Revealing the Surface Chemistry for CO₂ Hydrogenation on Cu/CeO_2–x Using Near-Ambient-Pressure X‑ray Photoelectron Spectroscopy

Catalytic reduction of CO₂ to valuable products is an attractive route for CO₂ recycling. CeO₂-supported Cu catalysts have shown high activity and selectivity for the hydrogenation of CO₂ to CO. To uncover the origin of their high performance, we prepared a practical and well-defined model of Cu/CeO_2–x catalysts with Cu nanoparticles dispersed on a CeO_2–x support. We studied the structure and catalytic activity of the practical catalyst and the evolution of the active phase and surface intermediates using near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) over the model catalyst under CO₂ hydrogenation conditions. For both model and practical catalysts, metallic copper and partially reduced ceria with oxygen vacancies were found to be active sites for the reduction of CO₂ to CO. Over the model catalyst, partial covering of Cu by CeO_2–x and diffusion of Cu into CeO_2–x was observed, suggesting that a strong interaction between Cu and CeO_2–x was favored during CO₂ hydrogenation. The surface analysis results indicated that the CO₂ hydrogenation proceeded through the following steps: activation of CO₂ in the form of carbonate on the surface of CeO_2–x, hydrogenation of carbonate to formate on the surface of Cu by the dissociated H₂, and conversion of formate to CO. This work provides direct experimental evidence on the surface properties of Cu and ceria during the RWGS reaction and the activation and hydrogenation processes of CO₂ over the Cu/CeO_2–x surface for a high RWGS catalytic performance.