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, TiO 2 , CeO 2 , 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 CeO 2 > ZnO ≈ TiO 2 , while that of partially hydrated samples after pretreatment at 20 °C changes in the order TiO 2 > ZnO ≥ CeO 2 ≈ 0. According to the results, the difference in the photocatalytic activity toward CO oxidation on the dehydrated ZnO, TiO 2 , and CeO 2 catalysts is attributable to different concentrations of oxygen vacancies, which are formed more readily after high-temperature treatment on ZnO and CeO 2 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 O 2 gives peroxides and hydroperoxides, with their concentrations decreasing in the series TiO 2 >> ZnO >> CeO 2 . Most likely, these species are active intermediates of CO photooxidation with oxygen in the presence of adsorbed water. The hydrophobization effect was detected upon TiO 2 modification with zinc, resulting in removal of surface acid sites capable of adsorbing water. The TiO 2 modification with zinc increases the activity of CO photooxidation with respect to the oxidation catalyzed by samples pretreated at low temperatures (20—60 °C).