SEMICONDUCTOR PHOTOCATALYSIS .17. ZNS-NANOCRYSTALLITE-CATALYZED PHOTOOXIDATION OF ORGANIC-COMPOUNDS

Freshly prepared ZnS (nano-ZnS) suspensions catalyze photooxidation of organic substrates under band-gap irradiation with water as a good electron acceptor, while H-2 evolves concomitantly. The organic substrates with hetero atoms or carbon-carbon double bonds (pi-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 alpha-carbon adjacent to the hetero atom or the pi-bond. The photooxidation in the presence of a larger quantity of water results in successive oxidation of the intermediary alpha-carbon radicals, giving the two-hole oxidation products, e.g., DEA and acetaldehyde from TEA, and formaldehyde from methanol. The formation of the intermediary alpha-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 alpha-carbon radicals through one-hole oxidation and deprotonation, and from change in the bond order of alpha C-H bond of the alpha-carbon cation radicals.