Synthesis of Pd nanoparticles decorated with graphene and their application in electrocatalytic degradation of 4-chlorophenol

Article abstract of DOI:10.1166/jnn.2014.8949

Pd/graphene catalysts were prepared in situ from graphite oxide and palladium salts by the hydrogen-reduction method and were then used for the construction of Pd/graphene gas-diffusion electrodes (GDE). The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential pulse voltammetry (DPV) techniques. In the Pd/graphene catalysts, Pd particles, with an average size of 3.6 nm and an amorphous structure, were highly dispersed in the graphene. The Pd/graphene catalysts accelerated the two-electron reduction of O₂to H₂O₂by feeding air, which favors the production of hydroxyl radicals (HO?). In the electrolytic system, HO? was determined in the reaction mixture by the electron spin resonance spectrum (ESR). The dechlorination degree of 4-chlorophenol reached approximately 90.5% after 80 min, and the removal efficiency and the average removal efficiency of 4-chlorophenol. in terms of total organic carbon (TOC) after 120 min, reached approximately 93.3% and 85.1%, respectively. Furthermore, based on the analysis of electrolysis intermediates by high performance liquid chromatography (HPLC) and ion chromatography (IC), a reaction scheme was proposed for the Pd/grapheme GDE catalytic degradation of 4-chlorophenol.

Full text of DOI:10.1166/jnn.2014.8949

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 7279–7285, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Synthesis of Pd Nanoparticles Decorated with
Graphene and Their Application in Electrocatalytic
Degradation of 4-Chlorophenol
Zhao-Yong Bian1ꢀ∗, Yu Bian1, Hui Wang2, and Ai-Zhong Ding1
1College of Water Sciences, Beijing Normal University, Beijing 100875, P. R. China
2College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, P. R. China
Pd/graphene catalysts were prepared in situ from graphite oxide and palladium salts by the
hydrogen-reduction method and were then used for the construction of Pd/graphene gas-diffusion
electrodes (GDE). The prepared catalysts were characterized by X-ray diffraction (XRD), trans-
mission electron microscopy (TEM) and differential pulse voltammetry (DPV) techniques. In the
Pd/graphene catalysts, Pd particles, with an average size of 3.6 nm and an amorphous structure,
were highly dispersed in the graphene. The Pd/graphene catalysts accelerated the two-electron
reduction of O2 to H2O2 by feeding air, which favors the production of hydroxyl radicals (HO•). In the
electrolytic system, HO• was determined in the reaction mixture by the electron spin resonance
spectrum (ESR). The dechlorination degree of 4-chlorophenol reached approximately 90.5% after
Delivered by Publishing Technology to: Chinese University of Hong Kong
80 min, and the removal efficiency and the average removal efficiency of 4-chlorophenol, in terms
IP: 50.51.96.170 On: Thu, 22 Oct 2015 09:43:06
of total organic carbon (TOC) after 120 min, reached approximately 93.3% and 85.1%, respec-
Copyright: American Scientific Publishers
tively. Furthermore, based on the analysis of electrolysis intermediates by high performance liquid
chromatography (HPLC) and ion chromatography (IC), a reaction scheme was proposed for the
Pd/grapheme GDE catalytic degradation of 4-chlorophenol.
Keywords: Electrocatalysis, Pd/Graphene Catalyst, 4-Chlorophenol, Degradation Mechanism.
1. INTRODUCTION
widely investigated because of its ease of control,
amenability to automation, high efficiency, and environ-
mental compatibility.9–16 Of the various electrochemi-
cal processes that have been developed, electrochemical
oxidation methods—such as anodic oxidation or the oxi-
dation by electrogenerated oxidizing agents—have been
the most extensively studied. Some high oxygen over-
voltage anodes, such as boron-doped diamond, Ti/SnO2,
Ti/IrO2, Ti/PbO2, Ti/Bi2O5–PbO2, and SnO2, have been
studied and showed potential for the degradation of
4-chlorophenol.17–21
Very recently, indirect electro-oxidation methods have
been developed for wastewater treatment involving H2O2
electrogeneration on the cathode. It has been reported that
H2O2 can be electrochemically generated by reducing dis-
solved oxygen in an acidic solution:22
Chlorinated organic compounds, especially chlorophenols,
have a significant impact on the environment and act as
mutagenic or carcinogenic pollutants.1–3 Chlorophenols are
easily introduced into the environment—primarily by the
chemical and pharmaceutical industries—during the chlo-
rination of municipal water or the degradation of other
organic compounds. In natural water, chlorophenols pose a
serious risk because of their persistence and accumulation
in the environment. However, conventional treatments for
decomposing chlorophenols, such as biological methods,
are slow or non-destructive due to chlorophenols’ resis-
tance to biodegradation.4
Advanced oxidation processes have received substan-
tial attention for their treatment of wastewater containing
organic contaminants.5–8 Notably, using electrochemical
oxidation for the treatment of wastewater has been
O2 +2H+ +2e− → H2O2
(1)
Carbon materials, such as carbon/polytetrafluoro-
ethylene,23–28 carbon nanotubes,29 graphite,30–32 carbon
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9
1533-4880/2014/14/7279/007
doi:10.1166/jnn.2014.8949
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Synthesis of Pd Nanoparticles Decorated with Graphene and Their Application
Bian et al.
felt,33–35 and reticulated vitreous carbon,36ꢀ37 are classified
as highly conducting electrode materials for the elec-
trochemical production of H2O2. Graphene, a one-atom-
thick two-dimensional monolayer of sp2-bonded carbon,
has attracted a tremendous amount of attention from
researchers38 due to its unique properties and potential
application in nanocomposites. Moreover, with its high
surface area and chemical stability, graphene will be an
important material in the design of next-generation cata-
lysts with enhanced activity.39ꢀ40 The cooperative oxida-
tion of active anodes and electrogenerated oxidant species
on the cathode is expected to significantly increase the
degradation rate of organic compounds.41 With a ratio-
nally designed electrochemical reactor, it is expected that
organic degradation from both anodic oxidation and H2O2
oxidation (electrogenerated on the cathode) will be con-
ducted in the same cell.
In this paper, the Pd/graphene catalyst used for the Pd/C
gas-diffusion electrode system was prepared and charac-
terized by X-ray diffraction (XRD), transmission electron
microscopy (TEM) and differential pulse voltammetry
(DPV) techniques. The degradation of 4-chlorophenol
in an undivided electrolysis device with a self-made
Pd/graphene gas-diffusion cathode and a Ti/IrO2/RuO2
anode was investigated. HO• was determined in the reac-
tion mixture by the electron spin resonance spectrum
(ESR). The degradation mechanism of 4-chlorophenol was
gas-diffusion cathode (16 cm2). A previous report contains
a schematic diagram of the experimental setup.44 A direct
current laboratory power supply with a current–voltage
monitor provided electric power. The experimental condi-
tions were as follows: the initial 4-chlorophenol concentra-
tion was 100 mg L−1; the current density was 39 mA cm−2
;
the concentration of the supporting electrolyte (Na2SO4)
was 0.05 mol L−1; the distance between electrodes was
2.0 cm; and the initial pH was 7.0. Before starting electrol-
ysis, air was fed for 5 min to keep the electrolysis solution
oxygen saturated. Furthermore, air was fed into the elec-
trolysis device at a flow rate of 25 mL s−1 until the end of
the electrolysis process.
2.3. Analytical Methods
The XRD patterns were used to determine the identity of
any phase present and their crystallite size. The Pd/C cata-
lyst was characterized by XRD with a Rigaku D/max-IIIA
X-ray power diffractometer using Cu Kꢁ radiation with an
Ni filter. The Pd particle morphology and size distributions
were determined by TEM (JEM-2010F TEM microscope)
operated at an accelerating voltage of 200 kV.
The DPV spectra were recorded using a potentio-
stat/galvanostat (EG&G Model 273A) with a standard
three-compartment cell consisting of a Pt wire as the
counter electrode, an Ag/AgCl electrode as the reference
electrode, and the Pd/graphene catalyst (or graphene) mod-
ified electrode as the working electrode. To prepare a cat-
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also investigated in the Pd/graphene gas-diffusion elec-
trode system. The purpose of this study was to use this
system to demonstrate effective methods for the formation
of hydroxyl radicals to completely oxidize 4-chlorophenol.
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alyst ink, 5 mL of Pd/graphene catalyst (or graphene) was
suspended in a mixture of 1 mL of ethanol and 50 ꢂL of
Nafion® solution (5 wt% Aldrich solution). Then, 10 ꢂL
of ink was transferred with an injector to the surface
of a clean glassy carbon disk electrode with an area of
0.126 cm2. Afꢀter the ethanol evaporated, the electrode was
heated at 75 C for 10 min. A Na2SO4 solution with a
0.5 mol L−1 concentration was used as an electrolyte,
which was saturated with oxygen (or nitrogen) by feed-
ing oxygen gas (or nitrogen gas) to the cell for 30 min
before the electrochemical measurement. Oxygen gas was
continuously fed to the solution throughout the electroly-
sis process. The temperature was kept at 25 ꢀC throughout
Copyright: American Scientific Publishers
2. EXPERIMENTAL DETAILS
2.1. Preparation of Pd/Graphene Catalyst and
Gas-Diffusion Cathode
Graphite oxide (GO) was prepared from natural graphite
flakes (99.99%, 325 mesh, Qingdao Zhongtian Com-
pany) according to a modified Hummers method.42 The
Pd/graphene catalyst was prepared using the hydrogen-gas
reduction method. Palladium chloride was first dissolved in
a concentrated hydrochloric acid solution and then diluted
with 15 mL of deionized water. The solution of PdCl2
was added dropwise to a vigorously stirred GO solution
the experiment. The scan rate was 100 mV s−1
.
The radicals were determined by ESR measurements,
which were performed in the X-band on a JES-FE3AX
ESR spectrometer (JEOL, Tokyo, Japan) at room tempera-
ture. The ESR spectrometer was set as follows: microwave
frequency, 9.44 GHz; microwave power, 20 mV; modu-
lation frequency, 100 kHz; modulation amplitude, 0.5 G;
center field, 3367 G; scan width, 100 G; receiver gain,
2×103; time constant, 0.3 s; sweep time, 4 min. The ESR
spin trapping agent was 5,5-dimethyl-1-pyrroline-N-oxide
(DMPO, 11.1 mmol L−1).
ꢀ
ꢀ
at 80 C. The mixture was kept at 80 C for 2 h and was
then evaporatedꢀto dryness. The dried catalyst was further
reduced at 250 C in a stream of hydrogen gas at a rate of
30 mL min−1 for 2 h and was then cooled to ambient tem-
perature under hydrogen atmosphere. A Pd/graphene cat-
alyst with a Pd load of 1.0 wt.% was obtained and stored
in a desiccator. The gas-diffusion electrodes were prepared
according to the method described in a previous report.43
The determination of 4-chlorophenol and its stable
degradation products in the electrolyzed solutions was car-
ried out using high-performance liquid chromatography
(HPLC, Waters, America) analyses by comparing their
2.2. Procedures
Electrolysis was conducted in a 100-mL undivided elec-
trolysis device with a Ti/IrO2/RuO2 anode (16 cm2) and a
7280
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Bian et al.
Synthesis of Pd Nanoparticles Decorated with Graphene and Their Application
retention times with those of the standard compounds.
Samples of 20 ꢂL, previously filtered with PTFE filters
of 0.45 ꢂm, were injected into the HPLC. The separa-
tion was performed using a Znertisl ODS-SP C18 column
(250 mm × 4.6 mm, 5 ꢂm) at a flow rate of 1.0 mL/min
successful formation of graphite oxide. The d-spacing of
graphite oxide was 7.34 Å, indicating that the order of the
layers had been disrupted. Oxygen functional groups were
inserted into the interplanar crystal spacing, resulting in an
increase in the distance between carbon sheets. This larger
distance between the GO sheets shifted the XRD peak to
smaller angles, causing the low broad peak presented at
12.1ꢀ.45 After using hydrogen gas to reduce the GO, the
XRD of the resulting graphene material showed that the
12.1ꢀ peak had disappeared, confirming that the reduction
of the GO sheets did in fact occur.46 The appearance of
a new, small, broad peak at approximately 2ꢄ = 25ꢀ with
a corresponding d-spacing of 3.82 Å in the Pd/graphene
suggested the presence of a minor component of multi-
layer graphene.47 This pattern was in contrast to the peak
patterns of the graphite and the GO, which were low and
broad because the formed metal nanoparticles prevent the
restacking of carbon sheets48 and enlarge the interlayer
spacing. The peak at approximately 40ꢀ was characteristic
of a diffraction peak of face centered-cubic crystalline Pd
and corresponded to its (111) planes. The average size of
the metal particles, calculated from the Pd (111) diffrac-
tion line using the Scherrer formula, was 3.6 nm.
ꢀ
and at a column temperature of 30 C. The aromatic com-
pounds were determined by HPLC, running with a mobile
phase of methanol/water (v/v) at 80/20 and an UV detector
set at a wavelength of 280 nm, except for benzoquinone,
whose concentration was measured at 254 nm. The car-
boxylic acids were determined by HPLC, running with a
mobile phase of methanol/KH3PO4 25/75 (v/v) at pH = 2ꢃ1
adjusted with H3PO4 and an UV detector set at 210 nm.
Small molecular carboxylic acids and the concentra-
tion of chloride ions in the electrolyzed solutions were
determined using ion chromatography (ICS-3000, Dionex,
America) by comparing the retention times of the stan-
dard compounds. Samples of 30 ꢂL, previously filtered
with PTFE filters of 0.45 ꢂm, were injected into the IC.
The separation was performed using an AS-11 column
at the flow rate of 1.2 mL min−1. The column tempera-
ꢀ
ture was 30 C. The mobile phase consisted of 5% (vol.)
5 mmol L−1 NaOH and 95% (vol.) deionized water in
the 0–6 min period; the volume of NaOH was increased
to 12% in the 6–41 min period, reduced to 5% in the
41–42 min period and remained at 5% in the 42–50 min
period.
The TEM image (shown in Fig. 2) of the Pd/graphene
catalyst and its metal particle size was measured to esti-
mate the geometry and the particle size of the Pd species.
No obvious aggregation was detected from the images.
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The TEM analysis showed a homogeneous distribution of
IP: 50.51.96.170 On: Thu, 22 Oct 2015 09:43:06
Copyright: American Scientific Publishers
Pd nanoparticles at the graphene surface, with an average
3. RESULTS AND DISCUSSION
particle size of 3ꢃ66 0ꢃ25 nm. This result helps confirm
the XRD result.
3.1. Characterization of the Pd/Graphene Catalyst
The XRD patterns of graphite, GO, and Pd/graphene cat-
alysts are shown in Figure 1. The initial graphite pow-
der showed a typical sharp diffraction peak at 2ꢄ = 26ꢃ6ꢀ
with the corresponding d-spacing of 3.36 Å. The sharp
diffraction peak revealed that the hexagonal system crys-
tal structure of graphite was stable. After chemical oxida-
tion, a lower broad peak, compared to the XRD pattern
of original graphite, appeared at 2ꢄ = 12ꢃ1ꢀ, indicating the
Figure 3 shows the DPV curves of the catalyst elec-
trode surface of different gases in a 0.5 mol L−1 Na2SO4
solution (pH = 12ꢃ8 adjusted with NaOH solution). There
was a high reduction peak (Pd/graphene, graphene: O2) at
about − 0.38 V when oxygen was present in the basic solu-
tion. However, this peak disappeared when the oxygen was
Graphite
GO
Pd/graphene
10
20
30
40
50
60
70
80
90
2theta (degrees)
Figure 1. XRD patterns of the graphite, GO and Pd/graphene catalysts.
J. Nanosci. Nanotechnol. 14, 7279–7285, 2014
Figure 2. TEM image of Pd/graphene.
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Synthesis of Pd Nanoparticles Decorated with Graphene and Their Application
Bian et al.
14
12
10
8
0.60
0.55
0.50
0.45
0.40
0.35
0.30
Pd/graphene (O2)
6
4
Graphene (O2)
2
Pd/graphene (N2)
–0.6 –0.3
0
–1.5
–1.2
–0.9
0.0
0.3
5
10
15
20
25
30
35
E (V)
Time (min)
Figure 3. DPV curves of the catalyst electrode surface using different
gases.
Figure 4. Relative intensity and ESR spectra of HO• radicals trapped
by DMPO in the electrolysis device. Concentration of DMPO:
11.1 mmol L−1; concentration of supporting electrolyte (Na2SO4):
0.05 mol L−1; current density: 39 mA cm−2
.
absent (Pd/graphene: N2). According to the redox poten-
tial, the two-electron reduction of O2 to peroxide anion
(HO−2 ) led to the formation of reduction current peaks
using catalyst-modified electrodes in a basic solution in
the presence of oxygen gas. The reduction current peak
of the Pd/graphene catalyst modified electrode was higher
than that of the electrode modified without catalyst. The
reduction was attributed to the Pd/graphene catalyst, which
accelerated the two-electron reduction of O2 to H2O2.
gas-diffusion electrode system accelerated the two-electron
reduction of O2 to H2O2 when air was fed into the cell,
which favored the production of HO• (see Figs. 3 and 4).
In the following experiments, the electrochemical degra-
dation of 4-chlorophenol was investigated in an undivided
electrolysis system, fed with air, using Pd/graphene gas-
diffusion cathodes.
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3.3. Electrochemical Oxidation of 4-Chlorophenol
Copyright: American Scientific Publishers
3.2. Hydroxyl Radical Electrogenerated on the
Changes in the UV absorption spectrum of 4-chlorophenol
Pd/Graphene Gas-Diffusion Electrode System
The Ti/IrO2/RuO2 anode was used herein. A direct
anodic electrochemical oxidation process, by MOx(OH•)
or MOx+1 produced on the anode surface, was involved in
the degradation of 4-chlorophenol on the anode.49 Further-
more, the gas-diffusion cathode catalyzed the two-electron
reduction of O2 to H2O2, which may be further converted
to HO•:50
with electrolysis time (0, 20, 40, 60, 80, 100, and
120 min) are shown in Figure 5. The result indicated that
4-chlorophenol has significant UV absorption at 225 nm
and 280 nm. The spectrum maximums of 4-chlorophenol
A
1.0
0.8
0.6
0.4
0.2
0.0
A 0 min
H2O2 +e− → OH− +HO•
(2)
B 20 min
C 40 min
D 60 min
E 80 min
F 100 min
G 120 min
B
C
The ESR was used to quantitatively determine the free
radicals in an undivided electrolysis device. Figure 4
shows the change of the HO•’s relative intensity-
concentration with the electrolysis time, as well as the typ-
ical ESR spectrum obtained after 15 min electrolysis in the
DMPO solution (inset). The spectrum was composed of
quartet lines with a peak height ratio of 1:2:2:1. The ESR
parameters (hyperfine constants aN = aH = 14ꢃ0 G and
g-value = 2ꢃ0065) coincided with those of the DMPO-OH•
adduct, as demonstrated previously,51 confirming that the
quartet signal was DMPO-OH• adduct. The adduct peak in
the Pd/graphene catalyst modified electrode system can be
attributed to the presence of HO•. The HO• concentration
matched the intensity of the adduct peak. The HO• con-
centration rose as the electrolysis time increased, reach-
ing its maximum after 25 min of electrolysis. This trend
indicated that the Pd/graphene catalyst in the Pd/graphene
D
E
F
G
200
250
300
350
400
450
Wavelength (nm)
Figure 5. The analysis of 4-chlorophenol by UV scanning at different
electrolysis times. Concentration of 4-chlorophenol: 100 mg L−1; concen-
tration of supporting electrolyte (Na2SO4ꢅ: 0.05 mol L−1; current density:
39 mA cm−2
.
7282
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Bian et al.
Synthesis of Pd Nanoparticles Decorated with Graphene and Their Application
100
80
60
40
20
0
30
25
20
15
10
5
100
(a)
100
80
60
40
20
0
80
60
40
20
4-Chlorophenol
Hydroquinone
Benzoquinone
Removal
TOC
Cl–
0
0
20
40
60
80
100
120
0
0
Time (min)
20
40
60
80
100
120
Time (min)
(b)
Formic acid
Acetic acid
Malonic acid
Succinic acid
Maleic acid
Fumaric acid
35
30
25
20
15
10
5
Figure 6. Removal of 4-chlorophenol and variations of TOC with elec-
trolysis times by the Pd/C gas-diffusion electrode system. Concentration
of 4-chlorophenol: 100 mg L−1; concentration of supporting electrolyte
(Na2SO4ꢅ: 0.05 mol L−1; current density: 39 mA cm−2
.
at 225 nm and 280 nm were reduced as the electrol-
ysis time decreased. This reduction indicated that the
concentration of 4-chlorophenol in the Pd/graphene gas-
diffusion electrode system decreased as the electrolysis
time increased. There are no obvious absorption peaks
after 120 min of electrolysis. The absorption peak for
benzoquinone (245 nm) appeared during the electrolytic
process. The color of the electrolyte in the anodic compart-
ment turned from colorless to yellow, which demonstrated
that the benzoquinone was accumulated during the elec-
0
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0
20
40
60
80
100
120
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Time (min)
Copyright: American Scientific Publishers
trolytic process. The degradation of 4-chlorophenol was
attributed to the cooperative oxidation processes, includ-
ing direct and/or indirect electrochemical oxidation at the
anode and H2O2 and HO• produced by the reduction of
oxygen at the cathode.
Figure 7. Change of the intermediate concentrations during the electro-
chemical oxidation of 4-chlorophenol. Concentration of 4-chlorophenol:
100 mg L−1
;
concentration of supporting electrolyte (Na2SO4):
0.05 mol L−1; current density: 39 mA cm−2
.
Figure 6 shows the removal efficiency of 4-chlorophenol
and TOC, and the electrolysis time in the Pd/graphene
gas-diffusion electrode system. The removal efficiency of
4-chlorophenol increased from 35.3% to 93.3% when the
electrolysis time was extended from 20 min to 120 min.
However, the TOC removal efficiency varied from 31.9%
to 85.1% during the same period. During the electrolysis
process, the total TOC removal was lower than the corre-
sponding degradation fraction of the target 4-chlorophenol.
This trend indicated that most of the 4-chlorophenol was
oxidized to intermediates, which were not further oxidized
to CO2 and H2O.
increased as the electrolysis time increased. The concen-
tration of chloride ions reached its maximum after 80 min,
and the degree of dechlorination reached approximately
90.5%. As the electrolysis time increased, the concen-
tration of chloride ions decreased. This trend indicated
that the chloride ion was already oxidized to Cl2 at the
anode. Similar findings were reported for the electrolysis
of chlorophene with a BDD anode,52 and Cl2 was imme-
diately driven away from the reaction system.53
These intermediates were unequivocally identified by
comparing their retention times and UV-Vis spectra with
those of pure compounds. Figure 7 shows the evolution
of the soluble species detected during the degradation of
4-chlorophenol. A fast formation and destruction of the
main intermediates can be observed. Figure 7(a) shows
that the concentration of the primary product of hydro-
quinone, benzoquinone, increases in the first 60 min and
then begins to decrease until its complete removal by
the 120 min marker. The hydroxylated derivative benzo-
quinone reached its peak concentration at approximately
80 min and then fell to below its detection limit at
3.4. Identification of Intermediates
and Their Evolution
The changes in the concentration of chloride ions
with electrolysis time during the degradation process
of 4-chlorophenol throughout the electrolysis process
are illustrated in Figure 7(a). The initial solutions
of 4-chlorophenol contained the dissociative chloride
ion. The chloride ion concentration of 4-chlorophenol
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Synthesis of Pd Nanoparticles Decorated with Graphene and Their Application
Bian et al.
120 min. Through the HPLC and IC analyses, the main
intermediates after 60 min were aliphatic carboxylic acids
(maleic, fumaric, succinic, malonic, acetic, and formic
acids). As shown in Figure 7(b), maleic, fumaric and
succinic acids attained their maximum concentrations at
80 min and were quickly degraded to malonic, oxalic and
formic acids, respectively. After 120 min, most of the
aliphatic carboxylic acids were transformed into carbon
dioxide and water. These results indicate that all of the
aromatic intermediates were transformed into carboxylic
acids because they reacted rapidly with the MOx(OH•) or
MOx+1 produced on the anode surface and the H2O2 and
HO• produced by the reduction of oxygen at the cathode.
Figure 8 presents a pathway for the degradation of
4-chlorophenol by the Pd/graphene gas-diffusion electrode
system. The degradation pathway is proposed as follows:
an HO• attacks at the C(4)-position of 4-chlorophenol
yields hydroquinone with the loss of Cl−. Hydroquinone
is subsequently dehydrogenated to benzoquinone. The oxi-
dation of benzoquinone, after the ring opening, leads to
the formation of aliphatic carboxylic acids such as maleic
acid and fumaric acid, which are degraded to succinic and
malonic acid and then form acetic and formic acid. The
final products are carbon dioxide and water. The degrada-
tion reaction of 4-chlorophenol in the electrolysis device is
believed to be cooperative oxidation, by direct or indirect
electrochemical oxidation at the anode and HO−, HO• pro-
duced by oxygen reduction at the cathode. Because of its
to degrade 4-chlorophenol to the final products of CO2 and
water. It may be more worthwhile to treat 4-chlorophenol
to its biodegradable stage-aliphatic carboxylic acids and
then seek an economically viable biological process.
4. CONCLUSIONS
A Pd/graphene gas-diffusion cathode that generated
H2O2by a two-electron reduction of oxygen gas fed to the
cathode was prepared and used to degrade 4-chlorophenol
in an undivided electrolysis device. In the Pd/graphene
catalyst, Pd particles with an average size of 3.6 nm
were highly dispersed in the graphene with an amorphous
structure. This characteristic indicates that the Pd/C cata-
lyst in the Pd/C gas-diffusion electrode system accelerated
the two-electron reduction of O2 to H2O2 in the pres-
ence of air, which favors the production of HO•. In the
Pd/graphene gas-diffusion electrode system, the dechlo-
rination degree of 4-chlorophenol reached approximately
90.5% after 80 min, and the removal efficiency and the
average removal efficiency of 4-chlorophenol, in terms
of total organic carbon (TOC) after 120 min, reached
approximately 93.3% and 85.1%, respectively. The main
intermediate of the 4-chlorophenol dechlorination was
determined to be hydroquinone. Hydroquinone was sub-
sequently dehydrogenated to benzoquinone. The further
oxidation of benzoquinone, after ring cleavage, led to the
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formation of aliphatic carboxylic acids, such as maleic,
IP: 50.51.96.170 On: Thu, 22 Oct 2015 09:43:06
fumaric, succinic, malonic, acetic and formic acids.
low current efficiency in practical use, it was not necessary
Copyright: American Scientific Publishers
Acknowledgments: This work was supported by the
Fundamental Research Funds for the Central Universities,
the National Natural Science Foundation of China (Grant
51278053 and 21373032), the Program for New Century
Excellent Talents in University (No. NCET-10-0233), and
the Beijing Natural Science Foundation (No. 8122031),
which are greatly appreciated.
OH
OH
O
HO
HO
Cl
OH
O
HO
COOH
HOOC
References and Notes
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Received: 23 November 2012. Accepted: 21 December 2012.
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