Theoretical design toward highly efficient single-atom catalysts for nitrogen reduction by regulating the “acceptance-donation” mechanism

Single-atom catalysts (SACs) show great potential for driving electrochemical nitrogen reduction reaction (NRR). However, the lack of physico-chemical understanding of the ambiguous activation mechanism impedes the development of electrocatalysts. In this work, using first principles calculations, we tuned NRR activity and elucidated regulation mechanism of N₂ activation by constructing SACs based on defective Ti₂CO₂ and Ti₂NO₂ MXenes. Interestingly, Mo@Ti₂CO₂-v and W@Ti₂CO₂-v show excellent NRR activity and selectivity with low limiting potentials of –0.27 and –0.27 V at the first hydrogenation (*N₂→*NNH) and the last hydrogenation (*NH₂→*NH₃) steps via distal pathway, respectively. The intrinsic activity tendency can be related to adsorption energy of intermediate *NNH (ΔE(*NNH)). Furthermore, electronic structure analysis demonstrates that number of unpaired electrons in d orbitals (N_un-d) determines occupation of d orbitals, thus regulating the “acceptance-donation” mechanism. Particularly, transition metals with N_un-d of 5 and 4 show moderate “acceptance-donation” interaction, thus achieving encouraging NRR performance. Moreover, ΔE(*NNH) and N_un-d can be considered as promising descriptors for screening and designing effective SACs. Finally, our investigation not only contributes to discovery of novel SACs toward NRR, but also provides insightful guidance for other electrochemical reactions with high bond-order molecules and multi-electron steps, such as ORR and CO₂RR. © 2023 Elsevier B.V.