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Chemical Effects at the Solid/Solid Phase Boundary GEORG-2\'IARIA S C H W A B Physikalisch-Chemisches, Institut der Universitgt, M~inchen 2 Sophienstrage 11, Mgnchen, Germany

Received March 20, 1970; accepted March 20, 1970 The phase boundary between solid/solid bodies affects kinetic or catalytic properties although, of course, thermodynamic potentials are unaffected. For example, the activation energy in the formic acid reaction over silver is lowered if supported on a p-type semiconductor and raised if supported on an n-type semiconductor. The effect is found if there is a much larger proportion of semiconductor so that the lower electron concentration can have an effect on the metal. Similar effects have been found for the inverse such as in the oxidation of carbon monoxide over an oxide supported on a metal. Also, similar boundary layer effects are found in the reactions of solids with each other. The experimental evidence for the boundary layer changes in the direction indicared by the catalytic studies is provided by two types of experiments. The one is based on rectifying effects and the other on conductivity measurements. The phase boundary between two solid bodies is not expected to change the thermodynamic potential and other thermodynamic properties of either phase nor its equilibrium with any third body. However, it is to be expected t h a t the phase b o u n d a r y itself and the adjacent zones of a solid m a y be changed b y the other solid in a kinetic respect. This is easily seen and well known in the ease of two phase catalysts. 5 i a n y conventional supported catalysts are such systems. I t can be shown t h a t in these eases the mutual influence is not just, one of texture and specific surface, b u t it is often of an energetic nature. ~{any years ago (1) this phenomenon was demonstrated in the ease of nitrous oxide decomposition over CuO-MgO (Fig. 1). The activation energy of the active component CuO is considerably lowered as compared with the activation energy over this pure component. Although in this special ease it later became probable t h a t an interracial chemical compound is formed, there is no doubt t h a t an electronic theory appears more general and more likely. I t would mean t h a t the Fermi-level of a metal can be changed b y spreading it on a semieonCopyright @ 1970 b y Academic Press, Inc.

ducting support and b y doping this support. The direct test was given (2) b y the catalytic action of nickel, cobalt, and silver on the surface of alumina. Alumina has been doped b y bivalent or quadrivalent additions of other oxides and it was shown t h a t the activation energy of formic acid dehydrogenation decreases with decreasing n-type conductivity of the support (Fig. 2). Another example is the hydrogenation of ethylene over nickel supported on zinc oxide, where the oxide was doped with lithium and gallium (3). Again the activation energy decreases with decreasing ntype conductivity (Fig. 3). A v e r y recent example is the formic acid reaction over silver deposited in thin layers on carborundum crystals SiC (4). Here, the activation energy is lowest on silver supported b y ptype SiC and highest with n-type SiC (Fig. 4). These effects are surprising for the following reason: A metal contains about 10 22 electrons per cubic centimeter; however, a semiconductor contains only at most 1018 electrons. I t is difficult to visualize t h a t this small number of electrons would shift considerably the energy of the very high

Journal of Colloid and Interface Science, Vol. 34, No. 3, November 1970

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FIG. 3. The rate of ethylene hydrogenation is plotted logarithmically over the composition of a reduced coprecipitated Ni on ZnO catalyst. The full line II designates undoped ZnO, the interrupted line I, Li-doped ZnO, and the dotted line III, Ga-doped ZnO. The reaction rate decreases with increasing n type character of the support ZnO. It has been shown that in the same series the true activation energy increases and the heat of adsorption decreases. been found. The other possibility would be that in the systems which have been studied, a thousand times more semiconductor is present than metal. This situation is the one which persists in all the examples cited. INVERSE

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similar influence on a semiconducting and catalytically active material. It would be necessary only that the work function of the metal differ from that of the semiconductor so that in a boundary layer many atomic layers deep an enrichment or depletion zone is formed. The best example of such an effect is the oxidation of carbon monoxide by nickel oxide supported by silver (5). The metal is a compact sheet and nickel oxide was deposited on it to different thicknesses. At thicknesses higher than 500 such preparations show the same activation energy as pure nickel oxide. In thinner layers, however, the activation energy rises about threefold. See Fig. 5. Obviously this means that in the latter case we are dealing with a semiconducting material consisting of the boundary layer only. Another example is the oxidation of sulfur dioxide by ferric oxide supported by silver alloys (6). Here the metal is a fine powder on which the oxide is supported as a fine powder on

which the oxide is supported as a very thin layer, thinner than a boundary layer. The experiment consists in changing the electron concentration of the silver by adding palladium (decrease of electron concentration) or mercury (increase of electron coneentration). The result shown in Fig. 6 is that pure ferric oxide shows the highest activation energy, then follows the palladium alloy, then pure silver, and last the mercury alloy, which induces in the ferric oxide an activation energy as low as 7 keM/mole. A third example has recently been studied in the oxidation of methanol over powder mixtures of silver and zinc oxide (Fig. 7). Here again the catalytic activity of the mixture is many times higher than is obtained with either component (7, 8). The explanation of these findings is that electrons go over from the silver into the boundary layer of the semieondueting oxides forming there a depletion layer of positive holes in nickel oxide and an enrichment layer of electrons in zinc oxide and ferric oxide. This transfer is possible because the work function of silver is smaller than that of all these oxides. A related and interesting phenomenon has been found by Steinbaeh (9): by irradiation of zinc oxide an electron transfer in the opposite direction can be enforced as shown 50 F

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Journal of Colloid and Interface Science, Vol. 34, No. 3, November 1970

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b y the fact t h a t with nonirradiated zinc oxide the activation energy of the carbon monoxide oxidation over silver decreases b y contact with zinc oxide and increases b y contact with illuminated zinc oxide. All these effects as measured b y the change in activation energy are much larger t h a n those found for metal catalysts supported b y semiconductors.

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The explanation based on the formation of b o u n d a r y layers has been concluded from purely chemical and catalytic experiI , I ments. However, direct confirmation b y I physical measurements was needed. Two I I types of experiments have been fruitful. One was the demonstration of a rectifying 103/ T 2 6 2~ 2!~ 2!3 2!2 ~' 2!0 [0 ,!~ & ,[0 ,!~ '~ ,!3 ~'2 ,', ,;0 I ] _1 ~ ' i effect (Fig. 8) of the interface silver nickel T3~ 40 ~,o ~ ~ ,;o ~-o & ~' ~' d~ o6o d~ 4° "3 "o ,ooo °c ,; ~]~ ~3 ,4~ ,~2 A d~ 2~3 d3 d~ ~2 d~ ,.;.2 4~ 4° d~ d~ oxide (10). And the other, a more conclusive one, was provided (11) b y conductivity Ag-Hg-Fe2Os; q ~ 7 R e a l / M o l e 12'7-31'7ac measurements of pellets of semiconducting Ag - Fe2Oa ; q~ 13 . . . . 24.3 - 3'73 ,, oxides with 10 %-20 % added metallic Ag = Pd - Fe203 ; q ~ 2 h . . . . 26'7- 333 ,, Fe2 O3 ; q~ 31 . . . . 4 9 7 - 59'/ ,, silver (Fig. 9). F r o m purely geometric conFro. 6. Arrhenius diagrams for SO2-oxidation siderations, one would expect t h a t addition over Fe20~ supported by silver powder. From of silver would increase the conductivity right to left: Unsupported, Ag-Pd alloy, pure of the semiconductor. This is indeed the silver, Ag-Hg alloy. In this series, as the electron ease with n-type semiconductors, but to a concentration of the metallic support increases, much higher degree t h a n would be exthe activation energy decreases drastically and pected simply b y conductivity of the silver. the reaction rate increases. If silver is added to p-type oxides, then the ~ 0 - , i ] conductivity actually decreases in spite of the higher conductivity of silver. This decrease can be understood only b y the transfer of electrons from the metal to the semiconductor. I n an oxide which can be

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Journal of Colloid and Interface Science, Vo]. 34, N o . 3, N o v e m b e r 1970

CHEMICAL EFFECTS AT THE SOLID/SOLID PHASE BOUNDARY 350 )[---

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FIG. 9. Logarithms of conductivity vs. reciprocal temperature for compressed pellets of semicondueting oxides with and withou~ ca. 10% of silver powder added. (Interrupted lines are extrapolations from measurements at higher temperatures. The conductivity of n-type semiconductors is increased by addition of silver (ZnO, Fe203) ; that of p-type materials is decreased because electrons transferred from the metal recombine with defect electrons. Cr20~ shows both kinds of behavior depending on the conduetion type produced by pretreatment. changed from p to n and vice versa by the atmosphere during preheating, e.g., chromic oxide, both effects can be produced depending on pretreatment. These experiments prove that electron transfer through the boundary layer indeed exerts a strong effect on the chemical properties of the adjacent phase, provided the particle size does not exceed the thickhess of the boundary layer. In view of the strong effect observed it can be said that here a new and promising method of catalyst synthesis has been found. SOLID STATE REACTIONS

in electrons. However, in most solid state reactions the rate-determining phenomenon is diffusion through the layer of the products. Even then, however, in the oxidation of germanium b y molybdenum trioxide (12) it has been shown that the reaction velocity can be influenced by doping in the sense mentioned through an influence on concentration gradients. In compressed powder mixtures of these two solids, n-doped germanium is oxidized more rapidly than p-doped germanium (see Fig. 10). The mechanism is very interesting. The diffusing species is the oxygen atom entering from the molybdenum trioxide phase into the layer of molybdenum dioxide and germanium dioxide, and there reacting with unmeasurable rate with the germanium surface. In the interface between germanium and its idoxide, the thermodynamic potential of the oxygen atoms depends on the electron concentration or electron free energy in germanium; it is higher, the lower is the electron concentration. Hence, n-type doping decreases the concentration of oxygen atoms at the interface and increases the activity gradient of oxygen atoms across the product layer and increases the reaction velocity as has actually been found. The experiments and considerations described show that indeed a solid/solid interface is a chemical system of very specific and peculiar properties which can be understood mostly in terms of an electronic

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In the experiments reported above the effect of the boundary layer and the effect of doping both change the catalytic activity. The question arises whether similar effects can Mso be exerted on the reactivity of the material itself as a reactant. For instance, one would expect t h a t a metal rich in electrons would be oxidized more easily b y a solid oxidant than a metal deficient

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Journal of Colloid and Interface Science, Vol. 34, No. 3, November 1970

342

SCHWAB

theory. It cannot a priori be denied that structural properties of both phases also play a role not yet investigated. Work is scheduled to collect new examples and to find new properties of such interfaces in the near future. REFERENCES 1. SCHWAB, G. M., AND SCHULT]~S, H., Z. Phys. Chem. B9 (1930) 265. 2. SCHWAB, G. M., BLOCK, J., MULLER, W., AND SCHULZE, D., Naturwissenschaflen, 44, 582 (1957); SCHWAB, G. M., Angew Chem. 73, 399 (1961). 3. SCHWAB,G. M., AND MUTZBAUER, G., Naturwissenschaften, 46, 13 (1959); Z. Phys. Chem. [N.F.] 32, 367 (1962).

4. PFAHLER, G., Dissertation, Universit~tt Mtinchen, Mtinchen, Germany, 1969. 5. SCHWAB, G. M., AND SIEGERT, R., Z. Phys. Chem. [N. F.] 50, 191 (1966). 6. SCHWAB, G. M., AND DERLETH, H., ibid. 53, 1 (1967). 7. SCHWAB, G. M., AND •OLLER, K., J. Amer. Chem. Soe. 90, 3078 (1968). 8. SEEMULLER, H., Dissertation, Universit/it Miinchen, Mfinchen, Germany, 1970. 9. STEINBACH,F., Private communication. 10. SCHWAB, G. 1V~., AND BBUNKE, F., Z. Naturforsch., A24, 1265 (1969). 11. SCHWAB, G. M., AND ~4~RITIKOS, A., Naturwissenschaflen 55, 228 (1968); Helv. Phys. Acta 41, 1166 (1968). 12. SCHWAB, G. M., AND GERLACH, J., Z. Phys. Chem. [N. F.] 56, 121 (1967).

Journal of Colloidand InterfaceScience,VoL 34, No. 3, November1970