Semiconducting catalysts

The preceding survey was not written for the purpose of giving a complete review of all important experimental results on the exchange of electrons between semiconducting catalysts and reacting gases. Its main... 

The oxidation of carbon monoxide on nickel oxide has often been investigated (4, 6, 8, 9, II, 16, 17, 21, 22, 26, 27, 29, 32, 33, 36) with attempts to correlate the changes in the apparent activation energy with the modification of the electronic structure of the catalyst. Published results are not in agreement (6,11,21,22,26,27,32,33). Some discrepancies would be caused by the different temperature ranges used (27). However, the preparation and the pretreatments of nickel oxide were, in many cases, different, and consequently the surface structure of the catalysts—i.e., their composition and the nature and concentration of surface defects— were probably different. Therefore, an explanation of the disagreement may be that the surface structure of the semiconducting catalyst (and not only its surface or bulk electronic properties) influences its activity. 

Doping. In the case of semiconducting catalysts, a small amount of foreign material dissolved in the original catalyst may modify the rate of a particular reaction. This phenomenon is sometimes called doping by analogy with the effect of similar materials upon semiconductivity. 

The inverse case, a semiconducting catalyst supported by a metal, termed inverse supported catalyst, has been studied systematically only in the last few years. Here, even more drastic effects can be expected because normally the number of free electrons in a metal is several orders of magnitude higher than in semiconductors. The effects are indeed considerably larger as will be shown below. However, the principles and the theory involved are more complex (6-8). 

By doping a semiconducting catalyst it is possible to alter the electron concentration, which leads of course to a redistribution of the electrons over the quantum states. However, it has been shown by measuring the work function that the Fermi potential at the surface of the solid is not influenced even by drastic shifts of the Fermi potential in the bulk 13-1S>. 

Dynamics of Electron Transfer Processes on Semiconducting Catalysts... 

Electron transfer from the conduction band of semiconducting catalysts across the interface into the accepting species in solution has also been investigated extensively with nanosecond laser-flash photolysis and time-resolved fluorescence spectroscopy (109). 

Z.R. Tian, W. Tong, J.Y. Wang, N.G. Duan, V.V. Krishnan, and S.L. Suib, Manganese Oxide Mesoporous Structures Mixed-valent Semiconducting Catalysts. Science, 1997, 276, 926-930. 

The adsorption of oxygen on certain semiconductors, such as NiO and Cu20 was studied in fair detail. The activation energies, heats of adsorption and kinetic laws for oxygen sorption on simple semiconducting catalysts are summarized in Table VI. 

Thus oxygen is chemisorbed on all simple semiconducting catalysts— metal oxides—and in a number of cases it is partly dissolved in the lattice. 

A molecule with a double bond adsorbed on a semiconducting catalyst surface converts into a radical bound with the lattice and having a free valence. A molecule with a single bond emerging from the gas phase may react with the free valence of such a radical and dissociate. 

Hydrocarbons are sorbed on semiconducting catalysts either weakly—reversibly, or strongly—irreversibly. The ratio of weak to strong adsorption is a function of temperature and the chemical composition of the catalyst. 

Let us consider schemes for certain oxidation reactions over metals and semiconducting catalysts. 

Various reactions, such as high conversion, low conversion with partial destruction of hydrocarbon molecules, and without essential distortion of the molecular structure may ocour in the oxidation over semiconducting catalysts. 

Tian, Z.-R. Tong, W. Wang, J.-Y. Duan. N.-G. Krishnan. V.V. Suib. S.L. Manganese oxide mesoporous structures Mixed-valent semiconducting catalysts. Science 1997, 276. 926. 

Tian ZR, Tong W, Wang JY, Duan NG, Krishnan W, Suib SL (1997) Manganese oxide mesoporous stmctures Mixed-valent semiconducting catalysts. Science 276 926-930 Tossell JA (1977) SCF-Xa scattered wave MO studies of the electronic stmcture of ferrous iron in octahedral coordination with sulfur. J Chem Phys 66 5712-19 Tossell JA, Vaughan DJ (1987) Electronic stmcture and the chemical reactivity of the surface of galena. Can Min 25 381-392... 

Manganese oxide mesoporous structures mixed-valent semiconducting catalysts. Science, 276, 926-930. 

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