Quantum Mechanical Screening of Single-Atom Bimetallic Alloys for the Selective Reduction of CO2 to C1 Hydrocarbons

Electrocatalytic reduction of CO2 to energy-rich hydrocarbons such as alkanes, alkenes, and alcohols is a very challenging task. So far, only copper has proven to be capable of such a conversion. We report density functional theory (DFT) calculations combined with the Poisson-Boltzmann implicit solvation model to show that single-atom alloys (SAAs) are promising electrocatalysts for CO2 reduction to C-1 hydrocarbons in aqueous solution. The majority component of the SAAs studied is either gold or silver, in combination with isolated single atoms, M (M = Cu, Ni, Pd, Pt, CO2 Rh, and Ir), replacing surface atoms. We envision that the SAA behaves as a one-pot tandem catalyst: first gold (or silver) reduces CO2 to CO2 and the newly formed CO is then captured by M and is further reduced to C-1 hydrocarbons such as methane or methanol. We studied 28 SAAs, and found about half of them selectively favor the CO2 reduction reaction over the competing hydrogen evolution reaction. Most of those promising SAAs contain M = CO2 Rh, or Ir. The reaction mechanism of two SAAs, Rh@Au(100) and Rh@Ag(100), is explored in detail. Both of them reduce CO2 to methane but via different pathways. For Rh@Au(100), reduction occurs through the pathway *CO -> *CHO -> *CHOH -> *CH + H2O(l) -> *CH2 + H2O(l) -> *CH3 + H2O(l) -> * + H2O(l) + CH4(g); whereas, for Rh@Ag(100), the pathway is *CO -> *CHO -> *CH2O ->*OCH3 -> *O + CH4(g) -> *OH + CH4(g) -> * + H2O(l) + CH4(g). The minimum applied voltages to drive the two electrocatalytic systems are -1.01 and -1.12 V-RHE for Rh@Au(100) and Rh@Ag(100), respectively, at which the Faradaic efficiencies for CO2 reduction to CO are 60% for gold and 90% for silver. This suggests that SAA can efficiently reduce CO2 to methane with as small as 40% loss to the hydrogen evolution reaction for Rh@Au(100) and as small as 10% for Rh@Ag(100). We hope these computational results can stimulate experimental efforts to explore the use of SAA to catalyze CO2 electrochemical reduction to hydrocarbons.