MoO3 nanosheets for efficient electrocatalytic N2 fixation to NH3

Article abstract of DOI:10.1039/c8ta03974g

The synthesis of NH₃ heavily depends on the energy-intensive Haber-Bosch process with a large amount of greenhouse gas emission. Electrochemical reduction offers a carbon-neutral process to convert N₂ to NH₃ at ambient conditions, but requires efficient and stable catalysts for the N₂ reduction reaction. Mo-dependent nitrogenases and synthetic molecular complexes have attracted increasing attention for N₂ fixation; however, less attention has been paid to Mo-based nanocatalysts for electrochemical N₂ conversion to NH₃. Herein, we report that MoO₃ nanosheets act as an efficient non-noble-metal catalyst for electrochemical N₂ fixation to NH₃ with excellent selectivity at room temperature and atmospheric pressure. In 0.1 M HCl, this catalyst exhibits remarkable NRR activity with an NH₃ yield of 4.80 × 10^-10 mol s^-1 cm^-2 (29.43 μg h^-1 mg_cat.^-1) and a faradaic efficiency of 1.9%. Moreover, this catalyst also shows high electrochemical stability and durability. Density functional theory calculations reveal that the outermost Mo atoms serve as the active sites for effective N₂ adsorption.

Full text of DOI:10.1039/c8ta03974g

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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DOI: 10.1039/C8TA03974G
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COMMUNICATION
MoO nanosheet for efficient electrocatalytic N fixation to NH
3
2
3
¶
a
¶
a
a
b
c
d
e
c
Jingrui Han, Xuqiang Ji, Xiang Ren, Guanwei Cui, Lei Li, Fengyu xie, Hui Wang, * Baihai Li *
and Xuping Sun *
a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
9
–11
The synthesis of NH heavily depends on energy-intensive Haber- wind, solar or nuclear energy-driven electricity.
3
However,
Bosch process with large amount of greenhouse gas emission. this process involves difficulty in breaking the strong N≡N
Electrochemical reduction offers a carbon-neutral process to triple bond of inert N2 and challenged by N2 activation,
convert N2 to NH3 at ambient conditions, but demands efficient underlining the need of high-performance electrocatalysts for
and stable catalysts for the N reduction reaction. Mo-dependent the N reduction reaction (NRR).
2
2
nitrogenases and synthetic molecular complexes have attracted
Noble metals-based materials are efficient NRR
increasing attention for N2 fixation, however, less attention has electrocatalysts, but the high price and limited availability
1
2–15
focused on Mo-based nanocatalysts for electrochemical N2 severely hinder their large-scale uses. Mo is an important
conversion to NH . Herein, we report that MoO nanosheet acts as element in nitrogenases that catalyze the N2 fixation at
3
3
1
6,17
an efficient non-noble-metal catalyst for electrochemical N2 ambient conditions in the biological system.
Great
fixation to NH with excellent selectivity at room temperature and attention has also focused on developing Mo-based molecular
3
1
8,19
atmosphere. In 0.1 M HCl, this catalyst exhibits remarkable NRR catalysts for N reduction.
2
However, these catalysts suffer
20
–10
–1
–2
–1
activity with a NH yield of 4.80×10 mol s cm (29.43 µg h
from limited stability under strongly reducing conditions and
21
mg cat.) and a Faradaic efficiency of 1.9%. Moreover, this catalyst it is challenging to effectively graft them onto electrodes.
also shows high electrochemical stability and durability. Density More recently, Yang et al. reported on the use of Mo nanofilm
3
–1
–
11
functional theory calculations reveal that outmost Mo atoms as a heterogenous NRR catalyst with a NH yield of 3.09×10
3
–
1
–2
22
mol s cm and a Faradic efficiency (FE) of 0.72%. Our
serve as the active sites for effective N adsorption.
2
recent work indicates that MoS nanosheet array is an active
2
2
3
Fixation of atmospheric N2 to NH3 is a fundamental
conversion to achieve hydrogen storage and earth nitrogen
NRR electrocatalysis with a higher FE of 1.17%.
In this communication, we report our recent finding that
MoO nanosheet is efficient for electrochemical N fixation to
1
,2
cycle. Industrially, reaction of N2 with H2 to form NH3 is
accomplished via the Haber-Bosch process but with inevitable
high temperatures (300-400 °C) and pressures (150-250
3
2
NH with excellent selectivity at ambient conditions. In 0.1 M
3
–
HCl, such catalyst attains a NH yield 4.80×10 mol s cm
10
–1
–2
3
3
–6
–1
–1
29.43 µg h mg cat.) and a FE of 1.9 %. It also shows high
atm).
The intensive energy input (1% of the worldwide
(
energy use) and large CO emissions (1.6% of the total global
2
electrochemical stability and durability. Density functional
theory (DFT) calculations indicate that the outmost Mo atoms
serve as the active sites for efficiently adsorbing N2.
CO2 production) push researchers to explore new NH3
7
synthesis technology. Electrochemical reduction of N (N +
,8
2
2
+
H + 6e → 2NH ) is attractive for artifical N fixation because
–
6
3
2
MoO3 was made by one-step hydrothermal reaction of
it can proceed at ambient conditions using renewable hydro,
molybdenum metal powder, H O2 aqueous solution and
2
ethanol (see Supporting Information for preparation details).
As shown in Fig. 1a, the X-ray diffraction (XRD) pattern for the
product presents diffraction peaks at 12.76°, 23.34°, 25.68°,
2
7.34°, 33.72°, 38.92°, 45.82°, 49.26°, 55.24°, and 64.54°
indexed to the (020), (110), (040), (021), (111), (060), (200),
(
002), (112), and (062) crystal planes of MoO
3
(JCPDS No. 05-
508), respectively. Fig. 1b shows the X-ray photoelectron
, confirming the
exsitence of Mo and O. Fig. 1c and 1d present the XPS spectra
2
4
0
spectroscopy (XPS) survey spectrum for MoO
3
of MoO
3
in the Mo 3d and O 1s regions. In the Mo 3d region
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(Fig. 1c), the peaks at 235.6 and 232.46 eV are assigned to sprayed Pt on sample surface to enchance the conductivity for
3
d
3/2 and 3d5/2 of the Mo cations in high oxidation states SEM characterizations). The EDX elemental mapping images
6
Mo ). The binding energy at 531.1 eV in the O 1s region (Fig. (Fig. 2e and 2f) further confirm that Mo and O elements are
+ 25
(
2
d) is assigned to lattice oxygen (O ), and the peak at 532.4 uniformly distributed in the product.
– 26
1
–
eV is attributed to surface adsorbed hydroxide species (OH ).
27
Fig. 3. (a) Average NH
test of MoO /GCE at the potential of –0.40 V. (c) Time-dependent current density
curves of MoO /GCE for NRR at different potentials. (d) Yields of NH and FEs under
3 3
yields and FEs of MoO /GCE at different potentials. (b) Recycling
3
3
3
2
different N flow rate at –0.40 V. All experiments were carried out in 0.1 M HCl under
room temperature and ambient pressure.
We evaluated the electrocatalytic NRR activity of MoO
3
Fig. 1. (a) XRD pattern for MoO
3 3
. (b) XPS survey spectrum for MoO . XPS spectra for
MoO in the (c) Mo 3d and (d) O 1s regions.
3
3
deposited on glassy carbon electrode (MoO /GCE) in 0.1 M
HCl, utilizing a two-compartment cell separated by Nafion
o
membrane under ambient temperature (25 C) and
atmospheric pressure (Fig. S2). All overpotentials were
reported on
Concentrations of produced NH
determined by the indophenol blue method and N
possible by-product was detected by the method of Watt and
a
reversible hydrogen electrode scale.
3
were spectrophotometrically
2
8
2 4
H as the
2
9
Chrisp. Corresponding calibration curve is shown in Fig. S3.
Fig. S4 shows the UV-Vis absorption spectra of the electrolytes
stained with indophenol indicator after electrolysis at a series
of potentials for 2 h. Obviously, NRR can be driven at applied
potentials ranging from –0.20 to –0.60 V. The highest
absorbance intensity of resulting electrolyte was obtained at –
0.50 V. Average NH3 yields and corresponding FEs under
different potentials are calculated and shown in Fig. 3a. The
highest FE (1.9%) is achieved at a potential of –0.30 V. As the
potential increases from –0.30 to –0.60 V, FE decreases to
–
1
–
0
.53%. Also note that a maximum NH yield (29.43 µg h mg
3
1
–10
–1
–2
cat.; 4.80×10 mol s cm ) can be obtained at –0.50 V, which
is much larger than reported aqueous-based NRR
–11
–1
electrocatalysts, including Mo nanofilm (3.09×10 mol s cm
–
2
22
, MoS /CC (8.08×10 mol s cm ), Fe O /CNT (3.6×10
–11
–1
–2 23
–12
)
2
2
3
Fig. 2. (a, b) SEM images for MoO
3 3
nanosheet. (c) TEM and (d) HRTEM images for MoO
–1
–2 30
–12
–1
–2 31
nanosheet. (f) EDX elemental mapping of Mo and O for MoO
3
.
mol s cm ), and Ru/C (3.43×10 mol s cm ), etc. More
detailed comparison is listed in Table S1. The corresponding
performance at a lower potential of –0.40 V is also impressive
Fig. 2a and 2b show the scanning electron microscopy (SEM)
images of MoO suggesting the formation of flake-like
nanostructures. The transmission electron microscopy (TEM)
image evidences the nanosheet nature of MoO (Fig. 2c). The
3
,
–
1
–1
with maximum NH yield of 28.50 µg h mg cat.. To verify that
3
detected reduction products is generated via the NRR process
of MoO /GCE, we performed electrolysis in N -saturated
3
3
2
high-resolution TEM (HRTEM) image reveals well-resolved
lattice fringes with interplanar distances of 0.40 and 0.36 nm
solution at open circuit potential and Ar-saturated solution at
0.40 V. The corresponding UV-Vis absorption spectra are
–
for the (100) and (001) lattice planes of MoO
3
, respectively (Fig.
presented in Fig. S5, enabling a more accurate assessment that
eliminates background contributions. We also performed
electrolysis in Ar-saturated solution at –0.40 V and almost no
2d). The energy-dispersive X-ray (EDX) spectrum (Fig. S1)
concludes the existence of Mo and O elements with an atomic
ratio of 24.63:75.37 (the signal for Pt element arising from
2
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apparent NH
COMMUNICATION
1
was detected (Fig. S6 and S7). N isotopic form *NNH
5
3
3
, the steps become downhill in free energy.
labeling experiment was performed as an alternative method Because of the hydrogen bond with the adjacent O atoms, the
to verify the NH originated from N in 0.1 M HCl electrolyte *NNH group prefers to locate the hollow position (structure
Fig. S8). The H NMR spectra identified the obtained a doublet c). Nevertheless, release of the first ammonia molecule meets
3
2
3
1
(
1
5
+ 5
15
+
coupling for
NH
4
.
Only
-saturated electrolyte. These results change of 2.25 eV for the process of from *NNH
in the electrolyte was indeed generated the adsorbed N atom moves back to the top site of the Mo
reduction by MoO /GCE. We also atom with the bond length of 2.89 (structure d).
NH
4
was observed after the potential determining step with the largest free energy
1
5
electrolysis in the
confirm that the NH
N
2
3
to *N, where
3
via electrocatalytic N
2
3
Å
investigated the catalytic ability of bare GCE. Obviously, bare Subsequently, the following reduction processes are exergonic
GCE has low activity for the NRR (Fig. S9). Furthermore, No steps.
N
2
H
4
was detected, indicating MoO
selectivity (Fig. S10).
Stability is another critical parameter to evaluate the excellent selectivity at ambient conditions. It achieves a NH
3
/GCE possesses excellent
In summary, MoO
3
nanosheet is proved as an efficient non-
noble-metal electrocatalyst for artifical N
2
fixation to NH with
3
3
–
10
–1
–2
–1
–1
performance of catalysts. Under sustained N gas flowing, 24-h yield of 4.80×10 mol s cm (29.43 µg h mg cat.) and a FE of
2
electrolysis at –0.40 V only leads to a slight decrease in current 1.9%, with high electrochemical stability and durability. DFT
density (Fig. S11), confirming its strong stability. After NRR for calculations reveal that the outmost Mo atoms act as the
24 h, the NH yield rate only shows a slight decrease compared active sites for effective N2 adsorption. This study not only
3
with those after
2 h electrolysis (Fig. S12), suggesting offers us an attractive earth-abundant catalyst material for
MoO /GCE still exhibit effective catalysis after long-term NRR. electrochemical NH3 synthesis, but would open exciting new
3
Consecutive recycling tests (Fig. 3b) were conducted at –0.40 avenues to the rational design of Mo oxides-based
V, showing almost no variation in NH3 yields and FEs, which nanocatalysts with enhanced performances for applications.
indicate high stability of MoO /GCE. Fig. 3c shows the long-
3
term electrolysis at different potentials in N -saturated 0.1 M
2
Acknowledgements This work was supported by the National
HCl solution, indicating high durability of MoO /GCE. By
3
Natural Science Foundation of China (No. 21575137).
varying the N flow rate, NH yield and FE at –0.40 V are also
2
3
steady without any apparent change (Fig. 3d), suggesting that
the N diffusion is a non-rate-determining step.
2
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Table of Contents Entry
MoO nanosheet acts as an efficient electrocatalyst for N fixation to NH with
3
2
3
excellent selectivity at ambient condition. In 0.1 M HCl, it shows high activity with a
–10
–1
–2
–1
–1
NH
3
yield of 4.80×10 mol s cm (29.43 µg h mg cat.) and a Faradaic efficiency
of 1.9%.