Comparative analysis of nitrogen adsorption kinetics on Fe(100) and Fe(111) based on applying the statistical rate theory

It is generally known that the initial sticking probabilities of nitrogen on iron single-crystal planes are extremely low although the corresponding activation barriers are not high or even negligible. It implies that the dissociative adsorption of nitrogen is accompanied by strong entropy barriers. In this work, these entropy barriers are interpreted in terms of a special orientation of nitrogen molecules necessary to undergo dissociative adsorption. The applied model is based on the statistical rate theory combined with the quasi-chemical interaction term in order to account for possible intermolecular interaction effects. Analysis of the model parameters led to a few important conclusions. Namely, adsorption on Fe(111) is more sensitive to the orientation of the nitrogen molecule than that on Fe(100); the frequency of getting the correct orientation is ca. 3 times greater on Fe(100) than that on Fe(111). However, the initial sticking coefficient is still ca. I order of magnitude smaller in the case of Fe(100) because of the considerable activation barrier (similar to 15 kJ/mol) which, in turn, is negligible in the case of Fe(111). It was also found that strong repulsive interactions are the main factor responsible for the increasing adsorbed amount with the increasing temperature, fast adsorption at small coverages followed by very slow adsorption when the coverage approaches its equilibrium value, and for the formation of ordered structures of the adsorbed nitrogen atoms.