Metal-organic framework derived Co3O4/C@SiO2 yolk-shell nanoreactors with enhanced catalytic performance

Article abstract of DOI:10.1039/c8ta03565b

The simultaneous rational optimization of structure and composition is extremely important for improving the catalytic performance of yolk-shell nanoreactors, but has been rarely achieved so far. In this work, yolk-shell Co₃O₄/C@SiO₂ nanoreactors (YSCCSs) are facilely synthesized by coordinating the growth of Co-zeolitic imidazole framework (ZIF-67) crystals with the hydrolysis/condensation of tetraethoxysilane (TEOS). In this process, the preliminarily formed ZIF-67 crystals play an essential role in the final structure of YSCCSs, which can be used as the template for silicon hydroxide growth and the precursors for carbon and cobalt species. To present the advantages of YSCCSs as a catalyst, sulfate-radical-based advanced oxidation processes (SR-AOPs) for bisphenol A (BPA) degradation were chosen as the model reaction. For comparison, three other catalysts, including yolk-shell Co₃O₄@SiO₂ nanoreactors (YSCSs), and Co₃O₄ and ZIF-67 derived Co₃O₄/C nanoparticles, were also tested. The results show that YSCCSs exhibit significant BPA degradation performance over YSCSs, and CO₃O₄ and Co₃O₄/C nanoparticles. This outstanding catalytic performance is ascribed to the rational optimization of the catalyst from the following two aspects: (i) confinement effect of yolk-shell nanostructure and the improved dispersity of carbon supported Co₃O₄ species enhance the catalytic activity and stability (structure optimization); and (ii) the existence of graphitized carbon accelerates the electron transport from the catalyst to peroxymonosulfate (PMS) and thus increases the reaction kinetics (composition optimization). The results of this research show that metal-organic framework (MOF)-derived nanoreactors with a rational structure and composition have a promising application prospect for environmental remediation.

Full text of DOI:10.1039/c8ta03565b

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Metal-organic framework derived Co3O4/C@SiO2 yolk-shell
nanoreactors with enhanced catalytic performance
Received 00th January 20xx,
Accepted 00th January 20xx
Ming Zhang, Chaohai Wang, Chao Liu, Rui Luo, Jiansheng Li *, Xiuyun Sun, Jinyou Shen, Weiqin
Han, Lianjun Wang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The simultaneously rational optimization of structure and composition is extremely important for improving the catalytic
performance of yolk-shell nanoreactors, but is rarely achieved so far. In this work, yolk-shell Co3O4/C@SiO2 nanoreactors
(YSCCSs) are facilely synthesized by coordinating the growth of Co-zeolitic imidazole framework (ZIF-67) crystals with the
hydrolysis/condensation of tetraethoxysilane (TEOS). In this process, the preliminary formed ZIF-67 crystals play an
essential role for the final structure of YSCCSs, which can be used as the template for silicon hydroxide growth and the
precursors of carbon and cobalt species. To present the advantages of YSCCSs as catalyst, sulfate-radical-based advanced
oxidation processes (SR-AOPs) for bisphenol A (BPA) degradation was chosen as the model reaction. For comparison, three
other catalysts, including yolk-shell Co3O4@SiO2 nanoreactors (YSCSs), Co3O4 and ZIF-67 derived Co3O4/C nanoparticles,
were also tested. The results show that YSCCSs appear significant BPA degradation performance over YSCSs, CO3O4 and
Co3O4/C nanoparticles. The outstanding catalytic performance is ascribed to the rational optimization of the catalyst from
the following two aspects: (i) confinement effect of yolk-shell nanostructure and the improved dispersity of carbon
supported Co3O4 species enhances the catalytic activity and stability (structure optimization); (ii) the existence of
graphitized carbon accelerates the electron transport from the catalyst to peroxymonosulfate (PMS) and thus increases
the reaction kinetics (composition optimization). This research results show that Metal-organic framework (MOF)-derived
nanoreactors with rational structure and composition have a promising application prospect for environmental
remediation.
half-life period (30-40 µs).8-11 Among SR-AOPs, different
catalysts such as transition metal ion (Fe2+, Co2+, Cu2+, and
Ce3+) and metallic oxides (CuFe2O4, Co3O4, and CoFe2O4) have
Introduction
Due to the large amount usage of chemical substances in
traditional agriculture and modern industry, serious
been extensively investigated.12-14 Particularly, cobalt-based
heterogeneous catalyst Co3O4 has attracted significant attention
because of its excellent catalytic performance. Unfortunately,
there are still some inherent defects such as the aggregation of
particles and leaching of cobalt ions, which limit their further
practical application. Hence, there is an urgent need to develop
stable and efficient cobalt-based heterogeneous catalysts for
SR-AOPs.
Yolk-shell Nanoparticles (YSNs), possessing a void space
between the core and the shell, have been extensively
investigated due to their remarkable physicochemical
properties.15-18 Especially, when used as catalysts, YSNs with
metal/metal oxide as the active interior core possesses
distinctive superiorities as follows: (i) stabilization of the active
sites by encapsulation of outer shell; (ii) enhancement of
catalytic/degradation efficiency by confinement effect, and thus
results in excellent catalytic performance.19-31 However, in
previous studies, the active cores of YSNs were usually
consisted of metal/metal oxide particles with serious
environment pollution is becoming
a
multidisciplinary
challenge.1-4 In recent years, advanced oxidation processes
(AOPs) have been generally recognized as an effective method
for degradation of organic pollutants, thanks to the creation of
highly reactive radicals, such as O2•−, OOH•, hydroxyl radical
(OH•) and sulfate radical (SO4•−).5-7Among them, SR-AOPs
have been extensively employed for wastewater treatment due
to its powerful oxidizing property. Compared with classical
hydroxyl radicals (•OH), sulfate radical has more merits, such
as (i) widely range of pH (2-9); (ii) higher oxidative potential
(2.5-3.1 V vs. NHE); (iii) more selective and efficient; (iv) long
Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School
of Environmental and Biological Engineering, Nanjing University of Science and
Technology, Nanjing 210094, People’s Republic of China. *E-mail: lijsh@
njust.edu.cn; wanglj@njust.edu.cn.
† Electronic Supplementary Information (ESI) available: SEM
images, N2 sorption isotherms. See DOI:10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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aggregation, leading to the loss of active sites. Moreover, the
Journal Name
YSCCSs: 500 mg of ZIF-67@SiO2 was ground into powder
View Article Online
DOI: 10.1039/C8TA03565B
monotonous component of metal/metal oxide greatly limits the and heated at 250 ˚C for 8 h in N2 atmosphere with a ramp rate
electron transport between reactants and catalyst, resulting in of 5 ˚C min-1, then continued to heat up to 600 ˚C for 3 h with a
slow reaction kinetics. Therefore, to further optimize the same ramp rate and atmosphere. The obtained black power was
structure and composition of YSNs is highly important to Co/C@SiO2. When the sample cooled down to room
enhance their catalytic performance, which is rarely temperature, heated again at 220 ˚C for 10 h in air atmosphere,
investigated so far.
the YSCCSs were obtained.
Recent works on metal/carbon based electrode materials
YSCSs: 500 mg of ZIF-67@SiO2 nanoparticles were ground
demonstrate that the existence of carbon component can into powder and heated at 600 ˚C for 8 h in air atmosphere with
significantly enhance the dispersity and stability of metal a ramp rate of 1 ˚C min-1. The YSCSs were obtained.
species in electrochemical applications.32-43 Except that, the
The preparation process of Co3O4, ZIF-67 and Co3O4/C see
introduction of carbon can obviously improves the electron the supplementary Data.
conductivity and thus accelerates the electron transport of
metal-based species.44-48 Inspired by the great success of
Characterization
XRD pattern analysis was performed to investigate the
metal/carbon composites in electrochemical applications, for
the first time, we reported a one-pot approach to synthesize
MOF derived yolk-shell nanoreactors with carbon supported
Co3O4 as active core and SiO2 as protective shell, which
possess optimized structure and composition for PMS
activation (as shown in scheme 1). By simply mixing TEOS
and Co(NO3)2·6H2O in the aqueous solution containing 2-
methylimidazole (2-MeIm), the core/shell nanostructure, in
which ZIF-67 and silicon hydroxide were core and shell
respectively, were formed. After subsequent pyrolysis, the
YSCCSs were obtained (Path 2). The catalytic efficiency of the
YSCCSs was evaluated by degradation of organic pollutant
BPA. In order to illustrate the synergistic mechanism of
structure and composition of YSCCSs, two other catalysts, i.e.,
YSCSs (Path 1), Co3O4 and MOF-derived Co3O4/C
nanoparticles were also applied to this catalytic system.
Furthermore, the cycling stability, cobalt leaching of YSCCSs
and possible BPA degradation mechanism were investigated.
structural characteristics by X-ray powder diffractometer
(Bruker D8) at 40 kV and 40 mA (monochromatized CuK (λ =
1.5418 Å) radiation). N2 adsorption and desorption isotherms
were measured using Micromeritics ASAP-2020 at liquid
nitrogen temperature (77 K). The pore size distribution and
specific surface area were calculated by using the Brunauer-
Emmett-Teller (BET) and nonlocal density functional theory
(NLDFT) method, respectively. Thermogravimetric (TG)
analysis was conducted by SDTQ 600 analyzer from 10 to 800
˚C under air atmosphere with a heating rate of 5 ˚C/min.
Scanning electron microscopy (SEM) analysis was performed
on a FEI Quanta 250 F system. Transmission electron
microscopy (TEM) analysis was performed on a FEI Tecnai 20
electron microscope operated at 200 kV. The X-ray
photoelectron spectroscopy (XPS) analysis was conducted on a
photoelectron spectrometer (PHI Quantera II) with Al Kα
(1486.6 eV) as the X-ray source. The leaching of cobalt ions
was performed by inductively coupled plasma (ICP). Infrared
spectra (IR) analysis was carried out on Perkin Elmer 100 with
the wavelength range is 4500 to 500 cm-1. The in situ reaction
process was monitored by pH meter (PHS-3C). Electrochemical
impedance experiment was conducted on Chenhua
electrochemical workstation (CHI660E, Shanghai, China)
Experimental
Materials
2-MeIm, BPA, PMS, Co(NO3)2·6H2O and TEOS were
purchased from Sigma–Aldrich. Anhydrous ethanol, anhydrous
methanol, sodium hydroxide (NaOH), potassium hydroxide
(KOH),tertbutyl alcohol (TBA), sodium carbonate (NaHCO3),
sodium nitrate (NaCl), HA and NaH2PO4 were obtained from
Nanjing Chemical Reagent Co., Ltd. 5,5-dimethyl-1-pyrroline
N-oxide (DMPO) was purchased from J&K Scientific.
Deionized (DI) water was used all the experiments. All the
reagents used directly without further purification.
Catalytic activity measurements
Organic contaminants are degraded by activation of PMS. The
catalytic performance of YSCSs, YSCCSs and Co3O4 were
evaluated by degrading selected contaminant model BPA.
Degradation measurements were conducted in a 250 mL beaker
with a constant stirring (400 rpm) at room temperature. In a
typical experiment, 100 mL of solution containing 20 mg of
BPA and 10 mg of PMS was prepared. 0.1 M of sodium
hydroxide was used to adjust initial pH of the degradation
system (range from 7.0 to 10.0). Then, 10 mg of catalyst was
added to the above solution and a 1 mL reaction mixture was
taken for analyzing at selected time intervals. The samples were
quenched with 1 mL methanol, filtered and analyzed by using
high-performance liquid chromatography (HPLC: Waters
e2695, America, Methanol/H2O = 7/3), which had a C18
reversed-phase column and a UV detector. The recycling
experiments were conducted in the same condition, except for
Sample preparation
Synthesis of ZIF-67@SiO2 nanoparticles: In a typical
experiment, 5.4 g of 2-MeIm was added to 50 mL of DI water
with stirring for 10 min. Then 0.75 mL of TEOS and 0.35 g of
Co(NO3)2·6H2O dissolved in 25 mL DI water were added into
the above solution with stirring for 3 h at room temperature.
The purple precipitate was collected by centrifugation with
5000 r/min for 3 min and washed with DI water and ethanol for
three times, respectively. The resulting sample was dried under
vacuum 6 h at 80 ˚C for using.
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using the regeneration catalyst, which was collected and The morphological structures of the prepared samples ZIF-
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DOI: 10.1039/C8TA03565B
washed with distilled water after each recycle. The leaching of 67@SiO2, YSCSs and YSCCSs were checked by SEM and
cobalt ions experiment was performed and analyzed by TEM. The SEM images (Fig. 1a and 1b) show that the prepared
inductively coupled plasma (ICP). The EPR experiments were ZIF-67@SiO2 and YSCCSs have a structure of rhombic
conducted by Bruker EMX 10/12, radical signals were dodecahedron. The enlarged SEM image (Top-right inset in
collected by using DMPO spin-trapping agent. The parameter figure 1b) shows that the Co3O4/C core is encapsulated
settings were the following: X-band, room temperature (298 K), obviously by the silica shell. It indicates that silica shell keeps
modulation frequency of 100 kHz, modulation amplitude of the undamaged dodecahedron structure after pyrolysis. A
1Gs, and microwave power of 20 mW.
similar morphological also can be found from Figure 1c.
However, the Co3O4 core is much smaller than the Co3O4/C
core. The result demonstrates the presence of carbon. (Fig. 1e
and 1f).
Results and discussion
Characterization of ZIF-67@SiO2, YSCSs and YSCCSs
Scheme 1 Illustration of the preparation process of the ZIF-67@SiO2, YSCSs and YSCCSs.
Fig. 1 (a-c) SEM images of ZIF-67@SiO2, YSCCSs and YSCSs. Top-right inset in (a-c): the enlarge SEM images of the broken structure; (d-f) TEM images of ZIF-67@SiO2,
YSCCSs and YSCSs. Top-right inset are the corresponding schematic diagram.
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Fig. 2 (a) TEM images of YSCCSs; (b) and (c) EDX line scan and elemental mapping revealing the elemental distribution of Si, Co and O in the yolk-shell structure; (d)
and (e) HRTEM images of a representative graphitized carbon and Co3O4.
To further investigate the internal structure of YSCCSs, and 65.2˚, corresponding to (111), (220), (311), (400), (511)
enlarge TEM image is indispensable. From figure 2a, an and (440) reflections, are well consistent with the characteristic
approximately 10 nm silica shell can be seen clearly, and peaks of Co3O4 (PDF: JCPDS 76-1802).51, 52 The formation of
diameters range of Co3O4 nanoparticles from 10 to 20 nm are amorphous silica shell is not obvious peak at the 22.8˚, which
dispersed in the MOF-derived carbon. Elemental mapping and likely originated from the fact that the amorphous silica shell is
line scan reveal the elemental distribution of Si, Co and O in too thin (about 10 nm) to obtain a strong diffraction signal. The
the yolk-shell structure (Fig. 2b and 2c). The presence of XRD analysis results are in accordance with the SEM and TEM
graphitized carbon is demonstrated by High-resolution TEM observation (Fig. 1 and 2). Here, we also examined the specific
(Fig. 2d and 2e). A lattice spacing of 0.343 nm is observed, surface area and porous structure of ZIF-67, YSCSs and
corresponding to the (002) plane of graphitized carbon. YSCCSs by Brunauer-Emmett-Teller (BET) gas sorption
According to previous report, the conductive graphitized carbon measurement (Fig. 3b and Fig. S2). The surface area and pore
can enhance the electron transport and suppress the volume were listed in table S1. According to the IUPAC
agglomeration of Co3O4 nanoparticles.40 Similarly, a lattice classification, these nitrogen adsorption-desorption isotherms
spacing of 0.202 nm can be contribute to the (400) plane of the are characteristic of type IV.53 The existence of large hysteresis
Co3O4.49
loop in the isotherm of YSCSs and YSCCSs define that the
The crystallographic structures of the ZIF-67@SiO2, YSCSs presence of unordered mesopores and macropore, which are
and YSCCSs were identified by powder X-ray diffraction imparted by the interior cavity between Co3O4 core and SiO2
(XRD) pattern (Fig. 3a and Fig. S1a). In the case of the ZIF- shell. The nitrogen sorption isotherm in figure S2 shows a
67@SiO2 pattern, the characteristic peaks of 7.5, 10.6, 12.9 and discontinuous hysteresis loop at 0.1 relative pressure, which can
18.3˚ corresponding to (011), (002), (112) and (222) reflections be ascribed to the typical sorption feature of polymers.54,55 The
match up with the simulated.50 However, the diffraction signal yolk-shell structures of YSCSs and YSCCSs have
a
intensity of ZIF-67 in ZIF-67@SiO2 is slightly decreased in considerable BET surface area of 173.4 and 194.6 m2g-1, and
comparison with pure ZIF-67 nanoparticles, which suggests the the total pore volume of 0.55 and 0.23 cm3g-1, respectively.
likely fact that ZIF-67 nanoparticles are fully encapsulated by Such a unique large cavity yolk-shell structure is beneficial for
the hydrolysis of silica. A similar situation also can be observed providing plenty of specific surface area to accelerate chemical
in the test of YSCCSs. The peaks of 19, 31.4, 36.9, 44.8, 59.4 reaction by improving mass transfer driving force.
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Raman spectra were used to investigate the degree of
graphitization (Fig. 3c). There are two typical peaks at 1348
and 1587 cm-1, which can be ascribed to D-band (breathing
mode) and G-band (E2g mode), respectively. The peaks
intensity ratio of D- and G-band (ID/IG) is a common index to
assess the graphitization degree. As we can see, the ID/IG value
of YSCCSs is 1.01, which indicates that YSCCSs have a
considerable graphitization degree. From the Fourier transform
infrared (FTIR) studies (Fig. 3d and Fig. S1b), we can see the
ZIF-67 and ZIF-67@SiO2 have a similar absorption peak.
However, the characteristic peak of Si-O-Si band is found at
1108 and 800 cm-1, which demonstrate the presence of silica
shell again. The characterize peak signal intensity of the Co3O4
at 1580 cm-1 sharply decrease in the sample YSCCSs compared
with YSCSs, which likely due to the fact that the presence of
MOF-derived carbon shield the Co3O4 characterize signal.56
The surface element bonding configurations and chemical
composition of the YSCCSs were investigated by X-ray
photoelectron spectroscopy (XPS) in the region of 0-1100 eV.
Sure enough, wide scans survey of the sample showed the
presence of C 1s, N 1s, O 1s, Si 2p and Co 2p (Fig. 3e). The
weak peaks of Co 2p at ~780 and ~795 eV can be observed,
which could be attributed to the Co3O4 characteristic peaks Co
2p3/2 and 2p1/2 level, respectively.57 However, the cobalt atomic
percent test by XPS (2.7 %) is less than the actual value by ICP
(13.9 %), which may be due to the fact that the presence of
silica shell shields the Co signals. Furthermore, the high
resolution Co 2p spectrum is given (Fig. 3f). Four significant
peaks could be observed, the peaks of 780.9 and 796.1 eV
correspond to Co2+ species and the peaks located at 779.6 and
795.1 eV are characteristic of Co3+. The high resolution Co 2p
spectrum could be regarded as the further demonstration for the
successful change cobalt atoms to cobalt oxide.58
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DOI: 10.1039/C8TA03565B
Fig.
nanoparticles
4
The proposed the formation process of the core-shell ZIF-67@SiO2
On the basis of above observations, we proposed the formation
process of the ZIF-67@SiO2 nanoparticles (Fig. 4). It was for the
first time used here to in situ monitor TEOS and ZIF-67 reaction
process by pH meter. The in-situ experiments were conducted by
added TEOS and Co(NO3)2 solution to the 2-MeIm solution,
respectively. As shown in figure S3, the pH value of system
decreased slightly from 10.62 to 9.95 when TEOS was added. The
whole hydrolysis process needed approximately 640 s. However, the
pH value was decreased rapidly (about 40 s) when Co(NO3)2
solution was added, which indicating the nucleation of Co-zeolitic
imidazole framework much faster than the hydrolysis/condensation
of TEOS.59-61 By zeta potential test, We found that in the alkaline
circumstance pure ZIF-67 nanoparticles appeared positive potential
(+3.8 eV). However, compound ZIF-67@SiO2 nanoparticles
appeared negative potential (-3.6 eV), which indicated that ZIF-67
and TEOS might be inter-attracted by superficial electrostatic
interaction. In the process of reaction, ZIF-67 was nucleate fast.
Subsequently, the TEOS on the surface of the ZIF-67 was
hydrolysis/condensation to form the SiO2 shell. Therefore, reaction
rate and electrostatic interaction are likely to the key to form of the
ZIF-67@SiO2 structure. Subsequently, catalytic experiments were
conducted to investigate the constitutive relationship between
structural constitution and catalytic performance.
Catalytic performance
Fig. 5 Degradation efficiency of BPA under different reaction systems with 25 min,
Reaction conditions: [BPA] = 20 mg/L, [PMS] = 0.1 g/L, [Catalyst] = 0.1 g/L, T = 298 K
The catalytic activities of various reaction systems were
evaluated by degrading BPA (Fig. 5). Approximately 13% of
BPA is removed with 25 min when only presence of PMS,
indicating that PMS of self-decomposition is weak. Meanwhile,
YSCCSs catalyst itself can remove only 5% of BPA by static
adsorption. As comparison, when 10 mg of Co3O4 and Co3O4/C
are added the degradation system containing 0.1 g/L of PMS,
about 75% and 80% of BPA can be removed within 25 min,
respectively. Under the same concentration of BPA and PMS
Fig. 3 The powder XRD patterns (a), N2 sorption isotherms (b), Raman spectra
(c), and FTIR spectra (d) of YSCSs and YSCCSs; the Wide-range XPS spectrum (e)
and Co 2p spectra (f) of YSCCSs.
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dose, catalyst YSCSs can remove 99.1% of BPA within 23 min, completely degraded within 2 min at 35 ˚C. AccVoiredwiAnrgticletoOntlhinee
indicating hydrophilic silica plays an important role in previously reported literature, the kDinOeIt:i1c0.10d3a9t/aC8TfAo0r356th5Be
dispersion of catalyst, which is demonstrated by water contact degradation of BPA is close to a first-order rate equation,10, 61
angles experiment (Fig. S4). Encouragingly, a significantly as follows equation (Eqn. (1)):
푐푡
ln = −푘푡
enhanced of degradation efficiency can be found when catalyst
YSCCSs was employed, which displayed much higher
efficiency and the degradation rate is more than 5 times
compared with Co3O4. By the ICP test, 0.1 g of YSCCSs,
YSCSs and Co3O4/C contain active component Co3O4 are 0.017,
0.033 and 0.0472 g, respectively. Therefore, when the reaction
is based on the same cobalt weight, YSCCSs will show better
performance. The comparison of different previous reported
catalysts for PMS activation are listed in table 1, this result
indicates that the YSCCSs show a remarkable improvement for
degradation of organic pollutant.
(1)
푐표
where k is the rate constant, ct and co is the real-time and
initial concentration of BPA, respectively. The temperature of
reaction system is 15, 25, 35 ˚C corresponding to the rate
constants are 0.57, 0.75, 0.96 min-1.The k values at the different
temperatures can be used to determine the activation energy
using the Arrhenius equation as follows (Eqn. (2)):
ln 푘 = ln푘0 − 퐸
(2)
1
푎
푅 푇
where Ea is Arrhenius activation energy (kJ mol-1); k0 is the
Arrhenius factor (g mg-1 min-1); R represent the universal gas
constant (8.314J mol-1K-1); and T is the reaction system
temperature (K). The Arrhenius activation energy Ea is 19.2 kJ
mol-1, which is lower than reported previously, indicating that
the YSCCSs have a significant catalytic activation in BPA of
degradation.62
Table 1 Comparison of different catalyst for PMS activation.
Reaction Conditions
Remove
efficienc
y (>95%)
Pollutant
Oxone
dose
Catalyst
Catalyst
Ref.
(ppm)
dose
(g/L)
Effects of catalyst dose, BPA dose and concentrations of BPA
(g/L)
In the process of BPA degradation, catalyst and PMS play a
crucial role in kinetic model. Thus, various catalyst doses and
PMS doses are investigated in detail. Under the same PMS dose
condition, degradation rate of BPA is rising with the catalyst
dose increasing, and the rate constant k is 0.42, 0.87, 1.7 and
3.1 min-1 corresponding to the catalyst concentration 50, 100,
150 and 200 mg/L (Fig. 6c). When catalyst dose concentration
is 50 and 200 mg/L, the degradation efficiency of BPA can
reach 99% within 10 and 1 min, respectively. The results
indicated that increase the amount of active site could
accelerate the reaction rate. Subsequently, different dose of
PMS are investigated (as shown in Fig. 6d). Approximately 85%
degradation rate can be obtained when PMS concentration is 50
mg/L (the dosage of PMS/BPA = 2.5/1). This results show that
YSCCSs have high efficiency for PMS activation. In addition,
by analyzing the rate constant k in different catalyst doses and
PMS doses condition, we can see catalyst have a more obvious
influence than PMS in the degradation system. In addition,
different concentrations of BPA in the system are also studied
(Fig. 6e). Satisfactory, approximately 95% degradation rate can
be achieved within 1 min when BPA concentration is 10 mg/L,
demonstrating that catalyst has a higher catalytic efficiency in
low concentration of organic pollutants. However, only 90%
BPA is degraded completely when BPA concentration is 40
mg/L, which may be the fact that PMS dose is too little to
FexCo3-xO4
Fe-DPA+
BPA(20)
BPA(15)
4-CP(100)
BPA(5)
0.2
2
0.1
0.5
60 min
15 min
30 min
5 min
68
69
Co3O4@MOFs
CuFe2O4-Fe2O3
0.25
0.36
0.1
0.2
0.5
0.1
0.5
31
0.2
12
Co3O4/CC
BPA(10)
Phenol(30)
AY(500)
0.1
7 min
70
CuO-Co3O4@MnO2
0.08
0.23
0.1
80 min
7 min
13
ZIF-67/PAN
71
YSCCSs
BPA(20)
5 min
This work
Effects of pH on the degradation of BPA
Various condition parameters of pH are indispensable to
evaluate the catalytic activity of the catalyst for BPA in
practical application. The initial pH plays a significant role in
the process of activate PMS.62-64 Hence, different pH values
from 7.0 to 10.0 are studied in the degradation system (as
shown in Fig. 6a). When the pH value is neutral, about 90% of
BPA can be degradation within 5 min in the system. With the
increasing of initial pH, the degradation efficiency of BPA has
an obvious improvement, and when the initial pH is 9.0
achieves the best. However, when pH is more than 10.0, PMS
suffer from self-decomposition (HSO5‾ react with OH‾)
generating •OH instead of SO4 ‾, diminishing the catalyst
•
efficiency.65-67 The results indicate that weak alkaline condition
in favor of promoting the efficiency of catalyst, and strong
alkaline is disadvantageous to catalytic reaction process.
•
generate enough SO4 ‾ for degradation of BPA.
Effect of anions and organic matter
−
−
Effects of temperature on the degradation of BPA
The anions (Cl−, HCO3 and H2PO4 ) and organic matter
would react with radical and reduce the efficiency of catalytic
in the degradation system.72 Therefore, it is necessary to
evaluate the effect of anions and organic matter (Fig. 6f). Cl− is
widespread in the natural water and generally believed that the
presence of Cl− demonstrate an inhibitory effect in degradation
system of organic contaminant. However, the degradation
In addition to the pH, temperature has been also investigated
as a crucial factor in degradation system. Thus, the temperature
range from 15 to 35 ˚C is selected to study the impact. As
shown in Figure 6b, with the increase of reaction temperature,
the degradation efficiency of BPA has
improvement. Encouraging, more than 80% of BPA is
a
significant
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curves of BPA nearly overlap with the control experiment when the PMS degradation system (Fig. 7b). As we canVsieeweA,rwticliethOntlhinee
the concentration of Cl− is 25 mM, indicating that presence of reaction time going on from 0 to 10 min, leaching of cobalt ions
DOI: 10.1039/C8TA03565B
Cl− shows a neglectable impact on catalytic efficiency. The increasing dramatically in the system of bare Co3O4-PMS. As a
−
same result can also be obtained when 25 mM of H2PO4 is comparison, the leaching value at a lower level of 0.066-0.093
existed in the degradation system. Further study identified only mg/L in the system YSCCSs-PMS, which has a significant
90% of BPA is decomposed within 10 min when there is a high improvement compared with previously reported. This result
concentration of HCO3 in the system. Compared to Cl− and suggestion that the responsible role of the SiO2 shell as a barrier
−
−
H2PO4 , the inhibitory effect is significant. HA is selected as to alleviate the external erosion toward Co3O4/C core. Hence,
organic matter in the system, only 67% of BPA is removed by SiO2 shell not only plays a protective role for Co3O4/C core, but
the catalyst within 10 min when the concentration of HA is 40 also inhibits the cobalt ion leaching to the reaction system.
mg/L, this result may be due to generate radicals is consumed
by the organic matter.
Fig. 7 (a) Consecutive runs of the catalytic activity of YSCCSs on BPA removal efficiency;
(b) The leaching of cobalt ions in the degradation process. Reaction conditions: [BPA] =
20 mg/L, [PMS] = 0.1 g/L, catalyst = 0.1 g/L, initial pH = 9.0, T = 298 K.
Probable degradation and synergistic mechanism
According the reported literatures,6 the mechanism of Co3O4
•
reacts with PMS to generate SO4 ‾as follows (Eqn. (3)-(5)):
•
Co3+ + HSO5 ‾ → Co2+ + SO5 ‾ + H+
(3)
(4)
(5)
•
Co2+ + HSO5 ‾ → Co3+ + SO4 ‾+ OH ‾
•
2 ‾
SO4 ‾+ OH- → ·OH + SO4
•
SO4 ‾/ ·OH + BPA → intermediates + H2O + CO2 (6)
In the activation process of PMS, electrons transfer from the
catalyst to PMS similar to the classical Fenton reaction.73 Co3+
of Co3O4 as active sites reacts with HSO5‾ to generate Co2+ and
SO5 ‾ (Eq. (1)), meanwhile, Co2+ reacts with HSO5• ‾ to yield
•
•
Co3+ and SO4 ‾. Co2+/ Co3+ can never be exhausted and just go
Fig. 6 effects of different initial pH (a); reaction system temperature (b); catalyst dose
(c); PMS dose (d); BPA concentration (e); ions and organic matter (f) on the BPA
removal efficiency. Reaction conditions: [BPA] = 20 mg/L (for a, b, c, d and f); [PMS] =
0.1 g/L (for a, b, c, e and f); catalyst = 0.1 g/L (for a, b, d, e and f); T = 298 K (for a, c, d, e
and f); initial solution pH = 9.0 (for b, c, d, e, f).
•
on changing in cycles. The generated SO4 ‾ possesses powerful
oxidizing property, which can oxidize the target organic
pollutant BPA to intermediates, innocuous CO2 and H2O.74
•
From the eqn. (5) we can see that OH is also formed in typical
chain reaction.75 Therefore, future probe the reactive species by
EPR experiments are essential (as shown in Figure 8a). Faint
signals of radical are detected in test when PMS are added to
the reaction system without catalyst, the result indicate that
PMS can decompose itself generate radicals slowly. However,
sharp characteristic radical signals with the intensity ratio of
1:2:2:1 are observed when catalyst and PMS are added, the
characteristic signals belong to DMPO−•OH product (αH = αN=
Recyclability and stability
Recyclability of catalyst is a crucial estimate to potential
application. The experiment was conducted by recovering the
catalyst material from the reaction system and reusing them in
the next run under the same condition. As shown in figure 7a,
we can see the recycled catalyst still maintain a highly active
and the degradation efficiency of BPA also exceed 90% within
7 min after 5 cycles. From the stability investigation and
reusability test we can see that the YSCCSs are responsible for
water treatment application. The magnetic property of YSCCSs
is shown in Fig. S5, the saturation magnetization of YSCCSs
reaching 14.5 emu g-1, which is benefit for the recyclability of
catalyst. The stability of the catalyst is a vital index for practical
application. The average cobalt ions concentration in urine and
serum of people are about 0.1 µg L-1 and 0.1–0.3 µg L-1,
respectively. Stability investigation was conducted by ICP-MS
measurements to detect the possible leaching of cobalt ions in
•
14.9 G). Beside the OH characteristic peaks, the peak of
•
DMPO−SO4 ‾ (αN = 13.2 G, αH = 9.6 G, αH = 1.48 G, αH =
•
0.78 G) also can be found, which indicated the presence of SO4
‾.76
It has been widely recognized activation of PMS can
•
•
generate OH and SO4 ‾, which possess powerful oxidizing
property. Radical quenching test was conducted using TBA
•
•
(mainly reacts with OH) and ethanol (mainly reacts with OH
•
and SO4 ‾) as scavenger to probe which one plays a major role
in the degradation system (Fig. 8b). When 0.2 M TBA as an
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Journal Name
•
effective radical scavenger for OH was added to the contain solved the important challenge in the practical appVliiecwatAiroticnle. OTnhlinise
DOI: 10.1039/C8TA03565B
YSCCSs and PMS system, the BPA of degradation efficiency work expands the development of MOF-derived nanoreactors
was slight decline. As a contrast, the same condition was for environmental remediation.
conducted except that TBA was used in place of ethanol, only
approximately 70% of BPA was degradation within 10 min.
Both Radical quenching test and EPR experiment demonstrate
Acknowledgements
•
•
the fact that, both OH and SO4 ‾ may be reactive species, and
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 51478224), the
priority academic program development of Jiangsu higher
education institutions.
SO4 ‾ plays a leading role in the PMS system.77
•
Notes and references
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Graphic abstract
The possible synergistic mechanisms of BPA degradation on the YSCCSs