Influence of calcination temperature on the catalytic performance of Co3O4/GO nanocomposites for Orange II degradation

Article abstract of DOI:10.1039/c5ra02680f

In this study, graphene oxide-supported cobalt oxide catalysts (Co₃O₄/GO), which are used to degrade Orange II from water via advanced oxidation processes based on sulfate radicals, are synthesized in situ as heterogeneous catalysts via a solvothermal method. The catalysts were calcined at different temperatures of 300 °C, 500 °C, 700 °C, and 900 °C for 2 h in N₂ atmosphere. The catalytic activities of both uncalcined and calcined catalysts were studied at the same time. The results show that all the formed catalysts exhibit high catalytic activity, and the catalysts calcined at 900 °C have the best activity for the degradation of Orange II. X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) are employed to study the mechanism behind the change in catalytic activity. The analysis indicates that the change in the quantity of hydroxylated cobalt (Co-OH), the valence state of cobalt oxide and the reduction degree of graphene oxide (GO), which are all caused by calcination, are responsible for the change in catalytic activity.

Full text of DOI:10.1039/c5ra02680f

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Page 1 of 36
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DOI: 10.1039/C5RA02680F
Influence of Calcination Temperature on the Catalytic
Performance of Co3O4/GO Nanocomposites for Orange II
Degradation
Penghui Shi,*a Xianli Wang,b Xuejun Zhou,a Yulin Min,a Jinchen Fan*a and Weifeng
Yaoa
aCollege of Environmental and Chemical Engineering, Shanghai University of
Electric Power, Shanghai, 200090, P.R. China, E-mail: sph1123@163.com;
jinchen.fan@shiep.edu.cn
bCollege of Municipal and Environmental Engineering, Henan University of Urban
Construction, Henan, 467036, P.R. China
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Abstract: In this study, graphene oxideꢀsupported cobalt oxide catalysts
(Co3O4/GO), which are used to degrade Orange II from water via advanced
oxidation processes based on sulfate radicals, are synthesized in situ as
heterogeneous catalysts via the solvothermal method. The catalysts were calcinated
at different temperatures of 300 °C, 500 °C, 700 °C, and 900 °C for 2 h in N2
atmosphere. The catalytic activities of both uncalcinated and calcinated catalysts
were studied at the same time. The results show that all the formed catalysts exhibit
high catalytic activity, and the catalysts calcined at 900 °C has the best activity for
the degradation of Orange II. Xꢀray diffraction (XRD), thermal gravimetric analysis
(TGA), Raman spectroscopy, Xꢀray photoelectron spectroscopy (XPS), and
transmission electron microscopy (TEM) are employed to study the mechanism
behind the change in catalytic activity. The analysis indicates that the change in the
quantity of hydroxylated cobalt (Co–OH), valence state of cobalt oxide and
reduction degree of graphene oxide (GO), which are all caused by calcination, are
responsible for the change in catalytic activity.
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1. Introduction
Advanced oxidation processes based on sulfate radicals (AOPsꢀSRs) have gained
extensive attention over the past years as a promising technique to completely
degrade most organic pollutants [1]. Thus far, peroxymonosufate (PMS) coupled
with Co2+ has been proven to be the best combination for the generation of sulfate
−
radicals (SO4 •) for Orange II degradation [2]. However, the degradation reaction in
a homogeneous system has several disadvantages because of cobalt leachate, which
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has biological toxicity and is recognized as a priority metal pollutant [3]. Moreover,
the Co2+ as catalysts cannot be reused. Therefore, great attention has been recently
given in the activation of PMS by heterogeneous cobalt sources [4]. Our previous
studies [5] found that cobalt oxide catalyst immobilized on graphene oxide
(Co3O4/GO) was successfully synthesized as a heterogeneous catalyst for PMS
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activation to generate SO4 •. Meanwhile, the experiments showed that Co3O4/GO
composites function more efficiently for PMS activation than either pure Co3O4 or
bare GO alone. These phenomena suggested a synergetic catalytic effect of Co3O4
and GO in the hybrid state. Whether hydroxylated cobalt (CoꢀOH) formed on the
surface of the GO sheet via direct interaction of Co species with nearby hydroxyl
groups or through the dissociation of H2O with Co2+ is a critical impact factor for
radical generation in heterogeneous activation of PMS [6].
The catalytic activity of catalysts is influenced by factors such as calcination
temperature, preparation procedure, nature of support, doping with foreign ions, and
the active phase precursor employed [7]. Specifically, the calcination temperatures
typically affect the morphology and chemical properties of the catalysts. Co3O4
belongs to the family of complexꢀmetal oxides known for their spinel structures that
consist of Co2+ cations in tetrahedral sites and Co3+ cations in octahedral sites. As
such, two kinds of oxygen ions exist, where one is bonded to three Co3+ ions and the
other is bonded to one Co3+ and one Co2+ ion as the nearest neighbors [8, 9]. In this
structure, accordingly, two species of cobalt oxides exist, which are CoO and Co3O4.
Both are stable in the natural environment [10, 11]. The catalytic activity of Co3O4 is
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related to the ratio of Co3+ and Co2+ on the predominantly exposed planes [12].
However, the morphology and chemical properties of the catalysts heavily depend on
the calcination temperature. High temperature calcination may greatly change the
special structure and surface redox reactivity of Co3O4. The graphite oxide support is
a solid with a C:O ratio between 2.1 and 2.9 and has multiple oxygenꢀcontaining
functional groups, such as hydroxyls and epoxides in the basal plane, and carboxyl
groups at plane edges [13]. Graphite oxide consists of loosely bound layers. Each
layer has a twoꢀdimensional arrangement of carbon atoms in the graphene pattern
with a thickness of approximately 1.1 nm [14]. The oxygenꢀcontaining functional
groups can easily fall off from the surface of graphite oxide through calcination in
inert gas atmosphere because of its special structure. Meanwhile, graphite oxide is
exfoliated and turns into graphene oxide, reduced graphene oxide (rGO), or graphene
based on different calcination temperatures. The present heterogeneous catalysts
Co3O4/GO are fabricated through supporting cobalt oxide on the surface of graphene
oxide. Calcination may significantly influence the catalytic performance of
Co3O4/GO, but the correlated research is insufficient.
In this study, we fabricated Co3O4/GO nanocomposites as a heterogeneous catalyst
for activation of PMS, which was used in the degradation of Orange II in water by
AOPsꢀSRs. We calcinated Co3O4/GO nanocomposites at different temperatures of
300 °C, 500 °C, 700 °C, and 900 °C for 2 h in N2 atmosphere. As the calcination
temperature was increased, the catalytic activity of the catalysts changed accordingly.
The relationship between structure and catalytic activity was investigated via some
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physical characterization. We highlighted the influence of calcination temperature on
the catalytic activity of Co3O4/GO catalysts, which has not been investigated before.
2. Experimental
2.1. Materials
Flake graphite (325 mesh, 99.99%) was supplied by Shanghai Yifan Graphite Co.
Ltd, China. PMS, available as a triple salt of sulfate commercially known as Oxone
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(2KHSO5 KHSO4 K2SO4, 4.5% to 4.9% active oxygen), was obtained from Shanghai
Ansin Chemical Co. Ltd and used as an oxidant. Orange II (98% purity) was
supplied by Shanghai Yiji Dye Chemicals. Other reagents including Co(NO3)2·6H2O,
H2SO4 (98%), NaNO3, KMnO4, and H2O2 (30%) of analytical grade were provided
by Sinopharm Chemical Reagent Co. Ltd. (China).
2.2. Preparation of catalysts
GO is exfoliated graphite oxide. Graphite oxide was prepared from purified natural
graphite with a mean particle size of 300 mesh according to the method reported by
Hummers and Offeman [5, 15]. Graphite oxide (50 mg) was dispersed into 30 mL of
hexyl alcohol through sonication for 3 h. The suspension was centrifuged (below
4000 r/min) to remove the sediment, and the supernatant liquid was stored for further
use. Meanwhile, 0.25 mmol Co(NO3)2·6H2O was dissolved into another 10 mL of
hexyl alcohol. The mixture was magnetically stirred for 2 h, and the resulting
mixture was heated to 140 °C under reflux and vigorous magnetic stirring for 12 h.
After the system was cooled to room temperature, the suspension was centrifuged,
washed with absolute ethanol and water several times until all the remaining hexyl
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alcohol and other sundries were removed, and dried in a vacuum oven at 60 °C for
24 h. The product was labeled as Co3O4/GO. Bare GO and pure Co3O4 were
synthesized with the same parameters for comparison. Finally, the dried Co3O4/GO
was calcined in tube furnace at 300 °C, 500 °C, 700 °C, and 900 °C for 2 h in a
flowing N2 atmosphere.
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2.3. Evaluation of catalytic activity
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The degradation experiments were conducted in a 250 mL vessel at room
temperature. In a typical reaction of Orange II degradation with 0.6 mM, 0.05 g/L of
catalyst and 6 mM of Oxone were added. Samples were obtained at regular intervals,
joined immediately with equal methanol as quencher [2], and then filtered. Orange II
concentration in the filtrate was measured in a UVꢀvis spectrophotometer (Shimadzu
2550) at 486nm, and make full spectrum scan from 200ꢀ700nm at the same time.
Commercial Oxone can be used to generate PMS, and 614.7 g/L of Oxone is
necessary to release 2 M of PMS. For the best results of degradation, the experiment
was conducted at neutral (pH 7.0, adjusted with 0.5 M phosphate buffer) conditions
[5]. And most of the experiments were conducted in triplicate.
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2.4. Characterization and analytical techniques
The structural features and the mineralogy of the catalysts were studied using X–ray
diffraction (XRD) patterns obtained on a RIGAKU XRD instrument, using filtered
CuKα radiation with accelerating voltage of 40 kV and current of 200 mA. The
sample was scanned at 2 from 5° to 90°.
Thermogravimetric analysis (TGA) was conducted by a NETZSCH simultaneous
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thermal analyzer TG 209. About 10 mg samples were load in an alumina pan, and
then heated from 25 to 900°C at a ramping rate of 10°C/min under N2 atmosphere.
The vacant alumina pan was used as a reference throughout the conduct.
The Raman spectrum was acquired on a LabRAM HR Evolution (Horiba
Scientific, France) with a Raman shift from 800 to 2000 cmꢀ1 using an excitation
wavelength of 532 nm diode laser excitation on a 300 lines/mm grating at room
temperature.
The nanoscale structures were observed using high resolution transmission
electron microscopy (FE–HRTEM, JEOL JEM–2100F) with an accelerating voltage
of 200 kV. The sample was prepared by dispersing a small amount of dry powder in
ethanol or water. Then, one droplet of the suspension was dropped on a
formvar–carbon–coated, 300 mesh copper TEM grids (Ted Pella) covered with thin
amorphous carbon film and allowed to evaporate in air at room temperature.
The atomic composition of the catalysts was detected by X–ray photoelectron
spectroscopy (XPS). The XPS spectrum was recorded on a ESCALAB 250
photoelectron spectrometer (Thermo–VG Scientific, USA) with Al Kα (1486.6 eV)
as the X–ray source. All XPS spectra were corrected using the C 1s line at 284.6 eV.
3. Results and discussion
3.1. Catalysts characterization
To understand the calcination temperature effect on the structural features and the
mineralogy of the catalysts, Co3O4/GO calcined at 300 °C, 500 °C, 700 °C, and
900 °C were determined via XRD. Fig. 1 shows the wide angle XRD profiles of
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uncalcined and calcined samples of Co3O4/GO. As shown in Fig. 1, in the case of
each sample, the patterns show reflection planes (111), (220), (311), (400), (511),
and (440) at 18.96°, 31.20°, 36.74°, 44.72°, 59.20°, and 65.10°, respectively. The
patterns indicate the presence of spinel Co3O4 [16]. These diffraction lines provide
clear evidence of the existence of Co3O4 nanoparticles in the samples, which is in
good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS)
card 43ꢀ1003 [17]. The different crystallinity degrees of Co3O4 were enhanced
depending on the increasing calcination temperature. No diffraction peaks related to
a Co2O3 or CoO phase were detected, which indicates that only Co3O4 was present in
the uncalcined catalysts. Compared with uncalcined catalysts, the calcined catalysts
had several characteristic diffraction peaks. In all the calcined catalysts, diffraction
peaks related to CoO were observed. Five diffraction peaks at 36.48°, 42.38°, 61.48°,
73.66°, and 77.52° can be assigned to reflection planes (111), (200), (220), (311),
and (222) of CoO crystal lattice, respectively, which is in good agreement with the
JCPDS card 43ꢀ1004 [18]. As the calcination temperature was increased, the
intensity of CoO diffraction peaks gradually increased, which indicates a high
crystallinity. In addition, when the calcination temperature was increased to 700 °C,
three new diffraction peaks were observed at 44.3°, 51.2°, and 75.8° related to
metallic Co, which is in good agreement with the JCPDS card 15ꢀ0806 [19].
Meanwhile, as the calcination temperature was increased from 700 °C to 900 °C, the
crystallinity of Co became more obvious. On the other hand, the typical diffraction
peak of graphite oxide (001) is weak in the uncalcined sample, which indicates that
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the graphite oxide was mostly exfoliated to the GO sheet. On the other hand, the
additional broad diffraction peak at 2 of 20° to 27° can be also indexed to the
disorderedly stacked GO sheets [20]. Calcination leads to the exfoliation of graphite
oxide and the disappearance of the (001) diffraction peak. GO sheets can be reduced
when calcined at high temperature. As the calcination temperature was increased, the
samples displayed a broader graphitic (002) and weaker (100) peak [21]. The
presence of these peaks imply that the interplanar carbon bonds of the pristine
graphite are broken, and graphene nanosheets are formed [22].
Fig. 1 XRD spectra of Co3O4/GO without calcination and calcined at 300, 500, 700
and 900 °C, respectively.
Fig. 2 shows the weight loss (TG) and the associated derivative thermogram
(DTG) curves of Co3O4/GO, which had been dried at 60 °C for 24 h. This figure
shows that the TG of the sample underwent continuous decline when the temperature
was increased from ambient temperature to 900 °C. The most obvious stage starts
from around 30 °C to 280 °C, where a mass loss of about 15% was observed. This
step can be ascribed to dehydration of the compound and several reactions in the
sample. Meanwhile, some oxygenꢀcontaining functional groups on the surface of GO
sheet were removed, and GO was reduced to reduced GO or graphene. As the
temperature was increased to 900 °C, no more obvious stage was observed except
for a continuous decline. This phenomenon indicated that the component of
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Co3O4/GO reacted continually. The reaction may have occurred as Eqs. 1ꢀ7. When
the temperature was increased to greater than 200 °C, the reduction reaction of
Co3O4 occurred as follows:
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Co3O4 + C
3CoO + CO↑
3CoO + CO2↑
(1)
(2)
(3)
Co3O4 + CO
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GO
rGO or Graphene
When the temperature rose to about 700°C, reduction reaction occurred inꢀdepth,
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Co3O4 + 2C
Co3O4 + 4CO
CoO + C
3Co + 2CO2↑
3Co + 4CO2↑
(4)
(5)
(6)
(7)
Co + CO↑
CoO + CO
Co + CO2↑
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The result is consistent with the XRD analyses.
Fig. 2 TG and DTG curves of Co3O4/GO.
Raman spectroscopy is a powerful nondestructive tool used to distinguish ordered
and disordered crystal structures of carbonaceous materials. The G band is common
to all sp2 carbon forms and provides information on the inꢀplane E2g phonon of sp2
bonded carbon atoms, whereas the D band is a breathing mode of the kꢀpoint
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phonons of A1g symmetry [23]. The D/G intensity ratio (ID/IG) is a measure of
disorder degree and average size of the sp2 domains. Fig. 3 shows the Raman spectra
of Co3O4/GO uncalcined, calcined at 500 °C and 900 °C. All of them exhibited a
strong D band at ～1346ꢀ1. However, uncalcined Co3O4/GO exhibited a G band
at ～1600 cmꢀ1, while the corresponding G bands of Co3O4/GO calcined at 500 °C
and 900 °C are ～1575 cmꢀ1. The red shifts of G band can be attributed to the high
ability for recovery of the hexagonal network of carbon [24]. The ID/IG of uncalcined
Co3O4/GO was about 1.04, whereas the ID/IG of Co3O4/GO calcined at 500 °C and
900 °C were 0.92 and 1.33. Compared with uncalcined Co3O4/GO, the decreased
ID/IG for Co3O4/GO calcined at 500 °C indicating the removal of oxygenꢀcontaining
functionalities because of calcination [25]. As the calcination temperature was
increased, the ID/IG for Co3O4/GO calcined at 900 °C gradually increased because of
the emission of CO2, CO and H2O (Eqs. 1ꢀ7), which resulting a large number of
carbon cavities and the increased disordered degree [26]. Meanwhile, the
introduction of CoO and Co nanoparticles (Eqs. 1ꢀ7) make the RGO or graphene
sheets monodisperse to keep large active specific surface area [27]. This
phenomenon also suggested an increase in the average size of the sp2 domains upon
conversion from GO to RGO or graphene (Eq. 3), which in turn can be explained if
newly created graphite domains are smaller in size than the ones present in GO [28].
The results were consistent with the XRD and TG analyses.
Fig. 3 Raman spectra of Co3O4/GO uncalcined, calcined at 500 °C and 900 °C.
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A detailed microstructural study was performed by TEM to complete the
characterization of the uncalcined samples and those calcined at high temperatures
on the nanometer scale. The TEM images of uncalcined and calcined Co3O4/GO at
500 °C and 900 °C are shown in Fig. 4. Some Co3O4 nanoparticles were distributed
on the surface of GO sheet in the uncalcined Co3O4/GO (Fig 4a). The average
particle size of Co3O4 is about 13.75 nm, which was calculated using the (311)
diffraction peak of Co3O4 on the surface of GO by the Scherrer formula [29]. After
Co3O4/GO was calcined at 500 °C in N2 atmosphere, some nanoparticles were fairly
homogeneously dispersed on the surface of the laminated structure (GO, rGO or
graphene) (Fig. 4b). The nanoparticles could be Co3O4 and CoO based on the XRD
analyses. When the calcination temperature was increased to 900 °C, the
nanoparticles were also uniformly distributed and the mean grain size of the
nanoparticles on the surface of the laminated structure was smaller than the size of
the nanoparticles in Co3O4/GO calcined at 500 °C (Fig. 4c). This result is due to the
generation of metallic Co caused by the reduction of Co3O4 or CoO. This result is
consistent with the XRD and TG analyses. Fig. 4d shows the highꢀmagnification of
Co3O4/GO calcined at 900°C. As shown in Fig. 4d, the particles on the surface of the
laminated structure had a lattice spacing of 0.467 nm, which corresponds to the (311)
planes of Co3O4. The lattice fringes at 0.213 nm of the particle with a softer contrast
correspond to the (200) planes of CoO. Particularly, the particle with a lattice
spacing of 0.203 nm corresponds to the (111) planes of Co, which indicates the
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generation of metallic Co. All these characterization results point to a progressive
reduction of Co3O4 to CoO and Co at the reaction conditions [30, 31].
Fig. 4 TEM images of Co3O4/GO uncalcined (a), calcined at 500°C (b), 900 °C with
low (c) and highꢀmagnification (d).
The surface element compositions, metal oxidation states, and adsorbed species of
solid materials were investigated via XPS. The XPS spectra of survey, C 1s, O 1s,
and Co 2p in the uncalcined and calcined catalysts at 900°C are shown in Fig. 5. The
binding energies obtained in the XPS analyses were corrected for specimen charging
by referencing the C 1s peak to 284.6 eV. The uncalcined and calcined catalysts at
900°C have different spectra for both C 1s, O 1s, and Co 2p. The survey spectra are
illustrated in Fig. 5a, which indicates that the XPS spectra of the uncalcined and
calcined catalysts at 900 °C are similar. Moreover, both spectra have C 1s, O 1s, Co
2p, Co LMM, O KLL, Co 2s, Co 3s, Co 3p, and O 2s. The C 1s XPD spectra of the
catalysts are shown in Fig. 5b. The C 1s XPD spectra of the uncalcined catalyst can
be deconvoluted into a considerable degree of oxidation with four components that
correspond to carbon atoms in different functional groups as follows: carbon in CꢀC,
C=C and CꢀH at 284.6 eV, carbon in CꢀO at 285.3 eV, CꢀOꢀC at 286.2 eV, and C=O
at 288.5 eV. On the other hand, the C 1s XPD spectra of the calcined catalyst at
900°C only present two peaks at 284.6 and 286.2 eV, which correspond to the
graphite structure and epoxy carbon. The disappearance of the CꢀO and C=O peaks
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in the XPS spectrum indicates that some or all of the epoxide, hydroxyl, and
carboxyl functional groups were removed because of calcination, which means GO
was partially or fully reduced to rGO or graphene [32]. The result is very consistent
with those obtained from XRD, TG and TEM analyses
As shown in Fig. 5c, the O 1s spectra of both uncalcined and calcined at 900°C
catalysts are clearly asymmetric, which indicates the existence of different oxygen
species on the surface [33]. In the O 1s pattern of uncalcined catalyst, three O 1s sub
peaks are resolved with binding energies at 533.0, 532.0, and 531.3 eV, which are
assigned to the oxygen in adsorbed water molecules, the hydroxyl oxygen (C–OH)
in GO, and the cobalt hydroxide (Co–OH) on the surface, respectively [34]. On the
other hand, the O 1s pattern of the calcined catalysts at 900 °C is distinctively
different. Aside from the main peak at 531.2 eV that corresponds to the Co–OH on
the surface, a shoulder at a binding energy of 529.7 eV is attributed to the lattice
oxygen species from cobalt oxide because of the hygroscopic moisture before testing.
Compared with the uncalcined catalytic, the calcined catalysts at 900°C had no XPS
peak at 532.0 eV and but had a peak at 529.7 eV, which indicates that the CꢀOH
disappeared and the lattice oxygen species from cobalt oxide appeared in the hybrid.
Meanwhile, the subdued XPS peak at 533.0 eV indicates a decrease in the amount of
adsorbed water molecules in the hybrid.
Fig. 5d shows the XPS spectra of Co 2p in the uncalcined and calcined catalysts at
900°C. Each spectrum has two major peaks with binding energies at 781.1 or 779.8
eV and 797.3 or 795.4 eV, which correspond to Co 2p3/2 and Co 2p1/2, respectively.
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Both spectra have two corresponding shakeꢀup satellite peaks located at
approximately 6 eV above the main peaks, which is characteristic of the Co3+ and
Co2+ phase [35, 36]. Compared with the catalyst calcined at 900°C, the uncalcined
catalyst had an XPS peak that is at a higher binding energy, which indicates a
stronger interaction between the graphite support and the cobalt oxide species [37].
Meanwhile, compared with those of Co 2p3/2 (778.10 eV) and Co 2p1/2 (793.30 eV)
of metallic Co, a small chemical shift toward higher binding energies occurred for
Co2p3/2 (779.8 eV) and Co2p1/2 (795.4 eV) (Fig. 5d), which indicates that the
metallic Co on the surface of the graphene sheets was oxidized [38]. Both XPS
peaks at 780.7 eV for the uncalcined catalyst and 778.0 eV for are attributed to the
surface Co3+ species and the peaks at 781.8 and 780.3 eV are attributed to the surface
Co2+ species [39]. Meanwhile, as shown in Fig. 5d, the XPS peak of Co3+ species in
the catalyst calcined at 900°C is weaker than that of uncalcined catalyst. On the
other hand, the XPS peak of Co2+ species is stronger. This phenomenon indicates an
increase in CoO and a decrease of Co3O4 in the calcined catalyst. This observation is
in good agreement with the XRD, TG, Raman and TEM results.
Fig. 5 XPS spectra of catalysts uncalcined and calcined at 900°C: (a) survey scan, (b)
C 1s region, (c) O 1s region, (d) Co 2p region.
3.2. Catalytic activities
The degradation curves of Orange II using Co3O4/GO of the uncalcined and calcined
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at 300 °C, 500 °C, 700 °C, and 900 °C as catalysts via AOPsꢀSRs are shown in Fig.
6. All experiments were done at neutral conditions (adjusted with 0.5 M phosphate
buffer). The pH value during the whole reaction process was almost constant. As
shown in Fig. 6, when the calcination temperature was increased, the catalytic
activity of the catalyst obviously declined and then increased when the calcination
temperature was above 700°C. Moreover, the catalyst calcined at 900°C had the best
catalytic activity. This phenomenon indicates that calcination results in a significant
change in the catalytic activity of the catalysts. The uncalcined Co3O4/GO had a high
catalytic activity, which indicates that calcination at low temperatures (< 500 °C) has
an inhibitory effect on catalyst. This result indicated that the structure of Co3O4/GO
was changed along with the change in calcination temperature, which leads to the
change in catalytic activity. Although many experts demonstrated that the activity of
the catalyst is greatly influenced by the increase in calcination temperature [7, 40ꢀ42],
no universally accepted standard is available because of the complexity of the
catalyst compound.
Fig. 6 Degradation curves of Orange II by using Co3O4/GO as a catalyst.
In order to better illustrate the degradation efficiency, the temporal evolution of
UV–vis spectra of the Orange II solution were test. The initial UVꢀvis spectra of
Orange II solution was shown in Fig. 7(a), three characteristic peaks at 230, 255 and
310 nm were response to aromatic rings (benzene and naphthalene ring). The
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characteristic peak at 403 nm was ascribed to the N=N group of azo form. The
shoulder at 485 nm was due to the transition nꢀð (the hydroxyl group toward the
nitrogen bridge of hydrazone form) [43]. The temporal evolution of UVꢀvis spectra
of the Orange II over uncalcined and calcined at different temperature were shown in
Fig. 7(b) and (c). As the experiment went on, the two characteristic peaks at 403 and
485 nm decreased simultaneously, suggesting that the two tautomeric forms react at
the same time. Finally, the two characteristic absorption peaks were disappeared. Fig.
7(d) reported the UVꢀvis spectra of the Orange II over Co3O4/GO (uncalcined,
calcined at 300°C, 500 °C, 700 °C and 900°C) at 2 min. It is more overtly
demonstrated that the result is in good agreement with the degradation curves of
Orange II (Fig. 6).
Fig. 7 UVꢀvis spectra of Orange II over Co3O4/GO: (a) initial UVꢀvis spectra of the
Orange II solution, (b) temporal evolution of UVꢀvis spectra of the Orange II over
uncalcined (b1), calcined at 300°C (b2), 500 °C (b3) and 700 °C (b4); (c) temporal
evolution of UVꢀvis spectra of the Orange II over calcined at 900°C; (d) the UVꢀvis
spectra of the Orange II over different catalyst at 2 min.
3.3. Mechanism analysis
Many reports are available on the mechanism of the catalytic activity of
heterogeneous catalysts in the degradation of organic pollutants. Many studies
attribute the catalytic reaction to the formation of CoꢀOH on the surface of catalysts
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because the catalytic reaction starts with Eqs. 8ꢀ10 [2, 6, 27, 44ꢀ46]. The formation
of Co–OH complexes is a critical step for radical generation in the heterogeneous
activation of PMS. In the presence of cobalt ions, PMS breaks up to generate active
sulfate radicals via electron transfer mechanism from cobalt cations (Eqs. 8ꢀ10).
Meanwhile, the reverse electron transfer from Co3+ to Co2+ makes the reaction
proceed cyclically until PMS is completely consumed at a sufficient reaction time [5,
47, 48]. Experiments have shown that the uncalcined catalysts present high catalytic
activity (Fig. 6).
In this work, the effect of calcination on catalytic activity was also studied. The
effects of calcination on the catalyst structure are shown in Fig. 8. Calcination results
in a change in the structure of the catalysts (Eqs. 1ꢀ7). According to previous studies,
calcination temperature has a key function in the catalytic activity of the catalyst.
When the calcination temperature is increased, the catalytic activity of the catalysts
first decreases and then increases. Compared with catalytic activity of the uncalcined
catalyst, those of the catalysts calcined at 300 °C and 500 °C decrease mainly
because of a decrease in the number of CoꢀOH on the surface of the catalyst
compound, which was verified via XPS (Fig. 5). The formation of CoꢀOH is crucial
for radical generation in the catalytic reaction (Eqs. 8ꢀ10). Meanwhile, another
reason is the decrease in Co3O4 and increase in CoO (Eqs. 1 and 2). Cobalt redox
cycling in the radical generation reaction takes place mainly in Co2O3 (contained in
Co3O4), which is an unstable oxide [46]. CoO is in dissolved form and did not
appear to react heterogeneously in redox reactions. As such, the decrease in Co3O4
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and increase in CoO in the catalysts result in the decrease in catalytic activity.
When the calcination temperatures were further increased, the catalytic activity of
the catalysts increased. Two aspects mainly contributed to the increase in catalytic
activity. First, highꢀtemperature calcination results in the exfoliation of GO and a
decrease in oxygenꢀcontaining groups on the surface of GO, which lead to the
formation of reduced GO (rGO) or even graphene. This phenomenon may result in
the enhancement of catalytic activity following five factors: [47] (i) rGO or graphene
−
has an important function in the generation of SO4 • through the electron transfer
between Co2+ and Co3+ because of its peculiar electronic structure and the high
migration efficiency of electrons. (ii) rGO or graphene is beneficial to the
homodisperse of nanoꢀparticles on its surface, which can enhance the catalytic
activity of the hybrid. (iii) rGO or graphene with cobalt oxide on its surface can form
strong interaction (CoꢀOꢀC), which favors the formation of CoꢀOH and promotes the
−
activation of PMS to generate SO4 • [6,49]. (iv) rGO or graphene has extraordinary
adsorption capacities because of its special twoꢀdimensional structure, which can
significantly increase the concentration of organic molecules near the catalytic
surface and then enhance the catalytic degradation rates [50]. (v) rGO or graphene
−
also can activate PMS to produce SO4 • for the degradation of Orange II (Eqs. 11
and 12). Therefore, the formation of reduced rGO or graphene because of calcination
could enhance the catalytic activity of the hybrid.
Meanwhile, high temperature leads to the reduction of cobalt oxide to metallic
cobalt (Eqs. 4ꢀ7). In other words, when the catalyst is calcined at high temperatures,
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the product is Co/rGO or Co/graphene. At particular conditions, metallic cobalt (Co0)
can release Co2+ (Eqs. 13 and 14), which was employed to activate PMS to generate
SO4 • for the degradation of Orange II in water (Eq. 15). Meanwhile, Co0 also can
−
−
activate PMS to generate SO4 • (Eq. 16). PMS can also reduce Co3+ to Co2+ and
generate SO5 • (Eq. 17). Thus, Co2+ exhibits redox cycling in the presence of PMS
−
−
and produces SO4 • [38]. Furthermore, Co3+ is deposited on to the surface of Co0 to
initiate the release of Co2+ (Eq. 18) [38]. In the presence of cobalt ions, PMS breaks
−
up to generate active SO4 •, which consequently aids in the oxidation of Orange II
(Eq. 19).
The reaction mechanism is proposed as follows:
Co2+ + H2O ↔ CoOH+ + H+
(8)
−
−
CoOH+ + HSO5 → CoO+ + SO4 • + H2O
(9)
CoO+ + 2H+ ↔ Co3+ + H2O
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
−
−
rGO + HSO5 → rGOꢀH + SO5 •
−
−
Graphene + HSO5 → GrapheneꢀH + SO5 •
Co0 → Co2+ + 2eꢀ
Co0 + O2 + 2H2O → Co2+ + 4OH−
−
−
Co2+ + HSO5 → Co3+ + SO4 • + OH−
−
−
Co0 + 2HSO5 → Co2+ + 2SO4 • + 2OH−
−
−
Co3+ + HSO5 → Co2+ + SO5 • + H+
Co0 + 2Co3+ → 3Co2+
−
SO4 • + Orange II → […many steps…] → CO2 + H2O
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Fig. 8 The effects of calcination on the catalyst structure.
4. Conclusions
Heterogeneous catalysts Co3O4/GO were prepared via solvothermal method. The
catalysts were then calcined at 300 °C, 500 °C, 700 °C, and 900 °C and
characterized via XRD, TG, Raman, XPS, and TEM. Both uncalcined and calcined
catalysts showed high catalytic activity at high temperatures in degrading Orange II
from water using AOPsꢀSRs. Calcination had a significant function in catalytic
activity. The effects of calcination temperature on the morphology, atomic
composition, and crystallite structure of the catalysts were investigated. The
relationship between calcination temperature and catalytic activity was discussed. In
accordance with the increase in calcination temperature, the number of CoꢀOH
decreased. This decline, which resulted in weakened catalytic reaction, was crucial
for radical generation in the catalytic reaction. At the same time, the amount of CoO
in dissolved form increased and did not appear to react heterogeneously in redox
reactions. When the calcination temperature was further increased, cobalt oxide was
reduced to metallic cobalt, and GO was partially or fully exfoliated and reduced to
rGO or graphene. The metallic cobalt can release Co2+ and activate PMS directly,
and rGO or graphene has a high electron migration efficiency and can also activate
PMS directly, all of which resulted in the enhancement of catalytic activity. In
summary, the catalytic activity of Co3O4/GO can be considerably influenced by
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calcination temperature. More study on the effects of calcination temperature should
be conducted for the preparation of highly efficient catalysts.
Acknowledgements
The study was supported by Shanghai Municipal Natural Science Foundation (no.
15ZR1417800), “Dawn” Program of Shanghai Education Commission (no. 11SG52),
Shanghai Key Project for Fundamental Research (no. 13JC1402800), Science and
Technology Commission of Shanghai Municipality (no. 14DZ2261000), and
Scientific Research Foundation of Shanghai University of Electric Power (no.
K2014–015).
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Fig. 1 XRD spectra of Co3O4/GO uncalcined and calcined at 300 °C, 500 °C, 700 °C,
and 900 °C.
Fig. 2 TG and DTG curves of Co3O4/GO.
Fig. 3 Raman spectra of Co3O4/GO uncalcined, calcined at 500 °C and 900 °C.
Fig. 4 TEM images of Co3O4/GO uncalcined (a), calcined at 500 °C (b), 900 °C with
low (c) and high magnification (d).
Fig. 5 XPS spectra of catalysts uncalcined and calcined at 900 °C: (a) survey scan,
(b) C 1s region, (c) O 1s region, (d) Co 2p region.
Fig. 6 Degradation curves of Orange II by using Co3O4/GO as a catalyst.
Fig. 7 UVꢀvis spectra of Orange II over Co3O4/GO: (a) initial UVꢀvis spectra of the
Orange II solution, (b) temporal evolution of UVꢀvis spectra of the Orange II over
uncalcined (b1), calcined at 300°C (b2), 500 °C (b3) and 700 °C (b4); (c) temporal
evolution of UVꢀvis spectra of the Orange II over calcined at 900°C; (d) the UVꢀvis
spectra of the Orange II over different catalyst at 2 min.
Fig. 8 Effects of calcination on the catalyst structure.
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Fig. 1 XRD spectra of Co3O4/GO uncalcined and calcined at 300 °C, 500 °C, 700 °C, and 900 °C.
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Fig. 2 TG and DTG curves of Co3O4/GO.
63x44mm (600 x 600 DPI)