Electrocatalytic Multielectron Nitrite Reduction in Water by an Iron Complex

Article abstract of DOI:10.1021/acscatal.0c03600

Catalytic reduction of nitrite by an iron complex in water near neutral pH to form hydroxylamine and ammonium is reported. The catalyst is an iron center coordinated by the pentadentate macrocycle 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene (FeN5H2). Catalysis is observed by cyclic voltammetry at a half-wave potential of Ep/2 = -0.98 V vs Ag/AgCl (1 M KCl) when FeN5H2, nitrite, and a buffer (pH 7.2) are present. Controlled potential electrolysis of FeN5H2 and nitrite in pH 7.2 buffer at -0.98 V produces hydroxylamine (faradaic efficiency > 90%). FeN5H2 catalyzes ammonium production by disproportionation of hydroxylamine with concomitant formation of nitrous oxide and dinitrogen. These results are a rare example of multielectron electrocatalytic nitrite reduction by an iron complex near neutral pH.

Full text of DOI:10.1021/acscatal.0c03600

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Letter
Electrocatalytic Multielectron Nitrite Reduction in Water by an Iron
Complex
Jesse R. Stroka, Banu Kandemir, Ellen M. Matson,* and Kara L. Bren*
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ABSTRACT: Catalytic reduction of nitrite by an iron complex in
water near neutral pH to form hydroxylamine and ammonium is
reported. The catalyst is an iron center coordinated by the
pentadentate macrocycle 2,13-dimethyl-3,6,9,12,18-
pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene
(FeN5H2). Catalysis is observed by cyclic voltammetry at a half-
wave potential of Ep/2 = −0.98 V vs Ag/AgCl (1 M KCl) when
FeN5H2, nitrite, and a buffer (pH 7.2) are present. Controlled
potential electrolysis of FeN5H2 and nitrite in pH 7.2 buffer at
−0.98 V produces hydroxylamine (faradaic efficiency > 90%).
FeN5H2 catalyzes ammonium production by disproportionation of
hydroxylamine with concomitant formation of nitrous oxide and dinitrogen. These results are a rare example of multielectron
electrocatalytic nitrite reduction by an iron complex near neutral pH.
KEYWORDS: ammonia synthesis, electrocatalysis, hydroxylamine disproportionation, hydroxylamine synthesis, nitrite reductase
−
itrite (NO2 ) plays an important role in the global
nitrogen cycle, where its transformation leads to a variety
a CoN4 center are particularly active and well-studied catalysts
for NO2− reduction.9,14−21 Ligand scaffolds represented in this
group include cyclams,21 the cyclam analog, 2,3-dimethyl-
1,4,8,11-tetraazacyclotetradeca-1,3-diene (DIM),14,15,19 phtha-
locyanines,9 diimine-dioximes,18 peptides16 and porphyr-
ins.17,20 Some of these catalysts also facilitate the reduction
of nitrate (NO3−).9,14,19−21 Hydroxylamine and
N
of products along different pathways. Human activity has
greatly disrupted this bio-geochemical cycle, with widespread
effects, including eutrophication and contamination of ground-
water.1,2 Studying the reactivity of NO2 will improve our
−
understanding of its interconversions relevant to the nitrogen
cycle. Furthermore, the reduction of the oxyanions of nitrogen
is a route to transforming groundwater contaminants into
useful products such as hydroxylamine (NH2OH) and
ammonia (NH3) or removal by forming dinitrogen (N2).3−6
9,14−17,19−21
+
NH4
are common reduction products, although
a cobalt diimine−dioxime complex produces N2O.18 Under-
standing the basis for product selectivity is an area of interest.
Product distribution has been found to depend on catalyst
structure,14,15 mechanism,18 applied potential,9,17,20 and pH.20
In many of these systems, however, the effect of pH is difficult
to assess due to the absence or interference of a buffer, leading
to large pH changes during the reaction.14,15,20,21 This pH
change impacts acidic disproportionation of NO2− to NO and
−
A biomimetic approach to studying the activation of NO2
has been taken through the investigation of Cu or Fe
complexes that mimic the reactivity of nitrite reductases,
which yield nitric oxide (NO) (Cu or Fe (heme) enzymes, eq
+
1) or ammonium (NH4 ) (heme or siroheme enzymes, eq
2).4,7 Copper complexes have been reported to catalyze the
11,12,17,22−24
−
NO3 ,
the differing reactivity of NH2OH and its
−
protonated form, NH3OH+,12,20 and catalyst stability.11,17
reduction of NO2 to NO, nitrous oxide (N2O), NH2OH,
8,9
+
and/or NH4 .
Among iron-containing compounds that
−
Herein, we report catalytic reduction of NO2 to NH2OH
transform NO2 to reduced products,10−13 iron porphyrins
have been reported as electrocatalysts for the reduction of
−
and NH4+ by an iron complex near neutral pH in the presence
of 3-(N-morpholino)propanesulfonate (MOPS) buffer. This
complex (FeN5H2) consists of an iron center coordinated by a
−
+
NO2 to mixtures of N2O, NH2OH, and NH4 , where product
distribution depends on applied potential, reaction time, and
pH.12,13
Received: August 17, 2020
Revised: October 22, 2020
NO2 + 2H+ + 1e− → NO + H2O
−
(1)
−
+
NO2 + 8H+ + 6e− → NH4 + 2H2O
(2)
Although cobalt is not implicated in Nature’s trans-
−
formations of NO2 , a number of cobalt complexes featuring
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Letter
macrocyclic ligand, 2,13-dimethyl-3,6,9,12,18-
pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene
(Figure 1)25−28 and was previously reported to catalyze the
reduction of carbon dioxide to formic acid with 75−80%
faradaic efficiency (FE) in DMF at −1.25 V vs SCE.29
linear relationship, consistent with these processes being
homogeneous.32
Controlled potential electrolysis (CPE) was carried out to
determine whether the irreversible feature observed in CV
−
corresponds to catalytic NO2 reduction and to identify the
resulting product(s). CPE was performed at the half-wave
potential of −0.98 V in the presence and absence of 500 μM
FeN5H2 in a solution containing 1.0 M NaNO2 and 1.0 M
MOPS at pH 7.2 over 1 h (Figures 3 and S6). With all
Figure 1. Structure of FeN5H2 as synthesized. Outer-sphere
perchlorate anion is not shown.
−
The reactivity of FeN5H2 toward NO2 was initially
investigated by cyclic voltammetry (CV) in the presence of
MOPS buffer at pH 7.2. The CV of FeN5H2 (Figure 2)
Figure 3. Controlled potential electrolysis at −0.98 V vs Ag/AgCl (1
M KCl) over 1 h of an aqueous solution containing 1.0 M NaNO2
and 1.0 M MOPS at pH 7.2 with FeN5H2 as indicated in the legend.
components present, 70 C of charge was generated in 1 h.33
Additionally, the solution pH increased from 7.2 to 7.5, despite
the high buffer concentration, indicating significant proton
consumption during the CPE experiment. In the absence of
FeN5H2, only a minimal charge (2.0 C) was passed and no
measurable change in pH was observed (Table S1).
Following CPE, product identification was conducted. The
+
presence of the six-electron-reduced product NH4 was
evaluated by the indophenol blue test34 (Figure S7).
Ammonium was formed with 8.1% FE. After confirming the
presence of NH4 , but accounting for only some of the charge
+
Figure 2. Cyclic voltammograms (100 mV/s) of 50 mM MOPS in
water (pH 7.2) with KCl or NaNO2 and FeN5H2 added as indicated
in the legend. The third scan is shown for each experiment.
passed, NH2OH was assayed by redox titration with K3[Fe-
(CN)6].35 Hydroxylamine is another multielectron reduction
+
product, which is also a proposed intermediate for NH4
−
features a single reversible redox event with a midpoint
potential (E1/2) of −0.10 V vs Ag/AgCl (1 M KCl)30
attributed to the chemically and electrochemically reversible
formation from NO2 by nitrite reductase enzymes (Scheme
1).36 Formation of NH2OH was indeed found to occur with a
FE of 91% in 1 h (TON = 65). In the absence of FeN5H2, no
31
−
+
FeIII/IIN5H2 couple (Figure 2). When NO2 is present, this
reversible feature persists, but an additional irreversible feature
is seen at half-wave potential Ep/2 = −0.98 V when 50 mM
MOPS, 100 mM NaNO2,, and 500 μM FeN5H2 are present.
The peak current for the irreversible feature has a linear
NH4 or NH2OH was detected.
+
The dependence of the product distribution between NH4
and NH2OH on CPE applied potential was evaluated to
compare to results reported for other catalysts9,12,13,17,20 (see
Figure S8 for results of CPE at a range of potentials). One-
hour CPE (500 μM FeN5H2, 1.0 M MOPS, 1.0 M NaNO2, pH
7.2) at −1.1 V passed 112 C with an FE of 6.8% for NH4+ and
93% for NH2OH, and at −1.3 V passed 188 C to yield an FE
−
dependence on [FeN5H2], [NO2 ], and [MOPS] before
eventually leveling off at higher concentrations of each
(Supporting Information Figures S1−S3). Solution pH also
impacts peak current, with lower pH translating to a higher
current response (pH 6.0−8.6; Figure S4). The Ep/2 value also
+
of 6.7% for NH4 and 89% for NH2OH (Table S1 and Figure
S9). While more charge was passed at lower potentials,
product distribution was not significantly affected by applied
potential. This observation contrasts with reports on electro-
−
depends on [FeN5H2], [NO2 ], and [MOPS] (Figures S1−
S3). The scan-rate dependence of the reversible and
irreversible features was also investigated (Figure S5); the
plots of peak current vs the square root of the scan rate for
both the reversible and irreversible features demonstrate a
−
+
catalytic NO2 reduction to NH2OH and NH4 by iron meso-
tetrakis(4-sulfonatophenyl)porphyrin (Fe(TPPS))12 and iron
meso-tetrakis(N-methyl-4-pyridy1)porphyrin (Fe(TMPyP)).13
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−
detection in this experiment relies on NMR, which has
relatively low sensitivity (Figure S11). Thus, FeN5H2-catalyzed
disproportionation of NH2OH is a route to form NH4 .
Scheme 1. Proposed Reaction Scheme for NO2 Reduction
to NH4 by Nitrite Reductases36
+
+
Scheme 2 summarizes the pathways observed in a model
consistent with our observations.
−
Scheme 2. Pathways Active during NO2 Reduction by
a
FeN5H2
The dependence of product distribution on potential seen for
Fe(TPPS) was found to result from stepwise reduction of
−
NO2 , where reduction of NH2OH occurs at a low potential
relative to its formation (Scheme 1).12
The effect of CPE time on product distribution also was
evaluated. CPE of FeN5H2 (500 μM FeN5H2, 1.0 M MOPS,
1.0 M NaNO2, pH 7.2 at −0.98 V) was performed for 5 h, and
a
−
FeN5H2 electrocatalytically reduces NO2 to NH2OH, which
+
undergoes direct reduction by the Hg electrode to form NH4 . In
addition, NH2OH undergoes FeN5H2-catalyzed disproportionation to
form NH4 , N2O, and N2.
+
[NH2OH] and [NH4 ] were assayed (Figure S10). A total of
+
128 C was passed, NH2OH was formed with 64% FE, and
NH4 was formed with 18% FE (Table S1). A dependence of
product distribution on time has been reported previously.12,20
For example, in the case of the Fe(TPPS) electrocatalyst,
Comparison of the dependence of FeN5H2 catalytic activity
+
−
longer CPE times increasingly favor NH4 formation at the
toward NO2 reduction on conditions highlights a number of
expense of NH2OH.12 The basis for this finding is that
unusual properties of FeN5H2 compared to other nitrite-
reducing catalysts. For FeN5H2, product distribution is not
dependent on potential, contrary to what has been seen for
other iron-12,13 and cobalt-containing17,20 catalysts as well as
metallophthalocyanines,9 likely because the FeN5H2-catalyzed
conversion of NO2− to NH2OH is fast relative to reduction of
12
+
NH2OH to NH4 .
The observation of increased FE for NH4+ and decreased FE
for NH2OH with time for Fe porphyrin catalysts is accounted
for by relatively slow conversion of NH2OH to NH4 . To
+
+
conversion of NH2OH to NH4 , N2O, and N2 occurs via
determine if a similar slow step is active here, a constant
potential of −0.98 V was applied to 1.0 M NH2OH and 1.0 M
MOPS (pH 7.2) in the presence and absence of FeN5H2 for 5
h. In the absence of the iron catalyst, consumption of NH2OH
was observed, resulting in formation of NH4+ (75% FE; Figures
S11 and S12).37 Notably, addition of FeN5H2 did not enhance
the amount of passed charge, indicating that FeN5H2 is not
active toward electrocatalytic reduction of NH2OH (Figures
S12 and S13). However, this result suggests that the mercury
electrode is active in this reaction.
disproportionation and thus does not require an applied
potential. Similar to Fe(TPPS),12 here the product distribution
depends on CPE time. For Fe(TPPS), this time dependence of
the product distribution results from the electrocatalytic
−
conversion of NO2 to NH2OH being faster than the
+
subsequent electrocatalytic conversion of NH2OH to NH4 ,
while here it is is attributed to the relatively slow formation of
NH4+ from NH2OH via FeN5H2-catalyzed disproportionation,
as well as NH2OH reduction at the electrode. Interestingly,
FeN5H2 shares a similar reactivity toward NH2OH dispro-
portionation as Fe(TPPS),12 but this reactivity is not reported
in cobalt-containing complexes confirmed to produce NH2OH
Disproportionation of NH2OH is another possible route to
formation of NH4 along with N2 and/or N2O.38−41 While
+
as an intermediate or product of NO2 reduction.17,20,21
−
uncatalyzed disproportionation of NH2OH is minimized at the
near-neutral pH used in our experiments,38,39 there are several
examples of iron complexes that catalyze this reaction.12,39,40 It
is worth noting that N2 and N2O would have eluded the
detection methods utilized thus far. Adding 500 μM FeN5H2
to a sealed reaction vessel containing 1.0 M NH2OH in 1.0 M
MOPS, pH 7.2 led to consumption of 1.5 mmol of NH2OH
over 5 h as determined by K3[Fe(CN)6] titration.35 NH4+ was
identified as a product by 14N NMR analysis (Figure S11).37 A
buildup of gaseous products was observed, and N2O and N2
were detected in the headspace by gas chromatography (GC)
(Figure S14). The detection of N2O was further confirmed by
infrared spectroscopy (IR; Figure S15). (Note that NO is not
detected as a product as shown in Supporting Information,
Figure S16).39−41 In the absence of FeN5H2, a minimal
amount of N2O is formed, as evidenced by the small N2O peak
In summary, we observe FeN5H2-catalyzed reduction of
−
NO2 to NH2OH at high faradaic yield; the NH2OH then
undergoes FeN5H2-catalyzed disproportionation to form
39−41
+
NH4 , N2O, and N2.
Similar metal complex-catalyzed
disproportionation pathways of NH2OH (NH3OH+) may play
a role in determining product distribution for other reported
catalysts for NO2 reduction. FeN5H2 stands out as a rare
−
−
example of an iron complex that catalyzes multielectron NO2
reduction near neutral pH, where uncatalyzed disproportiona-
−
tion reactions of both NO2 and the NH2OH product are
slow. Notably, the reactivity of FeN5H2 is supported by buffer,
allowing for controlled-pH studies, in contrast with reported
systems that do not use,20,21 or are inhibited by, buffers.14,15
Furthermore, the requirement that buffer be present to observe
catalysis indicates a role of buffer in the reaction, as has been
observed for catalysts for hydrogen production in water.42,43 In
future work, we plan to leverage this buffer requirement and
+
that is seen in GC (Figure S14). No NH4 is detected from
NH2OH disproportionation in the absence of catalyst, but its
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Letter
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utilize modifications of the ligand scaffold to gain insight into
the mechanism of the observed reactivity and to further
optimize activity. Investigation of the reduction of other
oxyanion species also is planned.
(8) (a) Woollard-Shore, J. G.; Holland, J. P.; Jones, M. W.; Dilworth,
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c03600.
■
sı
Experimental procedures, details on data analysis,
additional electrochemical data, absorption and IR
spectra, and tables of results (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
(9) Chebotareva, N.; Nyokong, T. Metallophthalocyanine catalysed
electroreduction of nitrate and nitrite ions in alkaline media. J. Appl.
Electrochem. 1997, 27, 975−981.
Kara L. Bren − Department of Chemistry, University of
Rochester, Rochester, New York 14627-0216, United States;
orcid.org/0000-0002-8082-3634; Email: bren@
chem.rochester.edu
Ellen M. Matson − Department of Chemistry, University of
Rochester, Rochester, New York 14627-0216, United States;
orcid.org/0000-0003-3753-8288; Email: matson@
chem.rochester.edu
(10) (a) Matson, E. M.; Park, Y. J.; Fout, A. R. Facile Nitrite
Reduction in a Non-heme Iron System: Formation of an Iron(III)-
Oxo. J. Am. Chem. Soc. 2014, 136, 17398−17401. (b) Cheung, P. M.;
Burns, K. T.; Kwon, Y. M.; Deshaye, M. Y.; Aguayo, K. J.; Oswald, V.
F.; Seda, T.; Zakharov, L. N.; Kowalczyk, T.; Gilbertson, J. D.
Hemilabile Proton Relays and Redox Activity Lead to {FeNO}x and
Significant Rate Enhancements in NO2− Reduction. J. Am. Chem. Soc.
2018, 140, 17040−14050.
(11) Toth, J. E.; Anson, F. C. Electrocatalytic Reduction of Nitrite
and Nitric Oxide to Ammonia with Iron-Substituted Polyoxotung-
states. J. Am. Chem. Soc. 1989, 111, 2444−2451.
(12) Barley, M. H.; Meyer, T. J. Electrocatalytic Reduction of Nitrite
to Ammonia Based on a Water-Soluble Iron Porphyrin. J. Am. Chem.
Soc. 1986, 108, 5876−5885.
Authors
Jesse R. Stroka − Department of Chemistry, University of
Rochester, Rochester, New York 14627-0216, United States
Banu Kandemir − Department of Chemistry, University of
Rochester, Rochester, New York 14627-0216, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c03600
(13) Barley, M. H.; Rhodes, M. R.; Meyer, T. J. Electrocatalytic
Reduction of Nitrite to Nitrous Oxide and Ammonia Based on the N-
Methylated, Cationic Iron Porphyrin Complex [FeIII(H2O)-
(TMPyP)]5+. Inorg. Chem. 1987, 26, 1746−1750.
Notes
The authors declare no competing financial interest.
(14) Xu, S.; Ashley, D. C.; Kwon, H.-Y.; Ware, G. R.; Chen, C.-H.;
Losovyj, Y.; Gao, X.; Jakubikova, E.; Smith, J. M. A flexible, redox-
active macrocycle enables the electrocatalytic reduction of nitrate to
ammonia by a cobalt complex. Chem. Sci. 2018, 9, 4950−4958.
(15) Xu, S.; Kwon, H.-Y.; Ashley, D. C.; Chen, C.-H.; Jakubikova, E.;
Smith, J. M. Intramolecular Hydrogen Bonding Facilitates Electro-
catalytic Reduction of Nitrite in Aqueous Solutions. Inorg. Chem.
2019, 58, 9443−9451.
ACKNOWLEDGMENTS
■
We thank D. M. M. Mevan Dissanayake and Alex A. Fertig for
assistance with complex synthesis and data collection. We
thank Rachel L. Meyer for assistance with GC-TCD samples
and Mikhaila D. Ritz for assistance with infrared spectroscopy.
We acknowledge the University of Rochester for funding, and
J.R.S. was supported by a Sproull Fellowship. E.M.M. is the
recipient of a Sloan Fellowship from the Alfred P. Sloan
Foundation, which has supported this work.
(16) Guo, Y.; Stroka, J. R.; Kandemir, B.; Dickerson, C. E.; Bren, K.
L. Cobalt Metallopeptide Electrocatalyst for the Selective Reduction
of Nitrite to Ammonium. J. Am. Chem. Soc. 2018, 140, 16888−16892.
(17) Cheng, S.-H.; Su, Y. O. Electrocatalysis of Nitric Oxide
Reduction by Water-Soluble Cobalt Porphyrin. Spectral and Electro-
chemical Studies. Inorg. Chem. 1994, 33, 5847−5854.
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ACS Catal. 2020, 10, 13968−13972