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

Abstract

In this study we examine photostimulated reactions taking place on the surface of wide band-gap metal oxides such as ZrO2 and TiO2. In particular, we explore the photostimulated adsorption (PhA) of oxygen on ZrO2 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 O2 PhA kinetics, dp/dt ∝ ρmpn, resides in the dependence of the order of the reaction on the order m with respect to photon flow (ρm) and on the order n with respect to pressure (pn). 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 O2 on ZrO2. 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 O2 on ZrO2 to processes that take place in the photocatalyzed oxidation of phenol in irradiated TiO2 aqueous dispersions and the interdependence of the rate of the photoreaction on the concentration of phenol and on the incident light intensity.