Diffusion-controlled reactions on porous silicas: Mechanisms, surface diffusion coefficients, and effects of geometry

The luminescence quenching reaction of Ru(bpy)₃²⁺ by molecular oxygen on a porous silica and on controlled porous glass was studied in the 88-353 K temperature range. Several distinct reaction mechanism ranges were observed. In the lowest temperature range the reaction is exclusively Langmuir-Hinshelwood (LH) and is controlled by the surface diffusion of O₂. The theoretical model of Freeman and Doll is applied to the experimental data, yielding, for the first time, surface diffusion coefficients in a gas-phase porous solid interface. At higher temperatures the reaction remains LH but crosses into a non-diffusion-controlled range. At still higher temperatures a modified Eley-Rideal (ER) reaction is observed, where although O₂ surface diffusion is minimal, surface residence times influence the reaction rate. The effects of surface geometry (average pore size and fractal dimension) are analyzed for each of the above mechanisms. We find that surface geometry plays a role only in the LH domain. The geometry of the surface affects the diffusion coefficient of the mobile O₂ reactant with respect to both the pre-exponential factors and the activation energies.