A MOLECULAR CLUSTER APPROACH TO THE STUDY OF THE BONDING OF CO AND NH3 TO A D(10) ION ON ZNO(0001) AND CUCL(111)

The local-density-functional approximation coupled to the molecular cluster approach is used here to compare the electronic structure of CO and NH3 molecules chemisorbed on the ZnO(0001) and CuCl(111) polar surfaces. For both substrates the interaction with the adsorbate is strongly dependent on the charge carried by the atom representative of the Lewis acid site. In particular, a realistic description of the surface-adsorbate bonding scheme is only obtained by forcing the partially occupied dangling bonds on ZnO(0001)/CuCl(lll) to be empty. The bonding of CO to CuCl(111) looks similar to that present in metal-carbonyl complexes, with a donation from the CO 5 sigma HOMO into the empty levels of the coordinatively unsaturated Cu surface ions assisted by a significant backdonation from the fully occupied Cu 3d orbitals into the CO 2 pi* LUMO. At variance to that, the bonding of CO to ZnO(0001) is limited to a donation from the CO HOMO into the empty levels of the surface Zn2+ ions. The surface-molecule electrostatic interaction, negligible for CuCl(111), plays for ZnO(0001) an important role in determining the relative energy position of CO based MOs. As far as the bonding of NH3 to CuCl(111) and ZnO(0001) is concerned, it has been found to be characterized in both cases by a donation from the NH3 3a(1) HOMO into the empty levels of the unsaturated metal sites. Any backbonding from the 3d orbitals of the Lewis acid sites is prevented by the high energy of the NH3 2e LUMO. Finally, for NH3 on ZnO(0001), the electrostatic interaction between the permanent NH3 dipole moment and the high value of the Lewis acid site effective charge plays a leading role in determining the binding energy.