Diffusion-controlled reactions of molecular oxygen on porous silica glass: Coverage dependence of reaction and diffusion rates and evidence for surface heterogeneity

At relatively low temperatures (T ≤ 130 K) and low coverages the bimolecular, fluorescence quenching, reaction of Ru(bpy)₃²⁺ by molecular oxygen on porous silica surfaces is essentially Langmuir-Hinshelwood (LH) as well as diffusion controlled. We have studied the reaction on controlled porous silica glass, with an average size of 95 Å (CPG-75), over the 80-253 K temperature range, varying the degree of O₂ coverage. An analysis of the second-order quenching rate constants was carried out based on the classical expressions for diffusion-influenced and diffusion-controlled reactions. As the temperature is increased above ̊130 K, the reaction turns from diffusion-controlled to diffusion-influenced, with substantial contributions from both diffusion and activation terms. Above 160-190 K (at high coverages) the mechanism becomes substantially Eley-Rideal (target annihilation) in nature, preventing the separation of the LH component from the overall rate constant. The rate constants in the predominantly diffusion-controlled range (75-125 K, at low coverages) were analyzed using the two-dimensional (Smoluchowski-type) diffusion model of Freeman and Doll. The treatment leads to the determination of the diffusion coefficient (D) of O₂ adsorbed on the porous surface. The diffusion-controlled rate constants and the corresponding diffusion coefficients are found to be markedly affected by the degree of O₂ surface coverage. This behavior is accompanied by an analogous coverage effect on the O₂ heat of adsorption (Q). The findings are interpreted in terms of the heterogeneity of adsorption sites which leads to the preferential occupation of high Q and, consequently, low D locations. We therefore demonstrate that the mechanism of diffusion-influenced LH reactions on amorphous solid-gas interfaces may be tuned by both temperature and degree of surface coverage.