Influence of photoadsorbed O2 On the photoreduction of CO2 with H2O at the surface of TiO2

The stoichiometry of the CO2 photoreduction using TiO2 (Degussa P25) as photocatalyst in the presence of water vapor was investigated in derail. It was observed that oxygen is missing in significant amounts on the product side and that the efficiency of the reaction decreases with time. Photoadsorption of O-2 at TiO2 was investigated as a possible explanation of these facts. It was found that the photoactivity is strongly hindered by photoadsorbed 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 reaction to oxygen photoadsorbed on the photocatalyst. Photoadsorption can therefore not be invoked to explain the fate of the missing oxygen in the reaction products.