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

Abstract

Understanding the relationship between the electronic state of active sites and N₂ 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 FeN₄ and MoN₄ coordination in polyphthalocyanine (PPc) organic framework to investigate the influence of the spin state of FeN₄ on NRR behavior. The neighboring MoN₄ can regulate the spin state of Fe center in FeN₄ from high-spin (d_xy ² d_yz ¹ d_xz ¹ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>d</mml:mi> <mml:msup><mml:mi>z</mml:mi> <mml:mn>2</mml:mn></mml:msup> </mml:msub> </mml:math> ¹ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>d</mml:mi> <mml:mrow><mml:msup><mml:mi>x</mml:mi> <mml:mn>2</mml:mn></mml:msup> <mml:mo>-</mml:mo> <mml:msup><mml:mi>y</mml:mi> <mml:mn>2</mml:mn></mml:msup> </mml:mrow> </mml:msub> </mml:math> ¹ ) to medium-spin (d_xy ² d_yz ² d_xz ¹ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>d</mml:mi> <mml:msup><mml:mi>z</mml:mi> <mml:mn>2</mml:mn></mml:msup> </mml:msub> </mml:math> 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 FeN₄ instead of MoN₄ , 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 N₂ . As a result, FeMoPPc with medium-spin FeN₄ exhibits 2.0 and 9.0 times higher Faradaic efficiency and 2.0 and 17.2 times higher NH₃ yields for NRR than FePPc with high-spin FeN₄ and MoPPc with MoN₄ , respectively. These new insights may open up opportunities for exploiting efficient NRR electrocatalysts by atomically regulating the spin state of metal centers.