Diffusion and flow of gases and vapours through micropores III. Surface diffusion coefficients and activation energies

Abstract

When less than a complete monolayer is present, surface-diffusion coefficients decrease rapidly with decreasing coverage. This is attributed to variation in heats of adsorption over the adsorbent surface. Diffusion is due to the most loosely bound molecules, and these tend to disappear first as coverage decreases. Only a limited range of coverage could be followed in a given system and, within this, activa­tion energies showed no marked variation. This should follow if molecules which require high activation energies play relatively little part in surface diffusion. In no system so far studied have activation energies been small enough to support the existence of ‘freely mobile’ or ‘gas-like’ adsorbed layers. A gas-like state is not approached at low coverages, since molecules adsorbed under these conditions are practically immobile. Surface diffusion coefficients compared at constant temperature and constant coverage differ for various molecules according to differences in molecular size, shape and polar character. When the monolayer is complete and multilayers are formed, diffusion coefficients are little dependent on coverage, and, though perhaps higher than for diffusion in the liquid state, are of a similar order of magnitude. Capillary condensation, in at least the initial stages, produces a rapid rise in surface-diffusion coefficients. Measurements in this region have only limited reproducibility. The importance of pore structure on the shape of adsorption isotherms is stressed. Its effect on surface-diffusion coefficients is indicated, but has not been followed in detail.