Semiconductor photooxidation of pollutants dissolved in water:: A kinetic model for distinguishing between direct and indirect interfacial hole transfer.: I.: Photoelectrochemical experiments with polycrystalline anatase electrodes under current doubling and absence of recombination

A kinetic model based on a set of reactions occurring at the illuminated semiconductor/electrolyte junction, under current multiplication (doubling) and external applied bias (absence of recombination), was developed as a tool for assessing mechanistic aspects of photoelectrochemical oxidation of water-dissolved pollutants. The model allows to distinguish whether direct or indirect interfacial hole transfer to the solute predominates. We apply the model to the photooxidation of aqueous solutions of formic acid and methanol on polycrystalline TiO2 (anatase) electrodes. Formic acid is mainly oxidized directly through photogenerated valence band free holes, while methanol oxidation occurs indirectly, via surface-bound hydroxyl radicals. The importance of the specific electronic interaction of pollutant molecules with the semiconductor surface in the photooxidation process is emphasized.