Boosting Selective Nitrogen Reduction via Geometric Coordination Engineering on Single-Tungsten-Atom Catalysts

Article abstract of DOI:10.1002/adma.202100429

Atomic interface regulation that can efficiently optimize the performance of single-atom catalysts (SACs) is a worthwhile research topic. The challenge lies in deeply understanding the structure–properties correlation based on control of the coordination chemistry of individual atoms. Herein, a new kind of W SACs with oxygen and nitrogen coordination (W-NO/NC) and a high metal loading over 10 wt% is facilely prepared by introducing an oxygen-bridged [WO₄] tetrahedron. The local structure and coordination environment of the W SACs are confirmed by high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure. The catalyst shows excellent selectivity and activity for the electrochemical nitrogen reduction reaction (NRR). Density functional theory calculation reveals that unique electronic structures of the N and O dual-coordinated W sites optimize the binding energy of the NRR intermediate, resulting in facilitating the electrocatalytic NRR. This work opens an avenue toward exploring the correlation between coordination structure and properties.

Full text of DOI:10.1002/adma.202100429

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Boosting Selective Nitrogen Reduction via Geometric
Coordination Engineering on Single-Tungsten-Atom Catalysts
Yu Gu, Baojuan Xi, Wenzhi Tian, Hua Zhang, Qiang Fu, and Shenglin Xiong*
selective for NRR.[14,15] For CO2 reduction
Atomic interface regulation that can efficiently optimize the performance of
single-atom catalysts (SACs) is a worthwhile research topic. The challenge
lies in deeply understanding the structure–properties correlation based on
control of the coordination chemistry of individual atoms. Herein, a new kind
of W SACs with oxygen and nitrogen coordination (W-NO/NC) and a high
metal loading over 10 wt% is facilely prepared by introducing an oxygen-
bridged [WO4] tetrahedron. The local structure and coordination environ-
ment of the W SACs are confirmed by high-angle annular dark-field scanning
transmission electron microscopy, X-ray photoelectron spectroscopy, and
extended X-ray absorption fine structure. The catalyst shows excellent selec-
tivity and activity for the electrochemical nitrogen reduction reaction (NRR).
Density functional theory calculation reveals that unique electronic structures
of the N and O dual-coordinated W sites optimize the binding energy of the
NRR intermediate, resulting in facilitating the electrocatalytic NRR. This work
opens an avenue toward exploring the correlation between coordination
structure and properties.
reaction, SACs usually promote the reduc-
tion of CO2 to CO by two-electron reac-
tion pathway,[16] while cobalt single atoms
immobilized by phthalocyanine ligands is
more inclined to generate methanol with
a Faraday efficiency as high as 40%.[17]
Therefore, there is an urgent need to
explore new catalytically active configura-
tions for inducing superior activities to
achieve distinctive electrocatalysis perfor-
mance.[18] However, due to the limitation
of synthetic approaches and the immatu-
rity of atomic-scale control technology, it is
difficult to engineer the targeted SACs and
adjustable structure.[19] Besides, difficulties
in judging the accurate coordination envi-
ronment and active sites is not conducive
to rationalize structure and performance
relationship.[6] Moreover, isolated metal
atoms tend to aggregate during high-
temperature treatment. The challenge is
posed on synthesizing high load (>10 wt%) SA electrocatalysts
with large-scale production.[8,19] Therefore, the necessary break-
through in SAC-based electrocatalysis is controlling the coor-
dination chemistry of metal single atoms, especially with high
metal loading, and in-depth insight into the structure–property
relationships.[20,21]
1. Introduction
Owing to the maximized atomic utilization and unique
quantum size effects, single-atom catalysts (SACs) have been
attracting intensive interests in the field of electrocatalysis.[1−4]
Meanwhile, the geometry and local electronic structures of
SACs can be regulated by changing the metal atoms (Pt, Mn,
Ni, Mo, etc.), adjusting the coordination anions (C, N, O, S,
etc.) and coordination numbers, which significantly improves
catalytic property of active centers and enables flexible reac-
tion tenability.[5−11] The adsorption of N2 in the N2 reduction
reaction (NRR) and the adsorption of H in the hydrogen evo-
lution reaction (HER) on catalytic active sites are competitive.
The atomically dispersed MoN1C2 catalyst possessed excel-
lent activity for HER,[12,13] while MoN3 is reported to be highly
The electrocatalytic NRR is a sustainable alternative to the
energy-intensive Haber-Bosch ammonia production process.[22]
The kinetic dilemma of NRR is derived from the antagonistic
relationship between N2 activation and NHx intermediates
(x = 0 to 2) transformation. According to previous theoretical
investigations, in a series of pure metal catalysts, the W atoms
have high NRR activity and selectivity for N2 fixation.[10,23]
Chen et al. theoretically studied a series of single metal atoms
on g-C3N4 substrate and found that W@C3N4 exhibit the out-
standing NRR catalytic activity with a low limiting potential,
and can well suppress the competing HER.[24] Du et al. reported
that the excellent NRR catalytic activity of W atoms anchored
on N-doped graphene is derived from the inherent proper-
ties of W atoms, such as significant positive charge, large spin
moment, moderate binding energy strength of NRR intermedi-
Dr. Y. Gu, Prof. B. J. Xi, Dr. W. Z. Tian, H. Zhang, Prof. Q. Fu,
Prof. S. L. Xiong
Key Laboratory of Colloid and Interface Chemistry
Ministry of Education
School of Chemistry and Chemical Engineering
State Key Laboratory of Crystal Materials
Shandong University
ates.[23] However, W atom catalysts can strongly activate N2, but
[10]
impede the conversion of NHx intermediates.
To the best of
Jinan 250100, P. R. China
E-mail: chexsl@sdu.edu.cn
our knowledge, there have been no reports to experimentally
investigate the application of single-atom W in the NRR. So,
much effort should be made to identify the active sites and
overcome the scaling relationship by exploring diverse coor-
dination structures of single-atom W. The introduction of O
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202100429.
DOI: 10.1002/adma.202100429
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
coordination weakens the W N bond strength, thereby opti-
mizing the binding energy of the adsorbed intermediate, and
reconciles the contradictory relationship between N2 activation
and intermediate transformation.
the source of metal in the catalyst which can introduce metal–
O sites and inhibit agglomeration of metal atoms during the
pyrolysis process. Due to its many secondary and tertiary amine
groups that can effectively chelate a large amount of metal ions
(Figure S1, Supporting Information),[26] the MFR is selected to
coordinate with W precursors to form a coordination polymer
named as W-MFR. The obtained W-coordinated polymer was
subsequently assembled with graphene oxide (GO) template via
intermolecular interactions,[27] and then underwent carboniza-
tion at 600 °C for 2 h under vacuum to afford the final high-
loading W single atoms coordinated by N/O and supported on
nitrogen-doped carbon (denoted as W-NO/NC).
As exhibited in Raman spectra (Figure S2, Supporting Infor-
mation), the ID/IG value increased from 1.13 for NC to 1.26 for
W-NO/NC, which indicates that the incorporation of W atoms
can arouse more intrinsic disorders in W-NO/NC. Further-
more, the Brunauer–Emmett–Teller surface areas of W-NO/
NC and NC are 126 and 76 m2 g−1, respectively (Figure S3a,c,
Supporting Information), and the main pore size is centered
at about 3.5 nm (Figure S3b,d, Supporting Information).
These results suggest that W atoms can effectively inhibit the
agglomeration between graphene sheets and MFR, and gen-
erate a considerable amount of voids, which provide abundant
In this work, we successfully prepared a new kind of W
SACs using Na2WO4 containing oxygen-bridged [WO4] tetra-
hedra[25] as the metal source by the combination of coordina-
tion polymer strategy and self-templating method. Melamine–
formaldehyde resin (MFR) is used to adsorb metal ions to fab-
ricate W atomically dispersed coordination polymer, which was
subsequently assembled with a graphene oxide (GO) matrix
via intermolecular interactions. Due to the combined resin-
chelating and self-templating method, the metal loading can
be higher than 10 wt%. The W-NO/NC configuration of O,N-
coordinated W single atoms is distinct from the reported M–N
configuration reported by traditional methods.
2. Results and Discussions
Synthesis of the high-loading W SACs is illustrated in Figure 1a
(see details in the Supporting Information). Specifically,
oxygen-containing metal salts with separated units are used as
Figure 1. a) Illustration of the formation of W-NO/NC. b) TEM image. c,d) Aberration-corrected HAADF-STEM images. e) HAADF-STEM image and the
corresponding EDX element mappings showing the distribution of C (red), O (blue), N (green), and W (yellow). Scale bars: b) 25 nm; c) 5 nm; d) 2 nm.
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coordination sites for the uniform distribution of W single
atoms. The X-ray diffraction (XRD) patterns (Figure S4, Sup-
porting Information) of W-NO/NC and NC both indicate two
broad diffraction peaks attributable to the (002) and (101) planes
of graphitic carbon. Notably, no diffraction peaks corresponding
to metallic W or its oxide crystal structures can be detected.
As can be seen from transmission electron microscopy (TEM;
Figure 1b; Figure S5, Supporting Information), the W-NO/NC
shows only some wrinkled graphene nanosheets and no nano-
particles are found in the selected detection range. High-angle
annular dark-field scanning TEM (HAADF-STEM) was applied
to investigate the existence forms of W atoms. Numerous bright
dots could be distinctly observed from the aberration-corrected
HAADF-STEM image with atomic resolution in Figure 1c,d,
indicating isolated tungsten atoms. The representative energy-
dispersive X-ray spectrum (EDX) elemental mappings of the
W-NO/NC sample in Figure 1e further confirm the uniform
spatial distribution of W atoms over the entire substrate along
with N, C, and O, coinciding with the X-ray photoelectron spec-
troscopy (XPS) results (Figure S6, Supporting Information).
The W content was about 10.2 wt% as determined by induc-
tively coupled plasma mass spectrometry (ICP-MS).
To further reveal the electronic structure evolution and the
local coordination chemistry of W-NO/NC, High-resolution
XPS and X-ray absorption fine structure (XAFS) analyses were
carried out. As shown in Figure 2a, the W 4f XPS spectrum of
W-NO/NC exhibits two peaks at 35.4 and 37.5 eV ascribed to the
[28]
W 4f5/2 and W 4f7/2
.
For comparison, W-N/NG sample was
synthesized via the similar preparation method except using
a metal-cation salt (WCl6) as the metal source (Figures S7–S9,
Supporting Information). The above two bands of W 4f have
some shift toward high binding energy in W-NO/NC compared
to W-N/NC, representing the higher oxidation state of W atoms
in W-NO/NC (Figure S10a, Supporting Information). The subtle
Figure 2. a–c) High-resolution W 4f (a), O 1s (b), and N 1s (c) XPS spectra of W-NO/NC and NC. d) XANES and e) k3-weighted FT-EXAFS curves of the
W-NO/NC at W L3-edge; f) WT-EXAFS of the W-NO/NC and the references; and g) k3-weighted FT-EXAFS fitting curves of the W-NO/NC at W L3-edge.
h) Atomic structure model of the W-NO/NC.
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displacement reflects the altering of local electronic structure
for W-NO/NC system. It is notable that in the high-resolution
in W-N2O2 with greater electronegativity enhances the locali-
zation around of W (Figure 3c). However, for W-N4, the nega-
tive charge is shared by the coordinated nitrogen atoms, thus
impeding charge occupy the anti-bounding orbitals of N2 mole-
cule. Therefore, W atom in W-N2O2 is more conducive to polar-
ization and activation of N2 molecules, and facilitate to trigger
the subsequent reduction reactions by a proton.
O 1s spectrum of W-NO/NC, an obvious peak corresponding
[25]

to W O bond is observed, which is not found in NC and
W-N/NC (Figure 2b and Figure S10b, Supporting Information).
Obviously, the metal atoms in W-N/NC are anchored in NC by
N atoms, while the W atoms in W-NO/NC has two coordina-


tions: W O and W N. Usually, the metal atoms are dispersed
on N-doped carbon substrate in form of metal–N–C. Tungsten
oxides tend to be converted into the corresponding nitrides
in NH3 atmosphere at high temperature.[29] As is well known,
with regard to the tungstate member, the structure of isolated
tungsten oxide remains extremely unstable; then the weaker
We then calculated catalyzed NRR pathways on W-N2O2 and
W-N4 models (Figures S15 and S16, Supporting Information).
The distal pathway would be more favorable because of higher
energy barriers (1.59 eV) of the alternative pathway (Figure S17,
Supporting Information). Therefore, we focus on the distal
pathway of W SAC (Figure 3d,e). Notably, since the free
energy level of *NNH is low, the first protonation step is not
the potential determination step (PDS) of W SAC. Instead, the
last hydrogenation step (*NH2 + H+ + e− → NH3(g)) becomes
PDS, which is consistent with previous theoretical studies for
W SAC.[10,23,24] Compared to W-N4 (1.32 eV), W-N2O2 showed

W O bonds cannot well survive during the high temperature
pyrolysis compared with W N bonds (Figure S11, Supporting

Information).[25,30] The single W atoms were partially reduced
by the generated ammonia during the pyrolysis of W-MFR
precursor and lost the binding O atoms.[31] W-NO/NC shows a
negligible difference in the XPS spectra of N and C from those
of the NC sample (Figure 2c and Figure S12, Supporting Infor-
mation), indicating the introduction of metal barely affects the
status of those elements.
the lower ∆G*NH (1.11 eV). It should be noted that though
3
the desorption of *NH3 is endothermic, the protonation of
+
*NH3 would form NH4 in solution, which is expected to be a
favorable step.[34] The HER is the major challenge for the NRR,
which can seriously reduce the Faradaic efficiency (FE) of NRR.
The difference between the limiting potentials for NRR and
HER is a descriptor for assessing the catalytic selectivity of W
SAC (Figure 3f). Compared with W-N4, although the UL(HER)
of W-N2O2 is slightly smaller (Figure S18, Supporting Informa-
tion), it has more positive [UL(NRR) − UL(HER)] values, which
demonstrate the higher selectivity toward NRR. The projected
density of states (PDOS) show that the 2π orbital near the
Fermi level of the N2 molecule is strongly split and obvious
overlap with the W d orbital (Figures S19 and S20, Supporting
Information). The powerful d–2π* coupling can activate the N2
molecule to be of radical nature, which is active for hydrogena-
tion. The nature of bonding between W and N can be further
understood by projected crystal orbital Hamilton populations
(pCOHP) analyses (Figure 3g,h). The bonding orbitals of W–N
in W-N2O2 (ICOHP: −5.31) are less occupied than in W-N4
(ICOHP: −5.89), indicating that the interaction between N and
W is weaker in W-N2O2. Therefore, W-N2O2 has weaker adsorp-
tion of *N intermediates (Figure 3i), which is more conducive
to the release of the second ammonia molecule than W-N4.
These further confirmed that the introduction of O coordina-
tion can enhance the activity and selectivity on the W active site.
Electrochemical measurements of W-NO/NC were con-
ducted in 0.5 m LiClO4 by a H-type two-compartment cell.
Before each NRR test, the feeder gas purification system was
employed to purify the N2 (14N2 or 15N2) and Ar gas to elimi-
nate the interference of impurities. All experiments were car-
ried out in strict accordance with the recommended protocol.[35]
The controlled experiments and isotope measurements were
performed to ensure that the N source originates. The yielded
NH3 and possible by-product (N2H4) were detected by the indo-
phenol blue method and Watt–Chrisp method.[36,37] The cor-
responding calibration curves are shown in Figures S21–S23
(Supporting Information). The current density of the LSV curve
under N2 is higher than under Ar, indicating that W-NO/NC
are active for NRR catalysis (Figure 4a). Figure S24 (Supporting
Information) shows time-dependent current density curves of
In Figure 2d, the XANES spectrum of W L3-edge revealed
that the white-line intensity of the W-NO/NC is situated
between those of W foil and WO2, indicating a cationic environ-
ment. The peak around 10,210 eV is due to the electronic tran-
sition from 2p3/2 to unoccupied 5d orbital.[32,33] More detailed
local coordination chemistry on W sites in W-NO/NC can be
obtained from extended XAFS (EXAFS) analysis. As displayed
in the FT-EXAFS spectra of W L3-edge (Figure 2e), W-NO/
NC possessed only one sharp peak located at 1.31 Å assigned
to the W-O/N coordination. Additionally, no W–W peak can
be detected for W-NO/NC, confirming the atomic dispersion
of single W sites, in line with the STEM observation. Wavelet
transform (WT) EXAFS was further conducted because of the
high sensitivity in both k and R ranges. The WT contour plots
of W-NO/NC exhibited one intensity peak at 5.2 Å−1, which is
close to that in WO3 and the signals derived from W-W con-
tribution are invisible (Figure 2f). Therefore, the W species in
W-NO/NC were confirmed to be isolated atomically distributed.
EXAFS fittings (Figure 2g, Figures S13 and S14, Supporting
Information) and the fitted parameters (Table S1, Supporting
Information) display that the isolated W atoms were coor-
dinated with two O atoms and two N atoms (Figure 2h). The
average bond lengths for W–N and W–O were 1.86 and 1.77 Å,
respectively. Here, it should be noted that the EXAFS analysis
obtained is the average coordination result. Other WNxOy with
slightly different coordination exists in the material inevitably,
due to the existence of disorder, edge and in-plane sites.
To reveal the relationship between NRR activity and local
coordination structure, we evaluate these two structures of
single-atom W sites from density functional theory (DFT) cal-
culations. The bond lengths of the N2 molecules adsorbed on
W-N2O2 and W-N4 are prolonged from 1.120 Å (molecular N2) to
1.158 and 1.151 Å (Figure 3a). The Bard charge analysis exhibits
that more electrons are transferred from W in W-N2O2 (0.50 e)
to adsorbed N2 molecules than W in W-N4 (0.47 e), while losing
fewer electrons than W in W-N4 (Figure 3b). The corresponding
charge density difference shows that the coordination atom O
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Figure 3. a) Adsorption configuration of an N2 molecule on W-N2O2 and W-N4 and b) corresponding Bader charge analysis and c) charge density
differences (cyan represents holes and yellow represents electrons). d,e) Free energy diagrams of W-N2O2(d) and W-N4 (e). f) Liming potential (UL)
versus [UL(NRR)−UL(HER)] of W-N2O2 and W-N4, in which the red dashed line indicates the corresponding value of the metal-based benchmark.
g,h) Projected density of states (PDOS) and projected crystal orbital Hamilton populations (COHP) of W-N2O2(g) and W-N4 (h). i) Illustration of the
correlation between integrated COHP (ICOHP) and the ∆G*N
.
W-NO/NC at various potentials for 2 h. Related UV–vis absorp-
tion spectra of the electrolytes are shown in Figures S25 and
S26 (Supporting Information), suggesting NRR process occurs
at potential ranges from −0.60 to −0.85 V. The corresponding
NH3 yields and Faradaic efficiency (FE) under these poten-
tials are plotted in Figure 4b. As observed, W-NO/NC offers a
activity (Figure 4c). Of note, NH3 is the only product without
the presence of N2H4 in the final product (Figure S27, Sup-
porting Information), implying W-NO/NC has excellent selec-
tivity for NRR.[38] Additionally, the isotope-labeling experiments
+
were carried out, and 15NH4 with the characteristic double
peaks was detectable in 15N2-saturated electrolyte (Figure 4d).
These results confirm that the detected NH3 was really pro-
duced from the electrochemical reduction of N2, rather than
maximum NH3 yield of 12.62 µg h−1 mg−1 and a highest FE
cat
of 8.35% at −0.70 V, superior to some single−atom catalysts
(Table S3, Supporting Information) and reported NRR electro-
catalysts in aqueous electrolytes (Table S4, Supporting Informa-
tion). Furthermore, W-NO/NC exhibited a far higher selectivity
and NH3 yield than that of W-N/NC and NC, indicating that
metal sites with N and O coordination are the main source of
−
from NH3 or NO3 contaminants. We also examined the sta-
bility of W-NO/NC, NH3 yields and FEs remained essentially
stable during recycling tests for five times (Figure 4e). The 24 h
electrochemical stability test of W-NO/NC at −0.70 V shows only
a slight decrease in current density (Figure S28, Supporting
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Figure 4. a) LSV curves of W-NO/NC in Ar- and N2-saturated 0.5 m LiClO4 with a scan rate of 2 mV s−1. b) Corresponding NH3 yields and FEs for
+
W-NO/NC at all potentials. c) Amount of NH3 generated over W-NO/NC, W-N/NC, and NC at −0.70 V. d) 1H NMR spectra of both 14NH4+ and 15NH4
produced from the NRR tests using 14N2 or 15N2 as N2 source. e) NH3 yields and FEs at −0.70 V during recycling test for five times. f) NH3 yields and
FEs at −0.70 V for 2 h over initial and post-NRR W-NO/NC subjected to 24 h operation.
Information). After 24 h of stability test of W-NO/NC, it was
found that the yield of NH3 and FE did not decrease signifi-
cantly (Figure 4f). Importantly, the control experiments were
performed in N2-saturated solution under open-circuit poten-
Acknowledgements
The authors gratefully acknowledge the financial supports provided
by the National Natural Science Foundation of China (Nos. 21871164,
21971145), the Taishan Scholar Project Foundation of Shandong Province
tial and Ar-saturated solution at −0.70 V; no NH3 was detected
(Figure S29, Supporting Information). In the NRR alternating
cycles test, NH3 is only generated in the N2-saturated solution
(Figure S30, Supporting Information).
(ts20190908), the Natural Science Foundation of Shandong Province
(ZR2019MB024), and Young Scholars Program of Shandong University
(2017WLJH15).
Conflict of Interest
The authors declare no conflict of interest.
3. Conclusion
We report here a new kind of W SACs with a unique O,N
coordination for elevating the NRR activity and depressing the
HER activity simultaneously. A high metal loading more than
10 wt% was achieved due to the combined resin-chelating and
self-template method. The isolation and local environment
were identified by HAADF-STEM and EXAFS characterizations
at the atomic level. The combination of electrochemical tests
and theoretical calculations confirm the W single atoms were
the activity origin of the highly NRR activity, and reveal the
critical role of the specific local coordination to influence the
adsorption behavior and reaction selectivity. This work poten-
tially offers a versatile synthesis approach for well-defined and
high-metal-loading SACs, and systematically investigates the
influence of local coordination environments on electrocatalytic
activity and selectivity.
Data Availability Statement
Research data are not shared.
Keywords
coordination chemistry, high loading, nitrogen reduction reaction,
single-atom catalysts, single tungsten atoms
Received: January 18, 2021
Revised: March 23, 2021
Published online:
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