Synthesis of surfactant-assisted FeVO4 nanostructure: Characterization and photocatalytic degradation of phenol

Article abstract of DOI:10.1016/j.molcata.2014.11.013

Pure triclinic FeVO₄ catalysts were synthesized by hydrothermal method and modified with Hexadecyltrimethylammonium Bromide (HTAB), Sodium Dodecyl Sulfate, Polyethylene Glycol (PEG) as surfactant. The materials were characterized by XRD, BET, FT-IR, DRS and SEM techniques. These catalysts were used for the photocatalytic degradation of phenol (Ph) under natural solar light irradiation. The effect of preparation parameters on the photocatalytic activity was also examined. It is found that the surfactant and pH value a significant influence on the particle morphology and even the crystalline structure of the product. Surfactant added products resulted an improve efficiency of the triclinic FeVO₄. With HTAB as surfactant at an hydrothermal temperature of 75 °C and the pH 3 value of the precursor solution the highest percentage of phenol degradation (100%) and highest reaction rate (1.24 mg l^-1 min^-1) were obtained in 45 min. It is concluded that the excellent photocatlytic activity of the HTAB-assisted FeVO₄ catalyst, with rod-like shaped morphology, was associated its small particle size, large surface area, the presence of more surface OH groups and lower band gap energy than pure FeVO₄.

Full text of DOI:10.1016/j.molcata.2014.11.013

Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis of surfactant-assisted FeVO4 nanostructure:
Characterization and photocatalytic degradation of phenol
Beysim Ozturk, Gulin Selda Pozan SOYLU∗
Istanbul University, Faculty of Engineering, Chemical Engineering Department, Avcilar, 34320 Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Pure triclinic FeVO4 catalysts were synthesized by hydrothermal method and modified with Hex-
adecyltrimethylammonium Bromide (HTAB), Sodium Dodecyl Sulfate, Polyethylene Glycol (PEG) as
surfactant. The materials were characterized by XRD, BET, FT-IR, DRS and SEM techniques. These cat-
alysts were used for the photocatalytic degradation of phenol (Ph) under natural solar light irradiation.
The effect of preparation parameters on the photocatalytic activity was also examined. It is found that the
surfactant and pH value a significant influence on the particle morphology and even the crystalline struc-
ture of the product. Surfactant added products resulted an improve efficiency of the triclinic FeVO4. With
HTAB as surfactant at an hydrothermal temperature of 75 ◦C and the pH 3 value of the precursor solution
the highest percentage of phenol degradation (100%) and highest reaction rate (1.24 mg l−1 min−1) were
obtained in 45 min. It is concluded that the excellent photocatlytic activity of the HTAB-assisted FeVO4
catalyst, with rod-like shaped morphology, was associated its small particle size, large surface area, the
presence of more surface OH groups and lower band gap energy than pure FeVO4.
Received 6 December 2013
Received in revised form 9 November 2014
Accepted 13 November 2014
Available online 25 November 2014
Keywords:
FeVO4
Phenol
Photocatalysis
Solar light irradiation
Characterization
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
proved to be a very stable and highly selective catalyst for partial
oxidation of primary alcohols and hydrocarbons [8].
Environmental applications of heterogeneous photocatalysis
have been intensively studied in the past decades. Heterogeneous
photocatalysis [1,2] has shown a high efficiency in the photooxi-
dation of many organic pollutants. Many efforts have been exerted
to the research of nanostructured titanium dioxide due to its non-
toxicity and chemical stability [3,4]. In order to make use of solar
light source in photodegradation reaction, a visible light active pho-
tocatalyst is desired.
Transition metal-based orthovanadates (AVO4) are the cohe-
sive class of materials, which have potential applications in various
fields [5]. Due to its various stable oxidation states (+2 to +5), vana-
dium combines with many elements leading to the preparation of
numerous new compounds [6].
In addition, FeVO4 catalysts were demonstrated to be useful
to treat various organic pollutants in water over a wider appli-
cable pH range. Until now, it has been not already reported on
removal of phenol and phenolic compounds from the industrial
wastes. However, there have been a few studies on the photodegra-
dation of dyes with FeVO4 powder [9–12]. For example, when using
goethite as heterogeneous Fenton catalyst in the degradation of 1,2-
benzenediol, 2-aminophenol and 2,3-dihydroxybenzoic acid, only
about 10% TOC removal were achieved after 2 h [13]. Deng et al.
prepared a heterogeneous Fenton-like catalyst by wet chemical
process and tested for the degradation of Orange II. The experi-
mental results indicated that the degradation efficiency of FeVO4
reached 92.9% after 60 min [14].
FeVO4 has been investigated for many years for its wide range
potential applications. FeVO4 is already known as a gas sensor
material for detecting H2S traces in air environment [7]. Particularly
FeVO4 is outlined with its large spectrum of valuable properties. It
In this paper, FeVO4 photocatalysts were synthesized via a
surfactant-assisted hydrothermal method and phenol was used as
a model pollutant to evaluate the photocatalytic activity of the
catalysts under natural solar light irradiation. The structure and
morphology of the prepared FeVO4 were characterized by various
analytical techniques. In addition, the effects of the parameters,
such as pH value of the precursor solution and different surfactant,
on crystal structure, morphology and spectroscopic properties of
FeVO4 catalysts were discussed in details. As the efficient solar-
light driven photocatalyst, the factors affecting the photocatalytic
∗
Corresponding author. Tel.: +90 212 473 70 70x17789;
mobile: +90 533 780 64 21.
E-mail address: gpozan@istanbul.edu.tr (G.S. Pozan SOYLU).
http://dx.doi.org/10.1016/j.molcata.2014.11.013
1381-1169/© 2014 Elsevier B.V. All rights reserved.

66
B. Ozturk, G.S. Pozan SOYLU / Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
activities for phenol degradation over different FeVO4 catalysts are
also explored.
Fourier transform infrared spectra were recorded on a Perkin
Elmer Precisely Spectrum One spectrometer at ambient conditions
by using KBr as diluent. All measurements were at 4 cm−1 resolu-
tion and 100 scans.
2. Experimental
2.4. Evaluation of photocatalytic activity
2.1. Materials
Photocatalytic activities of the FeVO4 powders were evaluated
by degradation of phenol in a quartz batch-photoreactor of cylin-
drical shape under natural sunlight irradiation. The reaction was
placed in an open place on a sunny day under natural sunlight
in June–August. In order to compare the light sources, photocat-
alytic activity of catalyst, which has the best photocatalytic activity,
was measured under UV and visible light irradiation. In a typi-
cal experiment, 100 mg catalyst was dispersed in 50 mL phenol
solution of initial concentration 25 mg L−1 and neutral pH (pH = 5)
under magnetic stirring. Before illumination, the mixed solution
was ultrasonicated for 5 min and magnetically stirred for 1 h in
the dark to ensure the establishment of the adsorption–desorption
equilibrium between the catalyst and the solution. 0.15 mL of H2O2
solution (30 wt%) were added to 50 mL of the phenol-containing
aqueous solution. After irradiation, the phenol solution was filtered
through a membrane filter (pore size 0.45 mm) and the filtrate was
used for TOC measurement with a TOC-V, Shimadzu equipment.
The concentration of phenol and products were analyzed by HPLC
equipped with C-18 column. The mobile phase used in HPLC was a
mixed solvent of methanol and water (60/40, v/v) with a flow rate
of 1 mL/min.
Ammonium metavanadate (≥99%) was purchased from Alfa
Aesar Company, phenol, hydroquinone, catechol were pur-
chased from Fluka Company. Ammonia solution was purchased
from Lachema Company. Iron (III) nitrate nona-hydrate (≥99%),
65%(w/w) nitric acid, ethanol (absolute), methanol (for HPLC, ≥99%)
were purchased from the Merck Company and used without fur-
ther purification. Degussa P25 TiO2 (consisting of 75% anatase and
25% rutile with a specific BET-surface area of 50 m2/g and primary
particle size of 20 nm) was used as photocatalyst. Deionized (D.I.)
water was used for the preparation of all the catalysts as well as to
dilute the phenol solution.
2.2. Catalyst preparation
FeVO4 catalysts were prepared via hydrothermal method using
ammonium metavanadate, iron (III) nitrate nona-hydrate and
deionized water as the starting materials.
0.77 g NH4VO3 (0.26 M) was dissolved into deionized water
to form
a transparent solution (S1). Following this, 2.66 g
Fe(NO3)3·9H2O (0.0427 M) was dissolved into deionized water to
forma transparentsolution(S2). Then pouredS2 into S1 undermag-
netic stirring. This mixture was precipitated by gradually adding
NH3 solution (25 wt%) or 65% (w/w) nitric acid used for pH adjust-
ment. After pH adjustment the mixture was sealed in 100 mL
Teflon-lined stainless steel autoclave (Parr Model 4843) allowed to
heat at 75 ◦C for 8 h under autogenous pressure. At the end of the
hydrothermal process, last products were cooled down to 30–40 ◦C.
The resulting precipitate was separated by filtration and sequen-
tially washed with ultrapure water, ethanol (absolute) and acetone
3. Results and discussion
3.1. Catalyst characterization
3.1.1. BET surface area
BET surface areas of catalysts were measured, and listed in
Table 1. For the pH series, surface area remarkably increased with
a increase in the pH 3–7. However, surface area dropped with
increase in the pH 7–11. These results demonstrate that the pH
value of the precursor solution exerted an important impact on the
surface area of the product. It can be inferred that surface area was
clearly affected by the change in pH.
Among the FeVO4 samples, FeVO4(7) catalyst showed the high-
est surface area with 30 m2/g. However, photocatalytic activity is
not only assigned to BET areas. Although surface area of FeVO4(7)
catalyst was comparable and even higher than FeVO4(3), it did not
show higher activity as shown in Table 1. This point was further
clarified by a series of detailed experiments.
to remove NO3 anions. Finally, the precipitate was dried at 50 ◦C
−
for 15 h and calcinated 500 ◦C for 4 h. The resultant FeVO4 samples
were ground at a constant vibration rate of 300 rpm for 10 min in a
Retsch MM 200 vibrant-ball mill by milling ball in milling container.
For the synthesis of surfactant-assisted FeVO4, 1 g of hexadecyl
trimethyl ammoniumbromide (HTAB) or Sodium Dodecyl Sulfate
(SDS) or Polyethylene Glycol (PEG) was diluted in the homogeneous
mixture before transferring into the autoclave. In this paper, the
samples will be denoted as FeVO4(P), FeVO4-S here P shows pH
and S shows added surfactant.
2.3. Catalyst characterization
With HTAB addition, surface area of FeVO4(3) increased from
15 m2/g to 35 m2/g. All of the used surfactants have increased
BET surface area of pure FeVO4(3) catalyst. The maximum sur-
face area was achieved for SDS addition to FeVO4(3) and surface
area increased 15–41 m2/g. Obviously, the surfactant (HTAB, SDS
or PEG) played a crucial role in generating high-surface-area FeVO4
materials.
Powder X-ray diffractions of samples were obtained using a
Rigaku D/Max-2200/PC diffractometer with the CuK␣ (ꢀ = 1.540)
radiation. Samples were scanned from 10 to 80 at a rate of 2◦/min
(in 2ꢁ). The sizes of the crystalline domains were calculated by
using the Scherrer equation, t = Cꢀ/B cosꢁ, where ꢀ is the X-ray
wavelength (A◦), B is the full width at half maximum, ꢁ is Bragg
angle, C is a factor depending on crystallite shape (taken to be one),
and t is the crystallite size (A◦).
The BET surface areas of the samples were determined by nitro-
gen adsorption–desorption isotherm measurement at 77 K. The
samples were degassed at 200 ◦C prior to the actual measurements.
The morphology and size distribution of the photocatalysts were
recorded by scanning electron microscopy (FESEM-QUANTA 450
FEG).
3.1.2. X-ray diffraction analysis
All the peaks were identified; indexed using the data available
from the Joint Committee for Powder Diffraction studies (JCPDS)
and the corresponding planes were indicated in Fig. 1. Fig. 1 shows
the XRD patterns of samples prepared using different pH value and
surfactant. It is found that the pattern of powder prepared at pH
3 shows the sharp and strong characteristic peaks associated with
the pure triclinic FeVO4 (JCPDS 38-1372). And also, the well-defined
peaks were observed without any impure phase. XRD pattern of
FeVO4(3) indicates high crystallinity than FeVO4(7) and FeVO4(11).
As seen in Fig. 1, FeVO4(3) shows clear characteristic peaks with 2ꢂ
Diffuse reflectance spectra were obtained using a Shimadzu
UV-3600. BaSO4 was used as the reflectance standard in the
experiments.

B. Ozturk, G.S. Pozan SOYLU / Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
67
Table 1
Physical and optical properties of FeVO4 materials and phenol degradation efficiencies over 45 min (%).
Catalyst
Crystallite size (nm)
Specific surface area (m2/g)
Band gap (eV)
Phasecomposition(%)
Phenol degradation efficiencies
over 45 min (%)
FeVO4
Fe3O4
FeVO4(3)
FeVO4(7)
FeVO4(11)
FeVO4(3)-HTAB
FeVO4(3)-SDS
FeVO4(3)-PEG
Degussa-TiO2 (P25)
18
11
7
13
8
15
30
17
35
41
32
50
2.55
2.05
1.77
2.41
2.47
2.51
3.12
100
82
51
100
100
100
–
–
18
49
–
–
–
55
50
22
100
80
60
16
32
–
27
at 16.5, 25.1, 27.2, 31.2, 34.7, 41.4 correspond to the lattice planes of
(1 1 0), (1 2 0), (0 1 2), (2 2 1), (2 2 1), (2 3 0), respectively, which infer
the formation of FeVO4 structure. On the other hand, the formation
of both FeVO4 (JCPDS 38-1372) and Fe3O4 (JCPDS 65-3107) were
3.1.3. UV diffuse reflectance spectroscopy
The UV–vis DRS of samples are shown in Fig. 2. From the DRS
spectra, the band gap values for all the catalysts were calculated
and given in Table 1.
observed for samples prepared at pH 7 and pH 11. Relative content
The band gap of catalysts decreases with an increase in the
pH value of the precursor solution. The lowest band gap energy
(1.77 eV) was obtained with the catalyst prepared at pH 11. This
result indicated that band gap decreases with increase in the
amount of Fe3O4 in the structure.
ꢀ
of each phase is estimated as the formula Ii/ Ii × 100%, where Ii
◦
is the strongest diffraction intensity of FeVO4 (I2ꢂ=27.0 ) and Fe3O4
◦
(I2ꢂ=35.8 ).
The average crystallite sizes (particle sizes) and phase compo-
sition of the catalysts are listed in Table 1. In Table 1, the crystallite
size decreased with increasing pH value of precursor solution. This
result clearly emphasized the role of Fe3O4 in decreasing the size
of nano FeVO4.
In Fig. 1, it can be seen that, the XRD patterns of all surfactant-
assisted FeVO4 nanoparticles present similar profiles and all the
diffraction peaks can be well indexed as triclinic FeVO4 (JCPDS
38-1372). Although all samples showed the same morphological
arrangements, crystallite size changed with the addition of surfac-
tant. As a result, the crystalline sizes of samples were evaluated
as 18, 16, 13, 8 nm for FeVO4(3), FeVO4(3)-PEG, FeVO4(3)-HTAB
and FeVO4(3)-SDS, respectively. This dissimilarity may lead to the
differences in their morphology and structure [15].
The calculated band gap of pure triclinic FeVO4 is 2.55 eV,
whereas the band gap is decreased to 2.41 eV by the addition of
1:1 (w/w) HTAB and strong red shift shown from DRS spectrum. In
addition, a small decrease in the band gap value was observed for
FeVO4(3)-SDS (2.47 eV) and FeVO4(3)-PEG (2.51 eV) catalysts. The
narrow band gap of the FeVO4(3)-HTAB sample indicates that this
sample has the lower conduction band level, which will increase the
absorption characteristic for solar light. There is a clear red shift in
the absorption band of all the materials synthesized with surfac-
tants and this could be occurred from the visible light absorption
in case of surfactant added samples must originate from taking of
nitrogen or carbon from surfactants (SDS, HTAB, PEG) into the mate-
rials during synthesis. Therefore, it is understood that the presence
Fig. 1. XRD patterns of FeVO4 powders prepared using different pH values and surfactants.

68
B. Ozturk, G.S. Pozan SOYLU / Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
FeVO (3)
4
(1)
(2)
(3)
(4)
(5)
(6)
(4)
FeVO (7)
4
FeVO (11)
4
FeVO (3)-SDS
4
FeVO (3)-PEG
4
(2)
(5)
FeVO (3)-HTAB
4
(6)
(1)
(3)
200
500
600
800
700
300
400
Wavelength (nm)
Fig. 2. The DRS spectra of FeVO4 catalysts prepared using different pH values and
surfactants.
of surfactants also favors the FeVO4(3) and hence the enhanced
visible light absorption.
3.1.4. Scanning electron microscopy (SEM)
SEM images indicate that the samples were composed mainly
crystalline phase. In Fig. 3a, SEM image of sample prepared at pH 3
shows that the particles are spherical in shape and homogeneous
in size. However, the increase in pH value of the precursor solu-
tion changed the structure of the catalyst. In Fig. 3b, two different
phases are observed for FeVO4(7). It can be seen that the shape of
particles are spherical and Fe3O4 particles scattered between FeVO4
particles. In addition, Fig. 3c reveals that the FeVO4(11) sample pos-
sesses a mixture of morphologies (FeVO4–Fe3O4) including micro-
and macro-clusters. This SEM results clearly show that pH is the
major effect for morphology of the FeVO4.
As shown in Fig. 3d, the shape of the particles was transformed
from spherical in shape to rod-like shaped morphology while
using the HTAB during synthesis. The morphology of the catalyst
prepared by PEG and SDS are different from FeVO4(3)-HTAB cata-
lyst. Fig. 3e, FeVO4(3)-PEG, the comprised of agglomeration of rod
shaped particles can be easily observed. In the SDS-assisted FeVO4,
the formation of rod shaped particles was not observed (Fig. 3f). In
addition, the interaction of SDS-covered nanoparticles could aggre-
gate into a mushroom-like architecture. Due to the van der Waals
interaction of surfactant molecules adsorbed on the nanocrystal
surfaces and the tendency to minimize the interfacial energy, these
nanoparticles would gradually self-assemble into aggregates with
a specific morphology [16,17].
Compare to FeVO4(3) (60–130 nm), the particle size of
FeVO4(3)-HTAB (40–110 nm) became smaller. In addition, surface
area of the FeVO4 increases drastically from 15 to 32, 35, 41 m2/g
due to the presence of PEG, HTAB, SDS respectively, during the
synthesis. The surfactants are the structure directing agents that
could reduce particle size and thus they influence the particle mor-
phology [5]. Therefore, surfactants greatly affect the morphology of
FeVO4, as well as the crystal structure predicted by the XRD results.
Fig. 3. SEM images of (a) FeVO4(3); (d) FeVO4(3)-HTAB.
FeVO (3)-HTAB
4
FeVO (3)
4
1630
FeVO (7)
4
509
673
911
3424
FeVO (11)
4
3.1.5. FT-IR spectroscopy
Fig. 4 shows the FT-IR spectra of samples in the range of
450–4000 cm−1. These spectra are taken in order to identify the
functional groups that are present in the synthesized sample.
Generally, triclinic FeVO4 consists of two FeO6 octahedra, one
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
FeO5 trigonal bipyramidal, three VO4 tetrahedra where Fe
O
polyhedra form a doubly bent chain to form a three dimen-
sional frame work [18,19]. For FeVO4(3) marked, strong and broad
Fig. 4. FT-IR spectra of FeVO4 powders prepared using different pH values and HTAB.

B. Ozturk, G.S. Pozan SOYLU / Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
69
band at about 954 cm−1 and 911 cm−1 are terminal V O stretch-
1.0
0.8
0.6
0.4
0.2
0.0
(a)
ings; 840 cm−1 and 735 cm−1 are bridging V O· · ·Fe stretchings;
694 cm−1, 673 cm−1 and 569 cm−1 are mixed bridging V O· · ·Fe
and V· · ·O· · ·Fe stretchings; 509 cm−1 is V
O V deformations
mixed with Fe O stretching modes. These data fits according to
literature [20,21], classified the bands of the crystalline FeVO4
powders in four groups: terminal V O stretching (range I), bridg-
ing V O· · ·Fe stretching (range II), mixed bridging V O· · ·Fe and
1.0
0.8
0.6
0.4
0.2
0.0
pH:11
pH:7
pH:3
(b)
V· · ·O· · ·Fe stretching (range III) and V
O V deformations mixed
with Fe O stretching modes (range IV). However, the intensity
of these peaks became less when the pH value of the precursor
solution was raised to 7.0 and 11.0, respectively. In addition, FTIR
spectra of FeVO4(7) revealed a broad band at around 3424 cm−1 and
a small band at 1630 cm−1 were assigned to O H stretching and
Adsorpsion
Photolysis
FeVO (3) (without H O )
0
30
60
90
120 150
4
2 2
Time (min)
FeVO (3) (with H O )
4
2
2
0
30
60
90
120
150
180
O
H bending mode, respectively. However, FeVO4(11) sample did
Time (min)
not show any peaks at around 3424 and 1634 cm−1 corresponding
to the OH group in the FT-IR spectra.
Fig. 5. Comparison of photolysis, photocatalysis and adsorption of phenol for sam-
ple FeVO4(3). Inset shows phenol photodegradation for FeVO4 powders prepared at
different pH value.
The concentration of the hydroxyl groups were measured as the
integrated areas of corresponding peaks. For FeVO4(3)-HTAB cata-
lyst, the peaks identified at 3425 cm−1 and 1629 cm−1 correspond
to the stretching and bending modes of molecularly adsorbed water
[22]. When the HTAB loaded, the shift to the higher wavenumbers
were observed. Furthermore, the concentration of the hydroxyl
groups was decreased with increase of pH value of the precursor
solution. This result indicated that the higher catalytic activity of
FeVO4(3)-HTAB sample was attributed to a higher concentration of
the hydroxyl groups of FeVO4(3)-HTAB.
the charge separation, being also able to decompose to produce HO•
radicals by absorption of light [27]:
H2O2 + h␯ → 2•OH
(1)
(2)
(3)
(4)
(5)
•
H
+ + OH− → HO−
− + O2 → O2
−
•
e
e− + H2O2 → HO• + OH−
RH + OH• → H2O + R• → Furtheroxidation
3.2. Evaluation of photocatalytic activiy
The photocatalytic degradation performance for the catalysts
synthesized at the different pH value of precursor solution were
monitored for the oxidative degradation of phenol and the results
are shown in Fig. 5b and Table 1. Catalytic activity of FeVO4 was
slightly affected by the solution pH values in the range of 3–7.
Only when the solution pH was over 7, the catalytic activity of
FeVO4 was apparently decreased. Thus, no pH adjustment of the
medium was needed for effective oxidation over the wider pH
range.
The performance of modified FeVO4(3) catalysts with varying
HTAB, CTAB and PEG loadings was also shown in Fig. 6a. The
degradation ability for the FeVO4(3) catalyst increased with the
surfactant loadings to FeVO4(3). 54% degradation of phenol was
achieved with the pure triclinic FeVO4 catalyst under solar light
irradiation for 45 min. The FeVO4(3)-HTAB photocatalyst showed
In general the diffusion rate of adsorbed reactive species on the
surface is faster than the photocatalytic reaction rate. Therefore,
the photocatalytic reaction is the rate control step. The Langmuir-
Hinshelwood model is usually used to describe the kinetics of
photocatalytic reactions of aquatic organics [23,24].
It basically relates the degradation rate (r) and the concentration
of organic compound (C) which is expressed as follows:
dC
dt
krkadsC
r = −
=
1 + kads
C
where kr is the Langmuir-Hinshelwood reaction rate constant
(mg L−1 min−1) and K is the Langmuir adsorption constant (L mg−1).
The adsorption (dark experiment in the presence of H2O2 with
catalyst), photolysis (experiment under illumination in the pres-
ence of H2O2 without catalyst) and photodegradation (experiment
under illumination with and without H2O2) of phenol on FeVO4(3)
are shown in Fig. 5a.
The experimental results showed that 17% and 35% of phenol
were removed for adsorption and photolysis, respectively after
150 min. It can be clearly seen that, both adsorption and photo-
lysis achieved equilibrium and had no positive influence on the
photocatalytic rate.
In addition, there was noticeable change in the reactivity with
FeVO4 under visible-light irradiation without H2O2. Only 32%
degradation of phenol was observed over FeVO4 as like photolysis
reaction. The decrease of phenol concentration in the degradation
of phenol in the absence of H2O2 under visible-light irradiation was
small, a combined result due to the poor adsorption of phenol in
the aqueous solution over the FeVO4 catalyst [25] and the weak
capability of oxygen to capture the photogenerated electrons [26].
It is noticed that the addition of H2O2 to FeVO4 suspensions
exhibits a much higher efficiency in degradation of phenol under
visible-light irradiation. This indicates that H2O2 plays a dual role
in photocatalytic reaction acting as electron scavenger, promoting
100
1.2
(a)
(b)
75
50
25
0
1.0
0.8
0.6
0.4
0.2
0.0
FeVO (3)
4
FeVO (3)-PEG
4
FeVO (3)-SDS
4
FeVO (3)-HTAB
4
0
15
30
45
Time (min)
Fig. 6. Time course of the adsorption of phenol by FeVO4(3), FeVO4(3)-HTAB,
FeVO4(3)-PEG and FeVO4(3)-SDS under solar light irradiation. Inset shows the
impact of surfactant on TOC removal in the phenol degradation.

70
B. Ozturk, G.S. Pozan SOYLU / Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
Table 2
phenol. The unsubstituted CeVO4 was found to be inferior while the
activity of the base metal-substituted compounds was comparable
to the commercial Degussa P-25 TiO2 catalyst [29].
According to activity results, the surface area, particle size and
surface OH groups were not the only contribution in high reactivity
for the degradation of phenol. The reaction with the catalysts is
more important beside the surface area, particle size and surface
OH groups.
HPLC analysis performed during the photocatalytic degrada-
tion of phenol. Benzoquione (BQ) and ring-opening products were
detected as intermediates. In addition, ring-opening products are
supposed to be short chain acids, such as, acetic acid, maleic acid,
oxalic acid, etc. [30].
Reaction rate constant (kr), adsorption equilibrium constant (Kads) and correlation
coefficient (R2) in the photocatalytic degradation of phenol.
Catalyst
kr (mg L−1min−1
)
Kads (mg−1 L)
R2
FeVO4(3)
FeVO4(7)
FeVO4(11)
FeVO4(3)-HTAB
FeVO4(3)-SDS
FeVO4(3)-PEG
Degussa-TiO2(P25)
0.45
0.36
0.14
1.24
0.78
0.61
0.21
0.312
0.595
0.323
0.878
0.943
0.892
0.191
0.94
0.945
0.83
0.99
0.84
0.85
0.88
the highest percentage of phenol degradation (100%) in 45 min.
In addition, there was noticeable change in the reactivity with
different surfactant loading of FeVO4 (Table 1). It can be clearly
realized that the photocatalytic performance decreased accord-
ing to the sequence of FeVO4(3)-HTAB (ca. 100%) > FeVO4(3)-SDS
(ca. 80%) > FeVO4(3)-PEG (ca.59%) within 45 min of reaction. Based
on the SEM photograph of FeVO4 modified with SDS and PEG,
particle agglomeration caused a drop in the photocatalytic activ-
ity.
We compared the TOC removal results on the photocatalytic
degradation of phenol with different light sources by the FeVO4(3)
catalyst. 43% TOC removal achieved on UV-B light irradiation in
150 min. Maximum TOC removal 59% can be achieved on solar light
irradiation in 45 min.
Fig. 6b presents the TOC removal results on the photocatalytic
degradation of phenol with different surfactants on solar light
irradiation. 25 ppm phenol can be totally degraded in 45 min by
FeVO4(3)-HTAB sample and 94% TOC removal can be achieved in
45 min. This result emphasizes the achievement of the total min-
eralization.
In Table 2 are the values of the adsorption equilibrium constant
(Kads), the reaction rate constant (kr) and correlation coefficient (R2)
determined by linear regression.
FeVO4(7) shows a higher adsorption equilibrium constant than
FeVO4(3) and FeVO4(11) due to the higher surface area of FeVO4(7).
However, highest reaction rate (0.45 mg l−1 min−1) was obtained
with the FeVO4(3) catalyst.
For the modified FeVO4 catalysts, the adsorption equilibrium
constant is increased with the surfactant loadings to FeVO4. It
can be explained in terms of increased specific surface area.
However, the adsorption equilibrium constants is increased with
an HTAB loading to FeVO4(3). In addition, highest reaction rate
(1.24 mg l−1 min−1) were obtained with the FeVO4(3)-HTAB cat-
alyst. One explanation lies in the strong interaction between the
FeVO4 and HTAB.
In fact, an increase in activity with different surfactant load-
ing could probably be explained by good dispersion of the active
species of FeVO4 due to the formation of larger BET surface areas.
The results showed that surface area and particle size of samples
were changed with the surfactant addition (Table 1).
Shang et al. investigated ultrasonic-assisted synthesized BiVO4
with polyethylene glycol (PEG) catalyst photocatalytic activity. The
BiVO4 samples were prepared under ultrasonic irradiation with 1 g
PEG. The sample prepared with PEG possessed a large surface area
and a small crystal size compared to pure BiVO4 sample. In addition,
PEG loading resulting in good visible-light photocatalytic activity
toward decoloration on Rhodamine B 8 [28].
The results of HPLC studies of photocatalytic reaction show
phenol concentration decays following a pseudo first order rate
law, reaching at the end of the photocatalytic run essentially zero
concentration and the concentration of the main reaction inter-
mediates initially rise and then decrease until almost complete
disappearance at the same time of complete phenol conversion.
Concentration profiles for phenol, hydroquinone and ring-opening
products obtained during the photocatalytic oxidation reaction
using FeVO4(3), is shown in Fig. 7a. Besides, the distribution of
intermediates also was compared with HTAB-modified FeVO4 cat-
alysts (Fig. 7b).
According to HPLC results, ring-opening products BQ was
observed during the degradation of phenol for FeVO4(3). In
addition to these products, BQ was also detected in very
low concentration. However, the concentration of BQ showed
remarkably increased with FeVO4(3)-HTAB catalyst. Moreover,
ring-opening products were found in higher concentration by using
25
Phenol
Benzoquinone
Others
(a)
20
15
10
5
0
0
15
30
45
Time (min)
25
20
15
10
5
Phenol
Benzoquinone
Others
(b)
In this study, the catalytic activity of surfactant-assisted
FeVO4(3) catalysts were found to be higher than P-25. Although the
decrease in surface area of FeVO4(3)-HTAB catalyst was comparable
even lower than TiO2 P-25, it showed high activity.
0
0
15
30
45
Time (min)
Deshpande and Madras investigated the photocatalytic activity
of metal-substituted orthovanadates using the photooxidation of
Fig. 7. Concentration profiles for intermediates obtained during the photocatalytic
oxidation reaction using (a) FeVO4(3) and (b) FeVO4-HTAB.

B. Ozturk, G.S. Pozan SOYLU / Journal of Molecular Catalysis A: Chemical 398 (2015) 65–71
71
HTAB modified catalyst to FeVO4(3) catalyst. These results indi-
Appendix A. Supplementary data
cate that FeVO4(3)-HTAB catalyst can be considerably activated
by solar light. These intermediates undergo further photo-
catalytic oxidation to ring cleavage to yield carboxylic acids
and aldehydes, which give CO2 and H2O due to decorboxyla-
tion.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.11.013.
It should be noted that, some structural features of catalyst play
a significant role during the photocatalysis. These results confirm
that FeVO4(3)-HTAB is more active than the other catalysts. These
results may also indicate that the photochemical reaction plays an
important role for the degradation of phenol under these condi-
tions.
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