Boosting Nitrogen Reduction to Ammonia on FeN₄ Sites by Atomic Spin Regulation

Abstract

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 d z 2 1 d x 2 −y 2 1 ) to medium-spin (d xy 2 d yz 2 d xz 1 d z 2 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.

Optimizing Atomically Dispersed Metal Electrocatalysts for Hydrogen Evolution: Chemical Coordination Effect and Electronic Metal Support Interaction

“…to explore the structure of atomic catalytic sites and the valence states of metal atoms. [100] In addition, Density functional theory (DFT) calculation and molecular dynamics simulations [39] have brought unprecedented progress to the discovery of catalytic reaction mechanisms and the prediction of catalytically active species.…”

“…In order to reveal the catalytic mechanism, a variety of atomic resolution characterization and analysis techniques are used, including X‐ray photoelectron spectroscopy (XPS), XAFS spectroscopy (X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS)), Mössbauer spectroscopy, etc. to explore the structure of atomic catalytic sites and the valence states of metal atoms [100] . In addition, Density functional theory (DFT) calculation and molecular dynamics simulations [39] have brought unprecedented progress to the discovery of catalytic reaction mechanisms and the prediction of catalytically active species.…”

Fe‐based catalysts for nitrogen reduction toward ammonia electrosynthesis under ambient conditions

“…Mo and Fe oxides, as well as PC in the MoFe‐PC, are both active during the NRR process, improving the NRR performance synergistically. Zhang et al 69 . investigated the influence of the spin state of FeN 4 on the NRR by designing the atomically dispersed Fe and Mo sites in the form of well‐defined FeN 4 and MoN 4 coordination in a polyphthalocyanine organic framework (denoted as FeMoPPC).…”

“…(H) Room‐temperature 57 Fe Mössbauer spectrum of FeMoPPc. Reproduced with permission 69 . Copyright 2021, Wiley…”

“…Reproduced with permission. 69 Copyright 2021, Wiley electrode. There was no observable change in the NH 3 yield rate or FE of FMO-NPs after six chronoamperometric cycles.…”