Theoretical study of the effects of surface Cu coordination environment on CO₂ hydrogenation to CH₃OH

The coordination environment of Cu (the coordination number and arrangement of surface atoms) plays an important role in CO₂ hydrogenation to CH₃OH. Compared with the extensive studies of the effects of coordination number, the comprehensive effects of coordination number and arrangement of surface atoms were seldom explored in literature. To unravel the effects of surface Cu coordination environment on CO₂ hydrogenation to CH₃OH, the adsorption and reaction behaviors of H₂ and CO₂ on Cu(111), (100), (110) and (211) with different coordination numbers and arrangement of surface Cu were systematically calculated by density functional theory (DFT) and kinetic Monte Carlo (kMC) simulation. It was found that the adsorption energies of intermediates in CO₂ hydrogenation on Cu surfaces increase with the decrease of coordination number. When the Cu coordination numbers are similar, the charge density on the open surface derived from the different atom arrangement becomes larger and leads to stronger interaction with intermediates than that on the compact one. DFT calculation and kMC simulation indicate that methanol formation pathway follows CO₂*→HCOO*→HCOOH*→H₂COOH*→H₂CO*→CH₃O*→CH₃OH* on four Cu facets; CO formation is via CO₂ direct dissociation on Cu(111), (100) and (110) but COOH* dissociation on (211). The low-coordinated surface Cu with more openness on Cu(211) is the highly active site for CO₂ hydrogenation to CH₃OH with high turnover of frequency (3.71 × 10^-4 s^-1) and high selectivity (87.17 %) at 600 K, P_CO2 = 7.5 atm and P_H2 = 22.5 atm, which is much higher than those on Cu(111), (100) and (110). This work unravels the effects of coordination environment on CO₂ hydrogenation at the molecular level and provides an important insight into the design and development of catalysts with high performance in CO₂ hydrogenation to CH₃OH.