Magnetic nano-sized cadmium ferrite as an efficient catalyst for the degradation of Congo red in the presence of microwave irradiation

Article abstract of DOI:10.1039/c5ra07591b

A highly active nano-sized CdFe₂O₄ catalyst was prepared by a hydrothermal process and characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), BET specific surface area method, and vibrating sample magnetometer (VSM) at room temperature. Application of the microwave-induced catalytic degradation method in the abatement of Congo red (CR) using the magnetic catalyst was studied. The degradation ratio of CR with CdFe₂O₄ reached 94.4% with 10 min microwave irradiation (MW), proving CdFe₂O₄ to be an excellent microwave catalyst. The intermediate products from CR degradation were investigated by UV-Vis, HPLC and ion chromatography. The reaction kinetics, effects of different ion species (SO₄^2-, NO₃^-, HCO₃^-, and CH₃COO^-), pH of the solution, dye initial concentration, dosage of catalyst, as well as degradation mechanism were comprehensively studied. Radical trapping studies and the fluorescence technique revealed the holes (h⁺) and hydroxyl radicals (OH) were involved as the main active species in the reaction. The band structure of CdFe₂O₄ was analyzed by UV-Vis diffuse reflectance spectroscopy and Mott-Schottky measurements. The mechanism of the degradation was discussed in detail. This work can provide an effective technology for dye wastewater treatment.

Full text of DOI:10.1039/c5ra07591b

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Magnetic nano-sized cadmium ferrite as an
efficient catalyst for the degradation of Congo red
Cite this: RSC Adv., 2015, 5, 51027
in the presence of microwave irradiation
*
Wen Shi, Xueyan Liu, Tingting Zhang, Qiong Wang and Lei Zhang
A highly active nano-sized CdFe2O4 catalyst was prepared by a hydrothermal process and characterized by X-
ray diffraction (XRD), scanning electronic microscopy (SEM), BET specific surface area method, and vibrating
sample magnetometer (VSM) at room temperature. Application of the microwave-induced catalytic
degradation method in the abatement of Congo red (CR) using the magnetic catalyst was studied. The
degradation ratio of CR with CdFe2O4 reached 94.4% with 10 min microwave irradiation (MW), proving
CdFe2O4 to be an excellent microwave catalyst. The intermediate products from CR degradation were
investigated by UV-Vis, HPLC and ion chromatography. The reaction kinetics, effects of different ion species
(SO42ꢀ, NO3ꢀ, HCO3ꢀ, and CH3COOꢀ), pH of the solution, dye initial concentration, dosage of catalyst, as
well as degradation mechanism were comprehensively studied. Radical trapping studies and the
fluorescence technique revealed the holes (h+) and hydroxyl radicals (cOH) were involved as the main active
species in the reaction. The band structure of CdFe2O4 was analyzed by UV-Vis diffuse reflectance
spectroscopy and Mott–Schottky measurements. The mechanism of the degradation was discussed in
detail. This work can provide an effective technology for dye wastewater treatment.
Received 26th April 2015
Accepted 2nd June 2015
DOI: 10.1039/c5ra07591b
www.rsc.org/advances
absorption ability of microwave. Some ferrites as MeFe2O4 (Me
¼ Zn, Mn, Co, Ni, Cu, Mg, etc.), appeared to be especially effi-
cient for the degradation and removal of organic pollutants.21–26
However, the performance of nanoscale ferrites in MW-induced
degradation process is still needed further research.
1. Introduction
Synthetic dyestuffs are used extensively in textile, paper-making,
and printing industries and dyehouses. The effluents of these
industries are highly colored and the disposal of these wastes
into receiving waters can cause damage to the environment. The
dyes generally have complex aromatic structures and thus most
of them are highly resistant to breakdown by chemical, physical,
and biological treatments.1,2 Microwave techniques have been
applied in various elds, such as organic synthesis,3,4 analysis,5
and military and environmental engineering.6,7 Researchers
have focused on the application of MW irradiation in environ-
mental wastewater treatment,8–10 particularly in the degradation
of dyes with complex aromatic structures, which are toxic and
nondegradable.11,12 The microwave-assisted degradation tech-
nique for pollutants can potentially reduce reaction time.
Over the past decade, MW absorbing materials, known as
dielectrics, have been the focus of much attention, particularly
for their signicant roles in wastewater treatment. Materials,
such as activated carbon,13,14 transition metal oxide,15,16
CNTs17,18 and polymers,19 are commonly used in microwave-
assisted degradation of organic pollutants. Among MW absor-
bents, ferrites are the most promising.20 Ferrite, which derived
from iron oxides, has attracted great interest for its strong
Now, we were interested in exploring another member of
ferrites, magnetic CdFe2O4. CdFe2O4 has normal spinel struc-
ture, excellent gas-sensing properties,27 better charge transport
characteristics and small bandgap (1.97 eV).28 Within literature,
most of the studies discussed its applications as gas-sensing
materials, but much less attention is given for dye degrada-
tion systems. To the best of our knowledge, there is no report on
the systematic investigation on dye degradation using nano-
sized magnetic CdFe2O4 as MW absorbing material.
In the present work, CdFe2O4 was prepared by a one-step
hydrothermal method. The catalytic activities of CdFe2O4 were
investigated by degradation of CR solution under MW. The focus
was concentrated on below aspects: (1) exploring potential possi-
bility about MW-induced CdFe2O4 catalytic degradation of dye; (2)
identifying intermediates via HPLC and ionic chromatogram
techniques and proposing MW-induced degradation mechanism
based on the exploration of generating chemically-active species.
2. Experimental section
2.1. Chemicals
College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang
110036, People's Republic of China. E-mail: zhanglei63@126.com; Fax: +86 24
62202380; Tel: +86 24 62207809
The target dye CR was purchased from Sinopharm Chemical
Reagent Co. Ltd., and used without further purication. All
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chemicals used in this experiment were analytical grade. oven temperature maintained at 30 ꢁC. The compounds were
Deionized water was used throughout the experiments.
eluted with methanol–water (38/62 (v/v)).
For further validating degradation of CR in aqueous solu-
tion, the ionic chromatogram was used. The other conditions
were as follows: AS9-HC column (250 mm ꢂ 4 mm i.d.),
9.0 mmol Lꢀ1 Na2CO3 eluent, 1.0 mL minꢀ1 ow rate and
conductivity detector.
2.2. Synthesis and characterization of CdFe2O4
CdFe2O4 nanoparticles were synthesized using a hydrothermal
process. Cd(NO3)2$4H2O (1.2339 g, 0.004 mol) and Fe(NO3)3-
$9H2O (3.2320 g, 0.008 mol) were dissolved in 80 mL of ethanol
under continuous stirring to get a homogeneous suspension.
The solution was then transferred into a 100 mL Teon lined
stainless-steel autoclave, which was sealed and maintained at
180 ꢁC for 10 h. Aer allowing the reactor to cool down to room
temperature, the precipitate was separated from solution by
centrifugation, washed with deionized water and dried at 60 ꢁC
for 12 h.
The morphologies of the catalyst were characterized using
scanning electron microscopy (HITACHI SU8000). Measure-
ment of BET surface area was performed using N2 adsorption–
desorption isotherms on a Micromeritics (Norcross, GA). The
X-ray diffraction (XRD) patterns of CdFe2O4 powders were
recorded on Siemens D5000 Diffractometer (Germany).
Vibrating sample magnetometer (VSM, Lakeshore 7407) was
used to measure the magnetic prosperities of CdFe2O4. A Mal-
vern Zetasizer Nano-ZS particle analyzer (Malvern, U.K.) was
used to determine the z potential of CdFe2O4. UV-Vis diffuse
2.4. Analysis of hydroxyl radicals
The formation of hydroxyl radicals on the surface is measured
by the uorescence method, which uses terephthalic acid as a
probe molecule.21,29 Terephthalic acid readily reacted with
hydroxyl radicals to produce highly uorescent product,
2-hydroxyterephthalic acid. The intensity of the uorescence
signal at 425 nm of 2-hydroxyterephthalic acid was in propor-
tion to the amount of hydroxyl radicals produced in water.
Fluorescence spectra of the generated 2-hydroxyterephthalic
acid were measured on a Cary Eclipse uorescence spectro-
photometer (VARIAN Co, USA).
3. Results and discussion
3.1. Characteristics of CdFe2O4
Fig. 1a shows the XRD pattern of as-synthesized CdFe2O4
powders. It is observed that the diffraction peaks could be well
indexed to the spinel-type of CdFe2O4 (JCPDS 22-1063). The
diffraction peak appeared below 25ꢁ indicated the presence of
a-Fe2O3. Fig. 1b displays SEM micrograph of the obtained
CdFe2O4, showing the spherical formation of the synthesized
nanoparticles with diameter in the narrow size range of
20–30 nm. Some agglomerates of the CdFe2O4 particles are also
observed because of the high surface energy and magnetic
interactions between the nanocrystallites. The specic area of
the sample was calculated using the Brunauer–Emmett–Teller
(BET) method. Fig. 1c displays the N2 adsorption–desorption
isotherm curve of the CdFe2O4. The BET surface area of
CdFe2O4 was estimated to be 85.80 m2 gꢀ1. The pore-size
distribution of the samples was calculated by Barreet–Juyner–
Halenda (BJH) method (see the inset of Fig. 1c). It can be seen
that the average pore diameter of CdFe2O4 is 24 nm. The mes-
oporous structures would help the adsorption and transition of
CR or its intermediates during the degradation process. Fig. 1d
presents the hysteresis loops of the CdFe2O4 (inset: the photo of
magnetic separation). The saturation magnetization of CdFe2O4
is 22.34 emu gꢀ1. Since saturation magnetization of 16.3 emu
gꢀ1 was enough to separate magnetic particles from solution
with a magnet,30 CdFe2O4 could be rapidly recovered from
water. The UV-Vis diffuse reectance spectrum of CdFe2O4 is
illustrated in Fig. 1e. The plot obtained by the transformation
based on the Kubelka–Munk function versus the energy of light
is displayed in Fig. 1f. The estimated band gap value (Eg) was
1.97 eV for CdFe2O4.
reectance spectrum (DRS) was acquired by
a UV2550
(Shimadzu Scientic Instruments Inc. Japan) and BaSO4 was
used as the reectance standard. A conventional three electrode
cells using a CHI660D electrochemical workstation (Shanghai
Chenhua, China) was used to determine the at-band potential
(V) of the sample. The working electrode was prepared by dip-
coating method as following: 10 mg sample was suspended in
10 mL deionized water, which was then dip-coated onto a 1 cm
ꢂ 1 cm uorine–tin oxide (FTO) conducting glass electrode,
while a platinum wire as counter electrode, and a standard
Ag/AgCl in saturated KCl as reference electrode. The Mott–
Schottky measurements were carried out from ꢀ1.0V to 1.0V.
2.3. Degradation tests and analytical methods
The experiments were carried out in a temperature-controllable
microwave oven (XH100B, Beijing XiangHu Ltd. China) equip-
ped with a reux condenser. At the beginning, 50 mL of CR
(20 mg Lꢀ1) aqueous solution and 0.05 g of CdFe2O4 were added
into a 250 mL round bottom ask with 3 necks. Then set up the
parameters of the MW oven: power (700 W), temperature
(100 ꢁC). CR solution samples during the degradation were
taken out at predetermined time intervals by magnetic separa-
tion. The degradation of CR under MW irradiation without any
catalysts and the CR adsorption on CdFe2O4 without MW irra-
diation were also carried out for comparison.
Cary 5000 UV-Vis-NIR (Varian, USA) was utilized to record the
spectra of aqueous CR solution aer treatment. The reaction
intermediates were detected by HPLC (Agilent 1100, USA)
equipped with diode array detector and a column oven. A
150 mm ꢂ 4.6 mm reverse-phase C-18 column was used for
3.2. Degradation of CR by CdFe2O4 with MW irradiation
separation. The injection volume was 20 mL, ow rate was Fig. 2a shows the change of absorption spectra of CR in the
1.0 mL minꢀ1, UV detector wavelength was 497 nm and column presence of CdFe2O4. The absorption peak at l ¼ 497 nm
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diminished gradually with increasing irradiation time and where C is the concentration of dye at various time (t), C0 is the
completely disappeared aer 10 min. No new absorption bands initial dye concentration and k is the apparent reaction rate
appeared in either the visible or the ultraviolet regions, which constant of dye removal. The results clearly demonstrated the k
indicated the complete decolorization of CR during the value was 26.71 ꢂ 10ꢀ2 minꢀ1 in MW/CdFe2O4 system.
reaction.
However, the k value was almost negligible with only MW irra-
The removal efficiency of CR over different conditions as a diation, proving CdFe2O4 to be an excellent microwave catalyst.
function of time was investigated (Fig. 2b). Self-degradation
of CR was almost negligible without catalyst. And the
3.3. Inuences of several factors
removal efficiency of CR was approximately 26.5% aer
10 min in CdFe2O4 dispersion without MW irradiation.
Whereas CdFe2O4 displayed high catalytic performance, as
almost 94.4% of CR was degraded under MW irradiation for
only 10 min.
Fig. 2c shows the kinetic studies of the degradation of CR
over different conditions. It was observed that the catalytic
reaction obeyed the pseudo-rst-order model according to the
Langmuir–Hinselwood model and may be expressed as:
3.3.1. Effect of initial concentration and catalyst doses. The
effects of different initial concentrations of CR on the degra-
dation ratio were examined in the concentration ranges from
20 to 70 mg Lꢀ1. Fig. 3a clearly shows that the degradation ratios
decrease with increased CR concentration, from 94.4% at initial
CR concentration of 20 mg Lꢀ1 to 46.8% at 70 mg Lꢀ1 aer
10 min MW irradiation. With the initial concentration increase,
more organic substances are adsorbed on the surface of the
catalyst, but the microwave irradiation time and catalyst are
constant.
ln(C0/C) ¼ kt
(1)
Fig. 1 Characterization of CdFe2O4. (a) XRD spectrum; (b) SEM image; (c) N2 adsorption–desorption isotherm. The inset showed the corre-
sponding pore-size distribution; (d) magnetic hysteresis loops (inset: the photo of magnetic separation); (e) UV-Vis diffuse reflectance spectrum
(DRS) of CdFe2O4; (f) plot of transformed Kubelka–Munk function versus the energy of light.
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Fig. 2 (a) The variation of UV-Vis absorption spectra at different irradiation time; (b) removal efficiency of CR at different conditions; (c) the
kinetic curves.
Fig. 3 The effect of the initial concentration (a) and dosage of CdFe2O4 (b) on the degradation of CR.
Fig. 3b shows the dependence of CR (20 mg Lꢀ1) degradation
The pH of the solution inuenced the adsorption behavior of
on the dosage of the CdFe2O4 catalyst. It was found that the the organic dyes on the catalyst surface. When the pH value is
degradation ratios increase with increased dosage of CdFe2O4. lower than isoelectric point (pH 5.4, Fig. 4b) of CdFe2O4, its
And when the dosage exceeded 0.05 g, further increasing the surface is positively charged. Whereas, when the pH value is
dosage had negligible effect on the degradation ratio of CR aer higher than the isoelectric point, its surface is negatively
10 min MW irradiation. Therefore, 0.05 g CdFe2O4 was used in charged. Therefore, in the acidic and neutral solution the CR
our experiments.
anions are close to the surface of CdFe2O4 particles or adsorbed
3.3.2. Effect of microwave power and initial pH. The MW on it. Hence, the results exhibited a high degradation ratio of
output power was regarded as the most important factor in CR before pH 7.0. However, at higher pH values the CR anions
the experiment, since the temperature that the CdFe2O4 were generally excluded away from the negatively charged
could reach was directly related to the power level. The surface of CdFe2O4. So the degradation ratio began to decrease.
investigated power levels were 200, 400, 600 and 700 W, and
3.3.3. Effect of anions. The existence of inorganic anions
other parameters remained constant, namely, CR aqueous such as chloride, sulfate, bicarbonate, nitrate, and acetate is
solution (20 mg Lꢀ1) and 0.05 g CdFe2O4, irradiation time considerably common in wastewaters and also in natural water.
10 min. The test results shown in Fig. 4a indicated that In this study, the initial pH 6.5 was employed and the effect of
degradation efficiency of CR gradually increased with the MW Clꢀ on the degradation of CR was studied by adding NaCl into
power from 200 to 700 W. So the output power of 700 W was the reaction solution at different concentrations of 0.001,
chosen in this study.
0.01 and 0.1 mol Lꢀ1. The results are shown in Fig. 5a. It can be
The inuence of initial solution pH on CR degradation was seen that Clꢀ has a detrimental effect at high concentrations of
also studied at catalyst dosage 0.05 g, CR concentration 20 mg chloride (>0.1 mol Lꢀ1). The degradation efficiency of CR
Lꢀ1. Different experiments were performed at pH values of 4.0, without additives is 94.4% aer MW irradiation for 10 min,
5.0, 7.0, 9.0, and 10.0 as shown in Fig. 4a. It was found that the while decreases to 76.6% at [Clꢀ] ¼ 0.1 mol Lꢀ1. The observed
degradation ratio of CR under MW irradiation changed scarcely inhibition effect can be explained with the scavenging of cOH
and maintained a high level from pH 4.0 to pH 7.0, and then fell radicals by ions.31
2ꢀ
slightly aer pH 9.0. In general, the natural pH of CR solution
Fig. 5b shows the effects of different anions (i.e. SO4
,
was close to 6.5. In this work, the CR solution without adjusting NO3ꢀ, HCO3ꢀ, and CH3COOꢀ) of their sodium salts at the same
pH was popularly adopted.
concentration of 0.001 mol Lꢀ1. Compared to the control test in
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Fig. 4 (a) The effect of microwave power and pH on the degradation of CR; (b) zeta potential of CdFe2O4.
Fig. 5 (a) Influence of Clꢀ concentrations on the degradation of CR (initial pH 6.5 and [C0] ¼ 20 mg Lꢀ1); (b) degradation of CR in the presence of
different anions (initial pH 6.5 and [C0] ¼ 20 mg Lꢀ1).
the aqueous CR solutions without anion, NO3ꢀ, SO42ꢀ, and
3.4. Stability and reusability of CdFe2O4
CH3COOꢀ have no obvious inuence onꢀthe CR degradation.
The reusability of the catalyst is crucial in the practical appli-
cation. To evaluate the catalytic stability of CdFe2O4, the parti-
cles were recovered using an external magnet to perform with
And the existence of 0.001 mol Lꢀ1 HCO3 leads to the signi-
cant retardation of CR degradation, which can be attributed to
the combination of radicals scavenging and competitive
successive tests of CR degradation. The morphologies of the
adsorption. HCO3ꢀ can be adsorbed by the catalyst. In addition,
CdFe2O4 aer reaction were then examined. As shown in Fig. 6a,
recycled CdFe2O4 showed strong activity in CR degradation and
h+ and cOH can be trapped by HCO3 (eqn (2) and (3)).
ꢀ
its activity remained almost unchanged in four cycles. The
degradation ratios in all four runs are more than 90.0%. And the
SEM image (Fig. 6b) shows that the sample still maintains the
original morphology and particle size. These results strongly
HCO3 + h+ / cHCO3
(2)
(3)
ꢀ
ꢀ
ꢀ
ꢀ
HCO3 + cOH / cHCO3 + OHꢀ
Fig. 6 (a) Cyclic tests of the CdFe2O4 magnetic catalyst; (b) SEM image of the CdFe2O4 after the experiment of CR degradation.
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Fig. 7 Variations of HPLC chromatograms of CR over CdFe2O4 with detection wavelength as 497 nm (a) and 345 nm (b); ion chromatogram of
CR solution during degradation (c).
indicate that the CdFe2O4 nanoparticles are stable and recov- radical scavengers on the degradation of CR over CdFe2O4
erable as catalyst under MW irradiation.
under MW irradiation was investigated. The effects of the
scavengers on the degradation efficiency of CR are shown in
Fig. 8a. It turned out that the catalytic degradation efficiency of
CR decreased notably from 94.4% to 49.5% in the presence of 1
mM sodium oxalate (Na2C2O4, h+ scavenger32). Meanwhile, the
rate of CR degradation (k) decreased obviously from 26.71 ꢂ
10ꢀ2 to 6.67 ꢂ 10ꢀ2 minꢀ1, indicating that the holes are the
main active species in the catalytic process. In addition, it has
been observed that, upon addition of tert-butyl alcohol (TBA)
(7.9 wt%), the rate of CR degradation decreases effectively (k ¼
3.5. Identication of the intermediates and nal products
In order to explore the intermediates in the degradation process,
the chromatography of HPLC analysis was carried out at 497 nm
and 345 nm, respectively. As shown in Fig. 7a, the matrix peak of
CR appeared at tR ¼ 4.28 min retention time, and it diminished
gradually along with the increase of the irradiation time, sug-
gesting CR was degraded obviously and rapidly. Synchronously,
the peaks with tR ¼ 4.28 min at 345 nm also signicantly
decreased (Fig. 7b). It could be seen that there was not any new
peak appearing compared to the initial CR solution at both
497 nm and 345 nm, indicating no byproducts was identied.
For further validating degradation of CR in aqueous solu-
tion, the ionic chromatogram was determined as shown in
Fig. 7c. The ionic chromatographic peaks corresponding to
ꢀ
2ꢀ
NO2ꢀ, NO3 and SO4 anions gradually increased along with
microwave irradiation. All of these results indicated that the
C–S, C–N and azo bonds in the CR molecule were destroyed
gradually. The sulfur and nitrogen atoms were oxidized and
transferred into NO2ꢀ, NO3ꢀ and SO42ꢀ anions, respectively. By
all means, the CR in aqueous solution could be mineralized to a
series of simple and innocuous inorganic ions in the end under
microwave irradiation in the presence of CdFe2O4.
3.6. Role of the reactive species
To detect the main active oxidative species responsible for the
degradation in the catalytic process, the inuence of some Scheme 1 Possible mechanism for CdFe2O4 under MW irradiation.
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Fig. 8 (a) Effects of a series of scavengers on the degradation efficiency of CR; (b) fluorescence spectral changes with MW irradiation time on
CdFe2O4 in a 3 ꢂ 10ꢀ4 M basic solution of terephthalic acid. (c) Mott-Schotty plots.
21.54 ꢂ 10ꢀ2 minꢀ1), which suggested that cOH is also Scheme 1. CdFe2O4 as microwave absorbent can strongly absorb
responsible oxidants in degradation of CR. To test the role of and transfer microwave energy. Under the microwave irradia-
cO2ꢀ, N2 or O2 was bubbled through the suspension. As shown tion, CdFe2O4 particle surface can produce great amount of “hot
in Fig. 8a, the negligible effect of N2 or O2 bubbling implies that spots”. Meanwhile, the electrons in CdFe2O4 could oscillate
ꢀ
cO2 is not the active species in this system.
under the microwave excitation. The “hot-spots” that MW
To further conrm the participation of cOH in degradation of generated could speed up the movement of electrons in
CR, the uorescence spectrophotometer was used to identify CdFe2O4 to excite electrons and therefore generate electron/
the generated cOH in the microwave-induced catalytic degra- hole pairs. The leover holes directly reacted with CR or inter-
dation process using terephthalic acid as a probe molecule. acted with the hydroxyl group or H2O to produce the cOH, which
Terephthalic acid readily reacts with cOH to produce highly was an extremely strong oxidant for the mineralization of CR. In
uorescent 2-hydroxyterephthalic acid (Ex: 315 nm, Em: addition, the holes can also directly oxidize CR to harmless
425 nm). Fig. 8b shows the changes of uorescence spectra from products. Thus, these evidences lead to the conclusion that the
3 ꢂ 10ꢀ4 M terephthalic acid solution with irradiation time. A holes (h+) and cOH should be the main factor that responsible
gradual increase in uorescence intensity at about 425 nm was for CR oxidation in this system.
observed, which further conrmed the production of cOH
radicals.
ꢀ
4 Conclusion
In order to make clear the role of cO2 and cOH, we also
used an electrochemical method to measure the at-band
positions (V) of CdFe2O4. As seen from Fig. 8c, the V value
is roughly 0.18 V versus Ag/AgCl (equivalent to 0.38 V vs. NHE).
Therefore, it can be determined that the conduction band
In this study, CdFe2O4 has been synthesized by a hydrothermal
process and characterized by various techniques such as SEM,
XRD, BET VSM and DRS. The as-prepared CdFe2O4 exhibited
excellent catalytic activity in the microwave-induced catalytic
degradation process and magnetic property for easy separation
using an external magnetic eld. The degradation percentage of
CR could reach up to 94.4% for 10 min. A possible degradation
mechanism is proposed based on the experimental results. This
work can provide an effective technology for dye wastewater
treatment.
potential (ECB) of CdFe2O4 is about 0.38 V. These accumulated
ꢀ
electrons on the CB of CdFe2O4 cannot reduce O2 to yield cO2
,
because the CB edge potential of CdFe2O4 (0.38 V vs. NHE) was
ꢀ
more positive than the standard redox potential Eq(O2/cO2
)
(ꢀ0.28 V vs. NHE).33 This explains why N2 or O2 bubbling has a
negligible effect on the catalytic performance. According to the
band gap energy (Eg ¼ 1.97 eV, Fig. 1f) obtained from DRS
measurement, the value band potential (EVB) of CdFe2O4 is
deduced to be about 2.35 V vs. NHE, which is more positive
than Eq(cOH/H2O) (2.27V vs. NHE).33 Therefore, the MW-
generated holes le behind in the VB of CdFe2O4 can theo-
retically oxidize the hydroxyl group or H2O to produce cOH
radicals.
Acknowledgements
This project was supported by the National Nature Science
Foundation of China (NSFC51178212), Liaoning Provincial
Department of education innovation team projects (LT2012001),
the Shenyang Science and Technology Plan (F12-277-1-69), the
Foundation of 211 project for Innovative Talent Training,
Liaoning University and the Program for Liaoning representative
3.7. Possible degradation methism
On the basis of the experimental results mentioned above, a office of China Environmental Protection Foundation
possible catalytic process for the degradation of CR in (CEPF2013-123-1-5). The authors also thank their colleagues and
MW/CdFe2O4 system could be proposed, as illustrated in other students who participated in this work.
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51034 | RSC Adv., 2015, 5, 51027–51034
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