In situ nano Au triggered by a metal boron organic polymer: Efficient electrochemical N2 fixation to NH3 under ambient conditions

Article abstract of DOI:10.1039/c9ta06573c

Ammonia has been obtained from the Haber-Bosch process for decades, which requires an unendurably high temperature and high pressure. Alternatively, electrochemical conversion of N₂ to NH₃ turns out to be a promising method with much lower energy consumption and accessible reaction conditions. In this work, a novel nano-gold catalyst supported on a boron organic polymer (Au/M-BOP) was prepared and achieved both excellent ammonia yield and faradaic efficiency at low potential (r(NH₃): 45.54 μg h^-1 cm^-2 or 75.89 μg h^-1 mg_cat.^-1 FE: 10.35%), results that are close to the top of the pyramid compared to published literature results. Moreover, the catalyst is conveniently accessible through a pre-immobilized in situ reduction, and can maintain its electrocatalytic activity for multiple uses, which could have potential implications in the industrialization of the electrocatalytic production of ammonia.

Full text of DOI:10.1039/c9ta06573c

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Number 12
March 2018
Pages 4883-5230
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J Po lue r an sa el od foMna ot et rai ad l js u Cs ht emm ai rs gt ri yn sA
View Article Online
DOI: 10.1039/C9TA06573C
ARTICLE
In-situ Nano Au Triggered via Metal Boron Organic Polymers:
Efficient Electrochemistry N Fixation to NH under Ambient
Conditions
2
3
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
b
,a,c
DOI: 10.1039/x0xx00000x
Xue Zhao , Chen Yao , Hao Chen ,Yunfan Fu , Changjun Xiang , Suhang He , Xiaohai Zhou* and
Haibo Zhang*,
a, d
Ammonia has been obtained from Haber-Bosch process for decades, which required unendurable high temperature and
high pressure. Alternatively, electrochemical conversion of N
2
to NH turned out to be a promising method with much
3
lower energy consumption and accessible reaction conditions. In this work, a novel nano-gold catalyst supported on boron
organic polymers (Au/M-BOPs) was prepared and has achieved both excellent ammonia yield and Faraday Efficiency at low
-
1
-2
-1
-1
potentials (r(NH ): 45.54 ug·h ·cm or 75.89 ug·h ·mgcat. FE: 10.35 %), was close to the top of the pyramid in the
3
published literature results. Moreover, the catalyst is conveniently accessible through a pre-immobilized in situ reduction,
and has its electrocatalytic activity maintained for multiple uses, which could have potential implications in the
industrialization of electrocatalytic production of ammonia.
2
mention that the starting reactant H is mainly from the
Introduction
Ammonia is ranked as one of the most important substances
that had involved in many aspects of human production
activities. Most of human produced ammonia is generally
reforming of fossil fuels. This process is not only accompanied
by a large amount of energy consumption, but also generates
a large amount of carbon dioxide to aggravate the Green
House Effect.16-20 Therefore, to develop a feasible and
environmental friendly methodology that can replace the
traditional Haber-Bosch process is not only an urgent task but
also a formidable challenge to chemists.21-23
So far many efforts have been made in the areas of
biomimetic nitrogenase, molecular catalysis, photo-
electrochemistry and electrochemistry.24-30 While running
equipment of bionic nitrogenase and molecular catalysis are
1
preferred to be consumed in agriculture as fertilizer. The
others were either participated in chemical reactions for
functionalization, or in pharmaceutical, textile, printing, dyeing
and other industries.2-6 Nitrogen (N
the air, which is a widely and readily available resource for
ammonia production. However, due to the high stability of
2
) contributes up to 78% of
-1
N
to NH
≡
N bond (bond energy: 940.95 KJ·mol ), the conversion of N
2
must overcome large energy barriers.7
-10
accompanied with massive energy consumption, the NH
3
3
Up to now,
conversion is still very low, so it is far from promising to
achieve large-scale production. On the contrary, the up-and-
coming electrochemistry nitrogen reduction reaction (NRR)
with low cost and environmentally friendly has shown the
industrial ammonia production still relies on the energy-
intensive Haber-Bosch process, where NH is primarily
generated under co-activation of N and H in the presence of
catalyst.11 To achieve this, the Haber-Bosch process requires
high temperature (350-550 ) and very high pressure (150-
50 atm.) to drive the equation N +3H 2NH . The energy
consumption could be up to 600 KJ·molNH3-1 12-15
not to
3
2
2
potential in industrial production of NH
The volcanic map of the theoretical limit potential and the
adsorption energy of N on different metal surfaces indicate
3
by many reports.31-34
℃
3
2
2
→
3
2
,
that transition metal as electrocatalyst can effectively enhance
the catalytic activity of NRR.19, 35, 36 Although the Ru, Rh, Ir, Mo
and Fe metal surfaces are at the top of the volcanic map, the
theoretical calculation shows that these metals have a
stronger promoting effect on hydrogen evolution reaction
a.College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,
P. R. China.
b.
Address here Department of Life Sciences and Chemistry, Jacobs University
Bremen, Campus Ring 1, Bremen 28759, Germany.
Engineering Research Center of Organosilicon Compounds & Materials Ministry of
c.
(HER: as a competitive reaction of NRR), thus often with low
Education, Wuhan 430072, P. R. China.
National Demonstration Center for Experimental Chemistry, Wuhan University,
Wuhan 430072, P. R. China.
d.
NRR Faraday Efficiency and would cause large amount of
energy consumption.35 On the other hand, it was found that
gold (which is not at the top of volcanic map) dramatically
showed good catalytic activity in the electro-catalytic NRR
*
Corresponding author: Haibo Zhang, E-mail: haibozhang1980@gmail.com
Xiaohai Zhou, E-mail: zxh7954@whu.edu.cn
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000
x
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ARTICLE
process.2, 7, 8, 31, 37-40 Bao D. et.al39 reported that electrocatalytic
Journal Name
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reduction of N can be achieved under ambient temperature
and pressure using tetrahedral gold nanorods as catalysts.
2
DOI: 10.1039/C9TA06573C
2
-1
-1
Wang Z. successfully achieved a yield of 25.57 ug·h ·mgcat.
8
with gold nanoflowers as NRR electrocatalysts. Li S-J. obtained
higher NRR catalytic activity by immobilizing Au on CeOx-RGO
and proved that CeOx is the critical. Through theoretical
calculations, Yao Y. 37 also found that NRR activity could be
effectively improved by the synergistic effect of the substrate.
Although theoretical and experimental studies have suggested
that Au might be the most promising catalyst for NRR,
Scheme 1 Synthesis route of Au/M-BOPs and electrochemical NRR experimental
operation
Materials preparation
however, it is still arduous to achieve both high NH
high Faraday Efficiency at the same time.
3
yield and Synthesis of M-BOPs: Solution A: 0.8148 g Cs [B H ] and
2 12 12
0.5816 g Ni(NO ) ·6H O were dissolved in 50 mL water.
3
2
2
Herein, a new metal boron organic polymers (M-BOPs) Solution B: 0.1321 g 1,10-phenanthroline (C12
8 2 2
H N ·H O) was
substrate was fabricated skillfully with substrate and reducing dissolved in 50 mL acidic aqueous solution (pH3, adjusted by
agent. Key to this approach is the usage of the dual-functional HCl(aq.)). Solution A was added dropwise into solution B under
12]2- (intrinsic reducibility and coordination), which
stirring for 30 min. The formed yellow precipitate was
separated by centrifugation, washed three times with ethanol
and dried under vacuum at 60 °C for 2 h to obtain the
substrate M-BOPs (Details of M-BOPs crystal culture cultivating
and single crystal structural analysis method can be found in
[
closo-B12H
plays an essential role of reductant in the AuNPs preparation
as well as a confinement effect for the tight immobilization of
AuNPs in the BOPs substrates. With a rapid in situ reduction of
HAuCl
material with inlaid structure (Au/M-BOPs). This material could
promote the electrochemical reduction of N to NH at
ambient conditions with high ammonia yield and high Faraday
4
in the substrate, we obtained a gold nanosphere
"Support Information", and the crystal structure field of view
was shown in Fig. S3).
Synthesis of Au/M-BOPs: 0.7401 g M-BOPs (2 mmol [B12H12] )
2
3
2-
was dispersed again in water to form a suspension, and then
-1
-2
3
Efficiency at the same time (r(NH ): 45.54 ug·h ·cm or 75.89
-1
8
4
2.0 mL of HAuCl solution (24.3 mmol·L ) was slowly added
-
1
-1
ug·h ·mgcat. FE: 10.35 %). The ammonia yield was close to the
top of the pyramid of the published literature results (shown in
Fig. 5). Its performance has surpassed most of the non-Au
nuclear NRR catalysts and a lot Au-based NRR catalysts
reported previously (listed in Table S1). It is worth mentioning
that Au/M-BOPs can work continuously for a long time and
also can be recycled for multiple times without notable
decrease of electrocatalytic activity. This in situ reduction
fabrication strategy can also extend out to design other
fascinating metal composites, such as Pt, Pd and Ag, which
would make an important step forward in the field of electro-
with vigorous stirring. After reacting of 30 min, the dark purple
solid was separated by centrifugation, then washed alternately
with water/ethanol three times and dried under vacuum at
6
0 °C for 2 h to obtain Au/M-BOPs. (Scheme 1
)
Materials Characterization
Field emission scanning electron microscopy (FE-SEM, Zeiss
SIGMA, UK) (acceleration voltage: 10.00 kV; signal acquisition
mode: InLens) and transmission electron microscopy (TEM,
HITACHI H-7000FA, Japan) were used to inspect the
microscopic morphology of Au/M-BOPs. X-ray photoelectron
spectroscopy (XPS, ESCALAB250Xi, USA), Energy dispersive x-
catalytic NRR and also show great potential for various ray spectroscopy (EDAX, Hitachi SU8100, Japan) and Fourier
catalytic reactions.
transform infrared spectroscopy (FT-IR, Thermo iS10, USA)
were used to qualitatively determine the composition of
materials. SEM image of element distribution was collected in
backscattered electron mode. Powder X-ray diffraction (XRD,
Bruker D8 Advance, USA) is used to determine the crystal
phase composition of materials. Single crystal X-ray diffraction
Experimental Sections
Chemicals and materials
2
Cesium-closo-dodecaborate (Cs [B12H12], 98%) was purchased
(SCD) was used to determine the composition of the precursor
from Strem Chemicals (Newburyport, USA). Carbon paper (CP)
was purchased from Shanghai Hesen Electric Co., Ltd. and
pretreated by ultrasonicated in water/acetone solution for
NRR experiments. Nafion solution (5 wt%) and nafion 115
proton exchange membrane were purchased from DuPont®
M-BOPs, and these data can be obtained from the Cambridge
Crystallographic Data Center via www.ccdc.cam. ac.uk/data_
request/cif (code: 1899654). The exact loading of Au in the
Au/M-BOPs material was determined by inductively coupled
plasma optical emission spectrometer (ICP-OES, Optima
Company. High purity N
2
(99.999%) and high purity Ar
5
4
300DV, USA). Ultraviolet-visible spectrometer (UV-Vis, UV-
802, Unico (Shanghai) Instrument Co., Ltd.) was used to
(99.999%) were purchased from Hubei Harvin (Group)
Chemical Co., Ltd. Other reagents were purchased from
Aladdin Chemical Reagent Co., Ltd and used as received unless
otherwise stated. Milli-Q ultrapure water (18 MΩ cm ) was
detect the absorbance of the electrolyte after indophenol blue
colorimetric.
Electrochemical measurements
-
1
•
used to prepare all solutions.
2
| J. Name., 2012, 00, 1-3
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J Po lue r an sa el od foMna ot et rai ad l js u Cs ht emm ai rs gt ri yn sA
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ARTICLE
Electrochemical NRR experiments were performed in a two- 455nm was obtained after colorimetric procVeieswsAinrtigcl.e OTnlhinee
compartment cell containing Nafion 115 proton exchange absorbance of the sample was compareDdOI:w10it.1h03t9h/eC9sTtAa0n6d5a73rCd
membrane isolation by CS 2350 electrochemical workstation curve to obtain the concentration of N H .
2 4
(Wuhan Koster). Electrochemical experiments were conducted
Preparation of the standard curve: N
2 4
H standard solutions
using a typical three-electrode system
as a counter electrode, and an Ag/AgCl electrode (filled with 0.6, 0.8 to 1.0 μg·mL , the absorbance for each solution was
，graphite rod was used were prepared with concentrations varied from 0, 0.1, 0.2, 0.4,
-1
saturated KCl, the potential is 0.197V calibrated by standard recorded at 455 nm. The obtained standard curve is y(Abs) =
2
hydrogen) was used as
a
reference electrode. The 0.10152x([N2H4]) + 0.0007 (R = 0.9995) (Fig. S2).
experimental conditions were maintained at ambient Faradaic efficiency (FE)
temperature (25 °C) and atmospheric pressure. The electro-
In our work, the Faraday Efficiency is defined as the ratio of
the amount of charge (QNH3) consumed for NH synthesis to
the total charge (Qtotal) through the electrode during
electrolysis. Synthesis of a NH molecule requires the provision
of three electrons, thus the Faraday Efficiency should be
calculated by equation (1). The rate of NH generation was
calculated with equations (2) and (3). (The unit obtained by
chemical NRR was carried out in 0.1 M HCl electrolyte solution,
and the working electrode was an Au/M-BOPs uniformly
coated CP. Working electrode was prepared as follows: 5.0 mg
Au/M-BOPs electrocatalyst was dispersed under 1 hour of
ultrasonication in 1 mL of isopropanol containing 120 mL
nafion solution (5 wt%, DuPont®) to form a uniform ink, then
3
3
3
3
0 μL ink was uniformly coated on CP (coated area of 0.5
-1
-2
equation (2) is: μg·h ·cm , and the unit obtained by equation
cm×0.5 cm) and let it naturally dried in the air. The loading of
Au/M-BOPs on the CP was 0.15 mg (approximately 0.6 mg·cm-
-1
-1
(
3) is: μg·h ·mgcat. ).
[
NH3 ]
9
65003
V
2
). The electrolyte was purged with N
2
for 30 min before test
Q
NH3
(1)
17
FE(NH ) =
=
3
to prepare N
isolation of N
2
saturated electrolyte (purge with Ar when
is required). Then the working electrode was
Qtotal
1000000 i dt

2
[
NH ]+V
t  A
3
(2)
(3)
activated by cyclic voltammetry (scanning 10 turns). Linear
sweep voltammetry (LSV) was obtained at a sweep speed of 30
r(NH ) =
3
mV/s. Potentiostatic polarization was carried out under N
2
[NH ]+V
3
r(NH ) =
3
flow (flow rate: 5 sccm), and Ar flow was used to purge anode
chamber throughout the electrocatalytic experiment to avoid
the electrolyte contaminated by air.
t  Mcat.
3
Where [NH ] represents the mass concentration of ammonia
-1
(
in μg·mL ), V is total volume of the electrolyte in cathode
Determination of NH
3
chamber (in mL), is instantaneous current of the
i
The NH content in the electrolyte was detected by indophenol
3
potentiostatic polarization test, t is total time of the
blue colorimetric method, and the absorbance detection was
realized on an UV-Vis. 2 mL electrolyte after potentiostatic
polarization (i-t test) for 2h was added to 2 mL 1 M NaOH
solution (containing 5% salicylic acid and 5% sodium citrate).
Subsequently, 1 mL NaClO solution (0.05 M) and 0.2 mL 1 w%
-2
potentiostatic polarization test, A is the effective area (in cm )
of Au/M-BOPs coated on CP, and Mcat. is the mass of Au/M-
BOPs (in mg).
sodium nitroprusside (CsFeN
to the above mixture. The mixture was kept for 2h to
sufficiently develop the color, and the UV-vis absorbance at
6
Na
2
O·2H
2
O) solution were added Results and Discussion
As the basis of Au/M-BOPs, the crystal structure of M-BOPs
was successfully resolved (Fig. S3 and Table S1). Each Ni2+ ion
is complexed in the form of six coordination with three 1, 10-o-
phenanthroline molecules, and the introduction of
6
55 nm was obtained (the electrolyte was tested three times
from color development to UV-vis spectroscopy tests, the
results were given in averages). Standard solutions of different
concentration gradients (converted to NH
2-
12
hydrophobic closo-[B12H ] can greatly enhance the stability
of the structure while balancing the positive charges. In the FT-
3
concentration
gradients of 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.6 and 2.0
IR spectra of Au/M-BOPs and M-BOPs (Fig. 1a), The
-
1
μg•mL ) were prepared using an accurate amount of NH
4
Cl,
-
1
characteristic peak around 2495 cm is attributed to B-H
stretching vibration, and a group of vibration absorption peaks
then [NH ]-absorbance standard curve was plotted by UV-vis.
3
The standard curve was obtained as: y(Abs) = 0.1542x([NH3])
-
-
1
at 1700-500 cm can be attributed to 1,10-o-phenanthroline.
2
0
.0002 with good linear correlation (R = 0.9996) (Fig. S1).
2
-
Closo-[B12
H12] as a reducing agent was significantly consumed
Determination of hydrazine (N
Watt and Chrisp methods were used to determine whether
was generated in electrolyte after potentiostatic
polarization (i-t test) for 2 h. Generally, concentrated
hydrochloric acid (30 mL, 12 M), ethanol (C OH, 300 mL) and
p-(dimethylamino)-benzaldehyde (p-C 11NO, 5.99 g) were
mixed to prepare the chromogenic agent for N . Then 5 mL
chromogenic agent was mixed with 5 mL electrolyte for 10 min
at 25 . Subsequently, the absorbance of electrolyte at
2 4
H )
after loading Au in situ. The FT-IR results also showed that
2-
12
closo-[B12H ] could be partially retained after the reaction
2 4
N H
was completed, maintaining the structure of M-BOPs to some
extent. XPS can be used to effectively determine the electronic
state of a metal. Fig. 1c gives the semi-quantatitive elemental
composition of Au/M-BOPs and the corresponding peaks. Fig.
2 5
H
9
H
2 4
H
1d shows the bonding energy area of Au. It can be observed
from the XPS results, all Au atoms in Au/M-BOPs are at zero-
valence. Compared with the bulk gold, the electron binding
℃
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ARTICLE
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silicon wafer oxygen, the distribution of oxygen in the sample
View Article Online
DOI: 10.1039/C9TA06573C
is for reference only.)
Although XPS and EDS can also give the content of elements
in material, these mothed are both based on surface analysis,
the atmospheric environment tends to cause some pollution
to the surface of the material, so XPS and EDS are only used for
semi-quantitative analysis. The exact content of Au in Au/M-
BOPs was given by ICP-OES based on bulk phase analysis. Fig.
S4 gives a test report for ICP-OES, and the loading of Au in
Au/M-BOPs is 12.07 w%.
Electrochemical NRR was carried out in two pools isolated
by proton conduction membrane (Nafion 115), using a typical
three-electrode system (working electrode: CP coated Au/M-
BOPs as cathode; reference electrode: Ag-AgCl/saturated KCl;
counter electrode: graphite rod; Electrolyte: 0.1M HCl solution)
to complete NRR test. All potentials of the NRR test were given
relative to the reversible hydrogen electrode (RHE). The
potential of the reference electrode Ag-AgCl/saturated KCl was
calibrated as 0.197 V by reversible hydrogen electrode. First,
the NRR activity of Au/M-BOPs was determined by linear
Fig. 1 (a) FT-IR spectra of M-BOPs and Au/M-BOPs, (b) XRD patterns of M-BOPs and
Au/M-BOPs, and (c-d) XPS spectrum of Au/M-BOPs
energy of 4f7/2 and 4f5/2 increased by 0.1 eV as a whole, which
indicates that Au in Au/M-BOPs obtain electrons and becomes
2
sweep voltammetry (LSV). In N saturated and Ar saturated
2
-
an electron rich state. In closo-[B12
bond is shown to exhibit certain electron-rich properties
while the zero-valent noble metal often exhibits some
H12] , the H atom in B-H
electrolytes, the LSV curve with the participation of Au/M-
BOPs showed a similar shape (Fig. 3a), but exhibited different
current values in specific regions. In the range of -0.1 V to -0.6
41, 42
,
2-
12
adsorption to the surrounding H atoms, thus closo-[B12H ]
2
V, the current at the saturation of N was significantly higher
can be regarded as an electron donor to make Au become an
electronic rich state.
than the current at the saturation of Ar, indicating an obvious
NRR reaction was occurred in this range. To follow this, Fig. 3b
shows the current density-time diagram of potentiostatic
polarization for 2 h at different potentials (-0.1 V, -0.2 V, -0.3 V,
Powder XRD is often used to determine the crystalline form
of a material. In Fig. 1b, we have overlapped the powder XRD
results of M-BOPs, Au/M-BOPs and also the well consistent
simulation results of M-BOPs from sing crystal XRD. Powder
XRD results show that both Au/M-BOPs and M-BOPs have a
-
0.4 V, -0.5 V, -0.6 V). Due to the difference in sweep speed,
the current value under LSV will be slightly higher than the
potentiostatic experiments under static. After the
electrocatalysis, the electrolyte was stained with indophenol
blue indicator, and then absorbance of the color developed
electrolyte was tested on UV-vis spectrophotometer (the
detailed test process was described in experiment section). Fig.
2-
12
similar structure, indicating that the remaining closo-[B12H ]
ions to some extent have maintained the M-BOPs structure
from collapse. In addition, the characteristic diffractions of 2θ
at 38.2°, 44.4°, 64.6°, and 77.5° correspond to the (111), (200),
(
220), and (311) crystal planes of Au, respectively, which is in
consistent with the standard diffraction pattern of Au (JCPDS:
4-0784). These shows the Au atoms are orientated in an
excellent face-centered cubic structure in Au/M-BOPs, and
3
c shows the UV-vis spectra of electrolyte at different
0
(111) plane is the dominant growth surface.
Electron microscope can effectively present the microscopic
morphology of material and obtain a further understanding
from visual. Fig. 2a and Fig. 2b are SEM images and TEM
images of Au/M-BOPs, respectively. They clearly show that Au
nanoparticles are uniformly dispersed both in the interior and
on the surface of the material with particle size of 6~10 nm.
Fig. 2c is the high resolution transmission electron microscope
(HRTEM) of Au/M-BOPs, it shows that the lattice spacing of Au
nanoparticles is 0.23 nm, which corresponds to the (111) faces
of Au face-centered cubic structure proved by powder XRD.
Fig.2d shows the element distribution and energy spectrum of
Au/M-BOPs. Taking the advantage of the in-situ nano-metal
reduction, B, C, N, Ni and Au atoms are uniformly distributed
in the carrier. (The sample for element distribution analysis
was prepared on silicon wafer. Due to the interference from
Fig. 2 (a) SEM image of Au/M-BOPs, (b) TEM image of Au/M-BOPs, (c) HRTEM image of
Au/M-BOPs, and (d) EDAX spectrum of Au/M-BOPs
4
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(
the detailed test process was described in experiment
View Article Online
DOI: 10.1039/C9TA06573C
section), and interpolation from standard curve to determine
whether N
that the presence of N
2
H
4
was present in electrolyte. It is worth noting
is not detected in the -0.1 V - -0.6 V
2 4
H
potential range (or the content was so small that it was
difficult to detect). This indicated that Au/M-BOPs had good
selectivity for N
process.
2
to NH
3
in the whole electrocatalytic NRR
In order to unambiguously confirm the source of NH
3
, we
conducted the following controlled experiments: (1) Using
Au/M-BOPs (coated on CP) as an electrocatalyst,
electrocatalyzed for 2 h in 0.1 M HCl saturated with N2 flow (In
the case of open circuit potential); (2) Using Au/M-BOPs
(
coated on CP) as an electrocatalyst, electrocatalyzed for 2 h in
.1 M HCl saturated with Ar flow; (3) Using original CP without
Au/M-BOPs as an electrocatalyst, electrocatalyzed for 2 h in
.1 M HCl saturated with N gas; (4) Using M-BOPs (coated on
the CP to form M-BOPs/CP) as an electrocatalyst,
electrocatalyzed for 2 h in 0.1 M HCl saturated with N . The
was obtained by interpolation corresponding electrolyte absorbance were measured by UV-
]-absorbance standard curve. Fig. 3d shows the vis, the NH yields were shown in Fig. 5a. Almost no ammonia
average NH
yields obtained at different potentials and the was detected under the open circuit potential, and the NH
corresponding Faradaic Efficiencies. As can be seen from the yield was almost zero in the case of an Ar flow saturated
0
Fig. 3 Electrochemical NRR result. (a) LSV test result, (b) potentiostatic polarization, (c)
electrolyte absorbance at different potentials, and (d) ammonia yield and Faraday
efficiency at different potentials
0
2
2
potentials, and the yield of NH
from [NH
3
3
3
3
3
-
1
-2
figure, when the potential increased from -0.1 V to -0.2 V, the electrolyte (0.5 ug·h ·cm : this extremely low value can be
NH
yield and FE presented an upward trend. When the regarded as caused by N , N is difficult to completely remove
potential is -0.2 V, the NH
yield and FE reach a maximum from the electrolyte), which guarantees the premise that the
3
2
2
3
-1
-2
-1
-1
value (r[NH
3
]: 45.54 ug·h ·cm or 75.89 ug·h ·mgcat. ; FE: experimental device and materials do not contain ammonia. In
1
0.35 %), and the NRR induced by Au/M-BOPs exhibits both the case of N flow saturated electrolyte, the NH yield of CP
high NH
3
yield and high FE compared with the reported and M-BOPs also remained extremely low (1.2 ug·h ·cm and
2
3
-1
-2
2
, 7, 8, 31, 37-40
-1
-2
15
+
supported Au.
electrocatalysts (as shown in Fig. 4
Au/M-BOPs also showed excellent NRR performance. However, indicating the ammonia was derived from the
Compared with other types of NRR 1.5 ug·h ·cm , respectively). Furthermore, only NH
4
was
1
5
,
Table S2 and Table S3), detected in NMR test using
N
2
as the feed gas (Fig. S5)，
N
2
yield and FE electrochemistry reduction under action of Au/M-BOPs.4
4-46
In
when the potential was lower than -0.2 V, the NH
value gradually decrease, which shows a similar trend with addition, durability was tested for Au/M-BOPs for eight times
other supported Au electrocatalytic NRR. This turnaround is at -0.2 V, as shown in Fig. 5b. NH yield still maintains a high
and hydrogen value of 40.5 ug·h ·cm after eight cycles, while FE slightly
When the potential was rises from 10.35 % to 10.98 % due to the small decrease of
3
3
-1
-2
attributed to the competitive adsorption of N
species on the catalyst surface.
2
3, 7, 15, 43
lower than -0.2 V, the hydrogen evolution reaction (HER) current. Time dependent experiment also showed that the
increased, and as the potential continued to decrease, HER
became main reaction of the catalytic system, so the NH
and FE both decreased significantly. In the ammonia industry,
is often the most main by-product. In order to judge
whether the Au/M-BOPs electrocatalytic NRR process was
accompanied by formation of N , we use Watt-Chrisp
3
yield
2 4
N H
2 4
H
chromogenic method to detect absorbance of the electrolyte
Fig. 5 (a) Ammonia source inspection results, (b) Au/M-BOPs electrochemical NRR cycle
test results, (c) Au/M-BOPs tolerance for electrochemical NRR, and (d) ammonia yield
from tolerance test
Fig. 4 Comparison of electrochemical NRR results between Au/M-BOPs and previously
reported materials. (a) Expressed by ug·h ·mgcat. , (b) expressed by ug·h ·cm
-
1
-1
-1
-2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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J Po lue r an sa el od foMna ot et rai ad l js u Cs ht emm ai rs gt ri yn sA
Page 6 of 7
ARTICLE
Journal Name
efficiency of Au/M-BOPs worked well after continuous working which can effectively prevent the aggregation of AViuewnAurtciclleeiOannlinde
for 10 h, with NH
yield only slightly reduced (Fig. 5c and Fig. the loss of active sites, making the Au/MD-OBIO: 1P0s.10e3le9c/Ctr9oTcAa0t6a5l7y3sCt
). After the electrocatalytic NRR, the Au/M-BOPs electrolyte excellent in durability.
3
5
d
was rinsed off with ethanol and collected for XPS and HRTEM
detection. These result (Fig. S6 and Fig. S7) showed that the
Au/M-BOPs are essentially the same as the original state,
proving the excellent stability of Au/M-BOPs at ambient
temperature and atmospheric pressure. In addition, the
infiltration of 1,10-phenanthroline in the electrolyte was not
detected by FT-IR spectrum (Fig. S8), which not only further
indicates that the material has good stability, but also reflects
the accuracy of the reported results of ammonia yield.
Conclusion
All in all, a rapid and effective method was used to synthesize
metal boron organic polymer at room temperature and in-situ
anchor nano-Au. The obtained Au/M-BOPs realized the
electrochemical reduction from N
conditions, and the ammonia yield and Faraday efficiency
2 3
to NH under ambient
-1
-2
-1
-1
reached “45.54 ug·h ·cm
or 75.89 ug·h ·mgcat. ”and
During the electrochemical reduction of N
BOPs, N was never detected in the electrolyte. Considering
the excellent selectivity of Au/M-BOPs for NH , we speculate
that N acted on Au/M-BOPs in the form of a hybrid distal-to-
2 3
to NH by Au/M-
“10.35 %”, respectively. This performance has surpassed most
2 4
H
of the non-Au nuclear NRR catalysts and a lot Au-based NRR
catalysts reported previously. The uniqueness of preparation
method makes Au/M-BOPs have good stability and tolerance
in NRR. This research not only provides a new way for the
design and synthesis of supported nano-metal, but also makes
the electrochemical ammonia preparation a step closer to
industrialization.
3
2
alternating pathway. According to the ammonia source
exclusion experiment in Fig. 5a, the presence of Au plays a
crucial role, and Au is considered to be the active site. Many
studies have proved that B-cluser can effectively promote the
4
7, 48
development of electrochemistry NRR.
In this work,
compared with the reported Au-nuclear NRR electrocatalysts,
Au/M-BOPs exhibit excellent performance, and thus the carrier Conflicts of interest
in Au/M-BOPs play an important auxiliary role. Furthermore,
There are no conflicts to declare
2
-
12
we speculate that [B12H ] as a large electronic system may
be able to serve as an electron supplier to facilitate the
occurrence of NRR 3e process. Based on this, we speculate
Acknowledgements
that the possible mechanism is shown in Scheme 2. First, N
2
This work was financially supported by the Fundamental
Research Funds for the Central Universities (no.2042016
HF1054) and Wuhan University Experiment Technology Project
Funding (no.WHU-2016-SYJS-06). We are grateful to Pei Zhang
of the Core Facility and Technical Support, Wuhan Institute of
Virology for her technical support in transmission electron
microscopy. Meanwhile, we also appreciated Dr. Qiquan Luo
for his discussions about mechanism of Au/M-BOPs promote
NRR.
was adsorbed on Au surface in Au/M-BOPs through
unsaturated coordination, and then protons (H ) tend to
combine with N atoms far away from the active center to form
Au@N=NH. The N far from the active center was completely
protonated while the N/N bond was destroyed, releasing one
+
molecule of NH
protonated, continuing to release another molecule of NH
3
. The N at the Au active point was further
. In
3
Au/M-BOPs, Au was anchored in the metal organic polymer,
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