Direct evidence of the key role of UV-formed peroxide species in photocatalytic gas–solid oxidation in air on anatase TiO2 particles

Abstract

IR spectroscopy was applied for the in situ investigations of surface intermediates formed on the surface of TiO₂ (anatase) and ZnO under UV light illumination in air and their reactivity in the elimination of simple pollutant molecules. UV-irradiation of TiO₂ (anatase) in air leads to the generation of peroxo-species with the peaks at 852 and 912 cm⁻¹, but the bands of O₂˙⁻_ads were not detected. This is, to our knowledge, the first direct in situ IR spectroscopic detection of O₂ photosorption intermediates in moist air. The formation of peroxo-species in these conditions is specific for TiO₂ (anatase), whereas on ZnO the predominant species under UV light illumination in air are O₂˙⁻_ads and H₂O₂, desorbing into gas phase. Adsorbed water and surface hydroxyl groups contribute to the formation and stabilization of peroxo-species on TiO₂ anatase during UV illumination in an oxygen atmosphere. If UV-irradiation is carried out in the environment of moist argon instead of moist air, the peroxo-species on TiO₂ anatase are formed from water in a negligible quantity. Peroxo-species formed after O₂ photoadsorption on TiO₂ anatase in moist air have band positions similar to peroxo-species formed after photodecomposition of H₂O₂ (with accompanying color change of this sample from yellow to white). Direct experimental IR-spectroscopic evidence of peroxo-species reactivity as oxidative intermediates on TiO₂ (anatase) in CO and ethanol vapor photooxidative processes is firstly obtained. These results confirm our early conclusion that peroxo-species formed under UV-irradiation in O₂ on the hydrated surface of TiO₂ (anatase) can be responsible for the surprising extreme dependence of the CO photooxidation rate on the adsorbed water coverage with the maximum at ∼0.5 ML. The ZnO sample was not active in the photooxidation of these molecules in air. It is concluded that UV formed peroxo-species are important diamagnetic oxidative intermediates in heterogeneous photochemical gas–solid oxidation processes on TiO₂ (anatase).