Orbital Dependence in Single-Atom Electrocatalytic Reactions.

Transition metal single-atom catalysts supported on N-doped graphene (TM-N-C) could serve as an ideal model for studying orbital dependence in electrocatalytic reactions because the atom on the catalytic active site has discrete single-atom-like orbitals. In this work, the catalytic efficiency of Fe-N-C for the oxygen evolution reaction (OER) under a small structural perturbation has been comprehensively investigated with density functional theory calculations. The results suggest that the subtle local environment of a single atom can significantly modulate the catalytic reactivity. Further analysis demonstrates that the energy level of the TM d orbital center, rather than the d-band center, is responsible for the OER catalytic efficiency as the d orbital participates mainly in the reactions. Essentially, the d-band theory can be extended to the sub-d orbital level, and a small perturbation of the crystal field, induced by lattice strain or -direction displacement of the TM atom, can prominently change the sub-d orbital associated with the reaction and in turn affect the catalytic activity.