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) [Formula: see text] (1) [Formula: see text] (1)) to medium‐spin (d(xy) (2) d(yz) (2) d(xz) (1) [Formula: see text] (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.