Novel perovskite-based composites, La1-: XNdxFeO3@activated carbon, as efficient catalysts for the degradation of organic pollutants by heterogeneous electro-Fenton reactions

Article abstract of DOI:10.1039/c8ra00244d

Perovskites, which have excellent electrocatalytic properties, are promising for use in heterogeneous catalysis. However, the design and development of green and effective electrocatalysts for environmental water treatment remains an arduous challenge. To

Full text of DOI:10.1039/c8ra00244d

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Novel perovskite-based composites,
La1ꢀxNdxFeO3@activated carbon, as efficient
catalysts for the degradation of organic pollutants
by heterogeneous electro-Fenton reactions
Cite this: RSC Adv., 2018, 8, 14775
Qijun Wang,ab Shu Zhou,a Song Xiao,a Feifei Wei,a Xuezhu Zhao,a Jun’e Quac
ac
*
and Hairen Wang
Perovskites, which have excellent electrocatalytic properties, are promising for use in heterogeneous
catalysis. However, the design and development of green and effective electrocatalysts for environmental
water treatment remains an arduous challenge. To overcome such difficulties, we present a facile sol–gel
method for the design and preparation of a series of perovskite-activated carbon (AC) composites
(La1ꢀxNdxFeO3@AC) for the degradation of methyl orange (MO) by heterogeneous electro-Fenton
reactions. Furthermore, the as-made La0.6Nd0.4FeO3@AC composite anode had the strongest oxidation
ability and stability, with MO wastewater and COD removal rates reaching 99.81% and 96.66% within
10 minutes, respectively. As far as we know, the La1ꢀxNdxFeO3@AC composites can be regarded as
a series of the most effective catalysts for the degradation of MO to date.
Received 9th January 2018
Accepted 28th March 2018
DOI: 10.1039/c8ra00244d
rsc.li/rsc-advances
Ln–O bond.12 The BE between a metal and oxygen, DH(M–O), is
usually estimated using the formula below:12
1. Introduction
Perovskite materials have fascinated scientists because of their
exceptional physical and chemical properties, such as high
temperature stability, high oxygen mobility and high stability of
the cation oxidation state.1–3 As one of the most common
perovskite-like oxides and a promising material with an abun-
dance of functionalities, lanthanum ortho-ferrite (LaFeO3) has
been widely applied in areas like environmental decontamina-
tion,4 multi-ferroic materials,5 gas sensors,6 solid oxide fuel
cells7 and photocatalysts.8 Moreover, LaFeO3 can produce free
radicals in the presence of hydrogen peroxide (H2O2), such as
hydroxyl radicals (cOH), and so can achieve efficient oxidation
degradation of organic pollutants.9,10
From the composition of perovskite oxide ABO3, it has been
found that the type of A site has great inuence on the lattice
parameters, cell volume and binding energy (BE) of ABO3.11 In
addition, when lanthanide ortho-ferrite (LnFeO3) is used as
a gas sensor material, replacing the ion at the Ln site with other
cations will effectively improve the sensing properties of LnFeO3
as its gas sensitive properties are closely related to the BE of the
DH(M–O) ¼ (Hf ꢀ mHs ꢀ nD0/2)/(CNm)
(1)
Here, Hf is the amount of material, MmOn is the formation
energy of the oxide, HS is the sublimation energy of the metal,
D0 is the dissociation energy for O2 and CN is the coordination
number for the metal. The BE of the Ln–O bond is typically less
than the absolute value of DH(M–O), leading to the easy disrup-
tion of the Ln–O bond. Therefore, the smaller the BE of the
Ln–O bond, the more oxygen will be generated by the dissoci-
ation process, and the accelerated oxidation process will
improve the gas sensing properties of LnFeO3.12,13 Signicantly,
the inuence of the introduction of rare earth metals (REM) has
been reported to authenticate that REM can enhance textural/
structural properties, increase the oxygen storage/release
capacity (OSC) and can improve the redox behavior of a ceria–
zirconia solid solution.14,15 Interestingly, as one of the REM
family, neodymium (Nd) shows its appeal in interatomic inter-
actions. The BEs of Nd–O and La–O bonds are about
ꢀ136 kJ molꢀ1 and ꢀ142 kJ molꢀ1, respectively.13 Since the
absolute value of the BE of Nd–O is less than that of La–O, it
indicates that Nd–O is much easier to dissociate than La–O. We
suppose that when La is replaced by Nd, theoretically, the
chemisorption of oxygen on the surface of LaFeO3, along with
the rate of the oxidation reaction, will increase.
aSchool of Materials Science and Engineering, Hubei University, Wuhan, 430062, P. R.
China. E-mail: whr9999whr@163.com
bSchool of Chemistry & Chemical Engineering, Huazhong University of Science &
Technology, Wuhan 430074, P. R. China
In terms of electrochemistry during the past few decades,
electrochemical advanced oxidation processes (EAOPs) have
cMinistry-of-Education Key Laboratory of Green Preparation and Application for
Functional Materials, School of Materials Science and Engineering, Hubei
University, Wuhan, 430062, P. R. China
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drawn scientists’ interest in the reclamation of water contami- nonahydrate (97%, AR), lanthanum nitrate hydrate (99.9%, AR),
nated with non-biodegradable organics16–18 because of the neodymium nitrate hexahydrate (99.0%, AR) were used as
capability of EAOPs in generating strong oxidizing radicals that catalysts. Citric acid (99.5%) was obtained from Tianjin Bodi
can completely oxidize organic pollutants into CO2, H2O and Chemical Co., Ltd and used as received. Deionized water, ob-
inorganic ions. Electro-Fenton (EF) is the most ubiquitous of tained with a Milli-Q water purication system (Millipore,
these EAOPs, in which the weak oxidant H2O2 is continuously France), was used for the preparation of solutions.
generated and supplied to an acidic contaminated solution
from the two-electron cathodic reduction of O2 gas, which
proceeds as follows:19
2.2. Modication of activated carbon and preparation of
La1ꢀxNdxFeO3@AC catalysts
2.2.1. Oxidation with nitric acid in liquid phase and func-
tionalization with urea. In a typical treatment, a certain quantity
of raw activated carbon was soaked for 24 h in dilute nitric acid
to remove impurities, and then the ACs were thoroughly washed
with deionized water until neutrality of the scrubbing solution
was reached. They were then further dried in an oven for 6 h at
O2 + 2H+ + 2eꢀ / H2O2
(2)
Then, it can be catalytically converted into highly powerful
hydroxyl radicals in the presence of an iron catalyst (e.g. Fe2+ or
iron oxides):20
H2O2 + Fe2+ / Fe3+ + cOH + OHꢀ
(3) 100 C. The obtained ACs were denoted modied ACs.
ꢁ
2.2.2. Preparation of La1ꢀxNdxFeO3@AC catalysts.
However, the classical EF process has some critical limita-
tions, like the need to operate at pH ¼ 2.0 or 3.0 for an optimum
run and the precipitation of soluble iron species used as
a catalyst.17,37 These imperfections could be overcome using an
Fe-containing solid catalyst as the source of Fe2+ ions instead of
soluble iron salt. Hence, it was practically signicant to develop
a solid heterogeneous catalyst with high activity and stability to
deactivation and to metal leaching.
In addition, activated carbon (AC) is a well-known material
which is widely used as an admirable adsorbent and supporting
material, due to its excellent properties in mechanical strength
and porosity.21
Therefore, taking into consideration the aforementioned
inconveniences of the electro-Fenton system, a strategy is
proposed here to prepare a heterogeneous catalyst containing
both perovskite oxide and activated carbon (AC). In this study,
we prepared La1ꢀxNdxFeO3@AC composites via a facile sol–gel
method. Organic pollutants in wastewater have had a great
negative impact on ecological environments, and the latest
research has all been devoted to the degradation of organic
pollutants.22,23 Methyl orange (MO), a typical azo dye in textile
wastewater, was used as the target pollutant, because azo dye-
rich textile wastewater is resistant to conventional biological
treatment due to the stability and toxicity of the dye. As an
important part of our research system, methyl orange degrada-
tion experiments, along with gas chromatography-mass spec-
trometry (GC-MS) analysis, have been completed precisely to
evaluate the electrochemical activities of the La1ꢀxNdxFeO3@AC
composites.
La1ꢀxNdxFeO3@AC was prepared by a sol–gel preparation
method. Fe(NO3)3$9H2O, La(NO3)3$2.5H2O, Nd(NO3)3$6H2O
and citric acid (C6H8O7$H2O) were used as raw materials. The
gel precursor was prepared as follows. Modied ACs were added
into a mixed aqueous solution of La(NO3)3$2.5H2O (La),
Nd(NO3)3$6H20 (Nd), Fe(NO3)3$9H2O (Fe) and citric acid (CA)
under vigorous stirring at ambient temperature and at the
composition in molar ratio of La/Nd/Fe/CA ¼ 1.0 ꢀ x/x/1.0/3.0
(x ¼ 0, 0.2, 0.4, 0.6, 0.8,1). Subsequently, the solution was stir-
red for 1 h at 80 ꢁC, and then heated to the boiling point of water
(100 ꢁC). The gel precursor was dried in air at 120 ꢁC for 6 h, and
then thꢁe resulting material was heated from room temperature
ꢁ
to 700 C under an argon atmosphere and calcined at 700 C
for 3 h.
2.2.3. Preparation of the air diffusion cathode. The air
diffusion cathode was prepared using the following proce-
dure.12 Aer undergoing stirring for 20 s, the mixture of 1.5 mL
distilled water, 1.5 mL PTFE (mass fraction 60%) and 0.27 g
carbon black was pasted uniformly onto one side of the PAN
carbon felt (3 ꢂ 5 cm2). Aer it had dried naturally, the coated
carbon felt was heated at 350 ꢁC for 30 min. Aer it had cooled
down, a mixture of 5.82 mL distilled water, 0.09 mL PTFE (mass
fraction 60%), 0.18 g carbon black and 0.5 mL isopropanol was
made and stirred for 20 s. The same procedure was utilized to
coat this mixture onto the same side of carbon felt. The rst
carbon black layer was waterproof and used for electric
conduction, while the second layer was for H2O2 production.
2.3. Electrochemical system
A cylindrical glass cell (0.5 L and a diameter of 5 cm) was used to
carry out the electrochemical experiments. A PAN air diffusion
cathode (3 cm ꢂ 5 cm) was tted on the inner side of the cell.
An anode basket made from a stainless steel wire mesh full of
2. Experimental section
2.1. Materials preparation
All of the chemicals used in this study were of analytical grade the LaFeO3@AC catalyst was centered into the cell. The elec-
and used as received, without further purication. Methyl trolysis solution was prepared by adding sodium sulphate into
Orange (MO) (Sinopharm Chemical Reagent Co., Ltd.), which is methyl orange solution. The pH of the solution was set to 2.0
widely used in the textile industry and as a biological stain, was using sulfuric acid solution (1 mol Lꢀ1) and a pH meter. Aer
regarded as the target pollutant. Sodium sulphate (99%, anhy- that, the solution was transferred to the electrochemical cell
drous) was the supporting electrolyte and iron (III) nitrate and stirred (500 rpm) with a magnetic stirrer. The oxygen
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demand of the system was provided by air bubbling with JSM6510LV) equipped with energy dispersive spectrometry
a compressed air system. The current was controlled with an (EDS).
electrochemical workstation.
X-ray photoelectron spectra (XPS) were recorded at room
Liquid samples were withdrawn periodically for UV-vis temperature on a Thermo Fisher Scientic 250Xi analyzer with
spectro-photometer analysis and chemical oxygen demand a magnesium anode under Ka (hn ¼ 1486.6 eV) radiation. Gas
(COD) measurements. Before each analysis, sample aliquots chromatography-mass spectrometry analysis was carried out
withdrawn from treated solutions were ltered with 0.45 mm using an Agilent 6890 GC/5975MSD GC-MS analyzer.
PTFE lters.
All of the investigations were carried out in triplicate to avoid
any discrepancy in experimental results and averages were re-
3. Results and discussion
3.1. Characterization of the composite La1ꢀxNdxFeO3@AC
ported. The removal efficiency of phenol could be calculated
using the following equation:
3.1.1. X-Ray Diffraction (XRD) characterization. XRD
characterization diagrams of modied AC along with
La1ꢀxNdxFeO3@AC (x ¼ 0, 0.2, 0.4, 0.6, 0.8 or 1.0) are shown in
Fig. 1. The diffraction peaks can be observed at the position of
2q ¼ 22.62ꢁ, 32.18ꢁ, 39.69ꢁ, 46.16ꢁ, 52.04ꢁ, 57.37ꢁ, 67.38ꢁ and
76.65ꢁ (Fig. 1a). This corresponds to the characteristic
diffraction peaks of orthorhombic LaFeO3 (JCPDS#88-0641).25
As is shown in Fig. 1b (aer the magnication of 2q between 31–
33ꢁ in Fig. 1a), with an enhanced ratio of Nd in La1ꢀxNdxFeO3@AC,
the diffraction peak at 2q ¼ 32.18ꢁ continues to dri to the right,
along with the continuous change for the lattice of La1ꢀxNdxFeO3.
By Bragg’s law,26 2d sin q ¼ nl, an increase in the diffraction angle
q will lead to a decrease in the crystal interplanar spacing d, which
simply means that the cell of La1ꢀxNdxFeO3@AC has a tendency to
shrink gradually with an increase in the doping ratio of Nd.
When x ¼ 1.0, the diffraction peaks which appear at 22.96ꢁ,
25.64ꢁ, 32.31ꢁ, 40.37ꢁ, 46.50ꢁ, 53.33ꢁ, 58.54ꢁ, 68.33ꢁ and 77.79ꢁ
(Fig. 1a) correspond to the characteristic diffraction peak of
orthorhombic NdFeO3 (JCPDS#25-1149).27 No characteristic
peaks of La2O3, Nd2O3 and Fe2O3 were detected, indicating that
the metal nitrates have been completely decomposed to form
single phase La1ꢀxNdxFeO3 composite oxides. Furthermore, as
shown in Table 1, the cell volume (V) and the average grain size
tended to be smaller with increases in the proportion of Nd in
La1ꢀxNdxFeO3@AC, because the radius of Nd3+ (0.983) is
Removal (%) ¼ [C0 ꢀ Ct]/C0 ꢂ 100%
(4)
C0 and Ct were the initial and nal absorbance values at 507 nm
of the methyl orange wastewater, or the initial and nal COD,
respectively.
2.4. Composite La1ꢀxNdxFeO3/AC characterization and
analytical method
The specic surface areas of the binding catalysts were deter-
mined by the nitrogen adsorption method (automated gas
adsorption apparatus, Micromeritics Inc. ASAP 2020 HD88),
performed at ꢀ196 ꢁC. Before the gas adsorption measurement,
ꢀ3
ꢁ
all of the samples were degassed at 250 C in a vacuum (10
torr) for 12 h. Surface areas and the micropore volume of
samples were determined using the Brunauer–Emmett–Teller
(BET) and Dubinin–Radushkevich (D–R) equations.
The pH at the point of zero charge (pHPZC) was determined
by mixing 0.05 g of each sample with 20 mL of 0.01 mol Lꢀ1
NaCl aqua-solution, with pH values adjusted between 2.0 and
11.0 by adding 0.1 mol Lꢀ1 HCl or 0.1 mol Lꢀ1 NaOH aqua-
solution. The nal pH was measured aer 48 h of shaking at
room temperature. Blank experiments (without the addition of
carbon) were also performed for each pH and the values
measured aer 48 h are considered as the initial pH, in order to
avoid the variation of pH caused by the effect of CO2 present in
the headspace. The pHPZC value of each carbon sample was
determined by intercepting the obtained nal pH vs. initial pH
curve with the straight line nal pH ¼ initial pH.24
The phases of the binding catalysts were identied by means
of X-ray diffraction (XRD), using a Rigaku D/max-IIIC under Cu
˚
Ka radiation (l ¼ 1.5418 A), at a scanning range of 2 theta (2q) ¼
10–80ꢁ and under a speed of 4ꢁ per minute. The average crys-
tallite size of the catalysts was estimated using the Debye–
Scherrer equation:
0:90l
D ¼
(5)
b cos q
where D is the crystallite size (nm), l is the wavelength (nm), b is
the corrected full width at half maximum (radian) and q is the
Bragg angle (radian).
The morphology and size distribution of the metal oxides as
well as the elemental analyses of the binding catalysts were
examined using a scanning electron microscope (SEM-JEOL
Fig. 1 XRD energy spectra of (a) modified AC; (b) LaFeO3@AC; (c)
La0.8Nd0.2FeO3@AC; (d) La0.6Nd0.4FeO3@AC; (e) La0.4Nd0.6FeO3@AC;
(f) La0.2Nd0.8FeO3@AC; (g) NdFeO3@AC.
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Table 1 The lattice parameters and pHPZC of modified AC and La1ꢀxNdxFeO3@AC
3
Samples
V (A )
a ¼ b ¼ g (ꢁ)
Average grain size (nm)
pHPZC
˚
a (A)
˚
b (A)
˚
c (A)
˚
Modied AC
LaFeO3@AC
—
—
—
—
—
90
90
90
90
90
90
—
2.82
6.56
6.63
6.90
7.20
7.36
7.58
5.9237
5.6241
5.5295
5.5216
5.8160
5.3326
5.7267
5.5751
5.7329
5.5280
5.6117
5.5479
7.1342
7.6785
7.5548
7.7742
7.2449
7.8760
242.0153
240.7594
239.4875
237.2951
236.4567
233.0093
309
251
207
189
185
210
La0.8Nd0.2FeO3@AC
La0.6Nd0.4FeO3@AC
La0.4Nd0.6FeO3@AC
La0.2Nd0.8FeO3@AC
NdFeO3@AC
slightly smaller than that of La3+ (0.114).28 Therefore, the cell in LaFeO3@AC is 10.81% and 11.68%, respectively. With an
3
˚
volume of NdFeO3@AC is the smallest one (233.0093 A ) in this increasing Nd ratio in La1ꢀxNdxFeO3@AC, the mass fraction of
series of La1ꢀxNdxFeO3@AC composites.
Fe and O decreased, however, the atomic fraction (or amount of
3.1.2. Scanning electron microscope (SEM) characteriza- material fraction) proportion of (La + Nd) : Fe : O is 1 : 1 : 3,
tion. The surface morphology of modied AC, such as AC and which is much closer to that of the theoretical value.
La1ꢀxNdxFeO3@AC (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0), was scanned
3.1.4. X-ray photoelectron spectroscopy (XPS) character-
using SEM, and the results are shown in Fig. 2. As shown in Fig. 2a, ization. The full XPS spectra of perovskite catalysts along with
the modied AC with dilute nitric acid has a smooth surface and the characteristic peaks of C-1s, O-1s, Fe-2p, La-3d and Nd-3d
holes on the surface are clearly visible, with no apparent granules are shown in Fig. 4. The C-1s peak corresponds to the C
or akes. The SEM images of La1ꢀxNdxFeO3@AC (x ¼ 0, 0.2, 0.4, element in the carrier AC in La1ꢀxNdxFeO3@AC, while the peaks
0.6, 0.8, 1.0) are shown in Fig. 2b–g. When x ¼ 0, the surface of of Fe-2p, La-3d and Nd-3d correspond to the Fe, La and Nd
LaFeO3@AC is mainly loaded with cubes (or chips) and particles. elements in the La1ꢀxNdxFeO3 oxide of the La1ꢀxNdxFeO3@AC
Nevertheless, when x ¼ 0.2, 0.4, 0.6 or 0.8 (Fig. 1b–f), the load ratio surface, respectively. The content of chemisorbed oxygen and
of cubes (or chips) on the surface of La1ꢀxNdxFeO3@AC decreases chemically adsorbed water in the O-1s characteristic peaks in
gradually with increasing proportions of Nd, and the particles La1ꢀxNdxFeO3@AC will be the decisive factor for the oxidative
gradually increased. Moreover, when x ¼ 1, which simply means performance of this series of La1ꢀxNdxFeO3@AC composite
that La1ꢀxNdxFeO3@AC changes to NdFeO3@AC, the cubes (or electrodes.
chips) can be observed on the surface again (Fig. 2g).
As shown in Fig. 5, aer the O-1s XPS peak deconvolution of
3.1.3. Energy dispersive spectrometry (EDS) characteriza- La1ꢀxNdxFeO3@AC, two peaks can be clearly observed at
tion. The results of the surface EDS elemental scanning of 529.37–529.64 eV and 531.30–531.47 eV. The former corre-
modied AC and La1ꢀxNdxFeO3@AC (x ¼ 0, 0.2, 0.4, 0.6, 0.8 or sponds to lattice oxygen in perovskite oxides (Oa) and the latter
1.0) are displayed in Table 2 and Fig. 3. The modied AC surface belongs to chemically adsorbed oxygen (Ob).29 The binding
is mainly C and O elements, while the mass fraction is 92.81% energy peaks of chemisorbed H2O between 532.86–533.07 eV
and 7.19%, respectively. As shown in Fig. 3b–g, Nd has been did not appear in Fig. 4a–f, or the peak intensity was quite weak,
successfully incorporated into the lattice of LaFeO3@AC. Table which coincided with the O-1s XPS energy spectrum of LaBO3
2 clearly indicates that the maximum mass fraction of O and Fe (B]Cu, Fe, Mn, Co, Ni) reported in the literature.8 As can be
Fig. 2 SEM images of (a) modified AC; (b) LaFeO3@AC; (c) La0.8Nd0.2FeO3@AC; (d) La0.6Nd0.4FeO3@AC; (e) La0.4Nd0.6FeO3@AC; (f) La0.2Nd0.8FeO3@AC;
(g) NdFeO3@AC.
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Table 2 The elemental content of modified AC and La1ꢀxNdxFeO3@AC
C
O
La
Nd
Fe
Wt%
Sample
Wt%
At%
Wt%
At%
5.50
12.67
5.80
10.53
7.20
8.31
4.80
Wt%
At%
Wt%
At%
At%
Modied AC
92.81
51.15
71.82
50.36
58.85
60.17
73.03
94.50
79.85
90.68
81.56
86.88
86.19
91.94
7.19
10.81
6.12
8.66
6.50
7.73
5.08
—
—
—
—
—
—
—
—
LaFeO3@AC
26.36
10.74
17.15
9.73
4.00
—
3.56
1.17
2.40
1.24
0.50
—
11.68
6.69
10.75
8.31
8.76
5.83
3.92
1.89
3.74
2.64
2.70
1.58
La0.8Nd0.2FeO3@AC
La0.6Nd0.4FeO3@AC
La0.4Nd0.6FeO3@AC
La0.2Nd0.8FeO3@AC
NdFeO3@AC
4.37
13.08
16.61
19.33
16.07
0.46
1.76
2.04
2.31
1.68
La0.2Nd0.8FeO3@AC and NdFeO3@AC (denoted as Ob/(Oa + Ob))
are 24.39%, 48.19%, 64.40%, 57.80%, 40.38% and 39.12%,
respectively. Furthermore, the analysis of the XPS peak splitting
could be seen to show that the addition of neodymium may
induce the formation of ferrous ions, due to the presence of
ferrous ions in lanthanum ferrite and oxygen vacancies (or
oxygen defects) can be produced while the chemisorbed oxygen
content is proportional to the oxygen defect concentration.30
Therefore, the addition of neodymium greatly increases the
catalytic activity of lanthanum ferrite.
To gain an in-depth understanding of the stability and metal
leaching of the composite anodes, we further performed an
extra XPS contrast measurement of the La0.6Nd0.4FeO3@AC
composite anode before and aer the electrocatalytic oxidation
process (Fig. 6).
As can be seen from Fig. 6a, the survey XPS spectrum of the
used La0.6Nd0.4FeO3@AC composite anode (before and aer
electrocatalytic oxidation) showed the same characteristic peaks
of C-1s, O-1s, Fe-2p, La-3d and Nd-3d. Moreover, Fig. 6b and c
show that aer O-1s XPS peak deconvolution, four peaks can be
clearly observed at 529.29–529.41 eV (lattice oxygen in perov-
skite oxides) and 531.32–531.40 eV (chemically adsorbed
oxygen). This same peak intensity and binding energy clearly
indicates that the La0.6Nd0.4FeO3@AC composite anode is of
perfect stability during the electrocatalytic oxidation process.
Particularly, aer electrocatalytic oxidation the characteristic
peaks of Fe-2p are the same as the original, which can further
demonstrate that the interference of the leaching of iron species
is negligible.
Fig. 3 EDS of (a) modified AC; (b) LaFeO3@AC; (c) La0.8Nd0.2FeO3@AC;
(d) La0.6Nd0.4FeO3@AC; (e) La0.4Nd0.6FeO3@AC; (f) La0.2Nd0.8FeO3@AC;
(g) NdFeO3@AC.
3.2. Adsorption and electrochemical catalytic oxidation
performance of the modied AC and La1ꢀxNdxFeO3@AC
composite electrodes
Under conditions without input current, the pH of MO waste-
water was adjusted to 2.0 using dilute H2SO4 solution, the
physical adsorption (Fig. 7a and b) along with electrocatalytic
degradation (Fig. 7c and d) performance of the composite
anodes was investigated according to the method described
above in the experimental section and the removal rates of MO
and COD in wastewater were taken as the research index.
As can be seen from Fig. 7a, using modied AC, LaFeO3@AC,
Fig. 4 Survey XPS spectrum of perovskite catalysts: (a) LaFeO3@AC;
(b) La0.8Nd0.2FeO3@AC; (c) La0.6Nd0.4FeO3@AC; (d) La0.4Nd0.6FeO3@AC;
(e) La0.2Nd0.8FeO3@AC; (f) NdFeO3@AC.
seen from Table 3, according to the ratio of peak intensity
between Oa and Ob, the relative amounts of Ob in LaFeO3@AC, La0.8Nd0.2FeO3@AC, La0.6Nd0.4FeO3@AC, La0.4Nd0.6FeO3@AC,
La0.8Nd0.2FeO3@AC, La0.6Nd0.4FeO3@AC, La0.4Nd0.6FeO3@AC, La0.2Nd0.8FeO3@AC and NdFeO3@AC composite electrodes to
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Fig. 5 O-1s XPS patterns of the perovskite catalysts: (a) LaFeO3@AC; (b) La0.8Nd0.2FeO3@AC; (c) La0.6Nd0.4FeO3@AC; (d) La0.4Nd0.6FeO3@AC; (e)
La0.2Nd0.8FeO3@AC; (f) NdFeO3@AC.
perform physical adsorption for 10 minutes, the removal rate of respectively (Table 1). According to the theory of pHPZC, if pH <
MO can reach 11.21%, 12.52%, 14.48%, 15.87%, 16.63%, pHPZC (in solution), the surface of the AC will have positive
17.94% and 18.53%, respectively. Moreover, as shown in Fig. 7b, charge.25 However, when the pH in MO wastewater is 2.0, the
under the same physical adsorption time, the COD removal surface positive charge in these composite materials can
rates of these composite electrodes are 33.05%, 34.46%, arrange in order and increase gradually, while the positive charge
34.78%, 35.17%, 35.86%, 36.53% and 37.49%, respectively. in anionic C14H14N3O3Sꢀ in MO gradually increases.32 Therefore,
The above results show that modied AC, LaFeO3@AC, a larger pHPZC value can lead to more La1ꢀxNdxFeO3@AC mole-
La0.8Nd0.2FeO3@AC, La0.6Nd0.4FeO3@AC, La0.4Nd0.6FeO3@AC, cules being adsorbed on MO, along with a higher removal rate of
La0.2Nd0.8FeO3@AC and NdFeO3@AC composite electrodes MO and COD.
have a gradually increasing trend in the physical adsorption of
Under the conditions of pH ¼ 2.0 in MO wastewater and an
MO. In addition, the abilities of the different composite elec- electrolyte concentration of Na2SO4 of 7 g Lꢀ1, the composite
trodes in the Fe–C primary cell process to remove organic electrodes were prepared and the electrocatalytic oxidation test
pollutants are very similar to each other,31 which suggests that
the primary cause of differences in MO adsorption capacity
between the composite electrodes is the different internal
adsorption capacities of AC.
The pHPZC of modied AC, LaFeO3@AC, La0.8Nd0.2FeO3@AC,
La0.6Nd0.4FeO3@AC, La0.4Nd0.6FeO3@AC, La0.2Nd0.8FeO3@AC
and NdFeO3@AC is 2.82, 6.56, 6.63, 6.90, 7.20, 7.36 and 7.58
Table 3 O-1s XPS results of the perovskite catalysts
Binding energy (eV)
Sample
Oa
Ob
Ob/(Oa + Ob), %
LaFeO3@AC
529.37
529.57
529.41
529.51
529.64
529.49
531.37
531.43
531.32
531.41
531.47
531.30
24.39
48.19
64.40
57.80
40.38
39.12
La0.8Nd0.2FeO3@AC
La0.6Nd0.4FeO3@AC
La0.4Nd0.6FeO3@AC
La0.2Nd0.8FeO3@AC
NdFeO3@AC
Fig.
6 Survey XPS spectrum and O-1s XPS patterns of the
La0.6Nd0.4FeO3@AC composite anode, before and after the electro-
catalytic oxidation process.
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Fig. 7 (a) The MO removal efficiency in MO wastewater of different composite anodes (physical adsorption); (b) the COD removal efficiency in
MO wastewater of different composite anodes (physical adsorption); (c) the MO removal efficiency in MO wastewater of different composite
anodes and air during the electrocatalytic process; (d) the COD removal efficiency in MO wastewater of different composite anodes and air
during the electrocatalytic process.
along with the direct aeration oxidation experiment were wastewater, the La0.6Nd0.4FeO3@AC composite electrode has
carried out. The results are shown in Fig. 7c and d.
the strongest activity, and the removal rates of MO and COD can
Fig. 7c and d show that the removal rates of MO and COD in reach 99.81% and 96.66%, respectively, within 10 minutes. In
10 min were 3.77% and 5.71% when the reaction system was the absence of surface adsorbed H2O, the highest content of Ob
only treated by aeration, which indicates that the air in the MO on the surface of La0.6Nd0.4FeO3@AC is 64.40%, leading to the
wastewater oxidation degradation capacity is quite limited.
Furthermore, the removal rates of MO and COD are greatly
strongest oxidation capacity.33,34
Additionally, in order to better understand the MO degra-
improved when using the composite electrodes, such as modi- dation procedure, color removal properties are investigated and
ed AC and La1ꢀxNdxFeO3@AC. The modied AC composite the results are shown in Fig. 8.
electrode is much weaker than the La1ꢀxNdxFeO3–AC series of
It can be clearly seen from Fig. 8 that with the electrocatalysis
composite electrodes in the resolution reaction of H2O2. The of La0.6Nd0.4FeO3@AC composite anodes, the decolorization
electrolytic current density is 400 mA dmꢀ2, and the removal rate of MO can reach 100% within 10 minutes, while the
rates of MO and COD in 10 minutes can only reach 40.15% and removal rate of COD is as high as 96.66%. These results
33.88%, respectively.
demonstrate that the chromophore in MO has been basically
Signicantly, the ratio of Nd in La1ꢀxNdxFeO3–AC has a great destroyed, and the organic species are degraded almost
inuence on the performance of the composite electrode. When completely, which corresponds with the GC-MS analysis.
the electrolysis current density is at 400 mA dmꢀ2, in 10
minutes the MO removal rate can reach 62.48%, 90.64%,
3.3. Infrared spectrum analysis of MO wastewater before
and aer reaction
99.81%, 94.67%, 86.18% and 73.36% using LaFeO3@AC, La0.8
-
Nd0.2FeO3@AC, La0.6Nd0.4FeO3@AC, La0.4Nd0.6FeO3@AC,
La0.2Nd0.8FeO3@AC and NdFeO3@AC composite electrodes. As shown in Fig. 9, the infrared spectra of MO wastewater
Meanwhile, the removal rates of COD can reach 72.85%, treated with different composite anodes, such as modied AC
87.31%, 96.66%, 92.57%, 82.93% and 80.24% in 10 minutes, and La1ꢀxNdxFeO3@AC (x ¼ 0, 0.2, 0.4, 0.6, 0.8 or 1.0), have
respectively. In terms of the MO and COD removal rates in MO been obtained in the range of 4000–750 cmꢀ1
.
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water samples were analyzed to produce a UV ultraviolet visible
spectrum and the results are shown in Fig. 10.
The latest result shows that the maximum absorption
wavelength of MO is affected by the initial pH.37 When the
initial pH is between 3.1–12.0, the maximum absorption
wavelength lmax ¼ 464 nm, and moreover, lmax ¼ 507 nm for
pH ¼ 0.0–3.1.
As can be seen from Fig. 10a, when the pH of MO wastewater
is 2.0, the maximum absorption wavelength (lmax) position is
basically consistent with 507 nm. Moreover, there are two
distinct absorption peaks at 270 nm and 507 nm when the
reaction time was 0 minutes. The former peak (270 nm) is in the
ultraviolet band, which corresponds to p / p* conversion of
two adjacent aromatic rings in MO molecules. The latter peak
(507 nm) is in the visible light band, which corresponds to the
transition from n / p* of the azo bond of the conjugated
structure in MO molecules.38 With an increase in reaction time
(0–10 minutes), the intensity of the absorption peaks at 270 nm
and 507 nm has been obviously weakened. At a reaction time of
7.5 minutes, the peak at 507 nm cannot be observed, and if the
UV spectrum of MO wastewater at 7.5 minutes reaction time is
magnied (Fig. 10b) only a weak absorption peak can be seen at
507 nm. However, in Fig. 10c, the peak at 507 nm has
completely disappeared, and there is no new absorption peak,
indicating that at 10 minutes reaction time, the MO in the
wastewater has been completely degraded.
Fig. 8 The decolorization rate of MO wastewater using different
composite anodes and air during the electrocatalytic process.
As shown in Fig. 9a, untreated MO wastewater appeared at
1607 cmꢀ1 with a double aromatic ring (benzene ring) stretching
vibration peak,35 while in Fig. 9e and f, the transmittance of the
stretching vibration peak at 1607 cmꢀ1 is almost close to 0, which
indicates that the aromatic ring of the MO molecule has been
completely destroyed aer the MO wastewater was treated using
La0.6Nd0.4FeO3@AC and La0.4Nd0.6FeO3@AC, and the destruction
of the aromatic rings in Fig. 9e is more violent than in Fig. 9f.
In Fig. 9f and g, the stretching vibration peaks of
aromatic rings can still be observed, indicating that the ability
of La1ꢀxNdxFeO3@AC (x ¼ 0, 0.2, 0.8 or 1) and modied AC
composite anodes to catalyze the oxidation of MO molecules is
less than that of x ¼ 0.4 and 0.6 in La1ꢀxNdxFeO3@AC. As can be
seen from Fig. 9b and c, the –N]N– vibration peak of MO can
be observed at 1424 and 1426 cmꢀ1 aer being treated with
a composite anode such as modied AC and LaFeO3@AC.36
However, as shown in Fig. 9d–h, the vibrational peaks of
–N]N– are already very weak or cannot be observed, and so it
can be seen that the composite anodes such as modied AC and
LaFeO3@AC have a weaker decoloration ability for MO waste-
water than La1ꢀxNdxFeO3@AC (x ¼ 0.2, 0.4, 0.6, 0.8 or 1.0).
When comparing Fig. 9g and h with 9b and c, although the
vibrational peaks of –N]N– in Fig. 9g and h have disappeared
or are not apparent, the stretching vibration peaks of the
aromatic rings in the MO molecule at 1607 cmꢀ1 are still quite
strong, which shows that the –N]N– bond in MO molecules is
more susceptible to destruction than the aromatic ring during
the electrochemical oxidation degradation process.
3.5. Gas chromatography-mass spectrometry (GC-MS)
analysis on the electrocatalytic oxidation degradation of MO
For the purpose of determining the intermediates during the
degradation process of methyl orange (MO), GC-MS was per-
formed to study the qualitative composition of the intermedi-
ates. 20 mL MO wastewater (aer degradation for 10 min) was
extracted using dichloromethane (30 mL) for 30 min, then an
amount of anhydrous sodium sulfate (Na2SO4) was added for
dehydration and condensed to 5 mL by a rotary evaporator to
satisfy the sampling conditions. The experimental results are
shown in Fig. 11.
As can be seen from Fig. 11, the MO wastewater was
degraded into a few intermediates during the degradation
process, which indicated that the La0.6Nd0.4FeO3@AC
composite anode shows excellent ability for the electrocatalytic
degradation of MO. When the time was 3.13, 3.96, 4.05, 4.38,
4.47, 7.59 and 8.93 min, the intermediates are the aromatics
and chain hydrocarbon species, which corresponds with the
literature.39,40 The results indicate that during the process of MO
electrocatalytic degradation by a La0.6Nd0.4FeO3@AC composite
anode, the large conjugated system formed by azo bonds and
benzene rings in methyl orange molecules was destroyed.
During the degradation process, at rst, the azo bond will
probably fracture to form benzene substances under the effect
3.4. UV-vis spectrum analysis of MO reaction products
degraded by the La0.6Nd0.4FeO3@AC composite electrode
Under the conditions of pH ¼ 2.0 in MO wastewater, an elec- of the La0.6Nd0.4FeO3@AC composite electrocatalyst. With the
trolyte concentration of Na2SO4 of 7 g Lꢀ1 and an electrolytic electrocatalysis, the benzene ring will be further opened to form
current density of 400 mA dmꢀ2, the La0.6Nd0.4FeO3@AC chain-like molecules, which are nally degraded to H2O and
composite electrode was used to catalyze the degradation of MO CO2. These results are consistent with the analysis of UV-vis and
wastewater. Aer sampling every 2.5 minutes for each day, the FT-IR spectra.
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Fig. 9 Fourier-transform infrared spectroscopy of MO using different composite anodes.
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Fig. 12 Mechanism of MO degradation by a La0.6Nd0.4FeO3@AC
composite anode.
Fig. 10 UV-vis spectral changes of the 500 mg Lꢀ1 MO solution during
the degradation process as a function of reaction time in the presence
of 7 g Lꢀ1 Na2SO4, I ¼ 400 mA dmꢀ2 and pH ¼ 2.0.
(I) Cathodic process – reduction of oxygen
O2(g) + 2H+ + 2eꢀ / H2O2
(6)
(7)
O2(g) + 4H+ + 4eꢀ / 2cO + 4[H] / 2H2O
(II) Fenton oxidation reaction in the middle of the reaction
system
(1) The iron wire supplied by the anode wire network
migrates between the anode and cathode (middle), and the
Fenton reaction occurs with the cathode H2O2
Fe ꢀ 2eꢀ / Fe2+
(8)
(9)
Fe2+ + H2O2 / Fe3+ + cOH + OHꢀ
Fe3+ + H2O2 / Fe2+ + cHO2 + H+
(10)
(2) Degradation process of MO in the middle of the electrode
cOH + MO / CO2 + H2O
HO2c + MO / CO2 + H2O
(11)
(12)
(III) Anode surface: the anode consists of two parts, the
outer steel wire mesh anode and the composite catalyst
(La0.6Nd0.4FeO3@AC) anode inside the wire mesh. Their elec-
trocatalytic oxidation process is as follows:
Fig. 11 GC-MS spectrum of MO wastewater after degradation by
a La0.6Nd0.4FeO3@AC composite anode for 10 min.
(1) Wire mesh near the anode
(i) The Fe2+ provided by the wire mesh reacts with the
convective migration of H2O2 to the anode (or the electro-
chemical reaction of the anode) to produce a Fenton reaction
3.6. Mechanism of electrocatalytic oxidation degradation of
the La0.6Nd0.4FeO3@AC composite anode
Fig. 12 shows the mechanism of
a La0.6Nd0.4FeO3@AC
Fe ꢀ 2eꢀ / Fe2+
(8)
(9)
composite anode in the degradation of MO wastewater.
The main part is divided into a cathode oxygen reduction,
a Fenton oxidation reaction system at the central part of the
surface of the anode and the electrocatalytic oxidation of the
MO reaction. Signicantly, the segmentation process can
probably be accounted for by a mechanism as follows:
Fe2+ + H2O2 / Fe3+ + cOH + OHꢀ
Fe3+ + H2O2 / Fe2+ + cHO2 + H+
(10)
(ii) The degradation process of MO
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signicance with efficiency in the application of the treatment
of MO. Furthermore, to the best of our knowledge, no research
on the electrochemical properties of a La1ꢀxNdxFeO3@AC
composite material has been reported. Our studies in this
current paper provide a series of novel perovskite–carbon
composites, and ingenious methods for the electrochemical
treatment of refractory pollutants, which are expected to play
more important roles in the treatment of organic pollutant
wastewater.
cOH + MO / CO2 + H2O
HO2c + MO / CO2 + H2O
(11)
(12)
(2) Electrochemical oxidation of H2O on the surface of the
La0.6Nd0.4FeO3@AC composite catalyst anode and steel wire
mesh
M + H2O / M(cOH) + H+ + eꢀ
2M(cOH) / 2MO + H2O2
(13)
(14)
Author contributions
(3) Catalytic oxidation of reactive oxygen species on the
anode surface of the composite catalyst (La0.6Nd0.4FeO3@AC). Q. J. W., S. Z., and H. R. W. conceived the idea and co-wrote the
(^Fe3+–OH means a xed carrier above the oxide containing manuscript. Q. J. W., S. Z., F. F. W. and X. Z. Z. carried out the
Fe3+)
materials synthesis and the electrochemical catalysis. Q. J. W.,
S. X., S. Z. and J’e Q. performed the materials characterizations.
H. R. W. proposed, planned and supervised the project. All of
the authors discussed the results and commented on the
manuscript. The manuscript was written through the contri-
butions of all authors. All authors have given approval of the
nal version of the manuscript. Q. J. W. and S. Z. contributed
equally to this work.
2+
^Fe3+–OH + H2O2 4 ^FeHO2 + H+
(15)
(16)
(17)
(18)
(19)
^Fe3+–OH + H2O 4 ^FeOH2+ + H+
2+
^FeHO2 / ^Fe2+ + HO2c
2+
^FeHO2 + ^FeOH2+ / 2^Fe2+ + O2 + H2O
^Fe2+ + H2O2 / ^Fe3+–OH + cOH + OHꢀ
Conflicts of interest
(4) Physical and electrochemical adsorption processes of MO
on the La0.6Nd0.4FeO3@AC anode surface of the composite
catalysts
The authors declare no conict of interest.
Acknowledgements
MO + La0.6Nd0.4FeO3@AC /
The authors gratefully acknowledge the nancial support of the
National Natural Science Foundation of China (Grant No.
51001045) and the Youth Foundation of Hubei Provincial
Education Bureau (Grant No. Q20120110).
C6H5OH ꢀ La0.6Nd0.4FeO3@AC (20)
(5) MO degradation process on the anodic surface
cOH + MO / CO2 + H2O
HO2c + MO / CO2 + H2O
M(cOH) + MO / CO2 + H2O
(11)
(12)
(21)
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