Resolving Active-Site Heterogeneity in Fe–N–C Catalysts by Integrated Multimodal Spectroscopy and Simulation

In this work, we combine Fe K-edge X-ray absorption spectroscopy (XAS), ⁵⁷Fe Mössbauer spectroscopy, density functional theory (DFT), and spectroscopy simulations to elucidate Fe-site structures before and after air exposure.

The as-pyrolyzed catalyst is best described by a heterogeneous distribution of FeN₄ sites, dominated by a defect-associated pyrrolic motif with third-shell nitrogen coordination and a smaller pyridinic contribution. We show that coordination beyond the first shell strongly influences Fe–N bond distances, local symmetry, and XANES features. Air exposure converts square-planar Fe(II)-like sites into oxidized, axially coordinated Fe(III)-like species.