Catalytic role of bridging oxygens in TiO<inf>2</inf> liquid phase photocatalytic reactions: Analysis of H<inf>2</inf>¹⁶O photooxidation on labeled Ti¹⁸O<inf>2</inf>

© 2017 The Royal Society of Chemistry. 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).