SURFACE AND MESOPOROUS DIFFUSION WITH MULTILAYER ADSORPTION

The theory by the authors on surface diffusion where multilayer adsorp tion occurs is reviewed and further discussed. The theory is derived f rom a kinetic-theory approach for homogeneous surfaces, but readily ex tendible to heterogeneous surfaces. The assumptions made in the BET th eory are used. The theory is capable of predicting the increase and th e subsequent decrease in surface diffusivity with surface concentratio n. The required information for the theory is the equilibrium adsorpti on isotherm. From the theory, one can understand the fact that when th e surface concentration approaches zero, the transport surface diffusi vity becomes equal to the corrected surface diffusivity or self-diffus ivity, which can be estimated readily from existing theories. On the o ther 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 availab le in the literature and the theoretical predictions. Finally, the con centration dependence of surface and mesoporous diffusivities is class ified into five types. The multilayer surface diffusion theory is used to predict all five types of concentration dependence. (C) 1998 Elsev ier Science Ltd. All rights reserved.