Structural dynamics of molybdenum vanadium oxide (MoVOx): Influence of activation condition

Article abstract of DOI:10.1007/s10973-017-6333-1

Molybdenum and vanadium oxides were known to be an effective catalyst for light olefin (propane) activation for conversion to value-added chemicals. However, it is difficult to control the selectivity to desired product whereby subsequent reaction can lead to coking and rapid catalyst deactivation. One of the key ways to improve on the above limitation is to optimise and control the molybdenum phase structure, particularly during catalyst precursor activation stage. This paper demonstrates the combination of optimal in situ activation under different condition and thermal analysis for structural control that can help to guide and gain an insight into the structure–activity relationship of the nanostructured catalyst system. In situ XRD analysis reveals the crystallization of molybdenum vanadium oxide was highly influenced by the activation condition hence exhibiting different structural properties. Activation under Air at 300?°C forms highly crystalline hexagonal phase and transforms to thermodynamically stable orthorhombic (o-MoO₃) phase at 450?°C. Activation under inert (helium) reveals the precursor remains amorphous until nanostructuring occurs at 450?°C. The precursor further transforms to the thermodynamically stable crystallized tetragonal phase (Mo₅O₁₄) at 500?°C. The obtained structural transition information is important in order to control and identify the catalytic active phase that is suitable for a particular reaction.

Full text of DOI:10.1007/s10973-017-6333-1

J Therm Anal Calorim
DOI 10.1007/s10973-017-6333-1
Structural dynamics of molybdenum vanadium oxide (MoVOx)
Influence of activation condition
1
2
1
Durga Devi Suppiah
•
Anna Komar
•
Sharifah Bee Abd Hamid
Received: 9 August 2016 / Accepted: 18 March 2017
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2017
Abstract Molybdenum and vanadium oxides were known
to be an effective catalyst for light olefin (propane) acti-
vation for conversion to value-added chemicals. However,
it is difficult to control the selectivity to desired product
whereby subsequent reaction can lead to coking and rapid
catalyst deactivation. One of the key ways to improve on
the above limitation is to optimise and control the
molybdenum phase structure, particularly during catalyst
precursor activation stage. This paper demonstrates the
combination of optimal in situ activation under different
condition and thermal analysis for structural control that
can help to guide and gain an insight into the structure–
activity relationship of the nanostructured catalyst system.
In situ XRD analysis reveals the crystallization of molyb-
denum vanadium oxide was highly influenced by the
activation condition hence exhibiting different structural
properties. Activation under Air at 300 °C forms highly
crystalline hexagonal phase and transforms to thermody-
Keywords In situ XRD Á Structure Á Activation Á
Molybdenum
Introduction
Molybdenum oxides were identified in activating C–H
bonds of alkanes, especially light olefins (propane) for
selective partial oxidation. The structural complexity, the
ability to occur at ?4 to ?6 oxidation states and oxygen
coordination geometry variation of molybdenum oxides
give rise to good catalytic performances. The capability to
do so creates high structural diversity of the mixed tran-
sition metal oxides. Vanadium also has rich coordination
geometries and oxidizing state ranging from ?3 to ?5. By
incorporating vanadium as a promoter into the molybde-
num oxide system, the desired active phase can be
achieved. Catalytic activity increased with vanadium
addition but less selective at iso-conversion [1–4]. The
catalyst synthesis applying a‘top down’ approach starts
with a bulk solid then structurally decomposed to obtain
nanostructures which are then subsequently assembled into
a new bulk material [5, 6]. In order to enhance catalytic
performance, studies of structure–activity relationship are
essential to understand the catalyst system, especially
during thermal induced activation [7]. In situ studies are
described as a technique that gives detailed structural and
chemical atomic scale insight in complex heterogeneous
catalyst in order to determine the structure–activity rela-
tionship [8]. Two criteria that influence the structural for-
mation of MoVOx catalyst are the vanadium insertion
namically stable orthorhombic (o-MoO ) phase at 450 °C.
3
Activation under inert (helium) reveals the precursor
remains amorphous until nanostructuring occurs at 450 °C.
The precursor further transforms to the thermodynamically
stable crystallized tetragonal phase (Mo O ) at 500 °C.
5
14
The obtained structural transition information is important
in order to control and identify the catalytic active phase
that is suitable for a particular reaction.
&
Durga Devi Suppiah
durgadevi@um.edu.my
1
Nanotechnology and Catalysis Research Centre
(NANOCAT), University of Malaya, Kuala Lumpur,
Malaysia
filling the molybdenum vacancies of the h-MoO structure
3
and followed by vanadium substituting molybdenum. The
addition of vanadium affected not only the morphology but
2
Institute of Research Management and Services (IPPP),
University of Malaya, Kuala Lumpur, Malaysia
123

D. D. Suppiah et al.
also the lattice parameters of the molybdenum salt crystal
structure [9].
broadening was determined using Quartz standard and
subsequently subtracted out from the XRD scan.
In this paper, the structural properties of molybdenum
vanadium oxide-based catalysts were investigated in cor-
relation with temperature and activation condition. In situ
XRD analysis was conducted under different activation
conditions to reveal the dynamics of structural evolution
which helps to determine the active catalyst state. The
structural evolution analysis obtained was then correlated
with thermal decomposition and heat transfer analysis to
further understand the mechanism hence determining the
most desirable structure for reactivity studies.
Thermal analysis and activation
Thermo-decomposition was analyzed by a Mettler Toledo
e
TGA/SDTA851 coupled with a Thermostar Mass Spec-
trometer (MS) to analyze the evolved decomposition gas as
it measures the mass changes with respect to temperature.
The MoVOx precursor was heated from 30 to 500 °C at a
-
1
heating rate of 5 °C min under both synthetic air and
-
1
also helium at a flow rate of 50 mL min . The thermo-
grams were evaluated, and the mass loss percentages were
determined. The crystallization and melting properties of
the precursor were analyzed using differential scanning
calorimetry (DSC) technique. The DSC instrument used
Experimental
e
Synthesis of molybdenum vanadium oxide
was a Mettler Toledo DSC822 equipped with a measuring
cell with ceramic sensors which measures the heat flow to
detect endothermic and exothermic effects [10, 11]. All
samples were heated from 30 to 500 °C at a heating rate of
Molybdenum vanadium oxides (MoVOx) precursor was
prepared using sol–gel technique to acquire the desired
catalyst structure. This was done by adjusting the method
established by Rodel [4]. Firstly 0.1 Mol/L vanadyl oxalate
-
1
10 °C min under both synthetic air and helium at a flow
-
1
rate of 50 mL min . The heat flow/energy of samples was
calculated and evaluated. The MoVOx spray-dried pre-
cursors were then activated under different gaseous envi-
ronment to investigate the activation effect on the catalyst
structural changes. One batch was calcined under static air
using muffle furnace (Barnstead Thermolyne) and another
batch under helium using Universal Temperature Pro-
grammer (UTP) at 500 °C for 4 h.
(
VO(C O )) solution was prepared by dissolving the
2 4
vanadium (V) oxide (98% purity) in oxalic acid (98%
-
purity). This was then added to a 0.05 Mol L solution of
1
ammonium heptamolybdate (AHM, 99%) with the rate of
-
1
addition of 1 mL min using an autotitrator at 80 °C.
After stirring at this temperature for 1 h, the solution was
spray-dried by atomizing with compressed air at 6 bar and
dried with hot air at 200 °C using a mini spray-dryer
(
Buchi). The spray-dried powder was then collected and
Morphological analysis
subjected to thermal analysis and activation followed by
structural and morphological characterization.
FEI quanta 200F Field Emission Scanning Electron
Microscope (FESEM) was used to investigate the
microstructure and surface structural defects of the pre-
cursors and the catalyst. The morphology observation was
carried out under low vacuum and accelerating voltage of
5.0 HV. The images were captured under magnifications
between ranges of 8–30 k. Spot analysis was done using
energy-dispersive X-ray (EDX) with INCA energy 400 to
quantitatively analyze the local metal content. Elemental
mapping was done to verify the metal homogeneity dis-
tribution in the MoVOx precursors and calcined catalyst.
In situ X-ray diffraction (in situ XRD) analysis
The structural phase changes with temperature were mon-
itored through an in situ X-ray diffractometer (Bruker)
which was equipped with Flowbus mass flow controllers.
The spray-dried catalyst precursors were heated from 30 to
-
1
5
00 °C at a heating rate of 5 °C min under both inert
helium) and synthetic air at 100 mL min . The diffrac-
-1
(
tograms were obtained using a theta/theta goniometer and a
position sensitive detector (PSD). The data sets were col-
lected in the range of 10° B 2h B 80° at 50 °C intervals.
Phase analysis was conducted using High Score Plus
software. Crystallite size was calculated from the XRD
peak broadening using Scherrer equation;
Results and discussion
Catalyst activation analysis via in situ X-ray
diffraction
Crystallite size ðDÞ ¼ kk=bstruc cos h
where k is the shape factor, k is the X-ray source wave-
length (Ka1 = 1.541 nm), and h is the peak position. Peak
profile fitting was conducted while the instrumental
The XRD diffractograms obtained via in situ activation
under air (20% Oxygen) are shown in Fig. 1a. The amor-
phous phase reflection at 12° starts to increase until 250 °C.
1
23

Structural dynamics of molybdenum vanadium oxide (MoVOx)
(
a)
(b)
5
4
4
4
4
00
80
60
40
20
5
4
00 °C
50 °C
400
80
360
40
320
00
280
3
4
3
3
00 °C
50 °C
00 °C
3
3
2
2
2
2
1
1
1
1
60
40
20
00
80
60
40
20
2
2
50 °C
00 °C
1
1
50 °C
00 °C
1
0
15 20 25 30 35 40 45 50 55 60
10
20
30
40
50
60
2θ
Position/°2θ (Copper (Cu))
Fig. 1 a In situ XRD. b Isoplot of In situ XRD of MoVOx precursor activation from 50 to 500 °C under air
At 300 °C, highly crystalline phase was obtained and this
can be observed in the thermal Isoplot in Fig. 2b. Analysis
of the diffractogram at 400 °C (Table 1) reveals hexagonal
crystal structure matching, vanadium molybdenum oxide,
Mo0.87 O2.94V0.13, (PDF-File 48-0766). The hexagonal
The phase then changes radically at 450 °C. Analysis of
the diffractogram at 500 °C reveals mixed phase structure
(Table 2) consisting mainly of orthorhombic phase that
correlates with the thermodynamically stable orthorhombic
molybdenum oxide, (MoO ), (PDF-File 05-0508), which
3
3
˚
structure unit cell volume is 359.39 A with unit cell
belongs to space group Pbnm (62) with unit cell volume is
3
202.99 A with unit cell parameter a = 3.962 A,
˚
parameter a = 10.592 A, b = 10.592 A, and c = 3.6990
˚
˚
˚
˚
˚
b = 13.858 A, and c = 3.697 A. Residue of the hexagonal
˚
A. This phase maintains until 450 °C.
(
a)
(b)
4
4
4
4
4
3
3
3
3
3
2
2
2
2
80
60
40
20
00
80
60
40
20
00
80
60
40
20
5
00 °C
4
50 °C
00 °C
4
3
3
50 °C
00 °C
2
2
50 °C
00 °C
200
1
1
80
60
1
50 °C
00 °C
140
20
1
1
1
0
20
30
40
50
60
1
0
20
30
40
50
60
2θ
Position/°2θ (Copper (Cu))
Fig. 2 a In situ XRD. b Isoplot of MoVOx precursor activation from 50 to 500 °C under helium
123

D. D. Suppiah et al.
Table 1 X-ray diffraction data of MoVOx activation under air
400 °C)
water is removed, leading to the vanadium expulsion into
secondary structure which corresponds to an intermediate
amorphous form [14]. At 400 °C, crystalline features of the
diffractogram appear though the hump is still broad and
this is presided to be the nanocrystalline phase of Mo O
(
2
q
Intensity/%
hkl
d
exp
d
ref
Phase
9
2
2
2
3
.60
43.79
100
100
210
101
111
310
9.218
3.483
3.427
3.036
2.543
9.193
3.470
3.428
3.033
2.547
Hexagonal
Hexagonal
Hexagonal
Hexagonal
Hexagonal
5
14
5.55
5.98
9.42
5.30
[
15]. Continuous controlled heating of the precursor to
50 °C creates a sharp reflection which is observed at 27°,
65.51
35.81
23.47
4
and this is in good agreement with the domain growth in
basal plane [16]. Further heating to 500 °C leads to highly
crystalline phase as observed from the thermal Isoplot in
Fig. 2b. The final thermodynamically stable phase obtained
(
Table 4) was vanadium molybdenum oxide (V0.07Mo
)
0.93 5
Table 2 X-ray diffraction data of MoVOx activation under air
500 °C)
O , PDF-File 31-1437 which corresponds to the tetragonal
1
3
phase with unit cell volume of 2106.87 A with unit cell
4
(
˚
2
q
Intensity/%
hkl
d
exp
d
ref
Phase
˚
˚
˚
parameter a = 22.853 A, b = 22.853 A and c = 4.03 A.
The highly crystalline structure can also be observed in the
Isoplot in Fig. 2b. This catalyst can be deduced to having a
very high structural stability as temperature does not affect
the particles bulk structure which is Mo O which is an
1
2
2
2
3
3
2.39
24.45
64.31
37.91
100
020
110
211
21
6.93
3.81
3.47
3.26
2.649
2.332
7.13782
3.81609
3.55812
3.2672
Orthorhombic
Orthorhombic
Hexagonal
3.29
5.01
5
14
7.2737
3.5763
8.0356
Orthorhombic
Hexagonal
advantage of activation under inert gas [16]. The inter-
planar spacing was larger indicating smaller refined crys-
tallite size due to the confinement of lattice deformations of
26.81
17.44
220
131
2.66693
2.36388
Orthorhombic
5
?
V
fivefold coordination [17]. The tetragonal structure of
Mo O is regarded as a suitable catalyst for alkane acti-
5 14
phase still exists. The activation temperature has influenced
the crystallite size of the MoVOx catalyst. The crystallite
sizes were calculated using Scherrer equation as shown in
Table 3. The nanostructure crystallite was stabilized by the
amorphous matrix attained during the spray drying process
vation because of the structure and channel network that
can accommodate heteroatom dopants and oxygen for
reaction. The structure does not have edge sharing octa-
hedral but instead has clusters of octahedral around a
5
?
4?
fivefold bipyramid containing Mo and V atoms which
[
12]. The crystallite size calculated was slightly larger with
are interconnected by a corner-sharing octahedral network
4
-
4?
the structural changes from hexagonal phase (50.6 nm) to
orthorhombic phase (55.2 nm). This was because of
exposure to air thus causing the complex hexagonal oxides
consisting of [Mo O ] [18, 19]. In the tetrahedral V
8 26
containing groups have oxygen ions which are known to be
easier to remove from the lattice during reaction thus
promoting the oxygen-containing products. The stability of
this catalyst with oxygen vacancies is essential, especially
for high dehydrogenation reaction selectivity [20].
breakdown to the o-MoO phase [13].
3
The XRD diffractograms obtained via in situ activation
under helium are shown in Fig. 2. Similarly, the amorphous
phase starts to change at 150 °C with the increase of the
highest reflection of the amorphous halo at 12° and 26°
regions. These broad amorphous peaks intensities grew with
the precursor’s degree of crystallinity. The precursor there-
fore grew in a perpendicular direction along (001) plane
which is the reflection of nanocrystalline Mo O [13].
According to the Scherrer equation which was calcu-
lated using the FWHM value (Table 5), the crystallite size
is smaller at the nanocrystalline phase at 450 °C as
Table 4 X-ray diffraction data of MoVOx activation under helium
(
5
14
500 °C)
These reflections at 26° continue to increase drastically
until 400 °C, while the reflection at 12° reduces and
eventually disappears. At this point, it is presumed the
2
h
Intensity/%
hkl
d
exp
d
ref
Phase
1
2
2
2
2
6.458
2.017
3.336
4.920
6.105
20
100
28
330
001
600
540
630
550
740
5.3820
4.0340
3.8089
3.5703
3.4107
3.2323
2.8365
5.3832
3.9900
3.8065
3.5669
3.4046
3.2299
2.8328
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Table 3 Crystallite size of MoVOx after in situ activation under air
49
Temperature
2h
FWHM
Crystallite size/nm
61
4
00 °C
00 °C
25.54
27.27
0.161
0.148
50.6
55.2
27.574
31.515
18
5
34
1
23

Structural dynamics of molybdenum vanadium oxide (MoVOx)
Table 5 Crystallite size of MoVOx after in situ activation under
helium
accordance with particle growth and coarsening of the
crystals [21]. As observed, the size which depends on the
degree of crystallization varies with different catalyst
activation conditions.
Temperature
2h
FWHM
Crystallite size/nm
4
50 °C
00 °C
26.089
21.995
0.250
0.094
32.60
85.80
5
Thermal analysis
In order to further understand the influence of thermal
activation condition on the structural changes of MoVOx,
the synthesized catalyst precursors were subjected to
thermal analysis under both activation conditions (static air
and helium). The catalyst was subjected to
compared to the final tetragonal phase structure at 500 °C.
This supports the previous argument that the tetragonal
phase growth starts from the nanocrystalline phase at
4
00 °C and at 500 °C the crystallite size increases in
Fig. 3 a Thermogravimetric
analysis. b Mass spectrometry
analysis of MoVOx from 30 to
170
280
330
425
(
a)
100
0.00
5
00 °C under 20 mL air
9
9
8
8
5
0
5
0
–
–
–
0.05
0.10
0.15
5
0
100
150
200
250
300
350
400
450
425
500
Temperature/°C
(
b)
170
280 330
m/e =18 (H O)
2
1
1
E–8
E–9
m/e =17 (H O)
2
1
1
E–10
E–11
m/e =44 (CO2)
m/e =45 (CO2)
m/e =46 (NO2)
5
0
100
150
200
250
300
350
400
450
500
Temperature/°C
123

D. D. Suppiah et al.
thermogravimetric analysis (TG) accompanied with mass
spectrometer (MS) evaluation, and the results were corre-
lated with Differential Scanning Calorimetry (DSC)
Analysis.
from 170 to 280 °C in the curve involving the desorption of
crystallized water (m/e = 17 & 18), carbon dioxide (m/
e = 44) and nitrogen oxides (m/e = 45 & 46) [22]. The
oxidation of ammonia and the reduction of vanadium and
molybdenum precursors generate the nitrogen oxides
fragments while carbon dioxide formation results from the
decomposition of vanadyl oxalate used as the starting
material [13]. Similar ion fragments were also observed
from the third mass loss of 2.75% from 280 to 330 °C. The
final mass loss of 7.3% was recorded at 330–425 °C. At
this section, the remaining fragments of water, oxides and
ammonium were released as crystallization process to the
Thermal analysis reveals a complex mechanism of
degradation during calcination. Figure 3 shows the TG-MS
of MoVOx catalyst precursor being activated under
-
1
2
0 mL min
air flow. Four steps of mass loss were
recorded. First mass loss of 5.45% was recorded at below
170 °C. Based on the MS evaluation in Fig. 1b, water (m/
e = 17 & 18) is released at this point. As the decomposi-
tion continues, a second mass loss of 7.68% was recorded
Fig. 4 a Thermogravimetric
analysis. b Mass spectrometry
170
320
387 425
(a) 100
0.00
analysis of MoVOx from 30 to
5
00 °C under 2 0 mL helium
9
9
8
8
7
5
0
5
0
5
–
–
–
–
0.05
0.10
0.15
0.20
50
100
150
200
250
300
350
400
450
500
Temperature/°C
1
70
320
387 425
(b)
m/e =16 (H O)
2
1
1
1
E–7
m/e =44 (CO2)
E–8
E–9
m/e =45 (CO2)
m/e =46 (NO2)
1E–10
50
100
150
200
250
300
350
400
450
500
Temperature/°C
1
23

Structural dynamics of molybdenum vanadium oxide (MoVOx)
Fig. 5 DSC plot analysis of
MoVOx conducted under air
and helium
1
70
280
380
450
Air
4
0
4
8
–
–
–
12
4
1
70
335
420
487
He
0
–
–
4
8
–
12
5
0
100
150
200
250
300
350
400
450
500
Temperature/°C
(
a)
(b)
Mo
2
0
0
O
V
Mo
V
5
µm
2 µm
2 µm
0
2
4
6
Fig. 6 SEM image at a8 K. b 30 k magnification for MoVOx before
activation (spray dried)
Fig. 8 Elemental mapping for MoVOx (activation under air)
(a)
(b)
(
a)
(b)
5
µm
2 µm
5
µm
2 µm
Fig. 9 SEM image at a8 K. b 30 k magnification for MoVOx after
activation under helium
Fig. 7 SEM image at a 8 K. b 30 k magnification for MoVOx after
activation under air
were recorded. The first mass loss was observed at tem-
perature below 170 °C which corresponds to the loss of
water. The largest mass loss (11.64%) was recorded
between 170 and 320 °C with the decomposition of mainly
carbon dioxide (m/e = 44, 45) and nitrogen oxides (m/
e = 45 & 46) besides desorption of crystallized water (m/
e = 16). Three more small mass losses were recorded at
320–387 °C (3.15%), 387–425 °C (0.81%) and
425–500 °C (0.47%).
thermodynamically stable tetragonal phase of the catalyst.
The huge mass loss recorded was due to exposure to oxi-
dizing medium such as air causes the complex hexagonal
oxides breakdown to the o-MoO phase [13].
3
The curve profile observed in Fig. 4a as the catalyst
-
1
precursor was being activated under 20 mL min
of
helium flow was different from the MoVOx catalyst pre-
cursor activated under air flow. Five steps of mass loss
123

D. D. Suppiah et al.
Table 6 EDX analysis of MoVOx catalyst precursors
Sample
Elements/atomic%
Molybdenum (Mo)
Oxygen (O)
Vanadium (V)
MoVOx
16.94
26.08
21.40
81.86
71.81
77.16
1.20
2.12
1.44
MoVOx (activated air)
MoVOx (activated He)
from DSC analysis. A broad exothermic peak was observed
above 420 °C indicating crystallization process. This
echoes the XRD thermal Isoplot findings whereby the
precursor crystallization leads to the thermodynamically
stable tetragonal phase (Mo O )of the catalyst [18, 24].
Mo
5
14
O
Microstructures of MoVOx
V
V
1
µm
0
2
4
6
8
Figure 6 shows the SEM images of spray-dried MoVOx (a,
b) catalyst precursor morphology at 8 and 30 k times
magnification. At lower magnification, the images show
clusters of spherical, smooth ball-like particles with no
discrete features which is consistent with any kind of
spray-dried precursors [25]. At higher magnification, the
ball-like structure appears to be spherical particles smooth
surface areas indicating the effect of drying process on
surface texture [26]. However, significant changes can be
observed for MoVOx catalytic structure after activation
under air flow as shown in Fig. 7a, b. At lower magnifi-
cation, the ball-like structure appears to be ‘cracked’ and
no longer has a smooth surface. At higher magnification,
new crystallite structure emerges featuring rough looking
hexagonal slices Fig. 8. This confirms the XRD data
analysis as shown in Fig. 1 whereby the hexagonal oxides
Fig. 10 Elemental mapping for MoVOx (activation under helium)
The DSC plot shown in Fig. 5 clearly exhibits the cal-
cination condition has influenced the structural properties
of the synthesized MoVOx precursor. The first broad
endothermic peak observed on both curves corresponds to
the release of water as supported by TG–MS evaluation. As
the melting process continues, a second endothermic peak
is observed which was associated with the second mass loss
in the curve, in which the desorption of crystallized water
(
m/e = 17 & 18) takes place. This curve was broader for
MoVOx calcined under helium indicating a slower
decomposition of ligands attached to the catalyst precursor.
The bulk nanostructured MoVOx catalyst creates a
barrier of heat and mass transfer during activation. Thus,
the structural changes were clearly influenced by the
degradation temperature. MoVOx degradation temperature
under Air was lower than under helium as supported by
in situ XRD and TG–MS [23]. Oxidative degradation of
MoVOx under air catalyst precursor shows an endothermic
and exothermic effect at region 280–380 °C whereby it
reflects that the melting is leading to the
metastable hexagonal phase. A sharp exothermic peak was
observed at region 380–450 °C, indicating a major struc-
breakdown to the o-MoO phase [13] hence leaving ‘slices’
3
of the structure. Figure 9a, b show images for MoVOx
catalyst precursor after calcination in helium. At lower
magnification, the ball-like structure appears to be uneven
but less ‘flaky’ unlike images in Fig. 7. At higher magni-
fication, a new finely dispersed phase appears to be com-
piling finer crystallite. The newly formed particles
morphology has long rods of tetragonal cross sections thus
further confirming the XRD analysis. Table 6 shows the
atomic percentage composition of molybdenum, oxygen
and vanadium in the synthesized MoVOx catalyst precur-
sor and calcined samples. Calcination affects the elemental
composition with small changes [13]. Based on the ele-
mental mapping analysis in Fig. 8 (MoVOx calcined under
air) and Fig. 10 (MoVOx calcined under helium), the ele-
ments appear to be highly dispersed in the bulk catalyst
surface. There are no aggregates of V O particles which is
tural changes took place whereby the h-MoO was oxidized
3
to the thermodynamically stable orthorhombic phase (o-
MoO ). This phase transition is irreversible. However, for
3
DSC curve for MoVOx catalyst precursor which was cal-
cined under helium reveals a higher degradation tempera-
ture. Hence, the amorphous structure undergoes two
structural changes as shown by the two exothermic peaks
2
5
also supported by XRD analysis [27].
1
23

Structural dynamics of molybdenum vanadium oxide (MoVOx)
7
. Bettahar MM, Costentin G, Savary L, Lavalley JC. On the partial
oxidation of propane and propylene on mixed metal oxide cata-
lysts. Appl Catal A Gen. 1996;145(1–2):1–48. doi:10.1016/0926-
Conclusions
The understanding of MoVOx phase transition during
thermo-oxidation (Air) and thermo-degradation (He) is
crucial to identify the active phase of the catalyst. Activation
under air reels highly crystalline hexagonal crystal structure
at 300 °C. However, at 500 °C, mixed phase consisting
mostly the thermodynamically stable orthorhombic molyb-
denum oxide was obtained. This was in accordance with the
thermo-decomposition profile exhibiting an endothermic
effect above 380 °C. Activation of MoVOx catalyst pre-
cursor under helium, however, demonstrates a different trend
of crystallization. The structural transformation was from
amorphous to nanocrystalline at 400 °C. The crystal growth
along the (001) plane indicates the reflection of nanocrys-
talline Mo O . At 500 °C, the thermodynamically
8
60x(96)00138-x.
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acterization of heterogeneous catalysts. Catal.
003;216(1–2):155–64. doi:10.1016/s0021-9517(02)00133-1.
J
2
9
. Mougin O, Dubois J-L, Mathieu F, Rousset A.
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Chem. 2000;152(2):353–60. doi:10.1006/jssc.2000.8663.
0. Wagner M. Thermal analysis in practice. Mettler Toledo col-
lected applications. Schwerzebach: Mettler-Toledo AG; 2009.
1. Gabbott P. A practical introduction to differential scanning
calorimetry. In: Gabbott P, editor. Principles and applications of
thermal analysis. Hoboken: Blackwell Publishing; 2008. p. 1–50.
2. Li X, Anton N, Arpagaus C, Belleteix F, Vandamme TF.
Nanoparticles by spray drying using innovative new technology:
1
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the b u¨ chi nano spray dryer B-90.
2
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010;147(2):304–10. doi:10.1016/j.jconrel.2010.07.113.
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Schl o¨ gl R, et al. The synthesis and structure of a single-phase,
5
14
stable tetragonal phase was achieved with high degree of
crystallization. All the particles obtained displayed
nanocrystalline properties and had been verified by mor-
phology characterization which also showed homogeneous
dispersion of the loading metals which provides the ‘site
isolation’ effect essential for catalytic propane activation.
This dynamics of activation profile provides an insight of the
catalyst reactive structural phase which can be further
exploited for modified dehydrogenation reaction such as
ODH reaction.
5
nanocrystalline MoVW mixed-oxide catalyst of the Mo O14 type.
J Catal. 2003;215(2):177–87. doi:10.1016/s0021-9517(03)00006-
x.
14. Ilkenhans T, Herzog B, Braun T, Schlogl R. The nature of the
active phase in the heteropolyacid catalyst H
used for the selective oxidation of isobutyric acid. J Catal.
995;153(2):275–92. doi:10.1006/jcat.1995.1130.
4
PvMo11O40Á32H
2
O
1
1
5. Giebeler L, Wirth A, Martens JA, Vogel H, Fuess H. Phase
transitions of V-Mo-W mixed oxides during reduction/re-oxida-
tion cycles. Appl Catal A Gen. 2010;379(1–2):155–65. doi:10.
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016/j.apcata.2010.03.022.
1
6. Zenkovets GA, Kryukova GN, Gavrilov VY, Tsybulya SV,
Anufrienko VA, Larina TA, et al. The structural genesis of a
complex (MoVW)5O14 oxide during thermal treatments and its
redox behavior at elevated temperatures. Mater Chem Phys.
Acknowledgements The authors would like to thank NANOCAT
technical service team for their contribution on sample characteriza-
tion and ROGS grant scheme (BR010-2014) for providing financial
assistance.
2
0
007;103(2–3):295–304. doi:10.1016/j.matchemphys.2007.02.
29.
1
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