Bonding geometry and mechanism of NO adsorbed on Cu₂O(111): NO activation by Cu⁺ cations

Abstract

An ab initio molecular cluster model approach has been used to investigate the adsorption geometry and the nature of the interaction of the NO molecule with the Cu2O(111) surface. The two possible NO orientations, N- and O-down, have been studied for adsorption on both onefold and threefold surface positions. We show that, in all cases, the most important contribution to the bonding is the electrostatic interaction, with negligible or small (depending on the orientation and position) contributions from chemical effects. In the monocoordinated position it is found that the two NO orientations exhibit opposite vibrational frequency shift with respect to the free NO molecule. For the three-coordinated site the vibrational shift is found to be nearly zero. In the first case, when NO is bound through the O atom, its shift is negative, as observed experimentally on Cu2O surfaces. Therefore, the present model calculations suggest that the species which is experimentally observed is O-down chemisorbed NO on a onefold unsaturated site. Also, it is shown that this negative shift arises from electrostatic effects because the other significant contributions to the frequency, i.e., the Pauli repulsion and the NO polarization, cancel each other. © 1994 American Institute of Physics.