Nanoclusters just by adding CO: The most closely packed surfaces of transition metals are usually stable under vacuum, but during catalytic reactions, energetic changes that result from adsorbing molecules could change the surface structure. Eren et al. present an extreme example for carbon monoxide (CO) adsorption on the (111) surface of copper at very low partial pressures. The surface decomposed into small nanoclusters (most containing 3 or 19 atoms). The surface was more reactive than the original and, for example, could dissociate adsorbed water at room temperature. Science , this issue p. 475

Atmospheric pressure X-ray photoelectron spectroscopy (XPS) is demonstrated using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude. The graphene serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu²⁺ oxidation state is stable in O₂ (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO₂, N₂, O₂) in the pressure range 10–1500 mbar including species with low photoionization cross sections (He, H₂). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect atmospheric pressure XPS based on this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis.

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.