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

• Adsorbed water on TiO2 surprisingly enhance CO photooxidation at moderate coverage.
• Peroxide species like TiOOTi and TiOOH are assumed to be key intermediates.
• The formation of stable C-containing species is controlled by water coverage.

Effect of water coverage on photocatalytic activities of CO oxidation was investigated on two TiO2 samples: Hombikat UV 100 and Degussa P25. Surprisingly, the initial rate for CO photocatalytic oxidation over both TiO2 samples shows the extreme dependence with maximum with the increase of water coverage from zero (for fully dehydrated TiO2 samples) to one monolayer. The maximum of CO photooxidation rate was at ∼0.5 monolayer of adsorbed water for both TiO2 samples, wherein the catalytic activity was higher for TiO2 Hombikat UV 100 than TiO2 Degussa P25. It was concluded that the role of water is more complex than just a competition for adsorption sites between H2O and CO, and with moderate H2O coverages the positive influence of adsorbed water on CO photooxidation rate occurs. Different mechanisms of CO photooxidation on a fully dehydroxylated and partially hydrated TiO2 with participation of different oxidizing species generated under UV-irradiation were suggested.