High-Pressure-Induced Pseudo-oxidation of Copper Surfaces by Carbon Monoxide

Instantaneous electronic and structural modifications of Cu surfaces by high-pressure CO were explored. At pressures over a millibar range the coverage of CO on the Cu(1 1 1) surface reached approximately one monolayer, and we observed several shake-up peaks in the Cu 2p photoemission spectra for the first time by using ambient pressure X-ray photoelectron spectroscopy. These shake-up peaks resembled those obtained for oxidized surface Cu atoms. We propose that these changes are related to the generation of a copper-carbonyl complex and subsequent Cu surface restructuring owing to lateral CO-CO repulsions. The generality of this finding was investigated in two model reactions, the water-gas shift and CO oxidation, on a Cu(1 1 1) surface, and we clearly identified the pseudo-oxidation of Cu. These reversible changes were found to be independent of the orientation of the Cu surface and are of general importance for Cu-based heterogeneous catalysis.