Hydrocarbon Selectivity in CO and CO2 Reduction on Al-Cu Single-Atom Alloy Electrocatalysts: Effects of Potential, pH, and Facet.

Electrochemical carbon dioxide (CO2) and carbon monoxide (CO) reduction reactions (CO(2)RR) are promising routes for carbon utilization. Previous work unraveled Al-Cu single-atom alloys to be thermodynamically favorable for CO2RR. Here, we explore the kinetics of acidic CO(2)RR to C1 products on Al-Cu(111)/(211) using first principles. We consider pathways involving CHO, COH, and HCOO intermediates. Ab initio molecular dynamics simulations provide insights into catalyst stability and interfacial water structure. Microkinetic modeling elucidates potential- and pH-dependent selectivity trends on Al-Cu(111), indicating that H2 evolution is suppressed. For CO2 reduction, HCOOH dominates across potentials and pH ≥ 2 $\ge 2$ , with hydrogen evolution competing at pH 0. In contrast, CO reduction yields higher current densities, with CH3OH prevailing at pH ≥ 2 $\ge 2$ . The CHO pathway exhibits a high activation barrier for CH4 due to protonation/deoxygenation of CH3O, leading instead to CH3OH. The COH pathway, though less thermodynamically favored, shows kinetic superiority, with COH dehydroxylation as the rate-determining step (RDS) for CH4 formation. This RDS, when analyzed over the stepped Al-Cu(211), has a lower barrier by 0.62 eV. These results highlight the influence of reactant species, electrochemical conditions, and material faceting on catalytic performance, offering insights into tuning selectivity for desired C1 products.