Surface Orientation and Pressure Dependence of CO2 Activation on Cu Surfaces

A fundamental understanding of interactions between catalysts and gas molecules is essential for the development of efficient heterogeneous catalysts. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) and density functional theory (DFT) simulation were employed to investigate the activation of CO2 on Cu surfaces, which acts as a key step in the catalytic reduction of CO2. APXPS results show that CO2 is adsorbed as CO2 delta- on the Cu(111) surface under a pressure of 0.01 mbar at 300 K. Adsorbed CO2 delta- gets partially transformed into carbonate with an increase of pressure to 1 mbar due to the disproportionation reaction between CO2 molecules. Subsequent annealing of the Cu(111) surface in a CO2 atmosphere leads to the dissociation of CO2 delta- and carbonate, and a transformation to a chemisorbed oxygen covered surface occurred at 400 K and elevated temperatures. However, on the Cu(110) surface, the CO2 delta- gradually dissociates to CO and chemisorbed oxygen in the presence of 1 mbar of CO2 at room temperature. The self-deactivation of CO2 adsorption due to the atomic oxygen generated by CO2 dissociation is observed on both Cu(111) and Cu(110) surfaces. Moreover, these experimental results indicate that the Cu(110) surface is more active than the Cu(111) surface in breaking C-O bonds, which is consistent with the results of DFT simulations. Our findings indicate that the activation of CO2 on Cu surfaces is strongly surface orientation- and pressure-dependent, which is an important step to clarify CO2 activation mechanisms on Cu-based catalysts.