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Qianli Zhang et al. / Chinese Journal of Catalysis 36 (2015) 975–981
electrode, its large-scale practical application is limited by its
high cost and difficulty in selecting a suitable substrate for the
thin diamond layer [14,15].
pared with twice-distilled water.
2.2. Electrode preparation
Alternatively, PbO2 electrodes have been used as anodes in
the chlor-alkali industry for over 70 years because of their high
electrical conductivity, which is similar to that of metals, good
stability, high overpotential for oxygen evolution, low cost, ease
of fabrication, and long service life. In recent decades, PbO2
anodes have been applied to oxidize organic pollutants
[10,16–19]. An SnO2–Sb layer between the Ti substrate and
PbO2 has been commonly introduced to enhance the adhesion
of the PbO2 layer [20]. Niu’s group has reported several suc-
cessful studies on the electrochemical mineralization of envi-
ronment contaminants such as perfluorooctanoic acid, per-
fluorocarboxylic acid [21,22], and sulfamethoxazole [13] using
PbO2 electrodes.
PbO2 displays two allotropic forms, orthorhombic and te-
tragonal (α and β). Compared with α-PbO2 that features a com-
pact structure, β-PbO2 is widely used in electrochemical degra-
dation owing to its porous structure that leads to high electro-
chemical efficiency and large surface area [23,24]. However,
direct electrodeposition of β-PbO2 on a Ti substrate lessens the
anode stability and activity because the porous β-PbO2 struc-
ture can easily detach from the Ti substrate.
A Ti metal net (2.0 cm 2.0 cm 0.1 cm) was used as the
substrate. Before deposition, the Ti substrate underwent sand-
blasting, ultrasonic cleaning in acetone for 10 min, ultrasonic
cleaning in deionized water for 10 min, immersion in 40%
NaOH at 80 °C for 2 h, etching in boiling 15% oxalic acid for 2 h,
and rinsing with deionized water [29]. After pretreatment, in-
terlayer α-PbO2 was prepared using the pretreated Ti substrate
as an anode and a Cu plate as a cathode in alkaline solution (0.1
mol/L PbO, 3.5 mol/L NaOH) at 40 °C and 10 mA/cm2 for 2 h.
Finally, top layer β-PbO2 was electrodeposited on the above
prepared substrate in acidic solution at 60 °C using a current
density of 20 mA/cm2 for 2 h. The acid solution consisted of 50
g/L Cu(NO3)2, 0.5 g/L NaF, and 150 g/L Pb(NO3)2 [23].
2.3. Electrode characterization
Scanning electron microscopy (SEM; HITACHI-4700, Japan)
was used to characterize the morphology of the electrode sur-
face. X-ray diffraction (XRD) patterns of the samples were rec-
orded on a D/max-RB X-ray diffractometer (Rigaku, Japan)
using Cu Kα radiation (40 kV, 100 mA).
Some methods have been reported to improve the adhesion
of β-PbO2 on the substrate. For instance, the electrodeposited
β-PbO2 on a platinized Ti (Ti-Pt) substrate was reported to be
stable in the electrochemical degradation of real textile
wastewater [25,26]. A fluoride-doped Ti/β-PbO2 anode showed
good performance in the electrochemical degradation of the
dimethyl phthalate ester [27]. β-PbO2 electrode modified by
either TiO2 or Co3O4 effectively electrochemically oxidized acid
orange 7 [28] and bisphenol A [12], respectively. Additionally,
to improve the activity and stability of β-PbO2 electrode, sever-
al layers of SnO2-Sb2O5-RuO2 and α-PbO2 were introduced be-
tween β-PbO2 and the substrate. Zheng et al. [10] investigated
the electrochemical degradation of 4-chlorophenol on
Ti/SnO2-Sb2O5-RuO2/α-PbO2/β-PbO2, and Chen et al. [23] in-
vestigated the influence of doped nano-CeO2 on Al/α-PbO2/
β-PbO2 electrode for enhancing electro-catalytic activity.
In this study, a Ti/α-PbO2/β-PbO2 electrode was prepared
simply by electrodeposition. The layer of α-PbO2 enhanced the
adhesion between the β-PbO2 layer and Ti substrate. The pre-
pared Ti/α-PbO2/β-PbO2 electrode displayed high catalytic
activity and long lifetime toward the degradation of 2-chloro-
phenol. The degradation of complex compounds, i.e., 2,4-di-
chlorophenol and bisphenol A, was also investigated to further
evaluate the efficiency of the Ti/α-PbO2/β-PbO2 electrode.
Polarization curves were obtained on the CHI 660C electro-
chemical workstation (Shanghai ChenHua Instrument Co., Ltd.)
using a conventional three-electrode system. A Ti/α-PbO2/
β-PbO2 electrode with an effective surface area of 1 cm2 was
used as the working electrode, a saturated calomel electrode
(SCE) was used as the reference electrode, and a platinum elec-
trode was used as the counter electrode. The measurements
were conducted at room temperature.
2.4. Procedure of electrochemical degradation
Electrochemical oxidation of 2-chlorophenol was performed
in a 150 mL beaker using a Cu plate (8 cm2) as the cathode and
Ti/α-PbO2/β-PbO2 (8 cm2) as the anode. The anode and cath-
ode were positioned vertically and parallel to each other with a
distance of 1 cm. Na2SO4 was chosen as the supporting electro-
lyte.
During electrochemical degradation, a portion of the reac-
tion solution was withdrawn from the reactor at certain time
intervals to determine the concentration of residual 2-chloro-
phenol in the test solution using a UV spectrophotometer
(TU-1901). The removal rate () was calculated according to
Eq. (1):
η = (A0 – A)/A0 100%
(1)
where A0 and Aare the absorbance values of the organic pollu-
tant at time zero and t (s), respectively.
2. Experimental
2.1. Materials
3. Results and discussion
2-Chlorophenol, 2,4-dichlorophenol, and bisphenol A were
obtained from Sigma-Aldrich. All other chemicals were pur-
chased from Shanghai Sinopharm Chemical Reagent Co., Ltd. All
chemicals were of analytical grade. The solutions were pre-
3.1. Electrode characterization
SEM was used to characterize the morphology and surface