Photostimulated Reactions at the Surface of Wide Band-Gap Metal Oxides (ZrO2 and TiO2): Interdependence of Rates of Reactions on Pressure−Concentration and on Light Intensity

In this study we examine photostimulated reactions taking place on the surface of wide band-gap metal oxides such as ZrO 2 and TiO 2 . In particular, we explore the photostimulated adsorption (PhA) of oxygen on ZrO 2 upon illumination with UV light (λ < 260 nm) and its postphotostimulated photoadsorption (PPSPhA) on irradiation with visible light (λ > 400 nm). The emphasis is on an examination of the decay channels of PhA-active surface centers to rationalize the interdependencies of the rate r ( ρ,p ) on pressure and light intensity The distinguishing feature of O 2 PhA kinetics, d p /d t ∝ ρ m p n, resides in the dependence of the order of the reaction on the order m with respect to photon flow (ρ m ) and on the order n with respect to pressure ( p n ). In other words, m → 1 if n → 0 whereas n → 1 if m → 0. The reaction orders m and n are interdependent in the case of PhA of O 2 on ZrO 2 . A general mechanism is described for the several stages implicated in both PhA and PPSPhA surface processes. A preliminary consideration is also given to expand the PhA pathway for O 2 on ZrO 2 to processes that take place in the photocatalyzed oxidation of phenol in irradiated TiO 2 aqueous dispersions and the interdependence of the rate of the photoreaction on the concentration of phenol and on the incident light intensity.