DFT elucidation of the catalytic mechanism for CO₂-to-CH₄ conversion by a NiFe biomimetic complex

The electrocatalytic reduction of carbon dioxide (CO₂) to methane (CH₄) remains a formidable challenge due to the requirement of eight proton-electron transfer steps and the propensity of many catalysts to favour competing pathways. In this study, we use density functional theory (DFT) calculations to elucidate the full mechanistic landscape of CO₂-to-CH₄ conversion catalysed by a biomimetic NiFe complex, known experimentally to exhibit remarkable selectivity toward methane when physisorbed on graphite. Through an in-depth investigation of multiple CO₂ insertion pathways, we identify proton-assisted insertion via a relay mechanism as the most kinetically accessible route, consistent with both thermodynamic considerations and experimental observations. Notably, the formation of a unique C-S bond, followed by oxygen bridging between the Ni and Fe centres, emerges as a decisive factor in suppressing the formation of CO or HCOOH intermediates while facilitating the critical C-H bond formation steps. Our results highlight the cooperative action of the Ni and Fe centers in facilitating electron transfer and stabilizing reactive intermediates throughout the 16-step catalytic cycle. Computational hydrogen electrode (CHE) analysis confirms the rate-determining nature of the CO₂ insertion step and supports the thermodynamic feasibility of the proposed pathway under experimental conditions. These findings reveal the unique mechanistic attributes of this NiFe catalyst and provide fundamental insight into the molecular design principles for selective multi-electron CO₂ reduction catalysis.