Catalytic Role of Surface Oxygens in TiO₂ Photooxidation Reactions: Aqueous Benzene Photooxidation with Ti¹⁸O₂ under Anaerobic Conditions

The important role played in TiO₂ heterogeneous photocatalysis by the two-fold-coordinated TiO₂ surface bridging oxygens (>O_s^2– or >OH_s^–) has been emphasized. Their ability to trap photogenerated holes and to act as main oxidizing agents, leaving the surface of the catalyst and being part of the intermediates and final reaction products, is probed by using isotopic labeling reactions. In particular, we have studied the aqueous photocatalytic mineralization of benzene in anaerobic conditions using Ti¹⁸O₂ as the photocatalyst. The reaction evolution has been followed by analysis of the concentration and the isotopic composition of CO₂, the final product of benzene photooxidation. The unique features of this experimental system provide the opportunity for in situ tracing of the fate of >O_s^2– or >OH_s^– species during the photocatalytic reaction. Experimental evidence shows that those oxygen atoms are incorporated into CO₂. Participation of surface-trapped holes (−OH_s^•/–O_s^•–) and the incorporation of surface oxygens into the oxidation products prove the main role played by those species in TiO₂-assisted photocatalytic processes.