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

Abstract
The stoichiometry of the CO₂ photoreduction using TiO₂ (Degussa P25) as photocatalyst in the presence of water vapor was investigated in detail. 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 O₂ 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 Θ = 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.