Web Release Date: October 19,
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
Departamento de Química Física e Instituto Universitario de Electroquímica, Universidad de Alicante, Ap. 99, E-03080 Alicante
Received: June 8, 2007
In Final Form: August 10, 2007
Abstract:
OH 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 (h
3.0
eV). The existing controversy on whether photogenerated valence band free holes are trapped either at adsorbed
water species to produce adsorbed OH radicals or at terminal oxygen ions (
) of the TiO2 surface to
generate terminal 
radicals is therefore solved in favor of the second hypothesis. The frequently proposed
photocatalytic oxidation mechanism involving the desorption of photogenerated OH radicals should be
refused because adsorbed water cannot be photooxidized. It is concluded that free OH 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.
Download the full text: PDF | HTML
