Thermally Stable Hierarchical Nanostructures of Ultrathin MoS2 Nanosheet-Coated CeO2 Hollow Spheres as Catalyst for Ammonia Decomposition

Article abstract of DOI:10.1021/acs.inorgchem.6b00265

MoS₂ ultrathin nanosheet-coated CeO₂ hollow sphere (CeO₂@MoS₂) hybrid nanostructures with a 3D hierarchical configuration were successfully constructed from a facile two-step wet chemistry strategy: first, CeO₂ formed on a silica core which served as a template and was subsequently removed by NaOH solution to attain hollow spheres, and then few-layered ultrathin MoS₂ nanosheets were deposited on the CeO₂ hollow spheres through a hydrothermal process. As a proof of concept application, the as-prepared CeO₂@MoS₂ hybrid nanostructures were used as catalytic material, which exhibited enhanced catalytic activity in ammonia decomposition for H₂ production at high temperature. It was demonstrated that, even with a structural transformation from MoS₂ to MoN_x under harsh conditions of ammonia decomposition at high temperature (700 °C), the 3D hierarchical nanostructures of the CeO₂@MoN_x were well kept, indicating the important role of the CeO₂ support.

Full text of DOI:10.1021/acs.inorgchem.6b00265

Article
pubs.acs.org/IC
Thermally Stable Hierarchical Nanostructures of Ultrathin MoS2
Nanosheet-Coated CeO Hollow Spheres as Catalyst for Ammonia
2
Decomposition
†
,‡,§
∥
†
†
,∥
,†,§
Xueyun Gong,
Ying-Qiu Gu, Na Li, Hongyang Zhao, Chun-Jiang Jia,* and Yaping Du*
†Frontier Institute of Science and Technology jointly with College of Science, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710054, China
‡
College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, China
§
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry &
Materials Science, Northwest University, Xi’an 710069, China
∥
Key Laboratory for Colloid and Interface Chemistry, Key Laboratory of Special Aggregated Materials, School of Chemistry and
Chemical Engineering, Shandong University, Jinan 250100, China
*
S Supporting Information
ABSTRACT: MoS ultrathin nanosheet-coated CeO hollow sphere (CeO @
2
2
2
MoS ) hybrid nanostructures with a 3D hierarchical configuration were
2
successfully constructed from a facile two-step wet chemistry strategy: first,
CeO formed on a silica core which served as a template and was subsequently
2
removed by NaOH solution to attain hollow spheres, and then few-layered
ultrathin MoS nanosheets were deposited on the CeO hollow spheres
2
2
through a hydrothermal process. As a proof of concept application, the as-
prepared CeO @MoS hybrid nanostructures were used as catalytic material,
2
2
which exhibited enhanced catalytic activity in ammonia decomposition for H2
production at high temperature. It was demonstrated that, even with a
structural transformation from MoS to MoN under harsh conditions of
2
x
ammonia decomposition at high temperature (700 °C), the 3D hierarchical
nanostructures of the CeO @MoN were well kept, indicating the important
2
x
role of the CeO support.
2
INTRODUCTION
thus opening the door of realizing practical applications of this
■
27−29
kind of nanostructure.
Ammonia (NH ), which can be
3
In recent years, two-dimensional (2D) layer structured
nanomaterials have attracted increasing interest due to their
unique properties and promising applications.
easily liquefied under moderate conditions, has been considered
1
−8
as an excellent storage medium for hydrogen compared with
As an
3
0
traditional carbonaceous materials. Catalytic ammonia
important two-dimensional (2D) layer structured nanomaterial,
decomposition is an efficient method to generate CO -free
x
MoS has been widely used for diverse applications, such as
2
9
−14
15
16
hydrogen. Up to now, Ru has been found to be the best
catalysis,
sistors,
solid-state lubrication, energy storage, tran-
31
1
7,18
19−21
22−24
ammonia decomposition catalyst. Transition metals based on
sensing,
and lithium batteries.
As far as
Fe and Ni have also been investigated as excellent catalysts for
the catalysis applications for MoS are concerned, both
theoretical calculations and experimental results have
indicated that the catalytic activity mainly originated from
2
32−35
2
5
26
NH decomposition.
Recently, molybdenum nitrides were
3
found exhibiting high performance as catalyst for catalytic
36
decomposition of ammonia to produce CO -free hydrogen.
x
active sites located along the edges of MoS layers. However,
2
Molybdenum nitrides can be obtained by ammonolysis of
the freshly prepared MoS2 layers would easily tend to
restacking and aggregation, especially during the high-temper-
ature reaction like ammonia decomposition, because of their
high surface energy and interlayer van der Waals attractions,
which severely reduced the active sites and damped the
performances concerning the practical applications. Therefore,
how to avoid the stacking and aggregation, thus increasing the
exposed active sites, is of special importance. Practically,
rational design and synthesis of three-dimensional (3D)
hierarchical architectures based on 2D nanosheets is an
effective approach for preventing stacking and aggregation,
37
MoS2, which gave the possibility for MoS used for catalytic
2
ammonia decomposition.
Ceria (CeO ) has been studied intensively for its unique
2
properties and found to act as a highly effective support or
promoter for catalytic ammonia decomposition to hydro-
3
8,39
gen.
Up to now, diverse morphologies of CeO
2
4
0
nanostructures have been fabricated, such as nanocubes,
Received: February 1, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00265
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Inorganic Chemistry
Article
4
1,42
nanotubes,43−45 etc. Among the various
nanorods,
morphologies, the porous hollow spheres
stainless steel autoclave and heated in an electric oven at 130 °C for 24
46,47
h. The autoclave was cooled down to room temperature. The SiO @
with remarkable
2
CeO particles were collected by centrifugation (10 000 rpm) and
interior space and pore structure are particularly attractive
because of their low density, high specific surface area, and
superior permeation.
Herein, for the first time, we reported the growth of 2D
ultrathin MoS nanosheets on uniform CeO porous hollow
spheres to form ultrathin MoS2 nanosheet−CeO2 hollow
sphere hybrid nanostructures with a 3D hierarchical config-
uration, which is denoted as CeO @MoS . The CeO @MoS
2
2
washed with absolute ethanol. The products were dispersed in 5 mol/
L NaOH solution for 2 days to attain CeO hollow spheres for further
2
use.
Synthesis of MoS Nanosheets. In a typical procedure, 50 mg of
2
2
2
sodium molybdate, 100 mg of thioacetamide, and 14 mL of deionized
water were added into a 25 mL Teflon-lined stainless steel autoclave.
After stirred for 30 min, the suspension solution in Teflon-lined
stainless steel autoclave was heated in an electric oven at 200 °C for 24
h. The autoclave was then cooled down to room temperature. The
black precipitate was collected by centrifugation, washed thoroughly
with absolute ethanol, and dried at 65 °C overnight in vacuum oven.
2
2
2
hybrid nanostructures and MoS nanosheets were used as
2
catalysts for the production of hydrogen via catalytic
decomposition of ammonia. It is found that, even through a
structural transformation from MoS to MoN under a harsh
condition of ammonia decomposition at high temperature (700
2
C), the 3D hierarchical nanostructures of the CeO @MoNx
The as-prepared MoS nanosheets were further treated at 600 °C in Ar
2
2
x
atmosphere for 4 h to obtain high crystallinity.
Synthesis of CeO @MoS Hybrid Nanostructures. In a typical
2
2
°
procedure, a 20 mg portion of the obtained CeO hollow spheres was
2
were kept well (Scheme 1), indicating the important role of the
dispersed in 14 mL of deionized water with ultrasonication for 30 min.
Then, 50 mg of sodium molybdate and 100 mg of thioacetamide were
added. After being stirred for 30 min, the suspension solution was
transferred into a 25 mL Teflon-lined stainless steel autoclave and then
heated in an electric oven at 200 °C for 24 h. The autoclave was then
cooled down to room temperature. The black precipitates were
collected by centrifugation, washed thoroughly with absolute ethanol,
and dried at 65 °C overnight in a vacuum oven. The as-prepared
CeO @MoS hybrid nanostructures were further treated at 600 °C in
Scheme 1. Schematic Illustration for the Synthesis of CeO @
MoS Hybrid Nanostructures and Its Use in Catalytic
Decomposition of Ammonia for H Production
2
2
2
2
2
Ar atmosphere for 4 h to obtain high crystallinity.
Characterization. The morphologies of the as-obtained products
were examined by the transmission electron microscopy (TEM,
Hitachi HT-7700) and scanning electron microscopy (SEM, FEI
Quanta F250). The detailed microscopic structure and the chemical
composition of the products were analyzed using high-resolution
transmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN)
operated at 200 kV. The crystallinity and crystal phase of the dried
powders of samples were examined by X-ray diffraction (XRD, Rigaku
D/MAX-RB) with a scanning rate of 5°/min from 10° to 80°, using
Cu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption−desorption
isotherms were measured on a Micromeritics TriStar 3000
porosimeter (mesoporous characterization) and Micromeritics ASAP
CeO support, which induced the enhanced catalytic activity.
2
2
020 (microporous characterization) at 77 K. The samples were
However, the pure MoS nanosheets exhibited poor catalytic
2
outgassed at 60 °C for 12 h under vacuum prior to measurements. The
specific surface areas were calculated on the basis of the Brunauer−
Emmett−Teller (BET) method, and the pore size distributions were
measured from the desorption branch of isotherms using the Barrett−
Joyner−Halenda method (BJH). Energy dispersive X-ray spectrometry
activity due to the serious aggregation during the catalytic
process. Although the synergistic effect between Mo- and Ce-
based materials is still not clear, the design and synthesis of
hybrid hierarchical materials spot a light on the preparation of
highly efficient and stable catalysts in heterogeneous catalysis.
(EDX) characterizations were performed on a transmission electron
microscope (Tecnai G2 F20 S-TWIN). X-ray photoelectron spectros-
copy (XPS) data were obtained using a Escalab 250 xi photoelectron
spectrometer using Al K radiation (15 kV, 225 W, base pressure ∼5 ×
EXPERIMENTAL SECTION
Materials and Reagents. Cerium(III) nitrate hexahydrate
■
(
(
2
10
Ce(NO ) ·6H O, 99.99%, Sigma-Aldrich), tetraethylorthosilicate
10 Torr).
3
3
2
Si(OC H ) , 98%, Acros Organics), sodium molybdate (Na MoO ·
In Situ XRD Experiments. In situ XRD experiments under
2
5
4
2
4
3
H O, 99%, Fluka), thioacetamide (CH CSNH , 98%, Sigma-Aldrich),
hydrogen atmosphere were carried out on a PANalytical X’Pert
2
3
2
ammonium hydroxide (NH ·H O, 25%), absolute ethanol (C H O,
Powder diffractometer operating in reflection mode with Cu Kα
radiation (λ = 1.54178 Å, 40 kv, 40 mA). The powder sample was
loaded into a ceramic tube which was attached to an in situ flow cell
(Anton Paar HRK-900 reaction cell). A small resistance heating wire
was installed right below the tube, and the temperature was monitored
with a chromel−alumel thermocouple that was placed inside the tube
3
2
2
6
9
9.7%), sodium hydroxide (NaOH, 96%), and ethylene glycol
(
C H O ) were purchased from Tianjin Zhiyuan Chemical Company.
2
6
2
All chemicals were used without any further purification. Deionized
water was used throughout this study.
Synthesis of CeO Hollow Spheres. The CeO hollow spheres
2
2
were synthesized via a slightly modified method developed by Stucky
near the sample. The in situ XRD tests (5% H /Ar, 30 mL/min) were
2
47
et al. In a typical procedure, a mixture of deionized water (56 mL)
and ammonium hydroxide (8.4 mL) was added into a flask containing
the mixture of ethanol (280 mL) and tetraethylorthosilcate (8 mL)
under vigorous stirring at room temperature for 24 h; the products
were then collected by centrifugation (8500 rpm), followed by drying
carried out following a temperature-programmed mode: 25 °C → 100
°C → 200 °C → 300 °C → 400 °C → 500 °C → 600 °C → 700 °C
→ 800 °C → cool down (ramping rate: 30 °C/min) with stabilization
at each temperature for 60 min. Data were collected with a step width
of 0.013°, and a counting time of 50 s per step (20 min/run). Data
obtained from the last run at each temperature was used for plotting.
at 60 °C for 6 h. A 100 mg portion of dried SiO was dispersed in 13
2
mL of ethylene glycol with ultrasonication in a beaker. Then, 0.5 g of
cerium nitrate hexahydrate and 0.75 mL deionized water were added,
and the mixture was stirred for 30 min to form a homogeneous
solution. The mixture was then transferred to a 25 mL Teflon-lined
Catalytic Testing. For a typical ammonia (NH ) decomposition
3
experiment, 100 mg (20−40 mesh) of the catalyst was loaded in a
quartz tube (I.D. = 6 mm) fixed bed reactor, and pure gaseous NH
3
was passed through the catalyst bed. The reactor temperature was
B
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Inorganic Chemistry
Article
Figure 1. (a) XRD patterns of CeO @MoS hybrid nanostructures, CeO hollow spheres, and pure ultrathin MoS nanosheets. In situ XRD patterns
2
2
2
2
collected under hydrogen atmosphere 5% H in Ar of (b) pure MoS nanosheets and (c) CeO @MoS hybrid nanostructures.
2
2
2
2
Figure 2. SEM images of (a) CeO hollow spheres and (b) CeO @MoS hybrid nanostructures. TEM images of (c) CeO hollow spheres and (d)
2
2
2
2
CeO @MoS hybrid nanostructures and (e, f) HRTEM images. (g) HAADF-STEM image of CeO @MoS hybrid nanostructures.
2
2
2
2
increased from 400 to 700 °C in 50 °C steps. At each step, the reaction
was allowed to equilibrate for 60 min to reach the steady-state
conditions, and data obtained from the last gas chromatography run at
each temperature were used to calculate the conversion value.
Reaction temperature was maintained at 700 °C for 58 h to evaluate
MoS nanosheets were examined with powder X-ray diffraction
analysis (XRD, Figure 1a). The diffraction peaks of CeO2
hollow spheres can be readily indexed to the cubic phase with
2
lattice constant of a = b = c = 5.41 Å (space group Fm3
̅
m,
JCPDS 34-0394). As for the pure ultrathin MoS nanosheets,
2
the stability of the catalyst, and the NH conversion was recorded
3
the crystal phase mainly consists of hexagonal MoS , but the
continuously. After the stability test, the temperature was decreased to
2
ambient temperature under NH flow and then increased from 400 to
discernible peaks cannot match the standard MoS card
2
3
7
00 °C in 50 °C steps. The NH conversion data were collected again
(JCPDS 37-1492) exactly. The diffraction peaks corresponding
3
during the heating procedure. The concentrations of outlet gases were
analyzed by an online gas chromatograph (Ouhua GC 9160), which
was equipped with a thermal conductivity detector (TCD) and
Porapark Q column (1.5 m of length).
to MoS were not observed in the XRD pattern of CeO @
2
2
MoS hybrid nanostructures, indicating that MoS nanosheets
2
2
coating on the CeO hollow spheres may consist of only few
2
9
,22,48−51
layers, which are too thin to be detected.
Due to the reducibility of produced H in catalytic NH
2
3
RESULTS AND DISCUSSION
■
decomposition conditions, it is necessary to monitor the crystal
The crystal phases of as-harvested CeO @MoS hybrid
nanostructures, CeO hollow spheres, as well as ultrathin
phase changes of catalyst under H atmosphere at the reaction
2
2
2
temperature. The corresponding in situ XRD results were
2
C
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Inorganic Chemistry
Article
shown in Figure 1b,c. The diffraction peaks of pure MoS2
nanosheets (Figure 1b) cannot match the standard MoS2
exactly at room temperature; with the temperature increase,
the peak shift occurred. When the temperature was higher than
3
00 °C, the detected peaks can be assigned to the (002), (100),
(103), and (110) planes in the hexagonal MoS phase. As for
2
CeO @MoS hybrid nanostructures (Figure 1c), diffraction
2
2
peaks assigned to hexagonal MoS phase also appeared when
2
the temperature was higher than 200 °C. The above results
demonstrated that the deviation of ultrathin MoS nanosheets
2
diffraction peaks from the standard card may be caused by the
5
2
low crystallinity. The crystallinity was improved as the
temperature increased and the low-crystalline MoS nanosheets
2
transformed to pure hexagonal MoS phase finally.
2
The morphology of as-obtained CeO hollow spheres and
2
CeO @MoS hybrid nanostructures is shown in Figure 2a,b,
2
2
respectively. As seen from the scanning electron microscopy
SEM) image of Figure 2a, the CeO hollow spheres were
(
2
composed of small CeO particles. Figure 2b showed the SEM
2
image of CeO @MoS hybrid nanostructures. As seen from
2
2
Figure 2b, the MoS nanosheets were uniformly coated on the
2
surfaces of CeO hollow spheres to form a 3D hierarchical
2
configuration. The inset in Figure 2b illustrated that the
Figure 3. EDX elemental mapping images of Ce, O, Mo, and S in the
selected area showing in Figure 2g.
CeO @MoS hybrid nanostructures possessed an inner hollow
2
2
structure. The structures of as-synthesized CeO2 hollow
spheres and CeO @MoS hybrid nanostructures were further
2
2
9
1 n
studied by transmission electron microscopy (TEM) (Figure
c,d). From Figure 2c it can be seen that the average thickness
eV) and Ce 3d 4f L (885.5 eV) states corresponded to v and
o
2
v . The u (u = u, u , u , u , u ) structures corresponding to the
1
0
1
2
3
of CeO was approximately ∼40 nm and the diameter of the
2
Ce 3d3 levels could be characterized by the same assign-
/2
53,54
hollow spheres was ∼260 nm. Figure 2d exhibited the typical
ment.
Therefore, the XPS results indicated that Ce in the
TEM image of CeO @MoS hybrid nanostructures; it was
2
2
as-synthesized CeO @MoS hybrid nanostructures was mainly
2
2
clearly demonstrated that the ultrathin MoS nanosheets have
cerium(IV) and cerium(III). The fitting of the O 1s region with
two-peak contribution elucidated that at least two kinds of
oxygen species were present in the near surface domain of
CeO @MoS hybrid nanostructures (Figure 4b). The peak at
2
covered the surface of CeO hollow spheres. As seen from
2
Figure S1, the pure MoS obtained by the same hydrothermal
2
method without the presence of CeO2 hollow spheres
presented the nanosheet morphology. From the high-resolution
2
2
about 530.0 eV was due to crystal lattice oxygen of CeO , while
2
transmission electron microscopy (HRTEM) images of CeO @
the peak at about 532.5 eV was due to chemisorbed oxygen on
the hybrid nanostructure surfaces. The binding energies of Mo
3d3/2 and Mo 3d5/2 peaks were located at 231.9 and 228.7 eV,
respectively, suggesting that the oxidation state of the Mo ion is
positive quadrivalence in the synthesized CeO @MoS hybrid
2
MoS hybrid nanostructures in Figure 2e,f, the lattice fringes
2
were clearly visible with a spacing of ∼0.27 nm, corresponding
to the spacing of the (200) planes of CeO , and the lattice
2
fringes with a spacing of ∼0.27 nm and ∼0.64 nm
corresponded to the (100) and (002) plane of hexagonal
2
2
28
nanostructures (Figure 4c). Meanwhile, the peaks at 161.6
and 162.6 eV in Figure 4d could be assigned to the binding
energies of S 2p3/2 and S 2p1/2, respectively; thus, the S was
negative divalent in the CeO @MoS hybrid nanostructures.
MoS (Figure 2f). High-angle annular dark-field scanning TEM
2
(
HAADF-STEM) image in Figure 2g indicated that the CeO @
2
MoS hybrid nanostructures assumed the well-defined 3D
2
2
2
hierarchical morphology.
Meanwhile, energy dispersive X-ray spectrometry (EDX)
Full nitrogen sorption isotherms of as-synthesized CeO @
2
MoS2 hybrid nanostructures were recorded to attain
information on the pore size distribution and specific surface
area. A type IV nitrogen isotherm shown in Figure 5a suggested
mapping analysis of the CeO @MoS hybrid nanostructures
2
2
also verified the core (CeO hollow spheres) and shell (MoS2
2
55
nanosheets) hierarchical structure (Figure 3). From the
elemental mapping images, it can be seen that the inner
hollow core consists of Ce and O elements, and Mo and S
elements are homogeneously dispersed in the shell, further
a characteristic of mesoporous materials. Accordingly, the
Brunauer−Emmett−Teller (BET) specific surface area of
2
−1
CeO @MoS hybrid nanostructures was about 54.9 m g .
2
2
The pore size distribution derived from adsorption data and
calculated by the Barrett−Joyner−Halenda (BJH) method was
shown in Figure S2. The plot indicated that most pores were in
the mesoporous structure with the pore diameter of
approximately ∼4 nm. Such a high specific area of hollow
structures with mesoporous structure could be desirable for
catalytic applications, which may facilitate the transportation of
mass and reactants.
confirming that the ultrathin MoS nanosheets were uniformly
2
coated on the surfaces of CeO hollow spheres.
2
X-ray photoelectron spectroscopy (XPS) was applied to
investigate the chemical states of Ce, O, Mo, and S in the
CeO @MoS hybrid nanostructures (Figure 4). As shown in
2
2
Figure 4a, the Ce 3d electron core level was characterized by
two series of peaks: 3d and 3d3/2. For 3d5/2 of Ce(IV), the
5
/2
n−2
9
2
9
1 n−1
peaks of the Ce 3d 4f L (882.8 eV), Ce 3d 4f L (887.3
eV), and Ce 3d 4f L (899.0 eV) states were labeled v, v , and
v3, respectively. For 3d5/2 of Ce(III), the Ce 3d 4f L (882.8
The catalytic activity of each sample for the hydrogen
production via decomposition of ammonia was tested in two
identical runs, in which the first heating run was regarded as the
9
0 n
2
9
2 n−1
D
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Figure 4. XPS spectra of CeO @MoS hybrid nanostructures: (a) Ce 3d, (b) O 1s, (c) Mo 3d, and (d) S 2p.
2
2
self-activation procedure. The catalytic tests were performed
over 100 mg of CeO @MoS samples with ammonia (NH )
obviously higher than that of the former, demonstrating that
the transformation of MoS to molybdenum nitride in Figure
2
2
3
2
3
−1 −1
space velocity of 12 000 cm g
h . After activation, the
5b was incomplete and a simple activated process would not
fully exhibit the actual catalytic ability of catalysts. It may be the
reason that the catalytic activity of CeO @MoS hybrid
cat
catalytic performance of CeO @MoS hybrid nanostructures
2
2
for NH3 decomposition reaction, tested as a function of
temperature, was presented in Figure 5b. As can be seen, the
2
2
exhibited lower activity compared with the previously reported
56
activity in terms of NH conversion increased with elevating the
MoO catalyst.
3
x
reaction temperature. The content of MoS in CeO @MoS
To investigate the phase changes, structure, and texture of
the CeO @MoS hybrid nanostructures, characterizations on
2
2
2
hybrid nanostructures was ca. wt 62%; however, the NH3
conversion of CeO @MoS hybrid nanostructures was
2
2
the reacted catalysts after the NH decomposition have been
2
2
3
obviously higher than that of pure MoS nanosheets when
the same amount of catalysts was adopted (100 mg), indicating
carried out. As seen from Figure 6a, the crystal phases of the
used CeO @MoS hybrid nanostructures and pure ultrathin
2
2
2
CeO serves as an excellent promoter which could effectively
MoS nanosheets after a stability test were examined with XRD.
2
2
improve the catalytic activity of MoS . To further demonstrate
the high-temperature stability of catalysts, long-duration
Ammonolysis of MoS at 700 °C for 58 h led to a mixture of
2
2
the molybdenum nitride phase, and the XRD diffraction
patterns were assigned to hexagonal MoN (JCPDS 25-1367)
and hexagonal Mo N (JCPDS 51-1326), respectively. There
stability tests were performed for pure MoS nanosheets and
2
CeO @MoS hybrid nanostructures at 700 °C with a gas
2
2
5
6
3
−1 −1
hourly space velocity (GHSV) of 12 000 cm g
h
over 58
still exists unreacted molybdenum sulfide phase which
cat
h. As depicted in Figure 5c, the catalytic activity of CeO @
corresponded to monoclinic Mo2S3 (JCPDS 40-972). For
2
MoS hybrid nanostructures was much higher than that of pure
CeO @MoS2 hybrid nanostructures, molybdenum sulfide
2
2
MoS nanosheets during the whole process. The conversion of
transformed to molybdenum nitride completely after reaction
2
two samples remained increased over the initial 34 h and
subsequently stabilized at nearly 100% conversion, indicating
at 700 °C for 58 h, and CeO phase was transformed to
2
hexagonal Ce O S (JCPDS 04-626). For Mo-based catalysts,
2
2
that nitridation of MoS to molybdenum nitride was a very slow
Tagliazucca et al. have performed in situ XRD to study the
2
process. Figure 5d showed the comparison of catalytic activity
for CeO @MoS hybrid nanostructures between activation by a
phase transformations of MoO catalyst during the NH
decomposition reaction. The phase transformations for
3
3
5
6
2
2
first heating procedure (Figure 5b) and by the stability test at
00 °C for 58 h. The NH conversion of the latter was
molybdenum oxide during the NH decomposition process
was very complex because nitridation of molybdenum and
3
7
3
E
DOI: 10.1021/acs.inorgchem.6b00265
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Inorganic Chemistry
Article
Figure 5. (a) N adsorption/desorption isotherms of CeO @MoS hybrid nanostructures. (b) Temperature dependent NH conversion at a GHSV
2
2
2
3
3
−1 −1
of 12 000 cm g
1
h
after activation by a first heating process. (c) Long-term catalytic stability of the catalysts measured at 700 °C at a GHSV of
cat
3
−1
h−1 (100 mg, 20 mL/min). (d) Comparison of catalytic activity for CeO @MoS hybrid nanostructures between activation by a
2 000 cm g
cat
2
2
first heating procedure and by a stability test at 700 °C for 58 h.
3
8
reduction of molybdenum oxide occurred simultaneously. They
concluded that many factors, including the active phase,
domain size, and surface species/area, could influence the
activity of the catalyst. It would be too simple to correlate the
from aggregating. In the CeO @MoS hybrid nanostructures,
2 2
we believe that the CeO spheres inside can also enhance the
2
dispersion of the active molybdenum sites and suppress MoS2
nanosheets from restacking and aggregation under harsh
reaction conditions. Also, the possible electron transfer on
the Ce−Mo interface could be accelerated, through which the
performance of the CeO @MoS hybrid nanostructures for
catalytic activity only to the phase composition. For the CeO @
2
MoS hybrid, the phase transformation was more complicated
2
than that for MoO , so it is impossible to distinguish the
3
2
2
contribution of each component. The morphology of CeO2
catalytic decomposition of ammonia was promoted.
(
Ce O S) hollow spheres had no substantial changes due to its
In addition, XPS was applied to investigate the surface state
of the corresponding catalyst after the stability test of Ce, O, S,
Mo, and N in CeO @MoS hybrid nanostructures (Figure
2
2
excellent thermal stability (Figure 6b). However, pure MoS2
MoN ) nanosheets seemed to severe restacking and
(
x
2
2
aggregation after the harsh reaction conditions (temperature
up to 700 °C, large concentration of ammonia and/or
hydrogen) as can be observed from the TEM and HAADF-
STEM (inset) image in Figure 6c. For CeO @MoS (Ce O S@
S3a−e) and pure MoS nanosheets (Figure S4a−c). As shown
2
in Figure S3a, only v (v = v , v ) and u (u= u , u ) for
o
1
o
1
characteristics of Ce(III), which could be observed in the Ce 3d
XPS spectrum, indicate that the oxidation state of Ce(IV) was
2
2
2
2
54
MoN ) hybrid nanostructures (Figure 6d), the CeO (Ce O S)
reduced to Ce(III). Meanwhile, XRD (Figure 6a) indicated
that CeO (Ce(IV)) was reduced to Ce O S (Ce(III)) by
x
2
2
2
hollow structure was well-maintained, and no major agglom-
eration occurred in the MoS (MoN ) nanosheets coated on
2
2
2
ammonolysis. The fitting of the O 1s region with the peak at
2
x
the surfaces of CeO (Ce O S) spheres. Noticeably, even after
about 530.2 eV is due to the crystal lattice oxygen of Ce O S
2
2
2
2
2
reaction at 700 °C for 58 h, the morphology of CeO @MoS2
(Figure S3b). Meanwhile, the discernible peak at 161.5 eV in
2
(
Ce O S@MoN ) hybrids were still well-maintained (STEM
Figure S3c could be assigned to the binding energies of S 2p
2
2
x
3/2
4
+
image in inset of Figure 6d). Meanwhile, the specific surface
of Ce O S. Figure S3d showed the peak at 229.8 eV (Mo
2 2
6+
6+
area of the used CeO @MoS (Ce O S@MoN ) hybrid
3d5/2), 232.28 eV (Mo 3d ), and 235.38 eV (Mo 3d3/2),
5/2
2
2
2
2
x
57
nanostructures calculated from nitrogen sorption isotherms
suggesting the mixed valence state of the Mo species. The
peaks at 394.4 and 397.9 eV can be assigned to Mo 3p3/2 and N
1s, respectively, which were characteristic of molybdenum
2
−1
was 37.5 m g . It has been reported that CeO acts as a
2
dispersant in the Ni−Al catalyst system and supplies “spacers”
to inhibit the mobility of Ni atoms, preventing the particles
58
nitride (Figure S3e), indicating that the molybdenum sulfide
F
DOI: 10.1021/acs.inorgchem.6b00265
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. TEM images of the reacted (a) XRD patterns of the reacted CeO @MoS (Ce O S@MoN ) hybrid nanostructures and pure MoS
2
2
2
2
2
x
nanosheets after stability test. (b) CeO (Ce O S) hollow spheres, (c) MoS (MoN ) nanosheets, and (d) CeO @MoS (Ce O S@MoN ) hybrid
2
2
2
2
x
2
2
2
2
x
nanostructures after two heating runs (insets in c and d represent HAADF-STEM images of corresponding catalyst after stability test).
transformed to molybdenum nitride after reaction at 700 °C for
surface area and exhibited enhanced activity in catalytic
5
3
8 h. Figure S4a−c showed the XPS signals taken from the Mo
ammonia decomposition for H production. The preparation
2
d, S 2p, Mo 3p, and N 1s regions of the corresponding catalyst
of CeO @MoS hybrid nanostructures and use of it as catalyst
2
2
after the stability test of pure MoS nanosheets. As seen from
for ammonia decomposition may pave the way toward facile
and efficient production of ultrathin transition metal
dichalcogenides and rare earth materials-based catalysts with
excellent performance.
2
Figure S4a, XPS signals at 235.6 and 232.5 eV corresponded to
Mo6 3d3/2 and Mo 3d , respectively. The peaks at 228.8 eV
+
6+
5/2
4+
in Figure S4a could be assigned to the binding energies of Mo
d5/2 of MoS . Double peaks at 162.8 and 161.7 eV (Figure
3
2
S4b), attributable to the core levels of S 3p1/2 and S 3p3/2,
ASSOCIATED CONTENT
■
2−
respectively, were characteristic of S in MoS . While the peaks
2
*
S
Supporting Information
located at 394.3 and 397.8 eV were ascribed to the levels of Mo
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00265.
3
p3/2 and N 1S of molybdenum nitride (Figure S4c), it is
noticed that the XRD results showed the samples after the
catalytic tests were mainly composed of Mo S and MoN ,
2
3
x
indicating that Mo should be reduced from +4 to even lower
oxidation state in the reductive reaction condition. However,
Mo6 was observed from the XPS spectra, which is perhaps
because the surface of the sample was oxidized when it was
exposed to air since XPS is a surface-sensitive technique.
Figures S1−S4 (TEM, pore size distribution, and XPS
data) (PDF)
+
AUTHOR INFORMATION
■
*
*
Corresponding Authors
E-mail: jiacj@sdu.edu.cn.
E-mail: ypdu2013@mail.xjtu.edu.cn.
CONCLUSION
■
In summary, for the first time, we reported the synthesis of 2D
Author Contributions
X.G., Y.-Q.-G., and N.L. contributed equally to this work.
ultrathin MoS nanosheets grown on uniform CeO hollow
2
2
spheres to form CeO2 hollow spheres@MoS2 nanosheets
hybrid nanostructures with a 3D hierarchical configuration. The
as-prepared CeO @MoS hybrid nanostructures possessed high
Notes
The authors declare no competing financial interest.
2
2
G
DOI: 10.1021/acs.inorgchem.6b00265
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
29) Chen, J. S.; Tan, Y. L.; Li, C. M.; Cheah, Y. L.; Luan, D.;
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We gratefully acknowledge the financial aid from the start-up
funding from Xi’an Jiaotong University, the Fundamental
Research Funds for the Central Universities (2015qngz12),
Fundamental Research Funding of Shandong University (Grant
014JC005), the Taishan Scholar project of Shandong Province
China), the NSFC (Grants 21371140, 21301107), the
National Science Funds of China for Excellent Young Scientists
Grant 21522106), and the Open Foundation of Key
Laboratory of Synthetic and Natural Functional Molecule
Chemistry of Ministry of Education (Grant 338080051). We
also appreciated Dr. Xinghua Li at Northwest University for his
kind help to obtain HRTEM images.
2
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DOI: 10.1021/acs.inorgchem.6b00265
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