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X. Liu et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 243–250
identification. Measurement of BET surface area and pore distribu-
tion was performed using N2 adsorption-desorption isotherms on
a Micromeritics (Norcross, GA). Vibrating sample magnetometer
(VSM, Lakeshore 7407) was used to measure the magnetic pros-
perities of CuFe2O4. The morphology of CuFe2O4 was determined
by scanning electron microscope (SEM) with energy-dispersive
X-ray spectrometry (EDS) analysis (Hitachi S-3400N, Japan). FTIR
spectra of CuFe2O4 were measured using FTIR 5700 (Nicolet Com-
pany, USA). The MG solution was irradiated using XH100B model
microwave apparatus (Beijing XiangHu Science and Technology
Development Co., LTD). UV–vis-NIR Cary 5000 (Varian, USA) was
used to check the degradation rate of MG. The dyes were also
detected by HPLC (Agilent 1100, USA) equipped with diode array
detector and a column oven. A 150 mm × 4.6 mm reverse-phase
C-18 column was used for separation. The injection volume was
10 L, flow rate was 1.0 mL min−1, DAD detector wavelengths were
254 nm and 618 nm, and column oven temperature maintained
298 K. MG was eluted with acetonitrile/50 mM HAC–NaAC (pH
4.5) buffer solutions (60/40 (v/v)). Ion chromatography (Dionex
ICS-900, USA) was also used to inspect the degradation prod-
ucts of MG. The conditions were as follows: IonPac®AS23 column
(250 mm × 4 mm i.d.), 4.5 mmol L−1 Na2CO3/0.8 mmol L−1 NaHCO3
eluent, 1.0 mL min−1 flow rate and conductivity detector. Cary
Eclipse Fluorescence Spectrophotometer (VARIAN Co, USA) was
applied to determine hydroxyl radical generated in MW/CuFe2O4
system. Operating conditions: The slit widths were set at 10.0 nm.
Sequential scans of the emission spectra were carried out between
200 nm and 600 nm at different excitation wavelengths ranging
from 240 nm to 400 nm. The spectra were recorded at every 2 nm
intervals.
Fig. 1. Three dimensional chemical structure of malachite green.
explanations on both the catalytic mechanism of MW and the gen-
eration of oxidizing substances (such as various active species)
during the process of microwave reaction have not been provided
yet in detail. The performance of ferrites in microwave-induced
degradation process still needs to be further researched.
In our present work, CuFe2O4 was chosen here as representative
ferrite with MG as target compound to investigate the behavior of
ferrites in microwave-induced catalytic oxidation. Meanwhile, sev-
eral scavengers were used to evaluate the kinds of generated active
species. The attention has been focused on active species genera-
tion mechanism in presence of CuFe2O4 under MW, and the ferrites
possible catalytic mechanism was revealed. Perhaps, the results
can offer some valuable references for the study on mechanism
of microwave induced catalysis process.
2.4. Degradation experiments
2. Experimental
The experiments were carried out in the temperature-
controllable microwave apparatus equipped with a reflux con-
denser. Typically, 4.0 g L−1 of CuFe2O4 catalyst was introduced into
50 mL of 20 mg L−1 malachite green solutions (initial pH 4.3) in a
250 mL boiling flask-3-neck. During the experiments, malachite
green solutions with catalyst were treated using the following
methods: including MW method, adsorption by CuFe2O4 and MW
combined with CuFe2O4. At each sampling event, the CuFe2O4 pow-
ers were separated from the solution with a magnet. The residual
solutions were determined by a UV–vis-NIR Cary 5000 spectrome-
try at 618 nm. The degradation percentage was calculated with the
equation:
2.1. Materials
The initial concentration of MG solution in all experiments
was 20 mg L−1. Cu(NO3)2·3H2O (purity > 99.0%), Fe(NO3)3·9H2O
(purity > 98.5%), terephthalic acid (purity > 99.0%),Vitamin C (VC,
purity > 99.0%), t-butanol (BU, purity > 99.0%), benzoquinone (BQ,
purity > 99.0%), and potassium iodide (KI, AR) were purchased from
Sinopharm Chemical Reagent Co., Ltd. (China). Acetonitrile (HPLC
Co., Ltd (China). The other chemical reagents were all of ana-
lytical reagent grade, and double distilled water was used for
solution preparation. Molecular structure of MG is as follows
(Fig. 1).
C0 − Ct
Degradation percentage (%) =
× 100
(1)
C0
where C0 is the initial concentration of malachite green (mg L−1
and Ct is the concentration of malachite green at time t.
)
2.2. Preparation of CuFe2O4
The effects of oxygen were investigated in which the reac-
tion solution was sparged with air or N2 for 2 h prior to the
initiation of the reactions and during the entire degradation
experiments.
Some experiments were repeated three times and the
results were reproducible within the experiments errors
CuFe2O4 was prepared by chemical co-precipitation with the
assistance of microwave irradiation in an aqueous solution. Some
Cu(NO3)2·3H2O and Fe(NO3)3·9H2O salts with a molar ratio of 1:2
were dissolved in 200 mL double distilled water to produce a clear
solution, which then added into 2.5 mol L−1 of NaOH in drops to
till pH around 11 with vigorous magnetic-stirring. A mixed solu-
tion was irradiated under microwave with output power of 600 W
for 10 min. The precipitate was filtered, washed with double dis-
tilled water, and dried at 383 K overnight. Then as-prepared sample
CuFe2O4 was annealed in a muffle furnace for 8 h.
(
4%).
2.5. Evaluation of active species
2.5.1. Evaluation of hydroxyl radicals (•OH)
It is well known that the lifetime of •OH is too short to
determine directly. Therefore, the generation of •OH was investi-
gated by the photoluminescence method by reacting terephthalic
acid with •OH immediately to produce highly fluorescent prod-
uct, 2-hydroxyterephthalic acid (Ex: 315 nm, Em: 425 nm) [16,17].
2.3. Analytical methods
Characterizations of CuFe2O4 were carried out using X-ray
diffractometer (XRD, Siemens D5000 Diffractometer) for crystal