Catalytic properties of CuMgAlO catalyst and degradation mechanism in CWPO of methyl orange

Article abstract of DOI:10.1016/j.apcata.2016.08.015

Cu-loaded MgAl hydrotalcite catalysts were synthesized by impregnation method and further used for catalytic wet peroxidation (CWPO) of methyl orange (MO) using hydrogen peroxide as oxidant. The synthesized catalysts have been characterized by XRD, FT-IR, SEM/EDS, TG/DTA, XPS, H₂-TPR and BET. The characterizations showed that a good dispersion of Cu species inside the MgAl hydrotalcite supports, where Cu²⁺ and Cu⁺ species were coexistence, mainly existed as Cu²⁺???Cu⁺ pairs. The CuMgAlO catalyst showed the highest catalytic activity due to the high content of Cu²⁺???Cu⁺ pairs, stable performance during reuse for five cycles with fewer copper leaching. The degradation mechanism of MO was determined by UV-vis, ATR-FTIR and GC–MS. The hydroxyl radicals (HO[rad]) induced strong oxidizing effect, where the azo groups ([sbnd]N[dbnd]N?) double bonds would be first attacked by the hydroxyl radical and other free radicals. With the continuous oxidization, these intermediates could be oxidized to the final oxidation products, such as water and carbon dioxide.

Full text of DOI:10.1016/j.apcata.2016.08.015

Applied Catalysis A: General 527 (2016) 72–80
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic properties of CuMgAlO catalyst and degradation mechanism
in CWPO of methyl orange
Jing Han, Hong-Yan Zeng∗, Sheng Xu∗, Chao-Rong Chen, Xiao-Jun Liu
College of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2016
Received in revised form 8 August 2016
Accepted 16 August 2016
Available online 27 August 2016
Cu-loaded MgAl hydrotalcite catalysts were synthesized by impregnation method and further used for
catalytic wet peroxidation (CWPO) of methyl orange (MO) using hydrogen peroxide as oxidant. The syn-
thesized catalysts have been characterized by XRD, FT-IR, SEM/EDS, TG/DTA, XPS, H2-TPR and BET. The
characterizations showed that a good dispersion of Cu species inside the MgAl hydrotalcite supports,
where Cu2+ and Cu+ species were coexistence, mainly existed as Cu2+ ↔ Cu+ pairs. The CuMgAlO catalyst
showed the highest catalytic activity due to the high content of Cu2+ ↔ Cu+ pairs, stable performance dur-
ing reuse for five cycles with fewer copper leaching. The degradation mechanism of MO was determined
by UV-vis, ATR-FTIR and GC–MS. The hydroxyl radicals (HO•) induced strong oxidizing effect, where the
Keywords:
CWPO
Degeneration process
CuMgAlO catalyst
Methyl orange
azo groups (
N
N−) double bonds would be first attacked by the hydroxyl radical and other free radicals.
With the continuous oxidization, these intermediates could be oxidized to the final oxidation products,
such as water and carbon dioxide.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Although the photocatalytic oxidation can offer good performance
for oxidation of refractory organic pollutants, the complexity, high
Different types of dyes are used in many industries such as tex-
tile, paint, ink, plastics and cosmetics. A certain amount of them
are lost in the process of their manufacturing and utilization and
often cause environmental problems. Due to its complex structure,
it is very difficult to successfully treat using traditional biological
processes [1]. Therefore, the treatment of the effluent containing
these compounds is important for the protection of natural waters.
Some physicochemical techniques are currently available for the
treatment of dye effluent. However, implementation of physical
methods such as flocculation, reverse osmosis, and absorption have
the inherent drawbacks of being non-destructive, since they merely
transfer dye from wastewater effluent, thus causing secondary pol-
lution. Chemical methods such as chlorination and ozonation are
also being used for the removal of certain dyes but being econom-
ically unfeasible and involving complicated procedures [2].
chemical consumption, and relatively high operation costs consti-
tute major barriers for their large-scale applications [8]. In recent
years, catalytic wet oxidation (CWO), by adding catalysts and oxi-
dants to improve the oxidation rate, has been found to be very
effective in the degradation of dye pollutants, which provide a con-
venient method for the treatment of undesirable pollutants [7,9].
Among all the possible CWO processes, catalytic wet peroxidation
(CWPO) is a well-known process that can be used for the degra-
dation of organic pollutants with the help of a homogeneous or
heterogeneous catalyst using hydrogen peroxide as oxidant under
ambient conditions in low cost installations with low energy con-
sumption [7,9–11].
Homogeneous catalysts are generally very efficient for such
reactions, however, these traditional catalytic systems have some
technological problems such as difficulties of separation and recov-
ery from the treated effluents. To eliminate these problems, the
attention was focused on to develop heterogeneous catalysts [7,12].
Copper as active species is well-known to undergo a Fenton-type
reaction and has been used successfully for treating various organic
pollutants, which still retains a high efficiency at a higher pH range,
and the use of Cu-based catalysts in the CWPO process is incipient
[13]. Even though such kinds of Cu-based catalysts such as Cu/13X
zeolite, Cu/SBA-15, AlCu-pillared clays and Al-Cu modified clay
have been used in CWAO process to degrade the dye wastewater
Thus, Advanced Oxidation Processes (AOPs) including photo-
electrocatalysis [3–5], photocatalysis [6], catalytic wet oxidation
(CWO) [7] and so forth, which rely on the generation of hydroxyl
and other radical species for environmental remediation, have been
successfully deployed for the treatment of organic dye wastewater.
∗
Corresponding authors.
E-mail addresses: hongyanzeng99@hotmail.com, hyzeng@xtu.edu.cn
(H.-Y. Zeng), xushengxtu2016@163.com (S. Xu).
http://dx.doi.org/10.1016/j.apcata.2016.08.015
0926-860X/© 2016 Elsevier B.V. All rights reserved.

J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
73
stream flowing at 20 mL min−1. The amount of H2 consumption
during reduction was monitored by a TCD.
[14–17], little information is available on the degradation mecha-
nism involved in the catalytic degradation of dye.
The aim of this work is to evaluate the potential application of
a synthesized CuMgAlO catalyst in CWPO using azo dye methyl
orange (MO) as a target compound. The physico-chemical prop-
erties of the CuMgAlO catalyst were characterized by XRD, FT-IR,
SEM/EDS, TG/DTA, XPS, H2-TPR and BET in order to clarify the rela-
tionship between structure and catalytic performance. Activity and
stability of the CuMgAlO catalyst were investigated in the CWPO
of MO, and the degradation mechanism of MO in the CWPO was
determined by UV-vis, ATR-FTIR and GC–MS.
2.4. MO dye degradation
0.3 g catalyst (MgAlO, CuO, CuMgAlO and so on) was added
into 200 mL methyl orange (MO) synthetic wastewater containing
100 mg L−1 MO. Then 6 mL of 30% H2O2 was added, and the reac-
tion was conducted at 55 ◦C, pH 6.0, magnetic stirrer (150 rpm) and
atmospheric pressure for 120 min. After the reaction, the reaction
solution was centrifuged at 7000 rpm for 3 min. The supernatant
was kept for MO measurement by the spectrophotometric method
(Hitachi U-2910) at 464 nm, and the precipitate was washed three
times. The solutions produced from the washing steps were stored
for MO determination. PE 5100 PC atomic absorption spectroscopy
(AAS) instrument was used to determine the contents of Cu element
in the supernatant. All the experiments were carried out in tripli-
cate under the same condition and average values are reported. The
MO degradation rate (ꢁ %) was calculated as follows [19],
2. Experimental
2.1. Materials
All the chemicals used in the study were of analytical
reagent grade and solutions were prepared with deionized water.
0.1 mol L−1 NaOH and HCl solutions were used for pH adjustment.
A pH electrode (Mettler Toledo S40 K) was used for pH measure-
ments.
A0 − Ae
ꢁ% =
× 100%
A0
2.2. Preparation of the CuMgAlO catalyst
where Ao and Ae were the initial and final absorbance values of MO
dye, respectively.
The CuMgAlO catalyst was prepared by coprecipitation and
impregnation. First, the precursor MgAl hydrotalcite with Mg/Al
molar ratio of 4.0 (MgAl-LDH) was prepared by urea hydrolysis
according to the literature previously described [18]. Part of the
precursor MgAl-LDH was calcined at 500 ◦C for 4 h, which was
denoted as MgAlO. Second, the CuMgAlO catalyst was prepared
by impregnation that 1.00 g L−1 of MgAlO was put into 500 mL
aqueous solution containing 0.01 mol L−1 CuCl2. The mixture was
maintained at 50 ◦C for 12 h under stirring (200 rpm) before it was
filtered. The filter was washed with deionized water to neutrality,
and dried at 80 ◦C for 10 h, which was denoted as CuMgAl-LDH.
Part of the CuMgAl-LDH precursor was calcined at 500 ◦C for 4 h
and cooled to room temperature, which was denoted as CuMgAlO.
2.5. Identification of reaction intermediates
During the CWPO, the identification of the intermediates
at specified time intervals was performed employing a USA
Agillent 6890/5793N gas chromatography-mass spectrometry sys-
tem (GC–MS). A HP-5 capillary column (30 m × 0.25 mm × 0.25 ␮m;
Hewlett Packard, USA) was used. The column temperature was held
at 50 ◦C for 2 min and then increased from 50 to 240 ◦C at a rate
of 10 ◦C min−1. The MS conditions were EI impact ionization 70 eV,
helium as the carrier gas, injection temperature 250 ◦C, source tem-
perature 150 ◦C, scanning from m/z 70–350 at 0.3 mL min−1 during
the recording mass spectra. When the reactions were completed
for 5, 15, 30, 60 and 120 min, respectively, the supernatant for each
experiment was analyzed by GC–MS.
2.3. Characterization of the catalysts
At the same time, the intermediates were also determined using
a Shimadzu UV-2550 spectrophotometer and a attenuated total
reflectance FT-IR (ATR-FT-IR) instrument, respectively. ATR-FTIR
spectra of the sample in the ATR cell was recorded at 2 cm−1 res-
olution and signal averaging of 64 scans on PE Spectrum One B
instrument. Background was subtracted using the Opus software.
Curve fitting was then performed using Origin 8.0 software and
PeakFit v4.12.
X-ray diffraction (XRD) patterns were collected on a Japan
Rigaku D/max 2550PC (␭ = 1.5405 Å) with CuK␣ radiation. The scan
step was 0.02 (2ꢀ) with a filament intensity of 30 mA and a voltage
of 40 kV. Fourier transform infrared (FT-IR) spectrum was recorded
on Perkin-Elmer Spectrum One B instrument using KBr pellet tech-
nique. Scanning electron microscopy (SEM, JEOL JSM-6700F) and
energy dispersive spectrometer (EDS, S-4800-I, Hitachi) were used
to characterize the surface morphology and the compositions of the
samples. The specific surface area, pore volume and diameter were
determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-
Hallender (BJH) using a NOVA-e1000 system (Quantachrome, USA),
respectively. Before being analyzed, the samples had to be heated
at 150 ◦C for 6 h to remove surface water. Samples were outgassed
at 110 ◦C in vacuum (1 × 10−4 Torr) for 8 h. Thermogravimetric and
differential thermal analysis (TG-DTA) was carried out in a nitrogen
atmosphere with a Seiko 6300 TG-DTA instrument with a heat-
3. Results and discussion
3.1. Characterizations of the catalysts
3.1.1. XRD analyses
To identify the structure change of the catalysts in the prepa-
ration, the samples were characterized by XRD and the results
are shown in Fig. 1. the MgAl-LDH precursor displays a typical
of MgAl hydrotalcite structure (JCPDS 70-2151) with sharp and
intense (003), (006), (009), (110) and (113) reflections. No other
crystalline phase is observed, where the interlayer distance (d003
0.788 nm) is typical of CO32− hydrotalcite [18,20]. The hydrotalcite-
like structure without any reflections of other possible phases is
also present in the CuMgAl-LDH, suggesting the complete incor-
poration of transition Cu species into the crystalline structure.
Compared to the MgAl-LDH, the CuMgAl-LDH exhibits broader and
less intense reflections with an unstable baseline, indicating a lower
ing rate of 10 ◦C min−1 under a He stream flowing at 60 mL min−1
.
X-ray photoelectron spectra (XPS) of samples were recorded on a
Thermo ESCALAB 250 spectrometer equipped with Al K␣ radiation
(1486.6 eV). The binding energies were calculated with respect to
C 1 s peak at 285 eV with a precision of 0.2 eV. H2-Temperature-
programmed reduction (H2-TPR) experiment was performed out
on a Quantachrome ChemBET equipment. For this, 25 mg of cata-
lyst (CuMgAlO) was exposed to argon (20 mL min−1) at 200 ◦C for
2 h. The sample was cooled to room temperature, and then heated
to 800 ◦C at a heating rate of 10 ◦C min−1 under a 10% H2 in argon

74
J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
LDH one except for some minor differences. Absorption peak at
about 3450 cm−1 is attributed to the stretching vibrations (ꢂ1−OH)
of structural hydroxyl groups in the brucite sheets. The peak at
3450 cm−1 is correlated with crystallinity and orderliness, where
the narrower half-width at the peak, the higher ordered cation
distribution inside the brucite sheets [25,30], suggesting that the
CuMgAl-LDH possesses lower crystallinity due to its broader band.
And the peak for the CuMgAl-LDH shifts toward lower wavenumber
(red-shift 6 cm−1), suggesting that the strength of hydrogen bond-
ing decreased. The result is well consistent with that of the XRD
analyses (Fig. 1), which also implies a bad long-range ordering (M-
OH arrangements, M: Cu, Al, Mg) in the brucite sheets. The peaks
at 1380 (ꢂ3) and 678 (ꢂ4) cm−1 are resulted from the asymmetri-
cal stretching vibration and the symmetrical stretching vibration
of CO32−, while the peak at 1630 cm−1 is due to the interlayer
H2O [18,20]. The peaks around between 500 and 1200 cm−1can be
attributed to the metal-oxygen vibrations [22].
3.1.3. SEM/EDS analyses
Fig. 1. XRD patterns of the MgAl-LDH, MgAlO, CuMgAl-LDH, CuMgAlO and CuO.
The semi-quantitative analysis of EDS was used to detect the
elements of Mg, Al and Cu in the MgAl-LDH and CuMgAl-LDH sam-
ples. As seen in Fig. 2, the analyses of EDS reveal that the Mg/Al
molar ratios in the MgAl-LDH and CuMgAl-LDH are 3.00 and 1.82,
respectively, and the CuMgAl-LDH contains Cu element. The results
is confirmed from the XRD and FT-IR analyses.
CuMgAl-LDH
678
The morphology of the samples is revealed by SEM and shown
in Fig. 3, where thin flat crystals indicate the layered structure for
all the samples. The MgAl-LDH and CuMgAl-LDH samples show
individually layered platelets, while MgAlO and CuMgAlO samples
had some tendency of platelets aggregating into a clumpy manner.
Some of the particles can be seen transversely, allowing to estimate
the platelet thickness of the MgAl-LDH and CuMgAl-LDH samples.
The platelet thickness of the CuMgAl-LDH is obviously thicker than
that of the MgAl-LDH. In particular, the surface morphology of the
CuMgAl-LDH particles appears the “worm-like” structures formed
independently on the edges or defects on the brucite sheets. It is
believable that part of the Mg2+ sites were replaced by Cu2+ through
isomorphic substitution (Jahn-Teller effect) to form the “worm-
like” structures [31,32]. On the other hand, it is evident that particle
sizes of the samples, especially CuMgAlO, are definitely decreased
after calcination.
MgAl-LDH
1640
1380
MgAl LDH
CuMgAl LDH
Mass (%) Atoms (%) Mass (%)
Atoms (%)
Mg
Al
22.09
11.22
-
18.56
6.18
-
13.18
10.72
11.55
10.28
3450
7.53
Cu
3.45
Waven2u0m00be2r500
(
cm–1
)
500 1000 1500
3000 3500 4000
3.1.4. Textural properties
In order to examine the textural characteristic of the catalysts,
BET gas sorptometry measurements were used to characterize pore
volume (VP), pore diameter (DP) and specific surface area (SBET). The
adsorption isotherms of the metal oxides CuO, MgAlO and CuMgAlO
are plotted in Fig. 4. The N2 adsorption/desorption isotherms of the
MgAlO and CuMgAlO belong to typical type IV with a H3 hystere-
sis loop at high relative pressures, which present a broad poresize
distribution in large mesoporous region due to the aggregates of
plate-like particles, giving rising to slit-like pores with nonuniform
size and almost without macropores [33]. From the nature of the
loop, it can be concluded that the pores of MgAlO and its corre-
sponding CuMgAlO are of uniform shape. The BET specific surface
area (SBET), total pore volume (VP) and average pore diameter (DP)
of the samples determined from these isotherms are summarized
in Fig. 4. MgAlO shows the highest surface area (206 m2 g−1), while
it decreased to 122 m2 g−1 when Cu is impregnated, namely CuM-
gAlO, but the SBET of the CuMgAlO is signifcsntly higher than that
(33 m2 g−1) of the CuO sample. Although the substitutions of Mg2+
in the brucite sheets by Cu2+ leads to decrease in the specific surface
area, the CuMgAlO still matains to possess a large BET surface area
(122 m2 g−1). It maybe because the CuO nanoparticles (detectable
by XRD) and/or Cu-O-Cu clusters finely dispersed on the surface and
filled in the mesostructure pores, resulting in the materials with
Fig. 2. FT-IR spectra and chemical composition (inset) of the MgAl-LDH and
CuMgAl-LDH.
crystallinity [21]. This can possibly be explained by the incorpora-
tion of Cu species into the MgAl-LDH resulting in a decrease of the
hydrotalcite crystallinity [22]. A slight expansion in the interlayer
space (d003 0.802 nm) of the CuMgAl-LDH due to the Jahn-Teller
effect of the Cu2+ ions [23,24]. The (200) and (220) reflections
attributing to the crystallization of periclase-like MgO (JCPDS 75-
1525) are observed in the MgAlO sample, which exhibits the typical
feature of a mixed oxide of Mg(Al)O due to transforming the hydro-
talcite phase into Mg(Al)O phase after calcination [25,26]. Although
little CuO phase occurs, no significant positional changes in the
basal plane reflections are noted in the CuMgAlO sample, indicat-
ing that Cu species are mainly distributed homogeneously in the
CuMgAlO oxide matrix [27–29].
3.1.2. FT-IR analyses
Fig. 2 displays the FT-IR spectra of the MgAl-LDH precursor and
CuMgAl-LDH in the region 400–4000 cm−1, where the FT-IR spectra
of the samples were typical of pure hydrotalcite-like structure. The
CuMgAl-LDH spectrum is generally similar to the precursor MgAl-

J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
75
Fig. 3. SEM images of the MgAl-LDH, MgAlO, CuMgAl-LDH and CuMgAlO, ×10,000.
Fig. 5. TG-DTA curves of the MgAl-LDH and CuMgAl-LDH.
Fig. 4. N2 adsorption/desorption isotherms and corresponding pore volumes, pore
diameters and specific surface areas (inset) for MgAlO CuMgAlO and CuO.
3.1.5. Thermal analysis
Thermal decomposition characteristics of the MgAl-LDH and
CuMgAl-LDH are compared in Fig. 5. The two TG-DTA curves are
basically similar and divided in two endothermic peaks, which
confirm the strucure of hydrotalcite [35]. The first peak at about
around 50–250 ◦C is due to the loss of the surface and interlayer
water, and the second peak is at about 252–600 ◦C corresponding
lower surface areas [34]. The textural parameters of MgAlO and
CuMgAlO catalysts do not change so greatly, indicating a similarity
in the porous structure of the two samples before and after impreg-
nation of Cu. The result presents that MgAlO is a good supports for
highly dispersed Cu species.
2−
to the decomposition of CO3 and dehydroxylation in the brucite

76
J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
300
934.7
931.9
962.3
CuMgAl-LDH
954.5
955.1
943.5
944.0
146
CuMgAlO
238
935.2
932.5
962.9
952.0
CuMgAlO
970
CuO
960
950
940
930
MgAlO
Blinding Energy (eV)
Fig. 6. XPS of Cu 2p spectra of CuMgAl-LDH and CuMgAlO.
sheets. The total weight losses at first peak for the MgAl-LDH and
CuMgAl-LDH are 17.8 and 17.3%, which at second peak are 21.1 and
22.4%, respectively. The decrease in temperature at both the first
and second endothermic peaks reveals the weakening interaction,
implying that the removal of water molecules and dehydroxylation
become more easy for the CuMgAl-LDH. This may be because Mg is
more stabilized in an octahedral coordination of the brucite sheets
than Cu, which is inherently less stable in octahedral coordina-
tion because of Jahn-Teller effect. These observations are consistent
with the results observed by XRD and FT-IR, where the strength of
hydrogen bonding in the brucite sheets and between the interlayers
decreased due to the presence of Cu species in the brucite sheets.
-300
-150
0
150
300
450
600
Temperature (oC)
Fig. 7. H2-TPR profiles for the MgAlO, CuMgAlO and CuO.
3.1.7. H2 −TPR analysis
The fact that copper species are hardly detected in the CuM-
gAlO sample indicates that the species are either highly dispersed
as small CuO crystallites detectable by XRD or forming a solid solu-
tion within the oxide lattice that can exist in Cu2+↔Cu+ pairs form
on the surface of the CuMgAlO oxide matrix (XPS). To get insight
into this issue, the reducibility of Cu species in the CuMgAlO sam-
ple calcined at 500 ◦C was investigated by H2-TPR. Fig. 7 shows the
reduction profiles of the CuMgAlO sample. For comparison, the H2-
TPR profile of pure CuO is also collected in Fig. 7, which shows a
single peak with maximum at 238 ◦C. For the MgAlO, no reduction
peak is detected until 800 ◦C. The profile of the CuMgAlO sample
shows one intense peak with a maximum at 300 ◦C and a shoul-
der at 146 ◦C in the same temperature range as that of pure CuO.
The intense peak is assigned to the reduction of Cu2+ ions (Cu2+
to Cu+) incorporated in Mg(Al)O phase, while the shoulder peak is
assignable to the reduction of CuO [41]. With the replacement of Cu
for Mg, the reduction peak shifted toward higher temperature com-
paring with that of CuO, which means that the interaction between
the copper species and the Mg(Al)O is enhanced [42]. This result
confirms the hypothesis from the analysis result by XRD, TG-DTA
and BET that Cu species are mainly distributed homogeneously in
the CuMgAlO oxide matrix, which results in difficult reduction of
Cu2+ and higher reduction temperatures. This result matches with
that obtained with the XRD analysis, where peaks associated to
CuO phase is the slender peaks observed in the CuMgAlO sample.
At the same time, the CuMgAlO had more Cu2+ and Cu+ species
(Cu2+ ↔ Cu+) and oxygen vacancies by the aid of the XPS analyses,
comparing with its precusor CuMgAl-LDH.
3.1.6. XPS analysis
The chemical states and surface compositions of the CuMgAl-
LDH and CuMgAlO were characterized by XPS. Elemental
compositions were detected from the areas of O 1s, Al 2p, Cu 2p,
Mg 1s and C 1s peaks, confirming the inference from the EDS anal-
ysis that the two samples contain Cu, Mg and Al elements. XPS
is a surface technique that preferentially detects species located
on the surface of a sample (sampling depth is about 3–10 nm). In
order to obtain more information about the Cu species in the two
samples, the core level XPS peaks of Cu 2p for the two samples
are shown in Fig. 6. The Cu 2p spectra show two main peaks that
are attributed to Cu 2p3/2 and Cu 2p1/2 at about 934.7 and 954.5 eV,
respectively. The Cu 2p3/2 peak at about 934.7 eV, companying with
a shake-up peak located at 931.9 eV assigning to Cu+ state, belonged
to Cu2+ state, meanwhile Cu 2p1/2 peak at about 954.5 eV, together
with the shakeup satellite structure centered at binding energies of
around 943.5 and 962.3 eV, also correspond to Cu2+ state [36,37].
It is also clear that Cu2+ and Cu+ species are coexistence, and oxy-
gen vacancies is also in the surface of the two catalysts. Especially,
an additional weak peak can be discerned at 952.0 eV according
with Cu+ state for the CuMgAlO (missing for the CuMgAl-LDH) [38],
implying that the amount of Cu+ species is more than that of the
CuMgAl-LDH. Especially after calcination, the Cu 2p3/2 and Cu 2p1/2
shift to a higher binding energy, also suggesting the increase of Cu+
and the presence of oxygen vacancies, offering the possibility for
oxygen storage and release, in the surface of the CuMgAlO [39].
It has been reported that Cu+ is the active intermediate for the
hydroxylation of phenol using H2O2 [40]. So, it is also clear that
Cu2+ and Cu+ species (Cu2+ ↔ Cu+) are coexistence in the CuMgAlO
and CuMgAl-LDH catalysts, and more Cu+ enhanced the catalytic
activity of the CuMgAlO in the CWPO of MO. The results indicate
that calcination facilitates the formation of Cu+ species, and the
CuMgAlO had higher catalytic activity than the CuMgAl-LDH due to
more amount of Cu2+ and Cu+ species (Cu2+ ↔ Cu+ pairs). The higher
catalytic activity of the CuMgAlO can be testified by subsequent
catalysis experiments.
3.2. Catalytic performances of the catalysts
3.2.1. Catalytic activity of the catalysts
In the work reported here, the activity of the catalysts
was investigated to determine its suitability for the degrada-
tion of azo dye methyl orange (MO) in the CWPO. Fig.
8
summarizes the color removal efficiencies of MO using CuO,
MgAl-LDH, MgAlO, CuMgAl-LDH and CuMgAlO as catalysts, respec-
tively. The CuMgAlO shows promising catalytic activity (close
to 100%) at 55 ◦C with high mineralization efficiency at atmo-
spheric pressure. The catalytic activity of the five samples is
in the following sequence: CuMgAlO > CuO > MgAlO ≥ CuMgAl-

J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
77
Fig. 9. Recycling and Cu2+ leaching of the CuMgAlO catalyst in the CWPO.
Fig. 8. Comparison of the catalytic activity and the Cu2+ leaching of the MgAlO,
CuMgAlO and CuO catalysts.
460
0.8
Initial
LDH > MgAl-LDH. When the reaction was performed in the presence
of the CuMgAl-LDH and MgAl-LDH, the degradation rates are only
20.8 and 36.7%, respectively. By using the mixed oxides (CuMgAlO
and MgAlO) as catalysts, they are elevated to 99.2% and 41.5%,
respectively, indicating that the mixed oxides are more effective
catalysts compared to their correspending precursors. Higher cat-
alytic performance of the CuMgAlO catalyst confirms the inference
from XPS analyses that the CuMgAlO has more Cu2+ ↔ Cu+ site pairs
on the surface of the CuMgAlO matrix. And the homogeneous inter-
dispersion of Cu species and high specific surface areas also make
the CuMgAlO highly catalytic activity, which is in good agreement
with the previous results [18,26,35,43,44].
5 min
0.6
270
0.4
15 min
30 min
60 min
0.2
120 min
0.0
200
300
400
500
600
700
800
It can be concluded that the removal efficiency can be greatly
improved through introducing Cu spices into the MgAlO catalyst.
The promoted catalytic activity can be attributed to the special
ability of Cu: (1) better dispersion of Cu spices achieved when CuM-
gAlO calcined at 500 ◦C increases the number of effectively active
sites, and the increase helps the instant production of more HO·
and HO2·, where the MgAlO can act as a good support for CuM-
gAlO catalyst; (2) the introduction of Cu, especially Cu2+ ↔ Cu+ site
pairs on the surface of the CuMgAlO matrix, increases the content
of chemisorbed oxygen on the catalyst surface, thereby improving
the redox properties, and so, Cu can act as an electronic promoter
of the CuMgAlO catalyst. With reference to above characterization
results and catalytic activities of the catalysts, the CuMgAlO is used
as the catalyst for further experiments.
Wavelength (nm)
Fig. 10. Changes in the UV–vis spectra of the intermediate species at different MO
degradation time.
CuMgAlO catalyst has high reusability and stability, which could be
attributed to the stable structure of the CuMgAlO catalyst.
3.2.3. Degeneration process of azo dye MO
3.2.3.1. UV–vis analyses. UV-vis absorption measurement is a very
simple method and applicable to the binding study of small
molecule substances, which is used as a sensitive measure of subtle
changes in molecule structure. The change of the UV–vis spectra of
the reaction intermediates at different reaction times are shown
in Fig. 10. The UV–vis absorption is characterized by one peak in
the visible region with maxima located at 460 nm and by one peak
in the ultraviolet region located at 270 nm, which are referred to
azo linkage and benzene ring, respectively [45]. The intensity of
the peak in visible region declined as the treatment time increased,
and almost totally disappeared after 120 min of catalytic degrada-
tion, demonstrating that the azo linkage of most methyl oranges
can be destroyed in 120 min. At the same time, the intensity of
the peak at 270 nm increases in the first 5 min and then decreases
until to the baseline of the spectra after 120 min. The results indi-
cate that the HO• radicals attacked the azo bonds in MO initially
3.2.2. Stability of the CuMgAlO catalyst
The recycling and reuse of catalysts have often been problematic
because of the instability and/or leaching. For practical applica-
tions, the stability and reusability of the CuMgAlO catalyst were
very important factors. To clarify this issue, the experiment was
established using the recycled CuMgAlO catalyst. We performed
five runs using of the catalyst after filtration, washing for removing
the reaction solution attaching the catalyst, and then calcined again
at 500 ◦C. The CuMgAlO catalyst was reused for the next run under
the same conditions, and the results in the five recycle experiments
were shown in Fig. 9. The slightly leaching of active Cu species was
also detected, giving the Cu concentration of 4.2 mg L−1 in the first
cycle, and Cu were not detected except of first cycle (Fig. 9). At the
same time, at the second cycle, the catalytic activity of the CuM-
gAlO (89.3%) declined mainly due to Cu species leaching. In the
subsequent cycles, the catalyst became deactivated after each run,
but the catalyst could be recycled without obvious loss in activity
and kept over 83.1% in the five recycles. The results show that the
and caused the cleavage of the azo groups (
N N ) double bonds.
In order to obtain more information on MO degradation, ATR-FTIR
was employed.
3.2.3.2. ATR-FTIR analyses. To review the degradation behavior and
molecular breaking of azo dye MO in the CWPO, the ATR-FTIR spec-
tra of the intermediates for different time lengths are shown in
Fig. 11. The peaks at 1033, 1124 and 1189 cm−1 correspond to the

78
J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
Scheme 1. Hydroxyl (HO•) and hydroperoxyl (HOO•) radicals generation mechanism.
15 min as the cleavage of the benzene ring. The results show that the
double bonds which is
1520
1370
HO· radicals caused the cleavage of
N N
Initial
5 min
15 min
1189
1124
easier to be destroyed than aromatic compounds because the elec-
tron rich azo bond is the most active site in the dye molecule, which
match the results from the UV–vis analyses. Meanwhile, the new
1420
1256
1669
peak at 1645 cm−1 attributed to the stretching vibration of
C O
1340
1157
groups of carboxylic acids is generated after 30 min, where there
is a significant increase in the intensity over degradation time, and
then decreases for 120 min. The changes of the new peak may be
due to different structures of carboxylic acids from different reac-
tion times. These results demonstrate the degradation process of
MO molecule and the formation of intermediates in CWPO using the
CuMgAlO as catalyst. The degradation intermediates were further
analyzed by using GC/MS.
1598
1645
30 min
60 min
120 min
1033
300
600
900
1200 1500 1800 2100
Wavenumber (cm–1)
3.2.3.3. GC–MS analyses. The GC–MS analysis was performed for
MO degradation, and the formation of intermediates during the
degradation process was identified by this technique. As radi-
cals play a significant role in the degradation of MO, the reaction
between Cu2+/Cu+ takes place in the presence of H2O2 forming reac-
tive hydroxyl (HO•) and hydroperoxyl (HOO•) radicals (Scheme 1),
which can be described as follows,
Fig. 11. ATR-FTIR spectra of the intermediate species at different MO degradation
time.
asymmetric stretching vibration in the −SO3Na group as well as the
coupling between the benzene mode and ꢂsSO3 [46]. The intensity
of the peak at 1033 cm−1 reduces over degradation time, and even-
tually vanishes for 30 min, demonstrating that the desulfuration
occurred.
Cu2+ + H2O2 ↔ CuOOH+ + H+
CuOOH+ → Cu+ + H+ + O−2
The new peak is found at 1157 cm−1 and it corresponds to C
O
groups of alcohols, while another new peak at 1340 cm−1 corre-
sponded to OH appears [45,46]. The new peak is also observed
at 1256 cm−1, with the vibration stretching modes ꢂ(C N) and
ꢂ(N N), respectively. The appearance of the three new peaks means
that the aromatic derivatives are very susceptible to oxidation, and
as a result, aliphatic and inorganic intermediates are produced. The
characteristic peak observed in 1370 and 1420 cm−1 is due to the
Cu++H2O2 → Cu2++ · OH + OH−
−
H2O2+ · OH → HOO · +OH
→
1O2+H2O
-H
O
2
N
N
bond and to the aromatic ring vibrations sensitive to the
MO + · OH → MO(HO·) → Intermediate + · OH → CO2+H2O
interaction with the azo bond, or to the bending vibration mode ı
(N N) of the hydrazone form of the azo dye [45,46], which disap-
pears after 5 min reaction due to the brokening of the azo bond. And
the peaks at 1520, 1598 and 1669 cm−1 are linked to benzene rings
and weaken with reaction time lasting until they disappear after
The sample before the MO degradation shows two m/z peaks
at 304 of high intensity attributed to ionization of the dye
MO molecule. The sample after 5 min of degradation shows
two m/z peaks of high intensity at 173 and 136 and two

J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
79
H+
O
S
O
O
S
H
N
CH3
N
CH3
N
O
CH3
N
N
N
O
HO
CH3
O
HO
HO
H2N
H
N
HO
H2N
HO
N
CH3
O
N
CH3
OH
CH3
CH3
HO
HO
O
S
H2N
SO3H
O
HO
HO
HO
HO
OH
O
NH2
HO
HO
HO
O
O
O
O
O
O
NH2
HO
HO
O
O
O
O
COOH
COOH
C
C
H3C
C
H
C
O
HO
OH
OH
OH
OH
HO
HO
H2O + CO2
Scheme 2. Proposed pathway for degradation of methyl orange.
lower intensities m/z peak at 151 and 121, which corresponds
to 4-aminosulfonic acid, N,N-dimethyl-p-phenylenediamine, 4-
Hydrazinyl-N,N-dimethylbenzenamine and N,N-dimethyl- pheny-
lamine, respectively. This shows that degradation proceeds through
4. Conclusions
The CuMgAlO was obtained from MgAl-LDH precursor where Cu
species were introduced in the brucite-like layers, which showed a
potential catalytic activity in the CWPO of MO by hydrogen perox-
ide as oxidant. Cu species incorporation into the framework of the
MgAlO support was confirmed by various characterization (XRD,
FT-IR, SEM/EDS, TG/DTA, XPS, H2-TPR and BET) studies, where Cu2+
and Cu+ species were coexistence in the CuMgAlO. Results showed
that the CuMgAlO catalyst exhibited best catalytic activity with
99.2% MO degradation rate, which had good operational stability
during recycling test with high catalytic activity and low copper
leaching. Combining with the UV–vis and ATR-FTIR techniques,
the GC–MS analysis showed that the degradation process could
be that the aromatic group connecting with azo double bond was
oxidized to benzene derivatives by hydroxyl radicals, where the
benzene rings were further destroyed after 15 min, and generated
new smaller molecular such as acetic acid and oxalic acid etc.
the cleavage of azo group
N
N
connecting two aromatic rings
to amines as shown in Scheme 2, which agree with the results in
the literature [47–49]. The sample after 15 min shows three m/z
peaks at 111, 110 and 94 of high intensity, which corresponds to
3,5-diallyl amide, 1,4-paradioxybenzene and phenol, respectively.
The sample after 30 min shows m/z peaks at 108, 71 and 56 may
be due to the formation of 2,5-cyclohexadiene-1,4-dione (or 3,5-
cyclohexadiene-1,2-dione), acrylamide and acrolein. After 60 min,
the intermediates again get converted into short chain organic
acids such as maleic acid (m/z 116), oxalic acid (m/z 90), crylic
acid (m/z 72), acetic acid (m/z 60) and formic acid (m/z 46) with
progress of time. Finally organic acids get destructed into CO2 and
H2O for 120 min, which is consistent with the previous studies
[50]. Based on the intermediates analyzed by GC–MS, ATR-FTIR and
UV–vis techniques, a probable degradation mechanism is proposed
in Scheme 2.

80
J. Han et al. / Applied Catalysis A: General 527 (2016) 72–80
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