Photochemical nitrogen conversion to ammonia in ambient conditions with femos-chalcogels

Article abstract of DOI:10.1021/ja512491v

In nature, nitrogen fixation is one of the most important life processes and occurs primarily in microbial organisms containing enzymes called nitrogenases. These complex proteins contain two distinct subunits with different active sites, with the primary N₂ binding site being a FeMoS core cluster that can be reduced by other nearby iron-sulfur clusters. Although nitrogen reduction to ammonia in biology does not require the absorption of light, there is considerable interest in developing catalyst materials that could drive the formation of ammonia from nitrogen photochemically. Here, we report that chalcogels containing FeMoS inorganic clusters are capable of photochemically reducing N₂ to NH₃ under white light irradiation, in aqueous media, under ambient pressure and room temperature. The chalcogels are composed of [Mo₂Fe₆S₈(SPh)₃]³⁺ and [Sn₂S₆]^4- clusters in solution and have strong optical absorption, high surface area, and good aqueous stability. Our results demonstrate that light-driven nitrogen conversion to ammonia by MoFe sulfides is a viable process with implications in solar energy utilization and our understanding of primordial processes on earth.

Full text of DOI:10.1021/ja512491v

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Article
Photochemical Nitrogen Conversion to Ammonia
in Ambient Conditions with FeMoS-Chalcogels.
Abhishek Banerjee, Benjamin D. Yuhas, Eric A Margulies, Yongbo
Zhang, Yurina Shim, Michael R. Wasielewski, and Mercouri G. Kanatzidis
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja512491v • Publication Date (Web): 15 Jan 2015
Downloaded from http://pubs.acs.org on January 20, 2015
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Photochemical Nitrogen Conversion to Ammonia in Ambient Condi-
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Abhishek Banerjee, Benjamin D. Yuhas, Eric A. Margulies, Yongbo Zhang, Yurina Shim, Michael
R. Wasielewski, and Mercouri G. Kanatzidis*
Department of Chemistry, Northwestern University, Evanston, IL 60208
KEYWORDS Nitrogen-fixation, Photocatalytic process, Chalcogels
ABSTRACT: In nature, nitrogen fixation is one of the most important life processes and occurs primarily in microbial
organisms containing enzymes called nitrogenases. These complex proteins contain two distinct subunits with different
active sites, with the primary N2 binding site being a FeMoS core cluster that can be reduced by other nearby iron-sulfur
clusters. Although nitrogen reduction to ammonia in biology does not require the absorption of light, there is considera-
ble interest in developing catalyst materials that could drive the formation of ammonia from nitrogen photochemically.
Here we report that chalcogels containing FeMoS inorganic clusters are capable of photochemically reducing N2 to NH3
under white light irradiation, in aqueous media, under ambient pressure and room temperature. The chalcogels are
composed of [Mo2Fe6S8(SPh)3]3+ and [Sn2S6]4- clusters in solution and have strong optical absorption, high surface area,
and good aqueous stability. Our results demonstrate that light-driven nitrogen conversion to ammonia by MoFe sulfides
is a viable process with implications in solar energy utilization and our understanding of primordial processes on earth.
Biological nitrogen reduction, catalyzed by the enzyme
nitrogenase1 is postulated to occur with concurrent
proton reduction to form H2, as suggested by the equa-
tion:1a, 2
thesized,5 and a great deal of them do show effective
catalytic ability in transformations related to nitrogen-
ase activity, such as the reduction of hydrazine to am-
5d
monia.5c,
Ammonia production from nitrogen has
been highly sought in molecular analogues of the ni-
trogenase active site (i.e., synthetic clusters bearing
Mo-Fe linkages), and some report of molecular analogs
have been shown to produce ammonia from N2 under
strongly reducing conditions created by chemical of
electrochemical means.6 Conversely, photochemical
ammonia production has been previously demonstrat-
ed using ultraviolet light in systems such as semicon-
ductor thin films (e.g., titania),7 diamond,8 and fuller-
enes.9 Molecular complexes of molybdenum, iron or
titanium, due to their high instability in air have to be
handled under carefully controlled inert atmosphere,
with nitrogen fixation reactions observed to be occur-
ring at extremely low temperatures (example –78oC for
[{tris(phosphine)borane}Fe(N2)][Na(12-crown-4)2],6e
and -30oC for [(C5Me4SiMe3)3Ti3(μ-H)7]6a. Conversely
fullerenes are reported to catalyze nitrogen fixation at
higher temperatures of 60oC.9 To the best of our
knowledge, we present here the first solid-state nitro-
genase analogue containing Fe/Mo/S capable of visible
light-driven ammonia production from nitrogen at
room temperature and ambient pressure.
N2 + 8 H+ + 8 e- + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi
(Eq. 1)
Unlike photosynthetic processes, biological nitrogen
fixation is an aphotic process — it does not require the
absorption of light to proceed. Light-driven biomimet-
ic catalysis is an attractive proposition, however, as it
can take full advantage of the abundant solar energy.
This concept is demonstrated successfully in the for-
mation of hydrogen and oxygen via photochemical
water splitting, known as artificial photosynthesis.3
Industrially, the reaction of N2 with H2 to form NH3 is
o
accomplished using high temperatures (300-400 C)
and pressures (~250 atm) with an iron catalyst, ie. the
Haber-Bosch process. Such a process is highly energy
demanding, as the N2 must react with the solid surface,
and consumes more than 1% of the world’s energy
supply.
The active sites of all nitrogenases are ob-
served to have an inorganic metal/sulfur cluster, in
most cases consisting of a central iron-molybdenum-
sulfur cluster, the so-called FeMoS co-factor.4 Many
inorganic molecular analogues of the nitrogenase
FeMoS cluster active site have been successfully syn-
Here, we report that biomimetic chalcogels10
are capable of photochemical reduction of nitrogen to
ammonia with light. The chalcogels are composed of
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double-cubane Mo2Fe6S8 cluster units linked by Sn2S6 mentary Information).
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adsorp-
Nitrogen
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ligands, forming a random, amorphous network with
strong optical absorption. Like nitrogenase enzymes,
our chalcogels are stable in aqueous environments,
and can thus be thought of as solid-state analogues.
Inorganic molecular analogues of the FeMoS
co-factor have already been known in the literature for
some time,5a, 5b but they are not functional analogs as
they are not known to bind and convert nitrogen. With
suitable terminal ligands, the FeMoS clusters, reported
by Holm and coworkers,11 can be cross-linked with
[Sn2S6]2- anions to form a polymeric and porous chal-
cogenide framework, as illustrated in Figure 1. The re-
action proceeds from the metathesis displacement of
the terminal Cl- ligands in the [Mo2Fe6S8(SPh)3Cl6]3-
cluster (2) by the terminal sulfides of [Sn2S6]4-.12 For the
preparation of FeMoS-chalcogels for the purposes of
tion/desorption measurements (Figure 2(C)) on the
dried chalcogels (aerogels) indicate a Type IV porosity
(average surface area of 118 m2/g).13 We note that once
the [Mo2Fe6S8(SPh)3Cl6]3- cluster is integrated into the
FeMoS-chalcogels it becomes stable against hydrolysis
contrary to the hydrolytically unstable precursor.
The clusters in the chalcogels were character-
ized by a variety of means.12 The stability of the FeMoS
cluster in the chalcogel matrix is confirmed with thiol
extrusion studies. The FeMoS-chalcogels are observed
to be stable in practically all solvents, including water,
without any dissolution. However, in the presence of
large excess of benzenethiol in solution the gel dis-
solves rapidly as the {Mo2Fe6S8(SPh)3} cluster is ex-
truded from the gel by the benzenethiol, yielding the
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[Mo2Fe6S8(SPh)9]3- anion in solution, showing charac-
12
nitrogen-fixation, we used
a
mixture of N-
teristic spectral features (Figure 2(D)).5c,
A semi-
methylformamide/N,N’-dimethylformamide for gel
formation (see Methods in Supplementary Information
for details). Upon standing at room temperature for 15-
20 days, solutions of this mixture form a black gel, re-
ferred to here as FeMoS-chalcogel.
quantitative estimation of the [Mo2Fe6S8(SPh)9]3- anion
(εM at 449 nm = 37000) concentration in solution gave
a value of 0.51 mM for 1.0 mM of gel used (see Supple-
mentary Information for details). The UV-Vis spectra
of an aqueous colloidal suspension of the gel, prepared
by sonication of the gel in water for ~2 min, also shows
a broad characteristic absorption in the visible spec-
trum peaking at 270 nm (Figure 2(D) inset).
Fig. 1 Chalcogel Synthesis. Schematic representation
of Mo2Fe6S8-Sn2S6 biomimetic chalcogel (FeMoS-
chalcogel), building block scheme (Mo=blue, Fe=red,
S=yellow, Sn=black), and a completed chalcogel shown
at right.
Fig. 2 Chalcogel Structural Characterization. A)
Scanning electron micrographs of a representative
FeMoS-Sn2S6 chalcogel. B) High resolution scanning
electron micrograph of the aerogel surface showing
sponge-like morphology.
C)
Nitrogen adsorp-
tion/desorption isotherm obtained at 77 K of FeMoS-
Sn2S6 chalcogels synthesized in NMF/DMF solvent
mixture. D) UV-Vis spectra of DMF solutions of
[Mo2Fe6S8(SPh)9]3-, [Mo2Fe6S8(SPh)3Cl6]3- and extruded
FeMoS cluster from chalcogel with excess benzenethi-
ol. Inset shows the UV-Vis spectra of a colloidal sus-
pension of FeMoS-Sn2S6 chalcogel in water.
Electron microscopy reveals the spongy, po-
rous nature of the FeMoS-chalcogels, always appearing
as an amorphous solid, as determined from TEM-SAED
and SEM experiments (Figure 2(A), (B)). Energy dis-
persive spectroscopy (EDS) was used to determine the
elemental composition of the chalcogels (see Supple-
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loid in the presence of nitrogen at 435 nm, showing the
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The FeMoS cluster in the chalcogel matrix is a
emergence of a new kinetic component.
strong chromophore. The ability of the FeMoS-
chalcogel to be photoexcited to sufficiently high ener-
gy capable with interacting with N2 was confirmed by
nanosecond transient absorption (nsTA) experiments.
Experiments were performed on the chalcogel sonicat-
ed in o-dichlorobenzene and spuncast onto a glass
slide, yielding a thin film. After photoexcitation at 416
nm, both ground state bleach and excited state fea-
tures are observed (Figure 3(A)). The ground state
bleach, monitored at 520 nm, is found to have a life-
time of around 200 ns (Figure 3(B)). The excited state
which arises at 660 nm decays with a similar lifetime
to the bleach.
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The chalcogels have very strong optical ab-
sorption in the ultra-violet to visible region of the
spectrum, as seen by UV-Vis studies of the gel in water
(Figure 2(D) inset). Because of the distinct role FeMoS
clusters play in nitrogenase enzymes, we explored the
ability of the FeMoS chalcogels to produce ammonia
from nitrogen photochemically. We thus performed
nitrogen fixation with our biomimetic chalcogels using
ultra-violet to visible-light. Slices of the chalcogels
were placed in aqueous solutions containing pyridini-
um hydrochloride (proton source) and sodium ascor-
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bate (electron donor).
Nitrogen was continually
To better understand these initial results,
flowed through the solution at a rate of 8-10 ml/min,
which was illuminated with a 150 W xenon lamp, with
the light intensity at the sample being 100 mW/cm2.
The reaction were monitored for typical periods of 32
or 48 hours, and at regular intervals of 2 and 4 hours,
respectively, aliquots were drawn from the reaction
solutions. In order to have maximum exposure of the
chalcogel to the light source as well as the reactants
the mixture was kept under constant stirring. The ali-
quots were then assayed for [NH4]+ ions in solution by
use of the direct method of ion chromatography,14 as
well as the indirect chemical indophenol method.15 Ion
chromatography analysis require no sample prepara-
nsTA was then used to probe the interaction of nitro-
gen with a colloidal species of the chalcogel in solu-
tion. These samples were prepared in the presence of
pyridinium chloride and sodium ascorbate, in order to
create conditions identical to those of the catalytic
studies (see Methods in Supplementary Information
for details). Sonication of this aqueous mixture of the
chalcogel for a few minutes yielded a fine colloidal
suspension of the gel in water. Photoexcitation of this
sample in the absence of nitrogen yields a broad
ground state bleach which decays with a lifetime long-
er than the instrument window of 4.5 μs (Figure 3(C)).
After bubbling the sample with nitrogen for one hour,
the ground state bleach was found to decay biexponen-
tially with a new 100-200 ns decay component in addi-
tion to a similarly long decay component as the nitro-
gen-free sample (Figure 3(D)). The observed results are
consistent with a portion of the FeMoS-chalcogel being
quenched by interaction with nitrogen, presumably
heralding one of the first steps in the catalytic activity.
+
tion. NH4 determination was performed with a cation
exchange column equipped with an electrolytically
regenerated suppressor and using 20 mM methanesul-
fonic acid as the eluent, with a flow rate of 1 mL/min.
Na+ ions were observed to elute first with a retention
+
time (Rt) of ~8.2 min, followed by NH4 ions with a Rt
of ~10.6 min. The amount of ammonium obtained
could be estimated very accurately with a calibration
curve, obtained from concentrations in the range of 2
μM to 2 mM (see Methods in Supplementary Infor-
mation). The indophenol method is a different ap-
proach based on the reaction of phenol and ammonia
in basic conditions and in the presence of hypochlorite
as the oxidizing agent and iron nitroprusside as a cata-
lyst. Each test requires 24 hours to complete. Its ad-
vantage is that it produces a distinct blue color due to
formation of the indophenol dye (λmax = 630 nm, FW:
199 g/mol) acting as a unique visual indicator and al-
lowing spectroscopic identification of NH3 in solution.
However, since this method employs a complex chem-
ical reaction toolkit, it is susceptible to interference
from many sources and requires elaborate handling of
the reaction products that can introduce losses and
errors.16 It is time consuming, cumbersome, subject to
interference and thus inadequate for an accurate quan-
tification of the amount of ammonia for our aliquots.
Fig. 3 Chalcogel Electronic Characterization. A)
Nanosecond transient absorption (nsTA) spectra of
chalcogel film. B) nsTA kinetics of chalcogel film at 520
nm. C) nsTA kinetics of suspension in the absence of
nitrogen at 435 nm. D) nsTA kinetics of chalcogel col-
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Ammonia was detected in all illuminated solu-
ited by alkyl phosphines known to bind to either Fe or
Mo (Figure S5).
In order to prove that nitrogen gas is the
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tions containing FeMoS-chalcogel saturated with ni-
trogen and its concentration continuously increased
with irradiation time (Figure 4(A)). The UV-Vis spectra
showing indophenol are shown in Figure 4(B) and the
ion chromatographs of the aliquots in Figure 4(D), re-
spectively. Figure 4(C) is a kinetic plot showing rising
ammonia production as determined from ion chroma-
tography. In a separate experiment run for 72 hours we
measured 12 µmols produced using ~1.5 μmols of cata-
lyst. Thus, for each equivalent of total catalyst, approx-
imately eight equivalents of ammonia were obtained,
clearly indicating the catalytic nature of the reaction.
Reactions were monitored for a maximum of 72 hours
and during this time no degradation or loss of activity
of the catalyst was observed.
source of ammonia in our experiments we used isotop-
ic labeling experiments. We thus performed two types
of isotope labeling experiments. One type involved the
15
use of N labeled pyridinium and the other type 15N2
gas. The use of 15N-labeled pyridinium chloride yielded
14NH3 ammonia, which was confirmed by the LC-MS of
the indophenol anion detected at 198 m/z and the ab-
sence of enhanced peak at 199 m/z. This eliminates the
possibility of a hydrodenitrogenation-type reaction of
the pyridine ring to yield NH3. The other type was the
use of 15N2 gas in the photochemical reaction. These
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15
experiments yielded NH3. We used two independent
methods to confirm the formation of 15NH3. First, we
1
used 2D H-15N heteronuclear single quantum coher-
ence (HSQC) NMR spectroscopy where 15NH4 was
+
clearly identified in solution with its characteristic 15N-
1
1H coupling JN-H = 72 Hz (Figure 5(A),(B)). Second,
indophenol assays of photocatalytically reacted 15N2
samples showed a strong 15N-labeled indophenol anion
mass spectroscopy signal at 199 m/z in LC-MS studies
(Figure S6), having relative intensities considerably
14
higher than the natural abundance ratio of N:15N nu-
clei.
Fig. 4 Catalytic nitrogen-fixation. A) Photos of ali-
quots obtained during nitrogen-fixation catalysis, after
performing the indophenol test. Note the steady in-
crease in the intensity of blue color with time. B) UV-
Vis absorption spectra of indophenol assays, showing
the characteristic absorption maximum at 630 nm of
indophenol. C) Kinetic plot of NH3 concentration evo-
lution in the assayed aliquots with time, using ion
chromatography. D) Representative ion chromato-
graphs of aliquots obtained during nitrogen-fixation at
2h and 32h showing increasing concentration of [NH4]+
ions. The Na+ ions are from ascorbate.
+
Fig. 5. NMR spectroscopic detection of 15NH4 in
1
the reaction solution. A) H-15N HSQC NMR data of
standard [15NH4]+ solution in 10% D2O/H2O. The peaks
are from a statistical mixture of [15NH4-xDx]+ species (x
1
= 0 - 4). B) H-15N HSQC NMR data of [15NH4]+ ob-
15
tained from photocatalytic N2-fixation (spectra taken
in 10% D2O/H2O). Inset shows the corresponding spec-
tra of the [15NH4]+ containing product solution with
DMSO-d6 added to avoid H-D exchange. The observed
In control experiments chalcogel containing
reaction solutions kept in darkness or solutions with-
out chalcogels showed no NH3 formation. Illuminated
chalcogel samples containing the proton and electron
sources but lacking nitrogen gas (saturated with argon)
also showed no detectable NH3. Finally, samples that
were not illuminated or illuminated under nitrogen
but lacked either the proton or electron source show
no NH3 formation either. Preliminary experiments per-
formed with ligands that can potentially bind to the
metals and act as inhibitors show that the activity of
the FeMoS clusters can be blocked. For example, we
observed that the NH3 production appears to be inhib-
15
1
doublet is from N-H coupling with JN-H = 72 Hz. Ex-
periments with [14NH4]+ solutions showed no such
spectra.
We have shown here that solid chalcogels con-
taining FeMoS clusters are capable of light driven ni-
trogen fixation and conversion to ammonia not only
under ambient conditions of temperature and pressure
but also in aqueous media. Although the
[Mo2Fe6S8(SPh)3Cl6]3- clusters are not exact FeMoS co-
factor analogs, when part of the chalcogel structure
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they seem capable of acting in a similar fashion to the chloride (50 mM) and sodium ascorbate (5 mM). Ni-
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biological systems. This is the first time a structural
analog of the nitrogenase enzyme is shown to also be a
functional analog at least in a qualitative sense. These
chalcogels feature multiple immobilized FeMoS active
sites in a design that features a very high spatial densi-
ty of such sites all closely and densely integrated into a
three-dimensional matrix. The high density of clusters
in the material enhances the prospect of multi-electron
transformations, particularly those that are photo-
driven as light energy accumulates creating highly ex-
cited cluster states. In that sense, our photoactive
chalcogels are good mimic of nitrogenases but with the
added property of strong light absorption that is gen-
erally a characteristic of photosynthetic centers and
semiconductors. The ease with which the amorphous
FeMoS-chalcogel converts nitrogen to ammonia in
aqueous solution at ambient temperature and pressure
in the presence of light, protons and mild reducing
agents raises intriguing implications for such activity
occurring in nature, especially in environments where
mineral sulfide surfaces with the proper transition
metal ingredients are adventitiously present. Although
such evidence has yet to be presented, such occurrence
may be particularly relevant to the less oxidizing con-
ditions of the early earth where ammonia generation
may have been catalyzed in this fashion on a global
scale.
trogen was continually bubbled through the solution,
which was illuminated with the light from a 150 W
xenon lamp. Aliquots of the solution were taken regu-
larly for a period of 32 hours and assayed for NH3 by
ion chromatography,14 and the indophenol method.15.
Experiments requiring the estimation of TON were
performed with minimalistic amount of catalyst (less
than 5 mg) with aliquots collected at the beginning
and after 72 h.
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ASSOCIATED CONTENT
Supporting Information. Detailed experimental pro-
cedures, including the synthesis of chalcogels, catalysis
experiments and analytical techniques for catalytic
product determination and estimation. This material is
available free of charge via the internet at
http://pubs.acs.org.
Author Information. The authors declare no compet-
ing financial interests. Correspondence and requests
for materials should be addressed to M.G.K at
m-kanatzidis@northwestern.edu
Acknowledgements. Electron Microscopy and Ele-
mental Analysis was performed at the Electron Probe
Instrumentation Center at Northwestern University.
We thank Prof. Andrew Jacobson of Earth and Plane-
tary Sciences at Northwestern University for use of
DX-120 instrument. This work was supported as part of
the ANSER Center, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Award
Number DE-SC0001059. We thank Tasha L. Drake for
help in performing the 15N-labeled experiments and
Debajit Sarma for the Cs-exchange and SEM-EDS stud-
ies.
Methods Summary
Clusters
(NBu4)3[Mo2Fe6S8(SPh)3Cl6]
(NBu4)3[Mo2Fe6S8(SPh)9]
(2),
(1)
well
and
as
as
Na4Sn2S6·14H2O, were synthesized according to litera-
ture procedures.17 Chalcogels were synthesized in a
nitrogen-filled glovebox by slowly adding a solution of
250 mg of cluster 2 (0.13 mmols) in 1.5 mL N,N’-
dimethylformamide to a solution of 150 mg of
Na4Sn2S6·14 H2O (0.195 mmols) in 1.5 ml of N-
methylformamide, with the mixture being manually
shaken after each drop, so that the entire addition took
place over 30-45 min. After all of 2 was added, the re-
action mixture was black and viscous. The reaction
was covered and left undisturbed under nitrogen for
15-20 days, after which the viscous solution hardened
into a rigid black chalcogel. The chalcogels were then
subjected to solvent exchange by washing with a mix-
ture of ethanol and water (4:1, v/v) four times, followed
by pure ethanol three times. The chalcogels were sub-
jected to supercritical drying with carbon dioxide for
surface area measurements; otherwise, the gels were
stored under ethanol.
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