Carbon monoxide on Ni{111}. Adsorption isotherms and intermolecular interactions

Abstract

The adsorption isotherms of CO on Ni{111} have been determined for CO pressures between 10^–8 and 80 Torr and temperatures of 120–260 °. On both a well annealed single crystal and a freshly reduced surface (which is probably rougher) the isosteric heat of adsorption is 134 kJ mol^–1 up to the saturation coverage. The isotherms obey a simple analytical equation, valid for a two-dimensional lattice gas, which implies large CO—CO repulsion for molecules on nearest-neighbour sites (ω > 30 kJ mol^–1) but no interaction between next-nearest neighbours. These results are corroborated by an ab initio calculation of the intermolecular potential as a function of distance. Based on these conclusions it seems that for the Ni/CO system interactions between adsorbed CO molecules are mainly responsible for the behaviour of the adlayer.