ADVANCES IN CATALYSIS
VOLUME 24
Advisory Board G . K. BORESKOV Novosibirsk, U.S.S.R.
M. BOUDART Stanford, Calijmnia...
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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
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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
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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 (,/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
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126
JAMES WE1
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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
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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).
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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. 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).
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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
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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.
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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.
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
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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
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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
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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
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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.
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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
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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).
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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.
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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
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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'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 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).
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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).
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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)
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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
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