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 (>18Obr2−) behave as real catalytic species of the global water splitting photocatalytic reaction (2H2O + 4h+ → O2(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 >Obr2− bridging oxygens as real photocatalytic species. In the primary step, one-fold coordinated −18Obr˙− radicals are generated from the direct photooxidation of >18Obr2− oxygens with valence band holes (>18Obr2− + h+ → −18Obr˙−). In the second step, a couple of adjacent −18Obr˙− radicals chemically react, giving rise to peroxo species (218Obr˙− → 18O22−), which are further photooxidized with photogenerated valence band holes, initially leading to 18O2(g) evolution according to the global photoreaction 18O22− + 4h+ → 2V[>18Obr2−] + 18O2(g)↑. Terminal oxygen vacancies (V[>18Obr2−]) become further healed via dissociative adsorption of H216O water molecules (2V[>18Obr2−] + 2H216O → 2(>16Obr2−) + 2H+), in such a way that >18Obr2− bridging ions are progressively substituted by >16Obr2− and the initially evolved 18O2(g) is further replaced by 16,18O2(g) and finally by 16O2(g).