Semiconductor Photocatalysis. ZnS-Nanocrystallite-Catalyzed Photooxidation of Organic Compounds

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Abstract
Freshly prepared ZnS (nano-ZnS) suspensions catalyze photooxidation of organic substrates under band-gap irradiation with water as a good electron acceptor, while H2 evolves concomitantly. The organic substrates with hetero atoms or carbon–carbon double bonds (π-bonds), such as triethylamine (TEA), diethylamine (DEA), methanol, ethanol, cyclopentene, cyclohexene, 2-methylfuran, toluene, and ethylbenzene, undergo effective one-hole oxidation. This leads to efficient carbon–carbon bond forming reactions between cumulatively formed radicals at the α-carbon adjacent to the hetero atom or the π-bond. The photooxidation in the presence of a larger quantity of water results in successive oxidation of the intermediary α-carbon radicals, giving the two-hole oxidation products, e.g., DEA and acetaldehyde from TEA, and formaldehyde from methanol. The formation of the intermediary α-carbon radical has been clarified by ESR analysis using 2-propanol as an organic substrate. Semi-empirical molecular orbital calculations suggest that the nano-ZnS-catalyzed photooxidation should be predictable from energetics in the formation of the α-carbon radicals through one-hole oxidation and deprotonation, and from change in the bond order of αC–H bond of the α-carbon cation radicals.

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