Establishing an orbital-level understanding of active origins of heteroatom-coordinated single-atom catalysts: The case of N2 reduction.

Heteroatom-coordinated single-atom catalysts (SACs) supported by porous graphene exhibit high activity in electrochemical reduction reactions. However, the underlying active origins are complex and puzzling, hindering the development of efficient catalysts. Herein, we investigate the active origins of heteroatom-coordinated Fe-XmYn SACs (X, Y = B, C, N, O, m + n = 4) toward nitrogen reduction reaction (NRR) as a model reaction, through comprehensive analysis of structural, energetic, and electronic parameters. Specifically, the number and arrangement of heteroatoms are found to significantly affect the degree of d-orbital splitting and magnetic moment of the Fe center. Moreover, d-orbital splitting energy (dSE), rather than the conventional d-band theory, explains the adsorption behavior of intermediates in multi-step electron-proton coupling (EPC) reactions. In addition, both s- and d-orbitals of Fe are found to be important for Fe-N bonding, which promotes charge transfer (CT) and N2 activation. Importantly, CT is thought to influence the Pauli repulsion and orbital interaction. Correspondingly, relationships are unveiled between limiting potential (Ulimit) and adsorption energy ΔE(*NNH), dSE, CT, Fe-N bond. In all, this work provides orbital-level insights into the active origins of Fe-XmYn SACs, contributing to the understanding of intrinsic mechanism and the design of electrocatalysts for multi-step EPC reactions.