Comparative periodic and cluster ab initio study on Cu2O(111)/CO

The adsorption of CO at the ''singly'' coordinated Cu ions of the regular nonpolar Cu2O(111) surface has been studied with different ab initio methods. The surface was modelled using infinite slabs and cluster models embedded in point charges. Effects of basis set, electron correlation, and surface relaxation on calculated properties like C-O stretching frequency, dynamic dipole moment, adsorption energy and adsorption geometry were investigated. The same physical picture emerges from the cluster and periodic calculations. An analysis of the contributions to the CO-surface bond was also performed. The interaction is partially electrostatic but, at variance with other oxide surfaces, there are important covalent contributions. In particular, we found that both sigma donation and pi* back donation significantly contribute to the strength of the bonding. On the other hand, the large blue shift in the CO vibrational frequency has a dominantly electrostatic origin as it is due to a combination of electric field-CO dipole interaction and wall effect. (C) 1997 Elsevier Science B.V.