Influence of photoadsorbed O-2 On the photoreduction of CO2 with H2O at the surface of TiO2

The stoichiometry of the CO2 photoreduction using TiO2 (Degussa P25) as pho tocatalyst in the presence of water vapor was investigated in derail. It wa s observed that oxygen is missing in significant amounts on the product sid e and that the efficiency of the reaction decreases with time. Photoadsorpt ion of O-2 at TiO2 was investigated as a possible explanation of these fact s. It was found that the photoactivity is strongly hindered by photoadsorbe d oxygen and ceases completely at a surface coverage of Theta = 0.05. This low value does not allow to attribute all the missing oxygen in the reactio n to oxygen photoadsorbed on the photocatalyst. Photoadsorption can therefo re not be invoked to explain the fate of the missing oxygen in the reaction products.