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 Ti¹⁸O₂ as the photocatalyst, we demonstrate that ¹⁸O_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 O_s 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 O₂ 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.