Efficient CO2 reduction to methane on Ru2-based adjacent vacant graphene catalysts: insights into bimetallic synergies, thermodynamics, and kinetics†

Abstract

Electrochemical conversion of CO₂ into value-added chemicals offers a clean pathway to utilize greenhouse gases along with the production of useful products. In this study, we aim to unravel a comprehensive reaction mechanism for the CO₂ reduction reaction (CO₂RR) into various C₁-based products such as CO, HCOOH, CH₂O, CH₃OH, and CH₄ using density functional theory (DFT). We analyse all possible reaction pathways and compute their thermodynamics on a novel two-dimensional catalyst system – two monovacant (adjacent-vacant) graphene sheet modified with two Ru atoms, referred to as 2MV-Ru₂. Our reaction mechanism investigation identified the potential-determining step (PDS) as CO* → CHO*, which demands a limiting potential of −0.88 V vs. SHE (standard hydrogen electrode) for the production of methane. We also examined the PDS on a series of bimetallic electrodes, generally referred to as 2MV-RuM (M = Ag, Au, Cu, Ir, Os, Pd, Pt, and Rh). We found the 2MV-RuPd electrode as the most effective catalyst for methane production as it lowered the limiting potential of PDS to only −0.13 V vs. SHE, which is a remarkable improvement over both 2MV-Ru₂ and conventional metallic electrodes. To supplement the thermodynamics, we also integrated kinetic analyses for the PDS to provide a more inclusive understanding of the reaction, which demonstrated the excellent capability of 2MV-RuPd electrode compared to 2MV-Ru₂. Finally, we examined the stability of all bimetallic electrodes to ensure their practical applicability, and obtained favourable formation free energies, which advocate that these electrodes are not only theoretically feasible but also experimentally synthesizable. Overall, our study offers a rigorous analysis of the CO₂RR on Ru-based adjacent-vacant (bimetallic) graphene catalysts. Superior thermodynamics and kinetics were demonstrated by the 2MV-RuPd electrode, thereby establishing it as a highly promising candidate for methane production. This work advances the understanding of CO₂RR mechanisms along with providing a robust foundation for developing next-generation catalysts for sustainable chemical synthesis.