Stabilization of Free Radical Intermediates

Free radicals intermediates are formed on the surface of metal oxide semiconductors, namely Ti02 and Fe203, following the light-induced charge separation upon band-gap illu-mination. These radicals appear to adhere strongly to the ~.urfaces 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-1,1,1trifluoroethane) is thus found to proceed with high quantum yields and very specific mechanisms on colloidal Ti02. The oxidation of sulfite on a-Fe203 particles yields the sulfite radical anion, S03', as initial intermediate. A subsequent electron transfer leading to the formation of sulfate occurs on the surface of the same catalyst particle.