Catalytic degradation of Acid Orange 7 with hydrogen peroxide using CoxOy-N/GAC catalysts in a bicarbonate aqueous solution

Article abstract of DOI:10.1039/c5ra13603b

The cobalt-based heterogeneous catalysts Co_xO_y-N/GAC were prepared by pyrolysis of a cobalt-phenanthroline complex on granular active carbon (GAC) in a nitrogen atmosphere, and tested for the degradation of Acid Orange 7 using hydrogen peroxide as a benign oxidant in a bicarbonate aqueous solution. Characterization by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electron spin resonance spectroscopy revealed the formation of a cobalt oxide nanoparticle shell with a metallic Co core on the surface of GAC; and the nitrogen substituted the selected carbon atoms during the pyrolysis process and bonded to cobalt. The catalysts were active for dye decolorization in an aqueous solution containing 10 mM H₂O₂ and 5 mM NaHCO₃ at room temperature. They also presented good stability with nearly no loss of cobalt ions after the reaction, in comparison with the high leaching of Co (0.25 mg L^-1) from the Co_xO_y/GAC catalyst without nitrogen. The production of intermediates, the formation of reactive radicals and the effect of HCO₃^- were also investigated to further explore the efficiency of the catalyst. This study can provide a promising way for the activation of the green oxidant H₂O₂ in a bicarbonate aqueous solution by heterogeneous cobalt catalysts for environmental remediation.

Full text of DOI:10.1039/c5ra13603b

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Catalytic degradation of Acid Orange 7 with
hydrogen peroxide using CoxOy-N/GAC catalysts in
a bicarbonate aqueous solution
Cite this: RSC Adv., 2015, 5, 84303
*
Lian Duan, Yanling Chen, Kaixuan Zhang, Huoyan Luo, Jianxin Huang and Aihua Xu
The cobalt-based heterogeneous catalysts CoxOy-N/GAC were prepared by pyrolysis of a cobalt–
phenanthroline complex on granular active carbon (GAC) in a nitrogen atmosphere, and tested for the
degradation of Acid Orange 7 using hydrogen peroxide as a benign oxidant in a bicarbonate aqueous
solution. Characterization by X-ray diffraction, transmission electron microscopy, X-ray photoelectron
spectroscopy, and electron spin resonance spectroscopy revealed the formation of a cobalt oxide
nanoparticle shell with a metallic Co core on the surface of GAC; and the nitrogen substituted the
selected carbon atoms during the pyrolysis process and bonded to cobalt. The catalysts were active for
dye decolorization in an aqueous solution containing 10 mM H2O2 and 5 mM NaHCO3 at room
temperature. They also presented good stability with nearly no loss of cobalt ions after the reaction, in
comparison with the high leaching of Co (0.25 mg Lꢀ1) from the CoxOy/GAC catalyst without nitrogen.
Received 11th July 2015
Accepted 29th September 2015
ꢀ
The production of intermediates, the formation of reactive radicals and the effect of HCO3 were also
investigated to further explore the efficiency of the catalyst. This study can provide a promising way for
the activation of the green oxidant H2O2 in a bicarbonate aqueous solution by heterogeneous cobalt
catalysts for environmental remediation.
DOI: 10.1039/c5ra13603b
www.rsc.org/advances
and found that the catalytic activity to the reaction was only
observed with Fe- and Cu-containing perovskites and LaCuO3
was more active than LaFeO3.17 At the same time, LaFeO3, in
addition to its high catalytic activity, was rather stable to
leaching of the active component during forty cycles of catalytic
tests.
1. Introduction
It is estimated that around 100 000 commercially available dyes
with over 7 ꢁ 105 tones of dyestuff are produced annually.1 As
they represent a serious danger for the aquatic environment,
many approaches such as adsorption/precipitation,2 electro-
coagulation,3,4 and biochemical degradation,5 have been
employed for the treatment of wastewater containing dyes. But
most of them proved to be limited in terms of cost and effective
application. Recently, advanced oxidation processes (AOPs)
have become increasingly important in the removal of organic
dyes.6–10 Among the approaches, the conventional Fenton
reagent composed of hydrogen peroxide (H2O2) and ferric ions
(Fe2+) has long been applied for the production of HOc and
pollutant removal.7 However, several drawbacks of the current
Fenton process limit the scale-up of its application, including
the narrow pH range for reaction (pH ¼ 2.5–3.5) and the accu-
mulation of iron-containing sludge, which is regarded as
a secondary pollution and the loss of catalyst. In order to
overcome these drawbacks, many Fenton-like catalysts
including heterogeneous metal oxides and homogeneous metal
complexes have been developed.11–16 For example, Taran et al.
studied the catalytic behavior of perovskite-like oxides LaBO3
(B ¼ Cu, Fe, Co, Ni, Mn) in wet peroxide oxidation of phenol,
In our recent published works, the simple Co2+ ions have
also been demonstrated to exhibit high catalytic capacity to
destroy organic dyes in the presence of H2O2 in bicarbonate
aqueous solution.18–22 For example, complete decolorization of
Acid Orange 7 could be observed with only 5 mM Co2+ ions and 4
mM H2O2 in 10 mM NaHCO3 aqueous solution within 10 min at
pH 8.3, which was much faster than that of Fenton reagent with
50 mM Fe2+ ions at pH 3.0.21 The system produces hydroxyl
radicals as the main reactive species from H2O2 by the in situ
formed complexes between Co2+ and HCO3ꢀ, and can even
activate dioxygen to the radicals at room temperature in the
presence of o-aminophenol and p-aminophenol.23,24 However,
in order to overcome the secondary pollution of cobalt ions, the
development of heterogeneous cobalt catalysts with high
activity and stability against serious leaching of active species in
the operation should be performed. But until now few works
have been done in H2O2 activation and pollutants degradation
in bicarbonate aqueous solution using heterogeneous cobalt
catalysts. Jawad et al. synthesized Co–Mg–Al layered double
hydrotalcite and tested its activity and stability for phenols
degradation using bicarbonate-activated H2O2 oxidation system
School of Environmental Engineering, Wuhan Textile University, Wuhan 430073,
China. E-mail: xahspinel@sina.com; Fax: +86-27-59367334; Tel: +86-27-59367334
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in the presence of the supported catalysts.25 However, the spectrophotometry. The surface compositions of the CoxOy-N/
leached cobalt still reached to about 0.2 mg Lꢀ1, which was very GAC catalysts were investigated by X-ray photoelectron spectra
close to the amount of Co2+ ions in the homogeneous Co2+– (XPS) on a VG Multilab 2000 spectrometer (Thermo Electron
ꢀ
HCO3 system.
Corporation). Electron spin resonance (ESR) spectra were
Solid carbon and N-doped solid carbon have been investi- recorded at room temperature using a Bruker ESR A-300
gated as supports in catalysis e.g. in the Suzuki–Miyaura reac- spectrometer.
tion,26 styrene oxidation,27 and the Fischer–Tropsch process.28
However, while the cobalt based catalysts supported on carbon–
nitrogen materials have been previously synthesized and
applied in selective oxidation of organic compounds,29,30 their
use as catalyst in bicarbonate–H2O2 oxidation system has not
been reported. In this study, the CoxOy-N/C catalysts were
prepared by wet impregnation of a combination of cobalt(II)
acetate and nitrogen containing ligands on commercial activate
carbon. Subsequent pyrolysis at 800 ꢂC led mainly to metal
cobalt and cobalt oxide in N2 atmosphere. The catalysts were
active for the dye decolorization in aqueous solution containing
H2O2 and NaHCO3 at room temperature. It also presented
a good stability with nearly no loss of cobalt ions aer reaction.
2.4. Catalytic degradation experiment
Degradation reactions were performed in a 100 mL open ask at
ambient temperature. The ask was putted in a shaker at
a stirring speed of 150 rpm. Aer the desired amount of organic
dye in 50 mL of the aqueous solution was added into the
reactor, the reaction was initialized by addition of the catalyst,
H2O2 and NaHCO3. For the recycling experiment, the catalyst
was separated without any treatment aer each recycle, and
then the next reaction was started by adding a fresh solution of
AO7, H2O2 and NaHCO3.
2.5. Analysis
To monitor the degradation process of organic dyes, solution
samples were taken out at given time intervals with solid
removal and measured immediately on a Varian Cary 50 Scan
UV-Vis spectrophotometer. All the measurements were con-
ducted in triplicate with a mean deviation lower than 0.01 to
ensure the reproducibility of experimental results. The
maximum absorption wavelengths of the dyes AO7 is 484 nm.
Initial rates of AO7 degradation were determined by the slope of
kinetics curves at the beginning of reaction. The extinction
coefficient of AO7 is 2.1 L (molꢀ1 cmꢀ1), and its molar
concentration was calculated by the Lambert–Beer's law. The Co
ion leaching was monitored by atomic absorption spectroscopy
(AAS, Analyst 300, P.E. Inc.). For identication of degradation
products, the samples were analyzed by mass spectrometry, and
the experiments were performed on an Esquire LC-ion trap
mass spectrometer (Bruker Daltonics, Bremen, Germany)
equipped with an orthogonal geometry ESI source. Nitrogen was
used as the drying (3 L minꢀ1) and nebulizing (6 psi) gas at
300 ꢂC. The spray shield was set to 4.0 kV and the capillary cap
was set to 4.5 kV. Scanning was performed from m/z 70 to 800 in
2. Experimental
2.1. Chemicals and reagents
Acid Orange 7 (AO7) was purchased from Sigma Chemical Co
and used without further purication (>85%) for all experi-
ments. Granular active carbon (GAC, 20–50 mesh, AR) was ob-
tained from Aladdin Industrial Corporation. Hydrogen peroxide
(30% w/w) was obtained from Sinapharm Chemical reagent Co.,
Ltd. Cobalt acetate, 1,10-phenanthroline and other chemicals
were of analytical grade if not noticed otherwise. The sample
solutions were prepared using deionized water throughout the
experiments.
2.2. Preparation of CoxOy-N/GAC catalysts
CoxOy-N/GAC was synthesized by a pyrolysis method similar to
that reported in literature.30 The typical procedure was
described as follows: a mixture of Co(OAc)2$4H2O and 1,10-
phenanthroline (Co : phenanthroline ¼ 1 : 2 mol ratio) in 50
mL ethanol was stirred for 20–30 minutes at room temperature.
Then, GAC was added into the solution and stirred for further
30 min. The whole reaction mixture was dried at 80 ꢂC until
ethaꢂnol was removed. Then the solid mixtures was pyrolyzed at
800 C for 2 hours in nitrogen atmosphere and then cooled to
room temperature. The same procedure was applied for the
preparation of carbon-supported Co catalysts (CoxOy/GAC) but
without adding 1,10-phenanthroline.
the standard resolution mode at a scan rate of 13 kDa sꢀ1
.
Before analysis, each sample was diluted ten times. Total
organic carbon (TOC) was determined by an Apollo 9000 TOC
analyzer.
3. Results and discussion
3.1. Catalytic characterization
2.3. Characterization
The amount of cobalt on CoxOy-N/GAC catalyst surfaces was
X-ray powder diffraction (XRD) pattern was obtained on approximately 11 mg Co/g GAC by AAS. The BET surface areas for
a Bruker D8 powder. The surface morphology was characterized GAC and CoxOy-N/GAC were approximately 661 and 633 m2 gꢀ1
,
on a eld-emission transmission electron microscope (FETEM, respectively. The slightly lower surface area of CoxOy-N/GAC was
Tecnai G2 F20). The BET specic surface areas of the samples also an indicator of the coating of cobalt on the GAC surface.
were carried out with a Quantachrome Autosorb-1. The amount Several technologies including XRD, EPR, TEM, EDS, BET and
of cobalt coating on the GAC surface was obtained by extracting XPS were then used to characterize the prepared CoxOy-N/GAC
the cobalt in a boiling, concentrated (10%) HNO3 solution for catalyst. The X-ray diffraction pattern is shown in Fig. 1(A). It
12
h
and then measured by ame atomic absorption can be seen that the catalyst showed a broad peak around 44.5ꢂ,
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Fig. 2(B) displays the EDS plot for CoxOy-N/GAC. As illustrated,
the composition of the catalysts was conrmed by the presence
of C, N, O and Co elements. The weight contents of the CoxOy-N/
GAC were evaluated to 65.68, 12.49, 21.07 and 0.76% for C, N, O
and Co, respectively.
X-ray photoelectron spectroscopy (XPS) was used to inves-
tigate the elemental composition of CoxOy-N/GAC. As shown in
Fig. 3(A), a set of peaks corresponding to C 1s (284.6 eV), N 1s
(401.0 eV), O 1s (531.6 eV) and Co 2p (780.0 eV) is observed.
The high-resolution spectra of Co 2p is shown in Fig. 3(B), in
which the typical Co 2p 3/2 and Co 2p 1/2 at 780.8 eV and 796.9
eV of cobalt oxides (CoxOy) are found. The satellite peaks at
787.4 eV and 802.6 eV are characteristic of the cobalt(II).30 The
dominant existence of Co2+ oxides in the XPS analysis indi-
cates that the Co NPs on the surface of the CoxOy-N/GAC
composite are severely oxidized. This is well-explained by the
fact that the exposure of Co NPs to ambient air could cause the
formation of a thin CoO shell since the cobalt(0) nanoclusters
are sensitive to aerobic atmosphere. Thus, those particles
observed in TEM spectra might consist of a Co core and a CoO
and/or Co3O4 shell. During the pyrolysis process the nitrogen
ligand might form graphene-type layers, in which selected
carbon atoms are substituted by nitrogen. XPS analysis
showed three distinct peaks in the N 1s spectra of CoxOy-N/
GAC with electron binding energies of 398.8, 400.3, and
403.4 eV, respectively. The lowest binding energy peak is
attributed to pyridine-type nitrogen, which is bound to a metal
ion.32 The electron binding energy of 400.8 eV is characteristic
for pyrrole-type nitrogen contributing two electrons to the
carbon matrix. Some of the nitrogen atoms are also bound to
a hydrogen atom. Such types of nitrogen are oen found aer
the carbonization of nitrogen-containing organic materials.33
Finally, the small peak at 403.4 eV is typical for quaternary
Fig. 1 (A) XRD and (B) ESR spectrum of CoxOy-N/GAC catalyst.
implying the presence of metallic cobalt in the catalyst.30 The
peak is very weak, indicating the high distribution of cobalt
particles in GAC surface due to the low Co loading and high
specic area of GAC. The characteristic peaks of CoO at 36.89ꢂ,
42.47ꢂ and 61.52ꢂ, and of Co3O4 at 36.35ꢂ are hardly seen,
suggesting that the cobalt oxides phases were much fewer. In
the carbon-thermal process, the activated carbon not only
served as a support but also acted as a reducing agent, upon
which cobalt acetate was reduced to cobalt metal at the cobalt–
carbon interface at a high temperature of 800 ꢂC, with some of
the carbon being simultaneously oxidized to COx. The EPR
spectrum of the catalyst given in Fig. 1(B) is characterized by
a very broad signal around 3000 G, which can be assigned to
metallic Co,31 in agreement with XRD data.
The morphology of CoxOy-N/GAC catalyst was examined by
TEM studies. As shown in Fig. 2(A), the images reveal that the
cobalt particles are uniformly distributed in the surface of GAC,
mainly with a size range of 10–30 nm. The particles are rarely
aggregated, indicating a good dispersion in the carbon struc-
ture. From the high-resolution TEM image focusing on single
Co nanoparticles (NPs), the incorporation of some particles into
the carbon layers was also identied. Such a structure of small
sized Co particles uniformly embedded in the carbon matrix
could effectively prevent the cobalt from agglomerating, and
might cause fast electron transport between the carbon matrix
and cobalt and thus the efficient chemical performance.
+
amine species (NR4 ).34 It has been reported that the N species
such as graphitic N and pyridinic N had positive effect on the
chemical performance of N-doped carbon materials.35 The
ratio between all Co atoms and all N atoms in the near surface
region of CoxOy-N/GAC is 1 : 2.3. Thus, the enriched N-doped
carbon in this work might enhance the performance of the
catalyst.
Fig. 2 HRTEM (A) and EDS (B) images of CoxOy-N/GAC.
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230 nm and 310 nm in ultraviolet region, which is attributed to
the p–p* transition of the benzoic and naphthalene ring
respectively.22 The addition of H2O2 into the aqueous solution
caused the decrease of the absorption bands of the dye in the
visible region with time and nally disappeared, indicating the
destruction of its chromophoric structure in the vicinity of the
azo-linkage. At the same time, the decrease of the two bands in
ultraviolet region was observed, due to the opening of the
benzene and naphthalene ring. In addition, the absorbance at
256 nm increased at the rst 5 min and then descended grad-
ually in the late reaction stage should not be ignored. The
change can be attributed to the formation and degradation of
anaphthalene type intermediates.22
As observed in above Fig. 4, HCO3ꢀ was absolutely necessary
in the heterogeneous cobalt catalyst–bicarbonate system; then
ꢀ
the effect of HCO3 concentration on AO7 degradation was
further studied with the results shown in Fig. 5. It can be seen
that with the increase of HCO3ꢀ concentration from 0 to 50 mM,
the degradation efficiency rst increased and then decreased.
The maximum rate was obtained at 10 mM, but even at a low
concentration of 2.5 mM, a fast degradation of the dye was still
observed. The initial enhancement of AO7 degradation
Fig. 3 (A) Wide survey XPS spectra and (B) Co 2p and (C) N 1s XPS
envelop of CoxOy-N/GAC catalyst.
3.2. Catalytic activity
The performance of different catalytic systems for AO7 degra-
dation is shown in Fig. 4. From Fig. 4(A), it can be seen that
CoxOy-N/GAC showed a high efficiency for AO7 degradation with
a removal of 94.4% aer 20 min in the presence of H2O2 and
NaHCO3; while for the NaHCO3/H2O2 system in the absence of
the catalyst only about 2.3% of AO7 removal was observed. For
the experiment with only CoxOy-N/GAC and NaHCO3, the
removal of AO7 was 14.7%, probably due to the adsorption of
AO7 on the surface of the catalyst. When using GAC instead of
CoxOy-N/GAC or using NaOH in place of NaHCO3, the degra-
dation of AO7 is less than 17%. Representative UV-Vis spectra
changes observed during AO7 degradation with CoxOy-N/GAC +
NaHCO3/H2O2 system are depicted in Fig. 4(B). Before reaction
it shows a main absorption bands at 484 nm, corresponding to
Fig. 5 Effect of NaHCO3 concentration on AO7 degradation. Condi-
the p–p* transition of the azo form, and another two bands at tions: CoxOy-N/GAC 1 g Lꢀ1, H2O2 10 mmol Lꢀ1, AO7 50 mmol Lꢀ1
,
25 ꢂC.
Fig. 4 (A) Degradation of AO7 with different systems: (1) CoxOy-N/GAC + NaHCO3/H2O2, (2) CoxOy-N/GAC + NaHCO3, (3) GAC + NaHCO3/
H2O2, (4) CoxOy-N/GAC + NaOH/H2O2 (pH 8.6), (5) NaHCO3/H2O2; and (B) UV-Vis spectra changes for AO7 degradation with CoxOy-N/GAC +
NaHCO3/H2O2 system. Conditions: CoxOy-N/GAC 1 g Lꢀ1, GAC 1 g Lꢀ1, NaHCO3 5 mmol Lꢀ1, H2O2 10 mmol Lꢀ1, AO7 50 mmol Lꢀ1, 25 ꢂC.
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efficiency was attributed to the increase of active complexes broken C–N bond between the benzene ring and the azo bond.
formed between cobalt and HCO3ꢀ and then the production of Positive ionization mode was also tested to detect positively
active oxygen ꢀradicals from the reaction with H2O2. However at charged species, and the results are shown in Fig. 6(B). At the
ꢀ
higher HCO3 concentrations, the number of HCO3 com- beginning of the reaction, an intense ion of m/z 373 was cor-
plexed to Co species increased and there were no equatorial responded to AO7 [M + Na]+. Aer 30 min of incubation, new
positions available to bind the dye and H2O2, leading the effi- peaks at m/z 197, 215 and 253 were observed. They could be
ꢀ
ciency to decrease. In addition, the free HCO3 in solution at attributed to 1,2-naphthaquinone and its further oxidized
higher concentrations may act as an cOH radical scavenger.
compounds. The toxicity of naphthaquinone from AO7
degradation is even higher than that of the azo dye;37 fortu-
nately, the compound can be easily degraded by the CoxOy-N/
GAC + NaHCO3/H2O2 system.
3.3. Intermediate products analysis
To further evaluate the efficiency of the catalytic CoxOy-N/GAC
+ NaHCO3/H2O2 system, the ESI-MS behavior of AO7 and its
degradation products at different incubation times was per-
formed. The results at negative ion mode are displayed in
Fig. 6(A). At the beginning of the reaction, an intense ion of m/z
327 corresponding to AO7 [M ꢀ Na]ꢀ was observed as ex-
pected. During oxidative treatment, the intensity of AO7 at m/z
327 decreased signicantly, indicating that the dye was
degraded into some intermediate products. Meanwhile, the
peak at m/z 173 emerged with regularly increasing of the
intensity. According to the values of m/z and the suggested
structures, this product can be regarded as p-phenolsulfonic
acid (p-PSA), which can be found as a common product that
comes from the destruction of AO7 in the reported literature.36
In addition, no other products such as 4-amino-
benzenesulfonate and 4-nitrosobenzenesulfonate were found.
All these implied that p-PSA was the only product from the
The smaller acid products such as formate, acetate, and
oxalate can be further detected by ion chromatography (shown
in Fig. 7). These products, known as ultimate organic products
from the aromatic ring-opening, are nontoxic and biodegrad-
able. Some inorganic ions such as sulfate and nitrate were also
observed in the gure. However, the TOC removal was only
about 13%, suggesting the difficulty of complete oxidation of
these products to CO2 or CO in the reaction system.
Based on above analysis, a schematic degradation pathway
of AO7 by the CoxOy-N/GAC + NaHCO3/H2O2 system was
proposed and is given in Scheme 1. During oxidation, AO7 was
attacked by the active species and then decomposed to and 1,2-
naphthaquinone and p-phenolsulfonic acid. Aerwards, the
1,2-naphthaquinone was further attracted to hydroxylated or
polyhydroxylated derivatives, and nally to carboxylic acids by
ring opening.
Fig. 6 (A) ESI (ꢀ) and (B) ESI (+) mass spectra of AO7 solution during degradation with the CoxOy-N/GAC + NaHCO3/H2O2 system. Conditions:
CoxOy-N/GAC 1 g Lꢀ1, HCO3ꢀ 5 mmol Lꢀ1, H2O2 10 mmol Lꢀ1, AO7 50 mmol Lꢀ1, 25 ꢂC.
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Fig. 8 (A) Variations of photoluminescence spectra of AO7 solution
during reaction in the CoxOy-N/GAC + HCO3ꢀ/H2O2 system and (B)
inhibited effects of scavengers on AO7 degradation by CoxOy-N/GAC
catalyst. Conditions: CoxOy-N/GAC 1 g Lꢀ1, HCO3ꢀ 5 mmol Lꢀ1, H2O2
10 mmol Lꢀ1, AO7 50 mmol Lꢀ1, TA 1 mmol Lꢀ1, 25 ꢂC.
Fig. 7 Ion chromatography after AO7 degradation with CoxOy-N/GAC
+ NaHCO3/H2O2 system (1: CH3COOꢀ; 2: HCOOꢀ; 3: Clꢀ; 4: SO42ꢀ; 5:
SO32ꢀ; 6: C2O42ꢀ; 7: NO3ꢀ). Conditions: CoxOy-N/GAC 1 g Lꢀ1, HCO3
ꢀ
5 mmol Lꢀ1, H2O2 10 mmol Lꢀ1, AO7 50 mmol Lꢀ1, 25 ꢂC.
uorescence spectra of the solution containing CoxOy-N/GAC,
HCO3ꢀ, H2O2 and NaTA. It can be seen that the uorescence
intensity increases sharply to 9 within 20 min, implying that
cOH radicals were indeed generated in the system. Another way
to detect radicals is adding different free radical inhibitor.
MeOH and TBA can inhibit cOH radicals,19 while superoxide
dismutase (SOD) can react with O2cꢀ radicals with a reaction
rate constant of 2 ꢁ 109 (mol Lꢀ1 ꢀ1
)
sꢀ1 39 As shown in Fig. 8(B),
.
in the presence of 1 mL MeOH or TBA, the catalytic activity was
inhibited; and the rate of AO7 degradation also decreased with
600 U mLꢀ1 SOD. These results implied that cOH and O2cꢀ
radicals were the main active species controlling the oxidation
reaction in this system.
According to these analyses, a possible mechanism of H2O2
ꢀ
activation by CoxOy-N/GAC catalyst in HCO3 solution was
ꢀ
proposed as follows (Scheme 2): H2O2 and HCO3 were rst
coordinated with the bonded metal cobalt; and then cOH
radical was produced via electron transfer from the metal to
H2O2. While metal cobalt was oxidized to Co(II). The Co(II)
species or the formed Co(II) species during catalyst preparation
can be further transformed to Co(III) by H2O2 with the formation
of another cOH radical. The Co(III) species would be reduced to
be Co(II) by H2O2 with the formation of O2ꢀc radicals, which also
partly contributed to AO7 degradation.
Scheme 1 Proposed pathways of AO7 degradation by CoxOy-N/GAC
+ NaHCO3/H2O2 system.
3.4. Radicals determination
3.5. Stability of catalyst
H2O2 is a common oxidizing agent and is oen converted to free
radicals with UV irradiation or the redox reaction of transition-
metal ions. In the CoxOy-N/GAC–HCO3ꢀ/H2O2 system, not only
HOc can be produced during the oxidation of low valence cobalt
species by H2O2, but also O2ꢀc can be formed by reduction of
cobalt with high oxidation states. In order to identify the reac-
tive oxygen species during AO7 degradation with the system,
disodium salt of terephthalic acid (NaTA) photoluminescence
probing technology and radical scavenging measurements were
carried out. TA could react with cOH to give 2-hydroxytereph-
thalic acid (HTA), which exhibits a bright stable uorescence.19
This reaction is unaffected by the presence of other reactive
species such as H2O2, HO2c and O2ꢀc, so it could be used as
a sensitive probe in detecting cOH radicals.38 Fig. 8(A) shows the
To test the stability and recyclability of CoxOy-N/GAC, the cata-
lyst was collected by ltration aer reaction, and the decolor-
ization reaction was reinitiated by adding a fresh solution of
AO7, H2O2 and NaHCO3. As shown in Fig. 9, the efficiency
obviously decreased during the next runs. Aer the h run, the
removal of AO7 decreased to 32.8%. The deactivation of the
catalyst could be attributed to intermediate deposition on the
surface, as the activity of CoxOy-N/GAC was fully recovered aer
heat treatment at 350 ꢂC for 1 h under N2, and complete
degradation of AO7 can be achieved within 20 min as the same
as the rst use.
The deactivation of the catalyst might also be caused by the
leaching of cobalt. To further evaluate the stability of CoxOy-N/
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ꢀ
Scheme 2 Mechanism of H2O2 activation and AO7 degradation by CoxOy-N/GAC in HCO3 solution.
GAC catalyst, the leaching of cobalt was measured in the time
course of AO7 decolorization. The results are shown in Fig. 10. For
comparison, the cobalt leaching from CoxOy/GAC catalyst was also
given. It can be found that under the conditions, there was no any
cobalt ions detected in the solution within 10 min; and aer the
time, the cobalt concentration increased with the reaction, but it
was only 0.02 mg Lꢀ1 aer 20 min. The value was much lower than
that reported in literature,12,25 dealed with the phenol degradation
by the solid cobalt catalysts–bicarbonate system, indicating the
excellent stability of CoxOy-N/GAC catalyst against leaching.
Another experiment was established to check whether the dis-
solved Co ions are responsible for the observed catalytic activity.
Aer the reaction of 5 min, the catalyst was ltered and the rate of
AO7 decolorization in the ltrate was measured. In this case, no
reaction proceeded within 20 min. These results indicated that
leaching of cobalt into the liquid phase was negligible and the
observed catalysis was intrinsically heterogeneous. For the CoxOy/
GAC catalyst, though the completely decolorization of AO7 was
observed within 10 min, the concentration of cobalt ions found in
Fig. 9 Degradation of AO7 with the used CoxOy-N/GAC catalyst (6th*:
the catalyst was thermal treated at 350 ꢂC under N2 for 1 h after the 5th
run). Conditions: CoxOy-N/GAC 1 g Lꢀ1, HCO3ꢀ 5 mmol Lꢀ1, H2O2 10
mmol Lꢀ1, AO7 50 mmol Lꢀ1, 25 ꢂC.
Fig. 10 (A) Comparison of AO7 degradation with different supported cobalt catalysts and (B) leaching of Co from the different supported cobalt
catalysts. Conditions: Cat 1 g Lꢀ1, HCO3ꢀ 5 mmol Lꢀ1, H2O2 10 mmol Lꢀ1, AO7 50 mmol Lꢀ1, 25 ꢂC.
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the solution increased with the increasing of reaction time. Aer 12 L. Zhou, W. Song, Z. Chen and G. Yin, Environ. Sci. Technol.,
20 min, the value was up to 0.25 mg Lꢀ1, suggesting that the
2013, 47, 3833.
leaching of cobalt species from the catalyst is much easier in 13 J. Wu, H. Zhang and J. Qiu, J. Hazard. Mater., 2012, 215, 138.
comparison with the CoxOy-N/GAC catalyst. In our previous works, 14 J. H. Ramirez, C. A. Costa, L. M. Madeira, G. Mata,
it has been shown that AO7 can be nearly complete decolorized in
10 min with 0.295 mg Lꢀ1 Co2+ in bicarbonate aqueous solution,
M. A. Vicente, M. L. Rojas-Cervantes, A. J. Lopez-Peinado
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attributed to the homogeneous reaction. Indeed, when the solid G. Mailhot, Environ. Sci. Technol., 2013, 47, 1952.
catalyst was separated aer 2.5 min of reaction, the further 16 A. N. Pham, G. W. Xing, C. J. Miller and T. D. Waite, J. Catal.,
decolorization of AO7 was still observed. From XPS result it could 2013, 301, 54.
be found that there was nearly no nitrogen species in the CoxOy/ 17 O. P. Taran, A. B. Ayusheev, O. L. Ogorodnikova,
GAC catalyst surface. Thus the high stability of cobalt species in
CoxOy-N/GAC catalyst might come from the interaction between
cobalt and nitrogen, as described in Fig. 3.
I. P. Prosvirin, L. A. Isupova and V. N. Parmon, Appl.
Catal., B, 2015, 180, 86.
18 A. Xu, X. Li, S. Ye, G. Yin and Q. Zeng, Appl. Catal., B, 2011,
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4. Conclusions
In summary, CoxOy-N/GAC is an efficient heterogeneous 20 Z. Yang, H. Wang, M. Chen, M. Luo, D. Xia, A. Xu and
Fenton-like catalyst that is capable of oxidizing highly stable Q. Zeng, Ind. Eng. Chem. Res., 2012, 51, 11104.
organic dye AO7 with the breaking of azo bonds and opening of 21 X. Long, Z. Yang, H. Wang, M. Chen, K. Peng, Q. Zeng and
naphthaquinone under mild reaction conditions. The HOc A. Xu, Ind. Eng. Chem. Res., 2012, 51, 11998.
radicals and O2ꢀc ions were produced as the main reactive 22 M. Luo, L. Lv, G. Deng, W. Yao, Y. Ruan, X. Li and A. Xu, Appl.
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activity can be fully recovered aer heat treatment under N2 24 W. Shi, Q. Cheng, L. Duan, Y. Ding, Z. Xiong, X. Li and A. Xu,
atmosphere. However, the detailed mechanisms are still not
well understood and should be studied in future work.
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Acknowledgements
This work was supported by the National Natural Science 27 C. B. Putta and S. Ghosh, Adv. Synth. Catal., 2011, 353, 1889.
Foundation of China (Grant No. 21207105).
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