Molecular Engineering of Active Fe Center in Metalloporphyrin Coupled with Polyoxometalates for Efficient Photochemical Nitrogen Fixation: Synergistic Effect of Multiactive Sites Strengthening Metal-N-N* Interactions

Abstract

Precisely modulating the chemical microenvironment of catalytic centers at the molecular level to achieve efficient photocatalytic nitrogen fixation remains a grand challenge. Herein, a polyoxometalates (POMs) metalloporphyrin organic framework Fe-PMOF {POM-TCPP(Fe)} is constructed by integrating the oxygen-rich unit POMs {ε-PMo₈^VMo₄^VIO₄₀Zn₄} and the photosensitive metalloporphyrin (Fe-TCPP) as a model to precisely regulate intermolecular electron transfer. Benefiting from electronic interactions, the optimized POM-TCPP(Fe) exhibits a favorable activity toward NH₃ production with a rate of 110.06 µmol g⁻¹ h⁻¹. The improved performance can be attributed to the effective regulation of the chemical microenvironment surrounding the active centers, enabling the synergistic interaction of multiple active sites (Fe and Mo) to facilitate the adsorption and activation of nitrogen. More specifically, oxygen-rich unit POMs exhibit strong electronegativity, which can attract electrons from Fe atoms, thereby decreasing the 3d orbitals’ electron density of Fe sites and elevating its unoccupied d-orbitals to facilitate N₂ adsorption. Moreover, the porphyrin units with high photosensitivity efficiently generate electrons under photoexcitation, which can rapidly migrate and inject them to the active Fe-N-N* sites to facilitate N₂ activation. Ultimately, the mimic nitrogenase active site intelligently integrates multiple active sites of transition metals Fe and Mo, thus improving the nitrogen fixation efficiency.