Insight into the Effect of Promoter Mn on Ethanol Formation from Syngas on a Mn-Promoted MnCu(211) Surface: A Comparison with a Cu(211) Surface

Density functional theory calculations have been employed to investigate the effect of promoter Mn on ethanol formation from syngas on a Mn-promoted MnCu(211) surface. Our results show that CO + 3H -> CHO + 2H -> CH2O + H -> CH3O is an optimal pathway for the overall CO conversion. Starting with CH3O, CH3 is formed via CH3O -> CH3 + O. Then, CHO insertion into CH3 can form CH3CHO, and further, CH3CHO is successively hydrogenated to ethanol via CH3CH2O intermediate. Meanwhile, CH3OH is formed via CH3O + H -> CH3OH. Compared to the pure Cu(211) surface, CH3 formation is found to be energetically compatible with CH3OH formation on the MnCu(211) surface, which can lead to more CH3 sources and less CH3OH; thus, the productivity and selectivity of ethanol can be improved. On the other hand, starting from CH3, the MnCu(211) surface is more favorable for CHO insertion into CH3 to CH3CHO in comparison with CH3 hydrogenation, dissociation and coupling to CH4, CH2, and C2H6 due to their high activation barriers; namely, the MnCu(211) surface exhibits a better selectivity toward C-2 oxygenates rather than hydrocarbons. As a result, we can show that, by introducing promoter Mn into Cu catalyst, the productivity and selectivity to ethanol from syngas can be effectively improved.