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

In this paper, we will theoretically deal with the concentration depen dence of the observed surface diffusivity due to the size distribution of the micropore. The mathematical model is derived based on the assu mption of a local adsorption isotherm and a local surface diffusive fl ux for a particular micropore of radius r. The observed quantities-amo unt adsorbed and the overall flux, and, hence, the observed surface di ffusion coefficient-are obtained from the integration over the full ra nge of micropore. Various micropore distributions, such as bimodal, un iform, shifted gamma distributions, have been used in the numerical si mulations. It is found that the micropore size distribution per se ind uces an increase in the observed surface diffusion coefficient with re spect to loading, and the rate of increase depends on the variance of the micropore size distribution as well as the shape of the distributi on. 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 variati on of the observed surface diffusion coefficient with fractional loadi ng, commonly observed in the literature. When dead-end pores are prese nt (i.e. pores do not contribute to the surface flux), the observed su rface diffusivity does not increase as drastically as that for the cas e 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'' po res.