Mechanistic Insights into the Synthesis of Higher Alcohols from Syngas on CuCo Alloys

Synthesis gas (CO + H₂) conversion is an important process in the transformation of coal, natural gas, or biomass into higher-value products. The explicit conversion into C₂₊ oxygenates on transition-metal-based catalysts suffers from a low selectivity, being a consequence of an imperative integration of C-O bond splitting and C-C coupling reactions. Recently, it has been demonstrated that a bimetallic CuCo catalyst has high higher alcohol selectivity under mild reaction conditions, but the details of the reaction mechanism on the surface are still elusive. In this work, we studied the formation of methane, methanol, and ethanol from syngas on a close-packed (111) and a stepped (211) CuCo surface combining density functional theory (DFT) and microkinetic modeling. We found the CuCo alloy to be a promising candidate catalyst, displaying the required coverage of CO and CH_x on the surface to facilitate C-C coupling. In addition, we found the selectivity to be very structure sensitive: the CuCo (211) surface is selective toward ethanol under certain reaction conditions, while the (111) surface is selective toward methanol. We identified the much lower C-O dissociation barrier and the higher rate of CH_x-CO coupling as the reason for the high activity and selectivity toward ethanol on the (211) surface.