Methanol synthesis from CO2: a DFT investigation on Zn-promoted Cu catalyst

The reaction steps during methanol synthesis from CO2/H-2 are influenced by the type of catalysts such as pure Cu and Zn-decorated Cu. In this study, density functional theory (DFT) calculations were carried out to illustrate the kinetic effect of the introduction of Zn atom which was expected to promote the catalytic reactivity of Cu catalyst. The results demonstrate that over Cu(211) surface, the primary reaction steps include CO2 + H -> HCOO, HCOO + H -> HCOOH, HCOOH + H -> H2COOH, H2COOH -> CH2O + OH, CH2O + H -> CH2OH, CH2OH + H -> CH3OH. Unlike the pure Cu(211) surface, the optimum reaction steps over Zn-decorated Cu, viz. ZnCu(211) surface, included formation of CH3O instead of CH2OH species which further hydrogenated to methanol. The rate-limiting step over Cu(211) was CO2 + H -> HCOO, whereas that over ZnCu(211) was HCOOH + H -> H2COOH which has been known as main intermediate for the methanol synthesis over Cu/ZnO catalysts. The results further revealed that the deviated kinetics on Zn-promoted Cu was mainly caused by the reduced activation energies of several critical reactions, viz. CO2 + H -> HCOO, CO2 + H -> COOH, and CH3O + H -> CH3OH, leading to the improved catalytic performance of ZnCu(211). Graphic abstract