Adsorption of Nitrogen on Rutile (110):  Monte Carlo Computer Simulations

Abstract

Grand canonical and canonical ensemble Monte Carlo computer simulations of the adsorption of N 2 on the (110) face of rutile at 77 K are reported. A novel ab initio adsorbate−adsorbent interaction potential is employed in conjunction with the X1 nitrogen−nitrogen potential to investigate the adsorption mechanism. It is demonstrated that at low pressures (1 Torr and below) the Ti adsorption sites within the depressed rows of oxides on the rutile (110) face (denoted by A) are completely occupied by nitrogen molecules in end-on orientations with slight alternating tilts perpendicular to the row axis that are produced by repulsive lateral interactions. At higher pressures, adsorption on rows of exposed oxides (denoted by B) commences, typically with a side-on orientation of the N 2 molecules. The calculated isotherm of adsorption exhibits type II behavior according to the Brunauer−Deming−Deming−Teller classification, in agreement with experimental findings. Although the experimental isotherms are often evaluated using the Brunauer−Emmett−Teller adsorption model, our simulations indicate that the assumptions of this model are not fulfilled. The implications of these discrepancies and their influence on surface area determinations are discussed.