J. Yang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 42–51
43
in TiO2 because of the comparatively high effect at much less cost
than noble ones. Vijayan et al. [11] found that the photocatalytic
activity of TiO2 doped with 0.5 wt% Fe exceeded that of non-doped
Co(NO ) ·6H O in the water by stirring before hydrothermal reac-
3
2
2
tion.
commercial and synthesized pure TiO . Venkatachalam et al. [20]
2.3. Apparatus
2
concluded that the photocatalytic activity in the degradation of 4-
2
+
2+
CP is higher for Mg and Ba doped nano TiO than those for both
The X-ray diffraction (XRD) spectra of TiO /Co nanoparticles
2
2
pure nano TiO2 and commercial TiO2 (DegussaP-25).
were collected on a Shimadzu XRD-6000 diffractometer with Cu
K␣ radiation (Shimadzu, Kyoto, Japan). UV–vis diffused reflectance
spectra of the powders were recorded to measure the absorbance of
the catalysts under the wavelength 200–800 nm with Shimadzu UV
2401 model ultraviolet and visible spectrophotometer (Shimadzu,
Kyoto, Japan) using BaSO4 white plate as reference. Energy dis-
persive X-ray spectrometry (EDX) was carried out with an EX250
spectroscope (Horiba, Kyoto, Japan) attached to an S-3400N II
scanning electron microscope (SEM, Hitachi, Tokyo, Japan). Trans-
mission electron micrographs (TEM) of the catalysts were taken by
a JEM-200CX (JEOL, Tokyo, Japan) microscope operating at a 200 kV
Cobalt is an abundant transition metal and it could be an attrac-
tive dopant for TiO . Cobalt doped TiO2 (TiO /Co) has shown high
2
2
activity for degradation of 2-chlorophenol (2-CP) [21], 4-CP [22],
Bisphenol A [22], acetaldehyde [23–25], acetonitrile [26], methyl
orange [16], methylene blue [27,28], rhodamine B [29] and azo
fuchsine [30]. To the best of our knowledge, few reports have been
published on TiO /Co nanoparticles for efficient photodegradation
2
of BPs and other CPs. The application of TiO /Co to photocatalytic
2
degradation of halophenols should be a profitable attempt. It is also
interesting to explore the degradation mechanism of TiO /Co to
2
halophenols. Meanwhile, the wastewaters are not only polluted by
only one type of phenolic compounds but also different substituted
and isomers of phenols [13], so it is of important significance to
investigate the difference of various halophenols during the pho-
tocatalytic degradation process.
accelerating voltage. The doping concentration of Co in TiO /Co was
2
measured by using an Optima 5300DV inductively coupled plasma
optical-emission spectrometry (ICP-OES, Perkin-Elmer, Fremont,
CA, USA). X-ray photoelectron spectroscopy (XPS) analysis was per-
formed on a PHI 5000 VersaProbe system, using monochromatic
Al K␣ radiation (1486.6 eV) operating at an accelerating power of
15 kW (ULVAC-PHI, Kanagawa, Japan). Fourier transform infrared
(FT-IR) spectra (4000–400 cm–1) in KBr were recorded on a Tensor
27 spectrometer (Bruker, Saarbrücken, Germany). The pH was con-
trolled by a Mettler Toledo SevenMulti pH meter (Mettler-Toledo,
Shanghai, China). The concentrations of CPs and BPs, as well as their
photocatalytic degradation products, were quantitatively analyzed
by high performance liquid chromatography (HPLC) on an Agilent
1200 equipped with a vacuum degasser, a quaternary pump, an
auto-sampler, a diode array detector (DAD), and an Agilent Chem-
Station (Agilent, Palo Alto, CA, USA).
In this present work, TiO /Co nanoparticles were prepared
2
by ultrasonic-assisted hydrothermal method. Well characterized
TiO /Co was used as catalyst for photodegradation of CPs and
2
BPs. The degradation rates of all the halophenols were carefully
estimated based on the reliable determination of the degradation
products. Moreover, the dehalogenation behaviors of 2,4,6-TCP and
2
,4,6-tribromophenol (2,4,6-TBP) were compared in the process
of photocatalytic degradation. In addition, the doping content of
cobalt in TiO /Co nanoparticles were optimized for degradation of
2
2
,4,6-TCP and 2,4,6-TBP.
2
. Experimental
2.4. Photocatalytic degradation of CPs and BPs
2
.1. Materials
Aqueous halophenol (400 mg/L) solution was adjusted to 5.0
by acetic acid–ammonium acetate buffer solution. Photocatalytic
degradation experiments were performed in a self-made reactor.
The irradiation was carried out by using an 18 W low pressure
mercury lamp in the center of the reactor and CP or BP solu-
tion was placed in quartz tubes 10 cm away from the lamp.
The lamp emits predominantly UV radiation at a wavelength of
254 nm. The substrate solution was first stirred for 30 min to reach
adsorption–desorption equilibrium, and then illuminated for a cer-
tain time according to the substance degraded. The solutions were
taken away and filtered through 0.45 mm filter membrane. The con-
centration of CPs or BPs and their degradation products were ana-
lyzed by HPLC, in which a C-18 column (5 m, 150 mm × 4.6 mm
i.d. Welch, Shanghai, China) was employed and a mobile phase of
methanol/1.0% acetic acid aqueous solution (60:40, v/v) was used
at a flow rate of 1.0 mL/min. The injection volume was 10 L and the
UV detection wavelength was 280 nm. The degradation percentage
through the experiment was obtained through Eq. (1):
Tetra-n-butyl titanate (Ti(OBu) ) and cobalt nitrate (Co(NO ) ·
4
3 2
6
H O) of analytical grade were purchased from Shanghai Chemical
2
Regent Company (Shanghai, China) and used as titanium and cobalt
sources, respectively, for preparation of TiO2 and TiO /Co photo-
catalysts. Acetic acid and ammonium acetate of analytical grade
from Shanghai Chemical Regent Company (Shanghai, China) were
used to adjust the pH of substrate solutions. 2,4,6-trichlorophenol
2
(
(
2,4,6-TCP), 2,4-dichlorophenol (2,4-DCP), 2,6-dichlorophenol
2,6-DCP), 2-chlorophenol (2-CP), 4-chlorophenol (4-CP), 2,4,6-
tribromophenol (2,4,6-TBP), 2,4-dibromophenol (2,4-DBP), 2,6-
dibromophenol (2,6-DBP), 2-bromophenol (2-BP), 4-bromophenol
(
4-BP) and phenol were obtained from Sigma–Aldrich (St. Louis,
MO, USA) with analytical grade. Purified water (Wahaha Group Ltd.,
Hangzhou, China) was used throughout the experiment.
2
.2. Preparation of photocatalyst
(
C − C)
0
Degradation percentage =
× 100%
(1)
C
0
TiO2 nanoparticles were prepared by minor modification of
Ref. [11] in ultrasonic-assisted hydrothermal method. Briefly, 5 mL
Ti(OBu)4 was mixed with 20 mL absolute ethanol and 40 mL water
with constant and vigorous stirring for 1 h, and then the mixture
was sonicated for 30 min. The resulting solution was transferred
into a 100 mL Teflon-sealed autoclave and heated at 373 K for
where, C0 is the initial concentration of CP or BP (mg/L); C is the
instant concentration of CP or BP (mg/L) when the degradation
was stopped.
2.5. Determination of Co content in TiO /Co catalyst
2
2
4 h. After cooled to room temperature, the mixture was trans-
ferred into an evaporating dish and dried at room temperature.
Finally, the resulting powders were calcinated at 773 K for 3 h.
About 0.10 g dried TiO2 or TiO /Co catalyst was digested with
2
HCl–HNO –HF (2:1:5, v/v/v) mixture. The solution from the catalyst
3
TiO /Co nanoparticles were obtained by adding about 30 mg of
digestion was transferred into a 25 mL volumetric flask and diluted
2