Effect of the Madelung potential value and symmetry on the adsorption properties of adsorbate/oxide systems

An ab initio Hartree-Fock self-consistent field cluster model approach has been used to analyze the effect of surface irregularities located at long distances of NO and CO adsorbed on a monocoordinated position of the Cu2O(111) surface. To perform this study different surface cluster models were used. One of these clusters has C-3v symmetry as in the perfect surface, In the two other cluster models the adsorption site local symmetry is still C-3v but to simulate surface irregularities the array of point charges used to include the Madelung potential has C-s symmetry. The NO/Cu+ interaction is always dominated by the electrostatic effects without important contributions from charge transfer mechanisms. The symmetry change between these two kinds of models does not practically alter the geometrical and vibrational parameters of the bond but affects the strength of the NO/Cu2O interaction in a non-negligible way. On the other hand the CO/Cu+ interaction is much larger with substantial contributions from sigma-donation and pi-backdonation. In this case the symmetry change does not have noticeable effects. The present study shows that, on adsorbate/oxide systems with electrostatic bonds, the energy of interaction may depend not only on the local electrostatic field strength felt by the adsorbate but also on the degree of perfection of the oxide surface. The implications of this result for the analysis of experimental data are discussed.