PHOTOCATALYTIC OXIDATION OF ORGANIC-ACIDS ON QUANTUM-SIZED SEMICONDUCTOR COLLOIDS

A detailed analysis of the reaction products and mechanisms of the pho tocatalytic oxidation of acetate in the presence of quantum-sized ZnO colloids (D(p) almost-equal-to 40 angstrom) is presented. The principa l oxidation products and reaction intermediates are determined to be C O2, HCO2-, CHOCO2-, HCHO, CH3OOH, CH3COOOH, and H2O2. Formate and glyo xylate, which are found as intermediates in the photooxidation of acet ate, also serve as effective electron donors on illuminated ZnO surfac es. The proposed relative reactivity of electron donors toward photoox idation is in the following order: CHOCO2- > HCO2- > HCHO > CH3CO2- > H2O2 greater-than-or-equal-to CH3COOOH > CH3OOH. Observed product dist ributions are discussed in terms of pathways involving direct oxidatio n of surface-bound acetate by valence band holes (or trapped holes) an d the indirect oxidation of acetate by surface-bound hydroxyl radicals . The product distribution observed at low photon fluxes is not consis tent with oxidation primarily by free hydroxyl radicals. A mechanism i nvolving the reaction of an intermediate carbon-centered radical with > ZnOH surface sites is proposed. When electron donors are strongly ad sorbed to semiconductor surfaces, surface-mediated reactions appear to play a dominant role in the determination of the time-dependent produ ct distributions.