A Spectroscopically Observed Iron Nitrosyl Intermediate in the Reduction of Nitrate by a Surface-Conjugated Electrocatalyst

Abstract

We report an iron-based graphite-conjugated electrocatalyst (GCC-FeDIM) that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode. A suite of spectroscopic methods, supported by the results of DFT calculations, reveals that the electrode surface is functionalized by high spin (S = 5/2) Fe(III) ions in an FeN₄Cl₂ coordination environment. The chloride ions are hydrolyzed in aqueous solution, with the resulting cyclic voltammogram revealing a Gaussian-shaped wave assigned to 1H⁺/1e^- reduction of surface Fe(III)-OH surface. A catalytic wave is observed in the presence of NO₃^-, with an onset potential of -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 X-ray absorption spectroscopy, 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.