Synergetic effects of graphene-CoPc/silk fibroin three-dimensional porous composites as catalysts for acid red G degradation

Article abstract of DOI:10.1039/c9ra03162f

The disposal of dye wastewater is one of the hotspots of scientific research. Upon combining the ability of graphene to accelerate the hydroxyl radical generation with the Fenton system, it has shown a faster degradation rate and can be recycled, showing greater degradation efficiency than the traditional dye treatment method. Herein, a catalytic system based on the regenerated silk fibroin (SF) gel integrated with cobalt tetraaminophthalocyanine (CoTAPc)-grafted-reduced graphene oxide (RGO) sheets were fabricated, and its catalytic activity was assessed via the degradation of acid red G (ARG) at varying catalyst and H₂O₂ dosages, pH values, and temperatures. The results revealed that the three-dimensional (3D) porous RGO-CoTAPc/SF gel exhibited a much stronger catalytic behavior than the other arbitrary components due to its high surface area and synergetic hydroxyl radical generation efficiency, with the dye removal ratio by RGO-CoTAPc/SF being higher in an acidic medium than in an alkaline medium. It also increases with the increase in temperature and RGO-CoTAPc/SF and H₂O₂ dosages. Further, the catalytic oxidation process of ARG was determined, and the possible degradation mechanism of ARG has been discussed. Our results suggest that the composite materials with high catalytic activity can provide a reference for future Fenton-like catalytic systems.

Full text of DOI:10.1039/c9ra03162f

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Synergetic effects of graphene–CoPc/silk fibroin
three-dimensional porous composites as catalysts
for acid red G degradation†
Cite this: RSC Adv., 2019, 9, 24751
*
Hui Ma,
Huanxia Zhang, Mingqiong Tong, Jianda Cao and Wen Wu
The disposal of dye wastewater is one of the hotspots of scientific research. Upon combining the ability of
graphene to accelerate the hydroxyl radical generation with the Fenton system, it has shown a faster
degradation rate and can be recycled, showing greater degradation efficiency than the traditional dye
treatment method. Herein, a catalytic system based on the regenerated silk fibroin (SF) gel integrated
with cobalt tetraaminophthalocyanine (CoTAPc)-grafted-reduced graphene oxide (RGO) sheets were
fabricated, and its catalytic activity was assessed via the degradation of acid red G (ARG) at varying
catalyst and H2O2 dosages, pH values, and temperatures. The results revealed that the three-dimensional
(3D) porous RGO-CoTAPc/SF gel exhibited a much stronger catalytic behavior than the other arbitrary
components due to its high surface area and synergetic hydroxyl radical generation efficiency, with the
dye removal ratio by RGO-CoTAPc/SF being higher in an acidic medium than in an alkaline medium. It
also increases with the increase in temperature and RGO-CoTAPc/SF and H2O2 dosages. Further, the
catalytic oxidation process of ARG was determined, and the possible degradation mechanism of ARG has
been discussed. Our results suggest that the composite materials with high catalytic activity can provide
a reference for future Fenton-like catalytic systems.
Received 28th April 2019
Accepted 16th July 2019
DOI: 10.1039/c9ra03162f
rsc.li/rsc-advances
addition to their thermal and chemical stabilities. The hetero-
geneous MPc-based catalysts have been widely developed to
1. Introduction
overcome the major drawbacks of metal leaching and non-
recycling ability of homogeneous MPc system, by loading on
solid supporters, such as TiO2, carbon nanotubes, cellulosic
bers (CFs), chitosan.13–16 Compared with the above carriers, the
reduced graphene oxide (RGO), a conductive support to improve
their electrocatalytic activity and stability, displays an ideal host
for CoPc complexation through – stacking, electrostatic inter-
action, and hydrogen bonding.17,18 Yang et al. organized cobalt
phthalocyanine (CoPc) onto/into graphene sheets through
a strong p–p interaction between CoPc and graphene, and it
showed a signicant improvement in the electro-oxidation of
dopamine.19 Wang et al. reported non-covalently graphene/
iron(II) phthalocyanine hybrid (Gr/FePc) and used it for the
photocatalytic degradation of phenols with enhanced activity.20
Liu et al. synthesized a novel FePc–Gr composite as a catalyst for
the oxygen reduction reaction that exhibited high activity and
good stability.21 Mukherjee et al. reported chemically synthe-
sized graphene/CuPc nanotube composites, which exhibited
a much stronger catalytic behavior for the degradation of
rhodamine B than for the pristine CuPc nanotube.22 Therefore,
the composite material, obtained by combining the graphene
and Fenton system, is expected to improve the degradation
efficiency of dyes profoundly.
In recent years, about 100 tons per year of dyestuffs have been
discharged into the water system, from the textile industry,
automobile industry, plastic industry and so on, which brings
enormous pressure on the environment.1,2 The conventional
treatment methods, include membrane ltration, adsorption,
Fenton oxidation, phytoremediation, and electrolytic precipi-
tation, which use metallophthalocyanines, TiO2, silver particles,
graphene, and bacteria as main materials.3–8 Among these
methods, the Fenton and Fenton-like oxidation, as one of the
prospective wastewater treatment technologies, are particularly
suitable for degrading the dyes into nal simple and non-toxic
products, such as H2O and CO2, which can hardly be degraded
by other chemical reagents.9,10
In the eld of Fenton and Fenton-like systems, metal-
lophthalocyanines (MPc) as analogous structures to metal-
loporphyrins, have been considered as promising catalysts in
recent years.11,12 MPc is a two-dimensional 18-p electron
aromatic macrocycle with a metal atom located at the central
cavity, which leads to their high electron transfer ability, in
China-Australia Institute for Advanced Materials and Manufacturing, College of
Material and Textile Engineering, Jiaxing University, Jiaxing, 314001, China.
E-mail: 13967327877@163.com; Fax: +86-573-83640322; Tel: +86-573-83641175
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03162f
Normally, the catalytic efficiency gets affected by the cata-
lyst's structure, surface atomic arrangement, and coordination,
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etc. Therefore, methods to effectively construct the graphene (60 mg mLꢀ1, dissolved in HFIP) was added to the above solu-
and Fenton system composite materials play a key role.23 The tion and a ternary mixture was obtained. The gel was then
formation of three-dimensional (3D) porous catalytic materials sonicated for 4 h to obtain a homogeneous gelation state.
can effectively prevent MPc from aggregation and retain the Brown GO–CoTAPc/SF gels were obtained aer freeze-drying at
ꢁ
high specic surface area that is necessary to achieve high ꢀ50 C for 3 days. Subsequently, the obtained gels were cross-
catalytic efficiency. Further, the regenerated silk broin (SF) gel linked with ethanol steam and reduced by hydrazine hydrate.
is considered to be an ideal 3D skeleton due to its large specic Another set of the bi-component nanocomposite was prepared
surface area and easy physical and chemical modications. The following the same procedure as that for RGO-CoTAPc, CoTAPc/
silk broin consists of 18 amino acids and has a typical natural SF, and RGO/SF composites. The as-synthesized samples were
protein structure, which is favorable for dye adsorption and used further for different characterizations and measurements
provides
a
reaction
microenvironment
for
metal- (Scheme 1).
lophthalocyanines to construct highly active catalysts.24 We do
believe that the highly interconnected 3D network will facilitate
electron transport, and the interpenetrating porous network
will allow rapid ion transport. However, fewer reports are
available on the usage of the degrading dye pollutants.
2.3. Catalytic activity test
Acid red (ARG), an azo dye, was selected as an assessment
system to investigate the catalytic activity of the RGO-CoTAPc/SF
gels. Typically, 30 mg of RGO-CoTAPc/SF and 0.5 mmol H2O2
were added to a 10 mL of ARG (5 ꢂ 10ꢀ5 mol Lꢀ1) aqueous
solution in a glass vial with a vibration rate of 100 rpm. During
the catalytic tests, 3 mL of solution was taken in every 30 min
and centrifuged to remove the catalyst. The supernatant was
analyzed using a UV-vis spectrophotometer (Shimadzu UV-2550,
Japan) at a wavelength of 510 nm. Further, the percentage of dye
removal (R) of ARG was calculated using the following equation:
In summary, a exible and porous 3D material was fabri-
cated using Co-tetraamino phthalocyanine (CoTAPc), graphene
oxide (GO), and silk broin (SF) as raw materials. In this system,
the SF shapes the honeycomb frameworks, providing functional
groups, such as –NH2, –COOH, and –SH, which could bind with
dyes; CoTAPc undertook the catalytic entity, and GO was
employed for the transfer of electrons. The morphology and
structure of the hybrid material and its application for the
degradation of acid red G were studied in detail. In addition, the
recycling ability of the material was also determined. Further-
more, a proposed synergistic mechanism of the acid red G
degradation by the GO–CoTAPc/SF composite has been pre-
sented here. Thus, the obtained materials can provide research
ideas for designing new catalytic materials in the future.
C0 ꢀ C
A0 ꢀ A
R ¼
ꢂ 100% ¼
ꢂ 100%
C0
A0
where R represents the removal rate of ARG, and C0 and C are
the initial concentration of ARG solution and concentration of
remaining ARG solution at time t, respectively. A0 is the absor-
bance of the initial ARG solution, and A is the absorbance of the
remaining ARG solution.
The effects of the mass and composition of the catalyst,
temperature, pH, and the dosage of H2O2 on the ARG degra-
dation rate were also studied.
2. Experimental part
2.1. Materials
Hexauoroisopropanol (HFIP), 99.5%, Meryer (Shanghai)
Chemical Technology Co., Ltd.; 1-(3-dimethylaminopropyl)-3-
ethylcarbodiimide hydrochloride (EDC$HCl), 98.5%, klamar-
reagent company; N-hydroxysuccinimide (NHS), 98%, klamar-
reagent company; graphite powder (<30 mm) was purchased
from the Sinopharm Chemical Reagent Co. Ltd., Shanghai; co-
tetraamino phthalocyanine (CoTAPc) was a gi sample from
Zhejiang Sci-Tech University; the regenerated silk broin (SF)
was made from silkworm cocoons in our laboratory; hydrazine
hydrate (HI), NaNO3, KMnO4, H2SO4 (98 wt%), and H2O2
(30 wt%) were of analytical grade and used as received.
2.4. Characterization
The as-synthesized products were characterized by scanning
electron microscopy (SEM) (Hitachi S-4800, Japan) equipped
with (EDS) energy dispersive X-ray spectroscopy and (WDS)
wavelength dispersive X-ray spectroscopy, transmission elec-
tron microscopy (TEM) (Talos F200X, USA), and X-ray diffrac-
tion (XRD) (DX-2600 X-ray diffractometer with Cu Ka radiation).
2.2. Preparation of RGO-CoTAPc/SF catalysts
Starting from the graphite powder, GO was synthesized by
a modied Hummers' method.25 The resultant product was
a stock GO aqueous suspension containing approximately
10 mg mLꢀ1 GO. The hybrid catalysts, including RGO-CoTAPc
and RGO-CoTAPc/SF, were prepared using the freeze-drying
method. In a typical run, 7.5 mg of CoTAPc, a little amount of
EDC$HCl and NHS were added to 10 mL of the obtained
aqueous suspension of GO (10 mg mLꢀ1), with the reaction
ꢁ
mixture being stirred at 30 C for 1 day. Then, 16.7 mL of SF Scheme 1 Illustration for the preparation of GO–CoTAPc/SF gels.
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FT-IR spectra were recorded on a VERTEX 70 spectrometer. The appeared at 2q ¼ 10.4ꢁ, which corresponds to the interlayer
specic surface area was measured using the Brunauer– distance of 0.84 nm, which indicated the good crystallization of
Emmett–Teller (BET) method (NOVA 1000e, Quantachrome the as-prepared GO.26 CoTAPc, tending to aggregate, exhibited
Instruments, USA) at liquid nitrogen temperature. For further a weak reection at 2q ¼ 27.1ꢁ, corresponding to the stacking
characterization of the products obtained aer ARG degrada- distance of 0.33 nm between the phthalocyanine moiety within
tion by RGO-CoTAPc/SF, the samples were collected aer 12 h. the column.27 The intensity of the characteristic peaks of the
Further, these samples were centrifuged and the supernatant GO–CoTAPc composite was lower than that of the pure GO,
was collected and dried to carry out the analysis by high- which may be attributed to the broadening of the inter-planar
performance liquid chromatography (HPLC) and mass spec- spacing due to the graing of the CoTAPc molecules onto the
trometry (MS) to distinguish the degraded products formed.
graphene sheets. As for GO–CoTAPc/SF gel, the diffraction peak
shied up to 2q ¼ 11.6ꢁ, and the intensity of the peak further
decreased due to the low percentage content and low crystal-
linity of GO in the GO–CoTAPc/SF sample. These observations
indicate the amorphous structure of the as-prepared catalysts
due to the insertion of SF and well dispersion of CoTAPc on the
surface of the GO sheets. Comparative FTIR spectra of the
samples are displayed in Fig. 1E. The spectra show the presence
of various functional groups in GO sheets. The peak at
1713 cmꢀ1 and 1618 cmꢀ1 corresponds to the stretching
vibrations of C]O and C–C. The peak at 1220 cmꢀ1 was
attributed to the bending and stretching vibrations of epoxy
groups. The band located at 1041 cmꢀ1 was related to the alkoxy
group.28 For pure CoTAPc, the peak located at 1663 cmꢀ1 was
assigned to C]O stretching vibrations. The vibrations of the
skeleton structure of phthalocyanine were located at 1390, 1102,
3. Results and discussion
3.1. Characterizations of catalysts
TEM images (Fig. 1A and B) of GO and GO–CoTAPc clearly show
that the graphene sheets stacked together with many wrinkles,
thereby forming a thin transparent layer. Compared with GO
sheets, the GO–CoTAPc composite exhibited
a different
morphology with many dark spots on the surfaces, indicating
that the CoTAPc molecules were anchored on the GO sheets and
had a relatively uniform particle size distribution. Besides, Co
was detected on the surface of the GO–CoTAPc/SF gel by the
energy-dispersive X-ray spectroscopy (EDX) (Fig. 1C). The above
analysis indicated that both graphene and CoTAPc existed in
the as-prepared composites. XRD was also employed to identify
whether CoTAPc was graed onto the graphene sheets. As
shown in Fig. 1D, for GO, a sharp and strong diffraction peak
and 1061 cmꢀ1
.
29 Compared with pure GO and pure CoTAPc, the
red-shi of the C]O carbonyl stretching vibration from
Fig. 1 (A) and (B) TEM images of GO and GO–CoTAPc, respectively; (C) the EDX spectrum of the as-prepared GO–CoTAPc/SF composite. (D)
XRD of GO, CoTAPc, GO–CoTAPc, and GO–CoTAPc/SF; (E) the FTIR spectra of GO, CoTAPc, and GO–CoTAPc/SF hybrid.
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1713 cmꢀ1 in GO to 1704 cmꢀ1 in CoPc–GO was observed, and
the new peaks appeared at 1415 and 1278 cmꢀ1 in the GO–
CoTAPc composite related to the C–N stretching modes.30 It
suggests that CoTAPc was graed onto the GO sheets through
amide bond, and the synthetic procedure of CoTAPc–GO/SF is
illustrated in Fig. 2B. In this study, the CoTAPc molecules were
graed onto GO through an amidation reaction between the
carboxyl groups of GO and the amido groups of CoTAPc, and
there also existed a p–p assembly between CoTAPc and gra-
phene to form the GO–CoTAPc composite. The CoTAPc nano-
particles could be anchored on graphene closely by covalent
bonding and p–p interactions, which would be benecial for
the oxygen reduction reaction.
The microstructure of the hybrid gels was analyzed by SEM,
and the images are shown in Fig. 2A–D. CoTAPc displayed
a granular morphology,31 while RGO showed a macroporous
structure with a thicker wall, which consists of folded graphene
sheets with some wrinkles, as seen in the upper right picture in
Fig. 2B.32 RGO/SF showed a porous structure with a smaller pore
size and smooth surface (Fig. 2C). Compared with RGO/SF, the
RGO-CoTAPc/SF hybrid presented the microporous inter-linked
network with a thinner wall (Fig. 2D). Also, there were CoTAPc
particles immobilized on the folded graphene sheets as seen
from the high magnication photo in Fig. 2D. Such morphology
is thought to be advantageous as it offers signicantly increased
surface area and open edge sites, which are much needed for
catalysis.33,34 With the introduction of SF, the surface area of
RGO/SF was increased to 392 m2 gꢀ1 compared with 212 m2 gꢀ1
of RGO. Due to the miniaturization and networking of the
pores, the specic surface area of RGO-CoTAPc/SF was dropped
to 63 m2 gꢀ1, but it was still much higher than the 2 m2 gꢀ1 of
CoTAPc, as shown in Fig. S1.†
3.2. Catalytic properties of the catalysts
Typically, the dye removal gets affected by many factors, such as
catalyst component, temperature, H2O2, and pH value. In detail,
we studied the inuence of these factors on the ARG dye
removal.
To investigate the effect of the catalyst component, the
degradation was performed at an equivalent catalyst amount
(49.43 mg SF, 18 mg RGO, and 2.59 mg CoTAPc in 70 mg RGO-
CoTAPc/SF) and pH 7, with the dye concentration of 5 ꢂ
10ꢀ5 mol Lꢀ1. The curves of CoTAPc, RGO, RGO-CoTAPc,
CoTAPc/SF, RGO/SF, and RGO-CoTAPc/SF versus time are
shown in Fig. 3A, showing three different trends. The removal
ratios of CoTAPc were determined to be only about 3.4%. RGO-
CoTAPc showed the degradation activity for ARG, but not more
than ꢃ40% of ARG was degraded in the measured period. The
same phenomenon was found in the study of Co3O4/graphene
as heterogeneous catalysts in Orange II degradation. However,
the degradation efficiency signicantly improved suggesting
a synergistic catalytic activity of Co3O4 and graphene in the
hybrid as it was more conductive than the individual semi-
conductor CoTAPc clusters because of the hybridization
between Co-4d orbital and graphene-p orbital.35 curves of RGO/
SF further revealed that the catalytic activities of the composite
for ARG degradation were relatively high, as more than ꢃ60% of
ARG was removed during 720 min. When 3D RGO-CoTAPc/SF
gel was used, almost 100% of ARG degradation products were
removed within 720 min. The results demonstrated that the
catalytic efficiency of RGO-CoTAPc/SF was signicantly
improved due to the remarkable synergistic effect of the
heterogeneous surface between CoTAPc, graphene sheets, and
SF.
Fig. 2 (A)–(D) SEM images of CoTAPc, RGO, RGO/SF, and RGO-CoTAPc/SF, respectively.
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Fig. 3 (A) Plot of the ARG removal by different samples (RGO-CoTAPc/SF 70 mg); (B) the pH effect on the degradation of ARG; (C) the
temperature effect on the degradation of ARG; (D) the effect of the dosage of RGO-CoTAPc/SF on the degradation of ARG; (E) the effect of H2O2
on the degradation of ARG. Reaction conditions: ARG-10 mL (5 ꢂ 10ꢀ5 mol Lꢀ1), initial pH ¼ 7 (nature pH), initial H2O2-0.5 mM, and RGO-
CoTAPc/SF-30 mg, temperature-20 ꢁC.
Due to the high catalytic efficiency of RGO-CoTAPc/SF, the
Usually, the catalytic reaction gets affected by temperature. To
following reactions choose the hybrid of RGO-CoTAPc/SF as the study the effect of temperature on the reaction, 20 ꢁC, 40 ꢁC, and
catalyst. The removal of the products obtained aer ARG degra- 60 ꢁC were set to study the effect of temperature on the removal of
dation by RGO-CoTAPc/SF gel was signicantly inuenced by the products obtained aer ARG dye degradation, as shown in
pH of the reaction medium. 0.1 M HCl and 0.1 M NaOH solutions Fig. 3C. The catalytic efficiency towards ARG increased with the
were used to adjust the pH in acidic and alkaline media of the increase in temperature. Moreover, the degradation rates of AꢁRG
reaction solution. The effect of pH on the removal of the products were determined to be 71.1%, 85.5%, and 95.4% under 20 C,
of ARG degradation in the presence of RGO-CoTAPc/SF 40 ꢁC, and 60 ꢁC, respectively. On one hand, the diffusion of the
composite is shown in Fig. 3B. It was obvious that the removal dye molecules increased with the increase in temperature, which
efficiency of RGO-CoTAPc/SF gel towards ARG increased with improved the catalytic efficiency by improving the contact of dyes
a decrease in the pH of the medium. At pH 2, the removal and catalysts. On the other hand, increasing the compatible
percentage of dye was 97.4%, which was higher than 71.1% in the temperature would increase the rate of the catalytic reaction.39
neutral region (pH 7) and 3.9% at pH 10. This could be due to the
The dosage of the RGO-CoTAPc/SF used in the reaction also
change in the surface charges of the RGO-CoTAPc/SF gel, which affected the degradation process, as presented in Fig. 3D. It can
was caused by the presence of various functional groups, such as be found that the percentage removal of the dye was increased
amide, hydroxy, and carboxyl, which were distributed on the with an increase in the catalyst amount, which reached 41.8%,
RGO-CoTAPc/SF gel surface. In an acidic medium, H+ ions were 71.1%, 87.8%, and 96.3%. This result was due to the fact that as
adsorbed, which in turn imparted NH3+ cationic properties to the the catalyst amount increased, more dye adsorbed on the
gel surface. With the increase in the pH value, the surface of the catalyst surface and the degradation efficiency was increased.
catalyst gradually became negative due to the ionization of the An oxidant is another important factor that affects the removal
–COOH groups in the presence of the OHꢀ ions. ARG is an of the products aer degradation of the dye molecules. Thus, we
anionic azo dye molecule, and the interaction of the ARG mole- compared the degradation removal without or with 0.25 mL
cules with the catalyst surface changed from the electrostatic H2O2, as shown in Fig. 3E. When H2O2 was added into the
attraction in the acidic medium to the electrostatic exclusion mixture solution, the degradation efficiency was enhanced from
under high pH. The electrostatic attraction accelerates the 71.4% to 94.3% at 720 min.
degradation of dye molecules, whereas the electrostatic repulsion
slows down the degradation of the dye molecules.36 Compared
3.3. Reaction kinetics
with the straight working pH value of 2.0–3.5 of the traditional
homogeneous Fenton (Fe2+/H2O2) systems, our system works
effectively in the 2–7 pH range.37,38
As seen in Fig. 3D and E, the rate of degradation in the rst
90 min was faster than in the later 120 min. In the whole
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degradation process, neither zero-order nor quasi-rst-order the reaction. At the end of the reaction, when the catalyst
kinetics can be used to t the kinetics. Therefore, the pseudo- concentration was 5 mg mLꢀ1, the removal rate of ARG was
rst-order kinetics was used to analyze the rst 90 min and similar to the rate obtained when the catalyst concentration was
the remaining process, using the formula expressed as:
7 mg mLꢀ1, which indicated that the consumption rates of the
Co(II) active species and OHꢀ ions were the same. This may be
due to the fact that the catalyst that did not appear in the
equilibrium global chemical equation could also affect the
reaction rate by reducing the activation energy, which led to the
increase in k. The effect of hydrogen peroxide on the apparent
reaction rate constant k1 was uniform and proportional to the
concentration of hydrogen peroxide. When the concentration of
hydrogen peroxide increased from 0 to 1 mM, the initial
degradation rate increased from 0.097 to 0.121 minꢀ1. For the
remaining time interval of ARG degradation, the k2 value
decreased with an increase in the concentration of H2O2. At the
catalyst dosage of 3 mg mLꢀ1, the dosage of 0.5 mM H2O2 had
a positive effect on the degradation efficiency and rate of ARG,
which was mainly due to the coordination of the proportion of
catalyst, oxidant, and the target compound.
ln(C0 ꢀ C/C0) ¼ ln R ¼ kt + y
where C0 and C are the initial concentration of the ARG solution
and concentration of remaining ARG solution at time t,
respectively, y is a constant, t is the reaction time (minutes), and
k is the apparent rate constant (minꢀ1).
Upon drawing ln R with t, as shown in Fig. 4A–D, the slope of
the linear relation curve, i.e., the reaction rate constant k (k1 for
the initial 90 min stage of degradation and k2 for the remaining
catalyzing process) was obtained. The k values and the corre-
lation coefficients (R2) are listed in Table 1.
Typically, k1 obtained for 90 min of the reaction provides
more information about the initial reaction rate, while k2 ob-
tained for the last time interval of the reaction provides infor-
mation about the equilibrium between the reactants and
products.40 From the apparent rate constants, it can be seen that
the ARG removal rate constants, i.e., k1 and k2 rst increased
and then decreased with an increase in the catalyst dosage from
1 to 7 mg mLꢀ1, whether in the rst 90 min or the remaining
catalytic reactions. However, when the catalyst concentration in
the system exceeded 5 mg mLꢀ1, the degradation of ARG begun
slower than at 3 mg mLꢀ1 concentration due to the presence of
enough Co(II) metal species in the solution at the beginning of
3.4. Degradation mechanism of dyes
According to the above results, a proposal of the catalytic
mechanism was established, as shown in Fig. 5. The ARG
molecules migrated from the bulk of the solution to the surface
of the catalyst via H bond and p–p interactions between ARG
and catalyst. Further, the heterogeneous Fenton-like reaction
(eqn (1) and (2)) occurred through the RGO-CoTAPc/SF activated
Fig. 4 The plot of ln R with t, both at the initial stage that took place in the first 90 min of the reaction, and the remaining catalyzing process of
different RGO-CoTAPc/SF dosage (A and B) and different H2O2 usage (C and D).
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Table 1 Kinetic constants assuming the first-order reaction kinetics
Catalyst dosage (mg mLꢀ1
)
H2O2 concentration (mM)
Rate constant k1 ꢂ 102 (minꢀ1
)
R2
Rate constant k2 ꢂ 102 (minꢀ1
)
R2
1
3
5
7
3
3
3
0.5
0.5
0.5
0.5
0
0.947
1.091
1.100
1.330
1.196
1.261
1.456
0.957
0.932
0.876
0.973
0.910
0.959
0.894
0.253
0.369
0.133
0.128
0.339
0.325
0.152
0.978
0.991
0.906
0.973
0.887
0.978
0.986
0.5
1
´
H2O2 to generate the HOc radical. According to H. GarcIa, RGO TACoPc/SF. The samples were centrifuged and the precipitant
exhibited high activity as Fenton catalyst generates the HOc was collected and dried to be analyzed via the FTIR spectros-
radical due to the involvement of hydroquinone/quinone (–OH/ copy. As shown in Fig. 6A, the signature peaks of ARG at 1600–
]O) subunits for H2O2 activation (eqn (3) and (4)).41 From the 1480 cmꢀ1 were the skeleton structure of the benzene ring. The
point of Ma, the electron transport pattern includes the transfer weak peak at 1450 cmꢀ1 was associated with the stretching
of electrons from the graphitic domain to the adsorbed/ vibrations of N]N group. Moreover,ꢀthe strong peak at
intercalated CoPc, and feedback from the Co ions through the 1034 cmꢀ1 was related to the –SO3 stretching mode.43
ligand-like attacking of oxygen functional groups of GO to the Compared with RGO-TACoPc/SF before usage for the ARG
central cobalt ions.42 Thus, the dual reaction centers were degradation, new peaks appeared at 1180 and 1100 cmꢀ1 for
responsible for the conversion of H2O2 on the RGO-CoTAPc/SF GO-TACoPc/SF aer usage, which was related to the asymmetric
2ꢀ
surface, which would improve the ORR catalytic activity. and symmetric stretchings of the SO4 bonds. The sharp and
Therefore, the degradation behavior of ARG could be catalyzed characteristic peaks of ARG disappeared/weakened, thereby
by the HOc radical, as illustrated by eqn (5)–(7). Besides these, affirming the degradation of ARG.
the larger surface area and a variety of amino acid residues of
High-performance liquid chromatography (HPLC) was also
the regenerated silk broin gel could benet in trapping more used to further identify the possible products, i.e., the oxidative
ARG and active radicals. Therefore, the results indicate that intermediates of ARG in this catalytic system. The comparative
each component of RGO-CoTAPc/SF plays an important role in HPLC chromatograms of ARG and ARG-GO–CoTAPc/SF (12 h
the substantial enhancement of degradation efficiency that is and 24 h samples) are shown in Fig. 6B. The chromatogram of
the synergetic catalytic effect.
ARG shows a sharp peak at the retention time of 1.93 min. Aer
Furthermore, the GO–CoTAPc/SF sample was collected aer 12 h of the ARG degradation by GO–CoTAPc/SF, it showed
720 min to ascertain the degraded products of ARG by GO- a decrease of up to ꢃ98% in the intensity of the main peak of
Fig. 5 The proposed mechanism of the H2O2 activation by RGO-CoTAPc/SF for the ARG degradation.
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Fig. 6 (A) FTIR spectra of ARG, RGO-TACoPc/SF (before used) and RGO-TACoPc/SF (after used); (B) comparative HPLC chromatogram of ARG,
degraded products of ARG after 12 h and 24 h.
the ARG molecules. In addition to the reduction in the peak
intensity of ARG, two newer peaks were seen at the retention
time of 2.41 min and 2.88 min, attributing to the initial
degradation of the ARG. Aer 24 h of the analysis, we did not
notice any peak related to the ARG, suggesting the complete
degradation of the organic aromatic skeleton of ARG.
3.5. Reusability study
The stability of the RGO-CoTAPc/SF composite catalyst is
meaningful for their reusability in environmental technology.
Therefore, repeated experiments were performed under iden-
tical conditions for the degradation of dye during a ve-cycle. As
shown in Fig. 7, the degradation rate of the RGO-CoTAPc/SF
composite gel was declined from 97.36% to 90.34% aer
a ve-cycle experiment, without no obvious variations. The
result indicated that the RGO-CoTAPc/SF hybrid was a reusable
catalyst in environmental technology. The Cornelia Marinescu's
results also show that reduced graphene mainly plays a catalytic
role in the degradation rather than in adsorption.39 Therefore,
RGO-CoTAPc/SF can realize the recyclable catalyst.
The oxidation products of ARG were also conrmed by mass
spectrometry (MS), as shown in Fig. S2.† According to the
relative molecular weight of the ARG dye, it can be seen that the
high molecular weight of the original ARG dye in the solution
was 512, as presented in Fig. S2A.† Aer 12 h of the reaction
time, as shown in Fig. S2B,† the peak of 512 decreased and the
peak of 96 increased. From the molecular structure of the ARG,
we speculated that the degradation of the dye molecules can
produce phenyl-ammonia, whose molecular weight was
approximately 96. It is noteworthy to point out that the peak of
512 almost disappeared aer 24 h of the reaction time, and the
peak of 96 was clearly increased. In addition, a peak at 166
appeared, which is due to the formation of the product of
phthalic acid, as shown in Fig. S2C.†44 It proved that several
biodegradable low-molecular-weight organic acids were ob-
tained due to the azo bond fracture.
4. Conclusions
In our study, a new type of RGO-CoTAPc/SF with high catalytic
activity was prepared. The results of SEM images show that the
hybrid possesses network porous structure with a large specic
surface area (63 m2 gꢀ1). With an increase in the dosage of the
composite materials, temperature, and the amount of H2O2, the
degradation ratio of dyes was almost 100% within the range of
pH 2–7 due to the synergetic effect of CoTAPc, RGO, and SF. In
addition, the results of FTIR, HPLC, and MS showed that dyes
were degraded in the presence of composite materials rather
than via adsorption. The composites we have prepared have also
been proved to be capable of dyestuff degradation aer repeated
use. Thus, our results show that a new type of catalyst can be
designed by combining graphene, catalyst, and skeletons.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 7 The recycling runs in the degradation removal of ARG.
We gratefully acknowledge the nancial support from the
National Natural Science Foundation of China (51703079 and
61704068).
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