Catalytic Role of TiO 2 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 Ti 18 O 2 as the photocatalyst, we demonstrate that 18 O s atoms behave as real catalytic species in the photo‐oxidation of acetonitrile‐dissolved aromatic compounds such as benzene, phenol, and benzaldehyde with TiO 2 . 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 O s species and their incorporation into the reaction products, with simultaneous generation of oxygen vacancies at the TiO 2 surface and their subsequent healing with oxygen atoms from either O 2 or H 2 O molecules that are dissolved in the liquid phase. According to the TOIET mechanism, the TiO 2 surface is not considered to remain stable, but is continuously changing in the course of the photocatalytic reaction, challenging earlier interpretations of TiO 2 photocatalytic phenomena.