A theoretical study of the parameters affecting the kinetics of gas adsorption on solid surfaces

Abstract

The nondissociative adsorption of gas phase molecules onto a spatially homogeneous square lattice was studied using a Monte Carlocomputer simulation method which accounts for the existence of an extrinsic precursor state. The parameters affecting the adsorption kinetics were varied systematically in order to compare our simulations with a model for adsorption originally proposed by Kisliuk [J. Phys. Chem. Solids 3, 95 (1957)]. Our studies indicate that in the presence of precursor mobility, the sticking coefficients obtained from our simulations are consistently lower than those obtained using Kisliuk’s equation. Upon further investigation, we found two reasons for this discrepancy, namely, islanding of the adsorbates on the surface, and site revisiting in the precursor state. We found that some degree of islanding within the chemisorption layer occurs as a direct consequence of the nature of the precursor mediated adsorption process and that the extent of this clustering is controlled by the competition between mobility within the physisorbed layer and desorption; when mobility dominates, islanding formation is enhanced. Site revisiting within the precursor state was also found to affect the adsorption kinetics; we were able to derive here an analytical expression for the sticking coefficient that accounts for this factor and that correlates well with results from the corresponding simulation. Finally, we show that sticking coefficients are affected by surface geometry.