Atomically Dispersed Molybdenum Catalysts for Efficient Ambient Nitrogen Fixation

Article abstract of DOI:10.1002/anie.201811728

NH₃ synthesis by the electrocatalytic N₂ reduction reaction (NRR) under ambient conditions is an appealing alternative to the currently employed industrial method—the Haber–Bosch process—that requires high temperature and pressure. We report single Mo atoms anchored to nitrogen-doped porous carbon as a cost-effective catalyst for the NRR. Benefiting from the optimally high density of active sites and hierarchically porous carbon frameworks, this catalyst achieves a high NH₃ yield rate (34.0±3.6 μg _NH3 h^?1 mg_cat.^?1) and a high Faradaic efficiency (14.6±1.6 %) in 0.1 m KOH at room temperature. These values are considerably higher compared to previously reported non-precious-metal electrocatalysts. Moreover, this catalyst displays no obvious current drop during a 50 000 s NRR, and high activity and durability are achieved in 0.1 m HCl. The findings provide a promising lead for the design of efficient and robust single-atom non-precious-metal catalysts for the electrocatalytic NRR.

Full text of DOI:10.1002/anie.201811728

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Atomically dispersed Mo catalysts for high-efficiency ambient N2
fixation
Authors: Lili Han, Xijun Liu, Jinping Chen, Ruoqian Lin, Haoxuan Liu,
Fang Lu, Seongmin Bak, Zhixiu Liang, Shunzheng Zhao, Eli
Stavitski, Jun Luo, Radoslav R. Adzic, and Huolin Xin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811728
Angew. Chem. 10.1002/ange.201811728
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Angewandte Chemie International Edition
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COMMUNICATION
Atomically dispersed Mo catalysts for high-efficiency ambient N
2
fixation
Lili Han[a,b]†, Xijun Liu[a]†, Jinping Chen[a]†, Ruoqian Lin , Haoxuan Liu , Fang Lü , Seongmin Bak ,
[b]
[a]
[a]
[c]
[c]
[d]
[e]
[a]
[c]
[f]
Zhixiu Liang , Shunzheng Zhao , Eli Stavitski , Jun Luo, * Radoslav R. Adzic, * and Huolin L. Xin *
Abstract: NH
3
synthesis by electrocatalytic
N
2
reduction
N
2
reduction reaction (NRR) offers a promising carbon-free
strategy toward cleaner and more sustainable NH
production.[8-
However, the efficiency of NRR suffers from a parallel
reaction (NRR) under ambient conditions is an appealing
alternative to the industrial method that requires high
temperature and pressure. Herein, we report single Mo atoms
anchored to N-doped porous carbon, which works as a cost-
effective catalyst for NRR. Benefiting from the optimally high
density of active sites and hierarchically porous carbon
3
2
7]
hydrogen evolution reaction (HER) in aqueous solutions on
traditional NRR electrocatalysts.[7]
Consequently, a variety of noble metal-based, non-noble
metal-based and metal-free materials have been developed for
NRR electrocatalysts at ambient conditions with aqueous
electrolytes.[8,14−26] Of them, electrocatalysts based on noble
metals, such as Au,[8,14] Ru,[15,16] and Rh,[17] show favorable
activities with hollow Au nanocages exhibiting the reported
highest Faradaic efficiency (FE) of 30.2
3
corresponding NH yield rate of 3.9 µg h cm ,
further advances would overcome the barrier of their high costs
for their widespread use. Non-noble metal-based catalysts have
also been extensively investigated to develop cost-effective and
frameworks, this catalyst achieves a high NH
3
yield rate (34.0 ±
mgcat. ) and a high Faradaic efficiency (14.6 ±
.6 %) in 0.1 KOH at room temperature. These are
–1
–1
3
.6 μgNH3 h
1
M
considerably higher values compared with those for previously
reported non-precious-metal electrocatalysts. Moreover, this
catalyst displays no obvious current drop during a 50,000-
second NRR. Similarly, high activity and durability are also
achieved in 0.1 M HCl by the catalyst. This work may provide a
promising lead for designing efficient and robust single-atom
non-precious-metal catalysts toward electrolytic NRR.
%
−2 [14]
and the
and their
−
1
2 3
O
-CNT,[18]
high-performance NRR electrocatalysts, including Fe
Mo 11/CeO
C/C,[19] Bi ,[20] MoS ,[21] Ti ,[22] etc. A notable
example involves the Bi hybrid for producing NH
where the highest FE and NH yield rate were achieved to be
2
4
V
2
O
2
2
3 2 x
C T
4
V
2
O11/CeO
2
3
,
Fixation of naturally abundant nitrogen (N
is one of the most important and challenging chemical
transformations required for eliminating hunger and generating
potential carbon-free condensed fuels.[1-4] From an energy-
saving perspective, green fixation methods for NH
strongly desired because the Haber-Bosch process, the main
industrial process for producing NH , requires extremely harsh
2 3
) into ammonia (NH )
3
0.16 % and 23.21 μg h−1 mgcat.−1, respectively.[20] Metal‐free
1
[
23]
porous carbon,[24] and their
catalysts, such as boron carbide,
derivatives[25,26] are another type of alternative materials, of
which boron carbide (B
3 2
from N are
4
C) nanosheets presented the highest FE
yield rate of 26.57 μg h–1
of 15.95 % and the corresponding NH
3
3
mg–1cat..[23] These significant breakthroughs guide the scientific
community to pay attention to developing cost-effective and
high-performance NRR electrocatalysts for achieving both of
reaction conditions (400−600 °C, 20−40 MPa) and causes
pollution and greenhouse gas emissions.[5-7] The electrocatalytic
high FE and high NH
conditions.
The activity of metal-based nanocatalysts is highly
influenced by their size.[27] It has been established that metal
atoms with unsaturated coordination are likely to the active
catalytic centers.[27] Therefore, when optimization catalysts, it is
highly desired to simultaneously decrease the size of a catalyst
and increase the fraction of metal atoms that has unsaturated
coordination. On the basis of this aspect, atomically dispersed
catalysts with mononuclear non-precious-metal complexes or
single non-precious-metal atoms anchored on supports would
be a promising candidate for NRR catalysts, because of their
3
yield rate in aqueous solution at ambient
[
a]
Dr. L. L. Han, J. P. Chen, Dr. X. J. Liu, H. X. Liu, F. Lü, Prof. J. Luo
Center for Electron Microscopy and Tianjin Key Lab of Advanced
Functional Porous Materials, Institute for New Energy Materials &
Low-Carbon Technologies, School of Materials Science and
Engineering, Tianjin University of Technology, Tianjin 300384,
China
E-mail: jluo@tjut.edu.cn
Dr. L. L. Han, Dr. R. Q. Lin
Center for Functional Nanomaterials, Brookhaven National
Laboratory, Upton, NY 11973, USA.
Dr. S. Bak, Z. X. Liang, Prof. R.R. Adzic
[
b]
c]
[
Chemistry Division, Brookhaven National Laboratory, Upton, NY
11973, USA.
E-mail: adzic@bnl.gov
Dr. S. Z. Zhao
maximum atom efficiency, unique catalytic performances, and
unsaturated metal coordination environment.[28-32] Considering
that nearly all natural N fixations are achieved with the enzyme
2
nitrogenase produced in bacteria and the active center of the
nitrogenase is an Mo-based structure,[33] single Mo atoms could
be a promising catalyst. Moreover, a density functional theory
[
[
[
d]
e]
f]
Department of Environmental Engineering, University of Science
and Technology Beijing, Beijing 100083, China.
Dr. E. Stavitski
National Synchrotron Light Source II, Brookhaven National
Laboratory, Upton, NY 11973, USA.
Prof. H. L. Xin
Department of Physics and Astronomy, University of California,
Irvine, CA 92697, USA.
E-mail: huolinx@uci.edu
(
DFT) simulation in 2017 has predicted that single Mo atoms
immobilized on defective boron nitride (BN) monolayers can be
a potential NRR electrocatalyst to yield NH
atomically dispersed Mo atoms bonded to
3
,[34] in which
atoms will
molecules and
2
species during the NRR.
[†
]
These authors contributed equally to this work.
N
contribute good performances in activating N
stabilizing N H while destabilizing NH
2
Supporting information for this article is given via a link at the end of
the document.
2
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This theoretical work shows that single-atom anchor sites can be
a possible alternative to the ambient condition enzymic reaction;
however, the poor electrical conductivity of BN should be
carefully considered if one wants to realize and use this material
for the NRR.
Brunauer-Emmett-Teller (BET) analysis gives its specific surface
area to be 302.3 m g and indicates the hierarchical existence
of meso- and macro-porous features (Figure S2). Such a 3D
2
–1
3
porous structure was etched by NH , which was produced from
decomposition of the hydroxylamine hydrochloride precursor. It
is of advantage to the accessibility of active sites and the mass
transport during electrochemical catalytic processes.[31] Further,
elemental mapping performed by energy-dispersive X-ray
spectroscopy (EDS) manifests the existence of Mo, C and N and
their homogenous distributions over the entire architectures
(Figures 1c and S3). High-resolution TEM (HRTEM) imaging
and selected area electron diffraction (SAED) demonstrate the
amorphousness of SA-Mo/NPC (Figure S4). This finding
coincides well with X-ray diffraction (XRD) measurements
(Figure S5), in which no reflection peaks exist for Mo-related
crystals and only humps from amorphous carbon are found.
Using aberration-corrected high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
technique with sub-Å resolution, many isolated bright dots on
carbon surfaces are observed (Figures 1d and S6). They are
identified by electron energy-loss spectroscopy (EELS) to be Mo
atoms (Figure 1e). In addition, the Mo loading was measured by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) analysis, and its value corresponding to Figure 1d is 9.54
wt%.
Herein, we fabricated and optimized single Mo atoms
anchored on N-doped porous carbon (SA-Mo/NPC) and
employed them as a catalyst (Figure 1a) for electrochemically
catalyzing the NRR. Due to the optimized abundance of single
active sites and the 3D hierarchically porous feature, the catalyst
exhibited a remarkable FE of NH
corresponding to an NH yield rate of 34.0 ± 3.6 μgNH3
mgcat. ) at –0.3 V relative to the reversible hydrogen electrode
vs RHE) in 0.1 M KOH at room temperature. This is among the
3
formation up to 14.6 ± 1.6 %
–
1
(
3
h
–1
(
best reported NRR electrocatalyst performances (Table S1).
Moreover, SA-Mo/NPC showed negligible activity decay in an
NRR electrolysis as long as 50,000 s. Meantime, similarly high
NRR activity and robust durability were also exhibited by SA-
Mo/NPC in 0.1 M HCl. This work provides a promising route for
the development of efficient and robust single-atom catalysts for
2
the N fixation.
Figure 2. Synchrotron radiation XANES measurements and XPS spectra. (a)
XANES and (b) FT-EXAFS curves of SA-Mo/NPC at Mo K-edge. (c) Mo 3d,
(d) N 1s and (e) C 1s XPS spectra of SA-Mo/NPC.
To explore the local structure of Mo speciation, synchrotron-
radiation-based hard X-ray absorption spectroscopy (XAS)
measurements were performed. The Mo K-edge X-ray
absorption near-edge structure (XANES) of SA-Mo/NPC, with a
2
Mo foil and Mo C as references, is displayed in Figure 2a. A shift
in the Mo K-edge to a higher energy compared with the
references indicates that the valence of Mo species in SA-
Mo/NPC is > +2. We compare the XANES spectrum of SA-
Figure 1. Structure of SA-Mo/NPC. (a) Schematic illustration of SA-Mo/NPC
and its corresponding atomic structure model. (b) TEM image. (c) Mo EDS
mapping revealing the homogeneous distribution of Mo on the carbon support,
in which the inset is the corresponding HAADF-STEM image. (d) Atomic-
resolution HAADF-STEM image. (e) EELS spectra from areas A and B in the
atomic-resolution HAADF-STEM image of the inset, in which areas A and B
does and does not contain single Mo atoms, respectively. The two orange
arrows point to the signals of Mo M4,5 and M2,3 edges, respectively.
[31]
3 2
Mo/NPC with MoO and MoO references and find that the
single site is predominantly +6 (Figure S7). To figure out the
component ratio of Mo6+ to Mo , which will be further verified by
X-ray photoelectron spectroscopy (XPS) results, we fit the SA-
4+
Mo/NPC XANES spectrum using the spectra of MoO
3
and MoO
2
6
+
references. The best fit gives about 82.5% of Mo and 17.5% of
4
+
2
Mo . The Fourier transform (FT) k -weighted extended X-ray
absorption fine structure (EXAFS) spectra of SA-Mo/NPC and
the references are shown in Figure 2b. The strongest peak of
SA-Mo/NPC is located at ~1.2 Å, which is associated with the
Transmission electron microscopy (TEM) and scanning
electron microscopy (SEM) characterizations show that SA-
Mo/NPC has 3D interconnected carbon frameworks with
randomly opened porous structure (Figures 1b and S1).
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Mo–C or Mo–N contribution,[31] and thus its existence indicates
3 3
The corresponding NH concentrations, FEs and NH yield rates
that the Mo atoms in SA-Mo/NPC are mainly bonded with C or N. were measured and obtained using the Nessler's test (see more
Meanwhile, the peak corresponding to the Mo-Mo signal at ~2.6
Å is very weak and much lower than the Mo–C/N peak,
manifesting that most of the Mo atoms are isolated from each
other. The above results are in a good agreement with Figures
details in the Experimental section and Figures S10 and S11,
which show that the values of FEs and NH yield rates are
derived from NH
concentrations and reaction times).[9,40] The
highest yield rate and its corresponding NH concentration and
FE of SA-Mo/NPC were achieved at –0.3 V vs RHE (Figure 3b),
3
3
3
1d and S6, which show that only a slight fraction of the Mo
–1
–1
atoms are close to each other and their positions are all
disordered. Therefore, it is rationally concluded that the Mo
atoms in SA-Mo/NPC with the Mo loading of 9.54 wt %, most are
in the form of single Mo atoms bonded with C or N atoms, and a
slight fraction have weak interactions with neighboring Mo atoms
and thus are in the form of clusters. XPS measurement was
further performed to investigate the surface composition and
valence states of SA-Mo/NPC. Figure 2c confirms the
which reached 34.0 ± 3.6 μgNH3
h
mgcat.
(corresponding to
34.0 ± 3.6 μgNH3 h–1 cm ; see Figure S11 and its caption for
details), 50.0 ± 5.3 μM at 1 h and 14.6 ± 1.6 %, respectively,
comparable to those of the best reported NRR catalysts under
–2
3
similar conditions (Table S1). Further, the NH concentration at –
0.3 V vs RHE was also measured by another independent
method, ion chromatography (see details in the Experimental
section), which directly gave the concentration value to be 47.6
μM at 1 h. Within error bars, this value equals the one (50.0 ±
5.3 μM) obtained by the Nessler's test, indicating the validity of
our experiments. Meanwhile, the high selectivity of SA-Mo/NPC
coexistence of Mo and Mo6+ [ref. 35] with 74% Mo and 26%
4
+
6+
4
+
of Mo , not far from the XANES result. Comparing the XPS
spetra of SA-Mo/NPC with the Mo loading of 9.54 wt % (Figures
1
d and S6) and the control sample containing clusters with the
for NH
pointed out that if all this extra increased reduction current is
used for the NRR under the N -saturated environment (Figure
3a), the NH yield rate at –0.3 V vs. RHE is calculated to be
119.6 μgNH3 h–1 mgcat.–1, higher than the actual value (34.0 ± 3.6
3
was verified, as no hydrazine was detected. It should be
Mo loading of 13.40 wt% (Figure S8), we find that the valence
state of Mo in SA-Mo/NPC is the coexistence of Mo4+ and Mo6+,
while Mo3+ appears in the control sample (Figure S9).[36] Thus,
the absence of the characteristic peaks dervied from Mo clusters
in SA-Mo/NPC reinforces that the quantity of Mo clusters in this
sample is ignorable. In the XPS spectrum of N 1s, the peaks at
2
3
–
1
–1
μgNH3
h
mgcat. ). This difference between the theoretical and
the actual ones is mainly ascribed to the coexistence of
competitive hydrogen evolution. Additionally, when N was
replaced with Ar under electrolysis, no NH was detected,
confirming that the bubbled N is the raw source of NH (see
Figures S12, S13 and their captions for details). Ammonia
contaminations in N , electrolytes and the catalyst are also
401.1, 400.1 and 398.5 eV indicate the presence of pyrrolic N,
2
[
37]
graphitic N and pyridinic N,
while the peak at 396.1 eV
3
corresponds to the N–Mo bonds (Figure 2d).[38] Additionally,
Figure 2e shows the formation of C–Mo bonds,[39] suggesting the
strong electronic coupling at the interfaces between Mo atoms
and the carbon support. Based on the above results, 3D N-
doped porous carbon frameworks bearing abundant atomically
dispersed Mo atoms are well confirmed.
2
3
2
excluded (see Figures S14–S16 and their captions for
details).[
41-43]
Further, 1H nuclear magnetic resonance (NMR)
detection with isotope-labelled 15N
(ref. 44) confirms that the
2
ammonia detected in the NRR experiments was produced from
the NRR (see Figure S17 and its caption for details).
To explore the effect of the Mo loading in the NRR,
potentiostatic tests for SA-Mo/NPC samples with different Mo
loadings were conducted under –0.3 V vs RHE. As shown in
Figure 3c, when the Mo content was changed from 0 to 13.40
wt%, the NH yield rate gradually increased at first, reached the
3
top point at 9.54 wt% and then decreased. This optimization
process suggests the following mechanism: The previous DFT
calculation in 2017 has proven that Mo-N sites are active for the
NRR,[ because they can activate N
34]
molecules and stabilize
species, which is an NRR
2
2 2
N H while destabilizing NH
improvement effect. Hence, it is rational to conclude that if the
Mo content in an SA-Mo/NPC sample is too low, the
improvement effect should be very slight. Thus, increasing the
Mo content, namely the density of the atomically dispersed
active sites, can make this effect more obvious. But, when the
Mo content is too large, we have found that Mo nanoclusters
appear, which decrease the amount of the Mo-N sites and thus
worsen the effect (see Figures S8 and S18 for details). This
worsening also indicates the core role of atomically dispersed
Mo atoms bonded to N atoms in the NRR electrocatalysis.
Figure 3. NRR electrochemical performances of SA-Mo/NPC in 0.1 M KOH.
(
a) Linear sweep voltammetric curves in an N
solution and an Ar-saturated (black line). j is current density. (b) NH
red) and FE (blue) at each given potential. (c) NH yield rates of SA-Mo/NPC
2
-saturated (red line) KOH
3
yield rate
(
3
samples with different Mo loadings. (d) Chronoamperometric curve and FE
stability. The samples corresponding to (a, b and d) have the Mo loading of
9.54 wt%. Each error bar represents
a standard deviation from six
measurements.
To verify the possible effect of the coexistence of Mo–C
moieties (Figure 2e) on the NRR activity of SA-Mo/NPC, we also
investigated the catalytic ability of Mo nanoclusters anchored on
carbon (NC-Mo/C) for comparison (see Figures S19 and S20 for
All NRR electrochemical tests were performed at room
temperature and atmospheric pressure. In an Ar-saturated KOH
solution (Figure 3a), the increase in the current density (j) after –
0
.25 V vs RHE was caused by the HER, which competes with
the NRR. By contrast, a clearly increased reduction current
density was observed in an N -saturated KOH, suggesting that
the catalytic reduction of N to NH is taking place in this system.
3
details). The highest NH yield rate and corresponding FE were
measured to be 1.6 ± 0.2 μgNH3 h–1 mgcat. and 2.1 ± 0.3 %,
–1
2
respectively, at –0.45 V vs RHE (Figure S21), suggesting that
Mo–C can catalyze the NRR to NH . This finding resembles the
3
2
3
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Angewandte Chemie International Edition
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COMMUNICATION
observed activity on the Mo
2
C nanodots embedded in carbon
Keywords: N reduction • electrocatalysis • single-atom
2
nanosheets.[19] Besides the NRR activity, the stability is another
critical factor to evaluate an electrocatalyst. As depicted in
Figure 3d, SA-Mo/NPC with the optimal Mo loading of 9.54 wt%
did not show any obvious decay in the current density during an
catalyst• molybdenum
[
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NRR process as long as 50,000 s, and its FE for producing NH
was always no smaller than 14.4 %, ranging from 14.4 % to
6.8 %, indicating its superior electrocatalytic stability. This
3
[
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J. G. Chen, R. M. Crooks, L. C. Seefeldt, K. L. Bren, R. M. Bullock, M.
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finding agrees well with the morphology observation (Figure
S22) and valence state analysis (Figure S23) of SA-Mo/NPC,
which shows the persistence of single Mo atoms.
Importantly, SA-Mo/NPC also displayed a good NRR
performance in acidic media (0.1 M HCl). As shown in Figure
S24a, an optimum FE of 6.8 ± 0.3% was achieved at –0.25 V vs
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rate were 46.3 ± 1.8 μM and 31.5 ± 1.2 μgNH3
3
concentration at 1h and NH
3
yield
–
1
–1
h
mgcat.
corresponding to 31.5 ± 1.2 μgNH3 h–1 cm ), respectively
Figures S25 and S26). These values were measured by the
–2
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error bars, also indicating the validity of our experiments. Again,
in 0.1 M HCl, SA-Mo/NPC demonstrated a remarkable durability
[
[
[
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Figure S24b), which shows that no obvious decays existed in
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In conclusion, we have demonstrated experimentally for the
first time that the electrocatalytic NRR is indeed feasible at
ambient conditions by using atomically dispersed non-precious-
metal-based active sites as heterogeneous electrocatalysts.
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high density of single Mo atoms anchored on N-doped porous
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Together with previously reported studies,[44-47] our work
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Angewandte Chemie International Edition
10.1002/anie.201811728
COMMUNICATION
COMMUNICATION
†
†
†
Lili Han, Xijun Liu, Jinping Chen,
Ruoqian Lin, Haoxuan Liu, Fang Lü,
Seongmin Bak, Zhixiu Liang, Shunzheng
Zhao, Eli Stavitski, Jun Luo,* Radoslav
R. Adzic,* and Huolin Xin*
Page No. – Page No.
Atomically dispersed Mo catalysts for
high-efficiency ambient N fixation
2
Single Mo atoms anchored on N-doped porous carbon were designed and synthesized for the
electrocatalytic reduction of N to NH . This catalyst exhibited high electrocatalytic activity and high
2
3
stability. This performance can be attributed to its structure with the conductive carbon support, high
porosity and existence of single-atom Mo.
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