Revealing the Role of CO during CO₂ Hydrogenation on Cu Surfaces with In Situ Soft X-Ray Spectroscopy

The reactions of H₂, CO₂, 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 H₂ is introduced prior to the addition of CO₂. Only on increasing the CO₂ partial pressure is CuO formation observed that coexists with metallic Cu. When only CO₂ is present, the surface oxidizes to Cu₂O and CuO, and the subsequent addition of H₂ partially reduces the surface to Cu₂O without recovering metallic Cu, consistent with a high kinetic barrier to H₂ dissociation on Cu₂O. 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 CO₂ activation and subsequent conversion to CH₃OH. These findings are corroborated by mass spectrometry measurements, which show increased H₂O formation when H₂ is dosed before rather than after CO₂. 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.