Predicting surface diffusivities of molecules from equilibrium adsorption isotherms

Abstract

It has been found that for SO 2 adsorbed upon a variety of different surfaces, both the characteristic heat of adsorption and the activation energy for surface diffusion are predominantly determined by the surface geometry of the substrate. A previously proposed [S.P. Rigby, Langmuir 18 (2002) 1613] fractal theory for the surface diffusion of molecules on heterogeneous surfaces has been expanded and applied to past literature data on the surface diffusion of SO 2 upon silica, porous glass and carbon. The theoretical predictions have been found to be in line with the experimental results. It has been found that the Arrhenius parameters for the surface diffusivity, at a monolayer coverage, are functions of the surface fractal dimension, as obtained by gas adsorption. The compensation effect predicted by the theory has also been observed. These findings suggest that once the relevant model parameters have been determined for the particular chemical species, the surface diffusivity at a monolayer coverage on a given surface may be determined from an equilibrium gas adsorption isotherm. The theory also allows the appropriate tortuosity factor to be calculated.