The adsorption isotherms of C O on N i { l l l } have been determined for C O pressures between lop8 and 80 Torr and temperatures of 120-260 "C. 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-' 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 ( w > 30 kJ mol-') 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 C O molecules are mainly responsible for the behaviour of the adlayer.

The thermal decomposition of ethylene on Ni(lll) at 250°C is shown to lead to carbon deposition on and -in a later stage -below the surface. Independent of the amount of carbon below the surface, CO is adsorbed with an isosteric heat of adsorption of 105 kJ/mol. The surface carbon reacts with oxygen at 25O'C. The reaction rate is independent of the surface carbon coverage and first order in oxygen pressure. The subsurface carbon segregates to the surface after removal of the surface carbon layer.