The photocatalytic oxidation of water to O2 over pure CeO2, WO3, and TiO2 using Fe3+ and Ce4+ as electron acceptors Original Research Article

We report here the results from a study on the photocatalytic decomposition of water to oxygen over pure WO3, CeO2, and TiO2. It has been demonstrated that Feaq3+ and Ceaq4+ species are efficient electron acceptors during the photoproduction of O2 over a variety of oxides, even at very low concentrations. The O2 yield was found to depend on the type and surface activity of the cation used as an electron acceptor, and the salt counter anion. While Ceaq4+ gives slightly higher initial O2 rates, Feaq3+ tends to give higher long-term O2 yields. The O2 yield was found to be mainly sensitive to the intrinsic properties of a given material, such as the type of the oxide used and its physicochemical characteristics, e.g. crystal structure and level of crystallinity. For the powders tested, O2 production was independent of the BET surface area and the activity does not correlate directly with the onset of light absorption of the powders. With a light having λ≥330 nm, O2 production activity decreased in the order TiO2-rutile>TiO2-anatase>WO3>CeO2⪢amorphous TiO2, whereas with a light having λ≥420 nm, it decreased in the order WO3>TiO2>CeO2. Small amounts of Sn on TiO2-rutile markedly improved its activity. In addition, the O2 yield strongly depended on the concentration of the electron acceptor and the pH of the suspension. During the reaction, small amounts of hydrogen were also produced. The reaction pathways for electron scavenging by Ceaq4+ and Feaq3+, and the process leading to O2 evolution will be discussed.