converter is a
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
85
device where the catalysts are placed in the annulus between two concentric cylinders, and where the exhaust gas flows into the inner cylinder and passes through the annulus in the radial direction. The pellet bed announced by General Motors is a shallow downflow design. The monoliths and metal screen beds are usually designed as axial flow cylinders. There are several designs which combine the two beds for reduction and oxidation into a single integral structure, which simplifies construction and installation and conserves heat during warm-up. One of the designs has a downflow center section for reduction, surrounded by a peripheral upflow section for oxidation. The high temperature encountered requires a high alloy metal for the converter, such as stainless steel or Inconel. After the catalysts have been deactivated with age, the free flowing pellets can be withdrawn and reloaded through a fill plug. For the factory packaged monoliths and metal screens, the entire converter assembly may have to be changed. The placement of the NO, bed ahead of the oxidation bed causes a delay of the warm-up of the oxidation bed from a cold start. Since many of the materials considered for the reduction of NO are also excellent oxidation catalysts, the NO, bed is often used as the oxidation bed by the injection of secondary air during the first two minutes from a cold start. After the oxidation bed is warmed up, the secondary air is diverted from upstream of the first bed to upstream of the second bed. This procedure helps the emission reduction when the catalysts are fresh, but hastens the aging of the NO, catalyst as it is being exposed repeatedly to oxidation and reduction conditions. There is only one exhaust manifold on a four or six cylinder engine. A V-8 engine has two exhaust manifolds, leading to two separate exhaust pipes that usually join at a "Y" some distance downstream. Placement of converters within two to three feet of the exhaust manifold can be achieved by using two separate converters for the two V-8 exhaust manifolds. The most convenient places for converters are inside the hood, and under the body of the passenger compartment. A flat converter shaped like a pancake or a cylinder with an elliptical cross section can fit under the car without causing a big reduction in road clearance. These converters operate with a bed temperature up to 1800"F, which generates a container skin temperature of up to 1100"F, and generates a great deal of heat within the passenger compartment. These high temperatures are achieved during idling after a hard drive, when the cooling effect of rushing air has ceased. By comparison, the thermal reactors operate at even higher temperatures, and are even more objectionable. Many devices have been proposed to speed up the warm-up of the catalytic converter during a cold start: a heat exchanger in the carburetor
86
JAMES WE1
to warm up incoming air, an insulated catalytic bed to keep the catalysts warm for a long period after cooling down, electrical heating of the catalysts, a propane and air flame that passes through the catalytic bed, and many others. They all help reduce the emission somewhat, but add to the mechanical complication ( 7 6 ) . A far more important task is the protection of the catalysts from very high temperatures in the exhaust gases, which can overheat the catalysts in a matter of 5-10 sec. A high temperature sensor and a bypass valve are probably the best solution. However, it is difficult to develop valves that will seal perfectly for gases at such a high temperature and velocity.
IV. Kinetics and Mechanisms A. OXIDATIONOVER BASEMETALOXIDES There are few studies in the literature on the kinetics and mechanism of oxidation over base metal oxides. Blumenthal and Nobe studied the oxidation of CO over copper oxide on alumina between 122 and 164°C. They reported that the kinetics is first order with respect to CO concentration, and the activation energy is 20 kcal/mole ( 7 7 ) . Gravelle and Teichner studied CO oxidation on nickel oxide, and found that the kinetics is also first order with respect to CO concentration (78). They suggested that the mechanism of reaction is by the Eley-Rideal mechanism GO
+
+ +
+
$0, Nia+ + 0-(ad) Ni*+, 0-(ad) Ni*++ GO2 NP+
+
On the other hand, the work of Roginski with manganese oxide suggested the Langmuir-Hinshelwood mechanism of (49) MnOa-GO + 0 2 (ad)4 MnOz.O GO,, MnOz.O
+ GO
+
---t
MnOz
+ GO,.
Schlatter et al. found that their data with copper chromite agrees better with 0.7 order for CO concentrations (63). For crystals of nickel oxide and chromium oxide, Yu Yao and Kummer have found that the kinetics depend on CO or hydrocarbon around 0.55 order and depend on oxygen around 0.45 order ( 7 9 ) . Hertl and Farrauto found evidence that CO adsorbs on copper as a carbonyl group, and adsorbs on chromium oxide as a unidentate carbonate. They found that the kinetics depends on CO to the first order, and depends on oxygen to the zero order (80). The most complete study on the oxidation of CO and hydrocarbons was reported by Kuo et al. ($1).Their study was done on a copper chromite catalyst under conditions that simulate exhaust gases. They found that CO oxidation kinetics is very accurately represented as first order in CO
87
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
Residence time, CM3 catolyst/CM3 fluid
FIG.13. Kinetics of CO oxidation over copper chromite.
and 0.2 order in 02.This first-order behavior persists from 500 to 900”F, as shown in Figs. 13 and 14. The kinetic rate constant follows an Arrhenius curve with an activation energy of 32.0 kcal/mole. Diffusional effects are felt at higher temperatures, despite the fact this catalyst has an “egg shell” construction where all the active ingredients are located in an outside
0.020.0 0.01 ~
0.04
0.08
0.12
0.I6
0.20
0.24
Residence time, CM3 catalyst/CM3 fluld
FIG.14. Kinetics of CO oxidation over copper chromite.
0.20
88
JAMES WE1
k-s-
I20 SCFM
---\+
30
--+----+--
051
1
,
I
1000 900 800
TOO
I0
I
L
1
600
500
400
FIG.15. Carbon monoxide activity of an eggshell catalyst.
shell of only 0.25 mm thickness. See Fig. 15. The oxidation of CO and hydrocarbons are independent of each other. Kuo grouped all the hydrocarbons into two groups labeled “fast oxidizing” and “slow oxidizing”; each group follows first-order kinetics while the fast group is 35 times faster. This procedure appears satisfactory in simulating the kinetic behavior of the converter. A more elaborate procedure would divide the hydrocarbons into more. numerous and more homogeneous groupings, each following first-order kinetics with a different rate constant: dci/dt =
- kici.
(15)
89
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
The total hydrocarbon concentration would be the sum
c(t) =
Cci(t) = C c i ( 0 ) exp ( - k i t ) . i
(16)
i
When the number of hydrocarbon groupings is sufficiently high, one could rank the hydrocarbon groupings according to the kinetic constant, and replace the sum in Eq. (16) by a Laplace integral
1 W
c(t) =
W
c ( t , k ) dk =
c(0, k)eektdk.
(17)
0
When the logarithm of c ( t ) is plotted against t, the slope which represents an apparent rate parameter ( k ) will gradually decline
-dc/cdt
(k) =
1
kc(0, k)e-kt dk
/I
c ( 0 , k)e+ dk.
(18)
B. OXIDATION OVER NOBLEMETALS The kinetics of oxidation over noble metals is dramatically different and much more complex. Every chemical species has an inhibiting effect on the rate of oxidation of another species. Carbon monoxide is a particularly strong self-poison, so that its oxidation kinetics usually proceeds at a negative order with respect to CO concentration. The kinetics also
%I
0
300
I
I
1
400
500
600
700
Furnace temperature (OF)
FIG.16. Bench test evaluation-base metal; solid line, base metal.
metal versus noble metal. Dashed line, noble
90
JAMES WE1
show a striking temperature dependence where the conversion could increase from 0 to 90% when the temperature is increased by 50"F, as shown in Fig. 16. This is due to the peculiar kinetic order of this reaction, and does not imply an unusually high activation energy. The kinetics of CO oxidation over platinum and palladium has been investigated by numeroui scientists, beginning with the pioneer work of Irving Langmuir in 1922 (81). Langmuir studied platinum wires at 230 to 450°C, where the oxygen and CO pressures were at 0.05 Torr. He found that the kinetics can be represented by an expression rate = k(O$)/(CO)
(19)
with an activation energy of 30 kcal/mole. He speculated that the rate-controlling mechanism is a combination of the adsorbed oxygen atoms and adsorbed CO. His temperatures are somewhat lower than exhaust gas temperatures, and his partial pressures are one to two orders of magnitude lower than exhaust gas partial pressures, so that extrapolations must be done with great caution. Eischens studied CO oxidation on platinum deposited on silica at 200"C, and he proposed that the rate-controlling step is the adsorption of oxygen molecules (82). Schwab and Gossner studied the oxidation of CO over silver and palladium, and confirmed the kinetics of Eq. (19) , with an activation energy of 22 kcal/mole (83).Margolis gave a survey on the catalytic "high conversion" of hydrocarbons to carbon dioxide. She reported that the kinetics over platinum could be zero to first order with respect to oxygen, and - 1 to second order with respect to hydrocarbon concentrations ( 6 5 ) . Tajbl et al. studied the oxidation of CO over palladium on alumina pellets from 200 to 234°C. They confirmed the kinetics of Eq. (19) with an activation energy of 28 kcal/mole (84). The major exception to these revults is given by Syzdykbaeva for CO oxidation over platinum on alumina. At temperatures of 200-300°C and CO partial pressures from 0.7 to 7 Torr, they found the reaction to be first order with respect to CO (86). Their kinetics may be diffusion limited. The platinum surface is intensively covered with adsorbed CO, especially at low temperatures and high concentration of CO where the Hinshelwood mechanism may dominate 0%(ad)
+ CO (ad)
+
However, at high temperatures and lower concentrations of CO, the Eley-Rideal mechanism may become more important 0%(ad)
+ CO
-+
Infrared absorption studies of CO on platinum shows two bands near
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
91
2000 cm-l, which suggests two different adsorption structures: a linear structure where the C atom is bonded to a single Pt atom, and a bridge structure where the C atom is bonded to two adjacent Pt atoms (86). Sklyarov et al. argued that at low temperatures and high concentrations of CO, the kinetics should be zero order to total pressure for stoichiometric mixtures, and negative order to total pressure with excess O2 (87). For higher temperatures and lower pressures, negative temperature coefficients should be expected. Oki and Kaneko studied CO oxidation a t 300-475”C and CO partial pressure at 120 Torr. They found the activation energy to be 64.2 to 90.0 kcal/mole (88). Cardozo and Luss discovered an instability of CO oxidation over platinum wires, and confirmed the kinetics of Eq. (19) (89). Hugo and Jakubith also found the same instability, and they argued that the kinetics must be positive order in CO a t sufficiently low concentrations of CO (90). Bonze1 and Ku studied CO oxidation on single crystals of platinum, and found several kinetic regions where the kinetic order of CO may be positive or negative (91). Ertl and Koch studied CO oxidation on palladium surfaces at pressures down to 4 X 10-7 Torr, and found that the rate reaches a maximum at 500°K with sharp declines at higher temperatures. They attribute this to the desorption of oxygen atoms at higher temperatures (92). Baddour and Modell studied the CO oxidation over palladium and platinum with simultaneous infrared measurements (93-95) . For CO oxidation over platinum on silica, the temperature range studied was 438483°K. They discovered that the kinetics is divided into two regimes: in the “normal regime,” where the partial pressure of CO is over 0.1 Torr, the reaction is -0.62 order with respect to CO and there is significant infrared absorption at 2100 cm-1; in the “low surface coverage regime,” where the partial pressure CO is below 0.1 Torr, the reaction rate is four times greater and follows a first-order dependence on CO, with the simultaneous disappearance of the infrared absorption peak. This is shown in Fig. 17. Their data agree with the Langmuir-Hinshelwood mechanism. A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form: Tco = T C I H ~=
-h(CO) ( O z ) / R ( @ ) ,
(20)
-h(C8H6) ( 0 2 ) / R ( @ ) ,
(21)
92
JAMES WE1
80
70
60
so
9
.I 8
4o
a
30
20
10
0
0.1
0.2
0.3
0.4
025
0.6
Pco ( Torr 1
FIG.17. Transition between kinetic regimes of CO oxidation over platinum.
where R(0) = [I
+ kai(C0) + k~&,(C3&)]~ .[I
+ kU3(C0)2(C~H6)2]*[1+ ka4(NO)'.'].
(22)
The first term in R (0) accounts for inhibition effects due to chemisorption of CO and C3Ha.The second term is required to fit the experimental data at higher concentrations of CO and CaH6.The third term accounts for the inhibition effects of NO,. Each rate parameter is of the form
ki = kj"exp[-(Ej/R)/(T
+ 460)],
(23)
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
93
where k," = 1.83 X
E/R
=
22,600,
k2" = 38.0 X
26,200,
kalo = 0.655,
- 1730,
ka2" = 2.08 X
-650,
ka3" = 0.398 X lW15,
- 20,900,
ka4" = 30.2,
6720.
The fit of these equations to the data is very good, as seen in Fig. 18. These equations are valid to very small values of CO concentrations, where the reaction becomes first order with respect to CO. In a mixture of CO with oxygen, there should be a maximum in reaction rate when the CO concentration is a t 0.2%, as shown in Fig. 19. When the oxidation of olefins and aromatics over a platinum loaded monolith is over 99% complete, the conversion of higher paraffins may be around 90% and the conversion of the intractable methane is only 10%. The concentration dependence of CO oxidation over Pt at ( 0 2 ) (CO)-l differs from the concentration dependence of CO oxidation over copper chromite at (02)o.2(CO).This can be explained by the fact that after the departure of a COz molecule, the reoxidation of platinum surfaces is slow but the reoxidation of base metal oxide surfaces is fast. On the other hand,
Space Velocity, Liter#(min)(gm cat.)
FIG.18. Inhibition effect of carbon monoxide on carbon monoxide conversion at 400°F over platinum. 100 ppm GHe, 4.5% 02,100 ppm NO.
94
JAMES WE1
/
3 4[
0
I
3
2
4
I
C. K
FIG.19. Comparison of first-order kinetics with highly self-poisoned kinetics.
the platinum surface is covered with CO and the reaction is inhibited by it; but this is not the case with copper chromite. The kinetic activities of noble metals and of base metal oxides are complementary, so that a mixture of the two would perform better than each class of material alone. We have already observed in Fig. 16 that noble metals have superior activity at high temperatures but base metal oxides have superior activity at low temperatures. Since the CO oxidation kinetics is negative first order with respect to CO over platinum but first order with respect to CO over copper chromite, the rates must be faster over platinum at low CO concentration but the reverse is true at high CO concentrations, as shown in Fig. 19.
C. DECOMPOSITION AND REDUCTION OF NO The decomposition of NO is a very slow catalytic reaction. Amiraami, Benson, and Boudart recently studied the kinetics over platinum and over oxides of copper, cobalt, nickel, iron and zirconium from 450 to 900°C. They found that the kinetics is first order in NO with concentrations from 1.5 to 15%, and that oxygen has a strong inhibiting effect. Even at these temperatures, the kinetics is about a factor of 1000 too low for automotive usage (97'). The kinetics of NO reduction by hydrogen and CO was studied by Ayen and Peters. Hydrogen reduction of NO over oxides of copper, zinc, and chromium was studied at 375425°C. The products formed include
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
95
nitrogen, NzO, and ammonia. The ammonia formation kinetics has the form (H2)1(NO)-0.2, and the nitrogen formation kinetics has the form (H2)1/2(N0)1/2 (98). The catalytic reduction of NO by GO over platinum on alumina, and over a number of base metal oxides was studied by Force and Ayen (99). The reaction temperatures were 240-500°C, and the NO concentrations ranged from 50 to 5000 ppm. As a first approximation, their kinetics is first order with respect to both NO and CO. A higher approximation of the kinetics is given by the form rN0
+
= ~!XNOPNOKCOPCO KNOPNOKCOPCO)~, /(~ (24)
where k, = 15.7 exp (- 10,00O/RT),
KNO= 0.034 exp (7560/RT), KCO= 3.15 exp(2900/RT), and the units of P are in atmospheres. They argued that the mechanism must be the combination of adsorbed NO and GO on adjacent sites.
0
200
400 600 TEMPERATURE. O C
800
FIG.20. Selectivity for “unfixed” nitrogen-containing products in NO-Hz reaction as function of temperature. Inlet concentration: NO, 1000-1200 ppm; Ht, 1.4%. S = [NO(in) - NO(out) - NH3(out)]/[NO(in) - NO(out)].
96
JAMES WE1
The twin problems of oxygen inhibition and ammonia formation remain the topic of many investigators. Meguerian and Lang demonstrated that the ammonia formed in the reduction bed would be reoxidized to NO in the following oxidation bed (44). Klimisch and Barnes argued that in the presence of a reasonable water shift catalyst, the hydrogen formed would be a more powerful reducing agent for NO (100). Klimisch proposed that the mechanism of hydrogen reduction of NO proceeds as a consecutive reaction : SH2 f NO + NH, f Hz0, NH3 -+ 4Nz f BHz,
where the first step is catalyzed by copper, platinum, palladium, and ruthenium, and the second step is catalyzed by nickel and ruthenium. Ammonia formation is serious between 300 and 6OO0C, temperatures encountered in the first few minutes after a cold start. In Figs. 20 and 21, selectivity refers to the percentage of NO converted t o N2. Generally speaking, HOreduction of NO tends to create more NHa, and CO reduction of NO tends to create more N2. Ruthenium is the exception and is by far the best catalyst in the suppression of ammonia formation. Unland suggested that surface isocyanates may be the intermediates that lead to NHs in NO reduction (101). 90 80
-
70
-
60 -
8
50-
.-.x .-3. t 409) cn Q
20 -
30
10 I
I
100 200
1
I
1
300 400 500 Temperature ("C)
I
I
I
600 700 800
FIG.21. Selrctivity for Nz in NO reaction.
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
97
Oxygen inhibition of NO reduction may operate by its greater affinity for hydrogen and CO. Several authors have found that when the molar ratio of Oz/CO is between the stoichiometric ratio of 0.5 and 0.7, the rate of reduction of NO rapidly falls to zero. They concluded that NO reduction can proceed only with an excess of hydrogen and CO (38, 39, 102). Jones et al. have found that below the temperature of 400"F, hydrogen prefers to attack NO instead of oxygen over platinum and palladium. This preference does not extend above 400°F (103). The NO reduction over Cu-Ni-Fe alloys has been studied recently by Lamb and Tollefson. They tested copper wires, stainless steel turnings, and metal alloys from 378 to 500"C, at space velocities of 42,000-54,000 hr-l. The kinetics is found to be first order with respect to hydrogen between 400 and 55,000 ppm, and zero order with respect to NO between 600 and 6800 ppm (104). The activation energies of these reactions are found to be 12.0-18.2 kcal/mole. Hydrogen will reduce both oxygen and NO when they are simultaneously present. CO reduction kinetics were also studied over monel metals by Lunt et al. (4 3 ) and by Fedor et al. (105). Lunt speculated that the mechanism begins by oxidant attack on the metal surface M
M
+ NO
+ 40,
---t
MO
+
MO,
+ )N2,
the metal oxide formed being subsequently reduced by CO and hydrogen. There is no published work on the kinetics of simultaneous redox catalysts, with precisely controlled stoichiometry in the gas. A catalyst that would selectively reduce NO in preference to oxygen is difficult to find and is unnecessary. A mixture of catalysts that is active in oxidation and reduction may be quite adequate to the task. The interaction of different catalytic sites with several gaseous species remains to be unraveled by future investigators.
V. Physical Transport Processes A. FLUIDMECHANICS The flow rate of gases into the exhaust pipe is the product of engine displacement volume, the engine revolutions per minute, and the volumetric efficiency. Secondary air is added to this stream, often in proportion to the engine revolutions per minute, before it enters the catalytic converter. The flow rate of gases can vary from 10 to 150 standard cubic feet per minute for a 4000 lb car going through the CVS-CH driving schedule, and may go up to 300 SCFM while pulling a heavy load upgrade.
98
JAMES WE1
Half the cylinders discharge their gases each cycle, so that an eightcylinder engine produces exhaust gases that pulsate at a frequency of four times the engine speed. The degree of mixing between exhaust gas and secondary air, the flow distribution of gases through the catalyst bed, the pressure drop through the bed, and the rate of mass and heat transfer from the gas to the solid surfaces all depend on the Reynolds number Re = d.u.p/p. In this equation, d is the dia.meter of the channel, pellet, or wire, u is the superficial velocity of the gas assuming that there is no packing in the bed, p is the density of the gas, and p is the viscosity of the gas. Through a 3 in. diam exhaust pipe, the Reynolds number varies between 3000 and 100,000, which produces a turbulent flow; good mixing would be assured if the secondary air is injected sufficiently upstream of the catalytic converter. Three catalytic beds are described in Table VI: a pellet bed, a monolith, and a metal screen bed. The Reynolds number in the monolith bed is sufficiently below 2100 to ensure a streamline flow, but the transition to turbulent flow in a pellet bed takes place at Re = 40. The monolith has the lowest pressure drop, since each stream of gas flows through a channel with no change of direction, and no mixing with gases from another channel. The pressure drops across the pellet bed and the screen bed are higher, since the gas streams are constantly splitting and going around packings, and constantly mixing with other streams. This random walk increases mixing and pressure drop at the same time. It is desirable for the gases to flow uniformly across the catalyst bed. Channeling is undesirable since some streams of gas would flow through too rapidly for adequate conversion. For instance, consider a first-order reaction in a uniform flow reactor where the exit stream contains 10% of the inlet unconverted. If three-quarters of the gas channels through the center half of the bed, then the unconverted portion rises to 16.5%. If the reaction is negative first order, which prevails for CO oxidation over platinum, channeling would cause the unconverted portion to rise from 10% to 43%. The cross section of a catalyst bed is many times larger than the cross section of the exhaust pipe, so that expansion and contraction cones are needed to connect the converter to the exhaust pipes. A very long and tapered connector would minimize pressure drop and promote uniform flow, but would be very space-consuming. The flow distribution across a monolith can be very uneven if the connectors are tapered at an angle of 45 deg, and is often parabolic in form-zero flow near the wall and very rapid flow in the middle. A flow distributor or a perforated baffle plate would improve flow uniformity, at the cost of increased pressure drop. Another cause of channeling is uneven void fractions in the bed, especially in the case of the stacked or rolled metallic screens.
TABLE VI Fluid Flow in Catalyst Beak
A (in.a, bed cross section area)
L (in., bed
0.10 diameter 0.05 channel width
100
0.01
20
3.0
60
0.75
20-420
0.6
wall thickness 0.03 wire diameter
12
4.0
48
0.85
20-420
4.0
d (in.)
Pellet bed Monolith
Screen bed
depth)
V (in.3, bed volume)
Void fraction
Reynolds number
Pressure drop (in. water at 100 SCFM)
1.5
150
0.35
10-170
6.0 -4 M
w
r
M
100
JAMES WE1
B. HEATAND MASSTRANSPORT IN THE POROUS CATALYST Diffusion effects can be expected in reactions that are very rapid. A great deal of effort has been made to shorten the diffusion path, which increases the efficiency of the catalysts. Pellets are made with all the active ingredients concentrated on a thin peripheral shell and monoliths are made with very thin washcoats containing the noble metals. In order to convert 90% of the CO from the inlet stream at a residence time of no more than 0.01 sec, one needs a first-order kinetic rate constant of about 230 sec-1. When the catalytic activity is distributed uniformly through a porous pellet of 0.15 cm radius with a diffusion coefficient of 0.01 cm2/sec, one obtains a Thiele modulus cp = 22.7. This would yield an effectiveness factor of 0.132 for a spherical geometry, and an apparent kinetic rate constant of 30.3 sec-' (106). If the same quantity of active ingredient is concentrated in an outside shell of thickness 0.015 cm, one obtains cp = 2.27. This would yield an effectiveness factor of 0.431 in a slab geometry, and the apparent kinetic constant has risen to 99.2 sec-'. If the active ingredient is further concentrated in a shell of 0.0025 cm, one obtains cp = 0.38, an effectiveness factor of 0.957, and an apparent kinetic constant of 220 sec-'. These calculations are comparable to the data given in Fig. 15. This analysis applies just as well to the monolith, where the highly porous alumina washcoat should not be thicker than 0.001 in. Oxidation kinetics over platinum proceeds at a negative first order at high concentrations of CO, and reverts to a first-order dependency at very low concentrations. As the CO concentration falls towards the center of a porous catalyst, the rate of reaction increases in a reciprocal fashion, so that the effectiveness factor may be greater than one. This effectiveness factor has been discussed by Roberts and Satterfield (106), and in a paper to be published by Wei and Becker. A reversal of the conventional wisdom is sometimes warranted. When the reaction kinetics has a negative order, and when the catalyst poisons are deposited in a thin layer near the surface, the optimum distribution of active catalytic material is away from the surface to form an "egg yolk" catalyst. Temperature gradients within the porous catalyst could not be very large, due to the low concentration of combustibles in the exhaust gas. Assuming a concentration of 5% CO, a diffusion coefficient in the porous structure of 0.01 cm2/sec,and a thermal conductivity of 4 X lo-* cal/sec"C cm, one can calculate a Prater temperature of 1.O"C-the maximum possible temperature gradient in the porous structure (107). The simultaneous heat and mass diffusion is not likely to lead to multiple steady states and instability, since the value of the p parameter in the Weisz and Hicks theory would be much less than 0.02 (108).
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
C. HEATAND MASSTRANSPORT FROM
THE
GASESTO
THE
101
SOLIDSURFACES
The rates of catalytic bed warm-up from a cold start and of destructive overheating are governed by the rate of heat transfer from the gas phase to the solid surfaces. In the highest flow rate of gases, the rate of mass transfer of pollutant molecules to the catalytic walls is inadequate in the monolith. The rate of mass transfer is given by (106, 109, 110)
Qm = h*S(c- c W ) ,
(25)
where Qm is the mass flux in moles/sec, h is the mass transfer coefficient in ft/sec, S is the solid surface area in ft2,c is the concentration in the gas phase in moles/ft3, and cw is the concentration a t the wall in moles/ft3. In a similar fashion, the rate of heat transfer is given by Qh
=
k,S(T - T W ) ,
(26)
where Q h is the heat flux in Btu/sec, Ic, is the heat transfer coefficient in Btu/ft2 sec O F , T is the gas temperature in"F, and T, is the wall temper-, ature in°F. The heat and mass transfer coefficients are given in the literature usually in terms of the Sherwood and Nusselt numbers Sh = h*d/D, NU = Ic,d/X, where D is the diffusion coefficient in the gas in ft2/sec, X is the heat conductivity of the gas, Btu/ft sec OF, and, d is the diameter of the pellet, channel, or wire. The Sherwood and Nusselt numbers are functions of the Reynolds number. The Sherwood number also depends on the Schmidt, number, and the Nusselt number also depends on the Prandtl number s c = P/PQ Pr = cpp/X, where c , is the heat capacity of the gas in Btu/OF lb. The heat and mass transfer coefficients are often correlated in the literature in terms of the Colburn j-factors
jD = (Sh) (SC)-"~(R~)-', j , = (Nu) (Pr)-1/3(Re)-1.
Heat and mass transfer in a channel with streamline flow has been thoroughly studied by many authors. The rates of heat and mass transfer are very rapid at the entrance of a channel, but decline to a steady value
102
JAMES WE1
after a length equivalent t o many times the channel diameter. These transfer coefficients would rejuvenate upon entrance to a new channel. This entrance effect is often exploited when compact heat exchangers are needed, but does exact a toll in increased pressure drop. Depending on the shape of the monolith channel, the values of the Nusselt number decline to a value of 2 4 when the value of L/d reaches 0.05 (Re) (Pr) (111, 112). The Nusselt number is highest for a circular cross section, and lowest for shapes with pinched corners, such as the sinusoidal channels. Given a channel diameter of 0.05 in., and a Prandtl number of 0.7, the value of L is calculated to be 0.03 to 0.7 in.-so the entrance region of increased transfer rate is only a small fraction of the whole channel. Eckert and Drake recommended using an average Nusselt number given by
Nu
= 3.65
+ [0.0668d*Re.Pr/L]/[l + 0.04(d.Re.Pr/L)2/a].
(27)
The average Nusselt number is not very sensitive to changes in gas velocity and Reynolds number, certainly no more than (Re)'I3. The Sherwood number can be calculated with the same formula as the Nusselt number, with the substitution of the Schmidt number for the Prandtl number. While the Prandtl number of nearly all gases at all temperatures is 0.7 the Schmidt number for various molecules in air does depend on temperature and molecular type, having the value of 0.23 for Hz, 0.81 for CO, and 1.60 for benzene. The correlation studies of heat and mass transfer in pellet beds have been investigated by many, usually in terms of the j-factors (113-115). According to Chilton and Colburn the two j-factors are equal in value to one half of the Fannings friction factor f used in the calculation of pressure drop. The j-factors depend on the Reynolds number raised to a factor varying from -0.36 to -0.68, so that the Nusselt number depends on the Reynolds number raised to a factor varying from 0.64 to 0.32. In the range of the Reynolds number from 10 to 170 in the pellet bed, j~ should vary from 0.5 to 0.1, which yields a Nusselt number from 4.4 to 16.1. The heat and mass transfer to wire meshes has received much less attention (110, 116). The correlation available shows that the j-factor varies as (Re)-0.411so that the Nusselt number varies as (Re)0,69. In the range of the Reynolds number from 20 to 420, the j-factor varies from 0.2 to 0.05, so that the Nusselt number varies from 3.6 to 18.6. The Sherwood number for CO is equal to 1.05 Nu, but the Sherwood number for benzene is 1.31 Nu. The other important factor is a, the geometric surface area exposed to gas per volume of reactor, which depends on the void fraction and the dimension of the packing. The product of the transfer coefficient and the surface-to-volume ratio governs the rate of heat and mass transfer per
103
CATALYSIS FOR .MOTOR VEHICLE EMISSIONS
TABLE VII Heat and Mass Transfer from Gas to Bed
a (ft2/ft3)
Pellets Monolith Screen Exhaust pipe
400 800 800 16
Sh for Nu at benzene at 100 SCFM 100 SCFM aL 12 2.4 16 100
15.7 3.14 20.9
-
50 200 266 48
Nu.aL/ exp( -NuaL/ exp(-ShaL/ Re-Pr RePr) ReSc)
12.2 4.03 35.7 0.229
<10-6
0.018 < 10-15 0.795
0.001 0.100 < 10-8
-
volume of reactor (94, 117).Table VII gives the values of a, the values of Nu at the gas flow rate of 100 SCFM, and the values of Sh for benzene. The rate of heat transfer per volume of reactor is given by U ~ C dT/dx , =
kga(T - Tw).
(28)
By grouping the variables, one obtains d T / ( T - Tw)dx = Nu.a/Re*Pr = j,(Pr)-2’3a.
(29)
If the wall temperature is a fixed constant, and To represents the inlet temperature to the bed, the integral of Eq. (29) is (TL - T w ) / ( T ,- T,) = e x p ( - N u d / R e * P r ) ,
(30)
where TL is the temperature at the outlet. It is seen from Table VII that the approach of the outlet temperature to the wall temperature is complete for the pellet bed and the screen bed, even at moderately high gas flow velocities. This approach is incomplete for the monolith, so that the gas leaving the monolith still carries 1.85% of the original heat content above the wall temperature. A similar derivation can be made for the rate of mass transfer from the gas phase to the solid phase.
u dc/dx = ha (c - c,)
(31)
and dc/(c
- h ) d z = Sh.a/Re.Sc = jD(Sc)-2’3*..
(32)
If the kinetics at the wall is infinitely fast, so that c, is equal to zero, the
104
JAMES WE1
GAS FLOW RATE (GI ( S C F Y )
m.at
0.02
1IG (5CFM.l)
FIG.22. Mass transport limitation and CO breakthrough in a monolith with high intrinsic catalystic activity.
integral of Eq. (32) is
.
c/c, = exp ( - Sh .aL/Re. Sc) = exp ( - Tu)
(33)
Provided that the catalyst is active enough, there will be sufficient conversion of the pollutant gases through the pellet bed and the screen bed. The Sherwood number of CO is almost equal to the Nusselt number, and 2.6% of the inlet CO will not be converted in the monolith. The diffusion coefficient of benzene is somewhat smaller, and 10% of the inlet benzene is not converted in the monolith, no matter how active is the catalyst. This mass transfer limitation can be easily avoided by forcing the streams to change flow direction at the cost of some increased pressure drop. These calculations are comparable with the data in Fig. 22, taken from Carlson (112).
When the gas velocities are increased, both the Reynolds number and the Nusselt number would increase, while the ratio Nu/Re decreases with (Re) to the -0.4 to -0.6 power. An increase in gas velocities would improve on the heat and mass transfer coefficients from gas to wall, but would also increase the fraction of heat that is not given up to the wall and the fraction of benzene that never goes near the wall due to the reduction in residence time. Heat loss in the exhaust pipe from the exhaust manifold to the converter can also be analyzed by Eq. (30). The flow is turbulent, and Nu depends on (Re)2/3.Assuming a tail pipe that is 3 ft long, and a Reynolds number of 30,000, the results of the computation are also shown in Table VII. Due to the low surface area, only 20% of the temperature above the wall temperature would be lost through the walls, and 80% would arrive a t the converter inlet. As the gas velocity increases, heat loss through the
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
105
pipe wall would increase, but the fraction of heat delivered to the converter inlet would also increase. This calculation agrees reasonably well with the data from Heinen where the logarithm of the gas temperature is plotted against distance in the pipe, shown in Fig. 23 (118). For a gas containing combustibles, the adiabatic flame temperature is given by
TAf =
T O
4- (co.H/pcp),
(34)
where To is the inlet temperature and, co is the inlet concentration of combustibles. Since the combustion takes place on the catalyst wall, the equation for simultaneous heat and mass transfer is hS(co - c,,.) O H = k,S(T,
- To),
where cw and Tware the wall concentrations and temperatures respectively. The maximum temperature attained on the wall is Tmax
=
To
= To
+ (h*co.H/kg)
+ (Sh-D*c,*H/Nu*X).
(3.5’)
The ratio of these two rises in temperature is (Tmm -
To)/(TAf
- T o ) = (Sh/Nu)*(Dpcp/X).
(36)
The dimensionless parameter Dpc,/X is called the Lewis number, which is the ratio of the diffusion coefficient of a gas through the mixture divided by the thermal diffusion coefficient of the gas mixture. The value of the Lewis number for HS is 2.4 while the value for CO and benzene are less than 1.0. Thus if the gaseous mixture contains much Hz,
-
60
C
40-
c a c 0
n
=-
-
20-
E
10: 8-
-
0
c
E
0 *
s j
’
6-
-
4-Speed (mphl-
200
400
idle 20
600
x)
40
50 60 70 80 90 100
800 1000 1200 Exhaust gos temperature
1400
1600
I800
(OF)
FIG.23. Exhaust temperatures at various locations.
106
JAMES WE1
the nonequilibrium maximum wall temperature may be much higher than the equilibrium adiabatic flame temperature of the gas.
D. HEATAND MASSTRANSPORT WITHIN
THE
CATALYST BED
There are several mechanisms in the catalyst bed to smooth out fluctuations in the inlet temperature and concentration of the gas, and to disperse local heat generation. These mechanisms include molecular diffusion of the gases, mixing cells created through the splitting and rejoining of gas streams around packings, conductivity of the solid packings, and thermal radiation (119). Some of these mechanisms only serve to disperse the disturbances in a forward direction, and some of them operate equally in both directions parallel to the gas flow. These mechanisms also operate in directions transverse to the gas flow direction, but with a considerably lower speed. Theories have been developed which assign effective or fictitious mass and thermal diffusion coefficients to the catalytic bed, both in the parallel and in the transverse directions. In much of the literature, the parallel direction is called the axial direction, and the transverse direction is called the radial direction. This usage is acceptable as long as we are dealing exclusively with axial flow reactors, and should be discouraged when radial flow reactors are also considered. These four dispersion coefficients would be many times larger than the diffusion and thermal conductivities in the gas phase. Parallel dispersions can be introduced by the mass and heat balance equations d2c dc De.p- - U - - R = 0, dx2 dx d2T dx2
(37)
- upcP--dT 4-R-H = 0, ax
where R is the reaction rate, H is the heat of reaction, D e . pand Ae.p are the parallel dispersion coefficients, and pc, refers to the thermal properties of the gas. These parameters are usually correlated in terms of the Peclet numbers PeM., = U * d / D e . p , PeH.p = u.d.pc,/X..,. The Peclet numbers decrease when the dispersion coefficients increase. In the Reynolds number range of 10-200, in a packed bed of pellets, PeM = 2 and Pen = 0.5 (119, 120). The dispersions in the transverse
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
107
direction could be important if there is a sufficient flux of heat and matter through the side wall of the reactor. The transverse dispersion coefficients can be introduced by the equations
a2T ar2
A,.,-+---
1 dT
rar
aT ax
u~c,-
+ R.H
= 0.
Naturally, there are two more Peclet numbers defined for the transverse direction dispersions. In these ranges of Reynolds number, the Peclet number for transverse mass transfer is 11, but the Peclet number for transverse heat transfer is not well agreed upon (121, 122). None of these dispersions numbers is known in the metal screen bed. A special problem is created in the monolith where transverse dispersion of mass must be zero, and the parallel dispersion of mass can be estimated by the Taylor axial dispersion theory (123).The dispersion of heat would depend principally on the properties of the monolith substrate. Often, these Peclet numbers for individual pellets are replaced by the Bodenstein numbers for the entire bed Bo = D,/Lu = d/(L.Pe). The Bodenstein number has the virtue of a linear dependence on the dispersion coefficient. As a rule of thumb, when L/d is greater than 50, the effects of parallel dispersion are negligible and the reactor behaves as a piston flow reactor. But L/d is often as small as 10 in the automotive converters, and these dispersion coefficients cannot be ignored. For instance, when Bo = 0.05, the degree of conversion for a first-order reaction can fall significantly so that the fraction unconverted for a plug flow reactor may be 0.01 but the fraction unconverted with Bodenstein number of 0.05 would be 0.022 ( l a & ) .Dispersion in the parallel direction always decreases the degree of conversion in a reaction with positive order, but would increase the degree of conversion in a reaction with negative ordersuch as the oxidation of CO over platinum. An alternative method to account for bed dispersion is to model the bed as a cascade of well stirred tank reactors, each with a uniform temperature and concentration (lag,196).Transverse dispersion can be accounted for by staggering the cells so that each cell feeds into two different cells in the forward direction (1.26).When the value of L/d is large, say above 20, the two models are not very different if the number of cells in the cascade is chosen to equal N = PeL/2d. When Pe = 2, this amounts to considering
10s
JAMES WE1
0'
'
BED LEN&
3
(IN)
FIG.24. Measured and calculated bed temperature profiles.
N
= L/d, or that each row of pellets amounts to a mixing cell. This is intuitively reasonable. However, the parallel dispersion for heat has a Peclet number equal to 0.5, which would argue that four rows of pellets should be considered to be a mixing cell. The heat balance equation for cell i in a cascade of N cells is Au(PCp)gas(Ti-~
- Ti) =
(PCp)aolid
(V/N)dTi/dt.
(41)
Consider a case where the initial temperature of all cells is T i ( 0 ) = To, and a t t = 0, the inlet gas temperature jumps to Ti,. The solution is ei(t) = 1 - e-x'
2
+
(Xt) +y ) , (42) - l)! 3-1
0 . .
(2
where
ei = (Ti- T o ) / ( T i n - To), = NAU(PC,),/~(PC,)a.
This model is supported by an experiment described by Wei where a pellet bed at 80°F is swept suddenly by air a t 800°F ( 1 2 7 ) . The temperature rise in the bed is illustrated by considering the bed to be a series of cells. Each cell consists of 3 to 4 rows of catalysts, as shown in Fig. 24. Thermal radiation becomes important at higher temperatures, especially above 2000"F, when thermal destruction of the monolith substrate probably takes place. Thermal radiation intensities are proportional to the emissivity of the surface multiplied by the absolute temperature raised to the fourth power. The thermal emissivity of the monolith may be close to 1.0 due to the blackened surfaces from deposition of platinum. Each point of the channel is completely visible from any other point of the channel. The
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
109
product of channel diameter and partial pressures of water vapor and COz in the channel is less than 0.001 ft-atm, so that we can safely neglect the absorption of radiation by the gas phase. Conduction and convection are short-range interactions, where the flux rates only depend on local gradients of temperature and concentration, giving rise to differential equations. Thermal radiation is a long-range interaction where the flux depends on temperatures of walls far away, giving rise to integral equations. As a first approximation, the radiated flux is sometimes considered only as a function of the local temperature gradient (128),
(d/dx)uT4 = 4aT3 dT/dx.
(43)
VI. Durability of Catalytic Converters The major objection to the use of catalysts in automobile exhaust is their lack of durability from the abuses of daily use for 50,000 miles without maintenance. The catalytic converter is not yet a device that can be attached to the exhaust pipe at the factory, and then forgotten for half the lifetime of a car while removing 90% of the pollutants. The two greatest enemies to catalyst longevity are poisons and high temperatures. The former can be minimized by rigorously excluding catalyst poisons from gasoline and lubrication oils at a cost, but the latter has no easy solution.
A. SLOWAGINGOF CATALYSTS FROM POISONS There is a great deal of evidence that lead can poison the oxidation catalysts, whether they consist of noble metals or base metal oxides (12'9,130). The tetraethyl or tetramethyl lead in the gasoline decomposes in the cylinders, and combines with ethylene dibromide and ethylene dichloride to form a mixture of halides and oxides with boiling points around 900°C. This mixture is partly retained in the engine and oils, partly emitted as gases, and partly left as particulates ranging in size from 0.3 to 10 p in diameter. The particulates of lead can block the pores and can cover the catalytic surfaces and render them inaccessible. Current refinery practices place 3.0 g tetraethyl lead in each gallon of gasoline. Assuming that an automobile consumes fuel at a rate of 10 miles/gal, the quantity of lead that passes over the engine in 50,000 miles would amount to 33 lb, quite sufficient to complete the coverage of the catalysts. To protect the oxidation catalysts from this barrage of lead, the Environmental Protection Agency has specified that lead-free gasoline must be available
110
JAMES WE1
with no more than 0.05 g lead/gal, which amounts to 18 ppm by weight (131). Even at this level, the quantity of lead consumed in 50,000 miles is still 250 g. In a catalytic bed, lead tends to deposit more heavily at the front rows of pellets or at the front end of a monolith, and at the outside of a catalyst pellet (132). The susceptibility of lead poisoning varies a great deal among catalysts. It is known that lead may volatilize from some catalysts during driving modes with high catalyst temperatures. The Universal Oil Product company has reported data showing that their catalyst can recover from lead poisoning from the inadvertant use of leaded fuels. They have shown that after a few tanks of lead-free gasoline had been used, the catalysts had essentially recovered their former activity ( 4 7 ) . The metallic NO, reduction catalysts that operate at above 1500°F most of the time may be immune to lead deposition. In the presence of phosphorus, some lead is deposited in the relatively innocuous form of lead phosphate. On the other hand, Chrysler Corp. recently announced preliminary engine dynamometer tests indicating ethylene dibromide, rather than lead, is the noble metal poison. Phosphorus is a common additive in gasoline to impart detergency and has been reported to collect on certain catalysts, and to be a catalyst poison. The EPA has ruled that the protection of oxidation catalysts would require that phosphorus-free gasoline be made available at no more than 0.01 g phosphoruslgal, which is less than 4 ppm by weight. Gagliardi et al. have shown that poisoning of catalysts with fuel containing 0.1 g phosphorus/gal is completely reversible after driving two thousand miles with phosphorus-free fuel (129).Phosphorus is also a common additive in lubricating oil, and its use may have to be restricted also. Sulfur is always present in gasoline a t about 0.04 wt.%, and has been reported to be a poison to oxidation catalysts. Twelve pounds of sulfur are burned in an engine over 50,000 miles. There is no current EPA rule on the maximum allowable sulfur content in gasoline. The mechanism of sulfur poisoning may be through sulfate formation, and such poisoning is reversible by heating to 1500°F which causes sulfate decomposition and volatilization of SO2and SO3. Somorjai has suggested that sulfur adsorption changes the surface free energy of platinum crystals, and causes rearrangement of surface structure (133). Other potential poisons include zinc, manganese, chlorine, and bromine. A number of metals may be deposited on the catalysts from engine erosion and wear, including copper, chromium, nickel, and iron. The mechanism of poisoning has been reviewed by Maxted (134) and by Butt (136).
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
111
B. THERMAL AGING AND DESTRUCTION OF CATALYSTS The greatest bane to catalyst durability is high temperature. Repeated cycling of temperature, presence of 15% water vapor, repeated cycling of the exhaust gas from oxidizing to reducing conditions, condensation and freezing of water vapor, mechanical vibrations and shakes, and many other factors all add to the premature aging of catalysts. Some of the immediate causes of catalyst aging can be listed as: (i) loss of support and active ingredient surface area due to sintering, (ii) reaction of active components with support material, (iii) volatilization of active components by reaction with gases, and (iv) destruction of support material by stresses, erosion, phase transition, and melting. There is little data available t o quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 d to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. The most spectacular failure of catalysts is in the melting of the substrate material and the metal container, sometimes obstructing the flow of exhaust gases and causing the engine to stall (112). This melting must take place at very high temperatures, as the melting point of alumina is about 3700"F, monel metal around 2400-250OoF, and the ceramic materials mullite 2800"F, cordierite 2750"F, and spodumene 2350°F. At temperatures much below the melting points, the support material may begin to flow under the impact of pulsating gas flow and mechanical vibrations. These temperatures are not reached during normal engine operations, as the temperatures of exhaust gases do not exceed 1700°F even during the highest speed operations. A short pulse or a prolonged flow of combustible mixtures can provide the chemical heat to increase the exhaust gas temperature. It was previously shown that 1% CO can increase the gas temperature by 141'F. Occasions may arise when combustible mixtures do not ignite in the cylinder, either because the spark plug is not functioning, or because the flame is choked due to the mixture being too rich or too lean. If all the cylinders misfire, the gasoline concentration in the exhaust gas would be 2.0% by volume, which would provide an adiabatic flame temperature rise of 4000"F, quite sufficient to melt any
112
JAMES WE1
of the substrates. The adiabatic flame temperature can be exceeded if the concentration of Hz is very high. Such extreme temperatures must be avoided for reasons of human safety, which is more important than catalyst durability. C. VEHICLEAGINGOF CATALYSTS The final authority on the durability of catalysts is performance in road vehicles. Such data have been rapidly accumulated by the various automobile manufacturers in recent months. This data takes into consideration all the accidents of everyday usage, serving to test how much abuse the catalyst can withstand and still perform its duty. Experience has shown that fresh oxidation and reduction catalysts by a large variety of formulations from many manufacturers would indeed perform their duty. Many oxidation catalysts perform well enough at 25,000 accumulated miles to satisfy the requirement of 0.41 g hydrocarbon/mile and 3.4 g CO/mile, but few would perform well enough at 50,000 miles without maintenance and adjustment of the engine. Many such vehicle endurance tests have to be terminated because of malfunction of the engine or the auxiliary equipment. Most of the NO, reducing catalysts in pellet or monolithic form begin to lose their activity at 2000 miles and fail to be effective at 4000 miles. This lack of durability may well be connected to the usage of the NO, bed for oxidation purposes during the cold start, which exposes the NO, catalysts to repeated oxidation-reduction cycles. Better catalyst durability can be anticipated in the single bed redox catalyst with a tightly controlled air-to-fuel ratio, since this oxidation-reduction cycle would not take place. Recent data indicates that the all metal catalysts of Questor and Could may be able to last 25,000 miles. Some of the vehicle aging data taken from the report of the National Academy of Sciences are shown in Table VIII. The CVS-CH and the’ Durability Driving Schedule are expensive and time-consuming. There is need for a quicker test of the performance and durability of catalytic converters in a vehicle. The most frequently used laboratory parameters for catalyst activity are : kinetic rate constants a t a given temperature, temperature needed for 50% conversion of CO a t a given space velocity, and percent conversion of CO at a given temperature and space velocity. These three parameters do not always rank all catalysts in the same order, and they represent only part of the information needed to predict CO emission from the converter in a vehicle going through the CVS-CH procedure. The problem of predicting vehicle performance from laboratory data is described in the next Section under Mathematical
TABLE VIII: Emissions as Function of Mileage for Durability Tests on Dual-Catalyst Systems ~
~
~
~~
~~~~~
Catalysts Emissions (g/mile) Manufacturer, vehicle
(a) HC/CO (b) NO,
Mileage
HC
co
0.32 0.39 0.52 0.21 0.47
-
NO, catalyst efficiency NO=
(%I
0.22 0.42 0.45 0.73 0.21 0.59
78 58 55 27 79 41
General Motors (1) 4500 lb
(a) UOP, noble, pellet
350 CID Chevrolet EGR (2) 4500 lb 350 CID Chevrolet EGR (3) 4500 Ib 350 CID Chevrolet EGR
(b) Gulf, noble, pellet (a) UOP, noble, pellet
0
1000 7000 13000 0 7000
1.7 3.0 4.8 1.0 1.8
(b) Johnson-Matthey, noble, monolith (a) UOP, noble, pellet
351 CID
Ford EGR
EGR
r
0
m
5
0
0 4Ooo
0.36 0.57
1.8 4.1
0.28 0.51
72 49
(b) General Motors Research, pellet
2
2
I?
m
; M
(a) Englehard, monolith (b) monolith 8-10 g platinum, dual-bed converter
(2) 5000 lb 351 CID Ford
4
B Y
Ford (1) 5OOO lb
t 5
(a) Pellet (b) Pellet Dual-bed converter
Low
3000 6000 9Ooo 12OOO 16OOO
20000 Low 1000 2Ooo 6000
0.3 0.33 0.48 0.72 0.66 0.66 0.82 0.35 0.61 0.59 0.68
1.5 1.5 2.6 1.9 3.6 5.4 3.8 3.8 3.3 3.6 4.2
0.56 0.49 0.70 0.89
0.75
78
80 71 63 64 46 37 70
1.3 1.5 0.68 0.99 1.25
-
1.72
25
2 0
3
w w
w
114
JAMES WE1
Modeling. There is as yet no rapid simulated laboratory aging test for catalysts that is recognized as a good predictor of catalyst aging in the vehicle.
VII. Reactor Engineering The purpose of automotive reactor engineering is the design of reactors to utilize fully the potentials of a given catalyst, to protect the catalyst from dangerous excursion, and to guide the development of new catalysts. An arsenal of auxiliary equipment can be deployed and coordinated in the task, including the secondary air pump, intake manifold heat exchanger, oxygen sensor, feedback mechanism to adjust the air to fuel ratio, and over-temperature protection bypass. Optimal design of the reactor system should be evaluated with the objective of performance over a long span of time while the catalysts deteriorate. Ease of maintenance, diagnosis, and replacement should be considered from the very beginning. A. MATHEMATICAL MODELING The performance of the automotive catalytic converter is governed by three sets of factors: engine output from a driving schedule, reactor system with auxiliary equipment, and the catalyst. There are many variables to be considered in an optimal design, and the interaction of these variables in determining the performance of a reactor is difficult to evaluate. For a given catalyst, it is very time-consuming and expensive to use it in a number of converter systems for each automobile model, and to measure the performance and durability over a long and arduous test procedure. It is far more efficient to have a reliable mathematical model that can predict the performance of many combinations, to discover the important variables, to explore fruitful directions in design modifications, and to screen out all but a few exceptional designs that should be experimentally constructed and tested. A predictive mathematical model must include a sufficient number of physical and chemical principles to explain all the important observed phenomena, to be extrapolated with confidence into regions where no experimental data exists, and yet to be simple enough for rapid computation. Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations: the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,
115
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
and the change in emphasis from maximizing profits to minimizing outputs in the ppm range. To evaluate each reactor design, there is a need for the development of a penalty function or functions to calculate the damages caused by each excursion away from the safe windows of operations, and for balancing short-term performance with long-term durability. Steady state models of the automobile catalytic converter have been reported in the literature (138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum (139).The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are: u(pcp), aTg/az = -ak,(T, (pcpIBaTs/at =
Ts),
+akg(Tg - T B ) .
(44)
It is impossible to solve these partial differential equations with the varied input conditions resulting from the California cycle. A computer was used in the study, where distance is divided into increments of 0.08 to 0.30 in. in thickness, and time is divided into increments of 0.4 sec. They obtained the temperature profile in the bed as a function of time from a cold start. This procedure could result in a very long computation, as the California cycle has a duration of 16 min, requiring 2400 time intervals and computations. A comprehensive mathematical model of the pellet bed was developed in the IIEC program, and described by Wei (127) and by Kuo et al. (21, 140). This model seeks to replace the catalytic bed by a series of cells with uniform temperatures and concentrations. The heat balance of the solid in cell i is given by Vi(pcp)a
dTB,i/dt =
u(pcp)g(Tg,i-1
- Tg,i)
+ Vi C
HjRi,j,
(45)
i
where V i is the volume of cell i, Ta,i and T g i. are the temperatures of solid and gas in ceIl i, R i , j is the rate of reaction of species j in cell i, and H j is the heat of reaction of species j . The heat balance of the gas in cell i is given by Au(pcp),(Tg,i-l- T g , 4 = V&,a(T,,i -
T8,i).
(46)
The mass balance for each combustibIe species j in cell i is Au(Ci-l,j - C i , j ) =
Of course,
Ri,j
ViRi,j.
(47)
depends on the temperature as well as all the gaseous
116
JAbIES WE1 l2M
MIbBEO TEMP. ("F)
600
TIME (SEC)
FIG.25. Experimental and predicted mid-bed temperature. Vehicle :Galaxie, 289 cu. in. engine with thermactor. Converter: radial, 392 cu. in. Catalyst: aged type 'IF" (12,500 miles; vehicle). Midbed temperature: -, experimental; 0 ,predicted T,;X, predicted
T..
concentrations. For a sudden change of inlet temperature, the relaxation time of a cell is given by V(pc,)./uA(pc,),, which may vary from 20 to 500 sec; while the relaxation time for a sudden change in inlet concentration is given by V / u A , which may vary from 0.01 to 0.25 sec. This model was used to predict the performance of a catalytic converter in an automobile going through the California cycle. The calculated mid-bed temperature is shown in Fig. 25. It closely matches the measured value, which is below 600°F during the first two minutes. The measured inlet concentration of CO is given by the solid line in Fig. 26, and the measured outlet concentration is given by the dashed line, which agrees very well with the calculated outlet concentrations given by the dots. The conversion of CO in the reactor is negligible during the first two minutes, and is nearly complete after that. Since the total emission of pollutants is the
FIG.26. Experimental simultaneous converter inlet and outlet and predicted outlet CO. Vehicle: Galaxie, 289 cu. in. engine with thermactor. Converter: 392 cu. in. radial flow. Catalyst: aged type "F" (12,500 miles; vehicle). FTP inlet = 0.91%. FTP outlet (exp.) = 0.29%. FTP outlet (pred.) = 0.25%.
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
117
integral of concentrations multiplied by gaseous flow rates over time, the overwhelming importance of the first two minutes from a cold start is evident. A significant decrease of total emission can be achieved by a catalyst that operates a t a lower temperature, or by a catalytic bed that would warm up faster. An examination of the curves will show that a catalyst with a “light-off’ temperature that is 100°F lower would make only a small improvement, but another catalyst with half the thermal mass may make a much greater improvement. Kuo et al. (140) have shown that if the catalyst kinetics pre-exponential factor is multiplied by a factor of 1, 10, 100, and 1000, the emission of CO is 9.1, 7.2, 6.0, and 4.5 g/mile. But if a catalyst with an activity factor of 0.1 is preheated to 800”F, the CO emission would be below 3.4 g/mile. In the first 100 sec, the outlet concentration of hydrocarbon is higher than the inlet concentration in Fig. 27, presumably due to the desorption of hydrocarbons previously adsorbed on the bed. After the engine is turned off, a catalytic converter usually returns to the ambient temperature in two hours, so that a restart of the engine would generate the same quantity of pollutants as the first cold-start in the morning. The IIEC model was also used to study the importance of various design parameters. Variations in gas flow rates and channeling in the bed are not the important variables in a set of first-order kinetics. The location of the catalytic bed from the exhaust manifold is a very important variable; when the bed is moved from the exhaust manifold location to a position below the passenger compartment, the CO emission averaged over the cycle rose from 0.14% to 0.29% while the maximum temperature encountered dropped from 1350 to 808°F. The other important variables discovered are the activation energy of the reactions, the density and heat
12w CYCLE
CYCLE
6m I CYCLE 7m 7 CYCLE
1OW
INPUT
OUTPUT 60’ 2w
TIME(SECI
FIG.27. Experimental simultaneous converter inlet and outlet and predicted outlet HC. Vehicle: Galaxie, 289 cu. in. engine with thermactor. Converter: 392 cu. in. radial flow. Catalyst: aged type “F” (12,500 miles; vehicle). FTP inlet = 152 ppm. FTP outlet (exp.) = 54 ppm. FTP outlet (pred.) = 49 ppm.
118
JAMES WE1
capacity of the catalysts, and the value of fast-warm-up. Variations in the inlet concentration of CO and hydrocarbons are not important in firstorder reactions, and variations in the inlet concentration of oxygen are not important unless they are less than stoichiometric. The volume and the shape of the bed are important variables. A similar pellet bed model was used by Harned to compute the performance of other kinetic expressions (141). For base metal catalysts, Harned considered the CO oxidation kinetics as zero order with respect to oxygen concentration and zero to first order with respect to CO concentrations, but the propylene oxidation kinetics is first order in propylene concentrations. Harned also considered CO oxidation over platinum where the kinetics is first order with respect to oxygen and negative first order with respect to CO concentrations. He computed the bed temperature profiles and outlet concentrations for various inlet conditions, and discovered a number of instabilities in the reactor for platinum kinetics. During steady-state conditions, the reaction zone is confined to a very narrow band of less than 10% of the total bed length, where the gas temperature leaps by 500°F and the CO concentration drops to zero. As the mass velocity of the gas flow is increased, the reaction zone moves toward the rear of the reactor with no change in the exit concentration of CO; finally, with a further increase of mass velocity, the reaction zone reaches the rear of the reactor and the exit concentration of CO makes a sudden leap upwards. During a bed warm-up, the total CO mass emission depends strongly on inlet gas temperature and CO concentration, as well as on the mass flow. As the catalysts age, mass emission will increase during the warm-up period, since a higher temperature is needed to reach 50% conversion efficiency; the phenomenon of (‘breakthrough)’is also observed, where CO is emitted from a fully warmed converter during high speed gas flow and possible channeling. Lead deposition and high temperature damage tend to occur at the front end of the converter bed, and on the outside surface of the catalyst-the two regions that are warmed up first from a cold start. This is unfortunate, since the active rear end of the converter is too cold to do much conversion during the first two minutes. A periodic circulation of the catalysts in a pellet bed would alleviate this problem by a more uniform deactivation of the catalysts. This circulation is not available in the ceramic and metallic monoliths.
B. ANALYSISOF ALTERNATIVE REACTORS The piston flow reactor has an advantage over a stirred tank reactor when the kinetics is of positive order, but the reverse is true when the
119
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
80-
-R 0.1
0.2
05
. A (I+&)*
2
I kI
FIG.28. Conversions in a piston flow reactor.
kinetics is of negative order, such as CO oxidation over platinum. The backmixing of outlet and inlet gases would decrease the CO concentration in the reactor, which increases the kinetic rate. Assuming an isothermal reactor, the relation between percentage conversion and space velocity in a piston flow reactor is shown in Fig. 28, where the kinetics is of first order, and of the form R = 5Okc/( 1 k’c)2. (48)
+
In Fig. 28, the abscissa kt is the product of the reaction rate constant and the reactor residence time, which is proportional to the reciprocal of the space velocity. The parameter k’c, is the product of the CO inhibition parameter and inlet concentration. Since k’ is approximately 5 at 600°F these three curves represent c, = 1, 2, and 4%. The conversion for a first-order kinetics is independent of the inlet concentration, but the conversion for the kinetics of Eq. (48) is highly dependent on inlet concentration. As the space velocity increases, kt decreases in a reciprocal manner and the conversion for a first-order reaction gradually declines. For the kinetics of Eq. (48), the conversion is 100% at low space velocities, and does not vary as the space velocity is increased until a threshold is reached with precipitous conversion decline. The conversion for the same kinetics in a stirred tank reactor is shown in Fig. 29. For the kinetics of Eq. (48), multiple solutions may be encountered when the inlet concentration is sufficiently high. Given two reactors of the same volume, and given the same kinetics and inlet concentrations, the conversions are compared in Fig. 30. The piston flow reactor has an advantage over the stirred tank
120
JAMES WE1
//--
u, -R m S O k C ( I + k'C)'
20 ,/A-
0
2
02
0.5
,
.
5,
,
.
,
I
kt
FIG.29. Conversions in a stirred tank reactor.
reactor for the first-order reaction, especially when the conversion is very high. For the kinetics of Eq. (48) , the stirred tank reactor is more advantageous, especially when the inlet concentration is high. A well designed reactor system should perform well not only within a narrowly defined set of windows, but should tolerate a good deal of fluctu-
0
20
40
60
% camnicm in Piston new
FIQ.30. Conversions of two reactors with the same kinetics, residence times, and inlet concentrations.
TABLE IX
5
Sensitivity of Conversion to Changes in kt and co c/co in piston flow reactor
Kinetics
&I&= -kc
e-kl
&/& = - k
1
- kl/co when kt < co
Skt
(c/co) In (Cleo) (c/co) - 1
sco
0 1 - (CICO)
0"s c/c. in stirred tank reactor
sk1
+
(1 k0-I 1 - kt/%
s
3
122
JAMES WE1
ation in the inlet conditions. A “robust” system is not easily upset, and its conversion levels are not too sensitive to kt, nor to c,. One may formally define the two sensitivities as S k f = d(c/c,)/d In kt,
S,, = d (c/c,) / d In c,.
These two parameters describe the change in fraction unconverted with a percentage change in kt or in c,. The first sensitivity is also the slope of the curves in Fig. 28. The values of these sensitivities are given in Table IX. In a piston flow reactor where the conversion level is c/co = 0.1, the value of S k t is -0.23 for the first-order kinetics, -0.90 for the zero-order kinetics, and -4.95 for the negative first-order kinetics. In the stirred tank reactor, the value of the sensitivities skf is -0.09 for the first-order kinetics, -0.90 for the zero-order kinetics, and 3-0.11 for the negative first-order kinetics. A positive sensitivity means that as kt is increased, the fraction unconverted also increases, clearly an unstable situation. The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use ( 8 4 ) . A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. The analysis of the transient behavior of the packed bed reactor is fairly recent in the literature (14.2-146).There is no published reactor dynamic model for the monolith or the screen bed, which compares well with experimental data.
VIII. Future Prospects The 1975 emission standards, even after the announced relaxation, will present problems. For the automobile manufacturers, the least disruptive solution would be minor design changes in the existing engines, which will not be adequate for the California cars nor for a large fraction of the cars in the other 49 states. The next preferred solution would be to keep the present engine design largely intact, and to install add-on devices such as catalytic converters and secondary air pumps. A major modification in engine design would involve long development time, heavy capital investment, and obsolescence of many of the present facilities. The catalytic converter may not be perfect, but it is the best short term solution. The biggest drawback of the oxidation catalytic converter is its lack of durability, unless great efforts are made to protect it from very high
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
123
temperatures and from poisons. The only noncatalytic solutions to the California standard in 1975 of 0.9 g hydrocarbon/mile and 9.0 g CO/mile are the Diesel engine, the Wankel engine, and the Honda stratified charge engine. These three engines all have their own shortcomings, and none of them can be produced in large enough quantities in 1975. The Diesel engine has excessive emission of smoke and odor from aldehydes and oxygenated aromatics, which are not regulated at this moment. It is approximately 50% larger and heavier than a gasoline engine of the same horse power. However, it does have the virtue of better fuel economy and easier maintenance. Wankel engines, such as the rotary engine mass produced by Toyo Kogyo for Mazda, need a thermal reactor to pass the 1975 California standards. But this two-rotor engine is designed only to power a subcompact car at the moment. Developments for a four-rotor engine, or a two-rotor engine with larger displacement, are not at a very advanced stage. Some experts believe that a rotary engine with twice the present engine displacement, powering an automobile with twice the weight of the Mazda, would produce twice the emission per mile and fail the standard. The stratified charge engine system of Honda, called the compound vortex controlled combustion (CVCC) , employs a small precombustion chamber with rich carburetion, connected to the main combustion chamber with lean carburetion. It is also designed for a 3000 lb subcompact car, and has no demonstrated emission capabilities and durability experience with larger cars. The more revolutionary engines such as the steam engine, the Stirling engine, the Rankine engine, the gas turbine, and the fuel cell and storage battery electric engines are not yet in intensive development and are not ready for mass production for at least ten years. With the arrival of the 1976 model year, NO, emission for new cars must be lowered to 0.4 g/mile, although the Environmental Protection Agency has decided to postpone the enforcement for one year. The technology for NO, reduction is far less advanced, and the NO, catalysts seldom endure more than five thousand miles. By operating the engine with very rich mixtures, and by the use of exhaust gas recirculation (EGR), the emission of NO, may come down to the level of 1.0 g/miIe and the rest of the reduction is up to the reduction catalysts. Rich mixtures and EGR also cause up to 30% increase in fuel consumption, and cause a deterioration of the drivability of the car. The use of feedback control on the air/fuel ratio, fuel injection, oxygen sensor, and a bed of redox catalysts would make an economical and drivable engine; the lack of alternating oxidation-reduction conditions may retard the aging of the catalysts. The durability of the sensor is yet to be demonstrated. The other potential solutions for 1976 are even more unattractive. The
124
JAMES WE1
Diesel engine has little prospect of attaining an emission of NO, below 1.0 g/mile. The inherently lean exhaust from Diesel makes an NO, reduction catalyst useless, and would require an NO, decomposition catalyst. The Wankel engine and the Honda stratified charge engine are also unable to reduce its NO, emission below 1.0 g/mile. There is a great deal of opinion from experts that the NO, emission requirement of 0.4 g/mile is unnecessarily stringent to ensure adequate ambient air quality, and a relaxation of the standard to 1.5-2.0 g/mile would be adequate. It would require a new Act of Congress to rescind the requirements of the Clean Air Act of 1970. The passage of a relaxed automotive emission standard for NO, will have profound effects on automotive catalysis. The need for a NO, reduction bed would be much decreased, since it is possible to tune the engine to be sufficiently rich and with enough EGR to keep the NO, level below 1.5 g/mile for virtually all cars. However, some automobiles may elect to use a NO, catalyst to improve on fuel economy and drivability. The Diesel engine may become the best long term solution, since its emission of hydrocarbons and CO is already below the required limits today without the use of any auxiliary devices. For scientists and engineers working on automotive catalysis, the existing challenge is to produce catalysts that can be abused with impunity, and to produce reactors that provide sure protection to the catalysts. If the job is done well, catalysis may be the long term solution to automotive emission, restoring the gasoline engine to the good graces of the public. LIST OF SYMBOLS a
A
C
CO
G,j
CP
D d
E F
H
geometric surface area per volume of reactor, ftZ/ft* cross section area of reactor in the direction of gas flow, f t z or in2 concentration of gas, lb mole/ft* or lb/ft* inlet concentration to the reactor concentration of gaseous species j in cell i heat capacity of gas, Btu/lb O F diffusion coefficient, cm*/sec diameter of particle, channel, or wire, in. activation energy, kcal/mole flow rate of gas, standard cubic feet per minute heat of reaction, kcal/mole or Btu/lb mole
mass transfer coeficient, ft/sec Colburn factor for mass transfer, (Sh) (Sc)-l'*(Re)-l Colburn factor for heat transfer, (Nu) (Pr)-l/*(Re)-l kinetic rate constant heat transfer coefficient, Btu/hr ftaoF bed depth, in. number of cells in the cascade model mass flux, moIes/sec heat flux, Btu/sec gas constant, 1.987 Btu/"R Ib mole rate of chemical reaction, mole/ ftS
distance in radial direction, in. solid surface area, ftP
CATALYSIS FOR MOTOR VEHICLE EMISSIONS Sbt, S,,
T 1 U
V W X
sensitivities of conversion to changes in kt and co temperature, O F time, sec velocity of gas, ft/sec bed volume, in.a total emission of pollutants distance, in.
Pe Bo
TU
125
Peclet number, ud/D, or udpcp/x, Bodenstein number, DJLu Number of transfer units, NuaL/ RePr or ShaLIReSc
Oreek Symbols
Dimensionless Parameters
D coH/XT, emissivity in thermal radiation effectiveness factor thermal conductivity, Btu/ft sec
“F Sh Nu Re sc Pr Le
Sherwood number, hd/D Nusselt number, k,d/X Reynolds number, dup/p Schmidt number, p / p D Prandtl number, cpr/X Lewis number, Dpcp/X
“WPcP>,/V(PcP). viscosity density, lb/fta Stefan-Boltzmann constant, 5.69 X erg/cm%ecOK’ Thiele modulus R(k/D)”*
ACKNOWLEDGMENTS This article was written with the support of National Science Foundation grant GK-38189. The author is grateful to the following organizations and individual for making avaiIable illustrations and tables: General Motors Corp. for Figs. 4, 7, 8, 16; Ford Motor Co. for Fig. 6; Chrysler Corp. for Fig. 23; American Lava, Corning Glass, DuPont, and Kali Chemie for providing samples used in Fig. 11; Gould for Fig. 12; Mobil Oil for Figs. 18 and 22; “Chemical Engineering Progress” for Figs. 2, 15, and 24; the Society of Automotive Engineers for Figs. 13, 14, 25, 26, 27, as well as Table 111; “Industrial and Engineering Chemistry” for Figs. 20, 21, and Table IV; the National Academy of Sciences for Table VIII; and Prof. R. F. Baddour for Fig. 17. REFERENCES 1 . “Report of the 65th General Motors Stockholders Meeting,” Detroit, May 25,1973. 1. “Report by the Committee on Motor Vehicle Emissions,” National Academy of
Sciences, Washington, D.C., Feb. 15, 1973. 3. Burke, D. P., Chem. Week, p. 23, Nov. 1, 1972.
4 . Annual Report, p. 212. Council on Environmental Quality, Washington, D.C., 1971. 5. “Statistical Abstract of the U.S.” Bureau of Census, Washington, D.C., 1972. 6. Williamson, S. J., “Fundamentals of Air Pollution.” Addison-Wesley, Reading, Massachusetts, 1973. 7. Pitts, J. N., Jr., J . Air Pollut. Conlr. Ass. 19, 658 (1969). 8. Patterson, D. J., and Henein, N. A., “Emissions from combustion engines and their control.” Ann Arbor Sci. Publ., Ann. Arbor, Michigan, 1972. 9. Haagen-Smit, A. J., Znd. Eng. Chem. 44, 1342 (1952). 10. Report by the Committee on Motor Vehicle Emissions, National Academy of Sciences, Washington, D.C., Jan. 1, 1972. 11. Fed. Regist. 35, 219 (1970); 36, 128 (1971).
126
JAMES WE1
12. Ebel, R. H., Advan. Environ. Sci. 1, 237 (1969).
13. Pazar, C., “Air and Gas Cleanup Equipment,” p. 541. Noyes Data Corporation, Park Ridge, New Jersey, 1970. 14. Paulus, H. J., in “Air Pollution” (A. C. Stern, ed.), 2nd ed., Vol. 111, p. 529. Academic Press, New York, 1968. 15. Hardison, L. C., in “Proceedings of the First National Symposium on Heterogeneous Catalysis for Control of Air Pollution” (B. R. Banerjee, ed.), p. 271. Nat. Air Pollut. Contr. Admin., Washington, D.C., 1968. 16. Kohl, A. L., and Riesenfeld, F. C., “Gas Purification,” p. 471. McGraw-Hill, New York, 1960. 17. Thomas, C. L., “Catalytic Processes and Proven Catalysts.” Academic Press, New York, 1970. 18. Houdry, E. J., Advan. Catal. 9,499 (1957). 19. Maga, J. A., Advan. Enrriron. Sci. Technol. 2, 57 (1971). 20. Starkman, E. S., Amer. Znst. Chem. Eng. Meet., Detroit, Michigan, June 1973 (1973). 21. Kuo, J. C. W., Morgan, C. It., and Lassen, H. G., SAE (Soc. Automot. Eng.), Pap. No. 710289 (1971). 22. Faith, W. L., “Air Pollution Control,” p. 200. Wiley, New York, 1959. 23. Ayres, R. U., and McKenna, R. P., “Alternatives to the Internal Combustion Engine,” pp. 16 and 89. Johns Hopkins Press, Baltimore, Maryland, 1972. 24. Harned, J. L., and Montgomery, D. L., SAE (Soc. Automot. Eng.), Paper No. 730561 (1973). 2.5. Lichty, L. C., “Combustion Engine Processes,” pp. 214 and 273. McGraw-Hill, New York, 1967. 26. Rose, A. H., Jr., in “Air Pollution” (A. C. Stern, ed.), 1st ed., Vol. 2, p. 40. Academic Press, New York, 1962. 27. Gleason, W. A., and Tuesday, C. S., Enuiron. Sci. Technol. 4, 916 (1970). 28. Jackson, M. W., SAE (SOC. Automot. Eng.), Pap. No. 660404 (1966). 29. Seizinger, D. E., and Dimitriades, B., J . Air. Pollut. Contr. Ass. 22, (1) 47 (1972). SO. Dimitriades, B., and Wesson, T. C., J . Air Pollul. Contr. Ass. 22, (1) 33 (1972). 31. Balzhiser, R. E., Samuels, M. R., and Eliassen, J. D., “Chemical Engineering Thermodynamics,” p. 670. Prentice-Hall, Englewood Cliffs, New Jersey, 1972. 32. Klimisch, R. L., SAE (SOC.Automot. Eng.), No. 730200 (1973). 33. Shelef, M., and Gandhi, H. S., Znd. Eng. Chem., Prod. Res. Develop. 11, 2 (1972). 34. Hougen, 0. A., and Watson, K. M., “Chemical Process Principles,” Vol. 1, pp. 212 and 223. Wiley, New York, 1943. 35. Campau, R. M., SAE (SOC.Automot. Eng.) Pap. No. 710294 (1971). 36. Boreskov, 0. K., Discuss. Faraday Soc. 41, 263 (1966). 37. Krylov, 0.V., “Catalysis by Nonmetals,” p. 191. Academic Press, New York, 1970. 38. Meguerian, G. H., Rakowsky, F. W., Hirschberg, E. H., Lang, C. R., and Schock, D. N., SAE (Soc. Automot. Eng.), Pap. No. 720480 (1972). 39. Bernhardt, W. E., and Hoffmann, E., SAE (Soc. Automot. Eng.), Pap. No. 720481 (1972). 40. “1973 Report on Progress in Areas of Public Concern.” General Motors Technical Center, Warren, Mich., 1973. 41. Acres, G. J. K., and Cooper, B. J., Platinum Metals Rev. 16, (3) 2 (1972). 4s. Bernstein, L. S., Lang, R. J., Lunt, R. S., Mussen, G. S., and Fedor, R. J., SAE (SOC.Automot. Eng.), Pap. No. 730567 (1973).
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
127
4. Lunt, R. S., Bernstein, L. S., Hansel, J. G., and Holt, E. L., SAE (SOC.Automol. Eng.), Pap. No. 720209 (1972).
4. Meguerian, G. H., and Lang, C. R., SAE (SOC.Automot. Eng.), Pap. No. 710291 (1971). 46. Bentley, D. R., and Schweibold, D. J., SAE (Soc. Automot. Eng.), Pap. No. 730227 (1973). 46. Zechnall, R., Baumann, G., and Eisele H., SAE (SOC.Automot. Eng.), Pap. No. 730566 (1973). 47. “Evaluation of Catalysts As Automotive Exhaust Treatment Devices,” Report of the Catalyst Panel to the Committee on Motor Vehicle Emissions, National Academy of Sciences, Washington, D.C., 1973. 48. Yolles, R. S., and Wise, H., Crit. Rev. Enuiron. Coontr. 2 , 125-146 (1971). 49. Dwyer, F. G., Catal. Rev. 6, (2) 261 (1972). 60. Snyder, P. W., Stover, W. A., and Lassen, H. G., SAE (SOC.Automot. Eng.), Pap. No. 720479 (1972). 61. Jagel, K. I., and Dwyer, F. G., SAE (SOC. Automot. Eng.), Pap. No. 710290 (1971). 61. Liederman, D., Volts, S. E., and Oleck, S. M., Amer. Chern. SOC.Dallas Meet., 1973, Div. Petrol. Chem. Preprints 18, (1) 15. 63. Schlatter, J. C., Klimisch, R. L., and Taylor, K. C., Science 179, 798 (1973). 64. Hancock, E. E., Campau, R. M., and Connolly, R., SAE (SOC.Automot. Eng.), Pap. No. 710292 (1971). 56. Margolis, L. Ya., Aduan. Catal. 14, 429 (1963). 56. Kats, M., Aduan. Catal. 5, 177 (1953). 57. Rushton, J. H., and Krieger, K. A., Aduan. Catal. 3, 107 (1951). 68. Roth, J. F., Ind. Eng. C h m . , Prod. Res. Develop. 10, 381 (1971). 69. Bauerle, G. L., Thomas, N. T., and Nobe, K., Chern. Eng. J . 4, 199 (1972). 60. Libby, W. F., Science 171, 499 (1971). 61. Pedersen, L. A., and Libby, W. F., Science 176, 1355 (1972). 61. Voorhoeve, R. J. H., Remeika, J. P., Freeland, P. E., and Matthias, B. T., Science 177, 353 (1972). 63. Voorhoeve, R. J. H., Remeika, J. P., and Johnson, D. W., Jr., Science 180, 62 (1973). 64. Osment, H. E., SAE (Soe. Automot. Eng.), Pap. No. 730276 (1973). 66. Kingery, W. D., “Introduction to Ceramics.” Wiley, New York, 1960. 66. Miller, M. R., and Wilhoyte, H. J., J . Air Pollut. Contr. Ass. 17, (12) 791 (1967). 67. Bagley, R. D., Doman, R. C., Duke, D. A., and McNally, R. N., SAE (SOC.Automot. Eng.), Pap. No. 730274 (1973). 68. “Torvex Ceramic Honeycomb,” product information. DuPont Co., Wilmington, Delaware, 1970. 69. “Thermacomb Bulletin No. 702.” American Lava Corporation, Chattanooga, Tennessee. 1971. 70. “Cercor Bulletin CHE-3.” Corning Glass Works, Corning, New York, 1971. 71. Briggs, W. S., and Graham, J. R., SAE (SOC.Automot. Eng.), Pap. NO. 730275 (1973). 71. “Platinum group metals,” from “Mineral Facts, 1971.” U.S. Bureau of Mines, Washington, D.C., 1971. 73. Ageton, R. W., and Ryan, J. P., “Mineral Facts and Problems,” p. 653. U.S. Department of Interior, Washington, D.C., 1970. 74. Sax, N. I., “Handbook of Dangerous materials,” p. 336. Van Nostrand-Reinhold,
128
JAMES WE1
Princeton, New Jersey, 1951; “Dangerous Properties of Industrial Materials,” 2nd ed., 1963. 76. Balgord, W. D., Science 180, 1168 (1973). 76. McDermott, J., “Catalytic Conversion of Automobile Exhaust 1971.”Noyes Data Corporation, Park Ridge, New Jersey, 1971. 77. Blumenthal, J. L., and Nobe, K., Znd. Eng. Chem., Process Des. Develop. 5 , (2)
177 (1966).
78. Gravelle, P.C., and Teichner, S. J., Advan. Catal. 20, 167 (1969). 79. Yu Yao, Y. F., and Kummer, J. T., J. Catal. 28, 124 and 139 (1973). 80. Hertl, W.and Farrauto, R. J., J. Catal. 29, 352 (1973). 81. Langmuir, I., Trans. Faraday Soc. 17,621 (1922). 8.2,Eischens, R.P., Pliskin, W. A., and Francis, S. A., J. Phys. Chem. 60, 194 (1956). 88. Schwab, G.M., and Gossner, K., 2.Phys. Chem. (Frankfurt am Main) [N.S.] 16,
39 (1958).
84. Tajbl, D. G., Simons, J. B., and Carberry, J. J., Znd. Eng. Chem., Fundam. 5 , 171 (1966). 86. Syzdykbaeva, M. B. et al., Zzv. Akad. Nauk Kaz. SSR, Ser. Khim: 17, (4)37 (1967). 86. Ford, R.R., Advan. Catal. 21, 51 (1970). 87. Sklyarov, A. V., Tretyakov, I. I., Shub, B. R., and Roginskii, S. Z., Dokl. Akad. Nauk SSSR 189, 1302 (1969). 88. Oki, S., and Kaneko, Y., J. Res. Znat. Catal., Hokkaido Univ. 18, (2)93 (1970). 89. Cardozo, M. A. A., and Luss, D., Chem. Eng. Sci. 24, 1699 (1969). 90. Hugo, P.and Jakubith, M., Chem-ZnpTech. 44, (6)383 (1972). 91. Bonzel, H. P., and Ku, R., J. Vac. Sci. Technol. 9, (2)663 (1972). 9.2. Ertl, G., and Koch, J., Pap., Znt. Congr. Catal., 6th, 1978 Paper No. 67, p. 969 (1972),Palm Beach, Florida. 93. Baddour, R. F., Modell, M., and Heusser, U. K., J. Phys. Chem. 72, 3621 (1968). 94. Cochran, H. D., Ph.D. Thesis in Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts (1972). 96. Sills, R. A., Ph.D. Thesis in Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts (1970). 96. Voltz, S. E., Morgan, C. R., Liederman, D., and Jacob, S. M. Znd. Eng. Chem. Prod. Res. Dev. 12,294 (1973). 97. Amirazmi, A., Benson, J. E., and Boudart, M., Amer. Inst. Chem. Eng. Meet., New York, 1979. Paper No. 26C. 98. Ayen, R. J., and Peters, M. S., lnd. Eng. Chem., Process D & D 1, 204 (1962). 99. Force, E. L., and Ayen, R. J., Amer. Inst. Chem. Eng., Symp. Ser. 68,80 (1972). 100. Klimisch, R. L., and Barnes, G., J. Environ. Sci. Tech. 6, 543 (1972). 101. Unland, M., Science 179, 567 (1973). 102. Baker, R. A.,and Doerr, R. C., Ind Eng. Chem., Process Develop. Des. 4, (2) 188 (1965). 103. Jones, J. H., Kummer, J. T., Otto, K., Shelef, M., and Weaver, E. E., Environ. Sci. Technol. 5, 790 (1971). 104. Lamb, A., and Tollefson, E. L., Can. J . Chem. Eng. 51, 191 (1973). 106. Fedor, R. J., Lee, C. H., and Makowski, M. P. Amer. Inst. Chem. Eng. New Orleans Meet., 1978 Paper No. 37B (1973). 106. Satterfield, C. N., “Mass Transfer in Heterogeneous Catalysis.” MIT Press, Cambridge, Massachusetts, 1970. 107. Prater, C. D., Chem. Eng. Sci. 8, 284 (1958).
CATALYSIS FOR MOTOR VEHICLE EMISSIONS
129
108. Weisz, P. B., and Hicks, J. S., Chem. Eng. Sci. 17, 265 (1962). 10.9. Eckert, E. R. G., and Drake, R. M., “Analysis of Heat and Mass Transfer,’, pp. 333 and 728. McGraw-Hill, New York, 1972. 110. Kays, W. M., and London, A. L., ‘Compact Heat Exchangers,” 2nd ed. McGrawHill, New York, 1964. 111. Sherony, D. F., and Solbrig, C. W., Int. J. Heat Mass Transfer 13, 145 (1970). 112. Carlson, D. W., Morgan, C. R., and Voltz, S. E., SAE (SOC.Automot. Eng.) Pap.
No.730569 (1973).
113. 114. 116. 116. 117.
McAdams, W. H., “Heat Transmission,” 3rd ed. McGraw-Hill, New York, 1954. Sen Gupta, A., and Thodos, G., AIChE J. 8,608 (1962). Petrovic, L. J. and Thodos, G., Znd. Eng. Chem., Fundam. 7, 274 (1968). Satterfield, C. N., and Cortez, D. H., Ind. Eng. Chem., Fundam. 9, 613 (1970). Hawthorn, R. D., Pap., Amer. Znst. Chem. Eng. Dallas Meet., 1972 Pap. No. 29C
(1972).
118. Heinen, C. M., Pap., Amer. Znst. Chem. Eng. N m York Meet., 1972 Pap. No. 4b
(1972). Froment, G. F., Advan. Chem. Ser. 109,l (1972). Votruba, J., Hlavacek, V., and Marek, M., Chem. Eng. Sci. 27, 1845 (1972). Votruba, J., and Hlavacek, V., Chem. Eng. J . (London) 4, 91 (1972). de Wasch, A. P., and Froment, G. F., Chem. Eng. Sci. 27, 567 (1972). Aris, R., Proc. Roy. SOC.,Ser. A 235, 67 (1956). Levenspiel, O.,“Chemical Reaction Engineering,” 2nd ed. Wiley, New York, 1972. Denbigh, K.,“Chemical Reactor Theory.” Cambridge Univ. Press, London and New York, 1966. 126. Deans, H. A., and Lapidus, L., AIChE J. 6,656 (1960). 187. Wei, J., Chem. Eng. Progr., Monogr. Ser. 6 Vol. 65 (1969). 128. Vortmeyer, D., Advan. Chem. Ser. 109, 43 (1972). 129. Gagliardi, J. C., Smith, C. S., and Weaver, E. E., Paper No. 63-72.h e r . Petrol. Inst. New York, 1972. 130. Campau, R. M., Stefan, A., and Hancock, E. E., SAE (SOC.Automot. Eng.), Pap. No. 720488 (1972). 131. Fed. Regist. 37, (36)Feb. 23 (1972). 138. Shelef, M., Dalla Betta, R. A., Larson, J. A., Otto, K., and Yao, H. C., Pap. A m . Znst. Chem. Eng. 1973 New Orleans Meet. (1973). 133. Somorjai, G. A., J. Catal. 27, 453 (1972). 134. Maxted, E.B., Advan. Catal. 3, 129 (1951). 136. Butt, J. B., Advan. Chem. Ser. 109, 259-496 (1972). 136. Ries, H. E., Jr., Advan. Catal. 4, 87 (1952). 137. Miyazaki, K.,J . Catal. 28, 245 (1973). 138. Bauerle, G.L., and Nobe, K., Znd. Eng. Chem., Process Des. Dwelop. 12, (2) 137 119. 120. 181. 122. 123. 124. 186.
(1973).
139. Vardi, J., and Biller, W. F., Ind. Eng. Chem., Process Des. Develop. 7, (1)83 (1968). 140, Kuo, J. C. W., Prater, C. D., Osterhout, I). P., Snyder, P. W., and Wei, J., FZSITA Congr., l&h, 1972 Paper 2/14, London (1972). 141. Harned, J. L., SAE (SOC. Automot. Eng.), Pap. No. 720520 (1972). 148. Lehr, C. G., Yurchak, S., and Kabel, R. L., AZChE J. 14, ( 4 ) 627 (1968). 143. Hoiberg, J. A., Lycke, B. C., and Foss, A. S., Advan. Chem. Ser. 109,48 (1972). 1 4 . Vortmeyer, D., and Jahnel, W., Chem. Eng. Sci. 27,1485 (1972). 146. Hansen, K.W., Chem. Eng. Sci. 28, (3)723 (1973).
This Page Intentionally Left Blank
The Metathesis of Unsaturated Hy d roca rbons Cata Iyze d by Tra nsition Metal Compounds J. C. MOL AND J. A. MOULIJN Znstitute of Chemical Technology University of Amsterdam Amsterdam, The Netherlands
I. Introduction.. . . . . . . . . . . . .
. . . . . . . . . 131
A. Type of Reaction
C. Structure of the Active Catalyst D. Metathesis and Cyclotrimerizati
C. Kinetics..
...................
.........
........
.....................
154
160
I. Introduction The metathesis reaction of alkenes constitutes a major development in the field of hydrocarbon chemistry in recent years. The first examples of the heterogeneously and the homogeneously catalyzed metathesis of linear alkenes have been published by Banks and Bailey (1) and Calderon et al. (d), respectively. By this reaction, linear alkenes are converted with high selectivity into equimolar amounts of two new alkenes, according to: 2 RI-CH=CH-R2
RI-CH=CH-RI
+ &-CH=CH-R2
(1)
Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly131
132
J. C. MOL AND J. A. MOULIJN
alkenamers : p CH=CH SfCH=CH-(CHz)n3p
0 (CHI),
Scott el al. (3)and Wasserman et al. ( 4 ) were the first to realize that this ring-opening polymerization, which had been known for several years, might be a special case of the metathesis reaction. The versatility of the metathesis reaction not, only gives many new pathways for organic syntheses, but also offers new openings for the chemical industry. A commercial application is the process for conversion of propene into ethene and n-butene (6). Other industrial applications of the metathesis of acyclic alkenes have been proposed, such as the production of detergent-range linear alkenes from lower alkenes, and the synthesis of isoamylene by mutual metathesis of isobutene and propene or 2-butene (6).The ring-opening polymerization of cycloalkenes results in a variety of polymers and copolymers, which may possess properties ranging from amorphous rubbers to highly crystalline polymers. For instance, cyclopentene can be converted into an elastomer with properties comparable with those of natural rubber (7, 8, 8a). The discovery of the metathesis reaction is also of importance from a theoretical and fundamental point of view, and has contributed to the development of new ideas about reactions of alkenes in the presence of transition metal compounds. Instead ef the name metathesis, the term disproportionation is frequently applied to the reaction, and sometimes the term dismutation. For historical reasons the name disproportionation is most commonly used for the heterogeneously catalyzed reaction, while the homogeneously catalyzed reaction is usually designated as metathesis. The name disproportionation is correct in the case of the conversion of acyclic alkenes according to Eq. (1); however, this name is inadequate in most other situations, such as the reaction between two different alkenes, and reactions involving cycloalkenes. Similar objections apply to the name dismutation. The name metathesis is not subject to these limitations and, therefore, is preferred.
II. Reactants and Catalyst Systems A. REACTANTS All the information to date (see Section 111) indicates that the metathesis reaction proceeds via the rupture and formation of carbon-carbon double bonds :
z
\I/
/1\
* )[+)(
(3)
THE METATHESIS OF UNSATURATED HYDROCARBONS
133
Various types of unsaturated hydrocarbons have been reported to undergo metathesis reactions by contact with appropriate catalysts. A short survey is given below. It is to be expected that in the near future still more examples will be found. 1. Acyclic Alkenes
Both terminal and internal acyclic alkenes can be metathesized, corresponding to Eq. (4) , where R is an alkyl group or a hydrogen atom.
The most thoroughly studied reactions are the metathesis of propene to ethene and 2-butene1 and the metathesis of 2-pentene to 2-butene and 3-hexene. Generally, the thermodynamic equilibrium ratio of the trans and cis components of the products is obtained. The reacting alkene molecules need not be identical, two different alkenes react with each other in the same way. The metathesis of acyclic alkenes substituted with other hydrocarbon groups, such as cycloalkyl, cycloalkenyl, or aryl groups, has also been observed. For instance, styrene is converted into ethene and 1,Zdiphenylethene (stilbene) (9, 9a). Recently, a few examples of the metathesis of alkenes carrying functional groups have been reported. According to a patent, acrylonitrile reacts with propene to crotononitrile (cis and trans) and ethene (10):
\= NC
F
+
>.I1
(5)
NC
It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene (11). A very interesting reaction is the conversion of methyl-9-octadecenoate into 9-octadecene and dimethyl-9-octadecenedioate (12):
Both the cis form (methyloleate) and the trans form (methylelaidate)
134
J. C. MOL AND J. A. MOULIJN
have been found to undergo the reaction. The metathesis of these and other fatty acid esters may be of technological interest. The metathesis of acyclic alkadienes and higher polyenes may involve both inter- and intramolecular processes. An example of an intermolecular reaction is the conversion of 1,Bhexadiene into 1,5,9-decatrieneand ethene:
1,5,g-Decatriene may, of course, react further to 1,5,9,l&tetradecatetraene, 1,5,9,13,17-octadecapentaene,etc. (13). Even the conjugated system 1,3-butadiene participates in metathesis reactions (14). An example of an intramolecular process is the reaction of 1,7-octadienq which gives cyclohexene and ethene (IS, 1 6 ) :
Because one might expect steric hindrance to be important, it is worth mentioning that the metathesis of alkenes branched at the double bond has been reported. Thus, isobutene gives (small) quantities of 2,3dimethyl-2-butene and ethene (16, 17') :
L
(r
r-
)CIl
(9)
The reverse reaction has also been shown to occur (18).Another example is the reaction of methylenecyclobutane to produce dicyclobutylidene and ethene (18a). 2. Cycloalkenes Metathesis of a cycloalkene initially yields a cyclic dimer, i.e. the size of the ring is doubled:
Reaction with additional monomers leads to the formation of larger rings (3,4 ) and eventually to high molecular weight polymers, namely poly-
THE METATHESIS OF UNSATURATED HYDROCARBONS
135
alkenamers :l
This ring-opening polymerization via metathesis may proceed in a stereospecific way, i.e. the double bonds of the resulting polyalkenamer can be exclusively (or principally) of the cis or of the trans type. The metathesis of cycloalkenes proves to be a general reaction with only a few exceptions, such as cyclohexene (21,23,23a)and certain fused-ring cyclopentenes (24). It has been demonstrated that in the case of large cycloalkenes, e.g. cyclododecene, interlocked ring systems (catenanes) can be formed in the following way ($6, 26) :
METATHESIS
380°
TWIST
M E TAT H E 818
Cyclic dienes and polyenes, monocyclic as well as bicyclic, can be metathesized in the same way as cyclic monoenes. As expected, cyclobutene (27), 1,5-cyclooctadiene, and 1 ,5,9-cyclododecatriene (28) yield the same polyalkenamer, in this case polybutenamer (1,4-polybutadiene), since these reactants consist of the same base units, i.e.-(CH2)&H=CH-: P
+
+CH-CH,-CH,-CH+
+CH-CH,-CH,-CH#
P
2P
(14)
The metathesis of alkyl- or aryl-substituted cycloalkenes provides a route to certain perfectly alternating copolymers. For example, metathesis of 5-methylcyclooctene leads to a polymer that may be considered as a 1 It has been suggested that these polymers are mainly linear, which may be a consequence of intermolecular metathesis reactions with traces of acyclic alkenes, or of other consecutive reactions (19-8.@).
136
J. C. MOL AND J. A. MOULIJN
highly regular copolymer of butadiene, ethene, and propene (28) :
+
~(cH?c - n=CH-
CH2)- (CH,-
C H 2 ) - (CH
I
- CH2)+
P
(16)
CH3
A chloro-substituted cycloalkene, 1-chloro-1,5-cyclooctadiene,has also been converted by metathesis into a polymrr, the perfectly alternating copolymrr of butadiene and chloroprene (29). Mutual metathesis of a cyclic and an acyclic alkene provides still more possibilities in synthesizing organic compounds. For instance, cycloalkenes are cleaved by ethene into a ,w-dienes. The reaction of 1 ,Bcyclooctadiene with ethene gives 1,5,9-decatriene (13) ; norbornene reacts with 2-butene to yield 1,3-dipropenyleyclopentane(SO) :
The metathesis of linear alkynes has also been reported, e.g. the metathesis of propyne, l-pentyne, 2-pentyne1 and 2-hexyne (31-33). This reaction can be visualized as the cleavage and formation of carbon-carbon triple bonds :
In Section 1II.D this reaction will be discussed further, particularly in connection with the mechanism of the metathesis of alkenes.
B. HETEROGENEOUS SYSTEMS Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1,34-38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products.
TABLE I Examples of Solid Catalysts for the Metathesis of Propene Conditions Temperature Catalyst system
(K)
Md&&-&oa M00~CrzO~Alp0~ Mo0&3iOz Mo(C0) A 1 5 0 8
436 433 811 344 700 811 811 298 373 811 811 811
WOsSiOl WOrAlPO, WSrSiOt Rez07-AlzOs Re,( CO),0-AlzO8 T&Os--SiOz TeOgSiOt Nb&-SiOz a
b
Pressure (106 Nm-) 32 7 1 35 32 8 1 1 32 32 32
Weight hourly space velocity (hr-1) 8.5 180d 3.5 1 to 2
40 7.5 2 6 16006 15 20 20
Equilibrium conversion: 42.3% (at 298 K)-47.8% (at 811 K). (Moles ethene and n-butenes/moles propene consumed) X 100%. Theoretical: 1.00. Gtls hourly space velocity.
ConversionD
(%) 42.9 36 28
25 44.8 34 18.3 30.2 20.4 18 20
30
Selectivityb Ethenebutene (%I molar ratioc 94 97 95 97 99 82 100 100 100
0.93 0.87 1.26 0.76 1.13 0.99 1.25 0.98 1.00
Reference 1 34 36
1 36
35 36 3Y 38
36 35 36
138
J. C. MOL AND J. A. MOULIJN
Generally, activation of the catalyst is achieved by passing a stream of inert gas or dry air over the catalyst at elevated temperatures. With some catalysts, e.g. MO(CO)~-A~~O~, activation is carried out by heating under vacuum. In a few cases it has been shown that the actual activation procedure does not yield a catalyst with maximum activity. This has been clearly demonstrated for the W 0 3 S i 0 2 catalyst; the activity gradually increases when the activated catalyst is brought into contact with the reactants. The initial activity and selectivity can be increased in this case by a controlled treatment with a reducing gas (H2, CO), hydrogen chloride catalyst an increase or a halogenated hydrocarbon. For the MO(CO)~--A~~O~ in activity by pretreatment with a halogenated alkene has been reported (39)* Selectivity to primary metathesis products is usually less than loo%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium (40), copper, or silver ions (41). For most solid catalysts more detailed information concerning composition, preparation, activation, and regeneration procedures, poisons and catalyst modifications is given by Bailey (42) and Banks (43).
C. HOMOGENEOUS SYSTEMS In general, soluble catalysts are composed of a transition metal compound and a nontransition metal compound, the so-called cocatalyst. In most cases, the cocatalyst is an organometallic compound, although catalytic activity has been reported with cocatalysts containing no carbonmetal bonds, e.g. AlCL (22, 44, 46). In some cases, addition of a third component improves the effectiveness of the catalyst system; an example is the system WC18-C2HsAIC12,of which the activity and reproducibility are increased by adding ethanol (46). Some studies report activity of 'the transition metal compound apart, although conversions are small in most cases (a%, 44, 46, 47, 47a). In Table I1 (2, 13, 48-56) a survey is given of a number of typical catalyst systems which show a high activity for the metathesis of pentene. Particularly striking is that near-equilibrium conversions can be attained with high selectivity under very mild conditions. Table I11 (8, 21, 24, 28, 66-68) gives some typical examples of catalyst systems for the ring-opening polymerization of cycloalkenes. Besides large differences in activity, large differences in selectivity also occur. Usually the reduction of selectivity is caused by the occurrence of side reactions, such as isomerization and alkylation of the solvent. The extent to which these side reactions occur depends upon the kind of reactant,
TABLE I1
Typical Ezamples of Catalyst Systems for the Honwgmea~sMetathesis of Pentene
Catalyst system
Y
Molar ratio of the components
Alkene : catalyst TemperaReactant (mol/mol) ture (K)
Reaction time
1:4:1 2:l 1:2 2:4:1 1:1 1:2
Zpentene 1o,OoO:1 Ambient 270:l 2-pentene 243 50: 1 Ambient 2-pentene 50:l Ambient 2-pentene loo: 1 Ambient Zpentene 50:l Ambient 2-pentene
1-3 min 30 min 4 hr 15 min 15 min 12 hr
1:8
Zpentene
4OO:l
1:2
1-pentene
2oO:l
5 min 273-278
50 min
Solvent Benzene Chlorobenzene Benzene Benzene Chlorobenzene Ether benzene Pentane benzene Chlorobenzene
+
+
Conversionu Selectiv- Refer(%) ity* (%) ence 49.9 51 50 48 50 31
99.6 96 100 93
24<
2
43 -@ 50 51 52 53
47 95
8
IS
rd
%M tr
X
1:2 1:1:20
2-pentene 1-pentene
90:1 Ambient 350: 1 Ambient
2:3 1:4
Zpentene 2-pentene
90:1 Ambient
50:1
Ambient
2 hr 2 hr
Chlorobenzene Chlorobenzene
14 6lC
24 hr 2 hr
Chlorobenzene Chlorobenzene
41 49
71
43 54
84 100
a
Equilibrium conversion: about 50%.
c
Conversion to octene ; because ethene escaped from the system, the calculated equilibrium conversion may be exceeded.
* (Moles primary products/moles reactant consumed) X 100%.
55
48
TABLE I11 Typical Examples of Catalyst S y s t e m for the Homogemma Ring-0penin.g Polymerization of Cyclopatene and C y c h t e n e
4
~~~~~~
Catalyst system WClsC9HEAlClr WClsC2H6A1ClALHfiOH WClAlBra 0
WCL[OCH(CH*Cl*)]r(C~H~)2AlCl
WOCG(CzH5)2AICl-(CsHsCOo)2 MoClr(C9Hs)sAl a
Molar ratio of the components
Reactant
1 :4 1:4:1 1:20 2:5 2:5:2 2:3
Cyclooctene Cyclopentene Cyclooctene Cyclopentene Cyclopentene Cyclopentene
Alkene : catalyst Tempera- Reaction mol/mol ture (K) time 6400: 1
51Oo:l 3OO:l 3700: 1
5OO:l 5OO:l
Ambient 273 Ambient 273 253 233
30 min 1 hr 60 hr 1 hr 15 min 4 hr
Recently, it has been questioned whether in this case the reaction proceeds via metathesis (58).
E
Conversion Solvent
(yo)
Reference
Benzene Benzene None Toluene None None
84 80 100 79 65 49
28 24 21
56 57 8
F ~
U
LI P
z
s
THE METATHESIS OF UNSATURATED HYDROCARBONS
141
TABLE IV Reactions of &Pentme at 298 K Catalyzed by WClsCd€6A1C1~(69)
WCls C1H&1Clo 2-pentene (mmol) (mmol) (mmol) Solvent 0.05" 0.05b 0.05b
.
Reaction time Conversion (min) (%)
0.1 0.1
45 45
Benzene Benzene
10 10-15
50 100
0.1
50
Toluene
10
100
Products 3-hexenel 2-butene pentylbenzene, butylbenzene, hexylbenzene pentyltoluenes
Catalyst formed in the presence of 2-pentene. Catalyst preformed.
the catalyst system, the solvent, and sometimes the procedure employed in carrying out the reaction. An illustration of the last is the reaction of 2pentene in the presence of a WClrC2HA1Cl2 catalyst (69). If this catalyst is formed in the presence of 2-pentene, metathesis occurs. However, if the catalyst is preformed in the solvent followed by addition of the 2-pentene, its behavior depends upon the solvent used: in benzene, alkylation of the solvent and metathesis occur simultaneously, whereas in toluene only alkylation takes place. Table IV summarizes these results. In accordance with this it has been found that in the case of competing metathesis and alkylation reactions a high benzene :alkene ratio favors alkylation (69a). In addition to those listed in Tables I1 and 111, a great number of active catalyst systems has been reported. Such catalysts can be derived from most of the transition metals, although, as in the case of the heterogeneous systems, those based on compounds of molybdenum, rhenium, and especially tungsten give the best results. More detailed information about the catalyst systems for the homogeneous metathesis of acyclic and cyclic alkenes is given by Hughes (60) and Dall'Asta (44), respectively.
111. Mechanism A. TYPEOF REACTION There are two plausible reactions which lead to the observed products of the metathesis of alkenes. The first possibility involves cleavage of a carbon-carbon single bond adjacent to the double bond; the second involves cleavage of the double bond itself. The following transalkylation
142
J. C. MOL A N D J. A. MOULIJN
scheme applies in the former case: a. for acyclic alkenes I
RI-CH=CH-Ri
R~-CH=CH+R~ I
t
+
e
R&CH=CH--%
(19)
R~-CH=CH-RZ
I
b. for cycloalkenes
The second possibility involves an exchange of alkylidene moieties (transalkylidenation) : a. for acyclic alkenes 6
Ri-CHfCH-RZ
Ri-CH=CH-Ri
I
i I
e
RI-CH~CH-RZ
+ R?-CH=CH-%
I
b. for cycloalkenes
Tracer experiments can be employed to distinguish between these two possibilities. Mol et al. (61) analyzed the products formed in the heterogeneously catalyzed metathesis of propene labeled with 14C. With propene labeled in the 2-position there will be, in the case of .a transalkylation reaction, an equal distribution of the radioactivity over the products : H-CH=*CH+CH~
t !
H+CH=*CH-CHs I
H-CH=*CH-H
e
+ CHg-CH=*CH-CHS
(23)
THE METATHESIS OF UNSATURATED HYDROCARBONS
143
In the case of transalkylidenation, radioactivity will be found only in the butene :
i H-CH=*CH-CHs
+
H-CH=CH-H
+
F!
I
H-CHYCH-CHa
(24)
CHs-*CH=*CH-CHa
In experiments with [2-14C] propene in the presence of a Re207-A1203 catalyst, the ethene formed showed no radioactivity, while the butene showed a specific radioactivity twice as high as that of the starting material. This result is completely consistent with a transalkylidenation scheme, and excludes a transalkylation reaction. Similar results were obtained by Clark and Cook (62) in experiments with radioactive propene on a Mo0&oO-fi&03 catalyst. Experiments with deuterated alkenes showed that during the metathesis reaction no hydrogen exchange takes place between the alkylidene moieties. Thus, in the presence of a Re20rA1203catalyst a mixture of CzH4and C2D4 only gave asymmetric CzHzDz(63): CHz=CHz
CHz=CDa
CDz=CDZ
CHz=CDz
+ * +
(25)
Calderon et al. (3, 46) obtained analogous results with the homogeneous system WCl6-CzH6AlCl2-C2H60H.In the reaction between 2-butene and 2-butene-d8, the only new product was 2-butene-d4, from which they concluded that a transalkylidenation reaction had taken place. The work of Calderon et al. had very important consequences for the understanding of the reaction mechanism of the ring-opening polymerization of cycloalkenes. They proposed that the same mechanism is operative in both the ring-opening polymerization of cycloalkenes, promoted by tungsten-based catalysts, and the metathesis of acyclic alkenes. I n contrast with Natta et al. (64), who had suggested earlier that the ring-opening polymerization proceeds by cleavage of a carbon-carbon single bond in the a-position with respect to the double bond, Calderon et al. considered the ring-opening polymerization to be a transalkylidenation reaction. Dall’Asta and Motroni (44, 57) provided direct experimental evidence for the transalkylidenation mechanism in the case of cycloalkenes. With a catalyst system consisting of WOCL, C2HsA1Cl2,and benzoyl peroxide they prepared a random copolymer of cyclooctene and cyclopentene, the cyclopentene double bond being labeled with 14C. The distribution of the radioactivity in the copolymer formed will depend on the site of ring opening.
144
J. C. MOL AND J. A. MOULIJN
In the case of cleavage of the a-single bond the radioactivity will be distributed as follows:
.
**
-CH=CH-(C€€~)r*CH=*CH-(CH,)a-CH=CH-(CH~)s-
a
Cleavage of the double bond leads to a different distribution: *
=CH-(CHi)~CH=*CH-(CHz)a-*CH=CH-(CH~)s-CH=*
**
After ozonization and reductive cleavage to the a,w-diols, essentially all the radioactivity was found in the Crdiol formed. This demonstrated clearly that the ring opening indeed proceeded via cleavage of the double bond. To provide experimental evidence for the general validity of the transalkylidenation mechanism in the ring-opening polymerization of cycloalkenes, Dall'Asta et al. (66)extended this investigation to the highly strained molecules cyclobutene and 3-methylcyclobutene, and to other catalysts, vie. molybdenum-, titanium-, and ruthenium-based catalysts. Also in these cases, only a transalkylidenation mechanism is consistent with the experimental results. Other possible reaction courses leading to the observed products have also been proposed. One possibility is a reaction via a n-allylic intermediate (62,66).In order to discriminate between a transalkylidenation reaction and a reaction involving a ?r-allylic intermediate, experiments have been carried out with radioactive propene labeled in either the 1- or the 3-position (61).Using [1-l4C] propene, all the radioactivity was found in the ethene formed; with [3-14C] propene as starting material only the butene was radioactive. These results show that the methyl groups retain their identity throughout the reaction. Thus, double-bond migration is not inherent in the metathesis of propene, so that the possibility of a n-allylic intermediate is excluded. For acyclic alkenes this result might have been expected from the fact that ethene undergoes the metathesis reaction [Eq. (25)]. The same conclusion can be drawn for the metathesis of cycloalkenes. This follows from, e.g., the metathesis of 3-phenyl-cyclooctene (28).It is known that in this compound the phenyl substituent promotes the doublebond migration. However, from the NMR spectrum of the product it could be concluded that even in this case during the metathesis reaction double-bond migration does not occur. A linear mechanism similar to the accepted mechanisms for ZieglerNatta polymerization, which has been proposed, e.g. by Marshall and Ridgewell (91), is also excluded by the tracer experiments. It can be concluded that the metathesis reaction of acyclic alkenes and cycloalkenes proceeds via the rupture and formation of carbon-carbon double bonds, i.e. that the metathesis of alkenes is a true transalkylidenation reaction.
THE METATHESIS OF UNSATURATED HYDROCARBONS
145
B. REACTION MECHANISM Bradshaw et al. (67) were the first to propose a reaction pathway that is compatible with a transalkylidenation scheme. They suggested that the reaction proceeds via a “quasi-cyclobutane” intermediate. Applied to linear alkenes, this is pictured as follows: RI-CH=CH-&
+ Rl-CH=CH-&
R1-CHF?
RI-CH-
- -CH--% - -AH-&
Ri-CH
CH-Rn Crk+dL-~ RI-
(26)
Many authors assume that the reaction indeed proceeds in such a way, with the specification that the “quasi-cyclobutane” intermediate corresponds with a complex of cyclobutane with C4-symmetry (3, 13, 22, 46, 49, 68-72). The role of the catalyst is described by these authors in terms of the “forbidden-to-allowed” concept of Mango and Schachtschneider (73, 7 4 ) , in which the assumption is made that the formation of the cyclobutane complex is the result of a concerted fusion of two alkenes. In the following this will be considered in more detail. Woodward and Hoffmann (76) showed that the ground state configuration of two ethene molecules correlates with a doubly excited state of cyclobutane. This means that the transformation of two ethene molecules into cyclobutane is “symmetry-forbidden”, which can be elucidated by a more detailed examination of this reaction. Let the ethene molecules approach each other with maximum symmetry which implies a parallel position leading to the geometry shown in Fig. 1. In the course of the reaction the ?r-orbitals of the ethene molecules are transformed into u-orbitals of the cyclobutane. A correlation diagram relhting these orbitals is depicted in Fig. 2. The symmetry is indicated as symmetric (S) or antisymmetric (A) with respect to plane 1 and plane 2; thus, AS means antisymmetric with respect to plane 1 and symmetric with respect to plane 2. By inspection of the correlation diagram it is clear that the SA-orbital transforms into a high energy orbital. In the absence of configuration interaction the
u /----
-
FIG.1. Geometry of the formation of cyclobutane from two ethene molecules.
146
J. C. MOL AND J. A. MOULIJN
c=c
c-c
c=c
c-c
I
I
FIQ.2. Correlation diagram for the formation of cyclobutane from two ethene molecules.
transformation of two ground state ethene molecules would result in a cyclobutane molecule with two of its electrons in the high energy SAorbital. In fact, configuration interaction will prevent crossing, but the intended crossing will manifest itself as a high activation energy barrier, which makes the reaction thermally “forbidden.” According to Mango and Schachtschneider, the presence of the catalyst permits a reordering of electrons; the metal atom accepts two electrons from the SA-orbital of the ethene molecules and simultaneously donates two electrons to the AS-orbital of the cyclobutane molecule. Instead of “forbidden” the reaction has now become “allowed” (Fig. 3). The reverse reaction can be treated in the same way. The valence isomerization of quadricyclene to norbornadiene in the presence of a Rh’ catalyst (76) is mentioned by Mango and Schachtschneider as an example of a forbidden-to-allowed process :
Thus, the catalyst serves to remove the symmetry restraints, which are essentially responsible for the stability of the quadricyclene. Application of the forbidden-to-allowed concept to the metathesis reaction implies that the metathesis is considered to be a concerted cyclo-
THE METATHESIS OF UNSATURATED HYDROCARBONS
147
addition of two alkene molecules to form a cyclobutane ring, which decomposes by a reverse process to two new alkenes:
More detailed and theoretical explanations of the role of the catalyst, based on this scheme, have appeared (72, 74, 77-82). In order to obtain experimental evidence for this scheme, some investigators did experiments in which 1,2-dimethylcyclobutane or cyclobutane were brought into contact with an active metathesis catalyst. However, 1,2-dimethylcyclobutane was stable under conditions where propene gave a high conversion to ethene and 2-butene (63).The experiments with cyclobutane led to the same conclusion (83).From this, and from the fact that cyclobutanes are not reaction products, although this can be expected thermodynamically, it follows that cyclobutanes are not free intermediates. This prompted Lewandos and Pettit (83) to propose a “tetramethylene” complex as the key intermediate :
However, because the symmetry of this complex is the same as that of the cyclobutane complex, the difference between these two intermediates
/
/
SA
-’
/
/
\ \
\
\.-AS
FIQ.3. Correlation diagram for the transition metal catalyzed transformation of two ethene molecules into cyclobutane according to the forbidden-to-allowedconcept.
143
J. C. MOL AND J. A. MOULIJN
seems not to be fundamental. Moreover, the experimental results may be fully consistent with the cyclobutane scheme if, for example, the metathesis of propene is represented in the following way, where reactions (a) and (b) are much faster than reaction (c) :
From the foregoing, however, it shouId not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the RhI-catalyzed valence isomerization of cubane to syn-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism:
,
r
Rh'"
Cassar and Halpern (86) provided evidence that the Rh*-catalyzed valence isomerization of quadricyclene to norbornadiene also proceeds through a nonconcerted mechanism:
&*& R hm
-Rh'_
&
(32)
Very recently, Fraser et al. (86a) proved, by isolating and analyzing the intermediate, that a similar reaction mechanism is operative in the transition-metal-catalyzed dimerization of norbornadiene.
THE METATHESIS OF UNSATURATED HYDROCARBONS
149
The most plausible inference from these studies is to consider for the metathesis reaction a nonconcerted reaction mechanism involving a five-membered metallocyclic intermediate. In the case of the metathesis of propene, this can be visualized as follows:
Grubbs and Brunck (86) have recently reported experimental evidence supporting this mechanism. They have made an attempt to synthesize the proposed metallocyclic intermediate for the metathesis of ethene. Starting from the assumption that a mixture of WCla and two equivalents of (C4Ha)Liforms an active metathesis catalyst (49), they treated WCIS with l14-dilithio-2 ,3-dideuterobutane. One may expect that the following reaction would take place:
(34)
Next, if the metathesis reaction occurs in conformity with scheme (33), complex I should break up into a mixture of CZH~, C~HSD,and CzHzDz:
I
11
Grubbs and Brunck found that ethene was formed and that this product They also consisted of 88% C2H3D,6% CzH4,and 6% symmetric CZHZDZ. demonstrated that CzH4and CzHzDzwere not formed by a secondary reaction. From this they inferred that a migration of 12% of the metal atoms had taken place. It can be concluded from the study of Grubbs and Brunck that indeed a metal-carbon u-complex might be the key intermediate in the metathesis reaction. For the conversion of I into I1 several reaction pathways can be
150
J. C. MOL AND J. A. MOULIJN
visualized, e.g. a direct 1 ,2-shift or a route via a symmetric intermediate comparable with a cyclobutane complex. However, the essential difference with the cyclobutane mechanism discussed before is its non-concertedness. Nevertheless, it remains possible that the critical reaction intermediate is not a metallocycle. For instance, the product distribution found by Grubbs and Brunck might also be explained by assuming transformation of complex I into a dialkene complex which undergoes the metathesis reaction according to Eq. (28) (87). As an alternative to the cyclobutane mechanism there have also been proposed mechanisms involving carbene complexes2 which cannot be considered as a more detailed description of the “quasi-cyclobutane” intermediate, as in the case of the tetramethylene complex. HBrisson and Chauvin (88) proposed the following scheme for the transalkylidenation step:
This scheme is based on kinetic studies which will be discussed in Section IV. It will be shown that these studies do not prove this scheme. Recently, Cardin et al. (89) reported the metathesis of electron-rich alkenes catalyzed by RhI-complexes :
These authors convincingly demonstrated that this reaction proceeds according to a mechanism involving carbene intermediates. At first sight, this is the same mechanism as supposed by HBrisson and Chauvin. However, it is questionable whether the study of Cardin et al. supports the scheme of HBrisson and Chauvin. The alkylidene groups in these kinds of electron-rich alkenes behave in a completely different manner from those of “normal” alkenes; strong donor groups, such as amino groups, generate a nucleophilic character and stabilize the carbene.
* According to the IUPAC rules the name “alkylidene complex” must be used; however, “carbene complex” is more usual.
THE METATHESIS OF UNSATURATED HYDROCARBONS
151
O’Neill and Rooney (90) found that the Mo03-Co0-A1203 catalyst converts diazomethane into nitrogen and ethene under conditions where propene undergoes metathesis. However, because many catalysts are active for this conversion (91),their results cannot be considered as supporting the hypothesis that the metathesis reaction of alkenes proceeds via carbene complexes. Contradictory to a carbene mechanism is the high selectivity which is typical for the metathesis reaction. In the case of carbene complexes, side reactions must be expected, such as addition and insertion [Eqs. (38) and (39) 1: R-CH=CH-CHa R-CH-CH-CH2-H
+ :CHg + R-CH-CH-CHa
(38)
+ :CHI
(39)
>Id
R-CH=CH-CH2-CH,
Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( 1 ) reported the formation of small quantities of C&,-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo (CO) e - A l 2 0 3 , which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere; most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. It should be noted that we have confined ourselves to the simplest reaction intermediates, namely, complexes involving only one transition metal atom and two alkene molecules. If the possibility of two transition metal atoms is taken into account the following complexes seem most likely : M
@ I
M
I:(
(40)
152
J. C. MOL AND J. A. MOULIJN
Another possibility might be a complex involving more than two alkene molecules; e.g. , analogous to the cyclobutane complex, a cyclohexane complex can be imagined. In the literature, evidence concerning these possibilities has not been provided so far. OF C. STRUCTURE
THE
ACTIVECATALYST
The ultimate purpose of mechanistic considerations is the understanding of the detailed reaction pathway. In this connection it is important to know the structure of the active catalyst and, closely connected with this, the function of the cocatalyst. Two possibilities for the action of the cocatalyst will be taken into consideration, namely, the change in the oxidation state of the transition metal and the creation of vacant sites. In the following, a few catalyst systems will be considered in more detail. The most thoroughly studied catalysts are the homogeneous systems based on WCla, with tetraalkyltin or CzHsAlC12as cocatalyst. Pampus et al. (92) interpreted the results of their studies on the ring-opening polymerization of cyclopentene (CaHs) with the system WCle-Sn(C2H5)4in terms of the following scheme: wcl(
+ 2 Sn(C2H6)(
WC14CzHs
-+
+
W C ~ C ~ H K2 Sn(CpH&C1 4- C ~ H K Q
+ 2 C K H ~ WCII(CSH& + CzH5. -+
(41) (42)
In agreement with this scheme, ethene and ethane were generated: 2 C ~ H K+ - CaHd
+ CZH6
(43)
The function of the tetraethyltin is to create vacant sites so that coordination of alkene molecules becomes possible, and to change the oxidation state of the tungsten atom from + 6 to +4. Similar behavior of the alumis not probable, because inum compound in the system WCl6-C2H6~Cl2 it has been demonstrated that WCh-AlCla is also an active catalyst (22, 44), which suggests that CzH5A1C12 functions as a Lewis acid. Vacant sites can be created by a Lewis acid as follows: WCls
+ AlCla +WClK+ + AlClr-
(44)
If the creation of vacant sites occurs in this way, it would be erroneous to conclude that the tungsten complex in its active form is not reduced, because reduction can also be accomplished by the reacting alkene molecules. The wide diversity of cocatalysts and transition metal complexes suggests that the oxidation state of the transition metal is not a critical parameter. More important seems the availability of vacant coordination sites. In agreement with this, in the case of heterogeneous systems also,
THE METATHESIS OF UNSATURATED HYDROCARBONS
153
the metal complexes appear to lose ligands. This has been exemplified by catalyst. They demonstrated Whan et al. (93-95a) for the Mo(CO),Y-A~~O, that an activation procedure is necessary, which consists of heating under vacuum, causing the loss of CO-ligands. Lewandos and Pettit (47) observed that W (co)S, Mo (CO) and toluene-W (CO), show activity towards metathesis without the presence of a cocatalyst. From data obtained in a careful experimental study of the metathesis of 4-nonene with toluene-W (CO),, they inferred that this complex loses the toluene ligand as well as a CO-ligand, giving a reacting complex with the following structure:
Some authors have tried to prepare an active catalyst by reducing WCl6 to WCld with hydrogen (2%) or with reducing metals, such as zinc, magnesium, or sodium amalgam ( 6 1 ) .This treatment did not result in an active catalyst. Similarly, it has been reported that 1 ,5-cyclooctadienetungsten tetracarbonyl is not active for the metathesis reaction ( 9 6 ) . This agrees with the study of Lewandos and Pettit, but is remarkable in view of the conclusion of Pampus et aZ., described above, that WC14(CbHs)Z is the catalytically active species. Perhaps this contradiction may be explained by the fact that the cocatalyst in the latter case participates in the active complex. For instance, it is conceivable that without a cocatalyst an inactive trans complex is formed, whereas in the presence of a suitable cocatalyst an active cis complex is obtained. Another reason for the inactivity of WCl, may be that it is insoluble in the solvent used (68). In this context, results from an investigation of the metathesis of propene over a W03-SiOz catalyst in a fixed-bed flow reactor are worth mentioning ( 9 7 ) . A significant increase of the activity of the fresh catalyst was observed during the first few hours after the propene flow was started, whilst the color of the catalyst changed from pale yellow to blue-violet. The change in color is an indication of loss of oxygen from the WO,. This suggests that propene reduces the catalyst to form active sites. The observation that pretreatment of the catalyst with a reducing agent, such as hydrogen or carbon monoxide, gave an increase of the initial activity and resulted in a blue colored catalyst supports this interpretation. It was suggested that in its active form the WO, is actually a slightly reduced oxide, (97). e.g. W Z O O ~ ~Analogous results have recently been published by Luckner and Wills ( 9 7 4 .
154
J. C. MOL AND J. A. MOULIJN
The same conclusions apply to results obtained with supported MOO, catalysts (97b-97e). From ESR studies it was concluded that during the formation of the active catalyst the MOO, is reduced (97c, 97e). Induction periods are also found in studies of homogeneous systems (61,69,88,97f). For the system MoCh (NO) 2 (C6H6N)z-CzH6AlC12this has been demonstrated by Hughes (69), who observed that the mixture of catalyst components achieved maximum activity towards the metathesis of 2-pentene after a reaction time of about one hour. From the above, it will be clear that the fragmentary results obtained YO far do not permit one to draw general and detailed conclusions about the transition state. The only conclusion, perhaps trivial, is that coordinative unsaturation is a prerequisite for metathesis.
D. METATHESIS AND CYCLOTRIMERIZATION OF ALKYNES A few publications have appeared concerning the metathesis of alkynes; so far only heterogeneous systems with acyclic alkynes have been reported (32-33). From experiments with [1-14C]2-hexyne this reaction was found to be analogous to the metathesis of alkenes, because it turned out to be a transalkylidynation reaction (33) :
*c-c=c-c*
*C-C~--cC-C
+ *c-c=c-c-c-c
F?
+
(45)
c-c-c-c=c-c-c-c
Moulijn et al. (32) studied the reactions of some linear alkynes over a WO3SiOZcatalyst in a fixed-bed flow reactor. Besides metathesis, cyclotrimerization to benzene derivatives occurred. Thus, propyne yielded, in addition to metathesis products, a mixture of trimethylbenzenes. From this an indication of the mechanism of the metathesis of alkynes can be obtained. For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three Tbonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,
THE METATHESIS OF UNSATURATED HYDROCARBONS
155
all three isomeric trialkylbenzenes will be formed, because of the symmetry of this complex.
AR
R
HR
R
2 R-CECH-
I
M
qR M
,
If, on the other hand, the reaction proceeds via a concerted fusion of three alkyne molecules, the 1,2,3-isomer cannot be formed. Similarly, in the case of a metallocyclic intermediate, it is to be expected that the 1,3,5- and the 1,2,bisomers will be formed exclusively (98). Because of the almost complete absence of the 1,2,3-isomer in the product mixture when propyne, 1-butyne, or 1-pentyne were passed over the W03-SiOz catalyst, it was concluded that cyclotrimerization over this catalyst does not occur via a cyclobutadiene complex (92). Since cyclotrimerization and metathesis of alkynes occur simultaneously, a common intermediate might be involved, which would mean that the metathesis of alkynes does not proceed via a cyclobutadiene complex. From the above, a parallel appears to exist between the metathesis of alkenes and alkynes. Both reactions result in a redistribution of, respectively, alkylidene and alkylidyne groups. Moreover, the results obtained so far suggest that in both cases the reaction might proceed via a metallocyclic intermediate.
IV. Thermodynamics and Kinetics A. THERMODYNAMICS A remarkable feature of the metathesis reaction is that the enthalpy difference between products and reactants ( A H R ) is virtually zero, because the total number and the types of the chemical bonds are equal before and after the reaction. Hence, ideally, the free enthalpy of the reac-
156
J. C. MOL AND J. A. MOULIJN
tion (AGR) is determined by the reaction entropy, and the product distribution a t equilibrium corresponds to a random distribution of the alkylidene moieties. A characteristic example of this is the metathesis of (2). At equilibrium, 2-pentene with the catalyst WCls-C~H6A1ClZ-CzH60H the molar ratio 2-pentene:2-butene:3-hexenewas found to be 2: 1: 1, which corresponds exactly to a random distribution of the two alkylidene moieties of 2-pentene. Further, it has been found that an identical product distribution can be obtained starting from 2-pentene1 or from an equimolar mixture of 2-butene and 3-hexene ( 2 , 69). Similarly, starting with either cyclooctene or the high molecular weight polymer formed by metathesis of cyclooctene, the same reaction products have been observed (3).Thus, a basic feature of the metathesis reaction is that true thermodynamic equilibrium can be attained. The question as to the driving force in the case of ring-opening polymerization has been discussed by many authors. In view of the above it can be understood that relatively strain-free rings, like cyclododecene (28), can be metathesized. For small rings up to cyclohexene the situation is different. Dainton et al. (99) reported calculations of the free enthalpy for the polymerization of cycloalkanes. The results of these calculations are also expected to hold, at least qualitatively, for the polymerization of cycloalkenes. According to these calculations the reaction entropy for the ring-opening polymerization of small rings is highly negative. Hence, in this case, the entropy cannot be the driving force. That small rings are, nevertheless, able to undergo ring-opening polymerization to their respective polyalkenamers is due to the ring strain of the monomer. This provides a substantial reaction enthalpy which compensates the unfavorable reaction entropy. The unreactivity of cyclohexene (Section 1I.A) may be explained by the fact that in this case the ring strain of the dimer is much higher than that of the monomer. The observation that cyclohexene can be a reaction product [Eq. (S)] supports the assumption that thermodynamic rather than kinetic limitations prevent cyclohexene from polymerizing. Calderon and Ofstead (24, 100) have observed that bicyclo-[2.2.2]2-octene can be polymerized via ring opening:
This monomer may be considered as a compound composed of two cyclohexene rings in the boat configuration. Presumably, the differencein energy between the cyclohexene rings of the monomer and the cyclohexane ring in the polymer, which is free to assume the less strained chair configuration, is responsible for the reactivity.
THE METATHESIS O F UNSATURATED HYDROCARBONS
157
TABLE V
Calculated Equilibrium Distributions for the Metathasis of Some Lower Alkenes at 998.16Ka Equilibrium distribution
Reaction 2a
AH, a b C (kJ/mol) AHR/TASR (mol %I (mol %) (mol%)
eb + c
+
2,3-di2 isobutene $ ethene methyl-2-butene 2 2-methyl-2-butene F! 2-butene 2 3-dimethyl-2-butene 2 propene F+ ethene 2-butene 2 1-butene F+ ethene 3-hexene 32 2-pentene F+ 2-butene hexene
+
)
+
+
+
19.53
-12.41
97.2
1.4
1.4
10.39
-3.39
85.1
7.45
7.45
1.28 1.07 -0.58
-0.25 -0.24 0.18
57.8 53.0 47.7
21.1 23.5 26.15
21.1 23.5 26.15
Thermodynamic data from Rossini et al. (101).
Of course, even in the case of acyclic alkenes reaction enthalpy is not exactly zero, and therefore the product distribution is never completely statistically determined. Table V gives equilibrium data for the metathesis of some lower alkenes, where deviations of the reaction enthalpy from zero are relatively large. In this table the ratio of the contributions of the reaction enthalpy and the reaction entropy to the free enthalpy of the reaction, expressed as A H ~ / T A & , is given together with the equilibrium distribution. It can be seen that for the metathesis of the lower linear alkenes the equilibrium distribution is determined predominantly by the reaction entropy, whereas in the case of the lower branched alkenes the reaction enthalpy dominates. If the reaction enthalpy deviates substantially from zero, the influence of the temperature on the equilibrium distribution will be considerable, since the high temperature limit will always be a 2: 1 :1 distribution. Typical examples of the influence of the temperature are given in Tables VI and VII.
B. STEREOSELECTIVITY Many authors have observed that the cis-trans ratio of the products of the metathesis reaction is equal to the thermodynamic equilibrium value. This suggests that the reaction is not highly stereoselective. However, under certain conditions the product distribution is influenced by kinetic factors. For instance, it proves to be possible to prepare from cyclopentene
158
J. C. MOL AND J. A. MOULIJN
TABLE VI Calculated Equilibrium Distributions for the Metathesis of Propene at Diferent Temperaturw Equilibrium distribution Temperature T (K)
AHR (kJ/mol)
AHRITASR
Propene (mol%)
Ethene (mol%)
2-Butene (mol%)
300 400 500 600 700 800 900 lo00
1.28 1.62 1.87 1.98 2.12 2.15 2.16 2.22
-0.252 -0.246 -0.231 -0.204 -0.189 -0.168 -0.150 -0.140
57.6 55.6 54.6 53.6 53.0 52.3 51.8 51.5
21.2 22.2 22.7 23.2 23.5 23.85 24.1 24.25
21.2 22.2 22.7 23.2 23.5 23.85 24.1 24.25
a
Thermodynamic data from Rossini et al. (101).
polymers varying from mainly trans microstructure to 100% cis microstructure, depending on the catalyst system (7, 8, 66, 64, 102) and the reaction conditions (103). Stereoselectivity has also been demonstrated for the metathesis of acyclic alkenes, although not as convincingly as for cycloalkenes. In a study of the metathesis of both cis- and trans-2-pentene in the presence of wcl6C2HAlCl2-CzH60H ( I S ) , it was found that from the very beginning substantial amounts of both cis and trans isomers of 2-butene and 3-hexene were formed. Regardless of the conformation of the initial 2-pentene1 a slight preference] with respect to the thermodynamic equilibrium ratio, for the cis isomers of the products was observed. The catalyst WCl6C4H9Liexhibits a higher degree of stereoselectivity. Starting with cis-2pentene, formation of cis-2-butene appeared to be favored. Starting with trans-2-pentene a slight preference for trans-Zbutene was observed (49). With both tungsten-based systems it was found that the 2-pentene gradually isomerized into its cis-trans equilibrium mixture. A study of the Z-CZHSA~CIZ metathesis of 2-pentene in the presence of MoC12(NO)2 ( CSHKN) revealed a striking stereoselectivity (104). In the early stages of the reaction cis-2-pentene gave mainly cis-2-butene and cis-3-hexene1 and trans-2-pentene reacted mainly to trans-2-butene and trans-3-hexene. For the metathesis of propene over some solid catalysts it was observed that the formation of cis-2-butene is favored, particularly at short contact times (106).
159
THE METATHESIS OF UNSATURATED HYDROCARBONS
Evidently, the structure of the transition state is such that cis as well as trans compounds can be formed with, more or less, a preference for the cis compounds. The reason for the latter may be that coordination of cis isomers is sterically favored. In agreement with this are the observations that cis compounds are more reactive than the corresponding trans compounds (49,106). The above studies are consistent with the hypothesis that the metathesis reaction itself brings about cis-trans isomerization (46). This hypothesis is further supported by the results of a kinetic study of the reactions of the three linear butenes on the metathesis catalyst MO(CO)~A 1 2 0 3 by Davie et al. ( l o r ) , who concluded that cis-trans isomerization for their system is a bimolecular reaction. Steric factors may also be important in situations where alternative modes of reaction are available. Dall’hta (44) examined the ring-opening polymerization of 3-methyl-cis-cyclooctene. By infrared analysis of the product formed, he obtained quantitative information about the occurrence of head-to-head and head-to-tail successions. More than 90% of the links in the polymethyloctenamer were of the head-to-tail type, but the sterically more hindered and, therefore, unfavored head-to-head links were also observed (about 5%). Ofstead (29) investigated the ring-opening polymerization of some 1 ,5-cyclooctadienes substituted at one of the two TABLE VII
Calculated Equilibrium Distributions for the Metathesis of Isobutene at Different Temperatures Equilibrium distribution
T (K)
AHg (kJ/mol)
300 400 500 600 700 800 900 lo00
19.53 19.03 18.25 17.84 17.26 17.08 16.86 16.94
Temperature
a
AHRITASR
Isobutene (mol%)
Ethene (mol%)
2,3-Dimethyl2-butene (mol%)
-12.31 -7.46 -4.24 -3.14 -2.40 -2.01 -1.70 -1.58
97.2 92.8 88.3 84.0 80.3 77.4 75.0 72.5
1.4 3.6 5.85 8.0 9.85 11.3 12.5 13.75
1.4 3.6 5.85 8.0 9.85 11.3 12.5 13.75
Thermodynamic data from Rossini et al. (101)
160
J. C. MOL AND J. A. MOULIJN
double bonds in the presence of WCla-CzHsAlC12-CzH~OH. Because two types of double bond are present, analysis of the products will give information about the influence of substituents upon the reactivity. When one of the double bonds was substituted with an ethyl group, a chlorine atom, or two methyl groups, it virtually did not participate in the metathesis reaction; e.g. l ,2-dimethyl-l ,5-cyclooctadiene reacted to the extent of more than 99.5% via cleavage of the unsubstituted double bond. Only in the case of l-methyl-l,5-cyclooctadienedid cleavage of the substituted double bond occur, although to a small extent (about 3%). Clearly, the reactivity of double bonds was strongly reduced or even completely suppressed by steric hindrance. However, this result cannot be generalized. For instance, in a study of the metathesis of a mixture of isobutene and propene, the double bonds of both propene and isobutene were found to participate to a substantial degree in the reaction (17). C. KINETICS 1. Catalyst Activity
Various investigators have tried to obtain information concerning the reaction mechanism from kinetic studies. However, as is often the case in catalytic studies, the reproducibility of the kinetic measurements proved to be poor. A poor reproducibility can be caused by many factors, including sensitivity of the catalyst to traces of poisons in the reactants and dependence of the catalytic activity on storage conditions, activation procedures, and previous experimental use. Moreover, the activity of the catalyst may not be constant in time because of an induction period or of catalyst decay. Hence, it is often impossible to obtain a catalyst with a constant, reproducible activity and, therefore, kinetic data must be evaluated carefully. Another phenomenon, that emerged during kinetic studies of the heterogeneously-catalyzed metathesis reaction, is surface nonuniformity. In a study of the metathesis of propene over a MoO3--C00-&03 catalyst Moffat and Clark (108) demonstrated clearly that the surface of this catalyst is heterogeneous. In a subsequent study Moffat et al. (109, 110) arrived at the same conclusion for the WO3--SiO2catalyst. A consequence of surface heterogeneity is that an increase in temperature can activate sites that are inactive at lower temperatures. This means that activation energies and heats of adsorption calculated from the temperature dependence of the rate constants and the adsorption equilibrium constants, respectively, may have little theoretical meaning because the number of active sites may vary considerably.
THE METATHESIS OF UNSATURATED HYDROCARBONS
161
From the early qualitative studies it was already concluded that the reaction rate can be very high. Calderon et al. ( 2 ) observed that in the presence of the catalyst system WCla-CzHsAICl~-CzHsOH 2-pentene is converted at room temperature into an equilibrium mixture of 2-pentene, 2-butene, and 3-hexene within a few minutes, at an alkene to catalyst ratio of 10,OOO:l. If it is assumed that every tungsten atom forms one catalytic center, the turnover number, i.e. the number of molecules reacting per active center per second, is about 100 sec-'. Since not every tungsten atom will form an active center, undoubtedly the turnover number will be even larger in this case. 2. Modeling Several investigations have been carried out to correlate conversion data with reaction rate equations in order to obtain a kinetic model for the reaction. Clark and Cook (62) and Ramain and Trambouze (111) applied the power function model to, respectively, the heterogeneous metathesis of propene and the homogeneous metathesis of 2-pentene. The initial rates were found to be first order in alkene and, in the latter case, first order in catalyst. For the metathesis of 2-pentene, besides first order in catalyst, Hughes (69) found the order with respect to 2-pentene to be between 0.7 and 1.7, depending on the alkene to catalyst ratio. Furthermore, from an investigation of the metathesis of propene by Davie et al. (94),it turned out to be impossible to discriminate between a first- or second-order reversible rate equation. From these data, although limited in number, it appears that with the power function model hardly any interesting information can be obtained for the kinetic mechanism. More indicative information comes from studies based on hyperbolic or Langmuir type models.
a. Heterogeneous Systems. For the heterogeneously catalyzed metathesis of propene, the Langmuir-Hinshelwood model, in which the reaction occurs between two chemisorbed molecules, was examined by several investigators. In this model the following reactions are assumed: P +8
* Ps,
+ Bs, + +
Es Es * E Bs a B
2Ps
8,
8.
These equations represent the adsorption-desorption reactions and the surface reaction; El P, and B are, respectively, ethene, propene, and 2butene, and s represents an active site. If the surface reaction is rate deter-
162
J . C. MOL AND J. A. MOULIJN
mining, the following rate equation can be derived: T =
=
-+d[P]/dt kKp2([P]*
- [E][B]/K,J/(l
+ KE[E] + KP[P] + KB[B])~, (52)
where k is the reaction rate constant, Keq is the equilibrium constant and KE, Kp, and KB are the adsorption-equilibrium constants of ethene, propene, and 2-butene, respectively. This rate equation leads to the following expression for the initial rate of the reaction : TO
= kKp2[P]o2/(f
+ Kp[P]o)'.
(53)
Equation (53) proved to be successful in correlating the initial rate data obtained by Moffat and Clark (108) and Lewis and Wills (112) for the heterogeneously catalyzed metathesis of propene on a Mo03-Co0-A1203 catalyst at 0.1 to 0.9 MNm-2 and 393 to 478 K. With a Mo(CO)a-A1203 catalyst at low pressures (0.5 to 20 kNm-2) and temperatures between 290 and 350 K, Davie et a2. (94) arrived at the same conclusion. In a subsequent paper Lewis and Wills (113)reported data obtained at high conversion levels, which were in accordance with the complete equation for the Langmuir-Hinshelwood model [Eq. (52) 1. Expressions for the reaction rate based on adsorption or desorption controlling mechanisms or derived from other common Langmuir type models were found to be unsuccessful in correlating the data. Moffat and Clark (108) observed that the reaction rate goes through a maximum with increasing temperature and that the temperature corresponding to the rate maximum decreases with decreasing pressure. This is in agreement with the Langmuir-HinsheIwood model and can be explained on the basis of Eq. (52). Earlier, Begley and Wilson (114) arrived at another kinetic mechanism for the metathesis of propene, although with a different catalyst, viz. W03Si02, and under different conditions (2 to 6 MNm-2 and 590 to 730 K) . They observed pseudo-first-order kinetics, from which they concluded that the Langmuir-Rideal model, in which the reaction occurs between a chemisorbed molecule and a molecule from the gas phase, provided a basis for representing their data. However, their results might have been governed by mass transfer effects (film diffusion), and it can easily be shown that first-order kinetics are to be expected if external mass transfer is rate determining. This objection is supported by recent results of Moffat et al. (109, 1lo),who observed severe interphase mass transfer limitations for the same system, in spite of calculations which predicted the mass transfer rate to be several orders of magnitude greater than the observed rate. As
THE METATHESIS OF UNSATURATED HYDROCARBONS
163
a possible explanation for this discrepancy it was suggested that the reaction occurs on a small number of very active site areas which are widely separated on the catalyst surface, the activities being limited by localized interphase diffusional effects. However, a high activation energy was found, while in the case of limitations by mass transfer a very low apparent activation energy is to be expected. According to Moffat et al. this high activation energy is due to the fact that the surface of the catalyst is heterogeneous. Aris (115) suggested the influence of surface diffusion from the inert support to the dispersed WOractive site areas as a possible reason for the observed behavior. Very recently, Luckner et al. (116) obtained initial rate data for the metathesis of propene using the WO3SiO2 catalyst at flow rates where mass transfer effects were found to be negligible. Their experimental data referring to measurements at 0.1 to 0.9 MNmV2and 672 to 727 K could be correlated by Eq. (53). A variant of the common Langmuir type models is obtained if it is assumed that both alkene molecules are chemisorbed (Langmuir adsorption) on the same active center. If it is further assumed that there are two different adsorption steps, the following set of reaction equations for the initial stages of the metathesis of propene is obtained:
+ s* P Ps*, Ps* + P P P2s*, P
P ~ s *P EBs*,
(54)
(55)
(56)
where s* is an active center with two active sites. With the initial rate approximation the following rate equation can be derived for the case of the surface reaction being rate determining:
where K1 and K2 are the adsorption-equilibrium constants of the first and second propene molecule. A comparison of Eq. (53) and Eq. (57) makes it clear that Eq. (53), from a mathematical point of view, can be considered as a special case of Eq. (57), namely when K 1 = 4Kz = 2Kp. Therefore, Eq. (57) must fit the experimental data at least equally well. Mol (9'7) tested Eq. (57) for the metathesis of propene on a RezOT-Al203 catalyst at 313-343 K and atmospheric pressures. Applying linear and nonlinear regression to the measured ro - [PIo relationship resulted in the conclusion that the data were well correlated by Eq. (57). Parameter estimation showed K1 to be larger than K2, which means that the first propene molecule is more easily adsorbed than the second. Of all other models examined, only a rate expression based on the Langmuir-Hinshelwood model was found to give a good correlation of the experimental results as well.
164
J. C. MOL AND J. A. MOULIJN
Applying the F-test it was impossible to discriminate between the two models a t a 95% confidence level. From this study and the studies mentioned earlier, it can be concluded that the metathesis of propene is well interpreted kinetically by assuming that the rate is controlled by the surface reaction between the adjacent adsorbed molecules, the two active sites being localized a t the same active center or at two neighboring active centers.
b. Homogeneous Systems. Hughes (69) was the first to propose Eq. (57) for the homogeneously catalyzed metathesis reaction. He investigated the metathesis of 2-pentene with the catalyst system MoClz (NO) ( CsHsN)2(CHJ3AlZCls and found that his experimental results could be correlated by Eq. (57) with Kl and Kz as the equilibrium constants for the formation of the mono- and dialkene complex. For the catalyst system W C ~ ~ - C Z H ~ A I C ~ ~ - CCalderon ~ H ~ ~ Het, al. (3,22, 46) also proposed a kinetic scheme in which one metal atom, as the active center, is involved. According to this scheme, which was applied by Calderon to both acyclic and cyclic alkenes, the product molecules do not leave the complex in pairs. Rather, after each transalkylidenation step an exchange step occurs, in which one coordinated double bond is exchanged for the double bond of an incoming molecule. In this model the decomposition of the complex that is formed in the transalkylidenation step is specified, whereas in the models discussed earlier it is assumed that the decomplexation steps, or the desorption steps, are kinetically not significant. For the metathesis of cycloalkenes the scheme of Calderon can be depicted as follows:
CATALYST
+
n-
,
2‘CH=CH’
J- (
4
THE METATHESIS OF UNSATURATED HYDROCARBONS
165
The first reaction is the initiation step and the other two form the propagation sequence. This sequence leads to the formation of macrocyclic compounds, which can be decomplexed by an exchange reaction of the following type :
CH=CH
(61)
In a study of the ring-opening polymerization of cyclooctene, high molecular weight polymers were observed during the early stages of the reaction ( 2 2 ) . Similar results were obtained in studies of the metathesis of cyclopentene (20,66).These results can be correlated with the scheme of Calderon by making the assumption that the propagation steps [Eqs. (59) and (SO)] are much faster than the alkene exchange step [Eq. (Sl)] (see Fig. 4 ) . If, on the other hand, the alkene exchange step were faster than the propagation steps, mainly low molecular weight products would be formed in the early stages. HBrisson and Chauvin (88) examined the metathesis between acycIic alkenes and cycloalkenes (telomerisation) in the presence of two other tungsten-based catalysts, namely WOCla-Sn(n-CaHg) 4 and wocI4-
F I ~4.. Kinetic model of Calderon applied t.0 the ring-opening polymerization of cyoloalkenes.
166
J. C. MOL A N D J. A. MOULIJN
C2HsAlCl2.The results of a typical experiment with 2-pentene and cyclopentene as the reactants are summarized in Table VIII. As is shown in this table, the reaction products can be arranged in groups consisting of three members, called “triads.” The first triad consists of 2-pentene and its metathesis products. The second consists of dienes which are formed by a reaction between a member of the first triad and a cyclopentene molecule. Subsequent triads originate from successive incorporation of additional cyclopentene molecules. The molar ratio of the telomers is strikingly simple; within each triad the molar ratio is approximately 1:2:1, while the total molar amounts of the triads themselves are in the ratio of 1:1/2: 1/4: 1/8. These simple ratios are independent of the conversion, which means that the telomers were not formed by consecutive reactions. Further, although the metathesis products of 2-pentene are formed, the dimers, trimers, etc. of cyclopentene were not observed. For the interpretation of these results, HQrisson and Chauvin proposed a mechanism involving carbene complexes [Eq. (36)]. This is depicted in Fig. 5 for the telomerization reaction. With this scheme the results from Table VIII can be explained in detail. However, the model of Calderon, when applied to the telomerization reaction, predicts the same product distribution as the scheme of HQrisson and Chauvin, including the absence of oligomers of cyclopentene. From studies of the telomerization between 1-alkenes and cycloalkenes another interesting result emerged. The expected triads were observed, but within each triad the ratio of the products varied between 1:10: 1 and 1:20:1 (88). This is not consistent with the scheme of HBrisson and Chauvin. However, with the scheme of Calderon, this can be explained if
FIG.5. Mechanism of HtSrisson and Chauvin for t.he mutual metathesis of a cyclic and an acyclic alkene.
THE METATHESIS OF UNSATURATED HYDROCARBONS
167
168
J. C. MOL AND J. A. MOULIJN
it is assumed that 1-alkenes react more slowly than internal alkenes. If the latter is true, which has in fact been demonstrated for related catalyst systems (117,118), it may be expected that the reacting 1-alkene and its metathesis products are not in equilibrium, so that the ratio within the triads will deviate from 1:2: 1;the expected ratio will be 1:N : 1 with N > 2. It may be concluded that on account of kinetic data the scheme of Calderon is to be preferred to that of HBrisson and Chauvin. 3. Concluding Remarks
The preferred kinetic model for the metathesis of acyclic alkenes is a Langmuir type model, with a rate-determining reaction between two adsorbed (complexed) molecules. For the metathesis of cycloalkenes, the kinetic model of Calderon as depicted in Fig. 4 agrees well with the experimental results. A scheme involving carbene complexes (Fig. 5) is less likely, which is consistent with the conclusion drawn from mechanistic considerations (Section 111). However, Calderon’s model might also fit the experimental data in the case of acyclic alkenes. If, for instance, the concentration of the dialkene complex is independent of the concentration of free alkene, the reaction will be first order with respect to the alkene. This has in fact been observed (Section IV.C.2) but, within certain limits, a first-order relationship can also be obtained from many hyperbolic models. Moreover, it seems unreasonable to assume that one single kinetic model could represent the experimental results of all systems under consideration. Clearly, further experimental work is needed to arrive at more definite conclusions. Especially, it is necessary to investigate whether conclusions derived for a particular system are valid for all catalyst systems. REFERENCES 1 . Banks, R. L., and Bailey, G. C., Ind. Eng. Chem., Prod. Res. Develop. 3, 170 (1964). d . Calderon, N.,Chen, H. Y., and Scott, K. W., Tetrahedron Lett. p. 3327 (1967). 3. Scott, K.W., Calderon, N., Ofstead, E. A., Judy, W. A., and Ward, J. P,, Advun.
Chem. Ser. 91, 399 (1969).
4 . Wasserman, E., Ben-Efraim, D. A., and Wolovsky, R., J. Amer. Chem. Soc. 90, 3286 (1968). 6 . Hydrocurbon Proc. % (ll),232 (1967). 6. Chem. Eng. News 48 (lo), 60 (1970). 7 . Haas, F., Nutzel, K., Pampus, G., and Theisen, D., Rubber Chem. Technol. 43,
1116 (1970).
8. Dall’Asta, G., and Motroni, G., Angew. Makromol. Chem. 16/17, 51 (1971). 8a. Amass, A. J., Brit. Polgm. J . 4, 327 (1972). 9. Bradshaw, C.P. C., British Patent 1,180,459 (1970). 9u. Lewandos,
G.S., Ph.D. thesis, Austin (1972).
T H E METATHESIS OF UNSATURATED HYDROCARBONS
169
10. Foster, G., German Patent 2,063,150 (1971). 11. O’Hara, J. I., and Bradshaw, C. P. C., British Patent 1,283,348 (1972). 12. Van Dam, P. B., Mittelmeijer, M. C., and Boelhouwer, C., Chem. Commun. p, 1221 (1972). 13. Zuech, E. A., Hughes, W. B., Kubicek, D. H., and Kittleman, E. T., J. Amer. Chem. SOC.92, 528 (1970). 14. Heckelsberg, L. F., Banks, R. L., and Bailey, G. C., J. Catal. 13,99 (1969). 16. Kroll, W. R., and Doyle, G., Chem. Commun. p. 839 (1971). 16. Heckelsberg, L. F., Belgian Patent 713,184 (1967). 17. Banks, R. L., and Regier, R. B., Znd. Eng. Chem., Prod. Res. Develop. 10,46 (1971). 18. Crain, D. L., J. Catal. 13, 110 (1969). 18u. Popov, A. M., Fridman, R. A., Finkel’shtein, E. Sh., Nametkin, N. S., Vdovin,
V. M., Bashkirov, A. N., Kryukov, Yu.B., and Liberov, L. G., Bull. A d . Sci. USSR, Chem. Div. 22, 1397 (1973). 19. Scott, K. W., Calderon, N., Ofstead, E. A., Judy, W. A., and Ward, J. P., Rubber Chern. Technol. 44, 1341 (1971). 20. Scott, K. W., Polym. Prepr., Amer. Chem. SOC.,Div. Polym. Chem. 13, 874 (1972). 21. Marshall, P. R., and Ridgewell, B. J., Eur. Polym. J. 5, 29 (1969). 22. Calderon, N., Accounts Chem. Res. 5, 127 (1972). 23. Natta, G., Dall’Asta, G., Bassi, I. W., and Carella, G.;Makromol. Chem. 91, 87 (1966).
23a. Hein, P. R., J. Polym. Sci., Polym. Chem. Ed. 11, 163 (1973). 24. Ofstead, E. A., and Calderon, N., Makromol. Chem. 154,21 (1972). 26. Wolovsky, R., J. Amer. Chem. SOC.92,2132 (1970). 26. Ben-Efraim, D. A., Batich, C., and Wasserman, E., J. Amer. Chem. SOC.92, 2133 (1970). 2‘7. Natta, G., Dall’Asta, G., and Porri, L., Makromol. Chem. E l , 253 (1965). 28. Calderon, N., Ofstead, E. A., and Judy, W. A., J. Polym. Sci., Part A-1 5, 2209 (1967). 29. Ofstead, E. A., Syn. Rubber Symp. 4, No. 2,42 (1969). 50. Singleton, D. M., U.S. Patent 3,530,196 (1970). 31. Penella, F., Banks, R. L., and Bailey, G. C., Chem. Commun. p. 1548 (1968). 32. Moulijn, J. A,, Reitsma, H. J., and Boelhouwer, C., J . Catal. 25, 434 (1972). 33. Mortreux, A., and Blanchard, M., Bull. SOC.Chim. Fr. p. 1641 (1972). 34. Atlas, V. V., Pis’man, I. I., and Bakshi-Zade, A. M., Sov. Chem. Znd. 1, (10) 17 (1969).
36. Heckelsberg, L. F., Banks, R. L., and Bailey, G. C., Znd. Eng. Chem., Prod. Res. Develop. 8, 259 (1969). 36. Heckelsberg, L. F., Banks, R. L., and Bailey, G. C., Znd. Eng. Chem., Prod. Res. Develop. 7, 29 (1968). 37. Howman, E. J., Turner, L., and Williams, K. V., British Patent 1,106,015 (1968). 38. Williams, K. V., and Turner, L., British Patent 1,116,243 (1968). 39. Davie, E. S., Whan, D. A,, and Kemball, C., Chem. Commun. p. 1202 (1971). 40. Kobylinski, T. P., and Swift, H. E., J. Catal. 26, 416 (1972). 41. Ellis, A. F., and Sabourin, E. T., US. Patent 3,595,920 (1971). 42. Bailey, G. C., Catal. Rev. 3, 37 (1969). 43. Banks, R. L., Fortschr. Chem. Fmsch. 25, 39 (1972). 44. Dall’Asta, G., Makromol. Chem. 154, 1 (1972). 46. HBrisson, J. L., Chauvin, Y., Phung, N. H., and Lefebvre, G., C.R. A d . Sci., Ser. C 269, 661 (1969).
170
J. C. MOL AND J. A. MOULIJN
46. Calderon, N., Ofstead, E. A., Ward, J. P., Judy, W. A,, and Scott, K. W., J. Amer. Chem. SOC.90, 4133 (1968). 47. Lewandos, G. S., and Pettit, R., J. Amer. Chem. SOC.93, 7088 (1971). 47a. Furukawa, J., and Mieoe, Y., J. Polym. Sei., Polym. Lett. Ed. 11, 263 (1973). @. Uchida, Y., Hidai, M., and Tatsumi, T., Bull. Chem. SOC.Jap. 45, 1158 (1972). 49. Wang, J. L., and Menapace, H. R., J. Org. Chem. 33, 3794 (1968). 60. Wang, J. L., Menapace, H. R., and Brown, M., J. Catal. 26, 455 (1972). 61. Chatt, J., Haines, R. J., and Leigh, G. J., Chem. Commun. p. 1202 (1972). 62. Raven, P. A., and Wharton, E. J., Chem. I d . (London) p. 292 (1972). 63. Bencze, L., and Mark6, L., J . Organometal. Chem. 28, 271 (1971). 64. Kroll, W. R., and Doyle, G., J. Catal. 24, 356 (1972). 66. Moulijn, J. A., and Boelhouwer, C., Chem. Commun. p. 1170 (1971). 66. Pampus, G., Witte, J., and Hoffmann, M., Rev. Gen. Caout. P l a t . 47, 1343 (1970). 67. Dall’Asta, G., and Motroni, G., Eur. Polym. J. 7, 707 (1971). 68. Hocker, H., and Jones, F. R., Makromol. Chem. 161, 251 (1972). 69. Kothari, V. M., and Tazuma, J. J., J . Org. Chern. 36, 2951 (1971). 69a. Hocks, L., Hubert, A. J., and Teyssi6, Ph., Tetrahedron Lett. p. 2719 (1973). 60. Hughes, W. B., Organometal. Chem. Syn. 1, 341 (1972). 61. Mol, J. C., Moulijn, J. A., and Boelhouwer, C., Chem. Commun. p. 663 (1968). 62. Clark, A., and Cook, C., J . Calal. 15, 420 (1969). 63. Mol, J. C., Visser, F. R., and Boelhouwer, C., J. CataZ. 17, 114 (1970). 64. Natta, G., Dall’Asta, G., and Mazzanti, G., Angew. Chem. 76, 765 (1964). 66. Dall’Asta, G., Motroni, G., and Motta, L., J. Polym. Sci., Part A-1 10, 1601 (1972). 66. Dolgoplosk, B. A,, Makovetskii, K. L., and Tinyakova, E. I., Dokl. Akad. Nauk SSSR 202, 871 (1972). 67. Bradshaw, C. P. C., Howman, E. J., and Turner, L., J . Catal. 7,269 (1967). 68. Zuech, E. A., Chem. Commun. p. 1182 (1968). 69. Hughes, W. B., J. Amer. Chem. SOC.92, 532 (1970). 70. Adams, C. T., and Brandenberger, G., J . Catal. 13, 360 (1969). 71. Pettit, R., Sugahara, H., Wristers, J., and Merk, W., Discuss. Faraday SOC.47, 71 (1969). 72. Mango, F. D., and Schachtschneider, J. H., J. Amer. Chem. SOC.93, 1123 (1971). 73. Mango, F. D., and Schachtschneider, J. H., J. Amer. Chem. SOC.89,2484 (1967). 74. Mango, F. D., and Schachtschneider, J. H., i n “Transition Metals in Homogeneous Catalysis” (G. N. Schrauzer, ed.), p. 223. Dekker, New York, 1971. 76. Woodward, R. B., and Hoffmann, R., “The Conservation of Orbital Symmetry.” Verlag Chemie, Weinheim, 1970. 76. Hoogeveen, H., and Volger, H . C., J. Amer. Chem. SOC.89,2486 (1967). 77. Mango, F. D., Advan. Catat. Relal. Subj. 20, 291 (1969). 78. Mango, F. D., Tetrahedron Lett. p. 505 (1971). 79. Van der Lugt, W. Th. A. M., Tetrahedron Lett. p. 2281 (1970). 80. Caldow, G. L., and MacGregor, R. A., Inorg. Nucl. Chem. Lett. 6,645 (1970). 81. Caldow, G. L., and MacGregor, R. A., J. Chem. Soc., A p. 1654 (1971). 82. Pearson, R. G., J. Amer. Chem. SOC.94, 8287 (1972). 83. Lewandos, G. S., and Pettit, R., Tetrahedron Lett. p. 789 (1971). 84. Cassar, L., Eaton, P. E., and Halpern, J., J . Amer. Chem. SOC.92, 3515 (1970). 86. Cassar, L., and Halpern, J., Chern. Commun. p. 1082 (1970).
86a. Fraser, A. R., Bird, P. H., Bezman, S. A., Shapley, J. R., White, R., and Osborn, J. A., J . Amer. Chem. SOC.95, 597 (1973). 86. Grubbs, R. H., and Brunck, T. K., J. Amer. Chern. SOC.94, 2538 (1972).
THE METATHESIS OF UNSATURATED HYDROCARBONS
171
Mango, F. D., Polym. Prepr., Amer. Chem. SOC.,Div. Polym. Chem. 13, 903 (1972). HBrisson, J . L., and Chauvin, Y., Makromol. Chem. 141, 161 (1971). Cardin, D. J . , Doyle, M. L., and Lappert, M . F., Chem. Commun. p. 927 (1972). O’Neill, P. P., and Rooney, J. J., Chem. Commun. p. 104 (1972). Wittig, G., and Schwarzenbach, K., Justus Liebigs Ann. Chem. 650, 1 (1961). Pampus, G., Lehnert, G., and Maertens, D., Polym. Prepr., Amer. Chem. SOC., Div. Polym. Chem. 13, 880 (1972). 93. Davie, E. S., Whan, D. A., and Kemball, C., Chem. Commun. p. 1430 (1969). 94. Davie, E. S., Whan, D. A., and Kemball, C., J . Catal. 24,272 (1972). 95. Whan, D. A,, Barber, M., and Swift, P., Chem. Commun. p. 198 (1972). 96a. Howe, R. F., Davidson, D. E., and Whan, D. A., J . Chem. SOC.,Faraday Trans. 1 68, 2266 (1972). 96. Wang, J . L., and Menapace, H . R., J . Catal. 23, 144 (1971). 97. Mol, J. C., P i D . Thesis, University of Amsterdam (1971). 97a. Luckner, R. C., and Wills, G. B., J . Catal. 28, 83 (1973). 97b. Furukawa, S., Kamiya, Y., and Ohta, N., Kogyo Kagaku Zasshi 74, 2471 (1971). 97c. Nakamura, R., and Echigoya, E., Bull. Jap. Petr. Inst. 14, 187 (1972). 97d. Ogata, E., and Kamiya, Y., Chem. Lett. p. 603 (1973). 97e. Henrici-OlivB, G., and OIiv6, S., Angew. Chem. 85, 148 (1973). 97f. Matlin, S. A., and Sammes, P. G., Chem. Commun. p . 174 (1973). 98. Whitesides, G. M., and Ehmann, W. J., J . Amer. Chem. Soe. 91, 3800 (1969). 99. Dainton, F . S., Devlin, T. R. E., and Small, P. A., Trans. Faraday SOC.51, 1710 (1955). 100. Calderon, N., J . Macromol. Sci., Rev. Macromol. Chem. 7 , 105 (1972). 101. Rossini, F . D., Pitzer, K. S., Arnett, R. L., Braun, R. M., and Pimentel, G. C., “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds.” Carnegie Press, Pittsburgh, Pennsylvania, 1953. 108. Giinther, P., Haas, F., Marwede, G., Niitzel, K., Oberkirch, W., Pampus, G., Schon, N., and Witte, J., Angew. Makromol. Chem. 14,87 (1970); 16/17,27 (1971). 103. Minchak, R. J., and Tucker, H., Polym. Prepr., Amer. Chem. SOC.,Div. Polym. Chem. 13, 885 (1972). 104. Hughes, W. B., Chem. Commun. p. 431 (1969). 106. Mol, J. C., Moulijn, J. A,, and Boelhouwer, C., J . Catal. 11, 87 (1968). 106. Dall’Asta, G., and Manetti, R., Eur. Polym. J . 4, 145 (1968). 107. Davie, E . S., Whan, D. A., and Kemball, C., Proc. Int. Congr. Catal., 6th, 1972 p. 1205 (1973). 108. MotTat, A. J., and Clark, A., J . Catal. 17, 264 (1970). 109. Moffat, A. J., Johnson, M. M., and Clark, A., J . Catal. 18, 345 (1970). 110. Moffat, A. J., Clark, A., and Johnson, M. M., J . Catal. 22, 379 (1971). 111. Ramain, L., and Trambouze, Y., C.R. Acad. Sci., Ser. C 273, 1409 (1971). 112. Lewis, M. J., and Wills, G. B., J . Catal. 15, 140 (1969). 113. Lewis, M. J., and Wills, G. B., J . Catal. 20, 182 (1971). 114. Begley, J . W., and Wilson, R. T., J . Catal. 9, 375 (1967). 116. Aris, R., J . Catal. 22, 282 (1971). 116. Luckner, R. C., McConchie, G. E., and Wills, G. B., J . Catal. 28,63 (1973). 117. Marlin, V. I., Shebaldova, A. D., Bol’shinskova, T. A., Khidekel’, M. L., and Kalechits, I. V., Kinet. Catal. USSR 14, 528 (1973). 118. Uchida, A., Hamano, Y., Mukai, Y., and Matsuda, S., Ind. Eng. Chem., Prod. Res. Develop. 10, 372 (1971). 87. 88. 89. 90. 91. 92.
This Page Intentionally Left Blank
0ne-Corn pone nt CataI ysts for Polymerization of Olefins YU. YERMAKOV
AND
V. ZAKHAROV
Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences Novosibirsk, USSR
I. Introduction. . . . . . . . . . . . ........................... 11. Chromium Oxide Catalyst ................................. A. Formation of the Propagation Centers. ....................... B. Kinetics of Polymerization. ................................. 111. Catalysts Based on the Use of Organometallic Compounds of Transition Metals.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Polymerization of Olefins in the Presence of Individual Organometallic Compounds. ....................................... B. Catalysts Formed by Interaction of Organometallic Compounds of Transition Metals with Oxide Supports. ....................... IV. Subhalides of Transition Metals. ................................ A. Preparation of Subhalides as One-Component Polymerization Cata-
..................................................
B. Data on the Kinetics of Polymerization. ...................... V. Determination of the Number of Propagation Centers. . . . . . . . . . . . . . A. Methods Used for the D ation of the Number of Propagation Centers. .............. ............................... B. Number of Propagation s in One-Component Catalysts. . . . VI. Some General Features of Propagation Centers in One-Component Polymerization Catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Coordinative Insufficiency of Transition Metal Ions in Active Centers. ................................................... B. Active Transition Metalearbon a-Bond. ..................... VII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . .... ..... ..
173 175 175 178 184 184 187 192 192 194 194 195 197 202 202 208 213 213
1. Introduction At the present time the concept of catalytic (or “ionic-coordination”) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 173
174
YU. YERMAKOV AND V. ZAKHAROV
The propagation center for catalytic polymerization is a chemical compound having an active bond between the catalyst and growing polymer molecule; the monomer insertion into this bond occurs as a propagation act. I n catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain makily on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. The propagation centers of the catalysts of olefin polymerization contain the active transition metal-carbon a-bond into which the insertion of olefin proceeds during the propagation. The catalysts for olefin polymerization may be divided into two vast classes according to the method of formation of the propagation center: two-component and one-component.’ Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with organometallic compounds of groups 1-111 elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organometallic cocatalyst. One-component catalysts cause polymerization without organometallic activators; in this case the formation of the propagation centers takes place at the interaction of the transition metal compound with the monomer, At the present time the following basic types of one-component catalysts are known : (a) Supported oxide catalysts. The supported chromium oxide catalyst is most active and best studied. It is used for the commercial production of high-density polyethylene [the process of the Philips Petroleum Co (S)]. The supported molybdenium oxide catalysts are less active in ethylene polymerization; catalysts containing vanadium and tungsten oxides are even less active. (b) Catalysts based on the use of organometallic compounds of transition metals (mainly formed by the interaction with oxide supports). (c) Subhalides of transition metals, Titanium dichloride is the most active system of this type. 1 The terms “one-component” and “two-component” for the catalysts of olefin polymerization were used in the review by Berger et al. ( 1 ) .
ONE-COMPONENT POLYMERIZATION CATALYSTS
175
Supported oxide catalysts were discovered at the same time (3-5) as the two-component Ziegler-Natta catalysts (6, 7) in the early 1950’s. The publications on other types of one-component catalysts [supported organometallic compounds of transition elements (8, 9, 9a) and titanium dichloride (lo)] appeared quite recently. Some data are also available (6) on the use of metallic cobalt and nickel supported on charcoal for high polymerization of ethylene. However, the application and investigation of these catalysts were not subsequently developed. A large body of investigations in the field of catalytic polymerization dealt with the study of two-component catalytic systems; the results of these investigations were regularly presented in reviews and monographs (1, 11-19). However, the study of these systems is hindered by the complication and variety of reactions taking place during the interaction of metallorganic cocatalysts with transition metal compounds. One-component catalysts, being simpler in composition, may be a convenient object for the study of the detailed mechanism of catalytic polymerization. The object of this review is to show recent results of the study of one-component catalysts of olefin polymerization.
II. Chromium Oxide Catalysts Since the publication by the discoverers (23) of chromium oxide catalysts a considerable number of papers devoted to this subject have appeared. Most of them (20-72) deal either with the study of the chromium species on the catalyst surface or with the problem of which of this species is responsible for polymerization. Fewer results have been published on the study of processes determining the polymer molecular weight (73-77) and kinetics of polymerization (78-99). A few papers describe nascent morphology of the polymer formed (100-10S) . Some results obtained have already been reviewed (104-107) ,2 so only the data of general interest in the problem of olefin polymerization by onecomponent catalysts will be touched upon here.
A. FORMATION OF THE PROPAGATION CENTERS So far the problem of active center formation in chromium oxide catalysts amounted mainly to a discussion of the oxidation number of chromium that is necessary for catalytic activity. As an “active species” chromium ions having practically every possible oxidation number-
* See also the review by Krauss (lO7a).
176
YU. YERMAKOV AND V. ZAKHAROV
Cr (VI), Cr (V) , Cr (IV), Cr (11)-have been considered [a concise review of this may be found in the paper by Eley et a2 ( 9 9 ) ] . Attempts have also been made to compare the catalytic activity with the content of Cr(II1) ions on the surface after reduction of the catalyst by hydrogen (57, 68). It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, e t a ) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity ( A ) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers ( N ) , but also on the specific activity of one center (propagation rate constant, K P )and on the effective catalyst surface (Serf)as well: A
-
KpNSert
I n the general case the change of some factor may result in changing every variable that determines the catalytic activity. The results of the investigation of chromium oxide catalysts accumulated up to now permit the following main stages in the process of the propagation center formation to be singled out:
1. Formation of the Active Component (the Precursor of the Propagation Centers) The active component of the chromium oxide catalyst is a surface compound of Cr(V1). In the case of silica as a support this stage may be presented by the schemes:
>
>S-o
Si-OH
+ CrO3
\ /
0
Cr
4
+ Ha0
(1)
3 s i - 0 / \o
\
7S i 4 H
0 7Si-OH \
o-i3s
+ 2CrO3 \
7S i 4 H
>
e
r=O
0 ‘
/
Si--O-cr=O
CJ
+ Ha0 (2)
The formation of the surface chromate- and dichromate-type compounds as a result of the reaction of CrOs with surface hydroxyk has been ascer-
ONE-COMPONENT POLYMERIZATION CATALYSTS
177
tained by optical methods (49) and chemical analysis (68). According to Hogan (69) the surface chromate is formed mainly by the reaction between CrOa and silanol groups [reaction (l)]. 2. Reduction of Active Component \
7 Si-0
\ //
0 reduotion
Cr -(=Ui--O)z
\
7 Si-0
/ (z
xo
LY 1 Cr
0z
(3)
+ oxidative product.? + y + z no more than 6)
The reduction is performed either by the components of the reaction medium under polymerization conditions or by a special treatment of the catalyst before its contact with the reaction medium. The ligands of the chromium ion formed are oxygen ions of the support (fragments 4 i 0-); in addition other ligands L (e.g. a reducing agent or its oxidation can be found. Though scheme products) and vacant coordination sites (0) (3) gives the reduction of the chromate, at present no results demonstrate without any doubt what Cr(V1) compound-chromate or dichromate type (or both)-is the precursor of the propagation centers. The value of the oxidation number of chromium ions resulting from stage (3) is in dispute. In Krauss and Stach (65, 67, 70) the conclusion about the formation of Cr(I1) after reduction by carbon monoxide has been drawn on the basis of the analysis of reduced chromium oxide catalysts. In Yermakov et al. (68) on the basis of the comparison of the average oxidation number of chromium in the catalyst, reduced by CO, with the concentration of the propagation centers it has been deduced that chromium ions with an oxidation number not higher than 3 are formed. Comparing the activity of the reduced catalysts with the average oxidation number of chromium (determined according to the quantity of COZevolved at catalytic reduction) , Eley et al. (71, 72) reached the conclusion that a reduction up to Cr(V) is necessary for the propagation centers to be formed. Baker and Carrick (108) have found that in the reduction of the chromium oxide catalyst by ethylene under mild conditions (125°C) formaldehyde was formed as an oxidation product. 3. The Formation of Active CT-C u-Bond
In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon u-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the
178
YU. YERMAKOV AND V. ZAKHAROV
catalyst after polymerization, the chromium organometallic complex whose mass spectrum showed fragments of the common formula Cr-(CH2),+ (with n = 0, . . ., 20). The Cr-C u-bond seems to result from the alkylation of low valent surface ions of chromium by the monomer; this process may be represented by the following overall scheme: j -Cr-n + C ~ H3, j -Cr-C (4) The problems of the mechanism of this process and the behavior of the transition metal-carbon bond apply equally to all one-component catalysts and will be touched on later (see Section VI) . The necessity of marking out the process (4) as a separate and final stage of the propagation center formation follows from the results obtained for the reduction of chromium oxide catalysts. It is known (55,59,68, 97) that the catalysts treated a t high temperatures by reducing agents containing no carbon (NHs, Hs, SO,) show rather high catalytic activity. In such catalysts the active bond Cr-C cannot arise just a t reduction but it is formed during the further reaction with ethylene. The process of the type (4) completes the formation of the propagation centers as a surface compound containing the active transition metal-carbon bond. It is likely that an analogy exists between the process of the active center formation in chromium oxide catalysts and catalysts obtained with the use of bis(triphenyl-silyl) chromate. I n the reaction of the latter with silica hydroxyls, according to (109) the surface Cr(V1) compound is formed: 0
j S i 4 H + (Ph&iO)&rOz --* 3 -Si+
&r-OSiPhs li
+ PhsSiOH
The following stage of the propagation center formation occurs through the reduction of Cr(V1) to the lower oxidation state. The compounds of Cr (11)seem to be active in polymerization in the solution of bis-triphenylsilyl-chromate (109).For the formation of these compounds the following scheme taking into account the results (110)concerning the study of the reaction of bis-triphenylsilyl-chromate with olefins was considered (109) : 0
PhrSi-0
’ ‘C/
Ph3Si-0
\o
CHI
PhsSi-0
+
>r]+2CH20
c
PhSSi-4
B. KINETICSOF POLYMERIZATION The following simple equation is basic for the analysis of the influence of various parameters on the rate of polymerization (V) by the heteroge-
ONE-COMPONENT POLYMERIZATION CATALYSTS
179
neous catalyst:
V = KpSeff-C *Nee
(5)
Here K , is the propagation rate constant, Seffthe catalyst effective surface, C the monomer concentration near the catalyst surface, and N , the surface concentration of propagation centers. The propagation reaction itself is of the first order with respect to the monomer concentration. This was demonstrated by measuring the propagation rate constants K, at different monomer concentrations (98). As for the dependence of the polymerization rate V on the monomer concentration some authors have also found first-order kinetics (84, 90, 96, 99),but sometimes deviations from the first order were observed (38, 51,88) that may be connected with a change in the number of propagation centers with monomer concentration. A maximum is typical for the dependence of the polymerization rate on temperature (38, 51, 84, 92). The activation energy of the propagation reaction (E,) determined by a temperature variation of the K , value is rather low (4-5 kcal/mole) (98); the dependence of the polymerization rate on temperature (i.e. the effective activation energy Eerr)is influenced by the change in the number of propagation centers with temperature. This change depends on the composition of the catalyst and its activation procedure [e.g. in the range of 30-75OC an E,rf of 6.5-16 kcal/mole was observed (98) depending on the type of the catalyst used]. I n addition, the activity drop with increasing temperature as a result of diminishing C (due to diffusional restrictions when a partly dissolving polymer is formed) is possible (98). Generally, the rate of polymerization by the chromium oxide catalyst varies with time. As a consequence, the average rate of polymerization (as polymer yield for a certain period of time) may yield incorrect information on the process kinetics. It is evident that true data on the kinetics of a nonsteady polymerization process may be obtained only when studying the reaction rate under isothermal conditions in a nongradient (perfect mixing) reactor. To study the developed process (not only the initial stage) it is also essential to obtain considerable polymer yield (at least tens of grams per gram of catalyst). The shape of the kinetic curves depends on the catalyst type and polymerization conditions (ethylene pressure, temperature, concentration of inhibitors in reaction medium) (89,97,98). The types of the kinetic curves obtained .at ethylene polymerization under various conditions are presented in Fig. 1. In the case of reagents with a low content of inhibitors a steady-state polymerization rate may be set up. Steady-state kinetics are also observed
180
YERYAKOV AND V. ZAKHAROV
10
20
30
40
50
60
Polymerirotion time ( m i d
FIG.1. Examples of the kinetic curves during ethylene polymerization by chromium oxide catalysts. Support-SiOz; temperature-W"C ;polymerization a t constant ethylene pressure in perfect mixing reactor. Curve 1-catalyst reduced by CO a t 300°C. Curve 2catalyst activated in vacuum (400°C); polymerization in the case of (1) and (2) in solvent (heptane); ethylene pressure 10 kg/cm*; 0 2 content in ethylene 1 ppm, H20 6 3 ppm. Curves 3,4, 5, 6-catalyst activated in vacuum (400°C); polymerization without solvent; ethylene pressure 19 (curve 3), 13 (curve 4), 4 (curve 5 ) , and 2 (curve 6) kg/cm2; 0 2 content in ethylene 6 1 ppm, HzO = 12 ppm.
<
a t a low absolute rate of polymerization (e.g. at low concentrations of monomer). In principle, the dependence of the polymerization rate on time may be determined by the change of any parameter on the right side of Eq. (5). Some information on the possibility of variation of these parameters with time may be given.
1. Propagation Rate Constant
If the catalyst active centers are nonuniform, a time variation of the average value of K , may be caused by the change of the proportion between the centers with various reactivity during polymerization. However, in the case of chromium oxide catalysts the experimental data show that the
ONE-COMPONENT POLYMERIZATION CATALYSTS
181
average reactivity of the propagation centers does not vary with time (98,111). 2. E$ective Surface of Catalyst
Large yields of polymer seem to be obtained only when polymerization proceeds on the outer catalyst surface, because the transport of high molecular polyethylene from catalyst pores is impossible (11.2). The “working” part of the specific surface of the catalyst can be expected to increase with diminishing strength of links between catalyst particles (11.2). Therefore, to obtain a highly active catalyst a support with large pore volume should be used (e.g. silica with pore volume 21.5 cm3/g). The splitting of catalyst macrograins during the polymerization of olefins is a well-known fact (93,113). Thus, the increase of the polymerization rate during the initial period can also result from an increase in Serf(93). However, when using supports with weak linkage between the primary particles of the catalyst, its splitting occurs quickly and it is unlikely to influence the shape of the kinetic curve. For example, in the case of chromium oxide catalyst reduced by CO supported on aerosil-type silica, steady-state polymerization with a very short period of increasing rate is possible (see curve 1, Fig. 1). 3. Monomer Concenlration at Catalyst Surface
In several papers (61, 84,96, 104) the decrease of the polymerization rate with time was assumed to be caused by the decrease of C as a result of diffusional restrictions due to the formation of a polymer film on the catalyst surface. However, as a matter of experience in work with heterogeneous catalysts for ethylene polymerization, it is known that ’ even for polymerization with no solvent, the formation of a solid polymer is possible at high rates (thousands of grams of polymer per gram of catalyst per hour) that are constant until large yields are reached (tens of kilograms of polymer per gram of catalyst). For the analysis of the role of monomer diffusion during ethylene polymerization while forming a solid polymer a model of the polymer grain (see Fig. 2) has been suggested (96).This model is consistent with the results of the study of nascent morphology of the polymer and its porosity (95,100,103).According to this model three levels are considered in the analysis of transport phenomena. a. Primary Particle (Fig. wa), that is a pellet of the catalyst covered with a polymer film. The size of the catalyst pellet is determined by the degree of splitting of the initial catalyst grains. This model results in the following expression for ethylene concentration (C) near the surface of the
182
YU. YERMAKOV AND V. ZAKHAROV
a
FIG.2. Models of the primary particle (a) and polymer grain (b) for the analysis of the role of monomer diffusion to the catalyst surface. (a) 1-catalyst; 2-polymer film. (b) 1-micrograin; 2-macropore. catalyst pellet (96) :
Here Ce, is the ethylene concentration equilibrium to the concentration in a gaseous phase, K , the propagation rate constant, N the concentration of the propagation centers on the catalyst surface, DPEthe diffusion coeficient of ethylene through the polymer film, G the yield of polymer weight unit per unit of the catalyst and yoat,yPEare the specific gravity of the catalyst and polyethylene. The calculation of C according to (6) shows (95) that if the catalyst splitting results in the formation of catalyst pellets about 1000 in size, then even under the most unfavorable conditions (the concentration of the active centers is equal to the total chromium content in the catalyst, 2r2 = 00) the diffusional restriction on the primary particle level is negligible. b. Micrograin of Polymer formed by primary particles (Fig. 2b). As a micrograin a region of the polymer grain can be considered in which the pores are less than 5 p in diameter. The extent of the diffusional restrictions on the micrograin level can be estimated according to the Thiele parameter (114). For a pseudohomogeneous model of the micrograin the Thiele parameter h in the case of polymerization is (95) : h =
Ta . A ki *'Yoat/
(*oat/yg
+ 1) CDeff.
(7)
Here rg is the radius of the micrograin, A k i n the catalyst activity in the kinetic region which can be calculated with the known values of K , and N ,
ONE-COMPONENT POLYMERIZATION CATALYSTS
183
ys the specific gravity of the grain, D,tr the effective diffusion coefficient for ethylene diffusion in polymer pores, and for ycat,C, and G see Eq. (6). cm according Due to the small dimensions of the micrograin [5 1 X to the porosity measurements (95)3, diffusional restrictions a t this level may occur only a t rather high (tens of kilograms per gram of catalyst per hour) polymerization rates.
c. Macrograin of Polymer that is formed by micrograins. At this level ethylene diffusion in macropores (2r, 2 5 p, Fig. 2b) is added. When the size of the macrograin (ds, see Fig. 2b) is 51 mm, the diffusion a t this level is not a rate-determining step. In polymerization with no solvent the transport of ethylene in macropores is provided for the pressure drop outside and inside of the grain. Thus, two factors may be pointed out that determine the possibility of obtaining high yields of crystalline polyethylene on a solid catalyst with no diffusional restriction: (1) the splitting up of the catalyst into small particles ( 5 1000 A), possible when using supports with low resistance to breaking; (2) the formation of polymer grains with polydispersed porosity. If the dissolving of a portion of the polymer takes place, diffusional restriction may occur as a result. Such a case was observed in (98) where a decrease of the polymerization rate (slurry process in cyclohexane) with temperature rise from 75" to 90°C was found despite the increase in the number of propagation centers. At a further increase of the polymerization temperature ( > 115°C) polymerization becomes a solution process that may also proceed with no diffusional restrictions (94).
4. The Change in the Number of Propagation Centers The most obvious reason for the polymerization rate variation with time is the change in the number of propagation centers (88, 89). According to Zakharov et al. (115) this change may be determined by the concurrence of the following reactions in the polymerization medium : 1. Reduction of the active component N. [a compound of Cr (VI) ] : N.
+ CzHi -+ Nr*A
(8) Here N , is the center containing a reduced chromium ion and A is the oxygenated compound that is adsorbed on the active center formed.
2. Formation of coordinative insufficiency at a chromium ion as a result of desorption of A:
(9) 3. Formation of the propagation center N, (i.e. a surface compound N,*A F?N, + A
184
YU. YERMAKOV AND
containing the active Cr-C
V.
ZAKHAROV
bond) : Nr
4. Propagation: N,'
+ C2H4 + CzHa
+
--t
Np Ni+'
where i is the degree of polymerization. 5. Temporary deactivation of the propagation centers by the inhibitor X: N,' + X + N x + P' ( 12) Here NX is the active center deactivated by X and Pi is the polymer molecule. 02,COZ, C2Hz appear to be X-type inhibitors. 6. Regeneration of the propagation centers: Nx CtH4 -+ Np (13)
+
7. Constant deactivation of the propagation center by the inhibitor Y: N,'
+Y
Ny
(14) Here Ny is the active center deactivated by Y. H20 is likely to be a Y-type inhibitor. To explain the steady-state period of polymerization it may be assumed that some quantities of Y are adsorbed on the support surface. In the case of catalysts activated with the help of reduction by CO, reactions (8) and (9) are excluded from the kinetic scheme. Such catalysts contain the centers N, before their contact with the polymerization medium. The change of shape of the kinetic curves with monomer and inhibitor concentration a t ethylene polymerization by chromium oxide catalysts may be satisfactory described (115) by the kinetic model based on reactions (8) - (14). -+
111. Catalysts Based on the Use of Organometallic Compounds of Transition Meto Is A. POLYMERIZATION OF OLEFINS IN THE PRESENCE OF INDIVIDUAL ORGANOMETALLIC COMPOUNDS In essence the active centers for catalytic polymerization of olefins are organometallic complexes of transition metals. For this reason a search for individual organometallic compounds that would possess catalytic activity in olefin polymerization is of great interest. The first attempts to use organometallic compounds of transition metals as catalysts for olefin polymerization were made long ago [e.g. CH3TiC13as a catalyst for polymerization of ethylene (11671. However, only in recent years as a result of the application of relatively stable organometallic compounds of transition
ONE-COMPONENT POLYMERIZATION CATALYSTS
185
metals (mainly *-ally1 and benzyl complexes) have some experimental data been accumulated in this field.
1. General Data on Ethylene Polymerization in the Presence of Individual Organometallic Compounds
a. a-Organometallic Compounds. Ethylene polymerization in solutions of benzyl compounds of Ti, Zr, Hf, and Th has been reported (9a, 117-120). The maximum polymerization rate was observed when using zirconium benzyl complexes (9a), though even in this case the activity was very low [the polymerization rate did not exceed 1.0 g CzH4/mmoleZr hr atm at 80°C (118)]. The introduction of substituents into the aromatic nucleus of the ligand may alter the activity of benzyl compounds (117, 118).The activity of halogen-containing compounds increases in the series Ti (CHZPh)4 < Ti ( CH2Ph3)I < Ti (CH2Ph) 3Br < Ti ( CH2Ph) &1 (117). The increase of the activity of titanium and zirconium benzyl complexes on the addition of A1(CHIPh) 3 was noted; the rise of the polymerization rate as a result of ultraviolet irradiation of zirconium benzyl compounds was reported (117).In the presence of zirconium benzyl compounds with very low rates polypropylene and poly-Cmethypentene were obtained (9a, 117) ; their fractions, insoluble in methanol, were partly crystalline. The ethylene polymerization was observed (9a) also in the presence of a-organometallic compounds of titanium and zirconium, containing such ligands as -CH2Si ( CH3)3, -CH (CsH6)Si (CH,) 3, -CH& (CH3) 3, -CH2SiCH3, and -CH20CHs. Tetrakis-a-bis-cycloheptylchromium (121) was found to be a one-component catalyst for ethylene polymerization after activation by exposure to light. b. r-Allylcompounds. The polymerization of ethylene to linear polyethylene in the presence of tris-a-allylchromium was mentioned by Wilke et al. (122). The ethylene polymerization also occurs in the presence of *-ally1 compounds of Zr, Ti, Hf, V, and Nb (gal 123, 124). a-Ally1 compounds of zirconium are most active, though in this case the polymerization rate is also low (2.0 g C2H4/mmoleZr-hr-atm at 8OoCin the presence of Zr (7r-C3H6)4). The substitution of some.of the ally1 ligands for halogens influences the polymerization rate and molecular weight of the polymer (123) (the rate increases in the case of zirconium compounds and decreases in the case of chromium compounds). The 2-methallyl compounds of Cr and Zr are more active than the a-ally1 ones (123) . According to Demin et al. (126, 126) the steady-state polymerization of ethylene occurs at 5-70°C in the presence of Cr(a-C3H6)3 and Zr (a-C3H6)4. In Ballard et al. (123) the induction period at ethylene polymerization using Zr (7r-C3H6)4 was observed; the introduction of hydrogen
186
YU. YERMAKOV AND V. ZAKHAROV
into the reaction system resulted in polymerization with no induction period. The steady-state polymerization in the presence of Cr (‘lr-C3H6)swas first order with respect to the monomer concentration (195); the effective activation energy was 4.7 f 0.5 kcal/mole. When the concentration of Cr (‘lr-CaH6)3 was varied, first a linear rise of the polymerization rate occurred with an increase of tris-?r-allylchromiumconcentration to the upper limit; then the rate does not depend on Cr(z-CaH6)3 concentration (126), The value of the upper limit of the polymerization rate increased with an increase in the water content of the solvent used. c. Arene and Cyclopentadienyl Compounds. Ethylene polymerization in the presence of chromate and dichromate of arene derivatives of chromium [(C,&Cr) 2cr04, (C&) zCr20~]at 60-100°C and pressure up to 40 kg/ cm* has been reported (127-1.29). Under similar conditions the products of oxidation of Cr (?r-CaHs)2 and Cr (?r-CbH6) showed some catalytic activity (130) for ethylene polymerization. But the attempts to initiate the polymerization of ethylene with homogeneous solutions of chromocene are reported to be unsuccessful ( 9 a ) . 2. The Problem of Active Center Formation during Polymerization in the Presence of Individual Organometallic Compounds In Ballard et al. (gal 118, 1.23) the formation of the propagation centers from benzyl and ally1 compounds of transition metals were supposed to be a result of the insertion of the monomer into the bond between the organic ligand and metal in the initial compound according to the following schemes:
-+
LnM-CHz-CHa-CHz-CH=CHz
HzC=CHz LnM-CHz-Ph
+ CHz=CHz
1
LnM-CHaPh
LnM-CHpCHz-CHzPh
( 16)
Here M is the transition metal and L, are other ligands of the initial organometallic compounds. In this case individual organometallic compounds are considered to be true catalysts, and the question of the dependence of the polymerization rate on the character of metal-ligand bonds in the initial organometallic compounds is discussed (123). However, initial organometallic compounds may be reagents in a more
ONE-COMPONENT POLYMERIZATION CATALYSTS
187
complicated process of the formation of the true catalyst. The latter may be formed by the reaction of organometallic compounds not with a monomer but with other compounds of the reaction medium. For the case of n-ally1 compounds of chromium and zirconium some data were obtained (126, 131) showing that solid insoluble products of decomposition of the initial complexes by water (that is always present in the reaction mixture in ppm quantities) are true catalysts. For example, the addition of 0.5 mole of HzO to a mole of Zr (r-C3H5)4 resulted in a tenfold increase of the polymerization rate; the precipitate formed in the reaction of Zr (n-C3H6) 4 with HzO was active, while the solution remaining after filtration was inactive (131). The addition of alcohol (CH30H)or organic acid ( C13COOH) as well as Lewis acids (AlBr3, TiC14) also resulted in the increase of the polymerization rate in the presence of n-ally1 compounds (126). The low rate of ethylene polymerization in solutions of r-ally1 compounds seems to be a consequence of the fact that: (a) only part of the initial compound is decomposed, and (b) only a small part of the decomposition products serves as active centers. The low activity of benzyl compounds in ethylene polymerization having been taken into account, in Giannini et al. (117) an opinion is expressed that the organometallic compound is “only a precursor of the true catalyst.” In this case, the individual organometallic compounds used so far are not the models of the active centers of polymerization catalysts, and the low polymerization activity of individual organometallic compounds of transition metals is likely to be determined by the low yield of the catalytically active species in its reaction with the polymerization medium.
FORMED BY INTERACTION OF ORGANOMETALLIC COMPOUNDS B. CATALYSTS OF TRANSITION METALSWITH OXIDESUPPORTS Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium (gal 132) , ?r-ally1 compounds of various transition metals (8, 9a, IS$), r-arene (134, 135) and n-cyclopentadienyl (9, 136) complexes of chromium]. 1. Catalytic Activity of Supported Organmetallic Compounds in Ethylene
Polymerization The activities of some supported catalysts formed by using different organometallic compounds are given in Table I. In all the cases the activity referred to one millimole of the transition metal increased sharply
188
YU. YERMAKOV AND V. ZAKHAROV
TABLE I
Examples of the Activity of the Catalyst Formed by the Reaction of Transition Metal Organometallic Compounds with Oxide Supports during Ethylene Polymerization
Organometallic compound
PolyNormalized polymerization merization activity * temperature (g CtH4/mmole ("C) Mshr aatm)
Support"
80 80 80 75
80 ')
80 80 90 60
References
390c 89 142e 57 58 600 25 18 24OC
132
132 13.2
133 132 13.2 13.2
136 9
The dehydration temperature of the support is given in parent,heses. M-transition metal. c In the presence of hydrogen. The support was dehydrated by boiling in toluene. b
as compared to the activity observed while using solutions of the same compounds. In ethylene polymerization by supported organometallic compounds linear polyethylene with high molecular weight is obtained; this polymer has mainly double bonds of the vinyl type. Some data reported on polymerization with catalysts obtained using different organometallic compounds may now be given. a. a-Organometallic Compounds. Zirconium compounds containing catalysts formed by supporting benzyl compounds are more active than titanium one (132). The use of halogen-containing benzyl compounds results in a decrease of the molecular weight of polyethylene formed. Highly active supported catalysts were also obtained by supporting ZrCCHi3i( CH3)3 1 4 and Z ~ C C H Z O C Hon ~ ] A1203 ~ (9a). b. T-Allyl Compounds. To prepare supported catalysts for ethylene polymerization 7-ally1 compounds of Cr, Zr, and Hf were used (8,9u,133). The activity of catalysts is dependent on the temperature of preliminary calcination of the support that determines the concentration and type of the surface hydroxyl groups. For the catalyst Cr(r-C3H6)3 SiOz the highest activity was observed (137) when silica was calcined a t 400-500"C, for the catalyst Zr (7-C3Hs)3 SiOz, a t 100-200°C. The temperature of
+
+
ONE-COMPONENT
POLYMERIZATION CATALYSTS
189
the catalyst after the reaction of Cr(r-CsHs)a with SiOz influenced the catalytic activity: when the heating temperature did not exceed 30°C catalysts with high initial but not stationary activity were obtained; catalysts heated a t 100°C showed lower but steady-state activity; heating at 250°C deactivated the catalysts (8). The rate of polymerization by the stationary catalyst "Cr (C3Hs)3 4-SiOz" was of the first order with respect to the monomer concentration; the effective activation energy was 5 f 0.5 kcal/mole (137). c. A rene Compounds. The supporting of r-mesitylenetricarbonyl- or rbenzenetricarbonylchromium and bis-benzenchromium on alumosilicate or silica resulted in the formation of a highly active catalyst for ethylene polymerization (134, 135). The decrease of the dehydration temperature of the support from 600" to 400" diminished the activity of the catalyst.
d. Cyclopentadienyl Compounds. The activity of the catalyst obtained by the reaction of Cr(r-CsHs)zwith silica increased with an increase of the dehydration temperature of the support up to 700°C (9). The first order of the polymerization rate with respect to the monomer concentration was observed; on varying the polymerization temperature the maximum activity was found to occur a t 60°C. It was shown that the catalyst had high reactivity in a chain transfer with hydrogen. This effect is thought (9) to be connected with the presence of the cyclopentadienyl ligand in the coordination sphere of the chromium ion of the active center. 2. The Reaction of Organometallic Compounds with the Support as a Stage of Propagation Center Formation
One of the necessary conditions for obtaining active catalysts by the reaction of an organometallic compound with the support seems to be the formation of surface compounds in which the transition metal is bound of the support (M' is the cation of with the fragment [(0),,-M'-0-1 the carrier). Up to now only limited investigations have been made of the reaction of organometallic compounds of transition elements with the surface of the oxides. In the reaction of cyclopentadienyl ( 9 ) , ally1 (138,139) and benzyl (13.2) compounds with oxides, cyclopentadiene, propylene, and toluene evolved. The stoichiometry of the reaction depends on the type of organometallic compounds and temperature of dehydration of the support. While supporting Cr (r-metallyl) s, Hf (r-allyl) 4, and Zr (r-allyl) 4 on silica dehydrated at 200"C, two propylene molecules per molecule of organometallic compound evolved (9a, 133). At a calcination temperature of SiOz of 40O0C, 1.5 mole of propylene was evolved per 1 mole of Cr(7rC ~ H S(137). ) ~ The reaction of N ~ ( T - C ~ H with ~ ) SiOz ~ gave about 0.9 or
190
YU. YERMAKOV AND V. ZAKHAROV
0.5 mole of propylene per mole of nickel compound at a dehydration temperature of silica of 300" and 700"C, respectively (139).
The following reactions with the surface of silica seem to be possible depending on the conditions of the support pretreatment and type of organometallic compounds: 3 Si-OH + MR, Si-O-M-Rz-I + RH (17)
+>
3 Si-0
5 Si-OH
+ MR. -,
bRz-2
3 S i - O/
3 Si-OH Si
(18)
0
OH
/
+ 2RH
+ MR,
---f
\MRz-2
Si'
+ 2RH
(19)
\O/
O 'H
The participation of siloxane groups in the reaction increases with the temperature of dehydration of Si02 and quantity of organometallic compound introduced in the reaction. According to the data of infrared spectroscopy (139), the allyl ligands formed in the surface organometallic complexes of Ni and Cr keep the r-ally1 character of the metal-ligand bond. The availability of organic ligands in the surface complexes obtained by the reaction of organometallic compounds with supports allows us to consider two possibilities of the initiation process: (1) the monomer insertion into the organic ligand-metal bond (bensyl, allyl, etc.) (9a) : Si+M(L,)R
+ CH2=CHs +>Si--O-M(L),,--CH2-
(21)
--CH*R
(here R is the organic ligand component of the initial organometallic compound of the transition metal; (L), are other ligands), and (2) the alkylation of a low-valent ion of the transition metal by a monomer with the formation of a new a-bond metal-carbon (the possibility of such a process is considered in Section V) . Some data allow the realization of the second possibility to be proposed (8, 140) a t least in the case of catalysts formed with the use of r-allylic ) the Si02 compounds of chromium. In the reaction of Cr ( T - C ~ H2 ~with surface, complexes are formed that may have vacant coordination sites
ONE-COMPONENT POLYMERIZATION CATALYSTS
191
due to the removal of the bidentate allylic ligand from the chromium atom. The treatment of the catalyst obtained with hydrogen at ‘200°C results in a complete removal of the allyl groups from the catalyst surface. However, the catalyst treated with hydrogen still shows high activity in ethylene polymerization. Thus, the presence of allyl groups in the catalyst of this type is not obligatory for the formation of the propagation centers. In the case of the catalyst “chromocene SiOz” Karol et al. (9) do not favor the interpretation of the initiation reaction as the insertion of a monomer into the Cr-CsHS bond either.
+
3. Application of the Supported Catalyst Obtained with the Use of Organometallic Compounds of Transition Metals for Nonpolymerization Reactions The catalysts formed by the support of organometallic compounds of transition elements are also of great interest for nonpolymerization reactions. Generally speaking, these catalysts can be used in three various states: (a) in the initial state, (b) after reduction, and (c) after oxidation (141). The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such catalysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of n-ally1 compounds with SiOz and A1203 were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial r-ally1 compounds. r-Ally1 complexes of Mo and W supported on silica were found to be active in olefin disproportionation (l@a). Some data have been obtained on the activity of the catalyst in a reduced state [for nickel (141, 143, 1&), palladium (1&a), and molybdenum (145, 145a)3. In the case of nickel catalysts the formation of nickel in the zero oxidation state takes place during the reduction of the surface organometallic compound by Hz. The infrared spectrum shows the total restoration of the concentration of Si-OH groups (139),so the reduction proceeds according to the scheme:
>
3 Si-O-NiC3Hs
+ H2+> Si-OH + Ni” + CsHe
122)
The nickel supported catalysts formed in this way have some specific features (144). The catalysts containing about 3% of Ni are paramagnetic. When varying the nickel content from 0.1 to 20%, all the nickel the reduced catalyst (the exposed surface area of nickel was about 600 m2/g Ni) is oxidized by oxygen. The activity in benzene hydrogenation is very high and increases in proportional to the nickel content in the catalyst. The reduction of the surface organometallic complexes of molybdenum
192
YU. YERMAKOV AND V. ZAKHAROV
results in the formation of Mo2+ions according to the scheme (146) :
> Si-O
3 Si-O ko(CaH&
3 Si-O
/
+ HZ
~
---t
Si-0
/
I
+ 2CsHs J
(23)
The catalysts thus prepared were active in the hydrogenation of ethylene and nitrogen (146u). Catalysts in an oxidized state showed high activity in the oxidation of carbon monoxide [nickel catalysts (146)] and hydrogen [molybdenum catalysts (146u)]. The data obtained up to the present time show that the method of catalyst preparation by the reaction of organometallic compounds with surface reactive groups may be applied to generate both isolated ions of transition metals (in various valent states) or superfine metal particles on the surface of the support.
IV. Subhalides of Transition Metals Among one-component polymerization catalysts subhalides of the transition metals are most similar in composition to the traditional ZieglerNatta catalysts. In this connection, the study of the simplest one-component catalyst of this type (especially TiCL) is of great importance for the clarification of still disputable problems of the mechanism of polymerizat ion by t wo-component cata1yst s.
A. PREPARATION OF SUBHALIDES AS ONE-COMPONENT POLYMERIZATION CATALYSTS The formation of high polymers of olefins in the presence of titanium halogenides with no specially added organometallic co-catalysts was discovered long ago [see (14‘7), and the references therein]. A complete description of various “alkyl-free” polymerization catalysts based on the use of transition metal chlorides may be found in the review by Boor ( 1 7 ) , where a comparison of these catalysts with traditional two-component systems is given. High molecular weight linear polyethylene was obtained from ethylene polymerization by Tic12 (148); this polymer contained only vinyl-type double bonds (less than 0.1 per 1000 carbon atoms). Polypropylene formed on Tic12 contained 25-30% of the isotactic fraction insoluble in hot heptane (148). In most cases one-component catalysts based on halogenides of transition metals were obtained with the help of various specific activation pro-
ONE-COMPONENT POLYMERIZATION CATALYSTS
193
cedures, the most common being: 1. Milling in a ball-mill (10, 149). In the activation of TiClz by ballmilling the average oxidation number of the titanium ions was changed; however there was no dependence evident between the catalytic activity and the content of Ti (11) in the catalyst; the proportional dependence of the activity on the specific surface was not observed either (10). 2. y-Irradiation (160, 151). When nonmilled samples of TiClz were y-irradiated an increase in activity was found ;y-irradiation of the samples of Tic13 resulted in the same activity as in case of the addition of AlEtZCI (151).
3. The activation by various nonorganometallic additives (14.9, 169-166) (metals, alkylhalogenides, diazocompounds, halogenides of transition and base metals, donor-type compounds). Recently samples of TiC12, active in polymerization without additional activation, were prepared (156-159). The activity of TiClz in ethylene polymerization was practically the same as the activity of a conventional two-component system TiC13 AlEttCl (see Fig. 3). The polymerization activity of TiClz depends to a large extent on the parameters tempera-
+
Polymerization time (min)
FIQ.3. Comparison of the rates of polymerization by one-component and two-component catalysts (75"C, ethylene pressure 5.5 kg/cmz, polymerization in hydrocarbon solvent). Curve 1-TiCh (specific surface 24 m2/g). Curve 2-TiCle AlEtrCl (A1:Ti = 4, specific surface of TiClo20 mz/g; the same sample of TiCla was used for the preparation of TiClz). Curve 3-TiCl1 AlEtZCl (A1:Ti = 4).
+
+
194
YU. YERMAKOV AND V. ZAKHAROV
ture, duration of the disproportionation of titanium trichloride when titanium dichloride was prepared (157); a temperature of 500-600°C was optimal. The specific surface and activity of Tic12 prepared from premilled samples of TiCL are greater than in the case of TiC12obtained from nonmilled samples, but no proportional dependence was found. ZrC12 prepared by disproportionation of ZrCls without ball-milling was also active (about 80 g C2H4/g ZrClzahr at 20 kg/cm2 and 75°C) in ethylene polymerization ( 157, 159). B. DATAON THE KINETICSOF POLYMERIZATION In Benning et al. (148) some data on the kinetics of ethylene polymerization in the presence of TiC12 activated by ball-milling are given. Polymerization was studied at 140-260°C (the solution process in cyclohexane) . The first orders of the polymerization rate on the monomer and catalyst concentrations have been established. The polymerization decreased with temperature; a sharp drop in rates at about 180-2OO0C was observed. The kinetics of ethylene polymerization at temperatures below 90°C (the slurry process) were studied in Bukatov et al. (157, 159). The steadystate polymerization rate was observed; the first order in the polymerization rate with respect to ethylene and the catalyst concentration was found. The polymerization rate increased on increasing the polymerization temperature from 20” to 80°C (E,rr = 7.5 f 0.5 kcal/mole). The addition of aluminumorganic compounds (AIEt3, AlEt2C1) during ethylene polymerization in the presence of highly active samples of TiClz results in a decrease of the catalyst activity (166) (see Fig. 3 ) . The same effect is observed when TiC12 is treated by aluminumorganic compounds before polymerization. But the treatment of low-activity samples of Tic12 may lead to an increase of the polymerization rate as a result of the reduction of the Ticla admixture in the samples, as well as due to the scavenging effect. In propylene polymerization by TiC12the addition of aluminumorganic compounds results in a fall of the polymerization rate and a concurrent increase of the isotactic fraction content in the polymer (158).A similar effect occurred when triphenylphosphine was added to TiC12. The content of the isotactic fraction decreased in the series AlEt3 > AlEtzCl > AlEtC12. The catalytic activity also decreases in the same row (159).
V. Determination of the Number of Propagation Centers The concept of an “active center” in catalytic polymerization is much more meaningful as compared with that referred to catalytic processes in
ONE-COMPONENT POLYMERIZATION CATALYSTS
195
which the reaction products are low molecular weight substances. The active polymerization center in a “working” state is bound to a growing molecule of the polymer, so the determination of the number of active centers implies measuring the number of growing polymer chains. According to the data on the number of propagation centers ( N ) the propagation rate constant (K,) is calculated:
K , = V/“MI, (24) where V is the polymerization rate and [MI is the monomer concentration. The value of K , as a measure of the reactivity of the active centers in the propagation reaction is the most fundamental characteristic of polymerization catalysts. The conclusions on the polymerization mechanism based on the correct values of N and K , are much more unambiguous than those made when considering only the data on the polymerization activity and molecular weight of a polymer. A. METHODSUSEDFOR
THE DETERMINATION OF THE NUMBER OF PROPAGATION CENTERS
To determine the number of propagation centers in one-component catalysts, in principle the same methods used t o study two-component catalysts of olefin polymerization may be applied [see (18, 160, ISOcr)]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are : 1. Kinetic methods based on the analysis of the dependencies between the polymerization rate and polymerization degree and the time and reaction parameters. 2. Inhibition methods where the number of propagation centers is calculated by considering the quantity of the inhibitor added and the resulting decrease of the polymerization rate.
3. Methods based on tagging the growing polymer molecules. The kinetic and inhibition methods have considerable limitations when applied t o both two-component and one-component catalysts (ISUa). The most reliable results can be obtained by the methods based on tagging the growing polymer chains. Therefore, it is advisable to note some details of this method. In studying two-component polymerization catalysts, beginning with Feldman and Perry (161),a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of “the anionic coordination” mechanism of olefin polymerization occurring
196
YU. YERMAKOV AND V. ZAKHAROV
as a monomer insertion into the polarized M+-C- bond. With the interaction of the active metal-carbon bond with alcohol the following reaction should occur. LnM+ * CHzP ROH* -+ LnMOR + H*-CHzP (25)
.
+
However, in olefin polymerization by two-component catalysts during polymerization not only active transition metal-polymer bonds are formed, but also inactive aluminum-polymer ones, as a result of the transfer process with the participation of a co-catalyst ( 1 1 , 162-164). The aluminumpolymer bonds are quenched by tritiated alcohol according to the scheme (25), so an additional tagging of the polymer occurs. The use of iodine (165, 166) as a quenching agent also results in decomposing inactive metal-polymer bonds. To obtain the correct data on the number of propagation centers in a two-component catalyst “specific” quenching agents should be used, reacting only with the active transition metal-polymer bonds and not reacting with inactive aluminum-polymer ones under polymerization conditions. For the study of olefin polymerization by catalytic systems based on titanium chlorides C140, C1402, S3202 can be used as “specific” quenching agents (157-159). The use of these inhibitors showed that in the case of polymerization by two-component systems the number of propagation centers is two orders less than the number of metal-polymer bonds measured by quenching polymerization with tritium-labeled alcohol (167, 158). In polymerization by one-component catalysts all the metal-polymer bonds are active, and the determination of the number of propagation centers consists of measuring the number of these bonds. However, it is necessary to take into account the possible change of the mechanism of the reaction between quenching agents and propagation centers while changing the catalyst composition. Therefore, to obtain the quantitative data on N and K , values the particular technique for each catalytic system should be elaborated. The elaboration of the method for the quantitative determination of the number of propagation centers includes the following main steps: 1 . The choice of a labeled compound, able to react with the active transition metal-carbon bond. This compound should have an inhibiting effect strong enough to result in completely stopping polymerization on its addition in a quantity comparable with that of the transition metal compound in the polymerization system.
2. The control of the presence of a label in the polymer after quenching polymerization by the agent chosen. 3. The verijication of the possibility of quantitative measurement of the
ONE-COMPONENT POLYMERIZATION CATALYSTS
197
number of propagation centers. To obtain quantitative data the quenching agent should react with all the growing polymer molecules through the same route. Besides, there should be no side reactions of the quenching agent with the catalyst resulting in radioactive contamination of the polymer. For such verification the determination of the number of propagation centers when varying the parameters that should not influence the propagation rate constant can be made. Particularly, the independence of the value of K , of the absolute polymerization rate (of the concentration of the propagation centers) a t the constant catalyst composition, of the monomer concentration, of the quantity of the quenching agent used, and the type of the quenching agent proves that the number of active bonds is determined quantitatively.
B. NUMBEROF PROPAGATION CENTERSIN ONE-COMPONENT CATALYSTS 1. The Chromium-Oxide Catalysts
It is evident [see Eq. ( 5 ) , Section 111that for catalysts of the same or similar composition the number of active centers determined must be consistent with the catalytic activity; it can be expected that only in the case of highly active supported catalysts a considerable part of the surface transition metal ions will act as propagation centers. However, the results published by different authors for chromium oxide catalysts are hardly comparable, as the polymerization parameters as a rule were very different, and the absolute polymerization rate was not reported. The data on the determination of the number of the propagation centers on chromium oxide catalysts by the inhibition method were given in several papers; water ( G I ) , carbon tetrachloride (167), and diethylamine (69)were used as inhibitors. It was found that the number of propagation centers is about 10% (GI),1% (167),and 20% (69)of the total content of chromium in the catalyst. In Hogan (69) it was supposed that in a highly active catalyst containing 0.01% of chromium all the chromium ions act as active centers. According to this it was calculated that in the catalyst containing 1% of chromium on silica the number of propagation centers reached 10% of the supported chromium, Several determinations of the number of propagation centers by the quenching technique have been carried out (98,112). As a quenching agent methanol, labeled C14in the alkoxyl group, proved to be suitable in this case. The number of active centers determined by this technique at relatively low polymerization rates (up to 5 X lo2g C2HJmmole Cr-hr in catalysts on silica was about at 75" and about 16 kg/cm2) (98,111,168)
198
YU. YERMAKOV AND
V.
ZAKHAROV
0.2% of the total chromium content. This result is consistent with the data obtained in Hogan (69) taking into account the difference in the catalyst activity. The number of active centers determined by the quenching technique was dependent on the polymerization temperature (98); that was the reason for the difference between the overall activation energy and the activation energy of the propagation step. The reduction of chromium oxide catalysts by agents containing no hydrogen (SOZ, CO) resulted in an increase in the number of propagation centers (97, 168). When the reducing agents (Hz, NH3, CzH4, HCN) responsible for the formation of water during their oxidation were used the number of propagation centers decreased. When the water formed was removed from the system (e.g. reduction in the flow of Hz or NH3 a t low pressure) the decrease in the number of propagation centers was less pronounced. The reactivity of the propagation centers in oxide polymerization catalysts depended on the nature of the transition metal, support, activation temperature of the catalyst, and type of reducing agent (168a). 2. Catalyst Formed by the Reaction of Tris-?r-Allyl Chromium with Silica
The number of propagation centers in ethylene polymerization by the catalyst Cr (7r-C3Hs) 3 SiOzwas found by the quenching technique (140); CHsOT and CI4Owere used for tagging growing polymer molecules. At a catalytic activity of about 1 X lo2g CzH4/mmole Cr-hr (5OoC,6 kg/cm2) the number of propagation centers was about 0.2% of the supported chromium. When the polymerization rate increased with time the proportional increase of the number of propagation centers was observed (see Fig. 4).
+
3. Titanium Dichloride The number of propagation centers in the polymerization of ethylene and propylene by TiCL was determined using radioactive quenching agents (157-159,169). When stopping polymerization by different quenching agents-propyl iodide labeled by C14,S360z, Cl4O2,CI4O,CH30H3-the coincident results were obtained. As the quenching agents used actually react with the propagation centers following different mechanisms, these results can support the quantitative character of the determinations performed. The stationary concentration of the propagation centers reached a maximole per mole of TiC12;in this way less mum value of about 1.5 X than 0.5% of the number of titanium ions adjacent to the surface take part in the formation of the propagation centers (for a sample of TiClz
ONE-COMPONENT POLYMERIZATION CATALYSTS
199
W
+
e
30
0
10
20
30 40 50 60 Polymerization time (min)
70
80
FIQ.4. Change of polymerization rate and number of propagation centers with polymerization time. Catalyst Cr (7r-C3H&/SiO2; ethylene pressure 6 kg/cm*, temperature 50°C. The symbols A,A, X, 0 correspond to different polymerization runs. Arrows show the moment of injection of Cl4O.
with the specific surface area of 25 m2,'g). The concentration of the propagation centers in propylene polymerization is still lower (about 0.5 X mole per mole of TiCL) . A proportional rise of the ethylene polymerization rate with the number of propagation centers was observed (see Fig. 5 ) . The propagation rate constant did not depend on the monomer concentration which corresponds to the first-order propagation step. The activation energy of the propagation calculated according t o the variation of K,, with temperature was found to be 6.5 f 0.5 kcal/mole. By quenching the polymerization with C1*02or C"O the determination of the number of propagation rate constants was found to be also possible for the two-component catalytic system TiClz AlEtzCl (158, 159). In contrast t o alcohols, carbon dioxide and carbon monoxide under polymerization conditions react only with titanium-carbon active bonds and do not react with inactive aluminum-polymer bonds. It was found (158,159)that the fall of the rate observed when aluminumorganic compound was added to TiClz during ethylene polymerization was due to the decrease in the number of propagation centers. The propagation rate constant remained unchanged. I n propylene polymerization the number of atactic propagation centers sharply diminished when the aluminum-
+
200
YU. YERMAKOV AND V. ZAKHAROV
20
10
40
30
60
Polymerization lime (min)
FIQ.5. Change of polymerization rate and number of the propagation centers with polymerization time. Catalyst TiC12; ethylene pressure 6 kg/cm*, temperature 60°C. The symbols A,0, f, 0 correspond to different polymerizationruns. Arrows show the moment of injection of C W 2 .
organic compound was added, while the decrease in the number of isotactic ones was not so large; that resulted in an increase in the stereoregularity of polypropylene. The propagation rate constant (both for the atactic and isotactic additions) did not change when the aluminum-organic compound was introduced into the polymerization system (158). The conclusion may be drawn that the data obtained of comparative studies of olefin polymerization by the one-component catalyst (TiCln) and two-component systems (Tic& AlEt,Cl,) confirm the concept of “monometallic” active centers on the surface of titanium chlorides developed by Cossee and Arlman (170-1 73).
+
4. Number of Propagation Centers at Maximum Activity of Catalysts
For technical reasons a quantitative experimental determination of the number of propagation centers by the quenching technique is difficult when catalysts with high activity (more than 500 g CzH4/g catalyst-hr under determination conditions) are used. But the number of propagation centers corresponding to the maximum activity ( A m a xcan ) be calculated using the value of the propagation rate constant :
N,,,
=
A,,/K,[M
1.
Here [ M I is the monomer concentration a t which A,,, was found. Table I1 represents the data on the reactivity of the propagation centers and their number corresponding to the maximum catalytic activity observed for three typical one-component catalysts. These data were ob-
20 1
ONE-COMPONENT POLYMERIZATION CATALYSTS
tained under similar conditions in the authors’ laboratory. For comparison, the activity of the two-component supported catalyst containing fine crystals of TiCI, on silica is shown. In the cases of CrOt/SiOz and Cr (‘lr-CsH6)3/Si02 systems a considerable part of the chromium contained in the catalyst is involved in the propagation center formation. In these catalysts all the ions of the transition metals are on the surface and the active component seems to be the main type of compounds present on the catalyst surface. In the case of TiClz the number of propagation centers do not exceed 0.5% of the number of surface titanium ions; this shows that the formation of the propagation centers proceeds at specific points on the surface of the crystalline catalyst (e.g. lateral faces, outlets of the spiral dislocations). The number of propagation centers in catalysts based on the transition metal chlorides may be raised by decreasing the size of the crystalline particles of the catalyst as a result of its being supported on the carrier. The supported catalysts containing a microphase of Tic13 show high catalytic activity (see the data of Table I1 for the system TiCla/SiOz A1Et3). At present there is no information concerning the number of propagation centers in systems of this type. However, if the value of K , for these catalysts is supposed to be close to the value of K , for TiCL [for propylene polymerization the equality of the propagation rate constants for the systems based on TiCL and Tic13 has just been established (l?‘4)] then in supported catalysts a part of the propagation centers with respect to the total content of the transition metal is still low (about 1%).
+
TABLE I1 Maximum Activity Observed for Ethylene Polymerization by Diflerent Catalysts and the Corresponding Number of Propagation Centers
Catalyst CrOa/SiOz, reduced by CO Cr (s-CsH&/SiOt Ticla TiCla/SiOz AlEts
+
Number of Normalized activity propagation KP (g C2Hd/mmole centers (mole/ (liter/mole sec). M .hr .atm)b mole M, X 100) 1.7 x 1 0 3 2 . 8 x 103 12 x 10s
-
3.4 x 103 2 . 3 X 10s 0.012 x 103 0.80 x 103
30 12 0.015 1. o c
Experimental data measured by the radio tracer quenching technique at 75’C. metal. Calculated on the assumption that K , is equal to that found for polymerization by TiCL.
* M-transition
202
YU. YERMAKOV AND
V.
ZAKHAROV
Should all the ions of the transition metal serve as propagation centers the upper limit of the theoretical activity of the catalyst of the given type may be reached. In the case of supported chromium catalysts Au.l. may be three to ten times higher than Au.i. obtained in the experiments the results of which are given in Table 11. But in the case of titanium chloride catalysts the catalysts with activity about 100 times higher might be obtained due to the total use of the titanium ions as propagation centers. However, the possibility of preparing a supported catalyst with isolated surface titanium ions with the same high reactivity of the propagation centers as in the phase of titanium chlorides is not evident.
VI. Some General Features of Propagation Centers in One-Component Polymerization Catalysts
Despite the difference in composition of various olefin polymerization Catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional ZieglerNatta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. The specific behavior of surface compounds, being the propagation centers of polymerization catalysts, are mainly determined by two of their features: the coordinative insufficiency of the transition metal ion and the presence of the transition metal-carbon bond.
A. COORDINATIVE INSUFFICIENCY OF TRANSITION METALIONS IN ACTIVE CENTERS 1. The Formation of Ions with Vacant Coordination Sites
The experimental evidence for the availability of the coordinative insufficiency of the transition metal ion in the propagation centers was obtained (i75)in the study of the deactivation of the propagation centers by “coordination” inhibitors. On the introduction of such inhibitors as phosphine and carbon monoxide into the polymerization medium, the reaction stops, but the metal-polymer bond is retained. It shows that in this case the interaction of the inhibitor with the propagation center follows the scheme:
x
LnM-CH&HzP
f X
1
LnMCHzCHzP
(26) So, the deactivation by such “coordination” inhibitors results from the --*
ONE-COMPONENT POLYMERIZATION CATALYSTS
203
formation of their stable complexes with the transition metal ion of the propagation center. The formation of complexes of various compounds (H20, alcohols, CO, NO, NO2, etc.) on their interaction with coordinatively insufficient ions of chromium in chromium oxide catalysts was observed by Matsumoto et al. (56), Kazanski ( l o b ) , Hogan (69),Krauss (70), and Eley et al. ( 7.2, 176). Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate a-complexes. The second reason is not so evident; for example, the data were obtained for a soluble catalytic system based on biscyclopentandienyltitaniumdichloride (178, 179) for which case the propagation center formed contains a Ti(1V) ion. At present a rather large number of compounds of transition metals with the metal-carbon a-bond is known (180, 181), where the metal is in various oxidation states and in different ligand environments, and the metal-carbon bond seems to have various strengths. At the same time individual organometallic compounds with high activity in olefin polymerization have not yet been found; in known cases of polymerization in the presence of solutions of individual organometallic compounds, the latter are most probably only the initial substances in a series of interactions resulting in the formation of the true propagation centers. It may be assumed that this results from the difficulty of obtaining and stabilizing organometallic complexes in which the transition metal ion has coordinative insufficiency and ability to form intermediate a-complexes with olefin. Three main possibilities for obtaining and stabilizing such complexes of transition metals may be singled out: 1. The formation of surface defects of a crystal lattice. It was observed while using crystal compounds of transition metals as catalysts [e.g. as was shown by Arlman (171, 173), for a Ticla surface defects appear on the lateral faces of the crystal]. In this case low surface concentration of the propagation centers should be expected, as is illustrated in the case of polymerization by titanium dichloride (158). The observed
204
YU. YERMAKOV AND V. ZAKHAROV
effects of the rise of catalytic activity at ball-milling and 7-irradiation seem to be caused by the increase in the number of defects on the crystal surface. 2. The formation of surface compounds of low-valent ions of transition metals on the surface of the support. In particular, fixing organometallic compounds on the support surface, it may be possible to stabilize coordinatively insufficient complexes of transition metals and to obtain highly active catalysts. In the ideal case a complete use of the transition metal in the formation of the propagation centers can be achieved. 3. The dissociation of coordinatively sufficient organometallic complexes in solution. For instance, for the system based on cyclopentadienyl complexes of titanium the active centers of catalytic polymerization [(CsH6)zTiR]+ are caused by the following process (178,179): (CaH6)2TiRC1.AlRC12 F? [(CsH&TiR]+ + [A1RC13](27) In the case of soluble catalytic systems all the complexes of the transition metal available in the system can serve as a potential source of active centers, but the actual number of propagation centers determining the catalyst activity is small. Therefore, in the case of heterogeneous catalysts, especially of supported ones, higher (per unit of the transition metal) catalytic activity is achieved than in the case of homogeneous systems. A strong tendency for the association of coordinatively insufficient organometallic complexes of transition metals seems to be mainly responsible for the low number of propagation centers in homogeneous systems. 2. Alkylation of Low-Valent Ions in the Process of Formation of the Active
Bond The formation of the active metal-carbon bond as a result of the interaction of low-valent ions of the transition metal with olefin is the most intriguing step of the polymerization process by one-component catalysts. The possibility of the formation of the transition metal-carbon bond resulting from the reaction of titanium low-valent ions with ethylene is shown in Dzsabiev et a2. (182):
+ C 2 H l 4 LnTi-CH2CH2-TiLn
(28) Two ions of the transition metal take part in this reaction. However, in the case of supported one-component catalysts the formation of the active bond seems to occur on the interaction of the monomer with isolated ions of the transition metal. That may be illustrated by the data showing that the activity of chromium oxide catalysts decreases linearly with decreasing chromium content (or even increases per chromium ion) to the rather low (0.01%) chromium concentrations on the catalyst surface (62, 69). In 2L,Ti
ONE-COMPONENT POLYMERIZATION CATALYSTS
205
(167, 183) the following general scheme of the reactions that may result during the formation of the u-bond metal-carbon of the interaction of a lowvalent ion of the transition metal having high coordinative insufficiency with the monomer is given:
M
‘a
+
/ CaH4 + M \
H -M
CH=CHz
\
+ H. CH=CHz
\
(29)
CHz=CH.
The radicals formed in these reactions may also participate in the alkylation of ions of the transition metal:
However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diaaocompounds according to the schemes : L,M
+ RX + L,M
/
R
\x
(31)
206
YU. YERMAKOV AND V. ZAKHAROV
3. Coordination Mechanism of the Propagation Reaction
The activation of olefins through the formation of the 7r-complex with the transition metal ion at polymerization was postulated as one of the stages of the propagation reaction in many works, beginning with those of Ludlum et al. (184) and Carrick (185) :
0 L.h!G-P
CHz=CHa K1
+ C2Hl K-1
CHz=CHa KO
1
L,M-P
-
1
133)
Lo-M-P
CHI-CHz-P
(34)
L.dO
The theoretical problems of the coordination mechanism of the propagation reaction were considered by Cossee et al. (170,186, 187). It should be noted that, similarly to olefin, the insertion of carbon monoxide in the active bond in the propagation centers of polymerization catalysts also follows the coordination mechanism (175). The insertion of carbon monoxide into the active bond was not feasible when a vacant coordination site of the metal ion had been occupied by phosphine. With a two-stage mechanism the propagation rate constant is:
Kp
=
Ki.K2/(Kz
+ K-1 + K i [ M J ) ,
(35)
For all one-component catalysts the first order of the propagation rate on the monomer concentration is observed. It can be consistent with two cases : (a) The rate-determining stage is the *-complex formation:
Kz >> K-i
+ KICM],
Kp
=
Ki.
(b) A low equilibrium concentration of *-complexes exists on the surface :
K1>> Kz
+ KiCM],
K,
=
KaK2.
where K = K1/K-l is the equilibrium constant of the ?r-complexformation. The possibility of the experimental determination of K , advances the problem of the influence of ligands on the reactivity of the propagation centers. It may be expected that ligands increasing the stability of the metal-olefin bond will lead to an increase of the K , value irrespective of whether the case (a) or (b) is actually realized. Figure 6 represents the energy diagram of a two-stage reaction. The change of the ligand environment of the transition metal ion in the propagation center may cause an increase in the heat of *-complex formation by the value Ag, which results
ONE-COMPONENT POLYMERIZATION CATALYSTS
207
Reaction coordinate
FIG.6. The energy diagram of a two-stage propagation reaction proceeding through the formation of the r-complex intermediate. Solid line-vcomplex formation with energy pi; dashed lines-r-complex formation with energy pl'.
in a rise of the equilibrium constant K up to the value K':
K'/K
= &/RT.
The heat of each stage (91 for the ?r-complex formation and 92 for the monomer insertion) will change to the new values: 411 = qi
4-Aq,
92' = qz
- Aq.
(36)
If for every stage of the propagation reaction the linear relationship between the change of the activation energy and the change of the heat of reaction is valid [for the exothermic stage A E = -aAq (188, 189)] the change of q1 and 92 causes the activation energies of two stages to be changed in the following way:
El'
=
El
E;
=
E2
- GiAq,
(37)
+ (1 - az)Aq.
(38)
Thus, if the change of the ligand environment results in an increase of the ?r-complex stability, the rise of the propagation rate constant up to the value Kp'will be observed according to the following:
if K, = K1,
then Kp)/Kp =
eolAglRT,
if Kp = K . K 2 , then K,'/Kp = eaaAgIRT.
(39)
(40)
In the case of chromium oxide catalysts the increase of K , was observed,
208
V.
YU. YERMAKOV AND
ZAKHAROV
in particular, while increasing the electronegativity of a cation of the carrier (M’) in the propagation center having the following composition ( 168a) : CHZ=CHz M’-%M-C
1
With the increase in electronegativity of the element M’ the degree of covalence of the bonds M’-0 and M-0 should increase, as a result of which an increase in electron density on the ion M can be expected. As in the formation of the 7-bond with olefin the 7-backbonding mechanism plays a large role, that should result in an increase in the 7-complex stability. The interpretation of data on the change of K , as a result of the reduction treatment of the chromium oxide catalyst (97) is hindered by the absence of precise data on the composition of the surface complexes being formed.
B. ACTIVETRANSITION METAL-CARBON +BOND The insertion of a monomer into the transition metal-carbon a-bond as a propagation step is now a generally accepted concept. Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. Some experimental data on the lifetime of the active metal-polymer in one-component catalysts and the polarization of the active bond can be presented. 1. The Mean Lifetime of the Active Metal-Polymer Bond
The mean lifetime of the active metal-polymer bond in one-component catalysts is limited by the following transfer processes (69,?’6,169) : ,,H
H
LnMf6&-AH-P-LmM-0
K8
I
+ CHz=CH-P
(41)
transfer with monomer: H C-CHz*--H
‘1 LnM-cH2
CHz-CHa Kaa
AH--P-L,M-n
I
+ CH2=CH-P
(42) These transfer processes seem to be p-hydride shift reactions, typical of
209
ONE-COMPONENT POLYMERIZATION CATALYSTS
a-organometallic compounds of transition metals. So the mean lifetime (i) of the metal-polymer bond in polymerization is determined for kinetic reasons by the rates of the processes (41) and (42) (not taking into account the possible decomposition of the active bond caused by the inhibitor contaminations) : i =
~/(KM[M]
+ Ks).
(43) In polymerization by one-component catalysts [chromium oxide catalyst (75) , titanium dichloride ( 1 5 9 ) ] at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role a t higher temperatures and lower concentrations of the monomer. The mean lifetime of the metal-polymer can be evaluated by the values of the polymerization degree (P,) and K , : i =
(44)
Pw/K,[M]-+y.
Here y = pw/P,,the symbols w and n referring to the weight and number of degrees of polymerization. The results of such an evaluation are given in Table 111. According to these results in ethylene polymerization by one-component catalysts i does not exceed one minute. At the same time these catalysts are active for many hours, hundreds of polymer molecules being formed on one active center. The active transition metal-ca.rbon bond can also exist a t rather TABLE 111 Mean Time of Growth of Polymer Chain on Different One-Component Catalysts at 76°C and Monomer Concentration of 1 mole/liter
Catalyst
Intrinsic viscosity" of polyethylene (dl/g)
M,*
7y (set).
Cr03/SiOz,reduced by CO Cr(C3H&/SiOz TiClz
16.0 15.0 10.5
2.95 X lo4 2.8 X 10' 1.7 X 106
76 33 6
Intrinsic viscosity was measured in decaline at 135°C.
* Calculated according to Ref. (190): [17] = y =
2.55 x 10-4M:74
Mw/M,, for polyethylene usually obtained by catalytic poly-
merization, y
> 3.
210
YU. YERMAKOV AND V. ZAKHAROV
high temperatures; the data are known for ethylene polymerization by molybdenum oxide catalyst (191) and by TiClz (148) a t temperatures up to 270°C. So, the active M-C bond in the propagation centers of heterogeneous catalysts is quite stable, its short lifetime being determined by its high kinetic lability as a result of the possibility of various reactions proceeding in the coordination sphere of transition metals. The formation of many polymer molecules on one active center is due to regeneration reactions, e.g. after the spontaneous transfer according to the scheme:
0
H
I LnM-• + CHa=CHz
I
---t
LnM-CHz-CHs
(45)
The propagation centers also react with the inhibitors inevitably present in the reaction medium. The interaction with “coordination” inhibitors may stabilize the transition metal-carbon bond, as the elimination of the coordinative insufficiency of the transition metal ion makes it impossible for the metal-carbon bond to rupture through the mechanism of the 8hydride shift. If the interaction of the “coordination” inhibitor with the propagation center is reversible it can result in stepwise growth (191~)of the polymer molecule according to the following scheme : Temporary deactivation :
X LnM-P’
+X
1
---t
LnM-P‘
(46)
Regeneration:
X
1
LnM-Pi
-+
+X
LnM-P’
(47)
Insertion of K molecules of monomer:
Temporary deactivation:
X LnM-PLP’
+X
.L
---t
LnM-P‘LP’
(49)
etc.
The data obtained while studying the role of aluminurnorganic compounds during polymerization by Tic12 (167-169)show that an aluminumorganic co-catalyst can be a reversible ‘‘coordination” inhibitor by itself. The decrease in the number of propagation centers by the addition of aluminumorganic compounds to titanium dichloride seems to be caused by the reversible adsorption of the aluminurnorganic compound on the titan-
ONE-COMPONENT POLYMERIZATION CATALYSTS
21 1
ium ion of the propagation center :
As a result the polymer chain growth seems to stop due to the impossibility of the monomer coordination on the titanium ion. The propagation centers are nonuniform and the adsorption of the aluminumorganic compound takes place mostly at sterically more accessible nonstereospecific active centers. The processes of reversible adsorption of the “coordination” inhibitors (including the adsorption of organometallic compounds) result in an increase in the lifetime of the transition metal-carbon bond. It is possible that due to this, in the case of propylene polymerization by two-component catalysts based on TiC13, at low temperatures a long-term increase of molecular weight with time was observed (192,193). 2. Problem of the Active Bond Polarization in One-Component Catalysts Olefin polymerization by catalysts based on transition metal halogenides is usually designated as “coordinated anionic,” after Natta (194). It is believed that the active metal-carbon bond in Ziegler-Natta catalysts is polarized following the type M+* --C. The polarization of the active metal-carbon bond should influence the route of its decomposition by some compounds (“polar-type” inhibitors), e.g. by alcohols. When studying polymerization by Ziegler-Natta catalysts tritiated alcohols were used in many works to determine the number of metal-polymer bonds. However, as it was noted above (see Section IV), in two-component systems the polarization of the active bond cannot be judged by the results of the treatment of the system by alcohol, as the radioactivity of the polymer thus obtained results mainly from the decomposition of the aluminumpolymer bonds. Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiClz (169) and the catalyst Cr(?r-C3H,)s/SiOz (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ?r-ally1complexes (?r-ally1ligands being removed from the active center) (140) C14 of the
-
212
YU. YERMAKOV AND V. ZAKHAROV
TABLE IV Results of the Determination of the Polymer Radioactivity Obtained at Quenching Polymerization by Differently Labeled Methanol" Quenching agent Catalyst
CH30HJ C'4HsOH
Cr03/Si0~ Cr (CsHs)3/Si02in initial state Cr (C3H6)3/Si02 reduced by Hz at 400°C Tic12 a
+ -
++
+
-
-
+, a radioactive polymer; -, a nonradioactive one.
alkoxyl group was found in the polymer. This result points to the fact that the active bond in these systems is polarized in an unusual way (M-.-.C+) which results in the following route of its interaction with alcohol:
+ C*HsOH
+ C*H30P
(51) Carbon-14 of the alkoxyl group in the polymer was also found during the treatment by other oxide polymerization catalysts containing the oxides 110,W, V ( 1 9 5 ) .By the character of the polarization of the active bond such systems may be designated "coordinated cationic." To explain the unusual reactivity of the active bond in chromium oxide catalysts in reactions with alcohols, the propagation center was earlier aswherein the role of the sumed (111) to contain the fragment Cr--0-C active bond may be played by the oxygen-carbon bond. But the data obtained for the catalysts prepared with the use of tris-?r-allyl-chromium prove that in chromium oxide catalysts the active bond is also the metalcarbon one. In the case of the catalyst Cr (r-C3H6)3/SiOp,reasoning from the propagation center genesis, the formation of the fragment Cr-0-C seems to be impossible. However, after the reduction of this catalyst by hydrogen the appearance of the carbon-14 alkoxyl group in the polymer is also observed while stopping the polymerization by alcohol. The route of the reaction of the active bond at the propagation centers of polymerization catalysts with alcohol seems to depend on the ligand environment of the transition metal ion. But at the same time the value of Kp characterizing the reactivity of the active center in the propagation reaction has a similar value for the catalysts containing the same transition metal but LnM-P
--*
LnMH
ONE-COMPONENT POLYMERIZATION CATALYSTS
213
differing in polarization of the active bond. For example, the reduction of the catalyst Cr(r-C3H6) by hydrogen results in a change of polarization of the active bond in the propagation center, but shows little influence on the propagation rate constant (diminishing K , from 2.8 X lo3 to 1 X lo3,liter/mole*sec). The reactivity of the active centers in the propagation reaction seems to be mainly determined not by the polarization of the active bond but by the possibility of olefin activation by its coordination with the transition metal ion.
VII. Conclusion The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the “coordination” mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by ZieglerNatta catalysts is often considered together with the processes of homogeneous catalysis. The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. In catalytic polymerization the possibility arises of determining the absolute values of the rate constants of individual reactions composing the total process. The urgent problem in studying catalytic polymerization is still, as before, to obtain more detailed data on the structure of the propagation centers and the mechanism of their formation. No doubt the further investigation of one-component catalysts will promote progress in this field; the study of heterogeneous catalysts prepared with the use of organometallie compounds of transition metals holds the greatest promise, as in this case the direct synthesis of surface compounds and more precise control of the oxidation degree of the transition metal ion and of its ligand environment are possible. REFERENCES 1. Berger, M. N., Boocock, G., and Haward, R. N., Advan. Culal. 19, 211 (1969). 2. Clark, A., and Hogan, J. P., in “Polythene” (A. Renfrew and P. Morgan, eds.), p. 29. New-York, 1960.
214
YU. YERMAKOV AND V. ZAKHAROV
3. Clark, A., Hogan, J. P., Banks, R., and Lanning, W., Znd. Eng. Chem. 48, 1152 (1956).
4. Field, E., and Feller, M., Znd. Eng. Chem. 49, 1883 (1957). 6. Peters, E. F., and Evering, B. L., Znd. Eng. Chem. 49, 1879 (1957). 6.
Ziegler, K., Holzkamp, E., Brcil, H., and Martin, H., Angew. Chem. 67, 541 (1955).
7. Natta, G., J. Polym. Sci. 16, 143 (1955).
8. Yermakov, Yu.I., Lazutkin, A. M., Demin, E. A., Zakharov, V. A., and Grabovski, Yu.P., Kinet. Katal. 13, 1422 (1972). 9. Karol, F. J., Karapinka, G. L., Wu, C., DOW,H. W., Johnson, R. N., and Carrick, W. L., J. Polym. Sci., Part A-1 10, 2621 (1972). 9a. Ballard, D. G . H., Advan. Catal. 23, 267 (1973). 10. Werber, F. X., Benning, C. J., Wszolek, W. R., and Ashby, G. E., J. Polym. Sci., Part A-1 6, 743 (1968). 11. Natta, G.,and Pasquon, I., Advan. Catal. 11, 1 (1959). 12. Gaylord, N. G., and Mark, H. F., “Linear and Stereoregular Addition Polymer.” Wiley (Interscience), New York, 1959. 13. Roha, M., Fortschr. Hochpolym-Forsch.4,353 (1965). 14. Henrici-Olive, G., and Olive, S., F O T ~ S CHochpo1ym.-Fmech. ~T. 6, 421 (1969). 16. Bawn, C. E. H., and Ledwith, H., Quart. Rev. 16, 361 (1962). 16. Reich, L., and Schindler, H., “Polymerization by Organometallic Compounds. Wiley (Interscience), New York, 1966. 17. Boor, J., Jr., in “Macromolecular Reviews” (A. Peterlin et al., eds.), pp. 115-262. Wiley (Interscience), New York, 1967. 18. Boor, J., Jr., Znd. Eng. Chem., Prop. Reg. Develop. 9,437 (1970). 19. Keii, T., “Kinetics of Ziegler-Natta Polymerization.” Kodansha Scientific Books, Tokyo, 1972. 20. Topchiev, A. V., Krentsel, B. A,, Perelman, A. I., and Miesserov, K. G., J . Polym. Sci. 34, 129 (1959). 21. Mihail, R., Corlateanu, P., and Ionesco, A. C., J. Chim. Phys. Phicos. chim. Biol. 56, 568 (1959). 28. Topchiev, A. V., Krentsel, B. A., Perelman, A. I., and Rode, T. V., Zzv. Akad. Nauk SSSR, Ser. Khim. p. 1079 (1959). 23. Topchieva, K . V., Sharaev, 0. K., Perelman, A. I., and Topchiev, A. V., Neftekhimiya 1, 780 (1961). 24. Scrgeev, G. B., Sharaev, 0. K., Topchicva, K. V., Perelman, A. I., and Topchiev, A. V., Nejtekhimiya 2,18 (1962). 25. Antuf’ev, V. V., Votinov, M. L., Savin, A. G., Cazsin, B. I., Semenova, A. S., and Leitman, M. I., Kinet. Katal. 3, 353 (1962). 26. Bukanaeva, F. M., Pecherskaya, Yu.B., Karanski, V. B., and Drisko, V. A,, Kinet. Katal. 3,358 (1962). 27. Sharaev, 0. K., Topchieva, K . V., Perelman, A. I., and Topchiev, A. V., Neftekhimiya 2, 187 (1962). 28. Miesserov, K. G., Nejtekhimiya 2, 581 (1962). 29. Kazanski, V. B., and Pecherskaya, Yu.B., Kinet. Katal. 4, 244 (1962). 30. Bukanaeva, F. M., Boreskov, G. K., and Dzisko, V. A., Kinet. Katal. 4,492 (1963). 31. Benbenek, S., Przem. Chem. 43, 28 (1964). 32. Boreskov, G. K., Bukanaeva, F. M., Dzisko, V. A,, Kazanski, V. B., and Pecherskaya, Yu.B., Kinet. Katal. 5,434 (1964). 33. Voevodski, v. v., Proc. znt. Congr. Catal., Srd, 1964 p. 89 (1965). 34. Habeshaw, J., and Hill, T., PTOC.Znt. Congr. Catal. Srd, 1964 p. 975 (1965).
ONE-COMPONENT POLYMERIZATION CATALYSTS
215
36. Clark, A., Finch, J. N., and Ashe, B. H., Proc. Int. Congr. Catal., drd, l96q p. 1010 (1965). 36. Matsumoto, A., Tanaka, H., and Goto, N., Bull. Chem. SOC.Jap. 38, 45 (1965). 37. Krauss, H. L., Weber, E., and Movik, N., Z . Anorg. Allg. Chem. 338, 121 (1965). 38. Ayscough, P. B., Eden, C., and Steiner, H., J . Catal. 4, 278 (1965). 39. Aleksandrov, I. V., Kazanski, V. B., and Mikheikin, N. D., Kinet. Katal. 6, 439 (1965). 40. Pecherskaya, Yu.I., and Kazanski, V. B., Dokl. A h d . Nauk. SSSR 162, 1104 (1965). 41. Benbenek, S., Malinowski, S., Kosek, S., and Iwicka, D., Przem. Chem. 44, 385 (1965). 4.2. Benbenek, S., Malinowski, S., and Kosek, S., Przem. Chem. 44, 441 (1965). 43. Matsumoto, A., Tanaka, H., and Goto, N., Bull. Chem. SOC.Jap. 38, 1857 (1965). 44. Tarama, K., Ioshida, S., and Doi, Ja., Shokubai 8, 269 (1966). 46. Van Reijen, L. L., and Cossee, P., Discuss. Faraday SOC.41, 277 (1966). 46. Matsuda, T., Ono, Yo., and Keii, T., J. Polym. Sci., Part A-1 4,730 (1966). 47. Shiba, T., Shih Chien Chow, and Takashima, K., J . Chem. SOC.Jap. Znd. Chem. Sect. 69, 1003 (1966). 48. Miesserov, K. G., J . Polym. Sci., Part A-1 4, 3047 (1966). 49. Karakchiev, L. G., Yermakov, Yu.I., and Kolovertnov, G. D., Kinet. Katal. 8, 188 (1967). 50. Pecherskaya, Yu.1.’ and Kazanski, V. B., Kinet. Katal. 8, 401 (1967). 61. Kazanski, V. B., and Turkevich, J., J . Catal. 8, 231 (1967). 68. Charcosset, H., Revillon, A., and Guyot, A., J . Catal. 8, 326 (1967). 65. Charcosset, H., Revillon, A., and Guyot, A., J . Catal. 8, 334 (1967). 54. Eden, C., Feilchenfeld, H., and Haas, Y., J . Catal. 9,367 (1967). 55. Tarama, K., Ioshida, S., and Doi, Ja., Catalyst (Tokyo) 9,48 (1967). 66. Percherskaya, Yu.B., and Kazanski, V. B., Probl. Kinet. Katal. 13, 236 (1968). 67. Turlier, P., Durrien, M., and Guyot, A., J. Catal. 9, 255 (1968). 68. Revillon, A., and Guyot, A., J. Chim. Phys. Physicoe. chim. Biol. 65, 845 (1968). 59. Tarama, K., Ioshida, S., and Doi, Ja., Congr. CataZ., dth, 1968 Preprint No. 13, p. 176 (1971). 60. Yermakov, Yu.I., Alt, L.Ya., Ivanov, L. P., Gelbshtein, A. I., and Anufrienko, V. F., Kinet. Katal. 9, 352 (1968). 61. Eden, C., Feilchenfeld, H., and Haas, Y., J . Catal. 11 (1968). 62. Holm, V. C. F., and Clark, A., J . Catal. 11, 305 (1968). 63. Guyot, A., and Durrien, M., J . Chim. Phys. Physiwchim. Biol. 65, 863 (1968). 64. Alt, L.Ya., Anufrienko, V. F., Tyulikova, T.Ya., and Yermakov, Yu.I., Kinet. Katal. 9, 1253 (1968). 65. Krauss, H. L., and Stach, H., Znorg. Nuel. Chem. Lett. 4,393 (1968). 66. Krauss, H. L., and Stach, H., 2. Anorg. Allg. Chem. 366, 34 (1969). 67. Krauss, H. L., and Stach, H., 2. Anorg. Allg. Chem. 366,280 (1965). 68. Yermakov, Yu.I., Zakharov, V. A., Grabovski, Yu.P., and Kushnareva, E. G., Kinet. Katal. 11, 519 (1970). 69. Hogan, J., J . Polym. Sci., Part A-1 8,2637 (1970). 70. Krauss, H. L., Proc. lnt. Congr. Catal., 6th, 1972, p. 207 (1973). 71. Eley, D. D., Rochester, C. H., and Scurrell, M. S., Proc. Roy. SOC.,Ser. A 329, 375 (1972). 78. Eley, D. D., Rochester, C. H., and Scurrell, M. S., J. Catal. 29, 20 (1973).
216
YU. YERMAKOV AND V. ZAKHAROV
73. Ivanov, L. P., Yermakov, Yu.I., and Gelbshtein, A. I., Vysokomol. Soedin., Ser. A
9,2422 (1967).
J. Chim. Phys. Physicochim. B i d . 65, 857 (1968). 75. Yermakov, Yu.I., Ivanov, L. P., and Gelbshtein, A. I., Kinet. Katal. 10, 183 (1969). 76. Yermakov, Yu.I., Ivanov, L. P., and Gelbshtein, A. I., Kinet. Katal. 10,411 (1969). 77. Yermakov, Yu.I., Ivanov, L. P., Zakharov, V. A., Kushnareva, A. I., and Gelbshtein, A. I., Kinet. Katal. 10,590 (1969). Y8. Boreskov, G. K., Dzisko, V. A., and Tyulikova, T.Ya., Dokl. Akad. Nauk SSSR 136, 125 (1961). 79. Yermakov, Yu.I., Boreskov, G. K., Dzisko, V. A., and Ivanova, L. I., Dokl. Akad. Nauk. SSSR 143, 1139 (1962). 80. Gurevitch, V. R., Azerb. Khim. Zh. No. 6, p. 37 (1963). 81. Clark, A,, and Bailey, G. C., J. Catal. 2,230 (1963). 86. Clark, A,, and Bailey, G. C., J . Catal. 8, 241 (1963). 83. Guyot, A., and Daniel, J. C., J. Polym. Sci., Part A 1, 2989 (1963). 84. Landau, M. A., and Shchekin, M. M., Neftekhimiya 3, 713 (1963). 85. Landau, M. A., Neftekhimiya 4, 53 (1964). 86. Gurevitch, V. R., Dalin, M. A., and Arytyunova, K. M., Azerb. Khim. Zh. N1, p. 69 (1964). 87. Guyot, A., J . Catat. 3, 390 (1964). 88. Yermakov, Yu.I., and Ivanov, L. P., Kinet. Katal. 6,889 (1965). 89. Yermakov, Yu.I., Boreskov, G. K., Slinko, M. G., and Skomorokhov, V. B., Kinet. Katal. 6, 909 (1965). 90. Guyot, A., Daniel, J. C., Durrien, M., and Pt8ack,M., J . Potym. Sci., Part A 3, 1765 (1965). 91. Clark, A., Znd. Eng. Chem. 59, 21 (1967). 96. Zakharov, V. A., Yermakov, Yu.I., Ivanov, L. P., and Skomorokhov, V. B., Kinet. Katal. 9, 605 (1968). 93. Whitaker, H. L., and Wills, G. B., J.Appl. Polym. Sci. 13, 1921 (1969). 94. Gurevitch, V. R., Plaksunov, T. K., and Dalin, M. A., Plast. Mass. No. 1, p. 3 (1969). 95. Yermakov, Yu.I., Mikhalchenko, V. G., Beskov, V. S., Grabovski, Yu.P., and Emirova, I. V., Plast. Massy No. 9, p. 7 (1970). 96. Vuillaume, G., Revillon, A., Spitz, R., and Guyot, A., J. Makromol. Sci.-Chem., Ser. A 5, 559 (1971). 97. Kushnareva, E. G., Yermakov, Yu.I., and Zakharov, V. A., Kinet. Katal. 12, 414 (1971). 98. Zakharov, V. A., and Yermakov, Yu.I., J. Polym. Sci., Part A-1 9, 3129 (1971). 99. Eley, D. D., Rochester, C. H., and Scurrell, M. S., Proc. Roy. Soc., SeF. A 329,361 (1972). 100. Emirova, I. V., Yermakov, Yu.I., Nevyantzev, I. D., and Ratner, I. D., Vysokomol. Soedin., Ser. B 12, 23 (1970). 101. Yermakov, Yu.I., Emirova, I. V., and Gul, V. E., Vysokomol. Soedin., Ser. B 12, 526 (1970). 102. Davidson, T., Polym. Lett. 8, 855 (1970). 103. Emirova, I. V., Yermakov, Yu.I., and Gul, V. E., Vysokomol. Soedin., Ser. B 13, 260 (1971). 104. Guyot, A., Rev. Gen. Cnout. Plast., Ed. Plast. 2, 37 (1965). 105. Kazanski, V. B., Kinet. Katal. 8, 1125 (1967). 74, Revillon, A., Daniel, J. C., and Guyot, A.,
ONE-COMPONENT POLYMERIZATION CATALYSTS
21 7
106. Clark, A., Catal. Rev. 3, 145 (1969). 107. Yermakov, Yu.I., “Chromium Oxide Catalysts for High Polymerization.” Nauka, Novosibirsk, 1969. 107a. Krauss, H., Advan. Catal. (to be published). 108. Baker, L. M., and Carrick, W. L., J . Org. Chem. 33, 616 (1968). 109. Carrick, W. L., Turbett, R. J., Karol, F. J., Kurapinka, G. L., Fox, A. S., and Johnson, R. N., J . Polym. Sci., Part A-1 10, 2609 (1972). 110. Baker, L. M., and Carrick, W. L., J . 078. Chem. 35, 774 (1970). 111. Yermakov, Yu.I., and Zakharov, V. A., Znt. Congr. Catal., 4th, 1968 Preprint No. 16, p. 200 (1971). 112. Gurevich, V. R., Dalin, M. A., Arutyunova, K. M., and Lagernaya, T. A., PTOC. Int. Congr. Catal., 4th, 1968 Symp. 111, p. 491 (1971). 113. Hock, C. W., J . Polym. Sci., Part A-1 4, 3055 (1966). 114. Wheeler, A, Advan. Catal. 3, 249 (1951). 116. Zakharov, V. A,, Druzhkov, V. N., and Yermakov, Yu.I., Kinet. Katal. 14, 998 (1973). 116. Beerman, C., and Bestian, H., Angew .Chem. 71,618 (1959). 117. Giannini, U., Zucchini, U., and Albizatti, E., Polym. Lett. 8, 405 (1970). 118. Bailard, D. G . H., and van Lenden, P. W., Makromol. Chem. 154, 177 (1972). 119. British Patent 1,265,747; Ref. Zh., Khim., No. 18 C 274P (1972). 120. German Patent (BRD) 2,026,032;Chem. Abstr. 75, 6669v (1971). 122. U. S. Patent 3,666,743;Chem. Abstr. 77,62538e (1972). 122. Wilke, G., Bogdanovic, B., Hardt, P., Heimbach, P., Keim, W., Kroner, M.,
Oberkirch, W., Tanaka, K., Steinriicke, E., Walter, D., and Zimmermann, H., Angew. Chern. 78, 157 (1966). 123. Ballard, D. G. H., Jones, E., Medinger, T., and Pioli, J. J. C., Makromol. Chem. 148, 175 (1971). 124. German Patent (BRD) 1,963,072;Chem. Abstr. 73, 77.850s (1970); British Patent 1,211,371;Ref. Zh., Khim. No. 11 C 209 P (1971). 126. Demin, E. A., Yermakov, Yu.I., Lazutkin, A. M., and Zakharov, V. A., Vysokomol. Soedin., Ser. B 12, 619 (1970). 126. Yermakov, Yu.I., Demin, E. A., Lazutkin, A. M., and Kuzyaeva, T. E., Kinet. Katal. 13, 787 (1972). 127. Jamazaki, H., Jamaguchi, M., and Hatihara, N., Mem. Znst. Sci. Znd. Res., Osaka Uniu. 20, 107 (1963). 128. Jamazaki, H., Matsumoto, Jo., and Hagihara, N., Mem. Znst. Sci. Ind. Res., Osaka Univ. 21, 131 (1964). 129. Taeima, Yo., Tani, K., and Juguchi, S.,Polym. Lett. 3,529 (1965). 130. Tazima, Yo., and Sadao, Yu., Bull. Chem. SOC.Jap. 39,2534 (1966). 131. Demin, E. A., Zakharov, V. A., Yermakov, Yu.I., and Ovtchinski, K.Sh., Kinet. Katal. 13, 248 (1972). 132. German Patent (BRD) 2,040,353;Chem. Abstr. 74, 126 364t (1971); French Patent 2,026,371. 133. German Patent (BRD) 1,963,256;Chem. Abstr. 73,77 820g (1970). 134. British Patent 1,264,393. 136. U. S. Patents 1,949,574 and 2,024,765. 136. British Patent 1,253,063. 137. Demin, E. A,, Zakharov, V. A., and Yermakov, Yu.I., Zzu. Vuzov (in press). 138. Schmonina, V. L., Stefanovska, N. N., Tinyakova, E. I., and Dolgoplosk, B. A., Vysokomol. Soedin., Ser. B 12, 566 (1970).
YU. YERMAKOV AND V. ZAKHAROV
215
1%. Yermakov, Yu.I., Kuznetzov, B. N., Karaktchiev, L. G., and Derbeneva, S. N.,
Kinet. Kalal. 14, 709 (1973). 140. Zakharov, V. A., Demin, E. A., and Yermakov, Yu.I., “Reaction Rate and Catalysis Letters.” Budapest 1, N3 (1974). 141. Yermakov, Yu.I., and Kuznetzov, B. N., Dokl. Akad. Nauk SSSR 207,644 (1972). 14% Yermakov, Yu.I., Grabovski, Yu.P., and Lazutkin, A. M., “Reaction Rate and Catalysis Letters.’’ Budapest 1, N3 (1974). l&a. Yermakov, Yu. I., Kuznetzov, B. N., and Startsev, A. N., Kinet. Katal. 15, 539
(1974). 143. Yermakov, Yu.I., and Kuznetzov, B. N., “Reaction Rate and Catalysis Letters.” Budapest 1,87 (1974). 144. Yermakov, Yu.I., and Kuznetzov, B. N., Kinet. Katal. 13, 1355 (1972). 144a. Yermakov, Yu. I., Kuznetzov, B. N., Rindin, Yu. A., andLazutkin, A. M., Kinet. Katal. 14, 1594 (1973). 14.5. Yermakov, Yu.I., and Kuznetzov, B. N., Rep, .Tap.-Sou.Semin. Catal., 2nd 197s (to be published). 146a. Yermakov, Yu. I., Kuznetzov, B. N., and Kuznetzov, V. L., Kinet. Katal. 14,
1085 (1973). 146, Yermakov, Yu.I., Kuznetzov, B. N., Bobrov, A. M., Ione, K. G., “Reaction Rate and Catalysis Letters.” Budapest 1,277 (1974). 146a. Kimkai, 0. N., Kuznetzov, B. N., Boreskov, G. K., and Yermakov, Yu. I., Dokl. Akad. Nauk SSSR 214, 146 (1974). 147. Fukui, K., Kagiya, Ts., Takeo, Sh., Yagi, T., Machi, S., Yuasa, S., Hirota, M., and Kodsma, Sh., J . Polym. Sci. 37, 341 (1959). 1.6s. Benning, C. J., Wszolek, W. R., and Werber, F. X., J . Polym. Sci., Part A-1 6,
755 (1968).
1 4 9 . Matlack, A. S., and Breslow, D. S., J . Polym. Sci., Part -4-1 3, 2853 (1965). 160. Oita, K., and Nevitt, T. D., J . Polym. Sci. 43, 585 (1960). 151. Bukatov, G.D., Zakharov, V. A., Yermakov, Yu.I., and Shirikov, N. G., Kinet. Katal. 12, 1357 (1971). 15%. Joyner, F. B., and Shearer, N. H., J . Polym. Sci. 58, 881 (1962). 165. Boor, J., Jr., Polym. Lett. 2, 265 (1964). 164. Razuvaev, G. A., Minsker, K. S., and Fedoseeva, G. T., Vysokomol. Soedin. 4,1495
(1965). 165. Duck, E. W., and Ridgewell, B. J., Eur. Polym. J . 2?37 (1966). 166. Bukatov, G.D., Zakharov, V. A., and Yermakov, Yu.I., Kinet. Katal. 12, 505
(1971).
157. Bukatov, G.D., Thesis, Novosibirsk, Institute of Catalysis (1973). 158. Yermakov, Yu.I., Bukatov, G. D., and Zakharov, V. A., Proc. Int. Congr. Catal., 5th, 1978 Preprint No. 24 (1973). 169. Bukatov, G. D., Zakharov, V. A., and Yermakov, Yu.I., Kinet. Katal. (in press). 160. Kern, W., and Schnecko, H., Chem. Ztg. 94,229 (1970). 16Oa. Yermakov, Yu.I., and Zakharov, V. A,, Usp. Khim. 41, 377 (1972). 161. Feldman, G.F., and Perry, E., J . Polym. Sci. 46, 217 (1960). 168. Kohn, E.,Schuurmans, H., Cavender, I. V., and Mendelson, R. A,, J . Polym. Sci.
58,681 (1962). W., Gullet, J., Combs, R., and Joyner, F. B., J . Polym. Sci., Part A 1, 2583 (1963). 164. Caunt, A. D., J . Polym. Sci., Part C 4,49 (1963). 165. Chien, J. C. W., J . Amer. Chem. Soc. 1, 86 (1958). 16% Coover, H.
ONE-COMPONENT POLYMERIZATION CATALYSTS
219
166. Schnecko, H.,Jung, K. A., and Grosse, L., Makromol. Chem. 148,67 (1971). 167. Yermakov, Yu.I., and Ivanov, L. P., Kinet. Katal. 8 , 357 (1967). 168. Zakharov, V. A., Yermakov, Yu.I., and Kushnareva, E. G., Kinet. Katal. 8, 1391 (1967). 168a. Yermakov, Yu.I., Zakharov, V. A,, and Kushnareva, E. G., J . Polym. Sci., Part A-1 9, 771 (1971). 169. Zakharov, V. A,, Bukatov, G. D., and Yermakov, Yu.I., Kinet. Katal. 12, 263 (1971). 170. Cossee, P., J . Catal. 3, 80 (1964). 171. Arlman, E. J., J . Catal. 3, 89 (1964). 172. Arlman, E. J., and Cossee, P., J . Catal. 3, 99 (1964). 173. Arlman, E. J., J . Catal. 5, 178 (1966). 17.4. Zakharov, V. A., Bukatov, G. D., Tchumaevski, N. M., and Yermakov, Yu.I., “Reaction Rate and Catalysis Letters.” Budapest 1,247 (1974). 175. Zakharov, V. A., Bukatov, G. D., Demin, E. A., and Yermakov, Yu.I., Dokl. Akud. Nauk SSSR 207,857 (1972). 176. Eley, D. D., Rochester, C. H., and Scurrell, M. S., J . Chem. SOC.,69,660 (1973). 177. Halpern, J., Accounts Chem. Res. 3, 386 (1970). 178. Dyatchkovski, F. S., and Shilov, A. E., Zh. Fiz. Khim. 41, 2515 (1967). 179. Grigorian, E. A., Dyatchkovski, F. S., Khvostik, G. M., and Shilov, A. E., Vysokomol. Soedin., Ser. A 9, 1233 (1967). 180. Wilkinson, G., Zh. Vses. Khim. Obshchest. 17, 377 (1972). 181. Green, M. L. H., in “Organometallic Compounds” (G. E. Coates, M. L. H. Green, and K. Wade, eds.), Vol. 11. Methuen, London, 1968. 182. Dzsabiev, T. S.,Dyatchkovski, F. S., and Shilov, A. E., Vysokomol. Soedin., Ser. A 13,2474 (1971). 183. Yermakov, Yu.I., and Zakharov, V. A., Rep. Sou.-Jap. Catal. Semin. lst, 1970 Preprint No. 26 (1971), Novosibirsk. 184. Ludlum, D. B., Anderson, A. W., and Ashleg, C. E., J . Amer. Chem. Soc. 80, 1380 (1958). 185. Carrick, W., J . Amer. Chem. SOC.80, 6455 (1958). 186. Cossee, P., Trans. Faraday SOC.58, 1926 (1962). 187. Cossee, P., Ross, P., and Schachtschneider, J. H., Proc. Znt. Congr. Catal., 4th, 1968 Preprint No. 14, p. 184 (1971). 188. Evans, M. G.,and Polanyi, M., Trans. Faraday Soc. 34, 11 (1938). 189. Semenov, N. N., Nekot. Probl. Kemitcheskoi Kinet. Reakt. Sposobnosti, 1958 p. 41 (1958). 190. De la Cuesta, M. O., and Billmeyer, J. W., J. Polym. Sci., Part A 1, 1721 (1963). 191. Friedlander, H.,J . Polym. Sci. 38, 91 (1959). 191a. Grigorian, E. A., Dyachkovski, I. S., and Shilov, A. E., Znt. Symp. Macromol. Chern., Budapest, 1969,preprint 4/13. 192. Bier, G., Hoffmann, W., Lehmann, G., and Seydel, G., Makromol. Chem. 58, 1 (1962). 193. Ingberman, A. K., Levine, I. J., and Turbett, R. J., J. Polym. Sci., Part A-1 4, 2781 (1966). 194. Natta, G., Danusso, F., and Sianesi, D., Makromo2. Chem. 30, 238 (1959). 195. Zakharov, V. A., Kushnareva, E. G., and Yermakov, Yu.I., Kinet. Katal. 10, 1164 (1969).
This Page Intentionally Left Blank
The Economics of Catalytic Processes J. DEWING Imperial Chemical Industries, Corporate Laboratory, Runcorn, Cheshire, England
AND
D. S. DAVIES Imperial Chemical Industries, Millbank, London, England
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Two Classes of Useful Chemical Transformatio ............. 111. The Network of Technological Factors and ..................... Catalyst Choice.. . A. Thermodynamics.. ........................................
221 222
225 226 228 228 C. Product Contamination. ...................... 228 D. Significance of Catalyst Life.. .............................. E. The Choice between Heterogeneous or Homogeneous Catalysts. . 230 IV. The Economic Factors and Constraints Affecting Catalyst Choice. . . . 231 A. Guidance from the Standard Cost Sheet: Straightforward . . . . . . . . . . 232 Improvements ............................. 233 B. Guidance from Cash Flow Models: More Bas 235 C. Guidance from Models of Competition.. . . . . D. Guidance from Models of Business Portfolios: “Entrance Fees”. . 236 V. Factors Involved in Economic Improvement to Typical Processes and 241 ssment ...................................... 243 ............................................
1. Introduction The purpose of this article is to indicate and illustrate the very wide range of factors, apart from and beyond questions of underlying chemistry and chemical specificity, that determine or constrain the utility of various investigative efforts in catalyst research; they also define the continuing and progressive contributions to the chemical industry which the catalyst chemist can make. The ultimate criteria are nearly always found in cost reduction and/or in product diversification and improvement. There is a steady flow of publication which contributes theories and information to help in the choice of catalysts, the improvement of their selectivity, or the improvement in reactor utilization through increase in 221
222
J. DEWING AND D. S. DAVIES
reaction rate. There is also a growing body of knowledge of chemical engineering science related to topics such as reactor design, bed layout, fluidization, heat transfer, catalyst/reactant contacting, etc. Many of these accomplishments are analogous to the craft inventions of the one-stringed fiddle and the shepherd’s pipe, the physicist’s explanation of simple sounds in terms of pure sinusoidal components and overtones, or the engineering study of (often oversimplified models of) resonant cavities. We now turn to the catalyst chemist’s analogue of a real Strad, pipe organ, orchestra, or string quartet; and then we need to seek ways for composing the analogue of the partita, toccata, or fugue, or even the specific setting of the primordial love drama of Tristan and Isolde. Indeed, the task of the industrial catalyst chemist is analogous to the act of composition, and the task of the production manager is indeed analogous to the act of creating the actual performance. The guidelines for value are analogous to those that have applied to Bach, Mozart, Wagner, Toscanini, and Bruno Walter-the supply of personal inspiration and creativity on the one hand, and the satisfaction of a paying audience on the other, that is, the customer, without whose interest and cooperation there is no meaningful existence for most artistic success. The catalyst chemist, like the composer, is an artist in the trade; he is concerned with the overall effect, and his criterion is the achievement of the most acceptable results per unit consumption of resources, as measured by what may be a clumsy but the only available scale: in his case, that of money. This in itself draws attention to one of the artistic aspects of the industrial catalyst designer’s job. Money values are neither absolute, invariant, nor always logically desirable entities. For example, resource producing nations can increase feedstock prices and they may do so for political rather than for hard, technological reasons. One very important consequence is the fact that a catalytic process that is economic in one year but not in the next is not as attractive as one that can adapt. The present uncertainties surrounding the value of money in the international business world, and the rapidly changing values of raw materials make a quantitative discussion of absolute values more difficult than was the case in the more stable situation which obtained until the last year or two. Therefore, we will frequently make use of figures that indicate a relative scale of values rather than definitive quantities.
II. Two Classes of Useful Chemical Transformations Broadly the output from the chemical industry can be considered as falling into two classes, materials and effect chemicals. I n the former class
223
THE ECONOMICS O F CATALYTIC PROCESSES
we include all commodities which are used for construction or combustion, e.g. plastics, fibers, fertilizers, fuels, and in the latter we include those bought because they produce specific sought after effects, e.g. catalysts, pharmaceuticals. The division is somewhat arbitrary and in many cases debatable but it provides a basis for drawing up Table I. Table I reflects two basic distinctions. First, work on catalysts in the small tonnage, high value effect chemicals and pharmaceuticals can make possible the design and commercialization of completely new product molecules, for the “entrance fee” (see below) for a novelty is still manageable. A new polypeptide catalytic system could open up whole new vistas of chemotherapy. Consequently, the laboratory chemist-perhaps in a university-may play a key role. But in the high tonnage, low value area of synthetic fibers, plastics, rubbers, and fertilizers, entrance fees are very high, for existing products are very good, capable of great, as yet unrealized diversification, and cheap because of large scale and efficient manufacture. A newcomer must have properties outstanding enough to command a big market quickly, and this is difficult and rare. Consequently, the catalyst chemist in this field is much more often concerned with the cheapening, improvement, and diversification of products already well known. The laboratory scientific component, though highly significant, is therefore necessary, but not sufficient. And the value of the catalysts themselves, TABLE I
The Principal Fields of Catalysis 1971 Output (million tons/yr)
Area of business
World
Total output value ( $million/yr )
U.S.A.
Price ($/ton)
Materials and fuels (1) Plastics synthetic fibers synthetic rubber Fertilizers (nitrogeneous as N-content ) Petroleum
30 6 5
8 2 2
350 1000 500
35 2400
8 470
225 20-30
Effect chemicals (2) Medicinal chemicals
N.A.0
0.1
Catalysts
N.A.
N.A.
a
N.A. indicates data not available.
6000
World
U.S.A.
10,OOO
3000 2000 1000
6Ooo
2500
8Ooo 1800 60,000 10,OOO
N.A.
600
N.A.
245
224
J. DEWING AND D. 5. DAVIES
TABLE I1
The Major Uses of Catalysts ( 3 ) Annual catalyst value, Reaction type Polymerization Hydrogenation etc.= Oxidation etc. Alkylation/acylation Catalytic cracking Catalytic reforming Hydrotreating Hydrodesulfuriaation Urethane formation a
U.S. industry ($million)
Elements used
Al, Ti, V, Cr Ni, Fe, Cu, Zn, Pt Pt, Rh, Ag, V, U, Sb, Cu, Mo AlCla; HzSo,, H F Zeolites, SiO /A20 AlzOs, Group VIII
}
Ni, Go, Mo, W
22 23 23 55 59
30 23
smines
10
Including steam reforming in ammonia and methanol synthesis.
though significant, are minor in relation to product values. Consequently, reduction in the cost of the catalyst itself is not the catalyst chemist's primary contribution. (Much the same is true of the pharmaceuticals chemist, the cost of making whose chemicals is small in relation to the cost of discovering activity and establishing safety.) Returning to the catalyst business, how does the $250,000,000 break TABLE I11 Catalyst Cost per Tan of Product Product value ($/ton)
Catalyst cost ($/ton product)
Homogeneous processes Terephthalic acid Vinyl acetate a-olefins
300 300 200
7" 4"
Heterogeneous processes Phthalic anhydride Formaldehyde Caprolactam Acrylonitrile Acrylic acid
300 40 400 300 400
2.2 4" 5 (NH,OH stage)
Product
5
15
20"
Reference
4 4 4
4 6 6 7
Figure represents catalyst and auxiliary chemicals and therefore represents an upper limit for catalyst cost.
225
T H E ECONOMICS OF CATALYTIC PROCESSES Oil t Gas
Auio exhaust purifier m u U
Naphtha
Sulfur
Zeolites Pi, Re,Ir
Aromatics
Phthalic
I
c4's
1
I
Eihylww
Propylsns
Maleic
chloride
Phenol
Thsrmaplariin, rubbers
Thermoset
FIG.1. The field of catalysis. The numbers in circles are approximate annual catalyst cost for principal uses, in $million. Total values: Catalysts ca. $200,000,000. Products (excluding fuel) ca. $100-200,000 million.
down? How is it shared between the various sections of the industry? I n Table I1 we give order of magnitude figures to answer the questions classified broadly under reaction type. Thus, the spread of catalyst cost is fairly even over the whole industry and about a third of the known metals are used in one way or another. Table I11 shows how catalyst costs vary in relation to product value. To complete the picture, Fig. 1 shows a map of the major catalyst usages in the petrochemical, fuel, polymer, and fertilizer fields.
111. The Network of Technological Factors and Constraints Affecting Catalyst Choice
Although primary catalyst cost is not a major factor in the price of the product, the work of the catalyst chemist of course crucially affects a wide variety of process costs that are of far greater significance. What goes on in the reactor dictates feedstock requirements, capital charges, downtime for catalyst recharging, and strongly influences the purification problems
1
226
J. DEWING AND D. 6. DAVIES
associated with product work-up. We must therefore define a process as embracing all the operations performed on the feedstock in converting it to a product of a suitable quality and a t a price which the customer will pay. The study of a generalized catalytic process may include feedstock purification, a catalytic reaction, operating routine, by-product production and credits, distillation or other purification, and disposal of effluents. All of these steps will require hardware with which to carry out these operations and all of them will contribute to the capital cost of the process and its operating costs. The process is therefore a network or system of interacting operations. The constraints in this system will vary according to the process, but many of the constraints will depend on the behavior of the catalyst/reactor combination and will be reflected in the economics of the process. A. THERMODYNAMICS In chemical processing the most fundamental constraint is that of the thermodynamics of the system. This constraint defines both the heat balance of the process and whether or not the processes in the reactor will be equilibrium limited. These constraints will limit the range of chemical engineering solutions to the problems of designing an economically viable process that can be found. The various processes involving carbon monoxide: steam reforming, water-gas shift, methanation, and methanol synthesis illustrate the operation of thermodynamic constraints and some of their attendant economic consequences. Steam reforming is represented by the chemical equation CnH2n+z
+ nH*O-*nCO + (2%+ 1)Hz
This transformation is only favorable at high temperatures, and at lower temperatures, e.g. 60O0C, the methanation reaction occurs. CO
+ 3Hz
---t
CH,
+ H20
The effect of pressure on the steam reforming reaction is to raise the temperature of equilibrium balance, but the processes using the hydrogen generated in this process, e.g. ammonia synthesis, require their feeds a t high pressure, again for thermodynamic reasons. Since it is cheaper to raise steam at high pressure and feed liquid hydrocarbons to the preheaters at high pressures than to compress hydrogen and carbon monoxide it is advantageous to run the process under pressure. At the temperature demanded by thermochemistry, around 900°C, a nonchemical constraint is encountered, viz. the life of the tubes in the multitubular reactor furnace.
THE ECONOMICS OF CATALYTIC PROCESSES
227
Under these very severe conditions, their life may be limited by their creep resistance and the process could not be economically developed to operate at pressure without the metallurgical development of steels able to withstand the conditions. Once the reactor constraint is removed there is immediately a demand for a catalyst which will operate satisfactorily under the more severe conditions now possible. This example illustrates one of the ways in which the catalyst chemist must study and use the work of others: it illustrates the importance of constraints not involving the catalyst directly. Other cases can be found where the catalyst performance has a more direct bearing on the easing of constraints. The synthesis of methanol provides such an example: CO
+ 2H2 @ CH30H
The desired reaction is favored by pressure, but not by high temperature.
If the temperature is raised the pressure must be raised to maintain the same equilibrium conversion. However, raising the temperature allows the equilibrium to be attained more rapidly. In this situation the development of more active catalysts allows the temperature of the reaction to be lowered. The process designer then has the option of either lowering the reaction pressure and maintaining the equilibrium conversion or maintaining the pressure and increasing the equilibrium conversion. The choice is not a simple one, depending as it does on the balance between many factors; the cost of compression to the required pressure on the scale required; the cost of reactor for the required output-lowering the pressure would reduce the space-time yield and increase the reactor capital cost; and the cost of recycling unreacted gases which would be proportionately less for the higher conversion process. Again there is no simple solution to the problem but it is significant that the newer methanol processes employ lower pressures and temperatures than the processes they are displacing. Initially low pressure processes (- 50 atm) were developed, but there is now a move to somewhat higher pressures as the size of plants is increased giving rise to the intermediate pressure processes (8). The key to these processes is the development of more effective catalysts but the economic advantage comes from the greater freedom it gives the process designers. Dehydrogenation processes also suffer from thermochemical constraints. Chemically the reaction is highly endothermic and also leads to an increase in entropy due to the increase in molecularity in undergoing the transformation. These changes are favored by high temperature and low pressure leading to processes running at low pressures, with all the penalties that this brings in terms of low space-time yields and high capital.
228
J. DEWING AND D. S . DAVIES
B. HEATTRANSFER So far, consideration has been limited to chemistry; physical constraints such as heat transfer may also dictate the way in which reactions are performed. Oxidation reactions are highly exothermic and effectively there are only two types of reactor in which selective oxidation can be achieved on a practical scale; multitubular fixed bed reactors with fused salt cooling on the outside of the tubes and fluid bed reactors. Each has its own characteristics and constraints. Multitubular reactors have an effective upper size limit and if a plant is required which is too large to allow the use of a single reactor, two reactors must be used in parallel. C. PRODUCT CONTANINATION The characteristics of the catalyst in a process may lead to economic advantage or disadvantage at the product end of a process. One of the stages in the typical Ziegler polyolefin process is washing the polymer to remove catalyst residues. This is an expensive process. Catalyst development has yielded several rival processes for the production of high density polyethylene which require no such step. Increases in catalytic activity have allowed the use of such low levels of chemically reactive components in the catalyst (especially chloride ion) that the resudies do not significantly effect the final properties of the polymer. The diversity of catalyst chemistry and of process type makes it impossible to generalize as to what constitues a better catalyst, except to say that it is one which leads to products that are cheaper or better, or both. In many cases this is likely to be a result of greater selectivity rather than activity, but in single pathway reactions activity, and also the pattern of its change with time, becomes all important. The “better catalyst” can only therefore be defined in terms of the detailed context of the process for which an improvement is sought.
D. SIGNIFICANCE OF CATALYST LIFE The ageing and decay characteristics of catalysts are of immense importance in defining the economics of processes. The simplest criterion that can be applied is that of total productivity during the life of the catalyst and also loss of productivity during the shut down required for catalyst replacement. Figure 2 illustrates notional performances for two catalysts A and B in hypothetical processes in which productivity is simply a measure of quantity of product produced. Catalyst A has a lower initial productivity but is more stable in use and dies off a t a much lower rate than catalyst B, which has a high initial productivity which falls relatively
THE ECONOMICS OF CATALYTIC PROCESSES
229
rapidly and eventually falls below that of A. At some point in the decay of a catalyst a decision must be taken to close the plant, discharge the catalyst, and install a new charge. On this simplistic basis the total integrated productivity of both catalysts may be the same or, as depicted in the figure, the productivity of the second charge of B is much higher than the aged catalyst A leading to higher total productivity for B at the direct cost of an additional catalyst charge. This simple picture may be totally misleading. The process designer faced with the performance figures for catalyst B must attempt to design a plant which will operate efficiently under a much wider range of productivities than for catalyst A. It may well be impossible for the process to operate a t the same efficiency throughout the life of catalyst B. The productivity of the fresh charge of catalyst is high and all the unit operations in the process must be sized to cope with this output, so that later in the life of the catalyst many of these units would be underutilized. Alternatively the die-off of the catalyst depicted could leave more unreacted feedstock in the product stream which would require larger clean-up and recycle units. The heat balance of the process will also vary during the life of the catalyst, which will again lead to nonoptimal use of the plant. Consideration of factory operation adds further advantages for the steady catalyst. The changing of the charge is often an expensive job and sometimes an unpleasant one, perhaps involving drilling out. It may also require the purchase of a separate reactor body so that a complete filled spare unit is always available. The extra capital inventory can be substantial. The
Time
FIG.2. Decay of catalysts.
230
J. DEWING AND D. €3. DAVIES
process workers have a varying task, and greater skill is needed. It may be difficult to provide a steady workload for them. Manning levels will be higher, and it is not impossible for conditions leading to industrial disputes to be more frequent. The attention given to the causes and control of catalyst die-off in industry is well illustrated by the behavior of different formulations of Cu-ZnO-Al203 catalysts for the low temperature water gas shift reaction. CO
+ HzO
+ COz
+ Hz
A detailed investigation described by J. S. Campbell (9) shows how precise control of the precipitation conditions is necessary if loss of activity by thermal sintering is to be avoided and that two types of poisoning can occur if the reactants are contaminated by sulfur or chlorine compounds. Sulfur leads to conventional poisoning while chlorine promotes rapid sintering of the active metallic copper by transport of volatile copper chlorides. Catalyst die-off may therefore be a complex physical and chemical phenomenon and assessment of its economic consequences may not be simple. The direct cost of lost production and determination of the optimum length of run before changing a catalyst can be expressed arithmetically. J. Happel illustrates this type of calculation (10) for catalytic cracking where the balance. of products changes with the age of the catalyst. However, many factors must be taken into account in selecting catalysts and in using them. It may well be advantageous to sacrifice some high initial performance in the interests of longer-term stability and predictability of performance. As computer monitoring and control of processes becomes more widespread, it is possible to monitor many more variables and assess their significance sufficiently rapidly to allow more efficient operation. The algorithms can take into account catalyst decay according to known patterns, but clearly unfamiliar patterns and irreproducible behavior from one charge to another are to be avoided if optimum conditions are to be achieved. Thus, catalyst life and death characteristics are important and must be treated in detail in choosing a catalyst for a process and designing the operating procedure; studies in this area can be difficult but very rewarding.
E. THECHOICEBETWEEN HETEROGENEOUS OR HOMOGENEOUS CATALYSTS The past fifteen years have seen evidence of great interest in homogeneous catalysis, particularly by transition metal complexes in solution; predictions were made that many heterogeneous processes would be replaced by more efficient homogeneous ones. There are two motives in these changes-first, intellectual curiosity and the belief that we can define the active center with
THE ECONOMICS OF CATALYTIC PROCESSES
231
precision in catalysts based upon known complexes and thus understand much more of the catalytic act than has been possible with traditional catalyst systems, and secondly, the belief that such catalysts may be more efficient and lead to economic advantage over traditional catalysts, but possibly at the expense of a more difficult product separation step. The situation has now changed and currently an area of considerable research interest is in “heterogeniaing” homogeneous catalysts. One such instance is to be found in the ethylene based manufacture of vinyl acetate (11). A homogeneous catalytic process based on palladium and copper salts was first devised, but corrosion problems were made much less serious in a heterogeneous system based on the same chemical principles. The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficultiesof catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two riva.1plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases.
IV. The Economic Factors and Constraints Affecting Catalyst Choice So far we have indicated the catalyst chemist’s job in qualitative terms only. We must now turn to the quantitative basis for deciding on the priorities of the different elements in any process opportunity or problem, and for recommending action. After this, we can summarize some typical stratagems that are suited to various typical situations. The first point to be clearly appreciated is that economic analysis does not lead to general or precise solutions. It leads to an approximate evaluation of a range of policy options, in terms that can then be judged in the light of the prosperity, objectives, and handicaps of the organization in which the work has been done. If circumstances are straightened, with recent profits poor and borrowing difficult, it is of no use to propose catalyst technology that requires complete plant rebuilding. If competitors are strong, it is necessary to be reasonably sure that the new proposals are competitive and not vulnerable to swift counterstrategy, such as pricecutting to delay the operation of the new process at full plant capacity. After a particular, and apparently successful research program, it might be entirely reasonable for one organization to translate the results into large scale practice, and much less reasonable for another organization, with a weaker position, or with better options for investing its funds, to do so.
232
J. DEWING AND D. S. DAVIES
A. GUIDANCE FROM THE STANDARD COSTSHEET: STRAIGHTFORWARD IMPROVEMENTS The simplest economic model for the guidance of the catalyst chemist is the standard cost sheet. This lists the variable costs (raw materials), fixed costs (capital charges) and semi-variable costs (conversion expense). Typically, these three elements may represent similar proportions of the overall cost per ton of product, but the circumstances following successful catalyst research can vary widely. The most universally applicable benefit is an improvement in raw material yield that can be achieved by the simple substitution of a new catalyst in the existing reactor. If the new catalyst works in a single tube trial unit, has the same sort of decay characteristics and life as the existing catalyst, and requires similar temperatures, it is likely that a large scale trial can be undertaken with very little risk and a good chance of success. If the purification train has been tightly designed, the trial may be best done a t a reduced feed rate, so that the output of the required product is no more than the original design rate. On this routine, capital charges and conversion costs do not change, and raw material requirements are less. But most practical catalytic processes are already high-yielding, so that the benefits cannot usually be more than marginal. Much larger benefits can be expected if there is a ready market for more product, if the work-up train has spare capacity, and if the reaction rate can TABLE IV Typical Eflects of Catalyst Incention on Cost Sheet Initial
50 25 75
Materials cost ($/ton product) Conversion cost ($/ton product) Capital charges ($/ton product)
~
~~
Cast
45 25 75
lo
Case %b
50 20 37
Case 30
30 20 50
Case 4d
30 10 25
-
__
-
-
-
150
145
107
100
65
~~
0 Case 1reflects a new catalyst, usable in the same plant, with 10% higher yield but no possible rise in throughput. Small but immediate benefit. Costing no extra capital. b Case 2 reflects doubled throughput with no market or purification limits. Large immediate benefit. N o extra capital. c Case 3 reflects a new process starting from cheaper materials, with a simpler process. Large benefit after substantial capital and development expense. Some risk. d Case 4 reflects the scale-up of the new process, without yield improvement but with a steadier catalyst. Benefits in operating and capital charges, after big capital expense and requiring big market increase.
T H E ECONONICS OF CATALYTIC PROCESSES
233
be increased. Under such conditions, a doubling of plant throughput could well be equivalent to a halving of raw material costs-or doubling of yield (very rarely conceivable). Sometimes a better catalyst will increase rate of both product output and yield, perhaps with a limited expenditure or better condensation equipment to deal with the extra product load. This will combine both the above benefits. A more steadily performing catalyst, requiring less attention and less frequent replacement, could permit a reduction in the semivariable costs for manning and maintenance stores. In the case of a catalyst system requiring frequent regeneration by burning off, a decrease in the carbon laydown (and consequent decrease in necessary burn-off frequency) may both increase throughput and reduce conversion costs. Table IV gives illustrations of these situations. It is very unlikely that any producer would lack the resources to conduct the necessary trials and realize the consequent improvements if scale-up went well.
B. GUIDANCE FROM CASHFLOW MODELS:MOREBASICCHANGES
If the new catalyst requires drastically different conditions, e.g. fluid bed operation instead of fixed bed operation, or if it needs substantial additions to the purification train, it is again possible to calculate the benefit in terms of the return (in reduced operating cost) on the new capital, but it is probably more informative to draw up a cumulative cash flow diagram. This is illustrated in Fig. 3. Cash flow diagrams simply involve the reckoning of the net money that has been cumulatively spent on, or saved by a project from its inception until a given time. When research and development are undertaken, the start is the spending of money (and increase of debt) until the results are implemented by improved manufacture and selling activities, after which time the debt can be recovered. It is usual to add interest payments to the debt, as interest becomes due. The maximum debt incurred during the project can be regarded as the “entrance fee” for that particular process or product. It is important to attempt to forecast this debt if there is any chance that there may be difficulty in borrowing such a sum, alongside other debts necessary at that time (for working capital, plant replacement, etc.). During the implementation of the improvement, money is spent, and debt is incurred, offset by operating margin as it improves. Interest is paid at intervals on the outstanding balance, and after a suitable payback time-which is shorter, the better the improvement-all the debt is recovered, and a better operation is running. This graph then gives three crucial pieces of information, viz. (a) the maximum risk in the form of new
234
J. DEWINQ AND D. S. DAVIES
+ Cash balance ($million)
cheaper operation
I L a b 8 pilot trial
( b)
Time (years) Cash balance 0 ( $ million) -I
-2
w\
t hrauqhout
i f
rIaNew plant items installed:
Process successful
some lost production
.I
(Cl
'Entrance fee"
Build second ploni
n
FIQ.3. Cash flow diagrams for catalytic successes.
Recover outlay
THE ECONOMICS OF CATALYTIC PROCESSES
235
debt, (b) the duration of extra debt, and (c) the profile of the ultimate benefit. It is also possible to visualize the staggering of the change, so as to reduce the overdraft at any given time, and to visualize various intervals before the extra capacity (if any) can be sold. It is possible to reflect the results of taxes and grants more exactly by showing the date of their payment. A further benefit is that this treatment makes the uncertainties more explicit. If the payback time is five years, then it is clear that the calculated benefit hangs on estimates of markets, costs, and prices up to five years ahead: the shorter the interval, the less the uncertainty. In troubled economic times, short payback strategies may be deliberately sought. FROM MODELS OF COMPETITION C. GUIDANCE
Catalyst chemists, like others, live and work in a competitive world. Competitors may react to new plants, or process improvements, in various ways, and it usually helps to look at the situation from their standpoint, and visualize dangers. In particular, if additional capacity can only be filled at the expense of a competitor’s market, his standard costs will rise, and he may be expected to react vigorously, by talking to customers, offering them benefits or security of supply and price, or price cutting. On the other hand, if the new capacity merely provides for market growth that no one else has the plant to supply, then there may be no adverse reaction. More generally, competitors themselves will also be making improvements that need taking into account. These improvements, naturally, will not be announced and must be guessed. Fortunately there is a very useful rough-and-ready method devised by the Boston Consulting Group (12 ) , based on the empirical observation that, for many manufacturers, cumulative experience generally permits an efficient operator to make the same sort of improvements. Plots of log (cumulative tonnage) against log (cost) are linear, with a slope that shows a 20-30oJo cost improvement for every doubling of cumulative manufacture. The causes of such improvements are varied, including bigger plant items, better operation, and better catalysts. It is therefore reasonable and prudent to plot a generalized experience curve for any process, and to identify the positions of one’s own organization and of all of the competitors by a knowledge of the cumulative tonnages and an inference of the costs. It is then possible to judge what pricing policies are open to others, and to avoid situations in which squeezes are likely. There is, however, a danger in this procedure. Knowing one’s own position it is easy to select one’s competitors’ most damaging counterstrategy; more often than not he himself will lack this clear view. Moreover, the classic progress down the experience curve depends on continuing management
236
J. DEWING AND D. S. DAVIES
Log
(cost)
Advantoge won by a through innovation Log (curnu lat ive manufacture)
FIG.4. Models of competition “learning curves.”
pressure on costs. Quite often, the market leader relaxes this pressure for substantial periods, and fails to reduce costs as fast as he might. Consequently, the well-known military error of missing opportunity by overestimating the enemy’s wisdom and competence occurs also in technological competition. Figure 4 shows some of the typical thinking arising from models of competition. Learning curves are based on the empirical observation that tight management leads, by various routes, to a 20-30% cost reduction for every doubling of cumulative output. The points on the graph are the costs deduced for two producers, A and B, at the end of years 1, 2, 3, 4,and 5. B has the larger capacity, and therefore builds experience faster. At the end of year 5, producer B could decide to price at level PI (which would give him a modest profit, but allow A none, so as to force A out of manufacture in due course) or at a higher level PZwhich allows A some margin, so that he finds it possible to trade but difficult to expand. Consideration of such curves help in decisions about scale of plant building and price strategy. Major innovation moves its author on to a new, lower learning curve: catalytic R. & D. is a good method for doing this. In the diagram, A saves his position in this way.
D. GUIDANCE FROM MODELS OF BUSINESS PORTFOLIOS: “ENTRANCE FEES” So far we have been considering catalyst inventions that call for little or no major extra capital expenditure, and pay off quite quickly. From time to time there is a major invention that leads to a completely new process, or (more rarely still) a completely new product. At this point, we enter the realm of major business decisions, for the sums of money needed to proceed
T H E ECONOMICS OF CATALYTIC PROCESSES
237
can soon become large in relation to the total sums that the organization can reasonably expect to borrow. To be sure, orthodox capitalist theory says that the money can always be found if the invention is good enough. What is missing from this statement is the standard of comparison: the invention has to be very much better than existing products of technology in order that borrowing can be drastically increased, and if existing products and processes are good, it is more difficult to establish crucially better or cheaper innovations. Furthermore, if existing manufacture is big, then economies of scale are considerable, and the new technology does not become attractive until it also is big. One cannot then run in new technology profitably on a comfortable, low-risk, small scale, for such a unit cannot be profitable and can only be regarded as a very expensive pilot plant. Figure 5 shows the way in which the requirement to invest heavily in new technology-thus loading it with a kind of “entrance fee”-has built up in the last few decades. The most spectacular case of products arising from a catalyst invention is that of the stereospecific hydrocarbon polymers made possible by the Ziegler-Natta work on aluminum alkyl/transition metal halide combinations around 1950. Until these catalysts existed, polypropylene, polyisoprene, and cis-polybutadiene could not be made, and linear polyethylene could not be made cheaply. For each of these products, very large investments were needed in big plants and in market development before they were competitive with the established, big thermoplastics and rubbers. Entrance fees ran into tens of millions of dollars. There have been several cases of new processes (requiring totally new Point of recovery of investment, ofter paying interest
1945 invention
( $ 100 million at 1972 values)
FIG.5. The growth of entrance fees. Reproduced with permission from D. S. Davies, Prac. Ray. SOC.A330, 149 (1972).
238
J. DEWING AND D. 6. DAVIES
plants) arising from catalyst inventions. One is the manufacture of vinyl chloride (for PVC) from ethylene, which depends on oxychlorination catalysts (developed from the copper systems used for the obsolete Deacon process) for converting HCl, air, and ethylene to ethylene dichloride. Another, and more spectacular case, is that of the Sohio process for manufacture of acrylonitrile by the so-called ammoxidation of propylene : CHsCH=CHz
+ NHa + lfOz-+ NCCHeCHz + HzO
This displaced more complex, capital-and-energy-intensive processes based on acetylene or ethylene oxide: CHzCHz
+ HCN
--t
CH*=CHCN
-+
CHzsCHCN
+ HzO
\ / 0
CH=CH
+ HCN
Ammoxidation illustrates several principles. First, it shows the benefit of “telescoping” two successiveprocesses into one reactor. The Sohio inventor, James D. Idol, Jr., observed that catalysts for two successive stages in an earlier ammoxidation procedure were very similar. He then found that the same catalyst could be used for both, thus eliminating a complete plant stage, at great saving of capital and operating cost: CHsCH=CHa CHz=CHCHO acrolein
+
4 0 2 -+
+ NHs +
0 2 --*
OHCCH=CHz acrolein CHtzCHCN
+ H20
+ 2H20
The second principle was the great defensive strength of an established, capital intensive procedure. I n the overall process for making acrylonitrile via acetylene, very big plants were needed for making the acetylene either by partial oxidation of methane or from carbide furnaces. Manufacture of HCN from methane involved further expense:
+ 3C -+ CaC2 + H20 --+ CHa + NHs + 1 3 0 2 CaO
+ CO CZHZ+ CaO HCN + 3Hz0
CaC2
-+
CzHz
+ HCN
-+
1
or 2CH4
+ Oz
--t
CzHz
+ CO + H 2 0
CHzaCHCN
The full cost of acrylonitrile manufacture based on methane was about 22$/lb in 1960 (allowing 15% return). But the marginal cost-with no capital charges or overheads-was only 7$/lb. The full cost of propylene ammoxidation (with 15% return) was about 13-16#/lb according to location. Thus, as soon as it was clear that ammoxidation was likely to
THE ECONOMICS OF CATALYTIC PROCESSES
239
succeed, the acetylene-based producers dropped the price to 14k/lb, a t which price they could afford to operate existing plants (but not build new ones). This greatly delayed the building of ammoxidation units in the less favored locations, and greatly extended the payback time of the earlier ammoxidation units (which, at 3040,000 tons/year, represented a big and adventurous scale-up) and required a further scale up to 100,000 tons/year for really attractive economics. Thus, the benefit from the Sohio catalyst invention was considerably transferred from the acrylonitrile producers to the fiber makers who were buying acrylonitrile and converting it t o acrylic fibers (and, in due course, transferred to the textile industry because of the progressive cheapening of the fibers). When catalytic invention thus has to be fitted in with other elements of an investment portfolio-plant extensions for existing technology, amenities, safety, effluent treatment, acquisitions-it is sometimes helpful to have models of the portfolio which help to see whether the organization is going to generate cash fast enough to go ahead as hoped, or whether cash constraints lie ahead. Figure 6a shows how the Boston Consulting Group recommend mapping portfolios (IS), and Fig. 6b shows a similar method (ICI) for assessing the pattern of relationships with customers and competitors. Neither yields definite answers, but both help to build up experience and images of situations that are adventurous or safe and clever or stupid. To decide whether to pursue a particular investment it is necessary to view likely demands from existing investments. The patches represent areas of business, or products, and represent estimates of profit at some future date. Projects in the COW square are profitable, because the producer is farther down the learning curve than his competitors. But low growth means that little or no new investment is needed. Hence cash is being generated by a strong, mature business. Projects in the DOG square face steadily increasing disadvantage, and reduction of profit, as producers with a higher market share pull ahead in experience and cost reduction. Projects in the * square are profitable, but require continuing and perhaps increasing investment. They are liable to be cash hungry. Projects in the ? square are usually in transition, and may subside dangerously into DOG or advance into *. Thus, invention may start in ?, move into * with expansion, and then move into COW if competitiveness is maintained or DOG if it is not. A good supply of COWS are needed to finance a substantial series of innovative enterprises starting in ?. Before investing, it is useful to consider whether strong customers may be
240
J. DEWINQ AND D. S. DAVIES
Boston
0 :
I
? 0
High
0'
oc"" 0"" 0 0
Rote of growth LOw
Low
High
Share of market (a)
ICI
Size relatlve to competitors
(b)
FIG.6. Portfolio analysis to guide investment in major catalyst innovation requiring new plant.
able to force prices of innovative products down quickly, or strong competitors may be able to outstrip a new product or process quickly. There are various strategies for moving out of the WEAK square to a stronger position. Overall strategies will usually involve a good combination of expensive but good new ideas, and lucrative but conventional "cash cows." Existing plants, for which the purchase price has already been absorbed, have great competitive strength : they can always be stretched, extended, and improved, and it is often very difficult for new technology, with its need to justify new capital, to beat the fighting, marginal cost of old, well optimized units. One then asks "Why innovate? Why not just become a cash dairy farmer?" The answer so far has been that the challenge of new technology has
THE ECONOMICS OF CATALYTIC PROCESSES
241
usually forced prices down to a level too low to justify new plants for optimized old processes, so that capacity increase can only be provided by new technologies. What usually happens is that there are periods when no one can afford to build a new plant. Shortages of product then develop, and prices rise to the point where plants can be built and are built to operate the new process. Then, as these are written off and improved, prices drop at which the old cash cows become very decrepit and, in due course, unprofitable. As they shut down, shortages again develop, prices rise, and the new process takes over completely-but usually after some unpleasantly unprofitable periods.
V. Factors Involved in Economic Improvement to Typical Processes and Guidelines for Assessment The history of the development of a wide range of catalytic processes illustrates the generality of the points we have made in the previous sections to such an extent that we can sum up with a set of precepts for the improvement of routes to, and of processes for, the manufacture of chemical products. Telescope the Process by Combining Stages. This has been done successfully in the conversion .of propylene to acrylonitrile by direct ammoxidation rather than oxidation to acrolein followed by reaction with ammonia in a separate stage, as was described in the earlier patent literature. The oxychlorination of ethylene and HC1 directly to vinyl chloride monomer is another good example of the telescoping of stages to yield an economic process. Make Useful Coproducts. The success of this approach to process development is heavily dependent on the market situation but in suitable cases where there is market demand for the total output of both products such processes can be successful. Examples here are the coproduction of phenol and acetone, which is essentially noncatalytic, and the more recently developed process for the cooxidation of ethyl benzene and propylene to produce propylene oxide and styrene. Cut Out Separations. This can produce significant savings regardless of the stage of the process to which it is applied, provided efficiency of other parts of the process can be maintained. Avoidance of treatments to feedstocks and intermediates is clearly advantageous as is the removal of the need to clean up catalyst residues in high density polyethylene. Cut Out Reactive Intermediates which Waste Energy. This precept is illustrated by the general move away from acetylene chemistry to ethylene over the past two decades.
242
J. DEWING AND D. S . DAVIES
Improve Catalyst Life and Steadiness. Regeneration or replacement of a catalyst is expensive both in direct cost and in lost production represented by the down time. Lowering the rate of deactivation of the catalyst whether by fouling, by sintering, or any other irreversible process will improve the economics of a process. Improve Selectivity. This is important at several levels ranging from achieving the highest efficiency in the use of feedstock to the elimination of harmful by-products which require additional processing steps for their removal. Extend Range of Feedstocks for the Process. Feedstock flexibility can enable maximum advantage to be taken of market fluctuations in the price and availability within the range of feedstocks which can be used. Obviously this is inappropriate in some cases where there is a close chemical link between feedstock and product, e.g. direct oxidation of ethylene to ethylene oxide, but in some processes, e.g. steam-reforming, flexibility is possible and may be advantageous. Improve Rate of Conversion. Increasing the activity of a catalyst may allow either higher output from a plant or operation of the plant under less arduous conditions thereby alleviating the constraints which limit the economic improvement of the process. Improve Chemical Engineering. Improvements in catalyst performance inevitably mean that the optimum plant operating condition will be different from that for the unimproved catalyst. Design changes may be needed to obtain the maximum benefit from improved performance. The cost of such changes must be taken into account when assessing the value of catalyst improvement. Use All Available Methods to Write Economic Scenarios. In assessing the cost and value of innovation two situations can occur if we take a simplistic, general view. First the innovation can be seen as being introduced in the form of expansion of capacity for the product, or second it can be seen as displacing existing capacity. The economic comparison in the former case is simply that of the full cost of the existing and the new technology, suitably weighted to take the uncertainties involved in the new technology into account. In the latter case, however, the full cost of the innovation must be compared with the marginal cost of the existing process. This is because the capital for the existing plant has already been spent and, in general, is irrecoverable and would thus be written off if the plant were replaced before the end of its useful life. Provided that the innovation looks attractive on this basis it is then necessary to consider the entrance fee in relation to the cash availability, and to consider all other known possibilities for technology replacement,
T H E ECONOMICS OF CATALYTIC PROCESSES
243
including conceivable improvements to the existing technology. These must be viewed within the framework of the future trends of raw materials prices and the likely pattern of realizations on the product over the next ten years. And, most important of all, it is vital to make a good assessment of the competitive position and the likely reactions of competitors. Bankruptcies, like military defeats, have resulted from the occurrence of a long and arduous period of trench warfare when preparation has been made for a quick Blitzkrieg. Only after all these matters have been clarified can the risk and likely payoff be properly assessed. Innovation depends on risktaking. Failure to assess the risk can bring condign retribution. REFERENCES 1 . Statistical Yearbook, United Nations, 1972. 2. United States Tariff Commission, 1971. 3. Burke, D. P., Chem. Week (1972), Nov. 1, p. 35; ibid., Nov. 8, p. 23. 4. Hydrocarbon Process. 49 (11), 113 (1971). 6. Tsao, U., Chem. Eng. (1970), May 18, p. 118. 6. Jones, M., Symp. Synthetic Fibre Processes, London,1972, Eur. Chem. News 22 (562), 29 (1972). 7 . Ohara, T., Hirai, M., and Shimiau, N., Hydrocarbon Process. 50 (II), 85 (1972). 8. Prescott, J. H., Chem. Eng. (1971), April 5, p. 60. 9. Campbell, J. S., Ind. Eng. Chem, Process Des. Develop. 9, 588 (1970). 10. Happel, J., “Chemical Process Economics.” Wiley, New York, 1958. 11. Stobaugh, R. B., Allen, W. C., and Stemberg, V. R. H., Hydrocarbon Process. 50 ( 5 ) , 153 (1972). 18. “Perspectives on Experience.” Boston Consulting Group, Boston, Mass., 1968. 13. Boston Consulting Group, Private communication.
This Page Intentionally Left Blank
Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA Institute of Physical Chemistry Polish Academy of Sciences Warstawa, Poland
.............
I. Introduction.. .... 11. Formation, Struct
245
d Nickel Hy-
. . . . . . . . . . . . . . . . . 247 ium and Its Alloys with
Gold or Silver. . . . . . ....................... IV. The Effect of Transformation into Hydride on the Catalytic Activity of Nickel and Its Alloys with Copper. .............................. V. Catalytic Activity of Other Metal Hydrides in Test Reaction of Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. General Remarks and Conclusions ............ References. ....................
253 268 283 285
1. Introduction The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals : nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations whieh are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. In addition to the surface physics and chemistry phenomena involved, a further effect may follow the interaction a t the hydrogen-metal surface, that is the absorption of hydrogen by the bulk phase of the metal. This absorption leads to the formation of a solid solution within a certain, usually low, range of hydrogen concentrations. However, with several transition metals, exceeding a certain limit of hydrogen concentration results in the formation of a specific crystallographically distinct phase of the 245
246
W. PALCZEWSKA
metal-hydrogen system, called a hydride phase. Some metals (e.g. Zr, Y) exhibit even more than one phase transformation of this kind. The solid solution of hydrogen in the original metal lattice has become designated the a-phase of the particular metal-hydrogen system. The hydride phase has been called the @-phaseof the system. Subsequent hydride phases, if formed, have been termed y, 6 etc. In the hydride the hydrogen occupies interstitial positions in the parent metal lattice. Such a system may be considered as an ordered "alloy" of two metals, one of which is hydrogen. There is evidence of hydride formation by many transition metals: in group I11 of the Periodic Table by Sc, Y (the lanthanides and the actinides as well) ; in group IV, by Ti, Zr, and Hf; in group V, by V, Nb, and Ta; in group VI, by Cr; and in group VIII by Ni and Pd. A number of monographs and reviews have dealt with fundamental problems in metal-hydrogen systems, among them being those by Smith (I), Barrer (6),Smiatowski (S),Libowitz ( 4 ) , Mackay ( l a ) , and Lewis (6). The recent conference of the Bunsen Gesellschaft and the Institut fur Festkorperforschung in Julich in 1972 was devoted to research on hydrogen in metal-fundamental and technological aspects of the phenomena involved currently studied in different scientific centers (6). It is tempting to associate the structural and electronic changes in a metal due to its transformation into the hydride phase with a respective change in the catalytic activity of the metal. Since many transition metals, able to form hydrides, are catalysts for various reactions involving hydrogen, one might thus expect the hydride under certain conditions to form spontaneously in situ during the reaction and in turn to change its mechanism and kinetics. The formation of a hydride may then in some cases be desirable, and in others not, to the aim of the catalytic reaction being studied. Among the three commonly used metal catalysts mentioned above which activate hydrogen, nickel and palladium form hydride phases of essentially the same type. The existence of a platinum hydride has not so far been proved. This review aims to present an account of the catalytic properties of palladium and nickel hydrides as compared with the metals themselves (or their a-phase solid solutions with hydrogen). The palladium or nickel alloys with the group Ib metals, known to form @-phasehydrides, will be included. Any attempts at commenting on the conclusions derived from experimental work by invoking the electronic structure of the systems studied will of necessity be limited by our as yet inadequate knowledge concerning the electronic structure of the singular alloys, which the hydrides undoubtedly are.
PALLADIUM AND NICKEL HYDRIDES
247
It. Formation, Structure, and Properties of Palladium and Nickel Hydrides
A short survey of information on formation, structure, and some properties of palladium and nickel hydrides (including the alloys with group IB metals) is necessary before proceeding to the discussion of the catalytic behavior of these hydrides in various reactions of hydrogen on their surface. Knowledge of these metal-hydrogen systems is certainly helpful in the appreciation, whether the effective catalyst studied is a hydride rather than a metal, and in consequence is to be treated in a different way in a discussion of its catalytic activity. The palladium-hydrogen system has been systematically studied since the 1930’s. A detailed account of these results and a bibliography of the subject is to be found in the monograph by Lewis (5). The nickel-hydrogen system was found by Baranowski and gmiaiowski (7) in 1959 during the electrolytic saturation of nickel wire with hydrogen in the presence of a catalyst of penetration. The @-phaseof nickel hydride was discovered and identified, by the X-ray diffraction method by Janko, as a face centered cubic phase analogous to the palladium hydride one. Its lattice parameters were also determined (8). The comprehensive set of isotherms characteristic of the palladiumhydrogen system may be found in the paper by Scholten and Konvalinka (9),reproduced in Fig. 1. The equilibrium pressure of hydrogen as a function of the system composition expressed as the atomic ratio H/Pd in the solid phase is represented there a t different temperatures. The absorption of hydrogen by palladium results at first in the formation of the solid solution; the atomic ratio H/Pd in the solid phase increases with hydrogen pressure; after attaining a limiting value of hydrogen content in the asolution, at a given temperature, the formation of the @-phasebegins and the horizontal section of the isotherm represents the pressure invariant coexistence of both phases. The H/Pd ratio finally reaches a certain value corresponding to the lower limit of the hydrogen concentration in the pure @-phase(e.g. at room temperature the H/Pd ratio in the p-phase is about 0.6, the equilibrium pressure of hydrogen is about 10 mm Hg). A further, even higher, increase of pressure leads to the continuous but comparatively small growth of the H/Pd ratio, the isotherm rising steeply up. On increasing the temperature the @-phaseappears a t increasingly higher hydrogen concentrations in the a-phase. Finally, at a temperature of about 300°C the two-phase region disappears and the isotherm has the appearance of a critical isotherm, analogous to that known, for example, for a one-component liquid-vapor system.
248
W. PALCZEWSKA
do3
4 O2
40
c
-
cE
4 N
z
0
4Q-4
40-=
\ \
104
,
I
,
I
I
0.2
0.4
0.6 WP d
0.8
FIG.1. Absorption isotherms of hydrogen in palladium within a large range of temperatures and pressures of hydrogen gas. Numbers denote temperature in ”C. Hydrogen pressure is given in the logarithmic scale. Broken line closes the area of the two-phase a fl region of the Pd-H system. Different shapes of experimental points denote different authors’ data, cited by Scholten and Konvalinka (9). After Scholten and Konvalinka (9).
+
The isotherms represented in Fig. 1 give a general idea of the equilibria in the Pd-H system under different p-T conditions. Most experimental /3 evidence shows, however, that the “equilibrium” pressure over Q coexisting phases depends on the direction of the phase transformation > p ~ (T, + ~H/Pd constant). This hysteresis effect at 100” process: p
+
PALLADIUM A N D NICKEL HYDRIDES
0.2
04
249
0.6
H/Pd
FIQ.2. Example of a hysteresis loop on the isotherm pn2 = f(H/Pd) obtained during absorption (upper curve) and desorption (lower curve) of hydrogen from palladium black at 100°C. After Sieverts and Danz (19).
is illustrated in Fig. 2. The phenomenon is explained as caused by a strain in the crystal lattice during its expansion in the @-phaseformation. It is accepted rather that the desorption isotherm represents the thermodynamic equilibrium (10, 11). The nickel-hydrogen system has not been studied in such detail. The isotherm at 25°C is presented in Fig. 3 on the basis of the results obtained by Baranowski and Bocheriska ( I l a ) .The P-phase of nickel hydride appears when H/Ni exceeds 0.04 at an equilibrium pressure of 3400 atm. The characteristic H/Ni ratio in the &phase then amounts to 0.6. Thermodynamic data characterizing the formation of palladium and 3500 -7
-t E c Q
n
a
2500
E m 2
z * 4500 (n
0.2 H/Ni
FIQ.3. Isotherm p~~ = f(H/Ni) at 25°C obtained during hydrogen desorption from nickel foil saturated with hydrogen. After Baranowski and Bochefiska (Ira).
250
W. PALCZEWSKA
TABLE I Standard Free Energies, Enthalpies, and Entropies of Formation of Palladium and Nickel Hydrides. Palladium Nickel hydride (10) hydride (f2) Go,cal/moIe Ha H", cal/mole Hz So,cal/"K mole Hz
-2820 -9325 -21.8
5640
-2100 -26.0
0 The standard free energies, enthalpies, and entropics calculated from the experimental data for the reaction 4Me Ha = 2MezH (where Me = Pd or Ni), at 1 atm of hydrogen pressure and 298°K.
+
nickel hydrides are compiled in Table I (10, 12). More data on the thermodynamics of the Pd-H system can be found in Ref. ( I S ) . As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni) , the a-phase, and 0-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the &phase exhibits a significant expansion of tfhelattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko {8),Majchrzak ( 1 5 ) , and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that a t moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium a t least up to about 100°C). Neutron diffraction studies have shown that in both systems Pd-H ( 1 7 ) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride "precipitation." In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. On uptake of hydrogen, at lower temperatures distant enough from that of the critical isotherm, the physical properties of palladium or nickel re-
PALLADIUM AND NICKEL HYDRIDES
25 1
main effectively not changed within the limits of the a-phase existence. The hydride P-phase, however, has certain markedly different physical properties. Among those of interest for catalytic research is the change in magnetic susceptibility. At room temperature, when the concentration of hydrogen in palladium reaches approximately 60 at. %, the paramagnetic susceptibility falls to zero (19). The saturation magnetization of nickel falls to zero at a H/Ni ratio of about 0.65, when the ferromagnetic nickel is converted into the hydride (20).The question whether this hydride is of paramagnetic or diamagnetic character still remains open (20a). In the Me-H systems one component of these “alloys,” namely hydrogen, has a simple electronic structure. Despite this apparent simplicity the nature of the bonding in transition metal hydrides remains a controversial subject ( 2 1 ) . From the change in the magnetic properties that has been observed one can assume that in these hydrides the hydrogen has a protonic character and its 1s electrons are transferred to the unfilled d bands in palladium (or nickel). Moreover the assumption of Mott’s rigid band theory is not necessary in this case. Following Switendick’s (22) considerations it can be assumed that the distribution of the density of states in the d band is different in palladium or nickel from those in the corresponding hydrides. The change in electronic structure does not consist solely of the filling of the d bands of the parent metal with additional hydrogen electrons and shifting the Fermi energy to higher values. Switendick in his calculations concludes that every atom of hydrogen introduced into palladium and kept there as PdHo.eshows the appearance of about 0.4new electronic states per unit cell below the top of the d band. This, together with the 0.36 holes already present in the palladium, allows for the filling of the band only when H/Pd = 0.7, which agrees quite well with the experimental evidence. For nickel hydride Switendick estimates that 0.90 electrons can be accornodated below the top of the d band. The screened proton model of nickel or palladium hydrides and Switendick’s concept of the electronic structure do not constitute a single approach sufficient to explain the observed facts. I n this review, however, such a model will be used as the basis for further discussions. It allows for the explanation and general interpretation of the observed change of catalytic activity of the metals, when transformed into their respective hydrides. Palladium-gold and palladium-silver alloys in absorbing hydrogen give rise to an a-phase similar to that of pure palladium. Subsequently, on exceeding the limiting hydrogen concentration under the characteristic physical conditions (i.e. p , 7’) they transform into the respective P-hydrides. The formation of a hydride phase has been observed a t room temperature with up to about 40 at. % of a group IB metal in the alloy, for
252
W. PALCZEWSKA
0s
a
3.600-
I
400
80
40
60
20
0
Nil% wt)
FIG.4. Lattice parameter changes of Ni-Cu alloys and of Ni-Cu hydrides from 100% by weight of Ni t o 100% wt. Cu. 0 , Ni-Cu; 8-Ni-Cu hydride phases of alloys with different Ni content. After Baranowski and Majchrzak ($5,,2513).
+,
example in the case of Pd-Ag-H system. The critical temperature for coexistence of a- and p-phases decreases with an increase in the content of group IB metal. Thus, e.g. in the Pd-Ag-H system with 10,20,30,40at. % Ag, the two-phase region, LY 0, extends respectively only to 144O, 63", -91", -219°C (23). The disappearance of the paramagnetism of palladium-silver alloys (rich in Pd) when the ratio ( H Ag)/Pd = 0.6 (24) illustrates that the effect of both these "alloying" elements in palladium is additive and each one contributes essentially in the same way to the change of magnetic susceptibility af palladium. As far as the lattice parameters arc concerned, the difference between those of a particular host alloy and of its respective hydride decrease as the group Ib metal content in the alloy increases. The hydrides of copper-nickel alloys have been studied by Baranowski and Majchrzak (25, 25a), who observed their existence up to a ratio Ni/Cu = 1. Figure 4 represents the lattice parameter of the alloys and their 8-phase hydrides as a function of the alloy content in nickel and copper. A rough estimation of the critical temperatures of coexistence of the (a B)-phases in two Ni-Cu-H systems containing 59 at. % and 63 at. % nickel was made by Majchrzak ( 2 6 ) .Both phases, a and p, were identified by the X-ray diffraction method. The presence of the @-phasewas not seen above 47°C for the alloy with 63 at. yo Ni and above 20°C for the alloy with 59 at.% Ni. Though this method gives only approximative numerical values, one can make conclusions of a general character, e.g. that the critical temperature of the Ni-Cu-H system increases sharply with a growing content of nickel in the Ni-Cu alloy, and that one might expect the critical temperature of the coexistence of the a- and p-phases
+
+
+
PALLADIUM AND NICKEL HYDRIDES
253
in the Ni-H system to be high (perhaps much higher than in the Pd-H system). It is worth mentioning that the hydrides of the alloys are formed with much greater ease than those of the respective pure transition metals. This is probably due to the fact that the increase in lattice parameters caused by the incorporation of hydrogen is smaller in the former case-less work is thus required to be done by the system and the process is energetically more favorable. The above account of the changes that take place when palladium, nickel, or their alloys are converted into their respective hydrides is of course not an exhaustive survey. Such was not the aim of this section. The conditions under which the hydrides can form and exist and some data on their structure have been presented here only in the detail necessary for further discussion of their role in the catalytic behavior of palladium and nickel.
111. Catalytic Activity of Hydride Phases of Palladium and Its Alloys with Gold or Silver The authors of numerous papers dealing with the catalytic activity of palladium (or of its alloys with silver or gold)’ in various reactions involving hydrogen have frequently drawn attention to a self-poisoning that occurs as the catalyst is sorbing hydrogen. Such observations were mostly reported without otherwise associating them with this specific ability of palladium and its alloys, which is the formation of a 8-hydride phase. Experimental observations were not always accurately recorded to enable us decide now, a posteriori, with which phase of the metal-hydrogen system the authors could actually be dealing. One is rarely able even to determine with certainty the exact conditions under which the self-poisoning occurred. From the thermodynamic point of view the hydrogen pressure, the temperature, and the alloy composition would be of importance here. In the kinetics of the process of penetration of hydrogen into the metal, the state and structure of the metal surface and the presence of some impurities enhancing or inhibiting this process play an important role. Many authors were not aware of the importance of these and other factors, as they did not concentrate their attention on the hydride phase formation, its thermodynamics, and kinetics. What is worse they have even sometimes erroneously interpreted a connection bekween the experimental conditions and the observed poisoning effect of the hydrogen. In this part of the review “palladium alloys” refers to those containing more than 40% Pd, i.e. such alloys which are able to form a 8-hydride phase.
254
W. PALCZEWSKA
FIG.5. Arrhenius plots for para-hydrogen conversion on palladium wire catalysts. 0,p~~ = 1.2 mm Hg; A,p~~ = 6.1 mm Hg; 0,after the exposure of a wire to atomic
hydrogen produced in rf discharges. Compiled after Couper and Eley (29).
However this was not always the case. It is possible to demonstrate, on the basis of selected examples from the literature representing the experimental evidence and the authors’ original interpretation, that the catalytic activity of palladium or its alloys changes sometimes dramatically, when there is a possibility of their being converted into the corresponding hydrides. As early as 1923 Hinshelwood and Topley (27) noted the “exceptionally erratic behavior” of palladium foil catalyst in the formic acid decomposition reaction within 140-200°C. The initially very high catalytic activity decreased 102 times during the exposure of palladium to hydrogen, which is a product of the reaction. Though the interpretation does not concern the 8-hydride formation, the authors’ observation deserves mentioning. When studying the kinetics of diffusion of hydrogen through palladium, Farkas (28) noticed the difference in catalytic activity of both sides of the palladium disks or tubes for the parahydrogen conversion; the energy of activation was greater on the inlet side than on the outlet side, where due to extensive desorption of the hydrogen its concentration could be lower. The poisoning effect of hydrogen when dissolved in palladium was for the first time properly described and interpreted by Couper and Eley (29) in 1950 in their study of the fundamental importance of the development of theories of catalysis on metals. The paper and the main results relate to the catalytic effect of an alloying of gold to palladium, on the parahydrogen conversion. This system was chosen as suitable for attempting to relate “catalyst activity to the nature and occupation of the electronic energy
PALLADIUM AND NICKEL HYDRIDES
255
levels of the catalyst.” In the interpretation of their results, the authors stated that a decrease in the catalyst activity was a consequence of a filling of the palladium d band by s electrons of gold. However, an increase in the activation energy of the reaction studied was observed not only on introducing gold as a component of the alloy, but also, quite analogously, on the absorption of hydrogen by palladium. The change of the rate of reaction on the palladium wire as catalyst was studied within the temperature range of 170-350°K. The hydrogen pressure was 1.2 or 6.1 mm Hg. Certain wire samples were exhaustively saturated with hydrogen by exposing them, before the reaction, to atomic hydrogen which had been produced by an electrodeless discharge. The set of results is compiled in Fig. 5. The highest rate of reaction together with the lowest activation energy can be observed for a palladium wire previously outgassed at 600°K and then used as a catalyst for parahydrogen conversion under a pressure of 1.2 mm Hg. Increasing the hydrogen pressure diminished the reaction rate. The activation energy increased from 3.5 kcal mole-’ for p = 1.2 mm Hg to 6.5 kcal mole-’ for 6.1 mm Hg and even to 11 kcal mole-’ for the standard p = 1.2 mm Hg, when the wire had been previously charged with atomic hydrogen. It should be added to the data given in the paper that in the temperature range 170-350°K the appreciated stationary hydrogen pressure over the two-phase Pd/H system ranges from mm Hg to 70 mm Hg, respectively. At the very beginning the reaction vessel containing the palladium catalyst filament was filled with para-hydrogen and then kept at liquid nitrogen temperature. At a certain moment (to = 0) the electrical heating of the palladium filament sample to the required temperature was begun. Thus the hydride formation to some extent could be already advanced under the initial conditions of low temperature. A similar, but much stronger, effect was achieved by previously exposing the palladium sample to the action of atomic hydrogen. Owing to the low desorption rate of hydrogen even at higher temperatures (as noted by Couper and Eley) the a p two-phase system of hydrogen in palladium could continue at the temperatures studied. Couper and Eley took into account the nonuniform saturation of the palladium filament with hydrogen and assumed such a high concentration of dissolved hydrogen in the surface layer that in effect the palladium hydride phase could be present there. The authors just attributed the observed increase in activation energy of the para-hydrogen conversion to the presence on the surface of the 0-hydride phase of the Pd-H system. The authors’ final conclusion is of fundamental importance for the mechanism of the poisoning effect of the “hydride” hydrogen for a palladium catalyst: “The d-band of palladium may also be filled by elec-
+
256
W. PALCZEWSKA
trons from dissolved hydrogen atoms which cause a similar increase in activation energy” (i.e. similar to that observed in gold-rich palladium alloys). The formation of hydrides of palladium or its alloys is a major complication in forming conclusions from the experimental work in the case of such studies, where it is the kinetics of a given particular reaction of hydrogen that is of primary interest. The process of the hydride “precipitation” in a solution of hydrogen in a metal, as well as the process of its decomposition, obey a kinetics of their own. Not only the given main reaction catalyzed by the palladium (or its alloys) but also, though in a specific way, the formation of a catalyst hydride phase depends on the state of the metal surface, the size of crystallites, the surface defects, and the presence of some particular poisons and promoters. In consequence a catalyst may itself change in time during the study of the given reaction of hydrogen. Moreover, in the case of hydride intervention, still a further factor, namely the kinetics of hydrogen diffusion into the metal, influences also the overall kinetics by removing a reactant from a reaction zone. In order to compare the velocity of reaction of hydrogen, catalyzed by palladium, with the velocity of the same reaction proceeding on the palladium hydride catalyst, it might be necessary to conduct the kinetic investigations under conditions when no hydride formation is possible and also when a specially prepared hydride is present in the system from the very beginning. Scholten and Konvalinka (9) in 1966 published the results of their studies on the kinetics and the mechanism of (a) the conversion of parahydrogen and ortho-deuterium and (b) hydrogen-deuterium equilibration. At first the a-phase of the Pd-H system was used as catalyst, and then the results were compared with those obtained when the palladium had previously been transformed into its p-hydride phase. The authors stated at the beginning of their work that to understand the mechanism of the reactions studied required an unambiguous determination of the influence of the hydrogen pressure on the rate of conversion or equilibration reactions. This could be possible only when dealing with a palladium catalyst incapable of absorbing hydrogen, i.e. with the palladium samples previously fully transformed into the 0-hydride phase, in which the H/Pd ratiq would be constant, almost independent of the hydrogen pressure. Then, for example, at room temperature: under p = 1 atm, H/Pd = 0.68; when under p = 10 atm, H/Pd = 0.70; and under p = 1000 atm, H/Pd = 0.80 only. The palladium was in the form of a sponge for investigations in the temperature range +40° to -4O”C, and in the form of wire for higher temperatures. The samples were activated by an oxidation-reduction procedure. It seems likely that Scholten and Konvalinka studied the effect of
PALLADIUM AND NICKEL HYDRIDES
257
temperature on the velocity of the conversion of para-hydrogen on palladium, beginning the kinetic measurements at higher temperatures and then proceeding to lower ones. When the pressures and temperatures were still remote from those characterizing the phase transition a! ---f @, i.e. when only an a-phase is present, a strong dependence of the Arrhenius plot on pressure is observed. This is especially marked for the highest of the pressures applied-265 mm Hg. The activation energy and the preexponential term increase, in accordance with earlier results by Couper and Eley concerning wires not preexposed to atomic hydrogen; Scholten and Konvalinka collated their results into Table 11, treating them as characteristic for the a-phase. A ten to hundredfold decrease in the velocity of the reaction, seen as a break down of the Arrhenius plot, is observed at a temperature which, for any given pressure, is always higher than that thermodynamically foreseen for the beginning of the &-@ transition (this discrepancy is smallest at 265 mm Hg pressure). The marked decrease of the rate of reaction is characteristic of the appearance of the 0-hydride phase. The kinetics of reaction on the hydride follows the Arrhenius law but with different values of its parameters than in the case of the a-phase. The catalytic activity of the “pure” @-palladiumhydride has been studied under the appropriate temperature and pressure conditions. The palladium sample was converted into the hydride in a manner which bypassed the area of coexistence of the phases. This was achieved by suitably saturating the metal with hydrogen at 35 atm above the critical temperature and then subsequently cooling the sample to the required temperature and reducing the hydrogen pressure. This method of sample preparation allowed one to avoid cracking the palladium crystallites, which would TABLE I1 The Arrhenius Equation Parameters for the ParaHydrogen Conversion on the u-Pd-H Phase (9) Activation energy p (mm Hg) (kcal/mole) 265 50 6
1.2
9.5 9.3 6 . 3 (6.3p 4.3s
Preexponential factor (molecule cm-* sec-1) 2 . 9 X 10Pa 1.4 X 1V8
1.5 X 10P2 (1.5 X 1W2P 5 X 1P”
Results of Couper and Eley (89).
258
W. PALCZEWGKA
TABLE I11 ~~~~
~
~~
~~
-
Reaction Kinetic equations on the 8-Pd-H phase
pHn = oHz oDz = pDa HZ= Da
5 . 8 X 102*pO.mexp( -12400/RT) 6 . 7 X lOz2’po~~exp(-12670/RT) 3 . 5 X 10zapo~64 exp( -12540/RT)
a Rate of reaction expressed in molecules.cm-2 sec-1. b Initial mixture 1 :1.
lead to an increase of specific area of the sample. This phenomenon appeared when the transformation into the &phase went via an (a phase. Table I11 lists the kinetic equations for the reactions studied by Scholten and Konvalinka when the hydride was the catalyst involved. Uncracked samples of the hydride exhibit far greater activation energy than does the a-phase, i.e. 12.5 kcal/mole, in good accord with 11 kcal/mole obtained by Couper and Eley for a wire preexposed to the atomic hydrogen. The exponent of the power at p amounts to 0.64 no matter which one of the reactions was studied and under what conditions of p and T the kinetic experiments were carried out. According to Scholten and Konvalinka this is a unique quantitative factor common to the reactions studied on palladium hydride as catalyst. It constitutes a point of departure for the authors’ proposal for the mechanism of the para-hydrogen conversion reaction catalyzed by the hydride phase. Assuming the composition of the hydride to be expressed by Pd3H2 (which corresponds to PdHo.a,) and bearing in mind the interstitial positioning of the hydrogen in the palladium lattice, the authors postulate the existence of the following equilibrium at the surface of the 8-hydride phase
+ e)-
K
= [H~1,2/[Pd)n~p~m
where [H,& determines a surface hydride, [Pdln a surface palladium atom, p~~is the hydrogen gas pressure, and K is an equilibrium constant for the palladium hydride formation. The concentration of free palladium atoms in the surface is then
In order for the reaction to proceed, hydrogen adsorption must be followed by its diffusion over the surface amongst other mobile adsorbed species
PALLADIUM AND NICKEL HYDRIDES
259
until a vacant site on the surface is found. Then one obtains the following expression for the rate of the hydrogen reaction
r = c(T)p~,[Pd], = c’(T)~&‘’ which is the product of the number of collisions of hydrogen molecules with the catalyst surface and the chance of an encounter with a “free” palladium atom. Such simple considerations led Scholten and Konvalinka to confirm the form of the dependence of the reaction velocity on the pressure, as had been observed in their experiments. Taking into account a more realistic situation, on the polycrystalline hydride surface with which a hydrogen molecule is dealing when colliding and subsequently being dissociatively adsorbed, we should assume rather a different probability of an encounter with a hydride center of a p-phase lattice, an empty octahedral hole, or a “free” palladium atom-for every kind of crystallite orientation on the surface, even when it is represented, for the sake of simplicity, by only the three low index planes. Nevertheless it does not change the principle of the mechanism proposed by Scholten and Konvalinka, i.e. the ability to act catalytically of only the superficial palladium centers released from the vicinity of the interstitial hydrogen. Bearing in mind the dynamic character of the equilibrium in a palladium-hydrogen system as a whole is to regard such centers as being mobile in the surface layer of the hydride. Rieniicker and Engels (30) investigated also the para-hydrogen conversion on palladium and on its alloys with silver. They aimed to relate the catalytic activity of the metals studied to their ability to dissolve hydrogen. Under the conditions of the experiments ( p = 200 mm Hg and the temperature range from the lowest 143”C, for 30% Ag, to the highest 200-3OO0C, for most alloys) one must exclude the possibility of hydride formation in the Pd-Ag alloys (23). The attempt of the authors to find a connection between the observed change of catalytic activity of Pd-Ag alloys in the para-hydrogen conversion (and in the benzene hydrogenation a t 150°C) with the hydride phase formation seems to be unfounded. The reaction of the heterogeneous recombination of atomic hydrogen on the metal, acting as catalyst, appears to be the simplest of all the reactions involving hydrogen. It can be regarded as an elementary test reaction for studying the catalytic activity of different surfaces with respect to hydrogen. Bearing in mind the results of kinetic studies by Smith (SI),L m e t t et al. (32, 33) and the mechanism proposed by Ehrlich (34) one may assume that a rate determining step of the overall heterogeneous reaction 2H
=
HZ
260
W. PALCZEWSKA
r.f. coil
I I lgm
\Thermostating
bath
To pumps
FIG.6. Side-arm tube of the apparatus for the determination of the coefficients of the heterogeneous recombination, y , of atomic gases previously dissociated in the rf discharge zone. The heterogeneous recombination proceeds on the inner glass walls of the horizontal side-arm tube and on a catalytically active cylindric sample of the metal investigated (Smith-Linnet t method).
is the reaction of hydrogen adatom with a free hydrogen atom
4-H = &, an example of the Rideal-Eley mechanism in the case of the above reaction. The kinetic equation obtained experimentally is of the first order, in the case of the Smith-Linnett method, thus serving for confirmation of the mechanism proposed. Dickens et aZ. (35) studied palladium and palladium-gold alloy foils as catalysts in the atomic hydrogen recombination. The gold content was 12, 31,45, 72, or 100 at. %. A stream of hydrogen gas, partially dissociated into atoms by radio frequency discharges, diffuses into the side-arm of the apparatus shown in Fig. 6. Part of the interior surface of the side-tube is lined with the foil whose catalytic properties are being investigated. On colliding with the walls of the apparatus the atomic hydrogen undergoes the recombination and its partial pressure, amounting a t the beginning to about 10% of the total hydrogen pressure (of about lo-' mm Hg), decreases. In a steady state, in the side-tube the rate at which atomic hydrogen is diffusing becomes equal to that with which i t disappears as a result of recombination. When all the rest.rictions necessary for the SmithLinnett method and valid considemtions are respected (31-36), one obtains the following relation from the analysis of the relevant kinetic equations n no exp[( - 7 ~ / 2 R D ) ~ ' ~ r ] , Had
-
where n is the concentration of atomic hydrogen in the side-arm a t a distance x from the discharge tube, no the concentration a t x = 0, y the co-
PALLADIUM AND NICKEL HYDRIDES
261
efficient of recombination, i.e. the ratio of the number of collisions which result in recombination to the total number of collisions with the given catalyst surface, D the diffusion coefficient, R the radius of the side-tube, and E the mean atomic velocity, The above equation is of course only a one-dimensional approximation of an exact general three-dimensional expression. Two thermocouples, Em at x = 0 and E, at a distance x, permit the monitoring of the atomic hydrogen concentration change along the sidetube. The atoms recombining on the thermocouple tip covered by the active catalyst evolve the heat of reaction and thus the thermoelectric power becomes a relative measure of the concentration of atoms in the gas phase. Finally, one obtains for the direct use in an experimental work the following equation y = Az (2RD)/E,
where A , an experimentally determined quantity, is the slope of a straight line plot obtained when the logarithms of the ratio of thermocouple readings are plotted against x
A
=
d[ln (E,/E,)]/dx.
The recombination of atomic hydrogen on palladium, gold, and a series of their alloys was investigated at room temperature. The values of the coefficient of recombination obtained after a short period of exposure of the metal catalyst to the atomic hydrogen action may be regarded as representing for these metals the catalytic activity in the reaction involved. However, to ensure the cleanliness of the inside walls, the catalyst surface, and thermocouple tips and to achieve the steady state of reaction, a longer exposure to atomic hydrogen is desirable. During such a procedure the authors noticed a steady decrease in the catalytic activity of palladium and its alloys with the low content of gold. The poisoning spread from the side nearer to the discharge toward the more distant parts of the catalyst sample. Foils which had not been previously annealed and those which were being investigated for the first time maintained their catalytic activity, when exposed to atomic hydrogen, for a longer period. On the other hand, foils removed from the apparatus after the kinetic experiment, then annealed in vacuo at 7O0-90O0C, and finally investigated for the second, third etc. time became poisoned much more quickly and efficiently. If the initial catalytic activity of palladium and its alloys with gold (rich in Pd) could be expressed by the order of lo+, after a sufficiently long exposure to atomic hydrogen the order fell to 10" (Fig. 7 ) . Table IV comprises the main results published by the authors in 1964.
262
A:
W. PALCZEWSKA
-2.0
I
I I I I
0
f
I
I I I
2.4
b
2
d
5.8
I
5.2
I
20
0
40
60
80
400
% A" FIG.7. Changes of the coefficient of recombination, y, of H atoms on the surface of Pd-Au alloy foil catalysts a t room temperature. 0 ,Initial values of log 7; 0 ,final values representing catalytic activity of Pd and its alloys containing absorbed hidroeen. Broken line denotes the alloy Pd40Au60 which represents the upper limit of gold content in Pd-Au alloys closing the region of Pd-Au hydride formation. After Dickens et al. (36). TABLE IV
Coeficients of Recombination, y , of Hydrogen Atoms on Pd and Pd-Au Foil Catalyst@
Metal catalyst
After a short exposure to H
Pd Pd88Au12~ Pd69Au31 Pd55Au45 Pd28Au72 Au
1 . 0 x 10-2 9 . 5 x 10-3 1 . 2 X 10-2 1 . 0 x 10-2 5.5 x 10-8 5.7 x 10-4
t = 20°C; p
FJ
After a long exposure to H * 1.2 x 2.5 x (1.3/2.0) X (1.2/2.1) x 1.5 X 5.8 x
0.2 mm Hg.
10-
10-0 lo-' 10-4
* Observed only for a first section of a cylindrical sample, near the atomic hydrogen source. 88 at. % Pd and 12 at. % Au.
PALLADIUM A N D NICKEL HYDRIDES
263
The poisoning effect exhibited by atomic hydrogen with regard to palladium and palladium rich alloys with gold can only be explained by these metals absorbing hydrogen and then being converted into the corresponding hydrides, much less active as catalysts for the reaction studied. The successive hydrogen absorption-desorption procedure (when the same sample was used for the second time after its annealing) results in disintegration of metal crystallites, thus activating the metal surface for the process of hydrogen penetration. This phenomenon is widely known and frequently used in order to obtain quickly a hydride sample with quantitatively reproducible characteristics. The formation of palladium hydride in situ in the side-arm of the apparatus at room temperature and a t a pressure of lo-’ mm Hg would seem to be inconsistent with thermodynamic data. According to these such a transition 01 + /3 at room temperatures would require the hydrogen pressure to be two orders of magnitude greater. However, this discrepancy is only apparent since the thermodynamic data relate to an equilibrium state involving molecular hydrogen, in which the partial pressure of atomic hydrogen should be only vanishingly small mm Hg). Under the experimental conditions related above the pressure of atomic hydrogen was comparatively enormously large and the corresponding “equivalent” equilibrium pressure of molecular hydrogen would be certainly adequate for the hydride formation. The similar phenomenon of poisoning in situ of a palladium catalyst by hydrogen which was in this case the product of a reaction was observed by Brill and Watson (37). The reaction studied was the decomposition of formic acid HCOOH
=
Ha
+CO,
one of the test reactions commonly used in investigat’ionsof the catalytic activity of metals. Palladium foil or the same foil cut into small pieces catalyzed the reaction in the range of temperature from 50 to 140°C. The reaction was kinetically of zero order; the logarithm of the reaction rate as a function of 1/T is represented in Fig. 8. Two characteristic sections with different energies of activation are clearly visible for all kinds of catalyst samples. The reaction when proceeding on palladium foils (within 100140°C) or on foil pieces (within 70-110°C) has an energy of activation of 32.9 kcal/mol. At a temperature range of 50-75”C, the value is much lower: 5.3 kcal/mole for palladium freed of presorbed hydrogen, or 12.3 kcall mole for palladium kept overnight in hydrogen ( p = 50 mm Hg) at 70°C. The authors conclude that the higher energy of activation is characteristic of palladium transformed into the /3-hydride phase owing to the absorption of hydrogen which is forming at the catalyst surface during the decomposition of formic acid molecules. The hydride appears at the higher range
264
W. PALCZEWSKA
2.6 2.8 3.0 3.2 YT ( 1 0 0 0 P K )
FIG.8. Arrhenius plots for the formic acid decomposition on palladium foil (1) and small pieces of this foil (2) at a higher temperature range, when hydrogen evolving as a product of the reaction was absorbed by Pd and transformed into the 8-Pd-H hydride phase. At the lower temperature range the reaction proceeds on the a-Pd-H phase, with a higher activation energy when the foil was “hydrogen pretreated” (2a), and a lower activation energy for a degassed Pd foil (3a). After Brill and Watson (97).
of temperature, when the rate of the reaction is great enough for a sufficiently large supply of hydrogen. Below 70°C the palladium sample surface is the solid solution with a different content of hydrogen. It seems justified to supplement the authors conclusions by adding that in the case of samples pretreated with hydrogen their higher energy of activation (12.3 kcal/mole) may result from the presence of a certain content of the 0-hydride phase in the a-solution phase. The formation and presence of both phases of the Pd-H system in the palladium catalyst samples investigated was confirmed by Brill and Watson by the values of the magnetic susceptibility of the samples investigated under the same conditions as in the kinetic studies. In order to follow further the effect that hydride formation has on the catalytic activity of palladium and its alloys it would be of interest to investigate a group of reactions involving the addition of hydrogen to a double or triple bond. Palladium itself has found a well-known wide application in such reactions. Nevertheless even where p-hydride formation is very probable it is still relatively rare to find considerations of this possibility in most publications. When studying the selective hydrogenation of acetylene on Pd (and Pd-Ag alloys), Bond et al. (58) observed that the prolonged exposure of palladium catalyst to hydrogen resulted in reducing its ability to hydrogenate acetylene and completely poisoned the hydrogenation of ethylene. The phenomenon was explained in terms of the poisoning effect of hydrogen dissolved in Pd and donating its s electrons to the unfilled d band of Pd.
PALLADIUM AND NICKEL HYDRIDES
265
Rennard and Kokes (39) in their paper stated directly that their purpose was just to study the catalytic activity of palladium hydride in the hydrogenation of olefins, in this case ethylene and propylene. Kokes (39a) in his article recently published in Catalysis Reviews summarizes the results of studies on such catalytic systems. Palladium sponge was sorbing hydrogen a t -78°C from the gas phase and then was used as the catalyst (38). The hydrogen concentration in palladium corresponded to resultant formuIas PdHo.ll, PdHo and PdHo .39. Although the authors call the samples of catalyst “palladium hydride” and their content of hydrogen “hydride concentration,” undoubtedly they are dealing in their investigations with the coexisting a B phases of the Pd-H system. The difference in hydrogen content thus is to be related t o the various ratios of both phases. Ethylene or propylene were hydrogenated within the temperature range -64°C to -98°C by the hydrogen desorbing from the Pd-H catalyst. It was stated that the rate of reaction of hydrogenation studied depended on the concentration of hydrogen in the catalyst. While being essentially a zero-order reaction with respect $0 hydrogen and hydrocarbon pressures, the hydrogenation remains a first-order reaction with respect to the concentration of the hydrogen in the palladium. Although the rate of hydrogenation via “hydride” hydrogen increases with the rising concentration of hydrogen in the palladium the corresponding rate constants show a fall as the hydrogen content increases. The activation energy values amount to 8.6 kcal/m,ole for PdHo.ll, 7.7 kcal/mole for PdHo.*4, and 7.5 kcal/mole for PdHo.40. From the consideration of the experimental results and the mechanism of the overall reaction as formulated by Rennard and Kokes, it follows that the addition of the surface hydrogen atom to the olefin admolecule is a rate determining step. Despite the fact that the reaction of hydrogenation is consuming the hydrogen from the Pd-H catalyst, it does not influence the rate of the reaction. The authors stated that the diffusion of hydrogen from the bulk to the surface was rapid enough to insure the independence of the hydrogenation rate from the velocity of diffusion. The catalytic system studied by Rennard and Kokes was in fact very complex. It can be expected that the satisfactory prolongation of the reaction should, however, result in a deviation from the formulated kinetics. Unfortunately no investigation comparable to that of Scholten and Konvalinka has been done in the case of olefin hydrogenation. Such a study of the catalytic activity of the “pure” p-phase of palladium hydride in comparison with the CY- or (a fi)-phases would supplement our knowledge concerning catalytic hydrogenation on palladium. Summarizing the conclusions formulated by Rennard and Kokes, it
+
+
266
W. PALCZEWSKA
1.2 50
100
It (mirll
450
FIG.9. Decrease of the catalytic activity of palladium on pumice with time. Acatalytic activity of Pd in initial measurements at 30°C; B-catalytic activity of Pd at 30°C after mercury vapor is frozen out; C-catalytic activity of Pd at 118°C after removing mercury vapor. (r& and (TO),, are the initial reactmionrates for the first and nth reactions (mm Hg/min). After Mann and Lien (41).
should be stated that the decrease in the rate constant as the hydride concentration increases can be regarded as further evidence of the poisoning effect of the hydride hydrogen. On the other hand, the increase of the rate of hydrogenation on palladium containing large amounts of hydride would have to be explained by the more rapid desorption of hydrogen from the Pd-H system more rich in hydrogen. A similar reaction was studied by Kowaka (40) who investigated the catalytic activity of palladium and its alloys with silver in the hydrogenation of ethylene. The author alluded to the poisoning effect of hydrogen pretreatment of the palladium catalyst. Among recently published results on the kinetics of hydrogenation of unsaturated hydrocarbons on palladium those by Mann and Lien (41) appear worthy of mention. They hydrogenated propylene on group VIII metals as catalysts and found the catalytic activity t o remain constant over a period of several days. Palladium, however, behaved in an exceptional way; inspite of being used in the same form as the other catalysts (i.e. dispersed on a pumice carrier) , the palladium quickly lost its activity. This, the authors ascribed t o poisoning by mercury vapor, but even its freezing out failed to eliminate the observed poisoning effect. Figure 9 illustrates these results, the poisoning of the palladium catalyst being quite apparent and even more pronounced as the temperature is raised. Since such poisoning appears solely in the case of palladium it seems far more justified to associate it with the formation of palladium hydride during the hydrogenation reaction. At a temperature of 30°C and a hydrogen pressure of 18 mm Hg in the reaction mixture the easy transformation of finely
PALLADIUM AND NICKEL HYDRIDES
267
dispersed palladium into the hydride can proceed. At 100°C the required pressure should be 200-300 mm Hg. Unfortunately the respective data are not available in the reported paper which would allow as t o comment fully on the experimental evidence represented in Fig. 9. The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm) . It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. Many other authors studied the catalytic activity of palladium in more complicated hydrogenatbn reactions because of being coupled with isomerization, hydrogenolysis, and dehydrogenation. In some cases the temperatures at which such reactions were investigated exceeded the critical p)-phases; in the other case the temperature for coexistence of the ( a hydrogen pressure was too low. Thus no hydride formation was possible and consequently no loss of catalytic activity due to this effect was observed. Ragaini and Somenzi (42) studied the hydrogenation and isomerization of l-butene on palladium at 50, 70, and 100°C. A t such temperatures the p-phases would amount to 23, equilibrium hydrogen pressures over a 55, and 200 mm Hg respectively or even 100-300 mm Hg, if the hysteresis effect is to be taken into account. Yet the pressures which the authors employed ranged from 20 to 700 mm Hg. Thus some of their results should be affected by the palladium hydride formation. However, in their considerations the authors only discuss the role of adsorbed hydrogen and of its equilibrium with hydrogen in the gas phase. The experimental evidence confirms, however, the derived kinetic equation which is a strong argument against the expected hydride formation. Quite recently Yasumori et al, (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains
+
+
268
W. PALCZEWSKA
in the large crystalline structure hinder the penetration of hydrogen into the bulk of a metal.
IV. The Effect of Transformation into Hydride on the Catalytic Activity of Nickel and Its Alloys with Copper
The effect that the presence of hydrogen in the lattice of nickel or nickelcopper alloys has on catalytic properties is much more difficult to trace in the literature than is the case with palladium and its alloys. Several factors contribute to this: (a) It is only recently that the existence of 8-hydride phases of nickel and its alloys with copper have been recognized. In the 1960’s experimental data led to the identification of these hydrides and also set out the thermodynamic conditions which govern their formation. The kinetic factors exerting influence on hydrogen to be built into the metal lattice are still not sufficiently well defined. However, the whole contemporary knowledge of the nickel-hydrogen systems appears to have been more familiar to physicists and physical chemists concerned with the study of metal-hydrogen interactions than they have to scientists working in the field of heterogeneous catalysis. If in the reactions of hydrogen on nickel or nickel-copper alloys the influence of presorbed hydrogen on the kinetics had even been noticed, the results were inevitably interpreted only in a rather general way, i.e. ascribed to the effect of preadsorbed or preabsorbed hydrogen. Some authors interpreted the phenomena observed in terms of Toya’s theory (44)by r and s hydrogen being alternatively present in the metal-hydrogen system. According to this approach r hydrogen denotes the state of the adatoms on the metal surface whereas the s hydrogen represents the hydrogen atoms dissolved in the metal, retained in its structure, and occupying positions in a surface plane which passes through the centers of the metal atoms in their first layer. Only r hydrogen was supposed to be catalytically active. This concept in principle has some features common with that of metal hydrides. (b) The discovery made in 1965 by Sachtler, and developed by him and co-workers ( 4 5 ) , of the nonhomogeneity of Ni-Cu alloys being in equilibrium below 200°C necessitated the revision of numerous papers previously published on this subject. Assuming that the segregation of Ni-Cu alloy leads to the coexistence of a rich in nickel phase with that rich in copper, one can take into account an eventual hydride formation only in the phase which is rich in nickel. Similarly, as with palladium, the absorption of hydrogen (leading to hydride formation) followed by its desorption
PALLADIUM A N D NICKEL HYDRIDES
269
results in a disintegration of metal crystallites (46, 47). In consequence the dispersion increases and the catalytic activity augments. However, with copper-nickel alloys (unlike palladium alloys) a sequence of successive absorption-desorption processes results in segregation of a previously homogeneous alloy (48). Thus the catalyst consists of altered phases, one rich in copper and the other rich in nickel formed adjacent to each other. The rich in copper alloy accumulates at the surface. The rich in nickel catalyst phase may eventually transform into a hydride and become poisoned for the given reaction. Thus the changes of the nickel-copper alloys phase composition lead to profound changes in the catalytic activity of these alloys, directly connected to different aspects of the initial phenomenon of the equilibrium phase segregation. Experimental evidence illustrating the effect that hydrides of nickel or its alloys with copper have on the catalytic activity of the respective metals is to be found in papers which discuss catalytic consequences of the special pretreatment of these metal catalysts with hydrogen during their preparation. One must also look very carefully into cases where self-poisoning has been reported as appearing in reactions of hydrogen with other reactants. Emmett and his co-workers have noted in several papers the poisoning or promoting effect of preadsorbed hydrogen in the reaction of ethylene hydrogenation on nickel or nickel-copper alloy catalysts. Hall and Emmett (49) studied the catalytic activity of nickel, copper, and their alloys using ethylene hydrogenation as the test reaction. The catalysts were prepared by the precipitation or coprecipitation of the respective carbonates, subsequently decomposed into oxides, and then reduced at 350°C with hydrogen. “Hydrogen treated” catalysts were cooled in a stream of hydrogen down to the reaction temperature. “Helium treated” ones were cooled in helium. In the low temperature hydrogenation of ethylene the authors observed that nickel suffered a significant poisoning as a result of the hydrogen pretreatment. On the other hand nickel-copper alloys exhibited an enhanced activity. Although these observations were hard to explain at the time, the authors nevertheless put forward a generally justified conclusion that absorbed hydrogen became part of the catalyst itself, modifying some of the catalyst basic properties, but not participating directly in the reaction. Following the authors’ opinion the preadsorbed hydrogen enters into the surface layer of the metal rather than into the bulk phase as its structural component. Hall and Hassell (60) continued these studies with the intention of proving that possible traces of oxide dissolved in the metal play no significant role in the poisoning or promoting effects arising from hydrogen which had been presorbed during the pretreatment procedure. The catalysts were prepared in essentially the same manner as before. The kinetics
270
W. PALCZEWSKA
of ethylene hydrogenation was studied as previously using a microcatalytic reactor a t about -90°C. With the identical pretreatment with hydrogen or helium the degrees of ethylene conversion on nickel or its alloy containing 72 at. yonickel were compared; hydrogen or helium were used as the carrier gas. When helium was the carrier gas an almost fivefold decrease in the degree of conversion of ethylene into ethane was observed in the case of a nickel catalyst which had been pretreated with hydrogen. Under identical conditions the degree of conversion increased almost threefold when the alloy was used as catalyst. Both effects were lower when hydrogen was used as a carrier gas. The authors also studied the effect that varying the conditions of reduction had on the total hydrogen content of the catalyst and on the reversible hydrogen sorption at 250°C. In conclusion the authors stated that the activity of the metal catalyst studied “is controlled not by large portions of hydrogen which are associated with residual hydrogen but rather by small portions which are absorbed on the catalyst surface or in solution in the metal lattice.” Thus, although not taking into account the possibility of a hydride phase being formed, the authors rely on data concerning the solubility of hydrogen in nickel which lead to the formation of a uniquely a-solution. The same authors also found that the conversion of para-hydrogen on nickel proceeded much more slowly after the pretreatment of a nickel catalyst with hydrogen, but more quickly on nickel-copper alloys undergoing a similar pretreatment. They referred to the similar effects observed earlier by Kowaka (40) on palladium and its alloys with silver : the hydrogen acted as a poison on pure palladium catalyst and as a promoter on alloys. However Hall and Hassell could not fully follow this analogy into a parallelism of the behavior of both metals, i.e. palladium and nickel (or their alloys), with respect to hydrogen, that is, by taking into consideration the formation of a- and p-hydride phases quite similarly in both metal-hydrogen systems. The first information about the discovery of the nickel hydride was too recent. The authors limited their discussion of the observed phenomena to general considerations on the effect that presorbed hydrogen had on electronic and geometric factors influencing the catalytic activity of the metals investigated. The discussion concludes with the statement that “the final solution of the problem lies in studies of the interaction of hydrogen with these surfaces.” In order that the possibility of contamination of catalysts with traces of oxides could be eliminated Campbell and Emmett ( 5 1 ) studied the catalytic activity of metallic films of nickel and its alloys with copper or gold. They were deposited under a high vacuum and then sintered (alloys also homogenized) in hydrogen at 5 cm Hg pressure a t 350°C or 500°C. The films were subsequently allowed to cool to room temperature and only
PALLADIUM A N D NICKEL HYDRIDES
27 1
then hydrogen was pumped off. The kinetics of ethylene hydrogenation was studied on these film as catalysts a t temperatures ranging from - 10°C to a maximum of 69°C. As before the nickel film cooled in hydrogen from 500°C down t o the reaction temperature was found to be poisoned, in contrast t o the similar film cooled in vacuo. The first-order rate constant a t 0°C was 0.025 min-I in the first case and 0.196 min-I (per 1000 cm2 surface) in the second one. An alloy containing 5.9y0 nickel cooled in hydrogen was twice as active as the same alloy cooled in helium. Unfortunately the comparison is not as meaningful here since as a result of the presence of some impurities in the helium, this gas itself showed a slight poisoning effect. Some films containing deposited nickel together with copper were annealed a t 500°C in order to ensure the homogenization of the alloys. After their cooling down t o room temperature the X-ray diffraction patterns demonstrated phase segregation of the alloys similar to that described by Sachtler et al. ( 4 5 ) . The attention of the authors was particularly directed toward the increased activity of the nickel catalyst film when copper was added. This increase is revealed in a change of the initial reaction rate of copper itself and of all the alloys (except those containing 25-350/, nickel) ; they are more active than nickel itself. A respectively similar difference was observed for the activation energy and the preexponential factor. Volter and Alsdorf (52) obtained a relation of a very similar character for the dependence of the catalytic activity in formic acid decomposition on the composition of the nickel-copper alloys. However, extending the times of the alloy annealing for their better homogenization caused the maxima on the catalytic activity curves to disappear. It seems therefore that a t equilibrium, and even more a t nonequilibfium, the multiphase composition of the alloys is a particularly complicating factor for a discussion of results. It not only affects the problem of the influence that the composition of an alloy has on the catalytic activity, but also the effect of the interaction of hydrogen with the same alloy during hydrogen sorption. Nickel-copper alloys react with hydrogen in different ways depending on their composition, eventually forming the respective hydride phases. The hydrides on decomposition cause a marked dispersion of the metal and alter the composition of the surface of the alloy by facilitating its equilibrium segregation (48). I n studies on the para-hydrogen conversion rate on nickel and its alloys with copper other authors also noted the poisoning effect of the sorbed hydrogen. Singleton ( 5 3 ) mentioned the poisoning of nickel film catalysts by the slow-sorbed hydrogen. Shallcross and Russell ( 6 4 ) observed a similar phenomenon for nickel and its alloys with copper a t - 196°C. At higher
272
W. PALCZEWSKA
TABLE V Activation Energy of Ha-Dt Equilibration Reaction on Nickel and Nickel-Copper Film Catalysts (a) Within Temperature Range -100"-+2O0C and (b) After Preheating in Hydrogen at 4ObO"C (66) E (kcal/mole) Film (wt.% Nil 100 83 73 42 30 20 13 11 4 2 0 (Cu only)
(a)
(h)
3.2 2.66 3.42 2.4 3.25 3.09 2.53
7.3 7.5 6.67 6.1 6.1 8.8 6.4 8.9 9.2 8.3 9.7
-
-
temperatures however (e.g. -20°C) the hydrogen exerted an activating action. The hydrogen-deuterium reaction is a further source of data. Zhavoronkova el al. (65) compared the activity of films of nickel and its alloys with copper as catalysts in this reaction. The velocity with which the reaction proceeded was measured by a static method. At the beginning the gaseous mixture was equimolecular with respect to hydrogen and deuterium. The reaction was studied under 0.5 mm Hg pressure and within a range of temperature of -100°C to +lOO"C. Nickel, copper, and their alloys with different composition were evaporated onto the reaction vessel inner walls in oacuo (10-8 mm Hg) a t 300°C and the deposits were then baked for one hour at 400°C. The authors observed for nickel and its alloys a poisoning effect due to hydrogen that had been adsorbed a t room and even higher temperatures (up to 60°C). The initial value of the activation energy of 2.4-3.5 kcal/mol increased when the film had sorbed hydrogen and amounted to 6.1-9.2 kcal/mol. Experimental data were compiled in Table V. On heating the film and pumping off the hydrogen it is possible to return to the initial Arrhenius plot. After mentioning the change in catalytic activity of nickel and its alloys under the influence of
PALLADIUM AND NICKEL HYDRIDES
273
sorbed hydrogen, the authors argue that the mechanism of the hydrogendeuterium exchange also undergoes a change. The results expressed as the dependence of the specific activity on the alloy composition show the poisoning effect of hydrogen (always called by the authors “adsorbed” hydrogen) to be particularly high in the case of nickel and alloys rich in nickel. If it were not for an accompanying compensating effect the rise in activation energy would result in a much greater decrease in catalytic activity. Quite unexpected is the marked poisoning effect of hydrogen pretreatment for alloys rich in copper. One might suggest that the initial heterogeneity and/or segregation of alloys under the influence of absorbed hydrogen and the presence of a rich in nickel phase would be responsible for the eventual hydride formation followed by catalytic poisoning of the alloy. Recently, other authors when studying the activation of hydrogen by nickel and nickel-copper catalysts in the hydrogen-deuterium exchange reaction concentrated for example only on the role of nickel in these alloys (66) or on a correlation between the true nickel concentration in the surface layer of an alloy, as stated by the Auger electron spectroscopy, and the catalytic activity (67). The quantitative results characterizing the process of sorption of hydrogen by nickel, copper, and their alloys, obtained by Cadenhead and Wagner ( 5 8 ) , were discussed by the authors without taking into account the possibility of a 13-phase hydride formation. They measured the amount of hydrogen sorbed by catalysts at -196°C and obtained such high values that to explain them it was necessary to assume a formation of four monoatomic layers of hydrogen adsorbed on the catalyst surface. The initial high temperature heat treatment in the reduction of oxides at sufficiently elevated temperatures yielded a 1: 1 ratio of H/Ni. However the results were poorly reproduced. The enhanced sorption of hydrogen by the metals investigated was attributed by the authors to the fine pore structure of the catalysts and/or the presence of numerous defects in their crystalline structure. Hardy and Linnett (69) studied the heterogeneous recombination of atomic hydrogen at room temperature on nickel and nickel alloy foils. They did not find any similarity to the behavior of palladium and its alloys with gold studied earlier (56).There was no evidence that, as a result of exposure to atomic hydrogen, hydride was formed in any metal catalyst investigated with a resulting change in the activity of the initial parent metal catalysts. In several papers dealing with catalytic reactions involving hydrogen and unsaturated hydrocarbons the observed self-poisoning of nickel or its alloys has been quite properly attributed to the presence of carbonaceous
274
W.
PALCZEWSKA
residues on the surface of the catalyst. This circumstance makes the search for the eventual poisoning effect of the hydride formation much more difficult. In summarizing the experimental material which has just been presented it must be stated that it is difficult to arrive at an unambiguous interpretation of the self-poisoning of nickel or nickel-copper alloy catalysts, pointing, for example, to a hydride phase in the metal catalyst acting as a unique poisoning agent in the reactions investigated. The analogy with palladium and its alloy catalyst fails. Undoubtedly the poisoning effect of presorbed molecular hydrogen appears in the case of nickel similarly as for palladium but rarely where alloys with copper are concerned. It might be supposed that when the poisoning effect of absorbed hydrogen was observed one was dealing with the formation of a layer of hydride on a t least some part of the surface area. The hydride would then be responsible for a decrease in the activity of the nickel (sometimes of its alloys with copper) and the comments of the authors quoted previously would remain essentially valid. The conditions under which this effect was observed in nickel do not exclude the possibility of hydride formation and its poisoning effect. There is some experimental evidence (60) that in finely dispersed nickel films deposited for example at liquid nitrogen temperature hydride formation is possible even under a very low pressure of molecular hydrogen far from equilibrium conditions. Each form of “active hydrogen” (atoms, protons) could be more effective in the hydride formation than is molecular hydrogen. It should also be taken into account that after the dissolution of significant amounts of hydrogen in nickel at higher temperatures, lowering the temperature can lead to the precipitation of a 8-hydride phase. However, in none of the above mentioned papers has the formation of a nickel hydride been directly proved; the authors did not even consider the possibility of its formation. Further, the poisoning effect of the hydrogen presorbed under those particular conditions can alternatively be explained by the presence of surface hydrogen in a form particularly nonreactive in the reaction studied. Again however there is no direct evidence for this. On the basis of information on the properties of the nickel-hydrogen and nickel-copper-hydrogen systems available in 1966 studies on the catalytic activity of nickel hydride as compared with nickel itself were undertaken. As test reactions the heterogeneous recombination of atomic hydrogen, the para-ortho conversion of hydrogen, and the hydrogenation of ethylene were chosen. In the initial investigations the samples of nickel or nickel-copper alloys were used in the form of foils transformed into their respective hydride phases by saturating them electrolytically with hydrogen (7). The presence of a hydride phase was confirmed by X-ray diffraction (8). The catalytic
PALLADIUM AND NICKEL HYDRIDES
275
activity of hydrides was compared with that of identical foils which, after being submitted to electrolytic saturation with hydrogen, were finally deprived of hydrogen as a result of a sufficiently long desorption process at room temperature. In order to obtain a catalytically highly active surface the foils were repeatedly saturated with hydrogen (up to even 20 times) and after each saturation were allowed to lose the “hydride” hydrogen during twenty-four hours of desorption at room temperature. Such repeated absorption followed each time by desorption results in a strong disintegration of the metal crystallites and the formation of a thick layer of fine crystalline metal catalyst on the foil surface. Its thickness depends on the depth of penetration of the hydrogen into the metal sample cathode leading to the formation of a hydride phase with the host metal. Such layers are characterized by a good reproducibility of their behavior with respect to hydrogen (61, 62) and therefore they were considered suitable for study as catalysts. In this series of investigations the heterogeneous recombination of atomic hydrogen, being the simplest test reaction, was used as the starting point in the study of the effect that hydride formation could have on the catalytic activity of nickel and its alloys with copper (63, 64, 64a). Using the Smith-Linnett method the values of the recombination coefficients were determined (a) for nickel and rich in nickel alloys with copper, and (b) for the identical foils which, after a multiple absorptiondesorption of hydrogen, were kept in the form of the respective hydrides, put into the side-arm tube of the apparatus, and there investigated as catalysts in the recombination. In order to ensure a sufficient stability of the hydride phase in the metal foil catalyst investigated the reaction was carried out a t temperatures low enough to maintain the hydride decomposition at an insignificant level. The reaction temperature was thus -78°C. The pressure of molecular hydrogen being dissociated in the highfrequency electromagnetic field amounted to lo-’ mm Hg, while atomic hydrogen content amounted for about 10% of the mixture of atomic and molecular hydrogen. The same reaction mechanism operated on the surfaces of both kinds of catalyst; on metals and their hydrides as well. The reaction proceeded according to the Rideal-Eley mechanism and was of first order with respect to the atomic hydrogen concentration in the gas phase. The coefficient of recombination of atomic hydrogen on nickel was about one order of magnitude higher than on nickel hydride at the same temperature. Even a partially decomposed hydride was still as inactive as the original hydride sample. Nickel alloys containing from about 1 to 2 wt. % ’ Cu were less active than nickel itself (at -78°C and a t room temperature as well), and the
276
W. PALCZEWSKA
TABLE VI Coeficients of H Atom Recombination, y, at -78°C on Nickel OT Nickel-Copper Foils and Their Respective 6-Hydride Phases Y
Catalyst Ni Ni99Cul Ni90Cu10 Ni75Cu25 Ni60Cu40 4
Hydride partly decomposed
Hydride phase
8.3 x 1.1 x 2.2 x 3.8 x 6.3 x 4.1 x
10-4 10-4 10-4 10-4 10-4 10-4
desorp. HZ-+ c absorp. H -+ desorp. HZ tabsorp. H +-+ desorp. HZ---t (5.0 x 10-3) -+
-+
1 . 0 x 10-3 7 . 5 X lo-' 1.5 X 1.4 X 2.1 X 2.1 X
Metal catalyst after hydride decomposition" 1 . 8 x 10-2 1 . 0 x 10-9 3.5
x 10-8
3.0 6.9
x x
Strictly: a-solid solution of hydrogen in a metal catalyst studied.
alloy hydride phases were also less active than was the nickel hydride. The experimental data chosen from the papers by Palczewska et al. (6'4, 64a) are presented in Table VI. A loss of activity brought about by the hydride phase presence in the alloys investigated amounted to about one order of magnitude (or even more). Alloys containing from 40 to 60 wt. % Cu were not poisoned by absorbed hydrogen. In spite of their saturation with hydrogen in the same manner as in the case of the other alloys, their catalytic activity remained unchanged, within the limits of reproducibility. Partial desorption of hydrogen led to a significant rise in activity, while a prolonged exposure to atomic hydrogen decreased y owing to an increase in the concentration of a /3-phase of hydride in the sample, as it could be proved by the X-ray diffraction method. The alloys were much more susceptible to hydrogen poisoning and much more easily regained their activity than the nickel itself. This can be accounted for by the greater ease with which the corresponding alloy hydrides are formed owing to the smaller increase of their lattice parameters during the transformation into hydrides. In these particular experiments it proved impossible t o investigate the effect of copper concentration on the catalytic activity of alloys free of the hydride phase. Figure 10 (53, 64a, 65) illustrates the changing values of the recombination coefficient on nickel-copper alloys related to the composition of the alloy at room temperature. The small amount of copper introduced into the nickel already distinctly decreased the catalytic ac-
PALLADIUM AND NICKEL HYDRIDES
277
I b 78 2.0 b 5.2
-8 -
2.4 2.6
2.8
3.0 0
20 40 60 80 ‘100
4 q N i
m%cu
FIG.10. Coefficient of H atom recombination on Ni-Cu alloy catalysts rn a function of the alloy composition, at 20°C. A, on Ni-Cu foils (59); 0 , on Ni-Cu evaporated fdms after their previous homogenization at 400OC (65,65a); 0 ,on Ni-Cu foils after a multiple hydrogen absorption-desorptiontreatment (64,).
tivity, but a further increase in the copper content did not have any marked effect. This result is in accordance with Sachtler’s et al. data (45) and points to a uniform composition of the alloy surface nonwithstanding the changes in the bulk composition. However, the poisoning of rich in nickel alloys by hydride formation proves that the rich in copper alloy phase does not completely cover the rich in nickel kernel of the alloy crystallites. The multiple absorption-desorption of the hydrogen procedure repeated at room temperature led finally to the segregation of both equilibrium phases of the copper-nickel alloy (48)’ a t least to some small extent as shown in Figs. 11 and 12. In the case of an alloy with 1 wt. % Cu the diffraction pattern (Fig. l l a ) represents the characteristic peaks of the nontransformed alloy and of its hydride phase present in the same sample. The peaks of low intensity (marked by arrows) situated between those of an initial aIIoy and its hydride correspond to the rich in copper equilibrium alloy phase found earlier by Sachtler et al. This phase became segregated from the initial alloy as a consequence of the multiple hydrogen treatment and remains (Fig. llb) after the hydride decompsoition. The respective coexisting rich in nickel equilibrium phase is hidden in the broad peak representing the main quantity of unchanged initial alloy phase. In the case of an alloy with 25 wt. yoCu, which was completely transformed into
278
W. PALCZEWSKA
FIG.11. X-ray diffraction pattern of a Ni99Cul alloy partially transformed into its 8-hydride (j3 NiCuH) before (a) and after (b) hydride decomposition. Arrows point to the diffraction peaks representing the rich in copper alloy phsae desegregated from the initial alloy after a multiple hydrogen absorption-desorption treatment. After Palczewska and Majchrzak (48).
hydride (Fig. 12a), its partial decomposition did not reveal the separation of the rich in copper alloy (Fig. 12b), but its peaks (marked with arrows) became clearly visible after the total decomposition of the hydride (Fig. 12c), whose diffraction reflections had previously hidden them.
FIG.12. X-ray diffraction pattern of a Ni75Cu25 alloy (a) completely transformed into its 8-hydride (j3 NiCuH), (b) after a partial hydride decomposition, alloy peaks appearing, (c) after a complete hydride decomposition, arrows pointing to the rich in copper alloy phase desegregated from the initial alloy after a multiple hydrogen absorptiondesorption treatment. The peaks had been revealed after the disappearance of the hydride peaks. After Palczewska and Majchrzak (48).
279
PALLADIUM AND NICKEL HYDRIDES
Metal foils used as catalysts in the experiments described above turned out to be ill-fitted to these investigations. The electrolytic transformation of alloy foils into alloy hydrides did not guarantee a sufficient purity of the samples. Copper rich alloys should be excluded from the experiments because they could not be hydrogen treated in the same manner as the other alloys, and consequently no active microcrystalline layer was developed on their surface. Thus nickel and nickel-copper alloy films evaporated in vucuo onto the inner walls of the reaction vessel have been chosen for further investigations. The films were deposited onto the inner wall of a lass tube kept a t 450°C; their thickness amounted to approximately 2000 . After annealing a t the same temperature in vacuo they were transferred into the side-arm of the Smith-Linnett apparatus in order for the recombination coefficients to be determined. The bulk homogeneity of alloy films prepared in this way was confirmed by X-ray diffraction (66, 66u,66). The values of the recombination coefficient of atomic hydrogen were determined for the following metal film catalysts: nickel, copper, and their alloys containing 3, 23, 43, or 80 wt. % of copper. The temperature of the heterogeneous hydrogen atom recombination ranged from +200 to - 60'C; each sample was investigated over the whole range in one experiment in the direction of decreasing temperatures. The temperature behavior of the alloy catalysts in the heterogeneous recombination of hydrogen atoms was different for rich in nickel alloys from one side and for rich in copper from the other. For the three alloy catalyst films, i.e. Ni97Cu3, Ni77Cu23, and Ni57Cu43 (numbers represent
d
I
-
Ni 77Cu23
0.5
c
2 0
F
d
-0.5
1
2
4/T &OOO/
"k
)
5
FIG.13. Arrhenius plots of the kinetics of H atom recombination on a Ni77Cu23 alloy film catalyst. Above room temperature-active NiCu film with low activation energy. Below room temperature-film deactivated owing to a 8-hydride phase formation; activation energy markedly increased. After Karpinski et al. (66).
280
W. PALCZEWSKA
here the percentage by weight of nickel or copper, respectively) the rate of the reaction represented by a plot in Fig. 13 does not conform to the simple linear Arrhenius equation within the whole range of temperature. Rather two straight lines are in keeping with the experimental evidence expressed as log ( y T ) as a function of 1/T. At some point near room temperature the relatively much more rapid decrease of the reaction rate with the lowering of temperature can be seen. The activation energy abruptly changes from about 0.5-0.8 kcal/mole characteristic of the range of 20020°C to about 4 kcal/mole a t low temperatures. This decrease of the catalytic activity was explained by the formation in situ of the hydride phase in the respective alloy as a result of the interaction of atomic hydrogen with the alloy. This conclusion was additionally confirmed by Palczewska and Janko (67)in separate experiments, where under the same conditions nickelcopper alloy films rich in nickel (and nickel films as well) were transformed into their respective hydride phases, which were proved by X-ray diffraction. The additional argument in favor of the transformation of the metal film into hydride in the side-arm of the Smith-Linnett apparatus consists of the observed increase of the roughness factor (-70%) of the film and the decrease of its crystallite size (-30%) after coming back from low to high temperatures for desorbing hydrogen. The effect is quite similar to that observed by Scholten and Konvalinka (9) for their palladium catalyst samples undergoing the (a+ @-phase transformation. Pure nickel films behaved in an unusual way, manifesting a constant energy of activation value of 1.7 kcal/mole throughout the whole temperature range. In keeping with the former interpretation such a result should be explained by the absence of the metal transformation into its hydride. The large crystallites of the nickel films studied, amounting to 2000 A, were probably too resistant to building in hydrogen. As mentioned and explained earlier nickel-copper alloys form hydrides much more easily. It should be added here, however, that quite recently when continuing studies on the recombination of hydrogen atoms a marked hydrogen poisoning effect was observed on ultrathin (below 100 A) nickel films (68). The recombination reaction proceeding on nickel-copper alloy films rich in copper and on copper itself maintained a constant value of the activation energy of about 1 kcal/mole. The Arrhenius plot for an alloy film Ni20Cu80 is represented in Fig. 14. On the basis of the related experimental evidence and its discussion one can regard the poisoning effect of the “hydride” hydrogen in nickel and its alloys with copper as normally accompanying the heterogeneous recombination of hydrogen atoms on these catalysts a t lower temperatures.
PALLADIUM AND NICKEL HYDRIDES
281
Ni20Cu80
I
1
2
3 4 I / T ( 1000PK)
5
FIG.14. Arrhenius plot of the kinetics of H atom recombination on a rich in copper alloy film catalyst: Ni20Cu80. Within the whole range of temperature the linear relationship holds; activation energy constant. After Karpinski (66a).
The full analogy with palladium and its alloys with gold should thus be emphasized once more. The range of observations concerning the direct comparison of the catalytic activity of nickel and rich in nickel alloys with their respective hydride phases has been further extended on reactions of a more complicated nature such as para-ortho hydrogen conversion and ethylene hydrogenation. On comparing the kinetics of conversion of para-hydrogen at -78°C on alloy films Ni97Cu3 and on the same films after their exposure to atomic hydrogen (66) it was stated that the catalytic activity of films decreased by about two orders of magnitude. The same effect was found for nickel films, much more marked for ultrathin films. This observation implies again the poisoning effect of hydride formation. The detailed results concerning this reaction will be elaborated and published. The hydrogenation of ethylene was studied by a conventional static method on nickel or alloy (Ni97Cu3) films as catalysts at -40°C (66,69). The kinetics of the reaction proceeding on those films preexposed t o molecular hydrogen was compared with the kinetics on the similar films preexposed to atomic hydrogen action. The second series of catalyst films thus may be treated as representing nickel or its alloy with copper partially transformed into the respective hydrides. The reaction temperature was chosen to keep the reaction rate sufficiently high and to diminish simultaneously the rate of hydride decomposition to a relatively negligible value. The results collected in Table VII permit a comparison of the catalytic activity of metal films investigated in their initial composition with
282
W. PALCZEWSXA
TABLE VII Rate Constants of Ethylene Hydrogenation at -4OOC on Nickel, and Nickel-Copper Alloy Before and After Their Exposure to Atomic Hydrogen
Film Ni Ni97Cu3
Rate const. X Mass of film lo2 (min-1 X (mfd 10 mg-1) 20 32.2 8.9 10.0
2.9 3.0 3.4 2.9
Rate const. Film exposed to H
Mass of film (mg)
x 102 (min-1 x 10 mg-1)
21.3 32.7 8.6 13.5
0.17 0.25 0.80 0.60
Ni-H (Ni97Cu3)/Ni-Cu-H
their catalytic activity after hydride formation. The respective kinetic plots are represented in Fig. 15 for nickel and nickel hydride films. The quite similar kinetic behavior is manifested by Ni97Cu3 alloy films. The initial rate constant values related to 10 mg of the metal film catalyst obtained from the kinetic plots (analogous to those represented in Fig. 15) diminishes by about one order of magnitude as a consequence of nickel film preexposure to atomic hydrogen (i.e. after transformation into 4.00
m = 19.3mg 3.90
.-v-.-w
3.80 N
r
a
m=32.2mg m = 20.0mg 3.50
LuI p , , , \ , , , , , , , I
0
2
4
6 8404244 t (min)
FIG.15. Kinetics of the ethylene hydrogenation on Ni and 8-Ni-hydride film catalysts; m denotes mass of films, which as known is connected with the thickness and crystallite sil;es of the films involved. Blank points-rate of reaction proceeding on Ni film catalysta; black points-rate of reaction proceeding on nickel previously exposed to the atomic hydrogen action, i.e. transformed to some extent into 8-Ni-hydride.
PALLADIUM AND NICKEL HYDRIDES
283
the hydride phase of at least a surface layer of film). The poisoning effect of hydrogen absorbed in the Ni97Cu3 alloy is smaller, amounting to a four-to-fivefold decrease of the reaction rate constant. Thus, the general conclusion derived from the direct experimental evidence reported is that hydrogen absorbed and then built into the structure of nickel or nickel-copper alloys in a form of a p-hydride manifests a poisoning effect by a marked decrease of the catalytic activity of the metals involved. This poisoning “hydride” effect observed in the heterogeneous atomic hydrogen recombination, the para-ortho hydrogen conversion, and ethylene hydrogenation is common for palladium and nickel or their respective alloys with the group IB metals. It must be pointed out, however, that in contrast to palladium and its alloys nickel and its alloys with copper, when acting as catalysts in hydrogenation reactions, are in common practice used under temperature and pressure conditions far distant from those thermodynamically predicted for hydride phase formation. In order to prove the influence which this transformation could have on the catalytic activity of nickel and its alloys with copper the hydrides were especially prepared under particularly “drastic” conditions. However, the possibility of catalyst poisoning by the hydride formation is opening a new way for the interpretation of observations of poisoning effects following various “hydrogen pretreatment” procedures. Moreover, the eventual formation of especially active (for penetration into metal lattice) forms of hydrogen should be taken into account when dealing with hydrogen as a substrate or a product and as an intermediate species as well. The conditions under which the /3-hydride phase of a metal catalyst may then form are not generally known and are not described by the thermodynamics of a metal-Hz system presented in the literature on the subject.
V. Catalytic Activity of Other Metal Hydrides in Test Reaction of Hydrogen
As mentioned previously in the introduction to the present review the ability to form the hydride phase is not characteristic solely of palladium or nickel. It would be of interest, therefore, to verify the results on the poisoning effect of hydride formation in the case of nickel or palladium by comparing with the other transition 3d, 4d, and 5d metals and the rare earth (4f)metals. The para-hydrogen conversion catalytic activity of the metals belonging to the first transition series: Ti, V, Cr, Mn, Fe, Co, Ni was compared by Eley and Shooter (70). The purpose of the research was not to discover
284
W. PALCZEWSKA
the influence of an eventual formation of a hydride phase on the catalytic activity of a metal. Nevertheless the paper provides an interesting contribution to the subject. The authors noted an instantaneous absorption of hydrogen by titanium and vanadium films a t room temperature, leading to values of the atomic ratios of H/Ti = 2.28 and H/V = 0.75. The results concerning the catalytic activity of these metal-hydrogen systems in the para-ortho hydrogen conversion as expressed by the Arrhenius constant at 293'K and 1.2 mm Hg are not different from those of iron or chromium films, which did not exhibit any marked absorption of hydrogen. However titanium left overnight in hydrogen a t room temperature lost its initial catalytic activity; it decreased about four times. This poisoning effect was reversible: the hydrogen being pumped off restored the original activity of titanium. In the authors' opinion the slow absorption of hydrogen appears to exert the same negative influence on the kinetics of the reaction involved as it did in the case of palladium (29).It may be presumed that during this hydrogen absorption process the formation of a Ti-hydride phase occurs, being responsible for the further observed loss of catalytic activity. In a recent series of papers Konenko et aE. (71-73) published the results of the studies which they conducted on scandium, yttrium, lanthanum, and some lanthanides : neodymium, samarium, dysprosium, erbium, and lutetium-as catalysts of the para-ortho hydrogen conversion in the high temperature range 190°C-3800C. The aim was to clarify the role of d or f electrons, respectively, in elementary catalytic reactions of hydrogen on those metal catalyst surfaces. The catalytic activity of the metals compared with that of their hydrides, carbides, or oxides demonstrated to what extent an involvement of d or f electrons in the formation of respective bonds in the compounds with H, C, or 0 changes the initial catalytic activity of the pure metals themselves. Scandium and yttrium samples were transformed into their hydrides by spontaneous absorption of hydrogen gas a t 70-300'C for 15 hr. The Arrhenius constants for scandium and yttrium were as follows: the activation energy for Sc--11.6 kcal/mole, ScH2-14.0 kcal/mole, Y and Y H p 6 . 8 kcal/mole; the preexponential factors were about one order of magnitude lower for hydrides, when compared with the respective metals. Sc: ScH2 = 1.0 X 101o:l.O X loQand Y: YH, = 3.3 X lO'z1.6 X 10'. The lanthanum and the lanthanides investigated were absorbing hydrogen during the reaction of para-hydrogen conversion. Therefore the initial kinetic characteristic of the reaction was related to the rare earth metal catalyst, and the final to its hydride with a formula Me-H2. I n the case of all metal catalysts investigated their transformation into the hydride phase did not change the values of the activation energy but de-
PALLADIUM AND NICKEL HYDRIDES
285
creased from two to fourfold the preexponential factor in the Arrhenius equation at 200°C. The activation energy was almost the same for all the rare earth metal catalysts, ranging from 10.2 to 11.1 kcal/mole (with the exception of erbium with its activation energy of 18.0 kcal/mole). The values of the preexponential factor for metals were: 0.5 X lo8to 4.6 X lo8 hr-l m-2 (with the exception of erbium with its value of 1.1 X 10"). The small, if any, difference in the activation energy of the conversion reaction catalyzed by the examined metals and their hydrides proves, in the authors' opinion, that surface transition complexes are similar in all the cases. However, the number of these complexes which form on the particular catalyst surface should change, diminishing in the case of hydrides. In proceeding with a discussion along these lines the authors arrive at the final conclusion that in a similar way the number of surface metal atoms able to form a covalent chemisorptive bond with the reacting hydrogen diminishes. That loss of active metal centers on the surface is a consequence of their being bound in a Me-H compound. Attention must be paid to the fact that the authors simplified the interpretation by assuming the electronic structure of free isolated atoms of catalysts investigated to be responsible for the surface reactivity of the metal or metal hydride phases. Eley and Norton (74) reported their results on the kinetics of parahydrogen and ortho-deuterium conversions, and the H r D z equilibration reaction on gadolinium. The metal films were deposited under ultrahigh vacuum conditions. They absorbed hydrogen rapidly, up to a ratio H/Gd = 1, which can account for a mixture of Gd metal and its hydride GdH2. The films saturated with hydrogen in a standard way up to that value of the hydrogen content partially lost their catalyst activity in all the reactions investigated.
VI. General Remarks and Conclusions For many years the catalytic activity of metal alloys, particularly those of group VIII and group IB metals, has been the subject of experimental and theoretical studies. Papers published by Couper and Eley (29) and Dowden and Reynolds (75, 76) suggested a possible relation between the electronic structure of a transition metal catalyst and its catalytic action. They initiated much fundamental research work on the above mentioned alloys permitting the gradual filling of the vacancies in the d band of a transition metal by s electrons of a group IB metal and following the catalytic consequences of such changes in the electronic structure of a metal catalyst. Palladium alloy systems appeared suitable for supplying experimental material confirming the electronic theory of catalysis on transition
286
W. PALCZEWSKA
metals, which emphasized the role of their unfilled d bands in catalytic activity (77). However, the experimental evidence collected during recent years, concerning mostly the nickel-copper alloy systems, complicated this almost currently accepted interpretation of the alloy catalytic behavior ( 4 5 ) . Chemisorptive and subsequent catalytic phenomena appeared to require a differentapproach for elucidation. The surface reactivity had to be treated as a localized quality of the atoms at the interface, influenced by their neighbors in the crystal lattice (78-8U). A detailed general discussion of catalysis on alloys is beyond the scope of this review. I n the monograph by Anderson (81) and in the review by Moss and Whalley (82), recently published, a broad survey of the catalytic reactivity of alloys may be found. Metal 8-hydride phases are alloys of an interesting specifity: one component, namely hydrogen, has small dimensions and a very simple electronic structure; it does not change much the spatial distribution of ions forming the host metal matrix; however it changes markedly the d band structure of the host transition metal by donating its s electrons to it. While being a compon’ent of an alloy catalyst hydrogen is also a component of a reaction miXture--“hydride” hydrogen and “free” hydrogen can interchange their situations. The experimental evidence presented proves that nickel and palladium (and also some other transition metals and rare earth metals) lose their high catalytic activity when transformed into the respective hydride phases. A similar, though weaker, poisoning effect was observed for nickelcopper hydrides and palladium-gold (or silver) hydrides. The poisoning effect of the “hydride” hydrogen was directly stated in simple reactions of hydrogen. Knowledge of the Ni-H, Ni-Cu-H, Pd-H, Pd-Au(Ag)-H systems might be helpful in explaining the changes in the catalytic activity of the respective metals. It could be useful in understanding the mechanism of various reactions involving hydrogen and proceeding on those catalysts. The hydride phase may be present in a catalyst as a result of the method of its preparation (e.g. hydrogen pretreatment) , or it may be formed during the course of a given reaction, when a metal catalyst is absorbing hyin drogen (substrate-e.g. in H atom recombination; product-e.g. HCOOH decomposition). The spontaneous in situ transformation of a metal catalyst (at least in its superficial layer) into a hydride phase is to be expected particularly when the thermodynamic conditions are favorable. Moreover, a specially active hydrogen species present in a reaction mixture (e.g. atomic hydrogen, protons) (83) or forming during the surface reaction (37) can penetrate into a metal catalyst lattice and become
PALLADIUM AND NICKEL HYDRIDES
287
incorporated into it. Coadsorbed promoters of hydrogen penetration into metals ( 3 , 8 4 ) catalyze the hydride formation, and our knowledge of these promoters and the mechanism of their action is far from complete. A specially prepared metal catalyst surface, itself properly activating hydrogen and enabling it to penetrate into the metal structure (GO),can also exhibit its importance. All the above mentioned circumstances may unfortunately contribute to a transformation of a metal catalyst into its hydride under conditions far distant from those learned from the thermodynamics of massive metal-molecular hydrogen systems. There are, however, cases when the hydrogen pretreatment results in an enhanced catalytic activity of an alloy. The phenomenon may be explained also in connection with a metal-metal hydride transformation, namely as a “post-hydride” effect. I n the particular case of nickel-copper alloys their hydrogen pretreatment may result in phase segregation (48), a t least at the surface. The desegregated rich in nickel alloy can display its relatively high catalytic quality and even keep it down to a certain temperature (lower than in the case of nickel itself), which would be the critical temperature of a given Ni-Cu-H system. Moreover, the absorption-desorption hydrogen by a metal, able to form the hydride phase, leads to a cracking of metal crystallites (9, 4?‘), the disclosure of new crystal planes, and increasing disintegration-the whole set of phenomena resulting in an enhancement of catalytic activity. The direct proof of hydride formation in situ in a reaction vessel is in principle possible. One can follow changes of resistance (of a film, a wire, etc.) or of magnetic susceptibility of a catalyst. Hydride identification by means of the X-ray diffraction method requires a catalyst sample to be taken out from a reaction vessel, and eventually frozen in order to avoid a rapid decomposition of the hydride under ambient conditions (67). Indirect methods used can profit by the thermodynamic data of a particular metal-hydrogen system. The determination of the H/Me ratio after complete desorption of hydrogen from a sample, despite an apparent simplicity of the method, gives adequate results only when the bulk metal sample was entirely saturated with hydrogen, and that is a very rare case. The metal catalyst crystallites can be saturated in a nonuniform way, not through their whole thickness. The surface of this polycrystalline sample varies to such extent in its behavior toward interaction with hydrogen that hydride forms only in patches on its surface. A sample surface becomes a mosaique of /3-hydride and a-phase areas (86). So far ignored, but perhaps the most important factor in catalysis by metals able to form hydrides, are the dynamical conditions of formation and decomposition of hydride phases.
288
W. PALCZEWSKA
I
30
35 t (rnin.)
40
45
(I
t
FIG.16. Example of a A s.p. = f(t) relation, manifesting surface potential changes in a nickel-hydrogen system as a function of time and amount of hydrogen introduced onto a surface of a nickel film deposited at liquid nitrogen temperature; hydrogen-nickel film interactions were studied by Tompkins-Eberhagen static condenser method at liquid nitrogen temperature. After Dug (60). Each dose of Hf= 2.5 X 1011 molecules.
Large crystals transform into hydrides slowly, whilc small crystallites, e.g. in ultrathin films, form hydrides instantly ( 6 8 ) . Nickel films freshly evaporated under ultrahigh vacuum conditions a t 78"K, exposed to molecular hydrogen a t the same temperature and extremely low pressure, not only adsorb hydrogen but also absorb it eagerly (60).Figure 16 illustrates the course of the absorption manifested by a sequence of positive increments to the surface potential changes observed, leading finally t o a distinct stable shift of the Fermi level of nickel to the higher energy that is characteristic of nickel hydride. The same phenomenon was observed for palladium films interacting with molecular hydrogen ( 8 6 ) . As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal ( 8 7 ) . Extensive studies are still needed on hydrogen-metal surface interactions, leading t o various forms of adsorbed hydrogen of different specific reactivity with the metal catalyst surface. Nevertheless, one can conclude on the basis of the experimental evidence presented that certain facts al-
PALLADIUM AND NICKEL HYDRIDES
289
ready observed and known reveal a new aspect of this interaction and its role in the catalysis on nickel, palladium, and rare earth metals. The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its 1s electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. The change in the electronic structure of a bulk metal catalyst, in consequence of its transformation into the hydride, influences respectively the metal surface atoms (ions) or, strictly speaking, their d orbitals. Recent achievements and the present knowledge of the subject only permit us so far to formulate such general conclusions. REFERENCES 1. Smith, D. P., “Hydrogen in Metals.” Univ. of Chicago Press, Chicago, Illinois, 1948. 2. Barrer, R. M., “Diffusion in and through Solids.” Cambridge Univ. Press, London and New York, 1951. 3 . Smialowski, M., “Hydrogen in Steel.” Pergamon, Oxford, 1961. 4 . Libowits, G.G., “Binary Metal Hydrides.” New York, 1965. 4a. Mackay, K. M., “Hydrogen Compounds of the Metallic Elements.” Spon, London, 1966. 6 . Lewis, F. A., “The Palladium/Hydrogen System.” Academic Press, New York, 1967. 6. Ber. Bunsen ges. Phys. Chem. 76, No. 8 (1972). 7. Baranowski, B.,and smialowski, M., J . Phys. Chem. Solids 12, 206 (1959). 8. Janko, A.,Naturwissenschaften 47, 225 (1960);Bull. Acad. Pol. Sci., Ser. Sci. Chim. 7,633 (1959);8, 131 (1960). 9. Scholten, J. J. S., and Konvalinka, J. A., J . Catal. 5 , 1 (1966). 10. Wicke, E., and Nernst, G. H., Ber. Bunsenges. Phys. Chem. 68, 224 (1964). 11. Baranowski, B., and Bochefiska, K., Rocz. Chem. 38, 1419 (1964). l l a . Baranowski, B., and Bochefiska, K., 2. Physik. Chem. (Frankfurt am Main) [N.S.] 45, 140 (1965). 12. Czarnota, I., and Baranowski, B., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 14, 191 (1966). 13. Nace, G . M., and Aston, J. G., J . Amer. Chem. SOC.79,3619,3623,and 3627 (1957). 14. Owen, E.A., and Williams, E. St.J., Proc. Phys. Soc., London 56, 52 (1944). 15. Majchrzak, S.,Bull. Acad. Pol. Sci., Ser. Sci. Chim. 14, 67 (1966). 16. Janko, A.,and Pielassek, J., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 15, 569 (1967). 17. Worsham, J. E., Wilkinson, M. G., and Shull, C. G., J . Phys. Chem. Solids 3, 303 (1957). 18. Wollan, E. O . , Cable, J. W., and Koehler, W. C., J . Phys. Chem. Solids 24, 1141 (1963). 19. Sieverts, A., and Danz, W., 2. Physik. Chem., Aht. B 38, 46 and 61 (1937). 20. Bauer, H. J., and Schmidbauer, E., 2.Phys. 164, 367 (1961);KoaIowski, L., and Kubiak, S., Bull. Acad. Pol. Sci., Ser., Sci. Math., Astron. Phys. 9, 409 (1961).
290
W. PALCZEWSKA
2Oa. Baranowski, B., in “Festkorperchemie” (V. Boldyrev and K. Meyer, eds.), p. 364, VEB Deutscher Verlag fur Grundstoffindustrie, Lsipsig, 1973. 21. Stalilidski, B., Ber. Bunsenges. Phys. Chem. 76, 724 (1972). 22. Switendick, A. C., Ber. Bunsenges. Phys. Chem. 76, 535 (1972). 23. Brodowsky, H., and Peeschel, E., 2. Physilc. Chem. (Frankfurt am Main) “3.1 44, 143 (1965). 24. Tsuchida, T., J . Phys. SOC.Jap. 18, 1016 (1963). 25. Baranowski, B.,and Majchrsak, S., Rocz. Chem. 42, 1137 (1968). 25a. Majchrsak, S.,Thesis, Inst. Phys. Chem., Warsaw (1968). 26. Majchrsak, S., unpublished results. 27. Hinshelwood, C. N., and Topley, B., J . Chern. Soc., London 123, 1019 (1923). 28. Farkas, A,, Trans. Faraduy SOC.32, 1667 (1938). 29. Couper, A., and Eley, D. D., Discuss. Faraday Soc. 8, 172 (1950). 30. Rieniicker, G., and Engels, S., 2. Anorg. Allg. Chern. 336, 259 (1965). 31. Smith, W., J . Chem. Phys. 11, 1110 (1943). 32. Linnett, J. W., and Marsden, D. G. H., Proc. Roy. Soc., Ser. A 234, 489 (1956). 33. Greaves, J. C., and Linnett, J. W., Trans. Faraday Soc. 55, 1355 (1959). 34. Ehrlich, G., J . Chem. Phys. 31, 1111 (1959). 35. Dickens, P. G., Linnett, J. W., and Palczewska, W., J . Catal. 4, 140 (1965). 36. Dickens, P. G., Schofield, D., and Walsh, J., Trans. Faraday Soc. 56, 225 (1960). 37. Brill, P., and Watson, A, M., 2. Physik. Chem. (Frankfurt am Main) [N.S.] 64. 245 (1969). 38. Bond, G. C., Dowden, I). A,, and Mackeneie, N., Trans. Faraday SOC.54, 1537 (1958). 39. Rennard, R. J., and Kokes, R. J., J . Phys. Chem. 70,2543 (1966). 39a. Kokes, R. J., Catal. Rev. 6, 1 (1972). 40. Kowaka, M., J . Jap. Znst. Metals 23, 655 (1959). 4 1 . Mann, R. S., and Lien, T. R., J . Catal. 15, 1 (1969). 42. Ragaini, V., and Somensi, G., J . Catal. 13, 20 (1969). 43. Yasumori, I., Shinohara, H., and Inone, Y., Proc. Znt. Congr. Catal. 5th, 1972 p. 771 (1973). 44. Horiuti, J., and Toya, T., in “Solid State Surface Science” (M. Green, ed.), Vol. 1, p. 1. Dekker, New York, 1969. 45. Sachtler, W. M. H., and Dorgelo, G. J. H., J . Catal. 4, 654 (1965); Sachtler, W. M. H., and Jongepier, R., ibid. p. 665;van der Planck, P., and Sachtler, W. M. H., ibid. 12, 35 (1968);Sachtler, W. M. H., and van der Planck, P., Surface Sci. 18, 62 (1969). 46‘. Tiedema, T. J., Kooy, C., and Burgers, W. G., Proc. Kon. Ned. Akad. Wetensch., Ser. B 62, 34 (1959). 47. Janko, A,, and Saummer, A., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 14, 885 (1966); Janko, A.,ibid. 10, 612 (19U). -48. Palczewska, W., and Majchrsak, S., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 17, 681 (1969). 49. Hall, W. K., and Emmett, P. H., J . Phys. Chem. 63, 1102 (1959). 50. Hall, W. K., and Hassell, J. A., J . Phys. Chem. 67, 636 (1963). 51. Campbell, J. S., and Emmett, P. H., J . Catal. 7, 252 (1967). 52. Volter, J., and Alsdorf, E., 2. Anorg. Allg. Chem. 380, 303 (1971). 53. Singleton, J. H., J . Phye. Chem. 60, 1606 (1956). 64. Shallcross, P. B., and Russell, W. W., J . Amer. Chem. SOC.81, 4132 (1959).
PALLADIUM AND NICKEL HYDRIDES
29 1
55. Zhavoronkova, K. M., Boreskov, G. X., and Nekipelov, V. N., Dokl. Akad. Nauk S S S R 171, 1124 (1967). 56. Clarke, J. K. A., J . Catal. 27, 412 (1972). 57. Takasu, Y., and Yamashina, T., J . Catal. 28, 174 (1973). 58. Cadenhead, D. A., and Wagner, N. J., J. Catal. 21, 312 (1971). 59. Hardy, W. A., and Linnett, J. W., Trans. Faraday SOC.66, 447 (1970). 60. DUB, R., Trans. Faraday SOC.70, 877 (1974). 61. Baranowski, B., Bull. Acad. Pol. Sci., Ser. Sci. Chim., Geol. Geogr. 7, 891 (1959). 6.2 Janko, A., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 10, 617 (1962). 63. Palczewska, W., and Frackiewicz, A., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 14, 67 (1966). 64. Palczewska, W., Frackiewicz, A., and Karpiliski, Z., Proc. Int. Congr. Catal., Ith, 1968 Paper 52 (1971). 64a. Palczewska, W., Frackiewicz, A,, and Karpiliski, Z., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 17, 687 (1969). 65. Karpifiski, Z., Palczewska, W., and Frackiewicz, A., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 21,203 (1973). 65a. Karpifiski, Z., Thesis, Inst. Phys. Chem., Warsaw (1972). 66. Frackiewicz, A., Karpihski, Z., Leszcsyfiski, A., and Palczewska, W., Proc. Znt. Congr. Catal., 5th, 19Y.2 p. 635 (1973). 6Y. Palczewska, W., and Janko, A., Thin Solid Films 18, 105 (1973). 68. Jablobski, A., and Palczewska, W., to be published. 69. Leszczyfiski, A., Frackiewicz, A., and Palczewska, W., PTOC.Symp. Mech. Hydrocarbon React., 1973 (to be published). 70. Eley, D. D., and Shooter, D., J . Catal. 2, 259 (1963). 71. Konenko, I. R., Nadjm, A., Tolstopiatova, A. A., Samsonov, G. V., and Makarenko, G. N., Zzv. Akad. Nauk SSSR, Old. Khim. Nauk p. 2710 (1970). 72. Konenko, I. R., Nadjm, A., Tolstopiatova, A. A., Samsonov, G. V., and Makarenko, G. N., Izv. Akad. Nauk SSSR, Old. Khim. Nauk p. 100 (1971). 73. Konenko, I. R., Gaifutdinova, R.K., Gorshkova, L. S., Tolstopiatova, A., Berg, L. G., and Moreva, N. I., Kinet. Katal. 14, 203 (1973). 74. Eley, D. D., and Norton, P. R., Z. Phys. Chem. N.F., 64, 145 (1969). ‘75. Dowden, D. A., J . Chem. SOC.,London p. 242 (1950). 76. Dowden, D. A., and Reynolds, P. W., Discuss. Faraday SOC.8, 172 (1950). 77. Eley, D. D., J . Res. Znst. Catal., Hokkaido Univ. 16, 101 (1968). 78. Dowden, D. A., J . Res. Znst. Catal., Hokkaido Univ. 14, 1 (1966). 79. Bond, G. C., Discuss. Faraday SOC.41, 200 (1966). 80. Dowden, D. A., Proc. Znt. Congr. Catal., bth, 19Y.2 p. 621 (1973). 81. Anderson, J. R., ed., “Chemisorption and Reactions on Metallic Films,” Vols. 1 and 2. Academic Press, New York, 1971. 8.2. MOSS,R. L., and Whalley, L., Advan. Catal. 22, 115 (1972). 83. Palczewska, W., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 12,817 (1964). 84. Smialowski, M., and Jarmolowicz, H., J . Catal. 1, 165 (1962). 85. Bovier, C., Janko, A., and Michel, P., Electron Microscopy 1968 Proc. Eur. Reg. conf., dth, 1968 (1968). 86. DUB, R., Surface Sci. 42, 324 (1973). 87. Pielaszek, J., Bull. Acad. Pol. Sci., Ser. Sci. Chim. 20,487 (1972).
This Page Intentionally Left Blank
Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species R. P. COONEY, G . CURTHOYS, AND NGUYEN THE TAM Department of Chemistry University of Newcastle New South Wales, Australia
I. Introduction. .... ........................................ 293 n Effect.. . . . . . . . . . . . 11. The Origin of the A. What is the Raman Effect?. . . . . . . . . . . . . . B. Quantum Mechanical Theory. . . . . . . . . . . . C. Classical Theory.. ...................... D. The Polarizability Ellipsoid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. An Example-The Stretching Vibrations of Carbon Dioxide.. . . . . 301 F. The Difference between Raman and Infrare G. The Principle of Mutual Exclusion.. ...... H. The Relationship of Spectral Activity to Sy 111. Instrumentation. ............................... A. The Laser Source.
C. Detectors.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
314
. . . . . . . . . . . . . . . . . . . 330 . . . . . . . . . . 333 .................... 333
C. Interfering Plasma Lines V. Raman Spectra of Adsorbed
. . . . . . . . . . . . . . . 336
References.
........................... ................ ......................
341
1. Introduction Our understanding of the nature of a solid surface and of the interaction between a molecule physically or chemically adsorbed on the surface is of 293
294
R. P. COONEY ET AL.
both theoretical and practical interest. Techniques available to study this interaction include Low Energy Electron Diffraction, Auger Spectroscopy, Field Electron Emission and Field Ion Emission Microscopy, E.S.R., Mossbauer and UV Spectroscopy, Neutron Inelastic Scattering, Infrared Absorption, and Raman Scattering Spectroscopy. Of these techniques, infrared spectroscopy has been the most widely used. It is particularly valuable for studying vibrations associated with the surface but has the disadvantage that most substrates absorb strongly in the infrared spectrum below 1300 cm-' and so vibrations of an adsorbed molecule cannot be examined in this region. Raman spectroscopy, on the other hand, does not suffer from this disadvantage and is most valuable for studying the vibrations of the adsorbed molecule itself. This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 4Lewis
acid site
i-O-Al-O-Si'
0I
I
Si-
I I
Bronsted acid site H+ H-O-di-o--LO-si-
-Si-
1
I
1 There have been some investigations into adsorption on zeolites (1,2) , and Greenler and Slager (3) have outlined a method for obtaining the Raman spectrum of a thin solid film deposited on a reflecting silver surface. In order to discuss the nature of the interaction between an adsorbed molecule and a surface it is important that the surface coverage be less than one monolayer since in multimolecular adsorption and capillary condensation the spectrum of the adsorbate molecule perturbed by interaction with other adsorbate molecules may mask the spectrum of the adsorbate molecule perturbed by interaction with the adsorbent. Surface coverages may be determined by obtaining an adsorption isotherm with the adsorbate
LASER RAMAN SPECTROSCOPY
295
under consideration and finding the monolayer coverage from the BET equation or from the Langmuir equation where these are applicable. In some cases, the surface coverage has been calculated from the surface area of the adsorbent as determined by nitrogen adsorption and the area occupied by the adsorbed molecule (determined from the density of the liquid or solid adsorbate), assuming the adsorbed species to be close-packed and all the adsorbent available for adsorption. It should be pointed out that these two methods may lead to quite different results. The specific surface area of Cab-O-Sil HS5 as found by the adsorption of nitrogen is 325 m2g-I, whereas l as determined by the adsorption of pyridine ( 4 ) . A further it is 154 m2 g method is to determine the orientation of the molecule to the surface from the Raman spectrum, to calculate the area of the molecule from the known molecular parameters, and calculate the surface coverage from the measured mass of adsorbate per unit mass of adsorbent. Before discussing specific examples of the application of Raman spectroscopy to studying adsorbate-adsorbent interactions, it will be necessary, a t this juncture, to explain the nature of the Raman effect.
II. The Origin of the Raman Effect A. WHATIs THE RAMAN EFFECT? The nature of the spectrum and the terminology of the Raman effectare summarized in Fig. 1. An incident photon, hvo, from an essentially monochromatic source, such Sample Loser Spectrometer Scottered hght
v'=v,
V'
I
800 15000
'
400
8
V'>Vo
.
I
15400
15800
I
400
.
I
800 Ramon shift (AP)
16200 16600
Wovenumber (cm-')
FIG.1. Terminology of the Raman effect (schematic).
296
R. P. COONEY ET AL.
as a laser, is scattered (hv’) both elastically and inelastically by a chemical sample. Further, there is always macroscopic or primary reflective scatter from the sample. The elastic (hvo = hv’) and inelastic (hvo > hv’ or hvo < hv’) processes constitute Rayleigh and Raman scattering respectively. By analogy with fluorescence spectroscopy that part of the Raman light with hvo > hv‘ is defined as the Stokes region, and the part with hvo < hv’ as the anti-Stokes region. The former is usually far more intense than the latter and represents the set of observations usually reported as the Raman spectrum. The individual frequencies are called Raman lines. The wavenumber displacement of each of these Raman lines from the unmodulated laser frequency is called the R a m n shift of that line and is designated AP. The number and intensity of the Raman lines and the magnitude of the Raman shift can be related to the identity, structure, and bonding of the molecules of the compound scattering the light. Both quantum mechanical and classical theories of Raman scattering have been developed. The quantum mechanical treatment of Kramers and Heisenberg (6) preceded the classical theory of Cabannes and Rochard ( 6 ) .
B. QUANTUMMECHANICAL THEORY (7-9) The quantum mechanical view of Raman scatering sees a radiation field hvo inducing a transition from a lower level k to a level n. If Vnk is the transition frequency, then the inelastically scattered light has frequency vo - Vnk. That is, the molecule “removes” energy hv,k from an incident photon. This process corresponds to Stokes scattering. Alternatively, a molecule underEnergy
level r
Third common
Energy
(01
FIG.2. The quantum mechanical view of Raman scattering. (a) Stokes scattering process; (b) anti-Stokes scattering process (8).
LASER RAMAN SPECTROSCOPY
297
going a transition from a higher level n to a level k induced by the radiation field hvo effectively “adds” energy h v n k to the incident photon. This process corresponds to anti-Stokes scattering (Fig. 2.) One aspect of the mathematical treatment of the quantum mechanical theory is of particular interest. The wavefunction of the perturbed molecule (i.e. the molecule after the radiation is “switched on”) involves a summation over all the stationary states of the unperturbed molecule (i.e. the molecule before the radiation is “switched on”). The expression for intensity of the line arising from the transition k + n involves a product of transition moments, M k & f , . n 1 where r is any one of the stationary states and is often referred to as the “third common level’’ in the scattering act. The third common level is often invoked in simplified interpretations of the quantum mechanical theory. I n this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level T. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by -V&. This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + v n k . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9).
C. CLASSICAL THEORY (7) The classical theory of scattering provides us with a relatively simple selection rule for Raman activity which can be compared with that for infrared activity. A diatomic molecule placed in an electric field of the type present in an electromagnetic wave experiences an induced dipole M at any instant due to the displacement of the electrons with respect to the relatively massive nuclei under the influence of the applied field E .
M = aE.
(1)
This induced dipole moment is independent of any dipole moment the molecule may possess in its equilibrium configuration. The molecular polarizability, a!, has the properties of a tensor because both M and E are vectors. The oscillating induced dipole may interact with the oscillations of the atomic nuclei during a molecular vibration. Hence we may expand a! as a
295
R. P. COONEY ET AL.
Taylor’s series in terms of the normal coordinate Q1 (Appendix). If the displacement is small we may ignore higher order terms in the expansion. a = ao
+ (aa/aQi),Qi.
(2)
The subscript zero in this expression refers to the equilibrium configuration. The normal coordinate Q1 is also a function of vibrational frequency u1 and of time t. &I = &Io cos (27rvlt). (3) yo
The applied field E is a periodic function of time t and has a frequency which is the frequency of laser emission.
(4)
E = Eo cos(27rvot).
On substitution in Eq. (1) followed by expansion and simplification we arrive at the expression
M = aoE0 cos(2rud)
+ +Eo&i(aa/aQi),[cos 2*(vo + d t + cos
~ T ( V O
-dt].
(5)
This expression may be extended to include the case of a polyatomic molecule by summing over all the different vibrations of the molecule. The implications of this treatment may be summarized with reference to Eq. ( 5 ) : (a) The first term aOE0 cos(2avot) describes the intense scattered radiation with unmodulated frequency yo. (b) The second and third terms +EoQ1(&/Y/aQi) ,[COS
ZT( vo
+
YI) t
+ cos 2 ~vo (-
t]
~ i )
represent the high frequency (anti-Stokes) and low frequency (Stokes) regions of the Raman spectrum. (c) In order therefore that (uo f v1) appear (i.e. the Raman spectrum) the coefficient (Y/aa/aQl), must be nonzero. A selection rule may therefore be stated for Raman scattering: For a vibration to be observable in the R a m a n spectrum there must be a change in molecular polarizability during the vibration. The molecular polarizability can be considered to be the cumulation of individual bond polarizabilities. The bond polarizability is known (in simple cases) to be an approximately linear function of bond length for small amplitudes of vibration. That is, polarizability is essentially a bond property and consequently is independent of direction along any axis (or independent of “sense”). For comparison the selection rule for infrared spectroscopy according to both classical and quantum mechanics may be stated: For a vibration to give rise to an infrared absorption there must be a change in the direction or magnitude of a dipole moment associated with that vibration. An important difference between the infrared and Raman selection rules
299
LASAR RAMAN SPECTROSCOPY
now becomes obvious. Unlike the molecular polarizability, the dipole moment behaves as a vector and so is dependent on direction along an axis.
MZ
azzazyazz
Mu
Mz
Ez
a y z ~ y y a*y zE, az;azyazz Ez
=
.'
(6)
Because an applied field in the y direction E, can induce a dipole M with a component in the x direction M , as well as the component in the y direction M,, it is necessary that we specify the components of the polarizability tensor by two subscripts (Fig. 3). If the bond A-B of a diatomic molecule stretches during a vibrational mode, M , and M,, will vary and therefore the corresponding polarizability tensor components will vary. The polarizability tensor may therefore be defined by a set of nine components which reduce in number to six because the tensor is symmetric. The physical significance of nlolecular polarizability is often explained in terms of the polarizabilily ellipsoid which is defined by the equation:
+ ffyvy2+
+ 2a,lixy + 2a,,yz + 2cuz,zx
1.
(7) This ellipsoid may be considered to diagramatically represent the molecular cY.zx2
t
cY,,z~
t
t
It
t
=
t
APPLIED FIELD Ey
INDUCED DIPOLE M -
t MY
I Fro. 3. Induced dipole moment. Charges in an anisotropic region such as a cheinical bond will tend to move in preferred directions. An electric field E,, might therefore produce an induced dipole M with components M. and Mu perpendicular and parallel to the field, respectively (11).
300
R. P. COONEY ET AL.
polarizability. When the Cartesian coordinate axes ( X , Y , 2) are chosen to coincide with the three principal axes of the ellipsoid the equation of the ellipsoid reduces to the form axxx2
+ ayyY2 + azzZ2 = 1.
*
(8)
With the new coordinate system only the three diagonal components axx,a y y , and ~ Z referred Z to as principal values of a are nonzero. The halfaxes of the ellipsoid are sip, .?:/', and a;'z/". If the polarisability ellipsoid
Symmetrical stretching, v,
Antisymrnetricol stretching, v2
Bending, us
FIQ.4. Polariaability changes during the vibrations of carbon dioxide (exaggerated) (7).
301
LASER RAMAN SPECTROSCOPY
is defined instead by the equation 9 -+
ffii
p
ffjj
k2 += 1.
(9)
ffkk
where aii, ajj, and (YM are the polarisability components along Cartesian axes i, j , and k then the appearance of the ellipsoid at some instant during a vibration can be more easily visualized. The half-axes of this ellipsoid are .I;?, and a:%.
E. AN EXAMPLE-THE STRETCHING VIBRATIONS OF CARBON DIOXIDE The symmetric stretching vibration. During this vibration the ellipsoid “breathes” (i.e. expands and contracts) at the frequency of the vibration (Fig. 4). The dimensions of the ellipsoid obviously change during the vibration and consequently the vibration is Raman active. The antisymmetric stretching vibration. The molecule loses its original symmetry during the vibration. A t the two extrema of the vibration the shapes of the molecule will be identical. Because the molecular polarizability is essentially the summation of all bond polarisabilities and is independent of direction along the internuclear axis, it will have identical values at the extrema. Consequently, the vibration is Raman inactive. An alternative way to view these changes in polarizability is illustrated in Fig. 5. During the symmetric stretching vibration (curve 1) ,the polaris-
Q1
FIG.5. Polarizability as a function of normal coordinates (schematic) (10).
302
R. P. COONEY ET AL.
ability is larger than the equilibrium value in one half-period and smaller in the other. For harmonic vibrations (i.e. small vibrational amplitudes) a changes linearly with changes in the normal coordinate. For the antisymmetric stretching vibration and the bending vibration the polariaability is the same at opposite phases of the vibration (curves I1 and 111) , and therefore its variation with the normal coordinates is as illustrated in Fig. 5. In these latter two cases, the tangent to the cwve at the equilibrium configuration is a t 0". To a first approximation, for small amplitudes, the polariaability does not change, and hence the antisymmetric stretching and bending modes are Raman inactive. Infrared activity of vibrations is readily deduced. The symmetric stretching vibration has no associated dipole moment change during the vibration and is, therefore, infrared inactive. The asymmetric stretching vibration has an associated dipole moment which fluctuates with the frequency of the vibration. The vibration is, therefore, infrared active. Application of similar reasoning to the case of the bending mode of COz would indicate that the vibration is Raman inactive, infrared active.
F. THEDIFFERENCE BETWEEN RAMAN AND INFRARED SPECTRA (10) The intensity of absorption or emission associated with a vibrational transition is proportional to the square of the transition moment integral (Appendix),
where $tr and +i are the wavefunctions for the final and initial states and 0 is an operator representing the mechanism responsible for the transition (dipole moment change, polarizability change, etc.) . It may be demonstrated (Appendix) that transitions can occur from a symmetrical initial state only to those states that have the same symmetry properties as the transition operator, 0. We may consider how this selection rule applies to the two cases of the infrared and Raman spectra : 1. Infrared Spectra For infrared absorption the operator 0 is the dipole moment M defined by M = ear which can be written in terms of the components
LASER RAMAN SPECTROSCOPY
303
where ei represents the charge on the ith particle and xi, yi, and ziare its coordinates. As the ei may be included with the proportionality constants, the transition moment integrals may be rewritten
For a fundamental transition to occur by absorption of infrared dipole radiation, it is necessary that one or more of these integrals (and consequently the intensity) be nonzero. It follows from the selection rule given above that in order that a transition be infrared active #f must have the same symmetry properties as at least one of x, y, or z. In general, the first excited state (i.e. the final state for a fundamental transition) is described by a wavefunction #f which has the same symmetry as the normal coordinate (Appendix). The normal coordinate is a mathematical description of the normal mode of vibration. Hence we may conclude: for a vibration to be active in the infrared spectrum it must have the same symmetry properties (i.e. transform in the same way) as, at least, one of x, y , or z. The transformation properties of these simple displacement vectors are easily determined and are usually given in character tables. Therefore, knowing the form of a normal vibration we may determine its symmetry by consulting the character table and then its infrared activity. 2. Raman Spectra
For Raman scattering the operator 0 is the polarizability a defined by
M
= aE
(see earlier). The method of determining the behavior of aij with respect to symmetry operations may be illustrated with reference to the simplest case, a nondegenerate vibration. Considering Fig. 3 we see that for a field applied along the y axis we may have M , = azvEu. Because M , and Ell can only remain unchanged or change sign under a symmetry operation, azywill remain unchanged or change sign depending on whether M , and E,, behave in the same or in the opposite way for the symmetry operation considered. Because M , and Ell are vectors having the same symmetries as the displacement vectors x and y , respectively, aZvhas the same symmetry as the product of the vectors, xy. In general, aij has the same symmetry transformation properties as ij. Hence we may conclude:for a vibration to be active in the Raman spectrum it must have the same symmetry properties (i.e. transform in the same w a y ) ,
304
R. P. COONEY ET AL.
as, at least, one of xx, yy, zz, xy, yz, or zx. The symmetry transformation properties of these binary functions are also usually given in character tables.
G. THEPRINCIPLE OF MUTUAL EXCLUSION (IS) The example of COz discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion: For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses B center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0,0,O),for every atom positioned at (xi,yil z i ) there will be an identical atom at ( -xi, -yi, -zi) . A square planar molecule XY, has a center of symmetry at atom X, whereas a trigonal planar molecule XY, does not possess a center of symmetry. When one of the Cartesian coordinates (i.e. x , y , or z ) of a centrosymmetric molecule is inverted through the center of symmetry it is transformed into the negative of itself. On the other hand, a binary product of coordinates (i.e. xx, yy, zz, xz, yz, zx) does not change sign on inversion since each coordinate changes sign separately. Hence for a centrosymmetric molecule every vibration which is infrared active has different symmetry properties with respect to the center of symmetry than does any Raman active mode. Therefore, for a centrosymmetric molecule no single vibration can be active in both the Raman and infrared spectrum.
H. THERELATIONSHIP OF SPECTRAL ACTIVITY TO SYMMETRY Centrosymmetric molecules represent a limiting case as far as molecular symmetry is concerned. They are highly symmetric molecules. A t the other extreme, molecules with very low symmetry should produce a set of Raman frequencies very similar to the observed set of infrared frequencies. Between these two extremes there are cases where some vibrations are both Raman and infrared active and others are active in Raman or infrared but not in both. Nitrate ion is an example of a molecule in this intermediate class. The relationship between Raman and infrared activity and molecular symmetry is summarized diagramatically in Fig. 6. We do not, in general, have to depend on conceptual approaches or on qualitative generalizations. Symmetry and group theory have provided us with a general method, called symmetry analysis, of determining the number of Raman active vibrations, the number of infrared active vibrations, and
305
LASER RAMAN SPECTROSCOPY
Point group C2D ,, e g CO,, trans CHCL = CHCL
Point group C2,,C, e g pyridine, CH,CL
FIG.6 . The relationship between Raman and infrared activity and molecular symmetry.
the number of coincident frequencies in both Raman and infrared spectra. The theory and methods of symmetry analysis are beyond the scope of this article. The type of information obtainable from such an analysis is illustrated in Table I for the cases of cis and trans-dichloroethylene. The latter is centrosymmetric and illustrates the Principle of Mutual Exclusion. TABLE I Symmetry Analysis of cis- and trans-Dichloroelhylene ~
~~
~~
Predictionso Symmetry"
Reduced representation*
+
trans (Ca) ~~
+
5A1(R,ir) +2Az(R) 4B1(R, ir) B*(R, ir) 5A,(R) 2A,(ir) BAR) 4B.(ir)
cis (CZJ
+
~~~~
+
+
TZR
n1
TZO
12
10
10
6
6
0
~
Point group symmetry in parenthesis. Reduced representation of normal modes : the symmetry species which describe the symmetry of the (3N 6) normal modes (where N is the number of atoms in the molecule). nR, the number of Raman lines predicted, nr the number of infrared absorptions predicted, and nc the number of coincident Raman-infrared frequencies predicted.
-
306
R. P. COONEY ET AL. Loser
\
-Interference filter
t
Focussing lens Monochrornotor
Recorder
Amplifier
I Detector
FIG.7. Block diagram of components of
a laser Raman spectrometer.
111. Instrumentation The first successful application of the continuous wave (CW) He-Ne gas laser as a Raman excitation source by KogeInik and Porto (14) was reported in 1963. Since that time, significant improvements in instrumentation have been continually achieved which have circumvented a great number of problems encountered with mercury lamp sources. The renaissance of Raman spectroscopy has also been due to improvements in the design of monochromators and photoelectric recording systems. A modern laser Raman spectrometer consists of four fundamental components: a laser source, an optical system for focusing the laser beam on to the sample and for directing the Raman scattered light to the monochromator entrance slit, a double or triple monochromator to disperse the scattered light, and a photoelectric detection system to measure the intensity of the light passing through the monochromator exit slit (Fig. 7). Som2 important aspects of each fundamental component will be discussed. A. THELASERSOURCE In general, the choice of a laser for use as a Raman excitation source is based on a number of considerations. The laser excitation wavelength, for experimental and theoretical reasons, must lie in the visible region, i.e. 400-700 nm. The laser should have many emission lines over a wide range of the visible region and the excitation frequency should not correspond
TABLE I1 Selection of Laser Lines for Colored Samples
Appearance of sample Green-yellow Yellow Orange Red Purple Violet Blue Green-blue Blue-green
Light absorbed Color k(nm) Violet Blue Blue-green Blue-green Green Green-yellow Yellow Orange Red
p = possible. ) s = suitable.
a
4w35 435-480 4W90 490-500 500-560 560-580 580-595 595405 605-670
457.9 476.5 (Indigo) (Blue)
488.0 (Blue)
496.5 (Bluegreen)
Exciting line (nm) 501.7 568.2 (Blue- 514.5 530.9 (Yellow- 632.8 green) (Green) (Green) green) (Red) Pa
Sb
P
*
r
tn
647.1 (Red)
S S
S
S
S
S
S
S
S
P
S
S
*iz 5
u1
w
M
2
!a
p s
0
tn d S
0
S
P
%¶
S
S
S
S
4
308
R. P. COONEY ET AL.
to any electronic transition of the system under investigation. This is to avoid absorption of radiation by the system thus enabling the studies of colored materials. It is desirable to use an exciting line whose color matches with that of the material. However, in general, one has greater flexibility and Table I1 serves as a rough guide in the selection of the laser excitation wavelength for colored materials. The laser should have an output power as high as possible yet it should be able to operate at reduced power without fluctuation to avoid decomposition of the material. Some of the principal lasers suitable for Raman spectroscopy are now discussed. 1. The Helium-Neon Laser
The He-Ne laser has a stable, narrow-beam output a t 632.8 nm in the red region. This type of laser typically has short term peak-to-peak fluctuation of less than 1%. Its long wavelength enables the studies of colorless materials and colored red-transmitting materials. The 632.8 nm line has a maximum output about 80 mW. 2. The Argon Ion Laser Since Raman scattered light intensity is very weak, of the order of lo-' of the excitation line intensity, more powerful laser sources than the He-Ne laser are often needed. The Ar+ laser emits various lines in the region from 457.9 nm to 514.5 nm, of which the most powerful lines (typically -700 mW) a t 488.0 nm (blue) and 514.5 nm (green) are preferred. Furthermore, two other factors which favor the use of the high frequency excitation lines are the peak sensitivity of the photomultipliers in this blue-green region (Fig. 8) and the fourth power Raman intensity law
I = const.
(yo
-V)~M~,
where (YO - v ) is the wavenumber of the Stokes line and M is the transition moment. As calculated from this law, the Ar+ 488.0 nm line gives rise to Raman signals at zero on the Raman shift scale 2.8 times as intense as corresponding signals from the He-Ne 632.8 nm line at equal power. Commercial Ar+ lasers may incorporate a light regulator accessory (also called a servo loop stabilizer) which minimizes fluctuations in the laser output power thus improving long term power stability to better than 0.5% rms. The Ar+ laser also has its disadvantages relative to the He-Ne system. It has an excitation line width about 0.15 to 0.25 cm-' which is broader than that of the He-Ne line (0.05 cm-I) . This broader excitation width im-
309
LASER RAMAN SPECTROSCOPY
1
2
3
4
5
6
7
8
9
0
Wavelength (Angstroms x D31
FIG.8. Response curves of some photomultiplier tubes versus excitation wavelength (see also Section 111,C).(Courtesy Spex Industries, Inc.).
poses a restriction on the resolution limit when the Ar+ laser is used as a Raman source.
3. The Krypton Ion Laser The Kr+ laser provides two strong emission lines a t 647.1 nm (red) and 568.2 nm (yellow). In terms of its power outputs (150 mW at 568.2 nm and 500 mW at 647.1 nm) and consequently in terms of Raman spectral sensitivity, it is to be preferred to the 632.8 nm line (80 mW) which is also in the red region.
4. The Ar+/Kr+ Mixed Gas Laser The mixed gas laser Ar+/Kr+ has all the principal Ar+ and Kr+ lasing lines through the visible region. In spite of its wavelength versatility, this
310
R. P. COONEY ET AL.
mixed laser has lower output powers at individual lines than those from the Ar+ or Kr+ laser separately. Control Laser systems have Ar+ and Kr+ plasma tubes which can be interchanged rapidly. For other laser systems, the interchange may also be carried out. Alternatively, two separate lasers, namely the Ar+ and Kr*, can be coupled to the same instrument. This system is considered the best combination Raman source available in terms of its wide choice of lasing lines over the whole visible region and individual line output powers.
5. Tunable Dye Lasers The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-') and a continuous background spectrum which requires the use of a tunable filter (16-18). A list of the principal laser lines useful as Raman excitation source is TABLE I11 Laser Lines as Raman Excitation Sources
Laser
Model
He-Ne Ar+
SP 125A CRL 52A and SP 165-00
Kr+
CRL 52K and SP 165-01
Ar+/Kr+
CRL 52 MG and SP 165-02
Wavelength (nm)
Color
Typical power (mW)
632.8 457.9 476.5 488.0 496.5 501.7 514.5 530.9 568.2 647.1 488.0 514.5 530.9 568.2 647.1
Red Indigo Blue Blue Blue-green Blue-green Green Green Yellow-green Red Blue Green Green Yellow-green Red
80 150 300 700 300 140 800 200 150 500 250 250 80 100 250
LASER RAMAN SPECTROSCOPY
311
I000 800 1
5 z
600
a"
400 200 0 400
450
500 Laser wavelength (nm)
FIG.9. Output powers of different laser wavelengths.
given in Table 111, and the output powers of different lasers are compared in Fig. 9.
B. MONOCHROMATORS The monochronomator in a Raman spectrometer must have excellent stray light and resolution characteristics. 1. Stray Light Discrimination (or Spectral Purity)
a. Origins of Stray light. The stray light discrimination of a monochromator is a measure of stray light levels present. When the nominal frequency setting of n spectrometer is exactly the same as that corresponding to a Raman transition, ideally the recorder should register only the Raman signals at this frequency. In the absence of Raman bands, the recorder reading should be zero. However, some stray signals are always present. They originate from varous sources. Since Rayleigh and Tyndall scattering at the excitation frequency are so much stronger than Raman scattering, a small amount of light at this frequency is accidentally and fortuitously reflected through the spectrometer ultimately reaching the detector, thus producing a recorder reading. Furthermore, imperfections in rulings of the gratings in a monochromator result in spurious lines, i.e. the so-called grating ghosts, in a Raman spectrum. b. Methods for Minimizing Stray Light. Several methods have been developed for minimizing stray light such as the introduction of double and triple monochromators and the use of iodine absorption filters.
R. P. COONEY ET AL.
312
Stray light
Level of Raman from gases liquids a single crystals
Double monochromatar
Triple monochromator
0
100
200
300
4
Raman shift ( A cm-'1
FIG.10. Scattered light including ghosts appearing in a single, double, and triple monochromator.Although scattered light plunges to the square root in a double monochromator, interference with Raman spectra still occurs occasionally. In a triple monochromator, instrumental scatter has never been known to interfere with Raman scatter (19).
(i) The triple monochromator. The stray light level in a double monochromator is equal to the square root of that in a single monochromator. Most modern Raman instruments employ double monochromators. The Gary 82 was the first to incorporate a triple monochromator which reduces stray light level to extremely low levels. Figure 10 shows the stray light levels in a single, double, and triple monochromator (19). Nevertheless, the advantage of obtaining greater spectral purity with a triple monochromator must be weighed against possible diminution of energy throughput and difficulties in optical alignment. Both drawbacks result from the increased number of optics and reflection loss from an ad-
313
LASER RAMAN SPECTROSCOPY
ditional grating. A t the blaze wavelength, 70% of the light would be transmitted by the single monochromator compared with 53% by a double monochromator. With a triple monochromator, only 37% is transmitted. A detachable monochromator (19)developed by Spex Industries, was another approach in minimizing stray light. It is a modified Czerny-Turner spectrograph which can be coupled to the exit slit of a double monochromator and function as a variable bandpass, variable frequency filter. This accessory, while providing the versatility of a triple monochromator, does not add much mechanical and optical complexity and can be removed when not wanted. (ii) The iodine filter (20, 21). This method involves placing a heated iodine cell between the sample and the monochromator. Iodine vapor has two transitions of identical frequency (19429.27 cm-' or 514.54 nm in air). One of these is a transition from the twelfth rotational level of the zeroth vibrational level of the ground electronic state to the eleventh rotational level of the forty-third vibrational level of the Bs& electronic state, i.e. 0-43P (12). The other is 0-43R (14) between the same electronic levels. The laser light must be single-moded and the frequency of the single mode is blocked within the wide rotational line width of the iodine vapor (Fig. 11). However, the method is necessarily unique for the 514.5 nm line from the Ar+ laser. 5145 36
1
SINGLE AMPLIFIED MODE OF LASER
(IN AIR) DOPPLER GAIN CURVE
IODINE ROTATIONAL LINE OF 0 - 4 3 VIBRATIONAL P(12),R(14) ROTATIONAL TRANSITION
MODE IS SWEPT THROUGH GAIN CURVE BY ADJUSTING ETALON
NEXT ROTATONAL LINE 20GK-
I
-8
-6
I - 4,
JL -2
\ 0
2
4
6
8
10
FREQUENCY SHIFT FROM CENTER OF GAIN CURVE (GHz)
FIG.11. Intensity of the single mode of Ar+ 514.5 nm as mode is swept through gain curve. The single mode will match the iodine transition line and will be absorbed (81).
314
R. P. COONEY ET AL.
2. Spectral Resolution The resolution of a monochromator is the smallest frequency interval the instrument can separate. The limiting resolution is the bandwidth measured at half height when scanning across an infinitely narrow intense source (22). As already mentioned, the broader excitation line width of Ar+ lasers (0.15 to 0.25 cm-l) compared to that of the He-Ne lasers (0.05 cm-l) means a lower resolution limit when the Ar+ laser is used as a Raman source. Table IV lists some commercial Raman spectrometers with their performance specifications. C. DETECTORS Photoelectric detection is incorporated in all modern Raman instruments. 1. Photomultipliers
The choice of a photomultiplier tube is dependent partly on the choice of the laser line. It is also based on two characteristics: (a) High quantum efficiency. The quantum efficiency is the ratio of the number of signal pulses which appear at the anode per second to the number of photons which reach the photocathode per second. It is a function of wavelength (23). (b) Low thermionic dark current. In the absence of light, few thermally excited electrons should leave the cathode of the photomultiplier tube (23). The ITT FW 130 photomultiplier tube with an 5-20 type response is often used. The 5-20 type response versus wavelength has been shown in Fig. 8. The quantum efficiency of the 5-20 type falls off rapidly at long wavelength. The RCA C313034 with a GaAs cathode has a cathode quantum efficiency far better than that of the 5-20 type in the red region. This photomultiplier tube allows the study of the 2700-3200 cm-l Raman shift region with greater sensitivity using the red line (Fig. 8). Thermoelectrical cooling,of the photomultiplier tube at about -30°C reduces the dark noise current to a very low level. However, as the quantum efficiency of the 5-20 type decreases as rapidly as the dark current in the red region, cooling brings only modest increases in the signal-to-noise ratio (233). 2. Signal Processing
DC amplification and pulse counting (often inaccurately called “photon counting”) are two types of signal amplifications often used.
TABLE IV
Some Commercial Raman Spectrometers
Spectrometer
Resolution (cm-l)
Stray light
Type
Focal length (m)
Gratings
~~
Cary 81
0.5 at 632.8 nm
Cary 82
0.25 at 632.8 nm
Cary 83
2.0
Spex Ramalog 4
0.18 at 17628 cm-'
10-lo (80 d from Littrow double gratings 632.8 nm) 10-10 (10A cm-1 from Triple gratings 632.8 nm) Littrow double gratings
10-lo (20A cm-' from 19,400 cm-l) Spex 1401 0 . 2 at 632.8 nm 10-9 (25A cm-1 from 632.8 nm) Spex Ramalab 1.O at 441.6 nm 10W1 (100A cm-l from 441.6 nm) 10-10 (25A cm-1 from Jarrell-Ash 25-500 1.0 at 632.8 nm 647.1 nm) Jarrell-Ash 25-400 0.25 cm-1 a t 632.8 nm 10-10 (15A cm-1 from 632.8 nm) JarreLl-Ash 25-300 0.25 cm-' at 632.8 nm 10-10 (25A cm-1 from 632.8 nm) 10-8 (25A cm-1 from Beckman 700 1.0 at 632.8 nm 488.0nm) Coderg PHO 0.25 10-11 Coderg T800
0.3 at 514.5 nm
Jasco R300
2
1.o
0.53 0.4
Czerny-Turner double gratings Czerny-Turner double gratings Czerny-Turner double gratings Ebert double gratings
0.85
Czerny-Turner double gratings Czerny-Turner double gratings Ebert double gratings
1.0
0.4
Ebert-Fastie double gratings
0.6
10-13 (50A cm-1 from Triple gratings 514.5 nm) 10-0 Czerny-Turner double gratings
0.75 0.50 0.50
1.o
0.8 0.4
1200 lines/mm, blazed at 500 nm 1800 lines/mm, blazed at 500 nm 1200 lines/mm, blazed at 500 nm 1200 lines/mm, blazed a t 500 nm 3600 to 8 lines/mm, blazed at 150-1125 nm 1200 lines/mm, blazed at 500 nm 1180 lines/mm, blazed at 500 nm 1180 lines/mm, blazed a t 500 nm 1180 lines/mm, blazed at 500 nm -
1200 lines/mm, blazed at 500 nm 1800 lines/mm, blazed at 500 nm
-
F
kM a a
kP
1: r P
2
2 a 0
z
cu c.. 01
R. P. COONEY ET AL.
316
TABLE V Relative Performance of a Spectrometer-Laser System Using Various Laser Sources.
A
Raman region (cm-1) He-Ne laser 0 (632.8nm) 3000 (781.1nm)
B
C
D
E
F b
Normaliring factor Molecular Power for scattering relative to Grating Quantum equivalent Raman efficiency 80 mw efficiency efficiencyc bandpassd intensity
1 .o
1.0
2.8
14.0
2.3
13.0
1.5
3.0
0.9
5.0
0.8
1.o
1 .o 0.25
1.0 1.6
1.2 1.1 1.2 1.1
2.3 1.5 2.2 1.2
0.59 0.81 0.65 0.91
64.0 51.0 51.0 36.0
1.1 1.0 1.0 0.75
1.5 0.73 0.9 0.2
0.8 1.1 1 .o 1.7
7.2 3.5 4.1 1.0
1.0 0.32
Ar+ laser
0 (488.0nm) 3000 (571.7nm) 0 (514.5nm) 3000 (604.8nm) Kr+ laser 0 (568.2nrn) 3000 (685.0nm) 0 (647.1nm) 3000 (830.0nm)
Freeman and Landon ($6). Blazed at 500 nrn. FW 130 photomultiplier. Constant luminosity conditions. " F = A X B X C X D X E.
In dc amplification, the signal pulses from the phototube anode are converted into photocurrent and the voltage drop produced is read and the output displayed on a recorder. In the pulse counting method, each photoelectron pulse arriving at the phototube anode is processed. The pulses are amplified and then used to trigger a pulse generator. The output pulses from the generator are integrated and displayed on a recorder. For large signals, dc amplification is preferred. On the other hand, pulse counting is more advantageous in the case of extremely low light levels. Hendra and Loader (2.4) considered dc amplification to be most suitable in the study of oxide surfaces because of the larger range of zero suppression available. Egerton et at. (26), however, have successfully employed pulse counting in recording spectra of surface adsorbed species. Tobin (23) has
LASER RAMAN SPECTROSCOPY
317
provided a review of signal processing (as well as sources and monochromators). It is worth comparing the relative performance of a spectrometer-laser system using various laser types. The data are summarized in Table V ($6).The data, though typical, depend considerably on the characteristics of the individual components of the spectrometer. From Table V, many features emerge as being important: (a) Molecular scattering efficiency is a fourth power function of the excitation frequency. (b) Lasers have different output powers at different excitation wavelengths. (c) Grating efficiencies, having a maximum value at a particular wavelength, fall off towards lower and higher wavelengths. (d) Quantum efficiencies, peaked at about 488.0 nm, decrease substantially towards longer wavelengths. (e) Dispersion of a grating spectrometer must vary considerably over the region 500-800 nm for equivalent bandpass, i.e. if constant spectral slit width is desired. (f) The overall Raman signal intensity would therefore be expected, in most cases, to be greatest for the 488 nm Ar+ line.
D. SAMPLING TECHNIQUES 1. Sample Illumination
Two principal methods are often employed whereby the scattered light is viewed either at 180" (coaxial) or 90" (right angle) relative to the incident laser beam. Figure 12 shows the optical arrangements of coaxial and right angle viewing employed in the Coderg PHO and Cary 81 spectrometers. For adsorption work, coaxial viewing has one main advantage in that it provides good spectra of opaque samples (e.g. silica, etc.) because the beam does not have to penetrate so far into the sample and hence problems associated with sample absorption are minimized. Coaxial viewing also facilitates high pressure studies using a diamond cell. Right angle viewing enhances the ratio of collected Raman to Rayleigh scattering and consequently enables the detection of low frequency vibrations. In Raman studies of adsorption on oxide surfaces where high background (see later) is a frequently encountered problem, right angle viewing has been found by the present authors to give better spectra, provided sufficiently high laser power is available. The optimum angle of illumination chosen depends on the physical size and shape of the sample and the fore-optics of the spectrometer,
318
k
LASER BEAM
IMAGE OF SLIT SLICER
t
7
UI
(b)
FIG.12. (a) The H1516 180"excitation unit for coaxial viewing for the Coderg PHO spectrometer; (b) coaxial; and (G) right angle viewing for the Cary 81 spectrometer. (Courtesy Coderg and Varian.)
LASER RAMAN SPECTROSCOPY
I
I
319
I
\SPIKE
I/-DIELECTRIC
FILTER
MIRRORS
2. Cells and Samples a. Cells. Since the wavelength of the laser line lies in the visible region from 400 to 700 nm, glass and silica can be used as cell and window materials. This is in contrast to cell and window materials used in infrared absorption spectroscopy which are often expensive and require special care when handling. The use of optical materials for infrared spectroscopy is also restricted by their transmission limits in the far-infrared region. In general, a Raman adsorption cell consists of a length of pyrex or silica tubing, one end of which is sealed with an optical flat, and the other either connected to a gas line for admitting the adsorbate or to a vacuum line for evacuating the cell. Activation of the samples may then be carried out in situ ( 2 7 ) . At the time of writing, in all papers published on adsorption studies on oxides surfaces, spectra have been reported of samples held at the ambient temperature of the sample compartment. It is obvious that when dealing with very volatile adsorbates, low temperature sample cells may be required to increase adsorption and also to prevent rapid desorption of the adsorbed species. In some instances, it is also desirable to record the spectra of species held at elevated temperatures for better correlation with industrial catalytic systems. It should be noted that there are only a few infrared spectra reported in the literature for high temperature studies of catalytic reactions. Sample emission at elevated temperature is a significant experimental complication in investigations of this type.
R. P. COONEY ET AL.
320
A large number of low and high temperature cells have been described in the literature ( 2 8 ) . b. Samples. Oxide samples are often used in the form of powders or broken pressed disks. For convenience of handling, the present authors (by) have made unbroken disks of Cab-0-Sil silica and zeolites by using a split die and a conventional KBr disk press.
IV. Recording Spectra of Adsorption Systems Laser Raman spectroscopy as it is applied to the study of surface adsorbed.species involves a number of experimental problems such as fluorescence, weak Raman lines, and. interfering plasma lines. Techniques of overcoming these problems have been continually improved and good
1700
1600
1500
1400
1300
1200
1100
1000
900
000
700
600
Wavenumber (cm-’) (a)
C
._ c
;
a n
v (cm-9 (b)
FIG.13. (a) Raman spectrum of a pretreated Cab-0-Sil disk recorded using a laser beam expander; (b) infrared spectrum of a newly pressed Cab-0-Sil disk. From Hendra and Gilson, “Laser Raman Spectroscopy,” p. 186. Wiley, New York, 1970.
LASER RAMAN SPECTROSCOPY
321
Raman spectra can now be obtained of numerous adsorbateadsorbent systems. A. SPECTRAL BACKGROUND 1. Oxide Spectra
The Raman spectrum of an oxide sample after adsorption may be considered to consist of the spectrum of the adsorbed species superimposed on the spectrum due to the oxide adsorbent. In general, the Raman spectra of oxide adsorbents are sufficiently weak or sufficiently simple that they allow the detection of Raman lines due to the adsorbed species. This is one major advantage of Raman scattering over infrared absorption spectroscopy. The infrared spectra of most oxide adsorbents show strong absorptions which may obscure those arising from the adsorbates (Figs. 13,14). 2. Fluorescence One additional feature which often complicates the recorded Raman spectrum of the adsorbent is strong emission background. The phenomenon, commonly called fluorescence, usually appears- as a broad emission band extending over a wide range of wavenumbers (Fig. 15). In some highly unfavorable cases, the emission background can be orders of magnitude more intense than those of Raman lines due to the adsorbed species. Emission background seems to be characteristic of a large number of oxide surfaces ranging from silicas, aluminas, silica-aluminas, and metal oxides to natural and synthetic zeolites. Hendra and Loader (24) reported that the background is fairly weak for silicas but more intense for most aluminas and silica-aluminas. However, they believed that the intense background does not arise from the adsorbent itself. The high fluorescent background due to the presence of small amounts of hydrocarbon impurities on the oxide surfaces has been reported by many authors ( 2 , 2 5 , 2 9 ) .Some organic compounds such as furan and acetone were found to cause an increase in fluorescent background of the oxide samples (SO). Piperidine on silica held a t high temperature generates a highly fluorescing material (27). In the case of aluminas and zeolites, fluorescence may arise from traces of transition metals (especially Fe3+) as has been found by Egerton et al. (SO). 3. Other Possible Origins of Spectral Background
Other explanations have been offered for the unusually high spectral background encountered in high surface area oxide materials. Buechler
r-
I
A
489
-
m
I
w
i
h3
X
h3
I
I
w z
c 7
F ‘d
M Y 4
1100
FREQUENCY (ern-'
I\ FREQUENCY (ern-’
FIG.14. Raman spectra of some zeolites A, X, Y, and B ( 1 ) .
i
r
323
LASER RAMAN SPECTROSCOPY 10
a
$ 4 0 3
0
2
ZOO0
2500
2000
1500
1000 500 AU (ern-')
0
-500
-1000
FIG.15. "Fluorescence" spectra of porous Vycor glass after heating (a) in air at looo, and (b) in oxygen at 550". The spectra were run under the same conditions except that the amplification for (b) was ten times higher than for (a) (85).
and Turkevich (2) considered the background originated from the highly porous materials used as adsorbent and suggested that Vycor glass (Corning 7930) with its small pore size compared to the excitation wavelength could minimize sample scattering. Careri et al. (31) correlated the background of aluminas with the presence of chemisorbed water molecules which are tightly hydrogen-bonded close to Lewis sites on the surface. Angel1 ( 1 ) , in his paper reporting the Raman spectra of about sixteen different types of natural and synthetic zeolites, proposed different explanations for a high scattering background such as the formation of colored centers, large dimensioned cavities in zeolite structures, etc. 4. Mechanism of Fluorescence
Though theories have been proposed (32-35) to explain this phenomenon, the mechanism of fluorescence is still not yet fully understood. Jankow and Willis ($6)proposed a mechanism which involves a direct excitation of the molecule or an impurity to an excited state, followed by internal conversion and then reversion back to the original state with emission of light. This mechanism can be explained as follows: A molecule in the lowest vibrational level of the ground state A is transferred t o a certain vibrational level in the excited state D. The molecule tends to cascade into the lowest vibrational level of state D by collisions with other excited molecules. It passes from state D to state C and then to state B by radiationless transi-
324
R. P. COONEY ET AL.
O-1
t
%
P
W C
'4
y L A
W
Interatomic distance
-
FIG.16. Mechanism of fluorescence. From Kauzmann, "Quantum Chemistry,', p. 697. Academic Press, New York, 1957.
tions at the intersections of the potential energy surfaces (Fig. 16). This process is known as internal conversion. Cascading into the lowest vibrational level of state B occurs before the molecule reverts to the original ground state A in the fluorescent emission process. 5 . Treatment of Fluorescence
a. Activation of Samples. It is necessary that both the adsorbent and adsorbate under investigation be of high purity and free from contamination. Purification of adsorbates is effected by conventional techniques such as gas chromatography, recrystallization, distillation, etc. There are several methods of activating an oxide sample prior to adsorption. Hendra and Loader (24) heated their samples in vacuum at about 300°C for about 12 hr. The following successful method of activating samples which effectively reduces the high fluorescent background has been reported by many authors (2, 25, 27, 29). The sample was heated in a stream of pure oxygen at 500°C for an extended period of time to burn out traces of hydrocarbon impurities, then cooled down to room temperature and finally evacuated overnight. It is also desirable to use greaseless joints during thermal treatments. Angel1 (1) reported that activation of zeolites a t temperatures up to 200°C always gives rise to an increase in fluorescence.
LASER RAMAN SPECTROSCOPY
325
Ij FIG.17. Changes in fluorescent background on changing the excitation wavelength. Raman spectra of o-xylene using different exciting lines: (a) Ar+ 488 nm; (b) Kr+ 647.1 nm; (c) Ar+ 514.5 nm; (d) Kr+ 568.2 nm. Fluorescent background was substantially reduced in spectrum (b). (Courtesy Spex Industries, Inc.)
However, treatment of zeolite 4A at 500°C in oxygen was found by the present authors to produce a satisfactory background.
b. “Drench-Quench” Technique. One method of reducing fluorescence is the so-called “drench-quench” technique. In this technique, exposure of the sample in the laser beam over an extended period of time in almost all cases causes a decay in fluorescence to an acceptable level. The rate of TABLE VI Log
e
of Barbituric Acid at Half the Laser Wavelength
Laser Laser wavelength wavelength
(A) 4880 5145 5682 6471
(1)
2440 2572.5 284 1 3235.5
Loge 3.10 3.22 1.10 1.83
R. P. COONEY ET AL.
326
3600
2800
2000
1600
1200
800
400
0
FIG.18. Raman spectra of barbituric acid using different excit,ation wavelengths. See Table VI (56).
fluorescence decay increases when a higher laser power and/or a shorter excitation wavelength is used. Very little is known about the mechanism of this self-quenching. The mechanism could possibly involve photodecomposition of the fluorescing material. c. Changing Excitation Wavelength.Another method is to remove the absorption band causing fluorescence from the Raman spectrum by changing the excitat,ionwavelength (Fig. 17). Jankow and Willis (36) proposed two criteria in selecting the laser wavelength. The wavelength should be long enough to avoid monophoton absorption and the compound should not absorb strongly at half the laser wavelength in order to minimize two-photon absorption (Table VI, Fig. 18). d. Time Discriminating Technique. Still another approach is a technique for reducing fluorescence on the basis of its lifetime. Yaney (37) emphyed a pulsed Raman technique involving a &-switched Nd:YAG laser and a pulse activated nanosecond photon counting detection system. Since
327
LASER RAMAN SPECTROSCOPY
,
Lamp sync
Sample Nd: YAG laser
11
Q- SW sync
. Trig Trig
=
Pulse gate
Pulse gen B-
Q Ratemeter
t
Recorder
FIG.19. Block diagram of the pulsed Raman apparatus (3’7).The photoelectron pulses from the photomultiplier (PM) are amplified, standardized, and sent to the ratemeter when the signal gate is activated by the output of the pulse generator B (PULSE GEN. B). Pulse generator B is active when there is an output from PULSE GEN. A to PULSE GATE input. The discriminator (DISC) is set to give no output when the AMP/DISC input is disconnected. The pulse generators have adjustsable pulse durations and delays to permit a wide choice of detect,ion intervals.
Raman scattering is essentially undelayed with respect to the arrival of the incident light, in this technique the detector is activated only during each laser pulse and deactivated a t all other times. This allows only Raman signals to be recorded but fluorescence signals and detector noise are gated out (Fig. 19). Improvement in Raman signal to fluorescence ratio has been achieved as illustrated in Fig. 20. The technique, however, a t present seems to be restricted by several instrumental limitations ( $ 7 ) .
B. WEAKRAMAN LINES
It is desirable that the oxide chosen for an adsorption study has a high surface area. This would potentially allow a greater number of adsorbate molecules to be adsorbed and consequently more intense spectra would be obtained. In general, the observed spectra of adsorbed molecules a t low coverages are weak. Further, some adsorbates (e.g. HzO) give rise to inherently weak Raman spectra even a t high coverage. For the detection of weak Raman lines, high laser power, high signal amplification, long pen period, and very slow scanning speed should be
328
R. P. COONEY ET AL.
-1200 counta/aec
+
-
rmin
FIG.20a. The pulsed Raman spectrum of Mn-doped ZnSe single crystal using a detection interval of 200 psec. Broad band fluorescence superimposed on a large instrumental scattered light component was observed. Recordings taken with ratemeter time constants (TC) of 1 sec and 10 sec are shown (37).
tA
INCREASING
FIQ.20b. The pulsed Raman spectrum of Mn-doped ZnSe with a 1 psec detection interval. The fluorescent background was significantly reduced from that observed with a 200 psec detect.ion interval in Fig. 20a (37).
used. Improvement can be achieved by using pulse counting electronics and a low dark noise photomultiplier. Another technique for improving the signal-to-noise ratio is to repeat scans over a frequency interval and signal averaging with a computer. In general, the signal-to-noise ratio is enhanced by the square root of the
LASER RAMAN SPECTROSCOPY
329
number of scans. Loader (38)utilized the time averaging computer-aided technique in recording the spectra of styrene adsorbed on silica gel a t low coverage. Hatzenbuhler et al. (39) reported the use of the Spex Ramalog I spectrometer with the Varian C-1024 computer in such a time averaging method but noted the major restriction was the channel storage capacity of this special purpose computer (Fig. 21). Most modern laser Raman instruments such as products of Varian, Coderg, Beckman, and Spex Industries have computer interfacing facilities. Such a system, laser Raman spectrometer plus general purpose computer, is expensive. The newly available system on the market, a Spex Industries product which includes the Spex Ramalog 4, Interdata model 74, and Ramacomp 0101 costs approximately US $65,000. In the Ramacomp 0101, some important information can be supplied by the computer after a spectrum has been run, such as: (i) data may be signal-averaged over multiple scans, (ii) data can be smoothed by the least squares method, (iii) peak intensities and peak positions can be automatically selected and
FREOUENCY
(cm-'I
FIG.21. Raman spectra showing improvement of signal-to-noise using multiple scans with computer time averaging over single scan. Lower traces: single scan; upper traces: multiple scans (10 scans) and computer output. (a) Y , of CCl,; (b) Hg emission line and YI of Li02; (c) YI of NaO2 with oxygen isotopic counterparts (39).
R. P. COONEY ET AL.
330
Focusing lens
W
v
Slit
FIQ.22. Optical diagram of the Cary 82 filter system. (Courtesy Varian.)
printed out as required, and (iv) ,previously stored or logged spectra can be combined by subtraction or ratioing. All of these facilities have special relevance in recording Raman spectra of adsorbed molecules.
C. INTERFERING PLASMA LINES A large number of nonlasing plasma lines emitted from the discharge plasma tube often interfere in the recorded Raman spectra. Loader (40) listed tables of plasma lines when using the argon ion and argon/krypton ion lasers as Raman sources. TABLE VII Transmissions of the Cary 81 Filter and Conventional Interference Filters for Typical Laser Lines Transmission (%)
X (nm)
Cary 82 filter
488
79
514.5
80
647.1
75
Interference filters (lower limit)
50 50 50
LASER RAMAN SPECTROSCOPY
331
FIG.23. The Oriel model B-34-40 laser beam expander. (Courtesy Oriel Corp.)
1. The removal of plasma lines is normally effected by using conventional interference filters, Interference filters, however, have several drawbacks in that they reduce transmission efficiency and do not withstand the intense laser output power over a long period of time. 2. The Cary 82 spectrometer employs an optical filtering system which is similar in some respects to the design by Claassen et aE. (41). This optical filtering arrangement is shown in Fig. 22. The Cary 82 filter system has higher transmission efficiency than conventional interference filters (Table VII). 3. The use of a laser beam expander as a spatial filter has also been found to be satisfactory (42). The beam expander consists of an interchangeable negative input lens and a positive output lens. Both the input and output lenses are designed for minimum spherical aberration. The expansion power may be varied by using a different input lens (Fig. 23.) The laser beam
FIG.24. The laser beam expander in the Cary 81 spectrometer (exaggerated). Only that part of the expanded plasma light transmitted through the expander is shown.
332
1600
R. P. COONEY ET AL.
I
1400
1300
1200
iioo
id00
goo
BOO
700
660
WAVENUMBER (CM-' )
FIG.25.Interference of plasma lines from the Ar+ emission in the Raman spectrum of a Cab-0-Sil silica sample.
and other plasma nonlasing lines are expanded by the input lens and recollimated by the output lens of the laser beam expander. The laser beam now illuminates the entire area of the sample which may be positioned in a Cary 81 spectrometer and the Raman scattered light from the sample occupies a complete section of the optical cone of the spectrometer. On the other hand, a very large fraction of the plasma discharge radiation falls outside the sample and little is collected within the optical cone (Fig. 24). In this manner, the spectrum recorded is truly representative of the entire sample and is not seriously affected by such localized effects as mechanical vibrations of the spectrometer (24). Figure 25 shows the interference of the plasma lines from the Ar+ emission in the spectrum of a Cab-0-Sil silica sample. The plasma line intensity was reduced in the spectrum of the same sample recorded using a Iaser beam expander (Fig. 13a). The laser beam expander will also be used to minimize local heating which may result from absorption by either colored adsorbates (e.g. some carbonium ions) or colored adsorbents (e.g. some vanadium oxides). Obviously, the lowering in intensity of the laser beam incident through the beam expander must be compensated for by increasing the source output power. However, it has been found that the intensity loss when using the beam expander is less than that encountered with interference filters.
333
LASER R A U A N SPECTROSCOPY
V. Raman Spectra of Adsorbed Molecules There are, at present, two overriding reasons an experimentalist would choose to employ laser Raman spectroscopy as a means of studying adsorbed molecules on oxide surfaces. Firstly, the weakness of the typical oxide spectrum permits the adsorbate spectrum to be obtained over the complete fundamental vibrational region (200 to 4000 cm-') . Secondly, the technique of laser Raman spectroscopy is an inherently sensitive method for studying the vibrations of symmetrical molecules. In the following sections, we will discuss spectra of pyridine on silica and other surfaces to illustrate an application of the first type and spectra of various symmetrical adsorbate molecules to illustrate the second.
A. PYRIDINE ON SILICA In studying the nature of the interaction between pyridine and a silica surface, the spectrum of the adsorbed pyridine is compared with the spectrum of liquid pyridine, hydrogen bonded pyridine (in which the strength of the hydrogen bond is varied) pyridinium ion, and coordinated pyridine (29, 4 3 ) . The most valuable spectral region for the investigation is around lo00 cm-' where the Raman bands are associated with the ring breathing modes of the pyridine ring. Frequencies and relative intensities of a number of pyridine systems and of pyridine adsorbed on chromatographic grade silica gel are given in Table VIII. It will be noted that as the hydrogen bond strength increases, Py(H20) > Py(CHzCl2) > Py(CHC13), the relative intensity ratio of the two ring modes increases and they may be used to assess the strength of the hydroTABLE VIII Characteristic Ruman Lines and Their Relative Intensities for Varwus Ppidine (Py) Species ($7, $9) High Low cover- Py coverage age liquid vj(cm-1) 1036 vj(cm-l) 1010 0.45 Z(ui)/ Ibj)
1031 1032 991 992 0.80 0.80
Py in CHCls
Py in CHlC12
1035 998 0.80
1032 992 0.77
Py in H20 1036 1003 0.56
PyH+BF,- Py:ZnCln 1032 1012 0.15
1050 1025 0.04
334
R. P. COONEY ET AL.
gen bond between the surface OH group and the adsorbed molecule. The two limits of hydrogen bonding may be represented as follows: N I
I
(I)
(5 I
-Si-
I
N H+
.
(11) 0-
I
S i -
I
Bands at 1000 and 1035 cm-' have been assigned (43) to a spectrum approximating to structure (I), a very weak hydrogen bond, and bands a t 1010 cm-1 and 1035 cm-l to a spectrum approximating to structure (11) in which the proton has become completely attached to the nitrogen atom. On Cab-O-Sil HS5 silica (@)-and the same is true for other silicas-at high coverage the evidence is for mainly physically adsorbed pyridine with Raman lines a t 991 cm-l and 1031 cm-'. The pyridine is adsorbed in multimolecular layers and capillary condensation may also be present. The spectrum of liquid-like pyridine predominates. A t less than monolayer coverage there is both a shift in the two spectral lines and a change in their intensity ratio, the spectrum of hydrogen bonded pyridine predominating, clearly indicating that when pyridine is adsorbed on silica gel adsorption o'ccurs through the pyridinium nitrogen atom and surface hydroxyl groups. B. ADSORPTIONOF PYRIDINE ON OTHERSURFACES The adsorption of pyridine on alumina surfaces has also been studied (43, 44). These surfaces are more difficult to examine due to the very high fluorescence associated with them. However, the presence of a line at 1019 cm-l at low coverages is attributed to pyridine chemisorbed on a Lewis acid site as Al-NC6Hs which is supported by its resistance to desorption. At intermediate coverages, there is evidence for hydrogen-bonded pyridine while at high coverages the spectrum indicates physically adsorbed liquid-like pyridine. Adsorption on TiOz (44) indicates Lewis pyridine at low coverages, there is no chemisorption on MgO (44), and adsorption on NH4+-mordenite (44) indicates pyridine hydrogen bonded to the surface at low coverage and physically adsorbed at high coverage. On porous glass (SO), the spectra of adsorbed pyridine indicate species similar to those on silica gel, and on NaY zeolite (SO) an upward shift of some of the Raman lines is attributed to pyridine coordination with cationic sites. Pyridine has proved to be a most useful probe in examining the properties of adsorbateadsorbent interactions.
335
LASER RAMAN SPECTROSCOPY
C. SYMMETRICAL ADSORBATES Earlier in this review, the relationship between the Raman and infrared spectra of molecules possessing high or low symmetry was considered. It was indicated that for molecules possessing a center of symmetry, no vibration is active in both the Raman and infrared spectra. Several adsorbates in this category and one of intermediate symmetry have been studied by laser Raman spectroscopy (Table IX) , and most of these spectra are considered in this section. Spectral changes on adsorption are of three types: appearance of inactive fundamentals (often coincident with infrared absorptions-see Table IX) , shifts in Raman line positions for active vibrations, changes in relative peak intensities, and changes in half-bandwidths. The first three types of change have been reported for centrosymmetric adsorbates. The most significant changes associated with adsorption which have been observed to date were the displacements (45) of Raman fundamentals of ethyne on adsorption on zeolite 4A (see Table IX). Such changes constitute a useful monitor of adsorbate-adsorbent interaction for various adsorbents. The appearance of the Raman spectrum of ethyne on zeolites A suggests an TABLE IX Raman Spectra of Symmetrical Adsorbates
Type
Adsorbate
A2 ABz
Brz COZ
A2Bz AZB4 ABC-ABC
CZHZ C2H4 trans CHCl=CHCl CCl, C6He
AB4 AeBe
Adsorbent
Possible symmet,ry change X Yn -+
Predicted coincidencesb Changes x Y observed
Ref.
cs2
SiOz (gel) Si02(gel) Vycor
c 2
0
12
Td-+civ
2
6
0
7
C2h
D6h
-+
+
C6v
None None Significant
(24) (34.6) (SO)
a The symmetry listed for Y is for a strong adsorbate-adsorbent interaction. The symmetry may vary with different types of adsorption. Infrared-Raman coincident frequencies in the spectrum of the adsorbate. Changes listed are associated with the appearance of inactive fundamentals, Raman line position, or with relative peak intensities.
336
R. P. COONEY ET AL.
adsorbate-adsorbent interaction more complex in nature than the ioninduced dipole mechanism proposed by Amaro and Seff (46), who have demonstrated that the adsorbed ethyne molecules are associated with Na+ ions. H-CeC-H I
N+ ,
Changes in relative peak intensity and marginal line shifts have been observed for benzene adsorbed on porous glass ( 2 5 ) . More significantly, infrared spectroscopic evidence had been found in the appearance of inactive fundamentals for the lowering of molecular symmetry of benzene on adsorption on zeolites (47). For other centrosymmetric adsorbates such as COz on zeolites X and Y ( 1 ) and ethene on porous Vycor gIass ( 2 ) , only marginal changes in line position were observed.
D. OTHERINVESTIGATIONS Among the important observations that have been made by studying the Raman spectra of molecules adsorbed on solid surfaces (Table X) , the following may be noted. The list is not intended to be exhaustive. 1. Kagel (29) found that whereas pyridine is hydrogen bonded to a silica gel surface 2-chloropyridine is not, the spectrum of 2-chloropyridine adsorbed on silica gel being identical with that of the liquid, and concluded that steric hindrance probably prevents hydrogen bond formation in this case. 2. Egerton et al. (SO) found that while the Raman bands of benzene adsorbed on porous glass are close in position to those in the liquid, the relative intensities of the different Raman bands of the adsorbed species are different from those in the liquid and point out that the decrease in intensity of the 994 cm-l band, which is assigned to the ring breathing mode of the benzene molecule, is consistent with a lowered polarizability of the ring electrons resulting from their interaction with surface hydroxyls. The authors also found that the Raman spectra of aniline and benzylamine, which are expected to hydrogen bond to the surface hydroxyls through the amino group, are very little different from those of the pure liquids in the spectral region under investigation (-1000 cm-') which is dominated by bands associated with ring vibrations. The bands associated with N-H stretching fall a t about 3400 cm-l and these could not be observed due to the fall in photomultiplier sensitivity in this region. 3. From the Raman spectrum of acetaldehyde adsorbed on silica gel
337
LASER RAMAN SPECTROSCOPY
TABLE X A Summary of Adsorbate-Adsorbent Systems Investigated Using Laser Raman Spectroscopy Adsorbate
Adsorbent
Pyridine
Chromatographic grade silica gel Cab-0-Sil Porous Vycor glass Aerosil Silica with excess Al3f 7- and ?-alumina Titanium dioxide NH1+-mordenite Magnesium oxide N a y zeolite 2-Chloropyridine Chromatographic grade silica gel Benzene Porous Vycor glass Chlorobenzene Porous Vycor glass CCL Brp CSP Chromatographic grade silica gel trans CHC1= CHCP Acetonitrile Chromatographic grade silica gel Aerosil 380 Zeolites X, Y Benzonitrile Chromatographic grade silica gel Acetaldehyde Chromatographic grade silica gel Trans-Stilbene Cab-0-Sil Styrene Chromatographic grade silica gel Cab-0-Sil Aerosil Propylenc Porous Vycor glass Zeolites A, X, and Y Ethylenea Porous Vycor glass Acrolein Zeolite Y Carbon dioxide Zeolites X, Y Ethynea Zeolite 4A Aniline Porous glass Benzylamine Porous glass a
For symmetrical molecules, see also Table IX.
Interaction Physical & chemical Physical & chemical Physical & chemical Physical & chemical Physical & chemical Physical & chemical Physical & chemical Physical & chemical Physical
Reference (29, SO, 43)
(29, 30,43) (25) (43)
(4.9) (SO,43) (43)
(4.9) (43)
(30) Physical Physical Physical
(29)
Physical
(24)
(2,25, 30) (300)
Physical (24) Physical (94) Physical (1) Physical & chemical (24) Physical (4@ Physical (24) Physical & chemical (38) Physical (38) Physical (38) Physical (2) Physical (1 1 (2) Physical (1 1 Physical (1) Physical (45) Physical (30) Physical (30)
338
R. P. COONEY ET AL.
Hendra and Loader (48) demonstrated that acetaldehyde trimerises on the surface to paraldehyde. 4. In another paper, Hendra and Loader (24) discuss the adsorption of some centrosymmetric molecules on silica gel and reference has already been made to these. They also discuss the adsorption of benzonitrile on silica gel at coverages ranging from 20 to 96 & molecule-'. Since the ring vibrations are relatively unaffected by adsorption, it was concluded that the molecule is adsorbed via the nitrogen and is perpendicular to the surface. From the known parameters of the benzonitrile molecule the area occupied by a molecule was calculated and it was found, assuming hexagonally close packing and assuming no rotation around the single carbon-carbon bond, If that the average area occupied by a benzonitrile molecule was 25 rotation occurs, the area is 39 Az. A t the lowest coverage, viz. 96 per molecule, there would be less than one monolayer on the surface. A t this coverage two lines were found centered a t 2238 cm-I and 2249 cm-' with relative intensities of 2.6 :1 and shifts of 10 cm-' and 21 cm-' respectively from the position of the line in the liquid. It was concluded that the two bands arise from benzonitrile chemisorbed through the nitrile group to two distinct types of surface sites. 5. Buechler and Turkevitch ( 2 ) investigated a number of catalytic surfaces and molecules adsorbed on them. Among a number of observations was the fact, using porous Vycor glass, the OH stretching vibration at 3750 cm-' does not change with the adsorption of substances and although its height increases slightly with decrease in temperature of dehydration, there is no change in shape as the dehydration is carried out, the band being strongly asymmetric. This is quite contrary to the behavior of this band in the infrared when adsorption of substances causes a shift, in some cases quite considerable, to lower frequencies and the shape of the band is greatly affected by the temperature of dehydration, becoming more symmetrical as the temperature of the dehydration is increased. Attempts to deuterate the OH group failed as the background scattering increased markedly. The adsorbent was treated with hexamethyldisilazane in order to methylate the OH group but the 3750 cm-' hydroxyl band did not decrease appreciably in intensity. Bands did appear due to Si(CH8)*groups and it was concluded that methylation of the hydroxyls, other than those seen in the Raman spectrum, had occurred. 6. Loader (38)studied the Raman spectra of styrene adsorbed on different silicas-chromatographic grade silica gel, Cab-0-Sil, and Aerosil380. The author utilized the fact that chemisorption will bring about marked changes in the spectrum whereas physical adsorption will cause only a broadening of the Raman lines accompanied in some cases by a frequency
A2.
LASER RAMAN SPECTROSCOPY
339
shift, to ascertain whether styrene was physically or chemically adsorbed by silica. On chromatographic grade silica gel the most marked change in the spectrum as the coverage was reduced was the gradual reduction in intensity of the line at 1631 cm-I assigned to the carbon-carbon double bond relative to the most intense ring modes at 992 cm-l and 1602 cm-l. This decrease in intensity is indicative of an interaction between that bond and the silica surface. New lines at 1277 cm-' and 1456 cm-' due to vibrations of a modified vinyl group appeared and it was concluded that styrene was chemisorbed by chromatographic grade silica gel. In the case of Cab0-Sil and Aerosil380, there were no dramatic changes in the spectrum and the adsorption was taken as physical. 7. Angel1 ( I ) has investigated the Raman spectra of acetonitrile, propylene, and acrolein on a number of zeolites and found that physical adsorption occurred. There are sufficient differences between the spectrum of the liquid and of the adsorbed species (e.g. the carbon-carbon double bond stretching in the case of propylene and the carbon-nitrogen triple bond stretching in the case of acetonitrile) to make it quite clear that it was not merely a case of condensation in the pores of the solid adsorbent.
VI. Conclusion Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. The fact that substrates do not substantially interfere with the spectrum of the adsorbed molecule itself makes Raman spectroscopy a most valuable method for examining vibrations of adsorbed species. Appendix Normal Coordinates, Vibrational Wavefunctions, and Spectral Activities (11,13)
Each normal mode of vibration can be described by a normal coordinate which is a linear combination of nuclear displacement coordinates of the molecule. For the symmetric stretching vibration v1 of COZ, the normal coordinate is of the form Qj
QI =
N ( Arl
+ Ard,
where N is a normalization constant (i.e. 2 - 9 and A n and Ar2 are called internal coordinates. They represent the changes in C=O bond lengths
340
R. P. COONEY ET AL.
which constitute the vibration. (The internal coordinates may be expressed as linear combinations of Cartesian coordinates.) In general, there are at least (3N-6) internal coordinates and (3N-6) normal coordinates and normal vibrations for an N atom nonlinear molecule. The vibrational wavefunctions may be expressed as functions of the j t h normal coordinate: ground state: 1Lj(0)
=
Nj
1st excited state: +j(l)
=
N j exp[- ( c Y ~ / ~ ) Q ~ ” ] ~ ( c Y ~ ) ~ / * Q ~ ,
exp[- (aj/2)Qi]
where N j is a normalization constant, a j = 2avj/h, and v j is the frequency of the jth normal mode. For nondegenerate vibrations all symmetry operations change Q j into f l times itself. Hence : Q is unchanged by all symmetry operations. In other words, Q; and consequently +j(0) behave as totally symmetric functions (i.e. the function is independent of symmetry). However, the wavefunction of the first excited state +j(l) has the same symmetry as Qj. For example, the wavefunction of a totally symmetric vibration (e.g. fJ1 of COz) is itself a totally symmetric function. The intensity of a spectral band is proportional to the probability that the associated transition could occur. The probability (and hence the intensity) of the fundamental transition: Sj(1)
t
+j(O)
is given by the quantum mechanical expression
1
2
/ + j ( l ) m j ( o ) dri
,
where 0 is an operator describing the mechanism causing the transition and d r is dxdgdz (i.e. the integral is taken over all space). The integral
1
+j(l)o+j(o)
is called the transition moment integral. Spectral band intensity is an experimental quantity which is independent of the renumbering of atoms belonging to an equivalent set. Therefore, the transition moment integral must be unchanged or at most change sign under the symmetry operation. However, if the integrand changes sign under a symmetry operation, it will have a value in one part of space equal and opposite to that in an equivalent portion of space. The integral over all space would then be zero. Therefore,
LASER RAMAN SPECTROSCOPY
341
the transition moment integral must be unchanged by a symmetry operation. It follows that the product
# A 1)o # m must behave as a totally symmetric function. As described above, the ground state vibrational wavefunction is totally symmetric for most common molecules. Therefore, the product $j(l)O must at least contain a totally symmetric component. The direct product of two irreducible representations contains the totally symmetric representation only if the two irreducible representations are identical. Therefore, transitions can occur from a symmetrical initial state only to those states that have the same symmetry properties as the transition operator, 0. REFERENCES
1. Angell, C. L., J. Phys. Chem. 77, 222 (1973). 2. Buechler, E., and Turkevich, J., J . Phys. Chem. 76,2325 (1972). 3. Greenler, R. G., and Slager, T. L., Spectrochim. Acta, Part A 29, 193 (1973).
4 . Peterson, D., Cooney, R. P., Nguyen The Tam, and Curthoys, G., unpublished work. 5. Kramers, H. A., and Heisenberg, W., 2. Phys. 31, 681 (1925). 6. Cabannes, J., and Rochard, Y., J. Phys. Radium [6]10, 52 (1929). 7 . Tobias, R. S., J . Chem. Educ. 44, 2 (1967). 8. Koningstein, J. A., “Introduction to the Theory of the Raman Effect,” Chapter I. Reidel Publ., Dordrecht, Netherlands, 1972. 9. Woodward, L. A., in “Raman Spectroscopy” ( H . A. Szymanski, ed.), Vol. 1, Chapter I. Plenum, New York (1967). 10. Herzberg, G., “Molecular Spectra and Molecular Structure,” Vol. 11. Chapter III. Van Nostrand-Reinhold, Princeton, New Yersey, 1945. 11. Bauman, R. P., “Absorption Spectroscopy,” p. 238. Wiley, New York, 1962. i d . Brand, J. C. D., and Speakman, J. C., “Molecular Structure,” Chapter VII. Arnold, London, 1960. 13. Cotton, F. A., “Chemical Applications of Group Theory,” p. 245. Wiley, New York, 1963. 14. Kogelnik, H., and Porto, S. P. S., J . Opt. SOC.Amer. 53, 1446 (1963). 15. Gelbwachs, J., Pantell, R. H., Puthoff, H. E., and Yarborough, J. M., Appl. Phys. Lett. 14, 258 (1969). 16. Collins, S. A., Jr., U. S. Patent 473,005 (1969). 17. Wolff, P. A., U. S. Patent 3,435,373 (1969). 18. Budin, J. P., Raffy, J., and Ernest, J., J . Quantum Electron 4, 558 (1968). 19. Landon, D. O., and Reed, P. R., Spex Speaker 17, No. 2 (1972). 80. Chase, L. L., Davis, J. L., Devlin, G. E., and Geschwind, S., Spex Speaker 15, No. 3 (1970). 2f. Hibler, G., Lippert, J,, and Peticolas, W . L., Spex Speaker 16, No. 1 (1971). 22. Sloane, H. J., Appl. Spectrosc. 25, 430 (1971). 23, Tobin, M. C., “Laser h m a n Spectroscopy,” pp. 44-56. Wiley, New York, 1971. 24. Hendra, P. J., and Loader, E. J., Trans. Faraday Soe. 67, 828 (1971). 25. Egerton, T. A., Hardin, A. H., Kozirovski, Y., and Sheppard, N., Chem. Commun. p. 887 (1971).
342
R. P. COONEY ET AL.
26. Freeman, S. K., and Landon, D. O., Spez Speaker 8, No. 4 (1968).
27. Nguyen The Tam, Cooney, R. P., and Curthoys, G., Appl. Spectrosc. 27,484 (1973). 28. Hester, R. E., i n ‘‘Raman Spectroscopy” (H. A. Szymanski, ed.), Vol. 2,pp. 141-155.
Plenum, New York (1970).
29. Kagel, R. O., J. Phys. Chem. 74,4518 (1970). 30. Egerton, T. A., Hardin, A. H., Kosirovski, Y., and Sheppard, N., J. Catal. 32,
343 (1974).
31. Careri, G., Mazzacurati, V., Sampoli, M., and Signorelli, G., J. Catal. 26,494 (1972). 32. Charlesby, A,, and Partridge, R. H., Proc. Roy. Soc., Ser. A 271, 188 (1963). 33. Cain, D.S., and Harvey, A. B., U.S. Nav. Res. Lab., Rep. 6792 (1968). 34. Byer, R. L., Opt. Spectra 4, 42 (1970). 36. Gall, M.J., Hendra, P. J., Watson, D. S., and Peacock, C. J., Appl. Spectrosc. 25,
423 (1971).
36. Jankow, R., and Willis, J. N., Jr., J . Mol. Spectrosc. 41, 412 (1972).
37. Yaney, P. P., J. Opt. SOC.Amer. 62, 1297 (1972). 38. Loader, E.J., J . Catal. 22, 41 (1971). 39. Hatsenbuhler, D.A., Smardzewski, R. R., and Andrews, L., Appl. Spectrosc. 26,479
(1972).
40. Loader, E. J., “Basic Laser Raman Spectroscopy,” pp. 32-35. Heyden, London,
1970.
41. Claassen, H. H., Selig, H., and Shamir, J., Appl. Spectrosc. 23, 8 (1969).
42. Cooney, R. P., Nguyen The Tam, Curthoys, G., and Peterson, D., Aust. Spectrosc. Con!., 9th, 1973 Paper D20 (1973). 4.9. Hendra, P. J., Horder, J. R., and Loader, E. J., J . Chem. Soc., A p. 1766 (1971). 44. Hendra, P.J., Horder, J. R., and Loader, E. J., Ghem. Commun. p. 563 (1971). 46. Nguyen The Tam, Cooney, R. P., and Curthoys, G., to be published. 46. Amaro, A., and Seff, K., J . Chem. Soc., Chem. Commun. p. 1201 (1972). 47. Galkin, G., Kiselev, A. V., and Lygin, V. I., Trans. Faraday Soc. 60,431 (1964). 48. Hendra, P. J., and Loader, E. J., Nature (London) 217,637 (1968).
Analysis of Thermal Desorption Data for Adsorption Studies MILOS SMUTEK, SLAVOJ GERNP, AND FRANTISEK BUZEK The J . Heyrovsk; Zmtitute of Physical Chemistry and Electrochemistry Czechoslovak Academy of Sciences M&chova 7 , Prague 2, Czechoslovakia
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Fundamental Definitions and Relationships. . . . . . . . . . . . . . . . . . . . . . . A. The Rate of Desorption and Readsorpt,ion.. . . . . . . . . . . . . . . . . . . . ......................... B. The Mass Balance. . 111. Temperature Schedules rption . . . . . . . IV. Fundamental Relationships for the Determination of Energy of Desorption, of the Order of Desorption and of the Preexponential Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Negligible Rate of Readsorption ( r d >> r.) . . . . . B. Rate of Readsorption Nearly Equal to the R
..................
343 347 347 354 361 365 365 371
the Kinetic Parameters of Desorption A. Direct Estimate ............................ B. Output Data Reflect Essentially the n,(t) Relationship.. . . . . . . . C. Output Data Reflect Essentially the dn,/dt Values. VI. Effects of the Surface Heterogeneity and of the Surface A. Discrete Distribution of the Activation Energies of Desorption on
............................. tion Energies of Desorption. C. Activation Energy of Desorption as a Function of the Surf Coverage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... List of Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
372 372 374 375 380 38 1 384 386 388 390 391
1. Introduction The identification of particles adsorbed on solid surfaces and recognition of their properties is one of the fundamental problems in research on adsorption and heterogeneous catalysis. Desorption of the adsorbed species from a surface and its subsequent analysis is an important method for solv343
344
MILO;
SMUTEK ET AL.
ing this problem. Desorption can be accomplished in several ways, each of which has special advantages and also limitations. A “chemical method” of desorption consists of admitting onto the surface covered with adsorbed particles a gas with a sufficiently high heat of adsorption and negligible reactivity to the adsorbed particles under the conditions of the experiment. Thereby, either direct displacement of the adsorbed species into the gas phase occurs or the adsorbed particles are made to migrate over the surface and then react with other adsorbed particles to form a product which readily desorbs (1, 2). Another possibility is, for example, hydrogenation of the adsorbed layer by gaseous hydrogen and desorption of the resulting product (1). A “physical method” of desorption is, for example, the desorption stimulated by the impact of ions or electrons (3-8).Another method is desorption by a strong electrostatic field (9). Irradiation by a current of photons can lead to a photodesorption process (lo-1.4). Even desorption by means of phonic energy has been described (16). Finally, the desorption by thermal energy falls into this group of desorption techniques. Thermal desorption is a widely applicable and frequently used method. The adsorbed particles are activated by the thermal energy supplied so as to overcome the energy barrier needed for their desorption. Prior to the desorption itself ,the particles become mobile [field emission microscopy has shown that the activation energy for surface diffusion is usually about 5 to 10 times lower than the activation energy for desorption (16-18)]. Consequently, the results of thermal desorption experiments reflect the kinetics of processes at the temperature and degree of surface coverage at which the desorption occurs, with the adsorbed particles generally in a nonequilibrium state. This can lead to an appreciable complexity and ambiguity in the interpretation of the results obtained, as will be outlined in Section VII. The inception of the thermal desorption method reaches back to the thirties when Urbach (19, 20) studied the rate of electron escape from continuously heated materials in his investigation of luminescence. A systematic application of thermal desorption to the study of the interaction of gases with solids occurred much later (21-28). The method was first worked out for rapid heating up of metal filaments and ribbons covered by adsorbed gas, with a heating rate of 10-1200 deg sec-1 (“flash-filament technique”). The desorption was monitored by measuring the pressure changes in the system by means of ionization gauges of various construction (26, 29, 30). The theory, techniques, and results of the flash desorption methods for the study of the interaction of gases with solid surfaces up to the beginning of the sixties was reviewed by Ehrlich (29). For further development of the quantitative treatment of the desorption kinetics, the work of Redhead (31) and of Carter (32) is of great impor-
THERMAL DESORPTION
345
tance. In the field of experimental technique, the undesired pumping activity of the ionization gauges and a need for the determination of the partial pressures of the individual gas phase components stimulated the employment of various types of mass spectrometers for the analysis of the gas phase in the system (68, 30, 33-37). Attempts at the elimination or control of the wall effects in the flash desorption technique which can introduce errors due to lags or distortion of the pressure changes detected, led to the development of .(1) a line-of-sight arrangement with a time-of-flight mass spectrometer detection (38-40) ; (2) the construction of different types of desorption cells, as for example, the “black chamber” with the walls kept at low temperature and covered with a getter (40) ; (3) the screening off of thermoelectrons emitted by the heated sample (41); and (4) to the development of a constant pressure technique based on automatic pressure control by means of a variable leak valve (46, 43). A discussion of these topics is beyond the scope of the present contribution. An attempt at overcoming the difficulties resulting from a strong adsorption of the ammonia gas by the walls of the system and the subsequent slow desorption was described by Peng and Dawson (35). The role of the wall effects, as well as a number of other points concerning the experimental technique and the evaluation and interpretation of the measured data have been recently dealt with in a comprehensive and stimulating review by PBtermann (4%). An excellent review of the thermal desorption method, covering the period up to about 1966, was published by Hansen and Mimeault (30). Much information also occurs in the relevant chapters of the book by Redhead, Hobson, and Kornelsen (44). Thermal desorption studies have been done so far mostly on polycrystalline filaments and ribbons. Of great importance is the application of this technique to single crystal surfaces, either in the form of thin disks cut from a single crystal rod, or in the form of single crystal filaments and ribbons, which has occurred in recent years (see Section VII). Attempts have been made to extend the thermal desorption technique to vacuumevaporated films (45, 45u-45f), to foils (46g, 45h), and even to powdered materials (45i-45m). An original approach has been advanced by Czanderna (46):the desorption process is monitored by direct weighing of the mass loss of a powdered sample with a sensitive microbalance. Thereby a more direct measurement and a possibility of working at higher pressures have been achieved. Chemisorption of simple diatomic molecules has usually been the object of thermal desorption studies. Recently, there has been a growing interest in the application of this method to the investigation of surface phenomena produced by more complex molecules which yield either fragment desorption products or catalytically formed species (35,46~-46h).Also, physisorp-
346
MI LO^
SMUTEK ET AL.
tion can be successfully studied by the thermal desorption method (4%
4w. In 1963, Amenomiya and Cvetanovid (47-49) suggested an adaptation of the thermal desorption method, consisting of a gradual desorption of gas adsorbed on powdered adsorbents or catalysts into the stream of an inert carrier gas, and monitoring of its concentration by a gas chromatography detector. The heating rate applied is very slow, about 10-30 deg/min, so that the system is kept near to an adsorption-desorption equilibrium. A term “temperature programed desorption” has been widely adopted for this method. This can be misleading since the flash desorption is of course a desorption by programmed temperature rise as well. However, in view of the general use of the said term for the method of Amenomiya and Cvetanovi6, we shall adhere to it in the same sense throughout the present article. When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters : the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section ILB, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. In the first one, the desorption rates and the corresponding desorbed amounts at a set of particular temperatures are extracted from the output data. These pairs of values are then substituted into the Arrhenius equation, and from their temperature dependence its parameters are estimated. This is the most general treatment, for which a more empirical knowledge of the time-temperature dependence is sufficient, and which in principle does not presume a constancy of the parameters in the Arrhenius equation. It requires, however, a graphical or numerical integration of experimental data and in some cases their differentiation as well, which inherently brings about some loss of information and accuracy. The reliability of the temperature estimate throughout the whole experiment with this
THERMAL DESORPTION
347
treatment is still more essential than in the following approach, in which the properties of the maxima on the desorption curve are analyzed in the first place. In most cases the authors prefer the second way of treatment of the desorption data, which is analytic in its nature: the Arrhenius equation, whose parameters are assumed to be constant, is solved either in a closed form or numerically. The resulting quantities determining the location, height, and shape of a maximum on the desorption curve are analyzed and expressed whenever possible, in at least approximately linear form, and then compared with the experimental results. A simple analytical expression of the time-temperature function is essential for this kind of treatment. The procedures mentioned will be dealt with in some detail in Sections IV and V.
II. Fundamental Definitions and Relationships A. THERATEOF DESORPTION AND READSORPTION Let us first consider an energetically uniform surface with all the sites having identical activation energies of desorption Ed and of adsorption E,, irrespective of how densely the neighboring sites are occupied (the Langmuir model). Then also the liberated heat of adsorption - A H = Ed - E , is independent of the degree of surface coverage e = n,/nBt.Here, ns stands for the number of moles of the adsorbed species on a suitably selected unit of the adsorbent surface. In the most precise concept, nntrepresents the total number of adsorption sites on this unit surface, divided by the Avogadro number N . Somewhat less exact is the identification of natwith the quantity nmax(T),representing the total number of moles per unit surface required to form an effective monolayer at a temperature T (usually the initial temperature of the desorption experiment). Clearly, nnt - nmax(T)2 0 and this difference increases with the temperature, since the ratio of the probabilities of desorption and of adsorption increases as the temperature rises [see Eq. (7)]. In the case of dissociative adsorption, the adsorbed particles are not identical with those in the gas phase and the quantity n, is higher than the number of moles nagadsorbed from the gas phase. In general, dissociative adsorption reduces the quantity nmsx(T ), for even in the simple model of an energetically homogeneous surface the sites which have not a sufficient number of free neighboring sites cannot be utilized in the dissociative adsorption (50). In the following treatment we shall deal mainly with the “overall reversible” systems, defined as systems in which the adsorbate is released into the gas phase in the form of the original molecules only (no new
348
MILO;
SMUTEK ET AL.
species appear in the gas phase). This limitation is obviously immaterial under such experimental conditions which make the rate of readsorption negligible as compared to the rate of desorption. Let us denote by x the number of particles formed from one molecule in its adsorption process. In the case of a nondissociative adsorption, x = 1 and n. = nag.Dissociative adsorption of simple molecules usually creates two particles on the surface, thus giving x = 2 and n, = 2nag.Finally, suppose there is a programed, homogeneous heating of the adsorbent surface ensuring that all its parts have the same temperature a t any time (see Section 111). For the model formulated by the above postulates, the specific desorption rate Td, i.e. the molar rate of release of the adsorbed species under consideration from the unit surface, is in general given by the product of four factors: (a) A function j ( y ) of the number of particles from which the species escaping from the surface is formed. This function reflects the number of considered adatoms which escape in a single desorption act. Thus, f(y) = y, if y like particles recombine while desorbing; f ( y ) = 1, if two unlike particles recombine; if the recombination pattern is AAB, thenfA(y) = 2 with respect to the species A, and jB(y) = 1 with respect to the species B; etc. For the “overall” reversible processes under consideration, y = 2. (b) The number n8 of moles of the considered adsorbed species per unit surface, related to its surface coverage by n. = enat.Only for like associating molecules, the quantity 0 is equivalent to the overall coverage. (c) The probability gI(8; y; conf) that the adsorbed particles are located in a configuration favorable for their recombination, i.e. that they occupy vicinal sites. For the nonassociative desorption, y = 1, and obviously gl(e; 1) = 1. (d) The rate kd’ of the actual desorption of particles in a favorable configuration. For an associative desorption, kd’ is clearly dependent on the arrangement of adsorbed particles in favorable configurations. Hence, the most general expression for the desorption rate in this model is gli(@;y) kii.
Td = f(y) 72s
(1)
i
The summation extends over all the configurations which give nonzero and different values of kd’. The rate factor kd’ can be factorized into three components: (i) the probability 7 that an attempt at escaping will be successful; (ii) the frequency kd” of all the attempts at recombination and escaping from the surface, when the configuration contains the minimum required number, i.e. y, of sites occupied by the proper components of the escaping molecule. When different geometrical arrangements of this group of y components of the escaping molecule lead to different frequencies, one of them has to be
THERMAL DESORPTION
349
chosen as the reference configuration; (iii) the factors gzi(conf; y) expressing the change in the frequency kd‘’ for other configurations, particularly with a higher number of nearest neighbors than the minimum required number (y - 1 ) . For a nonassociative desorption, evidently gzi = 1 and kd” is configuration-independent. In other cases, the functions gl(0; y; conf) and gz(conf; y) can be combined into a single coverage-dependent function g(0; Y> =
C ~ l i ( 0 y)gzi(y); i
Finally, the probability factor q, which is taken to be coverage-independent in the model of a homogeneous surface with no lateral interactions between adsorbed particles, will be expressed by means of the Arrhenius formalism based on the Boltzmann distribution, viz. q = exp (-
Ed/RT).
Then the relation (1) can be written in the form
rd
=
n$(Y)g(0; y)kd” exp(-Ed/RT).
(2)
Generally, and in analogy with bulk reactions, this expression is formally written as
rd = n # % d exp ( - Ed/RT) =
n,t,Beukd exp ( - Ed/RT),
(3)
where nst,g= nsg/O. In principle, the formal preexponential factor kd can be a function of 8 and T . However, often it is more or less correctly assumed that this dependence is but weak as compared with the terms By and exp( - Ed/RT), and therefore it cannot be accurately enough determined from the experimental data which usually exhibit a considerable scatter. Theoretical attempts to predict the desorption rates have not been so fa1 successful, particularly if applied to desorption by programed heating. This holds even for the simplest case of nonassociative desorption from a homogeneous surface with negligible lateral interactions between adatoms. The modern approach uses methods of statistical thermodynamics to describe the equilibrium properties of an adsorbed layer. An excellent survey of this subject was given by Clark (61). The limiting case of localized adsorption can only be dealt with rigorously in the whole range of the coverage. An appreciable surface mobility, which always occurs under the conditions of observable desorption rates, limits the rigorous treatment to low coverages only. It can be shown, however, that the free energy of adsorption decreases at a given heat of adsorption as the surface mobility increases
350
MI LO^
SMUTEK ET AL.
(61). [The problem was dealt with also by Scheer, Klein, and McKinley (62); according to our opinion, however, Clark’s approach is a more con-
sistent one.] The same effect takes place if the surface coverage is decreased under the conditions of desorption, because the surface mobility increases as the number of occupied sites decreases. If this fact is neglected, an apparent decrease in the heat of adsorption with increasing coverage can be found, simulating either energetic heterogeneity of the surface or occurrence of repulsions between adatoms. Current use of statistical thermodynamics implies that the adsorption system can be effectively separated into the gas phase and the adsorbed phase, which means that the partition function of motions normal to the surface can be represented with sufficient accuracy by that of oscillators confined to the surface. This becomes less valid, the shorter is the mean adsorption time of adatoms, i.e. the higher is the desorption temperature. Thus, near the end of the desorption experiment, especially with high heating rates, another treatment of equilibria should be used, dealing with the whole system as a single phase, the adsorbent being a boundary. This is the approach of the gas-surface virial expansion of adsorption isotherms (61, 63) or of some more general treatment of this kind. The theories of the desorption rate are based upon concepts of an equilibrium distribution of different states of adatoms and of their perturbation by the desorption. The application of the theory of absolute reaction rate is popular. It is usually employed in its most simple form, requiring the “activated complex” to be in statistical equilibrium with the reactants. It is not surprising, then, that the results are in general rather disappointing (61). Far more promising appears to us the refined stochastic approach, developed for the dissociation of diatomic molecules (64-67). By applying this method to nonassociative desorption, it can be deduced that the rate of desorption depends on the degree of interaction of adatoms with the adsorbent. If the interactions are weak, the desorption rate is considerably lower as compared ~ i t the h predictions of the theory of absolute reaction rates. The exponential term exp( - Ed/RT) is not affected by this model, but the preexponential term k d may vary substantially with the conditions of desorption. As long as direct excitations to the dissociative state do not prevail and stepwise excitations play a significant role, k d decreases with an increase in the desorption rate, due to the depletion of population in higher excited states. This effect will be most pronounced near the maximum desorption rate and will increase with higher heating rates. Also, the inweasing surface mobility due to the desorption proceeding will diminish k d , making it coverage-dependent even for the simple case of a nonassociative desorption. The experimental findings (68) indicate that these effects can be quite pronounced. In the paper cited, the activation energy of oxygen
THERMAL DESORPTION
351
desorption from silver was estimated to be 41.8 f 3.5 kcal/mole from isothermal runs and to be 34.5 f 1.5 kcal/mole from a linear heating rate of 2 deg/min. The discrepancy encountered between the measured and calculated O(T) curves demonstrated in Fig. 6 of the quoted paper can be readily explained by a decrease in the preexponential factor with increasing desorption rate and decreasing coverage. An interesting analysis of factors that may influence the value of the preexponential factor with first-order desorption kinetics, together with references to previous work, has been given by PBtermann (69). Summarizing the preceding remarks, we wish to emphasize that the results of the near-equilibrium theories of the desorption rate can be applied to experiments with a slow heating rate only. Many of the distortions of desorption curves attributed to changes in the activation energy of desorption are actually due to the incorrect assumption of a constant value of k d throughout the experiment. Near the end of a desorption experiment, the two-phase treatment of equilibria fails and this leads to a new increase in kd, since the relative importance of movements perpendicular to the surface increases as compared to, those along it. Rapid flash experiments exhibit the most pronounced deformations and may eventually become unsuitable for the purpose of drawing conclusions about the equilibrium properties of the adsorbed layer (see also Section 111). The theoretical difficulties in predicting the desorption rate increase for the case of associative desorption. Along with a sufficient activation, the desorbing species have to be in a favorable configuration, the minimum requirement being the occupancy of vicinal adsorption sites. This presents no problems with high coverages, when nearly every particle has one suitable neighboring partner at least, and only rarely and for a very short period is separated by more than one vacant site. With decreasing coverage, the deviations from the equilibrium distribution caused by desorption will be overcome more slowly, which can lead to a decrease in the desorption rate with respect to the equilibrium value. This effect will be stronger than that caused by the nonequilibrium occupancy of excited energy states, and the higher the desorption rate and the activation energy for migration, the sooner it will become appreciable. Attempts to deal with this effect (S4,60, 61) have not been very successful. Even for the near-equilibrium case attained by slow heating rates, and for not too low coverages, models with specified configurations and relative probabilities favorable for escaping have to be postulated for calculations of the term g(0; y) in Eq. (2). The recombination of two particles on desorption from a homogeneous surface incurs the least requirements on the model. They involve the coordination number of adsorption sites, the dependence of the desorption probability on the number of proper particles adjacent to the considered one, and the
352
MILO6 SMUTEK ET AL.
specification of lateral interactions between the adatoms. If it is assumed that the lateral interactions are insignificant and that the desorption probability is proportional to the number of nearest partners, one obtains with the coordination number z by a simple procedure g(e; 2) = (2/2)8, both for like and unlike associating particles, when expressed in terms of the desorbing gas species. Thus, y = 2 in this case and Eq. (2) becomes Td
,g
(Z/2) 8’kd’’ eXp ( - 8d/RT).
(4)
A comparison with Eq. (3) gives k d = (z/2)kd” for the formal preexponential factor. All the preceding statements on the variability of k d , particularly concerning its dependence on surface coverage due to the increasingly effective surface mobility with the decreasing coverage, are here again valid in an appropriately modified form. Sometimes an interesting feature of the desorption kinetics has been observed: the preexponential factor increases with an increase in the activation energy of desorption for a series of adsorbed modes of the same com. this increase can be somepound on the same metal ( 6 2 - 6 6 ~ ) Moreover, times fitted, usually poorly, by a linear relationship of the type In k d = const. Ed/RTi where Ti is an adjustable parameter called “isokinetic temperature.” The effect, observed both for the first-order and second-order desorption, has been termed the “compensation effect.” Attempts a t its theoretical elucidation (65, 67-69) have not been completely successful so far. Degras (66, 68) found some agreement of the experimental findings with a theory based on a coupling of adsorbed molecules by means of surface Rayleigh waves. An extension of this approach, utilizing the concept of surface phonons, has been suggested by Dobrzynski (7 0 ). A semiquantitative agreement of the same experimental data and some data by the authors (66-64) with a modified statistical-kinetic model, using surface coordination numbers as low as 3 (CO, 02)or even 2 (H2),was obtained by Roginskii et ul. (69). I n the following, the traditional treatment of the rate equation (3) will be adopted, taking the preexponential factor as a constant. Evidently, no other procedure is available at present. Even if a quantitative theory of the outlined problems were available, mathematical difficulties would render it possible to present only selected computerized data. Seemingly other problems arise in the treatment of the rate of adsorption, via. the probability that a gas particle will reach on impact a free adsorption site on a partially covered surface, and the probability that it will remain attached there. Since, however, the rates of adsorption and desorption are connected by the equilibrium requirements, essentially the same problems have to be solved in a theoretical evaluation of both the rates. In practice,
+
353
THERMAL DESORPTION
the main difference is that the adsorption rates are determined usually at much lower temperatures than the adsorption rates, so that the thermodynamic properties of the adsorbed layer may differ appreciably. The specific rate of adsorption (in the thermal desorption method, the equivalent term “readsorption” is used frequently) for the considered model of a homogeneous surface is given by
r,
=
kaPf(O; x) exp (- E,/RT) .
(5)
P stands for the partial pressure of the adsorbable particles in the gas phase to which the frequency of their collisions with the surface is usually proportional. f(0; x) expresses the probability of a transition of the impacting particle into the adsorbed state as a function of the coverage e and of the number x of fragments into which the adsorbed particle has decomposed. Even for a simple case of a nondissociative adsorption (x = l), this function is in general a complex expression which depends on the probabilities of capture of the impacting particle on the free and on the already occupied sites, on the degree of the surface coverage, on the type of covering process (random or patchlike covering), on the probabilities of migration and desorption from the covered surface, etc. Attempts a t analysis of the function f(0; x) can be found in the papers (71-74). A rough approximation is represented by the Langmuir model. It assumes that the impacting particle which decomposes on the surface into x fragments can be adsorbed only provided it hits a portion of the surface where x free neighboring sites are available. Then, f(e; z) = (1 - e)., and Eq. (5) can be replaced by r,
=
k , P ( l - 0). exp( - E , / R T ) .
(6)
The preexponential factor k , determines the rate of incidence of the gas particles onto the free surface at a unit pressure and has a dimension of (mole sec g-l cm-l) . For the equilibrium coverage a t temperature T , one obtains in this rough Langmuir approximation for the overall reversible systems (y = x), by combining Eqs. (3) and (6) ,
K = KOexp( - A H / R T ) , KO= (k,/n,t,,kd), net,, = n.t/f(x) ,f(x) = fb). In a nonequilibrium state when desorption dominates over adsorption, the coverage 0 is always higher than given by Eq. (7). The resulting depletion rate of the particles from a surface area A cm2 into the gas phase is given for the model described by Eqs. (3) and (6) as -A(dn,,/dt)
=
nst.Jcdexp(-&/RT)AIOz
- KP,(1 - e).].
(8)
354
MILOF)
SMUTEK ET AL.
B. THEMASSBALANCE Let us consider an experimental arrangement in which the desorption process is monitored by an ionization gauge or, better still, by a mass spectrometer, so that the dependence of the pressure P in the system on time t or temperature T may be calculated. The system has a constant volume V (cm3), is pumped out at a speed S (mole sec-I), and the flow rate of gas into the system is F (mole sec-l) . An adsorbent of area A (cm2) covered with adsorbed pwticles is placed inside the system. By heating the adsorbent, the adsorbed gas escapes gradually from the surface at a specific rate (subsequently, ‘(therate”) -dn,,/dt (mole sec-l cm-2). The heat supplied is removed by the walls of the system so efficiently that at the place where the pressure is measured and the system is pumped out, the temperature is constant throughout the experiment. At the beginning of the experiment, the system is in equilibrium: P = Po;dn,,/dt = 0;Fo = So.Finally, the gas under pressure P(t)is pumped from the volume V by a constant volume rate throughout the experiment. Assuming that the gas is ideal, we have S ( t ) = SoP(t)/Po. The system, as well as the gas flowing into it at the rate F , contains besides the adsorbable component (index a) also inert components (index i). Then, the flow rate is given by F ( t ) = F,(t) F i ( t ) . The total pressure in the system is a sum of the partial pressures: P ( t ) = P,( t ) Pi ( t ) , and the speeds of pumping off the individual comLet us further asponents are: S,(t) = SP,(t)/Po; Si(t) = SoPi(t)/Po. sume that P,(t) is essentially the same throughout the whole system of volume V which implies that the diffusing rate of desorbed molecules exceeds by orders of magnitude the pumping speed as well as the maximum desorption rate. This model system corresponds to the conditions under which flash desorption experiments are performed. The temperature programed desorption of Amenomyia and Cvetanovi6 is based on different model requirements as will be dealt with in Section 1V.B. Therefore, the following treatment in the present section is pertinent only to the flash desorption conditions. In the above defined system, the over-all mass balance gives for the change of the number of moles in the gas phase the expression dn,/dt = ( V / R T )dP ( t )/ d t = F ( t ) - Adn,,/dt - S$(t)/Po. (9) An analogous relation applies to the desorbing component: dn,,/dt = (V/R,T)dP, ( t ) /dt = F,(t) - Adn,,/dt - SoP,,(t)/Po. (9a>
+
+
THERMAL DESORPTION
355
In general, the flow rate F ( t ) consists of the following additive components: the controlled flow rate F D of the entering gas, the flow rate FL which is due to parasitic leaks and/or diffusion, and the flow rate Fw resulting from possible adsorption-desorption processes on the system walls (in Section I, references are given to papers dealing with the elimination or control of the wall effects in the flash filament technique). In each of these flow rate components a particular ratio of the investigated adsorbate and of the inert gas exists and all these components contribute to the over-all mean values F,(t) and F i ( t ) . If the pressure in the system is measured by an ionization gauge, the pumping speed Sa of this gauge must be added to the pumping speed of the pump. In some cases it is necessary to take into account also the pumping speed Sw = - Fww*hichis due to the adsorption on the system walls and can even differentiate to some extent between the individual components. A t the beginning of an experiment FO = SOand F,o = SOP,O/PO. The flash desorption technique is applied usually in ultrahigh vacuum conditions. Then all the mentioned contributions to S and F should be accounted for in the evaluation of the experimental desorption curves. The effect of SWon the results of desorption measurements is discussed in
FIG.1. Effect of pumping speed on a desorption peak at a fixed heating rate. Experimental parameters given in the text. Reproduced from Ehrlich (27), with permission.
356
MILO6 SMUTEK ET AL.
papers (75-77). A change of S over a wide range does not distort the experimental results above the limits of experimental error (78). Figure 1by Hansen and Mimeault (SO)illustrates the effect of the pumping speed on the normalized desorption rate (dn,/dt)/ (dn,/dt),,, with a hyperbolic heating schedule (see Section 111) given by the expression 1/T - 1.1 X t ( T is in "K and t in seconds). Second-order = 3.33 X desorption (z = 2), activation energy of desorption E d = 20 kcal/mole, negligible readsorption, and no inert gas in the gas phase are considered. The dimensions of S are liters per second. Increasing the pumping speed while keeping the heating rate constant results in a considerable reduction of the peak height and only a slight shift in the peak location toward lower temperatures. If the experimental conditions are such that the flow rate can be taken as constant throughout the experiment, we have F ( t ) = F = SOand F,(t) = Fa = SOP,O/PO. Then, Eqs. (9) and (9a), respectively, can be rewritten as -A(dn,,/dt) = -Anfit,,(d8/dt)
- A (dnfi,/dt)
+
=
(V/RT)dP(t)/dt So[P(t) - Pol/Po,
=
-An,t,,(dO/dt)
=
(V/RT)dP,(t)/dt
(10)
+ So[P,(t) - P,O]/PO. (1Oa)
In actual experiments we do not usually observe directly the desorbed amount, but rather the derived read-out quantities, as is the time dependence of the pressure in most cases. In a closed system, this pressure is obviously a monotonously increasing function of time. In a flow or pumped system, the pressuretime dependence can exert a maximum, which is a function of the maximum desorption rate, but need not necessarily occur a t the same time due to the effect of the pumping speed 8. If there are particles on the surface which require different activation energies E d for their desorption, several maxima (peaks) appear on the time curve of the recorded quantity reflecting the desorption process (total or partial pressure, weight loss). Thereby, the so-called desorption spectrum arises. It is naturally advantageous to evaluate the required kinetic parameters of the desorption processes from the primarily registered read-out curves, particularly from their maxima which are the best defined points. With flow systems, Eqs. (10) and (loa) apply also, provided steady initial conditions, especially the partial pressures, have been established before the run. Then the time dependence of P and dP/dt (or of the corresponding partial pressures) provides a direct means of estimating (dn,,/dt) as a function of time. Towards the end of desorption as the left-
THERMAL DESORPTION
357
hand side of Eq. (10) becomes much less than the second right-hand term, the P-t curve falls essentially in an exponential manner (the temperature at the outlet should be constant) (28,66). Thus the estimates of (dn.,/dt) , and of n,,(t) as well, become progressively less accurate toward the end of desorption. This also shows that the experimental conditions should be selected so as to have the left-hand term in Eq. (10) comparable to the larger of the terms on the right-hand side throughout the essential part of the desorption process. This caution applies especially if P-t curves only are recorded, because their numerical or graphical derivation for a further treatment is subject to rather lager uncertainties. There are three approaches to the evaluation of Eq. (10). The safest procedure is a complete evaluation of the equation which has been performed by some authors (27, 28, 66). In most cases, however, the experimental conditions are chosen so as to ensure that one of the terms on the right-hand side of Eq. (10) [or Eq. (10a) 3 is clearly predominant over a wide range of the desorption run, so that the other term can be neglected. In each particular case, justification of this neglect should be checked for low (dn.Jdt) values. The first term predominates at low values of the flow rate F (and thereby at low pumping speeds 8), and with high ratios of the system volume V to the initially adsorbed amount. Then, the pressure-time dependence is essentially the same as in a closed system, i.e. it has a monotonously increasing S-shape. The least distortion through the second term clearly OCcurs in the vicinity of the maximum desorption rate where P passes through an inflection point so that dP/dt has its maximum. Mostly, however, a more advantageous arrangement is used: the pumping speed S is chosen so high that the first term on the right-hand side of Eqs. (10) and (10a) becomes negligible as compared to the second one. Thus the pressure differences ( P - Po)observed are proportional to the desorption rate, and the pressure-time curve exhibits maxima which more or less closely coincide with the maximum desorption rates of the species released. The more significant the contribution of the first term, the greater is the discrepancy between the pressure-time curve and the desorption rate-time curve. This is evident from the derivative of Eq. (10) :
+
-AAn,t,g(d20/dt2) = ( V / R T )(d2P/dt2) (So/Po)(dP/dt).
(11)
If the left-hand side of Eq. (11) is zero, dP/dt must be positive and dzP/dtz negative, i.e. the maximum desorption rate is attained earlier than the maximum pressure. This distorting effect increases with an increasing value of d2P/dt2(which depends on the kinetics of the surface process and therefore among others also on the heating rate) and with the increasing ratio
358
MILO6 SMUTEK ET AL.
(Pol')/(SORT) = No/So ( N ois the number of moles in the gas phase before the experiment), and thus with a decreasing pumping speed So. Integration of the general expression (10) gives the following equation for the desorbed amount An,,, provided So and the temperature at the pumping gauge can be considered as constant through the integration limits (28) : AAn,, = ( V / R T ) [ P ( t )- PO]- Sd
+ (SO/PO)i ' P ( t ) d t .
(12)
The integral can be solved either numerically or graphically, or eventually by means of electrical circuits. In principle, it is possible in most experimental arrangements t o obtain a simultaneous registration of P ( t ), dP/dt, and/or Jot P(t)dt by suitable circuit connections and thus to get direct information about the time course of the desorption rate and/or the Ansg vs time dependence. Moreover, in the experimental arrangements with P ( t ) reflecting essentially the course of the rate of desorption, the dP/dt vs t curve can yield valuable information about the desorbing states, particularly in cases of poorly resolved successive desorption processes. Therefore, it is rather surprising that simultaneous recording of the mentioned P functions has not been, to our knowledge, described in the literature, except in the study (66) using a strictly closed system, where the recording of dP/dt proved to be very fruitful. With a rough model of adsorption leading to Eq. (6) and with a condition of constant F , one obtains after inserting Eq. (8) into Eq. (10) the following expressions for the P ( t ) function: Anat,,kdexp(*)[OX RT,
- KP,(l - O).]
=
=
P ( t ) - Po SO Po
+--RVT , dP(t) dt
V dPa(t) so Pa(t)Po- Pa0 +--. RT, dt (13)
Subscripts w and a are introduced to distinguish between the constant temperature of the spots where the pressure is monitored and the system is pumped out, and the changing temperature of the heated adsorbent, respectively. If the experimental conditions are such that the term with dP/dt can be neglected, Eq. (13) gives directly the dependence of the pressure in the system on the adsorbent temperature and even on the time t elapsed from the beginning of the experiment, if an analytical expression for the heating rate is available. The time derivative of Eq. (13) gives for
359
THERMAL DESORPTION
the maximum in the observed pressure, i.e. for dP/dt rangement using Eqs. (10) and (13), the expression
= X k d S o pm
-
Po
+"("). RT,
=
0, after a rear-
+ KmPam(l- Om)z-*]
exp(*)[el RTm
dt2
Similarly, for dPa/dt = 0; i.e. for the maximum concentration of the desorbed substance in the gas phase, we obtain 1
(dt),[ dT
RT,~
exp(-"") - AHAnat,gkdOmz
RTm
+ E,S~
-
Po
pa^]
Subscripts m denote values referring to the maximum P and P,, respectively. The conditions justifying the neglect of the term with dP/dt on the right-hand side of Eq. (13) are not sufficient for neglecting the negative contribution (d2P/dt2) in Eqs. (14) and ( 1 4 4 . More stringent demands m following from Eqs. (14) and ( 1 4 4 must be met. Provided that (d2P/dt2) is negligible, the term with (dP,/dt) pPmax in Eqs. (14) and ( 1 4 4 becomes negligible as well, and the expression is then simpler. In a further treatment we shall deal with Eqs. (14) and (14a) under such conditions only, which make the terms with (d2P/dt2)mand (dPa/ dt)pSmax negligible as compared with the other terms. Using Eq. (13) we can eliminate from Eq. (14) the unknown value of the surface coverage 0 and thus arrive, for a given pumping speed S and heating rate dT/dt, a t a relation between the measured data (i.e. the maximum pressure P , or the maximum partial pressure Pam,and the corresponding temperature T,) and the parameters k d , K, Ed, -AH, and x, characteristic of the surface
MI LO^
360
SMUTEK ET AL.
process under consideration. Thus two limiting cases are obtained, according to the ratio of the rates of adsorption and readsorption: (a) The desorption process at the maximum desorption rate will be practically free of the competitive readsorption provided the condition K P << 1 or K P ( 1 - 0 ) << 0”) respectively, can be met. Then, Eqs. (14) and ( 14a) , respectively, reduce to (Ed/RTm2)(dT/dt)m =
d&-’kd
eXp ( - Ed/RTm)
= 2 [ k d exp (
- Ed/R!!’,) ]l’ZISo/An,t,,](z-l)’”[ (Prn - P O ) / P O ] ~ ~ - ~ ) ~ ~ (15)
=
%[kdexp (- Ed/RTm)]l’z[So/Anst,g]‘“’’~z[ (Pam - P ~ o ) / P o ] ~ ~ - ~ ) ’ ~ * (154
(b) For the case of an essentially equilibrium desorption, there follows from Eqs. (14) and (14a)) respectively [with the equilibrium surface coverage 0, expressed by Eq. (7)]:
SO(Pm - PO) /PO = SO ( P a m - Pa01 /PO = - (An8t,,AH/RTm2) (dT/dt)m(KPa)F/z[1 ( K P a ) F 1 2 = - (Annt,,AH/RT2) ( d T / d t ) m [ L ( l - Om)/z]. ( 16) For the desorption processes that follow the first-order kinetics, we thus obtain [neglecting again (dzP/dt2) ,,, and (dP,/dt) or (dP/dt)m ] the general expressions
+
(174 Taking K , = 0, we obtain an equation for the desorption process alone, i.e. for the case where the rate of readsorption is negligible. On the other hand, if only the last term in the brackets is considered, one has the equation for the equilibrium desorption.
361
THERMAL DESORPTION
If the desorption process follows the second-order kinetics, it is necessary to solve a quadratic equation for 0 resulting from Eq. (8) and the general expression (17) becomes rather complex. From P , still further information about the parameters of the desorption process can be obtained. To this end, Eq. (8) must be solved. The solution, however, is accessible only in the case of desorption alone. If the contribution of the second term in Eq. (8) is appreciable, it is necessary to insert for P from Eq. (13). Thus, nonlinear differential equations result even for the most simple cases (z = 1, or the equilibrium desorption) , which can be solved by numerical methods only or by iterative methods provided the second term in Eq. (8) is small. 111. Temperature Schedules in Thermal Desorption Heating of the adsorbent in the thermal desorption method can be performed either in a continuous or in a stepwise manner. Most usually a continuous heating is applied. If treatment of the experimental data used by Ehrlich (27), by Ageev, Ionov, and Ustinov (28), and by Lapujoulade (66) is employed, a mere empirical knowledge of the time-temperature dependence is sufficient (see Section I). Most authors, however, use an analytical approach to the treatment of the desorption data, where a simple analytical expression of the time-temperature function is essential. In such a case, the following two schedules are employed due to their feasibility and relative simplicity from the mathematical point of view: (i) The temperature T of the adsorbent increases linearly with time t :
+
T = To at; dT/dt = O. (18) (ii) The reciprocal temperature of the adsorbent decreases in a linear way with time, i.e. its temperature increases in a hyperbolic way: 1/T = 1/To - Pt;
dT/dt
=
PT2.
(19)
a, B are the proportionality coefficients, To is the temperature of the ad-
sorbent prior to its heating, i.e. at t = 0, and T is the temperature of the adsorbent at time t. According to the value of a one can distinguish between rapid flash desorption (a = 200-1200 deg sec-l) , slow flash desorption (a = 10-80 deg sec-I), and the so-called temperature programed desorption (for this term see p. 346) (a = 10 deg min-'-lO deg sec-l). Carter et al. (79) have theoretically investigated the exponential increase in temperature expressed by
T
= Toexp(yt),
dT/dt
=
yT,
(20)
(y is the proportionality coefficient) which ranks between the linear and
362
3IILO;
SMUTEK ET AL.
the hyperbolic heating rate according to the steepness of the temperature rise. The said authors have pointed out some advantageous features of this schedule. Carter and Armour (80) suggested still other special temperature schedules which, however, appear to be of little practical importance. A different approach consists of stepwise changing the adsorbent temperature and keeping it constant at each of the prefixed values TI, Tz, . . ., T, for a certain time interval (e.g. 10 sec), thereby yielding the so-called step desorption spectra/(81-85). The advantage of this method lies in a long interval (in terms of the flash desorption technique) for which the individual temperatures Ti are kept constant so that possible surface rearrangements can take place (81-83). Furthermore, an exact evaluation of the rate constant kd’ is amenable as well as a better resolution of superimposed peaks on a desorption curve [see Section VI) . What is questionable is how closely an instantaneous change in the adsorbent temperature can be attained. This method has been rarely used as yet. Heating of adsorbents in the form of discs and platelets can be effected by electron bombardment (34, 86), by focused light (34, 37), or by the high-frequency induction technique (66).As the means for heating the films, a mere removal of the cold bath (45, 45a), or a heating coil wound onto the wall of the desorption vessel (46f)have been described. With wires, ribbons, and foils, most often direct electrical heating is employed. A constant current through the sample yields in most cases a hyperbolic temperature increase with sufficient accuracy over a wide range (87). With a constant potential difference across the sample, an approximately linear temperature increase results. Control of the adsorbent temperature and termination of the heating after a preset temperature has been attained can be effected with slow flash desorptions by manual setting of the feeding voltage (29). Branching of the feeding voltage from a motor-driven potentiometer can be used also (40, 64, 88). Electronic circuits (29-31, 88-90) permit flash desorptions to be accomplished over a wide range of heating rates and with variable heating schedules. The powdered adsorbents are usually heated by means of a furnace with a resistance heater. A substantial increase in the heating rate was achieved by a high-frequency induction technique (911.
A reliable determination of the adsorbent temperature is obviously a crucial requirement of the thermal desorption method. Thermocouples are mostly used to this end. With disks and foils, a thermocouple can be spotwelded onto the back or edge of the sample. Thermocouples can be attached also to ribbons as well as to the wall of the vessel containing an evaporated film. With powdered adsorbents, thermocouples are located in the layer of the sample. The adsorbents in the form of filaments and ribbons are frequently used simultaneously as resistance thermometers, switched
THERMAL DESORPTION
363
into a suitable circuit (26, 35, 46b, 4612, 61, 62, 87, 92). The relationship between the filament resistance and its temperature is established by calibration (26, 28, 30, 43, 61, 66a, 92). The reliability of determining the surface temperature of metal ribbons directly by means of a resistance measurement has been reported as questionable (26, 38,93). The assumption of a uniform temperature of the whole adsorbent surface at any instant of the heating period is actually not met. Temperature variations along the wire or ribbon arise due to irregularities in their cross section, inhomogeneity of cryst,allographic arrangement, etc. and even an ideally perfect wire exhibits a temperature distribution caused by the heat removal through leads. The temperature difference between the wire center and its ends is greatest in the steady state, and smaller under rapid heating (26, 27, 29). Obviously, end effects can also be limited by using long and fine wire specimens. The error in results caused by identification of the temperature of the whole surface with the temperature of the center of a wire 15 cm long is estimated to several tens of percent (26,27).A more via. that favorable report has been given by Pasternak and Gibson (@I), in their experiments the temperature of a filament 25 cm long and 0.02 cm in diameter was uniform within *20" between 900 and 2000"C, to closer than 1 cm from the leads. According to McCarroll (94), an error of 1% in the temperature calibration can lead to about the same error in calculating the value of E d . Correction of results for the end effects of wires and ribbons has been dealt with by Kisliuk (95) and Hickmott and Ehrlich (26). Matsushita and Hansen ( 4 6 ~compared ) the results of the desorption experiments with a 10-mil tungsten filament 20 cm long, which results were obtained when the end-to-end average temperature, and the center-section temperature, respectively, were considered. The resulting values of Ed were much the same in both cases. For some purposes, it was found useful to subdivide the continuous temperature distribution of the filament into several discrete areas with average temperatures and to consider the actual coverage corresponding to each temperature separately (35). Degras (65a) studied the temperature distribution of the surface of a molybdenum disk 0.2 mm thick and 20 mm in diameter, heated by electron bombardment. Two thin thermocouples spot-welded on the back of the sample 2 and 8 mm from its center, respectively, showed a temperature difference of less than 2% at 500"K, and less than 3% at 1200°K. Occurrence of the temperature inhomogeneity along a film during its heating was reported recently (45f).Different results were obtained when temperature measurement and control were effected only in one point, and in three points of the desorption cell, which illustrates that caution should be observed in this respect when studying thermodesorption from the films. This caution is obviously yet more essential when working with powdered materials.
MILO6 SMUTEK ET AL.
364
0.80 -
I 200 1200
0.10'
440
240
520
I~LO
280
1680
600
320 1920
I
360-1192 680 -0596 2160-0.199
t (msec)
FIQ.2. Effect of heating rate on a desorption peak at a fixed pumping speed. HyperK-l; S/V = 4.8 sec-'; E d = 80 kcal mole-'; bolic heating schedule, l/To = 9.95 X 5 = 1. Reproduced from Hansen and Mimeault (30),with permission.
I n Section 11,we mentioned the effect of a change in the pumping speed on the location of the maximum desorption rate. Changes in the heating rate d T / d t affect both the location and height of a maximum in the desorption rate (Fig. 2). Changes in the pumping speed and in the heating rate may compensate each other to some extent: the increasing pumping speed shifts the peak in the desorption rate to lower temperatures and reduces its height, whereas an increase in the heating rate has an opposite effect. A discussion of these phenomena can be found in the papers (27, 31 , 7 8 ) . In general, a high heating rate permits an easier recording of the output data (due to a high value of the desorption rate), and restrains the redistri,bution of the surface species which eliminates the danger of distortions of the desorption spectrum caused by activated conversion processes during the temperature rise (45).On the other hand, it can seriously impair the resolution of adjacent peaks and this can lead to an apparent simulation of a coverage dependence of E d or k d (&?a). A slow heating rate implies employment of a very sensitive detection of the pressure, or of a sufficiently big sample, facilitates the peak resolution, as well as the attaining of an equilibrium surface distribution prior to the desorption (this removes the necessity of accounting for the probability of a collision of the surface particles in the description of a higher order desorption), but complications may appear with possible considerable extent of readsorption.
365
THERMAL DESORPTION
IV. Fundamental Relationships for the Determination of the Activation Energy of Desorption, of the Order of Desorption and of the Preexponential Factor
Consider a system in which the desorbing particles have an activation energy of desorption Ed and the preexponential factor kd. Both these quantities are assumed to be independent of the surface coverage, i.e. constant during the whole desorption process (the case of E d depending on the coverage is dealt with in Section VI; the assumption of constant k d was discussed in Section 1I.A.). Consider further an experimental arrangement with values of the parameters V , S, d T / d t such that the readsorption can be neglected ( r d >> ra), and an experimental arrangement suitable for a near-equilibrium measurement ( r d = ra). In the following text, a treatment of the thermal desorption data obtained under the above conditions is presented in terms of the general and uniform approach advanced in the paper (96). A. NEGLIGIBLE RATEOF READSORPTION ( T d >> T a )
For the above simple model, the na(T ) relationship is easily obtained by integrating Eq. (8) from naoand Toto n. and T . Let us introduce the following notations: e =
Ed/RT;
€0
=
Ed/RTo;
em =
Ed/RT,.
(21)
The resulting integrals are expressed in terms of exponential integrals ~ , ( e )=
lrn u-zexp(-eu)du.
(22)
The results of the integration are then as follows: Order of desorption
Schedule hyperbolic exponential linear
11
+ [ E O ( ~) -To Eo(~o)
PT
x =1
x =2
x = 3
366
nm,o& SMUTEK
ET AL.
Assigning notations z = 0, 1, and 2, respectively, for the hyperbolic, exponential, and linear schedules, respectively, we can unify the left-hand sides of Eq. (23) to a single general expression (96) (k,/aZ) Tz-’[Ez(e) - ( T o / T )Z-lE,(~O)],
(24)
with a, standing for the respective parameter in the expressions (18), (19) , (20),i.e. a,8, or y. This formalism can be readily extended to any value of which results from a heating schedule z. For example, let us take z =
T-112 = T,”’
- alpt.
Then the exponential integral ElI2(e)can be expressed as erfc( de).In all cases when z has a value other than 1, however, Eq. (24) should be multiplied by ( z - 1) and the sign of usshould be respected. In the singular case z = 1 (sign of a1 undefined), corresponding to the exponential heating schedule, the factor, instead of ( z - I), is unity and corresponds to the change in sign of a,, i.e. to a transition from the reciprocal heating schedules to the direct ones. Now, Eo(E ) = exp ( -e) /el and for large e-values the following approximations can be used (97’) :
The leading term in the expansion gives sufficiently accurate results down to e = 5 (maximum error of 3.5%) , thus covering almost the whole region of observable desorption. With the first correction term, the range of evalues can be extended down to e = 1.5 (maximum error of 3.5%). Furthermore, it can be shown that Ez(eo)<< Es(e) for notable desorption rates as long as the experiment was started at a temperature with no observable desorption occurring. This condition is obviously always fulfilled, except perhaps for the most weakly bound physically adsorbed states. Thus, Eq. (25) together with Eq. (24) allow us to express Eq. (23), to a good approximation, as
n,
forz
=
1,
forz = 2,
(26)
367
THERMAL DESORPTION
Previous attempts at treating the linear schedule are presented by Ehrlich (29) and Redhead ( 3 1 ), and the exponential schedule by Carter et al. ( 7 9 ) . Most often, the primary experimental desorption data [mainly the P ( t ) or P(T) function] represent, after due corrections, the temperature dependence of the desorption rate, -dn,/dt = Nt vs T. The resulting curves exhibit peaks and their most reliable point is the maximum at the temperature T,, corresponding to the maximum desorption rate N , . Its location on the temperature scale under various conditions is essential for estimating the kinetic parameters of the desorption process. The theoretical expression for the relations at the maximum is obtained by putting (d2n/dt2) = 0. Differentiating Eq. (8), substituting into the result for N , the proper expression from Eq. (8), and equating the relationship obtained to zero, the maximum condition for order x results in the form x(hd/ao) T",-'[exp ( -Ern)
/ern]
= (nat/%m)
(27)
With x = 1, Eq. (27) gives directly the relationship being sought. With x = 2 and x = 3, the required expression for (nat/n,,) is inserted from Eq. (26), since the value of en, is surely such as to permit both the neglect of E,(e) and the approximation by the first term of the expansion ( 2 5 ) . Finally one gets forz = 1,
For the exponential heating schedule ( z = l ) , the quantities Ed and T occur only when grouped in the term e = Ed/RT, and thus particularly simple expressions for the temperature T, at the maximum desorption rate result, as was pointed out by Carter et al. (79): for the first-order kinetics and for the given quotient (kd/al), T, is exactly proportional to Ed; for the second-order kinetics, the same applies as long as the initial coverage (n,o/n,t) remains constant. For heating schedules other than the exponential one, the shift of T m with increasing Ed is not exactly linear, due to the term TZl. The expressions (28) can be further simplified as follows. The rate of the temperature increase a t T, can be expressed for the linear, hyperbolic, and exponential schedulesaccording to Eqs. ( 18), (19) , and (20) as (dT/dt) m = a,PTm2,and yTm,respectively. In our general notation, this can be written
368
MILO;
SMUTEK ET AL.
as aZ,, =
( d T / d t ) , = azT',".
If a generalization to an arbitrary order x of the desorption kinetics is also performed, the expressions (28) transform into
It is obvious that Eq. (30) holds for any heating schedule: the temperature T , of the maximum desorption rate depends but little on the type of continuous and monotonically increasing heating and is a function mainly of the rate of temperature change a,,, a t T,. Of some interest are the coverages remaining a t T,. They are easily derived from Eqs. (23) , (26) , and (28) and read ln(nam/nd = n,m/n,o
-Ern/(€,
e-'(em
%
narn/nso = +(em narn/%o =
[(em
+
1
f o r z = 1,
3 z ) / 3 ( e , -I- z)]'"
for 2 = 3.
Z)
;
+ 2z)/(ern + + 2 ~ ) / ( ~ m+
Z)
Z)
for x = 2,
(31)
Since generally em > z, these coverages a t T , increase only slightly as the heating schedule becomes less progressive and amount to about e-1 = 37%, 3 = 50%, and 1/3/3 = 59% of the initial coverage for the first-order, second-order, and third-order desorption, respectively. The maximum specific rate of desorption N , is obtained from Eqs. (15) and (31) as l/e f o r z = 1, N m = a,naoT~em(e, 2z)/(e, Z) times 1/4 forx = 2, (32) 1/3/9 for x = 3.
+
+
t
Again, using Eq. (29), the left-hand side of Eq. (32) can be written as amnaoTglem(em
+ 2z)/(ern + z ) ,
(33)
indicating but a slight dependence of N , on the actual heating schedule via 2z)/(e, z ) . With the exponential heating, the correction term (em z = 1, and provided a1 and kd are constant, the height of the desorption peak is in this case proportional to the initial coverage for the first-order desorption. For a higher-order desorption, the dependence on nB0enters through the parameter em [see Eq. (28)]. To obtain the relative value of coverage a t a temperature T with respect
+
+
369
THERXAL DESORPTION
to that at T,, let us introduce the abbreviation y’ = exp (em
- a).
(34) Then for E > 4 the following expressions hold with reasonable accuracy, independently of the initial conditions: In -
=
)m: (
1
1
+
Z/cm
-($>’&
forx
Of special interest is the hyperbolic heating schedule, z general expressions (35) reduce to simple forms:
=
1,
=
0, since the
fo rx = 1, for x = 2, for x = 3.
(36)
The relative coverages are in this case universal functions of the parameter y’ and thus of the distance from the maximum, expressed in (em - e), proportional to ( l/Tm) - (l/T). A direct consequence of this feature of the hyperbolic heating schedule is that the relative desorption rates p = N t / N mare also universal functions of y’, and their shape depends on the desorption order only. They read:
(37) for x
=
3.
Using the e-notation, we obtain (31) l n p = em - e p =
+ 1 - exp(em -
cosh[(e, - t)/2]-’ {exp(e, - e) ( 3 4 1
for x = 1,
B)
+ 2 exp(e,
- e)])3/2
f o r z = 2, for x = 3.
(38)
Thus, for the second-order desorption kinetics and the hyperbolic heating schedule, the peaks are symmetric about T, in the scale (1/T) . The firstorder peaks are asymmetric in this scale, exhibiting a steeper descent than ascent. These considerations suggest that the hyperbolic heating schedule is especially favorable for an analysis of the peak shapes and for the detection
370
MILO;
SMUTEK ET AL.
TABLE I Relative Rates of Deaorption for the Hyperbolic Healing Schedule Percent of the maximum
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5
First order em - €
Y'
0.01872 -3.9782 0.03822 -3.2644 0.05851 -2.8386 0.07968 -2.5297 0.10183 -2.2845 0.12507 -2.0789 0.14952 -1.9003 0.17536 - 1.7409 0.20276 - 1.5957 0.23196 -1.4612 0.26327 - 1.3346 0.29708 -1.2138 0.33392 - 1.0969 0.37449 - 0.9822 0.41987 -0.8687 0.47167 - 0.7515 0.53267 -0.6298 0.60934 -0.4970 0.71295 -0.3383 O.oo00 1.00000 1.3554 0.3041 1.5318 0.4265 1.6832 0.5207 0,6012 1.8244 1.9613 0.6736 2.0973 0.7407 0.8042 2.2350 0.8656 2.3764 0.9256 2.5235 2.6784 0.9852 2.8436 1.0451 3.0223 1.1060 3.2188 1.1697 3.4392 1.2352 1.3063 3.6926 1.3849 3.9943 4.3725 1.4753 4.8897 1.5871 5.7439 1.7481
Second order Y' em - €
0.01282 0.02633 0.04061 0.05573 0.07180 0.08893 0.10728 0.12702 0.14837 0.17157 0.19702 0.22515 0.25659 0.29222 0.33333 0.38197 0.44164 0.51949 0,63451 1 .ooooo 1.5760 1.9250 2,2642 2,6180 3.0000 3.4221 3.8973 4.4415 5.0757 5.8284 6.7405 7.8730 9.3213 11.2444 13.9282 17.9443 24.6261 37.9737 77.9872
-4.3565
-3.6369 -3.2038 -2.8873 -2.6339 -2.4198 -2.2323 -2.0634 - 1.9081 - 1.7627 - 1.6245 -1.4910 - 1.3603 - 1.2302 - 1.0986 -0.9624 -0.8172 -0.6549 -0.4549 0.0000 0.4549 0.6549 0.8172 0.9624 1.0986 1.2302 1.3603 1.4910 1.6245 1.7627 1.9081 2.0634 2.2323 2.4198 2.6339 2.8873 3.2038 3.6369 4.3565
Third order Y'
0,00991 0.02044 0.03165 0.04364 0.05649 0.07034 0.08531 0.10160 0.11941 0.13903 0.16083 0.18527 0.21303 0.24503 0.28269 0.32823 0.38560 0.46286 0.58203 1.00000 1.7783 2.3202 2.8976 3.5524 4.3197 5.2401 6.3673 7.7765 9.5774 11.9363 15.1165 19.5551 26.0220 35.9791 52.4857 82.8660 148.495 335.998 1348.50
,€
-
- 4.6142
- 3.8904 - 3.4530 - 3.1318 - 2.8736 - 2.6545 -2.4614 - 2.2867 -2.1252 - 1.9730 - 1.8274 -1.6859 - 1.5463 - 1.4064 - 1.2634 - 1.1140 - 0.9529 -0.7703 -0.5412 0.0000 0.5757 0.8417 1.0639 1.2671 1.4632 1.6563 1.8512 2.0511 2.2594 2.4796 2.7158 2.9732 3.2589 3.5829 3.9605 4.4172 5.0006 5.8171 7.2067
371
THERMAL DESORPTION
of any major distortions occurring either due to variable E d and/or k d , or due to interference with other closely adjacent peaks. Values of the reduced desorption rates p as functions of y’ and (em - e) are given in Table I.
B. RATEOF READSORPTION NEARLY EQUAL TO THE RATEOF DESORPTION (rd
%
ra)
In the near-equilibrium conditions, the coverage at each temperature is close to its equilibrium value. Thus
-dnag/dt = -n.t,,(dOeq/dt)
=
-nat,guT(dOeq/dT),
(39)
where again U T = dT/dt. As discussed in Section 11,the Oeq(T)relationship is in general rather complex, mostly as a result of the kinetics of adsorption. If the rough Langmuir approximation is used, the location of the maximum in the observed pressure P, in the temperature scale is obtained directly from Eq. (16). Introducing the abbreviation y’ = (1/22) In ( K P )
(40)
we can write Eq. (16) in the form 42RSo[(Pm - Po)/P,](Tm cash ym’)’
=
Anst,gam(-AH).
(41)
From the values of T , and P m obtained with various initial pressures PO and various heating rates a, at the maximum, the required estimates of KO,- A H , and x can be extracted. dn./dt) The near-equilibrium desorption in a closed system (dP/dt was used in practice, for example, by Procop and Volter (45h) and by Dawson and Peng (98). Due to the inherent uncertainty of the Langmuir model and difficulties in solving the transcendental equation (41), probably the most accurate treatment in the near-equilibrium cases is a numerical or graphical integration of the expression
-
-Anat ,g (dOeq/dt = (dt/dT,) ( V / RT,) (dP/dt)
+ SO( P - PO)/PO (42)
which is obtained by a combination of Eqs. (10) and (21). The resulting O e q ( t ) estimates are then easily converted into the required O,,(P,, T) values by inserting the proper values of P , and T for each t. The analysis of experimental data is clearly rather difficult in this approach. Therefore, an experimental arrangement on which the derived expressions are based is rarely used in practice for the quasi-equilibrium measurements. For powdered materials, a different experimental design advanced by Amenomiya and Cvetanovid (47-49) is widely employed.
372
MILOg SMUTEK ET AL.
Its main features are given by the use of a stream of inert carrier gas which percolates through a bed of an adsorbent covered with adsorbate and heated in a defined way. The desorbed gas is carried off to a detector under conditions of no appreciable back-diffusion. This means that the actual concentration of the desorbed species in the bed is reproduced in the detector after a time lag which depends on the flow velocity and the distance. The theory of this method has been developed for a linear heating schedule, first-order desorption kinetics, no adsorbable component in the entering carrier gas (Pa = 0), and the Langmuir concept, and has already been reviewed (48, 49) so that it will not be dealt with here. An analysis of how closely the actual experimental conditions meet the idealized model is not available.
V. Processing of the Experimental Data to Estimate the Kinetic Parameters of Desorption As already mentioned in Section I, there exist two principal approaches to the treatment of the experimental data obtained by the thermal desorption technique: (a) the desorbed amounts ns or the desorption rates dn,/dt measured at particular temperatures are substituted in the Arrhenius equation, and from the temperature dependence the required kinetic parameters are estimated; (b) the desorbed amounts or the desorption rates, measured on heating the adsorbent according to a known temperature-time function, are compared with those quantities in theoretical analytical expressions obtained by integrating the Arrhenius equation with parameters taken as constant. The corresponding mathematical background has been developed in Section IV. Here, the actual processing procedures will be outlined in some detail. Only the cases with negligible readsorption will be dealt with. The nearly-equilibrium version of the thermal desorption technique has been used in practice mainly in the arrangement of Amenomiya and CvetanoviC, and treatments of the data obtained by this method are given in the reviews (44 4.9).
A. DIRECT ESTIMATES OF dn,/dt
AND
n,
The minimum number of postulates of the model of a desorption process with no explicit analytical expression of the heating schedule are required if the primary output data are treated according to Eqs. (10) and (12), viz. by numerical or graphical derivations and integrations of the recorded pressure data. After an adaptation of the analyzer, these operations can be performed by means of electrical circuits. The known temperature-time relationship (either in the form of an analytical function or established
373
THERMAL DESORPTION
merely empirically) is then used to express an./& and n, as functions of temperature. The basic Arrhenius equation (8) is rewritten in the form log( -d9/dt) = log kd f
log 0 - (0.2172 Ed) ( 1 / T ) .
(43)
The simplest procedure then is to assume some value of the desorption order, usually x = 1 or 2, and to try to draw a straight line through the experimental points in the plot Dog( -d0/dt) - z log 01 vs 1/T. The slope gives the value of 0.2172 Ed, and from the extrapolated intercept with the log-axis for 1/T = 0 the value of k d can be estimated. With more substantial information, however, the treatment of Eq. (43) is given as a linear regression in two unknowns, y = a. alxl a2xZ)with y = log( -dO/dt) ; x1 = log 0; x2 = 1 / T ; and with parameters a0 = log k d ; a1 = z; uz = -0.2172 Ed, which are to be optimized. Using the conventional methods of the regression analysis, we can obtain not only the best estimates in the least-squares procedure in this way, but also the estimate of the variance along the correlating plane, the intervals of confidence, and the covariances of the parameters. Moreover, if the deviations between the input y data and the calculated y values exhibit a systematic trend, another linear term can be added to Eq. (43), based on an additional assumption for the model of desorption considered. For example, an assumption of a linear dependence of E d on 0 would result in the term xa = 0/T, with the corresponding parameter a3to be estimated. However, each additional term in Eq. (43) gives rise to an appreciable loss in accuracy of estimates: both the variances and the covariances of the estimated parameters increase thereby. The meaningful number of tested parameters is thus strongly limited by the number of triplets (n,, dn,/dt, )'2 introduced into the regression and by their accuracy. Also a decision between two alternate assumptions will be possible only if they differ appreciably. Thus the regression analysis can hardly decide whether the preexponential equals simply kd, or Ld/T as predicted by the theory of absolute reaction rates. Desorption from a heterogeneous surface is likely to be fairly well correlated when a h e a r dependence of Ed on 8 is assumed, etc. Obviously, if the confidence interval of the x estimate is such as to permit its unambiguous assignment to an integral order of desorption, such as x = 1 or 2, the regression with a diminished number of parameters [the term z log 9 subtracted from the left-hand side of Eq. (43)] will be recalculated, thus improving the confidence intervals of the remaining parameters. The primary evaluation of a number of runs under different conditions (initial coverage, rate of heating) will check whether any pronounced deviations from the basic Arrhenius model occur. When a significant disagreement between the estimates of n, and dn,/dt for the given temperature is encountered, its cause may be sought, for example, in the mobility of adsorbed particles.
+
+
MI LO^
374
SMUTEK ET AL.
B. OUTPUTDATAREFLECTESSENTIALLY THE n.(t) RELATIONSHIP In closed and quasi-closed systems, the term dP/dt in Eq. (10) dominates over a large range of the desorption and is always nonnegative. Thus the recorded P ( t ) curve essentially reflects the amount of desorbed gas up to the time t. After due corrections and conversion of the time values into the correspQndingtemperature values, Eqs. (23) and (26) can be used directly t0 evaluate the kinetic parameters of the Arrhenius equation, if they are assumed to be constant. In the widely used graphical method, the data are approximately linearized by taking logarithms of Eq. (23) and plotting 1/57 vs
(2 - z) InT
- lnpn(n,o/n,)]
(2 - z ) 1nT - ln[(n.t/n.)
- (nEt/nso)]
for x = 1, for x
=
2,
(44)
(2 - z ) 1nT - ln[(n,t/n,>2 - (nst/ns0)2] for x = 3. The effect of the correcting term E.(eO) decreases with increasing T and can soon be neglected. The remaining portion should give a linear plot in one of the expressions (44)containing n. data, depending on the desorption order involved. Then the slope gives the value of Ed/R and the extrapolated intercept for 1/57 = 0 equals the term ln(a,Ed/kdR) from which in principle the value of the apparent preexponential factor k d can be extracted. However, the value of kd obtained in this way is highly inaccurate due to the extrapolation errors and to the logarithmic form of the term. In principle, only the expressions for the correct desorption order should give a straight line at higher temperatures. I n practice, however, the experimental scatter, possible inaccuracy in corrections of the output data, inherent departures from the simple model considered (mainly the dependence of E d on e), together with a rather strong correlation which can be shown to exist between the functions In [(lln.) - (l/n,,)] and ln[ln(n,o) - In(n.)], can seriously impair the plot and make the estimate of the desorption order rather dubious. Statistical methods should be helpful in this case, but to our knowledge they have not been employed so far. The procedure using the plot (44)is especially suitable for treatment of the n.(t) data obtained in the experimental design of Czanderna (46). AS mentioned in Section I, the method consists of direct weighing of the Sample mass loss, so that the measurement is direct and simple; it is possible to work at higher pressures, no corrections for adsorption on walls are necessary, and the surface coverage can be controlled continuously. Since the technique required low heating rates CO.02-2.0 deg sec-I after (46)], careful consideration should be given to the role of readsorption. When experimental conditions enabling us to neglect it cannot be achieved, the
THERMAL DESORPTION
375
safest way is to obtain derivatives dn,/dt from the primary data and then to apply the procedures dealt with in Section V.A, with an appropriate subsequent analysis accounting for the readsorption.
C. OUTPUTDATAREFLECT ESSENTIALLY THE dnJdt VALUES Most authors choose such a high pumping speed in the experimental arrangement that the first term in Eq. (10) can be neglected and thus the recorded P ( t ) curve essentially reflects the dn,/dt values at the given time points. The treatment of these data is based on the formulas developed in Section IV. 1. Order of Desorption The order of the desorption process is estimated in the first place from the shape of the desorption peak, preferably in the 1/T scale. The firstorder peaks here are clearly asymmetric, the falling branch being steeper than the ascending one. The second-order peaks are near symmetric and are broader. The third-order peaks are even broader and are again asymmetric, but in this case the ascending branch is steeper than the falling one. As follows from the discussion of Eq. (36) and from Eq. (38) where T and E d occur only grouped in the term 6 = Ed/RT, the hyperbolic heating schedule is the most suitable one for the determination of the desorption order. With other heating regimes, the peak shapes are more complex. In linear heating, the peaks become narrower, and the narrowing of the falling branch is more pronounced than that of the ascending branch. For the first-order case, the differences between the peak shapes in the hyperbolic and the linear heating are of little significance. For the second-order case, the peak in a linear heating becomes somewhat asymmetric in the 1/T scale, but even under the worst conditions, it can be safely distinguished from the first-order shape. Appreciable deviations from the assumed simple model with constant E d and k d throughout a significant desorption range evidently impair the estimate of the desorption order and may make its value quite ambiguous. The results of the estimate of the desorption order from the peak shape can be corroborated by the behavior of the peak location with varying the initial coverage. As seen from Eq. (28), the peak temperature T, does not depend on the initial coverage-provided other conditions remain unchanged-for the first-order desorption, whereas for the second-order case it shifts about logarithmically towards higher T, values as the initial coverage decreases, and with the third-order kinetics this shift is still stronger. Again, it should be mentioned that serious variations of E d and/or k d with the coverage can largely invalidate the conclusions achieved.
376
MILO6 SMUTEK ET AL.
Even if the peak behavior fits well for a given apparent desorption order, the real kinetic situation may be a different one. As a rate controlling step in a second-order desorption, random recombination of two particles is assumed most frequently. However, should the desorption proceed via a nonrandom recombination of neighboring particle pairs into an ordered structure, the resulting apparent first-order desorption kinetics is claimed to be possible (36). The term pseudo-first-order kinetics is used in this instance. Vice versa, second-order kinetics of desorption can appear for a nondissociative adsorption, if the existence of a dimer complex is necessary before the actual desorption step can take place (99). A possibility of switching between the apparent second-order and first-order kinetics by changing the surface coverage has also been claimed (60, 99, 100). The first-order and second-order kinetics of desorption are by far the most common and practically considered cases. Less than first-order desorption kinetics indicates multilayer adsorption or transport limited desorption (101). An actual significance of the third-order kinetics in desorption has been found recently by Goymour and King (102, 103). 2 . Activation Energy of Desorption and the Preexponential Factor a. Utilization of the Peak Location For estimates of both Ed and kd in the Arrhenius equation, in principle two different points on a desorption peak or two runs with different heating factors a, are required. One obvious point is the maximum of the peak, and very often only this is used while the value of kd is supposed to be of the order of magnitude 1OI2 to 1013sec-I. As seen from Eq. (28) , the location of T, depends but weakly on kd as compared to its dependence on Ed, so that an uncertainty in the value of kd of one order of magnitude does not affect the estimated value of Ed appreciably. This has been clearly illustrated by analogue simulation of the thermodesorption processes (104). On the other hand, the said fact causes the estimates of k d to be very uncertain. A recently published computational analysis of the peak location behavior shows the accuracy of the obtained values of Ed (105). In estimating the value of Ed by means of the transcendental equations (28),the circumstance utilized is that the variation of em for a given change in T, is much less than the variation of exp(e,) (31). Until now, only particular solutions have been available for the hyperbolic and linear heating schedules and for the first-order and second-order desorptions. They can be found for example in the fundamental papers by Redhead (31) and Carter (32) or in the review by Contour and Proud’homme (106), and therefore will not be repeated here. Recently, a universal procedure for the
377
THERMAL DESORPTION
evaluation of Ed from the location of T, on the dn./dT vs T curve has been developed (96). It makes feasible a rapid estimate of Ed for any heating schedule and desorption order x and will be outlined in the following text. The procedure is based on the expression u = log em exp (em) =
log(kdTm/arn)
(z - 1) lOg(n.o/~at)
+ log[(em + xz)/(ern + z ) ] .
(45) The last term is generally so small that it can be replaced to a good approxiz ) . As shown in the above-mentioned mation by 0.43429(z - l)z/(e, paper (96),in the range 10 < em < 35 the following holds:
+
Ed
10-3(4.35737~- 3.5907)Tm.
(46)
Substituting for u from Eq. (45) and rearranging, we have 0.82405
log(am/Tm)
=
log k d - (0.22950/Tm) Ed
+ (x - 1) {log(n.o/n.t)
4-[0.434292/(em
+z)]}. (47)
For known or experimentally established values of the temperature T, a t the peak maximum, of the degree of the initial surface coverage n.o/net, of the temperature gradient a, = ( d T / d t ) , at T,, and of the parameter z characteristic for the heating schedule applied (see Section 1V.A) , and with a rough estimate of em in the correction term in Eq. (47), this equation represents a linear equation in Ed,(x - 1), and log k d . Thus, three experiments with sufficiently differing values of a, and nEomake it feasible to estimate the three kinetic parameters required. Of course, the accuracy of the results obtained depends on the precision of the input data. If more experiments are available, a statistical approach is possible, permitting the estimate of confidence intervals for the solved quantities. The procedure follows approximately the same lines as described in Section V.A. If the estimate of z gives unambiguously an integral order of desorption, this value will be adopted and the procedure repeated with the last term in Eq. (47) transferred to the left-hand side of this expression. If no reliable estimate of the density nStof the adsorption sites is available, then an estimate of k d is possible for the first-order desorption kinetics only. The actual temperature gradient a , at T, should be preferred to the assigned value of as. Their interchange is, however, easy, as a, = a,T:-*). If more than one peak appears on the dn,/dT vs T curve, the value of nSoshould be known for each peak separately. This can be achieved either by integrating the respective peak area or by deriving ns0from the peak height. The latter ap-
378
MILO;
SMUTEK ET AL.
proach is described in Section V.C.2.b and requires in general an iterative procedure. The former approach is straightforward. In the course of numerical integration, the desorbed amounts a t given time points and thereby a t corresponding temperatures of the sample are evaluated. Thus we obtain the pairs of dn./dt and n,(T) estimates, all that is needed for the direct analysis of the desorption mechanism according to the procedure in Section V.A. This method requires the minimum of assumptions and reveals the deepest insight into the desorption process, and therefore it is to be preferred. The analytical procedure outlined in this section should serve then only to corroborate the obtained results. For a rapid orientation in the kinetic parameters of a desorption process, the diagrams given in Fig. 3. can be used. The chart (a) gives directly the E d estimate for x = 1 and an adopted value of k d . The T , lines are spaced approximately equidistantly which permits an easy interpolation to the observed T , values for at least three runs with different a, (or a,) and 00 values. The experimentally established T , lines are drawn into the chart. Then a diagram as in Fig. 3(b) is drawn on transparent paper, tailored to the experimental values of a, and eo with the corresponding Tm values. The ordinate axis should match the log-ordinate axis of the chart; the scale of the abscissa axis is irrelevant. Then the transparent paper is shifted (its axes matching those of the chart) until its lines cross the corresponding T , lines on the chart on approximately the same vertical: its position on the chart gives the estimate of Ed, on the transparent paper it indicates the order of desorption x. The value of the ordinate of any cross-point on the chart serves to estimate log k d , by adding to the read-out value the corresponding term log a, - (x - 1) log e0. The range of the E d and 2 values which exhibit a reasonable fit indicates the accuracy of the estimates. The estimate of log k d depends moreover on the reliability of the value nBt.If no reasonable fit is possible, the assumption of constancy of the kinetic parameters in the desorption process has not been met. The chart clearly shows that the smaller the differences in the input values of a m and of n.0, the broader the range of a reasonable fit. Since Eq. (47) is in essential logarithmic, differences of about one order of magnitude in a, and in nd are necessary for good results. b. Utilization of the Peak Height Estimates of Ed can be obtained also from the height of the desorption peak. From Eqs. (32) and (33) we get
379
THERMAL DESORPTION
Ed (kcal /mole) (a )
I
~ogl~.m/amm).l
-2
----.--
,
log (al,,,/asml
,ml\
t
log (0,/9,1 (conditions a,m ,@J.)
(b)
FIG.3. Method for rapid graphical estimates of Ed, kd, and x. For an explanation, see the text.
Again, first E d is calculated without taking the term with tm into account. The resulting estimate of E d is used to evaluate the said term and then the calculation of Ed is improved by solving the complete equation (48). Since k d is not involved in this treatment, the resulting value of E d can be used in principle for an estimate of k d from Eq. (47), for example by revers-
380
41ILOi SJIUTEK ET AL.
ing the outlined graphical procedure. The accuracy of the estimates is much more affected by the accuracy of the quantities a, and nBothan in the case of the peak location analysis. Any distortion of the peak shape due to variations in E d and k d will also have an effect on the peak height N , = - (dn,/dt),. Thus, essentially only a rough estimate of k d can be obtained. c. Utilization of the Peak Shape The third independent possible estimation of E d is given by an analysis of the peak shape. To this end, the hyperbolic heating schedule is most convenient. According to Eq. (38), the width w of the peak at a given value of the relative rate of desorption p and for a given desorption order is a universal constant in the escale: w ( p ) = el - rz = f ( p ) , which leads directly to E d =
Rf ( p ) TiTz/(Tz - Ti).
(49)
The analysis can be performed for several values of the relative peak height using the appropriate values of f ( p ) taken from Table I. Thus several estimates of E d are obtained and either an average of them is calculated with its standard deviation, or provided a dependence of E d on p is encountered, conclusions on the variability of E d with coverage are drawn. As an alternative, only the half-widths of the peak are treated, as long as the experimental data are not distorted by some adjacent peak. Obviously, more information is obtained in such a case. The ratios of the half-widths taken at various values of p and compared with those given in Table I represent a criterion for the fit of the value of the desorption order. Since the estimates of E d are free of contributions of k d , the T, relations can be used to estimate grossly the value of k d , similarly as in Section V.C.2.b. p,
VI. Effects of the Surface Heterogeneity and of the Surface Coverage
As yet, only energetically homogeneous surfaces and activation energy independent of the surface coverage have been considered in our discussion. Most real surfaces, however, are energetically heterogeneous. Moreover, even on a homogeneous surface, adsorption states can occur whose formation depends on the degree of coverage (34, 83, 99, 107, 107a). Therefore, the desorption rate versus time curves may exhibit several peaks which are more or less separated. Analysis of such desorption spectra is based on the adoption of plausible models of the energetic structure of the system, with subsequent comparison of the predicted and experimental results.
THERMAL DESORPTION
381
The effect of simultaneous desorption from different adsorption sites on the shape of desorption peaks is dealt with in Sections V1.A and V1.B. In real systems, moreover, an intrinsic change in the peak shape as compared to the ideal one can occur, caused by variations in the adsorption energy and/or entropy with coverage. The variations in adsorption energy are due to lateral interactions between the adsorbed species, and possibly also to changes in the properties of the adsorbate (induced heterogeneity). The variation of the adsorption entropy and hence of the preexponential factor result from changes in surface mobility of the adsorbed species with the surface coverage and temperature, as mentioned in Section 1I.A. The treatment of these problems is covered in Section V1.C. A. DISCRETE DISTRIBUTION OF THE ACTIVATION ENERGIES OF DESORPTION ON THE SURFACE Let us consider a surface on which particles are adsorbed on sites with different activation energy of desorption, and the distribution of these energies over the surface is discrete so that nio particles are initially in a state with an activation energy of desorption Ed;,njo particles with an energy Edj, etc. Such a model corresponds to a concept of adsorption on different crystal planes each of which is homogeneous, or to a concept of different adsorption states of the particles adsorbed on a single crystal (26, 88). If the individual peaks in the curve of (dn,/dt) vs 1 or T are sufficiently separated and sharp so that they do not interfere appreciably, it is possible to analyze each peak separateIy by employing the foregoing relations derived for a homogeneous surface with Ed and kd independent of 0. Utilizing the analogy between optical and desorption spectra, Carter (32) applied the well established practical criterion for optical resolution to the desorption spectra. Accordingly, two adjacent peaks of equal height resulting from first-order desorption processes are considered as well resolved if the minimum of the valley formed between them is at least 20% less than the peak heights. A discussion of this approach was given by Czanderna et al. (108) who used for this purpose symmetrical Gaussian.curves which are suitable for approximating a second-order desorption. In terms of the desorption rate, the said criterion means that the rate of each of the desorption processes is, in the valley, equal to I/e = 37% (according t o Carter), or to 40% (according to Czanderna et al.) of the maximum rate. The distance of the peaks on the temperature axis is then given by
-
(Te
- T m ) + (Tm' - T:) 5 Tm' - T m
(50)
382
MILO;
SMUTEK ET AL.
T,
Te-Th
Thl
- - T
FIQ.4. Conditions for resolution of two adjacent peaks on a desorption curve according to Carter (98). T, and T,’ are the temperatures at maximum desorption rate for the first and second peak, respectively. T. and T,’are the temperatures at maximum desorption rate for the first and second peak, respectively. T. and T,‘are the temperatures a t l/e = 37 per cent of the maximum desorption rate for the first and second peak, respectively.
(see Fig. 4). The peak location, i.e. the value of T,, depends primarily on It is advantageous to express the condition for resolution of two nondistorted peaks by means of Ed and AEd (A& is the difference between the activation energies of desorption for the two subsequent desorption processes). The resulting expression for the resolution is given by Carter (32) as Ed/AEd = (0.91 - 1.8K1)/3K’. (51) Ed.
This equation was derived for a hyperbolic heating schedule. By substituting for K’ an empirical value of 1.5 X 10-2, the resolution turns out to be about 20 (31, 32). For a linear heating sweep, the use of an empirical value of K‘ = 1.7 X 10+ gave a resolution of about 16 (31, 32). Thus at Ed = 60 kcal/mole, differences of 3-4 kcal/mole are detectable and differences of 8 and 5 kcal/mole are adequately resolved (31, 3 2 ) . In his paper (109) Carter extended the criterion of resolution from the paper (32) to any order of desorption both under the linear and hyperbolic heating rate. Conclusions which are in agreement with the findings for the first-order case were obtained, viz. that the resolution decreases both as Ed and the heating rate increase, and further that an increase in initial surface population leads also to an increased resolution (except for the first-order desorption where the resolution is coverage independent). These criteria of resolution were deduced with the restrictive assumptions that the two peaks have the same height, i.e. that neOl= n.02, and that the
383
THERMAL DESORPTION
relation between Ed and T, is very nearly linear (31).In actual desorption spectra, however, adjacent peaks of different heights are usually encountered. As mentioned in Section V, T, in a first-order desorption is independent of the initial surface population n.0 and the shape of the - (dn,/dt) vs T curve is asymmetrical about T,. Figure 5 shows that the rising part of the peak is less steep than the falling part. In a case of two closely adjacent peaks of first-order desorptions, the contribution of the AEdi to the resulting over-all curve is second peak with energy Edi higher than the contribution of the preceding peak with energy Edi. The calculated model peaks in Fig. 5 fulfill Carter’s condition of a minimum separation needed for an adequate resolution and they have the same heights, i.e. the same initial populations %oi. In spite of this, the resulting desorption rate versus temperature curve (curve 1) is distorted since, due to the asymmetry of the peaks, a shifting of the maximum of the first peak to a higher temperature occurs. The curves 2 and 3 in Fig. 5 refer to analogous cases if the ratios of the initial populations amount to 1.54 and 1.85, respectively. The effect of the peak height on the resolution and on the shift of Tmiis shown. Thus, Edi determined from such a spectrum does not fully
+
[%) d
t
2 t
M,
1.56
1&6
136
126
IO’/TPK)
FIG.5. Effect of the initial populations ndi on a normalized desorption rate curve [criterion of resolution employed after Carter (SS)].Hyperbolic heating schedule, fl = 1.073 X deg-1 sec-1; Ed, = 40 kcal mole-’; z = 1; T, = 670°K; AT, = 15°K. Curves 1, 2,3,correspond to nam/neol= 1.00,1.54,1.85,respectively.
384
MILOB
SMUTEK ET AL.
coincide with the actual value, the difference, however, being small. For example, with a linear heating rate of 50 deg sec-I and with T, = 670°K, a shift of E d amounts to about 1%,i.e. from 40 to 40.5 kcal/mole. The model of discrete distribution of activation energies of desorption is often employed even for the analysis of desorption curves which by no means satisfy the mentioned criterions of resolution. An example of the procedure applied in such cases has been described by Winterbottom (110).The experimental P ( t ) curve is approximated by a superposition of theoretical single state curves, each characterized by a simple desorption mechanism. First a desorption curve for a single surface state is calculated by varying the relevant parameters until an approximate fit of the initial rise portion of the experimental P ( t ) curve is obtained. A first estimate for E d 1 results from the fitting procedure with assumcd values of k d l = loi3 sec-' and x = 1. Adjustment of the peak amplitude is accomplished by . calculated curve for the first desorption process PI theor(t) varying 1 ~ ~ 0 1The is then subtracted from the experimental desorption curve P ( t ) giving a new curve P'(1) which results from the overlap of the remaining desorption states. The location and further parameters of the second and all other peaks are tentatively found in the same manner until the whole desorption curve is approximately reproduced. In subsequent approximations more precise values for E d , k d , and x are determined and the fitting is improved until a reasonable agreement between theoretical and experimental desorption curves is achieved. It is convenient in'the case of complex spectra to simultaneously analyze several P ( t ) curves obtained under different heating rate, pumping speed, and initial surface coverage. In this way six surface substates were revealed by Winterbottom (110) for the beta phase of CO on polycrystalline tungsten. However, the reliability of conclusions resulting from this procedure is in general questionable, because its uncertainty increases with the increasing complexity and limited resolution of the desorption spectrum and with the increasing number of adjustable parameters used. This has been shown recently in an excellent paper by Pisani, Rabino, ) means of the statistical treatment of experimental data and Ricca ( 6 6 ~by for two poorly resolved peaks. Equally good fits were obtained with two simple alternative models largely differing from each other in the relative surface concentrations, E d and k d , of the two assumed surface species.
B. CONTINUOUS DISTRIBUTION OF ACTIVATION ENERGIES OF DESORPTION Let us consider that particles are adsorbed on surface sites whose activation energies of desorption form a continuous spectrum between certain limits. The problem now consists of finding the distribution of initial surface populations nsOiaccording to the energies E d i .
385
THERMAL DESORPTION
An analysis of the rate of release of adsorbed atoms from sites with a continuous energy spectrum for the case of an arbitrary distribution function of initial site populations was given by Carter ( 3 2 ) .The rate equation for the ith desorption process with x = 1 and negligible readsorption is
dn.i/dt = -kdneoi exp ( - Edi/RT).
(52)
The solution of Eq. (52) gives %i
=
n,oi exp[ -kd
[exp( -Edi/RT)dt1.
(53)
If the initial population naoiis an unknown function of Edi,the number of particles occupying the sites with energies between the limits Ed; and Edi AEdi is
+
The number of moles n, adsorbed on a unit surface with energies between Ed min and &i max is given by
Ld
Ed max
=
min
f(Edi) eXp[ -kd [ e x p ( - & i / R T ) d t
I
dEdi.
(55)
The desorption rate at any instant equals
1
Ed mas
dn,/dt =
-kd
f(Edi) exp( -Edi/RT)
Ed min
The required distribution of initial populations nsOican be obtained in the following manner ( 3 2 ) .Let us consider a system with Ed ni, = 20 kcal/ mole and Ed max = 45 kcal/mole. Assuming that kd = 1013sec-' and z = 1, we can calculate theoretical desorption rates dnSi/dtfor Ed = 20,21,22, . . ., 45 kcal/mole as a function of nsoi.With increasing temperature, 25 values of dn,/dt are measured at temperatures corresponding to Ed of 20, 21, 22, . . ., 45 kcal/mole. Since the total desorption rate at any moment must be equal to the sum of the individual desorption processes, we obtain 25 linear equations. Their solution permits the computation of the initial populations of the surface sites in the energy spectrum considered, i.e. the function n,oi(Edi). From the form of this function, desorption processes can be determined which exhibit a substantial effect on the experimental desorption curve.
386
MI LO^
SMUTEK ET AL.
An analysis of desorption for the case where the surface sites with a continuous energy spectrum have a uniform initial population was given by Grant and Carter (111) and by Erents, Grant, and Carter (112). In the paper (112), moreover, population distributions slowly varying with energy were considered. Methods based on an expansion into series were suggested for obtaining the initial population spectrum from a total desorption rate. From the model calculations performed it follows that the desorption rate from an initially uniform site population is, over a wide range of heating rates, independent of temperature but increases linearly with the heating rate, if a linear heating schedule is applied; for a hyperbolic heating schedule the desorption rate increases linearly with both temperature and heating rate. This indicates the necessity for carefully defining the heating schedules to determine the initial population density by the methods mentioned.
C. ACTIVATION ENERGY OF DESORPTION AS A FUNCTION OF THE SURFACE COVERAGE Some experimental desorption spectra can be fitted with the calculated curves only if the assumption of constant values of Ed and k d for all peaks is abandoned, and Ed and/or k d are considered to be functions of the coverage. Let us consider that Ed corresponding to a peak on the desorption curve is coverage dependent, while k d (and thus the adsorption entropy) remains constant. (For the variability of k d see Section 1I.A.) When seeking the required function Ed(0) we refer to Eq. (8) in which the term exp( -Ed/ R T ) exhibits the greatest variability. A set of experimental curves of the desorption rate with different initial populations nnomust be available. When plotting In( -dn./dt) - z ln(n.) vs 1/T, we obtain the function E d (n.) from the slope, for the selected n. as has been dealt with in Section V. In the first approximation which is reasonable for a number of actual cases, let us take a simple h e a r variation of E d with n. =
- An,
(57) (X is a constant). By combining Eq. (57) with Eq. (8) and neglecting the readsorption, we have Ed
Ed0
-dne/dt = ‘ % t , k d ( ~ a / n . t ) ” exp [-
- Xn,)/RTI.
(58) McCarroll (94)has demonstrated for the range of mild variations of E d with n. that the two most noticeable effects as X increases are a decrease in T , and an increase in the peak broadness. For the range of a strong dependence of E d on n., Hobert and Knappe (104) have visualized that with (Ed0
THERMAL DESORPTION
387
increasing X, the peaks become severely elongated, which effect becomes the more pronounced, the higher the X is. The paper by Dawson and Peng (98) can be quoted as an example of applying Eq. (58) to a kinetic analysis of both the first-order and secondorder desorptions with an activation energy varying linearly with the surface coverage. Hansen and Mimeault (SO) have mentioned two ways to determine the value of X. In the first method, Eq. (57) is solved for X at the point of inflection (i.e. d2n/dt2 = 0) of desorption curves with high initial coverages, using values of E d 0 and k d from low initial coverage experiments. More accurate is the second method in which the values of E d 0 and k d are again established from low coverage experiments. Using a computer to integrate Eq. (58) numerically, X is varied systematically until the sum of the squares of the differences between the calculated and experimental values of n, a t a set of values of t passes through a minimum. The parameters E d o and X can also be obtained by a simple graphical method described by Hansen and Matsushita ( l l b a ) . Their procedure, performed for a second order desorption and hyperbolic heating schedule, can be extended to other cases. Values of X for hydrogen peaks on individual crystal planes of tungsten and molybdenum were found to be about 1-3 kcal/monolayer (201).A stronger dependence of Ed and n, resulted for nitrogen on tungsten (98). The simple function (57) is based on the logarithmic Temkin adsorption isotherm which assumes a linear decrease in the heat of adsorption with increasing surface coverage, resulting from a uniform distribution of adsorption sites according to the individual values of E d ; . Yakerson el al. (85) considered some other models of surface heterogeneity as, for example, an exponential distribution of heats of adsorption on the surface sites leading to E d
=
Ed0
-vhle
(59)
( Y is the exponent in the power adsorption isotherm which corresponds to this distribution). None of the procedures outlined can claim any strict justification. Indeed, the deviations of experimental curves from the calculated ones based on simple assumptions can be due in general to a number of causes, some of which were dealt with in Section 1I.A. A principal ambiguity lies in the choice of whether to treat such departures in terms of either variable E d or k d , and in the former case often whether the changes in E d are to be attributed to nonequivalence of adsorption sites, or to lateral interactions between the adsorbed particles, or to yet some other factor (98).
388
hlILO6 SMUTEK ET AL.
With porous materials, a slow diffusion in the pores can sometimes control the rate of desorption. This may give rise to complications because diffusion in the pores may be complex and difficult to treat mathematically. Cvetanovid and Amenomiya (48) gave a model treatment for their modification of the thermal desorption technique.
VII. Conclusion As has been discussed in the preceding sections, the shape of a desorption curve depends on a number of different factors. Thus, it is in general hardly possible to estimate the required kinetic parameters of the desorption process considered, i.e. Ed,2, and k d unambiguously from a single desorption curve. Varying of the heating rate, flow rate, pumping speed, and initial coverage in the studied system is in most cases of essential importance for a reasonable reliability of the results obtained. A correct handling of the experimental data is naturally but a prerequisite for solving the proper problem, which is an interpretation of the physical meaning of the desorption curve and of the kinetic parameters extracted from it. For a long time, a distinct adsorption state was rather mechanically ascribed to each of the peaks in a desorption spectrum. As the experimental data on different adsorption systems were gradually accumulated, a series of peaks was encountered even in a number of systems which were expected to be rather simple. In recent years, a rapid expansion of work with single crystals occurred in adsorption studies. Because rather complex desorption spectra have been obtained frequently on single crystal faces too, the traditional preferential interpretation of multiple peaks in terms of heterogeneity of the investigated surface due to its polycrystalline character has become questionable. This situation has stimulated a search for other explanations, as for example the interconversion between adsorbed states upon heating (37, 60, 98, 107), order-disorder transition during heating (36,60), induced heterogeneity (107),bulk solution or incorporation of the adsorbate into the solid lattice (107, 113) , and interaction between chemisorbed species (36,37,103,107,114-116). A quantitative treatment based on the following approach has been recently given to the idea of explaining the multiplicity of desorption spectra by the existence of different desorption mechanisms rather than by different adsorption states (98, 117). Consider a surface on which an adsorption equilibrium has been established at a given temperature. On heating the surface, desorption occurs, the probability of which is composed of at
THERMAL DESORPTION
389
least two successive probabilities : a probability of migration from a state with higher adsorption energy to a state with lower adsorption energy, and a probability of subsequent desorption from this state, with an activation energy of Ed'. The overall probability of desorption then equals the product of these two probabilities, and for the overall activation energy of desorp4- Ed'. The dependence of the desorption tion we have E d = Emigration mechanism on the mobility of adsorbed particles increases with their surface density. Thus the variation in the particle translational mobility with the particle concentration plays an important role in the desorption mechanism (98, 117 ) . Similarly, this applies also to the preexponential factor, particularly for second-order kinetics. Recently, a quantitative lateral interaction model for desorption kinetics has been suggested (103).It is based on a statistical derivation of a kinetic equation for the associative desorption of a heteronuclear diatomic molecule, taking into account lateral interactions between nearest-neighbor adatoms in the adsorbed layer. Thereby a link between structural and kinetic studies of chemisorption has been suggested. In all probability, further attempts at elucidating the physical background of the phenomenon of multiple desorption spectra will appear in the near future. The outlined complexity of the analysis of the thermal desorption data and the resulting possible ambiguity of the conclusions deduced make a correlation with the results of different experimental techniques highly desirable. From a great number of examples of benefit brought by such an approach, only a few illustrative instances will be mentioned: a correlation with isotope exchange studies (34, 82, 83, 98) ; with sticking coefficient determination (82, 83, 107); with the adsorption of other gases used as a chemical probe for the study of adsorbed species (36,115) ; with the work function changes (2, 36, 98, lo?'); with LEED patterns (34, 113); with field emission measurements (118-1 20) ; with infrared spectroscopy (121, 162); with electron stimulated desorption (37, 106, 116, 117, 120, 123, 124) ; and with ion induced secondary ion mass spectroscopy (124). During the past twenty years, thermal desorption has become one of the most important methods in the investigation of adsorption phenomena. Both the experimental technique and the procedures of the data treatment have been considerably developed. In the future, it is likely that an ever growing emphasis will be laid on a deeper understanding of the detailed mechanism of the desorption processes and thereby of the actual physical meaning of their characteristic kinetic parameters. This will undoubtedly bring a new progress in the difficult but attractive field of investigation of the physical chemistry of solid surfaces.
390
MILO6 SMUTEK ET AL.
LIST OF SYMBOLS surface area of the adsorbent h a ) general parameter for heating schedules, characterized by the quantity 2 (see Section 1V.A) activation energy of adsorption (kcal mole-') activation energy of desorption (kcal mole-') activation energy of desorption for the ith desorption process (kcal mole-') exponential integral defined by Eq. (22) base of natural logarithms rate of gas flow into the system (mole see-'); an index may denote the component contributing to the total flow rate probability of transition of an impacting particle into the adsorbed state probability that the adsorbed particles are in a configuration favorable for their recombination liberated heat of adsorption (kcal mole-') equilibrium constant of adsorption preexponential factor of the equilibrium constant equilibrium constant of adsorption a t T = T, Boltzmann constant preexponential factor in the rate equation of adsorption preexponential factor in the rate equation of desorption (sec-1) rate constant of desorption rate constant of desorption in the maximum of the desorption rate frequency of attempts of a particle to recombine and escape from the surface Avogadro number
number of moles of a gaseous adsorbate at the time 1 = 0 (No = Anago) maximum desorption rate (mole s-1 cm-)) total number of moles per unit surface, which are required far an effective monolayer at the temperature T number of adsorbed moles on a unit surface (mole cm-2) number of adsorbed males on a unit surface at the time t = 0 number of moles of a gaseous adsorbate involved in the adsorption (n. = sn.,); the index g denotes that the adsorbate is in the gas phase number of adsorbed moles on a unit surface required for occupation of all adsorption sites number of moles of a gaseous adsorbate which will be required for occupation of all adsorption sites on a unit surface (n,t., = zn,t) pressure (Torr) stationary pressure a t the time t = 0 (Torr) partial pressure of the adsorbable component at the time t (Torr) partial pressure of the adsorbable component a t the maximum desorption rate (Torr) pressure a t the maximum desorption rate (Torr) gas constant effective rate of adsorption (mole sec-1 cm-2) effective rate of desorption (mole sec-1 cm-2) maximum effective rate of desorption (mole sec-1 cm-2) pumping speed (mole sec-1) pumping speed at the time 1 = 0 (mole sec-1)
THERMAL DESORPTION
T To
T,
temperature of the adsorbent at the time t (OK) temperature of the adsorbent a t the time t = 0 (OK) temperature of the adsorbent a t the maximum desorption rate
B 7 E
(OK)
t
V X
Y
Yl
time from the beginning of heating the adsorbent (sec) volume of the system number of particles into which a molecule decomposes upon adsorption; under the defined assumptions, also the kinetic order of the desorption process number of particles which recombine on the surface before desorption; in the accepted model, y = 2 defined parameter [y’ = exp(c,
7
e
x cc
- e\l
- I ,
2
ff
parameter defining the heating schedule (see Section 1V.A) proportionality coefficient of a linear heating (deg sec-1)
Y
391
proportionality coefficient of a hyperbolic heating (deg-1 sec-l) proportionality coefficient of a hyperbolic heating (sec-l) parameter defined by e = E d / R T ; possible indexes 0 and m refer to To and T,, respectively fraction of the successful attempts at desorption from the total number of attempts degree of the surface coverage (e = n./nSt);indexes 0 and m refer to t = 0 and to the maximum desorption rate, respectively proportionality coefficient in the function expressing a linear dependence of E d on 8 normalized desorption rate referred to the maximum desorption rate exponent in the power isotherm of adsorption; number of molecules adsorbed on 1 cm*
ACKNOWLEDGMENT6 We express our thanks to Drs. Z. Knor and Z. Bast1 for_stimulating critical discussions on the manuscript. In addition, we are grateful to Dr. V. Cerm&kfor kindly enabling one of us (M. S.) to take time off for the work on the present contribution. REFERENCES 1. Merta, R., Collect. Czech. Chem. Cammun. 36, 1504 (1971). 2. Madey, T. E., Surface Sci. 29, 571 (1972). 3. Menzel, D., Angew. Chem. 9, 255 (1970). 4. Madey, T. E., and Yates, J. T., Jr., J. Vac. Sci. Technol. 8, 525 (1971). 6. Leck, J. H., and Stimpson, B. P., J . Vac. Sci. Technol. 9, 293 (1972). 6. Benninghoven, A., Surface Sci. 28, 541 (1971). 7 . Benninghoven, A,, Appl. Phys. 1, 3 (1973). 8. Newsham, I. G., Hogue, J. W., and Sandstrom, D. R., J. Vac. Sci. Technol. 9,596 (1972). 9. Miiller, E. W., and Tsong, T. T., “Field Ion Microscopy.” Amer. Elsevier, New York, 1969. 10. Menzel, D., Kronauer, P., and Jelend, W., Ber. Bunsenges. Phys. Chem. 7 5 , 1074 (1971). 1 1 . Kronauer, P., and Menzel, D., in “Adsorption-Desorption Phenomena” (F. Ricca, ed.), p. 313. Adademic Press, New York, 1972. 18. PaignB, J., J. Chim. Phys. 69, 1 (1972).
392
MILOB
SMUTEK ET AL.
de Ribaupierre, Y., Surface Sci. 34, 732 (1973). McAlliater, J. W., and White, J. M., J. Chem. Phys. 58, 1496 (1973). Denison, D.R., J. Vac. Sci. Technol. 6, 214 (1969). Gomer, R., “Field Emission and Field Ionization.” Harvard Univ. Press, Cambridge, Massachusetts, 1961. 17. Ehrlich, G., Trans. 8th Nat. Vm. Symp., Vol. I, p. 126.Pergamon, Oxford, 1962. 18. Hayward, D. O., and Trapnell, B. M. W., “Chemisorption.” Butterworth, London, 1964. 19. Urbach, E., Sitzungsber. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Abt. 2A 139,365 (1930). 20. Urbach, E., in “Solid Luminescent Materials” (G. R. Fonda and F. Seitz, eds.), p. 115.Wiley, New York, 1948. 21. Apker, L., I n d . Eng. Chem. 40, 846 (1948). 22. Hagstrum, H. D., Rev. Sci. Instrum. 24, 1122 (1953). 23. Ehrlich, G.,J . Chem. Phys. 23, 1543 (1955). 24. Ehrlich, G.,J . Phys. Chem. 60, 1388 (1956). 25. Smith, A. W., and Aranoff, S., J . Phys. Chem. 62, 684 (1958). 26‘. Hickmott, T.W., and Ehrlich, G., J . Phys. Chem. Solids 5,47 (1958). 27. Ehrlich, G., J . Appl. Phys. 32, 4 (1961). 28. Ageev, V. N., Ionov, N. I., and Ustinov, Yu. K., Zh. Tekh. Fiz. 34, 546 (1964). 29. Ehrlich, G., Advan. Catal. 14, 255 (1963). 30. Hansen, R.S., and Mimeault, V. J., in “Experimental Methods in Catalytic Research’] (R. B. Anderson, ed.), p. 221.Academic Press, New York, 1968. 31. Redhead, P. A., Vacuum 12, 203 (1962). 32. Carter, G.,Vacuum 12, 245 (1962). 33. Rigby, L. J., Can. J . Phys. 42, 1256 (1964). 34. Tamm, P. W., and Schmidt, L. D., J . Chem. Phys. 51, 5352 (1969). 35. Peng, Y.K., and Dawson, P. T., J . Chem. Phys. 54,950 (1971). 36. Yates, J. T., Jr., and Madey, T. E., J. Vac. Sci. Technol. 8,63 (1971). 37. Yates, J. T., Jr., and King, D. A., Surjace Sci. 32, 479 (1972). 38. McCarroll, B., J. Chem. Phys. 46, 863 (1967). 39. Ptushinskii, Yu. G., and Chuikov, B. A., Surface Sci. 6, 42 (1967). 4ff. Ageev, V. N., and Ionov, N. I., Zh. Tekh. Fiz. 39, 1523 (1969). 41. Ageev, V. N., and Ionov, N . I., Zh. Tekh. Fiz. 38, 1149 (1968). 42. Gibson, R., Bergsnov-Hansen, B., Endow, N., and Pasternak, R. A., Trans. Nut. Vacuum Symp. 10, 88 (1963).. 43. Pasternak, R. A., and Gibson, R., A d a Met. 13, 1031 (1965). 43a. PBtermann, L. A., in “Progress in Surface Science” (S. G. Davison, ed.), Vol. 3, Part 1, p. 1. Pergamon, Oxford, 1972. 44. Redhead, P. A., Hobson, J. P., and Kornelsen, F,. V., “The Physical Basis of Ultrahigh Vacuum.” Chapman & Hall, London, 1968. 45. King, D. A., Surjace Sci. 9, 375 (1968). 45a. Savchenko, V. I., and Boreskov, G. K., Kinet. Katal. 9, 142 (1968). 45b. Erhardt, J. J., Vincent, L., Cassuto, A., and Malenge, J. P., Surjace Sci. 26, 490 (1971). 45c. Wedler, G., Fisch, G., and Papp, H., Ber. Bunsenges. Phys. Chem. 74,186 (1970). 45d. Wedler, G.,and Borgmann, D., Ber. Bunsenges. Phys. Chem. 78, 67 (1974). 45e. Volter, J., and Procop, M., 2. Phys. Chem. (Leip2ig) 249, 344 (1972). 45j. Benndorf, C., and Thieme, F., 2. Phys. Chem. Neue Folge 87,40 (1973). 45g. Procop, M., and Volter, J., Z.Phys. Chem. (Leipzig)250, 387 (1972). 13. 14. 16. 16.
THERMAL DESORPTION
393
Procop, M., and Volter, J., Surjace Sci. 33, 69 (1972). Smith, A. W., and Quets, J. M., J . Catal. 4, 163 (1965). Gay, I. D., J . Catal. 17, 245 (1970). Hobert, H., Knappe, B., and Weber, I., Ezp. Tech. Phys. 18, 49 (1970). 461. Durm, M., and Starke, K., Vak-Tech. 20, 129 (1971). 46m. Hoinkis, E., Vacuum 22, 525 (1972). 46. Czanderna, A. W., Vac. Microbalance Tech. 6, 129 (1967). 46a. Matsushita, K., and Hansen, R. S., J . Chem. Phys. 52,4877 (1970). 46b. Hansen, R. S., Arthur, J. R., Jr., Mimeault, V. J., and Rye, R. R., J . Phys. Chem. 70, 2787 (1966). 46c. Rye, R. R., and Hansen, R.S.,J . Chem. Phys. 50,3585 (1969). 46d. Cratty, L. E., Jr., and Hansen, R. S., J . Chem. Phys. 57,3661 (1972). 46e. Barford, B. D., and Rye, R. R., J . Vac. Sci. Technol. 9, 673 (1972). 46f. Yates, J . T., Jr., Madey, T. E., and Dresser, M. J., J . Catal. 30,260 (1973). 4668. McCarty, J., Falconer, J., and Madix, R. J., J . Catal. 30, 235 (1973). 46h. Cartier, P. G., and Rye, R. R., J . Calal. 32, 88 (1974). 46i. Yates, J . T., Jr., and Madey, T. E., Surface Sci. 28, 437 (1971). 46j. Dresser, M. J., Madey, T. E., and Yates, J. T., Jr., Surjuce Sci. 42, 533 (1974). 47. Amenomiya, Y., and Cvetanovi6, R. J., J . Phys. Chem. 67, 144, 2046, and 2075 (1963). -48. Cvetanovib, R. J., and Amenomiya, Y., Advan. Cdal. 17, 103 (1967). 49. Cvetanovib, R. J., and Amenomiya, Y., Catal. Rev. 6, 21 (1972). 50. Miller, A. R., Discuss. Faraday SOC.8, 57 (1950). 51. Clark, A., “The Theory of Adsorption and Catalysis.” Academic Press, New York, 1970. 62. Scheer, M. D., Klein, R., and McKinley, J. D., Surjace Sci. 30,251 (1972). 53. Steele, W. A., in “The Solid-Gas Interface” (E. A. Flood, ed.), Vol. I, p. 307. Dekker, New York, 1967. 64. Kim, S. K., J . Chem. Phys. 28, 1057 (1958). 66. Montroll, E. W., and Shuler, K. E., Advan. Chem. Phys. 1, 361 (1958). 56. Benson, S.W., and Fueno, T., J . Chem. Phys. 36, 1597 (1962). 67. Rice, 0. K., J . Phys. Chem. 67, 6 (1963). 58. Kollen, W., and Czanderna, A. W., J. Colloid Interjace Sci. 38, 152 (1972). 59. PBtermann, L. A., Nuovo Cimento, Suppl. 5, 364 (1967). 60. Clavenna, L. R., and Schmidt, L. D., Surjace Sci. 22,365 (1970). 61. Yates, J. T., Jr., and Madey, T. E., J . Chem. Phys. 51,334 (1969). 68. Ageev, V. N., Ionov, N. I., and Ustinov, Yu. K., Zh. Tekh. F ~ z 34, . 2056 (1964). 63. Ustinov, Yu. K., and Ionov, N . I., Zh. Tekh. Fiz. 35, 2099 (1965). 64. Ageev, V. N., and Ionov, N. I,, Zh. Tekh. Fiz. 35,2109 (1965). 65. Degras, D. A., Nuovo Cimento, Suppl. 5, 420 (1967). 66a. Degras, D. A., J . Chim. Phys. 64,405 (1967). 66. Lapujoulade, J., Nuovo Cimento, Suppl. 5, 433 (1967). 66a. Pisani, C., Rabino, G., and Ricca, F., Surjace Sci. 41, 277 (1974). 67. Armand, G., and Lapujoulade, J., Surjace Sci. 6, 345 (1967). 68. Degras, D. A., Le Vide No, 139, 11 (1969). 69. Roginskii, S. Z., Berkovich, M. A., and Shub, B. R., Dokl. Akad. Nauk SSSR 190, 1143 (1970). 70. Dobrzynski, L., Le Vide No. 139,8 (1969). 7 f . Kisliuk, P., J . Phys. Chem. Solids 3, 95 (1957). 72. Kisliuk, P., J . Phys. Chem. Solids 5, 78 (1958). @h. 46i. 46j. 46k.
394
MILO;
SMUTEK ET AL.
73. Ostrovskii, V. E., and Temkin, M. I., Kinet. Katal. 10, 118 (1969). 7 4 . Smutek, M., Collecl. Czech. Chem. Commun. 36,3415 (1971). 75. Earnshaw, J. W., and Hobson, J. P., J. Vac. Sci. Technol. 4, 257 (1967). 76. Hobson, J. P., and Earnshaw, J . W., J. Vac. Sci. Technol. 5 , 19 (1968). 77. Brzoska, K. D., E i p . Tech. Phys. 20, 169 (1972). 78. Durm, M., Stark, G., and Starke, K., Vak-Tech. 21, 111 (1972). 79. Carter, G., Grant, W. A., Farrell, G., and Colligon, J. S., Vacuum 18, 263 (1968). 80. Carter, G., and Armour, D. G., Vacuum 19, 459 (1969). 81. Bell, A. E., and Gomer, R., J. Chem. Phys. 44, 1064 (1966). 82. Kohrt, C., and Gomer, R., J . Chem. Phys. 52, 3283 (1970). 83. Kohrt, C., and Gomer, R., Surface Sci. 24, 77 (1971). 84. Pbtermann, L. A., in “Adsorption-Desorption Phenomena” ( F . Ricca, ed.), p. 227. Academic Press, New York, 1972. 85. Yakerson, V. I., Rosanov, V. V., and Rubinshtein, A. M., Surjace Sci. 12, 221 (1968). 86. Degras, D. A., Nuovo Cimento, Suppl. 5, 408 (1967). 87. Mimeault, V. J., and Hansen, R. S., J. Chem. Phys. 45, 2240 (1966). 88. Redhead, P. A., Trans. Faraday Soc. 57,641 (1961). 89. Pasternak, R. A., Fraser, E. C., Bergsnov-Hansen,B., and Wiesendanger, H. U. D., Rev. Sci. Znstrum. 33, 1320 (1962). 90. Pasta, M., and Soardo, P., Alta Freq. 40, 960 (1971). 91. Graham, H. C., and Triff, W. C . , Vac. Microbalance Tech. 6,63 (1967). 92. Ageev, V. N., Zh. Tekh. Fiz. 40, 1743 (1970). 93. Hill, M. P., Lecchini, S. M. A., and Pethica, B. A,, Trans. Faraday SOC.62, 229 (1966). 94. McCarroll, B., J . Appl. Phys. 40,1 (1969). 95. Kisliuk, P., J . Chem. Phys. 30, 174 (1959). 96. Smutek, M., Vacuum 24, 173 (1974). 97. “Handbook of Mathematical Functions,” Appl. Math. Ser. No. 55, p. 231. Nat. Bur. Stand., Washington, D.C., 1968. 98. Dawson, P. T., and Peng, Y. K . , Surfuce Sci. 33, 565 (1972). 99. Clavenna, L. R., and Schmidt, L. D., Surface Sci. 33, 11 (1972). 100. Schmidt, L. D., i n “Adsorption-Desorption Phenomena” ( F . Ricca, ed.), p. 391. Academic Press, New York, 1972. 101. Schmidt, L. D., J. Vac. Sci. Technol. 9, 882 (1972). 10%’. Goymour, C. G., and King, D. A,, J . Chem. SOC.Faraduy Trans. I , 736 (1973). 103. Goymour, C. G., and King, D. A., J. Chem. SOC.Faraday Trans. I, 749 (1973). 104. Hobert, H., and Knappe, B., Kinet. Katal. 13, 1060 (1972). 105. Lord, F. M., and Kittelberger, J. S., Surjace Sci. 43, 173 (1974). 106. Contour, J. P., and Proud’homme, R., Bull. SOC.Chim. Fr. [6] p. 2693 (1969). 107. Tamm, P. W., and Schmidt, L. D., J . Chem. Phys. 54,4775 (1971). 107a. Adams, D. L., Surjace Sci. 42, 12 (1974). 108. Caanderna, A. W., Biegen, J. R., and Kollen, W . , J . Colloid Interface Sci. 34, 46 (1970). 109. Carter, G., Vacuum 13, 89 (1963). 110. Winterbottom, W. L., J . Vac. Sci. Technol. 9, 936 (1972). 111. Grant, W. A., and Carter, G., Vacuum 15, 13 (1965). 112. Erents, K., Grant, W. A., and Carter, G., Vacuum 15, 529 (1965). Il%a. Hansen, R. S., and Matsushita, K., J . Chem. Phys. 52, 5965 (1970).
THERMAL DESORPTION
395
113. Yonehara, K., and Schmidt, L. D., Surface Sci. 25, 238 (1971). 114. Hill, M. P., Trans. Faraday SOC.66, 1246 (1970). 116. Yates, J. T., Jr., and Madey, T. E., J . Chern. Phys. 54, 4969 (1971). 116. Yates, J. T., Jr., and King, D. A., Surface Sci. 38, 114 (1973). 117. Simon, F. N., Lichtman, D., and Kirst, T. P., Surface Sci. 12, 299 (1968). 118. Plummer, E. W., and Bell, A. E., J. Vac. Sci. Technol. 9, 583 (1972). 119. Ustinov, Yu. K., Fiz. Tuerd. Tela 13, 558 (1971). 180. Goymour, C. C.,and King, D. A,, Surface Sci. 35, 246 (1973). 181. Yates, J. T., Jr., and King, D. A,, Surface Sci. 30, 601 (1972). 128. Nieuwenhuys, B. E., and Sachtler, W. M. H., Surface Sci. 34,317 (1973). 183. Baldwin, V. H., Jr., and Hudson, J. B., J . Vac. Sci. Technol. 8,49 (1971). l@. Benninghoven, A., Loebach, E., and Treitz, N., J . Vac. Sci. Technol. 9,600 (1972)
This Page Intentionally Left Blank
Author Index Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed.
A
Arlman, E. J., 200, 203, 219 Armand, G., 352(67), 993 Armour, D. G., 362, S94 Arnett, R. L., 157(101), 158(101), 159 ( i o i ) , in Arthur, J. R., Jr., 345(46b), 363(46b), 39s Arytyunova, K. M., 175(86), 181(112), 616, 217 Ashby, G. E., 175(10), 193(10), 614 Ashe, B. H., 175(35), 216 Ashleg, C. E., 206(184), 219 Aston, J. G.,2.50(13), 289 Atlas, V. V., 136(34), 137(34), 169 Ayen, R. J., 95, 128 Ayres, R. U., 63(23), 126 Ayscough, P. B., 175(38), 179(38), 216
Abduraimova, M. A., 39(105), 64 Abilov, A. G.,23(75a), 65 Acres, G. J. K., 73(41), 166 Adams, C. T., 145(70), 170 Adams, D. L., 380(107a), S94 Ageev, V. N., 344(28), 345(28, 40, 41), 352(62, 64), 357(28), 358(28), 361, 362(40, 64), 363(28, 62, 92), S96,S93, s94 Ageton, R. W., 81(73), 167 Albizatti, E., 185(117), 187(117), 217 Aleksandrov, I. V., 175(39), 616 Alekseev, Yu. A., 5(26a), 62 Alfani, F., 23(75b), 65 Aliev, V. S., 23(75a), 6S Alkhazov, T. G., 24(87c), 64 Allan, D. E., 24(89), 64 B Allen, W. C., 231(11), 243 Alsdorf, E., 271, 290 Alt, L. Ya., 175(60, 64), 616 Baddour, R. F., 91, 168 Amaro, A., 336, S4S Bagley, R. D., 80(67), id7 Amass, A. J., 132(8a), 168 Bagmanov, Z. A., 23(75a), 65 Amenomiya, Y., 346, 371, 372(48, 49), Bailey, G. C., 131, 134(14), 136(1, 31, 35, 388, S93 36), 137(1, 35, 36), 138, 151, 154(31), Amirazmi, A., 94(97), 128 168,169,17,5(81,82), 816 Anderson, A. W., 206(184), 819 Baker, L. M., 177, 178(110), 917 Anderson, J. R., 286, 291 Baker, R. A,, 97(102), 128 Anderson, R. B., 24(87h), 64 Baker, R. H., 39(108), 40(108), 66 Andrews, L., 329(39), S42 Bakshi-Zade, A. M., 136(34), 137(34), 169 Angell, C. L., 294(1), 323, 324, 335(1), Balandin, A. A., 8(43), 62 336(1), 337(1), 339, 341 Balashov, V. M., 23(68), 65 Baldwin, V. H., Jr., 389(123), 996 Antuf’ev, V. V., 175(25), 214 Anufrienko, V. F.,175(60, 64), 616 Balgord, W. D.,82(75), 168 Apker, L., 344(21), S92 Ballard, D. G. H., 175(9a), 185(9a, 118), 186, 187(9a), 188(9a), 189(9a), 190 Aranoff, S., 344(25), S92 (9a), 21 4,617 Ark, R., 107(123), 229, 163, 271 397
398
AUTHOR INDEX
Balzhiser, It. E., 68(31), 126 Banks, R., 175(3), 214 Banks, R. L., 131, 134(14, 17), 136(1, 31, 35, 36), 137(1, 35, 36), 138, 151, 154 (31), 160(17), 168, 169 Baranowski, B., 247, 249, 250(12), 251 (20a), 252, 274(7), 275(61), 289, 290, 291 Barber, M., 153(95), 171 Barford, B. D., 345(46e), 393 Barinov, N. S., 24(87g), 64 Barnes, G., 96, 128 Barrer, R. M., 246, 289 Bashkirov, A. N., 134(18a), 169 Basner, M. E., 24(87b), 64 Bassi, I. W., 135(23), 169 Basu, A. N., 23(77), 63 Batich, C., 135(26), 169 Bauer, H. J., 251(20), 289 Bauerle, G. L., 79(59), 115(138), 1.27, 129 Bauman, R. P., 299(11), 339(11), 341 Baumann, G., 73(46), 127 Bawn, C. E. H., 175(15), 214 BaBant, V., 26(96), 28(100), 29(96, loo), 30(100), 64 Beale, E. M. L., 28(99), 64 Beerman, C., 184(116), 217 Begley, J. W., 162, 171 Bell, A. E., 362(81), 389(118), 394, 396 Belousov, V. M., 39(111), 66 Benbenek, S., 17.5(31, 41, 42), $14, 216 Bencze, L., 138(53), 139(53), 170 Ben-Efraim, D. A., 132(4), 134(4), 135 (26), 168, 169 Benndorf, C., 345(45f), 362(45f), 363(45f), 392 Benning, C. J., 175( lo), 192(148), 193(lo), 194, 210( 148), 214, 218 Benninghoven, A., 344(6, 7), 389(124), 391, 396 Benson, J. E., 94(97), 128 Benson, S. W., 350(56), 393 Bentley, D. R., 73(45), 80(45). 127 BerAnek, L., 1(7), 12(51), 13(52, 53), 18 (52), 26(51, 95, 97, 98), 28(7, 51, 95, loo), 29(100), 30(100), 31(52, 53), 32(7), 33(51), 34(51), 38(98), 40(97, 98), 41(97, 98, 120), 42(120), 43(95), 44(121), 46(122), 47(123, 124), 48(97), 61, 62,64, 66
Berg, L. G., 284(73), 291 Berger, M. N., 174, 175(1), 213 Bergsnov-Hansen, B., 345(42), 362(89), 392,394 Berkovich, M. A., 352(69), 393 Bernhardt, W. E., 72(39), 97(39), 126 Bernstein, L. S., 73(42, 43), 80(42), 81 (42), 97(43), 126, f27 Beskov, V. S., 7(41a), 24(41a), 62, 175 (95), 181(95), 182(95), 183(95), 216 Bestian, H., 184(116), 217 Bezman, S. A., 148(85a), 170 Biegen, J. R., 381(108), 394 Bier, G., 211(192), 219 Biller, W. F., 115(139), 129 Billmeyer, J. W., 209(190), 219 Bindo, J. S., 3(12b), 8(12b), 24(12b), 61 Bird, P. H., 148(85a), 170 Blanchard, M., 4,61,136(33), 154(33), 169 Blumenthal, J. L., 86(77), 128 Bobrov, A. M., 192(146), 218 Boche&ka, K., 249, 289 Boelhouwer, C., 133(12), 136(32), 138 (55), 139(55), 142(61), 143(63), 144 (61), 147(63), 154(32), 155(32), 158 (105), 169,170, 171 Bogdanovic, B., 185(122), 217 Bol'shinskova, T. A., 168(117), 171 Bond, G. C . , 10, 11(47), 21(45, 46), 62, 264, 265(38), 286(79), 290, 291 Bonzel, H. P., 91, 128 Boocock, G., 174(1), 175(1), 213 Boor, J., Jr., 175(17, 18), 192, 193(153), 195(18), 214, 218 Boreskov, G. K., 72(36), 126, 175(30, 32, 78, 79, 89), 179(89), 183(89), 192 (146a), 21.4, 216, 218, 272(55), 291, 345(45a), 362(45a), 392 Borgmann, D., 345(45d), 392 Borisov, V. V., 23(62), 32(62), 63 Boudart, M., 10(44), 62, 94(97), 128 Boutry, P., 4, 15(16), 16, 30(16), 61 Bovier, C., 287(85), 291 Bradenberger, G., 145(70), 170 Bradshaw, C. P. C., 133(9, 11), 145, 168, 169, 170 Brand, J. C. D., 299(12), 341 Braun, R. M., 157(101), 158(101), 159 (ioi), i r i
AUTHOR INDEX
Breil, H., 175(6), 214 Brenner, W., 5(22), 23(22), 61 Breslow, D. S., 193(149), 218 Briggs, W. S., 80(71), 127 Brill, P., 263, 264, 286(37), 290 Brodowsky, H., 252(23), 259(23), 290 Brown, M., 138(50), 139(50), 170 Brunck, T. K., 149, 170 Brzoska, K. D., 356(77), 394 Bucka, H., 24(87a), 64 Budin, J. P., 310(18), 341 Buechler, E., 294(2), 321, 324(2), 335(2), 336(2), 337(2), 338, 341 Bukanaeva, F. M., 175(26,30,32), $14 Bukatov, G. D., 193(151, 156, 157, 158 159), 194, 196(157, 158, 159), 198 (157, 158, 159), 199(158, 159), 200 (158), 201(174), 202(175), 203(158), 205(157), 206(175), 208(159), 209 (1.59), 210(157, 158), 211(169), 218, 219 Burgers, W. G., 269(46), 290 Burke, D. P., 58(3), 126, 224(3), 243 Burwell, R. L., Jr., 21(57, 58), 63 Butt, J. B., 110, 129 Byer, R. L., 323(34), 348
C Cabannes, J., 296, 341 Cable, J. W., 250(18), 289 Cdenhead, D. A., 273,291 Cailingold, A. L., 24(87b), 64 Cain, D. S., 323(33), 342 Calderon, N., 131, 132(3), 134(3), 135(19, 22, 24, 28), 138(2, 22, 24, 28, 46), 139 (2), 140(24, 28), 143, 144(28), 145(3, 22, 46), 152(22), 153(22), 156, 158 (46), 159(46), 161, 164, 165(22), 168, 169, 170, 171 Caldow, G. L., 147(80, 81), 170 Campau, R. M., 71(35), 78(54), 109(130), 126, 127, 129 Campbell, J. S., 230, 243, 270, 290 Campbell, W. E., 39(103), 64 Carberry, J. J., 90(84), 122(84), 128 Cardin, D. J., 150, 171 Cardozo, M. A. A., 91(89), 128 Carella, G., 135(23), 169 Careri, G., 323, 342
399
Carlson, D. W., 102(112), 104, 111(112), 129 Carr&,S., 24 (82, 85, 86, 89a), 63,64 Carrick, W. L., 175(9), 177, 178(109, 110), 187(9), 188(9), 189(9), 191(9), 206, 214, 217, 219 Carter, G., 344, 361, 362, 367, 376, 381, 382,383,385,386, 392,394 Cartier, P. G., 345(46h), 393 Cassar, L., 148, 170 Cassuto, A., 345(45b), 392 Catry, J. P., 5(20), 22(20), 24(20), 61 Caunt, A. D., 196(164), 218 Cavenaghi, C., 24(85), 64 Cavender, I. V., 196(162), 218 Cazsin, B. I., 175(25), 214 Cervenf, L., 24(87), 64 Chang, C. C., 24(84a), 63 Charcosset, H., 175(52, 53), 216 Charlesby, A., 323(32), 342 Chase, L. L., 313(20), 341 Chatt, J., 138(51), 139(51), 1.53(51), 154 (51), 170 Chauvin, Y., 138(45), 150, 154(88), 165, 166(89), 167,169, 171 Chen, H. Y., 131(2), 138(2), 139(2), 1.56 (2), 161(2), 168 Chien, J. C. W., 196(165), 218 Chuikov, B. A., 345(39), 392 Claassen, H. H., 331,342 Claes, F., 22(61), 39(61), 63 Clark, A., 143, 144(62), 160, 161, 162,170, 171, 174(2), 175(3, 35, 62, 81, 82, 91, 106), 204(62), 213, $14, 216, 816, 217, 349, 350(51), 393 Clarke, J. K. A., 273(56), 291 Clavenna, L. R., 351(60), 376(60, go), 380(99), 388(60), 393, 394 Cochran, H. D., 91 (94), 128 Colligon, J. S., 361(79), 367(79), 394 Collins, S. A., Jr., 310(16), 3.41 Combs, R., 196(163), 218 Conner, W. C., 24(84a), 63 Connolly, R., 78(54), 187 Contour, J. P., 376, 394 Cook, C., 143, 144(62), 161, 170 Cooney, R. P., 295(4), 319(27), 320(27), 321(27), 324(27), 331(42), 333(27), 335(45), 337(45), 341, 342 Cooper, B. J., 73(41), 186
400
AUTHOR INDEX
Coover, H. W., 196(163), 218 Corlateanu, P., 175(21), 814 Corolleur, C., 46(122), 66 Cortez, D. H., 102(116), 129 Cossee, P., 175(45), 200, 206,207,216,219 Cotton, F. A., 304(13), 339(13), 341 Coull, J., 7(41b), 62 Couper, A., 254, 257, 284(29), 285, 290 Coussemant, F., 5(18), 7, 22(18), 23(18), 32(18), 61 Crain, D. L., 134(18), 169 Cratty, L. E., Jr., 345(46d), 393 Crombie, L., 12(49), 20(49), 62 Cunningham, R. E., 7(41), 24(41), 62 Curthoys, G., 295(4), 319(27), 320(27), 321(27), 324(27), 331(42), 333(27), 335(4.5), 337(45), 341, 342 Cvetanovi6, R. J., 24(80), 63, 346, 371, 372(48,49), 388,393 Czanderna, A. W., 345, 350(58), 374, 381, 393, 394 Cearnota, I., 250(12), 289
D Dainton, F. S., 156, 171 Ilalin, M. A,, 175(86), 181(112), 216, 217 Dalla Betta, R. A., 110(132), 129 Dalla Lana, I. G., 8, 62 Dall'Asta, G., 132(8), 135(23, 27), 138(8, 44, .57), 140(8, 57), 141, 143, 144, 152 (44), 158(8, 64), 159, 168, 165, 170, 171 Ilaniel, J. C., 175(74, 83, go), 179(90), 216 Danusso, F., 211(194), 219 Danz, W., 249, 2.51 (19), 289 Davidson, B., 24(91), 64 Davidson, T., 175(102), 216 Davie, E. S., 138(39), 153(93, 941, 159, 161, 162, 169, 171 Davis, B. H., 39(113), 66 Davis, J. L., 313(20), 341 Dawson, P. T., 345, 363(35), 371, 387, 388(98), 389(98), 392, 394 de Aguirre, I., 8(42), 62 Deans; H. A., 107(126), 129 de Boer, J. H., 14, 23(54), 62 Decroocq, D., 8(42), 62 Degras, D. A., 352, 358(65), 362(86), 363, 393, 394.
De la Cuesta, M. O., 209(190), 219 Demin, E. A,, 175(8), 185, 186(125), 187(8, 126, 131), 188(8, 137), 189(8, 137), 19O(P, 140), 191(8), 198(140), 202 (175), 206(175), 211(8, 140), 214,217, 218,219 Denbigh, K., 107(125), 129 Denison, D. R., 344(15), 392 Derbeneva, S. N., 189(139), 190(139), 191 (139), 218 Derbentsev, Yu. I., 8(43), 62 de Ribaupierre, Y., 344(13), 392 Derrien, M., 5(19), 22(19), 23(19), 61 de Ruiter, E., 22(59), 63 Devlin, G. E., 313(20), 341 Devlin, T. R. E., 156(99), 171 de Wasch, A. P., 107(122), 129 Dickens, P. G., 260, 262, 273(35), 290 Dimitriades, B., 68(29, 30), 126 Dmuchovsky, B., 23(74), 63 Dobrovolskij, S. V., 23(69), 63 Dobrzynski, L., 352, 353 Doelp, L. C., 5(22), 23(22), 61 Doerr, It. C., 97(102), 128 Doi, Ja., 175(44, 5 5 ) , 178(55, 59), 226 Dolgoplosk, B. A., 144(66), 170, 189(138), 217 Doman, R. C., 80(67), 127 Doraiswamy, L. K., 23(70), 63 Dorgelo, G. J. H., 268(45), 271(45), 277 (45), 290 now, H. W., 175(9), 187(9), 188(9), 189 (91, 191(9), 214 Dowden, D. A,, 264(38), 265(38), 285, 286 (78, 80), 290, 291 Doyle, G., 134(15), 138(54), 139(54), 169, 170 Doyle, M. L., 150(89), 171 Drake, R. M., 101(109), 125 Draper, N. R., 5(27), 62 Dresser, M. J., 345(46f), 346(46j), 393 Druzhkov, V. N., 183(115), 184(115), dl7 Duck, E. W., 193(1,55),218 Duke, D. A., 80(67), 127 Durm, M., 345(451), 356(78), 364(78), 393, 394 Durrien, M., 175(57, 63, go), 176(57), 179 (go), 216, 216 DUB,R., 274(60), 287(60), 288, 2991 Dwyer, F. G., 77, 86(49), 127
401
AUTHOR INDEX
Dyatchkovski, F. S., 203(178, 179), 204, Farrell, G., 361(79), 367(79), 394 210(191a), 219 Fedor, R. J., 73(42), 80(42), 81(42), 97, 126,128 Dzisko, V. A., 175(26,32, 78, 79),214,216 Dzsabiev, T. S., 204, 219 Fedoseeva, G. T., 193(154), 218 Feilchenfeld, H., 175(54, 61), 197(61), 116 Feldman, G. F.,195, 218 E Feller, M., 175(4), 214 Field, E., 175(4), 214 Filipenko, F. S., 24(87b), 64 Earnshaw, J. W., 356(75, 76), 394 Finch, J. N., 175(35), 216 Eaton, P. E., 148(84), 170 Finkel'shtein, E. Sh., 134(18a), 169 Ebel, R. H., 59, 186 Echigoya, E., 154(97c), 171 Fisch, G., 345(45c), 392 Eckert, E. R. G., 101(109), 129 Flock, W., 24(87a), 64 Eden, C., 175(38,54,61),179(38), 197(61), Fognani, F., 7,22(37), 23(37), 62 216 Force, E. L., 95, 128 Egerton, T. A., 316, 321, 323(25), 324(25), Ford, R. R., 91(86), 188 334(30), 335(30), 336, 337(25, 30), Forni, L., 7(40), 23(40), 24(82, 85, 89a), 62,63,64 341,342 Foss, A. S., 122(143), 129 Ehmann, W. J., 154(98), 155(98), 171 Ehrlich, G., 259, 260(34), 290, 344, 355, Foster, G., 133(10), 169 357(27), 361, 362(29), 363, 364(27), Fox, A. S.,178(109), 217 Fr$ckiewicz, A., 275(63, 64, 64a), 276(64, 367(29), 381 (26), 392 64a, 65), 277(64a, 65), 279(65, 66), Eischens, R. P., 90(82), 128 281 (66, 69), 291 Eisele, H., 73(46), 127 Eley, D. D., 175(71, 72, 99), 176, 177, 179 Francis, S. A., 90(82), 128 (99), 203, 216,216,219, 254, 257, 283, Fraser, A. R., 148, 170 284(29), 285, 286(77), 890,291 Fraser, E. C.,362(89), 394 Freeland, P. E., 79(62), 127 Eliassen, J. D., 68(31), 126 Ellis, A. F., 138(41), 169 Freeman, S. K., 316, 317(26), 342 Emirova, I. V.,175(96, 100, 101, 103), 181 Freerks, M. C.,23(74), 63 Freidlin, L. Kh., 39(104, 105), 64 (95, 100, 103), 182(95), 183(95), 216 Fridman, R. A.,134(18a), 169 Emmett, P. H., 269, 270, 290 Friedlander, H., 210(191), 219 Endow, N., 345(42), 392 Froment, G. F., 25(92), 64, 106(119), 107 Engels, S., 259, 290 (122), 1.99 Erents, K., 386, 394 Fueno, T., 350(56), 393 Erhardt, J. J., 345(45b), 392 Fukui, K., 192(147), 118 Ernest, J., 310(18), 341 Fukushima, T., 39(109), 66 Ertl, G., 91(92), 128 Furukawa, J., 138(47a), 170 Evans, M. G., 207(188), 919 Furukawa, S., 154(97b), 171 Evering, B. L., 175(5), 214 Evgrashin, V. M.,24(87f), 64
G
F Faith, L. E., 6(34), 62 Faith, W. L., 63(22), 126 Falconer, J., 345(46g), 393 Farkas, A., 254, 290 Farrauto, R. J., 86(80), 128
Gagliardi, J. C., 109(129), 110(129), 129 Gaifutdinova, R. K., 284(73), 291 GBI, D., 3(12), 8(12), 61 Galkin, G., 336(47), 342 Gall, M. J., 323(35), 342 Gandhi, H. S.,68(33), 116
402
AUTHOR INDEX
Gault, F. G., 46(122), 66 Gay, I. D., 345(45j), 393 Gaylord, N. G., 175(12), 214 Gelbshtein (Gelbshtejn), A. I., 23(71), 63, 175(60, 73, 75, 76, 77), 616, 216 Gelbwachs, J., 310(15), 341 Germain, J. E., 4, 61 Geschwind, S., 313(20), 341 Ghosh, A. K., 23(77), 63 Giannini, U., 185(117), 187, 217 Gibson, R., 345(42, 43), 363, 392 Giller, S. A., 23(75), 63 Gillespie, B., 6(35), 6.2 Gioia, F., 23(75b), 63 Gleason, W. A., 67(27), 126 Gomer, R., 344(16), 362(81, 82, 83), 380 (83), 389(82, 83), 392, 394 Gorelik, A. G., 7(41a), 24(41a), 66 Gorshkova, L. S., 284(73), 291 Gossner, K., 90, 128 Goto, N., 7(39), 23(39), 62, 175(36, 43), 203(36), 216 Goymour, C. G., 376, 388(103), 389(102, 103, 120), 394, 396 Grabovski, Yu. P., 175(8, 68, 95), 178(68), 181(95), 182(95), 183(95), 187(8), 188 (8), 189(8), 190(8), 191(8, 142), 211 (8), 214,216, 626, 218 Graham, H. C., 362(91), 394 Graham, J. R., 80(71), 127 Grant, W. A,, 361(79), 367(79), 386, 394 Gravelle, P. C.,86(78), 128 Greaves, J. C., 259(33), 260(33), 290 Greco, G., Jr., 23(75b), 63 Green, M. L. H., 203(181), 219 Greenler, R. G., 294, 341 Grigorian, E. A,, 203( 179), 204( 179), 210 (191a), 219 Grosse, L., 196(166), 219 Grubbs, R. H., 149, 170 Gunther, P., 158(102), 171 Gul, V. E., 175(101, 103), 181(103), 216 Gulevskij, E. K., 23(75), 63 Gullet, J., 196(163), 218 Gunn, D. J., 24(90), 64 Gurevitch, V. R., 175(80, 86, 94), 181 (112), 183(94), 216, 217 Guseinov, N. M., 23(75a), 63 Gut, G., 23(76), 63
Guyot, A,, 175(52, 53, 57, 58, 63, 74, 83, 87, 90, 96), 176(57, 58), 179(90, 96), 181(96, 104), 216,216
H Haag, W. O., 24, 63 Haagen-Smit, A. J., 58(9), 126 Haas, F., 132(7), 158(7, 102), 168, 171 Haas, Y., 175(54,61), 197(61), 616 Habeshaw, J., 175(34), 214 Hagihara, N., 186(128), 817 Hagstrum, H. D., 344(22), 392 Haines, R. J., 138(51), 139(51), 153(51), 154(51), 170 Hall, W. K., 6(36), 24(36, 81), 62, 63, 269, 290 Halpern, J., 148, 170, 203(177), 219 Hamano, Y., 168(118), 171 Hamilton, W. M., 21(57), 63 HanEil, V., 1(7), 12(51), 26(51), 28(7, 51), 32(7), 33(51), 34(51), 6 1 , 6 2 Hancock, E. E., 78(54), 109(130), 127, 129 Hanika, J., 23(64,72), 63 Hansel, .I. G., 73(43), 97(43), 127 Hansen, K. W., 122(145), 129 Hansen, R. S., 344(30), 345, 356, 362(30, 87), 363, 364, 387, 392, 393, 394 Happel, J., 3(12a), 7(12a), 61, 230, 24.3 Hardin, A. H., 316(25), 321(25, 30), 323 (25), 324(25), 334(30), 335(30), 336 (25, 30), 337(25, 30), 341, 342 Hardison, L. C., 62(15), 126 Hardt, P., 185(122), 817 Hardy, W. A., 273, 276(59), 277(59), 291 Harned, J. L., 65(24), 103(24), 118(141), 126, I29 Harvey, A. B., 323(33), 342 Harvey, E. A., 25(93), 64 Hashimoto, K., 7(39), 23(39), 62 Hashimoto, N., 7(39), 23(39), 62 Hassell, J. A., 269, 290 Hatihara, N., 186(127), 217 Hatzenbuhler, D. A., 329(39), 342 Haward, R. N., 174(1), 175(1), 813 Hawthorn, R. D., 103(117), 129 Hayward, D. O., 344(18), 392 Heckelsberg, L. F., 134(14, 16), 136(35, 36), 137(35, 36), 169
403
AUTHOR INDEX
Heimbach, P., 185(122),217 Hein, P. R., 135(23a), 136(23a), 138(23a), 169 Heinen, C. M., 105(118), 129 Heisenberg, W., 296, 341 Hendra, P. J., 316, 321, 323(35), 324, 332 (24), 333(43), 334(43, 44), 335(24), 337(24,43,48), 338,S4l,342 Henein, N. A., 58(8), 59(8), 126 Henrici-OlivB, G., 154(97e), 175(14), 171, 214 HBrisson, J. L., 138(45), 150, 154(88), 165, 166(88), 167, 169,171 Hertl, W., 86(80), 128 Hertwig, K., 24(87a), 64 Hersberg, G., 299(10), 301(10), 302(10), 341 Hester, R. E., 320(28), 342 Heusser, U. K., 91(93), 128 Hibler, G., 313(21), 341 Hickmott, T. W., 344(26), 363, 381(26) 392 Hicks, J. S., 100, 123 Hidai, M., 138(48), 139(48), 270 Hightower, J. W., 24(81), 63 Hill, M. P., 363(93), 388(114),394,396 Hill, T., 175(34),214 Hinshelwood, C. N., 254,290 Hirai, M., 224(7), 243 Hirota, K., 5(23), 23(23, 63), 61, 63 Hirota, M., 192(147), 218 Hirschberg, E. H., 72(38), 97(38), 126 Hlavacek, V., 106(120), 107(121), 129 Hobert, H., 345(45k), 376(104), 386, 393, 394 Hobson, J. P., 345, 356(75,76), 392,394 Hock, C. W., 181(113),217 Hocks, L., 141(59a), 170 Hocker, H., 138(58), 140(58), 153(58), 170 Hoffman, T. W., 24(87e), 64 Hoffmann, E., 72(39), 97(39), 126 Hoffmann, M., 138(56), 140(56), 158(56), 165(56), 170 Hoffmann, R., 145, 170 Hoffmann, W., 211(192), 219 Hogan, J., 175(3, 69), 177, 197, 198, 203, 204(69), 208(69), 214, 216 Hogan, J. P., 174(2), 213
Hogue, J. W., 344(8), 391 Hoiberg, J. A., 122(143),129 Hoinkis, E., 345(45m), 393 Holm, V. C. F., 175(62),204(62),216 Holt, E. L., 73(43), 97(43), 1.27 Holekamp, E., 175(6), 214 Hoogeveen, H., 146(76), 170 Hopper, J. R., 24(84c), 64 Horder, J. R., 333(43), 334(43, 44), 337 (43), 349 Horiuti, J., 268(44), 290 Hosten, L. H., 25(92), 64 Houdry, E. J., 62, 126 Hougen, 0. A,, 70(34), 126 Howe, R. F., 153(95a), 171 Howmrtn, E. J., 136(37), 137(37), 145(67), 169,170 Hubert, A. J., 141(59a), 170 Hudgins, R. R., 39(119a), 66 Hudson, J. B., 389(123), 396 Hughes, W. B., 134(13), 136(13), 138(13), 139(13), 141, 145(13, 69), 154, 156 (69), 158(104), 161, 164, 169, 170, 171 Hugo, P., 91(90), 128 Hunter, W. G., l(3, 5 ) , 61 Hussey, A. S., 39(108), 40(108), 66
I Imai, T., 24(87h), 64 Ingberman, A. K., 211(193), 219 Inone, Y., 267, 290 In Yuen-ken, 39(116), 66 Ioffe, I. I., 23(68, 73, 75), 24(87f), 63, 64 Ione, K. G., 192(146),218 Ionesco, A. C., 175(21), 214 Ionov, N. I., 344(28), 345(28, 40, 41), 352 (62, 63, 64), 357(28), 3.58(28), 361, 362(40, 64), 363(28, 62), 392, 393 Ioshida, S., 175(44, 55, 59), 178(55, 59), 216 Isagulyants, G. V., 8(43), 62 Ito, M., 7(38), 23(38), 62 Ivanov, L. P., 175(60, 73, 75, 76, 77, 88, 92), 179(88, 92), 183(88), 197(167), 208(75), 209(75), 216, 216, 619 Ivanova, L. I., 175(79), 216 Iwicka, D., 175(41),216
404
AUTHOR INDEX
J Jabloriski, A., 280(68), 288(68),291 Jackson, M. W., 67(28), 126 Jacob, S. M., 91(96), 128 Jagel, K. I., 77(51), 127 Jahnel, W., 122(144),129 Jakubith, M., 91 (go), 128 Jamaguchi, M., 186(127),217 Jamazaki, H., 186(127, 128), 21 7 Jambor, J., 47(123), 66 Janko, A., 247, 250, 269(47), 274(8), 275 (62), 280, 287(47, 67, 85), 288(62), 189, 290, 291 Jankow, R., 323,326,342 Jarmolowicz, H., 287(84), 291 Jelend, W., 344(10), 391 Jenkins, P. A., 12(49), 20(49), 68 Johnson, D. W., Jr., 79(63), 127 Johnson, M. M., 160(109, 110), 162(109, 110), 171 Johnson, R. N., 175(9), 178(109), 187(9), 188(9), 189(9), 191(9),,914,217 Jones, E., 185(123), 186(123),217 Jones, F. R., 138(.58), 140(.58), 153(58), 170 Jones, J. H., 97, 128 Jones, M., 224(6), 243 Jongepier, R., 268(45), 271(45), 277(45), 290
Jottrand, R., 4(14), 51 Joyner, F. B., 193(152), 196(163),218 Judy, W. A., 132(3), 134(3), 135(19, 28), 138(28, 46), 140(28), 143(3, 46), 144 (as),145(3, 46), 156(3, 28), 158(46), 159(46), 164(3,46), 168, 169, 170 Juguchi, S., 186(129),217 Jung, K. A,, 196(166), 219 Jungers, J. C., 5, 8, 11, 12, 20, 22(17, 18, 19, 20, 48, 59, 60, 61), 23(18, 19), 24 (20), 32(18), 39(17,48, 60,61), 61, 62, 63
K Kabel, R. L., 122(142),129 Kagel, R. O., 321(29), 324(29), 333(29), 336,337(29), 342 Kagiya, Ts., 192(147),218
Kalechits, I. V., 39(116), 66, 168(117),171 Ka116, D., 24(79, 83), 63 Kamiya, Y., 154(97b,97d), 171 Kamneva, L. S., 23(68), 63 Kaneko, Y., 91(88), 128 Kanemasu, H., 5(27), 62 Karakchiev, L. G., 175(49), 177(49), 189 (139), 190(139), 191(139),216, 218 Karapinka, G. L., 175(9), 187(9), 188(9), 189(9), 191(9), 214 Karol, F. J., 175(9), 178(109), 187(9), 188 (9), 189(9), 191, 214, 21 7 Karpiliski, Z., 275(64, 64a), 276(64, 64a, 65), 277(64a, 65, 65a), 279, 281, 291 Katz, M., 79(56), 127 Kawakami, T., 23(66), 63 Kays, W. M., 101(110), 102(110),129 Kazanski, V. B., 175(26, 29, 32, 39, 40, 50, 51, 56, 105), 179(51), 181(51), 214, 216, 216 Keii, T., 175(19, 46), ,914, 616 Keim, W., 185(122),817 Kemball, C., 138(39), 153(93, 94), 169 (107), 161(94),162(94),169, 171 Kern, W., 195(160),218 Keulks, G. W., 39(108), 40(108), 66 Khabibulaeva, 0. K., 5(24), 24(24), 61 Khidekel', M. L., 168(117), 171 Khvostik, G. M., 203(179), 204(179), 219 Kieboom, A. P. G., 39(110), 56 Kim, S. K., 350(54), 393 Kimkai, 0. N., 192(146a),218 King, I>. A,, 345(37, 45), 362(37, 45), 364 (45), 376, 388(37, 103, 116), 389(37, 102, 103, 116, 120, 12l), 392,394, 395 Kingery, W. D., 80(65), 137 Kiperman, S. L., 1(1), 47(1), 61 Kirst, T. P., 388(117), 389(117), 396 Kiselev, A. V., 336(47), 342 Kisliuk, P., 3.53(71, 72), 363, 393, 394 Kitamura, T., 39(119), 56 Kittelberger, J. S., 376(105), 394 Kittleman, E. T., 134(13), 136(13), 138 (13), 139(13), 145(13), 169 Kittrell, J. R., 1(3), 43(120a), 61,66 Klabunovskij, E. I., 24(87d), 64 Klein, R., 350,393 KleAha, V., 41(120), 42(120), 56 Klimisch, R. L., 68(32), 78(53), 86(53), 96, 116, 127, 128
405
AUTHOR INDEX
Klinzing, G. E., 7(41b), 68 Klopfenstein, E., 23(76), 65 Knappe, B., 345(45k),376(104), 386, 39S, 394 Kobylinski, T. P., 138(40), 169 Koch, J., 91(92), 128 Kodama, Sh., 192(147),818 Koehler, W. C., 250(18), 289 Kogelnik, H., 306, S4l Kohl, A. L., 62(16), 186 Kohn, E., 196(162),818 Kohrt, C., 362(82, 83), 380(83), 389(82, 83), S94 Kokes, It. J., 24(84a), 63, 2G5,290 Kolesnikov, I. M., 5(25a), 23(67), 24(88), 39(115), 61,63,64, 66 Kollen, W., 350(.58),381(108),S93,394 Kolovertnov, G. D., 175(49),177(49),116 Komarovskij, N. A., 24(87b), 64 Konenko, I. R., 284, 291 Koningstein, J. A., 296(8), 341 Konvalinka, J. A., 247, 248, 2.56, 257(9), 280, 287(9), 689 Kooy, C., 269(46), 290 Kornelsen, E. V., 345,398 Kosek, S., 175(41, 42), 216 Kothari, V. M., 141(59), 170 Kovalenko, T. I., 5(25a), 61 Kovalenko, V. I., 5(25a), 61 Kowaka, M.,266,270,290 Kozirovski, Y., 316(25), 321(25, 30), 323 (25), 324(25), 334(30), 335(30), 336 (25, 30), 337(25, 30), 341, 342 Kozorezov, Yu. I., 5(26, 26a), 61, 62 Kramers, H. A., 296,341 Kraus, M., 1(2), 47(2), 61 Krauss, H. L., 175(37, 65, 66, 67, 70), 177, 203, 816,817 Krentsel, B. A., 175(20, 22), 214 Krieger, K. A., 79(57), 127 Kroner, M., 185(122), 217 Kroll, W. R., 134(15), 138(54), 139(54), 169, 170 Kronauer, P., 344(10, ll), 391 Krylov, 0.V., 72(37), 126 Kryukov, Yu. B., 184(18a), 169 Ku, R., 91, 128 Kubicek, D. H., 134(13), 136(13), 138(13), 139(13), 145(13), 169 Kummer, J. T., 86(79), 97(103), 128
Kuo, J. C. W., 63(21), 86, 115, 117, 1.96, 129 Kurapinka, G. L., 178(109),217 Kurbanov, N. A., 24(87c), 64 Kuriacose, J. C., 22(60), 39(GO, 117), 63, 66 Kushnareva, A. I., 175(77), 316 Kushnareva, E. G., 175(68, 97), 178(68, 97), 179(97), 197(168), 198(168, 168a), 208(97, 168a), 212(195), 616, 216, 219 Kuznetzov, B. N., 189(139),190(139), 191 (139, 141, 142a, 143, 144, 144a, 145, 145a), 192(145, 145a, 146, 146a), 218 Kuznetzov, V. L., 191(145a), 192(145a), 218 Kuzyaeva, T. E., 185(126), 187(126), 217
L Lagernaya, T. A., 181(112),dl7 Lahiri, A., 23(77), 65 Lamb, A., 97(104), 128 Landau, M. A., 175(84, 85), 179(84), 181 (84), 216 Landon, D. O., 312(19), 313(19), 316, 317 (2611 341,348 Lang, C. It., 72(38), 73(44), 77(44), 96 (44), 97(38), 126, 127 Lang, R. J., 73(42), 80(42), 81(42), 116 Langmuir, I., 90, 118 Lanning, W., 175(3), 81.4 Lapidus, L., 1(4), 61, 107(126), 119 Lapin, V. B., 24(87c), 64 Lappert, M. F., 150(89),171 Lapujoulade, J., 352(66, 67), 357(66), 361, 362(66), 393 Larson, J. A., 110(132), 189 Lassen, H. G., 63(21), 77(50), 86(21), 115 (21), 126, 287 Lavrovskij, X. P., 5(24), 24(24), 61 Laxutkin, A. M., 175(8), 185(125, 126), 186(125), 187(8, 126), 188(8), 189(8), 190(8), 191(8, 142, 144a), 211(8), 614, 617, 618 Lecchini, S. M. A., 363(93), 394 Leck, J. H., 344(5), 391 Ledwith, H., 175(15), 214 Lee,C. H., 97(105), 1.B Lefebvre, G., 138(45), 169
406
AUTHOR INDEX
Lehmann, G., 211(192), 219 Lehnert, G., 152(92), 171 Lehr, C. G.,122(142),129 Leigh, G. J., 138(51), 139(51), 153(51), 154(51), 170 Leitrnan, M. I., 175(25),21.4 Lemcoff, N. O.,7(41), 24(41), 62 Leszczyhski, A., 279(66), 281(66,69), 291 Levenspiel, O., 107(124),129 Levine, I. J., 211(193), 819 Lewandos, G. S., 133(9a), 138(47), 147, 153, 169, 170 Lewis, F. A., 246,289 Lewis, M. J., 162,171 Libby, W. F., 79(60, 61), 127 Liberov, L. G.,134(18a), 169 Libowitz, G. G.,246, 289 Lichtman, D., 388(117), 389(117), 396 Lichty, L. C., 66(25), 70(25), 126 Liederman, D., 77(52), 91(96), 127, 128 Lien, T. R., 266,290 Linnett, J. W., 259, 260, 273, 276(59), 277 (59), 290,291 Lippert, J., 313(21),341 Lisovskij, A. E., 24(87c), 64 Loader, E. J., 316, 321, 324, 329, 330, 332 (24), 333(43), 334(43, 441, 335(24), 337(24,38,43,48), 338, 341,342 Loebach, E., 389(124),396 Lombardo, E. A., 6(36), 24(36), 62 London, A. L., 101(110),102(110),129 Lord, F. M., 376(105),394 LUCBB, K., 24(87a), 64 Luekner, R. C., 153, 163, 171 Ludlum, D. B., 206, 219 Lunt, R. S., 73(42, 43), 80(42), 81(42), 97, 126, 127 Luss, D., 91(89), 128 Luzarraga, M. G.,7(36a), 68 Lyapin, E. V., 7(41a), 24(41a), 62 Lycke, B. C., 122(143),129 Lygin, V. I., 336(47), 342 Lyubarskij, A. G.,7(41a), 23(73), 24(41a), 4 6 3 Lyubarskij, G. D., 23(62), 32(62), 63
M McAdams, W. H., 102(113), 129 McAllister, J. W., 344(14), 392
McCarroll, B., 345(38), 363, 386, 392,394 McCarty, J., 345(46g), 393 McConchie, G. E., 163(116), 171 McDermott, J., 86(76), 128 MacGregor, R. A., 147(80, 81), 170 Machi, S., 192(147),218 Mackay, K. M., 246,289 McKenna, R. P., 63(23), 126 Mackenzie, N., 264(38), 265(38), 290 McKinley, J. D., 350, 393 McNally, R. N., 80(67), 127 Madey, T. E., 344(2, 4), 345(36, 46f), 346 (46i, 46j), 351 (61), 363(61), 376(36), 388(36, 115), 389(2, 36, 115), 391, 398,393,396 Madix, R. J., 345(46g), 393 Maertens, D., 152(92), 171 Maga, J. A., 62(19), 126 Majchrzak, S., 250, 252, 269(48), 271(48), 277(48), 278, 287(48), 289, 290 Makarenko, G.N., 284(71,72), 291 Makovetskii, K. L., 144(66), 170 Makowski, M. P., 97(105), 128 Mdenge, J. P., 345(45b), 392 Malinowski, S., 175(41, 42), 616 Malkin, I. I., 4(13), 61 Malook, G. P., 5(25a), 61 Manetti, R., 159(106), 171 Mango, F. D., 145(72, 73, 74), 147(72, 74, 77,78), 150(87), 170,171 Mann, R. S., 266, 290 Mansour, A. H., 4(14), 61 Marek, M., 106(120), I29 Margolis, L. Ya., 79(55), 80(55), 127 Mar'in, V. I., 168(117), 171 Mariotti, J. F., 39(107), 40(107), 66 Mark, H. F., 175(12),$14 Mark6, L., 138(53), 139(53), 170 Marquois, J. C., 39(107), 40(107), 66 Marsden, D. G. H., 259 (32), 260(32), 290 Marshall, P. R.,13.5(21), 138(21),140(21), 144, 169 Martin, H., 175(6), 214 Marwede, G., 158(102), 171 Matlack, A. S., 193(149), 218 Matlin, S. A., 154(97f), 171 Matsuda, S., 168(118),171 Matsuda, T., 175(46),216 Matsumoto, A., 175(36, 43), 203, 216 Matsumoto, Jo., 186(128),217
407
AUTHOR INDEX Matsushita, K., 345(46a), 363, 387, 393, 394 Matthias, B. T., 79(62), f27 Maurel, R., 5(25), 39(25, 106, 107), 40, 61, 66 Maxted, E. B., 110,189 Mazzacurati, V., 323131). 342 Mazzantj, G., 143(64), 158(64), 170 Medinger, T., 185(123),186(123),217 Meguerian, G. H., 72(38), 73(44), 77(44), 96(44), 97(38), f26, 127 Mekhtiev, S. D., 7(38a), 23(38a), 62 Menapace, H. R., 138(49,50), 139(49,50), 145(49), 149(49), 153(96), 158(49), 159(49), 170, 171 Mendelson, R.A., 196(162),218 Mendiratta, A. K., 24(91), 64 Menzel, D., 344(3, 10, ll),391 Merk, W., 145(71), I70 Merta, R., 344(1), 391 Meyer, E. F., 21(58), 63 Mezaki, R., 1(5), 5(27), 61, 66 Michel, P., 287(85), 2991 Miesserov, K. G., 175(20, 28,48), 914, % f 6 Mihail, R., 175(21), 614 Mikhalchenko, V. G., 175(95), 181(91), 182(91), 183(91), 616 Mikheikin, N. D., 175(39),216 Miller, A. R., 347(50),393 Miller, M. R., 80(66), 197 Mimeault, V. J., 344(30), 345(30, 46b), 356, 362(30, 87), 363(30, 46b, 87), 364, 387, 399, 393, 394 Minachev, Kh. M., 24(88), 64 Minchak, R. J., 158(103),171 Minsker, K. S . , 193(154),218 Misono, M., 24(84), 63 Mitschka, P., 1(7), 28(7), 32(7), 61 Mittelmeijer, M. C., 133(12), 169 Miyahara, K., 3(9, 9a), 61 Miyamoto, K., 7(39), 23(39), 66 Miyazaki, K., 111(137), 129 Mizoe, Y., 138(47a), 170 Modell, M., 91, 128 Movik, N., 175(37), 816 Moffat, A. J., 160, 162, l 7 f Mol, J. C., 142, 143(63), 144(61), 147(63), i53(97), i58(105), 163, fro, 171 Montarnal, R., 4, 7, 15(16), 16, 22(37), 23(37), 30(16), 61, 62
Montgomery, D. L., 65(24), 103(24), 166 Montroll, E. W., 350(55), 393 Moreva, N. I., 284(73), 6991 Morgan, C. R., 63(21), 86(21), 91(96), 102 (112), 104(112), 111(112), 115(21), f26, f28, I29 Moro-oka, Y., 39, 43(118), 66 Morozov, E. A,, 23(67), 63 Mortreux, A., 136(33), 154(33), 169 Moss, R. L., 286, 691 Motroni, G., 132(8), 138(8, 57), 140(8, 57), 143, 144(65), 158(8), f68, I70 Motta, L., 144(65), 170 Moulijn, J. A., 136(32), 138(55), 139(55), 142(61), 144(61), 154, 155(32), 158 (105), 169, 170, f 7 f Muller, E. W., 344(9), 391 Mukai, Y., 168(118), 171 Mulhall, J., 39(112), 40(112), 66 Munch, R. H., 23(74), 63 Mushenko, D. V., 24(87g), 64 Mussen, G. S., 73(42), 80(42), 81(42), 166 Myint, A., 8(43a), 68
N Nace, G. M., 250(13), 689 Nadjm, A., 234(71, 72), 99f Nagata, S., 7(39), 23(39), 69 Nair, C. S . B., 23(77), 63 Nakamura, R., 154(97c), 171 Nametkin, N. S., 134(18a), 169 Natta, G., 135(23, 27), 143, 158(64), 169, 170, 175, 196(11), 211, 214, 619 Naaarova, N. M., 39(104, 105), 64 Neiman, M. B., 3(11, 12), 8(11, 12), 61 Nekipelov, V. N., 272(55), 2991 Nernst, G. H., 249(10), 250(10), 289 Nevitt, T. D., 193(150),218 Nevyantzev, I. D., 175(100),181(100),216 Newsham, I. G., 344(8), 391 Nguyen The Tam, 295(4), 319(27), 320 (27), 321(27), 324(27), 331(42), 333 (27), 335(45), 337(45), 341, 346 Nieuwenhuys, B. E., 389(122), 396 Nieuwstad, T. J., 12(50), 20(50), 62 Nihira, H., 39(109), 66 Nikolaev, Yu. T., 23(68), 63 Nishimura, S., 5(23), 23(23, 63), 61,63
408
AUTHOR INDEX
be, K., 79(59), w 7 7 ) , ii5(138), 127, 128, 129 Norton, P. R., 285, 291 Noyes, R. M., 3, 61 Nutzel, K., 132(7), 158(7, 102), 168, 171 Nukui, K., 23(62a), 32(62a), 63
0 Oberkirch, W., 158(102), 1'71, 185(122), $1 r
Ofstead, E. A., 132(3), 134(3), 135(19, 24, 28), 136(29), 138(24, 28, 46), 140(24, 28), 143(3, 46), 144(28), 145(3, 46), 156, 158(46), 159, 164(3,46), 168,169,
fro
Ogasawara, S., 24(80), 63 Ogata, E., 154(97d), 171 Ogino, A., 23(66), 63 O'Hara, J. I., 133(11), 169 Ohara, T., 224(7), 243 Ohi, N., 23(62a), 32(62a), 63 Ohta, N., 154(97b), 171 Oita, K., 193(150), 218 Okay, V. C., 43(120a), 66 Oki, S., 91(88), 128 Oleck, S. M., 77(52), 127 OlivB, S., 154(97e), 175(14), 171, 214 O'Neill, P. P., 151, 171 Ono, Yo., 175(46), %I6 Orlickas, A., 24(87e), 64 Osborn, J. A., 148(85a), 170 Osment, H. E,, 80(64), 127 Osterhout, D. P., 115(140), 117(140), 129 Ostrovskii, V. E., 353(73), 394 Ostrovskij, G. M., 4(13), 23(71), 61, 63 Otto, K., 97(103), 110(132), 128, 129 Owen, E. A., 250, 289 Ozaki, A., 39(109, 118, 119), 43(118), 66 Ozawa, Y., 6(35), 62
P PaignB, J., 344(12), 391 Palczewska, W., 260(35), 273(35), (63, 64, 64a), 276, 277(48, 64a, 278, 279(65, 66), 280, 281(66, 286(83), 287(48, 67), 288(68), 291
275 651, 691, 290,
Pampus, G., 132(7), 138(56), 140(56), 152, 158(7, 56, 102), 165(56), 168, 170, 171 Panchenkov, G. M., 23(67), 24(88), 39 (115), 63,64, 66 Pantell, R. H., 310(15), 341 Papp, H., 345(45c), 396 Partridge, R. H., 323(32), 342 Pasquon, I., 175(11), 196(11), 214 Pasts, M., 362(90), 394 Pasternak, R. A., 345(42, 43), 362(89), 363, 398, 394 Patterson, D. J., 58(8),59(8), 166 Paulus, H. J., 62(14), 126 Pazar, C., 62(13), 126 Peacock, C. J., 323(35), 8-42 Pearson, R. G., 147(82), 170 Pecev, N., 26(96), 29(96), 64 Pecherskaya, Yu. B., 175(26,29,32,40,50, 56), 214, 216 Pecque, M., 5@5J 39(25), 61 Pedenen, L. A., 79(61), 127 Peeschel, E., 252(23), 259(23), 290 Penella, F., 136(31), 154(31), 169 Peng, Y. K., 345, 363(35), 371, 387, 388 (98), 389(98), 392, 394 Perelman, A. I., 175(20,22, 23,24,27), 214 Perry, E., 195, 218 PBtermann, L. A., 345, 351, 362(84), 392, 393, 394 Peters, E. F., 175(5), 214 Peters, M. S., 95(98), 188 Peterson, D., 295(4), 331(42), 8-41, 8-42 Peterson, T. I., 61 Pethica, B. A., 363(93), 394 Peticolas, W. L., 313(21), 341 Petoyan, V. P., 7(41a), 24(41a), 62 Petrovic, L. J., 102(115), 129 Pettit, R., 138(47), 145(71), 147, 153, 170 Phillips, T. R., 39(112), 40(112), 66 Phung, N. H., 138(45), 169 Pielaszek, J., 250, 288(87), 289, 291 Pierron, E. D., 23(74), 63 Pimentel, G. C., 157(101), 158(101), 159 (ioi), i n Pines, H., 24, 63 Pioli, J. J. C., 185(123), 186(123), d l 7 Pisani, C., 352(66a), 363(66a), 364(66a) 384, 393 Pis'man, I. I., 136(34), 137(34), 169 Pitts, J. N., Jr., 58(7), 126
AUTHOR INDEX
Pitzer, K. S., 157(101),158(101), 159(101), 171 Pliskin, W. A., 90(82), 128 Plummer, E. W., 389(118), 396 Polanyi, M., 207(188), 819 Polinski, L. M., 25(93), 64 Polotnyuk, V. Ya., 23(69), 63 Polyakov, A. A., 24(87f), 64 Popov, A. M., 134(18a), 169 Popov, E. I., 8(43), 62 Porri, L., 135(27),169 Porto, S. P. S., 306, 341 PouEek, J., 23(72), 63 Prabhu, A. V., 24(91), 64 Prakash, G., 23(70), 63 Prater, C. D., 6, 17, 23(32), 34,49,62, 100, 115(140), 117(140), 128, 129 Prescott, J. H., 227(8), 243 Preszler, I., 24(83), 63 Procop, M., 345(45e, 45g, 45h), 371, 392, 393 Proud’homme, R., 376,394 Ptack, M., 175(90), 179(90),216 Ptushinskii, Yu. G.,345(39), 392 Puthoff, H. E., 310(15), 341
Q
Quets, J. M., 345(45i), 393 Quinlan, C. W., 43(120a), 66
R
409
Reitsma, H. J., 136(32), 154(32), 155(32), 169 Remeika, J. P., 79(62, 63), 127 Rennard, R. J., 265,290 Revillon, A., 175(52, 53, 58, 74, 96), 176 (58), 179(96), 181(96), 216, 216 Reynolds, P. W., 285, 291 Ricca, F., 352(66a), 363(66a), 364(66a), 384,393 Rice, 0. K., 350(57), 393 Ridgewell, B. J., 135(21), 138(21), 142 (21), 144, 169, 193(155), 218 Rienacker, G., 259, 290 Ries, H. E., Jr., 111(136), 129 Riesenfeld, F. C.,62(16), 126 Rigby, L. J., 345(33), 392 Rindin, Yu. A,, 191(144a), 218 Rizaev, R. G.,7(38a), 23(38a), 62 Rochard, Y., 296,341 Rochester, C. H., 175(71, 72, 99), 176(99), 177(71, 72), 179(99), 203(72, 176), 216, 816,219 Rode, T. V., 175(22),$14 Roginskii, S. Z., 91(87), 128,352,393 Roha, M., 175(13),214 Rooney, J. J., 151, 171 Rosanov, V. V.,362(85), 387(85), 394 Rose, A. H., Jr., 66(26), 126 Ross, P., 206(187), 219 Rossini, F. D., 157, 158, 159, 171 Roth, J. F., 79(58), 127 Rozental, A. L., 5(24), 24(24), 61 Rozhdestvenskaya, I. G.,39(104), 64 Rubanik, M. Ya., 39(111), 66 Rubinshtein, A. M., 362(85), 387(85), 394 Rushton, J. H., 79(57), 127 Russell, W. W., 271, 290 RkXiEka, V., 23(64, 72), 24(87), 63, 64 Ryan, J. P., 81(73), 127 Ryashentseva, M. A., 24(88), 64 RybBEek, L., 26(94), 28(94), 35, 37(94), 64 Rye, R. R., 345(46b, 46c, 46e, 46h), 363 (46b, 46c), 393
Rabino, G., 352(Ma), 363(66a), 364(66a), 384,393 Rader, C. P., 39, 64 Raffy, J., 310(18), 341 Ragaini, V., 24(86), 64,267, 290 Rakowsky, F. W., 72(38), 97(38), 126 Ramain, L., 161, 171 Rao, M. S., 39(119a), 66 Ratner, I. D., 175(100), 181(100),216 Raven, P. A., 138(52), 139(52), 170 Razuvaev, G. A., 193(154),218 Redhead, P. A., 344, 345, 362(31,88), 364 (31), 367, 376, 381(88), 382(31), 383 (311, 392,394 Reed, P. R., 312(19), 313(19), 341 S Regier, R. B.,134(17), 160(17), 169 Reich, L., 175(16), 214 Sabourin, E. T., 138(41),169 Reilly, P. M., 1(6), 24(87e), 39(119a),. 61, Sachtler, W. M. H., 268, 271, 277, 890, . 64,66 389(122), 396
410
AUTHOR INDEX
Sadao, Yu., 186(130), 91'7 Sadovskij, A. S., 23(71), 63 Safarov, M. G., 24(87c), 64 Sajus, L., 8(42), 62 Sakai, T., 23(62a), 32(62a), 65 Sammes, P. G., 154(97f), 1'71 Sampoli, M., 323(31), 342 Samsonov, G. V., 284(71, 72), 291 Samuels, M. R., 68(31), 126 Sandstrom, D. R., 344(8), 391 Sano, K., 7(38), 23(38), 62 Satt,erfield, C. N., 100, 101(106), 102(116), 128,189 Savchenko, V. I., 345(45a), 362(45a), 392 Savin, A. G.,17.5(25),914 Sax, N. I., 81(74), 82(74), 12Y Schachtschneider, J. H.,145(72, 73, 74), 147(72, 74), 170,206(187), 219 Schay, G., 24(79, 83), 63 Scheer, M. D., 350, 393 Schindler, H., 175(16), 814 Schlatter, J. C.,78, 86, 12Y Schmidbauer, E., 251(20), 289 Schmidt, L. D., 345(34), 351(34, 60), 362 (34), 376(60, 100, 101), 380(34, 107), 387(101), 388(60, 107, 113), 389(34, 107, 113), 392,393,394, 396 Schmonina, V. L., 189(138), 21Y Schnecko, H., 195(160), 196(166),218,219 Schneider, P., 1(2), 47(2), 61 Schock, D. N., 72(38), 97(38), 126 Schon, N., 158(102), 171 Schofield, D., 260(36), 262(36), 290 Scholten, J. J. S., 247, 248, 256, 257(9), 280, 287(9), 289 Schuurmans, H., 196(162), 218 Schwab, G. M., 90,128 Schwarzenbach, K., 151(91), lYl Schweibold, D. J., 73(45), 80(45), 127 Scott, K. W.,131(2), 132, 134(3), 135(19, 20), 138 (2, 46), 139(2), 143(46), 145(3,46), 156(2,3), 158(46), 159(46), 161(2), 164(3, 46), 165(20), 168,169, 170 Scurrell, M. S., 175(71, 72, 99), 176(99), 177(71, 72), 179(99), 203(72, 176), 216,216,219 Seff, K., 336,342 Seizinger, D. E., 68(29), 126
Selig, H., 331(41), 348 Semenov, N. N., 207(189), 219 Semenova, A. S.,175(25), 214 Sen Gupta, A,, 102(114), 129 Sergeev, G. B.,175(24), 214 Setlnek, K., 26(94, 96, 97, 98), 28(94, loo), 29(96, loo), 30(100), 35, 37(94), 38(98), 40 (97, 98), 41(97,98), 48(97), 64
Seydel, G., 211(192), 219 Shallcross, P. B.,271, 990 Shamir, J., 331(41), 34.9 Shapley, J. R., 148(85a), lY0 Sharaev, 0. K., 175(23,24,27), 214 Shaw, I. D., 24(87e), 64 Shchekin, M. M., 175(84), 179(84), 181 (84), 216 Shearer, N. H., 193(152), 218 Shebaldova, A. D., 168(117), 1Yl Sheinin, V. E., 7(38a), 23(38a), 62 Shelef, M., 68(33), 97(103), 110(132), 126, 188,129 Sheppard, N., 316(25), 321(25, 30), 323 (25), 324(25), 334(30), 335(30), 336 (25, 30), 337(25, 30), 341,342 Sherony, D. F., 102(111), 129 Shiba, T., 175(47), 816 Shigemura, D. S., 24(84c), 64 Shih Chien Chow, 175(47), 616 Shilov, A. E., 203(178, 179), 204(178, 179, 182), 210(191a), 219 Shimanskaya, M. V.,23(75), 63 Shimimu, N., 224(7), 243 Shinohara, H., 267, 290 Shooter, D., 283, 291 Shub, B. R., 91(87), 128,352(69), 393 Shuler, K. E., 350(55), 395 Shull, C. G.,250(17), 289 Sianesi, D., 211(194), 119 Sieverts, A., 249, 251 (19), 289 Signorelli, G., 323(31), 349 Sills, R. A., 91(95), 128 Silveston, P.L., 39(119a), 66 Silvestri, A. J., 6(29, 30, 31), 24(31), 62 Simon, F. N., 388(117), 389(117), 396 &monik, J., 26(95), 28(95), 43(95), 44 (1211, 64, 66 Simons, J. B., 90(84), 122(84), 188 Singh, H. B., 7(41b), 62
411
AUTHOR INDEX
Singleton, D. M., 136(30), 169 Singleton, J. H., 271, 290 Sklyarov, A. V., 91, 128 Skomorokhov, V. B., 175(89, 92), 179(89, 921, 183(89), 216 Slager, T. L., 294, 341 Slavinskaya, V. A., 23(75), 63 Slinko, M. G., 24(87b), 64, 176(89), 179 (89), 183(89), 216 Sloane, H. J., 314(22), 341 Small, P. A., 156(99), 171 Smardzewski, R. R., 329(39), 342 Smiatowski, M., 246, 247, 274(7), 287(3, 84), 289, 291 Smith, A. W., 344(25), 345(45i), 392, 393 Smith, C . S., 109(129), 110(129), 129 Smith, D. P., 246, 289 Smith, H. A., 39, 64 Smith, W., 259, 260(31), 290 Smutek, M., 353(74), 365(96), 366(96), 377(96), 394 Snagovskij, Yu. S., 4(13), 23(62), 32(62), 51, 63 Snyder, P. W., 77(50), 115(140), 117(140), 127, 129 Soardo, P., 362(90), 394 Solbrig, C. W., 102(111), 129 Somenzi, G., 24(86), 64, 267, 290 Somorjai, G. A., 110(133), 129 Speakman, J. C., 299(12), 341 Spitz, R., 175(96), 179(96), 181(96), 216 Sporka, K., 23(64, 72), 63 Stach, H., 175(65, 66, 67), 177, 916 Stalibski, B., 251 (21), 290 Stark, G., 356(78), 364(78), 394 Starke, K., 345(451), 356(78), 364(78), 393, 394 Starkman, E. S., 63(20), 126 Startsev, A. N., 191(142a), 218 Steele, W. A., 350(53), 393 Stefan, A., 109(130), 129 Stefanovska, N. N., 189(138), 217 Stefoglo, E. F., 24(87d), 64 Steiner, H., 175(38), 179(38), 216 Steinrucke, E., 185(122), 217 Stemberg, V. R. H., 231(11), 243 Stimpson, B. P., 344(5), 391 Stobaugh, R. B., 231(11), 243 Stover, W. A., 77(50), 127
Strelets, M. M., 23(62), 32(62), 63 Sugahara, H., 145(71), 170 Sukhareva, G. A., 23(68), 63 Swift, H. E., 138(40), 169 Swift, P., 153(95), 171 Switendick, A. C., 251, 290 Syzdykbaeva, M. B., 128 Szummer, A., 269(47), 287(47), 290
T Tada, T., 7(39), 23(39), 62 Tajbl, D. G., 90(84), 122(84), 128 Takagi, Y., 5(23), 23(23, 63), 61, 63 Takashima, K., 175(47), 216 Takasu, Y., 273(57), 291 Takeo, Sh., 192(147), 218 Tamm, P. W., 345(34), 351(34), 362(34), 380(34, 107), 388(107), 389(34, 107), 392,394 Tanaka, H., 175(36, 43), 203(36), 816 Tanaka, K., 39(109), 66, 185(122), 817 Tani, K., 186(129), 217 Tarama, K., 175(44, 55, 59), 178(55, 59), 216 Tashiro, M., 23(65), 63 Tatsumi, T., 138(48), 139(48), 170 Taylor, K. C., 78(53), 86(53), 127 Tazima, Yo., 186(129,130), 217 Tazuma, J. J., 141(59), 170 Tchumaevski, N. M., 201(174), 219 Teichner, S. J., 86(78), 128 Tellier, J., 39(106), 66 Temkin, M. I., 3(8), 61, 353(73), 394 TeyssiB, Ph., 141(59a), 170 Theisen, D., 132(7), 158(7), 168 Thieme, F., 345(45f), 362(45f), 363(45f), 392 Thodos, G., 102(114, 115), 199 Thomas, C. L., 62(17), 126 Thomas, G., 4, 15, 16, 30(16), 61 Thomas, N. T., 79(59), 127 Thomas, W. J., 24(91a), 64 Thonon, C., 5(17), 22(17), 39(17), 61 Tiedema, T. J., 269(46), 290 Tinyakova, E. I., 144(66), 170, 189(138), 217 Tobias, R. S., 296(7), 297(7), 300(7), 341 Tobin, M. C., 314(23), 316, 341
412
AUTHOR INDEX
Tollefson, E. L., 97(104), 188 Tolstopiatova, A. A., 284(71, 72, 731, 291 Topchiev, A. V., 175(20, 22, 23, 24, 27), 814 Topchieva, K. V., 175(23, 24, 27), 214 Topley, B., 254, 290 Toya, T., 268(44), 290 Trambouze, Y., 161, 171 Trapnell, B. M. W., 344(18), 392 Treitz, N., 389(124), 396 Tretyakov, I. I., 91(87), 128 Triff, W. C . , 362(91), 394 Tsao, U., 224(5), 843 Tsong, T. T., 344(9), 391 Tsuchida, T., 252(24), 290 Tsut8, K., 7(39), 23(39), 52 Tucker, H., 158(103), 171 Tuesday, C. S., 67(27), 126 Turbett, R. J., 178(109), 211(193), 217, 819 Turkevich, J., 175(51), 179(51), 181(51), 816, 294(2), 321, 324(2), 335(2), 336 (2), 337(2), 338, 341 Turlier, P., 175(57), 176(57), 816 Turner, G. E., 39(112), 40(112), 66 Turner, L., 136(37, 38), 137(37, 38), 145 (67), 169, 170 Tyulikova, T. Ya., 175(64, 78), 216, 816
U Uchida, A., 168(118), 171 Uchida, Y., 138(48), 139(48), 170 Uematsu, T., 24(84b), 64 Unland, M., 96, 128 Urbach, E., 344, 392 Ustinov, Yu. K., 344(28), 345(28), 352 (62, 63), 357(28), 358(28), 361, 363 (28, 62), 389(119), 398, 393, 396
V Valerio, A., 7(40), 23(40), 62 van Barneveld, J., 12(50), 20(50), 66 van Beekum, H., 12(50), 20(50), 39(110), 62,66 Van Dam, P. B., 133(12), 169 van de Putte, K. J. G., 39(110), 66
van der Borg, R. J. A. M., 14,23(54), 58 Van der Lugt, W. Th. A. M., 147(79), 170 van der Planck, P., 268(45), 271(45), 277 (45), 890 van Lenden, P. W., 185(118), 186(118), 21 7 Van Reijen, L. L., 175(45), 216 Vardi, J., 115(139), 189 Vasyunina, N. A., 24(87d), 54 Vdovin, V. M., 134(18a), 169 Vejrosta, J., 41(120), 42(120), 66 Vermeulen, T., 6(34), 62 Vincent, L., 345(45b), 398 Visser, F. R., 143(63), 147(63), 170 Volter, J., 271, 290, 345(45e, 45h), 371, 391,393 Voevodski, V. V., 175(33), $14 Volger, H. C., 146(76), 170 Volkova, A. N., 23(71), 63 Voltz, S. E., 77(.52), 91, 102(112), 104 (112), 111(112), 187, 188, 189 Voorhies, A., Jr., 7(36a), 24(89), 62, 64 Voorhoeve, R. J. H., 79(62,63), 187 Vortmeyer, D., 109(128), 122(144), 189 Voste, B., 23(64), 63 Votinov, M. L., 175(25), 814 Votruba, J., 106(120), 107(121), I89 Vuillaume, G., 175(96), 179(96), 181(961, 816
W Wagner, N. J., 273, 891 Wajc, S. L., 4, 61 Walsh, J., 260(36), 262(36), 290 Walter, D., 185(122), 817 Wang, J. L., 138(49, 50), 139(49, 50), 145 (49), 149(49), 153(96), 158(49), 159 (49), 170, 171 Wanke, S. E., 8(43a), 62 Ward, J. P., 132(3), 134(3), 135(19), 138 (46), 143(3, 46), 145(3, 46), 156(3), 158(46), 159(46), 164(3,46), 168, 169, 1 ro Wasserman, E., 132, 134(4), 135(26), 168, 169 Watson, A. M., 263, 264, 286(37), 290 Watson, C. C., 1(3), 61 Watson, D. S., 323(35), 342
413
AUTHOR INDEX
Watson, K. M., 70(34), 126 Wauquier, J. P., 11,12, 20,22(48), 39(48), 62 Weaver, E. E., 97(103), 109(129), 110 (129), 128, I29 Webb, G., 11(47), 62 Weber, E., 175(37), 216 Weber, I., 345(45k), 393 Wedler, G., 345(45c, 45d), 392 Wei, J., 6, 17, 24, 62, 108(127), 115, 117 (140), 129 Weiss, A. H., 5, 23(22), 61 Weisz, P. B., 17(56), 21, 24,62, 100, 129 Wells, P. B., 10, 11(47), 21(4.5,46), 52 Wepster, B. M., 12(50), 20(50), 62 Werber, F. X., 175(10), 192(148), 193(10), 194(148), 210(148), 214, 218 Wesson, T. C., 68(30), 226 Whalley, L., 286, 291 Whan, D. A., 138(39), 153, 159(107), 161 (94), 162(94), 169, 1Yl Wharton, E. J., 138(52), 139(52), 170 Wheeler, A., 182(114), 217 Whitaker, H. L., 175(93), 181(93), 216 White, J. M., 344(14), 392 White, R., 148(85a), 170 Whitesides, G. M., 154(98), 155(98), 171 Wicke, E., 249(10), 250(10), 289 Wiesendanger, H. U. D., 362(89), 394 Wilhoyte, H. J., 80(66), 12Y Wilke, G., 185, 217 Wilkinson, G., 203(180), 219 Wilkinson, M. G., 250(17), 289 Williams, E. St,. J., 250, 289 Williams, K. V., 136(37, 38), 137(37, 381, 269 Williamson, S. J., 58(6), 126 Willis, J. N., Jr., 323, 326, 342 Wills, G. B., 153, 162, 163(116), l Y l , 175 (93), 181(93), ,916 Wilson, R. T., 162, 171 Winterbottom, J. M., 11(47), 62 Winterbottom, W. L., 384, 394 Wise, H., 77, 127 Witte, J., 138(56), 140(56), 158(56, 10% 165(56), lY0, 171 Wittig, G., 151(91), 271 Wolff, P. A., 310(17), 341 Wollan, E. O., 250(18), 289
Wolovsky, R., 132(4), 134(4), 135(25), 168, 169 Woodward, L. A., 296(9), 297(9), 34f Woodward, R. B., 145, I70 Worsham, J. E., 250(17), 289 Wristers, J., 145(71), 1YO Wsaolek, W. R., 175(10), 192(148), 193 (lo), 194(148), 210(148), 214, 218 Wu, C., 175(9), 187(9), 188(0), 189(9), 191 (9), 214
Y Yagi, T., 192(147), 218 Yakerson, V. I., 362(85), 387,394 Yamashina, T., 273(57), 291 Yaney, P. P., 326, 327(37), 328(37), 342 Yao, H. C., 110(132), 129 Yarborough, J. M., 310(15), 341 Yasumori, I., 267, 290 Yates, J. T., Jr., 344(4), 345(36, 37, 46f), 346(46i, 46j), 351(61), 362(37), 363 (61), 376(36), 388(36, 37), 389(36, 37, 115, 116, 121), 391, 392, 393, 395 Yermakov, Yu. I., 175(8, 49, 60,64,68, 73, 75, 76, 77, 79, 88, 89, 92, 97, 98, 100, 101, 103, 107), 177(49), 178(68, 971, 179(88, 89, 92, 97, 98), l81(98, 103, l l l ) , 183(88, 89, 90, 115), 184(115), 185(125, 126), 186(125), 187(8, 126, 131), 188(8, 137), 189(8, 137, 139), 190(8, 139, 140), 191(8, 139, 141, 142, 142a, 143, 144, 144a, 145, 145a), 192 (145, 145a, 146, 146a), 193(151, 156, 158, 159), 194(156, 158, 159), 195 (160a), 196(158, 1591, 197(98, 111, 167, 168), 198(98, 140, 158, 159, 168, 168a), 199(158, 159), 200(158), 201 (174), 202(175), 203(158), 205(183), 206(175), 208(75, 97, 159, 168a), 209 (75, 159), 210(158), 211(8, 140, 169), 212(111,195), ~ l 4 , 6 1 6 , 9 1 6 , 2 1 7 , 1 1 8 , 219
Yermakova, A., 24(87d), 64 Yokoyama, S., 3(9), 61 Yolles, R. S., 77, 1.97 Yoneda, Y., 24(84), 65 Yonehara, K., 388(113), 389(113), 396 Yuasa, S., 192(147), 218
414
AUTHOR INDEX
Yurnhak, S., 122(142), la9 Yu Yao, Y. F., 86(79), la8
2 Zakharov, V. A., 175(8, 68, 77, 92, 97, 98), 178(68, 97), 179(92, 97, 98), 181(98, l l l ) , 183, 184(115), 185(125), 186 (125), 187(8, 131), 188(8, 137), 189 (8, 137), 190(8, 140), 191(8), 193(151, 156, 158, 159), 194(156,158, 159), 195 (160a), 196(158, 159), 197(98, 111, 168), 198(98, 140, l58,159,168,168a), 199(158, 159), 200(158),201(174), 202
(175), 203(158), 205(183), 206(175), 208(97, 159, 168a), 209(159), 210 (158), 211(8, 140, 169), 212(111, 195), a14,816,ais,air, ai8,ai~ Zanderighi, L., 24(82, 85), 26(98), 38(98), 40(98), 41(98), 63,64 Zdralil, M., 39(114), 66 Zechnall, R., 73(46), Id7 Zhavoronkova, K. M., 272, 991 Ziegler, K., 175, 914 Zienty, F. B., 23(74), 63 Zimmermann, H., 185(122), dl7 Zucchini, U., 185(117), 187(117),dl7 Zuech, E. A., 134(13), 136(13), 138(13), 139(13), 145(13, 68), 169, 170
Subject Index A
thermal aging and destruction, 111, 112 vehicle aging, 112-1 14
Acetylene in exhaust gases, 67 B hydrogenation of, on Pd, 264,267 Acrylonitrile manufacture of, using catalysts, 238,239 Base metal oxides, oxidation over, 86-89 Benzene in exhaust gases, 67 metathesis of, 133 Bromopentene, metathesis of, 133 Acyclic alkadienes, metathesis of, 134 Acyclic alkenes, metathesis reaction of, Butadiene, metathesis of, 134 133, 134 stereoselectivity, 158 C type of reactions, 142 transalkylation, 142-144 Carbon dioxide transalkylidenation, 142-144 from motor vehicles, 65 Acylation, use of catalysts in, 224 stretching vibrations of, 301, 302 Adsorbates, Raman spectra of, 333-339 Carbon monoxide symmetrical, 335, 336 Federal emission control requirements, Adsorbents, 361-364 59,60 Adsorption from motor vehicles, 58, 59,65 rate of, 352, 353,360 oxidation of, 86-94 thermal, see Readsorption Carboxylic acids, ketonization of, 35-37 Air pollution, 58 Catalysts, see also specific substances Alkenea activity of, 160, 161 aging and decay of, 228-230 deuterated, 143 metathesis of, equilibrium distributions cost per ton of product, 224, 233 for, 157-159 for coupled heterogeneous reactions, Alkylation, use of catalysts in, 224 26-28 heterogeneous, 230, 231 Alkynes cyclotrimerization of, 154, 155 homogeneous, 230,231 metathesis of, 136, 154, 155 major industrial uses, 224 Alloys as automotive catalysts, 80 one-component, 173-213 Automotive catalysts, 58, 77-82 output, 222, 223 bed configurations, 82, 83 propagation centers, see Polymerization fluid flow in bed, 98,99 self-poisoning, 253 heat and mass transport in, 100 structure of, 152-154 within bed, 106-109 supported oxide, 174, 175 from gases to solid surfaces, 101-106 two-component, 174, see also Zieglerkinetics and mechanisms, 86-97 Natta catalysts slow aging from poisons, 109, 110 Catalytic converter supports for, SO, 81 automotive, 71, 72 testing of, 78 single-bed, 72, 74 415
416
SUBJECT INDEX
Catalytic converter (continued) design of, 75-77, 83-86 dual-bed, 73 mathematical model, 114118 reactor engineering, 114-122 pkton flow, 118, 119 stirred tank, 120-122 single-bed, 72, 74 durability of, 109-114 of exhaust gases, 58, 59, 62, 63 Catalytic cracking, 224 Catalytic processes aging and decay of catalyst, 228-230 business portfolios, entrance fees and, 236-241 cash flow models, 233-235 competition and, 235, 236 learning curves, 236 computer monitoring, 230 economics of, 221-243 improvement and assessment, 241243 standard cost sheet, 232,233 heat transfer, 228 product contamination, 228 thermodynamics of, 226, 227 Catalytic reactions, see specific types Catalytic reactors, automotive, 58 Catalytic reforming, 224 Catenanes, 135 1-Chloro-l , 5-cyclooctadiene, metathesis of, 136 Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176, 177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 3 7 4 3 Copper chromite, oxidation of CO over, 86-88
Copper-nickel alloys, see Nickel-copper alloys Copper oxide, oxidation of CO over, 86 Coupled heterogeneous catalytic reactions, kineties of, 1-49, see also Kinetics coupling through catalytic surface, 9-13 experimental studies, 22-49 apparatus and procedure, 25, 26 catalysts, 26-28 nonsuitability of power-law type equations, 21, 22 selectivity and relative reactivity, 18-21 slow steps in, 13-17 Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-1 44 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5, BCyclododecatriene, metathesis of, 135 Cyclohexene, unreactivity, thermodynamics, 156 1,SCyclooctadiene, metathesis of, 135 stereoselectivity, 159, 160 Cyclooctene, metathesis of, catalysts for, 140 Cyclopentene, metathesis, catalysts for, 140
D 1,5,9-Decatriene, metathesis of, 134 Desorption activation energy of, 365-372, 376-380 distribution of, 384-386 on surface, 381-384 surface coverage, 386-388 associative, 351 chemical method, 344 effect of surface on, 380-388
417
SUBJECT INDEX
kinetics, 352 parameters, 372-380 mass balance, 354-361 nonassociative, 349 order of, 365-372, 375, 376 physical method, 344 preexponential factor, 365-372 rate of, 347-353, 370 effect of pumping speed on, 355, 356 vs. readsorption, 371, 372 theoretical predictions, 349, 350 thermal adsorption studies, 343-389 chemisorption and, 345 definitions and relationships, 347-361 temperature schedules in, 361-364 thermodynamic data, 350 Dichloroethylene, symmetry analysis of, 305 Diesel engine, 123
E Ethane in exhaust gases, 67 Ethene configuration of, 145, 146 metathesis of, 149 Ethylene in exhaust gases, 67 hydrogenation of using nickel or nickel-copper alloys &9 catalysts, 269, 270, 282 using palladium hydride as catalyst, 265, 266 polymerization of, 185, 186 number of propagation centers and maximum activity, 201 using r-ally1 compounds, 185, 186, 188, 189 using arene and cyclopentadienyl compounds, 186, 189 using a-organometallic compounds, 185, 188 using supported organometallic compounds, 187-189 Exhaust gases, see also specific substances automotive sir-to-fuel ratios, 65, 66 composition of, 65-68
gas flow rates and temperatures, 64,
65 properties of, 63-71 recirculation, 71 thermal properties, 69-71 thermodynamic equilibrium, 68, 69 transcience of, 63-65 catalytic converters of, 58, 59 equilibrium constants, 68-70 fluid mechanics of, 97-99 heats of combustion, 70, 71 physical transport processes, 97-109
F Fertilizers, 222, 223 Fibers, 222, 223 Flow systems, see Desorption Fluorescence, 321-327 mechanism of, 323, 324 treatment of, 324-327 Fuels, 222, 223
G Gold-palladium alloys, see Palladium-gold alloys
H Hafnium compounds as catalysts, 188 1,SHexadiene, metathesis of, 134 Hexyne, metathesis of, 136, 154 Hydrocarbons, see also specific compounds Federal emission control requirements, 59, 60 hydrogenation of, 20 from motor vehicles, 58, 59, 65-68 oxidation of, 88, 89 unsaturated, metathesis reaction of, 131-168 Hydrodesulfurization, use of catalysts in, 224 Hydrogen catalytic reactivity of, 245-289 from motor vehicles, 65 Hydrogenation, use of catalysts in, 224 nickel, 269, 270 palladium, 265-267
418
SUBJECT INDEX
Hydrotreating, 224 use of catalysts in, 224
I Infrared spectra differences from Raman spectra, 302304 molecular symmetry and, 304, 305 Iron oxide in catalytic converter, 62 Isobutene, metathesis of, 134 equilibrium distributions for, 159
K Kinetics elimination of time variable, 4-7 isolation of individual reactions, 7, 8 principles of analysis, 3-8 simultaneous solving, 3, 4
L Laser Raman spectroscopy, 293-341, see also Raman spectroscopy
M Manganese oxide in catalytic converter, 62 Metal hydrides, see also Transition metal h ydrides catalytic activity of, 283-285 Metathesis reaction of unsaturated hydrocarbons, 131-168 heterogeneous systems, 136-138 homogeneous systems, 138-141 kinetics of, 160-168 mechanisms, 141-155 reactants, 132-136 stereoselectivity, 157-160 structure of active catalyst, 152-154 thermodynamics, 155-157 type of reaction, 141-144 Methane in exhaust gases, 67 5-Methylcyclooctene, metathesis of, 135, 136 Methyl-9-octadecenoate, metathesis of, 13 Molybdenum compounds as catalysts, 136, 137, 141, 144, 151, 191, 192 activity of, 160 structure of, 153, 154
Molybdenum oxide as catalyst, 174 Motor vehicle emissions catalysis for, 57-125, see also Exhaust gases, automotive Federal control requirements, 60, 61
N Nickel compounds as catalysts, 191 Nickel-copper alloys, 252, 253 atomic hydrogen recombination, 273279 catalytic activity of, 268-283 para-hydrogen conversion, 270 poisoning effects, 271-274 X-ray diffraction, 277-280 Nickel hydride atomic hydrogen recombination, 273279 catalytic activity of, 268-283 catalytic reactivity of hydrogen on, 245-289 formation, structure, and properties of, 247-253 isotherms characteristic of, 249 neutron diffraction, 250 para-hydrogen conversion, 270 poisoning effect, 271-274 thermodynamic data for, 250 X-ray crystallography, 250 Nickel oxide in catalytic converter, 62 Nitrogen oxides decomposition and reduction of, 94-97 Federal emission control requirements, 59,60 from motor vehicles, 58, 59, 65 Noble metals as automotive catalysts, 79-81 oxidation over, 89-94 Norbornadiene, dimerization of, 146-148 Norbornene, metathesis of, 136
0 Olefins in exhaust gases, 66, 67 hydrogenation of, using palladium hydride as catalyst, 265, 266 polymerization of using chromium oxide catalysts, 175184
SUBJECT INDEX
using one-component catalysts, 173213 using organometallic compounds, 184-187 using transition metal organometallic compounds aa catalysts, 184-192 Oxidation, use of catalysts in, 224
P
419
Polymerization active center formation in organometallic compounds, 186, 187, 189191 effective surface of catalyst, 181 kinetics of, 178-184, 194 macrograins, 183 micrograins, 182, 183 monomer concentration at catalyst surface, 181-183 of olefins, see also Olefins catalysts for, 173-213 primary particles, 181, 182 propagation centers of catalysts in, 202-2 13 number of, 194-202 change in, 183, 184 maximum activity of catalysts, 200-202 method to determine, 195-197 in one-component catalysts, 197202 propagation rate constant, 180, 181 use of catalysts in, 224 Propane in exhaust gases, 67 Propene, metathesis of, 133 equilibrium distributions for, 158 heterogeneous, kinetic model, 161-164 reaction mechanism, 148 solid catalysts for, 137 transalkylation, 142 transalkylidenation, 143, 144 Propylene in exhaust gases, 67 hydrogenation of, using palladium hydride as catalyst, 265, 266 Propyne, metathesis of, 136 Pyridine, Raman spectra of, 333, 334
Palladium as catalyst, self-poisoning, 253-255, 263 oxidation of CO over, 90 Palladium-gold alloys, 251, 252 atomic hydrogen recombination, 260262 catalytic activity of, 253-268 para-hydrogen conversion, 254, 255 Palladium hydride Arrhenius plot for, 257 atomic hydrogen recombination, 260262 catalytic reactivity of hydrogen on, 245-289 formation, structure, and properties of, 247-253 isotherms characteristic of, 247-249 kinetic data for, 258 neutron diffraction, 250 thermodynamic data for, 249, 250 X-ray crystallography, 250 Palladium-silver alloys, 251, 252 catalytic activity of, 253-268 para-hydrogen conversion, 259 Paraffins in exhaust gases, 66, 67 Pentene, metathesis of catalysts for, 139, 141 homogeneous, kinetic model, 161, 164168 Q stereoselectivity, 158 Quadricyclene, isomerisation of, 146, 148 Pentyne, metathesis of, 136 Pharmaceuticals, 222, 223 Phenol, hydrogenation of, 31-35 R Plastics, 222, 223 Platinum Raman effect in catalytic converter 6 in carbon dioxide, 301, 302 oxidation of CO over, 90-94 normal coordinates, 339-341 Polyalkenamers, 134, 135 origin of, 295-305 Polyenes, metathesis of, 134 polarizability ellipsoid, 299-301
420
SUBJECT INDEX
Raman effect (continued) spectral activity, 339-341 terminology of, 295 vibrational wavefunetions, 339-341 Raman lines, 296 weak, 327-330 Raman scattering, 296 classical theory, 297-299 quantum mechanical theory, 296, 297 Raman shift, 296 Raman spectra, 296, 298, 303, 304 of adsorbed molecules, 333-339 of adsorption systems, 320-332 of Cab-0-Sil disk, 320 different from infrared spectra, 302-304 effect of fluorescence on, 321-327 molecular symmetry and, 304, 305 of oxides, 321 principle of mutual exclusion, 304 Raman spectroscopy, see also Raman spectra commercial spectrometers, 315 detectors, 314-3 17 photomultipliers, 314 signal processing, 314-317 instrumentation, 306-320 laser, 293-341 adsorbate-adsorbent systems, 337 interfering plasma lines, 330-332 lines, 310 source, 306-311 argon ion, 308 argon-krypton, 309, 310 helium-neon, 308, 309 krypton ion, 309 relative performance, 316 tunable dye, 310, 311 spectral background, 321-327 monochromators, 311-314 resolution, 314 stray light, 311-314 sampling technique, 317-320 cells, 319, 320 illumination, 317, 318 Readsorption, 347353 rate of, 360 vs. desorption, 371, 372 negligible, 365-371
Rhenium compounds as catalysts, 136, 137, 141, 144, 146, 148, 150 Ring-opening polymerization, 143 catalysts for, 140 thermodynamics of, 156 Rotary engine, 123
5 Silver-palladium alloys, see Palladiumsilver alloys Stokes scattering, 296, 297 Styrene, metathesis of, 133
T Tetralin, hydrogenation of, 12 Titanium compounds as catalysts, 188 Titanium dichloride, 192, 193 number of propagation centers, 198-200 Titanium trichloride, 193, 194 Toluene in exhaust gases, 67 Transalkylation, 141, 142 Transalkylidenation, 142 Transition metal compounds as catalysts, 174 coordinative insufficiency of ions, 202208 metal-carbon u-bond, 208-213 for metathesis reactions, 131-168 organometallics, 184-192 solid, 136-138 soluble, 138-141 Transition metal organometallic compounds, 184-192 catalysts formed with oxide supports, 187-192 for nonpolymerization reactions, 192 Transition metal subhalides as catalysts, 174, 192-194 preparation of, 192-194 Tris-r-allylchromium, number of propagation centers, 198 Tungsten compounds as catalysts, 136, 137, 141, 144, 149 activity of, 160, 161 structure of, 152, 153 Tungsten oxide as catalyst, 174
42 1
SUBJECT INDEX
U Urethane formation, use of catalysts in, 224
V Vanadium oxide as catalyst, 174 Vinyl chloride, manufacture of, using catalysts, 238
W Water vapor from motor vehicles, 65
X Xylenes hydrodemethylation of, 28-31 hydrogenation of, 12
Z Zeolites, 321, 322 Ziegler-Natta catalysts, 174 Zirconium chlorides, 194 Zirconium compounds as catalysts, 188
Contents of Previous Volumes Volume 1
Entropy of Adsorption CHARLES KEMBALL About the Mechanism of Contact Catalysis GEORGE-MARIA SCHWAB
The Heterogeneity of Catalyst Surfaces for Chemisorption HUGHS. TAYLOR Allcylation of Isoparaffins V. N. IPATIEFF AND LOUISSCHMERLINQ Volume 3 Surface Area Measurements. A New Tool for Studying Contact Catalysts Balandin’s Contribution to Heterogeneous P. H. EMMETT Catalysis The Geometrical Factor in Catalysis B. M. W. TRAPNELL R. H. GRIFFITH Magnetism and the Structure of CatalytThe Fischer-Tropsch and Related Proically Active Solids cesses for Synthesis of Hydrocarbons by P. W. SELWOOR Hydrogenation of Carbon Monoxide Catalytic Oxidation of Acetylene in Air for H. H. STORCH Oxygen Manufacture The Catalytic Activation of Hydrogen J. HENRYRUSHTON AND K. A. KRIEQER D. D. ELEY The Poisoning of Metallic Catalysts Isomerization of Alkanes E. B. MAXTED HERMAN PINES Catalytic Cracking of Pure Hydrocarbons The Application of X-Ray Diffraction to VLADIMIR HAENSEL the Study of Solid Catalysts Chemical Characteristics and St,ructure M. H. JELLINEK AND I. FANKUCHEN of Cracking Catalysts A. G. OBLAD,T. H. MILLIKEN,JR.,AND G. A. MILLS Volume 2 Reaction Rates and Selectivity in Catalyst Pores The Fundamental Principles of Catalytic AHLBORN WHEELER Activity Nickel Sulfide Catalysts FREDERICK SEITZ WILLIAMJ. KIRKPATRICK The Mechanism of the Polymerization of Alkenes LOUISSCHMERLING AND V. N. IPATIEFF Volume 4 Early Studies of Multicomponent CataChemical Concepts of Catalytic Cracking lysts ALWINMITTASCH R. C. HANSFORD Catalytic Phenomena Related to Photo- Decomposition of Hydrogen Peroxide by Catalysts in Homogeneous Aqueous graphic Development T. H. JAMES Solution J. H. BAXENDALE Catalysis and the Adsorption of Hydrogen Structure and Sintering Properties of on Metal Catalysts Cracking Catalysts and Related MateOTTOBEECK Hydrogen Fluoride Catalysis rials E. RIES, JR. HERMAN J. H. SIMONS 422
CONTENTS OF PREVIOUS VOLUMES
Acid-base Catalysis and Molecular Structure R. P. BELL Theory of Physical Adsorption TERRELL L. HILL The Role of Surface Heterogeneity in Adsorption GEORGE D. HALBEY Twenty-Five Years of Synthesis of Gasoline by Catalytic Conversion of Carbon Monoxide and Hydrogen HELMUT PICHLER The Free Radical Mechanism in the Reactions of Hydrogen Peroxide JOSEPH WEISS The Specific Reactions of Iron in Some Hemoproteins PHILIP GEORQE
423
Volume 6 Catalysis and Reaction Kinetics at Liquid Interfaces J. T. DAVIEB Some General Aspects of Chemisorption and Catalysis TAKAO KWAN Noble Metal-Synthetic Polymer Catalysts and Studies on the Mechanism of Their Action AND F. F. WILLIAMP. DUNWORTH NORD Interpretation of Measurements in Experimental Catalysis P. B. WEISZAND C. I). PRATER Commercial Isomeriaation B. L. EVERINQ Acidic and Basic Catalysis MARTINKILPATRICK Industrial Catalytic Cracking RODNEY V. SHANKLAND
Volume 5 Volume 7 Latest Developments in Ammonia Synthesis ANDERS NIELSEN Surface Studies with the Vacuum Microbalance: Instrumentation and LowTemperature Applications T. N. RHODIN,JR. Surface Studies with the Vacuum Microbalance: High-Temperature Reactions EARLA. GULBRANSEN The Heterogeneous Oxidation of Carbon Monoxide MORRISKATZ Contributions of Russian Scientists to Catalysis J. G. TOLPIN, G. S. JOHN, AND E. FIELD The Elucidation of Reaction Mechanisms by the Method of Intermediates in Quasi-Stationary Concentrations J. A. CHRISTIANSEN Iron Nitrides as Fischer-Tropsch Catalysts ROBERT B. ANDERSON Hydrogenation of Organic Compounds with Synthesis Gas MILTONORCHIN The Uses of Raney Nickel EUQENE LIEBERAND FRED L. MORRITZ
The Electronic Factor in Heterogeneous Catalysis M. McD. BAFERAND G. I. JENKINS Chemisorption and Catalysis on Oxide Semiconductors G. PARRAVANO AND M. BOUDART The Compensation Effect in Heterogeneous Catalysis E. CREMER Field Emission Microscopy and Some Applications to Catalysis and Chemisorption ROBERT GOMER Adsorption on Metal Surfaces and Its Bearing on Catalysis JOSEPH A. BECKER The Application of the Theory of Semiconductors to Problems of Heterogeneous Catalysis K. HAUFFE Surface Barrier Effects in Adsorption, Illustrated by Zinc Oxide S. ROYMORRISON Electronic Interaction between Metallic Catalysts and Chemisorbed Molecules R. SUHRMA”
424
CONTENT8 OF PREVIOUS VOLUMES
Volume 8 Current Problems of Heterogeneous Catalysis J. ARVIDHEDVALL Adsorption Phenomena J. H. DE BOER Activation of Molecular Hydrogen by Homogeneous Catalysts S. W. WELLERAND G. A. MILLS Catalytic Syntheses of Ketones V. I. KOMAREWSKY AND J. R. COLEY Polymerization of Olefins from Cracked Gases EDWIN K. JONEB Coal-Hydrogenation Vapor-Phase Catalysts E. E. DONATH The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts CHARLES D. P R A T E R AND RUDOLPH M. LAGO Volume 9 Proceedings of the International Congress on Catalysis, Philadelphia, Pennsylvania, 1956. Volume 10
Volume 11 The Kinetics of the Stereospecific Polymerization of a-Olefins G. NATTAAND I. PASQUON Surface Potentials and Adsorption Process on Metals R. V. CULVER AND F. C. TOMPKINS Gas Reactions of Carbon P. L. WALKER,JR., FRANK RUSINKO, JR., AND L. G. AUSTIN The Catalytic Exchange of Hydrocarbons with Deuterium C. KEMBALL Immersional Heah and the Nature of Solid Surfaces J. J. CHESSICK AND A. c. ZETTLEMOYER The Catalyt,ic Activation of Hydrogen in Homogeneous, Heterogeneous, and Biological Systems J. HALPERN Volume 12 The Wave Mechanics of the Surface Bond in Chemisorption T. B. GRIMLEY Magnetic Resonance Techniques in Catalytic Research D. E . O'REILLY Bare-Catalyzed Reactions of Hydrocarbons HERMAN PINESAND LUKEA. SCHAAP The Use of X-Ray K-Absorption Edges in the Study of Catalytically Active Solids ROBERTA. VANNORDBTRAND The Electron Theory of Catalysis on Semiconductors TH.WOLKENSTEIN Molecular Specificity in Physical Adsorption D. J. C. YATES
The Infrared Spectra of Adsorbed Molecules It. P. EISCHENB AND W. A. PLISKIN The Influence of Crystal Face in Catalysis ALLANT. GWATHMEY AND ROBERT E. CUNNINGHAM The Nature of Active Centres and the Kinetics of Catalytic Dehydrogenation A. A. BALANDIN The Structure of the Active Surface of Cholinesterases and the Mechanism of Their Catalytic Action in Ester Hydrolysis F. BERGMANN Commercial Alkylation of Paraffins and Volume 13 Aromatics Chemisorption and Catalysis on Metallic EDWINK. JONES Oxides The Reactivity of Oxide Surfaces F. S. STONE E. R. S. WINTER The Structure and Activity of Metal-on- Radiation Catalysis R. COEKELBERGS, A. CRUCQ,AND A. Silica Catalysts FRENNET G. C. A. SCHUITAND L. L. VAN REIJEN
CONTENTS OF PREVIOUS VOLUMES
425
Polyfunctional Heterogeneous Catalysis Electronic Spectroscopy of Adsorbed Gas Molecules PAULB. WEISZ A. TERENIN A New Electron Diffraction Technique, Potentially Applicable to Research in The Catalysis of Isotopic Exchange in Molecular Oxygen Catalysis G. K. BORESKOV L. H. GERMER The Structure and Analysis of Complex Volume 16 Reaction Systems JAMES WEI AND CHARLES D. PRATER The Homogeneous Catalytic Isomerization Catalytic Effect in Isocyanate Reactions of Olefins by Transitlion Metal ComA. FARKAS AND G. A. MILLS plexes MILTONORCHIN The Mechanism of Dehydration of AlcoVolume 14 hols over Alumina Catalysts HERMAN PINES AND JOOST MANASSEN Quantum Conversion in Chloroplast,s r Complex Adsorption in Hydrogen ExMELVIN CALVIN change on Group VIII Transition Metal The Catalytic Decomposition of Formic Catalysts Acid J. L. GARNETTAND W. A. SOLLICHP. MARS, J. J. F. SCHOLLEN,AND BAUMGARTNER P. ZWIETERING Application of Spectrophotometry to the Stereochemistry and the Mechanism of Hydrogenation of Unsaturated HydroStudy of Catalytic Systems carbons H. p. LEFTIN AND M. c. HOBSON, JR. SAMUEL SIEGEL Hydrogenation of Pyridines and Quinoof Siirface Groups Chemical Identification lines H. P. BOEHM MORRISFREIFELDER Modern Methods in Surface Kinetics: Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques GERTEHRLICH Catalytic Oxidat,ion of Hydrocarbons L. YA. MARGOLIS Volume 15 The Atomization of Diatomic Molecules by Metals D. BRENNAN The Clean Single-Crystal-Surface Approach to Surface Reactions N. E. FARNSWORTH Adsorption Measurement,s during Surface Catalysis KENZITAMARU The Mechanism of the Hydrogenation of Unsaturated Hydrocarbons on Transition Metal Catalysts G. C. BONDAND P. B. WELLS
Volume 17
On the Theory of Heterogeneous Catalysis JURO HORIUTI AND TAKASHI NAKAMURA Linear Correlations of Substrate Reactivity in Heterogeneous Catalytic Reactions M. KRAUS Application of a Temperature-Programmed Desorption Technique to Catalyst Studies R. J. CVETANOVIC AND Y. AMENOMIYA Catalytic Oxidation of Olefins HERVEYH. VOGE AND CHARLESR. ADAMS The Physical-Chemical Properties of Chromia-Alumina Catalysts CHARLESP. POOLE, JR. AND D. 8. MACIVER Catalytic Activity and Acidic Property of Solid Metal Sulfates Koao TANABEANI) TSUNEICHI TAKESHITA
426
CONTENTS OF PREVIOUS VOLUMES
Electrocatalysis s. SRINIVASEN, H. WROBLOWA, AND J. O’M. BOCKRIS
Volume 18 Stereochemistry and Mechanism of Hydrogenation of Naphthalenes on Transition Metal Catalysts and Conformational Analysis of the Products A. W. WEITKAMP The Effects of Ionizing Radiation on Solid Catalysts ELLISON H. TAYLOR Organic Catalysis over Crystalline Aluminosilicates P. B. VENUTOAND P. S. LANDIS On Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of r Complex Multicenter Processes G. N. SCHRAUZER
Volume 19 Modern State of the Multiplet Theory of Heterogeneous Catalysis A. A. BALANDIN The Polymerization of Olefins by Ziegler Catalysts M. N. BERGER, G. BOOCOCK, AND R. N. HAWARR Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts L. POLINSKI AND L. NAPHTALI Enhanced Reactivity at Dislocations in Solids J. M. THOMAS
Volume 20 Chemisorptive and Catalytic Behavior of Chromia ROBERTL. BURWELL,JR., GARYL. HALLER, KATHLEENC. TAYLOR, AND JOHN F. READ
Correlation among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activity KIYOSHIMORIKAWA, TAKAYASU SHIRASAKI, AND MASAHIDE OKADA Catalytic Research on Zeolites J. TURKEVICH AND Y. O N 0 Catalysis by Supported Metals M. BOUDART Carbon Monoxide Oxidation and Related Reactions on a Highly Divided Nickel Oxide P. C. GRAVELLE AND S. J. TEICHNER Acid-Catalyzed Isomerization of Bicyclic Olefins JEANEUGENEGERMAINAND MICHEL BLANCHARD Molecular Orbital Symmetry Conservation in Transition Metal Catalysis FRANK D. MANGO Catalysis by Electron Donor-Acceptor Complexes KENZITAMARU Catalysis and Inhibition in Solutions of Synthetic Polymers and in Micellar Solutions H. MORAWETZ Catalytic Activities of Thermal Polyanhydro-a-Amino Acids DUANEL. ROHLFINGAND SIDNEY w. Fox
Volume 21 Kinetics of Adsorption and Desorption and the Elovich Equation C. AHARONIAND F. C. TOMPKINS Carbon Monoxide Adsorption on the Transition Metals R. R. FORD Discovery of Surface Phases by LOW Energy Electron Diffraction (LEED) JOHN W. MAY Sorption, Diffusion, and Catalytic Reaction in Zeolites L. RIEKERT Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis CARLWAQNER
CONTENTS OF PREVIOUS VOLUMES
Volume 22
427
Volume 23
Hydrogenation and Isomerization over Metal Catalyzed Skeletal Reactions of Hydrocarbons Zinc Oxide J. R. ANDERSON R. J. KOKESAND A. L. DENT Chemisorption Complexes and Their Role Specificity in Catalytic Hydrogenolysis by in Catalytic Reactions on Transition Metals Metals J. H. SINFELT Z. KNOR The Chemisorption of Benzene Influence of Metal Particle Size in NickelR. B. MOYESAND P. B. WELLS on-Aerosil Catalysts on Surface Site The Electronic Theory of Photocatalytic Distribution, Catalytic Activity, and Reactions on Semiconductors Selectivity TH.WOLKENSTEIN R. VANHARDEVELD AND F. HARTOCI Cycloamyloses as Catalysts Adsorption and Catalysis on Evaporated DAVIDW. GRIFFITHSAND MYRONL. Alloy Films BENDER R. L. Moss AND L. WHALLEY Pi and Sigma Transition Metal Carbon Heat-Flow Microcalorimetry and Its. ApCompounds as Catalysts for the Polyplication to Heterogeneous Catalysis merization of Vinyl Monomers and P. C. GRAVELLE Olefins Electron Spin Resonance in Catalysis D. G. H. BALLARD JACK H. LUNSFORD
A
8 5 C 6 0 7 E E
F 9 G O
H 1
1 2 J 3
This Page Intentionally Left Blank