The development of
catalytic systems that selectively reduce O2 to water is
needed to continue the advancement of fuel cell
technologies. As an alternative to platinum catalysts, derivatives
of iron (Fe) and cobalt (Co) porphyrin molecular catalysts provide
one benchmark for catalyst design, but incorporation of these catalysts
into heterogeneous platforms remains a challenge. Co-porphyrins can
be heterogeneous O2 reduction catalysts when immobilized
on to edge plane graphite (EPG) electrodes, but their selectivity
for the desired four-electron reduction of O2 to H2O is often poor. Herein, we demonstrate substantial improvements
in the O2 reduction selectivity using a Co-porphyrin that
incorporates a 2-pyridyl group at one of the meso-positions of a Co-tetraarylporphyrin (cobalt(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin,
CoTPPy). The properties of CoTPPy immobilized on EPG were investigated
using cyclic voltammetry, rotating disk, and rotating ring-disk electrochemistry.
The presence of a single 2-pyridyl group in the CoTPPy gives rise
to the four-electron reduction of O2, as opposed to the
two-electron reduction commonly associated with cobalt porphyrins.
Detailed electrochemical studies of CoTPPy and related Co and Fe porphyrins
are described. Use of Co instead of Fe improves overpotentials by
over 200 mV with a factor of 2 increase in maximum turnover frequency
(TOFmax). This work demonstrates that a simple change in
catalyst structure can dramatically change the selectivity for O2 reduction.
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.
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