On the fundamental concepts underlying Henry-law adsorption and adsorbed gas transport in porous solids

Abstract

The basic concepts of the theories of adsorption and adsorbable gas flow (with particular emphasis on the latter) are examined critically in the light of (i) model calculations supplementing previous results of Nicholson and Petropoulos and (ii) new data on the permeability of a series of gases through a mesoporous colloidal graphite compact. The results of (i) indicate that the usual concepts of a constant surface diffusion coefficient and corresponding activation energy (E_s) are tenable, at least approximately, for sufficiently strong adsorption and narrow pores; but, even in this case, the parameters in question lack the physical meaning conventionally attributed to them. These results lead to notable new physical insights, particularly in connection with the relation between the values of E_s and of the energy of adsorption observed experimentally. The data of (ii) confirm the breakdown of the conventional surface flow theory in the weaker adsorption region, in accord with the predictions of the ab initio calculation approach of Nicholson and Petropoulos. The effect of pore size predicted by the said approach also helps to explain the difference between the flow behaviour reported here and that observed previously on similar graphite compacts of smaller pore size.