Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 3.—Role of H2O2 in photo-desorption of O2

Abstract

Photo-desorption of oxygen from “fully hydrated” anatase surfaces has been previously observed and ascribed to the formation and subsequent decomposition of hydrogen peroxide. The presence of HO₂ radicals has been detected by e.p.r. on these surfaces at 77K. To assess this process in detail adsorption of H₂O₂ from solution has now been studied on two types of samples outgassed at 400 and 200°C, respectively, (dehydroxylated and hydroxylated). Photo-decomposition of H₂O₂ into O₂ and H₂O on the dehydroxylated surfaces occurs which follows a diffusion law while the kinetics change to pseudo-zero-order at higher coverages of H₂O₂ on the hydroxylated surfaces. E.p.r. spectra show the existence of “residual O^– ₂” species on the surfaces in the dark while the formation of new O^– ₂ species and HO₂ occurs under u.v. illumination at 77 K. Results are discussed on the basis of our previous work confirming H₂O₂ as an intermediate in the photo-adsorption/photo-desorption process occurring on hydroxylated TiO₂ surfaces.