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

Electrocatalytic reduction of CO<sub>2</sub> 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. Here, 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 CO<sub>2</sub> 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, Co, Rh, and Ir), replacing surface atoms. We envision that the SAA behaves as a one-pot tandem catalyst: first gold (or silver) reduces CO<sub>2</sub> to CO, 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 CO<sub>2</sub> reduction reaction over the competing hydrogen evolution reaction. Most of those promising SAAs contain M = Co, Rh, or Ir. The reaction mechanism of two SAAs, Rh@Au(100) and Rh@Ag(100), is explored in detail. Both of them reduce CO<sub>2</sub> to methane but via different pathways. For Rh@Au(100), reduction occurs through the pathway *CO → *CHO → *CHOH → *CH + H<sub>2</sub>O<sub>(l)</sub> → *CH<sub>2</sub> + H<sub>2</sub>O<sub>(l)</sub> → *CH<sub>3</sub> + H<sub>2</sub>O<sub>(l)</sub> → * + H<sub>2</sub>O<sub>(l)</sub> + CH<sub>4(g)</sub>; whereas, for Rh@Ag(100), the pathway is *CO → *CHO → *CH<sub>2</sub>O→ *OCH<sub>3</sub> → *O + CH<sub>4(g)</sub> → *OH + CH<sub>4(g)</sub> →* + H<sub>2</sub>O<sub>(l)</sub> + CH<sub>4(g)</sub>. The minimum applied voltages to drive the two electrocatalytic systems are -1.01 and -1.12 V<sub>RHE</sub> for Rh@Au(100) and Rh@Ag(100), respectively, at which the Faradaic efficiencies for CO<sub>2</sub> reduction to CO are 60% for gold and 90% for silver. This suggests that SAA can efficiently reduce CO<sub>2</sub> 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). Lastly, we hope these computational results can stimulate experimental efforts to explore the use of SAA to catalyze CO<sub>2</sub> electrochemical reduction to hydrocarbons.