Effect of surface hydration on the photocatalytic activity of oxide catalysts in the CO oxidation

The effect of the state of hydrated surface of the bulk oxide photocatalysts, TiO₂, CeO₂, and ZnO on the rate of UV-induced oxidation of CO with atmospheric oxygen was studied. The activity of dehydroxylated catalyst samples evacuated at temperatures of >350 °C toward CO photooxidation decreases in the series CeO₂ > ZnO ≈ TiO₂, while that of partially hydrated samples after pretreatment at 20 °C changes in the order TiO₂ > ZnO ≥ CeO₂ ≈ 0. According to the results, the difference in the photocatalytic activity toward CO oxidation on the dehydrated ZnO, TiO₂, and CeO₂ catalysts is attributable to different concentrations of oxygen vacancies, which are formed more readily after high-temperature treatment on ZnO and CeO₂ and thus promote higher rate of CO photooxidation. Using a new technique for recording transmittance IR spectra, it was found that photoirradiation in the presence of adsorbed water and O₂ gives peroxides and hydroperoxides, with their concentrations decreasing in the series TiO₂ >> ZnO >> CeO₂. Most likely, these species are active intermediates of CO photooxidation with oxygen in the presence of adsorbed water. The hydrophobization effect was detected upon TiO₂ modification with zinc, resulting in removal of surface acid sites capable of adsorbing water. The TiO₂ modification with zinc increases the activity of CO photooxidation with respect to the oxidation catalyzed by samples pretreated at low temperatures (20—60 °C).