On the nature of photogenerated radical species active in the oxidative degradation of dissolved pollutants with TiO2 aqueous suspensions:: A revision in the light of the electronic structure of adsorbed water

oOH radicals photogenerated via hole trapping by water species adsorbed on terminal Ti atoms are widely considered as active species in primary, oxidizing photocatalytic reactions with TiO2 aqueous suspensions. On the basis of the electronic structure of surface-bound water obtained from electron photoemission spectroscopy techniques data reported in the literature, evidence is found that water species specifically adsorbed on terminal (surface) Ti atoms cannot be photooxidized under supra-band gap UV illumination (hv >= 3.0 eV). The existing controversy on whether photogenerated valence band free holes are trapped either at adsorbed water species to produce adsorbed oOH radicals or at terminal oxygen ions (O-s(2-)) of the TiO2 surface to S generate terminal O-s(-) radicals is therefore solved in favor of the second hypothesis. The frequently proposed S photocatalytic oxidation mechanism involving the desorption of photogenerated oOH radicals should be refused because adsorbed water cannot be photooxidized. It is concluded that free oOH radicals in the water layer close to the TiO2 surface may only be generated via the electroreduction of dissolved oxygen with photogenerated conduction band electrons because the photooxidation of nonadsorbed water molecules or solvated hydroxyl groups with valence band holes is hindered both thermodynamically and kinetically.