Hydrogenation of CO₂ to formic acid over a Cu-embedded graphene: A DFT study

Graphical abstract: - Highlights: • The H₂ molecule binds much more strongly on the Cu/dG than the CO₂ molecule. • H₂ dissociation occurs readily on the supported Cu atom. • The CO₂ conversion is significantly promoted by the Cu-H on the graphene. - Abstract: DFT calculations were used to investigate the properties of the atomic copper embedded in the surface of graphene (Cu/dG) and the catalytic reaction pathway for the CO₂ hydrogenation to formic acid (FA). The Cu/dG was active for the adsorption of the hydrogen molecule (H₂), and provided a reaction site for the heterolytic cleavage of H₂, leading to the formation of Cu-H deposited on a singly hydrogenated vacancy graphene (Cu-H/H-dG). The protonation of CO₂ takes place facilely over the generated metal-hydride species (Cu-H). Under the dilution of H₂, the catalytic process would be hampered by the formation of copper-formate deposited on the H-dG due mainly to the very high energy demand for the transformation of the copper-formate to FA through the protonation from the H-dG. It was further found that the presence of H₂ in the system plays a significant role in producing the FA on the Cu/dG catalyst. The copper-formate species can be converted into formic acid via the heterolytic cleavage of the second hydrogen molecule, yielding the FA and Cu-H species.