Probing the Reaction Mechanism in CO₂ Hydrogenation on Bimetallic Ni/Cu(100) with Near-Ambient Pressure X-Ray Photoelectron Spectroscopy

Bimetallic Ni-Cu catalysts feature high activity in CO₂ hydrogenation. However, the primary surface intermediates during reaction are still elusive, making the understanding of the reaction mechanism inadequate. Herein, taking advantage of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we focused on the mechanistic exploration of CO₂ hydrogenation on the Ni/Cu(100) model catalyst under millibar pressures. We show that CO₂ dissociates into CO and atomic oxygen on the Ni/Cu(100) surface and gives rise to the formation of chemisorbed O and nickel oxide (NiO). The CO₃* species is formed through the reaction of CO₂ with surface oxygen during CO₂ activation. With the presence of H₂, the conversion of adsorbed CO₃* into the formate intermediate, HCOO*, is unambiguously demonstrated by the C 1s and O 1s core-level spectra as well as ultraviolet photoelectron spectroscopy. Based on these observations, we conclude that the CO₂ hydrogenation route via CO₂ dissociation, the formation of CO₃*, the conversion of CO₃* to formate, and the ensuing hydrogenation of formate to methanol on the Ni-Cu catalyst are feasible.