Increase of CO photocatalytic oxidation rate over anatase TiO2 particles by adsorbed water at moderate coverages: The role of peroxide species

Effect of water coverage on photocatalytic activities of CO oxidation was investigated on two TiO 2 samples: Hombikat UV 100 and Degussa P25. Surprisingly, the initial rate for CO photocatalytic oxidation over both TiO 2 samples shows the extreme dependence with maximum with the increase of water coverage from zero (for fully dehydrated TiO 2 samples) to one monolayer. The maximum of CO photooxidation rate was at ∼0.5 monolayer of adsorbed water for both TiO 2 samples, wherein the catalytic activity was higher for TiO 2 Hombikat UV 100 than TiO 2 Degussa P25. It was concluded that the role of water is more complex than just a competition for adsorption sites between H 2 O and CO, and with moderate H 2 O coverages the positive influence of adsorbed water on CO photooxidation rate occurs. Different mechanisms of CO photooxidation on a fully dehydroxylated and partially hydrated TiO 2 with participation of different oxidizing species generated under UV-irradiation were suggested.