Medium spin Fe III regulating the peroxide selectivity in the heterogeneous oxygen reduction reaction of spin-polarized Fe-TAML complexes

Hydrogen peroxide (H₂O₂) can be produced cleanly and sustainably using electrosynthesis from the oxygen reduction reaction (ORR) via a 2-electron pathway, as opposed to the conventional anthraquinone technique. However, the rational design protocols for promoting the peroxide selectivity of ORR catalysts are still inadequate. This work synthesized a series of medium Fe^III complexes with tetra-amido macrocyclic ligands (Fe-TAMLs) using a simplified two-step procedure. A relationship between peroxide selectivity and the spin polarization of metal centers in these Fe-TAML complex ORR catalysts is revealed. Using X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES), the spin polarization of metal centers in Fe-TAMLs regulated by the ligand modification was studied. We found that the nitro group-substituted ligand Fe-TAML complex was regulated by its spin-polarization and offered the highest spin state enhancing peroxide selectivity about 55% to 78%. An operando XAS study is employed to verify the stability of the oxidation states and the robustness of the coordination structures of the Fe-TAMLs.