Production of peroxide species in Pt/TiO2 suspensions under conditions of photocatalytic water splitting and glycerol photoreforming

Photoinduced production of hydrogen and peroxide species has been investigated over irradiated aqueous Pt/TiO2 suspensions both in the absence and in the presence of glycerol in solution. Results obtained from pure water (water-splitting conditions) showed that evolution of H2 in the gas phase is accompanied by production of peroxide species on the photocatalyst surface and H2O2 in solution. Two kinetically distinguishable reaction pathways of peroxide formation were observed, one prevailing at low irradiation times (Route I) and another dominating after prolonged exposure to light (Route II). It is proposed that Route I involves oxidation of surface hydroxyl groups by photogenerated holes and subsequent dimerization of the so-formed hydroxyl radicals, whereas Route II proceeds via consecutive reduction of surface-trapped photogenerated oxygen by conduction band electrons. The rate of H2 production increases significantly in the presence of glycerol in solution due to efficient scavenging of photogenerated holes/hydroxyl radicals. This is accompanied by a substantial increase of the initial rate of peroxide formation, possibly due to an enhancement of the rate of the oxygen reduction pathway (Route II). Under all experimental conditions employed, the total amount of peroxide species bound to the photocatalyst surface was found to reach a steady-state coverage of ca. 260 μmol g−1 (3.8 molecules nm−2).