Structural Evolution and Underlying Mechanism of Single-Atom Centers on MoC(100) Support during Oxygen Reduction Reaction.

The single-metal atoms coordinating with the surface atoms of the support constitute the active centers of as-prepared single-atom catalysts (SACs). However, under hash electrochemical conditions, (1) supports' surfaces may experience structural change, which turn to be distinct from those at ambient conditions; (2) during catalysis, the dynamic responses of a single atom to the attack of reaction intermediates likely change the coordination environment of a single atom. These factors could alter the performance of SACs. Herein, we investigate these issues using MoC(100)-supported single transition-metal (TM) atoms as model SACs toward catalyzing the oxygen reduction reaction (ORR). It is found that the MoC(100) surface is oxidized under ORR turnover conditions, resulting in significantly weakened bonding between single TM atoms and the MoC(100) surface (TM@MoC(100)_O* term for SAC). While the intermediate in 2 e ORR does not change the local structures of the active centers in these SACs, the O* intermediate emerging in 4 e ORR can damage Rh@ and Cu@MoC(100)_O*. Furthermore, on the basis of these findings, we propose Pt@MoC(100)_O* as a qualified ORR catalyst, which exhibits extraordinary 4 e ORR activity with an overpotential of only 0.33 V, surpassing the state-of-the-art Pt(111), and thus being identified as a promising alternative to the commercial Pt/C catalyst.