Mechanistic study of methanol synthesis from CO₂ hydrogenation on Rh-doped Cu(111) surfaces

Self-consistent periodic density functional theory (DFT) calculations were employed to explore the adsorption and reaction mechanisms of CO₂ hydrogenation to methanol via the reverse water-gas-shift pathway on Cu(111) and three RhCu(111) surfaces with different doped-Rh atoms on Cu(111) surfaces, denoted as Rh₃Cu₆(111), Rh₆Cu₃(111) and Rh ML surfaces. All the possible favored adsorption sites, structures and adsorption energies of the relative species on Cu(111) and RhCu(111) were determined. H₂CO^*, HCO^*, CO^*, cis-COOH^* and trans-COOH^* adsorbing at Rh atoms through C atom are strengthened by the doped Rh, and the adsorption energies of species on Cu sites through O atom are not altered obviously, which can be explained by d-PDOS analysis. Through the analyses of Brønsted-Evans-Polanyi (BEP) relationships, it is found that the adsorption of trans-COOH^* and co-adsorbed CO^*/H^* is stronger on RhCu(111) surfaces, while the energy of CO^* in the TS geometry is dramatically decreased by doping with Rh atoms, leading to the alternation of rate-limiting step from CO₂ hydrogenation forming trans-COOH* on Cu(111) and Rh₃Cu₆(111) surfaces to CO hydrogenation forming HCO^* on Rh₃Cu₆(111) and Rh ML (monolayer) surfaces. The order of the highest activation barriers for the overall reaction is Cu(111) > Rh₆Cu₃(111) > Rh ML > Rh₃Cu₆(111). Moreover, due to the dissociative adsorption of H₂, the yield of CO byproduct is suppressed. The calculated results show that the overall reaction process of CH₃OH synthesis is facilitated kinetically and thermodynamically, especially on Rh₃Cu₆(111) surface. The present insights are helpful for the rational design and optimization of Rh-Cu bimetallic catalysts used in the process of CO₂ hydrogenation to CH₃OH synthesis.