Chemical and electrochemical hydrogenation of CO2 to hydrocarbons on Cu single crystal surfaces: insights into the mechanism and selectivity from DFT calculations

Abstract

Atomic level mechanistic insights into the chemical and electrochemical reduction of CO₂ on the Cu(111) and Cu(100) surfaces are presented based on DFT-based thermodynamic and kinetic calculations. On Cu(111), CO_ads is firstly formed by the dissociative hydrogenation of CO₂, and CHO_ads and CH₂O_ads are the key intermediates towards the chemical and electrochemical reduction of CO₂ into methanol and CH₄. Despite being thermodynamically or kinetically favoured, it is likely that CH₂OH_ads instead of CH₃O_ads is the intermediate for methanol and CH₄ formation. Based on the activation barriers, CH₂OH_ads intermediate either forms CH₃OH by direct hydrogenation or forms CH_2ads by hydrogenative dissociation, which may be a parallel path in the CO₂ reduction mechanism on the Cu(111) surface. Finally, the CH₂ intermediate leads to formation of the hydrocarbons. On Cu(100), CO₂ reduction takes a different pathway in the early stages; CO is formed through the direct dissociation of CO₂ rather than hydrogenative dissociation as on the Cu(111) surface due to stronger bonding of CO. Further reduction of CO also undergoes a different pathway, in which CO dimerization is more easy to achieve, whereas CO hydrogenation is difficult on the Cu(100) surface. This explains why C₂H₄ is formed more favorably on the Cu(100) surface and CH₄ is predominantly produced on the Cu(111) surface under both chemical and electrochemical conditions. In addition, DFT results showed for the first time that the electrochemical reduction would be expected to be highly favored at potentials of interest by CO₂ reduction compared with chemical reduction and that the carbon dioxide anion radical (˙CO₂⁻) is involved in the initial stage of CO₂ electroreduction. Simultaneously, the results also explain partly why CH₃OH is formed in gas phase chemistry and only CH₄ is observed in electrochemistry on copper surfaces. By analyzing the chemical and electrochemical reduction paths, important mechanistic information is deduced on the Cu single crystal surfaces. The study of CO₂ reduction mechanisms on copper will lead to a deeper understanding of the reaction chemistry and could eventually lead to the design of more efficient and selective catalysts.