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Photocatalytic oxidation of organic acids on quantum-sized semiconductor colloids.

A detailed analysis of the reaction products and mechanisms of the photocatalytic oxidation of acetate in the presence of quantum-sized ZnO colloids (Dp ≈ 40 A) is presented. The principal oxidation products and reaction intermediates are determined to be CO_2, HCO_2^-, CHOCO_2^-, HCHO, CH_3OOH, CH_3COOOH, and H_2O_2. Formate and glyoxylate, which are found as intermediates in the photooxidation of acetate, also serve as effective electron donors on illuminated ZnO surfaces. The proposed relative reactivity of electron donors toward photooxidation is in the following order: CHOCO_2- > HCO_2^- > HCHO > CH_3CO_2^- ≥ H_2O_2 CH_3COOOH > CH_3OOH. Observed product distributions are discussed in terms of pathways involving direct oxidation of surface-bound acetate by valence band holes (or trapped holes) and the indirect oxidation of acetate by surface-bound hydroxyl radicals. The product distribution observed at low photon fluxes is not consistent with oxidation primarily by free hydroxyl radicals. A mechanism involving the reaction of an intermediate carbon-centered radical with > ZnOH surface sites is proposed. When electron donors are strongly adsorbed to semiconductor surfaces, surface-mediated reactions appear to play a dominant role in the determination of the time-dependent product distributions.