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

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 Fe-aq(3+) and Ce-aq(4+) species are efficient electron accepters during the photoproduction of O-2 over a variety of oxides, even at very low concentrations. The O-2 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 Ce-aq(4+) gives slightly higher initial O-2 rates, Fe-aq(3+) tends to give higher long-term O-2 yields. The O-2 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, O-2 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 lambda greater than or equal to 330 nm, O-2 production activity decreased in the order TiO2-rutile>TiO2-anatase>WO3>CeO(2)much greater than amorphous TiO2, whereas with a light having lambda greater than or equal to 420 nm, it decreased in the order WO3>TiO2>CeO2. Small amounts of Sn on TiO2-rutile markedly improved its activity. In addition, the O-2 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 Ce-aq(4+) and Fe-aq(3+), and the process leading to O-2 evolution will be discussed. (C) 2001 Elsevier Science B.V. All rights reserved.