EFFECT OF MICROPORE SIZE DISTRIBUTION ON THE SURFACE DIFFUSIVITY IN MICROPOROUS SOLIDS

In this paper, we will theoretically deal with the concentration dependence of the observed surface diffusivity due to the size distribution of the micropore. The mathematical model is derived based on the assumption of a local adsorption isotherm and a local surface diffusive flux for a particular micropore of radius r. The observed quantities-amount adsorbed and the overall flux, and, hence, the observed surface diffusion coefficient-are obtained from the integration over the full range of micropore. Various micropore distributions, such as bimodal, uniform, shifted gamma distributions, have been used in the numerical simulations. It is found that the micropore size distribution per se induces an increase in the observed surface diffusion coefficient with respect to loading, and the rate of increase depends on the variance of the micropore size distribution as well as the shape of the distribution. For all distributions, the increase can be much higher, depending very much on the smallest micropore size. This study suggests that the micropore size distribution may be one of the sources for the variation of the observed surface diffusion coefficient with fractional loading, commonly observed in the literature. When dead-end pores are present (i.e. pores do not contribute to the surface flux), the observed surface diffusivity does not increase as drastically as that for the case of no dead-end pores. This is true irrespective of the capacity and affinity of the dead-end pores relative to those of the ''flowing'' pores.