Controllable synthesis of novel hierarchical V2O5/TiO2 nanofibers with improved acetone oxidation performance

Article abstract of DOI:10.1039/c5ra01001b

A controllable strategy to fabricate novel hierarchical V₂O₅/TiO₂ nanofiber catalysts was proposed. The catalysts, which comprised primary TiO₂ nanofibers and secondary V₂O₅ nanoparticles, were fabricated by combining electrospinning and hydrothermal growth. The controllable synthesis process and possible formation mechanism were also demonstrated through a series of time-dependent experiments. The hierarchical V₂O₅/TiO₂ nanofiber catalysts were further applied in the oxidation of volatile organic compounds for the first time and were found to present a high oxidation performance for acetone. The morphological, structural, chemical characterization and catalytic performance analyses illustrated the highest catalytic activity was obtained from the synthesized V₂O₅/TiO₂ nanofiber catalyst with 5 wt% V₂O₅. This finding could be attributed to the combined effect of the specific hierarchical nanofibrous morphology, abundant oxygen vacancies, and appropriate vanadium concentration.

Full text of DOI:10.1039/c5ra01001b

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Controllable synthesis of novel hierarchical V2O5/
TiO2 nanofibers with improved acetone oxidation
performance†
Cite this: RSC Adv., 2015, 5, 30416
*
Xuecheng Zhu, Jinghuan Chen, Xinning Yu, Xinbo Zhu, Xiang Gao and Kefa Cen
A controllable strategy to fabricate novel hierarchical V2O5/TiO2 nanofiber catalysts was proposed. The
catalysts, which comprised primary TiO2 nanofibers and secondary V2O5 nanoparticles, were fabricated
by combining electrospinning and hydrothermal growth. The controllable synthesis process and possible
formation mechanism were also demonstrated through a series of time-dependent experiments. The
hierarchical V2O5/TiO2 nanofiber catalysts were further applied in the oxidation of volatile organic
compounds for the first time and were found to present a high oxidation performance for acetone. The
morphological, structural, chemical characterization and catalytic performance analyses illustrated the
highest catalytic activity was obtained from the synthesized V2O5/TiO2 nanofiber catalyst with 5 wt%
V2O5. This finding could be attributed to the combined effect of the specific hierarchical nanofibrous
morphology, abundant oxygen vacancies, and appropriate vanadium concentration.
Received 17th January 2015
Accepted 12th March 2015
DOI: 10.1039/c5ra01001b
www.rsc.org/advances
properties of V2O5/TiO2 catalysts could be modied because of
the electronic interaction between TiO2 and VOx species.16
1 Introduction
Catalysts prepared via conventional methods such as wet
impregnation, sol–gel, and direct calcination display certain
deciencies like declining specic surface area and torpid mass
transfer process.17 Electrospinning is a remarkable and versatile
technique for catalyst fabrication, which endows the obtained
materials with controllable structures and nanobrous
morphologies that are favorable for the mass transfer process in
heterogeneous catalytic reaction.18,19 When electrospinning is
combined with hydrothermal growth, the resulting fabricated
hierarchical nanobers exhibit many merits, which leads to
their extensive application in electrochemistry,20 photo-
catalysis,21 and gas sensors.22 However, V2O5/TiO2 catalysts
synthesized via electrospinning combined with hydrothermal
growth have not been reported. Moreover, the as-obtained V2O5/
TiO2 nanober catalysts were applied on VOC oxidation for the
rst time. Both synthesis methods have been proven to be
comparatively versatile, low cost, applicable, and environment-
friendly techniques.20
Hence, in the present study, a hierarchical V2O5/TiO2 nano-
ber catalyst was synthesized by combining the electrospinning
technique and hydrothermal growth method. Moreover, the as-
obtained catalysts were supposed to exhibit enhanced perfor-
mance on VOC oxidation. In this study, acetone was chosen as
the representative VOC because it is relatively familiar in the
atmosphere. The morphological, structural, and chemical
characteristics of the catalysts were investigated by SEM, TEM,
XRD, Raman, EDS, N2 adsorption, and XPS method, and the
catalytic performance on acetone oxidation was also measured.
These analyses were carried out to investigate the inuence of
Under the pressure of increasing environmental awareness,
governments all over the world have developed stringent regu-
lations to limit the emissions of atmospheric pollutants from
automobiles and industrial activities.1,2 As signicant atmo-
spheric pollutants that are toxic to human health and are
involved in the formation of photochemical smog,3 volatile
organic compounds (VOCs), which are emitted from vehicle
exhausts, solvent evaporation, oil rening, and others, were
preferentially enrolled in the blacklist for emission reduction.4
Catalytic oxidation is one of the most efficient and cost-effective
methods in removing VOCs.5
Noble metals and transition metal oxides have been widely
studied for the catalytic oxidation of VOCs. Noble metal cata-
lysts present excellent oxidation performance at low-
temperature range,6 but their industrial application is signi-
cantly limited by the scarcity of resources, high cost, and the
characteristic of being easily degraded by Cl- or S-containing
species.7 Transition metal oxides such as Fe2O3,8 Co3O4,9 and
V2O5 (ref. 10) have attracted much attention. Among them,
vanadium oxide has been investigated to be effective in VOC
oxidation.11,12 In addition, V2O5/TiO2 catalysts with excellent
catalytic performance have been widely studied in NH3–SCR,13
VOC oxidation,14 and photocatalysis.15 Furthermore, the redox
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou
310027, Zhejiang, China. E-mail: xgao1@zju.edu.cn; Fax: +86-571-87951335
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra01001b
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the synthesis parameters on the morphological structure and recording on a SIRION-100 microscope. Energy dispersive X-ray
acetone oxidation performance and to reveal the structure– energy dispersive spectroscopy (EDS, GENESIS4000) combined
function relationship. This study may provide guidance for the with SEM was used to determine the element content on the
synthesis of novel metal oxide catalysts for application in broad surface.
elds that involve ltration devices.23–25
Transmission electron microscopy (TEM) was performed
using a Tecnai G2 F20 S-TWIN microscope operating with an
acceleration voltage of 200 kV.
2 Experimental
2.1 Catalyst preparation
X-ray powder diffraction (XRD) patterns were measured on a
PANalytical X'Pert PRO XRD system using Cu Ka radiation (l ¼
0.15418 nm) in the 2q range between 10 and 90ꢁ at a scanning
The hierarchical V2O5/TiO2 nanober catalysts were prepared by
the electrospinning and hydrothermal method. This synthetic
method involved the following steps: (1) electrospinning of the
precursor solution to obtain primary TiO2 nanobers aer
calcination; (2) hydrothermal growth of V2O5 nanoparticles on
TiO2 nanobers to obtain hierarchical V2O5/TiO2 nanober
catalyst aer calcination.
rate of 4ꢁ minꢀ1
.
Raman spectra were obtained using a Raman spectrometer
(LabRamHRUV, JDbin–Yvon, France) with Raman shi from 50
to 2000 cmꢀ1 under the 514 nm excitation laser light. All the
Raman spectra were recorded at room temperature with a
resolution of 1 cmꢀ1
.
The N2 adsorption–desorption experiments were performed
at liquid nitrogen temperature (77 K) using a Micromeritics
Instrument Corporation (Norcross, Georgia) Accelerated
Surface Area and Porosimetry (ASAP) 2020 system. The specic
surface areas were calculated from the N2 adsorption isotherms
by applying the Brunauer–Emmett–Teller (BET) equation. The
pore volume and average pore diameters were determined by
the Barrett–Joyner–Halenda (BJH) method using the desorption
branches.
X-ray photoelectron spectroscopy (XPS) were performed with
a Thermo ESCALAB 250Xi using Al Ka as the radiation source at
constant pass energy of 1486.6 eV. The binding energies for Ti
2p, V 2p, O 1s, C 1s were referenced to the energy of C 1s peak at
284.6 eV.
Firstly, 3.0 g polyvinylpyrrolidone (PVP, Mw ¼ 1 300 000) was
dissolved in a mixed solution containing 30 ml ethanol and 10
ml acetic acid. Aer magnetically stirring for 1 h, 10 g tetrabutyl
titanate was added into the mixed solution, and the obtained
yellow titania precursor solution was further magnetically stir-
red overnight at room temperature. Then the viscous sol–gel
solution was electrospun between a 10 ml syringe and a piece of
aluminum foil attached on a roller at a rotating speed of 50 rpm.
The applied voltage was 15 kV and the distance from the elec-
trode to collector was 15 cm. The injection speed was controlled
by a syringe pump at constant 1.0 ml hꢀ1. The primary TiO2
nanobers (denoted as pure TiO2) were obtained aer calcina-
tion in a muffle at 500 ꢁC for 3 h in air.
Secondly, 0.0348 g vanadium(III) acetylacetonate and 0.60 g
urea were dissolved in 75 ml deionized water. Under the
condition of magnetic stirring, 0.36 g hexadecyl trimethyl
ammonium bromide (CTAB) and 0.5 ml polyethylene glycol
(PEG, Mw ¼ 400) were added into the mixed solution drop by
drop. Then the resulting solution was transferred into a 100 ml
Teon-lined autoclave with 100 mg primary TiO2 nanobers.
The autoclave was sealed and heated at 120 ꢁC for 24 h, and
naturally cooled to room temperature. The lm was collected
out, washed successively three times each with ethanol and
2.3 Catalytic activity
The catalytic oxidation of acetone were performed in a xed bed
tubular quartz reactor with an inner diameter of 6 mm. 50 mg of
the catalyst was placed in the center of the reactor and held by
glass wool. Total feed ow was kept at 300 ml minꢀ1, leading to
the gas hourly space velocity (GHSV) of total feed holding at
360 000 ml gꢀ1 hꢀ1. The feed gas mixture contained 500 ppm
acetone, 5% O2, and balanced N2. The temperatures of the
catalyst bed and tubular furnace were monitored automatically
by E-type thermocouples. The inlet and outlet concentrations of
acetone were analyzed by a gas chromatograph (Agilent 7890A,
USA) equipped with a ame ionization detector (FID) and a
capillary column of HP-Innowax (Agilent, USA) operated at
60 ꢁC. The acetone conversion (uacetone) is calculated as follows:
ꢁ
deionized water, and then dried under vacuum at 50 C over-
night. Then the as-obtained lms were calcinated at 400 ꢁC for 3
h. Thus, the hierarchical V2O5/TiO2 nanobers were fabricated.
This sample was denoted as V2Ti, where the subscript is the
expectant mass percentage of V2O5 (in wt%). By adjusting the
weight of vanadium(III) acetylacetonate added into the
precursor solution proportionally, a series of V2O5/TiO2 nano-
bers with different V2O5 content were fabricated through this
synthetic method, which were denoted as V5Ti, V10Ti and V25Ti,
respectively.
cin ꢀ cout
uacetone
¼
ꢂ 100%
cin
cin, cout – inlet and outlet concentration of acetone.
For comparison, pure V2O5 powders (denoted as pure V2O5)
were also prepared by direcꢁt calcination of ammonium meta-
vanadate in a muffle at 500 C for 3 h in air.
3 Results and discussion
3.1 Morphology characterization
The morphology and microstructure of the hierarchical V2O5/
TiO2 nanobers (Fig. 1) were investigated by SEM. The primary
2.2 Catalyst characterizations
The morphology of obtained samples were characterized with TiO2 nanobers with diameters ranging from 400 nm to 600 nm
thermal eld-emission scanning electron microscopy (FE-SEM), interweave with one another into a compact micro network
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To obtain further information about the hierarchical V2O5/
TiO2 nanobers, TEM characterization was performed. Repre-
sentative TEM images of V5Ti are illustrated in Fig. 2A and B. As
shown in Fig. 2A, a single nanober possesses a slender
morphology with a diameter of approximately 400 nm. The
secondary nanoparticles could be distinguished from the
primary nanobers in the high-magnication image (Fig. 2B),
which reveals that a layer of secondary nanoparticles are evenly
distributed on the surface of the primary nanobers, in accor-
dance with the results from SEM characterization. In addition,
the inner-planar spacing was measured by high-resolution
TEM. As displayed in Fig. 2C, the lattice fringe spacing of
0.352 nm, which corresponds to the (101) plane of the anatase
TiO2 (JCPDS no. 21-1272), could be observed in different posi-
tions of the nanobers. In Fig. 2D, which was taken from the
interfacial region, the measured lattice fringe spacing of 0.348
nm refers to the (110) plane of shcherbinaite V2O5 (JCPDS no.
41-1426). To sum up, the morphology characterization results
reveal that the obtained hierarchical nanobers were composed
of TiO2 and V2O5 and that secondary V2O5 nanoparticles were
successfully grown on the surface of the primary TiO2
nanobers.
3.2 XRD characterization
The crystalline structure and phase purity of the V2O5/TiO2
nanobers were further characterized by XRD (Fig. 3). The XRD
patterns demonstrate that all the diffraction peaks could be
indexed to anatase TiO2 (JCPDS no. 21-1272), rutile TiO2 (JCPDS
no. 21-1276), and shcherbinaite V2O5 (JCPDS no. 41-1426).
As displayed in Fig. 3a, the detected peaks of pure TiO2
nanobers at 2q ¼ 25.3ꢁ, 36.9ꢁ, 37.8ꢁ, 38.6ꢁ, 48.0ꢁ, 53.9ꢁ, 55.1ꢁ,
62.1ꢁ, 62.7ꢁ, 68.8ꢁ, 70.3ꢁ, 75.0ꢁ, 76.0ꢁ, 82.6ꢁ correspond to the
(101), (103), (004), (112), (200), (105), (211), (213), (204), (116),
(220), (215), (301), and (224) planes of TiO2, indicating the
typical anatase crystal structure of TiO2 (JCPDS no. 21-1272). In
addition, the weak signal at 2q ¼ 27.4ꢁ demonstrates the
probable undesirable formation of rutile TiO2 (JCPDS no. 21-
1276). This nding may be attributed to the high calcination
Fig. 1 High-magnification and low-magnification SEM images of
hierarchical V2O5/TiO2 nanofibers: (A) pure TiO2, (B) V2Ti, (C) V5Ti, (D)
V10Ti, (E) V25Ti.
(Fig. 1A). In addition, the pristine TiO2 nanobers have smooth
surfaces without secondary nanostructures. Aer the hydro-
thermal growth of V2O5 nanoparticles, the expected hierarchical
nanostructures are obtained. As shown in Fig. 1B–E, the
secondary V2O5 nanoparticles are relatively evenly distributed
on the surface of the primary TiO2 nanobers. Moreover, the
V2O5 nanoparticles are distributed more densely, and their sizes
increase as the concentration of the precursor in the hydro-
thermal solution rises. In the case of V2Ti (Fig. 1B), the V2O5
nanoparticles have a distinct outline and a sparse distribution.
When the mass percentage increases to 10 wt% (Fig. 1D), a layer
of V2O5 nanoparticles with relatively regular shape and dense
distribution could be clearly observed on the surface of the
pristine TiO2 nanobers. As the concentration further increases
(Fig. 1E), the V2O5 nanoparticles overlap with one another,
eventually forming large particles and covering the pristine TiO2
nanobers completely.
Fig. 2 TEM images (A and B) and HRTEM images (C and D) of V5Ti
hierarchical nanofibers.
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Fig. 3 XRD patterns of hierarchical V2O5/TiO2 nanofibers: (a) pure
TiO2, (b) V2Ti, (c) V5Ti, (d) V10Ti, (e) V25Ti.
temperature because the phase transformation from anatase
phase to rutile phase could occur at 500 C.26 Aer the hydro-
ꢁ
thermal growth of low-concentration vanadium precursor, the
XRD patterns of V2Ti and V5Ti (Fig. 3b and c) remain the same
as that of pure TiO2 nanobers. No diffraction peaks of new
crystal species are further detected in V2Ti and V5Ti, which
indicates that V2O5 are highly dispersed in the nanobers.
Fig. 4 Raman spectra of hierarchical V2O5/TiO2 nanofibers: (a) pure
TiO2, (b) V2Ti, (c) V5Ti, (d) V10Ti, (e) V25Ti.
Nevertheless, as the concentration further increases (Fig. 3d spectra may be because the content of rutile TiO2 is too low to
and e), several new peaks appear at 2q ¼ 15.3ꢁ, 20.3ꢁ, 21.7ꢁ, show distinct peaks in the Raman spectra. Aer the hydro-
26.1ꢁ, 31.0ꢁ, 32.4ꢁ, 34.3ꢁ, 41.3ꢁ, 45.5ꢁ, 47.3ꢁ, 51.2ꢁ, 61.1ꢁ, which thermal growth of low-concentration vanadium precursor, the
reveal the (200), (001), (101), (110), (301), (011), (310), (002), Raman spectra (Fig. 4b and c) remain the same as that of pure
(411), (600), (020), and (321) planes of V2O5 and show the typical TiO2 nanobers because no new Raman bands are found. As
crystal structure of shcherbinaite V2O5 (JCPDS no. 41-1426). The shown in Fig. 4d, a very weak broad band centered around
diffraction peaks on behalf of V2O5 in V25Ti are stronger and 286 cmꢀ1, which might arise from the vibrations of bridging V–
sharper than that of V10Ti, implying that vanadium oxides are O–V,29 can be observed. As the concentration further increases
better crystallized. According to Chen and co-workers,27 metal (Fig. 4e), another two peaks are observed around 697 and
oxides can disperse spontaneously as a monolayer on the 990 cmꢀ1. The weak band around 697 cmꢀ1 may also refer to
surface of supports, which possess different utmost monolayer bridging V–O–V, whereas the sharp Raman band around
dispersion capacity for each metal oxide. Therefore, the 990 cmꢀ1, which reects the ordered lattice structure of crys-
increase in crystallinity might be because the high V2O5 talline V2O5, may be ascribed to the vibration of the vanadyl
concentration loaded on the surface has exceeded the utmost V]O bond. Furthermore, a remarkable decrease in the inten-
monolayer dispersion capacity of the primary supporter TiO2. sity of the peaks that arise from anatase TiO2 is shown in Fig. 4d
The spare V2O5 appears as crystal and gradually forms obvious and e, indicating the decrease in the crystallinity of the TiO2
nanoparticles on the surface of the nanobers.
supporter, which is possibly caused by the interaction between
With the exception of the three crystalline phases detected the TiO2 supporter and the loaded V2O5.
and described above, no other crystalline phase can be found in
In summary, Raman spectra uncovered the existence of both
the V2O5/TiO2 hierarchical nanostructures. These results anatase TiO2 and shcherbinaite V2O5 in the hierarchical nano-
conrm that the synthesis strategy adopted successfully ach- structures, thereby demonstrating the successful introduction
ieves V2O5/TiO2 hierarchical nanostructures that consist of of V2O5 onto the primary TiO2 nanobers.
primary TiO2 nanobers and secondary V2O5 nanoparticles.
3.4 Porous texture and EDS characterization
3.3 Raman analysis
The porous texture characterization results of the V2O5/TiO2
The Raman spectra of the samples are shown in Fig. 4. As nanober catalysts are listed in Table 1. The specic surface
illustrated in Fig. 4a, ve distinct peaks are observed at area of the samples decreases gradually with the increase in the
approximately 143 (Eg), 199 (Eg), 395 (B1g), 515 (A1g), and 636 concentration of vanadium ions in the hydrothermal precursor
(Eg) cmꢀ1, corresponding to the characteristic Raman modes of solution. For pure TiO2 nanobers, the specic surface area is
anatase TiO2,28 which is consistent with the XRD results. No 31.4 m2 gꢀ1, which is higher than the 19.5 m2 gꢀ1 of the TiO2
peaks that can be ascribed to rutile TiO2 are shown in the nanobers prepared in the similar way reported by Yuan et al.30
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Table 1 Porous texture results, EDS results and catalytic performance of hierarchical V2O5/TiO2 nanofibers
Surface area
Pore volume
Average pore
diameter (nm)
V2O5 content
(wt%, EDS)
Sample
(m2 gꢀ1
)
(cm3 gꢀ1
)
T50 (ꢁC)
T90 (ꢁC)
Pure TiO2
V2Ti
V5Ti
V10Ti
V25Ti
31.4
27.3
26.3
14.9
2.6
0.18
0.12
0.15
0.11
0.08
23.0
17.6
22.5
28.5
116.9
0
1.5
4.7
11.6
23.1
360
295
270
290
325
390
365
300
355
390
Moreover, the V2O5/TiO2 nanobers synthesized in our work are displayed in Table 2. For pure TiO2 nanobers and pure
display a much higher specic surface area of 26.3 m2 gꢀ1 at V2O5 powders, the Oads content accounts for a small part of
4.7% vanadium loading compared with the V2O5/TiO2 catalysts 13.60% and 19.13%, respectively. Aer introducing V2O5 onto
prepared by impregnation (6.8 m2 gꢀ1 at 5.0% vanadium the nanober surface, the Oads content rst rises to 28.35% and
loading), according to the literature.31,32 Furthermore, when the then gradually decreases to 18.95% with the increasing concen-
mass percentage is increased from 2 wt% to 25 wt%, the tration of vanadium precursor in the hydrothermal solution.
specic surface area decreases drastically from 27.3 m2 gꢀ1 to
The V 2p binding energy of the different catalysts is shown in
2.6 m2 gꢀ1, and the average pore diameter increases from Fig. 5B. As illustrated in the gure, two kinds of vanadium oxide
17.6 nm to 116.9 nm. This result may be attributed to the fact exist on the catalyst surface. The principal peaks at 516.6–
that the V2O5 nanoparticles generated from hydrothermal 517.0 eV and 515.5–515.7 eV can be identied as V5+ and V4+,
growth covered and occupied the pores of the primary TiO2 respectively.16 The V4+ contents (V4+/(V4+ + V5+)) on the surface of
nanobers, thereby leading to the sharp decrease in specic different catalysts are listed in Table 2, which shows that the V4+
surface area. Therefore, the catalytic oxidation performance content gradually decreases from 12.30% to 11.10% with the
could be a result of the mutual inuence of both the vanadium increase in the concentration of vanadium precursor. According
content and the specic surface area.
to Sipos and coworkers,35 vanadium pentoxide is always oxygen
The bulk composition of the V2O5/TiO2 nanober catalysts decient, and the oxygen vacancies are compensated by the
was determined by EDS analysis (Table 1). The results show that excess electrons on the vanadium sites, thereby reducing the
the vanadium content of the obtained V2O5/TiO2 nanobers is valence state from V5+ to V4+. Thus, the decreasing V4+ contents
enhanced as the concentration of the vanadium precursor may be attributed to the reduction of oxygen vacancies.
increases. However, the detected contents are not entirely
consistent with the expected ones. As the vanadium concen- played in Fig. 5C. For pure TiO2 nanobers, the principal peaks
tration rises, V2O5 in the catalyst basically increases according at 464.4 and 458.7 eV can be identied as Ti4+ 2p1/2 and Ti4+ 2p3/2
The Ti 2p binding energy of the obtained catalysts is dis-
,
to the growth of mass percentage, other than V2Ti. Therefore, in respectively. Aer introducing V2O5, both peaks move slightly
the hydrothermal process of V2Ti, the vanadium ions in the toward low binding energies within 0.5 eV, suggesting that the
solution were very sparse, such that they cannot form enough valence remains Ti4+ because the binding energy of Ti3+ is 1.8 eV
vanadium species on the surface of the nanobers within 24 h. lower.36 This result may be ascribed to the change in the chemical
environment around Ti4+.
The relative contents of V on the surface, as measured
by XPS, are also shown in Table 2. The results suggest that the
V/(V + Ti) ratios of the nanober catalysts are relatively higher
than that detected by EDS, demonstrating that V2O5 nano-
particles mainly formed on the surface of the TiO2 nanobers.
This nding is consistent with the conclusions in the SEM and
TEM analyses.
3.5 XPS characterization
To obtain detailed information about the surface composition
and chemical state of the V2O5/TiO2 nanober catalysts, the XPS
spectra of the samples with different vanadium content were
obtained and are shown in Fig. 5. The measured elements in
these samples are oxygen (O 1s), vanadium (V 2p), titanium
(Ti 2p), and carbon (C 1s) in the binding energy from 0 eV to
1000 eV. Oxygen, vanadium, and titanium are expected from the
chemical composition of the V2O5/TiO2 nanobers. The C 1s
3.6 Catalytic performance
peak was used for calibration. Table 2 displays more informa- The catalytic activities of the obtained V2O5/TiO2 nanobers for
tion about the different elements on the surface of the samples, the total oxidation of acetone under the conditions of acetone/
which include pure TiO2 and pure V2O5.
oxygen molar ratio ¼ 1/100 and GHSV ¼ 360 000 ml gꢀ1 hꢀ1
The O 1s curves of the various samples, which can be were measured at temperatures ranging from 200 ꢁC to 400 ꢁC.
resolved into two peaks by deconvolution, are shown in Fig. 5A. The conversion patterns are displayed in Fig. 6, and the light-off
As reported, the peaks at 529.6–529.9 eV can be assigned to temperature (T50, acetone conversion ¼ 50%) and the temper-
lattice oxygen (denoted as Olat), and the shoulder peaks at ature near complete oxidation (T90, acetone conversion ¼ 90%)
531.2–531.5 eV can be ascribed to the active surface-adsorbed are shown in Table 1.
oxygen (denoted as Oads).33,34 The active Oads contents (Oads
(Olat + Oads)) on the surface of the V2O5/TiO2 nanober catalysts display the lowest activity among the catalysts. However, aer
/
In the whole temperature range, pure TiO2 nanobers
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Fig. 5 (A) O 1s, (B) V 2p, (C) Ti 2p XPS spectra of hierarchical V2O5/TiO2 nanofibers: (a) pure TiO2, (b) V2Ti, (c) V5Ti, (d) V10Ti, (e)V25Ti, (f) pure V2O5.
Table 2 XPS results of hierarchical V2O5/TiO2 nanofibers
Sample
V (at%)
Ti (at%)
O (at%)
V/(V + Ti) (wt%)
O
ads/(Olat + Oads) (%)
V4+/(V4+ + V5+) (%)
Pure TiO2
V2Ti
V5Ti
V10Ti
V25Ti
0.00
1.13
1.71
2.35
3.01
16.91
22.65
19.68
16.43
14.36
15.47
0.00
47.58
48.52
43.46
43.55
44.61
60.07
0.00
5.76
9.97
14.83
17.15
100.00
13.60
28.35
26.87
26.20
25.20
19.13
—
12.30
12.04
11.58
11.10
5.72
Pure V2O5
ꢁ
introducing V2O5, the V2Ti sample shows a remarkable increase reduced by about 90 C compared with pure TiO2 nanobers,
in acetone oxidation performance. Moreover, when the mass which means that the catalyst possesses better oxidation
percentage further increases to 5 wt%, T50 and T90 are both performance. However, as the vanadium content further
increases, acetone conversion goes down obviously. For
instance, compared with the V5Ti sample, the T50 and T90 of
V25Ti increase by about 55 and 90 ꢁC, respectively. For the pure
V2O5 sample prepared via direct calcination method, the T50
ꢁ
and T90 are 315 and 385 C, respectively, under the same reac-
tion conditions as those of the V2O5/TiO2 nanober catalysts.
Apparently, the pure V2O5 sample is inferior to the V2O5/TiO2
nanobers in terms of catalytic performance, especially at a
temperature below 300 ꢁC. Based on the above results, the V2O5/
TiO2 nanober catalysts obviously perform well for the
complete oxidation of acetone. Furthermore, the products
detected are essentially CO2 and H2O, as conrmed by the
estimated carbon balance of over 99%.
In summary, V5Ti presents the best performance on acetone
oxidation, and 100% conversion is achieved at approximately
320 ꢁC. The catalytic activity over acetone conversion of the
obtained catalyst follows the sequence V5Ti > V10Ti > V2Ti >
Fig. 6 Conversion of acetone as a function of reaction temperature.
Reaction conditions: 300 ml minꢀ1 feed flow, 500 ppm acetone, 5%
O2, N2 balance, 50 mg catalyst, corresponding to a GHSV of 360 000
V25Ti z pure V2O5 > pure TiO2.
ml gꢀ1 hꢀ1
.
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are annotated on the image. First, the primary TiO2 nanobers
provided a negative zeta potential on the surface,20 which
attracted the trivalent vanadium cations (V3+) in the solution to
attach to the surfaces under the drive of electrostatic interac-
tions. Aerwards, the reaction of V3+ and urea took place on the
surface of the primary TiO2 nanobers and formed scattered
nuclei.37,38 As the hydrothermal process continuously pro-
ceeded, the nuclei grew large and dense and nally covered the
nanober surface, acting as the seed layer according to a similar
synthetic method reported by Kayaci et al.39 The closely and
uniformly packed nuclei determined the subsequent growth
kinetics, which has been well studied.20,40 Aerwards, the
oriented attachment growth mechanism caused the subsequent
growth of reticular nanowalls,41,42 which spread on the surface
of the seed-coated TiO2 nanobers. Upon calcination for 4 h in
air, the nanowalls decomposed gradually and nally trans-
formed into orange V2O5 nanobulks.
3.7 Formation mechanism
To have more insight into the actual evolution process of the
hierarchical V2O5/TiO2 nanobers, a series of time-dependent
experiments was performed. To verify the ultimate shape of
the secondary V2O5 grown on the surface of the TiO2 nanobers,
the concentration of the vanadium precursor in the hydro-
thermal solution was further increased to 0.06 mol lꢀ1, that is,
the mole ratio of vanadium precursor to TiO2 nanobers was
raised to 3.6 : 1. Fig. 7 displays the representative SEM images
of the different samples collected stepwise aer 3, 6, 12, 18, and
24 h of hydrothermal reaction. As shown in Fig. 7a, the small
nanoparticles nucleated on the surface of the nanobers dis-
played a misty outline during the rst 3 h. When reaction was
extended to 6 h, the nanoparticles could be clearly observed, as
shown in Fig. 7b. As the reaction time further increased to 12 h
(Fig. 7c), the nanoparticles were distributed more densely, and
their sizes increased. Moreover, 18 h of hydrothermal growth
resulted in the overlap of nanoparticles with one another,
forming a secondary layer that completely wrapped the primary
TiO2 nanobers (Fig. 7d). In the last 6 h of hydrothermal
process (Fig. 7e), two-dimensional reticular nanowalls instead
of nanoparticles were grown perpendicularly on the secondary
layer. Furthermore, aer calcination at 400 ꢁC for 3 h, the
morphologies of the as-obtained secondary nanostructures
signicantly transformed from nanowalls into V2O5 nanobulks,
as displayed in Fig. 7f.
4 Discussion
As stated above, the morphology characterization results reveal
the successful hydrothermal growth of secondary V2O5 nano-
particles onto the surface of primary TiO2 nanobers, forming
the pre-designed V2O5/TiO2 hierarchical nanobers, which
interweave with one another into a compact micro network. The
XRD results and Raman analysis further demonstrate that the
as-synthesized nanobers consist of TiO2 and V2O5, and the
vanadium content increases as the concentration of the
precursor rises. As reported in the literature,18 the hierarchical
nanobrous morphology is advantageous for catalytic reaction
because the special nanostructure can facilitate mass transfer
for ease of access of the reactant and allow more absorption
inside the structure. Qin et al.17 also proposed that a nano-
brous morphology could prevent the inhibition of the mass
transfer process, that is, the diffusion and adsorption–desorp-
tion of reactants/products. The obtained hierarchical nano-
bers are conducive to promote the diffusion of gas molecules
on the surface, which is signicantly important in heteroge-
neous catalytic reactions, accelerating the adsorption of reac-
tants as well as the desorption of products. In conclusion, the
material generated from a synthesis strategy that combines the
electrospinning technique and hydrothermal growth exhibits a
special hierarchical nanobrous morphology, which is one of
the pivotal factors that can enhance the mass transfer process,
as veried by the catalytic performance results that the V2O5/
TiO2 hierarchical nanobers behaved more actively than V2O5/
TiO2 nanoparticles prepared by traditional method (Fig. S1†).
Numerous literatures have demonstrated that introducing
vanadium oxide into titanium-supported catalysts could endow
the composite catalyst with superior catalytic oxidation perfor-
mance toward VOCs.10,12,16 As is reported, vanadium pentoxide
is always oxygen decient, and the resulting oxygen vacancies
are compensated by the excess electrons located on vanadium
sites, leading the reduction of the valence state from V5+ to V4+.35
Moreover, the enhanced catalytic oxidation activity benets
from the abundant presence of oxygen vacancies, which give
rise to numerous surface-adsorbed oxygen (Oads).43–45 In the
Based on the results of the time-dependent experiments, the
possible formation mechanism of the hierarchical V2O5/TiO2
nanobers is proposed and illustrated in Fig. 8. Various steps
Fig. 7 (a–e) SEM images of hierarchical nanofibers before calcination
with hydrothermal time of (a) 3 h, (b) 6 h, (c) 12 h, (d) 18 h and (e) 24 h. (f)
SEM image of calcinated V2O5/TiO2 nanofibers with hydrothermal
time of 24 h.
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Fig. 8 Schematic of the formation process of hierarchical V2O5/TiO2 nanofibers.
synthesis process of V2O5/TiO2 nanober catalysts, the vana-
To sum up, the specic hierarchical nanobrous
dium pentoxide crystal was structure defected. To retain elec- morphology, the abundant oxygen vacancies, and the appro-
troneutrality, considerable oxygen vacancies appeared.46 priate vanadium concentration collectively determine the
Aerwards, the formation of oxygen vacancies, on one hand, admirable oxidation performance of the obtained V2O5/TiO2
compelled the valence of vanadium to decrease gradually from nanober catalysts, and V5Ti sample displays the optimal
pentavalent to tetravalent.46 On the other hand, the gas-phase catalytic activity in acetone oxidation.
oxygen and water molecule in the atmosphere adsorbed on the
surface of the catalyst under the effect of oxygen vacancies,
inducing substantial surface-adsorbed oxygen, which is recog-
5 Conclusions
nized to be the most active oxygen species that takes main
A controllable method of fabricating a hierarchical V2O5/TiO2
nanober catalyst that combined electrospinning and hydro-
responsibility for the catalytic oxidation performance.47,48
Accordingly, the V4+ contents (V4+/Vtot) and Oads contents (Oads
/
thermal growth was proposed. The special hierarchical nano-
brous morphology was found to enhance the mass transfer
process. The XPS analysis indicates that sufficient oxygen
vacancies is another advantage of the as-obtained catalysts
because it could induce substantial surface-adsorbed oxygen.
An appropriate vanadium content of V2O5/TiO2 nanobers was
also proven to benet the catalytic performance. Among the
synthetic samples, V5Ti displayed the highest activity of acetone
conversion (T50 ¼ 270 ꢁC, T90 ¼ 300 ꢁC). Accordingly, this result
can be attributed to the combined effect of the specic hierar-
chical nanobrous morphology, the abundant oxygen vacan-
cies, and the appropriate vanadium content. Through the
proposed synthesis strategy, a noteworthy catalyst with excel-
lent acetone oxidation performance was successfully prepared,
which is promising for applications in broad elds that involve
the synthesis of novel metal oxide catalysts and paint coat of
ltration devices.
(Olat + Oads)) conformably illustrated the extraordinary signi-
cance of oxygen vacancies. As demonstrated in Table 2, aer
introducing V2O5 onto the surface of TiO2 nanobers, the content
of oxygen vacancies revealed a signicant increase, thereby
exhibited better activity for acetone oxidation compared to pure
TiO2 nanobers, as proven by the catalytic performance results.
The analysis was further supplemented by porous texture
and EDS characterization. For the V5Ti sample that displayed
the best acetone oxidation performance, the specic surface
area, which has an important function in the catalytic activity of
supported metal oxide catalysts,49,50 was slightly (1.0 m2 gꢀ1
)
declined compared with the highest value for V2Ti (27.3 m2 gꢀ1).
However, the vanadium content was found to be more than
tripled (EDS results). Therefore, a hydrothermal process with
low vanadium concentration does not have a signicant effect
on the specic surface area. Instead, appropriately increasing
the concentration of the vanadium precursor could enhance the
oxidation performance of the catalysts because vanadium oxide
performs a vital role in VOC oxidation.14,51,52 By contrast, further
increase in vanadium concentration would lead to a rapid
decline in catalytic activity, which is ascribed to the superuous
V2O5 nanoparticles that covered and occupied the pores of the
TiO2 nanobers, leading to severe decrease (the minimum was
2.6 m2 gꢀ1 in V25Ti sample) in specic surface area. For the
optimal V5Ti sample, the appropriate vanadium concentration
was conducive for acetone oxidation, and the content of oxygen
vacancies was maintained at a high level. In conclusion, the
sample that maintained the suitable concentration of vanadium
oxides and sufficient oxygen vacancies exhibited the best
activity for acetone oxidation, as proven by the catalytic
performance results.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grants no. 51406179 and 51125025) and
the Key Innovation Team for Science and Technology of Zhe-
jiang Province (Grant no. 2011R50017).
References
1 A. Barry and D. Corneau, For. Prod. J., 2006, 56, 38–42.
2 J. R. Woertz, K. A. Kinney, N. D. P. McIntosh and
P. J. Szaniszlo, Removal of toluene in a fungal vapor-phase
bioreactor, 1999.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 30416–30424 | 30423

View Article Online
RSC Advances
Paper
3 S. F. Zuo, Q. Q. Huang, J. Li and R. X. Zhou, Appl. Catal., B, 28 J. Y. Shen, H. Wang, Y. Zhou, N. Q. Ye, G. B. Li and
2009, 91, 204–209. L. J. Wang, RSC Adv., 2012, 2, 9173–9178.
4 C. Warneke, J. A. de Gouw, J. S. Holloway, J. Peischl, 29 I. E. Wachs, Dalton Trans., 2013, 42, 11762–11769.
T. B. Ryerson, E. Atlas, D. Blake, M. Trainer and 30 Y. Yuan, Y. C. Zhao, H. L. Li, Y. Li, X. Gao, C. G. Zheng and
D. D. Parrish, J. Geophys. Res.: Atmos., 2012, 117, D00V17.
5 S. C. Kim, S. W. Nahm, W. G. Shim, J. W. Lee and H. Moon, J. 31 M. P. Gimeno, J. Gascon, C. Tellez, J. Herguido and
Hazard. Mater., 2007, 141, 305–314. M. Menendez, Chem. Eng. Process., 2008, 47, 1844–1852.
6 B. Solsona, M. Perez-Cabero, I. Vazquez, A. Dejoz, T. Garcia, 32 C. R. Dias, M. F. Portela and G. C. Bond, J. Catal., 1995, 157,
J. Y. Zhang, J. Hazard. Mater., 2012, 227, 427–435.
J. Alvarez-Rodriguez, J. El-Haskouri, D. Beltran and
P. Amoros, Chem. Eng. J., 2012, 187, 391–400.
7 W. B. Li, J. X. Wang and H. Gong, Catal. Today, 2009, 148, 81–
87.
8 G. Picasso, A. Quintilla, M. P. Pina and J. Herguido, Appl.
Catal., B, 2003, 46, 133–143.
9 S. J. Jiang and S. Q. Song, Appl. Catal., B, 2013, 140, 1–8.
10 A. Nie, H. Yang, Q. Li, X. Fan, F. Qiu and X. Zhang, Ind. Eng.
Chem. Res., 2011, 50, 9944–9948.
11 C. Gannoun, R. Delaigle, D. P. Debecker, P. Eloy, A. Ghorbel
and E. M. Gaigneaux, Appl. Catal., A, 2012, 447, 1–6.
344–352.
33 X. S. Du, X. Gao, R. Y. Qu, P. D. Ji, Z. Y. Luo and K. F. Cen,
ChemCatChem, 2012, 4, 2075–2081.
34 X. S. Du, X. Gao, L. W. Cui, Y. C. Fu, Z. Y. Luo and K. F. Cen,
Fuel, 2012, 92, 49–55.
35 B. Sipos, M. Duchamp, A. Magrez, L. Forro, N. Barisic, A. Kis,
J. W. Seo, F. Bieri, F. Krumeich, R. Nesper and G. R. Patzke, J.
Appl. Phys., 2009, 105, 074317.
36 J. Fang, X. Z. Bi, D. J. Si, Z. Q. Jiang and W. X. Huang, Appl.
Surf. Sci., 2007, 253, 8952–8961.
37 J. Livage, Materials, 2010, 3, 4175–4195.
12 R. Delaigle, P. Eloy and E. M. Gaigneaux, Catal. Today, 2012, 38 C. Weeks, Y. N. Song, M. Suzuki, N. A. Chernova, P. Y. Zavalij
192, 2–9.
and M. S. Whittingham, J. Mater. Chem., 2003, 13, 1420–
1423.
39 F. Kayaci, S. Vempati, C. Ozgit-Akgun, N. Biyikli and T. Uyar,
Appl. Catal., B, 2014, 156, 173–183.
13 N. Y. Topsoe, H. Topsoe and J. A. Dumesic, J. Catal., 1995,
151, 226–240.
14 S. Krishnamoorthy, J. P. Baker and M. D. Amiridis, Catal.
Today, 1998, 40, 39–46.
40 C. R. Bullen and P. Mulvaney, Nano Lett., 2004, 4, 2303–2307.
15 S. T. Martin, C. L. Morrison and M. R. Hoffmann, J. Phys. 41 M. Li, F. Y. Kong, H. Q. Wang and G. H. Li, CrystEngComm,
Chem., 1994, 98, 13695–13704. 2011, 13, 5317–5320.
16 C. Gannoun, A. Turki, H. Kochkar, R. Delaigle, P. Eloy, 42 L. Ren, L. Jin, J. B. Wang, F. Yang, M. Q. Qiu and Y. Yu,
A. Ghorbel and E. M. Gaigneaux, Appl. Catal., B, 2014, 147,
58–64.
17 R. Qin, J. Chen, X. Gao, X. Zhu, X. Yu and K. Cen, RSC Adv.,
2014, 4, 43874–43881.
18 H. Bai, J. Juay, Z. Liu, X. Song, S. S. Lee and D. D. Sun, Appl.
Catal., B, 2012, 125, 367–374.
Nanotechnology, 2009, 20, 115603.
43 Z. Y. Tian, P. H. T. Ngamou, V. Vannier, K. Kohse-Hoinghaus
and N. Bahlawane, Appl. Catal., B, 2012, 117, 125–134.
44 T. Garcia, S. Agouram, J. F. Sanchez-Royo, R. Murillo,
A. M. Mastral, A. Aranda, I. Vazquez, A. Dejoz and
B. Solsona, Appl. Catal., A, 2010, 386, 16–27.
19 J. B. Mu, C. L. Shao, Z. C. Guo, Z. Y. Zhang, M. Y. Zhang, 45 B. de Rivas, R. Lopez-Fonseca, C. Sampedro and
P. Zhang, B. Chen and Y. C. Liu, ACS Appl. Mater.
Interfaces, 2011, 3, 590–596.
20 H. Wang, D. Ma, X. Huang, Y. Huang and X. Zhang, Sci. Rep.,
2012, 2, 701.
21 M. Shang, W. Wang, W. Yin, J. Ren, S. Sun and L. Zhang,
Chem.–Eur. J., 2010, 16, 11412–11419.
22 Z. Lou, F. Li, J. Deng, L. Wang and T. Zhang, ACS Appl. Mater.
Interfaces, 2013, 5, 12310–12316.
J. I. Gutierez-Ortiz, Appl. Catal., B, 2009, 90, 545–555.
46 Y. Wu, M. J. Forbess, S. Seraji, S. J. Limmer, T. P. Chou and
G. Z. Cao, J. Appl. Phys., 2001, 89, 5647–5652.
47 R. Spinicci, M. Faticanti, P. Marini, S. De Rossi and P. Porta,
J. Mol. Catal. A: Chem., 2003, 197, 147–155.
48 E. M. Cordi and J. L. Falconer, Appl. Catal., A, 1997, 151, 179–
191.
49 S. J. Xiong, Y. X. Tang, H. S. Ng, X. X. Zhao, Z. L. Jiang,
Z. Chen, K. W. Ng and S. C. J. Loo, Toxicology, 2013, 304,
132–140.
23 J. Li, F. Gao, L. Q. Liu and Z. Zhang, eXPRESS Polym. Lett.,
2013, 7, 683–689.
24 R. S. Barhate and S. Ramakrishna, J. Membr. Sci., 2007, 296, 50 G. F. He, Y. B. Cai, Y. Zhao, X. X. Wang, C. L. Lai, M. Xi,
1–8.
Z. T. Zhu and H. Fong, J. Colloid Interface Sci., 2013, 398,
103–111.
51 Y. Yang, G. Yu, S. B. Deng, S. W. Wang, Z. Z. Xu, J. Huang and
B. Wang, Chem. Eng. J., 2012, 192, 284–291.
52 D. P. Debecker, R. Delaigle, P. Eloy and E. M. Gaigneaux, J.
Mol. Catal. A: Chem., 2008, 289, 38–43.
25 X. H. Qin and S. Y. Wang, J. Appl. Polym. Sci., 2006, 102,
1285–1290.
26 N. Wetchakun, B. Incessungvorn, K. Wetchakun and
S. Phanichphant, Mater. Lett., 2012, 82, 195–198.
27 Y. Chen and L. F. Zhang, Catal. Lett., 1992, 12, 51–62.
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