FeN₄ Environments upon Reduction: A Computational Analysis of Spin States, Spectroscopic Properties, and Active Species

Abstract

FeN 4 motifs, found, for instance, in bioinorganic chemistry as heme-type cofactors, play a crucial role in man-made FeNC catalysts for the oxygen reduction reaction. Such singleatom catalysts are a potential alternative to platinum-based catalysts in fuel cells. Since FeNC catalysts are prepared via pyrolysis, the resulting materials are amorphous and contain side phases and impurities. Therefore, the geometric and electronic nature of the catalytically active FeN 4 site remains to be clarified. To further understand the behavior of FeN 4 centers in electrochemistry and their expected spectroscopic behavior upon reduction, we investigate two FeN 4 environments (pyrrolic and pyridinic). These are represented by the model complexes [Fe(TPP)Cl] and [Fe(phen 2 N 2 )Cl], where TPP = tetraphenylporphyrin and phen = 1,10-phenanthroline. We predict their Mossbauer, UV−vis, and NRV spectral data using density functional theory as windows into their electronic structure differences. By varying the axial ligand, we further show how well small chemical changes in both complexes can be discerned. We find that the differences in ligand field strength in pyrrolic and pyridinic coordination result in different spin ground states, which in turn leads to distinct Mossbauer spectroscopic properties. As a result, pyrrolic nitrogen donors with a weaker ligand field are predicted to show more pronounced spectroscopic differences under in situ and operando conditions, while pyridinic nitrogen donors are expected to show less pronounced spectroscopic changes upon reduction and/or ligand loss. We therefore suggest that a weaker ligand field leads to better detectability of catalytic intermediates in in situ and operando experiments.

Magnetic Order Transition of a Two-Dimensional Square-Lattice Electrocatalyst Assembled by Fe–N4 Units: Crucial Role on Oxygen Reduction

“…Meanwhile, Xu et al uncovered the spin polarization dynamics during the initial electron transfer step of the oxygen evolution reaction process . Krewald et al combined spectroscopic property and theoretical calculation to unveil the specific influences of ligand field strengths on the spin state of the Fe–N 4 catalyst . In general, previous studies only focused on the spin-state effect of a single Fe–N 4 site on the ORR.…”