S. Yanagida et al., SEMICONDUCTOR PHOTOCATALYSIS .17. ZNS-NANOCRYSTALLITE-CATALYZED PHOTOOXIDATION OF ORGANIC-COMPOUNDS, Bulletin of the Chemical Society of Japan, 68(7), 1995, pp. 1811-1823
Freshly prepared ZnS (nano-ZnS) suspensions catalyze photooxidation of
organic substrates under band-gap irradiation with water as a good el
ectron acceptor, while H-2 evolves concomitantly. The organic substrat
es with hetero atoms or carbon-carbon double bonds (pi-bonds), such as
triethylamine (TEA), diethylamine (DEA), methanol, ethanol, cyclopent
ene, cyclohexene, 2-methylfuran, toluene, and ethylbenzene, undergo ef
fective one-hole oxidation. This leads to efficient carbon-carbon bond
forming reactions between cumulatively formed radicals at the alpha-c
arbon adjacent to the hetero atom or the pi-bond. The photooxidation i
n the presence of a larger quantity of water results in successive oxi
dation of the intermediary alpha-carbon radicals, giving the two-hole
oxidation products, e.g., DEA and acetaldehyde from TEA, and formaldeh
yde from methanol. The formation of the intermediary alpha-carbon radi
cal 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 energ
etics in the formation of the alpha-carbon radicals through one-hole o
xidation and deprotonation, and from change in the bond order of alpha
C-H bond of the alpha-carbon cation radicals.