Water-promoting effect on reaction selectivity in the photocatalytic oxidation of sulfides to sulfoxides on TiO2

During TiO2-based photocatalysis, the presence of water usually leads to the unselective degradation of organic compounds due to the uncontrollable formation of water-derived radical species. Here we report that the presence of water dramatically enhances the selectivity in photocatalytic oxidation of sulfides to sulfoxides over TiO2. H218O-isotope labelling experiments reveal that both O2 and H218O contribute to the oxygen atoms in the formed sulfoxides and sulfones. Moreover, at the beginning of the oxidation of sulfides, water serves as the main source of the oxygen atom, while the contribution of O2 increases with the irradiation time. We uncover two roles of water in sulfides oxidation: to act as oxygen source in the oxidation of sulfides to sulfoxides; to provide protons to inhibit the further oxidation of sulfoxides. This study deepens our understanding of roles of water in photocatalytic oxidation of sulfides and offers a new strategy to optimize photocatalytic selective oxidation.