Preferential Orientation of CO Adsorption on Ni as Determined by Extended Hückel Calculations

The adsorption of CO on the nickel surface has been calculated using a modified extended Huckel technique for a number of different orientations of the CO with respect to the nickel surface. The calculations show that double site adsorption with the CO molecule normal to the surface and the carbon atom closest to the metal gives the most stable configuration (2.57 eV). The single site absorbs with an energy of 2.39 eV. However, other configurations also give fairly large bonding energies. The CO normal to the surface with oxygen closest to the metal gives a heat of adsorption of 0.37 eV. Two-center adsorption with the CO axis parallel to the surface and the molecule symmetrically placed between the nearest neighbors gives a chemisorption energy of 1.34 eV. These values compare with an experimental value of 1.98 eV. It is interesting to note that when the CO bond is stretched 10% in the latter configuration the total energy of the system decreases rather than increases.