Physical adsorption of gases on heterogeneous surfaces. Model study of the effects of simultaneous vertical and lateral interactions by monte carlo methods

Abstract

A model study has been undertaken to elucidate the interplay between vertical and lateral interaction effects in the physical adsorption of gases on heterogeneous solid surfaces. The surface model employed is a square lattice with a random distribution of adsorption sites of different energies on which adsorbed molecules interact only with their nearest neighbours. Isotherms are generated by simulating the system in the grand canonical ensemble by Monte Carlo methods. These isotherms are subsequently subjected to analysis by the adsorption integral equation using various local isotherm models, in an attempt to recover the adsorption energy distributions. When lateral interactions are ignored, the analysis yields an adsorption energy distribution which (a) is centred on a mean value which is too high and (b) has a ‘spread’ which is too small. These effects are correlated with the magnitude of the neglected lateral interactions and a possible physical interpretation of the mean energy calculated by such means is given. Differential heats of adsorption as well as isotherm ‘data’ are presented for the model system considered.