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

The reactions of H-2, CO2, and CO gas mixtures on the surface of Cu at 200 ?, 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-2 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 H-2 partially reduces the surface to Cu2O without recovering metallic Cu, consistent with a high kinetic barrier to H-2 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 H-2 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.

Automated summary

The reactions of H-2, CO2, and CO gas mixtures on the surface of Cu at 200 ? were investigated using AP-XPS and AtmP-NEXAFS spectroscopy. The order of gas dosing plays a crucial role in the catalyst chemical state, with metallic Cu preserved when H-2 is introduced before CO2. The addition of CO is essential for removing adsorbed oxygen and activating CO2 on the Cu surface, facilitating methanol synthesis.