p-d Orbital Hybridization Engineered Single-Atom Catalyst for Electrocatalytic Ammonia Synthesis

<正>The rational design of metal single-atom catalysts（SACs） for electrochemical nitrogen reduction reaction（NRR） is challenging. Two-dimensional metal–organic frameworks（2DMOFs） is a unique class of promising SACs. Up to now, the roles of individual metals, coordination atoms, and their synergy effect on the electroanalytic performance remain unclear. Therefore, in this work, a series of 2DMOFs with different metals and coordinating atoms are systematically investigated as electrocatalysts for ammonia synthesis using density functional theory calculations. For a specific metal, a proper metalintermediate atoms p-d orbital hybridization interaction strength is found to be a key indicator for their NRR catalytic activities. The hybridization interaction strength can be quantitatively described with the p-/d-band center energy difference（Δd-p）, which is found to be a sufficient descriptor for both the p-d hybridization strength and the NRR performance. The maximum free energy change（ΔGmax） and Δd-p have a volcanic relationship with OsC4（Se）4 located at the apex of the volcanic curve, showing the best NRR performance. The asymmetrical coordination environment could regulate the band structure subtly in terms of band overlap and positions.This work may shed new light on the application of orbital engineering in electrocatalytic NRR activity and especially promotes the rational design for SACs.