Diffusion in Porous Solids: The Parallel Contribution of Gas and Surface Diffusion Processes in Pores Extending from the Mesoporous Region into the Microporous Region

Abstract

When a species diffusing in a porous solid is adsorbed to some extent, the ratio of adsorbed molecules to gas molecules in the pores may be very high if there is a large surface area associated with the pores. In addition to transport of molecules through the pores by gaseous diffusion, transport by surface diffusion may then become important. Good examples have been reported in the literature. However, there are many cases in which the mode of transport of an adsorbing species is limited to gaseous diffusion. Although the overall process is simpler when gaseous diffusion alone must be considered, the experimental diffusion coefficient determined directly by classical nonequilibrium methods is still not a true diffusion coefficient. Because of the complication of the “adsorption capacity factor”, the experimental value is lower than the true value, and may be as much as orders of magnitude lower. For mesoporous solids this has been recognized by various investigators for some time. For certain microporous...