Understanding mechanisms of carbon dioxide conversion into methane for designing enhanced catalysts from first-principles

Conversion of carbon dioxide (CO₂) into methane (CH₄) on copper (Cu) surfaces were investigated to understand the fundamental mechanism of CO₂ reduction, and to suggest a key factor for designing promising catalysts for CO₂ conversion into hydrocarbon fuels. The density functional theory calculations revealed the lowest-energy reaction pathways on Cu(100), Cu(110), and Cu(111) planes, and determined that the potential limiting step for CO₂ reduction lies between the reaction intermediates CO^* and CHO^* (^* denotes the state adsorbed on the catalyst surface). The energy barrier to the potential limiting step is lowered in the following order: Cu(110) < Cu(100) < Cu(111). A key factor for obtaining the lowest energy barrier on Cu(110) may be the largest interatomic distance on the Cu(110) surface among the three surfaces, which enhances the interaction between the key intermediate CHO and the Cu surface, compared to that between CO and the surface. This finding may be applied to developing promising catalysts for CO₂ reduction by designing a Cu thin film on a supporting material with larger lattice constant than Cu. To demonstrate this, we evaluated the energy barriers to the potential limiting step on a single Cu thin layer supported on both palladium (Pd) and silver (Ag), and confirmed that the Pd supporting material can enlarge the interatomic distance of the single Cu(111) layer by 8.7% and thereby lower the energy barrier from 0.97 to 0.63 eV.