Interaction of CO with Cu+ cations: CO adsorption on Cu2O(100)

The interaction of CO with coordinately unsaturated Cu+ cations has been investigated on the Cu2O(100) surface with TDS and photoemission. CO adsorbs molecularly with a constant sticking coefficient for exposures up to 0.3 L, suggesting a precursor state and nonactivated adsorption at 120 K. The activation energy for desorption at low coverages is 16.7 kcalmol, significantly higher than most reported values for metallic copper surfaces, and in agreement with previous suggestions of a higher heat of adsorption for CO at Cu+ cations in the Cu/ZnO methanol synthesis catalyst. In contrast to CO adsorption on metallic copper surfaces, photoemission demonstrates a significant decrease in the surface dipole upon adsorption due to a net charge transfer to the surface and the resulting polarization of the adsorbed CO molecules. These electronic changes and the higher heat of adsorption are due to an increased σ-donor character for CO as evidenced by a direct interaction with Cu 4p-derived electronic states. No evidence is seen for a direct interaction with the Cu 3d bands. The enhanced σ bonding relative to CO on metallic copper surfaces can be rationalized in terms of the decreased Cu 4s/4p-derived density of states of Cu+ cations.