Stabilization of free radical intermediates on metal oxide semiconductors surfaces

摘要

Free radicals intermediates are formed on the surface of metal oxide semiconductors, namely TiO_2 and Fe_2O_3, following the light-induced charge separation upon band-gap illumination. These radicals appear to adhere strongly to the surfaces of the small colloidal particles (diameters between 5 and 20 nm) employed in this study. A second electron-transfer step yielding the observed end-products therefore preceeds desorption of the reactive intermediates. Oxidation of methylviologen (1,1'-dimethyl-4,4'-bipyridylium chloride) and reduction of halothane (2-bromo-2-chloro-1,1,1-udtrifluoroethane) is thus found to proceed with high quantum yields and very specific mechanisms on colloidal TiO_2. The oxidation of sulfite on α-Fe_2O_3 particles yields the sulfite radical anion, SO_3^- , as initial intermediate. A subsequent electron transfer leading to the formation of sulfate occurs on the surface of the same catalyst particle.