Surface and mesoporous diffusion with multilayer adsorption

The theory by the authors on surface diffusion where multilayer adsorption occurs is reviewed and further discussed. The theory is derived from a kinetic-theory approach for homogeneous surfaces, but readily extendible to heterogeneous surfaces. The assumptions made in the BET theory are used. The theory is capable of predicting the increase and the subsequent decrease in surface diffusivity with surface concentration. The required information for the theory is the equilibrium adsorption isotherm. From the theory, one can understand the fact that when the surface concentration approaches zero, the transport surface diffusivity becomes equal to the corrected surface diffusivity or self-diffusivity, which can be estimated readily from existing theories. On the other hand, when the surface concentration approaches infinity, that is, when surface condensation occurs, the surface transport diffusivity approaches zero. A fair agreement is obtained between the data available in the literature and the theoretical predictions. Finally, the concentration dependence of surface and mesoporous diffusivities is classified into five types. The multilayer surface diffusion theory is used to predict all five types of concentration dependence.