While the reduction of CO₂ has attracted much research effort that is driven by the need to address climate concerns, the global nitrogen cycle also demands significant attention. The Haber-Bosch process fixes large quantities of N₂ as NH₃ for use as fertilizer. However, the overuse of this fertilizer allows soil organisms to convert excess NH₃ to nitrogen oxides (e.g., NO₃^– and NO₂⁻), which results in the pollution of water bodies through eutrophication. The electrocatalytic reduction of aqueous NO₃^– represents one method for addressing these issues. Several homogeneous and heterogeneous electrocatalysts reduce aqueous NO₃^–, but all suffer from limitations, including large overpotential, poor product selectivity, and/or lack of catalyst tunability. Here, we report an iron-based graphite conjugated electrocatalyst that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode (GCC-FeDIM). A suite of spectroscopic methods is used to characterize the oxidation state, spin state, and environment of the iron atoms on the electrode surface. Notably, X-ray photoelectron spectroscopy reveals the presence of Fe(III) ions on the electrode surface, with the atomic ratio Fe:N of 1:4, consistent with the FeN₄Cl₂ coordination environment determined by X-ray absorption spectroscopy. In the absence of substrate, the cyclic voltammogram of GCC-FeDIM shows a Gaussian-shaped wave that is assigned to an Fe(III)-OH/Fe/II)-OH₂ surface process. A catalytic wave is observed in the presence of NO₃⁻, with onset potential −1.1 V vs. SCE. At pH 6.0, GCC-FeDIM rapidly reduces NO₃⁻ to ammonium and nitrite with 88 % and 6% Faradaic efficiency, respectively. Mechanistic studies, including in-situ XAS suggest that electrocatalytic NO₃⁻ reduction involves an iron nitrosyl intermediate. The Fe-N bond length (1.65 Å) is similar to that observed in {Fe(NO)}⁶ complexes, which is supported by the results of DFT calculations.

Sarah Braley

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