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 ∝ 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 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.

Photophysical Processes Related to Photoadsorption and Photocatalysis on Wide Band Gap Solids: A Review

“…The latter depends on different parameters, including the so called rate of surface recombination of carriers, which, typically, is not known. The tentative estimates of the recombination lifetime which is in agreement with that observed for ZrO 2 in the pressure range 0.1 ÷ 10 Pa have been presented elsewhere [75]. In general, one may conclude that the higher the actual pressure, the much number of short-lived active states of photoadsorption (photocatalytic) centers of different types can manifest themselves in a reaction studied.…”

“…Note, the concentration of photo carriers depends on light intensity, and so both decay constants for deep photoadsorption center depend on photon flux. The latter approach has been used to describe the interdependence of photoadsorption rate on pressure and on light intensity observed in the studies of the O 2 /ZrO 2 system [75]. The following generalization of eq.…”

“…The generation of electronhole pairs corresponding to fundamental photoelectric absorption has been simulated. In order to describe the formation of the post-sorption species M − (the pre-irradiation in vacuum and following contact with a gas, [75] and by (0.03 ± 0.003) s −1 [81] respectively, the UV irradiation of the ZrO 2 samples was nearly the same as for the case presented ( Figure 5). The condition k a p k 2 (eq.…”

Photo‐induced processes in heterogeneous nanosystems. From photoexcitation to interfacial chemical transformations

“…In fact, when βρ γ[M] equation (9) gives the linear dependence on light intensity observed in some recent studies by Emeline and coworkers [13] and by Basov et al [14], whereas when βρ γ[M] the reaction rate is independent of photon flow as reported by Ollis [15]. In other cases, when βρ is comparable to γ[M] the dependence of the reaction rate on photon flow is sub-linear; at certain ratios between these two terms the dependence may be similar to the square-root dependence reported by Emeline and coworkers [12,13].…”

“…Equation (9) completely describes the experimental dependencies of the rate of the photodegradation of phenol in TiO 2 dispersions on both photon flow and phenol concentration [12], and the dependencies of the rate of photostimulated adsorption of dioxygen on ZrO 2 particles on both oxygen pressure and photon flow [13]; they are presented in Figure 6. Note that equation (9) can be cast into the Langmuir-Hinshelwood form when the photon flow ρ is constant.…”