Journal of Natural Gas Chemistry 18(2009) –
Synthesis of self-assembled nanorod vanadium oxide bundles
by sonochemical treatment
Y. H. Taufiq-Yap1,2∗ ,
Y. C. Wong1,2 , Z. Zainal1,2 , M. Z. Hussein1,2
1. Centre of Excellence for Catalysis Science and Technology, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan,
Malaysia. 2. Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
[ Received March 12, 2009; Revised April 22, 2009; Available online September 10, 2009 ]
Abstract
Self-assembled nanorod of vanadium oxide bundles were synthesized by treating bulk V2 O5 with high intensity sonochemical technique. The
synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope
(TEM) and temperature-programmed reduction (TPR) in H2 . Catalytic behaviour of the materials over anaerobic n-butane oxidation was
studied through temperature-programmed reaction (TPRn). Catalytic evaluation of the sonochemical treated V2 O5 products was also studied
on microreactor. XRD patterns of all the vanadium samples were perfectly indexed to V2 O5 . The morphologies of the nanorod vanadium
oxides as shown in SEM and TEM depended on the duration of the ultrasound irradiation. Prolonging the ultrasound irradiation duration
resulted in materials with uniform, well defined shapes and surface structures and smaller size of nanorod vanadium oxide bundles. H2 -TPR
profiles showed that larger amount of oxygen species were removed from the nanorod V2 O5 compared to the bulk. Furthermore, the nanorod
vanadium oxide bundles, which were produced after 90, 120 and 180 min of sonochemical treatment, showed an additional reduction peak at
lower temperature (∼850 K), suggesting the presence of some highly active oxygen species. TPRn in n-butane/He over these materials showed
that the nanorod V2 O5 with highly active oxygen species showed markedly higher activity than the bulk material, which was further proven by
catalytic oxidation of n-butane.
Key words
nanorod vanadium oxide; sonochemical treatment; butane oxidation
1. Introduction
Vanadium is a key element in the formulation of catalysts utilized in the production of anhydride via selective
oxidation reaction in vapour phase with molecular oxygen
[1]. Today, vanadium phosphorus oxide-based (VPO) catalysts are still industrially employed for the selective oxidation
of n-butane to maleic anhydride (MA) [2], whereby vanadium
pentoxide is used as the starting material to synthesize such
valuable catalysts.
However, the low selectivity of this catalyst for MA remains a serious problem. Therefore, there is a need to develop new, functional forms of vanadium oxide-based catalysts to improve this oxidation process. Since the microstructure (shape and dimensions) of the vanadium oxide is generally considered to have great influence on the catalyst’s
performance, therefore control over the microstructure of
the vanadium oxide is a promising approach to improve the
catalytic performance [3].
∗
Recently, nanotechnology has gained substantial popularity due to the rapidly developing techniques both to synthesize and to characterize materials and devices at the nano
scale, as well as the promises that such technology offers to
substantially expand the achievable limits in many different
fields including medicine, electronics, chemistry and engineering. Nano-sized noble metal particles have occupied
the central place in heterogeneous catalysis for many years,
long before recognition of nanotechnology [4]. In this study,
nanorod vanadium oxide (V2 O5 ) catalysts were synthesized
via sonochemical treatment, and their physico-chemical properties were characterized by XRD, redox titration, chemical
analysis, SEM, TEM, H2 -TPR. TPRn and catalytic oxidation
of n-butane were also carried out to evaluate the catalytic behaviour.
2. Experimental
2.1. Preparation of self-assembled nanorod V 2 O5
Corresponding author. Tel: +603-8946 6809; Fax: +603-8946 6758; E-mail: [email protected]
Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
doi:10.1016/S1003-9953(08)60124-3
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Y. H. Tauf iq-Yap et al./ Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Vanadium pentoxide, V2 O5 (1.84 g, 10 mmol, 98.0%
Fluka), sodium fluoride, NaF (0.84 g, 20 mmol, 99.0%
Merck) and distilled water (200 cm3 ) were mixed and exposed
to high intensity ultrasound irradiation under ambient air for
30, 60, 90, 120, 180 and 240 min. The sonication was conducted without cooling so that the temperature of the reactant
mixture increased gradually. When the reaction was finished,
a dark orange precipitate was obtained.
After cooling the mixture to room temperature, the precipitate was filtered and washed with distilled water and
ethanol before dried at 373 K for 24 h to obtain nanorod V2 O5
and denoted as SX, where X refers to the duration of the sonochemical treatment.
effluent gas flowed via the heated gas line and was analyzed
by an on-line mass spectrometer.
The oxidation of n-butane was carried out at 673 K with
GHSV = 2400 h−1 in a fixed-bed microreactor with a standard
mass of catalyst (250 mg). n-Butane and air were fed to the
reactor via calibrated mass flow controllers to give a feedstock composition of 1.0% n-butane in air. The products were
then fed via heated lines to an on-line gas chromatograph for
analysis. The reactor comprised a stainless steel tube with the
catalyst held in place by plugs of quartz wool. A thermocouple was located in the centre of the catalyst bed.
2.2. Catalyst characterization
3.1. XRD
The total surface area of the catalysts was measured by
the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption at 77 K. This was done on a Sorptomatic 1990 Series,
Thermo Fischer-Scientific instrument.
The bulk chemical composition was determined by using a sequential scanning inductively coupled plasma-atomic
emission spectrometer (ICP-AES) (Perkin Elmer Emission
Spectrometer model plasma 1000).
The average oxidation numbers of vanadium in the sample bulk were determined by redox titration following the
method of Niwa and Murakami [4].
XRD analysis was carried out by using a Shimadzu
diffractometer model XRD 6000.
The electron microscopy techniques were used to obtain
the information of the morphology and size of the samples
by LEO 1455 Variable Pressure SEM. The morphology was
studied at an accelerating voltage of 30 kV. The particles were
attached on an aluminium stub by using double-sided tape.
The preparation was covered by using a thin layer of gold
coating by using BIO-RAD Sputter Coater. The SEM micrographs were recorded by using a digital camera at various
magnifications.
The particle size of the samples was examined using LEO
912AB energy filter TEM with an acceleration voltage of
120 keV.
H2 -TPR analysis was performed on ThermoFisher Scientific TPDRO 1100 apparatus equipped with a thermal conductivity detector (TCD). The experiment was done by following
the thermal conductivity of the outlet stream with TCD when
the temperature of the fresh catalysts was raised from ambient
to 960 K at 10 K/min in a H2 /Ar stream.
Figure 1 shows the XRD patterns of the sonochemical
treated and untreated V2 O5 with the presence of only the single phase of V2 O5 (JCPDS File No: 09−0387). The peaks
appeared at 2θ = 15.4o, 20.2o, 26.2o and 30.9o represent the
reflection of (200), (001), (110) and (400), respectively. These
patterns were in good agreement with those reported earlier
[5]. No peaks of other phases were detected, indicating the
high purity of the sonochemically treated V2 O5 . The sonochemical treated V2 O5 with different durations showed high
intensity compared to the untreated V2 O5 . This means that the
sonochemically treated V2 O5 products are more crystalline
than the untreated V2 O5 , which contains large amount of
amorphous phase.
3. Results and discussion
Figure 1. XRD patterns of sonochemical treated and untreated V2 O5
2.3. Catalytic test
The TPRn analysis was also performed using ThermoFisher Scientific TPDRO 1100 apparatus equipped with
TCD. The experiment was done by following the thermal conductivity of the outlet stream with TCD when the temperature of the fresh catalysts was raised from ambient to 960 K at
5 K/min in n-butane/He stream. On exiting the machine, the
3.2. BET surface area measurements and chemical analysis
The total surface area shown in Table 1 for bulk V2 O5
is 4.7 m2 ·g−1 . After undergoes sonochemical treatment for
30 min, S30 does not gave any significant change of the surface area (4.2 m2 ·g−1 ). However, prolonging the sonochem-
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
ical treatment to 60 min (S60) lead to a drastically reduction (96%) in the total surface area to only 0.2 m2 ·g−1 compared to the bulk. Further extending the treatment time to
90 and 120 min resulted in improved total surface area, 6.3
and 4.0 m2 ·g−1 , respectively. Interestingly, a significant increment to 13.0 m2 ·g−1 was observed for S180 compared
to the bulk. However, continuously sonochemical treatments to 240 min markedly decreased the surface area to only
5.6 m2 ·g−1 .
Table 1. Total surface area and chemical analysis
for nanorod and bulk V2 O5
V2 O5
Bulk
S30
S60
S90
S120
S180
S240
Total surface
area (m2 ·g−1 )
4.8
4.2
0.2
6.3
4.0
13.0
5.6
Oxidation of the vanadium
V4+ (%)
V5+ (%)
Vav
3
97
4.97
6
94
4.94
9
91
4.91
10
90
4.90
16
84
4.84
17
83
4.83
17
83
4.83
The average oxidation number of the vanadium and the
percentage of V5+ and V4+ oxidation state are summarised in
Table 1. The average oxidation state of the vanadium was observed to decrease with the increase of the sonochemical treat-
3
ment duration. Therefore, the sonochemical treatment technique in the presence of mineraliser promotes the formation
of V4+ phases in the V2 O5 . In other words, it reduced the
V5+ phase contain in V2 O5 to form more V4+ phase.
3.3. SEM
The morphologies of the V2 O5 that underwent sonochemical treament with different durations are examined by using
SEM as shown in Figure 2. All sonochemical treated V2 O5 at
different durations gave a mixture of small platelet and rodlike crystals morphology.
Bulk V2 O5 , which underwent sonochemical treatment
with various durations i.e. 30, 60, 90, 120, 180 and 240 min,
consists of small crystal platelets. However, for S30, S60 and
S120, these small crystal platelets formed are unstable, with
high surface energy and tendency to stack and agglomerate to
each other to form larger particles, hence lowering the total
surface area exposed to the surrounding environment. These
observations obtained were in good agreement with the BET
surface area measurements. Interestingly for S90, S180 and
S240, the small crystal platelets formed are somehow more
stable, and the tendency to stack and agglomerate to each
other to form larger particles are weak, hence increasing the
total surface area exposed to environment as shown in Table 1.
Figure 2. SEM micrographs of sonochemical treated V2 O5 (a) S30, (b) S60, (c) S90, (d) S120, (e) S180, and (f) S240
3.4. TEM
The TEM images (shown in Figure 3) further confirm that
the bulk V2 O5 , which underwent the sonochemical treatment
with different durations, consist of self-assembled V2 O5 bundles composed of the nanorods with diameters of 8−50 nm
and length of 150−420 nm except for S240, which consists
of small platelets with diameters 12−30 nm compared to the
bulk V2 O5 , which consists of bulky particles with diameters
∼430 nm.
It is obvious that the large V2 O5 particles were observed
in the initial reaction mixture (Figure 3a). As the reaction
time increased to 30 min, the nanorods V2 O5 began to selfassemble with each other and turned into bundled form (Fig-
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Y. H. Tauf iq-Yap et al./ Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
ure 3b). As the reaction time increased from 30 to 180 min,
more self-assembled nanorods V2 O5 bundles formed with
smaller diameter (Figures 3c–3f). As the reaction time in-
creased to 240 min, the self-assembled nanorod V2 O5 bundles became unstable and fractured into nano-sized particles,
which agglomerated (Figure 3g).
Figure 3. TEM micrographs of sonochemical treated and untreated V2 O5 (a) bulk, (b) S30, (c) S60, (d) S90, (e) S120, (f) S180 and (g) S240
The sonochemical formation of self-assembled nanorod
V2 O5 bundles underwent three steps in sequence: (1)
ultrasound-induced dissolution of V2 O5 and formation of nuclei, which further led to oriented attachment to form the
nanorods; (2) the individual nanorods were further attached
side-by-side to assemble into bundles; and (3) the unstable of
the self-assembled nanorod V2 O5 bundles led to formation of
the primary nanoparticles V2 O5 . A schematic illustration of
the development of self-assembled nanorod V2 O5 bundles is
shown in Scheme 1.
Scheme 1. Schematic illustration of the growth mechanism of self-assembled
nanorod V2 O5 bundles
3.5. TPR in H 2 /Ar
Figure 4 shows the TPR profiles of the nanorod and bulk
V2 O5 in H2 /Ar stream (5% H2 in Argon, 1 bar, 25 cm3 ·min−1 )
using a fresh sample of V2 O5 and raising the temperature from
ambient to 960 K at 10 K min−1 . The peak maxima temperatures and the amount of removed oxygen are summarised in
Table 2.
One distinct reduction peak was observed for bulk, S30
and S60 in the rate of hydrogen consumption at 933 and
942 K, respectively. Interestingly, prolonging sonochemical
treatment from 90 to 240 min induced the presence of a reduction peak at low temperature (∼80 K lower). The presence
of two reduction peaks indicated the presence of two kinetically different oxygen species in these materials, which correspond to the reduction of V4+ and V5+ phases, respectively.
Moreover, the first reduction peak at lower temperature only
appeared when the sonochemical treatment prolonged until
90 min with the amount of V4+ equal to 10%. This obser-
5
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
vation may suggest that these self-assembled nanorod V2 O5
may be more active and selective for n-butane partial oxidation to maleic anhydride.
25 cm3 ·min−1 ) over a fresh sample of nanorod and bulk V2 O5
with the temperature from ambient to 960 K at 5 K·min−1 ,
while following the CO2 (m/z = 44) signal on the mass spectrometer.
Figure 4. TPR profiles of nanorod and bulk V2 O5
Figure 5. TPRn of n-butane/He stream over synthesised V2 O5
Table 2. Total amount of oxygen atoms removed from nanorod
and bulk V2 O5 by reduction in H2 /Ar
This figure shows that the peak of CO2 production for
bulk V2 O5 centered at 763 K. Interestingly, nanorod S30 behaved to be more active with CO2 production at lower temperature (690 K). Additional CO2 peak produced at lower temperature was also observed for S90 and S120, at 615 and 608 K,
respectively (compared to the bulk). The total amount of CO2
produced in S90 and S120 also shows a significant increase
for more than 3 times compared to the bulk, 1.14×10−7 and
1.25×10−7, respectively.
Nanorod V2 O5 significantly showed higher rate of CO2
production at lower temperature, and the total amount of CO2
production also drastically increased compared to the bulk.
Hence, the nanorod V2 O5 really plays an important role by
producing more reactive and labile oxygen species on the
V2 O5 surface. Therefore, these nanorods V2 O5 are more active for n-butane oxidation.
V2 O5
Tmax (K)
Bulk
1
934
Total oxygen atoms removed
S30
1
933
Total oxygen atoms removed
S60
1
942
Total oxygen atoms removed
S90
1
848
2
926
Total oxygen atoms removed
S120
1
860
2
939
Total oxygen atoms removed
S180
1
847
2
921
Total oxygen atoms removed
S240
1
872
2
922
Total oxygen atoms removed
Oxygen atom
removed
(×10−3 mol·g−1 )
Oxygen atom
removed
(×1021 atoms·g−1 )
1.83
1.83
1.10
1.10
3.77
3.77
2.27
2.27
2.85
2.85
1.72
1.72
0.54
1.66
2.20
0.33
1.00
1.33
0.80
1.53
2.33
0.48
0.92
1.40
0.66
1.37
2.03
0.40
0.83
1.23
0.72
0.75
1.47
0.43
0.45
0.88
3.6. TPRn in butane/He
Figure 5 shows the TPR profile obtained by passing a n-butane/He stream (2% n-butane, 98% He, 1 bar,
Table 3. Total amount of CO2 produced from the nanorod
and bulk V2 O5 by TPRn in n-butane/He
V2 O5
Bulk
1
Total CO2
S30
1
Total CO2
S90
1
2
Total CO2
S120
1
2
Total CO2
Tmax (K)
CO2 produced (×10−7 a.u.)
763
produced
0.36
0.36
690
produced
1.26
1.26
615
880
produced
0.25
0.89
1.14
608
866
produced
0.25
1.00
1.25
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Y. H. Tauf iq-Yap et al./ Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
3.7. Catalytic oxidation of n-butane
shown to be the main contribute for the activation of n-butane
[6,7].
All the self-assembled nanorod and bulk V2 O5 catalysts
were tested in the selective oxidation of n-butane to maleic
anhydride. The results are summarised in Table 4.
Table 4. The catalytic performance of self-assembled nanorod
and bulk V2 O5 catalysts synthesized for oxidation n-butane
Catalyst
Bulk
S30
S60
S90
S120
S180
S240
n-Butane
conversion (%)
33
45
57
94
52
49
5
Product selectivity (%)
MA
CO
CO2
2.0
63
36
1.0
0
99
1.0
0
99
1.0
0
99
1.0
0
99
1.0
0
99
0
0
100
Bulk V2 O5 gave 33.0% of n-butane conversion with a relatively low MA selectivity (which is 2.0%), resulting in 0.7%
of MA yield together with 63.0% and 36.0% of CO and CO2 ,
respectively. An increment was observed in the n-butane conversion for S30, S60, S120 and S180 catalysts compared to the
bulk V2 O5 mainly due to their different morphology, whereby
the self-assembled nanorod V2 O5 contains more active sites
for the oxidation of n-butane compared to its bulk structures
as shown in SEM and TEM micrographs.
Interestingly, 94% of n-butane conversion was achieved
on S90 catalyst with 61% increment compared to the bulk.
Beside the different structure of the V2 O5 produced through
sonochemical treatment technique compared to its bulk, this
phenomenon occurred due to the presence of high kinetically
active oxygen, which easily removed at lower temperature
(848 K) as shown in H2 -TPR profile. Furthermore, the TPRn
in n-butane/He stream analysis had also proved that the S90
catalyst consisted of more active sites (with high reactive oxygen species) by consuming n-butane at a lower temperature.
However, the S240 catalyst showed a drastic decrement in
n-butane conversion to only 5% compared to the bulk V2 O5 .
This may be due to the morphology of the S240 catalyst,
where the self-assembled nanorod V2 O5 bundles become unstable after the ultrasound irradiation at ambient temperature
for 240 min and fractured into agglomerated nano-particles,
which directly destroyed the active sites of the V2 O5 catalyst. Moreover, this nano-particles are unstable, with high
surface energy and tend to agglomerated with each other to
form larger particles. This directly reduces the number of active sites of the V2 O5 catalyst.
Figure 6 shows the correlation between the catalytic performance (conversion) of the self-assembled nanorod and the
bulk V2 O5 catalysts with the duration of sonochemical treatment. An optimum of ∼90−100 min was obtained in order to
produce a highly active nanorod V2 O5 catalyst. This is mainly
due to the appropriate amount of V4+ (∼10%) present in the
nanorod V2 O5 in order to obtain an optimum V4+ /V5+ ratio
that gave high activity for n-butane oxidation as shown in Figure 7. Previously, the role of V4+ present in VPO catalyst was
Figure 6. n-Butane conversion as a function of the duration of sonochemical
treatment
Figure 7. n-Butane conversion as a function of percentage of V5+
4. Conclusions
All the nanorod V2 O5 that synthesised through sonochemical treatment technique with different durations showed
mixtures of small platelets and rod-like crystals, with high surface energy and tendency to stack and agglomerate with each
other to form larger particles. Hence, some of the nanorod
V2 O5 had lower total surface area. V2 O5 that undergo sonochemical treatment for 30 min (S30) had the highest amount
of total oxygen atoms, and S90, S120 and S180 had active
oxygen species, which were removed at lower temperature
(∼850 K). TPRn in n-butane/He stream have significantly
showed that nanorod V2 O5 had the rate of CO2 production at
lower temperature and drastically increased the amount compared to the bulk. An extremely high n-butane conversion
(94%) was obtained on S90 due to its morphology, which was
different from its bulk structures and with the present of kinetically reactive oxygen species. Hence, this shows that the
nanorod V2 O5 played an important role in producing more reactive oxygen species on the V2 O5 surface. Therefore, we can
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
further suggest that these nanorods V2 O5 are more active for
n-butane oxidation.
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