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 less than or equal to 130 K) and low coverages the bimolecular, fluorescence quenching, reaction of Ru(bpy )(3)(2+) by molecular oxygen on porous silica surfaces is essentially Langmuir-Hinshelwood (LH) as well as diffusion controlled. We have stu died the reaction on controlled porous silica glass, with an average s ize of 95 Angstrom (CPG-75), over the 80-253 K temperature range, vary ing the degree of O-2 coverage. An analysis of the second-order quench ing rate constants was carried out based on the classical expressions for diffusion-influenced and diffusion-controlled reactions. As the te mperature is increased above similar to 130 K, the reaction turns from diffusion-controlled to diffusion-influenced with substantial contrib utions from both diffusion and activation terms. Above 160-190 K (at h igh coverages) the mechanism becomes substantially Eley-Rideal (target annihilation) in nature, preventing the separation of the LH componen t from the overall rate constant. The rate constants in the predominan tly diffusion-controlled range (75-125 K, at low coverages) were analy zed using the two-dimensional (Smoluchowski-type) diffusion model of F reeman and Doll. The treatment leads to the determination of the diffu sion coefficient (D) of O-2 adsorbed on the porous surface. The diffus ion-controlled rate constants and the corresponding diffusion coeffici ents are found to be markedly affected by the degree of O-2 surface co verage. This behavior is accompanied by an analogous coverage effect o n the O-2 heat of adsorption (Q). The findings are interpreted in term s of the heterogeneity of adsorption sites which leads to the preferen tial occupation of high Q and, consequently, low D locations. We there fore demonstrate that the mechanism of diffusion-influenced LH reactio ns on amorphous solid-gas interfaces may be tuned by both temperature and degree of surface coverage.