A model is proposed for the hopping mechanism of surface diffusing molecules, adsorbed on porous solids, which allows a simple calculation of the mean hopping distance as a function of surface coverage.Surface permeabilities calculated with this model are compared to new experimental data. A satisfactory agreement is obtained, and the values of the parameters involved turn out to be very reasonable.
MARTA
SCOPEThe most commonly used models to describe the surface diffusion of physically adsorbed gases on porous solids are essentially the following.1. Fick's law, which considers the surface flux as being proportional to the concentration gradient, the proportionality constant being the diffusion coefficient. Several authors (Higashi et al., 1963;McIntosh, 1966;Perkinson, 1965;Sladek, 1967) introduced a number of corrections to take into account the dependence of the diffusion coefficient on the surface concentration of the adsorbate. 2. Hydrodynamic model, which pictures the adsorbed gas as being a liquid film slipping on the solid surface w3h a laminar flow (Flood et al., 1952;Gilliland et al., 1958;Babbit, 1950 These models show different degrees of validity for different regions of surface coverage values (Horiguchi et al., 1971), and, in general, we believe that a reliable theory for the interpretation and prediction of surface fluxes is still lacking.The purpose of the present work is to improve the mechanistic model introducing a simple method of calculating the mean hopping distance of an adsorbed molecule, to obtain new experimental data on surface diffusion, and to compare them with the theoretical model.
CONCLUSIONS AND SIGNIFICANCEThe hopping mechanism which we propose to describe surface diffusion permits a simple calculation of the mean hopping distance as a function of surface coverage which satisfactorily fits our experimental results on surface permeability.The essential parameter of this mechanism is the probability that an activated molecule is captured when it passes over an empty site. This probability evidently depends on the adsorption potential at each site, and we believe that this is a starting point toward a correct treatment of diffusion on energetically heterogeneous surfaces.A microphysical model to predict the dependence of activation energy on surface coverage is still lacking. However, an empirical relation is also proposed which seems to fit satisfactorily our experimental data.
BASIC KINETIC EQUATIONS1. The gaseous and adsorbed phases, both composed of to Smith and Metmer (1964) and Weaver a single gas, are in thermodynamical equilibrium characand Metzner (1966), the calculation of the surface flux is terized by adsorption isotherms. based on the following hypothesis.2. Adsorbed molecules migrate along the adsorbent sur-
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