Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 1.—Role of hydroxyl groups in photo-adsorption

Abstract

The photo-activity of TiO₂(anatase P. 25) samples with a “fully hydroxylated surface” was studied in the presence of oxygen. Photo-adsorption of oxygen was observed in the early stages of u.v. illumination reducing the pressure to < 0.1 N m^–2. After this fast adsorption, slow photo-desorption of oxygen was detected continuing for several hours. Thermal removal of water and hydroxyl groups reduced photo-adsorption and the process became diffusion controlled. Rehydration restored fast photo-adsorption and first order kinetics. Photo-desorption of water accompanied photo-adsorption of oxygen on these “fully hydrated surfaces”. A mechanism involving the formation of OH· radicals and their reaction to give H₂O₂, which decomposes leading to oxygen photo-desorption, is proposed to explain these facts.