Catalytic role of bridging oxygens in TiO2 liquid phase photocatalytic reactions: analysis of H216O photooxidation on labeled Ti18O2

Abstract

Experiments of photocatalytic oxidation of H₂¹⁶O with a suspended oxygen-isotope labelled Ti¹⁸O₂ photocatalyst are presented here for the first time. The photo-induced evolution of ¹⁸O¹⁶O demonstrates that bridging surface oxygens (>¹⁸O_br²⁻) behave as real catalytic species of the global water splitting photocatalytic reaction (2H₂O + 4h⁺ → O_2(g)↑ + 4H⁺). The experimental results are interpreted according to a previously developed water redox photooxidation (WRP) mechanism (Salvador, P. Prog. Surf. Sci. 2011, 86, 41–58), opening a new mechanistic pathway that involves the participation of terminal >O_br²⁻ bridging oxygens as real photocatalytic species. In the primary step, one-fold coordinated −¹⁸O_br˙⁻ radicals are generated from the direct photooxidation of >¹⁸O_br²⁻ oxygens with valence band holes (>¹⁸O_br²⁻ + h⁺ → −¹⁸O_br˙⁻). In the second step, a couple of adjacent −¹⁸O_br˙⁻ radicals chemically react, giving rise to peroxo species (2¹⁸O_br˙⁻ → ¹⁸O₂²⁻), which are further photooxidized with photogenerated valence band holes, initially leading to ¹⁸O_2(g) evolution according to the global photoreaction ¹⁸O₂²⁻ + 4h⁺ → 2V[>¹⁸O_br²⁻] + ¹⁸O_2(g)↑. Terminal oxygen vacancies (V[>¹⁸O_br²⁻]) become further healed via dissociative adsorption of H₂¹⁶O water molecules (2V[>¹⁸O_br²⁻] + 2H₂¹⁶O → 2(>¹⁶O_br²⁻) + 2H⁺), in such a way that >¹⁸O_br²⁻ bridging ions are progressively substituted by >¹⁶O_br²⁻ and the initially evolved ¹⁸O_2(g) is further replaced by ^16,18O_2(g) and finally by ¹⁶O_2(g).