Catalytic performance of phase-pure M1 MoVNbTeOx/CeO2 composite for oxidative dehydrogenation of ethane

Article abstract of DOI:10.1016/j.jcat.2018.07.016

The introduction of cerium oxide to phase-pure M1 MoVNbTeO_x with CeO₂ loading from 0 up to 90 wt.% results in highly active catalysts for oxidative dehydrogenation of ethane (ODHE). These catalysts are characterized with BET, ICP, XRD, TEM, SEM and XPS techniques and tested for ODHE process. The results show that the increase of active site, V⁵⁺, is one of the dominant factors to improve the ethane conversion. In addition, the easier re-oxidation of V⁴⁺ to V⁵⁺ due to the presence of Ce⁴⁺ makes an extra contribution to the increase of turn-over frequency. Among all the catalysts, M1 catalyst with 30 wt.% CeO₂ loading obtains the best productivity of 0.69 kg_C2H4/kg_cat·h while the corresponding performance of phase-pure M1 catalyst is 0.34 kg_C2H4/kg_cat·h at 400 °C with the contact time of 18.52 g_cat·h/mol_C2H6. It is further demonstrated that M1/CeO₂ catalyst (e.g., with 50 wt.% CeO₂ as a representative) loading remains highly active after 3 cycles of refreshment in situ, and its crystal structure and surface morphology keep stable compared to the fresh catalyst.

Full text of DOI:10.1016/j.jcat.2018.07.016

Journal of Catalysis 365 (2018) 238–248
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Catalytic performance of phase-pure M1 MoVNbTeOx/CeO2 composite
for oxidative dehydrogenation of ethane
1
1
⇑
Dan Dang , Xin Chen , Binhang Yan, Yakun Li, Yi Cheng
Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The introduction of cerium oxide to phase-pure M1 MoVNbTeOx with CeO2 loading from 0 up to 90 wt.%
results in highly active catalysts for oxidative dehydrogenation of ethane (ODHE). These catalysts are
characterized with BET, ICP, XRD, TEM, SEM and XPS techniques and tested for ODHE process. The results
show that the increase of active site, V5+, is one of the dominant factors to improve the ethane conversion.
In addition, the easier re-oxidation of V4+ to V5+ due to the presence of Ce4+ makes an extra contribution
to the increase of turn-over frequency. Among all the catalysts, M1 catalyst with 30 wt.% CeO2 loading
obtains the best productivity of 0.69 kgC2H4/kgcatꢀh while the corresponding performance of phase-pure
M1 catalyst is 0.34 kgC2H4/kgcatꢀh at 400 °C with the contact time of 18.52 gcatꢀh/molC2H6. It is further
demonstrated that M1/CeO2 catalyst (e.g., with 50 wt.% CeO2 as a representative) loading remains highly
active after 3 cycles of refreshment in situ, and its crystal structure and surface morphology keep stable
compared to the fresh catalyst.
Received 7 March 2018
Revised 5 July 2018
Accepted 6 July 2018
Keywords:
Oxidative dehydrogenation of ethane
(ODHE)
Ethylene
MoVNbTeOx
Mixed metal oxide
Ceria
Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction
sites [4,24,25]. The performance of MoVNbTeOx catalysts increases
with the purity of M1 phase [26]. It is reported that the MoVNbTeOx
Ethylene is one of the most important building blocks in
the chemical industry [1]. The oxidative dehydrogenation of
ethane (ODHE) to ethylene has received considerable attention
because of its distinct advantages (e.g., thermodynamically
favored, lower reaction temperature, no coke formation, etc.) over
steam cracking process [2–4]. In recent years, mixed metal oxides
seem to be the most promising catalytic system for ODHE reaction
[4–14]. Among them, MoVNbTeOx catalyst exhibits attractive
performances (i.e., activity, selectivity and productivity) at the
reaction temperature of about 400 °C.
Usually, MoVNbTeOx catalyst is prepared by slurry method
[14–18] or hydrothermal method [4,19,20]. This multicomponent
metal oxide is nanocrystalline solid mainly composed of phase-
M1 and phase-M2 [21]. The M1 phase has a needle-like crystal
morphology in which the (0 0 1) planes are arranged perpendicular
to the long axis of the needles [22]. While the M2 phase has char-
acteristic hexagonal rings hosting the Te-O units without any pen-
tagonal or heptagonal rings in the (0 0 1) planes [20,23]. Previous
studies have indicated that the M1 phase is the most efficient
phase in ethane oxidation, assuming V5+ ions as ethane activation
catalyst has a considerable potential for industrial application when
the catalytic activity obtains ethane conversion >80% and ethylene
selectivity >90% [27–30]. However, according to the reported
catalysts, the industrial requirements for commercialization (e.g,
1.00 kgC2H4/kgcatꢀh catalyst productivity) is far from achieved [29].
Since the catalytic activity of phase-pure M1 is related with the
amount of V5+ ions, increasing their amount could be an important
working direction. Meanwhile, because of the expensive raw
material for catalyst preparation and mass loss caused by post-
treatment, the high cost of MoVNbTeOx catalysts is one of the cru-
cial reasons hindering the industrial implementation in the ODHE
process [11]. As a result, introduction of cheap promoters naturally
becomes a simple and effective method for catalyst improvement
and cost reduction. At present,
a-Al2O3, SiO2, and ZrO2 have been
added into the MoVNbTeOx catalysts [11,31] as diluters with little
influence on the surface chemistry of the composite catalyst
[14,31]. In our previous work, the introduction of CeO2 significantly
improved the catalytic efficiency and reduced the cost of catalyst in
ODHE process, which provides a promising idea and preliminary
verification on improvement of MoVNbTeOx catalysts. However,
further detailed and thorough research on the addition of CeO2 to
MoVNbTeOx catalysts is still needed.
⇑
Corresponding author.
E-mail address: yicheng@tsinghua.edu.cn (Y. Cheng).
Dan Dang and Xin Chen contributed equally.
In this work, M1 MoVNbTeOx/CeO2 (M1/CeO2) catalysts are syn-
thesized for the ODHE reaction. As a matter of fact, pure CeO2
1
https://doi.org/10.1016/j.jcat.2018.07.016
0021-9517/Ó 2018 Elsevier Inc. All rights reserved.

D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
239
shows low activity of ethane conversion and low selectivity to
ethylene in the ODHE process at a temperature lower than
500 °C [11,34,35]. However, CeO2 itself has high oxygen-storage
capacity (OSC) and easy oxidation/reduction of the Ce4+/Ce3+ redox
couple [32,33]. Therefore, the essential idea of this paper is to use
CeO2 as an oxidant, added to the phase-pure M1 catalyst, to
increase the number of vanadium active sites so as to improve
the catalytic performance [23]. The catalytic performance is
evaluated for the ODHE in a laboratory-scale fixed-bed reactor
and the characterization of the catalysts are achieved by using
XRD, BET, ICP, XPS, SEM and TEM techniques.
The morphology of the synthesized catalysts are studied by
scanning electron microscopy (SEM, Zeiss, GeminiSEM 500) and
JEOL JEM2010 high-resolution transmission electron microscopy
(HR-TEM).
X-ray photoelectron spectra (XPS) measurements with a PHI
Quantera SXM system equipped with Al Ka X-ray source are made
to analyze the ion concentration on catalyst surface. Survey scans
(0–1200 eV) and high-resolution Mo (3d), V (2p), Te (3d), Nb
(3d), Ce (3d) and C (1s) spectra are obtained. During the scanning
for vanadium and cerium, the dwell time is increased from
300 ms to 500 ms and the number of scan times is increased from
6 to 10. The binding energy scale is corrected by setting the C (1s)
signal at 284.8 eV. The XPS data analysis for vanadium is per-
formed with XPSPEAK 4.1 software and that for cerium is per-
formed with Thermal Advantage 4.88 software. The binding
energy data of reference materials are obtained from NIST X-ray
Photoelectron Spectroscopy Database.
2. Experimental section
2.1. Catalyst preparation
MoVNbTeOx catalysts are prepared by hydrothermal synthesis
[11,20,36,37]. Ammonium heptamolybdate (Sigma-Aldrich,
99.0%), vanadyl sulfate (Sigma-Aldrich, 97%) telluric acid (Sigma-
Aldrich, 98%) and ammonium niobium oxalate (Sigma-Aldrich,
99.99%) are used to prepare an aqueous slurry comprising Mo, V,
Te and Nb at the molar ratio of Mo:V:Te:Nb = 1:0.25:0.23:0.18.
The slurry is put into a 100 ml Teflon autoclave, where the air
inside the autoclave is replaced by nitrogen for 15 min. Then the
autoclave is placed in an oven at 175 °C for 48 h. The obtained sus-
pension is filtered and washed by 500 ml deionized water, dried
overnight at 80 °C and calcinated in nitrogen at 600 °C for 2 h with
the heating rate of 5 °C/min. The formed lump solid is grinded to
powder form, which is the mixture of phases M1 and M2. After
removal of phase M2 by using 30% HNO3, the phase-pure M1 is
obtained accordingly.
The M1/CeO2 catalysts are synthesized by a sol–gel method
[11]. Cerium nitrate (Sigma-Aldrich, 99%) and citric acid (Sigma-
Aldrich, 99.5%) are dissolved in deionized water at a molar ratio
of 1:3 with continuous agitation for 24 h at 65 °C and obtained a
stable transparent sol with 10 wt.% CeO2. Powder phase-pure M1
is introduced to the CeO2 sol at different mass ratios, stirred in
60 °C water bath for 2 h, and dried overnight at 80 °C. Finally, a
series of catalysts with 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.%,
70 wt.%, 90 wt.% CeO2 are obtained after the activation by calcina-
tion in air for 6 h at 400 °C with the heating rate of 5 °C/min in
muffle furnace. The prepared catalysts are named M1/10CeO2,
M1/20CeO2, M1/30CeO2, M1/40CeO2, M1/50CeO2, M1/70CeO2,
M1/90CeO2, respectively. The pure CeO2 is also prepared by drying
of CeO2 sol and calcination in air at 400 °C for 6 h. Its catalytic
activity is also tested in the ODHE reaction in comparison with
other catalysts.
2.3. Catalyst test
All of the catalysts for ODHE reaction are evaluated in a fixed-
bed quartz tubular reactor (8 mm i.d., 750 mm in length) heated
by a furnace, in which the temperature in the middle of the catalyst
bed is measured with a thermocouple. The catalyst particles with
0.3 g in mass are diluted with 3.0 g quartz particles with the size
of about 200 lm to achieve the isothermal operation. The feed
composition is 30C2H6/20O2/50He. The contact time is defined as
W/FC2H6 (W is the catalyst mass and FC2H6 is the ethane molar
flow rate), and the total flow rate is varied from 22 ml/min to
66 ml/min.
The reactants and products are analyzed with an online
Shimadzu GC 2014 gas chromatograph equipped with A PorapakQ
column for identifying CO2, C2H4 and C2H6 and a 5A molecular
sieve column for O2, N2, CH4 and CO. A blank run is conducted by
loading the reactor with only quartz sands at the same reaction
conditions. No ethane conversion is detected, indicating that the
homogeneous gas phase reaction can be neglected.
The conversion of ethane and the selectivity to products are
calculated as follows:
!
2fC
2
2fC
2f
þ 2fC Hþ6 fCO þ fCO
XC
SC
¼
¼
1 ꢁ
ꢂ 100%
ð1Þ
2
H6
2H4
2
2
H6
C2H4
ꢂ 100%
ð2Þ
ð3Þ
ð4Þ
2
H4
2fC þ fCO þ fCO
2H4
2
fCO
þ fCO þ fCO
SCO
¼
ꢂ 100%
2fC
2H4
2
2.2. Catalyst characterization
f
CO2
SCO
¼
ꢂ 100%
2
Metal contents are measured by inductively coupled
plasma-optical emission spectrometry (ICP-OES, Varian Vista RL
spectrometer).
Specific surface areas of the samples are determined by nitro-
gen adsorption carried out at 77 K on a Quantachrome Autosorb-
6B analyzer. The data are calculated by multipoint BET analysis
method in the pressure range of P/P0 = 0.05–0.30. Prior to the mea-
surement, the samples are degassed in vacuum at 300 °C for 2 h.
X-ray diffraction (XRD) patterns of samples are obtained using a
2fC þ fCO þ fCO
2H4
2
where XC2H6 is the ethane conversion, S is the selectivity to a certain
product, and f is the molar fraction in the effluent gas.
3. Results and discussion
3.1. Catalyst performance for ODHE
Bruker D8 Advance equipment with Cu K
a
radiation. 2h scans are
3.1.1. Effect of contact time
run from 5 to 70° at a rate of 0.5 degree per minute. The spectra
are identified with JCPDS database (Joint Committee of Powder
Diffraction Standards) and the ICSD database (Inorganic Crystal
Structure Database). M1 phase (ICSD 55097) has characteristic
diffraction lines located at 2h = 6.6°, 7.7°, 8.9°, 22.1°.
The performances of catalysts M1 with and without addition of
CeO2 in ODHE process at 400 °C with different contact times are
presented in Figs. 1–3. The detailed experiment data tested at
400 °C and 18.52 gcatꢀh/molC2H6 are listed in Table 1. Each data is
collected after 2 h since the change of flow rate. Ethylene, carbon

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D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
Table 1
Catalytic performance of catalysts in ODHE process.a
Catalysts
C2H6 conversion / %
Products distribution/%
C2H4
CO
CO2
M1
24.2
54.4
49.3
44.8
18.4
6.9
94.6
83.9
83.5
81.9
60.4
5.5
3.5
1.9
5.8
5.9
6.3
38.1
93.3
M1/30CeO2
M1/50CeO2
M1/70CeO2
M1/90CeO2
CeO2
10.3
10.6
11.8
1.5
1.1
a
Reaction condition: reaction temperature of 400 °C, contact time of 18.52
gcatꢀh/molC2H6, C2H6/O2/He molar ratio of 30/20/50 at the reactor inlet.
monoxide and carbon dioxide are the major products in the ODHE
process. Meanwhile, little acetic acid exists in the product gases,
which is small enough to be ignored, just as reported in the litera-
ture [38].
The catalytic performances of these catalysts strongly depend
on the incorporation of CeO2 at the same reaction conditions.
Fig. 1 displays a volcano shaped behavior with the CeO2 addition,
Fig. 1. Space time yield of ethylene as a function of CeO2 content in ODHE process
with different contact times.
Fig. 2. Ethylene selectivity as a function of ethane conversion in ODHE process with
different contact times.
Fig. 4. Space time yield of ethylene as a function of CeO2 content on M1 in ODHE
process.
Fig. 3. Space time yield of ethylene as a function of CeO2 content normalized by M1
content with different contact times.
Fig. 5. Ethylene selectivity as a function of ethane conversion in ODHE process at
different temperatures.

D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
241
where the maximum is found at CeO2 addition of 30 wt.%. The
space time yield of ethylene increases from 0.34 to 0.69 kgC2H4
/
kgcatꢀh with the increase of CeO2 addition from 0 to 30 wt.% and
then decreases to 0.17 kgC2H4/kgcatꢀh with the further increase of
CeO2 addition up to 90 wt.% at the contact time of 18.52 gcatꢀh/
molC2H6. For comparison, López Nieto et al. conducted ODHE on
MoVNbTeOx system and reached a space time yield of 0.29 kgC2H4
/
kgcatꢀh at 400 °C and C2H6:O2:He molar ratio of 30:30:40 [39].
Meanwhile, the highest ethane conversion of 54.43% is achieved
over the catalyst with a CeO2 addition of 30 wt.% with ethylene
selectivity of 83.9%, while phase-pure M1 attains only 24.20%
ethane conversion under the same reaction conditions (Fig. 2).
For the same M1 catalyst, when the amount of CeO2 is added at
0–70 wt.%, the selectivity to ethylene drops with the increase of
CeO2 addition and ethane conversion. In contrast, a corresponding
increase in selectivity to carbon oxides is observed as shown
in Fig. 2. These results are consistent with the references [4,11,
20,37]. Pure CeO2 is not selective to ethylene, with ethane conver-
sion of 6.9% and ethylene selectivity of 5.5% at 400 °C. Therefore,
the lower selectivity of ethylene in M1/CeO2 system is mainly
caused by CeO2 existence. The ethylene selectivity of M1/90CeO2
Fig. 6. Space time yield of ethylene as a function of CeO2 content normalized by M1
content at different temperature.
Fig. 7. Ethane conversion and ethylene selectivity as a function of reaction time for M1, CeO2 and a series of M1/CeO2 catalysts.

242
D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
Fig. 7 (continued)
Table 2
Composition of the phase-pure M1 MoVNbTeOx/CeO2 catalysts.a
Catalysts
Mo-V-Nb-Te-Ce
atomic ratio of bulk
composition
Mo-V-Nb-Te-Ce atomic
ratio of surface
composition
M1
Used M1
M1/30CeO2
MoV0.23Nb0.25Te0.11
MoV0.23Nb0.25Te0.11
MoV0.23Nb0.25Te0.11
Ce0.44
MoV0.15Nb0.36Te0.24
MoV0.15Nb0.33Te0.23
MoV0.21Nb0.49Te0.32
Ce0.95
Used M1/30CeO2
M1/50CeO2
MoV0.23Nb0.25Te0.11
Ce0.44
MoV0.23Nb0.25Te0.11
Ce1.04
MoV0.20Nb0.52Te0.29
Ce0.93
MoV0.19Nb0.58Te0.34
Ce1.89
Used M1/50CeO2
MoV0.23Nb0.25Te0.11
Ce1.04
MoV0.21Nb0.55Te0.38
Ce1.71
Used M1/50CeO2 for 6 h MoV0.23Nb0.25Te0.11
Ce1.04
MoV0.21Nb0.53Te0.31
Ce1.84
Used M1/50CeO2 for 6 h MoV0.23Nb0.25Te0.11
MoV0.20Nb0.52Te0.34
Ce1.83
and refreshed by O2
for 12 h
Ce1.04
a
Reaction condition of used catalysts: reaction temperature of 400 °C, contact
time of 18.52 gcatꢀh/molC2H6, C2H6/O2/He molar ratio of 30/20/50 at the reactor inlet,
time on stream of 50 h.
sample plummets compared to other samples, which presents a
more similar property of pure CeO2.
We should note that the catalyst weight for each reaction is the
same, accordingly M1 content for ODHE decreases with the
increase of CeO2 addition. To justify the contribution of M1 itself,
we normalize the space time yield by M1 content and the results
are presented in Fig. 3. The normalized space time yield keeps
increasing before CeO2 addition reached 70%, and decreases when
CeO2 addition is 90%.
3.1.2. Effect of reaction temperature
Figs. 4–6 show the effect of reaction temperature (380–420 °C)
on the catalytic performance of M1/ CeO2 for ODHE at contact time
Fig. 8. C2H6 conversion, product selectivity (A) and space time yield (B) obtained
after refreshment of M1/50CeO2 in 3 cycles (reaction temperature of 400 °C,a
contact time of 18.52 gcatꢀh/molC2H6, C2H6/O2/He molar ratio of 30/20/50 at the
reactor inlet, time on stream of 4–6 h).
of 18.52 gcatꢀh/molC2H6
.

D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
243
As shown in Figs. 4–6, the reaction temperature has a signifi-
cant effect on the activity of the catalysts. When the reaction tem-
perature is ranged from 380 °C to 440 °C, the conversion of ethane
increases linearly with the increase of reaction temperature [40].
The normalized space time yield also shows a similar trend as
above.
catalyst before and after reaction has similar surface composition,
which indicates the stability of this catalyst system.
The BET measurements of samples are listed in Table 3. The
specific surface area and pore volume of phase-pure M1 catalyst
are relatively low, but increase considerably upon loading of
CeO2. This increase can be attributed to the addition of CeO2-gel
as starting compound for CeO2, which is much larger than the addi-
tion of other elements to M1 phase [15,46]. The decomposition
during calcination of precursors with organic nature causes a more
porous structure than M1 catalyst. The previous works have
proved that surface area is not related to catalytic performance
[11,20,47]. After refreshment, the surface area is similar to the used
catalyst, while the catalytic performance is higher, which also
proves this.
3.1.3. Catalyst stability
Stability is an extremely important property that may deter-
mine whether a catalyst has the potential for industrial application
or not. Time-on-stream experiments are carried out in order to
study the stability of the above-mentioned catalysts. As presented
in Fig. 7, the phase-pure M1 catalyst and the pure CeO2 exhibit
stable catalytic performance in 50 h time-on-stream tests. How-
ever, the ethane conversions for all the M1/CeO2 catalysts decrease
at different levels with the progress of the reaction time and the
ethylene selectivity has a corresponding increase.
Generally, oxidative dehydrogenation process based on
vanadium-based catalyst is described by Mars and van Krevelen
(MvK) mechanism: alkane molecules react with lattice oxygen of
the catalyst to produce alkene molecules and carbon oxides; the
gas phase oxygen replenishes the lattice oxygen by the re-
oxidation of the catalyst [41,42]. Considering pure M1 is quite
stable during reaction, we suppose that the conversion decrease
is not from structure damage but from the loss of lattice oxygen:
the oxygen in the reactant gas would not be sufficient to supple-
ment the loss of lattice oxygen in the catalytic materials. Therefore,
we have the plan to perform the following refreshment test.
3.2.2. X-ray diffraction (XRD)
Fig. 9 shows the X-ray diffraction patterns of the catalysts. Char-
acteristic peaks of M1(ICSD 55097) at 2h = 6.6°, 7.7°, 8.9°, 10.7°,
22.1°, 27.2°, 29.2°, 35.48° [48] are observed in all samples except
the pure CeO2. The presence of CeO2 diffraction signals, located
at 2h = 28.6°, 33.1°, 56.4°, 59.1°, 69.5°(JCPDS 43-1002), indicates
the presence of CeO2 to the surface of M1 material. No other
crystalline peaks are detected in all the samples, which can be
Table 3
Textural characterization of the phase-pure M1 MoVNbTeOx/CeO2 catalysts.
Catalysts
Specific surface
area m2/g
Pore Volume
cc/g
M1
Used M1
M1/10CeO2
Used M1/10CeO2
M1/30CeO2
16
12
39
14
42
21
32
30
29
0.056
0.052
0.166
0.080
0.133
0.095
0.081
0.076
0.073
3.1.4. Catalyst refreshment
Since the series of M1/CeO2 catalysts show the similar catalytic
performances in ODHE reaction, M1/50CeO2 is chosen as a repre-
sentative to test the reproducibility by refreshment after usage. A
used catalyst is refreshed by feeding 10 ml/min pure oxygen
in situ at 400 °C for 12 h, and then pure helium is switched on to
replace residual oxygen in the reactor before feeding the reactant
gases (i.e., a C2H6/O2/He molar ratio of 30/20/50 at 400 °C and a
contact time of 18.52 gcatꢀh/molC2H6). Fig. 8 shows the comparison
of the obtained C2H6 conversion, product selectivity and space time
yield. Overall, C2H6 conversion and product selectivity are stable in
the three cycles of refreshment with similar space time yields,
which indicates a good reproducibility of M1/50CeO2 catalyst
and a good repeatability of the experimental results. It could be
deduced that after a long period of reaction, the decrease of ethane
conversion is due to the loss of lattice oxygen, while the in-situ
supplement of oxygen could make M1/CeO2 system re-gain similar
activity as fresh catalyst.
Used M1/30CeO2
M1/50CeO2
Used M1/50CeO2 for 6 h
Used M1/50CeO2 for 6 h and refreshed
by O2 for 12 h
Used M1/50CeO2
CeO2
25
30
28
0.068
0.028
0.024
Used CeO2
3.2. Catalyst characterization
3.2.1. Elemental composition and textural properties
The bulk composition characterized by ICP-OES and surface
composition characterized by XPS of phase-pure M1 MoVNbTeOx
and M1/CeO2 catalysts are provided in Table 2. For the bulk com-
position, there is little difference in terms of the chemical compo-
sition compared to that of the M1 phase reported in the literature
[11,14,36,43,44]. It is reported that the metal content of phase-
pure M1 is not restricted to a specific molar ratio of the metals
in the structure, but covers the region of metal content for
phase-pure M1 in the quaternary phase diagram [45]. In this work,
all the pure phase M1 and M1/CeO2 catalysts are consistent with
the mentioned properties. The addition of CeO2 to phase-pure
M1 has no effect on the chemical composition. XPS surface compo-
sition analysis is semi-quantitative. We could see that the surface
composition is quite different from bulk composition. Molybde-
num seems to decrease a little after CeO2 addition. While the
Fig. 9. XRD patterns of the catalysts (reaction condition of used catalysts: reaction
temperature of 400 °C, contact time of 18.52 gcatꢀh/molC2H6, C2H6/O2/He molar ratio
of 30/20/50 at the reactor inlet, time on stream of 50 h).

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D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
Fig. 10. SEM images (A-E) and TEM images (F-I) of fresh and used catalysts (M1, M1/30CeO2, M1/90CeO2, reaction condition of used catalysts: reaction temperature of 400 °C,
a contact time of 18.52 gcatꢀh/molC2H6, C2H6/O2/He molar ratio of 30/20/50 at the reactor inlet, time on stream of 50 h).
attributed to the purification of MoVNbTeOx materials by HNO3
solution. The full width at half height of the peaks decreases as
the amount of CeO2 increases, indicating an increase in the size
of the crystalline domain of the ceria phase [49]. As presented in
Fig. 9, apparently, no significant change can be observed in XRD
patterns of fresh catalyst, used catalysts and the catalyst used for
3 cycles in the refreshment test, which illustrates a relative stable
crystal structure during reaction.

D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
245
3.2.3. Catalyst morphology
CeO2 has little influence on the valence states of these three ele-
ments in M1/CeO2 catalysts. Besides, there is no effect on the
change of valence states in those elements in the time-on-stream
experiments for 50 h.
Fig. 12(B) and (E) show the XPS spectra of vanadium and cer-
ium of the fresh and used catalyst samples (M1, M1/30CeO2,
M1/50CeO2 and CeO2). Table 4 shows the ratio variations of
vanadium and cerium valence states of catalysts mentioned
above. As illustrated in Fig. 12(B), the V 2p3/2 peak of catalysts
could be fitted into two components at 517.10 0.05 eV and
516.20 0.05 eV, with FWHM of both peaks as 1.3 0.1 eV.,
representing V4+ and V5+ species, respectively [51,52]. The Ce
The surface morphology of the synthesized materials is exam-
ined by SEM. Micrographs of the fresh M1 catalyst and M1/CeO2
catalysts with different CeO2 contents, together with the corre-
sponding used catalyst, are presented in Fig. 10(A–E). Electronic
images of the catalysts used before and after refreshment is also
presented in Fig. 10(F–I). It is obvious that the rod-like M1 phase
crystal has an average width of ꢃ100 nm and an average length
of ꢃ200 nm, just as reported in the literature [11]. A bronzelike
channel structure grows along the (0 0 1) direction and a rodlike
crystal morphology finally forms in which the (0 0 1) planes are
perpendicular to the axis length of the rod in the M1 phase [50].
The SEM images display a good dispersion of homogeneous CeO2
particles on the surface of fresh M1/30CeO2 and M1/50CeO2. It
can be seen that the more CeO2 content, the more CeO2 particles
on the surface of the catalysts, which is consistent with the result
of surface areas detected by the previous characterization tech-
niques (Table 3). However, for the M1/90CeO2 catalyst, the surface
is covered by CeO2 and almost no M1 could be observed, which
may result in the decrease of space time yield to CeO2 content nor-
malized by M1 content, as displayed in Figs. 3 and 6. In the TEM
images, two different crystals can be observed in the M1/CeO2 cat-
alysts, which represent M1 and CeO2 crystal, respectively. The
morphology of fresh and used catalysts (Fig. 11), the M1/50CeO2
catalyst before and after 3 cycles of refreshment, has almost no
change compared to each other, which indicates the structural sta-
bility of the M1/CeO2 catalysts in such long time-on-stream exper-
iments and refreshment test.
3d3/2,
spectra are composed of two multiplets (v and u) cor-
5/2
responding to the spin-orbit split 3d5/2 and 3d3/2 core holes. The
highest binding energy peaks, u’’ and v’’ respectively locating at
about 916.5 and 898.4 0.1 eV are the result of a Ce 3d94f0 final
state. The satellite peak u’’ associated to the Ce 3d3/2 is charac-
teristic of the presence of tetravalent Ce (Ce4+ ions) in Ce com-
pounds. The lowest binding energy states u0, v0, v’, u’
respectively located at 900.9, 882.2, 907.6 and 889.4 0.1 eV
are the result of Ce 3d94f2 and Ce 3d94f1 final states, which is
in good agreement with references [53–55]. The XPS characteri-
zation indicates that the Ce3+ and Ce4+ species can be fitted to
distinct line shapes corresponding to various final states: Ce
(III) = v + v’’ + u and Ce(IV) = v0 + v’ + u0 + u’ + u’’. The calculated
results can be seen in Table 4.
As listed in Table 4, a greater amount of V5+ can be achieved in
the fresh Ce-containing catalyst surface than that on the Ce-free
catalyst surface. In addition, the more the CeO2 content is intro-
duced, the more the ratio of V5+/ V4+ increases. At the same time,
the ratio of Ce3+/Ce4+ goes up accordingly. This phenomenon
implies that the increase of V5+ abundance on the catalyst surface
is benefitted from the transition of Ce4+?Ce3+ in the solid phase of
M1/CeO2 catalysts. Compared to the pure phase-M1 catalyst, the
catalytic performance of M1/CeO2 catalysts is enhanced effectively,
resulting from the increase of V5+ abundance on the catalyst sur-
face. This can be demonstrated by the experimental results in Sec-
tions 3.1.1 and 3.1.2.
3.2.4. X-ray photoelectron spectroscopy analysis (XPS)
XPS spectra of key elements in fresh and used phase-pure M1
and M1/CeO2 catalysts are measured to investigate the change of
element valence states on the catalyst surface with the addition
of different CeO2 content (Fig. 12). The binding energy data of
the reference materials are obtained from the NIST X-ray Photo-
electron Spectroscopy Database.
As shown in Fig. 12(A, C, D), the binding energy of Mo 3d (232.9
0.2 eV), Nb 3d (207.2 0.2 eV), Te 3d (576.3 0.2 eV) is the same
as reported, corresponding to valence states of Mo6+, Nb5+and Te4+
,
For the refreshment test, we can see after the in-situ oxygen
treatment, V5+/V4+ ratio of refreshed catalyst recovered, meaning
respectively [11,38]. These results suggest that the introduction of
Fig. 11. Characterization of XRD, SEM and TEM on the fresh and used M1/50CeO2 catalyst after 3 cycles of refreshment.

246
D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
Fig. 12. XPS spectra of fresh and used catalysts (M1, M1/30CeO2, M1/50CeO2). A: Mo 3d; B: V 2P; C: Nb 3d; D: Te 3d; E: Ce 3d. Reaction condition of used catalysts: reaction
temperature of 400 °C,a contact time of 18.52 gcatꢀh/molC2H6, C2H6/O2/He molar ratio of 30/20/50 at the reactor inlet, time on stream of 50 h.
that the oxygen supplement can make up the lattice oxygen loss
during reaction.
turn-over frequency. It could be inferred that the ability of oxygen
transfer from Ce4+ to V4+ is easier than gas phase transmission. The
transformation between V5+ and V4+ occurs dynamically during
the reaction [44], although the TOF calculation assumes that the
amount of active site (i.e., V5+) is constant. For pure M1, V5+ sites
oxidize ethane and then are re-oxidized mainly by gas phase oxy-
gen. In contrast, with the presence of CeO2, the re-oxidation of V4+
to V5+ is dominated by an easier way (i.e., by Ce4+), leading to an
increase of the turn-over frequency for this process. This
Assuming that surface V5+ is the only active site for ethane con-
version, we thus define the turnover frequency of this system as
the converted ethane molecules per second over a single V5+ site.
The calculated TOFs for these catalyst are listed in Table 5 using
BET measurement results, ethane conversion data, XPS deconvolu-
tion results and lattice parameter of M1 [56,57]. We can see that
adding CeO2 into this catalyst system obviously increases the

D. Dang et al. / Journal of Catalysis 365 (2018) 238–248
247
´
Table 4
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Catalysts
V5+/V4+
Ce3+/Ce4+
M1
Used M1
M1/30CeO2
Used M1/30CeO2
M1/50CeO2
Used M1/50CeO2 for 6 h
Used M1/50CeO2 for 6 h and refreshed by O2 for 12 h
Used M1/50CeO2
CeO2
0.75
0.82
1.84
1.27
2.13
1.80
1.95
1.31
–
–
–
0.70
1.07
0.53
0.70
0.69
0.74
0.65
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a
Reaction condition of used catalysts: reaction temperature of 400 °C, contact
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Table 5
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M1/30CeO2
0.61
M1/50CeO2
0.75
ꢁ1
TOF/molC2H6ꢀmol ꢀsꢁ1
0.29
V5+
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4. Conclusions
A series of M1/CeO2 catalysts have been prepared and evaluated
in ODHE process at different contact times and reaction tempera-
tures. It is found that phase-pure M1 with 30 wt.% CeO2 catalyst
has the best catalytic performance. Although the ethane conver-
sion decreases with reaction time, the activity could be re-gained
after refreshment by supplying oxygen in situ. It is shown that
the increase of V5+ amount is one of the dominant factors for
higher ethane conversion. In addition, by estimating the TOFs for
this complex system based on experiment results and characteri-
zation results, we found that the existence of Ce4+ makes the re-
oxidation of V4+ process easier and hence the turn-over frequency
is increased. This also explains the loss of lattice oxygen in M1/
CeO2 catalyst during ODHE reaction. The used catalyst after 3
cycles of refreshment maintains stable in crystal structure and sur-
face morphology. The results encourage further research on the
better understanding of the catalytic mechanism and performance
manipulation (i.e., activity, selectivity and stability) of M1/CeO2
catalysts during the application in ODHE process.
Acknowledgment
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This work is financially supported by the National Natural
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