Advances in Catalysis, Volume 30

ADVANCES IN CATALYSIS

VOLUME 30

Advisory Board

M. BOUDART Siunford, Culrfornru

V. B. KAZANSKY M o ~ t o n ,U S S R

G. A. SOMORJAI Berkeley, California

M . CALVIN Berkeley, Cuhjornru

A. OZAKI Tokvo, Japan

P. H. EMMETT Porrlund, Oreyon

G.-M. SCHWAB Munich, Cermuny

R. UGO Milun, Iruly

ADVANCES IN CATALYSIS VOLUME 30

Edited by

D. D. ELEY Department of Physical Chemistry The University Nottingham, England

HERMAN PINES Department of Chemistry Northwestern University Evanston, Illinois

PAULB. WEISZ Mobil Research and Development Corporation Princeton, New Jersey

1981 ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers

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COPYRIGHT @ 1981. BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART O F 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.

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ISBN 0-12-007830-9 PRINTED IN THE UNITED STATES OF AMERICA 81 82 83 84

9 8 7 6 5 4 3 2 1

Contents CONTRIBUTORS ............................................................... PREFACE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . JURO HoRIu~I(1901-1979) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii ix xi

Mechanisms of Skeletal lsomerization of Hydrocarbons on Metals F. G . GAULT I. 11. 111. IV. V. VI . VII. VIII.

.......................... Introduction . . . . . . . . . . . . . . . . . . . Approaches to Mechanis Bond S h i f t . . . . . . . . . . . . . . . . . . . . ............ Cyclic Mechanism . . . . . .............................. Hydrocracking . . . . . . . . . ................. Aromatization . . . . . . . . . . . . . . . . . . . . . . . . . . . . Skeletal Rearrangements Correlation between Met .......................... References . . . . . . . . . . . . . . . . . . . . . . .

1 5 16 28 48 52 58 72 90

Tin-Antimony Oxide Catalysts FRANK J. BERRY I. 11. 111. IV.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of the Catalyst . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of Bulk and Surface Properties on Catalytic Performance. . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . , , . . , . . . . . , . . . . . . , . . . . . . . . . . . . . . . . . . . .

97 99 116 128 129

Selective Oxidation and Ammoxidation of Propylene by Heterogeneous Catalysis ROBERTK . GRASSELLI A N D JAMES D. BURRINGTON I. 11. 111.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and Conclusions . . . . , , . . . . . . . . , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

133 135 161 162

vi

CONTENTS

Mechanism of Hydrocarbon Synthesis over Fischer-Tropsch Catalysts

.

P . BILOENAND W M . H . SACHTLER I. I1. I11. IV . V. v1. VII .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Product Distribution under Steady-State Conditions ...................... Kinetics ............................... Surface Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrocarbon Synthesis via Predeposited Carbidic Carbon . . . . . . . . . . . . . . . . . Fischer-Tropsch Synthesis via C ic Intermediates ..................... Suggestions for Future Research . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

165 169 178 187 195 206 212 214

Surface Reactions and Selectivity in Electrocatalysis GEORGE P . SAKELLAROPOULOS I. I1. I11. IV . V. VI . VII . VIII . I X. X.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... The Art of Electrocatalysis . .. Basic Electrocatalytic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption on Electrocatalysts ................ Electrocatalytic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrocatalytic Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrocatalytic Reactions and Mechanisms ..................... Techniques in Electrocatalytic Studies .................................. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

......................

218 220 223 240 264 280 290 299 309 321 322

Solvent and Structure Effects in Hydrogenation of Unsaturated Substances on Solid Catalysts LIBORCERVEN+ AND VLASTIMIL R~BICKA

.................................. Introduction n the Liquid Phase . . Kinetics of Ca Correlation Equations Describing the Effect of Structure of Reacting Compounds and Solvents on Reaction Kinetics Hydrogenation of Olefinic Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationships between the Effect of Structure of Substrates and Solvents on Reactivity ............................ Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of Symbols References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

335 336

AUTHORINDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECT INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONTENTSOFPREVIOUSVOLUMES ...............................................

319 397 413

I. I1. I11.

IV . V. VI . VII .

343 346 368 372 313 374

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

FRANKJ . BERRY,Department of Chemistry, University of Birmingham, Birmingham, England (97) P. BILOEN, Shell Research B. V., KoninklijkelShell-Laboratorium,Amsterdam, The Netherlands (1 65) JAMES D. BURRINGTON, Department of Research and Development, The Standard Oil Company (Ohio), Warrensville Heights, Ohio 44128 (1 33) LIBORCERVEN~, Department of Organic Technology, Prague Institute of Chemical Technology, I66 28 Prague 6, Czechoslovakia (335) F . G. GAULT,*Laboratoire de Catalyse, UniversitC Louis Pasteur, Strasbourg, Cedex 67008, France ( 1 ) ROBERTK. GRASSELLI, Department of Research and Development, The Standard Oil Company (Ohio), Warrensville Heights, Ohio 44128 ( 1 33) VLASTIMIL RPT~ICKA,Department of Organic Technology, Prague Institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia (335) W . M . H. SACHTLER, Shell Research B. V., KoninklijkelShell-Laboratorium, Amsterdam, The Netherlands (165) GEORGE P. SAKELLAROPOULOS,j’ Department of Chemical and Environmental Engineering, Rensselaer Polytechnic Institute, Troy, New York 12181(217)

* Deceased. Text prepared for publication by Y . Gault. t Present address: Department of Chemical Engineering, University of Thessaloniki, Thessaloniki, Greece.

vii

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Preface The memory of the recent 7th International Congress on Catalysis in Tokyo is still with us. It was one of the best organized and most flawless and efficient international meetings we have experienced. The world of catalytic science owes many thanks to the Japanese organizers and hosts of the Congress. The Congress radiated the glow of a very active growth of catalytic science and research around the entire world. There is an obviously mounting recognition of the importance to society as a whole of this broad field of molecular science. Man has recognized more explicitly than before the importance of the skills of selective conversion of molecular matter, to both the preservation and further evolution of man’s civilization. As we reflect over the time span since the first Congress in 1956, we perceive some substantial shifts in attitudes and approaches to the total bouquet of catalyst research. It is important to understand mechanisms and relationships in the functioning of the catalyst. Yet, to be soundly scientific or to progress useful understanding, we need not and cannot expect to understand all the really elementary steps; we need not explore ortho-para hydrogen conversion to “understand” every hydrogenation catalyst, or to know the molecular orbital description of the metal-carbon-oxygen-hydrogen system in order to improve Fischer-Tropsch catalysts. Perhaps we are overdramatizing by creating these examples, but only to illustrate a point. We seem to have arrived at a more balanced distribution of inquiry. It goes into a greater diversity of aspects of catalysis, each with appropriate scientific rigor. There is now an involvement of an even larger wealth and breadth of science, drawing on many “disciplines” from within and without the general area of chemistry. When we returned from Tokyo we found these observations reflected in the Table of Contents to this volume of this serial publication. Without deliberate design, here were six articles from six different countries. All are seeking to develop deeper understanding ; all dealing with catalyst materials and reactions of direct importance to our society : carbon-carbon rearrangement (Gault) ;controlled oxidation (Berry ;Grasselli and Burrington) hydrocarbon synthesis (Biloen and Sachtler) ; hydrogenation (Cerveny and RBZiEka) ; and a presentation (Sakellaropoulos) of electrocatalysis that reaches out to a field of catalysis whose growth and prime we believe is yet to come.

PAULB. WEISZ ix

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Juro Horiuti (1901-1979) Juro Horiuti was born in Sapporo, in northern Japan, on September 17, 1901 and graduated from the University of Tokyo in 1925. His interest in science was kindled at an early age, and he took up chemistry, a subject to which he devoted his later life. In his undergraduate days he selected physical chemistry as his major and studied under Professor Masao Katayama, who possessed a profound knowledge of thermodynamics. After his graduation, as a research student he studied the solubility of simple gases in organic liquids, primarily at the Institute of Physical and Chemical Research, Tokyo. Despite some lean years due to the unfavorable climate that prevailed for research at that time, he completed his work and received a Doctor of Science degree from his alma mater in 1932. In the same year he went to Germany to broaden his horizons. His move to Germany was timely since physical chemistry there was just beginning to make rapid progress due to the advent of quantum mechanics. He was able, therefore, to absorb the essence of the most advanced developments in the field from young researchers, some of whom later became outstanding scholars. He first studied in the laboratory of Professor A. Eucken in Gottingen. After completing his research on Fermi splitting of the Raman spectrum of chloroform, he came to the Kaiser Wilhelm I1 Institute, Berlin, in the spring of 1933 at the invitation of Professor M. Polanyi. By Horiuti’s own admission, Professor Polanyi’s invitation and collaboration proved to be a turning point in his life-Polanyi’s discourses and candid discussions with him about his doctoral thesis on solutions gave him the direction for his later career. Horiuti stayed in Berlin until August and learned of a new analytical microfloat technique for heavy water, after which he came to Manchester with Polanyi. At the University of Manchester Horiuti was welcomed as an Honorary Researcher. There he devoted all his time and effort to research, and as a result produced some of his most brilliant work. For example, some papers on deuterium coauthored with Polanyi were submitted to Nature as early as November 25 and December 16, 1933 and were published in Volume 132 (1934). In addition, he published other communications significant to chemical kinetics. This work was contemporary with that of Professors H. S. Taylor and E. K. Rideal on this subject. Among his communications, those describing isotopic exchange reactions of gaseous hydrogen with water and/or benzene on metal catalysts with enzymes or on enzymes alone may be the xi

xii

JURO HORIUTI

(1901-1979)

most noteworthy. Based on the exchange reactions of hydrogen with water, the mechanism for hydrogen electrode reactions was proposed, and, in addition, the so-called associated mechanism, based on the reactions of hydrogen with benzene, which assumed a half-hydrogenated state was also proposed. In 1935 the famous rule now named after Evans and Polanyi was proposed to develop the theory of prototropy. The fruitful collaboration of Horiuti and Polanyi came to an end when Horiuti was offered a professorship in physical chemistry at Hokkaido University, Sapporo. He returned to Japan in 1934 and held this position until 1965, when he reached the mandatory retirement age. In Sapporo he renewed his vigorous research activity on deuterium in collaboration with other zealous researchers. The first area he investigated, along with G. Okamoto, was the clarification of the mechanism of hydrogen electrode reactions. He did not attempt to clarify the single mechanism that was accepted as valid at that time (1935); rather he proposed a dual mechanism, i.e., one that made hydrogen recombination the rate-determining step on nickel electrodes, and this proposal was based on his theory and experimental data (1936). In the development of his theory, with the collaboration of G . Okamoto and K. Hirota, he extended the absolute rate theory proposed by H. Eyring in 1935so as to be applicable to heterogeneous reactions (1936) ; here he employed a semiempirical potential surface created through a method of his own. Research with platinum electrodes also was performed, with the assumption that a different mechanism from that with nickel electrodes would be involved. Based on these results the concept of stoichiometric number was proposed ; it later became well known in the course of research on the mechanism of ammonia synthesis in the 1950s. In 1940, the Imperial Prize of the Japanese Academy was awarded to him for his distinguished contribution to the study of chemical reactions. Perhaps this award helped lead to the establishment of the Research Institute for Catalysis, the first of its kind in the world, in Hokkaido University in 1943. During and after World War 11, Horiuti continued his research in chemical kinetics and its applications. His results were compiled in a voluminous paper entitled “A Method of Statistical-Mechanical Treatment of Equilibrium and Chemical Reactions” (1948). This method is applicable both to heterogeneous and homogeneous systems. Horiuti and his co-workers further attempted to apply the method to the study of a number of chemical syntheses and reactions, such as ammonia synthesis and ethylene hydrogenation. Nearly all of his research papers were published in the Journal of the Institute for Catalysis, of which he was the chief editor. His insatiable appetite for research was evident during the period from 1948 to 1965 while he was Director of the Institute, and it continued unabated after his retirement, except during the period from 1967 to 1971

JURO HORIUTI

(190 1- 1979)

...

XI11

when he served as President of Hokkaido University. His term of presidency unfortunately coincided with the period of countrywide student disturbances. However, he faced them resolutely and quelled them through the same indomitable determination that also characterized his approach toresearch. His keen perception and rigorous attitude toward research and its resultsmuch of this work quite new to the less advanced chemistry of Japan at that time-exerted a great influence, not only on the advancement of physical chemistry but also on related fields of chemistry, which proved to be the driving force in establishing the Catalysis Society of Japan in 1954. Horiuti became its first president. He is well known internationally also. He was a member of the following organizations : Council of International Congress on Catalysis (I.C.C.), 1959-1 972 ; Colloid and Surface Chemistry, Physical Chemistry Division of IUPAC, 1961-1969; Editorial Board of Journal of Catalysis, 1962-1975, and Zeitschrft fuer Physikalische Chemie, N . F., 1954-1979 ; and Advisory Board of Advances in Catalysis, 1962-1979. In 1966, he was elected a member of the Deutsche Akademie der Naturforscher Leopoldina. He died on June 27,1979 at the age of 77. Surely, the name of Juro Horiuti will be listed among the great ones in the history of catalysis.

Kozo HIROTA

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ADVANCES IN CATALYSIS, VOLUME 30

Mechanisms of Skeletal lsomerization of Hydrocarbons on Metals F. G. GAULT* Laboratoire de Catalyse UniversitP Louis Pasteur Strasbourg, France I.

. . . . A. The I3C Tracer Technique . , . . . . . . B. Kinetics. . . , . , . . . . , . . . Bond Shift . . . . . , . . . . . . . . A. The Various Proposed Mechanisms . . . . . . B. Evidence for Two Bond Shift Mechanisms . . . . Cyclic Mechanism . . . , . . , . . . . . A. Hydrogenolysis of Cycloalkanes . . . . . . . B. 1-5 Dehydrocyclization . . . . . . . . . C. The Multiplet Mechanisms . . . . . . . . Hydrocracking . , . . . . . . . . . . . Aromatization . . . . . . . . . . , . . Skeletal Rearrangements of More Complex Molecules . . A. Polymethylcycloalkanes . . . . . . . . . B. Substituted Aromatics , . . . . . . . . . C. Medium-Sized Rings . . . . . . . . . . Correlation between Metal Particle Size and Reaction Mechanisms . .

5 9 16 16 22 28 28 35 43 48 52 58 59 65 68 72

A. Isomerization and Hydrogenolysis of Hexanes and Pentanes on . . . . . . . . . . . . . Platinum-Alumina. B. Other Studies . . , . , . . , . . . . . . . References . . , . , . . . . . . . . . . . .

72 85 90

Introduction .

. .

. . . . . .

.

,

.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Approaches to Mechanisms of Skeletal Rearrangements

111.

IV.

V. VI. VII.

VIII.

I.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

. . .

1 5

Introduction

The skeletal rearrangements of hydrocarbons represent an important class of reactions catalyzed by metal surfaces, which have few counterparts in homogeneous catalysis. Although two reviews dealing with the subject (1, 2) have recently been published, this review seemed worthwhile on account

* Deceased. Text prepared for publication by Y.Gault. 1 Copynght 0 1981 by Academic Press. Inc. All rights of reproduction in any form reserved.

ISBN 0-12-007830-9

2

F. G . GAULT

of the significant strides recently made in this area. These are largely due to developments of experimental techniques for better characterization of supported metal catalysts, to use of single crystals, to 3Clabeling of reactant molecules, and also to progress in homogeneous catalysis, involving better knowledge of the structure and reactivity of some organometallic compounds, especially metallocarbenes and metallocarbynes. The first examples of skeletal rearrangements on metals were reported by the Soviet school of catalysis. A major step in hydrocarbon chemistry was the finding that platinum, unlike palladium and nickel, selectively catalyzes the hydrogenolysis of cyclopentane hydrocarbons. At about 300°C, on the classical Zelinskii platinum-charcoal catalyst, cyclopentane yields n-pentane as sole reaction product (3, 4, while palladized charcoal is completely inactive ( 5 ) and nickel-alumina produces all the possible acyclic hydrocarbons, from methane to pentane (5-7). Later on, Kazanskii and his co-workers, using Raman spectroscopy to analyze the reaction products, systematically studied the contact reactions of cyclic hydrocarbons, especially cyclobutanes and cyclopentanes. The important results that they obtained, none of which was invalidated when more sophisticated analytical tools were available, may be summarized as follows:

1. Unlike some other group VIII metals (8),platinum does not promote hydrogenolysis of cyclohexanes, but dehydrogenation to aromatic hydrocarbons. Cycloheptanes undergo ring contraction and aromatization rather than hydrogenolysis (9-12) (Scheme I).

0-(3

SCHEME1

0-6

2. In the hydrogenolysis of substituted cyclopentanes on platinized charcoal, an increase in the number of substituents decreases the rate of hydrogenolysis (13-15) (Scheme 2).

SCHEME 2. Relative rates of hydrogenolysis of substituted cyclopentanes.

SKELETAL ISOMERIZATION OF HYDROCARBONS

3

3. In monosubstituted cyclopentanes, the probability of rupturing tertiary-secondary C-C bonds is generally smaller than the probability of rupturing secondary-secondary C-C bonds (7, 26). 4. The rupture of the cyclopentane ring at a quaternary center is prohibited (17)(Scheme 3).

SCHEME 3

5. The hydrogenolysis of cyclobutanes very much resembles the hydrogenolysis of cyclopentanes, except for the reaction temperature, which is substantially lower (by about 80")and for the smaller selectivity (18, 19). In parallel studies, other skeletal rearrangement reactions were discovered and investigated: 6 . 1-5-Dehydrocyclization of straight-chain or branched acyclic alkanes-the reverse of the hydrogenolysis of cyclopentanes-takes place on the same platinum-charcoal catalyst under similar conditions. This reaction is often accompanied by aromatization (20-27) (Scheme 4).

--&

t Aromatics

SCHEME 4

7. Aromatization of substituted cyclopentanes occurs on various group VIII metals, especially palladium (28, 29). However, platinum remains the best metallic catalyst for this reaction. Most of these studies were made on a platinum supported catalyst, in which the metal was deposited on a catalytically inert carrier, charcoal. This shows that platinum is responsible for all these reactions, and indeed, replacing charcoal by silica or alumina does not substantially alter the results. In spite of the pioneering work of the Soviet scientists, it was believed, in the Western countries, until the 1960s, that skeletal isomerization on sup-

4

F. G. GAULT

ported metal catalysts necessarily occurs according to a “bifunctional” mechanism (30),including three consecutive steps : (1) dehydrogenation on the metal; (2) isomerization of the olefin on the support by a carbonium ion mechanism; and (3) rehydrogenation of the isomerized olefin on the metal (Scheme 5). Piatinurn

SO2

- A1203

SCHEME 5

However, new investigations showed the occurrence of skeletal isomerization on various metal films (31)and on supported platinum (32, 33) under conditions (200-300°C) such that the support (glass or alumina) is catalytically inactive. This clearly shows that the metal itself catalyzes the reaction. Two basic mechanisms were proposed for the skeletal isomerization of hydrocarbons on metals : 1. The bond shift mechanism (Scheme 6), which corresponds to a simple carbon-carbon bond displacement, accounting for the isomerization of short-chain paraffins (34).

A

Pt, Pd

8

-- /\J SCHEME 6

Pt

--A/

2. The cyclic mechanism (Scheme 7), which involves dehydrocyclization to an adsorbed cyclopentane intermediate C, followed by ring cleavage and desorption of the products, and is responsible for the isomerization of larger molecules on dispersed platinum-alumina catalysts (32, 33).

L

/ SCHEME 7

c,

SKELETAL ISOMERIZATION OF HYDROCARBONS

5

The cyclic mechanism was demonstrated by comparing the initial product distributions in the hydrogenolysis of methylcyclopentane and in isomerization of methylpentanes and n-hexane. For instance, the ratios 3-methylpentaneln-hexane, extrapolated to zero conversion, are the same in hydrogenolysis of methylcyclopentane and in isomerization of 2-methylpentane. Since cyclic type isomerization involvesfirst carbon-carbon bond formation and then carbon-carbon bond rupture, one does not expect hydrocracking of alkanes to occur by this mechanism. In contrast, as suggested early on (33), if bond shift isomerization involves first carbon-carbon bond rupture and then carbon-carbon bond recombination, a common intermediate should exist, leading to both the isomerization and the hydrocracking products.

II.

Approaches to Mechanisms of Skeletal Rearrangements

By tracing the displacements of the various carbon atoms during the isomerization process using 13C labeling, a rough classification of the reactions into two groups: bond shift and cyclic type, can be made. However, this technique does not provide any information about their detailed mechanism. In order to achieve a complete description of skeletal isomerization on a molecular scale, three different additional approaches have been used and very often combined. The first consists of changing the structure of the reacting hydrocarbon; with more complex molecules there are drastic changes in the selectivity and also in the relative contributions of the various mechanisms. In the second approach, the usual kinetic parameters (orders versus hydrogen and hydrocarbons, apparent activation energies) are determined for the various parallel pathways, including hydrogenolysis. Large differences in some of these parameters, activation energies, for instance, allow one to distinguish between different reaction mechanisms, and, conversely, the finding of identical kinetic parameters for several reactions point to a common reaction intermediate and interrelated mechanisms. A third method of gaining information about reaction mechanisms consists in systematically changing the structure of the catalysts, either by modifying the metal particle size or by alloying with a nonactive metal. Finally, as an ultimate refinement of the molecular description of the mechanism, reference is made, insofar as possible, to corresponding reactions which have been characterized or at least are supposed to occur in organometallic chemistry. A. THE13C TRACER TECHNIQUE

In the case of small molecules, bond shift is the only possible isomerization reaction. In the case of larger molecules, with at least five carbon atoms in the chain, the cyclic mechanism largely predominates on extremely dispersed

6

F. G. GAULT

catalysts, while on most other catalysts (films, bulk metals, supported catalysts of medium and low dispersion), both bond shift and cyclic type mechanisms compete. The first task is to evaluate their respective contribution to the overall reaction. The best way to do this is to label the reacting hydrocarbon with 13C. The 13C method of approach is illustrated by some examples: The isomerization of 2-methylpentane-2- 3C yields two different isomers of identical structure, 3-methylpentane~-3-'~Cand -2-13C, depending on whether the reaction mechanism is cyclic or bond shift (35)(Scheme 8).

SCHEME8

More generally, when more than two parallel pathways to the same molecule are possible, several reactant hydrocarbons labeled in various positions have to be used simultaneously. For instance, the isomerization of 2-methylpentane to n-hexane may happen in three different ways: either by a cyclic mechanism, or by a methyl shift (A), or by a propyl shift (B) (35).In this case, 2-methylpentane-2-' 3C allows one to estimate only the contribution of is required to the bond shift (A); another molecule, 2-meth~lpentane-4-'~C, estimate the contribution of the cyclic mechanism (36, 37) (Scheme 9).

f M

(Methyl shift A1

-

-)(ba -\--c-

"'

c-

&

(Propvl shift E l

Methyl or propyl shift

112

SCHEME9

The use of labeled molecules also allows a determination of the contribution of self-isomerization, that is, of molecules formed by isomerization but having the same structure as the reactant. The formation of such molecules

7

SKELETAL ISOMERIZATION OF HYDROCARBONS

may contribute an important part to the overall reaction and cannot be detected without 13C tracing. For instance, on a platinum catalyst of low dispersion, self-isomerized 2,3-dimethylpentane-3-' 3C is the major reaction product from 2,3-dimethylpentane-2-' 3C and can be detected only by labeling the reactant on the second carbon atom (38)(Scheme 10).Similarly,

SCHEME 10

for isopentane, methyl shift is ten times faster than chain lengthening, and can be observed only with the appropriated labeled hydrocarbon (39, 40) (Scheme 11). On the other hand, for n-pentane on a highly dispersed Pt/Al,03

M -

L -7

SCHEME 11

catalyst, the major reaction, self-isomerization by a cyclic mechanism, can be detected by label scrambling when n-pentane-2-13C is the reactant (39, 40) (Scheme 12).

,-+.,-.

-(o)-

-

M

+

1I5

215

+

-

2/51

SCHEME 12

In some cases, a complete estimation of the relative contributions of the various pathways of cyclic and bond shift types requires the simultaneous use of a number of13C-labeled molecules. Thus far the most complicated example is the isomerization of 3-methylhexane. This molecule, which dehydrocyclizes in three different ways, to 1,2-dimethylcyclopentane, 1,3-dimethylcyclopentane, and ethylcyclopentane (Scheme 17), may isomerize by 23 different pathways, consisting of both cyclic and bond shift types. In particular, four parallel pathways account for n-heptane, and five for self-isomerized 3methylhexane (41). Therefore, even when using all the possible labeled molecules, one cannot distinguish between all the isomerization pathways, since the complete location of 13C in 3-methylhexane cannot be completely achieved.

8

F. G. GAULT

During the past two decades, the isomerization of pentanes and hexanes has very often been used as a test reaction in the study of particle size or alloying effects. In these studies, the product distribution, and especially the percentage of cyclic molecules (cyclopentane or methylcyclopentane), have often been used for estimating the contribution of the cyclic mechanism. Such a procedure may be misleading. First, self-isomerization reactions, which may be predominant, are overlooked when nonlabeled molecules are used. Second, as will be discussed later, the percentage of cyclic products is highly dependent upon experimental conditions, increasing with decreasing hydrogen pressure, or with increasing hydrocarbon pressure, or temperature. A comparison of two catalysts is valid, therefore, only under strictly identical experimental conditions. Moreover, a third, less obvious and more fundamental reason for discarding the product distribution approach results from the mechanism itself; as shown for the isomerization of 3-methylhexane (41), the ratio p between cyclic and acylic products varies widely with the conversion, even at very small conversions, where the distribution of the acyclic isomers remains practically constant. This can easily be accounted for by assuming a simplified model (Scheme 13), where C represents a single cyclic intermediate, A, and A, two acyclic isomers, and A, a cyclic molecule, and where ki ( i = 1,2, or 3) represents the dehydrocyclization rate constants and k - i the hydrogenolysis rate constants.

SCHEME 13

Assuming further a steady-state concentration C for the cyclic intermediate and a pseudounimolecular reaction (42),the mole fractions of reactant and product may be expressed as Eq. (1):

xi = C ( k - i / k i ) ( l - e-ktr)

(1)

SKELETAL ISOMERIZATION OF HYDROCARBONS

9

The ratio, p, of the amount of cyclic (A4) to that of the acyclic products (A, and A,) is given by Eq. ( 2 ) :

and the ratio r (selectivity factor between the amounts of the two acyclic isomers), by Eq. (3): r=

k - , k , ( l - e-k3t) k-,k,(l - e-”“)

(3)

Since the “adsorption” rate constant of the cyclic product k , is always much larger (by two to three orders of magnitude) than the dehydrocyclization rate constants k , and k , , even at very small conversions where ( 1 - e - k 2 1 ) and ( 1 - e - k 3 1may ) be approximated to k,t and k,t, respectively, ( 1 - e - k 4 1 ) is already close to unity. Therefore, while the selectivity factor r remains constant, equal to k , / k , , the ratio p decreases sharply with the contact time. The only correct way, then, to compare the contribution of the cyclic mechanism for various catalysts is to determine the percentage of cyclic products at various contact times and to extrapolate to zero conversion. Even in this case, such a comparison is valid only for a given set of experimental conditions and may be misleading, since the desorption rate constant k - , may vary relative to the hydrogenolysis rate constants k - , and k - , from catalyst to catalyst. From the above discussion, it ensues that the tracer techniques are the most suitable for estimating the contributions of the various mechanisms. However, this method provides information only if skeletal rearrangement and not adsorption-desorption is rate determining (43). In the latter case, which happens at very low hydrogen pressures (1-10 Torr), a complete scrambling of the label throughout the molecule would be observed (44).O n the other hand, at moderate hydrogen pressures (100-200 Torr), a more complex process, referred to as a “repetitive process,” occurs, resulting in a partial scramble of the label (45).Therefore, the best experimental conditions for mechanistic purposes are those using high hydrogen pressures (from 500 Torr to several atmospheres), which, of course, make supported catalysts more attractive than films and single crystals, in spite of the ill-defined structure of the former.

B. KINETICS Before using kinetic data for mechanistic studies, a general kinetic scheme for skeletal rearrangement should be proposed. Very high negative orders

10

F. G . GAULT

versus hydrogen were generally found in hydrogenolysis of hydrocarbons, especially ethane (46-53). One could assume, then, that for isomerization as for hydrogenolysis, the rate-determining step is the skeletal rearrangement of a highly dehydrogenated species, obtained by a series of consecutive dehydrogenation steps, all equilibrated (Scheme 14).

(C$x+ 1 )a P (C,H,), + Ha (C,H,), 4 (iso-C,H,), (slow) H,

* 2H, SCHEME 14

According to this scheme, the same as that proposed by Cimino, Boudart, and Taylor (48) for ethane hydrogenolysis, negative orders versus hydrogen would correspond to a reactive species which has lost two hydrogen atoms. Thus, the orders, - 3.4 f 0.2, found for cyclic type isomerization of pentane and hexanes (40,5 4 ) would mean that the reactive species is obtained by rupturing seven or eight carbon-hydrogen bonds. Since it is difficult to reconcile such highly dehydrogenated species with any surface organometallic complex, another model has been introduced (44) (Scheme 15), involving bimolecular instead of unimolecular dehydrogenation steps.

*

(CnHX+ih+ Ha (CnHX),+ Hz (slow) (C,H,), -+ (iso-C,H, )a SCHEME 15

This model generalizes an early proposal made by Frennet and Lienard (55-57) for methane adsorption. These authors studied the surface composi-

tion of carbon and hydrogen atoms during the chemisorption of methane on metal films and found that the results were best explained by assuming that methane was adsorbed according to a bimolecular reaction between methane and adsorbed hydrogen: CH,

+ H,

(CH,),

+ H2

rather than by an unimolecular mechanism, CH,

4

(CH,),

+ Ha

Furthermore, accurate measurements showed that the surface hydrogen

SKELETAL ISOMERIZATION OF HYDROCARBONS

11

coverage, OH, depended mainly upon the surface coverage in hydrocarbon radicals, Oc, and therefore remained practically constant (Eq. 4): = e;(i

-

ec)

(4)

Scheme 15 leads to the rate equation (5):

where K i is the equilibrium constant for the ith dehydrogenation step, m the number of these steps, k the rate constant, and PHcand PH2the hydrocarbon and hydrogen partial pressures. When Eq. (4) is taken into account, Eq. (5) becomes Eq. ( 6 ) :

or Eq. (7) at low hydrocarbon coverage:

At low hydrocarbon coverage, therefore, this approach gives an order versus hydrogen of -in instead of the order -2m found from Scheme 14. Equations (6) and (7) not only account more satisfactorily for the very high negative orders that are observed, but also explain a number of facts. First, since all the dehydrogenation steps are endothermic, one expects the apparent activation energy E , to be larger than the true activation energy (Eq. 8): E, = E

+ CAHi

(8)

According to Eq. (8), where AHi is the enthalpy change for the ith chemical transformation, the more numerous the dehydrogenation steps in Scheme 15, and consequently the more negative the order versus hydrogen, the higher the apparent activation energy should be. Such a correlation between orders and apparent activation energies has been found indeed for the isomerization and hydrogenolysis of C , hydrocarbons on Pt/Al,O, catalysts (39, 40), as it was for ethane hydrogenolysis on various metals (49). Second, Eq. (9), also derived from Eq. (7),

AS, = A S

+ 1ASi

(9)

implies that the apparent activation entropy, AS,, increases with decreasing order versus hydrogen. Indeed, all the dehydrogenation steps, except the first one, are associated with an increase of the translational degrees of

12

F. G . GAULT

freedom, and therefore with an increase, AS,, of entropy. One explains thus the paradoxical result that the apparent activation energy for cyclic type isomerization ( - 3.4 f 0.2)is larger than for bond shift ( - 1.8) (40). Although Scheme 15 seemed to explain satisfactorily the negative orders versus hydrogen found for isomerization of pentanes and hexanes at 250"350°C under 200-1 100 Torr hydrogen pressure, it does not account for the positive orders versus hydrogen observed under low hydrogen pressures. Positive orders versus hydrogen were reported for both hydrogenation and hydrogenolysis of C, and C, hydrocarbons on platinum films under 40 Torr hydrogen pressure (34) and also for isomerization and hydrogenolysis of hexanes on Pt/Al,O, under 10-50 Torr hydrogen pressure (39).Moreover, the apparent activation energy ( - 12 kcal/mol) was much lower than those (45-70 kcal/mol) found at high hydrogen pressure. The same change of order versus hydrogen, from negative to positive, with decreasing hydrogen pressure, was observed on platinum black (58). On the other hand, in a systematic study of the isomerization and hydrogenolysis of various C,-C, hydrocarbons on palladium black, Sarkanyi, Guczi, and Tetenyi (59)found maxima in the curves representing the rates as a function of hydrogen pressure. The maximum rates of both hydrogenolysis and isomerization were shifted toward higher hydrogen pressures with increasing carbon chain length, decreasing branching and increasing temperature. These observations are in fair agreement with the finding that the orders versus hydrogen, which are negative in the isomerization of C5and C, hydrocarbons under high hydrogen pressures (39, 58), become positive in the isomerization of C, hydrocarbons under similar conditions (38, 43, 44). The positive orders versus hydrogen found at low hydrogen pressures could be interpreted by assuming that the rate-determining step is no longer the skeletal isomerization of dehydrogenated species, but adsorptiondesorption. If that is the case, one would expect that, between adsorption of the reactant and desorption of the products, a number of skeletal rearrangements, of bond shift or cyclic type, would occur. This would result, when using labeled molecules, in a complete scrambling of the labels on all the carbon atoms of the products. The label has been located in the isomerization products obtained from and 3-methylpentane-32-methylpentane-2-' 3C,2-methylpentane-4-' on 10% platinum-alumina and single crystals under 20 Torr hydrogen pressure (54, 60). Under these conditions, the scrambling of the label was found to be extremely limited: less than 10% of abnormal varieties are obtained. However, for these alkanes, on such catalysts and in these conditions, the selective cyclic mechanism is widely predominant and yields

SKELETAL lSOMERIZATlON OF HYDROCARBONS

13

mostly methylpentanes. Since only one cyclic intermediate (methylcyclopentane) can be formed, the scrambling of the label is not expected to be extensive (Scheme 16).

'y' SCHEME 16

In contrast, in the case of C , hydrocarbons, the scrambling of the label by consecutive skeletal rearrangements should be very easy on account of the large number of parallel pathways. For instance, 3-methylhexane can dehydrocyclize by three different pathways, to 1,2-dimethylcyclopentane,1,3dimethylcyclopentane, and ethylcyclopentane (41) (Scheme 17). In the

SCHEME 17

isomerization of 2,3-dimeth~lpentane-2-'~C, when considering only cyclic type isomerization, which involves dehydrocyclization to 1,2- and 1,3dimethylcyclopentanes and their interconversions via adsorbed 3-methylhexane, five consecutive steps are enough to obtain all the isotopic varieties of n-heptane (Scheme 18), and only two when envisaging the other reaction pathways including bond shift (44). However, the product distribution is

14

F. G. GAULT

very far from the statistical one (Table I) and is consistent with a one-step process. This consists of a cyclic type isomerization for n-heptane and of a mixture of cyclic and bond shift isomerization for 3-methylhexane (43, 44). This result and similar ones obtained with other labeled C, hydrocarbons (38,41)show that skeletal rearrangement and not adsorption-desorption is still the rate-determining step when the orders versus hydrogen are positive. It is very difficult to account for these positive orders by classical kinetic schemes such as Schemes 14 and 15. TABLE I Isomerization of 2,3-dimethylpentane-2-I3C on 10% Pt/AI,O, : Distributions of the 3-Methylhexanes and n-Heptanesa,b 3-Methylhexanes Observed Cyclic mechanism Bond shift Statistical n-Heptanes 0bserved Cyclic mechanism Statistical

4-'3C or 5-I3C or 6-I3C 24 0 100 43 1-13c

0 0 28.5

3-I3C

Me("C)

38 50 0 14.2

0 0 0 14.2

3-I3C 4 0 28.5

'3C 0 0 14.5

or 2-l3c 38 50 0 28.5 1 3 c

96 100 28.5

~

Data from (%), (44). * Conditions: P,,, 760 Torr; PHc,5 Torr; temp., 260°C. a

_

_

_

_

_

~-

SKELETAL ISOMERIZATION OF HYDROCARBONS

15

For hydrogenolysis, Leclercq, Leclercq, and Maurel(61) tried to account for the change in order with changing hydrogen pressure by the Cimino model (Scheme 14). They supposed that the rate-determining step involves molecular hydrogen, and they replaced all the equilibrated dehydrogenation steps with an overall equilibrium, with an equilibrium constant R : C.H2.+2

+ s * (CnH2n+2-2.)ads + aH2

(CnHZn+2-20) + H,

+

(10)

products

Assuming that the reactive species CnH2n+2-2a is by far the most abundant adsorbed radical, a rate equation (1l), that accounts satisfactorily for the results, may be derived from Eq. (10) :

The model proposed by Leclercq, Leclercq, and Maurel, however, like any classical model, does not account for the orders greater than unity that are currently obtained in hydrogenolysis. Moreover, it would account for the positive orders in isomerization only if the rate-determining step involves the participation of hydrogen, as in hydrogenolysis, which is very unlikely. An answer to the puzzling problem of the kinetics of skeletal isomerization may be found in a recent proposal, by Frennet and his co-workers (62), that hydrocarbon adsorption involves not a single site but an ensemble of several contiguous sites of the surface (seven to eight). When using the model of bimolecular dehydrogenation steps, one should always replace the first equation in Scheme 15 by C"H2"+2+ 2s -t H, -.+ (C"H,"+1)a

+ H2

(12)

The new rate equation

now includes the concentration of the free sites B S , equal to

e,

(14)

= (1 - e;)(i - 0,)

When taking into account Eq. (14), the rate may be expressed as u=knKi

P 80m(i- ey ; H C H

(1 -

&)"+Z

PH", Equation (15) includes two terms, f(0;, PH2)and g(0,) = (1 - 8C)m+z, whose formal orders versus hydrogen are negative and positive, respectively. At high hydrogen pressure and low 0,, (1 - 8;)' is an inhibiting term that contributes to the negative order versus hydrogen. This order may become i

16

F. G. GAULT

highly negative with moderate values of m, provided that z is large enough. At low hydrogen pressure and high 8,, the positive hydrogen pressure dependence of (1 - 8c)m+r,on account of z, overcomes the negative depenand large positive orders versus hydrogen may be found, dency of f ( O i , PH2) contrary to what would result from scheme 15 and Eq. (7). Moreover, g(Oc) contributes a negative term to the overall apparent activation energy, and that explains the decrease in apparent activation energy with decreasing hydrogen pressure (39). The characteristic plots expressing the rates as a function of the hydrogen pressure (59)may then be interpreted easily by using the Frennet model, and the shift of the maxima toward higher hydrogen pressures with an increase in the number of carbon atoms of the reactant means only that the adsorption strength and 8, increase accordingly. Although no accurate measurements have been made yet of the surface coverage with various hydrocarbon radicals in the presence of hydrogen, some indications may be obtained from the amounts of hydrocarbonaceous residues left on the surface after a catalytic reaction. Such amounts were determined by Auger electron spectroscopy after reaction, under catalytic conditions, of several C,-C, hydrocarbons on platinum single crystals (60).They were found to increase significantly, by about one order of magnitude, from pentanes to heptanes. One could therefore explain, by an increase of BC, the changes in order versus hydrogen, from negative to positive, that are observed in isomerization when going from C, to C, hydrocarbons. From the above discussion, it ensues that no straightforward conclusion can be drawn from any kinetic study. In particular, any attempt to deduce, from the orders versus hydrogen, the number of carbon-metal bonds in the reactive intermediate would be completely hopeless. The more significant results are those in which the function g(8,) does not interfere in the rate equation, that is, those for which negative orders versus hydrogen have been found. In this case, as suggested above, large differences between apparent activation energies most probably mean different reaction mechanisms. However, even when Oc is not negligible, comparison of the kinetic data obtained from several parallel pathways might also be significant, provided that z is the same for all the reactions. In this case, indeed, the rate equations include the same g(0,) function and differ only in the number of dehydrogenation steps m. 111.

Bond Shift

A. THEVARIOUSPROPOSED MECHANISMS Several reaction mechanisms have been proposed for bond shift isomerization. Although each of them accounts for some specific peculiarity of the

SKELETAL ISOMERIZATION OF HYDROCARBONS

17

reaction, depending upon the structure of the reactant, no definite proof could be given for any of them. In order to explain the isomerization of neopentane to isopentane on platinum films, Anderson and Avery (34)proposed a mechanism involving, as precursor, an a,a,y-triadsorbed species, and, in the transition state, a n complex of the Dewar type, attached to the surface by two carbon-metal bonds. By simplified Huckel molecular orbital (MO) calculations, they showed that hyperconjugative effect and partial charge transfer t o the metal could account for the relative isomerization rates of the various molecules studied (neopentane > isobutane > n-butane) (Scheme 19).

fi-K-Y

M

M

M M

M M

%NFME 19

A slight modification of this mechanism, involving a dissociatively adsorbed cyclopropane intermediate (63)and stabilization of this intermediate by methyl substituents explains the predominance of ethyl shift (Path B) over methyl shift (Path A) in the isomerization of isopentane to n-pentane (40) (Scheme 20).

6-, -+

m

m

m

-+

f5-6-f

m

m

Path B

m

m

m

Path A

SCHEME 20

In order to examine the possible participation of adsorbed cyclopropanes in the bond shift mechanism, the relative contributions of Paths A and B in chain lengthening were determined for a series of 2-methylalkanes (40, 43, 54). The contribution of Path B was found to decrease regularly from isopentane t o 2-methylpentane and 2-methylhexane. The decreasing contribution of Path B from isopentane to 2-methylpentane is readily explained by the decreasing number of methyl substituents in the cyclopropane intermediate, but the difference between 2-methylpentane and 2-methylhexane cannot be accounted for by the cyclopropane mechanism (43). In an investigation of the contact reactions of highly strained cage compounds, Rooney and his co-workers (64,65)found that bicyclo(3,3,2)octane, protoadamantane, adamantene dimer, and other similar compounds were isomerized on platinum- and palladium-supported catalysts at relatively low temperatures (15O"-25O0C on platinum, 200"-300"C on palladium). The ease

18

F. G . GAULT

of isomerization of these polycycloalkanes parallels the ease of their rearrangements by acidic catalysts. To explain the rearrangement of such compounds, it seems difficult to invoke either the Anderson-Avery mechanism (Scheme 19) or the Muller-Gault mechanism (Scheme 20). Indeed the formation of a,u,y-triadsorbed species is hardly conceivable on stereochemical grounds, and even less conceivable is the formation of n-allylic species, which are the normal final products in both mechanisms. Moreover, when the reactions are effected in the presence of deuterium (64, the deuterium distributions of both adamantene dimer and its isomers were binomial, showing that only one hydrogen atom was removed at a time during both the exchange and the isomerization processes. This latter result clearly shows that a-alkyl adsorbed radicals are enough to promote skeletal isomerization, and a mechanism was proposed (65) (Scheme 21, using neopentane as an example), in which the transient species

SCHEME21

involves three-center orbitals as in the nonclassical carbonium ions, with simultaneous n bonding to the metal. On account of the interaction with the metal, the second highest orbital in the organic moiety, normally antibonding, is lowered in energy and becomes sufficiently stable to be occupied by a third electron (Scheme 22). P

M

0

d

SCHEME 22

Although the Rooney-Samman mechanism nicely explains the peculiarities of the isomerization of caged molecules on platinum and palladium catalysts, its extension to simpler molecules is not completely obvious. As a matter of fact, none of the above mechanisms accounts in its original form for the different behaviors of platinum and palladium in skeletal rearrangements.

19

SKELETAL ISOMERIZATION OF HYDROCARBONS

As noted above, while isobutane is isomerized on both platinum and palladium, platinum but not palladium catalyzes the isomerization of neopentane to isopentane (34.' This inability of palladium to catalyze skeletal rearrangements involving a quaternary carbon atom has been confirmed in the case of more complicated molecules. The aromatization of 1,1,2- and 1,1,3-trimethylcyclopentaneson platinum involves a ring enlargement at the quaternary carbon atom, producing xylenes, while on palladium, the ring enlargement occurs at the tertiary carbon atom with formation of adsorbed gemdimethylcyclohexane and subsequent demethylation and dehydrogenation to toluene (63, 68, 69). The easiest way to explain these differences is t o assume that, in the case of palladium, the precursor species is attached to the metal by three consecutive carbon atoms, and in the case of platinum, by only two carbon atoms in the a-y positions. Making this assumption, a mechanism accounting for bond shift and ring enlargement on palladium has been proposed, which involves, as a precursor, a n-allylic species, easily formed on this metal, and, as the rate-determining step, the rupture of a carbon-carbon bond by hydrogen attack to form a metallocarbene and a n-adsorbed olefin (70). Demethylation or isomerization would then occur, depending on whether the two organic moieties are desorbed or react again after rotation of the nolefinic species (Scheme 23).

C,Y

I +

CH,

SCHEME23

Note that Scheme 23 should be divided into several elementary processes, each of which corresponds to a known reaction in organometallic chemistry (Scheme 24). The attack of the n-allylic species has its analog in the nucle-

/I m

-A

m

-2

m

//cH,

-~Mpz4y') -wT'

W

m

SCHEME 24

ophilic attack of n-allylic complexes by hydride ions (71, 72), while the dismutation of metallocyclobutanes into metallocarbenes and adsorbed However, it was recently shown that fresh palladium films do isomerize neopentane initially, but their activity dies off very quickly (66, 67).

20

F. G. GAULT

olefins is involved in the now well-established mechanism of olefin metathesis (73, 74). An interesting feature of Scheme 24, devised for palladium, is that it also provides a straightforward explanation for the bond shift isomerization on platinum. An adsorbed metallocyclobutane complex is similar to the trimethylene di-g-complex of platinum, which is readily formed from hexachloroplatinic acid and cyclopropane (75). Also, platinum is known to promote easily a-y exchange of some hydrocarbons with deuterium (31, 34, 76). Therefore, in the case of platinum, the direct formation of metallocyclobutanes, without intermediacy of n-allylic species, would explain the remarkable ability of this metal to promote the isomerization of neopentane to isopentane (Scheme 25).

SCHEME 25

In order to account for the possibility that not one but several platinum atoms, most probably in the (111) configuration, are involved in bond shift (77),a concerted mechanism, substantially the same as the one represented in Scheme 25, has been proposed (40) (Scheme 26), which involves a metal-

SCHEME 26

locyclobutane precursor and a transient species with carbenoid character. A similar transient species, but attached to one metal atom, was also proposed by Puddephatt et al. (78, 79) to account for the rearrangement of phenylsubstituted platinacyclobutanes (Scheme 27). Ph I

CH CH2<M>CH-Ph

=

CH

'M

M

SCHEME 27

In any event, according to our present conclusion, further developed in this review, most of the isomerization mechanisms involve one single metal atom, the relative contributions of the bond shift and cyclic type reactions being regulated not by the number of surface metal atoms involved, but by

SKELETAL ISOMERIZATION OF HYDROCARBONS

21

their electronic properties. We now favor the metathesis-like mechanism represented in Scheme 25 rather than the concerted mechanism in Scheme 26. Another attempt to account for the differences between the behavior of platinum and palladium in bond shift isomerization has been presented in a recent review by Clarke and Rooney (2). This new mechanism, which is also based on the ability of platinum and not of palladium to promote the formation of metallocyclobutanes, derives from Rooney's earlier mechanism but replaces the a-alkyl precursor by a metallocyclobutane, and, in the transition state, the n-olefinic bonding by a n-allylic bonding. As in the previous mechanism, it is assumed that on platinum the metallocyclobutane is formed directly, while in the case of palladium, it would result from a 1-2 hydrogen shift via a transient species of n-allylic character (Scheme 28).

H I CH2=C- CH2-CH,

SCHEME28

Having now established an (almost) exhaustive list of all the reaction mechanisms that have so far been proposed for bond shift isomerization, we shall define three criteria which allow a distinction to be made among them, and therefore (we hope) to make a choice.

1. Of the four basic mechanisms, two, the Anderson-Avery mechanism (Scheme 19) and the Garin-Gault mechanism (Scheme 25), should be associated with isomerization and hydrogenolysis; two, the Muller-Gault mechanism (Scheme 20) and the Rooney-Samman or the Clarke-Rooney mechanism (Schemes 22 and 28) should promote only isomerization. 2. Structural effects, such as those shown by the data in Table I11 (see Section III,B), might be very helpful; when the reactant becomes more complex, some parallel pathways should be favored and others hindered according to the mechanism envisaged. 3. On account of the extreme complexity of the rate equations (see Eq. (1 1) in Section II), it is not possible to use as a criterion a classification of the reactions according to their absolute rates. However, relative rates of several

22

F. G. GAULT

similar parallel pathways may be useful to preserve or to reject a reaction mechanism.

B. EVIDENCE FOR Two BONDSHIFT MECHANISMS The kinetic parameters (apparent activation energies, orders versus hydrogen and hydrocarbon) have been determined for all the isomerization and hydrocracking reactions of n-pentane and 2-methylbutane on a Pt/Al,O, catalyst of low dispersion (10% Pt; d = 90 A) (40). n-Pentane-2-13C and 2-methylbutane-2-' were used to estimate the contributions of cyclic type and bond shift isomerization, respectively. As shown in Table 11, the reactions TABLE I1 Reactions of n-Pentane and 2-Methylbutane on 10% Pt/AI,O, : Apparent Activation Energies and Orders versus Hydrogen"

---Reaction

Activation energy (kcal/mol)

Order versus hydrogen

Group I

71.4

+ 1.5

-3.4

+ 0.1

55.3

1.5

-1.8

+ 0.1

54.3 f 1.5

-2.3

k 0.15

- 1.9

* 0.2

Group I1

Group 111

L-T A/-X A/-- + C3HB

C3H8

Group IV

-

M

+

* 1.5 45 * 3

-1.65 If. 0.15

45 f 3

- 1.65 f 0.2

45.3

C2H6

C2H6

N

+CH,

+ CH,

44.5 f 3

-1.3

* 0.2

38.5 f 3

-0.6

* 0.2

35

*3

' Conditions: P H I 760 , Torr; PHc, 5 Torr; Temp., 260"-290°C.

- 0 . 7 f 0.1

SKELETAL ISOMERIZATION OF HYDROCARBONS

23

may be classified into four groups, according to their apparent activation energies. When the data in Table I1 are plotted, there is quite a good relationship between the negative orders versus hydrogen and the apparent activation energies. This is to be expected on the basis of the kinetic theories developed in Section 11. The reaction with the highest activation energy is the cyclic type isomerization of n-pentane, which leads to a complete scrambling of the label. The second group includes the isomerization of 2-methylbutane to n-pentane and the reverse reaction. The reactions belonging to the third group are the isomerization of 2-methylb~tane-2-'~Cto 2-methylbutane-3-13C and to neopentane and the hydrogenolysis of internal carbon-carbon bonds in n-pentane and 2-methylbutane. Last, demethylations are the least activated reactions. Noticeable in Table I1 is the splitting of the bond shift reactions into two groups, with apparent activation energies differing by 10 kcal/mol, and the fact that one of these groups (group 111) is associated with hydrogenolysis reactions, while the other (group 11) is not. Interesting also is the distinction, among the hydrogenolysis reactions, between internal fission and demethylation. According to the previous discussion, two mechanisms, the AndersonAvery mechanism, involving an a,a,y-triadsorbed precursor (Scheme 19) and the Garin-Gault mechanism, involving a metallocyclobutane intermediate (Scheme 25), could account for the reactions of group 111. That on palladium all these reactions are either negligible (internal fission, methyl shift) or nonexistent (neopentane isomerization)2 is the first argument in favor of the metallocyclobutane mechanism. Moreover, if this mechanism is operative in the isomerization of 2-methylbutane to n-pentane (Scheme 29), adsorbed ethylidene is expected to be formed instead of adsorbed methylene, as required by a metallocyclobutane mechanism for methyl shift and neopentane formation. It is well known from carbene and metallocarbene chemistry (80, 81) that substituted metallocarbenes are rapidly isomerized via 1-2 hydrogen shift to adsorbed olefins. Re-formation of the metallocyclobutane may then be slight, and bond shift isomerization is replaced by hydrogenolysis of the internal carbon-carbon bond. Scheme 29 would also explain the rapid isomerization of 2-meth~lbutane-2-'~C to 2-methylbutane-3-' ,C. In contrast, the Anderson-Avery mechanism does not discriminate among the various isomerization reactions. Moreover, for hydrogenolysis, it forecasts the rupture, in the 1,1,3-triadsorbed precursor, of the C,-C, bond next to the single metal-carbon bond. However, in a recent investigation of the hydrogenolysis of a number of hydrocarbons on platinum catalysts, Leclercq, Leclercq, and Maurel(82) showed that the C-C bonds in the fi position to a tertiary carbon atom are preferentially ruptured. As pointed out by these See Footnote I , p. 19.

24

F. G. GAULT

SCHEME 29

authors, this result is best explained by a mechanism involving a 1,1,3triadsorbed species (the precursor in the Anderson-Avery mechanism), but in which the rupture occurs at the C-C bond in the 01 position to the metalcarbon double bond. Hydrogenolysis in this case is therefore best represented as the rupture of a 1,1,3-triadsorbed species t o form a metallocarbyne and an adsorbed olefin (Scheme 30, taking as an example the hydrogenolysis of 2-methylpentane).

c, ,c-c C

J + c-c

SCHEME 30

Further evidence for the existence of two bond shift mechanisms and also for the metallocyclobutane mechanism are provided by the isomerization of 3C labeled C, hydrocarbons: 2,3-dimethylpentane (38),2-methylhexane (43),and 3-methylhexane ( 4 1 ) on Pt/AI,O, catalysts. In the case of 2-methylhexane, very different positive orders versus hydrogen, 1.75 and +0.15, are observed for methyl shift and chain lengthening, re~pectively.~ Although it is not possible to correlate easily the mechanisms and the orders versus hydrogen, especially when the latter are positive,

+

Although methyl shift can also result in chain lengthening, the term “methyl shift” is used here to describe the displacement of a methyl group to an internal carbon atom, without change of length of the main chain, and the term “chain lengthening” to describe any alkyl or methyl shift to a terminal carbon atom.

SKELETAL ISOMERIZATION OF HYDROCARBONS

25

such a large difference between the kinetic parameters of two parallel pathways clearly demonstrates that different mechanisms are involved. Moreover, the comparison of the respective contributions of bond shift and cyclic mechanism in the isomerization of several labeled C, and C , hydrocarbons on 10% Pt/Al,O, catalysts (Table 111) shows that the contribution of bond shift to the overall isomerization process decreases with an increase in the number of carbon atoms, but not to the same extent for methyl shift and chain lengthening. Thus, on going from methylpentanes to 2,3-dimethylpentane, the contribution of bond shift is divided by 1.5 for chain lengthening, but by 4 for methyl displacement. The comparison of 2-methylpentane and 2-methylhexane is less easy, because their cyclic type isomerizations likely involve different mechanisms, as will be developed in the next section. However, the contribution of bond shift is significantly decreased for methyl displacement, but slightly increased for chain lengthening. All these results are readily interpreted by assuming the existence of two bond shift mechanisms. The first one, which accounts for methyl shift, may be ascribed to the metallocyclobutane mechanism responsible for the group I11 reactions of n-pentane and isopentane. The second one, which accounts for chain lengthening (and chain shortening) is the same as the mechanism of higher activation energy (group 11) responsible for the interconversion between n-pentane and isopentane. The first is very sensitive to alkyl substitution, while the latter seems relatively insensitive to structural effects. The sharp decrease of the contribution of the metallocyclobutane mechanism with increasing substitution is probably connected with a n increase of the energy barrier for rotating (or displacing) the x-adsorbed olefin before C-C bond reformation. Taking as an example the reactions of the C, hydrocarbons (38,41,43),one observes that C, to C, methyl shift (b) and C, to C , propyl shift (c) in self-isomerization of 3-methylhexane are much faster than C, to C, methyl shift (a) in the isomerization of 3-methyl- to 2-methylhexane (or C, to C, methyl shift in the reverse reaction) (Scheme 31). Similarly, C, to C, methyl shift in the isomerization of 2,3-dimethyl- to 2,4dimethylpentane (d) is also a disfavored reaction. The difference between reactions (a) and (d), on the one hand, and (b) and (c), on the other hand, may be understood when considering the x-adsorbed olefins obtained by metallocyclobutane dismutation. In (a) and (d), these olefins have larger substituents (propyl and isopropyl) than in (b) and (c), and an increase of the energy barrier for rotation by a few kilocalories per mole above the normal value (10-15 kcal/mol) would explain the observed differences in rates (83). Although adsorbed propylidene in (c) is expected to rapidly isomerize to propene rather to recombine with adsorbed 2-butene and re-form a metallocyclobutane (cf. Scheme 29), the observed fast propyl shift is best accounted for by this mechanism. A possible explanation of this seeming contradiction

TABLE I11 Structural Effects in Bond Shift: Comparison of C , and C , Hydrocarbons".b,' Bond shift Methyl shift

(%)

Bond shift Chain lengthening

28

84

An L-T T-

79

41

22

(% )

T--

L n n n

ir-

29

35

20

Data from (38)and (43). Conditions: catalyst, Pt/AI,O, 10%; PHI, 760 Torr; Temp., 254°C (C6),26Oo-27O0C ( C , ) . ' For sake of simplification, the '3C-labeling methods used to determine the relative contributions of bond shift and cyclic mechanisms have not been indicated.

27

SKELETAL ISOMERIZATION OF HYDROCARBONS

'kE' Ei

P,

iPI

Me

CH \IM

la1

SCHEME31

is that metallocyclobutane re-formation involving symmetrical olefins is favored over the other reaction pathways. While there is good evidence in favor of a metallocyclobutane mechanism for the group 111 bond shift reactions of 2-methylbutane (Table 11), the mechanism for the bond shift reactions of higher activation energies (group 11) is not yet elucidated. Since no hydrogenolysis reaction belongs to group 11, likely mechanisms for these reactions would be either the cyclopropane mechanism (Scheme 20) or, better, a concerted mechanism such as theRooney mechanism in its original (Scheme 22) or modified form (Scheme 28). However, as already emphasized, the cyclopropane mechanism cannot account for the increase of methyl shift (Path A) relative to alkyl shift (Path B) with increasing the chain length of the reacting hydrocarbon. In contrast, the Rooney mechanism would satisfactorily explain this progressive increase, if one assumes that the rate of formation of the transient species with threecenter orbitals decreases with an increase in the size of the migrating alkyl group. In its original form, however, the Rooney mechanism does not explain the predominance of Path B over Path A in every case (Scheme 32). This objection can be overcome by assuming, as Clarke and Rooney d o (2), that vinyl or alkylvinyl and not alkyl groups are the migrating entities.

m

m

m

R

I

Cnz I* cn, - cn - cn, I

m

-

c&.;f':.,C.!+z *R : . cn - cn,

m

-

RC\H2

~~,-cn-cn, I

(Path BI

m

SCHEME32

Although both of Rooney's mechanisms, involving either a a-alkyl or a metallocyclobutane precursor (Schemes 21 and 28) could account for the bond shift reactions of group 11, the second one seems more likely; indeed, chain lengthening, unlike methyl migration, has an analog in coordination

28

F. G. GAULT

chemistry, namely, the isomerization of 2-phenyl- to 3-phenylplatinacyclobutane (78, 79).This reaction, like the corresponding one in heterogeneous catalysis, is not associated with any hydrogenolysis reaction and most probably occurs according to a concerted mechanism. Although this rearrangement could be explained by the carbenoid mechanism originally proposed by Puddephatt (Scheme 27), the very distinct properties of the two classes of bond shift reactions point toward a mechanism quite different from metallocyclobutane dismutation. Therefore Rooney’s proposal might very well be more applicable than the Puddephatt rearrangement mechanism.

IV.

Cyclic Mechanism

In a formal way, the isomerization of acyclic hydrocarbons according to a cyclic mechanism may be represented as the succession of three consecutive steps: (a) 1,5 dehydrocyclization to an adsorbed cyclopentane intermediate; (b) interconversion between isomeric adsorbed cyclopentyl intermediates, with displacement of the point@)of attachment to the metal; (c) ring cleavage, followed by desorption of the acyclic isomeric product(s). Steps (b), which allow the ring to be ruptured at different positions from where it was closed, are probably the ones which account for the multiple hydrogen-deuterium exchange of alkanes. The interconversions in Steps (b) are very fast reactions, while Steps (a) and (c) are rate determining. We shall discuss in turn Reactions (c) and (a), reviewing the most sigificant data, in order to deduce rational reaction mechanisms.

A. HYDROGENOLYSIS OF CYCLOALKANES Step (c), the reverse of Step (a), may be investigated separately by studying the hydrogenolysis of cyclopentanes and cyclobutanes. In these reactions, the effect of methyl substituents allows one to distinguish between several reaction mechanisms. As developed in the introduction, a number of important features in hydrogenolysis of cycloalkanes on platinum-charcoal catalysts emerges from the work of the Soviet school of catalysis. In a different approach, the hydrogenolysis of methyl- and 1,3-dimethylcyclopentaneswas investigated on a series of platinum-alumina catalysts with various metal loadings (0.220%) (84,85).It was found that the product distribution changed substantially with the percentage of platinum on the carrier. An almost selective

SKELETAL ISOMERIZATION OF HYDROCARBONS

29

rupture of the bisecondary C-C bonds was observed on the more concentrated catalysts ( >2% Pt), while one the less loaded ones ( < 1% Pt) the chances of rupturing the five C-C bond of the ring were approximately equal (Scheme 33). This effect, interpreted immediately as a particle size

1/5

Nonselective hydrogenolysis

Selective hydrogenolysis

SCHEME 33

effect, was first explained by assuming that, on large crystallites, several hydrocarbon molecules were simultaneously adsorbed, and that steric hindrance prevented adsorption at the tertiary-secondary C-C bonds. A more careful study of the hydrogenolysis of methylcyclopentane on two catalysts of extreme dispersion (0.2 and 10% Pt) showed that, in the temperature range 250”-310”C, the product distributions were temperature insensitive on the 0.2% Pt/A1,0, catalyst, but temperature sensitive on the 10% Pt/AI,O, catalyst (86). On the latter, all the observed distributions appeared as combinations of two limiting distributions, one of which includes only methylpentanes and therefore corresponds to a completely selective hybonds; the other one contains n-hexane, but drogenolysis of -CHz-CH2is different from the one obtained on the 0.2%Pt/Al,O, catalyst. Platinum films are intermediate between the two types of supported catalysts (86,87). These results were interpreted by assuming that the hydrogenolysis of substituted cyclopentanes can take place according to three distinct mechanisms: (1) the nonselective Mechanism A, occurring on highly dispersed catalysts and corresponding to an equal chance of breaking any C-C bond of the ring (but not of quaternary-secondary C-C bonds in gemdimethylcyclopentane); (2) the selective Mechanism B, allowing only the rupture of bisecondary C-C bonds ; and (3) the “partially selective” Mechanism C, competing with Mechanism B on catalysts of low dispersion. The occurrence of mechanism A on 0.2% Pt/Al,O, was confirmed for several substituted cyclopentanes and cyclobutanes (Table IV). On the other hand, while Mechanism C, which seems to occur on 10% platinum catalysts at high temperatures, could not be isolated on account of the fast consecutive isomerization of acyclic products that occurs above 320”C, Mechanism B was associated with the distribution of methylpentanes obtained at 220°C

30

F. G . GAULT

TABLE IV Product Distributions Corresponding to Mechanism A for Different Cyclic HydrocarbonPb-' (PtIAl,O,, 0.2%) Starting product

I

,r--

//\/\/

L-----

M L------

r-----t

Reaction product

Obtained (%)

Calculated (%)

n-Hexane

38

40

2-Methylpentane

41

40

3-Methylpentane

21

20

n-Pentane

53

50

lsopentane

41

50

23.5

25

46

50

30.5

25

26

20

31

40

43

40

n-Hexane

I

2,3-Dimethylbutane

L-----,

I , r---

n-Heptane

i

2,3-Dimethylpentane

L------

Data from (86).

* Catalyst, Pt/AI,O,,

0.2%.

' 1,2-Dimethylcyclobutane and 1,2-dimethylcyclopentanewere equilibrated mixtures of cis and trans isomers.

on 10% Pt/Al,O,. The distinction between these two reaction mechanisms (B and C) on the catalyst of low dispersion may appear formal, and the variations in the product distributions with the temperature, although not their expression as a linear combination, could be interpreted as the result of temperature-dependent steric interactions between surface and adsorbate. However, it was found recently that the hydrogenolysis of methylcyclopentane on iridium-supported catalysts is completely selective, whatever the temperature and whatever the dispersion of the metal on the carrier (88). Mechanism B, therefore, is real, and the temperature dependency of the

31

SKELETAL ISOMERIZATION OF HYDROCARBONS TABLE V Product Distributions from Hydrogenolysis of Cis- and Trans-I ,2-Dimethylcyclobutanes on Metal Films" Products

Metal Pt

Starting Temp. isomer ("C) Trans

Cis Ni Rh

Trans Cis Trans Cis

18 115 150 110 150 135 100 0 0

3-Methylpentane

2,3-Dimethylbutane

Conversion

n-Hexane

(%)

Trans/cis

7.3 9.2 11.1 23.1 22.5 2.8 18.7 0.9 6.5

21.4 24.3 26.5 41.7 39.3 17 29 8 26.5

11.3 66.6 62.4 34.6 38.2 80.2 52.3 91.1

33.3 52.6 70.5 51.1 19 74 8 49.5 81

93.516.5 9416 92.617.4 44.7155.3 61/33 9113 3/91

61

lOO/O OjlOO

Data from (89).

product distribution on 10% Pt/Al,O, shows that a third mechanism, C, must occur. Further information was then provided by the study of the hydrogenolysis of cis- and trans-dimethylcyclobutanes on metal films (89).In contrast to the rapid cis-trans interconversion of 1,2-dimethylcyclopentanes under any condition, the cis-trans isomerization of 1,2-dimethylcyclobutanes is slow on platinum films and negligible on nickel and rhodium films. This allows the initial product distributions to be determined. As shown in Table V, 2,3-dimethylbutane is the major reaction product in every case except for the cis isomer on platinum. Since both mechanisms A and B (nonselective and selective) compete on platinum films (86, 87), and probably as well on nickel and rhodium films, the excess of 2,3-dimethylbutane over the amounts expected by Mechanism A may be assigned to Mechanism B, involving the selective rupture of -CHz-CHzbonds. a,a,fl,fl-Tetraadsorbed species were suggested to be associated with Mechanism B (89),whose contribution increases with different metals in the same order, Pt < Ni < Rh, as that for multiple exchange of methane (90).Since multiple exchange of methane is usually taken as a criterion for the ability ofa metal to form adsorbed methylene, this result is in fair agreement with the proposed a,a,P,P-tetraadsorbed precursors (Scheme 34). The latter were

SCHEME 34

32

F. G. GAULT

presented recently, in a more modern version, as 1,2-dimetallodicarbenes (44,88,91).The higher yields in n-hexane and 3-methylpentane from cis than from trans-l,2-dimethylcyclobutanestrongly suggest that the rupture of tertiary-tertiary and secondary-tertiary C-C bonds involves some highly structure-sensitive intermediate species. These could be a,b-diadsorbed species or x-olefinic species, which are easily formed by cis elimination of two H atoms in an a,b-position (92,93),a process that is expected to occur more

SCHEME 35 H

qi-q?----?4\*q H

M

H

M

SCHEME 36

easily from the cis than from the trans isomer because of steric interaction of one of the methyl groups with the surface (Schemes 35 and 36). Since Mechanism A probably derives from x-bonded olefins, it can therefore operate with equal facility for each C-C bond in the C, ring of the cis isomer, but should be excluded for the tertiary-tertiary C-C bond and severely retarded for the secondary-tertiary C-C bonds in the trans isomer. Thus, mechanism A is dominant only on platinum and only for the cis-l,2-dimethylcyclobutane. The occurrence of a third mechanism, C, was suggested by a number of results that could be explained by none of the above mechanisms A and B, especially those involving the rupture of methyl-substituted C-C bonds, that is, the formation of n-hexane from methylcyclopentane on 10% Pt/AI,O, catalysts at high temperature (86),n-hexane from trans- 1,2-dimethylcyclobutane on metal films (89),n-heptane and 3-methylhexane from 1,2-dimethylcyclopentane, and methylhexanes from 1,3-dimethylcyclopentane on 10% Pt/A1,0, at relatively low temperature (94). Mechanism C, in our, opinion, is analogous to the metallocyclobutane bond shift mechanism (Scheme 25). Since the latter accounts for not only bond shift isomerization, but also C-C bond rupture, it might be operative for the hydrogenolysis of cyclic hydrocarbons. If one considers nonsubstituted cyclobutane and cyclopentane, one could object that there are two reasons why this mechanism is expected to be rather inefficient for ring cleavage. First, metallobicyclic compounds of the (2,1,1) and (l,l,l) series might be unfavored because of excessive strain. Second,

SKELETAL ISOMERIZATION OF HYDROCARBONS

33

after dismutation, isomerization of the metallocarbene end of the chain into a n-adsorbed vinylic group is required, and the resulting di-n-adsorbed diolefin is very unlikely in the cyclobutane and cyclopentane series (Scheme 37). However, such a mechanism is not introduced to explain the rupture of

(J-Q-0 M

SCHEME 37

secondary-secondary C-C bonds, but for the breaking of secondarytertiary and tertiary-tertiary C-C bonds. The participation to the metallocyclobutane intermediate of an exocyclic methyl or alkyl substituent should allow this mechanism to be operative, since the overall result of the rearrangement is the formation of a larger, less strained ring including the metal atom. Furthermore, isomerization of the carbenoid end of the chain to a n-adsorbed vinylic group is likely to occur via 1,2-hydride shift, thereby relieving some of the strain in the resulting carbene-olefin metal complex (Schemes 38 and 39).

SCHEME 38

SCHEME 39

In the hydrogenolysis of methylcyclopentane on 10% Pt/Al,O,, n-hexane is not detected as an initial product at 233"C, but appears in increasing amounts with increasing temperature (86).This suggests that the activation energy required for Mechanism C in this case is higher than that for Mechanism B. In contrast, at 230°C, the hydrogenolysis of 1,2- and 1,3-dimethylcyclopentanes afford, besides the acyclic products expected from Mechanism B (2,3- and 2,4-dimethylpentanes, respectively), appreciable amounts of acyclic isomers resulting from the rupture of secondary-tertiary and tertiary-

TABLE VI Product Distributions from 1,2- and I,3-Dimethylcyclopentaneson 10% Pti AI,O, at 2307C",b

PH*

Conversion (%)

2,3-Dimethylpentane

2,4-Dimethylpentane

3-Methylhexane

2-Methylhexane

2.4 5.85 19.3

47.7 62.9 78.2

-

7.5 10.6 9.75

-

760

-

33.6 39.4 38.2

&E

4 ii

5.7 56.8 55.7

-

-

Data from (94). Both dimethylcyclopentanes were equilibrated mixtures of cis and trans isomers.

31.4 29.2 28.3

~

-

35.2 31.3 32.9

n-Heptane

Toluene

25.5 19.5 9.15

19.3 6.6 2.8

~

-

0 0 0

35

SKELETAL ISOMERIZATION OF HYDROCARBONS

tertiary C-C bonds, assigned to Mechanism C (94)(Table VI) : The difference in the activation energies of Mechanisms B and C should therefore be less for dimethylcyclopentanes than for methylcyclopentane. Mechanism C, as will be developed in Section VI, may account for not only ring opening to acyclic products, but also may explain 1-6 ring closure, which leads to aromatization.

B. 1-5 DEHYDROCYCLIZATION Having characterized the three hydrogenolysis mechanisms by their precursor species : dicarbenes (Scheme 34), n-adsorbed olefins (Scheme-36), and metallocyclobutanes (Schemes 38 and 39), the knowledge of the overall mechanism of cyclic type isomerization requires the identification of the precursor species in 1-5 dehydrocyclization, the reverse reaction of hydrogenolysis of cyclopentanes. The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating, that is, on 0.2% Pt/Al,O, (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are cr,fl,y-triadsorbed species involving a single metal atom (33) (Scheme 40). C

M

SCHEME 40

This mechanism is analogous to that proposed by Shephard and Rooney (95) for the isomerization of o-ethyltoluene to n-propylbenzene on platinum, a reaction which most likely involves a “cyclopentane” intermediate and was assumed to consist of an alkene-alkyl insertion reaction of a 1,2~,5atriadsorbed precursor (Scheme 41).

q*Q---J&+* /

z

M M

SCHEME 41

._.‘

36

F. G. GAULT

This was an attempt at a more precise description of the same type of 1,2,6triadsorbed species invoked by Herington and Rideal (96) as key intermediates in aromatization of alkanes by 1-6 ring closure (Scheme 42).

P-Q M

*3- 0 M SCHEME 42

The common features in these three mechanisms (Schemes 40,41, and 42) is that dehydrocyclization requires previous dehydrogenation and formation of n-adsorbed olefins. Another approach to the mechanisms of dehydrocyclization was the comparative study of the reactions of 2,2,3-trimethylpentane (l), 2,2,4trimethylpentane (2) and 2,2,4,4-tetramethylpentane (3) (Scheme 43) on platinum and palladium films (70). On palladium, dehydrocyclization is

111

13)

(21

SCHEME 43

generally a minor reaction, while on platinum, substituted cyclopentanes are in every case the major reaction products. This shows that 1,2,5-triadsorbed species are not required for dehydrocyclization on platinum. Furthermore, the relative rates of dehydrocyclization of the three compounds indicate that different mechanisms must be involved on platinum and palladium. At 300°C on palladium films, the relative rates of dehydrocyclization of 1, 2,and 3 are 2.6, 0.2, and < lo-', respectively. This strongly suggests that, 3, on palladium, dehydrocyclization involves n-olefin f ~ r m a t i o n Indeed .~ which cannot dehydrogenate without skeletal rearrangement, does not undergo detectable dehydrocyclization below 360°C, and the low dehydrocyclization rate for 2 is probably due to steric hindrance arising from the P-methyl substituent on the required n-adsorbed olefin (96). The drastic decrease of the dehydrocyclization rate from 1 to 2 with increasing substitution is thus consistent with this mechanism (Scheme 44).

M-6 hindered

M 11)

(21

SCHEME 44 Recently, an organometallic reaction has been elucidated, which clearly confirms that ring closure can occur via alkyl-olefin insertion catalyzed by a Pd(I1) complex (97).

SKELETAL ISOMERIZATION OF HYDROCARBONS

31

In contrast, on platinum films under the same conditions, 1, 2, and 3 undergo dehydrocyclization with comparable rates (1, 1.3, and 1.1, respectively). This suggests that dehydrocyclization on platinum does not require n-olefin formation, but needs involve only two terminal carbon atoms in the 1,5-positions. Carbene insertion into a o-metal-carbon bond was proposed to account for these results (Scheme 45). An alternative route, maybe more

/

\ SCHEME 45

energetically favored, was also suggested, that is, the transient formation of an intermediate in which two p orbitals of the carbon atoms 1 and 5 are coupled together with a metal d orbital (Scheme 45). Indeed the carbene-alkyl insertion mechanism in Scheme 45 neatly explains why the rates of dehydrocyclization of 1, 2, and 3 are so similar. However, since 2-methylhexane also undergoes 1-5 dehydrocyclization, involvement of methylenic carbon atoms and not simply terminal carbon atoms must also be possible. The pathway for the C,-alkanes must be the reverse of nonselective hydrogenolysis of methylcyclopentane (Mechanism A), since it also results in isomerization to 2,4-dimethylpentane and 3-methylhexane, most likely via adsorbed 1,3-dimethylcyclopentane (scheme 46). It is

U

SCHEME 46

38

F. G. GAULT

emphasized that 2-methylhexane does not dehydrocyclize to gemdimethylcyclopentane, since neither 2,2- nor 3,3-dimethylpentanes are detected among the reaction products (43). Therefore, 1,5-dehydrocyclization involving a tertiary carbon atom, like the reverse process, the rupture of gemdimethylcyclopentane at the quaternary center, does not occur. Hence, three different dehydrocyclization mechanisms, corresponding to the three mechanisms of hydrogenolysis of methylcyclopentane, may be characterized : 1. the nonselective Mechanism A, accounting for ring closure between methylenic and methylic carbon atoms; 2. the selective Mechanism B, allowing ring closure exclusively between methylic terminal carbon atoms; 3. the Mechanism C, accounting for ring enlargement of cyclopentanes to aromatics and, more generally, for a number of aromatization reactions. We suggest that the precursor species in Mechanism A is a Wdicarbene, which undergoes recombination to a n-adsorbed olefin-the precursor species in nonselective hydrogenolysis of methylcyclopentane (44, 88, 91 ) (Scheme 47). The metallodicarbenes in Scheme 47 have stable analogs (98), and dicarbene recombination has been suggested as a transient step in organometallic reactions (99-101). The mechanism in Scheme 47 is now

-

Q-Q-R-P-V M

M

M

SCHEME47

preferred, for nonselective ring closure, to that described in Scheme 45. Whereas the monocarbene in Scheme 45 is most likely formed by a-hydrogen elimination (which is the sole possible way for 2,2,4,4-trimethylpentane), another possible path to adsorbed carbenes and dicarbenes is the isomerization of n-adsorbed olefins, either directly, or else indirectly via a o-vinylic complex (Scheme 48). The equilibrium in Scheme 48 is of course shifted far to the left, the more especially as the n-olefin is more substituted. Nevertheless, the concentration of metallocarbenes formed by this route could be raised enough, at high temperature, to permit the formation of dicarbenes and the occurrence of dehydrocyclization.

39

SKELETAL ISOMERIZATION OF HYDROCARBONS

ti -M

SCHEME48

Metallocarbene formation by hydrogen shift explains iLAe owservedselectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/Al,O, (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively ,C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydrocylization to gaseous cyclic molecules was determined. Relative rates of 3:2: 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-1073 and the temperature (320"-380°C). These relative rates can be explained neither by a simple 1-5 ring closure (Scheme 49), nor by an alkyl-alkene insertion (Scheme 45), which both

+-(&

; _________

~

; _________,6

+- (&).-pq - (& 7

. . . . . .

4

6

6

6

SCHEME 49

would result in relative rates of 1: 1: 1. Furthermore, if it is assumed that in the latter mechanism (alkyl-alkene insertion), ethyl- and dimethyl-substituted n-adsorbed olefins like A, and A, are unfavored because of steric hindrance (96) (Scheme 50), the calculated relative rates would be 2:1:1,

* x * $3 M

(A 1)

M

M

(A 2)

(A,)

+ M

(A,)

7 M

( A 5)

2 M

( A 6)

SCHEME 50

which disagree with the results. In contrast, according to Scheme 51 and assuming that the concentration of metallocarbenes originating from stable trisubstituted or up-disubstituted olefins is negligible, relative rates of 3: 2: 1 are calculated, in close agreement with the experimental data (41). However, energetic considerations may also be important.

40

F. G. GAULT

TABLE VII Relative Contributions of the Various Pathways in the Cyclic Type Isomerization of 3-Methylhexane on PtIAI,O,"

Cyclic intermediate

-

Catalyst

Reaction products

trans

e

cis

Contribution of Pathway 4

Contribution of Pathway 5

O

Contribution of Pathway 6

" Data from (41).

E

10% Pt

2% Pt

5.3

4.8

7.15

6

2.25

4

15.4

15.4

1.6

1.5

31.7%

31.7%

7.3

6.9

11.2

12

12.5

11.9

20

20

51%

50.8%

2.95

4.6

5.7

5.4

2.95

2.8

5.6

4.6

t

17.2%

17.4%

SKELETAL ISOMERIZATION OF HYDROCARBONS

41

42

F. G. GAULT

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86),the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (28O0-3O0"C), where it competes with another process, ascribed to Mechanism C (33).Fortunately, it was found recently that iridium supported on A1,0, or SiO, selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (102) (Scheme 52). Similarly,

SCHEME 52

on the same iridium catalysts at 150"-18O"C, rapid interconversion between 3-methylhexane and 3-ethylpentane takes place, while 2-methylhexane under the same conditions yields only cracked products (102) (Scheme 53).

SCHEME 53

Therefore, ring closure on iridium can occur only between two primary (methylic) carbon atoms, and a dimetallodicarbyne is suggested as an intermediate for Mechanism B (88, 91) (Scheme 54). Although metallocarbynes have been isolated (101, 103), dicarbynes thus far have no stable analogs in coordination chemistry. Such species, dimetallodicarbynes, on account of the linearity of the M-C-C bonds, necessarily involve two metal atoms instead of one, and this is in fair agreement with the fact that, on platinum, Mechanism B, unlike Mechanism A, takes place only on very large metal crystallites (see Section VIII). Selective dehydrocyclization of Type B is best pictured as a dicarbyne recombination to give a dicarbene, with each species multiply attached to two contiguous metal atoms (Scheme 54).

An

-c--c& I

c 111 m

I

c Ill m

-

C

C

\

I

;-F m m SCHEME 54

C

SKELETAL ISOMERIZATION OF HYDROCARBONS

43

Adsorbed carbenes formed by cc-hydrogen elimination are the most likely precursors of dicarbynes on iridium. On this metal, further dehydrogenation of metallocarbenes to metallocarbynes might also be easy, thus explaining why the selective Mechanism B for both dehydrocyclization and hydrogenolysis is the only one operating. Finally, the “partially selective” Mechanism C in hydrogenolysis of cyclopentanes has a counterpart in dehydrocyclization of methylpentanes and n-hexane. The intervention of this mechanism, involving metallocyclobutane intermediates, is strongly supported by studies of aromatization (see Section V). In Mechanism C , which is identical to one of the bond shift mechanisms already discussed (Scheme 25), the key intermediate is believed to be a 1,1,4,5-tetraadsorbed species. Dehydrocyclization then consists of a carbeneolefin addition resulting in a metallocyclobutane intermediate, that is, the reverse of the first step in Scheme 29. However, as discussed earlier, this mechanism, because of strain, should be energetically easier with all 1,1,5,6tetraadsorbed species.

C. THEMULTIPLET MECHANISMS The first mechanism proposed for the newly discovered hydrogenolysis of cyclopentane was the “doublet” mechanism (104), in line with the general “multiplet” theory of Balandin (105). According to this mechanism, cyclopentane is physically adsorbed on two metal atoms, and stands perpendicular to the surface in the vicinity of physically adsorbed hydrogen atoms (Fig. la). This mechanism, however, does not explain why cyclopentanes are hydrogenolyzed on platinum while cyclohexanes and paraffins are not.

(a)

(b)

FIG. 1. Doublet (a) and sextet-doublet (b) schemes of the hydrogenolysis of cyclopentane on the (1 1 I ) face of platinum (107, 108). Large circles, Pt atoms, intermediate circles, C atoms; small circles, H atoms.

44

F. G . GAULT

It was replaced later by the “sextet-doublet’’ mechanism (106), in which physically adsorbed cyclopentane lies parallel to the surface (Fig. Ib) (107, 108).In the transition state, the five carbon atoms of the ring are located over the interstices of the (111) plane of platinum. The distance between two contiguous interstices allows them to accommodate two consecutive carbon atoms, but one C-C bond in cyclopentane is necessarily stretched, and this favors the hydrogenolysis ot the ring. In contrast, all the carbon atoms of cyclohexane or paraffins may be brought in contact with the metal surface, which would explain the selectivity of platinum for C,-ring hydrogenolysis. In the first presentation of the model, the relative inertness of the tertiarysecondary C-C bonds in substituted cyclopentanes was explained by some kind of steric hindrance. Later on, when it was recognized that under some experimental conditions tertiary-secondary C-C bonds in methylcyclopentane could also be ruptured, the sextet-doublet model was slightly modified. It was assumed that hydrogenolysis takes place according to an Eley-Rideal mechanism, involving physically adsorbed methylcyclopentane and two chemisorbed hydrogen atoms also located in the interstices of the (1 11) plane. Hence, two modes of adsorption were considered, ads, and ads,, depending on whether or not the methyl group is in contact with the metal surface (109) (Fig. 2). In the ads, mode, favored at low hydrogen partial pressure, one of the adsorbed hydrogen atoms required for breaking the tertiary-secondary C-C bond is missing, and hydrogenolysis cannot take place. In the ads, mode, occurring at high hydrogen partial pressure or on a surface which has been precovered with hydrogen, only the five carbon atoms of the ring are in contact with the surface, the methyl group being directed away from it, and one expects an equal chance of rupturing any cyclic C-C bond. Liberman and his coworkers (109-111) explained by the unequal contributions of ads, and ads,

ads6

ads,

FIG.2. Two types of adsorption of methylcyclopentane on Pt/C (109). The arrows indicate the H atoms attacking the C-C bond.

SKELETAL ISOMERIZATION OF HYDROCARBONS

45

modes the difference in selectivity observed for hydrogenolysis of alkylcyclopentanes and dimethylcyclopentanes in pulse and flow systems. The sextet-doublet model was adapted for 1-5 dehydrocyclization, the reverse of cyclopentane hydrogenolysis, and it was proposed that physically adsorbed alkane reacts with chemisorbed hydrogen according to a push-pull mechanism (112) (Scheme 55).

n-n I

M

I

M

c-c i

M

'

n

I

hIM

SCHEME 55

As in the case of hydrogenolysis of cyclopentane, the change in selectivity observed in pulse and flow systems for the 1-5 dehydrocyclization of n-heptane to ethylcyclopentane and 1,2-dimethylcyclopentane,for instance, was interpreted by two modes of adsorption involving five or seven carbon atoms in contact with the surface (113) (Fig. 3). The model has been extended to cyclobutane hydrogenolysis and to cycloheptane reactions. Cyclobutane is assumed to undergo, like cyclopentane, planar adsorption on the surface (114). The stretching, however, is more pronounced than in the case of the C , rings and propagates to all four C-C bonds of the molecule. These can then be broken easily, and that accounts for the absence of selectivity in the hydrogenolysis of alkylcyclobutanes (114, 115). In the case of cycloheptane (116) it is assumed that in the stable twisted chair conformation, either two hydrogen atoms (at C, and C3, or at C, and C , ) are drawn into nearby interstices in the metal lattice, resulting in the formation of a new bond and 1-6 ring closure, or another combination of two hydrogen atoms (at C, and C , ) are drawn into the interstices and the C , - C , fragment is flattened, resulting in the rupture of a carbon-carbon bond as in the sextet-doublet mechanism for cyclopentane hydrogenolysis. Thus is explained both the aromatization of cycloheptane and the hydrogenolysis which occurs to a smaller extent (Fig. 4). Besides the sextet-doublet model, a slightly modified form of the doublet model was also presented by the Soviet school of catalysis to explain the main features of hydrocarbon hydrogenolysis on ruthenium, osmium, iridium,

46

F. G . GAULT

(a)

FIG.3. 1-5 Dehydrocyclization of n-heptane on Pt/C (113):(a) adsorption by contact of 7C atoms; (b) adsorption by contact of 5C atoms;

FIG.4. Adsorption of cycloheptane according to (116).

SKELETAL ISOMERIZATION OF HYDROCARBONS

47

and rhodium catalysts (107, 109, 117-129). These metals are very indiscriminate, allowing the hydrogenolysis of paraffins and cyclohexanes as well as of cyclopentanes. Moreover, the hydrogenolysis of cyclopentanes and cyclobutanes on these metals is extremely selective-only the secondarysecondary C-C bonds are broken-and this is accounted for by the steric interaction between substituent and metal when the molecule is adsorbed perpendicular to the surface, Although the Soviet authors at first denied the participation of the doubler mechanism on platinum, they were forced to introduce it, in addition to the planar (sextet-doublet) model, to explain the bonds in gemdimethylcycloselective hydrogenolysis of -CHz-CHzbutane (220). They now believe that two mechanisms, involving rib and planar adsorption, respectively, compete on platinum catalysts (222). The doublet and the sextet-doublet mechanisms resemble in many respects our Mechanisms A and B discussed previously, involving x-adsorbed cyclopentene and 1,2-dicarbene complexes, respectively. Mechanism A, like the sextet-doublet mechanism, implies a quasi-planar adsorption of the molecule and is in essence nonselective. In Mechanism B, as in the doublet model, the molecule stands perpendicular to the surface and hydrogenolysis is extremely selective. The main difference between the two ways of thinking is that we consider the precursor species to be chemically adsorbed, with previous rupture of several carbon-hydrogen bonds, while the Soviet scientists envisage only physically adsorbed molecules. Their interpretation of selectivity is therefore very different from ours. While we consider that the selectivity is primarily determined by the number of carbon-metal bonds that can be formed with a given molecule, the Russian scientists base their explanation on steric and often subtle conformational considerations. Besides the examples given above, one could quote some others, such as the interpretation of the product distributions of cis- and trans- l-methyl,3-ethylcyclobutaneson platinum, palladium, and rhodium catalysts (122).On the three metals, larger amounts of 2-methylhexane than of 3-methylhexane are obtained from either isomer, but the selectivity is higher for the cis than for the trans isomer. This result, which may be readily explained by a n-allylic model (89),is accounted for, in the multiplet model, by a 1-4 interaction arising between the P-hydrogen atoms of the ethyl group adsorbed in the interstices of the catalyst, and the hydrogen atoms at one of the carbon atoms of the ring (Fig. 5), resulting in weakening the cyclic C-C bond in the M position to the ethyl group. A second major difference, which we believe to be the essential one, between the sextet-doublet mechanism and the n-adsorbed olefin mechanism, is that our Mechanism A involves only one metal atom, while the sextet-doublet mechanism takes place on the whole metal surface and more specifically on the (111) faces of the crystallites. This very demanding con-

48

F. G. GAULT

Et

Me

W

FIG.5. Adsorbed trans-l-methyl-3-ethylcyclobutaneaccording to (122).

dition, which can be checked by using single-crystal model catalysts, is the only criterion that may allow an unambiguous choice to be made between the two proposed mechanisms. That on metals carbon-hydrogen rupture is extremely fast compared with carbon-carbon bond activation substantiates the mechanisms involving dissociatively adsorbed species, but does not prove them really. Conversely, the difference of selectivity in the experiments made with flow and pulse systems (109-111), presented as an evidence for physically adsorbed precursors, can also be explained by the existence of two mechanisms with different orders versus hydrogen, due to different dehydrogenation states of the reactive species. An alternative or simultaneous explanation is that each of these mechanisms is influenced to a different extent by the hydrocarbon-tohydrogen ratio in the feed, as explained in Section 11. Finally, the decreasing rate of 1-5 dehydrocyclization on replacing hydrogen by helium as carrier gas, or alkane by olefin as reagent (112, 123, 124), used as an argument against the mechanisms involving dissociatively adsorbed species, is readily explained by the formation of very strongly adsorbed hydrocarbonaceous residues and other kinetic considerations. In conclusion, the key problem concerning the multiplet mechanisms is the nature of the active sites, isolated atoms, or regular crystal faces. This is a very general problem indeed in heterogeneous catalysis on metals.

V.

Hydrocracking

The inability of most transition metals to promote the isomerization of acyclic alkanes, especially by the cyclic type mechanism, may be understood if one considers in some detail the mechanisms of hydrocracking. In Tables VIII and IX (39,102, 125-128) are reported the distributions of the products

49

SKELETAL ISOMERlZATION OF HYDROCARBONS

TABLE VIll Distributions of'Hydrocrackiny Products,frorn 2- Methylpentune on 10", MetulJAl,O, Cutul.vsts Distribution ot hydrocracking products Cracking T ( C ) r(",Y

Metal Pd

Pt

If

Ni co

6C,

249 265 300 350 350

6 5 3 7.9 7

50.5 70.5 68 43.6 42

254 254 300 350 350

15 6 4 0.9 1.4

48 47 27 23 23.5

150 170 195 195 220

1.5 2.5 1.4 5 23.3

79.3 77.3 69.4 75 76.6

280

30

98.8

215

2.9

100

3C2 -

~

-

~

8.1

-

~

4

-

~~

13.1

-

2.6

~

~-

0.8 -

1.1

4 1.4

~

~

1.7 0.7 1.2

2C3

C,

1.2 2.2 1.5 2.3 2.1

+ C4

+ c,

Ref.

4.7 6.6 6.5

96.8 93.6 85.7 91.1 87.3

I25 125 102 I26 I26

17.3 15.3 16 23.8 20

28.6 26.3 31.2 29.4 30.6

54.1 45.3 50.2 46.7 49.3

127 127 102

4. I 11.6 6.7 17.9

62 51.9 49.3 48 46.9

33 34.8 38.8 41.2 32.6

102 I02 102 102

10

2

CI

4. I

39 39

102

28.5

-

20.4

22.4

28.7

I28

87.9

3.5

3.1

3.3

2.2

102

~~

2, conversion.

TABLE IX Distributions of Hydrocrucking Products from 3-Methylpentune on 10:: Metul/AI,O, Cutulysts

Distribution of hydrocracking products Cracking (",,)"

("<,)

6C,

270

8.2

68.9

-

254 300 350

4.5 1.4 1.8

36 15.5 17

6.7

---

~-

-

-

.-

165 190

1.9 1.6

54.2 52.8

Metal

T("C)

Pd Pt

Ir

" a. conversion.

1.5 .~

3C2 ~~

2.8 2.1

2C3

C,

+ C,

C,

+ C5

Ret

-

1.6

98.4

I25

0.8 2.5 3.5

28.2 30.1 29.2

64.2 67.3 67.2

127

1

22.1 23.9

72.6 72.5

102 102

1.5

3Y 3Y

50

F . G . GAULT

from isomerization and hydrocracking of 2-methyl- and 3-methylpentanes on several group VIII metal catalysts (10% metal/Al,O,). On palladium, demethylation (primary-secondary or primary-tertiary C-C bond rupture) is the major hydrocracking reaction. On platinum, the demethylation is still favored, but the other cleavage modes (secondarysecondary and secondary-tertiary C-C bond rupture) become appreciable. On iridium, deethylation predominates, while on nickel, the initial hydrocracking distribution includes a large excess of methane relative to the simple demethylation and deethylation. Finally, on cobalt, extensive cracking to methane accounts for 100% of the overall reaction. These results may be rationalized in terms of metallocarbene chemistry and of the capacities of the different metals to form metallocarbenes. On palladium, the major hydrocracking reaction is demethylation, which is equally effective,in 2-methyl- and 3-methylpentane, for primary-secondary and primary-tertiary C-C bonds, and therefore, since metallocarbenes cannot be formed from tertiary carbon atoms, can in no way be ascribed to a 1,Zdicarbene mechanism. Since demethylation is also the major process in the reaction of neopentane on palladium (34,a possible precursor for demethylation could be a 1,3-diadsorbed species attached to two metal atoms (Scheme 56) instead of one as in the metallocyclobutane mechanism (see Scheme 29).

M

M

SCHEME 56

On platinum, the a,/?-dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section 111, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a /? position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section 111, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in 2,2,3,3-tetramethylbutane (77) likely implies the adsorption of two carbon atoms in the 1-4 position. On iridium, hydrocracking involves mainly the rupture of primarysecondary and secondary-secondary C-C bonds, resulting in deethylation. Hydrocracking on this metal is best accounted for by the rupture of a 1,2-

51

SKELETAL ISOMERIZATION OF HYDROCARBONS

dicarbene precursor, giving two metallocarbyne entities (Scheme 57), whereas isomerization likely involves a 1,5-dicarbyne precursor. Metallocarbene formation, however, is slow enough to permit attachment of only two carbon atoms to the metal in the adsorbed species (Scheme 57). R

72

‘\

R’

I

c

c-c / / \ \ m m

Ill

7 2

+

c

Ill

M

M

SCHEME 57

On nickel, and even more so on cobalt, the large excess of methane relative to the simpler cleavage modes (C, + C,, C, + C,) clearly shows that several consecutive C-C bond ruptures occur before desorption. Triple or multiple attachment to the metal may therefore be possible on these metals, resulting in a complete degradation to single-carbon species (CH,), which then desorb as methane (Scheme 58). c-c

c-c’ I/

m

\\

m

‘c’ II

m

c

-

c Ill

m

FC,

+

c

c-c

Ill

I / \ \

m

m

m

-

c- c C

+

I

I

C

C

Ill

1l1111

M

M

-

etc

M

SCHEME 58

In conclusion, the group VIII metals may be classified according to their increasing capacities to form metallocarbenes, and it is worth mentioning again that the above classification: Pd < Pt < Ir < Ni < Co, parallels that suggested by the multiple exchange of methane for adsorbed methylene formation (90). A last question that arises now is the mode of generation of the metallocarbenes. Besides the indirect formation by C-C bond rupture, in metallocyclobutane dismutation, for instance (Scheme 59a), two other possible ways are @-hydrogenelimination (Scheme 59b) and hydrogen shift in n-adsorbed olefin (Scheme 59c) (see also Scheme 48). While the first process is likely in the case of iridium, nickel, and cobalt, it should not be so easy on platinum, because of its competition with carbeneolefin isomerization (see Section 111, Scheme 29). We believe that the only way of explaining why 1,2-dicarbenes may account for the hydrogenolysis of cyclic hydrocarbons (Scheme 34), but only for a minor part for the hydrocracking of acyclic hydrocarbons, is the competition, for the latter, between carbene-dicarbene formation and carbene-olefin isomerization. Carbeneolefin interconversions are unlikely in the case of cyclic hydrocarbons, since a dicarbene species cannot transform into a 1,1,2,3-tetraadsorbed species (l-carbene-2,3-olefin) and further into a 1,1,3-triadsorbed species without C-C rupturing.

52

F. G . GAULT

\I/H C

I

M

-

\ /

C

II

M

(4

SCHEME59. Different modes of generation of metallocarbenes. (a) Metallocyclobutane dismutation; (b) a-hydrogen elimination; (c) a-8-hydrogen shift.

VI.

Aromatization

Two mechanisms of aromatization of alkanes on platinum catalysts have been proposed, depending upon the structure of the reacting alkane, direct 1-6 ring closure, and 1-5 ring closure followed by ring enlargement. The direct 1-6 ring closure mechanism is the major one for paraffins with at least six carbon atoms in a linear chain (129-131). This mechanism is evidenced by the initial formation of benzene from n-hexane, but not from 2-methylpentane (131), and by the aromatic product distributions from a number of methylheptanes and dimethylhexanes (130). However, besides the major aromatic products expected from this mechanism, minor amounts of other aromatic hydrocarbons were observed, which cannot be accounted for by direct 1-6 ring closure (Scheme 60). Some of them were ascribed to the occurrence of skeletal rearrangement (by methyl shift, for instance) of the reacting hydrocarbon before 1-6 ring closure.

53

SKELETAL ISOMERIZATION OF HYDROCARBONS

abnormal

ldring

31%

7.7%

-15%

210%

35%

e70%

=lo%

SCHEME 60

On the other hand, the 1-5 ring closure-ring enlargement process is supported by the initial formation of aromatics from a number of alkanes with only five carbon atoms in a linear chain (22,25,26,33, 70,132), by the easy aromatization of substituted cyclopentanes (63, 69, 132-134), and by the identical aromatic product distributions from 2,2,4-trimethylpentane and 1,1,3-trimethylcyclopentane(68, 132). According to Kazanskii (135), 1-5 ring closure-ring enlargement competes with direct 1-6 ring closure for the aiomatization of alkanes with more than five carbon atoms in a linear chain. The metallocyclobutane mechanism, already invoked to account for some aspects of the bond shift and cyclic mechanisms, allows one to rationalize the mechanism of 1-5 ring closure-ring enlargement. This mechanism is best represented by Schemes 61 and 62 for the aromatization of 1,1,3trimethylcyclopentane and 2,2,4-trimethylpentane, respectively. It is emphasized that in the former case carbene-olefin recombination must be favored over carbene isomerization to di-n-adsorbed olefin, since xylenes are the major reaction products while 2,4-dimethylhexane is not detected (69). Schemes 61 and 62, which include ring opening by a metallocyclobutane mechanism followed by carbene-olefin addition, account for the following results:

b M

SCHEME 61

b,

54

F. G. GAULT

SCHEME 62

1. On platinum films, gemdisubstituted cyclopentanes initially produce much larger amounts of aromatics than methyl- and ethylcyclopentanes. Relative rates of aromatization are given in Scheme 63. This is readily explained by either methyl stabilization of the .n-adsorbed olefin in the carbeneolefin species, or by the greater propensity for a,y,-bonding, that is, metallocyclobutane formation, across a quaternary center. I

58

7

25

56

SCHEME 63

2. On Pt/Al,O, catalysts, 1,2-dimethylcyclopentane produces both toluene and n-heptane. The amounts of both products increase with decreasing hydrogen pressure (94)(see Section IV, Table VI). However, under the same conditions, toluene is not formed from 1,3-dimethylcyclopentane or from ethylcyclopentane. This suggests that metallocyclobutanes are more easily formed (or cleaved in the appropriate fashion) when a tertiary carbon atom is involved in the bonding with the metal. 3. The initial formation, from isopropylcyclopentane, of ethylcyclopentane, 1-methyl-1-ethylcyclopentane, and ethylbenzene on platinum films or Pt/Al,O, catalysts (68, 231), illustrates these points concerning the metallocyclobutane mechanism and carbene-olefin addition (Scheme 64).

h c SCHEME 64

55

SKELETAL ISOMERIZATION OF HYDROCARBONS

4. The reactions of alkylcycloheptanes of Pt-C result in ring contraction and yield 1-methyl-1-alkylcyclohexanes,and monoalkyl- and dialkylbenzenes (10, 12). These results may be interpreted by the mechanisms shown in Scheme 65. The predominance of the products resulting from Path A over those resulting from Path B is explained by the easier formation of metallocyclobutanes involving tertiary carbon atoms. Furthermore, the attack of the metallocarbene on a disubstituted carbon atom in Path A might be faster than on a monosubstituted carbon atom in Path B.

-RB -Ra6 -Rv SCHEME 65

5. It is tempting to propose similar, slightly modified mechanisms for the aromatization of cyclopentanes and cycloheptanes on palladium (69, 134, 136), involving ring enlargement and ring contraction, respectively. Taking in account the known ability of palladium to promote the formation of n-allylic species, Scheme 66 may account for the aromatization of 1,1,3trimethylcyclopentane on palladium (69). Another possible mechanism, however, without intervention of metallocarbenes, is the Rooney-Shephard mechanism, involving alkyl-alkene insertion (95).

4/

6p.

M +

SCHEME 66

CH,

56

F. G. GAULT

The mechanisms presented in Schemes 61,62, and 64 have the advantage not only of explaining the aromatization of alkyl-substituted cyclopentanes and n-pentanes, but also of giving a simple picture of 1-6 ring closure. This appears indeed as a carbene-olefin addition, a very general reaction in organometallic chemistry. This mechanism is consistent with an intermediate olefin, and substantiates, in terms of carbon-metal bonding, the mechanism based on 1,5,6-three-point attachment proposed by Herington and Rideal more than 30 years ago (96) (Scheme 67). Another characteristic of this

SCHEME67

mechanism (Schemes 61 and 62) is that it considers that the 1-5 ring closurering enlargement process involves an intermediate step, C, ring opening. Such a sequence of rearrangements was already proposed by Herington and Rideal (Scheme 68).

SCHEME 68

An alternative mechanism is required to explain some peculiarities of the aromatization of n-heptane on Pt/Al,O, . The distribution, determined by microwave spectrometry, of the toluenes from n-heptane- l-”C, is : methyl(I3C), 50%; ortho-13C, 34%; rneta-I3C, 16% (94). These results can be compared to those obtained by Davis (137) with n-heptane-l-I4C. The formation of meta-labeled toluene can be explained neither by direct 1-6 ring closure, nor by cyclic-type isomerization of n-heptane to 3-methylhexane followed by 1-6 ring closure of the latter (94).We suggest that the “abnormal” aromatization process responsible for the formation of metalabeled toluene is initiated by a dicarbene as in the nonselective mechanism A (see Section IV, Scheme 47). Aromatization is not influenced by the dispersion of the platinum on the support ( I % ) , so that it may be assumed that aromatization involves a single metal atom. Isomerization of the dicarbenes (7)to the dicarbenes (8) via 7c-adsorbed cyclopentanes, followed by isomerization to the suitable carbene-olefin species (9), would result in 1-6 ring closure and aromatization (Scheme 69). However, metallocarbenes may isomerize in different ways to carbeneolefin species and further to adsorbed diolefins (Scheme 70). The latter may readily be desorbed as paraffins, and variations of the experimental con-

SKELETAL ISOMERIZATION OF HYDROCARBONS

57

SCHEME 69

3 4 *2H

M 2H

nA/ SCHEME 70

ditions (temperature, hydrogen pressure, catalyst, structure of the reactant), resulting in variations in the relative rates of steps 1-l’, 2-2’, 3, and 4, should have a drastic influence on the importance of the multistep aromatization mechanism. Some conflicting results concerning the relative contributions of the 1-5 and 1-6 aromatization mechanisms most probably result from different experimental conditions. One could explain in this way why, under high

58

F. G . GAULT

hydrogen pressure, which favors the desorption of the acyclic isomers, 2,4-dimethylpentane-3-'jC yields an amount of methyl-labeled toluene (24%) that is very close to the one expected from consecutive cyclic type isomerization to methylhexanes and 1-6 ring closure (Scheme 71), while the multistep mechanism (Scheme 62) would produce only ortho-labeled toluene. In contrast, at low hydrogen pressure, 3-eth~lpentane-l-'~C aromatizes by several competitive mechanisms (138).

SCHEME 71

In a similar fashion, the difference between Muller and Gault's results (33), obtained on platinum films under 50 Torr hydrogen pressure, and Dautzenberg and Platteeuw's results (131),obtained on supported platinum catalysts under high hydrogen pressure, may be explained. The first authors, in contrast with the second, observed initial formation of benzene from methylpentanes. In general, on platinum, both mechanisms, direct 1-6 ring closure and the multistep mechanism, compete for aromatization of hydrocarbons. Their relative contributions do not change with temperature, because 1-6 ring closure has a higher activation energy than 1-5 ring closure and controls the rate of the overall process in the multistep mechanism.

VII.

Skeletal Rearrangementsof More Complex Molecules

From the above discussion, it ensues that a number of reactions involving carbon-carbon bond rupture and formation may be accounted for by a limited number of adsorbed species and elementary steps. The predominant precursor species in skeletal rearrangements are metallocyclobutanes and metallocarbenes, which can be further dehydrogenated to metallocarbynes,

SKELETAL ISOMERIZATION OF HYDROCARBONS

59

dicarbenes, or carbene-olefin complexes, and react like the analogous species in coordination chemistry. Metallocyclobutane dismutation and the reverse reaction, carbene-olefin addition, are the two major steps in olefin metathesis (73,74),and dicarbene recombination also has an analogue in organometallic reactions (99-101). Therefore, we believe that for some of the proposed mechanisms, especially those involving two metal atoms, which have not yet a counterpart in homogeneous catalysis, parallel examples will be found in the near future. Besides metallocyclobutanes and metallocarbenes, simpler species, such as o-alkyl and n-allylic species, may also initiate skeletal rearrangements, especially on poor carbene-promoter metals like palladium. The corresponding reactions, 1,2-alkyl shift and alkyl-alkene insertion, also have analogs in coordination chemistry. On account of the variety of reactions that are initiated by these very reactive species, the chemistry of large molecules may become extremely complex. As a matter of fact, each compound may constitute a specific case. The nature of the predominant reactions depends primarily upon the structure of the reactant, and of course, upon the catalyst.

A. POLYMETHYLCYCLOALKANES This variety of possible paths is best illustrated by the variety of the reactions products from 1,1,3- and 1,1,2-trimethylcyclopentaneson metal films (63. 68. 69). As shown in Table X, the two major reactions of 1,1,3trimethylcyclopentane on platinum and palladium films are aromatization to xylenes or toluene and demethylation to gemdimethylcyclopentane. Neither demethylation at a quaternary center nor ring opening takes place. The two latter reactions. however, are significant for 1,1,2-trimethylcyclopentane. a molecule in which the cyclic C-C bonds are more accessible. On rhodium and nickel, the two major reactions of 1,1,3-trimethylcyclopentane are demethylation to gemdimethylcyclopentane and complete degradation to methane. In addition, ring cleavage takes place to an appreciable extent, and on nickel, all the possible acyclic hydrocarbons from C , to C , (mainly C , , C , , and C , containing gemdimethyl groups) are detected as initial products. On iron and tungsten, the predominant reactions are selective demethylation and complete degradation to methane. The latter is overwhelming on cobalt films at 300°C. However, at lower temperatures, the degradation to methane is slower, and, in the presence of deuterium, exchange and hydrogenolysis occur with comparable rates (139).

TABLE X Product Distributionsfrom I ,I ,3- and I ,I ,2- Trimethylcyclopentanes on Metal Films"

'4

Products

T ("C)

t(

(%)

Aromatics

Pt Pd Rh Fe W Ni co

300 320 300 300 250 300 300

5 5 5 5

66 35 14 7.6

5 17 20

-

Pt Pt Pd

300 300 300

10 10 (b)

31 42.5 2.1 '

SCH,

Or

34 54 24.2 46.7 54.6 26.4 2.7

4.8 -

40 28

23

-

-

-

Traces

-y-L

Acyclics

11 55 45.4 42.5 49.7 97.3

-

18 44.4

Data from (63),(68),and (69). Conversion not determined.

Distribution of aromatics

6.8 (C,) 0.3 (C,) 2.6 (C,) 19.1 (CZ-C,) -

6 (C7-Cs) 11.5 (C,-C,) 53.5 (C,-C,)

Xylenes Toluene Benzene

88 12.5 11.5 20 -

-

95.8 92.9

9.4 71.5 70 59 -

56

2.6 16 18.5 11 -

44

-

-

4.2 7.1 > 99

-

-

SKELETAL ISOMERIZATION OF HYDROCARBONS

61

The results from a careful study (139) of the reactions of a number of polymethylcycloalkanes (10-16) on cobalt films in the presence of deuterium at 1OO0-215"C may be summarized as follows:

1. Exchange and cracking of 14 are about twice as fast as of 13, showing the stabilizing effect of gemdimethyl groups. 2. The only significant products due to cracking, apart from methane (mainly d, ) still contained a gemdimethyl group, that is, gemdimethylcyclopentane. but neither 10 nor 1,3-dimethylcyclopentaneare obtained from 14. Similarly, 12 yields 1,1,3-trimethyl- and gemdimethylcyclohexanes, but neither 1,3-dimethyl- nor methylcyclohexane. 3. Demethylation at a tertiary center yields only the perdeutero demethylated compounds, for example, 10 yields d, and d,,-cyclopentanes. 4. When demethylation takes place at a quaternary center, as in 11, 12, and 13, the predominant deuterodemethylated species are either the d4 or the perdeutero species, while in the exchanged molecules, the maxima are at d4 or d, (Table XI). In contrast, exchange of 14,15, and 16 yields mainly the perdeutero isomers. These deuterium distributions were interpreted by assuming that in 11, 12, and 13, rapid interconversions between a,y-diadsorbed and a,a,y-triadsorbed species, attached to the metal at the two gemdimethyl carbon atoms, were followed by metallocyclobutane dismutation. The resulting adsorbed methylene-cycloalkane may be deuterated via either n-alkyl or tertiary alkyl species. In the former case, rapid desorption would occur, with the formation of a d, molecule, while in the latter case, a number of interconversions between c-alkyl and T-olefinic cyclic adsorbed species would yield the perdeutero compound. In the case of demethylation at a tertiary center,

62

F. G. GAULT

TABLE XI Distributions of Deuterioisomers in Some Exchanged and Demethylated Polymethylcycloalkanes on Cobalt Filmy

dl dZ

d3 d4 d5

d, d, d8 d9 dl0

d, I dl, dl 3 4

4

dl5

a

0.4 3.5 25.7 11 8.1 9.6 6.8 5.8 6.7 6.7 10.9

29.6 9.4 14.4 3.1 6.1 24.8 3 2.6 2.6 1.1 1.1 1.1

7.2 3.2 2.2 4.2 7.1 73.3 1.4 0.6 0.7

-

0.6 2.9 39.4 6.9 5.9 3.9 2.9 7.3 3 7.9 23.6

10.3 11.4 8.3 63.1 1.6 1.6 1.6 0.6 0.6 0.6

5.2 -

0.7 1.1 1.7 1.8 2.6 8.8 78.1

4.8

0.4 0.4 0.3 Data from 139.

only the second path is available, thus explaining the differences in the distributions of 11,12, and 13 on one hand, and 14,15, and 16 on the other hand (Scheme 72). The difference in the behavior of geminated and nongeminated adsorbed cycloalkanes was explained by assuming that in the former case, multiple attachment of the organic moiety to the metal, required for complete degradation to methane, was prevented by steric interactions between the surface and the gemdimethyl group. One could also explain the relative

I/

-

-H l / + D

-7"fi

CD'

$CD2-

'L

9=$

d,

C DI

CD,

+

M

SCHEME 72

SIC

d,,

SKELETAL ISOMERIZATION OF HYDROCARBONS

63

stability of geminated adsorbed cycloalkanes by replacing the metallocyclobutane in Scheme 72 by a species attached to two metal atoms (Scheme 73). Indeed, 1,3-dicarbenes are likely to be more stable than 1,Zcyclic dicarbenes, and demethylation would then appear as an extension of the Leclercq-Maurel mechanism (82).

II

M

II

1 I

M

M

I

M

-i+g M

SCHEME 73

Related to the demethylation of gemdisubstituted cycloalkanes is the aromatization of gemdialkylcyclohexanes, which takes place, on platinized carbon, with the elimination of an alkyl group and formation of monoalkylbenzenes (140, 141). However, 1-methyl-1-alkylcyclohexanes(alkyl = Et, Prop, But) yield more alkylbenzene than toluene, while for l-methyl-ltert-butylcyclohexane, toluene largely predominates over tert-butylbenzene (Table XII) (140-144). This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized 7c-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbeneolefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-tert-butylcyclohexane, dealkylation prevails over demethylation. TABLE XI1 Relative Ratios of Toluene and Aikylbenzene from 1-Methyl-1-aikylcyciohexaneson Pt-C

R = Et R = Prop R = n-But R = tert-But

46

54 85

1s 8.5

91.5

75

25

140,142 140,142 140, 143 144

64

F. G . GAULT

R--HZcQ

-

R-H&

HC II

M

M

H3CQ

Q

C

+ $

M

RCH=CH HJ!Q

R- CH,- C

tl M

M

M

M

SCHEME 14

Another related reaction is the hydrogenolysis of spiro(4,4)nonane (145) and other spiranes (146-148) on platinized carbon. Hydrogenolysis affects mainly the quaternary-secondary C-C bond, and is accompanied by ring enlargement to indane and o-ethyltoluene (Scheme 75). In the bicyclo (2,2,1)

Pro

\ /

lrogenolyrir

Ring enlargement

Et

3%

8%

SCHEME 15

heptane series, ring opening on platinized carbon was observed as long ago as 1933 by Zelinskii et al. ( 3 ) .The hydrogenolysis of norbornane was later investigated by Kazanskii et al. (149)(Scheme 76). 1,3-dimethylcyclopentane is the major reaction product, but toluene is also formed in small amounts. On Pt/Al,O,, isocamphane yields mainly 1,1,2,4,5-pentamethylcyclopentane (Scheme 76), while on nickel-kieselguhr, demethylation accompanies ring opening (150). It is possible that, because of the lack of adequate analytical tools at that time, skeletal isomerization (i.e., isocamphane-camphane interconversion) had been overlooked. This reaction does take place on

SKELETAL ISOMERIZATION OF HYDROCARBONS

65

palladium at 220"C, as reported by Rooney et al. (64,65)and is probably also linked with the 1,Zalkyl shift mechanism proposed for the isomerization of protoadamantane and other caged molecules.

SCHEME 76

B. SUBSTITUTED AROMATICS

A considerable amount of work has been devoted to the skeletal rearrangements of substituted aromatics, especially of alkyl-substituted benzenes. The first of these reactions that have been investigated are the cyclizations of diphenylmethane to fluorene and of dibenzyl to phenanthrene (151)(Scheme 77).

SCHEME 77

In these reactions, however, the formation of the fused rings takes place by 1-5 or 1-6 ring closure between two aromatic carbon atoms. In contrast, the formation of fluorene and 9-methylfluorene from 2-methyl- and 2-ethyldiphenyl, respectively (152) is much closer to the 1-5 ring closure observed with paraffinic hydrocarbons (Scheme 78).

w-q R

R

SCHEME 78

When the latter reaction was discovered, Kazanskii and Liberman (1.53, 154) systematically investigated the dehydrocyclization of alkyl-substituted benzenes : indane, 1-methylindane, and 2-methylindane were obtained from n-propyl-, sec-butyl-, and isobutylbenzenes, respectively (Scheme 79).

(yL(yJ m-m SCHEME 79

66

F. G. GAULT

Since 1-5 ring closure provides a route for the skeletal isomerization of alkanes, isomerization of substituted benzenes by a cyclic mechanism should also be possible. That was verified by Shephard and Rooney (95),who found that, on 0.5% Pt/Al,O,, interconversion of o-ethyltoluene and n-propylbenzene accompanied dehydrocyclization to indane (Scheme 80). In these

SCHEME 80

studies, the ratio of indane over isomer was larger from propylbenzene than from o-ethyltoluene, which suggests that the cleavage of the bisecondary C-C bonds (b) in adsorbed indane is easier than the rupture of the C-C bonds (a) adjacent to the aromatic ring. Hydrogenolysis of indane, indeed, yields o-ethyltoluene and propylbenzene in a ratio varying from 6.7 at 340°C to 2.5 at 430°C. As in the cyclic type isomerization of paraffins, the percentage of cyclic molecules (indane) increases with increasing temperature. The kinetics of 1-5 ring closure were investigated in parallel for aliphatic and aromatic hydrocarbons on Pt-C (155-157). The apparent activation energy for dehydrocyclization is always higher (by 7-15 kcal/mol) in the case of monosubstituted benzenes (n-propyl-, sec-butyl-, and isobutylbenzenes) than in the case of paraffins (ethylpentane, isooctane, n-hexane). The same is not true, however, for dehydrocyclization of o-ethyltoluene and isooctane, which occur with similar activation energies (157).This result is quite understandable if one considers that the first elementary step in the dehydrocyclization of monosubstituted benzenes but not of disubstituted benzenes results in a loss of aromaticity. The mechanism proposed by Shephard and Rooney (95) involved a cr-x-1,2,5-triadsorbed species for the dehydrocyclization of o-ethyltoluene and a 7c-allylic species for the dehydrocyclization of propylbenzene (Scheme 41). We suggest, instead, a carbene-benzene addition mechanism, in better agreement with the general picture we have given for 1-5 and 1-6 ring closure (Scheme 81).

SKELETAL ISOMERIZATION OF HYDROCARBONS

67

SCHEME 81

Besides cyclic type isomerization, alkylbenzenes may also undergo bond shift isomerization. Thus, small but noticeable amounts of isopropylbenzene were obtained from n-propylbenzene (95). When the lateral chain is lengthened, as in n-butyl- and n-pentylbenzenes, it may become difficult to distinguish among the isomers formed by bond shift and those formed by a cyclic mechanism. Among the reaction products of n-butylbenzene on Pt/SiO, (I%), isobutylbenzene is undoubtedly a bond shift product, but sec-butylbenzene can be obtained by either a cyclic or a bond shift mechanism. Since hydrogenolysis of C-C bonds next to the benzene ring is difficult, secbutylbenzene, obtained in larger amounts than o-isopropyltoluene, probably results mainly from a phenyl shift, a reaction which could be compared with the interconversion between 1-phenyl- and 2-phenylmetallocyclobutanes observed by Puddephatt et al. (78, 79).Moreover, a new reaction appears, 1-6 ring closure to naphthalene, which is more activated (by about 9 kcal/ mol) than 1-5 ring closure (Scheme 82).

58.8%

SCHEME 82

In the case of n-pentylbenzene and 2-phenylpentane (159),the same types of isomerization, via bond shift and cyclic mechanisms, proceed, but the situation seems complicated by consecutive reactions occurring without

68

F. G . GAULT

desorption from the surface. Indeed, at very low conversion, dimethylindanes are obtained, which may only result from two consecutive 1-5 ring closures, whereas 2-methylnaphthalene obviously stems from l-phenyl-3methylbutane, a bond shift isomer (Scheme 83).

SCHEME 83

Besides isomerization and dehydrocyclization, substituted benzenes also undergo hydrogenolysis of the side chain and homologation. This very interesting reaction, which consists in the growth of the side chain by one carbon atom, was demonstrated unambiguously by Csicsery and Burnett (160). o-Ethyltoluene leads to o-n-propyl- and o-isopropyltoluenes, o-diethylbenzene and cymenes, and the occurence of this reaction is, in our opinion, an argument in favor of carbene mechanisms.’

C. MEDIUM-SIZED RINGS Skeletal rearrangements of cycloalkanes containing 9- 18 carbon atoms were observed for the first time by Prelog et al. (162) on Pd/C catalysts at 400°C. Under these conditions, polycyclic aromatic and pseudoaromatic hydrocarbons are obtained (indene, azulene, naphthalene, phenanthrene, etc.). By carrying out the reaction on Pt/C under less drastic conditions, Kazanskii et al. (163) could observe the precursors of the aromatics as primary products. The latter are bicycloalkanes resulting from transannular 1-5 or 1-6 dehydrocyclizations (Scheme 84). For instance, cyclooctane yields

a-0 a - c o SCHEME 84

Recently (67, 161), evidence has been put forward for a carbene mechanism in the homologation of n-alkanes on metal films.

SKELETAL ISOMERIZATION OF HYDROCARBONS

69

n

d lai

K

c (bl

FIG.6. Conformations of (a) cyclononane and (b) cyclodecane (166).

cis-pentalane by elimination of two hydrogen atoms in the 1-5 position (163). The chief primary products from cyclononane, cyclodecane, and cycloundecane are hydrindane, decahydroazulene, and bicyclo(5,4,0)undecane, respectively, while hydrogenolysis products are obtained only in minor amounts (15, 164, 165). As pointed out by Kazanskii (166), the observed products result from the elimination of the two intraannular hydrogen atoms that are at the shortest distance from one another (Fig. 6). On increasing the size of the ring, and hence the distance between the transannular hydrogen atoms (Fig. 7), one should expect a decreasing ability

70

F. G. GAULT

FIG.7.

Conformation of cyclododecane (166).

of the cycloalkane to undergo transannular dehydrocyclization, and, simultaneously, an increasing ability to undergo hydrogenolysis. That is what is actually observed; while 50% of the reacted cyclododecane appear as dehydrocyclization products (l67),cyclopentadecane undergoes only hydrogenolysis (168). Transannular dehydrocyclization of methylcycloalkanes may also be regulated by conformational or other characteristics of the hydrocarbon. In the case of methylcyclononane, the predominant formation of cis-8-methylhydrindane with its methyl group in the angular position and of products derived from it (169) suggests that the transannular dehydrocyclization proceeds chiefly with elimination of the intraannular hydrogen atom located on the tertiary carbon atom, and therefore with the methyl group in the extraannular position (Scheme 85). But other bicyclic products are also obtained in appreciable amounts, so that it is not clear whether the direction of dehydrocyclization is determined by the presence of a hydrogen atom at a tertiary carbon atom or by some conformational peculiarities of the molecule.

b-cb SCHEME 85

In contrast, in the case of methylcyclooctane, 1-methyl-cis-pentalane and its hydrogenolysis products are only minor products, while 2-methyl-cispentalanes are the major ones (170) (Scheme 86). This suggests that the

SKELETAL ISOMERIZATION OF HYDROCARBONS

71

direction of dehydrocyclization in this case is determined by the conformation of the molecule, with the methyl group in the equatorial position, and not by the presence of a hydrogen atom on the tertiary carbon atom.

1 Methylcis penfalane

8% + H ~ d r ~ q e n o l yproduct3 s#~

2-Methyl

CIS

3 Methyl a s penlalanes

pen fa lane^

> 40%

28%

+ HydrogBnolYrm products 12%

11%

SCHEME 86

In any event, all these results suggest a mechanism in which the reactive species are formed by the removal of two intraannular hydrogen atoms and are attached to the metal by two c carbon-metal bonds in the 1-5 position. Because of conformational strain, indeed, the dicarbenes or carbene-olefin precursors invoked for the 1-5 ring closures of type A and C cannot be formed with medium-sized compounds. At this stage, one could question whether, in the case of simpler molecules (hexanes, heptanes, etc.), the dehydrocyclization mechanism could not also involve the simple ring closure of a 1,5 di-o-adsorbed species (Scheme 87).

0-0 M

SCHEME 87

This reaction, like dicarbene recombination, also has its analog in coordination chemistry, that is, reductive elimination of tetramethylene and pentamethylene ligands from platinum complexes yields cyclobutane and cyclopentane, respectively (I71). According to this direct ring closure mechanism, the observed selectivity for dehydrocyclization of n-alkanes on metals (nonformation of quaternary-secondary and tertiary-secondary C-C bonds in reactions of type A and B) should be interpreted in terms of simple steric effects. However, although, in the case of platinum, the concepts of steric hindrance could account for the change of selectivity that occurs with decreasing metal particle size (i.e., cyclization of n-hexane takes place on

72

F. G . GAULT

small but not on large particles, see Section VIII), they do not account either for the changes observed in the case of large particles or alloys of platinum and an inactive group IB metal, or for the change in selectivity when going from iridium to platinum and palladium. Therefore, it is better to consider the ring closure mechanism via 1,5 di-a-adsorbed species as restricted to medium-sized ring compounds, in which it is favored by transannular interactions. In any event, if ring closure via 1,5 di-a-adsorbed species occurs in the case of medium-sized ring compounds, the reverse step, C-C bond shift rupture in physically adsorbed cyclopentanes, should also take place on account of microscopic reversibility. As a matter of fact, the first mechanisms proposed for hydrogenolysis of cyclopentanes (the doublet and sextet-doublet mechanisms) involved physically adsorbed cyclopentane.

VIII.

A.

Correlation between Metal Particle Size and Reaction Mechanisms

ISOMERIZATION AND HYDROGENOLYSIS OF HEXANS AND PENTANSON PLATINUM-ALUMINA

1. Earlier Studies As already pointed out, the first particle size effect in skeletal rearrangement was found for the hydrogenolysis of methylcyclopentane (85).Nonselective hydrogenolysis takes place on highly dispersed catalysts, with a metal loading smaller than 0.6%, while selective rupture of bisecondary C-C bonds occurs on heavily loaded catalysts (more than 6% platinum on alumina). As soon as two distinct reaction mechanisms were envisaged for nonselective and selective hydrogenolysis (86),it was suggested that the nonselective mechanism involves a single metal atom. This view was consistent with the description given at that time of the highly dispersed catalysts. Indeed, the very high ratio H/Pt, around unity, between the number of chemisorbed hydrogen atoms and the surface platinum atoms in such catalysts was considered to be an indication of a quasi-atomic dispersion of the metal on the carrier. In contrast, the selective mechanism of hydrogenolysis occurring on concentrated catalysts was believed to involve every part of the crystalline lattice. The intermediate behavior of platinum films, between the supported catalysts of low and high dispersion, was attributed to the presence of both types of sites. A part of the activity of the films, that connected with the

SKELETAL ISOMERIZATION OF HYDROCARBONS

13

nonselective mechanism, was thought to be due to isolated exposed adatoms sitting on the regular low Miller index faces (87). A second type of structure sensitivity in the skeletal rearrangement of C, hydrocarbons was discovered when the isomerization of hexanes was investigated by using the 13C-tracer technique, which allows one to distinguish between cyclic and bond shift mechanisms. On a 0.2% Pt/AI,O, catalyst of high dispersion (H/Pt = l), the contribution of the cyclic mechanism is largely predominant (35);in the case of methyl migration (2-methylpentane + 3-methylpentane), 69-77% of the reaction takes place by this process, and in the case of chain lengthening (Zmethylpentane -P n-hexane), 90-98%. In contrast, on a 10%Pt/AI,O, catalyst of low dispersion (H/Pt = 0.04), the bond shift mechanism largely predominates (45). Although its contribution was difficult to estimate because of repetitive reactions in the adsorbed phase, it was in any case larger than 70%. Noteworthy also is the fact that the cyclic mechanism is selective (i.e., does not allow interconversion of methylpentanes and n-hexane) on large crystallites, while it is nonselective on small metal particles. This complete change of mechanism and selectivity with changing the particle size was attributed to the existence of two types of sites (35,45);the first, A, arising on very small platinum particles and responsible for the nonselective cyclic mechanism, was believed to consist of a single metal atom or a very small number of metal atoms (presumably two or three), one of which is involved in the bonding with the organic moiety. The second type of site, B, existing on the large crystallites and responsible for bond shift and the selective cyclic mechanism, was believed to consist of a group of several contiguous metal atoms, two of which participate in skeletal rearrangement. In the earlier papers of Corolleur, Gault et al. (35,45), it was emphasized that the nature of the reactive sites A or B is inherently due to the size of the crystallites, and that the sites B could not be identified at all when the atoms are regularly disposed on low-index faces. The latter statement was based on the observation that the sintering of a highly dispersed catalyst is accompanied by a substantial decrease in activity but not by a change in the selectivity. This result is quite understandable if one considers that small particles (A sites) have specific catalytic properties, and that only edge atoms (B sites) are active sites in large crystallites, as discussed later. Indeed, most of the catalytic activity in sintered low-metal-content catalysts is still due to small metal particles, and the reaction mechanism (nonselective cyclic mechanism) remains unchanged. The particle size effect in the case of supported platinum catalysts prepared from chloroplatinic acid was questioned by Dautzenberg and Platteeuw (131),who claimed that the difference of behavior between the 0.2% and the

74

F. G. GAULT

10%Pt/Al,O, catalysts was due to a difference in their chlorine content. This objection was ruled out by the results obtained with a series of Pt/Al,O, catalysts with low and high metal loading prepared by impregnation of an inert alumina with chloroplatinic acid, platinous tetrammine chloride and platinous tetrammine hydroxide. On these catalysts, the selectivity for methylcyclopentane hydrogenolysis depended only on the metal loading and not on the nature of the complex used for their preparation (172). Moreover, replacing alumina by silica or pumice did not alter appreciably the ability of the 10% platinum catalyst to promote selective hydrogenolysis. Finally, a 0.2% Pt/A1,0, catalyst was prepared by acidifying the alumina with gaseous hydrochloric acid at 200°C before impregnation, in order to simulate and exaggerate the chlorine incorporation that occurs during the preparation of the 10% Pt/Al,O, catalysts. The only difference in the product distribution with respect to a “normal” 0.2% Pt/Al,O, catalyst was the appearance, at 320°C,besides the hydrogenolysis products, of small amounts of benzene. Therefore, the change of selectivity with metal loading that is observed for methylcyclopentane hydrogenolysis is a true metal particle size effect and not an artefact due to bifunctionality. The Dutch workers (173) then reported experiments made with a 10% Pt/Al,O, catalyst, where the ratio 2-methylpentane/3-methylpentanein n-hexane isomerization was basically the same as with 0.5% and 1% Pt/Al,O, catalysts. To explain the discrepancies with Maire and Gault’s results, they questioned the pulse technique used by the French workers. However. the same selectivity changes were observed by the French team (174)on replacing the pulse system by a discontinuous or continuous flow system. Moreover, having noticed that the 10% Pt/A1,0, catalyst used by Dautzenberg and Platteeuw was much more dispersed than their own 10% catalyst, Corolleur, Gault et al. (174) investigated thoroughly the effect of the conditions of preparation of supported 10% Pt/AI,O, catalysts upon the average size of the metal crystallites. Methylcyclopentane hydrogenolysis was used again as test reaction, and in every case the correlation between particle size and selectivity was confirmed.

2. Recent Studies More recently, it was found that the hydroxylation state of the alumina has a determining influence upon the particle size of platinum crystallites. Controlled dehydroxylation of the alumina proved to be the most reliable method for preparing stable catalysts with gradual variations of the dispersion, and a continuous series of Pt/Al,O, catalysts with dispersions (characterized by the H/Pt ratio) ranging from 0.04 to 1.0 was prepared by

SKELETAL ISOMERIZATION OF HYDROCARBONS

15

TABLE XI11

Pi/AI,O, Caialysfs: Conditions of Preparution and Metal Dispersion Pt

R"Y

10 10 10 10 10 10 4 2 0.2 0.2 a

PI

On A1,0,'

10 8.5 8 8.4 1.5 7.1 4.1 2.5 0.3 0.2

A A B' B' B B' B' B'

B' A'

Calcinated (hr) 0

0 2 10 24 210 210 210 210 0

H/Pt 0.04 0.05 0.07 0.12 0.23 0.35 0.55 0.70 1 .o 1.o

Metal loading before impregnation.

* Metal loading after impregnation, determined by X-ray fluorescence. ' A and A' are two different batches of Woelm alumina with different OH contents. B' are samples of A' alumina calcinated in air at 600°C during 2-210 hr.

changing (1) the time of calcination (0 to 210 hr) of the alumina at 600°C, and ( 2 )the metal loading, from 0.2 to 10% in weight (Table XIII). In addition to chemisorption and oxygen-hydrogen titration, all these catalysts were simultaneously characterized by physical and chemical methods (127). The former included electron microscopy. X-ray line broadening. small angle X-ray scattering, and X-ray diffraction line profile analysis. The later consisted of a number of test reactions involving unlabeled and 13C-labeled hydrocarbons. These reactions and their dependence upon metal particle size will be described hereafter. a. lsomerizution of' 2-Metkylpentane to 3-Methylpentune (36,37,44, 127, 175). The isomerization of 2-methyIpentane-2-l3C to 3-methylpentanes3-13C and -2-13C allows one to distinguish between the cyclic mechanism (predominant on 0.2% Pt/Al,O,) and bond shift (predominant on 10% Pt/AI,O,) (35, 45) (Scheme 88). The most striking result obtained with the series of Pt/Al,03 catalysts with various dispersions was the constancy (18 & 3 % ) of the contribution of the cyclic mechanism on all the catalysts of low and medium dispersion (H/Pt

SCHEME 88

76

F. G . GAULT

-= 0.4) and its sharp increase with decreasing particle size, without however reaching 100% (Fig. 8). Electron microscopy showed that, in all the low-dispersed catalysts (H/Pt < 0.4), there were no metal particles smaller than 10 8, while such small particles were present in increasing amounts with increasing dispersion. This was confirmed by other techniques. The constancy of the percentage of cyclic mechanism on all the catalysts of low and medium dispersion allows one to rule out the mitohedrical theory (176),according to which some reactions take place on the edge and some others on the face of the crystallites. Indeed the mean size of the metal particles in the catalysts of low and medium dispersion ranges from 12 to 200 8, including therefore the 20-50 8 range where, according to Poltorak (176),the ratio between the number of face and edge atoms should vary widely. One could therefore reject the assumption that the bond shift and the cyclic mechanism take place on the face and on the corner (or edge) atoms of the crystallites, respectively (177). On the other hand, according to the classification of the active sites into two groups, A for the nonselective cyclic mechanism and B for selective cyclic and bond shift mechanisms, it may be erroneous to identify A sites with edge and B sites with face atoms. Indeed, the constancy of the bond shift mechanism throughout the 0-0.5 range of dispersion is understandable on this

0.25

0.50

L

0.75

I.(

H/Pt

to 3-methylpentanes: FIG.8. Particle size effect in isomerization of 2-meth~lpentane-2-'~C contribution of cyclic mechanism.

SKELETAL ISOMERIZATION OF HYDROCARBONS

77

basis only by assuming that the ratio between selective and nonselective cyclic mechanisms varies linearly with the ratio between face and edge atoms. One does not see why it would be so. The only obvious explanation, then, of the constant percentage of cyclic mechanism on the catalysts of low and medium dispersion is that the sites for both bond shift and cyclic type isomerization are topographically similar, that is, they involve both either edge or face atoms. Since isomerization also takes place on very small metal particles with practically no face atoms, one reaches the conclusion that isomerization by any mechanism takes place on the edge atoms of the crystallites and not on the regular low-index Miller faces. How, then, can the decrease of the bond shift mechanism and the corresponding enhancement of !he cyclic mechanism on the extremely dispersed catalysts, with particles smaller than 10 A, be understood? A possible explanation would associate bond shift isomerization with some specific sites in fcc crystallites, including edge atoms, which disappear below a critical size. Such sites-the B, sites of (110) or (311) configuration-arise when incomplete layers are added on the (111) or (100) faces of fcc crystallites (Fig. 9) (178),and the maximum concentration of these B, sites relative to face, edge and corner atoms have been calculated (I78a). According to these statistics, if one associates the cyclic mechanism A with edge or corner atoms and the bond shift mechanism with the B, sites, the percentage of bond shift should remain practically constant down to a particle size of 18 A and then decrease rapidly to zero. However, the critical limiting size below which bond shift disappears is much smaller than 18 A, and therefore the model involving B, sites cannot be retained.

FIG.9. Schematic drawing of (1 1 1 ) and (100) planes in incomplete cubooctahedron: ( 0 ) atoms of the filled layers; ( 0 )atoms added in the incomplete layers; crosses are B, sites (178).

78

F. G. GAULT

Another possible explanation of the special properties of the highly dispersed catalysts, based on Burton’s proposal (179),could be that the very small platinum particles do not retain the fcc lattice periodicity, but consist of pseudocrystals with uncommon D,, or icosahedral symmetry. It is worthwhile emphasizing that the crystallites having the limiting upper size of 10 8, are not very far from the small polyhedral clusters with 2-60 atoms, which, according to Hoare and Pal’s calculations (180),should be more stable than the normal fcc or hcp aggregates. Hoare and Pal noted that in the largest of their clusters (with 40-60 atoms), the D,,, T,, or I, symmetry of the central seed structure does not propagate to the last layers. The peripheral atoms sit irregularly, and “the lessening curvature of the physical surface makes it easier for them to migrate laterally, taking advantage of the low saddlepoints for this type of motion” (180, p. 190). The existence of these highly mobile quasi-isolated atoms (Fig. 10) could provide new possibilities for catalytic reactions, favoring, for instance, the occurrence of the nonselective cyclic mechanism. Although the potentials used in Hoare and Pal’s calculations (Lennard-Jones and Morse-Mye) may be considered as unsuitable for metal clusters, recent calculations (181), made with more realistic potentials, indicate that below 15 A, the clusters with icosahedral symmetry are more stable than the fcc cubooctahedra. Unfortunately, the size range in which the polyhedral metal clusters are supposed to be stable does not allow microdiffraction studies. Moreover, when platinum is deposited on an oriented rock-salt face, pseudocrystals with uncommon symmetry are present only in very small amounts, and in a particle size range of 80-120 A, where they obviously result from multiple twinning of fcc tetrahedra (182). Therefore there is so far no straightforward explanation, based on geometrical crystallographic considerations, of the specific catalytic properties of the small aggregates. On the contrary, both calculations and experimental data point to significant changes of the electronic properties in metal clusters smaller than 15 A. Calculations using a realistic potential showed

FIG. 10. Forty-five-atom cluster with broken icosahedral symmetry (180).

SKELETAL ISOMERIZATION OF HYDROCARBONS

79

that the local density of states and the asphericity of the charge density around a surface metal atom both vary with decreasing particle size, especially below 15 A (183). Also, the infrared absorption frequency of nitrous oxide adsorbed on platinum is shifted toward higher values with decreasing metal particle size, showing that besides the geometric factor, an electronic factor is certainly occurring in the range of particle size smaller than 40 A (184). Finally, the ultraviolet photoelectron spectroscopy (UPS) spectra of palladium particles deposited on silica are drastically altered when the size of the crystallites is smaller than 20-30 A (185); the narrow d-level emission and the nonobservation of a well-defined Fermi level clearly indicate that the small metal clusters no longer exhibit metallic properties, but should rather be considered as molecular type particles. The two different species, metallocyclobutanes and metallocarbenes, proposed as precursor species in bond shift and cyclic mechanisms, respectively, have different electronic requirements. In coordination cheinistry, the stability and reactivity of such species, especially metallocarbenes, are highly dependent upon the degree of occupation of the metal orbitals. Similarly, any modification, even slight, of the local density of states around a metal atom at the surface of a crystallite is expected to change substantially the relative concentration of the reactive species, and consequently the relative contributions of the corresponding mechanisms. Only in this way, with electronic and not geometrical factors, can the difference between the behavior of the different metals in skeletal isomerization be explained. We believe that the same type of explanation is also valid for the particle size effect that is observed on platinum. O n very small particles, the alteration of the valence bond would completely modify the relative concentrations of the various reactive species. O n the other hand, when the metal crystallites become large enough, their electronic properties should be the same as those of a very large crystal, and this would explain the constant contribution of the various mechanisms over a large range of particle size. b. Hydrogenolysis of Methylcyclopentane (36,44,127,175). In methylcyclopentane hydrogenolysis, the ratio of 3-methylpentane to n-hexane, which allows one to estimate the percentages of selective and nonselective cyclic mechanisms (see Scheme 33), first decreases sharply with increasing dispersion and then remains constant at a stationary value of 0.4 for dispersions higher than 0.5 (Fig. 11). Examination of the catalysts by physical methods showed that platinum particles larger than 20-25 A are present in all the catalysts of medium and low dispersion, but not in the more highly dispersed catalysts. There is therefore a limiting size of platinum particles, 25 A, below which the selective mechanism of hydrogenolysis cannot occur. Although we believe that the electronic properties of the metal primarily determine the nature, cyclic or bond shift, of the reaction mechanism, the

-

80

F. G. GAULT

* r\N

3-

FIG. 1 1. Particle size effect in hydrogenolysis of methylcyclopentane: 3-methylhexane/nhexane ratio.

geometrical factors should not be completely discarded, especially if one assumes that some intermediate species require not one, but two surface metal atoms. Geometrical factors then could become more important than the electronic ones, especially in the 20-30 A range of metal particles, where both hydrogenolysis of methylcyclopentane and cyclic type isomerization of methylpentanes change from selective to nonselective. A size of 20-25 A indeed corresponds to a cubooctahedron of four atoms on the edge, the smallest structure in which a diadsorbed species involving two contiguous edge atoms may be formed. Thus, while the change from selective to nonselective cyclic type isomerization of methylpentanes and hydrogenolysis of methylcyclopentane may be explained by some change in electronic properties, it could be more simply accounted for on simple geometrical grounds, if one assumes that (1) only edge atoms are used for forming the reactive species, and (2) the reactive species in nonselective cyclic mechanism, 1,1,5triadsorbed species (Scheme 45) or dicarbenes (Scheme 47), require only one metal atom, while the reactive species in selective cyclic mechanism, dicarbynes (Scheme 54), require two (Scheme 89). However, the electronic factor seems to be of key importance on iridium. Here, isomerization of 2-methylpentane t o 3-methylpentane occurs exclusively according to a selective cyclic mechanism, whatever the metal particle size (from 10 to 60 A) (88,102), which shows that the presence of two contiguous edge atoms is not really critical in this case. Moreover, even in

81

SKELETAL ISOMERIZATION OF HYDROCARBONS

the case of platinum, the limiting size 20-25 A is still low enough to be explained by a change in the d-band structure, if one extrapolates to this metal the observations of Takasu et al. (185) on palladium. c. Interconversion between Methylpentanes and n-Hexane (36, 37, 127). Since the selective cyclic mechanism does not allow interconversion between methylpentanes and n-hexane, two reaction mechanisms only, bond shift and nonselective cyclic mechanisms, are involved in these reactions. In the case of 2-methylpentane-4-13C, the contribution of the cyclic mechanism, which can be estimated as twofold the amount of n-hexane-3-13C (Scheme 90), is

M a ,A/? shift

+

& ( mechanism - &c

112

112

SCHEME 90

already high on catalysts of low dispersion, and continuously increases with decreasing metal particle size (Fig. 12). This increase may be considered as the result of two independent effects, the increase with dispersion of the contribution of the cyclic mechanism (electronic factor), and the change in the nature of this mechanism, from selective to nonselective (geometric factor). These two effects act simultaneously, and this explains why it is not possible to detect any break in the curve in Fig. 12, although the change of slope is noticeable. The complete disappearance of bond shift products on the catalyst of the highest dispersion suggests the existence, for chain lengthening as for methyl migration, of a limiting particle size below which the bond shift mechanism disappears.

82

F. G. GAULT

0.1

0.23 0.35

0.55

I.o

0.70

H/Pt

FIG.12. Particle size effect in isomerization of 2-methylpentane-4-13C to n-hexane: contribution of cyclic mechanism.

d. Isomerization of n-Pentane and 2-Methylbutane (39, 40). n-Pentane2-I3C is a convenient model compound for comparing the effects of metal particle size on the relative contributions of cyclic type isomerization and chain shortening by bond shift (Scheme 91). The contribution of the cyclic bond

-,hih

M cyclic_ mechanism

o--+"/\ 215

215

+1/5

SCHEME91

mechanism may be estimated from the percentage of cyclopentane and scrambled n-pentanes in the reaction products. As shown in Fig. 13, it remains constant for a dispersion range of 0.04-0.6 and then increases sharply for the more highly dispersed catalysts. The picture is basically the same as for 2-methylpentane + 3-methylpentane isomerization, except that the relative

H/PI

FIG. 13. Particle size effect in isomerization of n-~entane-2-'~C: contribution of cyclic mechanism.

83

SKELETAL ISOMERIZATION OF HYDROCARBONS

PlIAlfi,

N\

-

10%

N\

PtlAl,O,

(71.4 t0.151

--

0.2 %

1701

h - "(

Iv

Propane t Ethone

A/- x

--

N\

-

---

& -+ A )v

I

N\

35

I,

+

-

--

(701 168.5)

(621

Propone t Ethone N

(701

(56.51

t Methone 154.51 t Methone

154)

(45.3f 1.51 (45f 31 (45f31 (44.5k31

Propane t Ethone Propone Ethone

+

-

N

t Methone

138.5 231

t Methone (35")

FIG. 14. Reactions of n-pentane and 2-methylbutane on Pt/A103 catalysts, activation energies.

enhancement of the cyclic mechanism appears at a somewhat smaller particle size. On the other hand, 2-meth~lbutane-2-'~C allows one to distinguish, on catalysts of low dispersion, among two types of bond shift reactions differing in their activation energies, isomerization to n-pentane on the one hand, and to neopentane and 2-meth~lbutane-3-'~C on the other hand (Table 2 and Fig. 14). As shown in Figure 15, the ratio n-pentane to neopentane first increases slowly with increasing dispersion, and then rises sharply for the more highly dispersed catalysts, showing that one type of bond shift, the one with the lower activation energy, disappears faster than the other one on the very small particles. On the other hand, the ratio a-pentane to 2-methylbutane-3-' 3C first decreases with increasing metal dispersion, but then increases again by a factor of two on the most highly dispersed catalysts. All

84

F. G . GAULT

M T.,

1.20

3.10

I

0.04

I

I

0.35

0.70

I

I .o

H/Pt

FIG. 15. Particle size effect in isomerization of 2-meth~lbutane-2-'~C:( 0 )n-pentanel neopentane ratio; ( 0 )n-pentane/2-methylbutane-3-"Cratio.

these results agree with the conclusion that a new methyl shift reaction with a different mechanism takes place on the very small metal particles. One can notice that the easiest route on all catalysts is the isomerization of 2-methylb~tane-2-'~Cto 2-meth~lbutane-3-'~C,involving certainly a symmetrical intermediate. This result, and the earlier mentioned one, fast isomerization of 3-meth~lhexane-3-'~Cto 3-methylhexane~-2-'~Cand -4-13C (see Section 111, p. 27), support the view that, irrespective of the exact nature of the mechanism, bond shift reactions involving symmetrical intermediates are favored over the other reaction pathways. The appearance of a new methyl shift reaction on very small particles was confirmed by kinetic measurements made with a 0.2% Pt/Al,O, catalyst; the apparent activation energies for the various elementary steps were all higher than on the 10% Pt/Al,O, catalyst, as would be expected from Frennet's kinetic model, assuming that the hydrocarbon coverage OC is lower on small than on large particles (55-57), but the separation of the reactions into four distinct groups was still maintained (Fig. 14). However, some of the reactions had shifted from one group to another, and among the four reactions belonging to group 111 on the 10% Pt/A1,0, cata!yst, only one was left, isomerization to neopentane. The activation energy of methyl shift was raised to the same value as chain lengthening, and internal fission to the same as demethylation. Clearly the metallocyclobutane mechanism disappears on very small particles and is replaced by other processes. The fact that the extent of isomerization to neopentane decreases strongly (by about 4) while the other bond shift reactions still remain on catalysts of extreme dispersion suggests that, as on palladium, the precursor species in bond

SKELETAL ISOMERIZATION OF HYDROCARBONS

85

shift is a n-allylic species rather than an alkyl radical. The reaction would then develop as proposed by Muller and Gault (Scheme 20) and not according to Rooney’s mechanism (Schemes 22 and 28).

B. OTHERSTUDIES 1. Isoinerization of 2-Methylpentane and n-Hexane on Thick and

Ultrathin Films The isomerization of n-hexane and 2-methylpentane was investigated by Anderson et al. (177) on thick and ultrathin platinum films. The ultrathin films consisted of a discontinuous layer of very small metal particles with an average size of 20A, some of them being down to the limit of resolution of the electron microscope (8A).The isomerization of n-hexane, but not of 2-methylpentane, gave a noticeably higher percentage of methylcyclopentane on the ultrathin than on the thick platinum films. This increase in methylcyclopentane formation was attributed to an increase in the contribution of the cyclic mechanism due to the decrease in particle size when passing from thick to ultrathin films. Arguing from this effect, Anderson and co-workers proposed that the cyclic mechanism was taking place at the crystallite corner atoms, while bond shift was occurring on the face atoms. They also claimed that isomerization of 2-methylpentane, in contrast to n-hexane isomerization, was structure insensitive. The latter assumption is, of course, contrary to all that had been found previously on supported catalysts. Moreover, the percentage of n-hexane dehydrocyclized to methylcyclopentane was very irreproducible on thick films, varying from 27 to 52%, and the extent of this variation was larger than the one observed between thick and ultrathin films. Similarly, for the isomerization of 2-methylpentane on thick films, the percentage of methylcyclopentane was also irreproducible, varying from 35 to 70%. One could therefore question whether the increase in the percentage of methylcyclopentane on ultrathin films is a particle size effect at all. Indeed, it was recently found that the contribution of the cyclic mechanism as well as the amounts of carbocyclic products were tremendously increased by decreasing the hydrogen pressure below 100 Torr (39, 58). The use of 13Clabeled 2-methylpentanes allowed an estimate of the total contribution of the cyclic mechanism by adding to methylcyclopentane those acyclic isomers (3-methylpentane and n-hexane) that are formed via a cyclic intermediate. At 300°C. on a platinum black catalyst consisting of particles of a mean size of 220A, this contribution was around 30 and 85% under hydrogen pressures of 760 and 38 Torr, respectively (58).The latter value (85%) is close to the one (80%) found for the dehydrocyclization of 2-methylpentane on ultrathin films under 50 Torr hydrogen. At such a low hydrogen pressure, the cyclic

86

F. G . GAULT

mechanism predominates overwhelmingly, and it is difficult to detect any particle size effect. Such an effect was nevertheless detected in experiments made with 2-methylpentane-2-’ 3C on ultrathin and thick films under exactly the same experimental conditions (186),but it was rather limited; the total contribution of the cyclic mechanism was found to increase by only 20%. In any event the increased percentage of methylcyclopentane on ultrathin films should be considered not as the result of a decrease in the particle sizes, but as the result of an increase in the rate of methylcyclopentane desorption relative to ring opening and acyclic isomer desorption. As developed in Section 11, decreasing the hydrogen pressure is expected to increase the ratio between cyclic and acyclic products, and actually does so. The effect observed by Anderson et a / .(177)could be interpreted then in the same way, as an “indirect” particle size effect. It has been shown previously in this section that the reactive species responsible for skeletal isomerization are all located on the edge atoms of the crystallites, while hydrogen, as determined by chemisorption measurements, is adsorbed on every metal atom of the surface. A decrease in the particle size results then in a decrease in the ratio between the concentrations of hydrogen atoms and adsorbed hydrocarbon on the crystallites. This relative decrease of the local concentration of adsorbed hydrogen is expected to have the same effect as does, in a higher pressure range, a decrease of the hydrogen pressure, that is, it should increase the ratio of cyclic over acyclic products. The considerable irreproducibility of the percentage of methylcyclopentane on thick films could be considered, then, as the results of a large irreproducibility of the roughness of the films. It is interesting to note, in Anderson’s work, that the amounts of methylcyclopentane on polycrystalline thick films were always smaller (by a factor of 2) on the films condensed at 275°C than on those condensed at 0°C. Following an argument that was developed to interpret mechanistic changes in the exchange of cis-2-butene (187), one would expect that a large part of the defects such as edge, corner, and adatoms present on the films condensed at 0°C would be suppressed on those condensed at higher temperature, and this effect would explain the observed change in product distribution. 2. Reactions of’ Neopentane and Isobutane on Oriented Faces

A number of studies has been devoted to these reactions, first considered as taking place on every part of the metal crystallites. Using preferentially oriented platinum films, Anderson and Avery ( 3 4 ) found that the ratio, S, between the rates of isomerization and hydrogenolysis of isobutane was 3.5 times larger on (111) than on (100) faces. They attributed this result to the intervention of an additional symmetrical species triply attached to the metal (Scheme 92). Such a species, which is expected to favor isomerization over hydrogenolysis, would fit the (111) lattice, but not the (100) one.

SKELETAL ISOMERIZATION OF HYDROCARBONS

c M

A\ c

c

M

M

87

SCHEME 92

Boudart and Ptak (77) confirmed this enhancement of selectivity in isomerization on the (111) faces by comparing the reactions of neopentane on two 1% Pt/carbon black catalysts of exactly the same dispersion, 0.35. One of the catalysts had been fired at high temperature (SOOT), and such a treatment, according to the work of Lyon and Somorjai (188),is expected to favor the formation of randomly distributed (111) facets. It is remarkable that the selectivity in isomerization was five times larger on the sintered catalyst than on the nonsintered one. In the same paper (77) in which they introduced the concept of facile and demanding reactions, Boudart and Ptak noted that the extreme specific activities for isomerization and hydrogenolysis of neopentane varied by factors 15 and 300, respectively, on their different catalysts, Pt/A1,0,, Pt/SiO,, or Pt/carbon black of various dispersions. These considerable changes in turnover and selectivity, which do not necessarily follow the metal dispersion, were attributed to the faceting already discussed, and also to the decrease of the density of edge atoms and more generally of point defects with increasing particle size or roughening the surface. Triadsorption according to Scheme 92 is indeed possible on the (111) facets and, at point defects, edges, steps, on other crystallographic planes. It should be noted that in Boudart’s experiments, changes of selectivity and turnover are obtained by thermal treatment or by using different carriers (y- or ?-alumina, silica, carbon). In contrast, in Gault’s experiments (37) where the same carrier, y-alumina, was used, and the dispersion changed by modifying the hydroxylation state of alumina or the metal loading, the turnover remains remarkably constant for a wide range of dispersion (0.04-0.7) in isomerization of hexanes. The contrast between this constancy and the change in selectivity and reaction mechanisms may be explained by a general reaction model that includes repetitive processes. 3. Reactions of Neopentane on Pt-Silica and Pt-Zeolites Foger and Anderson (189)measured the selectivity and turnover number for isomerization and hydrogenolysis of neopentane on a number of catalysts where platinum was deposited on silica or Y zeolites. The selectivity did not vary widely with metal dispersion down to a limiting value of 10 A, below which isomerization disappears completely. The same trend toward increased selectivity as that observed by Boudart was noted when calcining the catalysts

88

F. G . GAULT

of medium dispersion. The limiting critical size of 10 A is in good agreement with that observed for the disappearance of bond shift in the isomerization of 2-methylpentane to 3-methylpentane and of 2-methylbutane to neopentane on Pt/Al,O, catalysts, and the constancy of the selectivity between 50 and 12 A is in favor of the above interpretation according to which all reactions take place on edge atoms. Foger and Anderson, however, interpreted their results by assuming that two types of bond shift reactions take place, one involving several surface metal atoms, occurring on (111) planes and operating in the case of catalysts of low and medium dispersion, the other occurring in the case of highly dispersed catalysts, involving any single metal atom of the surface, according to Rooney’s mechanism (65).In favor of this interpretation, they cited as evidence an increase, by 5-8 kcal/mol, of the activation energy when passing from large to small particles. This argument does not hold, however, since, as shown in Fig. 14, the activation energies of all the reactions of pentanes, hydrogenolysis, bond shift and cyclic type isomerization, are higher on highly dispersed than on lowdispersed catalysts. On the other hand, comparison of the activation energy of the reverse reaction, isomerization of 2-methylbutane to neopentane, and of the other reactions of 2-methylbutane (Fig. 14), shows that the mechanism of neopentane formation, and therefore of neopentane isomerization, does not change when passing from very large to very small particles. Another argument in favor of the 1,2-alkyl shift mechanism on highly dispersed catalysts is, according to Foger and Anderson (189),the percentage of adsorbed hydrogen atoms left on the surface as a function of the reaction temperature. This percentage, as suggested by temperature-programmed desorption experiments, varies roughly like selectivity. Anderson proposed then that the bridged intermediate in Rooney’s mechanism may react in two different ways, either rearranging into an adsorbed isomer, or reacting with hydrogen to yield hydrogenolysis products (Scheme 93). He thus explained

+iso-C,H,,

Pt

SCHEME 93

the drastic decrease in selectivity when passing from Pt-SiO, (d = 12 A) to Pt-Y zeolite (d = 10 A).6 The hydrogen temperature-programmed desorpRecently, Rooney (190) has pointed out that on the basis of molecular orbital theory, bridged species cannot undergo hydrogenolysis in the manner suggested in Scheme 93.

89

SKELETAL ISOMERIZATION OF HYDROCARBONS

tion profiles indeed show the presence of larger amounts of strongly adsorbed hydrogen in the latter than in the former catalysts. This should be correlated with the fact that hydrogen is retained in interstices of very small clusters (191,192). However, the difference between the two catalysts, Pt-SiO, and Pt-Y zeolite, is not in the TPD profiles, but in the reaction temperature (about 80" higher on the former than on the latter). Since the difference between the activation energies of hydrogenolysis and isomerjzation is not very large, the selectivity for the two types of catalysts, extraphated to the same temperature, would still be markedly different. Therefore, the rapid change in selectivity around 10 A may be considered as a true particle size effect related to a change in the electronic properties of the clusters when passing to very small metal particles, and not as a n artefact due to different hydrogen surface coverage. An interesting drop in selectivity from 10 to Ox, which parallels the increase in the concentration of electron-deficient species (Pt2+,Pt ), was observed in the series Pt-Na-, Pt-Ca-, Pt-La-Y zeolites. This decrease in selectivity could mean, whatever the reaction mechanism adopted for hydrogenolysis and isomerization, that the former reaction requires more electron-deficient centers than the latter. To summarize the previous investigations, most of the results indicate a remarkable constancy of the selectivities on all the metal particles down to a size of 10-12 A: the ratios of bond shift to hydrogenolysis, and of cyclic mechanism to bond shift, remain constant, the only exception being around 25 A crystallite size, where the change from nonselective to selective hydrogenolysis of methylcyclopentane occurs (Fig. 16).This constancy could mean that all reactions take place on edge atoms. However, in the case of neo+

'' I l i

NSCM

I

MAJOR MECHANISM

1

Mmor mechaniarn

-k

/ ,

///

I

activdion energy1

MAJOR MECHANISM I

FIG. 16. Particle size effects in isomerization of 2-methylpentane to 3methylpentane. SCM, selective cyclic mechanism; NSCM, nonselective cyclic mechanism.

90

F. G . GAULT

pentane, any treatment or preparation favoring the development of (111) facets favors bond shift over hydrogenolysis. In order to solve this apparent contradiction, well-defined surfaces such as single crystals exposing low-index Miller faces or stepped surfaces are required. REFERENCES 1. Anderson, J. R., Adv. Catal. 23, 1 (1973). 2. Clarke, J. K. A,, and Rooney, J. J., Ado. Catal. 25, 125 (1976). 3. Zelinskii, N. D., Kazanskii, B. A,, and Plate A. F., Ber. Dtsch. Chem. Ges. 66, 1415 (1933). 4. Kazanskii, B. A., and Bulanova, T. F., Izv. Akad. Nauk SSSR, Ser. Khim. p . 29 (1947); p. 406 (1948). 5 . Kazanskii, B. A,, and Bulanova, T. F., Dokl. Akad. Nauk SSSR 62, 83 (1948). 6. Kazanskii, B. A,, and Sergienko, S. R., Zh. Obshch. Khim. 9, 447 (1939). 7. Kazanskii, B. A,, and Rumyantseva, 2.A,, Izv. Akad. Nauk SSSR, Ser. Khim. p. 183 (1947). 8. Liberman, A. L., Bragin, 0. V., and Kazanskii, B. A., Dokl. Akad. Nauk SSSR 156, 1 1 I4 ( 1964). 9. Kazanskii, B. A,, and Liberman, A. L., Izv. Akad. Nauk SSSR, Ser. Khim. p. 265 (1947). 10. Khromov, S. I., Pik, E. I., Akishin, P. A., and Nikitina, L. M., Vestn. Mosk. Univ., Ser. Fir.-Mat., Estestv. Nauk 7, 97 (1952). 11. Balenkova, E. S., and Khromov, S . I., Neftekhimiya 2, 275 (1962). 12. Khromov, S. I., Balenkova, E. S., and Kazanskii, B. A., Vestn. Mosk. Univ., Ser. 2: Khim. 15, 36 (1960). 13. Kazanskii, B. A,, Rumyantseva, Z. A,, and Batuev, M. I., Izv. Akad. Nauk SSSR, Ser. Khim. p. 473 (1947). 14. Kazanskii, B. A,, Probl. Kinet. Katal. 6, 223 (1949). 15. Liberman, A. L., Russ. Chem. Rev. (Engl. Transl.) 30, 237 (1961). 16. Kazanskii, B. A,, Sovolova, 0. P., and Bashulin, P. A,, Izv. Akad. Nauk SSSR, Ser. Khim. p . 107 (1941); Chem. Zentralbl. 1, 1871 (1942). 17. Kazanskii, B. A., Rumyantseva, Z. A,, and Batuev, M. I., Izv. Akad. Nauk S S S R , Ser. Khim. p. 483 (1947). 18. Kazanskii, B. A,, and Lukina, M. Yu., Dokl. Akad. Nauk SSSR 74, 263 (1950). 19. Lukina, M. Yu.,Ovodova, V. A,, and Kazanskii, B. A., Dokl. Akad. Nauk SSSR 97, 683 (1954). 20. Kazanskii, B. A,, Liberman, A. L., Bulanova, T. F., Aleksanyan, V. T., and Sterin, Kh. E., Dokl. Akad. Nauk SSSR 95, 77 (1954). 21. Kazanskii, B. A,, Liberman, A. L., Aleksanyan, V. T., and Sterin, Kh. E., Dokl. Akad. Nauk SSSR 95, 281 (1954). 22. Liberman, A. L., Lapshina, T. V., and Kazanskii, B. A., Dokl. Akad. Nauk SSSR 105, 727 (1955). 23. Liberman, A. L.. Vasina, T. V., and Kazanskii, B. A,, Dokl. Akad. Nauk SSSR 117,430 (1957). 24. Liberman, A. L., Loza, C. V., Chan, M. N., and Kazanskii, B. A., Dokl. Akad. Nauk S S S R 120, 789 (1958). 25. Kazanskii, B. A., Liberman, A. L., Loza, G. V., Kuznetsova, I. M., Aleksanyan, V. T., and Sterin, Kh. E., Izv. Akad. Nauk SSSR, Ser. Khim. p . 1071 (1959).

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ADVANCES IN CATALYSIS, VOLUME 30

Tin-Antimony Oxide Catalysts FRANK J. BERRY Department of Chemistry University of Birmingham Birmingham, England

I. Introduction .

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Introduction

The oxidation of gaseous hydrocarbons over solid catalysts has been the subject of extensive industrial and academic research for many years. The mutual interest in these reactions reflects both their commercial importance and their value in studies of the fundamental features of catalytic processes. Indeed, the source of several theoretical investigations of catalytic reactions may readily be traced to developments in the modern chemical industry. Moreover, the numerous studies that have been conducted are a further reflection of the diversity of scientific interests embodied in this subject and the wide range of techniques currently available for their investigation. It is not surprising, therefore, that the heterogeneous catalytic oxidation of hydrocarbons is a subject in which a wealth of data has been accumulated and a diversity of opinions recorded. Hence, it is inevitable that in an article of this type any worthwhile inquiry into such a large field of scientific interest requires the consideration of a restricted aspect of the subject. In this respect it is interesting that Keulks et al. (1) have recently produced a valuable detailed review of a single aspect of hydrocarbon oxidation by 97 Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.

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examining the selective oxidation of propylene to acrolein. The study considered many important features of the process and attempted to relate some of the proposed mechanisms of selective oxidation to empirical observations. However, despite this extensive study of the reaction, it is quite clear that a large degree of confusion is still associated with the nature of the catalytic materials and their relationship with catalytic activity and selectivity. In principle it would seem reasonable that the bulk structure and surface properties of a solid would influence the catalytic performance. Verification of this view and an assessment of its importance may be more significant than first appears since it incorporates an implication that catalytic preparation should be designed to achieve the bulk structure and surface properties that give the optimized catalytic performance. Materials which have been shown to catalyze the conversion of propylene to acrolein have included metal oxides, mixed oxides, and more lately the multicomponent catalysts. A consideration of all these solids would require the assessment of numerous data and speculation. However, the mixed oxide catalysts have been associated with many of the more recent investigations of the course ofcatalytic oxidation, and these catalysts therefore seem to be worthy of detailed consideration. Although several mixed oxide catalysts have been developed commercially for the selective oxidation of propylene, the investigation of their fundamental physical and chemical properties has resulted in only a slow and steady accumulation of information. It also appears that attempts to correlate data from different investigations have frequently resulted in unsatisfactory interpretations. It seems that some of this uncertainty arises from correlations between results obtained from different catalysts subjected to different pretreatments and assessed under different evaluation conditions. Hence, the comprehensive description of the bulk and surface properties of a single catalyst, their interdependence, and their influence on catalytic performance is in most cases quite unclear. This article therefore seeks to examine in depth just one mixed oxide catalyst, tin-antimony oxide, which has been commercially developed (2-5) for the oxidation of propylene to acrolein as well as for the ammoxidation of propylene to acrylonitrile and the oxidative dehydrogenation of butenes to 1,3-butadiene. A recent book (6) and a subsequent review (7) have shown how little unanimity has been established about the fundamental properties of the material. In particular there seems to be much confusion as to the phase composition, the nature of the cationic oxidation states, the chemical environment of the cations, the charge compensation mechanism, the nature of the active sites, the distortion of the host tin(1V) oxide lattice by the dopant antimony atoms and whether any changes in the catalyst result from the adsorption and catalytic processes.

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In this article the tin-antimony oxide catalyst is considered in terms of 1. its preparation; 2. the physical and chemical properties of both bulk and surface as determined by physical and spectroscopic techniques ; 3. the dependence of the surface properties on the bulk structure; 4. the correlation between the properties of the catalyst and its catalytic character ; 5. the relationship between preparative procedure, the bulk and surface properties, and the catalytic character. Since the early studies of tin-antimony oxides have already been excellently reviewed (6, 7) only those reports that have a major bearing on more recent investigations and are directly related to the objectives of this article will be cited in this work. It should also be noted that the sections and subsections in this article represent an attempt to assess the available data on specific aspects of the material according to the objectives.However, some deviation from rigorous adhesion to individual themes has been inevitable when considering the interdependence of certain properties.

II.

Properties of the Catalyst

A. PREPARATION Lazukin et al. (8) prepared tin-antimony oxide catalysts by heating an alleged mixture of tin(I1) and antimony(II1) hydroxides at 1050°C in air, while Godin et al. (9), who were investigating the materials at a similar time, prepared their samples by mixing tin and antimony oxides in required proportions and heating in air at 850°C. Wakabayashi et al. (10) prepared tin-antimony oxides supported on alumina at 1 OOO"C, while Roginskaya et al. ( 1 1 ) heated mixtures of Sbz03 and SnO, at 500", 700", and 900°C. More recent studies (12-21) have involved materials prepared by the simultaneous addition of tin(1V) and antimonyw) chlorides to ammonium hydroxide solution, drying the precipitate at 120°C and calcining at temperatures from 400" to 1000°C. It would seem reasonable to presume that the preparation of tin-antimony oxides by coprecipitation would lead to a more intimate mixture of tin and antimony than would be achieved by solid state reactions between the respective oxides, and it would also seem reasonable that the close proximity

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of cations in the resultant product could be of significancein the behavior of the material as an oxidation catalyst.

B. BULKPROPERTIES A large degree of uncertainty surrounds such fundamental issues as the phase composition, the nature of the cationic oxidation states, and the electrical properties of the catalyst. Lazukin et al. (8) reported that calcination of mixtures of tin and antimony hydroxides in air at 1050°Cgave solid solutions even when the tin to antimony ratio was as high as 3 :1. Like many other investigations, the n-type semiconducting properties of the catalysts were identified. However, contradictory opinions on the extent of solid solution formation are common. The X-ray investigations by Godin et al. (9) reported little evidence for compound formation, and the materials were described as inhomogeneous mixtures of SnO,, a-Sb,O,, and small amounts of j?-Sb204.Godin's electrical studies, however, did go further than others and reported a threefold increase in electrical conductivity on addition of only 6.8% antimony to tin(1V) oxide. These observations were attributed to the substitution of Sb5+ ions for Sn4+ ions in the tin(1V) oxide lattice with the creation of Sn3+ ions which acted as electron donors to give free electrons and a rise in conductivity according to the scheme SnfZ,,Sb:+Sn~+022- == Snf',Sb:+022-

+ Xe-

It is interesting that this mechanism, which was suggested during one of the early studies of tin-antimony oxides, has received so little subsequent attention, since it is a process that could be applied to some of the mechanisms of catalytic oxidation proposed in later studies. Yet another illustration of the diversity of interpretation that can accompany results from the same technique is observed from the work of Wakabayashi et al. (lo),whose X-ray data of a supported catalyst containing 25% antimony and presintered at 1000°C for 3 hr were interpreted in terms of a solid solution of antimony in tin(1V) oxide. Further work (22) reported the increased electrical conductivity to be maximized at an antimony concentration of 3%. Another X-ray diffraction investigation that was interpreted in terms of the high solubility of antimony in tin(1V) oxide and compound formation at high temperatures was reported by Kustova et al. (23) in 1976. A more quantitative assessment of the X-ray and infrared data of Roginskaya's catalysts (11) reported that an increase in tin content gave a change in phase composition from the defective Sb,04 to a multiphase structure containing a solid solution of Sbz04in SnO, (the extent of which increased at temperatures from 500" to 900°C) to the tin(1V) oxide structure.

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The authors also suggested that an unidentified phase was formed at 900°C in compositions containing 50-80% antimony. The definition of the cationic oxidation states is another area of great confusion. Lazukin's study (8) discusses the formation of antimony(II1) or antimony(IV), while Suzdalev et al. (24)interpreted some Mossbauer spectra in terms of Sn4+ ions shifting the Sb3+-Sb5+ equilibrium in the initial catalyst toward Sb5+. It is surprising that the confusion over fundamental solid state properties such as phase composition and cationic oxidation states should have remained unclarified for so long, and it is disturbing that so many contradictory results and interpretations have been reported from supposedly similar materials. The lack of unanimity over the extent of antimony solubility in tin(1V) oxide is particularly surprising since many workers (9-11, 23, 25) have suggested that the formation of a solid solution is an important factor in the catalytic character of these materials. Hence, it is clear from these early studies that the extent and conditions for solid solution formation are completely uncertain. Before considering the most recent attempts to resolve this debacle it is pertinent to review our knowledge of the two binary systems and then assess the situations that might reasonably be expected to exist in the ternary tin-antimony-oxygen system. The antimony-oxygen system has been subjected to extensive investigation, and it now seems that a compound with formula Sb20, cannot be prepared (26,27) and that the pyrochlore Sb,O,, is the highest oxide of antimony that is normally formed at temperatures below 900"C, while Sb204is formed at higher temperatures (27, 28). Both these oxides contain Sb(V) and Sb(II1) species, and complete reduction to Sb,O, appears not to be achieved below these temperatures. It is quite surprising, therefore, that so many reports of catalytically active tinantimony oxides refer to the identification of Sb20,, and it would seem that only Kustova et al. (23) have identified Sb60,, in their low-temperature materials. As far as the tin-oxygen binary system is concerned, it appears that tin(1V) oxide is formed by the dehydration of tin(1V) hydroxide gel at 600°C and adopts a structure in which the oxygen atoms assume a slightly distorted, rutile type octahedral coordination around the tin atom (29, 30). It is now possible to consider the ternary tin-antimony-oxygen system. It would be reasonable to expect that multiphase systems would predominate in antimony rich materials although there is little information about the solubility of tin in antimony oxide. Since the metal-oxygen bond lengths in tin(1V) and antimony(V) and (111) oxides are all similar (26, 29), it is reasonable to expect that mixed oxide formation by coprecipitation is feasible and would involve the replacement by antimony of tin in the tin(1V) oxide

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rutile type lattice. Such phases might be most likely to exist in calcined materials containing low concentrations of antimony. It is relevant that attention should now be drawn to the recent work by Patterson, Pyke, Reid, Tilley, McAteer, et al. (12--16), who have examined several aspects of the structural and catalytic character of coprecipitated tin-antimony oxides. The structure characterization by Pyke, Reid, and Tilley (12) by X-ray diffraction techniques clearly showed that bulk equilibrium is difficult to establish in this system. This observation alone places some earlier data in a different perspective and implicitly demands a high degree of caution in the interpretation of physical and spectroscopic data recorded from these materials. The phase diagram (Fig. 1) defines the Sn1- XSbx02 2.SnO,+ Sb60,3

3. SnO, + Sb20,,

‘it,-

1. SnO,

_ _ _ _ _ - - -- - - - _-___----_ _ _ _ _ - - - - - - - b- - - - ,- - - - - - - - -aI

0

600

800 #loo0 Temperature ( C 1

FIG.1. The crystalline phases identified in catalysts calcined for 16-hr periods. Note: the compositions a to g correspond to Fig. 2 and the boundaries between phase regions correspond to maxima (0) or rapid changes ( 0 )in surface composition. [Reproduced from Y. M. Cross and D. R. Pyke (13.1

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crystalline phases identified in the catalyst following 16-hr calcination and shows that three corresponding regions of temperature and composition may be distinguished: (1) a region containing only the rutile (SnO,) type phase; (2) a region with bulk antimony concentrations greater than 60%, which contains both Sb,OI3 and the rutile type phases; (3) a region that is produced at high temperatures and contains Sb,O, and the rutile type phases. Since the solid solution phase has been considered as being of prime importance in the development of catalytic properties in these materials, it seems proper that this aspect of the study should be given first attention. The poor quality of the X-ray data of samples heated to 600°C was reported to preclude any degree of accurate phase analysis. Consequently, the sole appearance of rutile lines in the materials containing up to 70% antimony (12) strongly suggests that this criterion alone may not, as has sometimes occurred in the past, be taken as evidence of the existence of solid solutions. Further support for more cautious interpretation of X-ray data is obtained from studies by electron microscopy (12), which have given evidence of the coexistence of relatively pure SnO, with amorphous substances. It seems, therefore, that the formation of authentic solid solutions requires hightemperature calcination of materials containing low concentrations of antimony and thereby corresponds to the conditions under which one might expect a closer approach to equilibrium. Pyke et al. (12) suggested that an antimony concentration as low as 4% may indicate the upper limit of antimony incorporation in the tin(1V) oxide lattice. That this surprisingly low figure (as compared with other estimates of the solubility of antimony in tin oxide) might also be a rather generous assessment is suggested by the observation (12) that heating the materials to 1100°C or constant weight at 1000°C gives an equilibrium concentration of antimony in tin(1V) oxide that is closer to 3%. It is interesting that this figure corresponds to the level of antimony doping that has been reported to give the maximum conductivity (22). The assertion that solid solution formation is restricted to materials containing a low concentration of antimony and that short-period calcination is insufficient to achieve equilibrium is contrary to the conclusions of many earlier studies. This new interpretation is important since it requires a new degree of significance to be attributed to the role of the solid solution phase in the catalytic behavior of the materials. This theme will be taken up later when the relationship between the bulk and surface properties and the catalytic character is considered. As far as the multiphase materials are concerned, it seems that the possibility of any significant solid solution range for SnO, in Sb6013or Sb204 remains unclear. However, it is interesting to note the reported (12) existence of an unidentified phase at lower temperatures in the antimony rich region of the phase diagram following 14-day calcination in sealed tubes. It would

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be most reasonable to expect that at equilibrium no more than two solid phases should coexist; hence, this observation of a hitherto unidentified material suggests that there is scope for further investigation of its stoichiometry and structure and any role it may have in the catalytic process. The investigations by Pyke, Reid, and Tilley (12) are, as the authors claim, not unequivocal in regard to equilibrium structures. However, the authors' modesty must not be allowed to detract from the significant improvement that they have made to the understanding of this aspect of the tin-antimony oxide catalyst. Moreover, their work gives a much needed account of the effects of the preparative procedure on the structure of the catalyst. The white precipitate dried at 120°C is described as an amorphous solid consisting of a random assembly of octahedrally coordinated tin and antimony ions in an oxygen-hydroxyl-water matrix with the polyhedra associated through hydrogen bonds. The change in color to shades of blue-black or dull graygreen, which accompanies dehydration at temperatures exceeding 4OO0C, was attributed to charge transfer between Sb5+and Sb3+ions, as in blueblack (NH,),SbBr, (31). Pyke et d.(12) showed that the required close proximity of these ions in similar geometrical sites for the operation of this charge transfer process could be readily achieved in the largely noncrystalline matrix containing random arrays of Sn4+, Sbs+, and Sb3+ions in oxygen octahedra, which was formed at 600°C. It is interesting that the lattice parameters of the rutile type phase in tin antimony oxides containing a low antimony content and calcined at 600°C showed no significant difference from those of pure tin(1V) oxide. Much credit must be given to Pyke et al. (12) for their prophetic suggestion that crystallite formation by the aggregation of SnO, octahedra was accompanied by a counter diffusion of antimony ions away from the tin rich nuclei to the surface, a model that has been subsequently verified by X-ray photoelectron spectroscopy (XPS) studies (13,21) and appears to be directly related to the catalytic nature of the material. One of the few points of generalization that can be made from the work considered so far in this article is that the nature of each tin-antimony oxide depends critically on its composition and calcination treatment. It is interesting therefore that the increase in particle size and crystallinity of coprecipitated materials formed at high temperatures (12) and the small tolerance of the SnO, lattice for the incorporation of antimony are also properties of tin-antimony oxide single crystals formed by vapor phase deposition (32).Even more interesting are the presence of twinned microcrystals (12, 32), which suggest that the high-temperature properties of the system may persist in materials prepared by different methods at lower temperatures. This last observation deserves further attention since crystal

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defects such as these could, in principle, be relevant to the catalytic properties of the material. Although twinning is a form of stress relief in crystals, it is also a means of changing the anion to cation stoichiometry and, more importantly in this system, may be the means by which sites with different coordination to the normal crystal matrix are achieved. Such an alteration would change the chemical properties of this localized region and thereby be of potential influence in the catalytic properties of the material. In particular, it could relate to the accommodation of cationic species in unusual or lower oxidation states and be relevant to the formation of specific active sites. Attention must therefore be given to a detailed consideration of the cationic oxidation states in the tin-antimony oxide catalyst, particularly in the solid solution phase. Since the formation of blue, low-antimony-content, mixed oxide solid solutions by coprecipitation would involve the replacement of tin atoms in the white tin(1V) oxide rutile lattice by antimony, it might reasonably be expected that charge balance would be achieved by one of two simple processes : (a) the replacement of three tin(1V) species by two antimony(V) and one tin(II), or (b) the replacement of two tin(1V) by one antimony(V) and one antimony(II1). The strong blue color that is observed even in materials containing a very low concentration of antimony suggests that significant changes do take place in these solids. The resemblance of this color to that observed in the alkali metal antimony halides, together with the absence of any blue tin bearing solids for which a Sn2+-Sn4+charge transfer process has been reported (33), could indicate that the blue color in tin-antimony oxides may be attributed to charge transfer between Sb3+ and Sb5+in the tin(1V) oxide matrix. The origin of the Sb3+ species could then be readily assigned to the reduction of antimony(V) as a result of charge compensation. The operation of a Sb5+-Sb3+ charge transfer process in the solid solution would require the close proximity of both cations in similar geometrical sites (33), which in tin-antimony oxide would involve the occupation by antimony of either interstitial or substitutional positions in the tin(1V) oxide rutile lattice. It is at this point that a difficulty arises since antimony(III), like tin(II), possesses a lone pair of electrons and does not readily adopt octahedral coordination. Moreover, although the octahedral radii of antimony(V) and tin(1V) are defined with a fair degree of precision, the radii of species with lone pairs of electrons are larger and variable. Hence, the accommodation of either of the reduced species in the tin(1V) oxide lattice would be expected to alter the lattice parameters-an effect which has not been confirmed (12). Hence, reduction of the cationic species within the monophasic rutile type tin-antimony oxide lattice has little support from

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X-ray diffraction data. However, the observation by electron microscopy (12, 32) of twinned microcrystals in tin(1V) oxides containing a low concentration of antimony could be consistent with the accommodation of antimony(II1) in more favorable sites and would be compatible with the observed migration of antimony away from the tin rich species and the formation of a mixed oxide with similar lattice parameters to tin(1V) oxide. However, the charge transfer spectrum would still require some Sb3 ions to occupy octahedral sites, and whether or not such accommodation could be achieved at or near the twin boundaries is not clear (32). Hence, the unambiguous definition of the cationic oxidation states, particularly in the solid solution phase, assumes importance since it is the route to the characterization of the charge-balancing mechanism, confirmation of the charge transfer process and identification of species with possible catalytic influence at planar defects. Inspection of the early literature reveals that the definition of the cationic oxidation states is a confused and bewildering subject and the magnitude of the uncertainty resembles that which, until very recently, could be associated with the nature of the phase composition. Mossbauer spectroscopy is a technique that is ideally suited to the examination of this problem ;indeed, unlike many other spectroscopic techniques, which operate under conditions far from those of actual catalytic processes and which require the sample in a specially prepared form, Mossbauer spectroscopy is capable of examining authentic catalysts under realistic conditions. The technique is highly sensitive to features of catalytic relevance such as cationic oxidation states, phase composition, chemical environments of the cations, and lattice distortion, and is moreover capable of monitoring such features whether they occur on the surface of a bulk solid or within supported catalytically active small particles. The importance of Mossbauer spectroscopy to the investigation of the tin-antimony oxide system is enhanced by the amenability of both tin and antimony to Mossbauer investigation. Although a few Mossbauer studies (24,34,35)of rutile type materials included tin-antimony oxides, the inconsistencies in the data reflected the need for systematic investigations of materials prepared by coprecipitation under controlled conditions, and such studies have recently been undertaken ( I 7-19). The "'Sn Mossbauer spectra (17) of materials calcined at 600°C showed the chemical isomer shifts and quadrupole splittings to gradually increase from the values of tin(IV) oxide as the antimony concentration was increased to 10%. The trends were interpreted in terms of increasing electron density and environmental asymmetry at the tin(1V) nucleus with increasing antimony content and the formation of single-phase materials. The constant Mossbauer parameters of samples containing a higher concentration of antimony were attributed to the presence of two phases. The most significant +

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finding of this study was the lack of evidence for the formation of any reduced tin species and the consequent conclusion that charge balance was not achieved by the replacement of three tin(1V) units by two antimony(V) and one tin(I1) species. The l 2'Sb Mossbauer parameters (19) of tin-antimony oxides calcined at 600°C were related to the preparative procedure. The white precipitates, formed in alkaline media that contain hydrated tin(1V) and antimony(V) species, were considered to dehydrate after 16-hr calcination at 600°C to give blue, poorly crystalline, highly defective rutile type solids. The Mossbauer spectra revealed the presence of tin(IV), antimony(V), and antimony(I1I) species in oxygen environments and were consistent with the dehydration process inducing the reduction of antimony(V) as is observed in the pyrolysis of antimonic acid (26) and the transformation of the tin(1V) hydroxide gel to tin(1V) oxide (30). The coexistence of random arrays of such units in a noncrystalline monophasic solid is consistent with the X-ray diffraction study (12) that described these materials as single-phase amorphous solids. Despite the confusion in the literature over materials in which tin and antimony are in roughly equal proportions, the Mossbauer spectra gave little evidence for compound formation below 900°C and confirmed that bulk equilibrium is difficult to establish by short-period calcination at low temperatures. Prolonged heating at 600°C and short-period calcination at higher temperatures gave spectra that showed the formation of either Sb6013or Sb,04 second phases. The Mossbauer spectra therefore showed that blue tin-antimony oxides containing more than 20% antimony and calcined at 600°C for 16 hr could readily accommodate the close coexistence of antimony(II1) and (V) in the similar geometrical sites as required for the operation of the Sb3+-Sb5+charge transfer process. However, the materials containing less than 20% antimony were different. When heated in sealed tubes for long periods at 600°C or at higher temperatures (19a) these samples showed only an antimony(V) contribution to the spectrum, and, when subjected to prolonged or high-temperature calcination, gave parameters that were compatible with single-phase rutile type materials in which the antimony(V) species occupied a unique site within an octahedral array of oxygen atoms. No evidence was found for the reduction of antimony(V) or tin(1V) as a result of charge compensation. Closer inspection of the '"Sb Mossbauer parameters (19, 19a) showed that the decreasing antimony(V) chemical isomer shifts with decreasing antimony content in materials calcined for long periods or at high temperatures were consistent with increasing electron density at the antimony nuclei. Such data are compatible with the accumulation of electron density over the cationic species as the antimony content approaches 10%. It has

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been suggested (19) that charge balance in the monophasic solid solutions is achieved by the delocalization of electron density into a low-energy conduction band composed largely of 5s orbitals. Such a process conforms with the band structure (36-38) of tin(1V) oxide which has been used (39) to account for the increasing n-type semiconductivity, which has been reported (22) to reach a maximum on addition of 3% antimony. In this respect it is especially interesting that a material containing 3% antimony and calcined at 1000°C for 20 days gave a Mossbauer spectrum (19) which corresponded with a high electron density at the antimony nuclei and also showed optimized catalytic properties for the oxidative dehydrogenation of butene to butadiene (14). During the final preparation of this article, Figueras et ul. (39a) reported an X-ray investigation of the structural properties of tin-antimony oxide catalysts. In contradiction to the work of Pyke et al. (12), Figueras described materials containing up to 30% antimony and calcined between 500" and 850°C as solid solutions which give the separation of an additional Sb,04 phase at high temperature. Interestingly, however, Figueras et al. reported that only materials containing less than 5% antimony can solely accommodate antimony(V) in the tin(1V) oxide lattice and that above this antimony concentration antimony(V) is reduced to antimony(II1) as a result of a charge balancing process. In an attempt to clarify the structural and chemical changes which accompany the calcination process, a further 12'Sb Mossbauer study was conducted by Berry ( 1 9 ~ )This . showed that the detection of only antimony(V) in the Mossbauer spectrum did not justify the description of the material as a solid solution and that the presence of both antimony(II1) and antimony(V) need not indicate the operation of a charge balancing process. The Mossbauer data were consistent with the low antimony solubility in tin(1V) oxide which is achieved only at high temperatures, as suggested by Pyke et al. (21). It seems, therefore, that the recent characterization of the phase compensation and structural properties of tin-antimony oxides may readily be correlated with the Mossbauer determination of cationic oxidation states and lattice distortion, especially in materials containing a high concentration of antimony. The charge compensation mechanism, however, remains an intriguing aspect of this material and the nonlocalization of electron density may well be considered as a feature of potential catalytic relevance. In this respect a technique which is well suited to the study of the dependence of catalytic performance on the presence of any spin free species and semiconducting properties is ESR. It is interesting that catalytically inactive tin(1V) oxide is a broadband n-type semiconductor in which the conductivity has been attributed (40,41) to donor levels which result from defects such as impurity cations and anion vacancies. Although the large forbidden energy gap (38, 42) of 3.5 eV prevents intrinsic conductivity at ordinary temperatures, the introduction of

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antimony into the rutile lattice has been reported (9, 22, 43) to produce a donor level which is ionized at room temperature to give a marked increase in conductivity. The description of tin(1V) oxide as a controlled valence semiconductor (44, 45) suggests that doping with antimony is unlikely to produce defects that would be associated with nonstoichiometric semiconductors : hence, the enhanced catalytic properties would not be expected to depend on the defect concentration. A tin-antimony oxide containing a low concentration of antimony and prepared by coprecipitation has been found to give an ESR spectrum showing no evidence of hyperfine patterns as might be expected from signals originating from paramagnetic Sn(II1) or Sb(1V) (20). Given that the accumulation of electron density on the cationic species that was observed by Mossbauer spectroscopy did not give complete reduction to Sn(I1) or Sb(III), it is clear from the ESR spectrum that charge balance is not achieved by reduction to either of the intermediate paramagnetic lower oxidation states. It seems, therefore, that the model involving the achievement of charge balance by the delocalization of electrons into a low-energy conduction band deserves closer inspection. The retention of the signal when the sample was evacuated to Torr indicated that the signal was not due to a surface species undergoing dipolar interaction with adsorbed oxygen and could not be attributed to a superficially adsorbed paramagnetic oxygen species. Hence, the ESR spectra were interpreted in terms of the substitution process generating free electrons, which, in materials containing low concentrations of antimony, were equal in number to the dopant antimony atoms and then trapped at anionic vacancies. It was suggested that higher antimony concentrations give rise to the delocalization of extra electrons into conduction bands and a concomitant increase in electrical conductivity. The success of modern spectroscopic techniques in probing the solid state properties of catalytic materials is well illustrated in this section on the bulk properties of tin-antimony oxides. Given that the catalytic performance may reflect the fundamental properties, it is quite surprising that multispectroscopic investigations of catalysts have not been widely implemented in the past. It is also fortunate that recent developments in other spectroscopic techniques have placed several specifically surface properties within the range of experimental enquiry and that some studies have involved the tin-antimony oxides. C. SURFACE PROPERTIES

Although catalysis is a surface phenomenon, the spectroscopic investigation of the surface properties of tin-antimony oxides do not appear to have been pursued with an excessive degree of vigor. This may in part reflect the comparatively recent development of spectroscopic techniques with these specific powers, but it might also reflect the reluctance that some potentially

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interested workers may have felt when confronted with the confused understanding of the nature of the bulk. In this section particular attention will be paid to more recent spectroscopic studies of surface properties, and an attempt will be made to relate them to the nature of the bulk and the conditions of preparation, especially calcination temperature and composition. A convenient starting point for such an assessment involves two independent investigations of the surface composition of tin-antimony oxides by X-ray photoelectron spectroscopy (XPS) in 1979 (13, 21). The study by Pyke and Cross (13) followed their earlier work on the phase composition (12)and showed the extent of surface enrichment in antimony to depend on bulk composition and calcination conditions. For example, the surface of a material with a bulk antimony content of 20% and calcined at 600°C for 16 hr was found to be enriched by a factor of about 2.7, with the degree of enrichment decreasing rapidly with depth from surface until the bulk composition was attained at 50 A. Although the surface enrichment factor was found to vary with bulk composition at constant temperature, the dependence of the surface composition on calcination temperature represented an even more interesting feature of the study and is depicted in Fig. 2. It was found that catalysts containing less than 60% antimony retained a surface composition characteristic of their bulk until subjected to temperatures in excess of 400°C. It would appear that enrichment of the surface by antimony begins at this point and is maximized at about 1000°C.However, it is interesting that materials with high bulk antimony content show a marked decrease in surface antimony concentration at 700"-800°C. The materials heated for many days in air at 1000°C deserve special consideration since their surface compositions were found (13)to be directly related to the bulk phase changes (12). Catalysts containing more than 4% antimony segregated under these conditions into a rutile type phase and an Sb204 phase, the latter being volatilized at 1000°C to give equilibriated rutile type materials containing about 4% antimony. The XPS study (13) showed that volatile Sb204 is formed only when the surface antimony composition exceeds 25%. Samples with bulk compositions of less than 4% antimony appeared to be unable to give the separation of an Sb20, phase since they generated surface antimony compositions below the 25% critical level. Comparison of the influence of calcination temperature on surface composition and bulk phase composition allowed the three discrete regions depicted in Fig. 1 to be identified (13): (1) a region containing only the rutile type phase (designated as SnO, in Fig. l), which exhibited surface enrichment in antimony at increasing temperature; (2) a region with bulk antimony concentrations greater than 60% containing only the SbsOl and rutile type phase and in which surface composition varied little with

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I 40

40

111

Surface Composition ( a t . % Sb)

I/

400

6 00 8 00 1000 Calcination Temperature (C)

400

6 00 800 1000 Calcination T e m p e r a t u r e K )

FIG.2. Variation in surface composition with calcination temperature at constant bulk composition and constant calcination period (16 hr): (a) 0.5; (b) 4.0; (c) 12.0; (d) 17.5; (e) 39.4; (f) 68.0; (g) 83 at. % antimony. [Reproduced from Y.M . Cross and D. R. Pyke (13).]

temperature ; and (3) a region containing the rutile type phase and Sb204, which was produced at high temperatures and exhibited decreasing antimony composition with increasing temperature. The XPS study (13) clearly placed the X-ray investigation (12) by the same authors in an even more significant perspective. The authors suggested that the homogeneous amorphous and hydrated coprecipitated materials lost their water of hydration at elevated temperatures and partially crystallized into highly defective rutile-like structures containing antimony. It was suggested that this structure is metastable in compositions exceeding 20%

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antimony and that prolonged heating causes the slow evaporation of Sb204. The onset of surface enrichment in antimony at 400°C was correlated (13) with the effects of lattice diffusion and reconstruction. It was suggested that the preparative procedure produced antimony(II1) in unstable environments and that high-temperature calcination caused its migration to the surface and the occupation of more energetically favorable sites. Such a process in materials containing a high concentration of antimony was considered as the first stage in the formation of macroscopically distinguishable antimony oxide phases. The relationship between surface segregation of antimony and nucleation of the Sb204phase was (13) connected with the critical concentration of 25% surface antimony below which Sb204was not volatilized at 1000°C.Cross and Pyke (13) suggested a representation of the surface of a tin-antimony oxide catalyst as depicted in Fig. 3. Such an arrangement in a material containing less than 25% antimony permits the minimization of Sb-Sb interactions and allows each antimony cation to be completely surrounded by tin cations. By similar reasoning surface antimony concentrations exceeding 25% were considered as producing antimony cations in adjacent sites and thence a surface nucleus of Sb204.Hence, it was suggested (13) that diffusion in the bulk replenishes the vacant surface sites and vaporization of Sb204continues until the bulk antimony concentration reaches a level that is insufficient to maintain the critical surface concentration of antimony. This critical surface composition is also likely to be a characteristic of tin-antimony oxide single crystals formed by vapor phase deposition at 1300°C (32).

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FIG.3. Schematic representation of the (100) surface of (a) pure SnO, and (b) catalyst with 25% of the tin cations substituted by antimony: ( 0 )Sn; (0)Sb. [Reproduced from Y. M. Cross and D. R. Pyke (13.1

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Much credit must be given to Pyke and Reid for their study of tinantimony oxides since they have not only shown that surface composition depends on the calcination treatment, but have also established a clear connection between surface and bulk structural properties. The influence of this relationship on the catalytic properties of the materials will be discussed later. It is important at this point to note that the results of the other XPS study by Figueras et al. (21) are in agreement with those of Pyke and Cross (13) and, therefore, confirm that enrichment of the surfaces with antimony is an effect with potential catalytic relevance. It is interesting that the XPS investigations failed to provide definitive evidence for the formation of antimony(II1) in the equilibriated materials. However, the most recent '"Sb Mossbauer study (19a) of tin-antimony oxide solid solutions formed by high temperature calcination in air showed them to contain large concentrations of antimony(II1). Since antimony(II1) with a lone pair of electrons is reluctant to adopt octahedral coordination and given the similarity in lattice parameters between tin(1V) oxide and tin-antimony oxides (12), it seems unlikely that antimony(II1) may be accommodated within the tin(1V) oxide lattice. The Mossbauer spectra (19a) were interpreted in terms of high temperature crystallization of the tin(1V) oxide lattice and the counter migration of antimony as antimony(II1) to the surface. Since Sb204 volatilizes at 1000°C and given that X-ray diffraction (12) and X-ray photoelectron spectroscopy (13)have been unable to identify any Sb204in materials calcined at 1000°C the Mossbauer data were interpreted in terms of the occupation by antimony(II1) of asymmetric surface sites in which the principal contribution to the electric field gradient arises from neighboring tin(1V) species. The formation of antimony(II1) at the surface may be associated with the loss of oxygen which accompanies high temperature calcination (12) and may be related to the ESR evidence for the presence of electrons trapped at anionic vacancies. The loss of each molecule of oxygen could be envisaged as creating two surface 0 2 -vacancies and four electrons which would reduce two antimonyw) species to antimony(II1). Hence the surface may be depicted in terms of tin(IV), antimony(III), and oxygen vacancies containing trapped electrons. The situation is consistent with the observed failure of low antimony content tin-antimony oxides heated in sealed tubes to give an ESR signal and show the presence of antimony(II1) in their Mossbauer spectra. Presumably, under such conditions a suitable pressure is established to cause readsorption of oxygen and thereby preclude the formation of antimony(IZ1). It is also interesting that materials calcined in oxygen give ESR signals similar to those attributed to electrons trapped at anionic vacancies and also show the presence of antimony(II1). It therefore

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seems that tin-antimony oxides continue to lose oxygen even when heated in oxygen. Yet another feature of the tin-antimony oxide surface which could be relevant to its catalytic behavior concerns the presence and relative concentrations of acidic and basic sites. Indeed the surface acidity of oxides such as silica-alumina, metal-sulfates, and metal-phosphates has been related to their ability to catalyze cracking, isomerization and depolymerization reactions, while surface basicity has been associated with the activities of gallia and magnesia in the polymerization of propylene oxide and isomerization of butenes (46).A recent (15) study of the acidic and basic properties of tin-antimony oxides investigated the adsorption of pyridine and acetic acid. The dependence of the adsorption of these molecules on the bulk antimony concentration of catalysts calcined at 600°C (Fig. 4) showed that surface acidity decreased rapidly as the antimony concentration increased to about 20%and then remained constant until a concentration of about 50% was reached when a gradual decrease to zero acidity for the antimony oxide Sb6013 was observed. Surface basicity also decreased as the antimony content was increased to lo%, and it was suggested (15) that in this region the adsorption sites may be described as acid-base pairs in the rutile phase. The basicity increased rapidly with bulk antimony composition to reach a maximum at about 60% antimony and then decreased in those regions of composition in which the Sb6013phase had been identified (12). The surface enrichment in antimony in catalysts containing 6% antimony (13) was accompanied by a decrease in acidity and increase in basicity (Fig. 4). For other materials containing 19 and 33% antimony that also showed surface antimony enrichment (13), the variation of surface basicity remained similar ( 1 9 , although surface acidity kept an approximately constant and significant value. The material containing 64% antimony gave high values of surface basicity at each calcination temperature and a major increase in acidity above 8OO0C,where surface antimony content decreased. The relationship between the trends in acid-base properties (15) and surface cation composition (13) has been used to assign (15) the surface acid sites as either electrophilic oxygen associated with Sn4+ or as coordinatively unsaturated Sn4+ species. The concentration of these sites naturally decreases with increasing surface antimony composition. The basic sites were assigned (15) to superficial antimony(II1) species. The presence of these active sites with both acidic and basic functions, together with the other properties of the solid which have been discussed in this article, might reasonably be expected to influence the catalytic properties of these materials. However, before beginning such an assessment it is pertinent to conclude this section by noting the power of modern spectroscopic techniques for the examination of solid catalytic materials and how these investigations of the tin-antimony oxide catalyst have enhanced the description of the fundamental properties of the material. It is clear that such descriptions would be

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TIN-ANTIMONY OXIDE CATALYSTS

60r

'

10

20

30

40

50

60

70

80

90

100

(%) Sb

(a)

700

800

900

1000

Calcination temperature ( "C)

(b)

FIG.4. The adsorption of pyridine and acetic acid on tin-antimony oxides. (a) Graph showing adsorbed pyridine ( 0 )and adsorbed acetic acid (0)values as a function of bulk antimony content for catalysts precalcined at 600°C. (b) Variation of the amount of adsorbed pyridine and acetic acid as a function of calcination temperature ("C) of the tin-antimony oxides: (0) pyridine adsorption on 6%Sb catalyst; (A) pyridine on 19% Sb catalyst; (0) pyridine on 33% Sb catalyst; (+) pyridine on 64% Sb catalyst; ( 0 )acetic acid adsorption on 6% Sb catalyst; ( A ) acetic acid on 19% Sb catalyst; (w) acetic acid on 33% Sb catalyst; ( x ) acetic acid on 64% Sb catalyst. [Reproduced from J. C. McAteer (IS).]

difficult to achieve if only one technique were used, and it appears that the multispectroscopic investigation of these materials is a valid route by which accurate models might be developed. The studies of the tin-antimony oxides in particular have shown that the composition, structure, and properties of the surface depend on the nature of the underlying bulk structure, and it is

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interesting that this dependence may be correlated with the preparative procedure. An important application of this observation may be found in future attempts to optimize the catalytic performance of a given catalyst. 111.

Influence of Bulk and Surface Properties on Catalytic Performance

A. CATALYTIC CHARACTER There have been several studies of the catalytic character of tin-antimony oxides, and in this section an attempt will be made to relate the results to the solid state properties of the catalyst which have already been considered. Studies of the activity and selectivity of tin-antimony oxides for the oxidation of propylene to acrolein have been reviewed by Hucknall(6), and more recent investigations of hydrocarbon oxidation over the catalyst have been described by Higgins and Hayden (7). In this article only those studies that are directly relevant to the fundamental properties of the catalyst will be reexamined. The work of Godin el al. (9) is particularly important since it represents one of the original attempts to link catalytic properties with the fundamental character of the tin-antimony oxides. The increase in conductivity on doping tin(1V) oxide with 7% antimony was attributed to the substitution of Sb5+ for Sn4+ in the lattice and the production of Sn3+ species which by acting as electron donors gave free electrons. A selectivity of 60-70% for the oxidation of propylene to acrolein was found for catalysts containing between 7 and 807; antimony. Both Godin et al. (9) and Crozat and Germain (25) have suggested that octahedrally coordinated antimony(V) dissolved in the tin(1V) oxide matrix are associated with Sb(V)-Sb(II1) redox couples and constitute the active sites for the selective oxidation of propylene. Sala and Trifiro (47,48) also proposed that Sb(V)=O groups were instrumental in the oxidative dehydrogenation of butenes in a mechanism which involved the reoxidation of antimony(II1) by tin. Lazukin (8), who reported the presence of antimony(II1) and (IV) in a solid solution, also suggested that the adsorption of propylene and acrolein and their mixtures with oxygen occurs initially or antimony species. These reports are ,to some degree supported by the recent Mossbauer observations (18, 19) of the reduction of antimony during similar processes. However, as with other aspects of this catalyst, there is little unanimity as to the significance of specific cationic oxidation states in the formation of active sites. Indeed, Roginskaya et al. (IZ) reported that the catalytic activity for the oxidative dehydrogenation of butenes and ammoxidation of

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propylene was due to the presence of antimony oxides containing lowvalence antimony ions and suggested that their formation was connected with catalyst preparation at high temperatures. Tin(1V) has also been suggested as an active site (10, 49) in a mechanism in which antimony assisted the labilization of the Sn-0 bond during the abstraction of allylic hydrogen (49). Further, the function of tin(1V) as an acid center, which operates in conjunction with an antimony(II1) basic center (16), represents yet another opinion in this confused aspect of the catalytic properties of tin-antimony oxide. A similar lack of clarity pervades other areas concerning the relationship between the catalytic performance and fundamental properties of the catalysts. Wakabayashi et al. (10) reported that the optimized conversion of propylene to acrolein ( > 7%) over alumina-supported tin-antimony oxide (3 : 1) was dependent on the sintering temperature of the catalyst and was maximized after heating at 1000°C for 3 hr. Further work (22) showed that both electrical conductivity and surface area were maximized in the material containing 3% antimony and a close association between acrolein production and solid solution formation was suggested. The lower activation energy for the reduction of tin-antimony oxides with hydrogen as compared with that for the pure oxides has been confirmed by Sala and Trifiro (47), and, contrary to other studies, the activity reported to vary with calcination temperature. Some studies (50-52) have reported an increase in activity and selectivity for the oxidation of butene with increasing antimony content and to be maximized at an antimony concentration of about 20% (50). Sala and Trifiro also reported that materials calcined at 900°C showed peaks in activity for butadiene formation at antimony to tin ratios of 0.20 and 0.90 and that selectivity to butadiene increased with antimony content to 80% for the catalyst with a ratio of 0.40. Christie et al. (53)reported data on the catalytic performance which may also be related to the solid state properties of tin-antimony oxide. A material containing 6% antimony in a batch reactor showed maximized specific activity and low selectivity toward propylene. Above 10% antimony the selectivities exceeded 60% and increased little with increasing antimony content. It is interesting that the "'Sn Mossbauer parameters (17) showed similar trends, which were attributed to effects in the solid solution. Although catalytic oxidation of propylene has been found to be first order with respect to the olefin (8),a dependence on oxygen has also been reported (9,54). Investigations of the participation of lattice oxygen in the oxidation process over mixed oxides, which were thought to contain antimonyw), antimony(III), and tin(IV), reported no support for the redox mechanism observed with bismuth molybdate. The matter of oxygen participation has also been considered by Christie et al. ( 5 3 , who reported that the rates of

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propylene and isobutene exchange with 'H,O under conditions where a carbonium ion mechanism was followed gave pronounced maxima around 5% antimony. The rates of 1 8 0 2 exchange with the solid were reported to decrease in the same region. These observations, together with the enhanced incorporation of "0,into carbon dioxide and acrolein during propylene oxidation with "0,over catalysts containing low concentrations of antimony, may be indicative of a more limited supply of lattice oxygen or the participation of gaseous oxygen under such conditions. This work, as well as that of other workers (55,56), suggests the participation of lattice oxygen with a low bulk mobility. Although several factors have been considered as being influential in the function of tin-antimony oxide as a selective oxidation catalyst, the pretreatment of the material at high temperatures has been generally regarded as a desirable process, and this has sometimes been associated with the importance of the solid solution for catalytic activity. However, more recent work by Pyke, Reid, McAteer, and Patterson has cast doubt on the validity of this assumption. Pyke and Reid (14) investigated the oxidation of butene and obtained catalytic results that broadly confirmed those previously reported in the literature for materials prepared under similar conditions when used under corresponding evaluation conditions (47, 49). For example, materials calcined for 16 hr at temperatures exceeding 800°C were found to be both active and selective for the oxidative dehydrogenation of butene to butadiene. The results of Pyke et al. (14) showed that the specific activity for the formation of butadiene varied as a function of the catalyst calcination temperature, and the activity was therefore regarded as being dependent on bulk structure (12) and surface composition (13). The activity data for selected catalyst compositions were (14) plotted against surface compositions and phase analyses data and are reproduced in Fig. 5a-f. It was found that when the catalyst contained a single rutile-like phase the specific activity for butadiene formation increased with higher calcination temperatures as antimony segregated to the surface. The figure also shows that the pure antimony oxides are relatively inactive and that their presence in catalysts calcined at higher temperatures generally produces significant changes in catalytic activity. The authors (14) noted, however, that the close relationship between catalytic activity and the surface antimony concentration was not quantitative over the entire range of catalysts calcined for 16 hr, and this was partly attributed to the presence under certain conditions of multiphase nonequilibrated materials of poor crystallinity. Some of the most interesting results were recorded from materials subjected to prolonged calcination (14-21 days) in air at 1000°C when the additional Sb,O, phase was volatilized (13), and the resulting highly crystalline monophasic solid solutions of antimony in tin(1V) oxide contained surface antimony concentrations of up to 25% (13).The specific activity for the formation

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TIN-ANTIMONY OXIDE CATALYSTS

Sb

.'\

'1 400

Sb60,?

600

1

. x 0 ,4

800

IOOO'C

FIG.5. The relationship between specific activity for the formation of butadiene (solid line, pmol m-2 min- I), surface antimony composition (broken line, at. %), and catalyst calcination temperature: (a) 4; (b) 20; (c) 40; (d) 68; (e) 83; (f) 100 at. % Sb bulk composition. [Reproduced from H. Herniman, D. Pyke, and R. Reid (14).]

of butadiene from these catalysts varied almost linearly with surface antimony concentration and was maximized in materials bearing a surface composition of about 25% antimony. The trend was accompanied by a corresponding but nonlinear decline in activity for carbon dioxide formation with increasing antimony content. Hence, the high-temperature surface enrichment in antimony, which produces individual antimony cations surrounded entirely in nearest neighbor positions by tin cations, was associated with the increase in activity for butadiene formation. By extension the inactivity in selective oxidation of pure antimony oxides was attributed to the presence of antimony cations in adjacent nearest neighbor sites, and the decreasing yield of carbon dioxide with decreasing surface tin concentration was attributed to superficial tin atoms when surrounded by other tin atoms. Pyke and Reid (14) proposed that the formation of a solid solution is not essential to the production of an active and selective oxidation catalyst, and

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that the surface composition, which is dependent on the bulk structure, is a more important factor. It appears therefore that the formation of a catalyst with optimized activity and selectivity depends critically on the preparation and pretreatment conditions. This observation represents one of the principle justifications for in-depth investigations of single catalytic systems and is a feature that may lead to improvements in catalyst syntheses and be of general applicability in the future. At this point it is worth recalling that the poorly crystalline materials calcined for 16 hr (and which correspond to the samples most commonly investigated in the literature) are nonequilibriated materials. However, the catalyst containing 4% antimony should relate most closely to materials equilibriated at 1000°C since this composition corresponds to the reported (12) solid solution limit of antimony in tin(1V) oxide, and the surface antimony concentration and catalytic activity are clearly closely related (14) (Fig. 5a). Indeed, it is relevant that no significant increase in specific activity for butadiene formation was observed in materials of higher antimony compositions at lower calcination temperatures where the surface concentration of antimony would be greater. Pyke and Reid have proposed that coprecipitation produces a surface that, after low-temperature calcination, consists of a random array of tin and antimony ions in which there is a low probability of any individual antimony cation being surrounded entirely by tin cations. It seems, therefore, that high temperatures cause the migration of antimony to the surface, and a corresponding increase in specific activity for butadiene formation is observed. Although the formation and volatilization of Sb204effectively prevents the surface antimony concentration from exceeding 25% at 1000°C, the stage at which surface segregation processes give the formation of free Sb204 in materials calcined at low temperatures is not clear. It has been suggested (14) that up to 50 at.% antimony may be accommodated at the reaction surface without reducing the activity for butadiene formation if the antimony cations occupy non-nearest-neighbor surface sites. Moreover, although free Sb204is relatively inactive, Pyke and co-workers have pointed out that a surface layer need not inhibit activity since the facile sintering of the separated Sb204phase would remove the excess antimony. It is interesting therefore that the activity pattern depicted in Fig. 5e for a catalyst with a high antimony content does show that phase separation may result in the generation of new reaction surfaces which support suitable cationic environments for enhanced activity. Another suggestion by Pyke and co-workers was that the catalytic properties may be associated with the intense color change and migration of antimony which takes place when the white precipitate is heated above 400°C. It was suggested (12) that the Sb5+-Sb3+ charge transfer process to

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121

which the blue color has been attributed may be the key to the catalytic behavior of the material and therefore make it different from either mixtures of the oxides or related phases such as Sb,O,/TiO,. In this respect the disordered nature of the catalyst was considered to be of potential importance since such a relatively homogeneous noncrystalline assemblage of random tin-oxygen and antimony-oxygen polyhedra could exist over a range of tin to antimony stoichiometries, and thereby give rise to the wide spectrum of catalytic properties. However, the recent work shows that the relative roles of tin and antimony in the catalytic process are not completely clear. Although the tin may simply provide a disordered matrix that supports a high concentration of antimony in the required oxidation states and geometric sites, it is also possible that the tin cations may have a more fundamental role to play in the reaction. In this respect a recent study by McAteer (16) of the primary reactivities and selectivities in the partial oxidation of butene over tin-antimony oxides of different compositions and subjected to different calcination temperatures is relevant. McAteer proposed that the oxidation mechanism involves the presence of positively charged allylic and vinylic intermediates at specific acid and basic sites at the catalyst surface. The significance of such acidic and basic sites on oxide catalysts that are active and selective for the partial oxidation of olefins has been considered by other workers (49, 57-61). McAteer’s work (16) showed that increasing antimony content in tin(1V) oxide decreased the activation energy for the rate determining step in the oxidation of butene. The variation of specific activity and product selectivity as a function of antimony content in catalysts calcined at 600°C showed that the bulk structure (12) and acid-base characteristics of the tin-antimony oxide (15) are relevant in the interpretation of the catalytic properties. In poorly crystalline materials calcined at 600°C and containing low concentrations of antimony, the acidic and basic site concentrations both decreased and the activity maximized at about 5% antimony, while selectivity to carbon dioxide decreased rapidly and butadiene and methyl vinyl ketone selectivities varied slightly. The intermediate composition range between 10 and 60% antimony showed approximately constant acidity but rapidly increasing basicity. Optimal activity and selectivity to butadiene with minimal methyl vinyl ketone selectivity were obtained for the 33% antimony material, while carbon dioxide and minor product formation remained approximately constant. Beyond 60% antimony, where X-ray studies showed (12) the presence of Sb6OI3, McAteer (16) observed a rapid decrease in surface acidity, which was closely paralleled by a decrease in carbon dioxide minor product formation and catalytic activity. McAteer suggested that the presence of surface acid and base sites is necessary for activity in partial oxidation and that high surface basicity favored butadiene formation, while

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the formation of carbon dioxide appeared to be closely related to the presence of surface acidity. It also appears that the primary kinetics for the partial oxidation of butene over tin-antimony oxide catalysts (16) may be related to changes which occur on calcination at elevated temperatures and are consistent with the observations of Pyke et al. (14). Catalysts containing 6% antimony gave enhanced specific activity and constant product selectivities with increasing calcination temperature until lOOO"C, when selectivity to butadiene formation increased and selectivities to methyl vinyl ketone and carbon dioxide decreased. For materials containing 19% antimony, the increase in calcination temperature beyond 700°C was accompanied by increased selectivity to butadiene but decreased selectivity to methyl vinyl ketone. It is interesting that a peak in specific activity at 800°C corresponded to maximized surface basicity. Materials containing 33% antimony gave high activity and butadiene selectivity, with the former increasing with increasing calcination temperature to 900°C. High selectivity to butadiene and low specific activity were observed for 64% antimony at 600°C. It was observed that an increase in calcination temperature gave increased quantities of carbon dioxide coupled with increased activity, acidity, and surface tin content. These observations (16) are consistent with those of Pyke and Reid on surface composition (13) and its relationship with catalytic performance (14). For oxides of low antimony composition, activity was reported (16) to be enhanced by the increased surface antimony concentration on calcination. Selectivity to butadiene increased from -70 to 90% as surface antimony composition increased above 25% and was high for materials containing 33 and 64% antimony calcined at 600°C. Increasing surface antimony content was closely paralleled by increasing specific activity, but the presence of Sb204 phases in the mixed oxide was found to be detrimental to catalyst activity. Figueras et al. (21) have also recently studied the selectivity for acrolein as a function of bulk or surface content (Fig. 6). These workers concluded that (1) the change in selectivity is not directly related to a change in surface area as suggested by Godin et al. (9), and ( 2 ) the selectivity increases with surface content in antimony and is particularly striking when samples are calcined at high temperatures. Figueras et al. (21) associated the increased catalytic selectivity with surface antimony enrichment and proposed that this enrichment gave rise to the Sb204 phase. Figueras has concluded that good selectivities are obtained for propylene oxidation when the catalyst is biphasic and has suggested that the actual catalyst consists of an orientated film of Sb,04 supported by the solid solution of antimony(V) in tin(1V) oxide. Hence, despite the low activity of bulk Sb204 and the reported decrease in catalytic activity per unit area in materials with a high antimony

-

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TIN-ANTIMONY OXIDE CATALYSTS

0

20

10

30

40

Bulk Sb content

1

il c ._

-2

(b) 0 0

W

U

m

L

0

0

_II_/ 0

10

20 30 40 Surface Sb content

50

FIG.6 . Selectivity for acrolein formation at 380°C for Sb-Sn-0 catalysts. (a) As a function 825°C; (+) of nominal Sb content (calcination temperature: ( 0 )500°C; ( A ) 750°C; (0) 950°C; (A) 1100°C.(b) As a function of surface Sb content as determined by XPS (bulk content) : (A)0%; ( 0 )1.5%;(0)5%; ( x ) 10%; ( 0 )20%; ( A ) 40%. [Reproduced from Y.Boudeville, F. Figueras, M. Forissier, J.-L. Portefaix, and J. C. Vedrine (21).]

content, the work of Figueras has combined the presence of the Sb,O, phase with other assignments of active sites constituted by antimony in the solid solution. Hence, although the exact nature of the catalytic phase remains a matter of some uncertainty, the recent studies do convincingly show that the enrichment of the surface with antimony is an important feature of the catalytic properties of tin-antimony oxides and bears a closer correspondence with catalytic performance than the simple nominal composition.

FRANK J. BERRY

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B. MECHANISMS Studies of the selective oxidation of propylene, which is an important application of the tin-antimony oxide catalyst, have resulted in the description of several mechanisms and the subject was recently reviewed by Keulks et al. (1). It is not the purpose of this article to give a similar in depth consideration of this aspect of the catalytic properties of tin-antimony oxides. However, it is important that the improved knowledge of the bulk and surface properties of the catalyst and their relationship with the catalytic character should be considered in terms of the formulation of reaction mechanisms. Keulks et al. (1) discussed evidence for a model in which the selective oxidation of propylene proceeded via the formation of a symmetric allylic species with subsequent steps depending on the nature of the catalyst. In some catalytic systems the abstraction of a second hydrogen atom seemed to precede the insertion of oxygen, while others appeared to add molecular oxygen to form a hydroperoxide intermediate which subsequently decomposed into acrolein and water. As far as tin-antimony oxides are concerned, some early studies using l-13C-labeled propylene (9) were interpreted in terms of an oxidation mechanism involving a 71-ally1 species in a scheme similar to that proposed by Batist et al. (62) for bismuth molybdate catalysts:

+ AV + 0’--+C,H; + OHSb5+ + C3H5- + C 3 H 5 . . . . S b 4 + C 3 H 5 . . . . S b 4 ++ 20’- + C 3 H 4 0 + Sb3+ + OH- + 2e- + 2AV 2 0 H - -0’-+ AV + H,O to, + Sb3+ + AV + O ’ - + Sb5+ YO, + 2e- + AV -0’C,H,

(1) (2)

where AV is anion vacancy. Other workers (63) have reported that the adsorption of propylene on a tin-antimony oxide subjected to the irreversible preadsorption of oxygen yielded a positively charged surface complex containing an excess of carbon relative to oxygen and similar to that suggested for other selective oxidation catalysts such as Bi,O, * 3Mo0, and SnO, .MOO,. Similar mechanisms have been proposed by other reviewers of catalytic hydrocarbon oxidation and ammoxidation (64, 65). It is important to note that Figueras has recently investigated the oxidation of deuterated propylene CD,CHCCH, over bismuth molybdate and tin-antimony oxide catalysts. Although the results of this work are yet to be published (65a) and have only been communicated

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TIN-ANTIMONY OXIDE CATALYSTS

to the author of this article during its final stages of preparation, they do seem to provide an important contribution to the understanding of the working of the catalyst. It appears that percentage retentions of deuterium in the acrolein agree with literature data when bismuth molybdate is used as a catalyst. However, no isotope effect for the abstraction of the second hydrogen from the olefin has been noticed for the reaction over the tinantimony oxide catalyst. Figueras has suggested that the slow step of the reaction may not be the same on the bismuth molybdate and tin-antimony oxide catalysts and has proposed that the abstraction of the second hydrogen occurs before oxygen incorporation on bismuth molybdate, whereas the reverse process occurs on tin-antimony oxide. Figueras further suggests that these different reaction mechanisms may be connected with differences in the reducibility of the oxides which control the relative rates of oxygen incorporation and hydrogen abstraction. The experimental support (53, 55, 56) for the participation of lattice oxygen in these reactions has already been noted, and it is of interest that the results of one of these studies (53) have been correlated with a mechanism for the oxidation of propylene that involves the formation of a symmetric allylic intermediate n-bonded to the catalyst surface. Trimm and Gabbay (49) proposed that the oxidative dehydrogenation of 1-butene to butadiene proceeded by a mechanism in which an allylic hydrogen of an adsorbed molecule was extracted by surface oxygen in the rate-determining step. Trimm and Gabbay interpreted the butene isomer concentrations as indicating that the allylic intermediate was not the sole reaction pathway and that concurrent nonoxidative isomerization over weak protonic acid sites also took place. In view of these observations it would seem sensible that the influence of adjacent superficial antimony and tin ions should also be considered in terms of likely mechanisms. Immediately one would recall the suggestion (12) that the catalytic properties may be related to the blue color of the material, which has been associated with a possible Sb5+-Sb3+ charge transfer process. Such an association may then be related to the kinetics of butene oxidation, which have been interpreted in terms of the formation of allylic intermediates at active centres containing Sn4+ and Sb3 ions. Indeed, McAteer (16) has suggested that these active centers have acidic and basic functions and consist of surface oxide ions of different electron density as determined by the coordinated cations. McAteer described the pattern of selectivity for the formation of butadiene and a-ketone according to the depiction in Fig. 7a. The initial step was postulated as the formation at an acid center of a positively charged ally1 ion which is 71 or cr bonded at an adjacent basic site. The formation of butadiene was attributed to proton abstraction from the n-ally1 intermediate, its facile desorption at surfaces +

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FRANK J . BERRY

5l

0

FIG.7 . Mechanisms of olefin oxidation over tin-antimony oxides. [Reproduced from J. C. McAteer (16).]

of high basicity (59), and the reoxidation of the hydroxylated surface by molecular oxygen. The intermediate, which is 0 bonded to nucleophilic surface oxygen, was considered to give the formation of methyl vinyl ketone, which desorbs less readily on surfaces of high basicity (59). The patterns of selectivity for acrolein, methacrolein and furan were explained (16) in terms of the formation of vinyl intermediates by n-electron donation from adsorbed olefin to electrophilic oxygen (Fig. 7b), followed by rearrangement of the positively charged species and attack by nucleophilic oxygen. In view of the optimized specific activity for butadiene formation (14) in equilibriated materials calcined at 1000°C which has been correlated with surface enrichment by antimony(II1) and associated with the loss of oxygen during calcination, the presence of acidic and basic sites may be a significant feature of the catalytic character. However, further investigations may show that the concept of acidic and basic sites is not so fundamental as the role of the electron. Indeed, the description of catalytic properties in terms of semiconductivity theory has been applied to the oxidation and ammoxidation of propylene over Cu,O and BiMo (66), and the possible role of surface electronic properties in providing matching electron exchange levels has been emphasized.

c.

THECATALYST FOLLOWING ITS U S E IN

CATALYTIC PROCESSES

It is of interest to note that much of the recent structural work on tinantimony oxides has assumed that no significant structural changes occur in the oxides during the catalytic reaction. Mossbauer studies (18) have

TIN-ANTIMONY OXIDE CATALYSTS

127

shown, however, that changes in oxidation state and electric field gradients do occur when the materials are used in catalytic processes. The adsorption of acetic acid (up to 4.31 pmol m-') and pyridine (up to 2.69 pmol m-') has been found (18) to give no significant alteration to the Mossbauer parameters of the pure materials, and despite infrared evidence for the adsorption of bases to acid sites (15, 67, 68) it is clear that such processes are unlikely to involve the bulk tin cations in tin-antimony oxide catalysts in a charge-balancing role or give any significant modification to the rutile type lattice structures. However, tin-antimony oxides containing 20% antimony and reduced in a hydrogen-nitrogen gas mixture at 480°C by 4, 10, and 30% gave (19) '"Sn Mossbauer chemical isomer shifts, which were, within the limits of experimental accuracy, identical to those of the pure mixed oxide and showed no reduction of tin(1V) to tin(I1). The decrease in quadrupole splitting, which accompanied the increased reduction, is also significant since it reflects a diminishing electric field gradient at the tin nucleus and may be associated with the lZ1SbMossbauer spectra, which showed the reduction of antimony(V) to antimony(II1). The nature of these catalysts following exposure to olefins and oxygen has also been investigated. Margolis et al. (24) reported that the adsorption at 200°C of propylene, acrolein, and their mixtures with oxygen gave stable surface compounds that involved the chemisorption of the organic species on antimony and the formation of equal amounts of antimony(V) and (111). More recent "'Sn and '"Sb Mossbauer studies (19) of tin-antimony oxides following their use as catalysts for the oxidation of butene showed the process to resemble that which occurred during the reduction of the materials by hydrogen with no evidence being found for the reduction of tin(1V) to tin(I1). The '"Sb Mossbauer spectrum of the two-phase material containing 20% antimony confirmed the reduction of antimony(V) to antimony(III), although it was not clear whether the reduction concerned the substitutional antimony in the rutile phase or the antimony(V) in the second Sb,04 phase. The monophasic 2% material containing antimony in the tin(1V) oxide lattice gave, however, "Sn Mossbauer parameters that were similar to those of pure tin(1V) oxide. Although the '"Sb Mossbauer data gave no indication of the reduction of antimony(V) to antimony(III), the decrease in electron density at the tin nuclei and the apparent recovery of the lattice structure implied a decrease in both the lattice and valence contributions to the electric field gradient at the tin nucleus and was, by analogy with the materials reacted with hydrogen, consistent with the reduction of antimony(V) within the lattice to antimony(II1). The apparent absence of antimony(II1) in the "'Sb Mossbauer spectrum may reflect the low concentration of such a species. The problem remains, however, as to the accommodation of any such antimony(II1) species if created by such a mechanism. In this respect the electron microscopic evidence (32) for the presence of twin boundaries in tin-antimony oxides is significant since it

128

FRANK J. BERRY

could represent the means by which superficial antimony(II1) species could avoid octahedral coordination and enjoy more favorable sites. The failure of Mossbauer spectroscopy to identify any reduction of tin(1V) to tin(I1) and the proposed reduction of antimony(V) to antimony(II1) during the catalytic reaction may be correlated with mechanisms which have been proposed for the formation of n-ally1 intermediates during the chemisorption and catalysis process. It would be very interesting if future work could clarify whether other structural and chemical changes occur during the catalytic process so that the long held assumption that no significant changes occur in the catalyst as a result of the catalytic reaction may be confirmed or refuted.

IV. Conclusions The tin-antimony oxide catalyst is one that is most amenable to investigation by a wide range of spectroscopic and physical techniques. The more recent studies have shown that the properties of the system are very sensitive to composition, calcination temperature, and length of treatment. It seems that the coprecipitated catalyst is an initially homogenous and amorphous solid, which is slowly crystallized on heating. It seems that the range of compositions which give rise to the formation of solid solutions is small (<4% antimony) and only occurs when the materials are heated to high temperatures. The cationic species in the solid solution phase of antimony in tin(1V) oxide appear to be antimony(V) and charge balance seems to be achieved by the delocalization of electrons into a conduction band. Materials containing higher concentrations of antimony segregate at higher temperatures into the rutile type solid solution and a separate antimony oxide phase. The surface composition of the solid solution phase is enriched in antimony(II1) as a result of the high temperature calcination. The catalytic performance for the oxidation of olefins appears to be related to an enhanced antimony content in the surface, which is achieved at higher temperatures as a result of the migration of antimony through the bulk lattice. It seems that isolated antimony(II1) cations surrounded entirely by tin(1V) cations in nearest neighbor sites may constitute active centers. As far as the mechanism of catalytic oxidation is concerned, it seems that the reactive oxygen for the selective oxidation of olefins is lattice oxygen. It appears that olefin oxidation over tin-antimony oxides occurs via the formation of allylic intermediates by hydrogen abstraction with reoxidation of the surface by either bulk lattice oxygen or surface adsorbed molecular oxygen.

TIN-ANTIMONY

OXIDE CATALYSTS

129

It seems likely that under reaction conditions the catalyst itself undergoes solid state reactions that are a function of the reaction temperature and the reacting gases. It is clear that a close relationship exists between the bulk structure, surface aomposition, the formation of active sites, and the actual catalytic perlformance of the material. It follows that the optimization of relevant bulk and surface properties should be an important consideration in the preparation of a catalyst. In this respect a thorough understanding of such properties, their dependence on the preparative procedure, and their influence on catalytic performance is essential if catalysts with the desired structure are to be synthesized. Since many catalysts for a specific process enjoy common features (e.g., membership of a particular structural type), it is quite possible that specifically identified features may have general relevance and applicability. REFERENCES 1. Keulks, G. W., Krenzke, L. D., and Notermann, T. M., Adu. Catal. 27, 183 (1978). 2. Barclay, J. L., Bethell, J. R., Bream, J. B., Hadley, D. J., Jenkins, R. H.; Stewart, D. G., and Wood, B., British Patent 864,666 (1960). 3. Bream, J. B., Hadley, D. J., Barclay, J. L., and Stewart, D. G., British Patent 876,446 (1961). 4. Barclay, J. L., and Hadley, D. J., British Patent 902,952 (1962). 5. Bethell, J. R., and Hadley, D. J., U.S.Patent 3,094,565 (1963). 6. Hucknall, D. J., “Selective Oxidation of Hydrocarbons.” Academic Press, New York, 1974. 7. Higgins, R., and Hayden, P., Catalysis (London) 1, 168 (1977). 8. Lazukin, V. I., Rubanik, M. Ya., Zhigailo, Ya. V., and Kurganov, A. A., Katal. Katal. 3, 54 (1967); Chem. Abstr. 68,86799f (1968). 9 . Godin, G. W., McCain, C. C., and Porter, E. A., Proc. Int. Congr. Catal. 4th, 1968 p. 271 (1971). 10. Wakabayashi, K., Kamiya, Y., and Ohta, N., Bull. Chem. SOC.Jpn. 40, 2172 (1967). 11. Roginskaya, Yu. E., Dublin, D. A,, Stroeva, S. S., Kul’kova, N. V., and Gel’bshtein, A. I., Kinet. Katal. 9, 1143 (1968). 12. Pyke, D. R., Reid, R., and Tilley, R. J. D., J . Chem. Soc., Faraday Trans. 1 76,1174 (1980). 13. Cross, Y. M., and Pyke, D. R., J . Catal. 58,61 (1979). 14. Herniman, H. J., Pyke, D. R., and Reid, R., J . Catal. 58, 68 (1979). 15. McAteer, J. C., J . Chem. Soc., Faraday Trans. 175, 2762 (1979). 16. McAteer, J. C., J . Chem. SOC.,Faraday Trans. 175,2768 (1979). 17. Berry, F. J., and Maddock, A. G., Inorg. Chim. Acta 31, 181 (1978). 18. Berry, F. J., Inorg. Chim. Acta 39, 125 (1980). 19. Berry, F. J., Holbourn, P. E., and Woodhams, F. W. D., J . Chem. Soc., Dalton Trans.,2241 (1980). 19a. Berry, F. J., and Laundy, B. J., J . Chem. Soc., Dalton Trans., in press. 20. Berry, F. J., and McAteer, J. C., Inorg. Chim. Acta, in press. 21. Boudeville, Y., Figueras, F., Forissier, M., Portefaix, J.-L., and Vedrine, J. C., J. Catal. 58, 52 (1979).

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22. Wakabayashi, K., Kamiya, Y . , and Ohta, N., Bull. Chem. Soc. Jpn. 41, 2776 (1968). 23. Kustova, G. N., Tarasova, D. V., Olenkova, I. P., and Chumenko, N. N., Kinet. Katul. 17, 744 (1976). 24. Suzdalev, I. P., Firsova, A. A,, Aleksandrov, A. U., Margolis, L. Ya., and Baltrunas, D., Dokl. Chem. (Engl. Trunsl.)204,408 (1972). 25. Crozat, M., and Germain, J. E., Bull. Soc. Chim. Fr. p. 1125 (1973). 26. Stewart, D. J., Knop, O., Ayasse, C., and Woodhams, F. W. D., Can. J . Chem. 50, 690 (1972). 27. Schwarzmann, E., Rumpel, H., and Berndt, W., Z . Nuturforsch., B : Anorg. Chem., Ory. Chem. 32,617 (1977). 28. Stewart, D. J., Rumpel, H., and Berndt, W., 2. Nururforsch.. B : Anorg. Chem., Org. Chem. 32,690 (1977). 29. Baur, W. H., and Khan, A. A,, Acru Crystullogr., Sect. B27,2133 (1971). 30. Berry, F. J., and Maddock, A. G., Rudiochim. Actu 24, 32 (1977). 31. Atkinson, L., and Day, P., J . Chem. Soc. A p. 2423 (1969). 32. Pyke, D. R., Reid, R., and Tilley, R. J. D., J . Solid Stare Chem. 25, 231 (1978). 33. Robin, M. B., and Day, P., Adz,. Inorg. Chem. Rudiochem. 10, 247 (1967). 34. Skalkina, L. V., Suzdalev, I . P., Kolchin, I. K., and Margolis, L. Y., Kinet. Katul. 10, 378 (1969). 35. Birchall, T., Bouchard, R. J., and Shannon, R. D., Can. J . Chem. 51,2077 (1973). 36. Wright, D. A., Proc. Br. Ceram. Soc. 10, 103 (1968). 37. Arai, I., 1. Phys. Soc. Jpn. 15, 916 (1960). 38. Kohnke, E. E., J . Phys. Chem. Solids 23, 1557 (1962). 39. Leja, E. J., Actu Phys. Pol. A 38, 165 (1970). 39u. Portefaix, J.-L., Bussiere, P., Figueras, F., Forissier, M., Friedt, J., Theobald, F., and Sanchez, J. P., J . Chem. Soc., Faruduy Trans. 176, 1652 (1980). 40. Marley, J. A., and Dockerty, R. C., Phys. Rev. A 140,304 (1965). 41. Loch, L. D., J . Can. Cerum. Soc., 158 (1964). 42. Summitt, R., Morley, J . A,, and Borrelli, N. F., J . Phys. Chem. Solids 25, 1465 (1964). 43. Imai, I., J . Phys. Soc. Jpn. 15, 937 (1960). 44. Vincent, C. A., J . Electrochem. Soc. 113, 515 (1972). 45. Verney, E. J . W., in “Semiconducting Materials” (H. K. Hamish, ed.). Butterworth, London, 195 1. 46. Tanabe, K., “Solid Acids and Bases: Their Catalytic Properties.” Academic Press, New York, 1970. 47. Sala, F., and Trifiro, F., J . Cutal. 34,68 (1974). 48. Sala, F., and Trifiro, F., J . Cutul. 41, 1 (1976). 49. Trimm, D. L., and Gabbay, D. S., Trans. Furuduy Soc. 67,2782 (1971). 50. Sala, F., and Trifiro, F., Z . Phys. Chem. Wiesbuden 95, 279 (1975). 51. Lemberanskii, R. A,, Rostevanov, E. G., Annenkova, I. B., Liberman, E. S., and Mekhtiev, K. M., Azerb. Khim. Zhr. p. 24 (1974). 52. Lemberanskii, R. A,, Annenkova, 1. B., and Rostevanov, E. G., Azerb. Khim. Zhr. p. 10 (1975). 53. Christie, J. R., Taylor, D., and McCain, C. C., J . Chem. Soc., Furuday Trans. 172, 334 (1976). 54. Belousov, V. M., and Gershingorina, A. V., Kinet. Cutal. (Engl. Transl.) 11, 942 (1970). 55. Pendleton, P., and Taylor, D., J . Chem. Soc., Furaduy Trans. 1 72, 1114 (1976). 56. Sazonova, N. N., Venyaminov, S. A,, and Boteskov, G. K., Kinet. Cutul. (Engl. Trunsl.) 15, 364 (1974). 57. Ai, M., and Ikawa, T., J . Cutul. 40, 203 (1975).

TIN-ANTIMONY OXIDE CATALYSTS

58. 59. 60. 61. 62. 63. 64. 65. 65a. 66.

67. 68.

131

Ai, M., J . Catal. 40, 327 (1975). Ai, M., J. Catal. 40, 318 (1975). Ai, M., J . Caral. 52, 16 (1978). Ai, M., Bull. Chem. Soc. Jpn. 49, 1328 (1976). Batist, P. A,, Kapteijno, C. J., Lippeus, B. C., and Schuit, G. C. A., J. Catal. 7, 33 (1967). Krylova, A. V., Derlyukova, L. E., and Margolis, L. Ya., Dokl. Chem. (Engl. Transl.) 178,79 (1968). Margolis, L. Ya., Catal. Rev. 8,241 (1973). Kolchin, I. K., Russ. Chem. Reu. (Engl. Transl.)43, 475 (1974). Portefaix, J.-L., Figueras, F., and Forissier, M., J. Catal. 63, 307 (1980). Wise, H. in “Proceedings of the Symposium on the Mechanisms of Hydrocarbon Reactions,” p. 283. Elsevier, Amsterdam, 1975. Thornton, E. W., and Harrison, P. G., J. Chem. Soc.. Faraday Trans. 1 7 1 , 1013 (1975). Dewing, J., Monks, G. T., and Youll, B., J . Catal. 44,226 (1976).

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ADVANCES IN CATALYSIS, VOLUME 30

Selective Oxidation and Ammoxidation of Propylene by Heterogeneous Catalysis ROBERT K. GRASSELLI

AND

JAMES D. BURRINGTON

Department of Research and Development The Standard Oil Company (Ohio) Warrensville Heights, Ohio

I. Introduction . . . . . . . . . . . . . 11. Discussion . . . . . . . . . . . . . . A. Allylic Oxidation. . . . . . . . . . . B. Selective Oxidation and Ammoxidation of Propylene C. Development of Selective Ammoxidation Catalysts . D . Redox Properties of Catalysts . . . . . . . E. Selective Oxidation and Ammoxidation Mechanism . 111. Summary and Conclusions . . . . . . . . . References . . . . . . . . . . . . . .

1.

. . . . . 133 . . . . . 135

. . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

135 136 138 143 141 161 162

Introduction

An intimate relationship exists between the petroleum and chemical industries since about 85% of the primary organic chemicals produced today are derived from petroleum and natural gas sources. A comparison of petroleum utilization for energy and chemical industries illustrates that the major portion of every barrel of oil (90-92%) is used for energy production via total combustion to COz and water. In contrast to this, oxidation reactions in the chemical sector are made much more selective in nature by means of a catalyst that lowers the activation energy for the selected process and provides a facile path by which useful products can form. One such class of processes that is of great industrial importance is selective oxidation by heterogeneous catalysis. In these processes, organic feeds are converted in the vapor phase to useful products containing the same number of carbon atoms using solid phase catalysts. Of the important catalytic processes by which the major organic chemicals are produced (Fig. 1) (I, Ia), heterogeneous oxidation accounts for 21% of the total output. The major processes in this category include allylic oxidation to 133 Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007830-9

134

ROBERT K. GRASSELLI AND JAMES D. BURRINGTON

-L

25%

AROMAT I C ALKYLATION

HETEROGENEOUS OX I D A T I O N

METHANOL C6H6

,,

2n-Cu Chromate

-MeOH

,, , -,, 1

&,.,~, , -,.

\

OXIDATION

-x

FIG. 1. Important (top 20) industrial organic chemicals produced by catalysis. Reactions shown are the major process in each class. From ( I , l a ) .

FIG.2. Important (top 20) industrial organic chemicals produced by heterogeneous oxidation. From ( I , l a ) .

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

135

give aldehydes, nitriles, and acids, aromatic oxidation to acids and anhydrides, epoxidation of olefins, methanol oxidation to formaldehyde, and to a smaller extent, paraffin oxidation to anhydrides (Fig. 2) ( I , l a ) .

II.

Discussion

A. ALLYLIC OXIDATION Allylic oxidation, that is, the selective oxidation of olefins at the allylic position, represents a substantial portion of the production of important organic chemicals by heterogeneous oxidation. Furthermore, the development and study of selective catalysts for allylic oxidation has led not only to successful commercial processes, but also to important concepts concerning selective oxidation and the phenomena of catalysis in general. Examples of allylic oxidations (Eqs. 1-6) include ammoxidation, oxidation, dimerization, oxydehydrogenation and acetoxylation reactions :

+ NH, + i02 + O2 CH2=CH-CH, CH2=CH-CH3 + 40, 2(CH2=CH-CH3) + )02

CH2=CH-CH3

-.+

-.+

-.+

+

+ 3H20 CH,=CHCHO + H 2 0 CH2=CHC02H + H 2 0 CH,=CHCN

I

Ammoxidation Oxidation

(3)

CH2=CH-CH2CH2-CH=CH2

+ H20

Dimerization CH2=CH-CH2CH3

+ to2+.CH2=CH-CH=CH2

+ $0,+ HOAC

-.+

CH2=CH-CH20Ac

(4)

+ H20 Oxydehydrogenation

CH2=CH-CH3

(I) (2)

(5)

+ H20 Acetoxylation

(6)

In allylic oxidation, an olefin (usually propylene) is activated by the abstraction of a hydrogen CI to the double bond to produce an allylic intermediate in the rate-determining step (Scheme 1). This intermediate can be intercepted by catalyst lattice oxygen to form acrolein or acrylic acid, lattice oxygen in the presence of ammonia to form acrylonitrile, HX to form an allyl-substituted olefin, or it can dimerize to form 1J-hexadiene. If an olefin containing a P-hydrogen is used, loss of H from the allylic intermediate occurs faster than 0 insertion, to form a diene with the same number of carbons. For example, butadiene is formed from butene.

136

ROBERT K . GRASSELLI AND JAMES D. BURRINGTON

/>

f CH2=CHCN

NH,

CH2=CH-CH3

-H __t

CH2-CHr..

/

-"z,

-

-H

CH2=CH-CH2CH3

CH2=CHCH0

CH ;;;CH;;%H%H 2 3

CH2=CH-CH2X

-H

CH2=CH-CH=CH2

SCHEME 1 . Allylic oxidation processes.

B. SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE The most industrially significant and well studied allylic oxidations center around the formation of acrylonitrile from ammoxidation (Eq. 7), and acrolein from oxidation (Eq. 8) of propylene:

+ NH, + $0, ,~a&,*CH,=CHCN + 3 H 2 0 + H20 CH,CH=CH, + O2 &*CH,=CHCHO

CH,CH=CH,

* Catalysts:

(7) (8)

B i 2 0 , . nMoO, (n = I , 2, 3) USb3010 Fe203/Sb204 Bi/Mo/O, multicomponent systems

The ammoxidation reaction, which is by far the largest-scale industrial allylic oxidation process, was originally discovered and developed at Sohio (2) in the early 1960s. This process, by which more than a million tons of acrylonitrile are produced annually in the United States and more than 4 million tons worldwide, revolutionized the manufacture of this important monomer, displacing the more expensive acetylene-HCN-based route (Eq. 9): H-CrC-H + HCN CH,=CHCN (9) CuCi-NH4C',

Other processes (3),which involve more expensive and reactive organic feeds (ethylene oxide, acetaldehyde) and oxidants (NO) (Eqs. 10-12), were also

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

137

MAR

FIG.3. United States production ( 0 )and price (A)of acrylonitrile.

made obsolete by the ammoxidation process, by which virtually all the world acrylonitrile supply today is produced :

+ HCN CH3CH0 + HCN

4CH2=CHCH3

HO-CH,CH,CN

-+

AH

CH,CHCN

iy;g4

b

*CH~=CHCN

(10)

CH,=CHCN

(1 1)

600-700°C

+ 6 N 0 A*’o’sloz*4CHZ=CHCN + 6 H z 0 + N ,

(12)

The introduction of this process in 1960 resulted in a sharp drop in the price and increase in the supply of world acrylonitrile (Fig. 3), which allowed the discovery of many new high-volume applications in the fibers and plastics industries. The desired selective oxidation reactions are, of course, thermodynamically favorable. The discovery of appropriate catalyst systems is necessary to overcome kinetic limitations. This is a challenging task in that nonselective, complete, or deep oxidations (e.g., formation of CO,, H,O, and HCN) are thermodynamically more favorable than the selective oxidations or ammoxidations (Table I) (4). Therefore, it is necessary to intercept the desired products kinetically. TABLE I Thermodynamics of Oxidation Reuctions‘ Reactions (A) C3H6 + 0 2 CHz=CHCHO + H,O (B) C3H6 @z 4 CHZ=CHCOOH + HzO (C) C3H6 + 3 0 2 4 3CO + 3HzO (D) C3H.5 @, 4 3 c o z + 3HzO (E) C3H, + NH, + Wz-+ CH,=CHCN + 3H,O (F) C3H6 3NH3 g H 3 C N + 3H20 ( G ) C3H6 3NH3 + 302 3HCN + 6 H 2 0

+ + + +

From ( 4 ) .

-+

+ w2

-+

-+

AG&ac (kcaljrnol)

- 80.82 - 131.42

- 304.95 -463.86 - 136.09 - 142.31 - 273.48

138

ROBERT K . GRASSELLI AND JAMES

D.

BURRINGTON

C . DEVELOPMENT OF SELECTIVE AMMOXIDATION CATALYSTS

The study of selective oxidation catalysis at Sohio (2) began in 1952 with the concept that the lattice oxygen of a reducible metal oxide could serve as a more versatile and useful oxidizing agent for hydrocarbons than would molecular oxygen, which would serve to replenish catalyst oxygen vacancies. This overall oxidation-reduction cycle (Scheme 2) is necessary for selective catalytic oxidation to occur.

0x1DAT I ON PRODUCT

L

SCHEME 2. Catalytic selective oxidation-reduction cycle.

Early in this research, a working hypothesis was formulated, which consists of the following two postulates :

1. The oxygen atoms must be distributed on the surface of a selective oxidation catalyst in an arrangement that provides for limitation of the vumber of active oxygen atoms in various isolated groups (site isolation). 2. The metal-oxygen bond energy of the active oxygen atoms, at the conditions of the reaction, must be in a range where rapid removal (hydrocarbon oxidation) and addition (regeneration by oxygen) can occur (appropriate M-0 bond strength). The model can be explained on the basis of a completely oxidized surface grid (Fig. 4) from which oxygen is partially removed in stages. Using statistical methods (Monte Carlo) the distribution of oxygen (site population) on the surface can be calculated as a function of surface coverage (Fig. 5) (6). Assuming that oxygen clusters of 2-5 adjacent surface lattice oxygens lead to selective oxidation while clusters of more than 5 lead to waste products, one would predict that maximum selective oxidation should occur on a surface that is about 65% reduced (Fig. 6) (6). The behavior of solid CuO in propylene oxidation at 300°C is consistent with this model (Fig. 7) (6) the highest selectivity occurring on semireduced surfaces.

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

139

FIG.4. Reduction of an oxidized surface grid (a) SO%, (b) 60%, (c) 40%, and (d) 20% oxidized. From ( 6 ) .

The usefulness of a catalyst such as CuO, however, is limited since it is extremely difficult to maintain the oxidation state in the range of the selective acrolein-forming reaction. The development of catalysts that display a high order of selectivity in an essentially fully oxidized state was made possible by the synthesis of structurally isolated active sites, a discovery that led to the development of today’s highly selective commercial catalysts. One such catalyst which has been studied in detail is the U-Sb oxide system. Infrared and X-ray studies correlate catalytic activity for acrylonitrile formation to the presence of the USb3010phase (Fig. 8) (7). Another crystalline phase, USbO, , also exists over a wide range of Sb/U ratios, along with U 3 0 8 and Sb,04. Both the USbO, and USb30,, phases are catalytically active, but only the USb3OI0phase is selective. This can be traced to

140

ROBERT K. GRASSELLI AND JAMES D. BURRINGTON 100

90 80 70

-

60

50

In c

z 40 0

e

30

ln

2 20

I-

10 '0

10

20

40 50 60 70 Surface reduction ( O h )

30

00

90

100

FIG.5. Site population as a function of surface coverage (reduction of an oxidized grid. From ( 6 ) .

Surface reduction

(%I

FIG.6. Relative propylene conversion as a function of oxidation state (reduction of an oxidized grid).

Reduction (%)

FIG.7. Experimental propylene oxidation activity vs. catalyst oxidation state-copper oxide catalyst. Reaction temperature = 300°C. From ( 6 ) .

500

-

400

-

0

300-

I,

200-

m 4-l

115 111 5/1 S b / U ( A T O I1 R A T I O )

10/1

1/5 1/1 5/1 Sb/U (ATOM R A T I O )

10/1

0

100-

Ol

1/10

I

(b)

0.5

01

0

1/10

6

115 111 511 Sb/U ( A T O M R A T I O )

10/1

(C)

FIG.8. Correlation between structure and selective catalytic activity. (a) Catalytic activity; (b) X-ray diffraction; (c) low-frequency infrared. From (7).

142

ROBERT K . GRASSELLI AND JAMES D. BURRINGTON

Space group = Fddd 0

0

= 1.346 A

b = 12.72 C

= 15.40

! 1

z=8

FIG.9. Unit cell ofphase I (USb3OIo).From (8).

the solid state structure (Fig. 9) (8), which, for the USb3010 phase, was solved from powder diffraction patterns and by analogy to UNb3Ol0 and UTa,O,,, and was found to be orthohombic, belonging to the Fddd space group with Z = 8 and 112 atoms in the unit cell. An analysis of the heavy atom positions (Fig. 10) (9)shows that in USb3OI0every U atom is isolated by Sb atoms, that is, there are no adjacent U-0-U structures. However, in the USb05 phase, U-0-U-0-U rows, which are centers of waste formation, are present. Among the many other selective oxidation catalysts, those based on Bi-Mo oxides are probably the most extensively studied and serve as the basis for many of today’s highly active and selective commercial catalyst systems.

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

143

Phase I : USb,O,,, U-Sb-U

Sb-Sb-Sb

l S b Sb

Sbl Sb

U

/ S b

I S b Sb

I S b

Sb

Sb-Sb-Sb

u

u-u-u c.0,

Sb-U

-4

SbI

Sb

Sb-Sb-Sb

Sbl Sb

U

U I S b Sb Sb

I Sb

SbI -Sb

/ U Sb-Sb-Sb

Sb/

I

=o

u

U

U[

1Sb Sb-Sb-Sb

Phase I1 : USbO, u-u-u

1 I

-Sb

Sbl

I U Sb

SbI

U-Sb-U

c

Sb-U

u

/ U Sb

U

Sb

SbI

+

I U Sb-Sb-Sb

1 I

Sb

U

L A 4 , 4

FIG.10. Heavy atom positions. Phase I (USb,O,,) and Phase I1 (USbO,). From (8).

D. REDOXPROPERTIES OF CATALYSTS All selective oxidation and ammoxidation catalysts possess redox properties. They must be capable not only of reduction during the formation of acrolein or acrylonitrile, but also subsequent catalyst reoxidation in which gaseous oxygen becomes incorporated into the lattice as 0 2 -to replenish catalyst vacancies (Scheme 2). As mentioned earlier, the incorporation of such redox properties into solid state metal oxides was one of the salient working hypotheses on which the development of the Sohio ammoxidation process was based (2). Later, Keulks (10) confirmed the involvement of lattice oxygen in propylene oxidation by using 1 8 0 2 as a vapor phase oxidant. The results showed that the incorporation of18O into the acrolein (and COz) increases with time (Fig. ll), which is consistent with the above redox mechanism. Although allylic oxidations and ammoxidations proceed by the same general redox mechanism on all bismuth molybdate-based catalyst systems, distinct differences exist in the rate of reduction and subsequent reoxidation depending on the respective catalyst compositions. The differences in the efficiency of the redox cycle are generally attributible to the unique structural and chemical properties of a given catalyst composition. The existence of layer and shear structures are among the important factors contributing to the overall efficiency of the redox process.

144

ROBERT K . GRASSELLI AND JAMES D. BURRINGTON

Time of "02 addition (min) FIG. 11. Reoxidation mechanism determination by acrolein ''0 incorporation. (A) 0, from lattice inserted in oxidation products. (B) 0, from gas phase replenishes catalyst oxygen vacancies. From (10 and 13).

Among several bismuth molybdate catalysts studied (11), the rates of catalyst reduction by a feed consisting of propylene and ammonia in the absence of gaseous oxygen (i.e., reduction cycle) at 430°C decrease in the following order: multicomponent system > Bi,Mo,O,(fl) > Bi,Mo,O,,(a) > Bi,FeMo,O,, 2 Bi&OO,(y). The reoxidation of reduced catalysts by 0, (Lee, oxidation cycle) at 430°C follows a substantially different order: Bi,MOO, (y) > Bi,Mo,O,, (a) 2 Bi,Mo,O, (p) > multicomponent system > Bi,FeMo,O,, . All catalysts are single phase except the multicomponent catalyst, which is multiphase and has the general formula M,Z+Mi+Bi,MoyOz. In the catalytic cycle, the activation of propylene via a-hydrogen abstraction is rate determining. The reoxidation step, while it is essential to the catalytic cycle, is usually much faster for all bismuth molybdate-based catalysts. Therefore, a catalyst possessing a large number of active surface sites of proper structure and composition for selective oxidation, and one that is capable of reconstituting these surface sites rapidly with bulk lattice oxygen, which in turn are rapidly replenished by gaseous oxygen, will be the most efficient catalyst. Among those ennumerated here, the multicomponent system is clearly the best.

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

145

From a mechanistic standpoint, reoxidation of the catalyst involves two closely related processes : adsorption and activation of dioxygen, followed by the incorporation of the oxygen into vacancies of the solid. This transformation of 0, to 0'- requires electron transfer from the solid in order to activate and dissociate the dioxygen before incorporation into the oxygen vacancy. Reoxidation rate studies of bismuth molybdate-based catalysts (11) have shown that the overall rate is first order in the oxygen vacancy concentration of the catalyst and half-order in gaseous oxygen. This rate expression is consistent with a mechanism of reoxidation in which the ratelimiting step is incorporation of activated oxygen into the vacancies and where the adsorption and dissociation of the dioxygen occurs before the rate-limiting step. When the oxygen vacancies in the catalyst are located primarily near the surface, the reoxidation is very rapid. The activation energy for the reoxidation of these surface vacancies was found to be about 1-2 kcal/mol for the CI and B phases of bismuth molybdate. Reoxidation of subsurface vacancies is much slower, however, and is limited by the ability of the catalyst to transport the oxygen from the surface O2 chemisorption sites to these subsurface vacancies. For catalysts having structures that facilitate this type of diffusion the reoxidation proceeds rapidly. For example, in y-bismuth molybdate, layering in the structure (12, 13) (Fig. 12) results in low-energy pathways in which oxygen anions (and oxygen vacancies) can diffuse. In this case the reoxidation proceeds with an activation energy of only 8-9 kcal/mol. In more closed packed structures such as in cc-bismuth molybdate (13a), rapid diffusion does not occur and the overall reoxidation rate becomes diffusion limited with an activation energy of 25-26 kcal/mol. However, the presence of a redox couple in a catalyst can create a lower energy pathway for diffusion by promoting electron and oxygen transfer between the surface and the bulk. Thus, incorporation of iron into the scheelite structure of cc-bismuth molybdate (to give the single-phase Bi3FeMo,0,,) (14) reduces the activation energy for reoxidation to 7-8 kcal/mol. The presence of an Fe3+/Fe2+redox couple in the reduced catalyst promotes the rapid exchange of oxygen between the surface and the bulk. This is particularly facile in multicomponent bismuth molybdate systems (15). Although the reoxidation rate is usually not rate determining in the overall redox cycle, the ability of the catalyst to rapidly replenish its reservoir of lattice oxygen is clearly necessary in order to sustain the surface catalyzed reaction. The reconstitution of catalyst surface after reduction occurs by means of shearing processes (Fig. 13) (11, 16), which are essential for effective oxidation catalysts. While reoxidation at higher temperatures is generally rapid for all selective oxidation catalyst systems, it becomes more

corner shared -Moo2 u) .X

-Biz

0

I

9

02

FIG.12. Layered structure of Bi,MoO,. From (12, 13).

Very active selective Reduction

Very active very selective FIG.13.

Reduction

c3

Oxygen vacancy

Restructing of molybdenum tetrahedra into shears. From (11).

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

147

difficult at low temperatures, where selectivities to useful products are highest. Because of the incorporation of favorable redox properties in the multicomponent systems, these compositions are capable of rapid lowtemperature reoxidation, and thus are the most efficient catalysts for operation in the high-selectivity temperature regime. E. SELECTIVE OXIDATION AND AMMOXIDATION MECHANISM

The most well accepted feature of the mechanism is the formation of an allylic intermediate via a-hydrogen abstraction from propylene in the ratedetermining step. The structure of this intermediate and its subsequent steps involved in its conversion to selective products are much less well understood. It has been suggested from deuterium-labeling studies (17) that this intermediate undergoes a second hydrogen abstraction followed by 0 (oxidation) or N insertion (ammoxidation) (Scheme 3). The formation of an H

H

H

H

SCHEME 3. Selective propylene oxidation mechanism ( 1 7 ) . From (21).

allylic intermediate has been established by the experiments of Sachtler and deBoer (18) in which propylene labeled with 14C in the 1- or 3-position gave acrolein with I4C scrambled in both the aldehydic and methylene carbon (Eq. 13), while oxidation of 2-14C-propylene did not result in scrambling (Eq. 14):

14

6

Lo,-

Bi,O,- nMoO,

l4

@CHO

hv

-14 -

t

co

(14)

The rate-determining nature of the a-hydrogen abstraction was established by Adams and Jennings (17) in a series of experiments using deuterium-

148

ROBERT K . GRASSELLI AND JAMES D. BURRINGTON

TABLE I1 Oxidation of Deuterated Propjilene over Bismuth Molybdates”,b

Reactant

Relative rate of oxidation (450°C)b

A

1.oo

,4---wD

0.85

D *

0.98

’ From ( I 7). b k H / k D= 1.82.

labeled propylenes in which deuterium isotope effect of k,/k, = 1.82 was observed (Table 11). The distribution of deuterium in acrolein indicated a rapid second hydrogen abstraction from either side of the allylic intermediate (Eq. 15): Bi,O,. nMoO,

eD O2 45OoC, 81%

D@CHO 35.2%

@CDO 28.9%

e C H O 33.3%

(15) Experiments by Haber (19) suggest that the reaction on bismuth molybdates proceeds via allyl radical formation on oxygens associated with bismuth, since dimerization to 1,5-hexadiene and benzene occurs on Bi,O, (Eq. 16), while oxygens of molybdenum polyhedra are oxygen inserters, since acrolein is formed from the reaction of MOO, with “allyl radical” preformed from allyl iodide (Eq. 17):

Thus, the overall mechanism involves initial, rate-determining a-hydrogen abstraction by Bi-0 polyhedra, NH (from NH,) or 0 insertion from the

SELECTIVE OXIDATION A N D AMMOXIDATION OF PROPYLENE

149

lattice associated with Mo atoms, and replenishment of the resulting lattice vacancies by vapor phase 0, , via dissociation on oxygen chemisorption sites and subsequent 0 migration through the lattice (Scheme 4). Because

+

P

CHz=CHCN

M,

Bi

M2

Mo

+

H20

SCHEME 4. Allylic oxidation mechanism.

the initial a-hydrogen abstraction is rate determining, the experiments mentioned so far, in which propylene is oxidized, give relatively little information concerning the steps after allyl formation. One approach to this problem is the rapid generation of the allylic intermediate by an independent route with subsequent analysis of its rate-determining decomposition to selective products. Using this approach, free allyl radical was generated by thermal N, extrusion from azopropene over Mo oxide to determine its role in the formation of the allylic intermediate (Eq. 18):

150

ROBERT K . GRASSELLI AND JAMES D. BURRINGTON

The faster rate of allyl formation for azopropene (99.2% conversion to 1,Shexadiene) when compared with allyl iodide (30.7% conversion to lJ-hexadiene) at 320°C, reflects the lower activation energy for the azo decomposition (36.1 kcal/mol vs. 43.5 kcal/mol for allyl iodide) (20). Also, since the only by-product is an inert gas, N, , the azo compound serves as a very clean source of allyl radical. A summary of the reaction of MOO, with azopropene-when compared to that of allyl iodide (Scheme 5) (20)--reveals the formation of more CO, , 6 C H O t Cop 03%

+

6 3%

-

’

+

91‘10

t

trace

CH3CH0 1.4%

4

PhH 03%

T

/

G x S 26% E .

[A] -51x M%

C3HgN:N-C

C02 + 147%

6+ 76%

CH$HO+ p C H O + PhH 149%

515%

114%

H 3 5

CHEMISORBED SPECIES

--$

SCHEME5 . Azopropene and allyl iodide reactions over MOO, at 320°C. From (20).

acetaldehyde, and benzene, and less acrolein for the allyl radical reaction. The reaction of propylene with a- or /3-bismuth molybdates (Table 111) (20) under these conditions gives even higher selectivity to acrolein than does either allyl iodide or allyl radical generated from azopropene. In agreement with Haber’s results (19), the reactions of propylene with MOO, or Bi,O, are very slow, the latter resulting in only small amounts of 1,Shexadiene. The formation of acrolein from free allyl radical and MOO, can be explained by the formation of a n-ally1surface complex which forms a a-0-ally1 species followed by hydrogen abstraction (Scheme 6) (20). When ammonia is present, the formation of surface NH species followed by the analogous

TABLE I11 Pulse Reaction of Propylene over Bil Ma-Containing Carafysts, 320°C"

Catalyst

Contact Surface time area, tot. (set) (Mz)

6 (pmol)

G a l

MOO, Bi,O,. 3Mo0, Bi,O,. 2Mo0, Bi,O,. MOO, Bi,O, M: 'Mi +Bi,Mo,O, From (20).

4.0 4.4 4.0 4.2 4.0 4.4

~

~

~

5.6 6.1 5.8 5.9 2.9 6.1

11.2 11.2 11.4 10.9 11.1 10.3

-

-10.3 -10.3 -10.1

--/0.3 0.8/1.6

102 88.4 90.8 96.5 101 28.1

0.3 -

0.04

~

~~

~

-

-

-

6.5 5.7 0.7

-

0.1 0.3 0.6

~

~

~

90.3 90.5 50

-

0.2

-

-

63.6

-

0.5

95.7

102.0 95.6 97.9 97.9 101.5 94.6

152

ROBERT K . GRASSELLI AND JAMES D . BURRINGTON

SCHEME 6. Formation ofacrolein and AN from (1)

+ MOO,. From (20).

N-ally1 species and two hydrogen abstractions account for acrylonitrile formation. Thus, although allyl radicals are probably not the selective intermediate in propylene oxidation and ammoxidation, they can form acrolein or acrylonitrile via these selective 0-or N-ally1 intermediates. Based on these results, a general selective oxidation mechanism evolves (Scheme 7) (21). Initial hydrogen abstraction to form an allyl intermediate which is 7-c-bonded to a coordinately-unsaturated Mo, the 0-inserting site, -H

A 0

+ I\

0 0 \//

/””\

TT-ally1

Cr-ally1

SCHEME 7. Selective oxidation via n- and cr-0-ally1 molybdates. From (21).

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

153

followed by C-0 bond formation to give a o-0-ally1 intermediate (an allyl molybdate ester) and a second hydrogen abstraction produces acrolein. Alternatively, this o-0-ally1 molybdate could be formed directly from interaction of propylene with the catalyst. Such a a-allylic species has been observed in the selective oxidation of propylene by Boreskov (22) using infrared spectroscopy and proposed by Iwasawa (23) in the reaction of ethanol with silica-fixed MOO,. The formation of the C-0 bond activates the a-hydrogens toward abstraction, a well-known effect in many organic systems, and is consistent with the first hydrogen abstraction as rate determining. The importance of the Mo=O double bond in catalytic oxidation has been discussed by Trifiro (24). The feasibility of this mechanism was tested by the use of D- and "0-labeled allyl alcohols as precursors for the in situ formation of a-0-ally1 molybdates over MOO, and Bi-Mo-oxide catalysts (Eqs. 19 and 20, respectively)' :

'Mz-OH

0

0

---

Hp 04

&--./OH

+ -

c/o

Bi,MoO,

___)

,""\ BiO OB<, O+

0

HO,

0

Mi- OH B i0' OY

\

(20)

OB i

*O

In the experiments using l,l-dz- and 3,3-dz-allyl alcohol in the presence and absence of pyridine bases, the results (Table IV) show that reaction occurs on Brnnsted acid sites (Scheme 8), which results in diallyl ether formation with scrambling of the deuterium label via carbonium ion formation, and also on oxidizing sites on which both 1-d- and 3,3-dz-acrolein form via thz formation and interconversion of the two isomeric 1,l-dz- and 3,3-dz-allyl molybdates (22). These mechanisms are supported by data (Table IV) that show the suppression of ether formation and reduction of deuterium scrambling in the presence of base, which has little effect on the acrolein yield. The oxidation of '80-allyl alcohol to acrolein over MOO,, which Equations (19) and (20) were taken from (21).

TABLE IV Effeci of Base on Product and 1so;opic Distribution for Pulse Reaction o j Ally1 Alcohol-d, and Propylene-d, at 320 P h

Yields' Catalyst

MOO, MOO, MOO, Bi,O,. Bi,O,. Bi,O,. Bi,O,. Bi,O, Bi,O,.

I

AA'

+Octane 2-MPyr PYr n-Octane PYr PYr PYr PYr

I.l-dz l,l-dz 1,I-dz l,l-d2 1,I-dz 1,I-dz 1.1 /3.3-d,h 1 , I :3.3-dZh.' C;-I ,I-d2'

-

AA:solvent C y 1.7 1.7 0.4 1.7 1.7 0.4 0.4 0.4 -

4.4 0.6 0.0 25.1 19.8 20.4 26.9 6.0 91.5

Acrolein 30.1 27.4 48.8 37.0 35.1 44.2 54.0 20.0 8.5

Acrolein cx,)'

AA (XF

AA

HD

PhH

AE

I-d,

3.3-dz

38.2

7.1 6.9 4.7 3.8 4.5 5.4 8.9 3.6 0.0

1.5

0.0 0.0 3.1 1.3 0.5 0.0 0.0 0.0

19.0 0.8

41.8 56.0 57.0 52.0 69.8 66.8 31.6

58.2 44.0 43.0 48.0 30.2 33.2 68.4

56.7 72.2 77.3 73.0 82.3

43.3 27.8 22.7 27.0 17.7

9 9

9 9

9

9

55.7

44.3

29.4

70.6

-

-

64.5

46.5 30.2 38.8 29.5 10.3 70.4 0.0

From (21). 0.25 sec contact time, 0.7 mz catalyst. AA + solvent = 32 pmol total. unless otherwise stated. C;, propylene; AA, ally1 alcohol; HD. 1.5-hexadiene: PhH, benzene; AE, diallyl ether. 2-MPyr, 2-methoxy pyridine; Pyr. pyridine. Ratios by NMR. Masked by pyridine peak. Not enough collected to analyze by NMR. 55:45 (I.I-dz:3.3-d,) mixture used. Contact time = 0.025 sec, 0.07 m z catalyst. Feed = 24.5pmolC~-I,l-d,;3.0cm3(2.48 gm,4.2m2)Bi,03.3Mo03; 15 seccontacttime.

' j

3Mo0, 3Mo0, 3Mo0, 3Mo0, 3Mo0, 3Mo0,

Diluentd

(x)

f 0.8 0.4 0.0 0.0 0.0 0.0

1,l-dz 3.3-d~

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

155

ACID S I T E :

OX I D I 2 ING S I T E :

9; HO

A 7

Mo

A

y..

HO

OD

A

""\ /OH

3-

SCHEME8. Isomerization reactions of allyl alcohol. From ( 2 1 ) .

shows a slower drop in 6O incorporation with pulse time (Fig. 14) and a negligible effect of l60incorporation on addition of base for the acrolein formed, when compared to that for isolated alcohol (Table V), supports the C-3/C-1 interconversion of allyl alcohol on both acid sites, which is suppressed by base, and on acrolein-forming sites, which is not effected by base (21). A comparison of the ratio of l-d:3,3-d2-acrolein formed from the reaction of I,l-d,-allyl alcohol with Bi203.3Mo03 in the presence of pyridine (reaction only on acrolein-forming sites) and for the corresponding reactions of a 50: 50 mixture of 1,l:3,3-d2-ally1alcohol and 1,l-d,-propylene (Table IV) shows that only the 50:50 mixture produces the ratio observed for propylene-1,l -d, . In terms of the mechanism for selective propylene oxida-

156

ROBERT K. GRASSELL1 AND JAMES D. BURRINGTON

0.56 -

PULSE TIME (SEC)

FIG.14. I6O incorporation into acrolein (A)and ally1 alcohol ( A ) from reaction of "0-ally1 with MOO, at 320°C. 1.96 g ( I .O cm'. 1.4 m2) MOO,; fresh catalyst used for each injection Molar velocity = 83.5 mol sec-'. From (21). TABLE V

Effect of Busr on 'sO-AllyI Alcohol Oxidurion ouer Moo, at 320C"3h

Diluent

Acrolein

Ally1 alcohol

n-Octane Pyridine

0.142 0.123

0.229 0.670

~

~

~

~~

~

From (21) * 0 25 cm3 (0 49 gm) MOO, used, 0 06 sec contact time, alcohol diluent ratio = 0 33, 3 5 p of 3 2 M in alcohol 9 6 M diluent injected Ratio, by mass spectroscopy a

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

157

6coo 30 70

70 %

SCHEME 9. Results of isotopic experiments using l,l-d2-allyl alcohol in terms of selective propylene oxidation. From (21).

tion (Scheme 9) this means that formation of either a-ally1 molybdate directly from 1 ,I-d,-propylene cannot occur, but that initial formation of a n-ally1 species, which has equal probability of C-0 bond formation on either side of the allyl moiety, will produce the observed 30:70 I-d: 3,3-d,acrolein ratio via 0-0allyl molybdate isomerization (21). Ammoxidation of allyl alcohol on MOO, initially requires about 6 molecules of NH, per allyl alcohol in the vapor phase for maximum acrylonitrile yield (Fig. 15). However, after all surface Mo=O undergo ammonolysis to form Mo=NH, only a stoichiometric (or slightly higher) amount of ammonia is required to sustain maximum AN yield under catalytic conditions. These results suggest that N incorporation results from an 0 to N-allylic rearrangement (Scheme 10) (21). From these results a detailed mechanism for the selective oxidation and ammoxidation of propylene over Bi-Mo oxide catalysts evolves (Scheme 1 1) in which initial chemisorption occurs on coordinately unsaturated Mo centers of the active site 3, followed by allylic abstraction by Bi oxygens to form a n-ally1 complex 4, which then undergoes C-0 bond formation, resulting in a a-0-ally1 molybdate 5, the acrolein precursor and Mo(V) analog to the Mo(V1) esters formed from allyl alcohol and Bi203. MOO,. The incorporation of N in acrylonitrile formation from propylene results from formation of an analogous 71- and a- N-ally1 molybdenum complexes (6 and 7, respectively) after NH incorporation via condensation of surface Mo=O with NH, , and two subsequent hydrogen abstraction reactions. Lattice oxygen removed from the catalyst during the surface oxidation of

M O L A R R A T I O S NH3/AA

FIG.15. Yields vs. ammonia:allyl alcohol ratio for reaction with MOO, at 380°C. AA = allyl alcohol, 0.98 g (0.5 cm3, 0.7 m2) MOO, used; fresh catalyst used for each run; pulse size = 1.9 cm3 (35 pmol total). (m) CO, ( 0 )propylene, ( A ) acrolein, ( x ) acrylonitrile, ( A)allyl alcohol, ( 0 )1,s-hexadiene, and ( 0 )allyl ether. From ( 2 1 ) .

/

H2N\

,OH Mo: / \

7

O\

\

0

H N HN +

N C T

M’ 0 I

\

OH

/ ” 0\

SCHEME 10. Ammoxidation of allyl alcohol. From (21).

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

159

SCHEME 1 1. Mechanism for selective oxidation and ammoxidation of propylene over bismuth molybdate. From (21).

propylene to form acrylonitrile or acrolein (Scheme 11) must be replenished by gaseous oxygen in order to reconstitute the active site 3 and complete the redox cycle (21). As discussed previously, the relative rates of reduction of several bismuth molybdate-based catalyst systems using propylene decrease in the order : multicomponent system > Bi,Mo,O, (8) x Bi,Mo,O,,(a) > Bi,FeMo,O,, 2 Bi,MoO,(y) > MOO, Bi,03 (II,20). This is consistent with Scheme 1 1 , in which the rate-determining step requires activation of propylene by both chemisorption on Mo centers and hydrogen abstraction on Bi centers. The y phase and Bi,03 have too few chemisorption sites, while MOO, has no a-hydrogen abstracting sites, and thus, these catalysts are the least active.

-

160

ROBERT K . GRASSELLI AND JAMES D . BURRINGTON

The c1 and fl phases have a favorable balance of these two sites necessary to effect the rate-determining first hydrogen abstraction. The multicomponent system possesses the greatest number of active surface sites having the proper structure and composition and a solid state structure with the ability to rapidly reconstitute these surface sites with bulk lattice oxygen. For the oxidation of allyl alcohol to acrolein at 320°C, the relative rates decrease in the order multicomponent system > Bi,MoO, ( 7 ) > Bi,Mo,O,, (IX) > MOO, > Bi,O,. For the simple molybdates, the successive introduction of bismuth into the molybdate esters, formed in a fast step, facilitates the abstraction of hydrogen by oxygen associated with Mo in the rate-determining step to form acrolein. This corresponds to the second hydrogen abstraction in propylene oxidation. Bismuth oxide, however, is very inactive because it lacks the chemisorption site for allyl molybdate formation, while in the multicomponent system, both bismuth and other di- and trivalent metals facilitate this hydrogen abstraction (21). A comparison of this proposed selective oxidation mechanism (Scheme 1 1) with three other important mechanisms from the literature (Table VI) (21) shows that there is a considerable amount of discrepancy between the assignment of the role of the individual metallic components of the catalyst (13). Matsuura (25), based on low-temperature adsorption studies, attributes chemisorption and first hydrogen abstraction to Mo, while in Haber's TABLE VI Selective Oxidation Mechanism Comparisons" Step

Matsuura

Olefin Mo (B site) chemisorption Mo (B site) NH, chemisorption, NH formation 1st allylic H Mo (B site) abstraction 2nd (3rd) H Mo (B site) abstractions O(NH) insertion Bi (A site) Electron flow e - +Bi-+Mo+O, Remarks

From (22).

Site density (B) = 2 x site density (A); A-high AHads ; B-low AHa&

Haber

Sleight

Grasselli er al.

Bi

Mo

Mo

Mo

Mo

Mo

Bi

Mo

Bi

Mo

Mo

Mo

Mo e - -+Mo+Bi+O, If no Mo present, C,H,, forms from 2 allyls

Mo Mo M o 4 B i + O 2 e - +Mo+Bi-+O, [also Keulks (lo)] Bi (6p)/Mo (4d) Bi facilitates overlap serves as 2nd/3rd H abe - sink stractions by sink effect

e-

-+

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

161

mechanism (19, 26), based on in situ generation of the allylic intermediate, the roles are reversed. Sleight (27), however, attributes all the steps shown to groups, while bismuth supplies an overlapping Bi/Mo conduction band (6p/4d), which serves as an electron sink. The Grasselli et al. mechanism (21) is based on a consideration of the reactions of both propylene, which is incorporated in the mechanism of the rate-determining step, and the allylic intermediate, independently generated in situ from allyl radicals or allyl alcohol, which give information concerning the steps after initial a-hydrogen abstraction.

111.

Summary and Conclusions

The discovery of selective oxidation and ammoxidation of propylene over heterogeneous catalysts has made a major impact on both industrial and fundamental catalysis. The development of catalyst systems based on the concepts of active site isolation and appropriate metal-oxygen bond strength, has resulted in highly selective and active structures such as those based on U-Sb oxides and Bi-Mo oxides. The wealth of mechanistic information concerning the intermediates formed and their conversion pathways to selective products in the propylene oxidation/catalyst reconstruction sequence has greatly expanded the scope of understanding of the fundamental processes by which heterogeneous catalysis occurs. This in-

PARAFFINS SYNTHES I S GAS

z

I-

0 4

N

i I3 Y 0

E v)

0

w w

LL

1910

I950 FIG. 16. Trends in chemical feedstock.

I 980

162

ROBERT K . GRASSELLI AND JAMES D. BURRINGTON

formation will no doubt serve as a basis for the development of catalyst systems that will meet the demand for the conversion of less expensive, available feeds with high selectivity to the desired products in single-step processes. The history of major chemical feedstocks (Fig. 16) exemplifies this trend in the utilization of acetylene prior to 1950, dienes and olefins in the 1950-1990 era, and the current need for catalysts for the selective conversion of paraffins and synthesis gas to meet future world chemical needs. REFERENCES 1. Chem. & Eng. News 56,49 (1978). la. R. K. Grasselli, “Petrochemicals by Heterogeneous Catalysis,” presented at AAAS Ann. Meet., January 4, 1980, San Francisco, California. 2. Callahan, J. L., Grasselli, R. K., Milberger, E. C., and Strecker, H. A., Ind. Eng. Chem. Proc. Res. Dev. 9, 134 (1970). 3. Weissermel, K., and Arpe, H. J., “Industrial Organic Chemistry” (transl. by A. Mullen). Verlag Chemie, New York, 1978. 4. Stull, D. R., Westrum, E. F., Jr., and Sinke, G. C., “The Chemical Thermodynamics of Organic Compounds.” Wiley, New York, 1969. 5 . Mars, P., and van Krevelen, D. W., Chem. Eng. Sci., Suppl. 3, 41 (1954); Weiss, F., Marion, J., Metzger, J., and Cognion, J.-M., Kinet Catal. 14, 32 (1973); Dadyburjor, D. B. Jewur, S . S., and Ruckenstein, E., Catal. Rev.-Sci. Eng. 19,293 (1979). 6 . Callahan, J. L., and Grasselli, R. K., AIChE J. 9,755 (1963). 7. Grasselli, R. K., and Callahan, J. L., J . Catal. 14,93 (1969). 8. Grasselli, R. K., and Suresh, D. D., J . Catal. 25,273 (1972). 9. Grasselli, R. K., Suresh, D. D., and Knox, K., J . Catal. 18, 356 (1970). 10. Keulks, G. W., J. Catal. 19, 232 (1970); Keulks, G. W., and Krenzke, L. D., Proc. Int. Congr. Catal., 6th, 1976, Prep. B-20 (1977). 11. Brazdil, J. F., Suresh, D. D., and Grasselli, R. K., Prepr., Div. Petrol. Chem., Am. Chem. Soc. 24, No. 4, 1046; J . Catal. 66, 347 (1980). 12. van den Elzen, A. F., and Rieck, G. D., Acta Crystallogr., Sect. B 29,2436 (1973). 13. Gates, B. C., Katzer, J. R., and Schuit, G. C. A,, “Chemistry of Catalytic Processes,” pp. 352-360. McGraw-Hill, New York. 1979. 13a. van den Elzen, A. F., and Rieck, G. D., Acta Crystallogr., Sect. B 29,2433 (1973). 14. Jeitschko, W., Sleight, A. W., McClellan, W. R., and Weiher, J. F., Acta Crystallogr., Sect. B32, 1163 (1976). IS. Grasselli, R. K., and Hardman, H. F., U.S. Patent 3,642,930 (1972); Grasselli, R. K., Miller, A. F., and Hardman, H. F., Ger. Offen. 2,147,480 (1972), 2,203,709 (1972). 16. Grasselli, R. K.. “Structures as Related to Activity and Selectivity in Oxidation and Ammoxidation Catalysis,” presented at the Advances in Oxidation Chemistry Series, University of Manchester, U.K., April 17, 1975. 17. Adams, C. R.,and Jennings, T. J., J. Catal. 3, 549 ( I 964) ; 2,63 ( 1962). 18. Sachtler, W. H., and deBoer, N. H., Proc. Inr. Congr. Catal., 3rd, 1964 p. 252 (1965). 19. Grzybowska, B., Haber, J., and Janas, J., J. Catal. 49, 150 (1977). 20. Burrington, J. D., and Grasselli, R. K., J . Catal. 59,79 (1979). 21. Burrington, J. D., Kartisek, C. T., and Grasselli, R. K., J. Catal. 63, 235 (1980); Prepr.. Div. Petrol. Chem.. Am. Chem. Soc. 24, No. 4, 1034 (1979). 22. Davydov, A. A., Mikhaltchenko, V. G., Sokolovskii, V. D., and Boreskov, G. K., J . Catal. 55, 299 (1978).

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE

163

23. Iwasawa, Y., Nakano, Y., and Ogasawara, S . , J. Chem. Soc., Faraday Trans. I 14,2968 (1978). 24. Trifiro, F., Centola, P., Pasquan, I., and Jim, P., Proc. Int. Congr. Catal., 4th, 1968 p. 310 (1971); Trifiro, F., and Pasquan, I., J. Cutal. 12,412 (1968). 25. Matsuura, I., J. Cutul. 35,452 (1974); 33,420 (1974); Matsuura, I., and Schuit, G. C. A,, ibid. 25,314 (1972); 20, 19 (1971). 26. Haber, J., and Grzybowska, B. J. Cutul. 28,489 (1973). 27. Sleight, A. W., in “Advanced Materials in Catalysis” (J. J. Burton and R. L. Garten, eds.), p. 181. Academic Press, New York, 1977; Linn, W. J., and Sleight, A. W., Ann. N . Y. Acad. Sci. 272, 22 (1976).

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ADVANCES I N CATALYSIS, VOLUME 30

Mechanism of Hydrocarbon Synthesis over Fischer-Tropsch Catalysts P. BILOEN

AND

W. M. H. SACHTLER

Shell Research B. V . Koninklijkel Shell-Laboratorium Amsterdam, The Netherlands

I. Introduction . . . . . . . . . . . . . . 11. Product Distribution under Steady-State Conditions . . . 111. Kinetics . . . , . . , , . . . , . . . A. Steady-State Kinetics and the Nature of Slow Steps B. Transient State Kinetics and the Absolute Value of Rate Constants . . . . . . . . . . IV. Surface Spectroscopy, . . . . . . . . . . A. Electron Spectroscopy . . . . . . . . . B. In Situ Infrared Spectroscopy . . . . . . . C. Discussion . . . . . . , . . . . . V. Hydrocarbon Synthesis via Predeposited Carbidic Carbon A. Hydrogenation of Surface Carbon. . . . . . . B. Hydrogenation of Mixed Overlayers of Surface Carbon and CO,,, . . . . , . . . . . . . . C. Hydrogenation of Labeled Surface Carbon in CO/H, Atmosphere . , . . , . . . . . . . . VI. Fischer-Tropsch Synthesis via Carbidic Intermediates . . A. Propagation/Chain Growth . . . . . . . . B. Genesis of Carbidic Intermediates. . . . . . . C. Coverages, Sites, Selectivities, and Rates . . . . . VII. Suggestions for Future Research . . . . . . . . References . . . , . . . . . . . . . . .

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165 169 178 178

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200 206 207 210 211 212 214

Introduction

A reliable supply of liquid hydrocarbons is of vital importance to the industrialized world. The superiority of liquid over gaseous energy carriers, in particular for automotive purposes, resides in the high energy density of liquids and the relatively low weight (per unit of caloric value) of the required container. When compared to solid fuels such as coal, liquids have the 165 Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-1 2-007830-9

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P. BILOEN A N D W. M. H . SACHTLER

advantage that pollutants can be removed more easily, and transport through pipelines or in tankers is cheap and efficient. Coal is, however, much more abundant than the natural reserves of liquid hydrocarbons. Hence, there is a clear incentive to convert coal chemically into liquid hydrocarbons. Broadly speaking, this can be achieved along three lines: 1. Hydrogenative extraction and/or pyrolysis of coal. 2. Steam gasification of coal, followed by conversion of the synthesis gas obtained-a mixture of hydrogen and carbon monoxide-to methanol, which can subsequently by converted into other liquids, in particular hydrocarbons. 3. Steam gasification followed by conversion of the synthesis gas to hydrocarbons or a mixture rich in these.

Only the conversion of synthesis gas as described under 3 forms the subject of this review. The chemistry of the catalytic processes is, of course, independent of the way in which the synthesis gas was manufactured; besides coal gasification the steam re-forming of natural gas is a well-known route to produce synthesis gas, although with a higher H,/CO ratio. Within the conversions thus defined, it is convenient to distinguish methanation, where methane is the predominant product, and the FisckerTropsck process directed toward the manufacture of predominantly liquid hydrocarbons. However, as the product composition for a given catalyst largely depends on the conditions, in particular, the pressure, the temperature, and the HJCO ratio, we shall use the term Fischer-Tropsck catalysis also for those cases in which, owing to the variation of one or more of these parameters, the product is mainly gaseous or solid at room temperature and atmospheric pressure. Therefore, we define as a typical Fischer-Tropsch synthesis (process) the catalytic conversion of synthesis gas to a mixture of predominantly linear alkanes and alkenes, with the product distribution with respect to chain length displaying a recognizable pattern (cf. Section 11). Alcohols, if formed as by-products, are predominantly primary n-alcohols. For such syntheses the overall conversion can be described in good approximation by combinations of the equations nCO

mCO

+ 2nH,

+ (2m + 1)H2 p C 0 + 2pH2

+

-+

-+

CnH2. + nH,O C,H2,+2

(n

+ mH2O

CP-,H2,-,CH,OH

=

2, 3 , . . . )

(1)

(m = 1 , 2 , 3 , . . .) (2)

+ (p - 1)HZO

(p = 1 , 2 , 3 , . . .)

(3)

A Fischer-Tropsch catalyst, therefore, must catalyze the formation of carbon-carbon and carbon-hydrogen bonds and the rupture of carbonoxygen bonds.

FISCHER-TROPSCH REACTION MECHANISM

167

A secondary reaction, quite welcome when the H,/CO ratio of the feed gas differs from that required by the stoichiometry of the desired products, is the water-gas shift: CO

+ H 2 0 + C 0 , + H,

(4)

The history of the Fischer-Tropsch process has been adequately described in numerous articles, including two review papers by Storch ( I ) and Pichler (2) in Advances in Catalysis. We shall therefore refrain from repeating this exciting story here. It may suffice to mention the following highlights. 1. In 1902 Sabatier and Senderens (3)reported the synthesis of methane from carbon monoxide and hydrogen in the presence of nickel and cobalt catalysts. 2. In 1908 Orlov (4) reported the formation of ethylene from synthesis gas over NiPd catalysts. 3. In 1913 a patent (5)was granted to the Badische Anilin und Sodafabrik for the manufacture of a liquid “oil” from synthesis gas at 100-200 bars over cobalt or osmium oxide catalysts. 4. From 1923 onward Franz Fischer and Hans Tropsch ( 6 )developed the process defined above, operating at 10-20 bars and using iron and cobalt as the decisive catalyst ingredients. In 1938 Pichler (7) proved that with ruthenium as the catalyst synthesis gas can be converted at lo3 bars to a waxy material, which was later identified as linear polymethylene, indistinguishable from Ziegler polyethylene. Where the Fischer-Tropsch process has been used on an industrial scale, iron or cobalt are the essential catalyst components. Technical catalysts also contain oxidic “promoters,” such as alumina and potassium oxide. Ruthenium and nickel are most attractive for academic research since they produce the simplest product packages. Nickel is used for methanation (production of substitute natural gas and removal of carbon monoxide impurities from hydrogen). With iron it appears certain that the pure reduced metal is not the phase of highest activity: when the metal is exposed to synthesis gas at reaction temperatures, the activity slowly increases, indicating that the active phase is still being formed. Indeed, Pichler (8) reported that a pseudohexagonal carbide Fe,C is the active phase. Besides carbide formation the oxidation of the catalyst surface is of importance. As Eqs. (1)-(3) show, hydrogen is consumed and H,O formed. With respect to the oxidation-reduction equilibrium of the catalyst metal M, M

+ HZO * MO + H2

(5)

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P. BILOEN A N D W. M. H. SACHTLER

this means that the atmosphere is reducing at zero conversion. but may be oxidizing at high conversion. The simultaneous presence of metallic, carbidic, and oxide phases of the same element is one of the reasons why the chemistry of Fischer-Tropsch catalysts is so complicated. A warning is therefore appropriate with respect to the sometimes overstressed analogy between the Fischer-Tropsch and the ammonia synthesis. While it is certainly true that the same preparations of iron catalysts with the same promoters turn out to be active in both reactions, the phases responsible for the catalytic action under steady-state conditions may be completely different for these two processes. In ammonia synthesis the gas remains reductive irrespective of conversion, so that the metallic phase remains stable or nitrides are formed (9). In Fischer-Tropsch synthesis, the relative stabilities of metals, oxides, and carbides depend on the conversion. The nonuniformity of some catalysts under steady-state conditions and the nonuniformity of the product, approximated by the ( m y1 p - 1) organic compounds defined in Eqs. (1)-(3). where m, n, and p are each often of the order of 20. render the question of the reaction mechanism(s) quite intriguing. Without trying to describe all subtleties in detail, it is challenging to explain-at least for the simplest, nonpromoted catalysts-the rules of the game, how carbon-carbon bonds and carbon-hydrogen bonds are formed and carbon-oxygen bonds broken, while all these elementary steps have to be carried out at a high pressure of carbon monoxide, notorious as an efficient poison to hydrogenation catalysts. Much work has been devoted to these problems in numerous laboratories, and many interesting models have been proposed. In this contribution an attempt is made to summarize the experimental evidence of potential revelance to the reaction mechanism that is now available. l h e main sources from which information pertinent to the reaction mechanism has been obtained are:

+ +

1. analysis of the product distribution a. in steady state, b. in nonsteady state; 2. as in No. 1, with isotopic labeling; 3. reaction kinetics; 4. surface spectroscopy (XPS, IR, etc.); 5. chemical analogies (CO insertion in metal-organic complexes, C-C bond formation, hydrocarbon conversion reactions catalyzed by group VIII metals, etc.). The data obtained from sources (l), (2),and (3) have the advantage of being conclusive in the sense that they permit one to exclude some theoretical

FISCHER-TROPSCH REACTION MECHANISM

169

models. They remain ambiguous, however, where several models are consistent with the observed data. The information obtained via (4) and ( 5 ) can show what is possible chemically and what complexes are present on the catalyst surface: usually this information fails, however, to prove conclusively which is the kinetically preferred path of the reaction(s) under the conditions where the catalyst actually operates. In this review we shall be brief with respect to older data, adequately accessible to the interested reader in previous reviews ( I , 2, 10). For brevity we shall take the liberty of writing F T for Fischer-Tropsch.

II.

Product Distribution under Steady-State Conditions

As mentioned in the Introduction, we can distinguish “simple” FT catalysts, producing hydrocarbons exclusively with ruthenium as the outstanding example, and “complex” F T catalysts, such as “promoted” iron, wherein the steady-state metallic, oxidic, and carbidic phases can coexist. With the latter catalysts the product is a cocktail containing various oxygenates, in particular primary alcohols, as well as hydrocarbons. Following the approach of separating variables, we shall review here the models appropriate to describe the distribution of products over “simple” FT catalysts only. The products then consist of paraffins and cr-olefins with essentially unbranched carbon chains. As paraffins and olefins can easily be interconverted under F T conditions, the information most pertinent to the reaction mechanism has to be derived from the distribution with respect to the chain length. The formation of alcohols is briefly reviewed in Section VI. Basically, two models can be visualized to rationalize chain length distributions in the absence of secondary reactions: 1. Chain growth through lateral reaction between building blocks populating the catalyst surface. 2. Chain growth through stepwise insertion of one building block per step.

It is noted that the same choice of models has been discussed with respect to polymerization over heterogeneous catalysts. Indeed, the FT reaction can be visualized as a special type of polymerization, differing from other types by the fact that the “monomer” has to be produced in situ from C O and H, on the catalyst surface. An example for the lateral interaction concept is the model proposed in 1939 by Craxford and Rideal (II). These authors assumed that a surface carbide is first hydrogenated to CH, groups. These then interact with each

170

P. BILOEN AND W. M. H. SACHTLER

other, forming a giant (CH,), molecule lying on the catalyst surface. The gaseous hydrocarbons were then assumed to result from cracking of this molecule. This hypothesis failed to give a satisfactory rationale for the chain length distribution actually found. As Storch remarks in his review article ( I ) , it would be “very difficult to explain” on the basis of this methylene polymerization model that only a relatively small fraction of singly branched hydrocarbons is produced in the synthesis on iron and cobalt catalysts. Storch, Columbic, and Anderson (10) further remark that the rate of hydrocarbon hydrogenolysis is efficiently reduced by carbon monoxide and that the distribution curves of products from FT synthesis and hydrocracking differ significantly. The present writers agree that the lateral polymerization model is inadequate. The concept of stepwise insertion was introduced in 1946 by Herington (12). In the most general form this hypothesis assumes the presence of two kinds of species on the surface: chains Y, and insertable monomers X. The chains will be monoadsorbed with their terminal group. Upon applying the concepts of modern complex chemistry to this model, the insertion can be visualized as a cis migration, where the “chain” Y, forms a bond with the insertable species X, while the original bond between adsorbing metal atom and Y is broken:

In order to be able to write this down in symbolic units we have, admittedly, added two assumptions to the basic idea: 1. Chain and insertable species are bonded to the same metal atom. 2. Chain propagation is an act of cis insertion.

We believe that the first of these assumptions applies to most, if not all, reactions where bonds are formed between two adsorbed moieties. It is conceivable either that the resulting species Y,-X is identical to Y,, 1 , that is, a chemical homologue of Y, differing from it only in chain length, or that one or more chemical transformations have to take place to convert Y,-X into Y,, It is essential, however, that no insertion takes place before Y,-X has been transformed into Y,, 1 . Starting from some initial chain Y 1 , any chain Y, can be created by the succession of n - 1 insertion steps. Since each of the intermediates can be desorbed by some termination step, for instance, by addition of a hydrogen atom or B hydrogen abstraction, we arrive at the simple scheme yo-Y~

1

P,

*y2

1 P2

&y3

1

P,

&Y,

1 p,

-+2iy5-etc.

1

p5

(7)

FISCHER-TROPSCH REACTION MECHANISM

171

where P I , .. ., P, are the products. The chance for any intermediate chain to propagate rather than terminate is defined by the ratio of the rates =

k p / ( k p+ k,)

(8)

where k , is the rate of the reaction transforming Y, to Y,+ and k, is the rate of conversion of Y, to P,. If the former rate is of the first order in the surface concentration of X, this concentration is included k,. Since the propagation and termination reactions both affect only the terminal carbon atom of the chain, it is plausible that a is independent of n, expect for very small chains. As it follows that at steady state

[cf. Eqs. (31)-(33)], this concept leads to the conclusion that the product distribution must be that of a geometrical series. With c, being the relative molar concentration of product P,, Eq. (9) leads to c, = (In' @)an

(10)

where the first factor arises from normalizing the distribution by summation over all products. Of course, this distribution is not confined to F T synthesis, but is generally valid for all polymerization reactions with constant ratio of (one-sided) propagation and termination rates. It is, therefore, no wonder that Eq. (10) was already published in 1935 by Schulz (13) and in 1936 by Flory (14) in studies unrelated to F T reactions. In polymerization reactions one usually knows the molecular weight of the monomer and the composition of Po. It is not difficult then to define the number of insertion steps from the molecular weight of a product. As Eq. (10) can be written as log c, = 2 log(1n cc)

+ n log IX

(1 1)

the validity of the Schulz-Flory distribution can then be checked by plotting the logarithm of the concentration of products P I , P,, etc. against n. the number of insertions. The theory then predicts that a straight line is obtained with slope log cc; only the products with very short chains may deviate from that linear graph. Facing measured product distributions of FT reactions with this statistics requires, however, additional information or assumptions in order to define the number of insertion steps. The insertable species X in Eqs. (6) and (7) can be expressed by the parameters x,y, and z in X

=

CZH,O,

(12)

Much of the scientific work is aimed at elucidating the mechanism of the

172

P. BILOEN AND W. M. H. SACHTLER

F T synthesis and, by consequence, much of the material reviewed in the present paper is directed toward defining the true values of these parameters. The simplest assumptions one can make for a chain growth reaction of the type given in Eq. (7) are, of course: 1. The species Y , contains one carbon atom. 2. The species X contains one carbon atom, that is z

=

1.

In this special case, the number of insertions leading to a product with p carbon atoms should be given by n = p - 1. A straight line should then be obtained upon plotting the logarithm of the molar concentrations (= weight fraction divided by molecular weight) versus II or p . If the first assumption does not hold, that is, if Y, contains more than one C atom, a straight line shifted to the right with respect to the previous line will be expected. Figures land 2 show Schulz-Flory plots of FT synthesis products obtained over iron and cobalt catalysts, respectively (15, 16). The measured points do not deviate much from the linear graph predicted by Eq. (11). As there are also numerous data in the literature where Eq. (11) appears to be obeyed, the basic assumptions of (1) stepwise insertion, and (2) the insertable species containing one carbon atom, appear to permit a satisfactory description of

0 001 2

4

6

8

10

12 14 16 18 CARBON N U M B E R

FIG. 1. Log C, versus carbon number (cf. Eq. 11); catalyst, iron. From ( I S )

FISCHER-TROPSCH REACTION MECHANISM

0

1

2

3

4

5

6

173

7

CARBON NUMBER

FIG.2.

Log C, versus carbon number; catalyst, cobalt. From (16).

some of the basic reactions in the Fischer-Tropsch process. This conclusion is further supported by the observation that even under industrial conditions and with complex catalysts the overwhelming majority of the hydrocarbon products, prior to secondary reactions, consist of normal, that is, unbranched aliphatic, hydrocarbons. An obvious consequence of the Schulz-Flory and similar distribution laws is that of the numerous ( n + m p - 1) products of Eqs. (1)-(3) only methane and methanol can be produced with 100% selectivety (a = 0) by a FT catalyst. All other products with parameters differing from n = 1 and/or p = 1 can only be produced in packages with a wide range of these parameters. The Schulz-Flory distribution provides a simple means of formulating a mathematical model for the numerical values of n, m,and p , characterizing the product pattern. It is conceivable to include this “ideal” distribution law in the definition of an “ideal” F T catalyst. Actual product distributions that significantly differ from this simple pattern can then be rationalized in terms

+

I74

P. BILOEN A N D W. M. H . SACHTLER

of the contribution of non-FT reactions. While such a definition would have conceptual and practical advantages, it should not be used in too rigorous a way as one should remember that: 1. The value of c1 can be chain length dependent for short chains. 2. In Eq. (7) two monocarbon species Yo and X occur, each of which can be hydrogenated to methane. This would easily explain methane production exceeding that predicted by extrapolating a Schulz-Flory line defined by the heavier products. 3. There is no reason to exclude parallel reactions, where species with z > 1 are inserted into growing chains. With respect to the point mentioned last, special interest has been focused by several authors on the possible insertion of olefins. Of special relevance are results reported by Schulz and Achtsnit (17).These authors studied the F T catalysis on a cobalt catalyst and added ethylene marked with 14C to the feed. The product was analyzed; the specific molar radioactivities of the various fractions are shown in Fig. 3. It is seen that for higher carbon numbers the specific molar radioactivity increases linearly with the carbon number. Upon considering that, if ethylene can be inserted, the chance of this to happen increases linearly with the number of insertion steps that a chain undergoes during its life, this linear dependence is interpreted by the authors as proving ethylene insertion. Strictly speaking, this is not quite conclusive. One can also imagine that a mechanism exists by which ethylene transfers its I4C atom to a monocarbonic species which is then inserted. We shall discuss this possibility in Section VI. The fact that the linear graph in Fig. 3 when extrapolated to n = 0, does not pass through the origin is interpreted by Schulz and Achtsnit as an indication that ethylene can also act as initiator. The effect of ethylene on R E L A T I V E M O L A R ACTIVITY 8OC

-

-

2ot 0

2

FIG.3.

4 5 6 7 8 9 1 0 1 1 12 CARBON NUMBER OF PRODUCT FRACTION

3

Synthesis with ethylene, 14C in the synthesis gas. From (17).

FISCHER-TROPSCH REACTION MECHANISM

175

FT synthesis was also studied by Dry (18)and by Dwyer and Somorjai (19). All these authors agree that in the presence of ethylene the F T activity is increased. A large probability of ethylene insertion would explain why the C2 point of FT synthesis products is often found to be below the SchulzFlory line defined by the higher products, as, for instance, in Fig. 2. The insertion of @-olefinshigher than ethylene will give rise to branched molecules. The almost exclusive formation of straight chains on “simple” FT catalysts and the low fraction of branched molecules usually obtained even on “complex” FT catalysts, shows that this type of insertion is not very typical for F T catalysis. On cobalt catalysts Pichler and Schulz (20) showed that addition of ‘‘CH2=CH-C14H2, to a normal feed resulted in the formation of an appreciable number of hydrocarbon molecules with more than 16 carbon atoms and a high specific radioactivity. However, from the shape of the specific molar radioactivity graph in Fig. 4 it is not clear whether this concerns an insertion rather than a chain initiation with the labeled olefin as the starter. As for an insertion a straight line with positive slope would be expected, chain initiation is more likely. Another interesting feature of this graph is the regular pattern of the products containing 14C and having fewer than 16 carbon atoms in the chain. While the molar radioactivity is low in these low molecular weight fractions, the abundance of these fractions in the total product is high and 14.1% of all I4C atoms introduced as labeled C16H,, end up in these fractions. We shall return to this very interesting result in Section VI. At the present stage it can be stated that insertion of olefins, accompanying the insertion of a monocarbon fragment, is possibly significant for ethylene. =

MOLAR ACTIVITY ( n - C e

-

100)

b

?

500

I I

400

0

I

I

I

I

I

I

I

4

8

12

16

20

CARBON NUMBER

FIG.4. Molar activity of synthesis products as a function of C number in the Co normal pressure synthesis after addition of [l-’4C]-n-hexadecene-1 to the synthesis gas (relative molar activity of n-CI6in product: 291,000): ( 0 )n-paraffin; ( 0 )monomethyl-paraffin; ( 0 )n- and isoparaffins. From (20).

176

P. BILOEN A N D W. M. H . SACHTLER

This might result in an undershoot of the C, fraction of the total product below the Schulz-Flory line. For higher olefins this chance appears to be low in accordance with the abundance of unbranched products. The value of the parameters x and y in the insertable species C,H,O, cannot be derived from chain length distributions. In the literature three possibilities have received special attention and have become the subject of considerable speculation: 1. Condensation of surface oxymethylene species:

*

*

*

*

Starch, Columbic, and Anderson (10) proposed this propagation scheme as one of a detailed set of equations involving oxygenated intermediates. In a review article (15) Anderson remarks that the postulates “explain many of the characteristics of the synthesis products. Although most of these postulates were made without significant experimental evidence, except analytical data on the synthesis products, the postulates have been essentially substantiated by the mechanism experiments of Emmett and Kummer” (p. 359). We will discuss the experiments of Kummer, de Witt, and Emmett (21) in Section VI. 2. CO insertion, that is, x = 0, y = 1 : This possibility was in particular advocated by Pichler and Schulz (20). They write the reaction sequence leading to chain growth by one C atom as shown in Scheme 1 :

FISCHER-TROPSCH REACTION MECHANISM

177

SCHEME1

where M is the surface metal atom and (CO), an unspecified number of CO ligands on the same atom. The main argument in favor of this possibility is that in metal-organic complexes containing alkyl groups and carbon monoxide, cis insertion has been observed. Application of this mechanism to Fischer-Tropsch reactions is therefore essentially reasoning by analogy. Since these views have been reviewed properly, we can confine ourselves to referring to the relevant literature (20). 3. Insertion of CH, 0, = 0), in particular, of carbene or methylene CH, (22, 23):

As this mechanism has also been proposed in more recent work, to be discussed in detail in subsequent chapters, we shall devote considerable attention to it in Section VI. In concluding this section on product distribution we summarize that numerous data are consistent with a chain reaction of stepwise addition of one C,H,O, group to chains attached to the catalyst surface by one terminal carbon atom. The chain growth can be terminated at any chain length. the character of the termination reaction determining whether paraffins or olefins are formed. Alcohols are formed as by-products on “complex” catalysts, presumably on oxidic patches of the surface.

178

P. BILOEN AND W. M. H. SACHTLER

111.

Kinetics

This section deals with evidence regarding the mechanism of the FT reaction derived from kinetic experiments, that is, studies of the variation in rate of formation of various products with reaction conditions such as the carbon monoxide and hydrogen partial pressures. In Section III,A we consider to what extent conclusions-inferences on the nature of slow steps can be drawn from steady-state kinetics. In Section III,B recent inferences with respect to absolute magnitudes of rate constants derived from transient state kinetic experiments are discussed.

A. STEADY-STATE KINETICS AND

THE

NATURE OF SLOW STEPS

A large body of steady-state kinetic data, including those of commercial operations (10, 15, 24, 25), consistently shows that over a wide range of conditions and with a wide variety of catalysts the rate of CO conversion in FT is of significantly positive order in hydrogen partial pressure, and slightly negative order in the CO partial pressure. Further, it is generally observed that the tendency toward chain growth, as defined for instance via the parameter a in Eq. (8) of Section 11, decreases with increasing HJCO ratio and increasing temperature. Unfortunately, however, these data do not lend themselves readily to a detailed kinetic analysis, because they have in general been obtained at high conversions with catalysts of unknown dispersion. Vannice therefore reinvestigated the kinetic behavior of transition metal-catalyzed CO conversion, taking care to maintain differential conditions and performing chemisorption experiments in order to measure the degree of dispersion of his catalysts (26).Dalla Betta, Piken, and Shelef (27) and Rautavuoma (16)also performed kinetic studies of this type. Vannice obtained his data in a glass differential flow reactor, at 1 bar total pressure. Under these conditions methane is the major product, and his observations pertain primarily to the kinetics of methanation. However, the syntheses of methane and higher hydrocarbons are closely related (cf. Section VI). His data pertain to a comparative study, involving a wide variety of transition metals, and he integrates his observations in a simple kinetic model. It is on the meaningfulness of this kinetic model that we will focus our attention in this section. Relevant results of Vannice’s work are: 1. Quantitative data on the turnover numbers for methane production, and their variation with CO and H2 partial pressure, as summarized in Table

179

FISCHER-TROPSCH REACTION MECHANISM

TABLE I Kinetic Parametersfor the Methanation Reaction": NCH4 = Aexp(-E,,,/RT)P&PZ0

Catalyst

5% Ru/AI,O, 15% Fe/AI,O, 5% Ni/AI,O, 2% Co/Al,O, 1% Rh/AI,O, 2% Pd/AI,O, 1.15% Pt/AI,O, 2% Ir/A1203 4.75% Pd/SiO, 0.5% Pd/H-Y zeolite

CH4 formation at 275°C; turnover Em No. x lo3 (kcaljmol) 181 57 32 20 13 12 2.7 1.8 0.32 5.9

24.2 k 1.2 21.3 f 0.9 25.0 f 1.2 27.0 f 4.4 24.0 f 0.4 19.7 k 1.6 16.7 k 0.8 16.9 f 1.7 26.9 f 1.8 21.2 3.1

X

Y

1.6 k 0.1 1.14f0.10 0.77 0.04 1.22 f 0.18 1.04 f 0.1 1.03 f 0.05 0.83 k 0.01 0.96 f 0.02 0.71 f 0.05 0.84 f 0.12

-0.6 f 0.1 -0.05 f 0.07 -0.31 f 0.05 -0.48 f 0.28 -0.20 0.1 0.03 f 0.09 0.04 k 0.01 0.1 f 0.08 0.15 k 0.07 0.30 f 0.15

A (molecules site- sec-

5.1 x 2.2 x 2.3 x 9.0 x 5.2 x 1.2 x 1.6 x

*

10' 107 10' 10' 107

lo6

104 1.4 x 104 2.0 x lo6 5.7 104

From (26).

I. The parameters X and Y in Table I stand for the powers in the power rate law expression NCH4 = A exp( -E,/RT)P&P:,

molecules site-' sec-

'

(13)

in which the turnover number for methane production, N C H 4 , has been obtained from the rate of methane production in molecules per second divided by the number of surface-exposed metal atoms, as measured by hydrogen or CO chemisorption. 2. The existence of a compensation effect, that is, a relation between A and E m , as follows from Fig. 5.

10

15

20

25

30

Em (kcol/mol)

FIG.5. Compensation effect for the methanation reaction. From (26).

180

P. BILOEN AND W. M. H . SACHTLER

\ OPt lr

20

30

50

40

CO HEAT OF ADSORPTION

(kcaL/moll

FIG.6 . Correlation between methanation activity and AHa for CO. From (26).

3. A correlation between turnover numbers and heats of adsorption of the various catalysts for CO (Fig. 6). In later work copper was included, and the correlation widened to a full “volcano” plot (28).

Vannice demonstrates that Findings 1, 2, and 3 can be consistently explained in a simple kinetic model, involving a homogeneous surface exhibiting Langmuir adsorption, in which hydrogen-assisted C - 0 bond breaking is the rate-determining step. Specifically he inspects the kinetic consequences of the following sequence of elementary steps (26):

+ COACHOH,, + C ~ / ~ ) H Y . , B ~ , ~ C H+J HJ ,2,0, H,

CHOH,,

CHy,, W C H 4

(14) (15)

(16)

which leads to NCH4

=

kZeCHOH g yHi 2

(17)

With all steps preceding the rate-determining C-0 bond-breaking step [Eq. (1 5)] being essentially at equilibrium, and confining ourselves to Langmuir adsorption on a homogeneous surface, we can approximate the surfaces coverages 8 c H o H and OH, as

FISCHER-TROPSCH

REACTION MECHANISM

181

and, assuming the most abundant surface species to be CHOH.

We follow the analysis of (26), which proceeds with the simplification

which leads to

As shown in a short note (29) it is advantageous to retain the full ( n - 1) dependence in OH2, but this is not essential in our analysis of the original model. Substitution of the expressions for &-OH [Eq. (18)] and eH2 [Eq. (21)] in the rate expression (17) then yields NCH4

- k2K(“-Y/Z)KY/2pn H2 H2 p (CO n-Y/2)

(22)

In terms of the overall rate equation (13) Vannice therefore derives from this model

Y = n - yJ2

(25)

This model thus relates the measured parameters of the overall kinetics, (X, Y ) to the parameter y of the assumed elementary step (15) by Y

=

x - yJ2

(26)

with y being confined to integer values [cf. Eq. (15)]. Table I1 lists the Y values calculated from Eq. (26), with X taken as the experimentally observed order of the reaction rate in H2, and y used as a free “fitting” parameter, constrained, however, to integer values. Vannice concludes that the good agreement between Ycalc and Yexp is significant. Further, he argues that the correlation displayed in Fig. 6, which is indicative of inhibition of the surface reaction by too strong CO chemisorption, is a logical consequence of the model presented, because a high value for K,, would imply a high value for KCHoH = K , leading via Eq. (21) to low values OH2. Vannice also concludes that the model gives a plausible explanation of the compensation effect (Fig. 5), essentially because the rate expression (22) contains besides a rate constant ( k 2 ) equilibrium adsorption constants (KH2and K ) . The overall activation energy Em will therefore contain a con-

P. BILOEN AND W. M. H . SACHTLER

182

TABLE I1 Calculated and Experimental Y Values”

Pd/Si02 Pd/H-Y zeolite Ni/A1,0, Ir/Alz03 Pd/A1,0, Pt/AI,O, Rh/A1,0, Fe/AI,O, Co/AI,O, Ru/AI,O, a

0.7 0.8 0.8 1 .o 1 .o 0.8 1 .o 1.1 1.2 1.6

1 1 2

2 2 2 2 2 3 4

0.2 0.3 -0.2 0 0 - 0.2 0 0.1 -0.3 - 0.4

0.2 0.3 -0.3 0.1

0 0 -0.2 -0.1 -0.5 -0.6

From (26).

tribution from adsorption enthalpies in addition to the activation energy of the rate-determining elementary step (15), and the preexponential factor A will contain a contribution of adsorption entropies in addition to the activation entropy of Step (15). As adsorption enthalpies and entropies are in general interrelated (tighter binding leads to increased localization), a relation between A and E , results: the compensation effect. Besides this model Vannice discusses a variant, allowing for surface heterogeneity, and he concludes (26) that “the wide applicability of either kinetic model . . . over all Group VIII metals is strong support for its veracity” (p.471). However, with regard to the fit demonstrated in Table I1 it should be noted that the grid of fit values of Ycalchas a mesh size of 0.5 [Ay = 1 ;cf. Eq. (25)]. The distance between any Yexpand the nearest Ycalctherefore is 0.25 at maximum. The actual root mean square deviation between Yexp and Ycalc is 0.15, which seems statistically not very significant. Furthermore, we call attention to the appearance of y values larger than 2 in Table 11. As hydrogen participates as Hadsatoms, values larger than 2 for y imply participation of more than three species in Eq. (15), which is proposed to be an elementary step of the reaction. Three-body interactions, however, already have a low probability. Furthermore, the interpretation of y as reflecting the number of H atoms participating in the elementary step (15) implies that for different transition metals this step varies in nature, that is, from bi“mo1ecular” ( y = 1) to quadru“molecu1ar” ( y = 3), but remains the rate-determining step in an overall unchanged sequence. Finally, the overall kinetics are not very informative with regard to the nature of the elementary steps in a complex reaction, as demonstrated by

FISCHER-TROPSCH REACTION MECHANISM

183

the sequence:

co + H z A C , d , ‘ads

f

bI2IHZ

+ H20

*CHyads

CHY,,, *CH4

(27) (28) (29)

which leads to the overall rate equation NCH4 = k20cO~~

(30)

In the above we have deliberately stayed close to Vannice’s model, and changed only the nature of the rate-determining step from H-assisted C - 0 bond breaking (15 ) into (irreversible) hydrogenation of surface carbon (28). Consequently, the overall rate equation (30) differs from Eq. (17) only in that it contains the term 6c instead of OCHOH, When assuming further that Cadsinstead of CHOH,,, is the most abundant surface intermediate, the model can be made formally identical to that of Vannice, including the explanations it offers for the volcano plot and the compensation effect. We therefore conclude that several models are consistent with the kinetic data. For instance, positive and negative reaction orders in hydrogen and carbon monoxide, respectively, originate from the use of Langmuir-Hinshelwood kinetics quite independently of the nature of slow steps [cf. Eqs. (15)-(28)]. Probably they reflect the strength of adsorption of CO on the catalysts concerned, which is large compared with hydrogen. Also the compensation effect and the volcano curve pattern follow from very general laws interrelating heats of adsorption, entropies, and steady-state surface populations resulting from them. These correlations therefore d o not lend support to some special assumptions of Vannice’s models, which can easily be replaced by others without disturbing the relationships of kinetic and thermodynamic parameters. In particular, this pertains to the assumption that the reaction proceeds via (slow) hydrogenation of a surface enol complex [Eq. (15)i. The data disclosed by Vannice are highly valuable, as they allow a comparison on turnover basis. With the evidence now available regarding the development of extensive overlayers of surface carbon during FT synthesis (cf. Section IV), we would recommend that any forthcoming interpretation of the overall kinetics takes explicity into account the variations in surfacecarbon coverage with both the reaction conditions and the nature of the transition metal. Beeck (30)already pointed out that carbon deposition may well depend on the morphology of the surface, a point substantiated in more recent surface studies (31).This may underlie the large support effects disclosed in one of Vannice’s more recent kinetic studies (32).

184

P. BILOEN AND W. M. H . SACHTLER

One striking aspect of Vannice’s findings is that on a turnover-number basis the activities are far below those usually found for hydrocarbon conversion or hydrogenation reactions using the same metals. This aspect stimulated Dautzenberg et al. (33) to perform the transient state kinetic experiments described in the following section.

B. TRANSIENT STATEKINETICS AND THE ABSOLUTE VALUE OF RATECONSTANTS Space-time yields in FT are low compared with those observed in other metal-catalyzed reactions, such as hydrocarbon conversions, and the work of Vannice (26) has shown that this is due to low turnover numbers per surface-exposed metal atom. For methanation these are at 250°C of the order of about lo-’ C O converted site-’ sec-’, while, for instance, metalcatalyzed, hydrogenations of unsaturated hydrocarbons have turnover numbers one to two orders of magnitude larger. A priori two different reasons may be visualized. First, one or more steps in the FT synthesis reaction may have inherently low rates, resulting in a low overall activity despite the majority of gas phase-exposed metal atoms being active. Second, the Taylor fraction, that is, the fraction of all metal surface atoms that are actually active, may be low. The question what the actual value of the Taylor fraction is has been posed for many heterogeneously catalyzed reactions, but could seldom be answered since steady state kinetics yields results, depending on the product of the number of active sites and their inherent, that is, “absolute,” rate constant. For FT catalysis Dautzenberg et al. have found an in route ( 3 3 , using the characteristics of transient state polymerization kinetics (34). The essentials of the approach of Dautzenberg et al. can be recapitulated as follows. Assuming that the overall kinetics in F T is adequately represented by a stepwise chain growth:

the relative abundance of the different surface intermediates YT, YT+ steady state is governed by the ratio of k, and k,:

d dt

- [YT+

and therefore

=

k,[YT]

-

(k,

+ k,)[YT+ ‘3 = 0

(steady state)

. . . at (32)

FISCHER-TROPSCH REACTION MECHANISM

185

that is, the steady-state product distribution reveals nothing about the absolute value of surface coverages. This situation differs from that in transient state experiments, in which Eq. (32) is not satisfied and the surface coverages still build up in time: [YT] = [Y:],. Therefore, the ratio of the rates of production, (d/dt)[Pi+ ,I/ (d/dt)[Pi] also becomes time dependent. Boudart (34) has pointed out, in the general context of polymerization kinetics, that the evolution in time of this ratio, when normalized by its steady-state value k p / ( k p+ k,), is a function of k , only, provided that the rate constant for initiation ki is large with respect to both k , and k, (34). From a comparison of the transient state distribution with the steady-state one an absolute value for k , therefore can be found. The value of k, then follows from the steady-state value of GI = k p / ( k p k,) via Eq. (33). Dautzenberg et al. have exploited this characteristic chain growth aspect of the F T reaction. They subjected y-Al,O,-supported ruthenium to pulses of CO/H, with pulse durations of the order of 102-103 sec, well below the values of k p and k; '. Each CO/H, pulse was followed by a pulse of H, at elevated temperature, which wiped the surface clean of intermediates YT. The product of this hydrogenation pulse was added to that formed during th CO/H, pulse proper, and the sum was analyzed for its chain length distribution. The conditions prevailing in their experiment (CO/H, = 1, Ptotal= 10 bars, T = 210" ) led to GI values close to 1, that is, to a situation with k , >> k , . As a result the product from hydrogenation exceeded considerably the product formed during the CO/H, pulse, and the chain length distribution in the total product therefore closely resembles, a t least in principle, the chain length distribution in the adsorbing species at the moment of terminating growth, that is, at the end of the CO/H, pulse:

+

[y:l,,,,,l. . . [Yrl,,,,,, . . * Figure 7 depicts the chain length distribution in liquid product, collected with repeated CO/H, pulses (interrupted by hydrogenation pulses) with durations of 8 and 20 min. Steady-state distributions obtained with ruthenium exhibit highly linear Schulz-Flory plots, and the distribution in Fig. 7 is therefore substantially off steady state. Figure 7 also shows, as drawn curves, the fit obtained when performing the transient state kinetic analysis. Good fitting was achieved when assuming that the initiation is relatively fast, that is, [YT], is essentially at steady-state value within a time interval that is short as compared to the total pulse duration. The k , value corresponding to the fit, together with the value for k, derived from k , and the steady-state value of a , are given in Table 111. The authors conclude that the rather low turnover frequencies of F T catalysts must be interpreted, at least in the case of ruthenium, as a low 3

I

186

P . BILOEN AND W. M. H . SACHTLER

100-

86-

12 min PULSE

4-

210-1 -

868 mtn PULSE

4-

2-

10-2

I

'

'

I

'

I

I

I

'

FIG.7.

From (33).

intrinsic reaction rate per site participating in chain growth. As this statement, if correct, would pose an absolute limit. to the catalytic activity achievable by metal F T catalysts, a critical analysis of the implicit assumptions appears appropriate. These are:

1. CI is constant during the (CO + H,) pulse; 2. secondary hydrogenolysis can be neglected; 3. production of hydrocarbons during the H, pulse by hydrogenating a carbidic overlayer is ignored. TABLE 111 Kinetics of Fischer- Tropsch Reaction"*h

Pulse time Catalyst (min)

a

A

8

B

12

koverall

(sec-') 1.6 x -

kpropagation

ktermination

(sec-l)

(sec-')

1.4 x 1.6 x 1.6 x

10-2

7.3 x 1 0 - 4 8.3 x 8.3 x

From (33). Rate constants derived from transient state kinetics (cf. Fig. 7).

Space-time yield (mgml-' hr-l) 70

110

FISCHER-TROPSCH REACTION MECHANISM

187

With regard to Assumption 1, it may be argued that the surface coverage in ad-species builds up during the pulse. In consequence the rates of propagation and termination may both vary during the pulse, and this could result in a variation of 0: = k p / ( k p kt). With regard to Assumption 2, it is clear that a hydrogenation pulse may give rise to metal-catalyzed hydrogenolysis of the primary product, and there is recent evidence from infared spectroscopy (IR)that with Ru catalysts this actually occurs, at temperatures as low as 150°C (32). We are particularly concerned about assumption (3), as surface spectroscopy has recently shown that in addition to growing chains a substantial amount of carbidic carbon develops on the catalyst surface (35-37). While only part of it might be reaction intermediate under steady state conditions, it would be converted almost completely to methane and small amounts of higher hydrocarbons when the surface is exposed to hydrogen in the absence of co (37). Both hydrogenolysis and hydrogenation of surface carbon will distort the product distribution by an excess of lighter products, just as the transient effect does. The product pattern would, however, show characteristic minor differences. Hydrogenation and hydrogenolysis are expected to give a convex deviation from the Schulz-Flory line (i.e., an overshoot of lighter products, increasing with decreasing carbon number), while the interruption of chain growth by this pulse technique is expected to result in a concave deviation (i.e., an undershoot of “fully grown” chains increasing with increasing carbon number). The approach by Dautzenberg et al. emphasizes in our opinion a valid and hitherto hardly exploited inroad to the FT kinetics. In retrospect, with the knowledge now available from surface spectroscopy, an experimental verification of the complicating factors mentioned appears desirable. This would be valuable in particular because the main conclusion, viz. a high coverage of steady-state catalysts with growing chains, has not been confirmed by the in situ IR studies. discussed in the next section.

+

IV.

Surface Spectroscopy

The vast amount of work performed with surface sensitive spectroscopic methods, notably X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), has changed drastically our notion of the preferred modes of chemisorption of carbon monoxide on transition metals. Less than one decade ago Ford (38)in his authorative review still stated tungsten to be unique among the transition metals in being able to

188

P. BILOEN AND W. M. H . SACHTLER

break the strong CO bond, that is, to chemisorb CO dissociatively. Since that time UPS and XPS have proved that actually the majority of transition metals are able to d o so (39).With respect to the F T mechanism, this is highly relevant information, and it inspired Joyner (40) in 1977 to propose the participation of carbidic rather than oxygenated building blocks in the propagation stage of the FT synthesis. References to surface spectroscopic studies can be found in the aforementioned paper of Joyner and other recent articles (25, 41). Particularly interesting are the electron loss studies of Bertolini and Imelik (42) and the tunnel spectroscopic studies of Hansma et al. ( 4 3 , because they refer to coadsorption and reaction of carbon monoxide and hydrogen. In the present context attention will be focused on those studies performed with surface spectroscopy that dealt with hydrocarbon-producing systems. Broadly speaking these studies use either electron spectroscopy (UPS, XPS, and AES) or in situ IR as central technique, and accordingly the review is subdivided in two sections. In Section IV,C an integrating discussion will be attempted.

A. ELECTRON SPECTROSCOPY Electron spectroscopy involves the detection of electrons escaping from a catalyst surface under photon or electron bombardment. The conventional applications of these techniques therefore require the specimen to be situated Torr) or even lower pressure. The catalytic in a vacuum of lo-’ Pa ( % reaction thus has to be interrupted prior to spectroscopic analysis, so the information is confined to stable, that is, evacuation-resistant adsorbed layers present on the catalyst after interruption of the FT synthesis. Goodman. Kelley. Madey. and Yates (44, 45) used Auger electron spectroscopy (AES) to monitor ad-layer development on nickel; Dwyer and Somorjai studied iron and rhodium (35, 46) and Bonzel ef al. (36. 47) used both AES and XPS in a study of iron. The findings of the various studies are to a large extent corroborative. and we discuss in detail the work of Bonzel et al. to illustrate the findings typical for these studies. In the pertinent studies (36,47)an initially clean iron foil or iron (110)single crystal is being shuttled back and forth between a small microreactor. allowing for exposures to CO/H, at elevated temperature (460-750 K) and pressure (up to 600 kPa), and an XPS-AES spectrometer functioning at Pa base pressure. The reaction in the microreactor is allowed to proceed for some 100 sec. after which the sample is cooled in the CO/H, flow. and subsequently transferred to the vacuum tank of the spectrometer. After recording of the AES-XPS spectrum the sample is transferred back to the microreactor and the procedure repeated. This procedure yields a sequential

FISCHER-TROPSCH REACTION MECHANISM

189

series of catalytic rates together with a corresponding series of surface analyses of the vacuum-resistant ad-layer present on top of the catalyst upon interruption of the reaction. Figure 8 displays the rate of methane production found in the successive runs and Fig. 9 the corresponding series of XPS carbon C (1s) signals. Characteristic for the catalytic results with initially clean samples is an initial sharp rise in catalytic activity. Parallel with the initial rise in catalytic activity there is a sharp rise in the C (1s) signal, testifying to the rapid buildup of a carbon-containing overlayer (compare the initial CO, production observed by Araki and Ponec (48), and discussed in Section V). Further surface analysis shows this overlayer to be rich in hydrogen and poor in oxygen (OjC atomic ratios below 0.1). Detailed XPS and AES line shape analyses (47) suggest this adsorbing layer to consist of a family of surface species, ranging from amorphous surface carbon to mono- or polymeric CH,yspecies. When referring below t o this ad-layer as “carbidic carbon” we have in mind the complete family, including the partly hydrogenated species CH,. The initial sharp rise in catalytic activity is followed by a slow decline (Fig. 8). Decline rates are found to increase with increasing CO/H, ratio and increasing temperature. Surface analyses show a slow decrease in catalytic activity t o be reflected in a slow increase in carbon deposition. This carbon differs from the carbidic carbon, as shown convincingly by the NBS group CH,

01 0

TURNOVER NUMBER,

I00

I

I

200

300

5eC-l

I

400

I

I

500

600 t , sec

T = 560 K, and (0) T = 506 K. FIG.8. Turnover number versus time. ( 0 )T = 630 K, (0) From (36).

190

P. BILOEN AND W. M. H. SACHTLER INTEGRATED C ( I s ) - P E A K ARB UNITS

INTENSITY,

15

10

0

5

t 00

I

1

I

I

100

200

300

400

I

I

500 600

I 700

t , sec FIG.9. Surface carbon concentration as measured by XPS versus time (0) T = 560 K and ( 0 )T = 506 K. From (36).

(44. 45), in being completely dehydrogenated, crystalline, and graphitic in nature. Although the other electron spectroscopy studies involved different substrates and reaction conditions, the observations are mutually consistent, and can be recapitulated as follows: 1. Hydrocarbon synthesis over initially clean transition metal surfaces leads to the rapid buildup of an overlayer of carbidic carbon, often concurrent with an initial rise in the catalytic activity. 2. Depending on the reaction conditions, primarily on the CO/H, ratio, the carbidic carbon overlayer reaches a “steady-state’’ thickness, which may well exceed one monolayer. 3. The carbidic carbon is heterogeneous in nature, encompassing Cads and CH,,, and perhaps also CH2,ads, CH3,ads,and polymeric units involving C-C bonds. 4. The carbidic carbon is much more reactive towards hydrogen than graphitic carbon. When CO is removed from the CO/H, feed, methane and minor quantities of higher hydrocarbons are being produced at a rate substantially exceeding the rate of hydrocarbon production from the CO/H, feed proper. 5. Concurrent with the decline in catalytic activity, which follows the initial rise, graphitic carbon is being formed. Although a parallel pathway cannot be precluded, it originates most likely from carbidic carbon via a

FISCHER-TROPSCH REACTION MECHANISM

191

sequential reaction. Its rate of formation depends primarily on the temperature, higher temperatures favoring higher formation rates. These results lead different authors to draw essentially identical conclusions: (1)carbidic carbon may well be an intermediate in the F T synthesis reaction, and (2) graphitic carbon acts as a poison. In Section 1V.C we shall discuss these findings further.

B. In S i t u INFRAREDSPECTROSCOPY In the early 1960s in situ IR experiments were performed by Blyholder and Neff (49, 50). They studied the synthesis over Si0,-supported iron at C O plus H, pressure of approximately 3 bar and temperatures up to 180°C. IR revealed the in-growth of C-H bands during the synthesis (49), and the authors assigned these IR bands to reaction intermediates. With silicasupported nickel similar bands could not be observed, despite the gas phase products being detectable (50). More recently, Dalla Betta and Shelef (51) performed in situ IR measurements with Al,O,-supported ruthenium, exposed to 1 bar total pressure mixtures of H, :CO:He = 0.075:0.025:0.9 at temperatures from 250°C upward. U p to 250°C adsorbed CO was present at almost complete monolayer coverage. At higher temperatures the coverage decreased. As the phenomenon is irreversible with respect to a lowering of the reaction temperature, the authors conclude that it reflects surface blocking by a reaction residue rather than a temperature dependence of the carbon monoxide adsorption-desorption equilibrium. Hydrogen induces a decrease in the carbonyl stretching frequency of some 40 cm- At high temperature there is a further irreversible decrease, which the authors relate to a modification of the metal-CO interaction by surface residues. Exposure to CO/H, at 250°C results in IR-detectable adsorbed species. Bands at 1585/1378 cm-' and around 3000 cm-' are assigned to formate and hydrocarbon ad-species, respectively. The authors conclude that these surface species are not reaction intermediates. This very important conclusion is based on the observations that (1) the infrared bands continue to grow in intensity even after the FT reaction has reached steady state, and (2) substitution of D, for H, does not result in a significant rate of decrease in intensity of H-derived vibrations (Fig. 10). Ekerdt and Bell ( 3 7 ) likewise performed in situ 1R at subatmospheric pressures of CO/H,, contacted between 190 and 275°C with Si0,-supported ruthenium. Their results differ from those of Dalla Betta and Shelef (51) with respect to the behavior of the C O band (positioned at 2030 cm-',

'.

192

P. BILOEN AND W. M. H. SACHTLER ABSORBANCE

000

3000

2000

1600

1200 FREOUENCY, cm-1

FIG. 10. Isotope substitution during reaction, 57< Ru/AI,O, at 523 K : (a) background; (b) H, + CO for 6 hr; (c) substitute D2 for H, for 1 hr; (d) after 6 hr. From (51).

characteristic of linearly bonded CO) which (37) does not shift to lower frequencies when hydrogen is present. Both groups of authors agree. however, in the conclusion that the hydrocarbon adsorbing species developing during synthesis, as evidenced in the IR spectra by bands slightly below 3000 cm-', are not reaction intermediates, as follows from the substitution of D, for H,. Upon removing C O from the reaction mixture during steady-state operation, and this is one of the highlights of Ekerdt and Bell's (37) study, the production of methane and ethane continues for a significant period after the disappearance of all IR-observable CO,,, (Fig. 11). It therefore emerges, and this is the main conclusion of the authors, that during steady-state operation a reservoir is built up equivalent to several overlayers of carbon; upon hydrogenation this overlayer is readily converted to methane and higher hydrocarbons. The authors propose, therefore, that carbidic intermediates are relevant in methanation and FT synthesis, and they show that the assumption of hydrogenation of a carbidic intermediate being the slow step leads to consistent kinetics (cf. Sections I11 and VI). King (52) performed in situ IR spectroscopy of C O hydrogenation over S O , - and Al,O,-supported ruthenium and over SO,-supported iron. Compared to the two previous studies (37, 51) this author applied considerably higher pressures: up to 3 bars. With all the catalysts a substantial amount of C-H derived bands in the 3000 cm-' region developed. No evidence such as deuterium substitution has been presented to prove or disprove that these species are reactive intermediates.

FISCHER-TROPSCH REACTION MECHANISM

0

10

20

193

30

t,

rnin

FIG.11. Relative rates of methane and ethane production upon removal, at t = 0, of CO from the H, CO feed. Two experiments are being depicted, with steady state ratios of H,/CO of 6.0 ( 0 )and 3.1 (A).(Arrows indicate the time at which the CO band absorbance goes to zero.) From (37).

+

In short, it follows from in situ IR that: 1. Oxygenated building blocks such as

if at all present, have too low a steady-state concentration to be detectable by IR. 2. Under the low-pressure (and low-cc) conditions prevailing in the studies (37, 51), neither carbidic intermediates CH, nor growing chains are detectable by IR. 3. Fischer-Tropsch synthesis is accompanied by substantial buildup of reactive (“carbidic”) carbon, which does not give rise to C-H bands observable in IR.

194

P. BILOEN A N D W. M. H. SACHTLER

The implications of these findings, together with those of electron spectroscopy, will be discussed further in the next section.

C. DISCUSSION Although we will not attempt to place the findings of surface spectroscopy in a more general context until Section VI, we shall consider here some aspects concerning the compatibility of electron spectroscopic and IRderived findings, and moreover discuss to what extent conclusions regarding the role of carbidic intermediates can be drawn when considering these findings in isolation. There is a gratifying agreement between the results of electron spectroscopy and IR in that both indicate substantial buildup of carbon during the synthesis reaction. The existence of the carbidic overlayer, and its variation with the partial pressure of the reactants, in particular, hydrogen, can no longer be ignored when discussing the reaction kinetics or comparing the activities of various transition metals. Incidentally, the same conclusion was reached years ago by Otto Beeck (30) in the context of transition metalcatalyzed conversion of hydrocarbons; we note in passing that also in this field there is a recent upswing in studies concentrating on the structuresensitive aspects of ad-layer development (53). Whereas electron spectroscopy and IR both indicate the amount of oxygen in the adsorbing layer to be low, there is less consistency with regard to the amount of hydrogen. This remark applies specifically to the carbidic carbon, defined in this context as that part of the overlayer which easily hydrogenates. The aforementioned definition of carbidic carbon is not a very precise one, as shown by the temperature-programmed reduction work of McCarty and Wise (54).This inaccurate definition reflects the nature of the problem: the overlayers observed are heterogeneous and the reactivities reported are hard to compare. From XPS-AES lineshape analysis Bonze1 and Krebs (47)conclude that the carbidic carbon is rich in hydrogen, with a H/C ratio 2 1. I n situ IR, involving ruthenium instead of iron, indicates C-H bonds in the carbidic layer to be absent, and also fails to monitor an adsorbing layer of growing hydrocarbon chains (cf. 37, 51). The in situ IR studies (37, 5 1 ) have been conducted under conditions where a, characteristic of the probability for chain growth, was low. This may be one reason why growing chains were not observed. In this context it is regrettable that in the work reported by King (52),which was conducted under conditions favoring higher a values and led to abundant production of IR-observable C-H species, the reactivity check with deuterium was not performed, at least not reported. Further IR work at high-a conditions appears desirable.

FISCHER-TROPSCH REACTION MECHANISM

195

No trace is found (37,51)either of the C , intermediates, that is, the species Y, , and X defined in Eq. (7).Bell (37)considers equilibria of the type the plausibility of which is supported by the evidence to be discussed in Section V. The absence of IR bands characteristic of CH, might then be due to one of the following factors: 1. the C-H-derived vibrations in the overlayer directly attached to the catalyst surface are broadened beyond detection ; 2. the equilibrium (34) lies far to the left under the conditions prevailing in the IR studies in (37)and (54, or; 3. only a very small fraction of all surface carbon present actually participates in Reaction (34), the remainder being of a different nature. With regard to Possibility 1 the present authors, having limited IR experience, would gauge this to be a rather remote possibility. With respect to Possibility 2 we remind the reader that the studies of Eckerdt and Bell (37) and Dalla Betta and Shelef (51) were conducted at a rather high HJCO ratio, as is also reflected in the low values for the parameter ct (37).Low values of GI,corresponding to a relatively high methane production, are suggestive of a high degree of hydrogenation of the intermediates. Without further evidence Possibility 3 would therefore seem to be the most plausible one. Possibility 3 should be considered against the observation (37, 47) that in the absence of CO hydrogen readily converts “reactive” surface carbon in amounts that correspond to a monolayer or even more (37).This raises the question whether the hydrogenation of surface carbon, despite its capacity to discriminate between graphitic and carbidic surface carbon, is not too crude a reactivity test, leading to the overestimation of the role of carbidic carbon in methanation and the FT reaction. This aspect is one of the central issues in the next section, in which the conversion of carbidic surface carbon will be considered in more detail. V.

Hydrocarbon Synthesis via Predeposited Carbidic Carbon

Surface spectroscopy, as discussed in the previous section, has recently provided evidence that concurrent with the hydrocarbon synthesis a reactive “carbidic” overlayer develops, and that this reactive overlayer may contain the intermediates operative in the FT synthesis. However, the claims of surface spectroscopy regarding the relevance of carbidic intermediates have had a precedent in one of the very first publications of Fischer and Tropsch

196

P. BILOEN AND W . M. H . SACHTLER

(55).The uptake of considerable quantities of carbon from the reactant CO,

as evidence by a weight gain of the iron catalysts, led Fischer and Tropsch to suppose that a carbon-rich iron carbide, stable only at low temperature, constituted a n intermediate in the synthesis reaction. In subsequent years considerable effort was devoted to testing the reactivity of various carbides, culminating in the pioneering experiments with radioisotopes performed by Kummer, de Witt, and Emmett (21). In these experiments, reviewed by Pichler (2),it was found that in general the “artifically produced” carbides had too low a reactivity to be plausible intermediates. The carbide hypothesis therefore fell in disgrace, and was gradually supplanted by mechanistic proposals in which chain growth occurred via elimination of water from hydroxycarbene groups (10):

*

*

*

*

or via instertion of C O (2): R

R

I *-co

I I +*

c=o (36)

In this context it is worth noting that the majority of the carbides which proved irreactive in hydrogenation tests has been produced at a temperature exceeding that of the FT synthesis. Although Eidus (22) still adhered to the concept of C H 2 groups featuring in chain growth, the debate seemed to be decided, with CO insertion being the winner (20).In particular the radiotracer experiments (21),demonstrating that ‘‘CO-ex-’4COCH, did participate in chain growth, whereas I4CH2ex-CO 14CH, did not, were until very recently considered as rather conclusive evidence against participation of carbidic intermediates in chain growth. The issue of the role of carbidic carbon was reopened, however, by the simultaneous findings of Wentrcek, Wood, and Wise (56)and of Araki and Ponec (#8).Both groups deposited carbon on top of nickel at temperatures comparable to those prevailing in the FT reaction, via the Bondouard disproportionation reaction: 2CO+CO,

+ Cads

(37)

This “low-temperature’’ deposited carbon proved to be very reactive toward hydrogen, yielding methane. These studies (48, 56) triggered a number of other investigations, including some with surface spectroscopy. The sum

FISCHER-TROPSCH REACTION MECHANISM

197

total of the results has completely changed the outlook on the mechanism not only of methanation, but also of the FT synthesis. Whereas efficient methane production from surface carbon and the accumulation of surface carbon during the FT synthesis are indicative of a possible role of carbidic intermediates in the F T synthesis of higher hydrocarbons, there are several plausible alternatives. For instance, it would be compatible with the aforementioned findings to assume that carbidic carbon acts as an intermediate in methanation and as an initiator in FT synthesis (57), and that it plays only a minor role in the propagation stage of F T synthesis. Relevant in this context is the well-established fact that in many FT reactions methane is produced in quantities considerably in excess of those predicted by extrapolating the Schulz-Flory distribution of the higher hydrocarbons (20). Furthermore, we recall (cf. Section IV,C) that during steady-state hydrocarbon synthesis “reactive” (i.e., reactive vis-a-vis hydrogen) carbon often accumulates in quantities exceeding considerably the equivalent of one monolayer (37).What should be investigated, therefore, is how Cadscompares with CO,,, as competitive source of methane and higher hydrocarbons. This is the central issue of the work reviewed in this chapter, which is subdivided into three sections:

A. Hydrogenation of surface carbon: B. Hydrogenation of mixed overlayers of surface carbon and CO,,,: C. Hydrogenation of labeled surface carbon in CO/H2 atmosphere. A. HYDROGENATION OF SURFACE CARBON We shall here discuss studies in which carbon is first deposited on the surface of transition metal catalysts; after subsequent removal of coproducts of the deposition the carbon-covered surface is exposed to hydrogen. The C O disproportionation reaction (37) has been used as the carbon deposition pathway in the majority of studies (48, 54, 56, 58-60). Carbon has also been deposited via hydrocarbon hydrogenolysis (54, 62-63) and, notably, via the FT synthesis reactions (35-37, 45). Particularly the initial studies involved nickel substrates: ruthenium, cobalt, and iron were also studied. It has been established that the reactivity of surface carbon toward hydrogen is determined primarily by the temperature of carbon deposition. Deposition temperatures well above 250°C and heat treatments of lowtemperature deposited carbon at these temperatures lead to rapid deactivation of surface carbon (45, 64). In retrospect this observation reconciles the older findings of nonreactive, high-temperature produced carbides (2) with

198

P. BILOEN AND W. M. H . SACHTLER

the newer ones of reactive surface carbon. Temperature-programmed reduction studies further revealed that it is easy to produce a carbon layer with heterogeneous reactivity ( 5 4 , to be compared with the AES-XPS observed heterogeneity in structure of the ad-layer (47). In the absence of CO the predominant product of the hydrogenation is methane. The first of the recent proposals regarding the role of carbidic intermediates therefore related specifically to methanation (48, 56). In subsequent studies (36,37,58,61),however, it was found that smaller quantities of higher hydrocarbons, up to butane, are being coproduced (cf. Fig. ll), especially at low temperature, and/or low hydrogen pressures, stimulating the authors to propose the participation of carbidic species in chain growth. The above-mentioned evidence relating carbidic surface carbon to methanation and to FT synthesis raises the question how the reactivities of COadsand Cads compare. This additional information, discussed in the following two sections, is indispensable for deciding whether or not C - 0 bond breaking is a slow step in the synthesis, and whether or not hydrogenation of carbidic carbon is kinetically significant in the presence of CO, that is, in the FT synthesis proper. OF MIXEDOVERLAYERS OF B. HYDROGENATION SURFACE CARBON AND toad,

Wentrcek, Wood, and Wise (56)performed experiments with mixed overlayers of Cads and CO,,, on nickel, and Rabo, Risch, and Poutsma (58) extended these measurements to include cobalt and ruthenium. In the experiments of Wentrcek, Wood, and Wise, CO was pulsed at temperatures around 250°C over Al,O,-supported nickel. Part of the CO disproportionated via Eq. (37) to Cadsand CO,, as testified by the CO, in the outlet pulse. By monitoring the amount of CO consumed and the amount of C O , produced it was possible to quantify the amount of toad, and Cads coexisting on the nickel surface directly after the CO pulse. The relative amounts of these two ad-species could be varied by pulsing CO repeatedly. When subjecting the nickel catalyst, covered with CO,,, and Cadsin various proportions, to hydrogen pulses at 250°C it was found that the amount of CH, produced closely corresponded to the amount of Cads present rather than to the amount of C o a &present (Table IV). This finding strongly suggests that at 250°C in pure hydrogen the reactivity of nickel-supported Cadsexceeds considerably the reactivity of nickel-supported Coa&(note, however, the results (58)discussed below; see also Table V). According to this finding C - 0 bond breaking is a slow step in nickel-catalyzed methanation; the authors , however, conclude “that dissociative chemisorption of CO by

199

FISCHER-TROPSCH REACTION MECHANISM

TABLE IV Interaction of Hydrogen with Surface Carbon on NilAI,O, Catalysto at 553 K b Surface species deposited (mol x l o 6 )

Gaseous products (mol x lo6)

Pulse‘

co

C

cc

CH,

CO,

co

co co

1.15 0.41 0

1.34 0.67 0

1.31 2.01 0.05

0 0 1.96

I .34 0.67 0

1.90 3.98 0

0.56 0.27 0

1.40 0.67 0

1.45 2.12 0.02

0 0

1.40 0.67 0

2.37 4.12 0

0.67 0.13 0.17 0.20 0.17 0.31 0

1.23 0.64 0.47 0.39 0.35 0.32 0

1.23 1.87 2.34 2.73 3.08 3.40

1.23 0.64 0.47 0.39 0.35 0.32 0

2.60 4.32 4.62 4.75 4.86 4.78 0

H2

co co H,

co co co co co co H2

Mass of catalyst: 13.7 mg. From (56). CO pulse = 5.73 x mol; H, pulse

0.01

=

5.73 x

2.10 0

0 0 0

0 0 3.39

mol.

contact with a nickel surface represents an energetically possible mechanism for the formation of the surface carbon intermediate in methanation catalysis” (56, p. 365). Rabo, Risch, and Poutsma (58) extended these measurements to cobalt and ruthenium, and moreover varied the temperature of the hydrogenation. Their results clearly demonstrate that at room temperature Cads is much more reactive then toads. However, at higher temperatures both Cadsand co,d,are converted (Table V). In their concise discussion the authors state: “The pulse experiments at 200-300°C suggest that surface carbon is more reactive than non-dissociated CO. No conclusive proof regarding the relative reactivities of these two species could be obtained, however, because both of these species are present, and they both react with hydrogen in this temperature range” (58,p. 307). As we know that, whatever the rate may be, CO converts under actual synthesis conditions, this remark raises the crucial question whether it is CO,,, or Cadsthat is the source of the reaction products in actual synthesis. The one and only way to settle this question is to use isotope labeling to discriminate Cadsfrom toad,. The relevant experiments will be discussed in the final section.

200

P. BILOEN AND W. M. H . SACHTLER

TABLE V CO and H , Pulsed over 2.7%NilSiO, (Caialysi C)".b ~~

Gas pulsed

co

H, H, H, (two pulses)

co

H2 H2 H, (two pulses) CO (two pulses)

H2 H2 H2 (two pulses)

Catalyst temperature ("C)

Products (% of monolayer)

200

285 CO 21 .o co, 21 .o Cad, 37.0 CO,,, 36.0 CH, 13.0 CH, 1.2 CH, 282 CO 25.0 CO, 25.0 Cads 32.0 CO,,, 34.0 CH, 13.0 CH, 5.4 CH, 604 C O 42.0 CO, 42.0 Cads 40.0 CO,,, 49.0 CH, 15.0 CH, 1.6 CH,

200 200 200 200

200 200 200 200

200 200 200

Surface layer (% of monolayer)

58.0 22.0 9.0 1.8

57.0 23.0 10.0 4.9

82.0 33.0 18.0 10.1

' From (58). Pulse size, 364% of a monolayer.

C. HYDROGENATION OF LABELED SURFACE CARBON IN CO/H, ATMOSPHERE In order to expose clearly the nature of the problem [cf. citation from (58) in Section B], we did not always follow the chronological order of events. though being fully aware of the fact that the isotope studies of Araki and Ponec (48) were the first to compare the relative reactivities of Cadsand COad,,while the results by Wentrcek, Wood, and Wise convincingly proved the reactivity of surface carbon (56). Araki and Ponec used a continuous flow of CO/H,, with nickel films as the catalyst. When exposing initially clean nickel films to CO/H,, they observed that the methane production had a n induction period and was preceded by CO, evolution (Fig. 12). This led the authors to suspect that the order of events during the reaction was as follows:

cod,

-k

Oads

(38)

FISCHER-TROPSCH REACTION MECHANISM 0

w

20 1

~ 1 0 MOLECULES ' ~

LL

0

10

20

30

40

t, min

FIG. 12. Number of molecules formed of CO, (disproportionation in presence of H,) and CH4 (methanation) from the reaction mixture H,/CO, as a function of time. From (48).

followed by : Cads % CH4

(39)

o,,, =co,

(40)

and that is, an order of events in which CO dissociation precedes the attack of hydrogen. In retrospect the low pressures applied in these studies were rather fortuitous, because at higher pressures the pathway O,

% H,O

(41)

is known to prevail over Reaction (40). The finding that at low pressures Pathway (40) prevails is suggestive of Pathways (40)and (41) occurring via a Langmuir-Hinshelwood and Rideal-Eley mechanism, respectively, or, alternatively, of Pathway (40)being a transient phenomenon (cf. Section VII). In order to substantiate their ideas Araki and Ponec precovered their nickel films with '3Cads,and exposed the precovered catalysts to 2CO/H,. They observed that the evolution of l3CH4precededtheevolution of I2CH4 and " C 0 2 (Fig. 13). From this finding they concluded that the reaction pathway via Cadsis preferred.

202

P. BILOEN AND W. M. H. SACHTLER x IO'%notecutcs,

x 1OI5rnoleculas, "CH4 I2C02

FORMED

FORMED

t(rnin1

FIG.13. Number of molecules formed of compounds with indicated composition as a function of time. Ni film had been saturated by "C, formed by disproportionation of I3CO, then the reaction of l2C0and H, on the same film was followed. From (48).

When this work was subjected to a critical analysis, three points emerged which deserved further inspection. First, the results underlying Fig. 13 had been obtained at high precoverages with 13Cads,that is, close to one monolayer. One can argue that under these circumstances the adsorption of l 2 C 0 is inhibited by the almost complete precoverage of the catalyst surface with 13Cads.Removal of 13Cads(as 13CH,) then must precede catalytic hydrogenation of l 2 C 0 because free catalyst sites must be created first. Second, Araki and Ponec used very low pressures. When comparing the rates of two simultaneous parallel reactions, a small difference in reaction orders would suffice to render the reaction that is negligible at low pressures kinetically predominant at pressures three orders of magnitude higher. Therefore, the possibility could not be ruled out that under actual FT synthesis conditions a reaction via undissociated CO prevails. Finally, Araki and Ponec's results were confined to methane, and so did not provide information on the possible participation of carbidic carbon in the synthesis of higher hydrocarbons. These arguments induced Biloen et al. (59) to consider the deposition of

203

FISCHER-TROPSCH REACTION MECHANISM

submonolayers of 13Cads, to contact them with "CO/H, at (super)atmospheric pressures and to analyze both methane and the higher hydrocarbons for their 13C contents (59, 65, 66). SO,-supported nickel, cobalt, and ruthenium catalysts were precovered with submonolayer amounts of 3Cadsvia "CO disproportionation according to Eq. (37). Prior to the C O disproportionation the metal surface areas were being measured by hydrogen chemisorption, and dispersions were occasionally verified by measurement of the X-ray line broadening. 3COad, still residing on the catalysts after Reaction (37)was removed by flushing with excess "CO. A mass balance, counting the 1 3 C 0 molecules originally admitted and those removed unchanged by "CO flushing, was made in some experiments and revealed that the number of 1 3 C 0 molecules consumed during disproportionation was twice the number of 3C0, molecules produced, in accordance with the stoichiometry of Eq. (37). The accuracy of this material balance was such that the numbers of 13C0, molecules formed agreed within 3% with the number expected from the 1 3 C 0 consumed by disproportionat ion. Some 5 gm of supported catalyst were used, which exposed some 10'' metal atoms to the gas phase (measured by in situ chemisorption), on which an amount of 13Cadsvarying between 1 and 8 x 10'' atoms was deposited. The catalyst was contacted with a batch ofsome 2 x 10'' "CO and 6 x 10'' H, molecules, and the synthesis reaction was allowed to proceed until some 1 x 10' hydrocarbon molecules had been formed. With mass spectrometry it was then verified whether these hydrocarbon molecules has been formed reservoir. A reactor with a minimized from the 3Cadsor from the lZCO,ads) holdup of some 10 ml was used, -4 ml of it being catalyst pore volume. The reactant pressure initially exceeded 1 bar, while the number of reactant molecules was of the same order as the number of predeposited 3Cadsatoms. It was found that the methane in the product contained significant amounts of 13CH,. Table VI gives the results for five repeated exposures of a sample of 13Cad,-prec~vered Ni-SiO, to batches of "CO/H,. The I3Cadscoverage decreases from an initial value of 0 = 0.50 to a final value of 0 = 0.036. The results of this and a similar series have been plotted in Fig. 14 as percentage I3CH, in CH, versus OlsC,as (approximated as $(0,,,,, Ofinal). Essentially straight lines are observed, with slopes close to 45". From the linearity ofthese lines it is concluded that the predeposited 13Cadsis homogeneous as to its reis the fraction of the surface which activity. Considering that 1 - f313c,d8 under the conditions of the experiment is accessible to "CO, it is concluded 13CH,)] that from the numerical equality of 013c,a,and [13CH,/(12CH, 13Cadsand lZCOadsare "kinetically equivalent." For instance, if 60% of the surface sites is covered with 13Cadsand 40% is available to "CO adsorption,

'

+

+

204

P. BILOEN AND W. M. H. SACHTLER

TABLE VI Production of I3CH, with Varying 3Cad,Abundance".b

Series 20 batch

ITads

ITsds

I

(start)'

(end)'

(sec)

I3CHqC

''CHqC

59 29

29 18 12 6.7 4.3

480 660 720 960 1 I40

21 6.5 3.0 2.2

20 20

1.o

17

1 2 3 4 5

18

12 6.1

18 18

' From (59). ("Cad,-start) - (I3C,,,-end) # "CH, because the balance of is present in the higher hydrocarbons. Number of surface-exposed Ni atoms = I20 x Expressed in 10" atoms (molecules).

60% of the methane found is 13CH,. Implicit in this conclusion is the assumption that e12c. x 1 - e13cads (424 and "12COads" could result from rapid oxygen Identical reactivity of Tads scrambling : 13c,,

+ 1 2 ~ 0 , ~ + wo,, + 12cd8

(42b)

807060 50 40 30 20

-

10-

01 0

I 01

I

0 2

I

03

I

04

I

05

I

06

I

07 9

I3C

FIG.14. Percentage of I3CH, in total CH, versus f?~3c.0t,c is defined as (number of "C atoms present)/(number of surface-exp6sed N i atoms). The numbers in parentheses correspond to the serial numbers used in Table VI. From (59).

205

FISCHER-TROPSCH REACTION MECHANISM

but this possibility was ruled out on the basis of additional observations. It was therefore concluded that in the experiment lZCOadsand I3Cadswere being rapidly converted into a “common” intermediate, which reacted further to methane in a slow step:

It thus follows that methanation proceeds via rapid formation of a carbidic intermediate CH,, which is converted relatively slowly to methane: CO,, *CH,

*CH4

This result, obtained at high pressure, supports the propositions of Araki and Ponec (48).It should be noted, however, that the experiment gives no information on the nature of Step 1 (CO dissociation versus H-assisted C - 0 bond breaking) or on the value of x. Surprising in the light of the literature existing at the time was the finding (59) that I3C atoms were also present in the higher hydrocarbons. A significant amount of product molecules actually contained several 13C atoms in their molecular skeleton (Table VII). TABLE VII Isotopic Composition of Methane and Higher Hydrocarbonsavb ~

CH, Series Batch 11

5 20

fLc

0

C2Hb 1

0

1

C3H* 2

0

1

16 32 61 63

30 34 26 26

2

C4H10

3‘

1

0.62

44’ 56

2 4 5

0.50 0.32 0.24

42 58 76 24 82 18

45 48 61 75

1

2

0.36 0.27

38 62 54 46

49 30 21 51 30 19

31 27 28 11 55 26 14 6

2 3

0.20 0.13

75 25 86 14

60 23 17 78 22 -

61 25 73 21

46 9 39 13 29 10 25 -

1

2

3

4

13

-

-

-

37 17‘ 27 7 13 11 -

6 -

14

From (59).

* 4 wt % NiiSiO,;

0

T = 170°C; P = 1 bar; H,/CO molar ratio = 3. Number of 13C atoms per molecule. Data given in mole percent. 17 mol% of all C3H, molecules are 13C3H, molecules.

38 28 21 48 26 24

206

P. BILOEN AND W. M. H. SACHTLER

’Ihe higher hydrocarbons were predominantly straight-chain ones, with a close to linear Schulz-Flory chain length distribution. Therefore, it was concluded that the carbidic intermediates CH, leading to methane [cf. Eq. (43)] were also participating in chain growth. Moreover, the extent of 13Cads incorporation into the longer chains was, just as with methane, indicative of 3Cadsand “12COads”having essentially identical reactivities with respect to chain growth :

It follows that the chain growth in FT synthesis occurs via incorporation of carbidic intermediates CH,, originating from CO in a kinetically fast step:

According to this work the insertable C, species X of Eq. (7), denoted in Section I1 with the general formula C,H,O,, does not contain oxygen, that is, y = 0. It should be noted that the aforementioned conclusions are based on the results of transient state experiments, that they contain the implicit assumption that the total surface coverage in reactive intermediates was close to unity (essential only for the kinetic analysis), and that the inference that rapid oxygen scrambling (Reaction 42b) does not take place is crucial for the interpretation of the results (59,65).

Vi.

Fischer-Tropsch Synthesis via Carbidic intermediates

On the basis of the results that have been reviewed here, although not exhaustively, we will attempt to formulate a synthesis scheme which, on the strength of what has been presented so far, involves carbidic species CH, as the building blocks for higher hydrocarbons. We shall consider in particular to what extent the propagation can be detailed, whether it is plausible that oxygenated intermediates are involved, for instance, as precursors of the carbidic ones, and to what extent the available evidence allows any inferences to be made about sites, steady-state coverages, and intrinsic activities. We shall also briefly discuss the formation of oxygenates.

FISCHER-TROPSCH REACTION MECHANISM

207

A. PROPAGATION/CHAIN GROWTH As mentioned in Section 11, chain propagation makes use of the carbon building block C,H,O,, which, on the basis of the evidence presented, may be written as CH,: CO,,, +CH,

+Y*

lY;+

(47)

(cf. Reactions 7 and 46). It is of interest to analyze the available evidence to obtain information on the value of x,that is, to find out how many hydrogen atoms are attached to the carbon atom forming a C-C bond with the terminal carbon of the adsorbed chain, Y,*. It is clear that a value of x = 2 would permit a very simple formulation of the propagation step, attachment of a CH, unit to a C,H,,+ radical being the easiest way to create the homologous C,H,,+ radical with m = n 1. This is the most obvious way to formulate propagation reaction (6), converting Y, into y,+ as one elementary step. The presence of CH, groups on the catalyst surface had been postulated by Fischer and Tropsch (55,67) and by Craxford and Rideal(I1) when they proposed their carbide and surface carbide mechanisms. The first experimental evidence for the possible participation of surface CH, groups in F T catalysis was presented in 1940 by Eidus and Zelinskii ( 2 3 , who added benzene to the synthesis gas feed and identified toluene in the reaction product. When discussing the formation of alcohols, Eidus (23)argued that the most probable process is the stepwise attachment of methylene radicals, -CH,-, to the end of the growing chain. When, however, an oxymethylene group HCOH is added to the chain, the latter stops growing, while upon adding a hydrogen atom it will be desorbed as a primary alcohol molecule. A different type of argument in favor of CH, being CH, can be extracted from results obtained by Pichler and Schulz and reported in Section 11. When adding n-14CH,=CH-C,4H,, to C O + H, passing over a cobalt FT catalyst these authors found (20)that only 6.3%of the 14Catoms showed up in fractions containing more than 16 carbon atoms (chains initiated by labeled a-olefin or having it inserted) and 14.3% of the 14C atoms in low molecular weight fractions (the remainder, 79.4%,was simply hydrogenated to the C,, paraffin). However, it is clear from Fig. 4, taken from their paper, that the molar radioactivity of the low molecular weight fractions follows a straight line, typical for participation of 14Cin the propagation (i.e.,insertion) step. We must conclude that there exists a mechanism by which the 14C atom is passed from the hexadecene molecules to a smaller species, which is subsequently inserted into growing chains.

,

,,

+

208

P. BILOEN AND W. M. H . SACHTLER

Schulz and Achtsnit (17) state that this transfer process cannot be a secondary cracking, as this would lead t o a different distribution pattern. Moreover, the absence of cracking under these conditions had been established separately (20). They, therefore, conclude that a metathesis reaction must take place. This is a very interesting suggestion, as the mechanism of olefin metathesis is well established. There seems to be general agreement in the literature on metathesis (68, 69) that the mechanism involves metal carbenes and a metallocyclobutane intermediate, for instance,

CHR,=CHR,

+

+

M=CHR, CHR,=CHR,

+ CHRZ=CHR,

+

CHR,......CHR, . : M ......CHR, 2CHRi=CHR2

CHRl +

CHR,

I/ + / ICHR, M

(49) (50)

The statement made by Schulz et al. (17) that a metathesis reaction (50) takes place therefore implies that carbenes must be present on the surface and can react with olefins under the conditions of the FT reaction. Once this is accepted, it follows that Eq. (48) provides a potential reaction path to transfer the 14C atom from a terminally labeled cc-olefin to the surface carbene. This evidence, therefore, suggests that x = 2, the insertion of surface carbene resulting in a linear increase of the molar radioactivity with the number of insertions, in agreement with the evidence in Fig. 4. A possible argument against the hypothesis that x = 2 might be derived from the classical experiments by Kummer and Emmett (21), who added labeled ketene to synthesis gas and found that the carbon atom of the CH, group in CH,=CO was apparently not inserted into the growing chain. However, more recent evidence (41) has shown that with higher ketene concentrations, insertion of the CH, group does occur. Even more convincing are very recent results obtained by Pettit (70), who reported on an experiment very similar to that performed by Kummer et al. ( 2 4 , however with CH,N, instead of CH,CO. When CH,N, at atmospheric pressure was contacted at elevated temperature with supported Fe, Co, and Ru catalysts, ethylene was found as the only product. However, with hydrogen added to the feed the catalysts converted CH,N, efficiently to unbranched hydrocarbons, with a close to linear Schulz-Flory chain length distribution. These findings are very suggestive of CH,,,,, being the species involved in initiation, that is, Yf = CH3,ads,and CH, being the unit inserted in the propagation step. We will return to Pettit's experiments in the context of termination and alcohol formation. The totality of these data is considered as evidence in favor of x = 2, that is, an insertion mechanism that can be visualized as cis migration, where

FISCHER-TROPSCH REACTION MECHANISM

209

an alkyl group forms a bond with a carbene group attached to the same metal atom; in other words, Eq. ( 6 )is written as (66) R

\ cHz\M’cH2

R

-+

\CH,--CH, / M

(51)

This reaction mechanism is completely analogous to that proposed by Gault and Muller (71) for the formation of a cyclic intermediate from diadsorbed hydrocarbon :

M

The latter authors assume that this reaction starts with a hydrogen abstraction. Gault and Muller write this process as follows:

It is, of course, possible to formulate Reaction (51) in exactly the same way, omitting simply the bridging C, unit in Reaction (53). While a number of variants can be visualized in order to even better emphasize the analogy with modern results in metal-organic complex chemistry, we confine ourselves here to the statement that a substantial amount of evidence suggests that the two carbon atoms forming a new C-C bond in F T synthesis or in the formation of cyclic hydrocarbons on transition metal catalysts: 1. do not carry oxygen; 2. are probably hydrogen-deficient ; and 3. are probably attached to the same metal atom. Elsewhere (66) it was mentioned that these criteria would encompass not only the two types of reaction discussed but also metathesis and Ziegler catalysis as is briefly illustrated in Table VIII, which is self-explanatory.

210

P. BILOEN AND W. M. H. SACHTLER

TABLE VIII Catalyzed C-C Bond Formations lnlrolving One Metal Atom" ~

Proposed mechanism

Reaction 1.

R

Ziegler-Natta (Cossee)

I

H2C

I

R CH2

I

+

I1

I

MaCH, 2. Methathesis (Chauvin)

H,C

CH,

I/ M

II CH2

HZC-CHZ M-CH2 H2C=CH2

-+

M=CHR

3. Dehydrocyclization (Muller and Gaul0

4. Fischer-Tropsch propagation (Biloen and Sachtler)

CH2-R

-t

H,C-CH2-R

\M

* From (66)

B. GENESIS OF CARBIDIC INTERMEDIATES The predominant role of carbidic intermediates in chain growth proper does not preclude a role of oxygenated intermediate, for instance, as the precursors of the carbidic ones:

toads + yHads

+

CHyoads

+

CHx.adn

+ HZo

(54)

If Reaction (54) is fast, this would be consistent with CH,O not being observed with surface spectroscopy. Essentially, the question is whether hydrogen attacks before or after rupture of the CO bond. As discussed in Section 111, it is not likely that this can be deduced from the overall kinetics of the F T reaction (cf. Section 111,A).Araki and Ponec (48) addressed themselves explicitly to this problem. Mainly on the strength of oxygen at their low-pressure conditions being removed as CO,, they came to the conclusion that the prevailing step is one of CO dissociation proper: +

+ Oads

(55)

with Oadsremoval at low hydrogen pressure taking place predominantly via Oads -k

toads --* coZ

(56)

FISCHER-TROPSCH REACTION MECHANISM

21 1

and at high hydrogen pressure via Oads + H , k ) +HZO

(57)

Another indicative result is the finding of Poutsma et al. (72) that the triad of noble metals Pd, Pt, and Ir, when exposed to CO/H, in the appropriate pressure/temperature domain, produces methanol rather than FT products. In the discussion of this surprising finding the authors point out that this triad has a low tendency toward CO dissociation. On the other hand, these very metals are also good hydrogenation catalysts. Therefore, their unique selectivity vis-a-vis CO/H, , with methanol rather than hydrocarbons being the product, is compatible, with Reaction (55) rather than (54)being the preferred pathway for the genesis of the carbidic FT intermediates. Finally, as pointed out by Joyner (40),surface spectroscopy provided substantial evidence that FT catalysts, at least when present in a clean state, easily dissociate CO at the temperatures prevailing in FT catalysis. There is, therefore, no necessity to involve a special role for hydrogen in C-0 bond breaking. Summarizing, we conclude the available evidence to be compatible with the genesis of carbidic intermediates via Pathway ( 5 9 , that is, via CO dissociation. To the extent that hydrogen overall enhances the formation of surface carbon it is plausible that its role is primarily one of removing Oads after the dissociation step proper (37,48).

C. COVERAGES, SITES,SELECTIVITIES, AND RATES One of the questions which remains to be answered is whether the conclusion from the experiments with labeled carbon that CO dissociation is a fast step (cf. Section V,C) complies with the IR observation that toad, is abundantly present at the catalyst surface (cf. Section IV,B). A possible solution is to assume that CO storage and CO dissociation take place on different sites (48,59).When FT catalysts such as Ni or Ru are alloyed with an inert metal such as copper, the activity decreases drastically. The alloy studies of Bond and Turnham (73) and Araki and Ponec (48) consistently indicate that the decrease in activity originates from a decrease in the preexponential rather than from an increase in the activation energy in the Arrhenius equation (48, 57, 73, 74). This is indicative of an ensemble effect (75), arising whenever a step in the reaction pathway requires cooperation of several contiguous metal atoms of the active alloy partner. There is positive evidence that hydrogenation and dehydrogenation reactions do not need this cooperation (76),whereas CO dissociation apparently does (48).Thus, the evidence is compatible with genesis of the carbidic species in FT occurring on large ensembles of surface atoms. This also explains why mononuclear

212

P. BILOEN AND W. M. H . SACHTLER

transition metal-complex-catalyzed Fischer-Tropsch synthesis has not been confirmed (77). In view of the foregoing it is visualized that the fast dissociation of C O occurs on the sites coordinated by several metal atoms. Only on these sites is the coverage in undissociated cO,& low. Possible candidates for irreactive CO storage are the on-top sites; indeed, the position of the CO band in IR, peaking above 2000 cm- is in agreement with such a location of CO,,, . With regard to the synthesis of oxygenates-as this is significant in particular for catalysts containing Fe as the active ingredient-it seems possible to rationalize the available evidence by assuming that alcohols and hydrocarbons have the precursor [Yf] in common. An oxygenate molecule originates whenever undissociated CO or an oxymethylene group is inserted, which then blocks further chain growth (22,58).The insertion of a C,H,O, group with y = 1 may occur either on the same sites as the insertion of CH,, or on different sites, for instance, oxidic patches. We recall that the original findings of Fischer and Tropsch pertain to the application of strongly alkaline compounds for the suppression of excessive alcohol formation (6, 55). Electron donors may well stimulate CO dissociation by donating electron density in the CO-antibonding orbital. There is a further parallel between pretreatments/reaction conditions inhibiting CO dissociation (preoxidation, high CO pressures, etc.), on the one hand, and enhancing alcohol synthesis, on the other. Furthermore, the chain length distributions of hydrocarbons and oxygenates, although no identical, match closely, which is suggestive of a common intermediate being involved (78). The assumption that cis migration of an alkyl chain to a CH,O, species would block further chain growth, is in good agreement with the absence of secondary alcohols in FT products. Storch (1) stressed the preferential formation of alcohols in the synthol process. Here conversion is low and most of the gas is recycled. Since this must have as a consequence that the H,/H,O ratio of the gas is rather low, it would be consistent with the view that oxidic patches are formed on the iron surface and hence, in agreement with Eidus’ view, the formation of alcohols by terminating the growing chain with oxymethylene groups will be an indirect result of oxidizing the catalyst surface.

’,

VII.

Suggestions for Future Research

In the preceding chapters we have attempted to integrate the available evidence in a consistent scheme of FT reactions on simple or “ideal” catalysts.

FISCHER-TROPSCH

REACTION MECHANISM

213

This review cannot be complete, however, without mentioning some significant gaps in our present understanding of this reaction. As mentioned, the nature of the slow steps, apart from termination, is still somewhat obscure. The kinetic equivalence of Cadsand co,&would indicate that CO dissociation is fast compared with the subsequent steps of propagation and termination, but the high concentration of cO,& observed in IR spectroscopy renders additional assumptions necessary. We hypothesized that the surface contains active sites, where CO dissociates readily, and less active sites, where C o a &is stored, but this hypothesis has to be verified. Further, it is not clear to what extent spectroscopic studies are characteristic for the steady state of the FT reaction. Kolbel emphasizes the necessity to “run in” technical catalysts for several days to make sure that they show steady-state behavior. The spectroscopic evidence of carbidic adsorbates corresponding to several monolayers and Pichler’s analysis of an active bulk Fe carbide are suggestive of what might happen during the “running-in” period. To the extent that the solid state reactions that transform the original sample into the steady state of a working catalyst are of the first order with respect to the participating gases, the run-in time will increase inversely to the partial pressure of that gas. Therefore, the phenomena observed during the first 10 min in an experiment carried out with a film at a few Torr might be indicative of what happens in the first few seconds-and thus escaping observation-in an experiment at atmospheric pressure. The surprisingly high CO,/H,O ratios in the initial product gas observed by Araki and Ponec at low pressures might be indicative of the processes during this period. Spectroscopic experiments carried out with fresh catalysts should then better be repeated with reliably “run-in” catalysts. If the dissociation of CO is fast, but the subsequent attachment of hydrogen atoms to the carbon atom is slow, a kinetic isotope effect is expected upon replacing H, by D,. Surprisingly, Dalla Betta and Shelef (79) report an isotope effect to be absent, but Kellner and Bell recently (80)found a significant isotope effect. It is interesting which difference between the experimental conditions of these two groups could have shifted the rate-controlling step. Elucidation of this issue appears highly desirable. There is also considerable uncertainty as to the fraction of the surface atoms that act as active sites. As mentioned, results that support a Taylor fraction near unity are opposed by other results from which this fraction is concluded to be small. As pulse experiments such as those done by Dautzenberg et al. are open to, among other factors, the criticism that the hydrogenation pulse could have introduced artifacts, it would be desirable to check the product distribution in experiments in which chemically, the steady state is well defined, but “pulses” are used where normal synthesis gas is replaced

214

P . BILOEN AND W. M . H . SACHTLER

by an isotopically labeled substitute. In this way the replacement of H, by D, or l 2 C 0 by I3CO would show how fast various populations interchange. These suggestions will pertain to the small group of “simple” catalysts. When including the industrially relevant “complex” catalysts with bulk carbidic, nitridic, and oxidic phases, a vast amount of work seems required to clarify the role played by the various ingredients. In particular, the role of alkaline promoters is to be elucidated. Surface spectroscopic work by Broden showing that the adsorption of C O is strengthened by the presence of potassium (81) is encouraging but has to be extended to conditions simulating the steady state, where-at high conversion-H,O in the gas phase is an oxidizing agent for zero-valent potassium and binary solid solutions of coexisting oxides are expected. Another class of problems of which only the surface has been scratched is offered by mixed systems in which a Fischer-Tropsch type catalyst is combined with a solid acid such as a zeolite. Such systems have been used in recent attempts to produce narrower product distributions, and indeed deviations from the normal Schulz-Flory distribution have been reported (82-84). However, at the closing date of this review it was still unclear whether the results are characteristic of the running-in or of the steady-state behavior of the catalyst. In particular, the selective retention of the heavier products within the pores of the support might falsify the apparent catalyst selectivity. Only accurate mass-balancing and/or steady-state data can provide information on true product patterns unspoiled by this running-in phenomenon. As it appears preferable to clarify first the better defined systems before including more complicating factors, much work has been concentrated on the simple catalysts, and this review has been focused on that type of work, in an attempt to enable the reader to use his or her own judgment as to what extent the basic questions have been solved and focusing on additional variables is justified. REFERENCES 1. Storch, H. H . , Adu. Card 1, 115 (1948). 2. Pichler, H., Adu. Catal. 4, 271 (1952). 3. Sabatier, P., and Senderens, J. B., C. R. Hebd. Seances Acad. Sci. 514 (1902). 4 . Orlov, E. I., Zh. Russ. Khim. 0-ua.,Chast a i m . 40, 1588 (1908). 5. D. R. P. 293,787 to Badische Anilin und Sodafabrik. 6. Fischer, F., and Tropsch, H., Brennst.-Chem.4, 276 (1923). 7. Pichler, H., Brennst.-Chem. 19, 226 (1938). 8. Pichler, H., and Merkel, H., “Chemical and Magnetochemical Study of Iron Catalysts,”

Spec. Rep. U.S. Bur. Mines, Washington, D.C., 1947. 9. Ohya, A., Urabe, K., Aika, K., and Ozaki, A , , J . Catal. 58, 313 (1979).

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Wentrcek, P. R., Wood, B. J., and Wise, H., J . Caial. 43, 363 (1976). van Barneveld, W. A. A,, and Ponec, V., J . Catal. 51,426 (1978). Rabo, J. A,, Risch, A. P., and Poutsma, M. L., J . C a r d 53. 295 (1978). Biloen, P., Helle, J. N., and Sachtler, W. M. H., J . Catul. 58, 95 (1979). Sachtler, J. W . A,, Kool, J. M., and Ponec, V., J . Cutal. 56, 284 (1979). Dalmon, J. A., and Martin, G. A,, J . Chem. Soc., Faruduy Trans. 75, 165 (1979). Bernardo. C. A., and Trimm, D. L., Carbon 17, 115 (1979). Bonzel, H. P., Krebs, H. J., and Schwarting, W., Chem. Phys. Lett. 72, 165 (1980). Matsurnoto, H., and Bennett, C. O . , J . Catal. 53, 331 (1978). Biloen, P., Rec. Trau. Chim. Pays-Bas 99,33 (1980). Sachtler, W. M. H., Biloen, P., and Helle, J. N., Proc. Int. ConJ Heierog. Caial.,Varnu, 1979, Comm. of Depi. Chem. Bulgarian Acad. Sci. 23,3 (1980). 67. Fischer, F., BrennsiXhem. 11,489 (1939). 68. Herrison, J. L., and Chauvin, Y . ,Makromol. Chem. 141, 161 (1971). 69. Calderon, J., Ofstead, E. A,, and Judy, W. A., Angew. Chem. 88,433 (1976). 70. Brady, R. C . , and Pettit, R., J . Amer. Chem. Sac. 102, 6182 (1980). 71. Muller, J. M., and Gault, F. G., J . Caial. 24, 361 (1972). 72. Poutsma, M. L., Elek, L. F., Ibarbia, P. A,, Risch, A. P., and Rabo, J. A., J . C a r d 52, 157 (1978). 73. Bond, G. C . , and Turnham, B. D., J . Cutal. 45, 128 (1976). 74. Dalrnon, J. A,, and Martin, G. A,, Proc. Int. Congr. Catal., 7th, 1980 p. 67 (1980). 75. Sachtler, W. M. H., and van Santen, R. A,, Ado. Cutal. 26, 69 (1977). 76. Biloen, P., Dautzenberg, F. M., and Sachtler, W. M. H., J . C a r d 50, 77 (1977). 77. Thomas, M. G., Beyer, B. F., and Muetterties, E. L., J . Am. Chem. SOC.98, 1296 (1976). 78. Schulz, H., and Zein El Deen, A,, Fuel Process. Technol. 1, 31 (1977). 79. Dalla Betta, R. A., and Shelef, M., J . Catal. 49, 383 (1977). 80. Kellner, C. S., and Bell, A. T., J . Caial. 67, 175 (1981). 81. BrodCn, G., Gafner, G., and Bonzel, H. P., Ned. Tijdschr. Vacuum Tech. 16, 160 (1978). 82. Nijs, H. H., Jacobs, P. A,, and Uytterhoeven, J. B., J . Chem. Soc., Chem. Commun. p. 1095 (1979). 83. Van Hove, D., Makambo, P., and Blanchard, M., J . Chem. Soc., Chem. Commun. p. 605 (1979). 84. Fraenkel, D., and Gates, B. C., J . Am. Chem. Soc. 102,2478 (1980).

56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66.

ADVANCES IN CATALYSIS. VOLUME 30

Surface Reactions and Selectivity in Electrocata Iysis GEORGE P . SAKELLAROPOULOS* Department of Chemical and Environnzental Engineering Rensselaer Polytechnic Institute Troy. New York I . Introduction . . . . . . . . . . . . . I1 . The Art of Electrocatalysis . . . . . . . . . 111. Basic Electrocatalytic Concepts . . . . . . . A . Thermodynamics of Electrode Reactions . . . B . Electrocatalytic Processes and Operating Conditions C . Electrogenerative Processes . . . . . . . D . Electrode Kinetics . . . . . . . . . . E . Activity and Specificity Factors . . . . . . IV . Adsorption on Electrocatalysts . . . . . . . . A . Surface Properties and Interactions . . . . . B. Adsorption Isotherms . . . . . . . . . C . Hydrogen Chemisorption . . . . . . . . D . Oxygen Adsorption . . . . . . . . . E . Adsorption of Carbonaceous Species . . . . V . Electrocatalytic Factors . . . . . . . . . . A . Structural Effects . . . . . . . . . . B . Surface Stability . . . . . . . . . . C . Adsorbate-Electrocatalyst-Support Interactions . D . Redox Catalysis . . . . . . . . . . VI . Electrocatalytic Selectivity . . . . . . . . . A . Specificity Factors . . . . . . . . . . . B. Multiple Reaction Analysis . . . . . . . C . Selectivity Control . . . . . . . . . . VII . Electrocatalytic Reactions and Mechanisms . . . . A . Oxidations . . . . . . . . . . . . B . Halogenations . . . . . . . . . . . C. Hydrogenations . . . . . . . . . . D . Reduction of Functional Groups . . . . . . VIII . Techniques in Electrocatalytic Studies . . . . . . A . Electrochemical Techniques . . . . . . . B. Nonelectrochemical Techniques . . . . . .

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* Present address : Department of Chemical Engineering. University of Thessaloniki. Thessaloniki. Greece. 217 Copyright 0 1981 by Academlc Press. Inc . All rights o f reproduction in any form reserved. ISBN 0-12-007830-9

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GEORGE P. SAKELLAROPOULOS

IX. Electrochemical Reaction Engineering . . . . A. Electrochemical Reactors . . . . . . . B. Mass Transport Processes. . . . . . . C. Selectivity and Transport Processes . . . . D. Current and Rate Distribution in Electrocatalytic E. Steady-State Multiplicity . . . . . . . X. Conclusions . . . . . . . . . . . . References . . . . . . . . . . . .

I.

. . . . . . . . . , . . . . . . . . . . . . . . Reactors

. . .

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309 309 312 315 318 320 321 322

Introduction

The use of various electrode materials to explore chemical reactions in an electric field dates back to the beginning of the nineteenth century. Although the catalytic nature of some electrodes was only appreciated much later, Grove (1) recognized their “chemical or catalytic action” and the need for a “notable surface action” in early fuel cells, just a few years after the dawn of the notion of catalysis (2). When the term “electrocatalysis” was deliberately introduced by Grubb in 1963 (3),it did not reflect an unnecessary complication in nomenclature, but a real need to identify and comprehend the unique and characteristic features of catalytic electrode processes. How has this need been fulfilled to date? Where does the field of electrocatalysis stand compared to the development of conventional catalytic and electrochemical processes? What are the new directions and goals of this discipline? The most notable application of electrocatalysis has been in the area of electrochemical energy generation from which in essence the field emanates. Complete, fuel-cell electrooxidations of hydrocarbons, alcohols, aldehydes, hydrazine, etc. are the most well-studied reactions next to the classical hydrogen and oxygen electrocatalytic reactions. Deliberate use of electrocatalysts for other reactions, and especially for selective promotion of reaction paths for chemical processing, has only occasionally been considered. Although the generated information from fuel cell research is useful for some other electrocatalytic system, the goal of high-energy output quenched any detailed examination of incomplete and selective oxidations or reductions. With fuel cell electrocatalytic advances extensively reviewed in many excellent treatises (4-13), we shall limit our discussion to new trends, techniques, and directions in this area. Some preference will be given to the role and application of electrocatalysis to simple and multiple selective reactions for new, developing electroorganic (14, 15) and energy-conserving processes (16, 17). Early electrocatalytic progress was based upon classical experimental testing of catalysts, electrode designs, cell configurations, and operating conditions. Soon it became apparent, however, that the goal of further

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219

improving catalytic activity and selectivity required a clearer understanding of basic electrocatalytic principles. In addition, coupling of fundamental surface and interfacial phenomena with physical processes became necessary for the design of efficient,economical, practical electrocatalysts and processes. Thus, electrocatalytic investigations have been maturing to cover fundamental as well as technical problems, albeit applied to few systems. The need for this wide spectrum of investigations in a rational, technological world is very lucidly epitomized in Ionesco’s “Rhinoceros” (p. 83): Practice always has the last word; but only when it proceeds from theoretical understanding!. .. To understand is to justify. In order to understand a phenomenon and its effects one needs to work back to the initial causes by honest intellectual effort.

In view of this need, we discuss here a variety of electrocatalytic topics, ranging from basic and microscopic concepts to phenomenological principles. Thus, the origin of electrodic reactions, electrosorption, and electrode kinetics are introduced briefly for the benefit of the nonelectrochemist. Since electrocatalytic reactions take place at the electrode surface, attention is given to recent efforts to link catalyst activity with microscopic surface properties. These include surface crystallographic orientation, crystallite size and distribution, adsorbate-adsorbent-support synergism, multiple adsorption states, identification of surface intermediates, and electrocatalytic surface reaction mechanisms. For such studies, both electrochemical and nonelectrochemical experimental techniques have been developed. Several of them are outlined here: electrosorption methods, surface electron spectroscopies, and isotopic-mass spectrometric techniques, linking electrocatalysis to conventional heterogeneous catalysis. The spectroscopic and isotopic methods have been recently applied to a limited number of simple electrocatalytic systems. The exciting results that these methods have provided demonstrate their power for future electrode reaction studies. The application of these techniques to multiple reaction sequences would shed light into electrocatalytic selectivity phenomena. The latter are currently approached only macroscopically. Although recent developments suggest an appreciable specificity control by altering operating parameters, and especially the potential, the role of the electrocatalyst and its structure in selectivity remains obscure. The present discussion identifies needed information and gives a methodology for analyzing multiple reactions. Mass transport processes at the surface and in the pores of real and high surface area electrocatalysts can alter considerably the intrinsic activity and specificity as obtained from microscopic principles. This can lead to poor surface utilization, nonuniform distribution of reactants and currents, and decreased product yield and energy efficiency. Possible surface change and

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GEORGE P. SAKELLAROPOULOS

deactivation due to sintering and poisoning by impurities or intermediates also cause performance decline. These effects are often accounted for by electrode and reactor models of varying complexity and sophistication. Several existing models are analyzed in this report, while others are proposed for little understood processes, such as electrocatalyst deactivation, reactor steady state multiplicity, and selectivity control. The chemical engineer would recognize in the latter topics many familiar concepts. The catalycist would be already informed in surface and mechanism investigations and the electrochemist in electrodic phenomena. For an effective, synthetic evaluation of electrocatalysis, however, it is essential to transcend boundaries and transfer existing knowledge between these diverse groups of science and engineering. While this is already in progress for some fuel cell electrocatalytic research, it is hardly initiated in electrochemical processing investigations. By encompassing in this analysis both fundamental and applied principles, we hope to guide the uninitiated, entice the purist, and challenge the technologist.

II. The Art of Electrocatalysis

In anology to conventional catalysis, “an electrocatalyst participates and promotes (or suppresses) a reaction or reaction path involving electrontransfer, in the presence of an electric field, without itself being appreciably consumed.” Electron transfer occurs across an electrode-electrolyte interface, but the electrode per se may or may not induce the catalytic action. Redox metal ions and chelating or complexing agents can also act as catalysts in solution before or after the charge transfer. In all cases, the electrocatalyst acts as the meeting point and as an energy field modifier for ionic as well as molecular reactants. Since, in the vast majority of electrocatalytic reactions studied to date the electrode serves as the catalyst, we examine here mostly heterogeneous electrocatalysis, with only occasional reference to other electrocatalytic modes. Charge transfer is not unique to electrocatalysis as even a cursory survey of the catalytic literature can show. Indeed, oxidation (18-21), desulfurization (21), and reduction (22) mechanisms have been proposed, involving electron transfer between catalyst and reactant, to explain activity and selectivity effects. Electronic interactions between adsorbate bonds and d-band electrons of the catalyst are also used commonly to explain strength of adsorption (22,23,24).This electron exchange or transfer in conventional catalysis and electrocatalysis, and steps such as adsorption, surface reaction, and desorption, point toward expected similarities between the two catalytic

ELECTROCATALYSIS

22 1

processes. Thus, electrocatalytic (25, 26) and gas phase (27) reduction of alkenes on platinum black proceed via some similar steps, including alkyl radical formation and surface reactions with hydrogen atoms as the ratelimiting step. For electrooxidation of ethylene on palladium (28, 29) a mechanism analogous to that of the homogeneously catalyzed Wacker reaction (30) has been proposed. However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8).Thus, electrogenerative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33).However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). From this brief overview, it becomes apparent that selection of a suitable electrocatalyst should be based upon the following criteria : 1. optimal strength of reactant adsorption; 2. maximum specificity and control of the desirable reaction path; 3. fastest reaction rate and minimal electric energy losses ; 4. inertness toward reactants, products, electrolyte. and potential; 5. resistance to poisoning by molecular or ionic species and impurities; 6. optimal crystallite composition, size, and surface distribution; 7. resistance to sintering and deactivation ; 8. maximum surface area ; 9. complete surface utilization ; 10. inexpensive cost. Simultaneous satisfaction of these requirements is often impossible, and electrocatalyst or catalyst selection becomes a compromise. Thus, the

222

GEORGE P. SAKELLAROPOULOS

electronic d-band properties of transition metals, especially of the platinum group metals, and their inertness toward chemicals have often been the overriding factors in catalyst and electrocatalyst choice, despite high cost. Similarly, the degree of surface utilization may be sacrificed for crystallite stabilization, selectivity control or reduction of reactor size. While the use of catalysts and the attendant cost are well accepted in chemical processing, electrocatalysts are only rarely considered essential for electrochemical processing (16, 17, 28, 29, 31, 34). Even to date electrochemical processes are being evaluated (14, 15) without examining the possible benefits from electrocatalyst use, such as higher product selectivity, faster rate, smaller reactor size, and decrease in electric energy requirements. Electrochemical processing without use of electrocatalysts could perhaps be paralleled to thermal cracking about 40 years ago relative to present-day catalytic cracking. The classification of electrodes into low hydrogen overpotential and high hydrogen overpotential materials has probably contributed to the lack of appreciation of electrocatalysis for synthetic organic reactions. Thus, electrochemical reduction of organic reactants [e.g., benzene reduction (35)] is often attempted at electrodes of low catalytic activity for these reactions, to suppress simultaneous hydrogen evolution. Apart from consuming large amounts of electric energy, such reductions usually result in failure to form a desirable chemical. Yet, they can easily succeed on catalytic electrodes (36) with considerable energy savings, even with generation of some electric work (16, 17, 33, 36). Similar considerations apply to electrocatalytic oxidations. Langer and co-workers (16,32,33)have proposed the term “electrogenerative processes” for these electrocatalytic reactions, the free energy of which can be recovered as “by-product” electric work. Because of the unusual potentials at which electrogenerative catalytic reactions occur, such processes deserve particular attention for the selective production of chemicals (16, 17). The examples chosen above give a glimpse of the significance of electrocatalysis in prospective electrochemical processes and energy generation or conservation. Do we, then, understand the electrocatalytic action so that we can select and predict a priori the performance of an electrocatalyst? Although our present knowledge is well beyond the initial “art” stage, electrocatalysis still retains elements of uniqueness, uncertainty, mysticism, inspiration, awe, attraction, and stimulation. To remove the mysticism and the uncertainty, several investigations of microscopic phenomena underlying electrocatalytic activity, selectivity, and surface reactions have been initiated, entirely analogous to heterogeneous catalytic research. We wish to note that such studies should be coupled with an effort to compare and link electrocatalysis and heterogeneous catalysis. This would permit utilization by electrocatalytic research of the accumulated

ELECTROCATALYSIS

223

wealth of information in catalysis. It could also provide clues to uncertainties regarding electronic interactions at catalytic surfaces. While we shall attempt comparisons of the two disciplines, we should identify also the need for more data, with a variety of reactions under similar conditions. Use of wellcharacterized, simple catalytic reactions as models for electrocatalytic studies can provide more comparative information than the important, but complex, fuel cell oxidations of carbonaceous reactants studied to date. It would not be superfluous to emphasize the significance of maintaining well-defined, similar conditions in such studies. After all, electrocatalytic research has followed catalytic investigations faithfully, even into generating a hiatus of conflicting data by use of obscure procedures and electrode treatments. It is essential that fundamental studies for unambiguous unraveling of electrocatalytic principles be conducted with electrodes of known structure and history, at several potentials and under well-controlled experimental conditions.

111.

Basic Electrocatalytic Concepts

For the benefit of the would-be electrochemist or electrochemical engineer, we shall outline briefly some principles that would facilitate understanding of the origin of electrochemical reactions and the rate at which they proceed.

A. THERMODYNAMICS OF ELECTRODE REACTIONS 1. n e Double Layer lnterjace

When two immiscible phases containing charged species are brought into contact, an electric potential difference is established between them, indicative of charge distribution. Figure 1 shows the potential variation away from a metal electrode in contact with an electrolyte after equilibrium is reached. Although the interface does not resemble a parallel-plate capacitor with locally fixed charges (37, 38), the original concept of a charged “double layer” interface ( 3 7 )remains. Several models have been advanced to explain the observed potential capacity behavior of the double layer (8,38-44).The most refined and most satisfactory model (44), combines Stern’s compactdiffuse layer model (42) with Grahame’s specific adsorption model (43).In addition, it considers solvation of the electrode and of species in solution (44).

224

GEORGE P. SAKELLAROPOULOS

Diffuse

f U

c

3

w

c 0

DISTANCE

-

FIG. 1. Schematic potential distribution across the compact diffuse layer: &, metal potential; I&,,-~,potential at the outer Helmholtz plane (OHP); Oa, electrolyte potential.

The structure of the double layer according to this model ( 4 4 ) is shown schematically in Fig. 2. Specific adsorption of ions can take place in certain potential regions by displacing water molecules from the surface. Organic molecules are assumed to adsorb when the electrostatic forces holding water molecules to the surface weaken around the potential of zero charge. This picture can explain the often observed bell-shaped dependence of surface coverage on potential; however, it does not consider the polarizability of some organic reactants, the electronic interaction between substrate and catalyst during chemisorption, or possible direct electron transfer to or from the organic molecules. The above model, which was developed for aqueous electrolytes, may require modification for application to nonaqueous and organic solvent electrolytes. The potential distribution of Fig. 1 is typical of the compact-diffuse layer models (42-44). The potential varies almost linearly within the compact double layer, decaying exponentially within the diffuse layer. The thickness of the latter depends on the electrolyte ionic strength and becomes negligible in strong electrolytic solutions (42). This feature becomes important in electrocatalytic studies, since it is the potential difference (& - &) that can be measured or fixed experimentally versus a reference electrode, while reacting ions and molecules experience a potential difference (&, - (bMp)

ELECTROCATALYSIS

225

Adsorbed Anions

d

CI

c

eJ

I:

I

Primary Wo t e r Laye r

FIG.2. Double layer structure; water dipole model (44).

at the “outer Helmholtz plane” (OHP). Use of high ionic strength supporting electrolyte can suppress the potential drop in the diffuse layer. so that 4MP 2 &. Otherwise, a correction is necessary for the experimental value of the electrode potential using the Debye-Huckel theory to evaluate 4Ewith respect to &p (45). In the following sections the electrode potential E will since neither of these absolute potentials represent the difference (+M - 4Mp) is directly measurable. 2.

The Reversible Electrode Potential

For an electrochemical reaction a t equilibrium. the standard electrode potential EF( = 4; - 4iP)can be calculated from the standard free energy, AG; of the reaction at 1 atm, 1 N solution and 25°C: b

AG;

vj A G l j = -nFEF

=

(1)

j= 1

where AG;, is the standard free energy of formation of species j and vj the stoichiometric coefficient ofj. The latter is positive for products and negative

TABLE I Estimated and Observed Thermodynamic Reversible Potentials"

Reaction

$0,+ H, - H 2 0 C,H, C,H4

+ 3 0 , -2C0, + 2H,O + $0, CH,CHO

+ 0,+ N 2 + 2 H 2 0

C2H4

+ H,

C,jH6

3H2

CH,=CHCl

+ H, + H,

CH,=CHF C2H4

+ C1, + HZO

+

-+

4

+ HCI

Estimated from group contributions (46).

E," (V) (observed)

-68.3

-56.7

1.23

1.03

6-10

-318.2

1.15

0.85

6-10

-41.4

1.03

- 143.8

1.560

1.10

7, 9

-32.1

- 24.1

0.522

0.51

33

-49.2

- 23.4

0.169

0.145

36

-

+ HCI

References

- 148.7

- 30.2

C,H,F CH,Cl-CH,OH

E," (V) (estimated)

-52.2

C2H6

-+C,H,

AG; (kcal/mol)

- 337.2

+

NzH4

AH: (kcal/mol)

-

- 5.7

-

28,29

0.124

0.45

31

-24.5

0.530

0.45-0.6

31

-35.6

0.742

0.81

47

ELECTROCATALYSIS

227

for reactants. The standard electrode potential is usually referenced to the standard potential of the normal hydrogen electrode (NHE) which has been assigned the value of zero. Electrocatalysts often assist electrochemical reactions to reach equilibrium in the absence of current flow. Table I gives the calculated and measured electrode potentials at zero current for some typical electrocatalytic reactions (6-10, 28, 29, 31, 33, 36, 46, 47). Considering that most of these reactions become highly irreversible even at low currents, it is surprising to find, in general, good agreement between experimental and estimated values. One should note here that many seemingly simple reactions do in fact consist of a number of independent paths, whose irreversibility contributes to the observation of an apparent “mixed potential” (6, 39). In addition, the free energy of electroorganic reactions is often not known with any accuracy, while irreversibilities do not allow the exact measurement of their thermodynamic reversible potential. Finally, the potential of zero charge is usually difficult to determine for multiple organic reactions. Because of these reasons, the electrode potential versus a standard reference will be used in the following sections to evaluate electrocatalytic behavior under current flow. For conditions other than standard state, the reversible electrode potential can be calculated from the chemical potentials of reactants and products, p j :

E, = E: - (RT/nF)

1 v j In aj

(3)

j

where a j is the activity of species j . Deviations from the potential predicted by the Nernst equation (3) occur, especially under current flow, owing to (a) irreversible operation, (b) presence of impurities, (c) occurrence of multiple reactions, (d) chemisorption on the catalyst surface, and (e) formation of reaction intermediates (e.g., by cracking of carbonaceous species). Electrochemical processing requires the combination of two electrodes, the working electrode on which a desired oxidation or reduction occurs, and the counterelectrode. Equations (1)-(3) hold for each electrode separately. Their algebraic sum gives the cell thermodynamic voltage. The change of potential or cell voltage with temperature or pressure can be derived from the above equations using classical thermodynamic relations (6,8, 16).

PROCESSES AND OPERATING CONDITIONS B. ELECTROCATALYTIC From the participation of the electrolyte in the structure of the double layer, it is anticipated that the nature, concentration, and purity of the liquid

228

GEORGE P. SAKELLAROPOULOS

phase should play an important role in electode reactions. Indeed, electrooxidations and electroreductions in acidic and basic electrolytes proceed at different rates and give different products (7-9,16,31,33).The rate and yield is also affected by the presence of adsorbing impurities (7, 8), nonelectroactive adsorbing ions (7, 34), and solvents (31, 32, 35, 36). Therefore, care should be taken in electrocatalytic investigations to identify and eliminate possible complications arising from the electrolyte. For example, sulfuric acid may favor addition reactions to alkenes, phosphoric acid may catalyze polymerization reactions, chloride ions may adsorb and deactivate electrocatalytic sites, and acids corrode several metals that could act as oxidation or reduction electrocatalysts. Solvents and nonaqueous or aprotic electrolytes are often used to increase the solubility of organic reactants in aqueous solutions or to affect product selectivity. Such electrolytes, however, have low conductivities, which cause high ohmic losses of voltage due to current flow through the solution. To minimize ohmic losses, industrial electrochemical cells (reactors) should be designed with minimal spacing and resistance between the anodic and cathodic electrodes. Conductivities as high as 10 i2-l cm-' are desirable. High reactant solubility may not be always necessary for electrocatalytic reactions. Although the rate of many reactions increases with increasing concentration, reactions involving strong surface adsorption of reactants are often inhibited by high concentrations. For instance, the rate of hydrocarbon oxidations declines at high hydrocarbon and proton concentrations (8, 10). High solubility may also require the use of high-resistance diaphragms between the anodic and cathodic electrodes of a cell, to avoid extraneous reactions at the counterelectrode. Thus, possible rate increase at high concentrations may be offset by ohmic losses in the diaphragm. Nonetheless, low solubility and low reactant concentration may result in slow mass transport, which places an upper bound to the reaction rate (limiting current). Since electron transfer reactions proceed at the electrocatalyst-elecrolyte interface, the rate can be improved by increasing the electrode area. Thus, deposition of metals on a conductive support enhances the current density and the limiting current compared to the smooth, planar electrodes. Apart from the conventional parallel flat plate configuration, other electrode configurations are currently being tested for electrochemical processing and pollution control. These include fluidized bed, slurry, and flow-by or flowthrough electrodes, which increase the surface area and the rate of mass transport. Although most studies with these configurations involve ionic reactions, some of these designs appear attractive for electrocatalytic processes. With gaseous reactants (H2,O,, hydrocarbons, etc.), use of hydrophobized porous electrocatalysts has proved successful (6, 8, 33). The wet-proofing prevents pore flooding and creates a high-area, dynamic interface between

229

ELECTROCATALYSIS

gas-solid-electrolyte, which enhances the rate and compensates for the low solubility of the reactants. Such electrodes have been used in fuel cells and electrogenerative processes (6-10, 16,33). Present design efforts attempt to reduce the catalytic load without loss of activity to develop more efficient, more selective, and less expensive simple or mixed electrocatalysts and to prolong electrode stability and longevity. Recent fundamental electrocatalytic studies concentrate on unraveling the basic knowledge required for such improvements.

C. ELECTROGENERATIVE PROCESSES Equation (1) can serve as the basis for determining the highest reduction potential or the lowest oxidation potential of an electrocatalytic reaction. Figure 3 shows that spontaneous electroreductions result in a positive E: vs. NHE, often approached with the aid of a catalytic electrode (25,26, 31, 33, 36). For the example of ethylene shown in Fig. 3, reduction can occur below its reversible potential away from equilibrium. If the ethylene electrode is combined with a hydrogen anode, an “electrogenerative” cell is formed (16,17,25,26,31,33,36),similar to a fuel cell, that can generate low-voltage,

4

ELectrolysis

(a)

(b)

FIG.3. Representation of electrogenerative reduction (EGR) and oxidation (EGO) regimes versus conventional electrolytic ones: (a) reduction; (b) oxidation. The reversible potential ofthe organic species, noted with an arrow, is the maximum reduction potential or the minimum oxidation potential for that reactant.

230

GEORGE P. SAKELLAROPOULOS

direct current electric work. When the ethylene electrode potential becomes more negative than that of hydrogen, conventional electrolysis commences under electric energy consumption. The coupling of an oxygen cathode and a hydrogen anode would, of course, give a conventional fuel cell (3-13). Some typical electrogenerative and fuel cell potential-current density curves are given in Fig. 4 (25, 31, 48-49). The irreversibility of the cathodic reactions results in high overpotentials at low current densities. In the same figure the effect of electrocatalyst nature can also be seen for some reactions. Similarly, oxidation of ethylene can occur above its E: (3-13,28,29). With an oxygen counterelectrode, conventional electrolytic oxidation starts when the anode potential becomes more positive than the cathode. Halogenation of hydrocarbons is also possible electrogeneratively, above E,", on appropriate electrocatalytic anodes (47, 50, 51). The distinction between electrogenerative and fuel cell processes is made only to accentuate the emphasis of the latter on energy generation, regardless of usefulness of the reaction products. Electrogenerative processes, on the

mA/cm2 0 1.0

c

10

20

30

I

I

10

50

C U R R E N T DENSITY (mA/cm2) FIG.4. Potential-current density curves for some electrogenerative and fuel cell reactions : (0) benzene reduction on Pt (48);(A) vinyl chloride reduction on Pt (31); (A)ethylene reducoxygen reduction on Pt (49). (Retion on Pt (25); ( 0 )ethylene reduction on Pd (48a); (0) printed by permission of the publisher, The Electrochemical Society, Inc.)

ELECTROCATALYSIS

23 1

other hand, aim at the formation of useful chemicals, with possible energy recovery or conservation as a bonus (16, 17, 33, 52-54). Thus, complete oxidation of hydrocarbons to CO, would yield maximum energy output and is considered to be a fuel cell process. Incomplete, selective oxidation of an alkene to an aldehyde, however, would qualify as an electrogenerative process, since energy is sacrificed for the sake of production of an intermediate oxidation product. For electrogenerative processing it is often necessary to use a different, more selective catalyst than that used in fuel cells. Although the extensive screening of various electrocatalysts for fuel cells (3-23) can provide useful guidelines for choosing selective electrocatalysts, the specificity of such catalysts is not well characterized, with few exceptions (28, 29, 31, 54). We should emphasize here that most electroorganic reductions. oxidations halogenations, etc. are carried out in the conventional electrolytic potential regime (cf. Fig. 3), even when conducted on catalytic electrodes (55-59). Recent evidence shows that the product distribution can be entirely different and more favorable in the electrogenerative regime (31, 47, 50. 51, 60-62), often made accessible only because of the catalytic action of the electrode. Although the reaction rate might be slower than in electrolysis, it is the selective formation of a useful product that matters mostly in electrochemical processing, minimizing subsequent expensive separations. The tendency to evaluate electroorganic reductions only at very negative potentials versus NHE (or oxidations well above the oxygen electrode potential) may have led many such electrolytic reactions to failure. by not recognizing the combined catalytic and potential effect on selectivity.

D. ELECTRODE KINETICS 1. Activation Barriers

Treatment of activated electrochemical processes is in many respects analogous to treatment of conventional chemical reactions. The electrode potential shifts the potential energy-distance curves of reactants and products depending on its sign (63-65). Figure 5 exemplifies the potential energy change along the reaction coordinate, with product formation on the surface represented by curve 2. Imposing an electric potential E (i.e., a potential drop across the electrode-electrolyte interface), shifts the potential energy of the reactants to position 3 , and thus it modifies the activation barrier of the reaction. Here we assume nonionic products formed at the electrode. so that the product energy curve remains unchanged. For an elementary reaction of the form A

+ B + e*[AB']+C + D

(4)

232

GEORGE P. SAKELLAROPOULOS

) .

c3

a W

z W

+ U

z W

W2

c 0

e

REACTION

COORDINATE

FIG.5 . Potential energy diagram for an electron transfer reaction.

the initial free energy of activation in the absence of an electric field AGf, now becomes (5)

AG: = A G f , + P F E

The “symmetry factor” /? expresses the fraction of the contribution of electrical energy to the activation energy of the electrodic reaction. Its magnitude depends on the position of the energy barrier and varies between 0 and 1. Most often, a symmetrical energy barrier is assumed, for which = 0.5. The value of p is usually taken as constant with potential, a fair assumption for slow reactions over a wide range of potentials or overpotentials. With fast reactions, however, such as some metal ion discharge reactions, experimental evidence indicates that P is constant only at low overpotentials (66). A statistical mechanical treatment of reaction (4) gives the rate constant k, as a function of potential

k,

kT

(

y:)

=h exp - - = =

kT exp(%) h

kT -i;exp(

F)

ex,( - A H ;

-

g)

m)

exp( - BFE

F) = k,“ exp( - =) PFE ( 6 )

exp( - PFE

Similarly, the reverse reaction rate constant becomes

ELECTROCATALYSIS

233

where k,", k: express the rate constants in the absence of the electric field gradient at the interface. Comparing Eq. (6) with the usual Arrhenius rate constant gives a linear dependence of the activation energy on potential E A = AH6 + P F E

(8)

where EA is the Arrhenius activation energy at any potential. Equation (8) suggests that the electrode potential could, in principle, decrease or virtually suppress the activation barrier that the catalytic reaction exhibits at zero potential. Since electrochemical surface reactions involve electron transfer to or from the surface, a quantum mechanical approach becomes necessary to account for electron tunneling in such processes. Quantum mechanical treatments of electron transfer and adsorption have been reviewed recently (67-71).The Gurney treatment (68,72,73)assumes the transfer of an electron at the Fermi level of the metal to an H 3 0 + at its ground state at the outer Helmholtz plane. The electrode potential changes the minimum vibrational energy of the H+-OH, bond necessary to induce tunneling. Levich (67)has criticized the use of a Boltzmann distribution to characterize this bond and suggested activation via solvent dipole fluctuations around the ion. Despite their differences, both models predict reaction rate dependence on the final state, which, in turn, depends on the nature of the metal substrate, that is, on its catalytic properties. Although these analyses could provide in the future the fundamental background for understanding electrocatalytic effects, they are a t present far from being satisfactory or unequivocal for even the simplest electrocatalytic reactions. Other more phenomenological treatments could provide useful short- to midrange enlightening as concerns catalytic activity and selectivity. In view of present quantum mechanical discussions elsewhere (67-71) we shall remain here on more classical grounds. 2. T h e Electrochemical Rate Equation For the elementary reaction (4), the rate of electron transfer per unit area is directly proportional to the current density of the electrocatalyst : i = nFF = if - i, =

where Cj are the surface concentrations of reactants and products, n is the number of electrons transferred (here n = l),and if, i, are the current densities

234

GEORGE P. SAKELLAROPOULOS

associated with the forward and the backward reaction. Since current and rate are vector quantities, we denote as positive the current associated with electron transfer from the electrode to the reactant (cathodic) and as negative the anodic electron transfer to the catalyst. If the reaction is nonelementary, the rate equation becomes

where f ( C , ) and f”(Cj) can be functions of reactants, products. and other species as discussed later (Section III,D,3). Instead of the symmetry factor p, a ‘?transfer coefficient” for the cathodic (cI,) and the anodic (a,) reaction appears now to express the influence of more than one reaction step on the rate. The transfer coefficients are positive and can attain values larger than unity in contrast to the symmetry factor. When reaction (4) is at equilibrium,

(

i, = if,e = i,,e = nFk,“C,C, exp -

= nFkEC,C,

exp

YE)

~

[(‘ ‘:;“I

A similar expression is obtained for a nonelementary reaction. The “exchange current density” i, expresses the rate of the forward and backward step a t the reversible electrode potential and depends on concentration. Equation (9) is usually rewritten in terms of i, in electrode kinetics

{

(y;) [-

z‘ = zo. exp -

- exp

it)“”}

where q = E , - E is the “overpotential,” that is, the deviation of the electrode-electrolyte potential from its thermodynamic equilibrium value. For a nonelementary reaction i = i,{exp(%)

-

exp( -

g)}

When lql is larger than about 0.2 V, one of the exponential terms in Eqs. (11) or (12)becomes negligible and the reaction becomes essentially an irreversible cathodic (q > 0) or an irreversible anodic one (q < 0). Many electrocatalytic reactions of interest, with the exception of hydrogen oxidation or reduction, are irreversible when practical current densities are achieved(cf. Fig. 4). A semilogarithmic plot, then, of iversus q (or,!?) provides

235

ELECTROCATALYSIS

fl (or a) as the slope, assuming that it is independent of potential flF RT

(cathodic, elementary reaction)

(1 3a)

cccF

(cathodic nonelementary reaction)

(13b)

-~

T.CJ

RT

The exchange current density can also be estimated from Eqs. (12) or (12a), that is by extrapolating to y~ = 0 or E = E , , While a least square analysis can yield fl or u with a small error, the 95% confidence limits for i, values can range over several orders of magnitude because of the usually large extrapolation required. From the data of Fig. 4 for ethylene reduction on Pt and Pd in the Tafel region, we estimated the transfer coefficients and exchange current densities (Table 11). By comparing iO7s,as usually done in electrocatalytic studies, one might conclude that Pd is about a n order of magnitude better than Pt for this reaction. However, examination of the statistical 95% confidence limits of i,'s (Table 11) shows that the results for Pd lie entirely within those for Pt; that is, a claim on activity based on i, is unfounded statistically. Yet, Pd is a better electrocatalyst in the Tafel region, yielding about two orders of magnitude higher currents than Pt at the same potential (Fig. 4). For the above reaction, a shorter extrapolation to E = 0, to determine k", is somewhat better statistically, with overlap only of the lower confidence interval for Pd and of the upper one for Pt. In addition, the value of k" can always be defined for a reaction path, while estimation of i, requires knowledge of the thermodynamic reversible potential. The latter is not always TABLE I1 Statistical Estimates of Kinetic Parameters for Ethylene Electrocatalytic Hydrogenation" Palladium Tafel slope (V) Transfer coefficient, a Exchange current density, i,, (A cm-2) Upper 95% confidence limit for i , (A cm-2) Lower 95% confidence limit for i, (A cm-2) Rate constant, k"' (mol cm-' sec-')b Upper 95% confidence limit for k"' (mol cm-' sec-') Lower 95% confidence limit for k ' (mol cm-' sec-') Standard deviation Correlation coefficient a

Data obtained from Fig. 4 and (25) and (48a). (25).

* k"' = k"C2,

- 0.033

1.77 2.1 10-12 9.6 x lo-" 1.9 x 10-14 3.7 x l o - ' 5.9 x 10' 1.7 x 0.0206 0.9925

Platinum -0.037 1.59 4.2 x 10-13 7.6 x lo-'' 2.4 x 1 0 - 1 ' 1.4 x 1 0 - ~ 3.4 x lo-' 1.0 10-4 0.0263 - 0.9972

236

GEORGE P. SAKELLAROPOULOS

known or calculable, especially for reactions of complex organic molecules. For these reasons, we prefer the use of k” instead of i, in subsequent analyses of electrocatalytic reactions. Electrocatalytic reactions often involve several elementary steps some of which are not necessarily electrochemical. The transfer coefficient is, then, related to the symmetry factor of an electron transfer rate-determining step (rds) through (74) CI

= (s/v)

+ n,P

(14)

where s and n, are the number of electrons transferred in steps preceding the rds and in the rds and v is the “stoichiometric number.” The latter defines the number of times the rds takes place for one full completion of the overall reaction (23, 24). Although obtaining stoichiometric numbers is not easy with complex reaction schemes, their determination can provide mechanistic clues (75). 3. Estimation of’ Kinetic Parameters

The design of electrochemical reactors and fuel cells and the fundamental understanding of electrocatalytic properties and mechanisms requires kinetic information of electrodic reactions. Of course, kinetic analysis is rarely capable to lead to mechanistic evidence, but any proposed mechanism must satisfy the experimental kinetics. Since the rate of a single nonelementary reaction can be measured directly and accurately as current density (Eq. 9), electrochemical measurement of kinetic parameters has a distinct advantage over conventional concentration-time methods. Determination of the transfer coefficient from current-potential data was discussed earlier (Eq. 13b). The concentration dependence of the reaction rate can be easily obtained with simple order reactions at constant potential (76).With,f(Cj) = nC?, then

In changing the concentration of ionic species to determine reaction orders. one should remember to maintain a high constant ionic strength of the solution by using nonelectroactive supporting electrolytes. Otherwise the measured electrode potential would include the potential drop within the diffuse double layer, which is concentration dependent (cf. Section 111,AJ). Although some electrocatalytic reactions are first order in a key reactant [ e g , oxygen reduction ( 7 7 ) ] , several reactions of organic species exhibit other orders. Thus oxidation of hydrocarbons has a small fractional order in reactant and a negative order in H f concentration (78).Alkene reduction is

ELECTROCATALYSIS

237

zero order in hydrocarbon and second order in hydrogen ions in the investigated region of concentrations (25). Table 111 gives the orders of some electrocatalytic reactions (25, 48a, 77, 79-82). The simple orders observed reflect the adsorption and reaction characteristics of these systems under the applied experimental conditions. Since electrocatalysis involves adsorbed species. more complex (e.g.. hyperbolic) expressions of concentratiop AC,) are expected in certain potential and concentration regions (83. 8 4 ) :

Here K j is the adsorption equilibrium constant of species j. which can be a function of potential. In this case estimation of the best fit of kinetic parameters u j , m,a, k", K j , E A requires the use of nonlinear regression techniques ( 8 4 ~ )Although . experimental data can fit an equation similar to Eq. (16). mechanistic deductions from such information alone should be restrained. It can be shown that more than one mechanism can be devised. the rate expressions of which cannot be statistically discriminated ( 8 4 ~ ) . The activation energy of a simple irreversible reaction can be also obtained from electrochemical data. From Eq. (10)and for y~ > 0.2 V

Thus. similar to elementary electrode reactions (Eq. 8) E A = AH:

+ u,FE

(18)

Of course, in a complex kinetic scheme, k," will probably be a function of rate and equilibrium constants for elementary steps before the rds. Therefore, AHoand E A will be only apparent activation energies. This can lead to low experimental values for the activation energy, although a chemical step is rate determining. For example. Table IV gives the observed E , and the estimated (AH',) values for the rds of ethylene electroreduction on platinum (25),assuming Temkin and Langmuir adsorption. The activation enthalpy of the rds is consistent with proposed surface reactions as rds (25,261, while the apparent values would have implied a diffusion controlled process. Change of E A with potential is anticipated; however, A P 0 should be potential independent. Its variation in Table IV may have been caused by some nonlinearity of the Tafel lines from which the data were taken (26). In the above discussion it was implicitly assumed that transport of reactants and products to and away from the electrode was fast compared with

TABLE 111 Electrode Kinetic Parameters of Catalytic Reactions Reaction

Catalyst

z

Pt

1.9

-

1.8

Pt

0.4

-0.2

0.45

80

Pd

0.3

0.5

0.5

80

Pt

1. 0.5

0.5

0

81

UR

a"+h

Reference

A. Oxidation

H, C,H4

+ 4H,O

-

2e + 2 H +

12e -* 2C0,

+ 12H+

CzH4 + H,O - 2e +CH,CHO CH,OH

+ H,O

-

6e +CO,

+ 2H'

+ 6H+

25, 79

B. Reduction

+ 4H' + 4e -*2H,O + 2 H 2 0 + 4e + 4 0 H CZH4 + 2H+ + 2e +C,H, 0,

0,

Order with respect to the reactant. Order with respect to H + ,

Pt

1

1.7

77

Au

0.5

0

82

Pt

1.6

1.8

25

Pd

1.8

1.8

48a

239

ELECTROCATALYSIS

TABLE IV Observed and Estimated Activation Energies for Electrocatalytic Reduction of Ethylene on Platinum Black (25°-700C)’lsh Electrode potential versus NHE (V)

Activation enthalpy of rds ( A H : ) (kcaljmol)

Observed E, (kcal/mol)

Temkin adsorption

H addition

Disproportionation

0.200 0.170 0.145

I .5 2.5 4.9

15.3 17.6 21.2

13.8 16.1 19.7

12.3 14.6 18.2

Langmuir adsorption

~~

~~

~

~

From (25). (Reprinted by permission of the publisher, The Electrochemical Society, Inc.) Reaction rate: r = ko I3 C y exp( - A H ‘ / R n exp( - a,FE/RT) = k ; n C y exp( - E,/RT).

the reaction rate. This may not be always valid, especially at low reactant concentrations, high current densities, or with porous electrodes. Some transport considerations with multiple reaction schemes will be discussed later (Section IX).

E. ACTIVITY AND SPECIFICITY FACTORS From a multitude of experimental electrosorption and surface studies, the following properties emerge prominently as determining the activity and selectivity of electrocatalysts. 1. Surface activity: (a) strength and mode of adsorption; (b) adsorption

2.

3.

4.

5.

isotherms; (c) adatom formation; (d) substrate-catalyst interactions; (e) surface diffusion; (f) adsorbate spillover; (g) bulk electronic properties ; (h) surface electronic properties. Surface structure and history: (a) crystallographic parameters ; (b) crystallite size and distribution ;(c) defects and dislocations ;(d) nature of catalytic sites; (e) pretreatment and prior use of surface. Surface stability and deactivation: ( a ) sintering and redispersion; (b) poisoning by electrolyte ions; (c) poisoning by reactants, products, or impurities; (d) stabilization by support. Surface selectivity: (a)structural factors; (b) multiple surface reactions; ( c ) potential effects; (d) transport factors. Physical conditions: (a) concentration; (b) temperature; (c) pressure; (d) transport phenomena; (e) electric field.

Some of these principles have been the object of extensive investigation in electrocatalysis and conventional heterogeneous catalysis and they have

240

GEORGE

P.

SAKELLAROPOULOS

been reviewed in standard texts (8, 23, 24). Others are only recently being examined to understand microscopic surface phenomena. In the following sections we shall present relevant classical adsorption information and recent advances in electrocatalytic and selectivity factors. These include effects of crystallite size on rate, sintering, and redispersion of catalyst particles, electrocatalyst-support interactions, adsorbate spillover, alloying, and synergism. IV.

Adsorption on Electrocatalysts

A. SURFACEPROPERTIES AND INTERACTIONS The electrode surface participates actively in an electrochemical reaction sequence by providing adsorption sites for at least one reactant and for the reaction intermediates. Thus, the reaction rate and selectivity depend strongly upon the surface properties and its mode of interaction with reactive species and electrolytes. The existence, however, of the structured double layer interface and of the electric field under which electrosorption takes place distinguishes the latter from gas phase adsorption. Electrolyte ions, solvent molecules, and impurities may adsorb and compete with reactants for surface sites or they may poison the surface or contribute to surface changes under reaction. Despite the wealth of experimental information on the potential dependence of surface coverage and on the nature of some adsorbing species, a fundamental understanding of electrosorption mechanisms is still incomplete. Theoretical analyses of electrosorption are based on models for the double layer structure with or without ion specific adsorption and surface-solvent interactions (44, 85, 86). Although the surface is undoubtedly covered by solvent molecules, their significance in competitive chemisorption probably has been overemphasized (44) for certain potential regions. Competitive electrosorption of organic molecules and hydrogen or oxygen atoms in aqueous electrolytes is more likely at potentials at which electrocatalytic organic reactions occur (87). Both approaches can predict qualitatively the potential-surface coverage behavior of organic adsorbents. However, both analyses treat the catalytic surface as a homogeneous, uniform terrain with equal activity sites. Today there is ample evidence in catalysis that the assumption of a homogeneous surface is not valid. Thus, surface irregularities exist such as crystallographic dislocations, defects, and planes with different activity; adsorbing molecules may interact with the surface and with each other: several adsorption states of a species may exist on a single plane. These

ELECTROCATALYSIS

24 1

surface phenomena give rise to a coverage dependence of the adsorption energy (87a, 88) and have led to various models of heterogeneous surfaces (88-90). In such models, lateral molecular interactions on the surface are generally regarded as small (89, 91). The “intrinsic heterogeneity” model assumes a distribution of surface sites having a range of adsorption energies varying linearly with coverage (88). Boudart (89) introduced the “induced heterogeneity” model in which the energy of chemisorption is partly determined by the effect of the adsorbed dipole double layer on the work function of the metal. Recent studies on hydrogen electrosorption on Pt single crystals (90) support and refine the induced heterogeneity model. Thus, multiple states for adsorbed hydrogen (90, 91u, 92) were explained by hybrid multiple bonding to groups of Pt atoms and H-atom relocation dependent on coverage. Alternatively, longrange electronic interactions of surface-atom groups were assumed to create distinguishable adsorption states (90). Four different states were observed as compared with previously accepted two states of “firmly” and “weakly bonded” hydrogen (91a). The latter are equivalent to similar gas phase adsorption states of hydrogen (93, 94). Although the above models would explain qualitatively the observed multiple adsorption states, there is currently no fundamental treatment to predict such bonding. The possible existence of multiple states deserves attention for other adsorbents as well, including carbonaceous species. The potential dependence of such adsorbed states could explain reactivity and specificity of some reactants.

ISOTHERMS B. ADSORPTION A comparison of mechanistic models with observed kinetics is strongly dependent on the adsorption isotherm adopted for reactants, products, or intermediates. Because of simplifying assumptions and of the uncertainties about surface species and catalytic sites, more than one isotherm can yield kinetic expressions in accord with the experimental data. The Langmuir isotherm (94u) is still used extensively because of its simplicity. However, the assumption of a homogeneous catalytic surface with localized adsorption is often not valid, as shown above. Hence, the increasing efforts to use the more realistic Temkin isotherm (88). 1. Langmuir Isotherm

With simple electrosorption at equilibrium on z sites Z*

+ A + + e * [A]

242

GEORGE P. SAKELLAROPOULOS

the rate of adsorption of A will equal the desorption rate. As with other electrode reactions (cf. Eq. 9), then, kYCA+(1- 0,)"eXp

( y;)

-- =

k:HAexp

i"

(20)

where 0, is the fraction of surface coverage at equilibrium (0 I HA I1). Upon rearrangement, the electrochemical equivalent of the Langmuir isotherm is obtained. If HA << 1, Eq. (20) reduces to Henry's isotherm

UA

=

(

,"f)

K"CA+exp - __

where KO = (k,"/k:) is the adsorption equilibrium constant at E = 0. Similar expressions can be derived for multisite adsorption or for more than one adsorbing species, including gases, organics and inerts. The Langmuir isotherm is adequate for low (0, +0) or high (0, +.1) surface coverage. 2. Temkin Isotherm This isotherm was proposed to explain the linear dependence of the heat of adsorption on surface coverage (88).In the following derivation, however, induced surface heterogeneity will be implied rather than intrinsic heterogeneity, originally considered by Temkin. This isotherm is applicable for intermediate total surface coverage (0.2 I 8, I 0.8). Assume,j species adsorbing on a surface, each one with a surface coverage 0, and an adsorption energy r j . The free energy of activation for adsorption of one of them (m) on z , sites would change with coverage by a fraction, A,Z,,. of the total energy of interaction. f ( U ) = X j rjflj. Considering also the effect of electrode potential on the activation barrier, the free energy of activation of m. AGcf,, , becomes

+ A,z,f'(0) + BFE

AG:,,

=

AG&,

AGf,,,

=

AGLo,, - (1 - 3.,)z,f(O)

(22a) -

(1 - B)FE

(22b)

This equation can be used to derive an expression for the equilibrium surface coverage. For Eq. (19). then and z , = 1. f ( 0 ) = 1 . ~ 0 the ~ . adsorption isotherm is

0, ~~

1

-

-

0,

=

K"CA+ exp[:flB)/RT]

exp[-FEIRT]

(23)

For a homogeneous surface. .f'(O) = 0. and the Langmuir isotherm. Eq. (20), is obtained. Because of the appearance of HA in the exponential term. the variation of surface coverage will be more sensitive to this term than to the linear terms

ELECTROCATALYSIS

243

8,. (1 - 8,) for appreciable coverage ( 0 . 2 I 8, I 0.8).Therefore, the linear terms are often neglected, approximated to unity (25, 95, 96), to yield

f ( 8 ) = RT ln(K"CA+)- F E

(24)

Equation (24) predicts a linear dependence of the surface coverage on potential. Such behavior is observed with hydrogen. oxygen and methanol adsorption on platinum and other group VIII metal (97-100).

C. HYDROGEN CHEMISORPTION Since hydrogen atoms participate in electrocatalytic reductions ( 4 , 7, 9, 25, 26, 31) and in fuel cell anodic oxidation (3-13), we shall discuss here some hydrogen adsorption features. Detailed examination of classical adsorption results has been presented elsewhere (7, 99, 101). The nature of adsorbed hydrogen atoms is not precisely known. Polycrystalline platinum, palladium, and iridium show two major peaks for hydrogen upon potential sweep in the positive potential region (97),indicative of multiple adsorption states. Ruthenium and rhodium exhibit only one adsorbed state at the low-potential. weak adsorption region (102. 103). The surface coverage on the first group of metals varies approximately linearly with potential [Fig. 6 (97.98)].in accord with Eq. (24)for a Temkin isotherm.

'I ( V )

FIG.6 . Dependence of surface coverage on potential for hydrogen adsorption on Pt ( O ) , Pd (O),Ir (A),Rh (M), Ru ( O ) ,and 0 s ( A ) in H,SO, (7).

244

GEORGE P. SAKELLAROPOULOS

TABLE V Comparison of Electrosorption Isotherms for Hydrogen on Palladium and Platinum

Palladium"

Platinumb

Potential (V)

0"

AHrdr(kcal/mol)

6"

AHads(kcal/mol)

0.30 0.25 0.20 0.15 0.10 0.075

0.03 0.15 0.31 0.44 0.60 0.72

- 27.5 -27.5 - 27.5 -27.2 - 20.5 - 16.2

0 0.08 0.25 0.44 0.70 0.82

-21.0 - 15.5 - 12.5 - 9.0 -6.5 -5.5

~

Data from (98). Data from (97).

OVERVDLTAGL p [VOLTS]

FIG.7 . Hydrogen adsorption on the three main crystallographic planes of Pt (92).(Reprinted by peimission of the publisher, The Electrochemical Society, Inc.)

ELECTROCATALYSIS

245

Table V gives the change in the heat of adsorption with potential and coverage for polycrystalline Pt and Pd. While AH,,, varies with coverage on Pt as expected, it remains relatively constant on Pd for 0 < 0.4. This may result from firmly adsorbed hydrogen or hydrogen dissolution in the palladium lattice (104). This behavior of polycrystalline catalysts appears to result from a similar behavior at the microscopic level. Single-crystal studies with platinum, using the potential sweep method (90, 92, 101, 105), show that multiple adsorption states exist on all three main crystallographic planes [1001, [I lo], [l 113 (Fig. 7). The surface coverage-potential characteristics of monocrystals [Fig. 8 (101, 106)] confirm induced surface heterogeneity on each plane for intermediate coverages. Adsorption is stronger on the [1001 plane than on the [1111, the two planes that contribute primarily to hydrogen adsorption on polycrystalline platinum surfaces (106). Minor adsorption states on the latter may arise from sites of lower coordination number. The existence of various hydrogen species on platinum, palladium, and iridium surfaces has also been verified by in situ modulated specular and internal reflection spectroscopy and surface conductance measurements

POTENTIAL (V) FIG.8. Adsorption isotherms of hydrogen on [loo] (0) and [111] ( 0 )planes of Pt monocrystals. Dashed lines show theoretical curves assuming lateral interactions between surface H atoms (106). (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)

246

GEORGE P. SAKELLAROPOULOS

(107. 108).Two strongly adsorbed states of hydrogen were identified on Pt between 0.4 and 0.32 V and 0.32 and 0.2 V. respectively. The first was attributed to atomic hydrogen adsorption on grain boundaries, kinks, dislocations, or the [loo] plane and resulted in low coverage (107). The second was assumed to have a proton-like structure with an electron released into the metal conduction band (108). A weak adsorption state at 0.2-0.05 V was considered similar to gas phase weakly bound hydrogen (109).consisting of localized covalent bonds between surface metal atoms and adsorbent. Hybrid surface bonding and adsorbate relocation have also been proposed for hydrogen adsorption on platinum (90) as discussed earlier. Although these in silu results provide some additional insight into adsorption states, they should be coupled with theoretical orbital calculations to clarify the exact nature of the adsorbing species. At present, no theoretical analysis has been advanced in this area. The recent spectroscopic and monocrystal results are in agreement with earlier observations on the extensively studied hydrogen evolution reaction (HER). This reaction proceeds via the following steps:

*

+ Hf + e+[H] 2[H] -+ H2 + 2*

Volmer reaction

(2W

Tafel reaction

(2W

Heyrovsky reaction

(2Sc)

or [HI

+ H+ + e

+

H,

+*

and is catalyzed by metals in the order (110) Pt > Pd > W > Fe > Cr > Ag > Cu > Pb > Hg

(26)

This is approximately the order of increasing overpotential for the h.e.r. However, all three reactions are not important on all metals. Thus, a hydrogen ion discharge rate-limiting step would be enhanced by metals on which the metal-hydrogen bond is strong. On the other hand, a rate-limiting step involving [HI and H + reaction would be favored by low adsorption strength ( I l l ) . Since the strength of adsorption depends on surface electronic interactions between adsorbent and catalyst, a correlation of reaction rate and d-band character of simple metals is expected [Fig. 9 (112)] similar to gas phase catalysis (23,24).The correlation seems to fail within the noble metal group, and this may result from the existence of multiple adsorption states and the structure, history, and activity of the catalysts used. The d-band vacancies can be modified by appropriate alloying of metals. Figure 10 shows the dependence of i, for HER on the percentage of a component of a bimetallic electrocatalyst (113).Similar behavior is obtained with

247

ELECTROCATALYSIS

-E

-


0

a

-€

.*

Y

0

-g - 4 Pt (low C.D) -2 ~

36

.

44

40

'10 d - B A N D

40

52

CHARACTER

FIG.9. Dependence of the exchange current density of various electrocatalysts on d-band character (112).

-3

1n I"

,

0

I

20

I

40 O/O

I

I

60

00

100

cu

FIG.10. Effect of Ni alloying with Cu on the exchange current density of the hydrogen 75°C. evolution reaction (113): (0)10°C; (A) 38°C; (0)

248

GEORGE P. SAKELLAROPOULOS

other mixed catalysts (79), indicating that the lowest exchange current density (and catalyst activity) is reached when the d band is filled. Currently, little information exists on the surface structure, metal interactions, and adsorption characteristics of bimetallic clusters in an electric field. Recently developed electron spectroscopic techniques and comparison with similar studies on conventional mixed catalysts (114, 115) could shed some light on the catalytic action of bimetallic electrodes. The voluminous results on the overpotential of various electrodes for the h.e.r. led to early classification of metals into low (Pt, Pd, Rh, Ir), medium (Ag, Ni, Fe, Cu, Mo, W), and high (Hg, Pb, T1) overpotential groups. The Volmer-Tafel mechanism applies to the noble metal group, with Eq. (25b) as rds. The reaction mechanisms for other metals have been discussed widely (6, 8, 76). This classification should not be taken to imply that electroreductions require high-overpotential metals. Difficult organic reductions (25,26,31,33,36,116-118) have proven more successful on low-overpotential, catalytic cathodes without hydrogen evolution or participation of gaseous hydrogen in the reaction (25, 26, 33). Electrocatalytic reactions of hydrogen ions and atoms with other species will be discussed separately.

D. OXYGEN ADSORPTION Continuing interest in hydrogen-oxygen fuel cells has led into detailed investigations of oxygen adsorption and reduction on electrocatalytic surfaces. Although some phenomena involving surface species have been clarified, controversy still exists in the structure of the surface “oxygen layers.” The persistence in using widely varying, nonstandardized techniques and surface pretreatments, without recognizing their effect on the slow oxygen electrosorption, has contributed to this state of uncertainty. Depending on potential, oxygen is bound on the surface of noble metal electrocatalysts as a chemisorbed species or a surface oxide (7, 99). The formation of these surface oxygen layers is independent of 0, presence (7,119)and irreversible, with the exception of iridium (97,120,122).Irreversibility becomes more pronounced in the order: 0 s < Ru < Rh < Pd Pt ( 7 ) . Reductive removal of these oxygen layers is a slow kinetic process, commencing at potentials well below the characteristic potential for the layer formation on each metal. Thus, adsorption results based on the commonly used triangular potential sweep method can depend on the anodic potential excursions, the frequency of potential cycling and the number of cycles, that is, the catalyst surface history. Similarly, kinetic studies of oxygen reduction can be influenced by the dependence of the oxygen layer formation

-

249

ELECTROCATALYSIS

on potential and electrolyte concentration as well as by the inhibitive action of this layer on the reaction rate. Therefore, understanding the structure, nature, and extent of the surface oxygen layers is essential for evaluating the experimental evidence. Several surface oxygen species have been postulated based on kinetic parameters, stable intermediates, and in situ observations during oxygen reduction or evolution. As in the case of hydrogen, platinum is the most well-studied electrocatalyst. Optical measurements (122-126) show that a freshly developed oxygen layer on platinum behaves reversibly up to 0.95 V. However, rapid aging yields an irreversibly bound layer. Chemisorption of O H is assumed to occur in this potential region (123,124,127,128)formed by z*

+ H2O+[OHIZ + H i +

1’

(27)

where z is the number of sites occupied by the surface species. Three different O H species were proposed by Conway and co-workers (123, 129), with z = 1, 2, and 4, formed in successive reactions. Irreversible layers arise from reaction of [OH] to form [O]. which can lead to oxide growth (123.129.130) : [OH] +[O]

+ H’ + c

(28)

Above 1.0 V. a platinum-oxygen species is formed which. upon cathodic potential sweep, reduces to Pt in one step, without forming an [OH] intermediate (124). The conditions under which the surface oxygen is an adsorbed species or a true oxide are not well understood. Coulometric studies on platinum show that the oxygen surface coverage increases linearly with potential between 0.8 and 2 V (120.125.127.131). while at higher potentials a limiting coverage is observed (132, 133) (Fig. 11). The limiting coverage corresponds to an oxygen monolayer consisting of about two oxygen atoms per surface Pt atom (PtO,). Similar behavior is exhibited on rhodium with a possible surface structure of Rh,O, or R h o , (99, 134). From ellipsometric studies (122-126, 135, 136), it is found that the thickness of the oxygen layer on Pt reaches a limiting value of 0.8 nm (135),which is too large for a monolayer with both bound oxygen atoms above the metal surface. Parsons and Visscher (135)proposed. therefore, a surface structure with one oxygen atom above the surface and a second 0 sandwiched between two Pt atom layers (Pt-0-Pt-0). This model is not very different from Schuldiner’s (137, 138) and Hoare’s (139-141) models involving “dermasorption” of oxygen in the lattice and formation of a platinum-oxygen “alloy.” The latter models, however, assume possible adsorption of oxygen in deeper metal layers, with the top two atom layers available for easy cathodic reduction.

250

GEORGE P. SAKELLAROPOULOS

FIG.11. Oxygen electrosorption with potential: curve 1, Rh (134); curves 2 and 3, Pt (131, 132); curve 4, Pd (134).

The characterization of the surface oxygen film as chemisorbed or as a true metal oxide is not possible with electrochemical techniques. Recently developed electron. ion, or X-ray spectroscopic methods could prove useful, although they can be applied only ex situ. Schubert et al. (142) examined anodically treated electrodes by field ion microscopy. No platinum oxides were identified up to 2.2 V, a region in which limiting surface coverage develops (132). This technique has verified oxide formation on iridium (IrO,), which oxidizes and reduces reversibly (97, 120, 121). Schubert’s results on Pt support some X-ray photoelectron spectroscopic results (XPS or ESCA, electron spectroscopy for chemical analysis) that show PtO, formation only at high anodic potentials (143). The proposed hydroxide species on the Pt surface (143) has been discounted by Winograd and coworkers (144) on the basis of XPS spectra in the 0 1 s region and binding energy measurements. Surface oxides were also proposed for anodized Pd (143,Ru (146,146a),and Ir (247).On Au and Ni, oxides as well as hydroxides [Au(OH), and Ni (OH),] were formed upon anodization (147). These XPS results, obtained at 1-4 V potentials, suffer from some ambiguity owing to the strong hydration of the sample surfaces with concommitant contamination of the 0 1s spectrum. In other cases, electrodes have been exposed to the atmosphere before surface analysis (142), thereby complicating the interpretation of results.

ELECTROCATALYSIS

25 1

While a clearer picture starts emerging for oxygen binding on metal surfaces at highly anodic potentials. the oxygen layer in the low potential regime deserves attention. The latter prevails in oxygen reduction and fuel cells or electrogenerative oxidations of carbonaceous reactants. The surface coverage-potential curves for Pt, Rh, Pd, and Au exhibit an inflection at about 1.0-1.5 V [Fig. 11 (134,135)] corresponding to a surface monolayer of oxygen atoms (PtO). formed via Reactions (27) and (28) above. This structure is attributed to surface oxide formation on Pt and Pd around 0.9 V, and up to 1.5 V. where PtO, and PdO, are also formed (144,145).However, no oxide phase is detected by field ion microscopy on platinum. down to 0.5 V in sulfuric acid (142). In contrast to the high potential region. no sublattice or subsurface oxygen is observed by low-energy electron diffraction (LEED)and Auger spectroscopy (AES) on Pt [1001 and [1111 monocrystals (106, 148). The linearity of the coverage-potential plots (Fig. 11) can be attributed to either activated chemisorption (123-125, 127, 149) or oxide kinetics (131). Although the latter would not predict a limiting surface coverage, this is possible if oxide formation becomes transport limited after the formation of a saturated surface oxide monolayer. In view of the preliminary evidence on the surface species presented above, the characterization of the surface layer as chemisorbed oxygen would appear more likely. Irreversible adsorption under induced surface heterogeneity (Elovich equation) can explain both the steady state and the transient surface coverage results with surface species arising from Eqs. (27) and (28). Surface heterogeneity has been observed by Conway and co-workers (124)for oxygen species electrosorption on platinum anodes. Three oxygen states were identified below a monolayer of OH on platinum, possibly resulting from hybrid bonding and exchange on the surface. Various states of oxygen adsorption have also been considered for Ag electrodes ( 1 4 9 4 The proposed multiple states of oxygen bonding on Pt are analogous to those reported in gas phase catalysis (150-154). Collins et al. (150) identified two oxygen states around room temperature depending on length of exposure, using ultraviolet photoelectron spectroscopy (UPS) and thermal desorption techniques. These states were assumed t o arise from adsorbates competing for the same adsorbent electrons at high coverages and from delocalized chemisorption with a certain domain of influence of the adsorbate. At higher temperatures ( > 500°C) another, nonreactive oxygen state has been reported (151-153).The surface structure of these states, however, is not well understood despite more extensive studying of the gas phase interactions. While some investigators interpret their results as indicative of chemisorption or oxygen dissolution in the lattice or under the surface (150, 151,153,155,156), others propose oxide formation of stoichiometry PtO or

252

GEORGE P. SAKELLAROPOULOS

PtO, (152, 157), similar to some electrochemical oxygen layers. Figure 12 shows a possible structure of platinum oxides on various planes (152). The [loo] plane has a PtO, composition (152,157),while the bulk corresponds to a PtO oxide. Present information does not unambiguously point toward either surface oxide formation or chemisorption. Because of the apparent similarity of some surface oxygen species on catalysts and electrocatalysts, coordinated efforts in both fields using standardized techniques and procedures could resolve uncertainties. Because of the irreversible and not well-understood change of the electrocatalyst surface above 1.0 V, early mechanistic studies were conducted under ill-defined conditions. Thus, while anodic evolution of 0, takes place always in the presence of oxygen-covered electrodes, the cathodic reaction proceeds on either oxygen-covered or oxygen free surfaces with different mechanisms (77,158).The “electrochemical oxide path,” proposed for oxidecovered platinum metals in alcaline electrolytes (159,160),has been criticized by Breiter (7), in view of the inhibition of oxygen reduction by the oxygen layers. Present evidence points to the “peroxide-radical’’ mechanism (77,

-

d

FIG. 12. Lattice structure of bulk PtO; (0) Pt atoms; ( 0 )0 atoms (152).

253

ELECTROCATALYSIS

160-164) as the most probable for acidic or basic solutions and for various electrodes (Pt, Ru, Au, Ni, C ) . According to this mechanism, oxygen reduction in acids involves a concerted rate-limiting step, between 0, and H + with simultaneous electron transfer (77):

*

+ 0, + Hi + e +[HO,]

or

* [O,]

(rds)

(29)

(rds)

(31)

+ 0, = [O,]

+ H+ + e +[HO,]

This is followed by formation of surface [OH] and [H,O,] with possible desorption of the latter:

+ H + + e * [H202]* H,O, + * [H202] + * * 2[OH]

[HO,]

(32) (33) (34)

[OH]+H+ +r*H,O+2*

Since Steps (32)-(34) are in quasi-equilibrium and follow the rds. they are not proven mechanistic paths. However, evidence of H,O, in solution (164166) and the presence of surface [OH] discussed earlier support this mechanism. Kinetic evidence is consistent with the above mechanism, assuming Temkin adsorption on oxygen-free platinum surfaces at low potentials, < 1 V (77).Similar steps can be written for alkaline electrolytes, with reactions (29), (31), (32), or (34) involving H,O instead of H f . The above mechanisms hold for polycrystalline (148,167)as well as singlecrystal platinum [1001 and [lll] (148).The surface appears to be uniformly active for the oxygen reduction, although reaction intermediates are adsorbed more strongly on steps than on the flat surface (148).Kunz (168)suggested a different rds for porous, high surface area, supported platinum [HO,]

+ * +[HO] + [O]

(35)

However, Step (35) is not consistent with the reaction order in H'. In all cases, several surface species compete for catalytic sites. However, the adsorption characteristics of [HO,] appear to predominate and result in the observed kinetic parameters (77). In the mechanistic model, Eqs. (29)(34), nondissociative weak adsorption of molecular oxygen is postulated on platinum metals. with two different structures (13, p. 41): and

/"-"

* * Although isotopic labeling with O'* shows that the 0-0 bond remains intact during 0, reduction on carbon (169), no evidence exists for nondis-

254

GEORGE P . SAKELLAROPOULOS

sociative oxygen adsorption on noble metals. Such a step would be in contrast with gas phase adsorption on platinum (150-1-53), which is atomic except at low (1 10 K) temperatures (151). This difference should be clarified in future investigations since it would imply different reaction routes on platinum metal electrocatalysts.

E. ADSORPTION OF CARBONACEOUS SPECIES The relatively slow rate of hydrocarbon fuel cell oxidations prompted an intensive examination of the adsorption characteristics of organic reactants in the 1960s. Because of the low potential for the development of hydrocarbon fuel cells, such studies have largely subsided today and no modern surface analysis techniques have been applied to characterize intermediates. Conventional adsorption studies of carbonaceous species have been reviewed repeatedly (7, 9-12, 100); therefore, we summarize here only some essential adsorption features for fuel cell and selective electrocatalytic oxidations. 1. Hydrocarbon Adsorption

Heterogeneous catalytic studies show (23,24)that hydrocarbon adsorption on metals roughly parallels that of hydrogen, with some hydrocarbons bound stronger than hydrogen (e.g.. ethylene. acetylene, benzene). Thus, efficient electrocatalysts for the HER are expected to catalyze also hydrocarbon reactions. Indeed, noble metals promote anodic oxidation of ethylene (170); weak adsorption on Pd and Au was considered responsible for incomplete ethylene oxidation to aldehyde compared to C O , formation on Pt. Ir, and Rh. Platinum was the best electrocatalyst (171), exhibiting an optimal adsorption strength with ethylene. compared to simple and bimetallic electrodes (80). resulting from optimal intensity of adsorbate-adsorbent elctronic interactions. Figure 13 shows the familiar “volcano” shape of the rate of reaction with respect to the sublimation heat of metals (which is related to the adsorption strength), with Pt occupying the peak of the curve. The rate declines at the two ends of the volcano curve because of too weak or too strong adsorption at low and high L,. respectively. Alkanes that adsorb more weakly on Pt (23, 24, 172, 173) oxidize more readily than alkenes. Similarly, the difficulty for electrocatalytic. electrogenerative hydrogenation of alkenes on platinum parallels the strength of gas phase adsorption of the substrate ( 3 3 ) : acetylene > ethylene > propylene > cyclopropane. Palladium is a more active electrocatalyst for ethylene reduction than platinum (33).in agreement with adsorption strength on each metal. Selectivity and reduction rate of substituted alkenes also depends on adsorption

255

ELECTROCATALYSlS

n

(u

E

0 \

a

Y

>-

-

Iv)

z

W

a

I-

2 W

a a

I

60

I

100

I

I

140

I80

L, ( kc a I/mol) FIG. 13. Rate of ethylene electrooxidation versus adsorption energy (80)

strength (31). Table VI gives the potential of vinyl fluoride reduction over several simple catalysts at fixed rates. The lower the potential at a given rate, the more inefficient is the catalyst due to activation overpotential for the surface reactions. The potential values for the four metals follow a volcano type behavior, with very weak adsorption of the alkene on Ag and very TABLE VI Dependence of Potential and Selectivity of Vinyl Fluoride Reduction in 2 N HCIO, on Heat of Cathode potential vs. RHE (V)

Selectivity for CZHSF(%)

Catalyst

Heat of sublimation (kcal/gm atom)

8 10-4 (A/cm2)

2 x 10-3 (A/cmz)

8 x 10-4 (A/cm2)

2 x 10-3 (A/cm2)

Ru Pt Pd *g

160 135 91 68

0.05 0.23 0.21 0.02

- 0.03 0.17 0.18

3 0 53

12

Data from (31).

* Selectivity = rC2HIF/Zrprodue,s [cf. Eq. (67)].

0.002

-

0 62 54

256

GEORGE P. SAKELLAROPOULOS

strong adsorption on Ru. Selectivity, however, depends not only on the strength of adsorption but also on the intermediates stabilized on the surface of each metal. The nature and structure of surface intermediates in hydrocarbon adsorption has been investigated using galvanostatic (constant current) and potential sweep techniques (7, 10, 172-1 74) or radiotracer methods (175). Niedrach's (172, 173) galvanostatic results with C,-C, alkanes and with ethylene indicate the existence of common, partially oxidized surface species, despite differences in the initially adsorbed hydrocarbons. Methane adsorption is very slow, but higher saturated hydrocarbons adsorb faster and at similar rates. Potentiostatic adsorption followed by an anodic potential sweep gives two peaks [Fig. 14 (174)] corresponding to different adsorbed species. The intermediate responsible for the peak at low potentials (0.7ELECTROLYTE, IN HCIO, T. 60.C

n

5dl, U,= 0.30 V (DURING T,) ( 1 ) T t ' I O M S E C (BLANK\ (21 =20 SEC (31 = 4 0 SEC (41 :80SEC (5) z200SEC

50

0.5

150 Tr,MSEC

100

1.0

1.5 LINEAR ANODIC SWEEP POTENTIAL, V

15 sec A

10 sec B

C

D

E

PULSE SCHEDULE

FIG. 14. Response of smooth platinum anodes during linear anodic potential sweep for ethane adsorption at 0.3 V in 1 N HC10,(174). The anodic and cathodic pretreatment schedule of the electrode is shown at the bottom of the figure. The sweep follows ethane adsorption for time TE,corresponding to the various curves.

257

ELECTROCATALYSIS

0.8 V) is named type I or 0 type in the literature (174, 176) and does not desorb cathodically (177,178).This intermediate appears in about the same amounts with C,-C3 alkanes and is considered (176, 179)as a partially oxidized single-carbon species (CHO) occupying one to three surface sites. Unsaturated hydrocarbons do not appear to form type I surface intermediates. The broad peak of Fig. 14 around 1.2 V is attributed to a type I1 or CH-a intermediate (174, 176,178,180) which desorbs cathodically. Type I1 species probably are partially dehydrogenated hydrocarbons of a structure C,H, and their amount depends on the parent hydrocarbon. Propane forms two type 11 intermediates (180). CH-a and CH-j3 (or type 111), with the latter nondesorbable either anodically or cathodically : a polymeric surface residue is attributed to this species. Type CH-a is assumed to consist of adsorbed alkyl radicals of varying composition, depending on potential. Based on the above information, the following sequence has been proposed (172,173,181,182) for the formation of carbonaceous surface intermediates during alkane oxidation on platinum :

1--

hydrocarbon

/*

[C,H,] Type 11 (CH-a)

[C,H, - i 1 Type 111 (CH-p)

\

+ H,O\

[CHO] Type I

However, adsorption of two different types of species oxidizing a t different potentials has also been proposed (7, p. 137) to account for the different reactivity of species I and 11. Present information does not allow differentiation between these alternatives. Application of surface spectrometric techniques could resolve this ambiguity and could help identify possible structures of type I1 and I11 intermediates. Some insight into surface intermediates has been obtained by on-line mass spectrometry using I3C-labeled propane (183).The parent hydrocarbon does not only undergo dissociative adsorption with loss of H, but it can also yield lower C,-C, hydrocarbons by C-C scission. Thus, methane and ethane were detected at 0.4-0.8 V and 0.2-0.4 V, respectively. Methane was formed with equal probability from all carbon atoms of the original propane molecule. The observed product distributions with potential suggest that in the low-potential regime (<0.25 V) type I1 surface species consist of a mixture of C3H,, C3H,, C,H,, and C2H, and not of any less hydrogenated radicals. The latter are expected to reduce readily at these potentials as shown by electrogenerative hydrogenation of ethylene, propylene, and acetylene on platinum (25, 26, 33).

258

GEORGE P. SAKELLAROPOULOS

While saturated hydrocarbons could adsorb dissociatively (23,24),there is not enough evidence that alkene adsorption proceeds via loss of H atoms, at least below 0.5 V. Gilman (184,185)suggested that ethylene and acetylene adsorb on three catalytic sites on smooth platinum in HCIO,, forming identical intermediates. Figure 15 gives the potential dependence of surface coverage for these hydrocarbons, obtained by determining the occupied surface sites using cathodic desorption. Since hydrogen atoms adsorb and react with ethylene or acetylene below 0.2 V, surface coverage data in this region may not be reliable. If ethylene and acetylene had similar surface intermediates and coverage, their reduction would proceed at similar rates, in contrast to experimental electrogenerative results mentioned earlier (25, 26, 33, 186). In addition, dissociative adsorption of ethylene to form an acetylenic surface compound would result in surface hydrogen atoms in equilibrium with the electrolyte. However, electrocatalytic adsorption of ethylene on platinum black in a deuterated acidic electrolyte (26) shows no exchange and formation of deuteroethylenes at open circuit (around 0.52 V), suggesting associative ethylene chemisorption. Exchange at lower potentials ( 0.2 V) results from reversal of surface ethyl radicals [C,H,] and ethylene desorption (25, 26). The probabilities of H and D addition suggest that ethyl radicals form at low potentials by a concerted reaction of ethylene with hydrogen ions and simultaneous electron transfer N

-=

[C,H,]

+ Hi + e*

[C,H,]

(37)

Whether [C,H,] (y < 4) species are formed above 0.5 V is not clear, although such surface intermediates could arise in the oxidation sequence (78). N -

E

POTENTIAL (Volts) FIG.15. Potential dependence of acetylene and ethylene adsorption on smooth platinum electrodes in 1 N HCIO, at 30°C (184).The charge required for species removal from the surface is proportional to the surface coverage.

259

ELECTROCATALYSIS

The bell shape of the adsorption isotherm of Fig. 15 is quite characteristic of hydrocarbon chemisorption at the double-layer interface. Although such behavior might depend upon the strength of electrode-solvent dipole interactions and the ease of displacement of solvent molecules by the organic (44) (Section III,A,l), it can also arise from coadsorption of substrate and hydrogen or oxygen in aqueous electrolytes. Figure 16 shows the calculated surface coverage of platinum by [HI and [C,H,] in acidic electrolytes below 0.3 V, assuming Temkin adsorption of both species. For molecular ethylene adsorption with no electron transfer and for H + discharge, - we obtain similar to Section III,A,2:

0,

=

KiCH(1 - 0,) exp[ -f(O)/RT] exp[ - FE/RT]

8,

=

KECE(1 - 0,yexp[-z’(@/RT]

8, = 0,

+ 0,

and

f ( 8 ) = r,%,

+ rEO,

W CD

4 Qc Y

> 0

u

W

u

2

pc

a

v)

POTENTIAL vs. N H E 1 V 1 FIG. 16. Estimated surface coverage of hydrogen and ethylene coadsorption under Temkin conditions. Ethylene adsorption is assumed to occur on 4 ((1, u’) or 2 (b, b ) sites. Hydrogen surface coverage, curves (u, b). Ethylene surface coverage, curves (u’,b’). 2 N HC104 electrolyte; P of ethylene, 1 atm.

260

GEORGE P. SAKELLAROPOULOS

where H and E indicate adsorbed hydrogen and ethylene, respectively. The results of Fig. 16 were obtained for a two-site (23)or a four-site (187)ethylene adsorption, by a trial and error numerical solution of Eqs. (38)-(40). Adsorption constants were estimated from literature values of free energies of adsorption [AG; = - 3 kcal/mol in 1 N H 2 S 0 4 (187);AGG = - 8 kcal/mol for 0.5 N HClO, (91a)I. At potentials below -0.2 V, Temkin conditions (0.2 I OT 5 0.8) prevail and hydrogen adsorption with induced surface heterogeneity causes ethylene coverage to decline to zero. The more rapid decline of 8, observed experimentally (Fig. 15) results probably from surface reaction of [HI and [C,H,] known to occur at these potentials (25,26,33). Similar potential coverage behavior is also anticipated a t higher potentials where oxygen adsorption becomes important (0.6-0.8 V). The curve need not be symmetric with potential (cf. Fig. 15) since the adsorption constant for oxygen is different from that of hydrogen. One should notice that the results of Fig. 16 are only approximate. They are based on AGY in different electrolytes because of lack of information on ethylene adsorption in HCIO,; data in sulfuric acid may not be accurate because of addition reactions between olefin and H 2 S 0 4(26).These differences probably result in different maximum surface coverages of ethylene in Figs. 15 and 16. In the above derivation, OE is assumed relatively constant. This could arise, for instance, from strong ethylene adsorption. Independence of surface saturation from temperature (184,185,187)supports this assumption. Other unsaturated hydrocarbons and aromatics adsorb also strongly on platinum metals, in agreement with heterogeneous catalytic adsorption (23). Thus, strong benzene bonding t o platinum gives a relatively constant surface coverage with potential [Fig. 17 ( 1 7 5 ) ] .However, naphthalene adsorption yields a quite pronounced, bell-shaped surface coverage dependence on potential (188).This can arise again from solvent displacement at the double layer, or from hydrogen or oxygen coadsorption. Benzene adsorbs on platinum electrocatalysts associatively (189,190)with loss of its aromaticity (187).The same has been suggested for adsorption on conventional heterogeneous catalysts including nickel and palladium (191, 192). Deuterium exchange results on platinum black (193, 194) were interpreted as indicative of benzene adsorption on two types of catalytic sites between 0.45 and 0.2 V. One type promotes a six-site adsorption of the benzene ring with multiple exchange. The other type favors two-site u bonding and stepwise exchange. These results, however, were obtained on porous electrodes pretreated at length anodically in the surface oxide region. Superimposed diffusion (193,194)may have also complicated interpretation of this non-steady-state exchange on the surface.

26 1

ELECTROCATALYSIS

a W

>

0 0 W

-

0

I

I

I

POTENTIAL vs. RHE (volts) FIG.17. Benzene electrosorption on Pt in 1 N H,SO, at 50°C (175).

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 2. Adsorption of Oxygenated Species The adsorption of alcohols, aldehydes, and carbon oxides on metal electrocatalysts has been extensively studied because of the significance of their oxidation reactions for electrochemical energy generation (7,9,81,195). Particular attention has been payed to the surface intermediates of methanol oxidation on platinum. At least two adsorption states have been assigned to methanol, a weak one possibly associated with physisorption (196) and one or more states arising from dissociative strong adsorption of the reactant (197, 198). Breiter (199) proposed a parallel scheme for methanol oxidation

262

GEORGE P. SAKELLAROPOULOS

in acid electrolytes yielding electrolyte-soluble HCHO and HCOOH and a difficultly oxidized surface poison [C,H,lO,] :

CH,OH

-.

/ [C,H,,O,I

+

co,

[CH,OH]’

(41) [CH,O]

-+

[HCOOH]

1

1

CH,O

HCOOH

+

CO,

Similar steps have been proposed for CH,OH oxidation in alkaline electrolytes (200)or for HCOOH oxidation (201). Species [C,H,O,] is assumed to form by reactions (42) and (43) involving different adsorption sites (198): [CH,OH] [CH,OH]

+ [H,O]

-+

+

[CHO]

+ 3[H]

[COOH]

+ 5[H]

(42) (43)

With both reactions occurring at the same rate, a surface composition of C,H,O, would correspond to these strongly adsorbed species. However, other compositions with noninteger s, p , and q are likely, depending on the reactivity of each site. The oxidation of these intermediates is considered difficult, resulting in electrocatalyst deactivation (195).Formation of [CHO] and [COOH] has also been postulated for the reverse reaction, that is. the electroreduction of CO, (202),and for formic acid electrooxidation (201, 203,204). One should remember that species [CHO] was also proposed as a surface intermediate of alkane oxidations (type I), reacting only with difficulty (Section IV,E,l). It is, of course, anticipated that the dehydrogenation reaction of methanol to [CHO] proceeds in consecutive steps, with formation of intermediates [CH,OH] and [CHOH] : [CH,OH] -A[CH,OH] -a[CHOH]

-2 [CHO]

(44)

Surface poison [CHO] was tentatively identified recently by ex situ XPS (143).Tritium labeling of the methyl group indicates that rapid dehydrogenation after the first slow step yields species [CHO] in which H originates from the OH group (205).Other isotopic studies with HCHO, HCOOH, and CH,OH suggest the presence of a common C H O species in both acid and alkaline electrolytes (200, 202). Although Reactions (41)-(44) are usually viewed as electrocatalyst deactivation steps (195, 199), surface reactions of these intermediates with adsorbed [H,O] or [OH] havealso been postulated for the overall oxidation process of CH,, CH,OH, HCHO, and HCOOH (81, 206, 207). The presence of certain proposed intermediates such as [COOH], [C,O,H], and

ELECTROC ATA LY SIS

263

[C,O,H] has been deduced from isotopic studies of carbon oxide adsorption and reaction on platinum electrodes (208).At low potentials (0.05-0.15 V) a bridged surface species (A) was proposed for CO, adsorption, while at 0.2-0.25 V, intermediate (B) was formed on a hydrogen-covered surface: H...O=C=O... H

H...O=C=O

*I

*/

*I (A)

(B)

The latter could rearrange to form

on the surface (208).However, CO, “reaction” on the electrode was assumed to involve [OH], the surface concentration of which may be very low in acidic electrolytes. Previous electrochemical studies suggested that CO, and C O adsorb on platinum, forming an identical intermediate [CO] (209,210).In analogy to gas phase reduction of CO on platinum (23, 211, 212), two adsorbed states may exist corresponding to a bridged or a linear [CO] intermediate (213):

The linear adsorption of C O from the gas phase was associated with small, finely distributed crystallites, while the bridged form presumably occurred on a uniform catalysts surface (211). Two adsorption states have also been identified in gas phase CO adsorption on polycrystaline (213) and single-crystal (214, 215) nickel and ruthenium, using IR, XPS, and UPS spectroscopies. Recent isotopic (216),thermal desorption (217), and AES (218) studies indicate that even dissociative adsorption of C O can take place on nickel and rhodium to form surface carbon, which participates in hydrogenation reactions. This surface carbon appears t o have a carbidic structure (218), which, upon morphological rearrangement, can yield an inactive graphitic form that deactivates the catalyst. Participation of the surface carbide in gas phase C O reduction, however, contradicts IR spectroscopic evidence (22,219-221) of oxygenated intermediate [CHOH] as the main surface species. Possible formation of the latter in the reverse oxidation reaction of CH,OH on electrocatalysts has been considered in few investigations (81, 206, 207, 222).

264

GEORGE P. SAKELLAROPOULOS

Apart from single electrocatalysts, bimetallic catalysts of noble and nonnoble metals have been investigated for methanol oxidation. The synergistic effects arising from modification of the adsorption strength will be discussed in Section V,C,1. Macroscopic observations of adsorption of oxygenated species on Pt, Ir, and Rh show a Temkin behavior, with a logarithmic dependence of surface coverage on concentration (195,202-204,223-227). The rate of irreversible adsorption on Pt depends on the surface coverage by oxygen, hydrogen, and methanol in various potential regions between 0.2 and 0.7 V:

Similarly, the activation energy of adsorption depends on surface coverage E, = E i

+ AM fMRT&

(46)

where E i N 9.5 kcal/mol at OM = 0 and I , N 0.5 (100).Adsorption of carbon monoxide also follows a Temkin type isotherm in acidic electrolytes (228). On rhodium, irreversible adsorption on two different sites has been postulated for methanol (100).In contrast to these results, Langmuir adsorption was claimed for Pt based on radiotracer studies (208). The above discussion exemplifies the existing uncertainties and the conflicting postulates on surface intermediates of oxygenated carbonaceous species. It amplifies once more the need for a “synthetic” evaluation of such reactions on well-characterized, standard catalyst and electrocatalyst surfaces using a variety of techniques, possibly in situ.

V.

Electrocatalytic Factors

A. STRUCTURAL EFFECTS Electron microscopic studies in the 1940s proved that supported catalysts possess a crystalline structure, dispelling earlier conjecture of amorphicity. However, practical catalysts are never uniform, exhibiting particle size distribution, lattice defects (Frenkel or Schottky), and dislocations. The following questions then arise : Are all lattice surfaces equally active? Do surface clusters of particles and surface atoms have comparable activity? Does catalytic activity depend on particle size? Is there an optimal particle size or distribution? These questions remain, in general, still unanswered. However, in recent electrocatalytic studies concern about these effects is shown, following similar concern in conventional heterogeneous catalysis.

265

ELECTROCATALYSIS

Adsorption studies of hydrogen and oxygen on monocrystals (90,92,101, 105,106,148,152,157)indicate that all surface atoms are not equivalent and multiple adsorbate states exist (Sections IV,C and IV,D). Furthermore, all planes do not have similar activity; for example, hydrogen adsorbs stronger on a Pt [loo] plane than on Pt [lll] (106). Induced surface heterogeneity can, then, arise on each plane or edge because of different coordination numbers for the various metal atoms, which would result in different “environment” for the adsorbate at each atom site. For instance, atoms at the corners of a [1111 plane of a face-centered cubic lattice, like platinum, have a coordination number of 4; those at the edges and on the face have a coordination number of 7 and 9, respectively (229). As the length of the crystallite edge increases, the coordination number of surface atoms varies (Table VII). Thus, a dependence of activity on crystallite size may be anticipated. Experimental electrochemical results on activity variation with particle size are presently conflicting. Blurton et al. (230) reported a twentyfold decrease in Pt activity for 0, reduction with decreasing particle size below 15 A in diameter. Bett et al. (231) observed no change above 30 A particles and for similar conditions (H,S04 electrolyte, 70°C). Similarly, Kunz and Gruver’s (232)results indicated unimportant activity changes in H 3 P 0 4 for 30-400 A crystallites, while Bregoli (233) reported a fourfold decrease in activity with decreasing size for the same conditions; the largest decrease occured above 50 A. These discrepancies probably arise from different catalyst pretreatments, surface and electrolyte impurities, varying crystallite size distributions, or metal-support interactions. In order to unravel any crystallite size effects, uniform particles and controlled conditions should be used. Turkevich (234,235) gives procedures for deposition of controlled size particles on supports for heterogeneous TABLE VII Dependence of Coordination Number on Crystallite Size for a Pt Face-Centered Cubic Structure’ Number of atoms in edge

Average coordination number

49.5 137.5

2 3 5 10 18 50

co

a,

4.0 6.0 7.5 8.3 8.6 8.9 9.0

Crystallite length (A)

5.5 8.3 13.8 21.5

” Data from (229).

266

GEORGE P. SAKELLAROPOULOS

catalytic studies. Table VII indicates that for a face-centered cubic lattice the particle size should be below about 40 A if an appreciable change in coordination number, and thus in activity, should be observed. Although this size is based on a model lattice (229),other models do not alter this figure substantially (236).For comparison, the number of active sites for each particle size could be determined by surface titration or progressive surface poisoning methods (237,238).One should be aware that electrochemical pretreatment or potential cycling could cause surface rearrangements and roughening, as shown by LEED in Pt monocrystal studies (206).Finally, possible changes in electrical conductivity of supported electrocatalysts with crystallite size should be investigated, since they can affect charge transfer. The anticipated dependence of catalytic activity on particle size should not imply that all electrocatalytic reactions would exhibit such activity variation. A similar problem exists in gas phase heterogeneous catalysis and led to the distinction of reactions into “demanding” or “structure sensitive” and “facile” or “structure insensitive” (239,240).Thus, the hydrogenation of alkenes is in general facile, while isomerization or hydrogenolysis of hydrocarbons is demanding. Attempts to explain these differences are often based on the assumption of multiple bonding of dissociatively adsorbed hydrocarbons at edge and corner atoms of low coordination number (241, 242). Such an explanation, however, cannot account for activity differences observed for reactions on metals of similar structure. In addition, surface changes can make a reaction appear structure sensitive. Stonehart et al. (243) found the electroreduction of oxygen on platinum to be demanding only because of progressive oxide formation on the part of the surface sites.

B. SURFACE STABILITY 1 . Deactivation Processes The operation of electrocatalysts in an electrolyte and in the presence of an electric field imposes stringent conditions on maintaining activity. Avoiding catalyst aging or deactivation is of great economic importance for the successful application of electrochemical processes. Deactivation can arise because of:

1. poisoning by adsorption of ionic or molecular impurities or of solvent; 2. passivation and oxide formation; 3. metal dissolution or corrosion; 4. sintering of particles; 5. loss of porosity or flooding of gas-diffusion electrodes.

ELECTROCATALYSIS

267

Specific adsorption of anions on electrocatalysts competes with that of electroactive ions or molecules and retards reaction by blocking active surface sites. Thus, hydrogen adsorption on platinum group metals proceeds with decreasing heat of adsorption and surface coverage in the presence of coadsorbing anions (244, 245). The effect is stronger for the firmly bonded hydrogen than for the weakly adsorbed state (7, 91a) and it increases in the order OH- < C104- < SO4’- < C1- @ Br- < I-. The decline of adsorption strength and coverage, however, may not imply interactions of coadsorbing H atoms and anions but only redistribution of adsorbing states (90). Reduction of unsaturated hydrocarbons and alcohols is similarly retarded by C1-, Br-, and I - ions (34)to the extent that reaction may become diffusion controlled (246).The anions SO4’- and C1- affect hydrocarbon and alcohol oxidations by shifting the formation of C,H,O, intermediate to more positive potentials (199, 247). Of course, the Type I1 oxygenated intermediate does itself deactivate the electrocatalyst due to its low reactivity. Common catalyst poisons, such as H,S and HgCl,, also decrease disproportionately the reduction rate of alkenes and the working electrode potential under potentiometric operation (34). Although CO is a common poison of noble metal catalysts, its effect is associated with a one-to-one exclusion of H atoms from the surface (12, p. 1 1 5 ) and not with extensive blockage of neighboring sites, usual with strong poisons. Other catalyst poisons, such as traces of metals Al, Zn, Ti, Cu, Mo, and Co (248),that can exist in reactants or arise from gas lines and cell housings, have received only sporadic attention to date. Carbon monoxide, trace metals, and sulfur compounds, such as H,S, COS, mercaptans, and thiophenes, exist in hydrogen produced from coal gasification and used in molten carbonate HJO, fuel cells. In addition, nitrogen compounds from coal, such as HCN and HCNS can be present or they might oxidize to corrosive NO,. While carbon monoxide is reactive in these cells, the rest impurities can either poison the Ni anode or they can attack chemically cell and electrodes (249),for example, H,S sulfidizes nickel and stainless steel. H,S could also undergo oxidation to deposit sulfur (250):

+ 3C0,’- * SO, + H20+ 3C02 + 6e 2H2S + SO, * 2 H 2 0 + (3/x)S,

H2S

(47) (48)

in steps quite analogous to the conventional Claw reaction (17). The small concentration of poisons ( 1 - 2 0 0 ppm) is insufficient to cause large deactivation of catalysts and electrocatalysts via a chemical attack or a simple chemisorption route. Large molecules and particularly organosulfur compounds “anchor” on catalytic sites and “sweep” large surface areas rendering them inactive. Poison chemisorption at the mouth of fine pores

268

GEORGE P. SAKELLAROPOULOS

( - 100 A) of porous electrodes inhibits reactant diffusion into the active part of the pores, or it can sterically block the pore interior entirely. Chemical attack of the pore entrance (e.g., nickel sulfidation) may gradually decrease the pore diameter, and it can retard or totally restrict accessibility of the large internal surface area of a porous electrocatalyst t o the reactants. Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit 0, reduction (119,251,252)and oxidation of carbonaceous reactants (7, 78, 253, 254); however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255).Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). In addition to the above deactivation processes, classical particle sintering may become important in electrolytes even around 100°C (258),contrary to gas phase sintering which usually occurs >5Oo"C. This is not unexpected, since water (in the form of steam) is known to accelerate catalyst sintering in gas phase reactions such as methane re-forming or ammonia synthesis (259). The catalyst support, then, plays the role of a particle stabilizer. Indeed, if platinum is supported on preoxidized graphitic carbon, sintering in hot phosphoric acid is retarded (260). The independence of the sintering rate from reactant, for oxidation of aromatics or glycols, indicates that Pt ions are probably not involved in the deactivation process (258, 260). Sintering may then proceed via surface migration, similar to gas phase catalysis.

2. Surface Aging Models Particle sintering involves atoms that escape and diffuse away from crystallites, eventually being captured by other particles (261),or crystallite diffusion on the surface and subsequent collision and coalescence with larger particles (262). The latter model is unlikely for particles over 50 A (261). Since practical supported catalysts have a broad particle size distribution, a combination of atom and small crystallite diffusion is probable. That more than one process is responsible for sintering is also apparent from the power law kinetics of the aging process (261-264): dS - -kS" dt

_ -

( n = 2-13)

(49)

269

ELECTROCATALYSIS

where S is the total surface area. The sintering step is rate limiting at high temperatures, while at low temperatures the process is diffusion controlled (263). The reactive atmosphere plays an important role in sintering by modifying surface free energies, and thus the adhesion and the wetting angle between crystallites and support. The above-mentioned sintering models explain not only the aging process, but also the experimentally observed redispersion of sintered catalysts exposed to oxygen (265). Since oxygen can form surface oxides, PtO or PtO,, stresses may develop that can cause cracking of a sintered crystallite with subsequent dispersion of the fragments (266). This is not unlikely to occur to an electrocatalyst isothermally during a potential cycling into the surface oxygen layer regime. Surface roughening of platinum monocrystals by potential cycling (106)may be associated with such a redispersion process. Aging and redispersion phenomena of electrocatalysts are currently little understood, and no models exist for their quantitative description. In analogy to supported catalysts for gas phase reactions (266),we can write a continuity (mass balance) equation for the change of surface concentration nj of a particle consisting of j atoms, by capture and emission of i atom particles ( j 2 2): dn. J

j- 1

=

’Y (1 + Sij)Rj+i,inj+i+ t r2 Rj-i,inj-i 1

.

I

i= 1

-

m

i- 1

i= 1

i=l

1 (1 + S i j ) k J , p -j 3 1 kj,inj

Here k,,, and kJ,,are the rate constants of emission and capture, respectively, expressed in numbers of i-atom particles emitted or captured from aj-atom particle per unit time, and S i j is the Kronecker delta ( S i j = 1 for i = j or S i j = 0 otherwise). The first two terms represent the rate of formation of j-atom particles from those of larger and smaller size. The last two terms are the rates of loss ofj-atom particles to form particles of larger and smaller size. The coefficients are introduced to avoid duplication. At all times,

where N o is the total metal loading on the surface (atoms per unit area). For an electrocatalyst, kj,i and k;,, may be functions not only of temperature but also of the electrode-electrolyte potential : k,., = k7.i exp(cc’FE1RT)

ki,i

=

k;,: exp( - aFE/RT)

270

GEORGE P. SAKELLAROPOULOS

Therefore, the total surface area at any time (264) will be also a function of potential and initial loading:

Equations (50) and (51) can be solved numerically to obtain S ( t ) at various potentials. Figure 18 gives simulation results for gas phase sintering, also corresponding to electrocatalyst sintering at E = 0. Depending on M and M', the potential can favor sintering or redispersion in the anodic or the cathodic

..

1

0.'

t

s/s,

0.

'Yo-

0.

I

\ I

t, hours

-

FIG. 18. Rate of sintering of a supported catalyst for various values of surface concentrations N o . All sizes of particles can be captured but only single atoms are emitted. For condition see (266).

27 1

ELECTROCATALYSIS

regime. Such information is presently lacking in electrocatalysis. Electron spectroscopic and microscopic techniques could provide the above kinetic information to characterize crystallite sintering or redispersion and its dependence on electrolyte, potential, and reactants. 3. Pore Poisoning Model

The time-dependent decline of reaction rate (and potential) of a porous electrocatalyst could be modeled in analogy to gas phase deactivation (267268). Figure 19 shows a partly poisoned pore of a gas diffusion electrode. If wp is the surface concentration of the poison in the poisoned part of the pore (moles per unit area of the catalyst) and Cp(x)is the local poison concentration in the pore, the one-dimensional continuity equation for the poison yields

where rp is the pore radius (0.005-0.1 pm). Assuming Langmuirian adsorption of the poison, the rate of surface poisoning becomes

where kads and kdes are the adsorption and desorption rate constants, respectively, that may be potential dependent and w ~ is ,the~ maximum surface

Gas

i*

YEec

I---O

1

;P

FIG. 19. A schematic representation of a poisoned pore of a gas diffusion electrode

272

GEORGE P. SAKELLAROPOULOS

concentration of the poison. These equations are subjected to initial and boundary conditions: Att

=

Oandx 2 0:

A t t > Oandx

=

0:

A t t > Oandx

=

L:

C, = 0

and

wp = 0

c, = 1 2 ac = 0

(55a)

ax

In order to circumvent the complexity of this system of equations, and to obtain some qualitative picture of the poisoning process, one may consider the realistic but simpler case of a strongly and rapidly adsorbing poison (268). Under diffusion-controlled poisoning, then, the pore can be divided into two zones: Region (A) extending to a depth x, is completely poisoned and catalytically inactive. Region (B) consists of a fully active pore, for x > xp.Thus, Region (A): Region (B):

0 < x < x,, xp < x < L,

wp = w , , ~ ,

and

C, # 0

wp = 0,

and

C, = 0

(55b)

In Region (A), Eqs. (54) and (55) simplify now to

Equation (54) is meaningless at x = xp and can be substituted by a local mass balance, which yields from the above boundary conditions (267)

Upon integration, the rate by which the boundary xp shifts with time can be evaluated : xp = (t1’2 (58) When xp = L, the pore is totally poisoned. The rate of a slow reaction, then, in the unpoisoned pore, r(t), can be found relative to the rate of the completely active pore, ro (268): P = r(t)/ro = 1 - X , / L = 1 - ( & / L ) , / i

(59)

Figure 20 shows the decline of the rate in a catalytic pore with time. Such behavior is often observed in gas phase catalytic reactions (264). Similar behavior is anticipated for constant potential operation of electrocatalysts. However, if the reaction is diffusion controlled (h > 0), deactivation proceeds

273

ELECTROCATALYSIS

-0

0.5

FIG.20. Decline of rate with time in a poisoned pore. For h holds. For h = 10, the reaction is diffusion controlled (267).

1.0 =

0, intrinsic reaction kinetics

more rapidly, as indicated in Fig. 20. One should notice that the electrochemical Thiele modulus h is potential dependent (61, 62) and thus, deactivation becomes potential sensitive. For long pores, in which the potential may vary along the pore, an analysis similar to that for nonisothermal catalyst pellets (269) can be followed. Such analyses are presently lacking for electrocatalytic poisoning processes.

C. ADSORBATE-ELECTROCATALYST-SUPPORT INTERACTIONS 1. Polymetallic Cluster Electrocatalysts

Empiricism in early catalytic studies had shown that combining two or more metals could alter catalyst activity and stability. Electronic and structural interactions between the catalyst components were thought to be responsible for the observed changes by modifying the reactant strength of adsorption (115, 270). Electrocatalytic research exploited this idea for fuel oxidations and fundamental studies of electrocatalytic activity (78, 79, 80, 113, 271-274). Primarily, clusters of group VIII and group IB metals have

274

GEORGE P . SAKELLAROPOULOS

TABLE VIll Some Important Electrocatalytic Metal Groups” Group

Metals

Group

Metals

IB IIB IVB VA VB

Cu, Ag, Au Zn, Cd, Hg Ge, Sn, Pb V, Nb, Ta Sb, Bi

VIA VIIA VIII, VIII, VIII,

Cr, Mo, W Mn, Re Fe, Ru, 0 s Co, Rh, Ir Ni, Pd, Pt

a

Data from (23).

been tested. Table VIII gives some metal groups after Topkieff (23) that are important in catalysis. Their crystallographic, physical, and electronic properties are discussed by Bond (23). Depending on the d-band character of the metals or the clusters, adsorption strength can go through an optimum (see Fig. 13). Similar “volcano” behavior has been observed on simple metals for oxygen electroreduction (275),selective reduction of haloalkenes (see Table VI) (31)and for methanol oxidation (Fig. 21) (276).It is interesting to note in reference to Fig. 21 that the optimum activity is not the same at all potentials and it does not correspond to the same composition of the bimetallic Pt-Rh cluster. This variation in optimum activity may be caused by the different adsorption characteristics of oxygen on the two metals. Oxygen adsorbs on Rh at a lower potential than on Pt, thereby deactivating the electrocatalyst surface. Conversely, methanol adsorbs stronger on platinum than on rhodium. Based on these observations and on the experimental adsorption isotherms on the clusters, Rand and Woods (276)postulated that synergism between Pt and Rh resulted from the formation of a homogeneous surface alloy. Additional surface analysis would be required to support this assumption. Although alloying is possible with certain metal components, such synergistic effects are also known for supported catalysts that do not form alloys in the bulk (114).Even with alloying mixed catalysts, highly dispersed clusters consist of surface atoms with compositions not attainable in the bulk. Hence, the surface atomic composition can be quite different for large particles and for finely dispersed clusters, which, in turn, can cause activity dependence on metal dispersion. Such effects have been observed in the gas phase hydrogenolysis of ethane on Ni-Cu bimetallics (277). Thus, following Sinfelt’s terminology, we prefer the use of the term “multimetallic clusters” rather than alloys in referring to mixed catalysts. Synergism via deposition of metal atoms on the surface has been postulated for bimetallic clusters of Pt or Rh with Pb, T1, Bi, and Cd for HCOOH electrooxidation (278).The catalytic enhancement by the nonnoble surface

275

ELECTROCATALYSIS

C

~

20

40

60

80

0

% A t o m s Rh FIG.21. Activity of bimetallic Pt-Rh catalysts for methanol oxidation (276): curve (I), 0.58 V; (2), 0.63 V; (3), 0.68 V. (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)

atoms presumably results from suppressing hydrogen adsorption and its interaction with HCOOH on the surface, which forms a strongly adsorbed intermediate. Similarly, active ruthenium atoms were assumed to incorporate in the oxide film of Pt electrodes to cause a significant decrease in the overvoltage for 0, evolution (279). Synergistic effects owing to differences in adsorption on two metals, have also been observed in gas phase catalysis. Group VIII metals adsorb hydrogen strongly, in contrast to group IB metals, on which hydrogenolysis of alkanes is quite slow (280).Clustering, thus, of group VIII and IB metals gives a marked decrease in hydrogen adsorption strength and in activity for hydrogenolysis (114). Despite phenomenological understanding, the action of synergism is not resolved. It is presently uncertain whether true alloying occurs and what size

276

GEORGE P. SAKELLAROPOULOS

or arrangement of clusters would promote certain adsorption and surface reaction steps. In the presence of electrolytes and electric fields, the structure of such clusters is unknown. Possible oxide formation, local dissolution, and redeposition of one metal or establishment of localized surface galvanic cells could affect the electrocatalytic activity and selectivity. Today's surface spectrometric methods could enlighten our understanding of metal catalyst action. 2. Nonmetallic Electrocatalysts Metals and particularly the noble ones have always attracted attention because of their high catalytic activity. Although economic incentives prompted the examination of nonnoble metal electrocatalysts, few such efforts have proven fruitful. Electrocatalysts that show sufficient activity, but not longevity, include tungsten bronzes and solid organometallic complexes of transition metals, and are discussed in this section. The catalytic activity of phthalocyanine organometallic complexes in hydrocarbon oxidations (281) led to testing such compounds as fuel cell electrocatalysts (282). Phthalocyanine complexes have the structure of Fig. 22 with a multivalent metal, Fe, Co, Ni, or Cu, surrounded by four symmetric nitrogen atoms. These ligands (L) activate the 0-0 bond by forming an adduct with oxygen and thus promote reaction with hydrocarbons L

( a )

+ O2

6+ -+

L

6-

--- O2 R%L

+ R' + HO;

(60)

( b )

FIG.22. Structure of a Fe phthalocyanine (a) and of a Co(I1) porphyrin chelate (b).

277

ELECTROCATALYSIS

The electrocatalytic activity of Co(I1) phthalocyanine is somewhat lower than that of silver for oxygen reduction in fuel cells. Activity varies with the central metal atom in the order Fe > Co > Cu for acid and neutral electrolytes (283).This result is in contrast to Jasinski’s observation of lower activity for Fe than for Co ligands (282).The discrepancy can be explained by differences in the ligand structure and the supports used and possibly their conductivity. For instance, polymeric phthalocyanines show a fourfold increase in activity for oxygen reduction compared to monomeric ones, owing to a lo3-lo4 increase in their conductivity (283). Other metal ligands that can bind oxygen have also been tested for oxygen reduction, such as porphyrins, thiospinels, and diamine (Pfeiffer) complexes (284, 285). Porphyrin complexes (Fig. 22) were the most active, even better than phthalocyanines, with activity decreasing in the order Co(I1) > Fe(II1) > Ni(I1) N Cu(I1). Figure 23 shows potential-current curves for some phthalocyanine and porphyrin electrocatalysts. The higher the current at a given potential, the more active is the catalyst. Some of the observed catalytic properties of transition metal complexes may be explained by II bond formation between oxygen and the central metal atom (286). Electron releasing substituents would increase the electron density at the metal atom and they are expected to enhance activation of the 0-0 bond. Coporphyrin results support this assumption (286).Similarly, the use of supports that would act as electron donors could improve activity. In view of the present results, further research in metal complex electror

oe -

--+ Fe IIIII

0

50

--100

Fe (Poll

150

CURRENT (mA) FIG.23. Potential-current curves for some metal ligand electrocatalysts (284) : solid lines, porphyrins ; dashed line, polymeric phthalocyanine.

278

GEORGE P. SAKELLAROPOULOS

catalysis is desirable, especially in tailor-making stable, conductive, inexpensive chelates with the desirable electronic properties for adsorbate binding. An interesting application could be for selective electroodixations, an aspect that currently has not been exploited. Oxygenated bronzes of various metals are expected to be stable in acid solutions and some have been tested for 0, and H,Oz reduction (286,287)or hydrogen oxidation (288,289).Oxygenated bronzes arise by doping oxides such as WO, with alkali or alkali earth metals as well as rare earth elements. At present, the sodium tungsten bronzes (Na,WO,) are the most studied because of their high conductivity. For x < 0.25, these bronzes behave like semiconductors ;at higher sodium contents they become metal-like. Although several models have been proposed for the electronic structure of doped tungsten bronzes (290), they do not fully explain the properties of these materials. Doped sodium tungsten bronzes with platinum (0-400 ppm) exhibit marked improvement in catalytic activity for oxygen reduction, approaching that of platinum. Platinum appears to inhibit the removal of surface sodium atoms, which can give a semiconductor surface behavior to a metallic bronze (291). Platinized tungsten bronzes are also active for hydrogen electrooxidation by forming H,WO, on the surface, which permits further oxidation to give H + (288, 289). Other bronzes, such as Ce,,,WO, or Y b o , l W 0 3 ,show little activity for 0, reduction or H,. CO, and CH,OH oxidation (287). However, these bronzes proved better than Au or Ag for hydrogen peroxide reduction. Efforts to develop other tungsten electrocatalysts (e.g., tungsten carbides) for organic fuel oxidations have been thwarted by the high rate of corrosion of these electrodes, especially in alkaline electrolytes (292). 3. Catalyst-Support Synergism; Spillover

The notion that the support acts only as an inert surface extender or even as a physical stabilizer of catalyst particles is no longer sufficient to explain the observed effects of the support nature on catalytic activity. Conventional catalytic and electrocatalytic studies show electronic interactions and synergistic effects between catalyst and support. Thus, electrochemical oxidation of hydrogen on carbon-supported Pt proceeds faster if the carbon surface is preoxidized (293). Whether this may result from removal of inhibiting surface impurities is not clear. However, electron spin resonance (ESR) investigation demonstrates electron donation by platinum to the carbon support. X-Ray photoelectron spectroscopy studies show also that the metal acts as an electron donor to the support, with their interactions depending on the Fermi level of electrons in both (294).However, H-D exchange on Pt black and Pt on graphite was interpreted in terms on Pt6+-H6-

ELECTROCATALYSIS

279

formation, with the carbon support acting as the electron donor, thereby enhancing the reactions (295).Pyrolytic graphite also improved the rate of 0, reduction on Fe phthalocyanine, compared t o unsupported catalyst (296). Improvement was thought to result from synergistic n interactions between this organometallic catalyst (see Fig. 22) and the ring structure of the graphite. Other supports, such as metals, borides, and carbides proved inactive. In addition to electronic interactions, support-catalyst synergism can involve surface diffusion and “storage” of a reactant or intermediate on the support surface. This effect, termed “spillover,” often has been invoked in gas phase reactions involving participation of hydrogen (297). Thus, hydrogenation reactions are promoted by catalytic supports on which hydrogen spillover occurs readily: the support acts then as a hydrogen supplier to the catalytic sites. Further analysis of spillover phenomena and the techniques to study them in the gas phase are given in Bond‘s excellent review of the subject (297). In electrocatalysis some controversy still exists on the significance of hydrogen spillover. Bagotskii and co-workers (298)suggested that spillover may occur in hydrogen oxidation on carbon-supported platinum in acidic electrolytes. However, Lundquist, Stonehart, and others (249, 299, 300) claim no spillover for similar conditions. A more systematic examination of this important catalytic phenomenon is necessary, using electrocatalysts with well-characterized physical, chemical, and surface properties. Techniques such as hydrogen adsorption or desorption, hydrogen titration, and reduction of well-defined oxides (297)could prove useful in this investigation. The significance of hydrogen spillover synergism is well established for Pt-doped bronze electrocatalysts, involving the following steps: WO, + xPt-H H,WO, - xe

-+

H,WO,

(61)

+

WO, + xH+

(62)

Spillover occurs from the platinum atoms to the tungsten oxide to form a readily oxidizable intermediate (288, 289). Similarly, oxygen reduction on Pt-doped sodium bronzes may proceed via intermediate spillover from Pt (on which reaction occurs) to the bronze thereby freeing the platinum surface for reaction (301).

D. REDOXCATALYSIS Redox couples of multivalent ions (76)can act as rate or path modifiers for electroorganic reactions. Their action involves change of a metal ion valence by electron transfer to generate an active form. The latter reacts in

280

GEORGE P. SAKELLAROPOULOS

solution with the reactant, simultaneously restoring the metal ion to its original valence : for example (302), 4Ce'+ - 4r + 4Ce4+ 4Ce4'

+ H,O + C,H,CH3

+

C6H,CH0

(63)

+ 4H+ + 4Ce3+

(64)

Thus, the overall reaction does not change ideally the concentration of the metal ions, although irreversibilities and side reactions can decrease efficiency. Promotion of Reaction (64) occurs in solution. However, the rate of this reaction depends on the nature of the active metal ion as well as on the rate at which the latter is generated at the electrode. Thus, redox reactions can be considered as electrocatalytic. Several redox couples have been tested in organic oxidation including Ce, T1, Cr, and Fe (cf. Sections VI,A and VII,A). Nonetheless, an empirical, exploratory approach was maintained in all cases. Thus. the effect of redox catalysts on the rate and selectivity of electroorganic reactions is not well understood, despite extensive theoretical analyses of redox couples. In reactions of aromatics a n-electron transfer from the active ion to the aromatic ring has been postulated (303). Homogeneous catalysis by redox metals is also known for nonelectrochemical processes. Thus, ethylene is oxidized to acetaldehyde in the Wacker process in aqueous solutions containing Pd2-+ (304).Apart from complex formation and insertion (305),ionic oxidation and reduction may take place. It is noteworthy that palladium oxidation to form ions that act as homogeneous catalysts has been suggested as an important step in ethylene electrooxidation on solid palladium electrocatalysts (28, 29). In other redox, homogeneous catalytic reactions, palladium ions catalyze propylene oxidation to acetone (306).The Rashig process (307)is based on benzene oxidation with air in the presence of cupric and ferric chlorides. Toluene and xylene oxidize in solution containing organic salts of Co, Mn, and Mo (308,309).It is interesting to note that in some cases, reoxidation of the active metal ion to its original valence is assumed slow, for example, Cu(1) to Cu(1I) (310).It is conceivable that such steps could be assisted and accelerated electrochemically. Conventional processes, then, can provide a starting point for the study and development of new electrochemical redox processes. VI.

Electrocataiytic Selectivity

Catalyst specificity to promote a certain reaction path is of immense importance for process design and economic evaluation. Selective catalysts

ELECTROCATALYSIS

28 1

yield a desirable product with minimum waste of raw materials and reduce the requirements for subsequent expensive separations. The electric field at the electrolyte-solid interface of electrocatalysts constitutes a unique parameter for easy selectivity control compared to conventional processes. However, the combined effect of potential and electrocatalyst on product yield and specificity has been only sporadically explored for multipath reactions (31, 54). Multiple electrocatalytic reactions may involve adsorption on one or more active site, possible surface reorganization of the reactant or of an intermediate and surface reaction with one or several electron transfer steps. One reactant may undergo oxidation or reduction in two or more parallel or consecutive steps or in a combination of competing and series reactions. Alternatively, more than one reactant may oxidize or reduce independently. In the above categories belong oxidations of carbonaceous species, reductions of substituted alkenes and aromatics, halogenations and hydrohalogenations, solvent or electrolyte decomposition during an electroorganic reaction, and possible electrode corrosion. While the last two processes depend mainly on the potential and the electrolyte, most of the other reactions of interest depend, additionally, on the surface structure and catalytic properties of the electrode.

A. SPECIFICITY FACTORS The effect of electrocatalyst and operating conditions on selectivity were examined recently for the reduction of olefinic halides to saturated halides or cleavage products (hydrogenation versus hydrogenolysis) (31). These two steps proceed in parallel at different rates on various electrocatalysts. Thus. the ability for double bond reduction decreases in the order Pd >> Ru > Ag > Pt, although the overall rate is about the same on Pd and Pt (cf. Table VI). Figure 24 shows the extensive variation of reaction specificity with cathode potential as well as the smaller effect of electrolyte concentration. Similar behavior is exhibited by other halides and electrodes (31). The differences in selectivity between catalysts cannot be explained only in terms of the strength of reactant adsorption. A tentative explanation lies in the preference of platinum for concerted addition of protons to adsorbed alkenes with simultaneous electron transfer (25).The electronic structure of the surface intermediate of the concerted step appears to lead to halide cleavage. Palladium, on the other hand, can participate in insertion reactions (305)and promotes surface reaction between hydrogen atoms and adsorbed alkenes (48a). It is possible that palladium adsorbs vinyl halides on two different sites or at two different states, dependent on potential, one of which

282

GEORGE P. SAKELLAROPOULOS

IR-FREE VOLTAGE

VS.

NHE

(volts) FIG.24. Selectivity dependence on potential and electrolyte concentration for electroreductionof vinyl fluoride on Pd (31) : open symbols, 0.5 N HCIO, ;closed symbols, 2 N HCIO,.

gives cleavage while the other undergoes insertion. However, the extent and strength of hydrogen adsorption could also alter selectivity. Thus, at high positive potentials, low surface coverage of Pd by strongly bound hydrogen would favor a concerted reaction and halogen cleavage. At lower potentials, an abundance of weakly adsorbed hydrogen could promote insertion and double bond saturation. Insertion does not take place on Pt, which yields only cleavage products, even at low potentials. The selectivity of palladium and gold for alkene oxidation to aldehydes (28,29,170)was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process (304).Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts (321).In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene; large amounts of palladium chloride promote butene formation at the expense of acetaldehyde (322). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde (323);in the

ELECTROCATALYSIS

283

absence of water, however, vinyl acetate is produced (314), apparently via electrolyte-reactant interactions as in homogeneous catalysis. We should clarify here that the above cited studies are largely exploratory and the role of each parameter in reaction specificity is currently unclear. They show, however, the need for a fundamental understanding of molecular and electronic surface interactions that determine eiectrocatalytic as well as catalytic specificity. Thus, adsorption isotherms, surface states, molecular configurations, electronic distributions, dipole formation, and bond hybridization should be explored for well-characterized catalysts and model reactions in the presence and in the absence of an electric field. The dependence of reaction selectivity on surface structure and morphology is somewhat better understood, although not for electrocatalysts. Even for conventional, gas phase catalysis, most studies have been conducted on platinum surfaces (315).Reaction paths were found sensitive to the surface density of low coordination sites, such as kinks and steps, as well as to specific poisoning in the form of ordered or disordered layers. Thus, surface steps are mainly responsible for breaking C-H and H-H bonds, while kinks break additionally C-C bonds, initiating hydrogenolysis reactions (316). With reactants, then that may undergo hydrogenolysis, isomerization, or hydrogenation, the selectivity of the first two demanding reactions can be enhanced by deliberately tailoring catalysts to contain kinks or by alloying t o increase sites for multiple bonding (315,317). Selective surface poisoning may involve impurities or reaction intermediates, such as carbonaceous layers formed during hydrocarbon reactions. The order or disorder of this carbonaceous layer appears to affect selectivity, with demanding reactions favored by an ordered layer (315). Although alkene hydrogenation is assumed to occur on this layer (318),further characterization of its significance for catalyst selectivity is necessary. Apart from structural and molecular factors for selectivity control, we should also mention the significant role of reactor design, fluid mixing, and transport processes. We shall show later how these factors can be exploited to promote desirable paths.

B. MULTIPLE REACTION ANALYSIS For a comprehensive assessment of electrocatalytic specificity, a quantitative determination of the rate and the kinetics of each possible path is necessary under various conditions. We recently discussed a phenomenological analysis of multiple reactions in parallel or in series to obtain electrode kinetic information and criteria for selectivity control (60, 61). Previous

284

GEORGE P. SAKELLAROPOULOS

analyses have attempted only to explain fractional coulombic numbers and apparent Tafel slopes assuming first-order reactions (319-322). In the following analysis, a closed cell (batch reactor) is assumed for simplicity, in which R independent, nonelementary electrochemical reactions occur : S

C

vkjAj

+ nke = 0

(k

=

1,2,. . ., R )

(65)

j= 1

where S is the number of organic and ionic reactants and products ( A j )with stoichiometric coefficients v k j . Subscript k denotes the reaction path a n d j a reacting species. The reaction rate and current density associated with each path are given by Eqs. (10) and (16) for both chemical and electrochemical paths. For the former. the transfer coefficients and the number of electrons are zero. The total current density of the electrocatalyst is then R

R

i, =

it = k= 1

n,FF, k= 1

Because of its concentration dependence, it would vary with time in a closed reactor. The selectivity of a desirable product P,
where c(, and E,, are the transfer coefficient and the activation energy of the kth path. Equation (67) is the basis for determining electrode kinetic parameters, especially with simple-order, irreversible reactions (60, 61) For a parallel reaction scheme with one key reactant A and with irreversible steps

the specificity of B becomes

The reaction orders a,, a2 can now be determined readily similar to conventional kinetics (323,324).The potential dependence of each path can be found from a semilogarithmic plot of og vs. E at constant C , and temperature.

ELECTROCATALYSIS

285

Figure 25 shows such plots for vinyl chloride and fluoride reduction on palladium (31). Using this analysis, and data for one of the multiple paths, the transfer coefficients of each step were calculated recently (61). If the electrocatalytic reactions proceed consecutively, A XB+ZC 0

a similar approach gives for

0

(70)

&

At constant C , / C , (for instance in a well mixed reactor), a semilogarithmic plot of & versus Egives (a2 - a,)F/RTas the slope. This analysis was applied to the series oxidation of methanol and to the reduction of vinyl fluoride and chloride on platinum to ethylene and ethane (60). Knowledge of a2 for the ethylene reduction from separate experiments (25)permitted estimation of a, . The same methodology is applicable to more complex reaction schemes involving combinations of steps in series and in parallel (60, 61). If the

5

I .o

0.05

1 0 0.

I POTENTIAL vs. N HE ( V ) FIG.25. Specificity-potential relationship for vinyl fluoride (a) and chloride (b) reduction on Pd (61). (Data obtained from Ref. 31. Reprinted by permission of the publisher. The Electrochemical Society, Inc.)

286

GEORGE P. SAKELLAROPOULOS

electrode kinetic parameters of any step are not known, k l ,a l and a1 could be obtained from C,-t data, in analogy to conventional kinetics. The above analysis is, of course, based on the assumption of simple order reactions under Tafel operation and on the availability of sufficiently accurate data ( f5- 10%).With complex reaction kinetics, for example, those involving surface adsorption terms (Eq. 16), a nonlinear regression analysis would yield the best estimate of ak,k,, a j , and E,, for a possible kinetic model. In all cases, use of these parameters for predicting the performance of an electrochemical reactor or the selectivity of a reaction scheme should be restricted within the potential, concentration, and temperature range that they were determined, We should stress here that kinetic information is presently scanty for complex, multiple electrochemical reactions, yet it is essential for the design, optimization, and control of electrochemical processes.

C. SELECTIVITY CONTROL 1. Competing Reactions

Equations (67) and (69) or analogous ones, suggest that optimum yield of a desirable product for a given electrocatalyst can be achieved not only a t the often cited limiting current of one path but also in the kinetic regime. Results for organic halide reductions (31) demonstrate an appreciable selectivity change within a narrow potential range. Optimal conditions can be determined using the above relationships. Thus, if a2 - a, > 0 for two parallel paths, a reactor maintaining high concentration of reactant A would favor species C formation (323,324).A mixed flow reactor or dispersive transport then should be avoided for optimum production of C . The effect of potential on selectivity can override that of concentration (61).Figure 26 shows the sharp change of selectivity with potential for various differences of transfer coefficients (0 = FE/RT). If a1 > t12, C formation is promoted by positive electrogenerative potentials for a reduction. At the traditional negative reduction potentials (6'z -40), then production of C would be totally suppressed. This dramatic difference in selectivities between positive and negative potentials might have caused failure of some reactions studied at unfavorable potentials and concentrations. In the case of more than two paraller paths, one optimum potential or concentration can exist for maximizing the yield of a product. For three paths, the selectivity function (67) for species B becomes

287

ELECTROCATALYSIS

-8

-4

0

4

8

POTENTIAL, 8 FIG.26. Predicted variation of selectivity with potential for two parallel paths of different transfer coefficients: a2 - a, = 0.5; C l = 1.0, p = 1 (solid and dashed lines); p = 100 (dashdot line) (61). (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)

with an optimum potential dependent on the electrode kinetic parameters [Fig. 271 (62). Product B is eliminated completely at either high positive or very negative potentials. The sensitivity of reaction selectivity to small potential excursions for control purposes can be found from Eq. (69) or (72)by differentiation around the operating values O m , 4Bm(62). From Eq. (69) for two paths

where C: = CA/CAo,p = k;CT,,-"'/k;, and CAois the initial concentration of A. The tolerance in potential variation is then dependent on (a1 - a2). The larger this difference, the more sensitive is the specificity to a potential change. The latter should be considered carefully in the design of electrocatalytic reactors. Thick, porous flow-through or flow-by electrocatalysts may affect specificity adversely because of ohmic losses and potential variations in the pores. Because of the functional dependence of electrocatalytic rates on potential and activation energy, Eqs. (10) and (16)-(18), the temperature effect on selectivity will be different from that of conventional processes. In the

288

GEORGE P. SAKELLAROPOULOS

POTENTIAL, 8 FIG.27. Optimum selectivity with potential for three parallel reactions: GI, = 1, u2 = 2, GI) = 0.5 (62).

absence of an electric field, reactions of high activation energy are favored by high temperatures (323, 324). In electrochemical systems, however, the sign and magnitude of the term (al - a,)FE determines whether an elevated temperature would enhance, suppress, or even reverse the conventional effect of the activation energy (61).These features and easy adjustment make the electrocatalyst potential more versatile than temperature change for selectivity control. Of course, both temperature and potential can be used in combination, if desired, for specificity control purposes. 2. Consecutive Reactions

With two electrocatalytic steps in series, the concentration of the intermediate B (Eq. 70) goes through a maximum with time (or space-time for a flow reactor). Solution of the kinetic equations for each species (60) yields for the simple case of first-order reactions

289

ELECTROCATALYSIS

where I = u,k:C",',- l , ha = a2 - a1 and up is the surface-to-volume ratio of the reactor. A maximum, Cg, is exhibited at dCg/dt = 0 that corresponds to a time

for which

Figure 28 shows the intermediate B concentration with respect to the residence time in the reactor for a1 > a2 (Az* = At here). For a, > or2, the unusual behavior of decreasing t,,, and increasing Ci,,,, is observed, contrary to conventional reactions (325).Use of the most negative feasible potential, then, would maximize the yield of intermediate and at the same time it would minimize the necessary surface area of the catalyst. With a1 < a * , however, Cg,,,, increases with t,,, as the potential becomes more positive (60). In this case, an optimization sequence can be

I .o LOCUS of Maxima

0.8

c*, 0.6

0.4

0.2

C

1

I

I

2

I

I 4

I

1 6

I

I

I

8

xz* FIG. 28. The concentration of an intermediate in two reactions in series in a CER or BER. Results for a reduction with a2 - u1 < 0 ; u1 = 1, ci2 = 0.5, p = 0.1 (60).

290

GEORGE P. SAKELLAROPOULOS

devised in which the electrode potential can be changed with time (or with distance in a flow reactor) to maximize the yield of B (60).Such ideas have not yet been applied deliberately to the optimization of product distributions.

VII.

Electrocatalytic Reactions and Mechanisms

A. OXIDATIONS In early attempts to oxidize hydrocarbons electrochemically, organic solvents and corrosion-resistant electrodes (PbO,, C, Pt) were used to overcome low reactant solubility and anode dissolution at extreme potentials, + 1.8 V and up to 4.5 V (326,327).The primary anodic reaction was usually oxygen evolution or solvent decomposition. The electrode material, nonetheless, affected the product even at the small attainable yields. Thus, toluene oxidized to traces of aldehydes on PbO, (311),while on Pt it yielded up to 19% benzaldehyde (326).The catalytic effect of the anode, however, on rate and selectivity was not realized. In search of suitable electrode materials for the oxidation of methanol and formaldehyde, Miiller and Tagekami (328)and Tanaka (329)reported some of the earliest uses of electrocatalysts, although not explicity recognized as such. Hence, the significance of electrocatalysis remained obscure for another 30 years. The catalytic role of anodes to increase reaction rates was realized later, during the extensive catalyst screening program for fuel cell applications (4,6-12, and references therein). In the course of such investigations, extensive information was generated on the adsorption characteristics and intermediates of carbonaceous materials and on their reactivity under various operating conditions. In previous sections (IV,E) we discussed the present state of understanding of hydrocarbon electrocatalytic oxidations and the relevant mechanistic steps involved, on platinum [reactions (36),(41)-(44)]. The goal of maximum energy generation by oxidation of carbonaceous species often thwarted detailed examination of occasional selective oxidations, such as ethylene oxidation to acetaldehyde on Pd or Au (28,29, 170) or to ethylene oxide on Ag (330)or methanol and benzyl alcohol oxidation to formates and benzaldehyde, respectively (6-12,54,250,331).Product yields were usually determined at one potential only or even galvanostatically (330),and the combined effects of potential, catalyst, reactant concentration, and cell design or mixing on reaction selectivity are unknown at present. Thus, reaction mechanisms on selective electrocatalysts are not well understood with few exceptions. For instance, ethylene oxidation on solid pal-

ELECTROCATALYSIS

29 1

ladium appears to involve catalyst oxidation to form palladium ions, followed by homogeneous reaction in solution (28, 29) : Pd 2 pd+ 2 PdZ+

+ C2H42 Pd(C2H4)+ 2 Pd(C,H,)” Pd(C2H4)” + H,O Pd + CH,CHO + 2HC Pd

(79)

(80)

(81) Reaction (79) results in catalyst deterioration and Step (81) forms colloidal palladium particles in solution. Although the reaction is catalyzed by palladium ions, the use of Cu’ and Pd2+ ions, in analogy to the conventional Wacker process, has not been attempted. Mayell (332) exploited ideas from homogeneous and redox catalysis to electrooxidize olefins selectively to glycols, using ferrous-ferric and osmium catalysts in solution. By depolarizing the cathode with oxygen, conventional electrolysis proceeded under considerable electric energy savings. These results were essentially repeated recently for a Pt anode at similar potentials, about 1.6 V (333).Glycol yield decreased with electrolyte pH, but the role of anode potential and olefin concentration on selectivity were not examined. Propylene was similarly oxidized electrochemically to acetone and glycol in the presence of mercurous and thallium salts (334,335).It is noteworthy that increasing anodic potentials ( > 1.8 V) favored cleavage of the propylene chain to yield formic and acetic acid. It is conceivable, then, that unexplored low anodic potentials or electrogenerative oxidation at 0.61.0 V could minimize or eliminate these undesirable side reactions. The significance of the nature of the redox couple and its concentration in solution for controlling reaction selectivity has already been emphasized earlier. Apart from carbonaceous reactants, inorganic species have been oxidized electrocatalytically, albeit only in exploratory studies. Thus, H2S was oxidized to sulfur on platinized porous carbon (250,336) in a process that could be the electrochemical equivalent of the Claw process (337): -+

+

+

H,S

+ 2H,O - 6e +6H’ + SO, 2H,S + SO, +2H,O + (3/x)S,

(82)

(83)

Recently we suggested that these reactions could be used in treating industrial gases electrogeneratively (17), with simultaneous possible energy recovery owing to the spontaneity of reaction (82). By avoiding mixing of a flue gas with air as in a conventional Claus scheme, smaller cleanup reactors could be designed. Reaction (83) might also occur as an undesirable parallel step in molten carbonate fuel cells using carbon monoxide and hydrogen at the anode. In this case, H,S in the anodic stream (e.g., derived from coal)

292

GEORGE

P.

SAKELLAROPOULOS

could react and poison the electrocatalyst. Of course, it may also sulfidize and deactivate the anode (Section V,B,I). Shlatter’s anodic oxidation of SO2 to H 2 S 0 , on platinized carbon promoted with vanadium oxide deserves mentioning (250). Because of its high free energy, this reaction could be used for electrochemical production of sulfuric acid (338).This step could be combined with hydrogen production by water electrolysis (339) in an energy-conserving process. Electricity requirements could be reduced by about 50%. Despite the attraction of such reactions for large-scale application little is currently known about the surface reaction steps and possible side reactions.

B. HALOGENATIONS Electrocatalytic halogenations are in essence electrooxidation reactions with simultaneous introduction of one or more halogen atom into an organic molecule. Ibl and Selvig developed a conventional electrochemical process for chlorinating ethylene and propylene via brine electrolysis (340).Graphite anodes favored chlorohydrin formation while platinum gave glycols. Sodium hydroxide produced at the cathode can be used to produce propylene oxide from the corresponding chlorohydrin. The observed products on the above electrodes are in contrast to other conventional electrochemical chlorinations of olefins, where carbon favored glycol formation (341-343). The discrepancy may result from differences in contact patterns and cell designs. Chlorohydrins and dichloroalkanes are formed in electrogenerative chlorination of olefins on platinum black (50).The former predominate at high currents and low anode potentials, with no side reactions of chlorides, such as chlorine evolution. From the identified products, the following tentative mechanism has been proposed for ethylene chlorination (50):

+ CI[CHZCHZCI] + CI[C,H,]

-c

* [CH,CH,CI] + *

- e +CH2CI-CH,CI

(84)

+ 2*

(85)

+*

(86)

or [CH,CH,CI]

+ H,O

- e +CH,OH-CH,CI

The preference of the monochloroalkyl surface intermediate to react with C1- or H 2 0 is not presently clear. Whether reactions (85) and (86) occur on two different sites or whether species [CH2CH2Cl] undergoes surface reorientation or rearrangement that favors a particular step requires further study. It would be interesting to attempt to alter selectivity by modifying the

293

ELECTROCATALYSIS

adsorption strength of the intermediate, for example, by specific adsorption of ions other than C1-, or of inert organic molecules. The electrogenerative mode has also proved successful in direct and indirect electrocatalytic brominations and fluorinations under mild conditions. Vaporized bromine in nitrogen was used at the cathode to brominate alkenes at the anode, in a process similar to the chlorination above (47). Since bromine can form polyhalogen ions in solution, Br3-, Br5-, which could react nonelectrochemically, the electrolyte was flowed over the catalyst to remove such ions. As with direct chlorination, bromohydrins and dibromoalkanes were formed at platinum anodes. Indirect electrogenerative bromination involves use of an electrolyte containing Br-, but free bromine is replaced by chlorine at the cathode (47). Thus, chloride ions are formed in solution that can compete with bromide ions for reaction with the olefin:

+ 2C1RCH=CHR’ + CI- + BrRCH=CHR‘ + H,O + 2CIRCH=CHR’

- 212 -* RCH(C1)-CH(CI)R’

(87)

- 2r

--+

(88)

- 2r

-*

RCH(C1)-CH(Br)R‘ RCH(C1)-CH(0H)R’

+ HCI

(89)

It appears that surface reaction of the olefin with chloride ions forms a monochloroalkyl radical which further reacts with Br-, C1-, or H,O to give the observed products. The latter contain no glycols or bromoalcohols. The higher rates (or currents) observed in the presence ofC1- in the electrolyte further support the above opening reaction step. In general, the reactivity of Br- appears to be lower than that of C1- and dichloroalkane is formed in solutions of high CI- concentration. Based on the same principle, indirect fluorination of olefins is possible in aqueous solutions containing fluoride ions as well as C1- or Br- (51). Discharge of F- occurs at potentials about 1.5 V, below the reversible E;,/Fpotential, in overall steps like (87)-(89), to yield monofluorinated alkanes. Such reactions are apparently promoted by the electrocatalyst and the unusual electrogenerative potential of the anode. Although the reported yields are relatively low, the technique is intriguing if one considers that conventional fluorination is usually conducted in a very corrosive environment (free F,, HF, CoF,, molten cryolite) at potentials +4 to + 6 V. Information on conventional aromatic bromination and fluorination is at best incomplete, since the reported conditions usually do not give the electrode potential. Benzene was brominated to bromobenzene on graphite with Fe and KI salts as catalysts in solution (344).Toluene gave a mixture of 0- and p-bromotoluene (344); however, the selectivity of the reaction is unknown. Similarly, phenol was oxidized to 0- and p-bromophenol in HBr (345).

+

294

GEORGE P. SAKELLAROPOULOS

Because of the extreme conditions in conventional direct electrofluorination, porous carbon was used in most cases, occasionally promoted with nickel (346-349). The products of such fluorinations are quite interesting for industrial processing : ethylene yields vinyl fluoride and polyfluoroethanes, while acetylene forms difluoroethylene (348); acetone and alcohols give perfluorinated compounds on nickel (349).Unfortunately these studies have focused only on screening possible reactions, and thus little fundamental understanding exists for surface reactions and selectivity control.

C . HYDROGENATIONS Electrochemical reduction of unsuturated hydrocarbons is generally considered difficult, and high overvoltage cathodes have been used traditionally (55, 56). Farrington and Sawyer (350)reported some of the earliest successful reductions of liquid alkenes, 1-octene, 1-decene, and cyclohexene in ethanolic acid solutions, using platinum black electrodes. However, benzene failed to hydrogenate. Although relative rate constants were determined for the reactions, the effect of potential on rate was ignored. Sokol'skii's group (351, 352) examined a large number of catalysts for hydrogenations in solution using the potentiometric method. The rate was monitored by following the potential change of the catalysts. Because of supply of free hydrogen to the catalyst, however, both catalytic and electrochemical reductions proceeded simultaneously. Studies with 1-hexene, cyclohexene, and benzene showed that reduction was faster in acidic electrolytes than in bases similar to conventional hydrogenation (353).This was attributed to surface reaction with weakly adsorbed hydrogen atoms on supported and unsupported Pt, Pd, and Ni in acidic solutions (354).This is in agreement with hydrogen adsorption studies (97, 107), indicating the existence of a weakly bonded hydrogen in the low potentials ( ~ 0 . 1 V) 2 attained by this technique (cf. Section IV,C). The rate, however, of the potentiometric reductions depended on the catalyst history. Thus, reaction was faster and potentials were more positive on hydrogen-pretreated catalysts than on fresh ones, similar to gas phase catalytic hydrogenations (23, 24). Beck and Gerischer (34)used also the potentiometric method to study the kinetics of reduction of various simple-chain and cyclic olefins. Hydrogenation on a vibrating platinized platinum electrode was zero order in alkene and independent of pH in the region 2-8. In the presence of halide ions, specific catalyst poisoning caused decline of the reduction rate. Specific poisoning was not considered in the investigation of ethylene hydrogenation on platinized platinum and palladized palladium by Burke

ELECTROCATALYSlS

295

and others (246).The reaction was limited by reactant diffusion to the electrode. This is understandable at the low reactant concentrations and the geometric structure of the cathodes employed. In the same investigation reduction of acetylene to ethylene and ethane was reported, with the former unexpectedly, at high currents. To explain this, variation in the proportions of hydrocarbons dissolved in the electrolyte and adsorbed on the electrode was assumed. The latter is rather improbable considering the very small adsorption of ethylene and acetylene at the negative potentials of this study (cf. Fig. 15). Possible differences in the transfer coefficients and rate constants of the two consecutive steps of the overall reduction to ethylene and ethane could account for the selectivity variations with current or potential. In contrast to the above results, Davitt and Albright (355) reported kinetically controlled reduction of acetylene and ethylene-acetylene mixtures at positive potentials. Surface reaction of hydrogen atoms with associatively adsorbed acetylene was postulated, based on the negative reaction order in acetylene and on the experimental Tafel slope of about 120 mV. However, possible diffusion in the porous electrode structure was not considered. Pore diffusion could alter the order and the Tafel slope as shown later. Reduction of acetylene as well as ethylene, propylene, and cyclopropane, at positive potential was first reported by Langer and co-workers (25, 26, 33). Mass spectrometric and coulometric analyses showed quantitative hydrogenation of the unsaturates. Reductions were fastest in acidic electrolytes in agreement with the potentiometric results discussed earlier. The relative rates of the reactants on Pt black appeared to follow the strength of adsorption, with acetylene the most difficult to hydrogenate and with cyclopropane reducing readily. Deuterium exchange and kinetic analysis of ethylene electrogenerative hydrogenation on Pt black showed that, in the absence of pore diffusion, the rate is limited by surface addition of the second hydrogen to an adsorbed ethyl radical (25, 26):

+ H+ + e * [C,H,] z + H + + (,*[HI [CZHSI + [HI +CZH6 + 22

[CzH,]

2tCzHsI +[CzH,I

+ CzH6 + *

(90) (91) (92) (93)

The distribution of deuteroethylenes and deuteroethanes suggested that the opening step, reaction (90), should be a concerted reaction between adsorbed ethylene and hydrogen ions in solution with simultaneous electron transfer. The same experimental results indicated that the second step involved preferentially surface species, reactions (92) and (93), and not H + in

296

GEORGE P. SAKELLAROPOULOS

another concerted reaction. Disproportionation of ethyl radicals,, step (93), is probably favored at the upper potential range (> +O. 18 V). These results differ from other investigation on bright and platinized platinum cathodes in sulfuric acid (118). In the latter study, surface addition of the first hydrogen on an adsorbed ethylene was proposed as rate limiting, based only on kinetic data. This mechanism, however, did not explain the observed rate dependence on ethylene partial pressure. Use of sulfuric acid may have interfered with the hydrogenation reaction by forming addition products with the alkene. Furthermore, these electrocatalysts may not be as active as Pt black, especially with a gaseous reactant. This is further supported by inactivity of platinized Pt to reduce acetylene at positive potentials (356) while such electrogenerative reduction is possible on porous Pt black (33). Reduction of acetylenic and ethylenic compounds on catalysts other than platinum, such as Pd, Ru, Au, Ag, C, and W, are few (33,117,118,356a). The electrocatalytic mechanisms and the action of these electrodes are currently far from being understood. Inadequate understanding exists also for the reduction of aromatics on noble electrocatalysts, while mechanisms for high-overvoltage cathodes are well established (55,357). Langer and Yurchak (36) reported the first electrogenerative reduction of benzene, vaporized in a N, stream, in HC104 electrolytes. The rate of reduction decreased in the order Pt >> Pd Ru, in contrast to the activity of these metals for olefin hydrogenation but in accord with benzene catalytic hydrogenation (358). The reaction was assumed to involve hydrogen ion discharge and subsequent surface reaction between adsorbed benzene and hydrogen atoms. Addition of H to an adsorbed C,H, radical was proposed as the probable rds, based only on the Tafel slope of the reaction. This step differs from the rds proposed for gas phase hydrogenation of benzene, for which hydrogen adsorption is considered slow, inhibited by strongly adsorbed benzene (359). If a similar step were operative in the electroreduction, the Tafel slope would be twice the observed value. However, insensitivity of the electrocatalytic reaction to temperature (36) indicates strong benzene chemisorption, in agreement with catalytic results. To understand the electrocatalytic steps, a more detailed characterization of surface species and of the activity of surface sites for prospective catalysts is needed. Some adsorption results only on Pt, discussed earlier, indicate the existence of two differnet catalytic sites that contribute to benzene exchange reactions with deuterium (193,194). Adsorption and reaction on two different catalytic sites is not unknown in heterogeneous, gas phase catalytic reductions of olefinic and cyclic hydrocarbons (360). It is interesting that electrocatalysis promotes benzene reduction even at positive potentials versus NHE in contrast to conventional electrochemical N

ELECTROCATALYSIS

297

reduction at about - 3 V (35). Such a reaction, coupled with the reverse cyclohexane dehydrogenation reaction, could serve as a hydrogen storageenergy generation system (361).

D. REDUCTION OF FUNCTIONAL GROUPS Reduction of organic molecules containing substituent functional groups poses special interest because of the multitude of products that may result. Thus, reduction of nitrocompounds proceeds stepwise to amines, depending on potential. Most studies have focused on polarographic reduction of nitrocompounds with relatively little emphasis on electrocatalytic reduction. Allendoerfer and Rieger (362) showed that, on mercury cathodes in the presence of proton donors, nitroaromatics reduce stepwise, but readily, to hydroxylamines. Reduction to amines is more difficult. requiring more negative potentials. On Pt-Ru electrodes, nitrobenzene reduction proceeds via an electron transfer mechanism or surface reaction with adsorbed hydrogen atoms (363).However. rearrangement of intermediates is also possible on low hydrogen overvoltage cathodes (364).These results refer to conventional electrolytic reduction; at present no mechanistic or selectivity information exists for the positive potential region, although electrogenerative reduction of nitrocompounds is thermodynamically feasible (16). Aliphatic nitrocompounds reduce less readily than aromatic ones, since they lack the stabilizing effect of 71 electrons on the first anion radical formed in the reaction sequence. Addition of the first electron to nitroalkanes is irreversible; the resulting nitroso intermediate may undergo reduction to hydroxylamine or it may tautomerize to an oxime (365).For conventional electrolysis, cathodes of high hydrogen overvoltage (Hg. Pb) give complete reduction, while Cu, Sn, and Ni favor partial reduction to hydroxylamine (366). Noble metal electrocatalysts or positive potentials have not been explored for such selective reductions. Unfortunately, even catalytic hydrogenation of aliphatic nitrocompounds cannot provide reasonable extrapolations and guesses for the electrocatalytic behavior of these reactants because of contradictory results (367. 368). Differences in the history. load, support, solvent. and impurities in each study may account for the discrepancies. Reduction of carbonyl and carboxyl groups is also possible on electrocatalysts, although some of these reactions were believed previously to be difficult. For instance, aliphatic aldehydes and acids have been successfully reduced on Pt at positive potentials (116,369).However, these studies have not addressed reaction selectivity, catalytic action. and activity or identification of surface reactions and intermediates.

298

GEORGE P. SAKELLAROPOULOS

Electrochemical reduction of organic halides has attracted considerable attention. Conventional electroreduction on mercury results only in cleavage of the carbon-halogen bond without reduction of unsaturates (32).However, Cd, Zn, Sn, and Cu form also ethane from olefinic halides (370).An EEC mechanism has been proposed for these reductions [i.e., two electrochemical steps (E) and a chemical (C) step, consecutively] involving a direct electron transfer to the halide and formation of a carbanion (32): R X + e * R ' + X(94) R'

+ e*

R-

(95)

R- + H + * R H

(96)

or R - + H,O*RH + O H (97) Electrocatalytic cathodes, Pt, Rd, Ru, and Ag, do not follow the behavior of medium and high overpotential metals. Thus, electrogenerative reduction of vinyl fluorides and chlorides gives saturated halides or nonhalogenated olefins and alkanes, depending on catalyst, potential, and electrolyte or reactant concentration (31). In contrast to this, conventional electroreduction is pH independent, while fluorides are not reduced (372).Other differences between electrogenerative, catalytic, and conventional electrochemical reduction of organic halides have been outlined recently (31). To explain observed specificities in electrogenerative reduction. the following steps were considered (32): [CH,=CHF]

+ [HI * [CH,CH,Fl

(98)

or [CH,=CHF]

+ H+ + e

6+ --+

6-

[CH, = CH,--\

\

'*'

/

F]

(99)

/

and or

Reactions (98) and (100)could be considered analogous to insertion reactions in homogeneous catalysis. Palladium participates in insertion reactions

ELECTROCATALYSIS

299

readily (3051,and thus it favors double bond saturation. Concerted reactions like step (99), on the other hand, are known to occur on platinum (25.26), thereby promoting cleavage. With chlorides. electrocatalytic reduction might involve formation of a hyperconjugated intermediate. C’--C’-=Cl’+, which could cause hydrodechlorination (372). A comparison with gas phase catalysis reveals that the electrocatalytic reduction of organic halides operates between conventional electrochemical and catalytic processes (31). However, an examination of the adsorption characteristics, identification of intermediates, or multiple surface states and kinetic results are necessary for elucidating the role of electrocatalysis in cleavage and double bond reduction. VIII.

Techniques in Electrocatalytic Studies

Understanding the activity and selectivity properties of electrocatalysts requires the characterization of catalyst surfaces, determination of adsorption characteristics, identification of surface intermediates and of all reaction products and paths, and mechanistic deliberation for complex as well as model reactions. Electrochemical and classical methods for adsorption studies are well documented in the literature (5, 7-9. 23, 24,373). Here, we shall outline briefly some prominent electrochemical methods and some nonelectrochemical techniques that can provide new insight into electrocatalysis. Electrode kinetic parameters can be determined by potentionstatic methods using the methodology of Section 111,D,3.

A. ELECTROCHEMICAL TECHNIQUES 1. Galvanostatic Determination of Surface Coverage

The surface concentration of an adsorbing species can be estimated from the charge required for its electrochemical oxidation or reduction, using a high constant current pulse (37346). Initially, the electrocatalyst is equilibrated with the adsorbate at the desired potential, concentration, and temperature. A galvanostatic pulse is then applied, which causes potential variation with time, obtained on an oscilloscope. Oxidation or reduction of the adsorbate results in a slope change (arrest) of the potential-time trace. from which the duration of the oxidative or reductive process is determined. Thus, the charge (current x time) required for the removal of an adsorbed submonolayer is obtained. To calculate the fraction of the surface covered by the adsorbate, the maximum monolayer surface coverage is also needed. The latter is often determined from experiments not involving the adsorbing

300

GEORGE P. SAKELLAROPOULOS

species but hydrogen (or oxygen) monolayers. With n electrons then involved in the species oxidation or reduction the surface coverage is given by Qj =

qjlnqi

(101a)

where qj and q: are the charges required for the removal of species j and of a hydrogen monolayer, respectively. Although this galvanostatic method is easy and rapid, it often presents some ambiguity in interpreting experimental results. The potential-time arrest may not always be sharp, due to other simultaneous reactions, and the charge associated with the species of interest may not be clearly defined. Simultaneous reactions (e.g.. oxide formation)and possible partial oxidation of the surface species can also cause uncertainties in the number of electrons involved. When dealing with large organic molecules. the monolayer coverage of the adsorbing species may not be the nth fraction of the hydrogen monolayer, owing to steric hindrances. Slow diffusional transport and possible potential variation in the pores of porous electrocatalysts may lead to further ambiguities on surface utilization. Despite the uncertainties involved, this method has given satisfactory adsorption results with noble metal electrocatalysts. The surface coverage can be estimated alternatively from galvanostatic depostion of hydrogen on the surface, in the presence and in the absence of the adsorbing species (373c). IfqH and q i are the charges required to discharge and adsorb hydrogen atoms on the remaining vacant sites and on the whole surface respectively, the surface coverage of species j is (101b) In this procedure, possible reduction of the adsorbed entities by the discharged hydrogen is ignored. It is further assumed that the hydrogen coverage is not affected by the presence of other surface species, which may not be correct, as discussed earlier. Some desorption of the adsorbed molecules may also occur during the potential variation, despite the short duration of the experiments. With some electrodes. such as Pd, in which hydrogen absorption may take place, further inaccuracies are expected. 2. Potentiostatic Determination of Su$ace Coverage Because of the potential dependence of adsorption, it would be preferable to obtain the surface coverage potentiostatically rather than at constant current. In the potentiostatic technique the potential is rapidly changed to a very anodic (or cathodic) value after initial equilibration at the desired potential, concentration, and temperature (189).The resulting variation of current with time. obtained on an oscilloscope. is now a measure of

ELECTROCATALYSIS

30 1

surface concentration. If no other reaction occurs. integration of the current over the required time interval gives the charge for removal of the adsorbed species. From this, the surface coverage can be evaluated as before. In case of surface oxidation or other competing reactions. two potentiostatic measurements, in the absence and in the presence of the adsorbate, give by difference the charge associated with the adsorption process. The potentiostatic method is less ambiguous than the galvanostatic one. Its application, however, requires more sophisticated instrumentation. The rise time of the potentiostat should be fast enough to ensure rapid step change of the potential. Errors may arise from slow rise times as well as from current integration. With porous electrodes, all sites may not be under the same potential; diffusion of reactant into or out of the pores may be slow compared with the potential change, which can lead to incorrect estimates of surface coverage and utilization. 3. Potentiodynamic Determination of Surfuce Couerage

The difficulty of rapid rise times for the potential in the potentiostatic method could be circumvented by using a known rate of fast potential sweep. The latter is imposed upon the working potential with the aid of a signal generator and the current time behavior of the electrode is recorded on an oscilloscope. Thus, in the potentiodynamic method both potential and current change with time (174, 223). Figure 14 shows a typical potentiodynamic i-t trace for ethane adsorption on platinum. By obtaining such curves in the absence and in the presence of the adsorbing species. the charge required for adsorbate removal can be estimated in a manner similar to the potentiostatic method. In order to achieve clean catalytic surfaces before reactant adsorption and reproducible results, it is necessary to establish a repetitive pretreatment schedule of cathodic and anodic pulses of fixed duration for removal of impurities and of oxides. Figure 14 gives such a pretreatment procedure followed by a constant potential equilibration step. It is also important to establish the effect of sweep rates on the i-t curves. At slow sweep rates readsorption of the desorbed species is possible; at fast sweeps oxidative removal of the adsorbate may be incomplete or the capacitance current may not be separable from the reaction current. Because of the imposed potential variation, the potentiodynamic technique presents similar uncertainties as the galvanostatic method. Possible desorption of the adsorbate, owing to potential change, can complicate the results. Oxide formation in certain potential regimes may be more important in the potentiodynamic than in the galvanostatic method. Uncertainties from potential and concentration variations within porous electrocatalysts can be

302

GEORGE P. SAKELLAROPOULOS

intensified in the potentiodynamic method, especially at high sweep rates or with triangular variation (anodic and cathodic) of the potential. Thus, the potentiodynamic method rarely has been applied to porous electrocatalysts. The rapidity of the method has led, however, to its extensive use for adsorption studies with monocrystals as well as with polycrystalline catalysts (90, 99, 101, 172-174). It has proved valuable in screening electrocatalysts, studying surface intermediates, and identifying multiple adsorption states (cf. Sections IV,C-E).

B. NONELECTROCHEMICAL TECHNIQUES 1. Isotopic Exchange and Mass Spectrometry Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26)presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (153,193,194).Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). Evaluation of the electrocatalytic isotopic reaction of alkenes was facilitated by using Kemball’s statistical model to determine the origin of each species and the probability of each step (380,381).The model assumes olefin adsorption and reaction with deuterium or hydrogen to form ethyl radicals. These can revert to ethylene or they can add H or D to give ethane ( i = 0, 1,. . .(4):

[C2H,-iDil

a

tGH4 - P

itL

I

/72H4-iDit * \

~

[C2H,-iDil

(102)

/c2Hs-iDit1

\C2H6-,Di

This scheme does not specify an exact mechanism since the origin of H or D is not prescribed.

303

ELECTROCATALYSIS

At steady state, the surface concentration of each adsorbed species P,,, is given by I

where pImis the probability that species Pl will revert to P,,,. All probabilities plrncan be expressed in terms of four probability parameters (25,26,380,381):

where arrows indicate the probability of occurrence of a certain step such as desorption or addition of X, D, or H, and X represents both hydrogen and deuterium in the intermediates or the products. Parameters p and r express relative rates of the specified steps. while q and s give the selectivities of deuterium or hydrogen addition to adsorbed ethylene or ethyl radicals, respectively. All parameters are positive numbers and they can attain values larger than unity since they are ratios of probabilities. Equation (103) generates 12 simultaneous algebraic equations, which permit calculation of a theoretical isotopic distribution. An optimization algorithm campares this with the experimental distribution and searches for a minimum on a Gauss-Taylor surface to obtain the optimum values of the four probability parameters (26). Table IX gives the estimated statistical parameters for Pt at two different potentials (25).A large value of p suggests that adsorbed ethylene has little chance to desorb before it reacts to form an ethyl radical. Similarly, large I' indicates slow reaction of ethyl radicals relative to their rate of reversal to [C2X,]. These results are in agreement with electrode kinetic data, indicating that the second hydrogen addition TABLE IX Statistical Parametersfor Ethylene Reduction on Pt Black" Potential (V) Parameter P 4

r S

Data from (25).

E

=

0.130V

14.53 1.63 7.16 0.15

E = 0.11OV 3 1.92 0.89 8.40 0.10

304

GEORGE P. SAKELLAROPOULOS

is slow (25), and with conventional gas phase heterogeneous catalysis results (27). The observed values of 4 and s (Table IX) lead to the conclusion that H or D addition is not statistical. The first hydrogen addition should involve a proton. while the second addition should occur on the surface. A mechanism of alkene reduction can then be proposed, consistent with steps (86)- (89) discussed earlier (Section V1I.C). Although the information provided by the isotopic exchange model is unique and unavailable by other methods, this powerful technique has not yet been exploited for other mechanism studies. Transient isotopic studies are usually based on the assumption that the exchange reaction is first order and reversible (381). The mole fraction of all isotopic species is then given by dt = k,( 1 -

t)

where 0 is the extent of formation of isotopic products, Oe is the extent at equilibrium and k , is the exchange rate constant. From experimental values of 0 versus t the rate constant can be evaluated. Barger and Coleman (193, 194) adopted this approach to study the exchange of benzene with deuterium ions on Pt black. Results provided the interesting conclusion that two different catalytic sites exist on Pt favoring simple or multiple exchange (cf. Section IV,E,l ). This technique deserves further application to other surface reactions for characterizing catalyst sites and mechanisms. 2. Surface Spectroscopies In the last 15 years a vast number of techniques have been developed for the study of catalyst surfaces. Each one provides a “view” of the surface from a different perspective, one often complementing another. All are based on the principle of excitation in vacuo of the catalyst surface by electrons, X-rays, ions, or photons and subsequent detection of emitted electrons or ions. Figure 29 shows schematically a surface spectroscopy unit with excitation by any of the above four sources. The emitted electrons are subjected to a magnetic and/or an electric field to isolate narrow bands, 0.1-0.5 eV. and to measure their kinetic energy distribution. The electronic transitions from the valence or core states of the metal, caused by the excitation process, are depicted in Fig. 30. Table X gives the electronic transitions associated with some surface spectroscopic techniques and summarizes methods of detection. X-Ray photoelectron spectroscopy (or ESCA) and UPS are one-electron excitation methods using X rays and UV photons as sources, respectively.

305

ELECTROCATALYSIS

ANALYZER UV RESONANCE LAMP

FIG. 29. Schematic representation of a surface spectrometer with an electron, X-ray, ion, or UV-photon excitation source (382).

1. E

T

FIG.30. Electronic transitions involved in some surface spectrometries (382).

TABLE X Electronic Transitions and Detection Methods in Surface Spectroscopies

Technique

Acronym

X-Ray photoelectron spectroscopy or electron spectroscopy for chemical analysis 2. Ultraviolet photoelectron spectroscopy 3 . Auger electron spectroscopy

XPS or ESCA

4.

INS

1.

Ion neutralization spectroscopy

AES

5. Field emission spectroscopy

FES

6. Energy loss spectroscopy or ionization spectroscopy 7. Appearance potential spectroscopy

ELS or IS APS

8. Field ion microscopy

FIM

9. Secondary ion mass spectrometry

SIMS

10. Low-energy electron diffraction

a

UPS

Numbers refer to those of Fig. 30.

LEED

Probe or excitation method Monochromatic X-ray photon beam (- 1.5 keV) Monochromatic UV Photon beam (-20 ev) Electron beam (- 3 keV)

Low-energy He atoms (-5 eV) Electric field (-3 x 10' vjcm) Monoenergetic electrons ( - 1 keV) Monoenergetic electrons (10-1000 eV) High electric field ( - 5 x 1o8C/cm) Inert gas ions ( 1 keV) Monoenergetic electron beam (10-1000 eV)

-

Electronic transition"

Measurement

1,2

Electron emission vs. energy

2

Electron emission vs. energy Derivative electron emission vs. energy

3 and 4 5 and 6 I and 8 9, 10 11

12 and 13 14 and 15 14 and 15

Electron emission vs. energy Electron emission vs. energy Derivative electron emission vs. energy Derivative X-ray yield vs. incident electron energy Distribution,of fieldionized imaging gas Spatial distribution of sputtered ions Distribution of elastically scattered electrons

ELECTROCATALYSIS

307

In UPS, electrons from the valence band absorb the photon energy and are excited into an unoccupied state above the Fermi level. By using X rays instead of ultraviolet photons, deeper lying electronic states, below the bottom of the valence band, can be excited in XPS. Although X rays penetrate in considerable depth into the solid, the emitted electrons have a mean free path of only 5-15 A, and thus the technique probes in essence the surface. The information obtained depends on the surface chemical environment, which causes shifts in the core level binding energies. Such shifts are associated with charge and valence states of atoms and with extraatomic electronic relaxation induced by neighboring species. X-Ray photoelectron spectroscopy and UPS have been used in gas phase adsorption studies of 0,,CO, HCHO, N,, CzH4,C,H,, CzH6, C6H6,and CH,OH (150-153,214,215,382-385),some of which have been reviewed in Sections IV,D and IV,E. In the same sections, we also reviewed the few applications of XPS and UPS for electrosorption of 0,and for characterization of the oxygen layer on electrocatalysts (143-147, 163). Auger-type transitions involve two electrons. Bombardment of the specimen with high-energy electrons leaves a core hole, which is filled by radiationless rearrangement of higher energy electrons. Energy is conserved in this decay by the emission of an Auger electron or an X-ray photon. With the usual energy of the incident beam (2-3 keV), the Auger transition predominates. The energy of the Auger electron is fixed with respect to the Fermi energy of the solid, and it does not depend on the energy of the incident beam. Unfortunately, backscattered electrons and the two-electron transition complicate the unique, quantitative interpretation of Auger spectra. However, the ease with which this technique can be coupled with LEED, SEM, or other surface examination facilities has brought AES to the forefront for characterizing surface cleanliness down to 0.1% surface coverage. Electrocatalytic studies using AES are few (106,148,386)and have been discussed in Section IV,D. These and gas phase studies (228,256,387-390) demonstrate the importance of the qualitative information obtained with AES as well as the difficulties in interpreting the Auger fingerprints. Ion neutralization spectroscopy is similar to the Auger process except that the initial hole is created in the ground state of the atomic ion. The electronic rearrangement to a lower state here involves tunneling from the solid surface through the potential barrier between the surface atoms and the incident ions and neutralization of the ions. In this respect, INS is quite similar to FES, in which an electron tunnels through the surface potential barrier induced by an electric field. The energy distribution of the field-emitted electrons, which may originate from anywhere in the valence band, characterizes the valence density of states. Both techniques are very surface specific because of the tunneling requirements. However, few adsorption studies

308

GEORGE P. SAKELLAROPOULOS

of catalysts (382, 391), and none of electrocatalysts, have exploited these methods. The electronic transitions in ELS and IS are the inverse of those in Auger spectroscopy. An incident electron scatters from a valence or core electron which gains the amount of kinetic energy lost by the incident one. In particular, IS involves a core electron excited to the Fermi level, producing a weak edge in the distribution of backscattered electrons. By oscillating the energy of the incident electrons, and differentiating the energy distribution of the backscattered electrons, the latter can be distinguished from the abundant Auger electrons (392). Appearance potential spectroscopy involves detection of electronic transitions not of the backscattered electrons as in ELS, but of secondary processes. The latter include increase in soft X-ray (SXAPS)or Auger electron (AEAPS) emission or decrease in elastically scattered primary electrons (DAPS)(382).SXAPS is not as sensitive as AES for surface chemical analysis. However, SXAPS and IS spectra are easier to analyze than AES, since only one core transition is involved. This makes SXAPS and IS quite convenient for detecting heavy elements on catalyst surfaces. The development by Muller (393) of the field ion microscope gave a considerable thrust to studying surface structures. In a recent modification of this technique, Panitz (394)designed a field desorption spectrometer that promises determination of the crystallographic distribution of species on metal surfaces. Secondary ion mass spectroscopy can also provide some quantitative information of surface species by sputtering atoms with an inert gas ion beam and detecting them by mass spectrometry. Submonolayers of adsorbed gases can be studied by this method (395). For surface structure studies, perhaps the most popular technique has been LEED (373). Elastically diffracted electrons from a monoenergetic beam directed to a single-crystal surface reveal structural properties of the surface that may differ from those of the bulk. Some applications of LEED to electrocatalyst characterization were cited in Section IV (106,148,386).Other, less specific, but valuable surface examination techniques, such as scanning electron microscopy (SEM) and X-ray microprobe analysis, have not been used in electrocatalytic studies. They could provide information on surface changes caused by reaction, some of which may lead to catalyst deactivation (256,257).Since these techniques use an electron beam, they can be coupled with previously discussed methods (e.g. AES or XPS) to obtain a qualitative mapping of the structure and composition of a catalytic surface. A common objection to all surface spectroscopies is the examination of ideal catalysts under structural and operating conditions quite different from those of practical catalysis. This objection becomes even stronger for electro-

ELECTROCATALYSIS

309

catalysts, which normally operate in contact with an electrolyte. The adsorption results, discussed earlier, however, leave no doubt that new vital information has been obtained by these techniques even if they are applied by necessity ex situ. To bridge the gap between ideal and practical catalysts, optical spectroscopies, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and Mossbauer spectroscopy can be used. All have been reviewed recently (373, 396), and some examples have been cited earlier (107, 108). Electron spin resonance has been used in several studies of electroorganic reactions (357,371).It can detect short-lived radicals resulting from electron transfer. Recent application of Mossbauer spectroscopy in situ in electrochemical cells deserves mentioning, although it addressed only the anodic polarization and film stability of Co- and Sn-coated electrodes (397,398).Extension to electrocatalytic studies involving Mossbauer nuclides seems feasible.

IX.

Electrochemical Reaction Engineering

The successful development of electrochemical processes for some of the cited reactions requires an understanding not only of surface interactions but also of the changes in activity and selectivity that may incur from cell design, mixing, potential or current distribution, and transport processes. Here, we shall examine some simple reaction engineering principles that would be helpful for elucidating these effects.

A. ELECTROCHEMICAL REACTORS Several experimental and industrial reactors-cells have been devised, with widely different features, to exploit specific properties of particular reactants (399-401). Thus, one finds batch and continuous flow cells for liquid, dissolved, dispersed, or gaseous reactants, with planar and high-surface porous electrodes, fixed, suspended, or fluidized, and with electrolyte flow past the electrode or through it. Although operational analysis of many of these cells is feasible, it does not give a general perspective or guidelines for the choice and design of new electrochemical reactors. In order to rationalize the yield and selectivity of such reactors for complex catalytic reactions, we consider three basic, idealized, model reactors with different fluid contact patterns: a batch electrochemical reactor (BER) with thorough mixing of the electrolyte, a channel flow reactor (CER) with no axial but perfect lateral mixing (plug flow),and a well-mixed flow reactor (MER). Figures 31 and 32 give some

310

GEORGE P. SAKELLAROPOULOS

Control

Vduma

FIG.31. Typical channel flow electrochemical reactors (CER) with flow-by (a, b) and packed-bed (c) working electrodes. The control volume (d) includes the active electrocatalyst area (61). A, reactant; B, C, products; E, electrolyte; ce, counter electrode; we, working electrode. (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)

E A

n

we

e

Iwe

I

(C)

FIG.32. Typical mixed electrochemical reactors (MER). A CER with infinite recycling (c) behaves like a MER. Symbols as in Fig. 31 (61). (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)

ELECTROCATALYSIS

31 1

typical configurations of CERs and MERs (60-62). Reactor (b) of Fig. 32, with no input or output, could represent a typical BER. The continuity equation for each ideal model reactor yields for multiple reactions (60, 61) BER

CER : R k= 1

MER:

where steady state has been assumed for CER and MER and j = 1,2, . . .,R . Subscript 00 indicates bulk species concentration. The space-time z = &/V, express the ratio of the void reactor volume to the volumetric flow rate of the inlet stream. The term pbs,(1 - E ) converts the surface reaction rate F k , to a pseudohomogeneous, volumetric rate, by considering the bulk density pb, the specific surface area of the electrocatalyst S,, and the reactor voidage E . The operation of a BER with respect to time is equivalent to that of a CER with respect to distance along the electrode. The ideal plug flow characteristic of a CER yields an upper bound in conversion and selectivity. In a MER, because of thorough mixing, uniform concentration and rate prevail with time in the reactor and at its exit, equivalent to a conventional CSTR (325).This reactor model places, then, a lower bound to product yields. Real reactors with axial dispersion or imperfect mixing would operate between the two extremes (267,402,403).Channel flow with axial mixing has been considered in the analysis of flow-through electrodes for particular reactions (404-406); a diffusive transport term ( - V g j VC,) should be added to Eq. (106) to account for axial dispersion (60, 401, 404-406). Mixing in general decreases the reactant conversion but it may improve selectivity. Thus, the yield of a low-order path in a parallel reaction scheme would improve in a MER, as indicated earlier. The uniform concentration and rate in a MER and the simple algebraic form of the continuity equations (107) make this reactor ideal for kinetic analysis of simple and complex electrocatalytic reactions.

-

312

GEORGE P. SAKELLAROPOULOS

B. MASSTRANSPORT PROCESSES Electrocatalytic surface reactions may involve convective or diffusive transport of reactants and products external to the electrode surface or in the porous structure. If the rate of mass transport is comparable to or slower than the surface rate, the electrode kinetic and selectivity behavior will be altered (484 60-62, 407). The continuity equation for a molecular or ionic species diffusing at the external surface of the electrode (407-409) gives in dimensionless form

where D/Dt**

=

a/&* + u**.V*,

Sc

=

p p / D j , and

2 'is the characteristic length of the electrode, Cj, is the surface concentration o f j and D j its bulk diffusivity. Equation (108) can be solved at steady state for a quiescent electrolyte, with usual boundary conditions, to yield (407,409)

where Nu, is an electrochemical Nusselt number and 6 is the thickness of the diffusion layer. Thickness 6 is of the order of 0.03 cm for stationary electrolytes (407);it depends, however, on ionic strength. When reaction is much faster than the rate of mass transport ( C j , = 0), a limiting current is reached, dependent on the bulk concentration of the reactant. Convective mass transport around an electrode may arise from forced or free convection. Solution of Eq. (108) requires coupling with the corresponding equations of motion for forced or free convection (408, 409). For instance, laminar free convection on a vertically oriented electrocatalyst gives variation of the surface rate along the electrode with an average rate (407-410) Nui,av

=

iavy

njFDj(Cj, - C j , )

-

lo1(") -

dx*

dz*

=

~ ( S C G ~ ) ' . (112) '~

r*=O

where Cj. = CT(z*, Sc, Gr), k = 0.66 (410),and Gr = p2g(iY3(Cj,- C j , ) / p 2 is the Grashof number. For fast reaction compared to convection, the average limiting current then becomes proportional to the 1.25 power of the

313

ELECTROCATALYSIS

bulk reactant concentration (407). This behavior was observed experimentally for H + transport at low concentrations (0.1-0.5 N ) in electrogenerative hydrogenation of ethylene on Pt and Pd (25, 407) For surface reactions under developed forced convection at steady state R

where the mass transfer coefficient, k,, is given by (409, 410) k , 9 / D j = 0.67Re0.5S~0.33

(1 14)

At the limiting current, then, the rate would be proportional to the bulk concentration. For operation away from the limiting current. calculation of the surface concentration is necessary to evaluate the external surface rate. For an irreversible first-order reaction under slow convective mass transport, Eq. (113) yields

i = nF(

kok,e-Z8 )Cjm k , + koePa8

Thus, both the current density and its dependence on potential are affected. If pore diffusion is slow compared to reaction, the local concentration and rate in the interior of thin electrocatalysts can be estimated from a mass balance in the pores (411-415)

k= 1

with BC t+hj = 1 &i -0

dx*

where t+hj = C j / C j sin the pores, ?;(t,b) Thiele modulus (411, 412)

at x*

=

0

at x * = l = ?k/?ks

and hk is an electrochemical

314

GEORGE P. SAKELLAROPOULOS

The latter depends on reactant concentration, as in conventional kinetics (267, 402), as well as on the catalyst potential (412). Figure 33 shows the drastic decrease of the local concentration along a pore for a zero-order reaction as the Thiele modulus increases. Similar results apply to other order reactions but with concentration approaching zero only asymptotically. If external mass transport is fast compared to pore diffusion, C j , and r k s refer to bulk properties (i.e., Cj,, ?km). Otherwise, Eq. ( 1 17) should be coupled with Eq. (113) to determine the external surface concentration. In the case of single or parallel reactions, analysis of pore diffusion can be simplified by using an electrochemical effectiveness factor gk(41 1, 412) 8k --- _rk =

-'(*)

=

hi dx* ,\-*=o

rks

g[lO1

- ~;(I+!I)~I/I]

(119)

where x* refers now to the interior of the pores. For 8,+ 1, the electrocatalyst surface area is fully utilized, and intrinsic kinetics are observed. This implies that the reaction is slower than the diffusion process. If the reaction is fast (8,> 3), an asymptotic behavior is reached for which (267,412)

I .o

0.8

0.6

0.4

\

\ha=&

0.2 n

"0

0.2

0.4

0.6

0.8

I.o

X/L

FIG.33. Concentration change in porous electrocatalysts at various Thiele moduli for a zero-order reaction (407). (Reprinted by permission of the publisher, The Electrochemical Society, Inc.)

ELECTROCATALYSIS

315

The rate becomes, then, for an irreversible, simple-order reaction

Thus apparent electrode kinetic parameters are observed (412). In this analysis of Eqs. (117)-( 121), relatively thin electrocatalysts were assumed with high solid and electrolyte conductivity, so that the potential remained approximately constant along the pores. This assumption is adequate for strong aqueous electrolytes and for electrode thickness of the order of 0.04 cm, such as gas-diffusion electrodes. The potential variation in the pores is then only a few (5-10) millivolts (48~1,407, 412). The above approach was used to examine electrode kinetics at thin porous Pt electrodes (48a) and to evaluate the effect of pore diffusion on selectivity (61). It permitted also analysis of gas diffusion electrodes at the limiting current, when a change of the utilized surface area occurred at low reactant partial pressures due to the zero-order kinetics of the reactions (407).

c.

SELECTIVITY AND

TRANSPORT PROCESSES

The results of Sections VI,B and C for multiple reactions still hold for flow reactors. The selectivity function [Eqs. (67), (69), or (71)] apply exactly to an ideal MER, within the reactor and a t its exit. For an ideal CER, the same equations give the local selectivity along the reactor (60-62). The choice of suitable electrochemical reactors for parallel steps depends then on the reaction order of the desirable path with respect to the reactant. Although the surface and volume requirements of a MER are larger than those of a CER, the former would favor a low-order path. An economic trade-off exists, therefore, between reactor costs, subsequent separations of unwanted products, and waste of raw reactants. For a two-reaction sequence (Eq. 70) the batch cell equations (74)-(78) appIy also to an ideal CER, with time replaced by space-time. In a MER, however, mixing alters the yield of each product. Thus, Eqs. (107) yield for first-order series reactions in a MER (60) CX, = (1 + Le-'l')-'

with a maximum intermediate concentration

(122)

316

GEORGE P. SAKELLAROPOULOS

at

A",,

=1 exp[(i--)U] M2 + M1

VG

Comparison of Eqs. (77) and (12.5) shows that a CER always requires less catalyst area and space-time than a MER for maximizing the intermediate yield (60). Diffusive transport in thin porous electrocatalysts can change the above intrinsic selectivities. For example, the specificity of C in two parallel paths (Eq. 68), under slow pore transport of reactant (hk > 3), is given by (61)

Thus, the effects of concentration, potential, and temperature on specificity are weaker in the presence of pore diffusion [cf. Eqs. (69)and (126)l. The ratio of Eqs. (126) and (69) gives a measure of the change of selectivity under slow pore diffusion

This ratio can be less than or larger than unity, depending on p, u j , and c i k . If a, > a2, a positive potential will enhance B formation. Similarly, B is favored by slow pore transport if ky < k; and a, < a2 (61).One should notice that these conditions are detrimental to product B in the absence of diffusive transport. Selectivity enhancement in porous electrocatalysts deserves serious consideration in process design, despite increased surface-area requirements. In Eqs. (126) and (127),the potential was assumed constant within the thin porous electrode. With thick flow-by and flow-through electrodes, potential variation and slow transport in the pores could improve specificity, depending on the transfer coefficients of the parallel steps. Mass transport at the external surface of electrocatalysts can also be exploited for selectivity improvement. Solution of Eqs. (113) for some simpleorder parallel paths illustrates this effect (62).For a, = 1, u2 = 0,

ELECTROCATALYSIS

317

I.o

0.8 0.6

s" i

c>

0.4

I=

0 W

-I

g

0.2

POTENTIAL I 8 FIG.34. Selectivity of two parallel reactions with potential in the presence of slow external mass transport (62): a, = 1, a, = 0; solid lines, a, - a, = 0.5; dashed lines, ul = a,.

where 5 = k y / k M .Equation (128) suggests that q5B may become zero for a set of values of 8, Cx (Fig. 34). This is possible only assymptotically in the absence of transport limitations (5 = 0). Although this feature offers the means to eliminate B, if undesirable, by appropriate choice of potential, it also demonstrates possible failure to observe certain products in electrocatalytic studies at conventional electrolytic potentials. For instance, product B is formed in Fig. 34 under slow external transport only at positive reduction potentials electrogeneratively ; the potential of B appearance shifts to uncoventional, more positive potentials for reductions as the mass transfer coefficient becomes smaller (5 increases). A change in selectivity with potential becomes possible under slow mass transfer even for reactions with equal transfer coefficients, in contrast to intrinsic kinetic conditions (Fig. 34, dashed lines). This concept has not been exploited to our knowledge for selectivity control. In the above conditions, a monotonic change of 4Bwith potential is , a maximum in selectivity is expected at an observed. If a1 > c ( ~ however, optimum potential or concentration (62). These optima can be determined from Eq. (128), or similar ones, at d 4 , / d 8 and d+,/dC,. Figure 35 shows the

e,

GEORGE P. SAKELLAROPOULOS

318

0.8

c u W

c I\

/

0.4-

I

I

\ \\

\\

\

-J

3

0.2 -

I 0' ,C

Y

I

-

-A

I

,

I

--.

I

I ,

n

I

3

I

G

R

10

POTENTIAL, 8 FIG,35. Optimum selectivity with potential for two parallel paths under slow external transport: a , = 1, u2 = 0; a2 - a1 = -0.5; solid lines, p = 1 ; dashed lines, p = 0.1.

existence of such maxima with potential for # 0. The dashed lines indicate the anticipated shift of the selectivity curves for a lower p value. Formation of B is eliminated below a certain potential, although from intrinsic kinetics a high yield of B would have been expected. The particular results of Figs. 34 and 35 and of Eqs. (121)-(128) can be extended to other values of kinetic parameters and to oxidation reactions as well (60-62). The qualitative information here, however, demonstrates the significance of transport processes for electrocatalytic selectivity control and of correctly identifying reaction products at several operating potentials.

D. CURRENTAND RATEDISTRIBUTION IN ELECTROCATALY TIC REACTORS At steady state, the rate and current density of an electrocatalyst in a MER are uniform. In a CER, however, reactant concentration declines along the reactor and current decreases under potentiostatic control for non-zero-order, single, or multiple reactions. Current nonuniformity in a CER becomes more pronounced with decreasing reduction or increasing oxidation potentials (60-62). With slow diffusive transport in porous catalysts, significantly lower potentials are necessary to reach the same degree of nonuniformity as in the absence of pore diffusion (61).

ELECTROCATALYSIS

319

With galvanostatic instead of potentiostatic control, the potential would vary along a CER, which could compound the problem of selectivity variation caused by concentration changes. In both potentiostatic and galvanostatic operation, the nonuniform conditions may promote side reactions, including local electrode deactivation processes. If i" is the local current density at the inlet R

i" =

C

i,(z = 0)

k= 1

the dimensionless current density distribution in a CER for each one of two first-order consecutive reactions is (61)

- exp ( -pAz*e-a2e)}

(131)

and the total i: = i t / i o (cf. Eq. 66). A maximum in i: is observed (60) for (uz - ml) 0 < In p. These maxima depend on the relative magnitude of the first and second step, and they are not observed for single or parallel reactions. Maxima are also observed in the average current density, dependent on potential and space-time [Fig. 361 (60).

Figure 36 demonstrates the rapid decrease of current or energy capabilities of a cell with high depletion of reactants per pass. The shape and shifting of the curves obviates that any potential change in a CER (or BER) should be followed by an adjustment of the space-time for selectivity and energy or rate control. It is interesting that, despite the high nonuniformity of currents along a CER (Fig. 36), a plot of In ( i t ) versus 8 often yields a Tafel-like behavior from which an apparent transfer coefficient and rate constant can be extracted (60-62). Thus, potential-current density data are not sufficient to indicate multiple reactions. At long retention times in the reactor, however, an unusual maximum and subsequent decrease of the average current density with potential occurs for series reactions (60).This results from fast depletion of species A and B with potential at long space-times, but it is not related to zero concentrations or mass transport-limited reactions. Such maxima or limiting currents have been observed in the stepwise oxidation of unsaturated

320

GEORGE P. SAKELLAROPOULOS

2.0 1

p '0.01

Q8 -

0.4 -

lo-' XZ'

FIG.36. Dependence of average current density on potential and space-time for two consecutive oxidation reactions (60).

hydrocarbons (78,416), and they have been attributed to electrode passivation. The above results indicate that incidental choice of space-times (or residence times in BERs) could also yield apparent limiting currents with these reactions.

E. STEADY-STATE MULTIPLICITY Adsorption of more than one species and complex electrocatalytic surface reactions [Eqs. (10) and (16)] may result in nonunique steady-state operation and in current or potential oscillations (31, 78,417). We examined recently conditions for isothermal multiple steady states at planar, and porous, flow-by or flow-through electrocatalysts (418). If a surface reaction involving multisite adsorption exhibits a maximum with respect to concentration, slow reactant transport through the surface boundary layer can yield up to three steady states. The existence of a maximum is necessary but not sufficient for having multiplicity. The latter depends on the electrode potential, which can alter the shape and the position of the maximum, and on the magnitude of the mass transfer coefficient relative to the surface rate constant (418). Thus, as the potential becomes more negative for a reduction, the multiplicity region can be reached and oscillations may develop between two stable steady states. Oscillations could also arise from other simultaneous reactions such as oxide formation

ELECTROCATALYSIS

32 1

(417) from autocatalysis (419). Multiplicity in all these cases is analogous to similar behavior of conventional catalytic reactions under slow mass transport (420, 421 ), with an additional dependence on potential. The same complex kinetics gives multiple steady states isothermally in thin or thick porous electrocatalysts (418).A simple, graphical orthogonal collocation method (422) can show the existence of multiple solutions for concentration within a certain potential range (418). If ohmic losses in the pores cause a potential change within the electrode structure, multiplicity can also arise with respect to potential, even with simpler rate expressions (418). One might parallel the effect of potential on reaction multiplicity to that of temperature in conventional gas phase catalysis (423). In packed-bed, flow-through electrodes, concentration and potential variation within the bed can also give more than one steady state. The convective transport equation with axial dispersion, coupled with Ohm’s law for the electrode potential, was solved recently (418) by polynomial expansion and orthogonal collocation within the bed, to determine multiplicity regions. The question of multiple steady states is not simply an academic problem, since at certain operating conditions a small perturbation can shift the rate, output, and selectivity into a new unfavorable regime. Potential and current oscillations observed experimentally show that such regimes are within the usual operating conditions of electrocatalysts.

X.

Conclusions

The discussion of a number of topics in electrocatalysis, including adsorption phenomena, surface reaction mechanisms and investigation techniques, electrocatalytic activity and selectivity concepts, and reaction engineering factors, may seem at first too diverse. We believe, however, that fundamental principles cannot be divorced from their natural counterpart, praxis. Here, we attempt to establish ties between basic and applied electrocatalysis and with their conventional similes, catalysis, surface physics (and spectroscopy) and reaction engineering. By taking a vitae parallelae perspective, we hope that a “synthetic analysis” of the present state of the art emerges. Conventional heterogeneous catalysis and empiricism could provide a starting point in the selection ofelectrocatalysts for new unexplored processes for chemical production, energy generation or conservation, and environmental control. However, a fundamental understanding of adsorption characteristics, electrode kinetics, mechanisms, adsorbate-support interactions, and deactivation processes are needed for improved electrocatalyst

322

GEORGE P. SAKELLAROPOULOS

designs. Extensive information is available for the first three topics for fuel cell reactions (hydrogen, oxygen, and some simple carbonaceous reactants), but the other factors only recently attracted attention. Insufficient information exists currently for complex selective reactions, limited to phenomenological results with few electrocatalysts and reactants. The design of polymetallic clusters and of catalysts with controlled crystallite size, the exploration of redox catalysts, the tailoring of the physical catalyst structure, and the selection of reactors and operating conditions to enhance or suppress multiple reaction paths await further study. The exploitation of unconventional reduction or oxidation potential regimes for specificity control, which has been only occasionally attempted or appreciated, appears to be especially attractive. The opponents of fundamental studies with idealized electrocatalysts and reactions cannot deny the unique insight into surface molecular and electronic or energetic interactions that new surface and mechanistic techniques generate. A combination of surface spectrometries, isotopic reactions, and conventional electrode kinetics could help unravel some of the surface mysteries. The application of such methods in electrocatalysis is limited at present to hydrogen and oxygen reactants on simple catalytic surfaces. Extension to a variety of model and complex reactions should be attempted soon. The prospective explorer, however, should strive and attend with care the standardization of analytical methods for meaningful interpretations and comparisons. ACKNOWLEDGMENTS The author wishes to thank Rensselaer Polytechnic Institute and the University of Thessaloniki, in whose facilities parts of this article were executed. REFERENCES 1. Grove, W. R., Philos. Mug. J . Sci. 14, 127 (1839); 21, 417 (1842). 2. Berzelius, J. J., Juhresber. Chem. 15, 237 (1836). 3. Grubb, W. T., Nature (London) 198, 883 (1963). 4. Austin, L. G., NASA [Spec. Publ.] SP NASA SP-120(1967). 5. Srinivasan, S., Wroblowa, H., and Bockris, J. O’M., Adv. Cutul. 17, 351 (1967). 6. Liebhafsky, H. A,, and Cairns, E. J., “Fuel Cells and Fuel Batteries.” Wiley, New York, 1968. 7. Breiter, M. W., “Electrochemical Processes in Fuel Cells.” Springer-Verlag, Berlin and New York, 1969. 8. Bockris, J. O’M., and Srinivasan, S., “Fuel Cells; Their Electrochemistry.” McGrawHill, New York, 1969. 9. Vielstich, W., “Fuel Cells” (transl. by D. J. G. Ives). Wiley (Interscience), New York, 1970. 10. Cairns, E. J., Adv. Electrochem. Electrochem. Eng. 8, 337 (1971).

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ADVANCES IN CATALYSIS, VOLUME 30

Solvent and Structure Effects in Hydrogenation of Unsaturated Substances on Solid Catalysts LIBOR

CERVENY AND

VLASTIMIL

RCZICKA

Department of Organic Technology Prague Institute of Chemical Technology Prague, Czechoslovakia

. . . . . . . . . . . . . . . . B. Hydrogenation of Individual Substrates . . . . . C. Comparison of Reactivites of a Series of Substrates. .

I. Introduction

11. Kinetics of Catalytic Hydrogenations in the Liquid Phase A. Definition of Kinetic Region . . . . . . .

. . . . .

. . . . .

. . . . .

335 336 336 338 340

. . . .

343 343 345 346 346

111. Correlation Equations Describing the Effect of Structure of Reacting

IV.

V. VI. VII.

Compounds and Solvents on Reaction Kinetics . . . . . . A. Effect of the Structure of Reactants on Their Reactivity . . B. Effect of Solvents on Reaction Rate . . . . . . . . Hydrogenation of Olefinic Substrates . . . . . . . . . A. Difference in the Effect of Some Types of Catalysts. . . . B. Effect of Structure of Olefinic Substrates on Their Reactivity and Adsorptivity . . . . . . . . . . C. Effect of Solvents on the Hydrogenation Process . . . . D. Solvent-Free Systems and the Effect of Solvent Concentration. Relationships between the Effect of Structure of Substrates and Solvents on Reactivity . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . List of Symbols . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

1.

.

349 356 362

. . . .

368 312 313 314

Introduction

The catalytic hydrogenation of unsaturated compounds in the liquid phase on solid catalysts is the key step in a number of industrial processes. However, a fundamental understanding of the behavior of these reaction systems, especially in large-scale reactors and under changing reaction conditions, is still incomplete. A successful reaction course depends largely on the performance of the catalyst, its activity, selectivity, and lifetime. All of these parameters are considerably influenced by the composition of the reaction mixture, which, fixed in basic features by the goal of the process, can be 335 Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007830-9

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RMICKA

varied by using different solvents. In the case of raw materials, containing several substances that can react with hydrogen, the competitive reactivity demands special attention. This study is a comprehensive review of data reported on the effect of the composition of the reaction mixture on the hydrogenation of olefinic reactants in the liquid phase. It is mainly based on papers published by the authors, which deal with the effect of the structure of the reacting compounds on their reactivity and adsorptivity on hydrogenation catalysts, and with the effect of solvents on hydrogenation in the liquid phase. The majority of these studies were carried out with a view to quantify the particular effects, with the utilization of the LFER (linear free energy relationship) method. On the one hand, new possibilities for the application of these relationships appeared, but on the other, a number of limiting factors were found, connected predominantly with the considerably complex character of the systems involved in catalytic hydrogenation in the liquid phase. II.

Kinetics of Catalytic Hydrogenations in the Liquid Phase

A quantitative investigation of the effect of the composition of the reaction mixture on the hydrogenation of unsaturated compounds in the liquid phase on solid catalysts must be based on an unambiguous kinetic characteristic. To obtain this quantity, it is necessary to define experimental conditions adequately and in such a way that the data reflect the effect under investigation. Along with dissolved hydrogen and some admixtures, a liquid reaction mixture contains one or several reactants, reaction products, and sometimes also a solvent consisting of one or several compounds. All these compounds may affect the kinetics of the hydrogenation reaction. For the purposes of this study, the aim of which consists in a quantitative description of changes in composition of the reaction mixture, it is sufficient to employ the Langmuir-Hinshelwood kinetics ( 1 ) in its simplest form, where “the process of hydrogenation” is characterized by the rate constant and adsorption coefficient obtained from the simplest kinetic equations. A. DEFINITION OF KINETIC REGION Catalytic hydrogenation in the liquid phase with a solid catalyst is a complex process that may be divided (2) into several steps : 1. absorption of hydrogen into the surface film of the liquid, which is in contact with the gas phase;

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2. hydrogen transfer through volume of the liquid to the film at the catalyst surface; 3. molecular diffusion of hydrogen through the laminary film at the catalyst surface; 4. internal diffusion of hydrogen and reactant through the catalyst pores; 5. adsorption of reacting compounds on the catalyst surface; 6. reaction between adsorbed compounds or adsorbed compound and compound in the bulk phase; 7. desorption of the product from the catalyst surface; 8. diffusion of the product through the catalyst pores; 9. external diffusion of the product into the bulk phase. Kinetic studies of catalytic reactions should be carried out under conditions in which the effect of transport on the rate of the complex catalytic process is ruled out. Hence, each study of this type must be preceded by finding the so-called kinetic region, which is a state in which the reaction rate is determined by the surface processes (steps 5 6 , and 7). In earlier papers these requirements were not always met, but at present they are self-evident. The criterion indicating that a state has been reached in which the effect of external mass transport on the reaction rate is ruled out consists in the independence of the reaction rate of the intensity of mixing (3).By a suitable choice of the amount of catalyst, it is possible, with vigorously shaken ( 4 , 5 ) or stirred (6) reactors, to reach this region even for rapidly reacting substances. In those cases where selectivity of several substrates is measured in various media in which the hydrodynamic properties of the liquid phase may be varied to a considerable extent, it is necessary to guarantee the kinetic region for the most reactive compound in all media, because the value of the Reynolds number that characterizes mixing also depends, among other things, on the physical properties of the solvent (density and viscosity) (3).Problems involved in the definition of the kinetic region in hydrogenation under pressure are examined by Cerveny et al. (7), for example. With increasing hydrogen pressure the rate of hydrogenation usually increases in a linear way; at the same time, however, its concentration in the liquid phase also increases, so that requirements on the intensity of mixing when working under pressure are similar to those met with normal pressure. A more complicated problem is that of suppression of the effect of internal diffusion on the reaction rate. In hydrogenation processes the concentration of an unsaturated compound in the liquid state is usually higher by several orders of magnitude than that of hydrogen. If the concentration gradient of the substrate inside the catalyst grain due to diffusion is not too high, the internal diffusion of the substrate cannot have any essential influence on the

338

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RPI~ICKA

reaction rate, which in most cases approaches zero order with respect to the hydrogenated component. In order to reduce the effect of internal diffusion, finely pulverized and not too active catalysts are usually employed. The catalysts are usually obtained by impregnation of the carrier with a solution of an active metal salt, followed by reduction to metal (8).In testing the effect of the structure of substrates on the reaction rate, the values of relative activities may be only partly reduced, owing to internal diffusion compared with “purely kinetic” values, which in principle does not impede a successful application of LFER.

B. HYDROGENATION OF INDIVIDUAL SUBSTRATES Many kinetic equations have been suggested for the description of catalytic reactions (9, 10). The best approximations are usually seen (11)in relations of the Langmuir-Hinshelwood type (I), which assume the adsorption equilibrium of all the components present in the reaction mixture on the catalyst surface. Hydrogenations in the liquid phase are usually of zero order with respect to the concentration of the hydrogenated compound and first order with respect to the partial pressure of hydrogen. This experimental finding may be given various explanations. In the simplest case, the reaction rate of the type A+H2+P

(1)

may be expressed by the equation I/ daA W dt - ‘A

which readily degenerates, as shown below, to an equation of zero order with respect to the substrate and first order with respect to hydrogen. Its form is valid if there is no competitive adsorption of the unsaturated compound and hydrogen. Assuming an ideal behavior of the reaction mixture, the activities of components may be replaced by concentrations. In the kinetic region, the hydrogen concentration in the liquid phase is constant and equal to the equilibrium value of hydrogen solubility in the reaction mixture at a given temperature and pressure. If dilute solutions of an unsaturated compound are used, the hydrogen concentration in the pure solvent given by its solubility may be used in the first approximation instead of the hydrogen concentration

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339

in the reaction mixture. Even so, however, the adsorption coefficient of hydrogen KH remains unknown in most cases. For this reason, if Eq. (2) is applied for practical purposes, the coefficient KHcH is drawn into the kinetic constant, and Eq. (2) then becomes

dc,= rA = dt

~AHKACA

(1

+ KAcA + KHcH + Kpcp + K,c,)

(3)

where k A H is defined by Eq. (5). The catalytic hydrogenation of a number of unsaturated compounds in the liquid phase is adequately described by first-order equations with respect to the hydrogen pressure and by zero-order equations with respect to the concentration of the unsaturated compound A. With increasing hydrogen pressure, the hydrogen concentration in the liquid phase, and obviously also its surface concentration on the catalyst, increase linearly at pressures up to some lo5 Torr, as reflected in the linear rise in the reaction rate (5). If the composition of the liquid reaction mixture varies (e.g., with various solvents), and hence also its vapor pressure varies, the measured reaction rates must be corrected to the same partial pressure of hydrogen with respect to the partial pressure of the liquid mixture. A special situation may arise if reaction products considerably affect the hydrogen solubility, which then varies during the reaction. Such a phenomenon occurs most often in the hydrogenation of the substrate in bulk, without solvent, mainly where the chemical character of the hydrogenation product markedly differs from that of the initial compound [e.g., hydrogenation of nitrobenzene to aniline and water (12)]. In such a case the hydrogen concentration cannot be drawn into the constant, because its varying concentration in the liquid phase is reflected in the form of the kinetic equation. In many such cases the effect of reaction products is also reflected in the kinetic equation. Degradation of kinetic equations to zero-order equations with respect to the concentration of the substrate, which is very frequently met in catalytic hydrogenations in the liquid phase, is evidently due to the validity of the reaction While the concentration of the product cp during the reaction may be compared with that of the initial unsaturated compound, cA, the concentration of inerts, c,, which in the hydrogenation in the liquid phase are represented mainly by the solvent, is often higher by one or two orders of magnitude than the concentration of the hydrogenated compound. In spite of this however, neither the solvent nor the hydrogenation product are operative in equations of type (2) and (3) through their adsorption terms

340

LIBOR

CERVENPAND

VLASTIMIL RWICKA

(K,c,, Kpcp).This is obviously caused by their adsorption coefficients, which are much lower compared with that of the unsaturated compound. The hydrogen concentration in the liquid reaction mixture is usually lower than the concentration of the hydrogenated compound. Although the hydrogen concentration may be raised roughly by two orders of magnitude by increasing the pressure, the term K H c H still does not play any part in the equation of type (2) and (3). Hence, it may be inferred that the adsorption coefficient of hydrogen is much lower than that of the unsaturated compound. Sometimes it is assumed (13)that hydrogen is activated on sites other than those participating in the activation of the unsaturated compound. The reactivity of the compound in a heterogeneously catalyzed reaction depends on the rate constant of the surface reaction and on the adsorption coefficient. If the kinetic equation (3) is reduced to a zero-order equation with respect to the concentration of the initial compound, the rate constant k A H becomes the measure of reactivity: kAH = rA

= k,(KHcH)

(5)

This constant, however, is in itselfa composed parameter (2),as demonstrated by Eq. ( 5 ) .

C. COMPARISON OF REACTIVITIES

OF A

SERIES OF SUBSTRATES

If the reactivities of two compounds are compared on the basis of their reaction rates measured for each compound separately, the relative reactivity is given by the ratio of the rate constants expressed for both compounds by rA/rB

= kAH/kBH

=

kA(KHcH)A/kB(KHcH)B

(6)

If the ratio of the rate constants of surface reactions of compounds A and B is to be assessed, the expression K H c H must be the same for both measurements. If a number of compounds are hydrogenated in the same reaction medium, it may be assumed that the presence ofa low amount of the substrate does not significantly affect the solubility of hydrogen in the reaction mixture; in the first approximation, this solubility is equal to that of hydrogen in the solvent, and is canceled in the calculation of the ratio of reaction rates. No data have been reported on the extent to which the adsorption coefficient of hydrogen is affected by the other compounds present in the reaction mixture, and by the substrate and solvent in the first place. If one compares the reactivities of a number of similar compounds measured for each compound separately in the zero-order region with respect

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

34 1

to the concentration of the substrate in the same medium and under the same conditions, the equation kAH/kBH

= kA/kB

(7)

may be regarded as valid. If the reactivities of a single compound in various media are compared, the experimentally determined rA and rB comprise not only the rate constants of surface reactions but also the products of the adsorption coefficients of hydrogen and of its concentration in the bulk phase (KHcH), which may depend on the solvent used. If the relative reactivity value is obtained using the method of competitive reactions, the reactivity is given by the ratio of the rate equations of type (3) for both compounds (14); the denominators of the fractions cancel out: dcA _ -

kA(KHCH)A,BKACA

dcB

kB(KHCH)B,AKBCB

By measuring the existing concentrations C, and cB in the competitive hydrogenation of compounds A and B and using the integrated form of Eq. (8),it is possible to obtain the exact selectivity value of hydrogenation S A , B , given by the ratio of products of the rate constants and adsorption coefficientsof both compounds, because (KHcH)A,B and (KHCH)B,A cancel out. If the relative adsorption coefficient KA/KB is to be expressed by using the selectivity value S A , , , kA and kB must be substituted into Eq. (8). The measured rates of hydrogenation of the particular substrates yield only k A H and k g H [Cf. Eq. (6)];their relationship to k , and kBhas been discussed above. The substitution of kAHand k g H for kAand kBin Eq. (8) gives the relative adsorption coefficient K A / K B subjected to the same uncertainty as Eq. (7). Using the rates of hydrogenation of the particular substrates A and B, or the ratio of rate constants of A and B in competitive hydrogenation, the constants kA and kB, or the relative adsorption coefficient K A / K B , can be determined exactly only if ( K H c H ) , = ( K H c H ) B . For the evaluation of the effect of composition of the reaction mixture on the hydrogenation process, and above all of the effect of solvents on the relative reactivities of two substrates, the selectivity values defined by Eq. (8) appear to be those best suited; usually, they are determined with great accuracy because both compounds react in competitive hydrogenation under exactly the same conditions. Most authors, however, prefer to find out to what extent changes in the relative reactivity are produced by changes in

342

LIBOR

C E R V E N AND ~ VLASTIMIL RP~ZICKA

the rate constants or adsorption coefficients. Such information, although being of a higher quality, is, on the other hand, subjected to a greater error, because the error in determining k A H and k g H from two independent measurements, although performed under the same experimental conditions, is reflected also in the relative adsorption coefficient. Competitive hydrogenation processes in the liquid phase are mostly investigated by using chromatographic analysis of the reaction mixture. This procedure has certain limitations. According to Cerveny and REiCka (15), the selectivity values S A , B were calculated for various instantaneous compositions of a mixture of A and B in the competitive hydrogenation of their equimolar mixture (cf. Table I). If at a 99% conversion of B the conversion of A reaches 1% (the approximate level of a usual chromatographic analysis), the selectivity value at comparable rate constants k A H and k,, gives the relative adsorption coIf differences in the adsorptivity of A and B efficient K A , B = 2.18 x are still higher, the method of competitive reactions becomes unsuitable for the determination of the relative adsorption coefficient. An ideal case is represented by the state where both reactions proceed at comparable rates. Basically, the effect of composition of the reaction mixture on the hydrogenation process may be characterized by the reaction rates, rate constants, adsorption coefficients, or selectivities of competitive reactions. All these characteristics have been used in various studies. TABLE I Selectivity Values SA,Bfor Various Instantaneous Compositions of a Mixture of A, B in the Competitive Hydrogenation of Their Equirnolar Mixture Composition of the reaction mixture in dimensionless concentrations CA

cB

0.99 0.98 0.97 0.96 0.95 0.90 0.80 0.70 0.60 0.50

0.01 0.02 0.03 0.04 0.05 0.10 0.20 0.30 0.40 0.50

Selectivity x 103 2.18 4.87 8.69 12.68 17.12 45.67 138.65 296.24 557.50 1000.oo

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

343

111. Correlation Equations Describing the Effect of Structure of Reacting Compounds and Solvents on Reaction Kinetics

The procedure most frequently employed in heterogeneous catalysis consists in comparing the activity of catalysts determined by a standard testing reaction with some property of these catalysts. A less frequent procedure is based on the determination of the reactivity of several compounds on a single catalyst, which is subsequently related to some property of the reacting molecules (16). The third type of correlation of kinetic data involves the determination of the effect of solvents on catalytic reactions in the liquid state by means of a standard testing reaction on the same catalyst. This study is more concerned with correlations of the second and third types, in which the LFER method is often used. According to this method (17-19), it is assumed that a change in free energy due to an interaction with the molecule of the substrate or to a change in solvent may be expressed through

AF

=

AFo

+

AX

(9)

where A F o is the free energy change in the reaction of the chosen standard compound, A x expresses a change in the reaction medium or in the structure of the reacting compound, AF is the free energy change caused by the interaction with the molecule or by a change in the solvent used, and a is the proportionality constant. The basic condition observed in studies of this type consists in that all the values taken for comparison ought to be obtained under exactly identical conditions, and the structure of the substrate or solvent ought to be the only variable factor affecting the rate of the catalytic reaction. This is why it is very difficult, and for catalytic reactions even impossible, to compare data provided by different authors. In principle, the effect of composition of the reaction mixture on the hydrogenation process may be examined in two regions closely connected with each other: A. systems in which the structure of reacting compounds is changed; B. systems in which nonreacting compounds, most frequently the solvent, are changed.

A. EFFECT OF THE STRUCTURE OF REACTANTS ON THEIR REACTIVITY The correlation relations most widely in use for the description of the effect of structure of reactants on their reactivity are the Hammett (20),Taft

344

LIBOR CERVENY AND VLASTIMIL RPT~ICKA

(21),and Taft-Pavelich (22) equations:

log(k/ko) = PO log(k/ko) = p*a* log(k/ko) = p*a*

+ 6Es

(12)

where the parameters a* and Es express the polar or steric effect of substituents on the reaction site in the aliphatic series of structures, the parameter c expresses the polar effect of substituents in the aromatic series, and the parameters p*, 6, and p express the measure of sensitivity of the kinetic characteristic to the respective effects of substituents. In the field of heterogeneously catalyzed reactions these relations were first used by Kraus ( 2 3 , who stated the conditions permitting their application. With respect to the utilization of LFER, there are two rather important differences between homogeneous and heterogeneous reactions (23),viz. : (a) the concept of kinship between the compounds is more restricted for transformations occurring on solid catalysts than for reactions in solutions, because some substituents may change the way of adsorption; (b) steric requirements of the reaction on the catalyst surface are stricter than for homogeneous processes (23, 24). The analysis presented by Kraus (23) defined conditions that must be observed in the practical application of LFER in the field of heterogeneous catalysis: 1. to choose an adequate model reaction series, predominantly with respect to the sufficient extent and variability of the structural parameters of substrates; 2. to obtain consistent kinetic data not influenced by transport processes; 3. to employ for the given set a suitable kinetic or equilibrium characteristics; both the rate constants and the adsorption coefficients ought to be suited for correlation; 4. to bear in mind various possible specifications of the reaction site and substituents; 5. to check the statistical significance of the correlation thus obtained. Systems including compounds with substituents of various type not only extend the scale of the a*,E,, and (J values in the description of data by means of correlation equations, but may also help to reveal interactions not involved in the given equation (“nonreacting substituent”-active site of the catalyst), which may thus render the correlations in efficient. If a substrate then appears that does not obey the correlation only in a certain solvent, the cause should by analogy be sought in the interfering interaction between the substrate and the solvent.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

345

It was found (23) that the application of LFER in the field of heterogeneously catalyzed reactions more often brought positive results if the reactions were carried out in the gaseous phase. The failure of some correlations in the case of reactions in the liquid state has been assigned to specific interactions between the solvent and the catalyst. B. EFFECT OF SOLVENTS ON REACTION RATE Many equations have been suggested to express the effect of the solvent on the rate of chemical reactions (25-28). Quantitative correlations are based on various physical properties of solvents or on empirical and semiempirical parameters. The LFER has been applied in this field because it was obvious that no simple physical characteristic of the solvent could adequately describe all interactions between molecules of the substrate and solvent (28, 29). A semiempirical approach, similar, for examples, to the Hammett equation, does not require full understanding of complex molecular interactions in solution. A standard reaction or phenomenon is chosen, and the parameters of this reaction or phenomenon during changes in the solvent are examined. These parameters may be represented by the rate of equilibrium constants of the reaction, but also by, for example, shifts of the maxima in various spectra. The relations of this type most frequently used are the Grunwald-Winstein (30),Swain-Scott (3Z), Gielen-Nasielski (32),Berson (33),and Drougard-Decroocq (34) equations. The form of all these equations resembles that of the Drougard-Decroocq equation (34): log(k/k,) = AT

(13)

where T is the parameter characterizing the solvent and is the parameter of sensitivity of the reaction to a change in the solvent. Usually, equations of this type describe similar systems very adequately, but completely fail if another system (e.g., another type of reaction) is used. Koppel and Palm (27) theoretically justified the application of multiparameter correlations based on LFER in a quantitative expression of several types of interactions between the solvent and substrate. Their conclusion was that effects of the solvent on the chemical reactivity and on various physical and physicochemical phenomena are of similar nature and that there exist only several types of physical interactions between the solvent and substrate. Then it is possible to find a general approach to the evaluation of experimental data, that is, to express these interactions quantitatively. For this purpose they suggested a four-parameter equation (27) in

LIBOR E E R V E NAND ~ VLASTIMIL RPT~ICKA

346

which the parameters of the solvent are represented by the Lewis acidity, Lewis basicity, polarity, and polarizability. But the authors did not take into account the formation of hydrogen bonds, or charge transfer (CT) complexes, which often represent very strong interactions. Effects of the solvent on reactivity, quite complex already for homogeneous systems, in heterogeneous systems are in addition complicated by the presence of a further phase. The number of interactions between the solvent and substrate are supplemented by mutual interactions between the substrate, solvent, and another phase in the system. In the case of catalytic hydrogenations, the properties of the solvent alone may already determine the maximal surface concentration of hydrogen at which the reaction may proceed. Owing to the considerable complexity of the problem, many studies were restricted to a merely qualitative description, or to a more or less successful correlation of kinetic data with the physical properties of the solvents (35-45) without reaching any generalization. The effect of the solvent is obviously given by the whole bulk of its properties, with one or another property predominating as the case may be. For this reason, empirical multiparameter equations appear to be most promising in this case.

IV.

Hydrogenation of Olefinic Substrates

Most of the studies dealing with the effect of the composition of reaction mixture on the catalytic hydrogenation in the liquid state have been devoted to olefinic substrates. The reactions proceed at readily measurable rates already under normal conditions, so that their experimental investigation is not technically demanding. The system enables the structure of compounds undergoing hydrogenation to be varied within broad limits; it also makes possible the use of a broad variety of solvents, and is very suitable for the given purpose.

A. DIFFERENCE IN THE EFFECT OF SOME TYPES OF CATALYSTS In studies of structure effects it is necessary that the catalyst should appear as a constant parameter. Generally, such studies may be performed using a great variety of catalysts, but the possibility of generalization of the data is very restricted. This phenomenon is especially marked in the cases of hydrogenation of olefinic substrates, which on some catalysts proceed selectively, while on others they are accompanied by side reactions, mainly by isomerization and sometimes also by hydrogenolysis.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

347

The causes of the selectivity of hydrogenation catalysts, similarly to the mechanism of hydrogenation and isomerization reactions of olefins are complicated problems, which so far have resisted any satisfactory solution (46).

Papers (4, 47, 48) demonstrate that, while the character of the carrier (silica gel, active carbon) of the active component has no pronounced influence on the process of hydrogenation, there are distinct differences in the effect of the active components themselves. Side reactions occurred on rhodium and palladium catalysts, while on platinum catalysts they could not be observed in most cases (migration of the double bond, cis-trans isomerization). These reactions occurred only if a sufficient amount of hydrogen was present in the reaction mixture (part of hydrogen is irreversibly consumed by hydrogenation). Neither the carrier alone nor the catalyst in an inert atmosphere provoked any side reactions, which shows that hydrogen in one of its forms participates directly in the isomerization process. The observed phenomena may be adequately explained on grounds of the mechanism of gradual addition of hydrogen species to the double bond, the basic form of which is called the Horiuti-Polanyi mechanism (49): H,

\

ki + ** + 2H-* kz

/ \

k,\

c=c + * * *

/ H-*+

\

C-C

k4

/ k 5 \

* *

A

c-c

//

(14) /

I\

* * /

c-c

+**

*

Reactions (14) and (15) represent the chemisorption of hydrogen and olefin on the catalytic surface (**) as a reversible process; reactions (16) and (17) describe the gradual addition of hydrogen particles. According to reaction (16), a so-called semihydrogenated state is formed, that is, a monoadsorbed formation which can be stabilized in several ways. Cis addition of the second hydrogen particle gives rise to alkane, or the monoadsorbed formation is desorbed with formation of the original olefin or another isomer, or else it is desorbed reversibly and readsorbed on the initial or some other carbon atoms of the chain in such a way that eventually leads to the trans addition of the second hydrogen atom (SO). The process is probably more complicated than is indicated above (51), but the simplified mechanism just outlined allows us to elucidate many of the phenomena observed. This mechanism is also corroborated by an IR

348

LIBOR

C E R V E N AND ~

VLASTIMIL R B ~ I C K A

spectroscopical study by Eischens and Pliskin (52). The formation of cistrans isomerization products and migration of the double bond are most often explained just in terms of the formation of the semihydrogenated state (53).The limited number of isomers formed (migration of the double bond from position 1 virtually only to position 2) is explained by the fact that these processes take place in the adsorbed state. The isomerization products are most frequently assumed to arise in the ionic transition state (54-56). According to Cerveny and RdiiEka (47),the kinetics of hydrogenation and isomerization of 1- and 2-hexenes and 2-methylhexenes on Pd and Rh catalysts were investigated. The systems were described by formal power first-order equations with respect to the concentration of substrates. It was found that the activity of the catalyst did not affect the ratio of rates of hydrogenation and isomerization, which have the same kinetic order with respect to the hydrogenated component. The ratio was not affected by temperature; with increasing pressure of hydrogen, the selectivity of hydrogenation increased in respect to both the migration of the double bond and the cis-trans isomerization. Partial poisoning of the catalyst with 1-butanethiol did not lead to any changes in selectivity (57). Hydrogenation and isomerization obviously proceed on the same catalytic sites, with formation of the semihydrogenated state being the rate-determining step (57). Some authors (58, 59) relate the different isomerization ability of the individual metals to the position of metals in the periodic table; others (54, 55, 60, 6 1 ) attribute it to the existence of various forms of hydrogen sorbed on these metals. In all cases, the rhodium catalyst was more selective than the palladium one (47). A decisive role may be played by the strength of the bond between olefin and the catalyst (4,47,57).This view is indirectly supported by the degree of stability of organometallic complexes of transition metals with olefinic ligands, which decreases with increasing isomerization ability of the given metal. It may be assumed that the relative adsorptivity of an olefin on the surface of various metals corresponds to the stability of compounds formed by the olefin with salts of the respective metals (62-64). Comparison of the selectivities defined by the ratio of rate constants of hydrogenation and isomerization of olefins on rhodium and palladium catalysts has revealed (47) that the relative selectivity of rhodium with respect to palladium is always higher than unity. This value is four to five times higher for olefins in which migration of the double bond takes place than for olefins in which only cis-trans isomerization occurs, along with hydrogenation. These results agree with the view that the strength of the bond between the olefin (or semihydrogenated state) and the active site of the catalyst affects the selectivity of the hydrogenation. The several times higher relative selectivity value of rhodium compared to palladium for olefins

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

349

in which migration of the double bond takes place is obviously due to the fact that this process requires adsorption of the semihydrogenated species on yet another active site of the catalyst, which enhances the effect of bond strength between the reacting olefin and the catalyst on the selectivity of hydrogenation. These views are also in accordance with the reaction order with respect to the olefin concentration. While hydrogenations on platinum catalysts were also zero order at low olefin concentrations (48),the reaction order on rhodium and palladium catalysts under the same conditions approached unity (47), which allows us to.infer that the adsorption coefficient ofolefins on platinum catalysts is much higher than that observed on rhodium and palladium. Platinum catalysts were employed in most of the existing studies of the effect of structure of olefinic substrates and solvents on the course of hydrogenation. These systems are less complicated, and the effect under study may often be successfully quantified. On the other hand, however, one may expect a more important role of, for example, solvent in systems with those catalysts for which the results obtained indicate a lower adsorptivity of substrates on the surface (Pd, Rh), which just might stress interactions between the environment of the solvent and adsorbed molecules. In the future, qualitatively new findings may be expected in this field.

B. EFFECT OF STRUCTURE OF OLEFINIC SUBSTRATES ON THEIR REACTIVITY AND ADSORPTIVITY Quite a considerable number of papers deal with the effect of structure of olefinic substrates on their reactivity in the catalytic hydrogenation (65). Lebedev (66)attempted a generalization of the problem. His conclusion that the rate of hydrogenation ofolefins decreases in the order monosubstituted + symmetric disubstituted + asymmetric disubstituted +trisubstituted -+ tetrasubstituted ethylene derivatives is called “the Lebedev rule.” Campbell (67) supplemented it by demonstrating that the rate of hydrogenation decreases with the number and size of substituents on carbon atoms of the double bond, cis isomers are usually hydrogenated more quickly than trans isomers, and olefins containing the terminal double bond are more reactive than those with the double bond inside the chain. Such qualitative conclusions are only a rough expression of the dependence of reactivity on the structure of the hydrogenated compound. Not only the degree of substitution, but also the character of substituents, solvent, and catalyst are operative in this case. The structure of substrates may affect reactivity in heterogeneously catalyzed hydrogenations by affecting the rate constant of the reaction, and

350

LIBOR

CERVEN+ AND

VLASTIMIL

ROZICKA

also by acting upon the adsorption properties (adsorption coefficient). Most hydrogenations of the individual olefinic substrates on platinum catalysts proceed according to the zero-order rate equation with respect to the concentration of the substrate. The catalyst surface is completely occupied by the substrate, the structure of which, under otherwise identical conditions, determines the rate of hydrogenation by affecting the rate constant. If the system contains several competitively adsorbed substrates, the relative reactivities are given by the ratio of products of the rate and adsorption constants. In many papers in which the reaction rates or adsorption coefficients of a series of substrates were obtained the effect of structure is discussed only qualitatively. RBiiEka and Cerveny (5) found that the rate of hydrogenation of 1-olefins C,-CI7 in ethanol under usual conditions on platinum, palladium and rhodium catalysts decreased monotonically with the chain length. Since in this olefinic series the chain length did not affect (68) the relative adsorptivity of olefins, as demonstrated in Table 11, the observed phenomenon may be explained only by the effect exerted by the chain length on the rate constants. Since, moreover, the polar effect of substituents in this series is virtually the same, an explanation can be sought in the steric hindrance of the adsorbed double bond by the free end of the olefin molecule. The trans isomers were adsorbed much more weakly than the respective cis TABLE I1 Relative Adsorption Coefficients of Olefins on Pi Catalyst' ~~

~

~

Relative adsorption coefficient Olefin ~

~

4 , l ~

1 -Hexene 1-Heptene 1 -Undecene 1 -Tridecene cis-2-Hexene trans-2-Hexene cis-4-Methyl-2-pentene trans-4-Methyl-2-pentene cis-3-Heptene trans-3-Heptene

1 .oo 1 .oo 1 .oo 1.oo

0.15 0.06

0.53 0.03 0.69 0.07

' The values were measured (68) in the hydrogenation in ethanol on the catalyst 1.75% Pt/silica gel under usual conditions.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

35 1

isomers, and the latter were adsorbed more weakly than unbranched 1-olefins. These results were in good agreement with the adsorption coefficient of cyclohexene related to 1-hexene, 0.06, measured (69) in the competitive hydrogenation of a binary mixture of these substrates without any solvent on the catalyst 5% Pt/silica gel. Reference (70) reports the hydrogenation of 2-propene-1-01, 2-methyl-3butene-2-01, 1-hexene, and cyclohexene on the catalyst 5% Pt/silica gel in a series of 19 solvents. In all these solvents the strongest adsorption was observed for 2-propene- 1-01, the weakest was recorded for cyclohexene. The adsorptivity of 1-hexene was comparable to that of 2-methyl-3-butene-2-01 and varied markedly with the solvent. The weakest adsorptivity of cyclohexene may be assigned to the fact that here in fact we have a disubstituted double bond. By comparing the adsorptivities of 2-propene- 1-01 and 1-hexene, it can be seen that the adsorptivity is favorably affected by the presence of the OH group on the a-carbon atom next to the double bond. On the contrary, the adsorptivity of 2-methyl-3-butene-2-01 was weaker, probably due to the steric effect of the methyl groups. According to Cervenf et al. (71), the same catalyst was employed in the hydrogenation of a series of unsaturated alcohols in seven various solvents (cyclohexane, diethyl ether, toluene, methanol, benzene, ethyl acetate, 1,4dioxane). In all of these solvents the adsorptivity of unsaturated alcohols was affected by the position of both the double bond and the OH group. It decreased with the degree of substitution of the double bond, with the distance between the OH group from the double bond, and with the degree of substitution of the carbon atom on which the OH group was bound. It is of interest to compare the adsorptivities of compounds containing various functional groups capable of hydrogenation. Cerveny and RdiiEka (15)have found that in the hydrogenation on the 5% Pt/silica gel under usual conditions in an equimolar mixture of I-phenyl-2-propene and 4-nitrotoluene, 1-phenyl-2-propene is preferentially hydrogenated in all the seven solvents used. With respect to the comparable rate constants of both substrates, this finding must be assigned to the considerable difference in their adsorptivity, or in the adsorptivity of both functional groups. Although by changing the solvent it is possible to affect the rate and adsorption constants in the olefinic series and in the series of aromatic nitro compounds, this change does not cause a change in selectivity in the competitive hydrogenation of l-phenyl-2propene and 4-nitrotoluene, owing to the much higher adsorptivity of the double bond compared with the nitro group. Brown and Ahuja (72)reported a specially prepared nickel catalyst, extremely sensitive to a change in the structure of substrates, but they did not attempt any quantitative interpretation.

352

LIBOR

CERVENI AND VLASTIMIL R ~ ~ I C K A

The first attempt at quantifying the effectof the structure of olefins on the rate of their hydrogenation consisted in a successful application of the Taft equation to the rate constants of hydrogenation of four olefins of the type R,R,C=CH, on a nickel catalyst (73)carried out by Kraus ( 2 3 , who found a linear relationship between log kIe, and C o*.On the contrary, Jardine and McQuillin (74)found that the rate of hydrogenation of substituted pentenes on a palladium catalyst was independent of the polar effect of substituents. Using hydrogenation on the Pt catalyst, they found a relationship between the rate of hydrogenation and the equilibrium constant of formation of complexes of these olefins with Ag'. In an attempt to eliminate the steric effect of substituents, Kieboom and van Bekkum (75) hydrogenated substituted 2-aryl-3-methyl-2-butenes and 3,4-dihydro-l,2-dimethylnaphthalenes. They found that the polar effect of substituents on the rate of hydrogenation on a palladium catalyst was virtually negligible, and hence, that the character of the activated complex of the rate determining step was similar to the initial state. The adsorptivity of substrates was reduced by electron abstracting substituents, in agreement with the view that the substrate is adsorbed as a surface 71 complex. Maurel and Tellier (76) hydrogenated a series of C6-Cs olefins on a Pt/SiO, catalyst without solvent and found a linear relationship between log kIel and C o*. In the hydrogenation of a major series of olefins on the Ru catalyst, a linear relationship was found to exist between log(k,,,K,,,) and the Taft o* constants (77). A more detailed analysis of the possibilities of utilization of the Taft equation in the expression of the effect of structure of olefinic substrates on the rate of their hydrogenation is presented in papers by RdiiEka et al. (78)and cerveny and RdiiEka (79).A disadvantage of the reaction series in which substituents on the reaction site are limited to alkyls only consists in the relatively low variability, and thus in the partial interchangeability of the o* and E, values for the individual reaction components (80).By suitably choosing the structures, however, this disadvantage may be restricted to an extent that makes possible at least some resolution (78). With the mechanism of hydrogenation of the double bond not being known, the determination of the structural parameters presents a difficult problem. This is related to the question of a correct choice of the reaction site. RfiiiEka et al. (78) report correlation of the reaction rates of hydrogenation of 15 olefins on three platinum catalysts in ethanol (48). The existence of an associatively chemisorbed olefin was assumed in this correlation, and the mechanisms of simultaneous and stepwise addition of hydrogen particles were considered. In the first case, a pair of carbon atoms joined by the double bond was regarded as the reaction site. For the calculation of polar and steric constants of the Taft equation, the olefin was regarded as substituted ethylene de-

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

353

scribed by the formula R,R2-C=C-R3R4. In this case, the constants of the Taft equation are given by the sum of the r ~ *and E, constants of the individual substituents R, ,R, ,R, ,R4. Such a calculation procedure is only an approximate description of the actual state. While the g* constants are additive (81),the summation of E, constants is certainly an approximation, which may be justified only by practical results. The described model is an expression of the associative form of the complex between the active site and the olefin (82). In the case of a stepwise addition of hydrogen, one step must always be regarded as much slower than the other. Further distinction follows from the assumption (78, 79) that the attack on the olefin molecule may be initiated by the hydrogen radical, proton, or the hydride ion. The particle added may be directed in extreme cases either by a purely polar or a purely steric effect. Most likely, however, both effects are operative simultaneously, and the degree of their participation may be different. If the rate-determining step consists of the formation of a semihydrogenated state, the mechanisms are distinguished according to the character of the attacking particle and to the assumed orientation; if this step is represented by addition of the second hydrogen particle (formation of the alkane), one of the carbon atoms of the original double bond bearing three substituents, R, , R,, and R5,is regarded as the reaction site, as documented by the equation (+I R,R~-c=c-R,R,

-+

R,R,-C-C-R,R,

I I

+ H+

-+

R,R,-c-R,

I

(18)

* These speculations concerning the reaction mechanism resulted in four forms of the Taft equation differing in the procedure used in the calculation of polar and steric constants. Correlation analysis has revealed (78, 79) that kinetic data may be correlated in a single manner, namely, by employing the Taft-Pavelich equation in the form * *

4

log(k/ko) = p * C a * 1

4

+ SC Es

(19)

1

With respect to the established low value of the parameter p*, this equation may be reduced to

The above equation satisfies the mechanism of simultaneous addition; for stepwise addition, this equation may be employed if the rate-determining step consists in the reaction between the adsorbed olefin and the hydrogen radical or proton (48).No isomerization products that are supposed to arise

354

LIBOR

CERVENY AND

VLASTIMIL

RPT~ICKA

from semihydrogenated intermediates (49) were detected in the reaction mixture during the reaction. This finding supports the mechanism of simultaneous addition of hydrogen species. Figure 1 shows a correlation of the measured data, which were obtained (48) by the hydrogenation of olefinic substrates in ethanol on the catalyst 14% Pt/silica gel. Similar dependences were found with the catalysts 1.75% Pt on silica gel and 5% Pt on active charcoal and also (79) using data (83) provided by the hydrogenation of a series of olefins on the Ru catalyst. Polar and steric effects could not be distinguished in this case, however, because of their intercorrelation. Cis isomers, in which alkyls on the one side of the double bond may mutually affect each other (84,85),were not included in the series of model compounds (48). Cis derivatives were indeed hydrogenated more quickly (48)than trans derivatives, but the differences were not so large as to change markedly the dependences found for trans derivatives. The correlations outlined above are empirical, that is, their applicability is proved by their success. The finding that the carrier alone or the concentration of platinum on the carrier did not change the parameters of the Taft equation to any essential degree makes possible some generalization of the results. All alkyl-substituted ethylenes are hydrogenated under the same mechanism, most likely through a simultaneous addition of both hydrogen particles to the n-adsorbed olefin (48, 78).

0

2

1

3

Es FIG.I . Correlationof the rate data by means of the reduced Taft-Pavelich equation: 1, 1-hexene; 2, l-heptene; 3, l-octene; 4, 3,3-dimethyl-l-butene; 5 , 4-methyl-l-pentene; 6 , 4,4dimethyl-l-pentene; 7, 2,3-dimethyl-l-butene; 8, truns-3-heptene; 9, trans-4-nonene; 10, rrans-4-methyl-2-pentene; 11, 2-methyl-l-hexene; 12, trans-2-hexene; 13, 2-methyl-2-hexene; 14, 2,4-dimethyl-2-pentene; 15, 2,3-dimethyl-2-butene.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

355

With mixed Pt-Rh catalysts, there was an increase (86) in selectivity defined by the ratio of the rate constants of hydrogenation and isomerization with increasing Rh content in the catalyst. The values of the initial reaction rates could not be correlated by the Taft equation, although the conditions and compounds used were similar to those in Cerveny and RBiiEka (48).The causes of unsuccessful correlation [which was successful with the platinum catalyst (48)]should therefore be sought in the catalyst itself-in yet another type of independent interaction between the catalyst and substrates, specific adsorption, given by the new steric conditions, or possibly also in parallel side reactions occurring on the same catalytic sites as hydrogenation. The Taft-Pavelich equation was also employed for the correlation of data obtained (71) by the hydrogenation of five unsaturated substrates on the 5% Pt/silica gel catalyst. Correlation of the reaction rates was successful for data measured in cyclohexane, methanol, benzene, and ethyl acetate, poorer for diethyl ether, and unsuccessful for toluene and 1,Cdioxane. Correlation of the relative adsorption coefficients was successful in all the solvents mentioned above with the exception of diethyl ether. The substrate may be bound on the catalyst not only through the double bond, but also through the other “substituents.” This leads, as the case may be, to a change in the number of active sites that may be utilized in hydrogenation. In addition, the existence of interfering interactions of solvents with hydrogenated substrates may also be admitted. These phenomena may cause failure of the correlations. In the discussed series (71) of hydrogenated compounds the reaction rate and relative adsorptivity of substrates in most solvents were affected to a comparable degree by steric and polar influences. Negative values of the parameter p: and positive values of the parameter 6, were obtained in most cases in the correlation of the reaction rates by the Taft-Pavelich equation, while correlation of the relative adsorption coefficients gave opposite results. This can be seen as an example of an interesting compensation of the kinetic and adsorption terms. The results presented above confirmed that attempts at correlating the effect of the structure of substrates with kinetic parameters were very useful and might bring valuable information, for example, on the reaction mechanism. So far, however, these results cannot be regarded as general, because systems still appear in which the correlations fail. The present-day state of research in this field allows no more than a hypothetical explanation of the causes of unsuccessful correlations which, though much smaller in number than successful ones, nevertheless cannot be systemized. At the same time, it is just those systematic deviations that in the future might considerably contribute to the theory of the effect of the structure of substrates on the kinetics of their hydrogenation.

356

LIBOR

CERVENQ AND

VLASTIMIL

R~~ICKA

C. EFFECT OF SOLVENTS ON THE HYDROGENATION PROCESS The LFER method was used in the heterogeneous catalysis mainly in the investigation of the effect of the structure of reactants on the reaction rate; the effect of solvents on the hydrogenation process was interpreted for a long time on the qualitative level only. Such an approach, as well as the correlation of kinetic data with various physical characteristics of solvents, was of course unsatisfactory, because it was rather far from any potential generalization. A higher form of interpretation of the effect of solvents on the rate of heterogeneously catalyzed reactions was represented by the LangmuirHinshelwood kinetics ( I ) , in the form published by Hougen and Watson (2),where the effect of the solvent on the reaction course was characterized by the adsorption term in the kinetic equation. In catalytic hydrogenations in the liquid state kinetic equations of the Hougen-Watson type very frequently degrade to equations of pseudo-zero order with respect to the concentration of the substrate (the catalyst surface is saturated with the substrate), so that such an interpretation is not possible. At the same time, of course, also in these cases the solvent may considerably affect the reaction. As is shown below, this influence is very adequately described by relations of the LFER type. Cerveny et al. (69),attempted to apply the Drougard-Decroocq equation (34)to the data obtained in the hydrogenation of cyclohexene and 1-hexene on the catalyst 5% Pt on silica gel in 19 solvents. Since correlation of the reaction rates with the parameters of solvents r, originally obtained for the homogeneous Menschutkin reaction (88) between methyl iodide and tripropylamine was unsuccessful, an analogous definition was used for the parameters r', which were to characterize the solvent with respect to its effect in heterogeneously catalyzed reactions :

log(k,/k,) = I'r' Methanol (7' = 0) was taken as the basis of the correlation, and it was put by definition that I' = 1 for the hydrogenation of cyclohexene under the given conditions. Using this model reaction, the r' (cf. Table 111)were determined, used in a successful correlation with the initial rates of hydrogenation of 1-hexene in the same solvents on the same catalyst. Figure 2 shows the dependence thus obtained. The reaction rates of the hydrogenation of 2-methyl-3-butene-2-01 with the parameters r' were correlated by Cerveny et al. (89)in a similar manner. The dependences thus obtained for two catalysts of the same type (Pt), but with a different carrier, differed considerably. A much better correlation was found for data measured using a catalyst with the same carrier (silica gel) as

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

-2.0

-1.5

-1.0

-a5

357

o

Y’ FIG.2. Correlation of the rates of hydrogenation of I-hexene with the parameters T‘: 1, hexane; 2, cyclohexane; 3, dibutyl ether; 4, diethyl ether; 5, cumene; 6 , I-butanol; 7, ethyl benzene; 8, I-propanol; 9, ethanol; 10, toluene; 11, methanol; 12, benzene; 13, ethyl acetate; 14, tetrahydrofuran; 15, 1,4-dioxane; 16, benzyl alcohol; 17, diethyl malonate; 18, ethyl benzoate; 19, 1,1,2,2-tetrachloroethane.

that employed (69) in obtaining the standard values of z’ than for data measured with the catalyst 5% Pt on active charcoal. In both cases the I‘ values also differed considerably. Finally, hydrogenations performed on the catalyst with the silica gel carrier were zero order in most solvents with respect to the concentration of the substrate, while on the catalyst with active charcoal as the carrier the formal order in the power law rate equation was fractional in most cases. Cerveny et al. (71) report hydrogenation of nine olefinic substrates (1-hexene, ethyl acrylate, ally1 phenyl ether, allylbenzene, 3-butene-1-01, 2-butene-1-01, 3-butene-2-01, 2-methyl-2-propene-1-01,1-heptene-4-01) in seven solvents (cyclohexane,diethyl ether, toluene, methanol, benzene, ethyl acetate, and 1,Cdioxane) on 5% Pt on silica gel. No linear relation could be proved for any of the substrates when Eq. (21) was applied to the set of data obtained in the hydrogenations. In all cases correlation points for benzene and toluene did not fit. On omitting these points, experimental data satisfied Eq. (21), but linear regression gave a nonzero absolute term q on the righthand side of the equation: log(k,/k,)

=

I’z’

+q

(22)

The value of q gives the difference between the logarithm of the rate of hydrogenation in methanol calculated by correlation using Eq. (22), on the one hand, and the logarithm of the measured rate in methanol, on the other,

358

LIBOR

CERVENPAND VLASTIMIL R I ~ I C K A

thus reflecting the experimental error. Its magnitude exceeded the experimental error, but no acceptable interpretation could be suggested (71). It was found (71,89, 90) that the parameter A' depended on the structure of the hydrogenated compound. The highest sensitivity of the change in the reaction rate with a change in the solvent was observed with substrates containing the benzene ring; a lower sensitivity was exhibited by unsaturated alcohols and 1-hexene. For unsaturated alcohols the I' values decreased in the order: secondary alcohol with the monosubstituted double bond and with an OH group in the ci position, secondary alcohol with the monosubstituted double bond with an OH group in the p position, primary alcohol with the monosubstituted double bond and primary alcohol with the disubstituted double bond. The value of the parameter I' for 1-hexene(71)differed from that determined (69) for the original scale of 19 solvents. The result documents the necessity of comparing only consistent data. The value of A' for the same substrate was importantly affected by the catalyst used; the use of another catalyst (89, 90) (carrier) suggested a considerable limitation of the applicability of such type of correlations. The results described above lead to a conclusion that the parameter t' can characterize the solvent with respect to its influence on the rate of hydrogenation of olefinic substrates, but that the applicability of this parameter is far from universal. It has been found in many papers (92-97) that the solvent may considerably affect also the relative adsorptivity of substrates. The majority of these authors, however, only point out differences in the relative adsorptivities of substrates in competitive hydrogenations under various conditions, or attempt to correlate these adsorptivities with the physical properties of solvents. In order to predict the effect of solvents on the selectivity of heterogeneously catalyzed hydrogenations, it is of course very important to obtain more general information on the effect of solvents on the adsorption coefficients of reacting compounds, based on a qualitative basis. Cerveny, Prochazka, and RdiiEka (70)suggested, for correlation of the effect of solvent on the relative adsorptivity of compounds, an equation in the form

+

= Y+ log[(KA,B)nI(~A,B)O]

(23)

The parameters were defined by choosing y = 1 for a model pair of substrates A = 2-methyl-3-butene-2-01, B = 1-hexene, with methanol (+ = 0) as the standard solvent and 5% Pt/silica gel as the standard catalyst, similarly (69)to the definition of the parameter t'. The values of the parameters z' and 4 are given in Table 111. The adsorption coefficients of 2-methyl-3-butene2-01,2-propene-l-o1, and cyclohexene related to 1-hexene and determined by cerveny et al. (70) could be correlated neither with the ~ ( z ' )parameters of

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

TABLE 111 Values ofthe Parameters T' and No. 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19.

Solvent

4

5'

Hexane Cyclohexane Benzene Toluene Ethylbenzene Isopropylbenzene Methanol Ethanol I-Propanol I-Butanol Benzyl alcohol Diethyl ether Dibutyl ether 1,4-Dioxane Tetrahydrofurane Ethyl acetate Diethyl malonate Ethyl benzoate 1,1,2,2-Tetrachloroethane

359

-0.70 -0.16 - 0.77 - 0.60 -0.23 -0.07 0.00 -0.16 0.02 - 0.07 - 0.48 -0.01 -0.47 -0.34 -0.29 0.03 - 1.76 - 1.07 -0.23

4 -

0.78 1.14 1.14 1.23 1.08 0.00 -0.28 0.26 0.02 0.22 0.43 0.51 -0.18 0.37 0.24 0.71 -0.12 0.48

the original (34)or modified (69)Drougard-Decrooq equation, nor with the physicochemical properties of solvents (dielectric constant, dipole moment, molar polarization). The best results were obtained with the suggested Eq. (23), even though the correlation coefficients were not so high as in the case of correlations of the reaction rates with the parameters z'. The data thus obtained were also used in the calculation of the adsorption coefficients of 2-methyl-3-butene-2-01 related to cyclohexene and 2-methyl-3butene-2-01 related to 2-propene-1-01 assuming the validity of Eq. (24) for each particular solvent: sA,B

= s,4,C

sC,B

(24)

The validity of relations of this type has been experimentally confirmed in several cases (90).The relation would not be satisfied, if specific interactions occurred between the individual substrates. The results obtained so far indicate that such specific interactions are not so important as to invalidate Eq. (24).The relative adsorption coefficients may be expressed from Eq. (24) using Eq. (8). If a pair of substrates resembled by its character the definitional system (unsaturated alcohol-olefin), the correlation of data by means of Eq. (23)

360

LIBOR

C E R V E N ~AND VLASTIMIL RPT~ICKA

was successful (cf.Fig, 3), while for a pair of unsaturated alcohols or olefins the relative adsorption coefficients could not be correlated using this equation. Similar results were reported (71), where it was found that the adsorption coefficients of various unsaturated compounds related to 1-hexene may be correlated using Eq. (23)only in the case of unsaturated alcohols, and hence in systems similar to the one chosen by definition for the determination of 4. Similarly to the case reported in Ref. 69 and involving the il' values, also the y values depended on the structure of the substrates used (70,71) (in most cases their trend for the individual substrates was opposite to that of the A' values). This indicates an interrelation between the effect of structure of the substrate and of the reaction medium on the reaction rate and adsorptivity of these compounds. It was found (70) that the character of functional groups strongly affected the adsorptivity of substrates in various solvents. The effect of solvents on the relative adsorptivity of unsaturated compounds of a similar type (cyclohexene-1-hexene or 2-methyl-3-butene-2-ol-2-propene-l-ol) was less pronounced than in systems of compounds more differing in their structure (pair olefin-unsaturated alcohol). The most pronounced change in relative adsorptivity with a change in solvent was observed (71) for the pair of substrates unsaturated alcoholunsaturated hydrocarbon. The sorption of olefins was preferred in polar solvents (low values of the parameter 4), while in nonpolar solvents (high values of the same parameter) unsaturated alcohols were preferably sorbed. An explanation may be seen in an interaction between the solvent and the

I

-1.0

-a5

I

I

o

a5

cp

I

1.0

FIG.3. Correlation of the relative adsorption coefficients of 2-methyl-3-butene-2-01(D) and cyclohexene (C) with the parameters 4. Solvents are denoted similarly to Fig. 2.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

36 1

adsorbed substrate, which obviously affects considerably the bond strength between the substrate and the catalyst. Cerveny et al. (98) report the hydrogenation of 1,5-hexadiene-3-01using 5% Pt on silica gel as the catalyst, under usual conditions in seven solvents. The intermediates are 1-hexene-3-01 and 1-hexene-4-01,which are further hydrogenated to 3-hexanol. The rate constants of the individual steps were measured, and integrated kinetic equations were used in the determination of the relative adsorption coefficients. The original compound contains a double bond in both the c1 and p positions with respect to the hydroxy group, so that the system makes possible an investigation of the selectivity of hydrogenation of these bonds in one or two different molecules. The results obtained in the hydrogenation of 1-hexene-3-01and l-hexene4-01 were in accord with the conclusions in Cerveny et al. (71)concerning the effect of structure of unsaturated alcohols on their reactivity and adsorptivity. In most of the seven solvents used, 1-hexene-3-01was hydrogenated more rapidly, but differences in the reaction rates were unpronounced, similarly to differences in the adsorptivity. In the hydrogenation of 1,5-hexadiene-3-01the double bond in the fi position with respect to the hydroxy group reacted more rapidly in all solvents, but the differences were again not pronounced. The effect of solvents on the selectivity of hydrogenation of 1,5-hexadiene-3-01was small. The most marked effect of solvents was observed on the rate constants of reactions by which 1,5-hexadiene-3-01is consumed, and on the adsorption coefficients of both alcohols related to 1,5-hexadiene-3-01.In solvents with preferential adsorption of 1,5-hexadiene-3-01(methanol, ethyl acetate, 1,4dioxane) the rate constants had the lowest values. Such an interesting compensation of the kinetic and adsorption terms has also been observed in other cases (71). Correlations between the relative adsorption coefficients and the parameters 4 satisfied Eq. (23)with the absolute term q [similarly to Eq. (22)], the physical meaning of which remains obscure. Correlation analysis confirmed the more general character of the parameter 4 compared with the parameter T'. Using Eqs. (21) and (23), the authors (90) derived relations for the dependence of the selectivity of reactions occurring in parallel on the parameters z' and 4. The results obtained so far in the study of the effect of solvents on the kinetics of hydrogenation of olefinic substrates indicate the applicability of LFER. On the other hand, however, similarly to the results of the investigation of the effect of structure of substrates, they suggest the necessity of a complex approach to reaction systems, because the investigation of any particular effect regardless of the other effects leads to an exaggerated simplification,and the individual results cannot be applied to other systems.

362

LIBOR

CERVEN?

AND VLASTIMIL R

~~ICKA

D. SOLVENT-FREE SYSTEMS AND THE EFFECT OF SOLVENT CONCENTRATION Relationships between the structure and reactivity or adsorptivity of organic compounds in the catalytic hydrogenation in the liquid state have been studied, with a few exceptions (73, 76), in systems with solvents. The solvent is used in a great excess as a rule, so that the properties of the bulk phase are practically determined by its own properties. In solvent-free systems one usual component of the liquid reaction mixture is missing; hence, all the interactions of this component with the other components in the system are also missing. At the beginning, the bulk phase consists only of the substrate or of several substrates; during the reaction, hydrogenation products accumulate in the reaction mixture. At first glance the system seems to be simpler than the one with solvent, but consequences of the absence of the solvent must not be forgotten. In the hydrogenation of the individual substrates it may be expected that Eq. (7) is by far not so well satisfied as in the hydrogenation in a single solvent, because a change in the structure of the substrate means in this case that the bulk phase also changes appreciably, with all the resulting consequences. If the reactivity of the particular substrates determined under such conditions is to be only a function of their structure, in the first stage one must rule out the effect of different hydrodynamic properties of the bulk phase on the reaction rate and recalculate the reaction rate to the same partial pressure of hydrogen, which at a constant external pressure may differ because of the different tension of saturated vapours of the individual substrates. The reaction rates thus obtained must, of course, be regarded as reactivities measured in fact in different media, where, in addition to the actual rate constants of surface reactions, they also reflect values of the products of the adsorption coefficients of hydrogen and its concentration in the bulk phase ( K H c H ) , which obviously need not be the same for the particular substrates. Hence, when comparing the reactivities of compounds in solvent-free systems, only the directly measured k A H and kgH can be compared, bearing in mind that the expressions ( K H c H ) , and ( K H c H ) B , which include these values, may differ much more than if the reactivities are compared by separate measurements of the reaction rates in the same solvent. In a competitive hydrogenation of two substrates in a solvent-free system the relative reactivity is again given by Eq. (8). On the other hand, however, the properties of the bulk phase vary with each change in the substrate, and this change may of course variously affect the rate constants and the adsorption coefficients. Moreover, the rate constant of A cannot be directly measured in the presence of an unsaturated compound B. As a consequence, substitution of k A H and kgH into Eq. (8) instead of kA and kgr yields the values K A / K , subjected to the same inaccuracy. In this case, therefore, the re-

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

363

activities of competitively reacting compounds are more correctly compared using the selectivities S A , B . Measurements in binary mixtures of olefinic substrates without solvent showed (99)that the relation SA,Bx SB,c x

&,A

(25)

=1

which was successfully employed (90)in the recalculation of the selectivities determined in various systems with solvents, has a much more restricted validity. This finding resulted from a comparison between directly measured SA,Bvalues and values obtained from independent measurements of the selectivities of competitive hydrogenations of A and B with various compounds C by recalculation using Eq. (25). The differences thus obtained were quite important in some cases, exceeding considerably the possible experimental error. This limited validity of Eq. (25) is evidently due to a specific interaction between both adsorbing substrates (influence on the kinetic or adsorption constant) and the volume phase, which varies as the case may be [cf. with the validity of Eq. (24)!]. Cerveny et al. (ZOO) report an investigation of the hydrogenation of 12 olefinic substrates in the liquid state with 5% Pt on silica gel as catalyst under usual conditions and without solvents. The reaction rates related to 2,3dimethyl-2-butene and the relative adsorption coefficients obtained in systems of various pairs of substrates and by recalculation using Eq. (25) to 2,3-dimethyl-2-butene are given in Table IV. Comparison between the measured reaction rates and the rates of hydrogenation in solvents (71) has revealed that relations existing between the rates of hydrogenation in a solvent-free system approximately correspond to those determined in TABLE IV Relative Hydrogenation Rates und Adsorption Coefjicients of Substrates on Pt in Solvent-Free Systems

2,3-Dimethyl-2-butene I-Phenyl-2-propene 2-Heptene 2,5-Dimethyl-2-hexene 1,-Phenyl-2-butene I-Propenyl phenyl ether 2-Methyl-2-propene-1-01 2-Butene-1-01 1-Propene-3-01 2-Methyl-1-pentene 4-Methyl-2-pentene 1-Hexene

1.oo 11.85 19.33 2.92 8.39 4.57 5.28 6.12 4.33 11.07 4.94 14.01

1.oo 2.53 1.10 1.90 0.07 8.45 2.32 19.58 83.01 4.27 1.24 59.38

364

LIBOR

CERVEN+ AND

VLASTIMIL R ~ ~ I ~ K A

methanol. On the contrary, relations of the relative adsorption coefficients do not correspond to these relations in any of the solvents used. This suggests considerable interactions of the bulk phase with the adsorbed substrates. The highest reactivity and adsorptivity were exhibited by substrates with a mono- and disubstituted double bond. The high adsorption coefficient of 1-propene-3-01and 2-butene-1-01 was attributed (100) to the presence of the hydroxy group in the molecule of these substrates; on the contrary, the phenyl group had a negative effect on the adsorption coefficient of the phenyl-substituted olefin. The measured data also were used (100) in a quantitative representation of the effect of structure on the reactivity and adsorptivity of substrates by means of the Taft-Pavelich equation (22). The adsorption data suffered from a larger scatter than the rate data. No substrate or substituent could be detected that would fail to satisfy completely the correlation equations. In the correlation of the initial reaction rates and relative adsorption coefficients the parameter p* was negative, while the parameter 6 was positive. In correlations of the reaction rates obtained by the hydrogenation of a similar series of substrates on the same catalyst in a number of solvents, the parameters p: and 6, had the same sign as in the hydrogenation in solvent-free systems, while in the correlation of the adsorption coefficients the signs of the parameters p,* and 6, in systems with solvents were opposite to those in solvent-free systems. This clearly indicates that solvents considerably affect the influence of the structure of substrates on their reactivity. It has been reported in several papers (70, 71,91,98)that in some systems a change in the solvent may importantly affect the selectivity of hydrogenation of two substrates differing in their structure. At the same time, in this case (concentration range) the solvent did not play any role in equations of the Langmuir-Hinshelwood type, because the latter were mostly reduced to equations for pseudo-zero order. Hence, a change in selectivity was most likely caused by interactions of the solvent from the bulk phase with adsorbed molecules of substrates on the catalyst surface. An attempt was made (99) to find out to what extent the selectivity of hydrogenation of two substrates might be changed by the presence of a third compound that is equally adsorbed on the catalyst surface-which is, therefore, capable of competitive adsorption, does not cause a selective poisoning of the catalyst, but also participates itself in the reaction. Its effect upon selectivity may then be regarded as a change in the reactivity of the remaining two substrates, due mainly to the competitive adsorption of this compound, and hence predominantly to interactions of molecules adsorbed on the catalytic surface. At the same time, the initial concentration of this compound is comparable with the concentrations of the remaining two reacting substrates.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

365

Similarly to binary systems, in ternary systems without solvents the use of Eq. (8) in the calculation of relative adsorption coefficients is also questionable, because the effect of a third compound on the selectivity of hydrogenation of the remaining two may consist not only in its effect on the ratio of the adsorption coefficients, but also on the ratio of the rate constants. For this reason, the effect of a third compound on the hydrogenation of the other two was considered (99) as an effect on the ratio of reactivities of both compounds, and thus on the ratio of products of the rate and adsorption constants. The selectivity values measured in ternary systems very adequately satisfied the equation (SA,B)C x (SB,C)A x (SC,A)B = 1

(26)

Since all the selectivities (SA,&, ( S B , C ) A , and were measured in the same system under the same conditions, these interactions may affect the determined selectivity values, but Eq. (25) still remains valid. The (SA,B)C values measured in the presence of various C were not identical in most cases, owing to the interactions between C and both substrates A and B, thus making possible an investigation of the effect of compounds C on the selectivity values S A , B . It was found (99),that the selectivity of hydrogenation of two chemically similar compounds (compounds with the same functional groups, e.g., unsaturated alcohols) does not vary too much if a third compound with a different chemical structure (e.g., an olefin) is added. The following rule holds for a change in the selectivity of hydrogenation of two compounds with a different chemical character in the presence of a third substrate resembling by its chemical character one compound of the binary system: the selectivity of hydrogenation varies in all cases, in such a way that in the presence of a third compound, a compound resembling the third substrate by its chemical character is more strongly displaced from the catalytic surface, which results in a decrease in the reactivity of this compound compared with the second substrate. The results of cerveny et al. (101) were in agreement with these general conclusions. Addition of an unsaturated alcohol to a mixture of another unsaturated alcohol and olefin changed the selectivity of hydrogenation in such a manner that the reactivity of olefin relatively increased; the addition of an olefin to the same mixture resulted in a relative increase in the reactivity of the unsaturated alcohol. These effects are qualitatively similar to the effect of solvents on the selectivity of hydrogenation of two chemically different compounds. It was of interest to study the behavior of a ternary system in a range where the most rapidly reacting substrate disappears, and the system becomes a binary one. Most of the systems did not allow any experimental investigation of this type, because such a transition (disappearance of C) occurred only in

366

LIBOR CERVENY AND VLASTIMIL R ~ J ~ I C K A

the range of very low concentrations of A and B, as the hydrogenation of compounds C was not much faster than that of A and B. In spite of this, however, several systems were found (99-101) in which it was possible to observe a pronounced change in the selectivity of hydrogenation of the substrates A and B, due to the disappearance of the most rapidly reacting compound. This phenomenon was interpreted (99) in the following way. A general system of three substrates, A, B, C, was suggested, in which C was hydrogenated at the fastest rate, and it was assumed that hydrogenation products were not adsorbed and did not affect the further course of hydrogenation. If the ratio of reaction rates of competitivelyreacting compounds is proportional to the ratio of their concentrations, the concentration dependences of A and B may be described by the equation 10g(cA/ci) = (sA,B)C 10g(cB/cg)

(27)

which also describes the straight line 1 in Fig. 4.At the point X , C disappears, and if the latter also affected the selectivity of hydrogenation of A and B, the selectivity also changes, (SA,B)c--t SA,B.By the disappearance of C the ternary system becomes a binary one, with the initial concentrations of A and B being C i and C i . The binary system obeys the relation (straight line 2, Fig. 4): 10g(cA/ci) = sA,B 10g(cB/ci) (28) The coordinates of this system and the straight line 2 plotted in them are represented by a broken line in Fig. 4. In the original coordinate system, the 0

p

-

>a- 0.5

oa

I

CT,

0

d

-1.0 -1.0

-0.5 Log (c&)

0

FIG.4. Concentrationdependences of I-phenyl-2-propene(A) and 1-propenyl phenyl ether (B) in the competitive hydrogenation with 2-propene-1-01.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

367

straight line 2 cuts out the intercept on the axis log(cA/c:), as can be derived from Eq. (28): 4 = S A , B log(C3Ct) - lOg(C:/C3

(29) The straight line 3 in Fig. 4 represents the hydrogenation process in the system A, B, C after the disappearance of C, that is, in a binary system, in the original coordinates. It is a continuation of the straight line 4, which demonstrates the hydrogenation process only in the binary system, without a third compound. In the original coordinates the experimental point should follow the straight line 1 up to the point X , then the straight line 3, as documented by Fig. 4.In the figure, all experimental points are plotted in the original coordinate system. The coordinates of point X may be obtained by a common solution of equations for straight lines 1 and 2. From Fig. 4, (SA,B)c and (SA,B)C=O were read off as the slopes of straight lines 1 and 3. The selectivity value after disappearance of C was in good agreement with SA,Bmeasured in the binary system. Similar systems have also been described (100, 101). The results clearly indicate that the unsaturated compound adsorbed on the catalyst surface has a stronger effect on the selectivity of hydrogenation of two further substrates than the saturated compound with the same skeleton, which is operative only through interactions from the bulk phase. Of the cases described so far, the presence of a third compound had the strongest effect on the selectivity of hydrogenation of two olefinic substrates in the pair olefin-unsaturated alcohol. This influence appeared both in cases where the third compound was unsaturated, was adsorbed competitively and reacted on the catalyst surface, and in cases where the third compound was represented by an inert solvent not undergoing competitive adsorption (was not entering equations of the Langmuir-Hinshelwood type, which were degraded to pseudo-zero order) and obviously operated through interactions of molecules from the bulk phase with adsorbed molecules of the substrate. In all cases the addition of a compound resembling by its chemical character one of the compounds of the binary mixture shifted the selectivity of hydrogenation unfavorably with respect to the latter compound (71,99,100). While the effect of a third olefinic substrate on the selectivity of hydrogenation of a binary olefinic mixture could be investigated only at comparable concentrations of all three substrates, because the use of a single olefinic substrate in a considerable excess would lead to its preferential hydrogenation, the inert solvent was mainly used in a much higher concentration. An attempt to solve the problem of the effect of solvent concentration on the selectivity of hydrogenation was made (101). The measurements were again carried out in olefin-unsaturated alcohol systems,

368

LIBOR

CERVENQ AND VLASTIMIL RPT~ICKA

with methanol and cyclohexane as solvents. In all cases, the addition of methanol caused a change in the selectivity of hydrogenation in favor of the olefin, already at a solvent concentration comparable with that of compounds undergoing hydrogenation. Changes in selectivity caused by a 25-fold increase in solvent concentration were unpronounced. With cyclohexane a pronounced change in the selectivity of hydrogenation in favor of the unsaturated alcohol occurred only at higher concentrations of the solvent. This phenomenon may be explained by stronger nonbonding interactions of OH groups compared with interactions between nonpolar alkyls and nonpolar solvents. The order of relative reactivities of unsaturated alcohols with respect to 1-hexene was not the same (101) as that with respect to 4-methyl-2-pentene; hence, the relative reactivity of various alcoholic substrates related each time to one olefinic substrate depends not only on the structure of this olefinic substrate, but also on the structure of the reference compound. This finding shows that the resulting effect (relative reactivity) is affected by mutual interactions of both reacting substrates. The investigation of the effect of olefins on the course of hydrogenation in solvent-free systems, similarly to the investigation of these relations in systems with solvents, demonstrated the necessity of a complex view of the whole system, because the behavior of substrates in hydrogenation may be affected by all the components present in the reaction mixture.

V.

Relationships between the Effect of Structure of Substrates and Solvents on Reactivity

In spite of the large number of suggested relations describing the effect of solvents on reaction kinetics, only partial success has been observed in their application, mostly always in similar reaction systems; relations failed on passing to another system. The situation becomes much more complicated in heterogeneous systems. Their considerable complexity greatly impedes an adequate description of the effect of solvent on the course of the catalytic reaction using a one-parameter relation. Multiparameter equations were suggested that reflect the effect of solvent on the reaction rate (102). These equations were derived assuming the validity of the LFER type relations in each solvent. On the same assumption they may be applied to a system with a heterogeneous catalyst, where moreover the effect of solvents on the relative adsorption coefficients of substrates may be described in a similar way.

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

369

The rate of chemical reactions carried out under the same conditions is affected by the structure of substrates and by the properties of solvents. Assuming the validity of the Hammett (similarly, of the Taft and TaftPavelich) equation expressing the effect of the structure of substrates on their reactivity in each solvent

it may be written where

Equation (31) expresses the dependence of the relative rate (equilibrium) constant on the properties of the solvent characterized by the parameters R , and S , . The parameter R, is proportional to the differences between the relative Gibbs free energies for the nth and sth solvent of the given and standard substrates. The parameter S, characterizes the effect of a change of the substrate on the rate constant of the substrate with a standard substituent. In the Hammett equation this substituent is represented by hydrogen for which a was definitionally put equal to zero, while in the Taft and TaftPavelich equations methyl is used as the standard substituent (a* = E, = 0). The condition that is necessary and sufficient for the validity of Eq. (31) is the validity of the Hammett (Taft, Taft-Pavelich) equation with the same values of the constants a(o*,E,) of the respective substituents in each solvent. The equation is then independent of the reaction mechanism in each solvent, but the mechanism must be the same for the whole model series of substrates. A similar procedure may be employed in deriving the equation describing the dependence of the adsorption coefficientson the parameter of the solvents where R,, = P ~ - ,p K~, s ,from the Hammett equation for a heterogeneous system. Since the absolute adsorption coefficients are more difficult to measure than the relative adsorption coefficients,Eq. (33) was not adjusted (102) to a two-parameter form corresponding to Eq. (31). The parameter S,, analogous to the parameter S , introduced earlier is implicitly present in the respective equations. For the selectivity of the competitive catalytic reaction of A and B at the same concentrations of these compounds and the same reaction orders, it may be inferred that 10g[(sA,B)n/(sA,J3)s]

=

(Rp

+ RpK)a

(34)

370

LIBOR CERVENY AND VLASTIMIL R I ~ ~ I C K A

where A denotes the substituted substrate and B is the substrate with a standard substituent. Similar relations are obtained using the Taft or Taft-Pavelich equations (cf. Table V). In these equations, the solvent is characterized by two or three parameters. Some interesting facts may be derived (102) from relations of the LFER type for the same kind of reaction in the aromatic and aliphatic systems R,-C6H4-Yl (A) R2-Y2 (B)

+ Bl "-P

(35)

+ B2 -k*t P

(36)

which proceed separately in an nth solvent under the same conditions. The dependence of the rate constant k in scheme (35) assuming B, = constant on a change of the substituent R, is expressed by the Hammett equation; similarly, in scheme (36) the same dependence is expressed in terms of the Taft equation. By combining them, we obtain the expression Iog(knlk3 = PflO - d o *

+ log[(ko)fl/(k:)fll

(37) describing the dependence of the rate constant of a substrate of the R,-C,H,-Y type related to the rate constant of a substrate of the R,-Y2 type on a change in the structure of both types of substrates considered in this study. Let it be that in Schemes (35) and (36), R, = R , , B1 = B,, and Y1 = Y,. Ifk/k* depended only on an interaction with the molecule (A B) containing the reaction site Y, = Y,, this would imply that k/k* ought to be independent of the change in the reaction medium. Koppel and Palm (27) showed that --+

TABLE V Relations Expressing the Effect of Structure of Substrates and Solvents on Reaction Kinetics" Kinetic (equilibrium) parameter log k l k ,

Type of initial equation Hammett R,a

+ S,

Taft R,*u* Rp*,K6*

+ S,,

Taft-Pavelich R,.u*

+ DaE, + Spa

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

37 1

the slopes of the dependences of prel,obtained in the esterification of substituted benzoic acids and substituted benzylacetic acids as the ratio of p values in the given solvent to the respective value of the parameter p in methanol (standard solvent), on pre,for a-substituted acetic acids were not identical. As there is no special reason for a markedly different effect from the part of the aromatic or arylaliphatic system connected with the same reaction site on the parameter p(p*) compared with the aliphatic system, the authors (27) regard this fact as evidence of an interaction between the substituent and the solvent. In this case the ratio of rate constants is not dependent merely on a change in the structure of the organic compound. Similarity existing between Eqs. (31) and (37) suggests that a change in the solvent affects the rate or equilibrium constants similarly to a change in the structure. Using Eq. (37), it may be demonstrated that the equation log(kfllk3= pna - P P *

+ log[(ko)fl/(ko*)sl

(38)

is valid. Equation (38) combines the effects of a change in the solvent and in the structure of the substrate which lead to different effects on the reaction site of the molecule. It is a general form of Eqs. (31) and (37), documenting a close connection between the effect of the solvent and of the structure of the organic compound, and of their common influence on the reactivity and adsorptivity. The validity of suggested relations is self-evident, provided that the assumptions used in deriving them have been fulfilled. Cerveny et al. (102) report the measurements of values of the parameter S , used in the derived equations, for the hydrogenation of the double bond in aliphatic systems and for the hydrogenation of aromatic nitro compounds to aromatic amines. 2,3-Dimethyl-2-butene and nitrobenzene were the standard substrates used; the hydrogenations were performed in seven solvents using 5% P t on silica gel as catalyst. So far, there has been no suitable experimental material available for practical application of the suggested relations, which requires that the original equations of the Hammett and Taft type would be perfectly satisfied in a number of solvents. Cerveny et al. (102) provide a new view of the parameters of solvents suggested earlier in the modified Drougard-Decrooq equation (z’ and 4) and a more thorough discussion of the causes leading to their limited validity. Comparison between Eqs. (21) and (30) ought to give

Since the parameters T’ were defined without bearing in mind specific interactions of solvents with hydrogenated substrates, which, as demonstrated in

312

LIBOR

C E R V E N ~AND

VLASTIMIL

RPT~ICKA

this review, may considerably affect the reactivity and adsorptivity of substrates, it seems evident that Eq. (21) may be satisfactory only for series of compounds of similar structure, which indeed has been experimentally proved. The parameters of solvents, R , and S,, though obviously not transferable to other reaction systems, characterize nevertheless the solvent in the given system without being dependent on a change in the structure of the reacting compounds (with the same reaction site), which is their main advantage.

VI.

Conclusion

Attempts to determine the effect of the composition of the reaction mixture on the course of hydrogenation of unsaturated compounds in the liquid state on solid catalysts have been reported in many papers, without, however, significant generalized conclusions being reached. A qualitative change in the study of the effect of the structure of substrates on their reactivity and adsorptivity in heterogeneously catalyzed reactions has been brought about by the application of LFER, which provided a more reliable basis for the results obtained. Consistent application of these results made possible the quantification of many interactions occurring in complex three-phase and frequently multicomponent systems. A similar approach has been chosen also in the evaluation of the effect of solvents on the reactivity and adsorptivity of unsaturated substrates. Parameters of solvents, formally resembling those of substituents used in the evaluation of the effect of structure, were defined. These parameters adequately described the effect of solvents on the course of hydrogenation in systems of similar compounds, but became unsatisfactory for other model series. A detailed analysis of these parameters revealed that they could not be freed from the effect of the structure of substrates, which obviously is the cause of their nontransferhbility. The results obtained have led to a conclusion that a major generalization is feasible only by assuming a complex view of reaction systems, because all of the effects operative in the process are related to each other; even with the same reaction conditions and the same catalyst, the evaluation of the structure of substrates on the rate of hydrogenation in a single solvent (and hence a constant parameter in this case) may lead to conclusions different from those obtained in a different solvent (again a constant parameter). These differences are the greater, the greater the structural difference between the particular substrates in a given reaction series, or the greater the difference between the character of the solvents used. Since in the systems under

SOLVENT AND STRUCTURE EFFECTS IN HYDROGENATION

373

investigation there was no competitive adsorption of substrates and solvents, these phenomena are obviously due to interactions between adsorbed molecules of the substrates and the volume phase (solvent). Hence, the importance of these interactions is much greater than was assumed before. This is also the cause of the much larger number (23) of successful correlations of the effect of the structure of reactants on the kinetics of heterogeneously catalyzed reactions in the gas phase, compared with the liquid state. It seems that the effect of the composition of the reaction mixture on the course of hydrogenation of unsaturated compounds in the liquid state on solid catalysts can best be described in terms of equations that combine the effect of a change in the solvent with that of the structure of substrates. In the future, these equations may be applied on a major scale, and their use may be extended beyond the scope of catalytic hydrogenations.

VII.

List of Symbols

a thermodynamic activity (mol/liter) A general designation of substrate B general designation of substrate c concentration (moyliter) D parameter of solvent (defined in Table VI) E, steric parameter of the Taft equation (12) A F change in Gibbs free energy (kJ/mol) H general designation of hydrogen I general designation of inert k rate constant (dimension according to the form of kinetic equation) K adsorption coefficient (dimension according to the form of kinetic equation) P general designation of product r reaction rate (mol/min gm cat) R parameter of solvent (defined by Eq. 32) S selectivity (defined by Eq. 8) or parameter of solvent (defined by Eq. 32) t time (min) V volume of reaction mixture (liters) W weighed amount of catalyst (gm) * active site of catalyst CL proportionality constant y parameter of Eq. (23) expressing sensitivity of adsorptivity to a change in solvent S parameter of Eq. (12), expressing sensitivity of given reaction series to change in steric factor of substituent 1 parameter of Eq. (13), expressing sensitivity of reaction rate to change in solvent 1' parameter of Eq. (21), expressing sensitivity of hydrogenation rate to change in solvent p parameter of Eq. (lo), expressing sensitivity of given reaction series to change in polar factor of substituent p* parameter of Eq. (1 l), expressing sensitivity of given reaction series to change of polar factor of substituent

374

LIBOR

CERVENP AND

VLASTIMIL

R~~ICKA

parameter of Eq. (lo), expressing polarity of substituent parameter of Eq. (1 I), expressing polarity of substituent parameter of Eq. (13), characterizing effect of solvent on reaction rate 7’ parameter of Eq. (21), characterizing effect of solvent on hydrogenation rate $I parameter of Eq. (23), characterizing effect of solvent on adsorptivity of substrates on catalyst D

u* z

INDICES Superscripts 0 initial

Subscripts (

a adsorption in presence of compound A K related to adsorptivity n general designation 0 standard substrate r reaction re1 relative S standard 6 related to Taft equation P related to Hammett equation P* related to Taft equation )A

ACKNOWLEDGMENTS The authors thank Dr. M. Kraus, Institute of Chemical Processes Fundamentals, Czechoslovak Academy of Sciences, and Dr. J. Bartoii, Institute of Chemical Technology, Prague, for careful reading of the manuscript and valuable comments. REFERENCES 1. Hinshelwood, C. N., “Kinetics of Chemical Change.” Oxford Univ. Press, London and

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97. Smith, H. A., and Rader, C. P., Proc. In?. Congr. Catal., Znd, 1960 Vol. I , p. 1213 (1961). 98. Cerveny, L., Bartofi, J., and RbiiEka, V., Collect. Czech. Chem. Commun. 41,3572 (1976). 99. Cerveny, L., PlechaEova, D., and RfiiiEka, V., Collect. Czech. Chem. Commun. 43, 2387 (1978). 100. Cerveny, L., Nevrkla, V., and RbiiEka, V., Collect. Czech. Chem. Commun. 42, 2890 (1977). 101. Cerveny, L., Junova, R., and RbiiEka, V., Collect. Czech. Chem. Commun. 44,2378 (1979). 102. Cerveny, L., Bartofi, J., Nevrkla, V., and RbiiEka, V., Collect. Czech. Chem. Commun. 42, 3325 (1977).

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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 Abduraimova, M. A,, 352(77), 376 Abe, T., 294(347), 331 Achtsnit, H. D., 174, 208,215 Adams, C. R., 147, 148(17), 162 Adams, D. M., 20(75), 92 Adiic, R., 274(278), 329 Ahuja, V. K., 351, 376 Ai, M., 121(57, 58, 59, 60, 61), 126(59), 130, 131 Aika, K., 168(9), 214, 266(238), 328 Akishin, P. A., 2(10), 55(10), 63(143), 90, 94 Albright, L. F., 295,331 Aldag, A. W.. 20(77), 50(77), 87(77), 92 Aleksandrov, A. U., 101(24), 106(24), 127(24), I30 Aleksanyan, V. T., 3(20, 21, 2 3 , 53(25), 68 (163), 69( 163, 164), 90, 94, 95 Al-Essa, R. J., 20(79), 28(79), 92 Alkire, R., 31 1(404), 332 Allen, G. C., 250(143), 262(143), 307(143),326 Allendoerfer, R. D., 297, 331 Almon, J. J., 48(128), 93 Almy,D. B.,251(153), 302(153), 307(153),326 Alt, H., 277(284), 329 Alybina, A. Yu., 70(167), 95 Amano, A., 296(359), 331 Amir-Ebrahimi, V., 7(38, 41), 8(41). 9(43,44), 10(44), 12(38, 43, 44), 13(44), 14(38, 41, 43, 44), 17(43), 24(38, 41, 43), 25(38, 41, 43), 26(38, 43), 32(44, 91, 94), 34(94), 35 (94), 38(43,44,91), 39(41), 40(41), 42(91), 54(94), 56(94, 138), 58(138), 75(44), 79 (44), 91, 92, 94 Amis, E. S., 345(25), 375 Amundson, N. R., 321(423), 333 Anderson, J. R., 1(1), 4(31, 34), 12(34), 17, 19 (34), 20(31, 34), 32(92), 50(34), 76(177), 85,86,87,88,90,91,92,95,296(358), 302

(380), 303(380), 331,332 Anderson, R. A,, 169(10), 170, 176, 178(10), 196(10), 197(10), 215 Anderson, R. B., 172(15), 176, 178(15), 215, 299(373), 309(373), 332 Anderson, R. L., 38(100), 59(100), 92 Andreev, V. N., 262(202), 264(202), 327 Angerstein-Kozlowska, H., 241(90), 245(90), 246(90), 249(124, 129), 251(124), 267(90), 302(90), 325, 326 Annenkova, I. B., 117(51, 52), 130 Aonuma, T., 345(42), 375 Appleby, A. J., 253(162, 167), 274(275), 326, 327,329 Arai, I., 108(37), 130 Araki, M., 189, 196,197(48), 198(48),200,201 (48), 202(48), 205, 210, 211, 215, 263 (216), 328 Ariet, M., 313(413), 333 Aris, R., 31 1(402), 314(402), 332 Arpe, H. J., 162 Asada, A,, 275(279), 329 Ateya, B. G . , 311(406), 332 Atkinson, L., 104(31), 130 Augustinski, J., 250(146a), 307( 104a), 326 Austin, L. G., 218(4), 230(4), 231(4), 243(4), . 290(4), 31 1(406), 313(413), 322, 333 Avdeeva, T. I., 70(167), 95 Avery, N. R., 4(34), 12(34), 17, 19(34), 20(34), 50(34), 86, 91 Ayasse, C., 101(26), 107(26), 130

B Baba, H., 294(347), 331 Bagotskii, V. S., 237(81), 238(81), 261(81), 262 (81, 203, 204,206,207), 263(81,206,207), 264(203), 279,330,324,327 Baizer, M. M., 231(56), 294(56), 324

379

380

AUTHOR INDEX

Baker, B. S . , 4(31), 20(31), 91, 253(167), 327 Balandin, A. A,, 43(105), 93 Baldwin, R. H., 280(308), 330 Balenkova, E. S., 2(11, 12), 45(116), 46(116), 55(12), 63(140, 141, 142, 143, 144), 69 (164, 165, 166), 70(167, 168, 169, 170), 90, 93, 94, 95 Balsenc, L., 250(146a), 307(146a), 326 Baltrunas, D., 101(24), 106(24), 127(24), 130 Barber, W. A., 230(49), 324 Barclay, J . L., 98(2, 3,4), 129 Bard, A. J . , 284(319), 330 Barger,H. J., Jr.,260(193, 194),296(193, 194), 302(194), 304, 327 Baria, D. N., 297(369), 331 Barnes, K. K., 298(371), 309(371), 332 Barnett, A. E., 274(277), 329 Barrett, M. A., 249(122), 325 Barron, Y., 4(32,33), 5(33), 35(32,33), 42(33), 52(33), 58(33), 91 Barton, J., 361(98), 364(98), 368(102), 369 (102), 370(102), 371(102), 377 Bashulin, P. A., 3(16), 90 Basolo, F., 280(305), 281(305), 298(305), 330 Basset, J. M., 79(184), 95 Batist, P. A,, 124, 131 Batrakov, V. V., 243(100), 254(100), 264(100), 325 Batuev, M. I., 2(13), 3(17), 90,94 Bau, R., 89(192), 95 Baur, W. H., 101(29), 130 Beaty, E. M., 246(113), 247(113), 273(113), 325 Beck, F., 221(34), 222(34), 267(34), 294, 296 (356a), 323,331 Beeck, O., 183, 194,215 Behret, H., 277(285), 329 Bilanger, A,, 292(343), 331 Btlanger, G., 292(343), 331 Bell, A. T., 187(37), 191,192,193(37), 194(37), 195, 197(37), 198(37), 211(37), 213, 215, 216 Beloslyudova, T. M., 294(353), 331 Belousov, V. M., 130 Benedict, W. S., 10(46), 91 Bennett, A. J., 286(69), 324 Bennett, C. O., 197(64), 216 Benninghoven, A,, 308(395), 332 Beranek, L., 338(1 I), 374

Berl, W. G., 253(164), 326 Bernardo, C. A., 197(62), 216 Berndt, W., lOl(27, 28), I30 Berry, F. J., 99(17, 18, 19, 20), 101(30), 106 (17, 18, 19), 107(19, 19a, 30), 108, 109 (20), 113(19a), 116(18, 19), 117(17), 127 (18, 19), 129, 130 Berson, A., 345, 375 Bertolini, J . C., 188,215 Berzelius, J. J., 218(2), 322 Beskorovainaya, S. S., 264(227), 328 Bethell, J. R., 98(2, 5), 129 Bett, J . A. S., 265J231), 266(243), 268(258), 328,329 Beusch, H., 321(420), 333 Beyer, B. F., 212(77), 216 Bianchi, G., 268(255), 329 Biberian, 3. P., 251(154), 326 Biegler, T., 249(132), 250(132), 326 Bilous, O., 321(423), 333 Biloen, P., 197(59), 202, 203(59, 65, 66), 204 (59), 205(59), 206(59, 65), 209(66), 210 (66), 211(59, 76), 216 Binder, H., 263(222), 273(271), 277(284, 285), 278(292), 290(331), 328,329,330,331 Bionda, G., 293(344), 331 Birchall, T., 106(35), 130 Bird, R.B., 312(409), 313(409), 333 Blake, A. R.,221(28), 222(28), 226(28), 227 (28), 231(28), 268(28), 280(28), 281(28), 290(28), 291(28), 323 Blakeley, D. W., 194(53), 215 Blakely, D. W., 283(316), 330 Blanchard, M., 214(83), 216 Blaser, H. U., 25(83), 92 Blurton, K. F., 265(230), 268(260), 328,329 Blyholder, G., 191,215 Bobrov, A. V., 64(147, 148), 94 Bockris, J. O’M., 218(5, 8), 221(5, 8), 223(8, 44), 224(44), 225(44), 226(8), 227(8), 228 (8), 229(8), 230(5, 8), 231(5, 8, 65), 232 (66), 233(68), 236(75, 78), 237(79), 238 (79, 80), 240(8, 44),243(5, 8), 246(1 lo), 248(8, 79) 249(125, 136), 251(125), 252 (158, 159), 253(161, 166), 254(170), 255 (80), 256(175), 259(44), 260(175, 187, 189), 261(175), 268(78), 273(78, 79, 80), 277(286), 278(286, 2911, 279(301), 281 (170), 284(321), 290(8, 170), 299(5, 81,

38 1

AUTHOR INDEX

300(189), 322, 323, 324, 325, 326, 327, 330,332 Bold, M., 243(97), 244(97), 248(97), 250(97), 294(97), 325 Bold, W., 249(130), 326 Bogotskii, V. S.,264(226, 227), 326, 328 Bonze], H. P., 187(36), 189(36, 47), 190(36), 194, 197(36, 63), 198(36, 47), 214(81), 215,216, 307(389), 332 Bond, G . C., 211, 216, 220(23), 221(27), 236 (23), 240(23), 246(23, 109), 254(23), 258 (23), 260(23), 263(23,219), 274,279(297), 294(23), 299(23), 304(27), 323, 325, 328, 330, 347(46), 348(58, 59), 375, 376 Bonhoffer, K. F., 246(1 lo), 325 Boreskov, G . K., 153,162 Boronin, V. S., 76(176), 95, 265(229), 266 (229), 328 Borrelli, N. F., 108(42), 130 Bosch-Giral, P., 48(127), 49(127), 75(127), 79 (127), 81(127), 93 Boshoff, L. J., 178(24), 215 Bosvakova, E. N., 346(35, 36, 38, 39), 375 Bouchard, R. J., 106(35), 130 Boudart, M., 10,20(77), 50(77), 87,91,92, 184 (34), 185, 195(34), 215, 241, 266(239), 325, 328, 343(16), 375 Boudeville, Y . , 99(21), 104(21), 110(21), 113 (21), 122(21), 123, 129 Bradshaw, A. M., 79(185), 81(185), 95 Brady, R. C., 208(70), 216 Bragin, 0. V., 2(8), 43(107, 108), 44(107, 108, 109,110,111),45(112, 113, 114,115, 116), 46(113, 116), 47(107, 109, 117, 118, 119, 120, 121, 122),48(109, 110, 111, 112, 122, 123, 124), 66(155, 156, 157), 90, 93, 94 Branden, R.L., 348(64), 376 Brazdil, J. F., 144(1I ) , 145(1 l), 146(1l), 162 Bream, J. B., 98(2, 3), 129 Bregoli, L., 265,328 Breitenstein, A. M., 257(178), 327 Breiter, M., 241(91a), 243(97), 244(97), 248 (97), 250(97), 260(91a), 267(91a), 294 (97), 325 Breiter, M . W., 218(7, 12), 221(7), 226(7), 227 (7), 228(7), 229(7), 230(7, 12), 231(7, 12), 243(7,12), 248(7,119,121), 249(128,129), 250(121), 252, 254(7, 12), 256(7), 257(7), 261, 262(198, 199), 263(209), 264(223,

224,225,228), 267(7,12,199), 268(7, 119, 254), 290(7, 12), 299(7), 301(223), 322, 323,325,326,327,328,329 Brodd, R. J., 279(296), 330 Broden, G., 214(81), 216 Brown, C. A., 351,376 Bruckenstein, S., 257(183), 327 Brummer, S. B., 257(180, 182), 327 Brusic, V., 236(77), 237(77), 238(77), 252(77), 324 Buck, R. P., 231(54), 281(54), 290(54), 324 Bulanova, T. F., 2(4, 5), 3(20), 90 Burke, L. D., 267(246), 294, 295,328 Burnett, R. L., 68,94 Burrington, J. D., 147(21), 149(20), 150(20), 151(20), 152(20,21), 153(21), 154(21), 155 (21), 156(21), 157(21), 158(21), 159(21), 160(21), 161(21), 162 Burshtein, I. I., 297(361), 331 Burton, J. J., 78, 95 Burwell, R. L., 32(93), 92, 347(51), 348(53), 375 Bunvell, R. L., Jr., 296(360), 331 Bussiere, P., 108(39a), 130 Butler, J. A. V., 231(64), 324 Butt, J. B., 268(264), 270(264), 272(264), 273 (269), 329 Byrne, J. W., 25(83), 92 Byrne, M., 248(118), 296(118, 356), 325,331

C Cairns, E. J., 218(6, lo), 226(10), 227(6, lo), 228(6, lo), 229(10), 230(6, lo), 231(6, lo), 243(6, lo), 248(6), 254(10), 256(10), 257 (178), 290(6, lo), 322,327 Calderon, J., 208(69), 216 Calderon, N., 20(74), 59(74), 92 Callahan, J. L., 136(2), 138(2), 139(7), 141(7), 143(2), 162 Campanella, S., 89(192), 95 Campbell, B. K., 349(67), 376 Campbell, K. N., 349, 376 Capon, A,, 250(143), 262(143, 201), 307(143), 326,327 C a d r e , B., 252(157), 265(157), 326 Casey, C. P., 23(81), 38(100), 59(100), 92 Centola, P., 153(24), 163

382

AUTHOR INDEX

Cervena, J., 351(69), 356,357(69), 359(69), 360 (69), 376 Cerveny, L., 337, 338(8), 339(5, 12), 342, 348, 349(47,48), 350,351,352,353(48,78,79), 354(78, 79), 355, 356, 357, 358, 359(69, 90), 360(69, 70, 71), 361, 363, 364(70, 71, 98, 99, IOO), 365, 366(99, IOO), 367(71, 99, IOI), 368(101, 102), 369(102), 370 (102), 371, 374, 375, 376, 377 Cevinka, K., 338(8), 374 Chambellan, A,, 6(36), 48(127), 49(127), 75 (36, 127), 79(36, 127), 81(36, 127), 91, 93 Chan, M. N., 3(24), 66(155), 90, 94 Chapman, D. L., 223(40), 323 Charton, M., 354(85), 376 Chatt, J., 20(75), 92 Chauvin, Y., 20(73), 59(73), 92, 208(68), 216 Childs, W. V., 294(346, 348), 331 Chini, P., 89(192), 95 Chopey, N. P., 231(52), 324 Choplin, A., 32(94), 34(94), 35(94), 54(94), 56 (94), 92 Christie, J. R., 117, 124(53), 130 Chumenko, N. N., 100(23), 101(23), 130 Cimino, A,, 10, 91 Civera, M., 293(344), 331 Clark, D. N., 38(101), 42(101), 59(101), 92 Clark, J. K. A., 1(2), 19(67), 21, 27(2), 68(67, 161), 90, 92, 94 Clark, M., 253(169), 327 Cockrell, J. R., 302(375), 332 Coekelbergs, R., 10(55), 84(55), 91 Cognion, J.-M., 138(5), 162 Coleman, A. J., 260(193, 194), 296(193, 194), 302(194), 304, 327 Collins, D. M., 25 I , 254( 1SO), 307( ISO), 326 Colman, W. P., 230(49), 324 Columbic, N., 169(10), 170, 176, 178(10), 196 (lo), 197(10),215 Comeau, J., 257(183), 327 Comyn, R. H., 313(413), 333 Contour, J. P., 278(294), 330 Conway, B. E., 223(39), 241(90), 243(95), 245 (90),246(90,110,113),247(113), 249,251, 265(90), 267(90), 273(113), 302(90), 320 (417), 321(417), 323,325,326, 333 Cornet, D., 4(32), 35(32), 91 Corolleur, C., 6(35), 9(45), 29(87), 31(87), 48 (125), 49(125), 73, 74, 75(35,45), 86(186), 91, 92, 93, 95

Corolleur, S., 6(35), 48(125, 127),49(125, 127), 73(35), 75(35, 127), 79(127), 81(127), 91, 93 Covitz, F. H., 231(58), 324 Craxford, S . R., 169, 207, 215 Cross, Y. M., 99(13), 102, 103(13), 104(13), 110, 111, 112, 113, 114(4), 129 Crozat, M., 101(25), 116,130 Crucq, A,, 15(62), 92 Csicsery, S. M., 67(158, 159), 68, 94 Cusumano, J. A., 274(277), 329 Cyrot-Lackmann, F., 78(181), 79(183), 95 Czerwinski, A., 263(208), 264(208), 328

D Dadyburjor, D. B., 138(5), 162,269(266), 270 (266), 329 Dahms, H., 254(170), 260(188), 281(170), 290 (1 70), 327 Dalla Betta, R. A,, 178, 191, 192(51), 193(51), 194(51), 195, 213,215, 216 Dalmai-Imelik, G., 48(127), 49(127), 75(127), 79(127), 81(127), 93 Dalmon, J. A , , 197(61), 198(61), 211(74), 216 Damaskin, B. B., 240(86), 243(100), 254(100), 264(100), 325 Damjanovic, A,, 236(77), 237(77, 79), 238(77, 79), 248(79), 252(77, 158, 160), 253(160, 166), 273(79), 277(286), 278(286), 284 (321), 324,326,327,330 Dartigues, J. M., 6(36), 10(54), 12(54), 17(54), 48(127), 49(127), 75(36, 127), 79(36, 127), 81(36, 127), 91, 93 Dautzenberg, F. M., 52(131), 54(131), 58, 73, 74(173), 93, 95, 184, 186(33), 211(76), 215,216 David, R. E., 250(144), 251(144), 307(144), 326 Davies, M. O., 253(169), 327 Davis, B. H., 52(130), 56, 93, 94 Davitt, H. J., 295, 331 Davtyan, 0. K., 297(361), 331 Davydov, A. A,, 153(22), 162 Day, P., 104(31), 105(33), 130 Deans, H. A,, 283(318), 330 de Boer, N. H., 147,162 Debus, G., 358(93), 376 Debye, P., 225(45), 323

383

AUTHOR INDEX

Decroocq, D., 345, 356, 359,375 Degols, L., 15(62), 92 Delahay, P., 223(38), 253(165), 323,327 Delannois, Y., 10(55), 84(55), 91 De Maine, P. A. D., 246(113), 247(113), 273 (1 13), 325 Demuth, J., 307(384), 332 Denbigh, K. G., 284(323, 324), 286(323, 324), 288(323, 324), 330 DePauro, F., 358(94), 376 Derlyukova, L. E., 124(63), 131 Desjonqueres, M. C., 78(181), 79(183), 95 Despic, A., 232(66), 324 Dessau, R. M., 280(303), 330 Devanathan, M. A. V., 223(44), 224(44), 225 (44), 240(44), 259(44), 323 Deville, J. P., 252(157), 265(157), 326 Dewing, J., 127(68), 131 de Witt, T. W., 176, 196, 208, 215 Dey, A., 252(160), 253(160), 326 Dobroserdova, N. B., 348(55, 56, 61), 376 Dockerty, R. C., 108(40), 130 Dogonadze, R. R., 324 Dorling, T. A,, 263(21 I), 328 Downing, D. M., 273(269), 329 Draiic, D. M., 274(278), 329 Drougard, Y., 345, 356, 359,375 Druz, W. A,, 294(351), 331 Dry, M. E., 175, 178(24),215 Dublin, D. A.,99(11), 100(11), 101(11), 129 Ducros, R., 251(152), 252(152), 265(152), 307 (152), 326 Dudley, R. F., 218(13), 230(13), 231(13), 243 (13), 253(13), 323 Duic, L., 260(189), 300(189), 327 Dumesic, J. A., 309(396), 332 Dwyer, D. J., 175, 187, 188, 197(35),215

E Eady, C. R., 89(191), 95 Eastman, D., 307(384), 332 Eckert, C. A., 345(29), 375 Egorov, Yu. P., 3(29), 91 Eidus, J. (Ya.) T., 176(22, 23), 196, 207, 212 (22), 346(44, 45), 215, 375 Eischens, R. P., 348,375 Eisenberg, M., 313(410), 333 Ekerdt, J . G., 187(37), 191, 192, 193(37), 194 (37), 195, 197(37), 198(37), 211(37), 215

Elagina, N. V., 64(145, 146, 147, 148), 94 Elek, L. F., 21 1(72), 216 Emmett, P. H., 176, 196, 208,215 Enyo, M., 302(378, 379), 332 Epelboin, I., 321(419), 333 Ephritikine, M., 19(70, 72), 92 Erickson, N. E., 307(382a), 332 Escard, J., 278 (294), 330 Evans, G. E., 231(53), 324 Evnin, A. B., 220(18), 323

F Fadeev, V. S., 53(135), 94 Fahidy, T. Z., 291(336), 331 Farooque, M., 291(336), 331 Farrington, P. S., 294,331 F a d , W., 278(292), 330 Feiz, I . , 221(25), 229(25), 230(25), 235(25), 237 (25), 238(25), 239(25), 243(25), 248(25), 257(25), 260(25), 281(25), 285(25), 295 (25), 298(25), 302(25), 303(25), 304(25), 313(25), 323 Fellmann, J. D., 38(98), 92 Fichter, F., 280(302), 290(327), 330 Fieguth, P., 321(420), 333 Figueras, F., 99(21), 104(21), 108, 110(21), 113, 122, 123, 124(65a), 129, 130,131 Finkelstein, M., 231(59), 324 Firsova, A. A,, 101(24), 106(24), 127(24), 130 Fischer, E. O., 42(103), 92 Fischer, F., 167, 195, 196,207,212(6,55),214, 215,216 Fleishmann, M., 291(335), 331 Flentge, D. R., 280(310), 330 Flinn, D. R., 249(137), 326 Flory, P. J., 171,215 Flynn, P. C., 251(155, 156), 268(261), 326,329 Foger, K., 87, 88, 95 Ford, R. R., 187,215 Forissier, M., 99(21), 104(21), 108(39a), 110 (21), 113(21), 122(21), 123, 124(65a), 129, 130, 131 Fox, H. P., 294(346), 331 Fraenkel, D., 214(84), 216 Francis, G. A., 231(61, 62), 273(61, 62), 283 (61), 284(61), 285(61), 286(61), 287(61, 62), 288(61, 62), 310(61), 311(61, 62), 312 (61,62), 315(61,62), 316(61,62), 317(62), 318(61, 62), 319(61, 62), 324

384

AUTHOR INDEX

Frank, M. L., 70(169), 95 Franklin, T. C., 320(416), 333 Freidlin, L. C., 352(77), 376 Freidlin, L. Kh., 348(60), 354(83), 376 Freifelder, M., 297(368), 331 Frenkel, S. Z., 64(145), 94 Frennet, A,, 7(38), lO(55, 56, 57), 12(38), 14 (38), 15, 24(38), 25(38), 26(38), 84(55, 56, 57), 91, 92 Friedt, J., 108(39a), 130 Frumkin, A. N., 240(85, 87), 241(87a), 260 (190), 261(196, 197), 267(244), 325, 327, 328 Fuchs, R., 278(290), 330 Fuggle, J. C., 263(215), 307(215), 328 Fujikawa, K., 258(186), 327 Fujimoto, K., 218(312), 330

G Gabbay, D. S., 117(49), 118(49), 121(49), 124, 130 Gadzuk, J. K., 308(391), 332 Gadzuk, J. W., 188(39), 215 Gafner, G., 187(36), 189(36), 190(36), 197 (36), 198(36), 214(81), 215, 216 Garbowski, E., 79(184), 95 Gardener, P. D., 348(64), 376 Garin, F., 7(39, 40), 9(44), lO(40, 44), ll(39, 40), 12(39, 40, 44, 60), 13(44), 14(44), 16 (39,60), 17(40), 20(40), 22(40), 32(44,91), 38(44, 91), 42(91), 48(39), 49(39), 75(44), 79(44), 82(39, 40), 85(39), 91, 92 Garten, R. L., 183(32), 215 Garton, G., 265(234), 328 Gates, B. C., 144(13), 145(13), 146(13), 160 (13), 162,214(84),216,220(21), 267(248), 323,329 Gault, F. G., 4(32, 33), 5(33), 6(35, 36, 37), 7 (38, 40, 41), 8(41), 9 (43, 44, 45), lO(40, 44),11(40), 12(38, 40, 43, 44), 13(41, 44), 14(38, 41, 43, 44), 16(60), 17(40, 43, 63), 19(63, 69, 70), 20(40, 76), 22(40), 24(38, 41, 43), 25(38, 41, 43), 26(38, 43), 28(84, 85), 29(86, 87, 30(86, 88), 31(86, 87, 89), 32(44, 86, 88, 89, 91, 94), 33(86), 34(94), 35(32, 33, 94), 36(70), 38(43, 44, 88, 91), 39(41), 40(41), 42(33, 86, 88, 91), 47(89), 48(125, 126,127),49(125, 127), 52(33), 53

(63, 69, 70), 54(94), 55(69), 56(94), 58, 59 (63,69, 139), 60(63, 69), 61(139), 62(139), 64(150), 72(85, 86), 73, 74, 75(35, 37, 44, 45, 127, 175), 79(36,44, 127, 175), 80(88), 81(36, 37, 127), 82(40), 86(186, 187), 87, 91, 92, 93, 94,95, 209, 216, 353(82), 376 Gel’bahtein, A. I., 99(1 l), lOO(1 l), 101(1l), 129 Genshaw, M. A,, 249(125), 251(125), 253 (166), 284(321), 326,327,330 Gerischer, H., 221(34), 222(34), 233(73), 267 (34), 294(34), 323, 324 Gerlach, R. L., 308(392), 332 Germain, J. E., 64(150), 94, 101(25), 116, 130 Gershingorina, A. V., 130 Geske, D. H., 284(319), 330 Gielen, M., 345, 375 Gileadi, E. G., 236(78), 243(95), 256(175), 260 (175, 187, 189), 261(175), 268(78), 273 (78), 284(320), 300(189), 320(78), 324, 325,327,330 Gilet, M., 78(182), 95 Gilman, S., 249(127), 251(127), 256(174), 257 (174, 177, 181), 258, 260(184, 185), 263 (212), 264(223), 267(247), 301(174, 223), 302(174), 326,327, 328 Giner, J., 263(210), 313(415), 328,333 Gjostein, N. A., 268(262), 329 Godin, G. W., 99, 100, 109(9), 116, 122, 124 (81,129 Gonzalez-Tejuca, L., 266(238), 328 Goodman, D. W., 188, 190(44), 215 Goodridge, F., 221(29), 222(29), 226(29), 231 (29), 268(29), 280(29), 281(29), 290(29), 291(29, 334), 309(400), 323, 331, 332 Gordon, M. B., 78(181), 79(183), 95 Gossman, A. F., 250(145), 251(145), 307(145), 326 Gostunskaya, I. V., 45(112), 48(112, 124), 53 (135), 93, 94, 348(55, 56, 61), 376 Gould, R., 31 1(404), 332 Goursac, F., 276(281), 329 Gouy, A., 223(41), 323 Graham, J. H., 17(64), 18(64), 92 Grahame, D. C., 223(43), 224(43), 323 Grant, J., 307(388), 332 Granvik, J., 358(95), 376 Grasselli, R. K., 133(la), 134(la), 135(la), 136 (2), 138(2), 139(7), 141(7), 142(8, 9), 143 (2), 144(11), 145(11, 15, 16), 146(11), 147

385

AUTHOR INDEX (21), 152(21), 153(21), 154(21), 155(21), 156(21), 157(21), 155(21), 159(21), 160 (21), 161, 162, 220(2i)J,323 Greenberg, P., 265(230), 328 Green, H., 260(188), 327 CreL.1, M., 702(376. 377). 332 Green, M. L. H., 19(71, 72), 92 Griffith, L. R., 231(54), 281(54), 290(54), 324 Grottesfield, S. J. C. S., 249(123), 251(123), 325 Grove, W. R., 218.322 Grubb, W. T., 218,230(3), 231(3), 243(3), 254 (171). 322,327 Grunwald, E., 343(17), 345, 375 Gruver, G., 265,328 Grzybowska, B., 148(19), 150(19), 160(19). 161(19), 162 Guczi, L., 12, 16(59), 91 Gurney, R. W., 233,324 Gur’yanova, G . G., 66(157), 94 Gur’yanova, G. K., 47( 117), 93 Gutjahr, M. A,, 273(273), 329 Guy, R. G., 20(75), 92

H Haas, T. W., 307(388), 332 Haber, J., 148, 150, 160, 161, 162, 163 Hadley, D. J., 98(2, 3, 4, 5), 129 Hagstrum, H. D., 305(382), 307(382), 308 (382), 332 Hajek, M., 48(125), 49(125), 93 Haldeman, R. G., 230(49), 324 Halsey, G . D., 241(91), 325 Hamlet, Z., 345(33), 375 Hammett, L. P., 298(370), 343, 331, 375 Hanika, J.. 337(3), 374 Hansma, P. K., 188,215 Hardman, H. F., 145(15), 162 Harrison, P. G . , 127(67), 131 Hart, D. W., 89(192), 95 Hartog, F., 77( 178a), 95, 266(236), 328 Hatfield, N. A,, 262(205), 327 Hayden, P., 98(7), 99(7), 116, 129 Heal, M. J., 263(213), 328 Heath, C. E., 246( 112), 247( 112). 325 Hegedus, L. L., 271(267a), 329 Heiba, E. J., 280(303), 330 Heiland, W., 256(175), 260(175),26i(175),327

Heinemann, H., 4(30), 91 Helle, J. N., 184(33), 186(33), 197(59), 202 (59), 203(59. 66), 204(59), 205(59), 206 (59), 209(66), 210(66), 21 1(59), 215,216 Helmholtz, H., 223(37). 323 Henshelwood, C. N., 336, 338, 356, 374 Heral, V., 337(6. 7), 339(12), 374 Herington, E. F. G., 36, 39(96), 56(96), 170 (12), 92,215 Herisson, J. L., 20(73), 59(73), 92,208(68), 216 Herniman, H. J.,99(14), 102(14), 108(14), 114 (14), 118(14), 119, 120(14), 126(14), 129 Heumann, A,, 36(97), 92 Higgins, R., 98(7), 99(7). 116, 129 Hillenbrand, L. J., 278(293), 330 Hinden, J., 250(146a), 307(146a), 326 Hoare, J. P., 249, 326 Hoare, M. R., 78, 95 Hobbs. B. S., 278(288,289), 279(288,289), 330 Hohman, W., 268(257), 308(257), 329 Holbrook, L. L., 290(330), 330 Holbourn, P. E., 99(19), 106(19), 107(19), 108 (19), 116(19), 127(19), 129 Holloway, P. H., 307(387, 390), 332 Horanyi, G., 248(116), 297(1 16), 325 Horiuti, I . , 347, 354(49), 375 Horiuti. J., 231(63), 324 Horn, D. H. S., 55(136), 94 Hougen, 0. A., 227(46), 237(83, 84), 323,324, 336(2), 356, 374 Houston, J. E., 307(387), 332 Houston, R. J., 269(265), 329 Hovorka, F., 253(169), 327 Huang, J., 253(163), 307(163), 326 Hucknall, D. J., 98(6), 99(6), 116, 129 Hudson, J. B., 307(390), 332 Hiickel, E., 225(45), 323 Hulburt, H. M., 297(369), 331 Hull, M. N., 245(104), 325 Hunter, C., 313(415), 333 Hunter, W. G., 237(84a), 325 Hush, N . S., 231(55), 294(55), 296(55), 324

I Ibarbia, P. A., 21 1(72), 216 Ikariya, T., 38(99), 59(99), 92 Ikawa, T., 121(57), 130 Imai, I., 109(43), 130

386

AUTHOR INDEX

Imelik, B., 188,215 Iversen, P. E., 297(365), 331 Iwakura, C., 253(163), 275(279), 307(163), 326,329 Iwamoto, I., 346(42), 375 Iwasawa, Y., 153, 163

J

Jacobs, P. A., 214(82), 216 Jahnke, H., 277(283), 329 Janas, J., 148(19), 150(19), 160(19), 161(19), 162 Jardine, I., 352, 376 Jasinski, R., 276(282), 277(282), 329 Jeitschko, W., 145(14), 162 Jenkins, R. H., 98(2), 129 Jennings, T. J., 147, 148(17), 162 Jennison, D. R.,307(387), 332 Jewur, S . S., 138(5), 162 Jiru, P., 153(24), 163 Johnson, B. F. G., 89(191), 95 Jonassen, H. B., 348(63), 376 Joyner, R. W., 188, 21 I , 215 Juda, W., 292(339), 331 Judy, W. A,, 208(69), 216 Jungers, J. C., 358(91, 92,93,94,96), 364(91), 3 76 Junova, R., 365(101), 366(101), 367(101), 368 ( I O I ) , 377 Juttard, D., 74(172, 174), 95

K Kalinin, M. A,, 292(342), 33/ Kamiya, Y., 99(10), lOO(10, 22), 101(10), 103 (22), 108(22), 109(22), 130 Kanevsky, L. D., 279(298), 330 Kapteijno, C. J., 124(62), 131 Karpinski, Z., 19(66), 47(121), 92, 93 Kartisek, C. T., 147(21), 152(21), 153(21), 154 (21), 155(21), 156(21), 157(21), 158(21), 159(21), 160(21), 161(21), 162 Kasai, P. H., 220(18), 323 Kaska, W. C., 188(43), 215 Katzer, J. R., 144(13), 145(13), 146(13), 160 (13), 162,220(21), 323

Kazanskaya-Koperina, A. V., 44( I 1 1 ), 48 (1 1 I), 93 Kazanskii, B. A., 2(3,4,5.6,7,8,9,12, 13, 14). 3(7, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27), 43(104), 44(109, I I O ) , 45(114, 115, 116), 46(116), 47(109, 117, 118), 48 (109), 110, 123, 124), 52(129), 53, 55(12), 63(140, 142. 143). 64, 65, 66(155), 68, 69, 70(167, 168, 169, 170), 90, 91, 93, 94, 95, 348(56, 61), 376 Kazarinov, V. E., 262(200,202), 264(202), 327 Keating, K. B., 218(15), 323 Keii, T., 346(41,42), 375 Kelley, R. D., 188, 190(44, 4 9 , 197(45), 215 Kellner, C. S., 213, 216 Kemball, C., 10(47), 20(76), 31(90), 32(92), 51(90), 91, 92, 267(246), 294(246), 295 (246), 296(358), 302(380), 303(380), 328. 331, 332, 353(82), 376 Kemball, C. J., 302(381), 303(381), 332 Keulks, G. W., 97, 124,129, 143, 144(10), 162 Khafizova, N. A,, 70(169, 170), 95 Khan, A. A., 101(29), 130 Khazova, 0. A,, 262(206,207), 263(206, 207), 327 Khelkovskaya-Sergeeva, E. G., 45(1 IS), 47 (119, 120, 122), 48(122, 123), 93 Khromov, S. I., 2(10, 12), 55(10, 12), 63(140, 141, 142, 143, 144), 68(163), 69(163, 164, 165, 166), 70(167, 168, 169, 170), 90, 94, 95

Kieboom, A. P. G., 352,376 Kikuchi, E., 251(15S), 326 Kim, K. S., 250(144, 145, 146, 147), 251(144, 145), 307(144, 145, 146, 147), 326 Kim, S. H., 249(136), 326 King, C. J. H., 221(29), 222(29), 226(29), 231 (29), 268(29), 280(29), 281(29), 290(29), 291(29, 334), 309(400), 323, 331, 332 King, D. L., 192, 194,215 Kinoshita, K., 266(243), 268(258), 279(299, 300), 328, 329, 330 Kiperman, S. L., 338(9), 374 Kirmse, W., 23(80), 92 Kirsch, W. B., 348(63), 376 Kiss, L., 309(397), 332 Kita, H., 258(186), 327 Kittrell, J. R., 237(84a), 325 Klein, B., 348(54), 375

387

AUTHOR INDEX

Kluksdahl, H. E., 269(265), 329 Knozinger, H., 266(237), 328 Knop, O., 101(26), 107(26), 130 Knorr, C. A., 248(120), 249(120), 250(120). 268(253), 325, 329 Knox, K., 142(9), 162 Kobliansky, G. G., 349(66), 376 Koehl, W. J . , Jr., 280(303), 330 Kohling, A., 263(222), 273(271), 290(331), 328,329,331 Koetzle, T. F., 89(192), 95 Kohnke, E. E., 108(38), 130 Kolchin, 1. K., 106(34), 124(65), 130, 131 Kolodieva, E. V., 70(168), 95 Komuro, T., 281(313), 330 Kool, J. M., 197(60), 216 Koperina, A. V., 94 Koppel, 1. A., 345,352(80), 370, 371(27), 375, 376 Koscielski, T., 19(66), 92 Kozawa, A., 268(251), 279(296), 329,330 Kramers, H., 31 1(403), 332 Krasilshchikov, A. I., 253(161), 326 Kraus, M., 339(10), 344,345(23), 352,373(23), 374, 375, 376 Krebs, H. J., 187(36, 47), 189(36), 190(36), 194, 197(36,63), 198(36, 47), 215,216 Krenzke, L. D., 97(1), 124(1), 129 Krieger. K. A., 299(372), 332 Kroeker, R. M., 188(43), 215 Kromchenko, G . P., 297(363), 331 Kruschcheva. E. J., 268(252), 329 Krylova, A. V., 124(63), 131 Krylova, V. S., 297(363), 331 Ksouri, M., 321(419), 333 Kuhn, A. T., 221(28), 222(28), 226(28), 227 (28). 231(28), 237(80), 238(80), 248(118), 255(80), 268(28), 273(80), 280(28), 281 (28), 290(28), 291(28), 296(118, 356), 309 (399), 323,324,325,331,332 Kuhn, W . , 290(331), 331 Kul’kova, N. V., 99(1 I), lOO(1 l), 101(1l), 129 Kummer, J . T., 176. 196, 208, 215 Kunugi, T., 281(312), 330 Kunz, H., 265,328 Kunz. H. R., 253, 268(260), 327,329 Kurganov, A. A., 99(8), 100(8), 101(8), 117 (8), 129 Kustova. G . N., 100, 101, 129, 130

Kuznetsov, A. M., 324 Kuznetsova, I . M., 3(25), 53(25), 90

L Lacksonen, J . W., 278(293), 330 Lal, H., 260(190), 327 Lam, Y . L., 274(277), 329 Landsberg, R., 293(345), 331 Langer, S. H.,218(16, 17), 221(25, 26, 31, 33), 222, 226(31, 33, 36, 47), 227(16, 31, 33, 36, 47), 228(16, 31, 33. 36), 229(16, 17, 25,26, 31, 33, 36). 230(25, 31,48,48a, 49, 50, 51). 231(16, 17,31, 33,47, 50.51). 235 (25, 48a), 237(25, 26, 48a). 238(25, 48a), 239(25), 243(25,26, 31, 33, 36), 249( 133), 254(33), 255(31), 257(25, 26, 33), 258(25, 26, 33), 260(25, 26, 33), 281(25, 31. 48a), 282(31), 284(31), 285(25,31), 286(31), 291 (17), 292(50), 293(47, 51), 295, 296, 298 (25,26,31), 299(31), 302,303(25,26), 304 (25), 312(48a, 407), 313(25,407,412), 314 (412). 315(48a, 407, 412), 320(31). 323, 324,326,332,333 Langmuir, I . , 241(94a), 325 Lapshina, T. V., 3(22), 53(22), 90 Laundy, B. J., 107(19a), 108(19a), 113(19a), 129 Lawrence, J . P., 20(74), 59(74), 92 Lazukin, V. I . , 99, 100, 101, I17(8), 129 Lbl, N., 292(340), 331 Lebedev, S. V., 349,376 Leclercq, M., 15, 91 Leclercq, G., 15, 23, 63(82), 91, 92 Leclercq, L., 23, 63(82), 92 Leclere, C., 278(294), 330 Ledoux, M. J.. 86(187), 95 Lee, J . B., 251(150), 254(150), 307(150), 326 Lee, J . W., 273(269), 329 Leffler, J . E., 343(17), 375 Legare, P., 252( I57), 26% 157), 326 Leidheiser, H., 309(397, 398), 332 Leisegang, E. C., 263(213), 328 Leja, E. J., 108(39), 130 Lemberanskii, R. A., 117(51, 52), 130 Lester, G . R., 53(132), 93 Levanda, 0. G . ,221(30), 323 Levenspiel, 0.. 289(325), 31 1(325), 330 Levich, V. G., 233, 286(67), 324

388

AUTHOR INDEX

Lewis, F. A,, 245(104), 267(246), 294(246), 295(246), 325, 328 Lewis, J., 89(191), 95 Liberman, A. L., 2(8, 9, 15), 3(20, 21, 22, 23, 24, 25, 26, 27), 43(104, 107, 108), 44(106, 107, 108, 109, 110, l l l ) , 4 5 ( l l 2 , 113, 114, 115), 46(113), 47(107, 109, 117, 118, 119, 120, 122), 48(109, 110, I 1 1, 112, 122, 123, 124), 53(22, 25, 26),65,66(155, 156, 157), 69(164), YO, 91, 93, 94, 95 Liberman, E. S., 117(51), 130 Liebhafsky, H. A,, 218(6), 227(6), 228(6), 229 (6), 230(6), 231(6), 243(6), 248(6), 290 (61,322 Lienard, G., lO(55, 56, 57), 15(62), 84(55, 56, 57), 91, 92 Lightfoot, E. N., 312(409), 313(409), 333 Limido, G., 358(95), 376 Linder, W., 277(284), 329 Lippeus, B. C., 124(62), 131 Lishenok, 0. E., 69(165), 95 Litvin, E. F., 354(83), 376 Loch, L. D., 108(41), 130 Lohse, H., 293(345), 331 Lohse, U., 293(345), 331 Longoni, G., 89(192), 95 Loza, C. V., 3(24, 25, 26), 53(25, 26), YO, 91 Luck, F., 12(58), 85(58), 91 Lukina, M. Yu., 3(18, 19), 45(114), YO, 93 Lund, H., 297(365), 331 Lundquist, J. T., 265(231), 267(249), 279,328, 329,330 Lunsford, J. H., 280(310), 330 Luss, D., 268(256), 307(256), 308(256), 329 Lyon, H. B., 87, 95

M McAteer, J. C.. 99(15, 16, 20). 102, 109(20), 115, 120, 121, 122(16), 125, 126, 128(15), 129 McCain, C. C., 99(9), 100(9), 101(9), 109(9), 116(9), 117(53), 122(9), 124(9, 53), 129, 130 McCarty, J. G., 194, 197(54), 198(54), 215 McClellan, W. R., 145(14), 162 MacDonald, R. J., 76(177), 85(177), 86(177), 95

McDonald, R. T., 231(54), 281(54), 290(54), 324 McElroy, K. P., 292(341), 331 McHardy, J., 233(68), 278(291), 279(301), 324, 330 Machin, P., 89(191), 95 McIntyre, J. D. E., 246(107, 108), 249(126), 294( 107), 309( 107), 325,326 MacKenzie, R. E., 19(70), 92 McKervey, M. A,, 17(64, 65), 18(64, 65), 88 (65), 92 McNicol, B. D., 261(195), 262(195), 264(195), 327 McPartlin, M., 89(191), 95 McQuillin, F. J., 352, 376 Maddock, A. G., 99(17), 101(30), 106(17), 107 (30), 117(17), 129, 130 Madey, T. E., 188, 190(44, 45), 197(45), 215, 263(214, 215), 307(214, 215, 382a, 387), 328,332 Maire, D., 4(32, 33), 5(33), 12(60), 16(60), 29 (861, 30(86), 31(86, 89), 32(86, 89), 33 (86), 35(32, 33), 42(33, 86), 47(89), 48 (125), 49(125), 52(33), 58(33), 72(86), 74 (172, 174), 91, 92, 93, 95, 252(157), 265 (1 57), 326 Makambo, P., 214(83), 216 Makarova, L. I., 3(28), 91 Malatesta, M. C., 89(191), 95 Mann, C. A,, 290(326), 330 Mann, C. K., 298(371), 309(371), 332 Mann, J. L., 302(374), 332 Mannan, R. J., 237(79), 238(79), 248(79), 273 (79), 324 Margolis, E. I., 3(28), 91 Margolis, L. Ya., 101(24), 106(24, 34), 124 (63, 64), 127, 130, I31 Marhoul, A., 337(6), 338(8), 374 Marinsic, N., 320(417), 321(417), 333 Marion, J., 138(5), 162 Markby, R. E., 222(35), 228(35), 297(35), 323 Marley, J. A., 108(40), I30 Mars, P., 138(5), 162, 220(19), 323 Martin, G . A,, 197(61), 198(61), 21 1(74), 216 Martinyuk, G. A., 297(363), 331 Matsumoto, H., 197(64), 216 Matsushima, T., 251(153), 302(153, 378), 307 (1 53), 326,332

389

AUTHOR INDEX

Matsuura, I., 160, 163 Mathieu, M. V., 79(184), 95 Matusek, K., 47(121), 93 Maurel, R., 15,23,63,91,92,352,362(76), 376 Mayell, J. S., 249(133), 291, 326, 331 Mazza, F., 268(255), 329 Medwedowsky, W., 267(244), 328 Meislich, H., 354(85), 376 Mekhtiev, K. M., 117(51), 130 Menschutkin, N., 356,376 Menzel, D., 263(214, 215), 307(214, 215), 328 Merkel, H., 167(8), 214 Merrill, R. P., 251(152), 252(152), 265(152), 283( 152), 307( 152), 326, 330 Metzger, J., 138(5), 162 Mevesz, K., 188, 190(45), 197(45), 215 Meyer, J., 280(302), 330 Mignolet, J. C. P., 241(93), 325 Mikhaltchenko, V. G . , 153(22), 162 Milberger, E. C., 136(2), 138(2), 143(2), 162 Miller, A. D., 226(47), 227(47), 230(51), 231 (47, 51). 293(47, 51), 323, 324 Milliken, T. H., 4(30), 91 Mills, G . A,, 4(30), 91, 263(221), 328 Minachev, Kh. M., 3(29), 91 Minakshisundaram, M., 262(203, 204), 264 (203), 327 Mironova, V. A,, 348(61), 376 Mirzaeva, A. K., 64(146, 147, 148), 94 Mitchell, R. W., 281(31 l), 290(31 I), 330 Miyahara, K., 258(186), 327 Mochida, I., 344(24), 375 Mohilner, D. M., 222(35), 228(35), 297(35), 323 Moisev, I. I., 221(30), 323 Monks, G. T., 127(68), 131 Montfoort, A., 77(178), 95 Morawek, B., 48(127), 49(127), 75(127), 79 (127), 81(127), 93 Morikawa, K., 10(46), 91 Morley, J. A,, 108(42), 130 Mosevich, I. A,, 243(98), 244(98), 325 Moss, R. L., 263(21 I), 328 Moulton, D. M., 292(339), 331 Mueller, W. A., 345(33), 375 Miiller, E., 290, 330 Miiller, E. W., 308,332 Muetterties, E. L., 212(77), 216

Muller, J. M., 4(33), 5(33), 17(63), 19(63, 68, 69, 70), 29(87), 31(87), 36(70), 42(33), 52 (33), 53(63,68,69,70), 54(68), 55(69), 58, 59(63, 68, 69), 60(63, 68, 69), 73(87), 74 (174), 91, 92, 95, 209, 216 Miiller, K., 223(44), 224(44), 225(44), 240(44), 259(44), 323 Murray, R. W., 302(375), 332 Mussini, T., 268(255), 329

N Nagase, S., 294(347), 331 Nagy, F., 248(117), 296(117), 325 Nakano, Y., 153(23), 163 Namba, S., 266(238), 328 Nasielski, J., 345, 375 Nazarova, N. M., 352(77), 376 Neff, L. D., 191, 215 Nekrassov, L. N., 268(252), 329 Nevrkla, V., 363(100), 364(100), 366(100), 367 (IOO), 368(102), 369(102), 370(102), 371 (102), 377 Newman, J., 309(401), 311(401, 405), 312 (408), 332, 333 Niedrach, L. W., 254( 172, 173), 256, 257( 172, 173, 177, 179), 299(373a), 302(172, 173), 327,332 Nekitina, L. M., 2(10), 55(10), 90 Nijs, H. H., 214(82), 216 Nix, N. J., 348(64), 376 Norton, P. R., 251(151), 307(151), 326 Notermann, T. M., 97(1), 124(1), 129 Novak, M., 248(116), 297(116), 325

0 Oblad, A. G., 4(30), 91 O’Cinneide, A., 48(125, 126), 49(125), 93 O’Donohoe, C., 19(67), 68(67, 161), 92, 94 Ofstead, E. A,, 20(74), 59(74), 92,208(69), 216 Ogasawara, S., 153(23), 163 O’Grady, W. E., 218(13), 230(13), 231(13), 243(13), 253(13, 163), 307(153, 386), 308 (386). 323,326,332 Ohta,N.,99(10), 100(10,22), 101(10), 103(22), 108(22), 109(22), 129, 130 Ohya, A,, 168(9), 214

390

AUTHOR INDEX

Okamura, T., 243( 103), 325 Olenkova, I. P., 100(23), 101(23), 130 Olfer’eva,T. G.,44(109,111),45(113, 114),46 (113),47(109),48(109, I l l ) , 93 Oliver, R. G., 283(317), 330 Ollis, D. F., 181(29), 215 Olson, J. H., 267(248), 329 Orchin, M., 65(152), 94, 348(62), 376 Orlov, E. I., 167, 214 Osborn, J. A,, 25(83), 92 Oswin, H. G., 265(230), 328 Ovodova, V. A,, 3(19), 90 Ozaki, A., 168(9), 214

P Paal, Z., 47(121), 93 Pachta, J., 352(78), 353(78), 354(78), 355(86), 3 76 Page, C. L., 250(142), 251(142), 326 Paik, W., 249(136), 326 Pal, P., 78, 95 Palanker, V. S., 279(298), 330 Palm, V. A,, 345, 354(84), 370, 371(27), 375, 3 76 Panitz, J. A,, 308, 332 Paquot, C., 276(281), 329 Paravano, G., 296(359), 331 Parayre, P., 7(38), 9(43,44), 10(44), 12(38,43, 44), 14(38, 43, 44), 17(43), 24(38, 43), 25 (38,43), 26(38,43), 32(44, 94), 34(94), 35 (94), 38(43,44), 54(94), 56(94), 75(44), 79 (44), 91, 92 Parsons, R., 231(65), 236(74), 249, 250(143), 251(135), 262(143, 201), 284(322), 307 (143), 324,325,326,327,330 Pasquan, I., 153(24), 163 Paulson, P.M., 290(326), 330 Pavelich, W. O., 344, 364,375 Pearson, R. G., 280(305), 281(305), 298(305), 330 Peck, W. F.,246(107,108), 294(107), 309(107), 325 Pendleton, P., 118(55), 125(55), 130 Peover, M. E., 296(357), 309(357), 331 Petersen, E. E., 271(267, 267a), 272(267), 273 (267), 31 1(267), 314(267), 329 Petrii, 0.A., 243(100), 254(100), 264(100),325

Petry, 0. A., 260(190), 327 Pettit, R., 208, 216 Phillipson, J. J., 221(27), 296(360), 304(27), 323,331 Pickler, H., 167, 169(2), 175, 176, 177(20), 207 (20), 208(20), 214, 215 Piersma, B. J., 236(78), 268(78), 273(78), 320 (78), 324 Pietsch, S . J., 230(48, 51), 231(51), 293(51), 323,324 Pik, E. I., 2(10), 55(10), 90 Piken, A. G., 178, 215 Plate, A. F., 2(3), 43(104), 52(129), 53(133), 90, 93, 94 Platteeuw, J. C., 52(131), 54(131), 58, 73, 74 (173), 93, 95 PlechaEova, D., 363(99), 364(99), 365(99), 366 (99), 367(99), 377 Pletcher, D., 291(335), 331 Pliskin, W. A,, 348, 375 Plouidy, G., 29(86), 30(86), 31(86), 32(86), 33 (86), 42(86), 72(86), 92 Plummer, E. W., 307(385), 308(391), 332 Plump, R. E., 298(370), 331 Podlovchenko, B. I . , 260(190), 261(196, 197), 327 Polanyi, M., 231(63), 324, 347, 354(49), 375 Poltorak, 0. M., 76, 95, 265(229), 266(229), 328 Ponec, V., 188(41), 189, 196, 197(48, 57, 60), 198(48), 200, 201(48), 202(48), 205, 208 (41), 210,211,215,216, 263(216), 328 Ponteau, R. M. L., 279(295), 330 Popp, F. D., 297(366), 331 Portefaix, J.-L., 99(21), 104(21), 108(39a), 110 (21), 113(21), 122(21), 123, 124(65a), 129, 130, 131 Porter, E. A., 99(9), 100(9), 101(9), 109(9), I16 (9), 122(9), 124(9), 129 Potter, E. C., 236(75), 324 Poutsma, M. L., 197(58), 198, 199, 200(58), 21 I , 212(58), 216 Prater, C. D., 8(42), 91 Prelog, V., 68, 94 Preobrazhenskii, A. V., 43(107), 44(107, 109, 110),47(107, 109, 122),48(109, 110, 122, 123, 124), 93 Primet, M., 79(184), 95

AUTHOR INDEX Prochazka, A,, 351(70), 356(89), 358, 360 (70), 364(70), 376 Proudhomme, J. C., 29(86), 30(86), 31(86), 32 (86), 33(86), 42(86), 72(86), 92 Ptak, L. D., 20(77), 50(77), 87,92 Puddephatt, R. J., 20. 28(78, 79), 67, 92 Pulvermacher, B., 269(263), 329 Pyke, D. R., 99(12), 102, 103, 104, 105(12), 106(32), 108, 110, 111, 112, 113, 114(12), 118, 119, 120(12), 121(12), 122, 125(12), 127(32), 129, I30 Pyshnograeva, I. I., 326

Q Quinn, C. P., 221(25), 229(25), 230(25), 235 (25), 237(25), 238(25), 239(25), 243(25), 248(25), 257(25), 260(25), 281(25), 285 (25), 295(25), 298(25), 302(25), 303(25), 304(25), 313(25), 323 Quinn, H. A,, 17(64), 18(64), 92 Quyser, M. A,, 20(78, 79), 28(78, 79), 92

R Rabo, J. A , , 197(58), 198, 199, 200(58), 211 (72), 212(58), 216, 220(18), 323 Race, G. M., 291(335), 331 Rader, C. P., 341(14), 358(97), 375,377 Ragatz, R. A,, 227(46), 323 Ralph, B., 250(142), 251(142), 326 Rand, D. A. J., 249(134), 250(134), 251(134), 274,275(276), 326,329 Rank, J. S., 348(58), 376 Rapson, W. S., 55(136), 94 Rautavuoma, A. 0. I., 173(16), 215 Reddy, A. K. N., 249( 125), 25 1( I 25), 326 Reichardt, C., 345(26), 375 Reid, R., 99(12), 102, 103(12), 104, 105(12), 106(32), 108(12, 14), 110(12), 111(13), 112 (12, 32), 114(12), 118, 119, 120(12), 121 (12), 122(14), 125(12), 126(14), 127(32), 129,130 Reglier, M., 36(97), 92 Renou, A,, 78(182), 95 Renouprez, A,, 48(127), 49(127), 75(127), 79 (127), 81(127), 93

39 1

Reshetnikov, S. M., 294(354), 331 Respii-, A. R., 274(278), 329 Rhodin, T. N., 188(39), 215 Rideal, E. K., 36, 39(96), 59(96), 92, 169, 207, 215 Rieck, G. D., 145(12, 13a), 146(12), 162 Rieger, P. H., 297,331 Riffi, M. R., 231(58), 324 Risch, A . P., 197(58), 198, 199, 200(58), 211 (72), 212(58), 216 Robin, M . B., 105(33), 130 Roginskayd, Yu.E., 99, 100, 129 Rooney, J. J., 1(2), 17(64, 65), 18(64, 65), 19 (67), 20(76), 21, 27(2), 35, 55(95), 59 (139). 61(139), 62(139), 66, 67(95), 68 (67, 161), 88, 90, 92, 94, 95. 353(82), 376 Rosen, M., 249(137, 138), 326 Ross, P. N., 267(249), 279(249), 329 Ross, P. N., Jr., 243(101), 245(101, 105, 106), 251(106), 253(148), 265(101, 105, 106, 148), 266(106), 302(101), 307(148), 308 (148), 325,326,329 Ross, S. D., 231(59), 324 Rostevanov, E. G., 117(51, 52), 130 Rowe, J. E., 305(382), 307(382), 308(382), 332 Rowlinson, H. C., 32(93), 92 Roy, J. R., 348(62), 376 Rubanik, M . Ya., 99(8), 100(8), 101(8), 117 (81,129 Rubin, B. T., 260(187), 327 Ruckenstein, E., 138(5), 162, 269(263, 266), 270(266), 329 Rudd, E. J., 231(59), 324 Ruehlen, F. N., 294(346,348), 331 Rumpel, H., lOl(27, 28), 130 Rumyantseva, Z . A,, 2(7, 13), 3(7, 17), 90 Rupprecht, G. A., 38(98), 92 Rutt, D. R., 265(230), 268(260), 328,329 RuZiEka, V., 337(3, 5,6, 7), 338(8), 339(5, 12), 342, 348, 349(47, 48), 350, 351, 352, 353 (48, 78, 79), 354(78, 79), 355, 356(89), 357 (69, 71), 358, 359(69, 90), 360(69, 70, 71), 361(71, 90, 98), 363(71, 90, 99, IOO), 364 (70,71,98, 99, IOO), 365(99, lo]), 366(99, 100, IOI), 367(71, 99, 100, IOI), 368(101, 102), 369(102), 370(102), 371(102), 374, 375,376,378 Ryabinina, S. A,, 294(354), 331

392

AUTHOR INDEX

Rybinina, J. A., 346(35, 36, 38, 39), 375 Rye, R. R., 307(387), 332 Rylander, P. N., 297(367), 331

S Sabatier, P., 167, 214 Sachtler, J. W. A,, 197(60),216 Sachtler, W. H., 147, 162 Sachtler, W. M. H., 197(59), 202(59), 203(59, 66), 204(59), 205(59), 206(59), 209(66), 210(66), 211(59, 75, 76), 216, 275(280), 329 Sakellarpoulos, G. P., 218(16,17), 221(25,31), 222(16, 17), 226(31), 227(16, 31), 228(16, 31), 229(16, 17, 25, 31), 230(25, 31, 48, 48a), 231(16, 17, 31, 62), 235(25, 48a), 237(25, 48a), 238(25, 48a), 239(25), 243 (25, 31), 248(25, 31), 255(31), 257(25), 260(25), 273(61,62), 281(25,31,48a), 282 (31), 283(60, 61), 284(31, 60, 61), 285(25, 31, 60, 61), 286(31, 61), 287(61, 62), 288 (60, 61, 62), 289(60), 291(31), 295(25), 297(31), 298(25, 31), 299, 302(25), 303 (25), 304(25), 310(61), 311(60, 61, 62), 312(48a, 60, 61, 62, 407), 313(25, 407, 411, 412), 314(407, 411, 412), 315(48a, 60, 61, 62,407, 412), 316(60, 61, 62), 317 (62), 318(60, 61, 62), 319(60, 61, 62), 320 (31,60,418), 321(418), 323,324,332,333 Sala, F., 116, 117, 118(47), 130 Samman, N. G., 17(65), 18(65), 88(65), 92 Sanchez, J. P., 108(39a), 130 Sandstedte, G., 218(11), 230(11), 231(11), 243 (1 I), 254(1 I), 263(222), 273(271), 277 (284, 285), 278(292), 290(11, 331), 323, 328,329,330,331 Sankov, B. G., 63( 144), 94 Sarkanyi, A., 12, 16(59), 91 Sawyer, D. T . , 294,331 Sazonova, N. N., 118(56), 125(56), 130 Scheer, M. D., 348(54), 375 Schenker, K., 68(162), 94 Schlatter, M. J., 231(54), 281(54), 267(250), 290(54,250), 291(250), 292(250), 324,329 Schmidt, L. D., 241(94), 268(256), 307(256), 308(256), 329 Schmitz, R. A,, 321(421), 333 Schneider, P., 339(10), 374

Schrock, R. R., 38(98, 101), 42(101), 59(101), 92 Schubert, C. C., 250,251(142), 326 Schubert, U., 42(103), 92 Schuit, G. C. A,, 124(62), 131, 144(13), 145 (13), 146(13), 160(13), 162, 220(21), 323 Schuldiner, S., 249,299(373b), 300(373c), 326, 332 Schultz, H. P., 297(366), 331 Schultze, J. W., 249(131), 250(131), 326 Schulz, G. V., 171,215 Schulz, H., 174, 175, 176, 177(20), 196(20), 207(20), 208, 212(78), 215, 216 Schumann, D., 260(191), 327 Schwak, G. M., 273(270), 329 Schwarting, W., 197(63), 216 Schwarzmann, E., 101(27), 130 Scott, C. G . , 345,375 Sedova, S. S., 262(202), 263(207), 327 Segeri, N. M., 279(295), 330 Sell, C. D., 250(147), 307(147), 326 Selvig, A,, 292(340), 331 Selwood, P. W., 260(192), 327, 348(53), 375 Semrau, K., 331 Sen, R., 233(68), 324 Sen, R. K., 237(82), 238(82), 324 Senderens, J. B., 167,214 Sepa, D. B., 277(286), 278(286), 330 Sergienko, S. R., 2(6), 90 Sermon, P. A., 279(297), 330 Sexton, B. A., 263(217), 328 Shannon, R. D., 106(35), 130 Sharp, W. B. A., 241(90), 245(90), 246(90), 249(124), 251(124), 265(90), 267(90), 302 (90), 325, 326 Shelef, H., 178, 215 Shelef, M., 191, 192(51), 193(51), 194(51), 195, 213,215,216 Shepelin, V. A,, 291(333), 331 Shephard, F. E., 35, 55(95), 66, 67(95), 92 Sheppard, N., 20(75), 92 Shim, B. K. S., 32(93), 92 Shimoyama, Y., 76(177), 85(177), 86(177, 186), 95 Shingles, T., 178(24),215 Shirley, D. A,, 307(383), 332 Shnabel, K. Kh., 3(27), 91 Shokova, E. A., 68(163), 69(163), 94 Shuikin, N. I., 3(29), 53(134), 55(134), 91, 94 Shumilova, N. A., 251(149a), 326

393

AUTHOR INDEX

Sidarous, L., 320(416), 333 Siegel, S., 347(50), 349(65), 375, 376 Simic, D. N., 274(278), 329 Simmons, G. W., 309(397, 398), 332 Simons, J. H., 294(349), 331 Sinfelt, J . H., lO(49, 50, 51, 52,53), 11(49),91, 248(114, 115), 266(240, 241), 274(114, 277), 275(114, 115), 325, 328, 329 Sinke, G. C., 137(4), 162 Skalkina, L. V., 106(34), 130 Sleight, A. W., 145(14), 161, 162, 163 Slygin, A., 241(87a), 267(244), 325, 328 Smith, G. V., 347(50), 375 Smith, H. A,, 341(14), 358(97), 375, 377 Smith, C. E., 251(154), 326 Smith, R. E., 262(205), 327 Sobkowski, J., 263(208), 264(208), 328 Sokolovskii, V. D., 153(22), 162 Sokolskaya, A. M., 346(35, 36, 37,38,39,40), 375 Sokol’skaya, A. M., 294(354), 331 Sokolskii, D. V., 294,331,340(13), 346(35, 36, 38, 39), 374, 375 Sokol’skii, D. V., 294(353), 331 Somorjai, G. A,, 87,95, 175, 187(35), 188, 194 (53), 197(35),215,251(154), 263(217), 266 (242), 326,328 Somorjai, G. A,, 283(3 15, 3 16), 330 Soukup, J., 337(4), 346(43), 374, 375 Sovolova, 0. P., 3(16), 90 Sperner, F., 268(257), 308(257), 329 Spicer, W. E., 251(150), 254(150), 307(150), 326 Spitz, P. H., 280(309), 330 Srinivasan, S., 218(5, 8, 13), 221(5, 8), 223(8), 226(8), 227(8), 228(8), 229(8), 230(5, 8, 13), 231(5, 8, 13), 240(8), 243(5, 8, 13), 248(8), 253(13), 284(320), 290(8), 299(5, 8), 322,323, 330 Stanulonis, J. J., 267(248), 329 Steffgen, F. W., 263(221), 328 Steinkilberg, M., 263(215), 307(215), 328 Stender, V. V., 292(342), 331 Stenin, V. F., 261(196), 327 Sterin, Kh. E., 3(20, 21, 25), 53(25), 64(146. 147,148),68(163), 69(163,164), 90,94,95 Stern, E. W., 280(304), 330 Stern, O., 223(42), 224(42), 323 Sternberg, H. W., 222(35), 228(35), 297(35), 323

Stevenson, P. C., 265(234), 328 Stewart, D. G., 98(2, 3), 129 Stewart, D. J., lOl(26, 28), 107(26), 130 Stewart, W. E., 312(409), 313(409), 321(422), 333 Stocker, R., 290(327), 330 Stonehart, P., 265(231), 266, 268(258), 279, 328,329,330 Storch, H. H., 167, 169(1, lo), 170, 176, 178 (lo), 196(10), 197(10), 212, 214,215 Stracke, W., 221(32), 228(32), 298(32), 323 Strasbourg, 7(39), 11(39), 12(39), 16(39), 48 (39), 49(39), 82(39), 85(39), 91 Strecker, H. A,, 136(2), 138(2), 143(2), 162 Stroeva, S. S., 99(11), 100(11), 101(11), 129 Stull, D. R., 137(4), 162 Summitt, R., 108(42), 130 Sunderland, J. G., 221(28), 222(28), 226(28), 227(28), 231(28), 268(28), 280(28), 281 (28), 290(28), 291(28), 323 Suresh, D. D., 142(8,9), 144(11), 145(1I), 146 (1 I), 162, 220(20), 323 Sutlic, D. V., 218(15), 323 Suzdalev, I. P., 101, 106(24, 34), 127, 130 Swain, C. G., 345, 375 Swann, S., Jr., 297(364), 331 Sweeney, W. J . , 246(112), 247(112), 325 Swinkels, D. A. J., 302(376), 332 Szabb, S., 248(117), 296(117), 325

T Taft, R. W., 343, 344, 353(81), 364,375,376 Tagekami, S. Z., 290,330 Takahashi, M., 281(313), 330 Takasu, Y., 79(185), 81,95 Takeda, H., 281(312), 330 Tamura, H., 275(279), 329 Tamaru, K., 183(31), 187(31),215 Tanabe, K., 114(46), 130 Tanaka, S. Z., 290,330 Tannenberger, H., 278(287), 330 Tarasova, D. O., 100(23), 101(23), 130 Taylor, D., 117(53), 118(55), 124(53), 125(55), 130 Taylor, H., 10, 91 Taylor, H. S., lO(46, 47), 91, 241(91), 325 Taylor, W. F., lO(49, 51, 52), 11(49), 91 Telcs, I., 248(117), 296(117), 325

394

AUTHOR INDEX

Tellier, J.. 352, 362(76), 376 Teller, R. G., 89(192), 95 Teller, R. G., 89(192), 95 Temkin, M. I., 241, 242(88), 325 Tesche, B., 79(185), 81(185), 95 Tettnyi, P., 12, 16(59), 47(121),91, 93 Thacker, R., 249(139, 140), 326 Theobald, F., 108(39a), 130 Thomas, C. L., 330 Thomas, J. G. N., 243(96), 325 Thomas, J. M., 220(24), 236(24), 240(24), 246 (24), 254(24), 258(24), 323 Thomas, M. G., 212(77), 216 Thomas, W. J., 220(24), 236(24), 240(24), 246 (24), 254(24), 258(24), 323 Thonon, L., 358(92), 376 Thornton, E. W., 127(67), 131 Tiedemann, W., 309(401), 31 1(401), 332 Tilac, B. V., 249(129), 326 Tilley, R. J. D., 99(12), 102, 103(12), 104, 105 (12), 106(12, 32), 107(12), 108(12), 110 (12), 111(12), 112(32), 114(12), 120(12), 121(12), 122(14), 125(12), 127(32), 129, 130 Tilyayeff, S. K., 354(83), 376 Tipper, C. F. H., 20(78,79), 28(78, 79), 92 Tits, I. N., 65(151), 94 Tobias, C. W., 313(410), 333 Tochner, M., 257(179), 327 Tomanova, D., 9(45), 73(45), 75(45), 91 Topsne, H., 309(396), 332 Torrington, R. G., 263(213), 328 Tovmasyan, V. G., 45(112), 48(112, 124), 93 Towle, P. H., 280(308), 330 Tracy, J. C., 305(382), 307(382), 308(382), 332 Trainham, J. A,, 31 1(405),332 Trenner, N. R., 10(46), 91 Treshchova, E. G., 63(141), 94 Trifiro, F., 116, 117, 118(47), 130, 153, 163 Trimm, D. L., 117(49), I 18(49), 121(49), 124, 130, 197(62),216 Tropsch, H., 167, 195, 196, 207, 212(6, 5 3 , 214,215 Tseung, A. C. C., 278(288,289), 279(288,289), 330 Tucker, P. M., 250(143), 262(143), 307(143), 326 Tulupova, E. D., 3(29), 91

Turner, M. J., 327 Turnham, B. D., 211,216,263(219), 328 Turkevich, J., 265,266(238), 328 Tverdovsky, I., 243(98), 244(98), 325 Tysyachnaya, G. Y., 262(200, 202), 264(202), 327 Tyurin, Y. M., 267(245), 328

U Unwin, R., 79(185), 81(185), 95 Urabe, K., 168(9), 214 Urbach, H. B., 262(205), 327 Urbanovi, E., 351(71), 355(71), 357(71), (71), 360(71), 361(71), 364(71), 367( 376 Uytterhoeven, J. B., 214(82), 216

V Van Barneveld, W. A,, 188(41), 208(41), 215 Van Barneveld, W . A. A,, 197(57), 211(57), 216 Van Bekkum, H., 352,376 Van den Elzen, A. F., 145(12, 13a), 146(12), 162 Van der Plank, P., 275(280), 329 Van Hardeveld, R., 77(178, 178a), 95 Van Herdeveld, R., 266(236), 328 Van Hove, D., 214(83), 216 Van Krevelen, D. W., 138(5), 162,220(19),323 Vannice, M. A., 178(25, 26), 179(26), 180(26, 28), 181(26, 29), 182(26), 183(32), 184, 188(25),215,263(220), 328 Van Santen, R. A,, 184(33), 186(33), 21 1(75), 215,216 Vargaftik, M. N., 221(30), 323 Varsanyi, M. L., 309(397, 398), 332 Vasil’ev, Yu. B., 237(81), 238(81), 261(81), 262 (81,203,204,206,207), 263(81,206, 207), 264(203, 226, 227), 324, 326, 327,328 Vasina, T. V., 3(23), 3(26), 3(27), 53(26), 69 (164), 90, 91, 95 Vauquier, J. P., 358(91, 96), 364(91), 376 Vedenyapin, A. A , , 45(116), 46(116), 93 Vedrine, J. C., 99(21), 104(21), 110(21), 113 (21), 122(21), 123, 129

AUTHOR INDEX

Venuto, P. B., 52(130), 93 Venyaminov, S. A , , 118(56), 125(56), 130 Verbeek, H., 184(33), 186(33), 215 Verney, E. J. W., 109(45), 130 Vert, Zh. L., 243(98), 244(98), 325 Vertes, A., 309(397, 398), 332 Vetter, K . J . , 236(76), 249(131), 250(131), 279 (76), 324, 326 Vielstich, W., 218(9), 226(9), 227(9), 228(9), 229(9), 230(9), 231(9), 243(9), 254(9), 261 (9), 290(9), 299(9), 322 Vijh, A., 292(343), 331 Villadsen, J., 321(422), 333 Vincent, C . A,, 109(44), 130 Vinov, M., 278(287), 330 Visscher, W. H. M., 249, 25 1(13 3 , 326 Vlasenko, V. M., 220(22), 263(22), 323 Vogel, W., 267(249), 279(249), 329 Volintine, B. G., 320(418), 321(418), 333 Von Benda, K., 278(292), 330 von Stackelberg, M., 221(32), 228(32), 298 (32), 323 Vostokova, E. I., 47(120), 93

W Waegell, B., 36(97), 92 Wakabayashi, K., 99, 100, 101(10), 103(22), 108(22), 109(22), 117, 129, 130 Walborsky, H. M., 302(374), 332 Walker, R. D., 313(413), 333 Wanke, S. E.,251(155,156), 268(261),326,329 Warner, T. B., 249( l38), 299(373b), 300(373c), 326,332 Washington, E., 265(231), 328 Watson, K. M., 227(46), 237(83), 323, 324, 336(2), 356, 374 Webb, G., 348(59), 376 Webb, J . L., 302(374), 332 Wei, C. Y., 89(192), 95 Wei, J., 8(42), 91 Weiker, J. F., 145(14), 162 Weinberg, N. L., 218(14), 231(57), 323, 324 Weinberg, W. H., 283(318), 330 Weinstock, I., 257(177), 327 Weisang, F., 9(44), 10(44), 12(44), 13(44), 14 (44),30(88), 32(44,88,91), 38(44,88,91),

395

42(88, 91, 102), 48(102), 49(102), 75(44), 79(44), 80(88, 102), 91, 92 Weiss, A. H., 299(372), 332 Weiss, F., 138(5), 162 Weissermel. K., 162 Wells, P. B., 221(27), 283(317), 304(27), 323, 330, 347(46), 348(59), 375, 376 Wells, P. R., 343(18, 19), 375 Wender, I., 222(35), 228(35), 297(35), 323 Wentrcek, P. R., 196, 197(56), 198, 199(56), 200,216, 263(218), 307(218), 328 Westerterp, K. R., 31 1(403), 332 Westrum, E. F., Jr., 137(4), 162 Wheeler, A., 271(268), 272(268), 329 White, J. M., 251(153), 302(153), 307(153), 326 Whitesides, G. M., 71(171), 95 Whittle, V. J., 296(356), 331 Wiart, R., 321(419), 333 Wicke, E., 321(420), 333 Wiese, G . R., 249(139), 326 Wiesener, K., 292(338), 331 Wilke, C. R., 313(410), 333 Will, F. G., 241(92), 244(92), 245(92), 248 (120), 249(120), 250(120), 265(92), 268 (253,254), 313(414), 325,329,333 Williamson, W. B., 280(310), 330 Winograd, N., 250, 251(144, 145), 307(144, 145, 146, 147), 326 Winstein, S., 345, 375 Winterbottom, J. H., 348(59), 376 Winterbottom, J. M., 221(27), 304(27), 323 Wise, H., 126(66), 131, 196, 194, 197(54, 56), 198, 199(56), 200,215,216, 263(218), 307 (218), 328 Wojtowicz, J., 320(417), 321(417), 333 Wong, K. F., 345(29), 375 Woo, M . Y . C . , 307(386), 308(386), 332 Wood, B., 98(2), 129 Wood, B. J., 196, 197(56), 198, 199(56), 200, 216, 263(218), 307(218), 328 Wood, C . D., 38(98), 92 Wood, G. B., 313(413), 333 Woodhams, F. W. D., 99(19), 101(26), 106 (19), 107(19, 26), 108(19), 116(19), 127 (19), 129, 130 Woods, R., 243(99, l02), 248(99), 249(99, 132, 134), 250(132, 134), 251(134), 274, 275 (276), 302(99), 325, 326, 329

396

AUTHOR INDEX

Wright, D. A., 108(36), 130 Wroblowa, H., 218(5), 221(5), 230(5), 231(5), 236(78), 237(78), 238(80), 243(5), 255 (80), 268(78), 273(78, 80), 299(5), 302 (377), 320(78), 322, 324, 332 Wynblatt, P., 268(262), 329

Young, D. C., 283(314), 330 Yurchak, S., 222(36), 226(36), 227(36), 228 (36), 229(36), 230(50), 231(50), 248(36), 292(50), 296,323,324 Yuzefovich, G. E., 220(22), 263(22), 323

Z Y Yakubchik, A. O., 349(66), 376 Yamamoto, A,, 38(99), 59(99), 92 Yang, K., 237(84), 324 Yates, D. J. C., lO(49, 51,52, 53), 11(49), 91 Yates, J. T., 188, 190(44,45), 197(45), 215 Yates, J. T., Jr., 307(382a), 332 Yeager, E., 237(82), 238(82), 253(163, 169), 307(163,386), 308(386),324,326,327,332 Yoneda, Y., 344(24), 375 Yoshida, T., 346(41,42), 375 Youll, B., 127(68), 131 Young, C. B., 71(171), 95

Zaitseva, S. N., 70(168), 95 Zapletal, V., 337(4), 346(43), 348(57), 374, 375, 376 Zein El Deen, A,, 212(78), 216 ielezny, M.,. 356(89), 358(89), 376 Zelinskii, N. D., 2(3), 64, 65(151), 90, 94, 176 (23), 207,215 Zhigailo, Ya. U., 99(8), 100(8), 101(8), 117(8), 129 Zhutaeva, G. V., 251(149a), 326 Zilionis, V. E., 279(296), 330 Ziminova, N. I., 348(60), 376 Zotova, S. V., 47(120), 93 Zurilla, R. W., 237(82), 238(82), 324

Subject Index

A Acetic acid, adsorption, I15 Acetylene adsorption, potential dependence, 258 hydrogenation, 295 Acidity catalytic performance, 121 tin-antimony oxide, 114-1 15, 125-126 Acrolein catalytic selectivity, 122- 123 formation, 136-137, 150, 152 l60incorporation, 156 Acrylonitrile, 136- 137 n- Adsorbed cyclopentanes, 56-57 n-Adsorbed olefins, 35-37 dehydrocyclization, 38-39 hydrogen shift, 39,52 isomerization, 38-39 mechanism, 47 metallocyclobutane mechanism, 25 Adsorption acetylene, potential dependence, 258 activation energy, 264 anion, electrocatalyst, 267 carbonaceous species, 254-264 hydrocarbons, 254-261 coefficient functional group character, 360 olefin structure, 349-355 relative, 341 solvent effects, 359-361 solvent-free systems, 362-364 solvent parameters, 369 electrocatalyst, 240-264 electrode surface properties, 240-241 ethylene, potential dependence, 258 hydrogen ethylene coadsorption, 259-260

isotherms, 244-245 platinum crystallographic planes, 244245 surface coverage versus potential, 243245 isotherms, 241-243 hydrogen, 244-245 potential dependence, 258-259 modes, hydrogenolysis, 44 olefins, 35 1 oxygen, 248-254 solvents, effect of, 358-361 states, hydrogen, 246 unsaturated alcohols, 351 Aging process, power law kinetics, 268-269 Alcohols, unsaturated adsorption, 351 reduction, 267 solvent parameters, 358, 360 Aliphatics linear free energy relationship, 370-371 substituent effects, 344 Alkanes acyclic dehydrocyclization, 3 hydrocracking, 50 aromatization, 36 1-6 ring closure mechanism, 52-53 1-5 ring closure-ring enlargement mechanism, 53 formation, hydrogenation, 353 oxidation, carbonaceous intermediates, 257 Alkenes bromination, 293 isotopic reaction, 302 oxidation, catalytic selectivity, 282-283 Alkylbenzenes, isomerization bond shift, 67 1-methyl-l -alkylcyclohexane, 63

397

398

SUBJECT INDEX

Alkylcycloheptanes, aromatization, 55 Alkyl shift, 27, 88-89 Alkyl-substituted benzenes dehydrocyclization, 65 isomerization, cyclic mechanism, 66 Alkyl-substituted cyclopentanes, aromatization, 53-54 Ally1 alcohol ammoxidation, 157-158 isomerization, 153, 155 isotopic distribution, 153- I54 isotopic results, I57 l 6 0 incorporation, 156 oxidation, 155- 156 product distribution, 153-154 Ally1 iodide reactions, molybdenum trioxide, 150 Ally1 radical formation, 148 Ally1 species, 124- 126, 150, 152- 153 Allylic oxidation, see Oxidation, allylic Allylic species, 21 formation, isomerization, 18- 19 free allyl radicals, 149 a-hydrogen abstraction, 147 Alumina, hydroxylation state, 74 Ammonia synthesis, iron catalyst, 168 Ammoxidation, 136- 137 allyl alcohol, 157-158 ammonia-ally1 alcohol ratio, 158 catalyst development, 138-143 oxidized surface grid reduction, 139- 140 mechanism, 147-161 propylene, 157, 159 Anderson-Avery mechanism, 17, 23-24 Antimony(III), surface formation, 1 13 Antimony-oxygen system, 101 Appearance potential spectroscopy, 305-306, 308 Aromatization, 52-58 alkanes, 36 alkylcycloheptanes, 54 carbene-olefin addition, 56 cyclopentanes, 54-55 gemdialkylcyclohexanes, 63 isopropylcyclopentane, 54 rneta-'3C toluene, formation, 56 metallocyclobutane mechanism, 53-54 on palladium, 55

on platinum, 55 1-6 ring closure mechanism, 52-53 1-5 ring closure-ring enlargement mechanism, 53 Auger electron spectroscopy, 188- 189 ad-layer development, 188 electronic transitions, 305-307 Azopropene reactions, molybdenum trioxide, 150

B Basicity catalytic performance, I 2 1 tin-antimony oxide, 114-1 15, 125-126 Benzene electrosorption versus potential, 259-260 hydrogenation, 296-297 substituted carbene-benzene addition mechanism, 66-67 hydrogenolysis, 68 1-5 ring closure, 66 Bifunctional mechanism, 4 Bismuth molybdate, 124-125 ammoxidation, 159 iron in, 145 layered structure, 145-146 oxidation rates, 160 selective, 159 propylene reactions, 148, 151 redox mechanism, 144 rates, 145 reduction, relative rates, 159 Bond shift mechanism, 4, 16-28 alkylbenzenes, 67 apparent activation energy, 22-23 13C tracer technique, 5-6 criteria, 21-22 evidence for, 22-28 structural effects, 25-26 surface hydrogen coverage, 22-23 versus cyclic mechanism, 75-77, 89 Bondouard disproportionation reaction, 196 Bromination, 293 Bronze, oxygenated, electrocatalyst, 278 Butadiene, catalytic selectivity, 118-1 19, 122

SUBJECT INDEX

Butene oxidative dehydrogenation, 118-1 19 partial oxidation, 121-122

C "C tracer technique, isomerization, 5-9 Calcination temperature acidity, 1 14- 1 I5 catalytic character, I17 catalytic performance, 118-1 19, 122-123 versus surface composition, 1 10- 1 1 1 tin-antimonyoxide, 103-104,107,110-1 I 1 Carbene, insertion, Fischer-Tropsch synthesis, 177 Carbene--ally1insertion mechanism, 37 Carbene-benzene addition mechanism, 66-67 Carbene-olefin addition, 56 metal complex, 33 Carbidic intermediates, 189- 190, 194 Fischer-Tropsch synthesis, 196- 197, 206212 genesis, 210-21 I hydrocarbon synthesis via, 195-206 Carbon, surface carbon monoxide hydrogenation, 198-200 hydrogen interaction, 198- 199 hydrogenation, 197- I98 labeled, 200-206 Carbon dioxide, formation, 201 Carbon monoxide disproportionation reaction, 196, 203 dissociation, 201 -212, 263 Fischer-Tropsch synthesis, 178, 191 hydrogenation, 192, 198-200 linear adsorption, 263 mixed overlayers, 198-200 Carbonaceous intermediates adsorption, 254-264 hydrocarbons, 254-261 oxygenated species, 261-264 alkane oxidation, 257 Carbonyl, stretching frequency, 191 Catalysis heterogeneous linear free energy relationship method, 344

399

selective oxidation by, 133- 163 organic chemicals, production, I34 Catalyst activity, 189 ammoxidation, see Ammoxidation, catalyst electronic properties, 80-8 1 geometric factors, 76-78, 80 icosahedral symmetry. 78 incomplete cubooctahedron schematic, 77 hydrogenation effects, olefins, 346-349 mixed oxide, 98, see also Tin-antimony oxide nonuniformity, steady-state conditions, 168 olefin bond, strength, 348-349 palladium, see Palladium particle size activity and, 265-266 electronic properties, 79 hydrogenolysis, 79-80 isomerization, 81-82 reaction mechanisms and, 72-90 platinum, see Platinum poisons, 267-268 precovered, 203 redox cycle, 138 properties, 143-147 solid, hydrogenation, 335-377 supported sintering rate, 270 synergism, 278-279 tin-antimony oxide, see Tin-antimony oxide Chain growth reaction, 170-173, 184-185 CH, incorporation, 206 via carbidic intermediates, 207-210 via water elimination, 196 Chain lengthening, 25-28 Channel flow electrochemical reactor, 3 10 current density, 318-320 selectivity function, 315-316 Charge density, and particle size, 79 transfer electrocatalysis, 220-221 tin-antimony oxide, 105 Chemical feedstock, history, 161- 162 Chemisorption coverage-potential plot, 250-25 I

400

SUBJECT INDEX

hydrogen, 243-248 OH species, 249 Chlorination, 292 Coal, conversion, liquid hydrocarbons, I66 Cobalt films deuterioisomer distribution, 61-62 polymethylcycloalkane isomerization,

61-62 hydrocracking, 5 1 Compact-diffuse layer model, 224 Conductivity tin-antimony oxide, 100, 109 tin(1V) oxide, 108-109 Continuity equation electrochemical reactor, 3 1 1 mass transport, 3 12 Coordination number, platinum, 265 Copper, exchange current density and, 247 Copper oxide, propylene oxidation, 141 Crystal phases, 102-103 twinning, 105 Current density, electrocatalyst, 284,318-320 exchange density, 234,247 potential density, 230 Cyclic mechanism, 4-5,28-48 alkyl-substituted benzenes, 66 ''C tracer technique, 5-6 cycloalkane hydrogenolysis, 28-35 1,5-dehydrocyclization, 35-43 thick versus ultrathin film, 85-86 versus bond shift mechanism, 75-77,89 Cyclization, 65,see also Dehydrocyclization Cycloalkanes hydrogenolysis, 28-35 mechanisms, 30-32 product distributions, 30-31, 34 isomerization, 37,68-69 transannular dehydrocyclization, 68-70 Cyclobutanes, isomerization, product distributions, 31 Cyclodecane, conformation, 69-70 Cycloheptane, 2 adsorption, 45-46 Cyclohexanes, see also Hexanes hydrogenation selectivity, 367-368 in solvents, 356

hydrogenolysis, 43-44 relative adsorption coefficients, 360 Cyclononane, conformation, 69 Cyclopentanes aromatization, 54 hydrogenolysis, 2-3,43-44 as intermediates, 35 Cyclopropane adsorption, 17 mechanism, 17,27

D Deactivation process, electrocatalyst, 266-268 Dethylation, 50 Dehydrocyclization, 35-43 acyclic alkanes, 3 n-adsorbed olefins, 35-36,38-39 alkyl-substituted benzenes, 65 C-C bond formation, 210 carbene-alkyl insertion mechanism, 37 dehydrogenation, 35-36 iridium supported catalyst, 42 mechanisms, 38-39,42-43 palladium, 36 pathways, 40 platinum, 36-37 rate, 36-37,39 sextet-doublet model, 45-46 steric hindrance, 39 transannular cycloalkanes, 69-70 methylcycloalkanes, 70 methylcyclooctane, 70-71 Dehydrogenation, see Dehydrocyclization bimolecular, 10, 15 dehydrocyclization and, 35-36 methanol, 262 unimolecular, 10 Demethylation, 50 gemdisubstituted cycloalkanes, 59,63 polymethylcycloalkanes, 61 Deuterioisomer, distribution, 61-62 Diadsorbed diolefins, 33 Diadsorbed species, 61,71 Dibenzyl, cyclization, 65 Dicarbenes, isomerization, 56-57 Dicarbynes, 80-81 1,2-DimethyIcyclobutane,isomerization, 3 1

SUBJECT INDEX

Dimethylcyclopentanes aromatization, 54 isomerization, 34 2,3-Dimeth~lpentane-2-'~C, 13- 14 Diphenylmethane, cyclization, 65 Dissociation, carbon monoxide, 210-212,263 Double layer interface, 223-225 Doublet mechanism, 43,45,47 Drougard-Decrooq equation, 345,356,371

E Electric field, gradient, 127 Electrocatalysis anodic oxidation, ethylene, 254-255 carbonaceous species, see Carbonaceous species charge transfer, 220-221 electrode interactions, 240-241 kinetics, 231-239 electrogenerative processes, 229-231 electrolytic character, 228-229 operating conditions, 227-229 overview, 220-223 oxygen binding, 251 reaction engineering, 309-321 current distribution, 318-320 electrochemical reactors, 309-311 mass transport, 312-315 rate distribution, 318-320 selectivity, 315-318 steady-state multiplicity, 320-321 reactions, 290-299 halogenation, 292-294 hydrogenation, 294-297 oxidation, 290-292 reduction, functional groups, 297-299 redox, 279-280 Electrocatalyst adsorbate-support interactions, 273-279 adsorption, 240-264 isotherms, 241-243 bimetallic activity, 275 synergistic effects, 274-275 deactivation steps, 262 electric potential, 221 electrochemical reactors, 318-320

40 1

exchange current density, 247 kinetics, current density, 284 metal groups, 274 ligand, 276-277 nonmetallic, 276-278 polymetallic cluster, 273-276 porous, concentration change, 314 selection criteria, 221 selectivity, 280-290 consecutive reactions, 288-290 control, 286-290 multiple reaction, analysis, 283-286 parallel reactions, 286-288 specificity factors, 281-283 spillover, 278-279 structural effects, 264-266 coordination number, 265 study techniques, 299-309 electrochemical. 299-302 galvanostatic determination, 299-300 isotopic exchange, 302-304 mass spectrometry, 302-304 nonelectrochemical, 302-309 potentiodynamic determination, 301-302 potentiostatic determination, 300-301 surface spectroscopy, 304-309 supported, synergism, 278 surface concentration, adsorbed species,

303 surface coverage galvanostatic determination, 299-300 potentiodynamic determination, 301-302 potentiostatic determination, 300-30I surface oxygen layers, 248 surface properties, 240-241 surface stability, 266-273 aging models, 268-271 anion adsorption, 267 concentration charge, 267 deactivation, 266-268 pore poisoning model, 271-273 redispersion, 269 thermodynamics, 223-227 double layer interface, 223-225 reversible electrode potential, 225-227 Electrochemical effectiveness factor, 314 Electrochemical rate equation, 233-236 Electrochemical reactor, 309-311 continuity equation, 311

402

SUBJECT INDEX

engineering, 309-321 selectivity function, 315-316 Electrode gas diffusion, pore poisoning, 27 1 kinetics, 23 1-239 activation barriers, 231 -233 concentration function, 237 electrochemical rate equation, 233-236 ethylene hydrogenation parameters, 235 parameters, 236-238 rate constant, 232-233, 235 symmetry factor, 232 transfer coefficient, 234-236 overpotential, classification, 248 reactions double layer interface, 223-225 reversible electrode potential, 225-227 thermodynamics, 223-227 Electrogenerative processes, 229-23 1 Electrolyte, character, 228-229 Electron spectroscopy, 188-191 summary of findings, 190- I9 1 transfer potential energy diagram, 232 rate, 233-236 Electrooxidation, ethylene, 254-255 Electroreduction, vinyl fluoride, 282 Electrosorption, 240 benzene versus potential, 259-260 isotherms, 244 oxygen versus potential, 249-251 Eley-Rideal mechanism, 44 Energy activation adsorption, 264 dependence on potential, 233 electrocatalysis, 237 electrocatalytic reduction, ethylene, 239 free, 232, 242 isomerization, 83 mechanism requirements, 33 apparent activation bond shift mechanism, 22-23 chain lengthening, 25 hydrogenolysis, 23 isomerization, 11-12 free change in, 343 standard, 225-226

Energy loss spectroscopy, 305-306, 308 Entropy, apparent activation, 11-12 Ethane adsorption, potentiodynamic trace, 256, 301 rate of production, 192-193 Ethanol, olefin hydrogenation, 352-353 2-Ethyl-diphenyl, cyclization, 65 Ethyl shift, 17 Ethylene adsorption, potential dependence, 258 anodic oxidation, 254-255 chlorination, 292 hydrogen coadsorption, 259-260 hydrogenation, 294-296 electrocatalytic parameters, 235 oxidation, 290-291 reduction electrocatalytic, 239 parameters, 303-304 synthesis with, 174-175 o-Ethyltoluene, 66

F Fischer-Tropsch synthesis, 166-168 Bondouard disproportionation reaction, 196 carbidic intermediates, see Carbidic intermediates carbon-containing overlayer, I89 carbon monoxide coverage, I91 dissociation, 210-212 insertion, 176-1 77 removal, reaction mixture, 192- 193 carbonyl stretching frequency, 191 catalytic activity, 189 CH, insertion, 177 chain growth reaction, see Chain growth reaction ethylene, synthesis with, 174-175 hydrocarbon absorbing species, 191 192 hydrogenation, pulsed, 185- I87 insertion mechanism, 196, 208-209 isotope substitution, 192 kinetics, 178-1 87 rate constants, 184- 187 rate equation, 181, 183 slow steps, 178- I84 -

SUBJECT INDEX

steady-state, 178-184 stepwise chain growth, 184 transient state, 184- I87 turnover numbers, 183- 184, I89 lateral polymerization model, 169-1 70 mixed surface carbon-carbon monoxide, hydrogenation, 198-200 molar activity, 174- 175 methanation reaction, see Methanation reaction metathesis reaction, 208 oxygenates, 210-212 oxymethylene species, condensation, I76 production distribution, 169- 177 chain length and, I69 carbon monoxide-hydrogen distribution, 202 isotopic composition, 205 stepwise insertion, 171 transient conditions, 185-186 propagation reaction, 170-171,207-210 C-C bond formation, 210 spectroscopy, see Spectroscopy stepwise insertion, 170-1 71 surface carbon concentration, 190 hydrogenation, 197- 198 labeled, 200-206 simple versus complex. 167- 168 Fluorination, 293-294 Fuel cell electrooxidation, 21 8 potential-current density, 230

G Gahdnostatic technique hydrocarbon adsorption, 256 surface coverage, 299-300 Garin-Gault mechanism, 20, 23 Gemdialkylcyclohexanes, aromatization, 63 Gemdisubstituted cycloalkanes, demethylation, 59 Geminated adsorbed cycloalkanes, 62-63 Gold, alkene oxidation, 282-283

H Halides, organic, reduction, 298 Halogenation, electrocatalytic, 292-294

403

Hammett equation, 344, 369-370 Henry's isotherms, 242 Heptanes dehydrocyclization, 45-46 2,3-dimethylpentane-2-' 3C isomerization, 14 1,5-Hexadiene-3-01, hydrogenation, 361 Hexanes catalyst particle size, reaction mechanisms, 72-85 cyclic versus bond shift mechanisms, 73 dehydrocyclization, 35,42 interconversion, 8 1-82 isomerization, 8 thick and ultrathin films, 85-86 1-Hexene, hydrogenation reaction rates, solvent parameters and, 357 in solvents, 356 Hexene-ols, hydrogenation, 361 Heyrovsky reaction, 246 Horiuti-Polanyi mechanism, 347 Hougen-Watson kinetic equations, 356 1,2-Hydride shift, 33 Hydrocarbons absorbing species, 191-192 acyclic, isomerization, 28 adsorption, 254-261 surface intermediates, 255 type I1 species, 257 aromatics hydrogenolysis, 2-3 isomerization, 65-68 linear free energy relationship, 370-37 1 medium-sized rings, 68-72 substituents, reaction site, 344 isotopic composition, 205 liquid, from coal, 166 oxidation, 97-98 synthesis carbidic carbon, 195-206 Fischer-Tropsch catalyst, 165-21 6 unsaturated, reduction, 267 Hydrocracking. 48-52 deethylation, 50 demethylation, 50 metallocarbene formation, 51 -52 product distribution, 49 Hydrogen adsorption isotherms, 245

404

SUBJECT INDEX

platinum crystallographic planes, 244245 states, 246 surface coverage versus potential, 243, 245 chemisorption, 243-248 concentration, liquid phase hydrogenation, 338-340 ethylene coadsorption, 259-260 evolution reaction, 246 exchange current density, 247 galvanostatic deposition, 300 pressure, 12, 15-16 reaction rate, 338-339 solubility, reaction product effects, 339 solvent, effect, 356-361 spillover, 279 surface carbon, 198-199 surface coverage, 10-1 1, 16, 22 cc-Hydrogenabstraction, 147-149 cc-Hydrogenelimination, 52 Hydrogen sulfide catalyst poison, 267 oxidation, 291 Hydrogenation benzene, 296-297 binary systems, 363,366-367 bulk phase, solvent-free systems, 362 carbon monoxide, 192 carbon monoxide-hydrogen atmosphere, 200-206 catalytic performance, selectivity, 28 1-282, 348 competitive process, 341-342 electrocatalysis, 294-297 kinetic parameters, 235 ethylene, 294-296 Fischer-Tropsch catalysis, 183 hydrocarbon products, isotopic composition, 205 hydrogen concentration, 338-340 solubility, reaction product effects, 339 spillover, 279 kinetic equations, substrate concentration, 339 kinetics, liquid phase, 336-342 kinetic region, definition, 336-338

olefinic substrates, 346-368 catalyst effects, 346-349 Horiuti-Polanyi mechanism, 347 potentiometric reduction, 294 process, 336-337 product distribution, carbon monoxidehydrogen atmosphere, 202 pulsed, 185-187, 198-200 reaction rate constants, 340 solvents, effect, 345-346 reactivities, comparison, 340-342 selectivity, see Selectivity solvent concentrations, 362-368 structure, 343-346 solvent-free systems, 362-368 structure, reacting compounds and solvents, 343-346 surface carbon, 197- 198 labeled, 200-206 mixed, 198-200 ternary systems, 365-366 unsaturated alcohols, 351 H ydrogenolysis adsorption modes, 44 apparent activation energy, 23 aromatic hydrocarbons, 2-3 catalytic selectivity, 281-282 cycloalkanes, 28-35 mechanisms, 30-32 product distributions, 30-31, 34 equilibrium, 15 function, hydrogen pressure, 12, 15-16 indane, 66 mechanisms catalyst particle size and, 72-85 multiplet, 43-48 methylcyclopentane, 79-8 1 multiplet doublet, 43, 45, 47 mechanisms, 43-48 sextet-doublet, 43-45,47 norbornane, 64-65 on oriented faces, 86-87 platinum, 23-24, 87-90 rate equation, 1 I , 15

SUBJECT INDEX

selective versus nonselective, 29, 72 selectivity, 47 spiranes, 64 Hydroxylation state, 74

I Indane, hydrogenolysis, 66 Induced heterogeneity model, 241, 251 Infrared spectroscopy insitu, 191-194 IR bands, 195 summary of findings, 193 Intrinsic heterogeneity model, 241 Ion neutralization spectroscopy, 305-307 Iridium electronic properties, 80-81 hydrocracking, 50-51 supported catalyst, 42 Iron bismuth molybdate, 145 Fischer-Tropsch catalyst, 167-1 68 Isobutane, reactions, oriented faces, 86-87 Isomerization ally1 alcohol, 153, 155 catalyst activity, 348 particle size, 81 -82 complex molecules, 58-72 medium-sized rings, 68-72 polymethylcycloalkanes, 59-65 substituted aromatics, 65-68 cyclic-acyclic product ratio, 8-9 cycloalkanes, 68-69 function, hydrogen pressure, 12, 15- 16 interconversion, 81-82 isopentane, 17 label scrambling, 7, 12-1 3 mechanism, 5-16 u-alkyl adsorbed radicals, 18 bifunctional, 4 bond shift, see Bond shift mechanism "C tracer technique, 5-9 catalyst particle size and, 72-85 concerted, 20 cyclic mechanism, see Cyclic mechanism cyclopropane adsorption, 17 kinetics, 9-16

405

metathesis-like, 20 structural effects, 25-26 methyl shift reaction, 84 2-methylbutane, 82-85 2-methylpentane, 75-79 on oriented faces, 86-87 n-pentane, 82-85 on platinum-silica, 87-90 product distributions, 30-31, 34 rate-determining step, 10, 15 selectivity factor, 9 self-isomerization, 6-7 steric interaction, 32 surface hydrogen coverage, 10-1 1 Isopentane, isomerization, 17 Isopropylcyclopentane, aromatization, 54 Isotopic exchange, 302-304

L Langmuir-Hinshelwood equations, 364 Langmuir-Hinshelwood kinetics, 356 Langmuir isotherm, 241-242 Lateral polymerization model, 169-1 70 Lebedev rule, 349 Linear anodic potential sweep, 256-257 Linear free energy relationship method, 343344 aromatic and aliphatic systems, 370-371 solvents, effect, 356 Low-energy electron diffraction, 305-306,308

M Mass spectrometry, 302-304 Mass transport processes, 312-31 8 convective, 31 2-313 diffusive, 313-315 selectivity, 316 steady-state multiplicity and, 320-321 Mercury chloride, catalyst poison, 267 Metal ligand electrocatalyst, 276-277 Metallocarbenes electronic requirements, 79 formation, 38-39 isomerization, 56-57 Metallocyclobutane dismutation, 51-52

406

SUBJECT INDEX

electronic requirements, 79 formation, 21 mechanism, 23-27, 32-33,35,53-54 Metallodicarbenes, dehydrocyclization, 38 Metallodicarbynes, 42-43 Methanation, 178-182 carbidic intermediates, 205 carbon monoxide, heat of adsorption, I80 carbon monoxide-hydrogen atmosphere, 20 1 catalysts, 167 compensation effect, 179, 18 1 - 182 kinetic parameters, 179, 181-182 rate expression, 181 Methane adsorption, 10- 1 1 isotopic composition, 205 product ion labeled, 203-204 rate, 192- 193 Methanol dehydrogenation, 262 hydrogenation, selectivity, 368 oxidation, bimetallic catalyst, 275 surface intermediates, 261 --262 Metathesis reaction, 208, 210 1 -Methyl- 1 -alkylcyclohexanes isomerization, 63-64 toluene-alkylbenzene ratio, 63 2-Methyl-3-butene-2-01, relative adsorption coefficients, 360 2-Methyl-diphenyl, cyclization, 65 Methyl shift, 17, 25-27, 84 2-Methylbutane activation energy and reactions, 22-23, 83 isomerization, 22-23,82-85 n-pentane-neopentane ratio, 83-84 Methylcycloalkanes, transannular dehydrocyclization, 70 Methylcyclooctane, transannular dehydrocyclization, 70-71 Methylcyclopentane desorption, thick versus ultrathin films, 86 hydrogenolysis, 79-81 Methylene, insertion, Fischer-Tropsch catalysis, 177 3-Methylhexane dehydrocyclization, 13 isomerization, 7, 14, 39-40

Methylpentanes hydrocracking, product distribution, 49 interconversion, 81 -82 isomerization, 75-79 13C tracer technique. 6 particle size effects, 89 thick versus ultrathin films, 85-86 Mixed electrochemical reactor, 310 selectivity function, 315-316 Molybdenum, surface restructuring, 145- 146 Molybdenum trioxide acrolein formation, 152 ally1 iodide reactions, 150 azopropene reactions. I50 Mossbauer parameters, tin-antimony oxide, I07 Mossbauer spectroscopy, 106-109, 113, 127 Muller-Gault mechanism, 17 Multiplet mechanisms, 43-48 doublet, 43,45, 47 sextet-doublet, 43-45, 47

N Neopentane isomerization. 17, 20 react ions on oriented faces, 86-87 on platinum-silica, 86-90 o n platinum-zeolites, 87-90 Nickel, hydrocracking, 5 1 Nickel-alumina, 198-199 Nickel-silica, hydrogenation, 199-200 Nitrocompounds, reduction, 297 Norbornane, hydrogenolysis, 64-65

0 Olefinic halides hydrogenation versus hydrogenolysis, 28 1 282 reduction, 298 n-olefinic species, formation, 19 Olefins binary mixtures, selectivity, 363 bromination, 293 C6-C8, on platinum-silica, 352

407

SUBJECT INDEX

catalyst bond, strength, 348-349 Fischer-Tropsch catalysis, 174- 175 fluorination, 293 hydrogenation, 346-368 catalyst effects, 346-349 cis isomers, 354 Horiuti-Polanyi mechanism, 347 solvent concentration, 362-368 solvent-free system, 362-368 in solvents, 357 Taft-Pavelich equation, 353-354 oxidation, 125-126, 291 selective, 135-1 36 structure adsorptivity and, 349-355 reactivity and, 349-355, 368-372 ternary systems, 365-367 Organic chemicals, production catalysis, 134 heterogeneous oxidation, 134 Oxidation alkane, carbonaceous intermediates, 257 alkene, 282 allylic, 135-1 36, see also Ammoxidation mechanism, 149 catalytic, 97-98 selectivity, 282-287 electrocatalytic, 290-292 kinetic parameters, 238 electrogenerative versus electrolytic, 229 ethylene, 290-291 heterogeneous, organic chemicals, production, 134 hydrogen sulfide, 267, 291 methanol, bimetallic catalyst, 275 olefins, 125- 126, 29 I partial, 121-122 propylene, 291 selective, 136- I37 ally1 alcohol, 155-156 u-0-ally1 species, 152- 153 catalyst, 138-140 mechanism, 147-161 mechanism comparison, 160 propylene, 157, I59 rate, 148 states cationic, 101, 106, 116--117 following catalyst use, 127

propylene conversion, 140 propylene oxidation activity, 141 sulfur dioxide, 292 thermodynamics, I37 tin-antimony oxide, selective catalyst, 1 I71 I8 Oxidation-reduction cycle, 138 Oxide, kinetics, 250-251 Oxygen adsorption, 248-254 binding, 251 catalytic mechanisms, 124-125 dependence, catalytic character, 117-1 18 distribution, ammoxidation catalyst, 138140 electrosorption versus potential, 249-25 1 l6O incorporation, 156 oxide growth, 249 peroxide-radical mechanism, 252-253 surface layers, 248 chemisorbed versus oxide, 250 surface species, 249 vacancies, redox rate, 145 Oxygenated bronzes, electrocatalyst, 278 Oxygenated species, adsorption, 261-264 Oxygenates, synthesis, 210-212 Oxymethylene species, condensation, 176

P Palladium, 18-19 alkene oxidation, 282 aromatization, 55 dehydrocyclization, 36 electrocatalytic hydrogenation, 235 hydrogenolysis, 281 -282 z-olefin formation, 36 polar effect, substituents, 352 szlectivity, 348 Paraffins, hydrogenolysis, 43-44 n-Pentane activation energy, reactions and, 83 isomerization, 82-85 apparent activation energy, 22-23 Pentanes catalyst particle size, reaction mechanisms and, 72-85 isomerization, 8

408

SUBJECT INDEX

n-Pentylbenzene, isomerization, 67-68 Peroxide-radical mechanism, 252-253 Phase composition, tin-antimony oxide, IOO101

2-Phenylpentane, isomerization, 67-68 Phthalocyanine organometallic complexes, 276-277 Platinized bronzes, 278 hydrogen spillover, 279 Platinized carbon, spirane hydrogenolysis, 64 Platinum, 18-20, 22 activity, particle size and, 265 adsorption coefficient, olefins, 349 alkane oxidation, carbonaceous intermediates, 257 anode, linear anodic potential sweep, 256257 coordination number, 265 dehydrocyclization, 36-37 electrocatalytic hydrogenation, 235 ethylene electrooxidation, 254 films cyclopentane aromatization, 54 oriented faces, 86-87 thick versus ultrathin, 85-86 hydrocarbons, structural effects, 25-26 hydrogenation, 281 -282 olefins, in ethanol, 352-353 isomerization, 2-3 n-olefin, formation, 37 olefins hydrogenation, 352-353 relative adsorption coefficients, 350 oxides electrocatalysis, 250 structure, 252 oxygen species, 249 peroxide-radical mechanism, 253 supported, 3,28-29 Platinum-alumina, 24-25, 28-29 cyclopentane aromatization, 54 particle size isomerization, 76 reaction mechanisms, 72-85 selectivity, 72-73 preparation, 74-75 Platinum-rhodium, 274-275 selectivity, 355 Platinum-silica hydrogenation, olefins, 352

neopentane reactions, 87-90 particle size effects, 89 Platinum-silica gel, hydrogenation, 35 I Platinum-zeolites neopentane reactions, 87-90 particle size effects, 89 Po!ymeric phthalocyanine electrocatalyst, 277 Polymerization lateral model, 169- 170 product distribution, 171 Polymetallic cluster, electrocatalyst, 273-276 Polymethylcycloalkanes deuterium distribution, 61 -62 isomerization, 59-65 product distribution, 61 Pore poisoning model, 271-273 Potential versus current density, 230 electrochemical reactor, 319-320 dependence, activation energy, 233 distribution, double layer interface, 224 electrocatalyst, 221 oxygen electrosorption versus, 249-250 reversible electrode, 225-227 versus selectivity, parallel reactions, 286288 standard electrode, 225-227 versus surface coverage, hydrogen adsorption, 243, 245 vinyl fluoride reduction, 255 Potentiodynamic technique, surface coverage, 301 -302 Potentiostatic technique, surface coverage, 300-301 Propane, surface intermediates, 257 n-Propylbenzene, isomerization, 66 Propylene adsorption, tin-antimony oxide, 124 allylic oxidation, 135- 136 ammoxidation, 136-137, 157, 159 bismuth-molybdate, 151 catalytic selectivity, 124 conversion versus oxidation state, 140 deuterium labeled, 148 oxidation, 98, 291 to acrolein, 1 I6 activity versus state, 141 selective, 136-137, 147, 157, 159 product distribution, 153-1 54 Pyridine, adsorption, 1 15

SUBJECT INDEX

R Reaction rate constants Fischer-Tropsch catalysis, 184-1 87 solvent properties, 369 substrate structure, 349-350 correlation solvent parameters, 356-357 Taft-Pavelich equation, 354-355 poisoned pore, 272-273 ratio, ternary system, 366 ’ solvent effects, 345-346 structure, 368-372 solvent-free systems, 362-364 substrate structure, 368-372 Reactivity comparison, 340-342 single compound, various media, 341 competitive hydrogenation process, 341 342 linear free energy relationship method, 343344 olefin structure, 349-355 reactant structure, 343-345 solvent, effects, 345-346 Redox catalysis, 279-280 cycle, 138 mechanism, 143-144 oxygen vacancies, 145 properties, 143-147 rates, bismuth-molybdate, 145 Reduction electrocatalytic, kinetic parameters, 238 electrogenerative versus electrolytic, 229 ethylene, parameters, 303-304 functional groups, 297-299 potential, vinyl fluoride, 255 Reversible electrode potential, 225-227 Rhodium, selectivity, 348 platinum, 355 1-5 Ring closure di-0-adsorbed species, 71 substituted aromatics, 65 substituted benzenes, 66 1-6 Ring closure mechanism, 52-53, 65 1-5 Ring closure-ring enlargement mechanism, 53

409

Rooney mechanism, 18,21, 27, 88 Rooney-Samman mechanism, 17- 18

S Schulz-Flory chain length distribution, 206 Schulz-Flory distribution, 171-173 Selectivity binary systems, 363, 366-367 control, 286-290 consecutive reactions, 288-290 parallel reactions, 286-288 electrocatalyst, 280-290 multiple reactions, analysis, 283-286 specificity, 281 -283, 285 electrochemical reaction, 3 15-3 18 electrolyte concentration, 282 hydrogenation, competitive, 341 -342 intermediate concentration, 289 mass transport, 3 I6 mixed platinum-rhodium catalysts, 355 pore diffusion, 3 I6 potential, 282 parallel reactions, 286-288, 3 17-3 18 versus potential, 285 solvent concentration, 367-368 specificity electrocatalyst, 281 -283 versus potential, 285 ternary systems, 365-367 type of catalyst, 348 Self-isomerization, 6-7 Semihydrogenated state, 347-349, 353 Sextet-doublet mechanism, 43-44,47 dehydrocyclization, 45-46 Shearing, reconstitution, catalytic surface, 145-146 Silica-supported catalysts, 203 Sintering catalytic, 268 models, 268-269 rate, 270 Sodium tungsten bronzes, electrocatalyst, 278 Solid solution phase, tin-antimony oxide, 103, I05 Solvent-free systems, 362-368 adsorption coefficients, 362-364 hydrogenation rate, 362-364

410

SUBJECT INDEX

Solvents concentration, 362-368 selectivity, 367--368 ethanol, olefin hydrogenation, 352-353 hydrogenation effects, 356-361 olefins, 352-353 parameters, 358-359 correlation, reaction rates, 356-357 reaction kinetics, structure effect, 343-346 reaction rate, 345-346 structure, reactivity, 368-372 substrate, adsorptivity, 358-36 I Soviet school of catalysis, 45, 47 Spectroscopy, see specific method of spectroscopy surface electrocatalyst, 304-309 electron, 188-191 electronic transitions, 305-306 Fischer-Tropsch catalysis, 187- 195 spectrometer, schematic, 305 summary of findings, 194- 195 Spiranes, hydrogenolysis, 64 Stoichiometry, anion to cation, 105 Sulfur dioxide, oxidation, 292 Sulfuric acid, solvent, 296

T Tafel reaction, 246 Taft equation, 344, 352-353,369-370 Taft-Pavelich equation, 344, 353-354, 364. 369-370 Taylor fraction, I84 Temkin isotherm, 242-243 Tetraadsorbed species, 31 -32 Tin-antimony oxide, 97 I3 1 antimony content, 103, 107-108, 110, I12 acid-base properties, 114-1 I5 catalytic performance, I 18- I I9 calcination, 103- 104. 107. 1 10- 1 1 I temperature, 117-119, 122-123 catalytic mechanisms, I24 126 active centers, 125 n-ally1 intermediates, 124-126 olefin oxidation, 125-126 catalytic performance, I 16- I28 acid-base, I2 I , 125- I26 ~

antimony concentration, 118- 119. 122I23 calcination temperature, 117- 119, 122123 cationic oxidation states, 116-1 17 color change, 120- 12 I oxygen dependence, 1 17- 1 18 selective oxidation, 1 17- 1 I8 selectivity. 122- I23 sintering temperature, 117 solid state properties, I 17 surface composition, I 19- 120 coprecipitated, 102 Mossbauer spectroscopy, 106- I09 phase composition, 100-101 crystalline, 102- 103 solid solution, 103, 105 surface composition, 110 preparation, 99-100, 104, 107, 111-112 properties, 99- I 16 antimony concentration, 103, 107-108, 110, 112 antimony solubility, 100-101 binary systems, 101 bulk, 100- 109 catalytic character, 116-123 cationic oxidation states, 101, 106 charge balance, 105, 108-109 charge transfer, 105 color, 104-105, 120-121 conductivity, 109 electrical conductivity, 100 equilibrium concentration, I03 ESR spectrum, 109 following use, 126- 128 lattice parameters, 104-106 low-antimony content, 105-106 Mossbauer parameters, 107 reduction, antimony(V) to (HI), 127 sintering temperature, 1 17 surface, 109- 1 16 twinned microcrystals, 104-106 X-ray photoelectron spectroscopy, I 10I12 stoichiometry, anion to cation, 105 surface composition, 110, 119-120 critical, I I2 surface properties acid-base, 114-1 15 antimony(II1) formation, I 13

41 1

SUBJECT INDEX

bulk phase changes, 110 calcination temperature, I 10- I 1 1 catalytic activity, 118-1 I9 critical composition, I I2 enrichment factor, 110 schematic representation, I 12 X-ray photoelectron spectroscopy, 1 13 Tin-antimony-oxygen system, I01 Tin oxide conductivity, 108- I09 surface, schematic, I12 Tin-oxygen system, 101 Toluene meta-”C formation, 56 ratio, 63 Transannular dehydrocyclization, sce Dehydrocyclization, transannular Transient species, isomerization, 20 Transition metals, see specific metals comp1exes, catalytic properties, 277 metallocarbene formation, 5 I multiplet mechanism, 45-46 polymethylcycloalkane isomerization, 5960 promotion, isomerization, 48-50 trans- 1 -methyl-3-ethylcyclobutane, adsorbed, 47-48 Triadsorbed species, 35-36,61 Trimethylcyclopentanes aromatization, 53, 55 isomerization, product distribution, 59-60 Trimethylpentane, aromatization, 54 Twinning, tin-antimony oxide, 104-105

U Ultraviolet photoelectron spectroscopy, 305307 Uranium-antimony oxide system, 139, 141 143 heavy atom position, 143 structure, select catalytic activity, 141 unit cell, phase I, 142

-

V Vinyl chloride, specificity-potential plot, 285 Vinyl fluoride electroreduction, 282 reduction, 255, 298 specificity-potential plot, 285 Volmer reaction, 246

W Water dipole model, 224-225 Water-gas shift, 167

X X-ray photoelectron spectroscopy, 110-1 13, 188-189 electronic transitions, 305-307 surface oxygen, 250

Z Ziegler catalysis, 210

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Contents of Previous Volumes Volume 1 The Heterogeneity of Catalyst Surfaces for Chemisorption HUGHS. TAYLOR Alkylation of Isoparaffins AND LOUISSCHMERLING V. N. IPATIEFF Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. EMMETT The Geometrical Factor in Catalysis R. H. GRIFFITH The Fischer-Tropsch and Related Processes for Synthesis of Hydrocarbons by Hydrogenation of Carbon Monoxide H. H. STORCH The Catalytic Activation of Hydrogen D. D. ELEY Isomerization of Alkanes HERMAN PINS The Application of X-Ray Diffraction to the Study of Solid Catalysts M. H. JELLINEK AND I. FANKUCHEN Volume 2 The Fundamental Principles of Catalytic Activity FREDERICK SEITZ The Mechanism of the Polymerization of Alkenes Lours SCHMERLING AND V. N. IPATIEFF Early Studies of Multicomponent Catalysts ALWINMITTASCH Catalytic Phenomena Related to Photographic Development T. H. JAW Catalysis and the Adsorption of Hydrogen on Metal Catalysts OTTOBEECK Hydrogen Fluoride Catalysis J. H. SIMONS Entropy of Adsorption CHARLES KEMBALL

About the Mechanism of Contact Catalysis GEORG-MARIA SCHWAB Volume 3 Balandin’s Contribution to Heterogeneous Catalysis B. H. W. TRAPNELL Magnetism and the Structure of Catalytically Active Solids P. W. SELWOOD Catalytic Oxidation of Acetylene in Air for Oxygen Manufacture J. HENRYRUSHTON AND K. A. KRIEGER The Poisoning of Metallic Catalysts E. B. MAXTED Catalytic Cracking of Pure Hydrocarbons VLADIMIRHAENSEL Chemical Characteristics and Structure of Cracking Catalysts A. G. OBLAD,T. H. MILLIKEN,JR., AND G. A. MILLS Reaction Rates and Selectivity in Catalyst Pores AHLBORN WHEELER Nickel Sulfide Catalysts WILLIAM J. KIRKPATRICK

Volume 4 Chemical Concepts of Catalytic Cracking R. C. HANSFORD Decomposition of Hydrogen Peroxide by Catalysts in Homogeneous Aqueous Solution J. H. BAXENDALE Structure and Sintering Properties of Cracking Catalysts and Related Materials E. RIES,JR. HERMAN Acid-Base Catalysis and Molecular Structure R. P. BELL Theory of Physical Adsorption L. HILL TERRELL 413

414

CONTENTS OF PREVIOUS VOLUMES

The Role of Surface Heterogeneity in Adsorption GEORGE D. HALSEY 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 JOSEPHWEISS The Specific Reactions of Iron in Some Hemoproteins PHILIPGEORGE

Volume 5 Latest Developments in Ammonia Synthesis ANDERS NIELSEN Surface Studies with the Vacuum Microbalance: Instrumentation and Low-Temperature 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 QuasiStationary 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 EUGENELIEBERAND FREDL. MORRITZ

Some General Aspects of Chemisorption and Catalysis TAKAOKWAN Nobel Metal-Synthetic Polymer Catalysts and Studies on the Mechanism of Their Action AND F. F. NORD WILLIAM P. DUNWORTH Interpretation of Measurement in Experimental Catalysis P. B. WEISZAND C. D. PRATER Commercial Isomerization B. L. EVERING Acidic and Basic Catalysis MARTINKILPATRICK Industrial Catalytic Cracking RODNEY V. SHANKLAND Volume 7 The Electronic Factor in Heterogeneous Catalysis M. McD. BAKERAND 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 ROBERTGOMER Adsorption on Metal Surfaces and Its Bearing on Catalysis JOSEPHA. 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. SUHRMANN

Volume 6

Volume 8

Catalysis and Reaction Kinetics at Liquid Interfaces J. T. DAVIES

Current Problems of Heterogeneous Catalysis J. ARVIDHEDVALL

CONTENTS OF PREVIOUS VOLUMES Adsorption Phenomena J. H. DE BOER Activation of Molecular Hydrogen by Homogeneous Catalysts S. W. WELLER AND G. A. MILLS Catalytic Syntheses of Ketones AND J. R. COLEY V. I. KOMAREWSKY Polymerization of Olefins from Cracked Gases EDWINK. JONES Coal-Hydrogenation Vapor-Phase Catalysts E. E. DONATH The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts CHARLESD. PRATERAND RUDOLPHM. LAGO Volume 9

Proceedings of the International Congress on Catalysis, Philadelphia, Pennsylvania, 1956 Volume 10

The Infrared Spectra of Adsorbed Molecules R. P. EISCHENS AND W. A. PLISKIN The Influence of Crystal Face in Catalysis ALLANT. GWATHMEY AND ROBERTE. CUNNINGHAM The Nature of Active Centers 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 Aromatics EDWINK. JONES The Reactivity of Oxide Surfaces E. R. S. WINTER The Structure and Activity of Metal-on-Silica Catalysts G. C. SCHUITAND L. L. VAN REIJEN Volume 11

The Kinetics of the Stereospecific Polymerization of a-Olefins G. NATTAAND I. PASQUON

415

Surface Potentials and Adsorption Process on Metals R. V. CULVER AND F. C. TOMPKINS Gas Reactions of Carbon P. L. WALKER, JR., FRANKRUSINKO, JR., AND L. G. AUSTIN The Catalytic Exchange of Hydrocarbons with Deuterium C. KEMBALL Immersional Heats and the Nature of Solid Surfaces J. J. CHESSICK AND A. C. ZETTLEMOYER The Catalytic 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 and K-Absorption Edges in the Study of Catalytically Active Solids ROBERT A. VANNORDSTRAND The Electron Theory of Catalysis on Semiconductors TH. WOLKENSTEIN Molecular Specificity in Physical Adsorption D. J. C. YATES Volume 13

Chemisorption and Catalysis on Metallic Oxides F. S. STONE Radiation Catalysis R. COEKELBERGS, A. CRUCQ, AND A. FRENNET Polyfunctional Heterogeneous Catalysis PAULB. WEISZ A New Electron Diffraction Technique, Potentially Applicable to Research in Catalysis L . H . GERMER

416

CONTENTS OF PREVIOUS VOLUMES

The Structure and Analysis of Complex Reaction Systems D. PRATER JAMESWEI AND CHARLES Catalytic Effect in Isocyanate Reactions AND G. A. MILLS A. FARKAS

Volume 14 Quantum Conversion in Chloroplasts MELVINCALVIN The Catalytic Decomposition of Formic Acid P. MARS, J. J. F. SCHOLLEN,AND P. ZWIETERINC Application of Spectrophotometry to the Study of Catalytic Systems JR. H. P. LEFTINAND M. C. HOBSON, Hydrogenation of Pyridines and Quinolines MORRISFREIFELDER Modern Methods in Surface Kinetics: Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques GERTEHRLICH Catalytic Oxidation of Hydrocarbons L. YA. MARWLIS

Volume 15 The Atomization of Diatomic Molecules by Metals D. BRENNAN The Clean Single-Crystal-Surface Approach to Surface Reactions N. E. FARNSWORTH Adsorption Measurements during Surface Catalysis KENZITAMARU The Mechanism of the Hydrogenation of Unsaturated Hydrocarbon on Transition Metal Catalysts G. C. BONDAND P. B. WELLS Electronic Spectroscopy of Absorbed Gas Molecules A. TERENIN The Catalysis of Isotopic Exchange in Molecular Oxygen G. K. B O ~ E ~ K O V

Volume 16 The Homogeneous Catalytic Isomerization of Olefins by Transition Metal Complexes MILTONORCHIN The Mechanism of Dehydration of Alcohols over Alumina Catalysts HERMAN PINESAND JOOSTMANASSEN n Complex Adsorption in Hydrogen Exchange on Group VIII Transition Metal Catalysts J. L. GARNETTAND W. A. SOLLICHBAUMCARTNER Stereochemistry and the Mechanism of Hydrogenation of Unsaturated Hydrocarbons SAMUEL SIEGEL Chemical Identification of Surface Groups H. P. BOEHM Volume 17 On the Theory of Heterogeneous Catalysis JUROHORIUTIAND 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 HERVEY H. VOGEAND CHARLFSR. ADAMS The Physical-Chemical Properties of Chromia-Alumina Catalysts CHARLES P. POOLE, JR. AND D. S. MACIVER Catalytic Activity and Acidic Property of Solid Metal Sulfates Kozo TANABE AND TSUNEICHI TAKESHITA Electrocatalysis S. SRINIVASEN, H. WROBLOWA,AND J. O’M. BOCKRIS Volume 18 Stereochemistry and Mechanism of Hydrogenation of Napthalenes in Transition Metal Catalysts and Conformational Analysis of the Products A. W. WEITKAMP

CONTENTS OF PREVIOUS VOLUMES

The Effects of Ionizing Radiation on Solid Catalysts ELLISON H. TAYLOR Organic Catalysis over Crystalline Aluminosilicates P. B. VENUTOAND P. S. LANDIS On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of 8 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. BCOCOCK,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 JR., GARYL. HALLER, ROBERTL. BURWELL, KATHLEEN C. TAYLOR,AND JOHNF. READ Correlation among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activity KIYOSHI MORIKAWA, TAKAYASU SHIRASAKI, AND MASAHIDE OKADA Catalytic Research on Zeolites J. TURKEVICH AND Y. ONO 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

417

JEANEUGENE GERMAIN AND MICHELBLANCHARD 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 Mieellar Solutions H. MORAWETZ Catalytic Activities of Thermal Polyanhydroa-Amino Acids DUANEL. ROHLFING AND SIDNEY w . FOX

Volume 21 Kinetics of Adsorption and Desorption and the Elovich Equation C. AHARONI AND F. C. TOMPKINS Carbon Monoxide Adsorption on the Transition Metals R. R. FORD Discovery of Surface Phases by Low Energy Electron Diffraction (LEED) JOHNW. MAY Sorption, Diffusion, and Catalytic Reaction in Zeolites L. RIEKERT Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis CARLWAGNER

Volume 22 Hydrogenation and Isomerization over Zinc Oxide R. J. KOKESAND A. L. DENT Chemisorption Complexes and Their Role in Catalytic Reactions on Transition Metals Z. KNOR Influence of Metal Particle Size in Nickelon-Aerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity AND F. HARTOG R. VANHARDEVELD Adsorption and Catalysis on Evaporated Alloy Films R. L. Moss AND L. WHALLEY

418

CONTENTS OF PREVIOUS VOLUMES

Heat-Flow Microcalorimetry and Its Applications to Heterogeneous Catalysis P. C. GRAVELLE Electron Spin Resonance ill Catalysis JACKH. LUNSFORD

R. P. COONEY, G. CURTHOYS, AND NGUYEN THETAM Analysis of Thermal Desorption Data for Adsorption Studies MILOS SMUTEK, SLAVOJ CERNY, AND FRANTISEK BUZEK

Volume 23

Metal Catalyzed Skeletal Reactions of Hydrocarbons J. R. ANDERSON Specificity in Catalytic Hydrogenolysis by Metals J. H. SINFELT The Chemisorption of Benzene P. B. MOYESAND P. B. WELLS The Electronic Theory of Photocatalytic Reactions on Semiconductors TH. WOLKENSTEIN Cycloamyloses as Catalysts DAVIDW. GRIFFITHS AND MYRONL. BENDER Pi and Sigma Transition Metal Carbon Compounds as Catalysts for the Polymerization of Vinyl Monomers and Olefins D. G. H. BALLARD Volume 24

Kinetics of Coupled Heterogeneous Catalytic Reactions L. BER~NEK Catalysis for Motor Vehicle Emissions JAMES WEI The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. MOLAND J. A. MOULIJN One-Component Catalysts for Polymerization of Olefins Yu. YERMAKOV AND V. ZAKHAROV The Economics of Catalytic Processes J. DEWINGAND D. S. DAVIES Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species

Volume 25

Application of Molecular Orbital Theory to Catalysis ROGERC. BAETZOLD The Stereochemistry of Hydrogenation of qb-unsaturated Ketones ROBERTL. AUGUSTINE Asymmetric Homogeneous Hydrogenation J. D. MORRISON,W. F. MASLER,AND M. K. NEUBERG Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts J. K. A. CLARKE AND J. J. ROONEY Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces HELMUTKNOZINGER Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach ROGERA. SHELDON AND JAYK. KOCHI Volume 26

Active Sites in Heterogeneous Catalysis G. A. SOMORJAI Surface Composition and Selectivity of Alloy Catalysts W. M. H. SACHTLER AND R. A. VAN SANTEN Mossbauer Spectroscopy Applications to Heterogeneous Catalysis JAMESA. DUMESIC AND HENRIK TOPS@E Compensation Effect in Heterogeneous Catalysis A. K. GALWEY Transition Metal-Catalyzed Reactions of Organic Halides with CO, Olefins, and Acetylenes R. F. HECK

CONTENTS OF PREVIOUS VOLUMES Manual of Symbols and Terminology for Physicochemical Quantities and UnitsAppendix I1 Part I1 : Heterogeneous Catalysis Volume 27 Electronics of Supported Catalysts GEORG-MARIA SCHWAB The Effect of a Magnetic Field on the Catalyzed Nondissociative Parahydrogen Conversion Rate P. W. SELWWD Hysteresis and Periodic Activity Behavior in Catalytic Chemical Reaction Systems AND JAROSLAV VLADIMIR HLAVAEEK VOTRUBA Surface Acidity of Solid Catalysts AND B. H. C. WINQUIST H. A. BENE~I Selective Oxidation of Propylene GEORGEW. KEULKS,L. DAVIDKRENZKE, AND THOMAS N. NOTERMANN u-n Rearrangements and Their Role in Catalysis BARRYGOREWIT AND MINORU TSUTSUI Characterization of Molybdena Catalysts F. E. MASSOTH Poisoning of Automative Catalysts M. SHELEF,K. OTTO,AND N. C. OTTO Volume 28 Elementary Steps in the Catalytic Oxidation of Carbon Monoxide on Platinum Metals T. ENGELAND G. ERTL The Binding and Activation of Carbon Monoxide, Carbon Dioxide, and Nitric Oxide

419

and Their Homogeneously Catalyzed Reactions RICHARDEISENBERG AND DAN E. HENDRIKSEN

The Kinetics of Some Industrial Heterogeneous Catalytic Reactions M. I. TEMKIN Metal-Catalyzed Dehydrocyclization of Alkylaromatics SIGMUND M. CSICSERY Metalloenzyme Catalysis JOSEPHJ. VILLAFRANCA AND FRANK M. RAUSHEL Volume 29 Reaction Kinetics and Mechanism on Metal Single Crystal Surfaces ROBERT J. MADIX Photoelectron Spectroscopy and Surface Chemistry M. W. ROBERTS Site Density and Entropy Criteria in Identifying Rate-Determining Steps in Solid-Catalyzed Reactions RUSSELL W. MAATMAN Organic Substituent Effects as Probes for the Mechanism of Surface Catalysis M. KRAUS Enzyme-like Synthetic Catalysts (Synzymes) G. P. ROYER Hydrogenolytic Behaviors of Asymmetric Diarylmethanes YASUOYAMAZAKI AND TADASHI KAWAI Metal-Catalyzed Cyclization Reactions of Hydrocarbons

ZOLTIN PGL

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