Recycling of CO₂: Probing the Chemical State of the Ni(111) Surface during the Methanation Reaction with Ambient-Pressure X‑Ray Photoelectron Spectroscopy

Using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we studied the adsorption and reactions of CO₂ and CO₂ + H₂ on the Ni(111) surface to identify the surface chemical state and the nature of the adsorbed species during the methanation reaction. In 200 mTorr CO₂, we found that NiO is formed from CO₂ dissociation into CO and atomic oxygen. Additionally, carbonate (CO₃^2–) is present on the surface from further reaction of CO₂ with NiO. The addition of H₂ into the reaction environment leads to reduction of NiO and the disappearance of CO₃^2–. At temperatures >160 °C, CO adsorbed on hollow sites, and atomic carbon and OH species are present on the surface. We conclude that the methanation reaction proceeds via dissociation of CO₂, followed by reduction of CO to atomic carbon and its hydrogenation to methane.