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| - On the Nature of Photogenerated Radical Species Active in the OxidativeDegradation of Dissolved Pollutants with TiO2 Aqueous Suspensions: A Revision in the Light of the Electronic Structure of Adsorbed Water
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| - ◦OH radicals photogenerated via hole trapping by water species adsorbed on terminal Ti atoms are widelyconsidered 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 photoemissionspectroscopy techniques data reported in the literature, evidence is found that water species specifically adsorbedon terminal (surface) Ti atoms cannot be photooxidized under supra-band gap UV illumination (hν ≥ 3.0eV). The existing controversy on whether photogenerated valence band free holes are trapped either at adsorbedwater species to produce adsorbed ◦OH radicals or at terminal oxygen ions () of the TiO2 surface togenerate terminalradicals is therefore solved in favor of the second hypothesis. The frequently proposedphotocatalytic oxidation mechanism involving the desorption of photogenerated ◦OH radicals should berefused because adsorbed water cannot be photooxidized. It is concluded that free ◦OH radicals in thewater layer close to the TiO2 surface may only be generated via the electroreduction of dissolvedoxygen with photogenerated conduction band electrons because the photooxidation of nonadsorbed watermolecules or solvated hydroxyl groups with valence band holes is hindered both thermodynamically andkinetically.
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