Revealing the Role of CO during CO<sub>2</sub> Hydrogenation on Cu Surfaces with <i>In Situ</i> Soft X-Ray Spectroscopy
J. Swallow, Elizabeth Jones, Ashley R. Head, Joshua S. Gibson, Roey Ben David, Michael Fraser, M. A. Van Spronsen, Shaojun Xu, Georg Held, Baran Eren, Robert S. Weatherup
Abstract
High Resolution Image Download MS PowerPoint Slide The reactions of H 2, CO 2, 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 2 is introduced prior to the addition of CO 2 . Only on increasing the CO 2 partial pressure is CuO formation observed that coexists with metallic Cu. When only CO 2 is present, the surface oxidizes to Cu 2 O and CuO, and the subsequent addition of H 2 partially reduces the surface to Cu 2 O without recovering metallic Cu, consistent with a high kinetic barrier to H 2 dissociation on Cu 2 O. 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 2 activation and subsequent conversion to CH 3 OH. These findings are corroborated by mass spectrometry measurements, which show increased H 2 O formation when H 2 is dosed before rather than after CO 2 . 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.