Revealing the Role of CO During CO Hydrogenation on Cu Surfaces with Soft X-Ray Spectroscopy

The reactions of H, CO, and CO gas mixtures on the surface of Cu at 200 °C, relevant for industrial methanol synthesis, are investigated using a combination of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and atmospheric-pressure near edge X-ray absorption fine structure (AtmP-NEXAFS) spectroscopy bridging pressures from 0.1 mbar to 1 bar. We find that the order of gas dosing can critically affect the catalyst chemical state, with the Cu catalyst maintained in a metallic state when H is introduced prior to the addition of CO. Only on increasing the CO partial pressure is CuO formation observed that coexists with metallic Cu. When only CO is present, the surface oxidizes to CuO and CuO, and the subsequent addition of H partially reduces the surface to CuO without recovering metallic Cu, consistent with a high kinetic barrier to H dissociation on CuO. The addition of CO to the gas mixture is found to play a key role in removing adsorbed oxygen that otherwise passivates the Cu surface, making metallic Cu surface sites available for CO activation and subsequent conversion to CHOH. These findings are corroborated by mass spectrometry measurements, which show increased HO formation when H is dosed before rather than after CO. The importance of maintaining metallic Cu sites during the methanol synthesis reaction is thereby highlighted, with the inclusion of CO in the gas feed helping to achieve this even in the absence of ZnO as the catalyst support.