Catalytic Nitrogen Reduction on the Transition Metal Carbonitride (110) Facet: DFT Predictions and Mechanistic Insights

We present a comprehensive exploration of potential catalysts for the electroreduction of nitrogen at ambient conditions through theoretical analyses using density functional theory (DFT) calculations. Our focus is on the (110) texture orientations of rock salt (RS) structures of 11 transition metal carbonitrides (TMCN). The catalytic activity is assessed by constructing free energy diagrams for three different reaction mechanisms: associative, dissociative, and Mars-van Krevelen mechanisms. Additionally, we scrutinize the stability of these materials against poisoning in electrochemical media and decomposition to parental metals under operational conditions. Among the carbonitrides studied, VCN and NbCN emerge as stable and active catalysts for nitrogen reduction reactions (NRRs), exhibiting low onset potentials (-0.42 and -0.78 V vs RHE) at ambient conditions, while MoCN is active at nonambient conditions because of a relatively high kinetic barrier (1.18 eV) for N-2 splitting. However, TaCN and ScCN show activity for ammonia under ambient conditions but are found to be unstable during vacancy migration. An important discovery in our present research is that hydrogen evolution is predicted to have a higher overpotential than nitrogen reduction to ammonia on the carbonitride candidates studied herein.