Boosting Nitrogen Reduction to Ammonia on FeN 4 Sites by Atomic Spin Regulation

Understanding the relationship between the electronic state of active sites and N 2 reduction reaction (NRR) performance is essential to explore efficient electrocatalysts. Herein, atomically dispersed Fe and Mo sites are designed and achieved in the form of well‐defined FeN 4 and MoN 4 coordination in polyphthalocyanine (PPc) organic framework to investigate the influence of the spin state of FeN 4 on NRR behavior. The neighboring MoN 4 can regulate the spin state of Fe center in FeN 4 from high‐spin ( d xy 2 d yz 1 d xz 1 1 1 ) to medium‐spin ( d xy 2 d yz 2 d xz 1 1 ), where the empty d orbitals and separate d electron favor the overlap of Fe 3d with the N 2p orbitals, more effectively activating N≡N triple bond. Theoretical modeling suggests that the NRR preferably takes place on FeN 4 instead of MoN 4 , and the transition of Fe spin state significantly lowers the energy barrier of the potential determining step, which is conducive to the first hydrogenation of N 2 . As a result, FeMoPPc with medium‐spin FeN 4 exhibits 2.0 and 9.0 times higher Faradaic efficiency and 2.0 and 17.2 times higher NH 3 yields for NRR than FePPc with high‐spin FeN 4 and MoPPc with MoN 4 , respectively. These new insights may open up opportunities for exploiting efficient NRR electrocatalysts by atomically regulating the spin state of metal centers.