Photocatalytic degradation of phenol in water on as-prepared and surface modified TiO2 nanoparticles

Article abstract of DOI:10.1016/j.cattod.2015.03.048

The TiO₂ nanoparticles in the diameter of 10-23 nm prepared in this study offered high photocatalytic activity to continuously produce the higher concentration of hydroxyl radicals than that stoichiometric produced from the highly oxidizing agent H₂O₂ in the water. The initiate phenol degradation rate on the TiO₂ nanocatalyst was ca. 6 times higher than that in the phenol degradation only derived by H₂O₂. The addition of H₂O₂ with TiO₂ could enhance the initial concentration of hydroxyl radicals for the higher degradation rate. However, overloading H₂O₂ with TiO₂ could only slightly increase the degradation rate of phenol, and overloading TiO₂ decreased the phenol degradation rate immediately. Further enhancement for the phenol degradation rate has been realized by surface modification of TiO₂ via liquid acid treatment or hydrogenation. It did not change the bulk structure and the morphology/size of TiO₂, but strongly enhanced the photocatalytic performance for the phenol degradation. The formation of Lewis acid Ti³⁺ sites on the blue TiO₂ surface via hydrogenation contributed the higher phenol degradation rate than Br?nsted acid sites on acid-treated TiO₂. The preparation and regeneration of blue TiO₂ avoids the utilization of corrosive liquid acids but offers higher photocatalytivity, which is promising for the water treatment.

Full text of DOI:10.1016/j.cattod.2015.03.048

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Contents lists available at ScienceDirect
Catalysis Today
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Photocatalytic degradation of phenol in water on as-prepared and
surface modified TiO2 nanoparticles
Huajuan Linga, Kyungduk Kima, Zongwen Liua, Jeffrey Shia, Xunjin Zhub, Jun Huanga,∗
a Laboratory of Catalysis Engineering, School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW 2006, Australia
b Department of Chemistry, Hong Kong Baptist University, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
The TiO2 nanoparticles in the diameter of 10–23 nm prepared in this study offered high photocatalytic
activity to continuously produce the higher concentration of hydroxyl radicals than that stoichiometric
produced from the highly oxidizing agent H2O2 in the water. The initiate phenol degradation rate on the
TiO2 nanocatalyst was ca. 6 times higher than that in the phenol degradation only derived by H2O2. The
addition of H2O2 with TiO2 could enhance the initial concentration of hydroxyl radicals for the higher
degradation rate. However, overloading H2O2 with TiO2 could only slightly increase the degradation rate
of phenol, and overloading TiO2 decreased the phenol degradation rate immediately. Further enhance-
ment for the phenol degradation rate has been realized by surface modification of TiO2 via liquid acid
treatment or hydrogenation. It did not change the bulk structure and the morphology/size of TiO2, but
strongly enhanced the photocatalytic performance for the phenol degradation. The formation of Lewis
acid Ti3+ sites on the blue TiO2 surface via hydrogenation contributed the higher phenol degradation rate
than Brønsted acid sites on acid-treated TiO2. The preparation and regeneration of blue TiO2 avoids the
utilization of corrosive liquid acids but offers higher photocatalytivity, which is promising for the water
Received 30 November 2014
Received in revised form 24 March 2015
Accepted 28 March 2015
Available online xxx
Keywords:
Photocatalysis
Phenol degradation
Titanium dioxide
Hydrogen peroxide
Hydrogenation
Acid treatment
treatment.
© 2015 Published by Elsevier B.V.
1. Introduction
holes in the valence band [9]. The generated electrons and holes
can recombine or they can transfer to different positions on the
Phenol and phenolic derivatives are common contaminants in
industrial wastewater that pose risks to the environment [1,2].
The conventional technologies for phenol removal to meet its
safety discharge level which is in the range of 0.1–1.0 mg/L [3]
include adsorption, coagulation, biological treatment [2–4], and
enzyme oxidation [1,5]. However, these technologies do not actu-
ally destroy the organic pollutants and could generate secondary
pollutants. In the last decades, heterogeneous photocatalysis
employing transition metal semiconductors, in particular on tita-
nium dioxide (TiO2) has been proposed as a low cost and low energy
consumption for water treatment which ensures complete min-
eralization of organic contaminates without generating harmful
by-products [6–8]. Generally, the photocatalysis is initiated when
a photon with energy, hꢀ, equal to or greater than the band gap
energy, Ebg, reaches the surface of the photocatalyst, thus resulting
in the formation of electrons in the conduction band and positive
photocatalyst surface, then become trapped to form hydroxyl radi-
cals (•OH) which are essential for highly efficient photocatalysis
[10,11]. However, competition of the separation and recombination
of photo-generated electrons and holes leads to process inefficien-
cies [4,10,12] and long exposure time for complete mineralization
of phenol [6,13].
TiO2 photocatalysts are suitable for water treatment due to
its high photochemical stability, nontoxicity and inexpensiveness
[14,15]. For effective photocatalysts to overcome the above chal-
lenge, the structure and surface of TiO2 have been modified and
tailored. Nahar et al. have prepared iron-doped TiO2 by using
Degussa P25 TiO2 powder as the parent catalyst for degradation
of phenol [16]. However, the leaching of metals from TiO2 sur-
face would deactivate photocatalysts and make them hard to be
regenerated. Carpio et al. have investigated the phenol degradation
using TiO2 nanocrystals supported on activated carbon with 50%
degradation in 4 h [17]. However, the significant amount of phenol
absorbed on activated carbon took much longer time to reach target
degradation level. Colon et al. [8] have investigated the photocat-
alytic oxidation of phenol over acidic pre-treated TiO2 prepared by
∗
Corresponding author. Tel.: +61 2 9351 748; fax: +61 2 9351 2854.
E-mail address: jun.huang@sydney.edu.au (J. Huang).
http://dx.doi.org/10.1016/j.cattod.2015.03.048
0920-5861/© 2015 Published by Elsevier B.V.
Please cite this article in press as: H. Ling, et al., Photocatalytic degradation of phenol in water on as-prepared and surface modified
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nitric, sulphuric and phosphoric acids [18]. However, waste acids
from pre-treatment are harmful and corrosive pollutants for the
environment.
Recently, many researchers have reported that maximizing light
harness by the modification of TiO2 and improving acidity of the
semiconductor are feasible and cost-effective ways to improve
the photocatalytic activity of TiO2 [18–22]. Recently, Mao et al.
have produced high efficient disordered TiO2 nanocrystals for the
photocatalytic splitting of water via the combination of adding
impurities and hydrogenation treatment [19]. Wang et al. have
reported that hydrogenated TiO2 nanoparticles introduce a sur-
face disorder structure, extending the light absorption to the near
infrared range (∼1200 nm) [22]. This modification process is clean,
however, the production of black TiO2 requires the hydrogenation
under the high pressure with the relatively high operation risk.
In this work, effective TiO2 nanoparticles with a dominant
anatase phase and a very small fraction of rutile phase has been
prepared by a sol-gel method and tested for the photocatalytic
degradation of phenol with the addition of hydrogen peroxide
(H2O2). The kinetics of heterogeneous photocatalytic processes
was studied by investigating the effect of operating parameters.
Moreover, the blue TiO2 nanoparticles have been developed under
atmosphere pressure with improved charge separation and the
photocatalytic activity. The efficiency of phenol degradation on
this blue TiO2 was higher than that of acidic pre-treated TiO2 with
sulphuric acid (H2SO4) and hydrochloric acid (HCl).
Fig. 1. The XRD patterns of the as-prepared TiO2 and the further hydrogenated blue
TiO2 samples.
the illuminated TiO2 nanoparticles was mixed with 150 ml of phe-
nol solution with initial concentration of 55 ppm (0.58 mM). Prior
to irradiation this solution was magnetically stirred in the dark for
about 30 min to reach the adsorption equilibrium so that the loss of
compound due to adsorption can be taken into account. A proper
volume of H2O2 was added to the solution at the initiation of irradi-
ation and compressed air was purged into the solution to maintain
aerobic condition. The light was provided by a 230-W high pressure
mercury UV lamp which its strongest emission light is with wave-
length of 254 nm. The distance from the UV lamp to the solution
was 8 cm. All experiments were conducted at room temperature
and about 3 ml of the aqueous solution was collected at regular
intervals and analyzed by UV–vis spectroscopy at wavelength of
269.5 nm. The samples were filtered through a Millipore membrane
filter with pore size of 0.1 ␮m before the analysis.
2. Experimental
2.1. Catalyst preparation and characterization
The TiO2 nanoparticles were prepared by a sol-gel method.
Titanium (IV) isopropoxide (TTIP, >97%, Sigma–Aldrich) was firstly
dissolved in ethanol solvent (> 99.5%, Sigma–Aldrich) and distilled
water was added to the solution with a molar ratio of TTIP: ethanol:
water = 1:10:2. Hydrochloric acid (12.5 mol% in water, molar ratio
HCl:TTIP = 1:1) was added dropwise to adjust the pH under contin-
uous stirring for the hydrolysis process of the mixture in 30 min to
form sols. After aging for 24 h, the obtained gels were dried under
373 K overnight and then calcined at 773 K for 2 h to obtain TiO2
nanoparticles.
The Langmuir-Hinshelwood model is usually used to describe
the kinetics of photocatalytic reactions of aquatic organics
[7,23,24]:
The blue TiO2 nanoparticles were obtained after the hydrogena-
tion of TiO2 in H2 flow (80 mL/min) at 673 K for 24 h. The acidic
pre-treated TiO2 nanoparticles were produced by adding the par-
ent TiO2 into an acid solution (0.05 g TiO2 per mL 0.5 M H2SO4 or
1.0 M HCl) under sonication for 30 min and then dried overnight at
423 K for H2SO4 and 323 K for HCl.
The crystalline phases present in the TiO2 nanoparticles were
analyzed by a Shimadzu X-Ray Diffractometer (XRD-6000) with
CuK␣ radiation operated at 40 kV and 30 mA at scan range from 10
to 70 deg with continuous scanning mode at a rate of 2◦ min−1. The
surface area of the catalysts was measured by an Autosorb IQ-C sys-
tem according to the N2 adsorption isotherm at 77 K. An amount of
50 mg of the samples was degassed at 423 K for 12 h under vacuum
before the measurement. The morphology and size of solid catalysts
of the samples were observed with scanning electron microscopy
(SEM) which recorded on a FESEM, Zeiss Ultra+ and transmission
electron microscopy (TEM) by a Philips CM120 BioFilter. The crys-
tallographic structure of samples was imaged by a high resolution
transmission electron microscopy (HRTEM) by JEOL 2200FS.
dC
dt
krKadC
r = −
=
(1)
1 + Kad
C
where kr is the intrinsic rate constant, Kad is the adsorption equi-
librium constant and C is the concentration of aquatic organic.
When the initial concentration of phenol is low, Eq. (1) can be
simplified to the first-order kinetics with an apparent rate constant
kapp [7]:
ꢀ
ꢁ
C
ln
= −krKadt = −kapp
t
(2)
C0
The half-life time (degradation of phenol to its 50%) is calculated
by the following equation:
ln 2
t1/2
=
(3)
kapp
3. Results and discussion
3.1. TiO2 nanoparticles characterization
2.2. Photocatalytic activity test
The crystalline phases and crystallite size of both TiO2 and blue
TiO2 sample were determined from the X-ray diffraction patterns
(Fig. 1). The width of the peak obtained for both photocatalysts
was narrow which implies that crystalline structure is presented
Photodegradation of phenol was carried out in a 1.0 L capacity
cylindrical Pyrex-glass batch photoreactor. The proper amount of
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Fig. 2. SEM image of (a) the as-prepared TiO2, TEM images of (b) the as-prepared TiO2, (c) S-TiO2, (d) the hydrogenated blue TiO2, HRTEM images of (e) the as-prepared TiO2,
(f) S-TiO2, (g) the hydrogenated blue TiO2.
Please cite this article in press as: H. Ling, et al., Photocatalytic degradation of phenol in water on as-prepared and surface modified
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Fig. 4. Phenol (55 pm, 0.58 mM) degradation rate as a function of time at room
temperature. (a) UV irradiation only, (b) UV irradiation with 4.82 mM H2O2, (c) UV
irradiation with 0.5 g/L TiO2, and (d) UV irradiation with 0.5 g/L TiO2 and 4.82 mM
H2O2.
Fig. 3. Raman spectra of the as-prepared TiO2, HCl-TiO2, S-TiO2 and the hydro-
genated blue TiO2.
mineralization of phenol, the integrated approach of TiO2 photo-
catalysis and oxidation process at the presence of H2O2 has been
attempted to improve the efficiency of photodegradation of phe-
nol. As shown in Fig. 4, the direct photodegradation of phenol
without TiO2 had extremely low phenol removal and only 3% phe-
nol (Fig. 4a) was reduced after 24 h. It indicates UV itself is not
able to degrade phenol in water. Adding H2O2 for the advance UV
promoted oxidation process, the degradation rate of phenol was
obviously increased to 20% after 24 h (Fig. 4b). Phenol degradation
by the advance oxidation process by H2O2 involves the generation
and subsequent reaction of hydroxyl radicals (•OH). The hydroxyl
radical generated by hydrogen peroxide photolysis (Eq. (4)) oxi-
dizes phenol from hydrogen abstraction to produce a phenoxyl
radical (Eq. (5)) [3,32,33]. This organic radical quickly reacts with
dissolved oxygen to yield peroxyl radicals, initiating thermal reac-
tions of oxidative degradation, leading finally to carbon dioxide,
water and inorganic salts (Eq. (6)) [3,32].
in both TiO2 samples. The crystalline phases present were mainly
anatase (A) (1 0 1), (0 0 4), (1 0 3), (1 1 2), (2 0 0), (1 0 5), (2 1 1) and
(2 0 4) spacing [25–29] and small portion of rutile (R) (1 1 0), (1 0 1)
and (3 1 0) spacing [25–28,30,31], respectively. The broadening of
the peak at the 25.023◦ was chosen to estimate the grain size of
the sample which was approximately 21 nm by applying Scher-
rer’s formula [21]. No any change in XRD patterns were observed
for TiO2 after acidic pre-treatment with H2SO4 and HCl (not shown).
After hydrogenation to transfer white TiO2 to blue one, however,
the diffraction peak (1 0 1) of blue TiO2 (Fig. 1 right top) slightly
shifted toward the higher angle, which indicates the reduction of
the interplanar distance of anatase phase. This result clearly shows
that the change of surface structure occurs during the hydrogena-
tion process, but the bulk structure of particles kept the same. This
can be ascribed to the escape of O atoms from the lattice on the
surface, which often results in Ti3+ on surface.
The morphology and particle size of all TiO2 catalysts were
examined by SEM and TEM. The SEM image of TiO2 has demon-
strated spherical shapes of the particles (Fig. 2(a)) and the TEM
image shown in Fig. 2(b–d) has illuminated the circle shape crys-
talline structure of as-prepared, acid-pretreated, and blue TiO2 with
a size of 10–23 nm that is generally in agreement with the size
and main crystalline determined by XRD. Similarly, Raman spec-
tra (Fig. 3) also showed the similar bulk structure of as-prepared,
acid-pretreated, and blue TiO2 nanoparticles. There are five Raman-
active modes with frequencies at 143.7, 196.8, 395.5, 515.1 and
636.4 cm−1 presented Fig. 3. From the HRTEM images (Fig. 2(e–g)),
the TiO2 nanoparticles are highly crystallized as seen from the
well-resolved lattice, the acidic pre-treatment hardly changes the
surface crystallinity, while the blue TiO2 nanoparticles had unique
disordered surface structure after hydrogenation and the width of
the disordered outer layer surrounding of the crystalline core was
1–2 nm. It was also confirmed by the slightly shift of the diffraction
peak (1 0 1) during the formation of surface Ti3+ and the escape of
O atoms from the lattice on the surface.
H2O2 + hꢀ → 2•OH
•OH + C6H5OH → C6H5O• + H2O
C6H5O• + O2 → Intermediate → CO2 + H2O
(4)
(5)
(6)
Based on the stoichiometry of the advance oxidation process
by H2O2 (Eq. (7)), 14 moles of H2O2 are required for complete
degradation of 1 mole phenol [34]. In this study, the initial phe-
nol concentration was 0.58 mM and the concentration of H2O2
was 4.82 mM, 60% of the stoichiometry amount was used and 20%
degradation rate of phenol was obtained. Obviously, only adding
the highly oxidizing agent could not complete the degradation of
stable phenol in water.
C6H5OH + 14H2O2 → 6CO2 + 17H2O
(7)
However, only introducing 0.5 g/L TiO2 photocatalyst strongly
enhanced the reaction rate (without H2O2), and achieved the total
mineralization of phenol to CO2 and water in 23 h (Fig. 4c). It
indicates that the TiO2 photocatalyst could continuously produce
hydroxyl radicals for phenol removal. On TiO2, oxygen as electron
acceptor provides a natural sink for the photo-generated electrons.
Thus, hydroxyl ions (OH−) are traps for holes resulting in the for-
mation of hydroxyl radicals (•OH) via Eq. (9) and (10) that are the
major way of photo-generating •OH radicals in TiO2 photocatalysis
process [35]. While the adsorbed oxygen species are traps for elec-
trons, resulting in the formation of unstable superoxide species,
3.2. Optimize the reaction conditions on photodegradation of
phenol
3.2.1. H2O2 and catalyst addition for the generation of hydroxyl
radicals
In order to minimize the recombination of photo-generated
electron-hole pairs and long exposure time for complete
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Table 1
5
The kinetic parameters of different oxidation conditions on phenol degradation at
room temperature. (a) UV irradiation only, (b) UV irradiation with 4.82 mM H2O2,
(c) UV irradiation with 0.5 g/L TiO2, and (d) UV irradiation with 0.5 g/L TiO2 and
4.82 mM H2O2.
Condition
kapp (h−1
)
R2
r0 (mM/h)
Half-life (h)
(a) Direct photolysis
(b) UV/H2O2
(c) UV/TiO2
0.0015
0.0101
0.0583
0.1169
0.9624
0.9358
0.9963
0.9965
0.0009
0.0059
0.0341
0.0679
462
69
12
6
(d) UV/TiO2/H2O2
of phenol without TiO2, the advanced oxidation process by H2O2,
the photocatalysis on TiO2 and the addition of TiO2 with H2O2
were 0.0009 mM/h, 0.0059 mM/h, 0.0341 mM/h and 0.0679 mM/h,
respectively.
The reaction rates of the advanced oxidation process by H2O2,
the photocatalysis on TiO2 only, and the photocatalysis on TiO2
with H2O2 were increased by 7 folds, 38 folds and 76 folds, respec-
tively, compared to the direct photodegradation of phenol by UV
only. Moreover, the performance of the photocatalysis on TiO2 with
H2O2 was two times faster than the photocatalysis on TiO2 only. The
half-life of the direct photodegradation of phenol by UV only, the
advanced oxidation process by H2O2, the photocatalysis on TiO2
and the photocatalysis on TiO2 with H2O2 obtained from Eq. (3)
was 462 h, 69 h, 12 h and 6 h, respectively.
Fig. 5. The degradation kinetic of phenol (55 pm, 0.58 mM) on different oxidation
conditions at room temperature. (a) UV irradiation only, (b) UV irradiation with
4.82 mM H2O2, (c) UV irradiation with 0.5 g/L TiO2, and (d) UV irradiation with 0.5 g/L
TiO2 and 4.82 mM H2O2.
•−
•
such as O2 and HO2 (Eq. (11) and (12)) [10,36]. Hydrogen per-
oxide is produced via Eqs. (8)–(13) and further being reacted to
form ·OH radicals (Eq. (4)) [35]. Phenol can be hydroxylated by the
strongly oxidizing •OH radical that leads to successive oxidation
and eventual ring opening [37]. The resulted intermediates will be
further carboxylated to carbon dioxide and water (Eq. (5) and (6))
[37].
3.2.2. Effect of TiO2 catalyst loading, initial phenol concentration,
and hydrogen peroxide concentration
Normally, high catalyst loading always promotes the reaction
process. However, the optimal catalyst to phenol ratio is necessary
to clarify due to the control of the operation costs. Fig. 6(a) clearly
shows that increased in the amount of catalyst loading up to 1.0 g/L,
the degradation rate of phenol increased, while further increas-
ing in the amount of catalyst for the reaction, the degradation rate
decreased. A number of studies have demonstrated that the rate of
photodegradation for organic pollutants is strongly affected by the
number of active sites and the photo-absorption ability of the cata-
lyst used [7]. An increase in the amount of the catalyst increases the
number of active sites for the generation of more radicals to high
efficient of the process. However, excess amount of catalyst would
result in negative effect as high suspension inhibits the penetra-
tion of the photon flux and the tendency toward agglomeration,
resulting in a reduction in catalyst surface area available for light
absorption and hence a drop in the photocatalytic degradation rate.
The trade-off between these two opposing phenomena results in
an optimum catalyst loading for the photocatalytic reaction which
is 1.0 g/L.
TiO2 + hꢀ → e−CB + hV+B
(8)
(9)
h+VB + OH−
→
•OHads
ads
h+VB + H2Oads
→
•OHads + H+
(10)
− + O2,ads → O•
(11)
(12)
(13)
−
e
2
O•2− + H+ → HO•2
O2•− + H+ + HO•2 → H2O2 + O2
It was reported that the advance oxidation process by H2O2
resulted in an initially fast photodegradation as high initial H2O2
concentration, then followed by a slow degradation due to the loss
in the effectiveness of hydroxyl radicals as a result of saturation [7].
Using TiO2 photocatalyst, hydroxyl radicals could be continuously
produced and kept at a high level in the water, which contributed
the strongly enhanced degradation rate of phenol. Further adding
4.82 mM of H2O2 with 0.5 g/L TiO2, the mineralization of phenol
was completed in 15 h. Hence, the high initial hydroxyl radical
concentration would promote the high degradation of phenol [39].
As the initial concentration of phenol is low, the reaction was
proposed to obey the first-order kinetics with an apparent rate con-
stant (kapp). Plotting the negative of the natural logarithm of the
fraction of unreacted phenol with respect to the irradiation time
(for the oxidation conditions of TiO2 only and the addition of H2O2
with TiO2, the first 8 h reaction time was used) at different oxida-
tion conditions for a first-order reaction gives a straight line with
the slope of the line equals to kapp (Eq. (2), Fig. 5). The values of cor-
relation factor R2 obtained from the linear regression are all above
0.93 which means these processes obey the first-order kinetics. The
kap for phenol degradation under the four different oxidation con-
ditions ranged from 0.0015 h−1 by the direct photodegradation of
phenol without adding TiO2 and H2O2 to 0.1169 h−1 using TiO2
and H2O2 (Fig. 5), were used to estimate the initiate rate of phe-
nol degradation (r0) and the half-life of these processes. The results
are summarized in Table 1. The r0 of the direct photodegradation
Previous studies have reported that the photocatalytic reaction
rate of photocatalysis using TiO2 depends on the concentration of
organic pollutants [12]. Fig. 6(b) shows that increased in the con-
centration of phenol, the degradation rate increased up to 0.36 mM
and then slightly decreased with further increasing in phenol con-
centration. When the initial phenol concentration is extremely low,
the number of surface activity sites of the fixed catalyst extremely
sufficing for molecules of phenol, OH−, •OH and O2
adsorbed
•−
on the surface of the catalyst [3,12]. Increasing the initial con-
centration of phenol, the exposure time required for complete
mineralization of phenol to CO2 and water increases. But the degra-
dation rate increased up to a certain limited due to the total amount
of phenol degraded actually increases. Further increasing the con-
centration of phenol, the degradation rate slightly decreased. This
might be due to more and more molecules of phenol adsorbed
on the surface of the catalyst, the competitive adsorption of OH−
on the •s−ame site decreases and consequently the amount of •OH
and O2 on the surface of catalyst decreases [3,12]. Furthermore,
the formation of intermediates which are insoluble in water and
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Fig. 6. The optimization of operating parameters on phenol degradation. The degradation rate of catalyst loading (a), initial phenol concentrations (b) and hydrogen peroxide
concentrations (c) on phenol degradation at room temperature.
probably stick on the surface of TiO2 lower the rate of phenol degra-
dation at higher phenol concentration [37].
The slow reaction rates still remain a challenge in photodegra-
dation of organic pollutants. As previously described, adding H2O2
for the high initial concentration of hydroxyl radicals (•OH) could
significantly improve the photocatalytic degradation rate of phe-
nol in water. Fig. 6(c) shows that the degradation rate significantly
accelerated when the concentration of H2O2 was increased from
1.94 mM to 9.70 mM, and then slightly increased with further
increasing in the concentration of H2O2. This is mainly due to
the generation of more hydroxyl ions (OH−) which are the likely
traps for holes, leading to the formation of hydroxyl radicals (•OH).
Further increasing the concentration of H2O2 up to 48.3 mM and
77.6 mM, it will decrease the actual concentration of H2O2 in
the system as•hydroxyl radical efficiently reacts with H2O2 and
produces HO2 [40], also •OH radicals reacts with the generated
•
HO2 to produce water and oxygen [41]. Hence, there is a critical
hydrogen peroxide concentration for high efficient of photocat-
alytic degradation of phenol.
3.3. The degradation performance of surface modified TiO2
Fig. 7. Phenol (55 pm, 0.58 mM) degradation rate as a function of time at room
temperature over the as-prepared TiO2, the hydrogenated blue TiO2, S-TiO2 and
HCl-TiO2.
After hydrogenation or acidic pre-treatment, the bulk crystal
and morphological structure of TiO2 still maintained, evidenced
by the XRD patterns, Raman spectra, and TEM images. However,
the various functional active sites have been generated on the TiO2
surface. On blue TiO2, the removal of O atoms during the hydro-
genation would form the oxygen vacancies and active Ti3+ sites.
The formation of Lewis acid Ti3+ sites on surface could narrow the
band gap of the TiO2 nanoparticles and extend the light absorption
toward the infrared range (∼1200 nm) [22]. For the acid-treated
TiO2, Brønsted acid sites were incorporated to the TiO2 surface to
decomposition of the relatively stable side-on peroxide [42]. Both
hydrogenation and acid-treatment of TiO2 surface could enhance
the activity of catalysts in phenol degradation. As shown in Fig. 7,
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the degradation rate on the as-prepared TiO2 after 1 h reaction was
10.2%, while it was enhanced up to 24.6% on S-TiO2, and 38.8% on
HCl-TiO2, and 44.2% on blue TiO2, respectively. Among them, blue
TiO2 performed the highest degradation of phenol.
them, blue TiO2 performed the highest degradation of phenol. After
long-term running, the degradation rate on surface modified TiO2
increased slowly. It may be caused by the leaching of Brønsted
acid sites or the re-oxidation of Ti3+ sites and could be regener-
ated. Compared with acid treatment of TiO2, the preparation and
regeneration of blue TiO2 avoiding the utilization of corrosive liq-
uid acids but offering higher photocatalytivity is promising for the
water treatment.
After reaction performed in 4 h, the degradation rates were
continuously increasing on all samples such as 36.9% on the as-
prepared TiO2, 44.8% on S-TiO2, 49.9% on HCl-TiO2, and 55.7%
on the blue TiO2, respectively. The photocatalytic activity of the
S-TiO2, HCl-TiO2, and blue TiO2 nanoparticles were still higher
than the as-prepared TiO2, however, the difference between their
catalytic performance became small. It may be caused by the leach-
ing of Brønsted acid sites or the re-oxidation of Ti3+ sites. The
enhancement of the photocatalytic activity of S-TiO2 and HCl-TiO2
nanoparticles causes by an excess of adsorbed protons (Brønsted
acid sites) on the TiO2 surface after the acid treatment [18,42].
These protons do not dissolve in the organic solvent, but the leach-
ing of protons from the surface was hard to avoid. Also, the oxygen
vacancies on blue TiO2 surface would be slowly filled by O atoms
from H2O2 and Ti3+ sites could be transferred to Ti4+ sites accord-
ingly. Compared with acid treatment of TiO2, the preparation and
regeneration of blue TiO2 avoids the utilization of corrosive liquid
acids and the emission of waste water. The blue TiO2 provides a
green way to enhance the photocatalytic activity of the photocata-
lysts in water treatment.
Acknowledgment
Financial support by The University of Sydney Early Career
Research Scheme is gratefully acknowledged.
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Please cite this article in press as: H. Ling, et al., Photocatalytic degradation of phenol in water on as-prepared and surface modified
TiO2 nanoparticles, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.048