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

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.