Mechanistic Study of Pd-Cu Bimetallic Catalysts for Methanol Synthesis from CO₂ Hydrogenation

Density functional theory (DFT) calculations were carried out to explore the adsorptions of reactive species and the reaction mechanisms on Pd-Cu bimetallic catalysts during CO₂ hydrogenation to methanol. All the possible preferred adsorption sites, geometries, and adsorption energies of the relative intermediates on pure Cu(111) and three PdCu(111) surfaces were determined, revealing that both the adsorption configuration and corresponding adsorption energy are changed by doping with Pd atoms. The strengthened COOH∗ adsorption and the greatly weakened OH∗ adsorption change the rate-limiting step from CO₂ hydrogenation forming trans-COOH∗ on Cu(111), Pd₃Cu₆(111), and Pd₆Cu₃(111) surfaces to cis-COOH∗ decomposition forming CO∗ and OH∗ on Pd ML surface. Additionally, the highest activation barriers for the overall reaction pathway are reduced in the following trend: Cu(111) > Pd₆Cu₃(111) > Pd₃Cu₆(111) > Pd ML (monolayer). Compared to the reaction on clean Cu(111) surface, the complete reaction pathways for CH₃OH synthesis on PdCu(111) surfaces, especially on Pd ML, were facilitated and the yields of byproducts CO and CH₄ are suppressed, which corroborates well with experimental reports showing that Pd-Cu bimetallic catalysts have a strong synergistic effect on CO₂ hydrogenation to methanol. The present insights are helpful for the design and optimization of highly efficient Pd-Cu bimetallic catalysts used in CH₃OH formation from CO₂ hydrogenation.