Advances in Catalysis and Related Subjects, Volume 15

ADVANCES IN CATALYSIS AND RELATED SUBJECTS

VOLUME 15

Contributors to This Volume G. C. BOND G. K. BORESKOV D. BRENNAN H. E. FARNSWORTH KENZITAMARU A. TERENIN P. B. WELLS

ADVANCES IN CATALYSIS AND RELATED SUBJECTS VOLUME 15 EDITED BY

D. D. ELEY Nottiragham, England

HERMAN PINES Evamton, Illin&

PAULB. WEISZ JETS^^

Paulsboro, New

ADVISORY BOARD

A. A. BALANDIN Moecow, U.S.S.R.

P. H. EMMETT BaUimore, Maryland

G. NATTA Milano, Italy

J. H.

DE

BOER

Delft, The Netherlands

J. H O R ~ T I Sappro, Japan

E. K. RIDEAL Loradon, England

P. J. DEBYE Ithaca, New York

W. JOST Qdtingen, Germany

P. W. SELWOOD S a n t a Barbara, California

H. S. TAYLOR Princeton, New Jersey

1964 ACADEMIC PRESS, NEW YORK AND LONDON

COPYRIQHT

0 1964, BY

AUADEMIU PRESS

INO.

ALL RIGHTS RESERVED. NO PART OB THIS BOOK MAY BE REPRODUUED IN ANY FOR”, BY PHOTOSTAT, MIOROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION TROY THE PUBLISHERS.

ACADEMIC PRESS INC. 111 Fifth Avenue, New York, New York 10003

United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. Berkeley Square House, London W.1

LIBRARYOF CONURESS CATALOUCARD NUMBER: 49-7755

PRINTED IN THE UNITED STATES OF AMERXUA

Contributors

G. C. BOND,Johnson, Matthey and Companp, Ltd., Research Laboratories, Wembley, Middlesex, England

G. K. BORESKOV, Institut Kataliza, Novosibirsk, U.S.S. R.

D. BRENNAN, Department of Inorganic, Physicut, and Industrial Chemistry, University of Liverpool, Liverpool, England

H. E. FARNSWORTH, Barus Physics Laboratory, Brown University, Providence, Rhode Island

KENZITAMARU," Department of Chemistry, Yokohama National University, Yokohama, Japan

A. TERENIN,Physical Institute, Leningrad University, Leningrad, U.S.S.R.

P. B. WELLS, Department of Chemistry, University of Hull, Hull, England

* Present address: Department of Chemistry, The University of Tokyo, Hongo, Tokyo, Japan.

This Page Intentionaiiy Left Blank

Preface I n Volume I of this series, published in 1948, the founder editors, Frankenburg, Rideal, and Komarewsky, advanced the view that “in spite of the amazing practical successes of catalytic methods, and of an increasing knowledge of biocatalysts, only modest progress has been made in the scientific elucidation of the working mechanism and of the basic nature of catalytic action.” What is the present position, sixteen years later? There have been a number of important practical developments, the outstanding one, Ziegler-Natta catalysis, having brought the Nobel Prize to its discoverers. Many of these developments have been firmly rooted in basic chemical knowledge, and some, a t least, and especially the Ziegler-Natta system, have led to new ideas about working mechanisms. But what of the basic nature of catalytic action? The solution to this problem still remains as fugitive as the activated complexes which lie at the heart of the problem. By acting as a yearly forum for considered papers on catalytic topics, the editors hope that the Advances are still playing a part in the attack on this general problem. Increasingly these general problems of catalysis would seem to be slipping the net of the International Conferences. It may well be that the Faraday Society Symposium of 1950 was the most successful conference so far to be held on our subject. Certainly, subsequent international conferences have turned out more as conventions, valuable meeting places, but with too many papers and sessions t o lead to a really stimulating discussion. In such an age of large conventions, the Advances will play an important role, providing the material for the thoughtful consideration of “the basic nature of catdytic action.” Turning from generalities, with which we hope a t least some of our readers will agree, we present in this volume a series of articles progressing from studies of adsorption to catalytic mechanisms, which we believe all our readers will find stimulating and valuable. Quite a few of the topics were born in the 1930’s to 1940’s and scholarly minded readers may work back to the not always obvious early beginnings. After an induction period of some twenty to thirty years the techniques and methods described are now in full development, for which some thanks are due to the enterprise of scientific instrument manufacturers. Thus slow electron diffraction has long been in principle a choice method for examination of adsorbed films. Recent developments in technique (cf. Germer, in Volume 13) together with the availability of ultrahigh Vii

viii

PREFACE

vacuum are bringing the method within the range of the average laboratory. The article by Professor Farnsworth, a pioneer in this field, shows the advantages t o be accrued to the experimenter sufficiently able to combine this technique with that of the photoelectric work function method. Primarily a physicist’s approach, we may link this with Terenin’s article, where the classical methods of chemical spectroscopy are applied to the structure of adsorbed films on solid surfaces. It is to be hoped that the younger readers of this fine article will take the trouble to secure a copy of the book mentioned in reference 4, which still merits reading from cover to cover. By working with relatively high surface to volume ratios it is possible to study the exchange of atoms between gases and adsorbed layers. Professor Tamaru in his very interesting article reviews this and other techniques of studying adsorbed films during the actual progress of the chemical reaction. A step further takes us to Brennan’s study of atom production on metals, a fine example of the value of closely argued mechanisms based on kinetic studies, and a further step to Boreskov’s studies on isotopic oxygen exchange with oxides; a powerful approach to the whole chemistry of these solids. Finally, we come to the article by Bond and Wells on hydrogenation of unsaturated hydrocarbons, most classical of heterogeneous reactions. Here the idea of the surface n-complex is slowly taking shape, implied by recent studies in inorganic chemistry, and guided by efforts in laboratories at Belfast, Hull, and Sydney. Here we see a new formulation, with all its implications not yet fully established, but fitting into the framework of modern three dimensional chemistry. There are those workers who would think that as the test of catalytic theories that they should successfully predict powerful new catalysts. This is probably altogether too stringent a test to apply at the present time. It may reasonably be argued that present day theories should provide coherent interpretations of increasingly larger fields of catalytic phenomena, and fit them firmly into the general body of chemistry. True prediction must await complete descriptions of catalyst kinetics and thus in turn await the more penetrating models of adsorption and activation that only new techniques can give us. The present volume adds something more to both kinds of knowledge, and by its authorship continues to emphasize the truly international character of all scientific endeavor. October, 1964

D. D. ELEY

Contents CONTRIBUTORS

V

vii

PREFACE

The Atomization of Diatomic Molecules by Metals D. BRENNAN I. Introduction 11. Experimental Methods 111. Experimental Results IV. Discussion . V. Conclusions . References .

1 2 4 10 29 29

. .

The Clean Single-Crystal-Surface Approach to Surface Reactions H. E. FARNSWORTH I. Introduction 11. The Clean Surface 111. The Low-Energy Electron Diffraction (LEED) and WorkFunction Method IV. Vacuum Conditions . V. Results References .

31 32 33 38 38 62

Adsorption Measurements during Surface Catalysis KENZITAMARTJ

I. Introduction 11. General Scope of Adsorption Measurements during Surface . Catalysis 111. Experimental Methods . IV. Decomposition of Germane on Germanium . V. Decomposition of Formic Acid on Metal Catalysts . VI. Decomposition of Ammonia on Metal catalysts . VII. Ammonia Synthesis on Iron Catalysts . VIII. Concluding Remarks . References . ix

65 68 15

79 81 83 85 88 89

CONTENTS

X

The Mechanism of the Hydrogenation of Unsaturated Hydrocarbons on Transition Metal Catalysts G. C. BONDAND P. B. WELLS

I. Introduction . . 11. The Hydrogenation of Olefins . 111. The Hydrogenation of Alkynes and Dienes IV. The Hydrocarbon-Metal Bond in Catalytic and Organometallic . Chemistry References

.

.

. .

. .

92

-§e 155 205 221

Electronic Spectroscopy of Adsorbed Gas Molecules A.

TERENIN

.

I. Introduction 11. General Conaiderations . 111. Spectra of Physically Adsorbed Molecules . IV. Strong Spectral Perturbations V. Positive Ion Spectra of Adsorbed Molecules VI. Spectra of Anion Radicals on Siirfaccs VII. Radicals from Adsorbed Molecules . References .

. .

. . . . . .

227 '231. 236 246 256 274 277 280

The Catalysis of Isotopic Exchange In Molecular Oxygen G. K. BORESKOV

I. Kinetics of Isotopic Exchange in Molecular Oxygen . . 11. Some Expcrimental Data Relating Isotopic Exchange in Molecular Oxygen on Solid Catalysts . . 111. Conclusion . . References . .

286 --^--

293 337 338

AUTHORINDEX

.

.

341

SEBJEOT INDEX

.

.

351

The Atomization of Diatomic Molecules by Metals D. BRENNAN Department of Inorganic, Physical, and Industrial Cherniet~y Wnivereity of Liverpool, LiuerpooE, England

I. Introduction ........................................................ II. ExperimentalMethods ................................................ A. Atomization in a Static System. ..................................... B. Atomization in a Flow System ...................................... 111. Experimental Results ................................................ A. Half-OrderKinetics .............................................. B. The Transitionto First-OrderKinetics. ............................... C. The Activation Energies. ........................................... IV. Discussion .......................................................... A. Deductions from the Half-Order Dependence on Pressure of the AtomizationRate ................................................ B. The Transitionfrom Half-Orderto First-Order Kinetics C. The Nature of the Adsorbed State. V. Conolusions ........................................................ References

Page 1

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

2 2 3 4 6 7 9 10

11 16 23 29 29

1. Introduction The growing demand for more efficient heterogeneous catalysts to bring about chemical processes of ever increasing complexity has resulted in a large and expanding volume of descriptive literature without commensurate investigation of the basic elements of the processes involved. Indeed, the sophistication of many heterogeneous processes which are now commonplace has so outstripped our ability to give them an adequate analysis that there is widely current, in circles concerned with the practical aspects of heterogeneous catalysis, the view that it is virtually useless even to attempt a fundamental understanding and that a formal description of behavior under empirical conditions must suffice. While there can be much sympathy for this argument, it is to be regretted that there is frequently found associated' with it a belief that it is not profitable to devise and study exhaustively much simpler heterogeneous processes, which are perhaps not so useful from a purely chemical point of view. It is surely only in the elucidation of such greatly 1

2

D. BRENNAN

simplified systems that the key to the understanding of the more complex ones will be found. The atomization of diatomic molecules is one of the simplest heterogeneous processes that can be devised and it exemplifies with corresponding clarity the problems of heterogeneous catalysis. There is now an agreed kinetic description for the atomization of hydrogen over tungsten, derived from the application of independent experimental techniques. There is also available much information concerning the adsorbed state for this system. It is possible, therefore, to deduce the numerical consequences of certain limiting models and so develop a fairly definitive analysis of the reaction. Also available are descriptions of the kinetics of the atomization of hydrogen over platinum and gold, and of oxygen over platinum and, while these measurements have not received the benefit of independent confirmation, they give strong support to the discussion as developed for the hydrogen-tungsten system. Unfortunately, it has not been possible to study the heterogeneous atomization of nitrogen because of the very high temperature necessary for an appreciable rate (1).

II. Experimental Methods A. ATOMIZATION IN

A

STATICSYSTEM

Langmuir was the first to study heterogeneous atomization and the method adopted by him has been used subsequently, with modification, by several workers. The reaction occurs at an electrically heated filament hanging in a static atmosphere of the gas. All the atoms produced at the filament are trapped at the wall of the containing vessel, enabling the progress of reaction to be followed by means of the pressure change. Langmuir (2, 3) atomized hydrogen over tungsten. He relied on the glass of the reaction vessel wall, either at room temperature or cooled in liquid nitrogen, to trap the hydrogen atoms. The numerical data of Langmuir are incorrect because of the inefficiency of glass for the removal of hydrogen atoms in the quantities involved in his experiments; the data of Zaitsev (4)are unacceptable for the same reason. A great improvement in technique was made by Roberts and Bryce who used molybdenum oxide as the hydrogen atom trap. The oxide was deposited on the reaction vessel wall by heating a molybdenum filament in about 1 torr of oxygen, with subsequent baking in oxygen. The reaction was studied in the temperature range 1200-1400°K and at pressures in the range 10-2 to torr. Unfortunately, the data of Bryce (5) are incorrect because of contamination derived from stopcock grease and, for

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

3

over twenty years, analyses of possible reaction mechanisms were greatly hampered by attempts to accommodate these unreliable data. Mochan (6),and Ivanoiskaya and Mochan (7’) used a film of potassium as the hydrogen atom trap, and Mochan (8) tried a silver film to trap oxygen atoms formed at a platinum filament. None of these Russian experiments succeeded in providing reliable numerical characterization of the reaction, either because of inadequate atom trapping arrangements or inadequate vacuum technique, or both. Apart from the investigation of Fabian and Robertson (9), who measured the extent of atomization of oxygen occurring a t a platinum filament mass-spectrometrically, more recent work has relied on the development of the Langmuir method introduced by Roberts and Bryce. Thus, Brennan and Fletcher (10-12) applied improved vacuum technique and special care in the use of the molybdenum oxide to ensure its efficiencyas an atom trap to the atomization of hydrogen on tungsten, gold, platinum, and carbon. They found that a molybdenum oxide film which had adsorbed atomic or molecular hydrogen was capable of trapping oxygen atoms efficiently, while remaining virtually inert toward molecular oxygen at - 45’C. This property was used to study the kinetics of the atomization of oxygen over platinum. I n these investigations, the temperature was varied from 1200-1800°K and the pressure from 10-2 to torr.

B. ATOMIZATION IN A FLOW SYSTEM The atomization of hydrogen over tungsten has been studied by Hickmott (13) using an adaptation of flash-filament technique. I n this work, residual pressures of about torr were obtained and the reaction was studied in the pressure range 2 x lo-* to 1 x torr. An omegatron ion-resonance mass spectrometer was used to verify qualitatively that the observed pressure changes were in fact attributable to hydrogen and not to contamination. There can be no doubt that, from the point of view of surface and gas purity, this is the most definitive investigation of atomization kinetics available. A steady flow of hydrogen was established over the unheated filament at a pressure, p o , which was determined by the rate gas entered the reaction cell and the rate gas was removed by pumping due to the ionization gauge, the walls, and the port. When the temperature of the filament was raised rapidly to a value in the range 1100-1500°K, the pressure in the cell fell to a value l’k, which remained steady for about 30 sec before it began to increase. The new pressure l‘; remained constant as long as the glass walls of the reaction cell, which were kept at

4

D. BRENNAN

77"K, were able to trap all the atoms being produced at the filament. After the walls had adsorbed only about 1 x l O l a atoms cm-a, recombination of atoms began to occur and the pressure in the cell started t o rise in consequence. This very limited capacity of glass to adsorb hydrogen atoms with a sticking efficiency approaching unity is a further indication of the unreliability of any measurements made in a static system relying on glass as the atom trap. However, in this experiment, the time taken for atom recombination to become troublesome was long relative to the time taken to establish the new stationary state corresponding to PL.During this stationary condition, the only additional pumping in the system was that due to the removal of molecules by atomization a t the filament; let v1 (atoms cm-a sec-I) be this rate of atomization. Then

Av,

=

2Y(Po -

kT'

where Y (molecules cma dyne-l sec-l) is the known pumping speed due to the gauge, walls, and port in the absence of atomization; A is the area of the filament, which was put equal to the geometric area in the absence of reliable information about the roughness factor; T' is the temperature of the gas (77°K) and k is the Boltzmann constant. I n these and following equations, a parameter qualified by a prime refers t o a temperature different from that of the surface and, unless otherwise indicated, the cgs system of units is employed. Variation of the rate of admission of hydrogen to the cell permits the magnitude of p o , and therefore of Pi also, to be varied and the atomization can be studied as a function of pressure. There are two main corrections which have to be made to observed parameters in both static and flow systems. First, if the pressure gauge is at a temperature different from that of the reaction vessel, allowance must be made for thermal transpiration (14). Second, if a heated filament is in a steady state, there will be a temperature gradient a t the ends where it is attached to heavier gauge supports, and, especially for short filaments, the necessary correction (15) can be quite large.

Ill. Experimental Results It will be convenient to have most of the experimental results collected under the one heading, although the presentation of some data will be delayed until the following section. The pressure dependence of the atomization rate varies with the temperature of the metal and the pressure of the gas. For a given

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

6

temperature, the rate will change from being proportional t o dF2to a linear dependence on Pi as the pressure is lowered; a corresponding change in order of reaction would be observed if the pressure were held constant and the temperature of the metal raised sufficiently.

A. HALF-ORDER KINETICS If the temperature of the metal is sufficiently low, the pressure dependence of the atomization rate is accurately half order over a wide range of pressure, as can be seen from Fig. 1, which refers to the hydrogen-tungsten system (11);equivalent results have been obtained for the

FIG.1. The dependence on pressure of the rate of atomization of hydrogen over a tungsten filament at approximately 1200°K in the pressure range lo-* to 10-o torr; the differently styled points denote separate experiments.

6

D. BRENNIPN

7

THE ATOMIZATION O F DIATOMIC MOLECULES BY METALS

reaction of hydrogen over platinum and gold, and of oxygen over platinum (12) under similar conditions.

B. THE TRANSITION TO FIRST-ORDER KINETICS The probability, @l of a molecule's being atomized upon striking the metal surface is given by

where NL (collisions cm-2 sec-l) is the collision number of the gas at the temperature ( T ' )of the reaction vessel wall. Provided a Gaussian distribution of velocities prevails, the collision number of a diatomic molecule, X,, is related to the pressure thus,

z

=

3.50 x loaa

(3b)

where M x is the atomic weight of X. Hence, Pl can be obtained from the experimental parameters v, and Pk,,. It is useful to note that

and, for half-order kinetics,

a log 9;- - - 1 a log Pi - 2 while, for first-order kinetics, dlog 9; d log Pi = O I n Figs. 2 and 3 , the variation of the probability of atomization with pressure is shown for hydrogen over tungsten (11)and platinum (IZ), and for oxygen over platinum (12). It will be seen, now the temperature of the metal is very much greater, that there is a smooth transition from F I ~2.. The transition from half-order to first-order kinetics for the atomization of hydrogen; the points denote experimental determinations and the lines are theoretical curves derived from Eq. (39b). (a) 0, tungsten at TI= 1800" 100'K; (b) platinum at 1750' f 100°K.

A,

0 ,the same filament at T, = T, - 50".

8

D. BRENNAN

- 1.2

c

l61 - 1.4

I

%w

02/Pt

- 4.5

I

- 4.3

I

I

- 3.9

-4.1

1

- 3.7

I

- 3.5

Log pirnm

FIG.3. The transition from half-order to tlrst-order kinetics for the atomization of oxygan over pletinum at about 175OOK.

half-order kinetics at the higher pressures to a constant value for tYl at the lower pressures; the limiting values of PI are given in Table I. I n order t o keep the rate of reaction down to a measurable speed, it was necessary to use very much finer and shorter filaments in these experiments than in those of the preceding paragraph, with the result that the filaments were difficult to observe with the optical pyrometer and temperature measurements are therefore subject t o a rather large uncertainty. The limiting value of 0.058 for ,PIin the case of oxygen over platinum is in good agreement with the value of 0.068 reported by Fabian and Robertson ( 9 ) , but the value of 0.30 for hydrogen over tungsten has to be compared with the value of 0.05 obtained by Hickmott (13);reference to these values will be made later. TABLE I Limiting Valves of the Probability of Atomization, 8,, at about 1800°K

Limiting PI

System

0.30 0.24

Oxygen over

platinum

0.058

THE ATOMIZATION O F DIATOMIC MOLECULES BY METALS

9

0.4 0.2

0

- 0.2 57

0 -0.4 X

8 -0.8 _1

-1.0

- 1.2 - I .4

- 1.6 1

7.4

1

7.6

1

1

7.8

8.0

T

-’ x lo4

1

8.2

1

8.4

1

8.6

1

8.8

FIG.4. The dependence of the rate of atomization on temperature for hydrogen over tungsten platinum (A) and gold and for oxygen over platinum (0); the data are uncorrected for filament and cooling.

(e),

(n),

C. THE A C T I V A ~ OENERGIES N In Fig. 4 are given the plots of log[d(Pi ,,)l’a/dT] vs T-l for hydrogen and oxygen over the various metals (11,12) in the region of half-order kinetics. The temperature range accessible for gold is severely limited, on the one hand by the melting point of the metal and on the other by the slowness of the reaction. The equation of the straight line giving the best fit to the data was obtained by the method of least squares.

10

D. BRENNAN

The rate equations can be written in the general form

--)

v1 = A ( P ~ , , ) ~ eq / ~( - E RT

(5)

and the fully corrected values of A and E are given in Table 11. TABLE 11" Numerical Valuesfor the Constant8 of the Atomization Rate Equation v1 = A(Pimm)l'nexp

(- ?):

atoms em-' see-1

System

Oxygen aver

A

platinum

E (kcal mole-')

(1.5Itr 0.6) x loas (1.3 0.6) x loa6 (0.23f 0.17) x loa5

51.8 f 1.0 51.1 1.2 60.7 f 1.6

(1.6i 1.0) x loas

61.5 i: 1.3

'The roughness factor is put equal to unity in all cases except that of tungsten, for which the value of 1.4is used (16). Do(Ha)= 103 kcal mole-' and Do(O,) = 118 kcal mole-'.

IV. Discussion Two mechanisms for the atomization reaction are possible; they are: M - X

+ M + X

X , + M -+ M - X

+X

In these equations, M denotes the surface, without prejudice to the question whether the adatom is held at a surface site, or is freely mobile and therefore views the surface as an area of uniform potential. A choice between the two mechanisms can be made using experimental data restricted to the condition of an atomization rate which is slow relative to the rate of interchange of molecules between the gas phase and the surface; when this restriction is applicable, it is possible to write &X,+M

+M

- X

(111)

and so enable the concentration of adatoms to be expressed as an explicit function of pressure, subject to certain assumptions about the state of desorption. Prior to recent experimental work, attempts (17-21) to decide unequivocally between mechanisms (I)and (11),for this condition, failed because of the inadequacy and unreliability of the data

THE ATOMIZATION O F DIATOMIC MOLECULES BY METALS

11

relating to both the kinetics of the reaction and the state of the adsorbed layer. A detailed analysis of the confusion inherent in this situation has already been given (11)and will not be repeated here. Rather, the problems will be reviewed solely in the light of recent experimental work and the two parallel, and essentially concordant, discussions given by Ehrlich (22) and by Brennan and Fletcher (11, 12), together with some additional unpublished comment. The case for mechanism (I)will first be established, using data obtained under conditions for which equilibrium (111) is applicable and the discussion will then be extended to meet the situation for which equilibrium (111) no longer prevails.

A. DEDUCTIONS FROM THE HALF-ORDER DEPENDENCE ON PRESSURE OF THE ATOMIZATION RATE 1. The Extent of Adsorption during Reaction There are now available experimental estimates of the concentration (n, atomscm-2) of hydrogen atoms adsorbed on tungsten at high temperatures and, for the following discussion, it is useful to have rough values for surface coverage during atomization. For example, Hickmott (13) reports that during the course of an atomization reaction a t T = 1176'K and PLmm = 9 x lo-' torr, n, = 23.8 x lolo. For these conditions, the rate of adsorption and desorption of molecules is very much greater than the rate of formation of atoms (1.27 x 1014 molecules cm-2 sec-l as compared to 1.46 x 1012 atomscm-a sec-1, respectively) and the value of n, may be taken, therefore, as being the equilibrium coverage. Unfortunately, the pressures employed by Hickmott are considerably lower than those for which reliable data on the order of reaction are available and it is necessary to make an estimate of the change in coverage due to an increase in pressure to say 10-8 torr. To do this, it is obligatory to have a model of the adsorbed state. There are two limiting cases, namely, site adsorption and free mobility of adatoms, and the form of the equilibrium constant is affected accordingly, thus: site adsorption, free translation, ~ the ) concentration of molecules in the gas where m2 (molecules ~ m - is

12

D. BRENNAN

phase and n, (sites cm-I) is given by L - n,, L being the number of sites per unit area. I n both cases. it is supposed that there is not interatomic interaction on the surface. An attempt to distinguish between Eqs. (6a) and (6b) will be made later, but, for the purpose of deciding mechanism, it is necessary to estimate n, only for the case of the site model. From (6b) and Hickmott’s data, it follows that 6 = n,/L 5 x 10-3 at and 1176°K for hydrogen on tungsten. Hickmott obtained a heat of adsorption of 30 kcal mole-’, as compared with the more usual value of about 45 kcal mole-l, but, even allowing for possible variations in the heat of adsorption on different specimens, it seems very reasonable to conclude that, under a21 conditions of temperature and pressure for which atomization rates have been reported for hydrogen over tungsten, 9 was very much less than unity, assuming of course that a site model is applicable. Likewise, in the case of platinum and gold, on which the heats of adsorption of hydrogen are smaller, only values of 8 much less than unity are relevant under atomization conditions. For oxygen on platinum, the heat of adsorption is initially much greater, being about 70 kcal mole-l (23), but it falls rapidly with increasing coverage and at about torr, even at room temperature, 8 is only about 0.6. It is probable, therefore, that in the case of atomization of oxygen over platinum also, 8 is always much smaller than unity.

-

2. Comparison of the Kinetic Consequences of Mechanisms ( I ) and ( I I ) with Experiment Although the use of transition state theory is not necessary for the achievement of the objects of this paragraph, it will be convenient to use the theory so that the necessary equations will be available for later discussion. The difficulties which always attend the description, or even indeed the definition, of any transition state are preeent here in an especially acute form; however, we will proceed in the customary manner (21, 24). a. Mechanism ( I I ) . If E,, is the activation energy of the forward reaction and eII that of the reverse reaction, then &I

=

€11

+8 { W u -

&a)

(8)

where D ( X , ) is the energy of dissociation of the diatomic molecule and Q a is the heat of adsorption of one mole of gas (positive quantity). If

both the adsorbed atoms and the transition state occupy sites, then

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

13

and

vII = (1 - 8) Lm2

2RT

"'I)

(8b)

where F 2 is the partition function of one molecule, X,, in unit volume, f t refers to all the modes of the transition state except vibration in the direction of the reaction coordinate, and f8 is the partition function of a site. Remembering that m2kT = P , and 1 - 8 x 1 for small 0, it follows that mechanism (11)requires first-order kinetics and an activation energy considerably smaIler than QD(X,), whereas reference to Table I1 shows that the kinetics are half-order and the observed activation energy very close to * D ( X , ) . It is instructive to examine the condition for mechanism (11) to give the observed kinetics. From Eq. (6a), we have

and

We can also write

where fl is the partition function of an adsorbed atom. Use of Eq. (9a) in conjunction with Eq. (8b) gives

and, if 0 1 then K8m, > > 1, and Eq. (11) would become, on substituting Eq. (lo), N

Equation (12) has the correct pressure and temperature dependence, but the condition of 8 1 for its applicability is quite untenable. Further, the great constancy of dP1I2/dtover a wide pressure range (cf. Fig. 1, for example) rules out any possibility that Eq. (12) might just be applicable, within the limits of experimental error, for an intermediate value of 8. N

14

D. BRENNAN

If a freely mobile transition state is postulated, then

and

or

The grounds for rejecting Eq. (11) are now even more firm, since Eq. (13) does not so readily admit of special pleading on the basis of possible doubt about the value of coverage. Some extension of these arguments is necessary (11)if Qz is a function of coverage, which of course it is, but the rejection of mechanism (11) is not weakened any by this refinement.

b. Mechanism ( I ) . If E, is the activation energy for the adsorption of an atom according to mechanism I and er is the corresponding quantity for the reverse step, then

E,

= HmX2)

+ &,I +

€1

(14)

The form of the equilibrium constant for the transition state is now independent of whether it occupies sites or not, though its numerical value, of course, is affected:

and

vI

=

n, --f: exp

fi

2RT

If the adatoms are held on sites, Eq. (6a) is applicable in the form n1

so Eq. (15b) becomes

LK,m;" = r+=

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

16

Since 0 is known to be small, K s m 2< < 1, and Eq. (17) in conjunction with Eq. (lo), becomes

where

and accounts for the observed reaction order and activation energy. If the adsorbed atoms are freely mobile, then we must write fi K , =-exp PkIL

-

(ZT)

(19)

and Eq. (15b) becomes

where

Equation (20) is now of the correct kinetic form, regardless of coverage. We can conclude from the observed kinetics, therefore, that mechanism (I)is acceptable but mechanism (11)is not. An attempt will be made later to analyze the absolute rates predicted on the basis of mechanism (I) and different models of the adsorbed state.

B. THETRANSITION FROM HALF-ORDER TO FIRST-ORDER KINETICS 1. The Kinetic Equations Consider a surface at temperature T in contact with a gas a t temperature T' and pressure Pi.The condition for a steady state is that the rate of desorption of material as atoms and molecules should equal the rate of adsorption. The number of molecules adsorbing on unit area of the surface in unit time will be S i N i ; S; is the sticking coefficient referring to the adsorption when the temperature of the molecules is different from that of the surface. Hence,

+

SiN; = Q v ~ v 2 (21) where w 2 (molecules om-2 sec-1) is the rate of desorption of molecules.

16

D. BRENNAN

Hickmott (13) has shown €or the hydrogen-tungsten system that molecular desorption is second order in adsorbed hydrogen, which is the most direct evidence available that this process is to be described as a recombination of adsorbed atoms. Thus, we use this and other independent experimental evidence (34) to write for molecular desorption

- X 4M + X,

2M

(W)

and "2 =

&kan:

(22)

Substitution of this relation in Eq. (21)) along with Eq. (3a), gives 2z

n,

=

s; Pimrn

r(Mx!Z'r)1/2 -

"]

1/2

atoms cm-a

(23)

k2

The parameters on the right-hand side of this equation are all accessible experimentally and, indeed, are available for the hydrogen-tungsten system; the values of n,,obtained in this way, therefore, do not implicate any mechanism for the atomization reaction. Further, if

v1 << S;N;

(24)

then the values of n, obtained from Eq. (23) approximate closely to equilibrium coverages. If we accept mechanism (I),we have v1 = klnl

and Eq. (21) becomes

Hence,

and 1

When

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

17

that is, when condition (24) is applicable, then

If we separate each rate constant into an entropy and energy of activation, writing kl

= vl exp

(- $)

vSexp

(- $)

and

k,

=

then Eq. (29) becomes

This is the equivalent of Eqs. (18) and (20); it correctly describes the observed half-order kinetics and requires an activation of E, = El - &?3’2. The other limit of Eq. (28) is reached when

for which

and

Equation (28) can be developed further if it is supposed that the surface is in the same condition as if it were part of the wall of an isothermal box containing the gas, the whole being in a state of thermodynamic equilibrium. The pressure of molecules (P2) and of atoms (PI) in the gas phase would be determined by the equilibrium constant K for the reaction

x, + 2x

(V)

viz.,

K

=

Pf/Pa

(34)

18

D. BRENNAN

Further, the rates of adsorption of atoms and molecules would be exactly balanced by the respective rates of desorption, thus v1

=

V2 =

k,nl

= SINl

$ k p , = S2N,

8, and S, are the sticking coefficients of atoms and molecules, respectively, which are a t the same temperature as the surface. Hence

k; ---s; _

E2

2

S, ( 2 M X T ) l i 2Kmm

(37)

and Eq. ( 2 8 ) becomes

When the surface coverage is virtually undisturbed by the atomization process, then Eq. (29) becomes

and, similarly, Eq. (33) becomes

for the case when the atomization rate is relatively very large. Equation (39) thus not only implicates mechanism (I),but also requires the adatoms to be sufficiently mobile on the surface to achieve an equilibrium distribution, for under experimental conditions there must be no possibility of atoms returning to surface from the gas phase. 2 . Comparison with Experiment

There are several points a t which the discussion of the preceding paragraph can be checked by reference to experiment, in addition t o testing the equation describing the behavior of the system when the kinetics are intermediate between half-order and first-order.

a. The activation energy for the desorption of hydrogen atoms from tungAs already noted, Eq. (23) is a relation from which a value for surface coverage can be obtained which is independent of any supposi-

sten.

,

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

I

19

r

1400

(OK) 13pO 1250

1350

I

I

I

7.0

7.2

7.4

I

I

I

1200

I

I

I

7.6 7.8 8.0 8.2 8.4 8.6 T -I x lo4 FIQ.5. Temperature dependence of the rate constant for the formation of atomic hydrogen at pressures from 9 x lo-' to 3 x torr over tungsten at temperatures from 1176 to 1426°K.

tion about the atomization process. The use of this value of n, with the corresponding experimental value of v,, enables k, of Eq. (25) to be obtained. This has been done by Hickmott (13)for hydrogen over tungsten and the results are shown in Fig. 5. The linearity of the plot of log k, vs T-l strengthens the argument in favor of mechanism (I) and yields the relation

x 1013 exp

k,

= 2.2

El

= 67 kcal mole-,

(- 5)sec-1 (42b)

Since the activation energy for the adsorption of an atom i5 negligible, if not zero, it can be said that the energy of the surface bond is El = 67 kcal mole-'. Hickmott plots his measured rate of atomization as though it were directly proportioned to Pi,,, Eq. (28) shows that a more complex dependence on pressure is to be expeeted, and is indeed observed ( 2 1 , l Z ) . Hickmott's pressure range a t any one bmperature does not exceed sixfold and it is possible that this is nm wide enough to show a trend sufficiently marked to be neticeable over the scatter of

20

D. BRENNAN

the experimental points; even so, it is unexpected that these linear plots should actually pass through the origin.

b. The energy of activation for half-order kinetics. Hickmott found, quite independently of the atomization process, that

k,

=

E,

= 31 kcal mole-l

2.5 x 10-3 exp

(- $)cm*atoms-’ sec-1

(Ma) (43b)

The assumption of mechanism (I)requires that the activation energy in the region of half-order kinetics should be El - BE, = 51.6 kcal mole-’ [Eq. (31)]. This is in very good agreement with the result of Brennan and Fletcher (Table 11)and the theoretical expectation that the energy of activation E , should be &D(H,) eI, being less than the experimental uncertainty.

+

c. The pre-exponential factor for half-order kinetics. The use of Hickmott’s data to calculate the value of the pre-exponential factor in the region of half-order kinetics requires a knowledge of Si [Eq. (31)]. This may be obtained at the temperature of these reactions by measuring the rate of atomization under conditions for which Eq. (33) is applicable. Hickmott obtained 23; = 0.05 for hydrogen on tungsten, whereas Brennan and Fletcher obtained a value of 0.30 (Table I). Hickmott measured the rate of adsorption of hydrogen on tungsten at 77’K and obtained 23; = 0.07. However, Hickmott’s procedure is subject to considerable experimental error and the higher value of Brennan and Fletcher is more in accord with the value of 0.1 at temperatures between 77” and 194°K obtained by Hickmott from coverages determined by flash desorption after known exposures of the surface to gas. The only other value of Si for the hydrogen-tungsten system has been reported by Eisinger (25). Unfortunately, his value of S; = 0.2 at 300°K cannot be regarded as confirmatory because, in his experiments, no allowance was made for atomization during the flashing of the filament and at the ion gauge filament; there is also a serious possibility that contamination was present in significant amounts. The use of 23; = 0.05, and values of V, and U, from Eqs. (42a) and (42b), in Eq. (31) give v,

= 0.7

x

loas (P;mm)1/2 exp -

(

(44)

The value of the pre-exponential factor is in good agreement with that obtained by Brennan and Fletcher (Table 11),especially when it is recalled that they used a roughness factor of 1.4, whereas Eq. (44) im-

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

21

plies a value of unity, and that there is some uncertainty in the value of SL. It can be concluded, therefore, that the numerical deductions made from the data of Hickmott in relation to mechanism (I) are entirely self-consistent and, though there is no direct elucidation of the pressure dependence of the reaction in Hickmott's procedure, the agreement of these expectations with the experimental findings of Brennan and Fletcher is further confirmation of the validity of mechanism (I). TABLE 111 Numerical Value8 for the Constanta of the Equation log KSt, = a Hydrogen and Oxygen at 1300'K System

H,

b (kcel mole-')

a

+ 2H

0 , t 20

- ( b / R T )for

1.23 x 10'

107.1

7.7 x 106

121.4

For a small range of temperature, the constant for the equilibrium

(V) can be written in the form log K,,

=

a -

b (45)

RT ~

and, in Table 111,the values of the constants a and b are given (26) and and these values have been used to calculate the parameters A EequUibrlum of Eq. (40) recast in the form

For this purpose, S, has been put equal to unity, as has the ratio SiIS,; the results of the calculation are given in Table IV. Comparison of the values of Table IV with those of Table I1 invites the conclusion that the TABLE IV Numerical Valuesfor the Constant8 of the Equation ('1)equllibrium

System

=

(A)eguilibrium (p;mm)1'2

Aequilibrium

H, -+ 2 H

1.17 x

0, -+ 2 0

0.73 x 1 0 ~ 4

1034

exp (

Eequilibrlum (kcal mole-') 63.6 60.7

22

D. BRENNAN

- 0.4 OI

-0.8-1.2

-

.-5

0 .-

-a

-1.6 -

0

5 -2.0g l

1

-2.4

-

-la: -3.2 I

I

-12.0

I

-11.0 -10.0

I

I

-9.0 -8.0 -7.0 -6.0 Log Smm

I

-5.0 -4.0 -3.0 -2.0

FIG.6. The form of the transition from half-order to first-order kinetics calculated for hydrogen at 1200' and 1300°K by inttans of Eq. (39).

assumptions made in the derivation of Eq. (40)are valid, namely, that mechanism (I) is correct and that the degree of mobility of adatoms is sufficient to give an equilibrium distribution on the surface.

d. The transition from half-order to first-order kinetics. The properties of Eq. (39) are shown in Fig. 6 which refers to hydrogen at 1200' and 1300"K, for various values of and S,. At a given temperature, the transition in order occurs more rapidly the smaller Si and, for a given Si;the lower the temperature the lower the pressure necessary for the transition to occur. The form of the transition is controlled by the concentration of the adsorbed layer. When the concentration of adatoms is relatively large, the chance of an adatom's meeting another adatom with consequent adsorption of a molecule is relatively large. As the pressure falls, the adsorbed layer becomes more dilute and, eventually, the meeting of two adatoms becomes a relatively rare event and the absorption of atoms is then the dominant process. From Fig. 6 it is seen that, if the pressure is taken low enough, a well-defined value of S; = limp,+,,

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

23

(L P ~can be ) obtained. ~ ~ The ~ effect ~ of 8, ~ on ~the rate ~ is~also ~ shown~ in Fig. 6 [cf. Eq. (40)]where it is seen that, at a given temperature, diminishing X, moves the curve to lower pressures, and the transition to the first-order region is delayed, although this is not shown. The comparison of the requirements of Eq. (39) with experiment is a severe test of the validity of the assumption made in the derivation of the equation. In Figs. 2(a) and (b), the experimental data for hydrogen on tungsten and platinum are compared with the theoretical predictions based on filament temperatures calculated from the observed rates in the region of half-order kinetics and the data of Table 11.The temperatures obtained in this way are 1830°K for tungsten and 1750°K for platinum (cf. 1800" and 1750"K, respectively, for the nominal temperatures as measured with the optical pyrometer). The agreement between theory and experiment is seen to be very satisfactory over the very wide presdure range involved.

C. THE NATURE OF

THE

ADSORBEDSTATE

The object of this paragraph is to examine these experimental findings for possible conclusions about the state of the adsorbed layer. I n Eqs. (6a) and (6b), a distinction is made between a state in which adatoms occupy sites and one in which they are freely mobile. The latter state has the virtue of being easily described statistically, but despite its acceptance (11, 12, 13) as an adequate description of the surface layer during these reactions, there must be serious doubts as to whether free mobility is a realistic model. Free translation of a particle on a surface requires that the activation energy for surface diffusion, &, be very much less than R T and, while there can be no doubt that there is a very large measure of mobility a t the temperatures of interest here, the condition & << R T is not met. Thus, for hydrogen on tungsten, 10 < & < 16 kcal mole-l (27, 28), for hydrogen on nickel, & = 7 kcal mole-l (29), and for oxygen on tungsten, & = 30 kcal mole-] (30), whereas at 1500"K, R T = 3 kcal mole-l. Although values of d are not available for hydrogen and oxygen on platinum, it is very probable that they will be comparable to the values just cited. Even for hydrogen on gold, the heat of adsorption of atoms and the activation energy for their diffusion are not negligible (31, 32). Hence, even a t the highest temperatures used for atomization, movement of adatoms must occur as a sequence of jumps from site to neighboring site. It is probably an adequate approximation to suppose that an adatom spends most of its time occupying a site and the probability of its being found in transit from one site to the next is negligibly small; under these circum-

~

24

D. BRENNAN

stances, Eq. (6a) for the equilibrium constant K , is valid. A t still higher temperatures, it would be possible for movement to occur across several sites in one step, and the probability of finding an adatom in transit will increase; a situation intermediate between that defined by Eqs. (6a) and (6b)would then prevail. In the limit, at very high temperatures, a state of free translation would occur and Eq. (6b) for the equilibrium constant K , would be applicable 1. The Atomization Rate Equutions

For adsorption on sites and small 0, Eq. (18b) is applicable. The difference between gas and surface temperatures will be ignored. At 1300°K, only contributions to Fa from translation and rotation need be considered, the vibrational modes being virtually unexcited at this temperature and F H ,w 2 x loaa, and F,, w 1.5 x 10aa; L is about 10l6 sites cm-a. The quantities fe and ft are difficult to assess. It is usual to put f, = 1 on the assumption that the modes of a surface atom are not seriously perturbed by the presence of an adatom. This is clearly not true of a site which interacts with an adatom with an energy comparable to that with which it interacts with its own lattice, as is the case in chemisorption. Again, while the adatom may not be in a state of free translation, it moves with a very high hopping frequency. The statistical mechanics of a model in which the adatoms hops in a static periodic field has been considered by Hill (33) for physical adsorption, but the situation for chemisorption is more complex, not least because the field will vary with the motion of the adatom. For an immobile transition state ft would be unity, but, since there is appreciable mobility, ft must be considerably greater than this. If we put ft/f8 = 1, as one limit, we obtain d m m ( H 2= ) 2 x loa3 and dmm(02) = 0.8 x comparison with the experimental pre-exponented factors of Table I1 shows that the values of dmm are of the order IOa-104 too low, whereas one might have expected the difference to be much larger. If we consider the other limit, namely an adsorbed layer approxmating to a perfect two-dimensional gas, then Eq. (20b) is applicable with

f:

21r3kT =

(47)

where rn, is the mass of the atom X. For this model,d,,(H,) = 0.7 x loa6 and dn,,(02) = 4 x lOZ5 and comparison with Table I1 shows what might be considered to be disappointingly good agreement with experiment. It was this agreement, allied to a need for a very high degree of surface mobility which led to earlier conclusions that the adatoms were

THE ATOMIZATION O F DIATOMIC MOLECULES BY METALS

26

in a state of free translation. Despite this numerical agreement, the view that adatoms are hopping and are far from freely mobile is adhered to. Indeed, we are forced to the conclusion that the two situations are not really numerically distinguishable a t the low coverages under consideration. 2. The Desorption Rate Equations

a. Desorption of atoms. Rates are now available for the desorption of atoms and molecules as a function of concentration of the adsorbed species for several systems. Thus, in the case of the desorption of atoms, it is possible to use Eq. (15b), since it is no longer necessary to express n, as a function of pressure. Unfortunately, we still cannot use the experimental data to arbitrate between possible models of the adsorbed state, because, whatever this state may be, it will be a good approximation to write f t / f 8 = 1 and the rate constant will be simply kT/h z l0ls sec-1 in agreement with experiment [see Eq. (42), for example, and Ehrlich (36)]. 6 . Molecular desorption in terms of transition state theory. The rate of desorption of molecules should give a better criterion of the adsorbed state. If the mechanism of the process is the collision of two adatoms with sufficient energy for the formation of a molecule in a physically held state as in reaction (IV), then we can write, regardless of whether the adatom occupies sites or is freely mobile,

where eIV is the energy of activation, if any, for the adsorption of the molecular species. The state of rotation of the activated complex is unimportant, since, even if it were constrained to rotate only in the plane of the surface, the partition function will not be greatly affected; we will assume, however, that rotation is in three dimensions. I n the limiting case of free translation on the surface, we have

vIv

= dnf

(

exp -

where

and I is the moment of inertia of the molecule X,. The numerical agreement of d of this equation with experimental values (cf. Table V) is

26

D. BRENNAN TABLE V

Comparison of Calculated Values of d [Eq.(49)] and the Experimentally Determined Rate Constant k, for the Desorption of Molecules from Tungsten.

System

.d CEq. (4911 (cmzatoms-' sec-1)

x 10-2

k, (experiment) (cma atoms-' seo-1)

Hydrogen

1

x

(13)

Nitrogen

2 x 10-0

7 x 10-a

(34)

Oxygen

2.6 x lo-,

20

2.6

x 10-3 (35)

as good as we could expect; in this case, the numerical value of d is sensitive to the values used for the partition functions in Eq. (48). Even so, the validity of the free translation model cannot be accepted and we adhere to the belief that such agreement as this must be fortuitous,

c . Molecular desorption in terms of the interaction of hopping adatoms. An alternative description which dispenses with transition state theory, but incorporates the notion of a hopping adatom, will now be shown to account satisfactorily for the recombination of atoms on the surface. We have seen that agreement between theory and experiment is not an adequate criterion of validity, but the following discussion, unlike that of the preceding paragraph, has the merit of achieving this agreement without implicating objectionable hypotheses. A collision between two adatoms is said to occur when an adatom, in the course of hopping, finds itself a t a potential energy maximum, 8, adjacent to a potential well occupied by another adatom. Collisions cannot occur between two adatoms in transit; this is because, a t the temperatures used for these reactions, the great majority of the adatoms follow the profile of the potential energy curve of the surface and jump only to neighboring sites. The chance that, on its first jump, a n adatom will find the adjacent site vacant is 1 - 8. The chance that the adjacent site is occupied and, therefore, that a collision can be said to have occurred, is 0. If y is the probability that a collision between two adatoms results in the formation of a molecule, either in a physically adsorbed state or in the gas phase, then y o is the chance that a molecule will be formed as the result of the adatom's attempting to move into a n adjacent site which is already occupied. The chance that reaction does not occur as the immediate consequence of the collision is 0( 1 - y ) . The chance that the adatom will come to rest in a vacant site subsequent to this unsuccessful encounter is 1 - 0 and the overall probability for the sequence is 0( 1 - y ) ( 1 - 0). The probability that, immediately

THE ATOMIZATION OF DIATOMIC MOLECULES BY METALS

27

following the first unsuccessful collision, the adatom undergoes a further collision is 8, so the overall probability for the occurrence of reaction is now y8 y82( 1 - 7); if the second collision is also unsuccessful and the adatom comes t o rest in a vacant site, the overall probability for this sequence of events is

+

e(i -Y)(i- e) + eyi - y ) 2 ( i - e). Thus, the chance that an adatom undergoes reaction as the result of a hop is W

while that of its coming to rest on a site is W

There are n, adatoms on unit area of the surface, each with a hopping frequency w . Hence, the rate of recombination of atoms on the surface is (37) m

n-1

and v2=nw 1 Y i -

e

e+Ye

We can assume, without too much difficulty (28), that the hopping frequency conforms to the relation w = ooexp

wo

-

(- &)

l O l 2 to 1013sec-1

(51b)

The energy of a pair of adatoms in a collision configuration is Q2 - d and adatoms will need to collide with a t least this energy if they are t o be able to combine and appear in the gas phase; we write

28

D. BRENNAN

Since we are here concerned virtually with the interaction of atoms, the value of y o 1 cannot be far wrong. The term y6' is negligible relative to 1 - 8, so Eq. (50b) becomes N

and, for small 8, this approximates to

v,

=

unt exp

(-

z;)

(54)

where oc % 10-a

to 10-3

in agreement with experiment. Equation (50b) will not be valid for values of 6' approaching unity since the migrating adatom would then spend a significant part of its time in the act of migrating because of the dearth of vacant sites; under these circumstances, the model of a hopping motion of frequency w would no longer be valid. It is difficult to give an upper limit to 6' for which Eq. (50b) should be applicable, but 6' = 0.5, or even greater, might be a fair estimate. TABLE VI Equilibrium Conatants K,and K , f o r the Adaorption of Hydrogen on Tungaten at 1300°K State of adsorption

K (experimental)

Immobile

12x 10-10

0.24 x 10-lO

Freely mobile

1.6 x lo6

1.0 x 106

K (calculated)

3. The Equilibrium Constants Equations (6a) and (6b) can be used to calculate K , and K , for the two limiting cases of completely immobile and freely mobile adatoms. This has been done for hydrogen on tungsten using Hickmott's value of Q 2 = 30 kcal mole-1 and the experimentally founded values are compared in Table VI with those obtained by statistical calculation. Once more, it is not possible, for these low surface coverages, to distinguish even between these two extreme cases.

THE ATOMIZATION

OF DIATOMIC MOLECULES BY METALS

29

V. Conclusions There can be no doubt that atomization occurs by evaporation of adatoms from a dilute layer, as represented by mechanism (I). Every piece of crucial kinetic evidence points to this conclusion, However, there can be no clear deductions made from these data about the state of the adatoms. While it does give good numerical agreement with experiment in at least one instance where one might reasonably have expected an inappropriate model not to do so, the model of free translation can be rejected a priori, as argued above; it must be admitted, however, that we have not been able, a posteriori, to substantiate the argument, but the alternative model of hopping adatoms has been shown to be at least adequate in accounting for the experimental observations. REFERENCES 1. Farber, M., and Darnell, A. J.,J . Chem. Phys. 21, 172 (1953). 2. Langmuir, I., J . Am. Chem.Soc. 34, 1310 (1912). 3. Langmuir, I., J . Am. Chem. SOC.37, 417 (1915). 4. Zaitsev, N. S., J . Phys. Chem. USSR 14,644 (1940). 5. Bryce, G., Proc. Cambridge Phil. Sox. 32, 648 (1936). 6. Mochan, I., Uch. Zap. Leningr. Qos. Univ. Serv. Fiz. Nauk 38, 52 (1939). 7 . Ivanoiskaya, T., and Mochan, I., J . Phys. Chem. USSR. 22, 439 (1948). 8. Mochan, I., Chem. Abstr. 36, 30917 (1942). 9. Fabian, D. J., and Robertson, A. J. P., Proc. Roy. SOC.A237, 1 (1956). 10. Brennan, D., and Fletcher, P. C., Nature 183 249 (1959). 11. Brennan, D., and Fletcher, P. C., Proc. Roy. SOC.A250, 389 (1959). 12. Brennan, D., and Fletcher, P. C., Trans. Faraday SOC.56, 1662 (1960). 13. Hickmott, T. W., J . Chem. Phys. 32, 810 (1960). 14. Bennett, M. J., and Tompkins, F. C., Trans. Faraday SOC.53, 185 (1957). 15. Langmuir, I., Phys. Rev. 35, 478 (1930). 16. Taylor, J. B., and Langmuir, I., Phys. Rev. 44, 423 (1933). 17. Roberts, J. K., and Bryce, G. Proc. Cambridge Phil. SOC.32, 653 (1936). 18. Miller, A. R., “The Adsorption of Gases on Solids,” p. 65. Cambridge Univ. Press, London and New York, 1949. 19. Laidler, K. J., J. Phys. Colloid Chem. 53, 712 (1949). 20. Laidler, K. J . , J. Phys. CoZZoid Chem. 55, 1067 (1951). 21. Laidler, K. J., Catalysis 1, 75 (1954). 22. Ehrlich, G., J . Chem. Phys. %l, 1111 (1959). 23. Brennan, D., Hayward, D. O., and Trapnell, B. M. W., Proc. Roy. SOC.A256, 81 (1960). 24. Laidler, K. J., Glasstone, S., and Eyring, H., J . Chem. Phys. 8, 667 (1940). 25. Eisinger, J., J. Chem. Phys. 29, 1164 (1958). 26. Stull, D. R., and Linke, G. C., “Thermodynamic Properties of the Elements.” Am. Chem. SOC.,Washington, D.C., 1956. 27. Gomer, R., Wortman, R., and Lundy, R., J . Chem. Phys. 26 1147 (1957).

30

D. BRENNAN

28. Gomer, R., Discussions Faraday SOC.28, 23 (1959). 29. Wortman, R., Gomer, R., and Lundy, R., J . Chem. Phys. 27, 1099 (1957). 30. Gomer, R., and Hulm, J. K., J. Chem. Phys. 27, 1363 (1957). 31. Pritchard, J., and Tompkins, F. C., Trans. Faraday SOC.56, 540 (1960). 32. Pritcherd, J., Trans. Paraday Soc. 59, 437 (1963). 33. Hill, T.L. , J . Chem. Phya. 14, 441 (1946). 34. Hickmott, T. W., and Ehrlich, G., J . Phys. Chem. Solids 5 , 47 (1958). 35. Becker, J. A., Becker, E. J., and Brandes, R. G., J. Appl. Phys. 32, 411 (1961). 36. Ehrlich, C . , in “Structure and Propertios of Thin Films” (C. A. Neugebauer, J. B. Newkirk, and D. A. Vermilyea, eds.), p. 423. Wiley, New York, 1959). 37. Holland, B. W., private communication, 1963.

The Clean S i ngle-Crystal-Surface Approach t o Surface React ions H. E. FARNSWORTH Barus Physics Laboratory, Brown University, Providence, Rhode Island

I. Introduction ........................................................ 11. TheCleanSurface .................................................... 111. The Low-Energy Electron Diffraction (LEED) and Work-Function Method. IV. Vacuum Conditions .................................................. V. Results A. Structures of Clean Surfaces ........................................ B. SurfaceReactions ................................................ References

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

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

Page 31 32 . 33 38 38 38 42 02

I. Introduction It appears self-evident that surface reactions may be greatly influenced by surface contaminants and that an understanding of the fundamental mechanisms demands that observations be made with surfaces free of contamination. Since most solids of importance in surface reactions are crystalline, there is an additional question of the influence of the particular crystal plane which forms the surface and of the arrangement of the atoms in the surface monolayer, which may be different from that in a similar plane in the bulk. However, at a time when the defect solid state has received so much attention and has been so fruitful in explaining large numbers of observations, it might appear that lattice defects are the dominant factors in most surface reactions. Thus it is important to evaluate the separate roles played by defects and surface planes in any particular reaction. One needs more than a quantitative observation of the individual contributions to obtain an understanding of the mechanism involved. These conditions obviously require that experiments should be performed on atomically clean single-crystal surfaces of known surface structure and defect density. These requirements place a severe limitation on the experimental techniques which may be used to furnish the desired information. Although no single technique can furnish all of the information needed, it appears certain that low-energy 31

32

H. E. FARNSWORTH

electron diffraction (LEED), when combined with work function and other techniques, will play a dominant role because one first needs to know when a surface is clean and the nature of the surface structure. The present discussion is restricted to the immediate field of interest of the writer and is not to be considered as a general treatise. I n this work the LEED method has been used singly and in combination with other techniques. A discussion of the results of other workers is confined to cases in which there is a direct bearing on the results being presented. Brief accounts are given of the various types of experiments which have been performed to emphasize the range of possibilities considered thus far, the requirements which must be met, and the possibilities for future application.

11. The Clean Surface Since unknown contaminants in the form of stable chemical compounds and chemisorbed gases exist on surfaces subsequent to the most careful chemical cleaning, the production of atomically clean surfaces requires additional treatment. Methods which have been found to be effective are : (1) Exposure of the contaminated surface t o a suitable gaseous ambient in the proper temperature range for the promotion of a chemicd reaction whose product can be removed by heating. This method requires a knowledge of the contaminant and the existence of suitable reaction products and is therefore limited in its application. (2) Prolonged heating in an ultrahigh vacuum. This method is suitable for refractory materials of very high purity which require the removal of adsorbed and absorbed gases only. However, small amounts of impurity such as carbon may diffuse to and remain on the surface, thus increasing the surface contamination density. For example, it has been observed that carbon cannot be removed from nickel by heating in vacuum below the melting point. This method is obviously not suitable for solids having low melting points of a few hundred degrees centrigrade. Thermal etching may produce a roughened surface which is objectionable. (3) Production of a fresh surface in an ultrahigh vacuum by cleaving a single crystal. This method is limited to single crystals which permit cleaving in vacuum and is thus applicable only for certain crystal faces and to a relatively small number of cases. (4) Removal of contamination by ion bombardment with inert gas (usually argon) ions, followed by annealing ( I ) . This method can be applied to either single or polycrystalline materials and has been found

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

33

to be effective for materials with low melting points (below SOOOC), as well as for more refractory materials. It has also been found to be effective in removing a monolayer of carbon from nickel and silicon crystals. Because lattice defects are produced by ion bombardment, the conditions of bombardment and subsequent annealing are important and vary markedly with the material. Many precautions must be observed to insure that a clean surface is obtained. Although the voltage and current should be kept low to prevent excessive damage to the surface, they must be sufficient to overcome possible recontamination during the bombardment process. For example, some of the electrons formed in the discharge strike the anode and liberate gas atoms which may recontaminate the surface to be cleaned. The argon pressure should be kept low (less than 1 p), for a long mean free path, to prevent sputtered contamination from returning to the cleaned surface. Care must be taken if an electron beam is used to maintain the discharge. If there are electrically insulated metal surfaces surrounding the discharge region, these can become negatively charged by the ionizing electron beam and result in back sputtering from this film to the cleaned surface. To avoid this condition, the accelerating voltage in the electron gun should be kept below the sputtering threshold of the material in question or the insulated surfaces must be replaced by a conducting shield at the anode potential. When proper conditions are maintained, the ion bombardment method does produce clean surfaces except for the argon imbedded in the surface. This and the bombardment damage are removed by proper annealing. The amount of annealing required depends on the application. For example, the minority carrier lifetime of a semiconducting crystal (2) is much more sensitive to a low defect density than the work function, and hence requires more careful annealing of the crystal. I n an alternative method of minimizing these objectionable conditions, the discharge is confined in a separate envelope and a directed beam of ions is caused to strike the crystal by suitable collimation (3).

Ill. The Low-Energy Electron Diffraction (LEED) and Work-Function Method This method is suited to the investigation of the structure of surface monolayers of one or more components as well as the detection of a small fraction of a monolayer of contamination. This applicability is due to the extremely low penetrating power of electrons having energies of a few, to a few hundred, electron volts and also to the fact that the magnitude of the associated wavelengths are suitable for diffraction from the lattice grating of crystalline solids.

34

H. E. FARNSWORTH

A quantitative measurement of the thickness of the surface layer contributing to low energy diffraction was made by the writer by depositing silver vapor onto a gold crystal surface, using a calibrated silver source (4).Because the lattice structures are the same and the lattice constants differ by less than 0.4% the silver was found to deposit as a thin crystal on the gold surface. Because of the different indices of refraction and certain fine-structure characteristics for the two metals, the diffraction beams from silver and gold were readily distinguished, The results show that, for primary energies of 200 ev, the first monolayer contributes approximately 50% of the diffracted intensity and the first two monolayers contribute 90%. At 50 ev, the first monolayer contributes more than 75y0. Although gases have a lower atomic number than silver, and hence a lower scattering power, it is possible, nevertheless, to measure easily the diffraction pattern characteristic of a single gas monolayer (except hydrogen) on the surface of a solid if the gas atoms occupy a lattice somewhat different from that of the solid, as is usually the case ( 5 ) . If the gas atoms in the surface monolayer occupy the same lattice as that of the supporting crystal, it is possible to detect the presence of the gas atoms if the distance between the surface monolayer and the supporting atomic plane differs from that between two adjacent atomic planes (in the supporting solid) having the same Miller indices. This is true for chemisorbed oxygen and nitrogen on a (0001) titanium surface ( 6 ) .The presence of surface gas may be detected in amounts as small as several per cent of a single monolayer. When two or more monolayers of gas are adsorbed, the outer ones are amorphous and may completely prevent the observation of a diffraction pattern from the underlying crystal. Hence, in general, it is necessary to partially clean the solid surface by heat treatment or other means in high vacuum before any diffraction pattern can be detected. For example, for a copper crystal, one cannot obtain any diffraction pattern after the usual baking operation of the whole tube a t 300-400°C. For some crystals, even the first monolayer of gas has an amorphous structure. I n this case, the only effect of the adsorbed gas is to decrease the intensity of the diffraction beams from the supporting crystal. The low penetrating powers of the primary electrons result in an additional advantage of this method. The predominating effect of the surface monolayer of atoms causes the diffraction beams to grow and decay, as the wavelength is varied, in accord with the surface grating equation; thus a determination of the atomic plane forming the true surface can be made (7). Although the applicability of the low-energy diffraction method t o

35

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

the study of surface reactions was realized a t the time of the discovery of electron diffraction, few attempts by investigators other than the writer have been made to use it in the study of surface reactions, until recently. Although the LEED method reveals lattice structures, it does not readily identify the type of atom on the surface. Thus, by combining photoelectric work function and LEED measurements in a single experimental tube, more complete information on the mechanism of the various stages of interaction processes can be obtained since changes in work function occur when electronegative atoms penetrate a metal surface. I n the investigations to date the specimens under investigation have been in the form of opaque single crystals. A primary beam of electrons having small cross section strikes the crystal at normal or near-normal incidence and the back-reflection diffraction pattern is observed by means of a movable electron collector or a fluorescent screen. A diagram of an apparatus of the first type with provision for measuring work function is shown in Fig. 1. Some of the diffracted electrons enter a small opening (1 mm or less) in a double-walled Faraday collector which can be rotated by a magnetic control about an axis lying in the face of the crystal and perpendicular to the incident beam. The crystal can be m

i FIG.1 . Diagram of experimental tube. F is the gun filament; C is the collimator of the gun; M is the crystal mounting; D is the shielding drum; E is the electron collector; L is the direction of the incident ultraviolet light; W is a silica window attached to the Pyrex envelope with a graded seal. [From Farnsworth and Madden (27).]

36

H. E. FARNSWORTH

rotated about an axis coinciding with that of the incident electron beam. Thus by properly adjusting the crystal for the desired azimuth, the diffraction pattern is explored by measuring the electron current t o the inner Faraday box as a function of its angular position. This procedure is repeated for a series of different primary voltages separated by small intervals to obtain the complete diffraction pattern in a given voltage range. Only the elastically scattered electrons are measured, since a retarding potential is placed on the inner collector. To prevent contamination of the crystal surface, the cathode is mounted off the axis of the gun, and the electron path is deflected by an electric field. The external control and recording circuits have been developed t o permit rapid semiautomatic operation in the following manner. A small well-shielded permanent horseshoe magnet acts on the nickel bar through a side arm on the tube envelope. This nickel bar is directly coupled to the Faraday collector by a small vertical molybdenum rod so that the collector may be rotated about a vertical axis passing through the face of the crystal and the incident beam of electrons. The magnet is enclosed in a magnetic shield to prevent interaction of its field with the incident or diffracted electrons, and the earth’s field is compensated by Helmholtz coils. The magnet is mounted so that it may be rotated by means of a belt-and-pulley arrangement driven by a n elcctric motor which is similar to that used to drive the pen of a Varian recorder. The control mechanism of this motor is synchronized with that of a high-speed Brown recorder so that the horizontal pen motion of the recorder is coupled to the angular motion of the Faraday collector. The current of the Paraday collector enters the input of a vibrating-reed electrometer, the output of which is traced on the recorder chart. Hence, the recorder furnishes a trace of diffracted beam intensity as a function of colatitude angle. Typical recorder tracings are reproduced in Fig. 2. The fast response of the recorder makes it possible t o obtain a complete colatitude curve in 30 sec. After each curve is completed, the collector returns automatically and rapidly to its initial position. The primary voltage may be adjusted manually between traces of colatitude curves. With this arrangement a complete diffraction pattern in several azimuths over a considerable range in voltage may be obtained in a few hours. I n the apparatus of the second type, in which the diffraction pattern is observed on a fluorescent screen, the inelastically scattered electrons are excluded by a grid placed between the inner grid and the fluorescent screen (8). This method is being used in certain qualitative investigations in this laboratory but it does not furnish quantitative measurements of intensity which compare with those of the electrical method.

37

THE CLEAK SINQLE-CRYSTAL-SURFACE APPROACH

‘2

24

AFTER HEATING

CLEAN (110) NICKEL ( f l 0 ) AZIMUTH

21

-

74

OXIDIZED SURFACE

v

OF

( n o ) NICKEL

TO

2OO0C

(001)AZMUTH 72 V

18

15

8

i

12

X H c 0

8

9

8

6

3

4= 80

COLATITUDE ANGLE

60 COLATITLIDE ANGLE

80

FIG.2. Automatic recorder tracings of representative diffraction beams. Numbers on curves indicate orders of diffraction. [From Farnsworth (29).]

38

€I E..FARNSWORTH

A combination of the two methods in a single apparatus, or the equivalent, has some advantages.

IV. Vacuum Conditions Because of the chemical affinity of many gases for most solid surfaces, it is necessary to carry out the experiments in a vacuum usually referred to as ultrahigh vacuum, indicating a pressure less than lo-!’ Torr (mm Hg). This requirement has led many members of the younger generation to conclude incorrectly that only recently developed ultrahigh vacuum techniques, usually associated with the period since the development of the Baird-Alpert (BL) ion gauge, are adequate for this work. The techniques for obtaining pressures less than lo-!’ Torr were known as long ago as 1920 when mercury vapor pumps, getters, and liquid nitrogen traps and baking procedures were used. Although the gauges available then did not give an accurate measure of the pressure, it is now known that the procedures which were formerly found necessary to maintain a clean stable surface, as indicated by secondary electron emission and photoelectric characteristics, and a few years later also by LEED, are the same as those which now result in a pressure less than Torr. Thus the early results obtained by the writer on secondary electron emission arid LEED are characteristic of an approximately clean surface since the metals in use could be effectively cleaned by heat treatment as shown by LEED.

V. Results A. STRUCTURES OF CLEAN SURFACES 1 , Metals

For the metals copper, silver, gold, titanium, and nickel, the arrangement and spacing of the atoms in the surface monolayer is found to be approximately the same (within experimental error) as those for a similar plane in the interior of the crystal as determined by X-rays. Difficulties occur in attempting to evaluate the distances between atoms in a direction perpendicular to the surface. This is due to the fact that both the unknown inner potential and distance between atoms are involved. No information is available on the shape of the inner potential curve as one passes across the surface of thc metal. Many observations on the above metals have shown a complexity in the diffracted intensity as a function of wavelength or primary voltage which takes the form of

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

39

multiple peaks or fine structure. Laschkarew (9) suggested that this fine structure could be accounted for by multiple reflections from two or more planes within the crystal. Detailed computations in this laboratory failed to account for the results on this basis (10). More recently, MacRae and Germer ( 1 1 ) have reported observations for nickel which they interpret as due to a larger spacing between the first and second monolayers than the bulk value. According to their interpretation, each diffracted beam consists of three components or separate maxima when the beam intensity is plotted against voltage. The highest-energy component is associated with diffraction from lattice steps in the crystal surface which is not affected by the presence of an inner potential. The middle component is associated with diffraction from atoms below the surface of the crystal where the inner potential is effective and the atomic spacing has the bulk value. The difference in voltages of these two beams then gives the value of the inner potential. Using this value of inner potential and the wavelength for the lowest voltage beam, a depth spacing for this beam is computed. This value is obviously greater than the bulk value and is associated with the distance between the first and second monolayers. For this interpretation to have any significance, the following conditions must be satisfied: (1) all observed beams must have three components; ( 2 ) the values of inner potential obtained from observations on the corresponding diffraction beams must be the same within the experimental error; (3) the method should apply to all crystals. None of these conditions has been established by Germer and MacRae. They report a greater variation in values of inner potential than the experimental error and have reported observations for the ( 1 1 1 ) face of nickel only. The following statement (12) has been made. “An increase in the spacing between the surface atoms and atoms of the second plane of about the same amount has been observed also for other planes (than 11 1) of nickel.” However, the reference for this statement is the following (11).“The interpretation of the maxima from the Ni (110) face is not as obvious as is the interpretation in the case of the Ni (111) face. Resolution of the broad peaks into triplets is not observed in this case.” Although a value of inner potential of 16 volts was used in evaluating the increase in lattice spacing for the (111) face, a zero value of inner potential appears to have been used in considering the ( 1 10) face. Numerous observations in this laboratory on several crystals do not confirm any of these conditions. For (100) copper (13)the effective inner potential was found to vary from 6 volts a t 20 primary volts to 28 volts at 200 primary volts. For silver and gold (14) the diffracted beam structures were much more complex than for copper, resulting in complex fine structures which were very sensitive to small changes in

40

H. E. FARNSWORTH

angle of incidence. No well behaved value of inner potential could be found. Since the lattice structures are the same for silver and gold and the spacings are nearly identical, one must conclude that the differences in the two patterns must result from the different charge distributions. For a nickel crystal we do not confirm the presence of three components for each diffraction beam. It is clear that the concept of a larger spacing between the first and second monolayers has not been established at the present time. 2. Semiconductors

In the case of semiconductors, it was first shown in this laboratory that the arrangements of the atoms in the surface monolayers of (100) and ( 1 1 1 ) germanium and silicon are not the same as those for these planes in the bulk (15). The altered arrangements were revealed by the presence of fractional order beams for the surface gratings in certain azimuths. This was later found to be the case for all crystals tested which have a diamond-type lattice, including semiconducting diamond and several of the intermetallic compounds. The surface structure of silicon was observed to be much more complex than that of germanium. I n some azimuths, several fractional orders less than one-half were observed. The surface structure of diamond is of interest because of evidence that it should differ from that of other materials having similar lattice structures. Wolff and Broder (16),who found the microcleavage characteristics of Ge and Si to be different from those of diamond, attributed this difference to a change in structure in surface layers on cleaving of Ge and Si but not of diamond. From a consideration of the energetics of bonding, Green and Seiwatz (17)concluded that little or no rearrangement of the type found for Ge and Si should occur on (100) diamond. The success of an experimental test depends on one's ability t o obtain an atomically clean diamond surface. Although diffraction patterns characteristic of a diamond-type lattice have been obtained after mild heat treatment (18) (as little as 300" baking), previous experience with Ge and Si had shown that this condition was not a proper criterion of a clean surface. Further cleaning by thermal outgassing or ion bombardment and annealing was unsuccessful. A phase change from diamond to amorphous carbon was found to occur a t the surface when the crystal was heated in vacuum a t temperatures above about 450°C, thus causing an extinction of the diffraction pattern. This carbon could be removed by oxidation or by heating in hydrogen a t the proper pressure and temperature. Under these conditions it is presumed that carbon is removed as a hydrocarbon as rapidly as it is formed. If the crystal is

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

41

cooled in hydrogen, a clean surface is believed to result since the intensities of the integral-order diffraction beams are increased by a factor of 20 and very weak half-integral order beams appear in the same azimuths as in the cases of (100) and ( 1 1 1 ) Ge and Si. These results indicate a displacement of surface atoms for diamond similar to those for Ge and Si but of smaller magnitude. Some intermetallic compounds having the diamond or zinc blende structures, such as GaSb and InSb show an altered surface structure (19) as indicated by the presence of fractional order diffraction beams after cleaning by the argon-ion bombardment and annealing technique. For crystals of this type there is asymmetry in opposite directions perpendicular to (111) planes. This asymmetry results in atoms of type A on one surface and those of type B on the opposite surface (20).The possibility of detecting effects of the asymmetry of clean surfaces by electron diffraction has been considered. However, in the cases of GaSb and InSb, no difference in the diffraction patterns from these two surfaces has been detected. A hexagonal crystal such as bismuth telluride shows no change in surface structure (21) of the (0001) face due to cut bonds. This is to be expected if cleavage occurs between weakly bonded leaves perpendicular to the z-axis. Several suggested surface atomic arrangements have been proposed to account for the observations of fractional order beams in the diffraction pattern (15, 1 6 , 1 9 ) . All of these involve slight displacements of surface atoms in or perpendicular to the surface plane. Lander and Morrison (22) have recently repeated several of the above experiments on germanium and silicon with similar results. However, they have attempted to explain these observations and the observed intensity changes with changes in voltage by means of a model which requires many simplifying assumptions. We have attempted without success to apply their methods to results obtained in this laboratory. Their model requires relatively large changes in positions of surface atoms as well as appreciable changes in atomic positions in several of the underlying atomic planes with simplifying assumptions concerning inner potential and shadowing effects. Before the results of their computations can be accepted, it must be shown that the method is applicable to several crystals including metals. It is of considerable interest to learn if the surface structure of a cleaved crystal is similar to that of one cleaned by ion bombardment and annealing. An experiment of this type was first performed by Haneman (21) who made observations on the (0001) face of bismuth telluride. The results for the surfaces prepared by the two methods were

42

H. E. BARNSWORTH

in agreement. However, as noted above, no deviations from the normal structure were observed for this crystal face. Hence a more significant experiment has been performed by comparing (111) surfaces of germanium prepared by the two methods (15, 23). Although fractional order beams were observed for the cleaved ( 1 11) surface, there were appreciable differences in the intensity distributions for the two surfaces. Furthermore, pronounced changes due to annealing of the cleaved surface occurred, suggesting that the surface atoms were not in their equilibrium positions after cleaving at room temperature. It was also noted that the surface produced by ion bombardment and annealing was a much closer approximation to an ideal surface with one set of (1 11) surface planes forming the geometric surface than was the cleaved surface, Lander et al. (24) have recently repeated the cleavage experiment for silicon and have obtained similar results. They observed similar changes due to annealing of the cleaved crystal and noted that the observations after annealing agreed with those for the ion bombarded and annealed surface. These experiments raise considerable doubt concerning the significance of experimental results obtained on unannealed cleaved surfaces. In the light of the above observations on clean surfaces, it is of interest to determine possible variations of work function for various single crystal faces of semiconductors. Since the conventional method of measuring the photoelectric work function of a metal by the Fowler method does not apply to semiconductors, great care must be used in applying the contact potential method with the use of a reference metal, Results have been obtained in this laboratory (25) by combining contact potential and work-function measurements of the reference metal in a single experimental tube to avoid spurious changes of the reference and to permit determinations of absolute rather than relative values. Work-function values for atomically clean (loo), (1 1l),and (110) germanium surfaces were found to be the same within experimental error. For silicon, the values were in the sequence (100)> (1 10) > (111).

B. SURFACE REACTIONS The term “surface reactions” is used to include the interaction of one or more types of foreign atoms with or a t the surface atoms of the solid substrate. This includes epitaxy, adsorption, place exchange, oxidation, and catalysis. 1. Epitaxy of Copper on Titanium Using low energy electron diffraction, the arrangement of copper

THE CLEAN SINGLE-CRYSTAL-SURFACE

APPROACH

43

atoms on an atomically-clean (0001) titanium surface was determined for a caIibrated average thickness of 0.5 to 25 atomic layers (26). The copper formed at nucleation centers in the form of discrete oriented crystallites with a lattice spacing parallel to the substrate which was the same as that of the (111) plane in bulk copper, and with the (110) azimuth of copper parallel t o the (1 100) azimuth of the titanium crystal. No variation of lattice spacing with thickness was detected. Deposition of copper on a titanium surface, covered with a chemisorbed layer of oxygen, resulted in the formation of disoriented deposits of nucleation centers. These results show that the copper atoms diffuse over the titanium surface near room temperature to nucleation centers where they form in the normal copper lattice structure. Even though the substrate determines the orientation of the crystallites, there is no evidence that the distances between the copper atoms is affected by a different lattice spacing of the titanium even for a fraction of a monolayer. The pronounced effect of a small amount of contamination is significant and must be considered in all experiments on epitaxy. 2 . Adsorption, Place Exchange, and Oxidation

a. Metals. I n the interactions of gases, and particularly oxygen, with metal surfaces, one or more of the above three processes may be observed in order of increasing exposures (pressure x time). I n the initial process of adsorption, the gas, if diatomic, may remain on the surface in the form of (1) one or more layers in an amorphous molecular form, (2) a monolayer or less in which dissociated atoms occupy a two-dimensional lattice with the same atomic arrangement and spacing as those of the adsorbate or (3) with some larger spacing which is a simple multiple of this spacing. ( i ) Oxygen and nitrogen on (0001) titanium. An example of ( 2 ) is observed when the (0001) surface of a titanium crystal is exposed to nitrogen or oxygen ( 6 ) .I n both of these cases the arrangement and spacings of the gas atoms are approximately the same as those of the supporting (0001) titanium surface. However, the ability to detect the presence of the adsorbed gas is due to the fact that the interplanar spacing perpendicular to the surface or the inner potential or both have been altered as shown by changes in the curves of intensity vs voltage for the corresponding diffraction beams. These surface structures are closely related to those for T i 0 and TiN if one compares the chemisorbed monolayers with the (111) planes of T i 0 and TiN cubic crystals, respectively. (ii)Oxygen o n (100) and (110) nickel. An excellent example of all stages in the interaction process is found in the exposure of oxygen to

44

R. 1. FARNSWORTH

a (100) nickel crystal surface (27). For this work the photoelectric work function and electron diffraction measurements are combined in a single experimental tube. The results indicate that initial adsorption at room temperature occurs in an amorphous molecular form and that some of the adsorbed molecules diffuse over the surface to lattice defect sites where they dissociate into atoms which form in a double-spaced, facecentered or (2 x 2)c lattice structure on or in the surface of the adjacent nickel lattice. With continued exposure to oxygen, or with elapsed time under certain conditions, a pattern characteristic of a single-spaced, simple square structure (1 x 2) is observed with a slight increase in lattice constant. It will be shown below that this observation, when combined with others, indicates a replacement of certain nickel atoms by oxygen atoms although earlier observations of this structure (28) were interpreted as due t o a chemisorbed monolayer of oxygen on the surface in the form of a

LOG, PRESSURE x TIME- IN MM HG-MIN FIG.3. Peak current versus loglo(pressure x time) for five diffraction beams obtained after an intermediate anneal. Oxygen exposure occurred at 26OC. Curve 1: Typical beam from clean nickel in the (lT0) azimuth a t about 28 volts. Curve 2: Typical beam from clean nickel in the (001) azimuth at about 58 volts. Curve 3: Typical beam from 8 double-spaced, face-centered lattice of chemisorbed oxygen in the (110) azimuth at about 17 volts. Curve 4 : Typical beam from a ass lattice in the (001) azimuth at about 27 volts. Curve 5 : Typical beam from a NiO lattioe in the (110) azimuth at about 22 volts. [From Farnsworth and Tuul ( 2 8 ) . ]

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

45

( a ) Molecular adsorption and surface diffusion of oxygen: The evidence for molecular adsorption and surface diffusion at room temperature is obtained from the following observations (27, 29). (1) As oxygen adsorbs on the nickel surface, the intensity of the diffraction pattern from the nickel lattice decreases because of the low penetrating power of the electrons. This rate of decrease with increasing exposure is approximately the same for an ion-bombarded surface which has been well annealed as for one which has received a small anneal. However, the diffraction patterns from the gas-lattice structures on the surface with the small anneal are much more intense than those from the gas-lattice structures on the well-annealed surface because of the different defect densities in the two cases. If the extinction of the pattern from nickel were caused by the presence of the gas-lattice structures, one should expect a greater rate of extinction for the surface having a small anneal. Since this is not the case, the major p r t of the ex200

I50 E

f

0

G 100 0 C

.+ u

e

) .

U %

r 0

a" 50

Log,, pressure x time in mm Hg-min

FIQ.4. Curves similar to those for Fig. 3, obtained at room temperature after oxygen exposure et 16OOC. [From Farnsworth and Tuul (28).]

46

H. E. FARNSWORTH

tinction is the result of an amorphous structure on the surface which is independent of the defect density. (2) Intensities of the diffraction patterns from gas lattices formed a t 150" C are much greater than those formed a t 25OC. However, the rates of extinction of diffraction patterns from the nickel lattices &reapproximately the same in the two cases. The conclusion from these results agrees with that in (1) above, and the relevant curves are shown in Figs. 3 and 4.

;

Set I

200

c

S

150

i6

I

Il-1

.-+

e 0

'0 r

x

B

Set 3

IOC

I I

I\ t

5c

2

u

-4

-3 Log,,(

pressure x time) in mm Hg-min

FIG.5. Curves showing changes in beam intensity (indicated by the dashed lines) caused by allowing the crystal to remain in vacuum soveral hours subsequcnt to various exposures of oxygen. Multiply ordinate scales by 2 to obtain intensities of curves B. Curves A : A 17-volt diffraction beam in the (110) azimuth from a double-spaced, facecentcrod structure. Ciirves B : A 27-volt diffraction beam in the (001) azimuth from a singlc-spaced, simple-square structure. Cwve C in set 3: A 22-volt diffraction beam in the (110) azimuth from a NiO structure. [From Farnsworth and Madden (27).]

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

47

(3) Surface diffusion is indicated by changes which occur in diffracted beam intensity with elapsed time in vacuum subsequent to various oxygen exposures. Figure 5 shows several plots of peak diffraction beam current as a function of log of exposure. The curves in sets 1 and 2 show increases, I, in beam intensities of gas structures which have occurred as a result of conversion of the adsorbed amorphous molecular oxygen into atomic oxygen in a lattice structure. The curves in set 3 show two increases and two decreases in intensity, marked 11,I,, and 0,) D,respectively. The decrease, 0,) and the increase, 11, represent a conversion from one lattice structure t o another. The decrease D , and increase I represent a conversion from the place exchange structure (discussed below) to the oxide structure. Tests were made which show that these changes were not the result of adsorption from the residual ambient.

,

-6 -5 Log,o pressure x time in mm Hg-min

-4

FIG.6. Peak current versus log,, (pressure x time) for the five diffraction beams after ion bombardment of the crystal surface followed by a small anneal. [From Farnsworth and Tuul (ZS).]

48

H. E. FARNSWORTH

Log,, pmsure x time in mm Hg-min

FIO.7. Curves, similar to those for Fig. 6, obtained after a more complete anneal. [From Farnsworth and Tuul (28).]

( b ) Injiuence of lattice defect density: The lattice structures due t o oxygen are much more pronounced for a surface which has received only ; Ismall anneal subsequent t o ion bombardment. Figures 6 and 7 show diffraction beams characteristic of three different oxygen structures for two surfaces having different defect densities. The curves for the small anneal, and hence large defect density, are much more intense than those for the more complete anneal, and hence smaller defect density. Thus the oxygen lattice structures on the most perfect parts of the crystal surface are catalyzed by the lattice defects. ( c ) Place exchange of oxygen and nickel atoms*: The evidence for replacement of some of the surface nickel atoms by oxygen atoms is con*Additional evidence for t,he model of Ni,O, discussed in this section, haa now been obtained for the interaction of oxygen with a ( 1 1 1 ) nickel surface [Park, R. L. and Farnsworth, H. E., Appl. Phys. Letters 3, 167 (1963)l. These results are in agreement with the proposed model for Ni,O.

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

49

tained in curves shown in Figs. 8-10. The intense maximum, curve 4, and the maxima in the curves 1 and 2, Fig. 8, are characteristic of the exchange structure. Curve 3 in Fig. 9 shows how the photoelectric work function, 'p, of the crystal changes with exposure to oxygen over the range of exposures shown in Fig. 8. The initial rapid increase in 'p resulting from adsorption, to produce curve 3 in Fig. 8, is followed by a broad 0

0 0

-5

-1OOAZ

-4 Log,,(pressure x time) in mm Hg-min

FIG.8. Diffraction beam intensities as a function of oxygen exposure obtained after 8 small anneal of the crystal subsequent to ion-bombardment cleaning. Curve I: Typical beam, in the (110) azimuth at about 28 volts, from the clean nickel lattice. Multiply the ordinate scale by 2 t o obtain intensity. Curve 2 : Typical beam, in the (001) azimuth a t about 58 volts, from the clean nickel lattice. Multiply the ordinate scale by 6. Curve 3 : Typical beam, in the (1TO) azimuth at about 17 volts, from a double-spaced, face-centered lattice. Curve 4 : Typical beam, in the (001) azimuth at about 27 volts, from a single-spaced, simple-square lattice. Multiply the ordinate scale by 2 . Curve 5 : Typical beam, in the (1.10) azimuth at about 22 volts, from a nickel oxide lattice. [From Farnsworth and Madden (27).]

60

R. E. FARNSWORTH I

I

6.0 a > l

.-c .-6 5.5

g 1

5-8 Fi

5.0 f

4.5

0

C

I

-5

^

-

v I

-4 Log,,(pressure x time) in mrn Hg-min

Fra. 9. Cuwes 1 and 2: Peak voltage for beam from nickel lattice versus log,, exposure (small anneal). Curve 3: Work function versus log,, exposure (small anneal). [From Farnsworth and Madden (27).]

maximum and a subsequent rapid decrease. Although it is impossible to determine the exact exposure at which ‘p begins to decrease, this decrease occurs a t exposures too small for the formation of the oxide, as revealed by curve 3, Fig. 9. The results of more precise observations over a small range of exposure, in the region just preceding oxide formation are shown in Fig. 10. It is seen that ‘p is decreasing in the exposure range of the intense single-spaced, simple-square structure which just precedes the formation of the oxide structure. This suggests that oxygen has penetrated the nickel lattice in the exposure range just preceding oxide formation and that the intense single-spaced, simple-square structure is the result of this penetration. The presence of the maxima in curves 1 and 2 of Fig. 8, also indicates an exchange structure, as ex-

61

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

R

200

5.5

c

c

F

>

2 150

W

.-C

c 0 .c

-$ 0

>

5.0

.-0 c

W -0 0"

E

4.5 a

4.0

Log,o( pressure x time) in mm Hg-min

FIG.10. Curve 1: Work function versus log,, exposure. Curve 2: Intensity of 27-volt beam from single-spaced, simpled-square structure as a function of iog,, exposure. Multiply the ordinate scale by 4. Curve 3: Intensity of 22-volt beam from 8 nickel-oxide lattice as a function of log,, exposure. [From Farnsworth and Madden (27).]

plained below. Additional evidence for this penetration is furnished by the observation that the lattice constant of this structure is 2-5% greater than that of clean nickel. Curves 1 and 2 of Fig. 9 show how the voltages of two nickel-lattice beams decrease with exposure and indicate a change in inner potential or an increase in the depth spacing when the exchange structure is formed. The selection of a model for an exchange structure of nickel and oxygen depends to some extent on the relative effective sizes of nickel and oxygen atoms in the surface monolayer. These are probably not the same as those in the interior of a nickel oxide lattice since the bonding and electron transfer are obviously different because of the difference in

52

R. 1. FARNSWORTH

the number of nearest neighbors in the two cases, It is doubtful that a hard sphere model is applicable for the surface of a nickel crystal on which the surface monolayer is composed of both nickel and oxygen atoms. It should be noted that the photoelectric data for such a surface fit the Fowler curve for a metal while this is not true for the surface after the semiconducting oxide forms. I n the conventional picture of the NiO, where the oxygen exists as Oe- (diam. 2.80 A) and the nickel as Ni'+ (diam 1.44 A), the size of the 0 3 - is considerably greater than that of the NiB+. The following observations are in agreement with the model shown in Fig. 1l a for which every other row of nickel atoms in the (001) direction is replaced by oxygen atoms and each surface oxygen atom ha8 8 nearest neighbors of nickel atoms. (1) The No. 4 maximum in Figs. 3, 4, and 6-8 is expected if one assumes that the scattering power of nickel is greater than that of oxygen. This maximum is then due to the difference in Scattering powers of two types of atoms as in the case of x-ray diffraction from rock salt. (2) The maxima in curves 1 and 2 of Fig. 8 result in the in-phase scattering from the nickel and oxygen atoms in a manner analogous to the case of rock salt. In the above model, the minimum distance between nickel and oxygen atoms is only slightly greater than that of the Ni-Ni distance of 2.49 A in pure nickel. Since each surface oxygen atom has 8 nearest neighbors of nickel atoms, it appears improbable that the average diameter of these nickel atoms differs greatly from that of the Ni atom in pure nickel. Hence, the effective diameter of oxygen may not exceed appreciably that of the normal nickel diameter, i.e., the oxygen may not have the same ionic character as it does in NiO. This is consistent with the observation mentioned above that the exchange structure has the photoelectric characteristics of a metal rather than the semiconductor characteristics of NiO. Additional evidence for the above model is furnished by recent observations of Alessandrini and Freedman (30).In their experiments, forbidden 100 and 110 reflections from an expanded nickel lattice (parameter 3 . 6 0 8 ) were observed in a nickel-oxygen film by high energy electron diffraction. This result suggests that the plane lattice in Fig. I l a is the (100) face of a three-dimensional structure, in which a cube is constructed with surfaces similar to Fig. 11c. The arrangement of atoms in the (110) surface plane is shown in Figs, 1 l b and 12a. This is exactly the arrangement which has been found independently to account for LEED observations from an exchange structure of oxygen in a (110) face of nickel (29,31).The arrangement of atoms in the (111) surface

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

(a)

FIG.11.

(b)

53

(C 1

0 atoms (open circles) and Ni atoms (solid circles) on the (100) face. (b) Relative positions of 0 atoms (open circles) and Ni atoms (solid circles) on the (110) face. (0) Ni,O lattice with faces similar to (a). [From Farnsworth (a) Relative positions of

(304.1

FIG.12. (a) Ni,O lattice with (110) face exposed. Open circles indicate positions of 0 atoms. Solid circles indicate positions of Ni atoms. (b) Ni,O lattice with (111) face exposed. Open circles indicate positions of 0 atoms. Solid circles indicate positions of Ni atoms. [From Farnsworth (~OU).]

plane is shown in Fig. 12b. It is seen that a double spaced surface lattice occurs in all azimuths which would result in patterns in disagreement with observations which have been interpreted as due to a surface layer of chemisorbed oxygen (32). From Fig. l l a it is seen that one-half of the nickel atoms in alternate (100) planes are replaced by oxygen atoms and the remaining alternate (100) planes contain nickel atoms only. The oxygen atoms are located a t the corners of a simple cube so that each oxygen atom within the volume is surrounded by twelve nearest neighbors of nickel atoms. There are three times as many nickel atoms as oxygen atoms, thus forming Ni,O. Forbidden 100 and 110 x-ray reflections (but not forbidden 111 reflec-

54

H. E. FARNSWORTH

tions) are expected from this model. [All (111)planes contain three times as many nickel as oxygen atoms.] As mentioned previously the Ni,O structure has photoelectric characteristics of a metal rather than a semiconductor. The final location of the displaced nickel atoms when the exchange atructure is formed a t the surface is now evident. When one-half of the nickel atoms in the (100) surface monolayer are replaced by oxygen atoms, these replaced atoms should form on the surface into patches totaling one-half monolayer, since alternate (100) planes are composed entirely of nickel atoms. This does not mean that these surface nickel atoms are non reactive for further oxygen adsorption. The LEED observations of an expanded lattice of 2-5% for the exchange structure on the (100) face is in agreement with the observation of an expanded lattice of 2.3% by Alessandrini and Freedman. Similar reasoning concerning the location of displaced atoms applies to the (110) surface plane. Since all (111)planes contain the same number of oxygen atoms per unit area, the displaced nickel atoms should form Ni,O planes above the original surface plane. Observations on (110) nickel ( 3 1 , 3 3 )show the presence of a 1 x 3 structure which occurs between the 1 x 2 structure and N O . This 1 x 3 structure appears to consist of a surface exchange structure having two adjacent rows of oxygen atoms alternating with one row of nickel atoms. It is not known a t present if there is a three dimensional structure associated with this. (iii) CO o n (100) and (110) nickel. The adsorption of CO on a clean (100) surface is appreciably faster than the corresponding adsorption of Torr-min decreased the intensity of oxygen. A CO exposure of 3 x a 60-volt diffraction beam from nickel to about 20% of its original value. A weak half-integral order beam appeared in the (100) azimuth and did not become more intense with an exposure of lo-& Torr-min, thus suggesting a poorly formed simple-square structure. Subsequent to an additional exposure of about 3.5 x 10-4 Torr-min an additional diffraction beam appeared in the (110) azimuth which was not well defined in azimuth and hence was associated with a polycrystalline structure. Exposure of the (110) nickel surface to CO did not result in fractional order beams characteristic of an altered surface structure. However, a 1 x 1 structure was observed whose intensity distribution as a function of voltage was very different from that of nickel. Thus CO forms a (1 10) surface structure with the same lattice as that of nickel. I n principle, it should be possible to locate the positions of the carbon atoms from a detailed analysis of the intensity distribution versus voltage curves.

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

55

(iv) Comparison of published results on Ni-0 and Ni-CO. I n our results obtained prior to the simultaneous work function and LEED measurements, we interpreted the exchange structure in the (100) surface as a single-spaced, simple-square oxygen structure chemisorbed on the surface (28). Later publications by Germer and Hartman (34) have also interpreted this as a chemisorbed structure although we had previously published evidence that it is an exchange structure (27). It should be noted that the chemisorbed interpretation required the observation of diffraction from an oxygen lattice while Germer et al. (31)later state that diffraction from oxygen was not observed even though present in a (110) surface. Gerrner et al. (31) have reported that “carbon monoxide is adsorbed on (1 10) nickel, without rearrangement, in a structure which seems to be essentially amorphous.” The disagreement between their statement and the results obtained in this laboratory may be due to the fact that the most intense beams from the CO structure occur below 50 volts and apparently were missed by Germer and MacRae. At the higher voltages, the intensity of the pattern from the CO structure is much weaker than that from clean (110) nickel, and may have been erroneously interpreted as due to an “essentially amorphous structure.” However, the intensity distribution vs voltage curve is distinctly different from that for nickel, showing that the structure of CO is not amorphous. ( v ) Radioactive tracer method, CO on Ni. Although the applications of this method are limited to those gases which have suitable radioactive properties, the method can furnish quantitative information on the amount and coverage. By using this method in conjunction with ultrahigh vacuum techniques and single crystal surfaces of nickel, it became necessary to develop a technique which eliminates the requirement of removing the sample from the vacuum system for counting (35). This was done by constructing a bakable counter inside the adsorption chamber with a thin mica window which transmitted /Irays from 0 4 . Since the window would not withstand atmospheric pressure, the counter was initially evacuated with the chamber and subsequently filled with equal partial pressures of Freon-22 and argon at a total pressure of 2 cm Hg. Although these experiments were performed prior to the development of the argon-ion bombardment method of cleaning, they demonstrated the feasibility of applying the tracer method to adsorption measurements on atomically clean surfaces. b. Xemiconductors, Adsorption (i) Germanium and silicon. As mentioned above, the arrangement of atoms in the surface monolayers of these atomically clean elements weie found to be different from those of the corresponding planes in the

66

H. E. FARNSWORTH

bulk. The effects of oxygen exposure varied somewhat with the type of crystal and surface. For the (100) germanium surface, initial adsorption of oxygen a t low pressures extinguished all of the fractional order beams and some of the integral order beams which did not correspond to theoretical beams (15). The presence of the remaining intense integralorder beams indicates that the surface structure after adsorption was the same as the normal germanium structure and that the oxygen atoms satisfied the cut bonds a t the surface and restored the surface germanium atoms to their volume positions. The oxygen atoms also occupied the normal germanium lattice positions. Thus, this germanium surface, when contaminated with a surface monolayer of oxygen, reveals a surface diffraction pattern more like germanium than that of an atomically clean surface. Although Lander has repeated this experiment he has not reported this result for the (100) face. For the (111) surface of germanium, the integral order beams were weakened and occurred a t altered voltages, thus indicating a change in depth spacing, although the surface spacing remained the same. The behavior of the (1 10) face of germanium was similar to that of the (100) face. For the (100)silicon surface, an initial oxygen exposure extinguished all fractional order beams and weakened integral orders (36). The behavior of the (1 ll) silicon surface was similar to that of germanium. For all surfaces of both elements, continued oxygen exposure produced further extinction of all diffraction beams with no indication of a structure characteristic of an oxide. It is concluded that the oxides are amorphous and randomly arranged microcrystals. The rate of adsorption on the (100) Ge surface was found t o be 10-20 times greater than that on the (1 11) surface. The maximum value of sticking coefficient on (100) Ge was of the order of 2.4 x 10-3. The rate of adsorption and sticking coefficient on silicon depended on previous heat treatment of the crystal as well as the crystal face. After contaminating the above surfaces with oxygen, the clean surfaces could be regenerated by heating a t 500°C for Ge and 900°C for Si. From work function determinations it was concluded that oxygen diffuses into silicon (25). (ii) Diumond. The (100)and (111) surfaces (37)were observed t o be much more inert a t room temperature than those for Ge and Si. Oxygen exposures of 10-6 Torr-min to both faces had no appreciable effect on the diffraction pattern from a clean surface and exposures as large as 10-1 Torr-min produced decreases of the integral order beams of only 15 to 20%, with little change for exposures of 10 Torr-min. However, for the (1 11) face, the half-integral order beams were decreased as much as 40%, thus indicating an ordered structure for adsorbed oxygen on this face. This was not true for the (100) face, thus indicating a n

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

57

amorphous adsorbed layer. The sticking coefficients on both faces were less than a t room temperature. Observations a t elevated temperatures showed that an activation energy was required for adsorption and an ordered structure was observed for oxygen on both surfaces. The characteristics of CO, adsorption on the (100) surface at elevated temperatures were similar to those for 0, but were less pronounced. (iii) Intermetallic compounds. The effects of oxygen adsorption were not the same on GaSb and InSb surfaces (19). Exposures of both the (1 1 1 ) and (1 11) faces of InSb resulted in amorphous adsorption which caused a gradual decrease in intensity of both integral and half-integral order beams. The maximum value found for the sticking coefficient was about 10-5 for both the (1 11) and (1 1 I) surfaces and the clean surface pattern could be restored by heating at about 330°C. For GaSb, the sticking coefficient for 0, on the (111) or Ga face was found to be about 1 0 times the value for the (111) or Sb face. Secondary electron emission measurements indicated that multilayer adsorption of 0, and single layer adsorption of CO, occur on these surfaces. 3. Catalysis I n the consideration of some of the problems of catalysis as related to clean surfaces, two approaches have been considered in this laboratory: first, structure determinations by LEED subsequent to exposures to two different gases; second, examination by a mass spectrometer of reaction products formed a t the surface of a small catalyst which may be cleaned in ultrahigh vacuum and subjected to various treatments. a. Hydrogen-Oxygen Structure on Nickel. From a study of the electron diffraction structures on a clean crystal surface after exposure to two different gases, one may hope to learn something of fundamental significance concerning the mechanism of interaction. Although only a very small number of tests have been made, the value of this method is evident from the results obtained. Hydrogen cannot be detected directly because of its low scattering power but its presence may be observed by its effects on other structures. Thus, in the case of exposures of a (100) nickel surface to hydrogen and oxygen, i t was observed that the results were dependent on the order of exposure. No effect was observed when a n oxygen covered surface was exposed to hydrogen. However, when a hydrogen covered surface was exposed to oxygen, a new nickel hydride structure was observed. I n the first case, the bonding of the oxygen to the nickel prevents subsequent structure changes by hydrogen. b. Oxygen and CO on Nickel. It was shown above that, when a (1 10) nickel surface is exposed to oxygen, exchange structures having 1 x 2

58

R. E. FARNSWORTH

and 1 x 3 lattices are observed prior to the formation of NiO. As mentioned previously, exposure of clean (110) nickel to CO produces an ordered structure having the same surface lattice as that of nickel. However, if the clean (110) nickel is first exposed to oxygen to obtain a 1 x 2 structure and is then exposed to CO, a 1 x 3 structure is observed (33).This 1 x 3 structure is unstable and reverts to the 1 x 2 structure on remaining in vacuum for a few hours. No evidence of the dissociation of CO a t the surface was found. It is concluded that the oxygen in the CO enters into the 1 x 3 structure without dissociation. The position of the carbon in CO on the surface has not been determined thus far. The possibility of the formation and evaporation of C O , at the surface must be considered. The use of a mass spectrometer to examine the product leaving the surface is required to obtain this information. c. Hydrogenation of Ethylene on Nickel. By using a catalyst of small surface area, one may select more easily the type of catalyst and subject i t to various surface treatments. A mass spectrometer is suitable for monitoring the reaction velocity. The type of reaction chamber t o be used depends on the nature of the reactants. I n Fig. 13 is shown a reaction chamber which has been used for the hydrogenation of ethylene

Ki Soft iron

Top view

Side view of outgassing arm

and carriage

Shutter

FIG.13. Detailed construction of reaction chamber. [From Farnsworth and Woodcock (38)*1

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

59

a t the surface of catalysts in the form of thin sheets having a total surface area of 1.5 to 2.0 cm2. Contaminating effects were minimized by isolating the reaction chamber from the remainder of the system by cold traps and metal vacuum valves which could be baked. A magnetically operated carriage was used to transport the catalyst between position A, where it was cleaned by argon-ion bombardment, and position B, where i t was placed for activity determinations. During cleaning, the shutter could be moved into a position in the outgassing arm where it confined the sputtered film to the arm. Except for small tungsten wire hooks a t B, there was no metal in addition to the catalyst in the arm of the chamber where the reaction occurred. The horizontal arm containing position A, and metal films formed during cleaning, was rendered inactive by surrounding it with a dry ice-acetone bath during reaction runs. For the hydrogenation of ethylene, the activities of nickel and platinum were observed after various surface treatments of argon-ion bombardment, radiation cooling, and annealing (38). Activities for the ion bombarded surfaces were as great as 10 times those €or annealed surfaces while those for the radiation-cooled surfaces were 2-3 times the annealed values, thus indicating the role of lattice defects in this type of reaction. The activation energy for this reaction was determined as a function of surface treatment of nickel (39). A value of about 11.0 kcal/mole was observed for a surface which had been cleaned by argon-ion bombardment. Other treatments, such as outgassing and annealing in ultrahigh vacuum, outgassing and radiation cooling, argon-ion bombardment and annealing, resulted in values in the range 8-11.0 kcal/mole. Small amounts of surface contamination from the residual ambient resulted in lower activation energies, the lowest being 3.6 kcal/mole. These results suggest that some of the previously published data on activation energy may have been obtained on contaminated surfaces. A compensation effect was observed for ethylene hydrogenation in contact with nickel (40). A linear relationship was found between the logarithm of the frequency factor and the activation energy. The absolute rate constant and activation energy were only slightly dependent on the ethylene pressure in the range 0.02-21.7 Torr. d . Hydrogen-Deuterium Exchange on Nickel and Germanium. For reactions such as hydrogen-deuterium exchange, the above reaction chamber is unsuitable since the reaction on surface films, produced in the cleaning procedure, cannot be suppressed by cooling to the dry iceacetone temperature (41). A reaction chamber which was used to investigate the activity of germanium in the form of a single crystal as well as a sputtered film is shown in Fig. 14. The reaction chamber in the

60

H. E. FARNSWORTH

n

A

B

Fro. 14. Reaction chamber: M, magnetic controls; QV, glass valve; C, catalyst; E, electron gun;A and B,upper and lower catalyst supports, respectively. Inset shows detail of catalyst support. [From Shooter and Farmworth ( a l ) . ]

upper part of the figure could be separated from the cleaning chamber in the lower part by the nonlubricated ground glass valve, GV. After cleaning the sample in the lower chamber, it was transferred by magnetic control to the quartz support A. Thus, after withdrawing the carriage and closing the valve, no contaminating surfaces remained in the reaction chamber. The limit of sensitivity was determined by the leak rate of hydrogen through the valve. For the measurement of reactions a t B sputtered film surface, the experimental arrangement wa8 modified so that a sputtered film could be deposited on a glass surface in the lower compartment and was then transferred to the upper cham-

THE CLEAN SINGLE-CRYSTAL-SURFACE APPROACH

61

ber as in the above case. The results of these experiments showed no measurable activity of high purity clean (100) germanium crystal surfaces or of sputtered germanium surfaces for the hydrogen-deuterium exchange. e. Hydrogenation of Ethylene on Copper-Nickel Alloys. Hydrogenation of ethylene experiments with copper-nickel catalysts, each having a surface area 3.5 cm2, have shown a dependence of activity on argonion bombarding current used in cleaning the surface (42).The activities of the alloys increase initially with ion bombarding current, pass through a maximum and then decrease with further increase in current. The bombarding voltage of 500 and time of 10 min were kept constant. Maximum activities were observed for currents in the range 120-200 pa but none was observed for pure nickel. The activity of 60.5% (by weight) copper alloy was greater than that of pure nickel by a factor of I6 for bombardment current of 160 pa. The composition for maximum activity is probably between 60 and 80% copper. When the activated catalysts were annealed, the activity decreased a t a different rate for each catalyst. The critical temperature, below which no appreciable annealing occurred in 5 min, varied from about 500°C for a 21.4% copper alloy to about 300°C for 60.5 and 95% copper alloys. No critical temperature was found for pure nickel. These results have been interpreted to mean that defects resulting from bombardment of copper atoms which are nearest neighbors of nickel atoms produce the enhanced catalytic activity. This can result if preferential removal of copper atoms leaves the remaining nickel atoms in a more disordered state than that at the surface of pure nickel after similar ion bombardment because the lattice spacing in the alloy is not the same as that of pure nickel. Although annealing for short times subsequent to bombardment caused the activity to decrease abruptly a t a critical temperature, this effect was less pronounced with increase in annealing time. Previous observations (39)have shown that, during heating, copper diffuses to the surface of the alloy catalyst and reduces the activity. The abrupt decrease in activity a t a critical temperature for a short anneal suggests the presence of an additional mechanism, other than diffusion of copper to the surface, such as the annealing out of some active centers before appreciable diffusion of copper to the surface takes place. It is planned to examine the crystal of a Cu-Ni alloy single crystal, after various heat treatments, by low energy electron diffraction. It should be possible to determine if some of the activity of the surface is suppressed before a detectable fraction of a monolayer of copper appears on the surface. It does not appear probable that changes in surface area caused the

62

H.. E. FARNSWORTH

enhanced activity subsequent to ion bombardment because of the small amount of annealing required to decrease the activity and relatively low critical temperatures a t which the sudden changes of activities occurred. While changes in roughness undoubtedly occur with longer annealing times, comparisons with observations on diFusion and changes in roughness for pure nickel when heated for long periods of time in vacuum or in an ambient of hydrogen do not appear relevant. The concept of surface roughness, to which the BET method applies, loses its significance when roughness approaches atomic dimensions, since it merges with the concept of lattice defects at the surface. ACKNOWLEDGMENT Financial assistance for this work has been furnished by Research Corporation of New York, U.S. Office of Naval Research, Joint Services Contract with M.I.T. and Subcontract with Brown University, OfEce of Aerospaco Research, U.S.A.F., U.S. Army Electronics Research and Development Laboratory, U.S. Army Research Ofice (Durham), Advanced Research Projects Agency, National Science Foundation, and International Business Machines Corporation. Numerous present and former students and colleagues have made contributions as indicated in the references. REFERENOES

1. Farnsworth, H. E., Schlier, R. E., George, T. H., and Burger, R. M., J. Appl. Phyye. 29, 1150 (1968). 2. Madden, H. H., Jr., and Farnsworth, H. E., Phys. Rev. 112, 793 (1958). 3. Hagstrom, H. D., and D’Amico, C., J. Appl. Pkys. 31, 716 (1960). 4. Farnsworth, H. E., Phys. Rev. 49, 605 (1936). 5. Schlier, R. E., and Farnsworth, H. E., J. d p p l . Phy8. 25, 1333 (1954). 6. George, T. H., Farnsworth, H. E., and Schlier, R. E., J. Chem. Phys. 31, 89 (1959). 7. Farnsworth, H. E., Phy8. Rev. 49, 598 (1936). 8. Ehrenberg, W., Phil. Mag. [ 7 ] 18, 878 (1934); Scheibner, E. J., Germer, L. H., and Hartman, C. D., Rev. Sci. Instr. 31, 112 (1960). 9. Laschkarew, W. E., Trans. Paraday SOC.31, 1081 (1935). 10. Farnsworth, H. E.. and Johnson, W. E., Phys. Rev. 60, 168 (1941). 11. MacRae, A. U., and Germer, L. H., Ann. N. Y . Acad. Sci. 101, 631 (1963). 12. MaoRae, A. U., Science 139, 383 (1963). 13. Farnsworth, H. E., Phys. Rev. 34, 679 (1929). 14. Farnsworth, H. E., Phys. Rev. 40, 684 (1932); 43, 900 (1933). 15. Schlier, R. E., and Farnsworth, H. E., Semicond. Surface Phys., Proc. Conf. Philadelphia 1956 p. 3 (1967). Univ. Pennsylvania Press, Philadelphia, Pennsylvania. 16. Wolff, G. A., and Broder, J. D. Acta Cryst., 12, 313 (1959). 17. Green, M., and Seiwatz, R., J . Chem. Phys. 37, 458 (1962). 18. Farnsworth, H. E., Marsh, J. B., and Toots, J., Proc. Intern. Conf, Semicod. Phys. Ireter, 1962 p. 836 (1962). Inst. of Physics and The Physical SOC.,London. 19. Haneman, D., Phy8. Rev. 121, 1093 (1961). 20. Lavine, M. C., Rosenberg, A. J., and Gatos, H. C., J. Appl. Phys. 29, 1131 (1968). 21. Haneman, D., Phys. Rev. 119, 563 (1960).

THE CLEAN SINULE-CRYSTAL-SURFACE APPROACH

63

22. Lander, J. J., and Morrison, J., J . Chem. Phys. 37, 729 (1962); J. Appl. Phys. 33, 2089 (1962); 34, 1403 (1963). 23, Farnsworth, H. E., Ann. N . Y . Acad. Sci. 101, 605 (1963). 24. Lander, J. J., Gobeli, G. W., and Morrison, J., J . Appl. Phys. 34, 2298 (1963). 25. Dillon, J. A., Jr., and Farnsworth, H. E., J . Appl. Phys. 28, 174 (1957); 29, 1195 (1958). 26. Schlier, R. E., and Farnsworth, H. E., Phys. Chem. Solids 6 , 271 (1958). 27. Farnsworth, H. E., and Madden, H. H., Jr., J. AppE. Phys. 32, 1933 (1961). 28. Farnsworth, H. E., and Tuul, J . , Phys. Chem. Solids 9, 48 (1959). 29. Farnsworth, H. E., Trans. Am. Vacuum SOC.1962, Ninth Natl. Vacuum Symp., Lo8 AngeZes, CaZi,for.nia,p . 68 (1962). Macmillan, New York. 30. Alessandrini, E. I., and Freedman, J. F., Actu Cryst. 16, 54 (1963). 30a. Farnsworth, H. E., AppZ. Phys. Letters 2, 199 (1963). 31. Germer, L. H., MacRae, A. U., and Hartman, C. D., J. Appl. Phys. 32, 2432 (1961); Proc. Natl. Acad. Sci. U . S . 48, 997 (1962). 32. Germer, L. H., Scheibner, E. J., and Hartman, C. D., Phil. Mag. [S] 5, 222 (1960). 33. Park, R., and Farnsworth, H. E., J . Chern. Phys. 40, 2354 (1964). 34. Germer, L. H., and Hartman, C. D., J . A&. Phys. 31, 2086 (1960). 35. Dillon, J. A., Jr., and Farnsworth, H. E., Rev. Sci. Instr. 25, 96 (1954); J. Chem. Phys. 22, 160 (1954). 36. Schlier, R. E., and Farnsworth, H. E., J . Chem. Phys. 30, 917 (1959). 37. Marsh, J. B., and Farnsworth, H. E., SurfaceSci. 1 , 3 (1964). 38. Farnsworth, H. E., and Woodcock, R. F., Ind. Eng. Chem. 49, 258 (1957). 39. Tuul, J., and Farnsworth, H. E., J . Am. Ghem. SOC.83, 2247 (1961). 40. Tuul, J., and Farnsworth, H . E., J . A m . Chem. SOC.83, 2253 (1961). 41. Shooter, D., and Farnsworth, H. E., Phys. Chem. Solids 21, 219 (1981); J. phy8. Chem. 66, 222 (1962). 42. Yamashina, T., and Farnsworth, H. E., Ind. Eng. Chem. Prod. Res. Develop. 2, 34 (1963).

This Page Intentionaiiy Left Blank

Ad sorption Measurements during Surface Catalysis KENZI TAMARU* Department of Chemietry, Yokohama National University, Yokohama, Japan

I. Introduction .................................................... 11. General Scope of Adsorption Measurements during Surface Catalysis ...... A. Partial Equilibrium in the Reaction Scheme and the RateDetermining Step .............................................. B. Estimation of the Chemical Potential of Reaction Intermediates ........ C. UseofIsotopeTracers .......................................... D. Dynamic Treatment of Reaction Systems .......................... 111. Experimental Methods. ............................................. A. Gravimetric Method ............................................ B. Volumetric Method ............................................ C. Gas Chromatographic Technique. ................................. D. OtherMethods .................................................. IV. Decomposition of Germane on Germanium ............................ V. Decomposition of Formic Acid on Metal Catalysts ...................... VI. Decomposition of Ammonia on Metal Catalysts ........................ VII. Ammonia Synthesis on Iron Catalysts ................................ VIII. Concluding Remarks ................................................ References ........................................................

page 66 68 68 72 72 73 76 76 76

76 77 79 81 83 86 88 89

1. Introduction It is generally accepted that chemisorption plays an important role in surface catalysis and at least one of the reactants [or activated complexes (I)]should be chemisorbed on the catalyst surface (2). When we study the kinetics of the reaction on a solid catalyst, we analyze the data, in many cases, on the basis of the Langmuir-Hinshelwood, or sometimes, of the Eley-Rideal mechanisms, tacitly assuming that the elementary steps other than the rate-determining surface reaction are all in equilibrium (3). The adsorption on the catalyst surface during the reaction is accordingly estimated from the kinetic data of the overall reaction postulating that Langmuir adsorption isotherms are *Present addreee: Department of Chemistry, The University of Tokyo, Hongo, Tokyo, Japan. 66

66

KENZI TAMARU

applicable and that the rate of the reaction is proportional to the concentration of the reactants in their adsorbed state (or, in the case of Eley-Rides1 mechanism, one of the reacting spccics being gaseous or physically adsorbed, the rate is proportional to pressure of that species). The Langmuir adsorption isotherm is not always applicable, the heat and entropy of adsorption changing with coverage, and other adsorption isotherms such as proposed by Frumkin and Slygin ( 4 ) and Freundlich (5),sometimes, take the place of the Langmuir isotherm. The postulate concerning the deduction of adsorption data from the kinetic measurements undergoes, however, no fundamental change with these more elaborate treatments, though a better description of the adsorption may well be obtained. The adsorption of the several gases, which participate in the catalysis, onto the catalyst surface has been measured separately, and the thermodynamics and kinetics of chemisorption have been studied in various systems by many investigators. Chemisorption from gas mixtures, however, has been studied only in a limited number of cases and no measurements of adsorption on the catalyst in its working state has been carried out until recent years. In 1957, this author initiated a program of adsorption measurements during surface catalysis with simultaneous measurements of reaction rate ( 6 ) . The adsorption postulated from the reaction kinetics could consequently be compared with the observed results to examine the reaction mechanism. The state and the coverage of the catalyst surface during the reaction could be followed by direct measurements, including data on the pressures of the reacting species and on the reaction rate. Chemisorption on the catalyst surface during the progress of reaction cannot be estimated from the adsorption equilibria of reactants and products measured separately with each species. It depends not only upon the interaction among the adsorbed species and the catalyst Burface, but also upon the mechanism of the reaction, or the “kinetic structure” of the overall reaction. I n all cases the catalytic reaction proceeds through a certain number of elementary steps or reaction intermediates. The chemical potentials of those intermediates depend upon which of the steps is rate determining. Consequently by estimating the chemical potentials of the intermediates during the reaction, which is only possible by studying the reacting system in its working state, the rate-determining step may be identified. The properties of the catalyst surface such as, for instance, work function, heat of adsorption, reactivity of the adsorbed species, markedly

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

67

depend upon its coverage (Z), and the most important properties of the catalyst are not those of the bare surface, but those of the surface in its working state. The latter can be found only by an investigation of the catalyst in its working state. When a reducing and an oxidizing gas react on an oxide catalyst, for instance, the fugacity of the oxygen on the catalyst surface depends not only upon the pressures of the reacting gases, but also upon the kinetic structure of the overall reaction. The fugacity of oxygen over an oxide is one of its inherent thermodynamic properties and its change possibly influences the properties of the oxide itself, or its catalytic activity. In the case of acidic catalysts, the acidity of the catalyst surface, which should be correlated with the catalytic activity is that in the working state and not that under conditions far removed from those prevailing in the actual reaction. In the case of adsorption measurements during reaction the catalyst surface in its working state is treated as one of the reactants. The reactivity of the chemisorbed species depends upon the coverage of the catalyst surface and can be studied as a function of its coverage under reaction conditions. The activity of the chemisorbed species is generally not equal to its surface concentration. I n the case of homogeneous reactions, one of the most orthodox treatments of their kinetics is to measure all the possible elementary reactions separately, and on the basis of this information the overall reaction may be constructed and the kinetic structure of the overall reaction is elucidated accordingly. This method should also be employed in the case of contact catalysis, but the nature of the medium where the reaction takes place changes with surface coverage. If all the simpler processes, however, which make up the overall reaction are studied separately as a function of surface coverage and partial pressures, the kinetic structure of the overall reaction would be elucidated. This is also one of the main developments to be aimed a t by the adsorption measurements during surface catalysis. It is also one of the important fields where the theories of solids would play their important role in catalysis. The nature of the adsorbed species during reaction is not always revealed by the kinetic data analyzed following the ideas of Langmuir and Hinshelwood. For instance, when the reaction is zero order, it appears that the active part of the catalyst surface is saturated with such species as reactants, products, or intermediate compounds, the adsorption being independent of the ambient gas pressures. The kinetic behavior does not tell which of them is really adsorbed. One of the merits of the adsorption measurements during the reaction is to provide such an identification.

68

KEN21 TAMARU

In the example a t hand, a question also arises concerning the saturation of the catalyst surface. The zero-order kinetics only suggests saturation of the active part of the catalyst surface. This is not necessarily the whole surface, especially when the latter is heterogeneous. One of the most fundamental problems in catalysis is to estimate the area of the active part of the surface. Since 1925 when Taylor ( 7 )suggested his concept of “active centers,” many discussions of this problem have been presented from various points of view. Actually it is shown by the field emirrsion microscope that the heats of adsorption on different crystal faces are often largely different. Thus for a zero-order catalysis the adsorption on the whole surface may depend upon the partial pressures of the ambient gases, only a part of the adsorption being pressure independent. From this pressure-independent adsorption, the active part of the catalyst surface as well as the saturating species might be found. From adsorption measurements during reaction one can also examine whether the reaction rate is correlated with the amounts of adsorbed reactants or with the pressure of the reactants, i.e., whether the mechanism is of Langmuir-Hinshelwood type or of the Eley-Rideal variety. I n most discussions of surface catalysis it is tacitly assumed that all steps are in equilibrium except the rate-determining step. With this new approach, the validity of this assumption can also be verified, The importance of adsorption measurements during surface catalysis has been outlined so far in generality. I n the following sections, the fundamental principle of the kinetic study based on this approach will be discussed together with appropriate experimental methods and results. An attempt will be made to classify the application of this approach to various cases, and emphasis will be placed on the principles of this approach rather than the detailed discussion of each reaction.

II. General Scope of Adsorption Measurements during Surface Catalysis

A. PARTIAL EQUILIBRIUM IN THE REACTION SCHEMEAND THE RATE-DETERMINING STEP Let us suppose a heterogeneous catalytic reaction, L + R, where L stands for reactants and R, for reaction products. I n the course of the reaction, intermediate systems, E, F, G , . . . are formed and in some of these various kinds of adsorbed species are involved, such as

. . . +R

L f E + F + G + 1

2

9

4

n

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

69

The rates of the forward and opposing consecutive steps are designated +

c

+

- + +

c

as V,, V,, V,, V2,, . . V,, 8,with suffixes representing the number of each step. In the stationary state the overall reaction rate ( V ) is equal to the differences of forward and opposing rates of each step in the above reaction scheme: -

+

C

-

+

-

t

+

t

V+vI-v~IIvVa-v2+...~vn-v~ If step (2) is rate determining for the overall reaction, all the other steps being rapid enough in comparison with step (2), the rate of each step can be represented, for instance, as shown in Fig. 1 ( 8 ) . As the rates of step ( 1 ) -+

are sufficiently faster than those of step (2), V , becomes approximately t

equal to V,, which suggests that the step (1) is in equilibrium. The greater the difference in the rates, the closer it approaches equilibrium, Analogously it is concluded that all the steps except the rate-determing step are in equilibrium provided that they are rapid enough compared with the rate-determining step; in other words, no changes in free energy accompany those steps, and the free energy change of the overall reaction is accordingly concentrated at the rate-determining step.

I

I c

v,

I

I

FIG.1. Partial equilibrium in the reaction scheme.

I n the case of heterogeneous catalysis, let us take as an example a decomposition reaction, A(g) + B(g) 4-C(P)

and the reaction proceeds via the adsorbed state of the reactant, A(a), further, to those of the products, B(a) and C(a), where (g) and (a)represent gaseous and adsorbed states respectively. The reaction scheme is depicted in Fig. 2, each step being numbered as in the figure. (1) Case I . If the surface reaction, step (2), is rate determining, while

70

KEN21 TAMARU

FIG.2. The reaction scheme of a heterogeneous catalytic reaction,A(g) + B(g) -t- C(g).

all other steps are much more rapid than the step (2) so as to be equilibrated, the reactant A as well as the products B and C are all in adsorption equilibrium: A(g) + A(a), B(g) + B(a), C(g) + C(a). (2) Case 11. I n a similar way, if step (3) is rate determining, C(a) and A(a) are in adsorption equilibrium with C(g) and A(g) respectively, and B(a), on the other hand, with A(g) and C(g) in the following manner; B(a) C(g) + A(g). I n this case, the forward rate of the overall reaction (V,) can be expressed as follows according t o the LangmuirHinshelwood mechanism :

+

f'

=

kbl

- 1 $- b,Y,/P,

IpC

+ b,PA +b,P,

which can be expressed as follows when the denominator is approximately unity (the coverage of the catalyst surface is small): vf OC p A I p C

This rate expression can also be obtained on the assumption of a different rate-determining step. I n case I, the reaction rate can be expressed as follows:

If the adsorption of C(a) is so strong that biP, is much larger than 1 + b;PA+ bLP,, the following expression is obtained: v j CC

The same kinetic expression can be derived when the adsorption of A(g) onto the surface mainly covered by C(a) is rate determining. These treatments of the reaction kinetics clearly suggest that a kinetic expression is not enough to estimate the adsorption during the reaction or to elucidate the reaction mechanism, I n order to obtain more information, direct measurements of adsorption during reaction must be undertaken. If, moreover, both adsorption during the reaction and reaction

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

71

rate can be measured simultaneously, we may discriminate between alternative reaction mechanisms. In case I, when B(g) is removed from the reaction system, B(a) should drop to, or at least approach, zero, unless the adsorption of B is extremely strong, desorption of B(a) taking place. As B(a) is in adsorption equilibrium with B(g),in this case, as soon as B(a) is formed from A(a),it goes rapidly to B(g) to be removed from the system. I n case 11.on the other hand, B(a) is not in equilibrium with B(g), but with A(g) and C(g), and the removal of B(g) does not result in a rapid decrease of B(a). I n such a way, the two cases I and I1 could be distinguished by following B(a) with time during this operation. If B can be labeled with an isotope to give B*, and B(g) is replaced by B*(g) during the reaction, following the isotopic abundance in B(a) with time, would give the rate of step (3). I n case I, the mixing should be rapid, while in case I1 it should be slow. The labeled B*(a) would finally mix with A(g) with the corresponding rate. The rate of each of the steps, ( l ) ,( 2 ) , (3), and (4) may also be treated in the same manner. Consequently, the kinetic structure of the overall reaction could be elucidated. To explain the situation in a different way let us suppose a series of water tanks connected by means of tubes, ( l ) , ( 2 ) , (3), . . . of various sizes as shown in Fig. 3. The tank a t the extreme left corresponds to the initial reacting system, L, while that a t the right, to the final reaction product system, R. The reaction intermediate systems, E, F, G, . . . are located between them in order. The water level of L is higher than that of R and water flows from left to right, just as reaction proceeds. I n this case the water level of each tank corresponds to the chemical potential of each system. If the water levels of the adjacent tanks are equal, they are in equilibrium. The flow rate of water moving from left to right depends upon the size of each tube which connects the tanks. If tube (3) is much narrower than any of the other tubes, the water

L

F

G

R FIQ.3. A series of water tanks connected by mews of tubes as 6 model of reaction sequence.

E

72

KENZI TAMARU

levels of the tanks will adjust as shown in Fig. 3. All water levels preceding tube (3) are at the same height. The same goes for all water levels following tube (3). The difference of the water levels in vessels F and G is equal to that of L and R, which is the water level drop (free energy decrease) causing the overall flow (reaction) from L to R. The location of the drop in water level can accordingly be used as a criterion of ratedetermining step.

B. ESTIMATION OF CHEMICALPOTENTIALS OF REACTION INTERMEDIATES

A method to identify the narrowest connecting tube is to determine the water level in each tank. Similarly, a method to find the rate-determining step is to ascertain the chemical potential of each intermediate. An example of this sort is the decomposition of ammonia on a tungsten catalyst (9). During the reaction the amount of nitrogen chemisorbed on the surface is several times as much as that to form an adsorbed monolayer under the reaction conditions employed, This suggests surface nitride formation during the reaction. The adsorption of molecular nitrogen by the tungsten is not so strong as to form a nitride layer under identical experimental conditions. Consequently, the nitrogen chemisorbed during the ammonia decomposition has a higher chemical potential than the ambient nitrogen gas. Therefore the free energy cascade is located at the desorption process, provided that the whole surface of the catalyst takes part in the catalysis. Recently Apel’baum and Temkin (10) reported a new technique to measure directly the fugacity of the chemisorbed hydrogen during hydrogenation by means of a palladium membrane. This technique will be described in the section on experimental methods.

C. USEOF ISOTOPE TRACERS The size of each tube in Fig. 3 can also be examined by means of a dye. If we put a dye in tank R, it will become colored rapidly. So will tank G. But tanks L, E, and F will be colored after some delay, as tube (3) is very narrow. If the dye is added to tank G, a similar phenomenon will take place, but if it is in I?, tank L as well as E will be colored rapidly, while G and R will be colored at a much slower rate. In this way, by measuring at what rate each tank gets colored, the size of each tube may be estimated. Isotope tracers have been used to study the kinetic structure of the decomposition of germane on a germanium surface and of formic acid on a gold catalyst as will be described later,

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

73

The mixing of an isotope between a n adsorbed species and a reactant is also applicable for investigating the reactivity of the adsorbed species. I n this connection we should recall interesting experiments carried out by Bond (11)and Schuit et al. (12).They studied the hydrogen-deuterium exchange on platinum and nickel catalysts, respectively. Deuterium was first admitted to be chemisorbed by the catalyst, then the ambient gas was replaced with hydrogen and the mixing of hydrogen with the adsorbed species was followed. It was observed that part ofthe adsorbed deuterium is exchangeable very rapidly, while another part mixes at a measurable rate and a residual part virtually fails to undergo exchange. The distribution of those parts markedly depends upon reaction temperature. This behavior suggests that the reactivity of the chemisorbed hydrogen is not uniform. Similar experiments were carried out by Gundry (13)with nickel and tungsten catalysts, including evaporated films. These experiments gave clear evidence for heterogeneity in the reactivity of adsorbed species. The differential isotopic method initiated by Keier and Roginsky (14) belongs to a similar category, though it does not necessarily deal with the catalyst in its working state. Krylov and Fokina (15)developed the method to identify the active region of a catalyst surface.

D. DYNAMIC TREATMENT OF REACTION SYSTEMS The water levels of tanks, E, F, and G in Fig. 3 can also be treated dynamically to examine the sizes of some of the connecting tubes. The rate of response of each tank to 'a rapid change of water level in another tank can be followed to study the size of the tubes between them. When all the water levels are equal in height and the water in tank R is removed rapidly, for instance, the level of each tank undergoes corresponding changes according to the size of the connecting tubes. This is the method described above, when B(a) is followed with time. Tamaru (16)observed that when ammonia was rapidly removed from the equilibrated mixture of nitrogen, hydrogen and ammonia in adsorption equilibrium with a doubly promoted iron catalyst, the nitrogen adsorption underwent no appreciable rapid change as a result, which suggests that, if the major part of the adsorbing surface participates in the reaction, the hydrogenation of the chemisorbed nitrogen to form ammonia is not very fast as has been generally accepted. The development of the method consisting in following the time response to an abrupt change of one of the water levels of the tanks leads to a method with which to examine the rate of each stage in the process which the overall reaction comprises. This method corresponds

74

EENZI TAMARU

to a study of the size of the connecting tube in any given set of two water tanks. If the sizes of all the tubes throughout the reaction sequence are known, the kinetic structure of the overall reaction can be elucidated. Of course, generally speaking, it is not always possible to further separate the two sets of tanks with equal water levels, but the overall reaction may be separated into the simpler processes which it comprises. In the case of ammonia synthesis on a doubly promoted iron catalyst nitrogen adsorption (or desorption) and the hydrogenation of the chemisorbed nitrogen were separately measured as a function of the coverage and the pressure of reacting gases (17). The change in the properties of the catalyst surface with coverage has been taken into account. Thus a kinetic structure for the overall reaction could be set up and the nature of the rate-determining step under various reaction conditions could be determined as will be discussed later. These methods for elucidating the kinetic structure of the overall reaction have been briefly explained. They are not limited to heterogeneous catalysis, but are also applicable to reaction in general. The general method used hitherto t o study the reaction mechanism consists in following the material balance between L and R. On the other hand, very few kinetic observations on the intermediates have been carried out. Recently new tools, such as ESR, NMR, infrared and ultraviolet spectroscopy*, have been adapted to surface studies and detailed information on the catalytic reaction, as regards both adsorbed intermediates and their reactivity, is becoming available. It is to be expected, therefore, that the dynamic techniques that have been outlined will be effectively applied to elucidate the kinetic structure of the overall reaction. The existence of a certain adsorbed species on the catalyst surface does not necessarily imply that it is really a reaction intermediate which is involved in the reaction sequence. The identification of a reaction intermediate in the adsorbed state can be carried out by treating its kinetic behavior in the reaction as has been explained. I n this way, the elementary steps and reaction intermediates which the overall reaction comprises may be identified. The kinetic behavior of the overall reaction, such as order of reaction, activation energy and so forth, is determined by the constituent elementary steps and also by the kinetic mutual relation or kinetic structure of these steps as has been discussed in connection with Fig. 3.

* Recently Tachibana and Okuda ( 1 8 ) studied the electronic spectrum of cumene adsorbed on a silica-alumina catalyst during its cracking reaction arid suggested that a Bronstcd acid contributes to the cracking reaction through the formation of protoiiated cmen0.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

75

Similarly, the properties of a molecule are determined by the constituent elements and also by its structure.

111. Experimental Methods A. GRAVIMETRIC METHOD Mars et al. (19)and Scholten (20)constructed anelaborated apparatus for studying the adsorption of nitrogen during ammonia 'synthesis, measuring the weight of the catalyst. The apparatus used for this experiment consists of a gravimetric system; a beam balance is installed in a high-vacuum system and can be operated magnetically. The catalyst, placed in a platinum basket, is suspended on a thin glass wire. The temperature of the catalyst is measured directly with a Pt-PtRh thermocouple connected to the Pt-glass seals by means of 4-p wire loops to avoid weighing errors. Appropriate correction for buoyancy and flow are necessary. The reaction rate can be measured from the concentration of the reaction products in the exit gas. Elaborate studies were carried out by means of this apparatus, but the gravimetric method to measure adsorption under reaction conditions has some inherent disadvantages, only the weight change of the catalyst being measured. I n the case of ammonia synthesis, it was assumed that the hydrogen adsorption on the catalyst surface partly covered by nitrogen would be equal to that on the part of the surface not covered with nitrogen. Due to the low atomic weight of hydrogen the ambiguity in nitrogen coverage is obviously small. But, generally speaking, the ambiguity due to the mutual influence of the chemisorbed gases is not always negligible; it is especially important when the behavior of the adsorbed species depends sharply upon their coverages. B. VOLUMETRIC METHOD

A volumetric method to measure simultaneously adsorption during reaction and reaction rate was proposed by Tarnaru (6,21).It consists of a closed circulating system similar to those used in studying mixed adsorption. The amount of gases adsorbed on the catalyst surface can be calculated from the amount of the reactant introduced into the system and the pressure and the composition of the circulating gas. The composition of the circulating gas can be measured by thermal conductivity or mass spectrometry, and sometimes by condensing gases one by one with liquid nitrogen and solid carbon dioxide traps, successively; for instance, this is possible in the case of a mixture of hydrogen, carbon

76

EENZI TAMARU

dioxide, and formic acid vapor. Due allowance has t o be made for the analytical samples removed from the system. The amount of the catalyst is preferably large for accurate measurements and the adsorption should be measured under such conditions that the reaction takes place very slowly, while the rate of gas circulation is fast enough so that the composition of the circulating gas is virtually the same throughout the system and isothermal conditions prevail in the reactor. I n these measurements the adsorption is calculated from 8 material balance. The result yields the amount but not the kind of adsorbed chemical species. As to the reaction rate, it is to be noted here that the change in adsorption with time accompanies the change in the amount and composition of the reacting gas. This should be taken into account in the calculation of the reaction rate. I n Tamaru’s studies of ammonia synthesis, the produced ammonia was always trapped in a liquid nitrogen trap and its amount gave the reaction rate.

C. GAS CHROMATOGRAPHIC TECHNIQUE

A third method designed to measure adsorption during surface catalysis is a gas chromatographic technique initiated by Tamaru (22), and Nakanishi and Tamaru (23). In the usual gas chromatographic technique, inert gases such as nitrogen or helium are employed as carriers, but in the present modification, the reacting gases are used as a carrier gas and the catalyst is placed in the adsorption column maintained a t reaction temperature so that a stationary state of the catalytic reaction is established. The gases which participate in the reaction are introduced into the system a t the top of the adsorption (catalyst) column as gas samples and the extent of their adsorption on the catalyst surface in, its working state can be measured by the retention time. In this technique, the reaction rate can be measured simultaneously by analyzing the product in the exit gas. I n this way the adsorption on the catalyst surface in its working state and the reaction rate can be studied simultaneously under various reaction conditions. A necessary requirement is that the reaction proceeds slowly enough to keep the composition of the reacting gas as well as the surface condition of the catalyst virtually the same throughout the catalyst column. A variation of the technique, applicable t o such reactions as decomposition, isomerization or polymerization, consists in using the reactant as a sample gas and an inert gas as a carrier gas, keeping the catalyst column a t reaction temperature. The exit pulse of the sample gas gives

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

77

its retention time and its area, the amount of reactant unreacted in passing through the column. The apparatus employed for this experiment is similar to the usual gas chromatograph. The material of the column walls should be noncatalytic at reaction temperature. A requirement for this technique is that the adsorption be reversible, rapid, and moderate. The retention time in this case corresponds to A x / A N , where A x is the increase in adsorption at the increase of concentration of gas sample by A N . When the sample gas is one of the reactants, its adsorption also takes place from the carrier gas. When the surface is really saturated with a sample gas from the carrier gas, the retention time of the sample should be zero, no further adsorption taking place. Accordingly a small value of A x / A N for a reactant can be due ( 1 ) to weak adsorption or (2) to a strong and nearly saturated one from the carrier gas. I n the former case, A x / AN should increase as temperature becomes lower, while in the latter case, A x / A N should decrease, as the adsorption from the carrier gas approaches saturation so that A x / A N decreases provided the adsorption is equilibrated. Bassett and Habgood (24) deliberately applied this chromatographic method, in addition to the “microcatalytic chromatographic method” of Hall and Emmett (25), to the isomerization of cyclopropane on a molecular sieve. They were able to assess the heat of adsorption of the reactant, the activation energy and the order of the reaction. Ozaki, et at. (26) recently studied the “rapid and reversible” part of the adsorption of hydrogen on nickel-kieselguhr (50 wt-%) by means of a chromatographic technique. They measured the retention time of a deuterium sample using hydrogen as a carrier gas at various temperatures from - 195” up to 300”C,placing the catalyst in the adsorption column. The HD content in the exit pulse of the deuterium sample was analyzed, and the total uptake of hydrogen by the catalyst was separately estimated in a static system a t these temperatures. The amount of hydrogen adsorbed, which can exchange quickly with deuterium (or the fraction of rapid adsorption in total uptake), was thus estimated from the retention time. It sharply decreased with temperature up to -140°C, increased to pass a first maximum around 0°C and then a second maximum at about 120°C. This behavior was associated with different states of adsorption.

D. OTHERMETHODS Adsorption on the catalyst surface during the progress of reaction can be estimated in various ways. In the case of germane decomposition

78

RENZI TAMARU

on a germanium surface (27) it could be estimated in the following manner. The reaction system was cooled down rapidly to freeze in the state of the catalyst surface. Then, germane and ambient hydrogen were removed, and the catalyst surface with chemisorbed hydrogen was exposed t o higher temperatures to desorb hydrogen. This gave the amount of hydrogen chemisorbed during the reaction. The experiment was possible because the adsorption of hydrogen on a germanium surface is activated and reversible. It was thus concluded that the catalyst surface during the reaction is saturated with chemisorbed hydrogen, the

FIU.4. Sketch of a flow circulating system with a palladium membrane. A: circulation pump. B : palladium-membrane.

number of hydrogen atoms chemisorbed being equal to the number of surface germanium atoms.* I n a special case the fugacity of one of the adsorbed reactants could be measured directly during the reaction. For example, in the hydrogenation of ethylene on a palladium surface, Apel’baum and Temkin (10)successfully used a very thin palladium film through which hydrogen passes during the reaction. The apparatus employed is a flow circulating system schematically shown in Fig. 4. When the hydrogen pressure in the supply side is higher than the fugacity of the chemisorbed hydrogen, hydrogen passes into the reaction system through the palladium membrane, while if it is lower, hydrogen comes out of the reacting

* The analyses of an oxide catalyst after the oxidation of carbon monoxide, according to Voltz and Weller ( H ) ,gave an indication of the state of the catalylyst during the reaction.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

79

system through the membrane. I n this way the fugacity of the chemisorbed hydrogen during the reaction could be estimated. It turns out that the fugacity is substantially lower than that of hydrogen in the reacting gas a t 0-42", while a t 176°C they are almost equal. Recently the behavior of preadsorbed ethy1ene-C-14 on a nickel film could be followed by counting the /3 radiation from the nickel during the hydrogenation of C-12 ethylene (29). Only a fraction of the preadsorbed ethylene was removed from the film, while the remainder was firmly held during continued hydrogenation of C-12 ethylene. It was thus concluded that only a fraction of the chemisorbed species participates in the hydrogenation.

IV. Decomposition of Germane on Germanium The thermal decomposition of gaseous hydrides on their constituent elements to produce hydrogen is one of the simplest catalytic reactions (27,30). During the reaction the surface of the catalyst element is always renewed by the continuous deposition of fresh surface atoms and only two elements are involved including catalyst. In the case of germane decomposition on germanium it has been shown that (1) the decomposition is a zero order reaction and its activation energy is 41.2 kcal/mole; (2) during the decomposition the entire surface of germanium is virtually covered by chemisorbed hydrogen atoms the number of which is approximately equal to that of surface germanium atoms; (3) no hydrogen deuteride is formed during the decomposition of germane in the presence of an excess deuterium; (4) when a mixture of germanium hydride (GeH,) and deuteride (GeD,) is decomposed, abundant quantities of equilibrated hydrogen deuteride are produced, while (5) no exchange takes place between the two kinds of germane during the reaction to form GeH,D,; (6) the desorption rate of the chemisorbed hydrogen on germanium a t full coverage is equal to the decomposition rate of germane; ( 7 ) the ratio of the decomposition rates of germane and deuterogermane is 1.8 to 1; (8) the adsorption of hydrogen on the germanium surface is reversible with an activation energy of 14.6 kcal/mole for the initial adsorption,* and obeys the Langmuir isotherm for dissociative adsorption a t lower coverage, 0 = bP3:. This suggests a dissociative type of adsorption with a n initial heat of adsorption of 23.5 kcal/mole. At higher coverages the Freundlich adsorption isotherm is applicable, indicating a decrease of the heat of adsorption with coverage.

* Recently Bennett and Tompkins ( 3 1 ) observed that the activation energy for the initial adsorption on an evaporated $Zm is 16.0 kcal/mole.

80

KEN21 TAMAFtU

According to the adsorption isotherms, for the hydrogen pressures under reaction condition, the fraction of the surface covered with hydrogen would be about only one half, if adsorption equilibrium were established. Consequently, observation (2) leads to the conclusion that the fugacity of the hydrogen adsorbed on the germanium surface during the decomposition is much higher than in the ambient hydrogen gas. I n other words, desorption of hydrogen from the germanium surface is the rate-determining step. This conclusion is supported by observations (3) and (6) and is confirmed by the zero-order kinetics, I n fact, hydrogen from the gas phase can scarcely reach the surface during the decomposition, as shown by the lack of hydrogen deuteride production during the germane decomposition in the presence of deuterium, while exchange between Ha and D, proceeds when the decomposition is over. This lack of HD production during the decomposition of GeH, D, rules out a Eley-Rideal mechanism, involving collisions of deuterium molecules in the gas phase (or those in the van der Waals’ adsorption layer) with the chemisorbed species on the surface. Although the heat of adsorption decreases with coverage, the activation energy for desorption increases only slightly when one passes from a bare surface (14.6 23.5 = 38.1 kcal/mole) to saturation (41.2 kcal/mole). The rate of the hydrogen-deuterium (1:1 mixture) exchange on the germanium surface as HD production (re) is accordingly calculated from the hydrogen coverage of germanium surface (6) during the exchange reaction, obtained from the adsorption isotherm, and the rate of desorption a t full coverage ( r J . This calculation, by means of the formula r, = Oar,/2, gives results in good agreement with experiment.* The basic assumption behind this calculation is that the exchange can take place on all covered sites and not just on a limited number of active centers. Indeed, this agreement conforms to the observation that the surface is fully covered with hydrogen during decomposition, the germanium surface exhibiting no a priori heterogeneity, and all the germanium surface atoms seemingly participating in the decomposition. The fall in adsorption heats is consequently due to “induced heterogeneity” of a type akin to that suggested by Boudart (33) being due for instance, to the change in electronic properties of the surface with adsorption. It should be noted that the observation (5) suggests another free energy drop a t the chemisorption of germane on the germanium surface mostly covered by hydrogen. If the rate determining step is only hydrogen desorption from the germanium surface, the chernisorption rate of

+

+

* Kuchaev and Boreskov (32) studied the isotopic exchange of hydrogen on n- and p-type germanium, in which the density of free electrons or holes was deliberately altered in a wide range. The rate of the exchange waa the same for ell the specimens.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

81

germane is so strongly hindered by the chemisorbed hydrogen that it even becomes slower than the desorption rate of hydrogen. This finally results in two free energy cascades a t the step of the chemisorption of germane as well as that of hydrogen desorption, the hydrogen chemisorbed still covering most of the surface.

V. Decomposition of Formic Acid on Metal Catalysts The decomposition of formic acid on metal catalysts to form carbon dioxide and hydrogen has been studied extensively by many investigators. The reaction is of interest in connection with selective catalysis, as formic acid also decomposes to carbon monoxide and water on some “dehydrating” catalysts such as alumina. I n recent years, considerable progress has been made in the elucidation of the reaction mechanism using such tools as infrared spectroscopy (34),electric conductivity (35,36) isotopic tracer (37-39) and adsorption measurements during surface catalysis (21, 40). In the case of nickel, it was observed by Tamaru (21) that, when formic acid vapor gets into contact with a clean nickel surface a t 100°C, all the vapor is virtually chemisorbed at first until hydrogen comes off when a first saturation point is reached, no carbon dioxide, on the other hand, being evolved. When more and more formic acid is admitted, carbon dioxide is finally evolved a t the second saturation point where the ratio of hydrogen and carbon dioxide chemisorbed is 1 : 2, and the number of hydrogen atoms chemisorbed is approximately twice as large as the number of surface nickel atoms. These observations strongly suggest that a monolayer of nickel formate is formed on the nickel surface a t the contact with the acid vapor. I n other words, the nickel surface is first saturated according to HCOOH(g) + H(a)

+ HCOO(a)

and then following, HCOOH(g)

+ H(a)

--t

HCOO(a)

+ HAg)

until the second saturation with HCOO(a) takes place. This observation was confirmed by Fahrenfort, et at. (34)on a supported nickel catalyst. The formation of nickel formate monolayer can only be understood sterically if the superficial nickel atoms leave their place in the metal lattice. The formation of a formate layer on the catalyst surface, was originally imagined by Rienacker and Hansen (35),later confirmed by Hirota et al. (34)with infrared spectroscopy of the adsorbed species on copper,

82

KEN21 TAMARU

nickel, and zinc. Their results also agree with the observation of Fahrenfort et al. (34). Fahrenfort et al. (34) not only observed the formation of formate during the acid decomposition, but also studied the decomposition of the formate-covered surface by means of infrared spectroscopy and also by measuring the increase in gas pressure. They found that the rate and the activation energy of the decomposition of the surface formate both coincide with those of the overall reaction of formic acid decomposition. This was not necessarily the case in the data obtained by Tamaru (21) at lower temperatures. They accordingly concluded that the decomposition of formic acid on a nickel surface proceeds via nickel formate as a key intermediate. In the case of the decomposition on noble metals such as gold, the adsorption spectra showed only a weak absorption band which was ascribed to formate ion on the surface. Actually, during the decomposition of formic acid on a silver catalyst, according to Tamaru (Zl),hydrogen is adsorbed as much as carbon dioxide. Kinetically speaking, the decomposition on the catalyst is first order at higher temperatures and lower pressures, while it is zero-order at lower temperatures and higher acid pressures. The adsorption of the acid, on the other hand, seemed to conform to the reaction order, and was proportional to the pressure when it is a first order reaction and reached a saturation in the zeroorder region. We should here recall the work of Sachtler and De Boer (37) on the formic acid decomposition on a gold catalyst. They studied the reaction by means of deuterium tracer and found no hydrogen deuteride formation when HCOOH was decomposed in the presence of deuterium, while abundant (equilibrated) amounts of HD were observed when HCOOH and DCOOD were decomposed simultaneously. The hydrogendeuterium exchange does not proceed on the gold catalyst. These observations led to the conclusion that formic acid first decomposes to formate intermediate and hydrogen atom, the former decomposes yielding carbon dioxide and another hydrogen atom. Thus hydrogen atoms move round on the surface to combine with a partner with which to escape from the surface. The chemical potential of the hydrogen atoms on the gold surface is accordingly higher than that of ambient hydrogen molecule, which shows the hydrogen desorption to be rate determining. The formic acid decomposition on metal catalysts accordingly seems to proceed via surface formate, though the difference in basicity of the metal surface would result in different coverage during the reaction, which in turn leads to a different reaction order. It is widely known that Fahrenfort et al. (34)thus correlated the catalytic activities of the

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

83

individual metals with the heat of formation of their superficial formates. I n the case of a low heat of formation, such as gold, the surface formate is very unstable a t the decomposing temperatures. This results in slow overall reaction rate. With increasing stability of the surface formate, the reaction rate becomes faster until it reaches an optimum stability. If the surface formate is too stable, the catalyst surface is covered by the stable surface formate. The reaction rate, on the other hand, becomes slower, as the surface formate decomposes with difficulties, while the overall reaction becomes zero-order, the surface being saturated with formate. I n this way a “volcano-shaped” curve was obtained in a plot of catalyst activity versus heat of formation of metal formates. It is thus easily realized that the state of the catalyst surface during the decomposition depends upon the nature of the catalyst. I n the case of formic acid decomposition on a copper surface (40), adsorption measurements during the reaction revealed a characteristic behavior of the catalyst surface. The reaction is of zero-order, the rate being independent of the ambient gas pressures. The adsorption, on the other hand, is markedly dependent upon the partial pressure of formic acid. The amount of carbon dioxide adsorbed is comparable in size with that of hydrogen, which is in marked divergence from the case of a nickel surface, where carbon dioxide is adsorbed twice as much as hydrogen, forming a surface formate. The zero-order kinetics imply saturated adsorption on the active part of the catalyst surface, while the adsorption on the whole surface apparently depends upon the pressure of formic acid. The saturated adsorption of the copper surface calculated as one-site adsorption is shown as 8 = 1 in the results. It appears therefore that the catalytically active part is a minor part of the surface available for adsorption. I n this manner, adsorption measurements during surface catalysis in the case of a, zero-order reaction could lead to an estimate of the active part of the catalyst surface.

VI. Decomposition of Ammonia on Metal Catalysts The decomposition of ammonia on tungsten is one of the examples most frequently discussed in textbooks of catalysis (2, 3). It is accepted that the reaction is of zero-order in the initial stage, which is interpreted t o indicate the catalyst surface to be fully covered by ammonia during the reaction. According to Frankenburger and Hodler (41),on the other hand, a rapid first step of the decomposition is the formation of surface imide, NH, metal 4 NH-metal H,, which takes place

+

+

84

KEN21 TAMARU

at temperatures as low as 150°C and is followed by surface nitride formation a t about 200"C, 2(NH-metal) --f 2(N-metal) H,. These results clearly indicate that ammonia molecules are not the species that saturate the surface a t decomposing temperatures generally above 600°C. The decomposition of ammonia on iron catalysts has been extensively studied in relation to the ammonia synthesis and it is generally admitted that nitrogen desorption is the rate-determining step in the overall reaction (42). Consequently, it might also be possible to interpret the zero order kinetics on the basis of saturated adsorption of nitrogen on the catalyst surface, the nitrogen desorption being rate determining. However, this view seems contradicted by additional evidence. Jungers and Taylor (43)and also Barrer ( 4 4 )observed a kinetic isotope effect in the decomposition rates of ammonia and its deuterocompounds; on a tungsten surface NH, decomposed more rapidly than ND,. This isotopic effect of hydrogen cannot be explained by the saturated adsorption of nitrogen on the catalyst surface. One of the methods to cast light on this problem is to measure adsorption of the catalyst surface during the reaction (9). It is shown by the measurements that, a t reaction temperatures, the uptake of nitrogen is more than the monolayer and the formation of surface nitride layers was suggested.* The amount of nitrogen sorbed depends upon the reaction time and ammonia pressure and increases at higher ammonia pressures, or with the addition of ammonia. At lower temperatures, nitrogen comes off with difficulty and its chemisorption increases with reaction time, although the ammonia pressure becomes lower, and hydrogen pressure, higher, suggesting that the ambient ammonia is not in equilibrium with the nitrogen in the nitride and hydrogen. As to the adsorption of hydrogen during the reaction, virtually no hydrogen is adsorbed a t temperatures higher than 600"C, where the decomposition has generally been investigated. Consequently, if the discussion is based on the behavior of the whole surface, the interpretation of the zero-order kinetics as due to saturation with NH,(a) or NH(a) is not likely to be adequate, though this view has generally been accepted. During the course of the reaction, if the temperature was rapidly lowered to 150"C, all the ambient gas was removed and the temperature was then raised again, the pressure and the composition of the gas evolved from the catalyst with time give the rate of desorption of nitrogen (or decomposition of nitride layers). It is of interest to note that

+

* Logan and Kemball ( 4 5 ) also studied the decomposition of ammonia on a tungsten evaporated film and observed nitriding to various extent.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

85

nitrogen desorption separately measured in the absence of the reactant was almost exactly equal to the rate of nitrogen production in the overall reaction at equal nitrogen uptake, which suggests that nitrogen desorption is rate determining. Thus, the overall reaction can be considered to be a consecutive reaction comprising nitride formation followed by its decomposition,while the amount of the chemisorbed nitrogen (or the thickness of the nitride layers) during the reaction depends upon the relative rates of the two processes, It is accordingly suggested that the simpler processes of which the overall reaction is composed can be studied separately, and also that the rate of nitriding decreases, while that of nitrogen desorption increases, as the nitride becomes thicker, until the supply and the consumption of the chemisorbed nitrogen are dynamically balanced. The isotope effect in the rates of decomposition of NH, and ND, can be explained on the basis of the different rates of nitride formation from these two isotopic molecules, while no hydrogen is being chemisorbed during the decomposition at decomposition temperatures. The decomposition of ammonia on an ammonia synthetic catalyst is similar to that on tungsten (46). At lower temperatures, nitrogen is evolved with extreme difficulty and is increasingly chemisorbed with time and temperatures, though the pressure of ammonia decreases and that of hydrogen increases. At higher temperatures, on the other hand, the description of nitrogen takes place, while adsorption of hydrogen decreases with time and temperatures.

VII. Ammonia Synthesis on Iron Catalysts Ammonia synthesis is a model case of a fundamental investigation in surface catalysis and many efforts have been focused at this problem making use of a variety of the tools available (42, 47). Emmett and his group (48) found that the rate of chemisorption of nitrogen and the rate of ammonia synthesis are both of the same order of magnitude and that the exchange reaction of nitrogen, Nio NP = 2NgQ, proceeds at a speed comparable to that of the synthesis reaction. The exchange reactions between hydrogen and deuterium, and also between deuterium and ammonia, on the other hand, both take place at temperatures far below normal synthesis temperatures. These data have been considered t o support the view that nitrogen chemisorption is the rate-determining step. Temkin and Pyzhev (49) successfully derived a kinetic expression for the ammonia synthesis assuming that the chemisorption of nitrogen is

+

86

KENZI TAMARU

rate-determining . Their work was considered as another strong indication in favor of nitrogen chemisorption as rate-determining step. More recently Dutch workers a t the Staatsmijnen in Limburg (19,50), tried to estimate the adsorption during the reaction using a gravimetric method. After a careful study of nitrogen adsorption on a singly promoted iron catalyst, they compared the nitrogen coverage during the synthesis with that which would give a rate of nitrogen chemisorption equal to the observed rate of ammonia synthesis (at the same temperature and partial pressure). I n this comparison, a very good agreement is observed except for measurements a t very low temperatures. The next step in establishing the kinetic structure of the overall reaction is to confirm whether the chemisorbed nitrogen is in equilibrium with hydrogen and ammonia, N(a) gH,(g) = NH,(g). If it is equilibrated, the following equation will be valid,

+

p$€I*/(p;a * p3,J = K(eq) where P& is the pressure of nitrogen which should be in adsorption equilibrium with the nitrogen coverage under reaction conditions. As

= 7BPN,(1 -x)4

stm

as P,,(eq) = PEI(l- x) atm, PN,(eq)= PNI(l- 2) atm, where 7 = PNH,/PNH,(eq) and x is the molar fraction of ammonia in the 1:3 N2-H, mixture when it is in equilibrium. I n this way, Pi, may be calculated from the partial pressures of nitrogen, hydrogen, and ammonia in the catalyst bed, and, consequently, the nitrogen coverage which equilibrates with P i 9is obtained from the adsorption isotherm, postulating that hydrogen adsorption does not influence the nitrogen adsorption. Both values of nitrogen coverage thus obtained showed “perfect” agreement, in support of the current view that the nitrogen chemisorption is only rate-determining and all the other steps are equilibrated during the synthesis. at I n the calculation of P i 2 , however, the Dutch workers used PNHa the exit of the catalyst bed. I n other words, the ammonia pressure over the catalyst was postulated to be equal t o that a t the exit. I n the flow system P N H I a t the entrance of the catalyst bed is zero. Without knowing, consequently, the distribution of ammonia pressure throughout the catalyst bed, the calculation of Pk,is subject to error, especially since Pisis proportional to the square of PN1,$. Enomoto and Horiuti (51) suggested that the hydrogenation of the

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

87

chemisorbed nitrogen to form ammonia is rate determining. This conclusion was based on the determination of the “stoichiometric number,” of the rate-determining step, In general, the stoichiometric number of a step is the number of times that the step under consideration takes place in one overall reaction. In the case at hand, for the overall reaction N, + 3H, = 2NH,, the stoichiometric number of the ratedetermining step is two rather than one. Bokhoven et al. (52) repeated Horiuti’s experiments and reached an opposite conclusion (a stoichimetric number of one) which conforms to the current view. Their data have been reexamined by Horiuti and Takezawa (53) who insist that the conclusion of the Dutch group is not correctly derived. Recently Tanaka and Matsunaga (54)obtained the stoichimetric number of one by means of a singly promoted iron catalyst. They also confirmed that the rate of isotopic exchange, Nlo Ni8 = 2Nig,is slow during the reaction. Ozaki et al. (55)compared the rate of ammonia synthesis on a doubly promoted iron catalyst with that of deuteroammonia, and found that deuterium reacts markedly faster than hydrogen under the same reaction condition. From the kinetic data, as well as the isotope effect, they reached the conclusion that the rate-determining step of the overall reaction is the chemisorption of nitrogen on a surface mainly covered with NH radicals, and that the isotope effect is due to the fact that NH is adsorbed more strongly than ND. Tamaru (46)measured adsorption on a doubly promoted iron catalyst surface during the course of the synthesis. He found that nitrogen chemisorption undergoes no appreciable rapid change when ammonia is removed from the equilibrated mixture of nitrogen, hydrogen and ammonia all in adsorption equilibrium. This suggests that the hydrogenation step of the chemisorbed nitrogen is not as fast a process as has been accepted, if it is admitted that a significant part of the surface participates in the reaction. Tamaru studied adsorption and desorption of nitrogen in the presence

+

-+

c

of hydrogen (vl and vl, respectively) and also the reaction between the +

chemisorbed nitrogen and hydrogen to form ammonia (v2) all as a function of ambient pressures and coverages. The following expressions are thus obtained; -+

v,

=

AP,, exp( -aN(a))

=

B exp(PN(a))

t

211 3

v2

= CPH. exp(yN(a)1

A , B,C , a,8, and y are constants at a constant temperature, and N(a)

88

KENZI TAMARU

is the amount of nitrogen adsorbed. The criterion of the rate-determini---f

ing step can be the ratio, Vl/vz; if this ratio is much larger than unity, the latter step, the hydrogenation of chemisorbed nitrogen to form ammonia, is rate determining, while if it is much smaller than unity, nitrogen ad- or desorption is rate controlling. The values of fl and y empirically obtained are almost equal, while the activation energy contained in B is much larger than that contained in C. This leads to the conclusion that, at higher temperatures and lower hydrogen pressures, t 3

the ratio vl/va is much larger than unity; in other words, the hydrogenation of the chemisorbed nitrogen is rate determining, on the contrary at lower temperatures and higher hydrogen pressures, nitrogen chemisorption is rate determining.

VIII. Concluding Remarks A new approach, measuring adsorption during the course of catalytic reaction, has been described. The state of the surface which catalyzes the reactions is not that of the surface in the absence of reactants, but that which exerts under reaction conditions. In this sense, the properties of a catalyst surface to be studied should be those in the working state, rather than those of a bare surface. Adsorption measurements during the reaction can be one of the direct ways to obtain information on the working state. As this working state is dependent upon the mechanism of the reaction, it gives direct information toward the elucidation of the reaction mechanism. Methods to study the simple reactions (or elementary steps), as a function of the coverage of the surface and the pressures of reacting gases, would lead to the elucidation of the kinetic structure of the overall reaction, at least in a more straightforward way than has been generally possible thus far. I n recent years techniques to study the properties of adsorbed species have markedly developed, which may provide great assistance in measuring each adsorbed species quantitatively during the reaction. The new method discussed in the review has been fruitful in many respects, but it is still far from complete in itself. It is hoped that this discussion will contribute to the development of new tools to gain a fuller insight into the nature of contact catalysis. ACKNOWLEDQNENT The author owe8 a great debt of gratitude to Sir Dean Hugh Taylor, Professor Michel Boudert, Dr. W. M. H. Sachtler, Professor J. Horiuti, and Professor T. Kwan, a11 of whom read the manuscript, and whose many suggestions have markedly improved the article.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

89

REFERHINOES

1. De Boer, J. H., in “The Mechanism of Heterogenous Catalysis” (J. H. De Boer, ad.), p. 1. Elsevier, Amsterdam, 1960. 2. See, e.g., Trapnell, B. M. W., in “Chemisorption.” Butterworths, London, 1955. 3. See, e.g., Laidler, K. J., in “Catalysis” (P. H. Emmett, ed.), Vol. 11, p. 119. Reinhold, New York, 1954. 4. Frumkin, A., and Slygin, A., Acta Physicochim. URSS 3, 791 (1935). 5. Freundlich, H., in “Colloid and Capillary Chemistry” Methuen, London, 1926. 6. Tamaru, K., Catalysis Meeting, Tokyo, April, 1957; Tamaru, K., Bull. Chem. SOC. Japan 31, 666 (1958). 7 . Taylor, H. S., Proc. Roy. SOC.A108, 105 (1925). 8. See, e.g., Horiuti, J., in “Catalysis,” p. 28. Asakura-Shoten, Tokyo, 1953. 9. Tamaru, K., Trans. Paraday SOC.57, 1410 (1961). 10. Apel’baum, L. O., and Temkin, M., Zh. Fiz. Khim. 35, 2060 (1961). 11. Bond, G. C., J . Phys. Chem. 60, 702 (1956). 12. Schuit, G. C. A., De Boer, N. H., Dorgelo, G . J. H., and Van Reijen, L. L., in “Chemisorption” (W. E. Garner, ed.), p. 39. Butterworths, London, 1957; Schuit, G. C. A., and Van Reijen, L. L., Advan. Catalysis 10, 242 (1958). 13. Gundry, P. M., Proc. 2nd Intern. Congr. Catalysis 1960, p. 1083 (1961). Technip, Paris. 14. Keier, N. P., and Roginsky, S. Z., Dokl. Akad. Nauk SRSS 57, 151 (1947). 15. Krylov, 0. V., and Fokina, E. A., Kinetika i Kataliz 1, 421 (1960). 16. Tamaru, K., Proc. 2nd Intern. Congr. Catalysis 1960, p. 325 (1961). Technip, Paris. 17. Tamaru, K., 3rd Intern. Congr. Catalysis 1964, Amsterdam. 18. Tachibana, T., and Okuda, M., Bull. Chem. SOC.Japan 36, 462 (1963). 19. Mars, P., Scholten, J. J. F., and Zwietering, P., in “The Mechanism of Heterogeneous Catalysis” (J. H. De Boer, ed.), p. 66. Elsevier, Amsterdam, 1960. 20. Scholten, J. J. F., Chemisorption of nitrogen on iron catalysts in connection with ammonia synthesis. Thesis, Delft, 1959. 21. Tamaru, K., Trans. Paraday Soe. 55, 824 (1959). 22. Tamaru, K., Nature 183, 319 (1959). 23. Nakanishi, J., and Tamaru, K., Trans. Paraday SOC.59, 1470 (1963). 24. Bassett, D. W., and Habgood, H. W., J . Phys. Chem. 64, 769 (1960). 25. Hall, W. K., and Emmett, P. H., J . A m . Chem. SOC.79, 2091 (1957). 26. Ozaki, A., Nozaki, F., and Maruya, K., Catalysis Meeting, Tokyo, December, 1962. 27. Tamaru, K., and Boudart, M., Advan. Catalysis 9, 699 (1957). 28. Voltz, S. E., and Weller, 5. W., J . Phys. Chem. 59, 566 (1955). 29. Thomson, S. J., and Wishlade, J. L., Trans. Paraday SOC. 58, 1170 (1962). 30. Tamaru, K., J . Phys. Chem. 59, 777 (1955); Tamaru, K., Boudart, M., and Taylor, H., ibid. p. 801; Fensham, P. J., Tamaru, K., Boudart, M., and Taylor, H., ibid. p. 806; Tamaru, K., ibid. p. 1084; ibid. 60, 610 (1956); ibid. 61, 647 (1957); Bull. Chem. SOC.Japan 31, 647 (1958). 31. Bennett, M. J., and Tompkins, F. C., Trans. Faraday SOC.58, 816 (1962). 32. Kuchaev, V. L., and Boreskov, G. K . , Kinelika i Kataliz 1, 356 (1960). 33. Boudart, M., J . Am. Chem. SOC.74, 3556 (1952). 34. Hirota, K., Kuwata, K., and Nakei, Y., Bull. Chern. SOC.Japan 31, 861 (1958); Fahrenfort, J., Van Reijen, L. L., and Sachtler, W. M. H., in “The Mechanism of Heterogeneous Catalysis” (J. H. De Boer, ed.), p. 23. Elsevier, Amsterdam, 1960;

90

35. 36. 37. 38. 39. 40. 41. 42.

43. 44. 45.

46. 47.

48.

49. 50.

51. 52. 53. 54.

55.

KEN21 TAMARU

Eischens, R. E., and Pliskin, W. A., Proc. 2nd Intern. Congr. Catalysis 1960 p. 789 (1961). Technip, Paris. Reiniicker, G., and Hansen, N., 2. Anorg. AElgem. Chem. 285, 283 (1950). Suhrmann, R., and Wedler, G . , 2. Elektrochem. 60, 892 (1956). Sachtler, W. M. H., and De Boer, N. H., J . Phys. Chem. 64, 1579 (1960). Block, J., and Kral, H., 2. Elektrochem. 63, 182 (1959). Otaki, T., J . Chem. Soc. Japan 80, 255 (1959). Tamaru, K., Trans. Faraday SOC.55, 1191 (1969). Frankenhurger, W., and Hodler, A., Trans. Paraday Soc. 28, 229 (1932). Bokhoven, C., Van Heerden, C., Westrik, R., and Zwietering, P., in “Cat,alysis” (P. H. Emmett, ed.), Vol. 111, p. 265. Reinhold, New York, 1955. Jungers, J. C., and Taylor, H. S., J . Am., Chem. SOC.57, 679 (1935). Barrer, R. M., Trans. Faraday SOC.32, 490 (1936). Logan, S. R., and Kemball, C . , Trans. Paraday SOC.56, 144 (1960). Tamaru, K., Shokubai, 4 , 30 (1962). Frankenhurg, W. G., in “Catalysis” (P. H. Emmett, ed.), Vol. 111, p. 171. Reinhold, New York, 1955. Emmett, P. H., and Brunauer, S., J . Am. Chem. SOC.56, 35 (1934); Love, K. S., and Emmett, P. H., ibid. 63, 3297 (1941); Brunauer, S., Love, K. S., and Keenan, R. G . , ibid. 64, 751 (1942). Temkin, M., and Pyzhev, V., Acta Physicochim. URSS 12, 327 (1940). Scholten, J. J. F., and Zwietering, P., Trans. Faraday Soc. 63, 1383 (1957); Scholten, J. J. F., Zwietering, P., Konvalinka, J. A., and De Boer, J. H., ibid. 55, 2166 (1959); Scholten, J. J. F., Konvalinka, J . A., and Zwietering, P., ibid. 56,262 (1960). Enomoto, S., and Horiuti, J., J . Res. Inst. Catalysis, Hokkaido Univ. 2 , 87 (1953). Bokhoven, C., Gorgeles, M. J., and Mars, P., Trans. Faraday Soc. 56, 315 (1959). Horiuti, J., and Takezawa, N., J . Res. Inst. Catalysk, Hokkaido Univ. 8, 127 (1960); Kodera, T., and Takezawa, N., ibid. 8, 157 (1960). Tanaka, K., and Matsunaga, A., presented at the 16th Annual Meeting of the Chemical Society of Japan, Tokyo, April, 1963. Ozaki, A,, Taylor, H., and Boudart, M., Proc. Roy. Soc. A25.8,47 (1980).

T h e Mechanism of the Hydrogenation of Unsaturated Hydrocarbons on Transition Metal Catalysts G. C. BOND Johnaon. Matthey and Company. Ltd., Research Laboratories, Wembley. Middlesex. England AND

P. B. WELLS Department of Chemistry. University of Hull. Hull. England

Page 92 A . TheScopeoftheReview ............................................ 92 B The Variables of the System ......................................... 92 C . The Concept of Alternative Reaction Paths ............................ 94 D . What Is a Reaction Mechanism? ..................................... 96 11. TheHydrogenationofOlefins ........................................... 98 A . TheAdsorbedStateofOlefins ........................................ 98 B . Possible Reaction Mechanisms ....................................... 102 C The Treatment of Experimental Results .............................. 108 D . Reactions over Nickel .............................................. 110 E Reactions over Iron ................................................ 121 F. Reactionsover Cobelt .............................................. 124 G . Reactionsover Palladium ........................................... 124 H . Reactions over Platinum ........................................... 132 I . Reactions over Iridium ............................................. 143 J . Reactions over Rhodium ............................................ 146 K . Reactions over Ruthenium and Osmium ............................... 151 L Reactions over Other Metals ......................................... 163 M . Summary and Conclusions .......................................... 154 I11. The Hydrogenation of Alkynes and Dienes ................................ 155 A . Introduction ...................................................... 155 B The Hydrogenation of Acetylene ..................................... 159 C. The Hydrogenation of Monoalkylalkynes .............................. 173 D . The Hydrogenation of Dialkylalkynes ................................. 176 E . The Hydrogenation of More Highly Unsaturated Hydrocarbons ........... 183 F. TheHydrogenationofDienes ........................................ 184 I V The Hydrocarbon-Metal Bond in Catalytic and Organometallic Chemistry ..... 205 A . Patterns of Behavior in Catalytic Reactions of Hydrocarbons ............ 205 B . The Nature. Stability. and Reactivity of Hydrocarbon-Metal Compounds and the Relevance of This Information t o Heterogeneous Catalysis ........ 210

.

I Introduction

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

.

. .

.

.

.

91

92

Q. C. BOND AND P. B. WELLS

1. Introduction A. THE SCOPEOF

THE

REVIEW

The purpose of this chapter is to review the present status of the problem of the catalytic hydrogenation of unsaturated hydrocarbons over transition metal catalysts. This problem continues to attract the attention of those engaged in the study of heterogeneous catalysis, largely perhaps because of the rich variety of relevant systems and the wealth of attainable detailed information. Where the complexities are so great, it sometimes appears that little progress is being made. For example, Selwood ( I )has remarked: “No problems in surface chemistry have been more hotly debated than the adsorption and hydrogenation mechanisms for ethylene; and few debates have resulted in such meagre conclusions.” It is our contention, however, that a measure of order is now appearing, and we hope that this sense of incipient order will be conveyed in this article. We confine our attention largely to the hydrogenation of olefins, diolefins, and acetylenes over the nine metals of Group V I I I together with copper, gold, and tungsten: no results are available for any other of the transition metals. We shall refer only to those papers which contribute significantly to the understanding of reaction mechanisms, and we shall concentrate on advances made since the last major review (2). Measurements of the exchange of saturated hydrocarbons with deuterium will be referred to only where they illuminate our chief problem. In any case, this field has been reviewed quite recently (3).

B. THE VARIABLESOF

THE

SYSTEM

All catalytic systems are complex, but the system which now commands our attention has complexities peculiar to itself, and we must now consider their nature. We may distinguish at least six different types of unsaturation between carbon atoms: (1) acetylenic, -C=C; (2) olefinic, >C=C< ; (3) allenic, >C=C=C< ; (4) conjugated olefinic, >C=CH-CH=C< ; (5) nonconjugated olefinic, >C=CH(CH,),-CH=C<, (n 2 1); and ( 6 ) aromatic. We have thus at the outset six fundamentally different hydrogenation systems to contend with: we exclude from consideration the hydrogenation of small alicyclic rings ( 4 ) . There are in addition to these six main classes some minor variants in which novel features may appear; for example, cyclopropene and its derivatives, and molecules in which two types of unsaturation appear simultaneously (e.g., vinylacetylene).

HYDROGENATION OF UNSATURATED HYDROCARBONS

93

Each of these six unsaturated groups can exist in a variety of environments: we may take the olefinic double-bond as an example. (1) The number of substituent alkyl groups may be varied; (2) the chain length of a substituent alkyl group may be varied; (3) the double-bond may be located in a molecule containing one or more of a limitless number of other functional groups; or (4)the double-bond may be part of one or two alicyclic rings. The possible effects of substituents may be classified as follows. (a) Geometric effects, due to their size. Bulky groups may reduce the number of molecules which the surface can accommodate and may result in there being vacant spaces left between them on which hydrogen may adsorb noncompetitively. The size of the olefin molecule may therefore of itself affect the kinds of possible surface species. (b) EZectronic effects due to the electron-releasing or withdrawing properties of the substituents. These will affect the electron density within the doublebond, and may thus affect the strength with which the olefin is chemisorbed. There are at least fifteen metals (and a very large number of alloys) which will catalyze the hydrogenation of one or more of the types of unsaturation listed above, although nickel, palladium, and platinum have claimed a preponderance of attention up to now. Each metal may be used in one of a number of forms: (1)macroscopicforms (wires, foils, granules); (2) microscopic forms (powders by chemical reduction, smokes, skeletal powders, colloidal suspensions, blacks, condensed metal films); ( 3 ) supported forms, where the metal in varying concentration is dispersed to a varying degree on another more or less inert substance, usually an irreducible metal oxide or salt. Within each category there is scope for infinite variation. Even with metal in wire form, which might be imagined to be closely reproducible from batch to batch, the degree of surface roughness and the grain structure will depend on the specimen’s precise metallurgical history, and these factors may not be without effect on the rate and mechanism of a reaction which it is used to catalyze. We consider finally a few obvious variables which contribute to the overall complexity of our system. (1) Phase: hydrogenations may be carried out with the unsaturated molecule either as a vapor or as a liquid, with or without the presence of a solvent whose physico-chemical properties affect the rate and mechanism of reactions in a manner not yet understood. ( 2 ) Pressure: where the organic reactant is in the liquid phase, the hydrogen pressure can be varied from subatmospheric to 500 atm, and although the principal effect is on the rate, the product distribution is sometimes changed. I n gas-phase reactions, alteration of the ratio of the partial pressures of the reactants sometimes has a

94

a. C.

BOND AND P. B. WELLS

dramatic effect on the nature of the products formed. (3) Temperature: although here also the effect is chiefly on the rate, the temperature will certainly influence the relative proportions of the adsorbed species and their quantitative interactions. There is thus a very large number of potentially variable factors. Some of these, such as the chemical nature of the reactant and the metal, are readily controlled; others, such as the physical form of the catalyst, are difficult to control and their investigation requires elaborate equipment. With so many variables, one may forgivably be daunted by the magnitude of the total task of assessing the relevance of each variable to the reaction mechanism and of understanding with precision its mode of operation. It is therefore desirable to consider carefully what simplifications are possible, and what is the minimum of information we must have before a knowledge of the reaction mechanism can be claimed.

C. THECONCEPTOX ALTERNATIVE REACTIONPATHS Some simplification of our problem can be achieved by restricting the features of the reactions which we first seek to understand. We realized several years ago that to certain features of reaction mechanisms some of the variable factors are largely irrelevant (5);and most fortunately these factors are just those which are most difficult to observe. I n a “simple” reaction such as that of hydrogen with ethylene there is a very limited number of things which can be measured: orders of reaction, activation energy, and perhaps the simultaneous para-hydrogen conversion. Activation energies are notoriously capricious, for they (and orders of reaction) are influenced by surface contamination. The secret seems to lie paradoxically in studying more complex reaction systems, namely, those in which the reaction may proceed in two different directions, to yield two or more recognizably different products. I n such a system of competitive reactions, certain of the variable and uncontrollable factors seem to influence the alternative paths equally, so that the dijferences in rates, and hence also in orders and in activation energies, are of greater reproducibility and significance than absolute values. We hope to show in this review that this comparative approach is of considerable value. We consider now the easiest ways in which the main types of unsaturation may be studied so that alternative paths become discernable. These ways are illustrated by examples in Table I without mechanistic interpretation, which will be attempted subsequently. If by this technique extraneous factors are removed, a valid comparison of mechanisms for the different classes of hydrogenation should be possible.

w 5

TABLE I Alternative Reaction Paths i n the Hydrogenation of Unsaturated Hydrocarbons

E Unsaturated Hydrocarbon

Other reactant

Intermediate products

Final products

Designation of alternative path

*22 0

2

w

Acetylene

HZ

Ethylene; higher olefins

Ethane ; higher paraflins

Selectivehydrogenation to ole&

cl

Allene

HZ

Propylene

Propane

Selectivehydrogenation to olefin

cl

Butadiene

HZ

Butenes

Butane

Selectivehydrogenation to oleiin

Ethylene

DZ

Deuterated ethylenes

Deuterated ethanes

Olefin exchange

2-Butenes

Butane

Double-bond migration

1-Butene

HZ

35 w

!H 5 U

1d 0

brn

0

96

a.

C. BOND AND P. B. WELLS

The concept that certain factors affect two reaction paths equally arose from consideration of the literature and of our own experience (5). It is worth emphasizing that two factors are known which are not of this category. (1) Deliberate or excessive accidental poisoning, e.g., by mercury vapor ( 6 ) may result in the selective deactivation of one path. (2) Where the overall rate is determined by a transport process, an intermediate product may suffer further reaction before leaving the neighborhood of the surface (7’); it is therefore important to ensure that the catalytic steps are all slower than transport steps. D. WHATIS

A

REACTION MECHANISM?

We come now to the central problem of exactly what it is necessary to know before the mechanism of any particular reaction is generally regarded as established. Opinions on this will doubtless vary, but the following discussion is offered in the hope that it will clarify the issues involved. The establishment of the mechanism (however defined) of a heterogeneous reaction is an incomparably more difficult task than for a homogeneous reaction. The relative magnitude of the two tasks is shown by the fact that the mechanisms of many homogeneous reactions are agreed and their transition states quite well defined, whereas the mechanism of no heterogeneous reaction is as yet beyond dispute. The main cause of the greater complexity of heterogeneous reactions is that the surface is an active partner in the reaction and must be regarded as one of the reactants. The concentration of the active surface in the steady state is not directly accessible; moreover, the normal state of the free surface may be disturbed when adsorption occurs on it. The geometrical disposition of the surface atoms and irregularities therein will certainly be important, and for these reasons the transition state ofa heterogeneous process is inherently more difficult to describe (both in geometric and energetic terms) than that of a homogeneous reaction. Of necessity, therefore, we have to make our interpretations in terms of over simplified models, and to disguise our ignorance by crude symbolism. Any mechanistic analysis must be made in terms of a model; and every model is limited by its frame of reference. The kind of answer we get depends upon the language in which the question is framed; the value of the answer is determined by the care which has gone into defining the nature of the conceptual model and by the symbolism employed t o express it. We may ask the question, “Is ethylene associatively chemi-

HYDROGENATION OF UNSATURATED HYDROCARBONS

97

sorbed during its hydrogenation by a metal?” If we express the process as C*H4( g ) + CzH4 (4

we may (or may not) obtain a straight answer. But if the question is “What is the structure of the adsorbed ethylene?” we have t o abandon the use of simple formulas and express our answer in terms of structural formulas of which the following are examples: HSC-CH,

!I

H,C-CH,

T

It is a t this level of refinement, and with symbolism of this kind, that most mechanistic discussion is carried out, and the discussion in this review will prove no exception. We must recognize its limitations, however: it can, for example, lead one to propose steps which are sterically impossible, and the next stage of refinement must involve the use of scale drawings or molecular models. It is at this stage that considerations of surface geometry have to be introduced, and mechanisms discussed in terms of the precise disposition of surface atoms. We have, for example, to allow that certain processes may occur on certain crystallographic planes but not on others, but few discussions of mechanism have attained this level of refinement. There remains a third and perhaps final stage of refinement where the adsorbed species and their interaction are described precisely in quantum-mechanical terms. But this is for the future; for the present we must be content to seek agreement a t a much more elementary level, We propose for the present that a mechanism is understood if the following points are established beyond reasonable doubt. (1) The nature of all the species participating in the reaction. (2) The qualitative modes of their interaction contributing significantly to the total reaction. (3) Quantitative aspects of these interactions expressed on a relative but not absolute basis. There are two possible avenues to this information. A number of very useful direct physical methods are now being employed to establish the nature of adsorbed hydrocarbon species: the principal ones are infrared spectroscopy (8, 9) and the magnetic method ( I ) . These and other methods may suggest but cannot prove what species may exist during a reaction unless measurements are made under the appropriate conditions (see Section 11,A). For gaining information on what reactions occur on the surface there is no substitute a t the moment for the rational analysis of kinetic measurements, and it is with this indirect approach that the remainder of this article will be chiefly concerned.

98

a. C.

BOND AND P. B. WELLS

II. The Hydrogenation of Olefins A. THE ADSORBED STATEOF OLEFINS It is now clear beyond any doubt that adsorption of an olefin precedes its hydrogenation. Before discussing the mechanism of the hydrogenation of olefins a t metal surfaces, it is therefore convenient to summarize the present state of knowledge concerning the adsorbed state of olefins, and the possible ways in which they, and species derived from them, may interact with hydrogen-containing species. This is attempted in this and the following subsections. A considerable number of studies have been made on the interaction of ethylene with (‘clean)’metal surfaces, and these have been summarized elsewhere ( 1 , 2); information on other olefins is almost entirely lacking. In the absence of hydrogen, substantial disruption of ethylene occurs, and the formation of ethane by “self-hydrogenation” and of polymers has been noted. The techniques of magnetochemistry ( I ) and infrared spectroscopy (8)have contributed significantly to the solution of this problem. Unfortunately, most of the information so obtained is of doubtful relevance to the problem with which we are chiefly concerned, namely, the adsorbed state of an olefin during its hydrogenation. There are two reasons for this. First, a clean metal surface is highly reactive and becomes progressively less so as coverage increases: sites are available in abundance for adsorption of the inital product of the decomposition, namely hydrogen, and these site8 may be absent in an almost fully covered surface. Thus on energetic and steric grounds we may expect certain processes to occur on a nearly bare surface, while these processes would be unable to proceed on a highly covered surface. Second, the presence of reactant hydrogen in the system would be expected to maintain the adsorbed ethylene in a more fully hydrogenated state than in its absence, and there is evidence from infrared spectroscopic studies (8) that this is indeed so. For information on the adsorbed state of olefins during their hydrogenation, we are driven to rely almost entirely on the logical interpretation of the kinetics and product distributions observed. We shall, in fact, carry out our discussion in terms of two basic kinds of adsorbed olefin, namely (1) a-diadsorbed olefin and (2) n-adsorbed olefin. A u-diadsorbed olefin is supposed to be formed by rehybridization of the carbon atoms of the olefinic bond to sp3 hybridization followed by the

HYDROGENATION O F UNSATURATED HYDROCARBONS

99

formation of two a-bonds between the carbons and two metal atoms, which process may be formulated as R,CH=CHRa

+ 2* + R,CH-CHRa

1 I

Structure (A)

The geometric properties of such species have been fully discussed (2). We may distinguish between two forms of n-adsorbed olefin. (a) Any olefin may form a n-donor bond with a surface metal atom, the resulting species closely resembling the olefin complexes of palladium, platinum, and silver ( 1 0 , I I ) . This species may be represented as R,CH-CHRp,

T

Structure (B)

and it will be referred to as n-adsorbed olefin: the nature of the bonding is shown in Fig. 1. As the carbon atoms are still spa-hybridized, the

/

Filled bonding q 2p orbital antibonding

orbital

d-orbital -orbital

FIG.1. Diagramatic representation of the a-bonding of an olefin to a metal atom having a vacant d-orbital.

atoms and groups attached to the carbon atoms are almost coplanar in a plane parallel to the surface: only one metal atom in the surface is directly involved, although others may be obscured by the adsorbed molecule. (b) Olefins possessing one or more a-methylenic hydrogen atoms may lose one such, with the formation of what will be called a r-allyl-adsorbed species: RaCH= CH--CH,R,

+*

--H

R~CH-CHT-CHRI

I

Structure (C)

Again only one metal atom is directly involved, and the bonding is

100

Q.

C. BOND AND P. B. WELLS

assumed to resemble that in the now well-known rr-ally1 complexes of palladium salts (10).I n Structure ( C ) , all the atoms and the groups R, and R, will be coplanar in a plane parallel to the surface, and the C-C-C angle will be about 120". The n-methylallyl radical (R,=CH,, R,=H) will be capable of existence in syn- and anti-conformations, which viewed normal to the surface will appear as CH

CI H

/-\

CH8

bH* anti-conformation

syn-conformation

Free rotation between these conformers is prohibited. Species of this kind were first proposed in connection with interpretation of results concerning the exchange of polymethylcyclopentanes over palladium films ( 1 2 ) )but they may have a wider significance. Adsorbed species having n-electrons delocalized over four carbon atoms were also proposed (12).The fate of the hydrogen atom lost in the formation of Structure (C) from (B) (which is its most likely precursor) will be discussed in Section 11, B, 1. New effects arise when the adsorption of cyclic olefins is considered. I n the case of 1,2-dimethylcyclohexene,for example, both sides of the molecule are equivalent, and adsorption as a n-olefin species [Structure (B)] is possible. If, however, the molecule is adsorbed as Structure (A), the two methyl groups automatically assume a cis-configuration, and the ring assumes a boat or more probably a chair conformation. I n the case of 2,3-dimethylcyclohexene,however, the two aides of the ring are not equivalent if a change in hybridization occurs on adsorption. If adsorption occurs with the methyl group in position 3 pointing away from the surface, the product is o-1,2-diadsorbed cis-2,3-dimethylcyclohexane: if on adsorption this methyl group points toward the surface, the product has the trans configuration (13). Similar conclusions also apply to other dialkylcyclohexenes and to the octalins (13, 14). Motivated by the expectation that stereochemical analysis of products would elucidate reaction mechanisms, there have been a number of studies of the hydrogenation of alicyclic olefins reported recently (13-22):these will be reviewed later. It is now firmly believed that the hydrogenation of aromatic substances has certain steps in common with the hydrogenation of cyclic oIefins (16,23),and hence a word concerning the adsorbed state of the aromatic ring and of possible partially hydrogenated intermediates is in

HYDROGENATION O F UNSATURATED HYDROCARBONS

101

order. It seems likely that the aromatic ring is adsorbed flat on the surface, either by six a-bonds (with essentially complete loss of resonance energy) as in Structure (D) (16) or by a r-bond (with essential retention of resonance energy) as in (E)(23). Evidence for this latter structure is

*Q * *

Structure (E)

Structure (D)

accumulating. The hydrogenation may be visualized as proceeding from this structure through a series of intermediates until a r-adsorbed cyclohexene is formed (23) (see Fig. 2). The extent to which this last

q*C>l.-Q ._._-.' * z--adsorbed benzene

-H

7

'.

--H

'--____

*

'.---____

7- allylic adsorbed

1,3 -cyclohexadiene

*

a+@ -+H

cyclohexene

---___-

1

a-adsorbed cyclohexene

*

r-allylic adsorbed cyclohexene

FIG.2. Possible *-adsorbed intermediates in the hydrogenationof the benzene ring (23).

species (or its a-diadsorbed analog) is common to the hydrogenation of both dialkyl benzenes and dialkyl cyclohexenes will determine the connection between the stereochemistry of the resulting dialkyl cyclohexanes. Direct evidence for the participation of any of the foregoing species in hydrogenation reactions is scant. Structure (A) was believed to be formedLwhen ethylene is admitted to hydrogen-covered nickel-silica ( 8 ) . Evidence for Structure (B)is provided by the observation that the surface potential of ethylene on nickel film is t0.83 volts ( 2 4 ) and not negative as would be expected for Structure (A) on electronegativity considerations. Until such time as experimental methods for the direct observation of adsorbed species under reaction conditions are perfected, we must discuss possible reaction mechanisms in the most general way.

102

U. C. BOND AND P. B. WELLS

B. POSSIBLE REACTION MECHANISMS 1. Introduction

We start with the knowledge that interaction of an olefin with hydrogen or deuterium can lead to the occurrence of one or more of the following processes. (1) Hydrogenation (2) Olefm exchange (substitution of H atoms of the olefin by D atoms) (3) Hydrogen exchange (formation of HD and H,) (4) Double-bond migration (e.g., formation of 2-butenes from 1butene) (5) Cis-trans isomerization (e.g., formation of cis-2-butene from truns2-butene) Their occurrence may be explained by proposing a number of elementary steps in which a hydrogen atom (the term is used generically) is added to or removed from an adsorbed hydrocarbon species. We may classify the possible donors of hydrogen atoms in the following way, (1) Molecular hydrogen, either not adsorbed or at most weakly adsorbed. (2) Adsorbed atomic hydrogen, most probably arising from the dissociative adsorption of molecular hydrogen on pairs of vacant sites. (3) A hydrogen-rich hydrocarbon species. (4) Dissolved atomic hydrogen (most likely to be important in the case of palladium). The hydrocarbon species formed from a hydrogen-deficient species and an atom drawn from any of these sources is a hydrogen-rich species, and is itself a potential hydrogen atom donor [see (3)]. We may therefore summarize the several possibilities in terms of the general process: XH H atom donor

+

R H-deficient species

(T)X H atom acceptor

+

RH H-rich species

where X represents H in ( l ) ,a surface site in (2), R in (3) and the bulk metal in (4). A hydrogen-deficient species is thus ips0 facto also a hydrogen atom acceptor. R may represent either (a) adsorbed olefin having any of the Structures (A), (B), or (C), or (b) an adsorbed alkyl radical (see Section 11, B,2),in which case the reverse reaction will not occur. Although the first object of mechanistic studies is the identification of adsorbed hydrocarbon species, the nature and relative importance

HYDROGENATION OF UNSATURATED HYDROCARBONS

103

of the hydrogen donors and acceptors responsible for their interconversion is an important secondary objective. This is a particularly difficult matter for reasons which appear below. 2. Mechanisms of Hydrogenation

There is much evidence for the belief that the transformation of an adsorbed olefin into a saturated hydrocarbon occurs principally in two stages, and that a “half-hydrogenated state,” i.e., an adsorbed alkyl radical, is a relatively stable intermediate (2). The most impressive evidence for this view is the identity of products obtained when equilbrated and non-equilibrated hydrogen-deuterium mixtures are employed (2527). This is not to deny that a part of the reaction may proceed through the addition of a molecule of hydrogen essentially in one step (28). The paraffin once formed is quite unreactive, and its readsorption may be neglected. What is less certain is the manner of formation of the alkyl radicals. We have no means at the moment of deciding whether these may be formed by addition of H to Structure (B), as R,CH-CHR,

€

+R

+R,CH-CH,R,

1

or whether Structure (A) is a necessary intermediate: R,CH-CHRb

1

+ R,CH-CHRI

1 1

+H

---+ R,CH-CH,R,

i

(21

The observations (29) on the homogeneous reduction of coordinated olefins confirms the possibility of mechanism (1). It seems unlikely that r-allyl-adsorbed olefin [Structure (C)] will pass directly into an alkyl radical without either (A) or (B) intervening as in mechanism (1) or (2), although the process +H

P

-----I------I

R,,CH-CH-CHRI

RJ2H-CH,-CH2R,

(3)

cannot be discounted, The alkyl radical is finally hydrogenated by the process +H

R,CH-CH,R,

4

RaCH2-CH,R,

(4)

While the essential features of the hydrogenation of olefins are probably adequately described in terms of some or all of these general processes ( 2 , 3 0 ) ,the possibility that any of the sources of hydrogen atoms

104

AND P. B. WELLS

0. C. BOND

listed in the previous section may provide the hydrogen employed in mechanisms ( l ) , (2), and (4)greatly increases the number of possible elementary steps. Thus if we consider only mechanism (1) we have the following possibilities : R.CH-CHRb

1

+ H,

4 R,CH-CH,Rb

+H

+ R.CH-CH,Rb

R,CH-CHR,

T

RoCH-CHRb

T

I

1

+ R.CH-CCHRa

R,CH-CHzR,

1

T

1

*I

+ R,CH-CK,Rb

2RaCH-CH--CHzR,

+H

1

+ ------I------

+ R,CH-CH-CHR,

I

1

R,CH-CH,-CH,Rb

(10)

(Id)

Nor is this the end of the complication. We must admit the possibility that more than one of the possible sources of hydrogen atoms contributes to the formation of alkyl radicals, and that the relative extents of their contributions in process (4) may not be the same. We see, therefore, the justification of the statement made in the last section that the assessment of the r6les of the hydrogen donors in the hydrogenation process is a matter of some complexity; and indeed it is only the simplest systems where special techniques of interpretation are available that some resolution of this problem is possible. Orders of reaction never supply entirely unambiguous information on this point. A short comment is necessary on the formation of an alkyl radical from a cyclic olefin. If it is adsorbed as Structure (A) (o-diadsorbed species), it has already assumed the configuration of the product unless some unusual processes follow (16). If, however, i t is adsorbed as Structure (B) (7-olefin),the structure of the product is not assumed until i t becomes an alkyl radical, for the configuration-determining step is that in which the sp2-hybridized carbon atoms change their state of hybridization to that of the final product. 3. Mechanisms of Olejn Exchange

We discuss first the case of ethylene, where the r-allylic Structure (C) cannot exist. For simplicity we will suppose the formation of a monodeuteroethyl radical by addition of a deuterium atom to 7-adsorbed ethylene according to reaction (1). The process of olefin exchange is then accounted for by its reverse, followed by olefin desorption: H,C--CHz

T

-D f--

CH,-CHzD

1

--H

---+

H,C-CHD

T

(5)

HYDROGENATION OF UNSATURATED HYDROCARBONS

105

The rate of olefin exchange relative to that of hydrogenation will be governed by k5,/k4and also (together with the multiplicity of the exchange) by kd/kl, k d being the rate constant for the desorption of the olefin. The rate of olefin exchange is given by roe = k

d4

where 0, is the fractional coverage of the surface by olefin. This combination of steps may also account for olefin exchange in higher olefins, although there are other possibilities to consider. I n the case of propylene for example exchange could additionally occur through the r-allylic Structure (C): --H

+-------------CHS-CH-CH, I

CHa-CH-CH,

T

+D

+CHBD-CH-CH,

(6)

T

Reiteration of this sequence (either with or without the intermediate desorption of the olefin) could lead to complete exchange of the hydrogen on the terminal carbon atoms, but not that on the central atom. This therefore constitutes a critical test of this being the sole mechanism for exchange in this system. Evidence for the occurrence of this process should be sought in a comparative analysis of the exchange of ethylene and propylene. Its application to higher olefins is described in the next section. There is sometimes observed an extensive redistribution of deuterium atoms in the paraffin formed from an olefin and deuterium, although little olefin exchange is found (25, 28, 31, 32). This is understood if there is a facile olefin exchange [process ( 5 ) ] leading to adsorbed deuteroolefins which are, however, reluctant to desorb. 4. Mechanisms of Isomerization There are analogously two possible mechanisms for olefin isomerization. The first, which again we write with Structure (B) although (A) could be used equivalently, is as follows.

Double-bond migration -H

+H

RJ2H-CH-CH,Ra

RaCH

T

CH-CHZR,

R.CH,-CH-CCHR,

1

,-l

(7)

Cis-trans isomerization R,WCH-CH r R a

I

+H

+R.$H,-CH-Rb

I

--H

---+

r

R.WCH-CHb

R,

(8)

106

Q. 0 . BOND AND P.

B. WELLS

As in mechanism ( 6 ) for olefin exchange, an intermediate alkyl radical is again involved. The alternative mechanism, which is analogous to (6), can account for double-bond migration, as follows: -H

R,CH=CH-CH,RI,

I

+H

R,CH-CH-CHR, __----_-______ j R,CHa-CH=CHR,

I

1

(9)

However, cis-trans isomerization cannot be so explained, since free rotation is not possible in a n-allyl-adsorbed olefin as it is in an alkyl radical. This can only occur if a double-bond shift first occurs, as shown in Fig. 3. The mechanisms of olefin isomerization may be expected to be clarified by the use of deuterium, when it is possible to study the manner of the appearance of deuterium atoms in the isomerized olefin (31). It is possible to apply the methods of conformational analysis to predict the relative yields of the cis- and trans-isomers formed by isomerization of an a-olefin [mechanism (7): R,=H, R,=CH, in the simplest case]. On the assumption that the metal atom to which the 2-butyl radical is attached may be regarded as equivalent to another methyl group, it is found that there are three possible staggered conformations (see Fig. 4).Conformation (I)would lead only to cis-2-butene, (11)only to trans-2-butene, while (111)would be expected to give an equal mixture of both isomers. The energies of (11)and (111)are about equal, 1-Butene (9)

(anti -1 -methyl - r - a l l y l )

cis-2-Butene

(g)

(syn -1-methyl-r-allyl)

trons-2-Butene

(9)

FIG.3. The mechanism of the cis-tram isomerization of 2-butene involving n-allylia intermediates.

HYDROGENATION OF UNSATURATED HYDROCARBONS

107

while the energy of (I)is greater by about 0.8 kcal mole-1: the translcis ratio should therefore fall from 3 at very low temperatures [where (I) will not contribute] to a high-temperature limiting value of unity. Values of this ratio a t 0 and 150" have been computed to be 1.85 and 1.20, respectively, and values in this range are often encountered (see below). No quantitative prediction of the translcis ratio expected from mechanism (9) is possible. 5 . Mechanisms of Hydrogen Exchange

When deuterium is used in place of hydrogen, the formation of the species HD and H, is sometimes observed. If the rate of return of H atoms to the gas phase by hydrogen exchange equals the rate of return of D atoms by olefin exchange, then the deuteroparaffin will contain on average two D atoms: if the former rate exceeds the latter, the deuterium number (i.e., mean number of deuterium atoms) of the paraffin instantaneously formed will be less than two, and vice versa. I n principle hydrogen exchange may operate through the combination of a n H atom from LL hydrogen-rich species with a D atom or another H from any source, viz., RH

+D

--t

R

+ HD

:

RH

+H

+R

+ H,

The second H or D atom may be adsorbed as such, or may also belong to a hydrogen-rich species. These alternatives are exemplified as follows: CHZ-CH'D

1

+D I

2CHz-CH'D

--t

H C-CHD

'I

+ HD

(10)

-+ 2H

I

The principle of microscopic reversibility requires that hydrogen exchange proceeds only through those mechanisms whose reverse

FIG.4. Staggered conformations of the adsorbed 2-butyl radical.

108

0. C. BOND AND P. B. WELLS

reactions are also possible. Thus since the processes of formation of hydrogen-rich hydrocarbon species can rarely be specified, the mechanisms of hydrogen exchange generally remain uncertain.

C. THE TREATMENT OF EXPERIMENTAL RESULTS Although the ultimate objective of experimental studies is the establishment of reaction mechanisms, a certain amount of useful and interesting quantitative information emerges immediately from work of the kind we are here concerned with, without refined mechanistic analysis. It is for example a t once possible to establish broad trends of behavior with regard t o the ability of metals to catalyze olefin exchange and isomerization, and hydrogen exchange. Comparison of such trends with those shown in organometallic chemistry, as will be attempted in Section IV, takes us some considerable way toward an understanding of the mechanisms of catalytic hydrogenation. However, the purpose of this section is to review those techniques whereby experimental results may be caused to reveal the basic reaction mechanisms. It may be said a t once that the determination of orders of reaction can never by itself lead to the complete specification of mechanisms : possibilities may thereby be eliminated and probabilities suggested but full certainty never results. The attempted interpretation of such measurements in terms of fundamental theories such as the absolute rate theory (33, 34) and the power rate law (35) has been disappointing. The statistical mechanics approach adopted by the school of Horiuti (30, 36, 3 7 , 38) suffers from the limitations of the model, which is the simplest possible one : C,H, 2 HaC-CH,

1 1

CH2-CH8

1

:

H, 3 2H

I

+ H -+ C,H, I

The existence of all other processes is denied (13)and indeed in principle no more are necessary t o account for the observed processes. The scheme is readily extended to higher olefins (39),although calculations then become very laborious. This approach has been applied principally to reactions occurring over nickel [see, however, (40)and (41)for applic-

HYDROGENATION OF UNSATURATED HYDROCARBONS

109

ations over platinum], but perhaps because of its difficulty it has not found support outside Japan. A procedure of great utility in unraveling reaction mechanisms is that presented by Kemball ( 4 2 ) .This has the advantage that the minimum of mechanistic detail is imposed on the model, but it suffers from the disadvantage that i t is only applicable a t the moment t o the reaction of ethylene with deuterium, for reasons stated below. The assumptions of the model are that there exist adsorbed C,X4 and C2X, species which react with the following probabilities:

There are six possible adsorbed ethylenes and twelve possible ethyl radicals, and it is a relatively simple matter to set down eighteen simultaneous equations which may be solved for any values of p , q, and r to give the yields of the deuteroethylenes and total ethane. The ethyl radical distribution is converted into a n ethane distribution by means of the parameter s. This procedure gives more information that is yet experimentally attainable, for i t states the fraction of ethylene-do returned to the gas phase and the separate fractions of the positional isomers of deuteroethanes where those exist. It also of course gives the ratio C2X4(,/C,X, (a). The solution of the simultaneous equations for chosen values of the parameters has been greatly speeded by the use of a computer (31). An experimental distribution of products may then be compared by inspection with the computed distributions and that giving optimum agreement selected. The complexity of the analysis increases swiftly as the number of carbon atoms in the olefin rises: thus there are fifty-six distinguishable propyl radicals and twenty-four propylenes, making a total of eighty simultaneous equations to be solved, and although this is not impossible it has not yet been thought worthwhile. The analysis of the C4 system would be a formidable problem. A similar but rather simpler scheme has been applied to the reactions C2D,+H, and C,H,+D2 (see Section IIIB). The parameters p and r are especially interesting in that they give

110

a.

C. BOND AND P. B. WELLS

the relative rates of the surface processes involving hydrocarbon species: the parameters q and s give information on the nature of the hydrogen donors. The review of the experimental results, which now follows, will concentrate on assessing the extents to which the transition metals catalyze the processes of olefin exchange and isomerization and the mechanisms of these processes in the light of the subject matter of this section.

D. REACTIONS OVER NICKEL 1. The Reaction of Ethylene with Deuterium

The reaction of ethylene with deuterium has been studied using nickel wire (30, 43-45),nickel-kieselguhr (27, 32, 46) and evaporated nickel films ( 4 2 , 4 7 )as catalysts. The use of differing temperatures and differing partial pressure ratios makes a comparison between them difficult. The first detailed study (45) was carried out using nickel wire at 90" and equal pressures of reactants, and the following important conclusions emerged. (1) Olefin exchange was rapid, the deuteroethylenes being formed successively and each in turn passing through a maximum (see Fig. 5). This suggests that only one hydrogen atom is exchanged during each residence on the surface, and hence that the desorption of ethylene is rapid compared with other surface processes. For this to be

Percentage hydrogenation

FIG.5. The reaction of ethylcno with deuterium over nickel wire at 90": partial pressures of deuterated ethylenes as a function of conversion ( 4 5 ) .

HYDROGENATION O F UNSATURATED HYDROCARBONS

111

so, the ethylene cannot be very strongly adsorbed on the nickel surface. (2) HD and H, were also formed abundantly. (3) As a result of these exchange reactions the composition of the ethanes changed radically through the course of the reaction, the initially formed ethane being only lightly deuterated and the finally formed ethane being heavily deuterated. The initial ethane distribution is thus inaccessible, and the final distribution is of no particular significance except for comparison purposes (see Table 11). When the reaction is performed over nickel-kieselguhr with a tenfold excess of deuterium at 50" (32) the olefin exchange is, somewhat surprisingly, competely suppressed. The initial ethane distribution (see Table 111) has a deuterium number of 2.7, the excess deuterium being balanced by the occurrence of hydrogen exchange (5.2% H in deuterium after 94% conversion); there is thus little change in the ethane distribution through the reaction. A similar state of affairs also holds a t - 50", but because the activation energy for hydrogen exchange (Ehe)is less than that for addition (Eh) ( 4 3 , 4 4 )the rate of the former is now negligible (0.45% H in deuterium after 100% conversion) and the deuterium number of the initial ethanes (see Table 111) is close to the theoretical value of two. Both exchange reactions are of small importance a t - 78" (27, 46). Over a n evaporated nickel film at - 100' with a threefold excess of deuterium, some 60% of the total initial products are deuterated ethylenes (as),distributed as shown in Table IV. I n this respect the behavior of the film resembles that of nickel wire a t much higher temperatures, although hydrogen exchange is slight (1.5% H in deuterium after 100% conversion). A distribution computed by the method discussed in Section 11, C (see also Table IV) is in fair agreement with the experimental one. The values of the parameters show that each adsorbed ethylene molecule has only a 25% chance of desorbing, and that each ethyl radical has a 92% chance of reverting to ethylene; the appearance of some multiply exchanged ethylenes in the initial products is thus accounted for. Their nonappearance a t higher temperatures over other catalysts must be due to the readier desorption of adsorbed ethylene under these conditions. The high chance of ethyl reversal accords well with the known ( 3 ) ability of nickel films to give multiple exchange of ethane. The values of q and s show that H and D atoms are drawn on to comparable extents in both of the hydrogenation steps. Final deuteroethane distributions obtained from the reaction a t 0" are virtually unaffected by variation of the degree of sintering of the film (47). Matsuzaki (30) has recently made a detailed study of the hydrogen exchange reaction over nickel wire, confirming and extending earlier

TABLE 11 Intermediate and Final De&roethm Distributions from the Reaction of Ethylene d h Deuterium over Nickel C%fzt?y8t8

Catalyst" D,/olefin Temp. Conversion ("C) (%I

do

d,

d,

ds

4

d6

d,

(I

M

Ref.

? d W

1

90

100

8.9

23.4

27.1

18.5

13.7

5.8

2.6

-

2.32

(45)

k

10

50

94

7.7

17.5

28.0

18.1

10.6

9.7

8.4

-

2.69

(32)

k

10

-50

100

17.8

25.6

27.4

16.1

7.8

3.6

1.5

0.48

1.87

(32)

k

1

-78

55

10.7

32.0

32.0

13.2

6.3

4.0

1.8

0.52

1.92

(32)

Film

3

-100

100

17.9

26.8

23.2

14.0

9.6

5.9

2.6

0.62

1.99

(42)

Film

9

-92

100

12.4

29.9

25.4

14.3

9.7

5.9

2.4

0.62

2.06

(42)

Wire

(I

k = supported on kieselguhr.

u"U

8

F

E

TABLE I11 Initial Deuteroethane D.istributwmfrom the React& of Ethylene with Deuterium over Nickel Catalysts Catalysto

D,/olefin

Temp.

do

dl

4

4

4

4

d'

a

M

Ref.

10

k

10

Film

0

3

2

5.0

17.5

29.0

18.5

16.0

9.5

5.5

0.70

2.71

(32)

-50

18.0

25.5

27.5

15.5

8.0

4.0

1.5

0.51

1.88

(32)

-100

21.2

34.0

25.8

10.9

5.4

2.2

0.5

0.43

1.54

(42)

50

k = supported on kieselguhr.

eJ 4

("C)

k

0

59 0 !z 8 w

3 TI

IF

W

TABLE IV 0

Observed and Calculated Initial Distributions of Total Products from the Reaction of Ethylene with Deuterium over a Nickel Film at - 100' (42)

dl

d

z

W

tc

Ethane8

Ethylene d, 4

4

do

dl

d,

4

4

d5

4

M

b

2

tc Observed

46.7

13.2

2.7

0.7

7.8

12.5

Calculated'

41.4

18.8

6.4

1.0

2.8

9.9

9.5

4.0

2.0

0.8

0.2

1.55

+d

W

11.1

5.7

2.2

0.6

0.1

1.90

8

M F F

rm "Parametersused: p = 3, p = 2, r = 12,

8

= 1.

HYDROGENATION OF UNSATURATED HYDROCARBONS

115

work (44).Horiuti's treatment (39) of his results is claimed to provide further verification of the simple associative mechanism (see Section

11, C). 2. The Reaction of Higher Olejins with Deuterium

Only three studies of the reaction of higher olefins with deuterium over nickel catalysts have been reported in which mass-spectrometric analysis of the products was performed (32, 48, 49). There have been other separate studies of the isomerization reactions, to be described in the next section, but no simultaneous studies of both exchange and isomerization. Thus when results have been given for the deuterated butenes formed by olefin exchange (as), it is uncertain to what extent deuterated isomerixed olefins are contributing to the total effect. There is room here for much further work. The reaction of equal pressures of 1-butene and deuterium over nickel wire a t 90" (48)follows a course reminiscent of the ethylene-deuterium reaction under these conditions (see Fig. 6): butane-do is a substantial

Percentage hydrogenation (a )

Percentage 'hydrogenation

(b)

FIG.6. The reaction of 1-butene with deuterium over a nickel wire at 90" (48). (a) Change in the partial pressures of the deuteroethaneswith conversion. (b)Change in the composition of the deuteroethylenes with conversion.

116

G).

0. BOND AND P. B. WELLS

initial product. The reaction of cis-2-butene with a tenfold excess of deuterium over nickel-kieselguhr has been studied a t 50, 0, -48, and -778” (32). There is very little olefin exchange, and only butene-d, is formed. The near-final butane distributions are given in Table V: there is little change in these distributions with conversion, but only for the results at - 48’ is it possible to derive an initial distribution with any certainty. This is also shown in Table V. Hydrogen exchange increases with rising temperature as usual, and Eh,-EEhis -2.5 kcal. mole-’. The reaction of isobutene with deuterium over nickel-kieselguhr at low temperatures (32) is notable for the very high yields of isobutane-d, which result: it has been shown to be mainly isobutane -1, 2, -d2, and an explanation of this has been offered (2). No olefin exchange was observed. The reaction of 1-hexene with deuterium over nickel-silica at 105” yields hexanes distributed as shown in Table VI (49):no observations on isomerization or exchange were made. The hexanes become progressively more deuterated as the D,/hexene ratio is raised. Isomerization of n-hexenes has been observed during their hydrogenation over Raney nickel (50). It has been remarked that the tails of many of these distributions decline in a semilogarithmic manner (2, 28), and values of a [defined as (C,H 2n+14Dz+1) / (C,H,,+ z_zDz)]are included in the foregoing tables. An attempt has been made to explain the phenomenon (28). 3. The Isomerization of the n-Butenes There have been several studies of the kinetics of the isomerization although and exchange processes undergone by the n-butenes (39,48,51) none is completely satisfactory. Thus for example the analysis of the 2-butenes formed from 1-butene has only been easy since the advent of gas chromatography. There has also been confusion concerning the term “exchange reaction” (2). There has been as yet no mass-spectrometric analysis of the isomerized and the reactant olefins separately, and much work clearly remains to be done. In the remainder of this section we draw what conclusions we can from the reported rates and kinetics. Observed orders of reaction are summarized in Table VII; they are not particularly helpful, for they are temperature-dependent and there are other complications (48). The activation energies are shown in Table VIII. It would appear both from the orders a t 0’ and from the activation energies that the rate-controlling steps in the addition and in the exchange-isomerization reactions are not the same. The rates and activation energy for double-bond migration show an isotope effect,

TABLE

V

Dmrtcrobutane Distr&ut%muf r m the. Rcadion of Butcnes tdh a Tenfold Excees of Deuterium over Nickel-Kiesdguhr

Butene

Temp. ("C)

Final H content of D.

Conversion

(%)

d,

d,

d,

d,

d,

d.

d.

d,

d,

d.

d,,

M

a

(%)

0

kl

eiO-2-ButsM

-78

97

7.3

20.9

36.7

22.1

9.0

2.8

0.6

0.3

0.1

0.1

0.1

1.19

0.33

-

eiO-Z-Butene

-48

99

7.8

20.2

33.4

21.2

11.1

4.0

2.0

1.0

0.0

0.0

0.0

2.34

0.49

2.05

eiO-f-Bntene

-46

0

10.0

m.0

33.3

20.0

10.0

3.7

1.7

1.3

0.0

0.0

0.0

2.24

0.49

-

h-2-Butene

0

98

5.1

17.3

25.6

22.4

15.3

9.2

5.1

2.0

1.0

0.0

0.0

2.95

0.51

4.9

cia-2-Butene

50

96

2.1

9.4

21.8

21.8

19.7

14.6

9.4

5.2

2.1

1.0

0.0

3.98

0.57

10.2

Ieobutene

-70

82

0.0

0.0

97.0

2.4

0.5

Traces

2.03

-

-

Ieobutene

-46

100

0.0

0.0

85.5

8.5

1.6

Traces

2.03

-

-

TABLE VI Final Proahti DistributGm from. & M i o n of C . O&jimwith Deufetium uver Nickd CaMysts

? H content

Temp. ("C)

of D, d,

(%)

Ref.

22

(49)

d,

d.

d,

d,

d.

d.

d,

d.

d.

d,,

d,,

d,,

M

D

4.9

15.2

23.2

22.0

15.7

9.3

4.9

2.3

1.2

0.6

0.4

0.2

0.1'

3.05

0.54

105

1.4

6.1

16.3

21.3

21.0

16.8

8.0

4.5

2.2

1.1

0.9

0.4

0.0'

3.92

0.55

5.7

(49)

73

105

1.8

2.8

9.4

15.7

19.6

21.4

12.1

8.7

3.8

2.5

1.4

0.7

0.00 4.62

0.57

1.8

(49)

Cyclohexene

10

35

21.3

22.6

19.3

14.0

9.6

6.0

3.5

1.9

0.9

0.6

0.3

0.0

0.0

2.22

0.54

-

(52)

Cyclohexene

10

0

18.5

20.0

19.9

14.3

10.0

6.5

4.1

2.6

1.5

0.8

0.4

0.5

0.9

2.58

0.55

-

(52)

Catalyst

Oleh

D.lolefin

Ni-SiOt

1-Hexene

3.5

105

Ni-SiO,

1-Hexene

18

Ni-SiO,

1-Eexene

Ni5lm

Ni film

W 0 %

* ~

J Go

0

No more-highly deuterated hexanes observed.

119

HYDROGENATION OF UNSATURATED HYDROCARBONS

whereas for cis-trans isomerization there is no such effect. A possible explanation for this is as follows. If cis-trans isomerization proceeds via a 2-butyl radical [mechanism (S)] rather than via r-allylic intermediates (see Fig. 3), the atom removed to form the isomerized butene must be the one originally in the molecule and not the one added in forming the radical, otherwise isomerization would not occur (see Fig. 4).No isotope effect is therefore to be expected if alkyl reversal (step 5 or an analog thereof) is rate-determining. The same should be true of double-bond migration in 1-butene, but if as seems possible 1-butene is more strongly adsorbed than 2-butene, the formation of 2-butyl could now be the slow step in which an isotope effect is to be expected. The isotope effect cannot be explained in terms of a mechanism involving .rr-allylic species. The activation energy for exchange of 1-butene is similar to that for double-bond migration with deuterium as expected from the identity of the processes. There is, however, a difference in the case of the 2-butenes, but here it is important to distinguish between exchange in the reactant and isomerized olefin, which as noted above has not yet been done. The activation energy for hydrogen exchange is notably higher than those of the other processes. The slow step in the formation of butane may be the addition of hydrogen to butyl radicals (step 4). Amenomiya (39)has applied Horiuti's statistical mechanicalprocedure TABLE VII Orders of Reaction for the Processes Occurring in the Interaction of 1-Butene with Hydrogen or Deuterium

Catalylyst

1:

Y

m

n

Ref.

0

1

0

0.6

0.6

(39)

200

1

1

-

-

(39)

Temp.

("(3 Ni-A1 ,On Ni-A1 &On Ni wire Ni wire

-

-

100

N

0.5

100

N

0.5

.pH,,PD, > P,, and

> 10 cm.

N

-

0.5 0.5

N

0.6 0.6

N

0.6

(48)

0.6"

(51)

TABLE VIII Activation Energies (kcuZ.moZe-1) of the Proceseea OccuOring in the Interadion of the n-BPltew zoith Hydrogen or Deuteri26rn' Molecule

I-Butane

2.5

-

9.0

5.9

-

76-126

(51)

I-Butane

2.0

7.1

-

7.86

-

60-135

(48)

1-Butene

11.2

-

-

7.2"

-

- 30-0

(39)

2-Butene

3.3

-

10.0

-

-

55-120

(51)

ci.q-2-Butene

3.6

8.0

-

-

5.3bc

75-130

(48)

trans-2-Butena

3.5

8.0

-

-

4.7ae

75-130

(48)

5.0"

* Subscript abbreviations: h = hydrogenation, oe = o l e h exchange, he = hydrogen exchange,

'Using D,. Uaing H,.

dbm = double-bond migration, ct = o6e-tram homerimtion.

W 0

3

HYDROGENATION OF UNSATURATED HYDROCARBONS

121

to this system, and claims that all the results agree fully with the expectations of the simple associative mechanism (Section 11, C). 4.

Conclusions

Although many mechanistic aspects of hydrogenation reactions over nickel catalysts remain to be resolved, the broad picture is clear: alkyl radicals have a high probability of reverting by loss of a hydrogen or deuterium atom to adsorbed olefin, which in turn has a high probability of desorbing, even at low temperatures. This picture accounts qualitatively for the observed efficiency of nickel in olefin isomerization and exchange.

E. REACTIONS OVER IRON 1. The Reaction of Ethylene with Deuterium

This reaction has been studied over an iron film at - 100’ (42), with results (shown in Table IX) which are similar to those shown by nickel, although the product distributions differ in points of detail. The ratio of roe/rhis closely the same for the two metals. 2. The Reaction of Cyclic Olefins with Deuterium.

The reactions of cyclopentene, cyclohexene, and cycloheptene with deuterium have been studied over iron films (52). With unsintered films the addition reactions are rapid even at -35” and only final product distributions were obtainable. I n each case these were strongly temperature dependent in the sense that highly deuterated products were formed in progressively greater amounts with rising temperature. This phenomenon is exemplified by the results for cyclohexene, shown in Fig. 7a. The activation energy for this multiple exchange process is 4-5 kcal mole-’ greater than for that leading to the lightly deuterated products. A remarkable effect was observed when sintered iron films were used. The multiple exchange process disappears and a “normal” distribution is obtained (seeFig. 7b). The use of sintered films also enabled a course of reaction to be followed (cyclohexene at 0”, cyclopentene at - 35’): stepwise olefin exchange was observed, slightly more marked with cyclopentene than with cyclohexene, and the results bear a marked resemblance to those shown for the reactions of ethylene and of 1butene with deuterium over nickel in Figs. 5 and 6. Sintering also removes the ability of iron films to catalyze the disproportionation of cyclohexene to cyclohexene and benzene, and for this reason i t was

TABLE IX Product Dislrdbu*ions from the R M i m of Ethylene &A

Q

Deuterium over an I m FiJm a4

Ethylenea Conversion

d,

4

d,

9

- 100" (42)

Ethanes d,

do

4

d,

d,

U

4

ds

d,

M

0

%

k U

t'( 0

65.5

9.4

1.3

0.2

13.7

9.2

6.6

2.4

1.0

0.5

0.2

1.11

0.43

w

100

-

-

-

-

20.1

18.4

22.1

12.7

8.6

6.2

2.9

1.89

0.62

9z

HYDROGENATION OF UNSATURATED HYDROCARBONS

123

suggested that sintering removes sites on which the formation of a (possibly n-allylic) C,X,,-, species, an intermediate in the disproportionation and multiple exchange processes, can occur. The nature of these sites has not been specified. Sintering also radically alters the

4 Number of deuterium atoms

Number of deuterium atoms

FIO.7. Reaction of oyclohexenewith deuterium over iron films (52). (a)Final product distribution over unsintered films A, -35"; B, 0"; C, 28"; D, 89". (b) Final product distribution at 0' over A, unsintered film; B, film sintered at 196O.

distribution patterns of products from the exchange of alkylbenzenes with deuterium over iron films (53). Norbornylene, whose structure is given below, gives a very sharp product distribution: 87% of the d,-species is formed over an

Norbornylene

unsintered film at 0" and 94% over a sintered film at 55'. Redistribution produces species no more high deuterated than d,. 4-Methylmethylenecyclohexane behaves rather as ethylene at O", although exchange

124

G1. C. BOND AND P. B. WELLS

propagates around the ring, and species up to d,, are formed, at 88'. 4-Methylcyclohexene behaves similarly to cyclohexene itself (52). 3.

&?&ClUSiOnS

Although comparatively little work has been done with iron catalysts it is evident that iron is essentially similar to nickel in these reactions, and the conclusions regarding nickel (Section 11, D 4 ) probably also apply to iron.

F. REACTIONS OVER COBALT

No work relevant to this article has yet been published on the hydrogenation of olefins over cobalt catalysts. G. REACTIONS OVER PALLADIUM 1. The Reactions of Ethylene and of Propylene with Deuterium The reaction of ethylene and deuterium has been examined with palladium-alumina catalyst between - 36' and 67" (31):a selection of the results obtained after 5% conversion is shown in Table X. It is seen that there is much olefin exchange, whose extent rises only slightly with increasing temperature: at 37" some half the inital products consists of exchanged ethylenes. The relative of olefin exchange falls as the D,/CpH, ratio is increased. As with nickel and iron, ethane-dois one of the major initial products, but in contrast to the behavior of nickel and iron, the amount of hydrogen exchange is barely detectable. Table X contains three computed distributions obtained by the method outlined in Section 11,C: the parameters used are given below the table. The values o f p and r are very similar to those found appropriate to describe this reaction over nickel films at - 100" (see Table IV). Values of q and s, measuring the chance of acquisition of deuterium atoms in the hydrogenation steps, are, however, generally lower. It would appear that hydrogen atoms released by hydrogen-rich radicals are used for hydrogenation of other radicals in preference to their recombining or combining with deuterium atoms, which process is important in the case of nickel, It is worth noting that hydrogen exchange is more marked, although still slow, over platinum (Section 11,H) and rhodium (Section 11, I) than over palladium, in which case the availability of deuterium atoms on the surface may be restricted by their high solubility in this metal. This means that we must be prepared to permit the initial formation of alkyl radicals with the aid of dissolved deuterium.

TABLE X

*tlx

Prod& Distributions at 5% Gonuereion from t h Readion of Ethylene with Dsuterium mer PaUadi21m-Alumima(31) Ethylenes

- 36

1 Theoret. calc. 1

H in deuterium

Ethanes

2.2

0.2

21.1

22.0

7.6

1.6

1.1

0.5

0.0

0.91

0.05

38.7

8.4

1.1

0.1

19.8

21.1

8.5

2.0

0.3

0.0

0.0

0.88

-

0 4

3

41.5

8,9

1.6

0.1

24.5

18.1

5.6

1.3

0.3

0.1

0.0

0.69

0.1

67

1

45.4

8.6

1.1

0.0

22.0

16.8

5.4

0.6

0.1

0.0

0.0

0.66

0.1

44.6

7.1

0.8

0.0

22.0

18.6

5.3

0.9

0.1

0.0

0.0

0.69

-

20.6

5.3

1.9

0.7

17.2

27.5

17.1

5.2

3.0

1.4

0.1

1.36

0.05

23.2

4.0

0.4

0.0

18.7

32.7

II.0

3.4

0.5

0.1

0.0

1.10

-

5 Theoret. WlC. 3

Parametersused in calculations

1 2 3

2

6.6

1

-16

0

37.1

37

Theoret. calc. 2

T1p

P

q

r

s

3 2 3

0.5 0.5 0.5

9 0 4

0.25 0.1 1

5 0

?i

M tl

w

126

a. C.

BOND AND P. B. WELLS

An examination of the ethylene-deuterium reaction at the surface of a palladium thimble has been briefly reported (54): the gas diffusing through the thimble during the reaction was found to contain 10% H. Final ethane distributions from this reaction over palladium-silica and palladium-charcoal under high pressure a t - 78" have been reported (46). A preliminary study of the propylene-deuterium reaction over palladium-pumice (55) showed extensive olefin exchange: the reaction has since been re-examined using palladium-alumina (31). The progress of the exchange reaction at - 20" is shown in Fig. 8, from which it appears

FIG.8. Partial results of the course of reaction of propylene with deuterium over palladium-alumina at - 20' ( 3 1 ) .

that the exchange is substantially but not quite wholly stepwise. Deuteropropylenes account for about 55 yo of the initial products at - 20", but only about 38% at 20". E,,, - E, is therefore about - 3 kcal mole-l. There is comparatively little change in the propane distribution with increasing conversion at - 20", but the deuterium number of the propanes rises from 0.52 a t 10.5% conversion to 0.59 at 23.5% conversion because of the progressive exchange of the propylene (see also Table XI). The relative rate of olefin exchange is unaffected by deuterium pressure. These results are quite like those found with ethylene, and can therefore be accounted for without the introduction of .rr-allylio species.

Propylene composition Temp. D,/C.H. Conversion

do

d,

d,

Propane composition

d,

d.

d6

d.

d,

d,

d,

d,

8,

ds

d.

d,

d,

o

M

HiU deuterium

-20

3.0

10.5

86.0

10.9

2.5

0.6

0.1

0.0

0.0

61.9 26.6

9.1

2.1

0.4

0.0

0.0

0.0

0.0 0.38

0.52

0.2

-20

3.0

23.5

71.5

20.4

6.1

1.7

0.4

0.0

0.0

58.7

27.8

9.4

3.4

0.7

0.0

0.0

0.0

0.0 0.39

0.59

0.2

20

0.75

15.8

78.7

16.0

4.2

1.0

0.2

0.0

0.0

63.9

25.5

7.9

2.3

0.4

0.0

0.0

0.0

0.0

0.50

0.2

20

2.0

15.1

79.2 14.8

5.4

0.6

0.0

0.0

0.0

60.5 25.8

9.7

3.2

0.8

0.0

0.0

0.0

0.0 0.38 0.58

0.2

20

4.0

17.1

82.3

4.1

1.3

0.3

0.02

0.0

51.7

12.9

5.3

1.8

0.3

0.0

0.0

0.0

0.47

0.2

12.0

28.0

0.32

0.78

8

128

U.

C. BOND AND

P. B. WELLS

2. The Exchange and Ismerization of the n-Butenes

Palladium efficiently catalyses the isomerization of the n-butenes (32,56,57): the course of the hydrogenation of 1-butene over palladiumalumina at 37' is shown as an example (Fig. 9). At 18" some 50-60y0 of the initial products from 1-butene are isomerized olefins, which is comparable with the initial yield of exchanged olefins from ethylene and propylene a t or near this temperature: the initial translcis ratio is about 1.9 (31). When reactions are performed near room temperature, the butenes generally come t o equilibrium before they are completely hydrogenated, but the relative rates of isomerization fall with rising temperature 8s shown in Fig. 10 (compare the temperature coefficient for olefin exchange, last section). The kinetics of isomerization are similar for all three n-butenes: rates are virtually independent of hydrogen pressure and increase very slightly with butene pressure. This suggests that the slow step in isomerization may be the desorption of the isomerized olefin. The three isomers iaomerize at rates which are in the following ratios: cia-2-butene : 1-butene : traw-2-butene = 3 : 2 : 1.5

Equivalent results were obtained using deuterium (32). The advent of gas-liquid chromatography has rendered possible the separation of the individual isomers formed in the reaction of the butenes with deuterium, and their analysis by mass-spectrometry provides a fund of information concerning the details of the mechanism (31).It is possible here only to draw attention t o some of the more significant

_I_

cis-2- butene

0

20

40 60 80 Percentogt conversion

100

FIQ.9. Change in the composition of the butene fraction during the hydrogenation of 1-butene over prtlladium-aluminaat 37" ( 3 1 ): concentrations.

-:- indicates thermodynamia equilibrium

HYDROGENATION O F UNSATURATED HYDROCARBONS

129

observations. In all cases the isomerized butenes are found to contain very little deuterium: thus for example the 2-butenes formed from 1butene each have a deuterium number of about 0.5, and 2-butene-d, is by far the major product. Since hydrogen exchange is negligible, this points very strongly to the occurrence of intramolecular hydrogen transfer processes. The initial isomerized products from cis- and trans-2-butene are even more lightly deuterated ( M ~ 0 . 2 6 ) .As any isomer approaches its equilibrium concentration, and after it attains it, it exchanges further at a rate dependent on its structure and its concentration (see Figs. 11

'oor-----l

01 30°

I

40'

I

SOo

I

I

60'

70'

I

,

80'

sb'

Tempcrature

FIQ. 10. Temperature dependence of relative rates of butene isomerization over palladium-alumina ( 3 1 ) . 0: 2-butene from 1-buteneat 32 f 4% conversion. : tram2-butene from ci8-2-butene at 34 & 6% conversion (PB = 60 mm, Pna= 166 mm in each case).

Percentage hydrogenation

FIG.11. The exchange of ci8-2-butene and the deuterium oontent of the isomerized butenes as a function of percentage hydrogenation, over pelladium-&mine at 18' (31).

130

a.

C. BOND AND P. B. WELLS

and 12). Figure 12 shows that the relative rates of exchange of the reactant olefins are approximately trans-2-buteno : cis-2-butene : 1-butene = 12 : 4 : 1,

which is not the sequence of their reactivities in isomerization. The reason for this is briefly that the more likely an olefin is to isomerize, i.e. the further it is from its equilibrium concentration, the less likely is i t to reappear in the gas phase as its deuterated self after transiently becoming an alkyl radical. The effect of temperature is peculiar (see Fig, 13). The deuterium content of the reactant olefin, in this case 1-butene, at about 10% conversion falls with increasing temperature, paralleling its behavior in isomerization. However, the deuterium content of the isomerized 2butenes rises rapidly, as does also the deuterium number of the butane. This tells us ( 1)intramolecular hydrogen transfer becomes less important with rising temperature and is perhaps replaced by reactions involving adsorbed deuterium atoms (step 1. ii), and (2) butane is probably formed through the same intermediates as those which yield 2-butenes, viz., 2-butyl radicals. The same kind of behavior is shown by the other butenes. Partial pressure variation has no remarkable effect on the course of exchange and isomerization. 3. The Exchange and Isomerization of Higher OEejhs. The relative rates of isomerization of 1-pentene and of cis-2-pentene in the liquid phase have been shown to be largely independent of the nature of the support used for palladium, and of the presence of a num-

Percentage of butene isomer remaining

FIG.12, Olefin exchange in the reactant olefin over palladium-alumina( 3 1 ) : tran.92-butene at 37", cia-2-buteneand 1-butene at 18'. The dotted lines show the appropriate equilibrium concentrations of the isomers.

HYDROGENATION O F UNSATURATED HYDROCARBONS

131

ber of solvents (58). A few of the results are shown in Fig. 14. I n comparing the isomerization of 1-pentene with that of 1-butene (Fig. 9), it is interesting to note that the initial translcis ratio in the isomerized olefin is markedly lower in the former case, and indeed cis-Ppentene passes through a maximum concentration before attaining its equilibrium concentration. This appears to be characteristic of liquid phase reactions (57).

Temperature

FIG.13. The exchange of 1-butene with deuterium over palladium-aluminaas a function of temperature (analyses at 20% conversion) ( 3 1 ) . (J: cis-2-butene; : trans-% butene. 1

I

I

1

jPLF

~

cis-2 -pentene I

I

I

I

20

40

60

80

3

Percentage hydrogenation

FIG.14. Formation of 2-pentenes during the hydrogenation of 1-pentenein the liquid phase over Pd-charcoal(58). 0: in methanol; 0: in glacial acetic acid; : in benzene.

132

a.

C. BOND AND P. B. WELLS

Palladium also efficiently catalyzes the isomerization of cyclic olefins (15, 17, 19); 1,l-dimethylcyclohexene isomerizes to a small extent to

the less stable 2,3-isomer ( 1 7 ) , but 2-methylmethylenecyclohexane is rapidly isomerized, and has indeed disappeared after 30% hydrogenation. It is significant that the 2,3-dimethylcyclohexene is the major initial product, and this subsequently passes to the stabler 1,2-isomer (17). 4-tert-Butylmethylenecyclohexane similarly is converted rapidly to the stabler 4-tert-butyl-1-methylcyclohexene (15). Over palladiumcharcoal, 4-methylmethylenecyclohexaneisomerizes completely to 1,4dimethylcyclohexene, which itself does not isomerize (19).Table XI1 shows the olefin compositions which result after partial reduction of a number of related olefins: there are two comparably stable isomers (C and D), and these are both popular, but it is likely that neither of the two kinds of distributions shown are true equilibrium distributions. d l, g-octalin isomerizes to d9,lo-octalin over palladium-charcoal (19). The stereochemical analysis of hydrogenations over palladium is substantially complicated by this extensive isomerization. Final distributions from the reaction of cyclohexene with deuterium over palladium films have been briefly reported (52). 4. Conclusions

Palladium is characterized by its ability to catalyze very efficiently the processes of olefin exchange and isomerization while giving absolutely minimal hydrogen exchange. This behavior is accounted for by assuming quite efficient alkyl reversal coupled with relatively easy olefin desorption, as with nickel. Comparison of the behaviors of ethylene, propylene, and 1-butene reveals no startling differences, a n 0 there is no overwhelming case for invoking the participation of .sr-allylic species in the reactions of the last two olefins.

H. REACTIONS OVER PLATINUM 1. The Reaction of OleJins with Deuterium

The reaction of ethylene with deuterium has been studied over platinum catalysts (28, 31), with results which contrast strongly with those found with nickel, iron, and palladium catalysts. A selection of the results is presented in Table XIII. At low temperatures, there is very little olefin exchange or hydrogen exchange observed. There is, however, a, substantial redistribution reaction leading to the formation of all possible deuteroethanes. The extent of both exchange reactions increases with increasing temperature, and the following activation energy differences have been derived (31):E,, - Eh=6 kcal mole-', &a- Eh=6 kcal

w

3

TABLE XII

T1e

l a m e r Compo&tion after Partial Reduction (25-50%) of Various Unsaturated Cyclic Molecules over Palladium-Charcoal ( 1 9 )

: P

2

Reactant

A=

6

0

z

0

r 4

u

B=$

5FJ

c! m

A

0

3

75

22

0

C

0

3

75

22

0

D

0

0

19

81

0

E

0

0

23

77

0

tiM

U

a cc

U

0

P M 0 Ld

z

m

TABLE XI11

Produd Di&ibuZions from the Reaction of Ethylene with Deuterium a'er Platinum CalalydP

Ethylenes Support Temp.

D,/C,€I,

A1,0,

25

AI,O,

54 1 Theoret. calc. 1 201

d,

dl

Ethanes d,

d,

d.

d,

d.

M

27.2

23.2

10.5

6.4

3.0

0.5

1.68

1.2

(28)

10

9.2 11.9

1.2

0.2 1.7

0.0

19.3

28.2 26.7

11.9

2.3 0.6

0.4 0.1

1.55 1.59

(31)

13.8

4.9 3.7

0.3

0.2

25.6 23.2

8.7

6.2

-

(30

31.2

12.1

5.3

1.9

6.5

11.3

19.1

6.1

4.0

1.9

0.6

1.96

7.7

(31)

32.0

20.3

6.1

0.7

3.9

11.2

13.1

8.6

3.3

0.7

0.1

1.95

-

(31)

-

-

13.2

31.7

29.0

10.8

7.5

5.7

2.3

1.93

31.9

30.1

10.4

6.9

5.6

2.4

1.93

-

(28)

12.7 9.9

26.1

43.5

11.3

5.1

2.8

1.3

1.89

-

(28)

0.9

100

1.25

SiO,

0

1.1

100 100

-

_

-

_

_

-

_

_

-

_

~~

Parameters used in calculations

19 99

Q

19.2

10

0.5

Ref.

(%) 0.0

1

0.5

H in deuterium

0.8

0

49 49

d,

2.7

0

1

d,

7.4

Pumice Pd

2

d,

50

1.2

Theoret. calc. 2

a

d,

(%I

"(C)

A1.0,

Conversion

0.5 1

Platinized Pt foil.

(28)

9

HYDROGENATION OF UNSATURATED HYDROCARBONS

135

mole-’. At higher temperatures there appears to be some multiple exchange of the ethylene. The ethane distribution is sharpened by increasing the D,/olefin ratio, and the initial deuterium content of the ethylene decreases as P g 3 j (31). Initial rate expressions for the addition process are

- dp/dt

=

kPb: Ps’.~

(31)

-dp/dt

=

kP&fP$.5

(28)

or Results in essential agreement with the foregoing have been obtained using a number of different types of platinum catalyst (28):some are shown in Table XIII. Small differences in behavior were attributed to a secondary influence of the support on the course of the reaction. Table XI11 contains two distributions calculated by the method described in Section 11,C, and the parameters employed are given beneath the table. These show that the chance of ethyl reversal is high (9599%) and that the failure of deuterated ethylenes to appear extensively in the gas phase is due to the very low chance of ethylene desorption (1-2%). The initial appearance of multiply exchanged ethylenes is thereby explained, and the increase in the relative rate of olefin exchange exchange with rising temperature is due to the increasing ease of olefin desorption. The values of q and s suggest that ethyl radicals are an important source of atoms for forming both ethyl radicals and ethane. Similar studies made with propylene, using alumina- (31) and pumice-supported (25)platinum, have given similar results. Some final propane distributions are quoted in Table XIV. The extents of olefin and hydrogen exchange are comparable to those observed with ethylene, and deuteropropylenes up to propylene-d, are observed (31). The “tails” of the deuteropropane distributions again often decline logarithmically, and some values of u close to 0.5 are given in the table. It has been established that the use of equilibrated and non equilibrated hydrogen-deuterium mixtures gives identical products, showing that direct addition processes are not important (25). Keii (38, 40, 41) has offered an interpretation of some of these results in terms of Horiuti’s associative mechanism (see Section 11, C). 2. The Exchange and Isomerixation of Higher OleJins The relative rate of isomerization of the butenes is small over platinum-alumina between 0 and 100” (31). Cis-2-butene undergoes cistrans isomerization some fifteen times faster than the trans-isomer, while double-bond migration of 1-butene proceeds at an intermediate

TABLE XIV

is

Pmdprd Dirtributionsfmm the Renctkm of P w p y h wiu Dcutcrium OOCT Plafinum Catalynta

Temp. D.IC,H, (“C)

40

0.83

Conversion d .

d,

Propylene composition d. d. d, d,

d.

d.

d,

Propane composition d, d, d, dS

P W

d,

d,

d,

(I

M

Ref.

(Yo)

u b-

38

95.3

3.5

0.8

0.3

0.07

0.02

0.00

16.0

30.1

23.8

14.4

7.5

4.1

2.0

0.7

0.1

0.54

1.95

(31)

3 -W -

+d

0.08 0.03

73

0.83

51

91.2

6.0

1.8

0.6

0.3

75

1

100

-

-

-

-

-

-

-

42

100

-

-

-

-

-

-

-

100

97.0

3.0

0.0

0.0

0.0

0.0

0.0

18

18

0.03

14.2

26.5

25.0

14.9

9.2

5.0

2.8

1.0

0.2

0.60

1.99

(31)

18.7

26.1

22.0

15.0

8.7

5.2

2.8

1.2

0.3

0.63

2.04

(25)

0.0 18.0

46.2

19.0

7.9

3.9

2.0

1.7

1.3

0.53

2.53

(25)

b

15.8

5.6

2.4

1.4

0.8

0.3

0.1

0.43

1.01

(25)

ro

45.2

28.4

HYDROGENATION OF UNSATURATED HYDROCARBONS

137

rate, giving a translcis ratio of about unity. The rate expression for the hydrogenation of all three butenes is

- dp/dt

=

kP&P$ .

Accurate kinetics for the isomerizations could not be determined. The low rates of isomerization are not unexpected in view of results given in the last section. Evidence that the mechanisms of olefin exchange and isomerization are closely related is provided by a brief study of the isomerization of the butenes in the presence of deuterium (31). The rates of entry of deuterium into the reactant olefin were small, and in the case of cis-2-butene its deuterium number increased linearly with the fraction which had isomerized. Olefin exchange was, as expected, particularly slow in the case of trans-2-butene. Hydrogen exchange was also slight, especially with 1-butene. Similarly slow rates of isomerization have been observed in the hydrogenation of the n-pentenes in the liquid phase over platinum-charcoal (58)and of 1-hexene in the liquid phase over Adams platinum (14)(see Fig. 15). In the latter reaction, which was also studied using deuterium, the relative rates of addition, olefin exchange and double-bond migration were 1 : 0.3 : 0.03. The deuterohexane distribution was therefore slightly conversion-dependent (see Table XV). A possible reason for difference between the relative rates of exchange and isomerization is that addition of a deuterium atom a t the 2-carbon atom gives a 1-hexyl radical which may form an exchanged but not isomerized 1-hexene. It is however doubtful whether this mode is (Scale for 2-hexened

(Scale for I-hexene)

50

4og

P

L

30

6 ..-

+I

20 10

0 20

40

60

E

8 u m

trans -2 -hexene

0

ii

00

; u ? 8

100

Percentage hydrogenation

FIG.15. The isomerization of 1-hexene during its hydrogenation in the liquid phase over Adams platinum (14). 0 - H 8 ; @-Da.

25.6

13.5

35.2

31.1

10.7

5.5

2.0

1.4

0.8

0.0

0

0

0

0

0

0

0.42

1.76

68.3

16.1

35.2

29.5

12.1

4.3

1.7

0.6

0.2

0.1

0.09

0.07

0.05

0.05

0.03

0.02

0.37

1.65

10.8

31.8

31.3

14.4

6.7

2.7

1.2

0.5

0.26

0.17

0.11

0.06

0.02

0.01

0.01

0.47

1.95

100

HYDROGENATION OF UNSATURATED HYDROCARBONS

139

sufficiently favoured to account for a factor of ten difference in the rates. The addition of deuterium to methyl oleate catalyzed by Adams platinum leads to methyl stearates in which up to all but six of the thirty-six hydrogen atoms have been exchanged. Related compounds were also examined (59). The hydrogenation of a number of cyclic olefins over Adams platinum has been examined, largely to study the stereochemistry of the process (see next section), but it is instructive to note that minimal isomerization is generally observed. With 1,.Z-dimethylcyclohexene,no isomerization whatever is detected (13, 20); with 2,3-dimethylcyclohexene slight isomerization is found (1.6% of the 1,2-isomer at 50% conversion) which is independent of hydrogen pressure (13). Slight isomerization is also observed in the reduction of 2-methylmethylenecyclohexene (13), 4-tert-butylmethylenecyclohexane(15)and methyl-l,2-~yclohexenedicarboxylate (21).This work has been confirmed and extended to a wide range of substituted cyclohexenes and methylenecyclohexanes (18), only 4-methylisopropylidenecyclohexanegiving appreciable isomerization. No isomerization was observed in the hydrogenation of A's9octalin or dQ,l0-octalin(14).

co

A 9*'0-octa1in

The reactions of these octalins with deuterium, to yield deuterated cis- and trans-decalins, has also been investigated (14). I n each case the exchange rate is 10-20 times slower than the rate of addition, and deuterium contents of the two decalins are about the same. However, the decalins formed from A*, 1°-octalin were the more extensively deuterated (deuterium contents at 55% conversion, 2.85, 2.85: at 37% conversion, 2.02, 1.83). Hydrogen exchange was apparent during the reaction of A9~10-octalin but not with the A'*g-octalin. A theoretical scheme was developed to interpret the distributions of the deuterated decalins, the tails of which decline in the familiar logarithmic manner (see Fig. 16). 3. The Stereochemistry of the Hydrogenation of Cyclic Olefins The hydrogenation of disubstituted cyclohexenes and of substituted methylenecyclohexanes is generally observed to yield a mixture of the cis- and trans- isomers of the corresponding cyclohexane (see Section 11, A). The isomer composition is a function of hydrogen pressure (13,15, 20) (see Fig. 17) and of the nature and position of the substitu-

140

cf. C. BOND AND P. B. WELLS Number of deuterium atoms

2 4 6 8 I0 12 14 I

I

1

1

I

I

l

0

Number of deuterium atoms

FIG.16. Semilogarithmic plot of the partial distribution of deuterodecalins (14) formed from Asp 'O-octalin and deuterium at 81% conversion; catalyst. A d m s platinum.

v-A !

2 o_1.0

0

1.0

2.0

3.0

Log,, hydrogen pressure (atm

FIG.17. Dependence of the proportion of &-addition suffered by substituted cyclohexenee on applied hydrogen pressure (13, 15): catalyst, Adam8 platinum.

HYDROGENATION OF UNSATURATED HYDROCARBONS

141

ents (14,15, 20, 21) (see Table XVI). It should be noted here that cis-l,3-dialkylcyclohexanesare stabler than the trans-isomers, but that the reverse is true for 1,2- and 1,4-dialkylcyclohexanes.This is because substituents favor equatorial positions in the chair conformation. As noted above (Section 11, A), since an alkyl group attached to a n unsaturated carbon atom naturally moves away from the surface on adsorption as Structure (A) or on conversion of Structure (B) to an alkyl radical, the conformation of the product deriving from a 2,3dialkylcyclohexene depends on the disposition of the other substituent, and hence on the side of the ring a t which adsorption occurs. The TABLEXVI Percentage Yield of cis-Dialkylcyclohexanes Formed by Hydrogenation of Cyclohexenee and Methylenecyclohexanes over Adams Platinum at Atmospheric Pressure and Temperature Nature and positions of substituents

cia-Isomer

(%) 1,2-Dimethyl 2,3-Dimethyl 2-Methylmethylene 1,2-Dicarbomethoxy 2,3-Dicarbomethoxy [A9*'o-Octdin] [A9, lo-Octalin] 1,J-Dimethyl 1,3-Dimethyl 2,4-Dimethyl 3-Methylmethylene

1,4-Dimethyl 1,4-Dimethyl 1-Ethyl-4-methyl 1-1sopropyl-4-methyl 1-Methyl-4-ethyl 1-Methyl-4-isopropyl 1-Methyl-4-tert-butyl 1-Methyl-4-tert-butyl 4-Methylmethylene 4-Methylmethylene 4-Isopropylmethylene 4-tert-Butylmethylene 4-Methylisopropylidene

64 67 68 64 48 43 37 36 64 76 81 83 36

Ref.

142

0. C. BOND AND P. B. WELLS

occurrence of both cis- and trans-isomers in this case, therefore, requires no special explanation. I n the case of 1,2-dialkylcyclohexenes, however, where both sides of the ring are equivalent, normal processes should lead only to cis-1,2-dialkylcyclohexanes.I n fact, the yield of the cis-isomer from 13-dimethylcyclohexane is only about 80% at atmospheric pressure, rising, however, to over 96% a t very high hydrogen pressure (13).Some special mechanism, therefore, has to be invoked to explain this, and argument centers around its nature. Siege1 and his associates (13, 15) believe the trans-isomer arises through isomerization to 2,3-dimethylcyclohexene followed by desorption and then by readsorption on both sides randomly. Objections to this idea are (1) the required isomerization is in a direction which is thermodynamically unfavorable, and ( 2 ) even if isomerization occurs the olefin is unlikely t o desorb to the required extent. The Northwestern University school (14, 18,19)believes that a dissociatively adsorbed olefin or “stereochemically symmetrical intermediate” is involved [see also (52)]. I n the case in question, this would have the Structure (F). Among possible objections t o this theory is the

(-T= +*T’

+pij” 1 1 ‘ I

CHa

I

CH,

I* H

Structure (F)

claim that such a mechanism could not account for the observed pressure dependences (13). A third possible route to the trans-isomer is available if “topside” addition of molecular hydrogen to a .rr-olefin structure (B) is permitted

(12):

-

TH-(j a1

+

( x ; s 2

1 ‘CH,

H

1

bH,

but this mechanism also seems difficult t o accommodate with the observed pressure dependences. Thus, although the present situation is a somewhat confused one, it is evident that the stereochemical aspect of the hydrogenation of cyclic olefins is a potential mine of information. 4. Conclusions

The main features observed in the hydrogenation of olefins over platinum catalysts are consistent with the idea that alkyl reversal re-

HYDROGENATION OF UNSATURATED HYDROCARBONS

143

actions are facile, but that olefin desorption is difficult, especially at ambient temperatures. No evidence has been obtained for the intervention of rr-allylic adsorbed species.

I. REACTIONS OVER IRIDIUM 1. The Excknge and Isornerization of OleJins

The reaction of ethylene with deuterium over iridium-alumina has been studied between - 16 and 150" (31) and a selection of the results is given in Table XVII. Olefin exchange is seen to be slight at low temperature, although multiply exchanged ethanes are observed. At 150" olefin exchange is even less marked than with platinum (compare Table X I I I ) . Hydrogen exchange is also slight throughout. The following activation energy differences have been derived: E,, - Eh, 6 kcal mole-l; E h e - Eh,3 kcal mole-l. The rate expression for addition a t 60" is

- dp/dt = kPk: Pg4. The product distribution is not influenced by variation of partial pressures. The table also contains two calculated distributions in fair agreement with the experimental ones, the parameters used being given beneath the table. These parameters, which are numerically similar to those found successful in the case of platinum, show that adsorbed ethylenes have only a 1-5% chance of desorbing, this chance increasing with increasing temperature. The chance of ethyl reversal is again high (9095%). The chief difference between these parameters and those found appropriate in the case of platinum lies in the higher values of s, which suggests that molecular deuterium is an important agent in converting ethyl radicals to ethane. The reaction between propylene and deuterium over iridium-alumina has been similarly studied ( 3 4 , and some results are shown in Table XVIII. The chief distinction between these results and those obtained with ethylene lies in the effect of varying the D,/olefin ratio. The ethanes formed when this ratio is 4 . 3 have a deuterium number of 3.83, which excess is balanced by considerable hydrogen exchange. The relative rate of olefin exchange is inversely proportional t o deuterium pressure. It seems likely that when the D,/olefin ratio is 4 or higher, the surface species are largely propyl radicals plus hydrogen and deuterium atoms. The propane distribution then tends towards that observed in the exchange of propane with deuterium (60). The isomerization and exchange of the n-butenes over iridium-alumina has been investigated at and below room temperature (31). Rates of

TABLE XVII Product DistribvtPoRe from the Rrodwn of EtJtylenc with Deutmium (100 mm a h ) m r Iridiurn-dl~mina~ (31)

Ethyelens Temp.

Conversion

("C)

(%)

- 16

di

d,

ds

d,

5

2.9

0.6

0.4

0.2

1.5

0.4

0.0

do

4.4

d,

30.1

d.

36.9

Ethanes d,

14.7

d,

7.2

d.

2.2

d,

M

H in deuterium

0

(%)

z

0.4

1.98

0.2

-

Theoret. ealc. 1

3.3

3.0

30.2

34.8

19.2

6.5

1.2

0.1

2.00

86

5

8.1

2.8

0.7

0.2

4.6

25.7

30.0

12.5

9.4

4.8

1.2

2.18

1.3

152

5

19.6

7.9

3.4

1.1

0.0

18.5

27.0

9.8

8.1

3.9

0.8

2.33

1.9

Theoret. calc. 2

23.2

10.8

2.6

0.3

1.9

29.2

23.0

13.3

4.7

0.9

0.1

2.05

-

W

U

cd W

B

J a

Parameters used in calculations v 1

2

99 19

r

a

0.5 9 0.5 19

u

9 9

tr

E

Produd Distributionsfrom the Readion of Propyknc with Deutcriecm over Iridium Catalysts (31)

HYDROGENATION OF UNSATURATED HYDROCARBONS

TABLE XVIII

0

w

Propylenes Temp. D./C;R. ("C)

Conversion do

d,

d,

d.

Propanes d,

d,

d.

d.

d,

d,

d.

d,

d,

d.

d,

d.

(I

M

(%)

H in deuterium

(%)

2

5

d

16

0.83

53

97.6

2.0

0.4

0.0

0.0

0.0

0.0

15.0

28.5

26.1

15.4

8.5

3.9

1.9

0.7

0.08

78

0.83

49

96.7

2.2

0.7

0.3

0.1

0.3

0.0

13.4

23.1

22.7

17.5

11.2

7.1

3.4

1.5

78

4.3

99.3

0.6

0.1

0.0

0.0

0.0

0.0

3.2

12.1

17.9

16.1

13.4

11.0

11.0

10.4

5.5

0.52 1.97

1.9

0.2

0.65

2.34

-

5.0

-

3.83

12.2

!$

Y M

m

146

146

0. C. BOND AND P. B.

WELLS

isomerization were very low and no double-bond migration whatever was observed with 1-butene. Olefin exchange with the 2-butenes, observed after 20% conversion, was restricted to the formation of up to 2.6% butene-d,. No olefin exchange was observed with trans-2-butene, and hydrogen exchange was insignificant in all cases. The rate expression for addition at 0" was -dp/dt = kP&Pi. Similar results have been obtained in the hydrogation of the npentenes in the liquid phase using iridium-alumina as catalyst (58). 2. Conclusions

With respect to the reactions considered above, iridium behaves very similarly to platinum, and the explanation is as set forth in Section 11, I, 4. The relative rates of the exchange reactions are rather less for iridium than for platinum.

J. REACTIONS OVER RHODIUM 1. The Reactions of Ethylene and of Propylene with Deuterium

Table XIX presents a selection of the results obtained in a study of the reaction of ethylene with deuterium over rhodium-alumina (31), together with some calculated distributions obtained by the method previously employed. The proportion of deuterated ethylenes in the initial products rises from 30% at - 18"to 75% at 110". In contrast to the behavior of palladium, ethane-d, is the major ethane throughout and hydrogen exchange is significant at all but the lowest temperature studied. The parameters used in the calculations attribute the greatest effect of temperature to the variation of the chance of ethylene desorption, which rises from 25% at -18" to 62% at 110". The effect of temperature on the chance of alkyl reversal is relatively small. Another respect in which the reaction over rhodium differs from that over palladium is that the chance of acquisition of deuterium in the hydrogenation steps is higher, and indeed it appears that, as with iridium, molecular deuterium may be substantially responsible for the conversion of ethyl radicals to ethane. E,, - Eh is 3 kcal mole-l and Ehe- Eh is 4.5 kcal mole-l. The reaction is first-order in hydrogen and zero in ethylene. Table XIX also contains a distribution obtained from a reaction over a rhodium film at - 100" (42).The rates of olefin exchange and of addition are almost equal, and hydrogen exchange is significant. Ethane-d is the major ethane.

TABLE XIX Produel Dislribuions from tha Reodion of Ethu&ne wiih De&riunz over Rhodium-Aluminaa (5% Cmocrcion) ( 3 1 )

U

9

-18

1

26.2

4.2

0.8

0.1

7.9

19.7

26.5

8.1

4.4

1.8

0.2

1.83

0.4

Theoret. cak. 1

26.3

6.8

1.2

0.1

4.5

24.1

26.3

8.4

2.0

0.3

0.0

1.70

-

1

61.7

8.7

2.1

0.1

3.5

7.7

15.1

0.9

0.2

0.0

0.0

1.50

5.1

Theoret. ealc. 2

64.7

9.5

0.9

0.1

1.4

8.4

12.5

2.3

0.3

0.0

0.0

1.68

-

76

110

1

64.5

10.8

0.8

0.4

3.6

5.3

13.5

1.1

0.0

0.0

0.0

76

5

48.5

9.5

2.0

0.5

3.7

11.2

20.7

3.3

0.5

0.1

0.0

1.51 1.64

Theoret. oak. 3

47.9

9.3

1.3

0.1

2.1

13.2

20.2

4.9

0.9

0.1

0.0

1.75

-

34.4

7.0

1.7

0.9

8.4

14.4

15.8

7.4

5.0

3.6

1.4

2.05b

5

-ld

3

Parameters use in OalrmLStiou

1 2 3

P

e

r

8

3 0.6 1

1 2

4 9

2

5

4 4 4

Thisresult refers to a rhodiumfflm at - 100’ (42).

5.1

F Y

tr

148

U. C. BOND AND

P. B. WELLS

The propylene-deuterium reaction was studied (31) only between 88 and 128", in which range there is no great change in the nature of the reaction (see Table XX). From the progress of the olefin exchange reaction at 88" the exchange appears to be purely stepwise, and the temperature dependence of its rate is small, EOe-E , being only -2 kcal mole-'. Hydrogen exchange is appreciable and - E , is 5 kcal mole-'. The proportion of exchanged olefins in the initial products is about 50%, which is somewhat lower than the figure for ethylene in this temperature range. Propane-d, is the major propane (see Table XX). The product distribution is independent of deuterium pressure over a wide range. 2. The Reactions of the n-Butenes with Hydrogen and with Deuterium These reactions have been studied (31) over rhodium-alumina between - 20 and 175', and very marked changes of behavior have been observed in this temperature range. At low temperatures, relative rates of isomerization are small (see Fig. 18) and the behavior of rhodium is entirely reminiscent of that of platinum. However, with increasing temperature the relative rates of isomerization increase very quickly (see also Fig, 18)) and above about 80" the behavior of rhodium is reminiscent of that of palladium. The course of the reaction of 1-butene with hydrogen at 166' is shown in Fig. 19: the initial transleis ratio is about 1.6. The relative rate of olefin exchange also increases rapidly with increasing temperature, although at low temperatures exchange in the

FIG.18. Dependence of initial yield of isomerized olefin on temperature over rhodiumalumina ( 3 1 ) . 0-from 1-butene; @-from cie-Z-butene;.-from trans-2-butene.

TABLE XX Bcd& Distributionsfrom thc Rcadima of P r c p y h with Deuterium o w RJwdium-Alumina Catdysts (61)

Temp. D,JC,H.

Conversion d ,

d,

Propyleue composition d, d, d, d.

d,

do

d,

d.

Propane composition d, d, d.

d,

d,

d,

(I

M

(%)

(‘C)

Hin deuterium

(%)

128

3.0

12.7

77.5

16.0

4.9

1.3

0.3

0.1

0.0

11.8

18.4

45.0

17.7

8.0

1.1

0.0

0.0

0.0

0.33

1.91

2.8

90

0.70

8.7

76.8

17.8

4.1

1.1

0.3

0.06

0.0

36.6

31.1

24.9

5.5

1.2

0.7

0.0

0.0

0.0

0.22

1.06

12.7

90

1.0

9.0

76.0

17.6

4.7

1.3

0.4

0.04

0.0

32.8

29.9

25.4

8.0

2.8

0.8

0.3

0.0

0.0

0.33

1.21

8.9

90

3.1

9.9

61.2

27.1

8.7

2.4

0.5

0.03

0.0

23.5

27.0

31.8

11.8

4.4

1.5

0.0

0.0

0.0

0.36

1.51

4.6

Cj

;

5

0

5M W

3w

0 0

150

43. C. BOND AND P. B. WELLS

reactant olefin is more rapid than its isomerization. Thus, for example, with 1-butene a t 0" there are formed after 20% hydrogenation 0.7 mm of 2-butenes and 4.5 mm of deuterated 1-butenes, which therefore form some 85% of the changed olefins. This figure falls with rising temperature to a more normal figure of w10y0 at 125-175". This observation is somewhat difficult to explain, unless it is that a t low temperature the reaction of 1-butene with deuterium leads to a 1-butyl radical (from which exchanged 1-butene but not 2-butenes can be formed) in preference to a 2-butyl radical. I n the reactions of 1-butene with deuterium a t 30-60", the two 2butenes are almost equally deuterated but in each case 2-butene-do constitutes 50-60% of the product. The isomerized butenes are rather less deuterated a t higher temperatures. The general pattern of the high-temperature exchange results is very similar t o that described for palladium in Section 11,G, 2. Hydrogen exchange is, however, much more marked. 3. The Isomerization of Higher Olejns Studies of the liquid phase hydrogenation of the n-pentenes a t room temperature over rhodium-charcoal (58) have afforded results rather similar to those found with platinum, although relative rates of isomerization are a little faster. 4. Conclusions

Rhodium is anomalous in that relative rates of exchange and isomerization increase rapidly with rising temperature due to the increasing

Percentage hydrogenation

FIO.19. The isomerization of 1-butene during its hydrogenation over rhodiumalumina at 166" ( 3 1 ) . The dotted lines ehow the equilibrium concentretione expected at this temperature.

HYDROGENATION OF UNSATURATED HYDROCARBONS

161

probability of olefin desorption. Above about 80", rhodium behaves rather as nickel. At low temperatures it behaves as platinum. Inasmuch as molecular deuterium seems to be significant in the hydrogenation steps with ethylene, it resembles iridium.

K. REACTIONSOVER RUTHENIUM AND OSMIUM 1. The Reactions of Ethylene with Deuterium

These two metals are grouped together (1) because this crystal structure is close-packed hexagonal and not as in the case of the other Group VIII metals (excepting iron) face-centered cubic, and (2) because they have a number of common features in their catalytic properties. The reaction of ethylene with deuterium was studied (61) between 0 and 80" used alumina-supported catalysts. A selection of the results and some of the calculated distributions are shown in Table XXI. The addition reactions are first-order in deuterium and zero in ethylene. At -50" some 50% of the initial products are exchanged ethylenes over ruthenium and some 25% over osmium. These figures are unaffected by alteration in the ethylene pressure but are suppressed by increasing deuterium pressure. The relative rates of both exchange processes increase with rising temperature, and the following activation energy differences have been derived : Ru: 0s:

- Eh + Ehe - E, =i. 3 kcal mole-1 Eon- E, = Ehe - E, = 4 kcal mole-l E,,

The parameters used in the calculations are most instructive. With both metals q and s are relatively large and constant, showing SO-SO% chance of acquiring deuterium in the addition steps, which deuterium we must suppose is substantially molecular. It is interesting to note that this behavior occurs with four metals (Ru, Os, Rh, Ir) which lie adjacent both vertically and horizontally in Group VIII. The parameter p is 0.2 for ruthenium, indicating weak olefin adsorption, with 83% chance of olefin desorption. For osmium p is 0.8 and the chance of desorption is reduced to 55%. The vertical trend previously encountered in Groups VIII, and VIII, is thus maintained. Variation of temperature and deuterium pressure principally affects the chance of alkyl reversal. 2. The Isomerization of the n-Butenes

Both ruthenium and osmium catalyze the isomerization of the nbutenes quite effectively (61).Thus, for example, with I-butene, some 80% of the initial products a t 33" over ruthenium are 2-butenes, while

TABLE XXI Pmdud Disl+ibUtionsfrom the Reudion of Ethylene coiul Deutrrium m r Alumina-Suppmhd Ruthenium and Osmium* ( 6 1 ) -

~~~

Metal

~~

~

Temp.

D,/C,H,

Conversion

(‘C)

(%)

53

7

Ethylenes d, d,

d,

d,

d.

0.0 0.0

2.6 0.8 0.7 1.0 2.2 0.6 0.7 4.1 3.1 1.3 0.0 0.8 1.0 0.7 0.0 1.1

d,

d,

Ethanes d,

9.9 10.5 11.8 12.6 10.4 10.5 0.4 10.7 14.2 16.5 10.6 13.9 9.8 12.3 12.3 13.9

29.1 29.6 35.7 35.5 45.3 47.7 26.5 25.9 46.8 47.6 62.8 63.7 59.0 56.4 44.4 40.6

2.5 2.3 3.1 2.4 2.1 2.7 2.1 3.3 6.1 7.1 6.0 6.1 6.4 7.4 7.0 7.5

d,

dl

d.

M

H indeuterium (%)

0.0 0.1 0.0 0.1 0.0 0.1 0.1 0.0 2.5 0.8 1.8 0.5 3.0 0.8 3.0 1.1

0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 0.0 0.9 0.0 0.7 0.0 0.6 0.0

1.70 1.76 1.79 1.77 1.77 1.86 1.64 1.63 1.63 1.93 2.10 2.06 2.06 1.89 1.99 1.91

4.5

Q Ru

Ru Eu Eu 0s 06

0s

0s

0.54 Theoret. calc. 1 53 1.65 Theoret. calc. 2 32 1.o Theoret. calc. 3 80 1.0 Theoret. d e . 4 24 0.56 Theoret. talc. 5 24 4.0 Theoret. ealc. 6 17 1.0 Theoret. &tr. 7 47 1.0 Themet. distr. 8

52.1 53.5 45.1 46.0 39.8 36.7 57.5 52.8 24.8 23.7 15.9 13.9 18.9 20.1 30.7 30.8

7 7 7 16 16 15 15

3.9 3.1 4.1 2.4 0.0 1.6 3.5 3.0 1.6 2.8 1.1 1.0 0.4 2.1 1.5 4.5

0.0 0.1 0.0 0.1 0.0 0.1 6.2 0.0 0.0 0.2 0.0 0.1 0.0 0.2 0.0 0.5

0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Parameters used in calculations

1 2 3

4

p

q

r

r

0.2 0.2 0.2 0.2

9 9 9 9

2.5 1.8 1.2 3

4 4 9 4

5 6 7 8

p

q

r

s

0.8

9 9 9 9

1 0.5 0.8 1.5

4 9 9 4

0.8

0.8 0.8

0.0 0.0 0.0 0.0 0.0 0.0

0.6 0.1 0.9 0.2 0.7 0.1 0.6 0.1

-

P

1.8

s

-

2.5 4.5

-

8.5

0.5

-

3.0

-

5.2

-

td

U

* 3

w W

i

m

HYDROQENATION OF UNSATURATED HYDROCARBONS

153

over osmium at 96" the figure is about 60%. This difference tallies with the difference in olefin exchange activities described in the last section. With ruthenium E d b m - E h is about - 5 kcal mole-l, whereas over osmium it is $5.5 kcal mole-'. A significant different between the kinetics of double-bond migration and of cia-trans isomerization has been detected over ruthenium:

rh(1-butene), rh(cis-2-butene), rct = k P & , P ~ rdbm

=

kp!:PE .

This quite reliable result is not readily explained, unless it is assumed that the mechanism of cis-trans isomerization involves n-allylic species (see Fig. 3) and that two slow steps, each half-order in hydrogen, intervene. However, this mechanism is not easily reconciled with the observis greater than E,, over both ruthenium and osmium. ation that Edbm 3. Conclusions The vertical trends of strength of olefin adsorption, detected in Groups VIII, and VII18, are also observed in ruthenium and osmium. However, the horizontal trends (Rh > Pd, Ir > Pt for strength of olefin adsorption) are not continued, due perhaps to the change in crystal structure. In some respects these metals are entirely normal, although there is evidence for the participation of rr-allylic species in isomerization processes.

L. REACTIONS OVER OTHER METALS Fragmentary observations only have been made using other metals as catalysts for reactions which are pertinent to this review. The reaction of ethylene with deuterium over a tungsten film at - 100" yields only 7% deuterated ethylene in the initial products (42). The ethane distribution is sharp, the final ethane containing 70% ethane-d,. The observed initial distribution is matched by a calculated one on which p = 3 , q=5, r=0.4 and s=5. This suggests that olefin desorption is about as easy as over nickel, that deuterium (in this case not necessarily molecular) is substantially used in the addition steps, and most significantly that the chance of alkyl reversal is small (-30%). This is in harmony with the observed (62) difficulty of multiple exchange of ethane with deuterium on tungsten films. The reaction of cyclohexene with deuterium over a gold film between 150 and 240" produces a stepwise exchange reaction and a little hydrogenation (63).

154

0. C. BOND AND P. B. WELLS

M. SUMMARY AND CONCLUSIONS The foregoing sections have presented in some detail the available experimental results which are relevant to the mechanism of the hydrogenation of olefins on the surface of transition metals. They demonstrate that the metals of Group VIII fall into two broad groups. Metals in the first group (iron, nickel, and palladium) are active in isomerization and olefin exchange, while metals in the second (platinum and iridium) are almost inactive for these reactions. Ruthenium, rhodium, and osmium occupy an intermediate position. The characteristic properties of each metal are shown in a variety of reactions and are not affected by anumber of irrelevant variables. The general consistencyof the results is extremely pleasing. Their least satisfactory aspect is the paucity of precise mechanistic information which may be derived from them. One reason for this is the large number of elementary steps which may contribute. A further reason is the difficulty of thorough mathematical analysis when the reactant molecule contains three or more carbon atoms. The general picture of the hydrogenation of ethylene over the metals of Group V I I I is fairly clear. There are present on the surface adsorbed ethylene and ethyl radicals, hydrogen atoms, and probably no other species. The relative rate constants for the processes of their interconversion and reaction vary greatly between the metals, as does also in consequence their population. Most striking are the differences between the strengths of adsorption of ethylene on the various metals, and these determine whether isomerization and exchange reactions will be detected, The chief hurdle in the mechanistic analysis is the lack of extensive and accurate kinetics of the various possible exchange processes, but in the cases of iridium and platinum especially these are hard to come by. There are, however, grounds for optimism that satisfactory mechanisms for the hydrogenation of ethylene on the Group V I I I metals will soon be forthcoming. With the higher olefins, where other possible species may exist, the situation is naturally more complicated, but there is rarely any definite evidence for r-allylic intermediates (butene isomerization over ruthenium may be an exception). This is not to say that they do not exist, but rather that the observations may be rationally interpreted without them. It is worth noting that the predictive value of r-allylic mechanisms is somewhat small. In conclusion we may say that very marked behavioral patterns are emerging, and that the understanding of reaction mechanisms is proceeding apace.

HYDROGENATION OF UNSATURATED HYDROCARBONS

155

111. The Hydrogenation of Alkynes and Dienes A. INTRODUCTION Terminology The hydrogenation of multiply unsaturated hydrocarbons presents many features and poses many problems of a kind essentially different from those encountered in monoolefin hydrogenation. Thus, an acetylene or a diolefin will generally hydrogenate to give both monoolefin and paraffin, and the system may show any degree of preference for either product depending upon the catalyst and conditions used. Matters are further complicated if the parent hydrocarbon contains four or more carbon atoms, in which case a distribution of isomeric olefins is generally obtained and the olefin distribution is dependent upon the catalyst and the conditions employed. Throughout this section the term “selectivity” will be used to denote the extent to which an alkyne or diene will yield monoolefin as opposed to paraffin; thus 1.

+

Selectivity, 8 = Pc,H%, / (PC,H~,Pc,,,H%+J.

So defined, S may take values between zero and unity. Similarly, the term “stereoselectivity” will be used to describe the degree of preference which a system shows for the production of a particular olefin when the production of isomeric olefin is possible. Attention is henceforth confined to conditions where the degree of selectivity or stereoselectivity observed is that due to the inherent properties of the catalytic surface. Artificial factors, which when present may alter the normal product distribution include ( 1 ) deliberate or unintentional poisoning of the catalytic surface; (2) effects due to concentration gradients in the beds of flow reactors or in static systems where reactions exhibit very high rates; (3) the presence of pores in the catalytic surface of such a size that appreciable diffusion-controlled reaction takes place which differs significantly from that occurring on the exposed surface. Selective poisoning has been studied in some systems (6, 64-68) and is used industrially for the selective removal of diunsaturated hydrocarbons from monoolefin feedstock but it is unlikely that the systematic application of poisons will be possible before the factors governing inherent selectivity and stereospecificity are understood. The effects upon selectivity of diffusion in pores has been reviewed exhaustively (69)and will not be discussed here.

166

0. 0 . BOND

AND P. B. WELLS

Before turning to a detailed discussion of particular reaction mechanisms the factors which govern preferential product formation will be examined in general terms. 2. Selectivity If paraffin and olefin are both formed during one residence of the parent hydrocarbon molecule on the surface then a mechanistic factor is operative at every point in the mechanism where routes to products become independent, For example, adsorbed olefin may either undergo desorption or further hydrogenation [as shown below in Scheme (I)],and here the paths to the formation of products become independent. A knowledge of (a) the rate constants of steps (2) and (3) and (b) the

Scheme (I)

orders in ethylene and hydrogen for the ethane-forming step defines the mechanistic factor. However, at the present time our knowledge of these parameters, if it exists a t all, is only qualitative; more usually, even the number of routes available for product formation is not known with certainty. As soon as monoolefin is produced the system contains another potential adsorbate and a thermodynamic factor governs whether or not interference by products takes place to a noticeable extent. Consider two hydrocarbons, X and Y , whose surface coverages Ox and Oy are related to their pressures in the gas phase by the equations

Further, let their free energies of adsorption be AG, and dGy, respectively. Assuming that X and Y adsorb in competition with each other, their relative surface coverages are simply derived. 6 AC = AQ, - AGy

= -RT

In(kx/ky)

and hence ex -p x * exp( - 6 AGIRT)

_-

OY

P Y

Thus it can immediately be seen that a difference in the free energies of adsorption of X and Y of only a few kilocalories per mole will result in a very high surface coverage of the more strongly adsorbed hydrocarbon. For all systems reported to date alkynes and diolefins have been observed to adsorb considerably more strongly than the monoolefins

HYDROGENATION OF UNSATURATED HYDROCARBONS

167

which are produced by their hydrogenation. Consequently the readsorption of olefin in the presence of acetylene or diolefin is usually only of minor importance and in some cases is absent. While it is useful t o distinguish between mechanistic and thermodynamic factors in this way, these factors are not totally separable. Returning to Scheme (I), above, the desorption of olefin may be “assisted” by the adsorption of reactant. Useful analogies exist in organometallic chemistry in reactions where a weakly bound organic ligand to a metal atom is replaced by a more strongly bound ligand. Thus, the relative rates of olefin desorption and hydrogenation in the absence and in the presence of CnH(2n-2)may be very different, olefin formation being favored in the latter case. There is abundant evidence that this is so. 3. Stereoselectivity The hydrogenation of C,H,,,-,, when n 2 4 seldom produces one specific isomer of C,H,; more usually a distribution of isomeric olefins is observed. In such a case it is necessary to decide whether this distribution has arisen (1) by the direct addition of atomic or molecular hydrogen to the parent hydrocarbon, followed by the immediate desorption of olefin, or (2) whether isomerization of the reactant has occurred, or (3) whether olefin isomerization has intervened between the initial formation of olefin and its desorption. Three factors influence the distribution of olefins formed directly from an acetylene or diolefin. a. Reactants Having More than One Conformation. 1,3-Dienes distinguish themselves from alkynes and 1,2-dienesin that they exist in two conformations which are in dynamic equilibrium. The relative proportions of these conformations in the gas phase have been determined for butadiene (70). Consequently, the addition of a hydrogen atom to H

’

H

H

H

H ‘

/

H syn- 1,3-butadiene

anti-1,3-butadiene

each of the terminal carbon atoms of adsorbed syn-1,3-butadiene is reasonably expected to give adsorbed cis-2-butene, whereas addition to the adsorbed anti-conformation will give trans-2-butene. Alternatively, the hydrogenation of one double bond provides a route to

158

G . C . BOND AND P. B. WELLS

1-butene, and hence all three normal butene isomers may be directly formed from the parent hydrocarbon. It is interesting that all three isomers are indeed formed over all of the catalysts that have been studied to date, although the relative proportions in which they are produced varies widely, as will be shown in Section 111,F, 6 . The situation is further complicated if interconversion of the synand anti-conformations occurs when butadiene is adsorbed, since there can be no certainty that the equilibrium observed in the gas phase is still applicable. b. Molecular and Atomic A,dditionof Hydrogen. It is generally accepted that the reaction of a n adsorbed alkyne (see Section 111, B, I for notation) with two adsorbed hydrogen atoms in two consecutive steps yields an olefin in the cis-configuration. The hydrogen atoms, in this case, react by addition from below the axis of unsaturation:

Evidence from the kinetics of both olefin ( 2 7 , 3 1 )and acetylene (71) hydrogenation and from the exchange of cycloalkanes with deuterium (12) (in which rr-allylic intermediates are formed) has shown almost unequivocally that molecular hydrogen can interact with adsorbed hydrocarbon species. When this is the case, addition takes place from above the axis of unsaturation. The importance of molecular hydrogen interaction, as a general phenomenon, has yet to be determined. However, the evidence is sufficiently strong to compel the consideration of steps such as CHa-C-C

T-

*+

CH3

+H, CH3\

/H + H

*/=C\

CH,

I *

+

CHs\

H

c=c/H

/

\

CH,

for the direct formation of trccns-isomers of olefins from suitable alkynes. The postulate that alkynes and dienes are rr-bonded to metal surfaces (see Sections 111,B, I and 111, F, I ) provides an interesting clue t o the mechanism of hydrogen activation in such a process. The positive surface potentials observed for adsorbed ethylene and acetylene ( 2 4 ) show that the rr-electron system of the adsorbate is electron deficient; this is reasonably interpreted to mean that the electron donation from adsorbate to metal is not fully compensated by back donation into the antibonding orbitals of the adsorbate. Such positive polarization would tend, as the transition state is formed, to induce a weakening of the hydrogenhydrogen bond and the formation of a carbon-hydrogen (and a carbon-

HYDROGENATION O F UNSATURATED HYDROCARBONS

159

metal) bond. I n fact, compelling evidence for the participation of molecular hydrogen has only been obtained in a few instances, but similarly, the number of systems in which hydrogen definitely reacts in the atomic form only are also small; for the majority of systems the situation still needs to be clarified by careful experimentation. c. The Simultaneous Occurrence of Isomerization and Hydrogenation. Initial olefin distributions may be further complicated by (1)isomerization of the reactant, and (2) isomerization of olefins before desorption. Isomerization of reactant (e.g., the conversion of an alkyne to a 1,2diene, or vice versa) is readily detected if the isomer undergoes desorption. If this is not the case, however, then careful use of deuterium and the comparison of the distribution of deuterium in all of the olefinic products may be the only means whereby such isomerization can be detected or ruled out. Such considerations become important when discussing the formation of a 1-alkene from a 2-alkyne, for example. The complexities and uncertainties which surround the mechanism for olefin isomerization have been discussed in Section 11,where it was shown that isomerization may, in principle, occur by either an olefin/ alkyl interconversion or an olefin/allyl interconversion. Where isomerization of the product before desorption is suspected it is often informative to compare the characteristics of olefin isomerization over the same catalyst. This information is usually useful, as will be shown later, but such comparisons must be made with great care. I n the context of alkyne or diene hydrogenation, isomerization will depend upon a hydrogen atom concentration which is very different from that relevant to the ordinary isomerization of an olefin, and thus the product distributions may not be the same, and indeed, the mechanism itself may be different. I n the limit, an olefin may isomerize by olefin/alkyl interconversion when a plentiful supply of hydrogen is available (the formation of the half-hydrogenated state here requiring the addition of hydrogen) whereas if the availability of hydrogen is very low, due possibly to the demands of C,H,,,-,, for hydrogen, then conditions may favor the formation of a 7r-allylic species by hydrogen loss and isomerization may then be achieved by reversion t o olefin. Thus, the isomerization of olefin before desorption is a seriously complicating factor in alkyne or diene hydrogenation.

B. THE HYDROGENATION OF ACETYLENE 1. The Adsorbed State of Acetylene

There is a dearth of information concerning the adsorbed state of acetylene and other alkynes in the context of their reactions with

160

Q. C. BOND AND P. B. WELLS

hydrogen. Chemisorption studies have suggested (9)that the adsorbate forms two u-bonds with two surface metal atoms to give a structure which is ethylene-like [Structure (I)]. The geometry of this associatively H

- \

H

/-

*7=“\,

H-C--CH

Structure (I)

Structure (11)

** T

adsorbed state has been considered ( 6 , 72, 73) and the optimum metalmetal distances required in the catalyst surface are available in the commonly exposed planes. However, as was the case for the adsorbed state of ethylene discussed in Section 11, an alternative structure exists in which the adsorbate is n-bonded to the surface [Structure (11))above]. Here it is envisaged that each rr-election system interacts with a metal atom, so that two atoms in the surface constitute a “site” for adsorption. Such a structure is consistent with the surface potential of 1.0-volt measured for acetylene adsorbed on unoriented nickel film (24). Acetylene is more strongly adsorbed than ethylene, and this must be accommodated in the proposed structure for the adsorbed state. However, both Structures (I) and (11) fulfill this requirement. Di-ubonded acetylene is expected t o be more strongly bound to the surface than di-u-bonded ethylene because the first n-bond in acetylene is weaker than the second (2, 74). For the 7r-adsorbed structures, acetylene, can bond with two metal atoms whereas ethylene may only bond with one, so again acetylene is reasonably expected to be the more strongly adsorbed species. Since a choice between these notations has to be made, adsorbed acetylene and other alkynes will be presented in the following sections using Structure (11) and other structures analogous to it. Further evidence which lends support to this choice will be discussed in Section IV.

+

2. Introduction

All of the metals of Group VIII catalyze the hydrogenation of acetylene, This section will be concerned with three types of information which have been reported, namely, kinetics, the distribution of deuteroethylenes obtained when deuterium is added to acetylene, and the yields of ethylene and ethane obtained as initial products. Kinetic information has been available for some time (73, 75-78), and further reports have been made recently (71, 79-82); the nickel-catalyzed reaotion of dideuteroacetylene with hydrogen was reported in 1958 (83)

HYDROGENATION OF UNSATURATED HYDROCARBONS

161

and more recently studies of the deuteration of acetylene over the Group VIII metals of the second and third transition series have been reported (7.2). The following generalizations may be made. The initial rate order in hydrogen has always been observed to lie in the range 1.0-1.5, while orders in acetylene are zero or negative. When deuterium is used all possible isotopic isomers of ethylene are always observed, acetylene exchange is absent or only very slow, but the extent of hydrogen exchange varies from metal to metal. Ethylene, ethane, and higher hydrocarbons are always observed as initial products of the reaction. Thus, although the details of the reaction vary considerably depending upon the metal used as catalyst, certain broad features exist which are common to the reaction, whatever the metal catalyst employed. The elucidation of consistent mechanisms for acetylene hydrogenation is best achieved by considering first the results for acetylene deuteration, since these reveal information which influences the interpretation of the kinetics. 3 . The Reaction of Acetylene with Deuterium Table X X I I contains typical initial distributions of deuteroethylenes that have been reported. It is immediately clear that mixing of hydrogen and deuterium takes place on all of the catalysts; the extent of this redistribution is greatest over ruthenium and osmium, a little less over rhodium and iridium, and considerably less over nickel, palladium, and platinum. Here, therefore, the behavior of the metals shows a decided trend on passing from left to right across Group VIII. The interpretation to be placed on these distributions has been clarified by comparing them with those calculated from a steady state treatment (71,83). This treatment assumes that hydrogen atom addition occurs to both adsorbed acetylene and adsorbed vinyl and that hydrogen expulsion from the latter can take place, with the re-formation of adsorbed acetylene. The expulsion of a hydrogen atom or a deuterium atom from H:=CHD and D9=CHD was assumed to be equally likely. The solution of the steady state equations requires that numerical values be assigned to three parameters q, s, and p , where q is the percentage chance that adsorbed acetylene will react with a deuterium atom, s is the percentage chance that adsorbed vinyl (the half-hydrogenated state) will react with a deuterium atom, p is the percentage chance that adsorbed vinyl, one formed, will be converted to ethylene rather than revert to adsorbed acetylene (chance 100-p).

TABLE XXII Typicul Inaid D k t ~ b u t i o n sof Deuter& Ethyleneslobtained from C,H,+D, (for Six Metala) and from C,D,+H, (for Nickel) (71,83,84) (PDt/P4H*

= 2)

M

Metal

1

20 22 19

67 65 66

9 11

1 2 3

1.88 1.87 1.96

135 120

1 2

12 13

36 38

28 27

23 20

2.60 2.50

170 185

6 3

7 13

30 33

36 '35

21 16

2.59 2.48

Ni-p" Pd-ab Pt-8

95 15 89

2 2

Rh-8

fi-8 Ru-8 08-8

X-p = metal mpported on pumice.

'X - a = metal supported on alumina.

10

W 0

ktr fd td

8 E

HYDROGENATION O F UNSATURATED HYDROCARBONS

163

Calculated distributions, accurately reproducing those obtained experimentally, showed that the chance of surface species acquiring deuterium was in the region 60-90% for all of the metals (71,83,84). Table XXIII contains the extreme values of this parameter calculated TABLE XXIII Calculated Chances (yo)of Surface Species Gaining a Deuterium Atom i n C,H,+ D, (Six Metals) (71, 84) or of Gaining a Hydrogen Atom i n C,D,+H, (Nickel) (83)

Ru 70-90

(82q)

Rh76-88

0 s 60-90

(s2q)

Ir 78-88 ( 8 = q )

(8=q)

Ni 80 (s=q) Pd 79-86 ( S = q ) Pt 86-92

(8=q)

for experimental distributions obtained under a wide variety of conditions. For the ruthenium and osmium catalyzed reactions s 2 q, whereas the assumption s=q appeared valid for the remaining metals. Since the values of s and q are fairly independent of the metal, it is to be expected that the different distributions contained in Table XXII arise through a variation in the degree of “vinyl reversal” from one group of metals to another. Table XXIV shows that this is indeed so. The vinyl TABLE XXIV The Calculated Chance

(yo)that Adsorbed Vinyl Will Revert to Adsorbed Acetylene (71,83,84)

Ru 85-96

Rh 79-89

Ni -40 Pd 20-38

0 s 86-96

Ir 60-81

Pt 21-37

reversal reaction is the most likely fate for the half-hydrogenated state over ruthenium, osmium, rhodium, and iridium, whereas the hydrogenation of vinyl to ethylene is relatively more likely than vinyl reversal over nickel, palladium and platinum. This systematic variation of p is the more remarkable when it is recalled that the efficiency of these catalysts for the analogous “ethyl reversal” reaction decreases in the same way on passing from left to right across Group VIII (see Section 11). The origin of this trend is

164

U. C. BOND AND P. B. WELLS

open to speculation. Assuming that the mathematical analysis is correct, the rapidity of the vinyl reversal reaction over ruthenium, osmium, rhodium, and iridium suggests that the addition of hydrogen to vinyl may be rate-determining for these metals, whereas over nickel, palladium, and platinum this may not be the case. We turn now to consider the presence or absence of exchange reactions in the reactants. I n the past, Langmuir equations have been used to relate the surface coverages of each reactant to its pressure in the gas phase (80),the implicit assumption having been made that the rates of removal of adsorbed reactants by reaction are small compared to their rates of adsorption and desportion. The use of an isotopic tracer allows this assumption to be tested. The relative surface concentrations of C,H,, C,HD, and C,D, have been determined from the steady state treatment referred to above; typical values which represent the mean in each case of values obtained under a wide variety of conditions, are given in Table XXV. An TABLE XXV The Calculated Surface Concentrations of CnHn,CnHD, and C,D, Expeased as a Percentage of the Total Acetylene Coverage (71, 8 4 )

'0,Hs

b,,

b D s ~

Pd, Pt

90

9

1

Rh, Ir

70

25

6;

Ru, 0 s

40

40

20

acetylene exchange reaction should therefore be observed over all of the metals if desorption occurs. Experimentally, no acetylene exchange whatever was observed in reactions catalyzed by rhodium, iridium, palladium, or platinum (71);and only a very slow exchange was observed over nickel (83). Using ruthenium and osmium acetylene exchange was probably absent or of only minor importance under hydrogenation conditions (84). The conclusion was drawn, therefore, that acetylene adsorption must be irreversible, or virtually so, and consequently a Langmuir equation should not be used to relate acetylene coverage to its pressure in the gas phase. However, this does not seriously affect the interpretation of initial rate kinetics, because the magni-

166

HYDROGENATION OF UNSATURATED HYDROCARBONS

tude of the order in acetylene (zero or negative) indicates that acetylene coverage is high and pressure independent, or nearly so. Table XXIII has shown that, for each of the seven metals studied, the hydrogen available for reaction is composed of about 80% D and 20% H when acetylene reacts with deuterium. Therefore, if an adsorption/desorption equilibrium is set up for hydrogen, and is fast compared to the rate of hydrogen atom addition to adsorbed hydrocarbon species, then HD should be observed in the gas phase. The concentrations of HD observed in the gas phase at about 50% reaction are shown in Table XXVI. Most exchange was observed over ruthenium and osmium, less TABLE XXVI The Percentage HD Obeerved in the Gaa Phaee after 60% Conver8ion of Acetylene in Reaction C,H,+D,

Ni Pd Pt Rh

Ir Ru 0s

a

36"

9OG

(71,83,84)

60-120

60

100

0-46

60 60 60 76 80

100

60-110 134-200 30-120 133-183 147-201

100 100 170 210

Zero Zero 0-2 11-26 -16

21-30 28-67

Hydrogen isotopes reversed.

over rhodium and iridium, and virtually none over nickel, platinum, and palladium. Again, a systematic trend exists across the Group. The conclusion was drawn that a Langmuir equation is probably relevant for hydrogen in the case of those metals that promote hydrogen exchange. Over nickel, palladium, and platinum the adsorption of hydrogen must be virtually irreversible under reaction conditions. The implications of these conclusions upon the kinetics will be discussed later. The reversal of vinyl formation has a further interesting consequence because it governs the relative yields of the three isomers of dideuteroethylene. Tables XXIV and XXV have shown that, the greater the importance of vinyl reversal, the higher the concentration of monodeuteroacetylene adsorbed on the surface, Now the production of transand asym-dideuteroethylene depends, at least partly, upon the forma-

166

0. U. BOND AND P. B. WELLS

tion of adsorbed C,HD as shown in Fig. 20. Consequently, the yields of trans- and asym-dideuteroethylene will be expected to decrease in the sequence: RuzOs>Rh%Ir>PdrPt

The concentrations of the three isomers have been measured using infrared analysis (71,83, 84) and the results are shown in Table XXVII. H\

/H

*

\D

/c=c

H-CTC-H

**

cis-dideuteroethylene

)=c

0-C-fC-D

/D

**

‘H

)c=c

/H

*

\D

i I

)=C

/D

* H-C-C

D 7 ** -

\H

H

\

/D

/c=c * ‘D D

\

*

trons-dideuteroethylene

/c=c

asymmetric dideuteroethylene

/H \H

FIG.20. Routes to the formation of cia-,tram- and asymmetrio dideuteroethylene.

Expectations are largely justified. The &/trans ratios and the percentages of asymmetric isomer decrease on passing from left to right across the group for the elements of both the second and the third transition series. For rhodium, palladium, iridium, and platinum the steady state treatment referred to above, indicated that the chances of hydrocarbon species acquiring hydrogen or deuterium was the same in each of the two reaction steps. This being so, it would be expected from Fig. 20 that the yields of the trans- and asym-dideuteroethylene should be the same. That this is not so has been attributed (71)to the reaction X H x H X \ / 4 \ c=c f--‘Lc __j c=c

/”

*’

D ‘

1 PI) (X=H or D)

*’

h

167

HYDROGENATION OF UNSATURATED HYDROCARBONS

which is expected to occur since the existence of the vinyl group in normal and free radical forms in equilibrium has been postulated to account for the hydropolymerization of acetylene which always accompanies simple hydrogenation. The operation of the above reaction will TABLE XXVII Typical Distributions of cia-, trans-, and asymmetric Dideuteroethylene Observed i n the Reaction C,H,+D, (Siz Metals) ( 7 1 , 8 4 ) and i n C , D , + H , (Nickel) ( 8 3 )

Metal

Temp. (“C)

cia-C,H,D,

tram-C,H,D,

asym-C,H,D,

Ni Pd Pt

41 16 60

77

21

83 86

16 14

2 2 1

Rh

136 120

48 63

40 30

12 7

167 163

42 56

42 33

16 11

Ir Ru 08

cause additional trans-dideuteroethylene to be formed at the expense of the cis-isomer; thus H

HC-CH

** T

’4 ‘C=C’ ’*

H

H --f

H

1

!f’D

D

\

-+

& ‘ C‘

\D

H C=C

*’

H

/ ‘H

D

4. Kinetics and Mechanisms

The kinetics reported for acetylene hydrogenation are shown in Table XXVIII. Where comparison can be made, they seem to be independent of the physical form of the catalyst and of the support. The experimental observations detailed in the foregoing section, and the deductions made from them, have provided a basis for the following three mechanisms which have been proposed to account for the observed kinetics (2’1,84). a. Mechanism I . This is designed to explain an order of unity in

168

0. C. BOND AND P. B. WELLS

hydrogen and a zero or negative order in acetylene such that BC,B,+l. Hydrogen adsorption is assumed to be reversible, so that 9, K Pg:.

(3)

CaHa(g) -P HC-CH

T **

+H

HC-CH

P

(4)

p HC=CH,

I

(5)

I

+ H -+ (6)

HC=CH,

1

1

2C,H, (8)

H,C-CH,

T

TABLE XXVIII The Kinetics of the Hydrogenation of Acetylene over the Metale of Group V I I I Rate cc PH,PhH,

.

~~

~

~~

Catalyst

x

Fe-pa Fe-p Fe-pdrb

1.4 1

-

1

-

Ru-R'

0s-a

1.0

1.0

y

Ref.

Catalyst

m

y

Ref.

Catalyst m

0

(75) (82) (84

CO-p Co-pdr

1 1

-

-

(81) (81)

1 Ni-p Ni-p Ni-kd Ni-pdr 1 Ni-pdr -1 Pd-p 1.4 Pd-a 1.0 Pd-a 1 Pd-a Pd-sC 1

0.0

0.0

(84)

(84)

Rh-p Rh-a Rh-a

Ir-p Ir-s

-1

-0

-1 1.0-1.6

-1

(76) (79) 0.0 ( 7 1 )

-

0 (76) 1.0 - 0 . 3 ( 7 1 )

Pt-p Pt-R

Pt-a Pt-f'

p denotes that the metal was supported on pumice. pdr, powder form. Denotes alumina. k, kieselguhr. s, siliae. 8 f, foil. Complex hydrogen orders.

y

Ref.

0

(73, 75)

-0.1

(80)

0 0

(81)

-0.6

(71) (77)

(81) -0.6 (85) -0.6 ( 7 5 ) - (79)

-ive 0

-1.2 -0.7 1.6 -0.7

1.6

-

+ive -ive

(82) (78) (72) (79)

(86)

HYDROGENATION OF UNSATURATED HYDROCARBONS

169

The hydrogen atom involved in step (6) may arise by either of routes (1) or ( 5 ) ;in the latter case ethylene is being formed by the disproportionation of two vinyls [step (7)]. Equating the rates of formation and

* + 2HC=CH,

1

(7) --t

HC-CH

** T

+ HzC-CHa

T

removal of adsorbed vinyl,

+

k,ec,tllen = ~sec,tr, koec,,,eFI

*

For those reactions in which vinyl reversal is more rapid than vinyl hydrogenation ( p > 5 0 ) , k5 > k60H,and

The rate of ethylene formation, r is then given by

This mechanism seems adequate to describe the reactions over ruthenium, osmium, iridium, and rhodium (in certain instances) which exhibit an order of unity in hydrogen. b. Mechanism 11. This mechanism, like I above, is designed to explain an order of unity in hydrogen and a zero or negative order in acetylene. It differs from Mechanism I in that it assumes hydrogen adsorption to be irreversible. Steps ( l ) ,(3), (4), ( 5 ) , (6), (7),and (9) written down for Mechanism I are relevant. The rate of hydrogen adsorption will be proportional to the hydrogen pressure, as will the rate of formation of pairs of vinyls. In the absence of hydrogen desorption, hydrogen atoms liberated by vinyl reversal [step ( 5 ) ] must either combine with acetylene (which, as an overall reaction, is facile), or else combine with vinyl, which is the envisaged mechanism of step (7). Thus, if vinyl disproportion is the important route for ethylene formation then, equating again the rates of formation and removal of vinyl.

-

k p,, = k7e:sn8 = rate of ethylene formation.

This mechanism may adequately describe the reactions over nickel and palladium, where the hydrogen order is unity and hydrogen adsorption is apparently irreversible. The values of p (Ni: 40%, Pd-SO%) compare with the value of 50% expected if step (7) is the sole ethylene forming step.

170

Q. C . BOND AND P. B. WELLS

Iron and cobalt exhibit an order of unity, but since their abilities t o promote hydrogen exchange have not been measured, their mechanisms may correspond to either possibility considered above. c. Mechanism III. This mechanism is designed to explain an order in hydrogen of 1.5 and an order in acetylene that is zero or negative, so that 8c~lI,+-l, Hydrogen adsorption is assumed to be irreversible. For the platinum-catalyzed reaction, which shows this high order in hydrogen, it has been proposed that the rate-determining step involves the interaction of a hydrogen molecule with an adsorbed vinyl group [step (S)], the vinyls being present in a steady state concentration brought about by the operation of Mechanism I1 (71).Step (8)is a vinyl propagation reaction, because the liberated hydrogen atom will most HC=CH,

I

+ Ha (8)

3

HZC-CHa

r

&

+H

i

likely react with adsorbed acetylene t o give vinyl in a fast reaction. The steady state from Mechanism I1 gives &t,

.

... " Pdt2 2 = wclHlecIHaPfIl = vg. The order of 1.5 in hydrogen is thereby understood. 5. SELECTIVITY FOR ETHYLENE FORMATION

A remarkable feature of acetylene hydrogenation is that ethylene is obtained in high yield using metals which are extremely active for ethylene hydrogenation in the absence of acetylene. Unpoisoned palladium catalysts are a good example; a t room temperature their activity for ethylene hydrogenation is some ten to one hundred times that for acetylene hydrogenation and yet in the latter reaction the yield of ethane is of the order 1-5% in the initial products. This ethane is produced during one residence of the hydrocarbon unit on the surface, and all of the ethane-forming steps from ethylene are fast compared to the rate of formation of ethylene from acetylene. Let us now consider the experimental results in some detail. Selectivities are always dependent upon hydrogen pressure, as shown in Fig. 21, and upon temperature, as shown in Fig. 22. The figures refer only to the noble Group V I I I metals: the first row metals have been studied in less detail (75) and have shown the following selectivities; Fe-pumice, 0.9 a t 156", Co-pumice, 0.9 a t 197", and Ni-pumice 0.80 a t

171

HYDROGENATION O F UNSATURATED HYDROCARBONS

83". Two trends are evident from these results; first, the selectivity decreases with increasing atomic number down each triad of Group VIII (palladium or nickel falling out of line), and second, for the noble metals the selectivity increases from left to right across the Group. In Section 111,A, 2, thermodynamic and mechanistic factors were said to determine selectivity. The effectiveness of the thermodynamic factor depends partly upon the strength of ethylene adsorption. Heats of adsorption of ethylene on supported metals are not available, but it is known, however, (see Section IV, B, l a ) that the stability of ethylenemetal and other olefin-metal complexes increases with increasing atomic number down each triad of Group VIII. If it is assumed that the normal adsorbed state of olefin is that of a rr-complex with the surface, it would

0.8 0.9-

.-s .->

2

L L h P t Cll0"C (1520c

Ru (139OC

0.7-

Q)

0.5

0 s (117°C 0.4 o 0

-

100

200

)

300

Ir (133°C 1 400

)O

FIG.21. The dependence of selectivity upon hydrogen pressure for the reaction C,H, H, catalyzed by alumina-supported noble Group VIII metals; initial Pc,H,= 60 mm.

+

172

Q. C. BOND AND

P. B. WELLS

not be surprising to find ethylene more strongly adsorbed onto, and hence less easily displaced from, the heavier metals. This reduced chance of ethylene desorption would inevitably lead to a higher chance of paraffin formation, that is, a lower selectivity over the heavier metals. I.o

- 4 . 0

0

I

I

uu

1

O-Pd

50

100

150

Temperature

200

("C 1

+

FIG.22. The dependence of selectivity upon temperature for the reaction C,H, H, catalyzed by alumina-supported Group V I I I metals; initial Pcsnl= 50 mm, P,, = 200 mm.

In the absence of measured heats of adsorption, however, the operation of the thermodynamic factor in this sense must remain speculative. The second determining factor, that of the inherent activities of the metals for ethylene hydrogenation can be more firmly based on experimental evidence. Velocity constants for this reaction have been reported for metal films (87) and for silica-supported metals (88);Table XXIX shows that the relative activities observed by the two groups TABLE XXIX Values of th,e Logarithm of the Velocity Constant for Ethylene Hydrogenation (kRh=1)

Rh

Ru

Pd

Pt

Ir

Ni

W

Metalfilms(87)

0.0

-

-0.8

-1.6

-

-2.8

-3.0

metal (88)

0.0

-0.3

-0.9

-1.6

-2.0

-

-

IIYDROUENATION O F UNSATURATED HYDROCARBONS

173

of workers agree well where the same metals have been studied. The low value for iridium has been questioned (89).Table XXIX shows that for Group VIII the metals of the first transition series are the least active for ethylene hydrogenation ; those of the second transition series are the most active, and those of the third are of intermediate activity. Now, this sequence may be employed in discussions of acetylene selectivity since the adsorption of reactants is not rate-determining in ethylene hydrogenation. Iron, cobalt, and nickel may show a high selectivity because ethylene is most easily displaced from their surfaces, and because they have the lowest activities for ethylene hydrogenation. The lower selectivity given by ruthenium compared to iron probably reflects ruthenium’s higher activity for ethylene hydrogenation. Palladium, on the other hand, shows a higher selectivity than nickel, indicating that the thermodynamic factor assists ethylene desorption from palladium t o such an extent that it outweighs this metal’s higher activity for ethylene hydrogenation. Cobalt and rhodium show the same selectivity, possibly due to a cancellation of the opposing effects which are evident in Groups VIII, and VIII,. The thermodynamic factor must operate particularly weakly over osmium, iridium, and platinum because these metals show lower selectivities than the second row metals in spite of possessing lower activities for ethylene hydrogenation. This is readily demonstrated to be the case for osmium and iridium, where ethylene produced in the reaction is able to compete for the surface with remaining acetylene and the selectivity is thus conversion-dependent in a static system (71, 84); the same is not, however, true for the platinum-catalyzed reaction (71). Remarkably, this pattern of selectivity is largely reproduced for the hydrogenation of all acetylene and diolefins studied to date, and the factors discussed above must therefore have similar relevance in these systems as well. The results for the higher acetylenes and diolefins will be given in the sections which follow.

C. THE HYDROGENATION OF MONOALKYLALKYNES The hydrogenation of molecules of the type HCECR, where R is an alkyl group, is a largely neglected field. The only mechanistic studies reported to date are the hydrogenation of propyne in the gas phase (78) and of l-butyne (a)in alcoholic solution (57)and (b) in the gas phase (90). Only the last mentioned study has employed an isotopic tracer. 1. Propyne

The kinetics of propyne hydrogenation using pumice-supported nickel, palladium and platinum were invariably first order in hydrogen and

174

0. C. BOND AND P. B. WELLS

zero or of slightly negative order in hydrocarbon in the temperature range 45-90', showing that, as for acetylene itself, the hydrocarbon was strongly adsorbed and hydrogen was but weakly adsorbed. The selectivity for propene formation was always high, being 0.93 a t 91" for nickel, unity at 136" for palladium and 0.90 a t 75" for platinum. These figures are very similar to those reported for acetylene. The deuteration of this molecule has not been studied, so information on the reversibility of adsorption of the reactants is not available. However, it is likely that the mechanism of this reaction is analogous t o that described in Section 111, B, 4b for the nickel and palladium catalyzed hydrogenation of acetylene. 2. 1-Butyne I n the liquid phase a t room temperature, using alcohol as a solvent and palladium supported on barium sulfate as catalyst, the only products observed from 1-butyne hydrogenation were 1-butene (98%) and n-butane (2%) (57). The gas phase reaction using 0.03% palladium on alumina catalyst gave 1-butene (99.1yo),cis- and trans-2-butene (each 0.2%) and n-butane (0.5%) (90). These product distributions were maintained until a t least 75% removal of the parent hydrocarbon but isomerization and hydrogenation of the 1-butene occurred after complete removal of the alkyne. Thus, 1-butyne must displace l-butene from the surface before its isoinerization can occur, and it must prohibit the re-entry of 1-butene into the reacting surface layer. This represents the operation of a powerful thermodynamic factor. The mechanism may be simply written as shown in Fig. 23. The production of 2-butenes in only trace quantities shows clearly that isomerization of the reactant [steps (2) and (5)] and subsequent hydrogenation [step (S)] is a very slow process: H C,H, +m 1-butene / -11 \ C=C +CH,-CH=C=CHX d + (6) 2-butene \ (5) /

*

X

11

(adsorbed 1,2-butadiene)

H\

/c=c

*

H 2c5,,

\X

FIG.23. Reaction scheme for tho hydrogenation of 1-butyne to 1-butene; X or D.

E

H

HYDROGENATION O F UNSATURATED HYDROCARBONS

175

The gas phase study involved also the deuteration of 1-butyne. Deuterium was distributed in the major product and reactant as shown in Table XXX. Analysis of the 1-butene by NMR showed that no deutTABLE XXX Distribution of Deuterium in Reactant and Product Obtained in the Deuteration of 1 -Butyne at 350" Using Palladium-Alumina ( 9 0 ) Conversion = 21%

do

dl

d,

d,

1-Butyne

98.4

1.6

0.0

0.0

1-Butene

1.8

14.1

72.2

11.9

d24

0.0 0.0

erium was present in the ethyl group, and no signal was obtained for the -CH = proton. Steps (1) to (4)of the mechanism given above show how -do, -al, and -d, isomei s of but-1-ene may arise. The observed yield of the -d, isomer (see TiLble XXX) requires a mechanism for the exchange of the acetylenic hydrogen, since the third deuterium atom is not located in the ethyl group. Exchange of one hydrogen was observed in he parent hydrocarbon and it was suggested that it is the acetylenic hydrogen that has exchanged. The following mechanism was proposed to account for the 1-butyne exchange. 1-Butene-d, arises, on this

model, by the reaction of 1-butyne-d, with deuterium according to steps (1)to (4).The distribution of deuterium in the 1-butene was reproduced accurately by the investigators, and they showed that the rate of addition of deuterium to 1-butyne was some twelve times faster than the rate of formation of equilibrated 1-butyne-d,. Steps (8) and (8') must necessarily operate without the desorption of the 1-butyne from the surface, and thus i t must be assumed that the interaction of the triple bond with the surface is present before the carbon-metal bond is ruptured. The geometry involved in such an assumption is difficult to envisage. An alternative mechanism can be

a.

176

C. BOND AND P. B. WELLS

proposed for the exchange of l-butyne as shown in steps (9) and (10). The interaction of molecular hydrogen with adsorbed species was

discussed in Section 111,A, 36; its introduction here offers a neat route for the exchange of 1-butyne, although a choice between this and the originally proposed mechanism cannot be made on the basis of the experimental results so far published. The nickel-catalyzed hydrogenation of 1-pentyne and l-hexyne provided only the l-alkene and alkane, the selectivity being about 0.75 (91, 92). It must therefore be concluded that, generally, reaction sequences analogous to steps ( 5 ) and ( 6 ) are difficult.

D. THE HYDROGENATION OF DIALKYLALKYNES The hydrogenation of dialkylalkynes is widely used in organic chemistry for the preparation of olefins in the cis-configuration (93),but only recently have studies been reported which have sought to elucidate the mechanisms that occur in this type or reaction. Studies have now been reported on the hydrogenation of 2-butyne (56, 83, 94, 95), 2-pentyne (58, 91), 2-hexyne (91,92), and 3-hexyne (91, 92, $4). The characteristics of these reactions have been shown t o be fairly independent of the complexity of the molecule and of the phase in which the reaction was carried out. 1. 2-Butyne

We shall consider first the simplest reaction SO far reported (56, 9 4 ) , which is the hydrogenation of 2-butyne in a flow system at room temperature and a little above, catalyzed by alumina-supported palladium (0.03%). This reaction proceeds very selectively, only a trace of butane being formed in the presence of 2-butyne, as long as the catalyst is not completely fresh. Moreover, the reaction is highly stereoselective for the formation of cis-2-butene and only traces of trans-2-butene and 1butene were observed. After the removal of 2-butyne the cis-olefin both isomerized and hydrogenated, showing that a powerful thermodynamic factor is again operative (as was observed for l-butyne, propyne and acetylene) when alkyne is present. a. The Reaction of 2-Butyne with Deuterium. The study of this reaction using deuterium (94)has shown that cis-2-butene-2,3-d2accounts

HYDROGENATION OF UNSATURATED HYDROCARBONS

177

for 99% of the product as long as unreacted 2-butyne remains. Nuclear magnetic resonance spectra showed that the two deuterium atoms were present solely on the second and third carbon atoms. This is, without doubt, the most spectacular example of stereoselectivity so far recorded. Increasing the temperature from 14 to 58" only slightly reduced the selectivity and the stereoselectivity. , No exchange was observed in the 2-butyne7 but the presence of the alkyne inhibited the rate of hydrogen exchange by a factor of 40 or greater. The hydrogenation of 2-butyne using a 1 :1 mixture of hydrogen and deuterium gave random addition of hydrogen and deuterium atoms to the hydrocarbon, indicating that dissociative adsorption and mixing of H and D occurred before addition to the hydrocarbon. The mechanism of this reaction is adequately represented in steps (1)) ( 2 ) )and (3). The mechanism is simpler than that for the hydrogenation CH,

/

\

CH,

CH, cD

CH,

c=c

(3)

**

\

D

'

/

D '

of 1-butyne in that only one half-hydrogenated state is obtainable by hydrogen atom addition to the parent molecule. The reversal step (2) would not be noticed, and it must be assumed from the absence of isomerization of 2-butyne and the low yield of cis-2-butene-d, that step (4)is not favored. It is interesting and consistent that the formation of adsorbed 1,2-butadiene from both l-butyne and 2-butyne is difficult. CH,

\

*/

/

c=c

CH, -H

D '

We shall now consider briefly results obtained from the hydrogenation of 2-butyne in the liquid phase, using alcohol as solvent and Pd-BaSO, as catalyst (59). I n this study cis-2-butene was the only detectable product until the complete removal of the alkyne, the subsequent removal of the olefin by hydrogenation was accompanied by very fast isomerization. These results are in complete harmony with those obtained from the gas phase reaction and thus the mechanism of this reaction must be very similar in both the gas and liquid phases. The hydrogenation of 2-butyne has been studied over the other metals of Group V I I I and over copper (84, 95) using a static system and alumina-supported catalysts in the temperature range 100 t o 200". Under these conditions more complex distributions of products have been observed than was the case for palladium a t room temperature.

178

0. 0. BOND AND P. B. WELLS

Table XXXI contains mean initial product distributions which hardly varied with either initial hydrogen pressure (when it was altered by a factor of eight) or with temperature (425"). I n addition to the products listed about 2% 1,2-butadiene was produced in the reactions over rhodium and iridium. b. Selectivity. The trends in the selectivity across Group VIII are the same for 2-butyne as those reported for acetylene except that the absolute values throughout are higher. The highest selectivities have been recorded for the metals of the first transition series, indeed, these reactions were perfectly selective; lower values were observed for the metals of the second row, and Lower values still for the third row metals. Iron, cobalt, nickel, and copper were less active for the hydrogenation of butenes than for 2-butyne and thus the reason for their highly selective behavior may not have been due to the operation of a particularly strong thermodynamic factor, as was the case for palladium, Over the remaining metals butenes were more rapidly hydrogenated than 2-butyne and in all cases n-butane appeared as an initial product. An increase in selectivity on passing from left to right across Group VIII is observed and this again reproduces behavior observed for acetylene. c. Stereoselectivity. Although cis-2-butene is the most favored isomer produced over all of the metals, only copper resembles palladium in giving completely stereospecific formation of this isomer. 1-Butene is always produced as an initial product and its yield was especially high over iron, ruthenium and osmium. The mechanism of the reaction must exhibit reaction paths in addition to those already postualted [i.e., steps ( l ) , (2), and (311 and these have been investigated using deuterium in place of hydrogen ( 8 4 , 9 5 ) . Typical distributions of deuterium in the three butenes obtained using ruthenium are shown in Fig. 24, after the uptake of 0.15 mole of hydrogen per mole of 2-butyne and after the uptake of 1-25 moles. Initially, 100

80 Y

i

0

6o

40

20

~i~-2-bubna

trans-2-butene

1-butene

PIC.24. Distribution of deuterium in butenes obtained by the deuteration of 2butyne. Filled columns: uptake = 0 . 1 5 moles D, per mole C,H,; open columns: uptake = 1.25 moles D, per mole C,H,.

t-woo

dlt-t-

HYDROGENATION OF UNSATURATED HYDROCARBONS

ern

mln

179

a.

180

C. BOND AND P. B. WELLS

the distributions in the cis- and trans isomers of 2-butene are similar and show a sharp maximum a t -d 2, while that of the 1-butene is markedly different, the maximum here being present at -&. This information was used as evidence that the route to the 2-butenes was probably the same and that 1-butene was formed by a more complex mechanism. The distributions of deuterium in the olefin under conditions where they are isomerizing freely are closely similar, however, and it is therefore unlikely that the route to 1-butene involves the isomerization of cis-2butene. Morover, if the isomerization of cis-2-butene did occur before its desorption, it is unlikely, from our knowledge of its isomerization products (see Section 11), to yield trans-2-butene and 1-butene in the ratios shown in Table XXXI. The mechanism proposed (84, 95) is as follows. The formation of cis2-butene is adequately described by steps ( l ) , (2), and (3) proposed above for the palladium-catalyzed reaction. Since the initial cisltrans ratio in the butenes is almost independent of initial reactant pressures and temperature, and since the distribution of deuterium in these two isomers is similar, i t has been concluded that trans-2-butene is also produced directly from 2-butyne, and not by the subsequent isomerization of cis-2-butene. Steps ( 6 ) , (7), (8), and (9) describe the simplest route which satisfies the experimental observations and involves the addition of hydrogen to a free radical form of the half-hydrogenated state, which is envisaged t o be in equilibrium with the normal form. An analogous equilibrium was postulated in the mechanism for acetylene C H*

\

*,c=c\H

/

CH,

CH,

% \ . 4 ,C--C-H -0 * \*

CHs

-g +H

cis-2-butene

t~ans-2-butene

(9)

hydrogenation (see Section 111, B, 3). The hydrogen atoms taken up in steps (8) and (9) would be indistinguishable from those accepted in step (3) and hence the distribution of deuterium in the two 2-butene isomers would be the same or similar, as observed. 1-Butene is thought t o be formed by the initial isomerization of the reactant to adsorbed 1,2-butadiene, followed by hydrogenation (see Fig. 25). This mechanism predicts a maximum at -d, in the 2-butenes and a t -d in 1-butene if the chance of surface species aquiring a deuterium atom, as opposed to a hydrogen atom, is high. Moreover, the appearance of trace quantities of 1,2-butadiene in the rhodium and iridium catalyzed reactions lends further support to this route. A check may be made, since, if the 1,2-butadiene hydrogenates to yield approximately equal proportions of cis-2-butene and 1-butene (see Section

HYDROGENATION O F UNSATURATED HYDROCARBONS

181

111, F, 4 ) then the yields of cis-2-butene-d, and 1-butene-d, should be very similar. This was indeed found to be SO. While no evidence was obtained to show that olefin isomerization contributed significantly to the initial distribution of olefins, such isomerization became important as 2-butyne was removed in the static system experiments. This is shown in the rise in the yield of transolefin and the approach of 1-butene towards its equilibrium fraction. Also the yield of trans-2-butene-d, increased as the reaction proceeded, presumably due to the isomerization of 1-butene. To summarize, strong adsorption of 2-butyne on all metals and the low inherent activities for butene hydrogenation shown by iron, cobalt, nickel, and copper promote high selectivities for olefin formation in this reaction. Although complete stereoselectivity has been achieved using palladium and copper, the majority of metals are less stereoselective partly because of isomerization of the reactant which produces 1-butene, and partly because of the inherent nature of the half-hydro-genated state. The initial stereoselectivity is not impaired by the isomerization of olefin before its desorption, although this intervenes in some cases as the 2-butyne is progressively removed. 2. 2-Pentyne and 2- and 3-Hexyne

2-Pentyne has been studied in the liquid phase at about room temperature using Raney nickel and a variety of solvents (91),and also using carbon-supported palladium, rhodium, and platinum and aluminasupported iridium in the absence of a solvent (58).In the former study, cis-2-pentene was produced almost exclusively and the rate of hydrogenation of the pentene was slower than that for the 2-pentyne [this situation is similar to that recorded for the hydrogenation of 2-butyne

/

CH2X \

C ,H3

/c=c\x

,CX2-CH, H,C=C

\*

J Fro. 25. Reaction scheme for the hydrogenation of 2-butyneto butenes; X = H or D.

182

0. C. BOND AND P. B. WELLS

on nickel (see Section 111, D, I)]. The hydrogenation of pentene was arrested by the use of dimethylformamide as solvent, presumably because the solvent had a free energy of adsorption intermediate between those of 2-pentyne and cis-2-pentene. When no solvent was employed (58) the recorded initial selectivities were Rh, 0.80; Pd, 0.99; Ir, 0.57, and Pt. 0.90, Again, the pattern of selectivity found for other molecules is reproduced; Rh < Pd, Ir < Pt; Rh > Ir, Pd > Pt. The reaction was highly stereoselective, the yields of cis-2-pentene being Rh 96%; Pd 98%; Ir 92%; and Pt 93%. The nickel-catalyzed hydrogenation of 2- and 3-hexyne showed a selectivity of about 0.97 and the olefins were principally of the cisconfiguration (92). Using alumina-supported palladium (96) and no solvent, the initial olefin distribution from 3-hexyene was: cis-3hexene, 92%; trans-3-hexane, 5%; cis-2-hexene and 1.5%) trans-2hexene, 1.5%. No 1-hexene was observed and the selectivity was 0.94. Thus the features of the hydrogenation of dialkylacetylenes in the gas phase are largely reproduced when reactions are studied in the liquid phase; the cis-olefin is again observed as the major product and the yields of olefins by double bond migration are small. I n these reactions the high selectivity is again occasioned by a powerful thermodynamic factor because the isomerization of olefins to their equilibrium proportions and hydrogenation of olefin become important as each alkyne wa8 removed. In the deuteration of 3-hexyne at 0" the original hydrocarbon was devoid of deuterium at 35% conversion and no hydrogen exchange was reported. Reactant adsorption again appears to be irreversible; 87% of the cis-3-hexene contained two deuterium atoms and no isomers were observed containing more than four deuterium atoms. The degree of stereospecificity in the addition of deuterium to the original hydrocarbon is thus smaller for 3-hexyne than for 2-butyne, due presumably t o the presence of exchange reactions in the alkyl groups. The n-hexane obtained at 36% conversion of 3-hexyne was very fully exchanged indeed (do, d, < 1%; d , 1.6; d , 12.5; d, 51.2; d, 9.9; d, 6.1; d, 4.1; d, 5.6; d, 6.4; d,, 1.0; all, d,,, d,,, (1; dI4 1.5). This result was not commented upon explicitly by the investigators, but it suggests that once the hydrocarbon unit enters upon the hexane-forming step the desorption of olefin is prohibited; successive hydrogen atoms are then exchanged for deuteriums by the well-established cz-p mechanism. 3. 4-Undecyne Yields of trans-4-undecene of up to 68% have been reported in the hydrogenation of 4-undecyne using three types of palladium catalyst

HYDROGENATION O F UNSATURATED HYDROCARBONS

183

(10% WIWmetal on each of three supports) and ethyl acetate and cyclohexane as solvents (97). This result is exceptional and completely out of character with the picture developed from the hydrogenation of smaller acetylenic molecules. No mechanism was developed although these authors believe that the trans-isomer was formed by the isomerization of cis before desorption. One may only speculate over the reason for this remarkable result, but is a t least possible that these workers were using a sufficient weight of catalyst (weights used were between 9.8 and 17.4% of the weight of the acetylene) to reduce the concentration of the acetylene in the liquid phase to the point where the 4-undecyne was unable effectively to displace cis-4-undecene from the surface, with the consequence that it isomerized to the trans-isomer. No undecane or undecenes other than 4-undecene were reported in this study.

E. THEHYDROGENATION OF MORE HIGHLYUNSATURATED HYDROCARBONS It is not intended that the literature concerning the hydrogenation of alkenylalkynes and dialkynes shall be reviewed in detail. However, the hydrogenation of molecules as unsaturated as these provides further examples of the operation of the thermodynamic factor which are of interest. The palladium-, platinum-, and nickel-catalyzed hydrogenations of vinylacetylene (H,C =CH-C r C H ) provides 1,3-butadiene as the major initial product; butenes and butane are also produced (57). The product distributions are constant in liquid phase reactions until the parent hydrocarbon has been removed, showing that vinylacetylene is more strongly adsorbed than 1,3-butadiene and the butenes. The relative yields of butenes and butane resemble those obtained in 1,3butadiene hydrogenation over these metals (see Section 111, F, 6 ) . A recent report of the hydrogenation of 1,7-undecadiyne (I) using 10% palladium-charcoal (97) shows well how the hydrogenation of such an unsaturated hydrocarbon can proceed in clear-cut stages. When (I)

CH3-(CH,)Z-CEC-(CHZ),-C3CH +H.

(11)

CH3-(CH2),-C~C-(CHz)4-CH=CH, +H,

(111)

CH3-(CHz),-CH=CH-(CH2)4-CH=CH,

(IV)

CH3-(CH2),-CH=

+Ha

(V) (VI)

CH-(CH,)6-CH3

CH,-(CH,),-C~C-(CH,),-CzCBr CH3-( CH a) ,-CH=

.1CH-(

CH ,)4-C=CBr

184

a.

C. BOND AND P. B. WELLS

hydrogenation of this molecule was stopped after the uptake of one, two, and three moles of hydrogen, the products were found to be, respectively, undec-l-en-7-yne (11),1,7-undecadiene (111),and 4-undecene (IV). The vulnerability of the terminal triple bonds is unchanged when mixtures of alkynes are hydrogenated; for example, the partial hydrogenation of an equimolar mixture of l-octyne and 4-octyne gave 1octene and unchanged 4-octyne after the uptake of one mole of hydrogen (97). The terminal triple bond may, however, be sterically protected from attack by substitution of the acetylene hydrogen by bromine. Thus, 1-bromo-l-heptyne is very resistant to catalytic hydrogenation, and l-bromo- 1 , 7-undecadiyne (V) gave predominantly cis-l-bromoundec-7-en-l-yne (VI) after the uptake of one mole of hydrogen (97).

I?. THE HYDROGENATION OF DIENES 1. The Adsorbed State of Dienes No studies have yet been reported on the interaction of dienes with “clean” metal surfaces, and thus information on the adsorbed state of this class of molecule has to be deduced entirely from the characteristics of the reactions that they undergo. Information a t present available from studies of the hydrogenation of propadiene, 1,2-butadiene, 1,3-butadiene, and some higher adjacent and conjugated dienes indicates that these hydrocarbons are more strongly adsorbed than the monoolefins formed by their hydrogenation. Indeed, their strengths of adsorption are of similar magnitude to those shown by alkynes. Thus, propadiene and propyne react a t comparable rates when mixtures are cohydrogenated using a nickel catalyst a t 90°C (98).The following relative rates have been measured for the removal of C, hydrocarbons in the competitive hydrogenation of pairs, using palladium-alumina a t room temperature (90); 2-butyne : 1,3-butadiene : 4 11

:

1,2-butadieno : l-butyne 1 1

Assuming that the rate constants for the rate-controlling step in each reaction are of similar magnitude, which is reasonable, then 2-butyne appears to be the most strongly adsorbed of the C,H, isomers and 1,2butadiene and l-butyne the least strongly adsorbed. Let us first consider in detail the adsorption of 1,3-dienes and take the simplest member of the homologous series, 1,3-butadiene, as an example. The strong adsorption of 1,3-butadiene relative to l-butene (see Section 111, F, 6 ) strongly suggests that both olefinic linkages interact

HYDROGENATION OF UNSATURATED HYDROCARBONS

185

with the surface. Thus, by analogy with Section IIA, we may write two possible structures for the adsorbed state, one involving the formation of four carbon-metal bonds [Structure (I)]and the other involving two metal-olefin interactions [Structure (II)].At first sight, Structure H,C-CH-CH-CH,

?,I I

?,

Structure (I)

Structure (11)

(I)is expected to be very strongly adsorbed, four chemical bonds having been formed between the adsorbate and the surface. This stability would be offset, however, by the considerable internal strain inherent in this structure due t o the sp3hybridization of all of the carbon atoms. The actual strength of adsorption of 1,3-butadiene bonded as in Structure (I)is thus difficult to assess. Turning to Structure (11), the interaction of both olefinic linkages with the surface would certainly cause 1,3-butadiene to be more strongly held to the surface than 1-butene, assuming the latter to contain one such bond to the surface. Moreover, the sp2 hybridization of the free molecule is not likely to be greatly disturbed by the interaction with the surface and hence little strain will be present in the adsorbate. A second source of evidence of the adsorbed state lies in the manner in which adsorbed 1,3-dienes react, This will be discussed fully in Section 111, F, 6 ; it is sufficient for present purposes to state that, a t the surfaces of most metals, one olefinic linkage appears t o hydrogenate independently of the other to give adsorbed 1-butene. 1,2-Addition is thus generally preferred to 1,4-addition. The olefinic linkages retain their identity to a larger degree in Structure (11)than in Structure (I)and thus preferential 1,2-addition is more easily understood if the second structure is accepted. On the other hand, 1,kaddition would be expected to be a t least half as important as 1,2-addition if Structure (I)was correct. It appears, therefore, that Structure (11)represents a preferable notation, based on the evidence a t present available. I n Section 111,A, 3, it was noted that butadiene exists in two conformations in the gas phase, and it is expected that these conformations will also be present when the diene is adsorbed. Interconversion between the two conformations may take place via the formation of mono-.rr-adsorbeddiene and there can be no guarantee that the proportions of each on the surface will be the same as that existing in the gas phase. It has been suggested (12) that the syn-conformation of butadiene has a geometry such that it may be capable of adsorption to only one metal atom, as shown in Structure (111),by the delocalization of the

186

0. C. BOND AND P. 1). WELLS

n-elcctrons over the whole molecule; the experimental verification or negation of this suggestion will prove difficult, to say the least,

Structure (111)

Similarly, the adsorbed state of 1,2-dienes may be represented in two ways, as shown in Structures ( I V ) and (V). Since the two n-electron systems in a I,'l-diene are mutally a t right angles i t follows that the two metal-olefin bonds in Structure (V) must also be a t right angles to each other. The fission of one olefinic linkage and the subsequent H

CH a

b

,c-c H*'

/

*'

Structure (IV)

H,C =C =CH,

!!

Structure (V)

formation of two carbon-metal cr-bonds is sufficient to account for the strong adsorption, of, say, propadiene compared to propene, because this adsorption would be accompanied by a release of internal strain. Such a release of internal strain a t some point in the conversion of diene to inonene is indicated by the magnitude of the heat of hydrogenation of this reaction, which is typically some 10 kcal mole-' greater than the heat of hydrogenation of rnonoolefin to paraffin(96). Alternatively, if Structure (V) is accepted as the adsorbed state of the diene, it then follows that the relase of internal strain is associated with the formation of some reaction intermediate, probably the half-hydrogenated state. All reports of 1,2-diene hydrogenation to date have employed Structure (1V) to describe the adsorbed state, and Structure (V) does not appear to have been considered. It should, however, merit consideration in the future. 2 . General Features of the Hydrogenation of Dienes The characteristics of diene hydrogenation closely resemble those of alkyne hydrogenation. Thus, in the presence of excess hydrogen, the reaction exhibits two distinct stages. In the first stage alkene and alkane are often produced together, the former generally being the major product; usually, the selectivity is constant or decreases only slowly until the diene has been removed. I n the second stage, olefin hydrogenation takes place and this is accompanied hy isomcrization if the metal employed is itself a good olefin isomerization catalyst. As far as the information a t present extends, the second stage of the reaction ex-

HYDROGENATION OF UNSATURATED HYDROCARBONS

187

hibits a faster rate than the first stage when the noble Group VIII metals are employed; iron, cobalt, nickel, and copper, however, show the reverse order of activity for the two stages in the hydrogenation of butadiene (99).There is evidence for the existence of a powerful thermodynamic factor operating in the first stage of the reaction irrespective of the metal used as catalyst, with the result that olefin is usually produced with a high selectivity. 3. Propadiene Kinetic measurements are only available for the reactions catalyzed by pumice-supported nickel, palladium, and platinum (100).These reactions were zero-order in hydrocarbon and first-order in hydrogen, indicating in a superficial way the similarity of this reaction to that of alkyne hydrogenation and confirming the disparity in the strengths of adsorption of the reactants. The selectivities observed in the first stage of the reaction have been reported for most of the Group VIII metals (5, 84, 89, 100) and are shown in Table XXXII; only iron and cobalt have not received at least superficial study. The selectivities are very similar to those recorded for propyne (see Section 111,C, 1 ) and for acetylene (see Section 111, B, 5). Palladium is again found to be almost perfectly selective for olefin formation and iridium distinguishes itself by providing more paraffin than olefin in the initial products. The trends in selectivity across and down Group V I I I that were discussed in Section 111, B, 5 are not so apparent in Table XXXII; the reason for this is that propadiene hydrogenation has been studied a t temperatures which have varied considerably from metal to metal, and selectivity increased with increasing temperature in all cases. The trends observed for acetylene were generally obtained using catalysts within a narrower temperature range. The mechanism of propadiene hydrogenation is shown in Fig. 26.

Fro. 26. Reaction scheme for the hydrogenation of propadiene to propcne and propane.

c3

W W

TABLE XXXII 0

P CatC3lyst

Fe Ru-a

0s-a

S

Temp. ("C)

0.84

0.73

51

130

1

1

Catalyst

S

Temp. ("C)

Co

-

-

Xi-p

0.93

73

0.92

61

Pd-p Pd-s

1.oo

116

0.99

19

Catalyst

S

Temp. ("C)

+Z

U

Rh-a

Ir-a

0.36

2s

Pt-p Pt-a

0.80

89

0.89

79

HYDROGENATION OF UNSATURATED HYDROCARBONS

189

The addition of the first hydrogen atom must be assumed t o occur in one of two ways, giving two possible half-hydrogenated states. The addition of a second hydrogen atom gives propene, either adsorbed or directly in the gas phase. Mechanistic and thermodynamic factors then determine the relative changes of adsorbed propene undergoing desorption and hydrogenation, in a manner analogous to that described in Section 111,B, 5 for acetylene hydrogenation. It is t o be expected that hydrogen atom loss will occur from the halfhydrogenated states giving either adsorbed propadiene or its isomer, propyne. However, no propyne has been reported as a product of

H C-C-CH;

-1

/-” \-kI

L HC-C

** T

-

CH,

this reaction and this has been interpreted to mean that its desorption does not occur to a significant extent ( 2 ) ; such a conclusion agrees with other evidence presented in Section 111, B which shows that the rate of alkyne desorption is generally very slow indeed compared to normal hydrogenation rates. An interesting feature of this reaction mechanism is that the halfhydrogenated state obtained by the addition of a hydrogen atom to the central carbon atom of the alleiiic system, shown as H,F-CH=CH,

*

in Fig. 26, is isoelectonic with rr-adsorbed-ally1 [cf. Structure (C), Section 11, A]. This is the first reaction to be discussed in Section I11 for which a rr-allylic species has arisen as a possible half-hydrogenated state formed directly by hydrogen atom addition to the parent hydrocarbon. An anologous situation exists for all of the 1,2-dienes, and therefore this is one context in which evidence for the existence of these “noncIassica1” intermediates should be sought in the future. 4. 1,2-Butadiene The palladium-catalyzed hydrogenation of this molecule a t room temperature has been investigated both in the gas phase, using a flow-system ( g o ) , and in the liquid phase using a solvent (57). The products of the gas phase reaction were cis-%butene, 52%; trans-2-butene, 7%; 1butene, 40% and n-butane, 1%. Isomerization of the reactant was also observed, 2-butyne being produced to the extend of about 10% of the total olefin yield, together with traces of l-butyne and l13-butadiene.

190

a.

C. BOND AND P. B. WELLS

I n the liquid-phase the products were cis-2-butene, 52%; trans-2butene, 3% and 1-butene, 45%; no n-butane or isomers of the reactant were reported. The similarity between the product distributions shows that the solvent was substantially without effect upon the reaction mechanism. In forming 2-butene the reaction exhibits a marked preference for the therniodynamicslly less stable isomer, i .e., the cis-isomer. This stereoselectivity was attributed by the investigators (90) t o steric effects associated with the adsorption process. 1,2-Butadiene was assumed t o adsorb by the interaction of one olefinic linkage with the surface [see Structure (IV), Section 111, F, I ] it therefore follows that the molecule must approach the surface in one of three ways if adsorption is to occur. These three approaches are denoted by a, b, and c in Fig. 27. Assuming that the consecutive cis-addition of two hydrogen atoms to the adsorbed reactant takes place, cis-2-butene is generated by approach a, trans-2butene by approach 6 , and 1-butene by approach c, Approach b is stericslly hindered by the methyl group, whereas a is not and hence the thermodynamically less stable isomer of 2-butene is produced with a high stereoselectivity. The yield of 1-butene is high but a little lower than that of cis-2-butene, as is expected since approach c is slightly more hindered than approach a. Isomerization of olefin before its desorption appears to be absent in this reaction. A similar argument for the selective production of cis-2-butene may be constructed assuming the reactant to adsorb as a di-rr-adsorbed complex [cf. Structure (V), Section 111, F, I ] . All possible isomers of 1,2-butadiene were observed in the gas phase deuteration of this molecule. Such isomerization is an unusual feature of diene and alkyne hydrogenation because the desorption rate of diunsaturated hydrocarbons is usually very slow. A thermodynamic factor can hardly have been promoting this desorption because, from evidencc discussed in Section 11,l?, l , 1,2-butadiene appears to be most weakly adsorbed of the C,H, isomers. One mechanism of reactant isomerization involves simply the loss of a hydrogen atom from adsorbed C,H,, for which three relevant structures can be written, The 2-butyne, which was the major isomer produced, was largely unexchanged, how-

!

FIQ.27.

HYDROGENATION O F UNSATURATED HYDROCARBONS

191

-H

H,C-C=CH-CH,

--+ H C C-C

H,C=CH-CH-CH,

----f

, - T**-

CH,

--H

H

I

kH,c=c-cH,-cH, 1

-H

-.+

H

HC-C

CH,-cH,

7-

**

ever, apparently indicating that an intramolecular hydrogen transfer mechanism predominates over the mechanism suggested above. The other isomers contained deuterium, but the distributions were not recorded. However, it is reasonable to assume that their formation occurred as a result of the addition of a hydrogen atom to the reactant and the subsequent loss of a different hydrogen atom from the halfhydrogenated state, The distribution of deuerium in the cis-2-butene and 1-butene produced by the deuteration of 1,2-butadiene are shown in Table XXXIII. The majority of each product contained two deuterium atoms, and the most highly deuterated product was -a5. The authors calculated that deuterium and hydrogen atoms were available at the surface in the ratio 15.6:1. On this basis a t 8", '35% of the product is formed by the simple consecutive cis-addition of two hydrogen or deuterium atoms to the adsorbed diene, the addition being either 1,2 or 2,3. The cis-2-butene and 1-butene produced a t 8' were examined by NMR; the spectra and were consistent with the constitutions H,C-CK =CD-CH,D CH,-CHD-CD =CH,, in agreement with the proposed mechanism. 5. Cyclic 1,2-Dienes

For cyclic olefins containing up to about twelve carbon atoms, the cis configuration is thermodynamically more stable than the trans where the latter is obtainable a t all. The equilibrium proportions of the two configurations have recently been reported for some of these olefins (101). The palladium-catal yzed hydrogenation of 1,2-cyclononadiene and 1,2-~yclodecadiene(102)produced initially 1 7 and 32% of the transolefins respectively, whereas the equilibrium proportions of these components are 0.15 and 2.5%. Once again, this time in cyclic systems, we observe that the hydrogenation of a diene can produce a nonequilibrium mixture of olefins, the less stable isomer being formed in excess of its equilibrium proportion. Hydrogenation apparently gave no cyclonane or cyclodecane before complete removal of the diene which agrees with the high selectivity exhibited by palladium generally. Olefin production was, however, accompanied by some isomerization in the case of

TABLE XXXIII Deuterobutenea Obtained from the D e u k r a t h of 1,2-Butadiem Caldyzed by Patladium-Alunaincr( 9 0 ) W

Deuterobutenes

Product

0

z

(yo)

do

dl

4

ds

d4

d,

38 38

2.2 1.1

9.7 9.0

85.2 85.4

2.4 3.0

0.5 1.0

0.5

1.89 1.95

62 62

2.0 1.0

12.5 11.3

81.8 80.6

2.8 4.9

0.6 1.7

0.3 0.5

1.88 1.96

Temp. ("C)

Conversion

cis-2-Butene 1-Butene

8 8

&-2-Butene 1-Butene

40 40

M

(%) 0.0

U

HYDROGENATION OF UNSATURATED HYDROCARBONS

193

the Cl0 hydrocarbon since traces of cis, cis-l,3-cyclodecadiene were observed. The investigators assumed that these molecules adsorb by the interaction of one olefinic linkage with the surface [see Structure (IV), Section 111, F, I]. These dienes may then adsorb in one of four ways denoted by a, b, c, and d in Fig. 28, depending upon the manner in which the individual molecules approach the surface. Assuming again that the mechanism involves the cis-addition of two hydrogen atoms, the products would be cis-monolefin following adsorption by approaches a and d and trans-monolefin by b and c. Next, the investigators judged from molecular models that the observed yields of trans-olefin were too great to be accounted for by the adsorption of the reactant by b and c and proposed that adsorption occurred predominantly by modes a and d , and that this was followed by the formation of the two possible half-hydrogenated states. One of these, by rehybridization and the involvement of further surface sites was then postulated to produce the structure shown in Fig. 29. This structure has five carbon atoms which are coplanar with the surface (or nearly so) and may alternatively be written as 7r-allylic entity interacting with a single metal atom [see Structure (C), Section 11, A]. Addition of a hydrogen atom to this intermediate may take place at either C(iifor CQV)giving cis- or trans-olefin respectively and i t was proposed that the majority of the observed trans-olefins had been produced by this route. Reactant isomerization is accommodated, as usual, proposing that hydrogen atom expulsion occurs from the half-hydrogenated state, and in this case this would be effected by such loss from

FIG.28.

* FIG.29.

194

Q.

C. BOND AND P. B. WELLS

Co) or C,,, of the intermediate shown in Fig. 29. In support of their mechanism, the authors reported a yield of 38% trans-cyclodecene by the partial hydrogenation of cz's,cis-1,3-~yclodecadiene,which of course, may involve the intermediate shown in Fig. 29 as a half-hydrogenated state. A more comprehensive report of these interesting reactions would be welcome. 6. 1,3-3utadiene

The gas phase hydrogenation of 1,3-butadiene has been studied over all of the Group VIII metals (84, 90, 99, 103) and the reaction has also been studied in the liquid phase using the Group VIII, metals (57, 104, 105). The details of the gas phase studies will be reviewed first. a. Kinetics. The strong adsorption of this diene relative to hydrogen is shown by the reaction orders contained in Table XXXIV which are TABLE XXXIV Initial Rate Orders for the Hydrogenation of 1,3-Butadiene Catalyzed by Alumina-Supported Metals

Metal

co Ni

cu

Order i n 1,3-butadiene

Order in hydrogen

0.0

1.0 1.0

Ref.

0.0

1.0

(99) (99) (99)

0.0 0.1

(103) (103)

-0.3

Ru Rh Pd

- 0.7

1.0 1.o 1.7

0s Ir Pt

0.0 0.0 0.6

0.8 1.3

-

1.o

(84)

(84) (103) (103)

positive in hydrogen in each case and negative or zero in hydrocarbon. Over all metals except iridium and osmium the selectivities and olefin distributions observed in the first stage of the reaction were constant or nearly so until the diene was removed, indicating that the diene was also more strongly adsorbed than the butenes. The product distributions will be considered under three headings : (1) the selectivity for olefin formation, (2) the translcis ratio in the 2butene, and (3) the stereoselectivity for 1-butene formation.

HYDROGENATION O F UNSATURATED HYDROCARBONS

195

b. Xelectivity. The dependences of the initial selectivity upon hydrogen pressure and upon temperature are shown in Figs. 30 and 31, respectively, for some of the noble Group V I I I metals. The curves are of the same form as those shown in Figs. 21 and 22 for acetylene hydrogenation (see Section 111, B, 5). Results for the nobIe metals are compared with those obtained for metals of the first transition series in Table XXXV. The now familiar trends are again present: selectivities decrease on passing down each triad of Group VIII and there is a tendency for the selectivity to increase from left to right along each of the transition series. The latter trend is spoiled by iridium, whose avidity for the production of paraffin has been noted in propadiene hydrogenation. The factors governing the selectivity will be (a) the relative strengths of adsorption of 1,3-butadiene and the butenes on each metal and (b) the inherent activity of each metal for butene hydrogenation. For a detailed discussion of the interplay of these factors the reader is referred back to Section 111, B, 3 where the matter was discussed in full for acetylene. Comparison with the selectivities recorded for 2-butyne hydrogenation (see Section 111, DJ, a ) shows that the selectivities are often lower in

"1

0.80

FIG.30. The dependence of selectivity upon hydrogen pressure for the hydrogenation of 1,3-butadiene catalyzed by alumina-supported Group VIII metals ( 8 4 , 103): initial PcAlr, = 50 mm.

196

0. C. BOND AND P. B. WELLS

the hydrogenation of 1,3-butadiene, and the difference is not accounted for entirely by the higher temperature generally employed in the studies of the butyne. The difference is attributable to a less powerful thermodynamic factor being operative in diene hydrogenation, and this has been shown to be the case from the competitive reactions of these hydrocarbons reported in Section 111,F, 1where 2-butyne was shown t o be more strongly adsorbed than 1,3-butadiene on palladium. c. The Stereoselectivity in 2-Butene Formation. The formation of 2butene may occur either by the isomerization of adsorbed I-butene before its desorption or by 1 :4-addition of hydrogen to the adsorbed diene. I n the former case the trans/& ratio would be expected to be similar to that observed in the isomerization of I-butene, that is, about unity. The products of 1-4-addition will depend upon the populations of adsorbed butadiene in the syn and anti conformations. Since 1,3-butadiene in the gas phase consists almost entirely of the anti-conformer (YO), it is reasonable to expect that this conformer will be of major importance in the adsorbed state as well, although the possibility that the syn form may adsorb as Structure (111) (Section 111, F, 1) means that this conformation may be relatively more stable in the adsorbed state than in the gas phase, Consequently, 1 :4-addition is most likely to produce trans2-butene predominantly. The translcis ratios observed in both 1,3butadiene hydrogenation and 1-butene isomerization, each measured at

Temperature (Y)

FIU.31. The dependence of selectivity upon temperature for the hydrogenation of 1,3-butadiene catalysed by some noble Group VIII metals: initial PC,HI = 50 mm, PH, 100 mm. 0-alumina-supported metals ( 8 4 , 103) 0 - P d wire; a - R h wire; Q-Pt wire (106).

-

Fe

0.98

198

Ru

0.84

49

0s

0.58

56

1.oo

CO

1.00

75

Ni

1.00

Rh

0.84

52

Pd

1.00

.43

0.35

53

0.63

45

1

Pt

-

I-

-

-

-

-

c3 Y

tr

w

3 TI

198

G . C. BOND AND P.

B. WELLS

the same temperature, are compared in Fig. 32. Cobalt and palladium immediately distinguish themselves by producing high translcis ratios from 1,3-butadiene whereas the remaining metals provide a trans/& ratio similar to that observed in 1-butene isomerization under similar conditions of temperature and hydrogen pressure. There is p r i m a facie evidence, therefore, that the 2-butenes are generally formed by the preliminary formation of 1-butene and its subsequent isomerization before desorption, except in the cases of cobalt and palladium where 1:4addition may be operative. I n no cases do the translcis ratios correspond t o the thermodynamic equilibrium values which are lower than the values observed in the cobalt and palladium-catalyzed reactions and greater than those obtained using the remaining metals. d. The Stereoselectivity for 1-Butene Formation. If, as is proposed, 2-butene is produced by the isomerization of 1-butene it is t o be expected that the yield of 1-butene will be greatest over those metals which are poor olefin isomerization catalysts, and vice versa; this has been confirmed to be the case. Figure 33 shows the yields of 1-butene produced by l,%butadiene hydrogenation a t various temperatures using some Group V l l l metals and copper. Iridium, which is not included in the figure, gives 82y0 1-butene a t -20" (103). The metals fall in the

I Fe

Co

Ni

Cu

Ru

Rh

Pd

0s

Ir

Pt

FIG.32. The values of the ratio trans-2-butene/cis-2-butenc observed ( 1 ) in the hydrogentation ofl,3-butadiene (filled columns) and (2)in the hydroisomerization of I-hutene (open columns).

HYDROGENATION O F UNSATURATED HYDROCARBONS

199

following sequence as regards their activities for olefin isomerization: Cu w Ir

< P t < 0 s < R u < Rh (280")

Ni.

Comparison of this sequence with Fig. 33 shows clearly that the metals that are most active for olefin isomerization, rhodium ( 2 8 0 " ) and nickel, yield the least 1-butene in 1,3-butadiene hydrogenation; the reverse is also true in that copper and iridium, being poor isomerization catalysts, give the most 1-butene. Platinum, ruthenium, and osmium occupy intermediate positions in the expected order. Furthermore, the activity of all metal catalysts for isomerization increases with increasing temperature and the yield of 1-butene from 1,3-butadiene decreases as the temperature is raised; this agrees with expectation and helps to confirm the proposed mechanism. e . The Reaction of 1,3-Butudiene with Deuterium. Palladium-alumina has been used to study the gas phase deuteration of 1,3-butadiene in a flow system, a t 40" (90). The products a t this temperature consisted of 53.1% 1-butene, 42.0% trans-2-butene, and 4.8% cis-2-butene, and 0.1yo n-butane, in good agreement with the product distribution obtained by other workers (103). Butadiene exchanged only slowly, showing that its rate of desorption was probably also slow, thus providing another point of similarity between alkyne and diene hydrogenation. Hydrogen exchange was virtually inhibited by the presence of 1,3butadiene, and hence the surface coverage of "hydrogen" atoms must have been very low indeed, since the authors calculated the relative

-

0

cu

80 -

-

Temperature ('C)

FIG.33. 1,3-Butadiene hydrogenation: the dependence of the yield of 1-butene upon = temperature for six alumina-supported metal catalysts (84, 99, 103). Initial PC,Ha 50 mm; PHt200 mm.

200

C t . C. BOND AND P. B. WELLS

concentrations of deuterium and hydrogen atoms to be 3:l. Table XXXVI contains some distributions of deuterium in the reactant and olefinic products obtained in this work. The 1-butene and trans-2butene had similar deuterium contents whereas the deuterium content of the cis-2-butene was markedly lower. The origin of this lastmentioned observation was not explained by the authors and it remains obscure. The formation of the cis-isomer by the isomerization of 1butene would almost certainly not have resulted in its deuterium content being lower than that of I-butene. If it was formed by 1:Caddition, then it is difficult to see why its deuterium distribution differs from that observed in the trans-isomer. One could propose that separate areas of the catalyst promote the formation of cis- and trans- 2-butene by the 1:4-addition mechanism (i.e., some areas of the surface or some individual sites favor adsorption of 1,3-butadiene in the syn-conformation while other areas favor adsorption as the anti-conformation) and that the availability of hydrogen and deuterium atoms in these areas is different. Postulates that the surface is heterogeneous in this sense are to be avoided wherever possible because they cannot be easily substantiated by experiment. It is thus desirable that this aspect of the mechanism be given careful study in the future. In the same study the positions of deuterium atoms in the trans-2butene were examined using NMR. The spectrum corresponded t o the introduction of 0.8 deuterium atoms into each methyl group and 0.1 deuterium atoms into each of the C, and C, positions. This means that 90% of the deuterium has added by a 1:4-mechanism t o the terminal carbon atoms of the adsorbed diene. An essentially similar result has been obtained by the NMR analysis of the trans-2-butene obtained in the cobalt-catalyzed deuteration of 1,3-butadiene (99). To summarize, it may be said that the addition of hydrogen t o 1,3butadiene in gas phase reactions occurs partly by a 1-2-mechanism over all metal catalysts giving 1-butene in the gas phase. 2-Butene is either produced directly by a simultaneous 1-&addition process as in the cobalt- and palladium-catalyzed reactions, or it is produced indirectly by the isomerization of l-butene after its initial formation on the surface as is the case with the remaining metals of Group V I I I and copper. The fraction of adsorbed olefin which is hydrogenated to n-butane depends upon the manner in which the thermodynamic and mechanistic factors, discussed previously, operate in each particular reaction. We shall conclude our discussion of the gas phase reaction by speculating upon the reason for the operation of a 1:4-addition mechanism on cobalt and palladium. It has been suggested that an adsorbed 1methyl-rr-allylic species is formed on palladium by the addition of one

U

n

TA4BLEXXXVI

M

2 P

8Z

Deuterated Products Obtained from the Palladium-Catalyzed Reaction of 1,3-Butadiene with Deuterium (90) Conversion of diene t o olefin = 48%

1,3-Butadiene

83.0

12.1

4.1

0.9

0.0

0.0

0.0

-

0.23

1-Butene

15.8

30.1

32.3

13.0

6.3

1.9

0.6

0.0

1.72

truns-2-Butene

16.4

28.3

30.8

13.5

7.3

2.7

1.0

0.0

1.79

cis-2-Butene

23.4

35.2

28.7

8.6

3.1

1.1

0.0

0.0

1.36

a Pj ! i M U kd

2 U

a.

202

0. BOND AND P. B. WELLS

hydrogen atom to adsorbed 1J-butadiene (90). The process may be written as follows: H

H

H

H/

H '

Hydrogen atom addition to this l-methyl-r-ally1 will give either adsorbed l-butene or adsorbed 2-butene. This suggestion is the more worthy of consideration since palladium has been shown to promote the formation of r-adsorbed intermediates in other systems (12).If indeed palladium and cobalt have an ability to bond intermediates in a way that has not hitherto been appreciated and if these intermediates exhibit modes of reaction (e.g., hydrogen addition or exchange characteristics) which differ from those shown by the equivalent a-bonded structures, then investigators of chemical reactions have a point of departure, however modest, from which to design further studies of these species. f. Hydrogenation of 1,3-Butadiene in the Liquid Phase. Mention must be made of the liquid phase hydrogenation of 1,3-butadiene, although information here is more sparse than for the gas phase reaction. No evidence is available to indicate that solvent exerts specific directing influences upon the product distribution, although such evidence may be forthcoming in the future. Ethanol seems to have been used exclusively as solvent and only the Group VIII metals have been studied. The reported product distributions are shown in Table XXXVII together with specimen gas phase results for comparison. Distributions agree well with those reported from gas phase reactions in several cases and where such agreement does not occur it appears likely that olefin isomerization, probably by readsorption and secondary reaction, has taken place. This complication is likely to arise in liquid-phase work where solvents are used since the presence of a solvent will almost certainly reduce the influence of the thermodynamic factor by reducing the rate of collision of diene molecules with the surface. 7. Xubstituted 1,3-Butadienes

a. l-Methoxy-l,3-Butadiene. A recent study has been made of the liquid phase hydrogenation of trans-l-methoxy-l,3-butadiene catalyzed by Raney nickel and reduced platinum oxide at 30" (107).The incorporation of the methoxy group labels the ethylenic linkages so that their individual hydrogenation can be examined. All five possible methoxybutenes were produced over both metals during the addition of the

TABLE XXXVII Comparison of Selectivities, 8,and Product Distributions i n the Hydrogenation of 1,3-Butadiene i n the Liquid Phase (L) and the Gas Phase (G) Catalyst

Phase

Temp.

Selectivity

("C)

1-Butene

trans-2-Butene

cis-2-Butene

(%)

(%I

(%)

Raney Ni

L

28

0.34

21.5

61.0

17.5

Ni-alumina

G

35

1.00

44.9

37.8

17.3

Pd-suspension

L L

- 12

0.94

48.5

40.1

11.4

Pd-BaSO,

-8

0.40

6.0

75.0

19.0

Pd-BaSO,

L

20

1.oo

63.6

30.8

5.6

Pd-alumina

G

0

1.00

72.0

25.0

3.0

Pt suspension

L

72.1

18.4

9.5

G

- 12 - 20

0.61

Pt -alumina

0.50

70.0

17.0

13.0

Ref. 0 4

204

0. C. BOND AND P. B. WELLS

first mole of hydrogen although numerical yields have not been reported as yet. Some methyl butyl ether was also produced over both catalysts, indicating that the selectivity in each case was less than unity. The presence of small concentrations of cis-1-methoxy- 1-butene among the products must have occurred either by the isomerization of 1methoxy-2-butene (itself the product of an isomerization sequence) or, more likely, by the isomerization of the reactant. H CH,O

H

\

c-c

CH,O-CHa-C

/

/ T* \ H

C-C

/

H*

+H

/I\

H

H

trans- 1-methoxy-1,3-butadiene

'

H

*I \ c-c /

/I\

H

H

H -H --f

H

\

c-c

CH,O /

/

T \c-c/H H/ T \H

cis- 1-methoxy-1,3-butadiene

No cis-l-methoxy-l,3-butadiene was observed among the products, but this is hardly remarkable in view of the strong adsorption of dienes which makes their rate of desorption low. b. Allcyl Substituted 1,3-Butadienes. Several reports are extant in the literature giving product distributions obtained from a variety of alkyl substituted 1,3-butadienes. Precise mechanisms for the production of the products have not been established, however. A rationalization of the available information is achieved when it is realized that alkyl substitution at an sp2 carbon atom renders that carbon atom lesg vulnerable to attack by a hydrogen atom; the effect is presumably one of steric hindrance. Thus (a) 1,3-pentadiene produces more 2-pentene than I-pentene over carbon-supported rhodium, palladium, and platinum (58), (b) 2-methylbutadiene (isoprene) gives more 2-methyl-1-butene than 3-methyl-1-butene using Raney nickel and palladium and platinum blacks (108),and (c) 2,5-dimethyl-3-hexenes are only minor products in the hydrogenation of 2,5-dimethyl-2,4-hexadiene using the same catalysts as in (b) (109). In the latter reaction even the ability of palladium to promote a 1 :4-addition mechanism is abated. Conversely, platinum yields 2,3-dimethyl-2-butene in about 30% yield when 2,3-dimethyl-l,3butadiene is hydrogenated over this metal (105): the source of this product is uncertain, but it may be that steric factors are making l:4-addition a favored reaction in the substituted dienes, whereas such addition appears to be unimportant in the case of 1,3-butadiene itself. Where comparisons can be made in these reactions, Raney nickel and palladium black show higher selectivities for olefin formation than does platinum black, in agreement with the large body of results presented for other diolefins and acetylenes elsewhere in this section.

HYDROGENATION OF UNSATURATED HYDROCARBONS

205

8 . Cyclic 1,3- and 1,kDienes

Few studies have been made of the hydrogenation characteristics of these classes of molecule. The deuteration of cyclopentadiene (52) occurred very rapidly at - 34" at the surface of an iron film; cyclopentene was probably produced as an intermediate but full deuteration to cyclopentane was the only measurable process. A film sintered for 15 min at 200" was active for diene exchange at 90" and yielded product containing one deuterium atom, but it was not active for the addition reaction. Thin layers of palladium and platinum showed different properties when 1,3-cyclohexadiene interacted at their surfaces (110).I n the case of platinum, dehydrogenation to benzene was an important reaction whereas this was not the case with the palladium catalyst. Benzene was also produced, along with cyclohexenes in the iron-catalyzed deuteration of 1,Qcyclohexadiene using a film as catalyst at 20" (52). The benzene was highly deuterated when unsintered films were used but was undeuterated when the film had previously been sintered for 15 min at 200" and subsequently used at 0". These reactions show clearly that, in cyclic systems, loss of hydrogen atoms at carbon atoms adjacent to double bonds is a favored process. It is not known whether a special site is required for this process or whether this reaction can freely occur at the sites where hydrogen addition also takes place. If it is accepted that a benzene ring can exist as a n-adsorbed species, then the dehydrogenation and hydrogenation of these cyclohexadienes may take place in a manner which directly involves one metal atom per adsorbed hydrocarbon unit throughout the mechanism. The intermediates, all of which would be .rr-adsorbed, would be the same as those presented in Section I1 in connection with the hydrogenation and dehydrogenation of cyclohexene. The direct involvement of only one metal atom in the metal-adsorbate bond (although other metal atoms will presumably be sterically precluded as sites for hydrocarbon adsorption) renders the whole system very flexible and its simplicity is itself attractive, whereas a formulation in terms of sigma bonds and sp3 hybridized carbon atoms is less neat and convincing.

IV. The Hydrocarbon-Metal Bond in Catalytic and Organometallic Chemistry A. PATTERNS OF BEHAVIOR IN CATALYTICREACTIONS OF HYDROCARBONS In this concluding section some of the more important features and patterns of behavior contained in SectionsI1and I11will be summarized

206

0. C. BOND AND P. B. WELLS

and the probable contributing factors will be discussed or, if they have been examined already, they will be very briefly reiterated. Then, in subsection B, a specific attempt will then be made to explore the contention that the chemistry of the unstable compounds formed during the course of catalytic reactions a t metal surfaces is akin to the more 'conventional' organometallic chemistry of these metals.

in Olefin Hydroisomerization and Exchange Information in Section I1 has shown first, that those metals which promote olefin isomerization also promote olefin exchange, as is expected from the identity of mechanism, and secondly, the degree of eficiency shown by a metal catalyst for isomerization or exchange is of the same order for all olefins, assuming that the structural features of the olefin allow these processes to occur. This consistency is a very pleasing feature. Also, the study of some olefin hydrogenations over most of the metals of Group VIII now allows a sufficiently wide view to be taken for meaningful trends to be established. The following sequence for the relative activities of metals for olefin isomerization or exchange can be assembled from the references cited in Section 11: 1. Patterns

Activity for olefin isomerization } F e z N i z Rh(2 8 0 ° ) > P d > R u > O s > P t 2 1 r ~Cu or exchango at 100'C

The generalization can be made that iron, nickel, ruthenium, rhodium, and palladium are the best isomerization catalysts, whereas the corresponding metals of the third transition series and copper are relatively poorer isomerization catalysts. It must be remembered, of course, that the activation energy for isomerization is generally greater than that for hydrogenation, and that (E,- Eh)is not the same for all of the metals, nor has it necessarily a fixed value for a given metal over a n extended temperature range (cf. behavior of rhodium in butene isomerization) However, the above sequence is probably not impaired whatever the temperature; except for the position of rhodium, the difference between the activities of the metals merely diminishes as the temperature is raised. The factors which govern the observed yields of isomerized or exchanged olefin and paraffin are both numerous and interrelated, since the concentrations of all surface species (including hydrogen) and the rate constants of each elementary step are involved. We are sorely in need of experimental methods whereby the surface coverages of adsorbed species can be determined directly during reaction, and in this connection it is disappointing that the potentiometric method of studying

207

HYDROQENATION OF UNSATURATED HYDROCARBONS

hydrogen coverage (111,112) has not been more fully developed. At present our knowledge of the relative surface coverages of adsorbed species is indirect in that it has to be gleaned from the product distributions and the kinetics of the reaction. Using this method of approach, the mathematical analysis of the products of ethylene deuteration has shown that the chance of ethylene desorption (relative to the chance of hydrogen addition) is least favored from platinum and iridium surfaces and most favored from ruthenium and osmium surfaces; values for rhodium, palladium, and nickel are intermediate between these extremes. Metal

Ru

0s

Rh

Ni

Pd

Pt

Chance that ethylene will undergodesorptionratherthanhydrogenation (yo)

80

55

25-60

25

25

2

Ir 1-5

If a low chance of desorption is equated with strong adsorption of ethylene a t the metal surface (this is tantamount to assuming that the availability of atomic hydrogen is comparable at the surfaces of all the Group V I I I metals) then an inverse correspondence exists between the chance of ethylene desorption and the stability of metal-olefin complexes. The evidence from organometallic chemistry will be rehearsed fully in Section IV, B but brief mention must be made of some of it here. Platinum-olefin complexes can be prepared as stable compounds while the corresponding palladium complexes are usually less stable and the nickel complexes are unknown. This, we suggest, is related to the probable strong adsorption of olefins on platinum and their weaker adsorption on palladium and nickel. On this basis the high chance of olefin desorption and hence the comparatively weak adsorption of olefins on ruthenium, osmium, and rhodium should be paralleled by inherent instability of ethylene complexes of these metals, which is the case, as far as the evidence goes. By the same method of reasoning, iridium-olefin complexes should be stable, as are those of platinum; here unfortunately the parallel does not hold true. This correlation suggests that the bonding in olefin adsorption and in metal-olefin complexes is of the same type, i.e., it is r-bonding. As far as the information goes, those metals which form ethylene complexes will also form complexes with other olefins, and vice versa: this is paralleled in catalysis by the observation made in the first paragraph of this section, that the extent of olefin isomerization and exchange is always characteristic of the metal and substantially independent of the molecular weight of the olefin.

208

a.

C. BOND AND P. B. WELLS

2. The Selectivity Pattern of Alkyne and Diene Hydrogenation The selectivities for olefin formation observed in the hydrogenation of acetylene (Figs. 21 and 22), 1,3-butadiene (Figs. 30 and 31) and 2butyne (Table XXXI) over all of the Group VIII metals have been compared in Section 111. The information for propadiene using the noble metals and nickel is set out in Table XXXII and the selectivities observed in hydrogenation of other hydrocarbons where only a few metals have been studied fits into the pattern established with the more extensively studied molecules. When selectivities are compared a t or around the same temperature, they increase on passing along the three horizontal triads of Group VIII. and decrease on passing down the three vertical triads. The two exceptions to this rule are that palladium generally shows a higher selectivity than nickel, and that iridium sometimes gives a lower selectivity than osmium. If it is assumed that the discussion of the selectivity pattern put forward in Section I11 for acetylene is generally applicable to all of the systems studied, which is reasonable in view of the reproductibility of the pattern from molecule to molecule, then the following conditions hold. (1) Iron, cobalt, and nickel are highly selective for olefin formation because the olefins are least strongly bound and hence most easily displaced from their surfaces. and because these metals exhibit lower inherent activities for olefin hydrogenation than do the noble metals. (2) The lower selectivities afforded by ruthenium and rhodium reflect the higher activity of these metals for olefin hydrogenation, compared to iron and cobalt. (3) The high selectivity shown by palladium (usually unity or nearly so) must indicate that an unusually powerful thermodynamic factor is operative which outweighs the considerable activity of this metal for olefin hydrogenation. (4)Although osmium, iridium, and platinum probably possess lower inherent activities for olefin hydrogenation than the second row metals, they do in fact show lower selectivities, and this can only be attributed to the thermodynamic factor operating weakly. This accounts for the anomolously low selectivity sometimes afforded by iridium, because in these reactions the olefin is readsorbed and hydrogenated in the presence of the alkyne or diene, and the product distributions are considerably conversion-dependent. The same is true, to a lesser degree, for osmium. It is most gratifying that the selectivity pattern is reproducible from system to system, since the most useful catalyst for a given task may now be selected with some confidence. Thus, if it is required to reduce

HYDROGENATION O F UNSATURATED HYDROCARBONS

209

an acetylenic linkage to an olefinic linkage, and simultaneously to reduce paraffin formation to a minimum, then palladium will almost certainly be the best catalyst, whereas, if simultaneous paraffin formation is desirable, iridium or osmium is likely to be the best choice. If a conjugated diene is to be reduced, the hydrogenation of one double bond is best achieved by copper or platinum, whereas 1:Caddition is most likely to be successful using cobalt or palladium. However, care must be exercised. The trends that have been elucidated so far have, by necessity, been observed using the simplest molecules. A further stage in a rationalization of catalytic behavior would be to extend studies of hydrogenation to more complex systems, because further factors, especially steric, may then have to be taken into account. Perhaps a hint of this has already been obtained. For example, selectivities tend to increase with increasing molecular weight, those for dimethylacetylene being higher than for acetylene itself. While this may be due merely to a less powerful thermodynamic factor in the case of acetylene, it could alternatively reflect steric hindrance to the addition of hydrogen to the substituted olefin. Such a steric factor would also rationalize the lower selectivities observed in the hydrogenation of 1pentyne and 1-hexyne vis & vis 2-pentyne and 2- and 3-hexynes. Certainly, steric effects will almost certainly become more important the more complex the molecule being hydrogenated and this may modify the selectivity pattern as it appears from the hydrogenation of the simpler hydrocarbons. 3. Some Consequences of the Strong Adsorption of Alkynes and Dienes The experimental results described in Section I11 leave no doubt that alkynes and dienes are, generally, very strongly adsorbed on all metals. If the postulate is accepted that these adsorbates are .rr-bonded to the surface, then the displacement of adsorbed olefin by adsorbing alkyne or diene may be regarded as a displacement reaction somewhat analogous to the type known in organometallic chemistry where a weakly bound ligand may be displaced by an entity which becomes a more strongly bound ligand. An important result of strong alkyne or diene adsorption lies in its effect upon the hydrogen atom concentration on the surface during reaction. Hydrogen exchange generally takes place during olefin deuteration, but it is virtually absent or severely reduced in alkyne or diene deuteration, showing that the hydrogen atom concentration is lower in the latter cases. Thus the position of any olefin/alkyl equilibrium is displaced in favor of the adsorbed olefin when alkyne or diene is present compared to its position when these entities are absent. It is for this

210

G. C. BOND AND P. B. WELLS

reason that, in the platinum-catalyzed hydrogenation of 1,3-butadiene, the adsorbed 1-butene initially formed undergoes considerable isomerization to 2-butenes, whereas, the hydrogenation of 1-butene itself on platinum a t the same temperature is accompanied by only slight isomerization. I n this and many similar instances the thermodynamic factor fulfills a double role; first, the presence of 1,3-butadiene causes the displacement of an equilibrium in such a way as t o increase the surface concentration of adsorbed olefin at the expense of alkyl, and secondly, it causes the desorption of olefin from the surface by dint of its larger free energy of adsorption. Lastly, the strong adsorption of alkynes and diene results in their having a slow rate of desorption relative. to hydrogenation. Consequently, efficient preparative methods for their isomerization are unlikely to be found a t least until their modes of reaction are sufficiently understood for effective poisoning procedures to be developed which will eliminate the hydrogenation reaction.

B. THENATURE, STABILITY, AND REACTIVITY OF HYDROCARBONMETAL COMPOUNDSAND THE RELEVANCE OF THIS INFORMATION TO HETEROGENEOUS CATALYSIS The correlations in Section I V , A show that the proposed stabilities, and hence, the nature of some adsorbed species have direct parallels with the stabilities of organometallic complexes. I n this subsection we shall examine the matter from the opposite point of view and consider briefly some aspects of organometallic chemistry, the object being to assess the extent t o which this is helpful for the formulation of new ideas and concepts in catalytic chemistry. Compounds of the transition metals are well known t o form complexes with unsaturated hydrocarbons and their derivatives (113-116). Of the several types which exist, we shall choose to consider three: (1) n-olefin and .rr-diolefin complexes (e.g., K [PtCI,(C,H &], K [PtCl3(C4H6)]) ; (2) *-acetylene complexes (e.g., Pt(Ph,P),(C,R,) ); and (3) r- and o-ally1 complexes (e.g., PdCIz(n-C3H5) , Fe(CO),(.rr-C5H,)(~-C,H6) ). 1. Patterns of Behavior in the Formation of Diflerent Types of Hydrocarbon-Metal Complexes Certain well-defined trends in the ability of metals t o form complexes of a specified type with unsaturated hydrocarbons are now becoming apparent and will be briefly summarized in this section. There is, however, the difficulty that the failure to discover a particular complex to date is not evidence that its preparation is impossible, so that the con-

HYDROGENATION OF UNSATURATED &YDROCARBONS

211

clusions drawn may be subject to future modification. Nevertheless, we believe that the trends as seen a t the moment are of significance. a. T-Olejin and n-DioleJin Complexes. The ability t o form monoolefin complexes appears only in Groups VII, VIII, and IB. Manganese forms Mn(n-C,H,)(CO),(C,H,) and rhenium forms R~(T-C,H,)(CO)~(C,H,) (113):Ni (117), Ru, Rh (118) Pd and Pt also form stable monoolefin complexes, while those of Cu(1) and Ag are of low to moderate stability (115).A very unstable compound [Fe(CO),(1,3-butadiene)]in which only one of the double bonds is bonded to the metal atom has recently been reported (119). The ability of the transition metals to form complexes of moderate to high stability is summarized in Fig. 34. There is no obvious reason why, in due course, Tc should not be shown to form compounds analogous to those of Mn and Re, and 0 s and Ir compounds be formed, analogous to those of Ru and Rh. The ability to form complexes with chelating (nonconjugated) diolefins is widely possessed by the transition metals: the metals known to form such complexes (113,116,117),in which other n-bonding ligands such as n-cyclopentadienyl or carbon monoxide sometimes exert a stabilizing influence, are V, Cr, Mo, W, Cu, Ag, and the metals of Group VIII excepting Ir. Fe forms much stabler compounds with conjugated diolefins than with chelating diolefins, the reverse is true of Rh(1) and Pd(I1)and I r forms the stable complex Ir(n-C5H5)(C5H6) with the conjugated diolefin cyclopentadiene. The ability of the transition metals t o form complexes of moderate stability with chelating diolefins is also shown in Fig. 34. It is therefore correct to say that n-olefin and T-diolefin complexes are known for many of those metals which can be employed as metal catalysts for the hydrogenation of olefins and diolefins.

Monoolefins Diolefins

F'IG. 34. The ability of the transition metals to form complexes of moderate stability with monoolefins and chelating diolefins.

212

0.

BOND AND P. B. WELLS

6 . .rr-Acetylene Complexes. There are, in fact, comparatively few simple acetylene complexes of the transition metals known a t the present time. The simplest acetylene complexes are those formed by Pt using triphenylphosphine as the stabilizing ligand (113):the corresponding olefin complexes exist from which the olefin is readily displace by an acetylene (120). For example, tertiary butyl acetylene (TBA)give stable complexes such as [PtCl,(TBA)], which can be formed by the displacement of ethylene from [PtCl,(C,H,)], by TBA (113). Iron dodecacarbonyl and dicobalt octacarbonyl react with disubstituted acetylenes to give compounds of the type shown in Fig. 35a: a related compound is formed from [Ni(.rr-C,H,)CO], (see Fig. 35b) (113). Acetylene-copper complexes are polymeric in nature, and little is known concerning acetylene-silver complexes. Some unstable solid complexes are known, and in solution they are less stable than olefin complexes (125). There is a notable tendency to form oligomers when acetylenic substances interact with compounds of metals, and this tendency is also shown by butadiene (117) (see Section IV, B , 3 ) .This is particularly so with the carbonyls of iron and cobalt, and the oligomerization reactions are favored with nickel (121)and with palladium compounds (113, 122, 123). This phenomenon may be related to the hydropolymerization of acetylenes on metal surfaces, and it may be that such polymerization processes would be better described in terms of n-complexes. c . Allylic Complexes. r-Allylic complexes are of more recent discovery than those considered above, and it is certain that many more remain to

Fro. 35. Structures of acetylene complexes (113).

HYDROGENATION OF UNSATURATED HYDROCARBONS

213

be prepared. Nevertheless, the pattern revealed a t the moment is instructive. Numerous n-ally1 complexes of Pd(I1)and Co have been recognized (113,115,124):the palladium complexes are of the structure [PdX(7r-C3H,)], where X is a halogen or a pseudohalogen, Pd(n-C,H,) (n-C3N5),or Pd(n-C,H,), (117). The cobalt complexes have the structures Co(CO),(n-C,H,) or Co(n-C3H5),,the latter being unstable (117). n-Allylic compounds of nickel, corresponding to the three types of compound listed above for palladium, have been formed but are less stable than the corresponding palladium compounds (113,114, 117). Only one r-allylic platinum complex has so far been described, Pt(n-C,H,) (n-C3H6)(113); one has been described for chromium, Cr(n-C,H,), (117)) and some compounds containing the n-ally1 group have been reported for iron (117,125). The present situation is summarized in Fig. 36. It may reasonably be asked whether the failure to observe further 7r-allylic complexes of platinum is due to the right experiments not having been performed. There is, however, some evidence that palladium has the greater inherent propensity to form such compounds. For example, the reaction of PdC1, with allyl alcohol gives the n-allylic complex [PdCl(rr-C,H,)], -indeed, it was first prepared in this way-while PtCl, with allyl alcohol gives the diallylether complex PtCl,[(C,H,),O] (113).Again, the complex of mesitylene oxide with PdCl, has a rr-allylic structure (see Pig. 37a) whereas that with PtCl, has a polymeric rr-olefin structure (see Fig. 37b) (126). It will be recalled that cobalt andpalladiummay be the only two metals to catalyze the 1:4-addition of hydrogen to 1,3-butadiene, and that an adsorbed rr-allylic species proposed to feature in the mechanism; only time will show whether this is related to the above-mentioned propensity of these two metals to from n-allylic complexes or whether this correlation is merely a coincidence. 2. The Stability of Hydrocarbon-Metal Complexes

a. The Stabilities of Organometallic Compounds. The proposition that

Few complexes known

Many complexes known

FIG.36. The ability of the transition metals to form n-ally1 complexes.

214

Q. C. BOND AND P. B.

WELLS

the relative strengths of adsorption of a given olefin a t a variety of metal surfaces is related to the stabilities of the compounds formed by that olefin and metal salts of a given type requires that we examine briefly the quantitatively determined stabilities of hydrocarbon-metal complexes. Surprisingly little quantitative information is available. A large number of measurements of the stability constants of complexes of olefins, acetylenes, and aromatic hydrocarbons with silver ions have been reported, and these will be summarized below, but in only a few cases are enthalpies and entropies of formation known. There are similar but fewer results for the complexes formed by cuprous ion (127,128,129),but no results whatever for complexes of other metals (excepting the sandwich compounds). This is an astonishing omission in view of the novel, and indeed unique, nature of the bonding in these molecules. Some qualitative information on stabilities may be derived from observations such as displacement sequences and decomposition temperatures. The stability constants of silver ion complexes have been evaluated by classical partition techniques (130, 131) and more recently by the measurement of dissociation pressures (132,133)and the methods of gassolid (134) and gas-liquid chromatography (135,136).The use of supported solutions of silver nitrate as a stationary phase for the separation of olefins is now quite general. Enthalpies of formation have been recorded for olefin complexes of silver borofluoride (132),for silver nitrate complexes with cyclic olefins (131) and for silver nitrate-butadiene complexes (133):the results are summarized, together with some values for the ethylene-silver ion complex, in Table XXXVIII.

FIQ.37. Structures of mesitylene oxide complexes of (a) Pd and (b) Pt.

TABLE XXXVIII Eltthdp'm of Formation of Silver I o n - 0 k j n Complexes Ole&

Olefin:Ag+ in complex

Anion

Method

- A H (kcal .mole-')

Ref.

Ethylene

2:l

BF;

Dissociation pressure

9.5

(132)

Propylene

2:l

BF;

Dissociation pressure

11.0

(132)

cis-2-Butene

2:1

BF;

Dissociation pressure

12.0

(132)

trans-2 -Butene

2:l

BF;

Dissociations pressure

12.7

(132)

Isobutene

2:1

BF-4

Dissociation pressure

11.1

(132)

Cyclopentene

1:1

Cyclohexene Cy cloheptene

1:l

Partition Partition

6.6 5.7

(131) (134

Partition

6.5

(131)

Methylenecyclohexane

1:l

NO; NO; NO; NO;

Partition

3.1

(131)

1,3-Butadiene

1:l

NO;

Dissocistion pressure

10.8

(133)

1,3-Butadiene

1:2

NO;

Dissociation pressure

13.0

(133)

1:l

0

w

216

Q. C. BOND AND P. B. WELLS

The enthalpies of formation determined by any partition procedure employing metal ions in solution cannot yield directly the strength of the metal-olefin bond principally because of the unknown changes in the solvation of the silver ion on being complexed. The use of gas-solid chromatography is potentially a useful technique in this connection, but has not been exploited as yet. Measurements based on dissociation pressures are free from the solvation difficulty but such measurements are few. Notwithstanding the uncertainty in the partition method the results so obtained show trends which may be expected to hold for complexes other than with silver ion. The stability constants show great variation with molecular structure, suggesting that steric factors predominate in determining stability. I n the n-butene series, the stability constants of the silver nitrate complexes at 25°C are (130): 1-butena, 119.4; cis-2-butene, 62.3; trans-2-butene1,29.2

Although the sequence does not parallel the enthalpies of formation of the AgBF, complexes quoted in Table XXXVII, it is consistent with evidence from catalytic studies which suggests that this sequence also represents the relative strengths of adsorption of the n-butenes, 1-butene being the most strongly adsorbed and trans-2-butene the least strongly adsorbed at metal surfaces. There is, further, slight qualitative evidence to the same effect in that complexes of platinous chloride with cis-olefins are stabler than those with trans-olefins (137, 138). Thus, for example, the complex with cis-2-butene decomposes at 170°C, while that with trans-2-butene decomposes at 130°C (138). The logs of the stability constants for the silver ion complexes of a number of olefins are linearly related to their heats of hydrogenation (139),although the explanation of this is probably not simple. b. The &abilities of the Complexes Formed on Metal Surfaces. We must now consider the difference that may exist between the stabilities of the complexes formed a t catalytic surfaces during reaction and those shown by organometallic compounds that were discussed above. Certainly, a n isolable metal-olefin complex may have a stability that is greater than that of an olefin adsorbed at a metal surface by an order of magnitude. Indeed, it is a fundamental tenet of the theory of catalysis that very stable compound formation between reactant and catalyst surface is specifically not required. The isolable hydrocarbon-metal complex is almost invariably stabilized by the attachment of electron-withdrawing ligands to the metal atom. It has been pointed out to us (140)that the metal surface atom which acts as an adsorption site in a catalyst surface has a number of other metallic atoms, both in the surface layer and in the layer immediately below the surface, which can act as weak

HYDROGENATION OF UNSATURATED HYDROCARBONS

217

stabilizing ligands. On this view, the necessary “minimum unit” constituting an adsorption site may consist of the atom to which the adsorbate is bonded and its nearest neighbours. The electron withdrawing properties of these neighboring metal atoms will be weaker than that of the conventional ligands of organometallic chemistry (e.g., C1, CO, Ph,P), and hence the complexes formed a t metal surfaces will generally be less stable than the isolable ranges of organometallic complexes. Provided that this bonding is sufficiently strong t o allow reaction of the complex with hydrogen, then the situation may be expected to be conductive t o catalytic reaction. 3. The Reactivities of Hydrocarbon-Metal Complexes Under this heading we shall note, first, that the hydrocarbon moiety in a hydrocarbon-metal complex can be oxidized and reduced by chemical means to provide species which may correspond to the halfhydrogenated states that are postulated in catalytic reaction mechanisms; secondly, that organometallic compounds exhibit displacement reactions, and thirdly that organometallic compounds catalyze isomerization, hydrogenation, polymerization, and oxidation processes. The use of chemical oxidants and reductants has recently enabled the following sequence of reactions to be carried out ((125):

iHt ++HZCE*

co

CH, 60

(nil)

The reaction of (I) with 1,3-butadiene gives a u-allylic species (11) which can be converted to the 7r-allylic compound (111)by the action of ultraviolet radiation. The o-allylic species can be converted to the butene complex (IV) and the free olefin may be expelled from this compound by reaction with phosphine. (This provides another example of a strongly

218

Q. C. BOND AND P. B. WELLS

bound ligand displacing a weaker ligand.) Thus a sequence of elementary steps has brought about hydrogenation, although it cannot be classed as a catalytic sequence of course. A similar example of such oxidation and reduction procedures is the following: H,O, 75"

[PdClz(n-C& )I 2

T. [PdCl(n-C,H, )I a dry IICI

The interconversion of T - and o-ally1 groups has also been accomplished using palladium compounds (114): dimethylsulfoxide

[PdCl(d2,H,)] 2

(as solvent)

[PdCl(U-CSH,)(DMS)]

The reduction of a metal-olefin complex to metal-alkyl has been achieved using a molybdenum complex (141).The reduction of (V) to give (VI) was

NaBH, in THP+ Ph.CBP,

In CHCI,

co co

Q

/ Mo\ CO CH,CH, co co

/\

effected using sodium borofluoride in tetrahydrofuran, and the reverse step required oxidation by triphenylmethylfluoroborate in chloroform. I n the case of references (125) and (141) the investigators have realized the probable relevance of their work to the chemistry of catalytic reactions. The coordination of an unsaturated hydrocarbon confers upon it a reactivity which differs in kind from that which it formerly possessed, and in particular because of the reduction in the electron density about the relevant carbon atoms it is rendered more susceptible to attack by nucleophilic reagents (142). A number of reactions are now known in which hydrocarbon-metal complexes act as catalyst-substrate combinations, the hydrocarbon moiety being transformed in the reaction to a product which is not similarly coordinated. This process may be formalized as M

+ un -+

+X

M-un

d

M

+ X-un

where M is a metal atom in a complex and un is an unsaturated substance, generally but not always a hydrocarbon. The nature of Xis various, the principal reactions of coordination complexes being ( 1) isomerization,

HYDROGENATION OF UNSATURATED HYDROCARBONS

219

(2) hydrogenation, (3) polymerization, and (4)oxidation. One at least of these reactions, namely the Ziegler-Natta process for stereospecific polymerization, has achieved great commercial prominence, although it will not be further discussed here. Other processes, such as the Wacker process for oxidation of olefins, may also be expected to become important. The purpose of this section is not to enquire into the mechanisms of these reactions, relevant though this might be to the subject of metalcatalyzed processes. It is rather to note those complexes which are reactive in the reactions listed above and to add this information to our small sum of knowledge concerning the stability of hydrocarbon metal complexes. a. Isomerization Reactions. Olefin- and bisbenzonitrile-complexed palladium(I1) chloride are highly efficient catalysts for the double-bond isomerization of olefins. The isomerization of the hexenes, the methylpentenes, and the butenes have been studied in the temperature range 50-80" by simply adding catalytic amounts of the palladium compounds to the olefins in the liquid state (the butenes being under pressure) (143). Unsaturated ketones were similarly isomerized. I n all cases quantitative yields of the thermodynamic equilibrium mixtures of olefins were obtained. The rates of isomerization varied with the structure of the olefin, and stepwise migration of the double bond along the carbon chain took place by a mechanism of intramolecular hydrogen transfer. The process probably did not involve the formation of r-allylic species, since such complexes were found not to exhibit catalytic properties under these conditions. Ethylene platinous chloride had a definite but low isomerization activity in the same reactions. It is most revealing that this information corroborates that obtained using conventional metallic catalysts, it having been shown in Section I1 that palladium possesses a much higher hydrosomerization activity than platinum, due (according to Section IV, A) to the stronger complexing of the olefins with the platinum surface. b. Hydrogenation Reactions. Relatively few examples of homogeneous hydrogenations are known. The reduction of ethylene platinous chloride (144)by hydrogen may have been homogeneous a t low temperatures, but a reinvestigation (145)has suggested that the process is sometimes heterogeneous. The homogeneous hydrogenation of dicyclopentadiene is dimethylformamide solution by chlorides of Pt, Pd, and Ru has been reported (146):with other olefins, rapid reduction of the salt to the metal occurred. Complex ions formed from chloroplatinic acid and stannous chloride in methanol catalyze the hydrogenation of ethylene and acetylene under mild conditions (Jar),but Ru(I1) chloride in aqueous solution is reactive only toward olefinic acids where a carboxyl group is

a.

220

C.

BOND AND P. B. WELLS

conjugated with the double bond (29).Several other homogeneous hydrogenation catalysts are known which are not directly relevant to our considerations: these include the pentacyanocobaltate(I1) ion (148,149)and Ziegler-type combinations (150). c. Polymerization Reactions. Homogeneous polymerization (the terms includes the formation of dimers and oligomers) of unsaturated hydrocarbons is catalyzed principally by complexes of Ni, Pd, and Rh. The compound [Ni(CO),(Ph,P) ,] catalyzes the polymerization of butadiene (151)and of mono- and disubstituted acetylenes (121): dimethylacetylene reacts with [Ni(C0],]2- to form hexamethylbenzene (113)and other nickel compounds catalyze the trimerization of butadiene (117).The compound [PdCl,(tetraphenylcyclobutadiene)] results from the interaction of diphenylactylene with PdCl, (113) and its chemistry is being studied (152).Rhodium complexes catalyze the stereospecific polymerization of butadiene to trans-poly-l,4-butadiene(153-155) as does also IrC1, (155): palladium salts catalyze 1 ,%addition predominantly (155). The chlorides of Pd, Ru, Pt, and 0 s catalyze the polymerization of monosubstituted acetylenes in the presence of an hydridic reducing agent (221). d. Oxidation Reactions. The oxidation of ethylene to acetaldehyde catalyzed by PdCl,, commonly known as the Wacker process (156-159)) is not truly catalytic, for the chloride becomes reduced to metallic palladium, but in the presence of CuCI, it is reoxidized and a catalytic cycle ensues. The oxidation of higher olefins to ketones (160)is also feasible. The essential step appears to be the nucleophilic attack of OH- on the coordinated olefin, followed by a rearrangement: HO-CH,

11 .........PdC1,

-(Pd

+ -

i

3C1-)

+

HO-CH-CH3

+ CHS-CHO+H+

CHI4

Platinum chlorides have not been reported to behave analogously, although [PtF,I2- causes the oxidation of olefins in a noncatalytic process (161). 5. Summary

The correlations which exist between the organometallic chemistry of the transition metals and their catalytic chemistry has been pointed out in the predeeding sections. This material will now be summarized. First, n-olefin complexes are formed by virtually all of the Group VIII metals, and the strength of the olefin-metal bond may vary considerably depending upon the other ligands present in the complex. All of these metals adsorb olefins and are active in olefin hydrogenation. Second, n-acetylenic complexes are formed by many of the Group VIII

HYDROGENATION OF UNSATURATED HYDROCARBONS

22 1

metals, and all of these metals are active in acetylene hydrogenation. Both in catalytic and organometallic chemistry an acetylene molecule is able to displace an olefin molecule when the latter is either adsorbed at a metal surface or functioning as a ligand in a complex. Third, r-ally1 complexes are formed by palladium and cobalt; analogous complexes of nickel and platinum are less stable, while ruthenium, rhodium, and iridium are not yet known to form them. In catalytic reactions the deuteration of cyclic paraffins over palladium has provided definite evidence for the existence of rr-bonded multiply unsaturated intermediates, while rr-allylic species probably participate in the hydrogenation of 1,3-butadiene over palladium and cobalt, and of 1,2-cyclodecadiene and 1,2-~yclononadieneover palladium. Here negative evidence is valuable: platinum, for example does not form r-allylic complexes readily and the hydrogenation of 1,S-butadiene using platinum does not require the postulate that r-allylic intermediates are involved. Since both fields here are fairly well studied it is unlikely that this use of negative evidence will lead to contradiction in the light of future work. Fourth, the complexes of acetylenes and diolefins react chiefly in polymerization, this tendency being most marked with the complexes of nickel and palladium. Platinum complexes are not generally active in polymerization. In the catalytic hydropolymerization of acetylene, nickel displays the greatest activity, palladium takes second place and platinum third place. The remaining noble metals are less active than platinum. Thus, again, a correlation between the two fields is observed to hold. The conclusion must surely be drawn that the fields of metal-catalyzed reactions and organometallic chemistry have many features in common. It is hoped that this section has provided a useful summary of the available information and of the apparent correlations that exist. Certainly, if basically the same chemistry is common to both fields of chemical endeavor, a situation should develop in which discovery in one branch has direct implications and perhaps applications in the other. Such a situation would be of mutual benefit. REFERENCES 1. Selwood, P. W., “Adsorption and Collective Paramagnetism.” Academic Press, New York, 1962. 2. Bond, G. C., “Catalysis by Metals.” Academic Press, New York, 1962. 3. Kemball, C., Adwa%. CcctaZysis 11, 223 (1959). 4. Newham, J., and Burwell, R. L., J. Phys. Chem. 66,1431,1438 (1962); Newham, J., Chem. Rev. 63, 123 (1963). 5. Bond, G. C., Webb, G., Wells, P. B., and Winterbottom, J. M., J. CataZyaia 1, 74 (1962).

222

a.

C. BOND AND P.

B. WELLS

6. Bond, G. C., and Wells, P. B., Proc. 2nd Intern. Congr. Catalysis, Paris, 1960 p. 1169 (1961). Technip, Paris. 7. Ciola, B., and Burwell, R. L., J. Phys. Chem. 65, 1168 (1961). 8. Eischens, R. P., and Phkiir, W. A., Adwan. Catalysis 10, 1 (1958). 9. Little, L. H., Sheppard, N., and Yates, D. J. C., Proc. Roy. SOC.A259, 242 (1960). 10. Guy, R. G. and Shaw, B. L., Advan. Inorga. Chem. Radiochemi. 4, 77 (1962). 11. Bennett, M. A., Chem. Rev. 62, 611 (1962). 12. Rooney, J. J., Gault, F. G., and Kemball, C., Proc. Chem. SOC.p. 407 (1960); J . CUtUlysis 1, 265 (1962). 13. Siegel, S., and Smith, G. V., J. A m . Chem. SOC.82, 6082 (1960). 14. Smith, G. V., and Burwell, R. L.. J . Am. Chem. Soc. 84, 925 (1962). 15. Siegel, S., and Dmuchovsky, B., J. Am. Chem. SOC.84,3132 (1962). 16. Siegel, S., Smith, G. V., Dmuchovsky, B., Dubbell, D., and Halpern, W., J . Am. Chem. SOC.84, 3136 (1962). 17. Siegel, S., and Smith, G. V., J . A m . Chem. SOC.82, 6087 (1960). 18. Sauvage, J. F., Baker, R. H., and Hussey, A. S., J. Am. Chem. SOC.82, 6090 (1960). 19. Sauvage, J. F., Baker, R. H., and Hussey, A. S., J. A m . Chem. SOC. 83, 3874 (1961). 20. Siegel, S., and Dunkel, M., Advan. Catalysis 9, 16 (1957). 21. Siegel, S., and McCaleb, G. S., J. Am. Chem. SOC. 81, 3656 (1969). 22. Eigenmann. G. W., and Arnold, R.T., J. Am. Chem. SOC.81, 3440 (1969). 23. Rooney, J. J., J . Catalysis 2, 63 (1963). 24. Mignolet, J. C . P., Discussions Faraday SOC.8, 106 (1960). 25. Bond, G. C . , and Turkevich, J., Trans. Faraday SOC.49, 281 (1953). 2G. Douglas, J. E.,and Rabinovitch, B. S., J. A m . Chem. SOC. 74, 2486 (1952). 27. Twigg, G. H., Discussions Furaday SOC.8, 152 (1960). 28. Bond, G. C., Trans. Faraday SOC.52, 1236 (1956). 29. Halpern, J., Harrod, J. F., and James, B. R., J. Am. Chem. SOC. 83,753 (1961); Halpern, J., James, B. R., and Kemp, A. L. W., ibid. p.4097 30. See discussion following paper of Matsuzaki, I., Proc. 2nd Intern. Congr. Catalysis, Paris, 1960 p. 1121 (1961). Technip, Paris. 31. Bond, G. C., Phillipson, J. J.,Wells, P. B., and Winterbottom, J. M., submitted to T r a m . Faraday SOC.:see also Winterbottom, J. M., Thesis, Univ. of Hull, Hull, England, 1962. 32. Wagner, C. D., Wilson, J. N., Otvos, J. W., and Stevenson, D. P., J . Chem. Phya. 20, 338, 1331 (1952); I n d . Eng. Chem. 46,1480 (1953). 33. Laidler, K . J., Disc?tseions Faraday SOC.8, 47 (1950). 34. Eyring, H., Colburn, C. B., and Zwolinski, B. J., I)iscussions Paraduy Soc. 8, 39 (1950). 35. Schuit, G . C . A., and van Reijen, L. L., Bull. SOC. Chim. Belges 67, 489 (1968). 36. Horiuti, J., J. Rea. I m t . Catalysis, Hokkaido Uniw. 6 , 250 (1968); 7, 163 (1969); Horiuti, J., and Matsuzaki, I., ibid. 6,187 (1958); Miyahara, K., and Yatsurugi, Y., ibid. p. 197. 37. Horiuti, J., Proc. 2nd Intern. Congr. Catalysis, Paris,1960 p. 1191 (1961). Technip,

Paris. 38. Keii, T., J . Res. I m t . Catalysis, Hokkaido Univ. 3, 36 (1963); J, Chem. Phys. 22, 144 (1954). 39. Amenomiya, Y., J . Res. Inat. Catalysis, Hokkaido Uniu. 9, 1 (1961). 40. Keii, T.,J. Chem. Phys. 23,210 (1956). 41. Keii, T., J. Chem. Phys. 25, 364 (1966). 42. Kemball, C., J . Chem. SOC.p. 735 (1966).

HYDROGENATION O F UNSATURATED HYDROCARBONS

223

43. Farkas, A., Farkas, L., and Rideal, E. K., Proc. Roy. SOC.A146, 630 (1934). 44. Twigg, G. H., and Rideal, E. K., Proc. Roy. SOC. A171, 53 (1939). 45. Turkevich, J., Bonner, F., Schissler, D. O., and Irsa, A. P., Discussions Faraday SOC.8, 352 (1950); Turkevich, J., Schissler, D. O., and Irsa, A. P., J . Phys. Colloid. Chem. 55, 1078 (1951). 46. Schissler, D. O., Thompson, S. O., and Turkevich, J., Advan. Catalysis 9,37 (1957). 47. Crawford, E., Roberts, M. W., and Kemball, C., Trans. Faraday SOC.58, 1761 (1962). 48. Taylor, T. I., and Dibeler, V. H., J . Phys. CoZloid. Chem. 55, 1036 (1951); Taylor, T. I., and Weiss, A. R., unpublished work (see article by Taylor in Catalysis 5, 257 (1957).) 49. Burwell, R. L., and Tuxworth, R. H., J . Phys. Chem. 60, 1043 (1956). 50. Freidlin, L. Kh., Kaup, Yu Yu., and Litvin, E. F., Izv. Akad. Nauk SSSR, Otd. Khim. Nauk p. 1464 (1962). 51. Twigg, G. H., Proc. Roy. SOC.A178, 106 (1941). 52. Erkelens, J., Galwey, A. K., and Kemball, C., Proc. Roy. SOC.A260, 273 (1961). 53. Crawford, E. and Kemball, C . , Truns. Faraday SOC.58, 2452 (1962). 54. Kazansky, V. B., Proc. 2nd Intern. Congr. Catalysis, Paris, 1960 p. 1131 (1961). Technip, Paris. 55. Addy, J., and Bond, G. C., Trans. Faraday SOC.53,377 (1957). 56, Hamilton, W. M., and Burwell, R. L., Proc. 2nd Intern. Congr. Catalysis, Paris, 1960 p. 987 (1961). Technip, Paris. 57. Rieche, A., Grimm, A., and Albrecht, H., Brennstoff-Chem. 42, 5 (1961). 58. Bond, G. C., and Rank, J. S., paper submitted to Proc. 3rd Intern. Congr. Catalysis, Amsterdam, 1964. 59. Dinh-Nguyen, N., and Rhyage, R., J . Res. Inst. Catalysis, Hokkaido Univ. 8, 73 (1960); Acta Chem. Scand. 13, 1032 (1959). 60. Addy, J., and Bond, G. C., Trans. Faraday SOC.53, 383 (1957). 61. Bond, G. C., Webb, G., and Wells, P. B., submitted to Trans. Faraday Soc.; see also Webb, G., Thesis, Univ. of Hull, Hull, England, 1963. 62. Anderson, J. R., and Kemball, C., Proc. Roy. SOC.A223, 361 (1954). 63. Erkelens, J., Kemball, C., and Galwey, A. K., Trans. Faraday Soc. 59, 1181 (1963). 64. Pease, R. N., J . Am. Chem. SOC.45, 2296 (1923). 65. Maxted, E. B., Trans. Faraday. SOC.41, 406 (1945). 66. Corner, E. S., and Pease, R. N., Ind. Eng. Chem. Anal. Ed. 17,564 (1945). 67. Campbell, K. C., and Thomson, S. J., Trans. Faraday SOC.55, 306 (1959). 68. Herrington, E. F. G., and Rideal, E. K., Trans. Faraday SOC.40, 505 (1944). 69. Wheeler, A., Advan. Catalysis 3, 250 (1951). 70. Smith, L. B., and Masingill, J. L., J . Am. Chem. SOC.83, 4301 (1961). 71. Bond, G. C., and Wells, P. B., Submitted to J . Catalysis; see also Wells, P. B., Thesis, Univ. of Hull, Hull, England, 1961. 72. Herrington, E. F. G., Trans. Faraday SOC.37, 361 (1941). 73. Sheridan, J., J . Chem. SOC.pp. 133, 301 (1945). 74. Bond, G. C., Catalysis 3, p, 109 (1954) and reference contained therein. 75. Sheridan, J., J . Chem. SOC.pp. 373, 470 (1944). 76. Sheridan, J., and Reid, W. D., J . Chem. Soc. p. 2962 (1952). 77. Tamaru, K., Bull. Chem. SOC.Japan 23, 64 (1950). 78. Bond, G. C., and Sheridan, J., Trans. Faraday Soc. 48, 651 (1952). 79. Bond, G. C., Dowden, D. A., and Maokenzie, N., Trans. Faraday SOC.54, 1537 (1958).

224

0 . C. BOND AND

P. B. WELLS

80. Bond, G. C . , J. Chem. SOC.p. 2706 (1968). 81. Bond, G. C., and Mann, R. S., J . Chem. SOC.pp. 3566, 4738 (1959). 82. Grignon-Dumoulin, A., and Thonon, C., Rev. Inst. Franc Petrole Ann. Combuat. L4quides 14, 214 (1969). 83. Bond, G . C., J . Chem. SOC.p. 4288 (1958). 84. Webb, G., Thesis, Univ. of Hull, Hull, England, 1963. 85. de Pauw, F., and Jungers, J. C., Bull. SOC.Chim. Belges 57, 618 (1948). 86. Farkas, A., and Farkas, L., J. Am. Chem. SOC.61, 3396 (1939). 87. Beeck, O., D~cussionaFaraday Soc. 8, 118 (1950). 88. Schuit, G. C . A,, and van Reijen, L. L., Advan. Catalysis 10, 242 (1958). 89. Wells, P. B., Platinum Metals Rev. 7, 18 (1963). 90. Meyer, E. F., and Burwell, R. L., J. Am. Chem. SOC. 85, 2881, (1963). 91. Freidlin, L . K., and Kaup, Y. Y., Neftekhimya 2 , 164 (1962). 92. Freidlin, L. K., Kaup, Y. Y., Litvin, E. F. and Llomets, T. I., Dokl. Akad. Nauk SSSR 148, 883 (1962). 93. Burwell, R. L., Chem. Rev. 67, 895 (1957). 94. Meyer, E. F. and Burwell, R. L., J. Am. Chem. SOC.85, 2877, (1963). 95. Phillipson, J. J., Wells, P. B., and Gray, D. W., paper submitted to the 3rd Intern. Congr. CatalysG, Amsterdam, 1964. 96. Rossini, F. R., “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds.” Carnegie Univ. Press, Pittsburgh, Pennsylvania, 1963. 97. Dobson, N. A., Eglinton, G., Krishnamurti, M., Raphael, R. A., and Willis, R. G., Tetrahedron 16, 16 (1961). 98. Bond, G. C., and Sheridan, J., Trana.Paraday SOC.48, 664 (1962). 99, Phillipson, J. J., and Wells, P. B., unpublished work, 1962. 100. Bond, G. C., and Sheridan, J., Trana. Faraday Soc. 48, 658 (1962). 101. Cope, A. C., Moore, P. T., and Moore, W. R., J. Am. Chem. SOC.82,1744 (1960). 102. Moore, W. R., J. Am. Chem. SOC.84,3788 (1962). 103. Bond, G. C., Webb, G., Wells, P. B., and Winterbottom, J. M., submitted t o J. Chem. SOC. 104. Young, W. G., Meier, R. L., Vinograd, J., Bollinger, H., Kaplan, L.. and Linden, S. L., J . A m . Chem. 80c. 69, 2046 (1947). 105. Lebed’ev. S. V., and Yabubchik, A. O., J. Chem. SOC.p. 2190 (1928). 106. Wells, P. B., and Whitehead, M. L., unpublished work, 1962. 107. Bell, J. M., Garnett, R., and Kubler, K. G., Paper presented before the South Caroline Academy of Sciences, Columbia, South Carolina, April 27th, 1963. 108. Lebed’ev, S. V., and Yabubchik, A. 0. J . Chem. SOC.p. 823 (1928). 109. Kazanskii, B. A,, and Popove, N. I.,Izv. Ahad. NaukSSSR Otd. Khim. Nauk p. 442 (1952).

110. Gryaznov, V. M., Yagodovskii, V. D., Savyelyeva, E. A., and Sheemoolees, V. I., Kinetika iKataliz 8 - (1962). 111. Sokolskii, D. V., and Druz, V. A., Zh. Fiz. Khim. 26, 364 (1952). 112. Sokolskii, D. V., and Fasman, A. B., Dokl. Akad. Nauk SSSR 117, 846 (1967). 113. Guy, R. G., and Shaw, B. L., Advan. Inorg. Chern. Radwchem. 4, 78 (1962). 114. Miller, J. R., Advan. Inorg. Chem. Radioohem. 4, 133 (1962). 115. Bennett, M. A., Chem. Rev. 62, 611 (1962). 116. Fischer, E. O . , and Werner, H., Angew. Chern. Intern. Ed. Engl. 2 , 80 (1963). 117. Wilke, G., Angew. Chem. Intern. Ed. Engt. 2. 106 (1963). 118. Cramer, R.. Inorg. Chem. 1, 722 (1962).

HYDROGENATION O F UNSATURATED HYDROCARBONS

225

119. Murdoch, H. D., and Weiss, E., Helv.Chim. Actu 45, 1156 (1962). 120. Chatt, J., Shaw, B. L., and Williams, A. A., J . Chem. SOC.p. 3269 (1962). 121. Meriwether, L. S . , Colthup, E. C., Kennerley, G. W., and Reusch, R. N. J . Org. Chem. 26, 5155, 5763, 5169 (1961); Meriwether, L. S., Leto, M. F., Colthup, E. C., and Kennerley, G. W., ibid. 27, 3930 (1962). 122. Cookson, R. C . , and Jones, D. W., Proc. Chem. SOC.p. 115 (1963). 123. Luttinger, L. B., and Colthup, E. C., J . Org. Chem. 27, 3752 (1962). 124. Bertrand, J. A., Jonassen, H. B., and Moore, D. W., Imrg. Chem. 2, 601 (1963). 125. Green, M. L. H., and Nagy, P. L. I., J . Chem. SOC.p. 189 (1963). 126. Parshall, G. W., and Wilkinson, G., Inorg. Chem. 1, 896 (1962). 127. Andrews, L. J., and Keefer, R. M., J . Am. Chem. SOC.71, 2379 (1949). 128. Keefer, R. M., Andrews, L. J., and Kepner, R. E. J . A m . Chem.Soc. 71,2381 (1949). 129. Keefer, R. M., Andrews, L. J., and Kepner, R.E.,J. A m . Chem.SOC. 71,3096 (1949). 130. Hepner, F. R., Trueblood, K. N., and Lucas, H. J., J . Am. Chem. SOC.74, 1333 (1952). 131. Traynham, J. G., and Olechowski, J. R., J . Am. Chem. SOC.81, 571 (1959). 132. Quinn, H. W., and Glew, D. N., Canad. J . Chem. 40, 1103 (1962). 133. Kraus, J. W., and Stern, E. W., J . A m . Chem. SOC.84, 2893 (1962). 134. Duffield, J. J., and Rogers, L. B. Anal. Chem. 34, 1193 (1962). 135. Gil-Av, E., and Herling, J., J . Phys. Chem. 66, 1208 (1962). 136. Muhs, M. A,, and Weiss, F. T., J . Am. Chem. Soc. 84, 4697 (1962). 137. Roy, J. R., and Orchin, M., J . Am. Chem. SOC.81, 310 (1969). 138. Jonassen, H. B., and Kirsch, W. B., J . Am. Chem. SOC.79, 1279 (1957). 139. Gardener, P. D., Brandon, R. L., and Nix, N. J., Chem. Ind. (London)p. 1363 (1958). 140. Rooney, J. J., personal communication, 1963. 141. Cousins, M., and Green, M. L. H., J . Chew. SOC.,p. 889 (1963). 142. Stern, E. W., and Spector, M. L., Proc. Chem. SOC.p. 370 (1961); Stern, E. W., ibid. p. 111 (1963). 143. Sparke, M. B., Turner, L., and Wenhmn, A. J. M., paper submitted a t the Intern. Union Pure Appl. Chem. Meeting, London, 1963. 144. Flynn, J. H., and Hulbert, H. M., J . Am. Chem. SOC.76, 3393 (1964). 145. Cow, A. S . , and Heinemann, H., J . Phys. Chem. 64, 1574 (1960). 146. Ryltmder, P. N., Himelstein, N., Steele, D. R., and Kreidl, J., Engelhurd Ind. Tech. BUZZ.3, 61 (1962). 147. Cramer, R. D., Jenner, E. L., Lindsey, R. N., and Stolberg, V. G., J . Am. Chem. SOC.85, 1961 (1963). 148. Kwiatek, J., Mador, J. L., and Seyler, J. K., J . Am. Chem. SOC.84, 304 (1962). 149. De Vries, B., J . Cutulysis 1, 489 (1962). 150. Anonymous, Chem. Eng. News 41, 34 (1963). 151. Zakharkin, L. I., and Zhidareva, I. I., Izv. Akad. Nuuk SSSR Otd. Khim. Nauk p. 386 (1963). 152. Cookson, R.C., and Jones, D. W., Proc. Chem. SOC.p. 115 (1963) and other references therein. 153. Rinehart, R. E., Smith, H. P., Witt, H. S., and Romeyn, H., J . Am. Chem. SOC.84, 4145 (1962). 154. Teyssie, P., Compt. Rend. 256, 2846 (1963). 155. Canale, I. J., Hewett, W. A., Shryne, T. M., and Youngman, E. A., Chem. Ind. (London)p. 1054 (1962). 156. Smidt, J., Hafner, W., Jira, R., Sieber, R., Sedlmeyer, J., and Sabel, A., Angew. Chem. Intern. Ed. Engl. 1, 80 (1962).

226

Q.

U. BOND AND P. B. WELLS

157. Smidt, J., Chern. Ind. (London) p. 84 (1962). 158. Sherwood, P. W., Brennstoff-Chem. 42, 376 (1961). 159. Matveev, K. I., Osipov, A. M., Odyakov, V. F., Suzdal’nitshaya, Y. V., Bukhtoyarov, I. F., and Emel’yanova, 0. A., Kinetika i Kataliz 2, 661 (1962). 160. Smidt, J., and Krekelsr, H., presented to 6th World Petrol. Congr. Proc. 1963, Sect. IV, paper 40. 161. Kemmett, R. D. W., and Sharpe, D. W. A., J . Chem. SOC.p. 2567 (1963).

Electronic Spectroscopy of Adsorbed Gas Molecules A . TERENIN Physical Institute. Leningrad University. Leningrad. U.S.S. R

.

Page 227 231 A. Spectral Shifts .................................................... 232 B AbsorptionCoefficientChange ...................................... 232 C . Vibrational Frequencies Alterations ................................ 233 D . New Bands ...................................................... 234 E . Potential Energy Curves ........................................... 235 Spectra of Physically Adsorbed Molecules................................ 236 A . Benzene ......................................................... 236 B . Heterocycles ..................................................... 241 C . Phenol .......................................................... 245 Strong Spectral Perturbations ......................................... 246 A . Nitrobenzenes .................................................... 246 B . Surface Acidity Indicators ......................................... 249 C . Aromatic Aniines and Diamines ..................................... 253 Positive Ion Spectra of Adsorbed Molecules .............................. 256 A. Cation Radicals from Phenylated Amines ............................. 256 B . Carbonium Ions from Phenylalkanes ................................. 261 C. Cerbonium Ions from Phenylalkenes ................................. 264 D . Cation Radicals from Phenylalkenes ................................. 266 E . Positive Ions of Adsorbed Benzene .................................. 268 F. Carbonium Ions and Cation Radicals from Adsorbed Polyecenes . . . . . . . . . 269 G. Surface Reactions with Oxygen ..................................... 273 Spectra of Anion Radicals on Surfaces ................................... 274 Radicals from Adsorbed Molecules...................................... 277 References .......................................................... 280

I. Introduction ........................................................ I1. General Considerations ...............................................

.

I11.

IV .

V.

VI . VII

.

1. Introduction The classical series of investigations by de Boer and co-workers in the thirties (1-8) was concerned with absorption spectra of the halogens and some phenols. adsorbed from the gas phase on to sublimed salt films in a high vacuum . This pioneering work brought to light a wealth of information on the fundamentals of the interaction of photons with molecules bound to surfaces. 227

228

A. TERENIN

A sequence of studies followed by others on the spectra of gaseous iodine and nitric dioxide, when contacted with various adsorbents in 'uacuo. The systems studied were: I, sorbed by the porous mineral, chabasite (9),I, on sublimed T1 halide films (lo),I, on silica aerogel (II),NO, on glass plates ( 1 2 ) ,NO, on salt films (13).t After the war the interest shifted to the infrared vibrational spectra of adsorbed molecules and to the disturbance produced by them on the vibration of surface hydroxyl groups of microporous silicates, The progress in this field has been reviewed several times (17-21). A t present we are witnessing a strong renewed interest in this subject, as shown by the sharp increase in the number of parallel publications. Most of the recent spectroscopic work concerns adsorbates brought in contact with the adsorbent not from the vapor phase, but from inert organic solvents in their presence. They are also reviewed in this survey. The author has omitted in this review any reference to the wide and limitless subject of the adsorbed dye spectra, restricting the presentation to such compounds which can be adsorbed from the vapor state, without thermal destruction and other complications. The experimental methods used by the researchers in the field vary according to the optical properties of the adsorbent. The detection of a single monomolecular adsorbed layer with an average extinction coefficient, E = 1000 liters -mole-l.cm-l should require a photoelectric spectrophotometer with a photometric performance at the level of about 0.01% absorbance, which is far beyond the range of the ordinary commercial apparatus. Therefore either a multiple passage of the same beam through the layer must be achieved, or many monomolecular layers must be successively passed. Golub and Kondratiev (12) thus placed 200 microscope cover glasses in the light path in ambiant NO, gas at 1 mm Hg, but from all the evidence did not succeed in obtaining the spectrum of the adsorbed molecules (cf.10). The sublimed CaF, layers, used by de Boer ( 4 ) possessed a laminar structure, equivalent to an internal surface of 200 meteralgm, and some 100 successive monomolecular layers have been passed by light. It has been soon found advantageous to uae transparent microporous solids with a highly developed internal surface (200-600 meter2/gm like the mineral chabasite and silica aerogel ( 9 , I I ) . A t present most work is being done on adsorbents of this type (microporous glass, silica gel, silica-alumina, etc.), which, however, strongly scatter light when used in a granular or powdered form, and therefore do not conform with

t Surveys of the work done in the author's laboratory at that time have been published in Russian in the papers (14-16).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

229

the ordinary type of spectrophotometer in which transmittance is being recorded. To increase the transparency many researches resort to the technique of immersing the porous, practically opaque adsorbent into an inert solvent with a refractive index close to the latter, the adsorbate being dissolved. When the adsorbent is in the form of a translucent scattering slurry, suspended in an inert solvent, correct measurements of the adsorption spectrum require either a spectrophotometer with an integrating sphere, or a partial integration of the scattered light, with the opal-glass technique (22). As shown below, in the majority of the cases the results obtained by the immersion methods do agree with those, in which the adsorbate contacted the adsorbent from the gas phase in vacuo. A careful degassing and desiccation under evacuation of both, the adsorbent and the solution, had to be performed. Nevertheless, immersion experimental procedures might arise some suspicion as to the reliability of interpreting the spectral effects observed.? The principle of the measurement by diffuse reflection of the true absorption spectrum of a finely divided colored solid substance has been worked out by Kortuni (24). The substance is introduced under high dilution (mole ratio = 10-a to lo-*) into a very disperse powder (average particle diameter 0.1 p ) of a neutral “white” material, which should scatter unselectively and not absorb light in the spectral region concerned. The true shape of the adsorption spectrum is deduced from measurements for each wavelength A, of the relative diffuse reflectance, RdiR,defined as the quotient of the light flux @&, diffusely reflected from the solid mixture, to that, @it,of the “white” diluent, as standard, viz.,

The absorption coefficient k, or the extinction coefficient E of the colored substance, is defined as usually from the Bouguer-LambertBeer law: @ = @,.exp( - k d ) = @,.exp( - E C ~ ) ,

where d is thickness of the absorbing material, and c its concentration. In order to obtain the values of k, or E for a colored substance imbedded in a strongly scattering disperse medium, we have to apply the relation, first proposed by Schuster and later refined by Kubelka and Munk, and

t Adsorption carried out in open air, give instances of profound alteration of the adsorbed molecules’ spectrum, produced by the presence of water on the surface (23).

230

A. TERENIN

independently arrived a t in the U.S.S.R. by Gurevitch, Girin and Stepanov. For the strongly colored substance in the scattering diluent, it reads as follows:

or log F(&,,)

L

log

E

+ log c + const.

(2‘)

In this formula S is the scattering coefficient of the colored substance itself, which is assumed to be independent of the wavelength. The spectral dependence of the magnitude F(Rdi,) thus reproduces the true contour of the absorption spectrum of the diluted substance, giving log up to an indefinite constant. The relation (2) has been proved to adequately represent the spectrum of that substance, in cases when it was also available in such a form as to allow an ordinary transmittance measurement (24). As neutral “white” diluents Kortum and co-workers have used MgO, BaSO,, SiO,, and KBr powders. They obtained absorption spectra of the MnOj, CrO; anions, of HgI,, the molecular compounds of anthracene and pyrene with s-trinitrobenzene, and of trinitrobenzene, p-dimethylaminoazobenzene, triphenylchlormethane, hexaphenylethylene, etc. (see Sections IV, V and VII).? I n some instances a diffuseness and shift of the spectrum of these substances has been noticed, which depended on the diluent and has been explained, as due to adsorption. The drastic treatment experienced by the organic component during the grinding process precludes, however, a conclusive statement of the origin of these changes, more so since on desiccated SiO, a destruction of aromatic compounds has been ascertained. The same relations (21, or (2’) are valid for spectral measurements in the case of molecules adsorbed on stronly scattering adsorbents, as is the majority of instances. It is the only correct method to get the true contour of the absorption spectrum in the case of gaseous adsorption on such adsorbents which are not translucent at all. However, since the relation (2) gives the general aspect of the spectrum, which can be arbitrarily displaced along the ordinate axis, it is only the spectral positions of the absorption maxima, i.e., of E, which are of primary importance. But these are already observed in measuring t The technique of intimate mixing of the colored substance with tho powdered diluent, usod by Kortiim and co-workers,consists in a very long fine grinding of a mixture of the components, supplementedlately by careful drying. They noticed tho strong influence on the speotra obtained when air humidity was admitted to tho sample (23).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

23 1

the spectral dependence of the diffuse reflectance itself, RdiP According to formula ( 1 ) the minima in the spectral curve Rdiff(A) correspond to maxima of F(R)diff, as function of A. Therefore for simplification, absorption spectra of gaseous molecules adsorbed on powdered scattering adsorbents may be represented as spectral functions of l/R,,,, or - log Rdiff, without pretending to give relative extinction values, which is achieved by recalculating values of Rdi, into F( Rdiff). In measuring Rdi,, the light fluxes @' coming from two identical cells containing the adsorbent have to be used, only in one of which the vapor of the adsorbate is admitted. In this way the scattering by the adsorbent itself is compensated to a large extent, and only the spectrum of the adsorbate on its surface is measured [cf. reference (27)]. If the relative pressure of the adsorbate vapor, p / p 8 (pe being the saturation pressure a t the ambiant temperature), reaches the stage when a capillary condensation has taken place into the pores of the adsorbent, deformations of the spectrum in diffuse reflection can have a purely optical origin. As shown in the author's laboratory the bands might be artificially changed, and even fictious ones appear in those regions where the spectral dispersion curve of the liquid adsorbate and that of the adsorbent material intersect (25).One must be cautious not t o approach this stage in the adsorption isotherm and to keep a small fraction of the monomolecular coverage.

I I. Ge ne ral Con si de rat ions The aim of an investigation of the electronic spectrum of a n adsorbed molecule is twofold: first, to deduce from the spectral changes the perturbations, experienced by it in the surface field, and second, to obtain an information about the nature of the surface active sites, supplementing that from heterogeneous chemical kinetics. The inherent difficulty in the interpretation of the spectral changes noticed resides in that they are a resultant of two influences, affecting in general differently the two electronic levels implied in the photon absorption. I n fact, the ground level and that vibronic one, which is attained by the vertical Franck-Condon optical transition represent molecular species of different reactivity toward a perturbation. To approach a correct assignment, additional data are required and are obtained by other methods. Useful and decisive information can be obtained from the spectral changes experienced by the same molecule when dissolved in active solvents which possess a definite functionality, e.g., those containing protonic or aprotonic acids, oxidative or reductive agents, etc. Recently, the exclusive sensitive method of the electron

232

A. TERENIN

paramagnetic resonance (EPR) has been applied to the problem of the adsorbed species with great success. The very promising nuclear paramagnetic resonance method has already made its long expected start in the same direction. Four main spectral criteria are a t hand: (1) Displacement, or shift of the absorption spectrum towards either the lower frequencies (bathochromatic, or red shift), or to the higher ones as a whole. (2) Change of the absolute absorption coefficient and intensity redistribution in the bands. (3) Alteration of the vibrational frequencies and the appearance of new ones in those rare cases when the vibrational structures of the spectrum remains. (4) Appearance of new absorption bands in a spectral range, where the gaseous molecule does not absorb at all.

A. SPECTRAL SHIFTS It is customary to assess the magnitude and the direction of the shift from those experienced by the spectral absorption maximum only, which procedure is not quite correct. I n fact, it is the change in the 0-0 electronic transition which should be of primary concern. The A,, in the absorption spectrum depends, as is well known from the FranckCondon principle, on the position of the potential energy curve of the upper electronic state relative to the equilibrium minimum of the ground one. It might be and certainly does occur in particular cases that the adsorption process produces unequal displacements of the minima of both potential curves, which causes a shift of A,,,, not necessarily implying a change in the magnitude of the 0-0 electronic transition. In the assessment of the strength of the surface field from observations of a bathochromic shift, it must be kept in mind that any influence which causes a broadening and coalescence of the line components of a structured absorption band of a gaseous molecule, e.g., the mere addition of a foreign inert gas, enhances the averaged absorbance in this region, leaving an already present continuous portion of the spectrum unaffected (26, 28).

B. ABSORPTION COEFFICIENT CHANGE Besides the general phenomenon of an intensity increase due to the refractive index of the medium exceeding unity (29), there are instances of a wide change of the molecular extinction coefficient E , or the oscillator

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

233

strength, f, caused by adsorption. Thus, according to de Boer and coworkers (2, 4)the first I, molecules, adsorbed from the vapor on the sublimed CaF, (at a coverage e=0.005), manifest an anomalously large extinction exceeding by a factor of 500 that for the gaseous molecule. At the same time a considerable shift to the shorter wavelengths, viz., from 500 to 340 mp, i.e., approximately by 9400 cm-l, is observed. I n this adsorbed state the first I, molecules are tightly bound to the most active F- ions on the surface, producing a kind of chemisorbed species with a shared electron, like the anion I,. The state of the iodine molecule adsorbed on silica aerogel is quite different, as found by Kurbatov in the author’s laboratory (11). The absorption maximum is situated at 490 mp at low coverage, which corresponds to a “blue” shift of only 400 crn-l. The molecular extinction coefficient is about 10 times smaller than that in the gas phase or in a s o htion. For I,, adsorbed on CaF,, the extinction coefficient does decrease a t higher coverages, 8, after the most active F- sites have been filled. Concurrently a broadening and displacement of the spectrum toward the visible takes place. I n contradistinction to this behavior the spectrum of I, adsorbed on silica aerogel retains approximately the same spectral position, and the low value of the extinction coefficient at all coverages showing that no similarly active sites are present on the surface of such a gel. These large spectral differences are not reflected in the adsorption energies of the I, ground state, which are 13.8 kcal.mole-1 for CaF,, according to de Boer and Dippel (3), and 10.8 kcal-mole-1 for silica aerogel (11). C. VIBRATIONAL FREQUENCIES ALTERATIONS I n those rare instances when the vibrational structure in the absorption spectrum is preserved in the adsorbed state (C,H,, C,H,Cl, C,H,F, tetrazine) the structured spectrum is shifted as a whole, and no substantial change in the band intervals, i.e., of the total symmetric ring frequency of the upper state, is noticed in comparison with the gas phase (Section 111). One can anticipate that in these instances the corresponding vibrational frequencies of the ground state are also only slightly affected, which is confirmed by direct observations of them, either in the infrared or in the Raman spectra for the adsorbed molecules.7 This has its counterpart in the fact that benzene molecules interVibrational frequencies of benzene, adsorbed on silica gel, or porous glass, obtained in the infrared are to be found in reference ( 3 0 ) ,and those for naphthalene obtained with the Raman spectra technique in references (31, 32).

234

A. TERENIN

act with most solvents mainly with dispersion forces, which cause only a small shift and practically no change in the band intervals of the vibronic spectrum (33).The same is obviously valid for the adsorbed molecules of the same symmetry and unsensitivity to external influences. There are, however, observations that the asymmetry due to the unilateral polarizing field of an uniform surface can, even in the case of a loosely adsorbed molecule, cause the infrared forbidden symmetrical vibration transitions to appear. On the other hand, deformational vibrational modes with a component perpendicular to the surface can be frozen in the adsorbed state and disappear in the infrared spectrum, as found for multimolecular layers of naphthalene, 3-fluorbenzene, benzoquinone, etc. (34).The search for such low frequencies lying in the far infrared spectrum should be of substantial help in the assessment of the magnitude of the adsorption potential. According to Hirota and Nakai (35) the formate anion OCO-, adsorbed on silver, exhibits three low frequencies in the infrared: 410 (strong), 300, and 130 cm-l, the former ascribed t o & vibration of the anion, perpendicular to the surface.

D. NEWBANDS The cases when a new electronic spectrum emerges upon adsorption, presents most interest from the viewpoint of chemisorption, since it signifies that a new species has been formed and stabilized on the active sites of the surface. Instances are described below, which are of much value in assessing the nature of the various active sites present on the surface of catalysts. However, a drastic change of the spectrum and the appearance of entirely new bands does not necessarily imply that a strong chemisorption has taken place. The new bands, described below, are in most cases produced by electron exchange between the adsorbed molecule and a site on the surface. The weakness of the adsorption bond can be inferred from the fact that the ionized molecules formed exhibit a spectrum very similar to that of the same molecular ions obtained in solutions by homogeneous oxidation-reduction processes. This lack of specificity on different surfaces should imply either a low adsorption energy, or equal adsorption (respectively, solvation) energies in the ground and excited states. For large symmetrical aromatic hydrocarbon ions the extra charge appears to be evenly distributed in the extended conjugate bond system, so that a fractional charge only remains opposite to the charged site. For the unsymmetrical phenylated polyenes there is observed a marked bathochromic shift of the spectrum for the adsorbed molecular ions, with respect to that in solution (Section V, D). I n these instances we can presume, that the ionized

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

235

species is strongly disturbed by the polarizing fields emanating from the surface.

E. POTENTIAL ENERGY CURVES De Boer was the first to use the concept of potential energy curves and the Franck-Condon principle for the interpretation of the shifts of the adsorption spectra (1, 4 ) . According to his well-known presentation, the interaction of a molecule with the surface is represented by potential curves, which can be quite different for the ground, M , and the excited M* state of the molecule, as shown by the curves u and u+in Fig. 1.

R -

FIG.1. Diagram of potential energy curves for adsorption of a molecule in the ground M, and excited, M* states. Origin of the bathochromic spectral shift (hvad < hugas) (according to de Boer).

Their minima can be situated at different distances, R,, and RZ from the surface, and the respective adsorption energies Q and Q* may be different. In the act of light absorption a vertical transition occurs from the equilibrium position at the minimum of curve u to a mostly nonequilibrium one on curve u*, the magnitude of the absorbed photon, hv,, being determined by the energy difference. The photon hv,,,, absorbed by the molecule in the gas phase is obtained from the difference of the horizontal parts of the curves u, u* at large distances R. In a general qualitative way one can say that when Q* < Q and R,*> R,, there should be hv,, > hvgas,i.e., a hypsochromic shift of the absorption band. On the contrary, when Q* > Q and R,*< R,, the excited molecule being

236

A. TERENIN

tighter bound to the surface in comparison to the ground state, a bathhochromic shift should be expected in the adsorbed state, as shown in Fig. 1. An increased adsorption energy, &*, is expected when the excitation of the molecule leads either to an increased permanent dipole (Section IV, A), or to an increased polarizability of the molecule (Section 111, A), or to the splitting up of a covalent bond with the appearance of adsorbed atoms or radicals more tightly bound to the surface. It must be kept in mind, however, that the two-dimensional potential curves of Fig. 1 do not consider the possibility of a change in the interatomic distances r, in the molecule, due to the adsorption process. A chemisorption, in particular, implies an increase of the interatomic distance of those atoms which enter into covalent bonds with widely separated centers of the surface (14).The excitation process in the free molecule often leads to a loosening of the bonds manifested in decreased vibrational frequencies and, consequently, to increased interatomic distances. In the limit, for molecules like iodine, the absorption of photons near the band maximum corresponds t o a dissociation into atoms separated on the surface by a distance definitely larger than that when they were bound in the ground state. To keep sight of this intramolecular change, the two-dimensional potential curves plotted as functions of the distance R from the adsorbent must be supplemented by a third coordinate rA.A, giving the interatomic distance in diatomic molecules or between two key atoms in a polyatomic one. The corresponding three-dimensional diagram for a diatomic molecule has been represented by de Boer and Custers (2). It is advisable, however, to keep the usual two-dimensional curves, imagining them to be drawn in space as contour lines on the respective surfaces of potential energy.

111. Spectra of Physically Adsorbed Molecules A. BENZENE Since the pioneering work by Pavlova (36) several spectral investigations have been made on adsorbed gaseous benzene. I n accordance with adsorption isotherm measurements which indicate a purely physical interaction of this molecule with the surface of most adsorbents, it was found that the ultraviolet absorption spectrum of C,H, adsorbed on the porous silicates (silica gel, porous glass) is very close to that of the gaseous molecule (36-38). Its characteristic vibronic six-band progression belonging to the r-r* transition lAlg-lBauremains preserved with

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

237

only a bathochromic shift of 120 to 140 cm-l to lower frequencies and a a broadening of the bands with loss of rotational structure (Fig. 2). This

Fro. 2. Absorption spectra of benzene. Curve 1 : vapor adsorbed on a silica gel plate in vacuo; curve 2 : aqueous solution; curve 3: gaseous benzene; curve 4: liquid benzene. Photographic density records, from Pavlova (36). The strip in the middle helps to show the respective shifts.

aspect of the spectrum was observed a t a low coverage of 6 = 0.2 which is sufficient to record the absorption spectrum by photography in spite of its weakness (e = 200) in the gas phase. This lack in molecular absorbance was compensated by the high internal surface area of the porous adsorbents used, viz., 120-160 metera/gm (38)and 400 metera/gm (36), which represented plates 2-3 mm thick, transparent down to 220-230 mp as a limit. Figure 3 reproduces the spectrum of C6H6, adsorbed from the gas phase on to a translucent disk of pressed silica gel, in juxtaposition with that of the vapor, recorded by Okuda (37). The band half-widths reach 200 cm-l on the thermally cleaned and outgassed adsorbent samples, which are known to have surface silanol

238 A. TERENIN Si-OH groups, conspicuously demonstrated by infrared studies (1721). Ron et al. (38)have established that the width is decreased to about 75 cm-1 when the surface silanol groups are transformed on methylation groups, or eliminated by sintering at 1000°C in wacuo. into Si-0-CH, This result shows that the broadening and diffuseness of the C,H, absorption bands is primarily due to an interaction with the surface hydroxyl groups, and not to an initial inhomogeneity of the surface. This conclusion is corrobated by the large half-width of the bands for C,H, in water (Fig. 2), where an inhomogeneity of interaction cannot be presumed. The isosteric heat of adsorption a t low coverage on porous glass is definitely higher (17 kcal ernole-l) €or the untreated surface in comparison to the methylated or sintered one (10-12 kcal.mole-I) (38).These values are approximately the same as found for C,H, on silica gel (39).

1 .o

D

0.5

250

260

2

mP

FIG.3. Bathochromic shift of the benzene spectrum. Curve 1: vapor adsorbed on a silica gel disk (pressure 12.7 mm Hg); curve 2 : vapor at 13.6 mm Hg ( D scale x 5). Photoelectric records, from Okuda (37).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

239

According to Kisselev and co-workers the differential heat of benzene adsorption (at a half monomolecular coverage, 8 = 0.5) amounts to 10.2 kcal.mole-l (40).Of this figure about 30% is accounted for by the H-bond of the electronic cloud of the benzene molecule lying flat on the surface, with one silanol surface group (41). The mean separation between adjacent Si-OH groups even on a fully hydrated silica gel surface is in fact sufficiently large (-13 Aa)per silanol group (42)t o accommodate one C,H, molecule. The determination of the adsorption entropy from the isotherms (38) has unexpectedly established that benzene adsorption on porous glass is of a mobile type, with the properties of a two-dimensional ideal gas. Only at low coverages (less than 1 x mole-gm-l) the freedom of motion is decreased, and the molecules may be held on definite sites, which is also reflected by an increase in the adsorption energy. The nonlocalized benzene adsorption on silica aerogel has been also inferred from the infrared spectrum of the surface silanol groups (43). It appears that the adsorbed molecules are executing a two-dimensional translatory motion with a free rotation in the molecular plane. The large bandwidth, mentioned above cannot be, however, ascribed to the mobility of the adsorbed molecule, since it is not appreciably affected by a lowering of the temperature down to 77'K, the shift of the spectrum being additionally increased by -30 cm-l. It is known from infrared spectra that benzene adsorption on porous glass decreases the 0-H frequency of the surface silanol groups more than that of cyclohexane. This is considered as an indication that benzene molecules form H-bonds with the strongly acidic (protonic) Si-OH sites, their n--electron system acting as a base (17). It is of the outmost interest that in the adsorbed state the benzene spectrum reveals, the 0-0 vibronic transition, forbidden in the free molecule and missing in the spectrum of the vapour (38).Moreover there are present in the spectrum of the adsorbed molecules weak absorption bands, which accompany each member of the principal progression a t intervals of 135 and 2 x 135 cm-1 to lower energies and at 240, 2 x 240 cm-l to the higher ones. The appearance of these forbidden frequencies and their intensification with temperature lowering is an indication that the symmetry of the benzene ring is decreased (from D,, to CBV), the symmetry plane in the ring plane being removed, owing to the molecular plane being parallel to the adsorbent surface.7 The above-mentioned frequency 135 cm-l, combining with the others, is presumed by the authors (38)to belong to the vibration of the whole adsorbed molecule t It must be remarked that the forbidden 0-0 band of C,H, can be also observed for its solutions in H,O, CHC1, and CC1, (33, 44).

240

A. TERENIN

with respect t o the surface, as no such internal low frequency exists for

CeHe. The conspicuous lack of change in the vibrational frequency of adsorbed benzene as compared to the gas phase is additionally indicative of a purely physical interaction with the adsorbent surface.? As compared with the adsorbed state, benzene dissolved in a great variety of solvents (33, 44),manifests an even larger bathochromic shift (about 200 cm-1) of the entire band progression with respect to the gas. Sverdlova (33) established that the magnitude of this red shift and its regular dependence on the optical and dielectric properties of the solvent are quantitatively accounted for by a 25-30y0 increase of the polarizability OL of the benzene molecule in its excited state with respect to the ground one. The interaction with nearly all the solvents is proved to be due to dispersion forces only, no electrostatic mutual influences being present. The same conclusion is evidently applicable to the adsorption of benzene on the bare surface of silica gel, between the silanol groups, left after the high temperature dehydration, and also on the methylated surface. According to Pavlova (36) gaseous chlorobenzene, adsorbed on a silica gel plate, exhibits, in comparison with adsorbed benzene, a markedly smaller shift (40-90 cm-I), which is an additional indication of the absence of an electrostatic interaction with the surface. The shift is somewhat different for the consecutive bands of the same progression. In order to increase the transparency of silicic acid powder Robin and Trueblood suspended it as a slurry in cyclohexane, possessing approximately the same refractive index and thus obtained absorption spectra of adsorbed benzene and its derivatives (46).However, the adsorption from even dilute solutions produced poorly resolved broad bands, contrasting with the sharp spectrum of the cyclohexane solution itself with the same total number of dissolved molecules.$ In comparison with the cyclohexane solution, the maxima of the broad absorption bands of adsorbed benzene and its derivatives are seen to be shifted to the higher frequencies. However, relative to the gaseous state, the shift is to the red by ~ 1 3 cm-'. 0 The spectrum of benzene vapor, adsorbed on a purely ionic adsorbent,

t When the porous glass with adsorbed C,H, is brought in immediate contact with liquid air a series of broad absorption bands between 320 and 340 rnp has been observed by Ron et aZ. (38) which does not appear in liquid nitrogen, or when cooled without contact with liquid air. This spectrum has been presumably assigned to the forbidden ground singlet-triplet transition of C,H,, which has been previously observed in solutions under high 0, pressure ( 4 5 ) .

4 Cf. a similar result for phenol (Fig. 6) below.

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

24 1

a sublimed KCI film in vacuo, retained its vibronic structure on a continuous background ( 4 7 ) . The bathochromic shift is reported to be larger, than on silica gel and to increase for the bands a t the shorter wavelengths. A similar behavior was shown by C,H,CI, adsorbed on KC1. The spectrum in the adsorbed state was found to be similar to that of microcrystalline deposits of these compounds at - 180°C. This might be as indication of a condensation of the adsorbate vapors on the poorly developed internal surface of the sublimed salt film.

B. HETEROCYCLES Gaseous pyridine adsorbed on to a sublimed KCI film in vacuo displayed a broad absorption inaximum at 255 mp without the vibrational structure of the vapor spectrum (47, 47a). With respect to the onset of the latter, this band was located at higher frequencies and occupied the same range as that of a microcrystalline sublimed layer of pyridine a t - I80"C, which, however, exhibited a distinct vibrational structure of the spectrum. A similar behavior has been observed for a-picoline. The spectrum of pyridine vapor, adsorbed on to a disk of pressed silica gel, shows, according to Okuda (48, 48a) a progressive change with increasing ambient pressure (Fig. 4). A t the lowest coverage a broad unsymmetrical band appears with a narrow maximum a t 269 mp. With pressure increase it is gradually displaced to the red, and a t the same time a strong band appears in the shorter wavelength range with submaxima at 249, 254, and 261 mp. A similar enhancement of a structured band in the same range, recognized as the T-T*(l.Al-lBl)transition is observed on changing the solvent from a saturated hydrocarbon to ethanol or water (49).Therefore, it does not reflect a specific adsorbent action. The same enhancement of a band a t 255 mp has been observed for pyridine, adsorbed on KCI, and likewise for crystalline pyridine, as mentioned above. I n the spectral range, adjacent to the T-T* band from the long wavelength side, an n-r* transition of low intensity is present in pyridine (49). When an H-bond is formed between the basic nitrogen atom of pyridine and the solvent molecules, or the silanol groups of the adsorbent, this band is displaced to the shorter wavelengths, but being of low intensity, it cannot produce the enhancement effect and structure described. The 269 mp band in Fig. 4 shifting to the lower frequencies, cannot thus be assigned to an n-r* transition, but to a T-T*one displaced to the red by interaction with the acidic silanol groups, as is usually the case for transitions of this type.

242

A. TERENIN

I n contrast to gas phase adsorption, pyridine, adsorbed from a cyclohexane solution on silica gel, immersed as a mull into Nujol, displays remarkably sharp bands of the T-T* transition with only a small red shift (Fig. 4).This structured spectrum is nearly identical with that of pyridine in hydroxylated solvents, and in the solid state a t low temperature. When adsorption is carried out in the presence of a solvent, the molecules of the latter can decrease by competition the number of the most active sites available for the adsorbate. Moreover, coadsorbed hydro-

0 I5

D

D 0

0 10

005

mP

FIQ.4. Pyridine vapor adsorbed in wacuo on a silica go1 disk at different pressures. Curwe 1: 0.8; curue 2: 1.4; curwe 3: 8.3; curwe 4: 17.0 mm Hg; curwe 6: pyridine adsorbed on silica gel powder from cyclohexane (immersed in Nujol). Prom Okuda (48a).The scale on the right is for the dashed curve.

carbon molecules, by their fixation on the surface, seem to confer on the adsorbed pyridine an inert “rigid” environment in which the deformational modes of vibration, operative in the blurring of fine-structured spectra, are hindered. These factors should increase the spectrum sharpness, as observed. The marked red shift, mentioned above (Fig. 4),of the T-T*pyridine band, when the first gaseous molecules come into contact with the surface, is an indication of chemisorption on the Si-OH groups. I n contradistinction, pyridine (and a-picoline) molecules are easily desorbed as

ELECTRONIC SPECTROSCOPY OF ADSORBED UAS MOLECULES

243

are, similarly those of C,H, and C,H,Cl from the surface of KCI. They are also easily removed by coadsorption of NH, gas (47). The absorption spectrum of gaseous s-tetrazine,

'

N=N

H-C

\

N-N

L

H

/

consists of a system of sharp fine-structured bands in the visible, which has been obtained under high resolution by Konigsberger and Vogt already in 1913 (50). Only recently a complete analysis of its visible and ultraviolet spectra in the vapor state, and in various solvents at ordinary and low temperature has been made (51). The vibronic spectrum of s-tetrazine vapor, adsorbed in vacw on porous glass, silica gel, and silica alumina gel exhibits broad bands with a loss in the finer structure (52) (Fig. 5 ) .

55

j 3

)O

FIQ.5. a-Tetrazine vapor adsorbed in vactw. Curve 1: on a porous glass disk; curve 2: on the methylated porous glass [from Barachevsky and Terenin (52)J.On the top positions of the sharp bands of the vapor at 20DC, according to Mason (53),are indicated in the same scale.

244

A. TBIRBININ

The sharp bands of tetrazine vapor in the range from 560 to 450 mp belong to the sequence of 0-Of, 0-l‘, 0-2‘, 0-3’, 0-4’, and 0-5‘ vibronic n-n* transitions, the 0-0 one being the strongest (53).This general aspect of the spectrum is preserved when tetrazine vapor is adsorbed on microporous glass, but the bands are broadened and shifted toward the higher frequencies by -200 om-1 (Fig. 5, curve 1). There are only four distinct absorption peaks left. The maximum absorption is now situated close to the 0-1‘ band, i.e., has shifted by 1250 om-’, relative to the vapor. A “blue” shift is expected €or an n-v* transition and has been observed to a lesser extent for tetrazine dissolved in water (53). Here on the porous glass the shift is due to the H-bonds between the basic N atoms of the tetrazine ring and the acidic silanol groups, and is therefore substantially larger. The spectrum is further shifted to the higher frequencies by 100 to 150 om-1 when the temperature of the adsorbent is lowered down to 70’K. When the silanol groups on the surface are replaced, through methylation, by Si-0-CH, groups, the bands of adsorbed tetrazine are situated at longer wavelengths and more resolved (Fig. 5, curve 2). They are more closely situated to those in the vapor spectrum, the “blue” shift decreasing to about 50 cm-1. The maximum absorption still remains at the 0-1’ band. A similar decrease of the “blue” shift is observed when the porous glass with adsorbed tetrazine is brought into contact in wucuo with methanol and pyridine vapors, which evidently displace the adsorbed molecule and, in the limit, dissolve them in the liquid phase condensed in the capillaries. An additional adsorption of H,O vapor produces, on the contrary, no similar effect, and in contrast to the aqueous solution the spectrum retains its structure. In contact with NH, gas, the maximum of the adsorbed tetrazine spectrum is shifted to higher frequencies by 1980 cm-1 with a loss in structure. Such a perturbation and the sign of the shift was quite unexpected, since, a t first sight, ammonia was capable, like pyridine, to neutralize the silanol groups but not to enhance their protonization. A selective interaction with adsorbed tetrazine molecules is revealed when the adsorbent is brought into contact with the vapors of nitrobenzene, or benzoquinone. The spectrum is radically changed (52). This is presumably due t o the formation of intermolecular addition compounds between the electron donating and accepting molecules in the adsorbed state. The spectrum of tetrazine adsorbed on Aerosil and on some sulfates are, in the main, similar to that for porous glass. On the metal oxides and silica-alumina gel the visible bands are not observed, but only the strong ultraviolet one at about 380 mp (52),belonging to a second n-n*

ELECTRONIC

SPECTROSCOPY OF ADSORBED GAS MOLECULES

245

transition ( 5 4 . The specificity of the adsorbents is manifested by the different shifts of this ultraviolet n-n* and that of the n-n* transition, situated around 250 mp.

C. PHENOL Phenol, Ph-OH, and anisole, Ph-OCH,, when adsorbed on a silicic acid slurry suspended in cyclohexane, possess almost identical ultraviolet absorption spectra with maxima at 276, 270, 265, and at 274, 268, 262 mp, respectively (46). Both spectra are shifted to the red by -300 cm-l, which is less than in cyclohexane solutions. Benzene, chlorobenzene, and fluorobenzene adsorbed on silicic acid from cyclohexane, manifest a similar small shift, as mentioned in Section 111, A (46).The electronic transitions of phenol and anisole are of a n-n* type, like that of benzene. However, phenol is more strongly adsorbed on silicic acid, than anisole (by -2.5 kcal/mole), which can be attributed to a stronger H-bond with the surface silanol groups.? As this small difference in the adsorption energies (for the ground states) is not reflected in the absorption spectra it has been inferred that the bonding of the molecules to the adsorbent, occurs a t the substituent, -OH, or -OCH, and not at the benzene ring (46).This is confirmed by the infrared spectrum of phenol vapor, adsorbed on porous-glass, which exhibits a perturbation of the 0-H vibrational frequency, but not of the C-H one of the benzene ring (53a, 54). Figure 6 reproduces the spectra obtained by Kobayashi (54a) of phenol adsorbed on a silica, or, respectively, silica-alumina slurry immersed in solvents of a similar refractive index. The blurring of the band structure and decreased absorbance are an expected consequence of the light scattering in the still inhomogeneous medium. The similarity of behavior of phenol on the two adsorbents of widely different catalytic activity, like silica gel and silica-alumina, points to a blocking of the more active sites by the adsorbed solvent. We should expect a much larger red shift of the phenol spectrum than that observed if the adsorption sites were of a basic character, viz., presumably Si-0-Si, or Si=O centers, on account of the known increased protonic acidity of the OH group in the excited state of phenols (Section IV, B).

t Okuda (48a)pointed at the fact that the acidity of the silanol Si-OH group (pK,=10) is close to that of phenol (9.9), and therefore a double H-bond should be formed between them, like that in a phenol dimer, but for anisole this should be excluded.

246

A. TERENIN

IV. Strong Spectral Perturbations A. NITROBENZENES The absorption spectrum of nitrobenzene molecules adsorbed on silica gel from the vapor phase has been recorded by Okuda (55)(Fig. 7), and that from a cyclohexane solution in a slurry of silicic acid by Robin and Trueblood (46).The first author has found a shift from 240 in the gas to 260, and the second authors from 253 in cyclohexane to 270 mp, which is equivalent to about about 3000 cm-l. Such a large displacement does not necessarily indicate a chemisorption, since the position of the 260 mp band of adsorbed nitrobenzene is between those of its aqueous and ethanol solutions. However, Pig. 7 shows that after desorption the most firmly held molecules display a broad absorption band with a maximum a t 300 mp, which might indicate a kind of chemisorption. The rr-rr* 240 mp band belongs to an intramolecular charge-transfer between the benzene ring and the electrophilic -NO, group (56,56a). The large displacement observed should accordingly be ascribed to the resultant of the interactions between the dipole moments of the molecule in its excited and ground states, respectively, with the dipolar molecules of the solvent, or with the dipolar -OH groups of the adsorb-

mP

FIG.6. Phenol. Curve 1: dissolved in cyclohexane; curve 2: adsorbed on silica gel immersed in cyclohcxane; curve 3 : adsorbed on silica-aluminaimmersed in cyclohexanedecaline. From Kobayashi (540).

ELECTRONIC SPECTROSCOPY O F ADSORBED GAS MOLECULES

247

ent. The polarizability of the medium or of the adsorbent is here of secondary importance. According to Bakshiev (5?'),the charge-transfer imparts to the excited state of PhNO, an increased static dipole moment of 9 debyes as compared with the ground one, i.e., 4 debyes. Therefore in the excited state the molecule must experience a stronger electrostatic adsorption, than in the ground one, which fact should produce the observed large red shift, in accordance with Fig. 1 (Section 11, E). The infrared spectrum of nitrobenzene, adsorbed on silica gel, revealed a larger perturbation of the C-H vibrational frequencies (compared to a CS, solution), than of those of the -NO, group (55). The conclusion drawn from this was that it is the benzene ring, but not the nitro group which is involved in the interaction with the surface. This is just the opposite to what has been inferred above for phenol and anisole (Section 111,C). Indeed, if the protonized silanolgroups, -Si-OH, were the active sites for nitrobenzene adsorption, one should expect the contrary, the nitrogroup being much more basic than the benzene ring.

D

mP

FIG.7. Nitrobenzene. Curve 1: vapor ( D scale x 10); curve 2 : vapor adsorbed on a disk of pressed silica gel; curve 3 : after evacuation. From Okuda (55).

248

A. TJJRENIN

The bathochromic shifts of the nitrobenzene charge-transfer band, observed in several hydroxylated solvents (48u)does comply with the linear dependence of its frequency Y,,, vs the expression 2(D-l)/ D $2, first deduced by McRae (58),where D is the dielectric constant of the solvent. This should indicate, according to theories of the spectral displacementsfor solutes (57-59), that there is present mainly an electrostatic interaction between the dipoles of nitrobenzene in its ground and excited states and those of the solvent molecules.? It is difficult to bring over the same theoretical considerations from dissolved to adsorbed molecules. The unilateral interaction of an adsorbed molecule even with a smooth bare surface must be, in principle, treated rather as a binary association compound, than as a molecule halfimbedded in a continuous medium with a definite dielectric constant.$ A surface binary association of the polar nitrobenzene molecule with the silanol group should represent more correctly the actual situation, Actually, it has been found that a large bathochromic shift is produced by protonation of nitrobenzene in H,SO, (56a).A similarly large red shift of the order of 2500 em-l is displayed by the substituted nitrobenzenes, viz., p-nitroanisole, nitroanile, nitrophenol, etc., when adsorbed upon a silicic acid slurry immersed in cyclohexane (46). The colorless di- and trinitrobenzenes exhibit likewise anomalously strong bathochromic colorations upon adsorption from benzene on some adsorbents, which has another origin. Thus Cruse and Mittag (61)reported on a blue coloration of activated MgO, produced by the adsorption of rn-dinitrobenzene, and a red one for that of s-trinitrobenzene. hater the same effect was spectrally investigated by Kortiim et at. (62) with the technique of grinding the dried adsorbent together with the added adsorbate (Section I). For s-trinitrobenzene grinded with MgO the spectrum of the coloration, reproduced in Fig. 8 does display, in addition to the ultraviolet bands, a new strong broad absorption maximum at 465 mp. For mixtures of s-trinitrobenzene with CaF, or silica gel, no such coloration and new band appear. It is known that the same red coloration and a similar spectrum ca1: be produced for s-trinitrobenzene dissolved in an alkaline ethanol solution (cf. 62). In this case the effect has been ascribed to the inclusion of OH- ions into the benzene ring, with the formation of covalent bonds

t The relative participationsof the dipole-dipole,and the dipole-induced dipole kinds of electrostatic interaction for different transitions of nitrobenzene and its derivatives has been analyzed by Semba (60).

1The dielectric constant of silica gel isgiven as D = 7.5 (48a),whereas that of ethanol is 22. As mentioned above, the shifted band of nitrobenzene adsorbed on silica gel, 260, is nevertheless close to that observed in ethanol, 269.2 mp (55).

ELECTRONIC SPECTROSCOPY OF ADSORBED QAS MOLECULES

249

and of =NO; end groups. Under this assumption the ring has to be ruptured and conjugation between the =NO; and -NO, groups destroyed, which seems to be inconsistent with a shift to the red. But it is evident that no OH- ions could be present on carefully dried MgO or on AIO(0H) used by Kortum et al. unless the grinding process generated them. On the other hand, we have found that the same new 465 mp band can be observed upon adsorption of s-trinitrobenzene vapor on perfectly dry zinc oxide, or MgO thoroughly degassed under high vacuum conditions (63),when no alkaline surface can be formed.7

0

r-

I

/

/

I

3; /

--d -- I 0 0 m .c

0

.

,

-1.5 I

-2 0

I

~

/

4.0

f

W D

-0 3.5

I

I

I

20,

40,000

FIG.8. s-Trinitrobenzene. Curwe 1: grinded with MgO powder in air; curve 2: grinded with silica gel; curwe 3: dissolved in alkaline ethanol. Diffuse reflection spectra. From Kortum et al. (62). The scale on the right is for the dashed curve.

B. SURFACE ACIDITYINDICATORS One of earliest methods of obtaining information about the acidbase properties of the catalyst surface, in particular of the silicaalumina, was to adsorb from nonaqueous solvents on to them a series of known organic indicators and to notice the color change.: t I n a previous review of the subject (15)the author has suggested that this new band at 465 mp might be due t o the forbidden ground singlet to triplet transition which became allowed as the result of the perturbation caused by the activated adsorbent.

SCf. e.g. references (64, 65).

260

A. TERENIN

The most conclusive evidence has been obtained by de Boer and coworkers (3), who adsorbed in a high vacuum p-nitrophenol, phenolphtahalein, and thymolphtahlein vapors onto porous CaF, and BaF, films sublimed under strict elimination of traces of water. Under these conditions there appeared an intense coloration undoubtedly produced by direct contact of the molecules concerned with the surface. the absorption maxiFor adsorbed p-nitrophenol, OH-Ph-NO,? 0.3C

3 u)

.-

C 3

z

rr .e c

0.2c

0

.-C c

C 0

c v)

C 0

C

?

?

0.10

51

n

a

1

5

3

3 mP

FIG.9. pNitropheno1 vapor adsorbed in high vacuum on a sublimed BaF, layer. Curve 1: firstly adsorbed molecules (band 413 mp); curve 2: beginning of a second layer of molecules, adsorbed on the first ones by Van der W a d s forces; curve 3: second layer of physically adsorbed molecules formed (band 320 mp). Transmission spectra. From Custers and de Boer (3).

mum was situated close to, but was not exactly that of its anion, -0-Ph-NO,, in aqueous NaOH, where it is at 400 mp. I n fact, for this molecule adsorbed 011 CaF, it is at 365, and a t 413 m p on BaF, (Fig. 9). If we consider the absorption maximum of p-nitrophenol in aqueous HC1 at 316 mp, as characteristic of the neutral molecule, the t Here and below Ph designates the benzene ring in the substituted aromatic compound.

ELECTRONIC SPECTROSCOPY OF ADSORBED BAS MOLECULES

251

spectrum suffers an unusually large bathochromic displacement. The red shift in the case of BaF, is even stronger than that for the p-nitrophenolate anion. A similar behavior is shown by phenolphthalein, which absorbs in the ultraviolet, but which gives a bright red color when adsorbed in vucuo on to CaF,, with A,, = 475 mp, and a red-violet one, with A,, = 536 mp, when in contact with BaF,. The larger bathochromic shift has been explained by de Boer and co-workers as due to a lesser screening of the negative charges of the outer layer of F- anions by the positive charges of the underlying Baa+ cations, more voluminous than the Ca2+ ones. As both aromatic phenols are assumed to be adsorbed with their dipolar OH groups on the F- sites, the interaction will be stronger for the adsorption on BaF,, as actually observed. A complete proton detachment from the adsorbed molecule with the formation of H+F- on the surface evidently does not occur, as the spectrum is not identical to that of the respective phenolate anion, -0-Ph-NO De Boer and Houben ( 6 )have explained the anomalously large shift of the spectrum of these phenols to the red by the diagram of Fig. 1 (Section 11, E), implying markedly different adsorption energy for the ground and excited states respectively. They assumed that the large bathochromic shift observed in these and similar adsorption systems (cf. below) is caused solely by a stronger adsorption of the excited molecule, which lowers its level by a larger amount in comparison to the ground one. The adsorbed excited p-nitrophenol molecule has been represented as a limiting resonance structure interacting with a surface F- anion, viz.,

,.

(1)

the ring lying flat on the surface (6, 8 ) . We possess at present reliable data for the change in the electronic distribution of some polar molecules upon excitation to the first singlet r-r*,or to the n-r* level (57, 59). In particular, for p-nitroaniline the dipole moment of the molecule in the excited state is 16.1, as compared to 6.3 debyes in the ground one; for p-nitrophenol it is about 12 and 6 debyes, respectively (57).The negative end of the excited dipole clearly resides on the nitro group, the positive one on the -NR,, or -OH group, which both evidently are directed to the P- ion of the surface. In this respect scheme (I) correctly represents the orientation of the adsorbed molecule.

262

A. TERENIN

However, a larger electrostatic interaction of the adsorbent with the excited state of the adsorbate does not explain why the spectrum of such a proton-donating molecule should be situated very close to that of the respective phenolate anion, i.e., -0-Ph-NO,, possessing on the 0 atom an extra negative charge in contradistinction to structure (1). As found by Weitz and co-workers (66), and de Boer and Houben ( 6 )p-nitrophenol adsorbed on silica gel does not manifest the red coloration and consequently the spectral displacement referred to above. It was also absent on silicic acid, immersed as a slurry in a cyclohexane solution of this compound (46). I n fact a bathochromic shift of the band from 286 in the solution to 315 in the adsorbed state, has been noticed, which is far less than that on the F- anions. For p-nitroanisole, 0,N-Ph-OCH, the respective shift is from 294 to 316 m p , which shows that the acidic -OH group in nitrophenol is not decisive in producing this shift. The displacement from 315 on silica gel to 413 mp on BaF,, i.e., by about 7500 cm-l, is so abnormally large that an explanation based on the assumption of only a stronger dipolar adsorption in the excited state is insufficient t o account for it. It is legitimate t o presume that a new species has certainly been formed with an electronic configuration close to that of a nitrophenolate anion -0-Ph-NO,, which is the light absorbing entity, as was first proposed by Custers and de Boer (3). A similar red coloration is displayed by phenolphthalein when adsorbed on dried A1,0, from organic solvents ( 6 , 66). Traces of water reversibly remove it, which is consistent with a stronger adsorption of H,O molecules in comparison to the organic ones. This proves that the spectrum of the phenolate anion is not produced by the reaction with an alkaline humidity layer on the surface. This effect cannot be also attributed to the presence of hydroxyl ions OH- on the surface, since the coloration is strongest the less OH groups remains after thermal treatment of the alumina, whose surface initially had the Composition AlO(0H). The abstraction of H,O from two close OH groups leads to the formation of 0-bridges, largely ionic and basic. According to de Boer and Houben ( 6 ) ,these -0-sites cause the surface of the dehydrated alumina to adsorb predominantly organic molecules with positive end groups, as HO--, or R,N-. Analogous observation have been reported for the adsorption of basic indicators on some acidic adsorbents ( 6 4 , 6 5 ) . For example, p-dimethylaminoazobenzene, Me,N-Ph-N =N-Ph (Me stands for CH,), is strongly adsorbed on silica-alumina, known to possess considerable proton acidity, with a color change from yellow to red. For a methanolic solution the spectral change is from an absorption band at 400 to one

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

253

narrower one at 530 mp when the solution is acidified by HCI, the shift being ascribed to proton addition at the most basic N atom of the azogroup (62).This leads to formation of the cation

+

[Me,N= Ph =N-N-Ph]+

I

H

with an increased delocalization of the positive hole, as reflected in the possibility of positive charge transfer from the methylated N atom to one of the azo-group, viz., [Me&-Ph-N=

+N-Ph]+ I

H

The HO groups on the partially dehydrated surface of aluminium oxide, in contrast to the silanol, Si-OH groups, are not acidic, and adsorbed dimethylaminoazobenzene does not exhibit the color effect just mentioned. When, however, aluminium oxide is previously completely dehydrated a t high temperature, a surface red color somewhat different from that observed on the protonic surfaces is developed (6). This suggests the appearance of strong electron pair acceptor sites (Lewis acid) on the alumina surface after this treatment. It is still more remarkable that similar spectral change is manifested when p-dimehtylaminoazobenzene is ground together with dehydrated BaSO, powder, i.e., a quite inert neutral salt. Kortum and co-workers (62), who carried out this experiment, compared the absorption spectrum (in diffuse reflexion) of the surface coloration with that of an acidified solution of the azo dye in methanol, and found satisfactory coincidence of the shape and position of the bands in both cases (Fig. 10). When the red-colored dry adsorbent is put in contact with open air, the spectrum reverts to that of the normal unprotonated dye, as should be the case, since H,O molecules usually displace less strongly adsorbed organic ones. A renewed desiccation, brought about the restitution of the “acidic” spectrum. Protonic sites on BaSO, being excluded, the conclusion drawn by the authors was that it is the surficial Ba2+ cations which by entering into a coordination bond with the more basic N azo atom, give rise to the same change in the electronic distribution, as the proton does.

C. AROMATIC AMINESAND DIAMINES The amino- and imino-groups coupled with an aromatic ring represent a basic and equally a oxidizable (electron donating) part of the

254

A. TERENW

molecule, conferring to the latter a high sensitivity to spectral perturbations either by a solvent or by the surface of the adsorbent. These significant spectral changes have been frequently used to reveal either the acidic or the oxidizing properties of peculiar sites on various adsorbents. The system of bands at the longer wave-lengths in the absorption spectrum of aniline and its N-alkylated derivatives is centering around -290 mp in the vapor state or in a cyclohexane solutions. It represents a T-T* transition, similar to that of C,H,, but shifted to the lower frequencies and strongly intensified, owing t o the charge injection of the lone 2pz electron pair from the N atom into the electrophilic benzene ring. This produces a change in the valency configuration of the N atom which approaches the coplanar spa hybridization. When H-bonds are formed by the environment with the basic -NR, group of the amine, its lone electron pair is partially kept from the interchange with the ring, and the spectrum is shifted towards the position of that of C,H,. For example, in an aqueous solution, the aniline spectrum is shifted to higher frequencies by 1000 cm-l, as compared to a solution in cyclohexane. In a 0.1 M HCl solution the protonated anilinium ion, C,H,NH,. H+ exhibits an absorption spectrum which has completely reverted to that of C,H,. I n this case the lone pair of N

- 0.5 -1.0

, -1.5 z

-a“

LL 01

-0

-2.0

- 2.5 - 3.0

1 40,000

cm-‘

F ~ Q10. . p-Dimethylamino-azobenzeneground up with BaSO, powder in air. Cutwe 1: desiccated sample; curve 2: humidity admitted. Diffuse reflection spectra. According to Korttim et al. (62). Band 630 mp for the “chemisorbed” state, bond 416 m p for the physically adsorbed one.

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

266

remains localized in the coordinating bond with the added proton, and, consequently, its charge “injection” in the ring is precluded. The spectrum of aniline, adsorbed on silica gel either from the gas phase (27), or from a cyclohexane solution (46) is situated at higher frequencies with respect to that of the vapor, owing to the expected H-bond formation with the acidic silanol groups of the surface. The shift is larger than that observed in an aqueous solution but smaller than that characteristic of the anilinium ion. For aniline vapor, adsorbed on silica gel in W ~ C U Oa t low coverages (0 = 0.05-0.1) the first band is situated at 280 mp, whereas on the silica-alumina gels of different compositions, it is shifted to 260 mpl Fig. 11 (27). Within the precision limits the latter corresponds to the spectrum of the CBH,NH,.H+ion in an acidified aqueous solution. A similar behavior was shown by uand 8-napthylamine vapor adsorption on silica-alumina gel and bentonite (27). According to chemical evidence, the active sites of the silica-alumina

FIQ. 11. Aniline vapor adsorbed in wucuo. Curwe 1: on silica gel; curwe 2: on alumina gel; curwe 3: on silica-aluminagel (60:60);curwe 4: on Na-silica-aluminagel; curwe 6: on Li-silica-aluminagel. Diffuse reflection spectra, from Kotov and Terenin (27).

256

A. TERENIN

cracking catalyst are assumed to be protons, loosely held on the surface of the gel in close proximity to tetracoordinated A1 atoms (69-71). The shifts to higher frequencies of the aniline and naphthylamine spectra. upon their adsorption on the silica-alumina is consistent with the presence of protonic sites on the surfaces. In order to check this explanation, the adsorption of aniline vapor has been performed on silica-alumina samples for which the loosely held protons had been exchanged for Li+, or Na+ cations. This treatment is known to lead to a “poisoning” of the catalytic activity of the gel. It was established for such treated samples, that the 280 mp band of adsorbed aniline is shifted back to 260 mp, i.e., the position of the band for silica gel or to an intermediary position (Fig. 11). Thus the presence of protons is decisive for this spectral behavior. The spectrum of aniline, adsorbed on yAl,O, powder, or alumina gels of various kinds, exhibits a band at intermediary positions, at about 270 m p (27). Another widely accepted viewpoint on the catalytically active sites in the silica-alumina is that they are uprotonic acids, viz., electrophilic A1 atoms with unfilled p-shell, the electron density being shifted from them towards the three surrounding 0 atoms (72). Such an electron deficiency confers on the A1 atom an affinity towards an unshared electron pair of the basic adsorbed molecule, i.e., the properties of a Lewis acid. In fact, the addition compound, obtained on sorption of aniline vapor by a sublimed AlCl, film in a high vacuum exhibits the same shift towards the spectrum of benzene, as is the case with a protonic acid (73). It was of interest to compare the results described above with those obtained for aniline molecules adsorbed from the vapor phase on to sublimed KC1 layer in vucuo ( 4 7 ) . The spectrum shows even at a low coverage (corresponding to 0.1 mm Hg vapor pressure) the 280 mp band broadened and shifted to the lower frequencies by about 5000 cm-1. This is a kind of chemisorption, since, in contrast to pyridine, evacuation at 20°C is not sufficient to remove the adsorbed molecules, and for the disappearance of this spectrum heating is additionally required.

V. Positive Ion Spectra of Adsorbed Molecules A. CATIONRADICALS FROM PHENYLATED AMINES The previously described spectral changes observed for the adsorbed molecules reflect mostly perturbations of a physical, or semichemical kind (H-bond, protonizationf which leave the electron system of the

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

257

molecule essentially intact. There are known, however, addition compounds of unsaturated and aromatic compounds such as the polyenes, phenylethylenes, aromatic ketones, thio ketones, imines, etc., formed with electron deficient agents, e.g., BHal,, AlHal,, BeHal,, MgHal,, ZnHal,, CuHal,, and other Lewis acids. They exhibit the effect of halochrorr~y,i.e., the appearance of a coloration from initially colorless partners. In these complexes the interaction should not be limited to that of a simple base-acid type, but one has to presume a deeper perturbation of an oxidation-reduction kind, accompanied by the splitting of the shared electron pair. One electron can, in the limit, be transferred to the acceptor, and the molecule bound to the electrophilic agent can be transformed into a positive molecular ion-radical. Such a view on the halochromy has been put forward a long time ago by Pfeiffer, a pioneer in this field (74). Such aromatic amines and diamines, as diphenylamine, N-methyldiphenylamine, triphenylamine, benzidine, p-phenylenediamine, etc., are known to have low ionization potentials (75) and to yield univalent positive molecular ions (semiquinones)under ultraviolet irradiation in rigid media (76),or by oxidation in solutions (77).These molecular ions possess characteristic absorption bands in the visible range. The adsorption of the vapors of these amines in vacuo on carefully degassed silica gel, or porous glass at temperatures, ranging from 20 to 100°C does not produce any visible coloration of the adsorbent. The absorption spectrum reveals only the normal ultraviolet bands of the adsorbed molecules a t 280-300 mp. However, the adsorption of the same vapors on carefully degassed and evacuated silica-alumina invariably produces a more or less strong coloration of this adsorbent, the spectrum of which reveals bands undoubtedly belonging to the positive ion-radicals of the adsorbed molecules (27, 77a). I n Fig. 12 (curves 1 and 2) the double peaked absorption maximum, 520 and 560 mp is shown, characteristic of the positive ion-radical of dimethyl-p-phenylene diamine (Me,N-Ph-NH,)? (77), which was here obtained by adsorption of the vapor under high vacuum conditions (27, 78).The same interpretation is given to the absorption maximum 850 mp for benzidine vapor, adsorbed on silica-alumina gel (curves 3 and 4), which does not appear on silica gel. The band of the benzidine ion-radical, obtained by photoionization in the rigid EPA solvent at - 180°C is situated at 885 mp (76). The accompanying bands 760 and 450 seem to be intimately connected with that at 850 mp. From Fig. 12 another important fact can be inferred, viz., that the ion-radicals bands are present with undiminished intensity even on the

268

A. TERENIN

“poisoned” adsorbents, in which the available protons have been exchanged for alkali-metal cations. This means that the sites on silicaalumina, which produce the observed surface ionization of the diamine, are seemingly independent of the purely protonic centers, revealed by adsorbed basic spectral indicators on the same adsorbents (Section IV, B). For diphenylamine vapor adsorbed on silica-alumina gel, the band of the cation radical (H-NPH,)? appeared a t 650 (27, 77a), whereas it has been observed at 680 m p in photooxidation (76). I n Fig. 13 the spectrum of gaseous N-methyldiphenylamine, adsorbed on silica-alumina in vucuo at room temperature, reveals at 615 mp a band, which, according to the evidence mentioned (76, 77), belongs to the cation radical (H,C-JYPH,): . The strong and wide absorption band with a maximum at 1000 mp is probably due to a charge-transfer complex formed between the cation-radicals and coadsorbed phenylamine molecules. The colorations and visible bands described above are accompanied by a strong narrow EPR signal (10l6 spins/cms g=2.004, dH=14 oe), which gives an independent indication of the presence of univalent radicals on the surface of silica-alumina (78, 79, 80). On silica gel, or

1

2/

3-

900

)O

FIQ.12. Dimethyl-p-phonylenediamine vapor, adsorbed‘in wucuo. C u r v e 1 : on silicaalumina g d ( 5 0 : 5 0 ) ; curve 2: on Li-silica-aluminagel ( 5 0 : 5 0 ) . Benzidine vapor adsorbed in vacuo. C u r v e 3 : on silica-aluminagel ( 5 0 : 5 0 ) ;curwe 4: on Nrt-silica-aluminagel ( 5 0 5 0 ) . Diffuse reflectionspectre, from Kotov and Terenin (27).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

259

y-Al,O, powder the adsorption of the amine vapor produced neither a coloration, nor any EPR signal. With alumina gel a feeble coloration could be found for dimethyl-p-phenylene diamine, but none for benzidine (27). The chemisorption process observed can be represented by the following scheme: R Ph,N

:

site

4

R (Ph,N.)+(.site)-

(11)

being equivalent to an univalent surface oxidation, with electron transfer t o the adsorbent. The electrophilic site is considered to be an A1 atom on the surface with a vacant 3p orbital. Such an electron deficient Lewis acid center has been assumed for silica-alumina before (71, 72) (Section IV, C). As a demonstration that the interaction of the molecules with the adsorbent, leading to the coloration, is not due t o the formation of a

500

300

FIG.13. N-methyldiphenylaminevapor, adsorbed in vacua at 20' C . Curve 1: on silica, gel; c u w e 2: on silica-alumina (75:25) gel. Diffuse reflection spectra, from Barachevsky (77a).Band 615 belongs to the cation radical.

260

A. TERENIN

stable chemical compounds, dried ammonia gas (10 mm Hg), admitted to the evacuated system invariably caused the disappearance of the coloration, the ultraviolet spectrum of the unionized adsorbed molecules being restored (27, 77a). Similar spectra of molecular cation radicals have been obtained earlier in the author’s laboratory for diphenylamine, N-methyldiphenylamine, N , N’-dimethyl-p-phenylenediamine (77a)and also for benzidine (27), when adsorbed from their vapors on the natural alumosilicate, bentonite. However, in these instances the degassing and vacuum treatment of the adsorbent could not be pushed so far, as for silica-alumina. Therefore doubt might arise whether the coloration should not be ascribed to traces of active oxygen (cf. Section V, a). The 680 mp band of the diphenylamine cation-radical (650 on silicaalumina gel) has also been observed, when its vapor was adsorbed on a sublimed film of BiC1, in vacuo, but not on those of TlHal, AgC1, PbCl,, ZnCl,, SbCl,. A coloration could be also found upon the adsorption of diphenylamine vapor on CeO, and dehydrated CuSO, powders, but not on Fe(COOH),, A1,0,, MgO, ZnO, TiO,, ZnS, BaSO,, or other sulfates and phosphates (77a).Also, no coloration could be observed when Fe3+, Ce4+,and Cu2+cations were introduced into silica gel. Okuda (48a)observed for adsorbed p-dimethylphenylenediamine in the presence of oxygen the formation of a stable oxidation product with a band at 440 and, besides, a band at 705 mp which is equally present on silica gel and does change reversibly with 0, pressure. This latter band is explained by him as a charge transfer transition between the diamine and 0,. For diphenylamine the first absorption band at 280 mp (in the vapor and in hexane) is observed to be shifted to 300 mp when adsorbed on silica-aluminagel (77a).These relatively small bathochromic shifts point to the presence of physically adsorbed molecules. However, for the adsorbed state there appears also a subsidiary band a t 370 for diphenylamine (77a)and at 410 mp for benzidine (27),which point at other kinds of interaction with the surface of both silica gel and silica-alumina gel.? A similar absorption band in the visible has been observed by Okuda and Tachibana (79), when degassed silica-alumina was immersed as a slurry into a solution of p-phenylenediamine in cyclohexane. The band at 470 of the cation-radical (H,N-Ph-NH,)? appeared, accompanied t This band has been tentatively ascribed to a perturbation of the spin-orbital coupling which allows the normally forbidden ground singlet-triplet electronic transition in the molecule to appear in the absorption spectrum. There are also additional subsidiary bands in the visible at about 500 mp for diphenylamine, which can be obaerved also on silica gel, which origin remains to be explained (77a).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

261

by a single EPR line. A stronger band at 324 mp was that of the physically adsorbed diamine molecu1es.t Similarly to diphenylamine, which has been studied under conditions of high vacuum vapor adsorption (77a),triphenylamine displays the band at 660 mp of the cation radical Ph,Nt, when adsorbed on silica-alumina platelets immersed in an isooctane solution (8Oa). The colorations produced by aromatic amines and diamines on clays suspended in liquids (81)cannot be unambiguously attributed to cation radicals formed by interaction with the active sites of the surface, as no thoroughly preliminary degassing could be achieved under these conditions, Moreover, the view has been advanced (82) that these colorations are produced by the reaction of the adsorbed molecules with atomic oxygen, known to be released from the surface of silica gel and from all SiO, containing minerals, on dehydration at about 150°C. Such an alternative explanation of the formation of molecular cationradicals on the surface of silicates cannot concern the origin of the above described visible spectra, which were just absent on silica gel and were besides observed even at room temperature for the sufficiently volatile N-methyldiphenylamine under high vacuum conditions on carefully degassed adsorbents. Despite such reservation a large amount of work has lately been done by Hasegawa on the spectra of acid clays suspended in benzene solutions of the tertiary aromatic amines and benzidines, the adsorbent acquiring a strong coloration (83, 84). The observed bands are mostly assigned to the positive ion radical, produced by electron abstraction from the amine on the surface of the clay, e.g., that of triphenylamine, Ph,N+ , or tri-p-tolylamine (CH,C,H,) , N t . This interpretation could rely on the known spectra of such semiquinones, produced either by chemical oxidation (77), or by photooxidation in rigid media (76), referred to above. However, there were formed on the surface of the clays also quinoid stable compounds, produced evidently by reactions with oxygen, not removed from the clay and the solution.

B. CARBONIUMIONSFROM PHENYLALKANES

It has been observed a t an early date by Weitz and co-workers (85) that benzene or chloroform solutions of the colorless triphenylmethane

t The amount of the protonic sites on the sample surface has been assessed independently by adsorbing an acid indicator, benzeneazodiphenylamine, from the solution, which on proton addition experiences a shift of the absorption band from 440 to 640 mp. It was found that the electron acceptor (Lewis acid) sites increase upon thermal treatment of the catalyst in proportion with the increase of the total acid sites.

262

A. TERENIN

halides Ph,C-Hal, produce a characteristic surface coloration on dried silica gel, or on A1,0,, introduced into the solution. This coloration has been explained as belonging to the carbonium triphenylmethane ion, Ph,C+, since it coincided with that in the salts Ph,C+X-, where X is NO,, ClO,, etc., and in the complexes such as Ar,C+.AlCl;, Ar,C+. ZnCl,, etc. The necessity of a direct contact with the surface was inferred from the fact that ethanol, acetone, and traces of water, which are generally more tightly adsorbed, eliminated the coloration when added to the solution in which the adsorbent was suspended. KC1, MgO, and CaCO, powders, as adsorbents, did not produce similar effects, but grinding with NaCl or K,SO, unexpectedly did. Such an ionization of the easily split polar covalent bond Ar,C-X upon adsorption, does not represent a phenomenon peculiar for the latter. It could be observed with the same triphenylmethyl halides when dissolved in liquid SO, and in other solvents with high dielectric constants. The absence of any marked adsorbent specificity may presumably be masked by the solvent permeating the surface. These results can only mean that the adsorbents considered played the role of only a polar environment for the adsorbed molecule, similar to that of an ionizing solvent. Later Kortum and co-workers (62) have produced the double peaked band of the Ph,C+ cation by simply grinding desiccated BaSO, with admixed Ph,C-C1 in dry air, whereas MgO was inactive. The produced Ph,C+ cation is adsorbed on the surface, as can be surmised from the small red shift of its spectrum compared to Ph,C-C1 dissolved in

H,S04. In contrast t o the results described previously, the high specificity of the surface of the silica-alumina cracking catalyst compared to that of the silica gel has been amply demonstrated by the adsorption spectra of triphenylmethane and arylethylene vapors, using the high vacuum technique (86-89). Leftin (86) using thin platelets of the adsorbents and working under high vacuum conditions established that the first ultraviolet band in the region 280-250 mp of triphenylmethane, Ph,C-H, adsorbed from the vapor on silica gel has about the same position and the same vibronic structure as that in an ethanol solution, which together points to a purely physical adsorption. I n contrast to silica gel, the adsorption of the vapor on silica-alumina catalyst gives rise to a coloration and to the appearance of a strong visible band in the region 432-404 mp, with a characteristic double peak (Fig. 14) (886). The comparison with the known spectra of the

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

263

radical, Ph,C., the cation, Ph,C+, and the anion, Ph,C-, obtained in solutions, left no doubt that on silica-alumina, triphenylmethane is chemisorbed as a carbonium ion Ph,C+. The explanation given by Leftin for the formation of the species Ph,C+, is that a splitting of the covalent bond in the adsorbed triphenylmethane has taken place with the formation of the negative, hydride ion, viz., Ph,C-H -+ Ph,C+, H-. Another often-recurring explanation was that active oxygen present on the surface should react with the labile H in triphenylmethane. But this alternative has been disproved by a preliminary treatment of the catalyst with hydrogen at 5OO0C,which was expected to eliminate any chemisorbed oxygen. This preliminary treatment did not affect the formation of the carbonium spectrum. The spectrum of the Ph,C+ ion can easily be obtained also from Ph,C-OH adsorbed on silica gel under the action of additionally adsorbed HF vapor. However, for the adsorbed Ph,C-H this treatment is insufficient, and the adsorption of BF, is required (88b). An infrared vibration band at 7.32 p, which appears on the dehydrated, pressed silica-alumina catalyst, soaked with liquid triphenylcarbinol, Ph,COH, has been assigned to the Ph,C+ cation, the adsorbent acquiring a yellow coloration (90). Of other arylalkanes, there is an indication by Leftin and Hall (88c), that 1, I-diphenylethane vapor, Ph,HC-CH,, chemisorbed on silica-

mP

FIQ.14. Triphenylmethane vapor adsorbed in. vacuo. Curve 1: on a silica gel platelet; curve 2: on a silica-alumina catalyst platelet. Transmission spectra, from Leftin and Hall (88b).

264

A. TERENIN TERENIN

alumina at lOO"C, displays a band at 423 mp, assigned to the methyl-

+

diphenyl carbonium ion, Ph,C-CH3. It is assumed to be formed by the same abstraction process of a H- ion from the parent hydrocarbon. The spectrum of this carbonium ion is known from respective active solvents (91). C. CARBONIUMIONS IONS FROM PHENYLALKENES PHENYLALKENES According to Leftin and Hall (82%)and Webb (92), a band appears at 423 mp, when 1,l-diphenylethylene, Ph,C=CH,, vapor is adsorbed under high vacuum on a dehydrated silica-alumina catalyst but not on silica gel: Fig. 15. It was known before (91)that a similar band at 434 m p is obtained when the diphenylethylene is dissolved in concentrated H2S04(or the strong acid H,OBF,), in which a proton should be added to the ethyl-

+

enic double bond, viz., Ph,C=CH, + H+ --f Ph,C-CH,, the methyldiphenyl carbonium ion just mentioned above.

producing

1.0

u

Q)

C 0

n

5 0.5

n

a

0-

I

I

mCL FIU.16. 1,l-Diphenylethylene vapor adsorbed in wacuo. Cume 1 : on dry silicaalumina catalyst; curwe 2: on a hydrated silica-alumina. Transmission spectra, from Webb (92). Band 1 is that of the carbonium ion, band 2 is that of the cation radical.

Long before the experiments described, Evans (91) established that acid clays, like floridine, immersed in paraffin oil with dissolved 1,ldiphenylethylene showed a surface coloration with an absorption band at430-440mp.

ELECTRONIC SPECTROSCOPY O F ADSORBED GAS MOLECULES

265

Further results have been obtained by Leftin and Hall (88c) with cumene CH,

1

Ph-G-H

bH.¶

and a-methylstyrene, PhC= CH,

I

CH,

The vapors, adsorbed in vacuo on silica-alumina catalyst, displayed the same strong band at 393 mp, belonging to the carbonium ion: Ph

\+C-CH, /

H&

This band is likewise observed for a solution of a-methylstyrene in concentrated H,SO,. A dual possibility of formation is thus manifested: either the abstraction of a H- ion (cumene) or the addition of a proton, or Lewis acid (a-methylstyrene). On silica gel the 423 band, however, is made to appear (together with a more intense 595 mp band) by admission of anhydrous HF, or BF,, to the adsorbed diphenylethylene. This result serves to show the spectral indicator property of this diphenylalkene, demonstrating the strongly acidic character of the silica-alumina surface. We should be aware, however, that the alternative exists, that the carbonium 423 mp band is produced in 1,l-diphenylethylene not by proton addition, but by interaction with a strong electrophilic site, with the formation of an adduct of the type,

+

Ph2C-CHB

I 0

On such a concept, the adsorbed olefin should acquire the eIectronic configuration of a trivalent positive C atom in the Ph,C+ group, which was presumed to possess the spectrum of a carbonium ions, situated in the range of 400 mp. More exactly, it has been admitted (92) that the electronic configuration produced in the olefin by a coordinative valency bond with the electron-pair acceptor (Lewis)site in silica-alumina, viz., PhaC+-CH, J -Si-0--Al-0-Si-

bI

(420 mp band)

266

A. TERENIN

should have a spectrum identical that of a “classical” protonic form of

+

the carbonium ion, e.g., Ph,C-CH3.

D. CATION-RADICALS FROM PHENYLALKENES The most spectacular effect with the phenylolefin vapors, when adsorbed on silica-alumina catalysts, is the appearance of intense absorption bands at the longer wavelengths, 500-600 mp, in addition to the ones, described in Section V, C belonging to carbonium ions (8689). In the caae of Ph,C=CH, such a band, situated at 607 mp, confers to the surface of the adsorbent a blue coloration [Fig. 16 (band 2)]. There existed for some time a conflicting disagreement on the nature of this species (8&, 92), which has been resolved only recently. It was known before that Ph,C =CH,, dissolved in concentrated H,S04, yielded only the carbonium 423 mp band, whereas in H,S04 CH,COOH mixtures, and in a benzene solution with CC1,COOH added, both 423 and 607 mp appear (93). I n close analogy with the behavior of polynuclear aromatic hydrocarbons on silica-alumina catalysts (Section V, F), it was assumed that this band is that of the cation radical, (Ph,C=CH,)f , which is produced by electron abstraction from the olefin molecule. Additional water vapor adsorption reversibly eliminates the 423 band of the carbonium ion without any effect on the 607 mp one (88c, 92) (Fig. 15). This proves that these two bands belong to different species and are possibly formed on different sites. It is significant that the 600 mp band is equally formed on contact of Yh,C=CH, with sublimated in wacuo anhydrous AlCI,, in which no protons could be presumed, but only a strong electrophilic activity (92). To explain the prominent low frequency bands of the adsorbed olefins in the range of 600 mp, the assumption has been made by several authors (91, 92), that this is a charge-transfer band, the photon absorbed transferring an electron from the olefin to the electrophilic site8 (protonic, or Lewis acid). I n our notations this process can be represented by the scheme: + hv + P h 2 k C H *+Ph,C-CHx -----+ Ph&,CH2

+

S

S-

(excited state)

- heat

S

The excited state reverts t o the ground one with dissipation of the electronic energy into heat. The location of the positive hole left in the structure of the adsorbed olefin is left unspecified. Although absorption bands produced by such electron interchange with the adsorbent are of great interest and are to be sought in suitable

ELECTRONIC SPECTROSCOPY OP ADSORBED GAS MOLECULES

267

systems, nevertheless in the case considered here, strong arguments do arise against such an assignment, which have been advanced in full by Leftin and Hall (88b,c). I n particular, the high ionization potential of the phenylolefins (9.0 ev for diphenylethylene, 8.8 ev for styrene,t about 8.0 ev for stilbene) should require a much stronger electron acceptor to displace the charge-transfer band from the ultraviolet range, where it generally occurs, to the red wavelengths. The two alternatives relative to the interpretation of the 600 band have been resolved in favor of the olefin cation-radical (Ph,C=CH,)’, when recently, after many attempts, an EPR signal has been detected for chemisorbed diphenylethylene (89). The depressing action of additionally adsorbed oxygen disproved any suspicion on its active role in producing the signal. On a silica-alumina catalyst “poisoned” by the exchange of the protonic sites for Na+ the 420 band is eliminated, but the 330 mp one remains. This definitely disproved the assignment of the latter to the carbonium ion (92). It is more likely that just this band does belong to the charge-transfer optical transition sought for. I n fact it is situated in the same spectral range where charge-transfer bands of a variety of intermolecular and rr-complexes are known to lie. Identical bands at about 300 mp, which have been observed for the simplest alkylolefines,as butene-2, H,C-CH =CH-CH,, and ethylene, H,C=CH, (92),cannot have the same origin, since the ionization potentials of these molecules, are of the order of 10 ev (94),which is prohibitive for such an assignment. Notwithstanding this, the most puzzling feature are the strong and relatively narrow absorption bands, which appear for butene-2 and ethylene, adsorbed on silica-alumina, or dissolved in H,SO,. They are situated at 380, 460, and even at 550 mp, i.e., approximately in the same positions as those for the phenylolefins (92). There is no ground from the electronic structure, values of the polarizability, the basicity, and the ionization potentials, to expect such a close similarity in behavior and in spectra. Insufficient attention has been given to the possibility of a deeper catalytic transformation for these simplest alkylolefins, and therefore the assignments of the 300 bands to depolymerized carbonium ions and of those at 380 mp to charge-transfer surface complexes are inconclusive. The spectral chemisorption studies described above have been carried out at room temperature. Tachibana and Okuda (95) succeeded in observing a band at 335 mp of an unstable intermediate during the dealkylation cracking of cumene on silica-alumina at 15OOC. The band t Styrene, PhCH=CH,, &160 exhibits bands at 436 and 016 rnp in H,SO, and in H,SO,

+ CH,COOH,

respectively.

268

A. TERENIN

at 430 mp, shown by Leftin and Hall to belong to the stable dimethylphenyl-carbonium ion, Ph(CH3) 2C+ (Section V, B), was also present. The 430 band has been ascribed by the authors to the adsorption of cumene on the electrophilic sites, the 335 mp one to that on the protonated sites of the catalyst. In fact, cumene dissolved in concentrated H2S0, displays a similar band at 327.6 mp, where a proton adduct is formed. These findings support the dealkylation mechanism put forward by Johnson and Melik (96),who postulated a protonated cumene as intermediate. The ultraviolet spectra proved to be more valuable in this respect, than the infrared ones, as Tachibana and Okuda could not obtain any definite conclusion from these latter.

E. POSITIVE IONS OF ADSORBED BENZENE Some casual observations have been reported on the visible ooloration, when benzene vapor was brought into contact with degassed silica-alumina (97, 98). Two absorption maxima at 440 and 350 were observed, which exactly correspond to the bands at 420 and 300 mp of benzene, dissolved in the mixture HFSBF,, and assigned to the C,H, .H+ cation (99). This interpretation is corroborated by quantummechanical calculations, which yield transitions at about 400 and 320 mp for such a benzenium cation (100).The bands of the adsorbed benzenium cations could be removed by evacuation, in accordance with the weak basic properties of benzene. The stronger bases as NH,, or dioxane, displaced such chemisorbed benzene molecules (97). Benzene vapor, adsorbed on silica-alumina at 20°C displays a feeble coloration with a spectrum, shown in Fig. 16, curve 1. When silicaalumina with adsorbed benzene has been irradiated by the full ultraviolet light of a high pressure mercury lamp, there appeared a yellowgreen coloration, revealing absorption bands at 460 and 560 mp (Fig. 16, curve 2). The first band undoubtedly belongs to the C,H, * H+ cation, as mentioned before. The second one is to be assigned to the cation radical C,H,+ of benzene. I n fact, theoretical calculations yield for the electronic excitation energy of this cation 2.5 ev (495 mp), according to Pariser (102) and 2.28 ev (540 mp), according to Daudel (103).Hojtink (104)observed a band at 550 mp for C,H, dissolved in a strongly oxidizing solution. I n accordance with this interpretation the photochemically produced coloration of adsorbed benzene, gives rise to a single EPR line of an univalent radical.? t A blurring of the benzene absorption spectrum and a large bathochromic shift (about 7000 cm-1) in a CCl, solution, containing SnCI,, has been reported earlier (105). Thcre seema no connection between this finding and the benzenium cation or benzene cation-radial bands.

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

269

3’. CARBONIUMIONSAND CATIONRADICALS FROM

ADSORBED POLYACENES Coloration and absorption bands have been early noticed in the author’s laboratory, when napthalene and anthracene vapor were adsorbed under high vacuum conditions on to a silica-alumina catalyst. They had been ascribed at that time to an oxidation of the adsorbed hydrocarbons by traces of oxygen in the adsorbent. Later study disproved this preliminary interpretation. Meanwhile several authors reported on the appearance of visible colorations and of an EPR spectrum, when anthracene and perylene were adsorbed from evacuated solutions on to a degassed silica-alumina catalyst (106-108). The EPR findings gave indication that adsorbed radicals were involved, similar to those found in strongly oxidizing acid solutions, where the anthracene or perylene cation radicals were expected to be formed (109). The spectrum of the coloration of silica-alumina, when immersed into a decahydronapthalene solution of anthracene has been measured by Roberts, et al. (106), who found intense visible bands at 420 and 750,

Q

0

I

I

I

I

900

700

500

300

w FIG.16. Benzene vapor adsorbed at 20°C in vucuo. Curve 1:on silica-alumina(75:25); curve 2: after ultraviolet irradiation (10 hr); curve 3: on chromatographic alumina after ultraviolet irradiation (10 hr). Diffuse reflection spectra, from Barachevsky and Terenin (101).

270

A. TERENIN

and feeble ones at 658 and 640 mp. All the observed bands have been ascribed by the authors to the protonieed carbonium ion, anthracene H+. We have recorded the absorption bands and EPR spectrum which appear on contact of gaseous napthalene, anthracene, perylene, and tetracene under vacuum with the degassed surface of silica-alumina gels of different compositions (78,78a, 110). A similar research with ) evacuconcordant results, has been recently reported by Hall ( 8 0 ~for ated anthracene, perylene, pyrene, and 3,4-benzopyrene solutions in isooctane in which the adsorbent was immersed. The pink coloration, produced on silica-alumina in the presence of naphthalene vapor at 20"C, is due to a broad band with sharp maxima

-

500

300

mP

FIO.17. Anthracene vapor adsorbed in WQCUO at low coverage. Curve 1: on silicic acid; curw 2: on silica-alumina (76:25); eurwe 3: perylene vapor adsorbed on silica-alumina (76:26). Diffuse reflection spectra, from Barachevsky et al. ( I I O ) , and Terenin et al. (78, 78a). Anthracene 720 band belongs to the cation-radical that at 420 mp to the oarbonium ion. Perylene 634 band with those to the longer wavelengths, belong to the cation radical.

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

27 1

at 570 and 440 mp (78). No coloration appeared on silica gel. According to several authors (111,112), the carbonium ion, naphthalene .H+, observed in the strongly acid medium HF+BF, exhibits an absorption maximum at about 400 mp, i.e., in the proximity of the second peak observed on silica-alumina. The well-known spectrum of the anion, naphthalene; (113),which on theory must be identical with that of the cation radical, naphthalene’, does possess, however, a band a t 770, far from that at 570 mp, observed by us on silica-alumina. The structured band below 380 mp (Fig. 17) does belong to physically adsorbed anthracene molecules. . The surface species producing the visible bands at 720 and 420 mp? (Fig. 17), can easily be identified. The 420 band is in the same range as that at 420-440 observed for anthracene dissolved in strong acids, and ascribed to the carbonium ion AH+ of anthracene (111, 112) (letter A stands for anthracene),I n accordance with this assignment the 420 band is not present when anthracene vapor is adsorbed on “poisoned” silicaalumina, in which the surface protons are replaced by Na+ cations. The prominent 720 band has about the same position as that of the positive anthracene ion radical A’, obtained in strongly oxidizing solutions (109,113). It is accompanied there by a strong EPR spectrum (g=2.003). Similarly during the slow adsorption of the anthracene vapor (5OOC) at the very initial stage, when the surface coverage is low and the coloration is just discernable, a strong EPR spectrum is observed with hyperfine splitting (Fig. 18), which is blurred as the adsorption of the vapor is proceeding. Simultaneously, the absorption bands are also broadened as a consequence of the mutual disturbance and electron exchange between the ionized surface species, and shifted. The cation radicals of the polyacenes are evidently produced at the strongly electrophilic Lewis sites. We have established that, a t variance to the 420 band, the 720 mp one and the EPR spectrum remain, for an adsorbent, in which the available protons have been exchanged for Na+ cations. The assignment given above is corroborated by the observation of the 420 band on a purely protonic organic adsorbent, a strongly acidic exchange resin. On the other hand, the 720 band appears on contact of anthracene vapor with a AlCI, film, sublimed in vmuo, a strongly electrophilic agent (78,78a).$ t 720 and -400 according to Hall (80a). $ It has been shown by Rooney and Pink (114) that fresh dry AlCl,, known as coordination agent, has also electrophilic univalent oxidation properties. When brought in contact with anthracene, chrysene, and perylene in solutions, an EPR spectrum with hyperhe structure appears, characteristic of the cation radicals of these hydrocarbons.

272

A. TERENIN

The origin of the small 660 absorption band of adsorbed anthracene vapor (Fig. 17, curve 2), found at 640 (106) and at 665 (80a) for the adsorption from solutions, is uncertain. We ascribed it to a rr-coordinative bonding of anthracene with Al sites of the surface. This peak is the only one, appearing in the visible, when anthracene vapor is adsorbed on a y-Al,O, powder, or on an alumina gel, and is not accompanied by g = 2 0038

FIU.18. EPR spectrum of the anthracene cation radical, formed by adsorption of anthracene vapor in wucw on silica-alumina (95:5) at low coverage. According to Karakchiev, Barauhevsky and Holmogorov.

an EPR signal. It also remains after the proton exchange for Na+. Hall (80u) tentatively ascribes the 666 peak to a dipositive anthracenium ion Aa+,the spectrum of which has been previously computed (115). The existence of strong electrophilic sites on the silica-alumina catalyst is strikingly confirmed by the spectrum of adsorbed perylene vapor (Fig. 17). The sharp absorption peakf a t 634 is the same that has been observed at 580 mp for perylene in a strong oxidant (concentrated H,S04), and identified with the cation radical, perylene: (109,116). A strong EPR spectrum with a pattern of nine components, characteristic for this ion radical (116, 117), has been observed by us (78, 78u) for the adsorbed perylene vapor, in accordance with the EPR spectrum of perylene adsorbed on silica-alumina from solutions (107,108). t 646 adsorbed from isoctane (80a).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

273

Similar maxima at 680, 740, and 845 mp in the absorption spectrum of adsorbed teracene vapor are evidently due to the presence of the respective cation radical (2'8). According to Hall (80a) adsorption on silica-alumina in the presence of isoctane, displays for pyrene bands at 462 and 467, and for 3,4benzopyrene, the carcinogenic hydrocarbon, bands at 520 and 782 mp, corresponding evidently to similar cation radicals and to carbonium cations. It was presumed that these latter are not of the protonated form, but represent a positively charged configuration, covalently bound to an electrophilic (Al) site, like that schematically represented in Section VD for chemisorbed olefins.

G . SURFACE REACTIONS WITH OXYGEN

EPR spectra have been observed by Fog0 (108),together with surface colorations, when either anthracene vapor or perylene from an evacuated benzene solution were adsorbed on silica-alumina, previously carefully degassed in a high vacuum. On the ground that the spin intensity decreased after the catalyst has been reduced in hydrogen, he expressed the suspicion that it was just oxygen traces in the adsorbent, which acted as strong electron acceptors in producing the cation radicals from the adsorbed polyacenes, and consequently also from the phenylated amines and the other compounds. A quantitative assessment, made by Hall (Ma),of the oxygen concentration left in silica-alumina, has actually shown that the amount of 0,,removed by hydrogen reduction, equal to 3.3 x l O l a oxygen atoms per cm2 of the surface, is very close to the surface concentration of the paramagnetic radical species, equal to 2 x 1012/cmain the case of perylene. Moreover, when oxygen is partially removed, the decrease in the number of perylene cations corresponds to one cation per one 0 atom. These results seemed to prove that an electron abstraction from the adsorbate by the surficially held active oxygen was also possible. However, it was likewise shown, that after the removal by hydrogen of all the accessible oxygen, the EPR signal and the formation of cation radicals were decreased, but not entirely eliminated. As suggested by Hall, the reduction treatment might introduce some nonstoichiometry in the surface layer of the adsorbent, depressing the accessibility of the electrophilic Lewis sites, or their number. The most conclusive argument against the assumption of the active role of oxygen, instead of that of surface sites, is that on the silica gel no cation radicals are formed under identical conditions. But it was just

274

A. TERENIN

for the silicates that the presence of active atomic oxygen on the surface has been demonstrated (81,82). On the catalytically active silica-alumina oxygen, or air, reacts with the adsorbed cation radicals, e.g., those of N-dimethyl-p-phenylenediamine, already on staying at room temperature (48a). Irreversible oxygenated products and a presumed diamine * 0, complex are formed. It has been also shown for anthracene vapor, adsorbed on silicaalumina, that the coadsorption of oxygen does produce a continuous absorption spectrum into which the peaks of the cation radical are gradually merged (78). The change from a reversible green coloration to an irreversible brown one after oxygen was admitted to silica-alumina with adsorbed anthracene from heptane, has been reported earlier (106), and ascribed to a catalyzed oxygenation. The eluated product gave a red solution with an absorption maximum at 666 mp. Kortum and Braun (118) found that a ground-up mixture of silicagel or A1,08 with solid anthracene subjected to ultraviolet irradiation in open air displays very rapidly a new absorption band a t 276 mp (in diffusely reflected light) and acquires on further exposure a yellowbrown coloration. In similar mixtures with MgO and KC1 the photooxygenation of anthracene is much slower, which indicates a specificity of silica gel. It could be spectrally shown that anthraquinone is a primary product giving on further exposure 1,4-dihydroanthraquinone (quinizarine), which could be eluted, producing a red solution with an absorption maximum close to that observed on silica-alumina, as just mentioned above. The same strongly colored compounds (quinizarine, chrysazine) have been identified, together with anthraquinone in the final products of insolation of anthracene, adsorbed on active alumina of different previous treatment (119). On zinc oxide only anthraquinone was formed. Naphthalene produced naphthaquinone. Alumina and silica-alumina are not typical oxygenation catalysts; it is significant, therefore, that benzene, naphthalene, and anthracene vapors contacted in the presence of oxygen with vanadium pentoxide, a strong oxygenation catalyst, manifested neither the colorations of the cation-radicals nor the corresponding EPR spectrum. Only univalent dehydrogenated semiquinone seemed to form, producing a narrow EPR line (120).

VI. Spectra of Anion Radicals on Surfaces The formation of negatively charged semiquinone radicals has been first observed with the EPR method in the case of the quinones, adsorb-

ELECTRONIC SPECTROSCOPY OF ADSORBED UAS MOLECULES

276

ed from solutions on alkaline adsorbents, the paramagnetic signal being usually accompanied by a strong surface coloration. Thus, Bijl and associates (121) found that p-benzoquinhydrone (a), adsorbed from dioxane or benzene on to barium hydroxide, Ba(OH),. 8H,O confers on it a strong blue coloration, together with the appearance of a narrow EPR line (g=2.003, d H = 1 5 oe) without hyperfine structure, provided the hydroxide is hydrated. On anhydrous Ba(OH), no coloration or signal was detected, but it appeared on addition of water to the solution. Moreover p-benzoquinone or hydroquinone separately did not produce any such effect. This was ascribed to the loss of the two protons, held in common in the quinhydrone (a) by the two nuclei, and the splitting of the latter; upon electron exchange (b) into two separate anion radicals (c), viz., H 0

\

0

0

0

0

The necessity of the presence of H,O molecules, has been traced to their dipole moment, easing the proton abstraction from the weak quinhydrone acid (a). The adsorption on the surface should stabilize the two ion radicals (c), which are unstable in alkaline solutions. Similar results were obtained with the addition compounds thymoquinonehydroquinone, phenanthrenequinone-hydroquinone,giving blue-green coloration and likewise EPR single lines. It is important to note that phenanthrenequinone alone, without association with a hydroquinone could produce similar effects, i.e., could evidently form an adsorbed anion. I n a later work along this line (122) it was found from thermal measurements that a pair of the anion radicals can form a dimer a t the Baa+ cation site, since the spin intensity increased on heating, like that of a Na+(p-benzoquinone)- salt. Further, a similar EPR line has been observed (123) for p-benzoquinhydrone adsorbed from ether on A1,0,, likewise on a strongly basic exchange resin, and, moreover, on silicaalumina, which is surprising, owing to its strongly acidic properties. In contradistinction to the quinhydrone (a) the association compound (d) of phenazine with its counterpart dihydrophenazine, similarly held

276

A. TERENIN

by H-bonds (123),splits up on the acidic surfaces of silica-alumina, or strongly acidic exchange resin, with the formation of two halfdehydrogenated (semiquinone)positive radicals (e), viz.,

(4

(el

A seven component hyperfine structured EPR spectrum is observed, assigned to (e). From the absence of a EPR spectrum in the case of the adsorption of (d) on A1,0, it has been inferred that this adsorbent does not possess acidic sites. The absorption spectra of the unstable negative semiquinone radicals like (c) have been obtained in solutions in the presence of oxidizing agents with flow methods (124-126) and during the flash dehydrogenation of hydroquinones (127). In the author's laboratory (63)spectral measurements have been performed of the surface coloration of various adsorbents on to which the vapors of the quinone compounds were adsorbed under high vacuum in absence of any liquid solution. We expected to produce the anion radicals of these compounds, possessing a marked electron affinity, by absorbing the neutral molecules from the gas phase on to the surface of an electron excess semiconductor, like ZnO. In fact, the p-benzoquinone vapor adsorbed at 20°C on the degassed and thermally treated zinc oxide powder (63),gives a coloration and absorption bands at 436 and a t 700 mp,the latter appearing on increased coverage. Simultaneously with the former band a EPR single line (g=2.003, d H = 7 oe) appears. The absorption bands coincide with those observed for the semiquinones (c) in solutions (124-126), being shifted by about 10 mp towards the longer wavelengths. The EPR line decreases with increased surface concentration of the adsorbate, i.e., with the development of the 700 mp band. This should mean that either dimers or more probably association compounds are formed between the anion radical and a neutral adsorbed quinone molecule, the electron exchange between them producing a broadening of the EPR line. Similarly, for chloranil vapor adsorbed on ZnO ( S O O C ) , an absorption band at 452 mp appears coinciding with the band observed during the flow of oxidizing solutions and with the spectrum of the salt Na+(chlorani1)- (125, 126). On increasing the adsorption time, i.e., at higher coverages, a broad absorption band appears with a maximum at 540,

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

277

whereas in oxidizing solutions maxima at 540 and 575 with a shoulder at 620 mp have been observed (125,126). Naphthoquinone vapor adsorbed on ZnO gives a maximum at 445 mp and a single EPR line (g=2.003, A H =7 oe). With anthraquinone vapor, adsorbed on zinc oxide (at looo), a band at 490 has been observed, whereas in solutions the maximum of the band of the semiquinone (f) is situated a t 480 mp: 0-

b (f)

Like p-benzoquinone and chloranil vapors, adsorbed anthraquinone exhibits an EPR single line, when a visible coloration of the adsorbent begins. Thus such a mild electron-acceptor molecules as p-benzoquinone (electron affinity E 0.9 ev) and anthraquinone can, like the strong electron acceptor chloranil (electron affinity = 1.35 ev), abstract an electron from the ZnO, a semiconductor of the n-type. The abstracted electrons come from the local levels below and close to the conduction band, so that at room temperature they can migrate to the electron traps formed on the surface by the adsorbed electron acceptor. The accepting of an electron by the neutral quinone molecule, immediately leads to a redistribution of the valency bonds into the configuration of the anion radical (c).

VII. Radicals from Adsorbed Molecules Diphenylamine vapor adsorbed on bentonite, a natural silica-alumina, subjected to electrodialysis displayed a band at 720 mp in addition to those of the cation radical Ph,NH' (77a). We have assigned it to the radical Ph,N., on the ground that Lewis and Lipkin (76)have obtained such a radical by photodissociation of tetraphenylhydrazine Ph ,NNPh, in a rigid solvent at low temperatures, where a band at 730 mp was noticed. Such a dehydrogenation ability of bentonite is unexpected and it is more probable that traces of oxygen in an active form have been involved since bentonite did not permit a high temperature degassing. An instance of the formation of a neutral radical has been observed by Kortum et al. (62) for hexaphenylethane, Ph,C-CPh,, which when ground up as a liquid, together with dried MgO, and heated up to 80°C

278

A. TERENIN

exhibited the visible band, characteristic of the triphenylmethyl radical Ph,C. (Fig. 19). It must be remarked, however, that the grinding procedure, as mentioned in Section I cannot be equivalent to an adsorption process, since strong electrostatic fields are known t o be produced by friction, and a drastic modification of the surface might occur (128). A t an early date it was observed that photodissociation of adsorbed acetone (129) and biacetyl molecules (15) generated methyl radicals, which reacted with the surface of sublimed Bi or Sb, serving as adsorbents. From the spectral dependence of the metal erosion at the illuminated spot it could be ascertained that the maximum of the active spectral regions was markedly shifted to the red compared t o that for the gas phase photodissociation. This implied a decrease of the required photon energy by an amount equivalent to about 1 ev, which was explained as a compensation effect, produced by the much stronger adsorption of the radicals, primarily formed (Section 11, E). Similar photoerosion experiments have been carried out with CH ,I vapor, adsorbed on Pb, or Sn (130). With the advent of the suprasensitive EPE method of radical detection, it was natural that it should be used to observe and identify the radicals produced by the photodissociation of adsorbed gas molecules. Thus, silica gel with adsorbed methyl iodide subjected to a prolonged irradiation by ultraviolet light at 77'K and transferred to the resonance cavity of a EPR spectrometer displays the pattern, reproduced in Fig. 20a (131),which belongs to adsorbed methyl radicals, *CH, split

v' - 2.0

30,000

40,000

cm-'

FIG. 19. Spectrum of triphenylmethyl radicals, obtained by grinding hexaphenylethane with A1,0,. Curwe 1: at 25°C; curwe 2: at 140°C. Diffuse reflexion speotra. from Kortlim et al. (62).

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

279

by light from CHJ. Actually it has the right component number and magnitude of the splitting, known €or GH, radicals, produced by various means and stabilized in matrices at low temperatures. The intensity distribution between the line components being quite abnormal, this was explained by the loss of a part of rotational degrees of freedom in the adsorbed state. In a second similar experiment (132) the benzyl radical, PhkH, could be detected after‘the photodissociation of PhCH,Cl, adsorbed on silica gel, under experimental conditions similar to the previous case. The EPR spectrum of this radical in the adsorbed state is more complicated (Fig, 20b) consisting of a triplet, with components split additionally into quadruplets. The spectrum could be satisfactorily interpreted by a delocalization of the lone electron of the methylene group, which is partly “injected” into the benzene ring. The surface coverage in both experiments was less than monomolecular. Such photodissociation experiments with the sensitive EPR method have to be substantially improved by spectral efficiency measurements,

50 Oe

FIG.20. (a) EPR spectrum of the methyl radical, obtained after ultraviolet irradiation of methyliodidevapor adsorbed on silica gel ( 1 3 1 ) . (b) EPR spectrum of the benzyl radical, obtained after ultraviolet irradiation of benzylchloride vapor adsorbed on silica gel ( 1 3 2 ) .

280

A. TERENIN

since only this would permit an evaluation of the bathochromic shift of the photodissociation threshold for the adsorbed molecule, as compared with the gaseous state. ACKNOWLEDGXENTS The author would like to express his deep gratitude to his co-workers E. Kotov, V. Barachevsky, G. Lialin, and to his technical assistant S. Boglovskaja for their effective help in bringing this article to completion. REFERENCES

1. de Boer, J. H., 2.physik. Chena. B14, 163 (1931);B15, 281 (1932). 1. de Boer, J. H., I;. Physik. Chem. B18, 49 (1932);de Boer, J. H., and Custers, J. F. H., ibid. B21, 208, 217 (1933). 3. de Boer, J. H.,2. Physik. Chem. B16, 397 (1932);B17, 161 (1932);de Boer, J. H., and Custers, J. F. H., ibid. B25, 226, 238 (1934);de Boer, J. H., and Dippel, C. J., ibid., pp. 399, 408; Custers, J. F. H., and de Boer, J. H., Physica 1, 266 (1934); 8,407(1936). 4. de Boer, J. H., “Electron Emission and Adsorption Phenomena,” Chapt. VIII. Cambridge Univ. Press, London and New York. 5. de Boer, J. H., 2. Elektrochem. 44, 488 (1938). 6. de Boer, J. H., and Houben, G. M. M., Koninkl. Ned. Akad. Wetenschap. Proc. BS4,421 (1951). 7. de Boer, J. H., Advan. Catalysis 8, 79 (1966). 8. de Boer, J. H., Advan. Colloid Sci. 8, 1 (1960). 9. Chilton, D.,and Rabinowitch, E., Z. PhysBk. Chem. Bl9, 107 (1932). 10. Terenin, A., Acta Physiocochim. URSS 1, 407 (1934);Barakan, N. B.,Ruse. J . Phys. Chem. (English Transl.) 9,364 (1937);Kurbatov, L.N., ibid. 14,1049 (1940). 11. Kurbatov, L.N., Ruse.J . Phy8. Chem. (English Trans.!.) 14, 1111 (1940). 12. Golub, S. and Kondratiev, V. N., Physik. 2.Sowjetunion 1, 019 (1932). 13. Boitzova, Z., see references (14). 14. Terenin, A. N., Uch. Zap. Leningr. Cfo8. Uniu. 3 (No.17), 149 (1937);ibid. No. 38 Ser. Fiz. N a u k Part 6. 26 (1938). 15. Terenin, A. N., Russ. J . phy8. Chem. (English TraneZ.) 14, 1362 (1940); V’eatn. Leningr. Univ. Ser. Piz. i Khim. 11, 143 (1963). 16. Terenin, A. N., in “Heterogeneous Catalysis in Chemical Industq,” p. 179. MOSCOW, 1966. 17. Terenin, A. N., in “Surface Chemical Compounds and Adsorption Phenomena,” p. 206. Moscow Univ. Press, MOSCOW, 1967. 18. Terenin, A. N., Probl. Kinetiki i Kataliza. Ak&. Nauk SSSR 10, 214 (1969). 19. Eischens, R. P.,and Pliskin, W. A., Advan. CataZyA 10, 2 (1968). 20. Crawford, V. A,, Quart. Rev. (London) 14, 378 (1960). 21. Yates, D.J. C., Advan. CalaZyaie 12,266 (1960). 22. Shibata, K.,Methods of BiocLm. A d . 7, 77 (1959). 23. Schwab, ct. M., and Schneck, E., Z. physib. Chern. (Frankfurt) 18, 206 (1968); Kortiim, U., and Braun, W., ibid. p. 242;28,362 (1961). 24. Kortum, G., Spectrochim. Acta Suppl. p. 534 (1967);Kortiim, G.,and Schreyer, Angew. C b m . 67, 694 (1966);Kortiim, G., Trans. B‘araday SOC. 68, 1624 (lQ62). 25. Kotov, E.I., Opt. i Spektroslcop’ya 8, 368 (1967).

ELECTRONIC SPECTROSCOPY O F ADSORBED GAS MOLECULES

28 1

26. Kondratjev, V. N., Russ. J. Phya. Chem. (English Transl.) 7, 267 (1937). 27. Kotov, E. I., Opt. i Spektroskopiya 1, 500 (1966); 3, 116 (1957); Kotov, E. I., and Terenin, A. N., Dokl. Akad. Nauk SSSR 124, 866 (1959); Kotov, E. I., Thesis Leningrad University, 1969. 28. Rabinowitch, E., and Wood, W. C., T r a m . Fapaday SOC.32, 547 (1936). 29. Neporent, B. S., and Bakshiev, N. G., Opt. i. Spektroskopiya 5, 634 (1968). 30. Sidorov, A. N., Dokl. Akad. Nauk SSSR 95, 1235 (1964); Rusa. J . Phys. Chem. (English Transl.) 30, 996 (1956); Galkin, G. A., Kisselev, A. V., and Lygin, V. I., ibid. 36, 1764 (1962). 31. Karagounis, G., and Issa, R., Nature 195, 1196 (1962); 2. Elektrochem. 66, 874 (1962). 32. Pershina, E. V., and Rasskin, Sh., Dokl. Akad. Nauk SSSR. 150, 1022 (1963). 33. Sverdlova, 0. V., Opt. iSpektroakopiya 6, 349 (1959); Suppl. 2, p. 31 (1963). 34. Karagounis, G . , andPeter, O., 2.Elektrochem. 61, 827,1094 (1957); 63,1120 (1969). 35. Hirota, K., and Nakai, Y., “Kodzokogaku Toronkai (1969)” (in Japanese) Nippon Kagaku Zasshi 80, 700 (1969). 36. Pavlova, E. N., Dokl. Akad. Nauk SSSR 49, 272 (1945); Thesis, Leningrad University 1945. 37. Okuda, M., Nippon Kagaku Zaashi 82, 1116, 1118 (1961). 38. Ron, A., Folman, M., and Schnepp, O., J . Chem. Phys. 36, 2449 (1962). 39. Kisselev, A. V., in “Surface Activity” ( J . H. Schulman, ed.), Vol. 2, p. 179. Academic Press, 1968. 40. Kisselev, A. V., Quart. Rev. (London)16, 90 (1961); Djigit, 0. M., Kisselev, A. V., and Muttik, G. G., Colloid J. ( U S S R ) (English Trawl.) 23, 663 (1961). 41. Kisselev, A. V., and Poshkus, D. P., Dokl. Akad. Nouk SSSR 120, 834 (1968); Kisselev, A. V., Koutezky, Y., and GiZek, I., ibid. 137, 3 (1961). 42. Jdanov, S. P., and Kisselev, A. V., R w u . J . Phya. C h m . (EngJieh TvuMZ.)31, 2213 ( 1967). 43. Kisselev, A. V., and Lygin, V. I., Colloid J. ( U S S R )(English Tranal.) 23, 674 (1961). 44. Bayliss, N. S., and Hulme, L., Australian J. Chem. 6, 267 (1963). 45. Evans, D. F., Nature 173, 636 (1966). 46. Robin, M., J. Chem. Educ. 33, 526 (1956); Robin, M., and Trueblood, K. N., J. Am. Chem. SOC.79, 6138 (1967). 47. Fialkovskaia, 0. V., Opt. i Spektroakopiya 1, 696 (1956). 47a. FiJkovakaia, 0. V., and Terenin, A. N., Izv. Akad. Nauk SSBR Otd. Khim N a u k No. 3, 226 (1961). 48. Okuda, M., Nippon Kagaku Zasahi 82, 1287 (1961). 48a. Okuda, M., Progr. Rept. Electron. Processes Chem. (in Japanese) No. 4, 89 (1963). 49. Stephenson, H. P., J . Chem. Phys. 22, 1077 (1964). 50. Konigsberger. J., and Vogt, K., Physik. 2. 14, 1269 (1913). 51. Cf. Spencer, G. H., Cross, P. C., and Wiberg, K. B., J. Chem. SOC.85, 1926 (1961). 52. Barachevsky, V. A., and Terenin, A., Opt. ispektroskopiya 16, 967 (1964). 53. Mason, S. F., J. Chem. SOC. pp. 1240, 1263 (1969). 53a. Bartok, W., Lucchesi, P. J., and Suider, N. S., J. Am. Chem. SOC.84, 1842 (1962). 54. Yaroslavsky, N. G., and Terenin, A. N., Dokl. Akad. Nauk. SSSR 66, 886 (1949); Kurbatov, L. N., and Neuimin, H. G., ibid. 68, 341 (1949). 54a. Kobayashi, D., J. C k m . SOC. Japun, Pure Chem. Sect. 80, 1399 (1967). 55. Okuda, M., Nippon Kagaku Zasshi 82. 1121 (1961). 56. Nagakura, S., and Tanaka, J.,J. Chem. Phys. 22, 236 (1964). 56a. Bayliss, N. S.. and McRae, E. G., J . Phys. Chem. 68, 1006 (1964).

282

A. TERENIN

57. Bakshiev, N. G., Opt. i Spektroskopba 13, 43, 192 (1962). 58. McRm, E. G., J. Phys. Chem. 61, 662 (1957). 59. Lippert, E., et aZ., Angew. Chem. 73, 696 (1961). 60. Semba, K., BulC. Chem. Soc. Japan 34, 722 (1961). 61. Cruse, K., and Mittag, R., 2. Elektrochem. 54, 418 (1960). 62. Kortiim, G., Vogel, J., and Braun, W., Angew. Chem. 70, 651 (1968). 63. Lodin, V., Holmogorov, V., and Terenin A., Dokl. Akad. Nauk SSSR, 159, 187 (1964); Pimenov, Y., and Terenin, A., ibid., in press. 64. Benesi, H. A,, J. Am. Chem. SOC.78, 6490 (1966). 65. Okuda, M., and Tachibana, T., Kogyo Kagaku Zasshi 66, 1166 (1963). 66. Weitz, E., and Schmidt, F., Ber. Deut. Chem. Qea. 72, 2099 (1939); Weite, E., Schmidt, F., and Singer, J., 2. Elektrochem. 46, 222 (1940); Weitz, E., ibid. 47, 66 (1941). Kumler, W. D., and Strait, L. A., J. Am. Chem. SOC.66, 2349 (1943). Kortiim, G., 2. Phyaik. Chem. B42, 39 (1939). Thomas, G. L., Ind. Eng. Chem. 41, 2664 (1948). Ballod, A. P., and Topchieva, K. V., Usp. Khim. 20, 161 (1961); Diecussions Faraday SOC.8, 279 (1960). 71. Tamele, M. W., Dkcuaeions Faraday SOC.8, 270 (1960). 72. Oblad, A. cf., Milliken, T. H., and Mills, G. A., Adwan. Catalysis 3, 199 (1961). 73. Terenin, A. N., Izv. Akad. Nauk SSSR Otd. Khim. Nauk p. 69 (1940); dcta Physicochim. URSS 18, 226 (1948). 74. Pfeiffer, P., “Organische Molckiilverbindungen,” Enke, 1927. 75. Vilessov, F. I., Usp. Fiz. Nauk 81 (a), 669 (1963); Terenin, A,, and Villesov. F., Advan. Photochem. 2, 386 (1964). 76. Lewis, G. N., and Lipkin, D., J . Am. Chem. Boc. 64, 2801 (1942);Lewis, G. N., and Bigeleisen, J., ibid. 65, 2424 (1943). 77. Michaolis, L., Sohubert, M., and Granick, S., J. Am. Chem. SOC.61, 1981 (1939); Granick, S., and Michaelis, L., ibid. 62, 2241 (1940). 77a. Sidorova, A. I., and Terenin, A. N., Izw. Akad. Nauk SSSR Otd. Khim. N a u k No. 2, 162 (1950); Sidorova, A. I., Dokl. Akad. Nauk SSSR 72, 327 (1960); R w s . J . Phya. C h m . (English Franal.) 28, 626 (1954); Thesis, Leningrad University, 1962; Barachevsky. V. A., Thesis, Leningrad University, 1966. 78. Terenin, A., Barachevsky, V., Kotov, E., and Holmogorov, V., Spectrochim. Acta 19, 1797 (1963). 78a. Barachevsky, V. A., Kotov, E. I., And Terenin, A. N., Dokl. Akad. Nauk SSSR 143 362 (1962); 144, 378 (1962); Barachevsky, V. A., Holmogorov, V. E., Kotov, E. I., and Terenin, A. N., ibid. 147, 1108 (1962); cf. also ref. (110). 79. Okuda, M., and Tachibana, T., Bull. Chem. SOC. Japan 33, 863 (1960); Okuda, M., Nippon Kagaku Zaashi 82, 1290 (1961). 80. Holmogorov, V. E., and Baranov, E. V., Opt. i Spektroakopya 14, 827 (1963). 80a. Hall, W. K., J. Catalyeie 1, 53 (1962). 81. Hauser, E. A., and Legget, M. B., J . Am. Chem. SOC. 62, 1811 (1940). 82. Hauser, E. A., Le Beau, D. S., and Pevear, P., J . Phy8. Chem. 65, 68 (1961). 83. Hasegawa, H., J. Phys. Chem. 66, 292 (1961); 66, 834 (1962). 84. Hasegawa, H., J . Phya. C h m . 67, 1268 (1963). 85. Weitz, E., and Schmidt, F., Ber. Deut. Chem. &a. 72, 1740, 2099 (1939); Weitz, E., Schmidt, F., and Singer, J., 2. Elektrochna. 46, 222 (1940); Weitz, R., ibid. 47, 66 (1941). 86. Leftin, H. P., J . phya. O h m . 64, 1714 (1960).

67. 68. 69. YO.

ELECTRONIC SPECTROSCOPY OF ADSORBED GAS MOLECULES

283

87. O’Reilly, D.E., Leftin, H. P., and Hall, W. K., of. in ref. (86). 88a. Leftin, H. P., and Hall, W. K., J . Phys. Chem. 64, 382, (1960). 88b. Leftin, H. P., and Hall, W. K., Actes Congr. Intern. Catalyse, 2e Park, 1960 Vol. 1, pp. 1307, 1353 (1961). Editions Technip, Paris. 88,. Leftin, H. P., and Hall, W. K., Phys. Chem. 66, 1457 (1962). 89. Leftin, H. P., Hobson, M. C., and Leigh, J. S., J . Phys. Chem. 66, 1214 (1962). 90. Rubalcava, H., J. Phys. Chem. 63, 1517 (1959). 91. Evans, A. G.,Di8cussions Faraday SOC.8,302 (1950); J. AppZ. Chem. 1, 240 (1951). 92. Webb, A. N., Actes 2e Congr. Intern. Catalyse, Paris, 1960 Vol. 1, p. 1289 (1961). Editions Technip, Paris. 93. Evans, A. G., Jones, P. M. S., and Thomas, J. H., J . Chem. SOC.p. 1824 (1954); p. 104 (1957); Lavrushin, V. F., Zh. Obshch. Khim. 26, 2697 (1954); Grace, J. A., and Symons, M. C . R., J . Chem. Soe. p. 958 (1959). 94. Watanabe, K., J. Quant. Spectry. Radiative Transfer 2, 369 (1962). 95. Tachibana, T., and Okuda, M., Bull. Chem. SOC.Japan 36, 462 (1963); Shokubai (Tokyo) 4, 54 (1962). 96. Johnson, M. F. L., and Melik, J. S., J . Phys. Chem. 65, 1146 (1961). 97. Webb, A. N., Actes 2e Congr. Intern. CataZyse, Paris, 1960 1 , p. 1310 (1961). Editions Technip, Paris. 98. Kohn, H. W., J . Phys. Chem. 66, 1185 (1962). 99. Reid, C., J . A m . Chem. Soc. 76, 3264 (1954). 100. Muller, N., Pickett, L. W., and Mulliken, R. S., J . Am. Chem. SOC.76, 4770 (1954). 101. Barachevsky, V. A., and Terenin, A. N., Opt. iSpektroskopiya 17, 304 (1964). 102. Pariser, R., J . Chem. Phys. 24, 250 (1956). 103. Daudel, R.,private communication-calculations by Leroy and Lefebvre. 104. Hoijtink, G. I., Mol. Phys. 2, 85 (1959). 105. Schmidt, F., Ber. Deut. Chem. Ges. 74, 99 (1941). 106. Roberts, R. M., Barter, C., and Stone, H., J . Phys. Chem. 63, 2077 (1959). 107. Rooney, J. J., and Pink, R. C . , Proc. Chem. SOC.p. 70 (1961); Brower, D. M. Chem. Ind. (London)No. 6, 177 (1961). 108. Fogo, J. K., J . Phys. Chem. 65, 1919 (1961). 109. Aalbergsberg, W. I., Hoijtink, G. J., Mackor, E . L., and Weijland, W. P., J . Chem. SOC.pp. 3005, 3049 (1959). 110. Barachevsky, V. A., Holmogorov, V. E., and Terenin, A. N., Dokl. Akad. Nauk SSSR 152, 1143 (1963). 111. Gold, V., and Tye, F. D., J . Chem. SOC.pp. 2172 (1952). 112. Dallinga, G., Mackor, E. L., and Verrijn Stuart, A. A., Mol. Phys. 1, 123 (1958); Kitova, A. I., and Varshavsky, J. M., Dokl. Akad. Nauk SSSR 135, 1395 (1960). 113. Kon, H., and Blois, M. S., J . Chem. Phys.28, 743 (1958). 114. Rooney, J. J., and Pink, R. C., Proc. Chem. SOC.p. 142 (1961). 115. Balk, P., de Bruijn, F., and Hoijtink, G. J., MoZ. Phys. 1 , 151 (1958). 116. Hoijtink, G. J., and Weijland, W. P., Rec. Traw. Chim. 76, 836 (1957). 117. Yokozawa, Y., and Miyashita, I., J . Chem. Phys. 25, 796 (1956); Weissman, S. I., de Boer, E., and Conrade, J. J., ibid. 26, 963 (1957); de Boer, E., and Weissman, S. I.,J . Am. Chem. SOC.80,4549 (1958); Carrington, A., Dravnieks, F., and Symons, M. C . R., J . Chem. SOC.p. 947 (1959). 118. Kortiim, G.,and Braun, W., Ann. Chem. 632, 104 (1960). 119. Voyatzakis, E., Jannokoudakis, D., Dorfmiiller, T., and Sipitanos, C., Compt. Rend. 249, 1756 (1959); 250, 112 (1960); Voyatzakis, E., and Dorfmiiller, T., ibid. 251, 2696 (1960).

284

A. TERENIN

120. Hirota, K . , and Kuwata, K., BulL Chem. SOC. Japan 86, 229 (1963);Hirota. K., Kageyamct, Y.,and Kuwata, K., ibicl. p. 875. 121. Bijl, D., Kainer, H., and Rose-Innes, A. C., Nature 174, 830 (1954). 122. Golubev, V. V.,Boyarchuk, Y. M., and Evdokimov, V. E., Ruaa. J . Phye. Chem. (English Transl.) 34, 696 (1960). 123. Gragerov, I. P., Ponomarohuk, M. P., Strelko,V, V., Ganiuk, L. N., and Viesotsky, Z. Z.,Dokl. Akad. Nauk SSSR 147,867 (1962). 124. Harada, Y . , and Mattaunagct, Y., Bull. Chem. SOC Japan 34, 686 (1961). 125. Kainer, H., and Uberle, A., Chem. Ber. 88, 1147 (1066). 126. Eigen, M.,and Matthies, P., Chem. Ber. 94, 3309 (1961). 127. Bridge, N.,and Porter, a., Proc. Roy.8 0 0 . A244, 274 (1968). 128. Benson, R.E., and Castle, J. E., J . Phyya. Chem, 62,840 (1958). 129. Kudriavzeva, W.,and Prilejaewa, N., Acta Phyaicochim. URSS 8, 211 (1938). 130. White, P., €'roc. Chem. HOC. p. 337 (1961). 131. Parijsky, G.B., Zhidomirov, a. M., and Kazansky, V. B. Zh. Strukt. Khim. 4,364

(1963). 132. Tolkaohev, V. A., Tohheidze, I. I., and Buben, N. Y., Dokl. Akad. Nauk. SSSR

147, 643 (1962).

T h e Catalysis of Isotopic Exchange in Molecular Oxygen G. K. BORESKOV Inetitut Kataliza, Novosibirsk, U.S.S. R. I. Kinetics of Isotopic Exchange in Molecular Oxygen. ....................... 286 A. Methods of Investigation ........................................... 286 B. Kinetics of Exchange in Molecular Oxygen in the Absence of Isotopic Exchange with Catalyst ............................................ 286 C. Kinetics of Exchange in Molecular Oxygen with the Simultaneous Isotopic ExchangewithCatalystOxygen ..................................... 288 11. Some Experimental Data Relating Isotopic Exchange in Molecular Oxygen on Solid Catalysts .................................................... 293 A. The Catalytic Activity of Oxides. .................................... 293 B. The Principal Regularities of the Exchange in Molecular Oxygen on the Oxides with the Equilibrium Content of Oxygen. ....................... 306 C. The Relation of Catalytic Activity with Respect t o the Exchange in Mol310 ecular Oxygen t o Some Other Properties of Oxides. ..................... D. The Catalytic Activity of Oxides with the Nonequilibrium Content of Oxygen .......................................................... 319 E. Catalytic Activity of Metals with Respect t o the Kxchange Reaction of Molecular Oxygen ................................................ 322 F. Some General Conclusions Relating to the Catalysis of the Isotopic Ex326 changeofMolecularOxygen ........................................ G. The Relation of Catalytic Activity of Oxides with Respect to the Oxygen Exchange to the Activity with Respect to Some Other Oxidation Reactions 327 H. The Participation of the Oxygen of Oxide Catalysts in the Oxidation Reactions ........................................................... 333 111. Conclusion .......................................................... 337 References ........................................................... 338

The fact that investigation of isotopic exchange in molecular hydrogen has been the object of a number of experimental works has played an important role in the elucidation of the mechanism of hydrogenation catalysis. The reaction of isotopic exchange in molecular oxygen oy + 01,s + 20'80'8 (1) is not of lesser importance for the study of mechanism of catalytic oxidation reactions. The study of this reaction provides the information which is important for a judgment about the nature of oxygen bonds on the surface of oxida286

286

a. K. BORESKOV

tion catalysts. Some interesting information may be obtained in the simultaneous investigation of reaction ( 1 ) and the isotopic exchange with oxygen of solid catalysts (oxides or metals containing sorbed oxygen). Reaction (1)is the simplest one in which molecular oxygen participates. This reaction is also very convenient as a pattern reaction for the study of regularities of selection of catalysts because it proceeds without any change in the chemical composition of the reaction system. Nevertheless, there are but few works devoted t o the study of the reaction of isotopic exchange of molecular oxygen. This scarcity was probably due to the earlier difficulties in obtaining oxygen with a sufficiently high content of isotope OIE. It was not until 1954 that Winter published his first results relating the study of reaction ( 1 ) on magnesium oxide and zinc oxide ( I ) , and later on some other oxides (2). In past years a series of works devoted to the study of the given reaction was published by the Soviet authors (3-9). In the present paper an attempt is made to generalize the data obtained thus far and to outline the possible relations of the activity of various catalysts with respect to reaction ( 1 ) as well as to some other reactions proceeding with the participation of molecular oxygen.

1. Kinetics of Isotopic Exchange in Molecular Oxygen A. METHODS

OF

INVESTIGATION

The observations of reaction (1) are made with the aid of a mass spectrometer according to the change of intensity ratio of peaks corresponding to masses 32(0ia), 34(01s01a) and 36(0i8). Winter (20) was working with a static system having a continuous selection of a very small part of the reaction mixture flow through a thin capillary into the ionization chamber of the mass spectrometer. In a number of investigations the selection of gas samples for isotopic analysis was carried out in regular periods of time using ampulas which could be separated from the installation and transferred to the mass spectrometer. In order to eliminate the diffusional hindering possible at high rates of exchange, it is reasonable to carry out the investigations in a circulation system (5).

B. KINETICSOF EXCHANQE IN MOLECULAROXYGEN IN THE ABSENCEOF ISOTOPIC EXCHANGE WITH CATALYST

If the oxygen of a catalyst has the same content of 0 l 8 as the reaction mixture, the atomic isotopic composition of the reaction mixture re-

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

287

mains unaffected in the process of exchange while the relative content of molecular species with masses 32, 34 and 36 changes. The variation of the rate of this process with the depth of conversion is described, as in the case of any reaction of isotopic exchange, by the first-order equation regardless of the true mechanism of reaction (11). Let 2 be the overall number of oxygen molecules of arbitrary isotopic composition which has already exchanged atoms per time/unit on the surface of a catalyst (molecules/cm2-sec). I n order to form a molecule with mass 34(016018)molecules with masses 0;' and 0;' should participate in the exchange. The probability of such a phenomena to the accuracy of isotopic effects is equal to 2 c3

,c

36

Half of the exchanges with the participation of these two molecules 0 1 6 0 1 8 lead to the disappearance of these molecules, so that the probability of such a n exchange is

Here C32, C,,, C,, are the fractions of oxygen molecules with corresponding masses. The change of a number of molecules with mass 34 is Ng

dC34 =

d7

zs(2c32c3, -

where N g is an overall number of oxygen molecules in the system; S is surface of a catalyst (cma);T is time (see). If CL is the overall fraction of atoms of 0l8in the reaction mixture, then

The equilibrium fraction of molecules 0 1 6 0 1 8 is

c;,

=

2 4 -a)

Then 2

where

S Nfl c;, - Ci4

K = Z and on integrating

1

K =-ln 7

c;, -c,

(3)

288

Q.

K. BORESKOV

where C:* is the original fraction of molecules OIBO1*in reaction mixture. As has been already mentioned, this equation remains valid for any mechanism of exchange. The nature of a mechanism is shown only as a, dependence of the overall number of exchanges 2 on the oxygen pressure Po, and temperature T. C. KINETICSOF EXCHANGE IN MOLECULAR OXYGENWITH THE SIMULTANEOUS ISOTOPIC EXCHANGE WITH CATALYSTOXYGEN The equality condition considered above for the isotopic composition of reaction system and catalyst permits the simple and reliable treatment of experimental data using Eq. ( 6 ) . The fulfillment of this condition, however, requires a considerable amount of time and heavy oxygen for the preliminary carrying of the isotopic catalyst composition up t o the reaction mixture level. Moreover-and this is of prime importance-, the simultaneous measurements of rates of exchange in molecular oxygen as well as the isotopic exchange with catalyst oxygen provide the opportunity for finding the essential details of the mechanism of reaction. The study of reaction (1) with different isotopic oxygen composition of the reaction mixture and catalyst is necessary in many cases, although the estimation of the rate of reaction becomes, in this case, more complicated. The isotopic exchange with oxygen of the catalyst (for instance, oxide) involves (a) the dissociative adsorption of oxygen with the formation of adsorbed atoms or atomic ions, (b)the exchange of these adsorbed atoms or ions with oxygen ions of oxide, and (0) the desorption in form of molecules (12). Let us discuss the following versions: (1) The isotopic exchangein molecular oxygen [reaction (l)]is realized without the participationof catalyst oxygen. In this case,the process may be carried out following the adsorption-desorption mechanism, but the adsorption rate (desorption)is much higher than that of the exchange of adsorbed atoms or ions with catalyst oxygen. The exchange via the fouratom complex formed as a result of adsorption of the two oxygen molecules is not impossible. In both cases the overall content of isotope OI8in the reaction mixture remains constant dor - = 0, dr while the change in concentration of molecules OleO1*may be described by Eq. (4).

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

289

(2) The exchange in molecular oxygen occurs with the participation of a single atom of catalyst oxygen. This is possible if oxygen is adsorbed with the dissociation into atoms or ions. However, the desorption due to the low concentration or due to the insufficient mobility of adsorbed atoms or ions occur predominantly with their recombination into molecules with ions of oxide lattice. 0;;- --f 20; 0;

+

0'6/!Ja

0;

+0 s

--f 0 ' 8 0 &

--f

+ us

0'810s

One of the versions of this mechanism may be the assumption that adsorption of oxygen occurs on the surface anion defects. I n this case, the first oxygen atom fills the defect, while the second migrates along the surface and is desorbed in the recombination with the lattice oxygen.

oy, 0;

+0 s

--f

+ 0;:

0 1 ~ 1 0 8

+ 01e/os4.oleo& + OS

It is also possible that the adsorption of molecular oxygen and its exchange with oxygen of oxide lattice occurs in a single step via threeatom oxygen complex: 018OZ

+ O'y-J,

j.

/o'eo'eo'~/~8o;", + 0'8/rJs j.

The given versions are kinetically indistinguishable. I n all cases do! -= &Kz(o!-as) d7

and

or using relation (2)

Here K , is the number of oxygen molecules exchanging atome according to the mechanism considered in (molecules/cmBsec); as is the content of Ole in the oxygen of the catalyst surface. (3) The exchange in molecular oxygen proceeds with the participation of the two atoms of catalyst oxygen. This condition is realized in the case where the exchange of adsorbed oxygen atoms or ions with catalyst oxygen is carried out more rapidly than the dissociative adsorption (desorption). The isotopic composition of desorbed molecules in this case is

290

Q. K. BORESKOV

equivalent to the isotopic oxygen composition of the catalyst surface. The change of heavy isotop concentration in gas phase may, in this case, be expressed by the equation

- du dr

=

K3(a-us)

while the change of fraction of molecules equation

018018

is expressed by the

5'= K,[2a8(l-as)

- C,,]

(9)

dT

Here K , is a number of oxygen molecules exchanging atoms following the third mechanism. 2as( 1 -as) is an equilibrium fraction of O1eO1a molecules relative to the isotopic composition of the surface layer of oatalyst . If exchange will be realized simultaneously following all three mechanisms, then

where

R

K

= a-

2

+K,

These equations can be conveniently transformed by substituting the value us from (10) into (11) and by introducing the new variable y = 2a(l-a)

- C,,

- C,,

= C,*,

This variable has a physical sense of the motive power of isotopic exchange between oxygen molecules. If there were no isotopic exchange with catalyst oxygen the change of the fraction of the nonsymmetrical oxygen molecules (018018) would be proportional to y. On performing this substitution, Eq. (11 ) may be written as fol10ws:

where 8KS 'p

=

(K,+2Ks)2

and

K=K,+K,+K,

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

291

For the solution of the derived system of differential equations, one must know the dependence of as on the time of exchange. This dependence may be very complicated since it is determined not only by the rate of exchange of surface atoms of catalyst oxygen but also by the rate of diffusion of the exchanged atoms into the bulk of catalyst crystals. In general this problem can be solved only for certain special cases. Thus, for instance, if the diffusion in the solid phase proceeds very rapidly compared with the isotopic exchange of surface oxygen ions, then the content of the heavy oxygen isotope 0l8on the catalyst surface (as)as well as in its bulk (a#) remains the same and is given by as = av = ,@+I)

- ha

(13)

Here y is the average content of OlS in the entire system, which includes gas oxygen and catalyst oxygen. The relation h = ( N , / N K ) gives the ratio of the overall number of oxygen atoms (both isotopes) in the gas phase (N,) to the one of catalyst (NKh By substituting (13) into (10) we find

@= dr

(2+

K,) [y(A+1) - a ( h+ l) ]

= R1(y-d()

where

On integrating we get a =y

+ (ao-y)e-RIT

(14)

where a. is the original content of 0l8in molecular oxygen (as T=O). On substituting the derived value a into (12) we get

By means of the derived equations i t is quite easy to calculate, from the experimentally found values a,y, and T , the values for the constants of exchange K,, K , and K , for all three mechanisms. Generally speaking, the constants of exchange K,, K,, and K , can be calculated by means of differential equations (10) and (12) for the experimentally found values daldr and dC,,/dT at the different time of exchange. The reliable results can thus be obtained only with a large number of measurements and using very high accuracy. In order to elucidate the role played by the particular mechanisms, it is convenient to investigate the samples of catalyst containing a minimal

a. K. BORESKOV

292

amount of heavy isotope with the high overall content of Ole in molecular oxygen. Figure 1 shows the changes of isotopic composition of molecular oxygen in various mechanisms of exchange in the case when a, Cs4.This implies the possibility of elucidating the predominant mechanism of exchange according to the following qualitative indications : (a) The exchange proceeds, presumably, following the first mechanism if (da/dT),=, = 0 , as well as if (dCs2/&),,,, < 0. (b) If (dar/d~),,, < 0 and (dC&T),=o < 0, then the second mechanism prevails. (c) If (dcr/d~),,, < 0 and (dC,,/d~),,, < 0, the exchange is realized following, mainly, the third mechanism. All the deductions proceed from the assumption that the catalyst surface is uniform relative to the isotopic exchange. If the surface is nonuniform, then the constants for exchange K,, K,, and K , as well as the isotopic oxygen composition of catalyst surface a,, will be different for

-.-.-, 0

0.1

0.22 0.36 0.51 Q7 0.92 1.2

1.61 2.3

Q)

Kr

FIG.1. The change of isotopio composition of molecular oxygen at the different mecha n i s m ~of exchange; uo = 0.5; q4= 0.4; q4= 0.2; q,= 0.4. First meohmism, . . m y : )( -x -, u C,, Second mechanism,

-

- 0 - 0 - 9

-A-A-,

C8.3

C,,

Thirdmechanism.

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

-----

CATALYSIS OF ISOTOPIC

EXCHANGE IN MOLECULAR OXYGEN

293

various sections of the surface. However, taking into account the fact that isotopic exchange in this case occurs presumably on the most active sections of the surface, one can calculate the approximate value obtained for the constants of exchange for these sections from the experimental data, if 7 and h are small and by setting as a.

<

II. Some Experimental Data Relating Isotopic Exchange in Molecular Oxygen on Solid Catalysts

A. THE CATALYTICACTIVITYOF OXIDES Winter (10)relates isotopic exchange of molecular oxygen with magnesium oxide, zinc oxide, chromium oxide, nickel oxide, and iron oxide. He also compares the rates of isotopic exchange of these oxides with oxygen and the rates of adsorption and catalytic activity relating to the oxidation of CO and the decomposition of N,O. The comparison of these results with those of later investigations shows that catalytic activity of some oxides strongly depends on the conditions of preliminary treatment.This fact is obviouslyassociated with the change of oxygen content. I n general, the severe treatment in vacuum increases the activity while the oxygen treatment at higher temperatures abruptly reduces it. The difference in activity of some oxides subjected to different treatment is of several orders. Therefore, it is reasonable to compare the catalytic activity of various oxides separately for samples subjected to oxygen treatment at sufficiently high temperature (in order that these samples attain equilibrium composition of surface layer), and for samples whose surface layer composition differs from equilibrium one. Table I lists the principal data published after 1958, for oxides of equilibrium composition. Figure 2 shows the dependence of exchange rate on temperature of a number of oxides of the fourth period of Mendeleev Chart. Below we give the additional data for separate oxides. 1. Magnesium Oxide The data given in Table I are obtained for the sample of magnesium oxide treated in vacuum at 500"for 4 hr. Following this, oxygen was injected into a reaction vessel and the rate of isotopic exchange was determined beginning at 500". The rates both of isotopicexchange in molecular oxygen and of isotopic exchange with oxide oxygen were found to be very close. Contrary to this, Winter (I),in his first investigation, discovered that the rate of exchange in molecular oxygen was essentially higher than the one of the oxide oxygen. The first reaction was observable even at room temperature in the complete absence of exchange with the oxygen of magnesium oxide. The increased activity of

TABLE I Kinetic Churrreteristic.8 of the R e d i o n of Isotopic Exchange of ikfokc&r Oxygen on Vartiwa C d y 8 t 8

Variation range

Catalyst

Temp. ("C)

oxygen pressure (torr)

10 400-500 10 400-500 40 53CL600 5-240 45CL550 5-240 450-550 6-240 225-325 6-240 350-450 6240 125-250 5-120 225-300 4-240 250-350 13-120 425525 11-38 550-600 20G250 0.15-0.5 Under oxygen pressure 10 tom.

Surface (meter*/gm)

26 190 67 2.7 2.9 55 27 7.7 7.7 18.0 1.0 0.78 0.4, 1.6

Rate of exchange at 300" and 40 molecules

1.8 x 3 x 2 x 1x 6.1 x 2.1 x 9.4 x 5.2 x 6.7 x 7.0 x

107" 107'

106 107 107

10'8 109

10ls 10l1

lo1'

3.7 x 108 2.6 x los 6.7 x 1013

Activation energy

40 (32) (39) 46 42 22 33 16 24 26 40 69 16.5

Pre-exponential Order of factor @ion with molecules respect to

1.8 x 1022 3 x 10"

1.9 4.9 4.0 2.7 5.2 6.7 3.9

-

x 1024 x 10~3 x loza x loZ2 x 1019

x lozo x lOS1 3.7 x 1023 2.1 x 1029 1.0 x

lop0

0.7-0.8 0.6-0.7

0.4 0.6 0.4 0.4

0.4

0.9 1.0 0.5

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

295

magnesium oxide in Winter's experiments is evidently related to the conditions ofthe treatment which activates the surface. If the sample after treatment is cooled in vacuum, it develops high activity with respect to reaction ( 1 ) a t low temperatures. This activity, however, disappears due to heating a t 500" in oxygen (see Section 11,D). It is interesting that the rate of exchange of magnesium oxide with oxygen in the experiments of Winter (13)and Popovsky proved to be very close. The latter could not yet reproduce the abrupt change of activation energy at temperatures higher than 420" and in the range from 3 6 1 2 kcal/mole up to 7.7 kcal/mole. Within the range between 400"and 450" the activation energy was found to be constant and approaching 40 kcal/mole. The abrupt decrease of activation energy at elevated temperatures was observed by Winter also with zinc (14)and chromium oxides (15). He explained it by the fact that at higher temperatures the limiting phase of exchange represents the diffusion on the surface of oxides of those active defects where the exchange takes place. Dzisjak et al. ( 5 ) were not able to detect the change of activation energy for chromium oxide up to 550" and for zinc oxide up to 525". It is quite possible that the decrease of activation energy observed by Winter is connected in all the cases considered through the influence of diffusion hindering a t higher rates of exchange which was absent in the work of Dzisjak et al. ( 5 ) due to the forced circulation of the reaction mixture.

13

-I\

I

j

115

1.0 I

0

I

2.0 1

550' 4500 3500300025002000

215

+.lo3

I

150°

100' t°C

FIG.2. The dependence of rates of exchange of molecular oxygen on temperature.

296

0. K. BORESKOV

2. Aluminium Oxide Table I lists the results of the investigation of the sample yA1,Oa obtained by the dehydration of boehmite followed by the heating in vacuum at 600" for 4 hr. Before measuring the rate of exchange the sample was heated in oxygen of the natural isotopic composition at 500".

3. Vanadium Pentoxide According to the data obtained by Margolis (17)the exchange of molecular oxygen in the presence of vanadium pentoxide becomes observable at 435'. The data shown in Table I are found for the sample of commercial use ("analytically pure") with the size of original crystals being of the order 0.6 p. I n the temperature range 460-550" the rates, as well as activation energies and the order with respect to oxygen for reactions of exchange, coincide within the accuracy of measurements ( 4 ) . This permits the conclusion that the exchange of molecular oxygen proceeds with the participation of oxide oxygen, namely following the second and third mechanisms but not the first. As Cameron et al. (18)and Kasatkina et a2. (20)have shown, vanadium pentoxide belongs to a small number of oxides for which the inner mobility of oxygen in crystals is rather high and so the rate of isotopic exchange of oxide with oxygen is determined only by the rate of exchange on the oxide surface. JirPz and Novakova (21)have found that in the case where the isotopic exchange of vanadium pentoxide with oxygen starts with the equilibrium diatribution of isotopes in molecular oxygen (4, :C , , * C , = 4) this equilibrium is disturbed in the course of exchange, and the given expression for concentration ratio becomes much less than 4.At the same time one can observe the decrease of rates of exchange of vanadium pentoxide with oxygen as well as the deviation from the law of the first order (curve 2, Fig. 3). If the equilibrium of the gas phase will be attained again due to the insertion of the heated platinum wire into reaction system, the rate of exchange with oxide increases up to the initial value (curve 3, Fig. 3). The decrease of the rate of exchange would not be observed if, in using heated platinum wire, the isotopic composition of gas phase is maintained equilibric within the process of the whole experiment (curve 1, Fig. 3). These results indicate the hardly explicable dependence of the rate of exchange of oxide oxygen on the distribution of oxygen isotopes among the molecules of the gas phase. Muzikantov et al. (59) have not detected the influence of isotopic distribution in molecular oxygen on the rate of exchange with vanadium pentoxide.

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

297

Vanadium Pentoxide Promoted by the Sulfates of Alkali Metals. The additions of sulfates of alkali metals are used as promoters for a number of industrial vanadium catalysts, e.g., vanadium catalysts for the industrial production of sulfuric acid, phthalic acid, and others (22). They essentially increase catalytic activity in relation to the exchange of molecular oxygen ( 4 ) (Table 11) with the increase of promoting action in the series Li,SO, < Na,SO, < K,SO, < Rb,S04 < Cs,SO,

The addition of 0.1 mole of cesium sulfate to vanadium pentoxide increases the rate of exchange about 100 times (Fig. 4). The samples to be investigated were prepared by fusing the weighed amounts of vanadium pentoxide with sulfate in a quartz crucible at about 700°,followed by the rapid cooling of the fused material, grinding it to powder, and pressing it into tablets. The rate of reaction (I)and of the isotopic exchange with catalyst oxygen turned out to be close for the all promoted samples. The inner diffusion of oxygen in the bulk of the promoted samples as well as in vanadium pentoxide proceeds much faster than the surface exchange. Thermographical and X-ray analysis of the system V,O,-K,SO, lead to the conclusion relating the formation of compound which approaches 0

I

I

I

4

6

0.2

LL

-

0.4

Y

0

0

0.6

OX

0

2 t,h

FIU.3. The effect of isotopic composition of molecular oxygen on rate of exchange with oxygen of vanadium pentoxide after Jirb and Novakovs (21).

TABLE II

T h P r m t i a g Eff& of Sdfatea of Alkali Metals on Vanadium Pentoxide wdh Respect to the Isotopic Exchange in Molecular Oxygea

Rate of homomolecularexchange K Rate of isotopic exchange R molecules/cm, see x 10-11) (molecules/cmasec x 10-11) In the absence In simultaneous proceeding of I n the absence of

Sample characteristic Composition v205

V,O, - 0.1 Na,SO, V,O, 0.1 Li,S04

Surface

Temp.

of isotopic exchange

2.7

440 450 480 460

-

-

0.42 1.1

-

480

-

1.4

-

0.62

(metefl/gm)

1.7 1.4

1.2

V,Os * 0.1 KeS04

V,Os * 0.1 Rb,SO,

1.2

440 460 480

-

0.1 Cs,SO1

0.3

-

400 440

-

480

-

400

-

440

-

460

V,O,

-

400 420 440 450

6.55

-

33.3

homomolecularand isotopic exchange

-

-

homomolecular exchange

-

0.31

1.2

0.94

0.47

0.17 -

E

(E) mole 46 0

0.8

-

46 45

-

1.2

3.1

2.2

2.9

1.6 11.1 46.0

1.1 8.1 44.0

4.8 27.0

42

2.0 11 20.5

1.7 9.3 22.0

1.2

41

5.7 15.0 31.0

6.2 13.0 28.0

-

-

7.5

-

23.0 34.0

!3

1

c"

1.04

7.5

W

0

35

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

299

in its composition V,O,-K,SO, (24).Jird and co-workers (25) have pointed out to the formation of analogous compounds of vanadium pentoxide with rubidium and cesium sulfates. With the low content of sulfates in the samples the solid solutions of these compounds are apparently formed in vanadium pentoxide, and thus, the increased reactivity of oxygen holds only for oxygen ions directly bonded with the mentioned compounds. It would be interesting t o note that in the promoted samples the rapid exchange of oxygen holds not only for vanadium pentoxide but also for potassium sulfate while in pure potassium sulfates oxygen exchange with molecular oxygen is entirely absent under these conditions. 4. Chromium Oxide The characteristics of the activity given in Table I ( 5 )are related to the sample obtained by reduction of potassium bichromate with sugar powder followed by careful washing, pressing into tablets, and heating in E

logK

-I

%

c .5

0.:

z 0

W 0

c

0

c 0 W

-

W

0.E

+I

I

I

I

1

I

Li

No

K

Rb

Cs

FIQ.4. Activity of vanadium pentoxide promoted with sulfates of alkali metals in respect to isotopic exchange in molecular oxygen. KEY:@ -logarithm for rate of exchange;

0-electronegativity of atom of alkali metal.

-aotivation energy of exchange;

300

0.K. BORKISKOV

oxygen at 600'. These data approach those obtained by Winter who was working with the samples prepared by heating of chromium anhydride in the flow of air (2). As was already mentioned, we did not succeed in reproducing the abrupt decrease of activation energy at temperatures over 420". The rates of exchange in molecular oxygen and the rates of the isotopic exchange with oxygen of chromium oxide are very close. 6 . Manganese

Peroxide

The data of Table I (5)are obtained for the sample of active manganese peroxide (grade: pure) which is preheated in oxygen at 400" for 8 hr. Before investigating the activity, the sample was treated in vacuum at 350' and was heated further in oxygen at the temperature of the experiment. The sample is nonuniform in respect to chemical and phase composition; as X-ray analysis has shown, the sample contained a small amount of phase /3Mn20g together with the phase PMnO,, while the proportion of oxygen to manganese was 1.83. The change of the rate of exchange with catalyst oxygen (Fig. 6) indicates the great nonuniformity of oxygen (27); within 3% of monolayer, the rate of exchange changed fourfold. The increase in temperature of the pretreatment lowers the content of oxygen of the sample. As is clear from Table 111,the sample treated at 660" mainly consists of phase /3Mn,08. Nevertheless, the catalytic activity with respect to the exchange of molecular oxygen increases over tenfold with the elevation of temperature of treatment from 360" to 550"

X surfoce

O/O

FIQ.8. Relative reduction of rate of exohange of 0, and manganese peroxide oxygen with the increase of depth of exohwe.

TABLE In

0

TABLE III

k;l

Thc E J . o j T&mnt Temperature on the C m @ h a d G&ytk Actdvity of bfanganeee P~razide The Eff& of Treatment Temperature on the Composilion and CaWytic Activity of Mangame P E W

Temp. of

Temp. of SdaC€l treatment ChOmiCd treatment Surfece Chemical composition (mh+/gm) ("C) composition ("C) WtWP)

Phsse Phsse compoaition

composition

!s

ard 80

Rate of exchange Fbte of malecular Rate of molecular Rate of of oxide exchange R exahange of oxygen with oxygen m of oxide Icexchange x 10" of oxygen with R x oxygen 1wa K x BBC 10" R x 8w: 1W' (mokwdea/cm* molaoulea/cma (moleculea/cm*sec moleculw/cm' sec at 300"C,10 torr at 300°C. 10 torr at 30O0C,10 tom at 300°C.10 torr

cd

2

trl

8

ik2 i f

0.83

3.0

0.83

b5 w

2

tr

u

n

w

0 crr

w

0 c.l

302

c). K.

BORESKOV

Antoshin and Kasatkina (8)have investigated the isotopic exchange between molecules of CO , CO and O,, CO and manganese peroxide on the sample of manganese peroxide treated at 350". Table IV lists the kinetic characteristics of these reactions a t 300". The reaction of homomolecular exchange of atoms between molecules of CO, proceeds on the surface of manganese peroxide much faster than exchange in molecular oxygen. Evidently, this process is also realized through the intermediate exchange with catalyst oxygen, but this can not be completely proved.The oxygen of manganese peroxide in the exchange with CO turned out to be very nonuniform, so that the first experimental exchange rates were many times less than the maximal ones. The rate of isotopic exchange between 0, and CO, is much less compared with isotopic exchange rate between molecules of CO,; it is somewhat less than-but of the same order asthe exchange rate in molecular oxygen. This permits the assumption that exchange between 0 and CO is realized through the exchange of these gases with the surface oxygen of manganese peroxide. The limiting step of this process is the exchange of molecular oxygen. This fact is strongly supported by the independence of the rate of exchange of reaction between 0, and CO, on carbon dioxide pressure and by the same approximate dependence on oxygen pressure, as in the case of exchange between 0, and the oxygen of manganese peroxide. The decrease of the rate of exchange between 0 and CO as compared t o the rate of exchange of 0, with the oxygen of manganese peroxide is evidently related to the hindering effect of the adsorbed CO,.

,

,

,

,

,

,

6. Ferric Oxide Table I lists the data ( 5 )for the sample of ferric oxide prepared by the precipitation with ammonia of the ferric chloride solution followed by the heating of the ferric hydroxide at 500" in oxygen for 8 hr. The sample investigated by Winter (15) was prepared in similar way. The rates of exchange of molecular oxygen found in both works are close, while the order of reaction and the activation energy essentially differ; according to Winter, the activation energy in the range 290-400" is 19&2 kcal/ mole, and the order of the reaction with respect to oxygen is close to unity. It is quite possible that these differences are related to the conditions of sample pretreatment. The exchange rates of molecular oxygen and of the isotopic exchange with oxygen are very close. The surface oxygen of iron oxide is uniform.

7. Cobalt Oxide The sample of Co,O, investigated ( 5 ) was prepared by the thermal decomposition of cobalt nitrate followed by careful washing with water.

TABLE IV H

Kinetic Characterietiw of Various Exchange Reactions with Oxygen I80tqpes on Manganese Peroxide at 300°C

v,

0

9

Fl

-

Reaction of isotopic exchange

Rate of exchange at 40 torr pressure molecules x 10-11

Activation energy

Order of reaction With respect to With respect to

0,

CO 1

0 Y 0

8 M

w

9 Oa - co,

0,

6.6

- MnO,

oy + 0:s

--f

20'60'8

co, - Mno,

coy + coy + 2C0'6018 According to Dzisjak et aZ. ( 5 ) .

16

25

18.5

22

22

0.45

0.4

210

7.5

-

9300

4.0

-

E

3 8 0 0 W

304

G, E, BORESKOV

Before measuring the activity the sample was treated in vacuum ( lo4 mmHg) at 400' for 8 hr followed by the treatment in oxygen at the maximum temperature of activity measurements. Co,O, possess the highest catalytic activity of the all investigated oxides with respect to the exchange of molecular oxygen. The investigation of the isotopic exchange with oxide oxygen has shown the strong nonuniformity of the surface oxygen (28).ThiR should explain the fact that the observed initial rate of exchange with oxide oxygen is somewhat less than the rate of exchange of molecular oxygen. Even in the first measurements, the observed rate is obviously much less than the actual initial rate of exchange with the most active oxygen on the surface of Co,O,. 8. Nickel Oxide

The data represented in Table I are referred to the sample of nickel oxide obtained by the decomposition in vacuum of nickel carbonate (5). The investigation of the isotopic exchange with oxide oxygen has shown the radical nonuniformity of surface oxygen. The rate of exchange of molecular oxygen is very close to the initial rate of exchange with surface oxygen of NiO. With the action of oxygen on the sample heated in vacuum at 400' its activity eventually reduces up to the stable value, which is 30-40 times less than the initial one. Table I lists the values obtained for the stable activity. Winter (15),while investigating the samples of nickel oxide obtained by the decomposition of carbonate at 850' and of nitrate at 650", observed much less activity, especially in the case of carbonate (almost 100 times less); aha, a much higher activation energy was measured (36 kcal/mole).This difference may be obviously related to the different content of the excess oxygen in the samples to be investigated. Keyer (29) has investigated the isotopic exchange of 0%with nickel oxide oxygen obtained by the decompositionof carbonate at 900' and treated before measuring at 700'. The noticeable exchangewas discovered starting from 400'. It is very interesting that within the monolayer the rate of exchange changed insignificantly,which is the evidence of the uniformity of surface oxygen. The addition of 0.8 at -% of lithium increased the rate of exchange 7 to 8 times. Contrary to this, Winter has not discovered the noticeable effect of doping with small quantities of lithium oxide, silver oxide, and tungsten oxide on the rate of exchange in molecular oxygen because, possibly, of the incomplete solving of the dopents due to the low temperature of calcining (650').

Oxide Copper oxide obtained by the heating of hydroxide precipitated by

9. Copper

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

305

the alkali from copper sulfate solution followed by the careful washing has been investigated (5). Before carrying out measurements, the sample was treated in vacuum at 400”. When treated by oxygen at the temperature of the experiment the activity of this sample decreased up to the constant value. The surface oxygen of copper oxide, as shown by the investigations of isotopic exchange, is nonuniform (28). Margolis and Kiselev have investigated the effect of doping with several oxides (Cr,O,, Bi,O,, and Li,O). Some positive effects of lithium oxide and the negative effect of bismuth oxide have been discovered. 10. Zinc Oxide

The data of the Table I refer t o the sample of reactive zinc oxide (“luminophors”)heated in oxygen at 800”. Before measuring the activity the sample was treated in vacuum at 400” and treated with oxygen at 525”. The surface oxygen of oxide is uniform. The rates of exchange in molecular oxygen and of the isotopic exchange with oxide oxygen coincide. Winter in his earlier works (14) has investigated the zinc oxide obtained by the oxidation in air the pure metallic zinc. The sample was treated in vacuum at 630” for 18 hr and before carrying out the measurements of activity it was not treated with oxygen at high temperatures. Under these conditions activity appeared to be much higher ( 1 ) .The noticeable rate of exchange of molecular oxygen could be observed even at loo”, though the exchange with oxygen of oxide did not occur. It may be of interest that the rates of the latter process, according to various studies (5, 14, 28), coincide relative to the order of magnitude. The difference in rates of exchange with molecular oxygen may be ascribed to the difference in conditions of pretreatment of zinc oxide samples. I n more detail, this question is discussed below in the section dealing with the activity of oxides with the nonequilibrium content of oxygen. Gorgoraki and associates (7)studied the effect of doping with lithium and gallium on the activity of zinc oxide with respect to the exchange of molecular oxygen. The dopents were introduced by impregnation of zinc oxide (“luminophors”) with nitrate solutions followed by heating at 830” for 30 hr. It was discovered that the addition of 0.25 at -yoof lithium increases the activity 3-4 times, while the addition of the same amount of gallium reduces the rate of exchange 2-2.5 times. 11. Molybdenum Oxide

Table I lists the data for the sample of molybdene trioxide obtained by the thermal decomposition of ammonium molybdate at 450” for 5 hr. The rate of exchange of molecular oxygen is close to the rate of exchange

306

G . K. BORESKOV

with oxide oxygen. A marked exchange could be observed with oxygen inside the crystals of molybdenum oxide, but its rate was insufficient for the complete equilibration of the isotopic composition of oxide. Recently, molybdates of the transition metals have become widely used as catalysts of partial oxidation of alcohols and hydrocarbons. These catalysts are distinguished by the increased activity and selectivity. The investigation of iron molybdate obtained by the precipitation from the solutions of ammonium molybdate and iron nitrate followed by washing, drying and heating at 400" has shown that its specific activity with respect to the exchange in molecular oxygen is about two times higher than the activity of molybdenum oxide.

B. THE PRINCIPAL REGULARITIES OF THE EXCHANGE IN MOLECULAR OXYGENON THE OXIDES WITH EQUILIBRIUM CONTENT OF OXYGEN The catalytic activity of oxides with respect to the isotopic exchange in molecular oxygen, as it is quite clear from the above-listed data, very often strongly depends on the conditions of the pretreatment. The readily reproducible results may be obtained by preheating oxides in oxygen until attaining the equilibrium content of oxygen in the surface layer of a sample. Let us consider from the beginning the catalytic properties of oxides treated by the mentioned method. Winter has found earlier that on Cr,O,, NiO, and Fe,08 the rates of exchange ( K ) in molecular oxygen coincide with the isotopic exchange (R) with oxide oxygen. Later investigations have shown that this takes place for all oxides with equilibrium content of oxygen which have been studied (see Table V). This condition is manifested very distinctly in the oxides whose surface oxygen is uniform relative to the isotopic exchange with gas phase (V206,Cr203,ZnO, Fe,O,). For these oxides the experimental values for the rates of the processes under consideration coincide in the limits of the following inequality:

R I K S 2 R

It is much more difficult to compare the rates of exchange processes in the case of oxides whose surface oxygen is nonuniform and whose rate of isotopic exchange lowers within the monolayer. I n this case, the rate of exchange in molecular oxygen should coinicide with the maximum initial rate of isotopic exchange with oxygen of the surface catalyst, I n order to measure the latter, it is necessary to carry out the experiments with the least possible ratio of the quantity of oxygen in the gas phase to the quantity of oxygen on the catalyst surface. Figure 6 illustrates the results of these measurements for the isotopic exchange with oxygen of

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

307

manganese peroxide (8).The rate of exchange reduces very rapidly although the depth of exchange does not exceed 3% of the number of atoms of the sample surface. The initial rates of the isotopic exchange with oxide oxygen determined in such a manner are in a good agreement with the rates of exchange in rnoIecular oxygen as is clear from Fig. 6. An analogous result was obtained also for the exchange proceeding on cobalt oxide. It would be a more complicated task to compare the rates of exchange for nickel oxide and copper oxide whose catalytic activity strongly increases due to the treatment in vacuum and then slowly decreases while measuring the rates of exchange in the atmosphere of oxygen. However, with the introduction of corresponding corrections one may be convinced that the values K and R are quite close for this case also. This result permits the conclusion that on oxides with equilibrium content of oxygen, the simplest catalytic reaction, such as the isotopic exchange in molecular oxygen, proceeds with the participation of oxide oxygen. On oxides with uniform surface all surface oxygen participates TABLE V Comparison of Rates of Exchange in Molecular Oxygen and of Isotopic Exchange with Oxygen of Oxide

Rate of exchange molecules Oxide

VZO,

Temp. 550 600

x

10-11

In molecular oxygen With oxygen of oxide 13.5 2.4

13.0

16

16 0.61

0.32

1.7

15

ZnO

600

Fez08

400

6.8

4.7

NiO

300

6.8

6.8

cuo

300

6.7

4.7

MnO

300 200

22 6.2

26 4.2

29

308

0.K. BORESKOV

in the reaction; on oxides with nonuniform surface there participates only the most active “mobile” oxygen. From this direatly follows the fact that for oxides with equilibrium content of oxygen mechanism 1 may be excluded. It is much more difficult to establish whether isotopic exchange in molecular oxygen proceeds on these oxides followingthe second or third mechanism, that is, with the participation of one or two oxygen atoms of a catalyst. This question may be solved on the basis of the above-given (Section I, C) analysis of kinetic laws of exchange corresponding to the various mechanisms. The most distinct results may be obtained by treatment of the results of measurements carried out when as a. This condition may be successfully fulfilled for vanadium pentoxide due to relatively high mobility of oxygen inside the crystals of this oxide. With the validity of this inequality in the case of third mechanism the change of the relative concentrations of molecules OlsOlaare proportional to the relative concentrations of these molecules:

<

and their ratio in the process of exchange remains constant

13

FIU.6. Comparison of rate of exchange in molecular oxygen with the initial rate of its exchange with oxygen of manganese peroxide. KEY:0 exchange of moleoular oxygen; - exchange with oxygen of oxide.

-

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

309

In the case of the second mechanism

--dCa4 aT = K,(C,(-

a),

-

~

d r = K,C,,

(17)

and

that is, it rapidly rises in the course of exchange in accordance with the reduction of C36.Here C!4 and G6are the initial values for C,, and C,, as T = 0. Figure 7 represents the expected changes of the concentration ratio C,,/C,, in the course of exchange following the second and the third mechanisms as well as the experimental values found for vanadium pentoxide promoted with 0.1 mole of K,SO, at 480" and under 40 torr oxygen pressure. The location of experimental points indicates that the third mechanism predominates, i.e., exchange in molecular oxygen proceeds with the participation of the two atoms of catalyst oxygen. The second method of the clearing up of the mechanism is based on the exchange with oxide oxygen using gas oxygen with the equilibrium distribution of isotopic modifications, that is, when

Then in Eq. (12), yo = 0 and (dy/d.r),,, = qJ(da/d7)a. When the second mechanismisvalid, K , = 0 , =~0, and (dy/dr),,, = 0; thismeans that

0

I

I

I

30

60 r,min

90

1

120

FIG.7. Mechanism of exchange of molecular oxygen on vanadium pentoxide promoted with 0.1 mole K,SO,. Curve I-calculated with respect to third mechanism: curve II-calculated with respect to second mechanism; curve III--experimental date.

310

0.K. BORESKOV

equilibrium distribution of molecules in the gas phase in the process of exchange with oxide oxygen is not distorted. If the third mechanism is valid, then y~ > 0 and (dy/d~),,, > 0 which means that the equilibrium in the gas phase is disturbed in the process of exchange and j3 = (Ci4/C32C38) reduces within the initial period of exchange. As has been already mentioned, Jira and Novakova (ZI),while investigating the oxygen isotopic exchange on vanadium pentoxide, have found that the mixture of isotopic speces of oxygen molecules which are initially in equilibrium in the process of exchange becomes nonequilibrated, and j3 reduces from 4 to the value less than 2. This result also justifies the predominance of the third mechanism on the exchange of molecular oxygen on V,O,. Muzikantov and co-workers have acknowledged recently this conclusion. In the case of oxides with relatively slow diffusion of oxide inside crystals, and especially in the case of nonuniformity of surface oxygen, the determination of the predominant mechanism is more complicated. We have no sufficiently accurate information for the unique choice between the second and the third mechanisms for most oxides. Regardless of this lack, the coincidence of the rates of exchange of molecular oxygen with the isotopic exchange with oxide oxygen is strong evidence of the fact that the exchange in the case of oxides with equilibrium content of oxygen proceeds with the participation of oxygen of oxide, and its rate should, therefore, depend on the energy and nature of oxygen bonding in the surface layer of oxide.

C. THERELATION OF CATALYTIC ACTMTYWITH RESPECT TO THE EXCHANGE IN MOLECULAROXYGEN TO SOME OTHERPROPERTIES OF OXIDES The results of the preceding section indicate the dependence of catalytic activity of oxides with equilibrium content of oxygen on the oxygen bonding energy in the surface oxide layer. This energy is uniquely defined by the electronic structure of the metal’s atom that forms the oxide. Unfortunately, we have no calculations or experimental methods a t our disposal for the determination of this energy. If we make use of the formation energy relative to the single atom of oxygen calculated in accordance with the thermochemical data with the purpose of getting some rough estimate then, as it is clear from Fig. 8, the activation energy of exchange regularly varies with the increase of this magnitude. The optimum magnitude of bonding corresponds to that of Co,O, while in the copper oxide the oxygen bonding energy is apparently lower than optimum. On this basis, the investigated oxides of the fourth period may be classified into

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

311

two groups. The oxides of the first group, including TiO,, V,05, CraOs, Fe,O,, and ZnO are distinguished by high oxygen bonding energy and are characterized by a comparatively small activity as well as by the high activation energy of exchange. The order of reaction with respect to oxygen is between 0.6 and 0.8 while the surface oxygen is uniform. All these oxides, with the exception of Cr203, are weak n-type semiconductors. The nature of conductivity of chromium oxide is less clear; according to certain information, this oxide, after heating in oxygen, acquires the properties of a p-type semiconductor. For most oxides of this group, the number of electrons in 3d shell of cations corresponds to the most stable configurations: 0 for TiO, and V,O,, 5 for Fe203,and 10 for ZnO. The second group is formed by the oxides possessing the lesser oxygen bonding energy-Co30,, NiO, CuO, and MnO,. The exchange on these oxides proceeds very rapidly and is characterized by the low activation energy and by the small value of pre-exponential factor. The order of the reaction is 0.3-0.4. The surface oxygen is not uniform. Oxides of the second group are presumably p-type semiconductors possessing rather high conductivity. The number of electrons in d-shell of cations is: 3 for MnO,, 7 and 8 for Co304,8 for NiO, and 9 for CuO. Figure 9 shows the values obtained for the activation energy and the pre-exponential factor of the rate of exchange in molecular oxygen for the above mentioned oxides. The more active the oxide, the lower the

50

-

40

-

"2 '5 0

Fe203 0

w

20

0 Cr20,

0

ZnO

-

Mn02 0

10

-

01 30

COP4

I

40

I

50

I

1

I

60

70

80

I

90

I

100

kcol -Q gm-atom

FIG.8. Relation between activation energy of oxygen exchange and the oxide formation heat, referred to l gm-atomof oxygen.

312

0.K. BORESKOV

observed activation energy and the lower the pre-exponential factor. Proceeding from the most active catalyst C o 8 0 , to the less active V,O,, the activation energy increases by 30 kcal/mole and the pre-exponential factor increases by more than 4 orders. If one assumes that the limiting step of exchange is the sorption of oxygen proceeding on the entire oxide surface while the active complex of sorption is not localized, then the maximum possible value of preexponential factor is

c,

4:

For the conditions of the diacussed experiments it is loz*moleoules/cma sec. Here C, is the concentration (molecules/cm8)of the molecules to be sorbed, m is the mass of a single molecule, and lc is Boltzmann’s constant. With the localization of the activated complex on the surface, the theoretical value of the pre-exponential factor decreases. If the desorption realized from the whole surface is limited, the maximum theoretical value of the pre-exponential factor reaches 10%molecules/cm*sec. The experimental data shown in Fig. 9 illustrates that the observed activation energies for the first group of oxides are considerably higher than the activation energy of adsorption, and these energies approach the activation energy of desorption. Thus, for V20, the true activation energy of adsorption should be less than the observed activation energy

251

24

Zno 0 0

0

vzos

23 b

0

NiO

MnOz

I

10

20 E-

I

1

I

30

40

50

kcal mole

FIG.9. Activation energy and pre-exponential feotors of reaction of exchange in molecular oxygen on oxides of transition metals.

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

313

of exchange, at least, by 10 kcal/mole, so that the pre-exponential factor value would not exceed the theoretically possible value. This means that the surface of these oxides is filled, in principle, by oxygen, while the limiting step represents the elimination of oxygen. For the second group of oxides, the surface is evidently free to a great extent for sorption of oxygen, and the observable activation energy of exchange approaches the true one of the sorption. The decreased value of the pre-exponential factor compared with the theoretical value is most likely connected with the nonuniform surface oxygen of these oxides. The extent of the nonuniformity of surface oxygen increases in the following succession: CuO < NiO < MnO, < Co,O,

Accordingly, one should believe that the exchange in molecular oxygen proceeds only on a part of the surface of these oxides; this part will decrease as extent of the nonuniformity increases. This explains the reduction of the value of the pre-exponential factor proceeding from CuO to Co,O,. The reduction of K O values may be also associated with the localization of the active complex of oxygen sorption. The elimination of the translational degree of freedom along the surface as well as the rotation of active complex decreases the theoretical value K Oby almost 5 orders. Higher values of the surface oxygen bonding energies in the first group of oxides-compared with those of the second group of oxidescause certain difficulties in explaining the observed order of exchange in oxygen. If we assume that the limiting step represents the interaction of the oxygen molecule with oxygen defect of oxide surface, then the rate of this process may be expressed by the equation:

w

=

KP0J

where /3 is the relative concentration of oxygen defects on oxide surface. I n the first approximation, if one does not take into account the possible effect due to concentration of the surface oxygen defects on the energy of their formation, then

P

q6

and the total order with respect to oxygen should be 0.5. The experimental values obtained for oxides of the first group are somewhat higher: between 0.6 and 0.8. Such values are likely to be connected with the dependence of activation energy on the concentration of surface oxygen defects ( 6 ) . For oxides of the second group, the order of reaction of exchange in molecular oxygen is lower than 0.5. This value is likely to be related to

314

0. K, BORESKOV

the nonuniformity of surface oxygen of these oxides ( 6 ) .The more careful measurement of the order with respect to oxygen of the exchange reaction in molecular oxygen for various oxides at the variation of oxygen pressure within a wider range is of interest. On the basis of the concept relating the dependence of rates of exchange in molecular oxygen on the magnitude of the oxygen bonding energy on oxide surface, the effect of the promoting additions may be also explained. The addition of alkali metal sulfates to vanadium pentoxide leads to the formation of compounds close to V,O,.Me,SO,. These compounds are soluble in the excess of vanadium pentoxide. The oxygen bonding energy reduces in the formed solid solutions, and due to this fact, the activation energy decreases while the exchange rate in molecular oxygen increases. As is clear from Table 11, the oxygen mobility grows, depending on the nature of alkali metal proceeding from lithium to cesium. In other Words, there is a monotonous variation of oxygen bonding energy-in a series of promoted samples-with the increase of the ordinal number of an alkali metal, As is clear from Fig. 4, the rate and the activation energy of exchange for vanadium pentoxide promoted by sulfates on alkali metals undergoes a change similar to the change of electonegativity of alkali metal atom (26). 1. Oxygen Smption and the Exchange in Molecular Oxygen The adsorption and the desorption of oxygen are the necessary steps of isotopic exchange. I n the realization of the isotopic exchange reaction in molecular oxygen, the adsorption equilibrium is attained, and the rates of adsorption and desorption become equivalent. I n order to elucidate the mechanism of exchange the comparison of adsorption (and desorption) rates as well as the rates of isotopic exchange may be of interest. Winter (1,2) has investigated in detail the rate of oxygen adsorption on NiO, Cr203,and Fe,O, as well as on MgO and ZnO, comparing the obtained data with the rate of isotopic exchange. Dzisjak et al. (4) have investigated with the same purpose the oxygen sorption on vanadium pentoxide. The estimation of results of the given comparison requires, however, great care. The choice of conditions for the measurements of adsorption rate to be compared with the rate of exchange in case of solid oxides offers difficulties which almost definitely could not be overcome. The main difficulty is attaining a pure oxide surface containing no adsorbed oxygen. The treatment under severe conditions which is carried out in order to remove the adsorbed oxygen from the surface as well as other adsorbed substances may be accompanied by the loss of a part of oxygen which is in the crystal lattice or surface layers of oxide. I n this

CATALYSIS O F ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

315

treatment the reliable methods for the estimation of oxygen content in this layer are missing. Therefore, the measurable rates of oxygen aorption (desorption) may be consistent with the composition of the surface layer of oxide which strongly differs from the oxide composition in the process of exchange reaction. The elimination of adsorption oxygen without the distortion of the composition of the surface layer of oxide is obviously not always possible. Besides, the criteria enabling one to distinguish the adsorbed oxygen and the oxygen of the crystal lattice surface of oxide are not quite clear. Adsorption measurements, therefore, are still not very fruitful for the study of the mechanism of isotopic exchange of oxide. On the contrary, the measurement of the isotopic exchange rate in molecular oxygen is the only reliable method for estimation of the rate of adsorption (or desorption) of oxygen in the neighbourhood of adsorption equilibrium. 2. Activity Relating to the Isotopic Exchange and the Semiconductor

Properties of Oxides In the isotopic exchange on oxides possessing the equilibrium content of oxygen there occur the mutual transformations of molecular oxygen, and the atomic ions of oxygen which are connected with electronic transitions. The work function can, therefore, affect the value of the activation energy of isotopic exchange reaction. It may be of interest to testify whether there is any connection of catalytic activity of oxides with respect to the exchange reaction with their semiconductor properties. Table V I illustrates the comparison of catalytic activity with the magnitude of the work function of electrons for a series of transitionmetal oxides (6). The activity is characterized by the rate constant of exchange at 300°C while the work function may be specified by the contact difference of potentials, taken in volts, between the oxide and gold (30). The data of the Table VI is evidence of the lack of the simple relation between the catalytic activity and the work function. With the equivalent value of the work function, the catalytic activity of some oxides differs by 5 orders while with the equivalent activity, the work function, in particular cases, differs by 1.05 volts. From this fact follows that proceeding from one oxide to the other the change of the work function is not the only main factor determining the change of catalytic activity. The simple relation between the work function and the catalytic activity relating the oxygen exchange is also lacking for the promoted oxides. Thus, the addition of 0.5 at. - % of both lithium and gallium to zinc oxide leads to a decrease of the work function while the

TABLE M The Catalytic Activity and the Work Fum%Wpz of Oxidea of Metals of the Fourth Period 0

TiO,

V,O,

Cr,O,

Fe,O,

Co,O,

NiO

CUO

ZnO

Contact difference of potentials contacting with gold:

-1.36

-0.76

-0.10

-0.74

-0-13

+1-02

-0.03

0.0

T h e rate of exchange at 300"

2 x lo-'

0.001

0.0061

0.94

5200

68

70

0.37

Oxide :

molecules

p

CATALYSIS O F ISOTOPIC EXCRANGE IN MOLECULAR OXYGEN

317

catalytic activity of zinc oxide increases fourfold due to the addition of lithium and decreases fivefold due to the addition of gallium (7) (Table VIa). TABLE VIa

The Effect of Additions of Li and U a to Zinc Oxide on Catalytic Activity and Work Function V a h e

Work function I n vacuum tom, 436')

Sample

ZnO

I n oxygen (40 torr, 430")

Catalytic activity at 600°C molecules x 10-13

(XGZ

3.9

6.3

2.2

ZnO (0.6 at.-

yo Li)

4.0

4.3

8.2

ZnO (0.6 at.-

% Ga)

4.1

6.0

0.42

When the sulfates of alkali metals are added to the vanadium pentoxide both catalytic activity and electroconductivity increase (19). When vanadium pentoxide is promoted by the sulfates of various alkali metals, no dependence of the change in catalytic activity on the change of work function can be observed (Fig. 10). For instance, the 2 .c

Y

-g

1.0

0

FIG.10. Rate and activation energy of exchange of molecular oxygen and work function for vanadium pentoxide promoted with sulfates of alkali metals. Curve 1-logarithm of the exchange rate, curve 8-activation energy, curve 3-work function.

a. K. BORESKOV

318

addition of 0.1 mole of potassium sulfate to vanadium pentoxide increases the rate of isotopic exchange in oxygen fiftyfold while the work function of the electron is not appreciately changed. These results are not unexpected; the simple relation between the work function and catalytic activity resulting from certain theoretical concepts (31) have not found experimental support for many other reactions. From this fact we must not conclude that the change of the work function, defined by the shift of Fermi level, does not affect the catalytic properties of oxides. If the limiting step of exchange, for instance, the sorption of oxygen is associated with the formation of charged particles on catalyst surface (e.g., 0-) the heat of this step will include the work function as follows:

Q = A

- 0.60

-v

+W

Here, A is the affinity between oxygen atom and electron; q~ is the work function of electron determined by the level position of chemical potential; ,? is theI dissociation energy of oxygen molecule; and W is the interaction energy of the formed ion with catalyst. The latter value is determined by the properties of the adsorbed substance and of a catalyst, and should, in general, depend on the position of the adsorbed particle on the surface. The adsorption (and desorption) rate is determined by the energy of the activated complex of adsorption whose configuration is intermediate between the original molecule and the adsorbed particle. As Temkin (32) has shown, the adsorption laws always agree with the assumption analogous t o the Brensted rule saying that the change of activation energy of adsorption is a part of the change of heat of adsorption. Then, the activation energy of adsorption is

El = E , - cc A& while the activation energy of desorption is

E,

= E,,

+ (1-a) A&

where CL lies between the zero and unity and generally approaches 0.5. If one assumes that proceeding from one catalyst to the other W remains constant, then A& == - AP, I n the process of the isotopic exchange reaction, the adsorption equilibrium is attained and the rates of adsorption, desorption, and exchange are equivalent and equal to

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

319

where b is a constant independent of Ap,. From this follows that the rate of exchange increases with the decrease of Ap, at small degrees of surface coverage and passes through the maximum. The maximum rate corresponds to the value Ap, at which the degree of surface coverage 9 equals a, which is the ratio of the variation of activation energy of adsorption to the variation of heat of adsorption. But since such a simple relation between the change of the work function and the catalytic activity with respect to the exchange reaction is not supported by the experimental data, one should conclude that the assumption relating the constancy of W for various catalysts is not valid. With the variation of the composition of catalyst the energy W of the interaction of the charged adsorbed particles with catalyst changes as does the chemical potential p, of electrons. This energy ( W ) , including both the Colombian and the exchange constituents, depends in a very complex way on the electronic structure of a catalyst and cannot yet be determined in the framework of the zone theories. Therefore, proceeding only from the semiconductor properties, it is generally impossible to predict the catalytic activity, Only in those rare cases when variations of W are so small that they may be neglected as compared to the variation of p,, for instance, the doping with small quantities of altervalent ions which strongly affects the Fermi level, semiconductor theories permit to a certain degree of reliability prediction of the change of catalytic properties.

D. THECATALYTICACTIVITYOF OXIDESWITH THE NONEQUILIBRIUM CONTENTOF OXYGEN Winter (10)noted the effect of the conditions of the pretreatment of oxides on the catalytic activity relating to the isotopic exchange of molecular oxygen. The later investigations have shown that in particular cases the pretreatment in vacuum at high temperatures leads to a very strong increase of activity. This is most strongly manifested for zinc oxide. The samples of zinc oxide heated before testing the catalytic activity in oxygen possessed noticeable activity with respect to the molecular oxygen exchange at high temperatures (over 400'). At 450" and an oxygen pressure of 40 torr the rate of exchange is 4.8 x loll molecules/cmasec while the activation energy is about 40 kcal/mole. I n this case, the rates of exchange in molecular oxygen and the rates of isotopic exchange with oxide oxygen are very close. Winter ( 1 ) has investigated the activity of zinc oxide after treatment in vacuum for 16 hr at 630" and injecting oxygen after cooling the sample to the temperature of the experiment. Under these conditions, the noticeable exchange in

320

G . K . BORESKOV

molecular oxygen could be detected even at 120' while at 216" the rate of exchange reached 8 x lo8 molecules/cm*sec. A still more effective result was obtained by Barry and Stone (23).One of their samples of zinc oxide, obtained by the decomposition of carbonate, possessed the activity equal to 8.13 x loll molecules/cmasec at - 193°C. Unfortunately, the authors did not point out the conditions of the pretreatment. Gorgoraki, Boreskov, Kasatkina, and Sokolovsky subjected the sample of zinc oxide ("for luminophors") t o heating in the air at 860" for 6 hr. After cooling, the sample was powdered and pressed into tablets. Just before carrying out measurements of the activity the sample was treated in the reaction vessel at 400" for 8 hr. The isotopic exchange of molecular oxygen was proceeding very rapidly on the sample prepared by this method even at 25", but activity was unstable and was gradually decreasing, becoming unmeasurably small in about 6-8 hr (Fig. 11). The repeated treatment in vacuum at 400" completely restored the high original activity. At low temperature ( - 63" and - 194") the sample possessed low but stable activity which remained constant during a 3-hr test. The activation energy at low temperatures was 0.2 kcal/mole. The comparison of zinc oxide activities at high and low temperatures is demonstrated in Table VII. I t is seen that the zinc oxide sample activated after the abovementioned method possesses at - 63" similar activity as at 425", while the original activity at 26" is much higher than at 600". The heating of the activated sample in oxygen leads to the complete and irreversible loss of activity at low temperatures; its catalytic properties become identical

Minutes

FIQ.11. Activity reduction of activated sample of zina oxide at 26'.

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

321

TABLE VII

The Rate of Exchange i n Molecular Oxygen on Zinc Oxide Sample Activated at 850°C ~

~~

Temp. ("C):

-194

-63

0.043

0.09

25

450

475

600

26-tO.015 0.10

0.37

0.93

2.2

The rate of exohange molecules

( Z G Z x 10-18) Activation energy (kcal/mole) :

\---/

0.2

426

-

39.7

to the ones of the nonactivated sample. Air heating under the similar conditions as at the original activation temperature (850") also leads to the complete loss of activity at low temperatures, but after powdering and pressing into tablets the activity comes back to the initial value, This evidence permits the assumptions that the catalytic activity at low temperatures is conditioned by the accumulation of the excess zinc in the activated sample. The zinc oxide crystals, owing to prolonged heating at 850", can be enriched by the excess zinc to a considerable depth. When this sample is cooled the surface layer is oxidized while the low temperature activity is lacking. In order for this activity to be manifested, it is necessary to renew this surface by powdering and repeatedly pressing the sample into tablets. The reduction of activity at 25Ois due to the slow bonding of oxygen by the oxide surface. Under these conditions the oxygen is bonded weakly and can be removed by treatment in vacuum at 400". At higher temperatures the oxygen is bonded very stably and reacts with the excess zinc so that the catalytic activity disappears irreversibly. In order to test the above assumption the zinc oxide sample was treated at 450" with zinc vapors. As a result of this procedure we have obtained a catalyst approaching in its activity at low temperatures to the one described above. However, it still remains unclear what form assumes the excess zinc in zinc oxide responsible for its catalytic activity. The abrupt increase of catalytic activity at low temperatures due to the high temperature treatment is observable also for magnesium oxide. The comparison of data obtained by Winter (1)with other data (5)shows that the treatment at 540" for 16 hr makes the sample of magnesium oxide active with respect to the molecular oxygen exchange at room temperature and lowers the activation energy from 40 down to

322

a. K. BORESKOV

6.5 kcal/mole. There is no evidence concerning the concept of the nature of changes occurring in magnesium oxide responsible for the increase of activity at low temperatures. A sample of nickel oxide heated in the air at 800" for 3 hr with the powdering and pressing into tablets as well as treating in vacuum at 400" for 8 hr also possesses high activity at low temperatures (9).At 25' the rate of exchange in molecular oxygen reaches 1.2 x 1013 molecules/ cme sec with this sample. I n long-time testing, the activity gradually reduces but much more slowly than in the case of zinc oxide; within 27 hr the activity reduced threefold. The initial activity of this sample of nickel oxide at 25"is 28 times higher compared with the activity at 350". It may be possible that the increased activity of the sample treated by this method is related t o the formation of metallic nickel (34). I n all the cases considered the high activity of oxides at low temperatures is consistent with the nonequilibrium content of oxygen in oxide and disappears after the treatment with oxygen at elevated temperature, This isotopic exchange of molecular oxygen at low temperatures is not accompanied by the isotopic exchange with oxide oxygen. This means that the exchange of molecular oxygen proceeds under these conditions without the participation of surface oxygen of oxide, that is, following the first mechanism (see Section I, C). It may include the dissociative adsorption-desorption or surface formation of four-atom activated complex composed of two oxygen molecules. The evidence in favor of the choice between these two versions is still missing. The most remarkable and hardly explicable event is that the process of the exchange reaction itself is very rapid at the temperature of liquid nitrogen and possesses a very small activation energy, although it is related with the break-up of the stable bonding in molecular oxygen.

E. CATALYTICACTIVITYOF METALSWITH RESPECTTO THE EXCHANGE REACTION OF MOLECULAROXYGEN Catalytic action of metals on isotopic exchange of molecular oxygen has been investigated early by Margolis (17).The exchange was observable at 320' on platinum pretreated for a long period with oxygen at 300 to 400'. The exchange could be detected on silver only at 220'. Margolis and Kiselev (3) have investigated the effect of additions on the silver chloride and iodide. With the increase of chlorine content from 0.0015 up to 0.015%, namely, 10 times the exchange rate decreased up to 3 times. With the addition of iodide the exchange rate of molecular oxygen increased. Hasin and Boreskov (33) have investigated the isotopic exchange in

323

CATALYSIS OP ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

oxygen using platinum films obtained by the evaporation of platinum in vacuum. The adsorption of oxygen was first measured on the freshly prepared films. It included the fast and slow steps. At 20' the rapid adsorption reached 0.6 x 1015 atoms/cma, that is, about one-half of the monolayer; at 250" the rapid adsorption exceeded the value of the monolayer. The slow sorption of oxygen continued during the entire experiment. The total quantity of the sorbed oxygen before experiments of exchange was from 2 to 4 monolayers. TABLE VIII The Rates of Isotopic Exchange of Molecular Oxygen on Platinum Jilm under 0.5 tow Presaure

Preliminary treatment of film with oxygen

Surface Temp. of after treatment (cm?

Rate of Exchange

x

10-11

~~

Adsorption at 250"

200

200

3.7

Adsorption at 20" followed by the heating and adsorption at 250'

800

200

3.2

Adsorption at 250"

200

250

17

Adsorption at 20"followed by heating and adsorption at 260"

800

250

20

Table VIII lists the rates of exchange of molecular oxygen for various films under an oxygen pressure of 0.5 torr. The activities of films subjected to the different treatment with oxygen are very close. The activation energy of exchange is l6.5f 1.5 kcal/mole. The order of thereaction of exchange with respect to oxygen approaches 0.5. The exchange of the adsorbed oxygen with molecular oxygen has been investigated on the same films. I n order to increase the accuracy of measurements, oxygen, enriched with the heavy isotopes (38 % Ole) was adsorbed and the increase of concentration of 01* was measured after the filling of the reaction vessel with natural oxygen. The results of the measurements are given in Fig. 12. The break-up of curves is related to the addition of natural oxygen for the maintenance of the required pressure, which may decrease due to the selection of samples for analysis. The form of the curves indicates the strong reduction of the rates of exchange at small degrees of exchange (lessthan 0.1 of a monolayer) which,

324

Q.

K. BORESKOV

reveals the nonuniformity of the adsorbed oxygen. The activation energy of exchange of the adsorbed oxygen, with the depth of exchange equal to 0.15 of a monolayer, is 2 8 f 3 kcal/mole. Under the assumption of the considerablenonuniformity of the adsorbed oxygen, it is possible to calculate the change of the activation energy of the exchange AE with the time of exchange T according to the following equation ( 5 6 , 5 7 ) :

AE

=

2.3RT A log 7

(20)

Figure 13 (curve 1) shows the dependence of the activation energy of exchange AE found by the above-mentioned method on the extent of exchange X determined as a fraction of a monolayer of the adsorbed oxygen which became equilibrated with the gas phase. The dotted section of the curve corresponds to the extrapolation to X = 0. The results obtained permit us to conclude that the oxygen adsorbed on the surface of platinum film is nonuniform while the activation energy of its exchange increases by 6 kcai/mole within a 25% range of the monolayer. The difference of the experimental values of the activation energy of exchange in molecular oxygen ( E l ) and of the exchange energy of the adsorbed oxygen ( E , ) cannot only be explained by the nonuniformity of surface. The recalculations to the zero degree of exchange according to the data of Fig. 13 yield for E the magnitude of 28 - 6 = 23 kcal/mole while for El it is 16.5 kcal/mole. If one proceeds from the assumption that both processes of exchange follow the adsorption-desorption mechanism,

-

0

~

5

10

15

Hours

FIG.12. Isotopic exchange of molecular oxygen with the oxygen adsorbed on platinum; 1--200';

2-250".

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

325

the indicated difference may be explained by the fact that the activation energies El and E , have been measured for the sections of various degree of coverage with oxygen. Under these conditions the observable activation energy of the isotopic exchange is equivalent to the activation energy of oxygen desorption,

E,=E+q where E is the activation energy of adsorption while q is the adsorption heat. The exchange in molecular oxygen, as it has already been shown, proceeds, presumably, on the sections whose degree of coverage approaches u,where a is the ratio of the variation of activation energy of adsorption to the one of the heat of adsorption:

AE

=

-u

A ~

The experimental order of the exchange reaction with respect to oxygen pressure equivalent to 0.5 permits the conclusion that for the process

6

W-

a

y

I

0

0.1

I

I 1

0.2

0.3

0.4

X FIG.13. The change of activation energy of the oxygen adsorbed on platinum film with the growth of the depth of exchange.

326

G. K. BORESKOV

of oxygen adsorption, a is close to 0.5. The activation energy of exchange of molecular oxygen to be observed in this case is

El

=

E

+ 0.6q

The substitution into Eqs. (21) and (22) of the values El = 16,5 and = 23 kcal/mole which are found from the experimental data permits determination of the heat of oxygen adsorption. On the surface sections of platinum with the degree of coverage close to 0.5 a t 0.5 torr pressure and at 200" temperature, the heat of oxygen adsorption is about 13 kcal/ mole increasing in the range of 25% of a monolayer up to 25 kcal/mole. The reaction of exchange in molecular oxygen proceeds mainly on the sections constituting about 3% of the whole surface of platinum film. The rate of exchange of molecular oxygen on the silver film at 350" C is ten times less than that on the platinum film. The energy of activation is 19 kcal/mole. Oxygen adsorbed on the silver film is uniform. The higher rate of exchange of molecular oxygen discovered in the work just discussed, as compared to the data of Margolis (I?'),is possibly related to the purer surface of platinum film.

E,

F. SOMEGENERAL CONCLUSIONS RELATING TO THE CATALYSIS OW THE ISOTOPIC EXCHANUE OF MOLECULAR OXYGEN The catalytic activity of oxides with the equilibrium content of oxygen with respect to the considered reaction is stable and may be well reproduced. For all the oxides, the close coincidence of the exchange rates of the molecular oxygen and the isotopic exchange of oxide oxygen have been discovered. This implies the fact that the reaction of exchange proceeds with the participation of oxide oxygen surface. For some oxides, it is established that the exchange of the molecular oxygen is accompanied by the isotopic exchange of the two atoms of oxide oxygen. This means that the exchange of the molecular oxygen follows the adsorptiondesorption mechanism with the intermediate dissociation into atoms and the very fast and reversible reduction of them into anions of crystal lattice oxygen of the oxide surface. The specific catalytic activity of different oxides varies in the very wide range of about 108times. It is determined mainly by the oxygen bonding energy on the oxide surface depending on the electronic structure of metal cation. The oxides of transition metals with the number of delectrons of cation 3, 7, 8, and 9 possess the highest activity. The simple relation of catalytic activity with respect to the exchange to the work function of electrons of oxides is missing. A special treatment a t high temperatures leads, in particular cases, to

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

327

the abrupt increase of cataiytic activity of some oxides with respect to the molecular oxygen exchange at low temperatures. The isotopic exchange with oxide oxygen under these conditions does not take place. This low temperature activity is unstable especially at elevated temperatures and is obviously related to the distortion of the equilibrium content of oxide due to the treatment. The nature of distortions responsible for the occurrence of catalytic activity is not established; in the case of zinc oxide cause is probably the metallic zinc accumulating in the interstitial positions or precipitated in the form of adsorption layer on the surface of oxide crystals (35). Under the realization of isotopic exchange with metals (platinum and silver) a pronounced sorption of oxygen occurs in the surface layer of the metal corresponding to several monolayers. I n this form, the metals being investigated behave, with respect to the catalysis of molecular oxygen exchange, very similarly to the oxides. In the magnitude of activation energy and the absolute rate of exchange, platinum is very close to the most active oxide Co,O,. The oxygen sorbed on platinum as well as the surface oxygen Co,O, is not uniform in activity with respect to the exchange; the exchange in molecular oxygen proceeds with the participation of sorbed oxygen which occupies only 3% of catalyst surface.

G. THERELATION OF CATALYTIC ACTIVITY OF OXIDESWITH RESPECT TO THE OXYGENEXCHANGE TO THE ACTIVITY WITH RESPECT TO SOME OTHER OXIDATIONREACTIONS It was shown in the preceding sections that the catalytic activity of oxides and of the metals saturated with oxygen with respect to the isotopic exchange of molecular oxygen is mainly determined by the oxygen bonding energy in the surface layer of a catalyst. It is quite natural to assume that the oxygen bonding energy may be of essential importance also for the occurrence of some other oxidation reactions on these catalysts taking place with the participation of oxygen. It is of interest, therefore, to compare the catalytic activity of oxides to these processes. The data listed below are restricted by the oxides with equilibrium content of oxygen. Accordingly, for comparison we take the data with respect to the oxidation reactions realized in the excess of oxygen in the reaction mixture. It would be of interest to establish to what extent the oxides with nonequilibrium content of oxygen, being active with respect to oxygen exchange at low temperatures, possess the ability to accelerate other oxidation reactions, although the required data are lacking.

328

Q.

K. BORESEOV

1. Catalytic Oxidation of Hydrogen

The comparison with the activity with respect to the hydrogen oxidation is particularly reliable since this reaction was studied by Popovsky and Boreskov (36) on the same samples of oxides isotopic exchange in molecular oxygen had been investigated in other work (5). The investigation has been carried out using the stationary-circulation method (37) which allows the direct measurements of reaction rates at the excess of oxygen. The data for manganese dioxide are adopted from the work of Bruns (38).Figure 14 shows the comparison of specific catalytic activities expressed in gram moles of oxygen/meterahr for the oxides of transition metals of the fourth period with respect to the reactions of hydrogen oxidation as well as to the isotopic exchange in molecular oxygen. The complete similarity of the change in activity of the investigated oxides with respect to the said reactions is evident, Table IX lists the values for activation energies of hydrogen oxidation; they are much lower than the activation energies of isotopic exchange of oxygen on the same oxides (Table I). The similarity with respect to the catalytic activity in the reaction of oxygen exchange and hydrogen oxidation also appears on vanadium catalysts promoted by the sulfates of alkali metals (19). 2. The Oxidation of Carbon Monoxide

The abundant literature relating to the activity of different oxides

2 -

3 4 Y 0

-0

5 -

6 -

10

I

Ti02

I

I

V205 Cr,O,

I

I

Mn02 F%O,

1

Co304

I

I

NiO

CuO

I

ZnO

FIG.14. Specific catalytic activity of oxides of metals of the fourth period in the exchange of molecular oxygen and oxidation of hydrogen. Curve I: reaction of isotopio exchange of molecular oxygen; curve 11: oxidation of hydrogen in the excess of oxygen.

329

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

with respect to the oxidation of CO has been treated by Krilov (39). He made an attempt to take into account the reliability of data presented by various authors by introducing the corresponding coefficients into the statistical treatment. The results of such a treatment for the oxides of interest are listed in Table X where activity is characterized by the relative rate of reaction, taking the rate of reaction on nickel oxide as 100. They correspond to the following sequence of the change in activity of the different oxides: MnO, > Co,O, > NiO > CuO

> ZnO > TiO, > Fe,O, > V,O, = Cr,O,

that being very close to the experimental one for isotopic exchange of oxygen. Stone (all),on the basis of the analysis of literature data concerning the oxidation of CO, gives the following succession of the change of catalytic activity of oxides : COO > NiO z MnO,

ZnO > TiO, > Cr,O, > VaOs

CuO > Fe,O,

from 150' up to 400'

from 0" up to 150°

TABLE IX Activation Energy of Reaction of Oxidation of Hydrogen on Oxides of Metal8 of the Fourth Period

Oxide: Activation energy (kcal/mole):

V,O,

Cr,O,

MnO,

Fe,O,

Co,O,

NiO

CuO

ZnO

18

18

14

16

11

14

13

24

TABLE X Relative Catalytic Activity of Oxides of Metals of the Fourth Period with Respect to Cavbon Oxide Oxidation

Oxide

,

Ti0 VaOs Cr,O,

MnO a

FeaOa COS04

NiO CUO ZnO

Relative catalytic activity 18 11 11 310 16 200 100 83 22

330

G. K. BORESROV

The most active in this series are also the cobalt oxide, nickel oxide, manganese oxide, and copper oxide. 3. The Complete Oxidation of Hydrocarbons Gerald and Horwatisch ( 4 1 ) have recently investigated the catalytic activity of various oxide and metal catalysts with respect to the reaction of complete oxidation of methane in an excess of oxygen. The activity was characterized by the temperature a t which the reaction proceeds at a definite rate. According to the results of this investigation, catalysts may be grouped in the following succession with respect to the reaction of the complete oxidation of methane. Co,O, > MnOa > Pt > NiO z CuO > CraO, > Fa,O, > ZnO

This is very close to the succession of activities with respect to the reaction of exchange in molecular oxygen. Stein and co-workers (42) have been investigating the activity of a large number of oxides with respect to the complete oxidation of hydrocarbons with 5 or 6 atoms of carbon. The most active in all cases appeared to be Co,O,. High activity was discovered in most cases in manganese oxide and nickel oxide while the smallest activity was shown by zinc oxide and vanadium pentoxide. 4. Oxidation of Ammonium

Kurin and Zaharov (43) have found that the activity of oxides with respect to the oxidation of ammonium a t increased pressures lowers in the following succession: Co,O, > MnO, > Cr,O, > CuO > NiO > Fe,O, > ZnO > TiO,

For the convenience of comparison the data for the activity of all oxides are recalculated for 700" temperature. With the exception of chromium oxide this succession is very close to the experimental one for reactions of isotopic exchange in oxygen. 5. Oxidation of Sulfur Dioxide

For this reaction the comparison of activity of various oxides cannot be carried out because most of them transform in reaction conditions into nonactive sulfates. The exception is vanadium pentoxide whose activity strongly increases when promoted by sulfates of alkali metals. As is clear from Fig. 15, the catalytic activity of vanadium catalysts, with the addition of different sulfates of alkali metals, changes identically in reactions of isotopic exchange in molecular oxygen and in the oxidation of sulfur dioxide.

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

331

The examples cited clearly show that, generally, the changes of catalytic activity for molecular oxygen exchange and for some oxidation reactions are similar in series of oxide. The occasional deviations are due to the differences in the catalyst preparation techniques, degree of purity of the chemicals used, and of the testing conditions employed. I n many cases (as, for example, in oxidation of hydrogen), this similarity is so striking that the subject reactions appear to have a common limiting stage. I n hydrogen oxidation, chemisorption of oxygen may be such common limiting stage. However, the data in Tables I and I X show that the activation energies of oxygen exchange and of hydrogen oxidation reactions are substantially different. The rate of reactionoxygen pressure correlations of these reactions are also different, For example, Roiter and Yusa ( 4 4 )show that in oxidation of hydrogen over V,O,, the reaction rate is proportional to hydrogen pressure and that oxygen pressure has no effect upon the rate; a t the same time, Table I shows that the exchange rate varies as the 0.8 power of the oxygen pressure. These observations compel to discard the concept of a common reaction limiting stage. Thus, the unavoidable conclusion is that catalytic activity of the oxides studied is due to some identical, though quantitatively different, property. It seems logical to assume that this property is the bonding energy of the oxygen in the surface layer of an oxide.

Na2S04

Rb2S04

FIG.16. Catalytic activity of vanadium pentoxide promoted with sulfates of various alkali metals. Curve 1-reaction of exchange of molecular oxygen; curve 2-reaction of oxidation of sulfur dioxide.

332

Q.

R. BORESKOV

I n the isotopic exchange the magnitude of the bonding energy determines the activation energy of adsorption and desorption of oxygen, while in the oxidation of hydrogen it determines the activation energy of hydrogen interaction with oxygen on the oxide surface. Similarly, the activation energy of the interaction of the substance to be oxidized with the surface oxygen of oxide may depend on the oxygen bonding energy in the surface layer of oxide for other oxidation reactions. The oxygen bonding energy is, thus, one of the important factors determining the activity of oxides in the oxidation reactions. The rate of isotopic exchange in molecular oxygen, which can be easily determined experimentally, may serve as a convenient quantitative characteristic of the reactivity of surface oxygen of oxide. The task of predicting the catalytic action in the oxidation reactions remains, nevertheless, sufficiently complicated by the effect of the large number of some other factors. Thus, the catalytic activity may be limited by the stability of oxide phase under conditions of catalytic reaction; as an example, we may take the reaction of oxidation of sulfur dioxide which has been considered. In the process of this reaction the activity of most oxides is very low due t o the conversion of oxides into sulfates. If Co,O, were a more stable phase under reaction conditions compared with cobalt sulfate, then cobalt catalysts would perhaps be much more active than vanadium ones in the oxidation of SO,. A complex dependence on the reactivity of surface oxygen can be expected also for the process of partial oxidation when the output of the desired product is determined by the ratio for the rates of a series of concurrent and consecutive reactions. In some cases, the limiting steps of oxidation reactions may be the interaction of the substance to be oxidized with catalyst (chemisorption) but not the consequent reaction with surface oxygen of a catalyst. Furthermore, it is possible that some oxidation reactions are effected by the interaction of the substance to be oxidized with molecular oxygen. This possibility was considered in detail by Margolis ( 4 5 ) .I n the latter two cases, the dependence on the reactivity of surface oxygen of oxide may be absent or very small. Finally, even in the case when interaction with the surface oxygen of oxide is the limiting step its simple relation to the oxygen exchange reactivity may be complicated by the fact that the oxidation reaction decreases the oxygen concentration in the surface layer of oxide, and this fact may essentially affect the properties of surface oxygen. The reactivity of surface oxygen under reaction conditions may, therefore, strongly differ from the reactivity found in the oxygen atmosphere due to the measurements of the rate of isotopic exchange reaction. In a greater or lesser degree, this complication should be dis-

CATALYSIS O F ISOTOYIC EXCHANQE IN MOLECULAR OXYGEN

333

played in all cases. It can be eliminated, however, by the determination of the rate of exchange of molecular oxygen during the reaction under investigation. I n spite of the great number of the above mentioned complications, the oxygen bonding energy in the surface layer of oxide may serve as the basis for the generalization of experimental data as well as in the development of catalysts for the reactions involving the transitions of oxygen atoms between the molecules of reactants.

H. TEEPARTICIPATION OF THE OXYUENOF OXIDECATALYSTS IN OXIDATIONREACTIONS The oxides of transition metals have found a wide use as active components of oxidation catalysts. For the elucidation of the mechanism of oxidation reactions related to the transition of oxygen between molecules of reactants, it is very important t o establish whether catalyst oxygen participates directly in these transitions. One may suppose that the oxygen bounded to the reactant to be oxidized is taken from the surface layer of crystal lattice of a catalyst while the reduction of oxygen in the lattice is continuously compensated at the expense of the second reactant yielding the oxygen. It should be emphasized that these elementary processes of the bounding and yielding of oxygen are not connected with the phase conversions of the catalyst. The second version of the mechanism is based on the assumption that the transition of oxygen from the oxidizing reactant to the reactant to be oxidized occurs without the intermediate conversion of oxygen into anion of the surface layer of crystal lattice of a catalyst. The form of the intermediate interaction of oxygen with catalyst essentially differs in this case both in energy and in nature of bonding from the oxygen bonding form in oxide. Comparing these versions, one should not tend to obtain the general solution: the validity of the one or the other assumptions depends on the properties of the catalyst as well as on the nature of reactants and also on the conditions of realization of the oxidation reaction. A good illustration of this concept is the information given in the present paper concerning the most simple reactions of oxidation, namely, the isotopic exchange in molecular oxygen. For oxides treated with oxygen a t high temperature resulting in the equilibrium composition of surface layer with respect to oxygen pressure, the rate of exchange in molecular oxygen coincides in all cases with the rate of isotopic exchange with surface oxygen of oxide. This justifies the validity of the first given version of the mechanism of

334

0.K. BORESKOV

the reaction. The form of the intermediate interaction of oxygen with catalyst coincides in this case with the oxygen bonding form in the surface layer of oxide or it is so close to this form that the transitions between them may be realized very rapidly. The existence of other forms of atomically bonded oxygen is least probable under these conditions; in any case, the rate of the formation of such a form should be much lower as compared to the rate of bonding of the oxygen in the surface layer of oxide, since, otherwise, the exchange of molecular oxygen could be realized through this form and its rate would be higher than the exchange of oxide with oxygen. On the contrary, for oxides with nonequilibrium content of oxygen in the surface layer obtained due to the special treatment and being active with respect to the exchange a t relatively low temperatures, the rate of exchange of molecular oxygen strongly exceeds the rate of exchange with oxygen of oxide; this is the proof of the validity of the second version of the mechanism. I n this case, the intermediate form of bonding of oxygen with catalyst differs considerably from the oxygen bonding form in the surface layer of oxide. These results may be useful also for the solution of the question relating the mechanism of morc complex oxidation reactions. The rate of exchange of molecular oxygen specifies the rate of bonding of oxygen by catalyst. If the oxidation reaction proceeds via the same form of intermediate interaction of oxygen with catalyst, this step will be common to both reactions. The mere comparison of rates is insufficient, however, for drawing conclusions relating the mechanism of oxidation reaction. As a result of the interaction with the substance t o be oxidized the concentration of oxygen in the surface layer of oxide under oxidation reaction conditions may be lower, and the rate of bonding of oxygen on oxides accordingly much higher compared with the conditions of the exchange of molecular oxygen. A higher rate of oxidation reaction compared with the rate of exchange a t identical temperatures and oxygen pressure does not exclude the possibility of the reaction of oxidation proceeding via the interaction with oxide oxygen. The direct measurements of the rate of transition of oxide oxygen into the products of reaction are necessary. Such measurements can be carried out by introduction of a heavy isotope of oxygen into the catalyst composition but the numerous complications do not always permit unique conclusions. Most investigations were concerned with the oxidation reaction of carbon monoxide on manganese peroxide, copper oxide, and some other oxides. The pioneer investigators (46, 47) came to the conclusion about the participation of oxygen of oxide catalysts in this reaction. Contrary

CATALYSIS O F ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

335

to this, Wainstein and Turovsky (48, 49) by carrying out the oxidation of CO on manganese peroxide and copper oxide enriched with 01*, did not find any noticeable change in isotopic composition of catalysts. The sensitivity of the method enabled discovery of the exchange of about 5% of the oxygen of catalysts which corresponds to the several monolayers, The authors believed, due to the experiments with the stepwise oxidation of reduced oxides using fractions of oxygen of various isotopic composition, that the mobility of oxygen in the investigated oxides is sufficiently large for equilibrating the isotopic composition both in the bulk and on the surface of catalyst under experimental conditions. The direct measurements of oxygen mobility carried out for MnO, by Kasatkina (27), and for CuO by Popovsky (28),have shown, however, that the oxygen of these oxides does not possess the noticeable mobility under experimental conditions in the oxidation of CO. Therefore, the investigations by Wainstein and Turovsky proving the lack of the participation of the oxygen of the bulk of these oxides in the oxidation reaction of CO on MnO, and CuO leave yet open the question relating to the participation of surface oxygen of the catalyst in the given reaction and especially relating the most active part of this oxygen. Winter (2)investigating the mechanism of oxidation of CO on NiO and Cr,O,, has come to the conclusion that reaction proceeds with the participation of only a small part of surface oxygen of the catalyst which does not exceed 2.5%. I n the case of Cu,O about 10 to 40% of surface oxygen participates in the reaction, with the increase of this percentage with the elevation of reaction temperature. Popovsky and associates (50) have investigated the participation of surface oxygen of Co,O, in the reaction of hydrogen oxidation a t 75" and found that in the treating of a sample at 400°,2-20% of the oxygen participates in the reaction, while with the prelimina,ryevacuation a t 75" it is from 13 to 39%. A low accuracy of the determination of the amount of participating oxygen is connected with the necessity of introducing corrections to the exchange between catalyst, molecular oxygen and water vapor. Roiter and co-workers have investigated with the aid of 018, the participation of vanadium pentoxide oxygen in reactions of oxidation. I n the oxidation of naphthalene (51) on vanadium pentoxide enriched with 018, no decrease of the content of this isotope in catalyst was discovered within the accuracy of measurements (- 10%). If all oxygen required for oxidation of naphthalene was removed from the vanadium pentoxide surface and the intermixing of oxygen inside catalyst were full, then the final content of in vanadium pentoxide should decrease to up t o 10-30% of the original content. The authors have concluded from this

336

Q. K. BORESKOV

fact that vanadium pentoxide oxygen does not participate in oxidation of naphthalene. Some doubt as to the validity of this conclusion is caused by the evidence of the low mobility of oxygen inside vanadium pentoxide crystals a t the temperature of investigations (340-390") resulting from the considerable deviation from the first order of kinetics of exchange with water vapor at these temperatures (20). When reaction of oxidation of sulfur dioxide (52)is realized on vanadium pentoxide enriched with Ole, one could observe the decrease in the content of this isotope in catalyst, but it is still not a greater decrease than in the exchange with oxygen in similar conditions. If the oxidation of sulfur dioxide proceeded at the expense of surface oxygen of vanadium pentoxide then the extraction of 0lsfrom catalyst should be considerably accelerated in the process of reaction. The doubt as to the insufficient mobility of catalyst oxygen, in this case, due to high temperature of experiments (500-610") is out of the question. The data obtained by Kasatkina for the transition in the process of catalysis of 0 1 8 from vanadium pentoxide into the formed sulfur trioxide are in some contradiction to these results. However, one should not conclude from this that vanadium pentoxide oxygen participates in the oxidation of sulfur dioxide, since it has been shown that SO, exchanges with the oxygen of vanadium pentoxide with a rate much higher compared to the rate of oxidation of sulfur dioxide. This fact strongly complicates the studying of the participation of oxide oxygen in this reaction. Roiter and Uza (44)have measured the rates of reduction of vanadium pentoxide by hydrogen and the oxidation with oxygen separately, and have compared them with the rate of catalytic oxidation of hydrogen on V,O,; they came to conclusion that the oxygen of vanadium pentoxide does not participate in the catalytic reaction. These results permit with sufficient confidence the conclusion that the oxygen of the bulk of oxides does not participate in the reaction of catalytic oxidation. This conclusion is in agreement with the general concept that reactions associated with phase transformation cannot be the steps of catalytic processes. It is much more difficult t o solve the question about the participation of surface oxide oxygen in catalytic oxidation. For reactions of carbon monoxide oxidation and hydrogen oxidation on NiO, CuO, and Co,O, the most probable is, as this writer believes, the direct participation in the reaction of some of the moat active parts of the surface oxygen of oxides, The following arguments together with the direct experimental data tend to support this conclusion: (1) analogy in the relationships of activities of various oxides in these reactions with reactions of exchange in molecular oxygen for which

CATALYSIS O F ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

337

the participation of surface oxygen of oxides is already established; (2) a prominent nonuniformity of surface oxygen of these oxides in reactions of exchange with oxygen; and (3) the impossibility of oxygen sorption with noticeable velocity on the surface of these oxides in atomic form which essentially differs from the form of oxygen bonding in the surface layer of crystal lattice of oxide. The last conclusion follows from the equality of rates of exchange in molecular oxygen and isotopic exchange with oxygen of oxide, It is not impossible, however, that a t lower temperatures or with the considerable deviation of oxygen content in the surface layer of oxide from equilibrium with respect t o oxygen, oxidation reactions may proceed through some other forms of chemisorbed oxygen as occurs in the case of exchange of molecular oxygen on oxides subjected to special treatment. I n the case of reactions of catalytic oxidation on vanadium pentoxide there is no sufficient evidence for drawing the unique conclusion. If, in accordance with the opinion of Roiter we conclude that oxygen of V,O, does not participate in the reaction, then the disagreement with the conclusion about the impossibility of rapid sorption of oxygen in some other atomic form may be eliminated with the aid of one of the following assumptions: (1) reactions of oxidation proceed with the participation of oxygen sorbed in molecular form; (2) the reaction of oxidation changes the composition and the properties of oxide surface, thus enabling the new forms of oxygen sorption; (3) the rate of oxygen sorption in atomic form greatly increases due to the simultaneous adsorption of the substance to be oxidized. The later concept suggested by Roiter (53) is most probable if the adsorption of oxygen and of the substance to be oxidized is accompanied by the electronic transitions in opposite directions. There is some experimental evidence about the positive effect of presorption of some other substance on the rate of sorption (54, 55). Further study of participation of oxide oxygen in reaction of oxidation using a wider range of oxide catalysts and reactions is of utmost interest.

111. Conclusion The study of the catalysis of isotopic exchange in molecular oxygen has begun rather lately and covers a limited number of catalysts and a rather narrow region of variation of conditions for carrying out reactions. Neverhheless, the results obtained enable us to hope that this type of reaction wil1 be a useful method for the study of the mechanism of reactions of oxidation, and especially for the elucidation of nature of intermediate forms of oxygen interaction with solid catalysts. From the data obtained 80 far the author believes the equality of

338

U. K,BORESKOV

rates of exchange of molecular oxygen discovered for catalysts with equilibrium content of oxygen in a surface layer to be of essential importance. This equality confirms the participation of catalyst oxygen in the exchange reaction, and also the identity or closeness, according to the bonding energy of oxygen chemisorbed by the oxides, with the oxygen of the mrface layer of the crystal lattice of oxides. The activity with respect to molecular oxygen exchange may, therefore, serve as a characteristic of the general reactivity of oxygen, and can be useful for discovering regularities in selection of catalysts for oxidation reactions. The phenomena of high catalytic activity with respect to oxygen exchange at low temperatures of some oxides subjected to severe treatment are interesting but still insufficiently studied. In these cases the exchange proceeds through some special intermediate forms of oxygen interaction with the catalyst. Clarifying the role played by these forms in catalysis of oxidation reactions is very important. REFERENOES 1. Winter, E. R. S., J . Chem. SOC.p. 1622 (1954). 2. Winter, E. R. S., J. Chem. SOC.p. 3824 (1966). 3. Margolis, L. J., and Kiselev, V. A., Dokl. Akad. NaukSSSR 130. 1071 (1960). 4. Dzisjak, A. P., Boreskov, G. K., Kasatkina, L. A.. and Kochurihin, V. E., Kinetika i Kataliz 2, 386, 727 (1961). 5. Deisjak, A. P., Boreskov, G . K., and Kasatkina, L. A,, Kinetika i Kataliz 8,81(1962). 6. Boreskov, G. K., Dziajak, A. P., and Kasatkina, L. A., Kineiika i Kataliz 4, 388 (1963).

7. Gorgoraki, V. I., Kaaatkina, L. A., and Levin, V. J., Kinetika i Katuliz 4,422 (1963). 8. Kasatkina, L. A,, and Antoshin, G . V., Kinetika i Kataliz 4, 252 (1983). 9. Boreskov, 0.K., Gorgoraki, V. I., and Kasatkina, L. A., Dokl. Akad. Nauk SSb’R 160, 670 (1963). 10. Winter, E. R. S., Adwan. Catalysis 10, 196 (1968).

11. Avdeenko, M. A., Boreskov, G. K., and Slinko, M. G., Probl. Kinetiki i Kataliza Akad. Nauk SSSR 8 , 01 (1957). 12. Boreskov, G. K., and Popovsky, V. V., Kinetika i Kataliz 2,667 (1961). 13. Winter, E. R. S., J . Chem. SOC. p. 1509 (1964). 14. Winter, E. R . S., J . Chem. SOC.p. 1617 (1964). 15. Winter, E. R . S., J . Chem. SOC.p. 2726 (1966). 16. Winter, E. R. S., J . Chem. SOC.p. 1170 (1950). 17. Margolis, L. J., Izv. Akad. Nauk SSSR p. 226 (1969). 18. Cameron, W. C., Farkas, A., and Litz, L. M., J . Phys. Chem. 17, 229 (1963). 19. Boreskov, G. K., Kasatkim, L. A., Popovsky, V. V., and Balovnev, J. A., Kinetika i Kataliz 1, 229 (1960). 20. Kasatkina, L. A., Boreskov, G. K., Krilova, 2. L., and Popovsky, V. V., Izv. Vyaahykh Uchebn. Zavedenii Khim i Khim. Teknol. 1, 12 (1968). 21. JM, P., and Novakova, J., Collection Czech. Chem. Commun. 28, 1 (1963). 22. Boreskov, G. K., “Catalysis in the Indu&hd Production of Sulphuric Acid.” Ctoshknizdat, Moskwa-Leningrad. 1954. 23. Barry, I. J., and Stone, I?. S., Proc. Roy. SOC.A255, 124 (1960).

CATALYSIS OF ISOTOPIC EXCHANGE IN MOLECULAR OXYGEN

339

24. Boreskov, G. K., Illarionov, V. V., Ozerov, R. P., and Kildisheva, E. V., Zh. Obshch. Khim. 24, 23 (1954). 25. Jird, P., Tomkova, D., Jara, V., and WankovA, J., Zt. Anorg. Allgem. Chem. 303, 121 (1960). 26. Batzanov, S. S., “Eleotronegativity of Elements and Chemical Bonding,” p. 65.

I z d . Sibirskogo Old. Akad. NaukSSSR, Published by the Siberian Section, Novosibirsk, 1962.

27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Kasatkina, L. A., and Boreskov, G. K., Zh. Fiz. Khim. 29, 3 (1955). Popovsky, V. V., and Boreskov, G. K., Kinetika i Kataliz 1, 566 (1960). Keyer, N. P., Kinetika i Kataliz 1, 221 (1960). Morozova, N. D., and Popovsky, V. V., Kinetika i Kataliz 3, 439 (1962). Germain, J. E., “Catalyse H6tBrogene.” Dunod, Paris, 1959; F. F. Volkenshtein, “Electronic Theory of Catalysis with Respect t o Semiconductors.” Moskwa, 1960. Temkin, M. T., Zh. Fiz. Khirn. 14, 1153 (1940); 15, 296 (1941). Hasin, A., and Boreskov, G. K., Dokl.Akad.Nauk S S S R -, - (1963). Klemm, W., and Hass, K., 2. Anorg. Allgem. Chem. 219, 1, 82 (1934). Thomas, D. G., and Lender, J. J.,J . Phys. Chern. Solids 2, 318 (1957). Popovsky, V. V., and Boreskov, G. K., Probl. Kinetiki i Kataliza Akad. Nauk SSSR, 10,67 (1960). Boreskov, G. K., Slinko, M. G., and Filippova, A. G., Dokl. Akad. Nauk S S S R 92, 353 (1963).

38. Bruns, B. P., Dissertation, Moskwa, 1959. 39. Krilov, 0. V.. Kinetika i Kutaliz 3, 502 (1962). 40. Stone, F. S., “Chemistry of the Solid State (W. E. Garner, ed.), p. -,London, 1955. 41. Gerald, N. G., and Horwatisch, H., Mikrochim. Acta 1-2, 7 (1962). 42. Stein, K. O., Feenan, I. I., Thompson, G . P., Shutts, I. F., and Hofer, L. J. E., Anderson, R. S., Ind. Eng. Chern. 52, 113 (1960). 43. Kurin, N. P., and Zaharov, M. S., “Catalysis in Higher Schools,” Vol. 11.Izd. MQU., 1962.

44. Roiter, V. A,, and Yusa, V. A., Kinetika iKataliz 3, 343 (1963). 45. Margolis, L. J., “Heterogeneous Catalytic Oxidation of Hydrocarbons.” Gostoptehizdat, Moskwa, 1962. 46. Titani, T., Nakata, S., and Kanome, A., Bull. Chem. Sac. Japan 17, 288 (1942). 47. Karpacheva, C. M., and Rozen, A. M., Dokl. Akad. Nauk S S S R 68, 1057 (1949). 48. Wainstein, F. M., and Turovsky, G. J . , Dokl. Akad. Nauk S S S R 72, 297 (1950). 49. Turovsky, G. J. and Wainstein, F. M., Dokl. Akad. N a u k S S S R 79, 1173 (1951). 50. Popovsky,V.V.,Boreskov, G. K.,and Muzikantov,V. C., Zh. Fiz. Khim. 35,192( 1961). 51. Stukanovskaya, N. A., and Roiter, V. A., Kinetika i Kataliz Akad. Nauk S S S R Sb. Statei p. 216 (1960). 52. Roiter, V. A., Stukanovskaya, N. A., and Votkovskaya, N. S., Ukr. Khim. Zh. 24, 37 (1958).

53. Roiter, V. A., Kinetika i Kataliz 1, 63 (1960). 54. Keyer, N. P., Investigation of the importance and nature of nonuniformity of active surfaces in chemisorption and catalysis. Dissertation, Moskwa, 1959. 55. Tamaru, K., Trans Faraday SOC.69, 979 (1963). 56. Roginsky, S. Z., “Adsorption and Catalysis on Nonuniform Surfaces.” Tzd. Akad Nauk SSSR, Moskwa, Leningrad, 1948. 57. Roginsky, S. Z . , Zh.Fiz. Khim. 32, 731 (1958). 58. Boreskov, G. K., Dokl. Akad. Nauk S S S R 127, 591 (1959). 59. Muzikantov, V. C., Popovsky, V. V., and Boreskov, G. K., Kinetika i Kataliz 4, 624,745 (1964).

This Page Intentionaiiy Left Blank

Author Index Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italic show the page on which the complete reference is listed.

A Aalbergsberg, W. I., 269(109), 271(109), 272(109), 283 Addy, J., 126(66), 143(60), 223 Albrecht, H., 128(67), 131(67), 173(67), 174(67), 183(67), 189(67), 194(67), 203(67), 223 Allessandrini, E. D., 62, 6 3 Amenomiya, Y., 108(39), 116(39), 119(39), 120(39), 222 Anderson, J. R., 163(62), 223 Anderson, R. S., 330(42), 339 Andrenes, L. J., 214 (127, 128, 129), 225 Antoshin, G. V . , 286(8), 302, 307(8), 338 Apel’baum, L.O., 72, 78, 89 Arnold, R. T . , 100(22), 222 Avdeenko, M. A., 287(11), 338

B Baker, R. H., lOO(18, 19), 132(19), 133(32), 139(18), 141(18), 142(18, 19), 222 Bakshiev, N. G . , 232(29), 247, 248(67), 251(67), 281, 282 Balk, P., 272(115), 283 Ballod, A. P., 266(70), 282 Balovnev, J. A., 317(19), 328(19), 338 Barachevsky, V. 257(78), 268(78), 270(78), 271(78), 272(78), 273(78), 274(78), 282 Berachevsky, V. A., 243(62), 244(62), 267 (77a), 258(77a), 259, 269(101), 270 (78a, 110), 271(78a), 272(78a), 281, 282, 283 Barakm, N. B., 228(10), 280 Baranov, E. V . , 268(80), 282 Barrer, R. M., 84, 90 Barry, I. J., 320, 338 Barter, C., 269(106), 272(106), 274(106), 283

Bartok, W., 246(63a), 281 Bassett, D. W.. 77, 89 Batzanov, S. S . , 314(26), 339 Bayliss, N. S . , 239(44), 240(44), 246(66a), 248(66a), 281 Becker, E. J., 30 Becker, J. A., 30 Beeck, O., 172(87), 224 Bell, J. M., 202(107), 224 Benesi, H. A., 249(64), 252(64), 282 Bennett, M. A., 99(11), 211(116), 212(116), 213(116), 222, 224 Bennett, M. J . , 4(14), 29, 79, 89 Beneon, R. E., 278(128), 284 Bertrmd, J. A., 213(124), 225 Bigeleisen, J., 267(76), 268(76), 261(76), 277(76), 282 Bijl, D., 276, 284 Block, J., 81(38), 90 Blois, M. S., 271(113), 283 Boitzova, Z . , 228(13), 280 Bokhoven, C., 84(42), 85(42), 87, 90 Bollinger, H., 194(104), 203(104), 224 Bond, G. C., 73, 89, 92(2), 94(6), 96(6), 98 (2), 103(2, 26, 28), 106(26, 28, 31), 106(31), 109(31), 116(2, 28), 124(31), 125(31), 126(31, 66), 127(31), 128(31), 129(31), 131(31, 68), 132(28, 31),134 (28, 31), 136(26, 28, 31), 136(26, 31), 137(31, 68), 143(31, 60), 146(31), 146 (31, 68), 147(31), 148(31),149(61), 160 (31, 58), 161(61), 152(61), 155(6), 168 (31, 71), 160(2, 6, 74, 78, 79, 80, 81, 83), 161(71, 83), 162(71, 83) 163(71, 83), 164(71, 80, 83), 166(71, 83), 168 (71, 78, 79, 80, 81), 170(71), 173 (71, 78), 181(68), 182(68), 184(98), 187(6, 100) 188(6, loo), 194(103), 196(103), 196(103), 197(103), 198(103),199(103),

341

342

AUTHOR INDEX

203(103), 204(58), 221, 222, 223, 224 Bonner, F. 110(46), 111(46), 112(45), 223 Boreskuv, G. K., 80, 89 286(4, 6, 6, 9), 287 (11), 288(12), 294(4, 6, 33), 295(5), 296(4, 20, 59), 297(4, 22), 299(5, 24), 300(5, 27), 302(5), 303(5), 304(5, 28), 305(5, 28), 313(6), 314(4, a), 315(6), 317(19), 321(5), 322(9), 328 (5, 19, 36, 37), 335(60), 336(20), 338, 338 Boudart, M., 78(27), 79(27, 30), 80, 87(55), 89, 90 Boyarchuk, Y . M., 275(122), 284 Brandes, R.G . , 30 Brandon, R. L., 216(139), 225 Braun, W., 229(23), 248(62), 249(62), 253 (62), 264(62), 262(62), 274, 278(62), 260, 282, 283 Brennan, D., 3, 5(1I), 7(11, 12), 9(11, 12), 11, 12(23), 14(11), 19(11, 12), 23(11, 12, 13), 29 Bridge, N., 276(127), 284 Broder, J. D., 40, 41(16), 62 Brower, D. M.. 269(107), 283 Brunauer, G., 85(48), 90 Bruns, B. P., 328, 339 Bryco, G., 2, 10(17), 28 Buben, N. Y . , 279(132), 284 Bukhtoyarov, I. F., 220(169), 226 Burger, R. M., 32(1), 62 Burwell, R. L., 92(4), 96(7), 100(14), 115 (49), 116(49), 118(49), 128(66), 137 (14), 139(14),140(14),141(14), 142(14), 173(90), 174(90), 176(90), 176(56, 93, 94), 184(90), 189(90), 190(90), 191(90), 194(90), l99(90), 201(90), 202(90), 221, 222, 223, 224

C Cameron, W. C., 297, 338 Campbell, K. C., 155(67), 223 Canale, I. J., 220(155), 225 Carrington, A,, 272(117), 283 Castle, J. E., 278(128), 284 Chatt, J., 212(120), 225 Chilton, D., 228(9), 280 Ciola, R., 96(7), 222 kiiek, I., 239(41), 281

Colburn, C. B., 108(34), 222 Colthup, E. C., 212(121,123), 220(121), 225 Conrade, J. J. 272(117), 283 Cookson, R. C., 212(122), 220(162), 225 Cope, A. C., 19l(lOl),224 Corner, E. S., 166(66), 223 Cousins, M., 218(171), 225 Cramer, R., 211(118), 224 Cramor, R. D., 219(147), 225 Crawford, E., 110(47), 111(47), 123(63), 223 Crawford, V. A., 228(20), 238(20), 280 Cross, P. C., 243(61), 246(61), 281 Cruse, K., 248, 282 Custers, J. F. H., 227(2, 3), 233(2, 3), 236 (2), 260(3), 262, 280

D Dallinga, G., 271(112), 283 D’Amico, C., 33(3), 62 Darnell, A. J., 2(1), 29 de Boer, E., 272(117), 283 De Boer, J. H., 65(1), 86(60), 89, 90, 227 (1, 2, 3, 4, 57 6, 7, 81, 233(2, 3,4), 235, 260, 261(8), 252(1), 263(6), 280 De Bowr, N. H., 73(12), 81(37), 82, 89, 90 de Bruijn, F., 272(115), 283 de Pauw, F., l68(86), 224 De Vries, B., 220(149), 225 Dibeler, V. H., 116(48), 116(48), 119(48), 120(48), 223 Dillon, J. A., Jr., 42(25), 65(36), 66(25), 63 Dinh-Nguyen, N., 139(69), 177(59), 223 Dippel, C. J., 227(3), 233(3), 260(3), 262(3), 280 Djigit, 0. M., 239(40), 281 Dmuchovsky, B., lOO(15, 16), lOl(l6), 104 (16). 132(16), 139(15), 140 (16), 141 (15), 142(15), 222 Dobson, N. A,, 183(97), 184(97), 224 Dorfmullsr, T., 274(119), 283 Dorgelo, G. J. H., 73(12), 89 Douglas, J. E., 103(26), 222 Dowden, D. A., 160(79), 168(79), 223 Dravnieks, F., 272(117), 283 Druz. V. A., 207(111), 224 Dubbell, D., 100(16), lOl(l6), 104(16) 222 Duffield, J. J., 214(134), 225 Dunkel, M., 100(20), 139(10), 141(20), 222

343

AUTHOR INDEX

Dzisjak, A. P., 286(4, 6, 6), 294(4, 6), 296 ( 5 ) , 296(4), 297(4), 299(5), 300(5), 302(5), 303(5), 304(6), 305(5), 313(6), 314(4, 6), 316(6), 321(5), 328(5), 338

t Eglington, G., 183(97), 184(97), 224 Ehrenberg, W., 36(8), 62 Ehrlich, G., 11, 16(34), 25, 29, 30 Eigen, M., 276(126), 277(126), 284 Eigenmann, G. W., 100(22), 222 Eischens, R. P., 81(34), 82(34), 90, 97(8), 98(8), 101(8), 119(8), 222, 228(19), 238(19), 280 Eisinger, J., 20, 29 Emel’yanova, 0. A., 220(169), 226 Emmett, P. H., 77, 85, 89, 90 Enomoto, S . , 86, 90 Erkelens, J., 118(52), 121(52), 123(52), 124(52), 132(52), 142(52), 153(63), 205(52), 223 Evans, A. G., 264(91), 266(91, 93), 283 Evans, D. F., 270(45), 281 Evdokimov, V. E., 275(122), 284 Eyring, H., 12(24), 17(24), 29, 108(34),222

F Fabian, D. J., 3, 8, 29 Fahrenfort, J., 81(34), 82, 89 Farber, M., 2(1), 29 Farkas, A., 110(43), 111(43), 168(86), 223, 224, 296(18), 338 Farkas, L., 110(43), 111(43), 168(86), 223, 224 Farnsworth, H. E., 32(1), 33(2), 34(4, 5, 6, 7), 35(27), 37(29), 39(10, 13, 14), 40 (15, 18), 41(15), 42(15, 23, 25), 43(26), 44(27, 28) 45(27, 29), 46, 47, 48, 49, 50, 51, 52(29, 53), 54(33), 55(27, 28, 35), 56(15, 25, 36, 37), 58(33), 59(38, 39, 40, 41), 60, 61(39, 42), 62, 63 Fasman, A. B., 207(112), 224 Feenan, I. I., 330(42), 339 Fensham, P. J., 79(30), 89 Fialkovskaia, 0. V., 241 (47, 47a), 243(47), 256(47), 281 Filippova, A. G., 328(37), 339

Fischer, E. O., 211(116), 224 Fletcher, P. C., 3, 6(11), 7(11, 12), 9(11, 12), 11, 14(11), 19111, l2), 23(11, 12, 13), 29 Flynn, J. H., 219(144), 225 Fogo, J. K., 269(108), 272(108), 273, 283 Fokina, E. A., 73, 89 Folman, M., 236(38), 237(38), 238(38), 239(38), 281 Frankenburg, W. G., 86(47), 90 Frankenburger, W., 83, 90 Freedman, J. F., 62, 63 Freidlin, L. K., 176(91, 92), 181(91), 182 (92), 224 Freidlin, L. Kh., 116(50), 223 Freundlich, H., 66, 89 Frumkin, A., 66, 89

G Galkin, G. A,, 233(30), 281 Galwey, A. K., 118(52), 119(62), 123(52), 124(62), 132(52), 142(52), 163(63), 206 (52), 223 Ganiuk, L. N., 275(123), 276(123), 284 Gardener, P. D., 216(139), 226 Garnett, R., 202(107), 224 Gatos, H. C., 41(20), 62 Gault, F. G., lOO(12). 142(12), 158(12), 186 (12), 202(12), 222 George, T. H., 32(1), 34(6), 62 Gerald, N. G., 330, 339 Germain, J. E., 318(31), 339 Germer, L. H., 36(8), 39, 62(31), 63(32), 54(31), 56, 62, 63 Gil-Av, E., 214(135), 225 Glasstone, S., 12(24), 17(24), 29 Clew, D. N., 214(132), 216(132), 225 Gobeli, G. W., 42(24), 63 Gold, V., 271(111), 283 Golub, S., 228(12), 280 Golubev, V. V., 275(122), 284 Comer, R., 23(27,28, 29, 30), 27(28), 29,30 Gorgeles, M. J., 87(52), 90 Gorgoraki, V. I., 286(7, 9), 306, 317(7), 322(9), 338 Cow, A. S., 219(146), 225 Grace, J. A., 266(93), 283 Gragerov, I. P., 276(123), 276(123), 284 Granick, S., 267(77), 258(77), 261(77), 282

344

AUTHOR INDEX

Gray, D. W., 177(96), 178(96), 179(96) 180(96), 224 Green, M., 40, 62 Green, M. L. H., 213(126), 217(126), 218 (126, 141), 225 Grignon-Dumoulin, A.. 160(82), 162(82), 224 Grimm, A., 128(67), 131(67), 173(57), 174 (67), 183(67), 189(67), 1941671, 203 (67),223 Gryaenov, V. M., 206(110), 224 Gundry, P. M., 73, 89 Guy, R. Gr., 99(10), lOO(lO), 211(113), 212 (113), 213(113). 220(113), 222, 224

H Habgood, H. W., 77, 89 Hefner, W., 220(166), 225 Hagstrom, H. D., 33(3), 62 Hall, W. K., 77, 89, 261(80a), 262(87, 88a, b, c ) , 264, 266, 266(88a, b, c ) , 267, 270, 271, 272(80a), 273, 282, 283 Halpern, W . , 100(16), lOl(l6), 104(16), 222 Hdphern, J., 103(29), 220(29), 222 Hamilton, W. M., 128(66), 176(66), 223 Haneman, D., 41(19, 21), 67(19), 62 Hansen, N., 81(36), 90 Hctrada, Y., 276(124), 284 Harrod, J. F., 103(29), 220(29), 222 Hartman, C. D., 36(8), 62(31), 63(32), 64 (31), 65(31), 62, 63 Hasegawa, H., 261(83, 84), 282 Hasin, A., 294(33), 322, 339 Hass, K., 322(34), 339 Hauser, E. A., 261(81, 82), 274(81, 82), 282 Hayward, D. O., 12(23), 29 H e i n e m m , H., 219(146), 225 Hepner, F. R., 214(130), 216(130), 225 Herling, J., 214(136), 225 Herrington, E. F. O., 166(68), 160(72), 223 Hewett, W. A., 220(166), 225 Hiokmott, T. W., 3, 8, 11, 16(34), 19, 29 Hill, T. L., 24, 30 Himelstein, N., 219(146), 225 Hirota, K., 81(34), 82(34), 89, 234, 274 (120), 281, 284

Hobson, M. C., 262(89), 266(89), 267(89). 283 Hodler, A., 83, 90 Hofer, L. J. E., 330(42), 339 Hoijtink, GI. J.. 268, 269(109), 271(109), 272(109, 116, lie), 283 Holmogorov, V., 249(63), 267(78), 268(78), 270(78), 271(78), 273(78), 274(78), 276(63), 282 Holmogorov, V. E., 268(80), 270(110), 282, 283 Horinuti, J., 69(8), 86, 87, 89, 90, 108(36. 3?), 222 Horwatsich, H., 330, 339 Houben, G. M.M., 227(6), 261,262,263(6), 280 Hulbert, H.M., 219(144), 225 Hulm, J. K., 23130). 30 Hulme, L., 239(44), 240(44), 281 Hussey, A. S . , lOO(18, 19). 132(19), 133 (19), 139(18), 141(18), 142(18, 19), 222

I Illarionov, V. V., 299(24), 399 Irsa, A. P., 110(48), l l l ( 4 6 ) , 112(46), 223 Issa, R., 233(31), 281 Ivanoiskaya, T., 3, 29

J James, B. R., 103(29), 220(29), 222 Jannokoudakis, D., 274(119), 283 J a m , V., 299(26), 339 Jdanov, S. P., 239(42), 281 Jenner, E. L.. 219(147), 225 Jim, R., 220(166), 225 Jirfi, P., 296, 297, 299, 310, 338, 339 Johnson, M. F. L., 268, 283 Johnson, W. E., 39(10), 62 Jonassen, H. B., 213(124), 216(138), 226 Jones, D. W.. 212(122). 220(162), 225 Jones, P. M. S., 266(93), 283 Jungers, J. C . , 84, 90, 168(86), 224

K Kageyama, V., 274(120), 284 Kainer, H., 276(121), 276(126), 277(126), 284

346

AUTHOR INDEX Kanome, A., 334(46), 339 Kaplan, L., 194(104), 203(104), 224 Karagounis, G., 233(31), 234(34), 281 Karpacheva, C. M., 334(47), 339 Kaaatkina, L. A., 286(4, 5, 6, 7, 8, 9), 294 (45), 295(5), 296(4, 20), 297(4), 299 (5), 300(5, 27), 302(5, 15), 303(5), 304(5), 305(5, 7), 307(8), 313(6), 314 (4, 6), 315(6), 317(7, 19), 321(5), 322 (9). 328(5, 19), 335, 336(20). 338, 339 Kaup, Y. Y., 176(91,92), 181(91), 182(92), 224 Kaup, Yu Yu, 116(50), 223 Kazanskii, B. A., 204(109), 224 Kazansky, V. B., 126(54), 223, 278(131), 279(131), 284 Keefer, R. M., 214(127, 128, 129), 225 Keenan, R. G., 85(48), 90 Keier, N. P., 73, 89 Keii, T., 108(38, 40, 4 l ) , 135, 222 Kemball, C., 84, 90, 92(3), 100(12), 102(3), 109, llO(42, 47), 111(47), 112(42), 113 (42), 114(42), 118(52), 121(42,52), 122 (42), 123(52, 53), 124(52), 132(52), 142 (12, 52), 146(42), 147(42), 153(42,63), 158(12), 185(12), 202(12), 205(52), 221,222, 223 Kemmett, R. D. W., 220(161), 226 Kemp, A. L. W., 103(29), 220(29), 222 Kennerley, G. W., 212(121), 220(121), 225 Kepner, R. E., 214(127, 128, 129), 225 Keyer, N. P., 304, 337(54), 339 Kildisheva, E. V., 299(24), 339 Kirsch, W. B., 216(138), 225 Kiselev, V. A., 286(3), 322(3), 338 Kisselev, A. V. 233(30), 238(39), 239(40, 41, 42, 43), 281 Kitova, A. I . , 271(112), 283 Klemm, W., 322(34), 339 Kobayashi, D., 245, 246, 281 Kochurihin, V. E., 286(4), 294(4), 296(4), 297(4), 314(4), 338 Kodera, T., 87(53), 90 Konigsberger, J., 243(50), 281 Kohn, H. W., 268(98), 283 Kon, H., 271(113), 283 Kondretiev, V. N., 228(12), 232(26), 280, 281 Konvalinka, J. A., 86(50), 90

Kortiim, G., 229(23,24), 230(24), 248,249, 253(62), 254, 262, 274, 277, 278, 280, 282, 283 Kotov, E., 257(78), 258(78), 270(78), 271 (78), 273(78), 274(78), 282 Kotov, E. I., 231(25,27), 255(27), 256(27). 257(27), 258(27), 259(27), 260(27), 270(78a), 271(78a), 280, 281, 282 Koutezky, Y., 239(41), 281 Kral, H., 81(38), 90 Kraus, J. W., 214(133), 215(133), 225 Kreidl, J., 219(146), 225 Krekeler, H., 220(160), 226 Krilov, 0. V., 329, 339 Krilov% z. L., 296(20), 336(20), 338 Krishnamurti, M., 183(97), 184(97), 224 KrYlOV, 0.v.9 73, 89 Kubler, K. G., 202( 107), 224 Kuchaev, V. L., 80;89 Kudriavzeva, W., 278(129), 284 Kumler, w. D.9 282 Kurbat'Jv, L. N.9 228(10, 11), 233(11), 245(54), 280, 281 Kurin, N. p . 9 3309 339 Kuwata, K.9 81(34)7 82(34)7 89, 274(120)9 284 Kwiatek, J., 220(148), 225

L Laidler, K. J., lO(19, 20, 21), 12(21, 24). 17(24, 29, 65(3), 83(3), 89, 108(33), 222 Lander, J. J., 41, 42, 6 3 Langmuir, I., 2, 4(15), 10(16), 29 Lasohkarew, W. E., 39, 62 Lavine, M. C., 41(20), 62 Lavrushin, V. F., 266(93), 283 Le Beau, D. S., 261(82), 274(82), 282 Lebed'ev, S. V., 194(105), 204(105, 108), 224 Leftin, H. P., 262(86, 87, 88a, b, c, 89), 263(88b, c). 264, 265, 266(86, 87, 88a, b, c, 89), 267(89), 282, 283 Legget, M. B., 261(81), 274(81), 282 Leigh, J. S., 262(89), 266(89), 267(89), 283 Lender, J. J., 327(35), 339

346

AUTHOR INDEX

Leto, M. F., 212(121), 225 Levin, V. J., 306(7), 317(7), 338 Lewis, G. N., 257(76), 258(76), 261(76), 277(76), 282 Linden, S. L., 194(104), 203(104), 224 Lindsey, R. N., 219(147), 225 Linke, G. C., 21(26), 29 Lipkin, D., 267(76), 258(76), 261(76), 277 (76). 282 Lippert, E., 248(59), 251(59), 282 Little, L. H., 97(9), 169(9), 222 Litvin, E. F., 116(60), 176(92), 182(92), 223, 224 Litz, L. M., 296(18), 338 Llomets, T. I., 176(92), 182(92), 224 Lodin, V., 249(63), 276(63), 282 Logan, S. R., 84, 90 Love, K. S., 85(48), 90 Lucas, H. J., 214(130), 216(130), 225 Lucchesi, P. J., 246(53a), 281 Lundy, R., 23(27), 29, 30 Luttinger, L. B., 212(123), 225 Lygin, V. I., 233(30), 239(43), 281

M McCaleb, a. 8.7 100(21), 139(21), 141(21), 822 Maokenzie, N., 160(79), 168(79), 223 Mackor, E. L., 269(109). 271(109, 1 W , 272(109), 283 MacRae, A. U., 39(12), 52(31), 54(31), 66(31), 55(31),02, 62, 63 63 McRae, McRae, E. E. G., G., 246(66a), 246(56a),248, 248, 281, 281, 282 282 Madden, Madden, H. H. H., H., Jr., Jr., 33(2), 33(2), 36(27), 35(27),44(27), 44(27), 45(29), 45(29), 46, 46, 49, 49, 50, 50, 61, 61, 66(27), 65(27),62, 6 2 , 63 63 J. L., L., 220(148), 220(148), 225 225 Mador, J. Mador, Mann, Mann, R. R. S., S., 160(81), 160(81),224 224 Margolis, L. L. J., J . , 286(3), 286(3), 296(17), 296(17), 322, 322, 326, 326, Margnlis, 332, 338, 339 Mars, P., 75, 86(19), 87(52), 89, 90 Marsh, J. B., 40(18), 66(37), 62, 63 Maruya, K., 77(26), 89 Masingill, J. L., 167(70), 196(70), 223 Mason, S. F.,243, 244(53), 281 Matsunaga, A., 87, 90 Matsunaga, Y., 276(124), 284 Matsuzaki, I., 103(30), 108(30, 36), 110 (30), 111(30), 222

Matthies, P., 276(126), 277(126), 284 Matveev, K. I., 220(159), 226 Maxted, E. B., 155(65), 223 Meier, R. L., 194(104), 203(104), 224 Melik, J. S . , 268, 283 Meriwether, L. S . , 212(121), 220(121), 225 Meyer, E. F., 173(90), 174(90), 176(90), 176(94), 184(90), 189(90), 190(90), 192(90), 194(90), 199(90), 201(90), 202(90), 224 Michaelis, L., 257(77), 268(77), 261(77), 282 Mignolet, J. C. P., 101(24), 160(24), 222 Miller, A. R., 10(18), 29 Miller, J. R., 213(114), 218(114), 224 Milliken, T. H., 250(72), 269(72), 282 Mills, G. A., 256(72), 269(72), 282 Mittag, R., 248, 282 Miyahara, K., 108(36), 222 Miyashita, I., 272(117), 283 Mochan, I., 3, 29 Moore, D. W., 213(124), 225 Moore, P. T., 191(101), 224 Moore, W. R., 191(101, 102), 224 Morozova, N. D., 316(30), 339 Morrison, J., 41, 42(24), 63 l V l u I 1 8 , 1 V l . A., . 4 1 * ( l J O ) ,

6&J

&fuller, N., 268(100), 283 Mulliken, R. 8.,268(100), 283 Murdoch, H. D., 211(119), 225 Muttik, c. G., 239(40), 281 Muzikantnv, V. C., 298, 336(60), 339

N N Nagakura, S., S., 246(66), 246(66), 281 281 Nagakura, Nagy, p. P. L. L. I., I., 213(125), 213(125), 217(126), 217(125), 218 218 Nagy, "r (125), "225 Nakai, Y., 81(34), 82(34), 89, 234, 2611 Nakanishi, J., 76, 89 Nakata, S., 334(46), 339 Neporent, B. S., 232(29), 281 Neuimin, H. G . , 245(54), 281 Nowham, J., 92(4), 221 Nix, N. J., 216(139), 225 Novakova, J., 296, 297, 310, 338 Nozaki, F., 77(26), 89 ,10r\

347

AUTHOR INDEX

0 Oblad, A. G., 256(72), 259(72), 282 Odyakov, V. F., 220(159). 226 Okuda, M., 74, 89, 236(37), 237, 238, 241, 242, 245, 246, 247(55), 248(48a, 55), 249(65), 252(65), 268(79), 260, 267, 274(48a), 281, 282, 283 Olechowski, J. R., 214(131), 215(131), 225 Orchin, M., 216(137), 225 O’Reilly, D. E., 262(87), 266(87), 283 Osijsov, A. M., 220(159), 226 Otaki, T., 81(39), 90 Otvos, J. W., 105(32), 110(32), 111(32), 112(32), 113(32), 115(32), 116(32), 128(32), 222 Ozaki, A., 77, 87, 89, 90 Ozerov, R. P., 299(24), 339

Porter, G., 276(127), 284 Poshkus, D. P., 239(41), 281 Prilejaewa, N., 278(129), 284 Pritchard, J., 23(31, 32), 30 Pyzhev, V., 85, 90

Q Quinn, H. W., 214(132), 215(132), 225

R

Rabinovitch, B. S., 103(26), 222 Rabinowitch, E., 228(9), 232(28), 280, 281 Rank, J. S . , 131(58), 137(58), 146(58), 150(58), 181(58), 182(58), 204(58), 223 Raphael, R. A., 183(97), 184(97), 224 Rasskin, Sh., 233(32), 281 Reid, C., 268(99), 283 P Reid, W. D., 160(76), 223 Reinacker, G., 81(35), 90 Parijsky, G. B., 278(131), 279(131), 284 Reusch, R. N., 212(121), 220(121), 225 Pariser, R., 268(102), 283 Rhyage, R., 139(59), 223 Park, R., 54(33), 58(33), 63 Rideal, E. K., 110(43,44), 111(43,44), 116 Parshall, G. W., 213(126), 225 (44), 155(68), 223 Pavlova, E. N., 236, 237, 240, 281 Rieche, A., 128(57), 131(67), 173(57), 174 Pease, R. N., 155(64, 66), 223 (57), 183(57), 189(57), 194(57), 203 Pershina, E. V., 233(32), 281 (57), 223 Peter, O., 234(34), 281 Rinehart, R. E., 220(153), 225 Pevear, P., 261(82), 274(82), 282 Roberts, J. K., 10(17), 29 Pfeiffer, P., 257(74), 282 Phillipson, J. J., 105(31), 106(31), 109(31), Roberts, M. W., 110(47), 111(47), 223 124(31), 125(31), 126(31), 127(31), Roberts, R. M., 269(106), 272(106), 274 (106), 283 128(31), 129(31),131(31), 132(31), 134 (31), 135(31),136(31),137(31),143(41), Robertson, A. J. P., 3, 8, 29 145(31), 146(31), 148(31), 150(31), 158 Robin, M., 240(46), 245(46), 246, 248(46), 252(46), 255(46), 281 (31), 177(95), 179(95), 180(95), 187 (99), 194(99), 199(99), 200(99), 203 Rogers, L. B., 214(134), 225 Roginsky, S. Z . , 73, 89, 324(56, 57), 339 (99), 222, 224 Roiter, V. A,, 331, 335, 336(52), 337, 339 Pickett, L. W., 268(100), 283 Romeyn, H., 220(153), 225 Pimenov, Y., 276(63), 282 Ron, A., 236(38), 237(38), 238(38), 239 Pink, R. C., 269(107), 271, 272(107), 283 (38), 281 Pliskin, W. A., 81(34), 82(34), 90, 97(8), 98(8), 101(8), 119(8), 222, 228(19), Rooney, J. J., lOO(12, 23), 101(23), 142 (12), 158(12), 185(12), 202(12), 210 238(19), 280 (140), 222,269(107),271,272(107),283 Ponomarchuk, M. P., 275(123), 276(123), Rose-Innes, A. C., 275(121), 284 284 Rosenberg, A. J., 41(20), 62 Popova, N. I., 204(109), 224 Popovsky, V. V., 288(12), 296(20, 59), 304 Rossini, F. R., 182(96), 224 (28), 305(28), 315(30), 317(19), 328 Roy, J. R., 216(137), 225 Rozen, A. M., 334(47), 339 (19, 36), 335, 336(20), 338, 339

348

AUTHOR INDEX

Rubalcava, H., 263(90), 283 Rylctnder, P. N., 219(146), 225

S Sabel, A., 220(166), 226 Sachtler, W. M. H., 81(34, 37), 82(34), 89, 90 Sauvage, J. F., lOO(l8, 19), 132(19), 133 (19), 139(18), 141(18), 142(18, 19),222 Savyelyeva, E. A., 205(110), 224 Scheibner, E. J., 36(8), 53(32), 62 Schissler, D. O., llO(46, 46), 111(46, 46), 112(45), 126(46), 223 Schlier, R. E., 32(1), 34(5), 40(15), 41(16), 42(16), 43(26), 66(16, 36), 62, 63 Schmidt, F., 252(66), 261(85), 268(106), 282, 283 Schneck, E., 229(23), 230(23), 280 Schnepp, O., 236(38), 237(38), 238(38), 239(38), 281 Scholten, J. J. F., 75(19, 20), 86(19, 60), 89, 90 Schreyer, 229(24), 230(24), 280 Schubert, M., 257(77), 258(77), 261(77), 282 Schuit, c f . C. A., 73, 89, 108(36), 172(88), 222, 224 Schwab, G. M., 229(23), 230(23), 280 Sedlmeyer, J., 220(166), 225 Seiwatz, R., 40, 62 Selwood, P. W., 92. 98(l), 221 Semba, K., 248(60). 282 Seyler, J. K., 220(148), 225 Sharpe, D. W. A., 220(16l), 226 Shaw, B. L., 99(10), 100(10), 211(113), 211 (113), 212(120), 213(113), 220(113), 222, 224, 225 Sheemoolees, V. I., 205(110), 224 Sheppard, N., 97(9), 160(9), 222 Sheridan, J., 160(72, 76, 76 78), 168(73, 76, 78). 170(75), 173(78), 184(98), 187 (loo), 188(100). 223, 224 Shenvood, P. W., 220(168), 226 ShibatE, K., 229(22), 280 Shooter, D., 69(41), 60, 63 Shryne, T. M., 220(166), 225 Shutts, 0. F., 330(42), 339 Sidorov, A. N., 233(30), 281

Sidorova, A. I., 257(77a), 258(77a), 260 (77a), 261(77a), 282 Sieber, R., 220(156), 225 Siegel, S., lOO(13, 16, 16, 17, 20, 21), 101 ( l a ) , 104(16), 108(13), 132(15, 17), 139(13, 15, 20, 211, 140(13, 15), 141 (13, 21), 142(13, 16), 222 Singer, J., 262(66), 261(85), 282 Sipitanos, C., 274(119). 283 Slinko, M. G., 288(ll), 328(37), 338, 339 Slygin, A., 66, 89 Smidt, J., 2201166, lb7, 160), 225, 226 Smith, G. V., lOO(13, 14, 16, 17), 101(16), 104(16), 108(13), 132(17), 137(14), 138(1?), 139(13, 14), 140(13, 14), 141 (13, 14), 142(13, 14), 196(70), 222 Smith, H. P., 220(163), 225 Smith, L. B., 157(70), 223 Sokolskii, D. V., 207(111, 112), 224 Sparke, M. B., 219(143), 225 Spector, M. L., 218(142), 225 Spencer, G. H., 243(61), 246(51), 281 Steele, D. R., 219(146), 225 Stein, K. O., 330, 339 Stephenson, H. P. 241(49), 281 Stern, E. W., 214(133), 215(133), 218(142), 225 Stevenson, D. P., 105(32), 110(32), 111(32), 112(32), 113(32), 116(32), 116(32), 128 (32), 222 Stolberg, V. G., 219(147), 225 Stone, F. S., 320,329,338,339 Stone,H., 269(106), 272(106), 274(106), 284 Strait, L. A., 282 Strelko, V. V., 275(123), 276(123), 284 Stukanovskaya, N. A., 336(51), 336(52), 339 Stull, D. R., 21(26), 29 Suhrmrtnn, R., 81(36), 90 Suider, N. S., 246(63a), 281 Suzdal’nitshaya, Y.V., 220(159), 226 Sverdlova, 0. V., 234(33), 239(33), 240 (33), 281 Symons, M. C. R., 266(93), 272(117), 283

T Tachibana, T., 74, 89, 249(65), 262(65), 268(79), 260, 267, 282, 283

349

AUTHOR INDEX Takezawa, N., 87,90 Tamaru, K., 66, 72(9), 73, 74(17), 76, 76, 78(27), 79(27, 30), 81(40), 82, 83(40), 84(9), 86(46), 87, 89, 90, 160 (777, 168(77), 223, 337(66), 339 Tamele, M. W., 266(71), 269(71), 282 Tanaka, J., 246(66), 281 Tanaka, K., 87, 90 Taylor, H., 79(30), 87(66), 89, 90 Taylor, H. S., 68, 84, 89, 90 Taylor, J. B., 10(16), 29 Taylor, T. I., 116(48), 116(48), 119(48), 120(48), 223 Tchheidze, I. I., 279(132), 284 Temkin, M., 72, 78, 86, 89, 90 Temkin, M. N., 318, 339 Terenin, A., 228(lO), 280, 281, 282 Terenin, A. N., 228(14, 16, 16, 17, 18), 231 (27), 236(14), 238(17, 18), 239(17), 241 (47a), 243(62), 244(62), 246(64), 249 (16, 63), 266(27), 266(27, 73). 267 (27, 76, 77a, 78), 268(27, 77a, 78) 259(27), 260(27, 7 7 a ) ~261(77a), 269 (101), 270(78, 78% 110), 271(78, 784, 272(78, Wa), 273(78), 274(78), 276 (63), 278(16), 281, 282, 283 Teysaie, P., 220(164), 225 Thomas, D. G., 327(36), 339 Thomaa, G. L., 266(69), 282 Thomas, J. H., 266(93), 283 Thompson, G. P., 330(42), 339 Thompson, S. O . , 110(46), 111(46), 126 (46), 223 Thomson, S. J., 79(29), 89, 166(67), 223 Thonon, C., 160(82), 168(82), 224 Titani, T., 334(46), 339 Tolkachev, V. A., 279(132), 284 Tomkova, D., 299(26), 339 Tompkins. F. C., 4(14), 23(31), 29, 30, 79, 89 Toots, J., 40(18), 62 Topchieva, K. V., 266(70), 282 Trapnell, B. M. W., 12(23), 29, 66(2), 67 (21, 83(2), 89 Traynham, J. G., 214(131), 216(131), 225 Trueblood, K. N., 214(130), 216(130), 225, 240(46), 246(46), 246, 262(46), 266 (as), 281 Turkevich, J., 103(26), 106(26), llO(46,

46), 111(46, 46), 112(45), 126(46), 136 (26), 136(26), 222, 223 Turner, L., 219(143), 225 Turovsky, (3. J., 336, 339 Tuul, J., 44(28), 46, 47, 48, 66(28), 69(39, 40). 61(39), 6 3 Tuxworth, R. H., 116(49), 116(49), 118 (as),223 Twigg, G. H., 103(27), llO(27, 44), 111 (27, 44), 116(44), 116(6l), 119(61). 120(61), 167(27), 168(27). 222, 223 Tye, F. D., 271(111), 283

U Uberle, A., 276(126), 277(126), 284

v Van, Heerden, C., 84(42), 86(42), 90 Van Reijen, L. L., 73(12), 81(34), 82(34), 89, 108(36), 172(88), 222, 224 Varshavsky, J. M., 271(112), 283 Verrijn Stuart, A. A., 271(112), 283 Vilessov, F. I., 267(76), 282 Villesov, F., 267(76), 282 Vinograd, J., 194(104), 203(104), 224 Vissotsky, Z. Z . , 276(123), 276(123), 284 Vogel, J., 248(62), 249(62), 263(62), 263 (62), 262(62), 278(62), 282 Vogt, K., 243(60), 281 Voltz, S. E., 78, 89 Votkovskaya, N. S., 336(62), 339 Voyatzakis, E., 274(119), 283

W Wagner, C. D., 106(32), 110(32), 111(32), 112(32), 113(32), 116(32), 116(32), 128(32), 222 Wainstein, I. M., 336, 339 Wankova, J., 299(26), 339 Watanabe, K., 267(94), 283 Webb, A. N., 264, 266(92), 266(92), 267 (92), 268(97), 283

360

AUTHOR INDEX

Webb, G., 94(5), 96(6), 149(61), 151(61), Winter, E. R. S., 286(1, 2, lo), 293(1, lo), 152(61), 162(84), 164(84), 165(84), 295(13, 14, 16), 300(2), 302, 304, 306 167(84), 173(84), 176(84), 177(84), (1, 14), 314, 319, 321, 335, 338 178(84), 179(84), 180(84), 187(5, 84), Wintarbottom, J. M., 94(5), 96(6), 105(31), 188(6, 84), 194(84, 103), 195(84, 103), 106(31), 106(31), 124(31), 125(31), 196(84, 103). 197(84, 103), 198(103), 126(31), 127(31), 128(31), 129(31), 199(84, 103). 203(103), 221, 224 131(31), 132(31), 134(31), 135(31), Wedler, G., 81(36), 90 136(31), 137(31), 143(31), 145(31), Weijland, W. P., 269(109), 271(109), 272 146(31), 148(31), 160(31), 168(31), (109, lie), 283 187(6), 188(5), 194(103), 195(103), Weiss, A. R., 115(48), 116(48), 119(48), 196(103), 197(103), 198(103), 199 120(48), 223 (103), 203(103), 221, 222, 224 Weiss, E., 211(119), 225 Wishlade, J. L., 79(29), 80 Weiss, F. T., 214(136), 225 Witt, H. S., 220(153), 225 Weissman, S. I., 272(117), 283 Wolff, G. A., 40, 41(16), 62 Weitz, E., 252, 261, 282 Wood, W. C . , 232(28), 281 Weitz, R., 261(85), 282 Woodcock, R. F., 58(38), 69(38), 63 Weller, S. W., 78, 89 Wortman, R., 23(27, 29), 29, 30 Wells, P. B., 94(5), 96(6), 106(31), 106 (31), 109(31), 124(31), 125(31), 126 (31), 127(31), 128(31), 129(31), 131 Y (31), 132(31), 134(31), 135(31), 136 (31), 137(31), 143(31), 145(31), 146 Yabubchik, A. O., 194(105), 204(106, (31), 148(31), 150(31), 161(61), 152 108), 224 (61), 166(6), 168(31, 71), 160(6), 161 Yagodovskii, V. D., 206(110), 224 (71), 162(71, 83), 183(71, 83, 84), 164 Yamashima, T., 61(42), 63 (71), 166(71), 166(71), 167(71), 168 Yaroslavsky, N. G . , 245(54), 281 (71), 170(71), 173(71, 79), 177(95), Yates, D. J . C., 97(9), 160(9),222,228(21), 178(95), 179(95), 180(96), 187(6, 89, 228(21), 280 99), 188(5, 89), 194(99, 103), 195(103), Yatsurugi, Y., 108(36), 222 196(103, l06), 197(99, 103), 198(103), Yokozawa, Y., 272(117), 283 199(99, 103), 200(99), 203(99, 103), Young, W. G., 194(104), 203(104), 224 221, 222, 223, 224 Youngman, E. A., 220(166), 225 Wenham, A. J. M., 219(143), 225 Yusa, V. A., 331, 336, 339 Werner, H., 211(116), 224 Westrik, R., 84(42), 85(42), 90 Z Wheeler, A., 155(69), 223 Zaharov, M. S., 330, 339 White, P., 278(130), 284 Zaitsev, N. S., 2, 29 Wiberg, K. B., 245(51), 281 Wilke, G., 211(117), 212(117), 213(117), Zakharkin, L. I . , 220(151), 225 Zhidareva, I. I., 220(161), 225 220(117), 224 Zhidomirov, G. M., 278(131), 279(131), Wilkinson, G., 213(126), 225 284 Williams, A. A., 212(120), 225 Zwietering, P., 75(19), 84(42), 86(42), 86 Willis, R. G., 183(97), 184(97), 224 (19, 60), 89, 90 Wilson, J. N., 105(32), 110(32), 112(32), 113(32), 116(32), 116(32), 128(32), Zwoliiski, B. J., 108(34), 222 222

Subject Index A Absorption coefficient change, 232 Acetylene, adsorbed state of, 159 hydrogenation, 160 hydrogenation, kinetics and mechanism of, 167 reaction with deuterium, 161 Activation energy, for atomization, 9 for hydrogen desorpt;on from tungsten, 18 Activity of oxides, 315 Adatoms, “hopping” tmd molecular desorption, 26 Adsorbed acetylene, 169 benzene, positive ions of, 268 dienes, 184 gas molecules, electronic spectroscopy Of, 227-279 molecules, radicals from, 277 polyacenes, cation radicals from, 269 state, nature of, 23 state, of olefin during hydrogenation, 98 state, of 1,2-dienes, 186 Adsorption during atomization, 11 measurements, during surface catalysis, 65-68 molecular and surface diffusion of oxygen, 46 new electronic spectrum and, 234 of acetylene, 159 of alkynes and dienes, 209 of dimethylcyclohexanes, 100 of hydrogen, 28 of benzene, on porous glass, 239 on “clean” semiconductor surfaces, 65 on nickel, of oxygen and CO, 55 on single crystal surface, 43 spectra, of aromatic amines, 263 Alkynes, adsorption of, 209 Amines, adsorption spectra of aromatic, 253 Ammonia, decomposition on metal catalysts, 83 361

synthesis on iron catalysts, 85 Ammonium, oxidation of, 330 Aniline, spectra of, adsorbed on silica gel, 255 Aromatics, adsorbed state of, 100 Atomization, activation energy for, 9 heterogeneous, 2 in a flow system, 3 kinetics, pre-expontial factor, 20 of oxygen or hydrogen over platinum, 7, 8 rate equations for, 24

B Bathochromic shift, of adsorption spectra, 236 Benzene, adsorbed positive ions of, 268 p-Benzoquinhydrone, adsorption on barium hydroxide, 275 l,a-Butadiene, 1-methoxy-,hydrogenation of, 202 Butadiene hydrogenation, palladium catalyzed, 189 reaction with deuterium, 199 Butene-2, cis-trans isomerization in, 106 Butene isomerization, 129, 151 n-Butenes, exchange and isomerization of, 128 Butenes, hydrogenation of, 137, 148 n-Butenes, isomerization of, 116 Butyne-1, hydrogenation of, 174 Butyne-2, hydrogenation of, 176

C Calcium flouride, spectra of I, adsorbed on, 233 Carbon monoxide, catalytic oxidation of, 328 Carbonium ion formation, from naphthalene, over silica-alumina, 270 from anthracene, over silica alumina, 269

362

SUBJEUT INDEX

Carbonium ions from adsorbed polyacenes, 269 from phenylalkanes, 261 from phenylalkenes, 264 Catalysis, surface, adsorption measurements during, 66-68 on “clean” surfaces, 67 isotopic exchange in molecular oxygen, 286-338 Catalyst form for hydrogenation, 83 Catalysts, solid, isotopic exchange with molecular oxygen on, 293 transition metal, hydrogenation of unsaturated hydrocarbons on, 91-221 Catalytic activity compared t o oxygen isotopic exchange reaotions, 327 of metals for oxygen exchange, 322 of cxides in isotopic exchange, 293 relation of, to oxide properties, 310 work function and, 316 Cation radicals from phenylated amines, 366 from adsorbed polyacenes, 269 from phenylalkenes, 266 Chromium oxide, irvotopic oxygen exahange over, 299 Cie-2-butene,116 Clean surface, 32 Clean surface, structure of, 38 Cobalt catalysts, 173 Cobalt oxide, isotopic oxygen exchange over, 302 Complex, formation of hydrocarbonmetal, 210 Complexes, allylia, 212 hydrogenation reactions and, 219 isomerization reactions and, 219 oxidation reactions and, 220 polymerization reactions and, 220 reactions of hydrocarbon-metal, 217 stability of hydrocarbon-metal, 213, 216 a-oomplexes, 211 Copper, epitaxy of, on titanium, 42 Copper oxide, isotopic oxygen exchange over, 304 Cumene, adsorption on silica-alumina, 266 Cyclic dienes, hydrogenation of, 191, 194 Cycloheptene, reaction with deuterium, 121

Cyclohexadiene, hydrogenation of, on palladium and platinum, 206 Cyclohexene,reaction with deuterium over iron films, 123 reaction with deuterium, 121 hydrogenation over Adams platinum, 141 Cyclopentadiene, deuteration of, on iron film, 206 Cyclopentene, reaction with deuterium, 121

D Desorption, atomic, 26 molecular, 26 rate equation for, 25 Deuterium, react?on with acetylene, 161 butediene, 199 butenes, 137, 148 2-butyne, 176 cycloheptene, 121 cyclohexene. 131, 123 cyclopentene, 121 ethylene, 110, 121, 124, 146, 161 propylene, 146 Deuterium, reaction of cyclic olefins with, 121 reaction of higher olefins with, 116 reaction of olefins with, 132 Diamond, adsorption o f “olean” surface of, 66 Diatomic molecules, atomization by metals, 1-28 Dime, hydrogenation, 186 Dimethylcyclohexanea, adsorption of, 100 2,6-Dimethylhexadiene-2,4, hydrogenation of, 204 2,6-Dimethylhexene, hydrogenation of, 204 1, 1 Diphenylethylene, adsorption on silica-alumina, 264

E Electronic effects, in hydrogenation, 93 Epitaxy, of copper on titanum, 42 Equilibrium constants, for hydrogen adsorption on tungsten, 28

363

SUBJECT INDEX

Ethylene formation selectivity, 170 hydrogenation on copper-nickel alloys, 61

reaction with duterium, 146, 161

F Ferric oxide, isotopic oxygen exchange over, 302 Formic acid, decomposition on metal catalysts, 81 Franck-Condon principle, 232, 236

G Gas chromatographic method, of adsorption study, 76 Geometric effects, in hydrogenation, 93 Germanium, decomposition of germane on, 79

Gravimetric method of adsorption study, 76

of ethylene, 68, 61 of hexenes, 137 of hexynes, 181, 176 of isoprene, 204 liquid phase, of butadiene, 202 of monoalkylalkynes, 173 of octalin, 139 of octyne, 184 of pentadienes, 204 of pentenes, 160 of pentynes, 176 of propadiene, 187 of propyne, 173 of unsaturated hydrocarbons on transition metal catalysts, 91-221 of vinylacetylene, 183

with simultaneous isomerization, 169 Hypsochromic shift of adsorption spectra, 236

I

H 1-Hexene, hydrogenation over Adams platinum, 137 Hydrocarbons, complete oxidation of, 330 Hydrocarbon-metal bond, 206

Hydrocarbon metal complexes, stability of, 213 compounds and heterogeneous catalysis, 210

Hydrogen exchange mechanisms, 107 Hydrogen, catalytic oxidation of, 328 Ha-D, exchange, ethylene and, 104 mechanism of, 107 on nickel and germanium, 69 Hydrogen, molecular, interaction with hydrocarbon species, 168 molecular and atomic addition of, 168 Hydrogenation mechanisms, 103 Hydrogenation, of acetylene, 108, 169, 160, 169

of of of of of of of of

alkynes and dienes, 166 butadienes, 189, 196, 202 butenes, 148 butynes, 174-180 cyclohexenes, 141 cyclic dienes, 191, 194 didkylalkynes, 176 dienes, 184

Indicators, surface acidity, 249 Iridium-alumina catalysts, 146 reactions over, 143 Iron catalysts, 173 Isomerization, cis-trans, 106 of n-butenes, 116 mechanism, of olefin, 106 Isoprene, hydrogenation of, 204 Isotope tracers in kinetic studies, 72

K Kinetic transition, in atomization, 16, 22 Kinetics of acetylene hydrogenation, 168 of atomization, 6, 16 of 1,3 butadiene hydrogenation, 194 of catalyst-molecular oxygen exchange, 288

of isotopic exchange, 286

L Langmuir isotherm, 66 Lattice defect, influence of density, 48 LEED, low-energy electron diffraction, 33

M Manganese peroxide, isotopic oxygen exchange over, 300

35.4

SUBJEUT INDEX

Magnesium oxide, isotopic oxygen exchange over, 293 Mechanism, of atomization reaction, 10,12 of butyne hydrogenation, 180 a-Methylstyrene, adsorption on silicaalumina, 266 Molecules, spectra of physically adsorbed,

of hydrocarbons, 330 of hydrogen, 328 of sulfur dioxide, 330 Oxygen, exchange in molecular on oxides, 306

participation in oxidation reactions, 333 sorption and exchange, 314

236

Molybdenum oxide, isotopic oxygen exchange over, 305

N Nickel atom replacement by oxygen, 48 catalyst, in hydrogenation of pentyne-1 and hexyne-1, 176 catalysts, 173 hydrogen and oxygen on, 57 hydrogenation of ethylene on, 68 -kieselguhr catalyst, 111, 116 oxide, 322 oxide, isotopic oxygen exchange over, 304

oxygen and CO on, 67 pumice catalysts, 187 -silica catalyst, 116 wire, reaction of 1-butene with D, over, 115

ethylene reaction with D, over, 110 Nitrobenzene, spectra of adsorbed on silica gel, 246 Norbornylene, 123

0 Octalin, hydrogenation, 139 Octyne, hydrogenation of, 184 Olefin,cr-diadsorbed and r-adsorbed, 98 exchange, mechanism of, 104 Olefins, higher, exchange and isomerization of, 130, 135 exchange and isomerization of, 143 Olefin hydrogenation, 98 hydroisomerization and exchange, 206 Olefin isomerization, double band migration in, 105 Olefin reactions over nickel catalysts, 110 Osmium, reactions over, 161 Oxidation, of ammonium, 330 of carbon monoxide, 328

P Palladium catalysts, 191 on barium sulfate as catalyst, 174, 177 -alumina catalyst, 176, 182, 184, 124 -alumina catalyst, in butene-l hydrogenation, 128 -carbon catalysts, 181, 204 charcoal catalyst, 132 -pumice catalysts, 126. 187 n-Pentenes, hydrogenation of over rhodium-charcoal. 150 1,3 Pentadiene, hydrogenation of, 204 2-Pentyne hydrogenation, 181 Perturbations, spectral, 246 Phenol, adsorption spectra on silicio acid, 246

Place exchange, on single crystal surface, 43 Platimum, reactions over, 132 -alumina catalyst, 136 atomization of hydrogen over, 7 atomization of oxygen over, 8 -carbon catalyst, 181 -charcoal catalyst, 137 filma, isotopic exchange of oxygen on, 323

.pumice catalysts, 187 ions, of adsorbed benzene, 268 Pre-exponential factor for atomization kinetics, 20 Propadiene, hydrogenation of, 187 Propylene, reaction with deuterium, 146 Propyne, hydrogenation of, 173 Pyridine, adsorption spectra of, 241

R Radioactive tracer method for CO, on Ni, 56

Raney nickel catalyst, 181, 204 Rate determining step during surface catalysis, 68

355

SUBJECT INDEX Reactions of n-butenes with hydrogen and deuterium, 148 of ethylene with deuterium, 110, 121, 124 intermediates, estimation of chemical potentials of, 72 mechanism, 96 paths, alternate, 94 over iron, 121 over palladium, 124 possible mechanisms of olefin hydrogenation, 102 of propylene with deuterium, 124 single crystal approach, 31-62 systems, dynamic treatment of, 73 Rhodium-alumina catalysts, 146 -carbon, 181 -carbon catalysts, 204 reactions over, 146 Ruthenium, reactions over, 161

in 1-butene formation, 198 in hydrogenation, 167 Structure, of clean metal surfaces, 38 of clean semiconductor surfaces, 40 Sulfur dioxide, oxidation of, 330 Surface, “clean”, methods of producing, 32 reactions, with oxygen, 273 structure of oxygen and nitrogen on titanium, 43 structure of oxygen on nickel, 43

T Tetrazine, spectrum of adsorbed, 244 Transition metal catalysts, hydrogenation on, 91-221 Transition state theory in molecular desorption on, 26 Triphenylmethane carbonium ion, 262 Tungsten, atomization of hydrogen over, 7, 16

S Selectivity, in alkyne and dime hydrogenation, 208 of 1, 3 butadiene hydrogenation, 196 in butyne-2 hydrogenation, 178 in hydrogenation, 166 Semiconductor surfaces, 40 Silica-alumina, adsorption of aromatic h n e s on, 267 adsorption of triphenylmethane on, 262 nature of acids in, 266 Silicic acid powder, adsorption spectra of benzene on, 240 Spectra, of aniline adsorbed on silica gel, 266 of I, adsorbed on calcium flouride, 233 of physically adsorbed benzene, 236 positive ion, of adsorbed molecules, 266 of surface anion radica!s, 274 Spectral shifts, 232 Spectroscopy, electronic, of adsorbed gas molecules, 227-279 Static system, atomization in, 2 Stereochemistry of cylic olefin hydrogenation, 139 Stereoselectivity in butyne hydrogenation, 178 in 2-butene formation, 196

U 1,7 -Undecadyne hydrogenation on palladium-charcoal, 183 4-Undecyne, hydrogenation over palladium catalysts, 182

V Vacuum conditions needed for “clean” surfaces, 38 Volumetric method of adsorption study, 76 Vanadium pentoxide, isotopic oxygen exchange over, 296 Vibrational frequency alternations, 233 Vinyl acetylene, hydrogenation of, 183

w Work function method of surface investigation, 33

Z Zinc oxide, 319 Zinc oxide, isotopic oxygen exchange over, 306

This Page Intentionaiiy Left Blank