Modified carbon felt made using CexA1-xO2 composites as a cathode in electro-Fenton system to degrade ciprofloxacin

Article abstract of DOI:10.1039/c7ra03302h

Carbon felt (CF) was modified by Ce_xA_1-xO₂ (A = Zr, Cu and Ni) and the role of these Ce_xA_1-xO₂/CF (A = Zr, Cu and Ni) cathode materials in the oxidative degradation of antibiotic ciprofloxacin (CIP) was investigated in the electro-Fenton system. SEM, BET, TEM, XPS, TOC, HPLC-MS, CV and EIS were used to understand the characteristics of these cathode materials. The CIP degradation efficiency of the Ce_xA_1-xO₂/CF (A = Zr, Cu and Ni) cathode was better than that of pure CF, and 2.0 wt% Ce_0.75Zr_0.25O₂/CF was the most effective cathode material for degradation of CIP. It reached 100% degradation efficiency of CIP after 1 h and almost total mineralization (97.45%) after 6 h, owing to synergistic effects from the predominant role of homogeneous OH produced electrocatalytically through the Fenton reaction of Fe²⁺ and H₂O₂ and heterogeneous OH produced via the Fenton-like reaction of Ce³⁺ and H₂O₂ and the electrocatalytic activity of Ce_0.75Zr_0.25O₂ composite and the strong electrosorption of carbon felt. A possible path of CIP degradation was proposed in the paper.

Full text of DOI:10.1039/c7ra03302h

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PAPER
Modified carbon felt made using Ce A O
x 1ꢀx 2
composites as a cathode in electro-Fenton system
Cite this: RSC Adv., 2017, 7, 27065
to degrade ciprofloxacin†
a
a
b
c
a
a
Yi Li, * Jingjing Han, Xueyue Mi, Xinqiang Mi, Yanan Li, Suge Zhang
b
and Sihui Zhan
Carbon felt (CF) was modified by Ce
x
A
1ꢀx
O
2
(A ¼ Zr, Cu and Ni) and the role of these Ce
x
A
1ꢀx
O
2
/CF (A ¼ Zr,
Cu and Ni) cathode materials in the oxidative degradation of antibiotic ciprofloxacin (CIP) was investigated
in the electro-Fenton system. SEM, BET, TEM, XPS, TOC, HPLC-MS, CV and EIS were used to understand the
characteristics of these cathode materials. The CIP degradation efficiency of the Ce
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu
and Ni) cathode was better than that of pure CF, and 2.0 wt% Ce0.75Zr0.25 /CF was the most effective
O
2
cathode material for degradation of CIP. It reached 100% degradation efficiency of CIP after 1 h and
almost total mineralization (97.45%) after 6 h, owing to synergistic effects from the predominant role of
2+
Received 21st March 2017
Accepted 13th May 2017
2 2
homogeneous cOH produced electrocatalytically through the Fenton reaction of Fe and H O and
3+
heterogeneous cOH produced via the Fenton-like reaction of Ce and H
activity of Ce0.75Zr0.25 composite and the strong electrosorption of carbon felt. A possible path of CIP
degradation was proposed in the paper.
2 2
O and the electrocatalytic
DOI: 10.1039/c7ra03302h
O
2
rsc.li/rsc-advances
friendliness and computability and widespread applica-
1
Introduction
14–17
tions.
In the last few years, environmental pollution has been paid chemistry is the electro-Fenton (EF) process.
much attention in the world, and the treatment of antibiotic cathode material is crucial to the EF system. At present, it has
Recently, the most common EAOP based on Fenton
18,19
An efficient
1–3
wastewater is one of the major problems. Ciprooxacin (CIP) been reported that the cathode materials with excellent
2
0–22
is a uoroquinolone antibiotic and its concentration has been performance are gas diffusion electrode,
graphite elec-
ꢀ
1
23
24
detected up to 31 mg L in waste effluents from pharmaceu- trode and three-dimensional porous electrode. The biggest
4,5
tical manufacturers. And ciprooxacin is more difficult for advantage of the three-dimensional porous electrode is the
microorganisms to degrade and cannot be effectively removed larger specic surface area, which greatly increases the elec-
by conventional water treatment process. Therefore, it is trode reaction area, and thus increases the reaction rate.
6
important to nd an effective method for removing cipro- Carbon felt (CF), a kind of three-dimensional porous electrode,
2
5

oxacin from the environment.
Advanced oxidation processes (AOPs)
is oen used as electrode material in the EF process and all-
7–11
26
are based on in situ vanadium redox ow battery because of their low cost, high
production of hydroxyl radicals (cOH). As the second strongest stability, high conductivity, corrosion resistance, and high
1
2,13
oxidant aer uorine,
with persistent organic pollutants (POPs) and degrade them rial to load Ce
into CO , H
O and inorganic ions. Among AOPs, electro- using as cathode to degrade ciprooxacin by the electro-Fenton
chemical advanced oxidation processes (EAOPs) have received process.
much attention because of their efficiency in the destruction of
Over the past decades, cerium dioxide (CeO
toxic and/or bio-refractory organic pollutants, environmental extensive attention, which possesses considerable oxygen
cOH is able to non-selectively react surface area. So we choose carbon felt as the supporting mate-
composites (A ¼ transition metals) by
x
1ꢀx 2
A O
2
2
) has received
2
3
+
storage capacity owing to the redox cycling between Ce and
4
+
a
2
Ce . However, pure CeO shows poor stability and is prone to
Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of
Chemistry, School of Science, Tianjin University & Collaborative Innovation Center sintering at high temperature which leads to reducing the
27
of Chemical Science and Engineering (Tianjin), Tianjin 300072, China. E-mail: catalytic activity. In order to further improve the performance
liyi@tju.edu.cn
of oxygen storage, oxygen release and thermal stability, we
b
College of Environmental Science and Engineering, Nankai University, Tianjin
modied pure CeO . When the transition metal ions are doped
2
3
00071, China
into CeO2
producing CexA1ꢀxO2
(A ¼ transition metals)
c
Befar Group Corporation, Binzhou 256601, China
composites, the cerium ion will be replaced by the transition
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra03302h
metal ions which results in producing some lattice defects. This
1
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makes the migration channel of oxygen ions in the crystal 24 mg Ce(NO
3
)
3
$6H
2
O and 8 mg Zr(NO
3
)
4
$5H
2
O was dissolved
lattice become relatively large, which can effectively reduce the in 20 mL deionized water to prepare a 2.0 wt% precursor solu-
diffusion resistance of oxygen in the crystal lattice and improve tion. In order to get the best load capacity, 1.0, 1.5, 2.5, and 3.0
the ability of oxygen activity. And the conversion between wt% of Ce0.75Zr0.25O composites loaded on CF were investi-
2
different valence states of the transition metal ions is helpful to gated, respectively. According to the above description, the
form oxygen vacancies. Thus the electro-catalytic activity of Ce0.75Zr0.25
CeO is improved. Ce composites can be used for the aration of the Ce0.90Cu0.10
treatment of automobile tail gas, the transformation of vana- Ce0.85Ni0.15
O
2
/CF composite electrodes were synthesized. Prep-
/CF composite electrodes and the
2
/CF composite electrodes employed the same
2
x
A
1ꢀx
O
2
O
2
28
O
2
9
dium lithium battery and the degradation of antibiotics of method. However, the load capacity on CF was 1.0, 1.5, 2.0, 2.5,
carbon or graphite felt loaded iron oxide as cathode by EF and 3.0 wt% of Ce0.90Cu0.10O composite, respectively. The load
2
30–32
system.
However, it is not common that CexA O /CF capacity of Ce0.85Ni0.15O composite on CF was 0.14, 0.30, 0.45
1ꢀx 2 2
composites are used as cathode to degrade antibiotics in the EF and 0.60 wt%, respectively. CF without modication by Ce -
x
ꢂ
process.
In this paper, Ce
were obtained by impregnating the CF into Ce
solution and then dried and calcined. And the carbon felt
modied by Ce
A
1ꢀx
O
2
was also heated to 500 C for 5 h for comparison.
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu and Ni) composites
solid
x
1ꢀx 2
A O
2.3 Electrolytic system
The EF degradation of CIP was conducted in a cylindrical glass
x
A
1ꢀx
O
2
(A ¼ Zr, Cu and Ni) composites was used
ꢂ
cell of 250 mL capacity at room temperature (23 ꢃ 2 C) and pH
as cathode to degrade CIP in EF system, respectively. Compared
ꢀ1
¼
3.0. The aqueous solution was 50 mg L CIP solution and
to the pure CeO , Ce A O /CF (A ¼ Zr, Cu and Ni) composites
2
x
1ꢀx
2
ꢀ1
2+
ꢀ1
contained 0.1 mmol L Fe and 0.05 mol L Na
The solution was continuously saturated by bubbling
compressed O at the rate of 100 mL min . The distance
2
2 4
SO solution.
have many superior aspects: (1) Ce Zr O , Ce Cu
O
0.10 2
0
.75 0.25
2
0.90
and Ce0.85Ni0.15
O
2
catalysts have more oxygen vacancies and
higher oxygen storage capacity (OSC). (2) The electrocatalytic
activities of Ce0.75Zr0.25
composites are stronger than that of pure CeO
ꢀ1
between the Pt plate (1 cm ꢁ 0.5 cm) as anode and Ce
x 1ꢀx 2
A O /
2 2 2
O , Ce0.90Cu0.10O and Ce0.85Ni0.15O
CF composites as cathode was 1.0 cm. There was a magnetic
mixer to ensure the uniformity of the electrolysis during the EF
process.
2
. (3) CF has
strong electrosorption behavior. Above synergies gained the
efficient degradation of CIP. And intermediates of CIP degra-
dation were identied by using HPLC-MS analysis to propose
a reasonable mineralization pathway.
2.4 Analytical procedures
Electrochemical activities of Ce
x
1ꢀx 2
A O /CF composite elec-
trodes were evaluated by using cyclic voltammetry (CV) and
electrochemical impedance spectroscopy (EIS) in a traditional
2
Experimental
2.1 Chemicals
1ꢀx 2
three-electrode cell. CexA O /CF composites were the working
electrodes. Pt plate and saturated calomel electrode (SCE) were
used as the counter electrode and the reference electrode,
respectively.
The concentration of CIP at a certain time was detected by
reversed-phase high performance liquid chromatography
All chemicals are analytical grade and used without further
purication. Ciprooxacin, C H FN O ($98% purity), was
1
7
18
3 3
purchased from Alfa Aesar, UK. Na
Ce(NO $6H O, Zr(NO $5H O, Cu(NO
Ni(NO $6H O were purchased from Shanghai Aladdin Indus-
2
SO
4
,
FeSO
4
$7H
2
O,
3
)
3
2
3
)
4
2
3
)
2
$3H
2
O and
3
)
2
2
(HPLC) using a Merck Lachrom liquid chromatograph equip-
trial Co. Methanol and phosphoric acid used in preparation of
HPLC eluents were obtained from Sigma-Aldrich.
ped with a L-7100 pump, tted with a Purospher RP-18, 5 mm,
ꢂ
2
7
5 cm ꢁ 4.6 mm (i.d.) column at 40 C, and coupled with a L-
455 photodiode array detector selected at optimum wave-
2
.2 Preparation of the Ce
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu and Ni)
lengths of 250 nm. The mobile phase was a methanol/
composite electrodes
ꢀ1
0
.06 mmol L
3 4
H PO 55 : 65 (v/v). A ow rate was 0.8 mL
ꢀ
1
The Ce
x 1ꢀx 2
A O /CF composites were prepared by impregnating min . Samples of 20 mL were injected into the HPLC and the
the CF into Ce A O2 solid solution and then dried and measurements were controlled through EZ-Chrom Elite 3.1
x
1ꢀx
33
calcined. Before the experiment, the CF should be treated with soware.
34
acid to enhance its wettability and eliminate impurities. The
The formed intermediates of CIP degradation to get
tri-dimensional carbon-felt (6 cm ꢁ 3 cm each side, 0.5 cm in a reasonable mechanism were identied by the high perfor-
ꢂ
width-Carbone Lorraine) was soaked in 10% H
2
O
2
at 90 C for mance liquid chromatography-mass spectrometry (HPLC-MS,
ꢂ
3
h and then marinated in 10% HCl at 90 C for 1 h. At last, it Waters XEVO TQS analyzer) connected with ESI source. The
ꢂ
was rinsed with distilled water and dried at 60 C for overnight. Waters Acquity UPLC BEH C18 column (2.1 mm ꢁ 1.8 mm, 1.7
ꢂ
CF was immersed into Ce(NO
3
)
3
$6H
2
O and Zr(NO
3
)
4
$5H
2
O
mm) was employed. The temperature of the column was 40 C. A
solutions (quality ratio of 3 : 1) accompanied by addition of gradient elution was applied using the mobile phase formed by
NH $H O diluted solutions until the pH was 10 and the solu- 0.5% methane acid aqueous solution and acetonitrile
3
2
ꢀ1
tions went through ultrasound for half an hour. Subsequently, (80% : 20%) at the ow rate of 0.5 mL min , 10 min run. The
the sample was dried at 70 C for 12 h and then heated to 500 C injection volume was 20 mL and UV detector was set up at
for 5 h under a ow of N . When the quantity of CF was 1 g, 276 nm. ESI source conditions were as follows: capillary voltage,
ꢂ
ꢂ
2
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sample cone voltage and extraction cone voltage were 2000, 45
and 4 V, respectively; the source block and desolvation
ꢂ
temperatures were 100 and 250 C, respectively and the des-
ꢀ
1
olvation and nebulizer gas (N ) ow rate was 600 L h . Total
2
compounds mass spectra were obtained in the positive ion
mode between m/z 100 and 400.
Scanning electron microscopy (SEM) images and mapping
analysis were carried out on SU8010 eld emission to obtain the
x 1ꢀx 2
microstructures and the composition of Ce A O /CF
composite electrodes. Transmission Electron Microscope (TEM)
characterized the morphology and structure of the product. The
nitrogen adsorption–desorption isotherm was measured at 77 K
using the surface area and pore-size analyzer (Quantachrome
Autosorb iQ-MP). The total surface area was calculated from the
Brunauer–Emmett–Teller (BET) method and the pore size
distribution data was calculated using the density functional
theory (DFT) method based on the adsorption and desorption
data. X-ray Photoelectron Spectroscopy (XPS) was used ESCA X
PHI-1600 ray photoelectron spectroscopy of the United States
PE company to obtain the data of the binding energies of Ce 3d,
Zr 3d, Cu 2p, Ni 2p, C 1s and O 1s, and 284.6 eV was the
referencing of C 1s peak. The removal of the total organic
carbon (TOC, TOC-VCPH, Japan) can assess the mineralization
degree of treated solutions.
3
Results and discussion
3
.1 Characterizations of the CexA O /CF composite
1ꢀx 2
electrodes
3.1.1 SEM and mapping analysis. The surface morphol-
ogies of CF and CexA O /CF (A ¼ Zr, Cu and Ni) composites
1ꢀx
2
are characterized by SEM. As shown in Fig. 1a, we can see that
the surface of the carbon bers becomes excessively rough and
has many grooves aer the acid treatment. It can improve the
specic surface area of the electrode and thus increase the
active surface area, and further promote the oxygen reduction
reaction. As shown in Fig. 1b–d, Ce
nanoparticles are randomly loaded on the surface of the CF
bers. Fig. 1e–p presents the mapping analysis of 2.0 wt%
/CF composite, 2.5 wt% Ce0.90Cu0.10 /CF
composite and 0.45 wt% Ce0.85Ni0.15 /CF composite, respec-
tively. From Fig. 1e–h, C, Ce, Zr and O evenly disperses on the
scanning area, which indicates excellent distribution of CeO
and ZrO on CF. For Ce0.90Cu0.10 /CF composite, we can see C,
x
A
1ꢀx
O
2
(A ¼ Zr, Cu and Ni)

Ce0.75Zr0.25
O
2
O
2
O
2
2
2
O
2
Ce, Cu and O also disperses on the scanning area from Fig. 1i–l.
The corresponding elemental mapping of Ce0.85Ni0.15O /CF
2
composite shows uniform spatial element distribution of C, Ce,
Ni and O from Fig. 1m–p. By SEM and the mapping analysis,
carbon ber in carbon felt was successfully modied by
Ce0.75Zr0.25
O
2
, Ce0.90Cu0.10
O
2
and Ce0.85Ni0.15
O
2
, respectively.
.1.2 TEM analysis. As shown in Fig. 2, the morphologies
and structure of the 2.0 wt% Ce0.75Zr0.25 /CF composite, 2.5
wt% Ce0.90Cu0.10 /CF composite and 0.45 wt% Ce0.85Ni0.15
CF composite are investigated by HR-TEM. HR-TEM images
3
O
2
O
2
2
O /
Fig. 1 SEM images at different magnifications and mapping analysis of
CF (a), 2.0 wt% Ce0.75Zr0.25O /CF (b, e, f, g, h), 2.5 wt% Ce0.90Cu0.10O /
2 2
indicate that Ce0.75Zr0.25O , Ce0.90Cu0.10O2 and Ce0.85Ni0.15O2 CF (c, i, j, k, l), 0.45 wt% Ce0.85Ni0.15
2
O /CF (d, m, n, o, p).
2
nanoparticles have good crystallinity. From Fig. 2a, the lattice
fringes of 0.307 nm were assigned to the (1 1 1) plane of
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2
Fig. 2 HR-TEM images of 2.0 wt% Ce0.75Zr0.25O /CF composite (a),
2
2.5 wt% Ce0.90Cu0.10O /CF composite (b) and 0.45 wt%
Ce0.85Ni0.15 /CF composite (c).
O
2
35,36
Ce0.75Zr0.25
O
2
.
It was shown in Fig. 2b, the most frequently
observed reection with a d spacing value of 0.314 nm corre-
sponds to crystal lattice plane (1 1 1) of CeO . The reection with
a spacing value of 0.252 nm, attributable to the lattice plane (1 1
) of CuO, can also be identied. Fig. 2c showed lattice fringes
of CeO (d ¼ 0.314 nm and 0.270 nm) correspond to (1 1 1) and
2
1
2
2
(1 0 0) lattice plane of CeO . However, HR-TEM image did not
reveal any evidence of NiO particles which indicated that the
NiO crystallites were highly dispersed and the particle size was
37
too small not to be observed.
.1.3 XPS analysis. To understand the elemental composi-
tion and functional groups on CF and the CexA O /CF
3
1ꢀx
2
composite, the surface chemistry of the electrodes was investi-
gated by X-ray photoelectron spectroscopy (XPS) measurement.
Fig. 3 shows the XPS spectra of C 1s for different electrode
38
composites. On the basis of the reported literature, the main
peak at 284.7 eV is assigned to the graphitized carbon and the
rest of the peaks correspond to, the defective carbon (285.3 eV),
–
OH groups (286.0 eV) and C]O groups (287.0 eV). And we nd
that there is an irregular peak in 282.0 eV corresponding to
39
C–O–C groups. This may be due to the introduction of func-
tional groups in the treatment of CF.
The XPS spectra of O 1s for electrode composites are given in
Fig. 4. The O 1s XPS spectra are scientically resolved into two
XPS peaks centered at binding energies of 529.8 and 532.0 eV.
2
ꢀ
ꢀ
The main peak at 532.0 eV has been assigned for O
2
and O
ion deciencies in the subsurface layer of metal oxides due to
the formation of Ce0.75Zr0.25O2 solid solution, Ce0.90Cu0.10O2
Fig. 3 C 1s spectra of CF (a), 2.0 wt% Ce0.75Zr0.25
.5 wt% Ce0.90Cu0.10
Ce0.85Ni0.15 /CF composite (d).
O
2
/CF composite (b),
40–42
solid solution and Ce0.85Ni0.15O solid solution.
From Fig. 4,
we also can see that the other peak at 529.8 eV has been
assigned to the lattice oxygen in Ce0.75Zr0.25
2
2
2
O /CF composite (c) and 0.45 wt%
O
2
2 2
O , Ce0.90Cu0.10O
species). As shown in Table 1, the
2
ꢀ
and Ce0.85Ni0.15
O
2
(O
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x 1ꢀx 2 x 1ꢀx 2
Table 1 XPS analysis results of Ce A O composite and Ce A O /
CF composite
Cathode material
O
ads
O
Latt
Ce0.75Zr0.25
Ce0.90Cu0.10
Ce0.85Ni0.15
O
O
O
2
17.64%
41.50%
35.80%
78.33%
72.50%
73.52%
82.36%
58.50%
64.20%
21.67%
27.50%
26.48%
2
2
Ce0.75Zr0.25
2
O /CF
Ce0.90Cu0.10
Ce0.85Ni0.15O
2
O
2
/CF
/CF
surface adsorption oxygen contents of Ce0.75Zr0.25
Ce0.90Cu0.10O2 and Ce0.85Ni0.15O2 are 17.64%, 41.50% and
5.80%, respectively. And the surface lattice oxygen contents of
Ce0.75Zr0.25O , Ce0.90Cu0.10O2 and Ce0.85Ni0.15O2 are 82.36%,
2
O ,
3
2
5
8.50% and 64.20%, respectively. However, when the Ce
composite is loaded on the CF, the surface adsorption oxygen
contents of /CF, /CF and
/CF are 78.33%, 72.50% and 73.52%, respectively.
And the surface lattice oxygen contents of Ce0.75Zr0.25 /CF,
Ce0.90Cu0.10O /CF and Ce0.85Ni0.15O /CF are 21.67%, 27.50%
x 1ꢀx 2
A O
Ce0.75Zr0.25
O
2
Ce0.90Cu0.10O
2
Ce0.85Ni0.15O
2
O
2
2
2
and 26.48%, respectively. It is obvious that the surface adsorp-
tion oxygen contents of CexA O /CF composite is more than
1ꢀx
2
that of Ce
x
A
1ꢀx
O
2
composite. We can also see that
/CF and Ce0.85Ni0.15 /CF have similar adsorp-
tion oxygen and lattice oxygen contents. However,
/CF has higher adsorption oxygen content than
Ce0.90Cu0.10
O
2
O
2
Ce0.75Zr0.25O
2
that of other materials. The generation of rich adsorption
oxygen can be mainly attributed to the formation of oxygen
vacancies resulting from ZrO cooperation into the CeO lattice,
2
2
which can provide superior conditions for the adsorption and
activation of oxygen molecules and promote the formation of
adsorption oxygen species on the surface of Ce0.75Zr0.25O
2
catalysts. The results of O 1s XPS analysis indicate that the
existence of a large amount of adsorption oxygen on the catalyst
surface can produce abundant active oxygen species that
contributes to the high activity of the catalysts.
Fig. 5 shows XPS spectra of Ce 3d, Zr 3d, Cu 2p and Ni 2p for
CexA O /CF (A ¼ Zr, Cu and Ni) composites, respectively.
1ꢀx
2
Fig. 5a corresponds to Ce 3d XPS spectra of the prepared
samples at 875–925 eV. The Ce 3d XPS spectra exhibits eight
obvious XPS peaks at binding energies of about 882.4, 884.3,
8
3
87.7, 898.6, 901.5, 903.4, 905.8, and 916.8 eV. The four main
5/2 features at 882.4, 884.3, 887.7, and 898.6 eV correspond
d
to V, V , V and V components, while the 3d features at 901.3,
1
2
3
3/2
9
03.4, 905.8, and 916.8 eV correspond to U, U , U and U3
1 2
43
components, respectively. The presence of the ngerprint U
satellite peak at about 916.8 eV conrms the Ce oxidation
state. The appearance of characteristic bands of Ce labeled V
at 884.3 eV and U at 903.5 eV suggests that both oxidation
3
4
+
3
+
1
1
3+
4
+
states Ce and Ce co-exist on the surfaces of Ce
composites. As shown in Fig. 5b, the two peaks approximately at
82.5 eV and 184.9 eV correspond to the Zr 3d and 3d3/2 spin–
x 1ꢀx 2
A O /CF
Fig. 4 O 1s spectra of 2.0 wt% Ce0.75Zr0.25
O
2
/CF composite (a), 2.5
/CF
composite (c), the inserts in (a), (b) and (c) are O 1s spectra of
2 2
wt% Ce0.90Cu0.10O /CF composite (b) and 0.45 wt% Ce0.85Ni0.15O
1
5
/2
44
orbit states, respectively. Fig. 5c presents the Cu 2p XPS
Ce0.75Zr0.25
Ce0.85Ni0.15
O
O
2
composite,
composite, respectively.
Ce0.90Cu0.10
O
2
composite
and
spectra of Ce0.90Cu0.10O /CF composite. Three prominent XPS
2
2
peaks can be observed at binding energies of 934.3, 941.5 and
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9
9
54.5 eV, respectively. The XPS peaks centered at 934.3 and
54.5 eV correspond to Cu 2p3/2 and Cu 2p1/2, respectively. The
peak at 941.5 is satellites arising from Cu 2p1/2. The satellite
centered at about 939–944 eV and at a high Cu 2p binding
3
/2
2
+
45
Cu content on the surface of the catalysts. Fig. 5d shows Ni
/CF composite. The main peak
centered at 855.6 eV assigns to Ni species. In addition, a broad
peak at 861.8 eV is assigned as the shake-up satellite peak of
2p3/2 for the Ce0.85Ni0.15O
2
2
+
2
+
3+
46
2 3
Ni (NiO) and Ni (Ni O ).
To further identify the accurate element composition, the
XPS wide scan spectra in the binding energy range of 0–1200 eV
was performed on CF, 2.0 wt% Ce0.75Zr0.25O /CF composite, 2.5
2
wt% Ce0.90Cu0.10
O
2
/CF composite and 0.45 wt% Ce0.85Ni0.15
CF composite as shown in Fig. S1.† As shown in Fig. S1,† it can
be found that Ce0.75Zr0.25
nanoparticles successfully loaded on CF, respectively. And the
atomic oxygen content on the surface increases with Ce
2
O /
2 2 2
O , Ce0.90Cu0.10O and Ce0.85Ni0.15O
x
1ꢀx 2
A O
(
1
A ¼ Zr, Cu and Ni) composite load on CF (from 8.24% for CF to
3.49%, 12.71% and 13.15% for Ce Zr O /CF,
0.75 0.25 2
Ce0.90Cu0.10O /CF and Ce0.85Ni0.15O /CF composite). Moreover,
2
2
it revealed that the doping of Zr, Cu and Ni elements can
improve the catalytic properties of CeO
.1.4 CV and EIS analysis. The electrochemical perfor-
mance of the working electrode used for the CV and EIS tests is
shown in Fig. 6. CV curves of Ce
(A ¼ Zr, Cu and Ni)
composite, Ce A O /CF composite and CF electrodes at a scan
2
.
3
x 1ꢀx 2
A O
x
ꢀ
1ꢀx
2
1
ꢀ1
ꢀ1
2+
rate of 0.04 V s in 50 mg L CIP with 0.1 mmol L Fe and
ꢀ
1
0
.05 mol L Na SO at pH ¼ 3 solution are compared in Fig. 6a.
2
4
We can see that the redox peak current values of Ce
A ¼ Zr, Cu and Ni) composite and CF electrodes are three
orders of magnitude higher than that of Ce composite.
When Ce
x 1ꢀx 2
A O /CF
(
x 1ꢀx 2
A O
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu and Ni) composite and CF are
respectively used as cathodes, it is obvious that it is a typical
irreversible process. The cathodic peak current density of CF is
ꢀ
2
0.0858 mA cm
,
while those of Ce0.75Zr0.25O /CF,
2
Ce0.90Cu0.10O /CF and Ce Ni0.15O /CF increases to 0.1082,
2
0.85
ꢀ
2
2
0
.1234 and 0.0863 mA cm , respectively. It can be seen that the
electric conductivity of Ce0.75Zr0.25 /CF and
Ce0.85Ni0.15 /CF is better than that of CF. Due to the higher
peak current density, Ce0.90Cu0.10 /CF displays the best elec-
2 2
O /CF, Ce0.90Cu0.10O
O
2
O
2
trochemical activity. But the difference of the peak current
density for these three electrodes is very small. To further
investigate the reaction kinetics, the relationship between the
peak current and the square root of scan rate is demonstrated in
ꢀ1
Fig. S2.† As shown in Fig. S2,† the peak current in 50 mg L CIP
ꢀ
1
2+
ꢀ1
with 0.1 mmol L Fe and 0.05 mol L Na
2
SO
4
at pH ¼ 3
solution is proved to be linear variation with the change of the
square root of scan rate. It reveals that the electrochemical
behavior for all electrodes is controlled by diffusion process.
And the higher slope value indicates a faster diffusion process.
Fig. S2† shows that Ce0.90Cu O /CF exhibits the best electro-
33
0.10
ꢀ
2
1
ꢀ1
2+
chemical activity in 50 mg L CIP with 0.1 mmol L Fe and
ꢀ
1
Fig. 5 XPS spectra of Ce 3d for Ce
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu and Ni) 0.05 mol L Na
/CF composite (b), Cu degradation efficiency of Ce0.90Cu0.10
/CF composite (c) and Ni 2p for 0.45 wt%
2
/CF composite (d).
2
SO
4
at pH ¼ 3 solution. However, the CIP
/CF composite as
cathode is smaller than that of Ce0.75Zr0.25 /CF and
/CF, which demonstrates the electrochemical
composites (a), Zr 3d for 2.0 wt% Ce0.75Zr0.25
p for 2.5 wt% Ce0.90Cu0.10
Ce0.85Ni0.15
O
2
O
2
2
O
2
O
2
O
Ce0.85Ni0.15O
2
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and Ce0.85Ni0.15
O
2
nanoparticles are well dispersed on CF,
respectively. And Ce0.90Cu0.10O /CF has the best electro-
2
chemical behavior. However, the difference of the semicircle
radius for these three electrodes is very small.
3
.2 Analysis of inuence factors on the oxidative
degradation of CIP
.2.1 Effect of electric eld on the degradation of CIP. To
evaluate the feasibility of CF, Ce0.75Zr0.25
CF and Ce0.85Ni0.15 /CF cathodes in EF system, the perfor-
3
2 2
O /CF, Ce0.90Cu0.10O /
O
2
mance of CIP degradation by the electro-Fenton, electro-
catalysis and electrosorption effects are tested as a ref. 47.
Fig. 7 shows that CIP is rapidly degraded by electro-Fenton
with CIP degradation efficiency of 83.03%, 100%, 93.07% and
9
5.90% aer 1 h by using CF, Ce0.75Zr0.25
CF and Ce0.85Ni0.15 /CF, respectively. As shown in Fig. 7a, the
degradation efficiency is 88.91% for electrosorption of CIP on
2 2
O /CF, Ce0.90Cu0.10O /
O
2
2
+
x 1ꢀx 2
CF (no Fe and no Ce A O adding) aer 2 h which shows CF
has a strong electrosorption behavior. The reason why there is
a strong electrosorption behavior is that the carbon felt has
ꢀ
1
large specic surface area and can produce 0.7054 mg L H O
2
2
as a cathode aer 2 h (from Fig. S4†). From Fig. 7b–d, we can see
the degradation efficiency is respectively 96.24%, 92.64% and
95.75% for electrocatalysis and electrosorption of CIP by using
the Ce0.75Zr025
electrodes (no Fe adding) which shows Ce
catalytic activity. The result is that 0.4235 mg L , 0.3748 mg L
and 0.3980 mg L
O
2
/CF, Ce0.90Cu0.10
O
2
/CF and Ce0.85Ni0.15 /CF
O
2
2
+
x
A
1ꢀx
O
2
has a strong
ꢀ
1
ꢀ1
ꢀ
1
2 2 2
H O are produced when Ce0.75Zr0.25O /CF,
Ce0.90Cu0.10O /CF and Ce0.85Ni0.15O /CF electrodes are individ-
2
2
ually used as cathode. And what's more, a Fenton-like reaction
3
+
between Ce and H
2 2
O may be happened. Therefore, syner-
Fig. 6 (a) Cyclic voltammetry of Ce
x
A
1ꢀx
O
2
(A ¼ Zr, Cu and Ni)
ꢀ1
gistic effects from the predominant role of the Fenton reaction
composite, Ce
CIP with 0.1 mmol L Fe and 0.05 mol L Na
solution, the scan rate was 0.04 V s , all scans were performed from
the negative potential to positive potential and the initial potential was
x
A
1ꢀx
O
2
/CF composite and CF electrodes in 50 mg L
2+
of Fe and H
like reaction of Ce and H
and the electrocatalytic activity of Ce
2 2
O forming homogeneous cOH and the Fenton-
ꢀ
1
2+
ꢀ1
SO
2 4
at pH ¼ 3
3
+
ꢀ1
2
O
2
producing heterogeneous cOH
(A ¼ Zr, Cu and Ni)
x 1ꢀx 2
A O
ꢀ
1.0 V. (b) Nyquist plots of Ce
x
A
1ꢀx
O
2
(A ¼ Zr, Cu and Ni) composite, composites and the strong adsorption of CF contribute to the
Ce /CF composite and CF electrodes under a potential of 0.8 V
x
A
1ꢀx
O
2
degradation efficiency of CIP in electro-Fenton system.
.2.2 Effect of CexA O /CF composites (A ¼ Zr, Cu and
ꢀ
1
ꢀ1
2+
ꢀ1
in 50 mg L CIP with 0.1 mmol L Fe and 0.05 mol L Na
SO
2 4
at
3
1ꢀx
2
pH ¼ 3 solution.
Ni) with different ratios of Ce and A on the degradation effi-
ciency of CIP. To conrm the optimal constitution of the Ce
/CF composites (A ¼ Zr, Cu and Ni), quality ratios of Ce/A
were investigated through the degradation efficiency for CIP at
x
-
A
1ꢀx 2
O
behaviors of Ce
to degrade CIP solution.
Fig. 6b shows the Nyquist plots of CexA O (A ¼ Zr, Cu and
x 1ꢀx 2
A O /CF composites are not the main factor
ꢀ1
the experiment condition of 400 mA, pH ¼ 3 and 0.1 mmol L
1ꢀx
2
2
+
Fe solution.
Ni) composite, Ce
x
A
1ꢀx
O
2
/CF composite and CF electrodes
ꢀ
1
ꢀ1
From Fig. 8a, it can be seen that with the increase of Zr
doping amount, the CIP degradation efficiency increased and
then decreased when the load capacity of Ce1ꢀxZr O (the Zr
x 2
loading, x wt%) was xed at 2.0 wt%. The degradation effi-
ciencies of CIP for Ce/Zr quality ratio of 0.85 : 0.15, 0.75 : 0.25
and 0.65 : 0.35 were 94.78%, 100% and 92.66% aer 2 h,
respectively. So the Ce/Zr ratio of 0.75 : 0.25 exhibited the
under a potential of 0.8 V in 50 mg L CIP with 0.1 mmol L
2
+
ꢀ1
Fe and 0.05 mol L Na
2
SO
4
at pH ¼ 3 solution. It can be
found that there is a semicircular part at high frequency which
represents the charge transfer process. And the smaller of the
radius of the semicircle, the better electrochemical activity of
electrode and the faster of electron transfer rate. It can be seen
that the electrochemical performance of CF is much better than
highest
degradation
efficiency,
which
demonstrates
that of Ce0.75Zr0.25
electrochemical performance of 2.0 wt% Ce0.75Zr0.25
wt% Ce0.90Cu0.10 /CF and 0.45 wt% Ce0.85Ni0.15 /CF is better
O
2
, Ce0.90Cu0.10
O
2
and Ce0.85Ni0.15
O
2
. And the
Ce0.75Zr0.25O2 composite having a large amount of oxygen
vacancy and strong electrocatalytic activity. As shown in Fig. 8b,
it was found that the degradation efficiency for CIP increased
O
2
/CF, 2.5
O
2
O
2
2 2
than that of CF. This is because Ce0.75Zr0.25O , Ce0.90Cu0.10O ,

rst and then decreased with the increase of the Cu doping
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Fig. 8 Degradation efficiency of CIP in the presence of Ce
x 1ꢀx 2
A O /CF
composites (A ¼ Cu and Ni) with different Ce/Zr (a), Ce/Cu (b) and Ce/
ꢀ1
2+
Ni ratios (c) at the experiment condition: pH ¼ 3, 0.1 mmol L Fe
ꢀ1
2 4
and 0.05 mol L Na SO solution.
amount when the load capacity of Ce1 Cu O (the Cu loading,
ꢀx
x 2
Fig. 7 Degradation efficiency of CIP by electro-Fenton, electro-
catalysis and electrosorption (a) CF, (b) Ce0.75Zr0.25O /CF, (c)
2
x wt%) was xed at 2.5 wt%. The CIP degradation efficiencies of
CIP for Ce/Cu quality ratio of 0.95 : 0.05, 0.90 : 0.10 and
0.85 : 0.15 were 86.35%, 95.70% and 87.84% aer 2 h, respec-
tively. So the Ce/Cu ratio of 0.90 : 0.10 exhibited the highest
Ce0.90Cu0.10
pH ¼ 3, 0.1 mmol L Fe and 0.05 mol L Na
O
2
/CF, (d) Ce0.85Ni0.15
O
2
/CF at the experiment condition:
ꢀ1
2+
ꢀ1
2 4
SO solution.
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degradation efficiency, which indicates Ce0.90Cu0.10O
2
composite has a large amount of oxygen vacancy and strong
electrocatalysis effect. When Ce1 Ni O (the Ni loading, x wt%)
ꢀx
x 2
load capacity was xed at 0.45 wt%, it could be seen that the CIP
degradation efficiency also increased rst and then decreased
with increase of the Ni doping amount from Fig. 8c. The CIP
degradation efficiencies of CIP for Ce/Ni quality ratio of
0.80 : 0.20, 0.85 : 0.15 and 0.90 : 0.10 were 90.77%, 97.33% and
93.40% aer 2 h, respectively. Apparently, the Ce/Ni ratio of
0.85 : 0.15 exhibited the highest degradation efficiency which
showed Ce0.85Ni0.15O composite having large amount of oxygen
2
vacancy and strong electrocatalytic activity.
3
.2.3 Effect of Ce
dation of CIP. To research the effect of Ce
decay of CIP, we chose the Ce0.75Zr0.25 /CF and Ce0.90Cu0.10
CF and Ce0.85Ni0.15 /CF composite as cathodes to degrade CIP
x
A
1ꢀx
O
2
loading capacity on the degra-
x 1ꢀx 2
A O
loading on the
O
2
2
O /
O
2
at 400 mA and pH ¼ 3, respectively. It was shown in Fig. 9 that
the degradation of CIP was dependent strongly on the loading
quantity of CexA O (A ¼ Zr, Cu, Ni).
1ꢀx
2
As shown in Fig. 9a, there was a positive effect on CIP
degradation when Ce0.75Zr0.25
2
O load increased from 1.0 wt% to
2.0 wt%. Meanwhile, the CIP solution degradation efficiency
increased from 96.02% to 100% aer 1 h. This effect might be
due to the increase of oxygen vacancies with the increase of
2
Ce0.75Zr0.25O loading. Thus, more cOH could be formed by
2
+
Ce0.75Zr0.25O and Fe catalysis. That is to say, there is a synergy
2
+
2
3+
between Fe and Ce to make feasible a larger production of
32
hydroxyl radical. However, with the further increase of
loading, the degradation efficiency decreased to
4.54% and 94.24% for 2.5 wt% and 3.0 wt% Ce0.75Zr0.25
loading, respectively. It may be due to the too much loading of
, so that the conductivity of Ce0.75Zr0.25 /CF
electrode became poor.
Ce0.75Zr0.25O
2
9
O
2
Ce0.75Zr0.25
O
2
O
2
Fig. 9b shows the inuence of different loading capacity of
Ce0.90Cu0.10O2 on the degradation of the CIP. When
Ce0.90Cu0.10O loading increased from 1.0 wt% to 2.5 wt% on
2
the CF, we found that the degradation efficiency of CIP
increased from 86.86% to 93.07% aer 2 h. However, the
degradation efficiency of CIP of 2.5 wt% Ce0.90Cu0.10O
2
loading on CF only reached 96.65% aer 5 h. This may be
caused by the adsorption of a large number of CIP interme-
diates on the surface of Ce0.90Cu0.10O , which affected
2
production of oxygen vacancy and the electro-conductivity of
Ce0.90Cu0.10O /CF electrode. It could be seen that the removal
2
rate of CIP decreased to 87.08% of 3.0 wt% Ce0.90Cu0.10
loading from Fig. 9b. Because Ce0.90Cu0.10 content was too
high in Ce0.90Cu0.10 /CF composite which led to blocking
the pores of Ce0.90Cu0.10
effective area for O adsorption decreased which contributed
O
2
O
2
O
2
Fig.
9
Effect of (a) Ce0.75Zr0.25
O
2
,
(b) Ce0.90Cu0.10
O
2
,
(c)
O
2
/CF composite, and then the Ce0.85Ni0.15O load capacity on degradation efficiency of CIP at pH ¼
2
ꢀ1
2+
ꢀ1
3, 400 mA, 0.1 mmol L Fe and 0.05 mol L Na
SO
2 4
solution of the
2
experiment condition.
to producing less oxygen vacancy.
From Fig. 9c, we could see that the CIP degradation effi-
ciency increased from 87.59% to 95.90% aer 1 h with the
increase of Ce0.85Ni0.15
CF, respectively. And the CIP degradation efficiency of 0.45 wt%
loading on CF reached to 98.33% aer 4 h. That
maybe that more oxygen vacancies were produced when the
loading of Ce0.85Ni0.15 increased and more cOH was
2
O loading from 0.14 wt% to 0.45 wt% on
2
produced. However, when the Ce0.85Ni0.15O loading increased
to 0.6 wt%, the CIP solution removal rate reached 85.23% and
4.75% aer 1 h and 4 h, respectively. That was because the
Ce0.85Ni0.15O
2
9
conductivity of Ce0.85Ni0.15
O
2
/CF electrode became poor.
O
2
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Overall, Fig.
9 shows the optimal load capacity of
Ce0.75Zr0.25O , Ce0.90Cu0.10O and Ce0.85Ni0.15O on CF was 2.0
2
2
2
wt%, 2.5 wt% and 0.45 wt%, respectively. And their corre-
sponding CIP degradation efficiencies were about 100%,
9
3.07% and 95.9% at 1 h, respectively. The highest degradation
performance was measured for the Ce0.75Zr0.25 /CF electrode
at the experiment condition of 400 mA, pH ¼ 3, 0.1 mmol L
O
2
ꢀ
1
2
+
ꢀ1
Fe and 0.05 mol L Na
.2.4 Effect of applied current on the degradation of CIP.
The formation rate of H O is crucial for the oxidative degra-
2 4
SO solution.
3
2
2
dation of organic pollutants in EF process. And the optimal
generation rate of H O depends on the applied current, which
2
2
48
increases with the increasing applied current. However,
continuous increase of the current can also decrease the
generation of H
leading to the formation of H
the applied current is a main parameter in EF process. And it
2 2
O because of the promotion of 4e reduction of
14
O
2
2
O as side reaction. Therefore,
4
+
affects the electro-catalytic activity of the circulation of Ce and
3+
49
Ce .
Fig. 10a shows the effect of applied current on the oxidative
degradation of CIP by using CF as cathode. The degradation
efficiency increases from 61.84% to 83.03% aer 1 h when the
applied current increased from 100 to 400 mA. This enhanced
degradation efficiency of CIP is due to higher electrochemical
2
+
2 2
production of H O and Fe catalyst regeneration causing
a higher concentration of homogeneous cOH in the solution. In
addition, the increased current density leads to an enhanced
18
production of Pt (cOH) on the anode surface. However, further
increase in current to 500 mA, the CIP degradation efficiency
reaches 81.87% aer 1 h which is lower than that of 400 mA.
This is because the side reactions have an enhanced rate which
50
competes with the CIP oxidative degradation. The main side
reactions as follows:
ꢀ
+
O + 4e + H ¼ H O
(1)
(2)
(3)
2
2
ꢀ
ꢀ
H
2
O + 2e ¼ H
2
+ 2OH
+
ꢀ
H O ¼ O + 2H + 2e
2
2
2
Therefore, 400 mA is the optimal current value on the CIP
degradation.
As shown in Fig. 10b, the oxidation degradation performance
of the Ce0.75Zr0.25O /CF composites as cathode is tested under
2
the same operating conditions. Obtained results showed that
the CIP degradation efficiency is accelerated from 87.87% to
about 100% aer 1 h by increasing the applied current from 100
to 400 mA. This is probably related to higher electrochemical
2
+
3+
production of H
2
O
2
and Fe catalyst regeneration and Ce
/gas inter-
catalyst generation occurring at the Ce0.75Zr0.25
O
2
51
face causing a higher concentration of cOH. However, further
increase current to 500 mA, the degradation efficiency of CIP
decreases which can explained by acceleration of side reactions
Fig. 10 Effect of applied current on the degradation of CIP in the cell
2 2
of (a) Pt/CF, (b) Pt/Ce0.75Zr0.25O /CF, (c) Pt/Ce0.90Cu0.10O /CF, (d) Pt/
(
4
eqn (1)–(3)) at higher current values. So the optimal current is
00 mA for the degradation of CIP when Ce0.75Zr0.25 /CF
composites used as cathode.
ꢀ1
2+
Ce0.85Ni0.15
O
2
/CF experiment condition: pH ¼ 3, 0.1 mmol L Fe
O
2
ꢀ1
and 0.05 mol L Na
2
SO
4
solution.
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Fig. 10c shows the impact of different applied current on the 3.3 Comparison of degradation and mineralization
decay of the CIP by using as Ce0.90Cu0.10O /CF as cathode in the efficiency of CIP aqueous solution
2
EF process. Due to electrochemical production of H O from O
2
reduction and the electro-catalysis of Fe and Ce , the degra-
dation efficiency of CIP increases from 60.42% to 86.90% aer
2
3+
2
It is crucial to completely remove organic pollutants from
wastewater. The EF process can degrade and mineralize the
organic matter in aqueous solution and further achieve high
CIP degradation and TOC removal efficiency. Furthermore, the
degradation efficiency and mineralization degree of this tech-
nology depend on several parameters, one of the most crucial is
the nature of the cathode material used.
2
+
0
4
.5 h electrolysis when the applied current increases from 100 to
00 mA. However, when the applied current is 500 mA, the
degradation efficiency is reduced to 82.54% owing to fast
occurrence of side reactions (eqn (1)–(3)). So the optimal current
is 400 mA using as Ce0.90Cu0.10O /CF composite as cathode
2
The degradation of CIP solution was carried out by using
during the degradation of CIP.
different modied cathodes (Ce0.75Zr0.25
CF, Ce0.85Ni0.15
/CF) at 400 mA and pH ¼ 3. Fig. 11a showed
that the CIP solution degradation efficiency of 2.0 wt%
/CF reached about 100% aer 1 h. The degrada-
tion efficiency of 2.5 wt% Ce0.90Cu0.10 /CF was much higher
2 2
O /CF, Ce0.90Cu0.10O /
The results obtained with Pt as anode and Ce0.85Ni0.15O2/
CF as cathode are depicted in Fig. 10d. The CIP degradation
efficiency increases from 79.54% to 95.9% aer 1 h when the
applied current increases from 100 to 400 mA. The results
show that the higher the applied current, the higher the decay
in CIP concentration. This is because in addition to higher
O
2
Ce0.75Zr0.25O
2
O
2
than that of CF before electrolysis of 2 h and even its electrolytic
efficiency reached 93.07% aer 1 h. But the degradation effi-
2
+
electrochemical production of H O and Fe catalyst regen-
2
2
ciency of Ce0.90Cu0.10O /CF and CF was about the same aer 5 h.
2
4
+
3+
eration, as well as the circle of Ce /Ce which leads to
And the CIP solution degradation efficiency of 0.45 wt%
produce a large number of cOH. And it further brings about
2 2 2
the formation of H O from O reduction. However, the
degradation efficiency decreases to 95.90% under the
applied current of 500 mA owing to side reactions occurrence
(
eqn (1)–(3)).
Overall, the optimal current is 400 mA during the degrada-
tion of CIP for different electrodes. Due to the electrocatalysis of
Ce0.75Zr0.25O2 composite, Ce0.90Cu0.10O2 composite and
Ce0.85Ni0.15O2 composite, the CIP degradation efficiency by
using CF as cathode is lower than that of Ce0.75Zr0.25
composite, Ce0.90Cu0.10 /CF composite and Ce0.85Ni0.15
O
O
2
/CF
/CF
O
2
2
composite as cathode, respectively. And the degradation effi-
ciency of Ce0.75Zr0.25 /CF composite as cathode is higher than
that of Ce0.85Ni0.15 /CF composite and Ce0.90Cu0.10 /CF
O
2
O
2
O
2
composite as cathodes. This is because Ce0.75Zr0.25O compos-
2
ites have larger specic surface area, that is to say, it has a more
catalytic active sites, than Ce0.90Cu0.10O2 composite and
Ce0.85Ni0.15O composite which results in producing a small
2
amount of oxygen vacancies.
To compare specic surface area and pore volume of
Ce0.75Zr0.25
O
2
and Ce0.90Cu0.10
O
2
and Ce0.85Ni0.15
O
2
nano-
particles, the N
2
adsorption–desorption was performed. As
shown in Fig. S3a,† bimodal pore size distribution was found
in Ce0.75Zr0.25O2 material with average pore diameters of
ꢄ34 nm which was larger than that of Ce0.90Cu0.10O2 and
Ce0.85Ni0.15
pore volumes for Ce0.75Zr0.25
Ce0.85Ni0.15
2
O materials (Fig. S3b†). In addition, the obtained
O
2
and Ce0.90Cu0.10
O
2
and
3
ꢀ1
O
2
materials are ꢄ0.15 and 0.079 and 0.13 cm g
,
respectively (Table S1†). It was found that the Ce0.75Zr0.25
O
2
material exhibits a higher BET surface area compared to that
of Ce0.90Cu0.10O2 and Ce0.85Ni0.15O2 materials (Table S1†).
Overall, this observation indicates Ce0.75Zr0.25O nanoparticles
2
had lager specic surface area and pore volume than
Ce0.90Cu0.10O and Ce0.85Ni0.15O nanoparticles which caused
2 2
more exible response to the active site so that producing
more oxygen vacancies. And further it could produce a larger
number of H O by oxygen reduction which contributed to the
2 2
degradation of CIP.
Fig. 11 Effect of Ce
degradation rate (a) and the extent of TOC removal (b) of CIP aqueous
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu and Ni) cathode on the
ꢀ1
2+
ꢀ1
solution at pH ¼ 3, 400 mA, 0.1 mmol L Fe and 0.05 mol L
Na SO solution.
2
4
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Ce0.85Ni0.15
O
2
/CF attained 95.9% and 98.45% aer 1 and 3 h, can nd the amount of H
2 2 2
O produced by Ce0.75Zr0.25O /CF as
respectively. Further comparison of 2.0 wt% Ce0.75Zr0.25O /CF, cathode is similar to that of CF as cathode at 5 h which indicates
2
2
.5 wt% Ce0.90Cu0.10O /CF and 0.45 wt% Ce0.85Ni0.15O /CF the catalytic action of CexA O2 composites plays a very
2 2 1ꢀx
showed that 2.0 wt% Ce0.75Zr0.25O /CF was the outstanding one. important role in addition to the production of hydroxyl radi-
2
Although Ce0.90Cu0.10
than Ce0.75Zr0.25 /CF and Ce0.85Ni0.15
nanoparticles had lager specic surface area and pore volume Ce0.75Zr0.25
than Ce0.90Cu0.10 and Ce0.85Ni0.15
which resulted in having more catalytic activity sites to Ce0.85Ni0.15
contribute to the entry of oxygen to obtain a higher concentra- small amount of H O . Above all, Ce0.75Zr0.25O /CF electrode
O
2
/CF has a greater electrical conductivity cals. The production of H
2
O
2
2
of Ce0.90Cu0.10O /CF and
O
2
O
2
/CF, Ce0.75Zr0.25
O
2
Ce0.85Ni0.15
O
2
/CF as cathodes is lower than that of
O
2
/CF as cathode owing to the smaller specic
O
2
O
2
nanoparticles (Table S1†) surface area and pore volume of Ce0.90Cu0.10
O
2
and
O
2
nanoparticles (Table S1†) leading to producing
2
2
2
tion of hydrogen peroxide (Fig. S4†).
As it could be seen in Fig. 11b, the mineralization of CIP was rials which can be mainly attributed to the formation of oxygen
improved by Ce
(A ¼ Zr, Cu and Ni) loading on CF. vacancies (Table 1).
/CF was used as cathode in EF process,
there was a maximum value of the TOC removal. Because
nanoparticles had larger specic surface area
Table S1†) which led to having fast mass and electron transport
has a higher adsorption oxygen content than the other mate-
x 1ꢀx 2
A O
When Ce0.75Zr0.25O
2
3
.4 Degradation mechanism of CIP
Ce0.75Zr0.25O
2
(
ꢀ1
Samples withdrawn aer 1 h in 0.05 mol L Na
2
SO
/CF cell at
00 mA are analyzed by HPLC-MS. Apart from CIP, it can also be
4
with
ꢀ
1
2+
0
4
.1 mmol L Fe at pH 3.0 using a Pt/Ce0.75Zr0.25O
2
rate. Ce0.75Zr0.25O /CF composite had more catalytic activity
sites than that of Ce0.90Cu0.10O /CF composite and
2
2
found that the name, molecular structure and m/z values of
seven intermediates from Table S2.† These compounds are
formed via hydroxylation, deuorination, cleavage of C–N
bonds, deamination and breaking of the dihydroquinoline
moiety. Since hydroxyl radicals are the main oxidizing agents in
all the EAOPs tested, one can also expect the generation of the
same intermediates in Pt/Ce0.90Cu0.10O /CF, Pt/Ce0.85Ni0.15O /
Ce0.85Ni0.15O /CF composite. And Ce0.75Zr0.25O /CF composite
2
2
had more oxygen vacancies which resulted in the formation of
with higher concentration (Fig. S4†). From Fig. 11, the
mineralization degree of CIP aqueous solution by using
/CF during electro-Fenton system was 97.45%
aer 6 h owing to synergistic effects from the predominant role
2 2
H O
Ce0.75Zr0.25O
2
3
+
2
2
of homogeneous cOH and the Fenton-like reaction of Ce and
H O producing heterogeneous cOH and the electrocatalytic
CF and Pt/CF cells. However, the intermediate of m/z (201) only
2
2
can be found in the Pt/Ce0.75Zr0.25O /CF cell.
2
activity of CexA O (A ¼ Zr, Cu and Ni) composites and strong
1ꢀx
2
Based on the products of Table S2† and literatures of the
adsorption property of CF. It could be also found that removal
rate of TOC was fast during the early treatment time, but it
became slowly with the increase of time. The decrease of the
mineralization rate was related to the formation of the precip-
56,57
degradation mechanism of CIP,
a possible degradation path
of CIP is proposed. As shown in Fig. 12, the path is initiated by
the oxidation of CIP to generate either A via deuorination with
hydroxylation or D by cleavage of two C–N bonds of the piper-
azinyl group. Further the hydroxylation of A yields B, whereas D
produces E by the deamination. The cleavage of two C–N bonds
of the piperazinyl group of B leads to producing C, nevertheless,
F further is formed via the loss of the acetylene group of E. The
loss of the lateral groups of compound F along with the
breaking of the dihydroquinoline moiety yields G. C and G are
further oxidized by cOH to form short chain carboxylates which
52
53
itation of soluble iron species and the consumption of cOH.
It is well-known that the concentration of H O is a very
2
2
important parameter during CIP degradation in electro-Fenton
process. Herein, the concentration of H O is investigated by
2
2
potassium iodide spectrophotometry (l ¼ 352 nm, 3max
¼
ꢀ1
ꢀ1
2
6 400 M cm ). Aer applying current, the reduction reac-
3
+
2+
4+
3+
tions of Fe /Fe and Ce /Ce rapidly happen and more
oxygen vacancies will be produced. And the Pt anode allows to
generate supplementary hydroxyl radical from the oxidation of
can ultimately produce CO
2 2
, H O and inorganic ions owing to
54,55
strong oxidizing property of cOH.
water on its surface.
It can be seen that the H O concentration sharply increases
2
2
for the rst 10 min from Fig. S4,† which is consistent with the 3.5 Stability of Ce0.75Zr0.25O /CF cathode
2
results of Fig. 11. From Fig. 11, the CIP degradation efficiency
Because the reuse performance of modied cathode is vital for
industrial application, the stability of Ce0.75Zr0.25 /CF cathode
is tested 5 times aer 60 min. As shown in Fig. 13, the
and the mineralization degree also increase rapidly at this time.
Aer 10 min, the H concentration decreases gradually, but it
O
2
2 2
O
still remains the effective values and the degradation rate still
increases at this time. This maybe because a large number of
CIP intermediates adsorbed on the surface of CexA O and
1ꢀx
2
iron deposits and Fe(III)-carboxylate products adsorbed on CF
which causes the decrease of the production of oxygen vacancy
and the poor electrical conductivity. It can be also found that
the concentration of H
and Ni) as cathodes than that of CF. Because the strong electro-
2
O
2
is lower for Ce
x
A
1ꢀx
O
2
/CF (A ¼ Zr, Cu
3
+
catalysis of Ce
x 1ꢀx 2
A O and a Fenton-like reaction of Ce and
2 2
H O
can consume a large amount of hydrogen peroxide. We Fig. 12 Proposed degradation pathways for CIP.
27076 | RSC Adv., 2017, 7, 27065–27078
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and 2016XJ-0006), National Science and Technology Major
Project of the Ministry of Science and Technology of China
(
Grant No. 2016ZX05058-003-004), Natural Science Foundation
of Tianjin (Grant No. 15JCYBJC48400, 15JCZDJC41200 and
6YFZCSF00300) and by Innovation and Entrepreneurship
Training Program of Tianjin for College Students (Grant No.
01610056239).
1
2
Notes and references
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4
Conclusions
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