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.