Catalytic Role of TiO2 Terminal Oxygen Atoms in Liquid-Phase Photocatalytic Reactions: Oxidation of Aromatic Compounds in Anhydrous Acetonitrile

On the basis of experiments carried out with controlled amounts of residual oxygen and water, or by using oxygen-isotope-labeled (TiO2)-O-18 as the photocatalyst, we demonstrate that O-18(s) atoms behave as real catalytic species in the photo-oxidation of acetonitrile-dissolved aromatic compounds such as benzene, phenol, and benzaldehyde with TiO2. The experimental evidence allows a terminal-oxygen indirect electron-transfer (TOIET) mechanism to be proposed, which is a new pathway that involves the trapping of free photogenerated valence-band holes at Os species and their incorporation into the reaction products, with simultaneous generation of oxygen vacancies at the TiO2 surface and their subsequent healing with oxygen atoms from either O-2 or H2O molecules that are dissolved in the liquid phase. According to the TOIET mechanism, the TiO2 surface is not considered to remain stable, but is continuously changing in the course of the photocatalytic reaction, challenging earlier interpretations of TiO2 photocatalytic phenomena.