19 Electron Transfer and Catalysis

ELECTRONIC PROPERTIES AND CATALYTIC ACTIVITY

19

Electron Transfer and Catalysis W. E. GARNER University of Bristol,England

I. INTRODUCTION Progress in the study of heterogeneous reactions during the last thirty years has depended in a considerable measure on the evolution of our knowledge of the structure of the solid state and on the experimental techniques discovered in the course of its study. I n the surge forward, there has been a happy blend of theory and the new techniques. Knowledge about metals has not been gained along precisely the same track as for semiconductors and insulators, and on the whole it has had a more rapid and continuous growth. The electronic factor has been in the minds of those working on catalysis on metals throughout the last quarter of a century. By the early thirties, Langmuir had established that the alkali metals were bonded t o tungsten as ions, and Rideal and Wansbrough-Jones had suggested an interrelationship between the work function of metals and the speed of catalytic reactions. De Boer had added considerably t o our knowledge concerning ionic adsorption and its relationship t o the work function and the ionization potential of the adsorbed gas. Also Lennard-Jones had formulated the problem of the electron transfer process in chemisorption. The next advance came from the application of Fermi-Dirac statistics t o the electrons in metals, which led t o the band theory of a quasi-continuous series of energy levels, and t o the concept of Brillouin zones, which is of special value for alloys. This sets the stage for a detailed study of the electronic factor in catalysis on metals. The Pauling resonating-bond theory of metals opened up a radically new line of approach t o the study of the bonding of adsorbed gases and the im169

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W. E. GARNER

portant part played by the d-levels in the strength of the metal-substrate bonds. This led to significant advances by experimentalists, which first became evident as a contribution to knowledge at the Faraday discussion on catalysis in 1950. In the case of semiconductors, the major development had its origin in the theoretical ideas of Wilson, Schottky, Wagner, and Mott, and in the techniques developed in connection with the electronics and wireless industry. The presence of localized energy levels was demonstrated and these give rise, not only to semiconductivity, but also to changes in the rates of adsorption and desorption processes in heterogeneous reactions. Making use of the conceptions of Lennard-Jones, de Boer, and others, the views of H. S. Taylor on activated adsorption can now be expressed in more precise and often quantitative terms. There has been much discussion on the effect of surface heterogeneity on catalysis. At first the heterogeneity was ascribed to incompleted lattice planes, to corners, edges, cracks, etc. It is now clear that an important fraction of the surface imperfections are associated with dislocations. To these can be added lattice vacancies, which either in the surface or to a depth not exceeding 40 A into the solid, can behave as surface irnperfections as far as adsorption is concerned. Some of the effects previously attributed to heterogeneity, such as the fall of the heat of adsorption with coverage are now ascribed to more subtle effects, depending on the interaction between surface dipoles and to changes in the Fermi levels of the solid brought about by adsorption. As the molecular bond lengths became known, correlations were sought between the geometry of the surface and catalytic activity. There developed the multiplet theory of Balandin which was applied successfully to dehydrogenation catalysts. It also provided an adequate explanation of the work of Maxted and others on catalytic poisons and of the behavior of the different plane faces of crystals. There is no inherent conflict between the interpretations based on geometry and those based on the electronic potential of the surface. The two effects are probably complementary. More knowledge is, however, required about the influence of the electronic potential on the decomposition of complex molecules, before a decision can be made on their relative significance. Studies of the character of the bonding between the adsorbate and the surface have not yet yielded a final pattern, although the available evidence suggests that the chemical bonds may be charge transfer bonds of the Mulliken type. Further developments in this direction are badly needed to clarify the picture that has been delineated from the studies of electrontransfer processes. The studies by the field-emission microscope and of the changes in the

19.

171

ELECTRON TRANSFER AND CATALYSIS

magnetism of supported oxides offer considerable prospects for future development.

11. METALS 1. Electron Transfer Processes

The transformation of van der Waals adsorption into chemisorption can be described in terms of the Lennard-Jones potential energy diagram (1). He suggested that on a sufficiently close approach of the adsorbed molecule, surface energy levels would be created which would be lower than the occupied electron levels of the bulk of the solid. If these levels were sufficiently deep, they would facilitate the transference of electrons to give adsorbed ions, under circumstances where the ionization potential or chemical affinity of the gas molecule at large distances were otherwise unfavorable [see Eq. (1) and Fig. 1). He pointed out the possibility of exchange giving rise to homopolar bonds with the conductivity electrons of the solid, or with the deeper-lying, unfilled d-shells of the atoms. The latter would give rise to strong bonds, which the experimental work of Beeck (2) has substantiated. Also, the decrease in magnetization which accompanies the adsorption of gases, for example, hydrogen on nickel (S), supports the view that electron transfer can occur from the adsorbate to the d-levels. The pioneer work of Langmuir and Taylor (4) on the adsorption of alkali metals on tungsten and the further development of these and other researches by de Boer ( 5 ) has led to a detailed interpretation of the phenomena arising when molecules are adsorbed as ions. There exists on the surfaces of metals an electrical double layer with the negative charge outwards, which prevents the electrons leaving the metal. On adsorption, the metal double layer is modified both by the superposition of a double layer due to adsorbed ions and also by changes in the Fermi level due to the transference of electrons to or from the conductivity levels of the solid (6). In the event of the formation of covalent bonds, as Dowden E=O

N (El

+-

METAL,

ADSOi?BATE

FIG. 1. Positive ion formation on metal surfaces.

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W. E. GARNER

(7) has pointed out, the Fermi level will normally be lowered, and the metal double layer made more negative on the outside. The energy liberated in the formation of a positive ion is given by Eq. (I), which was used by Langmuir and de Boer,

H=*-I+-

e2 4do

where I is the ionization potential of the gas, cp is the work function of the metal, and e2/4do represents the interaction between the ion and its electrical image. Dowden (7) has discussed the conditions under which positive ion formation occurs on metal surfaces (Fig. 1). He concludes that positive ion formation is most favored if cp is large, the change in density of the energy levels at the Fermi surface is large and positive, the adsorbed molecule approaches as closely as possible to the metal atoms, as will occur if there are surface vacancies and if there is a large concentration of holes in the dband. For negative ion formation, an electron is removed from the highest occupied level in the metal to the lowest unoccupied level of the adsorbate. The treatment is similar to that outlined for positive ions. a. Heats of Adsorption. Equation (1) gives good agreement with the results for czesium on tungsten as shown by de Boer and by Higuchi, Ree, and Eyring (8) and by the latter authors for sodium or tungsten [see also Boudart (9)]. In the case of metallic ions, cp decreases with coverage, and Eyring and his co-workers taking account of the interactions between the dipoles make calculations of the heats of adsorption and obtain good agreement with the experimental values for the variation of the heats of adsorption with coverage. b . Adsorption of Hydrogen. Boudart (9) has shown that the same model gives the correct order of magnitude for the fall of heats of adsorption of nonmetallic elements, H, , 02,and N 2 , adsorbed on metallic surfaces. The adsorption of hydrogen is of special interest. Measurement of dipole moments have been made on a large number of metals (10, l l ) , and except for platinum, the double layer due to hydrogen has its negative side outwards. Mignolet ( I d ) interprets this as due to covalent bonding of the hydrogen with the conductivity electrons which reduces the Fermi level and increases the work function. On the other hand, Boudart (9) regards the binding as essentially metallic in character with resonating covalent bonds and the hydrogen carrying a positive charge and situated below the metal double layer. c. Conductivity. Some light has been thrown on this problem by Suhrmann and Schultz (IS), who show that hydrogen adsorbed on thin films of metallic

19.

ELECTRON TRANSFER AND CATALYSIS

173

nickel at, 90 and 293”K increases the electrical conductivity, which they attribute t o the transference of electrons from hydrogen to the conductivity levels of the metal. Water and organic substances with 7r bonds also transfer electrons t o the nickel. On the other hand, oxygen, carbon monoxide, and nitrous oxide abstract electrons from the conductivity levels. The effect with oxygen is seven times greater than with similar amounts of carbon monoxide, indicating that the oxygen bonding is much more ionic than that of carbon monoxide. Argon has no effect on the conductivity although Mignolet finds that on tungsten xenon produces a positive film. d. Dipole Moment. For the adsorption of hydrogen, there is a lack of agreement between the conclusions drawn from measurements of conductivity and of work function. This may be due t o the assumptions that = V = 4 m M , where V is the surface potential and M the dipole moment of the adsorbed gas. If the adsorption changes the Fermi level of the metal and if the work of removal of a n electron from the surface layer contains a quantum-mechanical term due to the formation of a covalent bond, then it is permissible to doubt whether the calculated dipole moment M has any real meaning. Mignolet, working on the assumption that the differential heat of adsorption dq = -A@, is able t o account satisfactorily for the variations in the heat of adsorption with coverage. The matter that is in doubt is not the relationship between the heat of adsorption and work function but the actual dipole moment of the adsorbed gas. e. Charge Transfer Bond. As regards covalent bonding, Pollard (14),using the stabilized surface levels of Tamm, has calculated the heat of adsorption of hydrogen on copper and decides in favor of the one-electron bond. Matsen, Macrides, and Hackermann (15) have applied the Mulliken charge transfer bond* (16) to the study of the relationship between the heat of adsorption and the ionization potential of a number of substrate molecules. This line of approach shows promise.

-+

2 . Rates of Adsorption and Rates oj Catalysis on Metals a. General. Accurate universal relationships for the rates of adsorption and free energy of activation, in terms of the metal work function and the ionization potential of the gas, have not yet been elaborated. Dowden (7) has given an approximate relationship for positive ion formation as follows:

kf

=

K + exp [--b(I - aAU+ - cp)/kT]

where I is the ionization potential of the gas, AU+ the value of the ionic adsorption energy, and cp the work function. From this equation, a decrease in the heat of adsorption and the work function or an increase in the elec-

*M states.

+H

-

M-H+ or M+H- with interaction between the “no bond” and ionic

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W. E. GARNER

tron concentration of the metal should increase the activation energy and decrease the rate of reaction. Schwab (17)has studied the rates of dehydrogenation of formic acid on a number of alloy systems based on gold or silver, to which metals with a higher valency were added. These additives increase the electron concentration of the gold and silver and increase the activation energy for dehydrogenation. This is in accordance with Equation (2).These alloys are complex systems giving Brillouin zones, and the activation energy approach maxima as these zones are filled. These results might be taken as an indication that the rate-determining step consists in the transference of electrons from the formic acid to the metal. This is, however, uncertain since, as Dowden has deduced, positive ion formation and covalent bond formation have a similar dependence on the properties of the Fermi surface. Another approach has been made by Eyring and his co-workers (8), who have derived equations for calculating the rates of formation of ions of the alkali metals on tungsten. These were based on the de Boer equation (1) and interaction between the ions, and tested against the rate measurements of Langmuir and Bosworth and Rideal (18). Agreement between theory and experiment is reasonably good. b. d-Metals. The strength of the binding of gases to metal surfaces and the activity of metals as catalysts are in general greater for d-metals than for s-p metals, and there has been a good deal of attention paid to such effects. Dowden suggests that only metals containing partially filled d-levels can adsorb gases rapidly below room temperature. It is suggested that the formation of d-bonds has a low activation energy. One possible explanation is that in forming a d-bond, an electron is transferred to the empty level giving a Mulliken charge transfer bond. The d-bond is not a very strong bond, but it can pass into a d-s-p bond, which is stronger. This process, however, requires an activation energy so that it usually does not take place until higher temperatures are reached. The formation of d-s-p bonds in the s-p metals would require the promotion of an electron to the conductivity level-an endothermic process, so that such bonds are weak. Exceptions are met with in specially activated metals, where the electron levels have been modified by defect structures (such as copper). Beeck (2) has brought out an interesting relationship between the heats of adsorption of hydrogen and ethylene, the rate of dehydrogenation of ethylene, and the percentage d-character of the metallic bond (Pauling) for a number of d-metals. He shows that the heats of adsorption of hydrogen and ethylene decrease as the d-character of the metal bond increases. This Beeck explained as due to bonding with empty orbitals in the d-band, which will decrease as the d-character of the metallic bond is increased. The rate of dehydrogenation decreases as the heats of adsorption of hydrogen and

19.

ELECTRON TRANSFER AND CATALYSIS

175

ethylene increase. This is in agreement with Beeck’s assumption that the rate-determining step is the removal of ethylene from the surface with adsorbed hydrogen, since the activation energy for this process would be expected to increase with increasing strength of bonding to the surface. Couper and Eley (19) in a study of the catalysis of the p-hydrogen conversion on palladium alloys have found that the activation energy for this reaction undergoes a sharp rise from 3.5 to 8.8 kcal at a molar composition containing 40 % palladium. This composition corresponds very nearly with that at which the alloys cease to be paramagnetic and at which the d-orbitals are filled. Dissolved hydrogen exerts a similar “poisoning” effect to gold. The authors conclude that in the palladium-rich mixtures the hydrogen is present on the surface as an Hs complex, held by d-orbitals. Dowden and Reynolds (20)have given a number of examples which show that the catalytic activity is considerably reduced as the positive holes in the d-band fall to zero. In styrene, hydrogenation at 20°, the activity of a series of nickel-copper catalysts decreases to zero as the ferromagnetic properties disappear. Also in the hydrogenation of styrene on Ni-Fe alloys, the effects of changes in the electron specific heat, i.e., changes in the energy density of electron levels at the Fermi surface, have been clearly brought out. Methanol and formic acid decompositions on Ni-Cu alloys decrease in speed as the 3d-band is filled. On the other hand, the rate of decomposition of hydrogen peroxide on Cu-Ni decreases as the vacancies in the d-levels appear. In this case, however, it is believed that an electron is transferred from the metal to the substrate. 111. SEMICONDUCTORS

Owing to the very high activation energy needed to move electrons or positive holes from one ion to another, semiconductors when in the stoichiometric condition have the low conductivity of insulators.* The conductivity can, however, be increased by the addition of an excess of either the cationic or anionic constituent, which introduces lattice defects either as interstitial ions or as lattice vacancies. The introduction of foreign altervalent ions also increases or decreases the concentration of the lattice defects. Thus the introduction of 2-mol. % LizO, in the presence of air, into NiO increases the conductivity about 10,000-fold (21). In terms of the band theory, the presence of lattice defects creates new energy levels in the gap between the full band of the cations and the conductivity bond. In the case of semiconductors, the presence of these levels reduces the activation energy for the transference of either electrons or positive holes, depending on the type of semiconductor (see Fig. 2). The

* CrzOI ,which is an amphoteric semiconductor, is a possible exception.

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W. E. GARNER

EMPTY LEVELS

---.

.

,r

E=O

\

-------- ---

IMPURITY

n-TYPE

LEVELS

p TYPE - - - -- - - - - - -

FULL BAND

high dielectric constant of solids causes overlap of the electronic orbits even a t relatively low concentrations of lattice defects, and the position of the energy levels therefore depends on concentration. Normally, increase in concentration of defects reduces the activation energy for electrical conductivity, and this is of significance in catalytic reactions. At high concentrations, the conductivity may even become metallic, as occurs in the case of ZnO and possibly CuzO. It is proposed t o limit the discussion of semiconductors t o oxides. The behavior of oxides is, however, very similar t o that of other semiconductors, such as halides or sulfides. I . Adsorption on Semiconductors

The dependence of adsorption on the energy levels of the solid and on the ionization potential or the electron affinity of the gas would be expected to be very similar t o that previously described for metals. Thus, the addition of the new electronic levels associated with the lattice defects will in general reduce the activation energies for chemisorption (Fig. 2). The first stage in chemisorption probably consists in the transfer of electrons either t o or from the cations of the lattice, leading t o the formation of a charge transfer bond. This bond will be more stable at low than at high temperatures. Secondary effects occur a t higher temperatures, such as reaction with the oxygen ions of the lattice or the creation or destruction of lattice vacancies. When the adsorbed gas is an ion, there will be a space charge or barrier layer which will affect the character of the adsorption isotherm, modify the rates of adsorption, and create lattice vacancies. a. Reversibk Chemisorption. At low temperatures, gases may be adsorbed reversibly. When the reversible adsorption is limited t o bonding with the lattice defects, on account of the limited number of localized sites available, the surface coverage is frequently much less than a monolayer. This may

19.

ELECTRON TRANSFER AND CATALYSIS

177

be described as adsorption on heterogeneities created by the presence of lattice defects. The reversible chemisorption is, however, not necessarily confined to the defect levels, but can occur over the whole of the surface atoms. This probably occurs for carbon monoxide on many oxides, since its adsorption is little affected by the state of oxidation of the surface. There is evidence from the work of Schwab and Block (22) that in the formation of the chemical bond, carbon monoxide donates electrons t o the cations of the lattice. The adsorption of hydrogen is often of a more specific character. At low temperatures it is preferentially adsorbed on n-type or stoichiometric oxides. The adsorption of hydrogen on zinc oxide is of considerable interest. At room temperature and below, there is very little adsorption unless the oxide has been heated in vacuum a t about 400", in which process water or carbon dioxide is lost. Treatment with hydrogen at 300" followed by evacuation is also effective. I n these processes the oxide is reduced. There are probably two steps in the process of reduction, in the first of which oxygen is lost from the surface, giving F centers, and in the second the F centers give rise t o interstitial zinc atoms, e.g., O=

+ Zn++ - Zn

The work of Taylor and Strother (23) has shown that there are two types of hydrogen adsorption, one a t low and another at higher temperatures. It is suggested that possibly these two types consist in adsorption of hydrogen on F centers and intersitial zinc atoms, respectively. The heats of adsorption of hydrogen and carbon monoxide a t low temperatures are very similar t o those found for these gases on metals, and the adsorption may therefore possess a very similar character. There is, however, reversible adsorption of another type which occurs a t high temperatures such as the adsorption of oxygen on zinc oxide above 500" (24). In this case, an equilibrium is established which includes the formation and destruction of lattice defects. b. Irreversible Chemisorption. In this type of adsorption, the adsorbed molecule on desorption leaves the surface in combination with one of bhe components of the lattice. The best-known example is probably the loss of WO3 from the surface of tungsten. At room temperature, carbon monoxide is chemisorbed reversibly on Zn0.Crz03with a heat of -20 kcal. and is desorbed unchanged on heating t o 100". Slightly above this temperature, it is readsorbed and can only be recovered from the surface as carbon dioxide. Hydrogen behaves similarly on this oxide (25). It would appear that on raising the temperature, the reversible chemisorption passes into adsorption as OH- or COT. On a number of oxides in their highest state of oxidation, it was shown

178

W. E. GARNER

that 1 g.-mol. of carbon monoxide, when adsorbed at room temperature, so reduces the surface that one half a g.-mol. of oxygen can be adsorbed inimediately afterwards (26). This points to the carbon monoxide reacting with two oxygen ions in the surface to give COT. Also, when CO and O2 are admitted together, they are used up in the ratio 2: 1. The following mechanism was suggested :

In stage I, a CO1 ion and an F center are produced, and in stage I1 tfheF center is destroyed by the adsorption of an oxygen atom. These F centers, however, decay with time, probably due to combination with cationir vacancies from the interior of the solid. c. Creation and Destruction of Lattice Defects. At sufficiently high temperatures, intersitial atoms and lattice vacancies can move freely through the lattice. This temperature should be approximately 0.3 to 0 . 5 T m ,but in the presence of a barrier layer or space charge, it may be considerably reduced. Thus, when CuzO is present as a thin layer on copper, the cationic vacancies in CUZOare mobile at 100". The temperature at which this happens for NiO is some 200" higher. It has been shown that at room temperature stoichiometric CuzO will adsorb considerably more oxygen than is needed to form a monolayer (27). Also, the reactivity of the adsorbed oxygen towards CO and COz decreases with time, which is explained as caused by the penetration of lattice vacancies so far below the surface that reaction is no longer possible. This process can be represented in two stages as follows: 02

+ 2M+

2

20-

+ 2M++

(1)

This gives a barrier layer which, above a critical temperature, aids the diffusion of cations to the surface, yielding cationic vacancies 0-

-e

-

0-

+ 2O+

(2)

where O+represents a cationic vacancy carrying one positive charge. This penetration of the surface gives only a thin zone of disordered lattice, and it is suggested that the process of incorporation ceases owing to the neutralization of the barrier layer due to (1). The formation and mobility of the interstitial atoms in zinc oxide have been discussed by Bevan and Anderson (24) and Morrison (28). d . Changes in Electrical Conductimty. In accordance with the FrederichMeyer rule, the electrical conductivity of n-type oxides decreases as the partial pressure of oxygen increases, and that of p-type oxides increases with increase in oxygen pressure. Reducing gases have a converse effect (29).

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ELECTRON TRANSFER AND CATALYSIS

179

Measurement of the changes in conductivity on adsorption are helpful in throwing light on the mechanism of the adsorption processes. A good example is the reversible and irreversible adsorption of carbon monoxide on cuprous oxide (30). When carbon monoxide is adsorbed reversibly at room temperature, the semiconductivity is decreased by the neutralization of positive holes : CO

+ M++

-

CO+

+ M+

(1)

and returns to its initial value on evacuation. At 200" the conductivity changes in three stages: first, an initial fall, probably represented by Eq. (I), which is followed by a rise to a maximum, possibly due to the CO reacting with adjacent oxygen ions to give C0,- and an F center, thereby setting free the positive holes and restoring the semiconductivity, and a final stage in which the cationic vacancies diffuse to the surface and neutralize the F centers and liberate COZ . The last process may occur under the influence of a negative barrier layer. It is of interest that COZ is not adsorbed on stoichiometric CuzO, but only on an oxide containing cationic vacancies. It is however, strongly adsorbed on CuO. The CO,-- probably forms a bond with Cu++ ions. I n this form it can be displaced from the surface by CO, which destroys the Cu++ions. Thus CO

+ COa--

-

2CO2

+ 2e

(2)

leading to the neutralization and ultimate destruction of the cationic vacancies. The chemisorption of water on CuzO at room temperature decreases the electrical conductivity (31). This is similar in character to the adsorption of CO, Equation (1).Other examples are given by Hauffe (32).Oxygen can be introduced into cssium chloride (33) or into sulfides (34,35),modifying the electrical conductivity. The changes in electrical conductivity occurring on the absorption of oxygen have been used by Gray and his colleagues (36) to study the reaction kinetics. The results obtained on CuzO at 100-200" and on NiO at 200-300" are consistent with the mechanism

+ 2Mez+ + 20- + 2Me3+ 20- + 2Me*+ 209- + 2Me3+ O2

which involves equilibrium in both stages. At low temperatures, the processes become irreversible owing to the back reactions becoming very slow. The irreversibility at low temperatures has been proved by isotopic exchange methods by Winter (37). Gray concludes that the second process, resulting in the incorporation within the lattice, occurs in a thin zone just below the surface.

180

W. E. GARNER

e. Barrier Layer. A s is the case with absorption on metals, the formation of a space charge or barrier layer on the surface of semiconductors produces changes in the work function and modifications in the adsorption isotherm and in the rates of adsorption. r h e effect of a barrier layer on adsorption has been discussed by Aigrain and Dugas (38) and by Weisz (3’9). Hauffe (32) has recently summarized the work of his school on this question. Employing electrochemical thermodynamics based on Boltzman statistics, he deduces the influence of a barrier layer on the work function. The relationship obtained is complex, involving several terms. One important term is the chemical work involved in changing the concentration of lattice defects. He deduces equations for the rate of adsorption and these are similar in form to the Elowich equations, viz., the amount adsorbed changes with time as log (t to).* The plots of volume adsorbed against log (t to) for oxygen adsorbed on NiO, for the results of Taylor and Struther (23) on the chemisorption of hydrogen on ZnO and in some other cases, give two rectilinear branches. According to the barrier layer theory, the first rectilinear curve is consistent with the view that it is due to chemisorption, and the second is due to incorporation of the gas in the lattice, which conclusion is contrary to the suggestion made in 111-1-a.

+

+

2. Catalytic Reactions

These may be divided into two groups: (a) those where the reaction utilizes the whole of the available area and (b) those which are localized on lattice defects or surface imperfections. In general (a) will possess the higher activation energies and frequency factors and hence will tend to be favored in the high-temperature range, unless other secondary processes intervene. In reactions of type (b), the rate of reaction will depend on a number of factors. Its activation energy will vary with the concentration of the lattice defects, since the electronic energy levels are dependent on concentration. There will also be effects due to a barrier layer. In addition, the actual concentration of lattice defects may be changed as the reaction proceeds to its stationary state. In no catalytic reaction have all these factors been thoroughly investigated. There is very little information available concerning the composition of the surface layers of the solid in the stationary state of any reaction. Much information has, however, been collected as a result of experiments in which the defect concentrations are artificially fixed by the addition of foreign ions. This minimizes the effects of some of the factors mentioned above.

* This relationship has also been derived by Eyring and co-workers (8) for adsorption on metals.

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ELECTRON TRANSFER A N D CATALYSIS

181

A few typical reactions to illustrate various aspects of electron transfer processes will he discussed individually. It is, however, too early to expect other than very tentative conclusions to be drawn. Insulators will only be referred to incidentally in the discussion of semiconductors. a. The Decomposition of Nitrous Oxide. It is possible to compile an activity series for this reaction (do), which divides the oxides into three groups. The p-type oxides, CuzO, COO, and NiO, are active at 200-300", insulators provide the middle range (350-550"),and n-type oxides are active at 600-800", being only a little more effective than the homogeneous decomposition. The p-type oxides are characterized by a capacity for yielding positive holes and cationic vacancies in the surface zone in the presence of oxygen, e.g*, 0 2

20-

+ 40'

(1)

The electronic levels associated with the cationic vacancies approach nearer to the full band as their concentration increases, decreasing the activation energy for the conductivity (36,4l).Thus, with the increase in the concentration of lattice defects, the activation energy for negative ion formation increases and that for the desorption of negative ions decreases. These considerations would also apply to the reaction NzO

2

NP

+ 0- + 2U+

(2)

where a negative ion is formed. Hauffe and his colleagues (32) have shown that the decomposition of nitrous oxide is retarded by the addition of trivalent cations to nickel oxide, which decreases the concentration of positive holes, and accelerated by the addition of monovalent ions, which has a converse effect. It does not appear therefore that Equation (2) can be the ratedetermining step. Dell, Stone, and Tiley (42) showed that one of the steps in the reaction was the interaction of NzO with oxygen ions on the surface. The over-all change is given by: N20+0--+2Uf-N2+0~

(3)

Hauffe suggests that this reaction is the rate-determining step, which is in accord with the above argument concerning the manner in which the defect electronic energy levels change with concentration. This follows, since the activation energy for the transfer of electrons from the oxygen ions to the solid will decrease with increase in the concentration of the positive holes. Stone (40)suggests that the electronic mechanism becomes less important at higher temperatures, ionic effects taking their place. The reaction on insulators occurs at temperatures at which oxygen ions are exchanged with

182

W. E. GARNER

the gas phase (3'7).The mechanism at high temperatures might therefore be

+ F center - Nz + 0-20-- - O2 + 2F centers

NzO

b. Oxidation oj Carbon Monoxide. The p-type oxides are active over the range 0-150" and these are followed at higher temperatures by n-type oxides and, finally, the insulators. The range for the last two classes is 150400",the temperature threshold being considerably lower than for the NzO decomposition. On p-type oxides, oxygen is adsorbed in a specially active form, which has an appreciable life time. This is usually explained as due to its adsorption as 0- ion 0 2

+ 2M+

2

20-

+ 2M++

(1)

Carbon monoxide donates an electron to the solid, CO

+ M++

-

CO+

+ M+

a dipole being formed with the positive charge outwards. The formation of carbon dioxide by (3): 0-

+ co+-

c02

(3)

is favored by Schwab and Block (22) who, from the effects of the addition of foreign ions conclude that (2) is the rate-determining step. There are, however, reactions which occur in the low-temperature range and involve the production and destruction of l a t h e vacancies in the surface layers, viz., 0 2

-

co + 20-0--

20--

-

+ 40+ + o--

COa--

+ 20+-0

co + CO,--

-

2CO2

+ 2e

At room temperature, the rates of reactions (4) and (6) are slow, which would be anticipated, since the activation energies for the movement of cationic vacancies are in the range 25-40 kcal. They are, however, sufficiently rapid to modify the surface electronic levels during the catalytic reaction. It may therefore take some time for the reaction to reach its stationary state. Changes in the electronic levels of the solid can be followed by measurements of electrical conductivity. Thus, for Cup0 it is shown by conductivity measurements that the concentration of cationic vacancies in the stationary state is low (3G). For NiO, the reaction is poisoned by the slow accumulation of carbon dioxide on the surface, owing to reactions (4) and (5) occurring and (6) and (7) being negligibly slow (42).

19.

ELECTRON TRANSFER AND CATALYSIS

183

The activation energies for the low-temperature oxidations of hydrogen or carbon monoxide on p-type oxides are usually a few kcal. The activation energies increase with increase in temperature, possibly because of an increasing part played by the defect-producing reactions. Over the range 0-300", the reaction changes from proportionality to po2 to proportionality to p,, at the higher temperatures, passing through a variety of relationships such as poZn,pcom in the intermediate range of temperatures (43). Also, the Fermi levels for NiO change about 5-6 kcal. in this range of temperatures (41). The reaction mechanisms may therefore prove to be difficult to sort out. Hauffe (32) and Parravano and Boudart (43) have discussed the position recently in the light of the known data. For n-type oxides and insulators, the complications due to reactions (4) and (6) do not arise. Neither is the oxygen adsorbed in a highly active form. The reactions may therefore proceed by an ionic mechanism [see Equation (1) in III-1-b]. Since, however, carbon monoxide is more strongly adsorbed on n-type oxides than on insulators, the former would be expected to be the better catalysts for the oxidation of carbon monoxide, as in fact they are. c . Hydrogen Deuterium Exchange. This has been studied on zinc oxide by Molinari and Parravano (44), and they find that the catalytic activity in the range 0-150" increases pari passu with the electrical conductivity. They made use of the addition of Liz0 to decrease the conductivity and of A1203 and Gh03 to increase it. The activation energy rose from 6.3 to >25 kcal. as the conductivity decreased. There was however a converse change in the frequency factor. Since the activation energy decreases with increase in electron concentration, it is clear that the rate-determining step must involve the formation of bonds which make use of the quasi free electrons of the zinc oxide. This could mean either the adsorption of negative ions 2e

+ H2 - 2H-

or the desorption of positive ions 2e

+ H+ + D+ -HD

was the rate-determining step. Hauffe (32) suggests the latter. This, however, does not explain the variation in the frequency factor, since the model based on interstitial zinc atoms requires that the frequency factor and the activation energy should increase or decrease together. If, however, there are two kinds of adsorption site, one possibly with a low frequency factor and a low activation energy, and the other with a high frequency factor and a high activation energy, then if the process of adsorption is composite, there is no difficulty in providing an explanation for the frequency factors (45). Voltz and Weller (46) have shown that at low temperatures the hydro-

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gen-deuterium exchange proceeds faster on reduced than on oxidized Cr203, which is in agreement with Parravano's work. d. Dehydrogenation and Dehydration of Alcohols. In spite of a very voluminous literature on this reaction, there are relatively few papers on the electronic aspects. Dowden and his colleagues (4'7) have recently studied the behavior of isopropyl alcohol on the systems magnesia-alumina, chromia-alumina, and zinc oxide-alumina, and have studied the electrical changes that occur. It was shown that dehydrogenation was favored with n-type conductors (excess ZnO), and dehydration by substances normally described as insulators or p-type oxides (e.g., CrzO3). There is no sharp division between semiconductors and insulators, and Weisz (48) has shown that the conductivity of hydrocarbon cracking catalysts, such as Al203-SiO2,is p-type. This might arise through the incorporation of excess aluminum, which has long been assumed to give acidity to the solid. It is broadly true, therefore, although there are some exceptions, that electron-excess lattices favor dehydrogenation and electron-defect lattices favor dehydration. It is therefore a reasonable hypothesis that the mode of decomposition of the adsorbed gas is determined by the direction in which electrons are transferred to form the charge-transfer bond between the adsorbate and the surface. It seems likely, therefore, that adsorption on surface cationic vacancies gives rise to dehydration and on surface anionic vacancies to dehydrogenation.

3. Supported Catalysts The work of Selwood (49) shows that changes in magnetism arise in a thin layer of a solid when it is deposited on another solid. These changes are interpreted as due to a change in the valency of the cations in the supported layer, i.e., to a process of valency induction. Thus, nickel oxide on y alumina is shown to contain appreciable concentrations of Ni+++ ions. Thus, the changes occurring when nickel oxide is supported on y alumina are similar to those found when lithium oxide is dissolved in nickel oxide. The catalytic activity of supported catalysts is at a maximum at an intermediate thickness of the absorbed layer (50). One of the consequences of the use of a support is therefore the production of changes in the electron levels of the catalyst. These electron levels have not yet been investigated in detail ( 3 2 ) . Received: March 14, 1956

REFERENCES 1 . Lennard-Jones, J. E., Trans. Faraday SOC.28, 334 (1932). 2 . Beeck, O . , Discussions Faraday SOC.No. 8 , 118 (1950).

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47. Alsop, B. C . , and Dowden, D. A., J . chim. phys. 61, 678 (1954); Garner, W. E., Dowden, D . A., and Garcia de la Banda, J. F., Anal. real SOC. espZnin. fis. y quim. Ser. 60B. 35 (1954). 48. Weisz, P. B., Prater, C. D., and Rittenhauser, K. D., J . Chem. Phys. 21, 2236 (1955). 4.9.Selwood, P. W., Bull. soc. chim. France p. 489 (1949). 60. Mooi, J., and Selwood, P. W., J . Am. Chem. SOC.74, 1750,2461 (1952).