Facile Synthesis of Stable MO2N Nanobelts with High Catalytic Activity for Ammonia Decomposition

Article abstract of DOI:10.1002/cjoc.201900016

In the present work, high quality γ-Mo₂N catalysts for ammonia decomposition were successfully synthesized via temperature programmed nitridation of α-MoO₃ nanobelts. The optimal conditions for the synthesis of MoO₃ precursors were obtained by using the orthogonal experimental method. The MoO₃ precursors and the corresponding fresh and used Mo₂N catalysts were characterized by various characterization techniques, including transmission electron microscopy, X-ray diffraction and N₂ adsorption-desorption. Furthermore, temperature- programmed desorption by N₂ or NH₃ and X-ray photoelectron spectroscopy analysis were performed to better understand the chemical properties of Mo₂N catalysts. The results revealed that Mo₂N catalyst has good NH₃ adsorption ability and facilitates the dissociation adsorption of N₂. Moreover, the morphology and structure of Mo₂N catalysts well maintained after the reaction. Therefore, among the three transition metal nitrides (Mo₂N, W₂N and VN) and some Mo-based catalysts previously reported, Mo₂N catalysts showed very high activity and stability. Nearly 94% conversion of NH₃ could be reached at 550°C with the gas hourly space velocity of 22000 cm³?g_cat^–1?h^–1 and no obvious deactivation was observed during a 72 h test.

Full text of DOI:10.1002/cjoc.201900016

molybdenum, nitrides, heterogeneous catalysis, ammonia
decomposition, catalytic properties
中
国 化 学
Facile Synthesis of Stable MO N Nanobelts with High Catalytic Activity
2
for Ammonia Decomposition
Jun Xu, Han Yan, Zhao Jin,* and Chun-Jiang Jia
Key Laboratory for Colloid and Interface Chemistry, Key Laboratory of Special Aggregated Materials, School of Chemistry and Chemical Engineering,
Shandong University, Jinan, Shandong 250100, China
Cite this paper: Chin. J. Chem. 2019, 37, 364—372. DOI: 10.1002/cjoc.201900016
Summary of main observation and conclusion In the present work, high quality γ-Mo
thesized via temperature programmed nitridation of α-MoO nanobelts. The optimal conditions for the synthesis of MoO
using the orthogonal experimental method. The MoO precursors and the corresponding fresh and used Mo N catalysts were characterized by various
characterization techniques, including transmission electron microscopy, X-ray diffraction and N adsorption-desorption. Furthermore, temperature-
programmed desorption by N or NH and X-ray photoelectron spectroscopy analysis were performed to better understand the chemical properties of
Mo N catalysts. The results revealed that Mo N catalyst has good NH adsorption ability and facilitates the dissociation adsorption of N . Moreover, the
morphology and structure of Mo N catalysts well maintained after the reaction. Therefore, among the three transition metal nitrides (Mo N, W N and
VN) and some Mo-based catalysts previously reported, Mo N catalysts showed very high activity and stability. Nearly 94% conversion of NH could be
reached at 550 °C with the gas hourly space velocity of 22000 cm ∙gcat ∙h and no obvious deactivation was observed during a 72 h test.
2
N catalysts for ammonia decomposition were successfully syn-
3
3
precursors were obtained by
3
2
2
2
3
2
2
3
2
2
2
2
2
3
3
–1 –1
Background and Originality Content
Hydrogen is the most plentiful element in the universe, which
is commonly regarded as a promising alternative to fossil fuels in
position reaction, and metal-nitrogen interaction is the core part
[23,24]
of such synthesis and decomposition of ammonia.
Generally,
the dissociation of N is known to occur on molybdenum surface
2
[25]
more easily than on that of iron or ruthenium. Therefore, due
to the assumed high activity and low cost, molybdenum nitride
has been considered as ideal catalysts for ammonia decomposi-
hydrogen economy. However, the storage of H
the greatest obstacles for the industrial applications. Ammonia,
with the advantages of high H storage density, low cost and
transportation safety, has been considered as a very suitable hy-
drogen carrier. The products of NH decomposition are pollu-
tion-free hydrogen and nitrogen, which do not contain carbon
2
remains one of
[1,2]
[
13]
2
tion reaction.
Our aim in this work is to develop efficient and stable unsup-
ported non-precious metal-based catalysts. Herein, MoO nano-
3
3
belt precursors have been synthesized on a large scale by hydro-
thermal method without employing any surfactant or templates.
[
3,4]
monoxide or any other catalytic poisons.
increasing demand for CO -free hydrogen to supply proton ex-
change membrane fuel cells, NH decomposition has drawn a
Therefore, with the
x
Mo
N catalysts were prepared through temperature programmed
. By using the orthogo-
2
3
nitridation of precursors with flowing NH
3
[
5]
great deal of attention. And the hydrogen production from cat-
alytic decomposition of ammonia is crucial to practical utilization
of hydrogen.
nal experimental design method, the optimal synthetic conditions
were determined as that pH was adjusted to 1 using HCl, hydro-
thermal temperature and calcination temperature were 220 °C
Although the Ru-based catalysts have exhibited very high cat-
and 500 °C , respectively. The obtained Mo
very high catalytic activity. Nearly 94% NH
obtained over Mo N at 550 °C using a gas hourly space velocity
GHSV) of 22000 cm ∙gcat ∙h . No observable deactivation oc-
curred during 72 h test. Our prepared Mo N catalysts also mani-
fested obviously superior catalytic performance compared to
other nitrides (VN, W N) and some Mo-based catalysts previously
From the TPD results, both the NH adsorption
capacity and the N desorption ability of Mo N catalyst were very
2
N catalysts showed
[
6-9]
alytic activity of ammonia decomposition,
the high cost and
3
conversion could be
limited availability hinder their large scale applications. Therefore,
economically viable non-noble catalysts are highly desirable.
Many less expensive catalysts (Fe, Co and Ni) have been tested for
ammonia decomposition, which were generally supported on
2
3
–1 –1
(
2
[
10-12]
2 3 2
supports, such as Al O , SiO , MCM-41 and ZMS-5 materials.
2
[
3,21,22,26]
Nevertheless, these representative metal catalysts still have two
major problems. First, the active component is easy to sinter at
high temperature; second, the recombinative desorption rate of
the adsorbed nitrogen atoms from active species is slow. These
problems might lead to low activity for the reaction. Thus, it is
essential to design a new catalytic system based on non-noble
metals that could solve the above two problems.
reported.
3
2
2
good and no obvious structural changes caused by denitrogena-
tion at high reaction temperature could be observed. By using
[
13]
HRTEM combined with XRD, we have identified that Mo
2
N cata-
lyst could keep its morphology and structure well after reaction.
Numerous experimental results showed that Mo
2
N catalyst
Transition metal nitrides (TMNs) with similar catalytic charac-
teristics to platinum group metals have received much attention,
due to their high activities in many hydrogen-related reactions
showed very high activity and stability.
Results and Discussion
The phase composition and particle size of the precursors and
catalysts were analyzed through X-ray diffraction (XRD). As shown
[
14-16]
such as hydro-denitrogenation (HDN),
hydro-desulfurization
[
17,18]
[19,20]
[21,22]
(HDS),
ammonia synthesis,
and decomposition.
Previous research has indicated that recombinative desorption of
nitrogen molecules is the rate-limiting step of ammonia decom-
in Figure 1a, for the MoO precursor, all diffraction peaks could be
3
*E-mail: jinzhao@sdu.edu.cn
For submission: https://mc.manuscriptcentral.com/cjoc
For articles: https://onlinelibrary.wiley.com/journal/16147065
364
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2
MO N Nanobelts for Ammonia Decomposition
Chin. J. Chem.
identified as the orthorhombic α-MoO
3
phase (space group: Pbnm
Table 1 Physical properties of MoO
peratures (100, 150, 180, and 200 °C) and corresponding fresh Mo
lysts
3
with different hydrothermal tem-
[
27,28]
JCPDS#5-508).
The intensity distributions of the diffraction
2
N cata-
peaks were slightly different due to different preferred orienta-
tions. When the hydrothermal temperature was altered, the
a
Specific surface area/ Pore volume/ Particles size /
Sample
2
−1
3
–1
phase of MoO
particle size calculated by Scherrer equation (Table 1). XRD pat-
terns of the fresh and used Mo N catalysts were shown in Figures
b and 1c. The reflection peaks corresponding to face-centered
3
did not change obviously, just the same as the
(m ∙g )
(cm ∙g )
nm
MoO
MoO
3
3
3
3
100 °C
4.7
5.7
0.05
0.06
0.08
0.06
0.15
0.16
0.19
0.18
38.7
35.1
37.9
34.3
5.8
2
1
150 °C
180 °C
220 °C
cubic γ-Mo
2
N (JCPDS#25-1366) could be observed. There were no
N, thanks to the
MoO
MoO
6.8
diffraction peaks of the oxide precursor for Mo
2
6.7
surface oxide layer provided by the passivation step in preparation
process. Compared to the fresh catalysts, no new phase was
found for the used catalysts. These results suggested that the
Mo
Mo
Mo
Mo
2
2
2
2
N 100 °C
N 150 °C
N 180 °C
N 220 °C
128.5
125.3
112.6
133.5
5.8
5.7
Mo
able to keep stable during reaction. The results of the physical
characterizations of MoO with different hydrothermal tempera-
tures and corresponding Mo N catalysts were also summarized in
Table 1. The hydrothermal temperature of the precursors has little
influence on both fresh and used Mo N catalysts. According to the
calculation results by Scherrer equation, the four types of Mo
2
N was the active phase for ammonia decomposition and was
5.9
a
The particle sizes of MoO
3
with different hydrothermal temperatures and
3
corresponding Mo N catalysts were estimated from the obtained XRD.
2
2
MoO
change significantly. The N
distribution of the fresh and used Mo
3
was changed, the SSA and pore volume of Mo
sorption isotherms and pore-size
N catalysts were evaluated
2
N did not
2
N
2
2
catalysts have similar crystallite sizes, which were in the range of 5
to 6 nm (Table 1). It could be easily found that the specific surface
2
with liquid nitrogen at –196 °C (Figure S1, Supporting Infor-
2
–1
mation). All samples showed type IV isotherms with H4 hysteresis
area (SSA) of four fresh Mo
which was much higher than that of the precursors (~6 m ∙g ).
The SSA of the fresh Mo N in this work was also higher than that
of Mo N in a former report (57 m ∙g ).
though the hydrothermal temperature during the synthesis of
2
N catalysts all exceeded 100 m ∙g ,
[
30]
2 –1
loop, which were typical for mesoporous materials (Figures S1a
and S1c). From the distribution curves, the corresponding pore
size of samples had a broad distribution in the range of 5—25 nm
2
2
–1 [29]
2
In addition, even
(
Figures S1b and S1d). More detailed textural properties of the
used catalysts were listed in Table S1. Obviously, after catalytic
tests, the SSA of catalysts decreased after the ammonia decom-
position reaction at high temperatures, but the particle size of the
used catalysts was almost the same as that of fresh catalysts.
To investigate the morphology of the precursors and catalysts,
transmission electron microscopy (TEM) images of the MoO
Mo N samples were represented in Figure 2. As shown in Figures
a—2d, it could be observed that α-MoO synthesized at different
hydrothermal temperatures had the similar nanobelt appearance
except the 100 °C one), with a diameter of 200—500 nm and
wide length range. The low hydrothermal temperature was unfa-
vorable for the crystallization of MoO , because the sample syn-
thesized at 100 °C could not form the stable nanobelt structure
Figures 2a and 2e). For the Mo N catalysts, it maintained the
same belt-shaped morphology as the α-MoO after nitridation, as
shown in Figures 2e—2h. It could also be seen that the Mo
3
and
2
2
3
(
3
(
2
3
2
N
catalysts retain their original morphology after the catalytic test-
ing (Figure S2).
Subsequently, we investigated the catalytic performance of
2
Mo N catalysts, of which the precursors were prepared with dif-
ferent hydrothermal temperatures. As shown in Figure 3, with the
temperature ranging from 350 °C to 600 °C, it could be clearly
seen that the NH
00% with a GHSV of 22000 cm ∙gcat ∙h . All the Mo
with different hydrothermal temperatures of precursors, reached
00% conversion at 600 °C. According to many reports, bulk and
supported Mo N catalysts can achieve 100% ammonia conversion
at 650 °C or higher temperatures.
3
conversion over Mo N increased from 3% to
2
3 –1 –1
1
2
N catalysts,
1
2
[
31-34]
2
Thus, our prepared Mo N
catalysts showed high catalytic activity upon ammonia decompo-
sition. It could be found that with the increase of the hydrother-
mal temperatures, the catalytic activity of the samples was slightly
promoted. This could be explained by the fact that stable nano-
belt structure cannot be formed at low hydrothermal tempera-
tures, while higher hydrothermal temperatures were beneficial in
enhancing the structure stability of Mo
the used Mo N catalysts showed similar catalytic activity with the
fresh catalysts, which indicated that our prepared Mo N catalysts
2
N catalysts. Meanwhile,
2
2
Figure 1 XRD patterns of (a) MoO
atures (100, 150, 180, and 200 °C) and corresponding (b) fresh and (c)
used Mo N catalysts.
3
with different hydrothermal temper-
have stable structure and activity (Figure S3). It also suggested
that our prepared Mo N catalysts might be promising catalysts for
2
2
ammonia decomposition reaction.
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we adjusted the calcination temperature of the precursors. With
increasing of the calcination temperatures for the precursor, the
SSA of the Mo
of the MoO
γ-Mo N (Figures 4a and 4b), respectively. Compared to the fresh
2
N slightly decreased (Table 2). The crystallite phase
3
and fresh Mo N catalysts remained α-MoO and
2
3
2
catalysts, it could be observed that no new phase was found for
the used catalysts (Figure 4c). The morphology of catalysts main-
tained the nanobelt shape (Figure S6). According to previous liter-
ature, when α-MoO and NH reacted with a temperature pro-
3 3
grammed reaction, the morphology of solid did not alter. Regard-
less of the shape and size of the parent crystal, the resulting
γ-Mo
α-MoO
2
N of the reactions turned out to be pseudomorphous with
[35]
3
.
This phenomenon was highly consistent with our ex-
perimental observation. The catalyst exhibited the highest cata-
lytic activity when the calcination temperature was 500 °C (Figure
5
b). After changing the above conditions, the obtained MoO
showed similar SSA, particle size and crystallite phase. The corre-
sponding structural properties of Mo N catalysts also remained
3
2
intact. However, these catalysts exhibited some differences in
morphology and activity. Therefore, the above results showed
that the optimum synthetic condition for the precursors were as
follows: pH = 1, HCl as the acid source, hydrothermal temperature
=
220 °C and calcination temperature = 500 °C.
[
36-38]
For VB and VIB transition metals, it is known that Mo,
[
39-41]
[42-44]
W
and V
can form stable nitrides. To evaluate the cata-
N among the TMNs, VN and W
lytic performance of Mo
2
2
N
Figure 2 TEM images of MoO
a) 100 °C, (b) 150 °C, (c) 180 °C , (d) 220 °C and corresponding (e—h)
Mo N catalysts.
3
with different hydrothermal temperatures
(
2
Figure 3 Temperature dependent ammonia conversion at a GHSV of
3
–1 –1
2
2
2000 cm ∙gcat ∙h over Mo N catalysts with different hydrothermal
temperatures (100, 150, 180, and 200 °C).
Moreover, some other factors of hydrothermal synthesis, such
as the acid source, pH and calcination temperature of precursors
were also altered for further investigation. The crystal structures
and activities of catalysts were tested (Figures 4 and 5). When we
changed the acid sources (HNO
3
, HCl) and pH (0.65, 1), the SSA of
2 –1
obtained Mo N catalysts was between 100—130 m ∙g and the
2
range of particle size was from 5 to 6 nm (Table S2). All the cata-
lysts showed similar morphology and crystallite phase (Figures S4
and S5). Among these samples, Mo
2
N catalysts of which the pre-
Figure 4 XRD patterns of (a) MoO
tures (400, 500 and 600 C) and corresponding (b) fresh and (c) used
Mo N catalysts.
3
with different calcination tempera-
cursors were prepared by using HCl to adjust pH to 1, exhibited
the best catalytic activity (Figure 5a). By applying HCl and pH of 1,
2
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2
MO N Nanobelts for Ammonia Decomposition
Chin. J. Chem.
2
Figure 5 Catalytic activity of Mo N catalysts prepared from MoO
3
with
2 2
Figure 6 XRD patterns of the fresh (a) and used (b) Mo N, W N, and VN
different (a) pH (0.65, 1) and acid sources (HCl, HNO
3
), (b) calcination
catalysts.
temperatures (400, 500, and 600 C).
2 2
Table 3 Physical properties of the fresh and used Mo N, W N, and VN
catalysts
Table 2 Physical properties of MoO
tures (400, 500, and 600 C) and corresponding fresh Mo
Specific surface area/ Pore volume/ Particles size /
3
with different calcination tempera-
2
N catalysts
a
Specific surface area/ Pore volume/ Particles size /
a
Sample
Mo
2
−1
3
−1
(
m ∙g )
(cm ∙g )
nm
Sample
2
−1
3
−1
(m ∙g )
(cm ∙g )
nm
2
N
123.1
18.7
21.4
0.25
0.22
0.30
5.6
7.8
MoO
MoO
MoO
3
3
3
400 C
500 C
600 C
6.7
5.9
4.3
133.5
123.1
106.6
0.06
0.05
0.04
0.18
0.25
0.19
34.3
38.5
38.3
5.9
5.6
5.5
2
W N
VN
16.5
Mo
Mo
Mo
2
N 400 C
2
N 500 C
2
N 600 C
2
Mo N-used
92.8
8.3
0.14
0.08
0.17
6.6
18.2
2
W N-used
a
VN-used
12.8
21.1
The particle sizes of MoO
3
with different calcination temperatures and
a
The particle sizes of the fresh and used Mo
were estimated from the obtained XRD.
2
N, W
2
N, and VN catalysts
corresponding Mo N catalysts were estimated from the obtained XRD.
2
catalysts were used as reference materials. The VN and W
lysts were prepared by the same preparation method as Mo
catalyst. XRD patterns of the as-prepared nitrides before and after
the catalytic test were shown in Figure 6. Although the peak of
TMNs showed similar profile, the XRD patterns were differently
indexed in accordance with the JCPDS card #: 25-1366, 25-1257,
2
N cata-
particles (16.5 nm) was the largest compared to the other two
catalysts (Figure 7c). After the catalytic tests, the nanobelt mor-
2
N
phology of Mo
However, severe agglomeration could be observed for W
VN catalysts (Figures 7e and 7f). It may be the reason for the low
catalytic activity for the W N and VN catalysts. The above obser-
vations correspond well to the particle size data calculated from
XRD pattern (Table 3). As listed in Table 3, the sizes of used W
and VN particles grew larger and sintered. However, the particles
sizes of used Mo N catalysts slightly rose, indicating that the
structure of Mo N was still stable after high temperature reaction.
2
N catalyst could be well maintained (Figures 7d).
2
N and
2
3
2 2
5-0768 for γ-Mo N, β-W N, and VN, respectively (Figure 6a).
There were only characteristic peaks of metal nitrides, no signals
of metal oxides or other phases were observed. The correspond-
ing particle size and SSA of these TMNs were listed in Table 3. The
2
N
2
2 2
particle size of Mo N, W N and VN catalysts were 6, 8 and 17 nm,
2
respectively. After catalytic test, the phase of each catalyst did not
The results of catalytic activity, long term stability and kinetics
tests over the three synthesized TMNs were presented in Figure 8.
As shown in Figure 8a, VN catalyst showed almost no catalytic
activity at temperature below 500 °C and only 42% ammonia
conversion at 600 °C. The NH conversion of W N catalyst was
2
–1
change (Figure 6b). Apparently, the SSA of Mo
higher than that of W N and VN. Meanwhile, the SSA of these
used catalysts decreased to about 76%, 44% and 60% comparing
with the fresh Mo N, W N and VN samples, respectively. These
might contribute to the sintering of particles during the catalytic
2
N (133 m ∙g ) was
2
2
2
3
2
similar to that of Mo N when the reaction temperature was below
2
[
45]
tests.
Figure 7 showed the TEM images of the fresh and used Mo
N, and VN catalysts. Among the three synthesized TMNs, the
fresh Mo N sample possessed the shape of nanobelts (Figure 7a),
while fresh W N and VN catalysts exhibited bulk crystal with ir-
regular morphology (Figure 7b). The average diameter of VN
400 °C. With the temperature rising, the NH conversion of Mo N
3
2
2
N,
increased rapidly, and surpassed that of W N since 450 °C . 100%
2
W
2
of NH conversion was achieved for Mo N at 600 °C, while the
3
2
2
conversion for W N was less than 70%. Obviously, among the
2
2
three studied catalysts, Mo N catalyst showed the highest catalyt-
2
ic activity. As shown in Figure 8b, long-term stability tests were
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Comprehensive Report
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Figure 7 TEM images of the fresh (a-c) and used (d-f) catalysts: (a, d)
Mo N, (b, e) W N, (c, f) VN.
2 2
2 2
performed on Mo N, W N, and VN catalysts at 550 °C with a high
3 –1 –1
GHSV of 60000 cm ∙gcat ∙h for 72 h. All tested catalysts exhibit-
ed stable activities at the test condition. The NH conversion of
Mo N maintained at 64%, which was much higher than that of
N (13%) and VN (8%) samples. Arrhenius plots for ammonia
decomposition and the obtained E values were presented in
Figures 8c and 8d. Obviously, the E value of Mo N catalyst (139.3
kJ∙mol ) was lower in comparison with that of W N (173.1 kJ∙
mol ) and VN (194.2 kJ∙mol ) catalysts. The results demonstrated
that Mo N catalyst was both highly efficient and stable for ammo-
3
2
W
2
a
a
2
–
1
2
–
1
–1
2
nia decomposition.
2
Table 4 summarized the activity of our prepared Mo N cata-
lyst, previously reported other typical Mo-based catalysts and
transition metal (Co, Ni) catalysts for ammonia decomposition.
2
The hydrogen yield on bulk Mo N catalyst used in this work exhib-
ited higher values than the listed yield values at the same tem-
perature. Meanwhile, the active components of the transition
metal catalysts with high metal loading were easily to sinter at
high temperatures, which might cause the catalyst deactivation.
Figure 8 (a) Catalytic activities of Mo
2
N, W
2
N and VN catalysts (GHSV =
3
–1 –1
22000 cm ∙gcat ∙h ); (b) Long-term stability of Mo
2
N, W
2
N and VN cata-
lysts measured at 550 °C (GHSV = 60000 cm ∙gcat ∙h ); (c) Arrhenius plots
and the corresponding E values for the Mo N catalyst in the range of
N and VN catalysts in the range
3
–1 –1
a
2
However, our prepared Mo
2
N catalyst possessed high catalytic
420—450 °C; (d) Arrhenius plots for the W
of 520—550 °C.
2
activity and excellent thermal stability for ammonia decomposi-
tion compared to these Mo-based, Co-based and Ni-based cata-
lysts. And the good catalytic performance of Mo
study should be related to the highly pure phases Mo
acted as active species for ammonia decomposition reaction.
Temperature programmed desorption by ammonia (NH -TPD)
measurement was used to evaluate NH adsorption ability of
three nitrides. The resulting TPD spectra of three transition metal
nitrides were shown in Figure 9. A broad desorption peak with
2
N catalysts in this
below 400 C was ascribable to the decomposition of NH that
3
2
N, which
was strongly adsorbed on the nitrides, indicating that Mo N sam-
2
ple could promote the dissociative adsorption of NH (Figure 9a).
3
3
A smaller NH adsorption peak could also be observed from W N,
3
2
3
2 2
but no signals of H and N were detected below 400 °C (Figure
9b). Moreover, only very weak adsorbed ammonia species were
present on the surface of VN catalysts (Figure 9c). The above re-
strong intensity was observed at 50—300 °C for Mo
manifested that the considerable amount of NH was adsorbed on
the surface of sample. Meanwhile, the N and H desorption
2
N, which
sults indicated that the NH adsorption capacity of nitrides in-
3
3
creased in the following order: VN < W N < Mo N, which was in
2
2
2
2
good agreement with the activity results.
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2
MO N Nanobelts for Ammonia Decomposition
Chin. J. Chem.
3
Table 4 NH conversion over various catalysts compared with the literature data
3
1 1
1
1
Sample
Temperature/°C
NH
3
conversion/%
GHSV/(NH
3
cm ∙gcat ∙h )
Yield/(mmol H
2
∙gcat ∙min )
Reference
Mo
Mo
Mo
2
2
2
N
N
N
550
600
550
550
550
550
550
550
550
550
550
600
600
600
550
550
94
100
67
80
69
94
83
75
37
22000
22000
6000
6000
6000
6000
6000
6000
15000
36000
2000
12000
30000
5000
5200
6000
23.82
25.44
4.48
5.35
4.62
6.29
5.56
5.02
6.19
13.26
1.7
12.3
11.4
3.3
3.5
6
This study
This study
3
3
Co
Mo
Co
3
Mo
3
N
2
N
21
21
21
21
22
26
46
47
6
3
Mo
Mo
Mo
3
N
Ni
Fe
3
3
N
3
3
N
Mo
Co Mo
Ni/Sp-C700
2
N
3
3
N
33
81.9
100
36.4
60
60
90
Ni/Y
Ni/SiO
Co-containing CNTs
Co-Cs/MSC-30
2
O
3
2
48
49
1
5
CMLa-N
2
3 2 2 2 2 2
Figure 9 NH -TPD of (a) Mo N, (b) W N and (c) VN catalysts and N -TPD of (d) three nitrides (Mo N, W N and VN) catalysts.
According to previous work, recombinative desorption for N
2
2 2
our prepared Mo N catalyst was more stable. Thus, Mo N catalyst
could facilitate ammonia adsorption and nitrogen desorption
process, which led to the high ammonia decomposition activity.
Apparently, our prepared Mo N catalyst has shown high cata-
2
lytic activity for ammonia decomposition reaction. In order to
further investigate the origin of its great catalytic performances,
occurred at low temperature was critical to obtain an optimum
ammonia decomposition catalyst since it was regarded as the
[
50]
rate-limiting step. Therefore, we carried out the temperature
programmed desorption by nitrogen (N -TPD) test to evaluate the
recombinative desorption rate of the adsorbed N over the three
transition metal nitrides (Figure 9d). The N desorption peak be-
low 600 °C was only observed on Mo N catalyst. It could be de-
duced that Mo N catalyst was able to make the recombinative
2
2
2
the optimum Mo
and X-ray photoelectron spectroscopy (XPS) techniques. The XRD
patterns and HRTEM images of the fresh and used Mo N catalysts
were presented in Figure 10a. Only the diffraction peaks corre-
sponding to cubic γ-Mo N (JCPDS#25-1366) could be observed for
Mo N sample before and after the catalytic tests, indicating that
the active γ-Mo N phases remain stable during high temperature
reaction. As shown in HRTEM images, the Mo N catalyst exhibited
mesoporous microstructure with microspores of about 10 nm
diameter. The average particle size of Mo N materials measured
2
N catalyst was characterized by XRD, HRTEM
2
2
2
desorption of nitrogen atoms more easily, providing the possibility
of the enhancement in the catalytic activity. The peak appeared at
high temperature (> 600 C) cannot be attributed to simple de-
2
2
sorption of N
arises from the liberation of N
line structure or the decomposition of nitrides at high tempera-
2
desorbed on nitrides. We suspected that N
2
peak
2
2
due to a change of nitrides crystal-
2
[
51]
ture. According to the TPD experimental results, we could also
easily found that the structures of W N and VN were changed by
denitrogenation at high reaction temperature under flowing He
gas, whereas that change was not observed on Mo N. The N
signal of NH -TPD was similar to that of N -TPD, which proved that
2
2
from the catalyst (~6 nm) was consistent with the average parti-
cle size calculated from XRD pattern. We could also find that the
2
2
2
particle size and morphology of the Mo N catalysts were not sig-
3
2
nificantly changed after the reaction, which suggested that the
Chin. J. Chem. 2019, 37, 364－372
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
369

Comprehensive Report
Xu et al.
structure of Mo
were performed for Mo
surface oxide layer provided by passivation step in the prepara-
tion process (Figure 10b). Four peaks of Mo-N bond at the binding
2
N catalyst was very stable. XPS measurements Experimental
N catalyst to confirm the presence of the
2
Chemicals
All reagents used in preparation process were bought from
commercial suppliers and could be used directly without further
purification. Ammonium paramolybdate [(NH ) Mo O ∙4H O] and
2
+
energy of 229.3, 230.3, 233 and 236.2 eV correspond to Mo ,
4
+
6+
6+
[3,52]
Mo , Mo and Mo , respectively. Previous reports
showed
that Mo , Mo and Mo oxidation states mainly originate from
Mo N, MoO and the oxidized form of γ-Mo N phase, respectively,
which suggested that more than one oxygen species present on
the surface of Mo N sample due to the surface oxidation. These
4
6
7
24
2
2
+
4+
6+
sodium tungstate dehydrate (Na WO ∙2H O) were provided by
2
4
2
2
2
2
Tianjin Kermal Chemical Reagent Factory. Sodium metavanadate
(NaVO ) was obtained from Shanghai Macklin Biochemical Co., Ltd.
3
2
Hydrochoric acid (HCl, 36%—38%) was obtained from Laiyang
experimental results clearly concluded that a ultrathin surface
oxide layer provided by the passivation step during the prepara-
Kantian chemical Co., Ltd.
Catalysts preparation
tion process prevents bulk oxidation of the active phase (Mo
2
N).
At the same time, the morphology and structure of Mo N catalyst
2
The precursor of Mo
2
)
N catalyst was prepared by hydrothermal
Mo O was dissolved with 52 mL
[27]
could be well maintained at high reaction temperature, and there
was no obvious structural change caused by denitrogenation.
Consequently, we could come to the conclusion that the stable
structure of the Mo N catalyst was the key to the high activity in
2
ammonia decomposition reaction.
method. 1.038 g (NH
4
6
7 2
O24·4H
–
1
millipore water in Teflon bottle. Then, 10 mL of HCl (2 mol∙L ) was
added to adjust the pH values of the solution to 1 and stirred for
30 min. Teflon bottle (100 mL) was sealed in a stainless-steel au-
toclave and held at different temperatures (100, 150, 180, and
2
20 C) for 15 h. The products were washed with millipore water
and ethanol, dried in air, and then calcined at 400 °C for 4 h in
muffle furnace. The precursors of the W N and VN catalysts were
prepared by using Na WO ·2H O and NaVO as raw materials,
2
2
4
2
3
respectively. The hydrothermal temperatures were set as 220 C
and then calcined at 500 C for 4 h. The other steps were the
same as the above procedure.
2 2
The Mo N, W N and VN catalysts were prepared by tempera-
ture-programmed nitridation method. First, 0.6 g corresponding
precursor was placed between two plugs of quartz wool in a tub-
–
1
ular quartz reactor and contacted with 50 mL∙min NH
3
gas, kept
at 20 C for 1 h and then heated to 650 C with heating rate of
–
1
2
°C ∙min . The final temperature was maintained for 4 h. The
products were cooled to room temperature and then passivated
in 1% O /90% He for 2 h to obtain the final metal nitrides.
2
Catalysts characterization
Transmission electron microscopy (TEM) images of the sam-
ples were obtained on a JEOL JEM-2100 microscope (100 kV).
High-resolution transmission electron microscope (HRTEM) imag-
es of the samples were acquired using a Philips Tecnai F20 in-
strument (200 kV). All samples were suspended in ethanol and
sonicated for 15 min. Then, the suspension was deposited onto
ultrathin carbon film-coated copper grids.
X-ray diffraction (XRD) patterns were derived at room tem-
perature by using a PANalytical X’pert3 powder diffractometer
(
1
λ
Cu Kα = 0.15406 nm, 40 mA, 40 kV) at a scanning range of
2
Figure 10 (a) XRD and HRTEM patterns of fresh and used Mo N catalysts;
0—90 (2θ).
2
(b) XPS profile of Mo 3d in the Mo N catalyst.
2
N -sorption isotherms were obtained by using a Builder sur-
face area analyzer (SSA-4200) measured at –196 C after de-
gassed at 200 °C for over 4 h under vacuum. Specific surface area
was calculated by applying the Brunauer-Emmett-Teller method.
Pore size distributions were calculated based on the Barrett-
Joyner-Halenda method.
X-ray photoelectron spectroscopy (XPS) analysis of the sam-
ples was recorded using an Axis Ultra analyzer with Al Kα radia-
tion (225 W) and corrected with a C 1s line at 284.6 eV.
Conclusions
In this paper, by the temperature programmed reaction of
α-MoO nanobelts with NH , we successfully synthesized nano-
belt-shaped Mo N catalyst for ammonia decomposition. The op-
3
3
2
timal synthetic conditions of precursor were determined as fol-
lows: HCl as acid source, pH = 1, hydrothermal temperature =
2
2
20 °C and calcination temperature = 500 °C. The bulk Mo N cat-
alyst could facilitate ammonia adsorption and the nitrogen de-
sorption process. Meanwhile, its morphology and structure could
be well maintained, and there were no obvious structural changes
caused by denitrogenation at high reaction temperature. These
results indicated that the structure of the catalyst was highly sta-
3
Temperature programmed desorption by ammonia (NH -TPD)
of the catalyst was performed on mass spectrometer (AMETEK,
DYCOR LC-D200). Before measurement, an amount of 300 mg of
catalyst (20—40 mesh) was pretreated in a 10% NH
3
/Ar mixture
(30 mL∙min ) flow at 500 C for 1 h. After cooling down to 20 C,
the catalyst was purged for 30 min under 10% NH /Ar mixture (30
mL∙min ) flow to obtain a saturated adsorption. Then, the feed
–
1
ble during the reaction. Therefore, Mo
high activity and stability for the ammonia decomposition reac-
tion. The facile synthesis method and high catalytic performance
indicated that the Mo
become a substitute for noble metals in the application for am-
monia decomposition to produce CO -free hydrogen.
2
N catalysts showed very
3
–
1
–
1
gas was switched to pure He (60 mL∙min ) until the stabilization
of baseline. Finally, desorption of NH was conducted in a stream
2
N catalyst we prepared may be possible to
3
–
1
–1
of pure He (60 mL∙min ) from 20 C to 600 C (10 C∙min ) and
the corresponding TPD curve was recorded. For temperature
x
370
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© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, 364－372

2
MO N Nanobelts for Ammonia Decomposition
Chin. J. Chem.
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Manuscript received January 9, 2019
Manuscript revised: February 11, 2019
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Chin. J. Chem. 2019, 37, 364－372