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 kcal/mol, 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 sigma-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 sigma-bonding relative to CO on metallic cooper surfaces can be rationalized in terms of the decreased Cu4s/4p-derived density of states of Cu+ cations.